CALPHAD-guided suppression of detrimental Fe-containing phases in secondary aluminum alloys

Recycling aluminum alloys provides significant economic and environmental advantages, yet the formation of Fe-containing intermetallic phases, particularly the brittle β-AlFeSi phase, in secondary (recycled) aluminum alloys presents challenges to their mechanical and corrosion properties. This study investigates the role of Cr and Mn additions in suppressing these detrimental phases in the Al-Si-Fe system based on CALPHAD modeling and experimental validation. The findings highlight the effectiveness of CALPHAD simulations in accurately predicting as-cast microstructures and sludge formation temperatures for the alloys examined in this study, as well as those reported in the literature. The addition of Cr and Mn promotes the stabilization of the α-Al₁₅M₃Si₂ phase and inhibits the formation of the detrimental β-AlFeSi phase. Furthermore, virtual experiments were carried out to identify optimal compositional and thermal conditions for efficient recycling. A model relating Fe content to optimal Cr and Mn additions for industrial recycling, expressed as $w\left(Fe\right)=\sqrt{0.9\ast {w\left(Mn\right)}^2+0.4\ast {w\left(Cr\right)}^2}+0.4$ is proposed to guide compositional adjustments. This integrated approach, coupling experimental validation with thermodynamic modeling, highlights the effectiveness of CALPHAD in addressing the Fe impurity challenges and enhancing the sustainability of aluminum alloy recycling.