Study On Numerical-simulation Predictive Methods Of Residual Stress And Buckling For Friction Stir Welding

Friction stir welding is a new-style solid-state joining process, which has unique advantage over other welding methods among the welding of the nonferrous metals, such as aluminum alloys. The residual stress existent in the friction stir welding joint has a serious influence on the mechanical property of joint, especially fatigue property. So it's necessary to make a detailed and thorough study to the residual stress of joint. In doing so we necessarily develop computer numerical simulation to set up the ideal model of FSW for exactly analyzing the distribution of residual stress and making clear the influential law of different factor's influence on the distribution and standard of residual stress, we can get the goal of controlling and optimization with the exact prediction of residual stress.The thesis adopts finite element analyzing method, which utilize sequentially coupled finite element model to predict the residual stress made in the FSW welding joint. The material adopted in the paper is AL-6061-T6. The size of model is 200×100×4.5mm and 200×100×2mm.First, setting up three-dimensional thermal model that can analyze temperature filed to get the temperature history in the welding. In the simulating process of the temperature filed,set up the moving thermal-generation model, only considering the heat generated by friction between the workpiece and welding tool. The meshing of entity model adopts the nonuniform scheme for the precision and less calculation. The model adopts thermo-physical properties of relationship between material and temperature, and then selects the rational welding process-parameter and boundary condition. The whole process is calculated by ANSYS ,obtaining the temperature range of joint is between 80% and 90% of the material melting point, which accord with the actual situation and can be as thermal load of structural analysis.Second, transforming the thermal model into structural model, which get the temperature filed result as thermal load exerted on the structure to analyze the residual stress. This method adopts logical thermo-mechanical properties, and brings to bear appropriate boundary conditions, takes the moving restraint condition of welding tool into consideration.Thus, the simulation is very close to actual case. The whole process is finished by ANSYS finite element software. Within the welding and HAZ, the result proves that residual longitudinal stress is major tensioning stress, and the peak value at the center of welding which is lower than yield strength. residual transverse stress is minor and Z stress is very little, fluctuates at zero point, can be ignored.At last, according to our prediction to residual stress, we employ the method of equivalent thermal load and non-linear buckling analysis to analyze the buckling of friction stir welded sheet. Result indicates that residual stress in the welding process of FSW can lead to buckling of large dimension sheet, which proves residual stress affect the stability of weld structure.In conclusion,the predictive methods are feasible to predict the residual stress and buckling deformation of FSW joint , but it will be perfected ulteriorly.