Cellular spacings in unsteady-state directionally solidified Sn–Pb alloys

Solidification thermal parameters, i.e. transient metal/mold heat transfer coefficient, tip growth rate and cooling rate, and cellular spacings have been measured in dilute Sn–Pb alloys solidified under unsteady-state heat flow conditions. It has been found that the cellular spacings decreased with increasing solute content up to the limit of the cellular range, at about 2 wt.% Pb. A sharp increase in spacing is observed when the growth morphology changes from cellular to cellular/dendritic. The Hunt–Lu’s model applied to cellular growth did not generate the experimental observations for the alloys examined in the present study. It is proposed the insertion of analytical expressions for tip growth rate and cooling rate into the resulting experimental equations permitting to establish empirical formulae relating cellular spacings with unsteady-state solidification parameters like: melt superheat, type of mold and transient metal/mold heat transfer coefficient.