The effect of solidification rate on microsegregation

Abstract

An X-ray diffraction technique has been utilized to determine the second phase content and average composition of the primary phase in aluminum-copper and aluminum-silicon alloys solidified at a cooling rate in the range of 0.06 to l05 K/s. For the Al-Cu samples, the normalized Θ phase content (ratio of the 6 content to the value predicted by the Scheil model) was found to be 0.71 at 0.1 K/s (solidification rate = 0.001 cm/s) and to increase with increasing cooling rate to 0.96 at about 180 K/s (1 cm/s). Beyond this cooling rate, it decreased with increasing the cooling rate to 0.44 at about 3.7 x lO4 K/s. The same trend was observed in the Al-Si samples, except that the normalized silicon content was much lower. Also, for both systems the normalized average composition of the primary phase was found to decrease progressively with increasing the solidification rate until it reached a minimum at 1 cm/s, beyond which it increased with higher solidification rates. The results are discussed with respect to the prevailing segregation models that include back-diffusion in the solid, dendrite tip undercooling, and the eutectic temperature depression. An equation which combines these effects at all cooling rates is given.