PowderTechnology_2

8/22/2019 PowderTechnology_2

1/13

1

P/M Processing:A Brief Tutorial on Powder Metallurgy andParticulate Materials

Randall M. GermanCAVS Chair ProfessorCenter for Advanced Vehicular Systems

Mississippi State University

[email protected]

copyright82006 R. M. German

IntroductionDefinitions:Powder Metallurgy a methodof producing components by

pressing or shaping metal powderswhich may be simultaneously orsubsequently heated to create acoherent object. It includes powderfabrication, testing, and handlingsteps. The common abbreviation isP/M.

Particulate Materials - smallsolid particles have fluid-likeproperties that allow easy forming,leading to unique products oftennot available via alternative

approaches. It is related to powdermetallurgy, but includes a broaderrange of compositions such asceramics, composites, cermets,and several high-performanceproducts.

Particle - a discrete solid with asize less than 1 mm. Particlescome in many sizes and shapes,ranging from the size of a virus toa grain of sand. Many engineeringparticles range from 0.1 m to 200

m in size, with ceramic particlestending toward the smaller sizesand plastic particles tendingtoward the larger sizes. Forreference, human hair is about 100m wide.

Powder solid particles thatcollectively exhibit fluid-likeattributes, such as flow and an

ability to conform to the shape of acontainer.

Overview of the P/MProcess:

linkages between subfields

Powder Technology dealswith powder size and shape,powder fabrication, and treatingpowders prior to consolidation.

Powder Processing dealswith the conversion of the powderinto an engineered shape especially compaction, sintering,and densification steps.

Product Characterization focuses on ensuring properperformance via testing quality,measuring properties, andcharacterizing the linkagesbetween performance to thepowder and process.

Why we use powders:bedsides food additives, paintpigments, catalysts, and inks,there are three clusters based onproduction cost benefits in largeproduction volumes (ferrous

automotive components), uniquemicrostructures in multiple phasematerials (self-lubricating bearings),or fabrication of materials difficultto process by other means (oxidedispersion strengthened refractorymetals).

three justifications for usingpowders

Industry Structure - shows whoworks and participates in the fieldand attends conferences, furthershowing the flow of materials andinformation

industry structure

PowderCharacterization

Concerns include thefollowing:1) particle size distribution2) particle agglomeration3) surface area4) interparticle friction5) flow and packing6) internal structure7) composition, homogeneity.

Single best tool is thescanning electronmicroscope (SEM) it givesthree-dimensional size, shape,packing, and agglomerationinformation.


2/13

2

example SEM of spherical powder

agglomerated, small spongepowder in SEM

Primary concern with particlesize is determination of acharacteristic size. Powderscome in a wide range ofsizes, shapes, chemistries spheres to sponges:

Particle Size Measurements:Microscopy

measure size of images

Screening (Sieve Analysis) measure weight of powder ineach screen interval,historical basis for othertechniques, but limitedaccuracy (" 4 to 8%accuracy) and size range(over 45 m).

schematic of sieving

mesh wires per inch (largermesh number is smaller openingsize)

example of a woven mesh sieve

example meshes openings -60 mesh =250 m100 mesh =150 m200 mesh =75 m

270 mesh =53 m

subsieve smaller than 325mesh (below 45 m)

minus sieve designatespowders that passed through thatsize (ex. -325 mesh 300 m)

mixed designation forexample -200/+325 indicatespowders below 75 m yet largerthan 45 m

Sedimentation measure particles based onsettling time or velocity in airor water or viscous fluid

terminal velocity balances forces

Stokes lawD =particle diameterV =steady-state settling velocityDM=solid theoretical density

DF=fluid density0=fluid viscosity

D = [18 V 0 / (g (DM - DF))]1/2

requires laminar settling ofparticles, can add centrifugal forcefor smaller particles.

Light Scattering measureparticles dispersed andpassing through laser or lightdetector; Fraunhofer and Miescattering are captured insingle computerized

instrument

disperse particles streamingthrough light sources provideangle-intensity data fordetermining particle size

most accurate, largest size range,and highly automated instruments,can measure from 20 nm to over 1mm in single instrument


3/13

3

Light Blocking thedispersion of particles in fluidand passage of particlesthrough detector region

dispersed particles cast shadowsproportion to size

measure shadow size, assumesphere cast the shadow tocalculate the size

Equivalent Spherical

Diameter most frequentbasis for particle size

based on assumption that theparameter measured wasproduced from a sphere; usuallyfrom particle volume or projectedimage area or surface area

example for projected image

solutions based on volume,surface area, projected area;give different results so basisof calculation needs to bespecified

Particle Size Distribution:Cumulative Size Distribution

shown here as both thecollective population (or number)of particles and weight (or mass) ofparticles (left axis is percent)smaller than size (on lower axis)

cumulative particle size distribution

key metrics - D10,D50, and D90corresponding particle sizes at

10%, 50%, and 90% on thecumulative particle size distribution

cumulative particle size distribution,showing median at 72 m sizecorresponding to 50% smaller

mean average not usedmuch

median central size (half largerand half smaller) used very often(same as D50)

mode most common size

log-normal distribution mostcommon for powders, thehistogram looks like a bell curvewhen the particle size is on alogarithmic scale (normal curve orbell curve or Gaussiandistribution); gives linear equation

when standard deviations areplotted versus log size

Particle Shape:Quantitative Descriptors * aspect ratio (longest overshortest measures)

* surface area to size ratio* curve fits to images

Qualitative Descriptors usually words based onstandard comparativeimageswords like sponge, sphere, andvarious fruits and vegetables suchas carrot; note special terms suchas flake, fiber, and dendritic (fernleaf shaped) and irregular

example particle shape terms

Surface Area:Permeation Approach relies on gas permeation andDarcys lawgas permeation (FSSS =Fishersubsieve size) widely used forsmaller (subsieve) powders inrefractory metal and cementedcarbide fields; usually surface areais converted to equivalentspherical diameter in m

test set up for measuring powdersurface area from air permeationrate


4/13

4

Gas Absorption Approach relies on chilled sample(liquid nitrogen) andexposure to gas (nitrogen)that absorbs on surfaceexternal area of powder, measuredby amount of gas that sticks or

absorbs (BET technique) andsurface area is usually given inunits of m

2/g

BET technique changes gaspressure to detect when surface iscoated with absorbed gas as ameans to determine surface area

approximate relationbetween particle size D50(m) and specific surfacearea S (m2/g) and material

theoretical densityD is

S = 6/(DD50)

assuming the particles arespheres

Interparticle Friction:Powder Packing and Flow used to measure how apowder will respond to

processing steps

density mass per unit volume(usually g/cm3)

apparent density powder inthe loose or poured state, novibration

tap density powder densityafter prolonged vibration

pycnometer density densityof the powder if pressed to void-free condition

flow time the time required for50 g to pass through a 60 funnel;some small powders are not freeflowing

angle of repose a measure ofthe resistance to particle slidingreferenced to horizontal

tests devices

Hall and Scott testers for apparentdensity (Hall used for free flowingpowders, Scott used for powdersthat do not flow)

Arnold meter fills bushing withpowder and slides the powder overa cavity to simulate die pressing,used to determine apparentdensity

angle of repose anothermeasure of interparticle friction,measured as angle from horizontalfor loose or vibrated powder

evidence of the angle of repose

Compressibility powder iscompressed to measurechange from apparentdensity; two forms, press tostandard pressure (say 550

MPa and report presseddensity or press to standarddensity and report requiredpressure

Compression Ratio measure of change indensity from apparent topressed = pressed densitydivided by apparent density

Chemical Tests:Standard Tests same asemployed in other fields;emission spectroscopy mostcommon

Special Tests loss on ignition determinehow much surface contaminant onthe powder via heating in hydrogenand measuring mass change

acid insoluble dissolvepowder in acid and measureresidue as basis for estimation ofceramic inclusions

Powder Fabrication

Main Approaches: milling,electrolysis, chemical


5/13

5

reduction, atomization ofmelts. Approach selectiondepends on material, costs,and desired purity. For alloysatomization is usually best.

Milling mechanical attritionof powderperformed using tumbling or fallingballs; crushers, impact devices,

jaw mill or ball mill; intense millingpossible with stirred ball mill(termed mechanically alloying orattritor milling)

rotating ball or jar mill formechanical attritioning of powder

grinding rate relatesenergy input W to particlesize output D;

W = g [DF-a DI-a]

subscriptF = final, I = initial,and a = 1 to 2

characteristics of milled powders angular, brittle, contaminated

milled, angular powder

attritor milling a stirred ballmill, can deliver mechanicallyalloyed powder

Electrolytic production ofpowder from chemicalsolution by use of directcurrent

powder produced by electrolysis,good for copper, gold, silver, and

similar metals; but not alloys

characteristics of electrolyticpowders sponge-dendritic, pureelements, some possiblecontamination from bath chemistry

electrolytic (dendritic) copperpowder

Chemical Reaction theproduction of powders fromprecursor chemicals such asoxides or compounds orsolutions, examples of thechemical reactions toproduce a powder are asfollows:

reduction (molybdenum, tungsten,iron, copper); example of tungstentrioxide reduced at 1000C byhydrogen to produce tungstenpowder and steam:

WO3 +3H26 W +3H2O

this process is applied to manyeasily reduced metals iron,copper, tungsten, andmolybdenum are most commonproducts

cubic molybdenum powder formedby hydrogen reduction of an oxide

reaction (compounds such asaluminides, carbides); for examplethe above tungsten powder ismixed with graphite and heated toproduce tungsten carbide (WC)

vapor decomposition (iron, nickel);

for example iron is reacted withcarbon monoxide and thencatalyzed to nucleate small ironparticles

agglomerated precipitated ironpowder from the carbonylreduction process carbonyl iron

growth rings inside carbonyl ironpowder

reaction techniques includecombustion or solid-state routes;for example solid powders are


6/13

6

heated to a reaction temperatureto produce a compound:

3Ni +Al 6 Ni3Al

porous chemically reacted orcombusted Ni3Al nickel aluminidepowder

precipitation techniques (changesin solution chemistry) to nucleateand grow very small powders;widely used for pure elements(cobalt, nickel, gold, silver)

chemically precipitated palladiumsponge powder

Atomization production ofpowder by the breakup andthen solidification of a streamof molten metal, good foralloys and pure metals thatmelt up to 1700C

gas atomization disruption ofthe melt stream based on high

pressure gas can be air, argon,nitrogen, helium

air-horizontal units for low meltingtemperatures (zinc, tin, solder)

inert gas units for high purityvacuum melted alloys (nickelsuperalloys)

air vertical units for intermediatequality (bronze, aluminum, copper)

schematic of vacuum inert gas

atomization for production of small,pure high purity spherical powder

characteristics of gas atomizedpowders ligaments and sphere,some splats, some satellites

SEM of gas atomized spheres withsplats and ligaments

superheat excess heating overmelting range of the alloy toensure no freezing

instability the cause of the meltstream disintegration first intoligaments then into spheres(Rayleigh instability)

satellite case where small solidparticles stick to larger particles tocreate agglomerates

laminar flow ability to sweepparticles away from nozzle to avoidsatellite formation

close-coupled nozzle gasoutlet is directly in contact withmelt stream

example of satellite particles

water atomization meltstream disintegration using highpressure water jets, good for largevolume production of lower meltingmaterials (iron, copper, bronze,

tin); scale up to large meltingbatches (like a steel mill)

melt pouring into atomizer nozzle

schematic of water jets being usedto atomize a melt stream

characteristics of water atomizedpowders irregular, larger (100m common), usually highly


7/13

7

contaminated by water 3000ppm of oxygen is typical; often canbe reduced after atomization byroasting in hydrogen or carbonmonoxide

SEM of water atomized ironparticle

centrifugal atomization molten material combined withcentrifugal force to throw offdroplets that solidify into particles

spinning cup centrifugal atomizer

characteristics of centrifugalatomized powders spherical,larger, often bimodal sizes

SEM of centrifugal atomizedpowder

Specialty AtomizationApproaches plasma,vacuum melt explosion,spark erosion, nichetechnologies

high temperature plasma sprayprocess for atomization

SEM of plasma atomized tungsten

Nanoscale Powders aregenerally smaller than 100nm (0.1 m in size) formedby vapor condensation,chemical reaction, explodingwires or intense millingroutes - characterized by highlevels of agglomeration, highsurface area, difficult handling

small nanoscale metal powder

Example Metal Powders:showing powder, productionroute, median size, andshapefactors include chemistry(especially purity), material

properties (especially chemicalstability and melting temperature),application requirements, andcosts

copper, electrolytic, 40 m,dendritic

iron, oxide reduction in hydrogen,50 m, irregular sponge

niobium, milled, 10 m, angularand irregular


8/13

8

tungsten, oxide reduced byhydrogen, 4 m, angular andagglomerated

Commercial Summary inUSA metal powderproduction is a $3 billionindustry, and global value isprobably just under $10billion; usually highest cost isassociated with lowest use

relative cost of metal powders

relative consumption of metalpowders

example costs water atomized iron =$1/kgcarbonyl nickel =$15/kggas atomized stainless =$15/kg

PowderMicrostructures

Microstructure:the internal arrangement ofphases, pores, and grains asseen in a microscope

example structures seen insidepowders

pore and dendrite structure insidegas atomized powder

possible variants includedendritic slower cooling, largegrains with segregation ofchemistry evident in microstructure

equiaxed mostly crystalline, butgrains are same size and shapedue to rapid cooling

nanoscale crystals below 0.1m, so high disorder in atomicstructure

amorphous no crystalstructure, no atomic order

Atomization Microstructure usually corresponds to howfast heat is extractedlarger particles favor dendritic andonly very rapidly cooled particlesgive amorphous with rapid cooling

microstructure measure -most common is grain size orsecondary dendrite spacing;cooling at a million degrees persecond gives about 1 m spacing

microstructure feature used formeasuring cooling rate

example particle size effect onmicrostructure

grain region of same atomicalignment as crystal

grain size measure, like

particle size, of crystal dimensions

grain boundary region ofdisrupted bonding at junction ofcontacting grains, usually about 5atoms thick

nanoscale grain size is small,generally below 0.1 m, so high


9/13

9

proportion of the atoms are ongrain boundaries

circles represent atoms, with highfraction of atoms at grainboundaries in nanoscale materials

fraction of atoms located oninterfaces as grain size becomesvery small

since grain size influencesproperties, a small grain size

opens new properties

amorphous state requires rapidcooling possible via coldsubstrate, small droplets in gasatomization, or intense mechanicalalloying

amorphous structure where theatoms (circles) have no repetitivepattern but are in random positions

production of amorphousmaterials relies on freezingin random (liquid like) atomicstructure

melt extraction for rapid cooling

example flakes by melt extraction

amorphous metals glassymetals; lack crystal structure,usually highly alloyed, produced byrapid cooling, atomization canproduce if particle size is small

Features for FormingAmorphous Metals

1) high alloys (such as Al-Ni-Mn)2) atoms of differing atomic sizes(such as Pd at 2.74 nm and B at1.96 nm)3) atoms of differing crystalstructures (such as face-centeredcubic and hexagonal closepacked)4) atoms with differing valences(such as Al at +3 and Li at +1)5) atoms with differing electro-negativities (such as Si at 1.8 andZr at 1.4)6) compositions near deepeutectics to suppress normalsolidification to low temperatures

basic concept formcomposition from atoms that arenot similar so solidification into acrystal requires considerableatomic sorting time; example54Au-26Pd-19Sb, 41Zr-23Be-13Ti-13Cu-10Ni, and 40Pd-40Ni-20P

critical cooling rate dependson alloy; defines heatextraction rate to produceamorphous phase:pure Ni 10,000,000,000 K/s62Ni-38Nb 2,000 K/s40Pd-30Cu-10Ni-20P 0.001 K/s

production of amorphous materialsby atomization favored by highalloy levels, rapid cooling, highdroplet velocities, and smallparticles

plot showing amorphous phaseformation in gas atomizationversus particle size andatomization gas (helium extractsheat faster than argon)

solidification time dependson particle size / particle velocityand is often in 0.1 to 0.4 s range;dominated by droplet size (smalleris faster) and rate of heat

extraction in atomization (turbulentgas is faster)

spheroidization time depends on particle size andviscosity of melt and is often in the0.1 s range

particle shape sphere ifspheroidization is faster thansolidification such as in gasatomization; ligament at

intermediate rates of cooling, andirregular if cooling is rapid

novel properties in theamorphous alloys -strength and hardnesscorrosion resistanceelastic modulusmagnetic responsewear resistance


10/13

10

comparison of strength and elasticmodulus, showing specialcharacter of amorphous alloys

example of Hall-Petch effectfor steel (strength depends oninverse square root of grain size):

30 m gives strength of 290 MPa5 m gives strength of 430 MPa1 m gives strength of 700 MPa

Tailoring Powders

Activities:these occur after powderfabrication targeted tomatching powder toproduction process attributes,but care is required to avoid

hazards, explosions, workerhealth issues, or fires

Definitions:pyrophoric a powder thattends to burn in air

mixing combination of powdersof different chemistries (such asiron, copper, and graphite aremixed to give a sintered steel)

blending combination ofpowders of same chemistry, butdifferent sizes, shapes, orproduction lots

feedstock a mixture ready forshaping or compaction

solids loading thecombination of powder as a

relative volume fraction in afeedstock (around 60 vol.% solid)

agglomeration process forjoining powders into clusters, usedto make small particles that feedand flow easily

deagglomeration ordispersion the breaking apartof agglomerated powders to attaindiscrete particles

dusting tendency of powder tospill fines during discharge or flow

saturation all of the pores arefilled with liquid

aggregate a hard cluster ofparticles that is not easilydispersed or broken into discreteparticles

packing the use of mixtures orblends of powders to improveapparent density, bimodalcompositions

lubricant polymer mixed with apowder to minimize tool wear inthe forming step (common

examples are zinc stearate, stearicacid, and lithium stearate)

binder polymer mixed with apowder to provide strength andtransport such as in extrusion(common examples are paraffinwax, polyethylene, or mixtures ofvarious oils and polymers)

J ustifications:several reasons for mixingand blending:eliminate inhomgeneitiesoffset separation during shippingadd lubricant or binderminimized dustingform new alloys or compositesto remove lot-lot variations

Mixing Variants:dry versus wet powderbatch versus continuous

Dry Mixing - mixing involvesshear, diffusion, andconvection

fundamental mixing processes diffusion, convection, and shear

double cone mixer for batch dryblending and mixing powders

mixture homogeneity is measuredby the uniformity of the ingredientsthroughout the powder lot

Mixing Rate end of mixingwhen segregation andmixing reach balance rateof mixing equals rate ofsegregation

mixing rate measured by themixture homogeneity (standarddeviation in composition overrepeated small samples):

M = Mo + exp(K t + C)

where M is the homogeneity, Moisthe initial mixture homogeneity, t isthe mixing time, and K and C


11/13

11

depend on the mixer design, mixeroperation, and powder

apparent density increases withmixing time, but is degraded byhigher lubricant contents

Mixture Density theoreticaldensity of powder-lubricantand powder-power mixturesfollows inverse rule ofmixtures:

1/DM = X1/D1 + X2/D2

DM=theoretical mixture densityX1=weight fraction of component1D1 =theoretical density ofcomponent 1X2=weight fraction of component2

D2 = theoretical density ofcomponent 2

Agglomeration: intentionalbonding of small particles toappear large

large agglomerates formed byspray drying

spray drying powder-solvent-polymer mixture is sprayed into hotchamber, solvent evaporates,producing large agglomeratedballs of small particles

fluid bed powder in chamberwith forces gas lifting the particlesagainst gravity as a polymer-solvent spray is added to themoving particles

granulation powder, polymer

and solvent are tumbled as solventevaporates to give agglomerates

agglomerated spherical particlesgranulated into a large cluster

spouted bed similar to fluidbed with particles traveling in acircular path to ensure all particlesare equally coated (superior tofluid bed for powders)

spouted bed to coating polymeronto powders such as foragglomeration

decrease in dusting with polymercoating

Wet Mixing: used to formslurries, pastes, andthermoplastic feedstock forinjection molding andextrusion

Examples: some polymer-solvent combinations usedfor agglomeration:paraffin wax-heptanecellulose- acetonepolyethylene glycol-water

variants include simple mixtures,intentional coatings or surfacebonds, and further diffusion bonds

examples of mixed, agglomerated,and diffusion bonded powders

Bonding Forces van derWaals (very weak), pendular(drops of liquid at contacts),and funicular (liquid nearlyfills voids between particlesso pores are spherical)

at saturation (all pores filled with

liquid) there is no bonding strength


12/13

12

pendular bond state with wettingliquid at particle contacts

pendular bonds provideagglomerate strength; estimatedpendular bonding force F

F = 3 (D

(=liquid-vapor surface energy

D =particle size

diffusion bond case; electronmicrograph and X-ray elementmap of diffusion bonded Ni on Feparticle

Wet Mixing:

Batch Mixing double conetwin shelldouble planetaryvarious tumbling containershigh intensity blades

batch mixer double planetary

continuous twin screw mixer

change in powder-polymer densitywith composition

critical solids loading -corresponds to peak in density inpowder-polymer mixtures; same asmaximum in mixture viscosity

viscosity resistance to shear;thick paint is hard to stir since thehigh particle content makes it moreviscous

plot showing how relative powder-binder mixture viscosity changeswith solids content

viscosity versus composition0M =viscosity of powder-polymermixture0B =viscosity of pure binderN =solids loading (vol. %)NC =critical solids loading (vol. %)

0M/0B =(1 - N/NC )-2

this model says infinite viscosity atthe critical solids loading

Particle Packing:Types of Packings orderedsuch as the stacking ofbricks and random such as

how balls fill a container

ordered packing of spheres

coordination number number of touching spheres orparticles

example ordered packings simple cubic, coordination of 6 anddensity of 0.52face-centered cubic, coordinationof 12 and density 0.74

Random Packing mosttypical of powder processingdense random correspondsto the tap density, highest densitypossible without pressure; forspheres coordination number near6, packing density near 0.60

loose random corresponds toapparent density, density on fillinga container without pressure orvibration; for spheres coordination

number near 7, packing densitynear 0.74

Improved Packing toimprove packing densityfill container with largerpowder first


13/13

13

1) fill holes in packing usingsmaller powder (withvibration)

2) holes require smaller powder isabout 15% size of largerpowder

3) optimal ratio is often 70% largeand 30% small

4) spheres pack best

density of packing is given bydensity of large powder plusdensity of small powder timesporosity in large powder

bimodal two modes in theparticle size distribution

multimodal multiple sizes, soparticle size distribution will show

many peaks

polydisperse broad size range

example of particle shape on

packing

packing density of cylinders based

on length to diameter ratio

bimodal packing behavior packing density versuscomposition

bimodal packing at optimalcomposition

Optimized Packing thepeak bimodal density f*

f* = fL + fS (1 fL)

were fL and fS are large and smallparticle packing densities

composition of peakXL* in terms offraction of large particles

XL* = fL / f*

Furnas Relation firstmodels for optimized packing,relied on wide particle sizedistributions

example of a high packing densitybroad particle size distribution

Percolation:Percolation long rangeconnections betweenparticles of the same speciesin a mixture; particles are in longcontacting strings within the

packing; example, make onepowder conductive and otherpowder nonconductive, thenpercolation corresponds toconditions where the mixture isconductive

schematic showing difference inpercolation (left) and isolation(right) giving differences inconduction

contiguity measure of thesame particle-particle contact areaper particle as a percentage of theparticle surface area or perimeter

upper drawing is no percolationversion, showing no contiguity,while lower version is connected tobe percolated

percolation limit compositionwhere first continuous string forms(how much conductor is added toinsulator to cause the insulator tobecome conductive)

PowderTechnology_2

Documents