Hot Tearing in Cast Aluminum Alloys: Measures and Effects of Process Variables by Shimin Li A Dissertation Submitted to the Faculty of the WORCESTER POLYTECHNIC INSTITUTE In partial fulfillment of the requirements of the Degree of Doctor of Philosophy in Materials Science and Engineering April 2010 APPROVED: _______________________________ Dr. Diran Apelian, Advisor Howmet Professor of Engineering _______________________________ Dr. Richard D. Sisson Jr. George F. Fuller Professor Director of Manufacturing and Materials Engineering ABSTRACT Hot tearing is a common and severe defect encountered in alloy castings and perhaps the pivotal issue defining an alloy‟s castability. Once it occurs, the casting has to be repaired or scraped, resulting in significant loss. Over the years many theories and models have been proposed and accordingly many tests have been developed. Unfortunately many of the tests that have been proposed are qualitative in nature; meanwhile, many of the prediction models are not satisfactory as they lack quantitative information, data and knowledge base. The need exists for a reliable and robust quantitative test to evaluate/characterize hot tearing in cast alloys. This work focused on developing an advanced test method and using it to study hot tearing in cast aluminum alloys. The objectives were to: 1) develop a reliable experimental methodology/setup to quantitatively measure and characterize hot tearing; and 2) quantify the mechanistic contributions of the process variables and investigate their effects on hot tearing tendency. The team at MPI in USA and CANMET-MTL in Canada has collaborated and developed such a testing setup. It consists mainly of a constrained rod mold and the load/displacement and temperature measuring system, which gives quantitative, simultaneous measurements of the real-time contraction force/displacement and temperature during solidification of casting. The data provide information about hot tearing formation and solidification characteristics, from which their quantitative relations are derived. Quantitative information such as tensile coherency, incipient crack refilling, crack initiation and propagation can be obtained. The method proves to be repeatable and reliable and has been used for studying the effects of various parameters (mold temperature, pouring temperature and grain refinement) on hot tearing of different cast aluminum alloys. In scientific sense this method can be used to study and reveal the nature of the hot tearing, for industry practice it provides a tool for production control. Moreover, the quantitative data and fundamental knowledge gained in this thesis can be used for validating and improving the existing hot tearing models. I ACKNOWLEDGEMENTS First and foremost I want to heartily thank my advisor Professor Diran Apelian for his guidance, stimulating advice, encouragement and patience during my research and study. My research life at WPI became a wonderful journey with a great mentor. My sincere thanks go to my thesis committee members Professor Richard D. Sisson Jr., Professor Makhlouf M. Makhlouf and Professor Diana Lados for encouragement, critical comments and stimulus questions. I thank my class professors Professor Satya S. Shivkumar, Yiming Rong, Jianyu Liang, Isa Bar-On and Mahadevan Padmanabhan for teaching me fundamental knowledge. I‟d like to gratefully acknowledge the member companies of the Advanced Casting Research Center (ACRC) to fund this project, especially the focus group and the group co-chairs John Jorstad and Ray Donahue, for their support and input. The support of CANMET Material Technology Laboratory is very important to this project. The hospitality of the lab during my six weeks stay there is very much appreciated. Special thanks go to Dr. Kumar Sadayappan for his guidance, valuable discussions and continuous support, as well as a member of my thesis committee. I would also like to thank Geethe Nadugala, Jim Thomson and other people involved in this project for their help with the initial trials conducted there. My fellow MPI groupmembers, Dr. Kimon Symeonidis, Dr. Brian Dewhirst, Dr. Daniel Backman, Ning Sun, Cecelia Borgonovo and Hao Yu made this group a source of friendship as well as good advice and collaboration. I‟m very grateful for their continuous encouragement and benefit a lot from those stimulating discussions and sharing ideas. Dr. Libo Wang deserves a special thank for his assistance in experiments and invaluable discussions. I greatly appreciate our department secretary Rita Shilansky, and MPI staff Carol Garofoli, Maureen Plunkett, Carl Raatikanen, and Bradford Lynch for their help. I thank all of those who supported me in any respect during the completion of my thesis. Finally, I owe my deepest gratitude to my family. My parents tried their best to support me in all my pursuits. My sisters and brother always give me love and encouragement. My husband Yonggang Zheng always stands beside me and encourages me. Thank you for your love and belief in me! II TABLE OF CONTENTS ABSTRACT ................................................................................................................................... I ACKNOWLEDGEMENTS ....................................................................................................... II TABLE OF CONTENTS .......................................................................................................... III EXECUTIVE SUMMARY ...........................................................................................................1 Introduction ..............................................................................................................................1 Objectives .................................................................................................................................1 Methodology ............................................................................................................................2 Outcomes/Conclusions .............................................................................................................3 CHAPTER I: Hot Tearing of Aluminum Alloys – A Critical Literature Review .....................6 CHAPTER II: Characterization of Hot Tearing in Cast Al Alloys: Methodology and Procedures ...........................................................................43 CHAPTER III: Why Some Al Alloys Hot Tear and others do not? – Part I: Effects of Mold Temperature and Pouring Temperature ...................61 CHAPTER IV: Why Some Al Alloys Hot Tear and others do not? – Part II: The Role of Grain Refinement ..........................................................87 APPENDICES ...........................................................................................................................102 APPENDIX A: Quantitative Investigation of Hot Tearing of Al-Cu Alloy (206) Cast in a Constrained Bar Permanent Mold ...............................................................102 APPENDIX B: Castability Measures for Diecasting Alloys: Fluidity, Hot Tearing, and Die Soldering .....................................................................................................110 III EXECUTIVE SUMMARY Introduction Hot tearing is a common and severe defect encountered in alloy castings and perhaps the pivotal issue defining an alloy‟s castability. It is identified as cracks, either on the surface or inside the casting. The main tear and its numerous minor offshoots generally follow intergranular paths and the failure surface usually reveals a dendritic morphology. Once hot tearing occurs, the casting has to be repaired or scraped, resulting in significant loss. Previous work by many researchers show that hot tearing is a complex phenomenon, it lies at the intersection of heat flow, fluid flow and mass flow, and various factors have influences on its formation. These factors include alloy composition, its solidification and thermo-mechanical characteristics, melt treatment, casting and mold design, mold material, and process parameters etc. Over the years many theories and models have been proposed and accordingly many tests have been developed. Unfortunately the tests that have been proposed are generally qualitative in nature; meanwhile, many of the prediction models are not satisfactory as they lack quantitative information, data and knowledge base. The need exists for a reliable and robust quantitative test to evaluate/ characterize hot tearing in cast alloys. WPI and CANMET - both members of the Light Metal Alliance - joined forces to address this need and to develop a quantitative means of measuring hot tearing. The premise is that one cannot control what one cannot measure. Objectives This work focused on developing a quantitative test method and subsequently using it to study hot tearing in cast aluminum alloys. The objectives were to: . Develop a reliable experimental methodology/setup to quantitatively measure and characterize hot tearing. Quantify the mechanistic contributions of process variables; investigate the effects of the variables on hot tearing tendency. 1 Methodology In order to achieve the above objectives, the following methodology and strategies were pursued: . Conducted an extensive and critical literature review; scrutinized the proposed mechanisms and identified the major factors which control the formation of hot tears. Phase I – Developed a reliable experimental methodology/setup to quantitatively characterize hot tearing in cast aluminum alloys. Established collaboration