Particle-based approach for modeling phase change and drop/wall impact at thermal spray conditions

The ability to model the impact of small, high-velocity molten droplets on a cold substrate is a key to designing the thermal spray process. Such drop-wall interactions involve complex phenomena such as droplet splash, phase change, solidification, substrate melting, and re-solidification. In this study, a particle-based approach, Smoothed Particle Hydrodynamics (SPH), is used to simulate the impact of molten droplets on a cold substrate at the thermal spray conditions, i.e., small droplets with high temperature and velocities. A solidification model is developed to simulate the phase change of the droplet and substrate. The present SPH method is validated against the experimental results using tin droplets on stainless-steel and aluminum substrate, and copper droplet on copper substrate. The histories of the spread factor, the substrate temperature, and the splat height at the impingement point are validated. A parametric study on the impact of the high melting point molybdenum droplets on different substrate materials (including tin, stainless steel, zinc, yttrium stabilized zirconia, and aluminum) is performed. The temporal evolution of the solidification interface and the height/thickness of the solidified splat are reported. Temperature distributions across the splat, substrate, and the corresponding melting/re-solidification are investigated. It was found that the cooling rates for the same impingement velocity were nearly the same for the same substrate regardless of the initial substrate temperatures but were higher for higher impingement velocities. A modified Biot number and thermal diffusivity were used to describe the heat transfer characteristics of the splat substrate combination. Results show that a higher modified Biot number for the splat-substrate combination indicates a faster cooling of the splat and a higher maximum substrate temperature. Furthermore, a substrate with high thermal diffusivity also causes heat at the impingement location to be dissipated quickly, resulting in a fast-cooling rate and accelerated solidification.