A numerical study of the combined effects of microsegregation, mushy zone permeability and fllow, caused by volume contraction and thermosolutal convection, on macrosegregation and eutectic formation in binary alloy solidification

Abstract

Numerical simulations of the columnar dendritic solidification of a Pb-20 wt% Sn alloy in a square cavity cooled from one side and fed by a rectangular riser are reported. Overall macrosegregation patterns predicted using Scheil and lever-rule type microsegregation models are found to be similar, although the predicted eutectic fraction is significantly higher with the Scheil-type model. The choice of mushy zone permeability function significantly affects the predicted number, length and orientation of segregated channels. The inclusion of shrinkage-driven flow leads to the prediction of the well-known inverse macrosegregation pattern. However, macrosegregation caused by thermosolutal convection readily masks the inverse segregation. The microsegregation models predict different solid concentrations and eutectic fractions, leading to different solid density distributions which, in turn, cause differences in the extent of contraction-driven flow.