Assessment of Post-eutectic Reactions in Multicomponent Al-Si Foundry Alloys Containing Cu, Mg, and Fe

Post-eutectic reactions occurring in Al-Si hypoeutectic alloys containing different proportions of Cu, Mg, and Fe were thoroughly investigated in the current study. As-cast microstructures were initially studied by optical and electron microscopy to investigate the microconstituents of each alloy. Differential scanning calorimetry (DSC) was then used to examine the phase transformations occurring during the heating and cooling processes. Thermodynamic calculations were carried out to assess the phase formation under equilibrium and in nonequilibrium conditions. The Q-Al₅Cu₂Mg₈Si₆ phase was predicted to precipitate from the liquid phase, either at the same temperature or earlier than the θ-Al₂Cu phase depending on the Cu content of the alloy. The AlCuFe-intermetallic, which was hardly observed in the as-cast microstructure, significantly increased after the solution heat treatment in the alloys containing high Cu and Fe contents following a solid-state transformation of the β-Al₅FeSi phase. After the solution heat treatment, the AlCuFe-intermetallics were mostly identified with the stoichiometry of the Al₇Cu₂Fe phase. Thermodynamic calculations and microstructure analysis helped in determining the DSC peak corresponding to the melting temperature of the N-Al₇Cu₂Fe phase. The effect of Cu content on the formation temperature of π-Al₈Mg₃FeSi₆ is also discussed.