The uniform droplet spray (UDS) process is a process in which
uniformly-sized, molten metal droplets are created and sprayed onto a substrate
to form a bulk deposit. The UDS process has been successful in producing
fully dense deposits of fine equiaxed microstructure from tin alloys. There
is growing interest to apply the UDS process to higher-melting-temperature
alloys such as aluminum alloys for the rapid production of near-net-shaped
parts.
Because of the high affinity of liquid aluminum to oxygen, an oxygen-reducing environment was developed to produce uniform droplet sprays of aluminum alloys. In the spray chamber below 1 ppm of oxygen, the sprays were obtained from 150 and 200 5m diameter orifices with mass flow rates of approximately 0.25 and 0.43 g/sec, respectively.
The quality of a sprayed deposit in terms of microstructure and bulk porosity depends on the thermal state of the incoming droplets as they impact the top surface of the deposit. To predict the temperature and solid fraction of the droplet as a function of flight distance, a droplet thermal model was developed by assuming Newtonian cooling and employing the Scheil equation and a droplet trajectory model. The droplet thermal model was experimentally tested by quenching droplets of Al-4.5wt% Cu and Al-4.3wt% Fe alloys at different flight distances. The volume fraction of solid in the droplets prior to quenching was measured to verify the droplet thermal model.
The evolution of microstructure during solidification was observed
in quenched droplets of Al-4.5wt% Cu about 295 5m in diameter. Secondary
dendrite arm spacing of the powder microstructure agrees with the cooling
rate predicted by the droplet thermal model within an order of magnitude.
However, the measured values of solid fraction appear to be inflated due
to a mushy zone from the solid solubility of copper in aluminum. The solidification
behavior of the 250 5m droplets of Al-4.3wt% Fe exhibited completely undercooled
droplets within 0.5 m of flight, thereby causing the equilibrium solidification
model to be inapplicable. Recommendations for future work are made.