This article addresses the effects of the Hartman cavity on the performance of the ultrasonic gas atomizer (USGA) used for spray forming. Numerical simulations of the gas flow field have been performed with the aim of establishing the effects of the Hartman cavity on flow development both inside and outside the air nozzles. Phase Doppler particle analyzer (PDPA) measurements have been made of gas velocity and turbulence intensity, drop mean and fluctuating velocity and drop size across planes at various distances downstream. High-speed imaging is used in the flow region near the orifice exit where recirculation zones are generated, and there is concern about drop deposition on atomizer surfaces. Shadowgraphic photographs show the presence of shock waves and cells in the emerging gas jets. It is shown that the nozzles with the Hartman cavity are able to sustain supersonic gas jets to lengths beyond those sustained with nozzles without the cavity. It is also found that, within the range of experimental conditions studied (100kPa < air nozzle stagnation pressure < 500 kPa), the Hartman cavity has little effect on the drop sizes generated. The Hartman cavity also has little effect on spray development. The rectangular slit orifices for the two gas jets and the liquid jet generate a spray, after impingement, that is somewhat rectangular in cross section. As the spray develops downstream, if changes shape under the influence of entrainment from the gas surrounding the spray. After a distance of 254 mm from the nozzle exit, the width and breadth of the jet are equal, but significant shape changes occur farther downstream. Gaussian velocity distributions result in liquid flux distributions and metal deposits with Gaussian shapes, instead of deposits with uniform thickness.