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.
Mg-containing high-Si aluminum alloys are heat-treatable alloys that are widely used in industry. Substantial attention has been paid to increasing the performance of such alloys by adding a small amount of Mn, which is an effective and common alloying element in aluminum alloys. In the present work, the solidification process of Mn-free Al-5Si-0.8Mg alloy and Al-5Si-0.8Mg-(0.45–1.97)Mn alloys are analyzed by the experimental results combined with thermodynamic calculation. The results showed that α-Al, Si, Mg2Si and π (Al8Mg3FeSi6) are predominant phases in the Mn-free Al-5Si-0.8Mg alloy while the π (Al8Mg3FeSi6) phase are transformed to α-Al(FeMn)Si phase with the addition of 0.45% Mn. With increasing Mn addition to 0.72%, the L→α-Al was replaced by L→α-Al(FeMn)Si and a primary α-Al(FeMn)Si phase appeared. Further increasing the Mn to 1.97%, the solidification reactions remained unchanged. However, the size and number of the primary α-Al(FeMn)Si phase gradually increased, while the divorced eutectic phenomenon of quaternary eutectic structure gradually weakened. Meanwhile, the Mg2Si phase in the quaternary eutectic structure gradually transformed from blocky to fine eutectic lamellar, and the quaternary eutectic structure was significantly refined. Primary blocky α-Al(FeMn)Si began to form when the Mn content was higher than 0.75%.
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