In-situ detection of weld defect during the welding process by laser ultrasonic technique

In manufacturing industry, in-process monitoring for quality assurance during the welding process is emerging. Conventional ultrasonic testing can be conducted only post-process because of the requirement of contact to target material where still hot in case during the welding process. Therefore, we employed a laser ultrasonic technique (LUT), which can be applied to a high temperature field like the welding process. In this study, we investigated the detectability of solidification cracks at single bevel groove during the arc welding process. A nanosecond pulsed laser was adopted as the generation laser and scanned at 100 Hz using a galvanometer mirror. After acquiring multiple transmit- and receive-points, synthesis aperture focusing technique (SAFT) was applied to obtain the indication image. This enabled to detect the presence of the solidification crack and the lack of penetration in the joint even during the welding process. The indicated position of solidification crack by LUT during the welding was 22.2 mm, which was deeper position than the actual value of 21.0 mm. This deviation can be explained by the difference between the assumed room temperature value for the ultrasonic velocity, and the actual value. However, the defect indication was clear enough even without considering the temperature influence.