See discussions, stats, and author profiles for this publication at: http://www.researchgate.net/publication/257540562Linear friction welding of a near- titaniumalloyARTICLEinACTA MATERIALIA JANUARY 2012Impact Factor: 4.47 DOI: 10.1016/j.actamat.2011.04.037CITATIONS9DOWNLOADS58VIEWS765 AUTHORS, INCLUDING:Elvi DalgaardPratt & Whitney Canada8 PUBLICATIONS 21 CITATIONS SEE PROFILEP. WanjaraNational Research Council Canada118 PUBLICATIONS 532 CITATIONS SEE PROFILEXinjin CaoNational Research Council Canada155 PUBLICATIONS 1,236 CITATIONS SEE PROFILEJohn J. JonasMcGill University468 PUBLICATIONS 8,882 CITATIONS SEE PROFILEAvailable from: Xinjin CaoRetrieved on: 12 August 2015Author's personal copyLinearfrictionweldingofanear-btitaniumalloyE.Dalgaarda,b,,P.Wanjarab,J.Gholipourb,X.Caob,J.J.JonasaaMaterialsEngineering,McGillUniversity,3610UniversityStreet,Montreal,QC,CanadabAerospaceManufacturingTechnologyCentre,InstituteforAerospaceResearch,NationalResearchCouncilCanada,5145DecellesAvenue,Montreal,QC,CanadaReceived8April2011;accepted18April2011Availableonline16November2011AbstractThe linear friction welding (LFW) behaviour of near-b titanium alloy Ti5Al5V5Mo3Cr (Ti-5553) was investigated by varying theprocessing conditions of frequency and axial pressure. The examined mechanical properties of the welded material included microhardnessand tensile properties. The maximum strains experienced by the material during LFW for each set of welding parameters were estimatedbasedontheprocessparametersandthenevaluatedusingAramis,athree-dimensionalopticaldeformationmeasurementsystem.TheLFWed Ti-5553 was examined with electron backscatter diraction techniques to relate the texture and phase changes to thethermomechanical conditions. Characterisationoftheweldsincludedanalysisofthemicrostructural featuresoftheweldregionandthe thermomechanically aected zone in relation to the parent material.CrownCopyright 2011PublishedbyElsevierLtd.onbehalfofActaMaterialiaInc.Allrightsreserved.Keywords: Linearfrictionwelding;Ti-5553;Microstructure;Tensileproperties1.IntroductionBeta(b) andnear-btitaniumalloys are of increasinginterestinthe aerospace industrydue totheirbetter form-ability and toughness as compared with the more commona+bTi6Al4Valloy. High-strengthmetastableballoyssuchasTi-5553havepotential toreplacesteel asthepre-ferredmaterial forlargecomponentssuchasthelanding-geartruckbeamonthelatestgenerationofairframes[1].Ti-5553was introducedin1997byTIMETandhas thenominal composition5 wt.%Al, 5 wt.%V, 5 wt.%Mo,3 wt.%Cr, withthebalancebeingTi. Table1listssometypicalphysicalandmechanicalpropertiesofthisalloyinthe solution treated and aged (STA) condition. The forgingbehaviour of Ti-5553 is similar to that of Ti10V2Fe3Al(Ti1023), thoughthehigherbtransustemperatureofthe former (856 C) vs. the latter (800 C) allows higherforgingtemperatures[2].WeldabilityisaclassicproblemwithTi anditsalloys.The metal rapidly reacts with atmospheric gases in the mol-ten state, requiring protective gas shielding in order to joinit successfully using fusion welding methods. The low ther-mal conductivityofTi leadstolongerweldtimesforlowenergydensityprocesses suchas TIGwelding. Thelongweldtimesleadtoslowcooling, requiringgasprotectionfor an extended time to protect the highly reactive Ti. Highenergydensitymethods,suchaslaserweldingorelectron-beamwelding, partiallysolvethisproblembyallowingarapid welding cycle (heating, melting and cooling) throughlocalised heat input, but still require protection (gas shield-ingforallareasabove350 Corvacuum)duringthetimethat the weld is molten. Clearly, Ti is a prime candidate forthe development and application of solid-state weldingmethods.Linear friction welding (LFW), a solid-state processconsistingofoscillatingonepartagainstastationarypartwhile applyinganaxial load(see Fig. 1), eliminates thenecessity for a protective environment when welding, sincethematerialdoesnotreachfusiontemperatures.Noaxialsymmetry is required and the parts can have quite complex1359-6454/$36.00CrownCopyright 2011PublishedbyElsevierLtd.onbehalfofActaMaterialiaInc.Allrightsreserved.doi:10.1016/j.actamat.2011.04.037Correspondingauthorat: MaterialsEngineering, McGill University,3610UniversityStreet,Montreal,QC,Canada.E-mailaddress:ecdalgaard@gmail.com(E.Dalgaard).www.elsevier.com/locate/actamatAvailable online at www.sciencedirect.comActaMaterialia60(2012)770780Author's personal copygeometriesinvolvingcurves. Withheatgenerateddirectlyattheinterfaceinfrictionwelding, ahighenergydensity(lowheatinput)comparabletothatdevelopedinlaserorelectron-beamwelding canbe achieved. This, combinedwith the low thermal conductivity of Ti and its alloys, cre-ates a very small heat-aected zone (HAZ) [3]. The processwas rst patented in 1969 [4,5], and in the early 1980s TheWelding Institute (TWI) demonstrateda working LFWmachine for metals. Since then the process has been devel-oped for a variety of materials such as Ti alloys [68], tita-niumaluminides [9], and Ni-based superalloys [1013],with a more detailed examination of the relationshipbetweenparameters andproperties. Nonetheless, totheknowledgeof the authors, littleworkhas beendone ontheLFWof bTialloys.The LFWprocess can be divided into four distinctphases[6].Therstphase,knownastheinitialorcontactphase, begins with contact between the two pieces in orderto initiate the wear of surface asperities. The second phase,known as the transition or conditioning phase, begins whenthelargewearparticlesthatwerecreatedduringtherstphasebegintobeexpelledfromtheinterface. Frictionalheatcreatesasoftplasticizedregionthatisnolongerableto support the applied axial load and begins to deform per-manently. When moving into the third phase, known as theequilibrium or burn-o phase, the ash begins to form [14].The axial pressure is increased and oscillation continues asin the prior phases. Frictional heat is conducted away fromthe interface and the plastic zone develops further, extrud-ing material away fromthe interface as ash. The lastphaseisknownasthedecelerationorforgephase, wherethe materials are brought to rest after the desiredshortening has been attained. Once the materials have beenbroughtto restandaligned, theaxialpressure is increasedandtheweldisconsolidated[14].Interest in manufacture and/or repair of aircraft compo-nentsusingemergingweldingtechnologiesof importanceto the aerospace industry has created an incentive for inves-tigatingthe LFWof Ti-5553. This researchworkformspart of a larger programme on the research and technologydevelopmentofLFWforthemanufactureofnear-a,abandnear-bTialloys.Microstructurecharacterisation,tex-ture analysis andmechanical propertyevaluationof thewelds were performed as part of the LFW process develop-ment trials to dene the optimum parametric window. TheintentofthispaperistoreportonthevariousndingsofthisresearchworkonLFWedTi-5553.2.ExperimentalprocedureThe equipment used for welding was an MTS LFW pro-cess development system, comprised of two hydraulic actu-ators: thein-planeactuatorthatoscillatesthelowerworkpiece horizontally; andthe forge actuator that applies adownward load through the top stationary workpiece.The technical specications of the equipment are describedelsewhere[6]. Table 2 showsthe experimental plan used inthisstudyaswell asthecalculatedmaximumstrainrateusingtheequationproposedbyVairisandFrost[15],themeasuredweldingtimeandthetotalmaximumstraincal-culated fromthese two latter quantities. The baseline(BL) values of fBL(frequency) andPBL(pressure) wereestablishedbasedonoptimareportedfor Ti6Al4V[6],sincethereislittleintheliteratureregardingthefrictionwelding of b or near-b Ti alloys and no specic parametershave been published [16]. Forging pressure was maintainedatthesamelevelasduringoscillationinordertoexaminethe eectof oscillation on the growth of the HAZwithoutthe complication of expelling extra material during theforgingphase. Ingeneral theaandbphasecontentofaTi alloyis verypertinent toits properties andhence totheselectionofthermomechanical processingparameters.InLFW, theweldingbehaviour is closelyrelatedtotheowstress; inthe case of Ti-5553, since the material isalreadymainlybphase, theuseof parameterswhichledto ow in the b phase of the Ti6Al4V material [6] was feltto be valid (since thermomechanical processing wouldoccur in the b phase in any case). Ti-5553 material in ingotformwassectionedtoobtainweldcouponsof 13 mminTable1PhysicalandtensilepropertiesofTi-5553intheSTAcondition[2].Average btransus, C 856Density,kg m34650Tensileelasticmodulus,GPa 112Compressiveelasticmodulus,GPa 113Ultimatetensilestrength(UTS),MPa 1236Yieldstrength(YS)@0.2%,MPa 1174Percentelongationatfracture(%El.),withgaugelengthof4D 13Horizontal H Reciprocal Motion (Bottom)Frictional PressureStationary (Top)LWaFig.1. Samplegeometryandoscillationdirection[6].Table2Samplesprepared.SampleIDFrequency Pressure Estimatedstrainrate(s1)Weldingtime(s)Estimatedmax.strainLFW-1 fBLPBL3.6 1.3 4.8LFW-2 Low(0.6fBL) High(1.5PBL) 2.2 1.6 3.5E.Dalgaardetal. / ActaMaterialia60(2012)770780 771Author's personal copywidth(W),35 mminheight(H)and26 mminlength(L),asillustratedinFig. 1. Priortowelding, thecontactsur-faces of the samples were ground and cleaned with alcohol.LFWed samples were sectioned transverse to the oscilla-tiondirectionthroughtheweldzone(asshowninFig. 2)for metallographic and electron backscatter diraction(EBSD)investigation(i.e. parallel totheplaneformedbythe transverse andthe normal directions). Conventionalpolishing procedures were used for optical microscopyand for preliminary preparation of the EBSDsamples.Final EBSDpolishingwascarriedoutinavibratorypol-isherfor12 h. Itisnoteworthythatthematerial requiredanextensiveandsensitivepolishingprocedure, aselectro-polishingisnot recommendedinordertopreservethebphase. For examination of the microstructure using opticalmicroscopy, etching was done using Krolls reagent.Microstructural examination was then performed usingan inverted optical microscope (OlympusGX71) equippedwith digital image analysis software (AnalySIS Five). Back-scatter imaging andEBSDmapping were performedat20 kVonaHitachi S-3000NVP-SEMequippedwithanOxford(HKL) EBSDdata acquisitionsystem(polishedsurface).MicrohardnesswasmeasuredusingaStruersDuraminA300machinewithafullyautomatedtestingcycle(stage,load, focus, measure). Aloadof 300 gwasappliedusingaloadcell withclosed-loopcircuit control, andhardnessprolesweredeterminedacrosstheweldregionusinganaverage of three measurements for eachpoint, withanindentintervalof0.2 mmandadwellperiodof15 s.For each weld condition, three tensile specimens havingastandardsub-sizegeometryof 25 mmingaugelength,6 mminwidthand4 mminthickness weremachinedinaccordance with ASTM E8M-01. All specimens were testedat roomtemperature using a 250 kNMTS 810 tensilemachineequippedwithanAramis3-Ddeformationmea-surement system. Before executing tensile testing, eachsamplewaspaintedwithahigh-contrast randompatternofblackonawhitebackground. ThefunctionalityoftheAramissystemdependsonthequalityofthisspecklepat-tern. The quality of the pattern was veried before mechan-ical propertyevaluationtoensurestrainrecordingalongthe entire gauge length. After examination for pattern rec-ognition, tensilepropertyevaluationwasconductedusingdisplacement control at arate of 2 mm min1uptotheyieldpoint andthen8 mm min1uptotherupturewiththeAramis systemset at anacquisitionrateof 2 framespersecond(fps).3.ResultsanddiscussionThemicrostructureoftheas-receivedmaterialrevealedbyoptical microscopyandscanningelectronmicroscopy(SEM) is shown in Fig. 3. At low magnication, extremelylargebgrainsaveraging100500 lmindiameterarevisi-ble. The SEM image at high magnication reveals an acic-ular microstructure within the b grains. One of the large bgrainboundaries canbe seencrossingthe corner of theframe(Fig. 3b). Thisas-receivedmicrostructureisconsis-tentwiththatexpectedforaningot structure[1] withoutFig. 2. Sectioningschematicformetallographicsamplesextractedfromwelded coupons: WD, welding oscillation direction; TWD, directiontransversetoWD, intheplaneof welding; ND, normal totheweldingplane.grain boundary(a) (b)Fig. 3. Parent material showing (a) a dark-eld image of the large equiaxed grains (Krolls reagent) and (b) high magnication backscatter electron imageofacicularsubstructureusingcompositionalcontrast(mirrorpolished).772 E.Dalgaardetal. / ActaMaterialia60(2012)770780Author's personal copyany secondary operations, such as thermomechanicalprocessingorsolutionizingand/orageingheattreatments.3.1.MicrostructuralevolutionVisual inspectionoftheinterfaceregionintheLFWedTi-5553showsanappreciableashfromall foursidesofthe joint (Fig. 4a), suggesting that the weld is integral[6,14]. The ash length was found to be larger in the direc-tionoftheoscillatorymovement,i.e.paralleltothespeci-men length as compared to that along the specimenwidth. However, unlikeLFWedTi6Al4V, whichexhib-ited a series of ridges on the ash extruded in the directionof the reciprocating motion, the LFWed Ti-5553 ash,though rough on the outer surface, displayed no regular rip-plesundertheprocessconditionsexaminedinthepresentwork. As theashlayer consists of plasticallydeformedmaterial extruded during the welding process, the dierencein the ow behaviours of the two alloys (a + b vs. near-b) isalmost certainly responsible for the dierence in ash mor-phology. WhileTi6Al4Vbehaves inlargepart likeananisotropic hexagonal close-packed material with very lim-ited slip systems, Ti-5553 is for the most part a body-centredcubicmaterialwithmultipleslipsystemsactivatedevenatroomtemperature, and certainly at the temperature atwhich ash is extruded. This leads to the very narrowspread and strong directionality of the ash ow inTi6Al4Vandthe more vertical spreadof the Ti-5553ash.ThemicrostructureofaweldedsampleisillustratedinFig. 5. At highmagnicationusing optical microscopy,faint indications of grain boundaries were seen, but, overall,the grains along the weld centre were not eectively revealedthrough chemical etching. The lowetching response isattributedtothehighlydeformedanddynamicallyrecrys-tallizedmicrostructure inthe weldzone. InFig. 5b, theregionadjacenttotheweldcentreispresented;hereafewmore equiaxed grains are visible, as well as some indicationof an acicular structure in certain regions between therecrystallizedgrains. InthemicrographofFig. 5c, repre-senting the structure about 0.4 mm from the weld line, thisacicularstructureisclearlyrevealed.Finally,inFig.5d,1mmfromtheweldline, theundeformedacicular micro-structurethat ischaracteristicof theas-receivedmaterial(Fig. 3b) can be seen clearly.3.2.MicrotexturemeasurementsTo reveal the details of the weld microstructure, includ-ing delineation of the grain boundaries and phase divisions,orientationdatawereobtainedfromtheas-receivedandLFWed samples. Fig. 6 depicts the inverse pole gureand phase fraction map of the as-received material, reveal-ingthatgrainsareequiaxedand100250 lmindiameter.Also observed is that the a phase comprises approximately3%ofthesurface, andbyextension, volumeofthemate-rial. This phase is mainly concentrated at grain boundaries.Inorder toimprove the accuracyof the phase fractiondata,pointsinthescanwithacondenceindexbelow0.2werediscardedbeforecalculation.InFig.7,aninversepoleguremapofsampleLFW-1canbeseen.Animagequalitymapisoverlaidinordertodene the grainboundary locations. Clearly, inthe as-weldedcondition, thisalloyundergoescompletedynamicrecrystallization in the weld zone (200 lmwide) underthe thermomechanical conditions employed, namely thecombination of straining at elevated temperatures and highstrainrates.Beyondtheweldregion,anabrupttransitionto athermomechanicallyaectedzone(TMAZ)consistingof mixedrecrystallizedanddeformedgrains is observed,followedby a further transitiontothe original large bgrains.Whilethelargeequiaxedanddeformedgrainsdis-playavarietyof orientations, it canbeseenthat, withintherecrystallizedzone, thegrainsarepredominantlyori-ented with their h1 1 1i directions normal to the sample sur-face. This represents the weldoscillationdirection. Thisreorientation to a preferred texture may be due to thealignmentduringoscillationoftheprimaryslipsysteminthis phase, {1 1 0}h1 1 1i, to be parallel with the oscillationdirection.Fig. 8 indicates an overall phase area fraction of 0.4% ain sample LFW-1. The a fractiondecreases to nearly none(estimated at 0.05%) in the recrystallized weld zone. This isin contrast to the 3% a observed in the as-received material,andindicatesthatthefastcoolingrateexperiencedbythematerial didnot permit anequilibriumphasefractiontoFig.4. (a)As-weldedLFW-1showing(b)rightand(c)leftsideashcross-sections.E.Dalgaardetal. / ActaMaterialia60(2012)770780 773Author's personal copydevelop and metastable b was retained in preference to theformationofa.Onceagain, basedontheaccuracyoftheTSLOIMAnalysis software suggestedin the literature[17,18], points with a condence index below0.2 wereremovedfromthe data set. It was observed that thesepoints were nearlyalways indexedas a, due perhaps tothelargenumberof linesavailablein thepattern, butthatmanualexaminationoftheKikuchipatternandtheindexassignedtoitledtotheconclusionthattheindexingwaserroneous.ThewidthoftherecrystallizedzoneinsampleLFW-2(see Figs. 9 and 10) averages 380 lm at the narrowest point(weldcentre), approximately50%wider thaninLFW-1.Once again, the recrystallized zone consists almost entirely(b) (a)(d) (c) grain boundary Fig. 5. LFW-1 (Krolls reagent): (a) weld centre at high magnication, (b) 100 lm from weld centre, (c) 400 lm from weld centre, and (d) 1 mm from weldcentre.Fig.6. EBSDmapsofas-receivedTi-5553:(a)inversepoleguremap;(b)phasefractionmap.774 E.Dalgaardetal. / ActaMaterialia60(2012)770780Author's personal copyofgrainsorientedwiththe h1 1 1idirectionnormaltothesurface. Aclose-upscanof this recrystallizedregioncanbeseeninFig. 11. Thephasefractionmapconrmstheearlier nding in the lower-resolution scan that there is lit-tletonoapresent inthesegrains, ascomparedwiththeTMAZ,whichcontained0.51 vol.% a.In Fig. 12, b pole gures are presented for the recrystal-lizedregionsofLFW-1andLFW-2.No a poleguresarepresented due to the negligible amount of a phase observedin the material. The texture intensity of these recrystallizedzones canbeseentobeveryhigh, approximately1015times random. This strong texture is based on a largeZ A H Z A M T r e t n e C d l e W Z A M T Z A H240 m Fig.7. LFW-1imagequalitycombinedwithinversepolegure.Fig.8. PhasefractionmapofLFW-1;pointswithcondenceindexbelow0.2removed.Weld Center TMAZFig.9. InversepolegureofLFW-2showingonesideofweldzonewithadjacentdeformedgrains.E.Dalgaardetal. / ActaMaterialia60(2012)770780 775Author's personal copynumberofsmall grainsandisquitereliable. Asexpectedfrom the inverse pole gure maps, many of these recrystal-lizedgrainsarealignedalongthe h1 1 1idirection.Duetotheverylargegrainsizeintheunrecrystallizedregionsofthe welded specimens, the textures will need to be measuredover larger scanned regions in order to be statisticallysignicant.3.3.MechanicaltestingThe microhardness proles showninFigs. 13and14reveal that the weldregionis somewhat softer thanthesurroundingTMAZ, whichinturnissofterthanthesur-rounding parent material; this is consistent with the micro-structural observations of adepletionin the weldment.SampleLFW-1, weldedatlowerpressureandhigherfre-quency, can be seen to display a pronounced hardness dropintheweldwithasharpdropover therange1.6 mm.Meanwhile LFW-2, the higher-pressure/lower-frequencysample, displays less softeninginthe weldcentre andaslightly less abrupt drop that, however, spans a wider regionas indicated by comparison with the parent metal hardnessrange. It is noteworthy that as the indentationspacing(200 lm to give ve indentation widths from centre to cen-tre) is comparable to the weld width, more detailed analysisof the eect of processing on the hardness distribution wouldrequire further investigation with higher-resolution methodssuchasnanoindentation. Nevertheless, thewidths of theFig.10. LFW-2phasemap;stepsize2 lm;pointswithcondenceindexbelow0.2removed.Fig.11. High-resolutionscanofequiaxedgrainzoneatstepsize0.10 lm:(a)inversepolegurecombinedwithimagequalitymap;(b)phasemap.776 E.Dalgaardetal. / ActaMaterialia60(2012)770780Author's personal copysoftened regions in the two samples are consistent with theweld zone widths observed in the EBSD scans given the res-olution of the hardness proles.The drop in hardness is readily explained by the fact thatnopost-weldheattreatmentwasperformedonthesesam-ples. Thesolutiontreatment of Ti5553is usuallycarriedoutbelowthe btransus,sothatsomeglobular aremains,muchof it ongrainboundaries. Without this treatment,thestrengtheningandgrainboundarypinningeects (aswell as the detrimental eect onfracture toughness) ofthe globular a that would have formed are absent.Althoughanincreaseinstrengthintheweldregionmighthave been possible due to the grain rening eects of heavydeformation combined with recrystallization at elevatedtemperature, this increase was not observed, perhapsobscuredbythegreatereectofsofteningduetothelossof a.Tensiletestingwasperformedonboththeas-receivedand the welded specimens. The as-received values in Table3areforthematerial that wasnot heat-treated, andarethus lower thanliteraturevalues quotedfor solutionizedand aged Ti-5553. The large variation in the %El. observedis consistent withprevious ndings for abillet structurewithout STA, and Fanning has reported [2] lower variationinthe elongationwhenthe microstructure of Ti-5553ishomogenisedusingSTA. Ascomparedtotheas-receivedtensileproperties,theYS,UTSand%El.valuesarelowerfortheLFWedsamples. Thereductionintensilestrengthof theweldedsamples canbeexplainedbythepresenceof a softened region in the weldment with a microstructuredepletedofa. Inaddition, thereisadistinctdierenceintensile properties betweensampleLFW-1, weldedat BLaxial pressureandfrequency, andsampleLFW-2, weldedFig. 12. b Pole gures of the recrystallized zones in (a) LFW-1; (b) LFW-2. The welding direction (WD) is normal to the gure (not shown); TD indicatesthedirectiontransversetoWD,andNDthedirectionnormaltotheweldingplane.Fig. 13. Microhardness proleacross theweldlinefor sampleLFW-1(baselinepressureandfrequency).Fig. 14. Microhardness proleacross theweldlinefor sampleLFW-2weldedatlowfrequencyandhighpressure.Table3Tensiletestingresults.YS(MPa) UTS(MPa) UniformEl.(%) TotalEl.(%)As-received 1046 13 1108 25 7.4 3.0 11.2 6.5LFW-1 1019 19 1058 23 3.0 0.5 4.0 1.0LFW-2 988 16 1013 10 2.0 0.1 2.9 0.9E.Dalgaardetal. / ActaMaterialia60(2012)770780 777Author's personal copyathigherpressureandlowerfrequency, withLFW-1dis-playinghigherYSandUTSaswell asgreaterelongation(see Table 3). The better mechanical performance ofLFW-1as comparedtoLFW-2canbeattributedtothesmaller amount of material aectedthermomechanically(weldzoneandTMAZ),asindicatedbytheEBSDmaps.The fracture surfaces of the welded tensile couponsrevealedverylargegrains(seeFig.15)thatareconsistentwith fracture occurring in the TMAZ adjacent to therecrystallizedweldzone. Bothunweldedandweldedsam-ples displayed ductile fracture characteristics withsomeareasof shear. Infact, all weldedspecimensfracturedinthe TMAZwithin1 mmof the weldzone, as showninFig. 16. Conversely, as-received(unwelded) tensilespeci-mens fractured at randomlocations along their gaugelengths. The consistent failure of the weldedsamples inthe TMAZ is most likely due to a depletion in thecoarse-grainedmicrostructure, whichwouldhave a ten-dency to be weaker than the similarly depleted ne-grainedrecrystallizedweldzone.Anexampleof thestraindistributionintheunweldedcondition is presented in Fig. 17. This should be comparedwiththatoftheweldedLFW-1inFig. 18. Theregionofstrain concentration indicated as the area of highest inten-sityintheweldedspecimencorrespondstothelocationofthefracturethatoccurredimmediatelyafterward.WhileitFig. 15. SEM images of fracture surfaces: (a) as-received, low magnication; (b) welded, low magnication; (c) as-received, high magnication; (d) welded,highmagnication.(a)WZ WZ Fracture surface Fracture surface (b)Fig.16. Polarized lightmicrographsoffractured tensile bars:(a)LFW-1and(b)LFW-2.778 E.Dalgaardetal. / ActaMaterialia60(2012)770780Author's personal copycan be seen that in the weld-freesample the averagestrainisapproximately12%, risingtoabout doublethat at theeventual fracturelocation, thelocalisationis muchmorepronounced in the welded specimen, with an average strainin the majority of the material of only 2%. This rises shar-plyto5%justoutsidetheTMAZandthencesteadilybutrapidlytothe maximumof 9.3%, almost ve times theaverage,atthefracturelocationadjacenttotheweldline.The resolution of the strain distribution image and the dif-culty of accurately positioning the weld line (which is notaltogetherstraight)inthecentreofthespecimenleadstosomeambiguityinFig. 18astotheexactpositionofthefracture with respect to the weld zones. However, the frac-turemicrographsinFig.16showclearlythatthefractureoccurrednexttotheweldlineintheTMAZ.Bothmicrohardness analysis andtensileevaluationofthe welded specimens lead to the conclusion that a softenedregion exists at and on either side of the weld line. This softregion, depleted of the strengthening a phase, as revealed bythemicrostructural andtextural examinations, representsan area where the strain concentrates. An object is strongestwhen the strain is evenly distributed over its area. Here, duetotheweldzonewithitscorresponding aphasedepletionandgrainrenement, andtheTMAZwithonlyaphasedepletion, there is strain concentration caused by the inho-mogeneityof thestrengthsof thesedierentregions, andtherefore in the owstress; this results in a localised increasein the strain. Of future interest for welded Ti-5553 is the useof a post-weld heat treatment to promote the precipitationofthestrengtheningaphase. Thepresentstudyindicatesthat asolutionizingandageingcyclewouldbebenecialin improving the properties of the weld.4.ConclusionsNear-b alloy Ti5Al5V5Mo3Cr alloy was weldedusinganMTSLFWprocessdevelopmentsystem.Processconditions were varied in order to examine the relationshipbetweenprocessparametersandthemicrostructural, tex-tural and mechanical properties. The following conclusionscanbedrawn:1. Theas-receivedmaterial displayedalargeequiaxedbgrainstructure of about 100500 lmdiameter grains.Thesegrainscontainedanacicularsubstructure. Theaphase in the as-receivedmaterial comprisedapproxi-mately3%of the total, mainlyconcentratedat grainboundaries.2. After welding, a very ne-grained (15 lmdiameter)recrystallized zone was observed in the weld centre,ranginginwidthfrom240to380 lmfor theprocessconditions tested. Withinthis recrystallizedzone, thegrainswerealmostall orientedwiththeir h1 1 1idirec-tionsnormal tothesamplesurface, i.e. parallel totheweldoscillationdirection.3. The TMAZ was observed to consist of deformed large bgrainswithsomerecrystallizationlocalisedatthegrainboundaries. Also, less than1%volume fractionof aphase was observed in the TMAZ and weld zone follow-ingwelding.Fig.17. Straindistributioninas-receivedTi-5553justbeforefracture.Weld LinePM PMTMAZTMAZFig.18. StraindistributioninLFW-1justbeforefracture.E.Dalgaardetal. / ActaMaterialia60(2012)770780 779Author's personal copy4. Microhardness testingrevealedasoftenedareawithin2mmoftheweldcentreline.5. Fractureoftheweldedspecimensduringtensiletestingoccurred in the TMAZ within 1 mm of the recrystallizedweldzone.6. Thestraininthefracturezoneoftheweldedspecimenswasapproximatelyvetimestheaveragestraininthetested specimen. By contrast, in the as-received material,the strain in the vicinity of the fracture was about doubletheaverage.AcknowledgementsTheauthorsaregratefultoStandardAeroLimitedforthematerials usedinthis study. 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