The Effect of the Forming Mode on Twinning and Springback in the Bending-Dominated Forming of Magnesium AZ31 Sheet

The sheet metal forming of magnesium is challenging due to the material's complex springback behaviour, which is due to the tension/compression yield mismatch. In this study, three different AZ31 grain sizes are produced by a special heat treatment, while maintaining the material strength in uniaxial tension at a similar level. Pure, V-die and channel bending tests are combined with roll forming to compare bending scenarios with and without tension applied transverse and parallel to the bending axis. This is complemented with electron backscatter diffraction to measure the twinning type and twinning area fraction (TAF) in the tension and compression bending zones. Our study shows that, like conventional steel, when bending magnesium, springback reduces with the increasing level of the outer fibre bending strain, i.e., when the bend radius is decreased and the TAF increased. It is further shown that when tension is applied, the TAF increases. However, while in some forming cases, the increase in TAF leads to a clear reduction in springback, in other forming cases the effect of the TAF on springback is less pronounced. Overall, this study provides clear evidence that the twinning behaviour in bending magnesium is influenced by the bend deformation mode and that this influences the springback behaviour.