Mechanical Characterization of Near-Isotropic Inconel 718 Fabricated by Laser Powder-Bed Fusion

Abstract

The characterization of Ni-based superalloy Inconel® 718 fabricated by powder-based laser fusion process was performed to study microstructural evolution and accompanying mechanical properties of as-deposited and post-heat-treated material. The current effort employs a low laser volumetric energy density during the deposition. X-ray diffraction, optical, and scanning electron microscopy were done for microstructural characterization. Examination of hardness values, tensile properties, and fractography based on sample orientation was also performed. X-ray diffraction studies facilitated qualitative confirmation of as-deposited samples possessing cube texture. The optical and scanning electron micrographs aided in examining the evolution of microscopic features after each post-processing stage. The as-deposited material possessed columnar grain morphology along with melt-pool marks, which on further magnification showed cellular-dendritic sub-grains along with Laves phases in the interdendritic grain boundaries. Solution heat treatment resulted in reducing these detrimental Laves phases and relief of thermal residual stresses caused by non-homogeneous recrystallization. Solution treatment also led to the formation of equiaxed grain clusters at various sample locations along with the existing columnar grains. The incoherent δ phase was formed at the grain boundaries along with coherent γ″ precipitates. The mechanical properties of as-deposited Inconel 718 alloy were near-isotropic in specimens oriented parallel and perpendicular to the build direction which is ascribed to low laser volumetric energy density. Micrographs of fractured surfaces taken for both directions of all sample conditions showed a ductile mode of fracture.