Semisolid metal processing

application of shearing during solidification to produce a non-dendritic semisolid slurry that can be Semisolid metal (SSM) processing is a relatively transferred directly into a mould or die to give a final new technology for metal forming. Different from product. Thixoforming is used to describe the near the conventional metal forming technologies which use either solid metals (solid state net shaping of a partially melted non-dendritic alloy processing) or liquid metals (casting) as starting slug within a metal die: thixocasting if in a closed die, materials, SSM processing deals with semisolid or thixoforging if in an open die. (These processes are slurries, in which non-dendritic solid particles are described in more detail below in ‘Technologies for dispersed in a liquid matrix. Semisolid metal component shaping’.) The technology was quickly slurries exhibit distinctive rheological guarded by a series of patents. During the early years, characteristics: the steady state behaviour is many alternative approaches to mechanical stirring pseudoplastic (or shear thinning), while the were suggested to achieve the required microtransient state behaviour is thixotropic. All the structures. These include isothermal coarsening of currently available technologies for SSM solid dendritic materials, mixing of powders, passive processing have been developed based on those unique rheological properties, which in turn mixing, electrical discharge through a semisolid denoriginate from their non-dendritic microstructures. dritic material, electromagnetic stirring, and intensive Year 2001 marks the 30th anniversary of the cold working of initially dendritic solid materials. concept of SSM processing. Today, SSM Those explorations were partly an attempt to circumprocessing has established itself as a scientifically vent a strong MIT patent position, but almost cersound and commercially viable technology for tainly also a result of intense curiosity from the production of metallic components with high foundry world and perceived disadvantages of integrity, improved mechanical properties, mechanical stirring. Of all those suggestions, however, complex shape, and tight dimensional control. only electromagnetic stirring has become a widely Perhaps more importantly, it has demonstrated its accepted method for the production of non-dendritic great potential for further technological development and commercial exploitation. In this feedstock materials on a commercial scale, although paper, progress made on the scientific small scale productions via intensive plastic deformaunderstanding, technological development, and tion are being used for some components today.4 industrial applications of SSM processing are The feasibility of SSM processing of various alloys reviewed. The areas for further research and has been investigated in the past 30 years. Initially, development are also discussed. IMR/335 the primary focus was on high temperature alloys, notably steels, and practically no attention was given © 2002 IoM Communications Ltd and ASM International. to aluminium and magnesium alloys. This was mainly The author is in the Wolfson Centre for Materials Processing, Brunel University, Uxbridge, Middlesex UB8 due to the drive for perfection of steel die casting 3PH, UK ([email protected]). technology for military applications. Semisolid processing was considered to be an effective means to reduce the casting temperature. However, because of the oil crises in the 1970s, and increasing environmenIntroduction tal concerns since the 1980s, automobile market forces Year 2001 marks the 30th anniversary of the concept have been pressing hard for weight reduction using of semisolid metal (SSM) processing. The original high performance light metal parts. As a consequence, experiment leading to the discovery of rheocasting since the 1990s, SSM processing has predominantly was performed in early 1971 by Spenser1 as a part of concentrated on aluminium alloys. In the past decade, his doctorate research under the supervision of SSM processing has experienced intensive research, Professor Flemings at MIT. Spencer was investigating development, and commercialisation. Today, SSM hot tearing during casting of steels. He used Sn–15Pb processing has established itself as a scientifically alloy* as a model system to evaluate the viscosity of sound and commercially viable technology for the partially solidified alloys using a Couette viscometer. production of metallic components with high integIn doing so, he was shearing the dendritic structure, rity, improved mechanical properties, complex shape, although that was not originally intended. He discoand tight dimensional control. Perhaps more importvered that when the dendritic structure is broken up, antly, it has demonstrated a great potential for further the partially solidified alloy has the fluidity of machine technological development and commercial exploitaoil and exhibits thixotropic behaviour.1,2 Flemings tion. and his team immediately recognised the importance Another significant aspect of SSM processing, of the discovery and, by the summer of that year, had which has not been clearly realised in the past, is that performed industrial trials demonstrating the feasibilit marks the first extensive manipulation of the solidification process. Historically, for thousands of years, ity of the two routes, which were termed ‘rheocasting’ since the birth of metal casting, solidification of alloys and ‘thixocasting’.3 In brief, rheocasting involves the of given compositions has been treated as a natural process, with the so called rapid solidification *Alloy compositions are in wt-% unless specified otherwise.