Consolidation at the Surface of the Deposit

The two critical unknowns in understanding the spray forming process are the sticking efficiency of droplets onto the growing deposit and the level of porosity in the final deposit (Fig. 11). As discussed, the droplet velocity and thermal models of the conditions in the spray can predict the velocities and the enthalpies of the individual droplets at the moment of impact. In most of the present models, the enthalpy of the spray is averaged over the full radial angle of the spray, ignoring the slightly different gas velocities and droplet size distributions in different radial directions. A correction for the different values of/(s) at different angles from the spray axis could be handled in the numerical models, but to-date, the resulting improved predictions have not been readily usable given the difficulties of experimental observation. Thus, although reasonable insight into the thermal condition of the spray is available, at present there is no physically based model that attempts to describe the complexity of the impact of a very high density of high-velocity, partially liquid droplets onto a similar partially liquid surface.

How much of the material arriving at the surface of deposition will stick? Also, how will the sticking efficiency vary with droplet size, impact angle, and the thermal condition (fraction of liquid) of the individual droplets and of the deposit surface? Other important and incompletely answered questions include what determines the resultant microstructure of the deposit, in particular the level of porosity and the grain size? These parameters are likely to be determined, at least in part, by the processes occurring during impact of the high velocity spray onto the deposit surface. Although not yet predictable, each of these phenomena are capable of investigation experimentally.

0 0

Post a comment