Effect of in-situ formation of AlP on solidification of hypereutectic Al–Si alloy

Abstract

Abstract The effects of in-situ AlP formation on the solidification process of an Al-20 %Si alloy were studied by examining the change law of the solidification microstructures after the addition of Cu–P alloy to the melt and stirring. During the early stage of stirring, the eutectic Si was clearly refined and Si mainly existed in a point-like shape. During the middle stage of stirring, the typical eutectic microstructure disappeared, and the microstructure consisted of fine primary Si uniformly distributed in the α matrix. Many AlP precipitates with sizes near the critical nucleation radius of primary Si were formed after the Cu–P alloy was dissolved in the Al-20 %Si alloy and stirred. These AlP precipitates became the nuclei of the primary and eutectic Si. The Si-poor layers around the primary Si provided the conditions necessary for the precipitation of the primary α phases when the primary Si grew. For finer primary Si, the distance between the primary Si was reduced, and lower temperature facilitated the formation of a complete α loop or continuous α phases around the primary Si. During the eutectic reaction, the ultrafine AlP caused the eutectic Si phases to nucleate alone and grow; the eutectic α phases grew based on the primary α phases, which resulted in the formation of divorced eutectic microstructures.