Microstructure and mechanical properties of curved AZ31 magnesium alloy profiles produced by differential velocity sideways extrusion

Lightweight curved profiles are widely utilised in the transportation industry considering the increasing need for improving aerodynamic efficiency, aesthetics and cutting emissions. In this paper, curved AZ31 Mg alloy profiles were manufactured in one operation by a novel process, differential velocity sideways extrusion (DVSE), in which two opposed rams were used. Effects of extrusion temperature and velocity (strain rate) on curvature, microstructure, and mechanical properties of the formed profiles were examined. Profile curvature was found to be more readily controlled by the velocity ratio of the bottom ram v2 to the top ram v1, whereas extrusion temperature (T = 250, 300, 350°C) and extrusion velocity (v1 = 0.1, 1 mm/s) slightly affect curvature for a given velocity ratio. A homogeneous microstructure with equiaxed grains (∼ 4.5 µm) resulted from dynamic recrystallisation (DRX), was observed after DVSE (v2/v1 = 1/2) at 300°C and v1 = 0.1 mm/s, where the initial billet had an average grain size of ∼25 um. Increasing extrusion temperature leads to grain growth (∼ 5 µm) at 350°C and v1 = 0.1 mm/s. DRX is incomplete at the relatively low temperature of 250 °C (v1 = 0.1 mm/s), and higher strain rate with v1 = 1 mm/s (T = 300 °C), resulting in inhomogeneous bi-modal necklace pattern grains ranging in size around 1-25 µm for the former and 2-20 µm for the latter. Grain refinement is attributed to DRX during the severe plastic deformation (SPD) arising in DVSE, and initiates at the prior boundaries of coarse grains in a necklace-like manner. Compared with the billet, micro-hardness and ultimate tensile strength of the profiles have been enhanced, which is compatible with grain refinement. Also, an obvious increase in tensile ductility was found. However, yield strength slightly decreases except for the complete DRXed case (300 °C, v1 = 0.1 mm/s), where a slightly higher value was found, indicating strengthening by grain refinement is greater than softening caused by texture modification. The initial billet had a strong basal texture wherein the {0002} basal plane is oriented parallel to the extrusion direction (‘hard’ orientation), while DVSE results in the profiles having weak basal textures and the {0002} basal plane oriented ∼ 5-10o to the extrusion direction (i.e. towards the orientation for easier slip). This significantly modified texture contributes to the softening of the profiles in the extrusion direction, in which tensile tests were performed, and the related elongation improvement.