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Investigation into Sacrificial Electrode Protection for High Volume Resistance Spot Welding Bola, A.
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Investigation into Sacrificial Electrode Protection for High
Volume Resistance Spot Welding
Ana Rita Gomes Bola
Instituto Superior Técnico – Departamento de Engenharia Mecânica
Avenida Rovisco Pais, 1096-001 Lisboa, Portugal
Abstract: The 𝐶𝑂2 emissions, due to the circulation of automobile vehicles, imposes a huge pressure on car companies. One
of the measures is to minimize these by decreasing the total weight of the vehicles by replacing some of its steel parts by
aluminium. With this, a problem arises. When resistance spot welding aluminium, the wear of the copper electrodes is higher
than when welding steel, which means that the number of the electrodes used for the same amount of spot welds is higher. As
the number of electrodes increase, so does the costs involved, and nowadays, even more, due to the continuous increasing of
the copper price. An attempt of solution for this problem is to create a protective layer on the electrodes surface, wearing only
the layer, keeping the electrodes body in good condition for a longer period. The proposed materials to coat the electrodes are:
zinc, silver based conducted adhesive, graphite and tin, and the coating process is different for the different materials. Zinc
and tin are welded to the electrodes surface, while the silver based coated conducted adhesive (SBCA) and graphite are
painted onto the electrodes surface. The main conclusions are that zinc, SBCA and graphite do not create good protective
layers for a wide range of parameters and for electrodes with different geometries and sizes. However, the tin layer has
demonstrated a better behaviour when performing welds on aluminium.
Keywords: tin, resistance welding, coated electrodes, aluminium
1. Introduction
Nowadays there is a strong political and economic
pressure to reduce the 𝐶𝑂2 emissions from automotive
vehicles [1] and a good way of doing that is by reducing its
total weight by producing some parts in aluminium instead
of steel. Alloyed aluminium has a low density (comparing
with steel) that leads to a decrease of the total weight of a
vehicle maintaining similar safety and strength levels as
steel [2]. The pressure imposed in this industrial field is also
leading to a development of specific solutions based on
intensive use of aluminium alloys [3].
With the reduction in weight the energy that is
necessary decreases and consequently the fuel
consumptions, which leads to a decrease in 𝐶𝑂2
emissions. By reducing the weight of a car in 10% it is
possible to save up to 8% in fuel, so it’s possible to
decrease the consumption of fuel in 3.4 to 5.3 litres per
1600 km per 45 kg of weight reduction [2].
Another advantage of using aluminium,
environmentally, is its possibility of being recycled over and
over again without losing its properties. About 90% of
aluminium that has been used in vehicles production is
recycled after its end of life. 60 to 70% comes, as raw
material, for production, from recycling. The use of recycled
aluminium instead of virgin aluminium can save up to 95%
of energy [2]. Besides all the advantages referred before,
aluminium is available in a large variety of semi-finished
forms, such as shape castings, extrusions and sheet,
which are very suitable for mass production and innovative
solutions in the form of compact and highly integrated parts
that meet the high demands for performance and quality
[1].
Although aluminium presents many advantages, there
is a problem associated with this material when performing
resistance spot welding (RSW), its tendency to alloy with
copper, the spot welding electrodes material (due to its
good characteristics). To keep the good quality of the welds
it is necessary to remove aluminium deposits from the
electrodes and for that, dressing equipment that has
hardened steel blades is available, however the amount of
copper that is possible to cut leads to only twenty
applications of the cutter [4]. Despite the cutting of the
electrodes being a good solution to keep the desired
properties of the weld, the amount of copper that is
consumed and the increase on copper’s price is
detrimental for its application.
As referred copper and aluminium easily bind to each
other. Currently the solution to this problem is to cut the
electrode after a certain number of welds (that reveal
damage in the electrode) removing the region that was
affected by the joining process. Depending on the
electrode shape and size, it is cut until it’s still safe to weld
with, i.e., when the electrodes are still thick enough to
support the pressure involved in the process.
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2. Methodology
The experimental procedure includes three main
steps: electrode preparation, coating process and
aluminium welding. The coating process is different
according to the layer’s material. After the coating process,
its performance is tested by welding aluminium and to
conclude the contact resistance is measured. The range of
parameters, time and force, used, were defined according
to ISO 18595:2007 – Annex B (informative) – Typical Spot
Welding Conditions, which was only a guidance on spot
welding conditions.
Parameters Coating Splash/Endurance
Electrodes Geometry
𝐴16
𝐵16/6
𝐴20
𝐵20/8
𝐴16
𝐵16/6
𝐴20
𝐵20/8
Tim
e [m
s] Squeeze 200 200 200 200
Weld 240 240 60 100 ( 120)∗
Hold 500 500 200 200
Fo
rce [kN
] Squeeze 2,5 2,5 3 6 (4,5)∗
Weld 2,5 2,5 3 6 (4,5)∗
Hold 4 4 3 6 (4,5)∗
Current Intensity [kA]
4 − 13 10− 14
8 − 26 12 − 32
Number of welds
5− 140
25− 100
− −
Materials Aluminium; Zinc/Tin Coated Steel; SBCA;
Graphite
Table 2-1: Parameters for the coating and welding trials
(* - parameters for the Graphite trials)
2.1. Electrode Preparation
The electrodes preparation was performed before
applying the coating. The same procedure was carried out
for all trials. To prepare the electrodes the following steps
were needed:
1. Turn on the machine and connect to the program;
2. Turn on the pump;
3. Clean both (Top and Bottom) surfaces of the
electrodes caps to remove the oxide;
4. Remove the old electrodes and place the cleaned
ones;
5. Apply a pre-load to the new caps;
6. Turn on the refrigeration system;
7. Apply an abrasive to the electrodes surface:
a. No/Zinc/Tin coating: apply an abrasive to have a
smoother surface;
8. Clean the electrodes’ surface to remove the particles
left by the abrasive.
1 Metal polish designed to remove tarnish from copper.
Note: For the Graphite and SBCA Trials the preparation of
the electrodes was the same as steps 1 to 3 from the Zinc
and Tin trials. The step number 3 was performed with
Brasso1.
2.2. Coating Process
The coating process depends on the material of the
layer and it is explained in the following sub sections how
each one of them was performed.
2.2.1. Zinc Coating
To apply a zinc layer onto the electrodes surface a
certain number of welds was performed in two sheets of
zinc coated steel so the zinc present on the sheets’ surface
could be deposited on the electrodes surface after each
weld (until the whole area of the electrode was coated).
The current used for these tests was chosen based on the
contact area and the material thickness and was
increased/decreased until the electrodes’ surface were all
coated.
2.2.2. Graphite and SBCA Trials
Graphite and SBCA were also tried as coatings,
separately and together, the steps of each trial are
described below.
Graphite
A. Dilute graphite in acetone using different
concentrations.
a. Put a certain amount of graphite (spoons) in a
mixture cup:
b. Add acetone using a syringe [ml];
c. Mix a. and b. using a spatula;
d. Apply the mixture to the electrodes surface with a
brush;
e. Dry the mixture with a dryer.
B. The same as process A but with light machine oil
instead of acetone.
Silver Based Conductive Adhesive (SBCA)
A. Paint the electrodes with SBCA and wait 30 minutes
to dry.
B. Mix acetone with SBCA and use a brush to paint this
mixture onto the electrodes surface and wait three
days to dry properly.
Graphite and SBCA
A. Mixture of SBCA and graphite:
a. Put a little portion of SBCA in a mixture cup;
b. Add graphite to the same cup;
c. Mix a. and b. with a spatula;
Investigation into Sacrificial Electrode Protection for High Volume Resistance Spot Welding Bola, A.
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d. Apply the mixture to the electrodes surface;
e. Dry the mixture for three days;
f. Use a fine abrasive to smooth the surface.
B. Mixture of SBCA, acetone and graphite: the same as
process A.
2.3. Aluminium Welding
After the coating process, it is necessary to evaluate
the coating applied. This step is the same for the different
coatings. First by performing a splash test and second with
an endurance test. The main objective of the splash test is
to find the right current values for the endurance test and
this is done by welding aluminium; it starts with a low value
of current and this value is increased by 1kA in each weld
until splash occurs. The purpose of performing an
endurance test is to evaluate how the coating behaves
when welding aluminium and to perform as many welds as
possible keeping the good quality of the welds and no
aluminium pickup on the electrodes; this test uses constant
current equal to the one obtained before splash occurs in
the splash test.
2.4. Contact Resistance measurements
The contact resistance measurements were
performed according to “DVS 2929-1 – Method for
determining the transition resistance basics, measurement
methods and set up”.
Figure 2-1: Scheme of the contact resistance measurements: (a)
Ohmmeter; (b) insulator [5]
2.5. Materials
During the coating process the materials used were:
Zinc Coated Steels:
― Dx56GI 0.8 mm;
― H340LAD+Z140 MBO 1.2 mm;
Tin;
Graphite powder (LECO);
Silver Based Conducted Adhesive;
Acetone;
Light machine oil.
2.5.1. Materials Selection
The materials layers were selected due to its high
availability (except SBCA). Zinc was chosen because it
alloys very easily with copper, zinc oxide theoretically has
a non-stick surface to liquid aluminium and also because
there is already zinc coated steel on the vehicle, which
would facilitate the implementation of this process in the
production line (after weld zinc coated steel, the electrode
would be already coated to weld aluminium, without a need
of the electrodes to leave the production line for the
coating). Tin was chosen due to its high tendency to alloy
with copper, to its high melting temperature when alloyed
with copper and because it is a low cost material. Graphite
is also a low cost material, has a high electrical
conductivity, high melting temperature and refractory
properties with liquid aluminium. The SBCA, although it is
expensive, it has a high electrical conductivity and
hopefully would behave as a consumable layer, protecting
the copper electrodes and avoiding damage. During the
Splash and Endurance tests the materials used were
Aluminium 5xxx and 6xxx.
2.6. Equipment
The machine that was used was Matuschek model
M800LL SMAX400kVA with Servo Studio software and
weld gun ServoSPATZ. It has a 1000 Hz DC power supply
and it is working in constant current mode. A caliper was
used to measure the weld spots size after the peel test. For
the graphite trials the following equipment was needed:
mixture cups, brush, spatula, syringe and a measuring
spoon. To measure the current intensity MNIYACHI weld
checker was used.
Figure 2-2: Matuschek model M800LL SMAX400 kvA (left)
MNIYACHI weld checker (right)
During the contact resistance measurements, it was
used an ohmmeter and a machine developed by TWI to
measure contact resistance. The electrodes used in these
experiments were ISO 5182: Class A2/2 (Cu, Cr, Zr). The
geometries used along the experiments are presented in
Figure 2-3. Two sizes were used for each geometry.
Investigation into Sacrificial Electrode Protection for High Volume Resistance Spot Welding Bola, A.
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Type A - Radius
Type B - Truncated
𝑑1 16 20 𝑑1 16 20
𝑅1 40 50 𝑅1 40 50
𝐿1 20 22 𝑑2 6 8
𝐿2 9,5 11,5 𝐿1 20 22
𝑑3 12 15 𝐿2 9,5 11,5
𝑑2 - - 𝑑3 12 15
(a) (b)
Figure 2-3: Type A (a) and Type B (b) electrodes and respective
dimensions [mm] [6]
3. Main Results and Discussion
3.1. Without coating – Control Testing
As a preliminary evaluation, aluminium 6061 was
welded to evaluate its behavior under different
circumstances and to set the parameters for the coating
trials. Aluminium has alumina on the surface which is
detrimental to the welding process, however it is possible
to remove it. Three tests were performed, in the first one
no cleaning was performed; in the second test, the
aluminium sheets were cleaned with acetone and in the
last test with abrasive and acetone.
The influence of the degree of cleanliness was
evaluated and it was possible to verify that the growth
curves of each test are different for each one, and also that
only the cleaned sheets provide acceptable values for the
average weld diameter (which is 5 mm according to ISO
5821). Cleaning the aluminium sheets decrease the
contact electrical resistance in the electrode/sheet
interface, which requires more current intensity to reach
the same generated heat, as it is possible to see on Figure
3-1. The aluminium sheets cleaned with acetone and
abrasive require higher current intensity for the same size
of spot weld.
Figure 3-1: Welding growth curves for different degrees of
cleanliness
To support the results obtained for the weld growth
curves, the electrical resistances for each case were also
measured and are presented in Table 3-1. It is possible to
see that the lowest electrical resistance, which is
associated to the highest degree of cleanliness (acetone +
abrasive), give the smallest spot welds and that the highest
values for the electrical resistance, that are presented for
the lowest degree of cleanliness (as received), give the
bigger weld spots.
𝑰 [𝒌𝑨] 𝑽 [𝒗] 𝑹 [𝝁𝛀]
As received 0,93 0,29 404
Acetone 0,95 0,29 302
Abrasive and acetone
0,98 0,12 122
Table 3-1: Non-standard resistance measurements (Matuschek
welder)
The results obtained for the control test are also
supported by the work done by L. Han et al. [7] where the
conditions on feasibility and quality of resistance spot
welding are studied. During this process, without any
coating to protect the electrodes, the copper from the
electrode and the aluminium rapidly alloy with each other.
3.2. Zinc Coating
3.2.1. Truncated Electrodes B16/6
The electrode geometry used in the first trials was
truncated (B16/6) with Dx56GI 0.8 mm zinc coated steel
(cleaned with acetone). It was possible to deposit a zinc
layer on the top electrode, by performing 100 welds with a
current intensity value of 8,5 𝑘𝐴, with an average thickness
of 18,2 𝜇𝑚, Figure 3-2, however the edges present lack of
coating. The cross section figure shows a yellow layer on
top, which corresponds to a brass layer (the result of
alloying copper and zinc [8]).
Figure 3-2: [B16/6] Zinc coated top electrode (left); detail
(middle); cross section (right)
The results of the bottom electrode are presented in
Figure 3-3. The bottom layer with 27,3 𝜇𝑚, is thicker than
the top layer.
Figure 3-3:[B16/6] Zinc coated bottom electrode (left); bottom
electrode detail (middle); bottom electrode (right)
By observing Figure 3-2 and Figure 3-3 although it is
possible to see that all area of both electrodes is covered
with zinc, the layer isn’t uniform, leading to an inconsistent
process [4]. When comparing top and bottom electrodes,
the average thicknesses are different. The discrepancy on
the values of the average thickness (between the top and
bottom electrodes) is due to two effects that happen during
the joining process: the Peltier effect [9] and the impact that
0
2
4
6
14 19 24
Avera
ge W
eld
Dia
mete
r [m
m]
Welding Current [kA]
Growth Curves - Control Test
● As received
● Cleaned with
acetone
● Cleaned with
abrasive
─ ISO 18595
× Splash
Investigation into Sacrificial Electrode Protection for High Volume Resistance Spot Welding Bola, A.
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occur between the top electrode and the material to be
welded.
To evaluate the layer behaviour while welding
aluminium 7 welds were performed in aluminium. Figure
3-4 shows how the bottom electrode looks like after
welding aluminium. It is evident, by observing the middle
figure that there is aluminium in the electrode. The cross
sections figure shows that both, the layer and the
electrodes, presents damage.
Figure 3-4: [B16/6] Zinc coated bottom electrode after welding
aluminium (left); detail (middle); cross section (right)
The results have shown that the state of degradation
of the top and bottom electrodes is not the same, although
they both perform the same number of welds on aluminium.
That is because the two effects happening during the
joining process already explained before, Peltier effect and
impact that occur between the top electrode and the
aluminium plate.
In addition to the results obtained for the electrodes
surface it is also possible to evaluate the weld growth curve
of the welds performed in aluminium. Figure 3-5 shows that
the average diameter of the welds obtained does not meet
the standard requirement of 5 mm for the average weld
diameter (imposed by ISO 5821). It is also possible to see,
that in comparison with the control test, the values used for
the current intensity to the zinc coated electrodes are much
lower. The reason for that is the contact resistance
experienced in the zinc coated electrode/sheet interface,
which is about 7,65 times higher than when any coating is
applied to the electrodes surface, requiring lower values for
the current intensity. The instability with the zinc coated
electrodes is visible for lower values of current intensity,
the splash occurs for 12,4 kA, while in the control test it is
possible to reach 19,5 kA of current intensity.
Figure 3-5: Weld growth curve of aluminium welded with [B16/6]
zinc coated electrodes
In the previous results (regarding the layer behaviour
and the weld quality), by analysing the cross section figure,
it is possible to conclude that only after 7 welds performed
in aluminium, the electrodes were too damaged to continue
(there are holes in both electrodes and respective coatings
resulting from the joining process). The average weld
diameters achieved were also a problem because, apart
from not meeting the requirements imposed by the
standard, they were inconstant and the average weld
diameter was not always increasing with the increase of
current intensity as it was supposed to.
3.2.2. Radius Electrodes A16
Due to the lack of coating on the edges of the
electrodes surface on the tests with truncated electrodes,
another geometry of electrodes was studied, radius
electrodes (A16), with a different zinc coated steel,
H340LAD+Z140 MBO 1.2 mm not cleaned (with the
protective oil still on the surface) as the use of lubricants is
associated to a slower pitting rate of the electrodes surface
[10]. In Test 1 (Figure 3-6), the current intensity value was
8 𝑘𝐴 and the number of welds 52 and in Test 2 the current
intensity value was 8,5 𝑘𝐴 but with 65 welds.
Figure 3-6: Weld growth curves of aluminium welded with [A16]
zinc coated electrodes
Comparing the two weld growth curves it is possible to
relate them with the electrodes preparation. The highest
intensity current value achieved before splash was on Test
2, in which the electrode preparation has the highest
number of welds performed on zinc coated steel. In Test 2,
the number of welds is lower, and so is the current intensity
value achieved before splash.
During the tests with truncated electrodes, it was
possible to conclude that the number of welds performed
on zinc, after a certain value, did not have much influence
on the layer’s performance when welding aluminium.
However, in radius electrodes, it shows some influence on
the values of average weld diameter. Figure 3-6 shows that
it was possible to reach higher average weld diameters
with electrodes coated with a higher number of welds, and
also that, for the first time, it was possible to reach the
standard requirements for the average weld diameter (5,3
mm). The problems of Test 2 were that although the
average weld diameter meets the standard requirement, it
happened for an expulsion weld and there is too much
aluminium in the electrodes surface.
0
2
4
6
8 10 12 14 16 18 20Avera
ge W
eld
Dia
mete
r [m
m]
Current Intensity [kA]
Weld Growth Curve - Zinc Coated Electrodes
0
1
2
3
4
5
6
5 10 15 20
Avera
ge W
eld
Dia
mete
r [m
m]
Current Intensity [kA]
● Test 1
● Test 2
─ ISO 18595
× Splash
Weld Growth Curves – Zinc coated electrodes [A16]
● Control Test
● Zinc Test
─ ISO 18595
× Splash
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3.3. Graphite and SBCA Coating
During the tests performed with graphite and SBCA
coatings, electrodes with [A20] geometry were used. The
procedure for these tests can be found in section 2.2.2.
Table 3-2 summarizes the quantities of each component
used during each test.
Test
Gra
phite
[spo
ons]
Solvent
No. o
f S
BC
A layers
Way / tim
e t
o d
ry
the layer
Oil
[sp
oo
ns]
Aceto
ne [
ml]
Gra
phite
A
1 - 1 - Dryer (2 min)
1 - 2 - Dryer (2 min)
1 - 6 - Dryer (2 min)
B 4 1 - - Dryer (2 min)
SB
CA
A - - - 1 * (30 min)
B - - 1 1 * (3 days)
Gra
phit
e a
nd
SB
CA
A 3 - - 1 * (3 days)
B 1 - 3 1 * (3 days)
Table 3-2: Specifications of the graphite and SBCA coating tests
(* Natural Drying)
To study the coating behaviour, aluminium was
welded after the electrodes were prepared. All welding
parameters were kept constant (values in section 2.)
except current intensity that was changed according to the
layer in study and is indicated in the next two tables. It was
only possible to perform one weld in aluminium, because
after the first one the coating was too damaged to continue.
Electrode Bottom Top Bottom Top
Before
welding
Al. 6061
After 1
weld on
Al. 6061
Graphite mixed with
acetone
[𝐼 = 18 𝑘𝐴]
Graphite mixed with
machine light oil
[𝐼 = 10 𝑘𝐴]
Table 3-3: [A20] graphite coated electrodes appearance before
and after welding aluminium
After the evaporation of acetone, the graphite turned
into powder again; the layer from the top electrode just fell
(due to gravity); the layer from the bottom electrode did not
fell, but, as it wasn’t adherent to the electrode surface, as
soon as the electrodes were pushed against each other,
the layer was expelled from the center; thus, after only one
weld on aluminium both electrodes showed a lot of
damage.
To complete the coating process, in SBCA case, after
applying the paint onto the surface (and to avoid the layers
burning) uncoated steel was welded with a very low value
of current intensity, 2 kA, with the purpose of burning only
the binder of the adhesive slowly, to avoid an explosion
when welding aluminium. When welding uncoated steel,
even with this low value of current intensity, all the layer of
SBCA was burned and the center of the electrode has been
exposed. The same thing happened when acetone was
mixed with the SBCA.
Electrode Bottom Top Bottom Top
Before
welding
uncoated
steel
After one
weld on
uncoated
steel
SBCA [𝐼 = 2 𝑘𝐴] SBCA mixed with
acetone [𝐼 = 2 𝑘𝐴]
Table 3-4: [A20] SBCA coated electrodes appearance before and
after welding uncoated steel
The same process used in SBCA trials was also
performed on SBCA mixed with graphite trials, and the
results were very similar.
It is possible to conclude that none of the experiments
with graphite, SBCA or both are the solution for the
problem in hand. Graphite mixed with acetone or with light
machine oil does not adhere to the electrodes surface. For
that reason, it does not act as a protective layer, it flows to
the sides as soon as the electrodes reach aluminium.
SBCA behaves differently, either by itself or mixed with
acetone or graphite. With this material, the chances of
adherence to the copper electrodes were higher. As SBCA
has, in its constitution, a binder, it easily adheres to the
electrodes surface, increasing the chances of success,
when compared to graphite. Although the binder helps the
layer to adhere to the surface, it was also expected that it
could possibly burn, and that was the reason why an
uncoated steel was welded before aluminium, as an
attempt of burning only the binder. As it did not work as
expected, it was not possible to weld aluminium with SBCA
coated electrodes, because when trying to burn only the
binder, the all depth of the layer was burned.
4. Conclusions
Different techniques were tried in order to extend the
electrodes life. However, although these techniques
present improvements in the electrodes life, there are still
problems in aluminium spot welding.
Investigation into Sacrificial Electrode Protection for High Volume Resistance Spot Welding Bola, A.
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The studies performed with tin were the most
promising to execute further investigations. The desired
improvements, such as extending the electrodes life, was
accomplished. In the projection life of the [A20/8] tin coated
electrodes, 9 000 welds were the result that stand out.
A single pair of electrodes, with the technologies
available on the market to dress them, can last up to 9 000
welds.
Regarding the other layers studied, they also allow to
weave some conclusions: The main ones are presented in
the four following sub chapters.
4.1. No coating
The level of cleanliness of a surface influences the
joining process leading to the cleanest surface to
confers the highest splash current and the highest
average weld diameter.
4.2. Zinc
Welding aluminium after coating the electrodes with
zinc causes a lot of damage on the electrode;
Zinc does not work as a barrier to the aluminium
because it sticks to it;
Changing the electrodes geometry (from B 16/6 to A16)
did not improve the results;
Due to the Peltier effect and to the impact that the top
electrode suffers, the bottom electrode is always more
damaged (in comparison to the top electrode);
Zinc is not a viable solution to increase the electrodes
life.
4.3. Graphite and SBCA
Graphite doesn’t adhere very well to the electrode,
either on its own or mixed with acetone or oil;
SBCA is burned very easily during the joining process
which makes it an unviable solution for the purpose of
project.
5. References
[1] J. Hirsch, Automotive trends in aluminium - The European perspective, Mater. Forum. 28 (2004) 15–23. doi:CCFBAADAA8178B7B1961FB26067826E1.
[2] J. Green, J. Litcher, L. Benton, Aluminum industry roadmap for the automotive market: Enabling Technologies and Challenges for body structures and closures, The Aluminium Association, Inc. with support from U.S. Department of Energy (1999).
[3] J. Hirsch, Aluminium in Innovative Light-Weight Car Design, Mater. Transactions Vol. 52 (2011) 818–824. <doi:10.2320/matertrans.L-MZ201132>.
[4] Developments towards high-volume resistance spot welding of aluminium automotive sheet component, lnternational Automotive Research Centre, Warwick Manufacturing Group and the University of Warwick (2006) 1–12.
[5] J.S. Hongyan Zhang, Resistance Welding, Fundamentals and Applications, Taylor & Francis Group, Boca Raton, 2006.
[6] International Standard, ISO 5821:2009 - Resistance welding - Spot welding electrode caps, (2009).
[7] L. Han, M. Thornton, D. Boomer, M. Shergold, Effect of aluminium sheet surface conditions on feasibility and quality of resistance spot welding, Journal of Materials Processing Technology, Vol. 210 (2010) 1076–1082.
<doi:10.1016/j.jmatprotec.2010.02.019>.
[8] BrazeTec, et al., (2004) Copper and Copper Alloys - Compositions , Applications and Properties Copper Development Association.
[9] A. Fernandes (2012), Conversão de Energia com Células de Peltier (Thesis) Universidade Nova de Lisboa, 1-88.
[10] M. Rashid, S. Fukumoto, J.B. Medley, J. Villafuerte, Y. Zhou, Influence of lubricants on electrode life in resistance spot welding of aluminum alloys, Welding Journal, Vol. 86 (2007) 62s–70s.