Mechanical Behavior And Numerical Modeling Of 7XXX Aluminum Alloy Welding Microzone

The application of lightweight materials is one of the most important factors to increase the speed of high-speed trains.At present,aluminum alloy is widely used as the body material for high-speed trains.Welding technology is the main way of connection,but the sharp temperature change during the welding process will lead to the transformation of microstructure,deformation and residual stress,which will seriously affect the service performance and safe operation of high-speed train aluminum alloy body structure.Therefore,the establishment of a complete mechanical constitutive model of welding and accurate prediction of residual stress after welding are of great significance to improve the welding process of aluminum alloy,improve the thermal processing level and extend the service life of components.Among them,the softening and hardening models are the key factors that affect the reliability of welding simulation results.Based on the evolution mechanism of welding heat cycle and precipitates,the dynamic yield strength evolution law of welding process of 7XXX aluminum alloy is revealed,and the softening model of aluminum alloy is established.The isothermal multistage load cyclic tension test method for aluminum alloy was developed,the cyclic hardening behavior characterization method for aluminum alloy was established,the relative internal stress was proposed as the characteristic parameter of cyclic hardening,the A-F nonlinear kinematic hardening model was improved,and the nonlinear mixed hardening model of 7XXX aluminum alloy was established.The micro-zone thermal simulation test of 7XXX aluminum alloy welded joint was carried out.The measured dynamic yield strength was compared with the predicted results of the softening model,and the accuracy of the softening model was verified.Based on the nonlinear mixed hardening model simulation,the elastic-plastic finite element method was used to simulate the isothermal cyclic tension test.Considering the softening model and hardening model,the welding thermal elastoplastic finite element method was used to simulate the multipass welding process of 7XXX aluminum alloy,and the influence of softening model and hardening model on the simulation results of welding residual stress was discussed.The results show that the calculated residual stress without considering the softening model is significantly higher than that considering the softening model.Under the condition of isotropic hardening model,the residual stress calculated is the highest,the kinematic hardening model is the lowest,and the mixed hardening model is in between.Combined with multi-pass welding test and weld residual stress after the test,the measured residual stress was compared with the simulated residual stress,the results show that the calculated residual stress is close to the measured residual stress under the conditions of both softening model and mixed hardening model.