Effects of Binary-Shielding-Gas GTAW on the Microstructure and Mechanical Properties of Inconel 690 Welded Joints

This study produced autogenous gas tungsten arc welds in 6 mm thick Inconel 690 plates using Ar-He shielding gas. The influence of helium content on arc characteristics, molten-pool geometry, microstructure, and mechanical properties was investigated systematically. With increasing helium fraction, the arc adopts a flattened, fan-shaped profile, leading to significantly greater penetration and bead width, accompanied by higher arc voltage and heat input. The weld-metal grain size is refined by up to 13.25%, while the solidification grain-boundary morphology shifts from cellular to a mixed equiaxed/cellular structure, and the fraction of high-angle grain boundaries increases. These microstructural changes reduce the ultimate tensile strength by a maximum of 12.21% and the elongation by 3.23%. Balancing weld quality and mechanical performance, gas mixtures of 60% Ar-40% He or 40% Ar-60% He are recommended as optimal shielding compositions for GTAW of Inconel 690.