Abstract
The atomization and spray forming of liquid metals was studied. Melt break up algorithms were developed to predict the size distribution of the spray generated as well as secondary aspects of the atomization phenomena. In the model, which is based on the Surface Wave Formation (SWF) theory, the relative velocity between the gas and melt phase was thought to induce a sinusoidal disturbance on the surface of the melt column. Depending on the flow conditions such a disturbance could grow in amplitude and cause certain parts of the surface to be torn off the liquid column. A number of different approaches to the problem of drop disintegration were also considered. Based on experimental observations of the critical Weber number made by other authors, a criterion was formulated, which allowed the secondary break up of drops to be predicted. In addition, an analytical model originally presented by Wolf and Andersen (1965), which was intended to describe the stripping mode of secondary disintegration, was also revised and incorporated into a computer routine. Finally a comparison of the models was made against the predictions of the empirical Lubanska (1970) equation. High Speed Photography studies of a water column atomized by gas revealed that the formation of a surface wave was the prominent mechanism perturbing and finally disintegrating the column into a fine spray of drops. Phase Doppler Anemometry studies of the water/gas jet produced during the atomization of a water column indicated that there was a gradient of particle sizes across the spray. The finer fragments were found in the close proximity of the centre axis of the conical flow, with particles becoming larger in size as the distance from the centre increased. Vaporization of the water drops near the centre due to the high gas velocities should be taken into account when interpreting these results. The break up algorithms were tested against experimental data for a number of different A1 and Fe alloys with various solute elements, obtained using a close coupled atomization facility. Case studies were made for the effects of gas injection pressure, initial melt stream diameter, initial melt stream exit velocity and number of atomizing gas jets on the mean powder particle size produced. The algorithms could predict the distribution of drop sizes in space, a feature that enabled the simulation of spray forming runs and the direct comparison of the numerical predictions to experimental data. The shapes of the Al-1.6wt%Hf and the Al-1.6wt%Hf-3.2wt%Li alloy preforms and the particle distribution along the radial direction of the Al-1.6wt%Hf preform were calculated and compared favourably with experimental data. Microscopic observation of Al-1.6wt%Hf and Al-1.6wt%Hf-3.2wt%Li preforms indicated that there was a variance of particle size as well as grain size along the radial direction of the spray. The grain size was found to decrease with increasing distance from the central axis of the preform, while the radial distribution of drop diameters did not reveal a distinct trend.