A comparative study between conventional fixed and ...
Transcript of A comparative study between conventional fixed and ...
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 TECHNOLOGYSCHOOL 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
1.1 Background ........................................................................................................................... 2
1.2 Purpose .................................................................................................................................. 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.
15
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]
16
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
34
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
285.
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.
Figure 18 Graph presenting the size decreasing the more spots welds that are welded. The process went
under robustness requirement after 285 spot welds.
285 spot
welds
35
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.
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.
Figure 19 Graphic window showing plotted values for series 1. The yellow line shows where the process went under
the robustness requirement.
Figure 20 Nugget index curve for series 1.
36
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
380 spot welds
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.
Figure 21 Robustness requirement fell under approved limit after 380 spot welds.
380 spot
welds
37
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.
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 23 Plotted values for nugget index for series 2.
Figure 24 Robustness requirement never fell below approved limits.
38
nearly impossible to peel. Expulsion occurred on one weld on coupon 19 and three welds on
coupon 25. After 65 coupons, 1 235 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.
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.
Figure 25 Graphic window from QA Analysis showing plotted values for series 3.
39
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.
5.1.3 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 cross-
section. 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.
Hardness measurement
Due to limited availability, hardness measurement was not made on combination 1. Cross-
sections of the nuggets are shown in appendix 1, 2 and 3.
Test 1 Test 2 Test 3
Spot nr 19 171 362 475 19 247 456 20 135 287 1009
Nugget
diameter
5.9 6 5.5 5.3 5.5 5.5 5.8 6.6 6.7 6.9 6.6
Peeled weld
nugget
7 - 5.8 - 7.1 7 6.1 7.3 7.2 7 7.3
40
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 90-110% of target diameter. Figure 27 shows the material combination.
5.2.1 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 11. 10 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
Energy
[J]
Weld size
HS-IF
220
[mm]
Weld size
HSLA 340
[mm]
Current
[kA]
Force
[kN]
Squeeze
time [ms]
Weld time
[ms]
Hold time
[ms]
6.5 3 100 350 150 4011 4.9 5.8
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.
Figure 27 Cross-section of material combination 2
41
Table 12 Parameters and result for Master mode weld schedule series
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
Energy
[J]
Weld
size
HS-IF
220
[mm]
Weld size
HSLA 340
[mm]
Current
[kA]
Force
[kN]
Squeeze
time
[ms]
Weld
time
[ms]
Hold
time [ms]
Weld
extent
6.25 3 100 350 150 100 4160 5.5 5.6
Pulse
Weld parameters
Q
[J]
Weld size
[mm]
Force
[kN]
Pause
time
[ms]
Time
[ms]
𝐼𝑚𝑎𝑥
[kA]
𝐼𝑚𝑖𝑛
[kA]
Dead
Time
[ms]
X
Y
Z
U
HS-
IF
220
[mm]
HSLA
340
[mm]
1 3 100
37
3963
5.2
6.2
2 3 5 0 5.00
3 3 5 70 7.00 20 1.20
4 3 5 70 8.00 6.00 20 0.5
5 3 5 70 8.00 6.00 20 0.35
6 3 5 70 8.00 6.00 20 0.35 1.10 1.50
7 3 5 70 8.00 6.00 20 0.35 1.05 1.50
8 3 5 70 8.00 6.00 20 0.35 1.05 1.50
9 3 5 70 8.00 6.00 20 0.35 1.05 1.50
10 3 5 70 8.00 6.00 20 0.35 1.05 1.50
11 3 5 70 8.00 6.00 20 0.35 1.05 1.50
12 3 5 150
42
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 418.
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.
418 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.
43
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
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.
Figure 29 Graphic window of QA Analysis showing plotted values for series 4. The vertical line shows where the
process went under robustness requirement.
44
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 4.0-4.3 kJ.
Figure 31 Graph presenting the size decreasing the more spots welds that are welded. The
process went under robustness requirement after 760 spot welds.
760 spot
welds
45
Nugget index is decreasing, it was only measured and not used in this trial. In Figure 33 the
plotted values of NI areis shown.
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 33 Nugget index curve for series 5.
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.
Figure 34 Robustness requirement never fell below approved limits.
46
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.
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.
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.
47
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.
Microscopic photographs of the nuggets are shown in appendix 5, 6 and 7.
Test 4 Test 5
Spot nr 19 171 361 475 19 475 931
Material 220 340 220 340 220 340 220 340 220 340 220 340 220 340
Nugget
diameter
4.7 4.9 4.9 5.3 4.6 4.8 4.9 4.7 4.6 4.9 4.8 4.9 4.3 5.1
Peeled weld
nugget
4.8 5.3 5.3 n/a 5 n/a 4.4 n/a 5.1 n/a 4.8 5.3 4 n/a
Test 6
Spot nr 20 135 287 800
Material 220 340 220 340 220 340 220 340
Nugget diameter 4.8 5 5.7 5.8 5.8 6 6 6.1
Peeled weld
nugget
4.6 6.7 n/a n/a 6.6 6.6 6.1 6.9
48
Nr of impressions: 33
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.
50
150
250
350
450
550
1 4 7 10 13 16 19 22 25 28 31 34 37 40
spot 19
HS-IF 220 DP 600 HSLA 340
50
150
250
350
450
550
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33
spot 475
HS-IF 220 DP 600 HSLA 340
Figure 37 Hardness measurement for series 4 spot 19 (left) and spot 475(right).
Nr of impressions: 41
49
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.
50
150
250
350
450
550
1 4 7 10 13 16 19 22 25 28 31 34 37 40
spot 19
HS-IF 220 DP 600 HSLA 340
50
150
250
350
450
550
1 4 7 10 13 16 19 22 25 28 31 34 37 40
spot 931
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).
50
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.
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 1. 300 ms 337 ms 466 ms
Weld Sch. comb 2. 350 ms 350 ms 359 ms
cv 50
150
250
350
450
550
1 4 7 10 13 16 19 22 25 28 31 34 37 40
HCC spot 20
HSLA 340 DP 600 HS-IF 220
cv
Figure 39 Hardness measurement for series 6 spot 20 (left) and spot 800(right).
Nr of impressions: 41
50
150
250
350
450
550
1 4 7 10 13 16 19 22 25 28 31 34 37 40
HCC spot 800
HSLA 340 DP 600 HS-IF 220
Nr of impressions: 41
51
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.
6.1.1 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
Figure 40 Bar plot showing when material combination 1 and 2 fell under
robustness requirement for each control system
0
200
400
600
800
1000
1200
combination 1 combination 2
Amount of approved spot welds for each combination and control system.
CCR Master mode HCC
52
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.
6.1.3 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
53
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.
6.1.4 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.
6.1.5 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
54
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
1 235 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.
6.1.6 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.
55
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.
56
The hardness of the materials was kept within hardness requirement as stated in the
Scania standard.
57
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.
58
8 REFERENCES
[1]: ”Scania Oskarshamn ställer om produktionen – inhyrda medarbetare berörs”, Scania, 2
february 2018, [Online] Available:
https://www.scania.com/productionunitoskarshamn/sv/home/media/nyheter/emma-en-
teknisk-lagspelare1811111112111.html [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
9781681731711
[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,”
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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
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
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
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
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
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)
TRITA ITM-EX 2018:541
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