Powder metallurgyAtomization Process

By,Nikhil borale (09mt21)Amit kumar (09mt04)

Atomization

Atomization process involves use of high pressure fluid jets to break up a molten metal stream into very fine droplets, which when solidify into fine particles .

Method was used on large scale during world war II .

High quality powders of Al ,brass ,iron , SS ,tool steel and super alloys are produced by the above method .

Types of atomization

Water atomization Gas atomization Vacuum atomization Centrifugal atomization Rotating disk atomization Ultrasonic atomization

Water atomization

This method uses high pressure water jets to dis-integrate the molten metal stream .

Water is used due to their higher viscosity and superior quenching ability .

The process is in-expensive and can be used both for small and large scale production .

Metals and alloys which do not react chemically with water cant be atomized by this process .

Gas atomization

Molten metal stream is dis-integrated by means of high velocity gas in this method .

The gases used are Ar , N2 ,He gas jets . The atomized powder are collected in

water bath . Fluidized bed cooling is used when certain

characteristics of powder are desirable for example, spherical shape or particles with little or no oxide(surface) content .

This process is costier than water atomization

Vacuum atomization

When molten metal supersaturated with gas ,under pressure is suddenly exposed to vacuum the outcoming gas of metal solution expands rapidly causing atomization of metal stream .

High purity powders obtained by this method.

Metal is saturated with H2 gas .

Centrifugal atomization ( REP ) This method uses centrifugal force to

break off molten metal drops from molten end of consumable electrode .

Method is used to produce powders of reactive metals of metal like titanium .

In this process one end of the metal bar is heated and melted by bringing it into contact with non consumable tungsten electrode , while rotating it longitudinally at high speeds.

Centrifugal force cause the metal droplets to be thrown off outwards , which when solidify as spherical shaped particles inside an evacuated chamber .

Rotating disk atomization this process involves the impinging of

stream of molten metal on the surafce of rapidly spinning disk ,which results in mechanical atomization of metal stream and causes the droplets to be thrown off the edges of disk .

Particles formed are generally spherical and their sizes decrease with increasing disk speed .

Use of vacuum /inert gas is mandatory because of purity requirements .

Ultrasonic atomization

This method involves use of ultrasonic waves to produce fine powders from a molten stream of metals .

Atomization set-up

1. Melting and superheating facility2. Atomization chamber 3. Powder collection tank

1. Melting and superheating facility

Standard melting furnaces are used to produce the liquid metal . This is usually accomplished by air melting, Inert gas or vacuum induction melting .

Complex alloys which are highly susceptible to contamination ( like Ni base super alloys) are usually melted in vacuum induction furnaces .

Metal from furnace transferred to tundish ( reservoir ) .

Atomization chamber

Consists of atomizing nozzles system located at base of tundish , which controls shape and size of molten metal stream .

The high velocity atomization medium breaks up the molten stream into fine droplets , which cool and solidfy as they settle at bottom of atomization chamber.

Inert gas like N2can be purged into tank to minimize oxidation of resulting powder .

Powder collection tank

2 ways – dry collection and wet collection .

Dry collection : in this particles solidfy before they reach the bottom of tank .The collection tank should have sufficient height to ensure complete solidification of particles before they fall to bottom .

Wet collection : powder is collected in water tank provided at bottom of atomization unit .the tank may be externally cooled .

Atomization nozzles and nozzle design

Function of atomization nozzle is to control the flow and pattern of atomizing medium to provide for efficient dis integration of powders .

For a given nozzle design , the average particle size is controlled by

1.pressure of atomizing medium2. Apex angle between the axes of gas

jets .

Higher apex angle lead to smaller particle size

Nozzle design controls the efficiency of dis integration which decides the final characteristics of powder .

Apex angle for water atomization is less than that of gas .

Nozzles used in atomization process(both water and gas ) are either annulart type or jet type .

Atomization medium

Selection of medium mainly on reactivity of the metal and cost of medium .

Water and air are inexpensive but they react with many molten metals .

Inert gases are useful but expensive . N2, Ar, He are inert gases are used to prevent contamination .

Synthetic oils are used as atomization medium in place of water or gas .

Oil atomization is used to produce powders of high carbon steel , high speed steels , bearing steels .

Process parameters

1. Pressure of metal head .2. Pressure of atomizing medium .3. Metal temperature .4. Orifice area 5. Molten metal properties .

Advantages

Processing capability : almost all metals that can be melted can be atomized .

Freedom to alloy : higher purity and pre-alloyed powders can be prepared directly from melt .

Cleanliness : powders can be produced with few or nil non-metallic inclusions .

Versatility : particle shape and size is possible to control by processing parameters such as

Metal temperature and pressure of atomizing medium .

• Productivity : higher productivity with lower cost of equipment as compared to many other methods .