[SY-F7] Prediction of the microstructure evolution in Electron Beam Melting Alloy 718 through phase field modelling
Electron Beam Melting (EBM) is a powder bed additive manufacturing process where powder material is being melted selectively by a layer-by-layer approach using an electron beam. It has some unique features when it comes to manufacturing of components in high-performance superalloys such as alloy 718 that are commonly used in gas turbines. EBM has high deposition rate due to 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 resultant microstructure of as-built EBM Alloy 718 is observed to have a microstructure gradient starting from the top of the sample. The aim of this study was to use modelling to create a deeper understanding of microstructure development during EBM and the homogenization that occurs during manufacturing in Alloy 718. A multi-component phase field modelling approach combined with thermodynamic modelling was used to predict the experimentally observed microstructure gradient. Of particular interest was to study the element segregation during the solidification and the subsequent “in-situ” homogenization heat treatment that occur due to the elevated powder bed temperature. The predicted element composition was then used in thermodynamic modelling to predict the changes in the CCT and TTT diagrams for Alloy 718. This helps to explain the observed phase evolution within the microstructure. The results indicate that the approach can be a valuable tool both for creating process understanding and for process development including subsequent homogenization and heat treatment.