Influence of melt convection on the columnar to equiaxed transition and microstructure of downward unsteady-state directionally solidified Sn–Pb alloys

Abstract

A combined theoretical and experimental approach is developed to quantitatively determine the solidification thermal parameters: transient heat transfer coefficients, tip growth rates and cooling rates during downward unsteady state solidification of hypoeutectic Sn–Pb alloys. For the growth conditions examined, solid in the top and melt below, with gravity pointing down, the rejection of solute into the melt during solidification results in increased melt density. The resulting thermosolutal convection can start in the melt both within the interdendritic region and ahead of the dendrite array. The experimental results have shown that melt convection may be causing pileup of fractioned dendritic arms, which must stimulate the CET occurrence. The results have supported a criterion recently proposed based on a critical cooling rate. For upward unidirectional condition, this critical value was found to about 0.014K/s for hypoeutectic Sn–Pb alloys. In the present study, in conditions of downward solidification, melt convection seems to favor the structural transition, which is anticipated and occurs for a critical cooling rate of about 0.03K/s, for any of three hypoeutectic alloys experimentally examined. Primary dendritic arm spacings have been affected by the direction of growth, decreasing in conditions of downward vertical solidification when compared with those grown vertically upwards. A tendency of reduction of secondary dendritic arms has also been observed for the Sn 5wt.% Pb alloy solidified downwards when compared with those grown vertically upwards.