Wire-Arc Additive Manufacturing and Wire-Arc ThermalSpray Methods to Clad Nuclear-Reactor Materials

The Department of Energy (DOE) requires the demonstration of bimetallic corrosion-resistant surfaces for structural and cooling components used in molten-salt 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. NanoCoatings, Inc. (NCI), along with our team members Colorado School of Mines (CSM) and Kymera International, propose to demonstrate and compare Wire- Additive Manufacturing (AM) methods, Gas-Metal-Arc-Welding (GMAW) and Twin Wire Arc Spray (TWAS), to deposit thick (mm-range) corrosion-resistant Ni-base alloys (e.g., Pure Ni and Haynes 244 onto qualified materials (e.g., IN617) in the ASME Boiler and Pressure Vessel Code. Oak Ridge National Laboratory (ORNL) scientists will be consulted to confirm cladding and substrate materials selection, candidate component geometries, DOE-Laboratory or University sources for molten-salt exposures, and selection of components for Wire-AM demonstration (in Phase II). These unique Wire-AM fabrication methods offer several attractive features, including: 1) high deposition rate of high-density material with good material utilization, 2) reduced contamination and porosity compared to powder-based AM methods, 3) excellent bond strength to the underlying substrate, 4) low-cost equipment suitable for large geometry processing, 5) ability to control heat-input to reduce part distortion, and 6) robotic control to ensure uniform clad deposits. Haynes 244 wire material for Wire-AM method comparison will be provided by Haynes Corporation. CSM will perform GMAW to demonstrate feasibility of this AM method, while Kymera will utilize their TWAS method. In general, both methods produce melted material that is transferred and bonds with the substrate with microstructures resembling those of cast products, dendritic/columnar and graded to the substrate. We will coordinate with Haynes personnel to determine if post-process heat-treatment will be required. In Phase I, flat-substrates of ASME qualified material (e.g., IN617) will be clad by the Wire-AM methods with Pure Ni and Haynes 244 to enable tooling and process-parameter development and to provide coupons for characterization and testing. NanoCoatings, Inc. employees will utilize South Dakota Mines (SDM) equipment to characterize / test Wire-AM clad materials under an equipment user-fee arrangement. This will include: 1) NDE x-radiography or ultrasonic C-scan (at South Dakota State) for interface bond-integrity and defect/porosity characterization, 2) Optical metallography as a complementary assessment of bond quality, defects/porosity, and microstructure; 3) X-ray diffraction for phase identification, 4) Adhesive pull tests for deposit bond-strength, and 5) Corrosion/oxidation in high-temperature air. After demonstration of Wire-AM deposited materials on flat surfaces, equipment design modifications will be identified in Phase I to enable cladding of pipe or container internal surfaces in Phase II. Phase II planning will include identification of a facility for molten-salt corrosion-exposures (e.g., ORNL, other DOE- Laboratories (e.g., Idaho National Laboratory), or University (Virginia Tech)) on Wire-AM processed flat and curved coupons. In addition, design/ fabrication / demonstration of subscale tooling and hardware required to enable Wire-AM demonstration of the inside-diameter surfaces of ASME Boiler and Pressure Vessel Code reactor pipe and container material will be perform.