Atomization(Powder Production)

Atomization:-
This uses high pressure fluid jets to break up a molten metal stream into very fine droplets, which then solidify into fine particles.
High quality powders of Al, brass, iron, stainless steel, tool steel, superalloys are produced commercially.
Mechanism of atomization:-
In conventional (gas or water) atomization, a liquid metal is produced by pouring molten
metal through a tundish with a nozzle at its base. The stream of liquid is then broken into droplets by the impingement of high pressure gas or water. This disintegration of liquid stream is shown in fig. This has five stages as-
i) Formation of wavy surface of the liquid due to small disturbances
ii) Wave fragmentation and ligament formation
iii) Disintegration of ligament into fine droplets
iv) Further breakdown of fragments into fine particles
v) Collision and coalescence of particles

• The interaction between jets and liquid metal stream begins with the creation of
small disturbances at liquid surfaces, which grow into shearing forces that fragment the liquid into ligaments. The broken ligaments are further made to fine particles because of high energy in impacting jet.
• Lower surface tension of molten metal, high cooling rate => formation of irregular surface => like in water atomization
• High surface tension, low cooling rates => spherical shape formation => like in inert gas atomization
• The liquid metal stream velocity, v = A [2g (Pi – Pg)ρ]0.5
where Pi – injection pressure of the liquid, Pg – pressure of atomizing
medium, ρ – density of the liquid

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 as follows-
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.

Atomizing nozzles:-

• Function is to control the flow and the pattern of atomizing medium to provide for efficient disintegration of powders
• For a given nozzle design, the average particle size is controlled by the pressure of the atomizing medium and also by the apex angle between the axes of the gas jets
• Higher apex angle lead to smaller particle size
• Apex angle for water atomization is smaller than for gas atomization
• Nozzle design: i) annular type, ii) discrete jet type;
i) free falling, ii) confined design
• In free falling, molten metal comes in contact with atomizing medium after some distance. Here free falling of metal is seen. This is mainly used in water atomization.
• In confined design used with annular nozzle, atomization occurs at the exit of the nozzle.Gas atomization is used generally for this. This has higher efficiency than free falling type. One has to be cautious that “freeze up” of metal in the nozzle has to be avoided.