Defects and 3D structural inhomogeneity in electron beam additively manufactured Inconel 718

• Published in: Materials characterization 2018-09, Vol.143, p.171-181
• Main Authors: Polonsky, Andrew T., Echlin, McLean P., Lenthe, William C., Dehoff, Ryan R., Kirka, Michael M., Pollock, Tresa M.
• Format: Article
• Language: English
• Subjects: Additive manufacturing; CARBIDES; CRYSTALLOGRAPHY; DISTRIBUTION; ELECTRON BEAM MELTING; ELECTRON BEAMS; INCONEL 718; Lack of fusion; LAYERS; MATERIALS SCIENCE; MICROSTRUCTURE; NITRIDES; POROSITY; SOLIDIFICATION; THREE-DIMENSIONAL LATTICES; Tomography
• ISSN: 1044-5803; 1873-4189
• DOI: 10.1016/j.matchar.2018.02.020

Material structure and defects created during additive manufacturing contribute to the variability in mechanical properties of these parts, limiting their use for critical applications. Three-dimensional crystallographic, structural, and chemical information in electron beam melted Inconel 718 was collected from the sub-micron to the millimeter scale using the TriBeam tomography system. The relationship of defects to the surrounding microstructure has been studied in detail. Lack of fusion defects created sharp discontinuities in the grain morphology that extended several build layers beyond the layer where the defect originated. Nitrides and carbides in the size range of 1.25 μm to 10 μm were observed throughout the volume, with pronounced nitride clustering. The similarities between solidification in additive manufacturing and traditional manufacturing processes are demonstrated by the comparable size and distribution of carbides measured for industrial-scale samples of 718. Defects created during additive manufacturing cannot be easily remedied via standard post-processing techniques, and will likely diminish the mechanical performance of additively manufactured parts. •A protocol for 3D characterization of structure and defects in additively manufactured EBM 718 is presented.•Lack of fusion defects disrupt local microstructure, creating isolated regions of equiaxed grains.•Clustering of micron-scale nitrides is attributed to a particle rafting process.•Normalized size-number density relationship of carbides matches that of industrial ingots.•Removal of porosity via post-processing will not correct microstructural inhomogeneities created by lack of fusion defects.