Study On Pretreatment And Compression Superplasticity Under High Strain Rate Of Commercial AZ31 Magnesium Alloy-Extruded

For the less density, high specific strength, specific stiffness, good heat conduction, easy recovery, and so on, magnesium alloys have been applied extensively in many fields, such as aerospace, automobile, electronic industry. Among all magnesium alloys, commercial AZ31 magnesium alloy has most widespread application in commercial application. However, the plastic forming properties of magnesium alloys is poor at room temperature as the hexagonal close-packed (HCP) crystal structure and coarser grains. As a result, the magnesium alloys is difficult to be plastic formed directly with forging forming method. However, with the process of pretreatment, the grain size of commercial AZ31 could be refined, In addition, the problem of poor forming ability can be resolved for the property of superplastic forming of magnesium alloy. In this paper, study on commercial AZ31 magnesium alloy-extruded mainly focuses on following two aspects.First of all, pretreatment process of commercial AZ31 magnesium alloy-extruded was optimized with orthogonal experiment. The result shows that, static recovery and recrystallization can refine grain size in the process of pretreatment. In condition of air cooling at 325℃, holding 20 min, the grain size is refined from the original size about 45.4μm to 26.3μm. With higher heating temperature and longer holding time, the grain size is refining firstly, but coarsening gradually.Secondly, the compression superplasticity of commercial AZ31 magnesium alloy-extruded is investigated with Gleeble3800 thermodynamic simulation tester. The forming ability of compression superplasticity is analyzed under the high strain rate, as well as the effect of forming temperature on the compression rheological stress and distribution of the microstructure resulting from compression deformation, which is in accordance with stress-strain curve and deformation of micro-structure. Finally, the compression superplastic deformation mechanism is researched. The results show that, with high strain rate and deformation temperature in the range of 150°C ~ 200°C, the compression superplastic performance is poor, but in the range of 250℃～300℃, the elongation of outer circumference is over 175.94%. It is possible to realize the Superplastic forming compression under conditions of low temperature and high strain rate. In the same strain rate and variable condition, compression rheological stress is gradually lowered with increased temperature. In addition, peak stress value decreases with the increase of forming temperature. When deformation temperature is in the range of 150°C ~ 200°C, its microstructure are scattered fracture after compression deformation, also lots of dislocation, twin crystaland twin crystal group emerge, meanwhile, obvious dynamic recrystallization organization distribution disappear. But when the deformation temperature is in the range of 150°C~200°C, obvious dynamic recrystallization organization distribution exists in the microstructure of the compression deformation, and dynamic recrystallization is more complete with increased temperature. The deformation mechanism of the compression superplasticity of commercial AZ31 magnesium alloy-extruded is grain boundary sliding mechanism in coordination with the dynamic recrystallization.