2. Definition of Powder Metallurgy
• Powder metallurgy may defined as, “the art
and science of producing metal powders and
utilizing them to make serviceable objects.”
OR
• It may also be defined as “material processing
technique used to consolidate particulate
matter i.e. powders both metal and/or non-
metals.”
3. Process of Powder Metallurgy:
Powder Production
Powder Characterization & testing
Mixing - Blending
Processing - Compacting
Sintering Operation
Finishing Operations
Finished P/M Parts
4. Characteristics of fine powder
1. Surface area
2. Density
I. True density
II. Apparent density
III. Tap density
IV. Green density
3. Flow rate
4. Green strength
5. Green spring
6. Compressibility and compression ratio
6. 8. Particle size
- dia of spherical particles
- Av. Dia of non spherical particles
divided into 3 classes
1. sieve
2. sub sieve
3. sub micron
9. Particle size distribution
shape factor= surface area/ Volume
Aspect ratio = largest dim/ smallest dim
7. PRODUCTION OF METAL POWDERS
The selection of materials in powder
metallurgy is determined by two factors.
i) The alloy required in the finished part.
ii) Physical characteristics needed in the
powder.
Both of these factors are influenced by the
process used for making powder.
8. i) There are numerous ways for powder production
which can be categorized as follows.
1) Mechanical methods of powder production:
i) Chopping or Cutting
ii) Abrasion methods
iii) Machining methods
iv) Milling
v) Cold-stream Process.
9. 2. Chemical methods of powder production:
i) Reduction of oxides
ii) Precipitation from solutions
iii) Thermal decomposition of compounds
iv) Hydride decomposition
v) Thermit reaction
vi) Electro- chemical methods
10. 3. Physical methods of powder production:
i) Water atomization
ii) Gas atomization
iii) Special atomization methods
The choice of a specific technique for powder
production depends on particle size, shape,
microstructure and chemistry of powder and also on
the cost of the process.
11. 1. Chopping or Cutting:
In this process, strands of hard steel wire, in diameter as small as 0.0313 inches
are cut up into small pieces by means of a milling cutter.
This technique is actually employed in the manufacturing of cut wire shots
which are used for peening or shot cleaning.
Limitations:
It would, however, be difficult and costly to make powders by this method and for
this reason it is not profitable to discuss the technique in detail.
2. Rubbing or Abrasion Methods:
These are all sorts of ways in which a mass of metal might be attacked by some
form of abrasion.
a) Rubbing of Two Surfaces:
When we rub two surfaces against each other, hard surface removes the material
from the surface of soft material.
* Contamination
12. b) Filing:
Filing as a production method has been frequently employed, especially
to alloy powders, when supplies from conventional sources have been
unobtainable.
Such methods are also used for manufacture of coarse powders of dental
alloys.
Filing can also be used to produce finer powder if its teeth are smaller.
* commercially not feasible.
c) Scratching:
If a hard pin is rubbed on some soft metal the powder flakes are
produced.
Scratching is a technique actually used on a large scale for the
preparation of coarse magnesium powders.
* scratching a slab of magnesium with hardened steel pins.
* a revolving metal drum to the surface of which is fixed a
scratching belt.
13. The drum, which is about 8 inches in diameter, rotates at a peripheral
speed of approximately 2500 ft./min. The slab of magnesium metal, 14
in. wide by 1.75 in. thick enters through a gland in the drum casing and
presses against the steel pins.
d) Machining:
A machining process, using for example a lathe or a milling cutter in
which something more than just scratching is involved, since the
attacking tool actually digs under the surface of the metal and tears it off.
On lathe machine by applying small force we get fine chips.
A large amount of machining scrap is produced in machining operations.
This scrap in the form of chips and turnings can be further reduced in
size by grinding.
* small scale production.
14. Disadvantages:
• Lack of control on powder characteristics, including
chemical contamination such as oxidation, oil and other
metal impurities.
• The shape of the powder is irregular and coarse.
Advantages:
•For consuming scrap from another process, machining is a
useful process.
•Presently the machined powder is used with high carbon
steel and some dental amalgam powders.
15. COMMERCIAL METHODS
These are the methods used for high production rate. Best examples
of mechanical production methods are the Milling Process and Cold
Stream Process.
Milling:
The basic principal of milling process is the application of impact and
shear forces between two materials, a hard and a soft, causing soft
material to be ground into fine particles.
Milling techniques are suitable for brittle materials.
Two types of milling are;
i) Ball Milling
ii) Attrition Milling.
16. Objectives of milling include:
Particle size reduction (comminution or grinding)
Shape change (flaking
Solid-state alloying (mechanical alloying)
Solid-state blending (incomplete alloying)
Modifying, changing, or altering properties of a material
(density, flowability, or work hardening)
Mixing or blending of two or more materials or mixed
phases
17. Ball Milling:
Ball milling is an old and relatively simple method for grinding large
lumps of materials into smaller pieces and powder form.
Principle of the process:
The principle is simple and is based on the impact and shear forces.
Hard balls are used for mechanical comminution of brittle materials
and producing powders.
Milling Unit:
The basic apparatus consists of the following;
• A ball mill or jar mill which mainly consists of a rotating drum
lined from inside with a hard material.
• Hard balls, as a grinding medium, which continue to impact the
material inside the drum as it rotates/rolls.
20. Important Parameters:
1. The most important parameter to consider is the speed
of rotation of the drum. An optimum/critical speed is
adjusted for maximum impact velocity.
* Critical speed is the speed above which the ball will
centrifuge.
• Very slow speed of rotation will not carry the balls to the
top, these will roll back down the drum sides.
• Very fast speed (higher than critical speed) will not let
the balls drop down as they will be carried around due to
centrifugal forces. Thus, an optimum speed is required.
This speed of rotation varies with the inverse square root
of the drum diameter.
21. 2. The material of grinding media and its size and
density.
• The size and density of the milling medium is
selected according to the deformation and fracture
resistance for metals.
• For hard and brittle materials large and dense
media is used. Whereas, small balls are used for
finer grinding.
• As a general rule, the balls should be small and
their surface should be a little rough. The material
of the balls and lining of the drum should be same
as that of the material being ground.
22. 3. The rate of milling of a powder is a function of
quantity in the total space between the balls.
4. Lubricants and surface active agents are used to
nullify the welding forces which causes
agglomeration.
Grinding Mechanism:
During milling the following forces cause fracture of
material into powder.
Impact Forces: These are caused by instantaneous
striking of one object on the other. (Impact is the
instantaneous striking of one object by another. Both
objects may be moving or one may be stationary).
Shear Forces: These are caused as one material
slides/rubs against the other.
23. Limitations:
• Rubbing action causes contamination of powder since
balls may also get rubbed.
• Working hardening of metal powder is caused during
milling.
• There is a possibility of excessive oxidation of final
powder.
• Quality of powder is poor.
• Particle welding and agglomeration may take place.
24. COLD STREAM PROCESS
• This process is based on impact phenomenon caused by
impingement of high velocity particles against a cemented
carbide plate.
• The unit consists of:
A feed container;
A compressor capable of producing a high velocity stream
of air (56 m3/min.) operating at 7 MPa (1000 psi);
A target plate, made of cemented tungsten carbide, for
producing impact;
A classifying chamber lined with WC while the supersonic
nozzle and target generally are made of cemented tungsten
carbide.
25. Mechanism of the Process:
The material to be powdered is fed in the chamber and
from there falls in front of high velocity stream of air.
This air causes the impingement of material against
target plate, where material due to impaction is
shattered into powder form. This powder is sucked and
is classified in the classifying chamber. Oversize is
recycled and fine powder is removed from discharge
area.
* Rapidly expanding gases leaving the nozzle create a
strong cooling effect through adiabatic expansion.
This effect is greater than the heat produced by
pulverization.
26. CHEMICAL METHODS
• Almost all metallic elements can be produced in the
form of powders by suitable chemical reactions or
decomposition.
Mostly chemical methods are based on the decomposition
of a compound into the elemental form with heating or
with the help of some catalyst.
In most cases such processes involve at least two
reactants.
(i) a compound of the metal
(ii) a reducing agent
27. REDUCTION OF METAL OXIDES
Manufacturing of metal powder by reduction of oxides is
extensively employed, particularly for Fe, Cu, W and Mo.
Advantages:
A variety of reducing agents can be used and process can
be economical when carbon is used.
Close control over particle size
Porous powders can be produced which have good
compressive properties.
Adoptability either to very small or large manufacturing
units and either batch or continuous processes.
28. Limitations:
Process may be costly if reducing agents are gases.
Large volumes of reducing gas may be required, and
circumstances where this is economically available
may be limited; in some cases, however, costs may
be reduced by recirculation of the gas.
The purity of the finished product usually depends
entirely upon the purity of the raw material, and
economic or technical considerations may set a
limitation to that which can be attained.
Alloy powders cannot be produced.
29. Production of Iron Powder
by Reduction of Iron Oxide:
(Direct Reduction Process)
Iron powders are commercially used for a large
number of applications such as fabrication of
structural parts, welding rods, flame cutting,
food enrichment and electronic and magnetic
applications.
The classical technique for production of iron
powder is the reduction of iron oxide.
30. Theory of the process:
It is the oldest process of production of iron
powder by using carbon as the reducing agent.
In this process pure magnetite (Fe3O4) is used.
Coke breeze is the carbon source used to reduce
iron oxide. Some limestone is also used to react
with the sulphur present in the coke. The mixture
of coke and limestone (85% + 15%) is dried in a
rotary kiln and crushed to uniform size.
** Hoganas Process
31. The ore and coke-limestone mixture is charged
into ceramic tubes (Silicon Carbide) with care so
that ore and reduction mixture are in contact with
each other but not intermixed. It can be achieved
by using concentric charging tubes with in the
ceramic tube.
Within the hot zone, several chemical reactions
occur and metallic iron is formed in the form of
sponge cake.
The main reaction is;
MO + R M + RO
32. If magnetite ore is used, then the following reactions
will take place:
Fe3O4 + 3CO FeO + 3CO2
FeO + CO Fe + CO2
C + ½ O2 CO
Decomposition of the limestone generates carbon
dioxide, which oxidizes the carbon in the coke to form
carbon monoxide. The ferrous iron oxide is further
reduced by the carbon monoxide to metallic iron.
Desulphurization occurs in parallel with reduction by
reaction between gas and sulphides present in the ore
resulting in gaseous sulphide compounds which in turn
react with lime to form calcium sulphide.
33. The sponge cake is removed from ceramic tubes and
dropped into a tooth crusher where this is broken into
pieces.
After these pieces are ground to desired particle size.
During grinding the powder particles are considerably
work hardened. The powder is annealed at 800 - 870
oC in the atmosphere of dissociated ammonia.
The powder is loosely sintered, but requires only light
grinding and screening to produce a finished product.
34. THE CARBONYL PROCESS
• The only method for the manufacture of metal
powder by the pyrolysis of a gaseous compound
which has been used industrially on a substantial
scale is the carbonyl iron or nickel process.
• When iron and nickel ores react under high pressure
(70 – 300 atm.) with carbon monoxide, iron
pentacarbonyl [Fe(CO)5] or nickel tetracarbonyl
[Ni(CO)4] is formed, respectively.
• Both compounds are liquids at room temperature.
• Fe(CO)5 evaporates at 103 oC and Ni(CO)4 at 43 oC.
35. Precipitate Formation:
This step of the process is carried out according to the
following scheme:
The liquid carbonyles are stored under pressure in tanks
submerged in water.
The distilled and filtered liquids are conveyed to steam
heating cylinders, where they are vaporized.
The vapors of liquid are sent to decomposers. The
decomposers are jacketed and heated, giving an internal
temperature of 200 – 250 oC. These cylinders are 9 – 10 feet
high with an internal dia of 3 feet, with conical bottoms.
The incoming stream of vapors meets a tangential stream of
ammonia gas. CO is removed here and precipitates of
metals are formed which are then sieved, dried and may be
milled to break up the agglomerates.
The CO gas arising from the decomposition is recovered
and re-used.
36. Carbonyl iron powder is used for the production of
magnetic powder cores for radio or television
applications.
In P/M it is used for the manufacture of soft
magnetic materials and permanent magnets.
Because of its high price and poor die filling
properties, it is not suitable for the manufacture of
sintered structural components.
The carbonyl process is also well suited for the
extraction of both metals from lean ores. The
process can be controlled so as to yield a spherical
metal powder.
37. Blending
• Thorough intermingling of different powders of
same composition or various grades of the same
powders
Mixing
• Intermingling of powders of more than one
material
• Blending & Mixing
– Preparation of alloys from elemental powders
– Production of dispersion strengthened alloys
– Production of porous bearings
– Electrical and magnetic materials
38. • Additives are added during blending
– Lubricants
– Binders
– Deflocculants
• Purpose of blending
– Uniform distribution of powder
• Problems faced
– Variation in particle size distribution
– Oxidation of powder
– Segregation of powder
39. • Excessive blending results in
– Segregation
– Change in powder characteristics
– Work hardening of particles reducing
compressibility
• Mechanism in mixing
– Convective mixing
– Diffusive mixing
– Formation of slip planes
• Wet mixing
• Dry mixing
40. Compacting(briquetting)
• Forms metal powder compacts of desired size
and shape with sufficient strength to withstand
ejection from the die and subsequent handling
up to the completion of sintering
• Techniques
– Pressureless shaping technique
– Cold pressure shaping technique
– Pressure shaping technique with heat
41. Pressure less shaping technique
• Loose sintering
• Slip casting
• Slurry casting
• Clay type moulding
• High velocity projection
42. Loose sintering
• Mechanical vibration in the mould followed by
heating to sintering temperature
• Simple and low cost
• Porosity varies from 50% to 90%
• Not suitable for complex parts
– Difficulty of part removal
– Flow characteristic of powder
– Shrinkage during sintering
• Die material – carbide, graphite, stainless steel, CI
• Poor dimensional accuracy
46. Isostatic pressing
• Pressure is applied in a flexible
mould.
• Glycerin, water, hydraulic oils,
gases, rubber or plastics etc.
• Even pressure & density
distribution
• Complex shapes
• Economical for very large products
• Excellent electrical properties
• Dimensional shrinkage is reduced
• Ductility is improved.
47. Powder rolling
• Uniformly dense
component
• High productivity
• Production of metallic
strips
• Porous material suitable for
fuel cell and alkaline
batteries
49. Pressure forming with heat
• Hot pressing
• Hot working
• Hot isostatic pressing
• Spark sintering
• Hot coining
50. Hot pressing
• applying pressure and temperature simultaneously
• compacting and sintering of the powder takes place at
the same time in a die.
• Fe and Brass powders
• Absorbed gases and volatile impurities are removed
• Increased die life
• Temperature gradient is reduced
• Low temp requirement
• Near net shape
51.
52.
53.
54. Sintering
• Imparts strength, densification and dimensional control
• Decrease in free energy due to decrease of surface area.
• 0.7 to 0.9 melting temperature of the major constituent
• Defined as the heating of loose or compacted aggregate of
metal powders below the melting temp with or without the
application of external pressure to transform it to a dense
material by interparticle bonding
• Progressive transition without melting
– Diffusion of particle to particle
– Formation of grain boundary
– Closing voids present in the green compact
55. Changes during sintering
• Dimensional changes
• Chemical changes
• Electrical property changes
• Phase changes
• Relief of internal stresses
• alloying
56. Stages in sintering
• Adhesion without shrinkage
• Densification and grain growth stage
• Final stage with closed pores or elimination of
the last isolated rounded pores
60. Adhesion mechanism
• Elementary bonding process
• No material transport
• Major contributor in initial stage of sintering
process
61. Material transport mechanisms
1. Recovery and recrystallisation
2. Plastic or viscous flow
3. Evaporation and condensation
4. Volume diffusion
5. Surface diffusion
6. Grain boundary diffusion mechanisms
• Depends on powder material, its
characteristics, sintering temperature and
atmosphere.
62. Recovery and recrystallisation
• Small displacement of atoms
• Observed in powders having severe lattice
distortions due to grain growth which occurs
along with densification.
• Stress removal promotes mutual bonding of
powders
63. Viscous flow
• Displacement of atoms over large distance
• Due to application of external pressure the
atoms flow under stress
• Assuming total surface tension of all pores is
equal to external pressure applied.
• Slow process.
64. Evaporation and condensation
• Not usually found in metal powders(
exception: chromium)
• NaCl
• Material transport through a gas phase.
• Evaporation of material from surfaces of
spheres and condensation in the neck
region.
• Higher Vapor pressure at convex surfaces
• Neck growth without shrinkage or
densification.
65. Volume diffusion mechanism.
• Most important mechanism
• Atom movement into vacant lattice sites
• Vacancy gradient occurs between
– Neck area and interior of the system
– Distorted and undistorted lattice
– Particle centers and mid point of contact with adjacent particles
• Vacancies diffuse from the neck area
• Atoms diffuse in opposite direction producing welding effect.
• Governed by
– geometrical arrangement of powder particles
– Type of vacancy sources and sinks
• Vacancy source- smaller pores and concave surfaces
• Vacancy sink- large pores, grain boundaries, flat or convex surfaces
• Occurs at high temp
66. Surface diffusion mechanism
• Movement of atoms on external surfaces
• Exchange of surface atoms and surface
vacancies
• Responsible for neck formation
• Significant for sintering fine powders at a
temp less than that of volume diffusion
• Shrinkage occurs
67. Grain boundary diffusion mechanism
• GB formed between adjacent particles in the original
sintered mass
• Vacancies are moving along with GB to external
surfaces
• Last stage of sintering
• Elimination of isolated pores.
• Atoms from external surfaces or locations along the GB
will diffuse towards the interior and this promotes
– Neck growth
– Rounding or shrinkage of the pore
• Found more effective with decreasing sintering temp.
68. PRE SINTERING
• PM parts will be hard after sintering process
• Machining will be difficult
• Parts are pre sintered at a lower temperature
than sintering temp
• Provides adequate strength for handling and
machining
• After machining sintering is done
• drilling is usually carried out in this method
• Pre sintering removes lubricants and powders
69. Sintering Atmosphere
• Sintering atmosphere selection depends on
– Characteristics of the material
– Properties desired in the sintered product
• Changes occurring prior to sintering
– Reaction of constituents among each other
– Reaction of atmosphere with material to be
sintered
– Reactions of atmosphere with furnace refractories
70. Sintering Atmosphere [contd].
• Functions
– Must prevent oxidation on the metal surface at
the sintering temp.
– Must avoid carburizing and decarburizing
reactions and nitriding conditions in certain
metals
– Must have tendency to reduce surface films such
as oxides on powder particles
– Must not contaminate the metal powder compact
at the sintering temperature
71. Sintering atmosphere types
• Reducing atmosphere
– Dry H2 or dissociated NH3
– Exothermic atmosphere having low or medium
carbon potential.
– Endothermic atmosphere enriched with
hydrocarbon gases to produce a carburizing
atmosphere
• Neutral atmosphere
• Oxidising atmosphere
72. FINISHING OPERATIONS
• Coining and sizing.
– These are high pressure compacting operations.
Their main function is to impart
• greater dimensional accuracy to the sintered part, and
• greater strength and better surface finish by further
densification.
73. • Forging.
– The sintered PM parts may be hot or cold forged
to obtain exact shape, good surface finish, good
dimensional tolerances, and a uniform and fine
grain size
– Forged PM parts are being increasingly used for
such applications as highly stressed automotive,
jet engine and turbine components.
74. • Impregnation.
– The inherent porosity of PM parts is utilized by
impregnating them with a fluid like oil or grease. A
typical application of this operation is for sintered
bearings and bushings that are internally
lubricated with upto 30% oil by volume by simply
immersing them in heated oil.
– Such components have a continuous supply of
lubricant by capillary action, during their use.
– Universal joint is a typical grease – impregnated
PM part.
75. • Infiltration.
– The pores of sintered part are filled with some low
melting point metal with the result that part's
hardness and tensile strength are improved.
– A slug of metal to be impregnated is kept in close
contact with the sintered component and together
they are heated to the melting point of the slug. The
molten metal infiltrates the pores by capillary action.
When the process is complete, the component has
greater density, hardness, and strength.
– Copper is often used for the infiltration of iron – base
PM components.
– Lead has also been used for infiltration of components
like bushes for which lower frictional characteristics
are needed.
76. • Heat Treatment.
– Sintered PM components may be heat treated for
obtaining greater hardness or strength in them.
– Carburizing
– Nitriding
– Carbonitriding
– temepering
77. • Machining.
– The sintered component may be machined by
turning, milling, drilling, threading, grinding, etc.
to obtain various geometric features.
• Finishing.
– Almost all the commonly used finishing method
are applicable to PM parts. Some of such methods
are plating, burnishing, coating, and colouring.
78. • Plating.
– For improved appearance and resistance to wear
and corrosion, the sintered compacts may be
plated by electroplating or other plating
processes.
– To avoid penetration and entrapment of plating
solution in the pores of the part, an impregnation
or infiltration treatment is often necessary before
plating.
– Copper, zinc, nickel, chromium, and cadmium
plating can be applied.
79. • Burnishing.
– To work harden the surface or to improve the surface
finish and dimensional accuracy, burnishing may be done
on PM parts.
– It is relatively easy to displace metal on PM parts than on
wrought parts because of surface porosity in PM parts.
• Coating.
– PM sintered parts are more susceptible to environmental
degradation than cast and machined parts.
– This is because of inter connected porosity in PM parts.
– Coatings fill in the pores and seal the entire reactive
surface.
• Joining.
– Electric resistance welding is better suited than oxy-
acetylene welding and arc welding because of oxidation of
the interior porosity.
– Argon arc welding is suitable for stainless steel PM parts.
80. Advantages of P/M for Structural Components:
These may be classified into two main headings;
(a) Cost advantages, and
(b) Advantages due to particular properties of sintered components.
Cost Advantages:
(i) Zero or minimal scrap;
(ii) Avoiding high machining cost in mass production as irregularly
shaped holes, flats, counter bores, involute gear teeth, key-ways
can be molded into the components;
(iii) Extremely good surface finish at very low additional cost after
sizing and coining;
(iv) very close tolerance without a machining operation;
(v) Assembly of two or more parts (by I/M) can be made in one
piece;
(vi) Separate parts can be combined before sintering.
(vii) High production rates
81. Advantages due to the particular properties of
sintered components.
(i) By achieving up to 95% density, the mechanical and
physical properties are comparable with cast materials
and in certain cases with wrought materials. In certain
cases 99.9 % dense structure can be obtained (liquid
phase sintering);
(ii) Platting is also possible directly at 90% density and
above and after impregnation of the pores at lower
densities.
(iii) Damping out vibrations and noise property with
controlled residual porosity;
(iv) Ability to retain lubricants such as lead, graphite and
oil giving less wear and longer life to bearings;
(v) Achieving a close control of porosity to give a
specified balance between strength and lubrication
properties (a superiority over wrought materials);
82. (i) Improved surface finish with close control of
mass, volume and density;
(ii) Components are malleable and can be bent
without cracking.
P/M makes possible the production of hard tools
like diamond impregnated tools for cutting
porcelain, glass and tungsten carbides.
Reactive and non-reactive metals (both having
high m.p &low m.p) can be processed.
