In the present work, cross-wavy bending at room temperature was carried out to tailor the microstructure and stretch formability of rolled AZ31 sheets. Wavy bending processing generates profuse {10–12} twins and a tilt basal texture. Subsequent recrystallization annealing causes grain coarsening and enhances the intensity of twin-orientation. The combined use of wavy bending and annealing can maintain high tensile ductility and remarkably enhances the stretch formability of rolled AZ31 sheet. It can be mainly attributed to the non-basal texture in the wavy bent sheet which increases the thinning capability during in-plane tension.

In this study, the influence of pre-tension on free-end torsion behavior and compression mechanical properties and micro-hardness of an extruded AZ31 Mg alloy was investigated using electron backscatter diffraction (EBSD), compression testing and micro-hardness testing. The result indicates that pre-tension can cause significant dislocation strengthening, which can increase the torsion yield strength and make the shear stress–shear strain curve of the pre-tension sample almost parallel to that of the as-extruded sample during plastic deformation stage. Texture in edge position on the cross-section of both the pre-tension and as-extruded samples can be rotated towards the extrusion direction by about ~30° by free-end torsion. The Swift effect is mainly responsible for the occurrence of massive extension twins in the central region. In contrast, normal stress is the main cause of extension twins occurring in the edge region. However, the effect of extension twins on micro-hardness is less than that of dislocations. The micro-hardness of both free-end torsion specimens increases almost linearly with increasing distance from center to edge on the cross-section. Nevertheless, the increase in micro-hardness of the pre-tension and then torsion sample is inconspicuous because pre-tension leads to dislocation proliferation and dislocation accumulation saturation. The result also indicates that both pre-tension and free-end torsion can lead to dislocation strengthening, which can obviously increase the micro-hardness and compressive yield stress. The underlying mechanisms were explored and discussed in detail.

This review summarizes the fundamentals of mechanics, microstructure evolution, mechanical properties improvement and numerical simulation of Mg alloys subjected to free-end torsion. The circular solid rod, thin-walled tubular and rectangular cross-sectional Mg alloy samples will produce linear, uniform and nonuniform shear strain distributions across the cross section during torsional deformation, respectively. This will ultimately lead to linear, uniform and nonuniform microstructure distribution. Most samples are dominated by dislocation slip and extension twinning during torsional deformation. In addition to the thin-walled tubular sample made from a rolled AZ31B plate, which is twisted around the ND, the sample has a unique twinning behavior, including extension twins, compression twins, extension-compression double twins and extension-compression-extension tertiary twins. The Swift effect, a distinctive manifestation of torsional deformation, is sensitive to the torsion axis direction, texture orientation and tension-compression yield asymmetry. Whether the sample is a circular solid rod or a thin-walled tubular, if the c-axis of most grains is perpendicular to the torsion axis, the sample will undergo axial contraction during torsional deformation. Conversely, if the c-axis of most grains is parallel to the torsion axis, the sample will undergo axial elongation. In addition, the Swift effect is related to the tension-compression asymmetry. If the TYS is greater than the CYS, the sample will be shortened when twisted in that direction, and vice versa. Moreover, free-end torsion has been shown to be an effective method for reducing tension-compression yield asymmetry and increasing microhardness. Some simulation methods can accurately capture the characteristics of torsional deformation of Mg alloys, such as the Swift effect, the activation of dislocation slip, twinning deformation behavior and texture evolution. Finally, the research trends and future challenges of Mg alloys undergoing free-end torsional deformation are proposed.