Effect Of The Powder Carrier Gas On Powder Stream Impact Characteristics In High-pressure Cold Spray

Cold gas dynamic spraying(CGDS) developed in recent 20 years is a relatively novel method of preparing coatings. Micron-sized particles(typically<50?m) can be deposited at a temperature below the melting point temperature. So CGDS can effectively minimize or eliminate the deleterious effects of oxidation, melting, evaporation and other common problems suffered in the thermal spraying. In general, when the particle flies at relatively higher impact velocity and temperature, the deposition efficiency is relatively larger, and the formed coatings are relatively better. Therefore, how to improve particle impact parameters is an important topic of CGDS. In order to feed safely and continuously sprayed materials, the powder carrier gas is characterized by higher pressure and lower temperature, compared to the main propulsion gas in high-pressure cold spray. The lower temperature is required to cool the powder injector tube and further to prevent the particle bonding the tube wall. However, the influence of the powder carrier gas on gas and powder stream acceleration is usually ignored in CGDS.First, it is found by combining numerical simulation and experiments that the particles located the substrate central tend to acquire an inferior impact parameter because of the axial injection of the colder carrier gas, which causes the nonuniform coating performance. As increasing the pressure of the cold powder carrier gas, the central regions of stacking coatings are scattered by cracks and pores. In addition, the preheating temperature of the substrate also decreases with the increasing of the pressure of cold powder carrier gas. As a result, the pores are shown at the interface and increase away the substrate center.Then, the parameters related to the power carrier gas are analyzed to examine their influence on the impact parameter distribution by the mean of numerical simulation. Our study found that the particles near the central region influenced by the cold powder carrier gas had an inferior deposited performance at a state of undesirable mixing degree of two gas streams. In accordance with the mixing degree, the exchange of momentum and heat between the gas streams and the particles, the gas flow and acceleration performance in nozzles, the particle trajectories and impact parameter distributions are quite distinct from the previous conclusions. The relation between the parameter related the powder carrier gas and the mixing degree are analyzed by numerical simulation methods. For the larger initial pressure differential, smaller diameter ratio or feeding particle with a longer pre-chamber, the vortexes can be triggered by the powder carrier gas, the heat and momentum exchange between the two gas streams becomes more intensively, and thus the gas streams mix stronger. Consequently, the particles in the central region benefit from the subsequent fully developed pipe flow rather than withstand the adverse influence of the cold powder carrier gas. However, when the larger initial pressure differential or smaller diameter ratio is adopted, the average impact velocity will decrease because of the increment of the mass flow rate percentage of cold powder carrier gas. However, increasing the pre-chamber length is a better method to increase the particle average impact parameter. It is also found that the particle dispersion has a certain relation with the powder carrier gas. The main propulsion gas is disturbed by the injection of the powder carrier gas, and turbulent is generated on the interface of the two gas streams. Powder stream is influenced by turbulent kinetic energy, and the value of turbulent kinetic energy determines the particle dispersion. So the spraying width can be controlled by the adjustment of the powder carrier gas.Moreover, the influences of carrier gas species are also studied. When helium is chosen as the powder carrier gas and air is the main propulsion gas, gas acceleration doesn't been influenced strongly by the low temperature powder carrier gas and has the characteristic of fully developed pipe flow in spite of no better mixing of two flow streams. That is because helium has a better acceleration performance. At a result, the gas inside nozzle and the particles near the central also have a better acceleration. By comparing analysis between the cases of selecting helium as powder carrier gas and helium-air gas mixture with different helium mass percentage as driving gas, the results show that the former is more economical and has a better particle acceleration performance.Above all, the effects of the carrier gas on coating quality have been examined. Furthermore, in order to provide basis for selecting suitably the parameters related particle carrier gas in practice, the results of the effect of particle carrier gas on impact characteristics of powder stream could provide a valuable guide for acquiring high-quality coatings.