Influence of part geometry on spatter formation in laser powder bed fusion of Inconel 718 alloy revealed by optical tomography

The metal powder used during the Laser Powder Bed Fusion (L-PBF) process is usually cycled for reuse in subsequent build jobs for cost-effectiveness and sustainability. Qualification guidelines are being established based on testing results of powder properties in terms of flowability, chemistry and rheological behaviors, etc. for making decisions on whether a batch of reused powder is suitable for producing parts that meet certain requirements. The current paper aims to develop experimental strategies for tracking powder history using novel design of specimens and on-line monitoring. Powder-capturing containers designed with internal lattice structures of varied beam lengths and diameters were manufactured by the L-PBF process using an Inconel 718 powder to investigate the influence of part geometry on the degradation of reused powder. The L-PBF experiment was monitored by a commercial Optical Tomography (OT) system which records the thermal emissions from the build area. Data were extracted from the OT images to evaluate the emissions of spatter particles introduced to the powder bed, which is influenced by the local layer profiles of the lattices and the overall geometries of the container. The collected powder samples were tested by combustion analysis for oxygen content and characterized by Scanning Electron Microscopy (SEM). Surface chemistry analyses of the powders were performed by X-ray Photoelectron Spectroscopy (XPS). Depending on the lattice structure geometry, the oxygen uptake in the powder collected from the containers was increasing by 10 ppm in case of empty container and up to as high as 118 ppm in case of container with larger areas of overhangs and higher surface-to-volume ratio. XPS results revealed the presences of Al-rich and Cr-rich oxides on the surface of powder samples collected from the container filled with lattices of high surface-to-volume ratio and the container filled with lattices of large overhangs, which agrees with the analyses of OT data.