Microstructure And Deep-Drawing Mechanism Of AZ31 Mg Alloy Sheets

Magnesium alloy sheet metal stamping forming products have excellent characteristics such as light weight,mechanical properties and good electromagnetic shielding properties.With these advantages,Mg alloy has a good prospect in 3C shell stamping field.Therefore,it is important to study the nature of microstructure evolution during deep drawing process so as to improve its microstructure and properties,which has of great engineering value to enhance the drawing proformance of Mg alloy sheet and expand its application in Mg alloy shell parts field,such as mobile phone shell.In this thesis,AZ31 Mg alloy sheets with thickness of 2mm and 1mm were prepared by extrusion-rolling combined forming process.After the sheet with a thickness of 2 mm was annealed,it was found that twins become disappeared on the rolled sheet along with the grains being smaller and the structure getting more uniform,and the elongation of the annealed sheet reached 22.5%.By testing the limiting deep drawing performance of 1mm AZ31 Mg alloy sheet at different temperatures,it is found that the temperature has a great influence on the drawing performance.From the room temperature to 200 °C temperature range,the limit drawing performance increases with increasing temperature.The ultimate drawing performance is best at 200 °C,and the limiting draw ratio LDR is 2.0.After the temperature exceeds 200°C,the limit drawing performance decreases.Through the physical simulation experiment of the deep-drawing process of the blank holder area,the plastic deformation mechanism of the plate in the blank holder area was studied by EBSD analysis combined with the VPSC metal plastic deformation model.The results showed that the deformation process is caused by dislocation slip.In combination with the twin shear,the start of the deformation,the base slip and the tensile twin start at the same time;with the increase of the strain,the cylinder slip gradually starts,played an important role in the deformation,and the role of the twinning gradually reduction.After deformation of this process,the texture of the basal surface of the rolled plate weakens,and a small portion of the grains deflect toward the RD direction.The XRD texture analysis of the process of deformation of the blank area,fillet area and sidewall area of the deep drawing cylinder specimen at room temperature and 200°C was carried out to study the mechanism of the twinning mechanism during the deformation process.The results show that the twinning mechanism is the same at room temperature and 200°C.The difference is that the deformation at room temperature is small and part of the crystals are twinned;at 200°C,the deformation is large and all crystal grains are twinned.During the entire deformation process,a twinning and corresponding untwisting occurs.The deformation of the sheet material in the blank holder area is stretched.The crystal grain rotates at the bottom of one side and rotates 86.3°from the ND direction of the sheet to the RD direction.The round corner zone and the side wall zone retreat,and the crystal grains return from the RD direction to the original plate base plane direction.