Segregation of Antimony along the Cross Section of Lead Castings as a Cause of Defects in Isostatic Pressing of Lead Alloy Products

Abstract

A comparative analysis of the macro- and microstructure of a lead casting and a lead rod obtained from it by isostatic pressing is reported. The effect of segregation of antimony as the main dopant of the investigated alloy on the quality of a product has been demonstrated using advanced nonferrous optical metallography methods. It has been established that the upper part of a casting with a surface crack caused by adding the crystallized casting with lead contains an increased amount of antimony, nonmetallic inclusions, and pores on the fusion surface. Inside the casting, there is a shrinkage cavity along the casting axis, which results from shrinkage of the material during its crystallization. The content of the main element of the alloy (antimony) in lead of the castings corresponds to the PbSb_0.9 alloy according to GOST (State Standard) 1292-81 “Lead‒antimony alloys. Specifications,” but the impurity content exceeds the value recommended by GOST the total impurity content is limited to 0.1%, while the total content of copper, cadmium, bismuth, and palladium in the investigated castings is ~1%. The casting microstructure investigations have shown that the main structural defect is a shrinkage cavity located along the casting axis. The occurrence of this cavity is related to an excessively high temperature of lead casting into a mold. This has led to a long crystallization time, as evidenced by the antimony segregation in the surface areas of the shrinkage cavity. The metal of the surface layers of the shrinkage cavity is characterized by the increased antimony contents, nonmetallic inclusions, and discontinuities, which significantly changes its mechanical properties as compared with the base metal, including the plasticity, which plays a decisive role in the plastic strain processing of the casting.