Si含量对Al-6.5Cu-0.6Mn-0.5Fe合金组织演变及力学性能的影响

1.SchoolofMechanicalEngineering,GuizhouUniversity,Guiyang550025,China;2.SchoolofMechanicalEngineering,DongguanUniversityofTechnology,Dongguan523808,China;3.SchoolofMechanicalandAutomotiveEngineering,SouthChinaUniversityofTechnology,Guangzhou5100,ChinaReceived13November2018;accepted5June2019Abstract:TheeffectofSicontentonthemicrostructuresandmechanicalpropertiesoftheheat-treatedAl−6.5Cu−0.6Mn−0.5Fealloywasinvestigatedusingimageanalysis,scanningelectronmicroscopy(SEM),transmissionelectronmicroscopy(TEM),andtensiletesting.TheresultsshowthatthemechanicalpropertiesofAl−6.5Cu−0.6Mn−0.5FealloysdecreaseslightlywhentheSicontentisbelow1.0%.Thiscanbeattributedtothecomprehensiveeffectofmicrostructureevolution,includingtheincreaseofnano-sizedα-Fe,thecoarsenedgrainsize,andanincreaseinAl2Cucontentatthegrainboundary.WhentheSicontentis1.5%,themechanicalpropertiesoftheAl−6.5Cu−0.6Mn−0.5Fealloysdecreasesignificantly,andthiscanbeattributedtotheagglomeratedsecondintermetallics,whichisresultedfromtheformationofexcessSiparticles.Keywords:Al−Cualloys;iron-richintermetallics;Si;tensileproperties1Introduction

Withtheincreasingrequirementsforenvironmentalprotectionandgreenmanufacturing,recyclingAlalloyshasbecomeanimportantdirectionfordevelopmentinthealuminumindustry.However,recycledAlalloyscontainmanyimpurityelements,suchasFe,Si,Ni,Zn,MgandMn[1,2].Ironisthemostharmfulimpurityelementinrecycledaluminumalloys.BecausethesolidsolubilityofFeinAlalloysislimited,FeusuallyprecipitatesintheformofhardandbrittleFe-richintermetallics,suchasChinesescriptα-Fe(Al15(FeMn)3(SiCu)2)[3],Al6(FeMn)[4],AlmFe[5],plate-likeAl3(FeMn)[6],andβ-Fe(Al7Cu2Fe)[7].Severalmethodshavebeeneffectivelyusedtopreventthedetrimentaleffectsofironinaluminumalloys.Thesemethodsinclude(1)reducingtheformationofFe-richintermetallicsbyloweringtheFelevelstobeaslowaseconomicallypossible,and(2)modifyingtheFe-richintermetallicsusingchemicalorphysicalapproaches.Inthechemicalapproaches,MnandSielementsareaddedtobreakuptheneedle-likeintermetallicsortotransformthemintoblockorChinesescript.Thephysicalapproachesincludetheuseofsuperheatedmelt,solidificationunderhighcoolingrate,andmelttreatment[8−11].Al−Cualloysarewidelyusedinaerospaceandautomobilemanufacturingindustrybecauseoftheirexcellentfatigueproperties,highspecificstrength,andgoodheatresistance.However,Al−Cualloyshavepoorcastingproperties,suchashotcrackingsusceptibilityandfluidityincastaluminumalloys[12].IthasbeenfoundthatSiandCualloyingofAl−Cualloysdecreaseshotcrackingsusceptibility[13−15].SABAUetal[15]foundthatAl−Cualloyshavethelowestthermalcrackingtendencywhenthecoppercontentis7.3%and8%.Inourpreviouswork,wefoundthatnano-sizedα-FecanbeformedinAl−CualloysthathavehighCuandFecontent,andthissignificantlyimprovesthemechanicalFoundationitem:Projects(51704084,51605106)supportedbytheNationalNaturalScienceFoundationofChina;Project(2017M623068)supportedbyChinaPostdoctoralScienceFoundation;Project(2015A030312003)supportedbytheNaturalScienceFoundationforTeamResearchofGuangdongProvince,China;Project(JC(2016)1026))supportedbytheScienceandTechnologyFoundationofGuizhouProvinceofChina;Project(KY(2017)101))supportedbytheYoungTalentGrowthFoundationofEducationDepartmentofGuizhouProvinceofChina;Project(RC2017(5788))supportedbytheScienceandTechnologyPlanofGuizhouProvinceofChinaCorrespondingauthor:BoLIN;Tel/Fax:+86-851-83627516;E-mail:linbo1234@126.comDOI:10.1016/S1003-6326(19)65065-X1584RuiXU,etal/Trans.NonferrousMet.Soc.China29(2019)1583−1591properties[16].Thenano-sizedα-Fephaseiscommonlyobservedin3xxxand6xxxalloyswithhighSicontent[17,18].Therefore,itcanbeexpectedthataddingSiwillpromotetheformationofthenano-sizedα-FephaseinAl−Cualloys.Also,SicontenthasasignificanteffectontheformationofFe-richintermetallics.However,SiisalsoanimpurityelementinAl−Cualloys.Asaresult,theSicontentisusuallylimitedtobelow0.1%forhigh-performanceAl−Cualloys[19].However,strictlycontrollingtheSicontentinaluminumalloyswillmakethesealloysexpensiveandisnotconducivetotherecyclingofwastealuminum.Therefore,theeffectsofdifferentSicontentsonthemicrostructureandpropertiesoftheAl−6.5Cu−0.6Mn−0.5Fealloywerestudiedforthepurposeofpromotingtheefficientuseofrecycledaluminum.2Experimental

ExperimentalalloyswithdifferentSicontentswerepreparedusingcommerciallypureAl(99.5%),Al−50%Cu,Al−10%Mn,Al−20%SiandAl−5%Femasteralloys.Thecompositionsofthealloysweredeterminedusingopticalemissionspectrometry,andtheresultsarelistedinTable1.First,pureAl,Al−5%FeandAl−20%Simasteralloyswerepreheatedinaclay-graphitecrucibleusinganelectricresistancefurnaceat400°Cfor1htoeliminatewatervapor.Therawmaterialswerethenmeltedat730°C.Al−50%CuandAl−10%Mnmasteralloyswereaddedat730°C.Finally,thetemperatureofthemeltwasmaintainedat730°Cfor30min.Approximately10kgofthemeltsweredegassedusingargontominimizethehydrogencontent.Themeltwasthenpouredintoacylindricaldie,whichhadasizeof80mminheightand50mmindiameter.Thedietemperaturewassettobe250°C,andthepouringtemperaturewas730°C.T7heattreatmentconditionswereusedinthisstudytostabilizethemicrostructures.Thesampleswerethensolution-treatedat535°Cfor12hbeforebeingquenchedinwarmwaterat100°C.ThesampleswerethenagedusingT7conditionsat215°Cfor16h.Formechanicaltests,sampleswithadiameterof10mmandaheightof80mmwereproducedusingawireelectricaldischargemachining.ThetensiletestwascarriedoutonanMTSCMT5105standardtestingmachine,andthereportedvaluesaretheaveragesofatleastthreesamples.Samplesforthemicro-hardnesstestandmetallographicobservationwerecutinthegaugelengthpartfromselectedtensilespecimens.Thelocationforthemicro-hardnesstestwasrestrictedtothecenteroftheα(Al)dendritenearthecenteroftheetchedspecimens.Themicro-hardnesswasmeasuredonatesterequippedwithaVickersdiamondindenterusinganindentationloadof100g.Thevaluereportedistheaverageofmorethan10readings.Thesamplesusedformetallographicobservationswereetchedin0.5%HFsolutionfor30s.Themorphology,theFe-richphase,andthefracturesurfaceswereanalyzedusingSEM(NovaNanoSEM430)attachedwithEDS.Precipitatesintheα(Al)matrixwereanalyzedusingTEM(JEOLJEM−3010)at200kV.TheareafractionoftheintermetallicsandprecipitationsofthealloyswerequantitativelycalculatedbyusingimageProPlussoftware.Table1Chemicalcompositionofalloy(wt.%)AlloyCuMnFeSiAlAl−6.5Cu−0.6Mn−0.5Fe−0Si(0Si)6.450.660.550.04Bal.Al−6.5Cu−0.6Mn−0.5Fe−0.5Si(0.5Si)6.470.620.540.58Bal.Al−6.5Cu−0.6Mn−0.5Fe−1.0Si(1.0Si)6.500.630.531.13Bal.Al−6.5Cu−0.6Mn−0.5Fe−1.5Si(1.5Si)6.520.630.561.50Bal.3Results

3.1Microstructureofas-castalloysFigure1showsthemicrostructureoftheas-castAl−6.5Cu−0.6Mn−0.5FealloywithdifferentSicontents.Table2givesEDSdataoftheFe-richphaseintheas-castAl−6.5Cu−0.6Mn−0.5FealloywithdifferentSicontents.TheFe-richphaseshowsChinesescriptmorphologyinalloftheexperimentalalloys.AsseeninTable2,theChinesescriptFe-richintermetallicisα-Fe(Al15(FeMn)3Cu2orAl15(FeMn)3(CuSi)2)[3,9].Inaddition,theamountofSiinα-FeincreaseswithanincreaseinSicontent,andthisisbecausetheAlatomissubstitutedbytheSiatomintheFe-richphase[20].OnthebasisofFig.1,theareafractionofα-Feisquantitativelycalculated.Theareafractionofα-Feincreasesfrom3.9%to4.3%andthento4.5%whentheSicontentisincreasedfrom0to0.5%andthento1.0%.Theresultsshowthattheareafractionofα-FeincreaseswithanincreaseintheSicontent,andthisissimilartotheresultsreportedintheworkbyLIUetal[10].3.2Microstructureofheat-treatedalloysFigure2showsthemicrostructureoftheheat-treatedAl−6.5Cu−0.6Mn−0.5FealloywithdifferentSicontents.Table3givesEDSdataoftheFe-richphaseintheheat-treatedAl−6.5Cu−0.6Mn−0.5FealloywithdifferentSicontents.Comparedwiththeas-castalloy,Al2Cuiscompletelydissolvedintotheα(Al)matrixaftertheT7heattreatmentinthe0Sialloy.Also,Cu-richβ-Fetransformsfromα-Feaftersolutionheattreatmentinthe0Sialloy.TheamountofAl2Cuincreases,andα-FeRuiXU,etal/Trans.NonferrousMet.Soc.China29(2019)1583−15911585Fig.1Microstructuresofas-castAl−6.5Cu−0.6Mn−0.5FealloyswithdifferentSicontents:(a)0Si;(b)0.5Si;(c)1.0Si;(d)1.5SiTable2CompositionsofFe-richintermetallicsinas-castcondition(at.%)Alloy0Si0.5Si1.0Si1.5SiPhaseα-Feα-Feα-Feα-FeAl78.99±2.0677.71±0.7274.70±2.4273.30±1.92Cu8.62±2.053.59±0.372.67±0.294.25±2.48Mn4.22±0.613.41±0.245.48±1.044.50±2.09Fe8.17±1.927.00±0.448.08±0.9310.55±1.92Si−8.28±0.199.07±1.567.41±0.88Fig.2Microstructuresofheat-treatedAl−6.5Cu−0.6Mn−0.5FealloyswithdifferentSicontents:(a)0Si;(b)0.5Si;(c)1.0Si;(d)1.5Si1586RuiXU,etal/Trans.NonferrousMet.Soc.China29(2019)1583−1591remainswithanincreaseoftheSicontent.WithafurtherincreaseintheamountofaddedSito1.5%,Siparticlesappearinthealloys.Also,excessiveSiparticlesagglomeratewiththeβ-FephaseandAl2Cuphaseatthegrainboundary.Figure3showstheeffectsofdifferentSicontentsonthegrainsizeoftheAl−6.5Cu−0.6Mn−0.5Fealloy.ThegrainsizeofthealloyincreaseswithanincreaseintheSicontent.ThegrainsizesofalloyswithdifferentSicontentsarequantitativelyanalyzed,andtheresultsareshowninFig.3(f).Thegrainsizeofthe0Sialloyisestimatedtobe540μm,muchlessthanthatofthe1.5Sialloy(2350μm).Figure4showsthemorphologyandEDSdataofthesecondphaseintheα(Al)matrixwithdifferentSicontents.AccordingtoEDSresultofthe0.5Sialloy,thesecondphaseprecipitationsintheα(Al)matrixareTphaseandα-Fe.Furthermore,theamountofTphaseTable3CompositionsofFe-richintermetallicsinheat-treatedcondition(at.%)Alloy0Si0.5Si1.0Si1.5SiPhaseβ-Feα-Feα-Feα-FeAl73.82±2.5174.15±0.7570.25±2.9171.47±1.54Cu17.78±1.804.52±0.434.07±0.223.23±0.63Mn2.09±0.024.66±0.195.57±0.516.12±0.81Fe6.32±0.708.13±1.299.17±1.467.±0.68Si−8.54±0.1910.94±2.0011.54±1.23Fig.3Macro-etchedcross-sectionmorphologyofsamples(a),micro-imagesofgrainsizefor0Si(b),0.5Si(c),1.0Si(d)and1.5Si(e)alloys,andquantitatively-analyzedgrainsizes(f)RuiXU,etal/Trans.NonferrousMet.Soc.China29(2019)1583−15911587Fig.4TEMimages(a,b)andEDSresults(c,d)ofalloyswithdifferentSicontents:(a,c)0Si;(b,d)0.5Sidecreasesandthatofα-FeincreasesinalloyswithanincreaseinSicontent.ThisindicatesthataddingSipromotestheformationoftheα-Fephaseintheα(Al)matrix.TheseresultsarealsoconsistentwiththepreviousfindingthataddingSipromotestheformationoftheα-FephaseduringT7heattreatmentinthe3xxxand6xxxalloys[17,18].OnthebasisofFig.4,theareafractionsofTphasesandα-Fephasesarequantitativelycalculated.TheareafractionsofTphasesandα-Fephasesinthe0Sialloyare12.6%and1.3%,respectively.And,theareafractionsofTphasesandα-Fephasesinthe0.5Sialloyare5.9%and10.2%,respectively.TheseresultsfurtherprovethattheSicontentpromotestheformationoftheα-Fephaseintheα(Al)matrix.Figure5showsmicro-hardnessofalloyswithdifferentSicontents.AddingSiobviouslyincreasesthemicro-hardnessofthematrix.Themicro-hardnessofthe0Sialloy(aboutHV92.7)ismuchlowerthanthatofthe1.0Sialloy(aboutHV110.5).Theseresultscanbeattributedtotheincreaseintheamountofnano-sizedα-FephasewithSiaddition.3.3MechanicalpropertiesofalloysFigure6showsthemechanicalpropertiesoftheheat-treatedAl−6.5Cu−0.6Mn−0.5FealloywithdifferentSicontents.ThestrengthandelongationdecreaseastheSicontentisincreased.ThemechanicalpropertiesoftheAl−6.5Cu−0.6Mn−0.5FealloysdecreaseslightlywhentheSicontentisbelow1.0%.TheultimatetensileFig.5Micro-hardnessofalloywithdifferentSicontentsFig.6MechanicalpropertiesofAl−6.5Cu−0.6Mn−0.5FealloyswithdifferentSicontents1588RuiXU,etal/Trans.NonferrousMet.Soc.China29(2019)1583−1591strength(UTS),yieldstrength(YS),andelongationoftheheat-treated1.0Sialloyarerecordedas280MPa,253MPa,and2.5%,respectively.WhentheSicontentis1.5%,themechanicalpropertiesoftheAl−6.5Cu−0.6Mn−0.5Fealloysdecreasesignificantly,andthiscanbeattributedtotheprecipitationofSiparticlesandtheagglomerationofthecoarsesecondphase.3.4FracturesurfacesofalloysFigure7showsthefracturesurfacesofalloyswithdifferentSicontents.Therearesomedimplesonthefracturesurfaces,andthisindicatesthatthefracturesofalloyexhibitcertainductilefracturecharacteristics.WhentheSicontentislessthan1.0%,thealloyfracturesurfacesarenotsignificantlydifferent,butwhentheSicontentis1.5%,brittlefracturecharacteristicsbecomeobvious.Inaddition,themorphologyofthesecondphaseofthealloyswithdifferentSicontentsisobviouslydifferentonthefracturesurfaces.WhentheSicontentis0%,therearesomecracksinthecylindricalβ-Fe(Al7Cu2Fe),andthisindicatesthatβ-Feactsascrackinitiationsitesandleadstoquasi-cleavagefracture(Fig.7(a)).WithafurtherincreaseintheSicontent,cracksarealsofoundintheα-FeandAl2Cuphasesonthefracturesurfaces,andthisindicatesthatthebrittleα-FeandAl2Cuactaspotentialcleavageinitiators(Figs.7(b,c)).However,theamountofcracksinα-Feislessthanthatinβ-Fe,andthisindicatesthatα-Feislessharmfulthanthecylindricalβ-Fe[21].Inthe1.5Sialloys,theagglomeratedsecondintermetallicphasesareclearlyseenwhenthefracturesurfacesundergocleavagefracture(Fig.7(d)).ThefracturesurfacesclearlyshowthattheplasticityofthealloydoesnotobviouslychangewhentheSicontentisbelow1.0%.However,theplasticityofthealloydecreasessharplywithanincreaseintheSicontentto1.5%.Figure8showsthelongitudinalfracturemorphologyofalloyswithdifferentSicontents.WhentheSicontentis0%,therearemanyβ-Fephasesonthefracturesurfaceofthealloy.Thisindicatesthatthebrittleβ-Feactsaspotentialcleavageinitiators(Fig.8(a)).WithafurtherincreaseintheSicontent(below1.0%),theα-FeparticlesandtheAl2Cuphaseareobservedatorbeneaththefracturesurfaceinthealloy,andthismeansthattheα-FeparticlesandAl2Cuactaspotentialcleavageinitiators(Figs.8(b,c)).Alargeamountofagglomeratedsecondintermetallicsprecipitateonthefracturesurfaceofthe1.5Sialloy,andacrackofagglomeratedsecondintermetallicsresultsintheformationofsecondarycracks(Fig.8(d)).4Discussion

Figure6showsthattheUTS,YSandelongationoftheheat-treatedalloydecreasewithanincreaseinSicontent,andthisismainlybecauseofmicrostructureevolution.First,addingSipromotesthetransformationfromT(Al20Cu2Mn3)phasetonano-sizedα-FeafterT7heattreatment.ThisphenomenoncanbeexplainedbytheSiFig.7FractographsofalloyswithdifferentSicontents:(a)0Si;(b)0.5Si;(c)1.0Si;(d)1.5SiRuiXU,etal/Trans.NonferrousMet.Soc.China29(2019)1583−159115Fig.8LongitudinalfracturemorphologiesofalloyswithdifferentSicontents:(a)0Si;(b)0.5Si;(c)1.0Si;(d)1.5SiatomreplacingtheAlatomintheFe-richphase.Asaresult,addingSipromotestheformationoftheα-Fephase[17,18].ThisresultisconsistentwiththepreviousfindingthatanincreaseinSicontentcanpromotetheformationofα-Fephaseintheheat-treated3xxxand6xxxaluminumalloys.Ithasbeenreportedthattheformationofnano-sizedα-FephaseismorebeneficialthanthelargeTphaseforthepropertiesofalloys[22].Inaddition,Sicontentobviouslyincreasesthegrainsizeofalloys(Fig.3).TheseresultscanbeattributedtothedecreasedamountofTphasesinthematrixwithanincreaseinSicontent.ManyresearchershavereportedthattheTphasespreventgraingrowthofAl−Cualloys[8,23].Finally,anincreaseintheSicontentstabilizesα-Feandinhibitsthetransformationofα-Fetoβ-Feduringheattreatment.InSi-freealloys,α-Fetransformsintocopper-richβ-Fe((Al15(FeMn)3Cu2+4Al2Cu→3Al7Cu2(FeMn)+2α(Al))[20,24,25].However,α-FeremainsstableandreducesCuconsumptioninalloyswithhighSicontent,andthisleadstoanincreaseintheAl2Cuphaseatthegrainboundaries.ThebrittleAl2Cuphaseactsasapotentialcleavageinitiator,andthisindicatesthatanincreaseintheAl2CuphaseinalloyswithhighSicontentdeterioratestheperformanceofthealloy.Inconclusion,thedecreaseinmechanicalpropertiescanbeattributedtothecomprehensiveeffectofmicrostructureevolution,includinganincreaseinnano-sizedα-Fe,thecoarsenedgrainsize,andanincreaseinAl2Cuatthegrainboundary.WhentheSicontentis1.5%,thealloypropertiesdecreasesharplybecauseofexcessSiparticlesandtheagglomerationofbrittlesecondintermetallicsatthegrainboundaries.TheseresultsdemonstratethattheSicontentshouldbeappropriatelycontrolledintheAl−Cualloys.Inthisstudy,acceptablemechanicalpropertiesareachievedbycontrollingtheSicontenttobebelow1.0%.Moreover,itisfeasibletoextendtheFeandSicontentsforthepurposeofusingrecycledaluminumalloys,whichgreatlyreducesmanufacturingcosts.Also,anincreaseinnano-sizedα-Feisbeneficialtoimprovingelevatedmechanicalproperties[26−28].Also,anincreaseinAl2Cuatthegrainboundaryisbeneficialtoimprovingelevatedmechanicalproperties[29].Therefore,itcanbeexpectedthattheAl−6.5Cu−0.6Mn−0.5FealloywithhighSicontentwillbebeneficialtothemechanicalpropertiesatelevatedtemperature.FurtherworkwillbecarriedouttoevaluatethemechanicalpropertiesatelevatedtemperatureforindustrialapplicationsofAl−6.5Cu−0.6Mn−0.5FewithhighSicontent.5Conclusions

(1)AnincreaseinSicontentintheAl−6.5Cu−0.6Mn−0.5FealloypromotesthetransformationoftheT(Al20Cu2Mn3)phasetonano-sizedα-Feinthematrix.AdecreaseintheT(Al20Cu2Mn3)phaseinalloyswithhighSicontentweakenstheinhibitionabilityofgraingrowth,andthisleadstocoarseningofgrainsize.(2)AddingSistabilizesα-Feandinhibitsthetransformationfromα-Fetoβ-Feafterheattreatment,andthisleadstoanincreaseintheAl2Cuphaseinthealloy.1590RuiXU,etal/Trans.NonferrousMet.Soc.China29(2019)1583−1591(3)MechanicalpropertiesoftheAl−6.5Cu−0.6Mn−0.5FealloydecreasewithanincreaseinSicontent.MechanicalpropertiesofAl−6.5Cu−0.6Mn−0.5FealloysdecreaseslightlywhentheSicontentisbelow1.0%.However,themechanicalpropertiesofAl−6.5Cu−0.6Mn−0.5Fe−1.5Sialloysdecreasesignificantly.ThiscanbeattributedtotheagglomeratedsecondintermetallicthatisresultedfromtheformationofexcessSiparticles.References

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林波1，李浩宇1，肖华强1，赵俞亮2，张卫文3

1.贵州大学机械工程学院，贵阳550025；2.东莞理工学院机械工程学院，东莞523808；3.华南理工大学机械与汽车工程学院，广州5100摘要：采用定量分析、扫描电镜、透射电镜和拉伸性能测试研究Si含量对高铁含量铝铜合金组织演变及力学性能的影响。结果表明：当Si含量低于1.0%时，Al−6.5Cu−0.6Mn−0.5Fe合金的常温力学性能稍有降低，这主要是由于组织演变的综合作用，包括纳米及α-Fe相的增多，晶粒的粗化以及晶界处Al2Cu相的增多；当Si含量为1.5%时，Al−6.5Cu−0.6Mn−0.5Fe合金性能急剧下降，这主要是由于多余Si粒子的析出导致第二相产生偏聚。关键词：铝铜合金；富铁金属间化合物；Si；拉伸性能(EditedbyBingYANG)