MASTER THESIS The Heat Treatment of Nickel Titanium - An

MASTER  THESIS The Heat Treatment of Nickel Titanium - An
Master's Programme in Mechanical Engineering, 60 credits
MASTER THESIS
The Heat Treatment of Nickel Titanium - An
Investigation Using Taguchi's Method of
Optimisation
Myles William Gibson
Thesis in Mechanical Engineering, 15 credits
Halmstad 2015-05-18
Abstract
This study was an investigation of the effect Heat Treatment has on the physical and
mechanical properties and characteristics of Nickel Titanium (NiTi). NiTi has a wide
range of uses, which are dependent on pre-defined, exact forms of the material. For
this reason, it is very important to be able to fully understand the processes used to
tailor the material to exact specifications. Taguchi’s method within the ANOVA
umbrella of variance analysis was used to design an experiment and analyse the data.
The method used was a series of tests of NiTi wire samples, which were subject to a
range of heat treatments with variable temperature, duration and cooling methods.
The samples were then subject to tensile tests to examine the effect the treatments had
on the mechanical properties of the material. An orthogonal array was used to
construct and define the experiments and provide a means of statistically analysing
the results in an efficient manner. The analysis showed that temperature had a
significant effect on the mechanical properties of the material, duration had no effect
and cooling effect had s minimal effect. The yield strength of the material was found
to be highest at 400°C, and the maximum possible yield strength of this material is in
the range 350-400°C. The 500°C heat treatment samples experienced the lowest yield
strength. These trends were caused by precipitate grain growth in the material. It was
also found that the cooling method had an effect on the extension of the samples.
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Contents
Abstract ...................................................................................................................................................................... i
Contents .................................................................................................................................................................... ii
1.
Introduction .................................................................................................................................................... 1
1.1 Background ........................................................................................................................................................................ 1
1.2 Aim of the study ................................................................................................................................................................. 1
1.2.1 Problem Definition ..................................................................................................................................................... 2
1.3 Study Environment............................................................................................................................................................ 2
1.4 Limitations ......................................................................................................................................................................... 2
2. Method ................................................................................................................................................................... 3
2.1 Alternative Method............................................................................................................................................................ 3
2.2 Chosen Methodology for This Project – Taguchi’s Method ........................................................................................... 3
2.2.1 Taguchi’s Method - Design of Experiment ................................................................................................................. 3
2.2.2 Taguchi’s Method – Analysis ..................................................................................................................................... 5
3. Theory ................................................................................................................................................................... 8
3.1 Summary of the Literature Study and State-of-the art................................................................................................... 8
3.1.1 Nickel Titanium .......................................................................................................................................................... 8
3.1.2 The Shape Memory Effect & Superelasticity .............................................................................................................. 9
3.1.3 Uses of Niti ............................................................................................................................................................... 12
3.1.4 Heat Treatment ......................................................................................................................................................... 14
3.1.5 Analysis of Variance (ANOVA) / Taguchi’s method ............................................................................................... 17
4. Results ................................................................................................................................................................ 19
4.1 Preparations and Data Collection................................................................................................................................... 19
4.1.1 Heat Treatment ......................................................................................................................................................... 19
4.1.2 Mechanical Testing ................................................................................................................................................... 20
4.2 Presentation of Experimental Results ............................................................................................................................ 21
4.2.1 Physical Appearance & Characteristics of Samples Post Heat Treatment ................................................................. 21
4.2.2 Ultimate Tensile Strength ......................................................................................................................................... 23
4.3 Presentation of Results Analysis ..................................................................................................................................... 24
4.4 Discussion ......................................................................................................................................................................... 24
4.4.1 R value ...................................................................................................................................................................... 25
4.4.2 Tensile Curves .......................................................................................................................................................... 26
5. Conclusions & Recommendations .................................................................................................................... 29
5.1 Conclusions ...................................................................................................................................................................... 29
5.1.1 Recommendations for future activities...................................................................................................................... 30
6. Critical Review .................................................................................................................................................... 31
6.1 Limitations ....................................................................................................................................................................... 31
6.2 Analysing the work from several points of view ............................................................................................................ 32
6.1.1 Ethical ....................................................................................................................................................................... 32
6.1.2 Social ........................................................................................................................................................................ 32
6.1.3 Economic .................................................................................................................................................................. 32
6.1.4 Environmental........................................................................................................................................................... 33
6.1.5 Occupational Health & Safety .................................................................................................................................. 33
References .............................................................................................................................................................. 34
Appendix I – Code of ethics for engineers ............................................................................................................ 37
Appendix II – The Millennium Development Goals ............................................................................................... 39
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1. Introduction
1.1 Background
Nickel-Titanium (NiTi) is a metal alloy with unique properties that allow it to
function in a special way. It is this capability that leads to it being defined as a ‘smart’
material. This function is known as the Shape Memory Effect (SME). Further to this,
it is said to be ‘superelastic’ (SE). These two capabilities open it to many possible
applications. Such uses are couplings, medical applications, actuators, heat engines
and orthodontic use. In each of these applications, this material is chosen due to its
ability to change shape or its impressive elastic strength.
Heat treatment is a process used to alter the physical and chemical properties of a
material, and it is most commonly used on metals. Heat treatment involves heating or
cooling materials to extreme temperatures and allowing them to cool in a controlled
manner in order to alter the microstructure of the material and modify its properties. It
most commonly has a softening or hardening effect on a metal. It is vital to gain a
comprehensive understanding of heat treatment process on a material, as it allows a
person to produce a material with the definite properties that they require. In order to
understand and explain how heat treatment works, it will be studied further in chapter
3.
Analysis of Variance (ANOVA) is a method of analysing data to understand the
reasons for variance of particular parameters within that data. In this investigation, the
objective is to understand the effect of different types of heat treatment on NiTi by
analysing a performance parameter of the treated material. In order to understand the
recorded data, a method of analysis will be used in this experiment. Methods created
by Genichi Taguchi will be adopted. Taguchi specialised in creating a method for
analysing variance in a process. This is used primarily for process improvement,
particularly within industry. In this case, his methods will be used in order to
understand how the variation of heat treatment will affect a performance parameter of
NiTi.
1.2 Aim of the study
The previous section has already touched upon the importance and many applications
of the material NiTi, and also the importance of heat treatment as a process. The two
are closely linked, as heat treatment is one of the most popular processes used to
modify NiTi. Other possible methods to modify NiTi are alloying and electrochemical
processes, however these are not as effective as heat treatment. The nature of the
material and its applications, mean it must be in a specific form in order to function as
intended. Heat treatment is a very important process for designers in order to create
the specification of NiTi they require in each application. It is for this reason that this
task is undertaken - To carry out an analysis of the heat treatment of NiTi, covering
the variation of heating temperature, treating duration and cooling rates. The data
presented in conclusion of this project will allow designers to gain knowledge of the
heat treatment they require in order to tailor the material for a particular use.
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1.2.1 Problem Definition
This investigation was to carry out a range of heat treatment experiments of NiTi
wire, before being subjected to mechanical testing. The data will be analysed using
Taguchi’s methods to understand the variance of one chosen performance parameter
and understand their relationship with various factors in the heat treatment method.
Further analysis of the data and material will present and explain the cause of the
changes.
1.3 Study Environment
All research and experimentation was carried out within Halmstad University, making
use of the laboratories and library.
1.4 Limitations
There were some limitations experienced by this experiment, due to not having access
to all the required equipment. This is detailed in section 6.1
2
2. Method
2.1 Alternative Method
An alternative method of carrying out the experiment is to use a full factorial array.
This involves carrying out a separate experiment for every possible combination of
the heat treatment variables. This is ideal if cost and time were not a consideration.
However, in reality it is difficult to do within the time constraints for this project. This
method would involve tripling the number of experiments. It would provide a very
thorough analysis, but the same conclusion can be drawn using a more efficient
analysis method.
There is available software, such as 3C Software, XL Stat or Modde. While these are
good options for variance analysis, they are not necessary for this investigation. These
still require the experiments to be carried out, and the small number of experiments
and data means it is quite easy to carry out all the analysis manually or ‘by hand’.
These software’s would be more useful on further investigations with much more
testing, variables and data to work through.
An alternative method of modifying NiTi is known as ‘cold work’ this is the same as
work/strain hardening. This involves applying a series of loads on a material in order
to strengthen it. This is another interesting topic, but is not as dynamic of flexablw as
heat treatment, so will not be within the scope of this study.
2.2 Chosen Methodology for This Project – Taguchi’s Method
The methodology used is this project was the Taguchi method. It is a suitable method
because it allows the experimenter to observe the variance of certain parameters
during an experiment. This means it can be used to analyse how various factors in the
experiments affect the output parameter. Its main advantage is to allow an
understanding to be gained in the relationship between factor and parameter, or input
and output.
2.2.1 Taguchi’s Method - Design of Experiment
Taguchi made use of orthogonal arrays in his methods. This was used to determine
the experiments in this project. An orthogonal array allows an equal assessment of
each factor through a specific Design of Experiment (DOE). Using this method allows
the experimenter to cut down the number of required experiments required, while still
obtaining valid and statistically sound results.
DOE with the Taguchi method has several steps;
1. Selection of independent variables / factors.
For this investigation, there are three aspects of the heat treatment that can be
altered to investigate their affects;
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•
•
•
Heat Treatment Temperature (°C).
Heat Treatment Duration (mins).
Cooling Medium.
2. Selection of number of levels within each independent variable
•
Heat Treatment Temperature.
o Level 1 - 300 °C
o Level 2 - 400 °C
o Level 3 - 500°C
These temperature ranges are used because they are around the range of the
expected best performance for NiTi. As discussed in chapter 3, the theoretical
highest yield strength for NiTi is a heat treatment temperature of 400°C. These
ranges should allow us to assess this and the variation with the temperature
either side of this.
•
Heat Treatment Duration.
o Level 1 - 30 minutes
o Level 2 - 60 minutes
o Level 3 - 90 minutes
These temperature ranges were chosen in order to establish if changing heat
treatment duration has any effect on the output parameter.
•
Cooling Medium.
o Level 1 - Air
o Level 2 - Water
o Level 3 – Oil
These cooling methods were chosen in order to establish if changing the
cooling rate of the material has any effect on the output parameter.
3. Selection of orthogonal array
With three independent variables, and three levels of each, the most suitable is
an L9 orthogonal array. The chosen orthogonal array is shown in table 2.1
4. Selection of Performance Parameters
Mechanical testing can be performed to obtain output measures to analyse the
heat treatment effects.
• Ultimate Tensile Strength (UTS)
4
Table 2.1 Orthogonal array for DOE with Taguchi Method
Independent Variable
Experiment
#
HT Temp
(°C)
HT Duration
(mins)
Cooling
Medium
1
2
3
4
5
6
7
8
9
300
300
300
400
400
400
500
500
500
30
60
90
30
60
90
30
60
90
Air
Water
Oil
Water
Oil
Air
Oil
Air
Water
Performance
Parameter
Ultimate Tensile
Strength
2.2.2 Taguchi’s Method – Analysis
Taguchi’s Method not only teaches the user Design of Experiment (DOE), but it also
provides a means of analysis of the experimental data. Taguchi’s method is within the
ANOVA collection of statistical models. These types of models specialise in
providing the tools to aid the analysis of variance within a set of data. In this
experiment, the objective is to understand the effect several variables have on the
output of a process. Therefore, this method is ideal in order to understand the
significance of each of the independent variables in the heat treatment. The measured
performance parameter (Yield Stress) from each experiment is used to analyse the
relative effect of the different parameters. Yield Stress is calculated from the
experimental data using equation 2-1.
𝜎𝜎 =
𝐹𝐹
𝐴𝐴
Where:
(2-1)
σ = Stress
F = Force in wire at fracture
A = Cross-sectional area of wire
In order to determine the effect each independent variable has on the performance
parameter, the signal-to-noise (SN) ratio needs to be calculated for each experiment.
The SN ratio is a measure which compares the desired signal or output against the
background noise or irrelevant data. It is essentially a measure of how strong a
response is received from a target parameter in an experiment. The range of the SN
mean for any set of target data (a variable in this experiment) is known as the ‘R’
value. An R value of greater than 1 signifies more signal than noise. This measure is
most commonly used for electrical signals, however it can be applied to any sort of
signal or experiment. In this experiment, the signal represents a correlation between
an independent variable and the performance parameter. The noise represents all other
causes of performance parameter variation. Therefore, an output value of greater than
1 will signify that the target independent variable has an effect on the performance
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parameter. The higher the number, the stronger the correlation and dependence
between the variable and output.
The following equations and procedure assume that the objective is to maximise the
output parameter (Yield Strength).
Equation 2-2 shows the formula for calculating the SN ratio in the case of maximising
the performance parameter;
1
𝑁𝑁𝑖𝑖
𝑆𝑆𝑁𝑁𝑖𝑖 = −10𝑙𝑙𝑙𝑙𝑙𝑙 � ∑𝑢𝑢=1
𝑁𝑁𝑖𝑖
Where:
1
𝑦𝑦𝑢𝑢2
�
(2-2)
SN = Signal-Noise Ratio
ἱ = experiment number
Nἱ = Number of trails for experiment
U = Trial number
y = Performance parameter mean
Once the data from the experiment has been obtained, the SN ratio for each
experiment is calculated and should be tabulated as shown in table 2.2.
Table 2.2 Experiment 1-9 with corresponding SN value.
Experiment #
1
2
3
4
5
6
7
8
9
Variable 1
1
1
1
2
2
2
3
3
3
Variable 2
1
2
3
1
2
3
1
2
3
Variable 3
1
2
3
2
3
1
3
1
2
SN
SN1
SN2
SN3
SN4
SN5
SN6
SN7
SN8
SN9
The next stage is to calculate the average SN value for each variable and its levels.
Equation 2-3, 2-4 and 2-5 show the equations used in order to evaluate variable 2.
Table 2.2 is also colour coded to show how each element relates to each other.
𝑆𝑆𝑁𝑁𝑉𝑉2,1 =
𝑆𝑆𝑁𝑁𝑉𝑉2,2 =
𝑆𝑆𝑁𝑁𝑉𝑉2,3 =
(𝑆𝑆𝑆𝑆1 +𝑆𝑆𝑆𝑆4 +𝑆𝑆𝑆𝑆7 )
(2-3)
(𝑆𝑆𝑆𝑆2 +𝑆𝑆𝑆𝑆5 +𝑆𝑆𝑆𝑆8 )
(2-4)
(𝑆𝑆𝑆𝑆3 +𝑆𝑆𝑆𝑆6 +𝑆𝑆𝑆𝑆9 )
(2-5)
3
3
3
In order to study one variable, the mean SN value for each of its levels is calculated
from equations 2-3, 2-4 & 2-5. The final value is the range of these three values (R).
The larger the R value, the larger the effect of the variable. This is because the same
change in signal causes a larger effect on the output variable being measured. This
6
information should be tabulated as shown in table 2.3. The method and equations
were sourced from Fraley et al. (2006).
Table 2.3 SN means for each level and corresponding range R value for each variable.
Level
1
2
3
∆
Rank
Variable 1
SNV1,1
SNV1,2
SNV1,3
RV1
…
Variable 2
SNV2,1
SNV2,2
SNV2,3
RV2
…
7
Variable 3
SNV3,1
SNV3,2
SNV3,3
RV3
…
3. Theory
3.1 Summary of the Literature Study and State-of-the art
3.1.1 Nickel Titanium
Nickel Titanium (NiTi) is metal alloy with roughly equiatomic percentages of
Nickel and Titanium. It has many properties which present it as an excellent
solution for many applications, but there are two principles which are particularly
useful and unique. These are the shape memory effect (SME) and superelasticity
(SE). These characteristics allow the material to function in unique ways, which
has led to it inheriting the label of a ‘smart’ material, since they can act as sensors
and actuators simultaneously (Otsuka & Kakeshits 2002). It is agreed that these
effects are characteristics of thermoelastic alloys (Otsuka & Ren, 2005).
Additional to these capabilities, NiTi has excellent corrosion resistance, abrasion
resistance and biocompatibility (Kim, Yoo & Lee 2008). This further opens its
potential medical applications. NiTi is much more ductile than other similar
materials, allowing it to be crafted and shaped more easily. They compare
favourably with other considerations as well, with elongations of 50-60%, and
tensile strength as high as 1000 MPa (Otsuka & Kakeshita 2002). NiTi also has a
high power/weight ratio and can be controlled with an electric current (Dilibal
2008). Huang (2001) carried out an investigation comparing NiTi with similar
materials, CuZnAl and CuAlNi. He found that NiTi was the overall winner in
most thermo-mechanic related performances with the one main drawback being
material cost.
Figure 3.1 Austenite and martensite microstructure. (Ryhänen 1999)
8
3.1.2 The Shape Memory Effect & Superelasticity
The microstructure of NiTi can take more than one form. Naturally, it exists in the
parent, austenite, body-centred cubic form denoted ‘B2’ (Seguin et al. 1999).
Under certain external influences it can change into an unnatural martensite ‘B19’
form. The microstructure has a translation and becomes monoclinic. Figure 3.1
shows the crystal structure of the austenite and martensite NiTi. Figure 3.1 also
illustrates that the martensite can exist in two forms. These two forms depend on
the mechanism of inducing the martensite phase.
The Shape Memory Effect (SME) was discovered in an Au-Cd alloy in 1951
(Chang & Read 1951), but research became much more active after it was found
in a NiTi alloy by Beuhler and his colleagues (Buehler, Gilfich & Riley 1963). It
has great potential for many applications, and considerable effort is still being
made to discover new materials (Huang 2001). It describes the transition from
austenite to martensite through a temperature change. The austenite form is the
high temperature form, and the martensite is induced when the material is cooled
below the ‘transformation temperature’. Figure 3.2 illustrates the transformation
between austenite and martensite.
Figure 3.2 The SME & SE process (Otsuka 2002)
In solids, there are two known types of transformations; displacive and
diffusional. Diffusional transformations involve atoms rearranging over long
distances, forming a new chemical composition. A displacive transformation
involves the movement of atoms as a unit. The bonds are not broken and
rearranged as with diffusional. The SME transformation in NiTi is a displacive
transformation (Santiago Anadón 2002). The transformation does not happen in
9
one instant, but transforms over a range of temperature. Figure 3 has MF, MS, AS,
AF noted along its axis. These mark the stages of transformation on either cooling
or heating. MS denotes the temperature in which the transformation from austenite
to martensite begins, and MF denotes where the transformation finishes. AS is in
the same respect, but for the martensite to austenite transformation on heating.
According to this principle then, the transformation is progressive. This means the
material contains proportions of both forms depending on the ambient
temperature when within the transformation range. For example, in the
transformation from austenite to martensite, twinned formations will grow in the
material. These will increase in size and quantity with as the temperature cools,
until MF when the microstructure is completely martensite. The area in the middle
of the graph signifies that there is a difference on transformation temperatures
between heating and cooling. This is a known as a temperature ‘hysteresis’.
The material exists in the twinned martensite form when induced through the
SME, as shown in figure 3.1. Figure 3.2 Illustrates how the three phases of NiTi
correspond and translate between one another. The high temperature form is
austenite. This is a rigid form of the material. When it is cooled below MF it will
transform into a twinned ‘incoherent’ martensite form. This form is very soft and
ductile in behaviour. Any load applied will deform the microstructure into the
untwined, coherent martensite. This will cause an external elongation of the
material. This is caused by the accumulation of small displacements of each atom.
This causes a microscopic change in shape, since all atoms move in the same
direction (Otsuka & Kakeshita, 2002). Applying temperature above AF will,
however return the material back to its parent form. This is the ‘shape-memory
path’ and utilises the shape memory effect. This mechanism allows NiTi to be
used as a control device with temperature being the input.
Superelasticity is the second mechanism which causes a change between the
Figure 3.3 Shape Memory Effect (Madoff et al. 2006)
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austenite and martensite form. The material changes directly from the austenite
form to the martensite form through an applied load. Upon release of the load, the
martensite will transform back to the parent austenite. This phenomenon occurs
only if the material is above the critical temperature, AF,,when the material is in a
totally austenite form. Otsuka & Ren (2005) went into a much deeper analysis of
the transformation mechanism, ‘Because the martensitic transformation is a shearlike mechanism, stress assists the martensitic transformation. The work done by
an external stress on the system is treated to be equal to the change of the free
energy of the system.’
The mechanism is illustrated in figure 3.2 by the ‘superelasticity path’. This
mechanism does not involve any change in temperature, and only occurs above
temperature AF. The ‘super’ element describes the ability of NiTi to go beyond
the elastic limit seen in normal alloys following Hooke’s Law. In this case, the
material is capable of undergoing a severe deformation and is able to return to its
original form without any plastic deformation. Figure 3.4 illustrates the
superelastic effects in comparison to a ‘normal’ alloy.
Figure 3.4 Elasticity in NiTi & Stainless Steel. (Ryhänen 1999)
The difference is NiTi can remain in an intact, martensite form whereas a normal
alloy would have plane slip and dislocation, causing a permanent deformation. As
Otsuka and Ren (2005) describe, ‘It is also observed in the early stage
investigation that the deformation of martensite must be twinning in order for the
shape memory effect to be realized, because slip is an irreversible process’.
The two processes described both involve a change from one form to another,
however their differences allow them to be used in distinct ways. Figure 3.5
illustrates a comparison between the two phenomenon. Superelasticity gives a
material an extra performance in tensile strength and cyclic fatigue, as it can
withstand more strain without being plastically deformed. The SME involves a
11
similar principle but allows the material to be useful for a different use. The
change in shape of material when subject to temperature change allows the
material to be used as a controllable device. This can be in the form of a motion or
extension when unrestrained. However, it can produce significant recovery forces
if it is constrained (Kim et al. 2008).
Figure 3.5 SME vs SE under loading. (Huang 2001)
3.1.3 Uses of Niti
The unique properties and capabilities of NiTi lead to it being a highly desirable
and useful material for a wide range of applications. The areas of these
applications range from medical uses to aeronautic uses to artistic uses. The SME
and SE give the material unique characteristics that provide it with capabilities to
perform unique tasks (Sreekumar et al. 2008). Dilibal (2008) had a brief look at
some of its applications. In medical applications, its biocompatibility allows it to
function harmlessly within the body. It utilises the SME to perform functions that
are possible through utilisation of the internal body temperature. One such use is
in orthodontics. The SME uses the temperature in the mouth to create a tension on
the teeth using a NiTi orthodontic wire. Another is a cylindrical device that
expands within the body for tissue expansion, allowing doctors better access to a
given area (Luo et al. 2010). It can be used as part of a prosthesis. Using NiTi can
offer an incredibly light but powerful device, instead of using motors or
compressors.
NiTi can be used as a coupling. A NiTi coupling is expanded in diameter in the
martensite form, it is then heated above AF, where it shrinks and secures the joint
(Otsuka & Kakeshita 2002). It was used for this on an F-14 fighter jet. It was
successful because it was reliable and cost was not a factor in a military
application.
One of the biggest uses is in actuation. This uses the NiTi as a controllable device
in order to induce a motion. It is particularly effective in this application because
it can behave as an actuator and a sensory device, in a compact, self-contained
mechanism (Santiago Anadón 2002). Essentially, NiTi offers excellent savings on
weight and size when compared to an alternative form of actuator. In the
application of SMAs to a thermal actuator, there are two basic components, a
temperature-sensitive SMA spring and a temperature-insensitive bias spring, both
12
set in series and thus resist each other. The SMA spring is stronger than the bias
spring, so it will override the bias spring when the temperature induces the SME.
One of the drawbacks of thermal actuators is their slow response, since the
response is restricted by heat conduction.
These applications use NiTi for its sensory ability. It can also be used as a microactuator; this induces the SME by applying a current through the wire. This uses a
‘servo actuator’ and provides more possible uses such as the following robotic
uses. An endoscope uses a radius of actuators in order to provide 360° motion
when inside the body (Kim et al. 2008) as shown in figure 3.6. Using the same
principle, it can be used for actuated legs in a robot, and many other uses in
robotics. One of the most exciting uses of a SMA was on the NASA Mars
Pathfinder. The advantages are large force/weight ratio, longer stroke, large
flexibility in design and environmental benefits. The drawbacks in this application
are the large current required to drive the device, and the difficulty in cooling the
NiTi wires. One way to address this is to use NiTi thin films. Their work output
per volume exceeds that of other micro-actuator mechanisms. They are capable of
recovering high strain or generating high force if constrained during recovery.
The large surface to volume ratio of the film favours relatively fast heat transfers,
allowing switching frequencies of up to 50 Hz (Sequin et al. 1999).
Further uses such as smart windows that open and close depending on the
temperature, flaps in air conditioners to the same effect, coffee makers, rice
cookers, drain systems in trains and vent control systems and for transmission in
cars (Stoeckel & Waram 1991). Otsuka & Ren (2002) take a much more detailed
look into the applications of NiTi.
Figure 3.6 SMA endoscope diagram (Haga et al. 2010)
13
It can also be used as a safety measure in devices, such as a shut off valve in a tap.
If the temperature becomes too hot, an actuator spring will expand due to the
SME and shut off the hot water supply.
It can even be used in a less functional application such as art. The SME can be
used to provide motion in a statue. A statue can be designed so that it will change
its form depending on the ambient temperature. Another example is shapechanging jewellery.
Predki et al. (2006) looked at an application using NiTi SME in a gearbox.
Utilising the SME allowed axial loads in the gearbox to be eased as temperature
increased. It solved a problem that had seen abnormally high bearing failure rate.
These uses all make use of the SME. There are also some uses that make use of
SE. Such as orthodontic wires, eyeglass frames, underwire for brassieres and
mobile phone antennas. They are also very useful as medical stents and guide
wires. This is due to their flexibility as well as their biocompatibility.
3.1.4 Heat Treatment
As mentioned previously in this paper, heat treatment is a popular topic within the
field of NiTi and the SME. Heat treatment is one of the most effective methods of
altering the properties of the material. It is therefore a very effective tool in
producing NiTi tailored to an application. This is a view shared by many,
“Thermal processing of NiTi is frequently used to optimise the mechanical
properties of NiTi applications.” (Frick et al. 2005). One of the properties that is
affected by the heat treatment is the transformation temperature for the SME. This
was the topic of a previous investigation by the author (Gibson, 2014). The
conclusion was that annealing raised the transformation temperature of the
material. The study found that an increase of 50°C in the annealing temperature
resulted in an increase of 10°C of the transformation temperature for a constant
heating duration. Otsuka & Ren (2005) explain that the transformation
temperature is dependent on the composition of the material. Therefore, the ability
to control the composition of the material is the ability to control the
transformation temperature. Annealing allows the user to vary the composition of
the material. They also found that the best SME and superelasticity characteristics
were obtained when the material was annealed at 673 K. The explanation they
found for this was that this was the highest temperature before which the material
would undergo recrystallisation.
Chan, Man and Yue (2012) also looked at the effects of heat treatment on NiTi
wires. Figure 3.7 shows the variation of the transformation temperature according
to annealing temperature. Their study also found that heat treatment clearly
affected the residual strain under cyclic loading. It found a pattern of increased
fatigue with increased annealing temperature. This finding would suggest the
superelasticity of the material deteriorates with increased annealing temperature.
Finally, they found that hardness also decreased with increased annealing
temperature. They stated that this was due to grain growth in the material after recrystallisation. This study looked only at a limited annealing range. It does not
14
give a full picture of the effect annealing has on NiTi. In order to truly understand,
a more systematic and complete study is required. Heat treatment is a process
through which a subject is heated and then cooled in a controlled way. This means
that not only the temperature of the treatment affects the material, but also time,
temperature change rate and cooling method. All these factors need to be
understood and controlled in order to understand their effects fully. This point is
summed up well by Paryab et al (2010), ‘Transformation temperature is a function
of chemical composition, heat treatment and quenching process on the alloys. In
the heat treatment process, rate of quenching, exposure time and heat treatment
temperature control the forward and reverse transformation temperatures of
austenite to martensite and [vice versa]’. They agree with the common statement
observed that the transformation temperature is defined by the microstructure of
the material, and the microstructure can be altered using heat treatment.
Figure 3.7 DSC curves show transformation temperatures for annealed NiTi.(Chan 2012)
Delville et al. (2010) also carried out an investigation on heat treatment of NiTi.
They used an alternative method of electrical pulse heating. This method was used
to solve the problem when NiTi cannot be put in a furnace. This could be when it
is joined with another material that cannot withstand extreme heats, another
reason is the slow response time achieved with a furnace. Their study concluded
that while the treatment conditions are very different, particularly in temperature
and exposure times, they could achieve similar functional properties with the
material. Duerig et al. (1990) commented that typically, the best annealing
conditions for NiTi are 300-400°C for 10-60 minutes. This depends on its
application, but gives all round good mechanical properties. Frick et al. (2005)
have similar findings as other papers regarding the effect on the mechanical
properties. They also comment on the growth of Ti3Ni4 precipitates, which
increases as the temperature of annealing increases, particularly above 350°C.
15
Mitwally & Farag (2009) also comment on this. They found that there are several
stable precipitates that form in the NiTi. These increase the strength of the
material, but they cause it to become more brittle, reducing its ductility. Krzysztof
& Sylwester (2012) carried out an experiment similar to that of this article. It
found that changing the annealing duration had a varied effect on each
experiment. Its effect was dependent on the annealing temperature. It was found
that annealing duration had no effect on the DSC curves at 400°C & 450°C.
However, when the annealing temperature was increased, the transformations
were affected by the annealing duration. There are many studies with relation to
this topic. The additional studies of Jiang et al. (2012), Sadiq et al. (2010) have
similar findings to what has been discussed already.
Another use of heat treatment is to train NiTi to ‘remember’ a new shape. This
will effectively reset the austenite or parent form of the material. This enables the
user to use heat treatment to teach the material its shape. For example, this is how
the NiTi is formed into a coil when used in an actuator application. During the
previous study of the author, (Gibson, 2014) this was demonstrated by forming a
NiTi wire into coil spring using a special apparatus as shown in figure 3.8.
Heat treatment can also help to repair the damage caused to the material under
strain, as Mitwally & Farag (2009) state; “Annealing restores SME by rearranging
the dislocations but causes the strength to decrease”.
Heat treatment also has a significant effect on superelasticity, as Otsuka &
Kakeshita (2002) describe; “This explanation clearly indicates that a high critical
stress for slip is vitally important for the realisation of superelasticity; it is in fact
possible to increase the critical stress for slip by thermo-mechanical treatments”.
Figure 3.8 (a) Coil spring induced from NiTi wire with HT. (b) apparatus and set up.
This is just one of the properties that can be changed with heat treatment. Another
important mechanical property that can be altered is the behaviour and
performance of the material under tensile load. This is an area which has been
looked at in previous studies, (Chan et al. 2011), (Gibson 2014).
16
3.1.5 Analysis of Variance (ANOVA) / Taguchi’s method
Analysis of variance (ANOVA) is an analysis technique that helps to look for
variation in average performance. Using this data, to make a comparison to
determine if these differences are due to normal statistical variation, or some
combination of statistical variation and other influences. This methodology should
consider both the spread of the data in each group, as well as the average for each
group. If the spread of the data for one group is about the same as the spread of
the data for another group, and if the average for the first group is about the same
as the average for the second group, then it seems logical to conclude that the two
groups are really not significantly different, and any differences that exist are due
to statistical randomness.
Taguchi takes the ANOVA concept several steps further, and offers a family of
designed experiment templates for evaluating the effects of several factors with
small numbers of test specimens. Taguchi formalized this approach through a
management philosophy he described as the loss function. In its simplest terms,
the Taguchi loss function is based on variability reduction. Taguchi teaches that
minimal variability in everything is inherently good (Berk & Berk, 2000).
ANOVA makes use of variable input parameters in order to study their affect on
variation and overall performance of some output. It is a process most commonly
used in process improvement. In that application it looks at how various
parameters in a production process affects the productivity. In this experiment, it
is a different objective and a different scenario, however applying the same
principles will show how altering the input variables of the experiment affect the
stated performance parameters. Taguchi took the existing theory of partial fraction
experiments and constructed a special set of general design guidelines for factorial
experiments that cover many applications (Bolboacā & Jäntschi 2007). Using
Taguchi’s method enables the experiment to be designed more efficiently.
Without Taguchi’s method, 27 experiments would be required. With Taguchi’s
method, 9 experiments are required. Using an orthogonal array and mathematical
equations, the same data can be deduced without the need to carry out all
experiments.
By carrying out a sensitivity analysis and an analysis of variance, one can
determine the prevalence of each parameter and their percentage contribution.
Antony et al. (2006) carried out an analysis of variation in the design of gearbox
parts. They concluded that TMED (Taguchi’s Method of Experimental Design)
was a success in that application, and successfully achieved the objective that was
set out for the investigation. Wu et al. (2014) carried out an investigation that has
a similar objective to that of this paper. They used Taguchi’s method to analyse
the variance in engine performance for several varied input parameters. The
results of that experiment proved that the predictions gained through Taguchi’s
method were indeed confirmed and validated. This method predicted the best
17
arrangement of parameters for optimal performance. Gopalsamy et al. (2009)
looked at a more conventional application for the Taguchi method. They used it to
analyse a processing machine. By varying the parameters they are able to
determine the most suitable set up for the performance characteristics they
require. They also found that results found from Taguchi’s method closely
matched those predicted by ANOVA. Another important objective of this study is
to confirm the validity of the Taguchi method, especially as an analysis of
variance tool as opposed to solely using at it as a process improvement tool.
18
4. Results
4.1 Preparations and Data Collection
4.1.1 Heat Treatment
The wire samples required some preparation ahead of the heat treatment. The
material used was 0.5mm diameter NiTi wire. Each sample was cut to the
required length for the mechanical testing post heat treatment. The gauge length
for the tensile testing was 60mm. However, further material is required in order to
fix the wire to the assembly outside the gauge length. This extra requirement leads
to an overall required length of 180mm per sample. Two samples were prepared
and treated for each experiment.
The process was as follows. The material was cut into the required 180mm
segments. It was then cleaned with white spirit to remove any dirt or surface
impurities. The pairs of wires were then placed into a pre-heated furnace at the
specified temperature per experiment. The time was precisely recorded, and upon
reaching the target time, the samples were immediately moved from the furnace
into their designated cooling area. Buckets were used to contain the oil and the
water respectively. Both were allowed to settle to room temperature. For air
cooling, a special set up was used to hold the wires and allow even cooling all
over.
Once the samples had been left to cool, they were cleaned and labelled according
to their respective experiment numbers.
19
4.1.2 Mechanical Testing
The next stage of the testing was to subject the samples to mechanical testing.
Ultimate Tensile Strength – This experiment involved applying a tensile load to
the wire. This load would steadily increase until the wire fractured, giving the
maximum tensile strength of the material. A strain rate of 1mm/min was used. If
the strain rate is too high, heat and friction generate in the material which will
reflect in the results.
Ideally, specialist equipment would be
used to test delicate wire such as was
used in this experiment. However, this
was not an option for the experimenter.
This has led to several situations within
this project that are not ideal. (1) The
equipment used was a 100KN tensile
tester. This is a machine intended for
much larger samples which require
much stronger loads. This experiment
would ideally have a 1KN machine.
The machine available was much less
sensitive to the small loads required for
this experiment. (2) Tensile testing of
wire requires specialist apparatus grip
attachments. These are specially
designed so that the ‘grip’ part of the
tensile tester holds the wire in such a
way that there are no stress
concentrations in the wire. This is
usually by mounting the wire on a
radius at each end. This ensures that the
wire fractures in a natural way. The
available equipment did not have this
apparatus, only standard clamps which
were not satisfactory. In order to solve
this, a new set up needed to be created.
Firstly a cylinder was used to try to
wrap the wire around. However, under
load the wire experienced a lot of ‘slip’
before eventually slipping off the end
of the cylinder radius. This was not an
acceptable solution. This set up is
shown in figure 4.1. The next concept
was to wrap the wire in the thread of a
bolt and clamp it in place. It was a
Figure 4.1 Concept 1 for wire grip. Cylinder
method.
Figure 4.2 Concept 2, bolt with clamp in
operation.
20
simple but very effective solution. The components were easily sourced in a local
hardware shop and the assembly was made. The assembly was trialled and proved
a success. It was not a match to specialist equipment, but it was the best possible
solution to the problem faced by the experimenter.
For each experiment the wire was fixed to the attachments before being put in the
tensile machine. The nature of the fixture meant that there was a small variable
between the corresponding gauge lengths of each test. This was only a small
amount and had very little impact on the UTS. The setup can be seen in figure 4.2.
Each experiment was run at a strain rate of 1mm/min. A pre-load of 5N was used
in each experiment to provide consistency. The experiments were run until
fracture, with the wire fragments then unloaded and stored. Data was exported
from the PC Excel for analysis.
The number of available samples allowed each experiment to be run twice. The
results showed very consistent results, which validated the experimentation. The
averages were taken from the two trials for analysis in section 4.4.
4.2 Presentation of Experimental Results
4.2.1 Physical Appearance & Characteristics of Samples Post Heat
Treatment
After the samples have been removed from the ovens and allowed to cool, it is
clear from initial inspection that they have altered from their condition prior to the
treatment. Untreated, the material exists in a relatively stiff, elastic form. Its
appearance is a shiny grey/silver colour. The treatments alter these properties in
several ways. They tend to be grouped together in their appearance by the heat
treatment temperature. The level 1 heat treatment samples (300°C) have a shiny
brass coloured appearance. They are still relatively stiff, close to the untreated
samples. Level 2 heat treatment samples (400°C) have a much darker, purple/blue
colour. They are much less stiff than the untreated samples, but still retain an
elastic form. The level 3 heat treatment samples (500°C) also have a dark
navy/blue colour. However, they behave much more plastically, and are easily
formed into shapes, with very little elastic resistance. The changes to the
elastic/plastic behavior of the samples are due to a change in the transformation
temperature, as explained in section 3.1.4. This change means that the proportion
of martensite has increased at room temperature. The increased martensitic
microstructure proportion gives a more plastic behavior in the material (Gibson
2014), (Chan 2012). The colour change is explained due oxidation in the heat
treatment process. The material gains an oxide layer on its surface, which explains
the change in colour. These colours can be seen in figure 4.3.
21
(a)
(c)
(e)
(b)
(d)
(f)
Figure 4.3 NiTi wire samples post heat treatment at 20x magnification. (a) untreated (b) expt
3 (c) expt 4 (d) expt 6 (e) expt 7 (f) expt 9.
22
4.2.2 Ultimate Tensile Strength
Figure 4.1 is a collated graph showing the force-extension curves for experiments
1-9. Table 4.1 shows this data in tabular form. It also shows the calculated Yield
Stress values for each experiment, using equation 2-1.
Tensile to Failure Tests (NiTi)
140
Force (N)
120
Expt #1
Expt #2
100
Expt #3
80
Expt #4
60
Expt #5
Expt #6
40
Expt #7
20
0
Expt #8
5
10 15 20 25 30 35 40 45 50 55 60 65 70
Extension (mm)
Expt #9
Figure 4.1 Collated graph showing force-extension curves for UTS experiments 1-9.
Table 4.1 Results of UTS experiments 1-9 and corresponding calculated Yield Stress.
Independent Variable
Performance Parameter
Ultimate Tensile Strength
Extension at
Yield Strength
failure (mm)
(MPa)
Experiment #
HT Temp
(°C)
HT
Duration
(mins)
Cooling
Medium
Load at
Yield (N)
1
300
30
Air
120,2
61,7
153,1
2
300
60
Water
112,3
59,4
140,0
3
300
90
Oil
125,5
69,5
159,8
4
400
30
Water
139,1
58,0
177,1
5
400
60
Oil
137,3
74,2
174,9
6
400
90
Air
132,9
64,5
169,3
7
500
30
Oil
92,13
30,1
117,3
8
500
60
Air
78,1
41,0
99,4
9
500
90
Water
72,0
33,7
91,2
23
4.3 Presentation of Results Analysis
The results were processed using Taguchi’s method as described in chapter 2.
Initially the SN ratio was calculated for each experiment, using equation 2-2. This
information is shown in table 4.2.
Table 4.2 SN ratio calculated for each experiment.
Independant Variables
Experiment #
HT Temp (°C)
HT Duration (mins)
Cooling Method
SN #
1
300
2
300
30
Air
43,70
60
Water
42,92
3
300
90
Oil
44,07
4
400
30
Water
44,96
5
400
60
Oil
44,86
6
400
90
Air
44,57
7
500
30
Oil
41,39
8
500
60
Air
39,95
9
500
90
Water
39,20
Next, calculate the mean SN value for each of levels within the variables using
equations 2-3,2-4 & 2-5. The resulting range is the output value of the analysis.
This is shown in table 4.3.
Table 4.3 Mean SN ratios for each level and resulting range for each independent variable.
Level
1
2
3
Δ (R)
Temperature
43,56
44,80
40,18
4,62
Duration
43,35
42,58
42,61
0,77
Cooling
42,74
42,36
43,44
1,08
The resulting R values are the basis for the investigation. They enable and
understanding to be gained in the importance of each variable.
4.4 Discussion
The methodology for this project was chosen so that the variables within the
experiments could be compared and evaluated. The objective was to gain an
understanding into the effects these variables have to the properties and
characteristics of NiTi. Taguchi’s method provided the Design of Experiment
(DOE) process as well as the analysis of the results. Firstly, the DOE method by
Taguchi was a very satisfactory method. It was straightforward to design an
efficient yet thorough experiment to study the effects of the heat treatment.
Without Taguchi’s method, the experiment would have had a substantial increase
24
in cost an time, yet it provided results as accurate and reliable as any other
method. Taguchi’s method also provides the tools to analyse the experiment. The
analysis enabled the experimenter to break down the raw results obtained in the
trials, and present them in a coherent and understandable way. It allows us to
directly observe and compare the effects each of the variables had on the output
material.
4.4.1 R value
The R value is a value obtained for each variable. It is the range of SN means
obtained for each level within the variable. The R value is a measure of the
variation of the SN value between each level. This means that it gives a measure
of how much effect each variable has on the performance parameter (Yield
Stress). From table 4.3, the temperature variable has an R value of 4.61. Duration
has an R value of 0.77 and Cooling has an R value of 1.08. In section 2.2.2, the
meaning of R value was explained. An R value greater than 1 shows a trend,
while less than signifies no significant correlation. If the R value for temperature
is addressed (4.61) this implies that the signal of this experiment is significantly
greater than the noise. This means that the hypothesis of this particular
experiment, which is to study if there is any relationship between temperature and
Yield strength, is true. The data shows that there is a significant correlation
between temperature and its effect on yield strength. This means that the
temperature of the heat treatment has a significant impact on the yield strength of
the resulting material. Applying this principle to the other R values has a slightly
different outcome. Duration has an R value of 0.77. This is less than the threshold
value of 1. This means that the signal is less than the noise for this experiment.
Therefore, there is no significant correlation in the data to support the hypothesis
that treatment duration has an effect on the yield strength of NiTi. This finding is
supported by Krzysztof & Sylwester (2012), who found that the heat treatment
duration had a limited effect on the material as discussed in section 3.1.4. Cooling
has an R value of 1.08. This number signifies that the signal from this experiment
is greater than the noise. However, the value of 1.08 is just only greater than the
threshold value of 1. This shows that it is a weak correlation. This means that the
cooling method does have an effect on the yield strength of the material; however
its effect is small enough to render it almost insignificant.
When the R values of the three variables are compared, it shows their relative
effect on the yield strength of NiTi. Temperature is a very significant variable, as
it has a significant effect on the material. Duration and cooling are relatively
insignificant for this performance parameter. This information allows a
conclusion to be made that in order to control the yield strength of the material,
the heat treatment temperature is the most important variable to achieve this.
25
4.4.2 Tensile Curves
Section 4.3.1 concluded that temperature has a much more significant effect on
the yield strength than the other variables. Looking at the data in more detail can
reveal more findings.
Tensile to Failure Tests (NiTi)
140
Force (N)
120
300°C
300°C
100
300°C
80
400°C
60
400°C
400°C
40
500°C
20
0
5
10 15 20 25 30 35 40 45 50 55 60 65 70
Extension (mm)
500°C
500°C
Figure 4.2 Collated graph showing load-extension curves for expt 1-9. Colour coded according to heat treatment level.
Figure 4.2 shows the same graph as figure 4.1, however it is colour coded to
indicate the curves according to heat treatment level. These curves show the effect
the heat treatment has on the results. Table 4.4 shows the mean values of yield
strength and extension within each level. Both figures support one clear
observation. The curves for level 3 (500°C) which are highlighted in green
perform much differently to the other levels. They are set alone in the graph, and
have much lower values for extension and yield load. In section 3.1.4, it was
stated that above a certain heat treatment temperature, there is increased grain
growth within NiTi. According to Mitwally & Farag (2009), this grain growth
begins above 350°C. Initially this increases the strength of the material, which
explains the maximum values recorded at Level 2 (400°C), but ductility also
decreases as temperature increases due to the growth of substrates. This explains
why the samples fracture much earlier at level 3 (500°C). Their ductility has
reduced to such a level that they have become much more brittle. This finding
shows that in order to improve characteristics of the material under tensile load,
the treatment temperature must be lower than 500°C. Looking at the curves for
level 1 (300°C) in red and level 2 (400°C) in blue initially show little difference.
Table 4.4 shows that there is a significant difference on the achieved failure loads
between level 1 and 2. Level 2 (400°C) has a higher yield strength than level 1
(300°C).The optimum balance would have small grain growth which increases
26
strength but does not reduce the ductility of the material. This optimum
temperature is likely to be in the range of 350°C to 400°C. This theory is
supported by the results, and is in agreement with the work of Otsuka & Ren
(2005). Time and budget constraints dictated a more sensitive temperature range
could not be considered. This investigation supports many other findings, but
what was not clear form other literature is the observed difference between the
variables. The results have showed clearly that temperature has a much bigger
effect than cooling method or duration.
Table 4.4 Mean yield strength and extension values of each heat treatment level.
Experiment #
Level
HT Temp
(°C)
Level Mean
Measured Parameters
Load at
Yield (N)
Extension at
failure (mm)
Yield
Strength
(MPa)
1
1
300
120,2
61,7
153,1
2
1
300
112,3
59,4
140
300
125,5
69,5
159,8
400
139,1
58
177,1
1
3
4
2
5
2
400
137,3
74,2
174,9
6
2
400
132,9
64,5
169,3
7
3
500
92,13
30,1
117,3
8
3
500
78,1
41
99,4
500
72
33,7
91,2
3
9
Yield
Strength
(MPa)
151,0
Extension
(mm)
174,8
65,6
102,6
34,9
63,5
As found in section 4.3.1, there is no significant correlation with heat treatment
duration and yield strength. However, it is worthwhile to look closer at the curves
according to cooling method.
Tensile to Failure Tests (NiTi)
140
Force (N)
120
100
Air
Air
80
Water
60
Water
40
20
0
Oil
5
10 15 20 25 30 35 40 45 50 55 60 65 70
Oil
Extension (mm)
Figure 4.3 Collated graph showing load-extension curves for expt 1-6. Colour coded according to cooling
method.
27
Figure 4.3 shows the curves highlighted by cooling method. This curve shows
only heat treatment level 1 and 2 (300°C & 400°C). The curves for level 3
(500°C) were not considered as they were severely altered by that temperature
and are not a useful comparison for cooling method. The yield point on these
curves is fairly evenly spread, which indicated the cooling method has an
insignificant effect on the yield strength of the material. Table 4.5 shows that
there is little difference in the mean values for yield strength within level 1 and 2
heat treatments according to the cooling method. However, looking at the
extension of these samples shows there is a correlation between extension length
and the cooling methods of these samples. Table 4.5 shows the means per cooling
type of the yield strength and extensions for these samples. This information
matches the observations from figure 4.3. The means for yield strength has a
range of 8.8 MPa, which equated to a variation within 5% of the max value.
Extension has a range of 14.2 mm, which equated to a variation of 19% from the
maximum value. This value indicates that there is a significant change in the
mean extensions values according to cooling method, supporting the hypothesis
that cooling the method has an impact on NiTi.
Table 4.5 Mean yield strength and extension values for cooling methods for heat treatments level 1 and 2.
Experiment #
1
6
Level
1
2
Cooling
Method
Measured Parameters
Load at
Yield (N)
Extension at
failure (mm)
Yield
Strength
(MPa)
Air
120,2
61,7
153,1
1
Air
132,9
64,5
169,3
2
Water
112,3
59,4
140
4
2
Water
139,1
58
177,1
3
1
Oil
125,5
69,5
159,8
5
2
Oil
137,3
74,2
174,9
28
Level Mean
Yield
Extension
Strength
(mm)
(MPa)
161,2
63,1
158,6
58,7
167,4
72,9
5. Conclusions & Recommendations
5.1 Conclusions
•
•
•
•
•
•
•
From the three independent variables that were included in the study, it
was found that heat treatment temperature had a significantly larger
impact on the mechanical properties of the material than the other
independent variables, heat treatment duration and cooling method. This
was determined from the R values of each variable calculated using the
Taguchi method.
Heat treatment temperature had an R value of 4.62. This shows a
significant correlation between heat treatment temperature and yield
strength of NiTi. Heat treatment duration had an R value of 0.77. This
shows no correlation between heat treatment duration and yield strength.
Cooling method had an R value of 1.08. This shows a weak correlation
between cooling method and yield strength.
The samples differ upon physical inspection post heat treatment. The
colour changes from grey silver to brass to dark navy as heat treatment
temperature increased due to the addition of an oxide surface layer. They
changed from a stiff, elastic form to a much more plastic form due to a
change in the transformation temperature.
Level 2 (400°C) heat treatment samples showed the highest performance
for yield strength. The level 2 samples had an average yield strength of
174.8 MPa (Table 4.4). The level 1 (300°C) samples showed the second
highest performance with an average yield strength of 151.0 MPa. The
level 3 samples (500°C) showed much lower performance in yield strength
than level 1 or 2. The mean value for level 3 was 102.6 MPa.
Studying the results shows that, the optimum heat treatment temperature
for yield strength is between 300°C and 400°C. This is because initial
grain growth above 350°C increases the strength of the material. Finer
margin testing is needed to determine a more accurate estimation.
The level 3 (500°C) samples lose so much performance in yield strength
because of increased substrate growth approaching 500°C. The material
loses its ductility at this temperature and becomes more brittle. This results
in fracture at a much lower load.
Comparing the heat treatment samples of 300°C and 400°C shows that
there is a correlation within the cooling methods (Figure 4.3). There
appears to be a relationship between cooling method and extension. Water
has an average extension of 58,7mm. Air has an average extension of
63,1mm and Oil has an average extension of 72,9mm.
29
5.1.1 Recommendations for future activities
This study made an initial study into the subject, but was limited by time,
resources and cost. While some understanding and conclusions have been
established, much more testing needs to be carried out in order to establish a full
understanding and profile on the heat treatment effects of NiTi. For example, only
three temperature ranges were experimented in this test. The range between these
was also 100°C. A much broader range of temperatures would be ideal with
smaller intervals. Yield strength was the only performance parameter studied. To
get a more complete picture, it would be beneficial to study several other aspects
of NiTi properties. This study established that heat treatment duration had an
insignificant effect on the material. There is a threshold value, but this was greater
than 30 minutes, as there was no difference observed between 30 minutes, 60
minutes and 90 minutes. Therefore, there is no need to look at duration as a
variable in any further testing. Cooling method has a slight effect on the
properties of NiTi, however we have established that heat treatment temperature
has a significantly higher effect on NiTi than any of the other variables.
Therefore, any further studies should maximize the focus on the heat treatment
temperature as this is the most important variable in terms of achieving a
particular specification of NiTi.
This study also had a limited resource of NiTi. This meant that there were limited
tests of each sample. With more available material, more tests could be carried
out, which will, in turn increase accuracy and eliminate errors in the data. Ideally
this would increase trials for each experiment, but would also increase the types
of testing. For example, with more material and better equipment, hardness,
fatigue and other performance parameters could be included in the study.
30
6. Critical Review
As with any project there is always room for improvement. For this reason it is
important to perform a review of the study once it has been completed. It is
particularly important to perform a review as the author, as they have experienced
the project first hand, and are in the best position to make an assessment of it.
6.1 Limitations
During the course of this project, some problems were encountered which have
had a detrimental effect on the overall outcome.
•
•
The available equipment was not ideal for this study.
o The tensile machine available at the university had a load rating much
too high for this study. This meant the sensitivity was poor for the
range of required. This has affected the data for the tensile curves. The
natural shape of the NiTi force-extension curve was not seen because
of the lower sensitivity.
o The tensile machine did not have the correct gripping attachments for
fine wire testing. This meant grippers had to be custom built. These
were the best solution possible, but still not ideal. Therefore, there was
a systematic error in the obtained results. However, while there was
indeed an error, it was consistent. This meant the comparison could
still be made between the samples, and the observations made are still
valid.
o The machine/software did not have the capability to successfully carry
out a ‘fatigue’ test on the samples. This would have involved a
‘tension and release’ loop to assess the fatigue over many cycles.
Although a basic test, the equipment did not provide the means, so this
test was not achieved.
o A machine to carry out a ‘hardness’ test was not available. This is one
of the parameters the experimenter sought to study, however it was
able to be tested as there was no such machine available to carry out
the test.
o These problems determined that only one out of the intended three data
sets were achieved. This has limited the scope of the project, from a
broad analysis to the analysis of one parameter.
High cost of the NiTi material meant that only a limited quantity was
available. Ideally, more material would be available and therefore more
testing could be carried out.
31
6.2 Analysing the work from several points of view
6.1.1 Ethical
The message that the code of ethics tries to promote is that good principles and
faithfulness will benefit everyone involved and allow better work output and
progress. It emphasizes building trust, relationships and reputation that will help
with mutual development.
To analyse the project with this in mind it is important to assess it against the code
of ethics as seen in Appendix I. This project does not apply to all the points, as it
is merely an individual research project, however there are some relevant points.
For example, point 5 must be adhered correctly, and has in this project. The
references section shows how much material is used from other authors. It is very
important to show respect to the authors and credit them appropriately with
whatever resources are used. Also, point 7 is about truthfulness in work. It is
important to ensure all the conclusions and findings in the project are correct as
far as the author can see. It would be unfair to misled readers with
misinformation. The report is in line with the rest of the code, while not
necessarily directly linked to it.
6.1.2 Social
In order to analyse this project from a social point of view, the best method is to
compare it to the goals of an existing social indicator system (About KTH, 2015).
One such indicator system is The Millennium Development Goals (MDGs)
(Appendix II). Firstly, it is important to remember that this study is a form of
scientific research. Its main objective is not to address a problem as such, but is
purely a study motivated by the will to learn and to gain more of an understanding
of a topic. For this reason, there may not be a particular motive of undertaking this
project for social benefit. However, when we look deeper at the objectives of
social development, it may turn out that there are indeed some social benefits
from this type of study. If we compare the benefits of our project to the objectives
of MDGs as stated in Appendix II, there are some interesting observations. While
most of the objectives have no relation to this study, points 4-6 address health
issues. The most common use of NiTi is in the medical field. Therefore, with the
goal of this study to understand NiTi better and promote more areas for its use,
this can in turn improve health issues solved by the capabilities of NiTi.
6.1.3 Economic
Economic Sustainability can be defined by two different statements (About KTH
2015);
•
The economic development that does not involve negative consequences
for ecological or social sustainability.
32
•
Economic sustainability is equated with economic growth, which is
deemed to be sustainable as long as the total amount of capital increases.
This study is regarding a clean material which has no negative ecological or social
impact; therefore this project satisfies point 1. About point 2, this study is helping
to enable a ground breaking material to be fully utilised across many lucrative
applications. This will of course have positive economic effects on the people,
companies and countries involved. This project satisfies these two points so can
be said to be economically sustainable.
6.1.4 Environmental
Firstly, NiTi is an environmentally clean material. It is biocompatible, so can
interact with other materials and bodies without causing a reaction. It is non
corrosive, and does not produce any pollutants. Secondly, throughout this project
there was no process which was directly adverse to the environment. There was
no machinery used which produced any pollutants or was unsustainable. The one
aspect of the study that comes to mind is the use of oil for the experiments. The
chosen oil was olive oil as it was cheap, easily available and suitable for the test.
This oil is more environmentally friendly than alternatives for this experiment.
However, there is most likely a more environmentally friendly way to test this
cooling method than using any oil. This would be readdressed in subsequent
investigations. All things considered, this investigation can be said to be generally
very environmentally sustainable. The applications and uses of NiTi that this
project aims to promote are also environmentally friendly.
6.1.5 Occupational Health & Safety
In any quantitative research project, there is going to be a form of
experimentation. In the majority of these experiments, there must be some form of
health & safety assessment. In this study, the experiments posed a level of danger
to the operator. There was a risk to health form the equipment used during this
study. Prior to the experimentation, the experimenter and the supervisor discussed
all the possible risks with this experimentation. This included high temperatures
with the furnace, which posed a risk of burns, fire, damage to equipment etc.
Measures where taken to ensure the operator took appropriate action to minimise
the risks as much as possible, such as a correct and safe procedure as well as
safety clothing such as oven gloves, handling tools and workshop overalls. The
tensile testing also posed some risks. The tensile machine experiences high levels
of force, so users must remain clear of the machine in operation. There is also a
risk of high velocity debris from the fracture, although in this case the wire did
not produce any fragments. By carrying out an assessment of each part of the
experiment prior to beginning, the risk of incident or injury is minimised. It was
important to remember this principle during this project.
33
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Chan, CW., Man, HC., Yue, TM. (2012) Effect of Postweld Heat Treatment on the
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current studied by transmission electron microscopy, Acta Material, 58, p4503-4515.
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aspects of shape memory alloys, Butterworth; London.
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36
Appendix I – Code of ethics for engineers
THE FUNDAMENTAL PRINCIPLES
Engineers uphold and advance the integrity, honour and dignity of the engineering
profession by:
I.
Using their knowledge and skill for the enhancement of human welfare;
II.
Being honest and impartial, and serving with fidelity their clients
(including their employers) and the public; and
III.
Striving to increase the competence and prestige of the engineering
profession.
THE FUNDAMENTAL CANONS
1. Engineers shall hold paramount the safety, health and welfare of the public in
the performance of their professional duties.
2. Engineers shall perform services only in the areas of their competence; they
shall build their professional reputation on the merit of their services and shall
not compete unfairly with others.
3. Engineers shall continue their professional development throughout their
careers and shall provide opportunities for the professional and ethical
development of those engineers under their supervision.
4. Engineers shall act in professional matters for each employer or client as
faithful agents or trustees, and shall avoid conflicts of interest or the
appearance of conflicts of interest.
5. Engineers shall respect the proprietary information and intellectual property
rights of others, including charitable organizations and professional societies
in the engineering field.
6. Engineers shall associate only with reputable persons or organizations.
7. Engineers shall issue public statements only in an objective and truthful
manner and shall avoid any conduct which brings discredit upon the
profession.
8. Engineers shall consider environmental impact and sustainable development
in the performance of their professional duties.
37
9. Engineers shall not seek ethical sanction against another engineer unless there
is good reason to do so under the relevant codes, policies and procedures
governing that engineer’s ethical conduct.
10. Engineers who are members of the Society shall endeavor to abide by the
Constitution, By-Laws and Policies of the Society, and they shall disclose
knowledge of any matter involving another member’s alleged violation of this
Code of Ethics or the Society’s Conflicts of Interest Policy in a prompt,
complete and truthful manner to the chair of the Committee on Ethical
Standards and Review.
38
Appendix II – The Millennium Development Goals
The Millennium Development Goals (MDGs) are eight international development
goals that were established following the Millennium Summit of the United Nations
in 2000, following the adoption of the United Nations Millennium Declaration. All
189 United Nations member states at the time (there are 193 currently), and at least 23
international organizations, committed to help achieve the following Millennium
Development Goals by 2015:
1.
To eradicate extreme poverty and hunger
2.
To achieve universal primary education
3.
To promote gender equality
4.
To reduce child mortality
5.
To improve maternal health
6.
To combat HIV/AIDS, malaria, and other diseases
7.
To ensure environmental sustainability
8.
To develop a global partnership for development
United Nations Publications. (2005)
39
I'm from Belfast, Northern Ireland. I
completed my Bachelor's Degree
there at Queen's University Belfast,
before coming to Högskolan i
Halmstad to study a Master for one
year. I aspire to travel more of the
world and have a successful career in
automotive engineering.
PO Box 823, SE-301 18 Halmstad
Phone: +35 46 16 71 00
E-mail: [email protected]
www.hh.se
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