| A friction stir welding tool, as a critical component of the process and welding machine, is a major factor affecting the weld quality. Experimental investigations have shown that the tool shape, including tool size and profile, has a significant influence on the macrostructure, microstructure and mechanical properties of the joint. A complex-shaped pin is found to be capable of improving the joint property and the stress condition of the pin, compared with the traditional cylindrical/conical pin. Therefore, it is of great theoretical significance and engineering application value to investigate the effect of tool pin profile on the thermal field and plastic material flow in friction stir welding, which will enrich the knowledge base of FSW process and help to implement the optical design of the tool/pin.A mathematic model of FSW process is established for the normal CT pin (Conical pin), and physical phenomena of heat generation, temperature distribution and plastic material flow in FSW process are analyzed. The influence of interfacial contact variables (slip rate and friction coefficient) on the numerical simulation results is studied.According to the stress characteristics at the tool-workpiece interface, the expressions of slip rate and friction coefficient are theoretically deduced. Among the variables and parameters in the expressions, the tool torque and axial force are monitored by experimental measurement, and the shear stress ration between the shoulder/pin bottom and pin side is obtained by comparing the simulated and experimental temperature values of several key points. Based on this measurement-calculation method, the slip rate and friction coefficient under various set of process parameters are obtained. Thus, the subjective randomness of the value selection is avoided, and it lays the foundation for improving the accuracy of numerical simulation.Through synthetically considering the characteristics of complex-shaped ST pin (conical pin with 4 flats) and TT pin (conical pin with 3 flats), a three dimensional "quasi-steady state moving reference"+"transient state moving mesh" model is established. The model is more approximate with the real FSW process, and both computation efficiency and accuracy are ensured. The heat generation, material velocity and stress conditions at the pin side flat area of ST and TT pin are discussed and analyzed seriously. The influence of pin profile on slip rate and friction coefficient is studied by using the measurement-calculation method.The heat generation, temperature distribution and material plastic flow behavior of CT, ST and TT are simulated by using the numerical model of FSW for complex-shaped pin. The transient plastic flow characteristics at various pin orientations of ST and TT are quantitatively analyzed. As demonstrated by the numerical results, the effect of pin profile on the total heat generation and heat distribution at the shoulder is negligible, while the effect on the heat distribution feature at the pin side is significant. The difference of temperature caused by CT, ST and TT is less than 30 K, and the fluctuation amplitude of the temperature around the pin is less than 10 K during the rotating of ST and TT pins. The maximum material flow velocity inside the shear layer with the ST and TT pins is 2-3 times that with the CT pin, i.e., ST and TT pins can "stir" more material and produce stronger material flow.The friction stir welding experiments are conducted, welding thermal cycles, material flow and weld cross-sections are measured. The predicted results basically agree with the experimental data. |
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