Numerical Simulation Of Residual Stress In Friction Stir Welds

Friction stir welding is a new solid-state joining technique with high welding quality, high energy efficiency,environmental friendliness and versatility.It has been widely applied to aerospace,bullet train,ship and automobile industries due to its advantages in manufacturing costs and welding quality.Weld distortion and residual stress are critical factors for the performance of FSW and become one of the key subjects in industries.Therefore,it is necessary and meaningful to investigate the weld distortion and the residual stress for the friction stir welds.A fully coupled thermo-mechanical model is adopted to study the effect of shoulder size on the temperature distribution and material deformation during the friction stir welding processes.Moreover,numerical models of residual stress in friction stir welds are established for the analysis of the reaction forces on the welding tool,the temperature distributions, material deformations and residual stresses.Five models with different mesh sizes are also used for the investigation of the mesh sensitivity for the prediction of residual stresses. Optimized mesh density is found for the current numerical model to investigate the effect of welding speed and rotating speed on temperature and residual stress distributions.The results correlate well with corresponding experiments.The first chapter is the brief introduction to FSW,including the characteristics,the applications and the current researches on temperature distribution,mechanical property and the controlling of welding parameters.The current work is briefly introduced at the end of this chapter.The second chapter is mainly about the related algorithms to solve thermal problems with finite element method.It consists of constitutive equations of elastic-plastic material, instantaneous model of heat conduction,dynamic explicit solving equations for coupled thermo-mechanical problems and techniques for mass scaling method and ALE meshes.The third chapter is about the effect of shoulder size on temperature distribution and material deformation in a fully coupled thermo-mechanical model base on solid mechanics.The last chapter is mainly about the analysis of residual stress distribution,the reaction forces on the welding tool,mesh sensitivity and the effect of welding parameters on residual stress. In conclusion,the main contributions of the dissertation arc summarized and the further work is suggested.The current thesis is supported by National Natural Science Foundation of China. (10802017)...