Welding quality evaluation for refill friction stir spot welding based on three-dimensional feature of ultrasonic image

Welding quality of refill friction stir spot welding directly affects the reliability of products. Ultrasonic non-destructive testing technology has advantages of high testing accuracy, large testing area, and fast scanning speed and is widely used in the evaluation of welding quality. In traditional ultrasonic testing, some single features are extracted from ultrasonic signal or image to evaluate welding quality. However, different features have different sensitivities to welding quality, which makes it difficult to accurately evaluate welding quality with a single feature. To solve the above problem, an integrated three-dimensional feature (3 DF ) is proposed to evaluate the welding quality of refill friction stir spot welding. In the new method, ultrasonic scanning microscope is used to detect welding specimen to obtain C-scan and B-scan images firstly. Then, multiple defect features are extracted from the above images to characterize the three-dimensional information of defects in welding specimen. Finally, the entropy weight method is used to fuse the defect features into 3 DF , and the welding quality is comprehensively evaluated. In order to verify the effectiveness of the 3 DF , 2524-T3 aluminum alloy plates with different welding rotational speeds in the refill friction stir spot welding process are used as specimens and the welded quality of specimens are evaluated by 3 DF . The experimental results show that the correlation coefficient between 3 DF and shear strength is up to 0.89, which is significantly higher than other single features. The effectiveness of 3 DF in evaluating welding quality has been proven. Highlights • 3 DF is a combination of ring width, average defect area, maximum defect area, and defect depth. • 3 DF is proposed to evaluate welding quality efficiently and non-destructively. • The entropy weight method assigns different weights to features with different importance. • New feature avoids the contingency and insensitivity of a single feature. • New method provides a reliable alternative for the destructive evaluate of welding quality.