Novel Additive Friction-Stir Deposition Method to Clad Nuclear-Reactor Structural Materials with Corrosion-Resistant Alloys

The Department of Energy requires the development and demonstration of bimetallic or corrosion- resistant surfaces for structural and cooling components in contact with liquid-salt coolants used in nuclear-reactor systems. Proposed coating materials must survive thermo-mechanical loading (e.g., no flaking or spalling) during exposure to the high-temperature conditions encountered during reactor operation and maintain metallurgical stability with no diffusion-induced composition changes or aging over extended time-durations. The objective of our research is to investigate a relatively new additive manufacturing method, Additive Friction-Stir Deposition, to deposit corrosion-resistant Ni-base alloys (e.g., Haynes 244, Hastelloy-N or others to be determined) onto specified materials in the American Society of Mechanical Engineers Boiler and Pressure Vessel Code. At the onset, we will consult with subject matter expert scientists for materials selection recommendations (including potential need for thermal-expansion buffer layer), candidate component geometries, sources for coupon-level liquid-salt exposures, and selection of hardware for Additive Friction-Stir Deposition demonstration in Phase II. This unique additive manufacturing method, being pioneered at a major United States University, exhibits several unique features, including: 1) deposition of millimeter-thick layers, 2) good coating adhesion thanks to plastic-deformation, material flow and mixing, and forging-action with the substrate, 3) low residual stress and low distortion, and 4) high build rate and good scalability. In general, this unique additive manufacturing process induces severe plastic deformation at elevated temperature which results in a uniformly recrystallized microstructure with minimal internal defects or porosity. Furthermore, the forged microstructure results in as-printed mechanical properties that are comparable to wrought material properties. Initially, flat-substrates will be processed with selected alloys to enable tooling and process-parameter development and to provide coupons for characterization and testing, which will include 1) Non Destructive Examination x-radiography for interface bond-integrity and defects/porosity characterization, 2) optical metallography as a complementary assessment of bond quality, defects/porosity, and microstructure; 3) x-ray diffraction for phase identification, 4) tensile testing to quantify the bond strength and failure mechanism (pre and post thermally annealed), and 5) corrosion/oxidation in high-temperature air. After development of acceptable Additive Friction-Stir Deposition material on flat surfaces, curved surfaces (representing a portion of a pipe, curvature to be determined) will then be printed to demonstrate application to complex-shaped surfaces. Preliminary characterization of Additive Friction-Stir Deposition processed curved surfaces will also be performed to assess the effects of curvature on the printed material. Phase II planning will include selection of liquid-salt for corrosion-exposures (in facilities to be selected) on Additive Friction- Stir Deposition-processed flat and curved coupons and development / fabrication of subscale tooling and hardware required to enable Additive Friction-Stir Deposition demonstration of the inside-diameter surfaces of American Society of Mechanical Engineers Boiler and Pressure Vessel Code reactor pipe material. Commercial applications include cladding of structural materials for high-temperature service required for future aerospace planes, protection of furnaces and other high-temperature equipment from corrosion, protection of low-cost steel cook stove components from oxidation/corrosion, and application of armor alloys to low-cost base alloys used in personnel shelters and transportation vehicles.