Refinement of primary silicon crystals in a hypereutectic Al-20%Si alloy by sulphur addition

Abstract

Sulphur addition was known to have a refining effect on primary silicon in hypereutectic Al-Si alloys. The purpose of this study is to clarify effects of an amount of S addition, holding time and superheating temperature on refinement of primary silicon crystals in an Al-20%Si alloy. Grain size of primary silicon and microstructure were observed by an optical microscope and an X-ray microprobe analyzer.When a melt was held at 900°C for 10min, addition of 0.3-0.4%S resulted in refinement of primary silicon. Ten minute holding at 900°C after a modification treatment increased a refinining effect significantly. However, longer holding did not introduce improvement of a refining effect. The finest silicon crystals were obtained when a melt, modified by 0.3%S, was superheated at 1100°C for 10min. In the S-treated specimens, grey colored particles were seen at the center and the boundary of the primary silicon and in the eutectic structure. A microanalysis showed these particles to contain two components, Al and S. On a cooling curve of a modified specimen, a clear break was observed and was identified with a point, at which the primary silicon was nucleated. These results were interpreted in a manner similar to the AlP nucleation theory of primary silicon crystals.