Theoretical And Experimental Investigation Of Warm Forming Of Magnesium Alloy Sheet

Magnesium alloy as the most development potential and competitive light metal is named as"the 21 Century's green engineering material"because of its low density, excellent specific strength and recyclebility. Warm deep drawing of magnesium alloy sheet not only meets the demands on the environmental protection and the lighter trend of products, but also can remarkably improve productivity and qualification of the products. The techniques of warm forming of magnesium alloy have been paid a great deal of attentions in recent years. At present, most investigations about warm deep drawing of magnesium alloy are focus on process and experiment. Few efforts are made on deformation mechanism in theory. So the theoretical investigation of warm forming technology also needs to be explored. However, the process of warm deep drawing of magnesium alloy sheet involving many processing parameters is a nonlinear producer of multi-fields coupled. Process design adopting try-and-error methods already have not made full use of favorable warm performance of magnesium alloy. Quantitative design of process based on numerical simulation is the development trend in the near future. So the research of warm deep drawing of magnesium alloy sheet is already became the most challenge project in the application technology of magnesium alloy.In this thesis, AZ31 magnesium alloy sheet is the main investigated subject. Theory and process study of warm deep drawing are the two main research threads, and the investigations are divided into three levels (basis level, theory level, application level). The combination research method of experiment with numerical simulation is adopted in general. The investigated route is from fundamental performance tests to warm forming theory, and then from warm forming theory to warm deep drawing experiments.AZ31 magnesium alloy sheet with good performance is fabricated through cross rolling and annealing heat treatment process. Through uniaxial tension tests and metallographic analysis, the mechanical behavior and the microstructure warm deformation characters of AZ31 magnesium alloy sheet are analyzed. A ductile fracture criterion of magnesium alloy sheet considering temperature effect is put forward. Forming Limit Diagrams experiments are also performed, and the forming limit curves of AZ31 magnesium alloy sheet are obtained under different forming temperatures. The material paramaters are calculated by the combination method of experiment with FE simulation. Thermo-mechanical coupled simulation of warm deep drawing of AZ31 magnesium alloy sheet is also performed. The effects of drawing temperature and blank holder force on the ductile fracture criterion are quantitative analyzed by numerical simulation method, and the deformation mechanism is disclosed. A new process of variable blank holder force and non-isothermal deep drawing of AZ31 magnesium alloy sheet is designed numerically. Finally, a hydraulic press with adjustable blank holder force and non-isothermal deep drawing die are manufactured. Real-time control of temperature and blank holder force is realized. Experimental verifications of variable blank holder force and non-isothermal deep drawing process are performed. The warm forming performance of AZ31 magnesium alloy sheet can be improved remarkably through the variable blank holder force and non-isothermal deep drawing process.The creatively work in this thesis as follows:1. The ductile fracture criterion of magnesium alloy sheet considering temperature effect is put forward. The difficulties of accurately deciding cracks in numerical simulation of warm forming are resolved. Based on this ductile fracture criterion,the deformation mechanisms of drawing temperature and blank holder force are disclosed. The change from qualitative to quantitative process design is realized.2. The clear meshes are fabricated by electrochemical etching method. Forming limit diagrams experiments of AZ31 magnesium alloy sheet are performed at different drawing temperature of 100℃, 250℃and 300℃, and the forming limit curves are measured by strain analysis technology. The warm forming performance evaluation criterion of AZ31 magnesium alloy sheet is put forward. The difficulty of quantitative evaluation of warm forming performance of AZ31 magnesium alloy sheet is resolved.3. Real-time control of punch load, punch velocity and drawing temperature are realized by variable blank holder force hydraulic press and non-isothermal die. The new process of variable blank holder force and non-isothermal deep drawing process of AZ31 magnesium alloy sheet are explored. The limit drawing ratio can reach 3.5.The research of this thesis is help to extend the application field of forming technology of magnesium alloy sheet,and develop the warm forming theory. It can promote the application of warm forming technology of magnesium alloy sheet.