Microstructure Development in Track-by-Track Melting of EBM-Manufactured Alloy 718

Electron beam meltingElectron beam melting (EBM) process (EBMElectron beam melting (EBM) process) is a powderPowder-bed fusion process within the group of additive manufacturingAdditive manufacturing (AM) technology that is used to fabricate high performance metallic parts. Nickel-Iron base superalloys, such as Alloy 718Alloy 718, are subjected to successive heating and cooling at temperatures in excess of 800 °C during the EBMElectron beam melting (EBM) process process. CharacterizationCharacterization of the dendritic structure, carbides, Laves and δ-phase were of particular interest in this study. These successive thermal cycles influence the microstructureMicrostructure of the materialMaterial resulting in a heterogeneous structure, especially in the building direction. Hence, the aim of this study was to gain increased fundamental understanding of the relationship between the processing history and the microstructureMicrostructure formed within a single layer. Different numbers of tracks with equal heights were for this purpose produced, varying from one to ten tracks. All tracks used the same process parameters regardless of number and/or position. MicrostructureMicrostructure characteristics (sub-grain structure, grain structure and phases) were analyzed by optical microscopy, scanning electron microscopy equipped with energy disperse spectroscopy and electron backscatter diffraction. The direction of dendrites changed in the overlap zones within the tracks due to re-meltingMelting of materialMaterial in the overlap zone. The primary dendrite arm spacings slightly increased along multi-tracks owing to a slight decrease in cooling rate by addition of the next tracks. Epitaxial growth of grains were observed in all samples due to partial re-meltingMelting of grains in previous layers and surface nucleation was also found to occur in all tracks.