Julio Gutierrez (Materials Technology, HSU)
In cold spray, the critical velocity defines the minimum velocity that the particles must reach in order to adhere to the substrate. The more particles exceeding this critical velocity, the higher are the efficiency of the process and the deposit quality. Individually needed process conditions to accelerate materials with different densities and melting temperatures could demand different nozzle designs, to transform the upstream process gas pressure and temperature into particle kinetic and thermal energy. In the present study a nozzle geometry optimization model based on 3D-CFD simulations was developed. Together with a design of experiments approach, the model seeks for the optimal nozzle geometry for predefined optimization objectives. In order to reach the highest particle velocity prior to impact upon the substrate, different geometry parameters were varied by means of the nozzle throat cross section, the nozzle divergent section length, the aspect ratio at nozzle full divergent length, and the aspect ratio at 50% of the nozzle divergent length. This allows the study not only of conical nozzles, but trumpet or bell-shaped ones. The cooling of the system was included for a higher accuracy. As sprayed materials, Al6061 and pure copper in mean particle diameters of 15 μm and 40 μm were taken as examples. The process gas was nitrogen with set stagnation pressure and temperature of 50 bars and 500 °C. For both materials, the simulation identified nozzle divergent section length as the most influential parameter, followed by the throat cross-section. The aspect ratio at nozzle full divergent length and the aspect ratio at 50% of the nozzle divergent length must be tuned to avoid gas over expansion in the nozzle, which could lead to a decreased powder acceleration.