Development of Electron Beam Freeform Fabrication for Inconel 718 Hot Structures

Electron Beam Freeform Fabrication (EBF³) is a recently developed metallic layer-additive manufacturing process. In EBF³ process, components are fabricated layer-wise by using an electron beam to create a molten pool on a substrate. Wire is fed into the molten pool and the substrate is translated with respect to the electron beam and wire to build a layer. The final part is, thus, built up layer by layer. Inconel 718 is a high temperature alloy with attractive mechanical and oxidation properties well-suited for hot structures applications in the aerospace arena. The present study examines how the microstructure, crystallographic texture, and mechanical properties of Inconel 718, typically used as a wrought alloy, are affected by melting and solidification during the EBF³ process. A block of Inconel 718 was fabricated using the NASA Langley EBF³ system. Specimens extracted out of this block were subjected to 1) microstructural characterization using optical and scanning electron microscopy; 2) tensile test evaluations as related to the effect of in-plane orientation on strength and elastic modulus of the EBF³ product, both in the as-deposited and the heat treated conditions; and 3) crystallographic texture characterization of the as-deposited and heat treated EBF³ products, using electron back scatter diffraction (EBSD). Salient conclusions stemming from these characterizations are: 1) mechanical properties of the EBF³-processed Inconel 718 are strongly affected by texture as evidenced by their dependence on orientation relative to the EBF³ fabrication direction, with the as-deposited EBF³ properties in general being significantly lower than those for the wrought Inconel 718; 2) of the two heat treatments employed, the one conducted at a higher temperature resulted in a significant improvement in both the strength and modulus of the EBF³ product to a level nearly equivalent to those for wrought Inconel 718. These properties were lower for the lower temperature heat treatment.