investigations on springback in v-bending...
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INVESTIGATIONS ON SPRINGBACK IN
V-BENDING OF TAILOR WELDED BLANKS
VIJAY GAUTAM
DEPARTMENT OF MECHANICAL ENGINEERING
INDIAN INSTITUTE OF TECHNOLOGY DELHI
DECEMBER 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
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
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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
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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
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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
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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.
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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
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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
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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
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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
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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
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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
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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
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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
Fig. 6.7 Microstructure of TWB of C-Mn steel with thickness combination of
1.6mmX0.9mm .................................................................................................. 101
Fig. 6.8 Microstructure of TWB of C-Mn steel with thickness combination of
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
Fig. 6.10 Microstructure of TWB of EDD steel with thickness combination of
1.5mmX0.8mm ................................................................................................ 103
Fig. 6.11 Microstructure of TWB of EDD steel with thickness combination of
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
thickness combinations: (a) 1.2mmX0.8mm, (b) 1.5mmX1.2mm and (c)
1.5mmX0.8mm ................................................................................................ 109
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Fig. 6.16 Microhardness profiles across the weld of TWB specimens of IF steel with
thickness combinations: (a) 0.8mmX0.8mm, (b) 1.2mmX0.8mm and (c)
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. 6.21 Stress-strain curves of C-Mn steel obtained from uniaxial tensile tests at 0°, 45°
and 90° to RD (thickness: 1.2mm) ................................................................. 117
Fig. 6.22 Stress-strain curves of C-Mn steel obtained from uniaxial tensile tests at 0°, 45°
and 90° to RD (thickness: 1.6mm) ................................................................. 117
Fig. 6.23 Stress-strain curves of EDD steel obtained from uniaxial tensile tests at 0°, 45°
and 90° to RD (thickness: 0.8mm) ................................................................. 118
Fig. 6.24 Stress-strain curves of EDD steel obtained from uniaxial tensile tests at 0°, 45°
and 90° to RD (thickness: 1.2mm) ................................................................. 119
Fig. 6.25 Stress-strain curves of EDD steel obtained from uniaxial tensile tests at 0°, 45°
and 90° to RD (thickness: 1.5mm) ................................................................. 119
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
Fig. 6.27 Stress-strain curves of IF steel obtained from uniaxial tensile tests at 0°, 45° and
90° to RD (thickness: 1.2mm) ........................................................................ 120
Fig. 6.28 Stress-strain curves of IF steel obtained from uniaxial tensile tests at 0°, 45° and
90° to RD (thickness: 1.5mm) ........................................................................ 121
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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
Fig. 6.30 ln(true stress)-ln(true strain) plots of C-Mn steel at 0°, 45° and 90° to RD
(thickness: 1.2mm) .......................................................................................... 124
Fig. 6.31 ln(true stress)-ln(true strain) plots of C-Mn steel at 0°, 45° and 90° to RD
(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
Fig. 6.33 ln(true stress)-ln(true strain) plots of EDD steel at 0°, 45° and 90° to RD
(thickness: 1.2mm) .......................................................................................... 126
Fig. 6.34 ln(true stress)-ln(true strain) plots of EDD steel at 0°, 45° and 90° to RD
(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. 6.36 ln(true stress)-ln(true strain) plots of IF steel at 0°, 45° and 90° to RD .......... 127
Fig. 6.37 ln(true stress)-ln(true strain) plots of IF steel at 0°, 45° and 90° to RD .......... 128
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. 6.40 Comparison of true stress-true strain plots of TWBs and weld zone of EDD steel
(1.5mmX1.2mm) at different orientations w.r.t. RD ....................................... 132
Fig. 6.41 Comparison of true stress-true strain plots of TWBs and weld zone of EDD steel
(1.5mmX0.8mm) at different orientations w.r.t. RD ....................................... 133
Fig. 6.42 Comparison of true stress-true strain plots of TWBs and weld zone of IF steel
(0.8mmX0.8mm) at different orientations w.r.t. RD ....................................... 133
xvii
Fig. 6.43 Comparison of true stress-true strain plots of TWBs and weld zone of IF steel
(1.2mmX0.8mm) at different orientations w.r.t. RD ....................................... 134
Fig. 6.44 Comparison of true stress-true strain plots of TWBs and weld zone of IF steel
(1.5mmX0.8mm) at different orientations w.r.t. RD ....................................... 134
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
Fig. 6.46 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
1.6mmX0.9mm ................................................................................................ 139
Fig. 6.47 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
1.6mmX1.2mm ................................................................................................ 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. 7.6 Effect of punch profile radius on springback in bending of 1.2mm thick IF steel
specimens oriented at (a) 0°, (b) 45° and (c) 90° w.r.t. RD ............................... 149
Fig. 7.7 Effect of punch profile radius on springback in bending of 1.5mm thick IF steel
specimens oriented at (a) 0°, (b) 45° and (c) 90° w.r.t. RD ............................... 149
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
Fig. 7.11 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 10mm ................. 153
Fig. 7.12 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 12.5mm ................................................................... 153
Fig. 7.13 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 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
thickness combinations: (a) 1.2mmX0.9mm, (b) 1.6mmX1.2mm and (c)
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
Fig. 7.24 Effect of punch profile radius on springback of TWBs of IF steels
(1.2mmX0.8mm) with weld line oriented at (a) 0° to RD, (b) 45° to RD and (c)
90° to RD ......................................................................................................... 166
Fig. 7.25 Effect of punch profile radius on springback of TWBs of IF steels
(1.5mmX0.8mm) with weld line oriented at (a) 0° to RD, (b) 45° to RD and (c)
90° to RD ......................................................................................................... 167
xx
Fig. 7.26 Variation of longitudinal stress at different points through the thickness (a)
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
Fig. 7.28 Variation of longitudinal stress at different points through the thickness (a)
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
Fig. 7.32 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
12.5mm ............................................................................................................ 173
Fig. 7.33 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
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
Fig. 7.35 Comparison of experimental and predicted punch force vs. displacement curves
of EDD steel sheets of different thicknesses with a punch profile radius of
10mm ............................................................................................................... 178
Fig. 7.36 Comparison of experimental and predicted punch force vs. displacement curves
of IF steel sheets of different thicknesses with a punch profile radius of 10mm
......................................................................................................................... 178
Fig. 7.37 Comparison of experimental and predicted punch force vs. displacement curves
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
punch profile radius of 10mm ......................................................................... 181
Fig. 7.39 Comparison of experimental and predicted variation of punch force with
displacement for TWBs of EDD steel of three thickness combinations with a
punch profile radius of 10mm ......................................................................... 182
Fig. 7.40 Comparison of experimental and predicted variation of punch force with
displacement for TWBs of IF steel of three thickness combinations with a
punch profile radius of 10mm ......................................................................... 182
Fig. 7.41 Comparison of experimental and predicted punch force vs. displacement curves
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)
C12, C22 Anisotropic constants for the blank-2(TWB)
C13, C23 Anisotropic constants for the blank-3(TWB)
R0, R90 Plastic strain ratio parallel to and perpendicular to
rolling direction
F Peak Bending Force