Undercooling and solidification of atomized liquid droplets

The overall analysis of the microstructures developed during atomized droplet solidification requires an understanding of the nucleation and growth kinetics that are operative during processing. Following the subdivision of a liquid volume into droplets, the basic nature of nucleation as a stochastic process and the activity of various catalysts play a major role in determining the droplet microstructure. Droplet thermal history under the influence of the external cooling rate and liquid undercooling is another important component of the solidification behavior and microstructure development in alloys and the particle incorporation modes that can be observed in composite processing. In addition, the kinetic competition near a phase transition can result in drastic microstructure changes that are sensitive to slight variations of the processing conditions. On a laboratory scale, the relevant processing/microstructure relationships can be simulated by controlled undercooling experiments on single droplets and droplet populations. The results that are obtained from these experimental simulations yield a valuable guideline for optimizing thermal spray processes and should be implemented into overall processing models.