2006 Volume 24 Issue 2 Pages 149-161
With the objective of interpreting penetration mechanisms and porosity formation mechanisms during laser or hybrid welding, Type304 stainless steel plates containing low S content were subjected to YAG laser only or YAG laser-TIG arc hybrid welding, and surface of the molten pool, keyhole behavior and melt flows during welding were observed through a high-speed camera and the X-ray real-time transmission apparatus. It was confirmed that melt flows near the molten pool surface were different between in air and in Ar atmosphere: the ones approached to the keyhole inlet and spread from the keyhole inlet to the surrounding pool, respectively. It is therefore considered in Ar atmosphere that the melt flows attributed to surface tension produce the "nail head" and the resulting weld bead displays a wine glass shape. In addition, it was revealed that the melt flows and keyhole behavior were greatly affected by arc current. In the hybrid welding at about 100A, there was a strong melt flow from the keyhole tip toward the rear part near the bottom part of the molten pool, resulting in deepened welds. It was observed that porosity reduction in hybrid welding at 200 A was due to reduced bubbles formation.
