Non-Destructive Evaluation of FSW Tool Plunge Depth in Thin Metallic Sheet Weld Using Ultrasonic Guided Waves

Abstract

Due to their high strength-to-weight ratio, thin aluminium alloy sheets are widely used in industries such as automotive, marine, railway, and aerospace. Welding has been discovered to be an effective method of joining, particularly in comparison to mechanical fasteners and semi-joining processes. However, fusion welding techniques can be vulnerable to hot cracking, which can result in solidification and liquation cracking. For lap welded joints in thin metallic sheets, friction stir welding (FSW) has proven to be very efficient. Non-destructive testing (NDT) methods are utilized to examine weld morphology. The ultrasonic method has demonstrated the ability to measure weld penetration depth in thick metallic plates using bulk and surface waves. However, the tool plunge depth (TPD) in FSW, which is analogous to the weld penetration depth of fusion welding, plays a crucial role in the weld strength of thin structures that has not been investigated using non-destructive methods. This study aimed to predict the optimum tool plunge depth for sound welding in thin structures by employing a guided Lamb wave-based non-destructive method. At a low frequency of 25kHz, only anti-symmetric (Ao) mode was transmitted through low TPD weld and through optimum TPD weld both Ao and symmetric mode (So) was getting transmitted. However, through high TPD weld both Ao and So was getting transmitted as well as extra anti-symmetric (AoAoc or SoAoc) mode was observed due to mode conversion. This different scattering behaviour of Lamb wave signals for varying tool plunge depths was analysed to predict the FSW tool plunge depths with accuracy.