Modelling and experimental development of the direct thermal method of rheocasting

Rheocasting processes produce a globular (non-dendritic) morphology in the primary phase of a metallic alloy. The authors have developed the direct thermal method (DTM) of obtaining this morphology, using only standard foundry equipment. In the DTM the alloy is poured, at low superheat, into a cylindrical thin walled metallic mould, and by careful selection of mould materials and geometry, a near isothermal situation occurs for sufficient time to render the primary phase globular. No active thermal management is required or used. The sensitivity of DTM rheocasting of aluminium alloy to variables such as mould inner and outer diameter, mould material, heat transfer at the system interfaces, and starting temperatures, is assessed in this paper. This is done through both experiment and computer modelling. It is concluded that steel is a superior mould material than copper for the DTM, both for operational reasons and to enable the casting of larger diameter billets.