Microstructure And Mechanical Properties Of AZ31 Magnesium Alloy Sheets Deformed By Differential Speed Rolling

The commercial hot-extruded AZ31 plates measuring 3mm in thickness were used in the study. The DSR process was performed on a mill of different upper and lower roller diameters, which was 135mm and 120mm, respectively. Different DSR processes, heat treatment processes and their influences on the microstructure and properties of AZ31 magnesium alloy sheets were investigated in this paper. Fracture behavior of sheets was also involved.The results showed that:(1) Under the experimental conditions, a critical deformationεC existed in one-pass large deformation DSR with the rolling speed ratio of 1.125 and temperature of 300℃. Microstructure and properties of sheets improved with increasing deformation and declined from the reached best condition when the deformation was≤εC and＞εC respectively.(2) Both of the one-pass and two-pass large deformation DSR processes with pass reduction of 38.1% gained fine equiaxed recrystallized grains. The grain sizes were refined from 13.6μm to 2.8μm and 1.0μm respectively, tensile strength increased from 275MPa to 328MPa and 350MPa respectively, elongation increased from 22.2% to 26.5% and 30.8% respectively, and textures of both were softened to some extent. After annealing at 300℃for 30min, the grains of the two DSR sheets were significantly homogenized. Grain sizes were 3.3μm and 2.1μm respectively, tensile strength decreasing rates were 3.96% and 4.0% respectively, and elongation increasing rates were 24.5% and 21.1% respectively. So it was possible to enhance ductility significantly while keeping the strength slightly reduced by differential speed rolling with subsequent annealing treatment.(3) Microstructure and properties of hot-rolled AZ31 magnesium alloy sheets had a close relationship with the annealing temperature and holding time. The results indicated that microstructure and properties were not undergone any significant changes when annealing at 200℃with increasing the holding time, because of the only occurance of static recovery in these annealing processes.30 minutes of annealing at 300℃were enough to fully recrystallize the microstructure and gain fine and homogeneous equiaxed grains. Part recrystallized grains grew up when the holding time increased to 60min at 300℃. Grains grew up after full recrystallization when annealing at 350℃for 30min and 60min.Significant changes had occurred in tensile strength and elongation when annealing at 300℃and 350℃with different holding time. An optimized mechanical property with tensile strength of 315MPa and elongation of 33.0% was achieved in sheets of 38.1% deformation after annealing at 300℃for 30 min.(4) The variation of average grain size, microhardness, tensile strength and elongation of AZ31 Mg alloy sheets with deformation of 38.1% and 44.5% was similar under different annealing process. When annealing at 350℃for 60min, there was somewhat different. A small number of abnormal grains occurred in sheets of deformation of 44.5% leading to the significant reduction in properties under this annealing process, while there were all normal grains in sheets of deformation of 38.1% under the same annealing process.(5) The SEM analysis showed that after the two-pass DSR process, fracture units reduced visiblely and the ductile rupture feature becomed more and more significant.