Three-dimensional Numerical Simulation Of Coating Formation In Plasma Spraying

Plasma sprayed coatings are built up by the accumulation of splats formed by thedeposition of individual molten droplets. A three-dimensional computational model ofsplats formation including heat transfer and solidification is established. The governingequations are discretized according to typical finite volume conventions. A volume of fluid(VOF) tracking algorithm is used to track the droplet free surface. A molten dropletimpacting, spreading, heat conducting and solidification on the steel substrate during theprocess of splat formation was simulated. Furthermore, the simulations of the process ofcoating formation under droplet/droplet and droplet/splat interactions and splat formationon substrates with different conditions were presented. Meanwhile, the simulated resultswere visualized.The simulation of the splat formation indicated that there are a high pressure zone onthe front of the flying droplet and a negative pressure zone at the anterior-lateral. This kindof pressure distribution leads to the flowing of fluid in the flying droplet. After the start ofimpacting, it was found that the pressure distribution inside droplets is in the shape of amushroom cloud, and the impact pressure decreases from the impact point which also is thearea of the maximum pressure to the back side of droplets. The maximum pressure Pmaxwas found at the impact beginning and the maximum spreading velocity of fluid Vmaxinside droplet delay for0.03μs. Both of the above are the function of impact velocity υ0,namely, Pmax=Cρυ0, Vmax=3υ0. The scalar quantity field of the spreading velocity at themoment of splashing occurrence changed from Gaussian distribution to asymmetricaldistributions. The splashing resulted in the formation of the final fingered splat. There isresidual compressive stress remains in the final splat.Different conditions of substrate and the position of impact point strongly influencethe formation process and the final shape of the splat. The peak profile on the surface ofsubstrate is more effective than the valley profile on the splat formation. The effects of peakprofile decrease with the reduction of the acuteness. As the thermal conductivity of thesubstrate increases, the droplet spreading decrease, the splat becomes thicker, fingers andsplash behaviors of the splat decrease. The splat aspect ratio, ψ, spread factor, ξ, and equivalent thickness, d, respectively is the quadratic function of the tilt angle of substrates θ.The trends of the change of ξ and d with θ are negative correlation.The impact dynamics simulations of droplet interactions revealed that protuberancebecomes smaller and smaller with the distance between two droplets increasing. Theinteractions between two droplets become negligible when the distance was increased to thecritical value. The changes of the radius ratio of two adjoining droplets in the impactvelocity direction directly influence the formation process and the final shape of splat.When the two droplets have the same radius, the interactions between two droplets reachmaximum, and the splashing happened seriously. Furthermore, those interactionssignificantly effect on the surface morphologies of the coating. When the difference betweentwo droplets size is larger, the interactions obviously become weaker.The simulation results of coating formation showed that the spreading of droplet on asolidified splat propagated uniformly, and interactions between the front of moving fluidand the edge of a splat resulted in the splashing and even the formation of a small satellitesplat. While a droplet impact on an unsolidfied splat, the significant splashing happenedand successive spreading of the droplet resulted in the formation of a coronary protuberance.The change of the relative position of droplets and splats leads to the variation of thedeformation process and final morphology of droplets, and finally directly affect the qualityof the coatings.