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Powder Composite Relatedto Powder Metallurgy

Ordinance Factory Institute ofLearning

Manojit Ghosh

Assistant ProfessorBengal Engineering and Science University,Shibpur Howrah

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References

Powder Metallurgy ScienceR.M. German

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History of Powder Metallurgy

The principles of powder metallurgy were employed about 5000 year ago tomanufacture solid objects from iron.

Egyptian implemented PM as early as 3000B.C.

PM rebirth on 1829. by an English Man cold-pressed and sintered some

platinum powder and produce first ductile platinum.

Commercial tungsten produced in 1916.

PM route used in Germany for producing tungsten carbide cutting-tool tipsfollowing the first world war.

PM recognized widely after 1945 and onwards. This is the only manufacturingprocess in some cases.

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Definitions

Powder MetallurgyA method of producing components by pressing or mouldingmetal powders which may be simultaneously or subsequentlyheated to produce a coherent mass.

PowderParticles of matter characterized by small size, less than 1mm in size.

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Powder metallurgy is the forming ofmetallic powders into parts by means ofhigh pressure and subsequent heattreatment called sintering. This process

takes place below the melting point of themetals and therefore guarantees themanufacture of compact, almostabsolutely dense work pieces avoidingmelting.

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DefinitionsSinteringSintering is the central technological step of powdermetallurgy. Sintering causes the transformation of porousgreen compacts of powder into metallic componentssuitable for further processing, by means of a combination

of diffusion and surface tension under high temperatures.

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Why PMWhy PM

High Strength (M, C)

Easy Formability (M, P)

High Ductility (M, P)

High Fracture Toughness (M)

Resistance to Corrosion and Oxidation (C)

Resistance to creep and Fatigue (M, C)

Insulation at high temperature (C)

M: Metal; C: Ceramic; P: Polymer

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Hyper EnvironmentHyper Environment

Temperature : -185rC to 2750rC

Heating rate : 23 E4 to 57 E7 J/m2 Sec

Total Heating : 11.5 E7 to 170 E7 J/m

Heating time : 1 to 3600 Sec

Pressures : 0.01 to 400 atm

Loading : 1 to 30 g

Shear Stress : 4 E-2 to 7E2

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Product

DESIGNER

(Properties)

MATERIAL SCIENTIST ENGINEER

(Fabrication)(Microstructure)

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Uniqueness of Powder MetallurgyUniqueness of Powder Metallurgy-- II

Highly porous structure - a real possibility , Providespredictable & consistent porosity

Mixing and blending different constituents

Mechanical alloying Utilizes a wide variety of alloy systems

Production to near shape bodies i.e, Eliminates or minimizesmachining

Amenable to elements with very high melting point

Amenable to spray or plasma forming

Good finish

Offers long term performance reliability in criticalapplications

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Uniqueness of Powder MetallurgyUniqueness of Powder Metallurgy-- IIII

The high precision forming capability of PM generatescomponents with near net shape, intricate features and gooddimensional precision pieces are often finished without the

need of machining.

By producing parts with a homogeneous structure the PMprocess enables manufacturers to make products that aremore consistentand predictable in their behaviour across awide range of applications.

Provides materials that may be machined, heat treated, platedor welded to further improve properties and performancecharacteristics.

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Economic AdvantagesEconomic AdvantagesThe growth of the P/M industry during the past few decadesis largely attributable to the cost savings associated with net(or near-net) shape processing compared to othermetalworking methods, such as casting or forging. In somecases, the conversion of a cast or wrought component to

powder metal provides a cost savings of 40% or higher.

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Why Cost EffectiveWhy Cost Effective

Better material utilisation with close dimensional tolerances.Conventional metal forming or shaping processes, against whichPM competes, generally involve significant machining operationsfrom bar stock or from forged or cast blanks.

The PM process has: the highest raw material

utilisation (over95%) the lowest energy

requirement perKgoffinishedpart

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Powder CharacterizationPowder Characterization

Necessity

Better structure property correlation

Particle size and distribution

Particle shape and variation with particle size

Surface area analysis

Inter particle friction

Flow and packing

Particle structure

Chemical characterization i.e. composition, homogeniety and

contamination

A complete characterization includes the following

parameters:

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Powder SamplingPowder Sampling

Filter Use

Necessity

Step prior to characterization

Powder characterized should be representative of total

Problem due to stratification

Remedy???

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Characteristics of metal powder

The performance of metal powders during processingand properties of PM products are highly dependent uponthe characteristics of metal powders that are used.

Purity of powders: No impunities like oxides must notbe at surface. Chemical composition: It refers to type and percentage of

alloying element and impurities and usually determinesthe particle hardness and compressibility.

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Particle Size: It isexpressedbythe diameter forspherical shapedparticlesandbythe average diameter fornon-spherical particlesasdeterminedbysievingmethodor microscopic.

Metal powdersusedinpowder metallurgyusuallyvaryinsize from4to200microns.

Particle size influencesmoldstrength,density/porosityof

the compact permeability,flowandmixingcharacteristics,dimensional stability, etc.

Characteristics of metal powder- II

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Characteristics of metal powder - III

Particle size distribution: It is specified in terms ofa sieveanalysis, i.e., the amount ofpowder passingthrough100-,200-, etc.,mess sieves. It influences packingofthe powderand its behavior duringmoldingand sintering.

Particle Shape: It influences the packingand flowcharacteristics ofpowders. Spherical (Condensed Zinc) Round (Atomised copper)

Angular (Mechanicallyatomised antimony) Acicular , Dendritic,flakes, irregular, etc.Spherical particles have excellent sinteringqualities,howeverIrregulars-shaped particles are superior for practical molding.

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Characteristics of metal powder - IV

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Characteristics of metal powder - v

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Characteristics of metal powder - VI

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Characteristics of metal powder - VII

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Characteristics of metal powder - VIII

Particle microstructure: It revels various phases, impurities,includingfissures andinternal porosities.

Apparent density: It is define as the weight ofloosely heapedquantity ofpowder necessary to fill a givendie cavity

completely. Apparent density is influencedby chemicalcomposition,particle shape, size, size ofdistribution, methodofmanufacturingetc.

Flowrate: It is definedas the rate at whicha metal powder willflowunder gravity from a container throughanorifice bothhavingthe specific shape andfinish.

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Characteristics of metal powder - IX

Flowratemeasuretheabilityofapowdertobetransferred. FlowrateisanimportantcharacteristicsbecausethediemustbefilledrapidlywithpowdertoachievehighrateofproductionandEconomy.Flowratedependsuponparticlesize,shapes,apparentdensity,etc. Sphericalshapedmetalpowderpossesmaximumflowrateswhereasdendriticonestheleast.

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PRODUCTION OF METAL POWDER

Manufacturing Process: Atomisation Reduction Elctrolysis Crushing Milling Condensation of metalvapour

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REDUCTION PROCESSI

nreductionprocess,thecompoundsofmetals(usuallyoxidese.g.,ironoxide)arereducedwithCOorH2attemperaturebelowthemeltingpointofthemetal(e.g.,iron)inanatmospherecontrolled. Thereducedproductisfurthercrushedandground.

Example:Ironpowder;Fe3O4+4C =3Fe+4COFe3O4+4CO=3Fe+4CO2

CopperPowderCu2+ H2=2Cu+ H2OSameprocesscanbeusedforW,Mo, NiandCoThisprocessisconvenient,economicalandflexible. Most

practicableprocess.

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Crushing Powder

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MILLING PROCESS

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CONDENSATION PROCESS

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Consider the use of metal powders in the fabrication oftungstenlamp filaments, dental restorations, oil-less bearings, automotivetransmission gears, electrical contacts, nuclear power fuelelements, high-temperature filters, aircraft brake pads, andjetengine components. Furthermore, metal powders find uses in suchproducts as paint pigments, porous concretes, printed circuitboards, enriched flour, explosives, welding electrodes, rocketfuels, printing inks, brazing compounds, and catalysts. Worldwidemarket share for P/M parts are given in the following pie chart.

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Raw Material

Mix

Form

Solid state

fusion

or

bonding

-Sintering

Optional or

secondary

operations

Finished

product

Elemental or alloy

metal powder

Other additions

(die lubricants, graphite

Mix

Pressureless

molding slip casting

Die Compacting

( Mech. / Hydr.)Isostatic compacting,

Roll compacting, High

energy formingGas pressure bonding

Hot Pressing

Spark sintering

Extrusion

sinteringPre-sintering

Finishing steps ( heat treat,

plate, tumble, machine, oil,

plastic impregnated )

Manufacturing steps ( metal

infiltration, size, coin, forge,

reform)

Finished PM product

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Mixing To form a homogenous blend. 0.5 - 1.5% lubricant is normally added in the mix.

Popular lubricants are stearic acid, stearin, metallic stearates,especially zinc stearate, and increasingly, other organic compoundsof a waxy nature. Function of lubricants: reduce friction between the powder massand the surfaces of the tool powder must slide during compactionleading to achievement of uniform density from top to bottom of thecompact easy ejection of the compact BUT- problem of over lubrication

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Sintering Sintering is a heat treatment wherein the pressed parts gain strength. The most common sintering temperature range for iron-based alloys is 1100 -1150C. In some cases, higher sintering temperatures up to 1250C are

employed. The time at temperature varies between 10 and 60 minutes, depending on theapplication. The most common type of furnaces is the mesh belt furnace. Components areplaced on a tray, or directly on the mesh belt, which transports them throughthe furnace. Mesh belt furnaces are limited to a temperature of maximum 1150C. Forhigher temperatures, walking beam or pusher-type furnaces are common. Thesintering cost increases significantly if sintering temperatures higher than1150C are used.An atmosphere, which prevents oxidation, is necessary in the sintering furnace.Dissociated ammonia, endothermic or nitrogen-based atmospheres are commonlyused. It is of great importance also to have a controlled carbon potential, inorder to ensure consistent mechanical properties and tolerances on thesintered components.

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Infiltration Analternativemethodofimprovingthestrengthofinherentlyporoussinteredparts istofillthesurfaceconnectedporeswithaliquidmetalhavingalowermeltingpoint.

The interconnected porosity is filled with an alloy having a melting point lowerthan the sintering temperature of the metal of which the component is made,e.g., copper-based alloys infiltrate ferrous parts, usually during the sinteringphase. Infiltration makes the components impermeable and there is someincrease in mechanical properties, but at expense of dimensional accuracy.

Clearly if the molten copper is already saturated with iron its ability to erodethe surface is lost.

Pre- Sintering Burn off stage

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Industrial Application

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Application to PMApplication to PM

1.1. PorousPorous ProductsProducts

(a)(a) BearingBearing

(b)(b) FiltersFilters2.2. FrictionFriction PartsParts

3.3. ElectricalElectrical MaterialsMaterials andand ProductsProducts

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