INVESTIGATIONS ON SPRINGBACK IN V-BENDING …eprint.iitd.ac.in/dspace/bitstream/2074/7200/1/TH-5126.pdfi CERTIFICATE This is to certify that the thesis entitled "Investigations on

INVESTIGATIONS ON SPRINGBACK IN

V-BENDING OF TAILOR WELDED BLANKS

VIJAY GAUTAM

DEPARTMENT OF MECHANICAL ENGINEERING

INDIAN INSTITUTE OF TECHNOLOGY DELHI

DECEMBER 2016

©Indian Institute of Technology Delhi (IITD), New Delhi, 2016

INVESTIGATIONS ON SPRINGBACK IN

V-BENDING OF TAILOR WELDED BLANKS

by

VIJAY GAUTAM

Department of Mechanical Engineering

Submitted in fulfillment of the requirements of the degree of Doctor of Philosophy

to the

INDIAN INSTITUTE OF TECHNOLOGY DELHI

DECEMBER 2016

Dedicated

to

my wife Usha for her unconditional love and support

and

my beloved Parents

i

CERTIFICATE

This is to certify that the thesis entitled "Investigations on Springback in V-bending of

Tailor Welded Blanks" being submitted by Mr. Vijay Gautam to the Indian Institute

of Technology Delhi for the award of the degree of Doctor of Philosophy in the

Department of Mechanical Engineering is a bonafide research work carried out by him

under my supervision and guidance. To the best of my knowledge the thesis has reached

the requisite standard. The research reports and the results presented in this thesis have

not been submitted in parts or in full to any other University or Institute for the award of

any degree or diploma.

(Dr. D. Ravi Kumar)

Professor

Department of Mechanical Engineering

Indian Institute of Technology Delhi

New Delhi-110016

iii

ACKNOWLEDGEMENTS

I would like to convey my deep sense of gratitude and sincere thanks to Prof. D. Ravi

Kumar, for giving me an opportunity to pursue this research work under his guidance. I

have learnt a lot from his remarkable acumen, dedication, leadership, focus and

perseverance while carrying out this work without which timely completion of the thesis

would have been nearly impossible. Words are really short to suffice his favour and

cooperation. I am grateful to him in all respects.

I express my deep sense of gratitude to Prof. N. Bhatnagar, Prof. S. Aravindan

and Prof. B. P. Patel for being part of my thesis committee. Their questions and valuable

suggestions during my presentations and examinations have been very useful in providing

direction to my research work. I have been fortunate enough to interact with Prof. K.

Hariharan who gave me valuable suggestions to complete my thesis work. I am also

grateful to Prof. S. K. Saha, Head of Department of Mechanical Engineering and other

faculty members for their kind support in carrying out my research work.

My special thanks to my wonderful friend Dr. Hariharan S. Subramanian for

his constant encouragement and unconditional support. Thanks to my senior and fellow

research scholars Dr. Bharatkumar Modi, Dr. Dhruv Anand, Mr. Satish Raja, Mrs.

Shefali and Mr. Fitsum Taye for their fruitful and productive association that made my

each and every visit to IIT Delhi full of pleasant and memorable experiences. My special

thanks to young budding researchers Mr. Ved Prakash and Mr. Amit Kumar for their

support.

I would like to thank Mr. Ram Chander and Mr. Subhash Chand and Mr

Ayodhya Prasad for their kind support for the experimental work carried out at IIT

iv

Delhi. I would like to express my immense gratitude to Mr. Aalok Vidyarthi, Associate

Vice President, Caparo Maruti Ltd., Bawal, Haryana for laser welding. I would like to

thank Mr. G. Manikandan, Researcher at R & D, Tata Steels, for providing high

strength C-Mn steel sheets.

I express my unfailing gratitude towards Delhi Technological University, Delhi

for allowing me to conduct research work as a part-time scholar. I heartily thank Prof.

Atul Agrawal, Department of Mechanical Engineering, DTU, for his constant and

immeasurable support and encouragement.

I take this opportunity to thank my all other friends especially Dr. Amit Pal and

Dr. Raj Kumar Singh who helped me directly or indirectly during this research work.

Finally, I am highly indebted to my family members for their unconditional love,

support, sacrifice and blessings. Special love to my kids Vibhu and Bhavye, who have

always been there to put a broad smile on my face in difficult times.

Last but not the least, I humbly thank the Almighty for being so kind to me and

pray that everyone is bestowed with opportunities and capabilities to fulfil their dreams.

(VIJAY GAUTAM)

IIT DELHI

v

ABSTRACT

The focus on weight reduction and better crash worthiness has led to significant increase

in applications of Tailor Welded Blanks (TWBs) for manufacturing of automotive sheet

metal parts. A tailor-welded blank is a combination of two or more blanks of different

thickness and/or mechanical properties that have been welded together in a single plane

prior to forming. The concept of combining various options into a welded blank has been

developed to enable design and manufacturing engineers to “tailor” the blank so that

metal’s best properties are located precisely within the part where they are needed. It not

only reduces the weight of the finished part, but also results in better part integration and

improvement in stiffness and crashworthiness, thereby eliminating many reinforcements

and stiffeners.

Tailor welded blanks present several challenges to the design and manufacturing

engineers because of several additional issues that arise due to forming of a pre-welded

blank. Springback phenomenon, usually observed in most of the sheet metal forming

operations, is more complex in the case of TWBs as compared to conventional blanks due

to the differences in material properties and/or thickness and the presence of weld zone.

Accurate prediction of springback in TWBs in a bending operation allows optimum die

design incorporating springback compensation as a corrective measure. It would lead to

optimum selection of material properties, blank design/weld orientation and design

variables to manufacture automotive parts from TWBs with minimum springback. In

view of this, in the present work, an existing analytical method for prediction of

springback in bending of plain sheets has been extended to TWBs considering the effects

of weld zone properties, punch profile radius and anisotropy of parent sheets. Numerical

simulations have also been carried out using a finite element (FE) method based software

to simulate V-bending and predict springback in both parent sheets and TWBs. The

predicted results have been validated with experimental work. TWBs of high strength

vi

C-Mn steel, Extra Deep Drawing (EDD) steel and Interstitial Free (IF) steel sheets of

three different thickness combinations were prepared by Nd-YAG laser welding. The

width of the weld zone was determined from microstructures and microhardness

variations. Tensile properties, strain hardening exponent and anisotropy of the parent

sheets and TWBs were characterised. Tensile properties of the weld zone were

determined from miniaturized specimens of width almost equal to the weld zone width

and were incorporated in the analytical model and the FE simulations. An experimental

setup with three punch profile radii to conduct V-bending experiments was developed for

measurement of springback on a UTM. The predicted results from analytical model and

numerical simulation were found to be in closer agreement with experimental results

when weld zone is considered to consist of three regions (parent sheets and weld zone)

indicating that incorporating the weld zone properties in the material model enhances the

accuracy of springback prediction. In the case of highly anisotropic sheets (EDD and IF

steels), it was found that orientation of the weld line in TWBs to rolling direction is

important and it influences the springback behaviour in bending. With increase in punch

profile radius, springback increased significantly in parent sheets and TWBs of various

thickness combinations. Higher coining force was observed towards the end for a smaller

punch profile radius resulting in lower springback. As the thickness ratio increases,

springback of TWBs decreases and is closer to the springback of the thicker sheets. FE

simulation results are closer to the values of experimental results than those predicted by

the analytical model due to assumption of plane strain condition and neglecting neutral

axis shift in the analytical model and more robust material model in FE simulations.

Keywords: Tailor Welded Blank, Springback, V-bending, Anisotropy, Weld Zone,

Analytical Model, Finite Element Simulation.

vii

TABLE OF CONTENTS

CERTIFICATE ..................................................................................................................... i

ACKNOWLEDGEMENTS ............................................................................................... iii

ABSTRACT ......................................................................................................................... v

TABLE OF CONTENTS ................................................................................................... vii

LIST OF FIGURES ............................................................................................................ xi

LIST OF TABLES ......................................................................................................... xxiii

ABBREVIATIONS ......................................................................................................... xxv

NOMENCLATURE ...................................................................................................... xxvii

CHAPTER 1 INTRODUCTION ......................................................................................... 1

1.1 Sheet metal forming ................................................................................................... 1

1.2 Sheet materials used in automotive applications ........................................................ 8

1.3 Tailor welded blanks ................................................................................................ 11

CHAPTER 2 LITERATURE REVIEW AND OBJECTIVES OF THE WORK .............. 19

2.1 Laser welding techniques to produce TWBs ............................................................ 19

2.2 Characterization of laser welded sheets ................................................................... 26

2.3 Effect of weld zone properties and width on formability of TWBs ......................... 29

2.4 Effect of thickness ratio and strength ratio on formability of TWBs ....................... 32

2.5 Springback in bending .............................................................................................. 34

2.5.1 Springback in conventional blanks .................................................................... 34

viii

2.5.2 Factors influencing springback ......................................................................... 35

2.5.3 Springback in TWBs ......................................................................................... 38

2.6 Need for further work ............................................................................................... 40

2.7 Objectives and scope of the present work ................................................................ 40

CHAPTER 3 DEVELOPMENT OF ANALYTICAL MODEL ........................................ 43

3.1 Analytical model for a normal blank ........................................................................ 43

3.2 Analytical model for a TWB .................................................................................... 49

CHAPTER 4 SPRINGBACK PREDICTION BY FINITE ELEMENT SIMULATION . 53

4.1 About the software ................................................................................................... 55

4.2 Modelling and simulations of bending process ........................................................ 55

4.3 Material model ......................................................................................................... 62

4.4 Contact and boundary conditions in bending simulations ....................................... 65

4.5 Springback simulations and measurement ............................................................... 66

CHAPTER 5 EXPERIMENTAL PROCEDURE .............................................................. 71

5.1 Material selection ..................................................................................................... 71

5.2 Preparation of TWBs ................................................................................................ 72

5.2.1 Laser cutting of blanks ...................................................................................... 73

5.2.2 Laser welding of blanks .................................................................................... 74

5.3 Determination of width of the weld zone ................................................................ 77

5.3.1 Microstructural examination of TWBs.............................................................. 78

5.3.2 Measurement of microhardness ......................................................................... 80

5.4 Tensile properties of parent sheets ........................................................................... 81

ix

5.5 Tensile properties of TWBs ..................................................................................... 83

5.6 Tensile properties of weld zone ............................................................................... 84

5.7 Determination of true stress-strain curves of weld zone by rule of mixtures .......... 86

5.8 Experimental procedure for v-bending and springback measurement ..................... 87

5.6.1 V-bending experiments ..................................................................................... 91

5.6.2 Measurement of springback .............................................................................. 92

CHAPTER 6 RESULTS AND DISCUSSION: MICROSTRUCTURE AND

MECHANICAL PROPERTIES ........................................................................................ 95

6.1 Chemical composition of parent materials ............................................................... 95

6.2 Microstructure and microhardness ........................................................................... 96

6.2.1 Microstructure of parent materials .................................................................... 96

6.2.2 Microstructure of TWBs ................................................................................. 100

6.2.3 Microhardness of TWB samples ..................................................................... 106

6.2.4 Determination of weld zone width .................................................................. 110

6.3 Tensile properties and anisotropy of steel sheets ................................................... 115

6.3.1 Tensile properties ............................................................................................ 115

6.3.2 Anisotropy ....................................................................................................... 128

6.4 Tensile properties of TWBs and weld zone ........................................................... 129

6.5 Tensile properties of TWBS and weld zone of edd and IF steel sheets ................ 131

6.6 Determination of true stress- true strain plots of weld zone by rule of mixtures ... 137

6.7 Summary ................................................................................................................ 140

CHAPTER 7 RESULTS AND DISCUSSION: SPRINGBACK AND BENDING FORCE

.......................................................................................................................................... 143

x

7.1 Springback in V-bending of parent sheets ............................................................. 143

7.1.1 Effect of sheet thickness .................................................................................. 145

7.1.2 Effect of punch profile radius .......................................................................... 147

7.1.3 Effect of anisotropy ......................................................................................... 155

7.2 Springback in TWBs .............................................................................................. 157

7.2.1 Effect of anisotropy ......................................................................................... 159

7.2.2 Effect of weld zone .......................................................................................... 162

7.2.3 Effect of punch profile radius .......................................................................... 164

7.2.4 Effect of thickness ratio ................................................................................... 172

7.3 Bending force ......................................................................................................... 175

7.3.1 Comparison of bending force vs. punch displacement curves for parent sheets175

7.3.2 Comparison of bending force vs. punch displacement curves for TWBs ....... 179

7.3.3 Analytical prediction of peak bending force for TWBs .................................. 183

7.4 Summary ................................................................................................................ 192

CHAPTER 8 CONCLUSIONS AND SUGGESTIONS FOR FURTHER WORK ........ 195

8.1 Conclusions ............................................................................................................ 195

8.2 Suggestions for future work ................................................................................... 198

REFERENCES ................................................................................................................ 201

PUBLICATIONS BASED ON THE PRESENT WORK ............................................... 213

BIO DATA....................................................................................................................... 215

xi

LIST OF FIGURES

Fig. 1.1 Common sheet metal forming operations ............................................................... 3

Fig. 1.2 Common bending operations .................................................................................. 4

Fig. 1.3 Springback in bending showing stress distribution before and after springback

through thickness (Custompartnet.com, 2013) .................................................... 5

Fig. 1.4 Methods of reducing springback in V-bending (Kalpakjian et al., 2008) .............. 7

Fig. 1.5 Different grades of steel with a wide range of mechanical properties ................... 9

Fig. 1.6 Different steps in the fabrication of a door inner using a TWB ........................... 12

Fig. 1.7 Various applications of TWBs ............................................................................. 14

Fig. 2.1 Different modes of laser welding (www.boconline.co.uk) .................................. 21

Fig. 2.2 Schematic arrangement for producing CO2 Laser (Naeem and Brandt, 2005) .... 23

Fig. 2.3 Schematic arrangement for producing Nd:YAG laser (Norrish, 2006) ................ 24

Fig. 2.4 Schematic arrangement of segmented punch and die set with clamps ................. 29

Fig. 2.5 Comparison of (a) elastic recovery in two different steels and (b) springback in

U-bending of two different steels (Billur and Altan, 2012) ................................. 37

Fig. 2.6 Springback seen in formed TWB rails for automobile applications .................... 38

Fig. 3.1 A schematic showing plane strain sheet bending (Hosford, 2007) ...................... 44

Fig. 3.2 Tailor welded blank with bending axis perpendicular to weld line ...................... 49

Fig. 3.3 Measurement of springback (change in included angle) ...................................... 52

Fig. 4.1 Blank surface modelled in FEA (dimensions in mm) .......................................... 58

Fig. 4.2 Punch surface modelled as analytical rigid .......................................................... 59

Fig. 4.3 Die surface modelled as analytical rigid ............................................................... 59

Fig. 4.4 Assembly of tools for bending simulation: (a) conventional blank and (b) TWB 60

Fig. 4.5 A TWB meshed with continuum shell elements .................................................. 60

xii

Fig. 4.6 Simulation of V-bending of TWBs ...................................................................... 62

Fig. 4.7 Initial and final stages in FE simulations of V-bending (a) and (b) unwelded sheet

and (c) and (d) longitudinally welded sheet ........................................................ 66

Fig. 4.8 FE simulation of springback in 0.9mm thick C-Mn steel with a punch profile

radius of 12.5mm ................................................................................................. 67

Fig. 4.9 Surfaces of blank plotted using CAE interface before and after springback ........ 68

Fig. 4.10 FE simulation of springback in bending of 1.2mm thick C-Mn steel with a

punch profile radius of 12.5mm ........................................................................ 68

Fig. 4.11 FE simulation of springback in TWB (1.2mmX0.9mm thickness combination)

of C-Mn steel with a punch profile radius of 12.5mm without considering weld

zone properties .................................................................................................. 69

Fig. 4.12 FE simulation of springback in TWB (1.2mmX0.9mm thickness combination)

of C-Mn steel with a punch profile radius of 12.5mm considering weld zone

properties ........................................................................................................... 69

Fig. 5.1 Laser blanking of sheets using CO2 laser ............................................................. 73

Fig. 5.2 Inside view of Nd:YAG laser system of Make: Oyabe Seiki ............................... 76

Fig. 5.3 Microstructures showing HAZ and fusion zone in a TWB with thickness

combination of 1.2mmX0.8mm ........................................................................... 79

Fig. 5.4 Variation of microhardness across the weld region of a TWB with thickness

combination of 1.2mmX 0.8mm .......................................................................... 81

Fig. 5.5 (a) Laser cutting of full size tensile test specimens and (b) Tensile testing using a

50kN UTM ........................................................................................................ 82

Fig. 5.6 Tested and untested tensile samples of (a) parent material and (b) TWBs .......... 84

Fig. 5.7 Tensile test specimens taken from TWBs with different ..................................... 84

xiii

Fig. 5.8 (a) Dimensions of the miniature tensile specimen taken from the weld zone (in

mm) and (b) actual tensile specimens cut using WEDM .................................. 85

Fig. 5.9 (a) 4-axis,CNC-WEDM and (b) Tensile specimen of weld zone being machined

by WEDM ......................................................................................................... 86

Fig. 5.10 Design of dies and punches for V-bending experiments (all dimensions in mm):

(a)-(c) Dies for a punch profile radius of 10mm, (d) Punch with profile radius

of 10mm, (e)-(g) Dies for a punch profile radius of 12.5mm, (h) Punch with

profile radius of 12.5mm, (i)-(k) Dies for a punch profile radius of 15mm and

(l) Punch with profile radius of 15mm .............................................................. 89

Fig. 5.11 Different sets of dies and punches fabricated for V-bending experiments ......... 90

Fig. 5.12 A V-bending experiment on UTM ..................................................................... 90

Fig. 5.13 Orientation of specimens of TWBs used in bending experiments ..................... 91

Fig. 5.14 Vision inspection machine with probe based measurement in progress ............ 92

Fig. 5.15 Tested EDD steel specimens of different thickness and TWB specimens of

different thickness combinations in V-bending ................................................. 93

Fig. 6.1 Microstructures of C-Mn steel sheets of different thickness: (a) 0.9mm, (b)

1.2mm and (c) 1.6mm .......................................................................................... 97

Fig. 6.2 Microstructures of EDD steel sheets of different thickness: (a) 0.8mm, (b) 1.2mm

and (c) 1.5mm ...................................................................................................... 97

Fig. 6.3 Microstructures of IF steel sheets of different thickness: (a) 0.8mm, (b) 1.2mm

and (c) 1.5mm ...................................................................................................... 97

Fig. 6.4 Average grain size (μm) of steel sheets of different thickness (in mm) ............... 98

Fig. 6.5 Scanning electron micrographs of three grades of steels used in the present work

........................................................................................................................... 99

xiv

Fig. 6.6 Microstructure of TWB of C-Mn steel with thickness combination of

1.2mmX0.9mm showing (a) transition zone between parent sheet and HAZ, (b)

HAZ and (c) fusion zone ................................................................................... 101


1.6mmX0.9mm .................................................................................................. 101


1.6mmX1.2mm .................................................................................................. 102

Fig. 6.9 Microstructure of TWB of EDD steel with thickness combination of

1.2mmX0.8mm showing (a) fusion zone, (b) HAZ and (c) transition zone

between parent sheets and HAZ ........................................................................ 103


1.5mmX0.8mm ................................................................................................ 103


1.5mmX1.2mm ................................................................................................ 104

Fig. 6.12 Microstructure of TWB of IF steel with thickness combination of

1.2mmX0.8mm showing (a) parent material, (b) HAZ and (c) fusion zone ... 105

Fig. 6.13 Microstructure of TWB of IF steel with thickness combination of

0.8mmX0.8mm ................................................................................................ 105

Fig. 6.14 Microhardness profiles across the weld of TWB specimens of C-Mn steel with

thickness combinations: (a) 1.2mmX0.9mm, (b) 1.6mmX0.9mm and (c)

1.6mmX1.2mm ................................................................................................ 108

Fig. 6.15 Microhardness profiles across the weld of TWB specimens of EDD steel with


1.5mmX0.8mm ................................................................................................ 109

xv

Fig. 6.16 Microhardness profiles across the weld of TWB specimens of IF steel with


1.5mmX0.8mm ................................................................................................ 110

Fig. 6.17 Measurement of weld width of TWBs (1.6mmX0.9mm) ................................ 112

Fig. 6.18 Measurement of weld width of TWBs (1.2mmX0.8mm) of EDD steel ........... 113

Fig. 6.19 Measurement of weld width of TWBs (1.2mmX0.8mm) of IF steels .............. 114

Fig. 6.20 Stress-strain curves of C-Mn steel obtained from uniaxial tensile tests at 0°, 45°

and 90° to RD (thickness: 0.9mm) ................................................................. 116





Fig. 6.23 Stress-strain curves of EDD steel obtained from uniaxial tensile tests at 0°, 45°






Fig. 6.26 Stress-strain curves of IF steel obtained from uniaxial tensile tests at 0°, 45° and

90° to RD (thickness: 0.8mm) ........................................................................ 120





xvi

Fig. 6.29 ln(true stress)-ln(true strain) plots of C-Mn steel at 0°, 45° and 90° to RD

(thickness: 0.9mm) .......................................................................................... 124


(thickness: 1.2mm) .......................................................................................... 124


(thickness: 1.6mm) .......................................................................................... 125

Fig. 6.32 ln(true stress)-ln(true strain) plots of EDD steel at 0°, 45° and 90° to RD

(thickness: 0.8mm) .......................................................................................... 125


(thickness: 1.2mm) .......................................................................................... 126


(thickness: 1.5mm) .......................................................................................... 126

Fig. 6.35 ln(true stress)-ln(true strain) plots of IF steel at 0°, 45° and 90° to RD .......... 127



Fig. 6.38 Comparison of true stress-true strain plots of TWBs and weld zone of C-Mn

steel .................................................................................................................. 129

Fig. 6.39 Comparison of true stress-true strain plots of TWBs and weld zone of EDD steel

(1.2mmX0.8mm) at different orientations w.r.t. RD ....................................... 132





Fig. 6.42 Comparison of true stress-true strain plots of TWBs and weld zone of IF steel


xvii





Fig. 6.45 Comparison of true stress-true strain plots obtained from ROM and tensile tests

of TWBs and weld zone of C-Mn steel for thickness combination of

1.2mmX0.9mm ................................................................................................ 138



1.6mmX0.9mm ................................................................................................ 139



1.6mmX1.2mm ................................................................................................ 139

Fig. 6.48 Comparison of true stress-true strain plots of C-Mn steel obtained from

experiments and ROM for different weld widths ............................................ 140

Fig. 7.1 Effect of sheet thickness on springback in bending of C-Mn steel with punch

profile radius (a) 7.5mm, (b) 10mm and (c) 12.5mm ........................................ 145

Fig. 7.2 Effect of sheet thickness on springback in bending of EDD steel with punch

profile radius 12.5mm and specimen oriented at (a) 0°, (b) 45° and (c) 90° w.r.t.

RD ...................................................................................................................... 146

Fig. 7.3 Effect of sheet thickness on springback in bending of IF steel with punch profile

radius 12.5mm and specimen oriented at (a) 0°, (b) 45° and (c) 90° w.r.t. RD 146

Fig. 7.4 Effect of punch profile radius on springback in bending of C-Mn steel ............ 148

Fig. 7.5 Effect of punch profile radius on springback in bending of 0.8mm thick IF steel

specimens oriented at (a) 0°, (b) 45° and (c) 90° w.r.t. RD ............................... 148

xviii





Fig. 7.8 Variation of longitudinal stress at different points through the thickness (a) before

and (b) after springback across the width in a 0.9mm thick specimen for a punch

profile radius of 7.5mm ..................................................................................... 151

Fig. 7.9 Contours of longitudinal stress (a) before and (b) after springback on the outer

surface of 0.9mm thick sheet for a punch profile radius of 7.5mm ................... 152

Fig. 7.10 Variation of longitudinal stress at different points through the thickness (a)

before and (b) after springback across the width in a 0.9mm thick specimen for

a punch profile radius of 10mm ...................................................................... 152


surface of 0.9mm thick sheet for a punch profile radius of 10mm ................. 153


before and (b) after springback across the width in a 0.9mm thick specimen for

a punch profile radius of 12.5mm ................................................................... 153


surface of 0.9mm thick sheet for a punch profile radius of 12.5mm .............. 154

Fig. 7.14 Variation of longitudinal stress before and after springback at the outer surface

along the length in 0.9mm thick sheet for different punch profile radii ......... 155

Fig. 7.15 Effect of anisotropy on springback in bending of EDD steel with punch profile

radius of 12.5mm for (a) 0.8mm, (b) 1.2mm and (c) 1.5mm thick sheets ...... 156

Fig. 7.16 Effect of anisotropy on springback in bending of IF steel with punch profile

radius of 12.5mm for (a) 0.8mm, (b) 1.2mm and (c) 1.5mm thick sheets ...... 156

xix

Fig. 7.17 Effect of anisotropy on springback in bending of IF steel with punch profile

radius of 15mm for (a) 0.8mm, (b) 1.2mm and (c) 1.5mm thick sheets ......... 157

Fig. 7.18 Effect of anisotropy on springback of TWBs of EDD steel with punch profile

radius of 12.5mm for different thickness combinations: (a) 1.2mmX0.8mm, (b)

1.5mmX1.2mm and (c) 1.5mmX0.8mm ......................................................... 161

Fig. 7.19 Effect of anisotropy on springback of TWBs of IF steel with punch profile

radius of 12.5mm for different thickness combinations: (a) 0.8mmX0.8mm, (b)

1.2mmX0.8mm and (c) 1.5mmX0.8mm ......................................................... 161

Fig. 7.20 Effect of anisotropy on springback of TWBs of IF steel with punch profile

radius of 15mm for different thickness combinations: (a) 0.8mmX0.8mm, (b)

1.2mmX0.8mm and (c) 1.5mmX0.8mm ......................................................... 162

Fig. 7.21 FE simulation of V-bending showing springback in TWBs considering weld

zone ................................................................................................................. 163

Fig. 7.22 Effect of punch profile radius on springback of TWBs of C-Mn steel with


1.6mmX0.9mm ................................................................................................ 165

Fig. 7.23 Effect of punch profile radius on springback of TWBs of IF steels

(0.8mmX0.8mm) with weld line oriented at (a) 0° to RD, (b) 45° to RD and (c)

90° to RD ......................................................................................................... 166



90° to RD ......................................................................................................... 166



90° to RD ......................................................................................................... 167

xx


before and (b) after springback across width of a TWB (1.6mmX0.9mm) for a

punch profile radius of 7.5mm ........................................................................ 169

Fig. 7.27 Contours of longitudinal stress (a) before and (b) after springback at the outer

surface of a TWB (1.6mmX0.9mm) for a punch profile radius of 7.5mm ..... 169


before and (b) after springback across width of a TWB (1.6mmX0.9mm) for a

punch profile radius of 10mm ......................................................................... 170

Fig. 7.29 Contours of longitudinal stress (a) before and (b) after springback at the outer

surface of a TWB (1.6mmX0.9mm) for a punch profile radius of 10mm ...... 170

Fig. 7.30 Variation of longitudinal stress before and after springback at the outer surface

along the length in a TWB (1.6mmX0.9mm) for different punch profile radii

......................................................................................................................... 171

Fig. 7.31 Effect of thickness ratio on springback of TWBs of IF steel with weld

orientation at (a) 0°, (b) 45° and (c) 90° to RD for a punch profile radius of

10mm ............................................................................................................... 173



12.5mm ............................................................................................................ 173



15mm ............................................................................................................... 174

Fig. 7.34 Comparison of experimental and predicted punch force vs. displacement curves

of C-Mn steel sheets of different thicknesses with a punch profile radius of

10mm ............................................................................................................... 177

xxi


of EDD steel sheets of different thicknesses with a punch profile radius of

10mm ............................................................................................................... 178


of IF steel sheets of different thicknesses with a punch profile radius of 10mm

......................................................................................................................... 178


of 0.9mm thick C-Mn steel sheets with different punch profile radius ........... 179

Fig. 7.38 Comparison of experimental and predicted variation of punch force with

displacement for TWBs of C-Mn steel of three thickness combinations with a



displacement for TWBs of EDD steel of three thickness combinations with a



displacement for TWBs of IF steel of three thickness combinations with a



of TWB (1.6mmX0.9mm) of C-Mn steel in bending with different punch

profile radii ...................................................................................................... 183

xxiii

LIST OF TABLES

Table 4.1 Details of blank mesh used in FE simulations ................................................... 61

Table 4.2 Yield stress ratios used in FEA to incorporate anisotropy of EDD steels ......... 64

Table 4.3 Yield stress ratios used in FEA to incorporate anisotropy of IF steels .............. 64

Table 4.4 Captured coordinates from loaded and unloaded frames for 0.9mm thick sheet

............................................................................................................................................ 68

Table 5.1 Technical specifications of resonator TruDisk 4002 ......................................... 74

Table 5.2 Thickness combinations used in preparation of TWBs ..................................... 77

Table 6.1 Chemical composition of selected materials (in wt%) ...................................... 95

Table 6.2 Width of weld zone measured in TWBs .......................................................... 114

Table 6.3 Tensile properties of C-Mn steel sheets ........................................................... 118

Table 6.4 Tensile properties of EDD steel sheets ............................................................ 122

Table 6.5 Tensile properties of IF steel sheets ................................................................. 122

Table 6.6 Tensile properties of TWBs of C-Mn steel with weld orientation .................. 130

Table 6.7 Tensile properties of weld zone of TWBs of C-Mn steel with weld orientation

transverse to RD .............................................................................................. 131

Table 6.8 Tensile properties of TWBs of EDD steel ....................................................... 135

Table 6.9 Tensile properties of weld zone of TWBs of EDD steel ................................ 136

Table 6.10 Tensile properties of TWBs of IF steel .......................................................... 136

Table 6.11 Tensile properties of the weld zone in TWBs of IF steel .............................. 136

Table 7.1 Springback in V- bending of C-Mn steel specimens ....................................... 144

Table 7.2 Springback in V- bending of EDD steel specimens ........................................ 144

Table 7.3 Springback in V- bending of IF steel specimens ............................................. 144

xxiv

Table 7.4 Springback in bending of TWBs of C-Mn steel with and without weld

properties with weld line orientation of 90° w.r.t. RD .................................... 158

Table 7.5 Springback in bending of TWBs of EDD steel with and without weld properties

.......................................................................................................................................... 158

Table 7.6 Springback of TWBs of IF steel with and without weld properties ................ 159

Table 7.7 Bending force for C-Mn steel .......................................................................... 186

Table 7.8 Bending force for EDD steel ........................................................................... 186

Table 7.9 Bending force for IF steel ................................................................................ 187

Table 7.10 Bending force for TWBs of C-Mn steel ........................................................ 189

Table 7.11 Bending force for TWBs of EDD steel .......................................................... 190

Table 7.12 Bending force for TWBs of IF steel .............................................................. 191

xxv

ABBREVIATIONS

AHSS Advanced High Strength Steel

AISI American Iron and Steel Institute

ASTM American Society for Testing and Materials

BIW Body In White

CAE Computer Aided Engineering

CAFE Corporate Average Fuel Economy

CNC Computer Numerical Control

CRCA Cold Rolled Close Annealed

CP Complex Phase

DD Deep Drawing

DP Dual Phase

EDD Extra Deep Drawing

FEA Finite Element Analysis

FEM Finite Element Method

HAZ Heat Affected Zone

HSLA High Strength Low Alloy

IF Interstitial Free

IFHS Interstitial Free High Strength

Nd:YAG Neodymium: Yttrium Aluminium Garnet

PPR Punch Profile radius

RD Rolling Direction

ROM Rule of Mixtures

RSM Response Surface Methodology

SEM Scanning Electron Microscope

SUV Sports Utility Vehicle

TRIP Transformation Induced Plasticity

TWB Tailor Welded Blank

TWIP Twinning Induced Plasticity

xxvi

UTM Universal Testing Machine

UTS Ultimate Tensile Strength

VHN Vickers Hardness Number

WEDM Wire-Cut Electrical Discharge Machining

XRD X-Ray Diffraction

Yb:YAG Ytterbium: Yttrium Aluminium Garnet

YS Yield Strength

xxvii

NOMENCLATURE

Elemental bending force along x-axis

dz Thickness of the element

σx(elastic), σx(plastic) Bending stress along x-axis in elastic region and plastic region

σox Yield stress along x-axis

σo' Yield stress in plane strain condition

w, w1, w2, w3 Width of the conventional blank, blank-1(TWB), blank-2(TWB)

and weld zone (TWB) respectively

t, t1, t2, t3 Thickness of the conventional blank, blank-1(TWB),

blank-2(TWB) and weld zone (TWB) respectively

r, r' Initial and final radii of curvature

z Distance of the element from neutral plane

ze, ze1, ze2, ze3 Distance of the elastic core from neutral plane for conventional

blank, blank-1(TWB), blank-2(TWB) and weld zone (TWB)

respectively

Ԑx True strain along x-axis

Ԑox Strain at yield along x-axis (parallel to rolling direction)

n, n1, n2, n3 Strain hardening exponent for conventional blank, blank-1(TWB),

blank-2 (TWB) and weld zone (TWB) respectively

E', E1', E2', E3' Elastic modulus in plane strain condition for conventional blank,

blank-1(TWB), blank-2(TWB) and weld zone (TWB) respectively

K', K1' , K2', K3' Strength coefficient in plane strain condition for conventional

blank, blank-1(TWB), blank-2(TWB) and weld zone (TWB)

respectively

M, M1, M2, M3 Bending moment for the conventional blank, blank-1(TWB),

blank-2 (TWB) and weld zone (TWB) respectively

C1, C2 Anisotropic constants for the conventional blank

C11, C21 Anisotropic constants for the blank-1(TWB)



R0, R90 Plastic strain ratio parallel to and perpendicular to

rolling direction

F Peak Bending Force

xxviii

θ Included bend angle

θo, θf Initial and final included angle

αi , αf Initial and final bend angle

Rd, Rp Die and punch corner radius

Wd Width of die opening

Q Die opening factor

Ks Springback ratio

L Width of the bend specimen

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