Publications

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The US Department of Energy's Office of Nuclear Energy is interested in developing supercritical carbon dioxide (sCO₂) power cycles that can achieve higher cycle efficiencies and lower costs than the traditional steam Rankine cycles. For the application of an sCO2 energy conversion system with a Very High Temperature Gas Reactor (VHTGR), turbine inlet temperatures over 850°C may be required. Consequently, it is necessary to demonstrate structural materials, including turbine inlet piping, that can be code certified at operating temperatures up to 900°C at sCO₂ pressures up to 42 MPa (6100 psi). There are very few metal alloys that retain their strength at these high temperatures, and that are chemically compatible with sCO₂.

Microstructures and mechanical properties are evaluated in austenitic stainless steel structures fabricated by directed energy deposition (DED) considering the effects of applied loading orientation, build geometry, and distance from the deposition baseplate. Locations within an as-deposited build with different thermomechanical history display different yield strength, while those locations with similar history have approximately the same yield strength, regardless of test specimen orientation. Thermal expansion of deposited material near the baseplate is inhibited by the mechanical constraint imposed by the baseplate, promoting plastic deformation and producing a high density of dislocations. Concurrently, high initial cooling rates decrease away from the baseplate as the build is heated, causing an increased spacing of cellular solidification features. An analysis of strengthening mechanisms quantitatively established for the first time the important strengthening contribution of high dislocation densities in the materials (166–191 MPa) to yield strength that ranged from 438 to 553 MPa in the present DED fabricated structures. A newly adopted mechanistic relationship for microsegregation strengthening from the literature indicated an additional important contribution to strengthening (123–135 MPa) due to the cellular solidification features. These findings are corroborated by the measured evolution of microstructure and hardness caused by annealing the DED material. These results suggest that the mechanical properties of deposited austenitic stainless steels can be influenced by controlling thermomechanical history during the manufacturing process to alter the character of compositional microsegregation and the amount of induced plastic deformation.

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Additive manufacturing (AM) offers the potential for increased design flexibility in the low volume production of complex engineering components for hydrogen service. However the suitability of AM materials for such extreme service environments remains to be evaluated. This work examines the effects of internal and external hydrogen on AM type 304L austenitic stainless steels fabricated via directed-energy deposition (DED) and powder bed fusion (PBF) processes. Under ambient test conditions, AM materials with minimal manufacturing defects exhibit excellent combinations of tensile strength, tensile ductility, and fatigue resistance. To probe the effects of extreme hydrogen environments on the AM materials, tensile and fatigue tests were performed after thermalprecharging in high pressure gaseous hydrogen (internal H) or in high pressure gaseous hydrogen (external H). Hydrogen appears to have a comparable influence on the AM 304L as in wrought materials, although the micromechanisms of tensile fracture and fatigue crack growth appear distinct. Specifically, microstructural characterization implicates the unique solidification microstructure of AM materials in the propagation of cracks under conditions of tensile fracture with hydrogen. These results highlight the need to establish comprehensive microstructure-property relationships for AM materials to ensure their suitability for use in extreme hydrogen environments.

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This SAND report fulfills the final report requirement for the Born Qualified Grand Challenge LDRD. Born Qualified was funded from FY16-FY18 with a total budget of ~$13M over the 3 years of funding. Overall 70+ staff, Post Docs, and students supported this project over its lifetime. The driver for Born Qualified was using Additive Manufacturing (AM) to change the qualification paradigm for low volume, high value, high consequence, complex parts that are common in high-risk industries such as ND, defense, energy, aerospace, and medical. AM offers the opportunity to transform design, manufacturing, and qualification with its unique capabilities. AM is a disruptive technology, allowing the capability to simultaneously create part and material while tightly controlling and monitoring the manufacturing process at the voxel level, with the inherent flexibility and agility in printing layer-by-layer. AM enables the possibility of measuring critical material and part parameters during manufacturing, thus changing the way we collect data, assess performance, and accept or qualify parts. It provides an opportunity to shift from the current iterative design-build-test qualification paradigm using traditional manufacturing processes to design-by-predictivity where requirements are addressed concurrently and rapidly. The new qualification paradigm driven by AM provides the opportunity to predict performance probabilistically, to optimally control the manufacturing process, and to implement accelerated cycles of learning. Exploiting these capabilities to realize a new uncertainty quantification-driven qualification that is rapid, flexible, and practical is the focus of this effort.

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