Effect of heat input on solidification behavior, microstructure evolution, and mechanical properties of melt-in-mode plasma arc welded inconel 625

Plasma Arc Welding (PAW) is an advanced fusion welding technique extensively employed for manufacturing components intended for high-temperature and corrosive service environments, such as aerospace expansion joints, metallic bellows, and exhaust system components. Among its variants, Melt-in-Mode Plasma Arc Welding (MIM-PAW) is particularly suitable for joining thin sheets with thicknesses ranging from 1 to 2.4 mm. It is recognized as highly effective for welding Inconel 625, an austenitic nickel-based superalloy known for its excellent mechanical strength, oxidation resistance, and thermal stability. Owing to these characteristics, Inconel 625 is widely used in liquid rocket engine components, gas turbine engines, and other high-temperature engineering applications. Although the alloy can be welded using several fusion welding processes, including gas tungsten arc welding, electron beam welding, and laser beam welding, MIM-PAW offers tremendous control and produces high-quality joints in medium-thickness sheets. The present study examines the influence of welding current on the metallurgical and mechanical behaviour of 2.4 mm thick Inconel 625 butt joints produced by MIM-PAW. Defect-free welds were fabricated using optimized parameters of 70 A welding current, 130 mm/min travel speed, 1 L/min plasma gas flow rate, and 4.5 mm arc length. The optimized joint exhibited a bead width of 5.748 mm, HAZ width of 0.1266 mm, fusion zone area of 7.915 mm², and HAZ area of 1.796 mm². The joint achieved an ultimate tensile strength of 872 MPa, yield strength of 566 MPa, elongation of 36%, notch strength of 918 MPa, notch strength ratio of 1.05, and joint efficiency of 92.96% at a heat input of 351 J/mm. Microstructural observations revealed that lower welding currents produced finer, equiaxed grains, whereas higher currents promoted grain coarsening and elongated grain morphology.