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.
