Abstract: Magnesium alloys are highly susceptible to cracking during the forming process, and accurate damage prediction can provide technical support for the plastic forming process. To this end, the Gurson-Tvergarrd-Needleman (GTN) damage model with a continuous medium shear damage factor was applied to predict the damage evolution of magnesium alloys during forming, and the flow stress and shear damage parameters of AZ31B magnesium alloy were identified by force-displacement curve with compressed specimen to predict and verify the surface damage and cracking of AZ31B magnesium alloy under upsetting process. The applicability of the model for predicting damage in the forming process under low stress triaxiality was extended. On this basis, the causes of damage and crack propagation were analysed for AZ31B magnesium alloy under compression conditions. The results showed that with the increase of compression displacement, The shear stress in the center area of magnesium alloy side increased, under the effect of which the voids inside the matrix grew and coalesced in the non-vertical compression direction, and eventually formed macroscopic cracks; the lateral crack direction of the upset specimen was in the range of 34°-46° with respect to the vertical compression direction, and the simulation results of the improved GTN model were 44°-54°, while the simulation results of the original GTN model were 90°; The improved GTN model could be applied to predict the damage evolution behaviour of magnesium alloys under rolling, hot stamping and other processes, laying the foundation for subsequent process improvement.

Key words: AZ31B magnesium alloy, GTN damage model, upsetting forming, low stress triaxiality, voids