Powder Metallurgy 1

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Powder Metallurgy (P/M) Processing Powder Metallurgy (P/M) Processing 
Powder Metallurgy (P/M)
 Powder metallurgy is an ancient technology of pressing  gy gy p g
metal powder in to specific shape. 
 E g the Iron pillar at Delhi (400 AD) was fabricated out of  E.g. the Iron pillar at Delhi (400 AD) was fabricated out of 
solid state reduced ferrous granules by sinter forging 
technique. 
 Modern P/M technology started in the 1920s with 
production of cemented carbides and mass production of  p p
porous bronze bushes for bearing applications. 
 P/M process is a rapid economical and high volume  P/M process is a rapid, economical and high volume 
production method of making components from powders
Stages of powder metallurgy process sing
 Production and characterization of powders:
 Treatment of powders  Treatment of powders
 Compaction of powders
 Sintering  Sintering
Production and characterization of powders
Chemical reaction and decomposition: Reduction of iron oxide
producing a sponge like iron poser. Fine powders of Ni and Fe are
produced by decomposition of their carbonyls
Electrolytic deposition: Metals like Cu, Be, Fe, Ni, etc can be
precipitated on the cathode of an electrolytic cell in the form of precipitated on the cathode of an electrolytic cell in the form of
sponge, powder or in a form which can be mechanically
disintegrated
A i i f l l A i i i l di i i Atomization of molten metals: Atomization involves disintegration
of molten metal in to fine droplets using high velocity gas ot water
jets. Gas atomization results in spherical powder while water
atomization gives powders of irregular shape.
Mechanical processing of solid materials: Mechanical processing
result in coarse irregular or angular particles by machining milling result in coarse, irregular or angular particles by machining, milling,
crushing or by impact.
Treatment of powders
• Several modification in structure , chemistry and size of
the powder sat necessary for best compaction.
• Includes removal of oxides and inclusions, mixing, addition
of lubricant and other sintering aids.
• Care should be taken to avoid agglomeration of Care should be taken to avoid agglomeration of
components or particle of very fine size. Mixing is very
important operation in P/M processing important operation in P/M processing.
C i f d Compaction of powders
Following are the major functions of compaction
 To consolidate the powders in to desired shape
 To provide the desired final dimensions to the components
 To impart the desired level and type of porosity to the 
component p
 To provide adequate strength for subsequent handling.
Die compaction is the widely used method of compaction.
Densities of up to 90% of full solid density can be achieved
by die compaction
Sintering
 It is heating the compacted billet at high temperature, leading
to decrease in surface area, increase the compact strength
and shrinkage in the compact.
 Often a pre‐sinter heating operation may be necessary for
removing the lubricant or binder.
 For metallic alloys, sintering is carried out in a protective
atmosphere, inside a furnace at temperatures generally at
75% of the absolute melting temperature of the alloy.
 I f l i li id h  In some cases some amount of melting or liquid phase
sintering may take place during the sintering operation.
Powder production
Chemical method of powder production:
These involve the following:
• Solid state : E.g. Reduction of iron or tungsten oxide with a
reducing gas. g g
• Gaseous state: Reduction of titanium chloride (TiCl4) vapor
with molten magnesium
A l ti th i it ti f t • Aqueous solution: e.g. the precipitation of cement copper
from copper‐sulphate solution with iron, or the reduction of
an ammoniacal nickel salt solution with hydrogen under
pressure (hydro‐metallurgy method).
Solid state Reduction:
In this oxides of Fe are heated in a reactor and reducing gases are g g
allowed to flow above the bed of oxide layers. The common iron
ore is magnetite (Fe
3
O
4
). In the presence of Co/CO2 gas mixture
(P + P = 1 atm) the reduction of higher oxides in to lower (P
CO
+ P
CO2
= 1 atm) the reduction of higher oxides in to lower
oxides and finally to iron may be expressed as:
3 Fe
2
O
3
(s) + CO ‐‐‐‐> 2 Fe
3
O
4
(s) + CO
2
(g)
Fe
3
O
4
(s) ) CO(g) ‐‐‐‐> 3 FeO(s) + CO
2
(g)
3 4
( ) ) (g) ( )
2
(g)
FeO(s) + CO(g) ‐‐‐‐> Fe(s) + CO
2
(g)
In addition to this Hydro‐metallurgical reduction techniques are
also used in which metals are directly reduced from aqueous
l b h d h h l ( ) solutions by either reduction with another metal (cementation) ,
or by gaseous reduction.
For each oxide and reaction temperature, a critical CO/CO
2
ratio p , /
2
in the gas mixture must be exceeded for the reaction to
proceed proceed.
The reduction of Fe
2
O
3
to Fe
3
O
4
occurs between 200‐500 °C and
/ f l /
4
d a minimum CO/CO
2
ratio of approximately 1/10
4
is required.
Fe
3
O
4
is reduced to FeO between 500‐900 °C, and FeO is
reduced to iron at temperatures between 900 ‐1300 °C.
Some specific powders produced by chemical methods are Iron
powders, Copper powders, Ni/Co powders, Ti powders powders, Copper powders, Ni/Co powders, Ti powders
Tungsten powder, Ta and Nb powder, Al2O3 , etc.
Electrolytic method:
In electro‐deposition of metal, dissolution of the impure metal
(anode) producing metal ions in solution and electrons takes
place. Adjustments in the chemical and physical conditions
during electro‐deposition makes it possible for the metal to
deposit loosely on the cathode of the cell, either as alight cake
or as flakes. These are readily crushed in to a powder form. This
yields very high purity metal with excellent properties for yields very high purity metal with excellent properties for
conventional P/M processing.
The process variables controlling the powdery deposit are
• High current density
• Weak metal concentration
• Additions of colloids and acids
• Low temperature
• High viscosity g y
• Avoidance of agitation
• Suppression of convection.
The electrolytic metal powders are generally dendritic in nature.
The common metal powders produced by electrolytic method are
Cu, Fe, Ti, , etc.
Atomization method
I hi l l i di i d i fi d l d In this molten metal is disintegrated in very fine droplets and
allowed to solidify. This technique is widely used due to the ease of
making highly pure metals and pre‐alloyed powders directly from
the melt. The molten metal is forced through a fine orifice, possibly
at the bottom of a crucible and high velocity gas or liquid stream
impinge the liquid stream causing the liquid to disintegrate in to a impinge the liquid stream causing the liquid to disintegrate in to a
very fine spray of droplets. The fine droplets during the flight,
solidifices into very fine metal powders.
The atomization process is carried out in the following stages:
• The molten alloy is prepared in a furnace and then it is
transferred to the tundish.
• The melt is poured from the tundish through the nozzle into
the chamber.
• The water (air, gas) jets break the melt stream into fine
d l droplets.
• The droplets solidify when they fall in the chamber.
Th d i ll t d t th b tt f th h b • The powder is collected at the bottom of the chamber.
• The powder is removed from the chamber and dried (if
necessary) necessary).
Fine particles are favored by
• Superheated metal
• Low metal surface tension
• High atomizing pressure
• Small nozzle diameter • Small nozzle diameter
• Low metal viscosity
h l • High atomizing agent volume
• High atomizing agent velocity
• Optimum apex angle.
The particle shapes of atomized powders can be tailored to
l f l h l h h l h b ll almost perfectly spherical to high irregular shape by controlling
the process variable. Sphericity is favored by
 High metal surface tension
 Narrow meting range  Narrow meting range
 High pouring temperature
 Atomizing gas especially inert gas  Atomizing gas, especially inert gas
 Low jet velocity
 Long apex angles in water atomization
 Long flight path.
Gas atomization
The common atomizing gas media are Nitrogen, argon or air. There are two
t f l th E t l i i l i hi h t t b t th types of nozzle: the External mixing nozzle in which contact between the
melt and the gas takes place outside the respective nozzle. This is used
exclusively for atomization of metals. Internal mixing nozzle are for
atomization of materials which are liquid at room temperature Gas atomization of materials which are liquid at room temperature. Gas
atomized powders are generally spherical with relatively smooth surfaces.
Higher pressure and smaller jet distance produce finer powder.
 pressures are generally in the range 14 x 10
5
Pa to 42 x 10
5
Pa
t l iti f 50
‐1
t 150
‐1
at gas velocities from 50 ms
‐1
to 150 ms
‐1
.
 Under this condition, the particle quench rate is ‐10
2
K s
‐1
.
 Gas atomization is generally used for preparation of super‐
alloys, Titanium, HSS and other reactive materials.
 The disadvantage is very low overall energy efficiency (~ 3%)
and expensive due to the inert gas other than nitrogen. and expensive due to the inert gas other than nitrogen.
Water atomization
In water atomization, a high pressure water stream is forced through the
nozzle to form a disperse phase of droplets which then impact the metal nozzle to form a disperse phase of droplets which then impact the metal
stream.
• The overall energy efficiency of the process is < 4%.
• This is generally used for low and high alloys steels including stainless This is generally used for low and high alloys steels including stainless
steels.
• Due to the oxide formation, water atomization is not used for
atomization of reactive metals such as Ti Al super‐alloys etc atomization of reactive metals such as Ti, Al, super‐alloys, etc.
 The water pressure is generally  in the range of 35 x 10
5
Pa to 210 x 10
5
Pa
 water velocities of 40 ms
‐1
to 15 ms
‐1
 particle cooling rates is of the order of 10
3
K s
‐1
to 10 
4
K s
‐1
.
 If the surface tension of the melt is high, once the droplet forms it assumes a 
spherical shape Higher viscosity, higher cooling rate and shorter time duration 
lt i l h di ti d h lt i i l h d results in slow spherodization process and hence results in irregular shaped 
powders. 
 In addition to these, other atomization process includes Liquid gas 
atomization centrifugal atomization vacuum atomization ultrasonic gas atomization, centrifugal atomization, vacuum atomization, ultrasonic gas 
atomization, etc. 
Mechanical Methods
These are not he primary methods of the production of metal These are not he primary methods of the production of metal
powder. This is generally being used for reducing the size of coarse
powders or flakes. They are used for the following cases:
 Materials which are relatively easy to fracture. i.e. relatively hard
and brittle metal alloys and ceramics
 Reactive materials such as Be and Hydrides Reactive materials such as e and Hydrides
 Common metals such as Al and Fe which are required in the form
of flakes powders, etc.
The equipments used for reducing the particle sizes are:
 Jaw Crusher
 Ball mill
 Disc grinder
 Attritor mill
Jaw crusher Rotary crusher Jaw crusher Rotary crusher 
Roller crusher
Attrition ball mill

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