Effect of oxygen in ambient atmosphere on penetration characteristics in single yttrium-aluminum-garnet laser and hybrid welding

Welding with a yttrium-aluminum-garnet (YAG) laser only or with hybrid source combined with a YAG laser and a TIG arc was carried out on Type 304 stainless steel plate with low S content in the air or in the chamber with Ar gas including 0%-15% oxygen gas. It was confirmed in both laser and hybrid welding that the penetration and geometry of welds depended upon the volume of the oxygen in the ambient atmosphere, and that a pronounced "nail head" disappeared in a weld metal containing a higher content of oxygen. An x-ray transmission real-time imaging apparatus revealed that melt flows depended on the volume ratio of oxygen in the atmosphere and there was a strong melt flow from the keyhole inlet to the surrounding pool near the top surface in pure Ar atmosphere. It is therefore considered that the melt flows induced by surface tension produces the "nail head" in pure Ar shielding atmosphere and the resulting weld becomes a wine glass shape. During YAG laser and hybrid welding in the air or in the Ar gas with the oxygen gas increased, the downward melt flows along the keyhole wall was more dominant to produce slightly deeper welds with narrower surface bead widths due to the effect of the surface tension with the positive gradient. On the other hand, the formation of shallower YAG-TIG weld beads produced in air was attributed to the mechanism that the Marangoni convection and electromagnetic convection induced by TIG arc constriction in the central part of the molten pool prevented downward melt flows from a keyhole tip resulting in deeper penetration than the tip. It was moreover confirmed that the porosity reduction in hybrid welding at a high current was due to the reduced formation of bubbles. (c) 2006 Laser Institute of America.