Effect of billet diameter on hot extrusion behavior of Mg–Al–Zn alloys and its influence on microstructure and mechanical properties

The effect of the initial billet size on the microstructural evolution and tensile properties of extruded AZ61 and AZ91 Mg alloys was investigated using homogenized billets with diameters of 50 and 80 mm. All the extruded alloys exhibit a bimodal structure consisting of fine dynamically recrystallized (DRXed) grains with numerous precipitates and relatively coarse DRXed grains with few precipitates. Although the size and hardness of the fine DRXed grains do not vary with the billet size, using a smaller billet results in a significant reduction in the size and area fraction of coarse DRXed grains because of an increase in the quantity of precipitates. This increase in both the fraction of fine grains and the number of Mg17Al12 precipitates improves the tensile strength of the extruded alloys. This size effect can be attributed mainly to the increase in the strain rate with decreasing billet diameter, which promotes dynamic recrystallization and precipitation during extrusion. The effect of billet size is more pronounced in the AZ91 alloy, which has a high concentration of alloy elements, than in the AZ61 alloy.