Mechanical properties of hybrid additively manufactured Inconel 718 parts created via thermal control after secondary treatment processes

Fractured tensile specimens of INCONEL 718 manufactured via directed energy deposition (DED) with operando thermal metrics and control were heat treated and their mechanical properties were compared as a function of the time spent in the solidification and cooling temperature ranges. Shorter solidification times (faster solidification rates) during the DED process led to higher concentrations of Nb, Al, Mo, and Ti supersaturated in the Fe-Cr-Ni matrix, leading to higher volume fractions of γ’’ strengthening phases and higher hardness after aging (720°/8 h, 620 °C/10 h). This trend persists through hot isostatic pressing (1180 °C at 150 MPa for 4 h), suggesting that secondary treatment processes are not sufficient to fully dissolve detrimental Laves and carbide phases, which initially formed during solidification and affect the alloy's hardness. However, samples with various post-solidus cooling times during the DED process exhibited equivalent hardness after aging, suggesting that the intrinsic heat treatment effect is entirely replaced by the post-processing aging treatment. This study shows that the solidification rate during DED has a lasting effect on the part's mechanical properties, that is not completely mitigated by post-processing treatments, while the initial effects of the post-solidus cooling rate (intrinsic heat treatment) disappear after secondary processes.