Powder Metallurgy handouts

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Powder Metallurgy

Dr. A R Dixit

Powder Metallurgy 1A R Dixit

History of Applications

• 3000 B.C. Egyptians made tools with

pow er me a urgy

• 1900’s tungsten filament for light bulb

• 1930’s carbide tool materials

• 1960’s automobile parts

• 1980’s aircraft engine turbine parts




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INTRODUCTION

Components can be made from pure metals, alloys, or

mixture of metallic and non-metallic powders

Commonly used materials are iron, copper, aluminum,

nickel, titanium, brass, bronze, steels and refractory

metals

Used widely for manufacturing gears, cams, bushings,

, , , .



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Powder Metallurgy (P/M)

• Competitive with processes such as

cas ng, org ng, an mac n ng.

• Used when

• melting point is too high (W, Mo).

• reaction occurs at melting (Zr).

• too hard to machine.

• very large quantity.

• Near 70% of the P/M part production

is for automotive a lications.

Powder Metallurgy 5

• Good dimensional accuracy.• Controllable porosity.

• Size range from tiny balls for ball-point

pens to parts weighing 100 lb. Most

are around 5 lb.

A R Dixit

Basic Steps In Powder Metallurgy

• Powder Production

• Blending or Mixing

• Powder Consolidation

• Sintering

• Finishing

Powder Metallurgy A R Dixit 6



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1. Powder Production

There are three main processes

for making metal powders:

1. Atomization

2. Chemical Methods

3. Electrolytic Processes




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Atomization

Atomization is a process of which a stream

o mo en me a s rans orme n o a spray

of droplets that solidify into powder.


Atomization

Produce a liquid-metal

stream by injecting

u

small orifice

Stream is broken by jets

of inert gas, air, or water

The size of the particle

formed depends on the

temperature of the metal,

metal flowrate through

the orifice, nozzle size

and jet characteristics



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Variation:

A consumable electrode is

rotated rapidly in a helium-

filled chamber. The

centrifugal force breaks up

the molten tip of the

electrode into metal

particles.

Fe powders made by atomization Ni-based superalloy made by

the rotating electrode process



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Chemical Processes

A process in which metal powders are

orme e ween me a ox es an

reducing agents.

Reduce metal oxides with H2/CO

have uniformly sized spherical or angular shapes


Electrolytic & Precipitating

1. The process of precipitating metal powders

the desired metal is the anode.

2. As anode is dissolved the desired metal is

deposited on the cathode.

3. Then metal deposit is then removed, cleaned,

and dried.




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Comminution

CrushingMilling in a ball mill

Powder produced

– Brittle: Angular

– Ductile: flaky and not particularly suitable for P/Moperations

Mechanical Alloying

Powders of two or more metals are mixed in a ball mill

Under the impact of hard balls, powders fracture and jointogether by diffusion

(a) Roll crusher, (b) Ball mill



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Characterization of

Powders


Size of powders 0.1 um – 1 mm

Sieve size quoted as mesh number

Particle D = 15/mesh number (mm)

325 mesh45 um

2. Blending or Mixing

Blending a coarser fraction with a finer fraction ensuresthat the interstices between lar e articles will be filled out.

Powders of different metals and other materials may bemixed in order to impart special physical and mechanicalproperties through metallic alloying.

Lubricants may be mixed to improve the powders’ flowcharacteristics.

Binders such as wax or thermoplastic polymers are addedto im rove reen stren th.


Sintering aids are added to accelerate densification onheating.



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BLENDING

To make a homogeneous mass with uniform distributionof particle size and composition

– Powders made by different processes have differentsizes and shapes

– Mixing powders of different metals/materials

– Add lubricants (<5%), such as graphite and stearicacid, to improve the flow characteristics andcompressibility of mixtures

Combining is generally carried out in – Air or inert gases to avoid oxidation

– ,explosion hazards

Hazards – Metal powders, because of high surface area to volume ratio are

explosive, particularly Al, Mg, Ti, Zr, Th

Some common equipment geometries used for blending powders

(a) Cylindrical, (b) rotating cube, (c) double cone, (d) twin shell



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3. Powder Consolidation• Cold compaction with 100 – 900

MPa to produce a “Green body”.

• Cold isostatic pressing

• Rolling

• Gravity

• Injection Molding small, complex

parts.

Powder Metallurgy A R Dixit 21Die pressing

COMPACTION

•using a hydraulic or mechanical press

• Pressed powder is known as “green compact”

• Stages of metal powder compaction:



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• Increased compaction pressure

• Provides better packing of particles and

leads to ↓ porosity

• ↑ localized deformation allowing new

contacts to be formed between particles



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• At higher pressures, the green density approaches

• Pressed density greater than 90% of the bulk density is

difficult to obtain

• Compaction pressure used depends on desired density

• Smaller particles provide greater strength mainly due to

reduction in porosity

• ze s r u on o par c es s very mpor an . or same

size particles minimum porosity of 24% will always be

there

• Box filled with tennis balls will always have open space between

balls

• Introduction of finer particles will fill voids and result in↑ density



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• Because of friction between (i) the metal particles and (ii)

between the punches and the die, the density within thecompact may vary considerably

• Density variation can be minimized by proper punch anddie design

(a)and (c) Single action press; (b) and (d) Double action press

(e) Pressure contours in compacted copper powder in single action press

Compaction pressure of some metal powders

Metal Powder Pressure (MPa)

Al 75-275

Al2O3 100-150

Brass 400-700Carbon 140-170

Fe 400-800

W 75-150

WC 150-400



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(a)Compaction of metal powder to form bushing

(b)Typical tool and die set for compacting spur gear

A 825 ton mechanical press for compacting metal powder



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Cold Isostatic Pressing

• Metal powder placed

mold

• Assembly pressurized

hydrostatically by

water (400 – 1000

MPa

• Typical: Automotive

cylinder liners →

Friction problem in cold compaction

• The effectiveness of pressing with a single-acting punch islimited. Wall friction o oses com action.

• The pressure tapers off rapidly and density diminishes awayfrom the punch.

• Floating container and two counteracting punches helpalleviate the problem.




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SINTERING

• Green compact obtained after compaction is brittle and

low in strength

• Green compacts are heated in a controlled-atmosphere

furnace to allow packed metal powders to bond together

Sintering

• Parts are heated to 0.7~0.9 T m .

• Transforms compacted mechanicalbonds to much stronger metallicbonds.


• Shrinkage always occurs:

sintered

green

green

sintered

V

V shrinkageVol

ρ

ρ ==_

3 / 1

_ ⎟⎟ ⎠

⎞⎜⎜⎝

⎛ =

sintered

greenshrinkage Linear

ρ

ρ



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Carried out in three stages:

•

volatile materials in the green compact that would

interfere with good bonding is removed

• Rapid heating in this stage may entrap gases and

produce high internal pressure which may fracture

the compact

• Promotes solid-state

bondin b diffusion.

Second stage: High temperature stage

• Diffusion is time-

temperature sensitive.

Needs sufficient time



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•Promotes vapour-phase

ranspor

•Because material

heated very close to

MP, metal atoms will

be released in the

vapour phase from the

•Vapour phase

resolidifies at the

interface



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• Third stage: Sintered product is cooled in a controlled

a mosp ere

• Prevents oxidation and thermal shock

Gases commonly used for sintering:

• H2, N2, inert gases or vacuum



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Liquid Phase Sintering

• During sintering a liquid phase, from the lower MP

component, may exist

• Alloying may take place at the particle-particle interface

• Molten component may surround the particle that has

not melted

• High compact density can be quickly attained

• Important variables:

• a ure o a oy, mo en componen par c e we ng,capillary action of the liquid

Hot Isostatic Pressing

• Produces powder metal parts to near

u ens y an s apes o vary ng

complexity.

• Performed in a pressurized fluid.

• Uses lower pressures to densify a

.




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HOT ISOSTATIC PRESSING (HIP)

Steps in HIP

• Simultaneous compaction + sintering

• Container: High MP sheet metal

• Container subjected to elevated

temperature and a very high vacuum to

remove air and moisture from the powder

• ressur z ng me um: ner gas

• Operating conditions

• 100 MPa at 1100 C



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• Produces compacts with almost 100%

• Good metallurgical bonding betweenparticles and good mechanical strength

• Uses

• Superalloy components for aerospace

industries• Final densification step for WC cutting

tools and P/M tool steels

Sintering Process Cont.

• Furnace provides time and temp.

con ro .

Continuous Furnace




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Sintering on Particles

The articles will

stretch and

densification will

form in places of

rapid shrinking.


Steel formed from HIP




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Advantages

• Ability to create complex shapes

• High strength properties

• Low material waste

• Good microstructure control


Disadvantages

• Creation of residual pores

• High tooling costs




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5. Finishing

• The porosity of a fully sintered part is still significant (4-15%).

• Density is often kept intentionally low to preserveinterconnected porosity for bearings, filters, acoustic barriers,and battery electrodes.

• However, to improve properties, finishing processes areneeded:

• Cold restriking, resintering, and heat treatment.

• Impregnation of heated oil.

•


. ., .

• Machining to tighter tolerance.

Special Process: Hot compaction

• Advantages can be gained by combining consolidation andsintering,

• High pressure is applied at the sintering temperature to bringthe particles together and thus accelerate sintering.

• Methods include

• Hot pressing

• Spark sintering

• Hot isostatic pressing (HIP)


• o ro ng an ex rus on

• Hot forging of powder preform

• Spray deposition



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Advantages and Disadvantages of P/M

• Virtually unlimited choice of alloys, composites, andassociated properties.

• Refractory materials are popular by this process.

• Controlled porosity for self lubrication or filtration uses.

• Can be very economical at large run sizes (100,000 parts).

• Long term reliability through close control of dimensionsand physical properties.

• Very good material utilization.


• High cost of powder material.

• High cost of tooling.

• Less strong parts than wrought ones.

• Less well known process.

POWDER METALLURGY

Properties similar to casting

Porosity related amount of compaction

Usually single pressed products have high

tensile strength but low elongation (brittle)

Repressing can improve elongation




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POWDER METALLURGY

Useful in making parts that have irregular

curves, or recesses a are ar o

machine.

Suitable for high volume production

Near Net Shape (very little waste)

eliminated


POWDER METALLURGY

Examples of typical parts

– Cams

– Ratchets

– Sprockets

– Pawls

–

(impregnated with oil)

– Carbide tool tips




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DESIGN CONSIDERATIONSPart must be so designed to allow for easy

Sidewalls should be perpendicular

Hole axes should be parallel to thedirection of opening and closing of the die

Holes, even complicated profiles, are

permissible in the direction of compressingThe minimum hole diameter is 1.5 mm(0.060 in)


DESIGN CONSIDERATIONS

The wall thickness should be compatible

w e process yp ca y . mm .

in) minimum

Length to thickness ratio can be up to 18maximum - ensures tooling is robust

Threads for screws cannot be made and

have to be machined later




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DESIGN CONSIDERATIONS

Tolerances are 0.3 % on dimensions. If

repress ng s one, e o erances can e

as good as 0.1 %. Repressing, however,

increases the cost of the product.


END


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