Abstract
There are few applications of laser welding to thick plates joining. One reason is attributed to narrow gap tolerance due to a small laser beam diameter leading to unacceptable seam defects such as underfill and burn through. Another is easy formation of weld imperfection such as porosity and cracking in a keyhole—type of deeply penetrated laser weld beads. Hybrid welding with a laser beam and an arc has been receiving considerable attention as one of the methods to suppress such defects and imperfection.
This paper reports fundamental investigation results of hybrid welding with combined high power CO2 laser and Metal Inert Gas (MIG) arc, and especially hybrid welding results of thick plates with such a wide gap that the majority of a laser beam can pass through. The behavior of laser-induced and arc plasmas near the molten pool during the hybrid welding was observed with a black-and-white CCD camera. Penetration depth were confirmed to depend upon the distance between laser beam irradiation location and MIG wire target. The different depths were interpreted in terms of the interaction between laser-induced plasma and molten metal, melt flows, and so on. Moreover, under the formation conditions of the deepest weld beads, the wide gap groove was fully filled with the molten metal. The observation suggests that the recoil pressure due to the laser-induced evaporation should force the molten metal fed with a MIG wire to fill the leading gap and a fully penetrated keyhole was formed during hybrid welding. Consequently, a sound hybrid weld bead could be produced in joining the plates of 22 mm thickness with 4 mm gap, and moreover some weld beads indicated the best grade of the inspection test result according to JIS Z3104.
