httpnptel.iitm.ac.pdf12

8/13/2019 Httpnptel.iitm.Ac.pdf12

1/3

Lecture 12

POWDER METALLURGY

Advantages: Metal in powder form is costlier than in solid form. Further, expensive dies and equipment needed to adaptthis process implies that the process is justified by the unusual properties obtained in the products. Powder metallurgyoffers the following specific advantages.

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

Production rates are high even for complex parts. This is primarily because of the use of automated equipment inthe process.

Near net shape components are produced. The dimensional tolerances on components are mostly such that nofurther machining is needed. Scrap is almost negligible.

Parts can be made from a great variety of compositions. It is therefore much easy to have parts of desiredmechanical and physical properties like density, hardness toughness, stiffness, damping, and specific electrical ormagnetic properties.

Parts can be produced with impregnation and infiltration of other materials to obtain special characteristics neededfor specific applications.

Skilled machinists are not needed, so labour cost is low

Parts with controlled porosity can be produced

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

Limitations: Powder metallurgy has the following limitations.

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

High cost of tooling and equipment. This is particularly a limitation when production volumes are small.

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

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

Uniformly high density products are difficult to produce.

Some powders (such as aluminum, magnesium, titanium and zirconium) in a finally divided state present firehazard and risk of explosion.

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

Applications of Powder Metallurgy:

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

Filters: Permanent metal powder filters have greater strength and shock resistance than ceramic filters. Fiber metal filters,

having porosity upto 95% and more, are used for filtering air and fluids. Such filters find use in dehydration for filtering airand fluids. Such filters find use in dehydrators for diffusing moisture laden air around some drying agent such as silicagel, Fig 12.1.

Page 1 of 3Lecture 12

12/11/2013http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-ROORKEE/MANUFACTURING...


2/3

Fig 12.1 Applications 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 from fuel system. Metalpowder filters are also used for arresting flame and attenuating sound.

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

Machinery Parts. Several machinery parts including gears, bushes and bearings, sprockets, rotors are made from metalpowders mixed with sufficient graphite to give to product the desired carbon content. The parts have nearly 20 percentporosity. The pores of the parts which are to rub against another surface in their use, are impregnated with oil to promotequiet operation.

Bearing and Bushes. Bearing and bushes to be used with rotating parts are made from copper powder mixed withgraphite. In small quantities, lead or tin may also be added for obtaining better wear resistance. After sintering, thebearings are sized and then impregnated with oil by vacuum treatment. Porosity in the bearings may be as high as 40percent of the volume. Other machinery parts made by PM include clutch plates, brake drums, ball retainers and weldingrods.

Magnets. Small magnets produced from different compositions of powders of iron, aluminum, nickel and cobalt haveshown excellent performance, far superior to those cast.

Electrical Parts. The possibility of combining several metal powders and maintaining some characteristics of each haspromoted PM for production of electric contact parts. These parts are required to have excellent electrical conductively, bewear resistant, and somewhat refractory. 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 forsecondary manufacturing and assembly operations. PM is therefore becoming increasingly competitive with conventionalmanufacturing processes like forging, casting, and machining. The high initial cost of dies, punches, and equipment for PMprocessing, 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 by PM

The shape of the part must permit ejection from the die.1.

The shape of the part must not require the powder to flow into thin walls, narrow passages, or sharp corners.2.


12/11/2013http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT-ROORKEE/MANUFACTURING...


3/3

The shape of the part should permit construction of strong and rigid tooling.3.

The shape of the part should make allowance for the length to which thin walled portion of the part can becompacted.

4.

The shape of the part should have the fewest possible change in section.5.

The special capabilities afforded by PM to produce certain part forms, should be utilized.6.

Goto Home


12/11/2013http://nptel iitm ac in/courses/Webcourse-contents/IIT-ROORKEE/MANUFACTURING

httpnptel.iitm.ac.pdf12

Documents