Mushy Zone Properties and Castability of Aluminium Foundry Alloys
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NET-N0--7D1 ARNE KRISTIAN DAHLE MO 9 70518 2 MUSHY ZONE PROPERTIES AND CASTABILITY OF ALUMINIUM Arne Kristian Dahle aj£x- fVo— nsi Mushy Zone Properties and Castability of Aluminium Foundry Alloys INSIBiSUTlON OF 1HIB DOCUMENT IS UNL&ffnSD m Thesis submitted to the Norwegian University of Science and Technology (NTNU) in partial fulfilment of the requirements for the degree Doktor Ingenipr. DISCLAIMER Portions of this document may be Illegible in electronic image products. Images are produced from the best available original document " <§Jm Afifimg war, die QdCfpetese" C. Dexter ACKNOWLEDGEMENTS The work presented in this thesis was started in January 1993 and finished in January 1996.1 am very grateful to theNorwegian Research Council, Elkem Aluminium ANS and Hydro Aluminium for funding this work. My advisor, actually for the last four years, has been Professor Lars Amberg. I would like to thank him for his stimulating and energetic support and advice, for providing opportunities, and for listening to my, sometimes, disputable ideas during these years. Stationed at the Department of Metallurgy, at the former Norwegian Institute of Technology, I have gained much insight from discussions with some of my predecessors doing their doctor ’s degree on casting and solidification. I would especially like to mention: Dr. Axel Kolsgaard, Dr. Stian Sannes and Dr. Per Arne Tpndel. Casting and moulding is not a straightforward task, and I am grateful to Mr. Arne Nordmark, Mr. Freddy Syvertsen and Mr. Alf Sandberg for sharing their knowledge and for their practical support. Without them I would have made spirals without a sprue.... I spent two months at the Aluminum Casting Research Laboratory at Worcester Polytechnic Institute, Worcester (MA), between February and May of 1995. The hospitality and involvement of Dr. Alauddin Ahmed, Professor Diran Apelian, Professor Makhlouf M. Makhlouf and Professor Peder C. Pedersen is very much appreciated. In addition to these, I would like to thank all my friends in Worcester for making the time there so enjoyable and memorable. The support of alloys from Fundo a.s., Hpyanger, was also very important, and I would like to thank Mr. Asbjpm Prestmo for providing an inexhaustible source of material and for some interesting discussions. Some material was supplied by Elkem Aluminium Mosjpen. I would like to thank my previous and present office mates, Dr. Egil Trpmborg and Mr. Jan Anders Saster, for providing interesting discussions and friendly surroundings. Dr. Axel Kolsgaard is acknowledged for helping me keeping track of time. - i - All my colleagues and friends at theDepartment of Metallurgy and SINTEF Materials Technology are acknowledged for creating a stimulating and friendly atmosphere. Finally I would like to thank my parents and my brother for their continued support. In addition to the articles included in this thesis, parts have been presented in the following publications: A.K. Dahle and L. Amberg, "Investigation of dendrite coherency in two commercial Al-Si foundry alloys", SINTEF Report, STF34 A93248, December 1993 A.K. Dahle and L. Amberg, "Relationship between dendrite coherency and castability of some commercial foundry alloys", SINTEF Report, STF24 A94558, April 1994 A.K. Dahle and L. Amberg, "Investigation of the Dendrite Coherency Point in Solidifying Al-Si Foundry Alloys", 4’th International Conference on Aluminium Alloys, Atlanta (Ga), 1994, T.H. Sanders Jr. and E.A. Starke Jr. (Eds.), pp. 91- 98 A.K. Dahle and L. Amberg, "Dendrite Coherency in Aluminium Alloys", TMS Materials Week 1994, Rosemont, Chicago (111.), 2-6.0kt. 1994, p. 112 A.K. Dahle, F. Syvertsen, A. Nordmark and L. Amberg, "Relationship Between Rheological Properties of the Mushy Zone and Feeding Mechanisms of Solidifying Aluminium Foundry Alloys", TMS Materials Week 1995, Cleveland (Oh), 29.Okt-2.Nov. 1995, p. 50 L. Amberg, A.K. Dahle, CJ. Paradies and F. Syvertsen, "Factors Affecting the Castability of Aluminium Foundry Alloys", Proceedings from the International Conference on Casting & Solidification of Light Alloys, Institute of Metals and Materials Australasia Ltd, Gold Coast (Qld.), Australia, 30-31 August, 1995, pp. 49-54 A. Ahmed, A.K. Dahle, D. Apelian, L. Amberg and M.Makhlouf, "Modeling of Solidification and Feeding of Solute-Rich Alloys", ACRL Spring Report, - it - Aluminum Casting Reseach Laboratory, Worcester Polytechnic Institute, Worcester(MA), May 1995, pp. 32-57 A.K. Dahle and L. Amberg, "The Effect of Grain Refinement on the Fluidity of Aluminium Alloys", accepted for publication at The 5’th International Conference on Aluminium Alloys, Grenoble- France, 1-5. July 1996 Trondheim, January 1996 Arne Kristian Dahle - m - - IV - ABSTRACT The expected growth in application and market share of aluminium castings is demanding an increased understanding of the mechanisms of defect formation during casting. Casting is a cost-effective production route, but the restricted utilization of castings is often a result of the inadequate reproducibility and quality of the cast structure. Today, much effort is concentrated on the development of a more complete understanding of how to produce the optimum microstructure at an optimum cost, involving the development of advanced computer models. The objective of this thesis was threefold: First to determine how the solidification conditions affect the rheological behavior in the partially solidified state. Second, to measure how alterations in solidification variables influence castability, and third to investigate the relationship between mushy zone rheology and castability. The development of mechanical strength in the mushy zone was measured as a function of chemical composition; content and type of main alloying element, grain refinement and eutectic modification. Measurements of the dendrite coherency point provided accurate determination of the point where the dendrite network is established. The strength measurements confirm that the dendrites are largely independent and free- floating before dendrite coherency. Both the point and rate of strength development in the subsequently established interdendritic network are strongly dependent on the size and morphology of the dendrites and fraction solid. The castability investigation was limited to evaluations of fluidity and feeding. Fluidity measurements showed a complex effect of increased grain refinement; first decreasing and then increasing or stabilizing the flow length. No direct correlation between dendrite coherency and fluidity was found for the effect of grain refinement, since dendrite coherency is continuously postponed with grain refinement. Alterations of the concentration and type of main alloying element gave a direct relationship between mushy zone rheology and fluidity. The range of the operating feeding mechanisms during solidification is directly related to the rheological properties of the mushy zone. Radiographs of cast Al-Si plates indicate that the amount and distribution of porosity may be influenced by burst feeding. The dendritic network collapses when the stresses upon it exceed its strength. - v - The mechanism and importance of burst feeding deserve future attention since it in combination with interdendritic feeding may be the most important feature controlling porosity formation. Some relationship between rheological properties and porosity was detected, but the quantification of porosity is difficult and puts a restriction on the significance of the results. This needs future focus. The data and new knowledge gained in this thesis have subsequently culminated in a comprehensive micro-macro model of shrinkage porosity formation, incorporating the basic principles of solidification and feeding. The model is currently in the state of numerical solution and programming, and no simulations are available at present. - vi - - Vll CONTENTS Acknowledgements i Abstract v Introductory Summary 1 Article #1 25 A.K. Dahle and L. Amberg: "On the Assumption of Additive Effect of Solute Elements in Dendrite Growth", submitted to Metallurgical and Materials Transactions Article #2 43 A.K. Dahle and L. Amberg: "Rheological Properties of Solidifying Aluminum Foundry Alloys", accepted for publication in Journal of Metals(JOM), March, 1996 Article #3 55 A.K. Dahle and L. Amberg: "Development of Strength in Solidifying Aluminum Alloys", submitted to Acta Metallurgica et Materialia Article #4 79 A.K. Dahle, P.A. Tpndel, C.J. Paradies and L. Amberg: "Effect of Grain Refinement on the Fluidity of Two Commercial Al-Si Foundry Alloys", accepted for publication in Metallurgical and Materials Transactions A, 1996 Article #5 97 A.K. Dahle, S. Karlsen and L. Amberg: "Effect of Grain Refinement on the Fluidity of Some Binary Al-Cu and Al-Mg Alloys", accepted for publication in Cast Metals, 1996 - viii - Article #6 121 A. Ahmed, A.K. Dahle, D. Apelian and L. Amberg: "Modeling the Feeding of Aluminum Alloy Castings", accepted for publication in Light Metals 1996, TMS, Warrendale (PA), 1996 Article #7 139 L. Amberg, A.K. Dahle, C.J. Paradies and F. Syvertsen: "Feeding Mechanisms in Aluminum Foundry Alloys", AFS Transactions, vol. 103, 1995, Paper 95-115 also published in Materials Australia, vol. 27, No. 7&8, 1995 Article #8 155 A.K. Dahle, A. Nordmark and L. Amberg: "Measuring Feeding and Porosity in Al-Si Plate Castings", SINTEF Report, STF 24 A96501, January, 1996 » - IX - INTRODUCTORY SUMMARY This introductory summary is intended to put the work reported in the articles comprising this thesis into a wider perspective