Song Pang

Influence of heat treatment on microstructures and mechanical properties of gravity cast Mg–4.2Zn–1.5RE–0.7Zr magnesium alloy

Abstract The microstructures and mechanical properties of Mg–4.2Zn–1.5RE–0.7Zr alloy were investigated under different heat treatment conditions. The as-cast alloy consisted of α -Mg phase, T -phase and Mg 51 Zn 20 phase. After aging treatment (single-step (325 °C, 10 h) and two-step (325 °C, 4 h)+(175 °C, 14 h)), neither T -phase nor Mg 51 Zn 20 phase dissolved into the matrix and the coarsening of α -Mg phase was not significant. When peak-aged at 325 °C for 10 h, dense short rod-like β′ 1 phase precipitated in the matrix. Further ageing at 325 °C led to coarsening of β′ 1 phase and a decrease in number density. Alloy aged at 325 °C for 10 h achieved the highest yield strength (YS) and ultimate tensile strength (UTS) of 153.9 MPa and 247.0 MPa, which were increased by 48 MPa and 23 MPa from as-cast condition, respectively. While the elongation slightly decreased to 15.6%. Comparatively, the YS and UTS of alloy two-step aged by (325 °C, 4 h)+(175 °C, 14 h) showed little difference from those of single-step aged alloy, but with a lower elongation of 13.4%. In addition, the fracture surfaces of Mg–4.2Zn–1.5RE–0.7Zr alloy under different thermal conditions were mainly characterized by quasi-cleavage feature, but with differences in the details.

Effects of processing parameters on microstructure of semi-solid slurry of AZ91D magnesium alloy prepared by gas bubbling

Abstract The semi-solid slurry of AZ91D magnesium alloy was prepared by gas bubbling process. The effect of processing parameters, including gas flow rate, cooling rate and stirring end temperature, on microstructure of AZ91D semi-solid slurry was investigated. With increasing the gas flow rate from 0 to 5 L/min, the average size of primary α-Mg particles decreases from 119.1 to 77.2 μm and the average shape factor increases continuously from 0.1 to 0.596. The formation of non-dendritic primary α-Mg particles during gas bubbling is the result of combined effects of dendrite fragmentation and copious nucleation. With increasing the cooling rate from 3.6 to 14.6 °C/min, the average particle size of primary α-Mg phase decreases from 105.0 to 68.1 μm while the average shape factor peaks at 9.1 °C/min. Both high and low cooling rates can induce dendritic growth of primary α-Mg particles. Changing the stirring end temperature from 590 to 595 °C has little effect on the average size and shape factor of primary α-Mg particles in AZ91D semi-solid slurry. The insensitivity of semi-solid microstructures to the stirring end temperature is attributed to the sufficient quantity of primary particles formed in the melt.