Simulation and optimization of a High Velocity Oxygen Fuel (HVOF) coating process
In HVOF processes, oxygen and a gaseous or liquid fuel are combusted to provide a desired level of total pressure and total temperature in the combustion chamber. Then, a coating powder in an inert carrier gas stream is injected into the uniformly heated hot gas stream generated by the combustion. Depending on the total temperature and the selected coating material, the powder transforms into a molten or semi-molten state. The high-energy gas/powder mixture accelerates through a converging-diverging water-cooled Laval nozzle and reaches supersonic speed. The supersonic jet discharges into a gaseous atmosphere and deposits the particles onto a target surface. Due to the high temperature (>1300 K) and especially due to the high kinetic energy of the particles, a continuous coating process of the target surface is achieved. Typical spray guns, as developed by Oerlikon Metco, are presented in the following schematic.
Extending the operation range of HVOF torches towards lower temperatures, but even higher particle velocities, to deposit copper or copper alloys will be important for upcoming applications in e-mobility for high power electric connectors and bus bars. Additionally, customers’ demands with respect to wider operating conditions and powder materials necessitates stable device operation with minimum down times due to in-nozzle erosion and unwanted in-nozzle particle deposition. Design optimization of spray gun devices for a broad range of operating conditions requires in-depth knowledge of all involved physical processes, such as the combustion, the mixing of combusted gas with particles and carrier gas, the nozzle design and its active cooling and the expected characteristics of the supersonic gas/particle jet. The scope of the project is to develop CFD (computational fluid dynamics) models that allow for predictive numerical simulations of HVOF processes and to assess the effects of design and operating condition variation on the particle deposition and the process stability.
