Microstructure evolution of Inconel 625 alloy during single-track Laser Powder Bed Fusion

Abstract

Elemental microsegregation and precipitate formation are inevitable during solidification of Additive Manufacturing (AM) parts. In this study, single-track Laser Powder Bed Fusion (LPBF) has been combined with optical and electron microscopy, as well as thermodynamic (CALPHAD) simulations, to evaluate the solidified microstructure and also the formation of the precipitates in an as-built LPBF microstructure of Inconel625 (IN625). It is shown that the microstructure consists mainly of columnar Nickel–Chromium ( γ -FCC) cell-like dendrites which grew epitaxially from the substrate. Utilizing Scanning Transmission Electron Microscopy (STEM) with Energy-Dispersive X-ray Spectroscopy (EDS) and High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM), we have detected NbC, γ ′ ′ -Ni 3 Nb, and laves precipitates embedded into the interdendritic regions. The level of microsegregation and the microsegregation patterns during the solidification of primary arms are obtained by STEM-EDS, and calculated using the Scheil–Gulliver (with solute trapping) and DIffusion-Controlled TRAnsformations (DICTRA) methods. Good agreement is seen between the Scheil–Gulliver predictions and STEM-EDS observations of microsegregation, however the level of elemental microsegregation was overestimated in DICTRA simulations as compared to the experimental result. The formation of precipitates was also evaluated computationally by calculation of driving force for the nucleation of the precipitates from the last solidified liquid where the composition and thermal information for the last solidified liquid extracted from the Scheil solidification simulation. The precipitates predicted via CALPHAD were compared with the precipitates identified via HAADF-STEM analysis inside the interdendritic region.