Microstructure and mechanical properties of AZ31 magnesium alloy sheets processed by constrained groove pressing

Constrained groove pressing (CGP) is of great potential for fabricating ultra-fine grained (UFG) sheet metals based on severe plastic deformation. However, researches on CGP of materials with poor formability and low ductility at room temperature (RT) are still very limited. Thus, in this work, an experimental setup with heating systems was developed for an improved processing efficiency by virtue of its new die design. CGP of AZ31 magnesium alloy sheets were performed at RT, 423, 473, 523 and 573 K. Their cracking behavior and the effects of pass number and processing temperature on the evolutions of their microstructure and mechanical properties were studied in detail. The results showed that the number of effective pressings gradually increased with processing temperature. Only one pass including four pressings could be finished at 423 K, and twins inside initial grains and dislocation cells were observed in the end. Two passes could be conducted to the materials at temperatures higher than 423 K, and bimodal structures formed due to the occurrence of dynamic recrystallization (DRX) and experienced various evolutions. The microstructure evolution of the materials was dominated by continuous DRX, and a relatively refined and homogeneous UFG microstructure was achieved at 473 K after two passes. In this situation, the average grain size was refined to 2.81 µm from the as-annealed 6.92 µm, and subgrains sized about 1 µm were observed. The influence of pass number on tensile properties gradually decreased with processing temperature. Work hardening, grain refinement and texture modification together decided the evolution of mechanical properties of the alloy sheets during CGP. The optimum comprehensive mechanical properties with yield strength of 240 MPa, tensile strength of 295 MPa, elongation to failure of 18.5% and average microhardness of 73.9 HV were achieved at 473 K during the second pass. In addition, the strong basal texture of the as-annealed sheets initially with a maximum intensity of 18.05 was decreased to 10.61 at 473 K while no significant change was made when processed at higher temperatures.