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1 Research on Corrosion Fatigue Crack Propagation Behavior of Welded Joint s of A7 N01P - T 4 Aluminum Alloy s J. A n M , J . Fhen M,N , G . Gou M * , F. Qin M , H . Fhen M , P . Ii c , Z. Ii d M School of MMPeriMl s Science Mnd Engineering, SouPhwesP JiMoPong UniversiPy, Fhengdu, 61 0031, FhinM N Fhengdu HndusPry Mnd TrMde Follege, Fhengdu, 611731, FhinM c FSR QingdMo SifMng Fo. IPd , QingdMo,266000 , FhinM d QuMnPum Design FhinM , Beijing, 100027, FhinM ANsPrMcP : Forrosion fMPigue fMilure is one fMilure form of sPrucPure under Phe service of cyclic loMd in corrosive environmenP s, NuP Phere is liPPle reseMrch on Phe c orrosion fMPigue properPy of A7N01P - T4 Mluminum Mlloy Mnd iPs welded joinPs, especiMlly Phe crMck propMgMPion NehMvior. FonsequenPly, Phe c orrosion fMPigue crMck propMgMPion Neh Mvior of welded joinP s of A7N01P - T4 Mluminum Mlloy s were invesPigMPed . M icrosPrucPures of welded joinPs were exMmined Ny opPicMl microscope (OM), elecPron NMck - scMPPered diffrMcPion (EBSD), μ - X360n porPMNle X - rMy residuMl sPress Mnd PexPure MnMlyzer , Mnd scMnning elecPron microscope (SEM) ; Phen Phe frMcPure s were exMmined . T he poPenPiodynMmic polMrizMPion meMsuremenP of Phe joinPs wMs sPudied. The resulP show s PhMP Phe propMgMPion rMP e of corrosion fMPigue crMck of NMse mePMl (BM) wMs higher PhMn PhMP of Phe heMP - MffecPed zone (HAZ) , Mnd Phe welding seMm mMPeriMl (JM) hMd M NePPer corrosion fMPigue resisPMnce. B esides, P he second phMse s hMd greMP effecP on Phe corrosion fMPigue crMck p ropMgMPion rMPe of A7N01P - T4 M luminum M lloy s . The chMin - like second phMses Mlong Phe rolling direcPion were likely Po develop inPo Phe microcrMck under Phe McPion of corrosive medium, which would greMPly increMse Phe corrosion fMPigue crMck propMgMPion rMP e of A7N01P - T4 Mluminum Mlloys . Keywords : Forrosion F MPigue , FrMck PropMgMPion , A7N01P - T4 Aluminum A lloys , Jelded joinPs, Second PhMses 1. HnProducPion Forrosion fMPigue fMilure is one of Phe fMilure forms of sPrucPures under Phe service of cyclic loMd in co rrosi ve environmenP . T his fMilure form ofPen occurs in 2xxx Mnd 7xxx series of Mluminum Mlloys of MircrMfP, vehicles Mnd oPher imporPMnP PrMnsporPMPion sPrucPures Mnd more PhMn hMlf of fMilures were in correlMPion wiPh corrosion fMPigue . As M resulP, Phe c orrosion fMPigue life of sPrucPures is fMr shorPer PhMn PhMP of Mir fMPigue , e speciMlly when Phe Mpplied sPress is much close r Po Phe fMPigue limiP of Phe sPrucPures . There Mre mMny fMcPors PhMP MffecP Phe corrosion fMPigue crMck propMgMPion rMPe of mMPeri Mls, including PemperMPure, PH, concenPrMPion of corrosive soluPion, Mnd so on. M.A. JMhMN 1 found PhMP increMsing sPress rMPio hMs M Pendency Po hMve negMPive effecP on fMPigue life of 2024 - T3 Mluminum Mlloys Mnd Phe wMPer vMpor reduces Phe fMPigue life. U. ZupMnc 2 discovered PhMP fMPigue resisPMnce of Phe corrode specimens drMsPicMlly decreMsed in compMrison wiPh Phe pMrenP mMPeriMl due Po mMPeriMl piPPing cor rosion. J. TMn found PhMP P he cumulMPive fMPigue dMmMge of mMPeriMls could Ne divided inPo Phe following process 3 : (1) cyclic plMsPic deformMPion; (2) micro - crMcks nucleMPion; (3) micro - crMcks propMgMPion; Mnd (4) mMcroscopic crMcks propMgMPion . And Ii Xu - Dong 4 found Phe 2 disPriNuPion of corrosion piPs hMd M sProng effecP on Phe fMPigue crMck propMgMPion NehMvior in mi cro scMle Mnd Phese piPs cMn chMnge Phe pMPh of fMPigue crMck propMgMPion. MMlM M. ShMrmM 5 found PhMP piPPing corrosion on Phe sMmple surfMce McPed Ms M sPress concenPrMPor in Mlloys Mnd crMck iniPiMPion siPes resulPing in fMPigue fMilure were MP second phMse pMrPicles Mnd inclusions. J.F. Ii 6 discovered PhMP exPended Mgeing Mlso led Po Phe coMrsening Mnd disconPinuous disPriNuPion of grMin NoundMry precipiPMPes Mnd Phese Pwo fMcPors conPriNuPed Phe evoluPion of corrosion mode. Previous resulPs showed PhMP pi P s would Ne preferenPiMlly formed Mround Phe impuriPies, which hMd M higher ionizMPion Pendency PhMn Mluminum suNsPrMPe Mnd would firsPly o ccur Phe elecProchemicMl dissoluPion effecP 7 Mnd p iP s were MlwMys ellipsoidMl 8 . Besides, Phe life of Mluminum Mlloys in corrosive soluPion would Ne shorPened Ny 2 - 3 Pimes PhMn PhMP in Mir E . The rising PemperMPure Mnd PH would generMlly reduce corrosion fMPigue sPrengPh 10 . T he pre - exisPing inPerfMciMl void NePween oxide film Mnd mMPrix mighP promoPe Phe formMPion of Mg( OH) 2 corrosion lMyer, which could provide PemporMry proPecPion 11 . MeMnwhile, M smMll numNer of reseMrch hMs Neen mMde MNouP fMPigue crMck propMgMPion NehMvior of Mluminum Mlloys welded joinPs . IHU Xue - song found 12 PhMP Phe difference of fMPigue crMck iniPiMPion life for NMse mePMl, weld mePMl Mnd H AZ is negligiNle. The rMPio of Ni Po Nf is independenP on sPress MmpliPude, NuP dependenP on microsPrucPure Mnd mechMnicMl properPy. R. SEETHARAMAN 13 found PhMP, JiPh Phe increMse in Phe chloride ion concenPrMPion, Phe corrosion rMPe of fri cPion sPir welded AA2024 Mlumin um Mllo y increMses in Phe sMlP sprMy corrosion PesP, N uP Phe rising rMPe of corrosion decreMses due Po Phe formMPion hydroxyl chloride lMyer. KMlendM MuPomNo 14 discovered PhMP corrosion piPs MppeMr Po Ne MssociMPed wiPh precipiPMPes in Phe Mlumin um mMPrix Mnd McP Ms preferenPiMl fMPigue crMck iniPiMPion siPes. This re duces Phe Pime required for fMPigue crMck iniPiMPion Mnd decreMses Phe PoPMl fMPigue life. A7N01P - T4 is a kind of Al - Zn - Mg alloy that has high strength, good extrusion , and good welding properties . Its yield strength and tensile strength can reach t o 295MPa, 407MPa respectively , and its elongation can reach to 11.8%. It has been selected to make the welded components of high - speed trains, such as under - frames and other key parts subjected to static and dynamic loading 9 . Jhen Mluminum Mlloys run wiPh loMding sPress , comNined wiPh residuMl sPress in corrosive environmenP s , iP is much more sensiPive Po fMilure PhMn in Phe Mir. Prior reseMrch is mosPly MNouP fMPigue properPies of welded joinPs in Mir environmenP, NuP Phere is liPPle reseMrch on Phe corrosion fMPigue properPies of Phe A7N01P - T4 Mluminum Mlloy Mnd iPs welded joinPs, especiMlly Phe crMck propMgMPion NehMvior. This paper reports the results from our investigation on corrosion fatigue properties , especially on crack propagation behavior. 2. MMPeriMl s Mnd MePhods 2.1 ． MMPeriMl s The experimenP mMPeriMl s Mre A7N01P - T4 Mluminum Mlloy plMPes ( P sPMnds for plMPe Mnd T4 sPMnds for M sPeMdy sPMP e MfPer - soluPion PreMPmenP Mnd nMPurMl cooling Mccording Po JHS H4000 - 2006 ) . Jelding wMs performed Ny Phe MePMl HnPer - GMs(MHG) Pechnique wiPh M TrMnspuls Synergic 4000 welding mMchine. The welding wires Mre ER5356 of 1.2mm diMmePer. The chemicMl composiPio n s of Phe A7N01P - T4 Al Mlloy BM Mnd Phe welding wire is lisPed in TMNle 1. The welding pMrMmePers Mre lisPed in TMNle 2. To remove Phe oxides Mnd reduce Phe porosiPy of Phe joinPs, Phe surfMce of Phe Mlloy wMs chemicM lly cleMned Nefore welding. 3 TMNle 1 FhemicMl composiPion of BM Mnd welding wire MMPeriMl Z n Mg Fu M n Ti Si Fe F r Al A7N01P - T4 4.0 ~5.0 1.0 ~1.8 0.10 0.20 ~0.7 0 0.01 ~0.06 0.35 0.40 0.06~0.20 BMl. ER5356 0.10 4.5~5.5 0.1 0.05~0.20 0.2 0.25 0.10 -- BMl. NoPe : 1. The chemicMl composiPion o f A7N01P - T4 refers Po JHS H4000 - 2006 : Temper designMPion sysPem for wroughP Mluminum Mnd Mluminum Mlloy. 2. The chemicMl composiPion of ER5356, which is similMr Po filler mePMl, wMs lisPed for fuPure MnMlysis. TMNle 2 Jelding pMrMmePers for Phe A7N01P Mlum inum Mlloy welded joinPs MMPeriMl ThicknessC mm PeMk currenPCA Jelding volPMgeCV Jire feeding speedC(mCmin) Jelding speedC( m mCs ) NoPe A7N01 P - T4 8 1E2 23.2 12 7 B Mcking welding 173 22.7 10 8 F over welding N oPe: Phe gMs used for welding is EE.EE E % pu riPy Mrgon. 2.2. MicrosPrucPure oNservMPion T he microsPrucPure of w elded j oinP s of A7N01P - T4 M luminum M lloy were exMmined Ny meMns of Zeiss.A1m opPicMl microscope(OM), elecPron NMck - scMPPered diffrMcPion(EBSD), μ - X360n X - rMy residuMl sPress dePecPor Mnd JSM - 64E0IV scMnning elecPron microscope (SEM) . A ll specimens were mechMnicMlly grinded using Phe differenP size of mePMllogrMphic sMndpMper s from 2 40 Po 2000 griP Mnd were polished on Phe MuPomMPic polishing mMchines unPil Phere were no scrMPches on Phe surfMce of specimens . AfPer polish ing , Phe specimens were cleMned in Mlcohol Mnd were corroded Ny Keller ’ s corrosives wiPh M proporPion of 2% HF, 3% HFH, 5%HNO 3 Mnd E0% H 2 O. Jhen EBSD wMs hired, MnMlysis wMs mMde Po sPudied Phe disPriNuPion of Phe second phMses. All Phe specimens were elecPropolished in elecProlyPe soluPion consisPing of 10% perch loric Mcid in E0% ePhMnol. T he elecPropolishing process rMn wiPh M volPMge of 25V Mnd polishing Pime of 38s. A mechMnicMlly polished specimen wMs used Po sPudied Phe deformMPion degree of grMins of A7N01P - T4 M luminum M lloy Ny μ - X360n . T he f rMcPure wMs oNserved Ny meMns of SEM Po sPudy Phe chMnges of Phe second phMses during Phe process of corrosion fMPigue crMck growPh. 2.3. P oPenPiody nMmic polMrizMPion meMsuremenP T he polMrizMPion curve wMs PesPed using FHH 660F elecProchemicMl corrosion workshop. The elecProchemicMl PesPing of specimen s wMs meM sured Ny Phree - elecP rode sysPem: Phe specimen iPself McPed Ms M working elecProde, plMPinum elecProde Mnd sMPurMPed cMlomel elecProde were respecPively used Ms MnMuxiliMry elecProde Mnd reference elecProde. The working MreM of specimen is 0.45cm 2 Mnd Phe specimen wMs polished on Phe MuPomMPic polishing mMchines . The elecProchemicMl PesPing medium wMs 3.5% wP. NMF l . The elecProchemicMl PesPing wMs conducPed wiPhin Phe scMnning poPenPiMl rMnge of - 0.3~0.3 MP 25 ℃ , wiPh M scMnning rMPe of 1 mVCs. 2.4. F orrosion FMPigue FrMck ing Experime nP Forrosion fMPigue crMck propMgMPion experimenP wMs performed Mccording Po HSO 12108: MePMllic 4 mMPeriMls — FMPigue PesPing — FMPigue crMck growPh mePhod . T he specimen s were improved from single edge noPched Pension specimen ( SENT ) in HSO 12108 . The speci men s of BM Mnd welding joinPs were mMchined Mlong Y - X( X sPMnds for Phe direcPion pMrMlleling Po Phe welding line Mnd Y sPMnds for Phe direcPion per pendiculMr Po Phe welding line ) . BM, HAZ Mnd JM respecPively sPMnd s for Phe specimen in BM , Phe heMPed - Mff ecPed z one Mnd Phe welded joinP. S Mmpling mePhod Mnd d imension of corrosion fMPigue specimen Mre shown in F i g.1. Fig.1 S Mmpling mePhod Mnd d imension of corrosion fMPigue specimen (M) S Mmpling mePhod (N) d imension of corrosion fMPigue specimen S pecimens in Ph e direcPion of Y - X (shown in F ig.1 ) for corrosion fMPigue crMcking experimenP were cuP Mnd mMchined from Phe BM, HAZ Mnd JM of Phe welded joinPs Mccording Po HSO 12108 . A noPch wMs cuP MP Phe edge of eMch specimen in Phe sMme wMy. The pre - sePPing noPch wMs 2mm long. T he pre - crMck lengPh wMs 1 - 2mm long, which wMs mMde Ny Phe f MPigue PesPing mMchine. To Mssure Phe MccurMcy of Phe experimenP dMPM, only Phe region of noPch wMs immersed inPo Phe soluPion , Mnd Phe exPensomePer wMs isolMPed from corrosive soluPi on Ny M sepMrMPion NoMrd Po prevenP Phe exPensomePer from corroding. A cyMnoMcrylMPe Mdhesive glue wMs used Po prevenP leMkMge. T he specimen s w ere clMmped in Phe corrosion soluPion conPMiner mMde of plexiglMss, which Mllows for oNservMPion of Phe crMck gro wPh process due Po iPs PrMnspMrency. T he specimen wMs clMmped on Phe PesPing mMchine Phrough Pwo fixPures which were locked PogePher wiPh s pecimen Ny Pwo pins, shown in F ig.2 . T he corrosion fMPigue experimenPs were conducPed under Mn MlPernMPing loMd c ondiPion wiPh M PriMngle loMd wMve on M PJS00 servo - hydrMulic universMl PesPin g mMchine . Specimens were immersed in 3.5 %NMFl soluPion MP Phe PemperMPure of 25 ± 1 ℃ Mnd experimenPs were conducPed in M closed lMNorMPory wiPh Mir condiPioning. The crMck opening displMcemenP wMs moniPored during Phe PesP wiPh Mn exPensomePer unPil Phe insPMnP NreMk occured. The sPress rMPio wMs R=0.1, R=0.2, R=0.3, respecPively. OPher experimenP condiPions Mre lisPed in TMNle 3. (b) (mm) (a) 5 F ig.2 F lMmping skePch of Phe specimen TMNle 3 TesP c ondiPion of Phe differenP specimen s No. R I oMd MmpliPude(KN) M Mximum loMd(KN ) FoncenPrMPion of NMF l( % ) Frequency (Hz) TesPed MreM 1 0.1 3.6 4 3.5 0. 5 J M 2 0. 2 3.6 4 . 5 3.5 0. 5 JM 3 0. 3 3.6 5 3.5 0. 5 JM 4 0.1 3.6 4 3.5 0. 5 HAZ 5 0. 2 3.6 4 . 5 3.5 0. 5 HAZ 6 0. 3 3.6 5 3.5 0. 5 HAZ 7 0.1 3.6 4 3.5 0. 5 BM 8 0.2 3.6 4 . 5 3.5 0. 5 BM 9 0.3 3.6 5 3.5 0. 5 BM 3. T he resulPs Mnd discussion 3.1. MicrosPrucPure Three di recPions of microsPrucPures of A7N01P - T4 Mluminum Mlloy were shown in Fig.3. From Fig.3, Phe grMins in X - Y plMne were MrrMnged uniformly Mnd hMd M negligiNle plMsPic deformMPion , w hile Phe grMins in Phe Y - Z plMne were “ squMshed . ” As M resulP, vimineous grM ins Mlong Phe X direcPion formed. There wMs Mlso M pMrPiculMrly s evere deformMPion of grMins in X - Z plMne. From Phe S pecimen Pin Extensom eter Separation board Corrosion testing box Fixture .olt 6 complePene ss of Phe 2D - Mnd 3D - mMp of Phe DeNye ring Mnd diffrMcPion inPensiPy of De Nye ring Ny μ - X360n porPMNle X - rMy residuMl sPress Mnd PexPure MnMlyzer , grMin orienPMPion Mnd disPriNuPion could Ne quMliPMPively MnMlyzed. T he more complePe Mnd Phe low er Phe diffrMcPion inPensiPy of Phe DeNye ring wMs , Phe more uniform Phe grMins were . The resulPs showed PhMP Phe DeNye ring of Phe X - Y plMne wMs MlmosP complePe (2D - Mnd 3D - mMps in ( M ) of Fig.3 ) , Phe DeNye ring of Y - Z plMne wMs complePe in pMrP (2D - Mnd 3D - mMps in (N) of Fig.3) NuP Phe DeNye ring wMs hMrdly formed in Phe X - Z plMne (2D - Mnd 3D - mMps in (c) of Fig.3). The diffrMcPion inPensiPy of Phe DeNye ring in Phe X - Y plMne wMs MP Phe highesP vMlue Mnd up Po E28K((M) in Fig.3 ) , Phe diffrMcPion inPensiPy of Phe DeNye ring in Phe Y - Z plMne wMs MP Phe medium vMlue Mnd up Po 583K( (N) in Fig.3 ) Mnd Phe diffrMcPion inPensiPy of Phe DeNye ring in Phe X - Z plMne wMs only 281K. Hn conclusion, grMins of A7N01P - T4 Mluminum Mlloy shMrply disPorP ed under Phe rolling loMd , Mnd Ph ere wMs greMP diversiPy in differenP direcPions, which led Po differenP mechMnicMl pro perPies in Phree differenP direcPions. Fig.3 OpPicMl 3D microgrMphs showing Phe PypicMl grMin sPrucPure of A7N01P - T4 Mluminum Mlloy Mnd Phe PesP resulPs of grMin size Ny μ - X360n porPMNle X - rMy residuMl sPress Mnd PexPure MnMlyzer : ( M ) T he PesP resulP of X - Y plMne. ( N ) T he PesP resulP of Y - Z plMne. ( c ) T he PesP resulP of X - Z plMne. T he micro sPrucPure s of welded joinP s w ere oNserved N y OM , shown in Fig.4 . The sMme pMrPs of NMcking welding Mnd cover welding hMve M similMr microsPrucPure. T he JM wMs mMinly Phe mixed sPrucPure of dendriPes Mnd equiMxed grMin ( F ig.4 M , N ). The microsPrucPure close Po Phe JM consisPed of fine equiMxed grMin (Fig.4 c , d ) , while Phe microsPrucPure close Po Phe HAZ w M s mMinly coMrse grMins ( Fig.4. c, d ) . The grMins of Phe HAZ hMd M disPincP preferred direcPion Mlong Phe rolling (a)X - Y plane ( c ) X - Z plane (b)Y - Z plane 7 direcPion (Fig.4 e, f) . However, Phe deformMPion of Phe HAZ wMs less PhMn PhMP of BM (Fig.3) NecMuse recovery Mnd recrysPMllizMPion of A7N01P - T4 Mluminum Mlloy hMd occurred during Phe welding. Fig.4 The microstructure of welded joint s : (a) the microstructure of WM of c over welding , (N) the mi crostructure of WM of backing welding , (c) the microstructure of fusion line of c over welding , (d) the microstructure of fusion line of c over welding , (e) the microstructure of the HAZ of c over welding , (f ) the microstructure of the HAZ of backing welding . 3.2 ． F orrosion FMPigue FrMck PropMgMPion RMPe ExperimenP T he relMPionship of corrosion fMPigue crMck propMgMPion rMPe dMCdN vs. sPress inPensiPy rMnge △ K were shown in Fig.5 . Hn order Po geP Phe sPMPisPics disciplinMriMn Mnd Prend disciplinMriMn, Phe sev en poinPs incremenPMl polynomiMl mePhod wMs used. The PMris equMPion (1) wMs used Po process Phe meMsured dMPM . da dN = C × ∆ （ 1 ） Where dM CdN is Phe fMPigue crMck propMgMPion rMPe ; △ K is Phe sPress inPensiPy rMnge ; F, m Mre Phe mMPeriMl consPMnP s. ( FL1 ) c ( FL2 ) d a(WM 1 ) b (WM 2 ) e ( HAZ1 ) f ( HAZ2 ) (c) (a) (e) (d) (b) ( f ) 15 mm 4 00 μ m 4 00 μ m 4 00 μ m 4 00 μ m 5 00 μ m 5 00 μ m 8 Fig. 5 dMCdN versus △ K curve s of BM Mnd welded joinP s : (M) corrosion fMPigue crMck propMgMPion rMPe (FFFGR) curves of JM under differenP sPress rMPio, (N) FFFGR curves of Phe heMP - MffecPed zone under differenP sPress rMPio, (c) FFFGR curves of BM under differenP sPress rMPio, (d) FFFGR curves of JM, Phe heMP - MffecPed zone Mnd BM under sPress rMPio R=0.1, (e) FFFGR curves of JM, Phe heMP - MffecPed zone Mnd BM under sPress rMPio R=0.2, (f) FFFGR curves of JM, Phe heMP - MffecPed zone Mnd BM under sPres s rMPio R=0.3. From Phe resulPs, Phe propMgMPion rMPe of corrosion fMPigue crMck of BM, Phe HAZ Mnd JM increMsed wiPh Phe increMse of sPress rMPio R (Fig.5) . Under Phe sMme sPress rMPio, the propagation rate of BM was the highest, the propagation rate of HAZ was the medium and the WM was the lowest (d, e and f in Fig.5) . T he microsPrucPure s of Phe frMcPured welded joinPs were shown in Fig.6. a b c d e f 9 Fig. 6 mePMllogrMphic sPrucPure Mnd frMcPure surfMce of welded joinP s : (M) The crMck propMgMPion pM P h of Phe JM under Phe OM , (N) PrMns granular fracture of Phe JM Ny SEM , (c) intergranular fracture of Phe JM Ny SEM , (d) microstructure of the HAZ under Phe OM , (e) Phe secondMry crMck of Phe HAZ Ny SEM. HP cMn Ne seen PhMP Phe crysPMllizing morphology of Phe JM is equiMxed grMin s Mnd Phe frMcPure model is predominMnPly PrMnsgrMnulMr, MccompMnied wiPh smMll MmounPs of inPergrMnulMr crMcking (Fig.6. ( M ) , (N) Mnd (c)). BecMuse of high conPenP of mMg nesium in ER5356 welding wire (T MNle 1), Phe precipiPMPes in JM Mre mMgnesium - rich . The precipiPMPes in Phe HAZ Mre m Min ly Phe zinc - rich . Due Po Phe polMrizMPion curves ( Fig. 7 ) , Phe HAZ hMs M higher corrosion currenP densiPy of 1.0E U 10 - 6 Amps Ccm 2 , compMred Po PhMP of JM , which is 1.78 U 10 - 8 Amps Ccm 2 . Besides, Ph e grMi ns of Phe HAZ Mre coMrse r PhMn PhMP of JM (Fig.6. (d)) . The long , conPinuous grMin NoundMry of Phe JM McP s Ms M chMnnel for corrosion fMPigue crMck propMgMPion Mnd second Mry crMcks Mre eMsy Po Ne formed 2 50 μ m 2 50 μ m 50 μ m 50 μ m 50 μ m (a) (b ) (c ) ( d ) (e ) 10 Mlong Phe grMin NoundMry (Fig.6. (e)). As M resulP, Ph e JM hMs M greMPer resisPMnce Po corrosion fMPigue crMcking PhMn Phe JM . F ig. 7 PolMrizMPion curves of Phe welding seMm(JM) Mnd Phe heMP - MffecPed zone(HAZ) F ig. 8 . MicrosPrucPure of Phe BM Ny SEM Mnd EBSD : (M) Phe rolling sPrucPure s of BM Ny EBSD , (N) s econdMry phMse disPriNuPed Mlong crysPMl NoundMry of BM Ny EBSD . (c) chMin - like corrosion piPs Mnd fMPigue sPriMPions on Phe frMcPure of BM Ny SEM , (d) Piny crMck on Phe frMcPure of BM Ny SEM . 11 Fig.E F omponenP MnMlysis of Phe second phMses The microsPru cPure s of BM were PypicMl rolling sPrucPure s Mnd P he second phMse s were uniformly MrrMnged in chMin - like formMPions on Phe grMin NoundMries (Fig. 8 (M) , (N) ) . F rom Phe resul P of componenP MnMlysis (Fig.E) , Phe second pMrPicle wMs rich in Fe, which wMs one of Phe mosP common impuriPies in indusPriMl Mluminum . F e would ineviPMNly uniPe wiPh Phe mMPrix Mnd form some impuriP ies like FeA1 3 , Al 4.5 FeSi Mnd Al 3 Fe 2 S i . From Fig.11d, A7N01P - T4 Mluminum Mlloy is rich in Fe, which enriched MP Phe grMin NoundMries. Those i mpuriP ies were hMrd , Ms well Ms NriPPle. The sPress concenPrMPe cMn Ne formed in some plMces where Phe impuriPies exisPed. A s M resulP, second phMses would eMsily Nre Mk or sepMrMPe from Phe mMPrix, which l eMd Po Phe forming of crMcks 15 , 16 Mnd greMPly MffecPed Phe locMlized corrosion NehMvior of Al Mlloys . The c orrosio n model of Phe second phMses is shown in Fig.10 . A P Phe eMrly sPMge, Al Mnd Fe would Ne dissolved Mnd form Al (OH) 3 Mnd Fe 2 + . T hen Phe corrosion piPs were formed. F urPhermore, wiPh Phe negMPive shifP of poPenPiMl, Fe 2 + would pMrPly deposiP in Phe co rrosion piPs , Mnd Phe resP forms inPo Fe(OH) 2 ouP of Phe corrosion piPs, which would PighPly wrMp Phe FeAl 3 , PogePher wiPh AH(OH) 3 . A s M resulP, FeAl 3 wMs proPecPed from furPher dissoluPion. O n Phe oPher hMnd, Phe hydrogen evoluPion reMcPion occur red in Phe corros ion piPs, PhereNy promoPing Phe Mnodic dissoluPion of Mluminum suNsPrMPe in Phe corrosion piPs. T he reMcPion could Ne summMrized Ms follows: 2H + +2e → H 2 (cMPhode); Al → A1 3+ 十 3e(Mnode). AfPer Phe sepMrMPion of second phMses from Phe mMPrix, c orrosion piPs were formed Mnd MrrMnged in chMin - like formMPions on Phe frMcPure surfMce , Mnd some of Phem even form ed M conPinuous line PhMP could Ne considered M Piny crMck ( Fig.8 (c) , (d)) , which would greMPly MccelerMPe Phe corrosion fMPigue crMck propMgMPion of A7N01P - T4 . So iP is very necessMry Po Mvoid Phe inProducPion of impuriPy elemenPs like Fe during Phe mMnufMcPuring process . The microsPrucPure of Phe NoundMry NePween BM Mnd Phe JM in X - Y plMne (Fig.3) is shown in Fig.11. The BM Mnd Phe JM Mre divided inPo Pwo pMrP s wiPh M cleMr line (Fig.11 (M)), which Purns ouP Po Ne Phe conPinuously MrrMnged second phMses MP high mMgnificMPion (Fig.11 (N)). Under Phe influence of Phe welding PhermMl cycle, Phe microsPrucPure s in HAZ Necome coMrser . The second phMse s were pMrPly r edissolved inPo Phe mMPrix in Phe process of welding. A P Phe sMme Pime, Phe second p hMse s PhMP hMd noP dissolved inPo Phe mMPrix were pushed, wiPh Phe expMnsion of Phe grMin NoundMry , inPo Phe MreMs neMr Phe BM. Therefore, Phe second phMses MrrMnged neMr P he NoundMry NePween BM Mnd Phe JM were dense (Fig.11 (c), (d)). As M resulP, Phe JM hMd fewer second phMses 12 Mnd wMs less likely Po form Phe corrosion piPs. Hn conclusion, Phe JM hMd NePPer resisPMnce Po corrosion fMPigue crMcking PhMn BM. (M). Nefore c orrosion ( N ) . MfPer corrosion Fig. 10 The Forrosion Model of Phe Second PhMse s: (M) Nefore corrosion , (N) MfPer corrosion Fig.11. MicrosPrucPure of Phe NoundMry NePween BM Mnd Phe JM Ny SEM Mnd EBS D: (M)OM microsPrucPure of Phe NoundMry NePween BM Mnd Phe JM MP low mMgnificMPion, (N)OM microsPrucPure of Phe NoundMry NePween BM Mnd Phe JM MP high mMgnificMPion, (c)EBSD microsPrucPure of Phe NoundMry NePween BM Mnd Phe JM MP low mMgnificMPion, (d) EBS D microsPrucPure of Phe NoundMry NePween BM Mnd Phe JM MP high mMgnificMPion( Phe second phMses in red were rich in Fe Mnd Phe second phMses in yellow were oPher second phMses wiPhouP Fe). (a) (b ) (c ) ( d ) 40 0 μ m 20 0 μ m 20 0 μ m 50 μ m 13 4. F onclusion T he NehMvior of corrosion fMPigue - crMck propMgMPion of BM Mnd welded joinP s of A7N01P - T4 Mluminum Mlloy in 3.5% wP. NMF l under Phe differenP sPress rMPio w Ms sPudied in Phis pMper. O n Phe NMsis of Phe PesP resulPs, Phe conclusions Mre drMwn Ms follows: (1) A7N01P - T4 Mluminum Mlloy hM d M rolling sPrucPure. T he grMi ns were elongM Ped Mlong Phe rolling direcPion Mnd Phe micros PrucPure in Phree direcPions were differenP from eMch oPher. (2) T he JM of A7N01P - T4 Mluminum Mlloy hM d M NePPer corrosion fMPigue resisPMnce PhMn PhMP of Phe JM in 3.5% wP. NMF l , Mnd BM hM d Phe worsP c orrosion fMPigue - resisPMnP performMnce. (3) T he second phMses in A7N01P - T4 Mluminum Mlloy were rich in Fe Mnd were disPriNuPed mMinly Mlong Phe grMin NoundMry . F hMin - like second phMses led Po Phe formMPion of microcrMck s during Phe process of corrosion fMPigue crMck propMgMPion , which greMPly reduce d Phe corrosion fMPigue - resisPMnP performMnce of A7N01P - T4 Mluminum Mlloy. So iP is very necessMry Po Mvoid Phe inProducPion of impuriPy elemenPs like Fe during Phe mMnufMcPuring process. AcknowledgemenP The res ulPs of Phis pMper were from mulPiple projecPs which include (i) “R eseMrch of Phe key Pechnologies Mnd equipmenP for nexP - generMPion rMilwMy PrMnsporPMPion in ciPies” Mnd (ii)“B Msic reseMrch of Phe design Mnd MdvMnced welding Pechnology for high speed PrMi ns in Phe wide region environmenP ” . The MuPhors Mcknowledge Phe finMnciMl supporP Ny Phe NMPionMl Science & Technology PillMr ProgrMm (No.20 15 BAG12 B 0 1 ) Mnd Phe NMPionMl Key BMsic ReseMrch Mnd DevelopmenP plMn ( No. 2014FB660807) . References ： 1. M. Wahab, J. Park, M. Alam, and S. Pang: 'Effect of corrosion prevention compounds on fatigue life in 2024 - T3 aluminum alloy', Journal of materials processing technology , 2006, 174 (1), 211 - 217. 2. U. Zupanc and J. Drum: 'Effect of pitting corrosion on fatigue performance of shot - peened aluminium alloy 7075 - T651', Journal of Materials Processing Technology , 2010, 210 (9), 1197 - 1202. 3. W. Shuo1a, C. Xu1a, Y. Yai, X. Wei, and F. 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