An improved method for assessing fillet weld fusion quality based on phased array ultrasonic testing

L-shaped fillet welds are widely used in shipbuilding, automotive industry, and energy sectors. The load-bearing capacity of fillet welds is directly determined by their fusion state. When lack-of-fusion defects exist between the base metal and filler metal, the mechanical properties of the joint can be significantly compromised. Moreover, such internal defects are often undetectable by visual inspection. Therefore, non-destructive testing of fusion quality in L-shaped fillet welds is essential for quality assurance and safety evaluation. Phased array ultrasonic testing (PAUT) has been extensively employed for fusion quality evaluation due to its high resolution and sensitivity. However, compared to electronic focusing, the phased array probe beam diverges along the length of the element. This results in severe dimensional distortion along the lateral axis of the inspection images and reduced resolution, compromising measurement reliability. To address this issue, an ultrasonic acoustic lens was designed for linear phased array probes. It enables ultrasonic waves to converge at designated positions within the workpiece through multiple refractions. The acoustic field after applying the acoustic lens was simulated using COMSOL, and the results demonstrate that the ultrasonic waves converge, with the main beam width reduced. For experimental validation, a straight weld with observable full morphology was fabricated and inspected using two methods. With the acoustic lens, the inspection images exhibited enhanced resolution, and the mean squared errors (MSE) for weld dimensions significantly decreased: Weld width MSE reduced from 1.4785 to 0.0141. Weld length MSE reduced from 13.4813 to 0.1128. Finally, the improved method with the acoustic lens was applied to inspect L-shaped fillet welds in test specimens, demonstrating its effectiveness.