Plasma welding of aluminium materials

Plasma welding of aluminium materials
Plasma welding of aluminium materials
Direct or alternating current?
D. Dzelnitzki, Mündersbach
Summary
After steel, aluminium is the most widely used metal. It has many positive properties that recommend it for diverse
applications. However, its high thermal conductivity, high thermal expansion and high-melting oxide layer make
welding difficult. In this respect, aluminium requires a welding process like plasma welding, which introduces the
heat in a controlled manner, eliminates the oxides and can be used efficienty. This report explains the principles of
this welding process and shows the different variants of the process. By considering the latest power source and
process technology, the user will be enabled to recognize his own possible uses and convert them into practice.
Current examples of use, technological performance data as well as future development trends complete the
picture of a joining technology which produces high-quality welds on aluminium materials.
1
Introduction
fundamental differences are found which must be
taken into consideration when processing by welding.
The thermal conductivity of aluminium exceeds that of
unalloyed and low-alloy steels by a factor of 3 to 4,
and it is even 12 times higher than that of high-alloy
steels. Its thermal expansion is about twice that of
steel and 1.6 times that of high-grade steel [3].
Such factors of course make it difficult to melt the
material, and the high-melting oxide layer additionally
reduces its suitability for welding. The user therefore
needs a welding process that introduces heat in a
controlled and concentrated manner, reduces
distortion of the components, eliminates the oxides
reliably, and can be used efficiently. In this respect,
everything suggests plasma welding.
To a large degree, metals determine industrial
development. Aluminium has proved to be particularly
versatile in this respect. Its positive properties allow it
to compete with other materials such as steel, copper
or wood and to rival them for the conventional fields of
use. Aluminium is lightweight, and easy to shape and
process. It has a high thermal and electrical
conductivity, is weatherproof and resistant to foods
and a wide variety of chemical substances. Alloying
elements increase its strength. At low temperatures
the notched impact strength decreases only slightly,
and aluminium does not become brittle. For these
reasons, after steel aluminium is the most widely used
metal [1, 2].
When the physical characteristics, table 1, of the two
materials are compared with one another,
Table 1
2
Plasma arc welding has developed from the TIG
process. While in TIG welding the arc burns freely
between a non-consumed tungsten electrode and the
workpiece, in plasma welding it is additionally
constricted by a nozzle and a gas stream.
Comparison of the important physical parameters of
aluminum and iron [1.2]
Physikal parameters
Unit of
measure
g / mol
3
g / cm
Al
1/K
24·10
Al2 O3
Fe
Atomic weight
Density
crystal lattice
modulus of elasticity
RPO,2
Rm
specific heat capacity
melting point
thermal conductivity
specific elektric
resistance
Extension coefficient
Oxides
26,9
2,70
kfz
2
3
N / mm
71·10
2
N / mm
10
2
N / mm
50
0,88
J / (g·K)
°C
660
W / (cm·K) 2,30
28 to 29
µΩ·m
Melting point of Oxides
°C
© 2000 EWM HIGHTEC WELDING GmbH
-6
2050
Principle of plasma arc welding
55,84
7,87
krz
3
210·10
100
200
0,53
1539
0,75
97
-6
12·10
FeO
Fe3O4
Fe2O3
1400
1600
1455
Figure 1. Principle of the plasma welding process
Because of the high arc temperatures, this small
stream of gas, also called plasma gas, is largely
ionized, and characterized by a high number of
charge carriers. When it flows through the plasma
nozzle it acquires a high exit speed, and the arc
acquires the form of a plasma stream.
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The temperature and field strength increase with the
volumetric rate of gas flow, and form an arc column of
very high energy. Different gases and changes in the
gas stream also influence the pressure effect and the
degree of contraction of the plasma arc. The
constricted plasma between the tungsten electrode
and the workpiece has a high energy density. In the
center of the stream the temperatures rise to 15,000
to 20,000 K [4].
In contrast to the TIG arc (11,000-16,000 K), which with its conical shape - carries a large proportion of
the energy in the marginal regions, the constricted
plasma jet transfers it directly into the workpiece. The
melting bath remains small, the heat-affected zones
are narrow, and the distortion is low.
Because of the substantially smaller beam divergence
of the stream, the plasma arc tolerates wider changes
in distance between the torch and workpiece. A 20%
increase in the cross-section of the plasma jet
compared with the TIG arc therefore allows a 10-fold
change in length, figure 2 [5].
Figure 3. Seam upper sides and fusion penetration in plasma
welding of AlMg3, t=3mm. no filler material
a) Plasma welding at the positive pole
I=35A, U=26V, Vs=40cm/min,
plasma gas: Ar, shielding gas: 70%Ar / 30%He
b) Plasma welding at the negative pole
I=70A, U=20V, Vs=90cm/min,
plasma gas: 30%Ar / 70%He, shielding gas: He
c) Plasma welding with alternating current
I=45A, U=26V, Vs=40cm/min,
plasma gas: Ar, shielding gas: 70%Ar / 30%He
In order to illustrate the physical principles in more
detail, the welding operations were performed so that
a continuous weld seam was formed with the same
gas flow on the upper side of the sheet, and no
incipient fusion was to be found on the underside of
the sheet. The welding-current, welding-voltage and
welding-speed settings and the choice of gas
depended on the requirements of the process.
2.1
Plasma welding with a positive-pole electrode has the
distinctive feature of a very good cleaning effect, and
produces a high-quality seam. Because of their
relatively high mass, the gas ions tear open the oxide
layer (melting point about 2,050ºC) of aluminium
alloys mechanically at the moment of impact.
According to current knowledge, the theory of this
oxide skin being destroyed by electrons emitted from
the aluminium is improbable, since the work required
by the electrons to leave the oxide is half that required
to leave pure aluminium [2, 4].
The emission of electrons therefore starts from the
oxide layer, and not from the metal underneath, figure
4.
Although the oxides ensure the good corrosion
resistance of aluminum, on the one hand, since they
regenerate immediately in the atmosphere, they
produce non-metallic inclusions by sinking in the
melting bath, on the other hand [2, 7].
Another problem in welding aluminium materials is the
sharp jump in the solubility of hydrogen. A good
solubility of hydrogen in the liquid state is
counteracted by a low solubility in the solid state.
Figure 2. Comparison of TIG and plasma arc [15]
This stability makes the plasma arc insensitive to
edge misalignment in the seam area. The ratio of
seam width to fusion penetration (seam depth) is
about 1:1 to 1:2 [6]. The weld profile is narrow and
deep, so that the amount of filler material and the heat
input can be reduced significantly.
In contrast to the MIG process, the energy input and
wire feed are entirely separate functions in plasma
welding. This advantage allows an exact optimization
of the process.
Plasma welding processes with direct current, with the
electrode at the positive or negative pole, and with
alternating current are described and compared with
one another.
The starting point of the reflections is the remelting of
a sheet metal surface (material: AlMg3, t=3mm) by the
three process variants, figure 3.
© 2000 EWM HIGHTEC WELDING GmbH
Plasma welding at the positive pole
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A sufficient heat concentration to liquefy the oxide skin
is generated during welding by using helium, which
has a considerably higher arc voltage for the same arc
length and a better thermal conductivity at high
temperatures than argon, or by using gases with a
high helium content [2, 4, 8]. The composition of the
oxide skin plays an important role here. Aluminiumsilicon alloys particularly favour plasma welding at the
negative pole, since the melting temperature of the
oxide layer decreases with increasing silicon-oxide
content. On the other hand, aluminum alloys
containing magnesium make this welding process
difficult, since the absorption of magnesium increases
the melting temperature of the oxide layer [16].
The thermal decomposition of the oxides in negativepole operation, figure 3, does not, however, lead to
the high quality of the seam surface characteristic of
plasma welding at the positive pole, although the
mechanical qualities of the weld join are very good.
In contrast to positive-pole operation, the negativepole electrode can be subjected to a significantly
higher current. It is not exposed to electron
bombardment, and is therefore exposed to less
thermal stress. In addition, more electrons are emitted
at the hot tip of the electrode than from the colder
workpiece surface with the reverse polarity [2]. The
arc voltage is therefore lower.
Since the plasma in the arc column flows almost
entirely from the cathode, the constricted plasma jet
has a very high energy density [4]. The pressure
effect of the arc is assisted additionally by the plasma
gas stream. A deep fusion penetration develops, even
more intensified by the larger amount of heat in
welding under helium or helium with a low argon
component. Under the same conditions, figure 3, the
welding speed must be increased.
To ensure an adequate thermal elimination of the
oxide, the welding current is increased and the
distance between the plasma nozzle and workpiece is
shortened, depending on the composition of the
oxides. This leads to a further reduction in the welding
voltage.
Plasma welding at the negative pole also takes place
without noise. The plasma arc is ignited with a pilot
arc which burns between the pointed electrode and
the plasma nozzle.
Figure 4. Cleaning effect and electron emission work in welding of
aluminium alloys [11]
During rapid solidification of the melting bath
(about 660ºC), gas bubbles containing hydrogen can
therefore easily be frozen in [8].
The hydrogen is formed at high arc temperatures from
moisture bonded in the oxides, and causes pores. An
intensive cleaning effect such as the effect that takes
place with plasma welding at positive pole, figure 3,
eliminates these oxides, vaporizes impurities on the
aluminium surfaces [4] and this way provides the
fundamental conditions for optimum weld seam
properties. However, this polarity is associated with
high thermal stresses on the electrode. The kinetic
energy of the electrons accelerated towards the
electrode is converted into heat there and causes
severe heating [2]. Thicker electrodes with
hemispherical ends which are capable of removing
the heat generated sufficiently rapidly must be used.
In this case, the plasma flow from the anode
determines the shape of the arc and therefore the
pressure effect on the melting bath. A relatively low
penetration depth is characteristic, figure 3. The
reason for this can be found in the low energy density
with the thicker positive-pole electrodes. The arc is
distributed homogeneously over their surface and is
stable with respect to time [4].
The arc constriction by the plasma nozzle and an
increase in the amount of plasma gas, however,
enable the user to increase the pressure of the arc to
such an extent that not only very small but also large
workpiece thicknesses can be welded. In this respect,
the fusion penetration depth can be varied ideally via
the volumetric flow rate of plasma gas. The high
temperatures of the plasma jet also have the effect
that the welding current required is only half that for
TIG welding with alternating current.
There is no noise nuisance during plasma welding at
the positive pole. The ignition operation is initiated via
high voltage pulses which jump from the electrode to
the workpiece.
2.2
2.3
Plasma welding with alternating current is a
compromise between the two direct-current variants
described. It combines an adequate cleaning effect,
figure 3, during the positive phase with the very high
energy density in the negative phase. Since the
electrode can cool down again each time in the
negative half-wave, its current-carrying capacity is
improved.
In plasma welding it is particularly important, in
addition to the field emission, that the high
temperatures required for thermal emission are
reached [4]. The very high temperature difference at
the alternating cathodes therefore has an adverse
effect on welding of aluminium, since the emission of
Plasma welding at the negative pole
Plasma welding with a negative-pole electrode is an
only little-used process variant for welding aluminium
materials. The cathodic cleaning effect of ions
described cannot take place at this polarity. The highmelting oxide layer must therefore be destroyed
differently.
© 2000 EWM HIGHTEC WELDING GmbH
Plasma welding with alternating current
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electrons from a composite material increases by
thermal emission with increasing temperature.
Electron emission from the relatively cold melting bath
(melting point of aluminum about 660ºC) is thus
counteracted by an easier emission at the highly
heated tungsten electrode (melting point about
3,350ºC). When the negative pole is applied to the
electrode, considerably more current therefore flows
than if the parent metal is the negative pole [4. 9]. In
the positive phase, the resistance of the arc
increases, on the other hand, because of the smaller
number of charge carriers. Lengthening the arc has a
similar effect, since it is likewise accompanied by an
increase in voltage.
Conventional welding transformers with falling
characteristics compensate the increase in the voltage
by a drop in current strength, so that the arc output
remains constant in the two half-waves. However, the
sinusoidal current-output wave form typical of this
technology is characterized by poor ignition properties
and an unstable arc. The low current strength in the
positive half-wave results in a negative direct-current
component in the alternating current, which is called a
rectifying effect. It impairs the cleaning effect, and by
premagnetizing the transformer coils causes greater
heating of the power source. The negative directcurrent content is therefore usually eliminated with a
filter capacitor.
In modern welding units, the welding current remains
unchanged in the two half-waves on the basis of
constant current characteristics. However, because of
the lower number of charge carriers, the arc voltage is
considerably higher in the positive half-wave [4]. The
arc voltage with a negative-pole electrode can be
about only half of that with positive-pole operation,
depending on the settings [10].
This effect also explains the different welding currents
between plasma welding with alternating current and
plasma welding at the positive pole, figure 3.
To achieve about the same welding output with the
same framework conditions, the welding current had
to be increased to 45 A, since at 35 A the sheet metal
surface could not be melted.
In this connection, another setting plays an important
role - the balance. This describes the ratio of the
duration of the positive half-wave to the negative halfwave based on the electrode. Values of about 30/70
to 50/50 have proved appropriate for plasma welding
with alternating current. Lower values reduce the arc
output, and higher balance settings place too great a
load on the electrode, figure 5 [10].
The almost vertical current output wave form when
passing through zero in particular is the decisive
advantage
Figure 5. Different arc voltages during the positive and negative
half-wave on the electrode lead to different arc outputs in
alternating welding current when the balance setting is
changed [10].
of this modern power source technology. The arc
dead times become so extremely short that high
voltage pulses are no longer necessary to assist reignition at each change in pole. Ionized gas residues
which still occur to a sufficient extent in the arc space
also ensure reliable ignition when the polarity on the
electrode is changed from negative to positive [10].
In spite of the emission of electrons thereby being
made difficult, burning of the arc is very stable.
In order to reduce the high noise nuisance associated
with plasma welding with alternating current, the
current output wave form used is not completely
rectangular, only when passing through the zero line.
However, the flatter increase in current leads to a
reduction in the arc output when the alternating
current frequency is increased beyond a certain value.
The fusion penetration with this process is also
controlled by the volumetric flow rate of plasma gas.
The arc ignition with a pilot arc between the electrode,
which is somewhat blunter compared with plasma
welding at the negative pole, and the plasma nozzle is
achieved with direct current (negative-pole electrode).
3
System requirements
3.1
Welding units
Plasma welding is a demanding production process.
The very close links between materials technology,
process technology and electrical engineering impose
high demands on the equipment.
Especially the requirement of being able to carry out
different welding processes with one welding unit
requires a complete system which allows the user to
perform his particular welding tasks to the optimum.
Three components form the basis of this system:
·
·
·
the power module
the control module
the torch connection module
In recent years, the Inverter has become accepted as
the power module of a welding unit. The advantages
of this principle are the relatively small dimensions,
the high efficiency, the insensitivity to variations in
mains voltage, and therefore a very good
reproducibility of the welding parameters.
The power source control must be capable, together
with the power unit, of switching the various types of
© 2000 EWM HIGHTEC WELDING GmbH
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The "tool" arc is finally managed from the torch
connection module and is optimized by the torch
specifically for each particular case. The connection of
either an electrode holder, TIG torch or plasma torch
determines the welding process. A compact system of
independent modules which act together to form a
welding unit results.
In the "TIG AC/DC-P" welding machine concept, a
series of machines exists to cover both MMA and TIG
welding and also the field of plasma welding, figure 7.
current, such as positive or negative pole electrode
and alternating current operation.
In addition, the control unit stores the preset welding
parameters.
The Inverter and control system of course also
determine the ability of the arc to react very rapidly to
external influences in order to keep the output
parameters constant regardless of the cable length in
the welding current circuit.
Above all, however, the control unit is that part of the
welding unit which enables humans to manage this
system.
The machine is easily handled with the operating
module, which contains all the necessary functions,
figure 6.
Figure 6. Operating module of a plasma welding unit for direct and
alternating current
To ignite the arc, the ignition current IS can be set as a
percentage of the main current I1. In addition to the
main current I1, a reduced welding current I2 can also
be selected to allow an intermediate lowering for
better control of the melting bath.
Both, the time taken for the welding current to rise
from the ignition current to the main current (up-slope)
and also the down-slope time for defined reduction at
the end of the weld seam, are freely adjustable, too..
Other elements are used to set the gas post-flow time,
the choice of operation (non-latched, latched, footoperated remote control and MMA welding) and for
the currentless program test.
Welding where there is an increased electrical danger
(safety sign) can be made safe with an optional
protection circuit which the welder can test before
starting his job.
The toggle switch singled out allows the polarity to be
changed (alternating current AC, direct current
electrode negative DC-, direct current electrode
positive DC+, alternating current AC with pilot arc),
figure 6.
Other potentiometers located in this area influence
alternating current operation: frequency (50-200 Hz),
balance and formation of spherical caps with TIG
ignition.
Electrical disturbances, lack of water in the torch
cooling system (internal and external) and voltages
exceeding the permitted voltage are displayed with
LEDs and cause the welding unit to switch off
immediately.
© 2000 EWM HIGHTEC WELDING GmbH
Figure 7. Multi-process inverter TIG 450 AC/DC-P welding unit
The power module is graduated in 300 A (35% duty
cycle) and 350 A/450 A (60% duty cycle).
A pilot-arc module takes care of ignition in plasma
negative-pole welding and plasma welding with
alternating current.
This system is completed by several remote controls,
e.g. pulsed and foot-operated remote control.
In addition, welding data can be documented with an
interface in combination with the measured-value
recording and monitoring software Q-DOC 9000,
figure 8.
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and machine use, the respective wire diameters of 1.2
mm to 3.0 mm have proved appropriate.
In the area of welding gases, a wide range of
variations are available to the user. In addition to
argon, argon with helium contents and helium, gas
mixtures to which further tiny amounts of a second or
third and fourth gas in the vpm range (vpm = volume
parts per million, 1 vpm = 0.0001 %) are added have
been developed in recent years. The aim of these
mixtures is to improve arc stability and the quality of
the surface, a reduction in porosity and improvement
in fusion penetration. These gases are oxygen (O2),
nitrogen (N2) and nitrogen monoxide (NO).
In contrast to hydrogen, oxygen and nitrogen are not
soluble in aluminium and therefore do not form pores
[2, 11].
The problem of such active additions to the shielding
gas possibly attacking the tungsten electrode [12] in
TIG welding does not exist in plasma welding if the
plasma gas consists only of inert gases. These protect
the electrode and ensure very long service lives. The
shielding gas can have any desired composition.
Argon has proved to be a suitable plasma gas in
plasma positive-pole and alternating-current welding.
The shielding gas is, as a rule, argon or an
argon/helium mixture (70% / 30%), which produces
very good weld qualities, figure 9.
Opt. Connection
PCINT 2
Interface
PC INT 1,2,3
Welding machine Plasma series
Windows Software
Q - DOC 9000
Printer for
Documentation
Figure 8. Graph of the welding parameters during plasma welding
3.2
Plasma torches
The plasma torch is a component of extremely high
importance in the plasma-welding process. Its
construction, together with the welding machine,
determines the reliability of the process. While a
number of torch types can be chosen for the negativeelectrode pole method, only a few of them can be
used with the positive-pole method or alternating
current. This is, above all, because the electrode is
exposed to high thermal stresses with the positivepole method and requires intensive cooling.
In all process variants the plasma nozzle undergoes
high exposure to heat. However, reproducible welding
results can be expected only under constant thermal
conditions in the torch head. For every plasma torch
there are several plasma nozzles, whose bore sizes
depend on the current strength and amount of plasma
gas. The nozzle bore determines the shape of the
plasma stream.
As well as having adequate cooling measures, the
electrode should, of course, not be positioned offcenter in the torch and grinding. The distance
between the electrode end and the plasma nozzle
should also be constantly adjustable.
The path of both the plasma gas and the shielding gas
must be optimal. The whole torch head should be
relatively small to ensure accessibility to the join even
in restricted places. Ideally, the torch is constructed in
a way that all three plasma processes can be carried
out with different forms of electrode and nozzle.
Manual and machine torches are used.
3.3
Figure 9. Seam construction in plasma positive pole welding with
the keyhole principle,
parent metal AlMg3, t=4mm, I=75AU=41V, Vs=27 cm/min,
filler material:AlMg5, d=1.6mm, plasma gas Ar,
shielding gas:70%Ar / 30%He (left),
shielding gas:Ar / 150vpm N2 (right)
More detailed studies, specifically with the plasma
positive-pole process using shielding gases with
additional contents of nitrogen (150 vpm), figure 9, or
nitrogen (70 vpm) and nitrogen monoxide (300 vpm)
have also produced good results.
It is known that admixtures of nitrogen in TIG welding
have the effect of a more concentrated fusion
penetration [11, 12]. A similar effect is also found in
plasma welding. The influence of these doped gases
probably consists in focussing of the plasma jet after
leaving the plasma nozzle. Both weld seams, figure 9,
were produced by machine by the keyhole process
and subjected to radiography examinations,
metallography analysis and hardness measurements.
Evaluation of the test reports shows evidence of very
good seam qualities. The seam surface in particular is
very fine and regular in structure in plasma positivepole and alternating-current welding.
Filler-material/shielding-gas
combinations and working areas
In principle, all weldable aluminium alloys can be
handled with the plasma welding process. The choice
of process depends on the composition of the
material, the geometry of the component and the
degree of mechanization or automation of the
application. The welding filler materials are
determined by the alloy type of the parent metal and
the mechanical properties of the weld join. For manual
© 2000 EWM HIGHTEC WELDING GmbH
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The cylinder base is about 4 mm thick and must be
welded through completely to be able to withstand the
very high operating pressure. The deep fusion
penetration profile at a relatively low welding current
and therefore the smaller torch qualify the process for
this application.
From a material thickness of about 3 mm, keyhole
technology can be applied to plasma welding at the
positive pole or with alternating current. By increasing
the stream of plasma gas, when sufficient energy is
introduced, the pressure effect of the plasma stream
is increased to such an extent that butt joints without
gap are melted through without blowing away the
liquid stream surroundings. If the plasma torch
undergoes a constant forward motion, the melt is
pushed aside by the penetrating plasma jet and flows
together again behind it, figure 10 [13].
The addition of oxygen (300 vpm) to the shielding gas
brings even further improvements here [11, 12].
The plasma gas in plasma negative pole welding
should not exceed a helium content of 90% in order to
ensure an adequate stability of the pilot arc. The
shielding gas is either helium or a helium-rich mixture
with argon contents of less than 30%.
All three plasma processes are flexible in use.
Different working areas can be covered by the various
gases and changes in the amount of plasma gas.
From the very soft plasma arc for surfacing and weld
joining with an inaccurate fit of the workpiece,
especially at small material thicknesses, to the plasma
stream with a high pressure effect for single-layer
welding of large material thicknesses, the appropriate
arc character can be adjusted for any application.
4
Welding applications
4.1
Manual plasma welding
The user should hold the plasma torch just like he
does in TIG welding. A longer torch distance can be
selected for plasma positive-pole and alternatingcurrent welding to make feeding of the filler material
easier.
The plasma nozzle does not only constrict the arc, it
also prevents the electrode from touching the welding
filler material or the melting bath.
Manual plasma welding depends on the torch size.
Small handy torches nowadays can be used in
positive-pole welding to a material thickness of about
3 mm (butt-weld, single-layer) at 35 A. The lower
loading on the torch in alternating current operation
allows welding to a workpiece thickness of 4 mm at
about 65 A under the same framework conditions.
Plasma negative-pole welding is reserved for larger
thickness ranges.
Comparing the features of the different processes,
positive-pole welding emerges as the best. The
current strength required is low, so that less heat is
introduced into the parent metal and distortion is
minimized. Production-related component tolerances
can be controlled and the welder is not exposed to
noise. Typical applications are processing of sections,
pipes and sheet metal.
4.2
Figure 10.
A weld seam of high quality is formed, as can be seen
both on the upper side and underside of the seam and
from the radiography, figure 11.
To close circular seams, the amount of plasma gas is
reduced sharply and the welding current is decreased.
Longitudinal seams are given run-off plates at the
ends [13]. This technology is very effective since it
allows single-pass welding to workpiece thicknesses
of about 8 mm. No production times for preparing the
weld seam are needed any more, and the costs for
expensive filler materials can be reduced.
Keyhole welding is used not only in the gravity
position (PA), but also in the horizontal-vertical
position (PC), figure 12, and the vertical-up position
(PF).
A slight porosity in this plasma welding with a positivepole electrode can be seen only in the upper limit of
the melt, due to rising of the gas bubbles, while the
remainder of the weld is virtually free from pores [14].
The reason lies in the poorer degasification conditions
in the melt in the PC position.
Plasma welding by machine
Entirely mechanical and automated plasma welding
can be used for all material thickness ranges. The
high concentration of heat and high power density
make high welding speeds possible. Welding is
carried out with and without filler material. A
cost/benefit analysis determines the use of the
particular plasma welding process within a complete
production plant. An example of use of plasma
negative-pole welding is gas-tight sealing of the bases
of shock-absorber cylinders.
Since this is fully mechanical welding with short cycle
times, ignition reliability is one of the most important
decision criteria. Only pilot arc ignition can meet this
requirement.
© 2000 EWM HIGHTEC WELDING GmbH
Principle of plasma keyhole welding [13]
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Figure 12.
Figure 11.
Plasma positive pole welding with the keyhole
principle,
I=100A, U=40V, Vs=32cm/min,
parent metal: AlMg3, t=5mm,
filler material: AlMg5, d=1.2mm,
plasma gas: Ar, shielding gas: 70%Ar / 30%He
a) seam upper side
b) seam underside
c) radiography
of this joining process concerning emissions of smoke
and noise, on the other hand. A high energetic
efficiency of more than 90 % ensures economicalt
utilization of energy.
However, this process can only find acceptance in
industry if it can be turned into a true highperformance technology with a high welding output.
The development of modern plasma torches must
therefore be pressed ahead with. They are the key to
opening up new fields of use.
The trend towards lightweight construction requires
welding processes in small material-thickness ranges
and alternative materials. Both, the existing aluminum
alloys and, for example, new magnesium alloys, can
be handled reliably with this plasma welding process.
As in TIG welding, especially fully mechanical and
automated
applications
are
advantageously
influenced by a pulsed welding current and filler
material.
The use of plasma welding processes by machines
with or without keyhole technology opens up diverse
possibilities in civil engineering, in car, railway vehicle
and ship construction, in pipe and profile production,
in machine, plant and container construction and in
the entire chemical industry.
5
Plasma positive pole welding with the keyhole
principle in the transverse position (PC), pulsed
current output wave form,
parent metal: AlMg3, t=6mm (left), t=8mm (right),
additive: AlMg5, d=1.2mm,
plasma gas: Ar, shielding gas: 70%Ar/ 30%He,
pore area: 0.48% (left), 0.23% (right)
Prospects
Plasma positive pole welding in particular will be the
focal point of interest in the coming years. The
reasons are its outstanding cleaning effect, on the one
hand, which results in an excellent weld seam quality,
and the good ecological friendliness
© 2000 EWM HIGHTEC WELDING GmbH
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WM009201.DOC; 08.00
6
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© 2000 EWM HIGHTEC WELDING GmbH
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WM009201.DOC; 08.00
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