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

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 x 0.99(t (day)). The YS reached a stable state after natural ageing for 8.3 months. The as-cast alloy comprised the intermetallic phases of Mg2Si, alpha-Fe and Mg-32(Al, Zn)(49). The diameter of the fibrous Mg2Si phase ranged from 0.08 to 0.25 mu m, while the size of the fine blocky alpha-Fe was measured as similar to 0.8 mu m. 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 eta'-MgZn2 precipitates with the number density of 23.6 x 10(23), 36.5 x 10(23) and 44.8 x 10(23)/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.