Flattening and solidification behavior of a metal droplet on a flat substrate surface held at various temperatures

Abstract

Abstract A free falling experiment, in which a metal droplet fell freely and impinged on a flat substrate, was conducted as a simulation of the thermal spray process. The effect of substrate temperature on flattening and the solidification of the droplet was investigated. The droplet may have a more regular ‘disk’- like shape, or may be less regular, with an appearance like a ‘splash’. The transition of splat morphology was recognized in the experiment. The splat morphology of Ni and Cu droplets was a splash on a room temperature substrate, whereas at high temperature it was a disk. The cross-section of the microstructure of the splat on the room temperature substrate was composed of isotropic coarse grains, whereas on the high temperature substrate it had quite a fine columnar microstructure. The grain size changed transitionally with increasing substrate temperature, indicating that the solidification rate on the room temperature substrate was lower than that on the high temperature substrate. A unique porous microstructure and flowing pattern were observed on the bottom surface of the splat on the room temperature substrate, whereas a flat microstructure without pores was observed on the high temperature substrate. It is concluded that, on the high temperature substrate, rapid solidification occurred at the interface between the splat and the substrate, and the inside of the splat solidified slowly. From the high-speed images, it was found that the droplet on the room temperature substrate flattened more rapidly. According to Rayleigh–Taylor instability, the more rapidly the droplet flattens, the more complex its shape is, i.e. splashing occurs.