This study examines the intricate relationships between strontium, antimony, and bismuth in recycled ADC12 alloys, with a focus on their impact on microstructure and solidification kinetics. Microstructural and thermal analyses show that specific Sb and Bi additions neutralise Sr's modifying effect, converting eutectic Si 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. EBSD and TEM analyses 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, preventing it from participating in Si modification. 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.
