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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
Powder Metallurgy 2A R Dixit
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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.
Powder Metallurgy 9A R Dixit
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
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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
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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.
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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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Powder Metallurgy A R Dixit 23
• 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.
Powder Metallurgy A R Dixit 34
• 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.
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Steel formed from HIP
Powder Metallurgy 48A R Dixit
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Advantages
• Ability to create complex shapes
• High strength properties
• Low material waste
• Good microstructure control
Powder Metallurgy 49A R Dixit
Disadvantages
• Creation of residual pores
• High tooling costs
Powder Metallurgy 50A R Dixit
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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.
•
Powder Metallurgy A R Dixit 51
. ., .
• 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)
Powder Metallurgy A R Dixit 52
• 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.
Powder Metallurgy A R Dixit 53
• 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
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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)
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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.
Powder Metallurgy 59A R Dixit
END
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