Effects of a modified heat-treatment on microstructure and mechanical properties of additively manufactured Inconel 718

The effectiveness of a modified heat treatment on the microstructure and mechanical properties of Inconel 718 (IN718) fabricated by the laser powder bed fusion (LPBF) technique was investigated in this work. A modified solution heat treatment (MSHT) conducted at 1160 °C for 4 h was applied to both a specimen printed in the X–Y orientation as well as another printed in the X–Z orientation. Following MSHT, the specimens were treated in two stages at 720 °C for 8 h and then again at 620 °C for 8 h. Microstructural data such as grain morphology, size, recrystallization, and texture were studied along with mechanical properties. The MSHT treatment was successful in removing the Laves phase, cellular-dendritic structure, and segregations but the metal carbides (MC-type carbides) remained around grain boundaries. Additionally, recrystallization and grain growth led to an equiaxed microstructure for both specimen orientations. Precipitated γ′ and γ″ phases were revealed within the microstructure via transmission electron microscopy (TEM) following the two-stage aging process. Micro and nanoindentation measurements were performed to validate mechanical property evolution. Hardness values decreased following the MSHT treatment but greatly improved following the aging procedure which is attributed to the dissolution of microstructural inhomogeneities during MSHT and subsequent precipitation of uniformly distributed fine γ′ and γ″ particles following the aging stages. The microstructure and mechanical properties of additively manufactured IN718 can be significantly improved through use of proper solutionizing and aging heat-treatments. • The rapid heating/cooling rate associated with LPBF resulted in the formation of columnar/cellular dendritic microstructure and Laves phases. • The modified solution heat treatment (1160 °C/4hr) entirely dissolved the Laves phases. • The size and distribution of γ′ and γ″ phases were affected by the modified heat treatments. • MSHT above the solvus temperature of δ-phase and Laves phases resulted in a more uniform microstructure..