A study of surface porosity formation in deposits sprayed onto flat substrates by the Uniform Droplet Spray process

Abstract

In the current state of spray forming processes, deposit surfaces formed on a substrate contain porosity. This surface porosity requires secondary operations to be removed. The added expense of these operations prevents the full industrialization of the spray forming process. Thus, a basic understanding of the surface porosity formation is desired. This thesis presents a fundamental analysis of surface porosity formation through modeling and experimentation. Individual splat formation and multiple splat interactions determine surface porosity in uniform droplet spray deposits. A strategy is proposed to minimize surface porosity, first, through control of individual splat formation for a suitable splat geometry, and second, through control of multiple splat interactions. Conditions for a suitable splat geometry are determined by experimentation on the effect of the droplet liquid fraction and the substrate thermal state on the final splat geometry. Pure tin droplets ( 416 [mu]m) are deposited on stainless steel substrates. A droplet liquid fraction of 83% and stainless steel substrate temperature between 150°C and 190°C results in suitable splat geometry. The effect of multiple splat interactions on surface porosity is studied through modeling and experimentation. The model explains the effect of droplet flux and splat solidification on surface porosity. One dimensional heat transfer between splat and substrate is assumed. Experiments are performed with pure tin droplets ( 416 [mu]m and 271 [mu]m) on stainless steel and glass substrates. Results show that slower splat solidification and higher droplet flux reduce surface porosity. A minimum surface porosity of 2.2% is achieved in the uniform droplet spray deposits with tin droplets (416 [mu]m) on a glass substrate.