Impact of Milling and Sintering on Growth of WC Grains in Liquid Co - and an evaluation of existing growth theories

of WC Grains in Liquid Co

-andanevaluationofexistinggrowththeories

EMANUEL EKSTRÖM

Master of Science Thesis, Stockholm 2007

School of Industrial Engineering and Management

Department of Materials Science and Engineering

Royal Institute of Technology

SE-100 44 Stockholm

Cemented carbides (WC-Co) are powder metallurgical products produced by

liquid phase sintering. WC-Co is widely used for making a large variety of

cutting tools, such as drills and insertsturning applications, due to its great

mechanicalproperties,where thehardnessof theWCgrains iscombinedwith

thetoughnessoftheoftheCobinder. WCgrainsizeandgrainsizedistribution

arethetwomostimportantfactorsto controlthemechanicalpropertiesofthe

products.

This study examinedthe grain growth dependence of dierent millingand

sintering times. Theresultinggrain size andgrain size distribution were mea-

suredusingimageanalysisonscanningelectronmicroscopyimages(SEM)and

by using electronbackscatter diraction (EBSD). Inaddition, the correlation

betweenhardnessand coercivity,themost commonindirectmeasures ofgrain

size,anddierentmethodsofcalculatingaveragegrainradiuswereinvestigated.

Anattemptwasalsomadetostudythecontributionofdefectstograingrowth.

This work also includes an overview of various grain growthequations and a

numericalimplementationofthese.

Experimental resultsshow that forshorter sintering times, powders milled

for short times (15 min and 1 h) have larger average grain radii. There is a

crossoverafter 6to 8h of sintering, wherethe powdersmilled foralongtime

(40hand200h), havelargeraverageradii. Themeasuredhardnessvaluescor-

relatewellwiththeaveragegrainradiuscalculatedfrom thegrain surfacearea

and the coercivity correlates with the establishedequations. EBSD measure-

mentsdetected boundariesthat could notbedetected by image analysis,and

that were not

Σ

Comparing heat-treated powder with the untreated resulted in a lower aver-

agegrain size after sintering for theheat-treatedpowder. Noneof thegrowth

equationsinvestigatedin thisworkcouldfullydescribetheexperimentalgrain

growth.

Through increased understanding of the grain growth, the growth can be

controlled and the end product can havethe desiredtool properties. The oc-

currence of abnormal grains in cutting tool applications can cause breakage,

which is especially important to avoid in applications such as PCB drills. A

correlation between hardnessand grain size provides further means forcheap

andfastindirectmeasures ofthegrain sizeinproduction.

Keywords: Cementedcarbide;WC-Co;Abnormalgrain growth;Nucleation;

Defects;Coarsening;Milling;Sintering;EBSD; Mechanicalproperties;

Hardness;Simulation;Modeling; Cuttingtools

Hårdmetallärettpulvermetallurgisktmaterialsomtillverkasgenomsmältfass-

intring och som kännetecknas av hårdhet, styvhet och god slitstyrka. Vol-

framkarbidens (WC)kornstorlekochkornstorleksfördelning ärtvåviktiga fak-

torerförattkontrollerademekaniskaegenskapernai hårdmetall.

I denhärstudien harkorntillväxtensberoendepåmalningochsintringun-

dersökts. WC-Comaldesochsintradesfyraolikatiderochkornstorleksfördelnin-

genmättesmedbildanalyspåsvepelektronmikroskopbildersamtmedelectron

backscatterdiraction (EBSD).I arbetetharävenkorrelationenmellan hård-

het,koercivitetocholikasättatt beräknamedelkornstorlekenundersökts. Ett

försökharocksågenomförtsförattstuderahurdefekterna idetmaldapulvret

påverkar korntillväxten. I arbetet har även ett ertal olika tillväxtekvationer

modelleratsnumeriskt ochför ochnackdelarmed deolikatillväxtekvationerna

harvägtsmotvarandra.

En långmaltid (40h och200h) visadesigge litenkornstorlekförsintring

kortare än 6h, men för sintringarlängre än 8h gavistället kort malning (15

min och 1h) den mindre kornstorleken. Det visade sig att uppmätt hårdhet

korrelerarbästmeddenmedelkornstorleksradiesomräknatsframfrånkornytan.

I EBSD mätningarna kunde man observeraett ertal korngränser, utöver

Σ

korngränser,somintehadedetekteratsmedbildanalys. Värmebehandlingenav

detmaldapulvretminskadekorntillväxtenunderefterföljandesintring. Ingenav

deundersöktatillväxtekvationernakundebeskrivadeexperimentellaresultaten

fulltut.

Genom ökad förståelse för korntillväxt kan man kontrollera tillväxten och

slutproduktenkanfåönskadeegenskaper. Förekomstenavabnormkorntillväxt

i skärverktyg i hårdmetall är en av de vanligaste kritiska defekterna och det

är speciellt viktigt är undvika korntillväxt i tillverkning av små verktyg, som

tillexempelkretskortsborrar. Hårdhetochkoercivitetärdevanligasteindirekta

mätmetodernaförattmätakornstorlekiproduktion. Enbrakorrelationmellan

kornstorlekochindirektamätmetodergerutökadeverktygförsnabbaochbilliga

mätningar.

Nyckelord: Hårdmetall;WC-Co;Abnormkorntillväxt;Kärnbildning;

Defekter;Förgrovning;Malning;Sintring;Mekaniskaegenskaper;Hårdhet;

EBSD;Simulering;Modellering;Skärverktyg

1 Introduction 1

1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Controllingtheproperties . . . . . . . . . . . . . . . . . . . . . . 3

1.3 Thecurrentwork . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 Literature review 5 2.1 Normalgraingrowth . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2 Newanalysismethods . . . . . . . . . . . . . . . . . . . . . . . . 6

2.3 Abnormalgraingrowth . . . . . . . . . . . . . . . . . . . . . . . 6

2.4 Growthmodels . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.5 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.6 Summaryanddiscussion . . . . . . . . . . . . . . . . . . . . . . . 10

3 Experimentalprocedure 11 3.1 Specimenpreparation . . . . . . . . . . . . . . . . . . . . . . . . 11

3.2 Measuringphysicalproperties . . . . . . . . . . . . . . . . . . . . 12

3.3 Analysispreparation . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.4 Analysistechniques . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.5 Removinggraindefectsbeforesintering . . . . . . . . . . . . . . 16

4 Data Analysis 17 4.1 Makingadistributionplot . . . . . . . . . . . . . . . . . . . . . . 17

4.2 Determiningthesmallestgrainsize . . . . . . . . . . . . . . . . . 19

4.3 Methodstocalculate theaveragegrainsize . . . . . . . . . . . . 20

5 Results 23 5.1 Averagegrainsize . . . . . . . . . . . . . . . . . . . . . . . . . . 23

5.2 Distributionplots. . . . . . . . . . . . . . . . . . . . . . . . . . . 24

5.3 Materialproperties . . . . . . . . . . . . . . . . . . . . . . . . . . 27

5.4 EBSDmeasurements . . . . . . . . . . . . . . . . . . . . . . . . . 28

5.5 Impactofdefects . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

6 Discussion 31 6.1 Eectofmilling. . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

6.2 Hardness,coercivityandaveragegrainsize . . . . . . . . . . . . 32

6.3 EBSD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

6.4 Impactofdefects . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

7 Numericalsimulations 39

7.1 Basicmodelforgraingrowth . . . . . . . . . . . . . . . . . . . . 39

7.2 ClassicLSW-theory . . . . . . . . . . . . . . . . . . . . . . . . . 40

7.3 Implementationofvariousgrowthmodels . . . . . . . . . . . . . 41

7.4 Generaldiscussion . . . . . . . . . . . . . . . . . . . . . . . . . . 49

8 Conclusions 51

9 Suggestionsfor future work 52

Acknowledgements 53

References 54

A Theoretical calculations 59

B Tables 63

C SEM images 69

D Additionalresults 77

E Additionalnumericalsimulations 83

Introduction

W

C-Cois interestingformakingcutting tools,dueit itsgreat mechanical

properties. This introduction aims to provide abackground about the

properties and productionof hardmetals. Theobjectivesand structure of the

thesisarealsostated.

1.1 Background

Metal machining is widely used in industry, becauseof the great possibilities

regarding shapeand itshigh precision. Cutting tools areusually produced as

asmallinsertof dierentshapesandcan bexedto atoolholder(seeFigure

1.1). Common cutting tool materials are hardmetals, ceramics and diamond.

Processessuchas cutting anddrillingrequirehigh performancematerialswith

themostimportant propertiesbeinghardness, toughnessand wear resistance.

Eachkindofcutting putsdierentdemandsonthetool,such assharpcutting

edgesor thermalfatigue. Unfortunately, high hardnessusually correspondsto

poor toughness. A successful combination of the two is found in cemented

carbides,whichwillbestudiedinthepresentwork.

Commercial hardmetals mainly consist of WC, TiC or TiCN. WC-Co ce-

mentedcarbidesconsistofWCgrainsembeddedinaCobinder. Thisstructure

oersgoodmechanicalpropertiescombiningthehardnessoftheWCgrainsand

the toughness of the Co binder. A ne homogenous microstructure provides

high potential for sharp cutting tools while a coarser structure is better for

applications working in a high temperature environment. The coarser grains

conduct heat betterthan the nergrains. Therefore, thevarious applications

requiredierentgrainsizes.

The process of making WC-Co cemented carbides begins with milling of

theraw materials into ane powder. Themilled powderis then pressedinto

thedesiredgeometricalshape. Finally,theproductis producedbyliquidstate

sintering. Thesinteringtemperaturerangesfrom 1300to1500

◦

C,andtypical

isothermal dwells are around 60 minutes long. During the sintering process,

somegrainsdissolve,whichmakesitpossibleforotherstogrow.

Changingthecuttingtoolinsertsinamachineinterruptsthemanufacturing

process,andisalargeeconomicfactor. Beingabletousehighercuttingspeeds

and thereby speedup production is anotherfactor for increased productivity.

Figure 1.1: Cuttingtoolinsert(yellow) foraturningapplication. (CourtesyofAB

SandvikCoromant)

Figure1.2: Aprintedcircuitboarddrill.

There is a high industry demand for longer-lasting tools that can withstand

higher temperatures and strongercutting forces. Today coatings are used on

almostallinserts,andthecarbideisnolongerincontactwiththecuttingpiece,

as longas thecoatingis intact. Thelifetime of theinsertis thereforedecided

eitherbythewearofthecoatingorbytheplastic deformationinthecemented

carbide.

1.2 Controlling the properties

The WC grain size and grain size distribution are two of themost important

parametersin thecontrolthemechanicalpropertiesof thetools. Factors often

studied to determine grain size are sintering temperature, pressure and time,

and also the choice of raw materials. Research during the past few decades

has been dedicated to improve the production of cemented carbides. Other

important factors in controlling the properties are the binder phasesand the

alloyingelementcontents.

In the grain growth process, grains tend to coarsendierently; individual

grainscanbecomemuchlargerthantheaverage,calledabnormalgraingrowth

(AGG). Theabnormal grains will reduce theperformance of theproductand

should therefore be avoided. There is also an industry demand for making

smallertools,such as printed circuitboard drillswith drilldiametersas small

as 75

µ

initiationpointsforbreakage.

ThephysicalmechanismofAGGisnotwellunderstoodandtherehasbeen

alotofresearchonhowtopreventAGG.Onewayistoadd smallamountsof

alloyelements, such asV or Cr,that work asinhibitors,preventingthegrains

from growing. Even thoughthis method is currently used, the physics of the

methodis notfullyunderstood. Anothermethodofreducingthegraingrowth

1.3 The current work

Theaim ofthis work was to studyhowthegrain size distribution depends on

and changes withvariation in themillingprocess. An attempt was also made

toseparatetheeectofinitialgrainsizefromtheeectofincreasednumberof

defects. Thegrain size and grain sizedistribution were measuredusingimage

analysis from a scanning electron microscope as well as electron backscatter

diraction.

Another objectiveof this thesis was to investigate thecorrelation between

indirect measures of grain radius, the hardness and coercivity, with dierent

waysofcalculatingtheaveragegrainradius.

This thesishasalsoevaluatedcurrentgrowthmodels,andinvestigatedhow

wellthemodelsaccountforthedefectsanddierenceingrainsizedistribution.

ThemodelswereimplementednumericallyinMATLABandcomparedwiththe

experimentaldata.

Chapter 2containsaliteraturereviewofthemostinterestingpapersinthe

eld andadiscussionofthendings. Thepaperswillinclude studiesofAGG,

variousgrowthequationsandprevioussimulationsofAGG.

Chapter 3willexplaintheexperimentalprocedure,themillingandsinter-

ingprocesses,andthetechniques usedforanalyzingthesamples.

Chapter 4will describethehow therawdata from theexperimentswere

analyzed. It will explain how the data were organized and dierent ways of

calculatingtheaveragegrain size. Theresultswill bepresentedin Chapter 5

andbediscussedin Chapter 6.

Chapter 7will explainhowthegrowththeorieswereimplementednumer-

icallyandit will alsoshowthe resultofthevarious growthequations. Finally

allndingsinthecurrentworkwillbeconcludedin Chapter 8.

Literature review

T

hischapterisareviewofthepastandpresentresearchintheeldofvarious

growththeoriesofWCgrains. Thereviewwillgiveabroaderbackground

totheareaofgrain growth. Thedierentgrowthequationsandtheirphysical

backgrounds will be covered and previous numerical simulations will also be

discussed. Inthiswork,whenevertheaverageradiusis mentioned,itrefersto

averagebasedonthenumberofparticles,unlessotherwisestated.

2.1 Normal grain growth

Duringsintering of cemented carbides,theaveragegrain size increasesdue to

coarsening of the grains. This phenomenon is called Ostwald ripening, and

the theory statesthat largegrains grow at the expense of thesmaller grains.

Diusionandtheinterfacialreactionsarelimitingfactorsofthecoarseningrate.

Ostwaldripeningwas studiedbyLifshitz, SlyozovandWagner [1,2]resulting

intheLSW-theory. Thetheoryproposesastationaryparticledistributionwith

the averagegrain radius increasing with time. The basic assumptionof LSW

theoryisthatthegrowthrateisproportionaltoitsdrivingforce. Theclassical

growthequationis

r

− r

= 2Kt,

where

r

n = 2

leavethesolid/liquidinterface,andavalueof

n = 3

rateislimitedbydiusion ofatomsofthesolidorliquidphase[2]. Mullins[ 3]

predicted that the distribution of crystal shapes does not aect the growth

exponent,

n

WC hasa simplehexagonal crystal structure and astoichiometric compo-

sition. Christensen et al. [4] derived a methodology for using rst principle

calculations to estimate the energy of an arbitrary unknown structure. This

methodwasapplied todierentinterfacialenergiesin aWC-Cocementedcar-

bide and to quantify theshape of theWC grains. Two newparameters were

denedtodescribetheequilibriumsize:

r

ofthe longandshort prismfacets and

k

theprismandtheheightofthetruncatedtriangle. Thearticleshowedalarger

accuracy in determining the

r

k

isthereforereasonableto usethesamevalueoftheinterfacialenergyforevery

grainboundary. Theinterfaceenergy

γ

J/m

2

to 1-2J/m

2

. ThecalculationsinthearticleshowedtheshapeofWC-Co

tobetruncatedtriangularprismswithatfacets.

2.2 New analysis methods

Kumar et al. [ 5] used electronbackscattering diraction (EBSD), further ex-

plained in Section 3.4.3, to characterize grain boundaries of WC-Co during

sintering. Theyobservedboundariesthat exhibitlowinterfacialenergy,called

Σ

areannihilatedathighertemperaturesandlongersinteringtimes. Theauthors

believe that

Σ

conrmedbyKimandKang[6] .

Mannesson et al. [ 7] have analyzed the WC grain size distribution after

milling and sintering at three dierent times, 0.25 h, 1h and 8 h. The sam-

pleswereanalyzedbyimageanalysisusingascanningelectronmicroscopeand

EBSD. Whenanalyzingthepowderbeforesintering, itbecomesclearthat the

EBSD techniqueistheonlytechniqueavailabletodetect

Σ

The resultof thestudy byMannesson etal. [ 7]was that EBSD, excluding

the

Σ

Therewasalsoalimitationin thedetectionofsmallgrainsfortheimageanal-

ysis. When including the

Σ

Σ

decreasedquicklyduring theinitial sintering. Theauthorssuggestedtheeect

of

Σ

growth.

When comparing the averagegrain size based on the number of grains in

the distribution to thehardness, alack of correlation hasbeen demonstrated.

Engqvistand Uhrenius[8]suggestan alternatewayof calculatingtheaverage

grain size, based on the volume or center of gravity. When calculating the

averageoftheobservedgrainsizes,moreweightisgiventothenergrains. By

usingtheequation

d

= P d

P d

,

where

d

The authors concluded that this method gives the larger grains more weight

in calculatingtheaveragegrainradius. This volume-basedaverageresultedin

abettercorrelation with hardnessvalues. Thesamples used byEngqvist and

Uhrenius[8]showedbimodaldistributions.

2.3 Abnormal grain growth

Abnormalgrain growth(AGG)iswhen fewgrainsgrowveryfastcomparedto

thegrowthofthesurroundinggrainsduringthesinteringprocess. Thisgrowth