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

Forming limit diagram (FLD) can reflects the forming property of metal sheet by showing its maximal major strain and minor strain before broken. It is the most simple and intuitive tool to predict and solve the problem during forming process. In recent years, researches on the FLD of magnesium alloy sheets have attracted more and more attention, but most studies were carried out with experimental method. So the study of FLD of magnesium alloy sheet by theoretical and numerical way or method at elevated temperatures has important significance to promote its development and application.In this paper, a prediction of FLD of AZ31magnesium alloy sheet at elevated temperatures was conducted by using M-K theory. The effects of material parameters and exponent parameters in anisotropic yield criterion to predicted results were discussed. The most ideal exponent parameters were obtained by comparison. A good theoretical computation model for AZ31magnesium alloy sheet at different temperatures was acquired. A calculation of FLD of AZ31magnesium alloy sheet at different temperatures was carried out by combining the ductile fracture criterion proposed by Jieshi Chen and finite element numerical simulation of the FLD experiment. The expression of parameters of the ductile fracture criterion for magnesium alloy was proposed. The change of the maximum value of general plastic work before broken was discussed.Following conclusions were drawn from the research:(1) It is shown that the predicted FLD was strongly influenced by the parameters and exponent parameters in anisotropic yield criterion. The predicted FLD would go up with the increase of strain hardening exponent and strain rate sensitivity index and the decrease of anisotropic index. The exponent parameters in anisotropic yield criterion make few differences to the left part of FLD, but for the right part, the raise of exponent parameters in Logan-Hosford yield criterion and the decrease of exponent parameters in Hill79yield criterion both lead to the rise of predicted FLD.(2)The most suitable yield criteria for predicting the FLD of AZ31magnesium alloy sheet is not the same at different temperatures. At150℃, the best predictions are obtained when Hill79yield criterion is applied, but at200℃and250℃, Logan-Hosford yield criterion is more appropriate. As a whole, Logan-Hosford yield criterion is most applicable to FLD prediction of AZ31magnesium alloy sheet at elevated temperatures.(3) The ultimate strain of AZ31magnesium alloy sheet during processing can be calculated accurately by using the ductile fracture criterion proposed by Jieshi Chen and the expression of parameters for magnesium alloy proposed in this article. The maximum value of general plastic work of the material before broken would decrease with the increase of temperatures, and the value of which is more sensitive to the change of temperature at low temperatures.