Research On Formability Of AZ31 Magnesium Alloy Sheets Within Medium Temperatures

Magnesium alloys have many advantages such as low density, high specific strength and stiffness, good damping characteristics as well as excellent castability. In recent years, magnesium alloys which are the lightest structural alloys are receiving more and more attention because of their features, so the prospects of magnesium alloy sheets are broad. Replacing steel, cast iron and even aluminum alloys by magnesium alloys, the weight of car components can be reduced, so that it can significantly contribute to greater fuel economy and reducing CO2 emissions. However, the use of magnesium alloys is seriously restricted by the limited ductility at room temperature due to their hexagonal close-packed (HCP) crystal structure. With the increasing of deformation temperature, the ductility of magnesium alloys is obviously improved, while the grain size will significantly grow up, at the same time, the cost of magnesium alloy products increases greatly, so studying on the formability of magnesium alloys within medium temperatures (130℃ to 210℃) receives increasing attention. As the most widely used commercial wrought magnesium alloys, it is of great significance to study the warm formability of AZ31 magnesium alloys, which can provide basic information for the real production.The AZ31 magnesium alloy sheets with the thickness of 0.5mm,1.0mm,1.5mm and 2.0mm were used for the present study. To investigate the warm formability of the sheets, the stretching tests were conducted at 130℃,170℃,210℃, with forming speeds of 10mm/min and 50mm/min, respectively. The formability of AZ31 magnesium alloy sheets at warm working conditions was evaluated by forming limit diagrams (FLD). Optical Microscope (OM) was used to examine the microstructure and the fracture morphology was analyzed by Scanning Electron Microscope (SEM). By microscopic observation, the forming mechanism of AZ31 magnesium alloys was also analyzed.The results show that AZ31 magnesium alloy sheets are suitable for stamping in medium temperature ranges. The FLD of AZ31 magnesium alloys is depended on the forming temperature, the formability of alloys can be obviously improved with the increasing of temperature. Non-basal slip system started by thermal activation is considered as the main reason which the plastic deformation of magnesium alloys can be greatly improved with the increasing of temperature. The effects of the thickness of the sheets on the formability are complex, under the constraints of the forming temperature and forming speed. It is also demonstrated that the formability of AZ31 magnesium alloy is on the decline with the increasing of forming speed. The higher the forming temperature is, the more obvious the effects of the forming speed. When the forming speed is 50mm/min, effective stretching tests can be carried out for AZ31 magnesium alloy sheets of 1.5mm only at 210℃. It is clarified that dynamic recrystallization occurs during the forming process of AZ31 magnesium alloys. With the increasing of forming temperature and the decreasing of forming speed, dynamic recrystallization is more likely to occur; the dimple is becoming bigger and deeper, so the formability of the sheets is improved. At 210℃, Friedel-Escaig cross slip is seen as the control mechanism of deformation and dynamic recrystallization, Continuous Dynamic Recrystallization is considered as the nucleation mechanism of AZ31 magnesium alloys.