Macrosegregation during solidification resulting from density differences in the liquid

Abstract

Macrosegregation has been observed in directionally solidified Pb-20 pct Sn alloys, over a range of freezing rates and temperature gradients. The macrosegregation was shown to result from the upward flow of less dense, tin rich, interdendritic liquid during solidification, using radioactive tracer techniques. For comparison, it was shown that macrosegregation occurred in the opposite direction in a Sn-4 pct Pb alloy, where the interdendritic liquid was lead rich, and consequently more dense. Shrinkage trails and pipes were observed in some of the Pb-20 pct Sn ingots, similar to “freckles” observed in directionally cast superalloys. A mathematical model for macrosegregation in vertically solidified ingots is presented, the driving force being the density differences in the interdendritic liquid during solidification. Liquid flow through the dendritic array is estimated by considering the partially solidified alloy as a porous medium of variable porosity. For simplicity, the model neglects backflow due to volume shrinkage (inverse segregation). The experimental results are compared to the model predictions.