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 d(90) = 10 mu m 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 mu m), 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.

Automated summary

This study reports on the adoption of TiB2 as an inoculant in the fabrication of IN718 through the direct energy deposition (DED) process. Effective grain refinement and low texture were achieved in the IN718/TiB2 samples with specific TiB2 powder size and mass fraction. Deposits made with low linear energy density were free from large grains, but had interlayer defects. Production of deposits without interlayer defects was possible by reducing the powder flow rate, although this resulted in a moderate increase in grain size. The addition of TiB2 as an inoculant reduced the Laves phase network and increased the tensile strength of IN718, but reduced its ductility.