Observation of keyhole behavior and melt flows during laser-arc hybrid welding

Hybrid welding was carried out on Type 304 stainless steel plate under various conditions using a YAG laser combined with a TIG arc, and the effects of various welding conditions on the penetration and porosity formation were investigated. In most cases the hybrid weld beads were deeper and wider in comparison with the YAG laser ones; however, this fact did not always take place. It was consequently found that a laser beam should be shot inside the molten pool made readily with a TIG arc alone to produce deeper weld beads effectively. It was also confirmed that the hybrid weld beads had a smaller amount of porosity than the YAG laser ones. In order to understand these reasons, keyhole behavior and melt flows were observed through the X-ray real-time transmission apparatus. It was consequently revealed that the keyhole was enlarged near the top surface but was still small in diameter in the middle and bottom parts. Nevertheless, keyhole behavior was rather stable inside the molten pool and only a few bubbles were generated. There was a strong melt flow toward the rear part of the molten pool from the keyhole tip. Moreover, the melt flow was steady and fast backwards along the bottom fusion boundary of the molten pool. It is therefore considered that the backward melt flows conveyed the heat to the rear molten pool, and the melt pool was elongated, resulting in the formation of a wider weld bead.