Mushy zone characteristics and macrosegregation during directional solidification

Abstract

Directional solidification of an alloy containing Pb-5.8 wt% Sb has been carried out over growth rates varying from 1 to 10 μm s-1 in a positive temperature gradient of 140 Kcm-1. The microstructural examination indicated cellular to dendritic transition at a growth rate of 1.5 μm s-1. An analysis of composition revealed a large macrosegregation of Sb along the longitudinal section of the directionally solidified alloys. The degree of macrosegregation was observed to increase with a decrease in the growth rate during solidification of the alloy. The microstructural analysis indicated a progressive increase in volume fraction of the solid phase, variation in shape factor of cells/dendtrites and decrease in the hydraulic radius as a function of distance from the solid-liquid interface in the mushy zone. These factors control the permeability of the mushy zone and resultant macrosegregation of the alloy. The macrosegregation behaviour of the alloy is discussed in light of thermosolutal convection in the mushy zone as well as in the melt ahead of the solid-liquid interface during directional solidification.