The mechanical properties of Al-Si-Mg-Cu cast alloys are heavily determined by Cu content due to the precipitation of relating strengthening precipitates during the aging treatment. In this study, the microstructures and mechanical properties of Al-9Si-0.5Mg-xCu (x = 0, 0.9, 1.5, and 2.1 wt.%) alloys were investigated to elucidate the effect of Cu content on the evolution of their mechanical properties. After T6 (480 °C + 6 h − 530 °C + 4 h, 175 °C + 10 h) treatment, Mg-rich and Cu-rich phases were dissolved in the matrix; the main aging-precipitates of the alloys change from the needle-like β″ phases in the base alloy to the granular Q′ phases in the 0.9Cu alloy, the granular Q′ phase in the 1.5Cu alloy, the granular Q′ phase, and θ′ platelets in the 2.1Cu alloy. The increase of Cu level results in difference of the type, number density, and morphology of the nanoscale precipitated phase. Because of precipitation strength, the yield strength was increased by 103–130 MPa depending on the Cu contents. The precipitation strengthening effect of the precipitates was quantitatively evaluated by the Orowan mechanism. The aging-treated Al-9Si-0.5Mg-2Cu alloy shows the good strength and ductility: yield strength 351 MPa, ultimate tensile strength 442 MPa, and elongation 8.4%. The morphologies of fracture surfaces of the alloys also were observed.

A comprehensive understanding of the structural characteristics and mechanical behavior of Fe-containing phases is important for high-Fe-level Al-Si alloys. In this paper, the crystal characteristics, thermal stability, thermophysical properties and mechanical behavior of multicomponent α-AlFeMnSi and α-AlFeMnCrSi phases are investigated by experimental studies and first-principles calculations. The results indicate that it is easier for Fe and Cr to substitute the Mn-12j site in α-AlMnSi in thermodynamics; Cr is preferred to Fe for substituting Mn-12j/k sites due to its lower formation enthalpy after single substitutions at Mn atom sites. The α-AlFeMnCrSi phase shows higher thermal stability, modulus and intrinsic hardness and a lower volumetric thermal expansion coefficient at different temperatures due to the strong chemical bonding of Si-Fe and Si-Cr. Moreover, the α-AlFeMnCrSi phase has a higher ideal strength (10.65 GPa) and lower stacking fault energy (1.10 × 10³ mJ/m²). The stacking fault energy evolution of the different Fe-containing phases is mainly attributed to the differential charge-density redistribution. The strong chemical bonds of Si-Fe, Si-Mn and Si-Cr are important factors affecting the thermophysical and mechanical behaviors of the α-AlFeMnCrSi phase.

In high-strength 6XXX-series aluminum alloys, Cu additions readily increase susceptibility to intergranular corrosion. Here, by introducing trace amounts of Sn, the Cu-induced intergranular corrosion is mitigated while maintaining high intensity. When the Sn content is maintained below 0.05 wt%, improved corrosion resistance is achieved, primarily due to the narrowing of the precipitation-free zone. However, excessive Sn additions (>0.05 wt%) increase the intergranular corrosion susceptibility of the alloys because of the formation of cathodic second phases such as Mg₂(Si,Sn) phases. Under high Cu conditions, Cu atoms exhibit a higher tendency than Sn atoms to replace Al sites in the β" phases, thereby promoting strengthening of alloys. Meanwhile, Sn additions accelerate the age-hardening process, advancing the peak hardness by approximately 1 h. The mechanical properties exhibit a non-monotonic dependence on Sn level. Among them, the alloy containing 0.025 wt% Sn achieves the optimal property combination, with a yield strength approaching 400 MPa, a tensile strength of ~445 MPa, and a maximum intergranular corrosion depth of only ~110 μm.