High power disk laser-metal active gas arc hybrid welding of thick high tensile strength steel plates

In this study, welding with hybrid heat sources combined with a high power disk laser and a metal active gas (MAG) arc was carried out on 780 MPa high strength steel plates of 12 mm in thickness. The effects of respective welding conditions, such as laser power, root gap and welding speed on the penetration, geometry, and defects of weld beads, were investigated. The results showed that the process window of welding conditions for the production of good welded joints was narrow. Also, the laser keyhole behavior, the molten pool geometry, and the melt flows inside the molten pool were observed by the high-speed x-ray transmission real-time imaging system. It was confirmed that the melt flows inside the molten pool during hybrid welding were different between humping and good weld beads. The melt flowed from the bottom tip of a keyhole to the humping portion and did not flow forward to the keyhole bottom tip, resulting in the formation of the humping defect. On the other hand, in the case of good weld beads, the tungsten particle flowed to the back part of the molten pool but then flowed to the forward keyhole. It was clarified that a good weld bead without humping nor underfilling could be produced due to the forward melt flow.