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Original
Article
Microstructural
characterisation
and
mechanical
properties
of
dissimilar
AA5083-copper
joints
produced
by
friction
stir
welding
Gihad
Karrar
a,∗,
Alexander
Galloway
a,
Athanasios
Toumpis
a,
Hongjun
Li
b,
Fadi
Al-Badour
caDepartmentofMechanical&AerospaceEngineering,UniversityofStrathclyde,GlasgowG11XJ,UK
bFacultyofMechanicalEngineeringandAutomation,ZhejiangSci-TechUniversity,XiashaHigherEducationZone,Hangzhou310018,
PRChina
cMechanicalEngineeringDepartment,KingFahdUniversityofPetroleumandMinerals,Dhahran,31261,SaudiArabia
a
r
t
i
c
l
e
i
n
f
o
Articlehistory: Received10May2020 Accepted16August2020 Keywords: IntermetalliccompoundsFrictionstirwelding
Mechanicalproperties
Dissimilarjoining
AA5083
Commerciallypurecopper.
a
b
s
t
r
a
c
t
Thisworkaimstostudytheinfluenceofthetoolrotationalspeedandtooltraversespeed
ondissimilarfrictionstirbuttweldson3mmthickAA5083tocommerciallypurecopper
plates.Complexmicrostructureswereformedinthethermo-mechanicallyaffectedzone,
inwhichavortex-likepatternandlamellarstructureswerefound.Severalintermetallic
compoundswereidentifiedinthisregion,suchasAl2Cu,Al4Cu9andthesedevelopedan
inhomogeneoushardnessdistribution.Thehighestultimatetensilestrengthof203MPaand
jointefficiencyof94.8%wereachievedat1400rpmtoolrotationalspeedand120mm/min
traversespeed.Placingthesoftermaterial(aluminium)ontheadvancingsideproducedan
excellentmetallurgicalbondwithnorequirementfortooloffsetting.
©2020TheAuthors.PublishedbyElsevierB.V.Thisisanopenaccessarticleunderthe
CCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).
1.
Introduction
Dissimilar aluminium/copper joints are commonly used
wherethereareadvantagesinpartiallyreplacingthecopper
withaluminiumforcertainengineeringapplicationssuchas
electricalconnectors,tubesofheatexchangers,transformer’s
foilconductorsandcapacitorfoilwindings[1–4].Thisislargely
drivenbythesimilaritiesintheelectricalpropertiesofeach
metal.Beyondthis,thereducedcostandthelowermassof
alu-miniumrendersitanattractivepartialsubstituteforcopper
∗ Correspondingauthor.
E-mail:[email protected](G.Karrar).
[5–7].Incircumstanceswherethereisaneedtoweldthe
sub-stitutealuminiumparttopurecopper,significantchallenges
emergeasaresultofthedifferencesinchemicalcompositions
and the mismatch inthe physicaland mechanical
proper-ties ofeach metal[8–10]. Basedon therelativeadvantages
againstconventional fusionwelding processes, i.e.absence
ofporosity,solidificationcracking,oxidationandlower
dis-tortion[11–14],thefrictionstirwelding(FSW)techniquecan
replacetheconventionaljoiningmethodsduetoitscapability
toproducedefect-freeweldondissimilarmetalsandalloys
[15–17].
https://doi.org/10.1016/j.jmrt.2020.08.073
2238-7854/©2020 The Authors. Publishedby Elsevier B.V. This is anopen access articleunder the CC BY-NC-ND license (http://
Ouyanget al. [18] studiedthe microstructuralevolution
duringfrictionstir buttwelding ofAA6061-T6tocopper,in
whichthelatterwasplacedonthe advancingside(AS).As
ageneralnote,theyclaimedthatFSWofaluminiumto
cop-perwasdifficultduetotheformationofbrittleintermetallic
compounds(IMCs)whichledtopoormechanicalproperties.
Intheirstudies[18],thedissimilarmechanicallymixedzone
i.e.stirzoneandthermo-mechanicallyaffectedzone(TMAZ)
exhibitedacomplexmicrostructurewithseveralIMCssuch
asAl2Cu,CuAlandAl4Cu9.Howeverdistinctive
microhard-nesslevelswereobservedatthestir zone,andtherewasa
lackoffurtherinvestigation onthe joint mechanical
prop-erties. Abdollah-Zadehet al. [19]studied the effectof tool
rotationalspeedandtooltraversespeedonthemicrostructure
andmechanicalpropertiesofthejointbyonlyconsideringthe
lapconfiguration.TheexistenceofAl2Cu,CuAland Al4Cu9
IMCswasalsoconfirmedasintheaforementionedstudy[18].
Thestudy[19]concludedthatasuitablerotationaltotraverse
speedratioresultedinmaximisingthejointmechanical
prop-erties.Furthermore,therelationshipbetweenIMCformation
andjointmechanicalstrengthhasbeeninvestigatedbyXue
etal.[20]atdifferenttooloffsets,toolrotationalandtraverse
speedsofdissimilarAA1060aluminiumtopurecopperjoints.
Noticeable improvements in the ultimate tensile strength
(UTS)wereobservedwhentheIMCthicknessincreased,
espe-cially at the interface between the aluminium base metal
andthestirzone.Theirfindings[20]wereinagreementwith
otherstudies [21–24],whereas otherpublishedwork [25,26]
proposedthatthejointmechanicalstrengthdependedonthe
volumefraction,geometryanddistributionoftheIMCs.
Anadditionalkeyfactorthataffectsthejointmechanical
propertiesinFSWofaluminiumtocopperistheplacementof
eachworkpiece[27].Numerousstudiesnotedthatdefect-free
buttjointsbetweenaluminiumtocoppercouldbeproducedby
placingthehardermaterial(copper)ontheAS[28–30].
Accord-ingtothesestudies[28–30],placingthecopperontheASleads
tosuitablemixingbetweenthealuminiumandcoppersinceit
iseasierforthesoftermaterial(aluminium)toflow.However,
tooloffsettingtowardseitherretreatingoradvancingsidewas
usuallyrequiredtoachievedefect-freejoints[31].Thevarious
rangesreportedforthetooloffsetsresultedinthis method
beingimpracticalforindustrialuse.
Incontrast,otherresearchersreportedthatsoundjoints
maybeobtainedbyplacingthesoftermaterial(aluminium)
onthe advancing side [32–35]. For example, Tanet al. [32]
successfullyjoined3mmthick5A02aluminiumtocopperby
placingthealuminiumontheadvancingsideandnegligible
tooloffsettowardstheadvancingside.Toolrotationalspeedof
1100rpmand20mm/mintooltraversespeedwerethe
weld-ingparametersthatresultedinahighUTSof130MPa(75.6%
jointefficiencyrelativetothealuminiumbasemetal).
Accord-ingtotheirfindings[32],thepresenceofathinandcontinuous
layerofIMCswasobservedatthealuminium/copperinterface.
TheformationoftheseIMCswasalsodetectedinsidethestir
zoneandresultedinaninhomogeneoushardness
distribu-tionacrosstheweld.Further,theynotedthatachanneldefect
developedatahighertooltraversespeedof40mm/min.
Despitethe advantageson thejoint mechanical
proper-ties when placing the softer material (aluminium) on the
advancing side, limited research has focusedon this
con-Table1–Chemicalcomposition(inwt.%)ofAA5083.
Si Mg Cu Mn Zn Ti Cr Fe
0.98 4.0 0.10 0.70 0.25 0.15 0.10 0.40
Table2–Chemicalcomposition(inwt.%)of commerciallypurecopper.
Cu Ag Fe Bi Sb As Pb S
Bal 0.035 0.05 0.001 0.002 0.002 0.005 0.005
Table3–Mechanicalpropertiesofbasemetals.
Materials Yield strength (MPa)
UTS(MPa) EL(%) Avg.HV
AA5083 163.97 225.66 67.57 70
Copper 257.26 273.66 114.14 90
Fig.1–Experimentalset-up.
figuration. Additionally, the relationship between the IMCs microstructureand themechanical propertiesrequires fur-ther investigations. In this paper, the influence of tool rotationalandtraverse speedonthejoint qualityhasbeen evaluatedwhenplacingthealuminiumontheadvancingside andwithout introducingthecomplexitiesoftooloffsetting. Using this configuration,threeparametersetsofrotational andtraversespeedwereemployedtoinvestigatethe relation-shipbetweenmicrostructureandmechanicalproperties.
2.
Experimental
programme
2.1. MaterialsandFSWdetails
AfullyinstrumentedHT-JM16×8/2staticgantryFSWmachine wasusedtobuttweld150×50×3mmthickAA5083to com-mercially purecopper plates along their lengths. Asimple highstrengthsteeltooldesignwasusedforwelding.Thetool pindiameter(Dp)andpinlength(plungingdepth,Pd)were 4.5mmand2.7mmrespectively,whiletheshoulder diame-ter (Ds)was 18mm.Thechemicalcompositions, aswellas themechanicalpropertiesofthetwomaterialsarepresented inTables 1,2and 3.A schematicillustrationofthe
exper-imental set-up is shown inFig. 1. After preliminary trials,
Table4–SummaryoftheFSWprocessparameters.
TestNo. Rotational speed(rpm) Traverse speed (mm/min) /ratio (rev/mm) 1 1000 80 12.5 2 1000 100 10 3 1000 120 8.3 4 1200 80 15 5 1200 100 12 6 1200 120 10 7 1400 80 17.5 8 1400 100 14 9 1400 120 11.7
material(AA5083)attheadvancingsidewith0mmtool off-set,therefore,themainpurposeofthisworkistooptimise theprocessparameterswhendifferentrotationalandtraverse speeds(/ratio)areused(Table4).DuringFSW,thetoolwas
tiltedbyanangleof2.8◦.
2.2. Metallographicexamination
Followingwelding,samplesweremetallographicallyprepared
usingstandardmetallographictechniques.Etchingwas
per-formedusingasolutionof1gofFeCl3,10mLHCl,and100mL
distilledwatertofirstrevealthecopperside,followingwhich
theAA5083sidewasetchedfor60sinasolutionconsistingof
1gNaCland50mLH3PO4dissolvedin125mLofethanol,
fol-lowedbya12sstepusingWecks’stint(4gofKMnO4and1gof
NaOHdissolvedin100mLofdistilledwater).Thereafter,each
etchedsamplewasexaminedwiththeaidofhigh-resolution
opticalmicroscopy.Interms ofcompositionalanalysisand
phaseidentification,energydispersivespectroscopy(EDS)and
X-raydiffraction(XRD)wereperformedatvariouslocationsof
theweldjoint.XRDwasperformedwitha40-mAoperating
current,40-Kvvoltageand1.5406-ÅCuK␣radiation.A
scan-ningrateof0.02deg./stepwithintherangeof20◦<2<100◦
wasusedthroughout.
2.3. Mechanicaltesting
Thejointmechanicalstrengthwasinvestigatedinaccordance
withASTM-E8,detailsofwhichareshowninFig.2.The
tech-niqueofhardnessmappingwasalsoemployedtoestablishthe
relationshipbetweenIMCs formationand jointmechanical
strength.
Fig.3–Effectoftoolrotationalandtraversespeedon macrostructure.
3.
Results
and
discussions
3.1. WeldappearanceandmacrostructureTable5showsthetypicaltopsurfaceweldappearanceand
cross-sectionalmacrostructuresofAA5083tocopper
dissim-ilar metal FSWjoints atdifferent welding conditions. The
symbolsforeachdefecttyperelatetoFig.3.Asexpected,the
weld surfacequalityisindicativeofthe tendencyfor
volu-metric defectstodevelop[26]. For instance,excessiveflash
formation, as well as material discontinuity,will generally
suggestvolumetricdefects[29].Fig.3summarisestheeffect
oftoolrotationalandtraversespeedontheweldappearance
andmacrostructure,inwhichAA5083wasplacedontheAS
without tooloffset.When therotationalspeed variedfrom
1000 to1400rpmand the traverse speed wasin the range
of80–120mm/min,visuallyacceptableweldswithnosurface
defectswereobtainedatthefollowingspecificparametersets:
• Lowrotationalspeedlevelof1000rpmat100mm/minand
120mm/minweldingspeeds(10and8.3/ratio)i.e.test
no.2and3respectively.
• Intermediaterotationratelevelof1200rpmand80mm/min
(15/ratio).
• Highrotationratelevelof1400rpmforthetworangesofthe
weldingspeed80,and120mm/min(17.5,and11.7/ratio),
respectively.
Parameter sets outside the above conditions led to an
unevensurfaceandtheformationofdefectssuchas
macro-crackstowardstheretreatingside(copper,Fig.4(a)),cavities
withacertaindefectareaonthecross-sectionlessthan0.02
mm2(Fig.4(b)),voids(Fig.4(c))andtunneldefectsinwhich
Fig.2–(a)DimensionsofthetensilesampleasperASTME8standards.(b)Exampleofsamplesthroughthewelding directionat1000rpmand100mm/min.
Table5–Weldappearanceandmacrostructureofdissimilarjoints(marksasinFig.3).
Symbol Defect AppearanceandMacrostructure Microstructure
None –
Crack Fig.4(a)
Cavity Fig.4(b)
Void Fig.4(c)
Tunnel Fig.4(d)
Table6–EDSresultsatweldstirzoneofTestno.2. Position Al Cu Non equilibrium solidsolution 1 75.14 24.86 – 2 62.76 37.24 – 3 90.35 9.65 Al(Cu) 4 85.84 14.16 Al(Cu) 5 61.98 38.02 – 6 68.08 31.92 – 7 67.18 32.82 – 8 79.26 20.74 – 9 37.72 62.28 – 10 35.22 64.78 –
thedefectareawaslargerthan0.05mm2(Fig.4(d)).Dueto
theinappropriatematerialflow,thesetypesofdefectshave
previouslybeenreported[31], wherethecracksare usually
relatedtotheformationoflargeIMCparticles,cavities,voids
andtunneldefects.
3.2. Microstructuralanalysis
Theweldmechanicalintegrityisdirectlyrelatedtothestir
zonemicrostructure.Aclassical“onionring”structureis
com-monlyfoundinthestirzoneofsimilarmaterialFSWjoints
[23].IndissimilarmaterialFSWhowever,aswirl-likepattern,
bandedorlamellastructure,aswellasvortex-type
microstruc-tures, are formed in the stir zone, TMAZ and also in the
heat-affectedzone(HAZ)[31].Fig.5(a)representsanexample
ofatypicalcross-section ofAA5083(AS) tocopper
dissim-ilar metalFSW jointwelded at1000rpmand 100mm/min.
Towardsthealuminiumside(Fig.5(b&e)),relativelysmall
cop-perparticleswereobservedasregularlydistributedbetween
thealuminiuminterfacezoneand theuppersurfaceofthe
stir zone. Fig.5 (c&f) illustratethat atthe stir zone,larger
copper particles (fragments) were stretched and irregularly
distributedalongthestirzoneandtowardsthebottomofthe
interfacialregionbetweenthestirzoneandthecopperside.
Theirregularcopperparticlescreatedalamellastructureof
copperandaluminiumatthebottomoftheTMAZtowards
thecopperside(Fig.5(d&g)).
EDSanalysiswasperformedtorevealthevariationonthe
aluminiumandcoppercontentat1000rpm,100mm/minand
10/v(seeTestno.2ofTable4).ItcanberevealedfromFig.6
(a)andTable6thatgoodintermixingbetweenaluminiumand
copperwasachieved.Table6alsoshowsthepresenceof
dif-ferentaluminiumsolidsolutionsatthislowlevelofrotational
speed.Fig.6(e)presentsanexampleofhowtheEDS
analy-siswasperformedtocapturethevariationonthealuminium
andcoppercontent.Unetchedmicrostructuresforthe
distinc-tiveregionsfromFig.6(a):Al-stirzone(rectangulari),inside
thestirzone(rectangularii)aswellasCu-stirzone
(rectan-gulariii),arepresentedinFig.6(b),(c)and(d),respectively.
Thedarkshadedlayerssurroundingthecopperparticlesand
fragmentsdemonstratetheformationoftheAl/Cuintermixed
regionasshownbythearrows.Thisembeddedlayerwas
pre-viouslyreportedtoaccompanytheformationofAl/CuIMCs
[29]. Moderate stirring actionwas observed on the copper
particlesandfragments,whichindicatestheabsenceofthe
Table7–EDSresultsatweldstirzonecondition7.
Position Al Cu 1 76.5 23.5 2 71.7 28.3 3 55.27 44.73 4 62.62 37.38 5 67.38 32.64 6 33.72 66.28
composite-like structureunderthis condition. Additionally, detachedcopper piecesfailedtoreact withthe aluminium matrixontheadvancingsideresultingintheabsenceofany lamellaorbandedstructuresattheinterfacezone.
Across-sectionofanAA5083tocopperdefect-freejointata higherlevelofrotationalspeedisshowninFig.7(a),as-welded
at1400rpmand80mm/minwithAA5083onASandnotool
offset.Althoughthreedistinctiveregionscanstillbeobserved
acrossthe weldjoint,theseregionswerecompletely
differ-entfromtheexampleoflowerweldingspeed,i.e.1000rpm.
Thecomplexstructurehasformedinthealuminiumsideof
the interface and towardsthe stir zone asshownin Fig.7
(b&e).Copperfragmentswere detachedfromtheretreating
sideandstirredwiththealuminiummatrixtocreatethis
com-plexstructure.Ahigherheatinputisthereasonbehindthis
complexstructure,wherethestirringactionwasinsufficient
tocreatethisstructureatalower levelofrotationalspeed.
Evidenceoftherelationshipbetweentheheatinputandthe
plasticstirringactionwasalsoobservedinsidethestirzonein
Fig.7(c&f)andthisresultedintheswirlandvortex-like
struc-ture.Unlikeothers[33–35],placingthecopperontheretreating
sidewithouttooloffsetproducedawiderTMAZattheAl/Cu
interfaceasillustratedinFig.7(d&g).
Likewise,EDSwasappliedatdifferentpositionsontheweld
zone asillustrated inFig. 8(a),whereTable 7summarises
theelementalcompositionsatthesepoints,Fig.8(e)
demon-stratesanexample oftheseEDSpoints. Table7shows the
variationinAl/Cucontentsasaresultofgoodintermixingand
theabsenceofnon-equilibriumsolidsolutions.Anenlarged
viewofrectangle(i)inFig.8(a)isshowninFig.8(b).Acomplex
structurecanbedetectedfromtheunetchedmicrostructure
ofthisregion,andthechemicalcompositionsofpoints1and
2inTable6showslightvariationinthealuminiumandcopper
contents.Thecompositelikestructureattheupperregionof
theCu-stirzone(rectangle(ii))resultedintwodifferent
con-tentsofAl/CuasillustratedinFig.8(c)andTable7.Thehigher
heatinput(1400rpm)andthetoolstirringactionformed
sim-ilarvariationinAl/CuatthebottomofCu-stirzoneasshown
inFig.8(d)ofrectangle(iii).Thedarkshadedlayersthat
sur-roundedthecopperparticlesandaccompaniedtheformation
ofAl/CuIMCsareshownbythearrows.
3.3. Interfacialelementaldiffusion
ThekeyfactortocriticallyanalysingthejointqualityinFSW
of dissimilar aluminiumtocopper is bycharacterising the
structure of the interfacial region [31]. The elemental
dif-fusion andstructureisable toconfirmareliablejoint [20].
Fig.9(a)representsamagnifiedviewoftheinterfacialregion
Fig.5–(a)Typicalcross-sectionofjointweldedat1000rpmand100mm/min.(b&e)interfacezonetowardsAA5083side. (c&f)stirzone.(d&g)interfacezonetowardsthecopperside.
1200rpmtoolrotationalspeedand80mm/mintoolwelding
speedasetchedbythealuminiumetchingsolution.
Contin-uouslayersofrefinedaluminiumgrainsareclearlyobserved.
Copperparticleswithdifferentsizesarediffusedalongwith
therefinedlayersofaluminiumasevidenceofgood
metallur-gicalbonding.Theresultantstirzone,asshowninFig.9(b),
elucidatesthatthecontinuousinterfaciallayersubsequently
leadstothelamellastructurethatsignificantlyimprovesthe
jointmechanicalstrength.
Theinterfacialregionformedacompositelikestructureas
aresultofincreasingtheheatinput,asshowninFig.9(c)of
testno.7at1400rpmtoolrotationalspeedand80mm/mintool
weldingspeed.Ithasbeenreportedpreviously[28]thatthe
resultantjointstrengthisgreatlyimprovedbythisstructure.
AsithasbeendemonstratedinFig.9(d),thiscompositelike
structurewasalsodominantinsidethestirzone.
3.4. Intermetallicphases
XRD analysiswas performedthrough the cross-sections to
identifythephasespresentinthestirzone.Fig.10presents
theXRDpatternsofthreetypicaldefect-freejointsoftestno.
2,4and7ofTable4.ThedominantIMCsonthestirzoneof
AA5083andcopperareAl2CuandAl4Cu9,andtheseare
con-firmedfromthethreepatterns,apartfromthefactthatAlCu
wasonlydetectedundertestno.7.Accordingtotheabove
microstructureanalysisoftestno.2and7,itcanbe
estab-lishedthatthenatureandquantityofIMCsareaffectedbythe
weldconditions.Peakintensitychangesbyvaryingthe
weld-ing conditions, where1000rpmand 100mm/min, 1200rpm
and80mm/minand1400rpmand80arethetoolrotational
speedand toolwelding speedoftestno.2,4and 7
respec-tively.Asobserved,thepeakintensityincreasesbyincreasing
thetoolrotationalspeed.Ithasbeenpreviouslyreported[22]
thatthevariationoftheintensitypeaksisattributedtothe
complexmixingbetweenAl-Cuoverall,whererelativelyhigh
intensitypeaksindicateahigherIMCquantity[34].
AccordingtotheAl-Cuphasediagram,theformation
tem-perature Al2Cu phaseis relatively low[18]. Therefore, it is
expectedthatAl2Cu willbepresent inthestir zoneasthe
temperatureduringweldingisknowntoreach0.8−0.9ofthe
aluminiummeltingtemperature[26],i.e.exceedingthe
for-mationtemperatureofAl2Cu.However,theIMCscannotbe
exclusivelypredictedonthebasisofanAl-Cuphasediagram,
wherethechemicalreactionsoccurringduringtheFSWunder
thethermalcyclesarefarfromtheequilibriumcondition[18].
InthecaseofAl4Cu9,thethermomechanicaleffectofFSW
explainsitsformationatthestirzone,wherethemelting
tem-peratureofthisIMCi.e.1030◦C[28]ishigherthanthepeak
temperatureduringFSW.
3.5. Microhardnessdistribution
Fig.11 demonstratestheVickershardnessdistribution
pro-filesofdissimilarjointsmeasuredacrossandatthemiddleof
theweldcross-section.Itisobservedthatthehardnessvalue
increasessignificantlyatthestirzonerelativetothebase
Fig.6–(a)SEMimageandEDSpointsattheweldzoneofTestno.2.MagnifiedviewofspecificregionsinFig.6(a):(b)region i,(c)regionii,(d)regioniii,(e)regioni.
innature[34],accompaniedwiththeformationofveryfine
recrystallisedgrainsandcopper-richdispersedparticles.
Ontheotherhand,thecombinedeffectofIMCformation
and grainrefinement duetothe recrystallisationincreases
thehardnessattheTMAZ.TheHAZindissimilarFSWof
alu-miniumtocopperismildlyaffectedbytherecrystallisation;
thisreducesthehardnessinbothHAZsides[31].Thehardness
variationsareadirectresultoftheheterogeneousdistribution
ofIMCsalongwiththesoftermaterials(aluminiumorcopper)
withinthestirzone.
3.6. Jointmechanicalstrength
Theperformanceofthedissimilarjointshasbeenevaluated
byassessingthe tensile properties.Fig. 12 showsthe yield
strength,UTSandjointefficiencyattheconditionsthat
devel-opeddefect-freejoints.Unlikeotherpublishedwork[28–30],
placingthe softermaterial(AA5083)onthe advancing side
resultedin higher tensile strength.The increase intensile
propertiescanbedirectlylinkedtothenatureandquantityof
IMCsinadditiontotheevolvedmicrostructure,whereproper
materialmixing isrequiredtoenhance the joint
mechani-cal performance[4].Moreover, significantimprovementsin
thejointtensilepropertieswereachievedcomparedtoother
studiesthatplacedthesoftermaterial(aluminium)ontheAS
[32–35].
ItisrevealedfromFig.12thattheeffectofthetool
rota-tional speed, in general, is higher than the effect of the
welding speed,asthisincreases theheatinputand
subse-quently improves the level of inter-mixing. Increasing the
welding speed for the same level of tool rotational speed
resultsinminorimprovementsinthejointUTS.The
evolu-tion ofthecomposite-likestructurethatwasproducedata
higher leveloftoolrotationalspeed(1400rpm) isthemain
reasonforthisimprovement.Thebenefitsofthisstructure
on the joint strength have also been reported previously
[26].
One ofthe mostimportantcriteria toidentifythe weld
joint performanceis byexpressing the joint efficiency, i.e.
theratiooftheweldtensilestrengthtotheworkpiecetensile
strength,whereajointefficiencylowerthan100%isgenerally
reportedduringFSWofdissimilar aluminium/copperjoints
[31].ThisefficiencyisalwaysrelativetothelowerUTSof
Fig.7–(a)Typicalcross-sectionofjointweldedat1400rpmand80mm/min.(b&e)interfacezonetowardsAA5083side.(c&f) stirzone.(d&g)interfacezonetowardscopperside.
Fig.8–(a)SEMimageandEDSpointsattheweldzoneofconditionno.7.(b)EnlargeviewofregtanulariinFig.8(a).(c) EnlargeviewofregtanulariiinFig.8(a).(d)EnlargeviewofregtanulariiiinFig.8(a).(e)EnlargeSEMimageofregtanulari inFig.8(a).
Fig.9–(a)Interfacialmicrostructureofthejointproducedattestno.4.(b)Lamellastructureinsidethestirzoneattestno.4. (c)Interfacialmicrostructureofthejointproducedattestno.7.(d)Compositelikestructureinsidethestirzoneattestno.7.
Fig.11–Hardnessdistributionundertestsno.2,4and7.
Fig.12–Yieldstrength,UTSandjointefficiencyatdifferent weldingconditions.
wereachievedattheconditionsthatyieldeddefect-freejoints.
Upto94.8%jointefficiencywascalculatedat1400rpmand
120mm/min,whichishigherthanthepreviouslyreported
effi-ciencyof75.6%whenconsideringthesoftermaterialonthe
advancingside[32].
The typical fracture surface for three different welding
conditions isshown inFig. 13.Failure occurred atthe
AS-TMAZattherelativelylowrotationalspeedof1000rpmand
100mm/minweldingspeed(Fig.13(a&d)).Thefailurelocation
graduallyshiftedtotheAS-HAZbyincreasingtherotational
speed asin Fig.13 (b&e)and (c&f) of1200rpm-80mm/min
and1400rpm-80mm/min,respectively.Thischangeinfailure
location,shiftingawayfromtheweldzoneandtowardsthe
AA5083parentmaterialisinfullagreementwiththegradual
increaseinjointefficiency,asdisplayedinFig.12.
4.
Conclusions
FSW of AA5083 to commercially pure copper was
experi-mentally investigated and the conditions that resulted in
successfuljointswere identified.Thefollowingconclusions
aredrawnfromtheresultsofthepresentstudy:
• Asuccessfulweldjointbetweenthetwodissimilar
mate-rialshasbeenachievedatdifferentrotationalandtraverse
speeds,wherethehardermaterial(copper)wasplacedat
theretreatingsidewithoutanytooloffset.
• An inhomogeneous microstructure was observed inside
andontheinterfacialzone,whencopperparticlesdetached
andintermixedwiththealuminiummatrix.
• Acompositelikestructurewasobservedatahigherlevelof
rotationalspeedandlamellaordispersedstructureswere
foundatthelowlevelofrotationalspeed.
• The predominant intermetallic compounds at the
aluminium-copperjointwereAl2CuandAl4Cu9.
• The volume fraction of the IMCs inside the stir zone
increasedbyincreasingthe toolrotationalspeed as
con-firmed by the high XRD peak intensities and higher
hardnessvalues.
• TheUTSreached203MPa,representingajointefficiencyof
94.8%ofthealuminiumalloyasaresultofthecomposite
likestructureandofanexcellentmetallurgicalbond.
Conflicts
of
interest
Theauthorsdeclarenoconflictsofinterest.
Acknowledgements
Theauthorsacknowledgethatpartofthisworkwascarried
outattheAdvancedMaterialsResearchLaboratory,housed
withintheUniversityofStrathclyde.
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