Impurity-induced phase transformations in AlMgZn(Cu) crossover alloys: Pathways to enhance recycling content and processability

Aluminum crossover alloys offer a broad property profile within a single composition, but due to the growing demand for recycling in the aluminium industry, they will be required to mitigate the impact of tramp elements such as Fe and Si. This study investigates the influence of Fe/Si ratios and cooling rates during solidification on phase transformations and microstructure evolution in AlMgZn(Cu) crossover alloys, aiming to increase recycling content and maintain processability. Thermodynamic simulations, coupled with experimental validation, reveal two critical phase transformations during homogenization: the 6-to-3 transformation (Al₆(Fe,Mn) → Al₁₃(Fe,Mn)₄) and the 6-to-α transformation (Al₆(Fe,Mn) → Al(Fe,Mn)Si). These transformations are governed by the Fe/Si ratio and cooling rate, significantly affecting intermetallic phase morphology. The 6-to-3 transformation can effectively decrease the size of intermetallic particles, facilitating processability in relevant industrial conditions. Higher cooling rates upon solidification (≈60 K/s) always result in small, spheroidized phases, ensuring rollability. In contrast, slow cooling rates (≤1 K/s) often promote coarse, stable phases that hinder processability. However, at cooling rates around 3 K/s the intermetallic phase morphology highly depends on the Fe/Si ratio. When Fe and Si levels are simultaneously high, the 6-to-α transformation yields hard-shell/soft-core structures that impair mechanical integrity, while a higher ratio governs a beneficial 6-to-3 transformation. This study provides new insights into impurity-induced phase transformations and their role in determining processability in industrially relevant conditions. By linking microstructural control to sustainable alloy design, the results serve as a foundation for the development of crossover aluminum alloys optimized for high scrap content.