Twist channel multi-angular pressing (TCMAP) as a new SPD process: Numerical and experimental study

Author's Accepted Manuscript

Twist channel multi-angular pressing (TCMAP) as a new SPD process, numerical and experimental study Radim Kocich, Adéla Macháčková, Lenka Kunčická

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S0921-5093(14)00805-3 http://dx.doi.org/10.1016/j.msea.2014.06.079 MSA31279

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Materials Science & Engineering A

Received date: 23 May 2014 Accepted date: 22 June 2014 Cite this article as: Radim Kocich, Adéla Macháčková, Lenka Kunčická, Twist channel multi-angular pressing (TCMAP) as a new SPD process, numerical and experimental study, Materials Science & Engineering A, http://dx.doi.org/10.1016/j. msea.2014.06.079 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Twist channel multi-angular pressing (TCMAP) as a new SPD process, numerical and experimental study Radim Kocich a, bӒ, Adéla Macháková b, Lenka Kunická b a

Department of Material Forming, Faculty of Metallurgy and Materials Engineering, VŠB TU Ostrava 17.listopadu 15, 70833 Ostrava-

Poruba, Czech Republic, [email protected], Tel. +420 59694455 b

Regional Materials Science and Technology Centre, VŠB TU Ostrava 17. listopadu 15, 70833 Ostrava-Poruba, Czech Republic



Abstract The paper proposes a new variation for the application of SPD methods. Suggested TCMAP (twisted channel multi angular pressing) technology obtains the larger imposed strain more effectively while increasing homogeneity of material. The number of passes needed to obtain the ultra-fine to nano-scale grains in bulk materials can be significantly reduced. Commercially pure Al (99.97%) was used for the experimental verification of the suggested process. The deformation behaviour of the material during the process was also described using the numerical simulation based on FE analysis. It was proved that the geometric parameters of the die, as well as the used individual deformation sections sequence, can significantly affect the size and homogeneity of the imposed strain or stability of the plastic flow of material. Predicted value of imposed strain, after a single pass, reached approximately 2.8. It is obvious that during the TCMAP process no dead zones occurred in any of the channel deformation sections. Due to designed shape of channel are both ends of processed sample defined by higher imposed strain and only negligible shape changes. A comparison between the FEM and experimental results of the required loads and the homogeneity of the imposed strain distribution showed good agreement. The high homogeneity of the distribution of the deformation was confirmed by

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micro-hardness testing, whereas a relative growth of 97% was documented after the first pass. The grain size, determined after TCMAP pass, averaged out 2.4 Pm.

Keywords: finite element method, aluminium alloys, bulk deformation, grain refinement, TCMAP

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Introduction

Oneofthecontemporaryresearchfocusesisontheincreaseofmechanicalpropertiesofmetallic materialsduetotheincreaseofrequirements.Amongthemethodstheresearchiscloselyfocusedon arethesevereplasticdeformation(SPD)processesthatenabletodecreasethegrainsizetothe nanoscale.ApplicabilityofmanyoftheSPDprocesseshasalreadybeenprovenbyquiteextensive investigation.Tothemostextensivelyexaminedbelongstheequalchannelangularpressing(ECAP) method[1],highpressuretorsion(HPT)process[2],twistextrusion(TE)[3]ormultiaxialforging(MAF) [4].ProbablythemosteffectiveprocessfromtheextremegrainrefinementpointofviewistheHPT, whichusuallyenablestoobtainthehighesthighanglegrainboundaryfraction,too.TheHPTmain disadvantageisthatverysmallspecimenscanbeprocessedonly.Eventhoughavariantforcontinuous HPT[5]hasalreadybeendesigned,alltheissuesarestillnotsolved(unwantedshapedeformationsofa specimenafterapass,etc.).AsimilarproblemisfortheTEmethod,theapplicabilityofwhichislimited primarilytorectangularspecimens.Inaddition,lowlevelsofmaximumpossibleimposedstrainarethe limitingfactorinthecaseoftheTEmethod.Whileitispossibletopartiallyaffecttheoverallimposed strainlevelchangingthetwistslopeangle,theresultinghomogeneityalongthecrosssectionremains considerablylow.Moreover,forasuccessfulapplicationoftheprocessaquitecomplicateddeviceis necessarytogetherwithalargernumberofpasses.

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However,itwouldbemorefavourableifsuchamountofstraincouldbeachievedduring single/continuouspassoperationtoincreasetheprocessefficiencywhiledecreasingtheprocesscost.In arecentlypublishedstudyfocusedonpossibleeliminationoftheabovementionedTEdisadvantagesa solutionhasbeenproposedbydesigningtheaxisymmetricforwardspiralextrusion(AFSE)process[6]. EventheefficiencyofMAFhasbeenincreasedbytherecentlydesignedrepetitiveforging(RF)process [7],forwhichamodifieddiegeometryincreasingthesheardeformationinthesampleisused.Contrary tothis,theECAPprocesshasalreadybeendesignedforbulkmaterials.DespitethefactthatECAPisone ofthefirstlydevelopedSPDprocessesitstillbelongstothemostfrequentlyusedSPDtechnologies.One ofthereasonsisarelativelyeffortlessapplicabilityofthemethodforvariouspurposes,suchasfor consolidationofpowdermaterials[8]or“recycling”ofwastematerial(e.g.intheformofchips)[9]. Nevertheless,aswasprovenbyseveralpreviouslypublishedstudies[10,11],itsefficiencyis,among others,stronglydependentontheselecteddeformationpath.DuringtheECAPonlythelimitedamount ofthestraincanbeimposedduringonepasswhilepreservingthedesiredlevelofhomogeneityinthe crosssection[1].Itisthereforeadvisabletofocustheattentiononthepossibilitiesofincreasingthe efficiencyofthistechnologyaswell.Amongthestillactualare,aboveall,effortstodesignthemost effectivedeformationpath[12],geometryofdiechannel(multiplebendingofthechannel)[13], introductionofbackpressure[14,15]ortodesignvariousmodificationsofthemethod,suchasthe torsionextrusion[16].

Ifthementionednegativeaspectsweresuccessfullydiminished,boththeECAPandTEmethods wouldprobablybethemostsuitablemethodsforthepotentialpracticalapplicationduetotheirrelative technologicalsimplicity.OneofthepossiblewayshowtoeliminatethedetrimentsoftheECAPandthe TEmethodswhilepreservingthepositiveaspectsiscombiningtheabovemethodstogether.Thiscanbe consideredintwobasicscenarios:1)ECAPandTEarelinedupassubsequentindependentprocessesor 2)ECAPandTEaremergedintooneprocesscontainingalltheprincipalpartsofbothprocesses.Thefirst

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caseassumesbothtechnologiestobedirectlyavailableinoneplace.However,itmayprovetoberather difficulttosynchronizethemutualconnectionofbothprocessesinsuchmannerthattheparametersof deformationand/orthehistoryoftheformingarenotinfluencedbyeachother.Alternatively,the solutionofferedbythemergingoftheECAPandtheTEmethodsobviouslyexcludesthesetypesof problems,andmoreover,itallowsforwiderrangeoftechnologicalsolutionssuchasthetwistchannel angularpressing(TCAP)[17].

Thelastmentionedprocessenablestoimposeintoamaterialaconsiderablyhigherstrainduringa single/continuouspassoperationthantheconventionalECAPmethod.Moreover,theequivalentstrain (ES)isdistributedmorehomogenouslythroughthecrosssectionoftheextrudedsample.Therefore,the totalnumberofpassescanbedecreasedandsothatefficiencycanbeincreasedwithlowercosts. Detailedinformationonthisprocesscanbefoundelsewhere[18,19].

Nevertheless,asisdocumentedbyseveralstudiestherecanbeotherpossiblewaysfornext increasingofimposedstrain.Oneofpotentialsolutionisthenextdeformationsection(e.g.multiple bending)incorporationintothediechannel.Djavanroodietal.reportedtheeffectofdiechannelangle onmagnitudeofimposedstrain[20]duringECAPwithparallelchannels.SimilarlyWangetal.studied thepositiveinfluenceofmultiplebendingondeformationbehaviourofMgalloyduringCshapeequal channelreciprocatingextrusionprocess[21].

Basedonthesefactsthetwistchannelmultiangularpressing(TCMAP)hasbeenrecentlydesigned withtheaimoffurtherTCAPprocessefficiencyincrease.ForthispurposetheTCAPconception,which wasmodifiedbyimplementationofanotherdeformationsection(multiplebending),wasused.

Theaimofthispaperwasoninvestigationofthenecessaryinformation,suchastheESvalueand itshomogeneitythroughoutthesample,influenceofthediechannelgeometry,thenecessarypunch

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loadorstabilityoftheplasticflowoftheextrudedmaterial.Forthesepurposesa3DFEanalysisand subsequentexperimentwereused.



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Experimental ThepurposeoftheexperimentwastodescribetheTCMAPprocessmoreindetailwithregardto

theinfluenceofthediechannelgeometryonthefinaltemperature,ES,deformationinhomogeneity throughoutthecrosssectionofthesample,stressdistributionorthenecessarypunchload.Plasticflow ofthematerialwas,amongothercharacteristics,monitoredaswell.TheTCMAPprocessprincipleis depictedinFig.1a.

Thefirstpartofthepaperdealswithanumericalanalysisofseveralmodelledvariantsofthe TCMAPprocess.TocomparetheefficiencyoftheTCMAPprocesstheTCAPprocesswassimulatedas well(Fig.1b);thedefinedconditionswerethesameforboththeprocesses. Deformationbehaviourofthematerialwasinvestigatedwithv=3mms1extrusionvelocityandP =0.02Coulombfrictioncoefficient.Thefrictioncoefficientvaluewaschosenconsideringpreviously publishedworks,inwhichmutualcomparisonsofvaluespredictedbynumericalsimulationsand obtainedexperimentallywiththehelpofmeasurementofpunchloadinghadbeensuccessful.Among themonitoredparameterswastheinfluenceoftheuseddiedefinedbytheangle(thetwistslope angle),Zangle(thetwistrotationangle),angle(theanglebetweentheindividualpartsofthe embeddeddeformationzone),angle(theanglebetweentheindividualpartsofthechannel)and\ angle(theoutercornerangle)(Fig.1a).Thestudywasalsofocusedondeterminationoftheinfluenceof theindividualdeformationsectionssequenceontheextrudedsampledeformationbehaviour.Bythis reason,twovariantsoftheTCMAPprocessweresimulated.AstheVariantItheTCMAPprocesswithA

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BCDEdeformationsectionssequence(Fig.2a)wasconsidered.TheVariantIIwasthendefinedwithB CDAEdeformationsectionssequence(Fig.2b). ThesecondpartofthepaperisfocusedonanexperimentalrealizationoftheTCMAPprocess.The objectiveofthispartwastoverifythemodelusedintheFEanalysis.Theselectedmaterialwasthe commerciallypureAl(99.97%)withchemicalcompositionof0.125Fe;0.10Si;0.020Zn;0.020Cu; 0.015Mn;0.015Mg;0.015Ti(inwt.%).Theextrudedsampleswerethesameasinthenumerical simulations,i.e.ofasquarecrosssectionwithdimensions12mmx12mmx130mm.Beforethe experimenttheAlsampleswereannealedat400°Cfor1hour.Withaviewtoobjectivelycomparethe resultsofthenumericalanalysesthedieusedfortheexperimentwasdefinedbyangleof40°,Zangle of90°,angleof110°,angleof90°andangleof20°(Fig.1d).Extrusionwascarriedoutatroom temperature(20°C)onahydraulicpressNMT300at3mms1extrusionvelocity;asalubricantMoS2was used.

During the practical experiment temperature of the extruded sample was monitored. Monitoring was performed using three thermocouples positioned in a 1 mm distance from the individual deformation zones (A section, C section, E section). At the same time, punch load was monitored as well. These parameters were subsequently used for verification of the results obtained using numerical simulations. To confirm the results, micro-hardness at cross-sections of the samples was measured as well. After deformation the microstructure of processed samples were evaluated. To compare the efficiency of the TCMAP process the microstructure after TCAP process was evaluated as well. The microstructure of the processed aluminum was investigated by OM (optical microscopy) using an Olympus GX51 optical microscope with computerized imaging system and evaluation

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software. The mean grain size was calculated using the “analySIS work” software package by Olympus Soft Imaging Solutions GmbH. 

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FEmodelling

ToobtainrequiredinformationonthenewlydesignedTCMAPprocesstheForge2009commercial softwarewasused.Simulationswerecarriedoutusingamodelthematerialgeometricdimensionsand mechanicalpropertiesofwhichweredefinedinthesamewayasforthesubsequentexperiment. Thereby,adirectcomparisonoftheresultspredictedusingtheanalysesandobtainedexperimentally wasenabled.DeformationbehaviourofAlafterasinglepasswaspredictedusinganelasticplastic materialmodel.Duetothepresuppositionofalargesheardeformationoccurrence,theNewton Raphsonconvergentalgorithmandautomaticremeshingwereactivatedduringthesimulation.Allthe componentsoftheassemblywerecreatedusingtheProEngineersoftware.Thedieandthepunchwere consideredtoberigidparts,whereasthebilletwasdefinedasadeformablepart.Thebilletwas characterizedbyameshwith48,675nodes.Theworkpiecewasmeshedusingtetrahedralelements. Thestressstraincurvefortheexperimentallyinvestigatedmaterial(Fig.1c)wasdeterminedusinga torsiontestperformedatroomtemperatureandthreedifferentstrainrates(0.1s1;1s1and10s1)ona SETARAMservohydraulictorsionplastometer.

Theexperimentallyobtaineddatawereinsertedintothematerialflowstressdatabaseofthe software.TheHaensel–Spittelequation(Eq.(1))wasusedtodescribethematerialbehaviourduring thedeformation.



Vf

Aem1T T m9 H m2 e m4 / H (1  H ) m5T e m7 H H m3 H m8T











(1)

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whereHistheequivalentstrain, H istheequivalentstrainrate,TisthetemperatureandA,m1,m2,m3, m4,m5,m6,m7,m8,m9aretheregressioncoefficients,thevaluesofwhichare151.32MPa,0.00253, 0.21142,0.03177,0.00654,m5÷m9are0. 

Theboundaryconditionsinthesimulationsweredefinedbytheparametersdescribing temperaturebehaviourofthealuminiumsampleandthesteeldie,bytheambienttemperature(20°C) andthenumberofpasses(singlepassinthiscase).Young’smodulus,Poisson’sratio,thermalexpansion, thermalconductivity,specificheat,emissivityanddensityweredefinedasthefollows:72(GPa),0.3,2.4 ×105(K1),228(W/(mK)),894(J/kgK),0.03and2700(kg/m3).

Tobetterdescribetheindividualmonitoredparametersthreespecificareas(points1,2,3)were selectedtobeobservedintheextrudedsample.Thesepointswerepositionedonaplanepassing throughthecentreofthesampleparalleltoitslongitudinalaxis(Fig.1a).Thispartoftheanalysiswas focusedonobservationoftheinfluenceofthediegeometryontheESvalue,itshomogeneity throughoutthecrosssectionofthesampleorthevalueofthepunchloadneededfortheextrusion.



4

ResultsandDiscussion

4.1 EquivalentStrain VariantI AfterasingleTCMAPpasstheaverageESvaluewas~2.76(Fig.3a).AscanbeseeninFig.3b, differencesintheESdistributionalongthelongitudinalsectionoftheextrudedsamplewerequitesmall. Eachofthedeformationsectionsinfluencedtheresultingstrainvalueinadifferentway.Theinfluence

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oftheindividualdeformationsectionssequenceontheESvalueischaracterizedbyitsgradualincrease; inthegraphicaldependenceinFig.3cnonequivalentstrainincrementsareobvious.Intheinitial extrusionstage(Asection)thedeformationinfluenceofthetwist(Asection)isobviousespeciallyinthe peripheralsampleregionsandthecentralsampleregionisdelayed,whereasinthetwofollowing sections(B,C)thecentralregionofthesampleisdeformedthemost.Duetoachangeinthematerial flowvectorintheBandCsectionsitcomestounificationoftheESdistributionthroughoutthecross sectionofthesample,thereforethecrosssectionstraingradientdecreases.AfterpassingthroughtheD section,arelativelylowerdeformationisnoticeableinthecentralregionofthesample(Fig.3b), whereastheperipheralregionsaredeformedmoreintensively,thereforetheESheterogeneity throughoutthecrosssectionofthesampleincreasesagain.Inthefollowingsheardeformationzone,i.e. theEchannelbendingsectiondefinedbytheangle,heterogeneitythroughoutthecrosssectionofthe sampledecreases,moreovertheESvalueincreasesonceagainduetothestrainimposedintheE section.Thereasonistheshapeandlocalizationofthemaindeformationzone(MDZ)inthedie.This facthadalreadybeenprovenbyvariousTCAPandECAPanalysescarriedoutbefore[13,18,22]. Followingthisdeformationsectionssequence,theEShomogeneitythroughoutthecrosssectionofthe sampleafterapassisveryhigh.ThisfindingisconformaltotheTCAPprocess.

CertaindifferencesfromtheconventionalECAPprocessareintheESdistributionandvalueinthe endpartofthesample.TheendpartisdefinedbyarelativelyhighESvalue(~1.7),whichissignificantly higherthanfortheTCAPortheECAPprocesses.AnalogicallytotheTCAPprocess,onlyalittlechangein theshapeoftheoutgoingendoftheextrudedsampleoccurredfortheTCMAPmethod.

 VariantII

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Aswasalreadymentioned,thesequenceoftheindividualdeformationsectionsisofasignificant influencenotonlyfromtheESdistributionhomogeneitypointofview.FromTCAPanalysesitis generallyknownthatthetwistsectionshouldnotbepositionedasthelastone.Otherwiseunintentional deformationofthesampletoaspiralshapeoccursandrepetitionoftheprocessisdisabled.Onthe otherhand,thesectionssequencechangeoverispossibleinthefollowingway;theAsectionpositioned behindtheB,C,Dsections(Fig.2b).

Theresultsofthesimulationsimplythatthetwomodelledvariantsarenotequivalent.Itis evidentthatafterthesectionssequencechangeover,theachievedESvalueanddistribution homogeneityforboththevariantsdiffered(Fig.4a).ForthevariantII,themaximalESvalue(~3.2)was achievedintheperipheralareasofthesample.Nevertheless,theESheterogeneitythroughoutthe crosssectionofthesamplewashigherthanforthevariantI(Fig.4a).ThetimedependenceinFig.4b confirmsalsoadifferentstraindevelopmentinthesample;asteeperESincreaseintheprimarystageof theextrusionpass,especiallyintheperipheralareasofthematerial(point1),canbeseen.Contraryto theaforementionedvariant,thesampleregionclosetotheoutercorner(point3)doesnotdelay,but thecentralregion.Obviousdifferencesbetweenthevariantscanbefoundespeciallyinthematerial flowduringthedeformation.Thiswasprovenbytheshapeoftheendofthesampleaswell(Fig.4a)and isdiscussedlaterinthesection4.4.Thesefactsconfirmedacompletelydifferentcharacteroftheplastic flowofthematerialforboththevariants.

Bypositioningthetwist(Asection)rightbeforethelastchannelbending(Esection)arelativeES increase,comparedtothevariantI,canbeobtained.OntheotherhandtheESheterogeneity throughoutthecrosssectionofthesampleishigher.Inthecaseofthetwistprecedingthelast deformationsection,thecentralareasofthesampleareofalowerESvaluethantheperipheralones, thereforetheEShomogenizationislowerthanforthevariantI.Anexplanationofthisphenomenoncan

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begivenbytheESinhomogeneityachievedinthepreviousB,C,Dsections,theperipheralareasofthe sampleinwhichweredeformedmoreintensively(Fig.4b).Subsequentdeformationinthetwistsection resultedinfurtherincreaseofthedeformationgradientthroughoutthecrosssectionofthesample, whichcouldnotbe“eliminated”intheEsection.

WhentheAsectionisthefirstdeformationsectioninthechannel(Fig.2a),peripheralareasofthe samplearedeformedbythesheardeformationduetotherotationofthesampleandduringtherestof thedeformationprocessthestrainisalwaysimposedinthesameshearplane.Ontheotherhand,when theAsectionisthesectionprecedingthelastsectionofdeformation(Fig.2b),thestrainfromtheB,C,D sectionsisimposedinoneshearplane,whereasthestrainfromtheA,Esectionsisimposedtoanother shearplane.Thereforeitcanbeassumedthatgrainrefinementofthedeformedmaterialissignificantly influencedbythepositionofthetwistsectionwithregardtotheB,C,Dsections,sincestrainintheB,C, DsectionsisimposedindifferentshearplanesthaninthesubsequentEsection(analogicallytothe deformationpathsoftheECAPprocess).

Variousanalysescarriedoutbeforerevealedthatifanydeformationsectionisprecedingthelast deformationsection(Esection),valueandhomogeneityoftheimposedstrainareinfluenced.Fromthe TCAPprocessisknownthatifthetwistsectionprecedesthebendingsection,strainofahomogenous distributionthroughoutthecrosssectionofthesamplecanbeachievedafterapass.Nevertheless, whenthematerialisofanonhomogenousstraindistributionbeforepassingthetwistsectionthe heterogeneityispreservedevenaftertheextrusionisfinished(dependingonactualESdistribution).In otherwords,nonhomogeneityoftheimposedstraincanbemodified(eliminated)bytheEsection, accordingtotheactualMDZshapeandposition,onlypartially.

Afterthesimulationshadbeencarriedout,theEStimedependencesinthecentralareasof sampleswerecomparedforboththeTCMAPvariantsandtheTCAPprocess(Fig.5a).Itisevidentthat

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thevaluesofindividualincrements,aswellasthetotalESvalue,weredifferent.FortheTCMAPvariantI theESincreaseintheprimaryextrusionstageisgradualcontrarytothevariantII.Thiscorrespondsto theAEsectionsequence,thecentralareasareinfluencedbythetwistsignificantlylessthanthe peripheralareas.ThisisprovenbythemoregradualESincreasefortheTCAPprocessaswell. Therefore,thedifferenceinthestrainhomogeneitybetweentheTCAPandtheTCMAPprocessesisnot onlyinthenumberofdeformationsections,butalsointheirsequence.

Bythereasonofhigherstrainheterogeneitythroughoutthecrosssectionoftheextrudedsample onlythevariantIwasmodelledinthesubsequentsimulations.

4.2 Temperature

Thetemperaturedevelopmentintheextrudedsampleimpliesthatonlyanegligibletemperature increaseoccurredduringtheTCMAPprocess.Thehighertemperatureareascorrespondedtotheshear deformationareas(Fig.5b)andthemaximalvalue(~25°C)wassituatedinMDZ(Esection).Comparedto theTCAPprocess,theTCMAPprocesstemperatureincreasewasrelativelylower(Fig.5b).Themost probablereasonisinadifferentincreaseoftheESvalueintimeforboththeprocesses.Contrarytothe TCMAP,intheTCAPdiethedeformationsectionsarepositionedrightbehindeachother.Duringthe TCMAP,alowerstrainisimposedintothematerialatfirst(ADsections)andthentheESvalue increases.Ontheotherhand,duringtheTCAPthematerialisdeformedwithinashorttimewithamore linearstrainincrementandthusahighertemperatureincreaseoccursintheextrudedsamplethan duringtheTCMAP.Moreover,thepossibilityofheattransferfromthesamplebeingextrudedthrough thewallsofthediechannelmustbetakenintoaccountaswell.ThelengthofchannelintheTCMAPdie islongerandthustheextrusiontime,inwhichthesampletemperaturecandecreasebyheattransfer throughthediechannel,islongerandmoreintensive.Thissuppositionwasconfirmedbythe occurrenceofseverallocaltemperaturemaximathatsubsequentlydecreased,whiletherewasonlyone

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temperaturemaximumfortheTCAPprocess(Fig.5b).ResultsofseveralTCAPanalysescarriedout beforerevealedthattemperatureoftheextrudedsampleincreasedinaconsequenceofincreasing extrusionvelocityaswell[18].Itshouldbestressedthatalowertemperatureincreaseduringextrusion isafavourablefeaturesincethetemperatureneededforactivationofsofteningprocessesisexpected nottobeoverreached.



4.3

Stressstate Thestressstateinthebilletduringextrusionisaveryimportantfeaturethatcanbe,among

others,usedtopredicatecriticalareasinthematerial.Distributionoftheprincipalstressesisoneofthe parametersusingwhichstressstatecanbedefined.Theaimoftheextrusionistoachieveconditions closetothehydrostaticpressure(eliminationoftensilestresses).Unfortunately,duetotheprocessing parameterssuchasfrictionandmaterialcharacteristics(e.g.strainratesensitivity)itisusually impossibletoachievesuchastateintheentiresample[1].ByvariouspreviouslyperformedECAP simulationswasproventhatsuchproblematicareas(withtensilestresses)areusuallypositionedright behindtheinnerchannelbendingintheupperhalfofthesample[22,23].Onthebasisofthe informationamodificationthatcanleadtoasignificantdecreaseofthetensilestressvalueoreven eliminationoftheareasoftensilestressoccurrenceduringtheprocesswasdesigned.Bythisreasona TCMAPvariantwithachamfercreatedatthebeginningoftheoutletchannelwassimulated(Fig.6b, detail).Thechamferwasdesigned5mmlongwithanangleof5°.

AscanbeseeninFig.6,therearelocaloccurrencesoftensilestress(positiveprincipalstress values)fortheoriginal(unchanged)TCMAPdieoutletchannelgeometry,too.Themainareaofits occurrenceissituatedjustbehindtheinnercornerofthechannelbendingandinfrontoftheouter corner(Fig.6)aswell.ForthemodifiedTCMAPdiethevaluesoftensilestressesdecreasedevenof70%

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duetothechamfer(Fig.6).Suchasubstantialdecreasemeansasubstantialdecreaseinthedangerof cracksdevelopment.Itisevidentthatbytheslightmodificationoftheoutletchannelthevaluesand sizesoftheareasoftensilestressesweresignificantlyinfluenced.Themodificationofthegeometrywas ofapositiveinfluenceinboththeabovementionedareasofthediechannel(behindtheinnercorner,in frontoftheoutercorner),butwasofnoinfluenceontheESdistributionorvalue,noronthepunch load.

4.4 Plasticflow

ToevaluatetheplasticflowofthematerialduringtheTCMAPprocessagridwassuperimposedon theplanepassingthroughthecentreofthesample.Bythereasonofamorepreciseevaluationthevery finegridwasmodelled,theindividualcellswerewithdimensions0.5x0.5mm.

Oneoftheoftendiscussedfactorsisthedeadzoneangle.Inmostofthestudiesperformedbefore negativeinfluenceofthedeadzoneontheESvalueandhomogeneitywassupposed[2326].Itis obviousthatduringtheTCMAPprocessnodeadzonesoccurredinanyofthechanneldeformation sections.AswasalreadymentioneddifferencesinthematerialflowbetweenthevariantsIandIIare evident.DuringthevariantIboththehalvesoftheextrudedmaterialacceleratedalternatively(Fig.7a). Beforeenteringthelastshearsection(Esection),theflowintherighthalfofthematerialwasslower thanintheleftone.Thisiscausedbythegeometryoftheverticalpartofthechannel;thematerialflow inthelefthalfofthechannelisfasterduetoashortertrajectory.Theflowchangedinthemomentof thematerialpassingthroughtheintersectionofthehorizontalandverticalpartofthechannel,thusin theMDZareatheupper(formerright)regionofthematerialwasacceleratedinrelationtothebottom (formerleft)region.Followingthistrendtheshapeoftheendoftheextrudedsamplewas“positively” influenced.Thismaterialflowdevelopmentisevidentfromthedeformation(slope)oftheindividual cellsofthesuperimposedgrid.Ascanbeseenonthesuperimposedgridhorizontallinesoscillationsthe

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celldeformationsareespeciallyinthebottomhalfofthesampleandlocallyintheupperone;theyare notlocalizedonthesurfaces,butinundersurfacelayers.

ContrarytothevariantI,forthevariantIIthecentralareasofthesamplewereofthefastestflow beforetheEsection.Duetothisaccelerationasignificantelongationoftheseareasoccurredanda sharpenddeveloped.Theoscillationsweresignificantlylowerandmorehomogenouslydistributed throughoutthewholecrosssectionofthesample.

DuetotheslightmodificationoftheoutletchanneloftheTCMAPdiewithachamferdifferences inthematerialflowareobvious;especiallyintheoutletchannelthematerialflowintheupperregionof thesamplewasmorehomogenouswhencomparedtotheversionwithnochamfer.Itis,mostprobably, causedjustbythechamfer.Inthechamferareathecrosssectionofthechannelisalittlewiderand thusarelativeaccumulationofthematerialcanoccur.Subsequently,thematerialisaccelerateddueto thecrosssectionchange(flowingintothechannelofasmallercrosssection).FortheTCMAPprocess theendofthesampleafterextrusionwascompletelydifferentwhencomparedtotheTCAPprocess.In addition,duringtheTCAPprocessoccurrenceofrelativelysmalloscillationsinthewholecrosssectionof thesamplecanbedetected.

4.5

Punchload OneoftheimportantfactorsinfluencingtheTCMAPprocessapplicationisthepunchload.Bythis

reason,thepunchloadduringtheextrusionprocesswasmonitored.Accordingtotheabovementioned, theoutletchannelchamferwasofapositiveinfluenceonthematerialflow,aswellasonthetensile stressesdistribution,thereforethevariantofTCMAPwithachamferwasevaluatedaswell.

AhigherpunchloadfortheTCMAPthanfortheTCAPprocessisevidentinthetimedependence inFig.7b.FortheTCMAPthepunchloadincreasedof58%whencomparedtotheTCAP,thehighest punchloadincreasecanbeobservedduringthesampledeformationintheEsection.Ontheother

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hand,intheareabetweenthedeformationsectionsDandEashorttimepunchloaddecreaseoccurred. ThedecreasewasveryprobablycausedbyfillingoftheEsectionwithmaterial;duetotheunfilled deformationzonethecontactsurfaceareabetweentheextrudedmaterialandchannelwallslocally decreased(i.e.decreaseoffriction).

Fortheabovementionedmodifiedoutletchannelgeometrydiethepunchloadincreasewas negligible,whereasitsinfluenceoneliminationoftheoscillationswasfavourable.Itisknownthatthe oscillationsarerelatedtothematerialplasticflowinstability[2224].InthevariantIwithnochamfera serratedcurveshape,whichisoftenconnectedtothematerialplasticflowinstabilitydevelopment,can beobserved.Forthemodifiedgeometryasmoothercurveshapeisevident.Inotherwords,the modifiedoutletchannelgeometrywasofapositiveinfluenceonthepunchloadaswell.

Aswasprovenbythepracticalexperiment,thesimulatedpunchloadvaluescorrelatedreasonably withtheexperimentallyobtaineddata.Comparedtotheanalyses,fortheexperimentalowerpunch loadincreasewasobtainedduringsamplepassingthroughtheA,B,Csections,whereasduringsample passingthroughtheEsectionthepunchloadwashigher.Itisnecessarytomention,thatafterreaching thepeakloadvaluetheloaddecreasedrelativelyfastduringthefollowingextrusionstage.Differences betweentheexperimentallyobtainedandpredictedvaluescanbeinfluencedbyseveralfactors(Fig. 7b).Amongthesearee.g.sensorsaccuracy,materialproperties,frictionvariability,ortheused mathematicalmodelandpossiblesimplifications.

DespitethefactthatthepunchloadfortheTCMAPprocesswasrelativelyhigherthanforthe TCAP,itcanbeinfluenced.Oneofthepossibilitiesis,e.g.,reductionoftheoutletchannelslength.By suchprecautionsfrictionandthusdeformationresistanceoftheextrudedmaterialcanbedecreased.



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4.6

Microhardness Microhardnesswasmeasuredonacrosssectionofthesampleafterasinglepass.Foramore

preciseevaluationmeasurementswereperformedinboththediagonals(Fig.8a).Fromthegraphical dependenceisevidentthatthehighestvalueswereobtainedintheupperregionofthesample(Fig.8b). Nevertheless,whencomparedtothecentralareasonlyaslightdifferencewasobservedbetweenthe maximalandtheminimalmeasuredmicrohardnessvalues.Ontheotherhand,aprobableinfluenceof themeasuredvaluesintheperipheralcrosssectionareas,wheretheindentationswerelocalizedvery neartothediagonalsends,mustbetakenintoaccount.Despitethisfactonlyalittlemicrohardness decreasewasobservedthere.Thisconclusionreferstoarelativelygoodcorrelationbetweenthemicro hardnessandthepredictedESdistributionintheextrudedsample.Itcanbestatedthatafterthe deformationhadbeencarriedoutthemicrohardnessincreasedtotheaveragevalueof90HV,which representsanincreaseof97%whencomparedtothestatebeforedeformation.

Onthebasisoftheobtaineddataitispossibletosummarizethefollowing:fromthepointofview ofachievementofahighESvalueanditshomogenousdistributiontheTCMAPprocesscanbe consideredasoneofthesuitableSPDprocesses.Amongthemainfactorsinfluencingthedeformation behaviourofamaterialdeformedusingSPDprocessesisthediegeometry.Byasuitableselectionofthe deformationsectionssequencetheachievedresultscanbesignificantlyinfluenced.



4.7

Microstructure

AsignificantgrainrefinementoccurredduringtheTCMAPprocess.InFig.9,thestatesbefore(Fig. 9a)andafter(Fig.9b)aperformedpassaredepicted.Inthedeformedstructure,grainsofsizelower than3Pmareprevalent(Fig.9c).Contrarytothis,theoccurrenceofgrainsofasizegreaterthan10 micronsisscarce.Thegrainsize,determinedafterperformedTCMAPpass,averagedout2.4Pm.

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ApplicationofTCMAPbroughtmarkedlyhomogeneousgrainrefinementthroughouttheprocessed sample.Inrelationtotheresultsofthesimulationcarriedoutbefore,thisfactisinquitegood correspondencewiththeimposedstraindistribution,bywhichonlyslightdifferencesintheimposed strainvalueswithintheindividualpartsofthesamplewereproven.Grainsizedistributioncorresponds totheabovementionedstatementaswell.Nevertheless,inthecaseofthesampleprocessedbythe TCAPtechnologythisissimilar(Figs.9d,9e).Thedifferencebetweenboththetechnologiesis,aboveall, indifferentvolumefractionswithintheindividualgrainsizegroups.Withinthesampleafterthe TCMAP,thevolumefractionofmostfinegrainsisapprox.40%higherwhencomparedtothesample afterTCAP.Onthebasisofthesefacts,theTCMAPismoreadvantageousfromthegrainrefinement efficiencypointofviewthantheTCAP.Thisconclusionissupportedbyahigherimposedstrainduring theTCMAPprocess.Ontheotherhand,thescaleandsizeofthelocaloccurrenceofthevortexlikeflow afterTCMAPmustbementioned.AsimilarconclusionaboutthepresenceoftracesoftwistafterTCAP processinghasalreadybeenpublished[19].Nevertheless,afterTCMAPthetracesweresignificantly weakerthanafterTCAP.ThissignifiesthatthestrainimposedduetotheBDzoneswasnotable“to erase”thetracesatall,howevertheyweresignificantlyreduced.Thisfindingisincorrespondencewith thepredictedflowpattern(Fig.7a).

ThemicrostructuralchangesoftheAlafterprocessingareproofofthehighefficiencyofboththe usedtechnologies.Itisevidentthatboththeinvestigatedmethodsleadtoachievementofaveryfine structureevenafterthefirstpass.Fromthegrainrefinementpointofview,boththemethodsaremore efficientthantheECAPprocess.InthecaseoftheapplicationoftheTCMAPprocess,asignificant reductionofthenumberofpassesnecessaryforachievementofthedesiredgrainsizecanbeexpected.



5

Conclusions

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TheworkdealswithnumericalsimulationsofanewlydesignedTCMAPprocess;severaldie geometryvariantsweremodelled.Thepaperisfocused,aboveall,onevaluationofefficiencyofthisSPD processconsideringthevalueofimposedstrainandotherfactors,suchastemperatureorstressstate. Thesubsequentexperimentwasaimedonverificationofthepredictedresults.Themainresultscanbe definedasfollows:

 -

ThestrainimposedduringtheTCMAPisdistributedwithahighhomogeneity.Themaximal valuesoftheimposedstrainareabout~3.

-

TheTCMAPgeometryenablestoobtainmaterialswithahighvalueoftheimposedstrain withoutsignificantchangesintheshapeofthecrosssectionandeventheendoftheextruded material.

-

DuringtheTCMAPprocessnodeadzoneoccurs,whichcontributestoamorehomogenous straindistribution.

-

ByanappropriateconstructionoftheTCMAPdiechanneltensilestressesintheextruded materialcanbeeliminated,sothatadangerofmaterialfailureisdecreased.

-

AhighvalueoftheimposedstrainfortheTCMAPiscompensatedwithahigherpunchload.

-

Thesubsequentlyobtainedexperimentalmicrohardnessvaluesthroughoutthecrosssectionof thedeformedsamplecorrelatedrelativelywellwiththepredictedimposedstrainvalues.

-

Thegrainsize,determinedafterTCMAPpass,averagedout2.4Pm. 

TheequivalentstraininTCMAPmethodwasinvestigatedandshowntohavepotentialasanew severeplasticdeformationprocess.Furtherexperimentsareneededtofindtheexactrelationsbetween

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theuseddeformationpathandgrainrefinementmechanismforagivenmaterial.Followupresearch shouldfocusonthesuitabilityofTCMAPutilizationforothermaterialsprocessing.

Acknowledgements

ThispaperwascreatedintheProjectNo.LO1203"RegionalMaterialsScienceandTechnology CentreFeasibilityProgram"fundedbyMinistryofEducation,YouthandSportsoftheCzechRepublic.



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Fig.1SchemesoftheTCMAP(a)andtheTCAP(b)includingmonitoredpointslocatedonthe“dividing” plane.1,2,3–monitoredplaces,–angleoftwistrotation,twistslopeangle,angle betweentheindividualpartsoftheembeddeddeformationzone,Mchannelangle,\angle associatedwiththearcofcurvaturewherethetwopartsofthechannelintersect,F–force.The stressstraincurvesusedforthesimulations(c)theTCMAPdieusedfortheexperiment(d)

 Fig. 2 Layout of TCMAP process – longitudinal section through the centre of the die with depicted individual deformation sections: variant I (a) variant II (b)

Fig.33Dsectionsandequivalentstrain(ES)contoursinthesampleafterTCMAP(variantI)(a)ES distribution(b)timedependenceofES(c)



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Fig.43Dsectionsandequivalentstrain(ES)contoursinthesampleafterTCMAP(variantII)(a)time dependenceofES(b)



Fig.5EStimedependenceduringTCAPandTCMAP(variantI,II)(a)distribution(3Dsection)andtime dependenceoftemperatureinthemonitoredpointsoftheextrudedsampleduringTCAPand TCMAP(b)



Fig.6ComparisonofmaximalprincipalstressfortheTCMAPwithunchangedchannel(a)modified channel(b)



Fig.7Plasticflowpatternformodelledvariants(a)dependenceofpunchloadforindividualvariants(b)



Fig.8Microhardnessofprocessedaluminum:measuredplaces(a)obtaineddependence(b)

Fig.9Microstructureofaluminumandcorrespondinggrainsizedistribution:initialstate(a)afterTCMAP (b)afterTCAP(c)



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