In high power laser welding, the slender keyhole is unstable and fluctuates. It is apt to form weld defects. Therefore, many types of monitoring system have been developed to judge the welding state. Especially, monitoring systems of plasma light emission is easy to use. However, the relationship between the plasma light emission and the keyhole behavior is not clear. Understanding of the relationship would be useful for monitoring and controlling the welding phenomena.
We observed keyhole behavior by a micro-focused X-ray transmission imaging system and plasma light emission by photodiodes in deep penetration laser welding simultaneously. It was found that the plasma light emission corresponds to the keyhole behavior and large plume plasma is generated by the interaction between the laser light and the molten metal at the top of the keyhole.