In-process monitoring of welding quality by robotic laser ultrasonic measurement system using microchip laser

ABSTRACT

Laser ultrasonic technique (LUT) is applicable even in a high temperature state such as during welding because the laser is irradiated remotely without contact. Recent studies have reported that LUT is applicable during welding, and it is effective for in-process monitoring. However, in the previous studies, there was a problem with the portability of the measurement device especially in a generation laser, so the scope of its application was limited. In this study, we constructed a robotic measurement system of laser ultrasonic with a small microchip laser mounted on a robotic arm. Microchip laser, which is a prototype product (1064 nm, 20 mJ/pulse, 1.5 ns, 20 Hz), was used to generate ultrasonic, and a laser interferometer (532 nm, 1 W) was used to detect the surface micro-vibration caused by the arrival of ultrasonic wave and was scanned using an uniaxial stage. After acquiring the B scope, which is composed of a generation point and multi-detection points, the synthetic aperture focussing technique (SAFT) is applied. SAFT image obtained by in-process measurement was evaluated to confirm the presence or absence of welding defect indication. Then, during gas metal arc (GMA) welding process in the mid-pass of a multi-layer welding for a single bevel groove, we attempted to detect welding defects generated by introducing slits before welding. As a result, in the second pass, defects were indicated in all areas where defects existed, and no indications were found in areas without defects. Therefore, it was shown that the robotic laser ultrasonic measurement system constructed in this study can detect defects in the process of welding. In conclusion, we were able to show the possibility of evaluating the welding quality in the process of welding even at the construction site by selecting the appropriate generation position and detection range according to the required accuracy and laser specifications.

KEYWORDS:

• Laser ultrasonic technique
• microchip laser
• defect of welds
• in-situ measurement
• robotic arm
• non-contacting monitoring
• synthetic aperture focusing technique

Acknowledgments

We are very grateful to UNITAC Co., LTD, for kindly providing the prototype microchip laser. The original work was funded by ImPACT Program of Council for Science, Technology and Innovation (Cabinet Office, Government of Japan).