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

In the present study, Phase Doppler interferometry (PDI) is utilized to mea sure droplet sizes and velocities, which are used as initial conditions in the following calculations. A numerical approach. along with the microstruc tural observation, is implemented to analyze the heat transfer, nucleation and growth of individual droplets during both flight and deposition. The re sults confirm that during the initial deposition stage, a second nucleation event can occur in the remaining liquid of impinging droplets. The two nuc leation events generate intermixed small and large grains at the end of sol idification, in agreement with the experimental observation. After a mushy layer forms, no second nucleation can occur. The deposited material's surfa ce behaves like a size modulator which regulates and equalizes the differen t-size nuclei within impinging droplets. Grains grow from the surviving nuc lei, and their growth fronts converge thereby producing a relatively equiax ed grain morphology. (C) 2001 Acta Materialia Inc. published by Elsevier Sc ience Ltd. All rights reserved.