Unit 5.ppt

Unit V:
Powder Metallurgy and
Non Ferrous Metals and
Alloys

In this unit we are going to study:
Steps in the making of Powder metallurgical
Component
Advantages and limitations of powder
metallurgy
Production of sintered structural
components:
Self lubricated bearing
Cemented carbides
Unit 5: Powder Metallurgy and Non
Ferrous Metals and Alloys

Production of sintered structural
components:
 Cermets
 Refractory metals
 Electrical Contact Material
 Friction Material
 Diamond Impregnated Tools
Non ferrous alloys
 Copper and its alloys
 Aluminium and its alloys
 Babbits
Unit 5: Powder Metallurgy and Non
Ferrous Metals and Alloys

Powder metallurgy is a branch of metallurgy
which deals with the production of metal and
non-metal powders and subsequently
manufacturing of components by using these
powders.
Powder metallurgy (P/M) components are
manufactured by mixing of metal or metal and
non metal powders, compacting with
simultaneous or subsequent heating at elevated
temperatures using a controlled atmosphere to
develop metal or metal like components with
satisfactory strength and density.
Unit 5: Powder Metallurgy

Steps involved in manufacturing powder
metallurgical component
Powder production
Blending or Mixing
Compacting (i.e. Pressing)
Sintering
Sizing or Impregnation
Testing and inspection
Powder Metallurgy

Powder Metallurgy

Powder production
Blending or Mixing
Compacting (i.e. Pressing)
Sintering
Sizing or Impregnation
Testing and inspection

Powder Production
 Mechanical:
 Machining
 Crushing
 Milling
 Shotting
 Automization
 Physical:
 Condensation
 Thermal decomposition
(or Gaseous Pyrolosis)

Powder Production
 Chemical
 Reduction
 Intergranular corrosion
 Precipitation from aqueous solutions
 Electro-chemical.

 To make a homogeneous mass with uniform distribution of
particle size and composition
◦ Powders made by different processes have different sizes
and shapes
◦ Mixing powders of different metals/materials
◦ Add lubricants (<5%), such as graphite and stearic acid, to
improve the flow characteristics and compressibility of
mixtures
 Combining is generally carried out in
◦ Air or inert gases to avoid oxidation
◦ Liquids for better mixing, elimination of dusts and reduced explosion
hazards
 Hazards
◦ Metal powders, because of high surface area to volume ratio are
explosive, particularly Al, Mg, Ti, Zr, Th
Blending and Mixing

Some common equipment geometries used for blending powders
(a) Cylindrical, (b) rotating cube, (c) double cone, (d) twin shell
Blending and Mixing

Compaction
 Press powder into the desired shape and size in dies
using a hydraulic or mechanical press
 Pressed powder is known as “green compact”
 Stages of metal powder compaction:

 The purpose of compacting:
To consolidate the powder into the desired shape
To impart the desired level and type of porosity and
To provide adequate strength for handling
Compaction

 Increased compaction pressure
Provides better packing of particles and leads to
↓ porosity
↑ localized deformation allowing new contacts to
be formed between particles
Compaction

 At higher pressures, the green density approaches density of
the bulk metal
 Pressed density greater than 90% of the bulk density is
difficult to obtain
 Compaction pressure used depends on desired density
Compaction

Sintering
Sintering is carried out to increase strength and
hardness of a green compact and consists of
heating the compact to some temperature under
controlled conditions with or without pressure for
a definite time.

The possible diffusion mechanisms are
Surface diffusion
Volume diffusion
Grain boundary diffusion
Evaporation and condensation
Sintering

 Promotes solid-state
bonding by diffusion.
 Diffusion is time-
temperature sensitive.
Needs sufficient time
Sintering

 Promotes vapour-
phase transport
 Because material
heated very close
to MP, metal
atoms will be
released in the
vapour phase
from the particles
 Vapour phase
resolidifies at the
interface
Sintering

Applications of PM
Automotive Applications

Applications of PM
High Temperature Applications
Aerospace Applications
Atomic Energy Applications
Other Applications

Advantages of Powder Metallurgy
 Metal plus metal components can be
manufactured by P/M.
There is almost no need of referring to their
equilibrium or phase diagrams. Components of any
desired composition can be manufactured.
 Metal plus non-metal components can be
manufactured
This is quite impossible to manufacture by the usual
methods.
 Controlled porosity can be obtained in the
components
This is essential for certain applications like liquid and
gas filters, self-lubricating bearings and insulating
bricks.

 It is possible to produce components with
properties similar to the parent metals
Whereas, if the components are manufactured by melting, the
alloy may have different properties from their parent metals.
 Production of refractory metals like W, Mo, Ti, Th,
etc.
This is possible without melting e.g. manufacture of ductile
tungsten in wire form for incandescent lamp filaments.
 Components from metals which are completely
insoluble in the liquid state can be manufactured
with uniform distribution of one metal into the
other.
However, if they are manufactured by melting and casting, the
distribution of one phase into the other is non uniform.

 Manufacture of cemented carbide cutting tools is
only possible by P/M.
The melting points of the carbides which are used for the
manufacture of these cutting tools are extremely high and
hence melting is not possible.
 Composite and dispersion hardened materials can
be manufactured
e.g. cermets and thoria dispersed tungsten filaments.
 There is a little chance for contamination of metal
powders during processing by P/M
The purity of the component remains the same as the original
purity of metal powders.

 P/M parts may be welded, brazed, machined, heat
treated, plated or impregnated with lubricants or
other materials.
 Close control over the dimensions of the finished
component can easily be obtained.
 No machining or minimum machining is required
and hence the scrap is minimum. This gives yield of
over 99.0%.
 Fast production of simple shaped components is
possible due to lesser number of steps involved in
P/M .

Limitation of Powder Metallurgy
 Fire Hazards
Most of the powders used in P/M are fine and fine powders of
some of the metals like Mg, Al, Zr, Ti, etc. are likely to explode
and cause fire hazards when they come in contact with air and
hence, they should be preserved carefully.
 Oxidation
Other metal powders are also likely to get oxidised slowly in air
and hence, they must be stored properly to avoid their
deterioration.
 High Capital Cost
It is not suitable to manufacture small number
of components because of high initial
investment on tooling and equipment.

 Large sized components
Large sized components can not be manufactured because of
the limited capacity of presses available for compaction.
 Complex shaped parts
Complex shaped parts can not be manufactured with ease by
P/M.
 Corrosion resistance
P/M parts have poor corrosion resistance because they
are porous. Due to this porosity, large internal surface
area gets exposed to corrosive environment

 Porosity
Due to the presence of porosity, mechanical properties such
as ductility, U.T.S. and toughness are poor as compared to
components manufactured by conventional methods. The
surface finish is also poor.
 Components with theoretical density can
not be manufactured.

Powder Production
 Mechanical:
 Machining
This method is used to produce filings, turnings, chips,
etc. which are subsequently pulverized by crushing and
milling.
Relatively coarse powders are obtained
The powder particles are of irregular shape
 Crushing
The solid materials are crushed by hammers, jaw
crushers, gyratory crushers, etc.
The powder particles of brittle materials are angular in
shape and ductile materials are flaky in shape.
Any material can be crushed to powder form; however,
the method is very much suitable for brittle materials.

Powder Production
 Mechanical:
(a) Roll crusher (b) Ball mill

Powder Production
 Mechanical:
Milling
 Milling is the most important and widely used
method for the production of powders of
required grade and fineness.
 Milling is done by using equipments such as ball
mills, rod mills, disk mills, eddy mills etc.
 In the ball milling method, the material to be
powdered is tumbled or rotated in a container
with large number of hard balls.
 The balls are made of steel, alloy steel, or white
cast iron.

Powder Production
 Mechanical:
Milling
 All the above three methods have low rates of
powder production.
 The particle shape is neither perfectly irregular
nor spherical but is intermediate to the above
two.
 This shape is suitable for the manufacture of
large number of components by P/M.
 The powders obtained from these methods are
in work-hardened condition and hence require
annealing heat treatment prior to their
compaction.

Powder Production
 Mechanical:
Shotting
 In this method, molten metal is poured on a
vibrating screen and the liquid droplets are
solidified either in air or a neutral gas.
 The size and character of the powder
depends on the temperature of molten
metal, size of openings in the screen and
frequency of vibrations of the screen.
 Shape of particles is nearly spherical.

Powder Production
 Mechanical:
Graining
 Graining involves the same procedure as the
shotting, the only difference being the
solidification of molten metal droplets is
done in water.
 The powders obtained by shotting and
graining methods are coarse and
subsequently other pulverization methods
are used for further reduction of size.

Powder Production
 Mechanical:
Automization
 Produce a liquid-metal stream by injecting
molten metal through a small orifice
 Stream is broken by jets of inert gas, air, or water
 The size of the particle formed depends on:
Temperature of the metal
Metal flowrate through the orifice
Pressure of jet
Nozzle size and jet characteristics

The process consists of main three stages
 Melting
 Atomization
 Solidification and cooling
Melting is done by induction, arc, plasma
or electron-beam technique to maintain purity
of melt.
 Atomization is done by high velocity water,
compressed air or inert gas.
 The disintegrated particles are solidified in
controlled atmosphere, vacuum , air or water.
Powder Production

Main two types of Automization Techniques:
Water Atomization
Gas Atomization
Powder Production

Gas Atomisation
Compressed air, nitrogen, argon or helium are used for
disintegration.

Water atomization technique for production of
powders
Water Automization

Powder Production
 Mechanical:
Automization

Powder Production
 Mechanical:
Automization
 Atomization is the method most frequently used for
metals having low melting points, such as tin, lead, zinc,
cadmium, and aluminum.
 Atomized products are generally in the form of sphere-
shaped particles.
 A wide range of particle-size distributions may be
obtained by varying the temperature of the metal,
pressure and temperature of the atomizing gas, rate of
flow of metal through the orifice, and the design of the
orifice and nozzle.
 The principal advantage of the atomization process is its
flexibility.

Fe powders made by atomization
Powder Production

Powder Production
 Physical:
Condensation
 In this method, metal vapours are
condensed to obtain metal powders.
 This method is highly suitable for
volatile metals because they get easily
transformed to their vapours.
 Large quantities of Zn, Mg and Cd
powders are manufactured by this
method.
 The powder shape is nearly spherical.

Powder Production
 Physical:
Thermal decomposition (or Gaseous Pyrolosis)
 Fine metal powders of some metals like Fe, Ni, W,
Mo, Co, Mg, etc. are manufactured by thermal
decomposition of their respective carbonyl vapours.
 However, the method is highly suitable for the
manufacture of Fe and Ni powders.
 Fe aid Ni carbonyls are produced by passing CO over
a spongy or powdered metal at some suitable
temperature (200 to 270°C) and pressure (70 to 200
atmospheres) as shown below:

Powder Production
Thermal decomposition (or Gaseous Pyrolosis)
These carbonyls are volatile liquids and their
vapours decompose at one atmospheric pressure
and temperature of 150 to 400°C, as shown
below

Powder Production
 Chemical
Reduction
 The largest volume of metalIurgical powder made by
the process of oxide reduction.
 Reduce metal oxides with H2/CO
 Powders are spongy and porous and they have
uniformly sized spherical or angular shapes which are
ideal for compacting
 The reduced powder is subsequently ground.
 The nature, particle size, and distribution of the raw
material and the conditions of reduction greatly
influence the form of the deposited particles.
 If the oxide powder is graded before reduction, a high
degree of size uniformity can be obtained in the
reduced powder.

Powder Production
 Chemical
Reduction
 This is the only practical method available for
producing powders of the refractory metals
such as W and Mo.
 Oxide reduction method is also an economical
method of producing powders of iron, nickel,
cobalt, and copper.

Powder Production
 Chemical
Intergranular corrosion
 It is a fact that the grain boundaries of any metal
corrode faster than the grains.
 In this method, grain boundary area of the metal
under interest is corroded by a suitable
electrolyte so as to separate out the grains from
the polycrystalline metal.
 The powder of stainless steel is made by this
process.

Powder Production
 Chemical
Precipitation from aqueous solutions
 This process is based on the principle that a less noble
metal displaces a more noble metal from an aqueous
solution containing the ions of more noble metal
 For example silver is displaced from a silver nitrate
solution by Cu or Fe, and Sn is displace from a
stannous chloride solution by Zn.
 The displaced metal separates in the form of powder.
 This method is commonly used for the production of
copper powder.
 Purity of the powder is excellent and the particle shape
is dendritic.

Powder Production
 Electro-chemical
 Metal powder deposits at the cathode from
aqueous solution
 Powders are among the purest available (99.99%)
 The particle shape is dendritic
 Method is generally used to manufacture powders
of Cu, Be, Fe, Zn,Sn,Ni, Cd, Ag etc
 The conditions which favour the powder formation
on cathode are:
1. High current density
2. Low metal ion concentration
3. High acidity
4. Low temperature

Characterization and Testing of Powders
Powder Characteristics:
 Chemical composition
 Porosity & Microstructure.
 Shape, size and distribution.
 Flow rate
 Specific surface
 Density

 Chemical composition
Chemical composition and impurities in
metal powders are determined by
standard techniques of chemical analysis
such as gravimetric, volumetric,
colourometric, etc.; or they can be
determined by spectroscopy.
The chemical composition and impurities
strongly influence pressing and sintering
characteristics.

 Porosity & Microstructure
→ For the determination and observation of
these properties, microscopy is used.
→ The powder is mounted in some suitable
medium for observation under microscope.
→ Depending on its suitability, either hot
mounting or cold mounting method is used.

 Porosity & Microstructure
(1) Hot mounting :
The metal powder in small quantity is mixed with bakelite
powder, mounted using a standard specimen mounting
press, polished carefully, etched in a suitable etchant,
washed with water and alcohol, and dried using blast of
hot air and examined under microscope.

Porosity & Microstructure
(2) Cold mounting :
→Small quantity of metal powder is mixed with some
suitable polymeric liquid and hardener.
→This medium is poured in a steel tube of a suitable size.
→This liquid from the medium polymerises and becomes
hard in a period of 10 to 15 min.
→ Subsequently the sample is removed and polished,
etched, washed, and examined under the microscope.

Particle Shape
The typical shape of powder are dendritic, acicular, fibrous,
flaky, shperoidal, granular as shown

Particle Size
Powder size is classified as fine powder and coarse
powder. Powder size is also determined by
microscope.
Usual particle size range of powders used in P/M is
between 1 to 1000 microns

Particle Size Distribution
Powder particle size distribution is classified
as wide distribution and narrow distribution.
Powder particle size distribution can be
measured by using one or more of the
following methods:
Sieve method
Microscopic method
Sedimentation method time
Elutriation method

Particle Size Distribution
Sieve method
Standard sieves of different mesh numbers are
used for this purpose.
The opening of a screen is expressed by the
number of meshes per linear inch.
Different sieves are arranged one below the
other as per their mesh numbers, the coarsest
being at the top.

Sieve method
100 gm of metal powder is placed on the top
sieve and the entire stack of sieves is vibrated for
15 minutes by a standard shaking machine which
gives circular and translatory motions to the
screens.
After this, the amount of powder retained on
each sieve is accurately measured.
From these weights, size and size distribution can
be found out.
Sieve method gives fairly accurate result when
the powder is in the size range of 44 to 840
microns

Microscope method
 Optical and electron microscopes are used for
measurement of size and distribution of
particles.
 Optical microscopes can magnify up to 2000 X
and electron microscopes up to 5,00,000 X.
 Optical or electron microscopy can be used for
the measurement of particle sizes above 0.1
micron; and electron microscopy is used for
particle sizes of smaller than 0.1 micron (1
micron = 10-3 mm) .

Sedimentation method
 In this method, classification on the basis of size
and size distribution of powder particles is done
according to their settling velocities in a fluid.
 Settling velocity v is given by
where ρs density of solid particle, ρf density of fluid,
D is diameter of spherical particle, η viscosity of fluid



18
)
( 2
gD
v
f
s 


 This involves suspending a small quantity of
powder sample in a fluid medium and allowing
the particles to settle for a suitable time.
 Settling velocity of a spherical particle is
proportional to the square of the particle
diameter and hence particles of equal sizes can be
separated on the basis of equal settling velocities.
 This is done by measuring the amount of particles
settling at different intervals of time.

 For irregularly shaped particles, particle size is
assumed to be the same as that of spherical
particle having the same settling velocity as that
of irregularly shaped particle under similar
conditions of testing.
 These methods are suitable for the measurement
of particle sizes in the range of 0.05 to 50 microns.

 This method is used for determination of size
distribution of fine particles.
 Here, the metal powder is allowed to settle in a
moving liquid or gas of a constant velocity.
 The particle with settling velocity of less than the
velocity of rising fluid will be carried upwards and
those with higher settling velocities will settle at
the bottom.
 By altering the velocity of medium, the particles
can be separated according to their sizes.

 This is a fractioning method and is used for
determination and separation of size fractions of
the powder.
 The method is suitable for powder sizes in the
range of 5 to 100 microns.

Particle Shape, size and distribution
Influence on the compacting and sintering
operations:
 The compacting and sintering characteristics are strongly
influenced by the contact area between the metal
particles.
 Dendritic, acicular, fibrous and flaky shape particles give
excellent compacting and sintering properties because of
better plastic deformation and mechanical interlocking.
They give high green density and green strength.
 On the other hand spheroidal and granular shape shows
poor compacting and sintering properties because of poor
mechanical interlocking.
 Fine particles have large surface area and give high
sintering rate which reduces the time of sintering.

operations:
 However, this results in the entrapment of air and other
gases. Due to this, the compact is likely to crack either
before or during sintering.
 Coarse particles have smaller surface area and hence
sintering characteristics are poor, and also these
components show poor mechanical properties.
 Wide distribution gives good green density and strength
due to “bridging effect” as shown in fig.
 Narrow distribution gives comparatively lower green
density and green strength.

operations:
Illustration of the” bridging effect” caused by small particles

Flow rate of powder
◦ The flow rate is a very
important characteristic
of powders which
measures the ability of a
powder to be transferred.
◦ It is defined as the rate at
which a metal powder will
flow under gravity from a
container through an
orifice having a specific
shape and size.
◦ Such an apparatus which
is used to determine flow
rate is called flow meter.

Flow rate of powder
 Flow meter consists of a standard and
accurately machined conical funnel made of
brass with smooth surface finish having an
internal angle of 60°.
The orifice situated at the bottom of the
funnel is either 1/8” (for ferrous powders) or
1/10” in diameter (for non-ferrous powders)
and has a length of 1/8”.
The time required to flow 50 gm powder from
this funnel is measured and reported as flow
rate in gms/minute.

Flow rate depends on
Particle shape size and size distribution
Amount of absorbed gases
Amount of moisture
Coefficient of friction.
In general, fine or irregular particles have poor
flowability and coarse or spherical particles have
better flowability.
Flow rate increases with decreased particle
irregularity and increased particle size.

Flow rate depends on
operations:
High the flowability, rapid is filling of the die
and uniform density of the cold compact.
There is a close relationship between
apparent density and flowability.

Specific surface :
It is defined as the total surface area of
a powder per unit weight (cm2/gm).
It depends on size, shape, density and
surface conditions of the particles.
It is evaluated either by permeability
method or adsorption method.

Specific surface :

Specific surface :
Permeability method
Here a fluid is passed with constant pressure through a
bed of packed powder contained in a chamber and the
pressure drop across the bed of the powder is measured.
From the observed drop of pressure, specific surface Sv is
calculated by :
where ε is porosity of powder bed, k is aspect factor, η is
viscosity of fluid,Δp is pressure drop and v is flow rate
V
p
L
A
k
Sv





 1
1
1
2

Specific surface :
operations:
 Fine particles have large specific surface area
and give high sintering rate which reduces the
time of sintering.
 Coarse particles have smaller specific surface
area and hence sintering characteristics are
poor, and also these components show poor
mechanical properties

Density:
(A)Apparent density :
The apparent density (or packing density) of a powder is
defined as the mass per unit volume of loose or unpacked
powder.
(B) Tap density:
The tap density is the apparent density of the powder
after it has been mechanically shaked or tapped until the
level of the powder remains constant.
This has a similar effect as apparent density on pressing
characteristics.
Apparent density is measured by using a standard
flowmeter funnel or volumeter, and tap density by Ro tap
machine.

Density:
operations:
 The lower the apparent density, the longer will be
the compression stroke and deeper dies will be
required to produce a compact of given thickness
and density.
 Higher the apparent density, higher is green
density and green strength and faster is sintering
process.

Characteristics of Compact
Compact Characteristics:
 Compressibility
 Compactibility
 Green Density
 Green Strength
 Green Spring

Compressibility
Compressibility is defined as the powders ability
to deform under applied pressure
It is measured as:
1. Ratio of the green density of compact to the
apparent density of powder
2. Ratio of the height of the uncompacted powder in
die to the height of the pressed compact
3. Ratio of the volume of powder poured into die to
the volume of the pressed compact

Compactibility
Compactibility is defined as the minimum
pressure required to produce a compact of given
green strength
Green Density
It is the density of a cold compact
Green density= Weight of compact/Volume of compact

Green Density
Green density increases as
1. Increase of compacting pressure
2. Increase of particle size
3. Increase of apparent density
4. Decrease of particle irregularity
5. Decrease of particle hardness
6. Decrease of compacting speed

Green Strength
 It is the mechanical strength of a green compact
 It is measured by transverse rapture test
 The strength is developed mainly due to cold
welding and mechanical interlocking of particles
Green strength depends on
1. Size ,shape and distribution of powder
2. Surface condition
3. Hardness and strength of powder
4. Pressure applied during compaction

Green Spring
 The compact expand as soon as they are ejected
out of the die cavity and this effect is called as
Green spring
 In general, the green spring amount to about
0.2% on the diameter and 0.5% on the length
side
Green spring depends on
1. Compacting pressure
2. Elastic recovery of the tools
3. Die design

Characteristics of Sintering
Sintering Characteristics:
 Dimentional change
 Density and porosity
 Mechanical Properties
 Microstructure

 Dimentional change
 %shrinkage=change in length/sintered length x 100
 %shrinkage=change in length/unsintered length x 100
First equation is used in carbide industries and second
used in bearing and other industries
 Density and porosity
By measurement of density, the total amount of porosity
can be calculated as
Where ρ is fractional porosity, ρv is the density of sintered
component and ρs is density of solid material
s
v


 
 1

 Mechanical Propertises
 Comptressive strength
 Hardness
 Y.S.
 UTS
Mechanical properties are determined by using
appropriate method of testing

Manufacturing of Typical P/M Components
 Oil Impregnated Porous Bearings
(Self Lubricating Bearings)
 Cemented Carbides
 Cermets
 Diamond Impregnated Tools
 Refractory Metals
 Electrical Contact Materials

Oil Impregnated Porous Bearings
 Controlled porosity of powder metal parts has led to
the production of ‘Oil Impregnated Porous Bearings’
(Self-lubricating bearings).
 Self-lubricating bearings are made of bronze, brass,
iron or aluminium alloy powders with or without
graphite.
 However, bronze bearings are widely used and are
made from Cu and Sn (90 : 10) with addition of
graphite. Graphite increases porosity and also
improves pressing characteristics.

These bearings must have the following
characteristics for their efficient working:
 Sufficient porosity (30 to 50%) to retain the
maximum possible amount of oil
 Inter-connected porosity in the largest
proportion and should be uniformly
distributed throughout the material
 Sufficient strength to sustain the loads
 Good dimensional accuracy

 When the porosity is more, the strength of the bearing
is less.
 Such bearings have more oil retaining capacity but less
load sustaining ability and hence they are suitable for
high speeds and low load conditions.
 When the porosity is less, the strength of the bearing
is more.
 Such bearing have less oil retaining capacity but more
load bearing ability and hence they are widely used for
low speeds and high load conditions.

The working of the bearing is as below :
 As the speed of shaft increases, the temperature of
bearing rises due to frictional heat.
 This results in decrease of viscosity and increase in
volume of the oil.
 Due to this, the oil is pulled out from the pores and
gets rapidly circulated along with the rotating shaft.
 With decrease of shaft speed, pressure decreases and
temperature also decreases; and due to this, the oil
goes back to pores by capillary action.
 There is no wastage of oil and working of the bearing
is smooth and silent.

The working of the bearing

The steps in the production of a porous bronze
bearing are as below:
(1) Mixing
(2) Cold compaction
(3) Sintering
(4) Repressing (i.e. Sizing) or Machining and
(5) Impregnation

(1) Mixing:
Metal powders of Cu and Sn with small amount of fine
natural graphite are blended or mixed to obtain the
desired alloy composition (90 Cu: 10 Sn).
(2) Cold compaction:
These powders are cold compacted at pressures between
20 to 50 kg/mm2 to form green compacts of desired
shape and size.

(3) Sintering:
These compacts are sintered in a reducing atmosphere at
a temperature of about 800°C.
A typical sintering cycle consists of holding the compact
at 400-450°C for the removal of part of the graphite and
diffusion of molten Sn into the copper, followed, by
further heating to 800°C for periods as short as 5
minutes.
At this temperature, a tin-rich liquid phase is formed
which is absorbed by the copper.

(4) Repressing (i.e. Sizing):
Distortions occurring during sintering can be eliminated
by repressing (i.e. sizing) or machining.
If the pore size is large, sizing can be done and if it is
small, machining should be done.
For small pore sizes, sizing should not be done because it
may result in closure of pores.
(5) Impregnation:
The repressed or machined components are impregnated
with cold or hot oil using pressure, vacuum or a
combination of these.

Applications:
 These bearings find applications in places which are
inaccessible or difficultly accessible. These are the places
which are impossible or difficult for regular lubrication.
 They are also used in certain applications where it is
desirable that the oil should not come in contact and
contaminate the products (e.g. in food and textile
industries)
 Self-lubricating bearings are used extensively in the
automotive industry and in washing machines,
refrigerators, electric clocks, and many other types of
equipment.

Cemented Carbide Tools
 These are important products of P/M and find
wide applications as cutting tools, wire drawing
and deep drawing dies, drills and stone working
tools.
 They are manufactured from carbides of
refractory metals such as W, Mo, Ti, Ta or Nb.
 These carbides are extremely hard (hardness
more than 3000 VPN) and retain their hardness
upto a very high temperature.
 However they are extremely brittle and hence
are likely to fail with slight shock loading.

 To increase their shock resisting ability, metals
such as Co, Ni, Cr or alloys of Co —Cr or Co —
Ni — Cr are used upto 20 % and the processing
is done by P/M.
 The hard carbide powders are bonded or
cemented together by these metals or alloys.
 For most of the common applications,
carbides of W and Mo are used and the binder
is Co.

 The steps in the manufacture of cemented carbides
are as below:
(i) Powder manufacture :
 Carbide powders of the refractory metals are
produced either from their respective oxides or
metals.
 Metal oxides can be reduced to metals by carbon or
hydrogen and subsequently the metals can be
converted to carbides by direct reaction with the
carbon, or the metal oxides can be directly converted
to carbides in a single step by reaction with carbon.
 Co powder is obtained by the reduction of the oxide
or oxalate by H2 at temperatures of 600 to 700°C

(ii) Milling:
 Carbide powders are mixed in the required
proportion along with the powder of metallic
binder by a wet mixing method.
 Lubricants such as paraffin wax dissolved in
petrol, camphor in ether or light
hydrocarbons, and glycerine in alcohol are
mixed to these powders just prior to
compaction which facilitates pressing and
avoids defects and cracks in the compacts.

(iii) Cold pressing and sintering :
 This mixture is compacted at a pressure of 35 to 45 kg/mm2 and the
compacts are heated to about 400°C for a sufficient period to remove the
lubricant by volatilisation.
 Sintering of these compacts is carried out in two stages. The preliminary
sintering is done using H2 atmosphere at a temperature between 900 to
1150°C.
 This is done to impart sufficient strength to the compacts.
 At this stage, they can be machined or cut to a shape and size to obtain
exact dimensions of the component after final sintering.
 Final sintering is done in the temperature range of 1350 to 1550°C for
about two hours using H2 atmosphere or vacuum.
 The liquid phase formed at this temperature binds the particles and hence
the name is cemented carbides.
 During this stage of sintering, large amount of shrinkage occurs in the
component and hence to obtain the final dimensions within tolerance
limit, the component must have oversized dimensions before sintering.

(iii) Machining :
 It should be done to the extremely close tolerances and is
done in two steps.
 First it is ground rough using silicon carbide grinding
wheels and finally with metal bonded diamond wheels.
 Electrospark or ultrasonic machining has also been used
for threading, boring or engraving of these cemented
carbide components.

Blending or Mixing of powder
 Blending is defined as the
intermixing of powders of
the same nominal
composition.
 It is used to achieve a
desired particle size
distribution. Mixing
implies intermingling
powders of different
chemical composition.

Que. Explain why P/M is indispensable for
manufacturing of certain components.
P/M is indispensable for manufacturing of certain
components like refractory metals, cemented carbide,
ceramics self lubricating bearing, liquid/gas filters
insulating bricks etc .
The melting points of the refractory metals, carbides,
ceramics are extremely high and hence conventional
casting process is not possible. In P/M, powders of these
components are manufacture, blended and
subsequently compacted and sintered at elevated
temperature but lower than melting point.
Controlled porosity is essential for applications like liquid
and gas filters, self-lubricating bearings and insulating
bricks. The amount of porosity along with control over
size, shape and distribution of pores can be obtained to
achieve the desired properties in the component only by
P/M. This is not possible by any of the usual techniques.

Cu-Zn System (Brass)
Cartridge brass:
70 wt% Cu

Unit 5.ppt

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Unit 5.ppt