Laser beam welding is used for small and thin parts that are required the high quality and precision. To weld large-scale and thick-wall products, high power laser beam must be transferred freely and deep penetration welding procedure must be developed. This paper will describe an optical fiber transmission system for 7 kW-class high power YAG laser and pulse modulated laser welding techniques to obtain deep penetration. The detailed observation of the weld pool and keyhole dynamics using high-speed camera and X-ray transmission system is carried out to understand high power YAG laser welding phenomena. It has been clarified that there are proper pulse duty and pulse duration for optimum-welding condition to obtain sound weld. Moreover it was revealed that bubbles, which were generated by the evaporation of metals near the bottom of keyhole, merged and disappeared into molten pool under N2 shielding gas. After confirmation of the high power YAG laser welding joint performance of stainless steel Type 304, this procedure has been applied to the welding of tanks for the nuclear fuel reprocessing plant.
