metal powder comp action

8/8/2019 Metal Powder Comp Action

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The resulting formed component after

compacting is called a green compact.

The compacting cycle is shown below;


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The ejection of the compact from the die is an

equally important which should be done

carefully to eliminate the spring back effect.

Spring back is the increase in compact size

after the compacting force is removed from

the die.

This release of elastic stored energy of the

compact can damage the compact.


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Parameters Influencing the Spring-Back include

Compacting pressure;

Compacting density, Powder properties,

Lubricants and alloying additions,

Shape and elastic properties of the compacting die


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Objectives of compacting powders consolidate the powder into desired shape

impart, to as high a degree as possible, thedesired final dimensions (with due considerationto any dimensional changes resulting fromsintering)

impart the desired level and type of porosity.

impart adequate strength for subsequenthandling, this is achieved by cold welding of

particles as particles plastically deform underpressure. The strength of the compact object iscalled green strength.


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How is this achieved?

Several approaches exist for achieving these

goals

In general the techniques can be categorized

according to

Pressure: It can be pressurized or pressure-less

compaction

temperature - room or elevated temperature;

pressure direction: uniaxial or Isostatic


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4.1 Effect of pressure in pressing

density of the green product largely depends on

the compacting pressure. The higher the compacting pressure the higher

the density of the compact


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Numerically, the variation of applied pressure,

P with the volume of powder, Vis represented

by the equation;

Where A and B are constants.

Equation however does not apply at extremelyhigh pressure


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Generally both mechanical and physical

properties improve with increasing density.

Thus method of powder compaction dependson the level of performance required from the

part

Many components are adequate whenproduced at 85-90% of the true density

high pressure compaction is mainly applied in

production of components that should be verydense


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Pressureless compaction: Entails filling the diecavity without the application of pressure.

materials move by force of gravity as well astapping of die cavity.

Therefore the density of the green compact isvery low.

Heat is applied in order to bring aboutshrinkage of particles at a temperature belowthe melting point of the material.

This reduction in surface area reduces theporosity of the material but the compact stillremain porous.


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Without pressure the only transport

mechanism that occurs is diffusion.

Because of the small driving force, relatively

long periods of time are required to achieve

high density of final product.

process is ideal for the production of lowdensity components such as metallic filters

and self lubricating bearings.


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4.2 Effect of Temperature in pressing

metal powder formed at elevated

temperature gives significant impact on the

entire compaction process and the quality of

the green compact as well as the final product

Mechanical properties of green compact arelargely influenced by the forming temperature

For most metal powder the compaction is

done in the range 100-200oC

Alternatively, only the die may be warmed

(50-100oC)


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4.3 Effects of direction of applied pressure

The direction of application of pressure on the

powder may be Uniaxial (in one plane) orIsostatic (uniform pressure from all directions)

4.3.1 Uniaxial or die compaction

It is the most conventional metallic and

ceramic powder pressing method.

This is achieved by pressing the powder in

rigid dies.

The applied pressure can be in one direction

or from both ends of the die.


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The pressing forces may be mechanical orhydraulic.

Practically, densities of up to 90 % of fulldensity can be achieved following thecompaction cycle,

Duration may be of the order of just a fewseconds for very small parts

Powders do not respond to pressing in thesame way as fluids and do not assume the same

density throughout the compact friction between the powder and die wall and

between individual powder particles hinders

the transmission of pressure


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A high uniformity in green parts can be

achieved depending on:

the kind of compacting technique

the type of tools

the materials to be pressed and the lubricant


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4.3.2 Isostatic compression

powder is sealed in a flexible envelope and the

assembly of envelope and powder is immersed

in a fluid which is then pressurized.

The hydrostatic pressure presses the powder

from all directions.


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Advantages

More uniform density, as compared to uniaxial

Shrinkage is unform and predictable

No die wall friction, thus no residual stresses

No lubricants and binders required

Near net shape production achievable

The surrounding fluid may either be cold,

resulting in cold Isostatic pressing or hot (hot

Isostatic pressing)


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4.3.2.1 Cold Isostatic pressing (CIP)

High fluid pressure is applied to a powder in a

confined mould at ambient temperature tocompact the powder into the required shape

Water or oil is usually used as the pressuremedium.

Compacting pressure ranges from 210 to 410 MPathough it can be as high as 760 MPa.

powder parts are densifies to anything between

60-80% of their true density. process can be divided into 2 types, both of which

use preshaped elastomeric moulds to hold thepowder that is pressed.


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Dry bag method

The elastomeric mould is fixed to the inside of

the pressure vessel.

After filling mould with powder, pressure is

applied by introducing pressurized oil between

fixed mould and vessel wall.


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process has fast cycle time and high production

rates.

It is used mainly to compact ceramic powders

in high output applications such as spark plugs.


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Wet bag method metal powder is filled in a mould made of

rubber urethane or PVC (out side the pressurevessel).

The metal powder loaded mould is then sealed

after evacuating air and then transferred tothe hydraulic pressing chamber to compact the

powder to desired shape.

Final shape of the mould is the final shape ofthe product.


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The method is used for making components

that are large, complicated in shape and are

produced in small quantities.


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4.3.2.2 Hot Isostatic pressing (HIP) High isostatic pressure is applied to a powder

compact at elevated temperature to produce acompact product.

The process effectively combines compaction

and sintering. results in the manufacture of fully dense

products (almost 100% of TD), although partiallydense bodies can also be intentionally

produced.

Metal powder is filled in a flexible of alloy sheetmetal and air is evacuated.


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The container is then sealed and heated to a

temperature ranging from 480oC (for Al alloys) to

1700o

C (for W powder). Hydrostatic pressure is maintained at about

100MPa.


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The pressing media is usually high density inert

fluids such as argon gas or vitreous liquids

Process is used in producing net shapes in Ni-based superalloy discs and shafts for aircraft

engines, tool steels and dies, rolls and other

wear resistant (WC-based) parts.


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4.3.3 Metal injection moulding (MIM)

makes use of very fine metal powders (


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Typical metal powders that are processed

include copper, bronze, carbon, Ti, stainless

steel and tool steels.

Product range include watches, automobiles,

surgical knives, small-callibre gun barrels.


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The sheet can go further processing by sinteringand re-rolling to produce end product of desiredproperties.

Density (and hence porosity) may be regulated.

Desirable properties of powders suitable forrolling include

Good flowability

Softness

Irregularity

Wide particle size distribution; to increase density These properties are common in powder

produced by atomization method.


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Typical applications of the sheet produced

include electronic equipment (integrated

circuits, resistor end caps, band terminators

etc) as well as sleeve bearings for internal

combustion engines.

4 3 5 C if l C i


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4.3.5 Centrifugal Compacting It is used for compacting heavy powders such as

WC.

The metal powder is filled in a mould which issubjected to centrifugal force to compact thepowder in the metal.

This achieves high density of the compact throughclose packing.

The compact is later sintered to further increasethe density.

Method is used to produce high abrasion and wearresistant tools e.g. cutting tool bits and die materialfor metal forming processes.

metal powder comp action

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