Types of atomization
Atomization of molten metal can be done in different ways depending upon the factors like economy and required powder characteristics. At present, water or gas atomizing medium can be used to disintegrate a molten metal stream.
The various types of atomization techniques used are,
1. Water atomization:
High pressure water jets are used to bring about the disintegration of molten metal stream. Water jets are used mainly because of their higher viscosity and quenching ability. This is an inexpensive process and can be used for small or large scale production. But water should not chemically react with metals or alloys used.
2. Gas atomization:
Here instead of water, high velocity argon, nitrogen and helium gas jets are used. The molten metal is disintegrated and collected as atomized powder in a water bath. Fluidized bed cooling is used when certain powder characteristics are required.
3. Vacuum atomization:
In this method, when a molten metal supersaturated with a gas under pressure is suddenly exposed into vacuum, the gas coming from metal solution expands, causing atomization of the metal stream. This process gives very high purity powder. Usually hydrogen is used as gas. Hydrogen and argon mixture can also be used.
4. Centrifugal atomization:
In this method, one end of the metal bar is heated and melted by bringing it into contact with a non-consumable tungsten electrode, while rotating it longitudinally at very high speeds. The centrifugal force created causes the metal drops to be thrown off outwards. This will then be solidified as spherical shaped particles inside an evacuated chamber. Titanium powder can be made using this technique.
5. Rotating disk atomization:
Impinging of a stream of molten metal on to the surface of rapidly spinning disk. This causes mechanical atomization of metal stream and causes the droplets to be thrown off the edges of the disk. The particles are spherical in shape and their size decreases with increasing disk speed.
6. Ultrarapid solidification processes:
A solidification rate of 1000C/s is achieved in this process. This results in enhanced chemical homogeneity, formation of metastable crystalline phases, amorphous materials.
