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Additive manufacturing (AM) is being considered as a primary manufacturing process for heat pipes. The Savannah River National Laboratory is researching AM to fabricate a hydrogen isotope separation unit for a thermal cycling absorption process that cycles from cryogenic to moderate temperatures and is a pressure boundary. AM is being explored as a replacement technology to improve heat transfer. Simple test samples with three internal geometries were designed and built from type 316L stainless steel using the powder bed fusion laser process. Nine test article geometries were prepared and subjected to tensile and burst testing and were interrogated using x-ray computed tomography and metallography. The parameters selected for processing the tubes produced a consistent product with acceptable tensile properties and microstructures; however, the process parameters used did not produce full density and minor modifications are needed to achieve full density.

In laser powder bed additive manufacturing processes, feedstock materials are often recycled after each build. Currently, a knowledge gap exists regarding powder reuse effects on powder size distribution, morphology, and chemistry as a function of part geometry and processing conditions. It was found during selective laser melting (SLM) of 316 stainless steel that a significant amount of (0.100 wt pct) oxygen pickup can occur in molten material (spatter) ejected from the powder bed surface. This value was significantly larger than the oxygen content of the as-received powder feedstock (0.033 wt pct). Furthermore, the powders in the heat-affected-zone regions, adjacent to molten pool, also exhibit oxygen pickup (≥ 0.043 wt pct). The oxygen content in unmelted 316L powder was found to vary as a function of its spatial position in the powder bed, relative to the heat source. Interestingly, the volume of melted material (i.e., thin vs thick walls) did not correlate well with the extent of oxygen pickup. Possible mechanisms for oxygen pickup in the powder during SLM, such as adsorption and breakdown of water, oxygen solubility, spatter re-introduction, and solid-state oxide growth, are discussed.

Components made by laser powder bed fusion (L-PBF) additive processes require extensive trial and error optimization to minimize defects and arrive at targeted microstructure and properties. In this work, in situ infrared thermography and ex situ surface roughness measurements were explored as methodologies to ensure Inconel® 718-part quality. For a given laser energy of 200 Watts, prismatic samples were produced with different exposure times (80 to 110 µs) and point spacings (80 to 110 µm). The infrared intensities from laser–material interaction zones were measured spatially and temporally. The conditions leading to higher IR intensity and lowest surface roughness values correlated well with less porosity and coarse solidification grain structure. The transition from highly columnar to misoriented growth is attributed to changes in thermal gradients and liquid–solid interface velocities. Hardness measurements and electron microscopy of the as-processed and post-processed heat-treated samples show complex transitions in microstructural states including the heavily dislocated FCC matrix, reduction of dislocation density, and copious precipitation, respectively. These results show that the geometry-process-structure-property correlations are dynamic, and they cascade depending on the transitions of phase states from powder to liquid to solid, as well as phase decompositions and deformations within the solid FCC phase. Validity of using analytical weld process models to describe the above phenomena is also highlighted.

Adhesion of thin films of epitaxial oxides to nickel-based metallic substrates is important for the successful development of high-temperature superconductor coated conductors. Detachment of epitaxial oxide buffer layers at the oxide/metal interface during either oxide growth or subsequent processing renders the conductor useless. In this study, thermal desorption spectroscopy (TDS) has been used to identify and understand one of the causes of buffer layer detachment, oxidation of carbon at the oxide–metal interface to form carbon monoxide. Results of TDS indicate that on the surface of a bare nickel-based alloy substrate, the rate of carbon oxidation depends on both the supply of carbon from the substrate and the supply of oxygen from the vapor. Sulfur at the surface of the alloy substrate reduces the rate of carbon oxidation. The effectiveness of various treatments of the bare substrate to eliminate CO formation and epitaxial oxide detachment has been demonstrated. TDS provides both a means to evaluate the kinetics of the oxidation reaction and a tool to assess the need and effectiveness of a substrate oxidation treatment.

The effects of conversion parameters on transport properties of YBa2Cu3O7-δ (YBCO) films on rolling assisted biaxially textured substrates (RABiTS) in the BaF2 ex situ process were investigated for total pressures Ptotal between 0.1 and 1.3 atm, water vapor pressures PH2O between approximately 7 and 70 Torr and processing temperatures TS between 700 and 790 °C. For this study, a 0.3-μm-thick Y–BaF2–Cu–O precursor film was deposited on a 1-cm-wide Ni=3 at.% W RABiTS with a buffer layer architecture of CeO2/YSZ/Y2O3/Ni deposited in single passes in various reel-to-reel systems for each layer. Under the conditions of Ptotal = 0.1 atm, TS = 740 °C and PO2 approximately 150 mTorr, JC > 2 MA/cm2 was obtained at 77 K and self field for PH2O ≤ 20 Torr. At higher PH2O (=70 Torr), however, the maximum attainable JC decreased. In addition, the JC at these higher PH2O dropped rapidly with increased dwell time. The highest JC, 2.5 MA/cm2, was achieved at 730 °C with Ptotal = 0.1 atm and PH2O approximately 7 Torr. Finally, the variation of IC with wet conversion time was performed at each processing temperature.

We investigated the influence of a chemisorbed S template with c(2 × 2) structure on the epitaxial growth of different oxide buffer layers on {100}〈100〉 Ni. The sulfur superstructure spontaneously forms on the Ni surface during the texturing anneal as a consequence of segregation. However, depending on the initial S bulk concentration and/or specific annealing conditions, the S layer can cover less than the entire substrate's surface. We show that an incomplete c(2 × 2) coverage causes degradation of the seed buffer layer texture as compared to the substrate texture. A simple step consisting of an H2S predeposition anneal can be used to control the superstructure coverage and optimize the seed layer texture.

In order for many large-scale bulk applications of high-temperature superconducting materials to be realized, the cost/performance of the superconductors needs to be optimized. From a performance standpoint, a long, flexible, single-crystal-like wire is required; from a cost-and-fabrication standpoint, an industrially scalable, low-cost process is required. Both of these critical requirements are met by rolling-assisted biaxially textured substrates, a conductor-fabrication technique that employs simple, scalable, thermomechanical processing techniques to obtain a near-single-crystal-like, flexible metal substrate in arbitrary lengths on which epitaxial oxide buffer layers and superconductors are then deposited.