EFFECTS OF Cu ADDITION AND SPHEROIDIZATION TREATMENT ON MICROSTRUCTURE AND MECHANICAL PROPERTIES OF Al-Si-Cu-Mg-Ti-Zr-Sr ALLOYS

Herein, six types of Al-7Si-xCu-0.35Mg-0.12Ti-0.24Zr-0.05Sr alloys were prepared with different Cu contents (x = 0, 1.8, 2.1, 2.4, 2.7, and 3.0). The effects of Cu addition and spheroidization treatment on the microstructure and mechanical properties of the alloys were examined. The findings demonstrated that the theta-Al2Cu phase tended to concentrate near the beta-Al5FeSi phase. Furthermore, Cu could refine the grain and passivate the eutectic Si. Among the as-cast alloys, the mechanical properties were found to be optimal when the Cu content was 2.7 wt%, with ultimate tensile strength (UTS), yield strength (YS), and elongation (El) values measuring 251.1 MPa, 159.5 MPa, and 5.7%, respectively. Despite the decrease in El, the UTS and YS improved by 23.6% and 23.5%, respectively, compared to the C0 alloy. However, excessive Cu content aggregated and adhered to the massive theta-Al2Cu phases, weakening the mechanical characteristics of the alloy. In addition, the spheroidization treatment caused significant spheroidization in the eutectic Si and promoted a more uniform size distribution. This treatment also facilitated the transition of the solid solution of numerous theta-Al2Cu phase into a matrix, leading to a uniformly dispersed precipitation of Al-Si-Cu and Al-Cu phases. Consequently, the mechanical properties of the alloys were considerably enhanced. When the Cu content was 2.7 wt%, the UTS, YS, and El of the alloy were 301.9 MPa, 174.4 MPa, and 9.1%, respectively, representing increases of 20.2%, 9.3%, and 59.7% compared with those of the as-cast alloy.