Microstructure and anisotropic mechanical properties of EBM manufactured Inconel 718 and effects of post heat treatments

Materials manufactured with electron beam melting (EBM) have different microstructures and properties to those manufactured using conventional manufacturing methods. A detailed study of the microstructures and mechanical properties of Inconel 718 manufactured with EBM was performed in both as-manufactured and heat-treated conditions. Different scanning strategies resulted in different microstructures: contour scanning led to heterogeneous grain morphologies and weak texture, while hatch scanning resulted in predominantly columnar grains and strong 〈001〉// building direction texture. Precipitates in the as-manufactured condition included γ′, γ″, δ, TiN and NbC, among which considerable amounts of γ″ yielded relatively high hardness and strength. Strong texture, directionally aligned pores and columnar grains can lead to anisotropic mechanical properties when loaded in different directions. Heat treatments increased the strength and led to different δ precipitation behaviours depending on the solution temperatures, but did not remove the anisotropy. Ductility seemed to be not significantly affected by heat treatment, but instead by the NbC and defects inherited from manufacturing. The study thereby might provide the potential processing windows to tailor the microstructure and mechanical properties of EBM IN718.