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Rheocasting

1976; Elsevier BV; Volume: 25; Linguagem: Inglês

10.1016/0025-5416(76)90057-4

1873-4928

M. C. Flemings, R. G. Riek, Kenneth P. Young,

Aluminum Alloys Composites Properties

This paper reviews what we know, and what we do not know, about the rheology of semi-solid metals, and about practical, metal-forming processes (Rheocasting Processes) which utilize semi-solid metals. The unique innovation which makes the semi-solid forming process possible is vigorous agitation of metals during the first fifty percent. or so of solidification. This produces a highly fluid slurry of solid spheroids dispersed in liquid. Our fundamental understanding of the mechanisms of formation and deformation of these slurries is as yet limited. However, it is known that the size of the solid spheroids and the viscosity of a slurry are functions of the process variables (e.g., shear and cooling rates) employed. Furthermore, it is known that such slurries are thixotropic, with a viscosity that decreases sharply with increasing shear rate. Rheocast slurries have been produced of a variety of alloys including aluminum alloys, bronzes, cast iron, stainless steel and super-alloys. Extensive research has been carried out to develop an economic machine (die) casting process which will utilize the semi-solid slurries as charge material. Potential practical advantages of such a system include (a) long die- and shot-chamber life, (b) sound, reliable casting, (c) ability to achieve precise, automated metal metering to the die-casting machine. A die-casting system has been developed which utilizes the thixotropic nature of the slurries and so is called "Thixocasting". The system combines a continuous slurry producer to provide Rheocast ingots, a reheating furnace, a "Softness Indicator" to determine when individual reheated charges are suitable for casting, and a commercial, cold chamber die-casting machine. A pilot plant scale system is currently operating, casting semi-solid stainless steel. At the present time, there is insufficient production experience to determine the extent to which the practical advantages of the process can be translated into economic and technical gains in the marketplace. Other potential applications of the Rheocast process are discussed. For example, non-metallic materials can be added to Rheocast slurries to produce composite material (Compocasting) and, we believe, continuous castings can be produced by the process that are more homogeneous, sounder, and of finer grain size than conventional ingots. So far we may only speculate that the technical advantages of these other applications of Rheocasting will be sufficient to lead to commercial application in the near future.