Investigation On Thermo-mechanical Process In Friction Stir Welding Of Aluminum Alloy Thin-walled Structure And Its Numerical Simulation

Friction stir welding (FSW) is an advanced solid state joining technology, which has great potential of application in many fields like aerospace industry. The thermo-mechanical effect of FSW on welded structure becomes more and more important in industry application. Meanwhile the thermo-mechanical effect of FSW is different from most arc welding processes for its lower temperature, lower residual stress and small distortion. Therefore, the interaction between stir tool and the material being welded, which is the most important character of FSW, was investigated in this dissertation. First, an equation was established to calculate the heat generation. Then, the effects of thermal load and stir tool's mechanical loads on welding residual stress and distortion of joint were distinguished. Finally, a high efficiency numerical simulation method was developed to predict the residual stress and distortion of engineering structures after FSW. The research in this dissertation could give powerful supports in some aspects of foundational research and application of FSW.FSW experiments on aluminium alloy sheets with different rotation speed, travel speed and down force were carried out, and the torques of stir tool and the welding temperature fields were also measured during welding process. Based on these measured results, the influence of rotation speed, travel speed and down force on heat generation was discussed. The results of experiment and analysis showed that there was a reactive-balanceable effect between heat generation and welding temperature in FSW, which resulted in the solid state welding process. Based on the transformation from mechanical energy to heat energy and the influence of welding parameters on heat generation in FSW process, an equation was established to calculate the heat generation using only welding parameters. The equation successfully represented the solid state welding process and the influence of rotation speed, travel speed and down force on heat generation in FSW. With this heat generation equation, the welding temperature field and the torque of stir tool during FSW process can be predicted through numerical simulation.The residual stress and distortion of aluminium alloy sheet after FSW are obviously different from those of the arc fusion weld. Their distortion directions are opposite, and the value of residual stress and distortion in FSW is lower than those in arc welding. According to this phenomenon, the trends and mechanisms of the stir tool's mechanical loads affecting on the transient stress and strain of welded structure were analyzed with numerical simulation. The investigation finds out that the down force of stir tool have the functions of increasing the components of hydrostatic stress in the stress state of the material in stir zone, increasing the friction between back plate and bottom surface of welded sheet, and bringing additional expansion in longitudinal and transversal direction of the sheet to the material in the upper part of stir zone besides the thermal expansion. These functions of down force are the essential reasons for the abnormal distortion phenomenon on the friction stir welded aluminium alloy sheet compared with the conventional arc welding. The torque of stir tool results in the asymmetrical distribution of residual stress in FSW, but it has little influence on residual distortion.According to the investigation of the interaction between stir tool and material being welded, a reasonable numerical simulation model on FSW was established to analyze the residual stress and distortion. In order to enhance the efficiency of the numerical calculation, a new welding numerical simulation technique called temperature function method was developed, in which the controlling variable was welding temperature, and this temperature was added into the model in subsection form. For the prediction of welding residual stress and distortion of large structure, the error between computed results from the temperature function method and the moving heat source method was no more than 15%, while the time cost in the temperature function method is less than 10% of that in the moving heat source method. At last, the temperature function method was applied to predict the residual stress and distortion of aluminium alloy thin-walled structure after FSW.