NUMERICAL-SIMULATION OF SUBSTRATE IMPACT AND FREEZING OF DROPLETS IN PLASMA SPRAY PROCESSES

In the present study, deformation, interaction and freezing during substrate impact of molten droplets in plasma spray processes are numerically investigated. The numerical simulation is conducted on the basis of the full Navier-Stokes equations and the volume of fluid (VOF) function by using a two-domain method for the thermal field and freezing problem and a two-phase flow continuum model for the flow problem with a growing solid layer. Important processing parameters, such as droplet temperature, substrate temperature and droplet velocity are considered and their effects on flattening and freezing are discussed. The numerical results reveal that a droplet spreads and solidifies into a splat of diameter 1.6 to 3.8 times the initial droplet diameter within a time of 0.12 to 0.44 mus. The droplet liquid separates from the solid/liquid interface when freezing occurs. Increasing initial droplet velocity and enhancing substrate temperature can significantly enhance the flattening extent of droplet. A fully liquid droplet impinging on a solid substrate may generate good contact between the splat and the substrate; a fully liquid droplet striking on another flattening splat produces ejection of the liquid; a fully liquid ring colliding with a flattening splat causes bounce of the liquid and formation of voids. A combination of a liquid droplet condition at a high initial velocity with a semi-solid or solid surface condition may produce good adhesion in sprayed deposits or coatings.