Microstructure homogeneity and mechanical property improvement of Inconel 718 alloy fabricated by high-deposition-rate laser directed energy deposition

The nonuniform microstructure of the as-built Inconel 718 superalloy fabricated by high-deposition-rate laser directed energy deposition (HDR-LDED) brings about heterogeneous room temperature tensile properties along the building direction (BD). Thus, it is imperative to explore suitable post heat treatment processes for homogeneous and better performances of HDR-LDED built Inconel 718 superalloy. In this work, three kinds of heat treatment schedules were employed to investigate the microstructure evolution and room temperature tensile properties. The results showed that the microstructure uniformities along the BD after three heat treatment processes were all significantly improved. In addition, the three heat treated samples present different microstructural characteristics which were reflected by the morphology, size and distribution of the Laves, δ and γ" phases. However, all the samples exhibited similar microhardness, indicating that the difference in the characteristic parameters of the second phases was not enough to influence the microhardness. Furthermore, the heterogeneity in room temperature tensile property along the BD was improved when compared with the as-built sample. And the S-30 sample demonstrated the best combination of strength and ductility (yield strength: 1013 ± 13 MPa, ultimate tensile strength: 1192 ± 11 MPa, elongation: 16 ± 6%). The fragmentation and the debonding of the “Laves+δ” phase contributed to the fracture in all the samples. The initial dislocation density inside the bulk sample may become one of the factors that have to be considered in the future when optimizing the heat treatment processes in HDR-LDED process.