Tailoring the tensile characteristics, hardness and impact strength of A356 hypoeutectic alloy via multi-step aging heat treatment

Abstract

Conventional single-step aging treatments, such as the T6 process, enhance the mechanical properties of A356 hypoeutectic aluminium alloy (Al–7Si–2Mg–1Cu), but often provide limited control over precipitate evolution, resulting in early peak aging and insufficient strengthening. This study investigates the effects of three novel multi-step aging regimes on the alloy’s tensile strength, yield strength, ductility, hardness, and impact toughness. The alloy underwent solution heat treatment at 540 °C/5 h, water-quenched, and aged under three conditions: single-step aging (T1) at 200 °C/0.5–8 h; two-step aging (T2) at 200 °C/0.5 h + 180 °C/0.5–8 h; and low-temperature two-step aging (T2L) at 200 °C/0.5 h + 160 °C/0.5–8 h. T1 achieved the highest early-stage properties—UTS 383 ± 5 MPa, YS 242 ± 2.9 MPa, hardness 165 ± 2 BHN, and impact strength 56 ± 1.5 J at 200 °C/0.5 h, but exhibited reduced ductility (20.8%) and rapid over-aging. T2 yielded a balanced profile with UTS of 340 ± 3.1 MPa, YS of 217 ± 2.4 MPa, hardness of 148 ± 3.2 BHN, and maximum impact strength of 42 ± 1.2 J at 200 °C/0.5 h + 180 °C/4 h. T2L achieved the highest hardness (208 ± 2 BHN at 160 °C/8 h) but lower impact resistance. These results demonstrate that multi-step aging, particularly T2 and T2L, offers a viable strategy for tailoring A356 alloys to meet the strength–ductility–toughness demands of high-performance automotive and aerospace applications.