Non-destructive evaluation of FSW tool plunge depth in thin metallic sheet weld using low frequency ultrasonic Lamb waves signal

Thin aluminium alloy sheets find widespread application in industries such as automotive, marine, railway, and aerospace due to their high load capacity-to-weight ratio. Welding has emerged as an effective joining method, surpassing mechanical fasteners and semi-joining processes. However, fusion welding techniques are susceptible to hot cracking, leading to solidification and liquation cracking. When it comes to lap welded joints in thin metallic sheets, friction stir welding (FSW) has demonstrated exceptional efficiency. To assess weld morphology, non-destructive testing (NDT) methods are employed, with the ultrasonic method proving capable of measuring weld penetration depth in thick metallic plates using bulk and surface waves. Nonetheless, the role of tool plunge depth (TPD) in FSW, analogous to the weld penetration depth in fusion welding, remains unexplored through non-destructive means, despite its critical impact on the load carrying capacity of thin structures. Thisstudy aimsto predict the optimal tool plunge depth for achieving high-quality welding in thin structures by utilizing a guided Lamb wave-based non-destructive method. At a low frequency of 80 kHz, the transmission percentage of the anti-symmetric mode (A0) was calculated for FSW specimens with different tool plunge depths. The findings revealed that the transmission percentage was higher for welds with the optimum tool plunge depth compared to those with low and high tool plunge depths. This distinctive behavior of the specimens with the optimal TPD towards Lamb wave signals proved useful in predicting the FSW tool plunge depths.