EFFECT OF COOLING RATE ON THE SOLIDIFICATION BEHAVIOR OF AL-7 PCT SI-SICP METAL-MATRIX COMPOSITES

The solidification behavior of two composites based on Al-Si alloy has been investigated as a function of cooling rate. Thermal analysis techniques have been used to establish the relationship between solidification history and the microstructure developed. The results of thermal analysis show that the characteristic parameters are influenced by the cooling rate. A marked difference in these parameters is observed between the reinforced and the unreinforced materials at all cooling rates studied. The cooling rates used in the present study range from 0.3 to 20 K/s. Increasing the cooling rate is shown to affect the undercooling parameters both in the liquidus and eutectic solidification region. The eutectic growth temperature of the composites is observed to be higher than that of the base alloy at all cooling rates. The depression in eutectic temperature DELTAT is found to decrease by 27 K for the unreinforced alloy (A356) and by 17 K for the composites (A356 + 10, 20 vol pct SiC) at a higher cooling rate of congruent-to 16 K/s. The presence of SiC reinforcement is observed to suppress the Mg2Si PreciPitate formation and decrease the amount of heat liberated during both primary and eutectic phase formation. Dendrite arm spacing (DAS) is correlated to the cooling rate by a relationship of the form DAS = A(T)-n, where n is found to be of the order of 0.33.