Powder Metallurgy

Definition of Powder Metallurgy

• Powder metallurgy may defined as, “the art and science of producing metal powders and utilizing them to make serviceable objects.”

OR

• It may also be defined as “material processing technique used to consolidate particulate matter i.e. powders both metal and/or non-metals.”

Process of Powder Metallurgy:

Powder Production

Powder Characterization & testing

Mixing - Blending

Processing - Compacting

Sintering Operation

Finishing Operations

Finished P/M Parts

Characteristics of fine powder

1. Surface area2. Density

I. True density II. Apparent densityIII. Tap densityIV. Green density

3. Flow rate4. Green strength5. Green spring6. Compressibility and compression ratio

Characteristics of fine powder7. Particle shape

- depends on powder production method

8. Particle size

- dia of spherical particles

- Av. Dia of non spherical particles

divided into 3 classes

1. sieve

2. sub sieve

3. sub micron

9. Particle size distribution

shape factor= surface area/ Volume

Aspect ratio = largest dim/ smallest dim

PRODUCTION OF METAL POWDERS

The selection of materials in powder

metallurgy is determined by two factors.

i) The alloy required in the finished part.

ii) Physical characteristics needed in the

powder.

Both of these factors are influenced by the

process used for making powder.

i) There are numerous ways for powder production

which can be categorized as follows.

1) Mechanical methods of powder production:

i) Chopping or Cutting

ii) Abrasion methods

iii) Machining methods

iv) Milling

v) Cold-stream Process.

2. Chemical methods of powder production:

i) Reduction of oxides

ii) Precipitation from solutions

iii) Thermal decomposition of compounds

iv) Hydride decomposition

v) Thermit reaction

vi) Electro- chemical methods

3. Physical methods of powder production:

i) Water atomization

ii) Gas atomization

iii) Special atomization methods

The choice of a specific technique for powder

production depends on particle size, shape,

microstructure and chemistry of powder and also on

the cost of the process.

1. Chopping or Cutting:

In this process, strands of hard steel wire, in diameter as small as 0.0313 inches

are cut up into small pieces by means of a milling cutter.

This technique is actually employed in the manufacturing of cut wire shots

which are used for peening or shot cleaning.

Limitations:It would, however, be difficult and costly to make powders by this method and for

this reason it is not profitable to discuss the technique in detail.

2. Rubbing or Abrasion Methods:These are all sorts of ways in which a mass of metal might be attacked by some

form of abrasion.

a) Rubbing of Two Surfaces:

When we rub two surfaces against each other, hard surface removes the material

from the surface of soft material.

* Contamination

b) Filing:

Filing as a production method has been frequently employed, especially

to alloy powders, when supplies from conventional sources have been

unobtainable.

Such methods are also used for manufacture of coarse powders of dental

alloys.

Filing can also be used to produce finer powder if its teeth are smaller.

* commercially not feasible.

c) Scratching:

If a hard pin is rubbed on some soft metal the powder flakes are

produced.

Scratching is a technique actually used on a large scale for the

preparation of coarse magnesium powders.

* scratching a slab of magnesium with hardened steel pins.

* a revolving metal drum to the surface of which is fixed a

scratching belt.

The drum, which is about 8 inches in diameter, rotates at a peripheral

speed of approximately 2500 ft./min. The slab of magnesium metal, 14

in. wide by 1.75 in. thick enters through a gland in the drum casing and

presses against the steel pins.

d) Machining:

A machining process, using for example a lathe or a milling cutter in

which something more than just scratching is involved, since the

attacking tool actually digs under the surface of the metal and tears it off.

On lathe machine by applying small force we get fine chips.

A large amount of machining scrap is produced in machining operations.

This scrap in the form of chips and turnings can be further reduced in

size by grinding.

* small scale production.

Disadvantages:

• Lack of control on powder characteristics, including chemical contamination such as oxidation, oil and other metal impurities.

• The shape of the powder is irregular and coarse.

Advantages:

•For consuming scrap from another process, machining is a useful process.

•Presently the machined powder is used with high carbon steel and some dental amalgam powders.

COMMERCIAL METHODS

These are the methods used for high production rate. Best examples

of mechanical production methods are the Milling Process and Cold

Stream Process.

Milling:

The basic principal of milling process is the application of impact and

shear forces between two materials, a hard and a soft, causing soft

material to be ground into fine particles.

Milling techniques are suitable for brittle materials.

Two types of milling are;

i) Ball Milling

ii) Attrition Milling.

Objectives of milling include:

Particle size reduction (comminution or grinding)

Shape change (flaking

Solid-state alloying (mechanical alloying)

Solid-state blending (incomplete alloying)

Modifying, changing, or altering properties of a material

(density, flowability, or work hardening)

Mixing or blending of two or more materials or mixed

phases

Ball Milling:

Ball milling is an old and relatively simple method for grinding large

lumps of materials into smaller pieces and powder form.

Principle of the process:

The principle is simple and is based on the impact and shear forces.

Hard balls are used for mechanical comminution of brittle materials

and producing powders.

Milling Unit:

The basic apparatus consists of the following;

• A ball mill or jar mill which mainly consists of a rotating drum

lined from inside with a hard material.

• Hard balls, as a grinding medium, which continue to impact the

material inside the drum as it rotates/rolls.

Figure: Tumbler mill used for milling metal powders

Important Parameters:

1. The most important parameter to consider is the speed

of rotation of the drum. An optimum/critical speed is

adjusted for maximum impact velocity.

* Critical speed is the speed above which the ball will

centrifuge.

• Very slow speed of rotation will not carry the balls to the

top, these will roll back down the drum sides.

• Very fast speed (higher than critical speed) will not let

the balls drop down as they will be carried around due to

centrifugal forces. Thus, an optimum speed is required.

This speed of rotation varies with the inverse square root

of the drum diameter.

2. The material of grinding media and its size and density.

• The size and density of the milling medium is selected according to the deformation and fracture resistance for metals.

• For hard and brittle materials large and dense media is used. Whereas, small balls are used for finer grinding.

• As a general rule, the balls should be small and their surface should be a little rough. The material of the balls and lining of the drum should be same as that of the material being ground.

3. The rate of milling of a powder is a function of quantity in the total space between the balls.

4. Lubricants and surface active agents are used to nullify the welding forces which causes agglomeration.

Grinding Mechanism:

During milling the following forces cause fracture of material into powder.

Impact Forces: These are caused by instantaneous striking of one object on the other. (Impact is the instantaneous striking of one object by another. Both objects may be moving or one may be stationary).

Shear Forces: These are caused as one material slides/rubs against the other.

Limitations:

• Rubbing action causes contamination of powder since balls may also get rubbed.

• Working hardening of metal powder is caused during milling.

• There is a possibility of excessive oxidation of final powder.

• Quality of powder is poor.

• Particle welding and agglomeration may take place.

COLD STREAM PROCESS

• This process is based on impact phenomenon caused by impingement of high velocity particles against a cemented carbide plate.

• The unit consists of:

A feed container;

A compressor capable of producing a high velocity stream of air (56 m3/min.) operating at 7 MPa (1000 psi);

A target plate, made of cemented tungsten carbide, for producing impact;

A classifying chamber lined with WC while the supersonic nozzle and target generally are made of cemented tungsten carbide.

Mechanism of the Process:

The material to be powdered is fed in the chamber and from there falls in front of high velocity stream of air.

This air causes the impingement of material against target plate, where material due to impaction is shattered into powder form. This powder is sucked and is classified in the classifying chamber. Oversize is recycled and fine powder is removed from discharge area.

* Rapidly expanding gases leaving the nozzle create a strong cooling effect through adiabatic expansion. This effect is greater than the heat produced by pulverization.

CHEMICAL METHODS

• Almost all metallic elements can be produced in the form of powders by suitable chemical reactions or decomposition.

Mostly chemical methods are based on the decomposition of a compound into the elemental form with heating or with the help of some catalyst.

In most cases such processes involve at least two reactants.

(i) a compound of the metal

(ii) a reducing agent

REDUCTION OF METAL OXIDESManufacturing of metal powder by reduction of oxides is extensively employed, particularly for Fe, Cu, W and Mo.

Advantages:A variety of reducing agents can be used and process can be economical when carbon is used.

Close control over particle size

Porous powders can be produced which have good compressive properties.

Adoptability either to very small or large manufacturing units and either batch or continuous processes.

Limitations:

Process may be costly if reducing agents are gases.

Large volumes of reducing gas may be required, and

circumstances where this is economically available

may be limited; in some cases, however, costs may

be reduced by recirculation of the gas.

The purity of the finished product usually depends

entirely upon the purity of the raw material, and

economic or technical considerations may set a

limitation to that which can be attained.

Alloy powders cannot be produced.

Production of Iron Powder

by Reduction of Iron Oxide:

(Direct Reduction Process)

Iron powders are commercially used for a large

number of applications such as fabrication of

structural parts, welding rods, flame cutting,

food enrichment and electronic and magnetic

applications.

The classical technique for production of iron

powder is the reduction of iron oxide.

Theory of the process:

It is the oldest process of production of iron

powder by using carbon as the reducing agent.

In this process pure magnetite (Fe3O4) is used.

Coke breeze is the carbon source used to reduce

iron oxide. Some limestone is also used to react

with the sulphur present in the coke. The mixture

of coke and limestone (85% + 15%) is dried in a

rotary kiln and crushed to uniform size.

** Hoganas Process

The ore and coke-limestone mixture is charged

into ceramic tubes (Silicon Carbide) with care so

that ore and reduction mixture are in contact with

each other but not intermixed. It can be achieved

by using concentric charging tubes with in the

ceramic tube.

Within the hot zone, several chemical reactions

occur and metallic iron is formed in the form of

sponge cake.

The main reaction is;

MO + R M + RO

If magnetite ore is used, then the following reactions will take place:

Fe3O4 + 3CO FeO + 3CO2

FeO + CO Fe + CO2

C + ½ O2 CO

Decomposition of the limestone generates carbon dioxide, which oxidizes the carbon in the coke to form carbon monoxide. The ferrous iron oxide is further reduced by the carbon monoxide to metallic iron.

Desulphurization occurs in parallel with reduction by reaction between gas and sulphides present in the ore resulting in gaseous sulphide compounds which in turn react with lime to form calcium sulphide.

The sponge cake is removed from ceramic tubes and

dropped into a tooth crusher where this is broken into

pieces.

After these pieces are ground to desired particle size.

During grinding the powder particles are considerably

work hardened. The powder is annealed at 800 - 870 oC in the atmosphere of dissociated ammonia.

The powder is loosely sintered, but requires only light

grinding and screening to produce a finished product.

THE CARBONYL PROCESS

• The only method for the manufacture of metal

powder by the pyrolysis of a gaseous compound

which has been used industrially on a substantial

scale is the carbonyl iron or nickel process.

• When iron and nickel ores react under high pressure

(70 – 300 atm.) with carbon monoxide, iron

pentacarbonyl [Fe(CO)5] or nickel tetracarbonyl

[Ni(CO)4] is formed, respectively.

• Both compounds are liquids at room temperature.

• Fe(CO)5 evaporates at 103 oC and Ni(CO)4 at 43 oC.

Precipitate Formation:This step of the process is carried out according to the

following scheme:

The liquid carbonyles are stored under pressure in tanks submerged in water.

The distilled and filtered liquids are conveyed to steam heating cylinders, where they are vaporized.

The vapors of liquid are sent to decomposers. The decomposers are jacketed and heated, giving an internal temperature of 200 – 250 oC. These cylinders are 9 – 10 feet high with an internal dia of 3 feet, with conical bottoms.

The incoming stream of vapors meets a tangential stream of ammonia gas. CO is removed here and precipitates of metals are formed which are then sieved, dried and may be milled to break up the agglomerates.

The CO gas arising from the decomposition is recovered and re-used.

Carbonyl iron powder is used for the production of

magnetic powder cores for radio or television

applications.

In P/M it is used for the manufacture of soft

magnetic materials and permanent magnets.

Because of its high price and poor die filling

properties, it is not suitable for the manufacture of

sintered structural components.

The carbonyl process is also well suited for the

extraction of both metals from lean ores. The

process can be controlled so as to yield a spherical

metal powder.

Blending • Thorough intermingling of different powders of

same composition or various grades of the same powders

Mixing• Intermingling of powders of more than one

material• Blending & Mixing

– Preparation of alloys from elemental powders– Production of dispersion strengthened alloys– Production of porous bearings– Electrical and magnetic materials

• Additives are added during blending

– Lubricants

– Binders

– Deflocculants

• Purpose of blending

– Uniform distribution of powder

• Problems faced

– Variation in particle size distribution

– Oxidation of powder

– Segregation of powder

• Excessive blending results in

– Segregation

– Change in powder characteristics

– Work hardening of particles reducing compressibility

• Mechanism in mixing

– Convective mixing

– Diffusive mixing

– Formation of slip planes

• Wet mixing

• Dry mixing

Compacting(briquetting)

• Forms metal powder compacts of desired size and shape with sufficient strength to withstand ejection from the die and subsequent handling up to the completion of sintering

• Techniques

– Pressureless shaping technique

– Cold pressure shaping technique

– Pressure shaping technique with heat

Pressure less shaping technique

• Loose sintering

• Slip casting

• Slurry casting

• Clay type moulding

• High velocity projection

Loose sintering

• Mechanical vibration in the mould followed by heating to sintering temperature

• Simple and low cost

• Porosity varies from 50% to 90%

• Not suitable for complex parts– Difficulty of part removal

– Flow characteristic of powder

– Shrinkage during sintering

• Die material – carbide, graphite, stainless steel, CI

• Poor dimensional accuracy

Cold pressure techniques

• Cold die compaction

• Isostatic pressing

• Powder rolling

• Explosive forming

• Cycle compacting

• Vibratory compacting

• Centrifugal compaction

• Powder extrusion

Die compaction technique

• Pressure applied mechanically or hydraulically.

• Single acting compaction– Thin parts like washers,

discs, thin rings etc

• Double acting compaction– Bushings , bearings

• Double acting floating die compaction

Die compaction

Isostatic pressing

• Pressure is applied in a flexible mould.

• Glycerin, water, hydraulic oils, gases, rubber or plastics etc.

• Even pressure & density distribution

• Complex shapes• Economical for very large products• Excellent electrical properties• Dimensional shrinkage is reduced• Ductility is improved.

Powder rolling

• Uniformly dense component

• High productivity

• Production of metallic strips

• Porous material suitable for fuel cell and alkaline batteries

Extrusion

• Super alloy powders

• Similar to rolling but density is increased

Pressure forming with heat

• Hot pressing

• Hot working

• Hot isostatic pressing

• Spark sintering

• Hot coining

Hot pressing

• applying pressure and temperature simultaneously

• compacting and sintering of the powder takes place at the same time in a die.

• Fe and Brass powders

• Absorbed gases and volatile impurities are removed

• Increased die life

• Temperature gradient is reduced

• Low temp requirement

• Near net shape

Sintering

• Imparts strength, densification and dimensional control• Decrease in free energy due to decrease of surface area.• 0.7 to 0.9 melting temperature of the major constituent• Defined as the heating of loose or compacted aggregate of

metal powders below the melting temp with or without the application of external pressure to transform it to a dense material by interparticle bonding

• Progressive transition without melting– Diffusion of particle to particle– Formation of grain boundary– Closing voids present in the green compact

Changes during sintering• Dimensional changes

• Chemical changes

• Electrical property changes

• Phase changes

• Relief of internal stresses

• alloying

Stages in sintering

• Adhesion without shrinkage

• Densification and grain growth stage

• Final stage with closed pores or elimination of the last isolated rounded pores

Sintering mechanisms

1. Adhesion mechanism.

2. Material transport mechanisms.

Adhesion mechanism

• Elementary bonding process

• No material transport

• Major contributor in initial stage of sintering process

Material transport mechanisms

1. Recovery and recrystallisation

2. Plastic or viscous flow

3. Evaporation and condensation

4. Volume diffusion

5. Surface diffusion

6. Grain boundary diffusion mechanisms

• Depends on powder material, its characteristics, sintering temperature and atmosphere.

Recovery and recrystallisation

• Small displacement of atoms

• Observed in powders having severe lattice distortions due to grain growth which occurs along with densification.

• Stress removal promotes mutual bonding of powders

Viscous flow

• Displacement of atoms over large distance

• Due to application of external pressure the atoms flow under stress

• Assuming total surface tension of all pores is equal to external pressure applied.

• Slow process.

Evaporation and condensation

• Not usually found in metal powders( exception: chromium)

• NaCl

• Material transport through a gas phase.

• Evaporation of material from surfaces of spheres and condensation in the neck region.

• Higher Vapor pressure at convex surfaces

• Neck growth without shrinkage or densification.

Volume diffusion mechanism.• Most important mechanism• Atom movement into vacant lattice sites • Vacancy gradient occurs between

– Neck area and interior of the system– Distorted and undistorted lattice– Particle centers and mid point of contact with adjacent particles

• Vacancies diffuse from the neck area• Atoms diffuse in opposite direction producing welding effect.• Governed by

– geometrical arrangement of powder particles– Type of vacancy sources and sinks

• Vacancy source- smaller pores and concave surfaces• Vacancy sink- large pores, grain boundaries, flat or convex surfaces• Occurs at high temp

Surface diffusion mechanism

• Movement of atoms on external surfaces

• Exchange of surface atoms and surface vacancies

• Responsible for neck formation

• Significant for sintering fine powders at a temp less than that of volume diffusion

• Shrinkage occurs

Grain boundary diffusion mechanism

• GB formed between adjacent particles in the original sintered mass

• Vacancies are moving along with GB to external surfaces

• Last stage of sintering• Elimination of isolated pores.• Atoms from external surfaces or locations along the GB

will diffuse towards the interior and this promotes – Neck growth– Rounding or shrinkage of the pore

• Found more effective with decreasing sintering temp.

PRE SINTERING

• PM parts will be hard after sintering process

• Machining will be difficult

• Parts are pre sintered at a lower temperature than sintering temp

• Provides adequate strength for handling and machining

• After machining sintering is done

• drilling is usually carried out in this method

• Pre sintering removes lubricants and powders

Sintering Atmosphere

• Sintering atmosphere selection depends on

– Characteristics of the material

– Properties desired in the sintered product

• Changes occurring prior to sintering

– Reaction of constituents among each other

– Reaction of atmosphere with material to be sintered

– Reactions of atmosphere with furnace refractories

Sintering Atmosphere [contd].

• Functions– Must prevent oxidation on the metal surface at

the sintering temp.

– Must avoid carburizing and decarburizing reactions and nitriding conditions in certain metals

– Must have tendency to reduce surface films such as oxides on powder particles

– Must not contaminate the metal powder compact at the sintering temperature

Sintering atmosphere types

• Reducing atmosphere– Dry H2 or dissociated NH3

– Exothermic atmosphere having low or medium carbon potential.

– Endothermic atmosphere enriched with hydrocarbon gases to produce a carburizing atmosphere

• Neutral atmosphere

• Oxidising atmosphere

FINISHING OPERATIONS

• Coining and sizing.

– These are high pressure compacting operations. Their main function is to impart

• greater dimensional accuracy to the sintered part, and

• greater strength and better surface finish by further densification.

• Forging.

– The sintered PM parts may be hot or cold forged to obtain exact shape, good surface finish, good dimensional tolerances, and a uniform and fine grain size

– Forged PM parts are being increasingly used for such applications as highly stressed automotive, jet engine and turbine components.

• Impregnation.

– The inherent porosity of PM parts is utilized by impregnating them with a fluid like oil or grease. A typical application of this operation is for sintered bearings and bushings that are internally lubricated with upto 30% oil by volume by simply immersing them in heated oil.

– Such components have a continuous supply of lubricant by capillary action, during their use.

– Universal joint is a typical grease – impregnated PM part.

• Infiltration. – The pores of sintered part are filled with some low

melting point metal with the result that part's hardness and tensile strength are improved.

– A slug of metal to be impregnated is kept in close contact with the sintered component and together they are heated to the melting point of the slug. The molten metal infiltrates the pores by capillary action. When the process is complete, the component has greater density, hardness, and strength.

– Copper is often used for the infiltration of iron – base PM components.

– Lead has also been used for infiltration of components like bushes for which lower frictional characteristics are needed.

• Heat Treatment.

– Sintered PM components may be heat treated for obtaining greater hardness or strength in them.

– Carburizing

– Nitriding

– Carbonitriding

– temepering

• Machining.

– The sintered component may be machined by turning, milling, drilling, threading, grinding, etc. to obtain various geometric features.

• Finishing.

– Almost all the commonly used finishing method are applicable to PM parts. Some of such methods are plating, burnishing, coating, and colouring.

• Plating.

– For improved appearance and resistance to wear and corrosion, the sintered compacts may be plated by electroplating or other plating processes.

– To avoid penetration and entrapment of plating solution in the pores of the part, an impregnation or infiltration treatment is often necessary before plating.

– Copper, zinc, nickel, chromium, and cadmium plating can be applied.

• Burnishing. – To work harden the surface or to improve the surface

finish and dimensional accuracy, burnishing may be done on PM parts.

– It is relatively easy to displace metal on PM parts than on wrought parts because of surface porosity in PM parts.

• Coating.– PM sintered parts are more susceptible to environmental

degradation than cast and machined parts. – This is because of inter connected porosity in PM parts. – Coatings fill in the pores and seal the entire reactive

surface.

• Joining. – Electric resistance welding is better suited than oxy-

acetylene welding and arc welding because of oxidation of the interior porosity.

– Argon arc welding is suitable for stainless steel PM parts.

Advantages of P/M for Structural Components:

These may be classified into two main headings;

(a) Cost advantages, and

(b) Advantages due to particular properties of sintered components.

Cost Advantages:

(i) Zero or minimal scrap;

(ii) Avoiding high machining cost in mass production as irregularly shaped holes, flats, counter bores, involute gear teeth, key-ways can be molded into the components;

(iii) Extremely good surface finish at very low additional cost after sizing and coining;

(iv) very close tolerance without a machining operation;

(v) Assembly of two or more parts (by I/M) can be made in one piece;

(vi) Separate parts can be combined before sintering.

(vii) High production rates

Advantages due to the particular properties of sintered components.

(i) By achieving up to 95% density, the mechanical and physical properties are comparable with cast materials and in certain cases with wrought materials. In certain cases 99.9 % dense structure can be obtained (liquid phase sintering);

(ii) Platting is also possible directly at 90% density and above and after impregnation of the pores at lower densities.

(iii) Damping out vibrations and noise property with controlled residual porosity;

(iv) Ability to retain lubricants such as lead, graphite and oil giving less wear and longer life to bearings;

(v) Achieving a close control of porosity to give a specified balance between strength and lubrication properties (a superiority over wrought materials);

(i) Improved surface finish with close control of

mass, volume and density;

(ii) Components are malleable and can be bent

without cracking.

P/M makes possible the production of hard tools

like diamond impregnated tools for cutting

porcelain, glass and tungsten carbides.

Reactive and non-reactive metals (both having

high m.p &low m.p) can be processed.

Applications

• Automobile components

• Aerospace components

• Industrial machine parts

• Electric motors

• Electronics industry

• Medical implants