An alternative evaluation method for friction condition in cold forging by ring with boss compression test

(1)

An

alternative

evaluation

method

for

friction

condition

in

cold

forging

by

ring

with

boss

compression

test

Chengliang

Hu

a

_,

_Hengan

_Ou

b,∗

_,

_Zhen

_Zhao

a,∗

a_Institute_of_Forming_Technology_&_Equipment,_Shanghai_Jiaotong_University,_Shanghai_200030,_China

b_Department_of_Mechanical,_Materials_and_Manufacture_Engineering,_University_of_Nottingham,_Nottingham_NG7_2RD,_UK

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received6February2014 Receivedinrevisedform 27September2014 Accepted12April2015 Availableonline22April2015

Keywords:

Friction RCT RCT-B

Calibrationcurves Frictiondifference

a

b

s

t

r

a

c

t

Ringcompressiontest(RCT)isaverypopularmethodusedtoquantitativelyevaluatefrictionconditions atthetool–workpieceinterfacebymeasuringthevariationsoftheinnerdiameteroftheringinmetal forming.TherearemanypossibilitiesformeasuringtheinnerdiametervariationsinRCTbecauseof non-uniformdeformationoftheinnerholeduringthetest.Suchnon-uniformdeformationoftheinnerhole causesdifficultiesinprecisemeasurementoftheinnerdiameterandhencetheaccuracyofthederived frictioncoefficient.ToavoidthedisadvantageindimensionmeasurementoftheconventionalRCT,an alternativemethodforevaluatingfrictionconditionsincoldforgingnamedringwithbosscompression test(RCT-B)isproposed.BytheintroductionoftheRCT-Bconcept,finiteelementsimulationresultsunder differentfrictionconditionswereobtained.Resultsshowedthattheshapeoftheouterbossremains sta-bleduringthecompressiondeformationandallowsthediameteroftheouterbosstobemeasuredmore easilyandaccurately.ThecalibrationcurvesoftheRCT-BconceptwereconstructedbyusingFE simula-tion,whichcovertherangeoffrictionconditionsincoldforgingprocess.Finally,theRCT-Bmethodwas successfullyappliedtodeterminethefrictionfactorsoffourdifferentlubricatingconditionsin compres-sionofaluminumrings.Furthermore,thephenomenawithdifferentlubricatingconditionsbetweenthe upperandlowerdie–workpieceinterfaceswerealsostudiedusingbothsimulationandexperimental testing.Theresultsshowthatitispossibletoquantitativelyassessthedifferenceoffrictionconditions attheupperandlowerdie–workpieceinterfacesbysimplycheckingtheinclinedangleoftheouterboss withtheRCT-Bmethod.

1. Introduction

1.1. Historicaldevelopment

Inmetalformingprocesses,frictionalwaystakesplaceatthe

interfacebetweentheworkpieceandthedieortool.Friction

con-ditionhasasigniﬁcanteffectonthematerialﬂow.Ononehand,

becauseofhighfrictioncondition,underﬁllingofmaterialintothe

diecavityoccurs.Ontheotherhand,severelocalfriction

condi-tioncouldbe beneﬁcial totheﬁlling of thediecavity. Friction

playsanimportantroleintheﬁnallyrequiredformingload,i.e.,

higher frictionresults in largertotal formingload required for

∗ _{Corresponding} _authors_at: _Institute_of _Forming _Technology_&_Equipment,

SchoolofMaterialsScienceandEngineering,ShanghaiJiaotongUniversity,1954 HuashanRoad,Shanghai,200030,China.Tel.:+862152580156/+441158467391; fax:+862162826575.

E-mailaddresses:[email protected](H.Ou),[email protected],

[email protected](Z.Zhao).

completingnecessarydeformationinametalformingprocess.

Fric-tionalsodirectlyinﬂuencesthesurfacequalityoftheformedparts

andexcessivefrictionleadstowear,pick-uporgallingofthetool

surfaceandhencereducestoollife.Therefore,frictionisan

impor-tantparameterinmetalformingandshouldbepreciselymeasured

andcarefullycontrolledforformingqualiﬁedproductsatlowcost.

Theringcompressiontest(RCT)hasproventobeoneofthemost

popularandcommonlyusedmethodsforquantitativeevaluation

offrictionconditionsinbulkformingoperations(RaoandSivaram,

1993).TheringcompressiontestwasﬁrstlyusedbyKunogi(1956)

asaqualitativemethodofcomparinglubricantsforcoldextrusion.

Inthetest,aﬂatring-shapedspecimeniscompressedto50%

reduc-tionusingﬂattool.Ifthelubricanthasagoodlubricationpropertyto

ensureverylowfrictioncoefﬁcient,theinnerdiameter(ID)expands

duringthedeformation,andwhenthefrictioncoefﬁcientishighthe

IDshrinks,asshowninFig.1.Itisquiteeasytorecognizethe

dif-ferentfrictionconditionsofdifferentlubricantsbyobservingthe

IDvariationduetoitssensitivitytofriction.

To determine the friction coefﬁcient for hot-working

pro-cess, the RCT was further developed by introducing a set of

http://dx.doi.org/10.1016/j.jmatprotec.2015.04.010

(2)

(a)

(b)

Fig.1.Ringcompressiontest:(a)lowfriction;(b)highfriction.

calibrationcurves by Male and Cockcroft (1964–1965). For the

calibrationwork,thering specimensof 6:3:2were compressed

understickingandzerofrictionconditions.Analyticaldatafrom

Schroederand Webster(1949) which were extracted fromthe

experimental results of compression of disc-shaped specimens

wereusedintheintermediatefrictionconditions.Byconducting

thetestusingarangeofmaterialswithdifferentdeformationand

lubricantsatelevatedtemperatures,RCTappearstobeareliable

methodindetectingthevariationsoffrictionconditionsduring

bulkplasticdeformation.

Based ontheassumptionofuniformdeformation aswellas

conformanceofMisesruleandconstantshearfrictionfactor,the

ﬁrst satisfactoryanalysis of thecompression of a ﬂat ringwas

madebyAvitzur(1964)throughanoptimumupperboundmethod

andlaterveriﬁedbyHawkyardandJohnson(1967)usingastress

analysisapproach,andthecurrentpositionoftheneutralsurface,

whichdividestheregionsofinwardandoutwardﬂowsof

mate-rial,wasdetermined.Bydividingthewholedeformationprocess

intoaseriesofsmalldeformationincrements,MaleandDepierre

(1970)providedapossiblemeansforthecalibrationofdifferent

ringgeometriesbycomputersolution.However,theactual

fric-tionfactorobtainedbyusingthetheorytoanalyzeexperimentally

determinedshapechangesappearstoproduceadegreeof

overes-timation.

To minimize the discrepancy between the theoretical and

experimentalresults, thebulging effectcaused bynon-uniform

deformation should be further considered. By using a simple

paraboliccurvewithonlyoneuncertaincoefﬁcienttorepresent

theproﬁle of thebulge,Liu (1972)theoretically calculated the

stress and strain components at the outerequator based on a

supposedvelocityﬁelddescribedbyatrigonometricfunctionand

presentedasetofcalibrationcurvesfor6:3:2ringgeometry.By

introducingaconstantaveragevalueofstressateachstepof

rel-ativesmalldeformation,LeeandAltan(1972)computedanother

setofcalibrationcurvesinconsiderationofbulging.Byadoptingan

assumedvelocityﬁelddescribedbyexponentialfunction,Depierre

andGurney(1974)developedareliablemethodfortreating

exper-imentalresults fromRCTwith andwithout bulgeformation so

astoobtainaquantitativeevaluationoffrictionfactor,inwhich

a paraboliccurve withtwo unknowncoefﬁcients wereusedto

describethebulgeproﬁle.Comparedtothecurvesdevelopedby

Maleand Depierre(1970),thecalibrationcurves consideringof

bulgingareclosertotheexperimentalresults.

Tofacilitatetheapplication ofRCTinindustry,Abdul(1981)

attemptedtodevelopacalibrationchart,whichprovidesfriction

factorlinesandheightreductionlinesagainstthegraduatedaxis

ofIDchange,withouttheconsiderationofthebulgingeffect.To

considertheactualnon-constantfrictionshearstressdistributions

duringRCT,Wang(2001)proposedanewwaytorecreate

calibra-tioncurvesbyusingthenon-constantfrictionstressexpressedby

afunctionoftheF-coefﬁcient,andtheobtainedcalibrationcurves

showedagoodagreementwithexperimentaldata.

Tosimulatethestateoflowandhighlevelsofnormalpressure

onthetool–workpieceinterfaceofmetalforming,twoalternative

ringcompressiontestswithconcave-shapedandconvex-shaped

cross-sectionintheringgeometrieswereputforwardasa

com-plementaryapproachofRCT.Thecorrespondingcalibrationcurves

wereobtainedusingFEManalysisandveriﬁedbytheexperimental

results(Petersenetal.,1998;Tanetal.,1998).

1.2. DimensionalmeasurementofRCT

ThepopularityofRCTcanbeattributedtoitspractical

conve-nienceandthefrictionconditioncanbequantiﬁedbymeasuring

thedimensionalchangeoftheinnerdiameter.Theaccuracyinthe

measurementsofthespecimendimensionsbeforeandafter

test-inghasasigniﬁcanteffectontheaccuracyoftheresultsoffriction.

Originally aimingattheslightlynon-circularshape upon

defor-mation,anenlargedtracingofeachspecimenwasmadeusinga

proﬁleprojector,theareaofthecentralholewasfoundbymeans

ofaplanimeter,andthemeandiameterwascalculatedassuming

thatthiswastheareaofatruecircle,andtheincurrederrorwas

foundtobe1in250(MaleandCockcroft,1964–1965).

AsrecommendedbyWangandLenard(1992),theinner

diam-eterwasmeasuredintwodirectionsatrightangles,onboththe

specimenendsandatthemiddle.Theaveragewasusedto

calcu-latethepercentagechange.Theheightreductionpercentagewas

alsocalculatedfromtheaverageoffourmeasurements.Inthiscase,

onlycaliperwasneeded.

MeasurementsoftheheightandIDweremadebyutilizinga

dig-italcaliper(Petersenetal.,1998),whereasdetailsofthegeometrical

proﬁleofthedeformedspecimenswereobtainedonatoolmaker’s

microscopeequippedwithanX–Ymicrometertableconnectedtoa

PC-baseddata-loggingsystem.Toallowdetailedcomparisonwith

thecomputedchangesingeometryforthecomplementaryRCT,

theexternalproﬁleofselectedspecimenswasdigitizedusingan

inductiveprobe.

Consideringthesoftpropertyoftheworkpiecematerial

‘Plas-ticine’,aftercompressiontheIDwasmeasuredbeforethespecimen

wasremovedfromthebottomplatentoavoidanychangeofshape.

Anaveragevaluewastakenfromthreemeasurementsonthetop

surfacefromthreearbitraryanglesacrossthecentreofthering

(Robinsonetal.,2004).

AsshowninFig.1,theIDattheﬂatfacesdiffersfromtheIDatthe

specimenmid-lineduetobarreling.Hartleyetal.(2007)proposed

amethodtomeasuretheIDbyplacingaball-bearingofknown

diameterontothespecimensothatitprojectsslightlyintotheinner

hole,andthistechniqueenablesmeasurementoftheIDattheﬂat

specimenfaces.FromthedimensionsdepictedinFig.2,allofthese

canbemeasuredaccuratelyusingastandardmicrometer,theIDis

simplycalculatedfrom

ID=2

D2

4 −

OH−h−D₂

2 (1)

As mentioned by Shahriari et al. (2010), a proﬁle projector

equippedwithanX–YmicrometertableconnectedtoaPC-based

(3)

Fig.2.Diagramillustratingthemeasurementtechniquewithball-bearing(Hartley etal.,2007).

Fig.3.Cross-sectionofdifferentRCTspecimen(Macaskill,2012).

ball-bearingwerebothusedtodeterminethefrictioncoefﬁcients

fortheNimonic115hotforgingprocessbyRCT.

For theconvex(high friction) proﬁle,thesmallestID atthe

equatorcouldbeproperlymeasuredbyusingtheproﬁleprojector

andplanimeter.Fortheconcave(lowfriction)proﬁle,theproﬁle

projectorcouldnotcatchthelargestIDattheequator.Although

themeasurementtechniquewithball-bearingmaybeused

conve-niently,theassumptionofthedeformedproﬁleasacircularshape

makesthetechniquenotsoprecise.Toobtaintheaveragevalue,the

measurementoftheIDbyusingacaliperisaneasybutnota

pre-ciseway,becauseitisdifﬁculttopositiontheequator.Togetmore

accuratevalueoftheIDusingasimplecaliper,thespecimenafter

deformationcouldbecutatacrosssectionthroughthe

axisym-metricz-axis,asshowninFig.3.However,itrequiresmoretimeto

completethecutting.

InadditiontothetwotypicaldeformedmodesshowninFig.1,

Abdul(1981)reportedthreeotherpossiblemodesof

inhomoge-neousdeformationduringhisexperimentalstudyatvariouslevels

offriction.Goetzetal.(1991)alsofoundthattheproﬁleoftheID

afterdeformationwasnotonlyconcaveorconvex,butalsostraight

orbuckled.Andinthepracticalexperimentalwork,therearemany

possibilitiesforthedeformedIDproﬁle,asshowninFig.3.This

indicatesthat theshapeof theIDof thering duringtestis not

souniform.Correspondingly,theshapeoftheouterdiameteralso

becomesunsteady.

Inthepresentwork,toovercomethedisadvantagein

measure-mentof theinner diameterinconventional RCT, analternative

methodtoevaluatethefrictionconditionnamelyringwithboss

compressiontest(RCT-B)wasproposed.Inthefollowingsections,

theRCT-Bconceptis ﬁrstdiscussed.InSection3,ﬁniteelement

(FE)simulationsarecarriedoutfordetailedevaluationofmaterial

deformationsunderdifferentRCT-BfrictionconditionsandtheFE

resultsareusedtoconstructthecalibrationcurvesoftheRCT-B.

Section4summarizestheproceduresforboththeRCT-BandRCT

experimentssoastovalidatetheRCT-Bmethodascomparedtothe

Fig.4. TheringgeometryofRCT-B.

conventionalRCTapproach.Detailedevaluationanddiscussionof

thefrictionconditionsobtainedfromboththeRCT-BandRCT

meth-odsaregiveninSection5.Thisisfollowedbytheconclusionsdrawn

fromthisresearchinSection6.

2. Theconceptofringwithbosscompressiontest(RCT-B)

Toachieveauniformlydeformedshape,theringwithanouter

bossisproposedforthecompressiontestbasedonthefollowing

considerations:(1)Theouterbosswouldnotbedeformedduring

thecompressionprocessbecauseitdoesnotcontactwiththedies;

(2)Theouterbosswouldbearigidzoneexpandingwiththeouter

diameterbecauseitisnotaffectedbythedeformationofthering.

The ring geometry 6:3:2 used by Male and Cockcroft

(1964–1965)iscommonlyusedforobtainingexperimental

cali-brationcurvesbecauseofpracticalconvenience(RaoandSivaram,

1993).Theproposedringgeometrywithbossisdesignedbasedon

thiscommonlyusedRCTgeometry.AsshowninFig.4,theratio

ofouterdiametertoinnerdiameterandtoheightis6:3:2,andthe

heightandwidthoftheouterbosshavethesamevalueof0.4.Based

ontheaboveconsiderationsofmaterialdeformationoftheRCT-B

specimenundercompression,itisassumedthatthevariationsof

thebosswouldbesufﬁcientlysensitivetodifferentfriction

condi-tions.SimilartotheconventionalRCTapproach,itisnecessaryto

constructasetofcalibrationcurvesforthemeasurementoffriction

conditions.FEmethodmaybeusedtoevaluatematerial

deforma-tionoftheRCT-Bspecimenunderdifferentfrictionconditionsand

toconstructthecalibrationcurvesoftheproposedRCT-Bmethod.

3. Finiteelementsimulation

3.1. Analysisofdeformationprocess

Inthis work,aluminum materialAl6082wasselectedasthe

materialfortheRCT-B.Toobtainmaterialpropertiestensiletests

werecarriedout,andthetruestressandstraincurvewasmodeled

usingthesimple powerlaw=Kεn_._After_regression_analysis_of

experimentaldata,theﬂowstressconstantKandthestrain

hard-eningexponentnwerecalculatedtobeK=191.88andn=0.181,

respectively.Thusthespeciﬁcmaterialmodelcouldbedetermined

andadoptedfortheFEmodel.

2Daxisymmetricmodelwithshearfrictionlawwasusedto

sim-ulatetheRCT-Btestsatroomtemperature.Thesimulationresultsof

deformationprocessunderdifferentfrictionconditionsareshown

inFig.5.Whenthefrictionfactoris0.0,thematerialuniformlyﬂows

outwardandtheinnerdiameterkeepsstraightwithheight

reduc-tion.Whenthefrictionfactoris0.1,thematerialﬂowsoutward

andaconcaveproﬁleofIDisobtained.Whenthefrictionfactoris

0.3,thematerialﬂowsinwardandaconvexproﬁleofIDisformed.

AllthesephenomenaontheIDdeformationarequitesimilartothe

conventionalRCT.Thissuggeststhattheouterbossdoesnotchange

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Fig.5.SimulationresultsofdeformationduringRCT-Bunderdifferentfrictioncondition:(a)frictionfactorm=0.0,heightreductionRH=30%;(b)m=0.1,RH=30%;(b)m=0.3,

RH=30%;(e)m=0.0,RH=50%;(f)m=0.1,RH=50%;(g)m=0.3,RH=50%;(e)m=0.0,RH=60%;(f)m=0.1,RH=60%;(g)m=0.3,RH=60%.

Whatismoreimportantisthatthereisalmostnodeformation

attheouterbossofthering,asshowninFig.5.Theequivalentstrain

inthatareaisverysmallduringthedeformationprocess.Evenat

veryhighfrictionconditionssuchasm=0.7andm=1.0,theshape

oftheouterbossstillkeepsunchangedasarigidzone,asshown

inFig.6.DuringtheRCT-Btest,theouterbossexpandswiththe

outerdiameterandthereisnotanybulgingorbarrelingeffectat

theouterboss.Therefore,theouterdiameterofthespecimencan

bemeasuredquiteeasilyandprecisely.

3.2. Constructionofcalibrationcurves

From the literature mentioned above, discrepancies exist

betweenthereportedcalibrationcurvesbecauseofmanyfactors

thataffecttheﬁnalshape ofthecalibrationcurves. Goetzetal.

(1991)examinedtheeffectsofstrainrate,innerdiameterproﬁle

andringgeometryuponringcurvesusingtherigidviscoplasticFE

programALPIDandexperimentaldata.Anderssonetal.(1996)

eval-uatedtheeffectoftheheat-transfercoefﬁcientinring-compression

testsusingacommerciallyavailableFEprogramDEFORM,andthe

effectcanbeconsideredsmall ascompared totheinaccuracies

involvedinthecalibrationcurves.Byinvestigatingtheeffectsof

materialproperties,strain-ratesensitivity,andthebarrelingeffect

onthecalibrationcurvesbyusingFEsimulation,SofuogluandRasty

(1999)concludedthattheuseofageneralizedcalibrationcurves

regardlessofthematerialtypeandtestconditionsmustbeavoided.

Itwasrecommendedthatspeciﬁccalibrationcurvesforaspeciﬁc

materialbeusedtoobtainreliabledataoffrictioncoefﬁcientsunder

thespeciﬁctestconditions.Forthesereasons,thecorresponding

calibrationcurvesforthenewRCT-Bconceptandtheconventional

RCTarebothconstructedbyFEmethod.

ToobtainthecalibrationcurvesofthenewRCT-Bconcept,the

displacementofthenodeatthemiddleoftheouterbossandone

nodeatthetopedgeareextractedfromtheFEsimulationresults

toquantifythedeformationoftheringwithbossspecimen.The

reductionintheouterdiameterofthebossafterdeformationis

calculatedasROD=(DB1−DB0)/D0andthereductioninheightis

cal-culatedasRH=(H0−H1)/H0,asshowninFig.7.Finally,thespeciﬁc

calibrationcurvescanbecreatedasshowninFig.8.

Tomakenecessarycomparisons,thecalibrationcurvesforthe

conventionalRCTarecreatedbasedonthereductionintheinner

diameteratthemiddleofthespecimen,asshowninFig.9.

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Fig.7.Reductioninouterdiameter.

0 10 20 30 40 50 60

0 10 20 30 40 50 60 0.60-1.00 0.50 0.20 0.15 0.10 0.40 0.30 0.05 0.00 Re du ct io n i n ou ter d ia me ter of boss( %)

Reduction in height(%)

Fig.8. CalibrationcurvesfortheRCT-B.

0 10 20 30 40 50 60

-30 -20 -10 0 10 20 30 40 0.20 0.15 0.10 0.05 0.00 Re du cti on i n i n te rna l di ameter (%)

Reduction in height (%)

0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00

Fig.9.CalibrationcurvesoftheRCTapproach.

Itisobservedthatofthechangeoftheouterdiameteroftheboss

inRCT-BislesssensitivethanthatoftheinnerdiameterintheRCT,

andwhenthefrictionfactorislargerthan0.5itwouldbedifﬁcult

fortheRCT-Btoquantitativelydeterminethefrictioncondition.

However,inmostcoldforgingprocessesthefrictionfactorwould

belessthan0.5,sotheRCT-Bmethodcouldbeusedtomeasurethe

frictionconditions.

Forfurtherstudy,FEsimulationswerecarriedoutwithdifferent

frictionfactorsbetweenthetopandbottomdie–workpiece

inter-faces.AsshowninFig.10,becauseofthefrictiondifferencethe

inclinedanglecanbeobservedeasily,asshowninFig.10dandf).

Asdiscussedabove,thesensitivitytotheinterfacialfrictionofthe

RCT-Breduceswhenthefrictionfactorbecomeslargerthan0.5.

Therefore,theinclinedanglesunderthesamefrictiondifference

betweenthetopandbottomsurfacesathigherfrictionconditions

(Fig.10i–k)areobviouslysmallerthantheresultsatlowerfriction

conditions(Fig.10a,b,gandh)asexpected.

AsshowninFig.10k,thefrictionfactorsatthetopandbottom

die–workpieceinterfacesaresupposedtobe0.70and0.75,

respec-tively,andtheinclinedangleissmall.However,whenthereduction

inheightisincreasedfrom60%to65%,theinclinedanglebecomes

larger.Theinclinedangleoftheouterbossalsoincreaseswiththe

increaseofdeformationduringtheRCT-BasshowninFig.10kand

l.

Basedontheaboveanalysis,theRCT-Bcanbeusedtoevaluate

thelubricationperformance(frictioncondition)byusingdifferent

lubricantsbetweenthetopandbottomdie–workpieceinterfacesin

onesimpletest.Accordingtothesimulationresults,ifthefriction

differenceismorethan0.02atlowerfrictionconditionandismore

than0.05atlargerfrictioncondition,itispossibletodistinguishthe

differencebysimplyinvestigatingtheinclinedangleoftheouter

bossofthering.

4. Experimentalworks

TheworkpiecesshowninFig.11weremachinedfromannealed

Al6082bar.Fortheringworkpieceswithouterboss,thenominal

dimensionsoftheouterdiameter,innerdiameter,height,aswell

astheheightandwidthoftheouterbossare18.00mm,9.00mm,

6.00mm,1.20mmand1.20mm,asshowninFig.4.Forthe

con-ventionalring workpiece,the nominaldimensionsof theouter

diameter,innerdiameterandheightare18.00mm,9.00mmand

6.00mm,respectively.

Twocylindricaldieswith100.00mmdiameterweremachined

and hardened by heat treatment, and then the diesurface

re-hardeningwascarriedoutbyTufftrideprocess.Theworkingsurface

of thedie wastreated by lapping and polishing processes and

themirrorsurface witha surfaceroughnessof Ra0.05␮mwas

achieved,asshowninFig.11c.

Threetypesoflubricatingoilswereadoptedasthelubricantfor

thetests,andthemainpropertiesareshowninTable1.Duringeach

test,thedieworkingsurfaceswerecleanedupwithalcoholatﬁrst,

thenalubricatingoilwasbrushedonthemasevenaspossible,after

thataworkpiecewasputinthemiddleandthecompressiontest

wasperformed.

Allthetestswereconductedona250kNInstron5985

mate-rialtestingmachineandthecompressionvelocitywascontrolled

at1mm/minbyaBluehill2system.Thedimensionsofworkpiece

beforeandaftercompressionwerecheckedusingadigitalvernier

caliper.DuringtheRCT-Btests,theinitialouterdiameterofboss,

outerdiameterandheightofeachworkpieceweremeasured,and

theouterdiameter ofboss and theheightwerealso measured

aftertest,eachdimensionwasobtainedfromtheaverageofthree

measurements.DuringtheconventionalRCTtests,theinitialinner

diameterandheightofeachworkpieceweremeasured, andthe

innerdiameterandheightwerealsomeasuredaccordingtothe

recommendationfromWangandLenard(1992),eachdimension

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Fig.10.Deformationresultswithdifferentfrictionfactorbetweenthetopandbottomdie:(a)RH=60%;(b)RH=60%;(c)RH=60%;(b)RH=60%;(d)RH=60%;(h)RH=60%;(i)

RH=60%;(j)RH=60%;(k)RH=60%;(l)RH=65%.

Fig.11. Workpiecesanddiesforexperimentaltests.

5. Resultsanddiscussion

5.1. Determinationoffrictionfactors

Fig.12showsthedeformationconditionoftheringworkpieces

withouterbossaftercompressionatdifferentheightreductions.

Itcanbeobservedthattheworkpiecesaftercompressionunder

thedrydiesstillkeptincircularshapeevenwhenthereduction

inheightreached58%.Andasexpectedaccordingtothe

simula-tionresultsdiscussedabove,littledeformationtookplaceatthe

outerboss.Thisconﬁrmsthattheshapeoftheouterbossremains

undeformedduringthetest.

Bycalculationofthereductionoftheouterdiameterofboss

andthereductionofheight,thefrictionfactorsunderfourdifferent

lubricatingconditionsincludingtheuseofthreelubricantsanddry

contactbetweentheworkpieceanddiesweredeterminedbased

onthecalibrationcurvespresentedinFig.8.AsobservedinFig.13,

thefrictionfactorswithlubricatingoilA,BandCare0.21,0.20and

0.22,respectively.Thefrictionfactorunderdrycontactcondition

is0.24.

FortheconventionalRCTcases,thereductionoftheinner

diam-eter and thereduction of heightwere alsocalculated, and the

corresponding data are shown in Fig. 14. From the calibration

curves,thefrictionfactorsfromtheconventionalRCTcaseswith

Table1

Lubricatingoilsusinginthetest.

No. Lubricatingoil Density@15◦_C/kg/m3 _Kinematic_viscosity_@40◦_C/mm2_/s _Kinematic_viscosity_@100◦_C/mm2_/s _Viscosity_index

A RandoHD68 – 64.6 8.4 98

B MagnaBD68 880 68 8.7 99

(7)

Fig.12.Theringworkpiecewithouterbossaftercompressionunderthedrydies.

0 60 6 0

5 0

4 10

20 30 40 50 60

0.00

0.05 0.15 0.20 0.30 0.40 0.50

Reduction in height(%)

Re

du

ctio

n in ou

ter

diameter

wi

th

b

o

ss(%

)

L-B (m=0.20) L-A (m=0.21)

Dry (m=0.24) L-C (m=0.22)

Fig.13.ExperimentaldatapointsonthecalibrationchartoftheRCT-Btests.

0 6 0

5 0

4 -30

-20 -10 0 10 20 30

Dry (m=0.28) L-B (m=0.21) L-A (m=0.23)

L-C (m=0.25)

Re

du

cti

on

in

inn

er

d

iam

ete

r(%)

Reduction in height (%)

0.20

0.15

0.10

0.05

0.00 0.30 0.40 0.50

Fig.14.ExperimentaldatapointsonthecalibrationchartoftheconventionalRCT tests.

Fig.15.Theringworkpieceaftercompressionunderthedrydies.

lubricatingoilA,BandCcouldbedeterminedas0.23,0.21and 0.25,respectively,andthefrictionfactoratdrycontactcondition canbedeterminedas0.28.

ComparingthetestresultsobtainedbyboththeRCT-BandRCT methods,themeasurementsoffourdifferentfrictionconditions fromtheRCT-Bcasesareinagoodagreementwiththatfromthe conventionalRCTmethodproducingamaximumerrorlessthan 15%inthedrycontactcondition,asshowninTable2.Ingeneral,

thespeciﬁcfrictionfactordeterminedbytheRCTmethodislarger

thanthatfromtheRCT-Bmethod.AsshowninFig.15,theinner

diameteroftheconventionalRCTringisnotcircularandinsteadit

becomesanovalshapeespeciallyatlargerreductionofheight.The

minimuminnerdiameterduetotheovalshapetoacertaindegree

contributestotherelativelargerreductionofinnerdiameterand

hencelargerfrictionfactorvalue.

5.2. Evaluationoflubricationconditions

Toevaluatethefrictionsunderdifferentlubricatingconditions

atthetopandbottomsurfacesbetweentheworkpieceanddies,

onlythelowerdiewasbrushedwiththebestlubricatingoilMagna

BD68andfollowedbycompressionsofthreeworkpieceswithouter

boss.AsshowninFig.16,theworkpiecesremainincircularshape

basically,andtheouterbossisinclinedtotheupperdieunderdry

condition,andtheinclinedangleoftheouterbossincreaseswith

thereductionofheightduringthetest.Theseexperimentalresults

areingoodagreementwiththesimulationresultsasdiscussedin

Section4,whichfurtherprovesthatthedifferenceinlubrication

conditionscanberecognizedbycheckingtheinclinedangleofthe

outerbossofthering.

Table2

Frictionfactorsdeterminedbydifferenttests.

No. Lubricatingoil Frictionfactorm(RCTtest) Frictionfactorm(RCT-Btest) Error(%)

A RandoHD68 0.23 0.21 8.7

B MagnaBD68 0.21 0.20 4.8

C Magna100 0.25 0.22 12.0

(8)

Fig.16.Theringworkpiecewithouterbossaftercompressionwithdifferentlubricatingconditionsbetweentheupperandlowerdie–workpieceinterfaces.

6. Conclusions

(1)Analternativequantitativeevaluationmethodforthefriction conditionsincoldforgingbyringwithbosscompressiontest wasproposedinordertoaddressthedifﬁcultiesencountered inthemeasurementoftheinnerdiameterinconventionalring compressiontest.

(2)FortheRCT-Btest,theproposedproportionofringgeometry oftheouterdiameter,innerdiameter,heightandtheheighto andwidthoftheouterbosswasdesignedtobe6:3:2:0.4:0.4. Thecompressionprocessesoftheringwithbossunderdifferent frictionconditionsweresimulatedandtheresultsshowedthat theshapeoftheouterbossremainsundeformedduringthe ringdeformation.Theundeformedouterbosswasveriﬁedby theRCT-Bexperimentalresultstoallowtheouterdiameterof bosstobemeasuredeasilyandprecisely,whichenablesmore accuratemeasurementoffrictionconditions.

(3)ThecalibrationcurvesofthenewRCT-Bconceptandthe con-ventionalRCTmethodsarebothconstructedbyFEsimulations. AlthoughthechangeoftheouterdiameterofbossinRCT-Bis lesssensitivethanthatoftheinnerdiameterinthe conven-tionalRCT,itisstillsufﬁcientforaccuratepredictionoffriction conditionwhenthefrictionfactorislessthan0.5.Therefore theRCT-Bmethodcanbeusedtomeasuretherangeoffriction conditionsincoldforgingprocess.

(4)TheRCT-Bmethodwassuccessfullyappliedtodeterminethe friction factors of four differentlubricating conditions with aluminumworkpieces.TheRCT-Bmeasurementresultswere veriﬁedbyconductingexperimentaltestsusingthe conven-tionalRCTmethod.ThetestingresultsfromboththeRCT-Band conventionalRCTwereina goodagreementintermsofthe measuredfrictionfactorswithamaximumerrorlessthan15%. (5)FEsimulationswithdifferentfrictionfactorsbetweenthetop andbottomdie–workpieceinterfacesarecarriedout.Theouter bossisinclinedtothediesurfaceofgreaterfriction,andthe inclinedangleoftheouterbossbecomeslargerwithgreater frictiondifferencebetweentheinterfaces.Theinclinedangle tendstoincreasewiththeincreaseofdeformationduringthe RCT-BasobservedbyRCT-Btestingresults.Thisobservation leadstotheconclusionthatthedifferenceoflubrication con-ditionscanbequantitativelyevaluatedbysimplyinvestigating theinclinedangleoftheouterbossunderringcompression conditions.

Acknowledgements

ThisworkwassupportedbytheNationalNaturalScience Foun-dation of China (No. 51475294), FP7-Marie Curie Action IRSES

MatProFutureproject(No.318968)andtheEngineeringand Physi-calSciencesResearchCouncil(EPSRC,EP/L02084X/1).Theuseful discussion with Prof. Niels Bay from Technical University of Denmarkonthetoolinglappingandpolishingduringthe46thICFG PlenaryMeetinginParisshouldbementioned.Theauthorswishto thanktheMr.TomBussoftheFacultyofEngineeringatNottingham forhissupportinexperimentaltesting.

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