The curling up of splats of molten metal deposited on a cold substrate was investigated both experimentally and numerically. An analytical model based on mismatch of thermal expansion between the splat and substrate was developed to calculate the deformation of the splat after curling up. The curling up angle at the edges of splats was predicted using the analytical model and compared with the experimental measurements. The prediction shows good agreement with the experiments.

A three-dimensional, time-dependent numerical model of free-surface flows and heat transfer including phase change has been used to simulate the impact of a liquid droplet on a solid particle and to predict the size of the void under the solid particle caused by incomplete filling by liquid landing on top of it. The solid particle was considered to be a protrusion on the substrate. Fluid flow in the impacting liquid droplet was modeled using a finite difference solution of the Navier-Stokes equations in a 3D Cartesian coordinates assuming laminar, incompressible flow. Heat transfer in the liquid droplet was modeled by solving the energy equation, assuming densities of liquid and solid to be constant and equal to each other. The free surface of the liquid droplet was assumed to be adiabatic. The porosity in this simulation was defined as the volume of the incompletely filled void under the solid particle to the volume of the solid particle. The simulation was repeated with different process parameters, and the results showed that process parameters play significant roles in determining the amount of porosity. A correlation is found to express the porosity as a function of the process parameters.

The impact and solidification of 4 mm molten aluminum alloy 380 droplets on a tool steel substrate was studied both analytically and experimentally. Temperature histories at different radial location on the substrate surface under impacting droplets were recorded using an array of thin film thermocouples with response times less than 1 µs. Photographs were taken of droplet impact onto the substrate. Initial substrate temperature was varied from room temperature to 300°C and average surface roughness from 0.5 to 5.0 µm. Estimates of thermal contact resistance were made by matching measured substrate temperatures with an analytical solution for surface temperature variation. A model of the true area of contact between molten metal and a rough surface was developed in order to predict how contact resistance changes with surface roughness and contact pressure. Impact of molten aluminum alloy droplets was simulated using a three-dimensional numerical. Using values of thermal contact resistance predicted by the model gave good agreement between computed and observed droplet shapes during impact.

In this paper, a three-dimensional stochastic model is extended to simulate the thermal spray coating process from droplet impact to coating formation on a solid surface. Process parameters of molten droplets landing on the substrate are generated randomly by assuming that these properties follow normal distributions with user-specified means and standard deviations; an empirical correlation is applied to relate particle velocity V to diameter D and Temperature T. Splat sizes after droplet impact are calculated from an analytical model and previous rules of droplets interactions are improved. Porosity is assumed to be caused by the curl-up of splats as a result of thermal stresses. We use a Cartesian grid to define the computational domain and to track the shape and position of the deposited coating. The surface of the coating and the location of pores within it are specified using a variable known as the “volume fraction”. The model is capable of predicting the variation of coating porosity, average thickness and roughness as a function of process parameters. Simulated predictions agree with experimental observation.