powder metallurgy 1

8/12/2019 Powder Metallurgy 1

1/25


2/25

PowderMetallurgy(P/M)

Powdermetallur isanancienttechnolo of ressin

metalpowder

in

to

specific

shape.

. .

solidstatereducedferrousgranulesbysinterforging

technique.

ModernP/Mtechnologystartedinthe1920swith

productionofcementedcarbidesandmassproductionof

porousbronze

bushes

for

bearing

applications.

,

production

method

of

making

components

from

powders


3/25

Stagesofpowdermetallurgyprocesssing Productionandcharacterizationofpowders:

Compactionof

powders

g


4/25

ProductionandcharacterizationofpowdersChemical 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 carbonylsElectrolytic deposition: Metals like Cu, Be, Fe, Ni, etc can be

sponge, powder or in a form which can be mechanically

disintegratedtom zat on o mo ten meta s: tom zat on nvo ves s ntegrat on

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

, , ,

crushing or by impact.


5/25

Treatmentofpowders

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.

components or particle of very fine size. Mixing is very

.


6/25

ompact ono pow ersFollowingarethemajorfunctionsofcompaction

To

consolidate

the

powders

in

to

desired

shape Toprovidethedesiredfinaldimensionstothecomponents

Toimpartthedesiredlevelandtypeofporositytothe

com onent

Toprovideadequatestrengthforsubsequenthandling.

Die compaction is the widely used method of compaction.

Densities of up to 90% of full solid density can be achieved

by die compaction


7/25

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 presinter heating operation may be necessary for

removing the lubricant or binder.

For metallic alloys, sintering is carried out in a protectiveatmosphere, inside a furnace at temperatures generally at

75% of the absolute melting temperature of the alloy.

In some cases some amount o me t ng or qu p asesintering may take place during the sintering operation.


8/25

PowderproductionChemicalmethodofpowderproduction:

These involve the following:

Solid state : E.g. Reduction of iron or tungsten oxide with a

reducin as.

Gaseous state: Reduction of titanium chloride (TiCl4) vaporwith molten magnesium

queous so u on: e.g. e prec p a on o cemen copperfrom coppersulphate solution with iron, or the reduction of

an ammoniacal nickel salt solution with hydrogen under

pressure (hydrometallurgy method).


9/25

SolidstateReduction:In this oxides of Fe are heated in a reactor and reducin ases areallowed to flow above the bed of oxide layers. The common iron

ore is magnetite (Fe3O

4). In the presence of Co/CO2 gas mixture

CO CO2

oxides and finally to iron may be expressed as:

3 Fe2O

3(s) + CO> 2 Fe

3O

4(s) + CO

2(g)

Fe O s CO

> 3 FeO s + CO

FeO(s) + CO(g)> Fe(s) + CO2(g)

In addition to this Hydrometallurgical reduction techniques are

also used in which metals are directly reduced from aqueous

solutions by either reduction with another metal cementation ,

or by gaseous reduction.


10/25

For each oxide and reaction tem erature, a critical CO/CO ratioin the gas mixture must be exceeded for the reaction to

.

The reduction of Fe2O

3to Fe

3O

4occurs between 200500 C and

a minimum CO CO2

ratio of approximately 1 10 is required.

Fe3O4 is reduced to FeO between 500900 C, and FeO is

reduced to iron at temperatures between 9001300 C.

Some specific powders produced by chemical methods are Iron

owders Co er owders Ni Co owders Ti owders

Tungsten powder, Ta and Nb powder, Al2O3 , etc.


11/25

Electrolyticmethod:In electrodeposition 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 electrodeposition 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

conventional P/M processing.


12/25

The process variables controlling the powdery deposit are

High current density

Weak metal concentration

Additions of colloids and acids

Low temperature

Hi h viscosit

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.


13/25

Atomization method

In t s mo ten meta s s ntegrate n very ne rop ets an

allowed to solidify. This technique is widely used due to the ease of

making highly pure metals and prealloyed powders directly from

the melt. The molten metal is forced through a fine orifice, possiblyat the bottom of a crucible and high velocity gas or liquid stream

very fine spray of droplets. The fine droplets during the flight,

solidifices into very fine metal powders.


14/25

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 finerop ets.

The droplets solidify when they fall in the chamber.

e pow er s co ec e a e o om o e c am er. The powder is removed from the chamber and dried (if

y .


15/25

Fine particles are favored by

Superheated metal

Low metal surface tension

High atomizing pressure

Low metal viscosity

Hig atomizing agent vo ume

High atomizing agent velocity

Optimum apex angle.


16/25

The particle shapes of atomized powders can be tailored to

almost perfectly spherical to high irregular shape by controlling

the process variable. Sphericity is favored by

High metal surface tension

High pouring temperature

z , y

Low jet velocity

Long apex angles in water atomization

Long flight path.


17/25

Gas atomizationThe common atomizing gas media are Nitrogen, argon or air. There are two

ypes o nozz e: e x erna m x ng nozz e n w c con ac e ween emelt and the gas takes place outside the respective nozzle. This is used

exclusively for atomization of metals. Internal mixing nozzle are for

.

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 105

Pa to 42 x 105

Pa a gas ve oc es rom ms o ms .

Under this condition, the particle quench rate is102 K s1.

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%)


18/25


19/25

WateratomizationIn water atomization, a high pressure water stream is forced through the

stream.

The overall energy efficiency of the process is < 4%.

steels. Due to the oxide formation, water atomization is not used for

, , , .

Thewaterpressureisgenerally intherangeof35x105 Pato210x105 Pa

watervelocities

of

40

ms1 to

15

ms1

particlecoolingratesisoftheorderof103 Ks1 to104 Ks1.

Ifthesurfacetensionofthemeltishigh,oncethedropletformsitassumesa

sphericalshapeHigherviscosity,highercoolingrateandshortertimeduration

resu s

n

s ow

sp ero za on process

an

ence

resu s

n

rregu ar

s ape

powders.

Inadditiontothese,otheratomizationprocessincludesLiquidgas

, , ,

atomization,etc.


20/25


21/25

Mechanical Methods

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 H drides

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


22/25


23/25

Rollercrusher


24/25

Attritionballmill


25/25

powder metallurgy 1

Documents