Solidification of Liquid Metal Droplets Impacting Sequentially on a Solid Surface

In this paper, a combined theoretical and experimental study is reported on the process of solidification of two liquid metal droplets impacting sequentially one on top of the other on a cold substrate. The process of interest is directly related to the splat-quenching regime in the spray deposition of metal droplets. The theoretical part of the study is focused on the heat transfer aspects of the solidification process and the difference in the behavior of the solidification of the first and second droplet. The experimental part of the study aims at the characterization of the structure of solidified splats composed of one or two droplets. It is found that the solidification of the second droplet exhibits drastically slower cooling rates compared to the first droplet. As a result, the grain structure of the top of a two-droplet splat is considerably coarser than the structure of the top of a single droplet splat. The findings of the present study imply that in splat cooling severe limitations need to be imposed on the thickness of the resulting solid product (ribbon or coating, for example) to ensure rapid solidification and fine grain structure. In addition, it is shown that the temperature field in the substrate is two dimensional and radial conduction in the substrate should not be neglected in the modeling of the process.