Influence of heat treatment and forming cycle on the precise forming of AZ31 magnesium alloy sheet

Precision forming of 3 mm thick rolled AZ31 magnesium alloy sheet at 90° results in severe cracking and a considerable rebound angle. The effects of different sheet heat treatments and changes in the forming cycle on the value of the forming angle were investigated using 90° precision forming tests. The reasons of sheet cracking were examined using tensile tests and numerical simulations. Microscopic analysis and characterization techniques such as metallographic experiments, SEM observations, and EBSD tests are used to clarify the mechanism of improvement and control of the forming angle value. The results indicate that annealing treatment exhibits high sensitivity to controlling sheet cracking, but low sensitivity to controlling rebound. Among them, after heat treatment at 250 °C+1 h, the sheet no longer cracks during forming. In incomplete annealing, the more small grains produced by recrystallization and the greater the number of dimples in the fracture, the closer the forming angle is to 90°. Full annealing can result in a small reduction in the forming angle value compared to incomplete annealing treatments. After extending the forming cycle for 1 h, the increase in the ratio of dual tensile twin boundaries and low angle grain boundaries resulted in a significant decrease in the value of the forming angle and no cracking of the sheet. The increase of recrystallized tissue on the tensile side and the increase of the KAM value led to the decrease of the forming angle value to the minimum value at 300 °C+1 h heat treatment condition.