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Mechanical and tribological properties of AA7075–TiC metal matrix composites under heat treated (T6) and cast conditions

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w w w . j m r t . c o m . b r

Availableonlineatwww.sciencedirect.com

Original

Article

Mechanical

and

tribological

properties

of

AA7075–TiC

metal

matrix

composites

under

heat

treated

(T

6

)

and

cast

conditions

Veeravalli

Ramakoteswara

Rao

a,∗

,

Nallu

Ramanaiah

b

,

Mohammed

Moulana

Mohiuddin

Sarcar

c

aDepartmentofMechanicalEngineering,RVR&JCCollegeofEngineering(A),Guntur,India

bDepartmentofMechanicalEngineering,AndhraUniversityCollegeofEngineering,Visakhapatnam,India cJNTUA,Ananthapuramu,India

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received11January2016 Accepted22March2016 Availableonline18October2016

Keywords: AA7075 TiC Stircasting

Castandheattreatment(T6)

a

b

s

t

r

a

c

t

Theslidingfrictionandwearbehaviorofaluminummatrixcomposites(AA7075–TiC)have beeninvestigatedunderdryslidingwearconditions.Thealuminummetalmatrix compos-ites(AMMCs)areproducedasAA7075matrixmetalandTiCparticulatesofanaveragesize of2␮masreinforcedparticlesthroughstircasting.AMMCsstudiedarecontained2–10wt.% ofTiCparticlesinbothascastandheattreated(T6)conditions.Allthecomposites

exhib-itedbettermechanicalproperties(hardness,tensilestrengthandpercentageofelongation) thanthematrixmetalinboththeconditions.Theweartestswerecarriedoutatasliding velocityof2m/s,slidingdistance2kmandatnormalloadof20N.Thewearresistanceof thecompositesincreasedwithincreasingweightpercentageofTiCparticles,andalsothe wearratewasnotablylessforthecompositematerialcomparedtothematrixmaterial.A detailedanalysisinas-castconditionandT6conditionoftheAMMCswascarriedoutusing

SEMinordertofindouttheinfluenceofTiCparticlesintheAMMCsformed.Ithasbeen observedthatunderT6heattreatmentcondition,matrixaswellascompositeshown

sig-nificantimprovementinmechanicalandtribologicalpropertieswhencomparedinascast condition.

©2016BrazilianMetallurgical,MaterialsandMiningAssociation.PublishedbyElsevier EditoraLtda.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http:// creativecommons.org/licenses/by-nc-nd/4.0/).

1.

Introduction

Theevaluationofmetalmatrixcomposites(MMCs)hasbeen one of the major revolutions in materials from the last

Correspondingauthor.

E-mail:vrkrao112880@gmail.com(R.R.Veeravalli).

3decades.Metalmatrixcompositesareobtainingwide impor-tance inseveralareas duetoits improvedmechanical and tribologicalpropertieswhencomparedwithbasicalloys, espe-ciallyinapplicationswherestrengthtoweightandstiffnessto weightratiosareofmostimportanceinautomobile,aerospace

http://dx.doi.org/10.1016/j.jmrt.2016.03.011

2238-7854/©2016BrazilianMetallurgical,MaterialsandMiningAssociation.PublishedbyElsevierEditoraLtda.Thisisanopenaccess articleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).

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particularlyunderpoor, partialorboundarylubricated con-ditions.Withparticlesreinforcedinthematrixofaluminum alloy,this materialexhibitsgoodpotentialforresistanceto wearandconsequentlybecomesmoresuitablefor tribologi-calapplications.Aluminumbasedmetalmatrix composites (AMMCs)havebecomeaveryvaluable additiontothefield ofnewermaterialsforhighperformancetribological appli-cations.Aluminumbasedcompositesarebeingincreasingly usedinautomobiles,aerospace,marineandmineral process-ingindustries[6].

AA7075isamatrixmetalalloy,withzincasthemajor alloy-ingelement.Itisstrong,withstrengthcomparabletomany steelsand has good fatigue strength and average machin-ability,buthaslessresistancetocorrosionthanmanyother aluminumalloys.Itsrelativelyhighcostlimitsitsuseto appli-cationswherecheaperalloysarenotsuitable.Thesuperior stress corrosion resistanceofthe T173 and T7351tempers makesAA7075alogicalreplacementfor2024,2014and2017 inmanyofthemostcriticalapplications.TheT6andT651

tem-pershavefairmachinability[7].Thepropertiesofthisductile AA7075alloycanbeimprovedbyreinforcingwithmicrosized ceramicparticles[8].

MainlytwocommonmethodsaretheretofabricateAMMCs inlargescales,powdermetallurgymethod(solidstate)and casting (liquid state). There are different methods of liq-uidstateprocessingtoproducemetalmatrixcomposites[9]. These methods include stir casting [10], pressure infiltra-tion [11], pressure lessinfiltration [12]andsqueezecasting [13].Unluet al.[14]proved that themechanical properties ofaluminummatrixcompositesreinforcedbyAl2O3andSiC

throughcastingmethodwasshownbetterthanpowder met-allurgymethod.

Themechanical alloying followed by hot pressing tem-perature and pressure shown an effect on hardness and distributionofTiparticlesinAl7075–TiCnanocomposite[15]. Venkataramanobservedthatthereexistsastrongcorrelation betweenthefriction,wearandtransition(frommildtosevere wear)behavioroftheAl7075alloyandAl-MMCs[16].Kumar examinationshaveshownthatthetremendousimprovement inthe wearresistancefor Al7075alloy canbe obtainedby increasingadditionofSiC,whichrestrictsthedeformationof thealuminummatrixmaterialwithrespecttoload[17].Kumar alsoinvestigatedAl–7SialloyreinforcedwithinsituTiB2

par-ticleswassynthesized byusing saltreactionroute andthe wearresistanceofthealloyalsosignificantlyimprovedwith theadditionofTiB2particles[18].

InpresentworkeffectofTiCparticleonmechanicaland wear behavior of AMMCs are studied in as cast and heat

Fig.1–Moltenmetalinfurnace.

treatedcondition.AMMCswere producedwith2–10wt.%of TiC throughstir casting technique asmention in previous work[19].

2.

Experimental

2.1. Materials

InthepresentinvestigationAA7075asmatrixmaterialandTiC particulatesofanaverageparticlesizeof2␮misusedasa rein-forcementmaterialandthechemicalcompositionofAA7075 alloyisshowninTable1.

2.2. Fabricationofcomposites

The composite, matrix material as AA7075 and reinforced materialastitaniumcarbide(TiC)with2,4,6,8and10wt.% TiCaboutanaverageparticulatesizeof2␮m,waspreparedby astircastingprocess.About0.750kgofAA7075matrix mate-rialwasfedintotheelectricfurnaceandwasmeltedat800◦C. TheexperimentalsetupisshowninFig.1.Themagnesium ribbonsareaddedathightemperaturestoincreasethe wet-tabilityofaluminumsothatthereinforcementaddedtothe metalisevenlydispersed.Anappropriateamount(2%ofthe wt.ofbasemetal)ofTiCparticulateswasthenaddedslowly tothemoltenmetal.TheTiCparticulatesaddedtothemolten metal waspre-heatedup to300◦C toremovethemoisture (if any)init.Simultaneously,the moltenmetalwasstirred

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Fig.2–Specimensofcomposite(a)castand(b)wear.

thoroughlyataconstantspeedof300rpmwithamechanical stirrer.Aftercastingthecompositespecimenswereobtained inthedimensionofØ15mm×150mmasshowninFig.2a.The sameprocedurewasfollowedtogettheAMMC’sofdifferent weightpercentages–4%,6%,8%and10%[19].

2.3. Heattreatment(T6)

Matrixmaterialanditscompositeswereheattreatedandaged atT6condition.TheT6temperisdonebyhomogenizingthe

materialat450◦Cfor2h,andthenagingat121◦Cfor24h.

2.4. Hardnesstest

Thehardnesstests were carriedout according totest pro-cedureIS1501:2002ASTMstandardsusingVickershardness testingmachinewitha10mmdiamondindenterand5kgload for25s.Thetestwasconductedatroomtemperature(28◦C) andthemeasurementofhardnesswastakenatthreedifferent placesoneachsampletoobtainanaveragevalueofhardness. 2.5. Tensiletest

AspertheASTMB557:2006standard,thetensilestrengthwas evaluatedonthecylindrical rodofas-castand heattreated condition(T6)composites.Thetensilespecimenswereshown

inFig.4withgagedimensionsof50mminlengthand10mm indiameter,respectively.Theemerypaperswereusedto pol-ishthe testspecimensinorder todecrease themachining scratchesandtheeffectsofsurfacedefectsonthesample.The universaltestingmachine(FIE/UTN-40)wasusedtoconduct thetensiletest.Thetensilestrengthwasevaluatedatcross headspeedof2mm/min.

2.6. Weartest

A wear and friction monitor shown in Fig. 3was used to investigatethedryslidingwearbehaviorofAMMCs.Standard wearpinspecimensof8mmdiameterand30mmheightfor weartestwerepreparedfromtheabovecompositesandwere retrievedthroughwirecutEDMprocessasshowninFig.2band polishedmetallographically.Theweartestshavebeen con-ductedunderheattreatedandcastconditionatafixedsliding

Fig.3–Wearandfrictionmonitor.

Fig.4–Tensilespecimens.

velocityof2.00m/s.Eachweartesthasbeencarriedoutfora totalslidingdistanceofabout2km.

2.7. Scanningelectronmicroscopy(SEM)

The wearmechanisms ofthe compositeswere established byscanningelectronmicroscopic(SEM)analysisofthe sur-facemorphologyofthetestsamples.Thescanningelectron microscopy(model:SEM–Quanta400,FEI,Netherlands)with EDAXenergydispersiveX-rayspectroscopy(EDS)wasusedin

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ordertoevaluatethemorphologicalchangesandthe elemen-talanalysisofcomposites.

3.

Result

and

discussion

3.1. Hardness

TheVickershardnesstestresultsofAA7075matrixmaterial andtheircompositescontainingTiCparticles(0–10wt.%)were presentedintheformofgraphandshowingtherelationship betweenthehardnessvaluesand%ofreinforcementofTiCat castconditionandheattreated(T6)conditioninFig.5.Itcanbe

observedthathardnessofallthecompositeswassignificantly higherthanthatofthematrixmaterialcharacterizedtothe hardnatureofTiCparticles.However,thehardnessdropsafter 8%TiCaddition.Thisincreasetrendwasobservedinascast andheattreatedconditionsfrom98.4VHNto118.6VHNand 181VHNto202.1VHNforAA7075matrixmaterialto8wt.% ofTiCreinforcedcomposite,respectively.Somevariationsin thetrendcouldbeduetotheagglomerationandnon-uniform distributionoftheparticlesinthemetalmatrix.Thehardness valuesof2,4,6,8and10wt.%TiCaremarkedlyhigherthanthat oftheAA7075.Duetotheincreasedstrainenergythehardness ofthecompositesisincreasedattheperipheraloftheparticles dispersedinthe matrix [20]. Thescanning electron micro-graphof8wt.% and10wt.%ofTiCreinforced compositeat heattreatedconditionwasshowninFig.6.Itshouldbenoted thatat8wt.%ofTiCreinforcedcomposite(Fig.6a)theTiC par-ticlesaredisperseduniformlyintheAA7075matrixmaterial andat10wt.%ofTiCreinforcedcomposite(Fig.6b)apartial agglomerationcanbedetectedinsomeregions.However,by increasingthe wt.%ofreinforcementatbothcastandheat treatedconditions,thehardnessandtensilestrengthofthe

reinforcement. 6.5 7 7.5 8 8.5 9 9.5 10 10.5 11 10 8 6 4 2 0 % of elongatio n % of reinforcement

Cast condition Heat treated condition (T6)

Fig.8–Variationofpercentageofelongationwith%of reinforcement.

10wt.%TiCcompositeisnotincreasedremarkably.Thismay bedemonstratingthattheeffectofagglomerationat10wt.% TiC reinforced composite, which leads to the formationof pores[21].

3.2. Tensileproperties

TheeffectofTiConthetensilestrengthandthepercentage ofelongationofthecompositeobtainedfromtensiletestare shown inFigs.7and8. Thetensile strength wasimproved withaddingTiCparticle.About130MPastrengthincreasewas determined for 8wt.% TiC reinforcementfollowing T6 heat

treatment. Thepercentageofelongationwas reducedwith increasing% ofTiC particlesinboththe conditions.Result oftensiletestsalsorevealedthat theadditionof reinforce-mentsignificantlydecreasedthepercentageoftheelongation from 8.341to7.140. TheadditionofTiC particles improves themechanicalpropertiesmainlybystresstransferencefrom

a

b

TiCp

TiC Agglomeration

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1 1.2 1.4 1.6 1.8 2 2.2 10 8 6 4 2 0 Wear rate, m m 3 /Km % of reinforcement

Heat treated Cast

Fig.9–Effectofwt.%ofreinforcementonwearrate.

0.2 0.22 0.24 0.26 0.28 10 8 6 4 2 0 Coefficent of friction % of reinforcement Heat treated Cast

Fig.10–Effectofwt.%ofreinforcementoncoefficientof friction.

the aluminum matrix to the reinforced particles TiC. This is because of Orowan mechanism by which a dislocation bypassesimpenetrableobstacleswhereadislocationbowsout considerablytoleaveadislocationlooparoundaparticle.A similar naturehas been observedbyMurali et al. [22]and O ˜noro[23]bythestresstransferencefromaluminummatrix toreinforcedparticlesTiO2andTiB2,respectively.The

interac-tionbetweenthedislocationsandTiCresultsinanimproved instrength[24].Thetensiletestresultsforascastcondition generallylowerthantheheattreatedconditionvaluesofthe samecompositionsduetothegoodhardnessatheattreated condition.

3.3. Wearbehavior

Thewearrateofthe AA7075matrix materialand its com-positeswithaddingTiCparticlesatheattreatedandascast conditionisshown inFig.9.Thewearrate decreases with increasingTiC contentanditwasfoundtobeminimumat 8wt.%TICascomparedtoothercompositions.Thismaybe attributedtotheincorporationofhardTiCparticlesresulting inanimprovementinthehardnessandreductioninrealarea ofcontact.Sincerealareaofcontactistakenastheratioof thenormalloadtothehardnessofthepinmaterial,therefore, wearratedecreaseswithdecreasingrealareaofcontact[25]. Beyond8wt.%,thewearrateincreasesanditwasfoundtobe lessthanthatofthematrixmaterial.Thisreductioninwear ratedueto10wt.%ofTiCcontentmaybe,thereby increas-ingtheporosityandcracksanddeteriorationofmechanical properties[26].Insummary,foragivenwt.%ofreinforcement, theheattreatedconditionoffershigherwearresistance. 3.4. Coefficientoffriction

Fig.10showsthevariationofaveragecoefficientoffriction overtheslidingdistancewithpercentageofreinforcement.It isobservedthattheaveragecoefficientoffrictiondecreases

with increasing wt.% TiC. However, the composites have shown a lower coefficient of friction as compared to that observed AA7075 matrix material. Thismay be dueto the higherhardnessofthecompositeresultinginlowerrealarea ofcontactandtherefore,smallernumberofjunctionswhich requirelessenergytogetshearedduringslidingascompared tothematrix.Itshouldbeobservedthat theeffectof%of reinforcement at heat treated and cast condition over the coefficient offriction are ofthe same nature, i.e.inversely proportional.

3.5. Wearsurfaceanalysis

Figs.11and12showSEMmicrographsat50magnificationsof thewornsurfacesofAA7075matrixmaterialandAA7075/TiC compositesinascastandheattreatedconditions.Wearrate isafunctionoftheamountofTiCparticlesincompositesand areeffectivetoenhancethewearresistanceofthecomposites inboththeconditions.

The interactions between dislocationsand TiC particles resist thepropagation ofcracksduringsliding wear. Strain fieldsarecreatedaroundTiCparticlesduetothethermal mis-matchbetweenthealuminumalloyandTiCparticleduring solidification.Thosestrainfieldsofferresistancetothe prop-agationofthecracksandsubsequentmaterialremoval.The homogeneousdistributionofTiC particlesprovidesOrowan strengthening [27]. The clear interface and good bonding delaythedetachmentofparticlesfromthealuminummatrix. Therefore,thewearresistanceoftheAMMCsisenhancedby TiC particles.Thegrain refining actionofTiC particles can furtherbeconsideredtoplayaroleinloweringthewearrate. AsshowninFig.11a,exhibitsdeeplonggroovesalongthe directionofslidingandmaterialdelaminationwereobserved onthewornsurfacesofmatrixmaterial(AA7075).At2and 4wt.%ofTiCparticlescomposites(Fig.11bandc)would con-firmthatmicro-ploughinganddelaminationarethetwomain dominantwear mechanisms.Ploughing isthe processesof displacingmaterialfrom groovetosideways toformridges adjacenttothegroveproducedandthusrepeatedslidingof hardasperityleadstometalloss[18].Byincreasingtheweight percentageofTiCto6and8wt.%,lessdeepgroovescanbe observed(Fig.11dande)andthesurfaceismuchsmoother comparedtotheoneitwasobserved(Fig.11a–c).Thesurface ofAA7075reinforcedbyTiCof10wt.%demonstratesrougher wornsurfacethanAA7075/8wt.%TiC.AtthisamountofTiC the debris are abletoeasily separateout from the surface whichcanbejustifiedbythepresenceofpores,asshownin Fig.11f.Thiscanbethemainreasonforincreasingofwear lossandCOFathighweightpercentageofTiC[8].

Fig.12ashowswithcoarserandthelargegroovesinthe matrix material indicating anabrasive wear. By increasing reinforced materialTiC from 2to4weightpercentage, the groovesarereducedandsomesmoothweartrackscouldalso beseeninFig.12bandc.Inthecompositewith6and8wt.%of reinforcement,smallsizegroovesareseenwithoxidepatches and discontinuous ridges(Fig. 12dand e) [28]. Itisfurther evidentfromthefigurethatthedepthofsubsurface deforma-tion reduceswhenTiCparticulatecontentisincreased.But furtherincreasesinreinforcementto10wt.%(Fig.12f)leadsto thedecreaseinthewearresistance.

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Fig.11–SEMmicrographsofwornsurfaceof(a)AA7075,(b)2wt.%TiC,(c)4wt.%TiC,(d)6wt.%TiC,(e)8wt.%TiC,and (f)10wt.%TiCcompositesatcastcondition.

Fig.12–SEMmicrographsofwornsurfaceof(a)AA7075,(b)2wt.%TiC,(c)4wt.%TiC,(d)6wt.%TiC,(e)8wt.%TiC,and (f)10wt.%TiCcompositesatheattreatedcondition.

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4.

Conclusions

Inthepresentwork,AA7075/TiCAMMCswerepreparedbythe stircastingprocessandtheeffectofTiCparticulatecontenton mechanicalandtribologicalpropertiesofthepreparedAMMCs wereinvestigatedbothascastandheattreatedconditions. Theresultscanbesummarizedasfollows:

• Thecomposites atcastconditionexhibits lesshardness, tensilestrengthandwearresistancethanheattreated(T6)

condition,duetofailuretoformgoodhardening.

• Themechanical(hardness,tensilestrengthandpercentage ofelongation)propertiesfortheTiCreinforcedcomposite specimensarebetterthanAA7075matrixmaterialinboth conditions.

• Thehighcontent ofTiCparticles inAMMCs leadtohigh wearresistanceupto8wt.%.

• ThewearrateofAA7075/8wt.%TiCcompositeatT6

condi-tionwasfoundtobeoptimalwearratecomparetothecast conditionsandAA7075matrixmaterial.

• Wearrateandcoefficientoffrictiondecreaseswithincrease ofwt.%ofTiCreinforcedparticle.

Conflicts

of

interest

Theauthordeclaresnoconflictsofinterest.

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