Seetharaman S. Fundamentals of metallurgy

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Table 12.1

Typical operating conditions of the different mills

Vibrating

Planetary Vertical

Horizontal

mill

ball mill attritor

attritor

ball mill

Cycle time (h)

0.01±0.5

1±10

0.1±10

Powder amount

> 1 kg (industry) 10

±100 g 100 kg (industry) 100 kg (industry)

> 1 ton

1 g (laboratory)

100 g (laboratory) 100 g (laboratory)

Mill speed (rpm) 1000 (industry) 80

±400 60 (industry) 1800

10±50

3300 (laboratory)

300 (laboratory)

Ball diameter (mm) 1

±10

6±30

6±10

6±25

Ball filling level

50±70% 70%

30%

50%

Powder filling level

1:10 powder 1:10 powder 1:10 powder

100% of ball interstices

to ball weight to ball weight to ball weight

ratio

Powder size (



m) 1

±10

5±100

±100

5±100

10±50

On a larger scale, the economic point of view prevails and the efficient use of

the equipment and raw materials, as well as energy consumption, become

important.

Coldstream process

In this process, a high-pressure air-stream (e.g. 7 MPa) containing the particulate

material, enters a vacuum chamber through a venturi nozzle where it impacts a

tungsten carbide target.

The pressure drop at the exit of the nozzle induces a

temperature drop thus chilling and embrittling the material inducing its fracture

when it impacts the tungsten carbide target. The mater ial is then transpor ted to a

first classifier, which allows oversized products to drop into a storage vessel for

subsequent impact against the target. This process applies to many body-centred

cubic metals that go through a ductile to brittle transition at low temperatures. It

is used for hard, abrasive, relatively expensive materials such as tungsten

carbide, tungsten alloys, molybdenum, tool steels, beryllium, and other alloys

(Inconel, nickel and cobalt high temperature alloys). This process allows a rapid

production (1 t/day) of irregular micron-sized particles.

12.2.2 Atomisation route

Atomisation is often used to produce metal powders and especially pre-alloyed

powders (brasses, stainless steels, superalloys, NiAl . . .). Moreover its high

cooling rates (10

to 10

K/s) allows producing alloys that cannot be obtained by

casting. The flexibility of this method, coupled to its applicability to many

alloys, easy process control and high production rate makes it a very interesting

route.

10,12

The world capacity of production is estimated to be higher than one

million tons per year.

Atomisation occurs when a jet of liquid is converted to very small droplets.

The different atomisation processes are (see Fig. 12.2):

· Two-fluid atomisation, where a molten metal is broken up into droplets by

impingement of high-pressure jets of water, oil or gas (a).

· Centrifugal atomisation, where a liquid-stream is dispersed into droplets by

the centrifugal force of a rotating electrode (c), disc or cup (b).

· Vacuum or soluble-gas atomisation, where a molten metal is supersaturated

with a gas that causes atomisation of the metal in a vacuum chamber (d).

· Ultrasonic atomisation, where a film of liquid metal (e) or the atomising fluid

(f) is agitated by ultrasonic vibration.

Atomised metal powders are generally found to follow a log normal distribution.

The particles size distributions are generally from few m up to 500 m and the

mean particle size from 10 to few 100 m. The geometric standard deviation 

measures the spread of particle size around the median mass diameter d

and

Understanding and improving powder metallurgical processes 477

typically varies from about 1.7 to 2.3 for water- and gas-atomised metal powders.

Compared to other types of powders, the atomised powders are relatively compact

(apparent density up to 65% of the theoretical density), with a high packing density

and a low specific surface area. This induces good flow characteristics, good

compressibility but low sintering activity. Particles shape varies from irregular to

12.2 Atomisation processes. (a) gas, water or oil atomisation, (b and c)

centrifugal atomisation, (d) vacuum atomisation, (e and f) ultrasonic

atomisation.

478 Fundamentals of metallurgy

spherical for respectively water and gas atomisation (Fig. 12.3c, d). The required

particle shape depends on the following process used to obtain the final product.

Spherical powders are us ed for therma l s prayi ng loose sint ering or hot

consolidation (e.g. extrusion, isostatic pressing). But irregular powders are needed

to guarantee a high enough green strength after cold pressing. The cleanliness of

the powder is also a very important parameter. The possible contaminants are bulk

dissolved impurities, surface impurities and inclusions (mainly from melt practice).

Surface oxidation is often considered as the main purity index.

The process parameters controlling the powder characteristics (size, distribu-

tion, shape and oxygen content) are related to the atomiser design (nozzle definin g

12.3 Morphology of powders obtained by (a) milling, (b) mechanical alloying,

Xingpu) and (f) precipitation (courtesy of Rudy de Vos).

Understanding and improving powder metallurgical processes 479

the geometry of the atomisation), the atomiser operating conditions (nature of the

fluid, pressure, temperature, atmosphere) and the material (melting temperature,

viscosity, surface tension, superheat). Some of these parameters have a great

influence on the powder characteristics. For example, the production of fine

powders is favoured by low metal viscosity, low metal surface tension, high

superheat of the metal (difference between the temperature of the molten metal

and its melting temperature), small nozzle diameter, high atomising pressure, high

flow rate and high velocity of the atomising fluid, short metal stream and short jet

length.

Spherical atomised powders are obtained by increasing the superheating

of the liquid metal (reduction of viscosity), increasing its surface tension by

addition of deoxidisers, such as B, P and Si and increasing the solidification time

of the particle (atomising with gas instead of water). More details about the

atomising parameters can be found in references 1 and 3.

In the following, the different atomisation methods will be described in more

detail.

Water atomisation ± oil atomisation

Water atomisation, with its capacity of at least 700 000 tons per year and

production rates up to 500 kg/min (8 kg/s), is the process mostly used for the

commercial production of irregular (Fig. 12.3c) metal powders (iron, stainless

steels, tool steels, soft magnetic alloys, nickel alloys and copper).

The

production costs are lower than the other atomisation methods. However,

limitations exist in relation to powder purity (relatively high oxygen content

(1 wt%)), particularly with reactive metals and alloys. The median particle size

decreases with increasing water pressure. Most industrial plants use a water

pressure in the range of 5 to 20 MPa, that results in mass median particle sizes of

30 to 150 m. Finer powders (median particle size from 5 to 20 m) can be

produced by using much higher water pressures (50 to 150 MPa).

Oil atomisation is similar to water atomisation. Oil is used as atomising

medium in order to decrease the irregular character of the particle shape and to

reduce oxidation, especially for molten metal containing elements such as Cr,

Mn or Si, which are readily oxidised.

12,14

Gas atomisation

The production of gas atomised powders is lower than water-atomised ones

because of its higher cost. It is about 300 000 tons per year for air atomisation

and around 50 000 tons per year for inert gas (nitrogen, argon and helium)

atomisation, the later ensuring a low oxygen content (100 ppm) of the spherical

powders (ferrous and non-ferrous alloys) (Fig. 12.3d).

11,12

In gas atomisation the powder size distribution strongly depend s on the

nozzle design (`free-fall' configuration or `confined'). Confined nozzle designs

480 Fundamentals of metallurgy

increase the amount of fine powder particles (<10 m) because of a higher

velocity and density of the gas on contact with the metal.

In practice, the mean

particle size decreases with increasing gas pressure and decreasing melt flow

rate. The pressures used in gas atomisation are generally in the range of 0.7 to

3 MPa. High-pressure gas atomisation (HPGA) allows the production of very

fine particles, e.g. Sn-5Pb with a mean size of 5 m was obtained at 12.5 MPa.

New trends in gas atomisation concern the nozzle design.

In conventional

nozzles, the atomising medium impacts the molten stream in the turbulent region

instead of the laminar region as for a Laval nozzle. The use of such a Laval

nozzle reduces the particle size and spread. A moderate gas pressure (e.g.

2 MPa) is sufficient to produce powders with d

 10 m with Ar or N

atomising gas or 5 m with He.

Another possibility is hot gas atomisation. As the hot gas exit velocity in the

supersonic gas nozzle increases subst antially with the temperature of the

atomisation die, a higher kinetic energy is available to disrupt the melt stream

into finer droplets and hence lower powder median size. But for each atomising

temperature, the nozzle design has to be optimised in the area of expansion of

the gas at supersonic velocity.

Vacuum atomisation ± tandem atomisation

In the vacuum atomisation, a molten metal supersaturated with gas (noble gas or

hydrogen) under pressure (1 to 3 MPa) is exposed to vacuum as a thin stream,

resulting in melt disintegration into fine droplets, as the dissolved gas bursts out

of the molten metal.

11,14

This method is mainly used to produce spherical

superalloy powders.

The combin ation of vacuum and ga s atomisation, known as tandem

atomisation, allows the production of fine super alloy powder.

After gas

atomisation, the droplets begin to cool down and the retained dissolved gas is

rejected from the solidifying metal dendrites. So the remaining liquid metal

becomes more and more supersaturated until a gas bubble nucleates that grows

explosively to produce a secondary atomisation event. The partially solidified

droplets shatter into much finer droplets that subsequently solidify into ultra-fine

powder particles.

Centrifugal atomisation (rotating elect rode, rotating disc)

In centrifugal atomisation, centrifugal forces break up the molten metal as

droplets that then solidify as powder particles.

In the rotating electrode process (REP), a consumable electrode is rotated

(1000 up to 50 000 rpm) while it is melted by an electric arc. The molten metal is

ejected at the edge of the electrode under the influence of centrifugal forces in

the form of droplets that solidify as spherical particles in an inert gas filled

Understanding and improving powder metallurgical processes 481

chamber. There is no liquid metal±crucible contact, which prevents the

contamination by ceramic particles. This is also an advantage for molten

titanium alloys that are corrosive to nearly all container materials.

The median particle size (about 250 m) decreases with decreasing melt

rates, increasing rotational speed and electrode diameter. Usually the spreading

of the particle size is much less by centrifugal than by gas atomisation.

Special methods ± ultrasonic atomisation

There are several ultrasonic atomisation techniq ues such as capillary-wave

atomisation (no commercial application) (Fig. 12 .2e), ultrasonic gas atomisation

(USGA) (Fig. 12.2f) and double ultrasonic atomisation (still in development).

In the capillary-wave atomisation, the liquid metal meets a vibrating surface

at ultrasonic frequencies and droplets (< 100 m) are ejected from the surface. In

ultrasonic gas atomisation (USGA) the molten metal stream is disintegrated by

impact with multiple high velocity gas pulses. The high-pressure gas (1.4 to

8.2 MPa) is accelerated by a shock wave tube (resonance cavities) to speeds of

up to Mach 2 at frequencies ranging from 60 to 120 kHz. The particle size is

usually less than 30 m. It is used commercially for production of low-melting

alloys such as aluminium alloys and on a lab-scale for stainless steels, Cu-, Ni-

and Co-based alloys.

In double ultrasonic atomisation, the metal stream is guided towards the inner

wall of a tubular resonator excited at ultrasonic frequencies. The molten metal

wets this vibrating wall and disintegrates according to the capillary-wave

atomisation process. Meanwhile, non-stationary shock waves are generated in an

inert gas flowing into the same tube. The pressure pulses, like in ultrasonic gas

atomisation (USGA), theref ore further disintegrate the capillary-wave-atomised

droplets. As the break-up of a molten metal occurs in two steps, the problem of

stream diameter in USGA is cancelled.

12.2.3 Aerosol routes

In aerosol-based processing techniques a liquid precursor (nitrates, acetates,

chlorides, alkoxides) is atomised into finely divided submicrometer liquid droplets

(aerosol) that are distributed in a gas medium. This aerosol enters a heated reaction

zone, where the solvent (methanol, ethanol, acetylacetone) is rapidly evaporated or

combusted and the intimately mixed chemical precursors are decomposed and/or

undergo chemical reaction to yield the desired products. The temperature, the

composition of the chemical precursors, the flow rate of the aerosol and the

atmosphere are the main parameters. The temperature should be high enough to

allow the complete reaction, but low enough to prevent excessive grain growth.

These simple and inexpensive processes can be adapted for the production of

ultrafine multicomponent powders with a well-controlled stoichiometry (ultrasonic

482 Fundamentals of metallurgy

droplet generators and electrostatic atomisation).

18,19

These powders have a narrow

size distribution and tend to be spherical and unagglomerated. This technology is

mainly applied to the fabrication of oxides. However, dense spherical palladium

particles

or 70 wt% Ni±30 wt% Fe alloy powders of 10±80 nm

were produced

by spray pyrolyse.

12.2.4 Physical routes

The main physical route to produce powder is physical vapour deposition

(PVD). Even if the PVD process is usually applied for coatings, some set-up

(inert gas condensation, electrical explosion wire, laser ablation) allows the

production of very fine (10 nm) powders with high purity (vacuum environment

process). However, the production rate is very low and the costs are very high.

During th e PVD process vapour phase species can be produced by

evaporation of the bulk material, sputtering, or ion plating. These vapour phase

species will collide with the inert-gas molecule and undergo homogeneous phase

nucleation to form powder particles that will be then collected.

When the

vapour phase species are produced by evaporation, this process is also called

inert gas condensation (IGC). It is used to produce Zn

and Ni

nano-powders.

However, the synthesis of multicomponent powder is difficult because the

different elements have different evaporation temperatures or sputtering rates.

This can be solved by using the laser ablation method.

12.2.5 Chemical routes

The numerous chemical processes can be classified as a function of the type of

chemical decomposition involved in the metallic powder production. These are

decomposition of a solid by a gas (oxide reduction), precipitation from a

solution (electrolysis, wet reduction, precipitation), condensation and thermal

decomposition (carbonyl process, hydrid-dehydird process).

Decomposition of a solid by a gas (oxide reduction)

This process consists of the reduction of the oxides by gasses such as hydrogen,

carbon monoxide.

12,14,25

A typical reaction is: MO (s) + H

(g) ! M (s) + H

O (g).

The process parameters are composition and flow rate of the reducing gas,

reduction temperature (main parameter), temperature profile in the furnace and

bed depth of the oxide if reduction is performed in a stationary system. Non-

stationary reduction, such as in rotary kilns or fluidised bed reactors favours

access of the reducing gas to the metal oxide particles and so improves the kinetics

of reduction.

Oxide-reduced powders are very porous and thus are called sponge powders.

They contain most of their residual oxides within the particles, instead of a

Understanding and improving powder metallurgical processes 483

surface enrichment as for atomised powders. This process is commercially used

for the production of iron, copper, tungsten and molybdenum powders, and on a

smaller scale, for cobalt and nickel powders.

Precipitation from a solution (electrolysis, wet reduction, precipitation)

Metal precipitation from a solution (obtained by leaching an ore) can be

accomplished directly by electrolysis or wet reduction (Sherritt Gor don process).

Indirect precipitation from a salt solution is done by first precipitating a

compound of the metal (hydroxide, carbonate, or oxalate) followed by heating,

decomposition, and reduction.

Electrolysis or electrodeposition from an aqueous solution consists in

depositing a pure metal on an electrode. The deposit can be a loose adhering

powder or sponge that can be disintegrated mechanically into fine particles (Cu,

Ag) or a coherent dense brittle layer of metal that can be ground into powder

(Fe, Mn).

Powder-like deposits are favoured by low cation concentration

(limits particle growth), low pH (favours conductivity), high current density and

frequent removal of the deposit at the cathode (brush-down interval). This

technique is mostly used to produce irregular shape (dendritic) powders such as

Fe, Cu and Ag, but it has also been employed for Sn, Cr, Be, Sb, Cd, Pb, Pd and

Zn. The main advantage of electrolytic powder is its high purity level after

removal of i mpu rities coming from the electrolyte. However, the high

production cost of, for example, electrolytic iron powders, limits their use to

niche applications such as catalyst or food additives (Fig. 12.3e).

The wet reduction (Sherritt Gordon process) is based on the separation and

precipitation of Cu, Ni and Co from a salt solution, by reduction with hydrogen.

High temperature and high hydrogen pressure are used to increase the reaction

rates. The pH value is increased by adding ammoni a in order to guarantee a

complete reduction. Powders with a good compactibility and different particle

shapes can be produced (e.g. Co with d

 60 m). Moreover, co-precipitation

or successive precipitation of different metals allows the production of alloyed

or composite powders.

Many metal powders are produced by the precipitation of their soluble salts

as insoluble hydroxide, carbonate or oxalate (indirect precipitation) (Fig. 12.3f).

Subsequent heating decomposes these compounds into the respective metals or

metal oxides and gaseous products.

Different precipitation methods can be

used: direct strike precipitation (addition of a precipitating agent), solvent

removal (sol-gel), hydrophilic non-solvent addition or precipitation from

homogeneous solution (PFHS) (use of a precursor whose decomposition

kinetics controls the rate of release of precipitating agent).

The parameters

governing the powder characteristics (morphology and agglomeration) are

concentration, pH, temperature, anion associated to the soluble metal cation,

choice of the precipitating reaction and aging conditions. This process is

484 Fundamentals of metallurgy

employed for platinum, selenium, tellurium, silver, nickel and silver-cadmium

oxide compounds through co-precipitation.

Condensation (precipitation from a gas)

The main processes are inert gas condensation (IGC) and chemical vapour

decomposition (CVD). CVD implies thermal dissociation and/or chemical

reaction of the gaseous reactants (halides, hydrides and halohydrides of metal s

and metalloids) on or near a heated surface to form stable solid products. The

decomposition of the gaseous reactants can be homogeneous and/or

heterogeneous reaction forming respectively powders or films. The different

heating methods lead to a variety of CVD methods such as plasma-assisted CVD

(PACVD), as well as different types of precursors, e.g. metalorganic CVD

(MOCVD) etc. . . . The CVD process parameters are deposition temperature,

pressure, input gas ratio and flow rate.

The main drawbacks of CVD are the toxic, corrosive, flammable and/or

explosive precursor gasses and the difficulty to deposit multicomponent

materials with a well-controlled stoichiometry because the different precursors

have different vaporisation rates. Ultra-fine powders (few nm up to few 10 nm)

can be produced.

Thermal decomposition (carbonyl process, hydride-dehydride process)

The carbonyl process was first used for the production of nickel powders (in

1889). Other possible metal carbonyls include iron, cobalt and, in fact, all of the

metals of the first, second and third transition metal series.

25,28

Thermal decomposition of gaseous cobalt, iron and nickel carbonyls occurs at

temperatures about 200ëC to have favourable reaction kinetics for acceptable

powder production rates. Fine spherical iron powder (less than 10 m) while fine

irregular porous nickel powder is obtained with this process. However, the

carbonyls are dangerous for the health even at very low concentrations (ppb-level).

Hydride decomposition (hydride/dehydride process) is used for metals that

are too ductile to be milled into very fine particles. The metal is embrittled by

hydrogenation during heating in hydrogen atmosphere at elevated temperatures.

Absorption of only a few per cent of hydrogen makes most transition metals so

brittle that they readily can be comminuted. After milling, the hydrogen can

simply be removed by heating the powder in a dynamic vacuum. This process is

used for Ti, Ta and Nb.

25,28

12.2.6 Plasma techniques

Gaseous plasma consists of a mixture of electrons, ions and neutral particles and

is electrically neutral (negative and positive charges balances each other) even if

it is electrically conducting due to the presence of free charge carriers.

Understanding and improving powder metallurgical processes 485