Formation of Phases and Microstructures in Al-8Si Alloys with Different Mg Content

Mg-containing high-silicon aluminum alloy is a heat-treatable aluminum alloy that is now widely used in the aerospace and automotive industries because of its high specific strength, high wear resistance and corrosion resistance, low thermal expansion coefficient, and low cost. More attention has been paid to optimizing the microstructure to increase the performance of this type of aluminum alloy. In the present work, the solidification processes of Mg-free and Mg-containing (0.33-1.32%) Al-8Si alloys were analyzed by the experimental results combined with the thermodynamic calculation. The results showed that α-Al, Si, and Al₅FeSi were in the Mg-free Al-8Si alloy ingots, while the Al₅FeSi phases in alloys with Mg additions were transformed into π phases (Al₈Mg₃FeSi₆) by the reaction L+Al₅FeSi→α-Al+Si+Al₈Mg₃FeSi₆. There was a binary eutectic reaction of L→α-Al+Al₅FeSi when the Mg content exceeded 0.51% and the Fe content was higher than 0.17%. With the increase of Mg content, the volume of Mg₂Si was gradually increased while the divorced eutectic phenomenon of the quaternary eutectic structure (α-Al+Si+Mg₂Si+Al₈Mg₃FeSi₆) was weakened and the eutectic structure was significantly refined.