© Indian Institute of Technology Delhi (IITD), New Delhi, 2012
INVESTIGATIONS ON FORMABILITY OF AA5182 ALUMINIUM ALLOY IN HYDROFORMING OF
SQUARE CUPS
by
Bharatkumar A. Modi
Department of Mechanical Engineering
Submitted
in fulfillment of the requirements of the degree of Doctor of Philosophy
to the
Indian Institute of Technology Delhi
August 2012
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Dedicated to Lord Swaminarayan,
HDH Pramukh Swami Maharaj and
Mother India
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CERTIFICATE
This is to certify that the thesis entitled “Investigations on Formability of AA5182
Aluminium Alloy in Hydroforming of Square Cups” being submitted by Mr. Bharatkumar Amrutlal Modi 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 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, 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. P.V. Rao, Prof. Puneet Mahajan and Dr. S. Arvindan 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. S. R. Kale, 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 am also thankful to Prof. Michael Worswick, Waterloo University, Canada for providing me with aluminium alloy sheets used in this work.
I would also like to thank the in-charge of Production Engg. lab Mr. Sri Chand Sharma, Mr. Tulsiram 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 while fabricating the tools for experimental work. I want to thank Mr. Subash Chand and Mr. Ram Chandar for their kind support for the testing and measurement work. My special thanks go to my senior research scholars and former post-graduate students Dr. Sushanta Kumar Panda, Mr. Rajvirendra, Mr. Nilesh Gajjar, Mr. Prajeesh and my fellow research scholars Mr. Dhruv Anand, Mr. Vijay Gautam, Ms. Shefali and Mr. Satish Raja for the
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fruitful and productive association with them in my research group. They always came up with constructive suggestions and encouraged me throughout. I express my gratitude toward my dear friends Dr. Kaushik Patel, Dr. Rahul Mulik, Dr. Senthil Kumar, Dr Kaushal Desai, Dr. T. C. Bera, Dr. Sanket Bhavsar, Hitesh Shah, Mitesh Shah, Patil Bhagatsingh and G. Kiran. Due their presence with me here in IIT, my whole stay has been a very pleasant, inspiring and a memorable experience.
I express my unfailing gratitude towards Institute of Technology, Nirma University, Ahmedabad for sponsoring me for research work under Quality Improvement Program. I heartily thank all the Saints of Swaminarayan Akshardham, New Delhi for their support during odd times and my family members (Jyoti, Mansi and Jaineel) for their sacrifice and moral support throughout my research work.
I take this opportunity to thank my all other friends and well wishers who helped me directly or indirectly during this research work.
(BHARATKUMAR A. MODI) IIT DELHI
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ABSTRACT
Importance of lightweight materials like aluminium and magnesium alloys in automobile applications has continuously increased in the recent years due to their high strength to weight ratio and excellent corrosion resistance. Currently, the use of aluminum alloys for sheet metal parts in automobiles is limited due to higher cost and lower formability. Sheet hydroforming, that uses fluid pressure for deformation of a blank, has high potential to manufacture complex auto body components, especially in the case of lower formability materials like aluminium alloys from materials with lower formability due to more uniform strain distribution. Successful production of parts using hydroforming mainly depends on design aspects of tooling as well as control of important process parameters such as closing force or blank holding force and fluid pressure.
In the present work, an experimental set up has been designed and developed for hydroforming of square cups from thin sheet materials. Square cups from 1 mm thick AA5182 aluminium alloy sheets have been deep drawn using the developed experimental set up. The optimum blank shape has been found through Finite Element Analysis. A methodology has been established to determine the variable closing force path for successful hydroforming of the cups with the assistance of programmable logic controller and data acquisition system. It has been found that it is possible to achieve better formability in terms of minimum corner radius and thinning in the case of variable closing force technique than in the case of constant closing force technique.
Influence of process parameters (peak pressure, pressure path and blank holding force) on formability has been investigated through experimental work and numerical
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simulations. The experiments have been designed using Taguchi method. Minimum thickness in the formed cups (usually at of the four bottom corners) and minimum corner radius that can be achieved have been considered as the criteria for evaluation of formability. It has been found that the peak pressure is the most important process parameter affecting thinning and corner radius that can be achieved. The variation of the pressure path has the least effect on formability. Regression models have been developed for prediction of minimum thickness in the cup and corner radius as a function of peak pressure and blank holding force.
Apart from the above process parameters, friction plays an important role in sheet metal forming. Influence of friction at blank-die interface on formability of AA5182 alloy in hydroforming has been studied by using two different lubricants (Tellus-46 oil and Teflon sheet). Higher minimum thickness at the corner (0.91 mm) and lower corner radius (20.4 mm) have been achieved. The material can withstand high peak pressure in lubricated conditions with Teflon.
Stress based forming limit criterion proposed by Stoughton has been used for predicting failure in square cup deep drawing of AA 5182 alloy in hydroforming.
Analytical procedure has been developed to predict forming limits by modifying Stoughton’s stress based criterion using Barlat’s 3-parameter yield function. Results showed that prediction based on stress based criterion agreed more closely with the experiments than the strain based forming limit diagram because of observed change in strain path at the corners.
Key Words: Sheet Hydroforming, Aluminium Alloys, Peak Pressure, Pressure Path, Blank Holding Force, Finite Element Analysis, Forming Limit Diagrams.
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Table of Contents
CERTIFICATE ... ii
ACKNOWLEDGEMENTS ... iii
ABSTRACT ... v
LIST OF FIGURES ... xi
LIST OF TABLES ... xix
NOMENCLATURE ... xxi
ABBREVIATIONS ... xxiii
CHAPTER 1 ... 1
Introduction ... 1
1.1 Conventional Sheet Metal Forming Processes ... 1
1.2 Limitations of Conventional Sheet Metal Forming ... 3
1.3 Importance of Light Weight Materials ... 4
1.4 Hydroforming Processes ... 5
1.5 Sheet Hydroforming Processes ... 7
1.5.1 Hydromechanical deep drawing ... 7
1.5.2 Dieless hydroforming or Hydrodynamic deep drawing ... 8
1.5.3 Punchless hydroforming or High pressure hydroforming ... 8
1.5.4 Advantages with sheet hydroforming ... 9
1.5.5 Applications of sheet hydroforming ... 10
CHAPTER 2 ... 13
Review of Literature and Objectives of the Present Work ... 13
2.1 Mechanics of Deep Drawing by Hydroforming ... 13
2.2 Important Aspects of Sheet Hydroforming ... 15
2.3 Component Geometry and Die Design ... 15
2.4 Process Control ... 18
2.5 Friction at the Tool- Blank Interface ... 21
2.6 Hydroforming Techniques ... 24
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2.7 Modeling of Hydroforming Processes ... 27
2.8 Product Quality ... 28
2.9 Formability of Sheet Metals in Hydroforming ... 29
2.9.1 Factors influencing formability ... 29
2.10 Prediction of Failure in Sheet Metal Forming ... 33
2.10.1 Effect of strain path and pre-strain on FLD ... 36
2.10.2 Stress based FLDs ... 38
2.11 Need for Further Work ... 39
2.12 Objectives of Present Research Work ... 41
CHAPTER 3 ... 43
Design and development of hydroforming set up and experimental procedure ... 43
3.1 Material Characterization ... 43
3.1.1 Tensile Properties ... 43
3.1.2 Anisotropy ... 45
3.2 Forming Limit Diagram ... 47
3.3 Development of Experimental Set Up for Hydroforming of Square Cups ... 49
3.3.1 Die Design ... 50
3.3.2 Design of blank holder ... 51
3.3.3 Design of pressure plate ... 52
3.3.4 Assembly of tools and control system ... 56
3.3.5 Preliminary hydroforming experiments with different materials ... 59
3.4 Closing Force for Sealing ... 59
3.5 Determination of Variable Closing Force Load Path Using PLC-DAQ System61 3.6 Design of Experiments ... 62
3.6.1 Levels of peak pressure ... 63
3.6.2 Pressure path ... 64
3.6.3 Levels of blank holding force (BHF) ... 64
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3.6.4 Contribution of process parameters on minimum thickness and radius
at the corner ... 66
3.7 Measurement of Output and Analysis ... 67
3.7.1 Measurement of blank holding force and fluid pressure ... 67
3.7.2 Strain measurement ... 67
3.7.3 Measurement of thickness variation in hydroformed cups ... 68
3.7.4 Measurement of corner radius of hydroformed cups ... 69
CHAPTER 4 ... 71
Numerical Simulation Methodology ... 71
4.1 Modeling of Tools and Blank ... 72
4.2 Material Model ... 74
4.3 Boundary Conditions ... 76
4.3.1 Contact between tools and blank ... 76
4.3.2 Pressure boundary condition in hydro forming ... 77
4.3.3 Tool motion ... 78
4.3.4 Closing force or Blank holding force ... 79
4.4 Analysis of Results in Post Processor ... 80
4.5 Prediction of Failure in FE Simulations ... 81
Results and Discussions ... 85
CHAPTER 5 ... 87
Mechanical Properties and Forming Limit Diagrams ... 87
5.1 Chemical Composition ... 87
5.2 Tensile Properties of AA5182 Alloy ... 88
5.3 Experimental FLD ... 90
CHAPTER 6 ... 93
Formability in sheet hydroforming ... 93
6.1 Comparison of Closing Force Techniques ... 93
6.2 Effect of Blank Shape ... 100
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6.3 Effect of Process Variables on Formability ... 107
6.4 Failure in Hydroforming of Square Cups ... 118
6.5 Main Effects Plot for Minimum Thickness ... 120
6.5.1 Analysis of variance and regression analysis for minimum thickness123 6.5.2 Main effects plot for radius at the corner ... 124
6.5.3 Analysis of variance and regression analysis for corner radius ... 126
6.6 Confirmation Tests for Maximum Thickness and Minimum Corner Radius . 128 6.7 Effect of Lubrication on Formability ... 130
6.8 Comparison of formability in hydroforming with FE simulation results of conventional deep drawing ... 141
CHAPTER 7 ... 147
Failure prediction with stress based FLD ... 147
7.1 Analytical Procedure To Develop Stress Based FLD ... 147
7.2 Failure Prediction Using Stress Based FLD ... 151
CHAPTER 8 ... 159
Conclusions and Suggestions ... 159
8.1 Conclusions ... 159
8.2 Suggestions for Further Work ... 161
References ... 163
Publications based on the present work ... 175
BIO-DATA ... 177
