Blown-powder direct-energy-deposition of titanium-diboride-strengthened IN718 Ni-base superalloy

This paper reports on the adoption of TiB₂ as an inoculant to fabricate IN718 via the direct energy deposition (DED) process. Effective grain refinement and low texture were achieved in IN718/TiB₂ using a TiB₂ powder size of d₉₀ = 10 μm and mass fraction of 1.5 wt%. The use of low linear energy density (33.08 J/mm) produced IN718/TiB₂ deposits free from large grains (>300 μm), however at the cost of the formation of interlayer defects. By comparison, the large grains were present in deposits made with a linear energy density of 78.74 J/mm and also in deposits manufactured without an included inoculant. Production of deposits at lower energies without interlayer defects was possible by reducing the powder flow rate to 7 g/min. However, this caused a moderate increase in grain size. The TiB₂ inoculant reduced the Laves phase network by replacing it with homogeneously distributed Cr-, Mo-, Nb-, and B-enriched needle-shaped precipitates. Tensile strength increased by 300–500 MPa with TiB₂ addition, but at the cost of significant ductility drop, regardless of the deposition conditions. The IN718 deposit displayed many micro-cracks at the network of Laves phase during tensile loading, whereas micro-cracks in the IN718/TiB₂ occurred at the interface between the needle-shaped precipitates and the matrix. The strength enhancement in IN718/TiB₂ was by a combination of strengthening mechanisms: grain boundary, dislocation structure formation, Orowan-type and load transfer related to the needle-shaped precipitates.