Characterizing Equiaxed Grain Formation in Inconel718 through Directed Energy Deposition

In this study, Directed Energy Deposition (DED) was employed to fabricate Inconel 718 (IN718) layers, which traditionally suffer from grain coarsening due to high thermal input, degrading mechanical properties. To address this, a novel approach involving the addition of ZrO ₂ nanoparticles to IN718 powder was investigated to refine grain structure and promote equiaxed grains, enhancing mechanical attributes. A 2 wt.% concentration of 200 nm ZrO ₂ nanoparticles was mixed using a swing planetary mixer. The deposition parameters, including laser power and energy density were varied. Optimal conditions of 250 W and 1000 mm/min led to the predominant formation of fine equiaxed grains. The study quantitatively assessed the impact of volume energy density, Marangoni convection, and the Fourier number on microstructural development. Increased volume energy density and Marangoni convection, coupled with a reduced Fourier number, were found to facilitate grain refinement and equiaxed formation. Transmission Electron Microscopy (TEM) analyses revealed that the addition of ZrO ₂ enhances nucleation, with the formation of L1â‚‚ structured Al ₃Zr intermetallic compounds contributing to heterogeneous nucleation and grain boundary pinning effects, thus accelerating grain refinement and promoting equiaxed grain formation. This microstructural enhancement correlated with improved hardness, with an average increase of 41-49 Hv across all regions. The transition from columnar to equiaxed grains, primarily induced by Al ₃Zr compounds, was most effective under specific conditions (250 W, 1000 mm/min). This research underscores the potential of incorporating readily accessible and cost-effective ZrO ₂ nanoparticles to achieve grain refinement and anisotropy reduction in IN718-ZrO ₂ layers fabricated via DED, leading to improved hardness and potentially broader applicability in additive manufacturing.