powder mwtallurgy final syllabus

8/17/2019 Powder Mwtallurgy Final Syllabus

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ELECTROCHEMICAL PROCESS

These methods are based on the electrolysis of molten

solutions of metals or fused salts.

The metals are electrically deposited on the cathode ofan electrolytic cell as a sponge or powder or at least in a

physical form in which it can be easily disintegrated

into a powder.

Advantages of the process:The technique has a number of advantages, e.g.

The product is usually of a high commercial purity.

A considerable range of powder qualities can be

obtained by varying bath compositions.

Frequently the product has excellent pressing and

sintering properties.

The cost of the operation may in some cases be low.


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Limitations:

Alloy powders cannot be produced.

The product of process is frequently in activecondition (presence of chemicals on powder particles)

which may cause difficulties in washing and drying it

(contaminationoxidation with atmospheric oxygen

may occur). The cost of operation may be high in some cases.

Basic principe of the process and e!"ipment "sed:

The equipment used is an electrolytic bath made of

steel, and lined from inside with rubber. Twoelectrodes are inserted in the bath.

!athode is made of lead while anode is made of the

same metal whose powder is being produced.


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Principe:

The basic principle is the electrolysis process in whichdecomposition of a molten saltaqueous solution into its

ions is obtained by the passage of electric current. The

metallic ions are deposited at the cathode which can be

removed with a brush and collected at the bottom.

The electrolytic tan"s have conical bottoms with a

valve. #uction pipes are connected to these bottoms and

powder is removed from the tan".The efficiency of the tan"process depends on the

deposition rate.


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Figure$ %lectrolytic !ell &peration for 'eposition of

owder #chematic.


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owder production at cathode is favored by$

high current density*

wea" metal concentration*

addition of acids*

low temperature* avoidance of agitation, and*

suppression of convection.

+ ery fine powder can be obtained when the current flowing is so

strong in relation to the strength of the solution that hydrogen is stronglyevolved from the cathode.

-ydrogen evolution is encouraged by$

(i) increasing cell voltage*

(ii) diminishing the sie of the cathode*

(iii) bringing the anode and cathode closer together*

(iv) increasing the temperature*

(v) wea"ening the strength of the metallic solution

(vi) adding acid


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+ /hen metal is deposited without evolution of

hydrogen, the deposit may be ductile and compact if the

current is 0ust not great enough to cause hydrogen

formation, or very hard with large crystals using strong

solutions and large quantities of electricity, or sandy and

brittle with little cohesion using very small current.


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#ESI$% CO%SI#ERATIO%S:

An outstanding characteristics of electrolytic powder

process is the large number of variables which eitherhave to be selected and fixed before plant is erected, or

which have to be controlled during operation. The most

important are*

(i) %lectrolytes(ii) %lectrodes

(iii) !urrent

(iv) Flow of electrolyte

(v) #tructural considerations

(vi) After treatment


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Eectro&tes:

The choice of the type of electrolyte will depend largely upon the

cost of the chemicals involved.

%lectrolyte should not corrode the apparatus i.e., it should be of non

corrosive nature.

!oncentration of the electrolyte should remain same with the passage

of time.

Cost: 1elatively pure salts of copper which are cheap and freely available

are uncommon, and therefore most copper powder production has

been derived from sulphatesulphuric acid baths.

#ome scientists are in favor of copper chloride bath because of bettercathode efficiency, lower cell voltage and less power consumption. 2t

is claimed that the chloride bath produces a more dendritic powder

with better pressing properties.


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2n the case of sulphate electroytes, the presence of a small amount of

chloride improves the anode current efficiency. #uch additions may,

however, cause corrosion problems in the cells and deterioration of the

"eeping qualities of the powder.

2ron powder sulphate or chloride baths. -aving selected the type of bath, the exact composition must then be

chosen and thereafter maintained with considerable care

/ith copper sulphatesulphuric acid electrolytes, it has been found that

cathode current efficiency improved as the copper content increased,reaching a maximum of 34.5 6 at 78 gm.liter, and decreased with

increasing acid, being 39.3 6 at :8 gm.liter. The apparent density of

powder produced increased to a maximum of ;.447 gm.ml. at


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The electrolyte composition does not necessarily stay

constant during electrolysis. ariations are usually caused

mainly by differences in anodic and cathodic current

efficiencies. 2n the case of copper, the concentration of metal in the

bath generally rises. #ubsidiary effects are caused by

evaporation, by dragout when the powder is removed, and

by the chemical solution of the electrodes when the current

is interrupted. 1eplace the electrolyte with fresh solution.

!ontrol of temperature is also important. 2t was found

that as the temperature increases from 98 to 4; !, thecurrent efficiency increased from 44.< to 39.5 6 and the

apparent density from ;.589 gm.ml. to ;.>54 gm.ml.


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Eectrodes:

The sie, shape and disposition of electrodes may

vary widely.

The anode may be soluble or insoluble and may be placeddirectly in the electrolyte or within a porous pot.

The anode may be of pure or impure metal, or in the formof scrap supported in a bas"et. ?nless, however, special

precautions are ta"en, impure anodes may cause operatingdifficulties or at least contamination of the powder by the

formation of slimes.

2t is unusual for the area of the anode to be larger or smaller

than that of the cathodes, for the purpose of balancing the

electrode efficiency.For similar reasons, in order to improve the distribution of

powder deposit on the cathodes, it is recommended to use anodes

with rows of holes bored in them


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2n the case of cathodes, the choice may depend upon whether the deposit

is going to be stripped off or allowed to fall off in the form of a sponge

or powder, or weather it is intended to ma"e a coherent brittle deposit.

2n the former case, the choice is mainly a matter of minimiing

corrosion, especially at the liquid level, and facilitating clean stripping.

For copper copper rod, Al sheets, b sheet.

For iron @b, o, Ta, / or b sheets /hen the deposit is of a brittle nature, it may be removed either by

"noc"ing it off or flexing the sheet cathode.

#ponge deposits may be removed using brushes.

Bayers of graphite paint or oils may be employed to facilitate the

separation. !astor oil oxidied with 97 6 perchloric acid applied by

preimmersion has been used.


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2t is not unusual to ma"e the deposition upon a cathode

starting sheet which is substantially crushed along with

the deposit. For example, iron gaue has been

recommended and used. This becomes embrittled

during the electrolysis and is readily crushed.

2t has even been proposed to employ coldpressed and

unsintered or sintered cathode which easily

disintegrate.

1otating electrodes


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C"rrent:

The choice of a specific operating current density will depend

mainly upon whether a coherent brittle or powdery spongy deposit is to

be made. 2n the former case the current density will be low, in the latter

it will be high.

2n each case there may be an optimum density which gives the

highest current efficiency, but this may not necessarily be the same

density which produces the most suitable grade of powder.

#ome wor"ers have found that rising temperature increases the currentefficiency.

Apparent density of the product is unaffected by current density.

The frequency at which the current is interrupted has a most important

influence upon the particle sie of the powder, and the longer theintervals the larger the particle.

The greater the interval between current interruptions, the higher

is the apparent density.


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'o( of Eectro&te:2n practice, convection and development of gas

bubbles cause a considerable flow of electrolyte overthe cathodes, and an important practical difficulty is tomaintain this reasonably constant. 2t would appear thata certain minimum forced circulation would be helpful

in attaining this. 2n an experiment it was found that stirring theelectrolyte coarsened the powder and increased theapparent density.As stirring is advantageous from the point of view ofevening out bath variables, but to some extentdisadvantageous in increasing the density andtherefore reducing the compressibility.


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Str"ct"ra:&wing to the substantial changes in behavior of anelectrolytic powder cell when its sie is increased, it is

advisable that, when such a process is advised in thelaboratory, it should be operated as a unit cell with fullsied electrodes before an attempt is made to design thefinal plant.#tructural design factors involve ta"ing decision upon

the sie and nature of the electrodes, whether theyshould be stationary or rotary, or be sheets, tubes orrods, etc., whether the cathodes should be lifted out ofthe cell for scrapping or not, whether the scrappingshould be manual or mechanical.&ther problems concern with the corrosive nature of theelectrolyte$ such as tan" construction and linings,contacts, electrolyte handling, cooling or heating, usedanode treatment, etc.


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After)treatment:

An electrolyte powder is generally in a reactive

condition, and is also wet with reactive electrolyte, thereare considerable problems in washing and drying it and

bringing it to a dry powder which is not only low in

oxide but reasonably stable on storage.

For example, with electrolytic iron powder, it was found

necessary to wash the cathode deposit with water, : 6

-:#&5, water, dilute citric acid, water, dilute ammonia,

and finally with distilled water before filtering, and thenmoistening with acetone before drying. %ven then it is

recommended that the powder should be annealed in

hydrogen to reduce the oxide content.


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T&rre* with copper powder, recommends annealing in a reducing

atmosphere. -e found, however, that treating the powder in a crac"ed

ammonia atmosphere often led to rapid subsequent deterioration onstorage. -e recommended treating the powder with suitable water

repellent chemicals and indicated that stearic acid dissolved in ammonia

was suitable for a commercial process.

any manufacturers avoid washing and drying difficulties byannealing the powder in a reducing atmosphere.

/hen a brittle electrodeposit is the first product, annealing may be

absolutely necessary in order to produce a powder having reasonable

pressing qualities, and is customary among iron powder producers.

&wing to the reactive nature of many electrolytic metal powders,

difficulties are frequently observed in preventing them from oxidiing or

corroding on storage. 2t is customary, at least with copper powder, to add

corrosion inhibitors to the powder.


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METAL PO+#ER TESTI%$

Characteri,ation of Meta Po(ders:• Cuality monitoring

• The following steps are involved in prior to processing into compact shape$

a) owder characteriation and testing b) owder handling and mixing


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a- Po(der Characteri,ation and Testing:

9. owder sampling

:. !hemical Testing

i) &xygen content of the powder

ii) Acid insoluble content of powders

7. articlerelated vs massrelated properties

5. article sie and particle sie distribution

i) #ieving

ii) icroscopic siing

iii) #edimentation methods

iv) !oulter !ounter and particle analysis by light obscuration

v) Baser light scattering

i l h d


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8. article shape and structure

4. #pecific surface area

>. !haracteristics determining the processing behavior of metal

powder$ i) Flow rate and apparent density

ii) !ompactibility

iii) 'imensional changes of powders due to

sintering

.- Po(der mi/ing and handing

9. #pecial precautions in handling and storing metal powders

:. owder ixing

i) ixing and demixing

ii) ixing apparatus


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PARTICLE SHAPE:• Figure shows various possible particle shapes of powders.• #pherical powders show excellent flow properties but give

poor green strength as compared to irregular powders.• /ater atomiation ranging from near spherical tohighly irregular.

• Das atomiation spherical powder particles.

• The reactivity of the metal or alloy essentially determinesthe particle shape. 2f the alloywater reaction produces astrongly adherent film then irregular particles are formed.#pherical shapes are produced when the oxide formed arehighly fusible at the melting point of the alloy as they have

no strength to overcome the forces of surface tension.• -igh melting metalsalloys have tendency to form

spherical particles because of long freeing times.• ery short freeing times for low melting metalsalloys

tend to form highly irregular particles.


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T . h P ti Sh d th M th d f P d P d ti


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Ta.e sho(s Partice Shape and the Method of Po(der Prod"ction

O%E #IME%SIO%AL

cic"ar Irreg"ar Rod)i0e

hemical decomposition !hemical decomposition

echanical comminution T+O #IME%SIO%AL

entritic 'a0e

lectrolytic echanical comminution

THREE #IME%SIO%AL

pherica Ro"nded

tomiation Atomiation

arbonyl Fe !hemical decomposition

recipitation from a liquid

reg"ar Poro"s

tomiation 1eduction of oxides

hemical decomposition

ng"ar

echanical disintegration !arbonyl @i

P d P ti


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Po(der Properties:

•rocessing conditions and final sintered properties are

determined to a very large extent by the characteristics of

the powder, such as* chemical composition

particle sie and sie distribution

particle shape

structure

surface condition

S i f P d


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Samping of Po(ders:• #tandard methods

A#T !ommittee E3

2F #tandard !ommittee (etal owder 2ndustries

Federation)

A#T #tandards E:98

2F #tandard 9

• A representative sample of the whole lot

• #amples from the entire cross section of the stream of

powder.• 1epresentative sample from a shipment consisting of

several drums.

• Thieve sampling

• ThievesG are devices to ta"e samples from different

CHEMICAL TESTS


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CHEMICAL TESTS

a- H&drogen Loss Test:

) A#T standard % 983, 2F standard :

for the socalled hydrogen loss of !u, / and Fe powder

•) A sample of powder is heated in a stream of hydrogen

for a given length of time and at a given temperature.

•) Boss of weight an approximate measure of theoxygen content of the powder.

•) -ydrogen loss values may be lower than the actual

oxygen content &xides not reduced by hydrogen

under the test conditions such as #i&:, Al:&7, !a&, etc

•) The hydrogen loss value may be higher than the actual

oxygen content in the presence of elements forming

volatile compounds with hydrogen, i.e. # or !.


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• #ome metals volatile at the test temperature, i.e., Hn, !d and

b.

• To avoid measuring the content of !, #, or volatile metals in

the metal powder, a modified hydrogen loss test is used.• The amount of water vapor produced by heating in a stream

of dry hydrogen is determined by titration.

• Total amount of oxygen in a metal powder including oxygenin refractory oxides, fuse a sample in a small singleuse

graphite crucible under a flowing inert atmosphere at a

temperature of :;;; o! or higher.

• The oxygen is released as !& and measured by infraredabsorption or alternatively converted to !&: and measured

by a thermal conductivity difference.

A id I . C t t f C d ' P d


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Acid Inso".e Content of C" and 'e Po(der:

• #amples of Fe powder are dissolved in -!l and those of

!u in -@&7 under specified conditions.

• The insoluble matter is filtered out, ignited in a furnace and

weighed.

• #ilica, alumina, clays and other refractory materials

• 2n Fe powder, the acid insoluble may also include insoluble

carbides.

Partice Si e and Partice Si e #istri. tion


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Partice Si,e and Partice Si,e #istri."tion

Methods:

(a) #ieving

(b) icroscopic #iing(c) ethods based on #to"esI Baw

i) the 1oller Air Analyer

ii) the icromerograph

iii) Bight and Jray (#edigraph) Turbidimetry(d) !oulter !ounter and article Analysis by Bight&bscuration

(e) Baser Bight #cattering* the icrotrac article Analyer

(f) Hetasier @anoparticle sie analyer

Si i


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Sieving:

A#T sieves sie,

and ?.#. standardsieve designation,Km

Tyler sieves sie,

and Tyler sieveseries designation,Km

95 (:;; mesh)

58 (@o 7:8) 55 (7:8 mesh)


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Figure: Schematic of sieveseries stacked in order of size

Figure: Stacked sieves on ashaker with rotaryand tapping action


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Figure: Sieved size of an irregularly shapedparticle


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Methods Based on Sto0es1 La(:

• #edimentation and %lutriation

• #to"esI Baw gives the settling velocity L of spherical particles with a diameter x and a density M in a fluidmedium with density MF and viscosity N

L O g (M MF ) 9< N + x:

/here g is the gravitational constant.

++ articles which are not spherical will also settle. Their

#to"esianG sie is defined by the diameter of a sphere ofthe material which has the same settling velocity as theirregular powder particle.

• !onvection currents in the suspending fluid must be


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• !onvection currents in the suspending fluid must be

avoided.

• The relative rate of motion between the fluid and the

powder particles must be slow enough to guaranteelaminar flow, which means the 1ynolds number should be

less than ;.:

x L MF

N

/here x is the particle sie, L is the settling velocity, MF

the density of the fluid and N its viscosity.

• The particles in the suspension must be perfectly

dispersed and the suspension must be dilute enough to

guarantee independent motion, which means maximum

concentration of about 9 6 by volume of particles in the

suspending medium.


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METAL PO+#ER

PROCESSI%$ TECH%I23ES

Compaction of Meta Po(ders:'ifferent ways of consolidation of metal powders*

4A- +ith appication of press"re (hich inc"des

i) ?niaxial pressing (single action or double action pressing)

ii) 2sostatic pressing

iii) 1oc"ing die compaction

iv) owder rolling

v) owder extrusion

vi) owder swaging

vii) owder forgingviii) owder 2n0ection olding

4B- +itho"t app&ing press"re s"ch as5

i) #lip mixing or slip casting

ii) ibrational compaction


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• Consoidation genera& occ"rs in three stages


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Consoidation genera& occ"rs in three stages

(a) rearrangement of particles.

(b) particles contacting by plastic deformation.

(c) mechanical loc"ing and cold welding of particles dueto surface shear strains.

2t is, therefore, some time easier to cold compact irregular

particles than spherical powder particles.


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Main varia.es in press"re compaction are:

(a) ethod of compaction

(b) !ompaction pressure, time and temperature

(c) 1ate of compaction

(d) !ompacting atmosphere

(e) Bubricants and other additives of the mix, and

(f) 'ie design()The main o.6ectives o.tained d"ring pressing are:

(9) To achieve the required part shape.

(:) To obtain the required green density.

(7) To secure sufficient green strength to permit safe handling of

the part.

(5) To provide particletoparticle contact which is necessary for

sintering.

The .asic t&pes of compacting presses are:


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The .asic t&pes of compacting presses are:

(i) echanical (single punch or rotary type) presses.

(ii) -ydraulic presses.

(iii) -ybridtype presses (mechanical presses may ma"e use ofauxiliary pneumatic or hydraulic devices).


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The powder metal must first fill the die orifice.

Filling may be done by hand or automatically from the

presshopper.

A constant volume or constant weight may be used.

ibration filling is introduced to create denser pac"ing to

avoid bridging and high porosity defects.

ressing may be done automatically.The pressure may be applied along more than one axis using

various punch and die sets designed to minimie defects.

%0ection after pressing may be carried out automatically or by hand.


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Minim"m re!"irements for an& po(der meta press:


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Minim"m re!"irements for an& po(der meta press:

(i) Adequate total pressure capability in the direction of

pressing and sufficient part e0ection capability.

(ii) !ontrolled length and speed of compression and e0ectionstro"es.

(iii) Ad0ustable die fill arrangements.

(iv) #ynchronied timing of press stro"es.(v) aterial feed and part removal systems.

$enera cassification of po(der meta"rg& parts:


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$enera cassification of po(der meta"rg& parts:

7- Cass I parts with a diameter (or thic"ness) up to

48 mm and one level parts of any contour that can

be pressed with a force from one direction.

8- Cass II parts are single level components of any

thic"ness and any contour that must be pressed

from two directions.

9- Cass III parts are two level components of any

thic"ness and contour that must be pressed from

two directions.- Cass I; parts are multilevel components of any

thic"ness and contour that must be pressed from

two direction.


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Compacting Presses and Attachments:

The presses systems used are*

(a) #ingle action press system consisting of$i) a die to form the outer contour of the part*

ii) an upper punch to form the top surface of the part*

iii) a lower punch to form the bottom surface of the part*

iv) if required, core rods to form any through holes

(for class 2 parts).

(b) 'ouble action opposed ram system consists of


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(b) 'ouble action opposed ram system consists of

a die, upper punch, lower punch and core rods (for class 2and class 22 parts).

(c) 'ouble action floating die system consists of moving upper punch, stationary lower punch, moving dietable and core rods (for class 2 = 2 parts).

Further during compaction tooling materials, clearances andtolerances require expertise and special attention is paid to(i) die design* (ii) die materials* (iii) punch* (iv) carbideinserts* (v) tolerances, clearances and finishes.

'orming and sintering in one step


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'orming and sintering in one step

• The simplest forming route for a metal powder is loose

powder sintering which is used for forming spherical

powder.

• 2n this process the metal powder is filled loosely or sha"en

into the mould by vibration and subsequently sintered in the

mould.• The mould can be made of steel or graphite and can be used

repeatedly.

• owders which are difficult to compact are often formed by

simultaneous application of pressure and temperature(pressure sintering).

'orming and Sintering in Separate Process Steps


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'orming and Sintering in Separate Process Steps

A/ia pressing

•The powder is charged by volume filling into a closed tool

consisting (in principle) of three parts* a die and an upper anda lower punch, and subsequently sub0ected to a pressure using

mechanical or a hydraulic press.

•The cycle begins with the tool in the filling position (9) in

which the upper and the lower punches are retracted so that a

defined filling space arises. The tool is filled with powder

from the filling shoe, the quantity of powder being determined

by the volume of the die cavity at this stage (:).

• The height of the filling space is determined by the apparent


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g g p y pp

density (filling volume) of the powder and by the required

final density of the component.

• 2t usually is equal to :.: to 7 times the height of thecomponent.

• After finishing the filling operation the powder is

compacted by the counter movement of the lower and the

upper punches until the tool has reached the pressing position (7 P 5).

• After completion of the pressing step the upper punch is

withdrawn and the compact is e0ected by moving the lower punch upwards (8). The e0ected component is pushed away

from the die by the filling shoe (4), and the tool is again

ready for a new pressing cycle.


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Compaction


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Compaction


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3nia/ia Pressing


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The e6ection s&stem


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6 &

• 'uring compaction according to the e0ection system with

two punches moving in opposite direction (figure E) the die

remains fixed and the two punches carry out thecompaction. The compact exhibits a symmetrical density

distribution with a neutral one in the middle. The counter

movements of the punches are effected by cams and toggle

0oints in the case of a mechanical press and by separatelycontrolled upper and lower cylinders in the case of a

hydraulic press.


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ISOSTATIC PRESSI%$


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ISOSTATIC PRESSI%$

• The metal powder is compacted uniformly in all directions

so that the compact becomes an accurate scale down of themould. uniform density a homogeneousmicrostructure

• For this purpose the powder is sealed in a flexible envelopeand the assembly (mouldpowder) is immersed in a fluidwhich is pressuried.

• There are virtually no residual stresses in the compactedmaterial, because there is no die wall friction.

• Figure9 shows the use of formers and use of containers withholes for support purposes.

Fig. 9$ %xamples of use of (a) container with holes and


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g p ( )

(b) (c) formers for producing shaped components by

isostatic pressing.


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Two types of processes:


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i) Cold Isostatic Pressing

ii) Hot Isostatic Pressing

< Cod Isostaic Process ))))) t(o methods are "sed $(a) free mould or Q(et .agQ process (Figure :) is suited for

(i) batch scale production*

(ii) the mold is filled and sealed outside the pressure vessel.

(iii) After the mold is introduced in to the pressure vessel, it iscompletely immersed in the pressure medium, usually watercontaining lubricating and corrosionpreventive additives*

(iv) complex parts*

(v) research and prototype wor"*(vi) several moulds in one runeven with*

(vii) differing shape, i.e. parts of different sies and shapes thatrequire the same process parameters can be pressed in the

same cycle.


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a) owder fill by weight or volume.

b) Filling of the mould from the top.


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c) Top lid put on the mould container.

d) ould is placed inside the pressure vessel.


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e) Top lid put on the pressure vessel after necessaryevacuation and filling with the pressuriing medium.

f) Top lid removed after required isostatic compaction for

removal of the mould.


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(b) fixed mould or Qdr& .agQ process (Figure 7) which is


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characteried by

(i) envelope is permanently fixed into the pressure vessel,

(ii) After the elastomeric mold is filled with powder, pressureis applied by introducing pressuried oil between the

fixed mold and the vessel wall,

(iii) only one compact at a time is used,

(iv) more simple shapes are made and

(v) more suited for mass production and faster production

rates.


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Fig.7$ #chematic of equipment for drybag isostatic pressing.


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Press"re $enerators


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• ressure is generated in the pressure medium through the

use of airdriven and hydraulically driven pumps and

pressure intensifiers. • The pressure medium typically is oil for dry bag processes*

water containing additives (watersoluble oil or rust

inhibitors) is used for wet bag processes. A filtering system

should be included with all systems to protect the pressuregenerating equipment from particulate contamination.

•#epress"ri,ation S&stems. 'epressuriation can be

accomplished with a single metering valve.

powder mwtallurgy final syllabus

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