Influence of nucleation and growth phenomena on microstructural evolution during droplet-based deposition

Abstract In the present study, Phase Doppler interferometry (PDI) is utilized to measure droplet sizes and velocities, which are used as initial conditions in the following calculations. A numerical approach, along with the microstructural observation, is implemented to analyze the heat transfer, nucleation and growth of individual droplets during both flight and deposition. The results confirm that during the initial deposition stage, a second nucleation event can occur in the remaining liquid of impinging droplets. The two nucleation events generate intermixed small and large grains at the end of solidification, in agreement with the experimental observation. After a mushy layer forms, no second nucleation can occur. The deposited material's surface behaves like a size modulator which regulates and equalizes the different-size nuclei within impinging droplets. Grains grow from the surviving nuclei, and their growth fronts converge thereby producing a relatively equiaxed grain morphology.