Effect of Expansion Sphere Diameter on Deformation Behavior of AZ31 Mg Alloy during Extrusion

A new process of expansion-shear extrusion (ESE) was proposed to fabricate AZ31 Mg alloy sheets. The effect of the expansion sphere diameter on the deformation behavior of AZ31 Mg alloy was analyzed by the finite element method and electron backscatter diffraction. The results showed that the dead zone range increases with the increase in the diameter of the expansion ball during the extrusion of the expansion ball. When the diameter of the expanded ball is 30 mm, the strain inhomogeneity and flow velocity non-uniformity parameters are as low as 0.262 and 0.049, respectively. The average grain size of AZ31 Mg alloy can be refined from 150 (±5) to 2.4 (±0.3) μm by the ESE deformation. The deformation mechanism of the microstructure of the expanded ball region is twin-induced dynamic recrystallization and continuous dynamic recrystallization. The low-angle grain boundaries in the coarse grains inside the expanded ball will further evolve into high-angle grain boundaries with the increase in strain of the subsequent extrusion process. In addition, the coarse grains activate the non-basal slips, which will further coordinate the subsequent extrusion ratio and shear deformation, and promote the occurrence of dynamic recrystallization.