Fully Coupled Thermo-mechanical Modeling Of Friction Stir Welding And Microstructure Analysis Of Weld For Railway Vehicle Structures

With the development of light-weight design of railway vehicles in China,aluminum alloy has been widely used in the field of railway vehicles.Friction stir welding(FSW)is very suitable for the welding of light alloys such as aluminium alloys.It has many advantages such as energy conservation,environmental protection,high welding quality,small post-weld deformation and so on.In this paper,the process of FSW is simulated by using DEFORM-3D software,taking the aluminium alloy sheets and profiles structure commonly used in railway vehicles as the research object.Based on the solid model of tool and plate,a fully coupled thermo-mechanical model is established for numerical simulation and analysis for FSW of two kinds of sheets with different materials.The rationality and correctness of the numerical simulation method are verified by comparing the simulation results of the temperature variation and the force variation of the tool with the corresponding experimental data.On this basis,the distribution and variation of temperature and effective strain in advancing side,retreating side and plate thickness direction during welding are analyzed,and the distribution law of temperature field and deformation field at weld seam is given.For FSW of complex profiles structure,the validated numerical simulation method is adopted to conduct fully coupled thermo-mechanical simulation.Based on the simulation results,the temperature field and material deformation are analyzed,and the influence of welding parameters on temperature field and deformation field is discussed.The results show that the distribution of temperature and deformation in the surface of wallboard is similar to that of the plate,but that in the thickness direction is determined by the structural characteristics of the profiled structure joints.The rotational speed and welding speed of the tool have obvious influence on the temperature field and deformation field,while the depth of tool sinking into workpiece has deeply effect on the temperature field.Based on the simulation results of fully coupled thermo-mechanical of profile structure,the material flow law in the advancing side,the retreating side and the thickness direction of wallboard are analyzed by point tracking method,and the effects of welding parameters on material flow and material penetration are discussed.The results show that the material flow in advancing side is complex,and the fluidity in advancing side is better than that in retreating side.The farther away from the shoulder,the worse the material particle fluidity.Increasing the rotational speed of the tool and decreasing the welding speed enlarges the range of "mutual penetration" of material particles,and improves the flow of welding materials.Based on the fully coupled thermo-mechanical simulation results of profile structure,the effective strain,strain rate and temperature at the weld seam are extracted,and the grain size of the weld seam is predicted by continuous dynamic recrystallization model.The results show that the grain size of the material is the smallest at the center of the weld,the grain size of the material far from the center of the weld increases gradually,and the grain size of advancing side is smaller than that of retreating side.In this paper,the thermodynamic mechanism of FSW is comprehensively studied,which provides theoretical guidance for FSW technology of vehicle structure and helps to promote the development of vehicle structure manufacturing.