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7/30/2019 Powder Metallography by Richa
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METALLOGRAPHY
RICHA TRIPATHI
3rd year
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Is the study of the metallic structures of metals
or alloys typically using microscopy so as to
relate the physical and mechanical properties of
alloys with the observed microstructure. It can determine:
Size and shapeof the grains
Presence of micro defects:segregation,haircracks, non metallic inclusions
Nature and distribution of secondary phases
What is Metallography ??
http://en.wikipedia.org/wiki/Microscopyhttp://en.wikipedia.org/wiki/Microscopy7/30/2019 Powder Metallography by Richa
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In Metallographic study of sintered products the type and
morphology of pores is studied. These pores affect the
various properties. The Metallographic preparationof such material can lead to changes in the specimen
surface
Some examples of such changes are:
Partial closing of pores by plastic deformation during
grinding
Break out of material around pore
Closing of pores with grinding debris
Rounding of pore edges
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Sample Preparation:
An abrasive cut off wheel with water as coolant may be used forsectioning purpose. Thorough rinsing with water must be carried out
in order to remove any cutting debris. Specimen mounting can be
done as usual practice.
Grinding:
This can be done using SiC paper of 220 grit size usingwater as a coolant or an automatic grinding wheel. A speed of 300
rpm with a load of 90, 60 and 30 N is used for three grinding steps.
After the grinding operation, the specimens are ultrasonically
cleaned in alcohol bath.
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Impregnation:
The process is necessary to seal the open porosity of thespecimen so that abrasives, water and etchants are not
entrapped later on. If the specimen is not moisture free,
bleeding out during etching may
occur which causes staining of the surface. Vacuum
impregnation is carried out with epoxy resin.
Regrinding:After impregnation, regrinding is carried out on 500 and
1000 grit silicon carbide paper.
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Polishing:This operation is generally carried out with 6, 3 and 1 mdiamond polishing spray on an automatic wheel using a load of 90,
60 and 30 N for one minute each. Polishing on a cloth with a
suspension of alumina may also be carried out. In unetched condition
total porosity, pore size and shape, nonmetallic inclusions, additions
like manganese sulphide for improved machinability, undissolvedalloying elements, etc., may be observed.
Etching:
Etching is generally performed by immersion. This facilitatesstudy of homogeneity of alloying, grain size and the presence of
different phases.
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General Summary of the Science
of P/M
1.powder production
2.powder compaction
3.sintering/ infiltration
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Morphology
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Different particle size are blended to reduce
porosity The particle shape is determined my rate
of solidification.
surface tension forces maintains the
spherical shape of the droplet until
solidification.
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Scanning electron micrographs of copper powder. Water atomized
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SEM picture of gas atomized powders
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Effect of milling
time on particle
shape
change of sphericalTi-6Al-4V alloy
particles.
(a) As-received
powder.
(b) After 1 h.
(c) After 2 h.(d) After 4 h.
(e) After 8 h.
(f) After 16 h
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Compaction
Impart shape to net or near net to powdermass
Net Shape: Die Compaction MIM
Near Net Shape:
CIP Hot isostatic Pressing
Extrusion
Rolling
MicrostructureParticles may be flattened and
distorted, depending on the nature of
the powder and the compacting
pressure.
Typically, uniaxial compaction createsflattened pores that subsequently
contribute to anisotropic dimensional
change in sintering.
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Isotatic Pressing
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Fine homogenous and isotropic microstructure
Improved wear and corrosion resistance,
through extended alloying possibilities
Shear strength fatigue life improved
Higher resistance to impact wear and abrasions
Desification of powder parts to 100% of theoretical density
Increased toughness
Advantages of HIP
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If heat is supplied during process, it
promotes intergranular growth and
diffusion, this process is known as
sintering.Temperature should be slightly above the
recrystallisation temperature but less than
its melting point
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Sintering Temperature and Time for
Various Metals
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Mechanical Properties
Hardness
This is determined by microhardness testers utilizing Knoop or diamond pyramid
hardness indentors. It measures the true hardness of the structure by eliminating the
effect of porosity, and this is a measure of resistance to wear.
Tensile Strength
The tensile properties of sintered products are directly influenced by porosity. Due to
presence of porosity, the tensile properties are somewhat lower than those of wrought
materials of the same composition and structure.
Crack initiation takes place at the preferred site like pore and then propagates
through it.
Pores are of two types: (a) interconnected and (b) closed or isolated.
interconnected porositymay give rise to internal corrosion.
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Forged on left; P/M on right
Powder Metallurgy: Connecting Rods
29
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MICROSTRUCTURE OF PM HIP PARTS
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MICROSTRUCTURE OF PM HIP PARTS
rapid solidification process, fine and regular microstructures can
be obtained thanks to the PM HIP technology, with
strength values similar to those of forged parts.
Cast, forged and PM HIP microstructures of duplex stainless steel
Cast ForgedPM HIP
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Produces a smaller grain size due to the high cooling rate.
Alloying elements do not segregate when the powder is handeld
Increasing the cooling rate of liquid material can result in the formation ofan ultrafine or microcrystalline grain size
Applying pressure drops the transformation temperature and increase
nucleation rate making it possible to obtain nano grain size sintered
sample.
H t St th M t l
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How to Strengthen Metals: Key: prevent dislocations from moving through crystal
structure!!!
Finer grain boundries can be done by recrystallizing(and cold working)
Increase dislocation density via COLD WORKING (strainhardening)
Add alloying elements to giveSOLID SOLUTIONHARDENING.
Add alloying elements to give precipitates or dispersedparticlesPRECIPITATION HARDENING (aka HeatTreat)
DISPERSION HARDENING fine particles (carbon)impede dislocation movement. Referred to as Quench Hardening, Austenitizing and Quench or
simply Heat Treat.
Generally 3 steps: heat to austenite T, rapid quench, thentemper.
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How do Metal
Crystals Fail??
Answer: Slip due
to dislocations
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The Effect of Grain Boundries:
Dislocations pile up at GB and cant go
further this effectively strengthens the
crystal!
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Valve system parts
clutch lever, shift-gear
Piston, Rod-guide, Valve case
Body parts ,ABS sensor parts,
Power steering pump.
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