The effect of interdendritic δ phase on the mechanical properties of Alloy 718 built up by additive manufacturing

Abstract

The effects of building direction and heat treatment on the tensile properties of superalloy IN718 fabricated by direct metal laser sintering (DMLS) were examined at room temperature and 650°C. It was noted that particle-shaped δ phases were localized in the interdendritic regions due to the segregation of Nb that occurs during the DMLS solidification process. To investigate the microstructure and mechanical properties of DMLS parts, vertical and horizontal specimens were cut parallel and perpendicular to the building direction, respectively. The tensile strengths of heat-treated materials produced by DMLS were comparable to those of cast and wrought specimens; however, when the horizontal- and vertical-direction specimens were subjected to solution treatment and aging (STA) at 650°C, the horizontal-direction specimen exhibited one-fourth the ductility of the vertical-direction specimen because of the interdendritic δ-phase precipitates, which were arrayed perpendicular to the stress axis in the former specimen. The morphology and a row of interdendritic δ-phase precipitates with incoherent interfaces were found to affect tensile ductility at a high temperature.