This investigation is purposed to clarify the influence of pilot gas composition to convective pattern on the weld pool surface in Plasma keyhole arc welding process. In order to clarify this, the convective pattern on the top surface of weld pool was investigated in both cases of pilot gas: pure Ar and Ar mixture with 10% hydrogen. For estimating the convective pattern, the zirconia particles with diameter of 0.03 mm were utilized as tracers. After welding, using the software (Dipp-motion, Detect Co., Ltd) for analyzing the movement of zirconia particles, the convective pattern was estimated. In case of pure Ar, zirconia particles were in circulated motions just behind keyhole on the top surface of weld pool, and transported to rear part of weld pool on the bottom surface. In case of Ar mixture with 10% hydrogen, zirconia particles on both surfaces (top surface and bottom surface) of weld pool were in circulated motions behind keyhole. Furthermore, the weld bead shape was narrow on both top surface and bottom surface in case of pure Ar. Meanwhile, the weld bead shape was wide on both top surface and bottom surface in case of Ar mixture with 10% hydrogen.

This investigation aims to develop a new Plasma-MIG hybrid welding process for butting welding joints of large thickness. In this welding process, Plasma torch and MIG torch are connected in electrode negative (EN) and in electrode positive (EP), respectively. Plasma torch is set up in the leading position, meanwhile the MIG torch is set up in the trailing position. The plasma welding is utilized in keyhole model. The results show that the successful single-sided welding in one pass is fully penetrated. The wettability of welding joints is improved and the penetration is increased in comparison with only conventional MIG welding process and only conventional Plasma welding process. In addition, in order to discuss the research results, the temperature field on the surface of weld pool is also measured. As a result, the temperature on the weld pool surface in Plasma-MIG hybrid welding case is higher in comparison with conventional MIG welding case, especially near the leading edge of weld pool.