Keyhole Behavior during High Power YAG Laser Welding

In laser welding, power density is very high at the processing points and a cavity called a keyhole is generated by the recoil pressure of evaporation. The authors examined keyhole and plasma/plume behavior using a microfocused X-ray transmission in-situ imaging system and a high speed video camera during spot welding and bead-on-plate welding of SUS304 stainless steel. The following results were observed. The keyhole was generated to a small degree by the first shot of a 26 J pulsed beam and became gradually deeper by increasing the number of shots in spot welding. When a pulsed laser beam of 1.1 ms in rise time and 7.6kW in peak power was used in bead-on-plate welding, the plasma/plume began to be generated in about 0.22ms after the initiation of laser beam irradiation, the keyhole began to form after about 0.6 or 0.7ms, became its deepest after about 1.5ms from the start of laser irradiation, and disappeared after about 1.3ms following the laser termination. The keyhole formation time was 0.6ms in rise time and the average velocity of keyhole growth was about 6m/s at this time.