Investigations On Residual State And Fatigue Life Of Friction Stir Welded Structure

During friction stir welding process, high temperature gradient in the weld zone can be observed and the temperature field changes when the welding tool moves. The uneven heating and cooling process in temporal and spatial scales during welding process can result in the residual stress and residual distortion. The residual stress has a significant effect on the damage tolerance and the fatigue life of the welded structure. The residual distortion can affect the assembly of the wled structure. Thus, the study on the residual stress, residual distortion and fatigue life of the friction stir welded structure is significant for the practical engineering application. Although the experimental method can be adopted to study the residual state and the fatigue life of the structure, experiment is sensitive to the environment, is in poor repeatability and needs much more time. Therefore, it is necessary to use the numerical method for the study of friction stir welding process, which can reduce the cost and time. Also, the design cycle of the production can be shortened. In brief, the numerical method is the key for the development and application of friction stir welding.In this dissertation, the numerical method is adopted to study the temperature distribution, residual stress and residual distortion of the butt joint of plate and T-joint of complex structures. The size effect on the residual distortion is analyzed to optimize the residual distortion. The effect of the residual stress on the fatigue life of the welded structure is studied through middle crack tension specimen and the eccentrically-loaded single edge crack tension specimen. A detailed analysis of the stress field around the crack tip is performed in order to highlight the influence of the residual stress on the fatigue life of the welded plate physically. The chepters are as follows:In chapter1, research developments of friction stir welding are reviewed,hich includes numerical modeling of friction stir welding process, the temperature distribution, the residual stress and the residual distortion of the welded structure and fatigue performance of the welded structrue. The main content of this work is also outlined.In chapter2, the basic theory of the sequencially coupled thermo-mechanical analysis and the the finite element expression are introduced in detail. Then the principles of linear elastic fracture mechanics and laws used for fatigue crack growth rate prediction are presented, including crack propagation modes, calculation of stress intensity factor and introduction of the empirical equations for fatigue crack growth rate prediction. In chapter3, a sequencially coupled thermo-mechanical model for friction stir welding simulation is established. The heat source model is programmed as a subroutine called DFLUX. With comparison to the experimental data, the model is validated. The sensitivity of the temperature to the meshing condition is discussed and the reasonable meshing size in the weld zone is given. Also, the effect of heat source pattern on the temperature distribution is studied. The relationship between the input power and the rotational speeds is studied by trial and error method. The welding heat input power is expressed as a function of the rotational speeds. The stress evolution during welding process are analysed in order to highlight the residual distortion. The size effect on the residual distortion is studied and a function is established to describe the relationship between the size of the plate and the residual distortion.In chapter4, the residual stress and distortion of T-joint is studied. Numerical model for T-joint is established to predict the temperature distribution, residual stress and residual distortion. The heat generation of the special designed welding tool is analytically studied and moving heat source is used to simulate the welding tool. The residual stress of T-joint is compared to that of plate butt-joint and the influence of the stringer size is also studied. The torque of the cross section before releasing the constraints is analysed to highlight the mechanism of deformation. The shape and size of the stringer are discussed and the residual distortion can be reduced by optimizing the size of the stringer. At last, the effect of the post-weld heattreatment on the residual stress is investigated.In chapter5, the effect of the residual stress in plate butt-welded joint on the fatigue life of the welded plate is studied. A modified numerical model for friction stir welding is established which can acurratly predict the residual stress in the weld zone. The fracture mechanical model for the welded plate is established and validated, which is used to calculate the residual stress intensity factor. The superposition method and the crack closure method are adopted to predict the fatigue crack growth rate and the fatigue life of the welded plate. A detailed analysis of the stress around the crack tip is performed to highlight the effect of residual stress on the fatigue crack growth rate of the welded plate physically. The effect of residual stress on middle crack and edge crack is discussed in order to find out the dangerous crack location. Finally the effect of the welding parameters on the fatigue life of the welded plate is studied and the fatigue life of friction stir welded plate is compared with that of TIG welded plate.In chapter6, the main contributions of this dissertation are summarized and a direction for further research in this field is discussed by proposing future works.