Limitation of bismuth residual in A356.2 Al

摘要

Minor element effects of bismuth on the microstructure and shrinkage characteristics of A356.2 aluminum alloy were investigated. The results showed that effective modification of the eutectic structure, using either 0.01 or 0.015% Sr additions, was counteracted with increased bismuth contents. With as little as 0.005% Bi (50 ppm) added, some undermodified eutectic was present in the structures; with as much as 0.1% Bi (1000 ppm), the modification effect of strontium was completely eliminated. It could be anticipated that the effect on mechanical properties would follow these microstructural changes. However, in other aluminum casting alloy, the tensile properties, elongation and hardness were not significantly affected with bismuth additions, even though there were marked influences on the microstructures. It was observed that more needle-like shaped iron-containing intermetallic phases were present with increased amounts of bismuth additions, but this characteristic was not apparent with all alloys. As measured using the Tatur test mold, an increased amount of shrinkage (pipe volume) was observed with increased bismuth additions, up to 0.02% (200 ppm).