Investigation of Microstructure Heterogeneity Using Multi-Length Scale Characterization of Additively Manufactured 316L SS Components for Nuclear Applications

Additive manufacturing allows for near net shape components to be manufactured with complex geometries and internal cooling channels while simultaneously allowing for microstructure control. Additive manufacturing has an added benefit of the possibility of removing the post processing needs associated with traditional nuclear component manufacturing. The microstructure of components built using laser powder bed fusion has been shown to be greatly affected by the build parameters. By altering the laser power, laser velocity, and the spot size the microstructure and, possibly, nanoscale partitioning may be tailored. In this study, nanoscale partitioning was confirmed to be the result of an abrupt transition of phase selection phenomenon from g - austenite (FCC) phase to d - ferrite (BCC) phase when moving from the outer edge of the melt pool to the interior, center region of the melt pool. This is inferred from a distinct shift in the Cr and Ni segregation in the inter – dendritic regions. This was achieved by studying four sample builds with varied build parameters, some of which underwent heat treatments. The samples were either built with Selective Laser Melting (SLM) or Concept Laser systems. Nanoscale segregation was identified in samples from the build varying parameters. All sample sets were confirmed ≥ 99% g-austenite (FCC) through X – ray diffraction. Solidification models, heat transfer models, and segmented etched optical microscopy images were performed and collected. This information, once gathered, led to the prediction of nanoscale partitioning patterns. The retained presence of nanoscale partitioning after massive transformation to ≥ 99% g-austenite (FCC) was confirmed through STEM/EDS analysis.