Novel Additive Friction-Stir Deposition Method to Clad Nuclear-Reactor Structural Materials with Corrosion-Resistant Alloys.
- NanoCoatings, Inc.
In this Phase I SBIR project, our team of NanoCoatings, Inc. (NCI) and Virginia Tech University (VT) evaluated Additive Friction Stir Deposition (AFSD) as a novel Additive Manufacturing (AM) method to deposit thick claddings of pure Nickel (Ni)and Haynes 244 (Ni-Mo-Cr-W) alloy, materials of interest as corrosion-resistant surfaces for ASME qualified nuclear-reactor pipe material. Both Ni and Haynes 244 material have shown excellent laboratory-test corrosion resistance to liquid fluoride salts, proposed for molten-salt reactors (MSR). AFSD is a unique AM method where intense plastic deformation induces high contact temperatures from frictional heating to enable the deposition transfer of a feed stock material onto another metallic substrate. This method has been shown to produce extremely fine-grained deposited material with higher hardness than the feed stock material and good adhesion to the underlying substrate. During AFSD processing of Ni, VT personnel observed seizure/adhesion of the Ni in the W-tooling, resulting in twisting of the square cross-section pure Ni feed stock material, which prevented plastic deformation and flow of this material onto the IN617 substrate material. This twisting deformation was attributed to the very low yield strength of the pure Ni feed stock. As a result, Ni was not considered for further investigation in this project. Haynes 244 alloy, a high-use temperature alloy originally developed with application for static parts in advanced gas-turbine engines that require low thermal expansion at elevated temperatures, has about a 2-3X higher yield strength than pure Ni. AFSD of Haynes 244 was demonstrated by VT personnel as an initial button deposition and then as a continuous ribbon deposition. The demonstration of a continuous ribbon deposition is critically important for cladding molten salt coolant pipe ID surfaces. AFSD deposits of Haynes 244 were about 1.5-2X harder than the AFSD feed stock material, presumably due to classic Hall-Petch grain size effects from the much finer grain size of the AFSD material. Bonding to the underlying IN617 substrate appeared to be of high quality, except for the observation of fine-scale porosity dispersed in the AFSD deposit and near the interface area. This porosity is somewhat troubling and could adversely affect the mechanical properties adhesion, bending-flexure, pressure-cycling, etc. It is believed that application of higher applied force and feed rate of the Haynes 244 material would be needed to effectively seal this porosity. Also, achievement of higher process temperature, e.g., >>0.7 AFSD temperature / Haynes 244 melting temperature is recommended to ensure continuous ribbon deposition. All of these conditions may be met using a more commercially-viable AFSD machine (e.g., manufactured by MELD), in contrast to the “home-made” unit employed by VT personnel. Microstructure characterization of the AFSD processed Haynes 244 material showed the presence of some fine-scale grain-boundary precipitates (assumed to be Nis(Cr,Mo,W) from Haynes 244 literature) at a higher concentration than for the feed stock material, presumably due to the much finer grain size. These precipitates were not revealed during XRD scans, which sampled a much larger area and the precipitates were very fine scale. After oxidation at 760°C for 215 hrs., these precipitates were much more pronounced populating grain boundaries for both Haynes 244 feed stock as well as AFSD processed Haynes 244 material. Scanning Electron Microscopy – Energy Dispersive Spectroscopy analysis revealed a preferential increase in Mo content and slight decrease in Ni, suggesting conversion to Ni-depleted, Mo-enriched Cr, W precipitation area along the grain boundaries. It is not clear at this time if there is a depletion of Mo from the zones adjacent to the grain boundaries. Surface oxidation of both Haynes 244 feed stock and AFSD Haynes 244 to a Cr-oxide protective film was observed, with a subsurface Cr depleted zone apparently providing the source of Cr in the oxide structure. This oxide and depleted zone widths were both quite thin, averaging about 0.38 µm (3,750 A) for the Cr-oxide thickness and about 7.38 µm for the penetration of the Cr-depleted zone. But the Cr-oxide layer did effectively prevent oxygen penetration into the AFSD structure, as observed from the oxygen EDS map underneath the oxide layer. Another important observation after the oxidation treatment was the higher density of porosity seen in the AFSD Haynes 244 deposit. Bond Technologies performed a short trade study to consider designs that would be required to enable actual AFSD cladding of pipe ID surfaces. Two design configurations were conceptualized with advantages and disadvantages being documented for both designs.
- Research Organization:
- NanoCoatings, Inc.
- Sponsoring Organization:
- USDOE Office of Science (SC), Engineering & Technology. Office of Small Business Innovation Research (SBIR) and Small Business Technology Transfer (STTR) Programs
- DOE Contract Number:
- SC0022489
- OSTI ID:
- 1923673
- Type / Phase:
- SBIR (Phase I)
- Report Number(s):
- DOE-NCI-489
- Country of Publication:
- United States
- Language:
- English
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