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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

[email protected]

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.


2/7

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;


3/7

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.


4/7

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


5/7

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]


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]


● Test 1

● Test 2

─ ISO 18595

× Splash

Weld Growth Curves – Zinc coated electrodes [A16]

● Control Test

● Zinc Test

─ ISO 18595

× Splash


6/7

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.


7/7

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.

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