Raw Material

Abstract

Non-dendritic slurries were originally produced at MIT by mechanical stirring during solidification of the alloy. This started out as a batch process, but was quickly developed into a continuous slurry generator, or rheocaster, in which liquid alloy was passed through an annular channel while being simultaneously cooled and stirred to exit as a semisolid slurry. In the laboratory, these devices were scaled up to operate all the way up to steel processing temperatures and over the course of some 4 years; MIT researchers cast several thousand steel, aluminium and copper alloy parts using both the rheocasting and thixocasting process routes. Later, as commercial organisations became more involved, continuous rheocasters capable of producing hundreds of pounds per hour of aluminium and copper alloy slurry were developed and used to cast large quantities of copper alloy parts or aluminium extrusion billet. However, the complexity of operating multiple mechanical stirring units in commercial foundries and concerns about degradation of the equipment and contamination of the product quickly led to the development of electromagnetic (magneto hydrodynamic or MHD) stirring deep in the sump of a DC casting unit. The attraction of the MHD process was the potential to avoid gas pick up while providing vigorous stirring. This has now become the standard industrial route for the manufacture of SSM bar feedstock for use in thixocasting. The originalMHD process, developed and patented by Olin Corp. and assigned to ITT Inc., involved rotational flow about the vertical axis of a DC caster (Fig. 10.1).