of parameters on the surface roughness of AA 6061 T6 ...

405 

AdvancesinProductionEngineering&Management ISSN1854‐6250

Volume13|Number4|December2018|pp405–416 Journalhome:apem‐journal.org

https://doi.org/10.14743/apem2018.4.299 Originalscientificpaper

Effect of process parameters on the surface roughness of aluminum alloy AA 6061‐T6 sheets in frictional stir incremental forming 

Azpen, Q.a,*, Baharudin, H.b, Sulaiman, S.c, Mustapha, F.d a,b,cUniversiti Putra Malaysia, Faculty of Engineering, Department of Mechanical and Manufacturing Engineering, Serdang, Malaysia dUniversiti Putra Malaysia, Faculty of Engineering, Department of Aerospace Engineering, Serdang, Malaysia aMiddle Technical University, Institute of Technology, Baghdad, Iraq 

A B S T R A C T   A R T I C L E   I N F O

IncrementalSheetForming(ISF)ischaracterizedbyessentialflexibility,greatformability, and low forming forces and cost compared to the conventionalsheetmetalformingprocesses.ISFwasbornasanadvancesheetmetalform‐ing process to perfectly fit previous requirements. Nevertheless, growingdemand to apply the lightweightmaterials in several fieldswas placed thisdeveloped process in a critical challenge tomanufacture thematerialswithunsatisfied formability especially at room temperature. Thus, utilizing theheatatwarmandhotconditioninsomeISFprocesseshasbeenintroducedtosolve this problem. Among all heat‐assisted ISF processes, frictional stir‐assistedSinglePointIncrementalForming(SPIF)waspresentedtodealwiththesematerials.Inthiswork,thisemergingprocesswasutilizedtomanufac‐turing products from AA6061‐T6 aluminum alloy. Experimental tests wereperformedtostudytheinfluenceofmainparametersliketoolrotationspeed,feed rate, step size and tool size on the surface roughness of the producedparts.ATaguchimethodandvaryingwallangleconicalfrustum(VWACF)testwereusedinthepresentwork.Theresultsfindthattooldiameterhasasig‐nificant impact on the internal surface roughness produced via the formingprocesswithapercentagecontributionof93.86%.Theminimumvalueofthesurfaceroughnesswas0.3µm.

©2018CPE,UniversityofMaribor.Allrightsreserved.

  Keywords:Frictionstirforming;Incrementalsheetforming(ISF);Heat‐assistedISF;Surfaceroughness;Aluminumalloy(AA6061‐T6)

*Correspondingauthor:[email protected](Azpen,Q.)

Articlehistory:Received11July2018Revised26August2018Accepted28August2018

1. Introduction 

Currently,thereisagrowingmarketinthemanufacturingofcustomized,rapidprototypingandlow‐costsheetpartswithsmalltomediumbatches(particularlyintransportation,artificialmed‐icalalternatives,andaerospaceindustries)[1,2].Themainreasonforemployingtoolrotationalspeed in SPIF is to improve the formability of lightweight and hard‐to‐formmaterials whichcharacterizedbylowformabilityatroomtemperature[3‐5].Inaddition,itleadstodecreasetheforcesviatheformingprocess[6‐8].Indeed,therearesomedrawbacksandtradeoffsthatinflu‐enceasa resultofemploying thespindlespeed inSPIF.Lowsurfacequality, lubricant failure,andhightoolwearratearethedisadvantagesoffrictionalstirincrementalformingprocess[9].Inaddition,SPIFathighrotationspeedpromotestheprobabilityfordevelopingtoolmarksontheworkedsheets[10].

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ThesurfacefinishorsurfaceroughnessisaseriousdrawbackinISF,whichlimitstheexpansionofthisprocessindifferentapplications.Toobtainabettersurfacetexture,itisimportanttocon‐trolseveralprocessesandmaterialfactorslikeformingangle,toolrotation,toolsizeandshape,stepsize,sheetthickness,andfrictionandlubricant.Thus,theresearchersconsideredtheinflu‐encesofthesemainfactorsonthefinalsurfacetopographyoftheproducedpartsinSPIF.AstudyconductedbyDuranteetal.[6]aimedtoinvestigatetheinfluenceofthetoolrotationalspeedsanditsdirectionsonthesurfacetextureofaluminumalloyAA7075‐T0formedbySPIF.Theexperimentalresultsprovedthatnosignificanteffectofthesetwoparameterswaspresentin the studied speed rangebetween0‐600rpm, and theobtainedvariedvaluesof the surfaceroughnessweremainlydependentonwhetherthetoolwasrotatedornot.

DuringSPIFof thealuminumalloyAA3003‐H14,amodelwasestablishedbyHamiltonandJeswiet[11]whichcanbeemployedtoimprovetheexternalsurfaceofmanufacturingSPIFpartsbyselectingadequateformingparametersviaaprocesssuchasfeedrateandtoolrotationspeedathighspeeds.Thispresentedmodelcanpredicttheorangepeeleffectandprovideagoodguidetoenhancethesurfacequality.Inaddition,thesurfaceroughnessforthepartswithhighrotationspeed/feedrateislessthanthoseofwithalowratio.

GoodsurfaceroughnessresultswereobtainedduringthemanufacturingofmedicalpartsbySPIFfromtheknowntitaniumalloyTi‐6Al‐4VbyOlesksiketal.[12].Theobtainedsurfacefinishoftheformedpartswereinfluencedbytheformingtoolroughnessandfrictioncaseatthetool‐sheetzone.

In fact, the final formed angle in SPIF is used as an index for both formability and surfaceroughnesswherethechangeinstretchingvalueintheformedpartleadstothechangeinboththeformingangleandsurfacefinish.Inthisway,Bhattacharyaetal.[13]investigatedtheimpactof toolsize(4mm,6mm ,and8mm),stepsize(0.2mm,0.8mm,and1mm),andwallangle(20ᵒ,40ᵒ,and60ᵒ)onthesurfaceroughnessofaluminumalloyAA5052viaSPIF.Theresultsofexperimentsshowedthatthesurfacequalityoftheformedpartsdecreasesastotheincreaseintoolsizesforallstepsizes.Inaddition,surfacefinishdecreasesduetotheincreasingoftheform‐ingangle.

PalumboandBrandizzi[14]provedthatboththesurfaceroughnessandthepart’saccuracyare influencedby the tool rotationspeedwhen the formingof the titaniumalloyTi6Al4Vwasstudied.Thespindlerotationrangeof800‐1600rpmwaswithtwovaluesofstepsizes;0.5mmand1mm.ThevalueofRabecame011.9μmcomparedtotheinitialsheetroughnessof0.5μm.Ambrogio et al. [15] performed an experimental study on three aluminum alloys, AA1050‐0,AA5754,andAA6082‐T6,withdifferentsheetthicknesses.Itwasproventhatthestepsize,form‐ingangle,andsheetthicknesshaveasignificantimpactonthesurfaceroughnessoftheshapedparts,whileithadaninsignificantimpactonthefeedrate.

Inthisregard,Silvaetal.[16]studiedtheinfluenceofboththestepsizeandfeedrateonthesurfaceroughnessofSAE1008steelmaterial.Itwasshownthatanadequateroughnesscouldbeobtainedwithafeedrateandstepsizeof8400mm/minand0.2mm,respectively.Lasunonetal.[17] examined the effect of some factors on the surface finish. Their results proved that theforming angle, step size, and its interaction affected the achieved surface texture,while therewaslittleinfluenceonthefeedrate.

The good surface finish canbe achieveddependingon the tool trajectory.Usually, the tooltrajectorywith a constant step depth leavesmarks at the end of each circle of the path and,therefore,producesapoorsurfacequality;especiallywithhighstepvaluescomparedtothespi‐raltoolpath[18].Skjoedtetal.[19]provedthatscarringcanberemovedbyusingaspiraltra‐jectoryduringSPIF.Luetal.[20]givenatoolpathalgorithmbasedonspecifiedcriticaledges.Asuperior surface roughness canbeobtainedbyusing this algorithmwith respect to the tradi‐tionaltoolpathemployedinISF.

AnempiricalresearchwasconductedbyLiuetal.[21]toinvestigatetheinfluenceoftoolsize,feedrate,stepsize,andsheetthicknessonthesurfacetextureofthefinalpartmadefromalumi‐num alloy AA7075‐T0. The response surfacemethodology and Box‐Behnken designwere ap‐pliedtoanalyzetheresults.Bettersurfaceroughnesswasachievedwithparametervaluesof25

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mmfortoolsize,6000mm/minforfeedrate,0.39mmforstepsize,and1.6mmforsheetthick‐ness.

Mugendiranetal.[22]builtaquadraticmodelwithsecondorderbasedonthreeprocesspa‐rameters(toolrotation,feedrate,andtooldiameter)toestimatetheinfluenceofthementionedvariablesonboththesurfacefinishandwallthicknessdistributionduringtheformingofalumi‐numalloyAA5052.OptimumvaluesofsurfaceroughnessRaandfinalsheetthickness(t)were2.45μmand0.753mm,respectively.Theseoptimalvalueswereobtainedatrotationspeed,feedrate,andstepsizeof1931rpm,654mm/rev,and0.65mm,respectively.

AnotherstudywasconductedbyLuetal.[23]todeterminetheimpactofthetooldesignonthesurfacequalityoffouraluminumalloysnamedAA6111,AA5052,AA2024,andAA1100.Theobtained results concluded thatbetter surface roughness couldbeachievedwithnewobliqueroller‐ball tool(ORB)ratherthantheconventional tool.TheemploymentofORBhelpedinre‐ducingthefrictionatthetool‐sheetzoneandat thesametime,reducedtheformingloadsandincreasedtheformabilityofthestudiedmaterials.

AdetailedexperimentalstudybyAzevedoetal. [24]aimedtoestimatetheeffectofseveraltypesoflubricantsonthesurfaceroughnessforsteelDP780andaluminumalloyAA1050‐T4.Itwasconcludedthattheexistenceoflubricantisanimportantfactortoobtainbettersurfacetex‐ture.Thisfindingsupportedtheresultsofpreviousstudies[25‐27].

Inthiswork,frictionstir‐assistedSPIFwasutilizedtomanufacturingAA6061‐T6sheetsthathavebeenutilizedinseveralapplicationsinindustrialsectors.Besidesthementionedbenefits,friction stir‐assisted SPIF shows superior profits,where, it does not need an exterior heatingsource and the surface finish is better than the other two heat‐assisted ISF types: electric‐assistedISFandlaser‐assistedISF.

2. Materials and methods  

2.1 Material 

Uniaxial tensile test was achieved to get the stress‐strain curve of AA 6060‐T6 sheet with athicknessof2mm.Fig.1presentsthespecimendimensionswhichareaccordingtoASTME8Mstandard.

Fig.2andTable1describethetruestress‐straincurveandthechemicalcompositionofthematerial used, respectively. It is clear that thematerial has a suitable total strain at fracture,whichispreferredinincrementalsheetmetalforming.

Fig.1Specimendimensionsoftheuniaxialtensiletest(dimensionsinmm)

Table1Chemicalcomposition(wt%)ofthematerial

Material Si Fe Cu Mn Mg Cr Ni Zn Ti AlAA6061‐T6 0.52 0.19 0.27 0.07 0.91 0.1 ‐ 0.02 0.01 97.91

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0.00 0.04 0.08 0.12 0.160

50

100

150

200

250

300

350

Tru

e st

ress

(Mp

a)

T rue strain (% ) Fig.2Thestress‐straincurveofAA6061‐T6

2.2 Experimental setup 

Thenecessarytaskofthejig,whichuseintheformingprocess,istightlyholdthesheetspecimenwith both clamping and backing plates. Forming jig includedof four clamping plates, backingplate,fourcolumnsandbaseplate.Thedimensionsofthebackingplateare170×170×20mmwithacentralholeof70mmindiameter,whichrepresenttheouterdiameteroffinalproduct.Inordertogetasmoothmaterialforming,theinnerdiameterofthebackingplatewasfilletedwith60mmradius.Ontheotherside,thealuminumsheetiswithdimensionsof150×150×2mm.ThewholejigassemblewasmountedtothebedofCNCmillingmachine(OKUMAMX45VA).Fig.3displaystheexperimentalsetupoftheformingjig.

Two forming toolswithhemispherical endswereemployed in theexperimental tasks.Thetoolsarewithtwodifferentdiameters,10mmand15mmandwithasameoflengthof110mm.Moreover, these toolswere hardened and temperedwith 60HRC andmade from high‐speedsteel(HSS)material.Inordertodecreasefrictioneffectatthecontactzonetherebyincreasethetoolslifeandsurfacequalityofthefinalproduct,tooltipswerepolished.Fig.4explainsthedi‐mensionsofthetoolsusedintheexperiments.

 Fig.3Theformingjig

Effect of process parameters on the surface roughness of aluminum alloy AA 6061‐T6 sheets in frictional stir incremental … 

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Fig.4Theformingtools(alldimensionsinmm)

2.3 Experiments 

Avaryingwallangleconicalfrustumtest(VWACF)wasusedtoachievethetestsbecauseofitshomogenousgeometrywiththesymmetricalparts[38].Theintendedmodeloftheproductwasdesigned to getmaximumdiameters (outer and inner of 70mm and 12mm, respectively), aheightof41mmandaradiuswith60mmofthevaryingslops.Fig.5explainsthedesigneddi‐mensionsofthetargetedcone.

Fig.5Theconicalprofile(dimensioninmm)

Aspiraltrajectoryoftheformingtoolwithacertainstepsizewasdesignedtogeneratethetoolpath.Thispathcanbecharacterizedbyapurestretchdeformationduringtheformingpro‐cess,whichhelpstocreateasheetthicknessthatuniformlydistributed[39].Moreover,itassiststoremovethepeaksoftheformingforcesandatthesametime,nostretchmarkscanleaveontheworkingsheetsurface.Ontheotherhand,thesesocketsregularlyhappenwithcountertype.TheCAD/CAMwasused tocreate theproductprofileandgenerate thespiral toolpathbyNCcode,asdisplayedinFig.6.

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Fig.6GeneratingthetoolpathbyCATIA

Thecontinualmotionoftheformingtoolviaformingprocess leadstoa localheatingatthecontact zone due to the local friction. In addition, this heating increases the rate of the toolswear. This will affect both surface roughness and geometric accuracy of the produced parts.Theseharmfuleffectscanbepreventedbyusingdifferenttypesoflubricants.Inthisstudy,lub‐ricant SAE 0W‐40was employed to diminish the friction effects. Taguchi technique was em‐ployedtohelpinthedesignofthetestswithaminimumnumberofrunstosavethetimeandoverallcost[40,41].Designofexperiment(DoE)whichcomprisesofselectionprocessparame‐tersandtheirinfluentiallevelsthatdependedonthepreviousstudies.Fromthesestudies,itwasconcluded these factors and their levels are extremely affected by thematerial properties. Inordertofindthecorrectandsuitableprocessparameter levelsthatcanbeusedtoobtainsuc‐cessfulsetsofexperiments,manyprimarytrialswereconducted.Fig.7(a)and(b),andFig.8(a)and(b)showthefirstfailedtrailsduetotheuseofhighrotationspeedandfeedrate,andsmalltoolsize,respectively.

(a) (b)

Fig.7Samplesfailedduetousehighlevelsofrotationspeedsandfeedrates

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(a) (b)

Fig.8Samplesfailedduetouseasmalltoolsize

Table2Processfactorsandtheirlevels

Description Factor Level1 Level2 Level3 Level4Toolrotationspeed(rpm) ω 50 400 800 1200Feedrate(mm/min) f 250 500 ‐ ‐Stepsize(mm) z 0.2 0.5 ‐ ‐Toolsize(mm) D 10 15 ‐ ‐

 Table3OrthogonalarrayL8(41.23)oftheexperimentstests

Test ω(rpm) f (mm/min) z (mm) D(mm)1 1 1 1 12 1 2 2 23 2 1 1 24 2 2 2 15 3 1 2 16 3 2 1 27 4 1 2 28 4 2 1 1

Tables2and3representtheprocessparameters,theirlevels,andtheorthogonalarray,respec‐tively.

3. Results and discussion 

Anumberofexperimentaltestswerecarryouttoassesstheeffectofthetoolrotationspeed(ω),feedrate(f),stepsize(z)andtoolsize(D)onthefinalsurfacetexturecreatedthroughtheSPIF.The experiments were stopped when the parts fracture.Where Fig. 9 (a), (b), (c) and (d) isdemonstratedthesamplesthatsucceededwiththecorrectselectionofparameterlevelsaccord‐ingtothementioneddesignedarray.

Oneof themaindrawbacks thataccompany incrementalsheet forming is thepoorsurfacequality of the produced components [23]. Thus, appropriate combination and optimization offormingparametersisachallengeandanimperativeissuetomanufacturepartswithexcellentsurface finishandotherdesirableprocess aspects; suchas formability and forming forces.Toachieve this goal, the Taguchi technique togetherwith analysis of varianceANOVA,were em‐ployedtoexaminetheinfluenceofthetoolrotation,feedrate,stepsize,andtoolsizeontheob‐tained surface roughness. These four forming parameters have significant effects on SPIF, asmentionedintheliterature.

TheexperimentalresultsforthesurfaceroughnessRaandthecongruousS/Nratiosarerec‐ordedinTable4.Moreover,thesurfaceroughnessvaluesfortheAA6061‐T6sheetsasreceivedare0.175µmand0.411µm,withandacrosstherollingdirection,respectively.

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(a)(b)

(c)(d)

Fig.9Samplesthatsucceededwiththecorrectselectionofparameterlevels

Table4TheDoEmatrixandtheresultsforsurfaceroughnessandS/Nratios

Runω

(rpm)f

(mm/min)z

(mm)D

(mm)Acrosstheformingtoolpath

Ra(µm) S/Nratio1 50 250 0.2 10 1.62 ‐4.19032 50 500 0.5 15 0.719 2.86543 400 250 0.2 15 0.581 4.71654 400 500 0.5 10 1.536 ‐3.72785 800 250 0.5 10 1.44 ‐3.16726 800 500 0.2 15 0.3 10.45767 1200 250 0.5 15 0.469 6.57658 1200 500 0.2 10 1.391 ‐2.8665

Fig.10Themaineffectsofthevariousparametersonthesurfacefinish

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ThemaineffectoftheconsideredfactorsonthesurfacefinishispresentedFig.10.Moreorfewerimpactsofthelevelsoftheseparametersontheoutputresponsecanbenoted.Thisgraphshowstheeffectoftoolsizeisasignificantonthesurfaceroughness.Theotherparameterssuchasrotationspeed,feedrate,andstepsizehavealessornegligibleeffectontheoutput.

Fig.11MaineffectforSNratiosforthesurfaceroughness

Fig.12Theinteractioneffectofvariousfactorsonthesurfaceroughness

Table5AnalysisofvarianceforthesurfaceroughnessSource DF AdjSS AdjMS F‐Value P‐Value Significant Contribution(%)

Regression 4 2.01239 0.50310 47.35 0.005 Yes ω 1 0.08094 0.08094 7.62 0.070 ‐ 3.96f 1 0.00336 0.00336 0.32 0.613 ‐ 0.16z 1 0.00925 0.00925 0.87 0.420 ‐ 0.45D 1 1.91884 1.91884 180.59 0.001 Yes 93.86

Error 3 0.03188 0.01063 1.56Total 7 2.04427 100

Table6StatisticalresultsofthedevelopedregressionequationofthesurfacesroughnessTerm Coef SECoef T‐Value P‐Value VIF

Constant 3.581 0.237 15.08 0.001 ‐ω ‐0.000234 0.000085 ‐2.76 0.070 1.00f ‐0.000164 0.000292 ‐0.56 0.613 1.00z 0.227 0.243 0.93 0.420 1.00D ‐0.1959 0.0146 ‐13.44 0.001 1.00

0 200 400 600 800 1000 1200

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Ra

(In

sid

e),µ

m

Rotation speed (), rpm

f-250 f-500 Z=0.2 Z=0.5 D=10 D=15

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Theoptimal conditionof the surface roughnessRa of theAA6061‐T6 sheet formedby fric‐tional‐stirassistedSPIFwasdeterminedbytheTaguchianalysis.Consistentwith thismethod,thegreaterthevalueoftheS/Nratioisthesuperiortheaggregateperformanceis.Thecaseindi‐catesthattheparameterlevelswiththehighestS/Nratioshouldbedesignatedasthebestlev‐els.Inthisstudy,theoptimalconditionfortheprocessparameters,whichprovidedaminimumRa,waswithintherunnumber6,asshowninTable4,andFigs.11and12.

Theanalysisof varianceANOVAhelped to create these relative impactsofparameters andtheir percentages contribution to the surface roughness, shown inTable5whileTable6pre‐sentsthecoefficientsoftheregressionequation.

This regression equationwas established based on the experimental results of the surfaceroughnessRa;the(Eq.1)candescribeit.

Inside μ 3.581 0.000234 0.000164 0.227 0.1959 (1)

ThefittingoftheregressionmodelisgivenbythedeterminationcoefficientR2.Thevalueofthiscoefficientreferstotheclosefittingoftheregressionequation.ThevaluesoftheR2,adjust‐edR2,andpredictedR2are98.44%,96.36%,and87.86%,respectively.Therefore,regardingthevaluesof thesecoefficients, theestablishedregressionequation fitswellanddescribes thesurfaceroughnessresponse.Lastly,normaldistributionplot,Fig.13,clarifiedthattheresidualstrackthenormaldistribution.Itcanbenotedthattheregressionequationhasagoodfittotheirexperimentaldataandarereliabletouse.

Fig.13Normaldistributionofthesurfaceroughness

4. Conclusion 

Inthepresentstudy,frictionstir‐assistedSPIFwasperformedtodeformAA606‐T6sheets.Thepurpose is to study the impactof certainprocess factorson thesurface roughnessof thepro‐ducedparts.Theresultscanbeconcludedinthefollowingworthypoints:

Thediameteroftheformingtoolhaveasignificantimpactontheinternalsurfacerough‐ness produced via the forming process of AA6061‐T6. To attain an acceptable surfaceroughness,thepercentagecontributionofthisparameterwas93.86%.

Anoptimalprocessparameterswasachievedforthesurfaceroughnessduringtheform‐ingprocess.Theminimumvalueofthesurfaceroughnesswas0.3µmatω=800rpm,f=500mm/min,z=0.2,andD=15mm.

ThevalueofthedeterminationcoefficientR2oftheestablishedregressionequationofthesurfaceroughnesswas98.44%.Thishighvaluerefertotheclosefittingofthesuggestedequation to describe the expected experimental data; it alsomeans the response valueshighlyadheretothenormaldistribution.

Effect of process parameters on the surface roughness of aluminum alloy AA 6061-T6 sheets in frictional stir incremental …

Acknowledgments The authors would like to thank Iraqi Government for the PhD scholarship and Research Management Centre, Universiti Putra Malaysia with the research grant (GP-IPS/2016/9479500), which enable the research to be car-ried out successfully.

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Azpen, Baharudin, Sulaiman, Mustapha

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