MM 357 – Ceramics and Powder Metallurgy

Powder Metallurgy: Lecture 2

Powder Manufacturing

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

German, P 89

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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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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German, P 100

German, P 101

Gas Atomization - Vertical • Typically 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 Atomization High 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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German, P 107 German, P 108

Centrifugal Atomization Use 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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German, P 111

German, P 113

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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German, P 90

Powder manufacturing - Chemical methods

• Reduction / decomposition of a solid

• Thermal decomposition (carbonyl)

• Hydride – De hydride

• Reaction from a liquid

• Precipitation from a liquid

• Pechini process

• Sol-gel method

Precipitation from a liquid

• Precipitate from a super saturated solution

• Large quantity of a soluble component is dissolved at a higher temperature

• At supersaturation, a large number of nuclei form suddenly

• All nuclei grow at same rate – uniform size distribution

• Precipitation of mixed oxides is possible:

– Fabrication of Nickel ferrite (magnetic) – aqeous solution of iron and nickel sulfate in solution at 80C, when pH is increased to 11 with Ammonium hydroxide, a mixed hydroxide precipitates – washed and dried.

Precipitation from a liquid

Pechini method

Sol-Gel synthesis

Sol-Gel method • Metal alkoxide is formed by reaction of meals with alcohol

• nROH + Me = (RO)nMe + (n/2) H2

• Eg Al reacts with isopropanol at 80C in presence of HgCl2 catalyst

• Alkoxides are commercially available.

• Shape formation

• Gelation

• Calcination

• High chemical homogeneity

Key variables:

• Rate of Hydrolysis and condensation influence the microstructure.

• Type of alkoxide, Reaction temperature, pH, Amount of water added (affects degree of polymerization)

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Sol-Gel method

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Rotary Calciner

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Moulson, P 102