BASIC PROCESSES OF POWDER METALLURGY 1

The basic operation of powder metallurgy techniques

• The four basic operations of the powder metallurgy are:

1.Manufacture of powder

2.Mixing or blending powder particles

3.Compacting

4.Sintering

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Summarized steps of power metallurgy

• Obtaining metal powder of required fineness and purity• Subjecting these particles to a sufficiently high pressure in a

die using a press to achieve cohesion among them• Sintering these compacted mass at a temperature high

enough to diffuse particles & cause inter-granular grain growth to form a single solid.• Taking out solid component & applying the secondary

operations such as finishing, sizing & finally inspecting the component.

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Manufacture of Powder Metallurgy• The size of powder particles normally ranges

from 1-100 µm, preferably 10-20 µm.• The processes:

1.Mechanical Process

2.Atomization

3.Electrolytic Deposition

4.Reduction

5.Hydride & Carbonyl Process

6.Condensation of Metal Powder4

Methods For Manufacturing of Metal Powder

• Mechanical Processes:• In mechanical processes, crushing or milling takes place.• This method is used for brittle metals/alloys like tungsten

carbide, iron oxide, Sb, Bi etc.

• Atomization:• In atomization process, the molten metal is disintegrated

in fine particles by using high pressure jets of air, water, inert gases and it is divided in particles being collected in collector.

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Methods For Manufacturing of Metal Powder

Ball MillingBall Milling AtomizationAtomization

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Methods For Manufacturing of Metal Powder• Electric Deposition:• This is method works on the principle of electrolysis and

mainly used to produce powder of copper.• In this method the anode of copper and the cathode of

aluminum are immersed in electrolyte of copper sulphate(acidic solution).

• Reduction:• Reduction method involves reduction of metals usually in

their oxide compound.• These compounds are heated with reducing agents such as

Carbon, Coal, CO, H2 etc.

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Methods For Manufacturing of Metal Powder

Electrolytic DepositionElectrolytic Deposition8

Methods For Manufacturing of Metal Powder• Hydride and Carbonyl Process:• Some metals such as tantalum, niobium etc, gets reacted

with hydrogen and makes hydrides that are stable ate room temperature.• These hydrides dissociates at 350°C in hydrogen and pure

metal.• Similarly many metals makes carbonyl compounds. at

suitable temperature and pressure they get dissociated and gives us the pure metal.

• Condensation:• This method is used for the metals such as zinc, cadmium,

magnesium etc. that are boiled and their vapour can be condensed to achieve their powder form.

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Mixing & Blending of Powders• Mixing:• The powder is mixed well to produce uniform distribution

of powder, particularly when the size of powders is different.• In wet mixing the better mixing of powders, water or

solvent is used for reduction of dust, reduction of chances of exploration and hazardous side effects and to prevent it from getting oxidized.

• Blending:• Blending is the operation to add lubricant and volatizing

agents and then thoroughly mixing of the powder.• The lubricants used are basically the metallic separators

such as zinc separate, graphite, oil, glycerin, paraffin etc.10

Mixing & Blending of Powders

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Powder Compaction• Compacting techniques are generally of 2 types:

A.Pressure techniques:

• This includes die pressing, roll pressing, extrusion methods, forging method and vibratory compaction.

A.Non Pressure Techniques:• This includes slip casting, continuous and gravity

methods.

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Pressure Technique

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ExtrusionExtrusion

Roll PressingRoll PressingDie PressingDie Pressing

Non Pressure Technique

Slip CastingSlip Casting14

Sintering• After compacting of the metal powder, the mass

obtained is called “Green Compact” or “Briquette”.• To achieve the final finished product, Sintering must be

done. Sintering is applied to obtain the all possible final strength and hardness that is needed.• The sintering process is consist of heating the Green

compact in a furnace to a temperature below it’s melting point. The heating takes place in reducing atmosphere.• Sintering is essentially a process of bending particles by

atomic diffusion.

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Sintering

• The Diffusion of atoms takes place at the boundaries of the atoms that are connected to another atom. At the boundaries of atom, while diffusion, the atoms are getting connected and sequentially grain boundaries are also diffused. 16

Sintering• There are basically 2 types of sintering:

1.Solid Phase Sintering:• In this process the sintering takes place across the cold

welded surfaces that are adjacent to each-other.

1.Liquid Phase Sintering:• In this process, the melt of one metal is penetrated

through another metal, thus alloying results in a continuous bonds between those two metals. 17