introduction to mechanical alloying

8/12/2019 Introduction to Mechanical Alloying

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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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3d o c s i t y


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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

cont..

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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

cont..

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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

d o c s i t y


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introduction to mechanical alloying

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