Theoretical Investigation And Thermo-mechanical Coupling Simulation Of Magnesium Alloys During Stamping Forming

As the lightest structural alloys,magnesium alloys have been paid more and more attention in the electronics,vehicles,aerospace and other fields for lightweight design,energy saving and emission reduction.Compared with magnesium casting alloys,wrought magnesium alloys have good comprehensive mechanical properties,so it has more application potential,while stamping forming is an important method for plastic forming of magnesium alloys.However,due to HCP(Hexagonal Close-Packed)crystal structure,magnesium provides only limited ductility for cold forming operations.But its formability is improved obviously at elevated temperature.In the present thesis,the anisotropic constitutive and failure model,forming limit theory of magnesium alloy under warm stamping forming conditions were systematically studied,which was funded by the National Natural Science Foundation of China.Meanwhile,the numerical simulation results were compared with the experimental data.The simulation of magnesium alloy forming based on the reasonable constitutive model plays an important role in accurate predicting the forming quality of magnesium alloy.By considering the yield surface evolution with the accumulated plastic strain,a coupled anisotropic yield and tension–compression asymmetric CPB06-GTN damage model with parametric evolution was obtained.The VUMAT subroutine was compiled in ABAQUS / Explicit based on the modified CPB06-GTN model and the subroutine was then tested using a single unit.The results show that the simulation predictions are in good agreement with the experimental results in uniaxial tension and compression.Using the written subroutine,not only the anisotropic yielding and irregular hardening in different directions but also the damage and fracture of magnesium alloys can be well predicted.The forming limit diagram is an effective criterion to evaluate the material's level of formability,but it is time-consuming and costly to obtain the FLD by experiment.The M-K criterion based theory algorithm for forming limit diagram of the anisotropic magnesium alloy sheet has been derived according to the proposed anisotropic yield criterions of magnesium alloy.The theoretical prediction program for the anisotropic forming limit diagram was compiled by C++ language.The effects of different yield criteria on theoretical predictions of forming limit diagram were also investigated by using the developed program.It is found that the theoretical predictions of forming limit diagram mainly depend on the shape of the yield surface in the first quadrant under the principal stress plane condition.In addition,the forming limit diagrams of magnesium alloys at different temperatures are predicted based on the anisotropic yield criterions.The deep drawing processes of magnesium alloy AZ31 at different forming temperatures were simulated by ABAQUS combined with the anisotropic yield CPB06-GTN damage model subroutine.The simulation results were compared with experiment data.The results show that the anisotropic deformation and failure of magnesium alloy can be well predicted by the proposed anisotropic damage model.Numerical simulation results including the failure strain of the magnesium alloy are consistent with the experiment data such as the FLD by experiment.Based on the results of isothermal modeling,the deep drawing process of magnesium alloy was optimized.The thermo-mechanical coupled simulation for the non-isothermal forming process of the magnesium alloy was finally conducted and it is verified that the non-isothermal forming is beneficial to improve the forming performance of magnesium alloys.On the basis of the above research,the deep drawing forming of carbon fiber reinforced magnesium alloy laminates under different temperatures and clamping forces was simulated.The results show that the most serious debonding occurred in the sidewall during its deep drawing.The smaller the clamping force,the more serious the debonding of the deep drawing part held by the clamp holder;while the greater the clamping force,the more serious the debonding of the bottom face of the punch.Thus,the proper clamping force is beneficial to improve the forming quality of the fiber/Mg laminates.