EVOLUTION OF MICROSTRUCTURE AND LOCAL THERMAL CONDITIONS DURING DIRECTIONAL SOLIDIFICATION OF A356-SIC PARTICLE COMPOSITES

Solidification microstructures of aluminium silicon alloy (A-356) cont aining 0, 10, 15 and 20 vol% silicon carbide particles formed during d irectional solidification from a chill have been studied and compared with the structures obtained during solidification of the base alloy u nder similar mould and chill conditions. Columnar dendritic structure was observed during solidification of the base alloy at all distances from the chill. In the case of composites, the presence of silicon car bide particles disturbs the orderly aligned arrangement of dendrites o bserved in the base alloy, under similar solidification conditions, ex cept near the chill surface where a particle-free zone is observed due to probable pushing of particles by the macroscopic solidification fr ont with cell spacings finer than the particle size. During the entire range of solidification conditions studied in this work, the silicon carbide particles are pushed by growing dendrites of alpha-aluminium i nto the last freezing eutectic liquid. The observations on pushing of silicon carbide particles have been examined in relation to existing m odels on particle pushing by planar solidification fronts. Even in the regions away from the chill, where silicon carbide particles are pres ent, there are large regions covering several dendrite arm spacings wh ere there are no particles representing another form of macrosegregati on of particles. It is observed that the secondary dendrite arm spacin gs (DAS) of alpha-aluminium are related to cooling rate by an equation DAS = b(T)(n) for the base alloy as well as for the composite. The co efficient b is generally higher for composites than for base alloy, an d it is found to be a function of particle content. The value of n for the composite is close to the value of the base alloy and is not sign ificantly influenced by the presence of particles. Cooling rate, tempe rature gradients and the rate of advancement of the solidification fro nt have been experimentally measured for the base alloy as well as for the composites during unidirectional solidification. The study indica tes that the presence of particles themselves alters the cooling rates , temperature gradients and growth rate of the macroscopic solidificat ion front under identical thermal surroundings during solidification. The possible influences of these alterations in growth condition on th e solidification microstructure due to the presence of particles are d iscussed together with the other possible direct influences of particl es on dendritic growth of aluminium-silicon alloys.