Segregation of mica particles in centrifugal and static castings of aluminium/mica composites

Abstract

Mica particles were dispersed in molten Al - 4% Cu - 1.5% Mg alloys and then poured into rotating permanent cast iron moulds to obtain hollow cylindrical castings. The centrifugally cast hollow cylinders showed two distinct zones - (1) an outer zone free from mica and (2) an inner zone, about 12 mm to 22 mm thick - where most of the mica particles had segregated. It was possible to segregate up to 9% mica near the inner periphery by centrifuging whereas it is difficult to obtain more than three percent in static castings. The mica particles near the inner periphery can serve as good solid lubricants when the cylindrical castings are used as bearings even after considerable machining, whereas the mica-free outer zone having nearly the same strength as the matrix alloy would serve as a good backing material. Effects of pouring temperature and particle size on the distribution of mica particles in the hollow cylindrical castings were studied. The mica-free zone width increased with an increase in the particle size of mica and the pouring temperature of the melt. Limited experiments on static casting are reported to establish that mica segregation is mainly due to density difference, and not to rejection by the solidifying front. Theoretical calculations give computed values of mica free zone widths which are in good agreement with those observed during centrifugal casting. © 1981.