Powder Metallurgy Chapter-16 - kuet.ac.bd Metallurgy.pdf · Powder Metallurgy Sintering: sintering...

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Powder Metallurgy Chapter-16 References Introduction to Physical Metallurgy By Sidney H Avner

Transcript of Powder Metallurgy Chapter-16 - kuet.ac.bd Metallurgy.pdf · Powder Metallurgy Sintering: sintering...

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Powder Metallurgy

Chapter-16

ReferencesIntroduction to Physical Metallurgy

By Sidney H Avner

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Powder Metallurgy

Powder metallurgy: powder metallurgy may be defined as the art ofproducing metal powders and using them to make serviceableobjects.

16.2 powder metallurgy procedures: (i) compacting (ii) sintering

Compacting: compacting or pressing consists in subjecting thesuitably prepared powder mixture at normal or elevatedtemperature, to considerable pressure.

The resulting powder compact is known as a briquette and is said,in this form, to be ‘green’. It can be handled but it is brittle.

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Powder Metallurgy

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Powder Metallurgy

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Powder Metallurgy

Sintering: sintering is an operation in which the greenbriquettes are subjected to heat, usually in an inert atmosphere,at a temperature below the melting point of the solid metal.

Sintering will give the required machine strength as well asother desired properties.

In addition to compacting and sintering and depending uponapplication, other accessory operations may be added to theprocess. These include pre-sintering, sizing, machining andimpregnation.

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Powder Metallurgy

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Powder Metallurgy

16.3 Preparation of metal powders:

Three most important methods for the preparation of metal powders are:

(i) Atomization (ii) reduction of oxides (iii) electrolytic deposition

Atomization: Atomization is the method most frequently used metals having lowmelting points, such as tin, lead, Zn, Cd and Al. As the liquid metal is forced througha small orifice, a steam of compressed air causes the metal to disintegrate andsolidify into finely divided particles.

Atomized products are generally in the form of sphere-shaped particles.

A wide range of particle-size distributions may be obtained by varying thetemperature of the metal, pressure and temperature of the atomizing gas, rate of flowof metal through the orifice and nozzle.

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Powder Metallurgy

Reduction of oxides: the reduction of compounds of the metal (usually an oxide) provides a

convenient, economical and flexible method of producing powders. The largest volume of

metallurgical powder is made by the process of oxide reduction.

The particles produced by oxide reduction are sponge like in structure and are ideal for

molding. The shape is generally jagged and irregular, and the particles are porous. This is

the only practical method available for producing powders of the refractory metals such as

tungsten and molybdenum.

Oxide reduction is also economical method for producing powders of Fe, Ni, Co and Cu.

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Powder Metallurgy

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Powder Metallurgy

(iii) Electrolytic deposition: The method of electrolytic deposition is most suitable for the

production of extremely pure powders of principally Cu and Fe. This process is essentially an

adaptation of electroplating. By regulation of current density, temperature, circulation of the

electrolyte and proper choice of electrolyte, the powder may be directly deposited from the

electrolyte. The deposit may be a soft spongy substance which is subsequently ground to

powder, or the deposit may be a hard, brittle metal.

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Powder Metallurgy

16.4 characteristics of metal powders:

200- mesh powder: all the powder passes through a 200- meshscreen/sieve.

325- mesh powder: all the powder passes through a 325- meshscreen/sieve.

325+ mesh powder: all the powder does not pass through a 325 meshscreen/sieve.

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Powder Metallurgy

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Powder Metallurgy

16.6 compacting:

Compacting techniques:

(i) Pressure techniques: such as die, isostatic, high energy rate forging,extrusion, vibratory and continuous.

(ii) Pressure less techniques: such as slip casting, gravity andcontinuous.

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Powder Metallurgy

Die compacting: diecompacting is the mostly usedmethod. The usual sequenceof operations in diecompacting (a) filling the diecavity with a definite volumeof powder (b) application ofthe required pressure bymovement of the upper andlower punches towards eachother and finally (c) ejectionof the green compact by thelower punch.

These operations are shownschematically in the fig.16.8

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Powder Metallurgy

High energy rate compacting techniques: These techniques may be either mechanical,pneumatic or explosive or spark-discharge methods applied in a closed die. The advantageof these method is the short time and high pressures that can be attained. In this process, itis possible to use low-grade and very cheap powders and some parts due to increasedstrength of the green compacts, may be used without subsequent sintering.

Disadvantages include high punch and die wear, limited tolerances and high cost.

Other compacting techniques:

(a) Forging and extrusion technique

(b) Vibration technique

(c) Slip casting

(d) Gravity compaction

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Powder Metallurgy Continuous pressure less compaction:..is used to produce porous sheet forelectrodes in Ni-Cd rechargeablebatteries. The powder may be appliedin the form of a slurry, to be createdon a metal screen or solid metal sheetto produce unusual composites.

Green density seems to increase withincreasing compaction pressure,increasing particle size or apparentdensity, decreasing hardness andstrength of particles, and decreasingcompaction speed.

The dependence of green density at30 tons/sq. in on apparent density isshown in fig 16.13.

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Powder Metallurgy

Increasing compaction pressure(fig 16.14), or green density,increases green strength becausemore particle movement anddeformation will promote greaterinterlocking.

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Powder Metallurgy Sintering: …is a essentially a process ofbonding solid bodies by atomic forces.Sintering forces tend to decrease withincreasing temperature. Evaluatedtemperature tend to favor the sinteringprocess. The longer the time of heatingor higher the temperature, the greaterwill be bonding between particles andresulting tensile strength.

The sintering process starts withbonding among particles as the materialheat up. Bonding involves diffusion ofatoms where there is intimate contactbetween adjacent particles leading to thedevelopment of grain boundaries.