Study On The Formability Of Magnesium Alloy AZ61 And Its Hot Forming Process For Bevel Gear

Magnesium alloys are currently the lightest ones among structural materials with high specific strength and stiffness, superior damping capacity and good electromagnetic shielding characteristic. Magnesium alloys are honored as "the green engineering materials in the 21st century", which can lighten weight, save energy and reduce comsumption if applied in the astronautical and automotive engineering fields. Due to hexagonal crystal structure, magnesium alloys have relatively low plasticity at room temperature while coarse grains and surface oxidation are prone to occur at elevated temperature. At present, the vast majority of magnesium products are in the form of die castings. In contrast with cast magnesium alloys, wrought magnesium alloys have fine and uniform grains and better mechanical properties. In order to investigate the hot forging process of magnesium alloys parts with complicated shape, in this paper, to begin with the hot deformation characteristics of magnesium alloy AZ61 were studied and the flow stress model and microstructure evolution model were put forward. Combined with microstructure evolution, a thermal-mechanical coupled finite element simulation system was developed. Based on the simulated results, hot forging process experiments of magnesium alloy AZ61 bevel gear were conducted.Based on the hot compression tests on Gleeble-1500 thermo- mechanical simulator, the relationship between the parameters of hot flow behavior and Zener-Hollomon parameter was deduced. Then two different methods were proposed to model the flow stress. By statistics, the mean errors of these two models are approximately 5.4%, so the predicted values agree well with experimental results.Based on analysis of the characteristics of flow stress, the dynamic recrystallization kinetics model was established. By the quantitative metallographic analysis and Vickers hardness test, the dynamic recrystallization grain size model and Vickers hardness model after forging were put forward.Two stage flow stress model and microstructure evolution model were integrated into 3D thermal-mechanical coupled rigid-plastic finite element software DEFORM. The hot forging process of magnesium alloy AZ61 bevel gear was simulated. The influences of forging temperature and velocity of top die to forging process of bevel gear were studied by analyzing the distribution of strain, stress, temperature and microstructure evolution during pre-forging and finish-forging.Forging experiments of magnesium alloy AZ61 bevel gear were completed. The simulation data correspond with the experimental results of load-stroke curves, microstructure and surface cracking on some pre-forging specimen under low temperature, which validates the accuracy of the flow stress model and microstructure evolution model in this paper.