Influence of Natural Convection During Dendritic Array Growth of Metal Alloys (Gradient Freeze Directional Solidification)

Purpose of this study was to examine the microstructural evolution of primary dendrites during Gradient Freeze Directional Solidification process in cylindrical Pb-5.8% Sb alloy samples to generate the ground- based research data to support a future microgravity experiment on the Space Station in a convection free environment. Pb-5.8Sb was selected for this study because of its ease of processing and availability of physical property data which will be required for predicting the dendrite morphology parameters, such as, primary dendrite spacing and dendrite trunk diameter. This alloy is also susceptible to thermosolutal convection caused by density inversion of the met in the mushy-zone during DS with melt on top and solid below (gravity pointing down). Two furnace cooling rates, 0.5 C/min and 4 C/min were utilized during the gradient freeze DS. Morphology of primary dendrites was observed to change from being branch-less (cellular) in the very beginning of DS, to those showing onset of side-branching, and finally to well-branched tree-like structure having tertiary and higher level side-branches as the solidification progressed from the cold to the hot end of the samples. Extensive macrosegregation was observed along the DS length, initially being solute poor and then becoming more and more solute rich as the solidification progressed. Experimentally observed primary spacings are smaller and the trunk diameter larger than those predicted from theoretical models which assume purely diffusive transport during solidification