PREDICTION OF NUGGET DEVELOPMENT

DURING RESISTANCE SPOT WELDING OF

2 MM THICK DUPLEX STAINLESS STEEL

SHEETS OF GRADE 2205

A thesis submitted in partial fulfilment of the requirements for the award of the degree

of

B.Tech

In

MECHANICAL ENGINEERING

By

C.HARISH (111111028)

CHENNUPATI LEELA SAI BHARADWAJA (111111029)

MALAVATHI GAUTHAM NAIK (111111053)

MECHANICAL ENGINEERING DEPARTMENT

NATIONAL INSTITUTE OF TECHNOLOGY

TIRUCHIRAPALLI-620015

BONAFIDE CERTIFICATE

This is to certify that the project titled Prediction of nugget development during

resistance spot welding of 2mm thick duplex stainless steel sheets of grade 2205

is a bonafide record of the work done by

C.HARISH (111111028)

CHENNUPATI LEELA SAI BHARADWAJA (111111029)

MALAVATHI GAUTHAM NAIK (111111053)

in partial fulfilment of the requirements for the award of the degree of Bachelor of

Technology in Mechanical Engineering of the NATIONAL INSTITUTE OF

TECHNOLOGY, TIRUCHIRAPPALLI, during the year 2014-2015.

DR.SANKARANARAYNASAMY Dr. T.SUTHAKAR

Guide

Project Viva-voce held on :

Head of Department

Internal Examiner External Examiner

i

ABSTRACT

Resistance spot welding is a complicated process, which involves the interaction of

electrical, thermal, mechanical, and metallurgical phenomena. The main advantage of

the spot welding process is the ease with which the process can be automated and

robotized in high volume and high production rate operations. The process involves

heating of two or more metallic sheets under the action of high amperage current for a

short duration through a pair of copper electrodes that hold the sheets under

compressive force. The compressive force ensures adequate contact between the

metallic sheets. Localized resistive heating at the contacting interface of the sheets

results in melting that on solidification provides with a joint in the form of a spot. In

order to obtain good weld nugget during spot welding, hit and trial welds are usually

done which is very costly. Therefore the numerical simulation research has been

conducted to understand the whole process. Extensive resistance spot welding tests

were conducted on Duplex stainless steel sheets of size 5*5 and later these were cut

using EDM exactly at the centre into two halves to view the nugget size under a

macro scope after the itching process. In this project, a 2D axisymmetric model of

thermo-elastic finite element method (FEM) is developed to analyse the mechanical

behaviour of resistance spot welding (RSW) process using commercial software

ANSYS which takes into account of temperature dependent thermalelectrical

mechanical properties, contact resistances, melting point, and effective heat transfer in

the fluid and electrode geometry. Contact conductance namely electrical contact

conductance is found using LCR device and thermal contact conductance is found

using the formulae. The welding current, electrode pressure and hold time affected the

thermo mechanical interactions of the welding process changed the final nugget

geometry. Predictions of the temperature distribution, associated stresses, and weld

nugget geometry were obtained from this model. The predicted nugget shape and size

agree well with experimental data. Weld thicknesses were then measured and

compared with the analytical results also generated in this work.

Keywords: resistance spot welding, localized resistive welding, 2D axisymmetric,

thermal contact conductance, and electrical contact conductance.

ii

ACKNOWLEDGEMENT

We take this opportunity to express our profound gratitude and deep regards to our guide

DR.SANKARANARAYNASAMY and co-guide Dr. N. SIVA SHANMUGAM, for their

exemplary guidance, monitoring and constant encouragement throughout the course of this

thesis. The blessing, help and guidance given by them time to time shall carry us a long way

in the journey of life on which we are about to embark.

Also, we would like to express our gratitude and appreciation to all those who gave us the

possibility to complete this report. A special thanks to our final year project co-ordinator,

Dr.V. Arul Mozhi Selvan and Dr. K.Pannirselvam, whose help, stimulating suggestions and

encouragement, helped us to coordinate our project especially in writing this report.

Lastly we we thank Almighty,our parents,our brother, sisters and friends for their constant

encouragement without which this assignment would not be possible.

iii

TABLE OF CONTENTS

Title Page no.

ABSTRACT .......................i

ACKNOWLEDGEMENTS..ii

TABLE OF CONTENTS..iii

LIST OF FIGURES....v

LIST OF TABLES.vii

CHAPTER 1: INTRODUCTION

1.1 Resistance spot welding..1

1.2 Mechanics of resistance spot welding2

1.3 Thermal phenomena and its significance..3

1.4 Duplex stainless steel...4

1.4.1 Chemical Composition, %.............................4

1.4.2 Fabrication.....................................................5

1.4.3 Corrosion Resistance.....................................5

1.4.4 Heat Resistance..............................................5

1.4.5 Heat Treatment..............................................5

1.4.6 Welding...........................................................5

1.4.7 Machining.......................................................6

1.4.8 Characteristics of Duplex 2205.....................6

1.4.9 Applications....................................................6

1.5 Objective of the present work.6

iv

CHAPTER 2: LITERATURE REVIEW

2.1 Introduction..8

2.2 Weldability Range8

2.3 Growth of Nugget.10

2.4 Research Works...10

2.5 Summary12

CHAPTER 3: THEORITICAL FORMULATION

3.1 Introduction..13

3.2 Thermal Analysis.14

3.2.1 Governing equation and boundary condition15

3.2.2 Heat Generation: Joules heating16

3.3 Electrical Analysis18

3.3.1 Governing equation and boundary condition.18

3.4 Estimation of electrical contact resistance......19

3.5 Estimation of Thermal contact conductance......19

CHAPTER 4: METHODOLOGY

4.1 Introduction21

4.2 Experimental setup22

4.3 Experimental procedure23

4.4 Measurements26

4.4.1 Contact resistance measurement26

4.4.2 Electrode dimensions measurement..27

4.5 Experimental results......28

v

4.6 Methodology of the simulation.....28

4.7 Material Properties....31

4.8 Boundary Condition......31

CHAPTER 5: RESULTS AND DISCUSSION

5.1 Experimental..32

5.2 Simulation...33

CHAPTER 6: SUMMARY AND CONCLUSION

6.1 Summary...39

6.2 Conclusion.....39

REFERENCES40

vi

LIST OF FIGURES

Fig. No. Title Page No.

1.1 Typical spot welding process 3

1.2 Microstructure of duplex stainless steel...4

2.1 Weldability range...9

3.1 Schematic view of sheet-electrode system in

cylindrical coordinate system..14

3.2 2-D Schematic view of part of sheet-electrode

geometry...16

3.3 Electrical Resistances in

the sheet electrode system.. ...17

3.4 Variation of resistance at the contact surfaces..18

4.1 Resistance spot welding machine....................22

4.2 Steps in resistance spot welding..23

4.3 Belt grinding of the sample ....23

4.4 Observing the microstructure under

Microscope24

4.5 Cutting the samples using EDM25

4.6 Microscopic view of nugget25

4.7 LCR meter.......................................................26

4.8 Measurement of electrode dimensions .26

4.9 Contact pairs......................29

4.10 Simulation Process.30

4.11 Thermal-Electric Boundary Conditions..31

vii

5.1 Nuggets Observed Under Microscope33

5.2

I. Fig 5.2(a) 9 kA, 7 weld cycles33

II. Fig 5.2(b) 9kA, 8 weld cycles.....33

III. Fig 5.2(c) 8kA, 7 weld cycles.33

IV. Fig 5.2 (d) 8kA, 8 weld cycles....33

5.3 Simulation for 8kA and 8 weld cycles.35

5.4 Diameter measurement36

5.6 Vonmises Stress.38

5.7 y component stresses.38

viii

LIST OF TABLES

Table No. Title Page No.

1.1 Chemical composition......4

4.1 Experimental Conditions....28

5.1 Experimental result.32

5.2 Experimental Results and error.37

1

CHAPTER 1

INTRODUCTION

The resistance spot welding process has been widely used in the mass production

industries, where long production runs and consistent conditions can be maintained.

The automotive industry is the major user of this welding process, followed by the

appliance industry. It is also used by many industries manufacturing a variety of

products made of thin gauge metals.

1.1 Resistance spot welding

Resistance spot welding process involves heating of two or more metallic sheets

under the action of high amperage current for a short duration through a pair of

copper electrodes that hold the sheets under compressive force. The compressive

force ensures adequate contact for the current to flow through the metallic sheets.

Localized resistive heating at the contacting interface of the sheets results in melting

that on solidification provides a joint in the form of a spot. Throughout the welding

process, the molten metal remains enclosed by the surrounding solid material and

thus, the chance of atmospheric contamination of the molten weld pool is very less. In

contrast to the other fusion welding processes, no filler materials or fluxes are used in

resistance spot welding. The copper alloy electrodes are shaped to provide required

current density and pressure at the point of welding. The duration of welding is

generally a fraction of second and the spots are produced in quick succession thus

making the process very fast. The rapid completion of a spot weld makes the process

very well suited for inclusion in an automated production line.

The resistance spot welding process is the principal method for joining of sheet metal

components in the automotive, building, transportation, office furniture and domestic

appliance industries. The main advantage of the spot welding process is the ease with

which it can be automated and robotized in high volume and high production rate

operations. It can readily be incorporated into assembly lines with other fabrication

and transfer operations. However, resistance welding can be difficult to control,

particularly due to the interaction between various controlling parameters and the

mechanical / electrical characteristics of the spot welding equipment as well as the

sheet materials. Resistance spot welding process is primarily used for joining of

2

uncoated and coated steel sheets, although materials such as stainless steel, aluminum

alloys, nickel alloys, titanium and copper base alloys are also spot welded for

numerous applications. In the recent years, the use of a wide variety of coated steel

and aluminum alloy sheets is increased significantly in the automotive industries with

an aim at enhancing the product life and fuel efficiency. Resistance spot welding of

both coated steels and aluminum alloy sheets involves rapid electrode wear and thus,

poses unique challenge. Continuous efforts are being made to keep pace with the

increasing demand of reliable spot welding process in these challenging areas. One

objective of the present work is to contribute to a better understanding and analysis of

resistance spot welding process to facilitate its effective use in some of those

challenging areas.

1.2 Mechanics of resistance spot welding

Resistance spot welding involves the coordinated application of electrical current and

mechanical force of appropriate magnitudes as depicted schematically in Fig.1.1.

Because of the relatively short current path in the work-pieces (sheet metals) and the

need to localize the heating, low voltage with relatively high amperage current is

necessary to develop the required amount of heating of the impending spot weld. The

sequence of the operation is first to develop sufficient amount of heat to melt a

confined volume of metal (Fig. 1.1). This is then allowed to cool while under

electrode force until adequate strength is developed to hold the parts together. The

current density and the mechanical pressure must be such that a weld nugget of

required diameter / size is formed, but not so high that the molten metal is expelled

from the weld zone. Expulsion of molten metal from the weld zone should be avoided

at all times if defect free welds are to be achieved. To counter the resistive heating and

subsequent thermal damage of the electrodes, cooling water channels are provided

within the electrodes. Thus, the development of a satisfactory spot weld depends on a

number of factors such as the magnitude and the duration of welding current,

magnitude of electrode force, and above all the nature and variation of electrical

contact resistance along the sheet to sheet and the electrode to sheet interfaces.

Furthermore, the variation in the physical and mechanical properties of the sheet

metals with temperature also plays vital role in the growth and the establishment of a

weld nugget. There occurs a complex interplay of all these factors and an unexpected

change in any of these may result in a defective weld. This h7as led the researchers to

3

investigate various types of interrelationship among these factors in order to develop a

proper understanding of the mechanisms of the process. Such approaches are reported

in details in the subsequent chapter.

1.3 Thermal phenomena and its significance

The basic phenomenon of resistance spot welding process involves a rapid thermo

electrical heating of the sheet materials up to the melting point and subsequent cooling

to the ambient temperature. Thus the process, in principle, poses a heat transfer

dominated problem. The heat transfer is influenced by several process dependent

factors. The correct knowledge of this heat transfer mechanism and the resulting in-

process thermal cycle can provide a mean of assessing the influence of various factors

affecting the process. The intermediate and the final temperature distribution in and

around the weld nugget, the rate of heating and cooling of the weld nugget and the

adjoining material, the changes taking place on the sheet surface in contact with the

electrode face and its dependence on interface temperature distribution are some of

the features that need to be understood thoroughly for proper analysis of the process.

The monitoring of the dynamic nature of the fusion zone temperature using

experimental means is a difficult task owing to the extremely short duration of the

Figure 1 .1 Typical spot welding process

4

process and the typical location of the impending weld nugget. Thus, there is a

necessity of accurate theoretical analysis based on the heat transfer phenomena.

1.4 Duplex stainless steel

Duplex 2205 is a nitrogen enhanced duplex stainless steel that was developed to combat

common corrosion problems encountered with the 300 series stainless steels. Duplex

describes a family of stainless steels that are neither fully austenitic, like 304 stainless, nor

purely ferritic, like 430 stainless.

Duplex stainless steels are called duplex because they have a two-phase microstructure

consisting of grains of ferritic and austenitic stainless steel. The picture shows the yellow

austenitic phase as islands surrounded by the blue ferritic phase. When duplex stainless

steel is melted it solidifies from the liquid phase to a completely ferritic structure. As the

material cools to room temperature, about half of the ferritic grains transform to austenitic

grains (islands). The result is a microstructure of roughly 50% austenite and 50% ferrite.

Fig 1.2 Microstructure of duplex stainless steel

1.4..1 Chemical Composition, %

Cr Ni Mo C N

22.0-23.0 4.50-6.50 3.00-3.50 .030 Max 0.14-0.20

Mn Si P S Fe

2.00 Max 1.00 Max .030 Max .020 Max Balance

Table 1.1 Chemical composition

5

1.4.2 Fabrication

The fabrication of this grade is also affected by its strength. Bending and forming of this

grade requires equipment with larger capacity. Ductility of grade 2205 is lesser than austenitic

grades; therefore, cold heading is not possible on this grade. In order to carry out cold heading

operations on this grade, intermediate annealing should be carried out.

1.4.3 Corrosion Resistance

Grade 2205 stainless steel exhibits excellent corrosion resistance, much higher than that of

grade 316. It resists localized corrosion types like intergranular, crevice and pitting. The CPT

of this type of stainless steel is around 35C. This grade is resistant to chloride stress

corrosion cracking (SCC) at temperatures of 150C. Grade 2205 stainless steels are apt

replacements to austenitic grades, especially in premature failure environments and marine

environments.

1.4.4 Heat Resistance

The high oxidation resistance property of Grade 2205 is marred by its embrittlement above

300C. This embrittlement can be modified by a full solution annealing treatment. This grade

performs well at temperatures below 300C.

1.4.5 Heat Treatment

The best suited heat treatment for this grade is solution treatment (annealing), between 1020 -

1100C, followed by rapid cooling. Grade 2205 can be work hardened but cannot be hardened

by thermal methods.

1.4.6 Welding

Most standard welding methods suit this grade, except welding without filler metals, which

results in excess ferrite. Adding nitrogen to the shielding gas ensures that adequate austenite

6

is added to the structure. The heat input must be maintained at a low level, and the use of pre

or post heat must be avoided. The co-efficient of thermal expansion for this grade is low;

hence the distortion and stresses are lesser than that in austenite grades.

1.4.7 Machining

The machinability of this grade is low due to its high strength. The cutting speeds are almost

20% lower than that of grade 304.

1.4.8 Characteristics of Duplex 2205

High resistance to chloride stress corrosion cracking

Resistance to chloride pitting and crevice corrosion

Good general corrosion resistance

Good sulfide stress corrosion resistance

High Strength

1.4.9 Applications

Chemical process vessels, piping and heat exchangers

Pulp mill digesters, bleach washers, chip pre-steaming vessels

Food processing equipment

Oil field piping and heat exchangers

Flue gas desulfurization equipment

1.5 Objective of the present work

The sheet-electrode set-up in resistance spot welding is mainly subjected to Joules

heating in different regions resulting from the flow of high current. A current density

is established in the sheet-electrode set-up that is mainly influenced by the

temperature dependent electrical resistivity of bulk material and the variation in

7

contact resistance along the sheet to sheet and the electrode to sheet interfaces. The

nature of the current density distribution in the sheet is also influenced by the extent

of the electrode to sheet contact region at any specific time instant that depends on the

electrode force, electrode to sheet interface temperature and the consequent plastic

flow at the interface especially for spherical tip electrode. Use of numerical

techniques such as finite element method is an effective alternative to solve such

nonlinear and complicated problem.

Thus, the present work is aimed at the development of a reliable process simulation

model and its validation for resistance spot welding based on finite element method

considering the coupled thermal-electric and thermal-mechanical phenomena. Special

emphasis is given to make the process simulation model general so that various

electrodes geometries and sheet materials can be included in the analysis.

Temperature dependent material properties are included in the model. The nature of

variation in electrical contact resistance along the sheet to sheet and electrode to sheet

interfaces are suitably converted to local electrical resistivity as a function of interface

temperature distribution at any time instant. The computed values of weld dimensions

from the developed numerical process simulation model have been validated with the

corresponding experimentally measured results in Duplex stainless steels sheets of

grade 2205. The computed results have provided quantitative information on the

current density and the temperature distribution in the sheet-electrode set-up at any

time instant for a given set of welding process parameters. The influence of the

process parameters on weld nugget diameter, penetration, and heat affected zone, the

resulting thermal cycles and other weld nugget characteristics are studied extensively

for the sheets. A resistance spot weld joint is primarily adjudged for its quality based

on the weld nugget dimensions (both nugget diameter and penetration), weld strength

(tensile shear, torsional failure, fatigue, etc.). In the present thesis, the weld nugget

dimensions are used as the basis to develop and validate the numerical process model

that can predict the weld nugget dimensions for any given input of sheet-electrode

geometry and material properties of duplex stainless steels (2205 grade).

8

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

A number of researchers have followed experimental approach to study the

correlation between the welding process parameters and weld quality in terms of weld

strength and weld nugget dimensions. Extensive work has already been carried out on

the modelling of spot welding process and study the effect of parameters on nugget

size. Electrically, thermally, and structurally coupled axisymmetric model considering

temperature-dependent properties and Joule heating were also done. Another group

of researchers have focused on the understanding of static and dynamic contact

resistance and their influence on weld nugget development.

The sections of the present chapter gives an over view of these already carried out

work in order to have a better understanding of the nugget formation in the RSW

process. This thesis mainly focuses towards the development of a reliable numerical

simulation model for the resistance spot welding process and its validation with

experimental results linking the process parameters.

2.2 Weldability Range

Weldability range (lobe) is the area where accept- able welds can be produced using a

specific combination of welding current and weld time. Welding range is limited by

the minimum acceptable weld size and splash limit.

In spot welding, weldability range is usually defined using coordinate axes where

weld time is located on one axis and welding current on the other. The electrode force

used, electrode geometry and cleanness, and the consistency and thickness of the

welded material affect the shape and size of the weldability range. Materials with

good welding properties have a large weldability range, which means that welding

parameters can be selected from a great number of different combinations.

9

Cold rolled metal sheets usually have a large weld- ability range. Welding current can

vary from 1.02.0 kA in common weld times. The alloying of the steel and thick zinc

coating, in particular, may decrease the weldability range. In this case, the correct use

of appropriate welding parameters is very important in terms of producing good spot

welds.

The area between the minimum acceptable weld diameter and splash limit is called

the weldability range. Welds produced in this range meet the common requirements

set for spot welds.

Fig 2.1 Weldability range

10

2.3 Growth of Nugget

The formation, size and growth rate of weld depend on the welding parameters used.

The increase in the weld nugget diameter as a function of welding current is presented

in Figure 2.1. The figure shows how the weld nugget diameter increases rapidly at the

beginning of the process, after which the growth slows down. At the end of the curve,

the nugget is too large for the electrodes to hold the weld pool between the welded

sheets, which causes a burst of molten metal expulsion from between the sheets.

A good spot weld has sufficient diameter and nugget penetration. The minimum

acceptable diameter of weld nugget is considered to be 3.5t, where t is work piece

thickness. Welds with smaller diameter do not have sufficient penetration and the size

of the weld is not enough to bear the calculated loads. In addition, too small welds are

created in the welding current range where the nugget size increases rapidly. In this

case, small variations in the work- piece surface quality, welding parameters or the

wear of electrodes greatly affect the variation of the weld size.

A recommended weld diameter is 5t. This value is usually achieved slightly under

the splash limit, where the weld nugget growth is stabilised and small variation in the

welding current or workpiece surface quality do not significantly change the size of

the weld.

2.4 Research Works

Yamomato et al. [1] studied extensively the spot weldability of thick mild steel sheets.

It was postulated that thicker sheets were less sensitive to the variation in sheet to

sheet contact resistance as greater resistive heating occurred due to bulk material

resistivity. The welding defects such as blowholes, shrinkage etc. in the weld nugget

were observed to be lesser when spot welds were made with high electrode force.

Dickinson et al. [2] first introduced the concept of weld lobe curves that, in reality,

was a graphical presentation of acceptable spot weld nuggets as a function of weld

current and weld time for a fixed value of electrode force [2]. Figure 2.1

schematically shows such a weld lobe curve. It is evident from Fig.2.1 that any

combination of weld current and weld time that lies on the lower side of the weld lobe

curves will not produce an acceptable weld nugget diameter. Similarly, any

11

combination of weld current and weld time that lies on the higher side of weld lobe

curve will lead to liquid metal expulsion from the impending weld nugget. H. A.

Nied[3] presented an analytical tool to predict the processing parameters needed to

produce a spot weld with sufficient joint penetration and conducted extensive tests on

Type 321 stainless steel sheets to validate the finite element model. . The electrical,

thermal and mechanical behaviors of the interface between two contact solids were

studied theoretically by Bowden and Williamson in 1958 [4]. Their study showed

that surface asperities condensed current density and restricted contact resistance

within the contact region, which caused temperature to rise at the interface when

electric current flow occurred. The spot welding characteristics of two HSLA steels

Mn-Mo-Vb and V-N were investigated and compared with that of SAE 1008 carbon

steel by Sawhill et al. [5]. It was reported that the minimum welding current required

for these HSLA steel sheets were lower compared to the low-carbon steel of same

thickness, which was attributed to the higher bulk material resistivity of the HSLA

steels. However, the HSLA steel sheets required greater electrode force due to their

tendency of high spring-back forces. Spot welded joints made in the HSLA steel

sheets had shown better mechanical strength compared to that of SAE 1008 steel

sheets of similar thickness. Sawhill et al. [6] also studied the spot weldability

characteristics of two more grades of HSLA steels one rephosphorised and the other

stressrelieved annealed (SRA) steel. Kaiser et al. [7] had conducted extensive spot

welding experiments with low-carbon and HSLA steel sheets under different surface

conditions. Several weld lobe curves were generated at different values of electrode

forces for different surface condition for each kind of steel. It was reported that

artificial increase in the surface contact resistance of HSLA steels could enhance the

permissible range of welding currents. Extensive studies on spot weldability of HSLA

steel sheets were reported by Jones and Williams [8] using both ac and dc spot

welding machines. Keef et al. [9] studied the spot weldability of titanium stabilized,

high columbium content and low columbium content stainless steel sheets. The

selection of the welding process parameters such as welding current, weld times, etc.

was performed depending on the material constituents in which titanium stabilized

sheets produced the best performance next to the columbium content steels. Sperle et

al. [10] studied the influence of weld nugget diameter on the weld strength for dual

phase high strength steel sheets in the thickness ranges 0.7, 0.8, 1.0 and 1.2 mm with

electrodes of different diameters.

12

2.5 Summary

A wide range of experimental studies are reported on resistance spot welding

of uncoated and coated low-carbon steels, stainless steels. These experimental

studies have been performed primarily with traditional ac spot welding

machines.

The quantitative knowledge on the nature and the variation of contact

resistances along the sheet to sheet and the electrode to sheet interfaces are

significant spot welding of metallic sheets with varying surface coatings and /

or treatments. Although experimental investigations are reported in this

direction, the results reported by various authors contradict considerably

especially with regard to the dependence of contact resistance on contact

pressure.

Numerical models are established as an effective route to realize the coupled

thermal-electrical and thermal-mechanical phenomena in resistance spot

welding process. The influence of the magnitude and the variation in initial

contact resistance depending on different surface conditions are rarely

included in the modelling calculations.

13

CHAPTER 3

THEORITICAL FORMULATION

3.1 Introduction

In the resistance spot welding process, the faying surfaces are fused and joined in spot

by Joule's heat due to the flow of electric current through a weldment held together

under electrode force . The complexity in this process arises from the interaction of

many factors such as the complex flow paths for heat and electrical current and the

variation in the properties of the materials with temperature and phase change.

The heat transfer mechanism in resistance spot welding involves primarily heat

conduction and to some extent heat convection within the molten welds pool.

However compared to convection, the conductive mode of heat transfer plays a

dominant role. This is due to the fact of rapid melting and solidification of the weld

nugget that remains confined within a solid containment of bulk material. Also since

the size of the nugget is relatively small, the effect of convective heat transfer on it

can be neglected.

The flow of current occurs through the region in which the electrodes and the sheet

surfaces are in contact. The amount of current flowing across the sheets is also critical

in determining the size of the nugget formed at the faying surface as heat generated

depends upon the flow of current. The heat generation due to flow of current and in

turn heat transfer occurring together gives rise to electro-thermal analysis. In this

analysis the properties of the materials vary with temperature. What adds on to the

complexity of analysis is the localized magnitude of contact resistance of appreciable

magnitude present along the sheet to sheet and the electrode to sheet interfaces that

also varies with the contact temperature, electrode force.

All the factors are thus taken into consideration to develop a comprehensive

modelling of resistance spot welding. Thus an Attempt is made in the present work to

include most of these features mentioned above in a realistic manner so as to keep the

simulation model tractable and sufficiently versatile. This chapter further contains

various governing equations involved in the electro-thermal and thermal-mechanical

phenomena.

14

3.2 Thermal Analysis

The resistance spot welding mechanism is illustrated schematically in Fig. 1. In the

welding, the required heat is generated by a passage of a high-current pulse across the

welding interface and base metal. Under the mechanical and electric loading

conditions in the weldment, the voltage potential field is formed within the base metal

and along the electrode and weldment interface. This voltage distribution causes a

current flow as follows:

where J is the current density vector, is the voltage potential. When the current

density in the weldment is high enough, the melting initiates at the interface of the

two workpieces. To prevent the electrode from sticking to the weldment, part of the

generated heat in the electrode interface is dissipated through the copper alloy of the

electrode to cooling water. Once melting is started, the contacts move closer together

through a squeezing action by the electrodes, enlarging the area of the contact. Thus,

contact resistance is altered, modifying in turn the distribution of voltage field in the

weldment. As a result, current density will be redistributed in the weldment and a new

temperature distribution in the weld is established.

Fig 3.1 Schematic view of sheet-electrode system in cylindrical coordinate system

15

Although the nature of heat flow is three-dimensional, the same can be modelled in a

two-dimensional axisymmetric form owing to the circular cross-section of the

electrode that applies both electrical current and compressive force (Fig.3.1). The

merit of the axisymmetric formulation is that the symmetry with respect to , the

azimuthal angle, permits to undertake the variation in heat transfer and current flow in

radial (r) and vertical (z) directions only. Hence, the thermal analysis is conceived as

an axisymmetric case of transient heat conduction in the present work .

3.2.1 Governing equation and boundary condition

The governing equation for as axisymmetric transient thermal analysis is given by:

It is convenient to introduce cylindrical coordinates to solve the problem. Assuming

symmetry of the processes with respect to the electrode axis it is possible to reduce

the number of dimensions to two without loss of generality .Where r and z are radial

and axial coordinates and is the density of the material, C is the specific heat

capacity, T is the temperature as a function of coordinates and time, t is the time, k is

the thermal conductivity, and qv is the rate of internal heat generation per unit

volume. All the material properties are considered to be temperature dependent.

The boundary conditions taken are as follows:

1. The symmetric boundary condition along z-axis is stated as:

2. Along the inner surface of the electrode (i.e. along GH and HK), the temperature

remains equal to the ambient temperature.

16

3. The exterior of the sheet-electrode geometry is adiabatic i.e.

Fig 3.2 2-D Schematic view of part of sheet-electrode geometry

3.2.2 Heat Generation: Joules heating

The total resistance across the electrodes could be considered as the sum of five

resistors in the series schematically represented in Fig. R1 and R5 are the interface

resistances between the electrodes and the sheet metal; R2 and R4 represent the

metals' bulk resistance. R3 is defined as the contact resistance at the faying interface

and is affected by the electrical, mechanical and thermal condition of the surface.

Since melting occurs at this last interface, heat production is the greatest at this

location, implying that R3 is much larger than the other resistances. However, this

value drops to zero as the weld nugget is formed.

17

Fig 3.3 Electrical Resistances in the sheet electrode system

Metallic surfaces usually contain a number of asperities widely varying in shape and

sizes. In the course of spot welding process, as the sheet surfaces meet under the

action of electrode force, the initial contact is established among these surface

asperities only. Depending on the magnitude of the electrode force, certain volume of

these asperities is locally collapsed and a real contact between the sheet surfaces is

established. As the electrical current is applied subsequently, the current conduction

happens through this real contact area only.

Fig 3.4 Variation of resistance at the contact surfaces

18

3.3 Electrical Analysis

The electrical analysis in the present work typically refers to the current conduction

analysis due to the flow of electric current through the sheet-electrode system. The

current conduction analysis is carried out in two-dimensional, axi-symmetric form

using cylindrical coordinates assuming no variation of electric potential in the

direction. Since the electrodes are cylindrical in shape and the electrode diameter is

usually much larger compared to the sheet thickness, it is possible to conceive further

that the nature of current flow will be predominantly axial through the sheet-electrode

system by the straight lines within the metallic sheets.

3.3.1 Governing equation and boundary condition

The governing equation for a steady-state current conduction through the sheet

electrode system can be expressed in terms of electrical potential, , as:

where, is the electrical resistivity. The appropriate boundary conditions for the

current conduction analysis can be presented as follows:

Where J denotes the uniform current density (flux) prevailing at the top surface of the

electrode due to the applied current. The equation (i) is expressed considering air

surrounding the sheet-electrode geometry as insulator to electric current. Equation

(iii) implies the symmetry of the domain of analysis along the radial axis.

19

3.4Estimation of electrical contact resistance

Reliable values of electrical contact resistance as a function of contact pressure and

temperature are always scarce in open literature especially in connection with

resistance spot welding. Several efforts are made in the recent past to provide with an

analytical relation that can estimate the value of electrical resistance along the

interface when two metallic surfaces are in contact.

For a given electrode force, the contact resistance between electrode-workpiece and

workpiece-workpiece interfaces has been measured for the entire welding cycles by

the following equation:

Where -

R(T), R(20 oC), (T), (20) are the contact resistance and average yield stress at T oC

and 20 oC respectively, L is the elemental length and Ac is the contact area.

3.5 Estimation of Thermal contact conductance

The thermal contact conductance of stainless steel sheets is calculated by taking the

impact of temperature, pressure and surface roughness into account. The simplest and

the most applicable equation to calculate this factor is:

Cc

Where hc is the thermal contact resistance, is surface roughness, m is the roughness

slope and P is the pressure in contact area. Furthermore, ks and E are equivalent

thermal conductivity and elastic modulus of the two materials in contact area which

are described by:

20

Where k1, k2, 1, 2, E1 and E2 are thermal conductivity, Poisson ratio and Young

modulus for materials 1 and 2 respectively.

21

CHAPTER 4

METHODOLOGY

4.1 Introduction

The present chapter outlines the experimental studies carried out on the resistance spot

welding of duplex stainless steel grade 2205 sheets, along with the methodology of

the finite element analysis on the same. As a part of this study, spot weld samples are

prepared at various combinations of welding currents and electrode force using five ac

and one medium frequency dc spot welding machines. The quality of the weld

samples are measured in terms of the final weld nugget dimensions (nugget diameter).

Based on the experimental results, the permissible ranges of process parameters for

each spot welding machine are evaluated. Simultaneously, the experimental results

have been used to validate the finite element based numerical simulation model of

resistance spot welding process developed in this work.

Conceptually, the rms value of welding current, Irms, can be specified as

where t is the total weld time and I(t) the instantaneous value of current at any time

within t.

When the value of welding current corresponding to Irms is used as an input to the

numerical computation of weld nugget dimensions, the calculated results of weld

dimensions tends to become general rather than specific to a particular spot welding

machine. Usually, the value of Irms is available at the machine controller interface

corresponding to a specific setting of the current controller. It is apparent from

equation that Irms cannot represent the temporal variation of the real-time current

waveform I(t), that is possibly significant in resistance spot welding due to the

transient nature of Joules heating involved in this process. In constant current ac spot

welding machines, a specific current setting (or percent heat input) is achieved by

utilizing only a portion of each ac cycle throughout the weld time. A pair of such

thyristors is normally used to allow only a portion of the positive and the negative half

(in symmetric manner) of each full ac cycle corresponding to a specific current

22

setting. Although, the phenomenon of resistive heating in sheet-electrode system is

directly proportional to the squared instantaneous current [~I2(t)], the same is highly

transient in nature and always in competition with the conductive heat dissipation

through the bulk material. Thus, the current waveform with larger peak values in each

half cycle will lead to greater rate of resistive heating in comparison to the waveforms

with lesser peak values. In addition, the instantaneous current waveforms with larger

peak values may also lead to uncontrollable melting and if the electrode force is

insufficient, expulsion of liquid from the fusion zone can result.

4.2 Experimental setup

The most common resistance welding machines use AC that has not been transformed

from the supply frequency. DC machines have become slightly more common than

before. Their welding current can be slightly lower than in AC machines. Some

welding machines tansform the supply frequency of the welding current to be higher,

which has several advantages e.g. smaller transformers. High frequency welding

current is better focussed on the connection point, which allows using significantly

lower welding currents. The resistance spot welding equipment is shown in the figure

4.1.

Fig 4.1 Resistance spot welding machine

23

The frame solutions for resistance welding machines can vary greatly. In larger

welding units and automated production, welding force is created by means of

pneumatic and hydraulic cylinders. Resistance welding machines load the power

supply network heavily during welding. The properties of resistance welding machine

affect the selected welding parameters. Most resistance welding machines are

powered by AC, in which case the welding current used depends on the size of the

workpiece and gap surface of the welding machine. Different kinds of transformers

and their location also have an impact on the selected welding current. Resistance

welding machines are equipped with a cooling system, which is most commonly

based on the circulation of cooling water. Components that are cooled down are

electrodes, electrode holders, transformers and contactors. The cooling of

transformers and contactors is usually separate from the other cooling circuit.

4.3 Experimental procedure

Resistance spot welding is the most commonly used resistance welding method. Spot

welding is used to join sheets together by means of lap joints. Spot welding produces

single spot-like welds, which are also called nuggets. Welding current is directed to

the workpieces through electrodes, which also generate pressing force. Electrodes are

usually located on both sides of the work piece and either one or both move and

transmit force to the work piece. The welding current of 420 kA is used for making a

single weld. The welding current depends on the material to be welded and work

piece thickness. A number of spot welds can be welded simultaneously, which is

called serial spot welding.

The stages of resistance spot welding resistance welding are as follows: electrodes

press the welded workpieces together => electrode force decreases the transfer

resistance of workpieces between the electrodes, which allows directing welding

current through the workpieces through the desired route. Welding current is

connected after the termination of the squeeze time. Welding current produces heat at

the faying surfaces and thus creates a weld pool between the workpieces. Welding

current is switched off as the weld time ends. Electrode force still presses the

workpieces together and electrodes cool the weld down. The weld pool must solidify

and the weld must achieve sufficient strength properties during the post-weld hold

time. After the end of the hold time, electrodes are retracted from the workpiece and

the total weld time required for the production of one spot weld ends, Figure 4.2.

24

Fig 4.2 Steps in resistance spot welding

In order to determine the diameters (D1,D2) of the nugget formed at the faying

surface, the samples were cut transverse to the faying surface using Electrical

discharge machining (EDM). Then the cross sectioned samples were polished using

different grades of emery sheets(Sheet 0-4) which was followed by alumina and

diamond fine polishing. After cleaning and drying the samples they were etched using

Kellens reagent(CuCl2 5g, Hcl 100ml, Ethanol 100ml). The samples were then tested

under a microscope to evaluate the nugget diameters. The images of the nugget

observed under microscope are shown in fig 4.3.

Fig 4.3 Belt grinding of the sample

25

Fig 4.4 Observing the microstructure under microscope

Fig 4.5 Cutting the samples using EDM

Fig 4.6 Microscopic view of nugget

26

4.4 Measurements

Various measurements have to be made so that they may be given as input in the

resistance spot welding simulation.

4.4.1 Contact resistance measurement

Previously, single sheet welding experiments were performed on the samples. The

voltage drop was measured in the welding electrodes using a computer assisted

measurement system which recorded values every 0.1 ms. The resistance was

calculated using the voltage drop measurement at the time of the peak in the welding

current. Cross sections were taken at the end of the welding experiments and

evaluated with respect to heat affected zone as a function of welding current.

Electrode imprints on the surface of single sheet experiments were examined with a

stereoscope for each welding experiment, and the diameter of the imprints were

recorded.

Our method employed measuring volume resistances of the sheets using LCR meters

available in Instrumentation and Control Engineering department and then calculating

the overall resistance between the sheets.

Figure 4.7 LCR meter

The LCR meter uses 4 wires to measure the resistance between any two points. The

measurement technique is as shown in the figures 3.7 and 3.8.

27

4.4.2 Electrode dimensions measurement

Since the electrode is filed off at regular intervals, it is necessary to measure its

dimensions. The various dimensions of the electrode were measured using digital

vernier calipers.

Figure 4.8 Measurement of electrode dimensions

4.5 Experimental results

Experimental trials of the resistance spot welding were conducted on 2 mm duplex

stainless steel sheets. The effect of welding time and on nugget size is determined

fixing one parameter and varying the other. Thereafter the samples were tested for

various conditions, which are shown in the table 4.1

28

S.No Diameter of the

Electrode (mm)

Welding Current

(kA)

Weld Time

( Weld Cycle )

1 6 7 7

2 6 7 8

3 6 7 9

4 6 8 7

5 6 8 8

6 6 8 9

7 6 9 7

8 6 9 8

9 6 9 9

Table 4.1 Experimental Conditions

4.6 Methodology of the simulation

This section explains the FEA simulation of the RSW process. It requires modelling

of complex interactions between electrical, thermal, metallurgical and mechanical

phenomena. A 2D axisymmetric FEM model has been developed to analyse the

transient thermal behaviors of process using ANSYS software and coupled structural-

thermoelectric analysis is performed by using advanced coupled field element

PLANE223 to simulate the thermal characteristics of RSW process. The objectives of

this analysis is to understand physics of the process and to develop a predictive tool

reducing the number of experiments for the optimization of welding parameters.

29

Fig 4.9 Contact pairs

Various dimensions of the electrode according to the specification are given as

follows:

t1=2 mm -thickness of workpiece,m

r1=12.7 mm -radius of workpiece section modeled

r2=3.13 mm -radius of electrode cavity

r3=6 mm -radius of electrode

r4=3 mm -radius of electrode flat at workpiece interface

h1=9.65 mm -height to electrode cavity from workpiece

h2=12 mm -overall height of electrode section modeled

=20o -taper angle of electrode

FEA models considers temperature dependent material properties, contact status,

phase changing and coupled field effects into the simulation of RSW. To solve the

coupled problem, iterative solution procedure is an often adopted method. Initially the

stress field and contact status are obtained from the thermal-mechanical analysis and

Contact 1 Between top

electrode and top surface

of first workpiece

Contact 2

Faying surfaces

Contact 3 Between

bottom electrode and

bottom surface of second

workpiece

30

then the temperature field is obtained from the fully coupled thermal-electrical

analysis based on the contact area at the electrode-workpiece interface and faying

surface. The calculated temperature field is then passed back to the thermal-structural

analysis to update the stress field and contact status. The objective here is to develop a

multi-coupled method to analyse the thermal and mechanical behaviors of RSW

process.

FEA model is axisymmetric about y axis since only half portion of the complete

model is analysed. The x axis is the contact surface of the two sheets called as faying

surface. The model is meshed using three elements => PLANE223, CONTA172 AND

TARGE169. The element PLANE223 with structural thermoelectric capabilities has

eight nodes with up to four degrees of freedom per node. It has UX, UY, TEMP and

VOLT degrees of freedom. The other elements are contact elements consisting of

contact pair of CONTA172 and TARGE169. Contact occurs when the element

surface penetrates one of the target segment elements (TARGE169) on a specified

target surface. Any translational or rotational displacement, forces, moments,

temperature, voltage and magnetic potential can be imposed on the target segment

element.

Fig 4.10 Simulation Process

31

4.7 Material Properties

Temperature varying properties are considered for copper electrode and duplex

stainless steel sheets. The properties assigned are thermal conductivity, resistivity,

Youngs modulus, specific heat and contact resistivity, density. In modeling RSW

process with the complicated thermoelectric behavior, several physical phenomena

must be considered. It is of great importance to define the parameters correctly to

obtain correct results. The current is imposed as an electric load on the top surface of

upper electrode. A force of 500000N is applied on the upper electrode which is

equivalent to the pneumatic pressure applied on the sheets. The most important

property in the simulation of RSW process is the contact resistivity of faying surface.

4.8 Boundary Condition

Thermo-Electric Analysis:

Figure 4.11 shows the boundary conditions imposed for the analysis. The upper face

of top electrode and lower face of bottom electrode are constrained in x and y

directions. A voltage difference is applied across the top face of upper electrode and

bottom face of lower electrode. The convection coefficient of air (21 W/m2 C) is

applied on faces of electrode and sheet which are open to environment. The

convection coefficient of water (300 W/m2 C) is applied on the inner faces of

electrodes which are in contact with the circulating cooling water with initial

temperature of 10C.

Fig 4.11 Thermal-Electric Boundary Conditions

32

CHAPTER 5

RESULTS AND DISCUSSION

This chapter explains the computed results of the simulation in Ansys to analyze

resistance spot welding. The FEM model is employed to simulate the RSW process in

order to quantitatively understand the effects of the process parameters on temperature

distribution and the nugget size at different cycles.

5.1 Experimental

Different welding conditions are applied to the samples and their corresponding

nugget diameters are obtained as tabulated.

S.No Electrode

Diameter

(mm)

Welding

Current (kA)

Weld Time

(Cycles)

Nugget

Diameter

D1 (mm)

Nugget

Diameter

D2 (mm)

1 6 7 7 4.413 1.983

2 6 7 8 4.884 2.934

3 6 7 9 5.144 3.098

4 6 8 7 5.596 1.970

5 6 8 8 6.230 2.282

6 6 8 9 6.108 2.696

7 6 9 7 5.830 2.856

8 6 9 8 6.258 3.133

9 6 9 9 6.110 3.337

Table 5.1 Experimental results

33

Fig 5.1 Nuggets Observed Under Microscope

5.2 Simulation

First the thermal-electric analysis was carried out by applying the appropriate

boundary conditions to the model and the simulation was carried out for different

welding conditions. Figures 5.5(a) to (d) show the computed temperature fields (in

terms of isotherms) in the sheet-electrode geometry corresponding to different

welding currents and different number of weld cycles. The effective rise in

temperature inside the copper electrode is observed to extend only up to a small

depth.. In Figs 5.2(a) to (d), each colour band represents a range of temperature in deg

C. In particular, the red coloured zone that is above a maximum temperature of 1450

deg C (liquidus temperature of duplex stainless steel) depicts the weld nugget.

Fig 5.2(a) 9 kA,7 weldcycle Fig 5.2(b) 9kA, 8weldcycle

34

Fig 5.2(c) 8kA, 7 weldcycle Fig 5.2 (d) 8kA, 8 weldcycles

At the start of the welding process, the temperature at the center of faying surface

increases very fast. The highest temperature remains at the center of the faying

surface throughout the whole welding process. Melting first occur at the faying

surface and then expand to the material near it. Due to the resistance offered to the

flow of current at the faying surfaces, Joule heat is generated at this surfaces which is

greater than the heat generated at other points on the weld surfaces. The nugget is

formed at 1450 C as it is the melting point of duplex stainless steel. By changing the

welding conditions, the temperature profile could be varied which in turn changes the

nugget size i.e. the welding quality. The following is the simulation for 8kA current

and 8weld cycles :

35

Fig 5.3 Simulation for 8kA and 8 weldcycle

The scale represents the maximum and minmum tempertures obtained at the work

piece with red and blue coloured zones correspoding to them respectively. Here in the

fig 5.3 the minimum and maximum temperatures are 10 deg C and 1892.17 deg C.

However melting occurs at 1450 deg C which corresponds to orange zone. This

determines the diameter of the nugget formed when measured along the coordinate

axis (in Symmetry) in x direction for D1 and Y direction for D2.

36

Fig 5.4 Diameter measurement

To measure the diameter D1 and D2, graphs of Temperature vs Distance is plotted in

x-axis and y-axis respectively. D1 is calculated as twice of R1 calculated from the

graph assuming the melting point of duplex stainless steel as 1450 degC ( as in Fig

5.5). Similarly, D2 is calculated as the difference of 2 points along the y-axis where

this temperature of 1450 deg C is attained.

37

S.No Electrode

Dia(mm)

Curren

t (kA)

Weld

Time

(Cycles)

Experimental Computed %Error

D1 D2 D1 D2 D1 D2

1 6 7 7 2.977 2.76 2.96 2.48 0.54 11.29

2 6 7 8 3.931 2.936 4.2 3.006 6.40 2.32

3 6 7 9 4.677 3.108 5.16 3.282 9.36 5.30

4 6 8 7 5.206 3.048 5.75 2.66 9.46 14.58

5 6 8 8 5.7 3.167 5.81 2.767 1.89 14.45

6 6 8 9 4.966 2.578 5.25 2.818 5.40 8.51

7 6 9 7 4.601 2.873 5.52 3.09 16.6 7.02

8 6 9 8 5.401 2.81 5.7 3.236 5.24 13.16

9 6 9 9 5.534 2.725 6.01 3.122 7.92 12.71

Table 5.2 Experimental Results and error

The stress and strain fields in the weldment during the RSW process are very complex

due to the combination of temperature and electrode force. At the squeeze stage, the

electrodes and work pieces are deformed under the application of the load.

Fig 5.6 shows the Von Mises stress distribution after the squeezing stage. The

maximum stress occurs at the edge of the contact surface between the electrode and

the work piece The welding residual stress is produced in welded joint as a result of

plastic deformation caused by non-uniform thermal expansion and contraction due to

non-uniform temperature distribution in the welding process. The deformation at the

end of hold step is extremely large than that of the squeeze step. This means much

deformation is produced in the RSW process due to the thermal expansion.

38

Fig 5.6 Vonmises Stress

Fig 5.7 y component stresses

Fig.5.6 and 5.7 show the distribution of normal stress y and Von-Mises Stress

during the squeeze step corresponding to a welding current of 9.0 kA and electrode

pressure of 6 Bar and 9 weld cycles . It can be seen in the figure that there was mainly

compressive stress in the contact area, and the maximum Von-Mises stress was about

10.4 MPa at the edge of the electrodeworkpiece interface.

39

CHAPTER 6

SUMMARY AND CONCLUSION

6.1 Summary

Based on finite element method, a numerical model to analyze coupled

thermal- electrical phenomena coupled with structural analysis in resistance

spot welding process has been developed using Ansys.

Important welding parameter such as weld current, welding time, electrode

force, contact resistance and dimensions of the sheet are taken as input in

order to give the output in the form of temperature distribution and stresses at

the weld.

Different welding conditions were taken, keeping one parameter fixed the

other parameter was varied and the nugget dimensions were found.

The results of weld nugget diameters are verified with experimental results for

spot welding of duplex stainless steel sheet and the error is identified.

6.2 Conclusions

A two-dimensional axisymmetric numerical simulation model to analyze the

resistance spot welding process is developed based on finite element method

considering the coupled electrical-thermal and thermal-mechanical phenomena

involved in the process.

The numerical simulation model considers the process parameters which

affect the weld e.g. welding current, electrode force and weld time as input

and computes the current density distribution and the temperature field in the

given sheet-electrode geometry at any instant throughout the weld time.

The weld nugget dimensions are obtained from the computed results of

temperature distribution.

Based on the modelling calculations, it is conceived that the computed values

of weld dimensions are sensitive to a certain extent to the initial value of the

initial contact resistance. It is further realized that the relation of contact

resistance with temperature along with an initial prescribed value is an

effective way to introduce the influence of contact resistance in numerical

modeling of spot welding process.

40

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