The Modification Effect of Sr and La-Ce Mixed Rare Earth on Microstructure and Mechanical Properties of AlSi9Cu3 Alloy

This study systematically investigates the synergistic effects of Sr and La-Ce mixed rare earth (RE) additions on the microstructure and mechanical properties of AlSi9Cu3 alloy. The results show that adding 0.02 wt.% Sr and 0.10 wt.% RE together produces the most pronounced refinement, reducing the secondary dendrite arm spacing (SDAS) from 40.12 μm to 26.98 μm and decreasing the aspect ratio of eutectic Si from 12.3 to 7.8. Sr modifies eutectic Si by inducing twin formation and suppressing anisotropic growth, while RE promotes the dispersion distribution of Al4Ce phases and enhances constitutional undercooling. The synergistic effect leads to a more uniform appearance of dendrites and secondary phases. Consequently, the tensile strength reaches 258.6 MPa, accompanied by a transition in fracture mode from brittle cleavage to ductile dimple fracture. This work provides experimental support for microstructural control and the strengthening–toughening design of AlSi9Cu3 alloys.