Characterization of SLM-fabricated Inconel 718 after solid solution and precipitation hardening heat treatments

The microstructure and hardness of solid solution heat treated (ST) and precipitation hardened Inconel 718 parts fabricated with selective laser melting are investigated. The temperature range for the ST is between 970 and 1250 °C, while the two-step precipitation hardening was done at 760 and 650 °C, each for 10 h. The result demonstrates the effects of homogenization and consequently the effects of aging on the microstructure and hardness of the samples studied. Complete recrystallization occurred for the specimens ST at and higher than 1180 °C. The grain structures of ST specimens qualitatively appear identical with those specimens ST and aged, implying that aging does not induce noticeable changes in the grain structures. Precipitation hardening generates uniformly distributed good yield of ellipsoidal γ″ precipitates with average size of minor and major axis of 11–17 nm and 48–81 nm, respectively. In addition, smaller quantities of γ′ precipitates with an average size of 24 nm are observed for the aged specimens. Increasing the hold time of ST for a particular temperature leads to coarsening of γ″ precipitates, which have a negative impact on the hardness of the material. After aging, the hardness of the specimens is increased by 32–43% relative to that of the as-printed specimen. The increments in hardness for the specimens ST at and lower than 1100 °C (and aged) are the result of the combined effects of hardening precipitates and strain associated with the lattice defects, such as dislocation networks and subgrain boundaries that remain undissolved. The microstructures of the specimens ST at higher temperatures (e.g., 1250 °C) have attained minimal lattice defects due to completed recrystallization. Hence, the increment in hardness for these specimens after aging is mainly due to the hardening precipitates. Needle-shaped δ phase is also precipitated along/near grain boundaries during solid solution heat treatment at 970 °C. Formation of δ phase can consume a lot of Nb, which otherwise be used for the precipitation of hardening phases.