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MECHANICALALLOYING
Process Variables of MA
Mechanism of MA
Introduction to Mechanical Alloying
Characterization and Applicationsd o c s i t y
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Introduction to MechanicalAlloying (MA)
Alloying of difficultto alloy metals
Amorphous phases
Crystalline
phases
Nanometergrain sizes
Extension ofsolid solubility
Fine dispersionof second phase
particles
Attributes
of MA
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Mechanical
Alloying
Milling of Mixture of powder(metal or alloys / compounds)
Material transfer =>Homogeneous alloy
Milling of uniform composition , i.e pure metals, intermetallics, or prealloyed powder
No material transfer for
Homogenization
MechanicalMilling
Introduction to MechanicalAlloying
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No PCA, Oxide + Carbide dispersionin Al
Graphite + Milling atmosphere control
(O2,Ar, N 2 ) => Al + Al 4C3 + Al 2O3
Milling of powder in Liquid Nitrogen.
Continuous flow of Liquid nitrogen Or liquid nitrogen can be introduced
into the milling chamber
ReactionMilling
Cryomilling
Introduction to MechanicalAlloying cont..
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Mechanical Alloying
MA is used to produce amorphous, nanocrystallineand composite powders
Mixing of starting powders proportions
Loading powders into the mill
Mill to steady state
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Processvariables
in MA
Ball-to-powderweight ratio
BPR
Type of mill
Milling time.
Milling energy /speed
Grindingmedium
Extent of vialfilling
ProcessControlAgent
Process Variables
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The planetary ball mills
Shaker millsAttritors ball millsCommercial mills
Types of Ball Mills
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The Planetary Ball mill
Clamping
Rotatingdisk
Vial
Grinding balls
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Planetary Ball Mill cont..
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Mechanical Alloying (MA)
Horizontal section Movement ofsupporting disk
Centrifugalforce
R o
t a t i o n o
f v i a
l
cont..
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Retsch PM 200Digital and
programmable
Planetary Ball Mill
WCvials
cont..
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Shaker mills
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With each swing of the vial, the balls impact against the sample andthe end of the vial, both milling andmixing the sample.
Because of the amplitude ( 5 cm)and speed ( 1200 rpm) of the clampmotion, the ball velocities are high( 5 m/s), and consequently the force
of the ball's impact is unusuallygreat.Vial is made up of hardened steel,
alumina, tungsten carbide, zirconia,stainless steel.
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Attritors ball mills
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Commercial mills
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PCA (lubricant or surfactant) is added to powder mixtureduring milling to avoid the cold welding.
PCA can be liquid, solid or gas.PCA are mostly organic compound, which acts as surface-active agents .
Function of PCA:PCA adsorbs on the particle surface and minimizes coldwelding.lower the surface tension of solid material
Energy for size reduction:E= .S
Process Control Agents (PCA)
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PCA Melting point ( C) Boiling point ( C)
Stearic acid 67-69 183-184
Ethyl acetate -84 76.5-77.5
Ethyl alcohol -130 78
Heptanes -91 98
Hexane -95 68-69
Methyl alcohol -98 64.6
Polyethylene glycol 59 205
Process Control Agents (PCA)cont..
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PCAs are low melting and boiling point compounds
The majority of these compounds decompose during
milling
Process Control Agents (PCA)cont..
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Hydrocarbons and carbohydrates introduce C, H and
oxygen into the powder particles that are uniformlydispersed in the matrix.
The presence of air can also acts as a PCA and
prevents welding.
Milling of the powders at very low temperatures(e.g., in liquid nitrogen) minimizes welding
Process Control Agents (PCA) cont..
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PCA is used at a level of 1 to 5 wt% of the total powder Amount of powder recovered increases with the increase ofquantity of PCAPCA reduces the particle size by 2 to 3 order of magnitude.Al powder milled for 5 hr 500m with 1 wt % stearic acid
10 m with 3 wt % stearic acid
Process Control Agents (PCA) cont..
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Choice of PCA depends powder being milled and purity offinal product desired.
No universal PCA
Process Control Agents (PCA)
Amount of PCA
Coldwelding Chemicalstability Amount ofpowder
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Milling Container
Milling container material can contaminate the powder or alter the chemistry of milled powderIf material of container is different from that of
powder being milled, powder may becontaminated.If materials of container and powder are same,
chemistry may be altered e.g. milling of Cu-In-Ga-Se in Cu container.
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Time is chosen to achieve a steady state betweenfracturing and cold welding of powder particles tofacilitate alloying.
Milling Time
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Milling Energy
Faster the mill rotate, higher will be energy inputinto the powder (E = mv 2)
Limitations
Ball Pinning
Temperatureincrease
wear of tools
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Impact energy depends on rotation direction ofvials and disksTalc was milled in PM. Highest valueobtained was 15 kJ/s.kg in same directionIn counter direction, it was observed 80 kJ/s.kgRatio of rotation speed and revolution speed
Milling Energy cont..
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Milling Energy
Milling Intensity: I = BPR V max f
Where BPR= M b / M p
cont..
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Also called charge ratio.BPR has strong effect on the time required toachieve a particular phase
10:1 BPR is commonly used.
Higher BPR can be obtained by increasingnumber of balls or by increasing density of
balls.
Effect of Ball-to-Powder Weight Ratio
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Effect of Ball-to-Powder Weight Ratio
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Refinement of particle and grain sizes with milling time.Rate of refinement increases with higher ball-to
powder weight ratios. d o c s i t y
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The density of grinding medium must be high.Stainless steel and WC are common grindingmediumUse, if possible, same material of grindingvessel and grinding medium to avoidcontamination
Size of grinding BallsFinal constitution of powder depends on sizeof grinding balls.
Grinding Medium
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Grinding Medium
Highest collision energy can be obtained if balls of different sizes are used.If large differences in sizes of balls thensmaller balls will be destroyed by larger onesUse combination of smaller and larger balls torandomize the motion of balls
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If the quantity of the balls and the powder is verysmall, then the production rate is very low.
If the quantity is large, then there is not enough spacefor the balls to move around and so the energy of theimpact is less.
Generally about 50% or a little more of the vial spaceis left empty.
Extent of vial filling
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During high-energy milling, the powder particles are repeatedly flattened, fractured, andrewelded.
Cold welding and fracturing depend ondeformation characteristics of powders
Mechanism of MA
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Mechanism of MA
Typicalstarting
Powder
Aftersingle
collision
Soft metal A
Dispersoid
Soft metal B
intermetallic
20 m 20 m 5 m5 m
Ball-powder-ball collision
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Around 1000 particles with an aggregate weightof 0.2 mg are trapped during each collision.
Ball-powder-ball collision
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(a)
(a)
Metal A
Intermetallic
Dispersoids
Metal B
0.5 micron
First stage of MA
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(b)
0.5 micron Metal B
Metal A
Dispersoids
Dispersoids
interdiffusionsIntermetallic
Dispersoids
Metastable phase
Precipitate phase
Secondstageof MA
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(c)
0.5 micron
dispersoid
Concentration of metal B
Concentrationof metal A
Remanent ofintermetallics
Equilibrium precipitates
Thirdstageof MA
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Mechanical alloying involves powders withvery small sizes and these should be handledwith caution and care.
Because of the large surface area, they arehighly reactive and can be pyrophoric and cancause health problems when inhaled.Precautions should be taken not to open the
powder to atmosphere immediately aftermilling since this can lead to oxidation of the
powders and in some situations they can even
catch fire.
Powder Handling
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