Numerical And Experimental Studies On Warm Deforming Of AZ31 Magnesium Alloy Sheet

Magnesium alloys are the lightest structural material therefore are very suitable for application in the transportation industry, aerospace and 3C industry. However, magnesium alloy has traditionally been considered one of the less shapable metals at room temperature due to its close-packed hexagonal structure, while its formability is excellent at warm condition. The previous investigations have shown that magnesium alloy might undergo the grain boundary (GB) sliding and dynamic recrystallization (DRX) pheomena during hot working processes. The grain refinement during DRX is insignificant at high temperatures due to the rapid grain growth. So controlling warm deforming condition is important to make the microstructure evolution of magnesium alloy to the expecting status.There are two main research methods to develop the sliding GB and DRX of magnesium alloy during warm deformation. However, it is different to observe GB sliding and DRX at warm deforming conditions of magnesium alloy due to the limitation of test conditions and research way. Therefore, based on the relationship between the deforming conditions and dynamic recrystallization, the macro-micro constitutive model including the effect of DRX has been developed. According to the experimental and numerical method, the quantitative influence of deforming conditions on GB effect and DRX evolution has been studied. The microstructure evolution has been simulated during forming process of magnesium alloy sheet. And the distribution of physical field has been obtained. According to the researches, the formability of magnesium alloy sheet can be predicted and the forming technology can be optimized.Through isothermal tensile test and metallographical observation, the DRX of AZ31 magnesium alloy sheet has been studied. The effect of deforming conditions has been explored, and the influence of deforming conditions on the volume fraction of DRX has been analyzed. This research can be used as the foundation to construct the constitutive model and simulate the evolution of DRX.Based on the analysis of effect of deforming conditions on the flow stress, stress exponential, deforming activation energy, peak strain, hardening ratio curves, critical strain and influence of deforming conditions on the volume fraction of DRX have been studied. According to these researches, a macro-micro constitutive model containing the effect of DRX has been put forward. This constitutive model can describe the effect of DRX on the macro-mechanical properties of magnesium alloy sheet, and it can be used as a theoretical method for formation of magnesium alloy sheet and improve the warm forming theory of thin sheet.The uniaxial tension of AZ31 magnesium alloy sheet at different temperatures is simulated using the thermoviscoplastic constitutive model. A representative volume element taking from the tension model is constructed to simulate the relationship between GB and deforming conditions. The results show the influence of deforming conditions on the GB and distribution of the stress and strain in grain and GB.Combining the Cellular Automata (CA) method with DRX theory, the DRX evolution model of magnesium alloy has been developed. The effect of deforming condition on the DRX kinetics during warm tension of magnesium alloy has been analyzed. The results show that the stress-strain curves, the structural transformation and the grain size distribution of ultimate tension agree with the test results. The phenomenon of DRX is obvious during warm tension of AZ31 magnesium alloy. When the deforming condition fits the critical condition of DRX, the grain boundary of DRX grains first nucleating and growing will become the position of next nucleation. This will make the grain fined and homogeneous. The results of analysis of DRX kinetics show that the DRX of warm tension of AZ31magnesium alloy does not complete at lower temperature and high strain rate. Compared with the classical recrystallization kinetics curve, the DRX kinetics curve is incomplete shape of S.Based on the above research, the constitutive model according to the CA method is introduced into the numerical simulation of stamping. The results show that the distribution of stress and strain of cup is homogeneous. Grain size at region of die profile is smaller than it at wall of cup. And the grain size of numerical simulation agrees well with the experimental results.The research of theoretical and numerical simulation of microstructure evolution of magnesium alloy at warm deforming conditon can make microstructure evolution rule obvious and improve the forming theory of magnesium alloy sheet. The forming properties of magnesium alloy sheet can be quantitatively predicted according to the study of DRX evolution. The research of DRX evolution can supply necessary theory of plastic processing technic and promote the forming technique of magnesium alloy sheet.