Numerical And Experimental Research On Superplastic Gas Blow Forming Ability Of AZ80 Magnesium Tube

In this paper,the formability and forming limit of AZ80 magnesium alloy extruded tube under different stress states are studied by the method of finite element simulation and experiment.The forming ability of AZ80 magnesium Alloy under three temperatures（320,350 and 380℃）and two strain rates(5×10^-3s^-1and 5×10^-4s^-1)was studied by means of hot tensile test.The material parameters m and K of AZ80 magnesium alloy tube at different temperatures were measured by the method of speed jump test.The free bulging of AZ80 magnesium tube with the size of 34.9mm×1.7mm×65mm and the ratio of height to diameter of 1 is simulated by quadrilateral shell element based on the experimental results of unidirectional hot tensile experiment with Marc finite element software in order to obtain the bulging height and the ultimate expansion rate.In order to obtain the main strain and secondary strain at the point of fracture to obtain the forming limit curve,the ellipse bulging with the long and short axis sizes of 40 mm-40 mm,40 mm-30 mm,40 mm-20 mm and 40 mm-10 mm of the magnesium tube with the size of 45 mm×2 mm×65 mm is simulated.By simulating the free bulging of magnesium tube under gas blow forming,the variation of wall thickness and equivalent strain rate with the forming process is analyzed,and the predicted values of forming pressure and forming time corresponding to the strain rate of hot tensile test are obtained.The predicted bulging height and ultimate expansion rate of the free bulging of tube under forming pressure and forming time are obtained.The higher the forming temperature,the smaller the strain rate and the larger the ultimate expansion rate.Through the simulation of ellipse bulging of magnesium tube,the predicted values of forming pressure and forming time corresponding to the target strain rate are accquired,and the corresponding predicted values of primary strain and secondary strain are got,and the forming limit curves under different temperatures and strain rates are accquired.Based on the simulation results,the gas blow forming experiments of AZ80magnesium alloy tube were carried out,and the influence of forming temperature and pressure on the forming ability of magenisium tube was studied.Under the same forming temperature,the ultimate expansion rate of the tube increases first and then decreases with the increase of the forming pressure.There is the optimum forming pressure.The error between the experimental value and the simulation value is between 37.93%and 15.74%,and the simulation value is higher than the experimental value always.With the increase of the forming temperature,the error between the simulation value and the experimental value decreases.Based on the predicted forming air pressure,the elliptical bulging experiment was carried out at 380℃and 5×10^-4s^-1,and the forming limit curve of magnesium alloy tube was drawn by measuring the size of strain circle printed on the tube surface.The results show that the trend of forming limit curve is consistent with that of simulation,and the error is less than 18.2%.