The uniform droplet spray (UDS) forming process has been developed
to enable precise control of droplet thermal states and the resultant material
microstructure of the deposit. By having a uniform droplet size throughout
the spray, all the droplets deposited onto a substrate will have the same
thermal state upon impact, allowing for precise control of the solidification
process within both the droplets and the deposit.
In this study, a one-dimensional, finite difference model was created to predict the temperature and liquid fraction of the deposit during the UDS process. The model employs an explicit temperature-enthalpy method to incorporate a variety of solidification assumptions. Experiments were conducted using Sn-15wt.%Pb binary alloy to validate the model. Temperatures were measured in the deposit and acceptable agreement with the simulation was obtained.
Modeling has shown that the deposit thermal state is highly dependent
on variations in spray conditions, which are predicted using droplet trajectory
and droplet thermal models. Further modeling of the individual droplet
splats immediately after impact revealed that three phases of solidification
exist: droplet solidification; droplet splat solidification; and consolidated
deposit solidification, each with significantly different cooling rates.
By manipulating process parameters, the percentage of solidification within
each of these three stages can be shifted, resulting in changes in final
microstructure. By using the deposit thermal model, the relationship between
process parameters and solidification behavior can be understood to obtain
the desired microstructure and material properties.