MM 357 Ceramics and Powder Metallurgy

Powder Metallurgy: Lecture 2

Sudhanshu Mallick

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German, P 85

Mechanical Powder Fabrication Techniques

Machining: Coarse powders are produced during

machining of metals. The powders can be further refined by grinding.

The rapid heating and cool down cycle that the machining produced particles can cause the particle surface to get oxidized.

It also results in the formation of various phases (different from the original metal these particles were machined from) which may give them different properties

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Mechanical Powder Fabrication Techniques

Milling:Milling is process of reducing particle size by mechanical

impaction using hard balls.

Useful for brittle materials. Ductile particles cold weld and stick together

Optimal conditions: Rotation speed fast enough to carry the balls to the top

of the jar. Too slow insufficient impact force, Too fast Balls stick to the jar surface by centrifugal force

Ball diameter approx 30 times the powder size Volume of balls approx half the jar volume Volume of powder approx 25% of jar volume fills the

interstitials between the grinding balls.

Contamination from jar and milling balls is a concern

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German, P 86

Mechanical Powder Fabrication Techniques

Mechanical alloying:Though not really a Powder

formation technique, Its being discussed here because of continuity.

Powders of different materials are taken in appropriate proportions and mixed in a stirring mill with the aid of milling balls.

The repeated impact, cold welding and fracture produce the desired composite powder

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G89

Gas Atomization - Horizontal

Typically used for low melting point metals

Gas emerging from the nozzle creates a siphon effect .

High velocity gas breaks up the metal into droplets

During flight the droplets loose heat, solidify and settle down

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G100

G101

Gas Atomization - VerticalTypically used for high melting

point metals

Closed inert gas filled chamber is used to prevent oxidation.

High velocity gas breaks up the metal into droplets

During flight the droplets loose heat, solidify before touching the chamber walls and settle down

Depending on the superheating, gas flow rate, nozzle geometry, particles of various shapes can be formed

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Water AtomizationHigh pressure jets are directed against stream of molten metals

causing disintegration and rapid solidification.

Synthetic oils or other non-reactive liquids can also be used instead of water to achieve desired particle shape and prevent oxidation

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G107

G108

G111

Centrifugal AtomizationUse centrifugal force to spray molten metal which solidifies into

a powder.

Better control over particle size

Typically done in inert atmosphere to prevent oxidation

Gives coarse particle sizes

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G113

G90

Powder formation by Electrolysis Used the conventional electrolysis principle (dissolution of Anode,

deposition at Cathode) to precipitate out powders

Metallic ions are generated by the dissolution of the Anode. Transport of the ions through the electrolyte Ions redeposit on the cathode Cathode is washed and ground to produce powders Depending on the conditions of the electrolytic cell (temp, voltage etc),

powders of different morphologies can be prepared

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