Study on the relationship between welding force and defects in bobbin tool friction stir welding

In this paper, the characteristics of traverse force (Fx), lateral force (Fy), and axial force (Fz) in bobbin tool friction stir welding (BTFSW) and their relationship with welding defects were studied. Fx, Fy, and Fz showed periodic oscillations and had sinusoidal waveforms in defect-free joints. Defect formation strongly influenced the variation of welding force, and Fy increased most noticeably in defective joints. The average value of Fy determined whether defects occurred in the weld. The material flow behavior during defect formation was analyzed and it was demonstrated that an increase in Fy was due to severe material deposition on the retreating side (RS), which significantly increased the extrusion force between the pin and the workpiece. Due to the good correlation between welding force and defects, a decision tree model based on the force features in the time and frequency domain was built to predict the welding defects. The accuracy of this model in predicting defects reached 95.3 %. Furthermore, the prediction results showed that Fy was the most sensitive force to the generation of defects, and the average value of Fy was the key feature in predicting whether defects occurred.