Predicting the Microstructural Evolution of Electron Beam Melting of Alloy 718 with Phase-Field Modeling

Abstract

Electron beam melting (EBM) is a powder bed additive manufacturing process where a powder material is melted selectively in a layer-by-layer approach using an electron beam. EBM has some unique features during the manufacture of components with high-performance superalloys that are commonly used in gas turbines such as Alloy 718. EBM has a high deposition rate due to its high beam energy and speed, comparatively low residual stresses, and limited problems with oxidation. However, due to the layer-by-layer melting approach and high powder bed temperature, the as-built EBM Alloy 718 exhibits a microstructural gradient starting from the top of the sample. In this study, we conducted modeling to obtain a deeper understanding of microstructural development during EBM and the homogenization that occurs during manufacturing with Alloy 718. A multicomponent phase-field modeling approach was combined with transformation kinetic modeling to predict the microstructural gradient and the results were compared with experimental observations. In particular, we investigated the segregation of elements during solidification and the subsequent ''in situ'' homogenization heat treatment at the elevated powder bed temperature. The predicted elemental composition was then used for thermodynamic modeling to predict the changes in the continuous cooling transformation and time-temperature transformation diagrams for Alloy 718, which helped to explain the observed phase evolution within the microstructure. The results indicate that the proposed approach can be employed as a valuable tool for understanding processes and for process development, including post-heat treatments.

Microstructure Evolution of Selective Laser Melted Inconel 718: Influence of High Heating Rates

“…In addition, a gradient of microstructure along the build direction, as well as from the dendrite cores to the interdendritic regions was observed. Similar observations were also reported by Kumara et al during the solidification of electron beam melted IN718 alloy [20]. Figure 4 shows the variation of primary dendrite arm spacing (DAS), λ, along with the height of the as-printed specimen.…”

“…At these temperatures, grain boundary liquation in HAZ could cause the constitutional liquation of Niobium (Nb)-rich particles such as Laves phase, carbides, and interdendritic γ/Laves eutectic formed during rapid solidification. These phases are usually found in high concentrations in the interdendritic regions in the microstructure of conventionally as-printed parts [6,[18][19][20]. Sui et al [18] investigated the formation of δ phase and proposed two possible mechanisms.…”

“…Moreover, the non-coherent interface between the Laves phase and γ-matrix enhances the concentration of vacancies, which facilitates the movement of solute atoms of constitutional elements to γ matrix thereby increasing the dissolution rate of the Laves phase. Nb and Mo are also the most severely segregated elements in the IN718 alloys, and thus it is relatively easy to measure their segregation [20]. An examination of the microstructure in the blocky zones revealed that the interdendritic Laves phase is dissolved in the γ-matrix.…”

“…The changes in Nb concentration exhibited an inverse relationship to the variations in the Laves volume fractions. This probably indicates that the Nb trapped inside the Laves phase in the "solidified" microstructure was released and diffused back into the dendrite core as a consequence of the very fast heating and cooling [20]. Therefore, there was still enough time for elemental diffusion for high heating rates (i.e., 20 s for 100 • C/s), but its severity decreased for the higher rate of 400 • C/s (12.5 s).…”

On the Microstructures and Anisotropic Mechanical Behaviours of Additively Manufactured IN718

“…That means, the as-built microstructure is spatially dependent, since the early solidified part of a build inevitably experiences longer in-situ 'heat-treatment' than the subsequently solidified part. Therefore, a microstructure gradient is observed in the as-built EBM IN718 component [94,[99][100][101][102]. However, such a segregation or Laves/δ-phases gradient [94,95,101] might be unfavourable for the mechanical properties, and it is also largely dependent on the process parameters.…”