Research Of Warm Forming Technology For6000Serious Aluminum Alloy Sheet Used For Automotive Body

Safety, energy saving and environmental protection has become the theme of the current automobile manufacturing industry development. Lightweight became the main trend of modern structure design, which is very critical to the auto body weight reduction, the improvement of vehicle fuel economy, the energy conservation and environmental protection. As the Directive2003/102/EC pedestrian protection laws in European Union constantly revised and execution, the pedestrian protection laws has had an significant effect on national automobile manufacturing industry, and proposed the new demand that the key body cover parts such as engine cover, luggage cover plate, roof cover, door, etc. should adopt the non-ferrous metal material instead of the traditional steels. Therefore, the use of light metals such as aluminum and magnesium alloy instead of traditional steel for the design, application and manufacturing research of automobile body covering parts is of great significance for the sustainable development of automobile industry and personal safety.As the representative of the lightweight metal materials, the aluminum alloy plate is featured with the typical characteristic of low density and high specific strength. However, its widely application is limited due to its low plasticity and poor ductility in room temperature. Aluminum alloy sheet warm forming technology is to heat the blank to a certain temperature when stamping, so as to realize its formability improvement since that the flowability of metal sheet can become much better when in high temperatures. Based on the reasonable technical process, parts with complex structures can be precisely formed, and the formability can be greatly improved as well.Among them, the strength and stiffness properties of aluminum alloy sheet, the rheological properties and formability under high temperatures, and the forming process, etc. are the key points for the research of aluminum alloy sheet warm forming technology. In this work, based on the auto body high strength6000serious aluminum alloy sheet, which is featured with the baking hardened behavior, the experiment and numerical simulation are carried out for the warm forming technology research, and the high temperature constitutive equation for6000serious aluminum alloy sheet is established. Based on the constitutive model, the numerical simulation to investigate the forming processes of forming temperature, blank-holder force, high temperature lubrication and friction are carried out, which provides an important theoretical and practical guidance for the industrial application of aluminum alloy sheet warm forming technology.Firstly, experiments to study the material bake hardening performance are implemented on auto body commonly used heat treatable6000serious aluminum alloy and the influence of pre-deformation was analyzed together with the micro mechanism and microstructure characteristics in bake hardening behavior. Then, discussion was carried out on the high temperature constitutive equation modeling methods and modeling means of aluminum alloy sheets, and the applicability of several yield criterions suitable for aluminum alloy sheet were analyzed according to the high temperature characteristics of6000aluminum alloy. Based on the flow stress characteristics in temperature range of25℃～400℃obtained in the high temperature thermal tensile test in different temperatures and different strain rates, the "hardening" and "soften" rheological property under different high temperatures were analyzed for6000serious aluminum alloy sheet. Based on the introduced Field-Backofen model and modified Field-Backofen model, the high temperature constitutive equation for6000serious aluminum alloy sheet was finally defined.Secondly, by analyzing the similarities, differences and difficulties of aluminum alloy forming limit test in room temperature and high temperatures, the laboratory self-developed high temperature forming limit test TFLD equipment was introduced. With this TFLD equipment, Nakajima test under four temperatures as room temperature,150℃,200℃and250℃were carried out on6000series of aluminum alloy sheet, and the thermal forming limit diagram in four temperatures were obtained, which is the basis for forming process simulation of warm forming technology.Finally, by introducing the high temperature constitutive equation based on test and the fracture criterion of thermal forming limit derived from experiment, the numerical simulation for6000series aluminum alloy was carried out in the FEM software of Dynaform. The simulation based on the punch model showed that the LDH of simulation was in good agreement with those obtained in test results, which verified the effectiveness of material constitutive model and the forming limit curve. Then, the simulation based on reverse draw model studied the process influence of forming temperature, blank holder force, high temperature lubrication and friction, which provides the technical reserve for further expand research of6000series aluminum alloy warm forming.