Second-order deformation behaviors of free-rotational tension after free-end torsion for rolled AZ31B magnesium alloy

The accompanying dimensional change caused by the second-order deformation behaviors severely limits the accuracy of the size and shape control of the structural parts. The effect of the microstructure evolution mechanism on the second-order deformation behaviors (e.g., Swift effect, inverse Swift effect) of magnesium (Mg) alloys has not been systematically reported. Therefore, in this study, in order to promote the precision forming of Mg alloy structural parts, the deformation mechanisms of the second-order mechanical behaviors of rolled AZ31B Mg alloy were systematically investigated by using the specifically designed multi-degree of freedom and multi-axial force field coupling loading mode. The free-end torsion (FET) of rolled AZ31B Mg alloy was carried out in the range of 30° to 90° along the transverse direction (TD) at room temperature and the free-rotation uniaxial tension was performed after FET. The Swift effect of axial shortening was observed in the FET stage, and the cumulative effect of misfit strain caused by tensile twinning is the main reason for the Swift effect. Meanwhile, obvious inverse Swift effect was detected in the free-rotation uniaxial tension. It is found that the external reverse residual shear stress dominates the rapid reverse rotation of the first stage in the inverse Swift effect, and the nonlinear distribution of the residual shear strain increases this reverse rotation. The competitive effect of detwinning caused by reverse rotation and prismatic slip leads to a slow reverse rotation. The prismatic slip provides the driving force for the inverse Swift effect in the positive rotation stage until the residual shear stress is completely released.