Effect of Cooling Rate on Fe-rich Intermetallic Phases in Solidification Microstructure of Al-Si-Fe-Mn Alloys

Al-Si cast alloys are widely used in automotive industrial applications as structural casting, wheels, and powertrain components. Recently, the recycling of Al-Si alloys has received specific interest in terms of circular economy. However, a major impurity element of Fe during the recycling process promotes the formation of the Fe-rich intermetallic phases (β-Al₅FeSi phase with a coarse needle-shaped morphology), which negatively affects the mechanical properties of the alloy. Therefore, Mn, which is also another major impurity element, is often added to avoid the formation of the β-Al₅FeSi phase. The Mn addition enhances the α-Al₁₅(Fe,Mn)₃Si₂ phase (script or polygonal morphologies) in Al-Si alloys. The α-Al₁₅(Fe,Mn)₃Si₂ phase is expected to improve high-temperature mechanical performance due to its high thermal stability. However, few studies have systematically investigated the solidification sequence of the α-Al₁₅(Fe,Mn)₃Si₂ phase in Al-Si-Fe-Mn quaternary alloys, and the effects of cooling rate on the solidification microstructure are still unclear. This study systematically investigated the solidification microstructure of Al-Si-Fe, Al-Si-Mn ternary alloys and Al-Si-Fe-Mn quaternary alloys solidified at different cooling rates. Based on thermodynamic calculations, we designed the alloy compositions of the Al-Si-Fe and Al-Si-Mn ternary alloys utilizing the eutectic reaction of the α-Al/Fe-intermetallic (β-Al₅FeSi or α-Al₁₅(Fe,Mn)₃Si₂) phases. The alloy compositions are Al-5%Si-1.5%Fe, Al-8%Si-1%Fe, and Al-5%Si-1.5%Mn (all wt% thereafter, unless otherwise stated). Also, since the addition of Fe to the Al-5%Si-1.5%Mn alloy was assessed to increase the volume fraction of the α-Al₁₅(Fe,Mn)₃Si₂ phase based on thermodynamic calculations, we designed quaternary alloys with 2.0%, 4.0%, and 6.0% Fe added to the Al-5%Si-1.5%Mn alloy.<br/>In this study, alloys designed based on thermodynamic calculations were prepared by solidification at two different cooling conditions. The metal raw materials were melted in a high-frequency induction furnace in the temperature range of 800-900 °C for 1800 s in an Ar atmosphere. Some were cooled by switching off the power source to the furnace (cooling rate: 0.3 °C×s^-1). The others were cast into an iron mold to form rod-shaped ingots at a higher cooling rate (145 °C×s^-1).<br/>The microstructural characterization revealed that needle-shaped β-Al₅FeSi phase and fine α-Al₁₅Fe₃Si₂ phase were formed as the Fe-rich intermetallic phases in the solidified Al-Si-Fe ternary alloys. The α-Al₁₅Mn₃Si₂ phase with Chinese-script morphology was formed in Al-5%Si-1.5%Mn ternary alloy. In the Al-Si-Fe ternary alloys, the volume fraction of the needle-shaped β-Al₅FeSi phase decreased with increasing cooling rate, and the fine α-Al₁₅Fe₃Si₂ phase was dominantly formed in solidification microstructures. Such Fe-rich intermetallic phases were dominantly formed as the primary solidified phase in a low cooling rate of 0.3 °C×s^-1, whereas the primary solidified α-Al phase was often observed in samples solidified at a high cooling rate of 145 °C×s^-1. Therefore, two-phase eutectic microstructures of the α-Al phase with Fe-rich intermetallic phases, which were predicted by calculated liquidus projections, were scarcely observed in the experimental alloys. Such a trend was found by differential scanning calorimetry (DSC) measurements of the solidification reactions at controlled cooling rates. These results suggest the reaction sequence in solidification may not follow the liquidus projections, likely due to the limited temperature range of liquid equilibrium with Fe-rich intermetallic phases. In this presentation, the solidification microstructure of the Al-Si-Fe-Mn quaternary alloy will be shown to discuss the influence of Fe and Mn combined addition to Al-Si alloys.