Study on the thermal history and the heterogeneities in microstructure and mechanical properties of directed energy deposition-arc Inconel 718 alloy

The non-uniformity of microstructure seriously affects the mechanical properties of alloys. In this study, the thermal history at different positions and the heterogeneities of microstructure and properties of directed energy deposition-arc Inconel 718 alloy (DED-Arc IN718) are studied. The results show that the microstructures are composed of Al 2 O 3 + γ(Ni) + (TiN) [nucleus]/(NbC + TiC) [shell] + NbC + Laves. However, the size and morphology of Laves phases along the build direction are obviously different due to the difference of the cooling rate. The thermal cycling process causes precipitation and dissolution of the γ'' phase, resulting in softening and hardening zones in DED-Arc IN718 sample. The presence of softening zone and hardening zone result in extremely heterogeneous mechanical properties along the build direction, with the maximum difference in ultimate tensile strength reaching 54   MPa. The long-striped Laves phases first fail internally during tensile loading to form a crack source, and then the crack propagates along expand along the γ(Ni) matrix. The granular or blocky Laves phase at the top and bottom is beneficial to improve the elongation, and the maximum difference of elongation is 20.1%. This study lays the foundation for further improving the mechanical properties of Inconel 718 alloy.