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
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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
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