AbstractAsymmetricincrementalsheetforming(AISF)could significantlyreducecostsincurredbythefabricationofcomplex industrialcomponentswithaminimalenvironmentalimpact.The AISFexperimentswerecarriedoutoncommerciallypuretitanium (Ti-Gr2),Timetal(15-3-3-3)alloy,andTi-6Al-4V(Ti-Gr5)alloy.A special testing geometrywas used to characterize the titanium alloys propertiesfromthepointofviewoftheformingzoneandtitanium structureeffect.Thestructureandpropertiesofthematerialswere assessedbymeansofmetallographicanalysesandmicrohardness measurements. The highest differences in the parameters assessedas afunctionofthesamplingzonewereobservedinthecaseofalpha-phaseTi-Gr2attheexpenseofthemostsubstantialsheetthinning occurrence.Aspringbackcausesasmallerstoreddeformationin Timetal(alloy)resultinginlesspronouncedmicrostructure refinementandmicrohardnessincrease.Ti-6Al-4Valloyexhibited early failure due to its poor formability at ambient temperature. KeywordsIncrementalforming,metallography,hardness, titanium alloys.I.INTRODUCTION OWADAYS,titaniumanditsalloysarewidelyusedin variousindustrialsectors(e.g.,aerospace,automotive, andbio-medicalmaterials)duetotheirexcellentproperties suchashighstrength,hotworkability,corrosionresistance, strength-weightratio,toughness,etc.[1][5].However,the highcostsofthetitaniumproducts,asaresultofdemanding formingandheattreatmentprocesses[1],limittheirusealso in other less sophisticated applications. This leads to extensive scientificandtechnologicalinterestindevelopingpotentially viableandeconomicallyaffordablemanufacturingmethods that aid in reducing the cost of the products.Oneofthesemethodsisanasymmetricincrementalsheet forming (AISF), based on the localized plastic deformation of theblankundertheactionofapunchtoolwhichfollowsa continuousandnumericallycontrolledpath[6][9].Main advantagesofthismethodarenodie,oronlyasimpleand cheapdieisrequired,andtheprocesscanbecarriedouton cheap machines that are often already available; this makes the process particularly suitable for low-series production.Inspiteofthehugeamountofpaperscontainingthe description and results of incremental sheet forming of various typesofmaterials[10][12],thereisalackofinformationon AISFoftitaniumalloys,especiallyfromthemicrostructure L.NovakovaiswiththeAerospaceResearchandTestEstablishment, Beranovych 130, Prague, 195 00 Czech Republic (phone: +420 225 115 499; e-mail: [email protected]).P.HomolaandV.KafkaarewiththeAerospaceResearchandTest Establishment,Beranovych130,Prague,19500CzechRepublic(e-mail: [email protected] [email protected]).and mechanical properties point of view. Titaniumisallotropicmetalwithahexagonalclosepacked (hcp)crystalstructureatlowertemperaturesandabody-centredcubic(bcc)structureattemperaturesabove882C. Generally,thetitaniumalloyscouldbeclassifiedintofour categoriesalpha(),near-alpha,alphaplusbeta(+),and beta()phasealloys[1].Thealpha-phaseandnear-alpha alloysexhibitlowerstrengthbutthebestcorrosionandcreep resistance andweldability. The alpha plus beta alloys havean excellentcombinationofstrengthandductility.Thebeta alloys(metastable)offergoodformabilityandincreased fracture toughness. This paper presents results of AISF experiments carried out onthreedifferenttitaniumalloyscommerciallypure(CP) titanium (Grade 2), Timetal (Ti-15-3-3-3) alloy and Ti-6Al-4V (Grade 5) alloy. In order to evaluate and compare behaviour of differentstructuretitaniumalloysduringAISFatambient temperatures,formingexperimentswereperformedusinga special testing geometry of the parts.Themaingoalofthispaperwastodescribetheproperties andstructureofthetitaniumalloyschosenfromthepointof view of the forming zone and titanium structure effect. II. EXPERIMENTAL A. Materials Unalloyedcommercialpuritytitanium(Grade 2),Timetal (Ti-15-3-3-3),andTi-6Al-4V(Grade5)titaniumalloysheets (TableI)of1.0mminthicknesswereusedforthe experiments.Theas-receivedsheetswereinannealed condition (700C/1h for the Grade 2 sheet 816C/5min for the Timetalalloyand790C/50minfortheGrade5sheet,allair cooled).B. Forming Experiments Two-pointAISFprocessingwasperformedinarobotic machine (1000 kg capacity) with a blank holder of dimensions of 870770mm using a ceramic wheel of 100mm in diameter. TABLE I COMPOSITION OF THE EXPERIMENTAL MATERIALS (WT.%) MaterialAlVFeCNHO+NTi Ti-Gr 2-- max. 0.3 max. 0.1 max. 0.03 max. 0.015 max. 0.25 bal. Ti-15-3*3.2914.550.100.0120.0060.0040.010bal. Ti-Gr 56.524.020.200.0060.0030.0010.174bal. *Moreover, Ti-15-3 alloy contains 3.01 wt.% Cr and 3.03 wt.% Sn Lucie Novakova, Petr Homola, Vaclav Kafka NEffect of Structure on Properties of Incrementally Formed Titanium Alloy SheetsWorld Academy of Science, Engineering and TechnologyInternational Journal of Mechanical, Aerospace, Industrial, Mechatronic and Manufacturing Engineering Vol:8, No:2, 2014 371 International Scholarly and Scientific Research & Innovation 8(2) 2014International Science Index Vol:8, No:2, 2014 waset.org/Publication/9997536 Fig. 1 Design of the formed parts used Anegativeformingconfigurationandfeedrateof1 m/min wereused.Acommercialmineraloilwasusedasalubricant duringtheprocessing.TheAISFformingtestsweremade usingspecialtestgeometry(abenchshape,seeFig.2)with varyingwallangles(max.35).Allexperimentswere performed at ambient temperature. C. Experimental Methods Thesamplesforthemetallographicanalysesandhardness measurementsweretakenfromthesignificantareas/zonesof theformedsheetshowninFig.1. Twospecimensweretaken fromtheformedflatzones(AandEzones),threefromthe corner zones (B, C, and D), one from the formed bottom zone (P), and one from the unformed part (1) of the sheet. Thesectioningofformedpartsformetallographicanalyses was performed bymeans of linear precision saw IsoMet 4000 using a blade speed of 3000 rpm and cutting rate of 4 mm/min.ThemetallographicanalyseswereassessedusingOlympus GX51opticalmicroscope.Specimenmicrostructurewas revealed by etching using solution of either the Krolls reagent (1.5ml HF, 4ml HNO3, and 94ml H2O) or the Wecks reagent (2gNH4HF2,25mlethanol,and100mlH2O).Theaverage grain sizes were measured by chord intercept method. Vickers microhardness measurements HV1 were performed onthepolishedcross-sectionsurfacesofthemetallographic samplesaftermetallographicanalyses.Microhardnessofthe sampleswasevaluatedusingtheWolpertWilson402MVD microhardness tester. Fig. 2 The arrangement during the forming experiments III.RESULTS AND DISCUSSION A. Forming Trials Fromthepointofviewofformability,TiGrade2(Ti-40) materialshapehasbeenproducedsuccessfullywithoutany problemsordefects(e.g.,bulge,wrinkleortear).Similarly, Timetal(15-3-3-3)alloyhasbeeneasilyformed;however,a muchhigherspringbackoccurredafterreleasingofthesheet clamping[13]. Thisbehavioriscausedbyhighyieldstrength and low Young's modulus of this alloy. In the case of Ti-6Al-4V (Grade 5) alloy, even the material wasintheannealedcondition,thesheethasshownearly failureduetopoorformabilityofthismaterialatroom temperature [14].B. Microstructure Characterization The microstructure comparison from selected zones of each material is shown in Figs. 3 to 5. The deformed microstructure ofthecornerzone(B)andflatzones(AandE)were compared with the unformed initial state of the sheet. Asatypical-typetitaniumalloy,CPtitaniumexhibits twins in deformed sheet areas (Fig. 3), due to hexagonal close packed(hcp)crystalstructureandthelackofsufficientslip systemstoaccommodatetheimposedstrain.Therefore, twinningispreferabledeformationmechanismforthehcp metals.TheformedbottomzonePcontainsalmost undeformedgrainsandthemicrostructureissimilartothe microstructureoftheinitialunformedzone(Fig.3(a)).The mostdeformedzonesoftheAISFsheetaretheflatzonesA andE(withhighslopeofthetestgeometrydesign) accompanied by the formed corner zones B (Figs. 3 (b)-(d)).InthecaseofTi-15-3-3-3titaniumalloy,thedeformation mechanism is different as compared to the CP Ti. Ti-15-3-3-3 alloyisakindofmetastable-titaniumalloywiththebody-centeredcubic(bcc)crystalstructureanddeformbyaslip. FromFig.4,agrainsubdivisionandfragmentationcouldbe seen,butnodeformationorshearbandsandsliplinesare visible by reason of etching reagent used (Kroll's reagent). The extensionofthemicrostructurechangesasafunctionofthe zoneareaintheAISFTi-15-3-3-3alloyissimilartotheCP titanium sheet microstructure. The most deformed zones of the sheet are the flat zones A and E (Figs. 4 (b), (d)), and the least deformed area is the formed bottom zone P containing almost undeformed grains. Asthe+phaseTi-6Al-4Valloyexhibitedearlyfailure, differencesofthemicrostructurechangeinvariouszonesof theAISFsheetarequitesmall.Theunformedzoneshowsa typicalannealedrecrystallizedmicrostructurewithequiaxed -phase and intergranular -phase (Fig. 5 (a)). The - grains boundaries are not completely defined. They are only outlined by the -phase due to a - and -phase interface contrast. The otherzones,exceptthebottomzonePthatwasnotreached due to earlyfailure, contain a deformedmicrostructurewith a slightlysmallerspacingof-phasegrainsascomparedtothe unformed zone. The early failure occurred in the zone E, i.e. in the zone with the mostly deformed microstructure and, hence, with the minimum grain size. World Academy of Science, Engineering and TechnologyInternational Journal of Mechanical, Aerospace, Industrial, Mechatronic and Manufacturing Engineering Vol:8, No:2, 2014 372 International Scholarly and Scientific Research & Innovation 8(2) 2014International Science Index Vol:8, No:2, 2014 waset.org/Publication/9997536 Fig. 3 Microstructure of the CP titanium in different zones AISF formed part. Etched by the Wecks reagent (magn. 500) Fig. 4 Microstructure of the Timetal (Ti-15-3) alloy in different zones AISF formed part. Etched by the Krolls reagent (magn. 200) World Academy of Science, Engineering and TechnologyInternational Journal of Mechanical, Aerospace, Industrial, Mechatronic and Manufacturing Engineering Vol:8, No:2, 2014 373 International Scholarly and Scientific Research & Innovation 8(2) 2014International Science Index Vol:8, No:2, 2014 waset.org/Publication/9997536 Fig. 5 Microstructure of the Ti-6Al-4V alloy in different zones AISF formed part. Etched by the Krolls reagent (magn. 1000) Itcouldbesummarizedthatthemostdeformedzonewith thefinestgrainmicrostructurecorrespondstotheflatzoneE withhighslopeofthetestgeometrydesign.Thiswasalso confirmed by grain size and thickness measurements. The results of thegrain sizemeasurement (measured in the directionofthickness)aresummarizedinFig.6.Fromthis graph it is obvious that the finest grain microstructure could be observedintheformedflatzonesAandEforallexamined materials. Fig. 6 Grain size measurement results for all three materials in different AISF deformation zones Onthecontrary,thezoneswiththemostcoarsegrain microstructurearetheunformedzoneandtheformedbottom zone P. This corresponds well with the microstructures shown in Figs. 3 to 5. C. Thickness Measurement Whenthegrainsizeiscomparedtothethickness measurementresults(relativethicknessvalue)inthe individualzones(Fig.7),itisobviousthatthegrainsize decreaseswiththedecreasingthicknessofthesheet.Onlyin thecaseoftheGrade5alloy,thethicknessofthesheetis almost the same for whole piece due to its early failure.Themaximumthinningoccursinthemostdeformedzones ofthepart,i.e.,thezonesAandE.InthesezonesoftheCP titaniumpartthesheetthicknessdecreasesabout20 %with regardtotheinitialsheetthickness.Similarlytothe microhardness results (Fig. 8), the thickness of the sheet in the zone C remains unchanged as compared to the initial thickness value. These facts also prove that no considerable deformation occurs in the zone C. FromthegraphsinFig.7,itisclearthatthehighest differencesinthethicknessoftheprocessedsheetasa functionofthesamplingzonewereobservedinthecaseof alpha-phase CP titanium (55% grain size change as compared totheinitialsheetthickness).Thesmallerstoreddeformation intheTi-15-3-3-3alloy,resultinginlesspronounced microstructurerefinement(25%forgrainsize),isprobably given by the significant springback occurring in this alloy. A 8 C u L G S 1 1 1AvWorld Academy of Science, Engineering and TechnologyInternational Journal of Mechanical, Aerospace, Industrial, Mechatronic and Manufacturing Engineering Vol:8, No:2, 2014 374 International Scholarly and Scientific Research & Innovation 8(2) 2014International Science Index Vol:8, No:2, 2014 waset.org/Publication/9997536 Fig. 7 Grain size and thickness comparison in different zones of CP Ti (top), Timetal (middle) and Ti-6Al-4V (bottom) AISF parts Fig. 8 Comparison of the microhardness of all three materials in different AISF deformation zones D. Microhardness Measurement Themicrohardnessmeasurementresults(Fig.8)arein goodagreementwiththegrainsizeandsheetthickness measurementsthehighestmicrohardnessvaluecorresponds tothelowestgrainsizeandthesheetthicknessvalues measured,andviceversa.Also,thehighestdifferencesinthe microhardnessoftheprocessedsheetasafunctionofthe samplingzonewereobservedinthecaseofCPtitanium.As could be expected, the least differences in microhardness were obtained in the early failed Ti-6Al-4V alloy.FromthecomparisonplotinFig.8,itisobviousthatTi-6Al-4V(Grade5)alloyexhibitsthehighestmicrohardness values and, conversely, the lowestmicrohardnessvalueswere measured in CP Ti (Grade 2) samples. IV.CONCLUSION Threetitaniummaterials,commercialpurity(Grade2), Timetal(Ti-15-3-3-3)andTi-6Al-4V(Grade5)alloysheets wereincrementallyformedusingspecialtestinggeometryat ambient temperature. Theresultsofthisinvestigationcouldbesummarizedas follows: Theparametervaluesevaluated(suchasgrainsize,sheet thicknessandhardness)aredependentonthesampling zone of the AISF part. From the point of view of the metallographic analyses, the mostdeformedzonewiththefinestgrainmicrostructure correspondstotheflatzonesAandEwiththehighest slope of the test geometry design. Thehighestdifferencesintheparametersassessedasa functionofthesamplingzonewereobservedinthecase ofalpha-phaseCPtitanium.However,themost substantialsheetthinningoccursinthealpha-phase material. AsmallerstoreddeformationinTimetal(alloy) resultinginlesspronouncedmicrostructurerefinement andmicrohardnessincrease,isprobablygivenbythe significant springback occurring in this alloy. InthecaseofTi-6Al-4V(Grade5)alloy,eventhe A 8 C u L k G S 1 G A 8 C u L k G S 1 1 1A 8 C u L G S 1AV G A 8 C D L M nVS 1Av C 1 1World Academy of Science, Engineering and TechnologyInternational Journal of Mechanical, Aerospace, Industrial, Mechatronic and Manufacturing Engineering Vol:8, No:2, 2014 375 International Scholarly and Scientific Research & Innovation 8(2) 2014International Science Index Vol:8, No:2, 2014 waset.org/Publication/9997536 materialwasintheannealedcondition,thesheethas shownearlyfailureduetopoorformabilityofthis material at room temperature. ItcouldbesummarizedthatregularAISFparametersare suitableonlyforthealpha-phasetitaniumalloys.Thebeta-phaseand+alloysneedtobedeformedathigher temperaturesinordertopreventthespringbackandcracking occurrence, respectively. The results of the experiments and analyses carried out will beusedforfurtherAISFdevelopmentandfiniteelement analyses. ACKNOWLEDGMENT Theresearchleadingtotheseresultshasreceivedfunding fromtheEuropeanUnionSeventhFrameworkProgramme (FP7/2007-2013)undergrantagreementn266208.The partnersfromAirbus(St.Eloi,France)areacknowledgedfor performing the forming experiments. REFERENCES[1]M. J. Donachie, TitaniumA Technical Guide. 2nd ed, ASM, 2000, ch 1. [2]R.R.Boyer,AnOverviewontheUseofTitaniumintheAerospace Industry, Mater. Sci. Eng., vol. A 213, pp. 103 114, Aug. 1996. [3]I. J. Polmear, Light Alloys from Traditional Alloys to Nanocrystals. 4th ed., Elsevier, 2006, ch. 1. [4]M.Niinomi,MechanicalPropertiesofBiomedicalTitaniumAlloys, Mater. Sci. Eng., vol. A243, pp. 231236, March 1998. [5]Z.Okazakietal.,CytocompatibilityofVariousMetalsand Development of New Titanium Alloys for Medical Implants, Mater. Sci. Eng., vol. A243, pp. 250256, March 1998. [6]W.C.Emmens,G.Sebastiani,andA.H.vandenBoogaard,The technologyofIncrementalSheetFormingABriefReviewofthe History, J. Mater. Process. Tech., vol. 201/8, pp. 981997, June 2010. [7]G.Hirt,etal.,FormingStrategiesandProcessModellingforCNC IncrementalSheetForming,CIRPAnn.Manuf.Technol.,vol.53,pp. 203206, Jan. 2004. [8]B.T. Araghi, et al., Investigation into a New Hybrid Forming Process: IncrementalSheetFormingCombinedwithStretchForming,CIRP Ann. Manuf. Technol., vol. 58, pp. 225228, Jan. 2009. [9]K.Jackson,andJ.Allwood,TheMechanicsofIncrementalSheet Forming, J. Mater. Process. Tech., vol. 209, pp. 11581174, Feb. 2009. [10]J.Jeswiet,AsymmetricIncremental SheetForming,Adv.Mater.Res., vol. 6-8, pp. 3558, May. 2005. [11]Z.Liu,Y.Li,andP.A.Meehan,ExperimentalInvestigationof MechanicalProperties,FormabilityandForceMeasurementfor AA7075-OAluminumAlloySheetsFormedbyIncrementalForming, Int. J. Precis. Eng. Manuf., vol. 14, pp. 18911899, Nov. 2013. [12]Q. Zhang et al., Influence of Anisotropy of the Magnesium Alloy AZ31 SheetsonWarmNegativeIncrementalForming,J.Mater.Process. Tech., vol. 209, pp. 55145520, Jan. [13]L.Novakova,P.Homola,V.Kafka,MicrostructureAnalysisof Titanium Alloys after Deformation by Means of Asymmetric Incremental SheetForming,Manuf.Technol.,vol.12,No.13,pp.201206,Dec. 2012. [14]G.Lutjering,InfluenceofProcessingonMicrostructureand MechanicalPropertiesof(ab)TitaniumAlloys,Mater.Sci.Eng.,vol. A243, pp. 3245, March 1998. World Academy of Science, Engineering and TechnologyInternational Journal of Mechanical, Aerospace, Industrial, Mechatronic and Manufacturing Engineering Vol:8, No:2, 2014 376 International Scholarly and Scientific Research & Innovation 8(2) 2014International Science Index Vol:8, No:2, 2014 waset.org/Publication/9997536
