Hot deformation and processing map of a typical Al

Y.C.Lina,b,⇑,Lei-TingLia,b,Yu-ChiXiaa,b,Yu-QiangJianga,babSchoolofMechanicalandElectricalEngineering,CentralSouthUniversity,Changsha410083,ChinaStateKeyLaboratoryofHighPerformanceComplexManufacturing,Changsha410083,Chinaarticleinfoabstract

Thehigh-temperatureflowbehaviorof7075aluminumalloywasstudiedbyhotcompressivetests.Basedontheexperimentaldata,theefficienciesofpowerdissipationandinstabilityparameterwereevaluated.Processingmapswereconstructedbysuperimposingtheinstabilitymapoverthepowerdissipationmap.Microstructuralevolutionof7075aluminumalloyduringthehotcompressionwasanalyzedtocorrelatewiththeprocessingmaps.Itcanbefoundthattheflowstressesincreasewiththeincreaseofstrainrateorthedecreaseofdeformationtemperature.Thehigh-angleboundariesandcoarseprecipitationsdistrib-utinginthegraininterior/boundaries,whichmayresultinthedeepinter-granularcorrosionandlargeareasofdenudationlayer,shouldbeavoidedinthefinalproducts.Theoptimumhotworkingdomainisthetemperaturerangeof623–723Kandstrainraterangeof0.001–0.05sÀ1.Ó2012ElsevierB.V.Allrightsreserved.Articlehistory:Received2August2012Receivedinrevisedform1October2012Accepted22October2012Availableonline2November2012Keywords:MetalsandalloysMicrostructureProcessingmap1.IntroductionDuringhotformingprocess,theeffectsofstrainrateanddefor-mationtemperatureonthemechanicalpropertiesofmetalsandal-loysaresignificant.Generally,thereareseveraltypesofmetallurgicalphenomenaduringthehotdeformation,suchasthedynamicrecrystallization(DRX)[1–7],metadynamicrecrystalliza-tion(MDRX)[8–11]andstaticrecrystallization(SDRX)[12–15],whichresultinthecomplexmicrostructuralevolutioninalloys[16–19].Inordertoobtaintheoptimumhotworkingprocess,agoodunderstandingofprocessingmapandmicrostructuralevolutionisveryimportantforthedesignersofmetalformingprocesses[20–23].Dynamicmaterialmodeling(DMM)aimstocorrelatethecon-stitutivebehaviorwithmicrostructuralevolution,flowinstabilityandhotworkability.Basedonthedynamicmaterialmodel(DMM),theprocessingmapwasdevelopedbyPrasadetal.[24].Processingmapsareusefultoidentifythedeformationtempera-ture-strainratewindowsforhotworking.Inrecentyears,process-ingmapsarebeingdevelopedtooptimizethehotworkingprocessesofthemetalsoralloys[25–35].PrasadandSeshachary-ulu[25]foundthatthestabledomainscanberelatedtodynamicrecrystallizationwithhighpowerdissipationefficiency,whiletheinstabilitydomainscorrelatedtotheadiabaticshearbandsforma-tion.AbbasiaandMomeni[26]studiedthehotworkabilityofFe–29Ni–17Coalloybythemechanicaltestingandmicrostructuralobservations,andestablishedtheprocessingmapsforthestudiedmaterial.MomeniandDehghani[27,28]characterizedthehot⇑Correspondingauthorat:SchoolofMechanicalandElectricalEngineering,CentralSouthUniversity,Changsha410083,China.Tel.:+86015200817337.E-mailaddresses:yclin@csu.edu.cn,linyongcheng@163.com(Y.C.Lin).0925-8388/$-seefrontmatterÓ2012ElsevierB.V.Allrightsreserved.http://dx.doi.org/10.1016/j.jallcom.2012.10.114deformationbehaviorof410martensiticand2205austenite–ferriteduplexstainlesssteelsusingconstitutiveequationsandprocessingmaps.Ramanathanetal.[30]developedtheprocessingmapforAl/SiCpcomposite,andtherelationshipbetweenmicro-structureandhotworkabilitywereinvestigatedthroughmicro-structuralobservations.Samantarayetal.[31]studiedtheprocessingparametersofanitrogenenhanced316L(N)stainlesssteelbasedonthehigh-temperatureflowbehaviorandmicrostruc-turalevolution,andtheoptimumwindowforthehotdeformationwereidentifiedas1350–1423Kand0.001–0.05sÀ1withpeakeffi-ciencyof50%andactivationenergyof150kJ/mol.Also,Samanta-rayetal.[32,33]optimizedthehotworkingparametersforthemodified9Cr–1Mo(P91)steelbythedynamicmaterialsmodel.TheoptimumhotworkingparametersforAl6063/0.75Al2O3/0.75Y2O3nano-compositewereidentified,andtheflowinstabilitycharacteristicwerevalidatedbyprocessingmapsandmicrographs[34].Senthilkumaretal.[35]developedtheprocessingmapoftheAlbasednanocompositebysuperimposingtheinstabilitymapoverdissipationefficiencymapinthetemperatureandstrainratespace,anddifferentdeformationmechanismssuchasdynamicrecrystal-lization(DRX),dynamicrecoveryandflowlocalizationwerevali-datedbythemanifestationofmanymicrostructuralfeaturesafterdeformation.Duetoitsexcellentstrength/weightratio,highstrength7075aluminumalloy,atypicalAl–Zn–Mg–Cualloy,iswidelyusedastheaircraftstructurecomponents.Inthepast,thehotdeformationbehaviorof7075aluminumalloywasinvestigatedbysomeresearchers.RajamuthamilselvanandRamanathan[36]studiedthehotdeformationbehaviorofstircast7075alloyusingprocess-ingmaps.Linetal.[37,38]foundthatsomematerialparametersaresensitivetothestrainrateanddeformationtemperature.TheyY.C.Linetal./JournalofAlloysandCompounds550(2013)438–4439developedthemodifiedJohnson–Cookconstitutivemodelstopre-dictthehotcompressiveflowbehaviorof7075aluminumalloy.Despiteanumberofinvestigationsinvestedintothebehaviorsof7075aluminumalloy,furtheranalysisshouldbecarriedouttooptimizethehotworkingprocess.Theobjectiveofthisstudyistocharacterizethehotcompres-sivedeformationbehaviorof7075aluminumalloyunderwiderangeofthedeformationtemperatureandstrainrate.Effectsofstrainontheefficiencyofpowerdissipationandinstabilityparam-eterwereinvestigated.BasedontheDynamicMaterialModel(DMM),theprocessingmapsofthestudiedalloywereconstructedtooptimizethehotworkingparameters.Besides,microstructureswereobservedtovalidatethedeformationmechanismsof7075aluminumalloy.microstructureswereobservedonLeicaDMIRMimageanalyzer.Transmissionelec-tronmicroscopy(TEM)analysiswascarriedouttoinvestigatetheprecipitatemor-phologyandsub-grainsofthecompressed7075aluminumalloy.ThefollowingsarethestepstopreparethethinfoilsamplesforTEMexperiments.Firstly,1.0–2.0mmthickfoilswerecutfromthedeformedspecimens.Then,thethickfoilsweregrin-dedinto0.7–0.8mmthinfoils.Finally,severaldiskswith3mmindiameterwerepunchedoutfromthesethinfoils,andsubsequentlyelectro-polishedusingasolu-tionofHNO3andmethanol(1:3involume).TEMfoilexampleswereexaminedusingaTecnaiG2-20microscopeoperatingat200kV.3.Experimentalresultsanddiscussion3.1.Truestrain–truestresscurvesTheforce-strokeofthespecimensduringcompressiontestscanbeautomaticallysavedbythetestingmachine,andtheexperimen-taldatacanbeeasilytransformedintothetruestress–truestraincurves,asshowninFig.1.Obviously,theeffectsofthedeformationtemperatureandstrainrateontheflowbehaviorof7075alumi-numalloyaresignificant.Thestressincreasessharplyuntilapeakstressataverysmallstrain.Inthefollowingdeformationperiod,thestressdecreasesuntilarelativelystablestressappears,show-ingadynamicflowsoftening.Generally,thetruestress-truestraincurvescanindicatetheintrinsicrelationshipbetweentheflowstressandthermal-dynamicbehavior.Duringtheinitialstageofdeformation,theworkhardeningresultsintherapidincreaseoftheflowstress.Withtheincreaseofthedeformationdegree,thedislocationdensityandthepotentialdrivingforceofrecoveryin-creases.Meanwhile,thenucleationandgrowthofnewgrainsoc-curs,i.e.,sub-grainsdevelop.Whentheworkhardeningandrecoveryreachadynamicequilibrium,thedislocationdensityre-mainsrelativelyconstantandsteadystateflowstressisobtained2.MaterialsandexperimentsInthisinvestigation,thecommercial7075aluminumalloy(atypicalAl–Zn–Mg–Cualloy)wasused,anditschemicalcomposition(wt.%)isshowninTable1.Cylindricalspecimensof10mmindiameterand12mminheightweremachinedforexperiments.Theflatendsofthespecimenwererecessedtoadepthof0.1mmdeeptoentrapthelubricantofgraphitemixedwithmachineoiltomini-mizethefrictionbetweenthespecimensanddieduringthehotdeformation.Inor-dertostudythehotcompressivebehavior,thehotcompressionexperimentswerecarriedoutonGleeble-1500thermo-simulationmachine.Fourdifferentdeforma-tiontemperatures(573,623,673and723K)andfourdifferentstrainrates(0.001,0.01,0.1and1sÀ1)wereusedintheexperiments.Beforethecompressiontests,thespecimenswereheatedtothedeformationtemperatureattheheatingrateof10K/s,andheldfor3minatisothermalconditionsinordertoobtaintheuni-formdeformationtemperature.Thereductionofthespecimen’heightwas60%andthetestedspecimenswerequenchedwithwaterimmediatelyattheendofcom-pressiontests.Thedeformedspecimenswereslicedparalleledtotheaxialsection.TheexposedsurfaceswerepolishedandetchedwithKeller’ssolution.TheopticalTable1

Chemicalcompositionsof7075aluminumalloy(wt.%).CompositionContent(wt.%)Zn5.8Mg2.3Cu1.5Cr0.21Fe0.16Mn0.05Ti0.02Si0.07AlBal.(a)True stress(MPa)140105703500.0Strain rate:0.001s-1(b)160True strss(MPa)12080400Strain rate:0.01s573K623K673K723K-1573K623K673K723K0.20.40.60.80.00.20.40.60.8True strainTrue strain-1(c)True strss(MPa)160120804000.00.20.4True strss(MPa)Strain rate:0.1s573K623K673K723K(d)2001501005000.0Strain rate:1s573K623K673K723K-10.60.8True strain0.2True strain0.40.60.8Fig.1.Typicaltruestress–truestraincurvesof7075aluminumalloyunderstrainratesof(a)0.001sÀ1;(b)0.01sÀ1;(c)0.1sÀ1;and(d)1sÀ1.440Y.C.Linetal./JournalofAlloysandCompounds550(2013)438–445[39].So,thesofteningphenomenonismainlyattributedtothedy-namicrecovery(DRV)andcontinuousdynamicrecrystallization(DRX).Forthecaseswiththedeformationtemperatureof573Kandallthetestedstrainrates,theflowstressesrapidlydecreaseswiththefurtherincreaseofdeformationdegreeafterthepeakstrain(correspondingtothepeakstress).Thisphenomenonmaybeinducedbythehighanglegrainboundariesformingviathecon-tinuousDRX.Ittakesalittlelongtimefortheflowstressandthemicrostructuretoreachasteadystatewhenthematerialunder-goescontinuousDRX[40],whichresultsinarelativelyquickde-creaseoftheflowstress.Increasingthestrainrate,thereisnotsufficienttimeforthegrowthofthemisorientationamongthesubgrains,leadingtotherelativelysteadyflowstresses.Also,itmayindicatethattheflowbehaviorofthematerialisunstablewhenthedeformationtemperatureis573K,whichwillbedis-cussedbelow.3.2.Establishmentofprocessingmapfor7075aluminumalloy3.2.1.TheprinciplesforprocessingmapsAccordingtotheprinciplesoftheDynamicMaterialModel[24],thematerialsundergothehotdeformationcanbeconsideredasapowerdissipater,andtheinstantaneouspowerdissipated(P)canbedividedintotwocomplementaryparts,i.e.,Gcontent(temper-aturerise)andJco-content(microstructuremechanisms),whichcanberepresentedasafunctionofflowstress(r),strain(e)and_Þ:strainrateðe_¼GþJ¼P¼reZ0_e_þrdeZr0_dreð1ÞForthegivenstrainanddeformationtemperature,theflowstresscanbeexpressedas_mr¼Keð2ÞwhereKisamaterialconstant,misthestrainratesensitivityusedtopartitionthepowerintoGcontentandJco-content:m¼dJ@ðlnrÞ¼_ÞdG@ðlneð3ÞJco-contentcanbeexpressedas,J¼Zr0_dr¼em_remþ1ð4Þ_=2.Foranideallineardissipationprocess,m=1andJmax¼reThepower-dissipationcapacityofthematerialcanberepresentedbytheefficiencyofpowerdissipation(g),g¼JJmax¼2mmþ1ð5ÞTakinguseoftheprincipleofthemaximumrateofentropyofproduction,acontinuumcriterionfortheoccurrenceofflowinstabilitiesisdefinedintermsofanotherdimensionlessparameter(n),(a)lnσ/MPa5.24.84.44.03.6-8-6-4-2 573K 623K 673K 723K0(b)5.24.8lnσ/MPa4.44.03.6-8-6 573K 623K 673K 723Kln(ε/s(c)lnσ/MPa5.24.84.44.03.6-8-6-4 573K 623K 673K 723K(d)lnσ/MPa5.24.84.44.03.6-8-6-4-2ln(ε/s(e)5.2lnσ/MPa4.84.44.03.6-8-6-4-2 573K 623K 673K 723K0(f)5.0lnσ/MPa4..03.5-8-6-4-2 573K 623K 673K 723K0ln(ε/s_withstrainsof(a)0.1;(b)0.2;(c)0.3;(d)0.4;(e)0.5;and(f)0.6.Fig.2.Fittingcurvestodescribetherelationshipbetweenlnrandlne(ln(ε/s(-1-1(ln(ε/s(-1-20-1( 573K 623K 673K 723K0(-1-4-1ln(ε/s-20Y.C.Linetal./JournalofAlloysandCompounds550(2013)438–4441_Þ¼nðe@lnðmmÞþ1þm60_@lneð6ÞThevaluesofmcanbeobtainedbydifferentiatingthethreeorder_Àlnrplots,asshowninFig.2.Then,polynomialfittinglineoflnethevaluesofgandncanbecalculatedbyEqs.(5)and(6).Thepowerdissipationmapisconstitutedbythethree-dimen-sionalvariationsoftheefficiencyofpowerdissipationwiththedeformationtemperatureandstrainrateatconstantstrain,andcanbeconsideredasacontourmaprepresentingiso-efficiencycontoursinadeformationtemperature–strainrateframe.Thismapdepictsthemannerinwhichthepowerisdissipatedthroughmicrostructuralchangesduringthehotdeformation,andhencere-vealsthedomaininwhichaspecificmechanismmaybecomeattractiveforminimizingtheenergyofthedissipatedstate.Thethree-dimensionalvariationoftheinstabilityparameter(n)withthedeformationtemperatureandstrainraterepresentstheinsta-bilitymap,inwhichtheshaderegionswithanegativeparameterindicatetheflowinstabledomains.Aprocessingmapcanbecon-structedbysuperimposingtheinstabilitymapoverthepowerdis-sipationmap,fromwhichindividualmicrostructureprocessesandthelimitingconditionsfortheregimesofflowinstabilitycanbeobtained.Thehighestefficienciesofpowerdissipationdonotnec-essarilymeanbetterworkabilityandmaybetheresultofsomevariationsofinstabilitysuchaswedgecrack.So,byprocessingun-derconditionsofhighestefficiencyinthe‘‘safe’’domainsandavoidingtheregimesofflowinstabilities,theintrinsicworkabilityofthematerialmaybeoptimizedandmicrostructuralcontrolmaybeachieved.3.2.2.EffectsofstrainontheefficiencyofpowerdissipationFig.3showstheeffectsofstrainontheefficiencyofpowerdis-sipation(g)of7075aluminumalloyunderthestudiedexperimen-talconditions.Obviously,thevaluesofgincreasewiththeincreaseofstrainunderthestrainratesof0.001sÀ1(Fig.3a)and0.01sÀ1(Fig.3b),exceptthatthereisadecreasingtrendunderthestrainrateof0.001sÀ1anddeformationtemperatureof723K.However,whenthestrainrateis1sÀ1,thevaluesofgdecreasewiththein-creaseofstrain,asshowninFig.3(d).Forthecasewithstrainrateof0.1sÀ1,theeffectsofstrainontheefficiencyofpowerdissipationarenotobvious(Fig.3c).Theefficiencyofpowerdissipation(g),whichisadimensionlessparameter,canbeusedtoindicatethedissipationofpowerin-ducedbythemicrostructuralevolution.Generally,thematerialshowsthegoodworkabilitywhendeformedinthedomainwiththehighefficiencyofpowerdissipation.However,thehighesteffi-cienciesofpowerdissipationdonotnecessarilymeanbetterwork-abilityandmaybetheresultofsomevariationsofinstabilitysuchaswedgecrack.Hence,onlythedeformationtemperatureandstrainratecorrespondingtothepeakefficiencyinthe‘safe’domainoftheprocessingmapcanbeconsideredastheoptimumhotwork-ingparameters.3.2.3.EffectsofstrainontheinstabilityparameterFig.4showstheeffectsofstrainontheinstabilityparameter(n)of7075aluminumalloyunderdifferentexperimentalconditions.Obviously,whenthedeformationtemperatureis573K,thevaluesofinstabilityparameterarenegativeunderrelativelylowstrainrates,andthepositivevaluesappearwiththeincreaseofthestrainrate.Thenegativenvaluesindicatethehotdeformationprocessinginthesecasesisnot‘‘safe’’.ThisphenomenonmaybeinducedbythehighanglegrainboundariesformingviathecontinuousDRX,andtheflowbehaviorofthematerialisunstablewhenthedefor-mationtemperatureis573K,asdiscussedinSection3.1.Increas-ingthestrainrateresultsintheinsufficienttimeforthegrowthofthemisorientationamongthesubgrains,andtherelativelystea-dyflowappears.Also,theincreaseofstrainmakesthenegativedo-mainsbecomemoreandmorelarge.Forexample,whenthedeformationtemperaturesare623and673K,thevaluesofinsta-bilityparametervaryfrompositivetonegative(thestrain>0.4).Theinterpretationofthesephenomenaisbasedonthefactthatthematerialpresentssometypeofunstableflowsmanifestation,suchastheadiabaticshearbandsorflowlocalization[21].Efficiency of power dissipation(%)Efficiency of power dissipation(%)(a)353025201510500.1Strain rate:0.001s0.20.30.4-1(b)353025201510500.1Strain rate:0.01s0.20.30.4-1 573K 623K 673K 723K0.50.6 573K 623K 673K 723K0.50.6True strainEfficiency of power dissipation(%)3530Strain rate:0.1s-125201510500.10.20.30.40.50.6True strainEfficiency of power dissipation(%)(c)(d) 573K 623K 673K 723K403020100-10-200.1 573K 623K-1Strain rate:1s 673K 723K0.20.30.40.50.6True strainTrue strainFig.3.Effectsofstrainontheefficiencyofpowerdissipationof7075aluminumalloywithstrainratesof(a)0.001sÀ1;(b)0.01sÀ1;(c)0.1sÀ1;and(d)1sÀ1.442Y.C.Linetal./JournalofAlloysandCompounds550(2013)438–445(a)Instability parameter,ξ(ε)0.0-0.6Strain:0.1 573K-1.2 623K 673K-1.8 723K-9-6-3lnε0Instability parameter,ξ(ε)0.6(b)0.60.0-0.6Strain:0.2 573K-1.2 623K 673K-1.8 723K-9-6lnε-30(c)Instability parameter,ξ(ε)Instability parameter,ξ(ε)0.60.0-0.6Strain:0.3 573K-1.2 623K 673K-1.8 723K-9-6(d)0.60.0-0.6Strain:0.4 573K-1.2 623K 673K-1.8 723K-9-6lnε-30lnε-30(e)Instability parameter,ξ(ε)Instability parameter,ξ(ε)0.60.0-0.6Strain:0.5 573K-1.2 623K 673K-1.8 723K-9-6-3lnε0(f)0.60.0-0.6-1.2Strain:0.6 573K-1.8 623K-2.4 673K 723K-3.0-9-6lnε-30Fig.4.Relationshipbetweentheinstabilityparameterandstrainrateunderthestrainof(a)0.1;(b)0.2;(c)0.3;(d)0.4;(e)0.5;and(f)0.6.3.2.4.EffectsofstrainonprocessingmapsandmicrostructuralevolutionFig.5illustratestheeffectsofstrainontheprocessingmapsgeneratedinthetemperaturerangeof573–723Kandstrainraterangeof0.001–1sÀ1.Thecontoursinthemapsrepresenttheper-centageefficiencyofpowerdissipation.FromFig.5,itcanbefoundthattheshadedareasdenotingtheinstabilitydomainsincreasewiththeincreaseofstrain.Meanwhile,theshapesoftheprocess-ingmapsaresimilarforthestrainrangingfrom0.1to0.3.However,thereexistobviousdifferencesintheshapeofthepro-cessingmapwhenthestrainislargerthan0.3,resultingfromtheoccurrenceofthedynamicrecrystallizationundertheseworkingconditions.Fig.6showstheopticalmicrostructuresofthede-formed7075aluminumalloy,whichindicatesthatthehigherthestrain,thefinerthegrains.ThisphenomenoncanbewellexplainedbythetheoryofcontinuousDRXforalloyswithhighstackingfaultenergy(SFE),i.e.,aluminumalloys.Theamountsofthestoreden-ergyanddefects(suchasdislocation)withinthecrystallatticein-creasewiththeincreaseofdeformationdegree,leadingtothesufficientmobilityofthedislocations.Additionally,thecontinuousDRXoccurssincethedislocationsrearrangementamongsubgrainboundariesandthemisorientationamongtheadjacentsubgrainsincreaseduringthehotdeformationof7075aluminumalloy.Asaresult,manysubgrainsappear,makingthelowanglegrainboundariestransformtohighanglegrainboundaries.Meanwhile,thegrainsarerefined.Fig.7(a)showstheTEMmicrographoftheas-received7075aluminumalloyandexhibitsthatthesub-grainsareequiaxedandrelativelylarge.Whenthematerialaredeformedunderthetemperatureof673Kandthestrainrateof0.1sÀ1,somefinesub-grainswithhighanglegrainboundariesappearalongtheserratedsub-boundaries,asshowninFig.7(b).Itrevealsthattheoriginalmicrostructurehasbeenreplacedbytherecrystallizationmicrostructures.FromFig.5(e),itcanbefoundthattherearethreetypicalpeakefficiencydomainsintheprocessingmap.Thefirstdomainisthedeformationtemperaturerangingfrom573Kto600Kandstrainraterangingupto0.002sÀ1.Themaximumefficiencyinthisdo-mainis34.4%.Thisregionisnotsuitableforthebulkmetalpro-cessingbecausethisdomainisveryclosetotheinstabledomainandrathernarrow.Theseconddomainisthedeformationtemper-aturerangingfrom690Kto723Kandstrainraterangingupto0.002sÀ1.Theefficiencyofthisdomainincreasesfrom23%to29.5%.Thethirddomainisthedeformationtemperaturerangingfrom710Kto723Kandstrainraterangingfrom1sÀ1to1.995sÀ1withapeakefficiencyof32.5%.Thisstabledomainisalsoclosetotheinstabledomainandflowinstabilitymayoccurifthedeformationtemperaturedecreasesunderaconstantstrainrateduringthehotdeformation.Thestabledomainmaycorrelatewiththesuper-plasticityordynamicrecrystallization.Theefficiencyofpowerdissipationforoccurrenceofsuper-plasticityisbeyond60%andthepeakefficiencyfor7075aluminumalloyisnohigherthan32.5%.Therefore,super-plasticitydoesnotoccurinthesedo-mains.Generally,dynamicrecrystallizationisabeneficialprocessduringthehotdeformationbecauseitcanproducethestableflow.Y.C.Linetal./JournalofAlloysandCompounds550(2013)438–4443(a)0-21516111316192530232622322428(b)0-21501324292822232516192121191122118.01315182124lnε-4-6lnε6.08.0-4-68.21113151918131525271601815132218212302527600630660690720600630660690720Temperature/KTemperature/K(c)0-211132624(d)292521230-28.2112524271601822lnεlnε8.20111618231922-4-68.2111315-4-61315190221815161921232425026021221915161821232425600Temperature/K5.0111318198.2630660690720600630660690720Temperature/K(e)0-28.21101626252423(f)0-20135.011168.22625248.2lnεlnε-4-61523-4-61518019021231916182122242522242118192221232425600630660690720600630660690720Temperature/KTemperature/KFig.5.Processingmapof7075aluminumalloyunderstrainsof(a)0.1;(b)0.2;(c)0.3;(d)0.4;(e)0.5;and(f)0.6.Thus,thedynamicrecrystallizationdomainisoftenchosenforoptimizingthehotformingprocessandcontrollingthemicrostruc-tures[30].Theprocessingmapexhibitsthattheoptimumhotworkingdomainsforthereasonabledynamicrecrystallizationisthetemperaturerangeof623–723Kandstrainraterangeof0.001–0.05sÀ1withapeakefficiencyof29.5%.Meanwhile,theshadeddomaininFig.5(e)representsthere-gimesofflowinstabilities(e.g.adiabaticshearbandsorflowlocal-ization)underthelowdeformationtemperatureandhighstrainrate.Themicrostructureinthisdomainisusuallyassociatedwiththeflowlocalization.Usually,theflowlocalizationoccursbeforeoratthepeakloadings.Thematerialmayexperiencethesofteningordegradationinitsloadcarryingcapacityafterthepeak.But,itstillcontinuestocarrythereducedload,whilethegrowthandcoa-lescenceofmicro-crackscontinue[30].However,evidencesoftheflowlocalizationorplasticinstabilitymanifestationsarenotfoundinthisstudy.Therearethehigh-angleboundariesunderthetem-peratureof573Kandstrainrateof0.001sÀ1,asshowninFig.8.Whenthestrainrateislow,anumberofsub-grainsgrowsup,whicheasilyenlargetheanglesofgrainboundaries.FromFig.8,itcanbefoundthatthesub-grainsarewelldeformedwiththehigh-angleboundaries,andcoarseprecipitationsaredistributedinthegraininteriorandboundaries,whichmayleadtothedeepinter-granularcorrosionandlargeareasofdenudationlayer[41,42].Generally,thesehigh-angleboundariesarenotexpectedtooccurinthemicrostructuresofthedeformedmaterial.There-fore,thedeformationtemperaturesandstrainratesintheinstabil-itydomainshouldbeavoidedduringthehotdeformation.4.ConclusionsThehotcompressivebehaviorsof7075aluminumalloywerestudiedunderwidedeformationtemperaturesandstrainrates.Basedontheexperimentaldata,theprocessingmapswerecon-structedbysuperposingtheinstabilitymapoverthepowerdissi-pationmapataseriesofstrains.Itcanbeconcludedthat,firstly,thedeformationtemperatureandstrainratehaveagreateffectontheflowstressof7075aluminumalloy.Increasingthestrainrateordecreasingthedeformationtemperaturecanincreasetheflowstress.Secondly,theeffectsofstrainontheprocessingmaparesignificant.Thevaluesofefficiencyofpowerdissipationin-creasewiththeincreaseofstrainwhenthestrainratesare0.001sÀ1and0.01sÀ1.However,theincreaseofstraindecreasesthepowerdissipationvaluesunderthestrainrateof1sÀ1.Forthecasewiththestrainrateof0.1sÀ1,theeffectsofstrainonthepowerdissipationarenotobvious.Thirdly,withtheincreaseofstrain,theinstabilitydomainsintheprocessingmapincrease,indicatingthatsometypesofunstableflowmanifestation(e.g.adiabaticshearbandsorflowlocalization)occurduringthehot444Y.C.Linetal./JournalofAlloysandCompounds550(2013)438–445Fig.6.Themicrostructuralevolutioninthedeformedspecimenswithdeformationdegreesof(a)20%;(b)30%;(c)40%;(d)50%;(e)60%;and(f)70%(e=0.1sÀ1,T=673K).Fig.7.TEMmicrographsof(a)as-receivedand(b)deformedspecimenunderthetemperatureof673Kandstrainrateof0.1sÀ1.Fig.8.TEMmicrographsofthespecimendeformedunderthetemperatureof573Kandstrainrateof0.001sÀ1.Y.C.Linetal./JournalofAlloysandCompounds550(2013)438–4445deformation.Fourthly,thehigh-angleboundariesandcoarsepre-cipitationsdistributinginthegraininteriorandboundariesmayleadtothedeepinter-granularcorrosionandlargeareasofdenu-dationlayer,andarenotexpectedtooccurinthemicrostructuresofthedeformedmaterial.Finally,thetemperaturerangeof623–723Kandstrainraterangeof0.001–0.05sÀ1canbeconsideredastheoptimumhotworkingparameters.AcknowledgmentsThisworkwassupportedbytheProgramforNewCenturyExcellentTalentsinUniversity(No.NCET-10-0838),Sheng-huaYu-yingProgram,FundamentalResearchFundsfortheCentralUniversitiesofCentralSouthUniversity(2012zzts076),andtheGraduateDegreeThesisInnovationFoundationofCentralSouthUniversity(No.2011ssxt094),China.References[1]S.Mandal,P.V.Sivaprasad,R.K.Dube,B.Raj,Mater.Sci.Forum550(2007)601.[2]M.Mirzaee,H.Keshmiri,G.R.Ebrahimi,A.Momeni,Mater.Sci.Eng.A551(2012)25.[3]K.L.Wang,M.W.Fu,S.Q.Lu,X.Li,Mater.Des.32(2011)1283.[4]B.Oberdorfer,E.M.Steyskal,W.Sprengel,R.Pippan,M.Zehetbauer,W.Puff,R.Würschum,J.AlloysComp.509(2011)309.[5]Z.Zhang,M.Wang,Z.Li,N.Jiang,S.Hao,J.Gong,H.Hu,J.AlloysComp.509(2011)5571.[6]A.Momeni,K.Dehghani,G.R.Ebrahimi,J.AlloysComp.509(2011)9387.[7]F.Otto,J.Frenzel,G.Eggeler,J.AlloysComp.509(2011)4073.[8]F.Chen,Z.S.Cui,D.S.Sui,B.Fu,Mater.Sci.Eng.A0(2012)46.[9]Y.C.Lin,M.S.Chen,J.Zhong,J.Mater.Process.Tech.209(2009)2477.[10]Y.C.Lin,L.T.Li,Y.C.Xia,Comput.Mater.Sci.50(2011)2038.[11]B.Ma,Y.Peng,Y.F.Liu,B.Jia,J.Cent.SouthUniv.Tech.17(2010)911.[12]M.S.Salehi,S.Serajzadeh,Comput.Mater.Sci.49(2010)773.[13]M.S.Salehi,S.Serajzadeh,Comput.Mater.Sci.53(2012)145.[14]Y.C.Lin,M.S.Chen,J.Mater.Sci.44(2009)835.[15]Y.C.Lin,M.S.Chen,J.Zhong,Comput.Mater.Sci.44(2008)316.[16]K.P.Rao,Y.V.R.K.Prasad,C.Dharmendra,N.Hort,K.U.Kainer,Mater.Sci.Eng.A528(2011)69.[17]S.Mandal,P.V.Sivaprasad,B.Raj,V.S.Sarma,Metall.Mater.Trans.A39(2008)3298.[18]B.Bradaskja,B.Pirnar,M.Fazarinc,P.Fajfar,SteelRes.Int.82(2011)346.[19]J.Luo,M.Q.Li,Mater.Sci.Eng.A538(2012)156.[20]Y.C.Lin,M.S.Chen,J.Zhong,Comput.Mater.Sci.42(2008)470.[21]Y.C.Lin,G.Liu,Mater.Sci.Eng.A523(2009)139.[22]B.Eghbali,Mater.Sci.Eng.A527(2010)3402.[23]K.P.Rao,Y.V.R.K.Prasad,K.Suresh,Mater.Des.32(2011)4874.[24]Y.V.R.K.Prasad,H.L.Gegel,S.M.Doraivelu,J.C.Malas,J.T.Morgan,K.A.Lark,D.R.Barker,Metall.Mater.Trans.A15(1984)1883.[25]Y.V.R.K.Prasad,T.Seshacharyulu,Mater.Sci.Eng.A243(1998)82.[26]S.M.Abbasi,A.Momeni,Mater.Sci.Eng.A552(2012)330.[27]A.Momeni,K.Dehghani,Mater.Sci.Eng.A527(2010)67.[28]A.Momeni,K.Dehghani,Mater.Sci.Eng.A528(2011)1448.[29]S.Ramanathan,R.Karthikeyan,G.Ganasen,Mater.Sci.Eng.A441(2006)321.[30]S.Ramanathan,R.Karthikeyan,M.Gupta,J.Mater.Process.Tech.183(2007)104.[31]D.Samantaray,S.Mandal,V.Kumar,S.K.Albert,A.K.Bhaduri,T.Jayakumar,Mater.Sci.Eng.A552(2012)236.[32]D.Samantaray,S.Mandal,A.K.Bhaduri,Mater.Sci.Eng.A528(2011)5204.[33]D.Samantaray,S.Mandal,A.K.Bhaduri,Mater.Des.32(2011)716.[34]H.Ahamed,V.Senthilkumar,Mater.Sci.Eng.A539(2012)349.[35]V.Senthilkumar,A.Balaji,R.Narayanasamy,Mater.Des.37(2012)102.[36]M.Rajamuthamilselvan,S.Ramanathan,J.AlloysComp.509(2011)948.[37]Y.C.Lin,L.T.Li,Y.X.Fu,Y.Q.Jiang,J.Mater.Sci.47(2011)1306.[38]Y.C.Lin,L.T.Li,Y.Q.Jiang,Exp.Mech.52(2012)993.[39]D.Samantaray,S.Mandal,C.Phaniraj,A.K.Bhaduri,Mater.Sci.Eng.A528(2011)8565.[40]F.Montheillet,J.P.Thomas,Metall.Mater.HighStruct.Effic.146(2004)357.[41]X.H.Chen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