Effects of Specimen Orientation and Heat Treatment on Microstructure and Crack Propagation Path of the IN718 Alloy Built by the Additive Manufacturing Process

The effects of the heat treatment and the specimen orientation relative to the building platform on the microstructural evolution and mechanical properties were investigated in an IN718 alloy built by the selective laser melting process. The specimens were built in the perpendicular and the parallel directions to the building platform. Optical microscope, scanning electron microscope, transmission electron microscope and electron backscatter diffraction analysis of the as-built specimen showed a melt pool boundary, a very fine cellular structure, the delta-phases at the melt pool boundary and in the intercellular region regardless of the specimen orientation. After heat treatment, the melt pool boundary and the cell boundary were hardly observed, and the delta-phases were still observed at the melt pool boundary and the high-speed cell boundary. The gamma '- and gamma ''-phases precipitated during the aging heat treatment. The application of heat treatment improved hardness, yield strength and tensile strength, but reduced the ductility. To investigate the fracture mode, an iso-stress test was performed under constant stress and temperature. It was worthwhile to note that the crack in the vertically built specimen propagated through the melt pool boundary, and that in the horizontally built specimen propagated through the high-speed cell boundary. These boundaries were arranged perpendicular to the tensile loading direction.