Sedimentation during liquid processing of metal matrix composites

During the incorporation of ceramic particles into metallic alloy melts for the production of metal matrix composites, the particles tend to float or sink, depending on their density. In order to study the rate of hindered settling in these systems, a novel electrical resistance technique has been developed. A current is passed between two electrodes, and the potential drop over a fixed distance is measured with two other electrodes. Experiments were carried out in an aluminum-silicon foundry alloy (A356) containing up to 30 volume percent 82 pm silicon carbide particles. The settling rates were much slower than Stokes Law predicts. The particles' behaviour was compared with hindered settling in aqueous systems. It was found that the particles settle in a non-flocculating manner to approximately 55% after passing through a region of intermediate density. The implications for fabrication and remelting of metal matrix composite material are discussed.