FORMABILITY OF TAILOR-WELDED BLANKS OF LOW-CARBON STEELS IN STRETCH FORMING. Sushanta Kumar Panda

Transcription

1 FORMABILITY OF TAILOR-WELDED BLANKS OF LOW-CARBON STEELS IN STRETCH FORMING by Sushanta Kumar Panda Department of Mechanical Engineering Submitted in fulfillment of the requirements of the degree of Doctor of Philosophy to the Indian Institute of Technology, Delhi July 2007

2 TH bl I.

3 Dedicated to My Parents, Younger Brother Babuki and Teachers

4 CERTIFICATE This is to certify that the thesis entitled "Formability of Tailor-Welded Blanks of Low-Carbon Steels in Stretch Forming" being submitted by Mr. Sushanta Kumar Panda to the Indian Institute of Technology, Delhi (India) for the award of the degree of Doctor of Philosophy in 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. D. Ravi Kumar Associate Professor Department of Mechanical Engineering Indian Institute of Technology Delhi New Delhi i

5 ACKNOWLE DGE ME NTS I would like to convey my deep sense of gratitude and sincere thanks to Dr. D. Ravi Kumar, my research guide, for giving me an opportunity to pursue this research program in IIT Delhi. I have learnt a lot under his guidance and leadership, and through freedom that he gives me to take the initiatives while carrying out this work. Without his guidance, inspiration, constant encouragement and constructive criticisms, timely completion of this thesis was nearly impossible. No amount of words would suffice in return to his favor and cooperation. I am grateful to him in all respects. I express my deep sense of gratitude to Prof. R. Sagar, Dr. P. V. Madhusudan Rao and Prof. R. K. Pandey for being part of my thesis committee. Their questions and valuable suggestions during my presentations and examinations were very useful in giving direction to my research work. I am also very grateful to Prof. Arun Kanda, Head of Department of Mechanical Engineering and to the other faculty members of the department for their kind support in carrying out this research work. I thank Dr A. K. Nath, Director Industrial CO2 Laser Section in Raja Raman Center for Advanced Technology, Indore for extending the laser welding facility to carry out my research work. I will never forget the help of Mr Harish Kumar, Scientific Officer and other staff members for their help during laser welding of all the low carbon steels and for arranging a comfortable stay in the guest house during my visit. I express my thanks to Dr. Debashish Bhattacharjee and his colleagues at the R&D Centre of TATA STEEL LTD., Jamshedpur and BHUSHAN STEEL LTD., Shahibabad for providing the necessary steel sheets of special grades used in the present work. I have pleasure in thanking Dr. Phani Kumar and Dr V.S. Sarma of HT Madras for helping me to get the microhardness readings. ii

6 I would also like to thank the in-charge of Production Engg lab Mr. Sri Chand Sharma and in charge of Sheet Metal Lab Mr L. C. Soni, who have been very generous and very helping towards me in maintaining a suitable atmosphere and in extending all the facilities required. I want to thank Mr Subash Chand and Mr Ayodhya Prasad for his kind support while fabricating the tools for experimental work. I also thank all the staff members of Material Science and Strength of Materials laboratory in the Department of Applied Mechanics. My special thanks goes to my senior research scholars and former post-graduate students Dr Swadesh Kumar Singh, Mr D. V. Raju, Mr Vidya Sagar Reddy, Mr Vikram Patrikar and Mr Vinayak Raut, and my fellow research scholar Mr Bharat Kumar Modi and post graduate student Rajvirendra for the fruitful and productive association with them in my research group. They always came up with constructive suggestions and encouraged me throughout. I express my unfailing gratitude towards my dear friends Bulu Pradhan, Deba Satpathy, Punyadarshini Punam Tripathy, Asok Sethi, Rupraj Mohapatra and my uncle Ramakanta Panigrahi who were always ready to help me when ever I needed. Whether it be formatting of my thesis or preparing a presentation or collecting some useful material for my research, they were always very eager to help me in all possible ways. Due their presence with me here in IIT, my whole stay has been a very pleasant, inspiring and a memorable experience. I take this opportunity to thank my all other friends and well wishers who helped me directly or indirectly during this research work. (SUSHANTA KUMAR PANDA) IIT DELHI iii

7 ABSTRACT The use of new manufacturing concepts and advanced materials is of major interest to manufacturers of automobile parts. The technology of "tailor welded blanks" has gained a lot of importance among most of the major car manufacturers and steel companies worldwide. The term "tailor welded blank (TWB)" refers to a blank where multiple sheets of materials are welded together to create a single blank prior to the forming process. When creating a TWB, designers are able to tailor, hence the name, the location in the blank where specific material properties are desired. TWBs offer significant benefits in terms of reducing manufacturing cost, decreasing vehicle weight and improving part performance. However, this technology presents several challenges due to additional forming issues that arise in stamping of a 'pre-welded blank'. Because of the importance of stretch forming process in making complex stampings for autobody components, formability of three different types of TWBs in biaxial and plane strain stretch forming modes has been studied in the present work. The TWBs are laser-welded samples of low carbon and ultra low carbon steel sheets with difference in thickness, grade and surface conditions. Weld parameters have been optimized to eliminate defects like porosity and lack of penetration. The mechanical properties of the parent sheets have been determined by tensile tests. In the case of TWBs, sub-size longitudinal specimens have been used to determine the tensile properties of the weld zone. In transverse tensile tests, fracture has been found to take place on thinner/weaker side due to non-uniform deformation and early strain localization. The formability of tailor-welded blanks has been studied by conducting limiting dome height (LDH) tests and measuring major and minor strains along and across the weld line in the deformed samples. In TWBs with difference in thickness, the limiting dome height decreases as the thickness ratio increases and the thickness of the thinner side is also crucial. A high thickness ratio causes two major strain peaks on thinner side in biaxial stretch forming of TWBs. Fracture takes place due to strain localization at the peak close to the pole and as the thickness ratio increases, the fracture location shifts towards the weld. The weld ductility and the extent of difference in properties are the two crucial parameters for formability in TWBs with difference in properties. In both these TWBs, the fracture takes place perpendicular to the weld line and propagates towards the stronger side. iv

8 Limiting dome height of the TWBs with coated-uncoated combination is marginally lower than that of both the coated and uncoated sheets. In this type of TWB, lower friction leads to more uniform strain distribution on the coated side as indicated by lower strain gradients and fracture on the uncoated side but away from the weld line. In plane strain stretch forming, which is an important mode of deformation, the effect of weld orientation with respect to major principal strain on formability has been studied. In TWBs with difference in thickness and properties, the LDH is higher in transverse cases when compared to longitudinal cases. The weld properties play a major role in the deformation of longitudinal specimens. But the extent of difference in properties, thickness and surface characteristics have a strong influence on the location of fracture in transverse specimens. The effect of weld line movement on strain distribution has also been analysed. Significant weld line movement occurs towards the thicker/stronger side in biaxial stretch forming of TWBs. The maximum weld line movement occurs at the pole and it increases with increase in thickness ratio and becomes constant beyond a certain thickness ratio. In the case of plane strain stretching of TWBs, the extent of weld line movement is higher in the transverse samples than in the longitudinal samples. Because of the growing importance of application of hydroforming technique to sheet forming, an attempt has been made to improve formability of TWBs with difference in properties (but of same thickness) by application of hydraulic counter pressure in biaxial stretching. The effect of counter pressure and lubrication between the punch and the blank on LDH and strain distribution has been investigated. It has been found that simultaneous application of counter pressure and lubrication leads to higher stretchability and greater uniformity of strain distribution. The weld line movement and the effect of counter pressure and lubrication on variation of load with punch displacement have also been studied in hydro-mechanical stretch forming. Both conventional and hydro-mechanical stretch forming processes have been simulated using a finite element method based software and the results have been found to correlate well with experimental data. Reasons for minor discrepancies have been discussed. Keywords: Tailor Welded Blank, Formability, Stretch Forming, Limiting Dome Height, Strain Distribution, Hydroforming, Finite Element Simulation.

9 TABLE OF CONTENTS CERTIFICATE ACKNOWLEDGEMENTS ABSTRACT TABLE OF CONTENTS LIST OF FIGURES LIST OF TABLES NOMENCLATURE ABBREVIATIONS ii iv vi xx xxi xxiii CHAPTER 1 1 INTRODUCTION 1.1 Sheet metal forming processes Formability of sheet metals Tailor welded blanks and their applications Challenges in application of tailor welded blanks 9 CHAPTER 2 11 REVIEW OF LITERATURE AND OBJECTIVES OF THE PRESENT WORK 2.1 Evaluation of formability of sheet metals Forming limit diagram and strain distribution Factors influencing formability Design variables Process variables Material variables Formability issues in TWBs Effect of weld properties on formability Forming parameters Weld Line Movement in TWBs 28 vi

10 2.5 Application of numerical methods for TWBs Hydroforming and its application to sheet metal forming Hydroforming applied to deep drawing Hydroforming applied to stretch forming Concluding remarks Objectives and scope of the present study 41 CHAPTER 3 43 EXPERIMENTAL PROCEDURE 3.1 Material Selection Laser welding Preparation of tailor welded blanks by laser welding Microstructure and mechanical properties of parent sheets and tailor welded blanks Microstructure and microhardness Erichsen cup test Tensile properties Stretch forming tests Biaxial stretch forming Plane strain stretch forming Hydro-mechanical stretch forming Measurement of LDH, strain distribution and weld line movement 68 CHAPTER 4 69 SIMULATION OF STRETCH FORMING USING FINITE ELEMENT METHOD 4.1 Introduction Modeling of tooling and TWB Material model for tooling and blank Boundary conditions Failure criteria 82 vii

11 CHAPTER 5 87 CHARACTERIZATION OF MICROSTRUCTURE AND MECHANICAL PROPERTIES 5.1 Chemical composition of parent materials Microstructure and microhardness Mechanical properties of parent materials Mechanical properties of tailor welded blanks Erichsen cup test 110 CHAPTER FORMABILITY 6.1 Limiting dome height in biaxial stretching Limiting dome height in plane strain stretching Strain distribution in biaxial stretching TWBs with difference in thickness TWBs with difference in properties TWB of coated-uncoated combination Comparison of strain distribution at equal dome height Strain distribution in plane strain condition Weld line movement Variation of load with displacement 157 CHAPTER HYDRO-MECHANICAL STRETCH FORMING OF TAILOR WELDED BLANKS 7.1 Limiting dome height Strain distribution Weld line movement Variation of peak load and peak pressure 178 viii

12 CHAPTER CONCLUSIONS AND SUGGESTIONS FOR FURTHER WORK 8.1 Conclusions Suggestions for further work 185 REFERENCES 186 APPENDIX PUBLICATIONS BASED ON THE PRESENT WORK 199 BIO-DATA 200 ix