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.

The 2319-Al alloy is widely used in aviation industry. The crack-free 2319 alloy thin-walled sample was fabricated utilizing the laser-CMT composite additive manufacturing technique, achieving a material utilization rate of 96.43%. The impact of deposition parameters and deposition height on the microstructure and mechanical properties was studied. The microhardness of the additive manufacturing samples exhibited a gradual decrease from construction direction, with values reaching 90 HV, 78 HV, and 72 HV, respectively. The tensile property also exhibited a gradual decrease from the bottom to the top; the highest tensile strength was 296 MPa. The grain size along the construction direction of the deposited sample gradually increased, exhibiting respective sizes of 34.7 um, 36.6 um, and 45.7 um. With the increase in the height of the second phase, the segregation at the grain boundary is intensified, and as the size inside the grain increases, the corresponding density decreases. The good laser-CMT composite additively manufactured 2319 aluminum alloy samples could be obtained under the optimized deposition parameters.

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.