KimUniversitogy8Warmlasershockpeening(WLSP)isathermomechanicaltreatmenttechniquecombiningtheadvantagesoflasershockpeeninganddynamicstrainaging(DSA).ThroughDSA,WLSPofsteelincreasesthedislocationdensityandstabilizesthedislocationstructurebypeening(LSP)hasbeensuccessfullyusedtoimprovetheafterLSPanddeeprolling(DR).Hu5investigatedLSPofAISI1045steelbyANSYS,validatedbyexperiment.Chu6comparedthemicrostructure,hardnessandresid-formationofahighdensitye-martensitephase.eectoffatiguelifeimprovementbyLSPislimited.Thus,itisveryimportanttostabilizethemicrostructureandthecompressiveresidualstressgeneratedbyLSP.Dynamicstrainaging(DSA)andDPcanbothimprovethemicrostructurestabilityofmetallicmaterials.DSA12,13,thediusionofC(carbon)andN(nitrogen)atomsCorrespondingauthor.E-mailaddress:(G.J.Cheng).AActaMaterialia59(2011)10141025fatigueperformanceofmetalliccomponents1.Bygener-atingaworkhardenedlayerandintroducingcompressiveresidualstressinthematerialsurfacethespeedofcrackini-tiationandpropagationduringcyclicloadingissloweddown,whichresultsinafatigueperformanceimprovement.LSPisaneectivewaytoimprovesurfacehardness,fatigueperformance,corrosionresistanceandwearresistance2.Steelsarewidelyusedinindustry.LSPofsteelhasbeenextensivelystudiedintheliterature.Forexample,Nikitin3,4comparedthenearsurfacemicrostructurechangeandfatiguelifeimprovementofAISI304stainlesssteelHowever,thecompressiveresidualstressgeneratedbysurfaceprocessingtechniques(SP,LSP,DP,etc.)isnotsta-bleduringcyclicloading7,8,especiallyathightestingtemperatures3,4,9,10.Forexample,Altenbergeretal.11investigatedthethermalstabilityofthecompressiveresidualstressandsurfacenanostructuregeneratedinAISI304stainlesssteelandTi64alloybydynamicprecipitation(DP)andLSPbyinsitutransmissionelectronmicroscopy(TEM)study.Itwasobservedthatcompleteresidualstressrelaxationat550600C176Cwasrelatedtothethermalinsta-bilityofthenearsurfacemicrostructure.Inthisway,thepinningofmobiledislocationsbycarbonatoms.Inaddition,WLSPgeneratesnanoscalecarbideprecipitatesthroughstrain-inducedpre-cipitation.Thecarbideprecipitatesstabilizethemicrostructurebydislocationpinning.Thisresultsinhigherstabilityofthedislocationstructureandthusimprovesthestabilityofthecompressiveresidualstress.Inthisstudythemechanismoffatigueperformanceimprove-mentinAISI4140steelbyWLSPisinvestigated.ItisfoundthatmicrostructuresformedafterWLSPleadtoahigherstabilityofdis-locationstructuresandresidualstress,whicharebeneficialforfatigueperformance.C2112010ActaMaterialiaInc.PublishedbyElsevierLtd.Allrightsreserved.Keywords:Warmlasershockpeening；AISI4140steel；Dynamicstrainaging；Dynamicprecipitation；Carbide1.IntroductionAsasuperiorsurfaceprocessingtechnique,lasershockualstressgeneratedbyLSP,DRandshotpeening(SP)onHadfieldmanganesesteel.InChusstudyitwasfoundthatLSPresultedinalargehardnessincreaseduetotheFatigueperformanceimprovementstrainaginganddynamicprecipitationChangYea,SergeySuslovb,BongJoongaSchoolofIndustrialEngineering,PurduebSchoolofMaterialsEngineeringandBirckNanotechnolReceived30July2010；receivedinrevisedformAvailableonlineAbstract1359-6454/$36.00C2112010ActaMaterialiaInc.PublishedbyElsevierLtd.Alldoi:10.1016/j.actamat.2010.10.032inAISI4140steelbydynamicduringwarmlasershockpeeningb,EricA.Stachb,GaryJ.Chenga,y,WestLafayette,IN47906,USACenter,PurdueUniversity,WestLafayette,IN,USA13October2010；accepted13October2010N/locate/actamatrightsreserved.todislocationcoresinthetemperaturerange150300C176C,isanimportantstrengtheningmechanism14insteel.InDSAtheinteractionbetweendislocationsandsoluteatomsresultsinrepeatedpinningofdislocationsandthusleadstoenhancedworkhardening13,15.AttheDSAtemperaturethesoluteatoms(carbonandnitrogen)migratetodisloca-tioncores,whichformso-calledCottrellclouds16insteel.TheCottrellcloudsexertapinningforceondisloca-tionsandinhibitdislocationmovementduringplasticdeformation.Forplasticdeformationtocontinue,newmobiledislocationsmustbegenerated.Thisleadstodislo-cationmultiplicationandresultsinahigherdislocationdensityandamoreuniformdislocationarrangement.Sub-stantialeortshavebeenmadetotakeadvantageofDSAintreatingsteel.Forexample,Chen17improvedthefati-gueperformanceofAISI304stainlesssteelbyplasticdeformationattheDSAtemperature.Kerscheretal.18increasedthefatiguelimitofSAE52100steelbyTMTattheDSAtemperature,andidentifiedtheoptimaltempera-ture(335C176C)thatledtobestfatigueperformanceimprove-ment.Huangetal.19comparedthefatigueperformanceofSA533B3steelatroomtemperatureand300C176Candfoundthatthebetterfatigueperformanceat300C176CwasacombinedeectofDSAandtheformationofcarbideprecipitatesduringcyclicloading.Dynamicprecipitationduringhotdeformationisalsoknownasstrain-inducedprecipitation(SIP).Dynamicpre-cipitationdiersfromstaticprecipitationinthattheformerresultsintheformationofnanoscaleprecipitatesdynami-callyduringwarmdeformation.Indynamicprecipitationthedislocationsgeneratedbydeformationactasfavorablenucleationsitestogrowprecipitatesdynamically.Com-paredwithstaticprecipitation,dynamicprecipitationismoreecientinstrengtheninginthatittakesamuchshortertimetoreachpeakhardness.Tiittoetal.20inves-tigatedtheeectofdynamicprecipitationinsteelonthehotflowbehaviorofalloysteel.Itwasfoundthatthepeakpinningforceresultingfromdynamicprecipitationleadstoapeakintheflowcurveduringhotdeformation.Asdis-cussedearlier,DSAcanincreasethedislocationdensitygeneratedbydeformation.Thehighdensitydislocations,inturn,canprovidenumerouspotentialnucleationsitesfordynamicprecipitation.Thus,theeectivenessofDPcanbeimprovedthroughDSA.Liaoetal.21proposedanucleationmechanismtoexplaintheultrahighdensenano-precipitationduringWLSP,andfoundthatdisloca-tionsafterhighstrainratedeformationandelevatedtem-peraturesarethetwomostimportantfactors.Thenucleationmodelwasvalidatedbyexperiments.Theperformanceofsurfaceprocessingtechniques,includingLSP,DRandSP,canbeimprovedbytakingadvantageofDSAandDP.Matlock15comparedtheeectofDRofAISI4140steelatroomtemperatureand260C176C(DSAtemperature).ItwasfoundthatDRattheDSAtemperaturesignificantlyincreasedthecorehardnessC.Yeetal./ActaMaterialiaandalsoledtoamorestabledislocationstructureandthusimprovedthefatigueperformance.HightemperatureDRofaluminumalloyswasalsoproventobemoreeectiveinfatigueperformanceimprovementthanroomtempera-tureDRbyJuijerm2224.Harada25comparedshotpeeningofspringsteelatroomtemperatureandelevatedtemperatures(100C176C,200C176C,300C176Cand400C176C).ItwasfoundthatSPattheoptimaltreatmenttemperature(200C176C)tendstoincreasethenearsurfacecompressiveresidualstressmagnitudeandhardnessduetothedecreaseinflowstressathightemperature.Inaddition,itwasfoundthatthemagnitudeoftheresidualstressgeneratedbySPdecreasedduetorecoveryattreatmenttemperatureshigherthan200C176C.ThoughitwasnotmentionedbyHarada,theincreaseinhardnessat200C176C(intheDSAtemperatureregime)couldalsobepartiallyattributabletoDSA,whichledtothepinningofdislocationsbyCottrellcloudsandresultedinahigherdislocationdensityandgreaterworkhardening.InthewarmshotpeeningworkonAISI4140steelcarriedoutbyWick26andMenigandSchulze27itwasdemonstratedthatSPatelevatedtemperature(around300C176C)improvedtheresidualstressstabilityandledtobetterfatigueperformance.AccordingtoWick26,inthewarmpeeningsamplesstaticanddynamicstrainagingoccursimultaneouslyduringandafterwarmpeening,whichleadstoahighersurfacehardness.Inaddition,DSAinwarmshotpeeningleadstotheformationofahighden-sityofdislocationsandmoreuniformdislocationarrange-ment,whichcontributetoahigherresidualstressstabilityduringcyclicloading.AsasuperiorsurfaceprocessingtechniqueLSPcanalsotakeadvantageofTMTbytreatingsteelintheDSAtem-peratureregime(150300C176C).Thus,itisofinteresttoinvestigatetheeectoftreatingtemperatureonthefatigueperformanceimprovementbyLSP.Inapreviousstudybyourgroup28itwasfoundthatwarmlasershockpeening(WLSP)cansignificantlyimprovethestabilityofthecom-pressiveresidualstressinAA6061alloysthroughthepin-ningofdislocationsbytheformationofahighdensityofnanoscaleprecipitatesgeneratedbydynamicprecipitation.InthisworkWLSPofAISI4140steelwascarriedoutanditseectsonfatigueperformancewerestudied.Themicro-structureofthesamplestreatedafterLSPandWLSPwascharacterizedbytransmissionelectronmicroscopy(TEM).TheresidualstressanddislocationdensityweremeasuredbyX-raydiraction.2.Experiments2.1.MaterialsSampleswerecutandmachinedfromaAISI4140steelplatewiththechemicalcomposition0.41C,0.21Si,0.83Mn,0.025P,0.027S,0.91Cr,0.18Mo,theremainderFe(allwt.%).Thesampledimensionswere76.2C210C22.38mm.BeforeLSPthesampleswereausten-itizedfor20minat850C176C,oilquencheddownto25C176C,59(2011)101410251015temperedat450C176Cfor2handcooledinavacuumfurnace.ThisprocedureresultsinsteelwithaVickershardnessof310VHandamicrostructureoftemperedmartensite(Fig.4).2.2.WarmlasershockpeeningexperimentsAschematicoftheWLSPprocessisshowninFig.1.BK7glasswasusedastheconfiningmediumduetoitshighshockimpedanceandhighmeltingpoint,makingitsuit-ableforLSPatelevatedtemperatures.Inthiscasewatercannotbeusedastheconfiningmediumduetoitslowevaporationpoint.Inpractice,siliconeoil(type710)couldalsobeusedforconfinement,duetoitshighvaporpoint(C24300C176C)comparedwithwater.Thinaluminumfoilisusedasanablativecoatingmaterialtoprotectthetargetmaterialfromsurfacemelting.TheworkingtemperaturesforWLSParemanipulatedusingahotplate.Athermom-eterisusedtomonitorthesampletemperature.Thelaserbeamsizeusedis1mm.Theoverlapratiois75%.FurtherdetailsoftheWLSPexperimentcanbefoundinYeetal.28.2.3.Characterization2.3.1.Micro-hardnessThemicro-hardnesschangeofthesamplesbeforeandafterLSPorWLSPismeasuredusingaLecoM-400-Hmicro-hardnesstestmachinewitha200gloadanda10sFig.1.Schematicofthelasershockpeeningprocess.1016C.Yeetal./ActaMaterialiaholdingtime.Theaverageoffivemeasurementswasusedforeachdatapoint.2.3.2.ResidualstressABrukerD8-DiscoverX-raymicro-diractionsystemwasusedtomeasuretheresidualstressofthesample.TheX-raycollimatorusedinthisworkis0.1mmindiam-eter.The220peakwasusedforstressanalysis,whichcorrespondstoa2hangleof123.916C176intheunstressedstate.Theinterferencelinesofthesteelphaseweredeter-minedat11wanglesfromC050C176to+50C176usingCoKa1radi-ationandanalyzedbythesin2wmethod29.TheX-raypeakbroadeningswereevaluatedfromthefullwidthathalfmaximum(FWHM)integralvaluesafterremovaloftheKa2signal.TheFWHMvalueatthe90C176X-rayinci-denceangleoftheBraggdiraction220peakswasusedasameasureoftherelativedislocationdensity29,orworkhardeningrate.Tomeasurethecoreresidualstressthematerialwasremovedlayerbylayerbyanelectrolyticpolisher(ProtoManufacturingInc.).TheelectrolyticpolishingmediumwastheA1solutionfromProtoManufacturingInc.Toinvestigatethethermalstabilityofthecompressiveresidualstressthesampleswereputinafurnaceat350C176Cfordier-entannealingtimesandthentheresidualstressmeasured.Toinvestigatethecyclicstabilityofthecompressiveresid-ualstresstheresidualstresswasmeasuredafterdierentnumbersofroundsofcyclicloading.2.3.3.TemTheTEMsampleswerepreparedbythefocusedionbeam(FIB)lift-outmethod30inaFEINovaLab200FIBsystem.TEMwascarriedoutinanFEITitanoper-atedat300keV.2.3.4.FatiguetestA100KNMTSservo-hydraulicfatiguetestingmachinewasusedtocarryoutthethree-pointbendingfatiguetest,inloadcontrolmode.Theloadingprofileisasinewavefunctionwithafrequencyof5Hz.ThestressratioRis0.1forallthefatiguetests(i.e.R=rmin/rmax,whererministheminimumstressandrmaxisthemaximumstress).Themaximalbendingstresswascalculatedbyr3PL2bh2,wherePistheappliedload,Listhespanforthebendingfatiguetestset-up,bisthewidthofthespecimenandhisthethicknessofthespecimen.Allthetestswerecarriedoutatroomtemperatureandinalaboratoryenvironment.3.Resultsanddiscussion3.1.Processconditionsforwarmlasershockpeening3.1.1.LaserprocessingconditionOneofthemostimportantparametersinLSPislaserintensity,whichcontrolstheshockpressure.InthisstudyBK7glass(shockimpedance1.44e6gcmC02sC0131)wasusedastheconfiningmedium,whichhasamuchhighershockimpedancecomparedwithwater(shockimpedance0.1655e6gcmC02sC0132).AccordingtoFabbroetal.33thelaser-inducedshockpressurecouldbeestimatedas:PGPa0:01a2a3pZg=cm2spI0GW=cm2p,whereaisthatportionofabsorbedenergycontributingtothether-malenergyoftheplasmaandZ(2Z1Z11Z2)isthereducedshockimpedancebetweenthetargetmaterial(steel4140shockimpedance3.96gcmC02sC01,estimatedasZ=qD,whereqisthematerialdensityandDistheshockvelocity34)andtheconfiningmedium.FromourcalculationstheshockpressureusingBK7astheconfinementwasabout2.7timeshigherthanthatusingwaterastheconfinement.Inthisstudythelaserintensitiesusedwerefrom1.5to4GWcmC02witha0.5GWcmC02interval.Itwasfound59(2011)10141025thattheconfiningmedium(BK7glass)crackedatlaserintensitiesabove4.0GWcmC02.TheresidualstressesforstressareveryclosebetweenLSPandWLSP,i.e.WLSPFig.2.SurfaceresidualstressesforLSPandWLSP(250C176C)atdierentlaserintensitiesandcorrespondingpeakplasmapressures.Fig.3.Hardnessatdierenttemperatures(laserintensity4GWcmC02).C.Yeetal./ActaMaterialia59(2011)101410251017laserintensitiesfrom1.5to4.0GWcmC02underLSPandWLSPconditionsweremeasured(Fig.2).TheestimatedpeakplasmapressureatdierentlaserintensitieswerealsoplottedbasedonFabbrosmodel33(seeFig.2).ItwasfoundthattheresidualstressmagnitudesincreasedalmostlinearlywithincreasinglaserintensityforbothLSPandWLSPfrom1.5to4.0GWcmC02.Inaddition,theresidualstressmagnitudesforLSPandWLSP(250C176C)areverycloseatalllaserintensi