In our first paper, we have proposed the monitoring method using reflected Ar+ laser beam in YAG laser welding. It has been shown that this method was effective to monitor the changes of the welding conditions in YAG laser welding. It is also very important to detect the welding defects in YAG laser welding. In this paper, signals of the light and acoustic emission in addition to that of reflected Ar+ laser were measured at the same time. We investigated changes of these monitoring signals due to welding defects, such as lack of penetration, underfill by a wide gap and misalignment in butt joints. Main results are as follows;
1) The signal of the light emission was suitable to detect the penetration was full or partial.
2) The reflected Ar+ laser signal was suitable to detect the underfill due to a wide gap and the misalignment.
3) Only one signal was changed when these welding defects were occurred. While various signals were altered at same time when the welding conditions were varied.
4) The signal of the acoustic emission was not changed by these welding defects.
