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Lecture 12

POWDER METALLURGY

Advantages: Metal in powder form is costlier than in solid form. Further, expensive dies andequipment needed to adapt this process implies that the process is justified by the unusual properties

obtained in the products. Powder metallurgy offers the following specific advantages.

i. Parts can be produced from high melting point refractory metals with respectively lessdifficulty and at less cost.

2. Production rates are high even for complex parts. This is primarily because of the use ofautomated equipment in the process.

iii. Near net shape components are produced. The dimensional tolerances on components aremostly such that no further machining is needed. Scrap is almost negligible.

iv. Parts can be made from a great variety of compositions. It is therefore much easy to have

parts of desired mechanical and physical properties like density, hardness toughness,stiffness, damping, and specific electrical or magnetic properties.

v. Parts can be produced with impregnation and infiltration of other materials to obtain specialcharacteristics needed for specific applications.

vi. Skilled machinists are not needed, so labour cost is low

vii. Parts with controlled porosity can be produced

viii. Bi-metallic products, sintered carbides and porous bearings can be produced only by thisprocess.

Limitations: Powder metallurgy has the following limitations.

i. High cost of metal powders compared to the cost of raw material used for casting or forging acomponent. A few powders are even difficult to store without some deterioration.

ii. High cost of tooling and equipment. This is particularly a limitation when production volumesare small.

iii. Large or complex shaped parts are difficult to produce by PM process.

iv. Parts have lower ductility and strength than those produced by forging.

v. Uniformly high density products are difficult to produce.

vi. Some powders (such as aluminum, magnesium, titanium and zirconium) in a finally dividedstate present fire hazard and risk of explosion.

vii. Low melting point metal powders (such as of zinc, tin, cadmium) give thermal difficultiesduring sintering operation, as most oxides of these metals cannot be reduced at temperaturesbelow the melting point.

Applications of Powder Metallurgy:

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There is a great variety of machine components that are produced from metal powders, manyof these are put to use without any machining operation carried out on them. Following are some ofthe prominent PM Products.

Filters: Permanent metal powder filters have greater strength and shock resistance than ceramicfilters. Fiber metal filters, having porosity upto 95% and more, are used for filtering air and fluids. Such

filters find use in dehydration for filtering air and fluids. Such filters find use in dehydrators for diffusingmoisture laden air around some drying agent such as silica gel,Fig 12.1.

Fig 12.1Applications of powder metallurgy parts. Filiers can be used for diffusing or for separating.

These filters find wide usage also in petrol / diesel engines for separating dirt and moisture fromfuel system. Metal powder filters are also used for arresting flame and attenuating sound.

Cutting Tools and Dies. Cemented carbide cutting tool inserts find extensive applications in machineshops. These are produced by PM from tungsten carbide powder mixed with cobalt binder.

Machinery Parts. Several machinery parts including gears, bushes and bearings, sprockets, rotors aremade from metal powders mixed with sufficient graphite to give to product the desired carbon content.The parts have nearly 20 percent porosity. The pores of the parts which are to rub against anothersurface in their use, are impregnated with oil to promote quiet operation.

Bearing and Bushes. Bearing and bushes to be used with rotating parts are made from copperpowder mixed with graphite. In small quantities, lead or tin may also be added for obtaining betterwear resistance. After sintering, the bearings are sized and then impregnated with oil by vacuumtreatment. Porosity in the bearings may be as high as 40 percent of the volume. Other machineryparts made by PM include clutch plates, brake drums, ball retainers and welding rods.

Magnets. Small magnets produced from different compositions of powders of iron, aluminum, nickeland cobalt have shown excellent performance, far superior to those cast.

Electrical Parts. The possibility of combining several metal powders and maintaining somecharacteristics of each has promoted PM for production of electric contact parts. These parts are

required to have excellent electrical conductively, be wear resistant, and somewhat refractory.

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Several combinations such as copper tungsten, cobalt tungsten, silver tungsten, copper-nickel,and silver molybdenum have been used for production of these parts.

Economics of Powder Metallurgy:

Since it is possible to produce near net shape parts by PM, there is usually very little scrap and

also no need for secondary manufacturing and assembly operations. PM is therefore becomingincreasingly competitive with conventional manufacturing processes like forging, casting, andmachining. The high initial cost of dies, punches, and equipment for PM processing, however,requires sufficiently high production volume to make this process cost effective.

Design Considerations for PM Parts

The following recommendations should be kept in mind while designing parts to be made byPM

1. The shape of the part must permit ejection from the die.2. The shape of the part must not require the powder to flow into thin walls, narrow passages, or

sharp corners.3. The shape of the part should permit construction of strong and rigid tooling.4. The shape of the part should make allowance for the length to which thin walled portion of

the part can be compacted.5. The shape of the part should have the fewest possible change in section.6. The special capabilities afforded by PM to produce certain part forms, should be utilized.

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