Dual-channel welding torch with CO₂ regulation for high-strength Ti/Fe joints: Weld formation and microstructure control

A dual-channel welding torch was innovatively employed in this work to enable separate delivery of Ar shielding gas and CO₂ active gas flows. By rotating the torch body, four distinct CO₂ incorporation modes relative to the molten pool were achieved: front-side, backside, 304SS-side, and TC4-side modes. The CO₂ gas served dual functions of arc ionization and surface oxidation, each of which significantly affected molten pool behaviors. A CO₂ gas flow of 1.0 L/min was determined as the threshold for effective arc ionization, promoting inward Marangoni flow and substantially improving joint back reinforcement. Surface oxidation of TC4 and 304SS happened at flow rates of 0.5 L/min and 1.5 L/min, respectively. Due to the differing oxidation resistances of Ti and Fe, the molten pool exhibited asymmetric spreading across the substrates, resulting in joint inclination. The inclination phenomenon caused uneven heat distribution across the joint, leading to excessive dissolution of Ti or Fe. Among the four incorporation modes, the front-side mode optimized the benefits of CO₂ ionization for molten pool regulation while minimizing oxidation risks. This configuration produced optimal weld formation and homogeneous microstructure, achieving a remarkable tensile strength of 525.5 MPa—a 183 % increase compared to pure Ar shielding under identical welding parameters.