Effect of Melt Stirring on Microstructure Formation During Solidification of Sr-Modified Near-Eutectic Al-Si Alloys

Near-eutectic Al-11 wt pct Si alloys containing 0.005-0.02 wt pct Sr were solidified in conjunction with electromagnetic stirring, and the effect of this melt stirring on eutectic evolution was elucidated. Specimens solidified in the absence of electromagnetic stirring exhibited both lamellar and fibrous modified eutectic phases whereas those solidified while stirred showed spherical eutectic grains containing acicular unmodified Si phases. The spherical eutectic grains were found to evolve far from the eutectic front at the periphery of each such sample. Two different eutectic morphologies were identified in these grains: acicular in the outer parts and fibrous in the central regions. The eutectic Al in the center was determined to have essentially a single orientation whereas that at the periphery showed multiple crystallographic orientations. These results suggest that the two eutectic phases were evolved by different growth mechanisms. Thermal analysis of the cooling curves established that electromagnetic stirring decreased the eutectic depression temperature, indicating that eutectic modification resulting from the addition of Sr was inhibited by stirring. Melt stirring evidently fragmented the eutectic front after which the fragmented eutectic front served to nucleate spherical eutectic grains.