powder metallurgy -2

8/12/2019 Powder Metallurgy -2

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Powder:

aracter zat on,

treatment,

compact on

andsintering


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Powdercharacterization

Powdersampling

Chemicalcompositionandstructure

ar c es ze

Seiving method

Fishersubseive anlaysis

Lightscatteringmethid

Sedimentationmethod

Particlesurfacetopography

Surfacearea

rue,apparen an ap ens y Compressibility

Greenstrength


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PowderTreatment

Needforpretreatment

Makepowderamenabletoforming

Softeningofthepowderincaseitishard

Obtainparticlesofaparticularsize:sieving

Lubricantadditive:Tominimizefrictionbetween owder

particlesandbetweenparticlesanddiewallsduring

compaction.Zincstearate,orstearates ofothermetalslike, , , .

Binderadditive: Toprovidestrengthtothemoldedgreen

body.Variouspolymersandwaxesusedare.e.g Polyvinyl

alcohol PVA ,polyethyleneglycol,paraffinwaxorwatersolublewaxes.

Im urit articlesin owdersaretoberemoved

Removeagglomerationofpowdersforeasyflow

Degassingofpowders


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Powdercompaction

Diecompaction

Wetcompaction

Isostatic ressin

Powderrolling

Injectionmoulding


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Packing density ranges from 52% for simple cubic packing to

or e ra e ra an pyram a c ose pac ng an s

independent of the size of the spheres. The number of

particles per unit volume(N in a system of packed spheres

of uniform size is

Thenumberof articlecontacts erunitbulkvolume N is

dependentonthepackinggeometryandistheproductofNpandhalfcoordinationnumber(CN)


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, . . . . .

Inbimodalpowdermixture,largefractionoflarge sizeparticlesand

smallparticles aremixed.Formaximumpacking,themixtureshould

contain26.6wt.%o sma erpartic esan ractionpac ing PF densitywouldbearound86%whichgreaterthanthemonosized

powder. Fromfigure thepackingdensityismaximumatacritical

densityX.Inthisconditionthelargerparticlesareincontactwith

eachotherandthesmallerparticlesfilluptheinterstitialvoids.


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Powderbedunderload


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Initial repacking of particles. sliding with restacking.

Elastic deformation of the particles.

.

Compression of the solid crystal lattice then occurs.


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Initial repacking of particles. During this stage, sliding of

. Elastic deformation of the particles takes place until the

elastic limit (yield point) is reached. Here surface

e orma on s e as c.

Plastic deformation and/or brittle fracture predominate.Progressive flattening of the contact brings particle

centres closer to each other and particles originally in

contact undergo plastic deformation. As the distance,

contacts are generated. The neighboring contacts at a

given sphere will start to impinge until all the voids are

v r ua y e m na e . Compression of the solid crystal lattice then occurs.


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Dur ng t e ens cat on o t e pow er mass n a e r ct ona

forces at the wall of the compacting die restrain the densification

of the powder because they act against the compacting punch

and the axial stress available for the compaction processdecreases.

The axial compressive stress in the powder decreases

exponentially with increase in distance from the face of movingupper punc .

This effect is more pronounced with larger frictional coefficient

and smaller inner diameter of die


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Diepressing


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axial pressing in a steel or carbide die under pressures of

- . It is possible to press parts with complicated shape up to 25

part / minute.

handled. Bonding by interlocking and cold-welding between the

.

Can achieve 93% theoretical density at pressure of 800 MPa

for Iron.

Warm Compaction

Powder mix with special lubricant and the tool sets are

ea e o an respec ve y. Warm compacted components have a strength high enough

to allow some machining operations before sintering, which

drastically decreases tool wear.


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Cold isostatic compaction

A change in the pressure of an enclosed incompressible fluid is

conveyed undiminished to every part of the fluid and to the

surface of its container.Powder materials are sealed in a forming mold with low

deformation resistance like a rubber bag to apply liquid

pressure.Then, the molded body is compressed uniformly over its entire

surface by transmitting the liquid pressure

This consists of a pressure vessel or cavity, closure system,

reservoir with filtering system, high pressure generator,

epressur sng sys em, u rans er sys em an con ro s


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Wetbagpressing

Powder is filled in a formin mold and sealed airti ht outside the

highpressure vessel before direct immersion into a pressure

medium. Then, isostatic pressure is applied to the outer surfaces

of the mold to compress the powder into a shape.


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Advantages of CIP:

High green density

High green strength

Reduced internal stresses

Can be compacted without binders or lubricants

ratio parts can be compacted

Dimensional control of green compacts is less precise

than in rigid die pressing

Surface is less smooth Low production rate

Flexible moulds have ver low life com ared to ri id

steel or carbide molds.


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Powder roll compaction

Roll compaction, or powder rolling, is a process that can

manufacture continuous lengths of metal strip, or sheet, from

powders.


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Powder

extrusion Injectionmoulding


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Sintering

Sinterin is a thermal rocess for consolidatin owderparticles in to a coherent structure via mass transport on

the atomic scale. The bonding leads to improved

properties like strength, electrical and magnetic propertiesand integrity.

The temperature used for sintering is below the melting

material.

,

together by cold welds, which give the compact sufficient

green strength to be handled. At sintering temperature,

diffusion processes cause necks to form and grow at

these contact points.


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Solidstatesintering:Pressureless

Powderparticlesareneverflat.Theyhavecurvature.Acurved

surfacealwayshavesomestress.TheLaplaceequationforstress

associatedwithacurvedsurfaceis

where, isthesurfaceenergyandr1andr2aretheprincipleradiiof

curvatureforthesurface.

Whenthesurfaceisconcave(i.e.whentheradiusislocatedin

thevaporphase),theconventionistoassignanegativesignand

thesurfaceisundercompression

Duringsintering,theatomsfromtheconvexregionare

expectedtomovetotheconcaveregions


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Due to the curvature of the surfaces, the chemical potential of the

atoms and vacancies within the curved surfaces are altered. The difference in the chemical potential across these regions leads

to diffussional flux of atoms thereby reduce the free energy.

The flux of atoms are from the convex surface to the concave

surface. Chemical potential of the vacancy as well as the atoms

of the surface. The curvature will produce the same effect as an

equivalent external applied stress. The driving force of sintering isthe excess surface free energy


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Stagesinsintering

Intheinitialstageofsintering, aneckformsbetween

twopowderparticlesatthecontactregion.Thisneck

closedporesinthefinalstage. Duringthesintering transportofatomsfromregionsof

higherchemicalpotential totheregionsoflower

chemicalpotentialoccurs.

Themajormechanismsofmasstransport

Evaporationandcondensation:formaterialswithhigher

.

Diffusion:surface,grainboundaryandlattice.

Plasticflow

Viscousflow.


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Initial stage:

Consider two e ual sized s heres in contact.As the sintering proceeds, a neck will form and grow.

The curvature of the neck is reciprocal of the neck radius, .

(b) is the case of densifying.The geometrical parameters are

the radii of the curvature,

area of the surface A, and

the vol. of materials transported in to the neck.


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u y

xistheradiusoftheneck

risthesphereradius,

tistheisothermalsinteringtime

.

Cisaconstant.


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Table Initialstagesintering

Mechanism n m

Viscousflow 2 1

Plasticflow 2 1

Evaporationcondensation 3 2

att ce vo ume us on

Grainboundarydiffusion 6 4

ur ace us on


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Differentoperatingmechanismsduring sintering


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In actual case of sintering , additional neck forms with progress in

of the compacts. The shrinkage is approximately related to the

neck size by the expression

Where,Listhechangeinlengthofcompact,Loistheinitial

lengthofthecompact.


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Intermediatestagesintering

dominating mechanism depends on the rate at which each

mechanism operate.

The interaction between each mechanism is important

Densifying mechanism is by grain boundary diffusion and or

lattice diffusion.

As long as grain boundary remains attached to the pores, grain

s ze w ncrease an poros ty ecreases.

A stage will come when the continuity of the grain boundary is

os an pores ecomes n epen en . The above mechanism will still continue and the pore size

.


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Finalstage

After prolonged sintering, pores becomes closed

.

Their shape changes to lens shaped and finally to

Material shrink by lattice or volume diffusion.

Difference in vacancy concentration and porecurvature lead to pore coarsening , where the

growth of the large pore occurs at the expense of

t e sma er an ess sta e pores.


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LiquidPhaseSintering

Animportantfactorthatdeterminesthedensificationrateis

t egra n oun ary us oncoe c entmu t p e yt e

boundarythickness.

diffusionrateinaliquidishigherthaninasolid

Ifgrainboundarycontainsaliquidfilm,thedensificationrateis

fast

.

Foreffectingliquidphasesintering,inadditiontopresenceof

liquidphaseatthegrainboundary,theliquidshouldbeableto

we eso pow ersur ace


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Transientliquidphasesintering

In a compact that contains only iron powder particles, the solid

state sintering process would generate some shrinkage of the

. ,

with ferrous PM materials is to make an addition of fine copperpowder to create a transient liquid phase during sintering.

At sintering temperature, the copper melts and then diffuses into

the iron owder articles creatin swellin .By careful selection of copper content, it is possible to balance

this swelling against the natural shrinkage of the iron powder

dimensions at all during sintering.

The copper addition also provides a useful solid solution

strengthening effect.


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Permanentliquidphasesintering

For certain materials, such as cemented carbides or hard metals, a

sintering mechanism involving the generation of a permanent

.

the use of an additive to the powder, which will melt before thematrix phase and which will often create a socalled binder phase.

The process has three stages:

a Rearran ement

Astheliquidmelts,capillaryactionwillpulltheliquidinto

packingarrangement


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c ema c o e m cros ruc ure c anges ur ng s ar ng w m xe

powders and pores between particles. During heating the particles sinter.

But when a melt forms and spreads, the solid grains rearrange.

u sequent ens cat on s accompan e y coarsen ng. or may

products there is pore annihilation as diffusion in the liquid accelerates

grain shape changes that facilitate pore removal


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(b)Solutionprecipitation

In areas where capillary pressures are high, atoms will

preferentially go into solution and then precipitate in areas of

lower chemical potential where particles are not close or in

contact.

This is called contact flattening and densifies the system in a

way similar to grain boundary diffusion in solid state sintering.

Ostwald ripening will also occur where smaller particles will go

into solution preferentially and precipitate on larger particles

leading to densification


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Final densification

Densification of the solid skeletal network, liquid movement

from efficiently packed regions in to pores.

For permanent liquid phase sintering, the major phase should

be at least slightly soluble in the liquid phase and

the solid particulate network occurs,

o erw se rearrangemen o e gra ns w no occur


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a. Contactflatteningisthefirstmechanism

b. Diffusionintheliquidisthecontrollingtransportmechanismc. Thethirdmechanisminvolvesgrowthoftheintergrain contact

powder metallurgy -2

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