Simulation And Optimization Of FSW Residual State For Subway Floor Profile Structure

Friction stir welding technology is widely used in the manufacture of subway car bodies.The residual state of welded components directly affects the performance of equipment,which in turn affects driving safety.Therefore,it is of great significance to analyze and optimize the residual state of friction stir welded components.At present,the research on the residual state of welded components mostly adopts the experimental method,and the investment cost is high.Qualitative analysis is often used to study the influence of processing parameters on residual state,and the optimal process parameters under corresponding working conditions are less given.Based on this,this thesis takes the subway floor profile structure as the research object,and uses numerical simulation technology to simulate the friction stir welding process and post-weld heat treatment process.Based on the surrogate model method and intelligent optimization algorithm,the welding and post-weld heat treatment process parameters are optimized to improve the residual state of welded components.The main research contents are as follows:(1)Numerical simulation of friction stir welding of subway floor profile structure.The finite element model of local profile structure and whole profile structure of subway floor is established.According to the forming mechanism of friction stir welding,the mathematical model of thermal load and mechanical load is established.The welding simulation calculation of local profile structure is completed,and the welding residual state of components is obtained.Based on the calculated plastic strain of the local profile structure,the inherent strain method is used to simulate the welding of the overall profile structure.According to the relevant manufacturing standards in the industry,the welding residual stress and welding deformation are used to evaluate the quality of the welded components.(2)Optimization of friction stir welding process parameters for deformation control.The local profile structure of subway floor is selected as the research object,and the reduction of welding deformation is taken as the optimization goal.The welding speed,stirring head speed,stirring head offset and shoulder pressure are selected as design variables.Based on the response surface surrogate model,the optimization model of welding process parameters is established.Aiming at the problems of slow convergence speed and insufficient solution accuracy of the basic particle swarm optimization algorithm,two strategies of random inertia weight and shrinkage factor are used to improve the basic particle swarm optimization algorithm.The improved particle swarm optimization algorithm is used to solve the optimization model.Based on the obtained optimal friction stir welding process parameter combination,the welding simulation analysis of the local and overall profile structure is carried out to verify the optimization effect.(3)Post-weld heat treatment simulation and process parameter optimization of friction stir welding components for residual stress control.Based on the analysis results of the structural stress field of subway floor profiles under the optimal combination of welding process parameters,a numerical calculation model of post-weld heat treatment is established and solved to explore the reduction effect of heat treatment process on welding residual stress.In order to further reduce the residual stress,the heating temperature,heating time,holding time and cooling time are optimized.In view of the low prediction and fitting accuracy of BP neural network,a BSO-BP neural network model is proposed to accurately describe the highly nonlinear response relationship between process parameters and residual stress,and an optimization model of post-weld heat treatment process parameters is established based on this model.The improved particle swarm optimization algorithm is used to solve the optimization model,obtain the optimal post-weld heat treatment process parameters and verify the optimization effect.In this thesis,an effective method for improving the residual state of friction stir welded components is proposed,and the optimal combination of friction stir welding and post-weld heat treatment process parameters is obtained.The forming quality and mechanical properties of friction stir welded components are improved,which provides theoretical support and technical guidance for the formulation of welding structure process and residual state control of metro vehicle body.