Numerical Simulation Of Friction Stir Processing

Friction stir processing is an emerging processing technique based on the principles of friction stir welding. It has been widely used in many areas such as improve the microstructure of metallic materials and refine its surface grain, fabricated metal matrix composites, make the metallographic structure more homogeneous, and so on. However, the research on the intrinsic mechanism of friction stir processing is still lacking. Some key problems such as the mechanism of heat production and heat transfer, thermoplastic material flow and the distribution of stress and strain have not yet been studied sufficiently and block the application of this advanced technology. Based on the above considerations, this paper established the thermal mechanical coupling model of friction stir processing.The temperature field is analyzed in detail. It is found that the highest temperature region is located at the midpoint of the center of pin and its edge. There is a diminishing marginal in the temperature rise, as the temperature increases, the rise of it will gradually decline. As the size and material of workpiece were determined, there must be a stable temperature. When reaching the stable temperature, the prolongation of the pin’s dwell time has no significantly contribution to the value and contributes of temperature. According to the simulation orthogonal experiment, the rotation speed of the pin has a biggest influence on the temperature.The residual stress field of friction stir processing was investigated. The results showed that the longitudinal residual stress on the surface of the workpiece was saddle shaped, and the residual stress of the forward side was slightly larger than that of the back side. In the thickness direction, with the increase of the distance from the surface of the workpiece, the peak value of longitudinal residual stress becomes smaller.The distribution of equivalent strain field within the workpiece were calculated, and found it is very similar with the distribution of temperature. However, the distribution of equivalent plastic strain in the thickness direction is relatively shallow, and the gradient is larger. The microstructure of the cross section of the working area is analyzed, it is found that the equivalent plastic strain is the main factor to determine the thickness and distribution of the friction stir zone.At the end of the paper, the effect of ultrasonic wave on the temperature field of friction stir processing is also discussed. There usually have two kinds of ultrasonic loading modes, which are horizontal and vertical. For each of them,the numerical model is established. The simulation results show that the transverse ultrasonic vibration can significantly improve the temperature of the area which near the center of the processing area, make the temperature in the processing area more homogeneous. In contrast, the longitudinal ultrasonic vibration has no significant effect on the temperature field.