Numerical study on the effect of the cold powder carrier gas on powder stream characteristics in cold spray

• Colder carrier gas can either damage or regain the fully developed pipe flow.
• The better mixing of two gas streams is, the more energy is utilized to accelerate particles.
• Central particle acceleration depends mainly on particle injection locations.
• Particle dispersion is determined by the turbulence kinetic energy triggered on the interface of two gas streams.

The quality of coatings prepared by cold spray is largely determined by the primary parameters of the gas-powder stream. In this investigation, the parameters related to the power carrier gas, including the initial pressure differential between the main propulsion gas and the powder carrier gas, the diameter ratio of the nozzle throat to the powder injection tube and the particle injection location, are analyzed to examine their influence on the deposition characteristics. The results indicate that the particles in nozzle center region have relatively low impact velocity and temperature on substrates, which would bring about nonhomogeneity and nonuniformity of formed coatings. The larger the initial pressure differential, the smaller the diameter ratio and the longer the pre-chamber, the more intensive the heat and momentum exchange and mix between the two gas streams become. This intensive exchange and mix can compensate the effect of cold powder carrier gas to the impact velocity and temperature of the particles in the vicinity of nozzle center area. However, the overall impact parameters depend upon the mass flow rate percentage of each gas stream and the mixing degree of the two gas streams. Moreover, it is found that the particle dispersion has a certain relation with the turbulent kinetic energy generated on the interface of the two gas streams.