A comparative study between conventional fixed and advanced adaptive control system for resistance spot welding

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1 DEGREE PROJECT IN MECHANICAL ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2018 A comparative study between conventional fixed and advanced adaptive control system for resistance spot welding CAROLINE BOHLIN KTH ROYAL INSTITUTE OF TECHNOLOGY SCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT

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3 Abstract Resistance spot welding is the main welding method used in the automotive industry to weld thin sheet metal. Today adaptive control systems have been developed for RSW, which means it can adjust the parameters in the weld process automatically during welding. The control systems can register the parameters and properties of the weld in real-time and from that calculate with algorithms how to adjust to give optimal weld conditions. This project is performed at Scania CV AB, Oskarshamn. Conducted in the part of body in white, where an adaptive control system called HCC is used in all weld processes. In this project, HCC was compared to the fixed control system CCR and another adaptive control system named Master mode. First step in the comparison was to create a weld schedule for each control system and test them on two different material combinations. The aim was to quantify gains and benefits that adaptive resistance spot welding systems have on the welding process. Benefits are quantified by examining the parameters and factors such as: weld time, expulsion, robustness, electrode wear and parameters in the control system. The tests were performed by welding as many approved spot welds as possible without tip-dressing the electrode. The experiment followed the requirements from international standards and the Scania standard for resistance spot welding. The results from the experiment showed that HCC was the most robust process and the spot welds never decreased in size, which CCR and Master mode did. It is possible to weld several different material combinations with HCC, it increases flexibility in production and reduces the time needed to develop new weld schedules. The same schedule can handle many combinations with the same thickness. HCC allows the process to use several pulses and each pulse adds in time. Therefore, the weld schedule should be well developed and optimized to avoid waste in terms of long weld times. The results will give Scania knowledge about the processes and how to further optimize the welding processes in production. The result can also be used as foundation for selection of products or future investments. Keywords: RSW, Resistance spot welding, HCC, Heat Capacity Control, adaptivity, automotive

4 Sammanfattning Motståndspunktsvetsning är den huvudsakliga svetsmetoden som används inom fordonsindustrin för att svetsa tunn plåt. Idag har adaptiva styrsystem utvecklats för RSW vilket innebär att de automatiskt kan justera parametrarna i svetsprocessen under svetsning. Styrsystemen kan registrera parametrarna och egenskaperna hos svetsen i realtid och därmed beräkna med algoritmer hur de bör justeras för att ge optimala svetsförhållanden. Detta projekt är resultatet av ett examensarbete på Scania CV AB, Oskarshamn. Det utfördes i den nya karossfabriken, där ett adaptivt styrsystem som heter HCC används i alla svetsprocesser. I projektet jämfördes HCC med ett konstantströms styrsystem CCR samt ett annat adaptivt styrsystem kallat Master mode. Den primära metoden var att skapa ett svetsschema för varje styrsystem och testa dem på två olika materialkombinationer. Syftet var att kvantifiera vinster och fördelar som adaptiva punktsvetssystem har på svetsprocessen. Testerna utfördes genom att svetsa så många godkända punkter som möjligt utan att formera elektroden. Fördelarna kvantifieras genom att man undersökte parametrarna och faktorerna svetstid, sprut, robusthet, elektrodslitage och parametrar i styrsystemen. Experimentet följde kraven i enighet med internationella standarder och Scania-standarden för punktsvetsning. Resultaten från experimentet visade att HCC var den mest robusta processen och punkterna minskade aldrig i storlek, vilket CCR och Master mode gjorde. Det är möjligt att svetsa flera olika materialkombinationer med HCC, det ökar flexibiliteten i produktionen och minskar den tid som krävs för att utveckla nya svetsscheman eftersom samma schema kan hantera många kombinationer med samma tjocklek. HCC tillåter processen att använda flera pulser, och varje puls adderar tid och svetsschemat bör därför vara välutvecklat och optimerat för att undvika slöseri med avseende på långa svetstider. Resultaten kommer att ge Scania mer kunskap om processerna och hur man kan optimera processerna ytterligare i produktionen. Resultatet kan också användas som grund för val av produkter eller framtida investeringar.

5 Acknowledgements This thesis is the final examining part of the master s program production engineering and management at the Royal Institute of Technology (Kungliga Tekniska Högskolan), Stockholm. This master thesis was conducted at Scania CV AB in Oskarshamn. I want to share my gratitude to Marie Allvar and Sebastian Danielsson at Scania CV AB for great supervising during the thesis. Sharing their time, knowledge and interest during the progress has been very appreciated. Many thanks also to my supervisor at KTH Joakim Hedegård from Swerea Kimab who has been a good support during the project. Fredrik Svensson, Manager MBBEN, Scania Oskarshamn for giving me the opportunity to complete my Master s thesis at MBBEN. Stefan Borg, Svetsrådet, for sharing useful information and Erik Tolf, Scania Södertälje, which contributed to yielding results. Caroline Bohlin Oskarshamn, May 2018

6 Nomenclature Notations Symbol Description Unit Q Energy [J] U Voltage [V] I Current [ka] R Resistance [µω] d n Nugget diameter [mm] d w Weld diameter [mm] t Thickness [mm] T Weld time [ms] F Force [kn] Abbreviations RSW CCR HCC NI BIW UT AHSS UHSS DP HS-IF HAZ Resistance spot welding Constant Current regulation Heat capacity control Nugget index Body in white Ultrasonic testing Advanced high strength steel Ultra high strength steel Dual phase High strength interstitial free Heat Affected Zone

7 CONTENTS 1 INTRODUCTION Background Purpose Research questions Research methodology Delimitations Disposition FRAME OF REFERENCE Joining in Automotive industry Resistance spot welding Equipment for resistance spot welding Control systems for resistance spot welding Standards and regulations for RSW Non-destructive and destructive testing MATERIAL EXPERIMENTAL METHODS Welding Set-up Welding experiments Analysis RESULTS Material combination Material combination Weld time at spot DISCUSSION AND CONCLUSION Discussion Conclusion... 55

8 7 RECOMMENDATIONS AND FUTURE WORK REFERENCES... 58

9 1 INTRODUCTION This chapter introduces the thesis. It describes the background, the purpose, research questions, the methodology and the limitations used in the presented project. The demand for a highly efficient production makes resistance spot welding the main joining method in the automotive industry. The process is fast, efficient and able to weld high strength steel in a short amount of time. To be competitive in the market, Scania CV AB in Oskarshamn strive to improve and decrease fuel consumption of the trucks. The weight of the truck has a direct relation to fuel consumption and therefore are advanced high strength steels (AHSS) increasingly introduced in the design of the vehicle. It enables the use of thinner steel sheet with retained strength. The cab body is, now more than before, constructed with different materials, from mild steel to AHSS with different thicknesses. This requires variation of the parameters and puts higher requirements on the welding procedures. The variation in sequence in production also makes it important to have a welding process that can adapt flexibly between the different components. Cycle time is one of the most critical factors to consider in mass production. A truck contains about 3500 spot welds and by improving the process the timesaving generated from each spot-weld can add up to a large timesaving for each truck. At Scania, it is important to know how the process works and acts. Today Scania uses adaptive process control, to control the spot welding processes. By using adaptive control system in the production the process can adjust the parameters during the welding process. The control system is fairly new and the benefits that are given are not yet investigated. It is known that the control system has a advantageous impact on the welding process, but the benefits have not been properly quantified. Scania wants to know how the adaptive control system handles important spot welding properties such as: expulsion, time, robustness and the microstructure and hardness of the weld, and to see if the process handles different materials and combinations differently. This will give them knowledge that will be useful in process development and for new investments of equipment in the production. 1

10 1.1 Background Scania CV AB is an international company manufacturing trucks in premium class and selling to customers all over the world. At the plant in Oskarshamn the cab is manufactured. This thesis will be executed in the part of Body in white (BIW). The new factory containing BIW was built in 2015 to improve the production by investing in new technology and to meet the increasing demand since the new generation of cabs was launched. During the spring (2018) the new factory is taking over all production for BIW when the old BIW factory shuts down. The production is still striving to improve and the company needs to keep updated of how their processes work to keep production as efficient as possible [1]. The main method to join sheet metal in the automotive industry is resistance spot welding (RSW). The control systems can either use fixed- or adaptive weld schedules, the latter is used at Scania. The adaptive control systems can detect when the weld is complete by knowing the leading parameter or property. One version of adaptive systems utilizes ideal curves that have been determined for the varying parameters and the system adjusts so that the parameters follow the curve. Following adaptive system that is used in the production is Matuschek s HCC Heat capacity control. Another adaptive Matuschek system available is Master mode. The reference welds are made by using fixed schedule CCR Constant current regulation. Control systems for resistance spot welding is further described in chapter 2.4. Adaptivity keeps the welding parameters optimal for each individual weld and the defects are therefore reduced. The gain is specific for each application and tests need to be run for each one to know in what way and how much better it is to use adaptivity for a specific application. Development of computational control systems for RSW has grown a lot over the last years. Today it is possible to customise the process by taking into account which material, parameters, machine and electrodes that are used in the process. This enables a more cost and time efficient process when the machine can adjust itself. The computational system can also give important information in real-time of what happens in the weld process [2]. Science and information in this area are limited, earlier investigations and reports regarding a comparative study like this topic has not been found. Scania is the only company using HCC in almost all processes, they also have a special license to review the program and change 2

11 parameters. Since no other company uses HCC to the same extent, earlier investigations and research in this area is lacking. However, a result of tests on boron steel using fixed weld schedule system and adaptive weld schedule has been published. It showed that the hardness in HAZ was different and changing unexpectedly when using adaptive control system [1]. Therefore, it would be interesting to study if it is possible to see differences in the material properties in HAZ when evaluating the adaptive systems at Scania. This report is the first output of finding the actual benefits adaptive spot welding has on the production at Scania. Scania is interested in learning more about how the materials and the combinations used in the cab are affected. The theoretical advantages are known, such as less weld expulsion and optimal welding time, but quantifying investigations are lacking. The cycle time for implementing the welded spots is one of the most important aspects to consider. The large amount of spot welds in the truck means that a time gain for each spot weld can generate a large total time gain. 1.2 Purpose The purpose of the thesis is to increase understanding and knowledge about the true benefits of the adaptive systems. The result will be used as a foundation or motivation for continued work or selection of products. It can also be used to support further investments. The objective is to identify and quantify a few of the gains and benefits that the adaptive processes give. Problem definition Investigate the actual gain it allows to use adaptive systems on the welding process compared to the traditional system, for a few selected applications. The properties and areas that will be investigated are: The robustness of the process: How many completed welds fulfil the requirement stated by the standard? How does the result deviate between traditional and adaptive system? Weld expulsion: How do the different control systems handle weld spatter? Weld time: Investigate the difference in weld time between the control systems. 3

12 Surface defects and electrode wear: Control if the surface and electrode appear differently when using different control systems. Other parameters that will be investigated to help answer previous areas are current, voltage, end resistance, energy levels and nugget index. The aim is to answer how these areas are affected by using the three different control systems available at Scania. Another aim is to answer how some of the properties of the material and different combinations can affect the outcome. 1.3 Research questions I. What are the advantages/disadvantages with using the adaptive control systems, HCC and Master mode in RSW compared to the fixed system CCR, for selected applications? II. Are there differences in weld properties due to the control system used? III. Does the result comply with the Scania standard? 1.4 Research methodology In the beginning of the project, a workplan stating the workflow and schedule were created, where milestones and sub-goals was defined. To answer the research questions, investigations were made in several steps. Deep and broad knowledge in fundamentals and physics about RSW has been collected by performing a literature study regarding relevant standards, articles and literature. Literature were found from KTH library and books from previous courses in education. It was also important to understand the BIW production and requirement on the process stated by Scania standard and other international standards. Beyond this, internal education was necessary to use experimental equipment and interviews were held with people with relevant knowledge within and outside Scania. The experimental phase included physical weld tests and laboratory analysis of welds and electrodes, to evaluate the different outcomes and find results. 1.5 Delimitations Project delimitations will regard control system, type of material and material thickness. The electrode that will be used is ISO 5821: Cap B A2/2. Also, the investigated 4

13 result will be limited and prioritised starting with evaluating the first prioritised combination. At least two material combinations and at least two properties will be evaluated. Properties that will be evaluated in the report are shown in Table 1. The different systems will be compared with reference to the nugget size. The same nugget size will be the aim for the samples and thereafter it will be possible to make a comparison. Table 1 Specification of process- and material properties Process properties Robustness Time Weld expulsion Electrode wear Weld properties Hardness HAZ Control system The thesis will be limited to the three control systems, CCR, Master mode and HCC. Material The material combinations will consist of combinations existing in the cab body. Two to three sheet combinations with different materials will be used. The work will to the extent possible, comply with STD4429 Resistance Spot Welding - Requirements, Scania standard. 1.6 Disposition In this chapter the Introduction is described, it holds an overview of the subject of the thesis and the workplan and method for accomplishing it. The second chapter gives the Frame of reference; collection of all fields of theory which the thesis is based on regarding process, physics and state of the art. In chapter 3, Material, the material combinations used in the experiments are described. In Experimental methods, chapter 4, all steps of creating a weld schedule and the experimental method is described, as well as the analysis. The Result from the experiment is given in chapter 5. Thereafter, the result and findings from the compared control systems are compared and discussed in chapter 6, Discussion, this chapter also answers the thesis question stated in the beginning and conclusions from the results are drawn. Recommendations and future work is given in chapter 7. Last chapter is references, 5

14 where the literature and data from theory stated in chapter 3 is summarised. At last, all data and information that did not fit in result are appended in appendix. 6

15 1 FRAME OF REFERENCE In frame of reference, the theory regarding resistance spot welding and control systems are presented. 2.1 Joining in Automotive industry There are several joining methods used to join the body parts together. At Scania s Body shop, they use laser-brazing, MIG-brazing, stud welding and RSW to join and combine different materials in the best way. RSW is the most commonly used method in the production. Resistance welding is a group of processes that uses resistance, current and force to weld pieces together. [2] Most common of these is RSW, which is used widely in steel sheet production, due to its high efficiency. Adhesives are used in between some sheets to distribute the load and increase fatigue life [3]. Adhesives are normally a factor to consider, but will not be investigated in this project since the joint will not contain adhesive. This thesis will focus on resistance spot welding and the following theory will therefore only concern RSW. 2.2 Resistance spot welding A resistance spot weld is accomplished by forcing the workpieces together by two electrodes while an electrical current flows between the electrodes through the material. When the current passes through the material, the highest resistance will occur in the middle between the sheets, and that is where the weld will start to form. The resistance and the current creates high energy and a localised heat. The heat makes the material melt, and a spot weld is formed. In Figure 1, a picture shows the cross-section of a spot weld between two metal sheets. The workpiece has higher resistance than the copper electrode and therefore only the workpiece melts. [4] Figure 1 Cross-section of a typical spot weld 7

16 RSW is used mainly to weld thin sheet of steel during a short amount of time. It is possible to weld two-, three- and four-sheet combinations, or even more. The process is highly suited for automation and robotics allowing highly efficient production. It therefore makes it suitable for the automotive industry, enabling high strength steel to bond efficiently and without adding extra material keeping the weight as low as possible. [5] It also requires little training for operators and the process does not need filler material or shielding gas which makes it more environmentally friendly and cost efficient. Most of the heat is restricted to the direct area and therefore RSW gives little deformation of the workpiece [6]. Some heat is transferred away by surrounding material due to the materials conductivity [2]. The physical fundamentals of resistance spot welding is dependent of the material property resistance (R) across the weld, the parameters time (t) and current (I). Heat input (Q) is defined by Joule s law as a function of resistance, current and time. As seen in equation (1). Q = RI 2 t (1) Normally current and resistance is not constant during RSW, as further described in 2.2.3, resistance changes as the material melts, this leads to a change in current during the process to comply with the resistance. Because the resistance changes, the expression in equation (1) must be integrated to get the correct heat input Parameters in the process The process is depending on the parameters time, force and current and when the weld schedule is created, these parameters must be carefully considered and chosen. Changing one parameter will affect another one and therefore they need to be weighed against each other. Parameters are able to be adjusted and changed. A materials resistivity is vital in developing the heat-effect. Therefore, current and weld time will be adjusted to suit the specific material properties. Time Force Current Time. The total time is divided into different phases. The first one is squeeze time when the electrodes squeeze the material to develop the right pressure and joint fit before welding. The 8

17 sheets must be in the right position before the current is run, otherwise the weld will be inadequate. The second phase is weld time when the current flows. During this time the material melts and the nugget is created. The third phase is hold time, when the electrodes hold pressure without flowing current. This keeps the sheet in place during the solidification. Before and after the phases there is off time, when the electrodes have time to move to another spot and find the right location for the weld.[8] In Figure 2 the different phases can be seen. Figure 2. Spot welding cycle [21] Electrode force. The electrode force has an impact on the resistance. When the force is increased the sheets are pressed tighter and surface contact is improved. The surface resistance will be lower due to this and the welding energy will be lower. That means less heat to the weld and it gives a higher risk of loose spots. If the force is low, resistance will become higher and the process will get more heat. This can lead to expulsion that is described further in chapter Commonly used force values in thin steel sheet spot welding processes are 3-6 kn. [7] Welding current. Normally, the current amplitude for resistance spot welding of thin steel sheet is 5-10 ka [7]. If current is not set to correct values it can lead to poor strength, too small weld nugget or discontinuities in the weld. Current is the parameter that influences the energy the most. To keep the current in an acceptable interval, a weld lobe can be created that generates acceptable welds with the right quality. The lobe curve shows the minimum weld current to create a nugget of right size and the maximum weld current to avoid expulsion, e.g. 9

18 in relation to weld time. A lobe curve is created by making test welds and noting size while systematically varying the parameters, and thereby find minimum and maximum of current and time with an approved size and avoiding expulsion. [6] Resistance Resistance in the weld rely on the resistivity of the material. Resistivity is the materials ability to pass current through a conductor and it varies for every steel. The resistance is given by formula (2) R = ρl/a (2) where ρ is the resistivity, l the length of the conductor and A the area of the conductor, which in a resistance spot weld is the area of the electrodes tips. The higher resistance there is the more heat is developed. The reason the workpiece melts and not the electrode is that copper has very low resistance and the highest heat will always develop at the interface with the highest resistance, i.e. between the sheets being welded. Therefore the nugget starts to grow there. Total resistance between the electrodes for a two-sheet stack up is expressed by formula (3). R tot = r 1 + r 2 + r 3 + r 4 + r 5 (3) where r 1 and r 5 is the contact resistance between electrode and metal sheet. r 2 and r 4 is the resistance in the workpiece and r 3 is contact resistance between the metal sheets [6]. Figure 3 r 4 r 5 Figure 3 Principle of total resistance between electrodes. shows the principle of resistance between electrodes and metal sheets. The resistance changes as the material melts, and is therefore called dynamic resistance. The resistance is high in the beginning due to oxides and rough surfaces and it drops when the material melts. Because of this, the nugget is formed rapidly after the oxides have been 10

19 broken through and surfaces evened out. When the nugget starts to form, the resistance increases and at last, the resistance drops again when the current density is spread over the nugget. [2] Resistance End is one of the parameters measured in the control system at Scania and it indicates if the nugget has grown. It gives the value of the resistance curve when the weld time is stopped. Therefore, the resistance end can be different for the same material and material combinations depending on when the weld time is stopped. If the process is stopped early, the resistance is in an early stage on the resistance curve and resistance end becomes high. If weld time is longer, the resistance curve has had time to decrease and resistance end becomes lower. Resistance end indicates the size of the nugget, if it is high the nugget probably did not grow enough. If it is low the nugget has had the time to increase in size Shunting effect Shunting effect is the phenomenon that occurs when a spot is welded close to another spotweld, parallel resistances occur and the current takes a path through the first weld. Figure 4 shows how the current flows through the first weld when the second weld is welded. Normally spot-welds are welded in series so shunt effect usually occur. The result from this is that when the subsequent spot is welded and current takes a path through the first spot the welding current gets too low for the second spot. This needs to be considered in automotive since there is almost always a spot close by when welding a body, and shunting effects are considered as the normal condition. The first spot is therefore not considered when creating weld schedules and the current is adapted to match subsequent welds. [8] Figure 4 Electrical shunting between welds. Current is diverted due to the close spacing between welds. [9] Weld expulsion Weld expulsion is a problem that the automotive industry is generally striving to avoid in all spot welding processes. Expulsion leads to loss of material, and in worst case holes in the 11

20 sheet. It also gives marks on the metal sheet which can be unacceptable if they are visual and can be seen on the surface of the finished body. It can therefore be a quality problem. Expulsion happens due to too high energy and there are several reasons that it occur. One reason is that the weld becomes too large for the electrode which causes the material to eject out of the weld. Expulsion then happens when the weld-current and/or weld-time is too high. Another cause is when the electrode force is too small. When the material melts and expands it needs to be held back by the force from the electrodes/welding gun. Otherwise, the material will press the sheet apart allowing the material to eject and create a weld expulsion. On the other hand, if current or weld time is too small or the electrode force is too high, it will lead to a small nugget not being qualified according to standard. [9] Gaps between the sheets in production, dirt on the surface or too high current in the beginning are also factors that can lead to expulsion. 2.3 Equipment for resistance spot welding Machine type The variety of machine types is wide and the equipment is adapted to suit the application depending on efficiency, complex shape and high production rates. From the beginning of the RSW technology, AC was the most common electrical power source using 50 Hz, today that technology is in many cases not efficient and precise enough. Nowadays, the power source used in the automotive industry is normally medium frequency direct current machines (MFDC). By using MFDC, the welding current can have a frequency between Hz. Higher frequency enables more control over the process and better quality since more advanced algorithms can be used in the weld control as it enables fast feedback from the process. By getting feedback for the outcome of the parameters the process can be adjusted to get the right values. [10] Weld guns There are mainly two different types of weld guns, pneumatic and electrical servo guns. Pneumatic guns have been widely used in the industry. Now, many industries have changed to electrical servo guns that generally are more powerful and more precise. Weld guns can be operated manually or automatically. The latter option is used at Scania installed on robots. 12

21 Weld guns are normally either linearly moving or X-shaped gun chosen to suit the applications. [7] Electrodes Electrodes are used to lead the current through the metal sheet, cool the material after welding and to fixate and put pressure on the sheets during welding. [9] The electrode consist mainly of copper but can have a variety of alloying elements, such as chrome, to increase the hardness and be able to carry the force under high pressure. Copper has low resistivity and also a good electrical conductivity that causes the electrode to not melt since the material will melt where the resistance is the highest. Inside the electrode, a water coolant flows to cool the electrode and protect it from getting overheated, to avoid high resistance build up and excessive wear. [2] The electrode comes in different shapes and sizes to fit different applications. Figure 5 shows a picture of electrodes with different shapes. The geometry of the electrode changes during welding due to the pressure and heat. Deformation can lead to lower current density that causes loose spot welds. Electrode wear, possibly leading to weld expulsion on the surface, can occur more quickly when welding coated material since the resistance becomes higher on the surface from alloying and oxidation. By tip-dressing the electrode regularly, it is kept in good condition. Tip-dressing means that the outer surface is grinded away and the electrode becomes as new on the surface. When the electrode has been tip-dressed several times the electrode becomes shorter and needs to be replaced. [4] Figure 5 Different shapes of electrodes [18] In automotive the positioning of electrodes can be difficult. Many components that are welded together require tight tolerances when they are fixed against each other. Gaps between the sheets are hard to avoid due to the many components and it is one reason for weld expulsion in production. In automated production, the misalignment can also be caused by poor programming. 13

22 2.4 Control systems for resistance spot welding There are generally two different types of control systems, fixed and adaptive. A fixed system uses the parameters that are set in the program during the entire process. An adaptive system has the ability to adjust and change the parameters during the process to avoid expulsion and an incomplete weld. Adaptive control systems are now used in most automotive industries. Earlier, each sheet combination has required a unique optimization of parameters. Now with adaptive control systems, each program can handle a variety of sheet combinations and the amount of weld schedules can be decreased for the production plant CCR Constant current regulation is a fixed control system that works in such a way that the parameters which are set in the weld timer will be the parameters used in the process. CCR works after a fixed schedule. CCR uses a predetermined current to create a target for current used in the process. The limit is transformed to a target phase angle by the control system. When the target phase angle is set, the second current is measured and compared to the target current. The phase will adjust the current in consideration between the target current and the measured current. Therefore, the same values of time and current will be used in the process. [11] Master mode Master mode is an adaptive control system that can adapt some parameters during welding to create the best weld after variation in circumstances. This enables welding of different material combinations with the same schedule. To create a weld schedule with Master mode, first a reference weld is created using CCR. The optimised parameters are used to weld a reference weld and create good master curves. Master curves are the curves showing current, time and resistance. When it has been created the system is switched to Master mode and the current and voltage curve is saved as a function of time. Figure 6 shows an illustration of the procedure for creating a weld schedule with Master mode. 14

23 Figure 6 Procedure of creating weld schedule with Master mode. [14] During welding, the parameters are adjusted to reproduce the parameters of the reference weld. Master mode is expanded with a parameter called weld extent. Weld extent is set to a value in percent that the process is allowed to extend the time to be able to fulfil the values of the reference weld. When welding in master mode the process can adjust current and time. Current will be the prioritised parameter to change and time will change if correct energy levels are not fulfilled. Master mode can respond to changes in welding in a way that CCR cannot. [12] HCC Heat capacity control (HCC), is an adaptive control system that compares the outcome of actual values to the programmed parameters value of the input. It can regulate itself to match output with input. An interval of current values is set for each pulse to let the system know what to relate to and act within. The system uses several pulses and can adjust the amount of pulses depending on if the weld is complete. The time from previous pulse will decide how the next pulse will act and regulate current. Time is regulated to avoid expulsion and if it is noted to occur the machine shuts of the current. The program has the ability to adapt weld time and if shorter time than maximum time is used in one pulse, the current is reduced in next pulse. If time reaches maximum in one pulse, current will be increased in the next pulse. Master mode has been replaced by HCC at Scania and is used in all welding processes except 15

24 for a few. Scania is the only company using HCC in almost all processes, and having the permission to access the software and be able to program and change the parameters. HCC uses the guiding quality parameter called nugget index, NI, to end the process when the weld is considered complete. NI is calculated from the shape and amplitude of the weld curves. The values are automatically generated from the reference curves knowing when the nugget signifies a good size. If Nugget index ends outside the interval an error occurs and it is considered to be a weld failure. Nugget Index can be set for each pulse and normally the value is decreasing with every pulse. [9] Dead time is also a parameter that is set to neglect the time in the beginning of the pulse when the curves peak. 2.5 Standards and regulations for RSW The Scania standard STD4429 regulates the requirements on resistance spot welding in Scania s production. It states which requirements there are on design and construction of resistance spot welded structures exposed to dynamic or static load. Nugget diameter d n is the size of the diameter of a metallographic sample on an etched cross section, it is calculated by formula 3. Approved nugget diameter that has to be fulfilled for all spot welds is: d n 3.5 t (4) The robustness requirement has a target nugget size of dimension d n target 5 t (5) d w is the reference value of the weld diameter measured on the nugget. Due to uncertainty in measuring, a peeled nugget, 15 percent is added to the diameter d w = 1,15 D n target (6) Minimum size to achieve the robustness requirement is d n robust 0,9 5 t (7) 16

25 where t is the thickness of the thinnest sheet. In production, d n robust has to be fulfilled in 75% of all welds. For joints with sheets of different thicknesses, the thinner sheet is decisive for determining the nugget size for each joining plane. [13] Measuring the diameter of the peeled weld nugget is made in agreement with SS-EN ISO 14329:2004 Resistance welding Destructive tests of welds Failure types and geometric measurements for resistance spot, seam and projection welds. In Figure 7 the different plug failures are illustrated. Figure 7. The different plug failure modes that can occur. a) illustrates a symmetrical plug, b) asymmetrical plug and c) a partial plug failure.[24] a and b, symmetrical plug and asymmetric plug d w = d p = (d 1 + d 2 )/2 (8) c, partial plug d w = (d 1 + d 2 )/2 and (9) d p = (d 1 + d 3 )/2 (10) 17

26 SS-EN Continuously hot-dip coated steel flat products for cold forming Technical delivery conditions is the international standard which states the properties and chemical composition for continuously hot-dip coated steel flat products for cold forming. SS-EN ISO 10447:2015 Resistance welding Testing of welds Peel and chisel testing of resistance spot and projection welds regulates how the peel tests of the sheets should be done. SS-EN ISO 14271:2017 Resistance welding Vickers hardness testing (low-force and microhardness) of resistance spot, projection, and seam welds is the standard that regulates procedure of hardness measurement. According to Scania standard STD 4429 the maximum hardness in the weld nugget and the HAZ must not exceed 550 HV Non-destructive and destructive testing For cost reasons, it is important to keep track of the quality of the welds and make sure to find defects in an early stage to eliminate waste and correct the problem in the beginning. To be able to control the weld it must be tested by either non-destructive or destructive testing. For control in production, ultrasonic testing (UT) is used at Scania. UT is a non-destructive testing method that is able to detect defects under the surface. Ultrasonic sound waves with high frequencies are sent into the material and waves are reflected back to the transmitter if it detects a defect. [13] A display shows an image of the spot weld and measurements of the diameter. If porosity or other defects exist in the weld it will show on the screen. The negative side of UT is that the precision is important but still difficult to control while using it. The transmitter need to be placed right in the middle of the spot weld, which is difficult to estimate for the operator. Destructive testing that is conducted on the cab body is peel testing. In Figure 8, a peeling tool is shown. Complete cab tear down is performed regularly to control the nuggets. Peeling is performed by attaching a roller tool to the metal sheet and then tearing the welds apart when rolling it. Another destructive testing method that can be used to control the weld is chisel testing were the joint is pressed apart by a chisel. 18

27 Figure 8 Peel testing using a vise and a roller. [19] 19

28 3 MATERIAL In this chapter the materials and material combinations that are used in the experiments are presented. Also weldability of the materials and RSW on coated steels are described. Due to the importance of keeping the vehicle as light as possible, the construction of the trucks is made out of a variety of different material that will suit the applications best. Low weight is a request but it must also match the requirement of strength. Therefore, many different materials are combined and welded together. The combinations can contain materials ranging from mild steel to AHSS. In Figure 9, a graph shows different steels and illustrates the trade-off between strength and elongation. Some parts of the body need to show proof of strength, e.g. the A-pillar that has high strength Boron steel to support the frame and also enables low weight. Other parts need to be ductile due to the forming it goes through during manufacturing. On those parts mild steel or dual phase is used, if it also needs increased strength. [4] Figure 9. Relation between elongation and tensile strength for material groups. [20] Further down, tables of the materials and its properties considered in this thesis are presented. The first combination consists of two sheets of interstitial free, mild steel with the same thickness that is included in the firewall. The second combination consists of three sheets of different materials. One IF steel from the rear wall panel, also a DP steel from the outer sill member and the last one is a HSLA from the upper/inner side member. The different combinations represent two types of material combinations that exist in a Scania truck. Table 20

29 2 presents the material combinations that is used in the experiment in chapter 4 and its coating, which thickness it has and which component it is a part of. Table 2 Material combinations Nr Definition Designation Thickness Coating Part 1. HS-IF 260 HX260 YD 1.5 mm Z100MB Firewall HS-IF 260 HX260 YD 1.5 mm Z100MB Firewall 2. HS-IF 220 HX220YD 0.8 mm Z100MB Rear wall panel DP 600 HCT590X 1.2 mm Z100MB Sill member outer HSLA 340 HX340LAD 0.9 mm Z100MB Side member, upper inner The chemical composition for the steels are presented in Table 3. [14] Table 3 Chemical composition Definition C max. Si max. Mn max. P max. S max. Al total Nb max. Ti max. HS-IF HS-IF HSLA C max. Si max. Mn max. P max. S max. Al tot Cr + Mo Nb + Ti V max. B max. DP to Mechanical properties for the steels are presented in Table 4. [14] Table 4. Mechanical properties Definition Steel number Proof strength Tensile strength Elongation Plastic strain ratio Strain hardening exponent Rp0,2 R m A 80 % r 90 n 90 MPa MPa min. min. min. HS-IF to HSLA to to HS-IF to to DP to

30 High strength IF, interstitial free steel - HX220YD and HX260YD IF is a conventional steel with very high ductility. The high strain hardening keeps a good indentation in deep drawn parts and the interstitial free microstructure gives it high drawability. HS-IF is suitable for complex parts that require high mechanical strength such as skin parts and structural parts. [15] High strength low alloy, HSLA, mikro-alloyed steel - HX340LAD High strength low alloy steel is steel that is cold rolled, precipitation hardened and goes through grain-size treatment to receive a fine grained microstructure of ferrite. The steel has high strength and a low alloy content. Due to the high strength and the possibility to reduce weight in applications, this sheet metal is suitable for structural components such as cross members and chassis components. HSLA has good weldability. [16] Dual phase steel, DP - HCT590X Dual phase steel is an Ultra High Strength Steel (UHSS) and has a microstructure that consist of two phases, ferrite and martensite or bainitic particles. Ferrite makes the steel ductile and hard martensite make the steel keep a high strength. The structure is achieved by annealing the steel and keeping the temperature under a certain time during the austenitic and ferritic phase. It thereby ends with a microstructure of dual phases. Specific properties of dual phase steel is high fatigue strength and energy absorption. Drawability also increases as the steel is hardened during forming. DP steel is suitable for applications such as structural and reinforcement components. [15] Weldability of the material During welding it is necessary to keep track of the parameters and control the welds in case hard martensite is formed. The higher carbon content of the material, the higher is the risk of forming martensite. Martensite will make the nugget brittle and decrease ductility. If the cooling from the electrode is too fast it also leads to higher risk of creating hard martensite. Uncoated mild steel is the simplest steel sheet to spot weld. Advanced high strength steel is more difficult to weld due to the increased amount of alloying elements that requires a more thorough developed weld process. Higher forces and larger electrodes are needed to keep contact between the surfaces. Welding in boron steel requires not only higher forces, but also more time. [4] 22

31 RSW on Coated steel Steel for automotive industry is kept corrosion resistant by coating it with, most commonly, zinc. The coating contributes to more contamination and the risk of getting defects increase. It also leads to more electrode wear when the zinc gets stuck on the electrode. Hot-dip and electro galvanised material is used at Scania. Hot-dip steel is rolled through a zinc-bath and electro galvanised steel rolls through a zinc electrolyte bath connected to a current. Electro galvanizing has a more complicated manufacturing process that enables a thinner layer of coating. The process is more expensive and is used more in special applications that requires higher quality. Therefore, hot dip galvanizing is used in most cases. Galvannealing can be done after coating to decrease electrode wear. It is a subsequent heat treatment to create an iron-zinc layer instead of just zinc. It makes it easier to weld and gives less electrode wear. [4] 23

32 4 EXPERIMENTAL METHODS In this chapter, experimental methods for the three control systems CCR, Master mode and HCC are described. 4.1 Welding Set-up The welding Set-up that was used in the experiments was partly consisting of the hardware and the software. Furthermore, fixtures and test coupons had to be constructed to be able to execute the experiment Welding equipment and robot The equipment used was a robot and a resistance spot welding gun with MFDC (1000Hz). The inverter was a Matuschek Spatz+ M400 with a maximum short circuit control of 18 ka. The maximum electrode force was 3.5 kn and the weld control unit was a PC based Matuschek Spatz+ M400L. Control systems possible to use with the equipment are CCR, Master mode and HCC. The spot welding gun was of X-type from ABB. Figure 10 below presents an image and data of the equipment used in this experiment. Robot: IRB /2.70 (ID;100017, BS 010 R01) Machine type: Spot welding gun ID weld gun: Max electrode force: 3.5 [kn] Short circuit control: 18 [ka] Throat depth: [mm] 800 Current type: MFDC (1000 Hz) Weld control unit: PC-based Matuschek SPATZ Water cooling: [l/min] 6 Transformer: Roman TDC kva Inverter: Matuschek Spatz+ M400L (Type nr: M04PLM002; Serie nr: SP-M40L ) Electrodes: Cap B A2/2 Figure 10 Table of information and data for welding equipment 24

33 Software The outcome of each spot weld is visualized in QA analysis in Spatz studio. Useful values and curves that can be seen in QA analysis are time, current, voltage, resistance, NI and energy. In Figure 11 the graphic window for the parameter curves for one spot weld is shown. It illustrates how the values change dynamically over time in the weld process. Figure 12 illustrates the graphic window where weld time changes for different spot welds. Figure 11 Graph in Last event showing parameter curves during the welding process for one welded spot. Figure 12 Graph in QA analysis showing average parameters during the welding process for several welded spots Specimen manufacturing Flat coupons were cut in sizes 600x48 mm and 150x48 mm. The smaller coupons were used when weld schedules were created, and the larger coupons were used in the experiment when welding larger series to be more efficient by welding many spots in a series. Measurements are shown in Figure 13. Figure 13 Measurements for coupons used in experiments. Large coupon to the left and small coupon to the right. The fixtures were manufactured at Scania to keep the coupons in the right place during welding with the robot. One fixture for large coupons and one for small coupons, to be placed directly on to the weld gun, were created. Figure 14 shows pictures of the fixtures. 25

34 Figure 14 Fixture for large coupons (left) and small coupons (right), with coupons placed in the small fixture. Thereafter, a robot program was created to follow the path for the spots over the large fixture. The large fixture fit five large coupons side by side and each coupon fit 19 spot welds. The spot spacing between the welds were set to 25 mm due to requirements from the Scania standard if 1.5 mm thick materials are welded. It is important to have a sufficient overlap (14 mm in this case) between the upper and lower coupons, to enable tightening of the coupons in a vice when doing destructive test by peeling. The large fixture is always placed in the same position following the markings on the floor. The small fixture was placed directly on the electrode arm enabling fast welding of small coupons. Some unpeeled coupons were measured with UT-equipment and all peeled tests were measured with slide calipers. The electrodes were tip-dressed right before each test started Weld Schedule A weld schedule was created for each material combination and control system. Every trial ended with peeling the coupon and measuring the nugget diameter. The value was documented in a data sheet. Table 5 shows an overall layout of the different series included in the experiment, numbered based on which material combination it is and which control system is used. 26

35 Table 5 Series in welding experiment. CCR Master mode HCC Weld Sch. comb Weld Sch. comb On each small coupon, three spot welds were welded. To avoid misleading results from shunting effects, as mentioned in 2.2.3, the first spot weld was not included in the result. The first spot weld to consider in the test was therefore the second spot weld on the coupon. Starting parameters were based on the graph shown in Figure 15. The graph shows corresponding parameters for welding time and force for a certain thickness. It is a general graph that can be used as a guide to find parameters to start with when creating the weld schedule. To use the graph, the thickness of the material combination was measured. The graph was handed out to Scania from Matuschek. It is only used as a starting point and shorter times were strived for. Figure 15 General graph for selecting start parameters for welding time and force. 27

36 When creating the weld schedules, the parameters did not follow a structured DOE due to the limited amount of time. Instead each weld test was evaluated and from that the parameters were altered accordingly. The parameters were optimized until they met the requirement for 10 approved spot welds without expulsion. Target nugget size, d w, that is the aim for the weld schedule is calculated from formula (6). d w for this project was 7 mm for combination 1 and 5,1 mm for combination 2. Values are presented for both material combination 1 and 2 in Table 6. Min value were set to 90 % and calculated from formula (7) to see when the robustness requirement fell under approved limits, 75% must be over d n robust. Maximum value was set as an indication where the weld spots started to get too large and as a limit to use when creating weld schedule. Though it was not necessary to stop the trial if the size is greater than 110%. Table 6 Minimum, target and maximum diameter for material combination 1 and 2 Material thickness [mm] Min [mm] Target size [mm] Max [mm] CCR Squeeze time was set to 100 ms and hold time to 150 ms for all weld trials. From the schedule in Figure 15 the force was set to 3 kn and the weld time to 400 ms for both combinations. One pulse was used in the weld schedule. Master mode For Master mode, the same parameters as for CCR were tested to get a comparative result. In this control system it is possible to select weld extent. It was set to 100, which means the process can adapt with up to doubled time. One pulse was used in the weld schedule. HCC The force was set to 3 kn and intervals for current and time were chosen to let the process adapt within. HCC uses several pulses, with a minimum of three pulses. A maximum of 12 pulses were programmed in Spatz studio. 28

37 4.2 Welding experiments For combination 1, coupons were fixed, five in the bottom and five on top in the large fixture. For combination 2, HSLA 340 was placed in the bottom, DP600 in the middle and HS-IF220 on the top. The plates were offset by 14 mm on the long side, to accommodate peeling by hand. 19 spots in a row were welded, the first spot was as earlier mentioned neglected due to shunt effects. The robot welded the coupons (in total 19 spots x 5 coupons per set). Figure 16 shows one picture of how the fixture was placed in the robot cell and another picture of a set of coupons fixed in the fixture as it is being welded. Figure 16 To the left the fixture is placed in place for experiment and to the right the robot is welding a set of coupons. During welding the following was observed in QA Analysis and visually: Robustness, when the nugget diameter was smaller than minimum allowed diameter When expulsion occur Electrode wear Time taken to weld one spot Current Voltage Resistance end curves Energy levels 29

38 Observations were documented in a data sheet and thereafter the last coupon of the set was peeled and nuggets measured. If the nuggets were still approved, the test continued with one more set. The first control system to test was CCR, series 1 and 2. Thereafter series 3 and 4 with Master mode and at last series 5 and 6 with HCC. 4.3 Analysis The analysis was conducted after the trials to collect data and results from the experiment Ultrasonic testing Non-destructive testing A number of coupons were analysed with ultrasonic testing to evaluate nugget diameter. The UT is calibrated against the material to establish which thickness it has and thereafter the transducer is placed above the nugget. The transducer was rotated three or four times to find and record a relevant measurement of the nugget. Due to factors explained in section 2.6, it requires experience and training to perform UT measurements. From experience at Scania it is known that UT consistently shows smaller nuggets than what is seen in destructive tests, and measured values are seen as guiding and relative rather than absolute. For material combination 2, both sides of the nugget were inspected since the material thickness varies on the sides. Defects were noted and documented Metallographic investigation and hardness measurements Chosen samples of spot welds were prepared by cutting a cross-section through the middle of the nugget. It is important to make the cut in the middle, otherwise the diameter will show a smaller value than it is. After cutting the piece, it was mounted into a plastic cylinder, thereafter it was grinded and polished to diamond size 3 µm and etched with Nital 2 %. Thereafter the sample welds were examined in microscope, studying the measurements of the nugget diameter and surface indentation. A photography was taken of the nugget. The metallographic investigation was completed with hardness measurements using a Q-ness hardness tester with test load HV

39 4.3.3 Peeling destructive testing Coupons were fastened in a vice and peeled with a roller tool according to the Scania standard. Every fifth coupon was peeled, the mean weld diameter was measured and calculated according to formula (8), (9) or (10) depending on geometry of the peeled plug. 31

40 5 RESULTS In this chapter the results from the experiment are presented. The aim of this work is to evaluate the welding quality by investigating the parameters, properties and factors; robustness, current, expulsion, time, energy and resistance. 5.1 Material combination 1 Material combination 1 consist of HS-IF mm. A cross-section of a spot weld is shown in Figure 17 below. Figure 17 Cross-section of material combination Weld schedule Series 1. CCR Combination 1 For series 1, a weld schedule with one pulse was developed. The weld parameters and test results are presented in Table samples were welded and gave approved weld size without expulsion. Table 7 Parameters and results for weld schedule series 1. Weld parameters Current [ka] Force [kn] Squeeze time [ms] Weld time [ms] Hold time [ms] Energy [J] Weld size [mm] Series 2. Master mode Combination 1 Master modes parameters were set to the same as for CCR, series 1. Requirements were fulfilled when welding 10 subsequent approved weld sizes. Table 8 shows the chosen parameters and results for the weld schedule. 32

41 Table 8 Parameters and results for series 2. Weld parameters Current [ka] Force [kn] Squeeze time [ms] Weld time [ms] Hold time [ms] Weld extent [%] Energy [J] Weld size [mm] Series 3. HCC Combination 1 Creating weld schedules with HCC required many tests with different parameters. The current intervals were kept constant at ka. It is higher than for CCR and Master mode since the current is pulsed and it was not possible to select lower values of current because then the weld nuggets became too small. 12 pulses were set with different parameters, where the first pulse 1 is squeeze time. 10 subsequent spot welds were welded with approved size and no expulsion. It was noticed that the weld nuggets seemed to grow when welding test spots on long coupons. Table 9 presents the parameters and results for the weld schedule for series 3. Some of the parameters definitions have been replaced with X, Y, Z and U due to company secrecy. In appendix 7 definitions are shown. Table 9 Parameters and results for weld schedule series 3. Pulse Force Paus Time e [kn] time [ms] [ms] Weld parameters I max I min Dead Time [ka] [ka] [ms] Energy Weld X Y Z U [J] size [mm]

42 5.1.2 Experiment Series 1. CCR Combination 1 The robustness requirement was not fulfilled after 16 coupons. The test was therefore considered stopped at the last spot weld on the previous coupon. Figure 18, presents a graph of the measurements from the peeled coupons and the sizes are decreasing the more spot welds that are welded. The red line illustrates the minimum size 6.3 mm. The blue vertical line shows where the robustness requirement fell below 75 % of approved welds after spot spot welds Figure 18 Graph presenting the size decreasing the more spots welds that are welded. The process went under robustness requirement after 285 spot welds. The weld parameters set gave expulsion on the shunt spots on coupons 6, 11 and 12. Since the parameters time and current are fixed when using CCR, current and time did not change during the trial. In Figure 19, the graphic window illustrating the plotted values of time, current, voltage, resistance end and energy are presented. It represents figures from 475 spot welds. The increasing resistance had a direct impact on energy. During the experiment when the electrodes were more worn, resistance end became higher and went from 234 µohm to 256 µohm. This indicates that the nuggets had not grown enough. The energy levels decreased slightly from 4.8 kj to 4.4 kj. 34

43 Figure 19 Graphic window showing plotted values for series 1. The yellow line shows where the process went under the robustness requirement. Nugget index is measured for CCR since it is run in a program that also handles HCC. It is only measured and not making any impact on the process. The curve for NI is shown in Figure 20. The curve decreased during the experiment. Figure 20 Nugget index curve for series 1. Series 2. Master mode Combination 1 When welding with Master mode, it showed that the process was more robust than CCR and it fell under the robustness requirements after 380 spot welds. Figure 21 shows when the process fell below the limit for a robust process. Some coupons show results from only a few spot welds because the steel sheet broke when peeling and therefore the peeling had to be interrupted for that coupon. 35

44 380 spot 380 spot welds welds Figure 21 Robustness requirement fell under approved limit after 380 spot welds. During the experiment the control system compensated with regulating time and current. Weld time varied between 301 and 348 ms and current varied between 7.62 ka and 7.94 ka. In Figure 22 the graphic window presents the parameters in the process for each spot in the series. The figures in the graph represents 465 spot weld. Spots that stands out in the graphs are shunt welds that show peaks of time and drops in current. In the beginning, the current had a small decreasing trend and after 110 spots the trend was slightly increasing. In the figure, an arrow shows where the falling trend turns and start to increase. However, there was no Figure 22 Graphic window of QA Analysis showing plotted values of spot welds in series 2. The arrow shoes where the current starts to increase and the vertical line shows where the process went under the robustness requirement. 36

45 apparently increasing or decreasing trend of the parameters. From what could be seen from the graphs the most obvious relation was that when current decreased time increased, and vice versa. In the beginning of the experiment when the electrodes were tip-dressed, the welding parameters kept the same value as programmed in the master curve. But very quickly as the electrodes got worn, weld time increased and then stabilized. As the electrodes got further worn, at the end of the experiment, time regulation started to vary more. The energy varied with time since current was relatively stable. Master mode did not use the entire interval of weld extent to regulate time. Resistance end went from 231 µohm to 250 µohm. Nugget index is showed in Figure 23. It had a decreasing trend and as well as for CCR, nugget index does not have an impact on the process while running Master mode. Figure 23 Plotted values for nugget index for series 2. Series 3. HCC Combination 1 When welding with HCC, the weld diameter never came under the robustness requirement. The longer the experiment went, the spot welds grew larger instead of decreasing as CCR and Master mode did. The weld size reached a diameter of up to 8 mm. It was never under minimum diameter value and the lowest diameter measured was 6.5 mm. Figure 24 shows plotted values for peeled nuggets. Due to breakage of the sheet metal only a small number of spots could be measured on each coupon, because when the welds got larger size they were Figure 24 Robustness requirement never fell below approved limits. 37

46 nearly impossible to peel. Expulsion occurred on one weld on coupon 19 and three welds on coupon 25. After 65 coupons, welds had been welded without showing any decreasing trend of the weld size. Therefore, the experiment was stopped with the motivation that the spot welds never decreased in size as well as it didn t occur any more expulsion. HCC regulated current and weld time more than Master mode did. Since it has the possibility to add several pulses it went from using 4 pulses in the beginning to use 7 pulses at the end. HCC has a large spread in current and time between the spots, because it adapted more during the process. Weld time varied between 325 s to 627 s and current varied between 7.76 ka to 8.80 ka. Heat input has high values ranging from 5.6 kj up to 11.4 kj, due to the pulsed process. Energy levels varied as the weld time varied. As seen in Figure 25, the plotted values are more spread than the values for CCR and Master mode were. HCC has a slightly decreasing trend in the resistance end, which is the opposite from what CCR and Master mode had. The decreasing trend is a consequence of that weld time is longer. The algorithm calculates from the known values when to stop. Figure 25 Graphic window from QA Analysis showing plotted values for series 3. The NI curve, shown in Figure 26, only has a slightly decreasing trend which differentiates it from the other control systems. It is kept stable, since it is the guiding quality parameter that HCC uses. Figure 26 Plotted vales for nugget index for series 3. 38

47 Other observations that was noticed was that the electrode arm got very warm and the electrode tip changed in a colour that earlier tips welded in other control systems did not Analysis - Combination 1 In table 15 results and measures of the nugget diameter from microscopic photographs and measurements from the peeled nuggets adjacent to the analysed nugget is presented. Since the nugget can only go through either peeling or cross-section test a comparison were made from a nugget close by. Nugget 171 and 475 was not possible to measure due to failure when peeling. The results showed that the size varied depending on which method was used. The uncertainty between a cross-section and peeled nugget is added with 15 %.This assumption was not correct for all welds, some of the peeled nuggets were a lot larger than the crosssection. However, the mean value of the uncertainty for the spots in the table is 12 %. Since the sizes are not from the same nuggets the results are not fully comparative. UT and cross-section were performed on the same nugget. The result for UT turned out to vary a lot depending on operator. Also, the figures measured were much smaller than the real value, which is also a known condition. No specific trend could be seen and because of the uncertainty, the figures are not included in the result. Table 10 Nugget and peeled weld diameter for series 1, 2 and 3. Test 1 Test 2 Test 3 Spot nr Nugget diameter Peeled weld nugget Hardness measurement Due to limited availability, hardness measurement was not made on combination 1. Crosssections of the nuggets are shown in appendix 1, 2 and 3. 39

48 5.2 Material combination 2 It was difficult to find parameters that resulted in welds close to target on both sides of the nugget in the 3 sheet stack. To get weld diameters that were approved on both sides, it had to be accepted that the nugget on the side with HSLA 340 had a diameter larger than the original target of % of target diameter. Figure 27 shows the material combination. Figure 27 Cross-section of material combination Weld schedule Series 4. CCR Combination 2 For series 4, a weld schedule with one pulse was created. The weld parameters and test results are presented in table spot welds were welded with approved weld size and without expulsion. It was accepted that HSLA 340 became larger than target diameter because it was not possible to find parameters for both materials that were on or close to target. Table 11 Parameters and result for weld schedule series 4. Weld parameters Weld size Weld size Current [ka] Force [kn] Squeeze time [ms] Weld time [ms] Hold time [ms] Energy [J] HS-IF 220 HSLA 340 [mm] [mm] Series 5. Master mode Combination 2 The same parameters as for series 4 were chosen to get a comparative result. The parameters were tested on long coupons and they showed to give a lot of expulsion. Therefore, current was decreased to 6.25 ka and 10 test welds were run. It showed to give no expulsion and weld diameters within target size for both materials. Weld extent was set to 100 so the process in the experiment would have the ability to adapt with doubled time. In table 12, weld schedule for series 5 is presented, showing the parameters and result for heat input and weld size. 40

49 Table 12 Parameters and result for Master mode weld schedule series Weld parameters Weld Weld size Current [ka] Force [kn] Squeeze time Weld time Hold time [ms] Weld extent Energy [J] size HS-IF HSLA 340 [mm] [ms] [ms] 220 [mm] Series 6. HCC Combination 2 On this series, weld schedule optimization tests were run on long coupons to clearer see the result when changing some of the parameters. Selected weld parameters and results in energy and weld size for weld schedule series 6 are shown in table 13. Time intervals were set around the parameters chosen for CCR and Master mode. Some of the parameters definitions have been replaced with X, Y, Z and U due to company secrecy. In appendix 7 definitions are shown. Table 13 Parameters and results for weld schedule series 6. Weld parameters Weld size Pulse [mm] Force [kn] Pause time [ms] Time [ms] I max I min Dead Time [ka] [ka] [ms] X Y Z U Q [J] HS- IF HSLA [mm] [mm]

50 5.2.2 Experiment Series 4. CCR Combination 2 The robustness requirement was not fulfilled after 22 coupons and the test was therefore considered stopped at the last spot weld on previous coupon 21. As illustrated in the graph in Figure 28, the robustness requirement fell below 75 % of approved welds after spot spot welds Figure 28 Graph presenting the size decreasing the more spots welds that are welded. The process went under robustness requirement after 418 spot welds. The weld parameters set gave expulsion on the shunt welds on coupons 6, 11 and 12. Time and current levels were constant (CCR). As seen in Figure 29, energy levels increased a bit in the beginning as the voltage increased, and after about 80 welds it stabilises and then decreases. This is also where the weld size start to get smaller. The graph represents figures from 499 spot welds. Since the parameters time and current are fixed when using CCR, current and time did not change during the trial. Energy levels were relatively stable, but varied some due to changes in resistance that affected the voltage values. Resistance end increased and went from 244 µohm to 293 µohm. The energy levels decreased slightly from 4.2 kj to 3.9 kj. This series acted similar as series 1 did with material combination 1, CCR. 42

51 During the test a drop in the voltage- and energy curves occurred. The wrong material had by mistake been exchanged in the middle of the sheet stack with a 1.5 mm HS-IF 260 instead of 1.2 mm DP 600. During welding it showed no specific behaviour, the exchanged material was discovered from the voltage curve that changed because of a different resistance in the Figure 29 Graphic window of QA Analysis showing plotted values for series 4. The vertical line shows where the process went under robustness requirement. material. Nugget index is measured for CCR since it is run in a program that also handles HCC. The curve for NI is shown in Figure 30. The NI decreased during the experiment. Figure 30 Nugget index for series 4. Series 5. Master mode Combination 2 During the experiment the process kept the spot welds above robustness requirement until coupon 40 and the experiment was stopped on spot 760. No expulsion occurred during the trial. As seen in Figure 31 weld size varied a lot but had an overall decreasing trend. 43

52 760 spot welds Figure 31 Graph presenting the size decreasing the more spots welds that are welded. The process went under robustness requirement after 760 spot welds. Master mode varied both time and current during the experiment to compensate for the electrode wear. Time and energy were relatively stable while current and resistance changed opposite to each other. It had the same relation in series 2, Master mode combination 1. Master mode prioritises to adapt current which is a function in the program. In this case it can be seen that current is adapted and varies a lot, while time is kept stable in the beginning and starts to adapt the longer the experiment went. The plotted values from series 5 can be seen in Figure 32. The graph represents figures from 866 spot welds. Time was mostly stable at 351 ms and with some peaks of 370 ms. Current dipped in the beginning to 6.19 ka and then increased and stabilised in current values round 6.7 ka. Energy is relatively stable during the trial on levels of kj. 44

53 Figure 32 Graphic window from QA Analysis of plotted values for series 5. The vertical line shows where the process went under the robustness requirement. Nugget index is decreasing, it was only measured and not used in this trial. In Figure 33 the plotted values of NI areis shown. Figure 33 Nugget index curve for series 5. Series 6. HCC Combination 2 In the beginning of the trial, the spot welds were at its smallest and after 20 spot welds they grew larger. As for series 3, HCC combination 1, the welds never came below the robustness requirement. In Figure 34 the weld size is represented for the peeled coupons. No expulsion Figure 34 Robustness requirement never fell below approved limits. 45

54 occurred during the experiment. After 43 coupons the series went out of material and since the process didn t show any expulsion or trend of decreasing weld size, the experiment was stopped. Weld time increased gradually as the process added more pulses. From the beginning the process used 3 pulses and weld time were kept round 210 ms. For each added pulse, weld time increased 70 ms as programmed in the weld schedule and at the end of the trial weld time was kept round 450 ms and 7 pulses. Energy increased as time increased from 3.1 kj to 6.1 kj. Plotted values for the parameters can be seen in Figure 35 below. The graph represents figures from 816 spot welds welded in the series. Current varied between 6.8 ka and 7.3 ka. Resistance end had a decreasing trend and went gradually from 280 µohm to 245 µohm which made impact in voltage that also kept a decreasing trend. Figure 11 Robustness requirement never fell under allowed limit for series 6. Figure 35 Graphic window from QA Analysis of plotted values for series 6. Nugget index is calculated from the shape and amplitude of the weld curves. HCC uses NI as a parameter to control the other parameters and NI is more stable in this series than the other series with CCR and Master mode. It had a slightly decreasing trend as seen in Figure 36 but was kept relatively stable. Figure 36 Nugget index for series 6. 46

55 5.2.3 Analysis Combination 2 In table 14, results and measures of the nugget diameter from microscopic photographs and measurements from the peeled nuggets adjacent to the analysed nugget is presented. Since the nugget can only go through either peeling or cross-section test a comparison was made from a nugget close by. Material HSLA 340 does not have any available measurements because HS- IF 220 was the only one peeled and measured. This was a decision based on the trend that the nugget of HSLA 340 was consistently larger than HS-IF 220. Uncertainty between a cross-section and peeled nugget is added with 15 %. As seen in table 14, the difference in measures between the methods are not as large as for combination 1. The mean value of the uncertainty for the spots in the table is 6.7 %. Since the sizes are not from the same nuggets, the results are not fully comparable. The results from UT varied and were smaller than nugget diameter. Therefore, they are not included in the analysed results since they were not correct. Table 14 Measurements for nugget and weld diameter for series 4, 5 and 6. Test 4 Test 5 Spot nr Material Nugget diameter Peeled weld nugget n/a 5 n/a 4.4 n/a 5.1 n/a n/a Test 6 Spot nr Material Nugget diameter Peeled weld nugget n/a n/a Microscopic photographs of the nuggets are shown in appendix 5, 6 and 7. 47

56 Hardness measurement Hardness measurement were completed for all control systems for material combination 2. Series 4. CCR combination 2 For series 4, hardness measurement was made on spot 19 and 475 to compare difference in hardness in the beginning and the end of the series. In Figure 37, the results are plotted. The results showed that for series 4 hardness increased from 216 HV0.3 to 367 HV0.3. In appendix 4, macroscopic photographs are appended of the spot welds. It can there be seen that for spot 475 the weld has low penetration. Hardness indentation might have been measured on different places of the welds for spot 19 and 475. Overall the results between spot 19 and 475 were similar. spot 19 spot HS-IF 220 DP 600 HSLA 340 HS-IF 220 DP 600 HSLA 340 Nr of impressions: 41 Nr of impressions: 33 Figure 37 Hardness measurement for series 4 spot 19 (left) and spot 475(right). 48

57 Series 5. Master mode combination 2 HS-IF 220 and DP 600 had similar hardness measurements for spot 19 and 931. The results are plotted in Figure 38. HSLA 340 had a softer zone in the middle where it decreased from 376 HV0.3 on spot 19 to 265 HV0.3 On spot 931 for indentation 3. Microscopic photographs are appended in appendix 5. spot 19 spot HS-IF 220 DP 600 HSLA 340 HS-IF 220 DP 600 HSLA 340 Nr of impressions: 41 Nr of impressions: 41 Figure 38 Hardness measurement for series 5 spot 19 (left) and spot 931(right). 49

58 Series 6. HCC combination 2 For series 6, hardness measurements were completed for spot 20 and 800. The results showed that the hardness were similar for spot 20 and 80. Figure 39 illustrates the plotted values from the indentations. The only thing that can be seen changing is for spot 20 where a drop in hardness has occurred in the weld nugget. However, it is still within the approved limits stated by the Scania standard. Macroscopic photographs of some spots from series 6 are appended in appendix 6. HCC spot 20 HCC spot cv cv HSLA 340 DP 600 HS-IF 220 HSLA 340 DP 600 HS-IF 220 Nr of impressions: 41 Nr of impressions: 41 Figure 39 Hardness measurement for series 6 spot 20 (left) and spot 800(right). 5.3 Weld time at spot 300 The time measured for weld time at spot 300 for the series are presented in the table below. Table 15 Weld time at weld spot 300 for respective series. CCR Master mode HCC Weld Sch. comb ms 337 ms 466 ms Weld Sch. comb ms 350 ms 359 ms 50

59 6 DISCUSSION AND CONCLUSION In this chapter a discussion of the results from the experiments in the thesis are presented. In the conclusions, research questions that are presented in Chapter 1 are answered. Answers are based on the information from the results and discussion. 6.1 Discussion One of the aims with this master thesis was to investigate how the areas stated in purpose, chapter 1.2, are affected by using the three different control systems available at Scania. Another aim was to answer how some of the properties of the material and different combinations could be affected by the different control systems. Those areas are discussed in the following sections and the research questions are answered Robustness of the process The experiment showed that HCC did not fall below the robustness requirement and it could weld many more spot welds than CCR and Master mode. As seen in Figure 40, robustness increased for both material combinations with master mode and further more with HCC. Important to notice is that series 6, HCC combination 2, was stopped because of material shortage and if the trend is right it would be able to handle the robustness for a longer time. The graph also shows that robustness is affected by which material combination that is used. In this case, material combination 1 required higher current and it might be therefore the processes got lower results in robustness. The higher current made the electrode get worn quicker. 1 4 Amount of approved spot welds for each combination and control system combination 1 combination 2 CCR Master mode HCC Figure 40 Bar plot showing when material combination 1 and 2 fell under 51 robustness requirement for each control system

60 6.1.2 Expulsion Expulsion occurred for all control systems but Master mode seemed to have easier to expuld since the weld schedule for combination 1, series 2, could not be run on the same current as series 1, therefore both weld schedules had to be reprogrammed and lowered. Also series 5, Master mode combination 2, could not be programmed with the same parameters as CCR. When creating weld schedule for combination 1 in CCR, current were first set to 8 ka but when running the same parameters for Master mode the process gave expulsion on all spot welds. Therefore the current were set to 7.75 ka for both weld schedules. CCR only spattered on shunt spots, it could be a sign of that the parameters were a bit high programmed. For combination 2, Master mode had to have lower current values than CCR 6.25 instead of 6.5, to be able to handle the process without expulsion. For HCC it was more difficult to anticipate expulsion. It occurred during series 3 on 4 spot welds on coupon 19 and 25. Thereafter no more expulsion occurred for HCC Weld time - CCR held constant time during the experiment - Master mode adapted weld time a little bit but did not use its entire interval of weld extent. - HCC was the control system adapting weld time the most. But also being able to weld for a short time and get approved size. CCR held constant time during the experiment. The positive side with constant time is if the time is critical and very important to know. E.g. in the car industry it would be important to keep track of time but in truck manufacturing, cycle times are generally longer and therefore more time-consuming weld processes can be accepted. Still there is a trade-off between robustness and weld time. From the results, it is seen that HCC is more robust and can handle welding of a large number of welds without tip-dressing. If the interval between tip-dressing is increased from today s 150 to 300, it would save production time. The quality of the welds have shown to be maintained for the size and hardness at 300 spot welds with HCC. Weld extent was used when running Master mode but the process did not use the parameter to its full extent. It increased the weld time with 16 % for series 2 and 5 % for series 5. This 52

61 might be a result of the curves of the reference welds being too narrow and not optimised to allow the curves to act within its full capacity. HCC was the control system that adapted time most between short and long weld times. The weld schedule could have been further optimized but it took a lot of time to create weld schedules for HCC and due to that, the selected weld schedules were the best developed. In the beginning of the HCC series, the processes held weld times that were low and if the weld schedules had been further optimized the times would perhaps have been able to keep lower. Most increase in time was HCC series 3, where it at the end at spot weld 1200 had weld times of up to 675 ms Surface defects and electrode wear Neither of the control systems showed surface defects that were not approved by the standard. HCC showed larger electrode indentation that was less visual attractive, since the nugget grew, the longer the process went. It could be a problem if a nugget with that kind of indentation were seen on some visual surface of the cab. The electrode got warmer for HCC than for the other control systems since it used more energy for the spot welds. Coolant in the weld gun was not working well on the lower electrode-tip and since the time between spot welds were round 2.5 to 4 s it did not have time to properly cool the tip between the welds. This was a machine problem and not something that could be caused by HCC. This problem was assumed to not have a significant impact on the results Factors affecting the results A likely reason for the robustness of HCC is the use of a guiding quality parameter that controls the duration of the welding. NI is kept stable for HCC and adapted the other parameters such as time and current. It is seen in the results for Master mode and CCR that resistance end is increasing and for HCC it is decreasing. This can be an indicator of nugget size changes during the welding trials. When the electrodes got worn the area of the electrode tip increased and the current density between the electrode and the material decreased. The current is spread over a larger area and not enough current is used to melt material and get a 53

62 complete weld. This is the main reason that CCR nuggets decrease in size faster than adaptive systems. It was more difficult to create a weld schedule for the three sheet stack, combination 2. The sheets had different thicknesses with different target sizes and when creating weld schedule the 340 steel became larger than the 220 steel for each parameter set up. This was a difficulty for each control system. It was hard to know when the process was optimised and when it was time to select final parameters. The weld process acted differently in the beginning when the electrode-tips were tip-dressed. The process welded with very few pulses with newly dressed electrodes and with an increased number of pulses for the more worn they got. When creating the weld schedule the electrodes became a bit worn for every weld parameter that was tried and due to limited time it was not possible to tip-dress between every parameter optimisation. So, when the weld schedule had been selected and the experiment started with new-dressed electrodes the first welds were welded with only 3 pulses and got small weld sizes. This was a repeatedly behaviour and might be something that is important to consider when creating weld schedule for the production. After HCC series 3 had been run, the control system was switched to CCR. HCC had welded approved spot welds and a test was performed to see what would happen if the material now were welded with CCR, with the same worn electrodes from the HCC trial. The weld nuggets from the test were incomplete or non-existing Hardness No specific differences could be seen in hardness between the different control systems. The result that were found in the article [3] cannot be substantiated by this report. 54

63 6.2 Conclusion The research questions have been answered and the following conclusions can be made in this study: I. What are the advantages/disadvantages with using the adaptive control systems, HCC and Master mode in RSW compared to the fixed system CCR, for selected applications? Advantages with adaptive processes: o More robust process o The same weld schedule for HCC can be used for several material combinations. It is more flexible and time is saved on not having to create weld schedule for each material combination in the production. o HCC has high reliability creating the right nugget size. Disadvantages with adaptive processes: o More difficult and time consuming to program the adaptive control systems to find optimal parameters. o Master mode showed to be more inclined to expuld. o Less control of the weld time. II. Are there differences in weld properties due to the control system used? From what can be seen in these experiments, the material had hardness results that normally can be expected. The hardness curves had a normal appearance and behaviour. No further microstructural investigations were made. When creating weld schedule, the parameters selected for HCC should be tested and evaluated with both tip-dressed and worn electrodes. Since the process can act very different depending on how worn the electrodes were when creating the weld schedule. III. Does the result comply with the Scania standard? All processes managed to fulfil the robustness requirement within 150 spot welds (today s interval for tip-dressing). For all series except series 1 they were robust over 300 spot welds. Spot 300 HCC had an approved nugget size and robust process for both material combinations. 55

64 The hardness of the materials was kept within hardness requirement as stated in the Scania standard. 56

65 7 RECOMMENDATIONS AND FUTURE WORK It would probably be possible to increase the interval between tip-dressing in the production. Approximately to start with increasing the interval from 150 to 300 spots. Further investigations can be done for different materials. To see how the time increase caused by HCC can have an impact on different materials. Due to problems in delivery it was not possible to investigate boron steel as the plan was from the beginning. Investigate how weld schedules in production handle the processes. Further development and optimisation would probably give better weld times and less expulsion. Investigate the electrode and do further evaluation. It could be seen changes in electrodes colour when using HCC that was not seen with CCR and Master mode. Find out if it can lead to any problem or if it is unimportant. 57

66 8 REFERENCES [1]: Scania Oskarshamn ställer om produktionen inhyrda medarbetare berörs, Scania, 2 february 2018, [Online] Available: [Accessed: 19 april, 2018] [2]: Kimchi, M. and Phillips, D, H., (2018), Resistance Spot Welding: Fundamentals and Applications for the Automotive Industry, Ohio, Morgan & Claypool publisher, ISBN [3]: N. D. Raath, D. Norman, I. McGregor, R. Dashwood, and D. J. Hughes, Effect of Weld Schedule on the Residual Stress Distribution of Boron Steel Spot Welds, Metallurgical and Materials Transactions A, vol. 48A, s , june 2017 [online] Available: Springer US, [Accessed: 30 january, 2018] [4]: Svetskommisionen, motståndssvetsning, Kungälv: Goterna, 1987 [5]: R. Karlsson, Adhesive expert, Scania CV AB, Oskarshamn, interview, may 2018 [6]: G. Davies, Materials for Automobile Bodies, Oxford: Elsevier, [7]: K. Weman, Welding Processes Handbook, Stockholm: Liber AB, [8]: M. A. Omar, The Automotive Body Manufacturing Systems and Processes, Wiley: Chichester, West Sussex, [9]: O. Andersson, Process planning of resistance spot welding, pp. TRITA-IIP, [10]: C. V. Nielsen, W. Zhang, W. Perret and N. Bay, Three dimensional simulation of resistance spot welding, Proceedings of the Institution of Mechanical Engineers Part D Journal of Automobile Engineering, pp. 1-13, July [online] Available: ResearchGate, [Accessed: 5 march 2018] [11]: S. Danielsson and M. Allvar, Welding engineers, Scania CV AB, Oskarhamn, motståndspunktsvetsning, internal document, february [12]: L. Wei, D. Cerjanec, and G. A. Grzadzinski, A comparative study of single-phase AC and multiphase DC resistance spot welding.(author Abstract), Journal of Manufacturing Science and Engineering, vol. 127, no. 3, pp , [13]: M. El-Banna, D. Filav and R. Chinnam, Online qualitative nugget classification by using a linear vector quantization neural network for resistance spot welding, The International Journal of Advanced Manufacturing technology, vol. 36, no. 3, pp ,

67 [14]: U. Matuschek, K. Poell, Spot welding with adaptive process control, Matuschek Messtechnik GmbH, IIW Annual Assembly Meeting 2005 in Prague, Alsdorf, Germany, [15]: Scania STD 4429, Resistance spot welding - requirements, Scania, Södertälje, [16]: SS-EN 10346:2015, Continuously hot-dip coated steel flat products for cold, Swedish Standard Insitute, [17]: E. Tolf, J. Hedegård and M. Lundberg, Possibilities with Advanced Control of the Resistance Spot Welding Process, Corrosion and Metals Research Insitute, Stockholm, [18]: ArcelorMittal, High strength low alloy (HSLA) steels for cold forming, Automotive Worldwide, May 2018[Online] Available: [Accessed: 10 may, 2018]. [19]: SS-EN ISO 10447:2015 Resistance welding Testing of welds Peel and chisel testing of resistance spot and projection welds, Swedish Standard Institute, [20]: WSA, Spot Weld Cap Electrodes, Weld suppliers, [Online] Available: [Accessed:13 february, 2018]. [21]: Steel definitions, Steel Strength Ductility Diagram for Today s AHSS Grades (includes comparison of traditional low-strength and high-strength steels), WorldAutoSteel,[Online]. Available: [Accessed: 21 february, 2018]. [22]: M. Rhadakrishnan, Manufacturing technology, april 2017 [Online]. Available: [Accessed: 21 february 2018]. [23]: S. Danielsson, Welding engineer, Scania CV AB, Oskarshamn, interview, march [24] SS-EN ISO 14329:2004 Resistance welding Destructive tests of welds Failure types and geometric measurements for resistance spot, seam and projection welds, Swedish Standard Institute,

68 Appendix 1 Photographs of the spots from series 1 that were examined in microscope are appended in appendix 1. The photographs show the cross-sections of the two-sheet combination. The measurements show the diameter of the nugget and the depth of the electrode-indentation. Series 1, spot 19 Series 1, spot 171 Test 1, spot 361 Test 1, spot 475

69 Appendix 2 Photographs of the spots from series 2 that were examined in microscope are appended in appendix 2. The photographs show the cross-sections of the two-sheet combination. The measurements show the diameter of the nugget and the depth of the electrode-indentation. Series 2, spot 19 Series 2, spot 247 Series 2, spot 456

70 Appendix 3 Photographs of the spots from series 3 that were examined in microscope are appended in appendix 3. The photographs show the cross-sections of the two-sheet combination. The measurements show the diameter of the nugget and the depth of the electrode-indentation. Series 3, spot 21 (17), spot 135 (18) Series 3, spot 287 Series 3, spot 1009

71 Appendix 4 Photographs of the spots from series 4 that were examined in microscope are appended in appendix 4. The photographs show the cross-sections of the three-sheet combination. The measurements show the diameter of the nugget and the depth of the electrode-indentation. Series 4, spot 19 Series 4, spot 171 Series 4, spot 361 Series 4, spot 475

72 Appendix 5 Photographs of the spots from series 5 that were examined in microscope are appended in appendix 5. The photographs show the cross-sections of the three-sheet combination. The measurements show the diameter of the nugget and the depth of the electrode-indentation. Series 5, spot 19 Series 5, spot 475 Series 5, spot 931

73 Appendix 6 Photographs of the spots from series 6 that were examined in microscope are appended in appendix 6. The photographs show the cross-sections of the three-sheet combination. The measurements show the diameter of the nugget and the depth of the electrode-indentation. Series 6, spot 20 (21) Series 6, spot 135 (22) Series 6, spot 287 (23) Series 6, spot 800 (24)