Effect of cold spray particle conditions and optimal standoff distance on impact velocity.

The impact velocity of particles plays an important role in coating formation of cold spray. A proper standoff distance from the nozzle exit to the substrate surface can ensure sprayed particles to achieve relatively high impact velocities. Numerical simulations were conducted by using a CFD software, Fluent, to predict the supersonic flow field outside the nozzle. Based on the simulation results, the effect of particle conditions on impact velocity is studied and also the way to determine the optimal standoff distance is proposed. It is found that particles with small size, low density or irregular shape can be significantly affected by the dilatational waves, oblique shock waves and bow shock waves. With the energy dissipation or passing through the bow shock waves, these particles are decelerated sharply. Moreover, there exists an optimal standoff distance for different conditions. The determination of the optimal standoff distance requires two essential considerations: One is that the substrate must be located at the place where the carrier-gas velocity is relatively low to weaken the strength of the bow shock waves; the other is that particles should be accelerated twice by dilatational waves before entering the bow shock waves, which means there must be two groups of dilatational waves in front of the bow shock waves. [ABSTRACT FROM AUTHOR]