Coupled Thermal-mechanical FEM Analysis Of Friction Stir Weding Whole Process

In this thesis, based on solid mechanics theory, a coupled thermal-mechanical3D FEM model of friction stir welding of whole process was developed with the platform of ABAQUS nonlinear finite element software. Based on this model, the temperature field, equivalent plastic strain field, velocity field and metal migration behavior of7075aluminum alloy friction stir welding process were studied.Based on the ALE adaptive mesh technique, element mesh and boundary conditions were appropriately conducted, and the problem of highly mesh distortion in FEM analysis of friction stir welding process was solved. It successfully achieved finite element simulation of the whole process, and verified the reliability of the model.The simulation results indicate that the peak temperature of weld metal materials is about81.8%of the material melting point during the whole process of FSW, which proves that the process is a solid connection. With studying the change and distribution of weld temperature field during the whole process friction stir welding, the following results are achieved. At the stage of plunging, the weld metal material temperature is increasing as double linear curve, and the ratio of weld metal material temperature after friction stir tool contact weld surface increasing higher than before contacting. At welding stage, distribution of the weld high-temperature region on cross section looks like a bowl-shaped, and the temperature field on upper surface is wide, whereas the lower surface. The weld high-temperature region limits in the pin shoulder. The peak temperature of metal materials locates at the inner edge of the shoulder. The research results have theoretical guidance meaning for revealing the mechanism of heat producing and temperature field distribution studying of friction stir welding.With particle tracking technology, the migration characteristics of plasticity metal materials in the weld zone are revealed. In the direction of horizontal, flow ability (migration amount is the largest) of the metal material in front of the tool and from advancing side of the weld are better than retreating side. The metal materials at advancing side of the weld migrate mainly in the form of rotating flow around tool and laminar flow, and a few metal materials at retreating side of the weld in the form of rotating flow. At thickness direction, under the influence of rotation friction and extrusion of friction stir tool, the metal material in front of the tool and at the advancing side of the weld both have obviously spiral upward migration movement. The metal materials from the surface and bottom of the weld both have no obvious migration in thickness direction. Additionally, with welding speed increasing, the region of metal plastic flow is decreasing, and the displacement of periphery metal materials in radial direction and tangent direction decreasing, against feeding the transient cavity behind the pin, and easy to produce welding defects. The tool shoulder shape has greatly effect on metal materials flow at thickness direction and the forming of joint. It reflects that a tool with concave shoulder is advantageous to feed the transient cavity behind the tool and acquires good joint. The above achievements on the plastic flow behavior of metal materials can provide a theoretical basis to reveal the mechanism of the weld and defects formation, and the tool optimization.