Development of Brazing Alloy for SiC Brazing - EWI · Development of Brazing Alloy for SiC Brazing...
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Drew Shipley, Project Engineer Alber Sadek, Senior Engineer EWI Development of Brazing Alloy for SiC Brazing Introduction Silicon carbide (SiC) is an important structural ceramic used in the aerospace, automotive, electrical, and nuclear industries. Its exceptional thermal and environmental stability, high resistance to radiation and thermal shock, high strength, and toughness make SiC a widely used ceramic material. Resistance to fretting wear and hydrogen resistance, as well as a lower neutron penalty than zircaloy, make SiC an economical choice for advanced nuclear fuel programs. Joining SiC For use in industry, SiC is commonly joined to itself or other metals. SiC-to-SiC joint manufacturing regularly requires very high temperatures and pressures. In nuclear environments, processes such as displacement reaction bonding, diffusion bonding, and brazing are currently used to join SiC-to-SiC. However, the general method for each process creates joints that do not typically pass irradiation or flowing water tests that simulate in-service conditions. These processes can also require high pressure and extensive heating times, making joint assembly difficult and costly. Due to these process shortcomings, EWI has developed a patent-pending technology using a multiphase, braze-alloy interlayer consisting of silicon and aluminum that has the potential to meet the requirements for nuclear fuel programs. SiC Brazing Method EWI’s novel SiC brazing method was developed to produce hypereutectoid braze alloy compositions derived from the aluminum-silicon binary phase diagram. Compositions with high silicon content and with high aluminum content were made. With the creation of an alloy braze paste, the paste was placed at the interface of the SiC-to-SiC joint. The SiC specimens were brazed at various temperatures between the liquidus of aluminum and solidus of silicon. With a high silicon content in the braze paste, a higher temperature is required during the brazing cycle. Whereas, high aluminum content braze pastes used lower temperature brazing cycles. Whether the paste had high aluminum or silicon content, the same dwell time was used for the brazing cycle. Compression shear testing using an “ad hoc” modification of the ASTM D905-89 test procedure was done to analyze the strength of the SiC brazed joint. Results All aluminum-silicon braze pastes showed good wettability with the silicon carbide interface, allowing for successful brazing of the joint. In both high-aluminum and high-silicon content, eutectic aluminum-silicon precipitates occupy the bulk of the brazing interlayer microstructure, which also contains plates of silicon. The two-phase microstructure provides crack arresting paths to enable high toughness. The image below shows the two-phase aluminum-silicon microstructure. As the silicon content of the braze paste was increased, the volume fraction of aluminum-silicon precipitate decreases, changing the silicon phase from lathes to block-like shapes. The difference between the microstructure containing silicon lathes and block shapes can be seen below. At a higher temperature, the eutectic aluminum- silicon precipitates become finer after the brazing cycle. Along with a finer aluminum-silicon microstructure, the silicon lathes were less prevalent after a higher temperature brazing cycle. Microstructures with silicon lathes (left) and with silicon block shapes (right).
Transcript of Development of Brazing Alloy for SiC Brazing - EWI · Development of Brazing Alloy for SiC Brazing...
Drew Shipley, Project EngineerAlber Sadek, Senior EngineerEWI
Development of Brazing Alloy for SiC Brazing
IntroductionSilicon carbide (SiC) is an important structural ceramic used in the aerospace, automotive, electrical, and nuclear industries. Its exceptional thermal and environmental stability, high resistance to radiation and thermal shock, high strength, and toughness make SiC a widely used ceramic material. Resistance to fretting wear and hydrogen resistance, as well as a lower neutron penalty than zircaloy, make SiC an economical choice for advanced nuclear fuel programs.
Joining SiCFor use in industry, SiC is commonly joined to itself or other metals. SiC-to-SiC joint manufacturing regularly requires very high temperatures and pressures. In nuclear environments, processes such as displacement reaction bonding, diffusion bonding, and brazing are currently used to join SiC-to-SiC. However, the general method for each process creates joints that do not typically pass irradiation or flowing water tests that simulate in-service conditions. These processes can also require high pressure and extensive heating times, making joint assembly difficult and costly.Due to these process shortcomings, EWI has developed a patent-pending technology using a multiphase, braze-alloy interlayer consisting of silicon and aluminum that has the potential to meet the requirements for nuclear fuel programs.
SiC Brazing MethodEWI’s novel SiC brazing method was developed to produce hypereutectoid braze alloy compositions derived from the aluminum-silicon binary phase diagram. Compositions with high silicon content and with high aluminum content were made. With the creation of an alloy braze paste, the paste was placed at the interface of the SiC-to-SiC joint. The SiC specimens were brazed at various temperatures between the liquidus of aluminum
and solidus of silicon. With a high silicon content in the braze paste, a higher temperature is required during the brazing cycle. Whereas, high aluminum content braze pastes used lower temperature brazing cycles. Whether the paste had high aluminum or silicon content, the same dwell time was used for the brazing cycle. Compression shear testing using an “ad hoc” modification of the ASTM D905-89 test procedure was done to analyze the strength of the SiC brazed joint.
ResultsAll aluminum-silicon braze pastes showed good wettability with the silicon carbide interface, allowing for successful brazing of the joint. In both high-aluminum and high-silicon content, eutectic aluminum-silicon precipitates occupy the bulk of the brazing interlayer microstructure, which also contains plates of silicon. The two-phase microstructure provides crack arresting paths to enable high toughness. The image below shows the two-phase aluminum-silicon microstructure. As the silicon content of the braze paste was increased, the volume fraction of aluminum-silicon precipitate decreases, changing the silicon phase from lathes to block-like shapes. The difference between the microstructure containing silicon lathes and block shapes can be seen below. At a higher temperature, the eutectic aluminum-silicon precipitates become finer after the brazing cycle. Along with a finer aluminum-silicon microstructure, the silicon lathes were less prevalent after a higher temperature brazing cycle.
Microstructures with silicon lathes (left) and with silicon block shapes (right).
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Development of Brazing Alloy for SiC Brazing
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Drew Shipley, Project Engineer, is a member of EWI's materials engineering team. He is involved in developing and optimizing the bonding of similar and dissimilar materials using EWI lab furnaces and also works with solid-state bonding technologies such as friction stir welding. Before joining EWI, Drew was an undergraduate research assistant at The Ohio State University where he studied creep life of nickel-based superalloys and gained experience in microscopy (SEM, TEM, EBSD), sample preparation, and mechanical testing.
Alber Sadek, Senior Engineer, works in EWI's Materials group. His current work involves the selection of metals and alloys as they are applied in different industrial sectors, covering their physical properties, material characterization, weldability (similar/dissimilar alloys), corrosion, wear, fatigue, and creep properties. He investigates the effect of welding parameters, welding processes and shielding gas composition on the microstructure and mechanical properties of different alloy weldments and has extensive experience in failure analysis and troubleshooting for a variety of industrial sectors.
After shear testing, the average strengths of different braze paste compositions were found. Braze paste containing a high silicon-content had a greater average shear strength than the braze paste with a high aluminum content.
Conclusions EWI’s patent-pending technology meets the requirements of advanced nuclear fuels programs. Using a hypereutectic mixture of aluminum and silicon as the braze material, silicon carbide joints are made without the need for high pressures
and temperatures. This technology can be used to braze newly developed tubes to silicon carbide multilayer structures for nuclear cladding application, retaining high strength and corrosion resistance while also providing a cost-saving advantage to materials currently being used. This technology is useful for applications in which applying pressure to a joint during heating can cause deformation or fracture. With a lower temperature and no need for pressure, it requires less energy to braze the silicon carbide joints with this technology.
