Interactive Effect of Bismuth and Antimony on Modification Efficiency of Strontium in ADC12 Die-Cast Alloy

This study examines the interactions between strontium (Sr), antimony (Sb), and bismuth (Bi) in recycled ADC12 alloys and their impact on eutectic Silicon and solidification kinetics. Microstructural and thermal analyses show that specific Sb and Bi additions neutralise Sr's modifying effect, converting eutectic silicon from a modified to an unmodified morphology. This transition is marked by a shift in eutectic Si growth temperature from 564 °C (fibrous) to ≈571–572.6 °C (flake-like). A critical Sr/(Sb + Bi) ratio was identified: ratios below approximately 0.38 result in an unmodified structure, while ratios above around 0.42 lead to modification. Electron backscatter diffraction and transmission electron microscopy demonstrate that modified Si exhibits an independent eutectic Al orientation, distinct from primary Al, with a higher twinning density. The interaction mechanism involves the preferential pre-eutectic formation of a quaternary Mg2(Sb, Bi)2Sr intermetallic phase, which consumes free Sr. As a result, further alloying beyond saturation mainly increases this intermetallic phase without additional Si refinement. Excessive Sr additions, intended to counteract this neutralisation, can lead to harmful Sr-rich phases, such as Al2Si2Sr. These findings highlight the importance of carefully controlling tramp elements in recycled Al–Si alloys to ensure consistent microstructure and predictable mechanical properties.