Investigation into Sacrificial Electrode Protection for ...With this, a problem arises. When...

Investigation into Sacrificial Electrode Protection for

High Volume Resistance Spot Welding

Ana Rita Gomes Bola

Thesis to obtain the Master of Science Degree in

Mechanical Engineering

Supervisors: Prof. Maria Luísa Coutinho Gomes de Almeida

Prof. Eurico Gonçalves Assunção

Examination Committee

Chairperson: Prof. Rui Manuel dos Santos Oliveira Baptista

Supervisor: Prof. Eurico Gonçalves Assunção

Member of the committee: Prof. Inês da Fonseca Pestana Ascenso Pires

Prof. Rosa Maria Mendes Miranda

October 2016

i

Acknowledgment

I would like to thank to Prof. Luísa Coutinho and to Prof. Eurico Assunção for their support and

guidance through the dissertation, and for the opportunity for being involved in such project.

I also would like to thank to TWI, especially to Sullivan Smith, for all the guidance through all

the experimental work and integration in the company.

More important, to my parents, grandparents and to my brother and sisters for their

unconditional support.

And for all the support during all this journey, to Diogo.

ii

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 of 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 the best behaviour when performing welds on aluminium, acting as a protective

layer and extending the electrodes life.

Key-words: tin, resistance welding, coated electrodes, aluminium

iii

Resumo

As emissões de 𝐶𝑂2, devido à circulação de veículos automóveis, impõem uma grande pressão

às companhias de produção dos mesmos. Uma das medidas para as minimizar é a diminuição do peso

total dos veículos substituindo algumas das suas partes em aço por alumínio.

Com isto surge um problema. Ao soldar alumínio, usando soldadura por resistência por pontos,

o desgaste dos elétrodos de cobre é superior ao experienciado quando se solda aço, o que significa

que o número de elétrodos usados para o mesmo número de soldas é superior. Como o número de

elétrodos aumenta, os custos de produção também aumentam, e hoje em dia, ainda mais, devido ao

contínuo aumento do preço do cobre.

Uma tentativa de solução para este problema passa por criar uma camada protetora no topo

dos elétrodos, desgastando apenas essa camada, mantendo o corpo dos elétrodos em boas condições

por mais tempo. Os materiais depositados na superfície dos elétrodos foram: zinco, adesivo de prata,

grafite e estanho, e o método de deposição variou consoante o tipo de material. O zinco e o estanho

foram soldados aos elétrodos, enquanto o adesivo de prata e a grafite foram pintados.

Conclui-se que o zinco, o adesivo de prata e a grafite não criam uma boa camada protetora,

para uma grande variedade de parâmetros e para elétrodos de diferentes tamanhos e geometrias. No

entanto, a camada de estanho demonstrou comportar-se melhor ao soldar alumínio, agindo como

barreira protetora permitindo estender a vida dos elétrodos.

Palavras-chave: estanho, soldadura por resistência, proteção de elétrodos, alumínio

iv

Index

Acknowledgment ..........................................................................................................................i

Abstract ........................................................................................................................................ ii

Resumo....................................................................................................................................... iii

Figure Index ................................................................................................................................ vi

Table Index ............................................................................................................................... viii

Equation Index .......................................................................................................................... viii

Abbreviation List ......................................................................................................................... ix

1. Introduction .......................................................................................................................... 1

1.1. Objectives .................................................................................................................... 2

1.2. Thesis Structure........................................................................................................... 2

2. Literature Review ................................................................................................................. 3

2.1. History .......................................................................................................................... 3

2.2. The Process ................................................................................................................. 3

2.3. Parameters of the process .......................................................................................... 4

2.3.1. Spot weld time cycle ............................................................................................... 4

2.3.2. Squeeze and hold time ........................................................................................... 4

2.3.3. Weld time ................................................................................................................ 5

2.3.4. Pulsation of the weld current and Heat treatment .................................................. 5

2.3.5. Heat Input ............................................................................................................... 5

2.3.6. Contact Resistance ................................................................................................ 5

2.4. Equipment .................................................................................................................... 6

2.5. Electrodes .................................................................................................................... 7

2.6. Testing of Spot welds .................................................................................................. 8

2.7. Common defects on RSW ........................................................................................... 9

2.8. Resistance Welding Processes ................................................................................. 10

2.8.1. Resistance Seam Welding (RSEW) ..................................................................... 10

2.8.2. Projection Welding ............................................................................................... 10

2.9. Difficulties of RSW of aluminium ............................................................................... 11

2.10. Existing Technologies for increasing life of electrodes.............................................. 13

2.10.1. Delta Spot Technology ....................................................................................... 14

v

2.10.2. Electrode dressing .............................................................................................. 14

2.10.3. Electrodes Coating ............................................................................................. 16

2.10.4. Multi-Ring Domed Electrodes............................................................................. 18

2.10.5. Lubricants ........................................................................................................... 19

2.10.6. CapClean by Matuschek .................................................................................... 19

2.11. Copper and Copper alloys ......................................................................................... 20

2.12. Summary of the literature review ............................................................................... 21

3. Experimental work ............................................................................................................. 22

3.1. Methodology .............................................................................................................. 22

3.1.1. Electrode Preparation ........................................................................................... 23

3.1.2. Coating Process ................................................................................................... 23

3.1.3. Aluminium Welding ............................................................................................... 24

3.1.4. Contact Resistance measurements ..................................................................... 24

3.1.5. Summary of the methodology .............................................................................. 25

3.2. Materials .................................................................................................................... 26

3.2.1. Materials Selection ............................................................................................... 26

3.3. Equipment .................................................................................................................. 27

3.4. Electrodes .................................................................................................................. 28

4. Results and Discussion ..................................................................................................... 29

4.1. Without coating – Control Testing ............................................................................. 29

4.2. Zinc Coating ............................................................................................................... 32

4.2.1. Truncated electrodes B16/6 ................................................................................. 32

4.2.2. Radius Electrodes A16 ......................................................................................... 37

4.3. Graphite and SBCA coating ...................................................................................... 39

5. Conclusions ....................................................................................................................... 42

5.1. No coating .................................................................................................................. 42

5.2. Zinc ............................................................................................................................ 42

5.3. Graphite and SBCA ................................................................................................... 42

6. Suggestions for future work ............................................................................................... 43

7. References ........................................................................................................................ 44

vi

Figure Index

Figure 1-1: Evolution of the copper price [5] ........................................................................................... 1

Figure 2-1: Evolution of temperature and resistance in RSW of two sheets [8] ...................................... 3

Figure 2-2: Spot weld time cycle [9] ........................................................................................................ 4

Figure 2-3: Welding Periods .................................................................................................................... 4

Figure 2-4: Scheme of a spot welding machine [7] ................................................................................. 6

Figure 2-5: Electrode Scheme: (1) Water jacket; (2) Electrode body ..................................................... 7

Figure 2-6: Types of electrodes cap: (1) Pointed; (2) Dome; (3) Flat; (4) Angled; (5) Truncated; (6)

Radius; (7) Irregular; (8) Graft; adapted from [10] ................................................................................... 7

Figure 2-7: Peel Test [11] ....................................................................................................................... 8

Figure 2-8: Cross section of a spot weld: (d) nugget size; (h) indentation; (t) nugget penetration; (X) gap

between sheets; (Dc) diffusion joint area; (Dhaz) heat affected zone diameter; [7] ............................... 8

Figure 2-9: Typical defects in spot welding joints [13] ............................................................................. 9

Figure 2-10: (a) Expulsion; (b) Shrinkage void at weld nugget; (c) Solidification crack at the transition

region from HAZ to base metal; (d)magnified view of solidification cracks – adapted from [14] ............ 9

Figure 2-11: Seam welding scheme (left); types of seam welds (right); adapted from [16] .................. 10

Figure 2-12: Projection welding scheme ............................................................................................... 10

Figure 2-13: Relative and absolute shares of Aluminium in some European cars [1] .......................... 12

Figure 2-14: Average use of aluminium per car in Western Europe [1] ................................................ 13

Figure 2-15: Development of aluminium consumption for automotive application in Europe [22] ........ 13

Figure 2-16: Delta spot welding [23] ...................................................................................................... 14

Figure 2-17: Amount of copper consumed [10] ..................................................................................... 15

Figure 2-18: Tip dresser (left); cutter (right) [27] ................................................................................... 15

Figure 2-19: Coated electrode [29] ........................................................................................................ 16

Figure 2-20: (a) unFSPed sample and (b) FSPed sample [32] ............................................................. 17

Figure 2-21: Protection device with a copper foil .................................................................................. 18

Figure 2-22: Multi-Ring Domed Electrode: (a) top view; (b) cross section ............................................ 18

Figure 2-23: (a) Fresh electrode; (b) Electrode after 100 welds on aluminium ..................................... 19

Figure 2-24: Cap Clean by Matuschek [38] ........................................................................................... 19

Figure 2-25: Phase Diagram (Cu-Zn) [41] ............................................................................................. 20

Figure 2-26: Phase Diagram (Cu-Sn) [41] ............................................................................................. 21

vii

Figure 3-2: Scheme of the contact resistance measurements: (a) Ohmmeter; (b) insulator [42] ......... 24

Figure 3-3: Coating applied onto the electrodes surface ...................................................................... 25

Figure 3-4: Matuschek model M800LL SMAX400 kvA (left) MNIYACHI weld checker (right) .............. 27

Figure 3-5: (a) Ohmmeter; (b) TWI machine to measure contact resistance ........................................ 27

Figure 3-6: Type A (a) and Type B (b) electrodes and respective dimensions [mm] [43] ..................... 28

Figure 4-1: Welding growth curves for different degrees of cleanliness ............................................... 29

Figure 4-2: [B16/6] Non coated top electrode after welding aluminium (left) and top electrode in detail

(right); .................................................................................................................................................... 30

Figure 4-3: [B16/6] Non coated bottom electrode after welding aluminium (left) and bottom electrode

detail (right); ........................................................................................................................................... 31

Figure 4-4: [B16/6] Zinc coated top electrode (left); top electrode in detail (middle); top electrode cross

section (right) ......................................................................................................................................... 32

Figure 4-5: [B16/6] Zinc coated bottom electrode (left); bottom electrode detail (middle); bottom

electrode cross section (right) ............................................................................................................... 32

Figure 4-6: Peltier effect scheme .......................................................................................................... 33

Figure 4-7: Monitoring of the zinc coating process along 100 welds: B16/6 (a) top and (b) bottom

electrodes .............................................................................................................................................. 34

Figure 4-8: [B16/6] Zinc coated top electrode after welding aluminium (left); top electrode detail (middle);

top electrode cross section (right) ......................................................................................................... 34

Figure 4-9: [B16/6] Zinc coated bottom electrode after welding aluminium (left); bottom electrode detail

(middle); bottom electrode cross section (right) .................................................................................... 35

Figure 4-10: [B16/6] electrodes; (a) after being coated; (b) after being coated and perform 7 welds in

aluminium .............................................................................................................................................. 35

Figure 4-11: Weld growth curve of aluminium welded with [B16/6] zinc coated electrodes ................ 36

Figure 4-12: Monitoring of the zinc coating process along 25 welds: B16/6 (a) top and (b) bottom

electrodes .............................................................................................................................................. 36

Figure 4-13: Monitoring of the zinc coating process along 50 welds: B 16/6 (a) top and (b) bottom

electrodes .............................................................................................................................................. 37

Figure 4-14: [B16/6] electrodes appearance after the splash test: (a) coting process with 25 weld; (b)

coating process with 50 welds; .............................................................................................................. 37

Figure 4-15: Weld growth curves of aluminium welded with [A16] zinc coated electrodes .................. 38

Figure 4-16: [A16/6] top and bottom electrodes before (a) and after (b) welding aluminium – Test 2.. 38

viii

Table Index

Table 2-1: Consumable volume from each electrode type [mm]............................................................. 7

Table 2-2: Properties of mild steels and aluminium alloys [13] ............................................................. 11

Table 2-3: Colour description according to the %Zn; adapted from [41] .............................................. 20

Table 2-4: Colour description according to the %Sn; adapted from [41] .............................................. 21

Table 3-1: Parameters for the coating and welding trials (* graphite trials) .......................................... 22

Table 4-1: Non-standard resistance measurements (Matuschek welder) ............................................ 30

Table 4-2: Specifications of the graphite and SBCA coating tests ........................................................ 39

Table 4-3: [A20] graphite coated electrodes appearance before and after welding aluminium ............ 39

Table 4-4: [A20] SBCA coated electrodes appearance before and after welding uncoated steel ........ 40

Table 4-5: [A20] SBCA mixed with graphite coated electrodes appearance before and after welding

uncoated steel ....................................................................................................................................... 40

Equation Index

Eq. 2-1: Heat Generated ........................................................................................................................ 5

Eq. 2-2: Average diameter...................................................................................................................... 8

ix

Abbreviation List

SBCA – Silver Based Conducted Adhesive

RSW – Resistance Spot Welding

AC – Alternating Current

DC – Direct Current

𝜙𝑎𝑣 – Average diameter

RSEW – Resistance Seam Welding

ESD – Electrostatic Discharge

HDG – Hot Dip Galvanized

PVD – Physical Vapour Deposition

ISO – International Organization for Standardization

I – Current Intensity [𝑘𝐴]

V – Voltage [𝑣]

R – Electrical Resistance [Ω]

SEM – Scanning Electron Microscope

EDX – Energy Dispersive X-Ray Spectroscopy

𝐶𝑂2 – Carbon Dioxide

𝐴𝐿2𝑂3 - Alumina

𝑁𝑖/𝑇𝑖𝐵2 – Nickel / Titanium diboride

𝑇𝑖𝐶 𝑀𝑀𝐶- Titanium Carbide reinforced with metal matrix composite

𝐶𝑢 - Copper

𝐶𝑟 – Chromium

𝐴𝑙 – Aluminium

𝑍𝑛 – Zinc

𝑆𝑛 – Tin

𝑍𝑟 – Zirconium

1

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 confers [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 so 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 on 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.

Figure 1-1: Evolution of the copper price [5]

2

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.

1.1. Objectives

Due to what was referred before it is intended to extend the electrodes life by creating a layer

that can protect the electrode from sticking to aluminium and that can be replaced when aluminium

starts to damage it. The layer is applied either by welding the material that is intended to cover the

electrode or painting a solution on the electrodes surface. This would increase electrodes’ life because

a cut to the electrode to remove the aluminium wouldn’t be needed, hopefully the only thing to be cut

and replaced would be the layer. The materials that are going to be used to coat the electrodes are:

Zinc, Tin, Graphite and Silver Based Conducted Adhesive (SBCA).

1.2. Thesis Structure

The present document is structured in seven chapters and they are the following:

1. Introduction;

2. Literature review;

3. Experimental work;

4. Results and Discussion;

5. Conclusions;

6. Suggestions for future work;

7. References

The introduction intends to initiate the document making reference to the main reasons for light

weighting vehicles, as they are directly related to the purpose of this invention. Chapter 2 introduces the

principles of the process and highlight the technologies that have been developed along the years that

have the same objectives as this thesis and their limitations.

In chapter 3 the methodologies used during the experimental work are presented along with the

materials and equipment. Chapter 4 shows the results obtained and its analysis. Chapter 5 presents the

main conclusions of this investigation. This document is closed in chapter 6 with suggestions for further

developments on this topic.

3

2. Literature Review

The main objective of the literature review is to explain the process and to present the work what

developed so far in this area of study. It is structured in four main parts and a summary in the end.

History, main principles, parameters and equipment used in RSW;

Difficulties of resistance spot welding aluminium;

Alternatives developed to overcome the difficulties and the reasons for them to fail;

Brief introduction to copper and its two main alloys: Brass (Zn) and Bronze (Sn);

Summary.

2.1. History

Resistance Spot Welding was developed by Elihu Thompson accidentally, when performing

other experiences that ended in fusing two copper wires (in 1885). Patents for resistance welding dated

from 1900. Some years later, he merged his company with Thomas Edison’s creating the well-known

General Electric. In 1930 this technology starts to be used in the automotive industry, increasing its

productivity in manufacturing and repair [6].

2.2. The Process

Resistance Spot Welding (RSW) is the most popular method to join sheets due to its highly

efficiency and well-suitability for automated production lines and mass production [7].

A spot weld is performed with two copper electrodes that are pressed against each other, while

two sheets (or more) of metal stays between them. As soon as the electrodes reach the sheets the

circuit is closed and an electrical current can flow through; the material is melted due to the Joule effect,

in which heat is generated due to the passing of current. As the resistance is higher in the interface

between sheets that is the place where the weld pool starts. The reason for that is that copper has a

low resistance, and the materials in between have usually the double of its resistance. That leads the

interface between sheets to be the place with the highest resistance.

Figure 2-1: Evolution of temperature and resistance in RSW of two sheets [8]

4

2.3. Parameters of the process

This sub-chapter includes the main parameters of the process, such as: the spot weld time cycle

and how it is divided, including a detailed description of the squeeze, weld and hold times; weld current;

heat input and contact resistance.

2.3.1. Spot weld time cycle

The spot weld time cycle corresponds to the total time that is necessary to perform one spot

weld and it is divided in four main parts as suggested by the figure below.

Figure 2-2: Spot weld time cycle [9]

2.3.2. Squeeze and hold time

The squeeze time is the one that precedes the weld time (corresponds to the period 1 on Figure

2-3); during this period of time the current doesn’t flow because its main objective is allowing the

electrodes to reach the necessary pressing force to perform the weld, however the properties of the

weld are not affected by it. When the squeeze time is not set properly molten metal expulsions from the

weld or even expulsions between the electrode and workpiece may occur.

During the hold time (period 3 on Figure 2-3) current stops flowing through the

electrodes/workpiece, and its main objective is to cool down the weld (using the cooling system that

exists inside the electrodes) so it can get sufficient strength. Higher thicknesses require longer hold

times so the weld can be cooled down properly. Shorter hold times are used for materials with the

tendency to be brittle.

Figure 2-3: Welding Periods

5

2.3.3. Weld time

The biggest slice of the time cycle is the weld time that corresponds to the period of time that

the current is flowing through the material. The weld time has a direct influence on the nugget size

because if it increases, the heat input is higher and the amount of material melted also increases,

originating bigger weld spots. Increasing the weld time may bring some disadvantages such as a more

rapid wear to the electrode (due to the temperatures reached on its surface) and more indentation on

the workpiece too, however in materials with a tendency to be brittle or harden the increase on the weld

time will lead to longer cooling times which can be useful to prevent cracks.

The weld time is usually measured in cycles, but it can also be measured in milliseconds. One

cycle corresponds to 20 milliseconds in the 50 Hz power frequency.

2.3.4. Pulsation of the weld current and Heat treatment

Pulsation of the weld current or heat treatment are two other adjustable parameters which are

mostly used for thicker sheets. Heat treatment can be used before or after welding, depending on the

application or purpose of the weld. Pulsation of the weld current allows to focus energy input to the weld

better, so higher currents can be used.

2.3.5. Heat Input

As referred before the material is melted due to the Joule effect so the heat generated depends

directly on three factors: current intensity, resistance of the workpiece and period of time that the current

is flowing (weld time), and can be calculated as following:

𝑄 = 𝑅𝐼2𝑡 [7]

Eq. 2-1: Heat Generated

𝑄 – Heat Generated [𝐽]

𝐼 – Current Intensity [𝐴]

𝑅 – Resistance of the workpiece [Ω]

𝑡 – Weld time [𝑠]

Not 100% of the heat generated is used to melt the material, there is a percentage of it that is

conducted to the surroundings and to the electrodes. As the resistance of the workpiece is not an

adjustable parameter, it depends only on the material to be welded, so the only parameters to control

the heat generated are the current intensity and the weld time.

2.3.6. Contact Resistance

The resistance to the flowing of current due to surface conditions is defined as contact

resistance [7]. The presence of oxides and impurities increases the contact resistance increasing the

difficulty to weld components (and it is more problematic when welding aluminium). The contact

resistance is also influenced by the contact area between the electrodes and the workpiece. When

electrodes get dirty or alloyed with the parent metal the contact resistance increases, which lead to

6

higher temperatures on the electrode’s surface (because the heat generated is higher) and consequently

the electrodes wear also increases.

Materials are usually protected with oils to avoid corrosion and dirt, however in RSW, the surface

conditions largely influence the weld quality, that’s the reason why materials should be cleaned before

welding. The presence of dirt and oxides can confer not only bad mechanical properties to the weld but

also inclusions or make the parent metal stick to the electrode easier. The main problems associated

with inclusions, in RSW, are the difficulty in finding them and the capability of joining fractures from

different places on the weld.

2.4. Equipment

There are two main type of machines to perform RSW, AC and DC machines, the most popular

ones are AC.

These machines are composed by several items as suggested in Figure 2-4. To press the

electrodes against the material there is a pneumatic or hydraulic cylinder (depending on the amount of

force required to perform the weld, higher forces requires hydraulic cylinders), connected to it there is a

push bar that makes the link between the cylinder and the electrodes. There is also a control unit to set

the parameters of the welding process.

Although Figure 2-4 doesn’t show, these machines are equipped with two separate cooling

systems, one to refrigerate the transformer and other to refrigerate the electrodes (and the weld

consequently). The main objective of the first one mentioned is keeping the transformer within a range

of temperatures that allow it to work properly without overheating. The cooling system of the electrodes

influences directly the weld, its purpose is not only to keep the electrodes with a low rate of wearing,

increasing its life, but also to help the weld to cool down properly [7].

1 – Pneumatic/hydraulic cylinder

2 – Push bar

3 – Electrodes and electrode holders

4 – Throat area

5 – Control Unit

6 – Transformer

Figure 2-4: Scheme of a spot welding machine [7]

7

2.5. Electrodes

The electrodes has the configuration presented in Figure 2-5 and have the main role on

performing a spot weld, they are the ones conducting the current to the workpiece and also to support

the force that holds the parent metal in place. For these reasons they have to have specific

characteristics such as high electrical and thermal conductivity, which allows a good heat dissipation,

and high hardness values so they don’t break due to the high pressure forces applied to perform the

weld. The material that is mostly used in electrodes is alloyed copper as it meets the requirements

exposed before.

Figure 2-5: Electrode Scheme: (1) Water jacket; (2) Electrode body

The only problem with copper electrodes is their tendency to form alloys with the parent

materials, mainly with zinc coated steels or aluminium, and for that reason, when welding these

materials, the electrodes are cleaned after a certain number of welds to ensure the quality requirements

are meet. Table 2-1 presents the common industrial practice performed by companies in the amount of

copper that is cut in radius and truncated electrodes.

A16 A20 B16/6 B20/8

Table 2-1: Consumable volume from each electrode type [mm]

The geometry of the electrodes tips can vary between different types as Figure 2-6 suggests,

depending on the weld that is intended to perform.

Figure 2-6: Types of electrodes cap: (1) Pointed; (2) Dome; (3) Flat; (4) Angled; (5) Truncated; (6) Radius; (7) Irregular; (8) Graft; adapted from [10]

8

2.6. Testing of Spot welds

The best way to inspect the weld quality of a weld spot on RSW is by doing a destructive test to

the weld, the Peel Test. The welds are open to be measured and inspected visually after that. The way

to open the welds is to clench one of the sheets in a vice and apply a force, by using a plier, to the other

sheet, perpendicular to its position when welded.

After the weld is opened the weld spot must be measured in two different directions (90 degrees

from each other), the direction that gives the smallest and the largest measurements. After that the

average diameter,𝜙𝑎𝑣, must be calculated with Eq. 2-2.

𝜙𝑎𝑣 =𝐷+𝑑

2 [11]

Eq. 2-2: Average diameter

Figure 2-7: Peel Test [11]

Another way to inspect the quality of the weld is by making a cross section to the weld and

analyzing it with a microscope. Figure 2-8 shows a scheme of a cross section of a spot weld and the

specific zones of it. There is also another destructive test that is possible to perform in order to evaluate

the strength of the weld, tensile test. It requires more resources than the peel test such as specific

machinery and specific specimens.

Figure 2-8: Cross section of a spot weld: (d) nugget size; (h) indentation; (t) nugget penetration; (X) gap between

sheets; (Dc) diffusion joint area; (Dhaz) heat affected zone diameter; [7]

9

2.7. Common defects on RSW

There are some defects than can occur in resistance spot welding leading to have less strength

in the weld or even to total destruction of manufacturing parts of car bodies. The most common

unconformities are cold weld, small-diameter nugget, bad shape of welding nugget, cracks

inside/around the welding nugget and deep indentation of welding electrodes in sheets. [12]

The shape of each defect is presented in the scheme of Figure 2-9.

Figure 2-9: Typical defects in spot welding joints [13]

The figures below presents photos of some of the defects in the previous scheme

(a) (b)

(c) (d)

Figure 2-10: (a) Expulsion; (b) Shrinkage void at weld nugget; (c) Solidification crack at the transition region from

HAZ to base metal; (d)magnified view of solidification cracks – adapted from [14]

10

2.8. Resistance Welding Processes

Resistance welding is composed not only by Resistance Spot Welding but also by two main

more processes. They are briefly described above with some examples of applications.

2.8.1. Resistance Seam Welding (RSEW)

Resistance Seam Welding shares the principle as resistance spot welding with different

electrode geometry. Two rotating disc electrode wheels are used to apply current and force, with them

rotating either continuously or intermittently [15]. Seam welding allows the joining of long sheets with no

interruptions, however it is possible to choose between continuous or intermittent welds (see Figure

2-11 (b)).

Figure 2-11: Seam welding scheme (left); types of seam welds (right); adapted from [16]

2.8.2. Projection Welding

In resistance projection welding, pressing force and welding current are localized to the

workpiece through projections prepared before the welding process. The projections are made at the

same time as the production of the workpieces and can have different shapes [7].

This process is on the peak of implementations for pressure vessel fabrication and similar

applications. Research has been developed so this process can transition from the fundamental stage

to the real world application [17]. The main advantage of projection welding is its ability of making several

welds simultaneously and relatively close to each other without the harmful impact of stray currents [7].

Figure 2-12: Projection welding scheme

11

2.9. Difficulties of RSW of aluminium

Pure aluminium has a very low mechanical strength but when alloyed with some elements, such

as silicon, magnesium, copper or zinc, this property drastically increases above mild steel level.

Aluminium has a high electrical and thermal conductivity; although a good electrical conductivity is a

desirable property on resistance spot welding, a high thermal conductivity is not, and that is because it

makes heat dissipate too fast. Both properties referred before requires high current intensities and short

weld times, which is also a disadvantage as higher welding currents requires more energy. [13]

Another difficulty when welding aluminium is its oxide film, commonly called alumina (𝐴𝑙2𝑂3),

that appears very quickly on the surface. Its melting temperature is about 2000℃, while aluminium and

its alloys have a melting temperature that ranges from 480℃ to 660℃, for this reason the oxide film

should be removed chemically (with some solutions) or mechanically (with an abrasive). Despite alumina

makes aluminium very difficult to weld, it presents an advantage as it protects the surface of aluminium

from corrosion and avoids the using of protective oils [13]. Table 2-2 shows a comparison between mild

steel and aluminium alloys physical properties.

Melting

Temperature [℃]

Electrical Conductivity

𝟏𝟎𝟔[𝑺. 𝒎]

Thermal Conductivity

[𝑾

𝒄𝒎. 𝒌]

Coefficient of thermal

expansion

𝟏𝟎−𝟔[𝟏/𝑲]

Density

[𝒈 𝒄𝒎𝟑⁄ ]

Mild Steel 1560 5 - 10 0.32 - 0.66 11.4 7.8

Aluminium

alloys 480 - 660 14.3 - 37.7 1.2 - 2.37 22-23 1.7-3.0

Table 2-2: Properties of mild steels and aluminium alloys [13]

Another issue when resistance spot welding aluminium is its tendency to alloy with copper, so

after a certain number of welds the electrodes need to be cut in order to avoid spatter between

electrodes surface and the workpiece and to keep the good quality of the weld.

The electrodes degradation happens in four basic steps. The first step is aluminium pickup, it

starts right on the first weld when drops of molten aluminium are transferred from the sheet surface to

the electrode tip face, after that electrodes alloy with aluminium (second step) when molten aluminium

alloy adheres to and reacts with the workpiece’s material forming complex regions of Cu-Al alloys. The

electrode tip face pitting is the third step where there is a breaking up of the local bonds through

transference of the molten Cu-Al mixture or brittle fracture of solidified Cu-Al intermetallic phases. The

brittle fractures start on a ring near the periphery of the contact area and then grows to the interior or to

the exterior of the ring to form larger cavities by combining smaller pitted areas. The formation of this

cavities is the last step of the electrodes degradation [18].

Resistance Spot Welding depends a lot on the surface conditions of the workpiece, which is

supported by [19], where its effects on the weld quality were studied. This study makes a comparison

between two sheets of AA5457, a full cleaned surface (variant 1) and a reduced cleaned surface (variant

12

2); it was concluded that variant 1 allows wider welding windows, less tendency to stick to electrodes

and less indentation of the electrodes on the surface.

Ihsan K. Al Naimi et al [20] in their study on the influence of surface pre-treatment in resistance

spot welding of aluminium AA 1050 conclude that surface condition of the aluminium sheets has

significant influence on the weldability and the electrode lifetime and that is possible to obtain a lower

contact resistance by glass blasting (a process that removes the original oxide film and provides a

rough surface, decreasing the contact resistance due to breakdown of oxide layer by asperity

deformation).

Another problem that emerge when RSW aluminium alloys is revealed in [21] where the

electrode degradation along its life on electrodes with tip-face diameter of 10 mm and radius of curvature

of 50 mm is study. During the tests performed along the study it was observed that the electrodes life

when welding 1.5 mm thick sheet aluminium alloy 5182 ranged from 400 to 900 welds (keeping all the

parameters constant along the electrodes life). It was found that, as the contact area between the

electrode and sheet and also sheet to sheet increased, and as the current was kept constant along the

welds, the current density decreased and undersized weld nuggets were achieved.

Although aluminium welding presents that many difficulties, many solutions have been

developed by different companies, which are going to be presented later on this document. Aluminium

is applied as castings, extrusions and sheet in different parts of the car body, in engine blocks, power

train parts, space frames in Audi A2, A8, BMW Z or in Lotus Elise, sheet structures in the Honda NSX

or in Jaguar or as closures and hang-on parts, for example in DC-E-class, Renault or Peugeot. The

following graphic shows the amount of aluminium weight in the total weight [1].

Figure 2-13: Relative and absolute shares of Aluminium in some European cars [1]

The tendency is to increase the amount of aluminium used in each car, because in the last

decade the amount of aluminium used in passenger’s cars doubled its value and it is expected to do so

in the next decade. In 2000 an average of 102 kg of this material was used in automotive parts in

Westerns Europe, but only 5 kg of it was for the body-in-white, however this value has been increasing,

as it is possible to see in Figure 2-14 [1].

13

Figure 2-14: Average use of aluminium per car in Western Europe [1]

The graphics below presents the amount of aluminium consumed for automotive applications in

1994 and 2005.

Figure 2-15: Development of aluminium consumption for automotive application in Europe [22]

2.10. Existing Technologies for increasing life of electrodes

To extend the electrode’s life, when performing RSW of aluminium, is necessary to make a

selection of the weld parameters that minimise the occurrence of aluminium melting at the electrode-

sheet interface so that build-up of aluminium on the electrode can be prevented. That is because

accumulation of aluminium on the electrode face leads to an increased heating resistance, increasing

even more the temperature on the interface electrode-sheet which makes more aluminium to stick to

the electrode [4].

Increasing the life of the electrodes is something that have been investigated in different

companies and some technologies have been developed due to this investigations, such as “Delta Spot”

and “Smart Dress”.

14

2.10.1. Delta Spot Technology

Delta Spot Welding is one of the existing alternatives to increase electrodes life, it works with

the same principle as resistance spot welding do, the main difference is that the electrodes are not in

direct contact with the workpiece because a running process tape, that transfers the welding current,

runs between the electrodes and the sheets to be welded (after every spot weld the process tape spools

on to its next position [23]), as Figure 2-16 suggests. The tape movement is continuous which leads to

an uninterrupted process producing constant quality [24].

Figure 2-16: Delta spot welding [23]

With the development of this technology regular cap cutting is no longer necessary, which

means that they are effectively protected against wear and also from sheets coatings [24]. The process

tape needs to be replaced, in normal use after 7000 welding spots.

According to [25], it was possible to conclude that when welding advanced high-strength steels

(AHSS):

No cracks or failures are observed in the microstructures of the welded joints;

The process tape is effective in the protection against wear and deposits from the sheet

coatings.

2.10.2. Electrode dressing

Electrode tip dressing is a maintenance technology for increasing electrodes life with the

purpose of keeping its shape and free from contamination from the welded material, which leads to

maintain the weld quality each time. The dressing is performed by cutting the surface of the electrodes

with a single or multi blade, or in less severe cases is used a rotating abrasive to clean up the surface.

A study from the International Automotive Research Centre shows that the electrodes

maintenance has a huge impact on the weld quality and in the number of welds that are possible to

perform with quality. The electrodes surface that did not receive any maintenance has a really bad

appearance with some holes and non-uniform shape and after 700 welds the component outer surface

shows evidences of surface explosion and the weld spot does not meet the requirements (small

diameter and not circular). However, when the electrodes surface receives frequent maintenance it is

possible to achieve 10 000 welds where the component outer surface is in good conditions and the weld

spot have 6 mm diameter. [4]

15

Dressing an electrode every few hundred welds works very well when RSW steel, but for

aluminium is not the best solution. Aluminium needs more frequent attention to maintain the weld quality.

The number of times that copper can be re-machined with a cutter is limited, so when substantial

changes in its geometry are visible it needs to be replaced. A potential solution for RSW of aluminium

is to use a less aggressive means for maintaining the electrode. [4]

Figure 2-17 shows how an electrode is consumed along the time. According to [10] it is possible

to achieve more welds in the last 1/16 of the electrodes length than in the first ones, due to the better

conditions that are achieved by the cooling system.

Figure 2-17: Amount of copper consumed [10]

An example of a technology developed for dressing the electrodes is “Smart Dress”. In this

particular case the dressing is integrated in a complete automated solution and it presents some

innovations when compared with other dressing systems such as: dressing the electrodes with an

abrasive, removing the minimal amount of copper possible, extending its life; an optical sensor able to

monitor the electrode condition providing vital data regarding the state of wear and an associated control

system that determines the level of electrode cleaning; a mechanical dresser designed to minimise

copper removal during tip dressing and an automated tip changer able to replace worn tips during

production without the need for a line stop and manual intervention. [26]

The results obtained by the implementation of this technology in a production line are the

following: increased production life of electrodes, in three times when welding zinc coated steel,

achieved by minimal material removal using the abrasive dresser development; regular electrode

maintenance of RSW of aluminium leaded to a dramatically decrease in the copper consumption;

monitoring and adaptive control of the electrode during spot welding production, able to adapt electrode

maintenance schedules to compensate for unexpected electrode damage and control of spot weld

quality through monitoring and maintaining welding electrodes improving quality assurance of the

production line. [26]

Figure 2-18: Tip dresser (left); cutter (right) [27]

16

2.10.3. Electrodes Coating

Besides the technologies presented above, the coating of electrodes is another attempt to

extend the electrodes life. These studies were first tried for welding galvanized steel and when

aluminium started to emerge in RSW another coating trials were performed for welding it. The advantage

of coating electrodes in comparison with other technologies is the possibility of using current electrodes

and current materials, with no need of altering its composition so it does not alloy with each other. Peter

Jasko, Peter Baksa and Stefan Emmer [28] studied the changes on and under the surface of welding

electrodes coated with a 𝑁𝑖/𝑇𝑖𝐵2 (as suggested on Figure 2-19) during resistance spot welding of

galvanized steel sheet after making 0, 1, 5, 20 and 100 welded joints.

Figure 2-19: Coated electrode [29]

As conclusions for this study, related to the stability of the coating and interaction with the welded

material it was observed that with the coated electrodes (𝑁𝑖/𝑇𝑖𝐵2) after performing 100 welds and in

comparison with non-coated electrodes, the increase in deformation has been significantly reduced, the

stabilization of the average contact area after 100 welds, although before weld number 100 higher

thermal loads were verified. The 𝑁𝑖/𝑇𝑖𝐵2 coating shows substantial destruction right after the first weld,

mainly on the edges where there was a delamination of the nickel layer from the copper electrode. On

the subsequent welds it was possible to see the removal of the layer from the outer ring. The most

probable cause for the layer destruction is the presence of faults in the coating, tensile stresses induced

by the ESD deposition and contact tension peaks on the edge of the seating surface of the electrode

during RSW [28]. Although all the visible destruction of the layer deposited on the surface, it was possible

to identify 𝑇𝑖𝐵2 in the central part of the contact area, after 100 welds [28].

According to Kevin Randall Chan [29] it is proved that coated electrodes can reduce the degree

of wear and degradation of electrodes improving tips life, in his study regarding the weldability and wear

mechanisms affecting a 𝑇𝑖𝐶 𝑀𝑀𝐶 coated cap electrode for the RSW of zinc HDG steels. In this study it

was possible to weave a lot of conclusions concerning the weldability and also the failures presented by

this coating. The electrode tip life was increased from 300 to 1100 welds due to the use of coating and

the formation of alloy layers was slow down, which lead to a decrease on the amount of material loss

and reduction length [29]. The most likely reason for the electrode failure was the eventual penetration

of zinc and breakdown and loss of the 𝑇𝑖𝐶 coating. During the experiments of this study it was possible

to conclude the following: the coating cracked easily due to defects present from the coating process,

these cracks allowed the penetration of zinc in localized areas, forming a layer of brass underneath the

coating.

17

A study from Finlay et al, [30], showed that a coating of PVD of chromium using unbalanced

magnetron sputtering and filtered arc was used to coat a Cu-Cr electrode can extend the life of

electrodes in 100% when welding Al-45%Zn coated steel sheet.

The Department of Mechanical Engineering from the University of Waterloo together with Huys

Industries Limited have studied coatings on resistance welding to extend electrodes life and they

conclude that the deposition of 𝑇𝑖𝐶𝑃/𝑁𝑖 coating onto the surface of the copper electrodes causes

extensive cracking within the coating and delamination at the interface between coating and the

substrate. On the other hand, when applying multi-deposition of 𝑁𝑖 − 𝑇𝑖𝐶𝑃/𝑁𝑖 − 𝑁𝑖 on the electrodes

surface it produces dense coatings and a well bonded interface and although 𝑁𝑖 does not react

chemically with 𝑇𝑖𝐶𝑃 it acts as a barrier and increase the toughness of the coating. With the multi

deposition coating it was also possible to significantly reduce the electrodes erosion [31].

According to [32] an improvement of the 𝑇𝑖𝐵2 − 𝑇𝑖𝐶 coating (regarding morphology,

microstructure, phase composition and resulting mechanical properties) of the electrode was achieved

with modification of the coated surface with Friction Stir Processing (FSP). The results showed that it

was possible to decrease the number of cracks on the coating and enhance the interfacial binding

between the coating and the substrate. Figure 2-20 shows the evidences of this experiments.

(a) (b)

Figure 2-20: (a) unFSPed sample and (b) FSPed sample [32]

Another attempt of improvement to the 𝑇𝑖𝐵2 − 𝑇𝑖𝐶 coating (to increase electrodes life when

welding zinc coated steels) was performed. In this case by changing the coating parameters. It was

found that it was possible to reduce the previously found defects by using an Argon atmosphere during

the deposition. Also when using a pre-existing Ni interlayer, it was possible to increase the average

hardness of the coating [33].

In order to minimise the electrode wear in RSW of aluminium alloys, RR Patil et al [34], used a

carbon black paste in fluidic form in the electrode-sheet (ES) interfaces, so the electrical conductivity

could increase reducing the resistive heating. This paste in a fluidic form was chosen due to its

chemically inertness to copper and aluminium. The results of this study showed that with this

methodology it is possible to double the electrodes life in comparison with uncoated electrodes, however

it was possible to see a very thin aluminium build-up on the ES interface.

18

M. Kondo et al investigated the electrodes life and the degradation characteristics during

continuous resistance spot welding of aluminium alloy sheets, and it was found that the copper of the

electrodes and the aluminium from the sheets rapidly alloyed. To prevent this from happening an

electrode protection device with a copper foil, as Figure 2-21 show, that does not allow direct contact

between the electrode and the worksheet was conceived. It was possible to extend the electrodes life

to 5000 welds, however after weld number 100 the electrodes were remarkably deformed [35].

Figure 2-21: Protection device with a copper foil

2.10.4. Multi-Ring Domed Electrodes

Another technology to solve the problem of RSW of aluminium was developed by General

Motors (GM), a Multi-Ring Domed (MRD) electrode that is presented in Figure 2-22. The electrode

design incorporates several protruding concentric rings on the weld face that act to deform the

aluminium sheet surface on contact and break through oxides on the surface, however, developing

dressing blades to cut this specific geometry of the electrodes have been critical to implement this

solution into GM plants.

(a) (b)

Figure 2-22: Multi-Ring Domed Electrode: (a) top view; (b) cross section

The analysis of the electrodes consisted of visual examination, peak height measurements and

SEM examination. The electrodes wear during RSW with this electrodes occurred in several steps,

beginning with deformation of the small concentric rings, when the rings were flattened the wear

reactions accelerated forming an aluminium contamination layer. It was concluded that it is necessary

to cut the electrodes after performing 100 welds on aluminium because at this point either pitting or

flattening of one or both inner rings occurred. The electrodes appearance after 100 weld is presented

on Figure 2-23 (b).

19

(a) (b)

Figure 2-23: (a) Fresh electrode; (b) Electrode after 100 welds on aluminium

However, dressing the electrodes after 100 welds would lead to large cut depth and this would

tend to shorten the electrodes life. [36]

2.10.5. Lubricants

Rashid et al [37], in order to extend electrodes life, studied the influence of lubricants when RSW

of aluminium 5182. During the experiments different metal-working lubricants were placed between the

electrodes and the aluminium sheets, and one of them gave good results and for that reason was further

investigated. For the same welding conditions and failure criteria, one lubricant was found to extend the

electrodes life to almost double (730 welds) than when no lubricant was used (393 welds). It was

concluded that the lubricant can reduce the thickness of the oxide layer present on aluminium surface

reducing the heat generated in the interfaces and consequently the pitting rate and alloying areas.

2.10.6. CapClean by Matuschek

Matuschek developed a technology regarding the contamination that aluminium causes onto

the copper electrodes surface. The accumulation with aluminium on the electrodes leads to deterioration

of the conductivity and affects the heat generation until the electrodes are unserviceable.

To perform reliable spot welding of aluminium it is imperative to avoid impurity layers, which is

achievable by the use of “drive in and get cleaned” device by regular cleaning of the electrodes. The

specific form of grinding of the Cap Clean ensures up to 10 000 spots per pair of electrodes. Figure 2-24

presents the stages of the grinding tool, which rotates (with a relative angle to the electrodes axis) and

have a positive and negative movement (in the same electrodes axis) [38].

Figure 2-24: Cap Clean by Matuschek [38]

20

2.11. Copper and Copper alloys

Copper importance to human society dates back to prehistoric times [39], it was actually the first

metal used by man in any quantity [40]. Even before it was possible to extract it from the ores, it was

already used as it was available in Earth’s crust (so people used to use it to make ornaments and tools).

The earliest workers in copper soon found that it was easily hammered into sheets and shapes.

Iron emerged and became the basic metal to civilisation, however copper was the chosen material when

strength and durability were required [40].

Copper main alloys are bronze and brass, which corresponds to add tin (Sn) and zinc (Zn),

respectively, to it. When zinc is added to copper its mechanical resistance and strain increases with the

increase in the percentage of zinc (until 30%). According to each percentage, copper acquires a different

colour. Its description is presented in Table 2-3 [41].

% Zn Colour Description

<5 Copper colour

5 – 20 Different shades of gold

30 Yellow brass

40 Light yellow

Table 2-3: Colour description according to the %Zn; adapted from [41]

Figure 2-25 presents the phase diagram for brass.

Figure 2-25: Phase Diagram (Cu-Zn) [41]

The increase of tin content, increases the properties related to mechanical resistance. As brass,

bronze also have different colour according to the amount of tin present. Table 2-4 presents a general

idea of the colours that are reached with each percentage of tin.

21

% Sn Colour Description

5-15 Red

15-25 Yellow

>25 White

Table 2-4: Colour description according to the %Sn; adapted from [41]

Figure 2-26 presents the phase diagram for bronze.

Figure 2-26: Phase Diagram (Cu-Sn) [41]

2.12. Summary of the literature review

As explained during the literature review, resistance spot welding of aluminium presents several

difficulties, one of them is the rapid electrode wear along the process that leads to short electrodes life.

Several technologies and procedures have emerged along the years attempting to solve the

problem of Resistance Spot Welding of coated steels and aluminium. The reason why the solutions for

coated steel are also included in the literature review is to evaluate if these solutions would be viable to

implement for aluminium.

It is possible to conclude that each technology presented in the literature review can improve

the electrodes life, however either they still consume the electrodes or require new welding equipment.

It is expected that the technology that is going to be developed can overcome these difficulties

and hopefully, the copper electrode would not be consumed at all, only the layer deposited onto its

surface, and as the electrodes that are going to be used has the same size and geometry that the ones

used regularly by the automotive industry, no further equipment would be required to use the technology.

22

3. Experimental work

This chapter is structured in four main parts which are:

Methodology;

Materials;

Equipment;

Electrodes;

Summary of the methodology.

The methodology is composed by the different steps needed to accomplish all the experiments,

such as the electrodes preparation, the coating process for each material, and, to study the behaviour

of the layers produced when welding aluminium (there is also a subchapter in the methodology for the

contact resistance measurements, 3.1.4). In the section 3.2 Materials, a complete list of all the materials

used during the experiments can be found, and also an explanation for the reasons why each material

was chosen. In both sections, 3.3 Equipment and 3.4 Electrodes it is possible to find information about

the equipment needed for the experiments and the types of electrodes used.

3.1. Methodology

The experimental procedure followed 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

Time [ms]

Squeeze 200 200 200 200

Weld 240 240 60 100 (120)∗

Hold 500 500 200 200

Force [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 3-1: Parameters for the coating and welding trials (* graphite trials)

23

3.1.1. 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.

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.

3.1.2. 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.

3.1.2.1. Zinc and Tin 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.

The deposition of tin on the electrodes surface uses the same principle as zinc deposition.

3.1.2.2. 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;

1 Metal polish designed to remove tarnish from copper.

24

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;

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.

3.1.3. Aluminium Welding

After the coating process, 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.

3.1.4. 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 3-1: Scheme of the contact resistance measurements: (a) Ohmmeter; (b) insulator [42]

25

3.1.5. Summary of the methodology

The work plan intended to study various coatings onto the electrodes surface. For that reason,

the study was planned and guided according to the results that had been achieved during the

experiments. Figure 3-2 presents a scheme with the three coatings.

Figure 3-2: Coating applied onto the electrodes surface

Coatings

Zinc SBCA & Graphite Tin

26

3.2. 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.

3.2.1. Materials Selection

The materials selected for the coating were: zinc, SBCA, graphite and tin. The reason for

choosing each material is explained below.

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)

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.

Graphite is also a low cost material, has a high electrical conductivity, high melting temperature

and refractory properties with liquid aluminium

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.

During the Splash and Endurance tests the materials used were Aluminium 5xxx and 6xxx.

27

3.3. 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 (the master adapter control is not being used). A caliper was used to measure the weld

spots size after the peel test. To the graphite trials the following equipment was needed: mixture cups

(acetone resistant), brush (to paint the electrodes with the different mixtures), spatula, syringe and a

measuring spoon. To measure the current intensity MNIYACHI weld checker was used.

Figure 3-3: 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, they are both are presented in Figure 3-4.

(a) (b)

Figure 3-4: (a) Ohmmeter; (b) TWI machine to measure contact resistance

28

3.4. Electrodes

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 3-5. Two sizes were used for each

geometry.

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 3-5: Type A (a) and Type B (b) electrodes and respective dimensions [mm] [43]

Chapter 4 will present the results obtained during the experimental work, together with the

discussion.

29

4. Results and Discussion

This chapter presents the results obtained during the experimental work along with the

discussion of each result. It starts with the results of the control testing, where no coating was applied

to the electrodes in order to evaluate the process as it is typically used and also to evaluate how the

degree of cleanliness of aluminium can influence the results obtained.

After that, the results for zinc coated electrodes are presented. Zinc was tested in two

geometries of electrodes under different values of parameters. After, the results for graphite, SBCA and

a mixture of both are presented.

As the results for tin coated electrodes very soon showed potential, unlike the other materials,

this one was further investigated.

4.1. Without coating – Control Testing

First of all, and before trying any coatings, aluminium 6061 was welded to evaluate how it

behaves under different circumstances and to set the parameters for the coating trials. As referred

before, aluminium has alumina on the surface which makes the welding process more difficult, however

it is possible to remove it. Three tests were performed, in the first one no cleaning was performed, it was

welded as received; 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 ascertained and it was possible to verify (by

consulting Figure 4-1) 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 heat generated, as it

is possible to see on Figure 4-1. The aluminium sheets cleaned with acetone and abrasive were the

ones that required higher current intensity to achieve the same weld diameter, this was an indication

that with this cleaning process the contact electrical resistance was the lowest.

Figure 4-1: Welding growth curves for different degrees of cleanliness

0

1

2

3

4

5

6

10 15 20 25Avera

ge W

eld

Dia

mete

r [m

m]

Welding Current [kA]

Growth Curves - Control TestAs received

Cleaned withacetone

Cleaned withabrasive andacetone

ISO 18595

Splash

30

To support and validate the results obtained for the weld growth curves, the electrical

resistances for each case were also measured and are presented in Table 4-1. 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 (when the

current intensity is fixed 17 kA) however, the process unsettles first for the cases with the lowest degree

of cleanliness. The biggest degree of cleanliness allows to reach higher current intensities before splash

than the other two cases. Nevertheless, this cleaning process allowed achieving the required weld

diameter (according to ISO 18595) before occurring splash, which is what is desired in industry.

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 4-1: Non-standard resistance measurements (Matuschek welder)

Despite being able to achieve the necessary weld diameter the damage to the electrodes was

clear. Figure 4-2 and Figure 4-3 show the electrodes appearance after performing 11 welds on

aluminium. The respective cross sections show the damage caused on the surface of the electrode. The

detailed figures of both electrodes show that the contact area surface is almost all covered with

aluminium. This justifies the reason to create a coating layer to protect the electrodes surface and

increasing its life.

Figure 4-2: [B16/6] Non coated top electrode after welding aluminium (left) and top electrode in detail (right);

31

Figure 4-3: [B16/6] Non coated bottom electrode after welding aluminium (left) and bottom electrode detail (right);

The results obtained for the control test are also supported by the work done by L. Han et al.

[19] 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.

32

4.2. Zinc Coating

4.2.1. Truncated electrodes B16/6

When first trying to coat the electrode with zinc coated steel (2xDx56GI), cleaned with acetone,

the zinc coated steel sheets were sticking to the electrodes. So it was decided to weld another zinc

coated steel (H340LAD+Z140 MBO 1.2 mm) that contained the oil, which comes from the producer, to

understand if the oil would improve the deposition of zinc without sticking that much to the electrodes.

The deposition of coating was improved when the zinc coating steel was changed, however, the

improvement achieved was not enough to meet the main objective, keeping aluminium away from the

electrodes surface, and further tests were carried out with 2xDx56GI as the improvements observed

with H340LAD+Z140 MBO 1.2 mm were not significant.

The electrode geometry used in these first trials were truncated [B16/6] coated using 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, as it is possible to see on Figure 4-4, 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 [44]).

Figure 4-4: [B16/6] Zinc coated top electrode (left); top electrode in detail (middle); top electrode cross section (right)

The results of the bottom electrode are presented in Figure 4-5. The bottom layer is thicker than

the top layer with 27,3 𝜇𝑚.

Figure 4-5: [B16/6] Zinc coated bottom electrode (left); bottom electrode detail (middle); bottom electrode cross

section (right)

33

By observing Figure 4-4 and Figure 4-5 although it is possible to see that all area of both

electrodes is covered with zinc, the layer isn’t uniform, which lead to an inconsistent process [4], and,

when comparing top and bottom electrodes, they present different average thicknesses.

The difference on the values of the average thickness (between the top and bottom electrodes)

is due to two effects that happen during the joining process:

Peltier effect [45];

Impact that occur between the top electrode and the material to be welded.

The Peltier effect describes that the existence of a voltage in a closed circuit (in which an

electrical current flows) composed by two junctions (A and B) of two different materials (1 and 2,

conductors) causes a temperature gradient. One of the joints cools down by dissipating energy, A, and

the other junction becomes hotter by absorbing energy from the surroundings, B. In RSW and

accordingly to Figure 4-6, the material presented with 1 is copper and the material presented with 2 is

the one to be welded. This effect happens independently from the room temperature. The arrows

represent the flow of the current intensity.

Figure 4-6: Peltier effect scheme

As the top electrode is the one with movement, it impacts with aluminium (the same do not

happen for the bottom electrode, as it is fix during the joining process) breaking the alumina layer present

on the aluminium surface. Breaking the layer of alumina reduces the electrical resistance on the top

electrode/sheet interface (in comparison with the electrical resistance on the bottom electrode/sheet

interface) as alumina presents a much higher melting temperature (around 2040 ℃ [46]) than aluminium

(between 582 − 652 ℃ [47]). As the electrical resistance is lower on the top electrode/sheet interface,

the heat generated is also lower (the current intensity and the period of time that it is flowing is the same

in both cases, 𝑄 = 𝑅𝐼2𝑡).

These two factors conduct to less generation of heat on the top electrode, which leads to less

vaporization of zinc and consequently to lower values of average thickness of the layer deposited.

34

It is possible to see a darker zone at the edges that results from carbon (present on the zinc

coated steel surface) that is burned during the joining process, and also that the darker area is higher

also on bottom electrode.

The cross section images (in Figure 4-4 and Figure 4-5) show the layer deposited onto the

electrodes surface. Its yellow colour indicates a 30 to 40% zinc range [41].

The coating process of the test presented before was monitored and the results are presented

in Figure 4-7. The monitoring process was performed taking a picture of the top and bottom electrodes

each x welds (x was decided according to the test), in this case each 20 welds.

(a)

(b)

0 welds 20 welds 40 welds 60 welds 80 welds 100 welds

Figure 4-7: Monitoring of the zinc coating process along 100 welds: B16/6 (a) top and (b) bottom electrodes

This test showed that from weld number 60 to weld number 100 the coating is worse (looks like

some part of it is gone). That’s because with this joining process (RSW) the electrodes are subjected to

a lot of impact, breaking the layer that was formed in previous welds.

To evaluate the layer behaviour while welding aluminium, another electrode was prepared with

the same procedure as the electrode presented above. After that, 7 welds were performed in aluminium.

Figure 4-8 and Figure 4-9 show how the electrode look like after welding aluminium. It is evident, by

observing the middle figures that there is aluminium in the electrodes (either on top and bottom). The

cross sections figures show that both, the layer and the electrodes, presents damage.

Figure 4-8: [B16/6] Zinc coated top electrode after welding aluminium (left); top electrode detail (middle); top

electrode cross section (right)

35

Figure 4-9: [B16/6] Zinc coated bottom electrode after welding aluminium (left); bottom electrode detail (middle);

bottom electrode cross section (right)

It is also possible to observe 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 aluminium.

Figure 4-10 presents a comparison between two bottom electrodes, before and after performing

7 welds on aluminium.

(a) (b)

Figure 4-10: [B16/6] electrodes; (a) after being coated; (b) after being coated and perform 7 welds in aluminium

Comparing the two pictures presented in Figure 4-10 it is possible to see that (b) presents

damage (more than one hole on the electrodes surface) and (a) do not (flat surface). The difference

between both is that (a) was taken before welding aluminium and (b) after. For that reason, it is possible

to conclude that the coating process is less aggressive to the electrodes (as it was supposed to be) than

welding aluminium, being the main cause of degradation of the electrodes.

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 4-11 shows that the average diameter

of the welds obtained do 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

36

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 4-11: Weld growth curve of aluminium welded with [B16/6] zinc coated electrodes

In the previous results (regarding the layer behaviour and the weld quality - Figure 4-7 until

Figure 4-10), by analysing the cross section figures, 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, beyond 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.

To evaluate how the number of welds performed on zinc coated steel during the coating process

influenced the quality of the coating layer, the coating process was monitored during two tests, where

the only variable changing was the number of welds, all the other parameters were kept constant (and

can be found in Table 3-1), and the value for the current intensity was between 8,5 and 9 kA. The first

test was performed with 25 welds, and stopped each 5 welds to register the coating layer both on top

and bottom electrodes; the images of that test are presented in Figure 4-12. The second test was

performed with 50 welds, but in this case it was monitored each 10 welds and the results are presented

in Figure 4-13.

(a)

(b)


Figure 4-12: Monitoring of the zinc coating process along 25 welds: B16/6 (a) top and (b) bottom electrodes

0

1

2

3

4

5

6

8 10 12 14 16 18 20

Ave

rag

e W

eld

Dia

me

ter

[mm

]

Current Intensity [kA]

Weld Growth Curve - Zinc Coated Electrodes

ISO 5821

Control Test

Zinc Test

Splash

ISO 18595

37

(a)

(b)


Figure 4-13: Monitoring of the zinc coating process along 50 welds: B 16/6 (a) top and (b) bottom electrodes

Comparing the results, it is possible to observe that there is not so much difference in the visual

appearance of the layers after weld number 20, the only thing happening differently is the increasing of

the dark area of burned carbon with the increase in the number of welds performed.

After the coating process, a splash test was performed for both pair of electrodes. The

electrodes that performed 25 welds during the coating process reached a splash current intensity of 13

kA, while the electrodes that performed 50 welds during the coating process, reached only 10 kA of

splash current. The electrodes appearance after performing the splash test is presented in Figure 4-14;

the number of welds performed on aluminium with the 25 welds coated electrodes was six and with the

50 welds coated electrodes only two.

(a) (b)

Figure 4-14: [B16/6] electrodes appearance after the splash test: (a) coting process with 25 weld; (b) coating

process with 50 welds;

It is possible to see aluminium pick up [18] in both tests (and in both electrodes of each test)

making this procedure not suitable for the process. With this comparison it is possible to conclude that

increasing the number of welds during the coating process is not the solution to get a layer that can give

a stable process when welding aluminium.

4.2.2. 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 [37]. In the first test

(Test 1 in Figure 4-15), with A16 geometry, the current intensity value was 8 𝑘𝐴 and the number of welds

52 and in the second one (Test 2 in the same figure) the current intensity value was 8,5 𝑘𝐴 but with 65

welds.

38

Figure 4-15: 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 1 the

number of welds is lower, and so it is the current intensity value achieved before splash. The electrodes

appearance of Test 2 before and after welding aluminium is presented in Figure 4-16. It is possible to

see aluminium on the surface of both, top and bottom, electrodes.

(a) (b)

Figure 4-16: [A16/6] top and bottom electrodes before (a) and after (b) welding aluminium – Test 2

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 4-15 shows that it was possible to reach higher average weld diameters with

electrodes coated with a higher number of welds. A higher number of welds during the coating leads to

higher amounts of zinc onto the surface, and also to thicker layers which improves the welding process

because a thicker layer would be more stable during the welding process. For the first time, it was

possible to reach the standard requirements for the average weld diameter (5,3 mm). The problem of

Test 2 was that although the average weld diameter meets the standard requirement, it happened for

an expulsion weld (when the welding pool exits the sheets to be welded) and there is too much

aluminium in the electrodes surface (see Figure 4-16). It is possible to conclude that to weld aluminium

6xxx, the A16 zinc coated electrodes are the more suitable ones, when compared to the B16/6 zinc

coated electrodes. That is due to the weld spots average size reachable with each type of electrodes.

A16 allows to reach the standard requirements while the B16/6 do not.

0

1

2

3

4

5

6

5 10 15 20Avera

ge W

eld

Dia

mete

r [m

m]

Current Intensity [kA]

Weld Growth Curves - Zinc coated electrodes [A16]

Test 1

Test 2

ISO 18595

Splash

39

4.3. 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 3.1.2.2. Table 4-2 summarizes the

amount of each component used during each test. The amount of SBCA was not possible to measure

in milliliters because the paint was too thick to put in a syringe, so it was measured in number of layers

painted onto the electrodes surface.

Test Graphite [spoons]

Solvent Number of SBCA layers

Way/time to dry the layer Oil [spoons] Acetone [ml]

Graphite A

1 - 1 - Dryer (2 min)

1 - 2 - Dryer (2 min)

1 - 6 - Dryer (2 min)

B 4 1 - - Dryer (2 min)

SBCA

A - - - 1 Natural drying

(30 min)

B - - 1 1 Natural drying

(3 days)

Graphite and SBCA

A 3 1 Natural drying

(3 days)

B 1 3 1 Natural drying

(3 days)

Table 4-2: Specifications of the graphite and SBCA coating tests

To study the coating behaviour, aluminium was welded after the electrodes were prepared. All

welding parameters were kept constant (values in section 3.1) 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

aluminium 6061

After one weld on

aluminium 6061

Graphite mixed with acetone

[𝐼 = 18 𝑘𝐴]

Graphite mixed with machine light oil

[𝐼 = 10 𝑘𝐴]

Table 4-3: [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

40

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. Although, when welding uncoated steel, even with this low value of current intensity, the all

depth of 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 4-4: [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.

The electrodes appearance before and after welding uncoated steel are presented in Table 4-5

Electrode Bottom Top

Before welding

uncoated steel

After one weld on

uncoated steel

SBCA mixed with graphite [𝐼 = 2𝑘𝐴]

Table 4-5: [A20] SBCA mixed with graphite coated electrodes appearance before and after welding uncoated steel

41

It was possible to evaluate that none of the approaches, being with graphite, SBCA or both were

the solution for the problem identified in the beginning. 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.

The reason for graphite to not adhere to the copper electrodes was that it was in powder, and

there was no binding element to join them. Acetone was used as a binder, but as it was expected it does

not react with graphite, evaporating form the surface, leaving only the powder onto the electrodes

surface. With light machine oil, although it did not evaporate, it also did not react with graphite, or copper,

and the mixture deposited onto the surface had any sticking/binding element to hold the layer when the

electrodes were pressed against each other.

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.

42

5. Conclusions

Along the years, different techniques were tried in order to extend the electrodes life. First when

welding coated steels, and more recently for aluminium. However, although these techniques present

improvements in the electrodes life, there is still problems when performing resistance spot weld of

aluminium. The solution developed during this study is no exception. Although, the problems with this

technology are less than the ones presented in the literature review.

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 conclusion. The main ones

are presented in the four following sub chapters.

5.1. No coating

The level of cleanliness of a surface influences the joining process;

The cleanest surface confers the highest splash current and the highest average weld

diameter.

5.2. Zinc

After a certain number of welds the deposition of zinc does not increase;

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.

5.3. Graphite and SBCA

Graphite doesn’t adhere very well to the electrode, either as 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.

43

6. Suggestions for future work

As referred before, the technology developed along this project needs some improvements so

it can be implemented in a production line.

For that reason, future work on these developments is required, such as:

― Automate the cleaning and coating process that is performed manually, in order to make

it viable for a production line;

― Develop a technology that allows to remove only 10 𝜇𝑚 of the electrodes surface;

― Try to produce Cu-Sn electrodes in order to avoid the cleaning and coating process;

― Review the possibility of protect the intellectual property;

― Cost analysis to conclude about the economic viability of the technology.

44

7. References

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Technologies and Challenges for body structures and closures, The Aluminium Association, Inc.

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