A novel upcycling strategy to Al alloys: Si removal via Mg-assisted suspension based fractional crystallization

With the electrification and lightweighting of the automotive industry, Al Si cast alloys scraps from end-of-life internal combustion engine vehicles is rapidly accumulating, severely disrupting the aluminum recycling cycle. This trend underscores the urgent need for advanced upcycling strategies that can remove Si with low energy and cost, thereby enabling the sustainable recycling of casting alloys. In this study, we proposed a Mg-assisted suspension based fractional crystallization method to achieve selective separation of Si from Al Si alloys. Thermochemical modeling first guided alloy selection and process windows. A systematic experimental study evaluated the effects of Mg content and cooling rate on Si removal. The optimal process parameters are 20 wt% Mg addition with a cooling rate of 30 °C/h, achieving a Si removal degree of 63.0%. In addition, microstructural characterization demonstrated the separation mechanism. In Al Si systems with high Mg content (e.g., 15 wt% Mg and 20 wt% Mg), polyhedral primary Mg 2 Si precipitates as a single solid phase during the initial solidification stage and floats upward due to its lower density than the melt, thereby enabling selective Si separation. In contrast, eutectic Mg 2 Si forms with α-Al, where it becomes entrapped within the α-Al dendritic network and thus cannot separate. Comprehensive assessment indicates that Mg can be recovered via vacuum distillation, thereby reducing net material costs. Additionally, this process has the potential to effectively control Si and Mg content while maintaining low energy consumption. Overall, this approach provides a technically feasible, economical, and environmentally friendly pathway for large-scale upcycling of Al Si cast scraps.