Thermodynamics-guided design of sustainable secondary Al-Si alloys for enhanced Fe-impurity tolerance and optimized Mn doping

The accumulation of Fe impurities throughout the lifecycle of secondary scrap-based Al-Si alloys can degrade their mechanical properties. This often necessitates neutralizing the detrimental Fe contamination with Mn addition. However, some reports suggest that specific ranges of Fe concentrations have minimal deleterious effects, and Mn introduction can yield only minor or even adverse impacts. Our working hypothesis is that the tolerances for Fe and Mn can be rationalized based on the underlying microstructure-property relationships. Calculations of non-equilibrium solidification paths, evaluations of precipitation driving force, and estimations of interfacial energy were employed to predict the resulting microstructure characteristics. The findings indicate that in moderately-contaminated alloys with 0.8 wt.% Fe, near-eutectic compositions demonstrate greater Fe tolerance than far-from-eutectic alloys. This is because near-eutectic alloys require less undercooling to promote the preferred formation of the metastable Chinese-script α phase and inhibit the undesired platelet-shaped β and δ phases. Conversely, in severely-contaminated alloys with 2.0 wt.% Fe, far-from-eutectic compositions display better Fe tolerance. The enhanced tolerance is attributed to the higher nucleation rate of the platelet β phase in far-from-eutectic alloys, leading to a more refined size distribution. Sufficient undercooling of the platelet phases facilitates the formation of the Chinese-script α phase in moderately-contaminated alloys and the polyhedral γ phase in severely-contaminated alloys. Consequently, Mn addition for neutralizing Fe contamination becomes redundant. This study investigates how Si, Fe, Mn, and cooling rate affect the formation of Fe-rich intermetallic phases. Moreover, it provides general recommendations for designing sustainable Al-Si alloys with less compromised properties.