Microstructure and corrosion of SS316L–IN718 functionally graded structure deposited using twin-wire gas tungsten arc-based wire and arc additive manufacturing

In the present study two different filler wires, Stainless Steel 316L and Inconel 718, are fed into a weld pool generated by a Gas Tungsten Arc (GTA) process to deposit material layers whose composition is governed by the independently controlled wire feed rates. The in-house fabricated twin-filler GTA welding system enables layer-by-layer deposition of welded structures, with each layer engineered to achieve a desired chemical composition. A functionally graded block was deposited with 100% SS316L on one side and 100% IN718 on the other side, with intermediate layers comprising mixed compositions. Layers of the mixed composition of 25%, 50%, and 75% Inconel 718 were achieved by suitably setting the individual wire feed rates. Compositional evaluation of the FGM block from one end to the other has been carried out which confirmed a gradual variation in the percentage of the alloying elements. Microstructural evolution, hardness distribution, and intergranular corrosion behaviour was comparatively evaluated along the build direction under aggressive test conditions. It is found that there is a gradual decrease in hardness and corrosion resistance as the material composition transitions towards SS316L. A significant observation from this study is that the layer containing 75% Inconel 718 and 25% stainless steel (Layer 4) exhibits properties closely comparable to those of the 100% Inconel 718 outer layer under experimental conditions. This finding is particularly prominent, as it suggests that the graded component may retain its functional capabilities under tested conditions even if the outer IN718 layer experiences wear, damage, or partial degradation during service.