15. Application of Wire Arc Additive Manufacturing for Inconel 718 Superalloy

The wire arc additive manufacturing (WAAM) is one of the advanced manufacturing processes to fabricate full-density 3D Inconel 718 (IN718) metal parts in an open freeform environment. Thus, there is no size restriction of the fabricated parts using this process which is suitable for industry-led medium to large production supply chain. So far, the use of WAAM process in the fabrication of IN718 parts is solely focused on the structure–property relationship under heat-treated conditions. Therefore, the present study is attempted to investigate the effects of welding parameters, heat-treatment, and high-oxidation temperature on the processing–microstructure–property relationship of IN718 alloys manufactured via gas tungsten arc welding (GTAW)-based WAAM process. A wrought IN718 alloy was also studied for comparison.

It was observed that increasing the arc current increased the width and reduced the height of the walls as a result of higher surface tension and arc pressure acting upon a constant volume of material under constant wire feed speed and travel speed. A complete opposite trend was seen with increasing wire feed speed under constant arc current and travel speed. Increasing the travel speed adversely affected both the width and height of the walls due to the deposition of lower volume of material. Irrespective of welding conditions, a highly textured and homogeneous microstructure of γ-matrix was developed parallel to the build-up direction. Due to the elemental segregation of heavy elements, the matrix microstructure was mostly composed of Nb-depleted dendritic core region (DCR) along with Nb-enriched interdendritic region (IDR). The mechanical properties in terms of microhardness and tensile strength were found to be similar and independent of the effect of processing parameters. A modified homogenization (1100 °C for 1 h/air cooling)-annealed (720 °C for 8 h/furnace cooling at ~71.2 °C/h to 620 °C for 8 h/air cooling) condition was performed on WAAM IN718 alloys to dissolve laves phase and precipitate out strengthening phase of γ″. The heat-treated WAAM parts showed weakly anisotropic tensile properties at room temperature and exceeded the minimum requirements for cast IN718, but not that of wrought IN718 due to its large columnar grain structure. The high-temperature oxidation study at 1000 °C revealed that the kinetics of oxidation followed the parabolic rate law and were independent on the thermal history, microstructural, and compositional heterogeneities of WAAM parts. Both AF and HA alloys formed oxide scales that were identical in nature. The external oxidation of the protective Cr₂O₃ scale was formed at the air/alloy interface, which was covered by an outermost thin layer of rutile-TiO₂ and spinel-MnCr₂O₄ at air/scale interface. The internal oxidation of Nb-rich rutile-Ti_0.67Nb_1.33O₄ scale at the scale/alloy interface and subscale of Al₂O₃ within the alloy was observed. Based on the thermodynamic data and kinetics abilities of metal cations, a mechanism of oxide layer formation was suggested.