High speed imaging of the flow during close-coupled gas atomisation: Effect of melt delivery nozzle geometry

Abstract

We describe the application of a high-speed still imaging technique to the study of close-coupled gas atomisation. A pulsed Nd:YAG laser is used to obtain pairs of still images with an effective 6 ns exposure time, from which velocity maps of the flow of the atomised fluid can be reconstructed. We demonstrate directly that the melt spray cone consists of a jet precessing around the surface of a cone. Further, we demonstrate that the width of this jet is directly related to the geometry of the melt nozzle. By applying Particle Image Velocimetry techniques we are also able to map the flow field in both the primary and secondary atomisation zones, demonstrating an asymmetric recirculation eddy exists at the circumferential edge of the gas-melt interface in the primary atomisation zone. Keywords: Gas atomisation; Discrete Fourier transform; Particle Image Velocimetry IntroductionClose-coupled gas atomisation (CCGA) is an important production technique for fine, spherical metal powders. Such powders have a variety of uses, such as for pigments, catalysts, metal injection moulding (MIM) feedstock, solder pastes for 'flip-chip' type circuit board fabrication and solid rocket propellant. In principle, CCGA is straightforward: high pressure gas jets impinging upon a molten metal stream are used to disrupt the stream, breaking it up into a series of fine droplets. However, in practice the complex interaction between the high velocity gas jets and the metal results in a turbulent, and often chaotic, flow with the result that the details of the process are far from well understood. Consequently, early work into gas atomisation, such as that by Klov and Schfer (1972) or Bradley (1973), focused on empirical correlations between median particle size and process parameters such as gas pressure, gas flow rate and melt flow rate. The most widely quoted of these empirical relationships is that due Lubanska (1970), which correlates particle size with (1+G)

Log-Normal Melt Pulsation in Close-Coupled Gas Atomization

“…Conversely, we have postulated that the high frequency variation, which generally has a well defined peak spectral power in the frequency range 300 -1000 Hz (depending upon the atomization parameters and the thermophysical properties of the atomized fluid), is related to the motion of melt filaments within the spray cone. This observation appears to be confirmed by recent observations using ultra high speed (20 ns) pulsed laser imaging which has allowed the highly regular motion of such filaments to be observed directly [13]. However, as this high frequency variation in the optical intensity is related to the motion of the melt at the nozzle tip, and not to the quantity of melt present, this high frequency component of the signal is filtered out before we make any further analysis to understand the pulsation behavior of the melt.…”

“…Ting et al [7] reported the wake closure pressure in the system they studied as being around 4.0 -4.5 MPa. Aspiration pressure measurements for the die/nozzle configuration used here [13] suggest that in this system wake closure occurs between 3.0 -3.5 MPa. It is therefore tempting to speculate that the transition we observe here between the distribution of optical intensities being described by a single log-normal distribution and being described by two superimposed distributions is a consequence of the atomizer moving from being in the open-wake condition to being in the state where it is alternating between the open-and closed-wake conditions.…”

“…Conversely, in the analogue atomization experiments imaging was performed in reflected light provided by two high power halogen lamps mounted in the base of the atomizer. Our previous high speed imaging studies [11,12,13] have shown that the analogue atomizer is a very good model for the metal atomizer, displaying many qualitatively similar phenomena including low frequency pulsation and high frequency precession of the second (atomized) fluid. In the context of the investigation conducted here performing experiments on an analogue atomizer has a number of advantages over performing metal atomization experiments, not least that the analogue atomizer can be run over a much wider range of gas inlet pressures, which is useful in elucidating the mechanism for the pulsation effect.…”

Estimation of Cooling Rates During Close-Coupled Gas Atomization Using Secondary Dendrite Arm Spacing Measurement

“…In practice the complex interaction between the high velocity gas jets and the metal results in a turbulent, and often chaotic, flow with the result that the details of the flow are far from well understood. In particular, high speed imaging studies of the gas atomization process have revealed that atomizers are subject to quasi-periodic fluctuations on time scales from 0.1-10 -3 s [22,23], while the application of Particle Image Velocimetry (PIV) techniques to gas atomization [24] has revealed complex recirculation patterns both within the melt plume and in the adjacent gas. Moreover, due to the complexity of the process details of the flow have generally not been well captured by models of the atomization process.…”

Numerical and experimental modelling of back stream flow during close-coupled gas atomization

“…The outer surface of the melt nozzle thus acts as the inner surface of the gas outlet manifold. This gas die design has a large exit area and subsequently has higher gas flow rates when compared to the discrete jet design [3,4]. This type of gas die configuration is often used for moderate inlet gas pressure atomizers to prevent excessive gas consumption.…”