5. powder metallurgy

Powder Metallurgy

ME 312 Manufacturing Technology
Vikrant Sharma, Mechanical Engineering Department. FET. MITS

Introduction:
Powder metallurgy can be described as an art of manufacturing
commercial articles from powdered metals by placing these powders
in moulds and applying pressure. These compressed parts are then
heated to bind the particles together and improve their strength and
other properties.

Vikrant Sharma , FET. MITS ME 312 Manufacturing
Technology

 The products made through this process are very costly on account
of he high cost of metal powders as well as the die used.

 The application of the process is, therefore economically feasible
only when the number of required products is very high.

 A few typical examples of such products are tungsten carbide cutting
tools, self lubricating bearings, turbine blades having high
temperature strength.

 Small, intricate parts with high precision are required.

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Historical Note
 Powders of metals such as gold and copper, as well as some of the
metallic oxides, have been used for decorative purposes since
ancient times.
 It is believed that the Egyptians used PM to make tools as far back
as 3000 BC.
 Around 1815, Englishman William Wollaston developed a technique
for preparing platinum powders, compacting them under high
pressure, and baking (sintering) them at red heat. The Wollaston
process marks the beginning of powder metallurgy as it is practiced
today.
 U.S. patents were issued in 1870 to S. Gwynn that relate to PM
self- lubricating bearings. He used a mixture of 99% powdered tin
and 1% petroleum, mixing, heating, and finally subjecting the mixture
to extreme pressures to form it into the desired shape inside a mold
cavity.

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 In the 1920s, cemented carbide tools were being fabricated by PM
techniques.
 Powder metal gears and other components were mass produced in
the 1960s and 1970s, especially in the automotive industry.
 in the 1980s, PM parts for aircraft turbine engines were developed.

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The powder metallurgy process:
The entire powder metallurgy process mainly consists of the following
basic stages,

1. Production of metal powders.
2. Mixing and blending of metal powders in required proportions.
3. Pressing and compacting the powders into desired shapes and
sizes.
4. Sintering the compacted parts in a controlled furnace atmosphere.
5. Subjecting the sintered parts to secondary processing, if required.

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Main characteristics of metal powders:
1. Particle shape: There will be a variation in the particle shapes in a
collection of powders. It effects compactness, porosity and strength
of the part.

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2. Particle Size and Distribution: Particle size refers to the dimensions
of the individual powders. There are various methods available to
obtain particle size data. The most common method uses screens of
different mesh sizes. The term mesh count is used to refer to the
number of openings per linear inch of screen. Higher mesh count
indicates smaller particle size. A mesh count of 200 means there are
200 openings per linear inch.
It has an appreciable influence over compressibility, density, porosity
and also shrinkage during sintering. Use of particle of similar size
increases porosity and of dissimilar size decreases it.

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3. Interparticle Friction and Flow Characteristics: Friction between
particles affects the ability of a powder to flow readily and pack
tightly. A common measure of interparticle friction is the angle of
repose, which is the angle formed by a pile of powders as they are
poured from a narrow funnel.
Flow characteristics are important in die filling and pressing.
Automatic die filling depends on easy and consistent flow of the
powders. In pressing, resistance to flow increases density variations
in the compacted part; these density gradients are generally
undesirable. A common measure of flow is the time required for a
certain amount of powder (by weight) to flow through a standard-
sized funnel.

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Production of metallic powder:
1. Atomization:
This method involves the conversion of molten metal into a spray of
droplets that solidify into powders. It is the most versatile and popular
method for producing metal powders today, applicable to almost all
metals, alloys as well as pure metals.

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2. Reduction process:
It includes a variety of chemical reactions by which metallic compounds
are reduced to elemental metal powders. A common process
involves liberation of metals from their oxides by use of reducing
agents such as hydrogen or carbon monoxide. The reducing agent is
made to combine with the oxygen in the compound to free the
metallic element. This approach is used to produce powders of
iron, tungsten, and copper.
3. Mechanical methods:

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Blending and mixing of powders:
To achieve successful results in compaction and sintering, the metallic
powders must be thoroughly homogenized.
Blending refers to when powders of the same chemical composition but
possibly different particle sizes are intermingled. Different particle
sizes are often blended to reduce porosity.
Mixing refers to powders of different chemistries being combined. An
advantage of PM technology is the opportunity to mix various metals
into alloys that would be difficult or impossible to produce by other
means.

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Compacting:
It is the process of converting loose powder into a green compact of
accurate shape and size. It is done in steel dies and punches.

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 The dies and punches used should be highly polished and the
clearance between them should be kept at the minimum in order to
maintain proper alignment.
 The clearance should be sufficient to allow a free movement.
 High carbon steel and tungsten carbide are the principal die material.

Technology

 As a result of compaction, the density of the part, called the green
density is much greater than the starting material density, but is not
uniform in the green. The density and therefore mechanical
properties vary across the part volume and depend on pressure in
compaction.

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Sintering:
Sintering is a heat treatment operation performed on the compact to
bond its metallic particles, thereby increasing strength and hardness.
The treatment is usually carried out at temperatures between 0.7 and
0.9 of the metal’s melting point. Compressed metal powder is heated
in a controlled-atmosphere furnace.

Because PM applications usually involve medium-to-high
production, most sintering furnaces are designed with mechanized
flow-through capability for the work parts. The heat treatment
consists of three steps, accomplished in three chambers in these
continuous furnaces: (1) preheat, in which lubricants and binders are
burned off; (2) sinter; and (3) cool down.

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Main objectives of sintering
1. Achieving high strength.
2. Achieving good bonding of powder particles.
3. Producing a dense and compact structure.
4. Producing parts free of oxides.
5. Obtaining desired structure and improved mechanical properties.

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Design Considerations
 The shape of the parts must be as simple as possible.
 Hole and grooves must be parallel to the direction of ejection.

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 undercuts, threads, knurling and other similar impression should not
be included in die, but produced later through machining.
 The designed dimensions of parts should carry adequate allowances
to compensate for the likely changes in dimensions due to shrinkage
during sintering.

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5. powder metallurgy

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