A hybrid manufacturing approach, integrating additive manufacturing (AM) with electric field-assisted sintering (EFAS), is developed for fabricating compact heat exchangers (CHXs) from refractory metals. The methodology employed additively manufactured sacrificial channel molds (SCMs) as shapeholders, which are embedded in metal powders using EFAS. Following embedding, the SCMs are chemically dissolved to form CHX internal channels. SCMs are fabricated from chemically reactive, calcium-based ceramic feedstocks. The microstructure, phase composition, and dissolution behavior of both as-printed and embedded SCMs are investigated. The shrinkage of the SCMs embedded in refractory metals and the SCM-metal interfacial characteristics are studied. The results showed that the SCMs containing sufficient chemically reactive ceramics dissolved effectively. The as-printed SCMs retained the phase composition of their feedstocks, but the embedded SCMs experienced phase changes during embedding. Shrinkage after embedding is strongly dependent on SCM density, with lower density SCMs exhibiting greater shrinkage. A thin SCM-affected zone is observed at the metal matrix surface, characterized by increased porosity compared to the bulk matrix. This effect is attributed to infiltration of the SCMs into powder particle boundaries under pressure, followed by their dissolution. This study demonstrates the feasibility of manufacturing CHXs from hard-to-process refractory metals for use in harsh environments.

Li7La3Zr2O12 (LLZO)-based solid-state electrolytes (SEs) are promising materials for next-generation solidstate batteries. In this work, digital light processing (DLP), an emerging additive manufacturing technology, is employed to produce porous Ta-doped LLZO (LLZTO) scaffolds. The self-standing scaffolds are 100 mu m thick and have 40% porosity. The scaffolds demonstrate symmetric cell cycling stability exceeding 1,500 h at 0.1 mA/cm2 current density, with a capacity of 0.1 mAh/cm2 (1 h for each half cycle). At higher current densities, reversible soft shorts frequently happen, while immediate hard shorts are prevented due to Li dendrite growth being hindered by the tortuous pore network. In addition to the cycling stability, the phase stability of LLZTO is investigated during the post-printing thermal process for printing resin removal. We discovered that the LLZTO partially decomposes into Li2Zr2O7 and other impurity phases from 400 degrees C to 800 degrees C, but the pure LLZTO phase is restored upon the completion of resin removal beyond 800 degrees C.

Refractory alloys, such as molybdenum-based systems, are attracting growing interest for applications in extreme environments, such as in the nuclear and aerospace industries. Recent advances in sintering technologies, coupled with mechanical alloying, have enabled the tailored design of these alloys by leveraging powder characteristics to control final microstructures and mechanical properties. In this study, Mo–30W alloys were fabricated using electric field-assisted sintering (EFAS) from ball-milled powders with and without hydrogen treatment to investigate the influence of surface oxides on material properties and sintering behavior. The results revealed that samples processed from as-ball-milled powder contained a high density of oxides within the microstructure, whereas oxide presence was significantly reduced in samples fabricated from hydrogen-treated powders. Interestingly, the two powder types led to opposite trends in grain size distribution: samples from untreated powders exhibited grain refinement from sample periphery to the center, while samples from hydrogen-treated powders showed grain coarsening toward the center. This behavior is attributed to temperature gradients present during sintering due to electrical percolation pathway differences during Joule heating. The powder surface oxides may have influenced the temperature distribution and grain evolution. Microhardness profiles measured along both axial and thickness directions were consistent with the grain size distribution. Furthermore, oxide films on powder surfaces have delayed densification by hindering particle necking and atomic diffusion during sintering.

Embedding fiber optic sensors in critical components is a key step for real-time monitoring of structural conditions during service and supporting autonomous system operations. Successful integration of these sensors necessitates effective interfacial bonding between the fiber and matrix, good integrity and functionality of the embedded sensors, robust mechanical strength of the matrix materials, and the ability to retain these properties during transient thermal and stress events. This study demonstrates the encapsulation of fused silica optical fibers in stainless steel and nickel through the electric field-assisted sintering (EFAS) process. Copper-coated and gold-coated single mode optical fibers were embedded under different EFAS conditions. The resulting components with embedded sensors were evaluated using advanced microscopy and optical frequency domain reflectometry (OFDR) to assess the aforementioned critical aspects of embedding. The results indicate that both copper- and gold-coated fibers can be successfully embedded in stainless steel and nickel with good fiber integrity and fiber-matrix bonding. Samples fabricated under optimal conditions passed helium leak testing, confirming effective interfacial bonding. Microstructural characterization revealed excellent fiber-matrix adhesion and interdiffusion of elements across the interface. The functionality of the embedded fibers was evaluated through OFDR scans, which revealed signal insertion loss of 0.43 – 0.52 dB for nickel samples and 0 – 0.75 dB for stainless steel samples at the embedding sites. Additionally, the embedded fibers underwent cyclic thermal treatment between 500°C and 700°C. The fibers maintained good integrity and interfacial characteristics, demonstrating their ability to survive cyclic thermal events for sensing in harsh environments.

This study investigated the microstructural characteristics and mechanical behavior of diffusion welded nickel-based Alloy 617 obtained by electric field-assisted sintering (EFAS) using various parameters. The interfacial microstructure exhibited different characteristics including good grain boundary (GB) migration across the interface in the samples diffusion-welded at 1100°C and a flat interface in the samples joined at 1000°C and 1050°C. The interface consisted of fine Al2O3 oxides, while precipitation of interfacial M23C6 carbides was not observed. Grain boundaries migrated across the Al2O3 oxides, leaving these oxides within the grains. Graded grain size was observed, with grain coarsening being more significant near the sample surface due to the temperature gradient induced by EFAS. Tensile testing revealed that the specimens fractured in the matrix away from the interface, indicting strong diffusion-welded joints. The peak tensile strength of 807 MPa was obtained in the samples welded at 1000°C due to minimal grain growth. The materials obtained at 1100°C exhibited reduced tensile strength but improved ductility. Strain maps revealed by digital image correlation showed alternating high and low strain segments in the samples produced at 1000°C and 1050°C, indicating that the flat interfaces with no GB migration were less ductile compared to the matrix. A greater strain uniformity was observed along the bond interfaces with improved GB migration. The hardness reduced near the sample surfaces due to enlarged grains induced by temperature gradient. This study demonstrates that GB migration and enhanced mechanical strength can be achieved in diffusion-welded Alloy 617.

The integration of fiber optic sensors into high-temperature materials is critical for real-time monitoring and autonomous operation of engineering systems. This study demonstrated a spark plasma sintering (SPS)-assisted embedding process for integrating sapphire fiber optic sensors into stainless steel components during part fabrication. Optical fibers were encapsulated in stainless steel 316L powders which were sintered at different fabrication conditions using SPS to investigate the effects of sintering parameters on the embedment. Measurements of optical transmittance, combined with microstructural analysis (X-ray computed tomography and scanning electron microscopy) and mechanical testing (tensile and microhardness), were conducted to examine the fiber functionality, fiber-matrix bonding quality, and properties of the sintered materials. The results show that under suitable fabrication conditions, intact optical fibers can be encapsulated in highly-densified (>98 % relative density) stainless steel components. These conditions also led to a good bond at the fiber-matrix interface with micron-sized material interdiffusion across the interface. The sintering parameters were observed to affect fiber optical attenuation, where high temperature, pressure, and hold time during SPS enhanced fiber-matrix bonding and adversely affected optical transmission. Tensile testing confirmed the superior tensile strength and ductility of the matrix fabricated by SPS. Furthermore, the materials exhibited limited strength reduction (similar to 70 MPa) upon the integration of fibers. This study demonstrates the effectiveness of SPS for fiber material integration for high-temperature applications.

As Electric Field Assisted Sintering (EFAS) gains more industrial acceptance and use, it becomes more important to develop more efficient means to implement this technology. To this aim, 3D printed continuous carbon fiber reinforced carbon (CCC) was manufactured and fabricated into tooling for EFAS systems as an alternative to traditional graphite tooling. The impact of fiber orientation on the thermal and electrical properties of the CCC was characterized. Sample material was sintered in Tokai G535 graphite tooling, under common processing conditions and compared with CCC tooling. There was nearly 50 % energy savings compared to graphite while maintaining equivalent sample density and microstructure plus keeping ram temperatures 39 % cooler. This is due to spatial control of generated heat and thermal diffusivity within the molds, by means of fiber orientation anisotropy. Finite element modeling of the tooling design supported the experimental results as well as displays the effect of optimization of this 3D printed CCC material.

Developing graded microstructure and mechanical properties is critical for accelerating the design and optimization of structural materials for a wide range of applications. In this study, Fe-Cr alloys were fabricated by spark plasma sintering (SPS) technique. The microstructure, microhardness, and tensile properties of the as-fabricated and thermally annealed variants were investigated. Graded grain structures were created in the as-fabricated cylinders along both axial and radial directions. Grain size was gradually reduced from the sample periphery to the center. Microhardness measurements reveal a gradual decrease of hardness towards the periphery of the samples. The as-fabricated Fe-Cr alloys show a desired combination of tensile strength and elongation, primarily due to the formation of a high density of oxides, voids, dislocations, and grain boundaries. After in-situ thermal annealing at 600 degrees C under 60 MPa for 2 h using SPS, the Fe-Cr alloys underwent minimal grain growth, and the graded grain structures were retained. The hardness was more uniformly distributed in the annealed variants, and the tensile strength was reduced with an increase in the total elongation, which is attributed to the dissolution of nano-sized oxide particles and the relief of the residual stress. This study demonstrates that SPS coupled with subsequent heat treatment can tailor the graded microstructure and control the mechanical properties of FeCr alloys, showing potential applications in other alloy systems.

The development of compact heat exchangers (CHXs) has gained increasing interest in many industries owing to their high thermal efficiency and reduced size. Diffusion bonding (DB) is an advantageous technique for fabricating CHXs. Alloy 617 is a candidate for manufacturing CHXs for high-temperature advanced nuclear reactors due to its elevated-temperature properties. Previous endeavors in DB of Alloy 617 were conducted by hot pressing (HP), which reported precipitates at the diffusion-bond interface, limited grain boundary (GB) migration, and significantly reduced high-temperature mechanical properties. To overcome these challenges, this study investigated DB of Alloy 617 using electric field-assisted sintering (EFAS). Stacks composed of three sheets were bonded with EFAS using different temperatures, pressures, and hold times. DB using HP as the zero-current analog of EFAS was also performed for comparison. The result shows that Cr- and Mo-rich precipitates were formed at the interface of the hot-pressed samples. The electric current and temperature in EFAS play a significant role in precipitation and GB migration. The electric current coupled with correct temperatures can effectively prevent precipitate formation at the interface and achieve excellent GB migration. Nanoscale Al-rich oxide was formed at the interface of the samples made by both HP and EFAS, but grain boundaries can ignore the nanoscale Al-oxide and migrate across the interface. The temperature, pressure, and hold time also affected diffusion. The temperature is a prerequisite for a successful GB migration, and GB migration can be enhanced by increasing pressure and hold time.

Pratt & Whitney wants to work with INL to develop modeling capabilities around their use of EFAS/SPS to bond materials for their custom geometries and conditions. These are example slides of the type of work we can do for them that they have asked for.

Structural health monitoring is critical for components working in harsh environments to assessing their safety and integrity. This can be realized with advanced sensors, such as fiber optic sensors, embedded in these components. Recent studies have explored the use of ultrasonic additive manufacturing and laser-based additive manufacturing for fiber embedment. Fiber embedding using these techniques has been challenging for high-temperature materials commonly used in extreme environments, resulting in large gaps and voids around the embedded fiber. A good fiber-matrix bond is critical to guarantee free gas/liquid leaks as well as good temperature sensing and strain coupling. To this aim, this study proposed a novel fiber embedding technique using electric field-assisted sintering (EFAS). Sapphire fibers with a diameter of 125 μm and length of 55 mm were placed in SS316L powder, which was subsequently sintered using EFAS. To optimize the processing parameters for fiber embedment, a parametric study was scoped. The as-fabricated samples were first examined using optical transmission testing to inspect fiber integrity. After that, the samples were cross-sectioned for microscopy analysis to evaluate the quality of bond between the embedded fiber and the matrix. The density, microstructure, and hardness of the sintered SS316L were also studied to optimize EFAS parameters. The result shows that intact sapphire fibers can be embedded in high-density SS316L with good metallurgical bonds using the optimized EFAS parameters. Optical transmission inspection demonstrates the successful transmission of light through the embedded fibers.

Electric Field Assisted Sintering (EFAS, also referred to as spark plasma sintering) is a powerful technology for the consolidation of powder materials. The high heating rate during the sintering process is critical for minimizing energy consumption, but it can also cause microstructure heterogeneities in sintered parts, such as spatially varied porosity. The examination of localized porosity usually requires the use of a scanning electron microscope with a carefully prepared surface. In this paper, photothermal radiometry is used to measure local thermal diffusivity and extract localized porosity of EFAS-sintered parts by using a percolation-threshold model. Applying this approach, we identified the radial position-dependent porosity variation in EFAS parts, which is likely formed due to the large temperature gradient during the sintering process. This approach has a unique advantage because it can measure samples with minimal or no surface preparation, enabling the possibility of in situ characterization in EFAS with proper system modification. Necessary modifications on the measurement approach for EFAS deployment and in situ characterization are also discussed.

Compact heat exchangers are of interest for a number of applications including advanced reactors. Alloy 617 is one of the top candidate materials for the gas-cooled reactor intermediate heat exchanger. Previous endeavors to diffusion weld Alloy 617 utilized hot pressing (HP). It was reported that grain boundary migration across the interface was hindered by extensive precipitation. Bonds of this nature have been observed to reduce the elevated-temperature mechanical properties compared to the wrought-product form. It was hypothesized that the electric current applied during electric-field-assisted sintering (EFAS) can overcome these challenges, resulting in improved diffusion welding (DW). This study investigated DW of Alloy 617 via EFAS. Stacks composed of three sheets that were 20 mm in diameter were welded using EFAS. Specimens were welded with an applied electric current, a pressure of 30 MPa, hold time of 30 min, and temperatures of 1050°C, 1100°C, and 1150°C. DW using HP as the zero-current analog of EFAS was also performed at the most promising EFAS conditions. Results revealed that both the applied electric current and temperature played a key role in precipitation and grain boundary migration in diffusion-welded Alloy 617. Precipitates were observed at the interface of the hot-pressed samples which limited grain boundary migration. Electric current was found to prevent precipitate formation along the interface at 1150°C. The electric current coupled with a temperature of 1150°C during EFAS resulted in significant grain boundary migration across the interface.

Magnesium aluminate spinel was sintered and annealed at 1300°C under an applied 1000 V/cm DC electric field. The experiment was designed such that current could be removed as a variable and just the effect of a noncontact electric field was studied. Enhanced grain growth was observed for both samples that were sintered or annealed after densification in the presence of an electric field. Grain‐boundary character distributions revealed that no microstructural changes were induced due to the field. However, the electric field was found to enhance the kinetic movement of cations within the lattice. Energy‐loss spectroscopy experiments revealed cation segregation resulting in regions of Mg‐rich and Al‐rich layers adjacent the grain‐boundary cores. The defects generated during segregation supported the generation of a space charge gradient radiating from the grain‐boundary core out into the bulk, which was significantly affected by the applied field. The interaction between the field and space charges effectively reduced the activation energy for cation movement across boundaries thereby enhanced grain‐boundary mobility and resultant grain growth.