Formability of Magnesium Alloy AZ31B from Room Temperature to 125 °C Under Biaxial Tension

Magnesium AZ31B sheets of 2 mm thickness were stretch formed using a 101.6 mm diameter punch at room temperature and subsequent increments from 25 to 125 °C. Surface strains were measured using a digital image correlation method in order to ensure that biaxial stretching was achieved. The punch height versus load curve was found to be the same for temperatures of 25 and for 50 °C, while at 75 °C the load for a given punch height was less. This difference seems to indicate a change in deformation mechanism between 50 and 75 °C. Electron Backscatter Diffraction (EBSD) was used to quantify features of the microstructure in the as-received and the strained specimens. Rather than a sudden transition from twinning to slip at low temperatures, it appears that twinning gradually decreases and slip activity increases as temperatures rise across the range from 25 to 125 °C. This confirms recent predictions found in the literature. The twin activity predominantly involves the formation of compression twins which rapidly transform further to create secondary twins for easier strain accommodation.