Interplay between solidification conditions and post heat treatments on properties of a nickel-based superalloy processed by wire directed energy deposition

• Nb-rich interdendritic zones covered with secondary phases Laves phase, NbC and NbC + TiN are formed in the as-built microstructure. • Higher annealing temperature of 1200 °C/1h is required to entirely dissolve the Nb-rich interdendritic zones (K = 1.00) and NbC. • Direct ageing utilizes chemical heterogeneities of the interdendritic zones and promote localized precipitation in segregated zones of [Ni3Nb] γ” at earlier stages. • Short-term direct aging at 10 h promotes a rapid strengthening by γ” of ΔUTS = 116 MPa (16 %) compared to solution annealing with ageing. • The effect diminishes with increasing ageing time (100 h, ΔUTS = 24 MPa, 3 %), as homogeneous precipitation of γ“ takes place during solution annealing with aging. The intrinsic solidification conditions in additive manufacturing affect microstructure formation and require new heat treatment procedures. The present study aims to identify heat treatment routes and in particular to evaluate the performance of direct ageing (DA) compared to established solution annealing with ageing (SA + A). Structures of nickel-based superalloys were therefore additively manufactured using wire-arc directed energy deposition and subjected to various heat treatments to determine the structure–property relationship. In the as-built state, the microstructure is characterized by its solidification structure, consisting of linear-shaped Cr-Ni-rich dendritic zones and segregated Nb-rich interdendritic zones filled with secondary phases. Solution annealing at 1200 °C/1h is required to entirely dissolve the segregated zones and secondary phases. The microstructure formed during ageing strongly depend on the initial conditions. In DA, the precipitation of strengthening γ“ takes place at earlier stages, is limited to the Nb-rich interdendritic zone while the dendritic zone remains γ”-free, and leads to a directionally graded microstructure. In contrast, SA + A requires longer ageing times, however, leads to a homogeneous distribution of γ“. These variations in the microstructure are also reflected in the mechanical properties. While SA + A leads to high hardness and isotropic tensile properties, even higher tensile strengths can be achieved with DA.