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2877 |
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Cryo-gas cooling method and system for thermal spray coatings |
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Maximus Akuh* / Air Products GmbH, Germany Zbig Zurecki / Air Products and Chemicals, Inc., USA Lisa Mercando/ Air Products and Chemicals, Inc., USA |
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Numerous thermal coating applications require significant heat input during the coating operation and cooling combined with well controlled temperature for the substrate in order to achieve optimum performance of the final part. Processes, such as high-velocity oxy-fuel (HVOF), plasma spray, and spray deposition of sputtering targets, are only just a few that face this challenge. This presentation concludes a comprehensive effort to develop a versatile and cost-effective, cryo-gas cooling method and system for high heat input, thermal spray coated substrate parts. The Air Products LIN-Cooling System comprises thermal spray-dedicated, gas-liquid aerosol cooling nozzles and a multi-zone, infra-red thermal imaging controller. The system enabled halving the spraying time and consumption of feed powder, hydrogen, and oxygen during the HVOF coating of an airplane landing gear with WC-CoCr. Hard-facing of metallic work parts with WC-Co-type coating offers a performance and cost alternative to toxic chromium (Cr6+) plating. However, the optimum wear, fatigue, and corrosion performance of the sprayed coatings can be reached only when the substrate temperature is fully controlled during spraying. Maintaining a narrow temperature range of 50oC150oC is typically required while coating shot-peened, aerospace-grade AISI 4340 steel components, and a further reduced top temperature limit is needed during coating of aluminum components. The use of interpass cooling breaks (ICBs) is therefore necessary, even in the HVOF operations involving the conventional, forced air or CO2 cooling. The LIN-Cooling System eliminated ICBs, thus, bringing significant time, feed powder and process gas savings. Additional labor savings were realized by the use of soft, flexible masking, not feasible with the traditional cooling methods. The operation of the system and its effect on coating microstructure, surface oxidation and composition, powder deposition efficiency, properties of substrate material, and evolution of interfacial residual stresses will be presented and discussed. Keywords: HVOF, thermal spray, automated cryogenic nitrogen cooling, production rate, WC-Co coatings, thermal imaging, substrate oxidation, residual stress, deposition efficiency. |