Enhancement of Solidification Microstructure and Mechanical Properties of Al-5Si-Cu-Mg Alloy Through the Addition of Scandium and Zirconium

Although low-silicon Al-Si alloys have been extensively studied, further improvement in their mechanical performance remains a critical challenge. This study examines the synergistic effects of scandium (Sc) and zirconium (Zr) additions on the solidification behavior, microstructural evolution, and mechanical properties of Al-5Si-Cu-Mg alloys. The Sc/Zr additions refine the α-Al grains and modify the eutectic Si morphology, with the most uniform microstructure obtained at 0.5 wt.% due to the formation of coherent Al3(Sc,Zr) dispersoids. These additions also suppress the formation of needle-like β-Al5FeSi phases and promote the transformation to compact α-Al15(Fe,Mn)3(Si,Zr,Sc)2 intermetallics, optimizing the solidification process. The yield strength increases with Sc/Zr content owing to grain-boundary and precipitation strengthening. However, the alloy without Sc/Zr exhibits the highest ultimate tensile strength and elongation, likely due to its finer secondary dendrite arm spacing and the absence of casting-induced cracks in this investigation. Although Sc/Zr additions of 0.25–0.5 wt.% contribute to microstructural refinement, the concurrent formation of porosity and coarse intermetallic compounds leads to a deterioration in ductility. Excessive Sc/Zr additions further coarsen grains and degrade the overall mechanical integrity.