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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