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Bobbin tool friction stir welding (BTFSW) is a new solid-state welding technology that has been developed successfully in many joining applications, such as thick plate and hollow component welding. The existence of the gap can possibly cause an unstable welding, and the variety of the torque is closely related to the existence of the gap. Compared with the conventional friction stir welding (FSW), BTFSW has the different tool and welding process, especially some new feature of the periodic vibration signal arises when welding two sheets in the presence of gaps. Compared with vibration-accelerated signal, torque signal is capable of reflecting the vibration of the welding process. The aim of this study was to develop an effective algorithm to detect gaps in BTFSW operations. Experimental studies were conducted to determine the features of welding process parameters and the gap. Time-frequency (TF) analysis provides a display of subtle torque features, shows the corresponding duration of low frequency signal clearly. Followed by a low-pass filter, gauss wavelet transform (WT) algorithm would improve the detection and identification of the gap presence. Several experimental studies with a variety of process parameters and proposed algorithm was shown to be able to reliably detect the presence of gaps in BTFSW operations for traverse speeds below 200 mm/min and gap sizes above 0.3 mm. This gauss WT algorithm could become a useful noninvasive tool for welding quality assessment in practice.

Study on the characteristics of the torque vibration and the detection of the gap