syed safiuddin samad final thesis me cadcam

7/27/2019 Syed Safiuddin Samad Final Thesis Me Cadcam

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AN INVESTIGATION OF NUGGET FORMATION AND

SIMULATION IN RESISTANCE SPOT WELDING

A dissertation submitted in partial fu lf il lment of the requi rements for the award of the degree

of

MASTERS OF ENGINEERING (CAD/CAM)

BY

SYED SAFIUDDIN SAMAD1604-11-765-017

DEPARTMENT OF MECHANICAL ENGINEERING

MUFFAKHAM JAH COLLEGE OF ENGINEERING AND TECHNOLOGY

(Affiliated to Osmania University)

HYDERABAD-500034

2013


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CONTENT

CHAPTER 1: INTRODUCTION11.1 RESISTANCE SPOT WELDING (RSW) PROCESS 2

1.2 PRINCIPLE OF RESISTANCE SPOT WELDING 4

1.3 TERMINOLOGY 5

1.4 WELDING MACHINES 6

1.5 ADVANTAGES OF SPOT WELDING 6

1.6 OBJECTIVES 7

1.7 INVESTIGATION APPROACH 7

CHAPTER 2: LITERATURE REVIEW AND CRITICAL ISSUES 9

2.1 EFFECT OF ELECTRODE FORCE 14

CHAPTER 3: SPOT WELDING FACTORS AND BEST PARAMETERS 16

3.1 MATERIAL USED FOR THE STUDY 16

3.2 RANGE OF MATERIAL FOR SPOT WELDING 17

3.3 CHOICE OF THICKNESS 17

3.4 ELECTRODES FOR SPOT WELDING 18

3.4.1 ELECTRODES SETTING FOR SPOT WELDING OF SHEETS 183.5 SPOT WELD NUGGET DIAMETER 19

3.6 EXPULSION 21

CHAPTER 4: EXPERIMENTAL STUDY 23

4.1 SAMPLE PREPARATION AND THE TEST PLAN 1 25

4.2 SAMPLE PREPARATION AND THE TEST PLAN 2 26

4.3 SPOT WELDING MACHINE 26

4.3.1

ELECTRICAL SYSTEM 26

4.3.2 MECHANICAL SYSTEM 27

4.3.3 AIR OPERATED MACHINES 27

4.3.4 CONTROL SYSTEM 28

4.3.5 COOLING SYSTEM 28

4.3.6 A.C. RESISTANCE WELDING MACHINE 28

4.3.7 A.C. SPECIFICATION OF RESISTANCE WELDING MACHINE 29

4.4 SPOT WELD NUGGET DIMENSION CHECKING 31

4.5 SPOT WELDED SAMPLE OF TEST PLAN 1 33


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4.6 SPOT WELDED SAMPLE OF TEST PLAN 2 34

4.7 PREPARATION OF ETCHED SAMPLES 34

CHAPTER 5: INTRODUCTION TO SIMULATION OF RSW PROCESS 39

5.1

FINITE ELEMENT MODEL 40

5.1.1 HEAT TRANSFER ANALYSIS 40

5.1.2 ELECTRICAL FIELD ANALYSIS 41

5.2 MODELING AND PARAMETERS 42

5.2.1 MODEL AND MESH 42

5.2.2 MECHANICAL BOUNDARY CONDITIONS 45

5.2.2a INTERFACE ELEMENTS 45

5.3 WELDING PARAMETERS AND MATERIAL PROPERTIES 46

5.4 SIMULATION OF 2 LAP SHEETS OF 1 MM THICKENESS 50

5.5 SIMULATION OF 2 LAP SHEETS OF 1.5 MM THICKENESS 61

5.6 SIMULATION OF 3 LAP SHEETS OF 1 MM THICKENESS 74

5.7 SIMULATION OF 3 LAP SHEETS OF 1.5 MM THICKENESS 84

5.8 MEASURED VALUES OF THE NUGGET 94

CHAPTER 6: RESULTS AND DICUSSIONS

6 RESULTS AND DICUSSIONS 97

CHAPTER 7: CONCLUSIONS AND FUTURE SCOPE OF WORK

7.1 CONCLUSIONS 98

7.2 FUTURE SCOPE OF WORK 99

REFERENCES

100


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LIST OF TABLES

Table 3.1 : Chemical compositions (IS 513: 2008) 16

Table 3.2 : Mechanical Properties at Room Temperatures in as Delivered condition 17

Table 3.3 : Suggested electrode tip diameter corresponding to the plate thickness

for RSW 18

Table 4.1 : Experimental data for 2 sheets spot welding process with 1.0 mm thickness 24




Table 4.5 : Experimentally obtained values of nugget Height and Diameter for 2 lap

sheets spot weld of 1mm thickness under varying welding conditions 36

Table 4.6 : Experimentally obtained values of nugget Height and Diameter for 2 lap

sheets spot weld of 1.5 mm thickness under varying welding conditions 36

Table 4.7 : Experimentally obtained values of upper nugget Height and Diameter

for 3 Lap sheets spot weld of 1mm thickness under varying welding

conditions 37

Table 4.8 : Experimentally obtained values of lower nugget Height and Diameter

for 3 Lap sheets spot weld of 1mm thickness under varying welding

conditions 37

Table 4.9 : Experimentally obtained values of upper nugget Height and Diameter

for 3 Lap sheets spot weld of 1.5 mm thickness under varying welding

conditions 38

Table 4.10: Experimentally obtained values of lower nugget Height and

Diameter for 3 Lap sheets spot weld of 1.5 mm thickness under

varying welding conditions 38

Table 5.1 : Model Dimensions 42

Table 5.2 : Element types and degree of freedom options 44

Table 5.3 : Thermal and electrical properties of materials 47

Table 5.4 : Mechanical properties of materials 48

Table 5.5 : Employed spot welding conditions for 2 Lap sheets simulation of 1 mm

thickness 50

Table 5.6 : Employed spot welding conditions for 2 Lap sheets simulation of 1.5 mm

thickness 61


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thickness 74


thickness 84

Table 5.9 : Measured values of nugget diameter and height from the 1 mm thickness

of 2 lap sheets simulation 94

Table 5.10 : Measured values of nugget diameter and height from the 1.5 mm thickness

of 2 lap sheets simulation 94

Table 5.11 : Measured values of upper nugget diameter and height from the 1 mm

thickness of 3 lap sheets simulation 95

Table 5.12 : Measured values of lower nugget diameter and height from the 1 mm


Table 5.13 : Measured values of upper nugget diameter and height from the 1.5 mm


Table 5.14 : Measured values of lower nugget diameter and height from the 1.5 mm



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LIST OF FIGURES

Figure 1-1 : Principle of resistance spot welding 4

Figure 1-2 : Schematic of a typical spot weld sequence 5

Figure 2-1 : Schematic illustration of computational procedure used in FEA

Technique 12

Figure 2-2 : Electricalthermalmechanical contact model 13

Figure 3-1 : Electrode geometry to the thickness of the plate 19

Figure 3-2 : A & B shows the types of expulsion 21

Figure 4-1 : Two sheets spot weld coupons (specimens) 25

Figure 4-2 : Three sheets spot weld coupons (specimens)

26

Figure 4-3 : Overview of mechanical system of spot and projection welding

Machine

27

Figure 4-4 : Various parts of RSW machine

30

Figure 4-5 : Resistance spot welding machine 30

Figure 4-6 : The spot welding machine selected for the experiments 31

Figure 4-7 : Schematic view of nugget size diameter 31

Figure 4-7a: Different material zones around the spot weld 32

Figure 4-7b: Etched specimen of 1mm thickness

33

Figure 4-8 : Two sheets spot weld coupons (specimens) after welding 33

Figure 4-9 : Three sheets spot weld coupons (specimens) after welding 34

Figure 4-10: Nugget size for different direction (Above: Vertical,

Below: Horizontal) 35

Figure 5-1 : The FEA model of RSW process 43

Figure 5-1a: Boundary conditions for simulation 43

Figure 5-2 : FEM model for spot welding process with contact elements 44

Figure 5-3 : Contact elements 46

Figure 5-4 : 2D Axisymmetric Quarter model geometry of 2 and 3 lap sheets 48

Figure 5-5 : 2D Axisymmetric Quarter meshed model geometry of 2 lap sheets 49


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Figure 5-6 : 2D Axisymmetric Half model geometry of 2 lap sheets 49

Figure 5-7 : The predicted temperature distribution from the simulation for the

Sample 1 50

Figure 5-8 : The predicted nugget geometry from the simulation for the sample 1 51

Figure 5-9 : The predicted temperature ( ) distribution graph with respect totime in seconds (1cycle = 0.02 seconds) from the simulation for the

sample 1 51

Figure 5-10: The predicted temperature distribution from the simulation for the

Sample 2 52

Figure 5-11: The predicted nugget geometry from the simulation for the

sample 2 52

Figure 5-12: The predicted temperature ( ) distribution graph with respect totime in seconds (1cycle = 0.02 seconds) from the simulation for the

sample 2 53


sample 3 53


sample 3 54Figure 5-15: The predicted temperature ( ) distribution graph with respect to

time in seconds (1cycle = 0.02 seconds) from the simulation for the

sample 3 54


sample 4 55


sample 4 55


sample 4 56


sample 5 57


sample 5 57

Figure 5-21: The predicted temperature ( ) distribution graph with respect to


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time in seconds (1cycle = 0.02 seconds) from the simulation for the

sample 5 58


sample 6 59


sample 6 59


sample 6 60


sample 7 61


sample 7 62


sample 7 62


sample 8 63

Figure 5-29: The predicted nugget geometry from the simulation for thesample 8 63


sample 8 64


sample 9 64


sample 9 65


sample 9 65


sample 10

66



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sample 10 66


sample 10 67


sample 11

68


sample 11 68


sample 11 69


sample 12 70


sample 12 70


sample 12 71Figure 5-43: 2D Axisymmetric Quarter meshed model geometry of 3 lap sheets

with loading

71

Figure 5-44: 2D Axisymmetric Quarter meshed model geometry of 3 lap sheets 72

Figure 5-45: 2D Axisymmetric Half model geometry of 3 lap sheets 72

Figure 5-46: A practical model of resistance spot welding for three pieces of sheets 73


sample 13 74

Figure 5-48: The predicted upper & lower nugget geometry from the simulation

for the sample 13 75

Figure 5-49: The predicted temperature ( ) distribution in upper & lower weldgraph with respect to time in seconds (1cycle = 0.02 seconds) from the

simulation for the sample 13

75



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sample 14 76

Figure 5-51: The predicted upper & lower nugget geometry from the simulation for the

sample 14 76

Figure 5-52: The predicted temperature (

) distribution in upper & lower weld

graph with respect to time in seconds (1cycle = 0.02 seconds) from the

simulation for the sample 14

77


sample 15 77


sample 15 78


simulation for the sample 15 78


sample 16 79


sample 16 79

Figure 5-58: The predicted temperature ( ) distribution in upper & lower weldgraph with respect to time in seconds (1cycle = 0.02 seconds) from thesimulation for the sample 16 80


sample 17 80


sample 17 81

Figure 5-61: The predicted temperature (

) distribution in upper & lower weld




sample 18 82





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sample 19 84






sample 20 86






sample 21 87


for the sample 21 88Figure 5-73: The predicted temperature ( ) distribution in upper & lower weld




sample 22 89






sample 23 90



Figure 5-79: The predicted temperature ( ) distribution in upper & lower weld


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sample 24 92






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CHAPTER 1

1.

INTRODUCTION

Resistance spot welding (RSW) is widely utilized as a joining technique for

automobile structure due to flexibility, robustness and high-speed of process

combining with very high quality joints at very low cost. Resistance spot welding

(RSW) process is also used in sheet metal joining process, due to its high speed,

suitability for automation and inclusion in high-production assembly lines with other

fabricating operations. It is a complex process in which coupled interactions exist

between electrical, thermal, mechanical, metallurgical phenomena, and even surface

behaviors. In order to well understand the mechanism of such a complex process,

numerous researches have been performed on all kinds of welding conditions and

materials, using both theoretical and experimental methods. In recent years, numerical

method provides a powerful tool in studying these interactions and much related work

has been done on the numerical modeling of RSW In particular, the FEA which can

dea1 with nonlinear behaviors and complex boundary conditions has become the most

important method for the analysis of RSW process.

The aim of this project work is to develop a numerical model of the resistance spot

welding process enabling to predict accurately the weld geometry development,

thermal history distribution, deformations and residual stresses in the work piece.

Studies have been focused on the electrical/thermal/mechanical modeling. The

simulation takes into account the temperature dependency of the electrical, thermal,

and mechanical material databases. The established welding parameters and boundary

conditions corresponding to the practice, (e.g. welding current, welding force, and

welding time), are employed in the model. However, modeling of such coupled

process combined electrical, thermal, mechanical phenomena is a highly nonlinear

problem and requires the precise material properties ranging from room temperature

to fusion state.


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Furthermore, understanding of the electrical and thermal contact characteristics under

various loads and elevated temperatures is obviously important for a predictive RSW

model. The contact properties can be differed markedly regarding different sheet

surface coatings. In addition, it is widely agreed for the influence of contact resistance

as well as that of contact size variation on the nugget development in the literature.

Benefits of its process advantages including joining performance, flexibility,

robustness and hi-speed of process with very high quality joints at very low cost,

RSW has found to be a major joining technique utilized for automotive assembly

fabrication. However, due to a very short operation time being commonly less

thanone second, it is not therefore an easy task to assess or disclose entirely the

internal process characteristics by practical means when the variability of welding

parameters or that of sheet configurations is encountered. Numerical modeling can be

thereforeconsidered as another approach to obtain a better understanding in process

characteristics and consequently helps improving the joining quality as well as

process performance.

1.1 RESISTANCE SPOT WELDING (RSW) PROCESS

Resistance welding was invented a century ago, and has evolved into a simple,

straightforward manufacturing process that is fast, easily automated and easily

maintained. These characteristics make resistance welding a preferred process in mass

production manufacturing situations. Resistance spot welding is one of the most

common methods of the resistance welding processes. It is used widely in the

automotive, appliance, furniture and aircraft industries to join sheet materials. In

automotive applications, resistance spot welding is used to manufacture small

reinforcing bracket as well as complete outer panels. It is conservatively estimated

that several thousands of resistance spot welds are used in an average size vehicle.

Because of the extensive usage, even a small process improvement would bring

significant economic benefits. This large potential payoff has attracted a significant

amount of research in general resistance spot welding and in specific sub-field of

resistance spot welding control.


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The basic sequence of resistance spot welding is as follows. Water-cooled copper

electrodes are used to clamp the sheets to be welded into place. Then, the force

applied to the electrodes ensures intimate contact between all the parts in the weld

configuration. An electric current is passed across the electrodes through the work

pieces. Because of the imperfect contact condition, there is an extremely high contact

resistance at the faying surface of the work pieces. This resistance generates a

substantial amount of joule heat on the contact interface which melts the metal to

make a weld.

Several authors have reported the electrical parameters vary continuously during the

welding of mild steel sheet. At the end of the first cycle, there is an increase in voltage

across the copper electrodes and a reduction in current flowing through the weld zone

until a peak stage is reached. Throughout the remaining portion of the weld cycle, the

voltage decreases to a constant value as the current increases also to a constant value.

These changes in voltage and current have also been represented as dynamic

resistance.

The contact resistance, which is the electrical resistance at the joint faying surface, is

relatively high compared to the bulk material resistance of the joint material causing

fast heating at this contact interface. The combination of heat extraction by the chilled

electrodes and rapid contact surface heating causes the maximum temperature to

occur roughly around the faying surface. As the material near the faying surface heats

up, the bulk resistance rises rapidly, and the contact resistance falls. Again, the peak

resistance is at the faying surface, resulting in the highest temperatures. Eventually

melting occurs at the faying surface, and a molten nugget develops. On termination of

the welding current, the weld cools rapidly under the influence of the chilled

electrodes and causes the nugget to solidify, joining the two sheets. In general, the

control parameters of the resistance spot welding process are the electrode force,

electric current, weld time, and surface conditions.


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1.2 PRINCIPLE OF RESISTANCE SPOT WELDING

In Resistance Spot Welding, passage of a relatively high welding current I [kA]

throughout a locally compressed work piece area (by means of an electrode force F

[kN]) during a properly defined period of time t [cycle, ms] heats this area due to

resistive heating following Joules law (Figure 1-1),

() () ()

Q = the heat generated [J];

R = the ohmic resistance [ohm];

i = the welding current [A];

t = time [s];

T= total weld time [s];

Figure 1-1: Principle of resistance spot welding


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Figure 1-2: Schematic of a typical spot weld sequence

1.3 TERMINOLOGY

The automotive industry has established some common terminology that can be used

when describing the spot welding process. This includes references to the welding

apparatus, welding parameters, and welding specimens. To start, the apparatus used

for spot welding is often called a welding gun. It can be modified by the type of

actuator used. In this investigation pneumatic actuator is used which can be controlled

by air pressure and will be referred to as an air gun. The actuator controls the position

of, and force applied by the electrode tips. These are the parts of the welding

apparatus that actually make contact with the material to be welded. They are

typically made of copper, because of its high electrical and thermal conductivity.

When the electrodes contact the specimen there is slight deformation of the electrode

tip. This results in a larger contact areabetween the tips and specimens than would be

found by simply measuring theelectrode face diameter. This contact is referred to as

the electrode diameter and is typically assumed to be circular. The force with which

the electrodes are pressed together is called the electrode force. This force cause anarea on the contact surfaces of the specimens to be pressed very close together. This

area is defined as the contact diameter. The entire surface between two specimens,

where the weld will form, is called the faying surface.


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The metal first begins to melt at this surface within the contact diameter. The metal

that melts, and subsequently solidifies to bond the specimens, is known as the weld

nugget. The nugget has two critical dimensions. The first is the nugget diameter

which is represented by the width of the nugget at the faying surface. The second is

the penetration depth which is the overall height of the nugget at its center. The area

around the nugget that has had its microstructure changed due to the heat of welding

is known as the heat affected zone (HAZ). A current must be passed through the

electrodes to cause the heating and melting of the specimens necessary for nugget

formation. This current is specified by the operator and is called the welding current,

the duration that the current is applied to specimen is called the weld time.

1.4 WELDING MACHINES

There are various types of welding machines used in industry today. Their size and

shape depends on their intended application. Many machines in the automotive

industry are mounted on robotic manipulator arms to move about the body of a

vehicle during assembly. Welding machines can be built for use with either DC or AC

currents. The automotive industry predominantly uses AC machines. For this

investigation AC resistance spot welding was used. Detailed description of the

machine and its operation can be found in another chapter 4.

1.5 ADVANTAGES OF SPOT WELDING

Spot welding provides many benefits over other existing welding techniques. One of

its greatest benefits is that it does not require additional material to form a weld. The

nugget formed is a combination of the material from the specimens to be welded. In

most other welding processes, such as arc-welding and metal-inert gas(MIG) welding,

a wire or rod of material must be fed into the weld area to have enough material to

form a weld. Additionally, attaching sheets of metal together is faster with spot

welding because only certain areas to be welded to establish the necessary bond

strength.


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1.6 OBJECTIVES

The goal of this investigation was to develop a credible numerical modeling scheme

and analysis procedure, verified by the experimental data, to investigate the electrical,

thermal, and mechanical phenomena in resistance spot welding of low carbon steel.

Create finite element models for studying nugget geometry in a spot welded metal

sheet joints using the commercial ANSYS finite element code. Use the finite element

model to simulate the nugget size in a spot welded joint for currently used material

low carbon steel in the automotive industry. The results of this study will be used to

identify critical parameters to be used in future quality monitoring and control

routines for resistance spot weldingtimeThe other objective of this work is to

develop a multi-coupled method to analyse the thermal and mechanical behaviours of

RSW process, reduce the computing time with the minimum loss of accuracy and get

more adequate information of the process, Improve the quality monitoring and

process control of RSW

1.7 INVESTIGATION APPROACH

To achieve this objective, it was first necessary to obtain welding data that would

characterize the various welding conditions that might be encountered in an industrial

setting. This data consisted of both the physical weld specimens produced and the

current, voltage, and force data taken during the welding process. The data was first

examined in an attempt to find any characteristics that could be used as an indicator of

an impending splash. Since the data alone did not produce any such indicator, it was

used as input to a finite element program.

The results of this program were compared to the actual experimental results of the

welds to verify the programs accuracy. After the program was proven to be accurate,

the results were examined for feasibility of use in a control routine. It was expected

that if the results of a finite element simulation proved accurate, the time required to

produce these results would not be acceptable for a real-time control mechanism.

Therefore, it was intended that the results of the simulation be used as further

verification of future control routines that would be developed. The final focus of this


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investigation involves the verification of one of these welding models. The model

uses the heat balance equation of a welded part to predict the mean temperature of the

weld. It is believed that there is a characteristic mean temperature during welding,

beyond which splash will occur. The verification of this model is the final step of this

investigation.


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CHAPTER 2

2. LITERATURE REVIEW AND CRITICAL ISSUES

Finite Element Method (FEM) has been routinely applied in automobile industries for

the analysis of structural strength and component design because of the spread of

computers with high performance, low price and development of FEM software. It is

also playing a significant role in predicting the behavior of the weld cracking, weld

distortion and residual stress in a welding area as described by Ueda et al. (2007).

To assemble various components of automobile bodies, the resistance spot welding

process (briefly named as spot welding process in this paper) for both basic two

pieces of sheets and complicated multi-pieces of sheets is widely used because of its

low cost and high productivity. In order to determine spot welding conditions before

production, many experiments have to be performed. However, the experimental

approach to clarify the detail behaviors of the spot welding process is difficult

because the spot welding time is very short. The simulation approach has

advantagesto investigate and to visualize the detail phenomena induced in the nugget

formation process.

The basic numerical simulation for the spot welding process was started from the

1960s. Archer (1960) calculated temperature change with alternative current in spot

welding using a simple one-dimensional model. Greenwood (1961) calculated

temperature change in the spot welding using an axisymmetric model. Rice and Funk

(1967) considered the change of resistance with temperature in the spot welding using

a one dimensional model. Yamamoto and Okuda (1971) imported the measured

contact diameter in the spot welding to the simulation model and temperature

distributions are computed using an axisymmetric model.

The early simulations done in 1960s and 1970s were mainly limited to the

approximate prediction of the thermal history in the spot welding process using the

one dimensional models or the axisymmetric models based on the finite difference

method (FDM) and the nugget growth was not predicted.


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In the 1980s, the electrical contact resistance was considered in the simulation

models. Nied (1984) proposed a contact model between worksheets and analyzed the

thermal deformation of stainless sheets in the spot welding using FEM. This contact

model was based on a simple elastic deformation theory and was not accurate enough.

Nishiguchi and Matsuyama (1987) proposed an elasticplastic contact model and

investigated the conditions when the expulsion occurred in spot welding process. Han

et al. (1989) considered the change of electrical resistance with contact pressure in the

heat transfer analysis using an axisymmetric FDM model. Cho and Cho (1989)

developed an axisymmetric FDM model with the consideration of the change of

electrical resistance with hardness of steel. These models neglected the changing of

the contact diameter in the spot welding process.

From the early 1990s, the coupling of the electric filed, the thermal field and the

mechanical field for the spot welding of the two pieces of sheets were gradually

considered in the simulation models. Tsai et al. (1992) developed an user subroutines

for commercial software ANSYS to simulate the spot welding process and predicted

the displacement and voltage between two electrodes. Vogler and Sheppard (1993)

considered the electrical resistance with both temperature and contact pressure in the

FEM model, and predicted the weld lobe for two pieces of mild steel sheets. Huh and

Kang (1997) analyzed thermal field and electrical field using a three-dimensional

FEM model. Matsuyama (1997) considered the effect of surface unevenness on

contact resistance in the axisymmetric FDM model. Murakawa et al. (1995) analyzed

the effects of the spot welding conditions on nugget growth for two pieces of

aluminum sheets.

Murakawa and Zhang (1998) simulated the expulsion in the welding process and

discussed the influence of initial gap between two pieces of steel worksheets. A

research group of Li et al. (1997), Sun et al. (1997) and Dong et al. (1998) analyzed

the weldability of steel sheets with surface coating, aluminum sheets and predicted the

electrode displacement and face extrusion in the spot welding process. Xu and Khan

(1999) considered the change of contact resistance with pressure in the spot welding

for a steel sheet and an aluminum alloy sheet in the FEM model. The researches in the

1990s covered the contact models, the nugget growth and occurrence of expulsion in


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the spot welding for two-piece sheets of mild steels, aluminum alloys and stainless

steels.

From the 2000s, the simulation for the nugget growth and the expulsion in the spot

welding process became possible using commercial FEM codes. Khan et al. (2000)

developed an electro-thermal FEM solver and an interface program linking thermal

mechanical solver of ABAQUS. The nugget growth during the spot welding of

aluminum alloys was predicted considering phase change and convection in the weld

pool using an axisymmetric model. The simulated results showed that the effect of

phase change and convection in the welding pool on nugget development is not

significant. Disadvantage of this type of approach is that the computed results have to

be transferred between two solvers at each iteration step. Zhang (2003) developed a

FEM software SORPAS for the spot welding process simulation and a graphical

user interface for easier industrial applications. Feulvarch et al. (2006) presented a

model considering the electrical and thermal contact resistances using SYSWELD

which is a software dedicated to the simulation of welding and heat treatment.

Furthermore, the electrical and thermal contact resistances were measured by a

developed experimental device. Loulou et al. (2006) proposed an inverse methodusing

the measured temperature histories at some points in order to determine the detail

electrical and thermal contact parameters for the simulation of the spot welding

process.

Hou et al. (2007) used ANSYS and simulated the stresses in two worksheets and the

electrode displacement in the spot welding process. Eisazadeh etal. (2010)

investigated the nugget formation in spot welding for the two pieces of worksheets

and the effects of welding parameters on temperature of contact surface were studied

using ANSYS.


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Figure 2-1: Schematic illustration of computational procedure used in FEA

technique

Although above commercial FEM codes are convenient for industrial users, the

academic researchers may have some problems if a new contact interface and a new

material model for spot welding simulation need to be developed. For these reasons,

there are still many researchers using in-house FEM programs in their research works.

Murakawa et al. (2000) developed an in-house FEM software JWRIAN and

predicted the electrode displacement and clarified its correlation with the nugget

formation. Matsuyama (2001) imported both the measured current and the voltage to

the simulation model which was named as the Hybrid-Simulation. The Hybrid-

Simulation model could predict the nugget diameter with a short time. However, it

was not a full simulation model and phenomena could not be predicted without

experimental measurement.


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Figure 2-2: Electricalthermalmechanical contact model

De et al. (2003) developed an in-house software and estimated contact diameter,

nugget diameter and thickness for different welding conditions in the spot welding of

aluminum alloy.

However, the contact pressure in this model was assumed to be equal to the

temperature dependent flow stress and discontinuous contact points in the radial

direction were not allowed. Dilthey and Ohse (2007) developed a software package

SpotSIM with various database for the prediction of nugget dimensions and the

electrode indentation in spot welding. Nodehet al. (2008) predicted the residual

stresses due to spot welding by simulation which is important for the analysis of

structure strength.

Feulvarch et al. (2004) presented a general finite element formulation of electrical

thermal contact between two surfaces. This contact algorithm could be available to

the arbitrary mesh for 2D and 3D applications. Feulvarch et al. (2004) also gave a

simulation example of the spot welding process for the three pieces of sheets using

this contact algorithm and considering the fraction of the power density dissipated at

the two contact surfaces. However, the details of the nugget formation process for the

three pieces of sheets at the different welding conditions were not the main interests in

this article. The published simulation results mainly focused to the phenomena of the


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spot welding process for the basic two pieces of worksheets. There are many spot

welds for multi-pieces of worksheets in the industrial applications such as automobile

body assemble shown in Hondas (2008) web site. The spot welding for multi-pieces

of sheets is more difficult than two pieces of worksheets and it needs to be studied in

details. In this study, the nugget formation processes under various spot welding

conditions for the two and three pieces of low carbon steel sheets used in the real

automobile bodies were focused. The real shape, real size of electrodes and real

welding conditions were used in FEM model. A thermal-elasticplastic material

model for both worksheets and electrodes were employed in simulation shown in

fig.2. The simple and reliable contact elements between the multi-piece worksheets

and between a worksheet and an electrode with independent material properties were

used. The nugget formation process was investigated firstly by experiments.

Then, the simulations for electric current flow, heat generation and nugget formation

in the spot welding process were carried out the schematic illustration of

computational procedure used in FEA technique is shown Fig.1 By comparing the

simulation results with the experimental ones, the simulation accuracy was verified.

Lastly, the weld lobe for three pieces of low carbon steel sheets was estimated based

on simulation results.

2.1 EFFECT OF ELECTRODE FORCE

The electrode force in resistance spot welding functions to ensure electricalcontact

and to retain weld nuggets from expulsion. In the process, the forcereaches a preset

value during the squeeze stage, theoretically remains constantduring weld cycles,

holds for a short period after the current terminates, and isthen released. In reality,

however, the force varies during weld cycles primarilydue to thermal expansion of the

weld joint. It is also affected by the mechanicalcharacteristics of the welding machine,

the process parameters and the thermaland mechanical characteristics of the work

piece material. However, the electrode force is to be assumed constant during and

after the spot welding process.This research project consisted of both experimental

investigations and numerical simulation studies undertaken in order to develop a

comprehensive simulation of a weld nugget during the resistance spot welding

process. Welding tests were conducted to establish the required welding parameters


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for the numerical simulations, and verify the accuracy of the simulation results in

terms of the development of weld geometry and the deformations and stresses in the

work piece. The dominant parameters of this process for spot welding a sheet

configuration are welding current, force, time, and electrode.


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CHAPTER 3

3. SPOT WELDING FACTORS AND BEST PARAMETERS

3.1 MATERIAL USED FOR THE STUDY

If the material whether sheet or rolled sections, is clean, little difficulty shouldbe

experienced. Low-carbon steel can be satisfactorily resistance weldedusing a wide

range of time, current, and electrode force parameters. Themetal referred to as mild

steel is that in which the carbon content does notexceed 0.15%. Carbon content has

the greatest effect on weldability ofsteels; weld hardness increases rapidly with asmall rise in carbon content.To obtain acceptable weld performance, carbon content

should be keptbelow 0.10% + 0.3t, where t is the sheet thickness in inches. For

materialsabove this range, postweld tempering may be necessary.

The material used for this study was a cold rolled formable IS 513: 2008: CR3 Low

Carbon steel. The sheet metal was provided with skin passed deep drawing properties

and a general purpose surface finish. Typically this sheet metal contained up to 0.10%

of carbon. Other than carbon there are also some other particles present in the

chemical composition of the steel used for manufacturing this sheet metal. The details

of the chemical composition are provided in the in the table given below.Furthermore

according to the manufacturer data sheet this general purpose sheet metal is typically

used for unexposed drawn parts for automotive and appliance end applications. Hence

this sheet metal was chosen to conduct the present study.

Table 3.1: Chemical compositions (IS 513: 2008)

S.No. Designation

Maximum Percentage of alloying elements

Carbon Manganese Sulphur Phosphorus

1. CR3 0.10 0.45 0.03 0.025


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Table 3.2: Mechanical Properties at Room Temperatures in as Delivered

condition

S.No DesignationYield

Stress

Re(MPa)

Tensile

Strength

Rm(MPa)

Elongation

PercentHardness

LO=80mm LO=50mm HRB HR(30T)

1. CR3 220 Max. 350Max. 34 35 57 55

3.2 RANGE OF MATERIAL FOR SPOT WELDING

Spot welding facilitates if;

There is sufficient contact resistance sheet-to-sheet for heat to be generated bythe heavy current flow.

The heat is not conducted away too rapidly from the point at whichwelding is

desired.

Therefore, those good conductors of heat and electricity such as copper, aluminium or

silver present greater difficulties than do iron and steel, which are moderate

conductors in comparison.

3.3 CHOICE OF THICKNESS

The thickness of the sheet metal for making the coupons acts as an independent

variable in case of the determination of the coupon dimensions. The spot weld nugget

diameter depends on the thickness gauge of the sheet metal, which is presented in the

next paragraph. The thickness of the sheet metal chosen for this study was 1.5 mm

(averaged experimental value 1.49mm). The reason behind choosing this particular

thickness gauge was because most of the spot weld nugget diameter - to - thickness

expressions were derived and tested either for this particular thickness value, or this

value was near the median value for the range of thickness dimension used.


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3.4 ELECTRODES FOR SPOT WELDING

Resistance spot welding electrodes should be made of the materials

havinghighthermal andelectrical conductivities and sufficiently low contactresistance

to prevent burning of the workpiece surface or alloying at theelectrode face.

In addition, the electrode should have adequate strength toresist deformation at

operating pressures and temperatures. Electrodematerials for resistance spot welding

have been classified by RWMA and inInternational Standards Organization (ISO)

standard ISO 5182.

Using the proper electrodes for the spot welding application is necessary inorder to

achieve the best results in any spot welding operation. Selection ofthe alloy is

important since this can help modify the heat balance or reducethe tip wear. The tip

face diameter and contour must also be consideredsince these factors control the

welding pressure and current density whichmust be within an acceptable range for

satisfactory results. Incorrect tip facegeometry will also result in increased surface

marking.Although there are many alloys, types, sizes and shapes of

electrodescommercially available, there are six standard nose configurations and,

ofthese, there are three that are most frequently used for spot welding. Thereare: flat,

radiused and domed. Most of the welding schedules are based onthese three shapes.

Other sizes and shapes are often required to conform tothe contour of the weldment or

to suit other conditions. Each of theelectrodes are manufactured using a number of

different alloys to provide thebest combination of electrical and mechanical properties

for a particularwelding operation.

3.4.1 ELECTRODES SETTING FOR SPOT WELDING OF SHEETS

Table 3.3: Suggested electrode tip diameter corresponding to the plate thickness

for RSW

PLATE THICKNESS (mm) 0.5 0.8 1 1.5 2 2.5 3 3.5 4 4.5 5

REQUIRED ELECTRODE

DIAMETER (mm)

4 4.5 5 6 7 7.5 8.5 9.5 11 12 13


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Suggested electrode tip angle is 120 degrees. If the thickness of the two plates in

different, the electrode tip must have the diameter corresponding to the one required

by the thinner of the plates to which it contacts.

Figure 3-1: Electrode geometry to the thickness of the plate

3.5 SPOT WELD NUGGET DIAMETER

The most critical dimension to be determined for this study is the spot weld nugget

diameter since it plays the vital role in determining the mode of failure of the welded

joint. Several standards are set to determine the nugget dimension for a particular

sheet metal thickness. Several researchers have also proposed mathematical equations

for the calculation of a desired spot weld nugget diameter. Most of these standards

and calculations were based upon the lap shear coupon configuration. Hence, the

desired spot weld nugget diameter in this study is calculated for the lap shear coupon

configuration.

Ewing et al. (1982) and Chao (2003) have reported such standards concisely.American Welding society (AWS), American National Standards Institute (ANSI)

and Society of Automotive Engineers (SAE) jointly recommended the size of the spot

weld nugget diameter for steel according to the following equation.

D = 4 t (3.1)

Where Dand t are the nugget diameter and sheet thickness in mm respectively.

Apart from the above mentioned equation, the following two equations are widely


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used in the industry for the minimum nugget diameter and nominal nugget diameter

respectively.

D = ( ) (3.2)

D = ( ) (3.3)

whereD and t are in inch respectively. All these formulas provide a general idea

about the dimension but they cannot distinguish between the failure modes of the spot

weld nugget. VandenBossche (1977) first introduced such kind of formula to identify

the nugget diameter in conjunction with the material property and coupon width

value. The formula he proposed for transition weld diameter is given in the following

form.

=

(3.4)

WhereD, wandt are the nugget diameter, coupon width and sheet thickness in

mm respectively. SYPMis the yield stress of the base metal. Chao (2003) later proposed

a very simple form of equation to predict the critical nugget diameter for the failure

from the interfacial mode to nugget pull out mode.

The critical nugget diameter proposed by Chao (2003) is

D = 3.41t3

4

(3.5)

whereD and t are the nugget diameter and sheet thickness in mm.

+8.48

(3.6)

Wheret is the thickness of the sheet metal (in mm), Hmax and Hmin are the

maximum and minimum hardness value of the spot weld joint area.


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Possible causes for the undersized weld issues are:

Dirty material

Electrode misalignment

Insufficient cooling Weld current low

Weld force high

Weld time short

Wrong tips.

3.6 EXPULSION

Expulsion is the forceful ejection of molten metal from the weld. Severe expulsion

may eject enough material to create a through hole in the work piece, commonly

termed burn through. When the high current combines with inadequate electrode

force, improperly faced electrodes, or inadequate follow-up of the electrodes,

expulsion occurs due to overheating. Expulsion may occur at any interface, i.e., at the

electrode tip to work piece interface (Fig. A), or at any faying surface (Fig.B).

Fig. A Fig. BFigure 3-2: A & B shows the types of expulsion

Expulsion is caused by lack of containment of the expanding molten metal material

between the electrode tip faces, and excessive expulsion is undesirable. Expulsion

results in internal cavities and generally reduces the strength of the weld. This

tendency is so pronounced that the maximum current is normally limited to a value

where the expulsion will not occur.


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Expulsion is a very significant issue for the spot welded product quality as expulsion

at the weld interface may displace or may damage adhesives or sealers. Whiskers may

prevent the installation of seals, or may damage them during installation. Corrosion is

more likely to occur when burs/whiskers are present.


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CHAPTER 4

4. EXPERIMENTAL STUDY

The material used in this project work is low carbon steel sheet metal, with a

thickness of 1.0 mm and 1.5 mm for both the thicknesses the length of the coupon is

80 mm and width of the coupon is 40 mm. The experiments involved joining of two

and three sheets layer of sheet metal. In this experimental study the electrode size,

squeezing force and squeezing cycle were constant throughout the study. The

parameters vary only on welding current, welding time and sheet layers as shown in

the following tables 1 to 4.

Resistance spot welding conditions include three parameterswhich are force (axial

squeezing force), electric current and welding time. The welding conditions for the

two pieces of low carbon steel sheets with 1mm and 1.5 mm thickness of each work

piece are shown in Table 4.1 and Table 4.2. Three levels ofwelding current (3.0 kA,

3.5 kA, 4.0 kA) were selected in experiments.The welding time is indicated by current

cycles which arewidely used for both alternative and direct current. One cycle is0.02

seconds and alternative current was used in the experiments. The maximumcycles of

welding current were 75 cycles (time is 1.5 seconds) and 100 cycles time is 2.0

seconds). Theaxial force applied to electrodes was 2000 N and it was kept constant

for all the experiments of sheet metal thickness 1 mm and 1.5 mm for both 2 sheets

and for 3 sheets.The similar welding conditions were applied for 3 sheets spot

welding process as per the data shown in Table 3 and Table 4. The nugget diameter

d and thickness his measured in all the experimentswith different welding

conditions for both 2 sheets and 3 sheets.


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Table 4.1: Experimental data for 2 sheets spot welding process with 1.0 mm

thickness

S. No Sheet metal thickness

(mm)

Spot Welding Parameters

WeldingCurrent (kA) ElectrodeForce (kN) Welding time(Cycles)

1. 1 3.5 2 75

2. 1 4.0 2 75

3. 1 4.5 2 75

4. 1 3.5 2 100

5. 1 4.0 2 100

6. 1 4.5 2 100


thickness


(mm)


Welding

Current (kA)

Electrode

Force (kN)

Welding time

(Cycles)

1. 1.5 3.5 2 75

2. 1.5 4.0 2 75

3. 1.5 4.5 2 75

4. 1.5 3.5 2 100

5. 1.5 4.0 2 100

6. 1.5 4.5 2 100


thickness


(mm)


Welding

Current (kA)

Electrode

Force (kN)

Welding time

(Cycles)

1. 1.0 3.5 2 75

2. 1.0 4.0 2 75

3. 1.0 4.5 2 75

4. 1.0 3.5 2 100

5. 1.0 4.0 2 100

6. 1.0 4.5 2 100


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thickness


(mm)


WeldingCurrent (kA) ElectrodeForce (kN) Welding time(Cycles)

1. 1.5 3.5 2 75

2. 1.5 4.0 2 75

3. 1.5 4.5 2 75

4. 1.5 3.5 2 100

5. 1.5 4.0 2 100

6. 1.5 4.5 2 100

4.1 SAMPLE PREPARATION AND THE TEST PLAN 1

Figure 4-1: Two sheets spot weld coupons (specimens)


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4.2 SAMPLE PREPARATION AND THE TEST PLAN 2

Figure 4-2: Three sheets spot weld coupons (specimens)

4.3 SPOT WELDING MACHINE

In making a resistance spot weld, the machine used must deliver the correct amount of

current, localize it at the point where welding is desired and apply the proper pressure at

the correct time. The transformer and electrode system must also be cooled due to the

heat generated using high current in resistance welding. Therefore, a resistance welding

machine is basically composed of following systems:

Electrical system

Mechanical system

Control system

Cooling system

4.3.1 ELECTRICAL SYSTEM

The electrical system of resistance welding machine supplies electrical power to theweld in high current and low voltage, the major parts are the welding transformer and


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the secondary circuit including the electrodes which conduct the welding current to

the work. According to the electrical operation, the welding machines are classified as

AC, DC and CD machines, which utilize alternating current, direct current and current

from capacitor discharge, respectively.

4.3.2 MECHANICAL SYSTEM

The mechanical system of resistance welding machines is used to hold the workpiece

and apply the welding force. For all types of welding machines, the mechanical

system and secondary circuit designs are essentially the same, the major parts include

pneumatic or hydraulic mechanisms, machine frame and some associated accessories.

A general overview of each part is shown in Fig. 3. The following sections will

mainly focus on the characteristics of resistance welding machines with air-operated

mechanism and hydraulic mechanism.

4.3.3 AIR-OPERATED MACHINES

Air-operated machines are the most popular type. It is mainly used for small size of

machines.

Figure 4-3: Overview of mechanical system of spot and projection welding

machine


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A - air or hydraulic cylinder BramC - spot welding attachmentD - upper platen

E - lower platen F - knee G - flexible conductor H - transformer secondary J - knee

support

The air machines can operate very rapidly and are easily set up for welding, providing

much faster electrode follow-up because of the compressibility of air. The fast

electrode follow-up is particularly important when spot and projection welding

relatively thin sections. In addition, air-operated machines are low in noise during the

operation.

4.3.4 CONTROL SYSTEM

The objectives of the control system of welding machines are basically to:

Provide signals to control machine actions, making the machine working

automatically following the sequence of welding steps.

Start and stop the flow current to the welding transformer.

Control the magnitude of the current.

Furthermore, in recent years, the new-developed control systems even have the

functions of on-line monitoring the welding quality. In servo-driven spot weld gun,

the force is also controlled using the feedback control system.

4.3.5 COOLING SYSTEM

Most resistance welding machines use water cooling, the elements needed cooling

include: SCR, secondary coil of welding transformer, welding circuit and electrodes.

A closed-loop cycling system with distillation water is commonly used as the cooling

system for resistance welding equipments.

4.3.6 A.C. RESISTANCE WELDING MACHINE

Most resistance welding machines are single-phase AC machines. This is the type ofmachine most commonly used, because it is the simplest and least expensive in initial


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costs, installation and maintenance. The electrical circuit is shown in Fig. 4. and Fig.5.

The power from the single phase of main power is applied to the primary side of the

welding transformer through a switch (anti-phase dual silicon-controlled rectifiers),

converted by the transformer and output high current (low voltage) on the secondary side.

In AC resistance welding, the welding current flows with positive and negative half

cycles, there is zero heat or current between these two half cycles. This is called cycling,

which can cause some undesirable effects in welding smaller and thinner parts, where the

weld time is typically under 3 cycles, because the weld may cools effectively between the

half cycles, this will result in loss of the heat required to make a good weld.

Another negative effect of cycling is that when the heat is not applied constantly

throughout the duration of the weld, the nugget growth can be irregular. Variations in the

weld nugget are directly related to the quality and strength of a weld. Other AC

disadvantages are unbalanced line loading and lower power factors due to the inherent

inductive reactance in the machine.

4.3.7 A.C. SPECIFICATION OF RESISTANCE WELDING MACHINE

For spot welding process a spot welding machine with rated configuration of rated

capacity 15KVA, the maximum short circuit secondary - 18 kAmps, with a supply

voltage of 415V and the rated frequency 50/60 Hz, control panel for the welding current,

welding time and squeeze time controller was used. The maximum electrode force is

3000 N (pneumatic loading). The welding electrodes were made of copper alloy with a

conical shaped tip surface geometry. All the welding parameters were set to obtain a

reasonably good spot weld nugget. It should be noted here that the weld lobe was not

constructed in this study by varying the welding current and welding time during the spot

welding process.


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Figure 4-4: Various parts of RSW machine

The variation was performed to obtain the maximum possible weld nugget diameter that

the spot welding machine can produce. After performing the spot welding operation, the

obtained spot weld nugget diameter was checked. This procedure is described in the next

section.

Figure 4-5: Resistance spot welding machine


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Figure 4-6: The spot welding machine selected for the experiments

4.4 SPOT WELD NUGGET DIMENSION CHECKING

The coupons were checked after spot welding them together to ensure the desirable

nugget diameter was attained. Welding quality is primarily depends on the nugget

size. There are two critical nugget size parameters. The first is the nugget diameter

which is represented by the width of the nugget at the faying surface. The second is

the penetration depth which is the overall height of the nugget at its center. The figure

below is the schematic view of the welding nugget diameter.

Figure 4-7: Schematic view of nugget size diameter


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During the welding process a large amount of heat is applied on the material, it

undergoes severe changes in its micro structural format. So the hardness profile

changes along the radial axis of the spot welded (nearly) circular nugget changes

according to the applied heat. The three separate zones namely the spot welded

nugget, the heat affected zones and the base metal are clearly identified in the

following figure. These three zones have different levels of hardness values. So

investigation of the hardness profile will definitely reveal the actual dimension of the

spot weld nugget.

Figure 4-7a: Different material zones around the spot weld


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Figure 4-7b: Etched specimen of 1mm thickness

4.5 SPOT WELDED SAMPLE OF TEST PLAN 1

Figure 4-8: Two sheets spot weld coupons (specimens) after welding

Height of the nugget

Nugget Diameter


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4.6 SPOT WELDED SAMPLE OF TEST PLAN 2

Figure 4-9: Three sheets spot weld coupons (specimens) after welding

To investigate the effect of welding time and current on nuggetsize, two series of test-

specimens of the same material werewelded, fixing one parameter and varying the

other. In order tochoose the best parameters for spot welding joint.

After welding, the samples were prepared for metallographicexamination using

standard metallography procedures. In thisstudy, Nital,which is the

recommendedEtchant for low carbon steels, was used formacrography and

investigation of thenugget size.

4.7 PREPARATION OF ETCHED SAMPLES

One method to determine the nugget diameter of a spot weld is to cut the welded

material through the centre of the weld; the piece is then mounted (in epoxy) and

polished. The polish specimen can then be etched to reveal the microstructure of the

weld. This etching will show the boundary of nugget formed during welding as well

as the boundary of the heat affected zone (HAZ).


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The samples to be etched must first be trimmed down to a size by using either

shearing or hacksaw cutting. Since some material will also be removed from the cut

surface during the polishing process. After the specimen has been cut, it is cleaned

thoroughly with soap and warm water and dried. This removes the oil used in the

cutting process from the part. It also removes the contaminants that can become

lodged between the non-welded sections of the work piece during the cutting process.

Polishing is done on the samples and the faces are etched for metallography

examination to measure weld nugget size and micro structure examination of the

nugget and HAZ. The experimental results of nugget size are shown in the following

tables 4.6 to 4.11:

Figure 4-10: Nugget size for different direction (Above:Vertical, Below: Horizontal)

The microscope was used to observe the nugget size. The sample preparation has

several steps.

1) Sectioning: limited the sample size to be examined by microscope.

2) Cold mounting: the sample is embedded in epoxy type of materials.

3) Grinding: removes the damages on the surface produced by sectioning,

grindingmaterials: abrasive paper


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4) Polishing: to produce a flat and scratch-free with high reflectivity

5) Etching: using chemical to dissolve selectively the surface of materials inorder

toreveal the inhomogeneous nature in microscopic scale.

Table 4.5: Experimentally obtained values of nugget Height and Diameter for

2 lap sheets spot weld of 1mm thickness under varying welding

conditions

Table 4.6: Experimentally obtained values of nugget Height and Diameter for

2 lap sheets spot weld of 1.5 mm thickness under varying welding

conditions

Plates -2 Cycles

(1c=20ms)

Current

(KA)

Diameter

(mm)

Height

(mm)Thickness

1mm

75

3.5 3.8 0.85

4.0 3.8 0.84

4.5 4.0 0.85

100

3.5 4.1 0.87

4.0 4.4 0.96

4.5 4.8 0.87

Plates-2 Cycles

(1c=20ms)

Current

(KA)

Diameter

(mm)

Height

(mm)Thickness

1.5 mm

75

3.5 3.80 0.85

4.0 3.88 0.89

4.5 4.24 1.06

100

3.5 4.36 1.10

4.0 4.48 1.18

4.5 4.52 1.22


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Table 4.7: Experimentally obtained values of upper nugget Height and Diameter

for 3 lap sheets spot weld of 1mm thickness under varying welding

conditions

Table 4.8: Experimentally obtained values of lower nugget Height and Diameter

for 3 lap sheets spot weld of 1mm thickness under varying welding

conditions

Plates-3 Cycles

(1c=20ms)

Current

(KA)

Diameter

(mm)

Height

(mm)Thickness

1 mm

( lower

eld)

75

3.5 3.43 0.75

4.0 3.62 0.75

4.5 3.70 0.77

100

3.5 4.00 0.86

4.0 4.04 0.88

4.5 4.20 0.90

Plates -3 Cycles

(1c=20ms)

Current

(KA)

Diameter

(mm)

Height

(mm)Thickness

1mm

(Upper

weld)

75

3.5 3.70 0.88

4.0 3.80 0.89

4.5 4.20 0.92

100

3.5 4.22 0.92

4.0 4.24 1.20

4.5 4.51 1.25


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Table 4.9: Experimentally obtained values of upper nugget Height and Diameter

for 3 lap sheets spot weld of 1.5 mm thickness under varying welding

conditions

Table 4.10: Experimentally obtained values of lower nugget Height and

Diameter for 3 lap sheets spot weld of 1.5 mm thickness under

varying welding conditions

Plates -3 Cycles

(1c=20ms)

Current

(KA)

Diameter

(mm

Height

(mm)Thickness

1.5 mm

(upper

eld)

75

3.5 3.40 0.82

4.0 3.43 0.82

4.5 3.80 0.87

100

3.5 3.92 0.96

4.0 3.96 0.95

4.5 4.02 0.96

Plates-3 Cycles

(1c=20ms)

Current

(KA)

Diameter

(mm)

Height

(mm)Thickness

1.5 mm

(Lower

weld )

75

3.5 3.30 0.78

4.0 3.35 0.80

4.5 3.53 0.83

100

3.5 3.58 0.86

4.0 3.70 0.90

4.5 3.88 0.89


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CHAPTER 5

5. INTRODUCTION TO SIMULATION OF RSW PROCESS

The RSW process has been widely employed in sheet metal fabrication owing to its

high speedsuitability for automation and inclusion in high production assembly

lines with other fabricating operationsIt is a complex process in which coupled

interactions exist between electrical, thermal mechanical, metallurgical phenomena,

and even surface behaviours. In order to well understand the mechanism of such a

complex processnumerous researches have been performed on all kinds of welding

conditions and materials, using both theoretical and experimental methodsIn recent

yearsnumerical method provides a powerful tool in studying these interactions, and

much related work has been done on the numerical modelling of RSW. In particular,

the FEA, which can dea1 with nonlinear behaviours and complex boundary

conditions, has become the most important method for the analysis of RSW process

Nied developed the first FEA model for RSW process, investigated the effect of the

geometry of electrode on work piece and predicted the deformation and stresses as a

function of temperatureHowever, the model developed was restricted to elastic

deformation, and not calculated the contact areas at the electrode work piece and

faying surfaceFurthermoremany researchers developed more sophisticated FEA

models that considered temperature dependent material properties, contact status,

phase changing, and coupled field effects on the simulation of RSW. To solve a

common coupled problem, the iterative solution procedure is an often adopted

method, in which the equations describing different domains are solved separately

using dedicated solvers, and the data exchanged at every time step until convergence

of iteration is reached.

This information is transferred forward to the next time step and the numerical

scheme of study repeated. This iterative method can also be employed to simulate the

interactions between coupled electrical, thermal, and structural fields of the RSW

process. Initially the stress field and contact status are obtained from the thermal-


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mechanical analysis, and then the temperature field is obtained from the fully coupled

thermal-electrical analysis based on the contact area at the electrode work piece

interface and faying surfaceThe calculated temperature field is then passed back to

the thermal-structural analysis to update the stress field and contact status

Even if the iterative method can provide the temperature field, the electric potential

field, the stress and strain distributions in one calculation, the modelling of transient

processes with such a methodology would probably require tremendous computing

time. The objective of this project work is to develop a multi-coupled method to

analyse the thermal and mechanical behaviours of RSW process, reduce the

computing time with the minimum loss of accuracy and get more adequateinformation of the process, improve the quality monitoring and process control of

RSW.

5.1 FINITE ELEMENT MODEL

5.1.1 HEAT TRANSFER ANALYSIS

Heat transfer in resistance spot welding process involves convective heat transfer as

well as heat conduction in bulk of the sheet-electrode system. The transient heat flow

in resistance spot welding process has been modeled as a case of axisymmetric heat

conduction problem.

t

T=q+

r

k+

z+ v

C

T

rz

Tk

r

Tk

r

(1)

Where , Cand kare density, specific heat and thermal conductivity respectively. All

the material properties are considered to be temperature dependent. The term qv

refers to the rate of internal heat generation per unit volume.

The thermal boundary conditions can be decomposed from the nonlinear isotropic

Fourier heat flux constitutive relation:


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q = (2)

On the boundary surface, there is

q = n

T (3)

Where q is the heat flux through the boundary surface; n is the outward normal to the

surface.

5.1.2 ELECTRICAL FIELD ANALYSIS

The governing equation of the electrical analysis is

0+

+Ce

zC

zrr

C

rre

e

(4)

Where, Ce is the electrical conductivity; is the electrical potential.

The coupled thermal electrical problem is solved by the following matrix equation:

[ ] { }{ } [

]

(5)

Where is the thermal specific heat matrix; the thermal conductivity matrix;the electric coefficient matrix; temperature vector; the electric potentialvector; {Q} the heat flow vector; and {I} is the current vector.

For the structural analysis, the stress equilibrium equation is given by

( ) (6)

Where is the stress; bis the body force, r is the coordinate vector.


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The constructive equation of the material based on the thermo-elastic-plastic theory is

given by

(7)

(8)

Where {} is the stress vector; [D] is the elastic-plastic matrix; {} is the strain

vector; [De] is the elastic matrix; and {} is the coefficient of thermal expansion.

5.2 MODELING AND PARAMETERS

5.2.1 MODEL AND MESH

Figure 5-1 illustrates the 2-dimensional axisymmetric FEA model of RSW process

built in ANSYSprogram, where X and Y represent the faying surface and the

axisymmetric axis respectivelyIts corresponding dimensions are tabulated in Table

5.1Since the model is also mirror symmetric about thefaying surface, only the

values of the upper half of the model are listed.

Table 5.1: Model Dimensions

Dimensions OE=HI

(mm)

OI=EH

(mm)

PA=FG

(mm)

PB

(mm)

AG

(mm)

EF

(mm)

ED

(mm)

OP

(mm)

Values 2 15 5 11 18 12.5 3 32 30o


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Figure 5-1: The FEA model of RSW process

Figure 5-1a: Boundary conditions for simulation
http://www.sciencedirect.com/science/article/pii/S026130691000498X


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Figure 5-2: FEM model for spot welding process with contact elements

2-D axisymmetric models of two- as well as three-sheet joining with the application

of round-face electrodes are constructed. Both electrical-thermal and mechanical

contact elements are specially treated at the electrode-to-sheet and sheet-to-sheet

interface. The imposed boundary conditions and representative mesh model can be

found in Figure 5-2.

In each analysis, the model is meshed using contact elements as shown in figure 5-2

the solid element is employed to simulate the coupled interaction between the sheets

and electrodes. In order to correctly couple and transfer the data the model must have

identical mesh both in the electrical thermal analysis and in the thermo-elastic and

plastic analysis. Whereas the element types are different or have different degree of

freedom options, as shown in Table 5.2.

Table 5.2: Element types and degree of freedom options

Analysis Solid Contact element type

Degree of freedom

(for contact

element)

Electrical-Thermal PLANE 223 CONTACT172/TARGET 169 TEMP, CURRENT

Thermo-Elastic-

PlasticPLANE 223 CONTACT172/TARGET 169 UX, UY


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5.2.2 MECHANICAL BOUNDARY CONDITIONS

Uniform load was applied at the top of the copperelectrode during welding and

holding cycle. The electrode was removed at the end of theholding cycle. At the

faying surface between the electrode and the workpiece, a contact 172element was

used. At the faying surface between workpiece and workpiece, the

verticaldisplacement of the part of faying surface under electrode was set to zero, and

asubroutine program was used to determine if the other part of faying surface is under

contact or not. If some nodes are under contact and are under pressure stress, a

zerovertical displacement was applied here.

5.2.2a INTERFACE ELEMENTS

The contact between an electrode and a sheet or between two sheets was modeled by

the interface elements (Ma and Murakawa, 2009) as shown in Figure 5-2., and Figure

5-3. If the strain n of the interface element in the normal direction n of the contact

interface is larger than 1.0, the state of the interface is the non-contact state. In the

initial state and non-contact state, the electrical conductivity Ce,thermal conductivity k

and Youngs modulus E will be zero. This means that curren t flow through the

interface element and heat generation will be zero at the inter-face element. If the

strain n of the interface element in the normal direction n of the contact interface is

equal to or less than 1.0, the state of the interface is the in-contact state. In the

contact state, the electrical conductivity Ce, thermal conductivity k and Youngs

modulus E will be a given value, respectively. This means that current flows through

the interface element and heat is generated at the interface element. In the contact

state, the interface element has a strong stiffness. The material properties of interface

contact elements used for the analysis of the electrical field, thermal field and

mechanical field are independent from worksheets and electrodes. Therefore, the

contact resistance can also be considered if it is known. The formulation of the

interface element has no much difference from the ordinary element except the

material properties and their changes with the contact states. Therefore, it is relatively

simple and reliable to deal with the electricalthermalmechanical contact between

two faces for the spot welding process simulation.


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Prior to welding, the electrical initial conditions are set equal to zero, while the

temperature of entire structure is maintained at temperature of 20C. During the

welding cycle, the welding current is applied at the top of the upper electrode and zero

potential is imposed at the bottom surface of the lower electrode. Consequently, the

current flows from the upper electrode, passes through work piece and terminates at

the bottom annular section of the lower electrode. Both force and current are modelled

from practical welding signals and defined as a time-dependent function.

Figure 5-3: Contact elements

5.3 WELDING PARAMETERS AND MATERIAL PROPERTIES

The welding parameters used in this analysis arewelding current 50 Hz sine wave

AC current of 3.5kA.4kA and 4.5kAweld time, 75cycles (1.5 S): 100 cycles ( 2 S )

electrode force2000 Nhold time3 cycles (0.06S)The thermal, electrical and

mechanical properties of electrode and work piece are given in Table 3 and Table

4Because the materials are subjected to a wide range of temperatures, most of these

properties are considered as temperature dependent.

The most important property in the simulation of RSW process is the contact

resistivity of faying surface. Generally speaking, the contact resistivity is a dependent

function of contact pressure, temperature, and average yield strength of two contact

materials. It is pointed out by Vogle M, and Sheppard S., that the contact resistance

decreases as the contact pressure increases. Babu S.S., Santella M.L. developedan

empirical model is for the pressure and temperature dependence of electrical contact

resistance, a curve fitting procedure is used and the desired relationship between

contact resistance and pressure and temperature is established


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During the RSW process, the contact resistivity distribution influences the current

density pattern, which affects the temperature field through Joule heating while, the

temperature fieldthen influences the mechanical pressure distribution through thermal

expansion, related to the interface resistivity. Therefore, this is a highly non-linear

problem involving the complex interaction between thermal, electrical and

mechanical phenomena. To simplify the problem, any researchers take the contact

resistivity as a function of temperature. This simplification is reasonable because,

firstly, the load is constant in a specified RSW process; secondly, the yield strength of

the materials, which determines the contact status in the contact area, is essentially

influenced by temperature with this simplification, the computing time can be greatly

reduced. Therefore, in the present work, the temperature dependent contact resistance

is imposed on the faying surface.

Table 5.3: Thermal and electrical properties of materials

Temperatur

e

C

Thermal

conductivity

W/m.C

Electrical

resistivity

.m 10-8

Contact

resistivity

.m2 10

-7

Specific Heat

J /(kg.C)

Mild

Steel

Copper

Electrode

Mild

Steel

Copper

Electrode

Faying

surface

Mild

Steel

Copper

Electrode

21 64.75 390.3 14.2 2.64 2.38 443.8 397.8

93 63.25 380.6 18.6 3 2.31 452.2 401.9

204 55.33 370.1 26.7 4 2.25 510.8 418.7

316 49.94 355.1 37.6 5.05 2.12 561.0 431.2

427 44.86 345.4 49.5 6.19 1.93 611.3 439.6

538 39.77 334.9 64.8 6.99 1.79 661.5 452.2

649 34.91 320 81.1 8 1.31 762.0 464.7

732 1004

760 30.50 315.5 101.1 8.98 0.567 2386 477.3

774 1004

799 1189

871 28.41 310.3 115.5 9.48 0.492

982 27.66 305 115.8 9.98 0.417

1093 28.56 300.1 117.9 0.342

1204 120.9 118

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