Gravitational macrosegregation in unidirectionally solidified Lead-tin alloy

Abstract

Small laboratory samples of binary lead-tin alloys, about 20 mm diameter, 50 mm high, and weighing about 180 grams, were solidified unidirectionally upward, with both cooling rate and thermal gradient being closely controlled. A total of nine ingots were produced; six of these ingots had a nominal composition of Pb-15 wt pct Sn, and the remaining three were Sn-15 wt pct Pb. Detailed thermal measurements, chemical composition measurements (dendrite composition, fraction of interdendritic eutectic formed), and microstructural characterization (primary and secondary dendrite arm spacing measurements) were carried out. Normal macrosegregation was observed in the Pb-rich alloys, with Sn content being highest at the top of the ingot. In the Pb-15 wt pct Sn alloys, macrosegregation was found to increase with increasing thermal gradients, and also with decreasing cooling rates. In some of our experiments with Pb-rich alloys it was possible to induce severe segregation, in the form of deep freckles. The freckles were in the form of a column of tin-rich material which extended over the upper half of the ingot. No significant macrosegregation was evident in the Sn-rich alloy for the growth conditions employed here. Segregation behavior of all ingots was correlated with the measured thermal history, compositional data, and the fraction eutectic phase in the finally solidified samples.