LAMB WAVE BASED NON DESTRUCTIVE EVALUATION OF WELD QUALITY IN THIN SECTION FRICTION STIR JOINTS

Abstract

Weld integrity assessment is critical for determining safety and dependability in the aerospace, civil, automotive, petrochemical as well as mechanical sectors. Non- Destructive Evaluation (NDE) based on Ultrasonic Guided Waves (UGW) is one of the finest emerging techniques for Structural Health Monitoring (SHM) systems, offering continual monitoring of similar, dissimilar, and composite materials. Traditional approaches need costly, time-consuming point-by-point examination, but Lamb wave based NDE has proven to be useful for wide-range, economical transmission, throughthickness, and in-situ inspection. The proposed study will conduct theoretical and experimental examinations of wave propagation through Al6061-Al5083 dissimilar Friction Stir Weld (FSW) of thin sections lap joints. In this investigation, two plates with thicknesses of 2mm each and dimensions of 250mm × 180mm are welded in a lap configuration with a fixed 30mm overlap length. Retro-reflective tape is attached at 30mm intervals on opposing sides of the stir zone. A Laser Doppler Vibrometer (LDV) is used to measure out-of-plane displacement of wave propagation. A high-voltage amplifier is utilized to generate 120V signals, along with that damping pastes are also used at sample boundaries to increase signal-to-noise ratios. Following NDE testing, weld samples were cross validated using SEM, Stereoscopy and CT scan enabling additional weld characterization. Using a single variable (at a time) method, both fundamental modes i.e., antisymmetric (A0) and symmetric (S0) were employed for weld characterization through process parameter optimization, which included tool rotational speed (RPM), transverse speed (mm/min), and corresponding sound weld window. Frequency wavelength (f-k) filtering techniques have been utilized for mode separation and further signal processing. The operative signal is a modulated sinusoid with central burst at Gaussian packet signal of 60, 80 and 100kHz, respectively. A preliminary investigation reveals the frequency response dependence of rotational and transverse speed in signal transmission. When rotational speed increases and transverse speed decreases, signal transmission enhanced up to a point due to improved material mixing capability, i.e., weld quality, but then declines due to excessive heat input, resulting in weld discontinuities such as tunnel defects, wormholes, voids etc. Our experiment discovered a direct correlation between Lamb wave scattering and defect size, which dictates weld quality; as we surpass the welding window, the possibility of defects escalates. The S0 mode conversions produce abrupt transmission responses at 80kHz for 1300RPM rotation speed, 75mm/min transverse speed and corresponding joint W2.