محاضرة رقم 7

Water atomization: In water atomization, a high pressure water stream is forced through nozzles to form a disperse phase of droplets which then impact the metal stream. In this method, large quantities of energy are required to supply the water at high pressure. It is estimated that the overall energy process efficiency is ?4 %. This production method is significant for low and high alloy steels, including stainless steel. Because of oxide formation, water atomization is not likely to be used in the atomization of highly reactive metals such as titanium and the super alloys. In general, water atomized powders are irregular in shape, with rough oxidized surfaces.
The advantages of atomized powders and more specifically, of high pressure water atomized powders are summarized by Gummeson. These are:
1. Freedom to alloy
2. All particles have the same uniform composition
3. Control of particle shape, size and structure
4. Higher purity
5. Lower capital cost.
Fine particle sizes are favoured by:
(a) Low metal viscosity
(b) Low metal surface tension
(c) Superheated metal
(d) Small nozzle diameter, i.e. low metal feed rate.
(e) High atomizing pressure
(f) High atomizing agent volume
(g) High atomizing agent velocity
(h) High atomizing agent viscosity
(i) Short metal stream (F)
(j) Short jet length (E)
(k) Optimum apex angle (ao).