Effects of Sr–Ti–B–La/Ce and Graded Cu Microalloying on Microstructure and Strength–Toughening of Al–7Si–0.35Mg Casting Alloy

A systematic study was conducted on commercial Al–7Si–0.35Mg cast alloy modified by a multielement microalloying strategy: fixed additions of Sr and Ti–B, 0.3 wt.% La or Ce, and a graded Cu addition (0–3.5 wt.%). Microstructure and mechanical properties were characterized using optical microscopy, scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, electron backscatter diffraction, X‐ray diffraction, Vickers hardness testing, and room‐temperature tensile testing. The results show that the Sr–Ti–B–La/Ce composite modifier effectively rounds/ spheroidizes corners of eutectic Si and reduces the area fraction of large Si particles (>300 μm 2 ) from 47.8% to 21.0%, while markedly increasing the proportion of finer Si particles in the 1–50 μm 2 range. Secondary dendrite arm spacing and mean grain radius were refined by ≈54% and 38.5%, respectively (average grain radius decreased from 351 to 216 μm). With increasing Cu content the alloy behavior transitions from predominantly solid‐solution strengthening to second‐phase particle strengthening controlled by Al 2 CuMg. At about 1.5 wt.% Cu, a fine dispersion of Al 2 CuMg particles forms and the mechanical properties reach an optimum, with the ultimate tensile strength of 271.9 MPa and elongation (EL) of 6.7%. When Cu exceeds ~2.5 wt.%, Al 2 CuMg phases coarsen and form continuous networks, causing ductility to fall sharply (EL down to 2.94% at 3.5 wt.% Cu) despite a continued rise in hardness with Cu content (maximum ~109 HV). Fractographic analysis indicates a uniform ductile dimple morphology at the optimal composition, whereas coarse second‐phase particles induced by excess Cu act as preferred crack‐initiation sites. Based on multiscale characterization, this work reveals synergistic interactions between Sr–Ti–B–La/Ce and Cu in terms of interfacial adsorption, solute redistribution, and precipitation kinetics. An optimized composition represented by 0.3 wt.% La/Ce + 1.5 wt.% Cu is proposed, providing insights into the microstructure and engineering guidance for the alloy design and industrial application of high strength‐and‐toughness cast Al–Si–Mg alloys.