Electrical connection for aluminium conductors in automotive

Electrical connection for aluminium conductors in automotive
EXAMENSARBETE INOM MATERIALDESIGN,
AVANCERAD NIVÅ, 30 HP
STOCKHOLM, SVERIGE 2017
Electrical connection for
aluminium conductors in
automotive applications
Prestudy of available solutions for electrical
connection methods of aluminium cables
EMILIA HAMEDI
KTH
SKOLAN FÖR INDUSTRIELL TEKNIK OCH MANAGEMENT
Abstract
Due to increasing weight of electrical component and wiring harnesses in a vehicle contrary to the
demand of light constructed vehicles as well as the constantly increasing and fluctuating price of
copper compared to aluminium’s stable and far lower price, the use of aluminium conductors as an
alternative have been promoted.
This thesis work lay theoretical research of the available methods used for electrical connection of
aluminium conductors in order to increase the knowledge about the available termination
techniques.
Due to aluminium’s characteristics such as lower conductivity and strength, tendency to form oxides
and relax over time, differences in thermal expansion coefficient and high potential for galvanic
corrosion, there is a risk of deterioration and degradation of the connection if the termination of
aluminium conductors is not done correctly without being aware of the challenges when it comes to
aluminium connection.
The founded solutions are different welding and soldering techniques such as friction welding,
ultrasonic welding, resistance welding, plasma soldering and many other modifications of
conventional crimp.
A robust termination system that faces all those challenges and ensure a reliable connection during
the entire life length of the vehicle and in order to inhibit corrosion different type of sealing of the
contact interface will be required.
In order to evaluate the performance of the founded connection method, testing with evaluation of,
tensile strength of conductor to contact attachment, tightness demand, corrosion resistance,
vibration and heat evolution at the contact attachment have to be conducted.
Keywords: Connector, electrical contact, termination, aluminium conductor
I
Definitions and Abbreviations
Contact area
Area of metal-to-metal attachment between the conductor and terminal
Voltage drop
Voltage across a conductor or component because of the electrical current flow
Contact
Resistance
Resistance at the contacting interface
Alternator
Converter of energy/power by transfer of engines mechanical work into electricity
and recharging the battery
Chassis
Part of the vehicle which includes brake system, frame, electrical system, fuel
tanks, suspension and wheels
Cab
An enclosed space in crew compartment, trucks and busses
ISO
The International Organization for Standardization
Powertrain
Power providing component which deliver to consumers that include gearbox,
propeller shafts, engine and etc.
Mechanically deformation and compression of splice, terminals or connectors to
the conductor with the aim to ensure mechanical and electrical connectivity
Crimping
Welding
Connector
Process for joining materials, usually metals by heating the surface to the melting
point.
A connecting device for termination of conductor and fixed to the cable for
establishing an electrical connection
A device used to create electrical circuit and join electrical termination.
Galvanic
corrosion
Electrochemical process when two metals are in electrical contact with each other
and one metal have higher tendency to corrode
Voltage drop
Electrical current flow in a conductor or component resulting into voltage across
the conductor or component.
Dimensional changes owing temperature changes or temperature shock
Terminal
Thermal
expansion
Cycling
Repeated process
Serration
CSA [mm2]
Asperities on terminal surface with the aim to ensure additional contact area and
fixed gripping of conductor
Cross section area
IACS
International Annealed Copper Standards
SEM
Scanning Electron Microscope
DC
Direct current, electron flow in one direction
AC
Alternating current, when the electrons change their direction
IDC
Insulation displacement connection
II
Table of Contents
1
2
Introduction .................................................................................................................................... 1
1.1
Aim and objective of the study ............................................................................................... 1
1.2
Outline of the Study ................................................................................................................ 1
Background ..................................................................................................................................... 3
2.1
Automotive cables .................................................................................................................. 3
2.2
Cable conductors .................................................................................................................... 5
2.2.1
2.3
3
Conductivity .................................................................................................................... 5
Conductor materials ............................................................................................................... 7
2.3.1
Copper and copper alloy as conductor material............................................................. 8
2.3.2
Aluminium and its alloys as conductor material ........................................................... 10
2.3.3
Aluminium as an electrical conductor .......................................................................... 11
2.4
Comparison of aluminium and copper conductors .............................................................. 13
2.5
Application of Aluminium wires in automotive industry ...................................................... 16
2.6
Current load capacity ............................................................................................................ 18
Current electrical connection methods ........................................................................................ 20
3.1
Contact physics ..................................................................................................................... 20
3.2
Contact area .......................................................................................................................... 20
3.3
Automotive Connector contacts ........................................................................................... 22
3.4
Cable Termination ................................................................................................................. 23
3.4.1
3.5
Crimping ........................................................................................................................ 25
Welded connectors ............................................................................................................... 27
3.5.1
Thermite (exothermic) welding .................................................................................... 27
3.5.2
Friction welding............................................................................................................. 27
3.5.3
Resistance welding ........................................................................................................ 28
3.5.4
Resistance brazing......................................................................................................... 28
3.5.5
Resistance butt welding ................................................................................................ 28
3.5.6
Ultrasonic welding ........................................................................................................ 29
3.6
Parameters affecting the power connections performance................................................. 30
3.7
Parameters affecting the contact area ................................................................................. 30
3.8
Factors affecting the reliability of power connections ......................................................... 31
3.8.1
Plastic and elastic deformation of power connectors .................................................. 33
3.8.2
Corrosion ....................................................................................................................... 34
3.8.3
Thermal expansion ........................................................................................................ 35
III
4
3.8.4
Creep ............................................................................................................................. 36
3.8.5
Creep and stress relaxation .......................................................................................... 37
3.9
Fretting .................................................................................................................................. 38
3.10
Intermetallic compounds ...................................................................................................... 39
3.11
Degradation of connectors ................................................................................................... 41
3.11.1
Economical consequences of contact degradation ...................................................... 43
3.11.2
Prognostic models for contact remaining life ............................................................... 44
Study of electrical connection, connectors and termination techniques ..................................... 46
4.1
Electrical connections made of aluminium ........................................................................... 46
4.1.1
4.2
Types of connectors .............................................................................................................. 47
4.2.1
Plug-and-socket connectors.......................................................................................... 48
4.2.2
Common features of connectors .................................................................................. 49
4.2.3
Electrical Terminals ....................................................................................................... 50
4.3
High power connectors ......................................................................................................... 52
4.3.1
Improvement of electrical connection.......................................................................... 52
4.3.2
Electrical connections made of aluminium ................................................................... 57
4.4
Other parameter of electrical contacts................................................................................. 57
4.4.1
4.5
5
Aluminium wiring connection ....................................................................................... 46
Connections in high-vibration environments ............................................................... 58
Termination technique of aluminium cables ........................................................................ 63
4.5.1
Terminal with integral oxide breaker............................................................................ 65
4.5.2
Favourable electric connection of wire harnesses in automobiles .............................. 66
4.5.3
Plasma soldering of copper connector to an aluminium conductor ............................ 67
4.5.4
Compression ................................................................................................................. 67
Methodology for testing electrical connectors in Scania trucks and busses ................................ 68
5.1.1
Tensile strength of conductor to contact attachment .................................................. 68
5.1.2
Tightness demand and corrosion resistance ................................................................ 69
5.1.3
Thermal aging and contact resistance .......................................................................... 70
5.1.4
Vibration and temperature ........................................................................................... 70
5.1.5
Heat evolution at the contact attachment ................................................................... 70
5.2
Experimental procedure ....................................................................................................... 70
6
Discussion...................................................................................................................................... 71
7
Conclusion ..................................................................................................................................... 73
7.1
Challenges of electrical connections for aluminium conductors .......................................... 74
IV
8
Future work ................................................................................................................................... 75
9
References .................................................................................................................................... 76
V
1
Introduction
The weight of electrical and electronic components and wire harnesses are increasing due to added
functions such as safety and comfort in all type of vehicles. Weight and energy saving are the main
driving force for development of alternative cables used in vehicles [1] [2]. It is estimated that an
approximate weight reduction of 10 % of a passenger vehicle can contribute to 3-4 % less fuel
consumption, and about 5 % less fuel consumption can be obtained for heavy vehicles such as
busses and trucks if substituting heavy material by lighter materials [3].
The commonly used conductors and wiring harness systems are made of copper which compared to
aluminium is expensive with volatile prices and heavier than aluminium. The demand for
environmental friendly vehicles has led to research and development of high voltage wiring harness
system that result into weight saving improved fuel efficiency, low carbon society and more cost
efficient vehicles [4]. The average weight of wire harness system of a vehicle from 1960 is 3 to 5 Kg
compared to 50 to 70 Kg in today’s vehicles. Although the weight of the wiring harness is about 30
Kg per vehicle, a weight saving of 30 % is possible if copper is replaced by aluminium. Cross sectional
area of minimum 1.5 mm2 enables weight reductions up to 48 % if the copper conductors are
replaced by aluminium conductors [5]. An average passenger vehicle has potentials of weight
reduction of 8.8 Kg if the current, copper made electrical system with CSA range 2.8-80 mm2 is
replaced by aluminium cables. Due to this the potential weight saving is expected to be higher in
case of bigger vehicles such as busses and trucks.
It should be mentioned that aluminium conductors with large cross section area which support high
current are mainly being used in battery cables aluminium conductors in cable uses is growing and
being used in in many successful electrical power distributions. A variety of stranded wires and
conductors are being used in modern vehicles, but high voltage wire harnesses with newly kind of
electrical connection is being developed in order to meet the demand of weight reduction [6] [7]. In
order to achieve a consistent conductance in the wire harness and a reliable electrical connection
the different materials properties must be taken into account when developing a new electrical
component.
1.1 Aim and objective of the study
Different areas of the vehicle are exposed to different environments which require a robust
electrical connection that last the entire life length of the vehicle. The main objective of this study is
to put forward state of the art regarding the available connection of aluminium cables used in
vehicles. The objectives consist of:
•
•
•
Investigate the available methods used for electrical connections of aluminium cables and if
those methods are applicable solutions that fulfil requirements of Scania.
Increase the knowledge about different aluminium conductors and the available electrical
connection methods
A comparison between those methods in order to list pros and cons of each approach
1.2 Outline of the Study
Scania has one of the world’s most sustainable transport solutions which always develop and
optimize new solutions in order to meet the market demands and offer the most sustainable
1
solutions. When it comes to electrical and electromechanical components Scania have done some
attempts to lower the average cable harness components by for instance a new interface design
between the cab and chassis which may result into an average weight saving of 3.96 Kg. An
approximate cost saving of 98 SEK based on today’s electrical system, current cable conductors and
the present copper prices if a change to aluminium would be performed [3].
The electrical and electromechanical group at Scania are responsible for cable harness components
such as cables, connectors, terminals and protection hoses for Scania vehicles. Development of new
cable harness solutions in terms of weight saving, fuel economy, low carbon society and gas
emissions, Scania is investigating the possibilities of replacing copper cables by aluminium cables and
the challenges are to find a suitable electrical connection method that fulfil the requirements in
different environment of the vehicle. Scania vehicles can be divided into three different zones for
cable harness systems, namely chassis, powertrain and cab with different environment i.e. different
vibration levels, operating temperatures and CSA. The battery is located in the chassis zone where
the cables needs to cope with temperatures up to 100 °C, high vibrations and have higher current
carrying capacity since it is the battery cable that carry the current which starts the motor, single
core cables with large CSA is required for the chassis zone. Power train is the zone that has to cope
with the highest temperatures and vibrations. Cables in this zone are both single core and multicore
cables with size range of 25-125 mm2. The cab zone is the area with lowest vibration and current
carrying levels where the temperatures do not exceed 80 °C. The cables for cab zone are in the
range of 0.5-6 mm2 except the sensor cables which can have even lower CSA.
Due to this the cable harness system poses challenges when solving the electrical connection if the
copper cables are replaced, the new type of connecting method must be robust enough and last the
entire life length without breakdowns furthermore be easy to use in service and aftermarket.
Evaluation of the consequences, performances and potential risks according to internal Scania
product requirements needs to be done in order to be able to determine if it is possible to replace
copper cables by aluminium cables.
2
2
2.1
Background
Automotive cables
Electrical cables in vehicles have to provide a compact and reliable method to transmit energy and
power from alternator or energy accumulators (i.e. batteries) to consumers such as electrical panel
and lightening system. It is also the connecting medium for the embedded system which distributes
the power and signals within the vehicle [8]. Depending on where in the vehicle a cable is aimed to
be used, the length, current carrying ampacity and acceptable voltage drop will vary. Those
properties are given by
𝐺𝐺 = 𝜎𝜎𝜎𝜎/𝐿𝐿
Equation 1
and depend on, electrical conductivity σ, cross section area A and length L. Society of automotive
engineers (SAE), standardize the cable sizes for different current carrying ampacity. A cable assembly
consist of an electrical conductor with different insulating material depending on operating
condition. Connection of those cable assemblies are done by different terminating connectors in
solder or solderless condition [9]. Classification of the cables are done according to international
standards and divided depending on their ability to withstand temperature range shown in Table 1
[10] [11].
Table 1. Classification of the cable depending on temperature to withstand.
Class
Temperature range
A
-40°C to 85°C
B
-40°C to 100°C
C
-40°C to 125°C
D
-40°C to 150°C
E
-40°C to 175°C
F
-40°C to 200°C
G
-40°C to 225°C
H
-40°C to 250 °C
There are two type of cables in which they can be divided into, solid cable which is a single stranded
conductor or stranded cable which consist of a number of solid conductors that are twisted to a
single conductor. The stranded cables are the most common cables used in vehicles due to their
flexibility, long flex-life and reliability. The difference between single-core and multi-core cables are
the core number of stranded cable. When the flexibility is not the prime priority, single-core cables
are used, for instance as engine starter cables, battery cables and power cables, while multi-core
cables are used for airbag release and antilock system, where the cables need to be customized for
specific requirements and be accommodated in small installations. Double insulated multi-core
cables can be used in trucks in order to increase reliability, vibration resistance and safety. The
3
shielded cables are used for components where protection from electrical noise and
electromagnetic radiation without any effect on signals is essential. In addition to those cables, other
cables like high-voltage cables, flat cables, data cables, ignition cables, coaxial cables and special
cables are used in automotive vehicles. For instance special cables which need a special protection
are used for communications within the vehicle i.e. pressure, temperature, gearbox and sensors
while flat cables are used in areas where the installation space is limited but an intensive amount of
devices have to be installed, for instance bumpers, door harnesses and roof liners dashboard.
When it comes to the insulating materials of the cables, the most commonly used materials are
extruded electric type with core material component of, polyamide (PA), polyvinyl chloride (PVC),
polytetrafluorethylene (PTFE), thermoplastic polyester (TPE), polyethylene (PE), polyethylene
crosslinked (PE-X), polypropylene (PP ) etc. [12]. According to SAE standards, there are two main
categories of cable insulators used in automotive vehicles, cross-linked and PVC. The difference
between those two is the temperature range, PVC cables can be used in lower temperatures, up to
80 °C compared to cross-linked which can withstand temperatures up 125°C and is more resistant to
aging, heat and abrasion. In addition to insulation material, when selecting the insulating layer one
have to consider the required performance factors according to Table 2 [13]
Table 2. Performance factors of insulating layer.
Performance factors
Characteristics
Thermal performance
Expansion and contraction
Compatibility with ambient
Electrical properties
Insulation resistance
Dialect constant
Mechanical characteristics
Tensile strength
Toughness
Abrasion resistance
Chemical resistance
Acids
Oils
Stability when exposed to flame
In addition to the SAE standards there are Japanese and Germany standards which are in accordance
with international standards ISO 6722 which specify the automotive cables type and name, technical
parameters, conductors and insulation materials.
For cable termination there are two main methods, solder type or solderless type where solderless
types refers to mechanical termination and solder type is when the electrical conductor is fastened
to a terminal and a strong electrical connection is provided. Crimping of stranded cables or solid
cables is a simple installation method where the conductor is compressed and attached to a terminal.
Crimping method creates an electrical path with high current carrying ability and low resistance
while the electrical and mechanical connectivity depends on type of solder. Crimping termination
compared to solder termination has the advantages of selecting terminal, cable and insulating
4
material without out weighting the thermal characteristics and the material effect on humans since
the main concern of crimping method is to select the right tool for connectors/terminals and cables.
However loosening of the terminal and disturbances of electrical path cannot be ignored since
occurrence of stress relief, shock and vibrations of terminating point due to applied mechanical
forces is possible.
2.2
Cable conductors
An assembly compromising of a conductor which is the current carrying part, protection, insulation,
shielding and termination is a conductor system. The conductor can be a solid wire or strands which
is a combination a wires that are not insulated from one another or a cable which is a combination
of conductors insulated from one another with the main function to yield electrical current through
the system while the insulation’s function is to restrain the current flow to the conductors in the
system. The current carrying part is isolated from external influences by the protective part
(example sheath) while the shielding’s function is reducing the effect of electric and magnetic field
on the cable [14].
2.2.1 Conductivity
Conduction in terms of path of free electrons makes an element conductive and the larger number
of free electrons the better conductor. In addition to that the degree of conductivity is affected by
for instance plastic deformation, alloying elements, structural defects and reduction of the mean
free path of electrons which result in heating. Resistivity, conductivity and physical changes are
temperature dependent and the conductor materials property varies linearly with the temperature.
These temperature dependent changes for a linear conductor can be expressed as
𝑅𝑅𝑇𝑇 = 𝑅𝑅0 [1 + 𝛼𝛼𝑅𝑅 (𝑇𝑇 − 𝑇𝑇0 )]
Equation 2
𝑙𝑙 𝑇𝑇 = 𝑙𝑙0 [1 + 𝛼𝛼𝐿𝐿 (𝑇𝑇 − 𝑇𝑇0 )]
Equation 3
Where RT is resistance, and lT is the length of the conductor at a temperature T, R0 is the
temperature at 20 °C and l0 is the length at 20 °C, αR is the electrical resistance coefficient while αL is
the coefficient for lineal expansion [14]. The electrical characteristic of a material and its ability to
accommodate electrical charge is specified by electrical conductivity σ. Reciprocal to conductivity is
resistivity, a materials ability to resist electrical current flow, which convert the electrical energy into
other form of energy, most commonly into heat. Materials which have high resistivity are good as
insulators and for a material to be a good conductor it must have low resistivity with order of 10-8
Ωm. All metals and their alloys can be used as electrical conductors in different applications such as
electrical contacts, power distribution lines, resistor and heating elements and electricity
transmission. Electrical conductivity is usually stated as percentage of international annealed copper
standards (IACS), where annealed copper conductivity is 5.8001⨯107 S/m is 100 % IACS at 20°C. Gold,
silver, copper and aluminium are considered as good electrical conductors where gold has a
conductivity of 200 % IACS. The electrical resistivity of some solid materials is shown in Table 3.
5
Table 3. Electrical resistivity of solid materials at 20°C
Material
Order of resistivity
Gold
10-9
Silver
10-9
Copper
10-9
Aluminium
10-8
Nickel
10-7
Iron
10-6
Glass
105
Polyethylene
1011
PVC
1015
Conductivity and resistivity are properties that depend on the materials alloying elements, impurities,
plastic deformation and temperature and are usually stated at room temperatures. The temperature
dependency for instance can be expressed by
𝜌𝜌1 = 𝜌𝜌2 �1 + 𝛼𝛼(𝑇𝑇1 − 𝑇𝑇2 )�
Equation 4
Where ρ1 and ρ2 are resistivity at two different temperatures and α is temperature coefficient
which is positive for conductors, meaning that resistivity is raised if the temperature is increased
[14].
2.2.1.1 Effect of lattice imperfection on conductivity
Addition of alloying elements and impurities decreases the conductivity in the lattice much more
than any other lattice imperfection. The degree of reduction depends on the metallurgical state of
the impurities, type and concentration of them for instance the presence of impurities in solid
solutions have larger conductivity reduction effect than as incorporated in, a second phase of the
microstructure. Impurities in solid solution that result into disturbances in the lattice periodicity on
atomic scale increase the electrical resistance more than perturbations caused by a second phase on
a macro scale. For electrical purposes, purity of a solid conductor is essential since there is a linear
relationship between electrical resistivity and concentration of the impurities even though there is a
limit of solubility of the impurities. However, a high-purity conductor material is not the solution for
prime quality conductors with low resistivity, since increasing the purity of conductor material lower
the mechanical properties and increase processing cost significantly. Due to this limited addition of
particular solutes may substantially enhance the mechanical response of the conductor without
deteriorating its conductivity [14].
2.2.1.2 Grain boundaries
According to previous studies, grain boundaries and impurities species or segregated alloy at the
grain boundary have a significant effect on the transport properties and performance of the
conductors since electron scattering at the grain boundaries contribute to the resistivity. When an
6
electron crosses the boundary and enter a new region, it cannot continue in the same direction and
the same velocity which is due to anisotropy of elastic and electronic properties of the solid at the
grain boundary [14].
2.2.1.3 Vacancies
The concentration of vacancies in a solid conductor can be appreciated by rapid quenching from a
higher temperature and by irradiation with high energy particles. Produced vacancies by this method
enable pronounced effect on the electrical resistivity in pure metals [14].
2.2.1.4 Dislocations deformation
Plastic deformation leads to reduction of the metal ductility, harden the metal, increase its electrical
resistivity and tensile strength. For many types of conductors, an increase in tensile strength is useful
and is achieved by cold working. The increase in electrical resistivity in conductors is mostly caused
by plastic deformation when scattering of conduction electrons which is due to introduction of
dislocation into the lattice. The relation between, increasing in resistance∆ρ, due to plastic strain γ is
given by
∆𝜌𝜌 = 𝑎𝑎𝛾𝛾 𝑛𝑛
Equation 5
Where n and a is characteristics of the conductor material. A plastically deformed conductors’
tensile strength and electrical resistance can be reduced by annealing which in turn increase the
ductility [14].
2.3 Conductor materials
Conductors used for cables should maintain high thermal and electrical conductivity, have good
wear and abrasion resistance, high melting point and low cost. Changes in voltage drop and power
losses when the temperature changes, requires low temperature resistance coefficient. A higher
resistance of conductor leads to voltage drop and increase of power loss which affects workability
and solder ability of the conductor which is essential for connection and termination of the
conductor.
In addition to this, cable conductors should maintain high corrosion resistance in order to avoid
degradation caused by corrosion; further a mechanical strength that withstand the thermal stresses,
mechanical load and vibration; likewise enough ductility and flexibility which enables them to be
drawn in different sizes and shapes. The most common material used for electrical conductors are
usually copper or aluminium because of their suitable properties such as conductivity, tensile
strength and accessibility of the raw material. Copper is one the most commonly used material for
electrical conductors in e.g. flexible cabling, high/low voltage power cables, wiring of
motors/alternators/transformers, underground cables while aluminium is used in domestic wiring,
overhead transmission lines and busbars. Basic properties of copper and aluminium conductors and
their applications are summarized in Table 4.
7
Table 4. Basic properties of Aluminium and Copper [14].
Aluminium
Copper
EC-0
(A)
Al-Mg
(5005) (B)
Density (g cm3)
2.7
2.7
Al-MgSi
(6201)
(C)
2.69
8.94
Phosphor
Bronze
(95/5) (E)
8.86
Brass
(70/30)
(F)
8.53
Melting point (°C)
Thermal Conductivity (W
cm-1 K-1)
Linear Thermal expansion
Coeff.
(10-6 K-1)
Thermal Electrical
Resistivity Coeff.
(10-3 K-1)
Electrical resistivity µΩ cm
660.0
652.0
654.0
1083
1060
955.0
2.34
2.05
2.05
3.91
0.84
1.2
23.6
23.7
23.4
17.0
17.8
20.30
4.46
4.03
4.03
3.93
4.0
1.0
2.80
3.32
3.20
1.70
8.7
6.4
Tensile Strength (MPa)
83.0
200.0
330.0
220.0
345.0
330.0
Yield Strength (MPa)
28.0
193.0
310.0
69.0
140.0
110.0
Elastic Modulus (GPa)
Current Carrying Capacity
(%)
Specific Heat Capacity
(J/g/K)
Hardness (×102N MM-2)
69.0
69.6
69.6
115.0
110.0
110.0
80.0
OFHC
(D)
100.0
0.9
0.9
0.9
0.38
0.38
0.38
2.3
5.1
9.5
4.2
5.0
6.0
2.3.1 Copper and copper alloy as conductor material
Copper is a metal with high conductivity, solderability, weldability and formability. A variety of
electrical products such as shaped and flat busbars, tubes, wires and sheets can be manufactured by
drawing and rolling but in order to have high conductivity and be useful for electrical application
electrolytic refining for removal of Au, Ag, Au, Sb and impurities have to be applied. Electrolytically
tough pitch coper (ETP), or C11000 which is electrolytic refined copper are the most used in power
industry. The embrittlement of ETP copper is the main deficiency which occurs when the metal is
heated in hydrogen to temperatures above 370 °C since the present oxygen in the metal react with
the hydrogen and build steam which leads to internal cracking, using copper with lower oxygen
content prevent this problem. Phosphorus can be used as an effective copper deoxidizer but it is not
suitable for electrical applications since it decreases the conductivity and due to this electrolytic slab
that are melted and refined in oxygen free, inert gas and no metallic oxidizer process used instead. A
99.8 % pure copper with <0.005 % impurity called oxygen free high conductivity copper (OFHC) can
be produced. The copper conductivity is set according to the international annealed copper standard
(IACS), meaning % IACS is equal to 100 and IACS has a resistivity of 1.7241 µΩ cm. It is common to
8
define metals purity by ratio of resistivity at 5.2 and 273 K which varies between 150-500 for OFHC
copper. Copper has high corrosion resistance under atmospheric conditions and at room
temperature an oxide layer Cu2O forms at the surface which prevent further oxidation and at higher
temperatures an oxide layer of CuO forms at the surface when it is exposed to air. If copper is
exposed to air containing chlorine or ammonia compounds a considerable corrosion of copper may
occur. Improvement of the mechanical properties of copper is essential in order to be useful in
electrical applications. This usually result into reduction of the electrical conductivity hence
strengthening can be done by either additional alloying elements or cold working. By annealing the
cold-drawn pure copper at temperatures of 200-325 °C, softening can be achieved but presence of
impurities and the previous cold deformation may alter the annealing range. Additional alloying
elements or presence of impurities rises the annealing temperature while high degree of prior cold
deformation requires lower annealing temperature range. The effect of alloying elements and
impurities on coppers electrical conductivity is shown in Figure 1 [14].
Figure 1. Effect of alloying element and impurities on conductivity of copper.
9
Addition of alloying elements such as Mg, Ni, Ag, Cr and Mn and further treatment such as annealing
and hard drawing, makes the copper conductors usable in various applications. For example the
hard drawn conductor have higher mechanical strength than annealed copper, hence more suitable
for voltage cables, overhead transmission lines and underground cables, while annealed copper have
higher ductility and flexibility can be used as low voltage power cables and serve bending without
failure [15].
The main advantages of copper and the properties that makes it suitable as conductor material is
high electrical and thermal conductivity, low voltage drop and resistivity which compared to other
metals provide better voltage quality and less voltage losses for the same cross section. It has higher
chemical stability and corrosion resistant which helps avoiding oxidation accidents even though
copper is usually by nickel, tin and silver. High tensile strength flexibility and anti-fatigue enable
withstanding mechanical loads, long flex life, high stress levels, and high fatigue strength value of 62
MPa for annealed copper. Due to coppers low thermal expansion coefficient, it will not go through
many contraction and expansion cycles which minimize the stresses in terminating joints and this
makes copper conductors one of the most preferred electrical conductors and it has a wide field of
applications.
2.3.2 Aluminium and its alloys as conductor material
Due to aluminium’s light weight, availability, moderate cost, relatively good thermal and electrical
properties it has been considered as an alternative to copper for conductor applications in electrical
systems in recent years. [14]. Aluminium is normally not used in its pure state, alloying elements
such as Mg, Mn, Cu, Fe and Zn is usually added in order to enhance different properties. Aluminium
alloys are categorized as either cast alloys or wrought alloys where casting as treatment is used for
cast cat alloy to determine their shape while wrought alloyed are extruded, forged and rolled into
shapes such as foil, plate and sheet products. The wrought alloys are usually differentiated by a fourdigit number as shown in Table 5 where the first number stands for a series that is characterized by
the main alloying element. A prefix, EN AW (European standard) or AA (Aluminium association) is
used to denote the standards. There are eight different series with different physical, mechanical
and corrosion properties hence different behaviour and properties during service forming, welding
and surface treatment.
By heat treatment processes of the alloys, different properties can be achieved and the aluminium
alloys are also differentiated by being heat-treatable or non-heat-treatable alloys. Heat treatment
strengthens the alloy and the elements get homogenously distributed which changes the
microstructure and age hardening or precipitation may occur. The non-heat-treatable alloys can be
strengthened by cold-working during forging, rolling or annealing. Further, mechanical deformations
at ambient temperatures increase the metal resistance and strengthen it. Formation of vacancies
and dislocations in the structure which inhibit the atom movement and strengthen the alloy [3].
10
Table 5. Main properties and application of aluminium alloy series.
Series
No.
1xxx
Primary
alloying
elements
None
Heat
treatment
Non-heattreatable
Properties
Excellent workability
and corrosion
resistance, high
electrical and thermal
conductivity
No atmospheric
corrosion resistance,
good combination of
high toughness and
strength
2xxx
Cu
Heat-treatable
3xxx
Mn
Non-heattreatable
Good workability and
enough strength
4xxx
Silicon
Non-heattreatable
Lower melting point
Non-heattreatable
Good weldability and
corrosion resistance in
marine environment,
moderate to high
strength
5xxx
Mg
6xxx
Mg and
silicon
Heat-treatable
Highly formable and
weldable, high strength
and corrosion resistance
7xxx
Zn
Heat-treatable
High strength alloy
8xxx
Other than
the other
series
Depend on the alloying element; e.g.
8006 (Al-Fe-Mg): a combination of
strength and ductility at both room
temperature and higher temperature
8001(Al-Ni-Fe): High corrosion resistance
at higher pressure and temperature
Applications
Transmission, power grid,
lines (1350)
Aircraft alloy after
cladding with 6 b xxx
series or painting for
higher corrosion
resistance (2024)
Heat exchangers,
beverage cans, cook
utensils (3004)
Brazing alloys, welding
wires
Pressure vessels, storage
tanks and marine
applications (5083),
constructions and
building (5005), in
electronics (5052)
Marine frames and trucks
(6061), structural and
architecture applications
(6010)
Aircraft industry (7050
and 7075)
Domestic wiring(8177),
bearing alloys in trucks
and cars (8280 and 8081),
Al-Li alloys for aerospace
applications, power
generation (8001)
2.3.3 Aluminium as an electrical conductor
For the same length and resistance, aluminium conductor should have 60 % larger cross section area
compared to copper, while the weight of aluminium conductor is 48 % of copper with a current
carrying capacity of 80 % of that of copper conductor. Aluminium is ductile, it is softer than copper
and can be rolled into several µm thin foils with relatively high electrical and thermal conductivity.
11
Since aluminium’s mechanical strength is low it cannot be drawn into extremely thin wires. The
purity of aluminium and grade of cold work affect mechanical durability and resistivity. Commercial
aluminium has a resistivity of 2, 78 µΩ cm whereas that of high pure aluminium (99.999 %) has a
resistivity of 2,635 µΩ cm at 20°C. The commercial aluminium contains <0.015% Ti, Cr, Mn, V, <0.1%
Si and <0.02 boron. Addition of Ti and V minimize the effect of impurities on conductivity while
adding boron leads to transformation of those impurities into borides with low effect on electrical
conductivity. Alloying elements is essential for improvement of creep and tensile strength when the
metal is hard-drawn, since pure aluminium results into insufficient mechanical properties. The most
common used alloys for electrical applications are Al-Mg-Si or Al-Mg containing Co or Fe. The
drawbacks of aluminium conductors, preventing wider usage is their lack of a reliable and
economically viable termination. Oxidation of aluminium occurs immediately when it is exposed to
oxygen which generate oxide layer, Al2O3 on the surface which are an electrical insulator. The oxide
layer weakens the terminal and conductor connection which leads to a softened surface layer and a
significant weakened metal to metal contact which results into separation of the layers in long term.
The main challenges due to nonconductive surface are therefore to strand conductivity to the
terminal, the strand to strand conductivity and intermediate wire size [5]. Different methods such as
ultrasonic welding, friction welding, plasma welding, plating and brazing have been established in
order to overcome this problem. Most of those methods are either too expensive or require bigger
operator care with marginal mechanical or electrical operations in some cases [14].
Disadvantages of aluminium conductors can be summarized as:
•
•
•
•
•
•
Overheating of the connections may occur because of connection problem that generate
heat under electrical load.
Oxidation on the surface of the conductor.
The insulating layer increase overheating which challenges the safety and reliability of
connection.
Loosening of the terminal due to aluminium’s tendency to cold flow under pressure.
Thermal expansion owing coupling of dissimilar metals.
Aluminium should have larger gauge than copper for the same current carrying capability.
The weight and price of aluminium metal are the primary factors that make aluminium attractive as
conductor material. The advantages of aluminium are summarized as:
•
•
•
•
•
Aluminium compared to copper is available in ample supply while copper is resource limited.
This result into constantly stable and lower price of aluminium.
The conductors retain its intrinsic qualities after removal of the insulation material since it
does not adhere to aluminium which facilitates the recycling treatment.
Aluminium conductor unlike copper conductor has no effect on compound containing
rubber since no stearates is produced even under presence of oils.
Corrosion is attributed to the oxide layer formed at the surface of the conductor and usually
traced to connection joint between two different metals when it is exposed to air or
moisture. This targeted point can be approached by different protection methods.
Aluminium is a light metal with a mass of 2700 Kg/ m3 and for equal electrical conductance
as copper, two times less relative weight of aluminium is required. This weight reduction
12
•
•
results into reduced fuel consumption, hence reduced CO2 emissions. Even the logistic
processing and installation operation benefits from a weight reduction.
Even though pure aluminium is 65 % IACS, aluminium’s conductivity in relation to the same
weight as copper is two times larger.
Aluminium is ductile with low melting point which enables customary processing and
excellent workability such as bending, machining, stamping and forming with a variety of
surface treatments.
2.4 Comparison of aluminium and copper conductors
A conductor’s capability to carry current can be calculated by conductivity, resistivity, length and CSA
of conductor metal expressed by resistance formula:
𝑅𝑅 =
𝜌𝜌𝜌𝜌�
𝐴𝐴
Equation 6
Where ρ is resistivity, L is length and A is area of the conductor. Since coppers conductivity is greater
than aluminium, about 1.5 times conductive as aluminium a larger cross-section area of aluminium is
required for the same current carrying capacity. The raised volume owing the bigger CSA of
conductors results into increased weight of armouring and insulation materials which are one of the
main drawbacks of aluminium conductors and have to be considered when comparing and
calculating weight reduction of the assembly. Additionally, the need of larger cable accessories, for
instance cable straps, cable ties and cable tapes results into the need of another installation
processes since the grater size of aluminium cable require larger space for a greater bending radii
and installation of cable assembly. The bending radii of aluminium cables increase with 58 % which
requires 27 % more space for installation of the cable [16]. This means that it is not only
characteristics differences between these two cable materials, one have to consider how the whole
system and other components might be influenced by replacing copper with aluminium.
When substituting copper to aluminium one should take into account their differences in e.g.
mechanical strength, resistivity and density, physical properties etc. , a comparative characteristics
of aluminium and copper are expressed in Table 6.
13
Table 6. Properties of Aluminium and copper conductors [17], [18] [19]
Characteristics
Copper
Aluminium
1350
(Hard drawn)
Weight for the same conductivity
100
54
Cross section for the same
conductivity
100
164
Electrical resistivity (µΩcm)
1.72
2.83
Aluminium
8000
(half hard)
118 (annealed)
118-132 (half hard)
200-250 (annealed)
260-300(half hard)
169-200 85100b
103-145
Mass density (g/cm3)
8.89
2.705
2.710
Modulus of rigidity (torsion) at 20°C
(Shear Modulus) (KN/mm2)
0.2 % proof strength annealed
(N/mm2)
44 (annealed)
44-49 (half hard)
50-55 (annealed)
170-200 (half hard)
62 (annealed)
115 (half hard)
Elastic modulus (KN/mm2)
Tensile strength (N/mm2)
Fatigue Strength (N/mm2)
Fatigue No of cycles
a
a
70
28
a 20-30
b 35 (annealed)
50 (Half hard)
300 x106
50 x106
a
Coefficient of expansion(/°C)
(20-200°C )
17.3 x 10-6
23 x 10-6
(alumina 7.6 x 10-6)
Thermal conductivity at 20°C ( W/mK)
397
230a
Temperature coefficient of electrical
resistance
0.00393 (annealed, 100 % IACS )
0.00381 (fully cold worked, 97%
IACS)
0.00429a
Temperature for creep of
0.22%/1000h under typical
termination stress
150 °C
Specific heat(J/kgK) at 20°C
386 (at 20°C)
393 (at 100°C)
Thermal diffusivity (mm2/s) at 300K
117
Thermal conductivity (Wcm/cm2K)
3.94 (at 20°C)
3.85 (at 100°C)
Corrosion (Zero potential at 20°C)
+0.34V
a
20°C
a
900
98.8
a
2.05
-1.67V
Cu2O (conductivity 10s/cm)
Al2O3
copper sulphate conductance)
(conductivity 10-7 s/cm)
Conductor price (May,2016) USD/Kg
4.98
1.67
a
Approximated value for high conductivity hard drawn aluminium or pure aluminium
b
Values for annealed conductor
Surface oxide film
14
The mechanical properties, stress-strain relation of a conductor metal are expressed by Hooke’s law:
𝜎𝜎 = 𝐸𝐸𝐸𝐸
Equation 7
𝜏𝜏 = 𝐺𝐺𝐺𝐺
Equation 8
Where σ is stress, E elastic modulus is the stress-strain curve in the elastic deformation zone which
can evaluate the stiffness behaviour. The stress produced by shear stress, called shear modulus is
expressed by:
The metal can return to its previous dimension in the elastic deformation zone without being
permanently deformed after unload. The higher stiffness of a conductor the higher load it can resist
without fracture or necking. Therefore, a conductor with less stiffness and bending radii require
more caution during installation. The conductor stiffness is affected by its flexibility class, a smaller
bending radii is required for conductors with high flexibility class. As shown in Table 6, conductors
having less stiffness than coper with respect to their elastic modulus values require cautions during
installation. If the minimum bending radii is exceeded, unwanted and unmovable kinks with hot
spots and potential weakness forms and remains in the aluminium conductor. Equivalent creep
occurs at 20°C for aluminium conductors and at 150°C for copper conductors while the creep rate at
0.022 % occurs at much higher temperatures for copper conductors compared to aluminium where
aluminium’s tendency for creep is higher at normal operating temperature and room temperature.
Creep is a deformation that is irreversible and affects conductor termination by decreasing contact
pressure with potential overheating issues. Likewise, cold flow which is a permanent deformation of
the material when exposed to force or pressure during mechanical termination. Creep coupled with
cold flow is a factor challenging termination of aluminium conductors. Due to this aluminium should
have its own regulations and standards for torque settings for terminals, connection and terminating
methods.
A metals ability to conduct heat is defined as thermal conductivity. Metals with high thermal
conductivity have a better heat dissipation which results into less temperature rise and hot spots at
terminating joint. The specific heat is the conductors’ ability to absorb heat while the heat generated
by joule effect is stored in the conductor and expressed by
𝐽𝐽 = 𝐼𝐼 2 𝑅𝑅𝑅𝑅
Equation 9
Where R is resistance, t is the time period the conductor conduct heat which is equivalent for
conductors with same length and resistance value and the temperature rise owing heat storage is
expressed by
𝐽𝐽
∆𝑇𝑇 = 𝑐𝑐𝑐𝑐
Equation 10
Where c is the conductor metals specific heat value. Comparison of copper and aluminium’s
temperature rise shows that the temperature rise for aluminium conductor is smaller than copper
conductor. Due to this, thermal diffusivity which indicates the capability to conduct relative store
thermal energy, aluminium conductors with lower thermal diffusivity has a higher tendency to
absorb thermal energy while copper favours thermal conductance.
As indicated in Table 6, aluminium has larger thermal expansion coefficient than copper, this may
result into incompatible expansion between the terminal and conductor. Brass and copper is the
typical terminal metals used for copper conductors which have much lower thermal expansion
coefficient than aluminium. Using those metals as terminal metals for aluminium conductors will
result in connection problems where the connection tends to loosen over time owing the different
15
expanding degrees. Loosening of the connections results into increased contact resistance which in
turn leads to arcing and overheating.
Regarding the mechanical properties of conductors shown in Table 6, required stress for 0.2 %
plastic deformation before losing elasticity is defined as proof stress and copper conductors ability to
withstand proof stress is about 3 to 6 times higher than aluminium conductors. This means that
copper conductors have lower risk of breaking or necking during processes where mechanical pulling
is employed. It is more likely for aluminium cables to neck-down or stretch when exposed to high
pulling forces resulting in reduction of CSA, worsen current carrying capacity and overheating issues.
Pulling force serve greater mechanical deterioration and irreparable stresses on multicore cables
consisting few small sized cables which limit application of aluminium based cables with small CSA or
diameter.
With respect to corrosion issues indicated in Table 6, aluminium conductors have higher tendency
for corrosion owing its low zero potential. Connecting aluminium with metals having higher zero
potential results into galvanic corrosion. Formation of insulating Al2O3 is one of the most damaging
factors since it inhibit current flow, consumes the conductor metal and cause formation of hot spots
in the contact spots.
In addition to those properties, the constantly increasing and fluctuating price of copper compared
to aluminium’s stable and far lower price, about one third or copper makes this to one of the main
differences between these conductor materials.
2.5 Application of Aluminium wires in automotive industry
There are many suppliers in the market offering aluminium wires in different dimensions which can
be used with conventional terminals where copper made lugs are used as connection method.
Aluminium in combination with copper leads to potentials of galvanic corrosion [7].
Earlier studies have shown that the weather conditions have a significant role when it comes to
degree of corrosion, since galvanic corrosion occurs when the metal comes in contact with
electrolysis for instance snowmelt salt, chloride and squalls depending on whether the vehicle is
used in North America or Middle East for instance. Due to this development of corrosion protection
and different solution is essential in order to prevent corrosion and provide wires with electrical
connections that last the entire lifetime of a vehicle.
Although there was no international standards for regulation of aluminium cables characteristics in
automotive applications, usage of aluminium cables in automobiles started in 2000. An non-ISO
standard, LV122-2 developed by German car manufactures for aluminium cables in automotive was
released, until 2013 when the international standard ISO 6722-2 was created where standardization
of CSA conversion with the aim to minimize the impact on termination system. The standard
includes aluminium cable size with a range of 0.75-120 mm2, including specification of battery cables
and primary cables for equivalent current carrying capacity [20]. The investigation of using
aluminium cables in automobiles started in Europe followed by the Japanese automakers who
expected a weight reduction of 40 kg by replacing copper cables with aluminium cables [21] [22].
The larger cable being replaced, the more weight saving, according to a study done for battery
cables [21], a 6 m long cable with CSA of 95 mm2 and insulated by PVC would require an aluminium
cable with 150 mm2 for equivalent current, even though this results in 22.9 % increased cable
diameter, a weight saving of 46.3 % is possible. As indicated in Figure 2, the Japanese and European
automakers have implemented aluminium cables such as battery cables and power cables where
large sized cables represent cables with CSA larger than 25 mm2.
16
Figure 2. History of Aluminium cables in automobiles [3].
The issues related to larger cable size of aluminium are fatigue life, flexibility and strength of
stranded conductors since next generation of automobiles demand reduced size of cable conductors
and according to a study done by Kuypers [23], application of cables for high voltage systems e.g. 48
V are not recommended if the vibration cannot be relieved and if the short circuit cannot be
protected.
Development of aluminium cables started for more than 30 years ago where it was used in airbus
where large sized cables was firstly used before development of small gauges signal cables. Due to
this the small sized cables which counts for major part of cable harness weight, cables with small CSA
have been investigated by [24] [25] [26] [27]. Those studies recommended substituting aluminium
cables with CSA range of 0.5-15 mm2, in areas where high vibration is not present for instance
replacing copper conductor by aluminium conductors in engine compartment with presence of high
vibrations is not recommended.
Apart from this, aluminium cables with large CSA for instance battery cables are being used on a
mass production basis but the termination techniques faces some challenges and have to be
improved. When it comes to the further usage of aluminium cable harnesses in automobiles, battery
cables and cables with CSA over 15 mm2 will reach a wider use, while signal cables with small CSA
and midsized cables (2.2 mm2-8 mm2) need further investigation before being recommended [3].
The difficulties of aluminium conductors is to provide an stable and reliable termination and the
challenges as indicated in Table 6 owing aluminium’s characteristics which represent incompatibility
with the terminal, corrosion potentials and high thermal expansion which challenge termination and
reliable connection of aluminium cables. The challenges regarding application of aluminium cables in
automobiles are summarized as:
17
•
•
•
•
•
•
•
The increased diameter of aluminium conductor requires larger volume and space which
may affect the present termination system and parameters that are standardised and the
strategies of cable harness makers have to be in agreement with terminal makers.
A greater diameter and bending radii of aluminium cables challenge the installation of the
cable harness and is controversial to minimization trend that claim high flexibility and
lowered volumes.
Aluminium conductors may face some mechanical strength issues, especially the small sized
conductors where fracture crack or kinks during service or installation may occur, in fact
reduced flex life and break strength lead to cable failure, hence, cables with smaller CSA
than 0.75 mm2 is not recommended in current vehicles.
A reduction of contact pressure under termination and crimping owing the applied force
result into non-gastight contact interface where corrosion and oxidation can occur even if
the termination joint is sealed. This lead to loosening of mechanical connection and
increased contact resistance which generate heat and further creep and cold flow of the
conductor.
The surface of aluminium tends to quickly oxidize and form an electrically resistance layer
when exposed to external atmosphere. This surface layer worsens the metal to metal
contact and if a conventional termination technique, as shown in Figure 3 is used, the
insulating layer will prevent current flow and as a result, higher contact resistance and
unacceptable heating of the joint can occur.
Galvanic corrosion which occurs when two dissimilar metals are in contact with each other
and in presence of an electrolyte is another issue that affect the contact behaviour, weaken
the conductor and lead to contact failure.
Using different metals at the termination join hampers handling high temperature, high
vibration conditions and thermal shock since aluminium have high thermal expansion
coefficient which along with those conditions generate incompatible thermal expansion of
the terminal and conductor which in turn deteriorate the connection with a failure as result.
•
Figure 3 Comparison of conventional crimping of copper and aluminium [28].
2.6 Current load capacity
Due to characteristics of aluminium a larger cross section of aluminium is required compared to
copper. A comparison between aluminium and copper, in terms of cross section area for the same
current carrying capacity, is shown in Table 7 below [5].
18
Table 7. The relation between aluminium and copper for the same current carrying capacity [2].
Wire cross section of aluminium
Wire cross section of copper
2.5 mm2
1.5 mm2
4.0 mm2
2.5 mm2
6.0 mm2
4.0 mm2
Equation 11 describes the cross section needed for aluminium cables to get the same equivalent
electrical current as in copper. Since the cross section of the conductor is reversely proportional to
electrical conductivity, the aluminium conductors have to be 1.6 times larger than copper in order to
carry the same current [5].
𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝐴𝐴𝐴𝐴
𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝐶𝐶𝐶𝐶
𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶
58.5 𝑆𝑆𝑆𝑆/𝑚𝑚𝑚𝑚2
= 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐶𝐴𝐴𝐴𝐴 = 35.5 𝑆𝑆𝑆𝑆/𝑚𝑚𝑚𝑚2 ≈ 1.6
19
𝐶𝐶𝐶𝐶
Equation 11
3 Current electrical connection methods
3.1 Contact physics
The conduction of electricity occurs at asperity point even called a-spots which raise the contact
resistance. The contact resistance can be described by:
𝜌𝜌2 𝜂𝜂𝜂𝜂𝜂𝜂�
𝑅𝑅𝑐𝑐 = �
4𝐹𝐹 �
1�
2
Equation 12
Where ρ is resistivity, 𝜂𝜂 (≈1) is empirical coefficient of surface cleanliness, H is the metal hardness
and F is the contact force. In the case of connector application, a stable and low contact resistance is
required during the entire life of the connector. The contact material determines the hardness and
resistivity while the contact force is a design factor. The degree of surface corrosion, contaminations
in the surrounding environment and type of protection determine surface cleanliness𝜂𝜂. In general a
material with low H (soft material), and low resistivity ρ and great contact force F provides larger aspot size. There are challenges regarding connectors with low cost, small size, light weight and
operating in extreme environment but those challenges can be faced by different approaches [28].
3.2 Contact area
All solid surfaces, on a microscale are rough and this roughness consists of variation in high and
geometrical characteristics that depend on details of surface generation process [29]. Contact
between two bodies occurs at discrete spots that are produced by mechanical contact of asperities
on two surfaces shown in Figure 4.
Figure 4. Bulk Electrical interface and constriction of electric current.
The true contact for a material is a small fraction of a nominal contact area for a range of contact
load [29]. The real surface of the contacting area is not flat and contains many asperities, shown in
Figure 5, deformation of those contacting asperities can plastic, elastic or a mix of plastic-elastic
deformation depending on the stresses in the local mechanical contact and on the materials
properties, for instance the hardness and elastic modulus of the material.
20
Figure 5. Contacting asperities [14].
The contact surface of metals in an electrical interface is often covered with electrically insulating
layers or oxides. In general a surface is electrically conductive when metal to metal contact spots is
produced, which is possible if the insulating film is displaced at the asperities of contacting surface or
getting ruptured. Due to this the real area of electrical contact in an electrical junction is smaller
than the area of mechanical contact. The electrical current lines in an electrical junction become
distorted when the contact interface is approached and the flow lines together passes through the
contact spots (or “α-spots”) [30]. The apparent contact area, shown in Figure 6 include, real contact
area, oxides, contaminant films and the α-spots which provide the conductive paths for electrons
and is formed by small cold welds.
Figure 6. Contact area and α-spots
As shown in Figure 6 the α-spots which are the physical contact area, where the passage of
electrons occurs are much smaller than the real contact area. The properties of those α-spots are
related to the contact metal material and environmental dependent. The electrical resistance
increases since constriction of electrical current by α-spots reduces the volume of the material. This
increase in interface resistance is defined as constriction resistance and often, presence of
contaminant films on the contacting surface, increase the resistance of α-spots. The interface
contact resistance in turn is determined by total interface resistance owing the constriction and film
resistance [30]. The relation between apparent contact area Aa and applied normal load Fc and
hardness is given by
𝐹𝐹𝑐𝑐 = 𝜀𝜀𝜀𝜀𝐴𝐴𝑎𝑎
21
Equation 13
where ε is pressure factor that is determined by the amount of asperities on the contact areas and
H is the hardness of a metal with estimated ability to resist deformation under pressure load up to
three times of yield stress (𝐻𝐻 = 3𝜎𝜎𝑦𝑦 ).
The constriction resistance of a single α-spot is given by
𝑅𝑅𝑠𝑠 = (𝜌𝜌1 + 𝜌𝜌2 )/4𝑎𝑎
Equation 14
Where a is the radius of a α-spot ,ρ1 and ρ2 are electrical resistivity of conductor and connector
which means that a minimum constriction resistance can be obtained if the contact metals are the
same, i.e.,
𝜌𝜌
𝑅𝑅𝑠𝑠 = �2𝑎𝑎
Equation 15
If greater force F applied, the radius of the contact area increases which in turn lower the
constriction resistance. Other factors contributing to resistance of electron movements are the
sulphides, oxides and inorganic layers that form at the surface. The total contact resistance is the
sum of film resistance Rf (𝑅𝑅𝑓𝑓 = 𝜎𝜎� 2 ) where σ is resistance per area of the film and Rs which is
𝜋𝜋𝑎𝑎
constriction resistance. During formation of contact spots, mechanical breakage of the films occurs
hence resistance contributed by film resistance is almost unneglectable [3].
Taking operating temperature into consideration, the contact resistance value can be expressed as
Wexler resistance
4𝜌𝜌
𝜌𝜌
𝑅𝑅𝑤𝑤 = 3𝜋𝜋𝜋𝜋 𝐾𝐾 + 2𝑎𝑎 Г(𝐾𝐾)
Equation 16
Where Г(𝐾𝐾) is the temperature related function and = 𝑙𝑙�𝑎𝑎 , l is the mean free path of electrons [31].
The formation of insulating intermetallic compounds on the contact interface result into increased
resistance that continues increasing if hardness of the formed layer is higher than the contacting
material and current conductivity is possible only when this film layer is broken. Due to this coatings
for electrical material is used in order to prevent formation of this insulating film layer, mechanical
wear and corrosion, decrease the hardness and promote conductivity. Using tin and nickel as plating
material for aluminium conductors leads to that, the thickness of coating and α-spot’s diameter and
their conductive ratio affect the contact resistance. [3]
Contact resistance is the most important characteristics of electrical contacts and changes in contact
resistance cause significant drawbacks since it might be greater than the previous value owing to
considerably differing with variables in contact pressure load, real contact area and resistive film
which is an fatigue indicating factor since accumulation of strain leads to fatigue behaviour, this
means that an increased resistance results into fatigue behaviour and high voltage drop that is
directly measurable in a DC potentiometers [32].
3.3 Automotive Connector contacts
The use of electrical devices in modern vehicles has grown tremendously. A vehicle in 1940s needed
a 6Volt DC electrical system for providing the required power for ignition, starter motor, wiper and
lighting circuit while vehicles today require currents up to hundreds of amperes and multiple voltage
levels. Typical voltage level in a vehicle is described in Table 8. An electrical connection is required
22
for all these electrical systems which have to withstand performing in extreme environments where
the temperatures varies between -40°C in arctic conditions and up to 150°C at engine mounted
sensors [28]
Table 8. Typical voltage level of different application in a vehicle [28].
Type
Circuit
Current
(A)
Voltage (V)
Power
Appliance/commercial/residential
10-100
120-240 ac
Lightening
Appliance/commercial/residential
>10
120-240 ac
Control
Appliance/commercial/residential
<2
24-36 ac
Power
Automotive
>1
12-36 ac
Power
Automotive, Electric Drives
~300
~300 ac, dc
Control
Automotive
<1
5.12 dc
Low power
Electronics
<1
5-12 dc
Signal
All
0.1
<1
3.4 Cable Termination
Cable termination is a non-separable connection between the terminal and the cable where
mechanical (crimped or soldered) or metallic joint provide the mechanical and electrical connection.
The interface between the terminal and cable is permanent, it provide the mechanical and electrical
link between the terminal and the cable. Further it has the main role in, performance and assembly
of the terminal which is a part of the connector. Degradation of cable or terminal interface result
into degradation of female or male terminal contact interface. The mostly common terminal-cable
interfaces used for appliances and automotive wiring is barrel crimps and F-crimps, shown in Figure
25. The termination process consist of, stripping the wire insulation, insertion of the bare section
into the barrel region or crimp wing where the crimping tool rolls the wing and deform or press the
barrel which deform the wire, loosen the surface oxides, induce metal flow which forms a tight
bundle and create electrical and mechanical bonding. The assembly is under high compression, so
when the tool is released, the assembly rebounds. The barrel or crimp wing are relaxed which makes
the metal to bounce back from the compressed state. This in turn reduces the contact force
between the crimp wing and the wire so the crimp should be designed in a way where back
bouncing with remaining residential force is allowed and low contact resistance with suitable
mechanical joint can be obtained [33] [34].
23
Figure 7. Barrel and F-crimp (a) Insertion of the cable into barrel crimp, (b) The crimping is finished, (c) Cross section of
barrel crim, (d) Insertion of the cable into F-crimp, (e) F-crimp is finished, (f) Cross-section of F-crimp [28]
The highest mechanical strength and the most optimum electrical performance do not take place at
the same crimp connection level since a crimp with optimal electrical performance might have poor
mechanical strength while a crimp connection with high mechanical strength lead to poor electrical
stability. Due to this a trade-off where the best combination of long-term mechanical strength (pull
strength) and electrical stability have to be found. A combination of best electrical performance and
mechanical strength and their relationship is shown in Figure 8. At different crimp compaction level,
the pull strength (shown with the dashed line in the figure) varies while the electrical performance
(shown as solid line in the figure) varies with crimp compaction. Discernment of different
compaction levels is performed at various environment conditions with SAE USCAR-21 as an good
environmental exposure reference where crimped connections in automotive are specified
[35] .With respect to Figure 8, at compaction levels, tighter than highest pull strength point, occurs
the best combination of mechanical and electrical performance. A controlled crimping process is
essential for reliable wire termination where in the case shown in Figure 8 it is ± 50 microns.
Therefore, terminal manufactures select material, coating, tool geometry, gauge, pressing force
profile, internal barrel surface serration and final crimp deformation wisely in order to maximize the
operating range [34]. Crimping is not suitable for small gauge wires since it will be subjected to high
operating temperature which makes the crimp unreliable, hence alternative termination such as
soldering or welding which are more expensive can be used since it provide higher mechanical
strength and stable electrical interface.
24
Figure 8. Crimp compaction versus pull strength and electrical resistance of the crimp [28]
3.4.1 Crimping
The connection of a cable to a terminal have a structure shown in Figure 9 below where a crimped
wire with a terminal is inserted to a pair of connectors, male or female connector which provide the
electrical connection between wire harness and equipment/component.
Figure 9. Crimped wires and connectors [36].
To ensure the retention force, crimping as a connection method is being used; the wire with stripped
insulation is crimped in the terminal. A special press tool is used in order to achieve a deformation
that is predefined and a mechanically strong electrical connection between the connection element
and conductor is achieved. The crimping condition i.e. the strength of the crimped area affects the
25
retention force and the electrical connection, generally the retention force decreases when the
crimping force increases and the electrical connection stabilize. Due to this the manufactures have
to take into account the required range of electrical connection and retention force in order to fulfil
the standards. [37]. During crimping the cable and the connection element material is exposed to
high mechanical pressure which leads to exceeded yield point. Crimping is correctly executed when
the metal flow is mutual which means that the combination of connection element and cable area
has to be suited with the crimping tool being used [38].
Crimping connection method is being used for copper wires but the property differences between
copper and aluminium leads to challenges that require improvements and other solutions in order to
enable the method for electrical connections of aluminium wires. The aluminium surface is covered
by an insulating oxide film which has the biggest detrimental effects in the crimping properties and
according to previous studies the electrical connection might be improved if the insulating oxide film
layer is removed. In one of the study’s experiment a conductor wire was pushed against the terminal
and the contact resistance were measured while load was applied. The results showed that in order
to stabilize contact resistance, aluminium required more load than copper. In order to stabilize the
contact resistance for aluminium wires, a higher crimping compression compared to copper wires is
required. The wire retention force is then lower since stabilization of aluminium wire requires higher
crimping compression than copper, the retention force lowers which means that the contact
resistance needs to be improved at lower compression or improve the wire retention force at higher
crimping compression which enables electrical connection of aluminium by crimping. A comparison
of contact resistance, crimping strength and wire retention force between copper wires and
aluminium wires is shown in Figure 10 [6].
Figure 10. Contact resistance, crimping strength and wire retention force of copper and aluminium
Few modifications of the terminal design and connection technology crimping is essential for
enabling use of aluminium cables.
26
Inside the core crimp the geometry is knurled which together with the flowing characteristics of
aluminium ensures that the surface oxide layer is broken and the terminal can strand conductivity
electrical connection is achieved [5].
The strand to strand conductivity is achieved by ultrasonic welding of single aluminium strands to
each other before crimping the wire to a terminal. A cable geometry called “nugget” with electrical
strand to strand connection that can be crimped to a terminal is then provided. The nugget
geometry will result into a more robust mechanical characteristic due to prevention of movement of
the strands. A crimped, nugget that can be connected to a terminal is shown in Figure 11.
Figure 11. An ultrasonic welded cable with "nugget" shape that can be crimped to a terminal [5].
3.5 Welded connectors
Connections with all type of copper or aluminium conductors can be done by welding which is a
highly accepted method since it creates an efficient, permanent electrical connection which is
economical with a good appearance and most suitable for connection of two members with
different cross sections. From the electrical standpoint, a joint that is properly welded is the most
reliable joint since the contact resistance will be reduced which in turn prevent heat generation from
high current [39]. There are different welding techniques for different applications but the most
commonly used for power connections are thermite (exothermic) welding, friction welding,
explosion welding, and resistance welding and ultrasonic welding and resistance brazing.
3.5.1 Thermite (exothermic) welding
Fusion process where two metals are heated and the overheated metal undergoes an aluminotermic
reaction and the metals become bonded is called thermite welding. The melted metal from the
reaction between aluminium and metal oxide acts as metal filler and make a molecular weld since it
hangs around the conductor. The advantages of the thermite connections are that a high mechanical
strength and good corrosion resistance can be achieved; the current carrying capacity of the
connector is greater than the conductor’s capacity and a high stability during repeated short circuit
current. Disadvantages of this process is the intense heat anneals the conductor which lead to that,
the thermite connectors is not suitable in tension applications [14].
3.5.2 Friction welding
A combination of different nonferrous metals can be welded by a technique called friction welding.
It is a solid state welding process where mechanical energy is converted to thermal energy at the
contact interface without any external heat or energy. The wire is prepared by crimping the
aluminium made sleeve on the striped wire before cutting the end of the sleeve and getting a clean
surface. A rotating work piece is in contact with a non-rotating piece under pressure until the
welding temperature is reached at the interface. A friction between the pieces generates frictional
27
heat which rises the temperature at the interface. When the interfaces are in contact with each
other, atomic diffusion occurs and metallurgical bond between two pieces forms [14].In order to
ensure a surface without any pollution with residues of coating or aluminium oxide, a thin layer of
the end of the connecter is removed by a turning tool. [7]. Both annealing of the carrier and
segregation of metal oxides which occurs during melting when welded by other techniques can be
minimized by this welding process. Production practicality and economics of this process have
limited usage of this contact welding to low volume application [40]. A high voltage aluminium
conductor welded to aluminium connector by friction welding technique is shown in Figure 12.
Figure 12. Aluminium connector welded to an aluminium conductor by friction welding [7].
3.5.3 Resistance welding
Welding process where heat is generated by resistance to the electrical current flow through the
conductors under force by electrodes and are held together and varying surface are joined by heat
that is generated by a shot time pulse of high amperage current and low voltage which forms a fused
nugget of welded metal. The electrode force is preserved when the current flows ceases and weld
metal cools and solidifies. The resistance of materials weldability by this process is inversely
proportional to its thermal and electrical conductivity.
3.5.4 Resistance brazing
A resistance welding process where a filler material is inserted in-between two pieces that are
heated locally and melted by the heat from resistance to the electrical flow through the joint is
called resistance brazing. An efficient localized heating method can be provided by using of high
resistivity electrodes. In order to minimize or provide oxidation of the joint during heating a flux
which provide a coating is used, in addition to that it dissolves the present or formed oxide during
heating and assist the filler metal to promote capillary flow [14].
3.5.5 Resistance butt welding
Wires with various materials and cross section area can be welded by resistance butt welding but is
usually used for wires of the same type. The material is melted by the electrical flow on both joint
partners and the pollutions are flushed by pressing them together. A sleeve with base metal of
aluminium needs to be crimped in order to ensure reliable electrical contact and clamp the wire, for
a cleaner and more flat surface, the face side of the wire is cut off. This welding technique results
into less damages in coating compare to other welding techniques but as in friction welding the end
of the cable is cut of and cleaned before welding in order to get a clean and flat surface. According to
previous studies it seems to be possible to weld similar wires without any sleeve where a connecter
can directly be joined to a wire. An example of a resistance but welded connector to conductor is
shown in Figure 13 [7].
28
Figure 13. Resistance but welding of aluminium made connector and conductor [7].
3.5.6 Ultrasonic welding
For years the only method to make a splice of wires was the clip and dip method where a metal
terminal was applied over the wire strands in order to hold the wires together, which then was
dipped in a cleaning solution to remove oil and oxidation. It was dipped in molten solder and shaken,
to remove the extra solder and then cooled in neutralizer acid, a cleaning agent and dip-solder. The
disadvantages of dip soldering are the introduction of acid and limited quality control. An more
accurate mothed is ultrasonic welding where high frequency sound or ultra sound causes rapid
vibration within the wires that is welded, the vibrations which can be up to 20 000 vibration per
second causes the wires to rub against each other. This friction rises the surface temperature and
sets the condition for the wire to form a molecular bond to one another and splices the wires. No
terminal, no acid and no solder are used and important parameters such as wire quantity, gage,
orientation to the weld, weld height before and after the weld, width of the weld, required time to
make the weld, pressure, amplitude and energy put in to weld are monitored in a software used
together with the ultrasonic welder which control welding parameters and ensure that they are
repeatedly accurate. A tool compress the wires, then a ultrasonic are applied and when the tooling is
opened a splice nugget of solid aluminium with fixed and durable connection after the ultrasonic
splice is made it is covered with tape or rubber mould according to purchasers specifications [41]
[42]. This welding process might damage the structure of contact material during welding and is
sensitive to surface conditions, due to this coating and predefined cable head geometry is required
while the oxide layer and coating from the surface have to be removed before processing in order to
avoid unbounded and broken areas, an example of an ultrasonic welded aluminium cable is shown in
Figure 14 [7].
Figure 14. Ultrasonic welded aluminium conductor [7].
29
3.6 Parameters affecting the power connections performance
According to previous studies a reliable connection of aluminium cannot be achieved by established
methods and applications for joints with copper conductors. The difficulties with aluminium is that
whenever two dissimilar metals are in contact to each other, due to their differences in mechanical,
metallurgical and physical properties and reaction of those under different conditions determine
their degree of compatibility [14]. The interface between the conductor and its terminal is the
electrical contact interface where a good metal to metal contact is essential in order to assure
continuity of electricity [3]. This requirement for aluminium is not easily achieved owing to the
insulating oxide layer that is ever-presented at the surface. In addition to the external factors that
affect the electrical conductivity, the microstructure of the connector metal also have an important
role on electrical conductivity. Several phenomena related to the electrical contact interface such as
temperature distribution, deterioration in contact which may result into increase in electrical
contact resistance [3]. Aluminium’s propensity to undergo stress relaxation and creep, high tendency
for galvanic corrosion and having a large thermal expansion coefficient that may result into fretting
at the contact interface can result into failure of the connection and should be an awareness of [14].
A course of a cycle describing the complexity of failure mechanism is shown in Figure 15 [14].
Figure 15. Schematic of degradation mechanism in aluminium power connections.
3.7 Parameters affecting the contact area
Parameters such as shape and size of the contact surface, contact resistance and pressure,
protection against external environment all together effect if the current can cross through the
contact interface without being interrupted. A continuous passage of electrical current can be
measured and there are three way, as shown in Figure 16, for improving the contact performance in
order to ensure a reliable passage of current.
•
•
•
Improvement of contact design
Developing new contact material, lubricants and coatings
Involving the state and structure of the interface
30
Figure 16. Schematic illustration of main means of improving reliability of electrical contacts.
The stiffness of contacting members, the current density through the contact and the applied force
are generally factors that define the contact area and a sufficiently large contact area can prevent a
temperature rise of the interface under different conditions. The contact temperature itself on the
other hand is an effect rather than a cause that occurs in a joint as a function of the contacts
geometrical dimensions, current density and voltage drop throughout the contact. The temperature
of the contact interface can increase more than the connector or conductors bulk temperature
without making the contact interface electrically instable. On the other hand an increase of contact
voltage even if to moderate values cause increase of contact temperature above that of connector
or conductor bulk temperature. Due to this, the changes in voltage and contact temperatures with
the connector operating time have to remain small <10 mV. This requirement can be met if, the real
contact is large enough regardless deterioration and there are still enough of contact spots (a-spots)
which guarantees that overheating in the joint is not reached. Previous studies [43] [44], have shown
that lubrication and mechanical abrasion (brushing) of aluminium is one of the most simple and
efficient method for achieving large number of contact points. Brushing together with application of
a contact aid compound (grease) to the contact interface prevent oxidation of the metal. In order to
ensure a large contact surface area between the conductor and connector, serration of the
connector contact surface for splicing conductors can be used [45] [46].
3.8 Factors affecting the reliability of power connections
The probability of a process or equipment to function for a given period, without failure when
operated correctly under stated conditions is the definition of a reliable connection. The discrete
nature of the interface is one of the main problems when providing reliable electrical contacts. An
electrical contact within the contacting interface in the discrete region is formed between the slides.
It is the formation of conductive contact areas that control the efficiency and reliability of electrical
contacts which in turn is dependent on several interrelated or independent factors. Those factors
can be divided into factors determined by contact units’ fabrication characteristics, design31
technological factors, or the performance factors that are dependent on operating conditions which
are divided into external or internal groups shown in Figure 17 [14].
Figure 17. Effect of performance factors on reliability of electrical contacts.
The external factors are variation in time, temperature, atmospheric pressure, humidity which are
uncontrollable while the internal factors are electric, (operating voltage, strength and type of
current) mechanical contact load and characteristics and type of motion such as, the sliding velocity.
The performance factors affect the surface films and the contact materials properties. If chemical or
physical process occurs in the contact zone, formation of wear particles influencing the interface, the
contact resistance and finally the reliability of electrical contact [14]. The effect of designtechnological factors on the quality and reliability of electrical contacts are show in Figure 18.
32
Figure18. Design and technological factors effect on the performance of electrical contacts [14].
The selected contact material, geometry, the intermediate layer that separate contacting surfaces,
the contact surface microrelief and quality of selected coating all together determine the number,
size and distribution of contact spots that influences the real electrical contact area, surface film
resistance and the reliability of contact area [14].
3.8.1 Plastic and elastic deformation of power connectors
Plastic deformation of asperities occurs if the contact force is higher than few Newtons and
formation of α-spots occurs. For lighter contact forces elastic deformation of asperities occurs
instead and for some regions of contact force a combination of elastic-plastic deformation takes
place [47] [48].
33
3.8.2 Corrosion
An electrochemical or chemical reaction between a meat and its surrounding environment leading
to significant changes and deterioration of the material properties and function is the definition of
corrosion. It starts with a progressive change in the geometry without any change in the materials
microstructure or other chemical composition. Degradation starts with formation of a corrosion
product layer and continues till one of the reactants can sustain a reaction and spread through the
layer. The characteristics and the composition of the corrosion product layer have a huge influence
on corrosion rate and the most common that could have an effect on metallic component of power
equipment are localized, atmospheric, pitting, dust and galvanic corrosion [14].
3.8.2.1 Galvanic corrosion
Galvanic corrosion is an electrochemical process where the differences in materials properties such
as electrode potentials and the presence of electrolyte is the basis of galvanic corrosion. Galvanic
corrosion is considered as one of the most dangerous degradation mechanisms in a bimetallic
system [49]. The voltage differences between two metals is the driving force behind electrons flow
where the direction is dependent on which of the metals is more active. The less noble metal is
more active and become anodic, corrosion occurs and the other metal that is less active becomes
cathode [14]. A power source starts when moisture comes in between the metals and an electrode
potential of at least 5 mV is required in order to start a current flow [49]. In the case of copperaluminium connections, the most non noble metal aluminium behave as anode, dissolves and
deposited at copper in a complex hydrated aluminium oxide form while hydrogen forms at cathode
copper. This degradation process continues until all aluminium is consumed and as long as an
electrolyte is present although the erosion rate at the surface is limited by the corrosion products.
Corrosion can affect aluminium to copper connections in two ways, either a mechanical failure
caused by a severely corrosion of the connector or an electric failure caused by a drastic reduction of
contact area [14]. Galvanic corrosion of aluminium and copper is shown in Figure 19.
Figure 19. Galvanic corrosion of aluminium and copper [36].
3.8.2.2 Localized corrosion
Localized corrosion have an relatively fast attach rate with a small affected area which is hard to
quantify and detect since the surface defects tends to be small and a good indication of the extent of
those defects is hard.
34
3.8.2.3 Atmospheric corrosion
Altering of a material or gradual degradation of a material when it gets in contact with e.g. water,
oxygen, vapour, carbon dioxide, chloride and sulphur components presented in the air is called
atmospheric corrosion. Degradation is accelerated considerably by a thin film of water owing the
electrolytic nature of corrosion. Even though the corrosion rate is dependent on the temperature,
humidity, sulphate and chloride levels, it is not constant over time, it decrease when exposure time
increases [14].
3.8.2.4 Pitting corrosion
Degradation of a metal surface confined to a small area or localized to a point that forms cavities are
pitting corrosion. Those pits have an irregular shape and may get filled by corrosion products.
Coated metals are usually affected by pitting where the pits forms at a weak spots where the coating
is mechanically damaged and cannot self-repair. Metals such as aluminium, copper, cobalt,
chromium, and their alloys are prone to pitting corrosion. A local breakdown of surface corrosion
films, initiates pits which accelerate the corrosion rate, those pits are commonly observed in H2S and
CO2 environments. The corrosion products are usually black in colour and adhering to the metal
surface [14].
3.8.2.5 Dust corrosion
Dust corrosion have been investigated by Zhang [50], occurs because of the presence of water
soluble salts in dust that comes in contact with the metal. Zhang indicated that one the most
influencing factors on dust corrosion is humidity and pH factor. As shown in Figure 20, corrosion of
dust particles increases linearly with humidity. Typical product appearing around the dust particle is
shown in Figure 21 [14].
Figure 20. Corrosion ratio of dust particles having different PH,
as a function of humidity.
Figure 21. SEM picture of corrosion product around dust
particle.
3.8.3 Thermal expansion
Thermal expansion is another degradation mechanism caused by differences in thermal expansion
coefficient between two different contact metals. In the case of aluminium to copper connections,
35
when the connector is exposed to increase in temperature, aluminium expands at a higher rate than
copper which results into lateral movement in the metal-contact bridges zone with a reduced
contact area, or plastic deformation of contact interface. This lead to increase in contact resistance
which rise the connection temperature but a matrix recovery where the stresses are relieved is
feasible at higher temperatures. However, those thermal stresses build up again when the material
is cooled and further plastic deformation and interracial shearing with small recovery possibilities
occurs since potentials for matrix recovery and stress relieves at lower temperatures are small. If this
process is repeated and if the generated thermal stresses are higher than aluminium’s yield stress,
plastic deformation in the contact zone occur which accelerate degradation of the connection until
failure. If the metals are annealed, the thermal stresses in aluminium can be assumed to be
negligible and an estimation of magnitude of the maximum generated elastic stresses in aluminiumcopper connections in thermal cycles, at peak temperatures can be calculated by
∆𝑑𝑑 = 𝜀𝜀𝑡𝑡 ∆𝑇𝑇
Equation 17
∆𝑑𝑑𝑑𝑑 = ∆𝑇𝑇[𝜀𝜀𝑡𝑡 (𝐴𝐴𝐴𝐴) − 𝜀𝜀𝑡𝑡 (𝐶𝐶𝐶𝐶)]
Equation 18
Where Δd is the amount of contraction for both copper and aluminium when cooled, εt is the
thermal expansion coefficient and ΔT is the change in temperature. The differential strain because of
the constraint is then described by
Aluminium- copper contacts differential strain at 100 °C, 150°C and 200 °C, are 6.8, 10.2 and
13.6×10-4 (1/°C), since for copper αt=17.2×10-6(1/°C), and for aluminium αt=24.0×10-6(1/°C). The
elastic modulus of aluminium is 70 GN/m2 and the tensile yield strength is 55 MN/m2, which gives a
yield strain of 7.8×10-4 (1/°C). Those values compared to the calculated differential strain values,
indicates that during cooling of contact, aluminium should yield [14]. In addition to this,
thermoelastic ratcheting is another consequence of great thermal expansion of aluminium where
excessive tightening of bolts in aluminium to copper joints can deform the conductor plastically
during heating cycles and cannot regain their original shape after cooling cycles. Due to this
loosening of the joint occurs after repeated heating and cooling cycles which in turn increase contact
resistance and joint temperature [51].
3.8.4 Creep
A metals plastic deformation during creep is related to the viscous diffusion flow and dislocation
dynamics phenomenon. At temperatures close to the melting temperature occurs viscous diffusion.
The creep rate gradually decreases and vanishes at low applied stresses and medium temperatures,
which can be described by the logarithmic law:
𝜀𝜀 = 𝜀𝜀0 + 𝛼𝛼𝛼𝛼𝛼𝛼(𝛽𝛽𝛽𝛽 + 1)
Equation 19
Where ε0 is the initial deformation, α and β are constants and can be used for studying the rate of
plastic deformation of metals which is valid for materials with different type of crystals and at
temperatures below 0.2-0.3 Tm . At lower temperature, creep is mainly affected the dislocation
mechanism that is result of applied stresses and thermal fluctuations which unhamper the motion of
dislocations through the lattice. Klypin [52], studied creep of metals under the influence of electrical
current and founded that creep rate is strongly affected by the electrical current and the creep rate
increase significantly when current is applied. The growth of creep in connectors and consequent
process occurring in the contact is shown in Figure 23.
36
Figure 22. Schematic illustration of creep evolution.
3.8.5 Creep and stress relaxation
Cold flow or creep in a metal occurs when it is exposed to constant external force over time. Creep is
higher for aluminium then copper and the rate depends on temperature and stress. Even stress
relaxation depends on temperature, time and stress but in contradiction to creep it does not result
into dimensional changes. Stress relaxation occurs at higher stress levels and metallurgical structure
changes result into reduced contact pressure. Failure of contact joint occurs when the elastic strain
changes to plastic strain which reduce residual contact pressure significantly which in turn increase
the contact resistance. Acceleration of initial contact pressure loss occurs when the temperature is
raised which shorten the time of contact area loss. Due to this, high stresses owing the connector
system and deformation of the conductor. If no residual mechanical loading is provided for the
contact interface, an acceleration of stress relaxation resulting in to failure of contact joint will occur
[14].
Farrell [53] [54] investigated the effect of temperature and metallurgical state on copper and
aluminium’s strain relaxation and the result showed that an increased hardening and temperature
increase the rate of stress relaxation and that metal forming, stress relaxation, and flow stress is
affected by the flow of the electrical current and the interaction between dislocations and electrons.
3.8.5.1 Effect of electrical current on stress relaxation
Silveira and co-workers [55] investigated the effect of metals mechanical properties on electrical
current and founded that, stress relaxation rate of polycrystalline copper and aluminium near 0.5 Tm
(melting temperature) can be increased by applying 1.6 A/mm2 continuous AC or DC current. It was
37
found that DC current changed the arrangement of dislocation in copper specimens which
restructured the structure of the cells while no changes in dislocation structure were found for
aluminium. Application of current increases the stress relaxation rate considerably, Braunovic [56]
studied the differences of aluminium conductors stress relaxation at room temperature and low
initial stresses (20 N), when influenced by low-density current (3 A/mm2) and conductors at higher
initial stresses (260N) at a higher temperature (150°C), it was clear that aluminium’s stress relaxation
under cycling conditions and at higher temperatures is similar. It means that a wires contact force
(initial force Fi=20N) under current cycling decreases at the same rate as if the wire would be
exposed to stress relaxation at 150 °C and higher force, F0=260N.
It is not only the effect of temperature that justify the effect of current cycling on stress relaxation
since the temperature difference owing passage of low current density is about 2-3°C. one
explanation to this phenomenon could be that thermal fatigue in aluminium wires is caused by
current cycling which reduce the materials overall durability and increase the rate of stress
relaxation. Additionally, the contact zone that is under compression during cycling, work-hardens
because of the wires thermal expansion which increases the contact stress and it is known that the
rate of stress relaxation is increased by work hardening, hence the rate of stress relaxation can
augment.
A metals stress relaxation is usually related to its structural defects for instance solutes, grain and
subgrain boundaries, impurities, precipitates etc. so when the electrical current passes through the
wire, it will cause heating, weakening of the binding forces between the dislocations and obstacles
that prohibit the motion of the dislocation. This increases the density of dislocations and alters their
arrangement. Repeated current cycling in the wire reduce the dislocation density, enhance mobility
and free the dislocations from pinning defects which in turn increase the rate of stress relaxation
[14].
3.9 Fretting
Fretting is an accelerated damage of surface that occurs at the contacting interface when exposed to
small oscillatory movements produced by mechanical vibration, load relaxation, differential thermal
expansion of contacting metals and junction heating when the power is switched on and off. This
kind of fretting damage includes fretting corrosion, fretting fatigue and fretting wear. Fretting is a
time related process and the effect of it is not easily recognizable since a power connections failure
is usually associated with contact zones’ destruction by arcing [14]. Factors affecting fretting can be
divided into environmental conditions, material properties and behaviour and contact condition,
those factors and how they affect each other are schematically illustrated in Figure 23 [57].
38
Figure 23. Schematic illustration of factors affecting fretting [14].
Those factors can interact and influence each other which have an impact on degree and nature of
fretting, for instance under some conditions the environmental effect can be neglected from the
contact area, hence have no strong effect on fretting.
Fretting is one of the factors that contribute to electrical instability and joint failure, it is one of the
major deterioration mechanism in dry connections and appears in conductor and contact materials
such as copper, aluminium, tin, nickel and silver [58] [59]. Fretting of aluminium made connections is
of considerable importance since the aluminium oxide particles degrade the metal which initiates
oxidation and accumulation of fretting products [60]
3.10 Intermetallic compounds
Usage of bimetallic welds of aluminium to copper by for instance pressure welding, friction welding
and ultrasonic welding. Frequent current surges; generate conditions for nucleation and growth of
intermetallic at the interface which decrease the mechanical strength and electrical stability due to
the intermetallic lower mechanical strength and higher electrical resistance. Previous investigations
of aluminium to copper joints have shown that if the intermetallic phase’s thickness exceeds 2-5 µm,
the joint loses its mechanical integrity. Figure 24 shows microstructure of intermetallic phases
formed at Al-Cu contact interface owing thermal gradient.
39
Figure 24. Microstructure of intermetallic phases formed at Al-Cu interface [14].
The process of diffusion can be described by
𝐷𝐷1 = 𝐷𝐷0 𝑒𝑒𝑒𝑒𝑒𝑒 �
−𝑄𝑄
−𝑄𝑄
�𝑅𝑅𝑅𝑅 � 𝐷𝐷2 = 𝐷𝐷 = 𝐷𝐷0 𝑒𝑒𝑒𝑒𝑒𝑒 � �𝑅𝑅𝑅𝑅�
1
Equation 20
Where D0 is a constant, R is the universal gas constant and Q is the required activation energy for
diffusion. Aluminium’s activation energy for diffusion is Q= 40 Kcal/mole and diffusion at a
temperatures T1 and T2 (T2= T1+∆T) is,
𝐷𝐷0 𝑒𝑒𝑒𝑒𝑒𝑒 �
𝐷𝐷
−𝑄𝑄
𝑄𝑄
�𝑅𝑅𝑅𝑅 � 2�𝐷𝐷 = 𝑒𝑒𝑒𝑒𝑒𝑒 �𝑅𝑅 �1�𝑇𝑇 − 1�𝑇𝑇 �
2
1
1
2
Equation 21
Due to current constriction, development of α-spots may occur at higher temperatures with
diffusion rate than in the bulk. Required temperature rise ∆T, to double diffusion rate D2/D1=2 at
T1=60°C gives ∆T=4°C. If the α-spots is assumed to form at T2=300°C and the bulk at T1= 60°C will
result into diffusion rate of α-spot is 1012 faster than in the bulk, this means that formation of
intermetallics is likely to occur under these conditions. The characteristics and composition of
common intermetallic phases formed in aluminium to copper joints are shown in Table 9.
40
Table 9. Characteristics and composition of intermetallic phases formed in aluminium-to-copper joint
Symbol
Composition
Cu
(wt %)
Al
(Wt %)
Hardness
x102
(N/mm2)
Resistivity
(µΩ cm)
𝐃𝐃𝟐𝟐𝟎𝟎
(cm/s)
Q
(Kcal/
mole)
Phase
1
Γ2
Cu2Al
80
20
3.0
14.2
3.2X10-2
31.6
Phase
2
δ
Cu3Al2
78
22
18.0
13.4
2.6 X10-1
33.5
Phase
3
ξ2
Cu4Al3
75
25
62.4
12.2
2.7 X106
61.2
Phase
4
η2
CuAl
70
30
64.8
11.4
1.7 X10-6
19.6
Phase
5
θ
CuAl2
55
45
41.3
8
9.1 X10-3
29.3
Phase
3.11 Degradation of connectors
The electrical connections should have a sustainable operating condition over a long period of time.
Their life span, age and life cycle management are affected by operating conditions, design criteria,
manufacturing process, safety consideration and maintenance procedure. The fluctuations in
humidity and temperature, the reactive gaseous composition, seasonal changes and the different
environment within the vehicles are all together parameters affecting the degradation mechanism.
Their remaining life can be calculated by measuring the total damage and analysing their history. The
connectors expected remaining life, is the time period after which probability of failure is
unacceptably high.
The “bathtub” curve, shown in Figure 25 is a reliability curve by which a components life expectancy
and usefulness can be expressed. As shown in Figure 9 the failure rate during the first operation time
(Break-in period) is high, this is usually because of installation or manufacturing problems. The
probability of failure is then constant during “normal life” until it sharply increases during the wear
out time. The original design life may not match the sum of expected remaining life and operating
life time hence an expected remaining life which is a probability distribution will still be more precise.
When a component of an electrical system approaches to its end of operating life span, failure of the
component is likely to occur. The design and manufacture of an electrical component determines its
lifetime which affected by service conditions. The useful life of a component is shortened at a high
rate if the operating condition of a component becomes more serve [14].
41
Figure 25. Probability of failure, "bathtub"
Additionally, one of the most dangerous degradation mechanisms is oxidation of metal to metal
contacts within the contact interface occurring in mechanical connectors. A chemical process where
the oxygen content of the base metal increases and results into losing its electrons is oxidation
process that is considered as the most dangerous degradation mechanism in mechanical connectors
[14].
Degradation mechanism is a less likely mechanism in the case of aluminium contacts since growth of
oxides is self-limiting. For aluminium it reaches 10 nm thickness which is much less than the contact
spots diameter, after a short period of time. When the bare aluminium surface is exposed to air or
moisture, the oxide forms as a duplex film that consist porous bilk layer on top. A maximum
thickness of the barrier layer is reached within microseconds since it is temperature dependent
while the bulk film is dependent on both temperature and relative humidity is developed more
slowly. Oxidation kinetics of the most comment contact materials is described in Table 10 [14].
Table 10. Oxidation kinetic of common electrical contact material.
Metal
Cu
Ambient
Air
Product
Cu2O
Al
Air
Al2O3
Sn
Air
SnO
Ni
Air
NiO
Characteristic Features
At
Thickness
(Nm)
103 hr
Thickness
(Nm)
105 hr
Temperature
dependent, Oxide forms
immediately, Initially
slow growth rate
Humidity and
temperature
dependent, Oxide forms
immediately (2 nm in s)
Self-limiting, Weak
temperature
dependency
Weak temperature
dependency
100 °C
15.0
130.0
20 °C
2.2
4.0
20 °C
4.2
6.1
Selflimiting
growth
-
-
20 °C
1.6
15.0
100 °C
25.0
36.0
100 °C
3.4
34.0
42
The formed oxide on aluminium’s surface provides a corrosion protection up to melting point of
aluminium. Aluminium oxide have a resistivity of 1024 µΩ cm, it is adamant, hard brittle and
transparent which means that event the clean appearance of aluminium conductors does not assure
low contact resistance without a suitable surface prep. The oxide film can either be broken
mechanically or electrically, thick oxide film is broken by fritting at high voltages. In the case of
aluminium, particularly if both contact members are aluminium based, rupture of the oxide film is
essential for enabling current flow. While in the case of copper, a continuous oxidation of metal to
metal contacts occurs when it is exposed to oxygen, this results into increase of contact resistance.
The copper oxide grow and flake of the base metal and from 40 °C to 200 °C in air the, a continuous
thickness growth of Cu2O that is temperature dependent grows and above 200 other copper oxides
that consume the metal forms. Copper oxide is softer then aluminium oxide which easier gets
disrupted by applied contact force [14].
It is known that making electrical contact through Al2O3 is more difficult than forming a metal to
metal contact through Cu2O3, because of the aluminium oxide’s hardness. According to Tylecote [61],
cold welded aluminium require lower deformation for initiating welding in aluminium compared to
cold welded copper and that for cold-forged metals a lower deformation is needed for initiating
welding. It is generally known that annealed metals have higher ductility and larger α-spots can be
formed since plastic flow through fractures in oxides occurs easier. Deformation is more
concentrated in under plane-strain conditions which leads to larger cracks in the oxide if aluminium I
scold-worked.
A study done by Braunovic [62], showed that thermal cycling in aluminium containing 0.5 at %
magnesium cause segregation of impurities to free surface and have a considerable effect on contact
resistance, resistivity and hardness. He suggested that vacancies that are formed near metal-oxide
interface, diffuses into the metal and start a solute flow, moving from the metal to the surface which
results into higher magnesium content in the oxide. It is known that this flow of solute lowers the
contact resistance but there are other unknown, complex reactions such as clustering of vacancies,
polygonization and dislocations that could occur at the same time and affect the electrical and
mechanical properties [62].
The quality of distributed power is a function of equipment’s characteristics, supply system and end
user system. The equipment should be designed so it can withstand variation in power quality
variations. An crucial prerequisite for maintain a reliable power connector with structural integrity
all through their service life is to control their aged-related degradation which can be accomplished
by a systematic management process of age-related degradation consisting of:
•
•
•
•
•
Understanding of failure mode
Inspection, monitoring and assessment
Minimizing degradation
Replacement, maintenance and repair
Utility development of maintenance program
3.11.1 Economical consequences of contact degradation
Properties of connector material may deteriorate due to influence of deterioration mechanism or
aging in-service which reduce the margin of operating safety or useful lifespan. Using an old
43
connections beyond their originally expected life enable economic benefits. An evaluation of the
connectors remaining useful lifetime has to be done in order to guarantee that the structural
integrity and safety are held even during the extended operating time. A continuous and valid
monitoring of a reliable extended life of the critical equipment is required. If an error and its
evolution is discovered and monitored then the defects severity can be measured where decisions
on what action should be taken, can be considered [63]. Early detection of faults, evaluating and
monitoring a fast developing defect and its progress provide necessary information which can be
used for reacting on time and reducing the overall damages. Power components are susceptible to
different deterioration modes and ageing, the cause, potential impacts and their cost are shown in
Table 11 [14].
Table 11. Power components susceptible to ageing [14].
Application
Cause
Impact
Conductors
Broken conductor strand
Conductor strands broken- overheatedoverhead line could come down
Circuit breakers
Overheating, overloading
Splices
Loose/ corroded/ improper
connection and splices
Replacement safety consideration and
expensive repair
Power distribution
Poor breaker connections
Overheating, burning, fire, arcing
Disconnect
switches
Miscellaneous
power
components
Loose/ corroded/ improper
connection and splices
Replacement safety consideration and
expensive repair
Overheating, burning, fire, arcing, 25 % of
all power equipment failures are caused by
loose electrical connections
Switches breakers
Overheating, overloading
Loose/ corroded
connections, poor contacts
Expensive replacement and cost of repair
3.11.2 Prognostic models for contact remaining life
Estimation of a components, for instance a connectors life time can be done by prognostic model
where data collection of the components performance from initiation to the final stage is an
important prerequisites. Development of those prognostic models requires derivation and validation
since a statistically valid model provide subjective predictions. Models that are based on genes,
related to the component performance are likely to be correspondingly predictive in other context
as well while simple models are likely to be integrated into predictive practices and utility
maintenance with nominal disruption.
44
In the case of remaining life a connection, the contact interface is assumed to be homogenous with
circular shaped a-spots, as shown in Figure 26 [64]. Additionally it is assumed that oxygen intrusion
and growth of oxide film is the main factor affecting conductivity of the surfaces. A calculation of the
contact life time as a function of time and variation of contact resistance is shown in Figure 26.
Figure 26. Circular shaped contact spot [14].
45
4 Study of electrical connection, connectors and termination
techniques
This chapter is state of the art of different electrical connection methods, connectors and
termination techniques. The principles and most essential aspects affecting the electrical contacts
are discussed in this chapter. The current connectors, terminals and termination techniques that are
used today are discussed in 4.2-4.3 while termination techniques for aluminium conductors are
discussed in chapter 4.5. This is a literature study based on information from patents, books, reports,
articles and presentations at companies working with electrical conductors and connectors.
4.1 Electrical connections made of aluminium
The electrical connection of an aluminium cable can either, as mentioned earlier be by conventional
terminals with different copper made lugs but, in order to reduce the chemical potentials and
corrosion potentials between copper and aluminium, a fully aluminium based connector for
electrical connection of aluminium cable can be used. Implementation of new electrical connection
requires that one take into account that different environment may affect in different way if another
material or method is used. A study done for high voltage connectors made of aluminium based
their requirement on electrical hybrid vehicles EHV, where the ambient temperature in the engine
can be up to 140 °C while the temperature limit for insulating polymers used is about 180 °C. Due to
this, the maximum allowed temperature increase in the connector is about 40 °C which means that
testing of artificial ageing and effect of creep should be considered at this temperature. There are
other requirements such as electrical safety, electromagnetic compatibility, assembling and
manufacturing that have to be considered when implementing new electrical connection method [7].
4.1.1 Aluminium wiring connection
Due to coppers high conductivity, high corrosion resistance and good strength, it has been used for
most wiring in vehicles. Additionally the electrical connection of copper is relatively easy, with
mechanical means, welding, brazing, soldering and crimping. The need of reducing vehicle weight
and coppers violate prices makes aluminium as an attractive alternative to copper even though
aluminium faces challenges related to aluminium’s properties such as conductivity, strength
properties of electrical contact and low resistance to corrosion. As mentioned earlier aluminium
wires conductivity is about one-half of copper which means that a larger diameter of aluminium for
the same current carrying capacity as copper will be required. Further aluminium’s density is about
one-third of copper which means that even if larger diameter is used, the mass will still be less than
copper. Increasing the diameter of the conductor require a bigger connector housing and packing of
those in an already limited space, in modern vehicle is difficult. Another factor that has to be
considered when replacing copper cables by aluminium cables is corrosion resistance since
aluminium is prone to galvanic corrosion when it is connected to dissimilar metals in presence of
electrolyte. A crimped connection between aluminium conductor and copper alloy terminal before
and after corrosion is shown in Figure 27, part of the cable strand have corroded away after a severe
corrosion. Sealing of the copper terminal from electrolytes can be used as a protection method for
this type of corrosion [28].
46
Figure 27. Galvanic corrosion of an aluminium cable crimped to a copper terminal [28].
4.2 Types of connectors
The purpose of an electrical connection is to permit passage of electrical current across the contact
interface which requires a good metal to metal contact in order to achieve an uninterrupted
electrical current passage. Although the function of a connector is to complete the circuit by
providing electrical interconnection, complex processes such as degradation of the contacting
interface which is due to changes in temperature, load and contact resistance occurs in the contact
zone and effect the contact behaviour. Different factors such as physical and electrical properties of
connector material, contact area, mechanical properties, oxidation tendency of the contact material
and contact pressure affect the contact resistance. In addition to that, the type of connector or
connection system is determined by non-electrical factors, for instance the size, shape, mating and
unmating force, mounting and frequency have to be considered. Environmental factors such as
temperature cycles, humidity, contaminants, vibration and shock require a complicated selection
process. In order to have a reliable performance of a connector during its service life and meet the
electrical and mechanical requirements and sustain a specific operating condition different type of
connector devices have been developed. In general it is accepted that a good mechanical joint also is
a good electrical joint but there are other operating conditions, deterioration mechanism and design
features of a particular connector have an impact on connection performance [65].
Depending on complexity of the electrical system, different type of connectors such as device
connectors, junction splices, in-lines, headers and separable contacts with male and female
terminals used. The “architecture of the wiring harness, determine the number and location where
the circuits will be connected. Design and application of a connection needs consideration of where
the devices is going to be connected in which determine the voltage and current level; and the
functional, mechanical, electrical and environmental requirements which determine the connection
system used. For instance if the connection is directly to the motor then a sufficient retention force
is required to withstand the vibration, while a ribbon cable require a large-gauge round cable from
47
a power feed line. Connectors can be classified according to their functional operation and currentcarrying capacity into three groups: light, medium and heavy duty connectors [65].
Light duty connectors are devices operating at voltages up to 250 V and carrying at flows below 5 A.
A stable and low contact resistance and right connector material is essential for a successful
operation of the connectors.
Medium duty connectors operates at voltages up to 1000 V and carries currents above 5 A, the
electric wear is prime importance.
Heavy duty connectors operate at voltages up to hundreds of kV and carry currents up to tens of kA.
4.2.1 Plug-and-socket connectors
This type of connector compromise contact base and contact finish material intended for quick
engagement and disengagement of electronic units which have to maintain satisfactory operation
over period of time. The selection of connector material is based on requirement to sustain
mechanical flexibility, rigidity, electrical, contact force deflection and contact design. The most
common connectors that are frequently used in automotive applications are, rack and panel,
terminal-terminal and plug and receptacle.
When one part of the connector is fixed or stationary and the other part of the connector is
removable, rack and panel connectors for mounting of equipment used. The connector is mated
since the removable part is installed on the “rack” and in order to ensure alignment of contact
terminal, a floating connector is mounted which in turn prevent damages due to position variations
during insertion. An example of rack and panel connector is shown in Figure 28 [66].
Figure 28. An example of rack and panel connector [66].
The simple terminal-terminal connector requires manual connection and no insulation is present to
protect the terminal and is used within an enclosure for instance appliance’s housing. Low contact
resistance even with the non-noble coatings is provided by a sufficient contact force and the
adequate mechanical force prevent the contacts from parting. A simple blade-box connector is
shown in Figure 29 [14].
48
Figure 29. A basic example of terminal-terminal connector [66].
The most widely used type of connector is plug and receptacle, it requires manual installation which
is used in vehicles where complex circuits and harnesses require flexibility in order to support the
large number of functions and electrical features. Since this type of connector is manually installed
mating can be difficult, due to this some type of mechanical assistance e.g. bolt-screw is
incorporated. Figure 30 is an example of plug and receptacle connector [66].
Figure 30. An example of plug and receptacle connector [14].
4.2.2 Common features of connectors
The design of the connectors varies depending on type of application it will be used for but the
necessary parts of a connector are: cable or wire termination, contact terminal, enclosure and line
insulation.
Cable termination: An inseparable connection between the terminal and the cable conductor is
provided by termination where a mechanical or metallic joint (welded, soldered or crimped) provide
a permanent mechanical and electrical connection which is the link between the cable and the
terminal.
Contact terminal: It is the conducting members which consist of female and male terminals where
the mated current passes. The terminal is attached to the contact interface on the one end and the
cable on the other.
49
Enclosure: Connector parts are assembled into a housing where mating is done by joining connector
enclosure together to and. The enclosure protects the internal parts and provide mechanical guide
for mating.
Line insulation: It is a part of plastic enclosure which serves to ensure the alignment and location of
the terminal. This kind of insulation is essential as electrical insulation, multiple lined connectors and
isolation between adjacent lines.
4.2.3 Electrical Terminals
Electrical contacting points are provided by the electrical terminals where a stable and low electrical
resistance during the connector’s service life at different operating conditions are required. Contact
force, contact material and wire terminations are the most important parameters. Depending on
what application it is going to be used in, different combination of terminal configuration such as
form of termination, alloying material, type of crimp, coating, cladding and spring members varies.
Additionally the selected material and terminal design have to consider the connector assembly and
overall fabrication process. The most common terminal types are summarized in Table 12 [28].
Table 12. The most common electrical terminals.
Type
Contact
form
Base
material
Coating
Typical
current
(A)
Press-fit
Pin-hole
Cu-alloy
Sn, Solder
≤5
Bump-flat
Butt
Sn, Pd, Ag,
Au
≤2
Pinhyperboloid
Wire-pin
Cu, Cualloy
Cu, Cualloy
Sn, Ag, Au
≤2
Pin-sleeve
Multiple
beam
Cu-alloy
Sn, Ag, Au
≤ 10
Blade-leaf
Beam
Cu-alloy
An, Ag, Au,
Ni
≤ 30
Blade -box
Row
Cu-alloy
None, Sn, Ni
≤40
Tuning fork
wire-blade
None
≤ 20
IDC
Wire-blade
None
≤ 20
Plug-screw
or Wirescrew
Wire-wire
twist
Cu, Cualloy
Cu, Cualloy
Termination
Crim or
solder
None or
solder
Crim, IDC or
solder
Crim, IDC or
solder
Crimp or
solder
Crimp or
solder
None or
crimp
None or
crimp
Flat-flat or
wire-flat
CU, Al
None, Sn
≤ 100
None or
crimp
Multiple
wire-wire
Cu, Al
None
≤ 20
None
50
Common
usage
Commercial
automotive
Commercial
automotive
Commercial
Commercial,
appliance,
automotive
Automotive
appliance,
commercial
Appliance,
automotive
Commercial
automotive
Commercial
automotive
Household
wiring/
appliance
Household
wiring
Pin-sleeve terminals have reduced size where the contacts are smaller with low contact force and
high circuit density. These types of terminal are used for appliance, automotive and commercial
electronics. Metal coating of terminal might be required since those terminals control circuits and
deal with all or some electronic signals. A female and male pin-sleeve terminal is shown in Figure 31.
Figure 31. Female and male pin-sleeve terminals [28]
Blade-leaf terminals, shown in Figure 32 evolve from blade-box type and are used in automotive
where high circuit density and low insertion force is required. Smaller blades cannot be
accommodated in the “box” if the connection density increases but various leaf designs are possible.
There are challenges regarding connector component used in vehicles since they are exposed to
fluids from lubricating oils, mechanical vibrations, high pressure washer jets and extreme
temperatures
Figure 32. Blade-leaf terminal [28].
The blade-box terminals have a robust construction and have ability to carry a wide range of current.
These types of terminals are usually used for commercial electrical circuits and appliances and can
be applied for high current range due to its large contact area. Large contact areas in combination
with high contact force these terminals are resistant to high vibration and if a suitable coating is
selected, the contact is gas-tight, hence resistant to gas corrosion.
Insulation displacement connection terminals (IDC) are used for connecting an insulated wire to a
device in a single operation which eases the installation but the design of the terminal is sensitive to
gauge. A suitable strain relief should be a part of the terminal design in order to avoid disconnection
and maintain wire-slot orientation. Since the wire- blade contact is usually small, the current an IDC
can carry is limited and therefore not suitable for high current connectors but.
Tuning fork type is similar to IDC terminals and due to its groove design, it can be used in a wider
range of contact force.
The plug-screw and wire screw are designed for manual mating of circuits. It is for connecting an
equipment or device to a wire where the wire is crimped to a lug. The current carrying capacity of
51
this type of terminals are high due to high contact force that might be applied but changes in
vibration, thermal contraction or expansion and relaxation under stress lead to drop of contact force
to significantly lower levels.
The wire-wire twist terminals are used for joining the electrical wiring in household. The low contact
resistance is provided by copper-copper contacts. High number of contacting point ensure a durable
and reliable connection and the formed oxides are fritted through a voltage supply. Even though it is
difficult to establish good electrical contact for aluminium based wires, owing the formed oxides on
the surface, it is still acceptable for similar applications.
4.3 High power connectors
An average vehicle with internal combustion engine and a 12-volt electrical system have a peak for
high-power electrical circuit under 2 kW. The charging circuit carries about 100 A during some
conditions while the starter connection hundreds of amperes during few seconds at a time and
simple bolted ring terminals are usually used for connection of those. Electric and hybrid vehicles
have to operate at hundreds of volts during some conditions where some of the circuits carry
hundreds of amperes during extended period. Hence the connections have to retain a stable and
resistance over time with additional requirements. In order to protect the nearby electronic
components from being influenced by the high power circuits, an electromagnetic shielding of the
connection is required. Additionally an environmentally sealing of the connector which prevent
dielectric breakdown between circuits where a high isolation resistance between the neighbouring
circuits is essential. Electric motors, DC/AC inverters, DC/DC converters, and battery packs all require
reliable connections that can perform in demanding environments within the vehicle. A typical high
power connector used in vehicles is shown in Figure 33. Special features such as HVIL (high voltage
interlock circuit), electromagnetic shielding and isolated terminal cavities are highlighted. The
purpose of HVIL is to protect from hot disengagement of contact and avoid damage of contact
terminal and the operator [28].
Figure 33. High-voltage connector used in vehicles [28]
4.3.1 Improvement of electrical connection
In order to narrow the contributing factors affecting the crimping connection of aluminium wires
and enhance the electrical connection, cause and effect diagram shown in Figure 34, with respect to
serration can be used. An asperity on the terminal wire barrel that is usually formed by three
grooves and contributes to stability of the mechanical and electrical connection is called serration.
52
The surface oxide film is usually broken at serrations which prevent the wire coming off after
crimping since it is caught in the serration after crimping [36].
Figure 34. Cause and effect diagram of crimping properties [36].
Serration contribute to stabilization of electrical connection which enhance crimping properties of
the terminal. Deformation of the wire at the serration occurs during crimping which leads to
breakege of the surface oxide film. Applying more load to this section result into formation of the
surface, tin adheres to aluminium and bond the wire and the terminal which establish the electrical
connection. Due to this, increasing the volume of serration will result into increased bonding section
of the wire and terminal which improve the electrical connection.
The retention force of the wire is another affecting factor that have to be taken into acount and
improved in order to enable aluminium wire connection by crimping method which can be done by
crimping at a higher compression. If the serration can be modified so the finest aspiraties is spread
over a broad range and if a sufficient serration edges is secured, both the wire retention force and
the electrical connection can be improved with a reliability eqvivalent to the copper wires used in
vehicles [36].
4.3.1.1 Anti-corrosion of the crimped section
Deep corrosion can be caused by slight amount of electrolysis solution which means that the best
way to protect the crimped section from corrosion is to protect it from coming in contact with
electrolysis solution which means preventing moisture from entering crimped sections. As shown in
Figure 35, moisture may enter through either from path 1, 2 or 3, therefore it is important to protect
the end part of the terminal to avoid any clearance except the exposed section of the conductor.
53
Figure 35. Paths for intrusion of moisture into crimped section [36].
With respect to reasons mentioned above, R&D laboratories at Sumitomo Electric group company
[36] have developed a mould structure as anti-corrosion technique shown in Figure 36, that covers
the whole crimped section with optimized resin material regarding heat resistance, adhesion to the
terminal and assuming actual use in vehicles.
Figure 36. Mould structure of crimped wire section [36].
4.3.1.2 Corrosion protection
In order to protect the crimped area from corrosion different type of protection can be used
depending on where the cable is going to be installed. A vehicle can be divided into three different
zones depending on degree of required corrosion protection. Figure 37 shows the location of each
zone were zone 1 include for instance the instrument panel and without any need for corrosion
protection since the zone is considered as “high and dry” while zone 2 and 3 are interior and
exterior compartments which demands corrosion protection. Selective metal coating, SMC is a
treatment method for the terminals which accidentally can be exposed to corrosion atmospheres
while applications located in wet areas are protected by connector housing with a single seal design
[5].
54
Figure 37. Recommendation of corrosion protection for Aluminium terminals [5].
4.3.1.3 Selective metal coating (SMC)
Galvanic corrosion protection of an aluminium cable connection to a copper terminal can be done by
selective metal coating SMC, where a layer of electroplated brass (CuZn) with an overlaying layer of
tin is coated at the punched edge of the terminal at the core crimp which mainly is formed of copper
based lead material and is located next to the aluminium wire as well as on hot dip tin that is copper
based material. Figure 38 below shows an example of SMC protection layer [7].
Figure 38. Corrosion protection by SMC layer [7].
The SMC protection is then tested by sodium chloride solution that is used as an electrolyte where
chloride is the element promoting the corrosion. Dissociation of sodium chloride into a conductive
Na+ and Cl- ions solution which is a requirement for galvanic corrosion occurs. When aluminium
dissolves formation of hydrogen gas H2 which increase pH level in sodium chloride solution and
accelerate dissolution of aluminium. Chloride which is the reactive element reacts firstly with
aluminium and zinc and then with more noble metals i.e. copper and tin, and then dissolves these
metals. Inspection of reflection electron microscope (REM) pictures from the end of the corrosion
55
test shown in Figure 39, shows that chloride diffuses into the CuZn intermediate layer, react with
zinc and dissolves it slowly. Chloride will also diffuse homogeneously into the outer Sn layer, but not
beyond that [5].
Figure 39. REM picture of Selective metal coating protection layer after the corrosion test.
The connection between the terminal and aluminium cable have an electrochemical potential, ΔE
(mV) that initiate the galvanic corrosion and is the electromotive force (EMF) of the corrosion.
According to a study done at Delphi Deutschland [5], the ΔE between aluminium and copper is
measured to 435 mV where the SMC protection layer have a potential to reduce ΔE by 306 mV and
in order to prevent galvanic corrosion, a direct contact between the aluminium, copper and chloride
should be avoided by for instance SMC coating and avoiding the conductive contact between
aluminium and copper [5].
4.3.1.4 Tin layer
According to previous study where a corrosion test were performed test at Delphi Deutschland
[5]the galvanic corrosion slows down if the terminals e.g. at the copper punched edges are re-tinned,
since copper and ten tends to form intermetallic phases such as, Cu6Sn5 and Cu3Sn which can diffuse
into the outer layer of tin. The copper base material can be protected by the intermetallic CuSn
phase, due to the electronegativity differences between copper and CuSn, penetration of chloride
from the outside into tin can be prevented [5].
4.3.1.5 Additional corrosion protection
Only pre-tined as corrosion protection is not enough for preventing contact corrosion, a layer of
brass (CuZn) on the copper, as an additional corrosion protection can be used. Earlier study done at
Delphi Deutschland shows that if a 5 µm thick layer of tin is deposited on copper and on brass as
shown in Figure 40 and stored in 25 days at a storage temperature of 170 °C, the layer on the
copper interlayer will dissolve due to tin’s diffusion behaviour with brass and copper. The diffusion
of copper from brass into tin is slower than pure copper into tin, since the endeavour of the copper
56
from the brass to alloy with tin is significantly lower than concentrated copper to tin. The CuZn
interlayer slows the diffusion of copper in the overlaying tin and there is no formation of
intermetallic phase of zinc and ten. It takes about 60 days to dissolve the copper layer, as shown in
Figure 40 below [5].
Figure 40. Copper diffusion into a layer of tin [5].
4.3.2 Electrical connections made of aluminium
The electrical connection of an aluminium cable can either, as mentioned earlier be by conventional
terminals with different copper made lugs but, in order to reduce the chemical potentials and
corrosion potentials between copper and aluminium, a fully aluminium based connector for
electrical connection of aluminium cable can be used. Implementation of new electrical connection
requires that one take into account that different environment may affect in different way if another
material or method is used. A study done for high voltage connectors made of aluminium based
their requirement on electrical hybrid vehicles EHV, where the ambient temperature in the engine
can be up to 140 °C while the temperature limit for insulating polymers used is about 180 °C. Due to
this, the maximum allowed temperature increase in the connector is about 40 °C which means that
testing of artificial ageing and effect of creep should be considered at this temperature. There are
other requirements such as electrical safety, electromagnetic compatibility, assembling and
manufacturing that have to be considered when implementing new electrical connection method [7].
4.4 Other parameter of electrical contacts
Even though contact resistance and terminal design are the most important parameters affecting
the contact performance there are other parameters such as pressure, bulk resistance, force, wipe,
coating and plating that are important as well. The frictional force that holds the mated terminal
together is provided by the contact force. Contact force together with wipe remove non-conducting
particles and film of the surface and provide a cleaner surface at mating which is necessary for low
initial contact resistance. The amount of required wipe force depends on contact material, type of
57
surface film and contact geometry. For example the surface film of bare copper is thick and hard to
remove, hence the required for removal will be several Newton with a wipe distance of several
millimetres while tin-coated contact with soft tin and hard tin oxide require a wipe force less than 1
N with wipe distance of tenths of a millimetres [67]. The contact force is usually distributed over the
available asperity points but if the asperities are concentrated in small area the contact pressure
increases significantly, hence one have to consider the pressure in contact material when designing
small contacts so the deformation in the contact does not affect the performance of the contact.
A terminals bulk resistance is particularly important for high current ratings. The bulk resistance is in
the range of few micro ohms to tens of micro ohm in relation to the contact resistance which have
to be 10-100 times larger. When the wiring harness is attached to a device that generates heat, for
instance a light source or a motor, the terminal and the wire forms a major heat sink at the attached
part of the circuit. The terminal should not be a heat source and cause heat flow into the wiring.
Imbalance of heat flow may rise the contact temperature considerable in some circuits which result
into degradation owing stress relaxation of the contact and shortened life or failure of the contact as
result [28].
4.4.1 Connections in high-vibration environments
Multiple of sensors such as knock sensors, positioning sensors and temperature sensors are directly
mounted on the engines and supply the needed information for an optimum performance of the
engine. Connections to the sensors have to tolerate vibrations for the life of the engine where a
good performance can be achieved if the influence from the vibrations on the contact interface can
be eliminated. Using suitable lubricant and plating of the contacts in high-vibration environments
and relief of the strain in wiring harnesses are important factors for reducing vibrations and relative
movement at the contact interface.
4.4.1.1 Wire connectors
Devices for connecting cables and wires to electrical equipment, the wire connectors can be crimped
or thermos compression bonds, wrapped or screwed joints, soldered or welded. The most used are
straight coupling types, clamp-on types, lugs, splices, crimps, compression-screw lugs, binding-head
screw terminals and eyelets.
4.4.1.2 Requirements on the aluminium made connectors
A new high voltage aluminium connection method that fulfil the requirements mention above have
been developed and patented by German institute of metal forming and casting [7]. The connector’s
essential part is the symmetrical wedge with an angle a amplify the external force F and ensure an
enough contact force for a minimal contact resistance. The external force is kept constant while an
extra distance d is kept in order to compensate the creep effect during the connector’s lifetime.
Different welding techniques w can be used in order to connect to the cable c. Figure 41 below is an
schematic illustration of an high voltage aluminium connection [7].
58
Figure 41. Schematic illustration of a high voltage aluminium connector [7].
Since aluminium is not an optimum material for elastic spring at higher temperatures, an external
steel made spring and housing where the reaction force of the spring is carried out by the housing is
used. Due to high voltages which can be above 400 V, a protection against electrical shock hazard is
provided by slots in contact area with an extra space for insulating fins. As shown in Figure 42 below,
the width of the fins is half of the slots which mean that it can be connected from both parts and all
the touchable areas are covered by either insulating material or housing [7].
Figure 42. Fins as protection against electrical shock hazard [7].
4.4.1.3 Manufacturing high voltage connectors made of aluminium
The connectors are manufactured by extrusion moulding which is an economically favourable
process for high quantities. The extruded bars are made of an AlMgSi alloy that is suited for
machining and extrusion moulding with a proof stress of 215 MPa that is high enough for mechanical
loads that may appear in the application and an electrical resistivity of 32-36 nΩ/m. Required
geometries can be achieved by adjustment of sawing, milling and welding techniques used. The
components are deburred by vibratory grinding after the machining and prepared for further coating.
The coating is done in order to lower electrical resistivity which is due to the oxide layer on the
surface of aluminium and can lead to high contact resistance. Nickel and silver is used as coating
materials, 20 µm of nickel is for diffusion barrier to aluminium but since nickel is a hard and brittle
material with high contact resistance, a 10µm layer of silver is coated on nickel in order to keep the
contact resistance low at higher temperatures as 180 °C. After the coating, the connector can be
welded to the conductor by different welding techniques [7].
59
4.4.1.4 Alloy composition
Pure aluminium with purity of 99.6 % have a enough high conductivity of 62 % IACS (international
annealed copper standards) while the tensile strength is about 70 MPa which is quite low and have
to be improved by for instance designing an more suitable alloy for usage in wiring harnesses. The
electronics and material R&D laboratories at Sumitomo Electric group company [36] has developed a
new aluminium alloy that is suitable for wiring harnesses in vehicles and fulfil the IACS.
The alloy elements where selected by using misfit strain(MS) which is an index that specify the
degree of deformation due to atomic rearrangement in the base metal which occurs due to
dissolution of an solid element. MS is based on first principle calculations by which materials physical
properties can be calculated using the basic law of quantum mechanics and the solubility of a solid
element at room temperature. The solid solution state in this case is when Mg or Fe is melted in the
base metal Al, in an atomic state. The conductivity decreases while the strength is improved
significantly and MS can be used as an index for indicating the strength improvement effects since
the larger MS becomes, the bigger strength improvement effects. This means that the alloying
element with large MS and small solid solubility will result into improved strength and conductivity
which is required for wiring harnesses in vehicles. The research done at R&D laboratories at
Sumitomo Electric company examined the solid solubility and MS of different elements and founded
that Fe is the most suitable element and at least 1.5 mass % Fe is required for the eligible properties,
the variation of tensile strength and conductivity with the amount of Fe is shown in Figure 43 and
the relation between MS and solid solubility of different elements are shown in table 13 [36].
Figure 43. Relation between material property and Fe content [36].
60
Table 13. The relation between misfit strain and solid solubility of different elements [36].
Element
Misfit strain
Solid solubility (mass %)
Fe
3.9
0.03
Mn
3.5
0.62
Cr
3.2
0.62
Ni
2.9
0.37
Sn
2.2
0.00
W
2.0
0.05
Mo
2.0
0.06
Pt
1.8
0.00
Cu
1.6
2.48
Ti
1.0
0.70
Mg
1.0
18.60
Li
0.7
14.00
Si
0.6
1.50
Zn
0.4
67.00
Au
0.3
0.60
Ag
0.2
23.50
If the regular continuous billet casting and extrusion method is used for manufacturing,
crystallization of coarse Al-Fe compounds occur and lead to brittle and poor in processability.
Another continuous casting and rolling system called the Properzi method where the aluminium
alloy is cooled after casting and sent to continuously hot rolling without any need of reheating.
Precipitation of fine form compounds resulting into exceptional processability is enabled by this
method. However, further research showed that processability of the wire decreased if Fe content
is more than 1.2 mass % , therefore Fe have to be partly replaced by another alloying element. A
comparison of the elements in Table 13 shows that an element with large solid solubility which
enhances the strength and small MS since processability is lowered if MS is large, should be suitable
as a second additive, for instance Mg. The study showed that an alloying composition of Al-1.05 %
Fe-0.15 % Mg would fulfil the required processability and result into a conductivity of 60 % IACS and
tensile strength of 120 MPa which is the eligible, predetermined target [36].
Wiring harnesses flexibility is essential for processability and mountability in vehicles, therefore
annealed materials are usually used since annealing decrease hardness and increase ductility.
Annealing can either be done by batch annealing or continuous annealing. Batch annealing is when
the wire reel is placed in a furnace with temperature about 350 °C , held for approximately 4 hours
and then slowly cooled while continuous annealing is when the wire from the reel passes through a
furnace with temperatures about 500 °C or higher and then rapidly cooled.
61
Inspection of transition electron microscope (TEM) pictures, shown in Figure 44 have shown that, if
the metal is heated up to 500 °C or higher temperature and then rapidly cooled, considerable
precipitation of Al-Fe compounds occur in comparison to annealing temperature is around 350 °C
and slowly cooled where Al-Fe compound precipitate in fine form. At higher annealing temperatures
Fe supersaturates, exceed the solid solubility and lower the conductivity. The effect of different
annealing condition on conductivity is shown in Figure 45. Due to this batch annealing is preferable
in order to obtain suitable properties for conductors used in vehicles [36].
Figure 44. Transition electron microscope pictures of different annealing treatment [36].
Figure 45. Effect of different annealing condition on conductivity [36].
According to the result of the study done at Sumitomo Electric Group Company it is possible to
manufacture aluminium wires that can be used for wiring harness in vehicles using methods
mentioned above and thereby replace the copper wires with wires made of aluminium alloy if the
CSA is larger. The effect of weight reduction by replacing copper wires by aluminium wires are
shown in Table 14 [36] .
62
Table 14. Effect of replacing wires by available aluminium wires [36].
Cooper
Type
Ultra-thin
wall
Thin Wall
Size
(mm2)
0.5
Aluminium
Weight
(g/m)
5.4
0.75
7.6
1.25
13.1
2
21.2
Type
Size
(mm2)
Weight
(g/m)
Weight
reduction
(g/m)
Ultrathin wall
0.75
3.1
2.3 (43%)
Thin wall
1.25
2
2.5
5
9.1
11.7
2.6 (34%)
4.0 (31%)
9.5 (45%)
4.5 Termination technique of aluminium cables
A reliable and effective metal-to metal contact between the terminal and conductor where
corrosion or deterioration of the contact is prevented is essential for a good termination.
Conventional termination techniques that are used for copper conductors are not suitable for
aluminium conductors since aluminium have high tendency for galvanic corrosion and is vulnerable
to oxidation when two dissimilar metals are in contact with each other. Due to this, other solution
such as welding, soldering or modified conventional crimp where the oxide layer is cracked overtime
by arranges morphology of the surface or by additives needs to be adapted. Welding and soldering is
probably the preferable solution even though conventional crimping is still being used. Additionally
coating with copper, tin and nickel as treatment before crimping is used as corrosion protection of
termination joint, such as using sleeve or shrinkable tube for additional corrosion resistance. Yuichi
[68] tried to improve the conventional crimping, by means of crimping one portion to the conductor
and the other portion to the insulation where the conductor is protected from being exposed to
humidity environment. The difficulty of this method is to crimp onto insulation where moisture
might still enter through formed gaps.
Later, Sakaguchi [69] invented another solution where sealing was applied in order to protect the
crimp and connect the crimping portions and fully integrate both parts. Other methods such as
sleeve or shrinkage tube which is similar to application of sealer have been adopted in order to
protect the conductor in connection part from being exposed to external environment.
Nölle and lietz [70] invented a copper made terminal with adopted aluminium sleeve where the
aluminium conductor can be connected to brass or copper in for instance cable lug or battery
terminal applications. The purpose of the sleeve was to enable welding with a suitable fitting which
can be regarded as extension of the cable at termination point and increase the connection point.
Hino [71], suggested use of tubular joint terminal where cables can be injected into the tubular joint
and then mechanically pressed in order to form a metallic bond
Takashima and co-workers [72] designed a new connection structure where the uninsulated portion
were inserted into the terminal where molten solder was injected in order to pressure bond with the
terminal and caulk the conductor. Galvanic corrosion can be prevented by this method. In the case
of battery terminals, an electrically conductive adhesive can instead of the molten solder be filled
63
between the conductor strands and the terminal in order to seal and prevent galvanic corrosion.
However, those approaches do not work for dissimilar metals being in contact with each other since
galvanic corrosion might still occur then [73] [74]. Applying protective sleeve and conductive
adhesives in termination have shown to offer an optimized termination of aluminium conductors.
Termination method by applying conductive adhesives is shown in Figure 46 [21].
Figure 46. Termination by applying conductive adhesive [21].
Further, another approach invented by TE connectivity [75] involves, sealing before crimping and
using an additionally intermediate cape that is made of the same material as the terminal in order to
shield the exposed aluminium strands. This method is quite complicated, expensive and only suitable
for larger cables. Therefore they redesigned the connecting structure and used a crimp terminal
which includes an insulation barrel and wire barrel which ensure pressure bonded contact between
the conductor strand and the terminal. This method is applicable for cables with small CSA, about
2.5 mm2.
Termination of aluminium conductors is challenged by the formed oxide layer which must be broken,
TE connectivity [75] developed a crimp barrel solution for connection of aluminium conductors
where the surface have the same properties as F-crimp barrel but it is featured with a “shark fin
shaped serration”, shown in Figure 47. The surface oxide layer is broken by the serration, during
crimping and electrical contact is further established by cold welding.
Figure 47. Shark fin shaped serration of crimp barrel with the aim to break the oxide layer [75].
The connectors in this case is made of copper and their study showed a crimping connection
between an aluminium conductor and copper connector is mechanically stronger than connection
between aluminium conductor and aluminium connector. For example, a wire with CSA of 1.5 mm2
64
has pull-out strength of 80 N and a residual surface crimp pressure of 180N/mm2 with no existing
condition that could cause creep. In order to prevent corrosion, additional material such as sealing
agent is rolled into the front end of the crimp barrel [75].
4.5.1 Terminal with integral oxide breaker
As mentioned before aluminium oxidize when it is exposed to moisture and build an oxide layer on
the surface which does not flake off and protect the non-oxidized aluminium from corrosion which is
an problem when using aluminium as conductor since the conductivity decreases and the metal to
metal contact is deteriorated. Due to this the connection between for instance power strips, lug or
terminal and the wire end have to be ensured in order for the interface to be able to efficiently
conduct the electricity that the cable is meant to carry. High resistance at the interface leads to
inefficient conductance and overheated interface and since the fastening of the wire to the terminal
usually is done by a heat process such as soldering or welding which in turn may deteriorate
properties in the area where an efficient metal to metal contact is essential for ensured conductivity.
The terminal can also be mechanically fastened to the aluminium conductor and to ensure a proper
electrical contact the insulation on the wire, in the area in which is going to be mechanically crimped
firstly removed or penetrated. Due to aluminium oxidation and corrosion, crimping of aluminium
wire compare to copper wires is more complicated and requires that one have to consider how to
get good electrical conductivity and resistance of air infiltration and moisture between the
aluminium wires. In addition to this the cable have to be smooth and straight in order to avoid stress
concentrations, further provide a wire and terminal combination with a coherent and strong
geometry [76].
A patented invention shown in Figure 48 with a shape known as a CRN terminal in the industry. It is
based on a one piece integral electrical terminal made of aluminium which have a mount portion B
and a wire receiving portion A, that is cylindrical shaped but can be made of a variety of shapes and
an aluminium stranded wire C, with an insulating sheath D, an abrasion sheath with two holes where
the terminal can be fastened. The receiving portion has a sealing portion with an integral seal ring
that work as sealing [76].
Figure 48. Aluminium terminal with integral oxide breaker [76].
The contact portion with an integral oxide breaker with several tapered protrusions that breaks
through the oxide layer of the conductor strands and the receiving portion have a seal portion with
integral sealing for protection of the insulator on the wire [76].
65
4.5.2 Favourable electric connection of wire harnesses in automobiles
A Japanese invention relates the excellent electrical connectivity of a crimped contact of a stranded
aluminium wire or aluminium cable end structure for electrical connection of wire harnesses such as
battery cables in automobiles. An aluminium cable is composed by aluminium stranded wire which is
the electrical conductor and a crimped contact is used as the connection part. The crimped section is
then equipped with a connection terminal at each end of the cable in order to be able to connect it
to different electric instruments. The contact shown in Figure 49, has a U-shaped cross section 1
with a bolt fastening part 4, in the inner crimping section 1, the serration 3, is formed of several
concave grooves which prevent the stranded wires from coming out since the stranded wires are
fitted into the grooves 2 of the serration 3. A hole 5 in which bolt can be pierced is drilled in the
fastening portion 4 [77].
Figure 49 Crimp contact for an aluminium stranded wire [73].
The stranded wire is inserted into the crimping portion 1, the side walls 6, are pressed from the
outside which crimp the wire an d the crimping portion to each other.
Good electrical connection is achieved when the oxide film on the stranded wire is broken. In order
to improve the electrical contact between the crimped contact and aluminium stranded wire and
prevent the aging deterioration in electric connectivity and mechanical connectivity. A crimp contact
with a serration in an inner face of a crimping portion, of the crimp contact with a d/e ratio of at
least 0.33, in which e represent the diameter of the wire while d represent the depth of a groove
forming the serration with 3 or more grooves in order to achieve a stable electric connectivity and
low contact resistance. The properties of a copper or copper made alloy crimping portion of the
crimping contact, made for aluminium wires is shown in table 15 [77]
Table 15. Properties of a copper made crimping contact.
Stress relaxation
70 [%]
Electrical conductivity
25 [% IACS]
Tensile strength
400 [MPa]
Vickers Hardness
90 [N/mm2]
66
4.5.3 Plasma soldering of copper connector to an aluminium conductor
A patented method by Gebauer & Griller for connecting a copper or copper alloy made connector to
an aluminium made conductor is based on coating the surface of the connector by nickel and then
welded to aluminium conductor by way of zinc, therefore a direct contact of aluminium and copper
can be avoided. It is known that if aluminium conductors is welded or soldered to a terminal and in
particular cable lugs they get adhesively bonded by means of electrically conducting plastic material
which connect them to one another in a locking manner by compression or deformation. Due to this
it is necessary that the aluminium conductor is not in direct contact to the copper terminal since
presence of a liquid act as an electrolyte that effects the connection, as an electrochemical process
takes place and break the mechanical and electrical connection. The terminal and conductor can be
connected by different methods such as plasma welding and crimping. The present zinc will fuse into
the conductor when the terminal and conductor is connected by plasma welding process. If the
terminal has a sleeve where the conductor is fitted onto, zinc is applied at the end face of the
conductor, the conductor and sleeve can be connected by mechanical deformation of the sleeve or
by crimping the sleeve. Due to this, zinc will fuse both to the conductor and terminal element [78].
4.5.4 Compression
An electrical connection method patented by Mecatraction proposes a terminal made of aluminium
for electrical connection that include a tubular shaped shank and a recess that partially is filled with
contact grease with a stopper which keep the grease in recess. The grease volume is chosen to
correspond with the free volume of the recess after cable insertion and crimping. The crimping
portion can be deformed radially and the stopper has a transverse membrane that can be torn by
the electric cable when it is inserted into recess and have one portion placed in crimping portion and
the grease is filled in bottom of membrane and recess. The grease enable breakage of the aluminium
film formed on the surface of the recess that is exposed to air and the recess’s opening is then
sealed by silicon. The purpose of the stopper is to retain the grease and work as sealing to prevent
water penetration, silicon as sealing material [79]. An example of compression as crimping method
and how the cross section in the crimped area changes after crimping is shown in Figure 50.
Figure 50. An example of compressed terminal [80].
67
5
Methodology for testing electrical connectors in Scania trucks and
busses
Evaluation of the connector’s performance and behaviour is one of the most essentials for reliability
and stability control of electrical contact in order to assess potential risks of failures. The chemical,
thermal, mechanical and electrical effect on the electrical connectors have to be tested in order to
guarantee high conductance between cable and connector and a good electrical contact which fulfil
the requirements of ISO standards. Testing methods for assessment of electrical connector’s
behaviour are proposed with respect to ISO 8092:2005 that is an international standardization test
method in which testing methods and performance requirements for single and multi-pole
connections used with electrical wiring harnesses in vehicles are specified.
The requirements and main assessments principles of electrical connectors used in vehicles is
summarized as follows:
•
•
Reliability and life-cycle durability of electrical connectors
Corrosion resistance and tightness test for sealed interfaces in order to prevent galvanic
corrosion.
• Good metal to metal contact between the conductor and connector to ensure continuity of
electricity.
• High mechanical tensile strength that meet the demand of handling connectors, plugs and
terminals.
• Ensure that the terminals and the conductor is in contact to each other during thermal shock
cycles which is due to the thermal expansion which may lead to increased resistance and
heat.
• Stress relaxation and creep, relaxation over time may result into higher resistance
All the testing are performed with respect to internal, general regulation that take into account the
requirements for environmental factors affecting electrical and electromechanical devices with
additional product description information that is need which all, is in accordance to ISO standards.
5.1.1 Tensile strength of conductor to contact attachment
Mechanical tensile strength of the conductor to contact attachment is done using a test apparatus
operated at a constant speed with a range 25-100 mm/min. Each sample is attached to
corresponding cable as specified by connector manufacturer and if more than one cable is attached
the force applied should be according to Table 16 in which the minimum tensile strength of
conductor crimp with non-specified nominal cross sectional area can be determined by interpolation.
The isolation of the cable should be mechanically ineffective so the test is performed on the contacts
and the conductor crimp should withstand the minimum tensile strength of conductor crimp
specified in Table 16 [81].
68
Table 16. The minimum tensile strength of conductor crimps [81].
Nominal cross section area of cable [mm2]
Minimum tensile strength [N]
0.22
30
0.35
50
0.5
60
0.75
90
1
100
1.5
150
2
175
2.5
200
3
260
4
310
5
355
6
360
10
380
5.1.2 Tightness demand and corrosion resistance
If the connectors are sealed and used with one part that is attached to a component, the testing
condition is then exposing the connector and the component to submersion in water. The
requirement is that no intrusion of water is allowed and the leakage current at a 48 V applied
voltage should not exceed 50 µA which is measured during the last exposure cycle when the
connectors are in water. Current leakage is then measured according to ISO 809 where the samples
are immersed into deionized water with 5 % NaCl, 0.1 g/l wetting agent and a temperature of 23
±3 °C. A colouring agent is included in order to visualize and check the ingress of liquid into the
sample after the electrical test [82] [81].
If the connector is permanently attached to a component, both the connector and the component is
exposed to corrosion test with respect to the requirements set for the component otherwise
corrosion resistance testing of all the connectors is done for 500 hours in 40°C with a humidity of ≥
93 % but if the component is known to withstand exposure to corrosive environment the testing can
be reduced to 240 hours. Salt spray test is applied for sealed connectors in 168 hours and then
placed in a temperature chamber at +80°C for 48 hours. The salt spray is a sodium chloride solution
with a concentration of 50 g/l and a conductivity of 20 µS/cm is used in order to simulate a corrosive
environment. The corrosive behaviour of the connector metal is evaluated and measured by
morphological approaches such as SEM/EDS or metal loss. The requirement on the connectors is
that no corrosion on the pins is allowed and it should be possible to un-mate the connector after
test termination. No water intrusion is allowed in the sealed connectors and an insulation test to be
performed after the termination of the corrosion test [81].
69
5.1.3 Thermal aging and contact resistance
In order to test the voltage withstand and contact resistance an voltage of 1600 V DC is applied at a
humidity of 45-75 % for 1 minute across the contacts with the requirements that, no dielectric flashover or break down is allowed, different conductive demands depending on usage purpose of the
connector, for instance contact resistance of ≥ 2 mΩ for low conductive demands while 0.02-0.05
mΩ is a typical contact resistance value for ring bolted connections in Scania [82].
5.1.4 Vibration and temperature
If the connector is permanently attached to a component, then the connector together with the
components is exposed to a vibration test. Otherwise the connector is exposed to random vibration
type at different frequencies and levels depending on if the if the connector is aimed to be used in
chassis, cab or power train. The temperature range is -25 to + 85°C with a temperature change range
of 1-5 °C/min and a exposure time of 72-210 hours depending on if the connector is going to be used
in the cab, chassis or powertrain [82].
5.1.5 Heat evolution at the contact attachment
The conductor to contact attachment’s tensile strength is evaluated by attaching the sample to the
corresponding cable or the cable specified by the connector manufacturer and then then tested at a
constant speed within the range 25-100 mm/min. the effect of cable insulation should be
mechanically rendered and the test should be performed on the contact only. If more than one cable
is attached to the connector, the applied force should be according to Table 16, by using different
samples [81] [82].
5.2 Experimental procedure
Testing of the tensile strength of conductor to contact attachments has been performed. The tested
aluminium cables were from TE connectivity [75] as described in section 4.5, it is a modified crimp
featured with “shark fin shaped serration”. The tests were carried out at Scania laboratory and the
result is evaluated according section 5.1.1. Tensile strength test were performed on cables with CSA
of 2,5 mm2 and 4 mm2. The result showed that the 2.5 mm2 cables had 18 % lower tensile strength
then the minimum tensile strength requirement while the 4 mm2 cables had 36% lower tensile
strength then the minimum requirement. The setup of the test is shown in figure 51 and 52.
Figure 51. Tensile strength test of aluminium cable with CSA
of 2.5 mm2
Figure 52. Tensile Strength test of Aluminium cable with
CSA of 4 mm2.
70
6 Discussion
Creep is an irreversible deformation which effect the conductor termination by raising the potentials
for overheating. As indicated in Table 6, aluminium’s tendency for creep is higher than copper at
normal operating temperature and room temperature. Additionally aluminium’s tendency to cold
flow under pressure is high, this leads to loosening of the terminals [14]. Creep coupled with cold
flow is a factor challenging termination of aluminium conductors. Due to this aluminium should have
its own regulations and standards for torque settings for terminals, connection and terminating
methods [17].
Aluminium has larger thermal expansion coefficient than copper [17], this may result into
incompatible expansion between the terminal and conductor. Coupling dissimilar metals with
increase the thermal expansion leading to loosening of the connection over time and increased
contact resistance which in turn leads to arcing and overheating of the surface which challenge the
safety and reliability of the connection [17]. Additionally galvanic corrosion which occurs when two
dissimilar metals are in contact with each other and in presence of an electrolyte is another issue
that affect the contact behaviour, weaken the conductor and lead to contact failure. Using different
metals at the termination join hampers handling high temperature, high vibration conditions and
thermal shock due to aluminium’s thermal expansion coefficient which along with those conditions
deteriorate the connection with a failure as result.
With respect to corrosion issues indicated in Table 6, aluminium conductors have higher tendency
for corrosion owing its low zero potential. Connecting aluminium with metals having higher zero
potential results into galvanic corrosion. Formation of insulating Al2O3 is one of the most damaging
factors since it inhibit current flow, cause formation of hot spots in the contact spots.
A force is applied during termination and crimping, this applied force reduces the contact pressure
which results into non-gastight contact interface where corrosion and oxidation can occur even if
the termination joint is sealed. This result into loosening of the connection and increased contact
resistance which generate heat and further creep and cold flow of the conductor. Additionally
increased resistance result into fatigue behaviour and high voltage drop [32].
Aluminium is a ductile metal with low melting point which enables customary processing with
excellent workability but since it has one third of mechanical strength of copper one have to take
that into account dimensioning the conductor in order to achieve the desired mechanical strength
for both wire and enough pull out strength of the connection. Additionally, aluminium forms an
oxide layer which have an insulating function and protect the material from corrosion but for a good
electrical connection the oxide layer have get destroyed during termination [78]. The formed oxide
layer can be destroyed methods such as serration [78] during crimping, an additional oxide breaker
with tapered protrusions which breaks through the oxide layer [79] or an electrical connection
including tubular shaped shank that is filled with contact grease which enable breakage of the oxide
film formed at the surface [82].
Coating of the connection material is done for avoiding formation of insulating surface layer,
mechanical wear and corrosion decrease the hardness and promote conductivity [3].
Conventional crimping that is used for copper is not suitable for aluminium conductors. Aluminium,
according to study done by Otsuka [6], for stabilization the contact resistance, aluminium require a
71
higher crimping compression than copper in order to improve the wire retention force that enable
electrical connection.
The conductive area is essential for a reliable electrical contact which in turn is affected by both
internal and external factors, shown in Figure 17 but according to [43] one of the most simple and
efficient way of achieving large contact points are by lubrication and mechanical abrasion which
together with application of a contact aid (grease) prevent oxidation of the metal.
72
7 Conclusion
The conventional crimping that is used for copper is not suitable for aluminium conductors. Other
solutions such as
• Welding
• Soldering
• Modified conventional crimp
Have to be adapted for enabling a safe and reliable usage of aluminium conductors in vehicles.
Welding and soldering is the preferable solution since they offer a reliable contact that last during a
long period of time with less damage and degradation. The most common for aluminium conductors
are:
•
•
•
•
Friction welding
Ultrasonic welding
Resistance welding
Plasma soldering
Even though welding and soldering is the most preferable solution, modification and enhancement
of the conventional crimp by for example applying sealing, shrinkage tube or sleeve for protection of
conductor from being exposed to external environment.
Breaking the oxide layer by crimping can also be done by featuring the F-crimp barrel with serration
or adding extra elements such as “oxide breaker and sealing ring into the terminal. However, there
are many different patented solutions when it comes to enhancing the conventional crimp and make
it more suitable for aluminium’s properties where the most preferable depend on application it is
going to be used for.
Additionally, coating with copper, tin and nickel as treatment before crimping is used as corrosion
protection of termination joint.
73
7.1 Challenges of electrical connections for aluminium conductors
If Scania wants to replace copper cables by aluminium cables, an awareness of potential problems
such as corrosion, creep and relaxation that leads to increased contact resistance and failure is
essential. As shown in figure 53, the implementation of aluminium conductors faces 6 main
challenges which Scania have to be aware of.
Figure 53. Six main challenges of implementation of Aluminium conductors in trucks and busses.
It means that that there is no best solution and it is not easy to determine whether welding or
crimping is the best solution since aluminium is a demanding material. Depending on the size of the
cable, the required and suitable technology varies. When it comes to choosing the “right” supplier
for the selected component one have to consider if the supplier is aware of the difficulties with
aluminium contacts and how do they solve these challenges to ensure a liable connection that last
and meet the Scania standards.
74
8 Future work
In order to be able to determine the most suitable electrical connection method for different
aluminium conductors, Scania will need:
•
•
•
•
A closer collaboration with supplier that Scania is working with at the first stage in order to
see what kind of solutions they offer.
Order samples that can be tested at Scania in order to see if they fulfil the requirements of
Scania vehicles.
Cost assessment of each approach.
Evaluation of the most suitable connection method for each application and if it is usable,
even in aftermarket and service.
75
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