Study on non-contact measurement method of resistance spot weld nugget diameter using laser ultrasonic technique

Resistance spot welding can instantaneously join two or more plates by the resistance heating of the metal and is in high demand owing to its high productivity and high-work efficiency. For the quality control of resistance spot welding, the size and quality of the welded part, called the nugget, are important. Therefore, this study aims to establish a highly efficient and high-speed resistance spot-weld inspection method that can be applied to wholelot inspection, which is currently a difficult process. The objective of this study is to measure the nugget diameter using laser ultrasonic technique that enable remote, non-contact ultrasonic inspection. An investigation of the available ultrasonic waves using simulated test specimens demonstrated the feasibility of estimating the distance from the generation/detection point to the joint using the diffraction of the Lamb wave, which can propagate long distances in a thin plate. By measuring the actual resistance spot welding specimens, it was determined that differences in the nugget diameter of approximately 0.5 mm could be clearly distinguished from the arrival time of the diffraction waves. It was also inferred that the nugget diameter could be calculated by determining the propagation velocity of the diffraction wave with a similar accuracy to that of the measurement using a contacttype probe.