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INTRODUCTION
Earliest use of iron powder dates back to 3000 BC.
Egyptians used it for making tools
Modern era of P/M began when W lamp filaments were
developed by Edison
Components can be made from pure metals, alloys, or
POWDER METALLURGY mixture of metallic and non-metallic powders
Commonly used materials are iron, copper, aluminium,
nickel, titanium, brass, bronze, steels and refractory
metals
Used widely for manufacturing gears, cams, bushings,
cutting tools, piston rings, connecting rods, impellers etc.
PROCESS
Powder production
Blending
Compaction
Sintering
Finishing/secondary operation Operations
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1. POWDER PRODUCTION
Atomization
Machining
Reduction
Crushing and milling
Electrolytic deposition
Shotting
Granulation
Condensation of metal powder
Thermal decomposition
Atomization
Produce a liquid-metal Variation:
stream by injecting
A consumable electrode is
molten metal through a
rotated rapidly in a helium-
small orifice
filled chamber. The
Stream is broken by jets centrifugal force breaks up
of inert gas, air, or water. the molten tip of the
electrode into metal
Standard dust collector is particles.
used to collect the
powder.
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Reduction
Reduce metal oxides with H2/CO at elevated temperature.
The pure metal is then crushed and milled to a powder.
Powders are spongy and porous and they have irregular
shape
Fe powders made by atomization Ni-based superalloy made by
the rotating electrode process
Electrolytic deposition Thermal Decomposition
Metal powder deposits at the cathode from React high purity Fe or Ni with CO to form
aqueous solution . gaseous carbonyls
Powders are among the purest available Carbonyl decomposes to Fe and Ni
Small, dense, uniformly spherical powders
of high purity
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Crushing and milling
Milling in a ball mill
Powder produced
Brittle: Angular
Ductile: flaky and not particularly suitable for P/M
operations
Mechanical Alloying
Powders of two or more metals are mixed in a ball mill
Under the impact of hard balls, powders fracture and join
together by diffusion
(a) Roll crusher, (b) Ball mill
shotting
molten metal is poured through a sieve or
orifice and cooled by dropping into the
water .
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Granulation Metal powder Characteristics
Molten metal is converted into small 1. Surface area
particles by rapidly stirring the metal while
it is cooling.
Metal powder Characteristics Metal powder Characteristics
2. Density 3. Compressibility and compression ratio
True Density
Apparent density:- depends on particle size Its defines as the volume of initial powder
is defined as the ratio of volume to weight (Powder loosely filled in cavity) to the
of loosely filled mixture. volume of compact part. Depends on
Tap Density particle shape & size distribution.
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Flow rate 5. Particle shape, size and distribution
Shape:
It is influenced by the way its made. The shape
The ability of the powders to flow readily and may be spherical (atomization) (Electrolysis) flat
or angular (Mechanical crushing). The particle
conform to the mould cavity. The flow rate
shape influences the flow characteristics of
helps to determine to possible production powders.
rate. Particle Size (Fineness) and size distribution:
Particle Size and Distribution are important
factors which controls the porosity,
Compressibility and amount of shrinkage. Proper
particle size and size distribution are determined
by passing the powder through a standard sieves
ranging from 45 to 150 micrometer mesh.
BLENDING
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 Some common equipment geometries used for blending powders
Metal powders, because of high surface area to volume ratio are (a) Cylindrical, (b) rotating cube, (c) double cone, (d) twin shell
explosive, particularly Al, Mg, Ti, Zr, Th
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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:
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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Smaller particles provide greater strength mainly due to
At higher pressures, the green density approaches reduction in porosity
density of the bulk metal Size distribution of particles is very important. For same
Pressed density greater than 90% of the bulk density is size particles minimum porosity of 24% will always be
difficult to obtain there
Compaction pressure used depends on desired density Box filled with tennis balls will always have open space between
balls
Introduction of finer particles will fill voids and result in density
Because of friction between (i) the metal particles and (ii)
between the punches and the die, the density within the
compact may vary considerably
Density variation can be minimized by proper punch and
die design
A 825 ton mechanical press for compacting metal powder
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Compacting Methods 1. Pressing
1. Pressing
2. Centrifugal Casting
3. Slip Casting
4. Extruding
5. Gravity Casting
6. Rolling
7. Iso-static Moulding
8. Explosive Compacting
9. Fibre Metal processes
Different die punch systems
(a) and (c) Single action press; (b) and (d) Double action press (a) Compaction of metal powder to form bushing
(e) Pressure contours in compacted copper powder in single action press (b) Typical tool and die set for compacting spur gear
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2. Centrifugal Casting 2. Centrifugal Casting
4. Slip Casting Extruding
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Rolling:
Cold Iso-static Pressing
Metal powder placed
in a flexible rubber
mold
Assembly pressurized
hydrostatically by
water (400 1000
MPa)
Explosive Compacting
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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
Carried out in three stages:
First stage: Temperature is slowly increased so that all
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
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Second stage: High temperature stage
Promotes solid-state
bonding by diffusion.
Diffusion is time-
temperature sensitive.
Needs sufficient time
Third stage: Sintered product is cooled in a controlled
atmosphere
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:
Nature of alloy, molten component/particle wetting,
capillary action of the liquid
HOT ISOSTATIC PRESSING (HIP)
Produces compacts with almost 100%
density
Good metallurgical bonding between
particles and good mechanical strength
Uses
Superalloy components for aerospace
industries
Final densification step for WC cutting tools
and P/M tool steels
Steps in HIP
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Secondary operations 2. Coining:
1. Sizing: Repressing the sintered component in a die to
Repressing the sintered component in a die to increase the density and to give additional
meet required tolerances. strength.
5. Machining:
Removing excess material by using cutting
tool to imparts specific features such as
3. Infiltration: Threads, Grooves, Undercuts etc, which are
Filling the pores of sintered product with not practicable in powder metallurgy process.
molten metal to improve the physical
properties.
4. Impregnation:
Filling of Oil, Grease or other Lubricants in a
Sintered components such as Porous Heating
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6. Heat Treatment Applications of PM
Process of Heating & Cooling at a desired rate
to improve Grain Structure, Strength &
Hardness.
Applications of PM Applications of PM
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Applications of PM Applications of PM
Abrasives (Grinding and Polishing wheels
and Discs);
Electrical, Electronic and Computer
parts (Permanent magnets, Electrical
contacts).
END
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