Effect of prolonged natural ageing on microstructure and tensile properties of a high strength die-cast Al-Mg-Si-Zn alloy

Abstract

A two-year prolonged natural ageing effect of a high strength and heat treatment-free Al5.5Mg2Si3Zn die-cast alloy was investigated, and the microstructure was characterized to understand the underlying mechanisms of high strength in the as-cast state and the strong natural ageing strengthening. The die-cast alloy exhibited the high yield strength (YS) of 211 ± 2.3 MPa, ultimate tensile strength (UTS) of 334.7 ± 14.4 MPa and the good elongation (EL) of 5.8 ± 1.28 % in the initial as-cast state. After natural ageing for 24 months, the die-cast alloy delivered the high YS of 290.9 ± 2.7 MPa, UTS of 375.3 ± 6.8 MPa and the industrially acceptable EL of 2.58 ± 0.24 %, which shows 38 % improvement in YS over the die-cast alloy under as-cast condition. The intrinsic connection between the YS and the natural ageing time of 24 months can be expressed as an exponential model, i.e., YS (MPa)= 291.83–71.93 × 0.99t (day). The YS reached a stable state after natural ageing for 8.3 months. The as-cast alloy comprised the intermetallic phases of Mg2Si, α-Fe and Mg32(Al, Zn)49. The diameter of the fibrous Mg2Si phase ranged from 0.08 to 0.25 μm, while the size of the fine blocky α-Fe was measured as ∼0.8 μm. Strengthening by refined primary and secondary phases and solid solution are responsible for the high as-cast YS over 200 MPa. The precipitation strengthening of nanoscale η'-MgZn2 precipitates with the number density of 23.6 × 1023, 36.5 × 1023 and 44.8 × 1023/m^2 contributed to the strong enhancement of 34.2, 68.0 and 79.8 MPa over the as-cast YS after natural ageing for 1, 7.2 and 24 months, respectively.