In situ measurement of ultrasonic behavior during lap spot welding with laser ultrasonic method

Understanding the joining process in real time during welding can help improve welding quality, reliability, and manufacturing process efficiency. For this purpose, a laser ultrasonic method that can detect in situ welding quality without contact during welding would be useful. However, the behaviors of ultrasonic propagation as well as melting and joining with increasing temperature remain unclear. Therefore, this study aimed to experimentally investigate the in-process ultrasonic behavior when using the laser ultrasonic method. Lapped tungsten inert gas (TIG) arc spot welding was used for the ultrasonic measurements, wherein the ignition of the TIG arc from the specimen's top surface and laser irradiation for the ultrasonic measurement from the bottom surface occur simultaneously. In this welding, the melting width at the interface of the lapped plates is an important parameter. We investigated the type of ultrasonics used to measure the melting width and examined whether the melting behavior during welding could be experimentally captured. The results show that the ultrasonic velocity decreases as the temperature increases. The echo from the interface between the plates disappears, and the echo from the top surface appears. The disappearance and appearance of each echo facilitate the understanding of the melting behavior during welding.