Effect of heat treatment on the microstructure and mechanical properties of Inconel 718 processed by selective laser melting

The microstructural and mechanical properties of Inconel 718 were determined on the specimens manufactured by selective laser melting (SLM) of prealloyed powder. High- density (99.8%) cylindrical specimens were built with four orientations (0°, 45°, 45°×45° and 90°) in relation to the building and scanning directions. Because of directional, dendritic-cellular grain growth, microstructure of the as-built specimens was characterized by columnar grains of supersaturated solid solution with internal microsegregation of Nb and Mo, demonstrated by fractions of Laves eutectic or its divorced form in interdendritic regions. Such a heterogeneous microstructure is unsuitable for direct post-process aging and makes the alloy sensitive to subsolidus liquation during rapid heating to the homogenizing temperature. In homogenized and aged condition, the alloy received a very good set of mechanical properties in comparison with the wrought material. In heat-treated condition, like in as-built condition, weak anisotropy of properties was found, manifested by lower Young's modulus, yield strength and tensile strength of the specimens extended along the build direction in comparison to the values for the other variants of the specimens. This is attributed to the fact that the grains maintained their geometric and crystallographic texture obtained during solidification.