The effect of wake-closure phenomenon on gas atomization performance

The relationship between melt flow rates, gas-only aspiration of the atomizer nozzle and operating pressures are elusive, and most often these parameters are interdependent. The understanding of these interactions, however, is crucial to the efficient production of fine metal powder. Atomization yield above and below a condition known as wake closure pressure can produce median powder size (D50) variation as large as 42%. Here, a pulsatile atomization model is presented to elucidate the interaction between the atomizing gas and the melt around the melt pour tube. The model reveals that melt flow rate is determined by the time-averaged stagnation pressure formed in front of the melt orifice. This stagnation front dictates the behavior of the melt exiting the melt orifice and how it interacts with the atomizing gas. It is noted, however, that the wake-closure phenomenon is highly sensitive to atomizer and melt pour tube geometry and it possesses a hysteresis type aspiration behavior that suggests that there is an energy of formation associated with the wake-closure structure. Furthermore, the atomizing gas has to possess the necessary energy to overcome this energy barrier in order to form the wake closure structure. Concurrently, the entropy of the system increases with the formation of Mach disk and oblique shocks, leading to a lower equilibrium state for the gas dynamics.