New insights into the effects of powder injector inner diameter and overhang length on particle accelerating behavior in cold spray additive manufacturing by numerical simulation

Application requirements, especially additive manufacturing, have forced cold spray technology to strengthen the precise control of particle behavior and optimize the spray gun for long working hours. Our recent studies show that the powder injector, as a small component inside the spray gun, greatly disturbs the stream fields and the impact parameters of particles. A model dedicated to the hydrodynamic numerical calculation was established which takes into account the influence of changed parameters of powder injector and the mixing effect between the powder feeding gas and main working gas, enabling more accurate simulations and greater insight into the particle acceleration procedure. Simulation results reveal that the particle distributions near the nozzle throat and in front of the substrate are significantly influenced by the particle diameter, the inner diameter of powder injector, and the original pressure discrepancy between the powder feeding gas and main working gas. Employing lower differential pressure and a smaller inner diameter of the powder injector can not only make the particles more concentrated near the nozzle axis, to reduce the possibility of nozzle clogging, but also improve the particle impact velocity. At the same time, for higher precision of cold spray additive manufacturing, particles with diameters of 20–40 μm should be considered. Besides, the impact velocity and temperature of particles are extremely affected by the interaction between the powder feeding gas and the main working gas, which is more sensitive to the particle diameter of 15–45 μm. In addition, the mixing state of the two gases shows significant effects on the stream fields in the upstream of the nozzle, but little influence around the nozzle exit. In the case of the powder injector extending into the nozzle throat, the flow field is greatly influenced because of the obvious disturbance of gas flow at the powder injector end.