Mono-size droplet production by the uniform-droplet spray process at high temperature -- With applications to ASTM F75 and silicon

The use of the uniform-droplet spray (UDS) process, a controlled capillary jet break-up of molten metal stream at a precise frequency, was extended for the production of mono-sized droplets of high melting-point materials, with particular interest in developing ASTM F75 droplets suitable for the production of bio-implants with porous mating surfaces for effective osseointegration and silicon droplets applicable to efficient photovoltaic energy harvesting.;To facilitate stable UDS spraying of high melting-point materials, charging of UDS droplets at high temperature, required to prevent in-flight merging of droplets, was first investigated through literature survey, theoretical modeling and experiments with copper droplets. Nitrogen gas, a non-attaching gas having a high breakdown temperature, has been successfully used as the chamber gas in which to charge copper droplets effectively. The charging of the droplets, experimentally confirmed by droplet deposition experiments, was in qualitative agreement with model predictions. The model, however, somewhat overestimates the spray width, indicating that ionization of the gas molecules decreases the charge induced on the droplets at high temperatures. Effective droplet charging, nonetheless, can be achieved at temperatures < 1200°C.;The ASTM F75 and silicon droplets were characterized and their solidification behavior was investigated using theoretical models. A nucleation kinetics model, with the aid of experimentally obtained reference data, was used to construct continuous cooling transformation (CCT) curves for the heterogeneous nucleation of traveling ASTM F75 and silicon droplets. UDS conditions for the formation of microcrystalline structures in solidified droplets were investigated with the aid of a dendrite fragmentation model and experimentally confirmed. The amount of droplet supercooling, required in the dendrite fragmentation model, was determined on the CCT curves for droplet nucleation, while the dendrite tip radius and the plateau duration, also required in the dendrite fragmentation model, were calculated with a free dendritic growth model and a droplet in-flight solidification model developed in previous studies.;Helium gas-cooled 640 microm UDS droplets of ASTM F75 alloy, which underwent a supercooling of 92 K, solidified to have a well developed dendritic microstructure, whereas smaller 460 microm and 300 microm droplets, cooled in helium gas for supercoolings of 179 K and 377K, respectively, solidified into a fine equiaxed microcrystalline structure, all in line with prediction by the fragmentation model. 550 microm silicon UDS droplets, solidified in helium gas, were characterized by well-developed <110> dendrites showing no indication of fragmentation, which was also predicted by the fragmentation model at the supercooling of 81 K that the 550 microm droplets underwent. Smaller 390 microm silicon UDS droplets cooled in helium gas, which underwent a supercooling of 172 K, were characterized by a finer microcrystalline structure indicative of fragmented orthogonal <100> dendrites, which was also predicted by the fragmentation model. However, in contrast to ASTM F75, marginal conditions for the fragmentation in silicon UDS droplets led to somewhat incomplete fragmentation of the <100> dendrites.;The overall consistency between model predictions and metallographic observations is excellent, indicating that the developed procedure is useful in correlating the microstructure with the process parameters in droplet-based manufacturing processes.