AZ91 alloy was cast in a steel mould pre-exposed to three different temperatures: -196 ºC, 20 ºC and 650 ºC. The aim of the study was to determine the difference in the microstructure and mechanical properties between the castings formed in a cold mould and those solidifying under near-equilibrium conditions in a mould pre-heated to 650 ºC. Solidification at a low temperature led to dispersion of the structure elements as well as supersaturation of the solid solution of aluminium in magnesium. The heat treatment results indicate that the alloy solidified in the mould pre-exposed to 20 ºC can be successfully aged (heat treated to the T5 temper). It was found that the effect of the ageing process (T5 temper) was greater than the effect of the microstructure fragmentation, which was due to rapid solidification. The ageing results were assessed by comparing the microstructure and mechanical properties of AZ91 brought to the T5 condition with those obtained for the material in the T6 condition.
Effects of solution treatment on room temperature mechanical properties were studied in cast AZ91 (Mg-9%Al-1%Zn-0.2%Mn) and AZ91-0.5%Ca alloys. In as-cast state, the Ca addition contributed to the suppression of discontinuous β phase precipitation and the formation of Al2Ca phase. After solution treatment, the AZ91 alloy had only a small amount of Al8Mn5 particles, while β and Al2Ca phases were still present in the Ca-containing alloy. In as-cast state, the AZ91-0.5%Ca alloy showed better yield strength and hardness than the AZ91 alloy. The solution treatment increased the elongation in both alloys, which eventually led to the increase in ultimate tensile strength. The solution treatment resulted in a marked decrease in yield strength and hardness in the AZ91 alloy, whereas the decrements in those values were relatively negligible in the Ca-containing alloy due to the residual phases and solution hardening effect of Ca.
The objective of this study was to investigate the dependence of the room temperature tensile properties on the volume fraction of discontinuous precipitates (DPs) in a cast AZ91 magnesium alloy. In order to obtain various volume fractions of DPs, the solution-treated alloy was aged at 428 K for up to 48 h. The volume fraction of DPs increased from 0% to 72% with an increase in the aging time up to 24 h; for aging times longer than 24 h, discontinuous precipitation was substantially inhibited owing to the occurrence of significant continuous precipitation within the α-(Mg) grains. YS and UTS of the alloy increased with the volume fraction of DPs, whereas the elongation showed a reverse trend. A relatively rapid change in the tensile properties with increasing volume fraction of DPs up to ~40% was noted, which would be due to the reduction of the effective α grain size in response to the formation of DPs along the grain boundaries.
In this study, the effect of gas pressure on the shape and size of the AZ91 alloy powder produced by using the gas atomization method was investigated experimentally. Experiments were carried out at 820°C constant temperature in 2-mm nozzle diameter and by applying 4 different gas pressures (0.5, 1.5, 2.5 and 3.5 MPa). Argon gas was used to atomize the melt. Scanning electron microscope (SEM) to determine the shape of produced AZ91 powders, XRD, XRF and SEM-EDX analysis to determine the phases forming in the internal structures of the produced powders and the percentages of these phases and a laser measuring device for powder size analysis were used. Hardness tests were carried out to determine the mechanical properties of the produced powders. The general appearances of AZ91 alloy powders produced had general appearances of ligament, acicular, droplet, flake and spherical shape, but depending on the increase in gas pressure, the shape of the powders is seen to change mostly towards flake and spherical. It is determined that the finest powder was obtained at 820°C with 2 mm nozzle diameter at 3.5 MPa gas pressure and the powders had complex shapes in general.