Experimental And Numerical Study Of Forming Limit Diagram Of AZ31Magnesium Alloy Sheet At Elevated Temperatures

Magnesium alloys, light weight metallic structural materials with advantages of low density, high specific strength and specific stiffness, high thermal conductivity, excellent electromagnetic shielding and so on, their sheets will be comprehensively used in aviation and spaceflight, transportation vehicles, Electronic communications fields and so on. Due to the close-packed hexagonal structure of magnesium, the magnesium alloy sheets have poor formability under room temperature and low rate of finished products, which has restrict its application.Forming limit diagram (FLD) is the most simple and intuitive method to estimate and evaluate the formability of sheet metal, and also can be used to predict and judge the success or failure of stamping process, to assist for the determination of stamping process parameters. So it is a very effective tool to solve the problem of sheet metal stamping. In recent years, FLDs were used widely to evaluate the comprehensive properties of magnesium alloy sheets by domestic and foreign scholars, but most focus on experimental study and little investigation on their application. So the study of FLD of magnesium alloy sheet at elevated temperatures has important significance to promote its application.The hot sheet formability test machine and the grid measurement&analysis system which developed by BUAA were used to determine the FLD of AZ31magnesium alloy sheet at the forming temperature range of150～250℃ and the influence of forming temperature, stress state and strain rate on the formability of AZ31magnesium alloy were analyzed. The FLD experiments of AZ31magnesium alloy at the temperature range of150～250℃were simulated using DYNAFORM, and the influence of judging criterion on the acquisition of limit strain data as discussed. In order to research into the application of FLD in finite element simulation, the deep drawing processes were simulated using DYNAFORM with the FLDs and Keeler’s function as fracture criterion respectively in the forming temperature range of150～250℃.Following conclusions were obtained from the research:(1) The forming temperature has a great influence on the FLD of AZ31magnesium alloy, the height of FLD curve increases with the increase of forming temperature. The punch speed has little influence on the formability of AZ31magnesium alloy at150℃, yet the influence at the temperature between200℃to250℃is rather huge. The stress state has great influence on the formability of AZ31magnesium alloy at150℃, while it has less influence at the temperature range of200～250℃(2) The forming process of AZ31magnesium alloy can be predicted accurately using DYNAFORM at different temperatures, and the limit strain date can be determined more precisely by using the maximum load judging method and the strain path judging method in the process of simulation test. The effect of friction condition on the FLD of AZ31magnesium alloy is remarkable, the height of FLD curve decreases with the increase of friction coefficient. The influence of strain rate on the formability of AZ31magnesium alloy is related to forming temperature, it increases with the forming temperature increasing from150℃to250℃, which agrees well with the experimental results.(3) It can be predicted more precisely that whether the fracture will happen or not in the forming process by using the FLD as the fracture criterion than by auto Keeler’s fracture criterion when simulating with DYNAFORM.