Numerical Simulation Of Material Behaviors And Mechanical Responses In Friction Stir Welding Process

Friction stir welding (FSW) is a new solid state welding technique under the actions of mechanical forces and frictional heat. FSW has many advantages on the aspects of mechanical properties and producing efficiency, such as high strength, low defect, no pollution, etc. FSW can be used to join the buttcover plate and lap jointed sheeting of many materials, such as aluminium alloy, magnesium alloy, copper alloy, etc. Moreover, FSW can even join dissimilar metals. FSW has been applied to aerospace, automobile and ship-building industries.During the FSW process, a rotating pin is inserted into the welding plates. Due to the heat generated by the friction between the pin and the welding plates, the material becomes thermoplastic. Therefore, the thermoplastic material is transported due to the movement of the rotating pin. The compact joint can be formed due to the frictional heat generated between the pin and the welding plates and the action of forging. So, the study of the transport process of the material around the pin becomes very important for the establishment of FSW theory and the development of FSW in the future. Although the transportation of the material during the FSW process can be studied by experiments, numerical modeling is still necessary because the material flow process can not be revealed directly from experiments. The transportation of the material, the materials deformation and the residual stress distributions can be studied in details from the numerical model of FSW. The numerical model and the experiments can develop in a complementary way to avoid the duplicate tests, which is beneficial to the development and the application of FSW.Numerical method is used in the current dissertation to analyze the effect of the welding parameters on the FSW process and the flow patterns of the material around the rotating pin in details. The recrystallization process in FSW is investigated and then the effect of the variations of the welding parameters is studied. The hardness distribution in the friction stir weld is obtained from the variation of the grain size in the weld. The research work will be introduced with the following chapters.In chapter 1, the research developments since the invention of FSW are surveyed, which includes the welding parameters, the material flow patterns, the microstructure, the mechanical properties, the temperature distribution and the joining of dissimilar metals. In the last section, the brief description of the research in the dissertation is presented. In chapter 2, the physical nature of the plastic deformation is introduced. The Hall-Petch equation and the equation of the grain size evolution are also given. The rate independent constitutive model with its algorithm and the rate dependent constitutive model with its return mapping algorithm are introduced. The computational costs are discussed to accelerate the convergence. Numerical examples are given to show the validity and the efficiency of the method.In chapter 3, the results of the two dimensional rate independent constitutive model of FSW are given. The different material flow patterns on the retreating side and the advancing side are analyzed. The numerical results can correlate well with the experimental ones to validate the model established in the chapter. The formation of the swirls, the laminar flow, the stress/strain distributions around the pin, the energy dissipations and the residual stress distributions are investigated in details. The effect of the variations of the welding parameters on the stress distributions of the pin is studied.In chapter 4, the results of the three dimensional rate independent constitutive model of FSW are given. The three dimensional material flow and the material deformation on the cross section perpendicular to the weld are analyzed. The effect of the welding parameters on the material flow and deformation is investigated. The comparison of the microstructure and the equivalent plastic strain shows that there is a good correlation between them. So the effect of the variation of welding parameters on the weld quality can be obtained theoretically from the equivalent plastic strain distributions on the cross section.In chapter 5, the numerical model of FSW is established by using rate dependent constitutive model. The return mapping algorithm is used for the solving process in the generalization of the classical yon Mises criterion for rate dependent materials. The material behaviors and the stress distributions around the pin are herein investigated. The effect of the welding parameters on the recystallization process is also studied.In the conclusion, the main contributions of the dissertation are summarized and the further work is suggested.The research of the dissertation is supported by the National Natural Science Foundation of China (10302007, 10225212 and 10421202), the Program for Changjiang Scholars and Innovative Research Team in University of China (PCSIRT) and the National Key Basic Research Special Foundation of China (2005CB321704).