In laser welding, monitoring is an important measure to assure the reliability of welding. Various monitoring methods have been proposed in CO2 laser welding. However, the research of monitoring methods in YAG laser welding is still few. We propose here a monitoring method in YAG laser welding. This monitoring method is based on an idea that Ar+ laser irradiates keyhole or molten pool and the intensity of the reflected Ar+ laser is measured by monochromator. The authors investigated the proper irradiation condition of Ar+ laser in this monitoring method. As a result, the signal of reflected Ar+ laser showed the size and oscillation of molten pool under the proper condition. Keyhole behavior was also observed by high speed camera at the same time. By analyzing the high speed video pictures, keyhole profile was detected under the various welding conditions. According to these results, this method is useful to monitor the YAG laser welding.
