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DR D H KIRKWOOD, THIXOFORGING RESEARCH GROUP, UNIVERSITY OF SHEFFIELD DESCRIBES A SOFT FORGING PROCESS FOR THE NEAR-NET SHAPING OF ALLOYS.
Although it has been described as a 'soft' forging process, semisolid metal forming (SSM) is really a hybrid process possessing some of the advantages of both casting and forging. Initially the material behaves in the semisolid condition like a solid (it can be handled and holds its shape unsupported), but when it enters the die it flows like a liquid to take accurately the die shape as in a die-casting. However, the structure is fine and uniform, virtually free of porosity, and may be heat treated to give mechanical properties superior to castings.
SSM relies on the peculiar flow behaviour of slurries containing non-dendritic solid, which was first studied at MIT by Spencer Mehrabian and Flemings in a tin alloy and published in 1972. It was discovered by these workers diat if the alloy was uniformly stirred during freezing, discrete rounded particles of solid are formed rather than the normal dendrites, and the slurry is highly fluid (like machine oil) up to at least 50% solid.
Furthermore, diese slurries were said to be thixotropoic in that the apparent viscosity is dependent on shear rate and time: the more vigorously it is stirred, die more fluid it becomes; however, on standing without stirring it begins to stiffen and become more...