Effects of Sr on Fe-rich intermetallics in recycled Al–Si–Cu alloys

Sr has been widely used to convert the faceted needle-like eutectic Si to fibrous Si particles for producing automotive engines, pistons, and wheels made of Al–Si–Cu alloys. However, its application to recycled Al–Si–Cu alloys has not been investigated in details, especially how needle-like Fe-rich intermetallics can transformed into less harmful phases. In this study, the influence of minor additions at 0.015 and 0.03 wt% Sr has been quantified by using X-ray computed tomography (XCT) at a micrometer resolution. It has been found that half of the needle-like β-Fe intermetallics have transformed into a compact α-Fe phase as a result of 0.015 wt% Sr addition. Further increasing the Sr level to 0.03 wt% resulted in the majority of the needle-like phases transforming into less harmful crystal structures including fibrous Si and blocky α-Fe, leading to an increase in elongation by 90.47%. In addition, the first-principles calculations have shown that Sr has a strong affinity to vacancy and interfaces, prohibiting the nucleation of eutectic Si and porosity. Therefore, the porosity number density decreased from 274.72 to 91.26 mm⁻³, and the eutectic Si number density increased from 13,061.27 to 42,449.13 mm⁻². First-principles calculations and quantification of blocky Fe-rich intermetallics replacing needle-like morphology as a function of Sr addition uncover that Sr–Al–Si co-segregation to the low interfacial energy α-Al/Fe-rich intermetallics interface cut the diffusion path of Fe along faceted planes at elevated temperature, promoting twinning by alternating stacking sequences along the high Miller index planes.