Powder Metallography by Richa

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/Microscopy


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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

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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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