Microstructures and mechanical behavior of Inconel 718 fabricated by selective laser melting

In this study Inconel 718 cylinders were fabricated by selective laser melting in either argon or nitrogen gas from a pre-alloyed powder. As-fabricated cylinders oriented in the build direction (z-axis) and perpendicular to the build direction (x-axis) exhibited columnar grains and arrays of gamma '' (body-centered tetragonal) Ni3Nb oblate ellipsoidal precipitates oriented in a strong [200] texture determined by combined optical metallography, transmission electron microscopy, and X-ray diffraction analysis. Fabricated and hot isostatic pressed (HIP) components exhibited a more pronounced [200] columnar gamma '' phase precipitate architecture parallel to the laser beam or build direction (spaced at similar to 0.8 mu m), and a partially recrystallized fcc grain structure. Fabricated and annealed (1160 degrees C for 4 h) components were similar to 50% recrystallized and the recrystallized regions contained spheroidal gamma' precipitates distributed in a dense field of fine gamma '' precipitates. The gamma '' precipitates were always observed to be coincident with {100} planes of the gamma-fcc NiCr matrix. Some delta phase precipitates in the unrecrystallized/recrystallized interfaces and recrystallized grain boundaries were also observed in the annealed samples. The microindentation (Vickers) hardness was 3.9 GPa for the as-fabricated materials, 5.7 GPa for the HIP material, and 4.6 GPa for the annealed material. Corresponding tensile properties were comparable with wrought Inconel 718 alloy. Published by Elsevier Ltd. on behalf of Acta Materialia Inc.