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.
Types:
Water atomization, gas atomization, soluble gas or vacuum atomization, centrifugal atomization, rotating disk atomization, ultrarapid solidification process, ultrasonic atomization.
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
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)]0.5 where Pi – injection pressure of the liquid, Pg – pressure of atomizing medium, - – density of the liquid.
