IN
DEGREE PROJECT 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
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
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.
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
Nomenclature
Notations
Symbol Description Unit
Q Energy [J]
U Voltage [V]
I Current [kA]
R Resistance [µΩ]
𝑑𝑛 Nugget diameter [mm]
𝑑𝑤 Weld diameter [mm]
𝑡 Thickness [mm]
T Weld time [ms]
F Force [kN]
Abbreviations
RSW Resistance spot welding
CCR Constant Current regulation
HCC Heat capacity control
NI Nugget index
BIW Body in white
UT Ultrasonic testing
AHSS Advanced high strength steel UHSS Ultra high strength steel
DP Dual phase
HS-IF High strength interstitial free
HAZ Heat Affected Zone
CONTENTS
1 INTRODUCTION ... 1
1.1Background ... 2
1.2Purpose ... 3
1.3 Research questions ... 4
1.4 Research methodology ... 4
1.5 Delimitations ... 4
1.6 Disposition ... 5
2 FRAME OF REFERENCE ... 7
2.1 Joining in Automotive industry ... 7
2.2 Resistance spot welding ... 7
2.3 Equipment for resistance spot welding ... 12
2.4 Control systems for resistance spot welding ... 14
2.5 Standards and regulations for RSW ... 16
2.6 Non-destructive and destructive testing ... 18
3 MATERIAL ... 20
4 EXPERIMENTAL METHODS ... 24
4.1 Welding Set-up ... 24
4.2 Welding experiments ... 29
4.3 Analysis ... 30
5 RESULTS ... 32
5.1 Material combination 1 ... 32
5.2 Material combination 2 ... 40
5.3 Weld time at spot 300 ... 50
6 DISCUSSION AND CONCLUSION ... 51
6.1 Discussion ... 51
6.2 Conclusion ... 55
7 RECOMMENDATIONS AND FUTURE WORK ... 57 8 REFERENCES ... 58
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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.
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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
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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.
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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 B0-16-20-40-6-45 A2/2. Also, the investigated
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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
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,
Process properties Weld properties
Robustness Hardness
Time HAZ
Weld expulsion Electrode wear
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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.
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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
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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 = 𝑅𝐼2𝑡 (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.
2.2.1 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. 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
Time
Force
Current
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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.
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 2.2.4. 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.
Figure 2. Spot welding cycle [21]
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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]
2.2.2 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).
𝑅𝑡𝑜𝑡 = 𝑟1+ 𝑟2+ 𝑟3+ 𝑟4+ 𝑟5 (3)
where 𝑟1 and 𝑟5 is the contact resistance between electrode and metal sheet. 𝑟2 and 𝑟4 is the resistance in the workpiece and 𝑟3 is contact resistance between the metal sheets [6]. Figure 3
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
Figure 3 Principle of total resistance between electrodes.
𝑟4
𝑟5
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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.
2.2.3 Shunting effect
Shunting effect is the phenomenon that occurs when a spot is welded close to another spot- weld, 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]
2.2.4 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
Figure 4 Electrical shunting between welds. Current is diverted due to the close spacing between welds. [9]
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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
2.3.1 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 1000-2000 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]
2.3.2 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.
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Weld guns are normally either linearly moving or X-shaped gun chosen to suit the applications. [7]
2.3.3 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]
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.
Figure 5 Different shapes of electrodes [18]
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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.
2.4.1 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]
2.4.2 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.
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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]
2.4.3 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
Figure 6 Procedure of creating weld schedule with Master mode. [14]
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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 𝑑𝑛 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:
𝑑𝑛 ≥ 3.5√𝑡 (4)
The robustness requirement has a target nugget size of dimension
𝑑𝑛 𝑡𝑎𝑟𝑔𝑒𝑡 ≥ 5√𝑡 (5)
𝑑𝑤 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
𝑑𝑤 = 1,15 × 𝐷𝑛 𝑡𝑎𝑟𝑔𝑒𝑡 (6)
Minimum size to achieve the robustness requirement is
𝑑𝑛 𝑟𝑜𝑏𝑢𝑠𝑡 ≥ 0,9 × 5√𝑡 (7)
17
where 𝑡 is the thickness of the thinnest sheet. In production, 𝑑𝑛 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.
a and b, symmetrical plug and asymmetric plug
𝑑𝑤 = 𝑑𝑝 = (𝑑1+ 𝑑2)/2 (8) c, partial plug
𝑑𝑤 = (𝑑1+ 𝑑2)/2 𝑎𝑛𝑑 (9) 𝑑𝑝 = (𝑑1+ 𝑑3)/2 (10)
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]
18
SS-EN 10346 “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 HV0.2.
2.6 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.
19
Figure 8 Peel testing using a vise and a roller. [19]
20
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]
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
Figure 9. Relation between elongation and tensile strength for material groups. [20]
21
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.
Altotal Nb max.
Ti max.
HS-IF 220 0.01 0.30 0.90 0.080 0.025 ≥ 0.010 0.09 0.12 HS-IF 260 0.01 0.30 0.90 0.080 0.025 ≥ 0.010 0.09 0.12
HSLA 340 0.12 0.50 1.4 0.030 0.025 ≥ 0.015 0.10 0.15
Mechanical properties for the steels are presented in Table 4. [14]
Table 4. Mechanical properties
Definition Steel number
Proof strength
Rp0,2
MPa
Tensile strength
Rm MPa
Elongation
A80
% min.
Plastic strain
ratio r90
min.
Strain hardening
exponent 𝑛90 min.
HS-IF 260 1.0926 260-320 380 to 440 30 1.4 0.16
HSLA 340 1.0933 340 to 420 410 to 510 21 – –
HS-IF 220 1.0923 220 to 280 340 to 420 32 1.5 0.17
DP 600 1.0996 330 to 430 590 20 – 0.14
C max.
Si max.
Mn max.
P max.
S max.
Altot Cr + Mo
Nb + Ti
V max.
B max.
DP 600 0.15 0.75 2.50 0.040 0.015 0.015 to 1.5
1.40 0.15 0.20 0.005
22
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]
23 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]
24
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.
4.1.1 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.
Figure 10 Table of information and data for welding equipment
Robot: IRB 6700-300/2.70
(ID;100017, BS 010 R01) Machine type: Spot welding gun
ID weld gun: 91401244
Max electrode force:
[kN]
3.5 Short circuit control:
[kA]
18 Throat depth: [mm] 800
Current type: MFDC (1000 Hz)
Weld control unit: PC-based Matuschek SPATZ Water cooling: [l/min] 6
Transformer: Roman TDC-5080 91 kVA
Inverter: Matuschek Spatz+ M400L
(Type nr: M04PLM002; Serie nr: SP-M40L 240153)
Electrodes: Cap B0-16-20-40-6-45 A2/2
25 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.
4.1.2 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.
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.
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.
Figure 13 Measurements for coupons used in experiments. Large coupon to the left and small coupon to the right.
26
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.
4.1.3 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.
Figure 14 Fixture for large coupons (left) and small coupons (right), with coupons placed in the small fixture.
27
Table 5 Series in welding experiment.
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.
CCR Master mode HCC
Weld Sch. comb 1. 1 2 3
Weld Sch. comb 2. 4 5 6
Figure 15 General graph for selecting start parameters for welding time and force.
28
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, 𝑑𝑤, that is the aim for the weld schedule is calculated from formula (6).
𝑑𝑤 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 𝑑𝑛 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
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.
Material thickness [mm] Min [mm] Target size [mm] Max [mm]
1.5 6.3 7.0 7.7
0.8 4.6 5.1 5.7
0.9 4.9 5.5 6.0
29
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.
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
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.
30
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.
4.3.1 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.
4.3.2 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 HV0.3.
31 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.
32
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 260 1.5 mm. A cross-section of a spot weld is shown in Figure 17 below.
5.1.1 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 7. 10 samples were welded and gave approved weld size without expulsion.
Table 7 Parameters and results for weld schedule series 1.
Weld parameters
Energy [J] Weld size [mm]
Current [kA]
Force [kN]
Squeeze time [ms]
Weld time [ms]
Hold time [ms]
7.75 3 100 300 150 4745 7.3
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.
Figure 17 Cross-section of material combination 1.
33
Table 8 Parameters and results for series 2.
Series 3. HCC – Combination 1
Creating weld schedules with HCC required many tests with different parameters. The current intervals were kept constant at 8.25-10.00 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
Weld parameters
Energy [J]
Weld size [mm]
Force
[kN]
Paus e time [ms]
Time
[ms]
𝐼𝑚𝑎𝑥
[kA]
𝐼𝑚𝑖𝑛
[kA]
Dead Time [ms]
X Y Z U
1 3 100
25 5941 7.2
2 3 5 65 7.50
3 3 5 100 9.50 20 0.60
4 3 5 100 10.00 8.25 20 0.40
5 3 5 100 10.00 8.25 20 0.40 1.40 1.70
6 3 5 100 10.00 8.25 20 0.35 1.30 1.70
7 3 5 100 10.00 8.25 20 0.35 1.20 1.70
8 3 5 100 10.00 8.25 20 0.35 1.10 1.70
9 3 5 100 10.00 8.25 20 0.35 1.10 1.70
10 3 5 100 10.00 8.25 20 0.35 1.05 1.70 11 3 5 100 10.00 8.25 20 0.35 1.05 1.70
12 3 5 150
Weld parameters
Energy [J]
Weld size [mm]
Current [kA]
Force [kN]
Squeeze time [ms]
Weld time [ms]
Hold time [ms]
Weld extent
[%]
7.75 3 100 300 150 100 4718 7.3
