Welding with a 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 a low S content in air or in the chamber with Ar gas including 0% to 15% oxygen gas. It was confirmed in both laser and hybrid welding that the penetration and shape 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.
X-ray real-time transmission 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. On the other hand, the formation of shallower YAG-TIG weld beads 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 producing deeper penetration than the tip. Moreover, porosity reduction in hybrid welding was confirmed to be due to reduced bubbles formation at a high current.