Microstructural Evolution of Electromagnetically Stirred Feedstock SSM Billets During Reheating Process
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TECHNICAL ARTICLE
Microstructural Evolution of Electromagnetically Stirred Feedstock SSM Billets During Reheating Process S. Nafisi • R. Ghomashchi
Received: 6 January 2013 / Revised: 11 February 2013 / Accepted: 21 February 2013 / Published online: 8 March 2013 Springer Science+Business Media New York and ASM International 2013
Abstract Semi-solid metal (SSM) processing is an effective alternative to the classical manufacturing processes of casting and forging. The key issue in SSM is the production of suitable feedstock with guaranteed continuous supply. To produce good quality billets, the current work studied the thixocastability of electromagnetically stirred (EMS) Al–7Si alloy billets and compared them with conventionally cast billets to establish the effect of EMS in improving the quality of feedstock. In all cases, the intention was to study the microstructure of the billets just before feeding it into the high pressure die casting machines and therefore the billets were reheated to thixocasting temperature and isothermally held for 10 min before being quenched in water. The outcome of EMS application was the formation of refined and more globular SSM billet structures with less entrapped eutectic. The implication of pursuing such work is to guarantee continuous supply of feedstock to enable the establishment of manufacturing facilities, similar to mini mills for steel industry, where the feed stock is shipped to casting site to be shaped in high pressure die casting machines. Keywords
Stirring EMS Thixocasting Superheat
S. Nafisi (&) EVRAZ INC. NA, P.O. Box 1670, Regina, SK S4P 3C7, Canada e-mail: [email protected] R. Ghomashchi School of Mechanical Engineering, The University of Adelaide, Adelaide, SA 5005, Australia e-mail: [email protected]
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Introduction The concept of semi-solid metal (SSM) forming is over 40 years old. The original experiment leading to the invention of semi solid processing of metals was performed in early 1971, as part of a doctoral thesis at MIT, USA [1]. Since then, a large number of investigations have been carried out to both understand the process scientifically [e.g., 2–4] and find ways to implement the concept industrially [e.g., 5–10]. Basically, the process of SSM casting is carried out directly from the melt in one of two methods. In the rheocasting process, the slurry (liquid and solid mixtures) is prepared by stirring the superheated molten metal as it cools down to the mushy zone (the region between liquidus and solidus on phase diagram). In the thixocasting process, reheating of the alloy to temperatures above solidus and holding it isothermally within the mushy zone induces the desired structure [11]. One of the important issues in deciding whether to switch to one specific SSM fabrication route is the supply of feedstock. The term slurry-on-demand (SoD) describes the slurry-making operations to provide a constant supply of slurry for shaping operations. This may include the billets (slugs) produced via rheocasting, stored and used
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