Improved Stretch Formability of AZ31 Magnesium Thin Sheet by Induced {10–12} Tension Twins

In order to improve the stretch formability of magnesium alloy sheets, {10–12} tension twins were introduced by pre-compression along the rolling (RD) and transverse directions (TD) with 1.6%, 3.3%, and 5.4% strain levels. Subsequent annealing at 473 K was conducted for 6 h to preserve twinning lamellae. In order to avoid bending–buckling, a special fixture for thin sheet compression has been developed. In addition, hemispherical tests were performed at room temperature. The mechanical properties were improved, while the Lankford value (r-value) decreased and the strain hardening exponent (n-value) increased as the pre-compression level increased compared with the as-received sheet. The stretch formability was also improved. The Erichsen value was dramatically improved the most, from 2.83 mm in the as-received to 5.36 mm and 6.78 mm in TD and RD pre-compressed 5.4% Mg specimens, respectively, which increases by 89% and 139%. The stretch formability was enhanced much more with the pre-compression along the RD as compared to the TD.