Influence of Nozzle Throat Cross Section on Microstructure and Properties of Cold Sprayed Coatings

In cold spray, the coating microstructure and properties are strongly dependent on the particle velocity before impact. Particle velocity for a given powder and thereby energy consumption during the process are mainly dependent on process pressure, process temperature and nozzle geometry. Of the several aspects of nozzle geometry, the throat cross section determines the mass flow rate and thereby the total energy for the process. While reduction in throat cross section reduces the energy consumption per unit time of operation, viscous boundary layer effects become significant at smaller throat cross sections causing reduction in gas and particle velocities. This work explores the effect of throat cross section on the particle velocity distribution and thereby the coating microstructure and properties. Computational fluid dynamic simulations are also carried out to rationalize the observations. Our results corroborate the theoretical findings of previous work based on nozzle exit or throat width h to divergent length L ratio, i.e., h/L. Moreover, it was found that nozzles with lower throat cross section (h/L < 0.02) consume significantly higher energy per unit mass of coating due to longer coating duration and significant boundary layer effects in addition to yielding relatively poorer coating property.