Influence of shielding gas composition on molten metal flow behavior during plasma keyhole arc welding process

The purpose of this investigation is to elucidate the behavior of molten metal flow inside weld pool during welding of Plasma Keyhole Arc Welding (PKAW). An observation of the behavior of three-dimensional (3D) molten metal flow inside weld pool during welding was carried out with the support of an advanced X-ray observation system. The results showed that the metal flow in pure Ar shielding gas case was in upward direction from bottom surface toward top surface of weld pool behind the keyhole. Meanwhile, the molten metal flow in Ar shielding gas mixed with 0.5 % O2 case was in downward direction from top surface toward bottom surface. The variation of convection flow inside weld pool as described above is a result of the variation of keyhole diameter and inclination angle of keyhole wall (keyhole profile) with (1) large around the top surface but narrow around the bottom surface in case of pure Ar and (2) narrow around the top surface but large around the bottom surface in case of Ar mixed with 0.5 % O2, which affect the direction and magnitude of shear force acting on weld pool surface. Consequently, only the very slight increase of oxygen content in argon shielding gas due to lowering of shielding effect is found to significantly affect material and heat transport process in PKAW. The results also imply that when argon shielding gas with small amount of oxygen is used, deep and narrow weld penetration is more easily obtained similarly with AA-TIG welding process. However, its mechanism is suggested to be mainly related to the change in shear force due to keyhole shape deformation by the decrease of the surface tension rather than the change in direction and magnitude of Marangoni force.