Synergistic Effects of Liquid Solute Concentration and Cooling Rate on Secondary α2-Al Formation in High-Solid-Fraction Rheo-Diecast Al-Si Alloys: An Integrated Experimental and Phase-Field Study

The synergistic effects of solute concentration and cooling rate on the evolution of secondary α2-Al during high-solid-fraction rheo-diecasting of Al-xSi (x = 1, 4, 7 wt.%) alloys was studied. Combined gradient-cooling experiments (100 vs. 10 K/s) and phase-field simulations show that the population and morphology of secondary α2-Al are co-governed by initial Si content and cooling rate. Higher cooling rates promote finer, more uniform secondary α2-Al in Al-1Si and Al-4Si, while lower cooling rates cause coarsening and coalescence. In addition, the formation of α2-Al is severely suppressed in Al-7Si. Crucially, a lower initial solute concentration significantly amplifies cooling rate-induced solute enrichment, quantitatively evidenced by the final liquid concentration difference (Al-1Si: 0.83 wt.% > Al-4Si: 0.29 wt.% > Al-7Si: 0.13 wt.%). This enrichment governs the dynamic competition between constitutional and thermal undercooling, contributing a substantially greater driving force for early-stage nucleation in Al-1Si compared to Al-7Si. As solidification progresses in all three systems, the enrichment of the residual liquid narrows the solidification interval, thereby progressively elevating the role of thermal undercooling.