Microstructure and Mechanical Properties of IN718 Alloy Deposited by Directed Energy Deposition

This study investigates the influence of build height variation and heat treatment on the microstructure evolution and tensile properties of IN718 alloy produced by Direct Energy Deposition (DED). Microstructural evolution and tensile properties were evaluated at room temperature and 650°C for samples taken from three build heights: 5 mm (bottom region), 21 mm (middle region), and 45 mm (top region) from the substrate. The microstructural observation indicate that the bottom and middle samples exhibit a higher fraction of γ′/γ″ phases and fine Laves/δ phases in the as-built condition due to in-situ heat treatment from repeated thermal cycles, leading to enhanced strength compared to the top sample, which primarily contains coarse Laves phases. Post-heat treatment (HSTA) leads to dissolution of the fine Laves/δ phases in the matrix, promoting recrystallization in the bottom sample and resulting in reduced tensile strength at both room temperature and 650°C compared to the top and middle samples. The tensile fracture mechanism of IN718 alloy involves fracture and debonding of the Laves phase, highlighting the inhomogeneous microstructures and corresponding mechanical property differences along the build direction. These findings emphasize the need for further research on post-heat treatment processes to optimize material performance.