Effect of dipped cryogenic approach on thrust force, temperature, tool wear and chip formation in drilling of AZ31 magnesium alloy

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

Availableonlineatwww.sciencedirect.com

Original

Article

Effect

of

dipped

cryogenic

approach

on

thrust

force,

temperature,

tool

wear

and

chip

formation

in

drilling

of

AZ31

magnesium

alloy

Ugur

Koklu

_,

_Himmet

_Coban

a_{DepartmentofMechanicalEngineering,KaramanogluMehmetbeyUniversity,70100Karaman,Turkey}

b_{NaturalandAppliedScience,KaramanogluMehmetbeyUniversity,Karaman,Turkey}

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Articlehistory:

Received11December2019 Accepted10January2020 Availableonline22January2020

Keywords:

Cryogenicapproach AZ31magnesiumalloy Drilling

Thrustforce Drillwear Temperature

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Magnesiumalloystendtohaveinflammablenatureandchipself-ignitionathighcutting speedsunderdrymachiningcondition,althoughtheycanbeeasilymachinedwithgood surfacequality.Inthedrillingprocess,coolingandlubricationhaveacriticalimpactasit controlsheatgeneration,toolwear,surfacequality,andcuttingforce.Inthepresentstudy, drillingtestsonAZ31magnesiumalloywereperformedwithdryandcryogenicconditions atvariousfeedratesandcuttingspeeds.Theeffectofdippedcryogenicapplicationduring drillingonthrustforce,temperature,toolwear,andchipformationwereinvestigated.The resultsshowedthattheappliedcryogenicdrillingmethodprovidedlesstoolwear,smaller chipsandreducedamountofadhesions.Drillingtestsperformedinthecryogenic envi-ronmentincreasethethrustforcesby32%–39%comparedtodrycutting.Sparkandchip ignitionwerenotobservedevenathighcuttingspeedsduringdrycutting.

©2020TheAuthors.PublishedbyElsevierB.V.Thisisanopenaccessarticleunderthe CCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1.

Introduction

Magnesium alloys attract significant attention in most engineeringapplicationssuchaselectronics,aerospace, auto-mobile,structuralandbio-medicalindustries.Mostofthefinal productsusedinmanyengineeringapplicationsare manu-facturedbymachiningprocesses[1,2].Itispossibletoeasily machine,andgoodsurfacefinishcanbeachieved.However, underdrycuttingcondition,inflammablenatureofthe mate-rial may cause self-ignition offinechip particles athigher cutting speeds. Built-up edge formation in metal cutting, whichresultsinpoorsurfacefinishanddimensionalaccuracy,

∗ _{Correspondingauthor.}

E-mail:[email protected](U.Koklu).

iscausedbylowmeltingpointofthesealloysandmaterial adhesiononthecuttingtool[1].Itisalreadyknownthatthe majorityoftheworkduringmachiningisconvertedtoheat andendsupwithariseinthetemperatureoftool,workpiece, andchip,whichhasanimportanteffectonchipformation, cuttingtoolwear,andmachinedsurfacefinish.Temperature isalwaysofconcerninmachining.Forthepurposeof decreas-ingthecuttingtemperature,applyingcryogenicsasacoolant isthegeneralproceduretoeliminatetheeffectofthe temper-ature,whichisknownascryogenicmachining[3,4].

Baloutetal.[5]investigatedtheeffectofsubjecting vari-ousmetallicmaterials(magnesium,aluminumandbrass)to pre-coolingandpreheatingonthedrillingprocess.The exper-imentalstudywascarriedoutatmanydifferenttemperature values(15, 10,5,0, –20,–30,–40,–50,and −60◦_C)andthey stated thatthrustforce andtorquedecreasedwith increas-https://doi.org/10.1016/j.jmrt.2020.01.038

2238-7854/©2020 The Authors. Publishedby Elsevier B.V. This is anopen access articleunder the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

ingmaterialtemperature.Kakinumaetal.[6],Mishimaetal. [7],andKakinumaetal.[8]experimentallyinvestigatedthe micro machinability offrozen polydimethylsiloxane elastic material.The materialwas machinedwhen the mold was filledwithliquidnitrogen.Theauthorsstatedthatmicro chan-nelsweremilledeasilyandaccuratelyusingthismethod.In addition,itwasemphasizedthattestsundercryogenic con-ditionyieldhighercuttingforcethanroomtemperaturetests. Songetal.[9]investigatedthedirectmechanicalmachiningof polydimethylsiloxaneusingcryogeniccooling.Themachined surfacewas analyzed using various machiningparameters such as spindle speed and feed rate, and their effects on thecutting temperaturewere examined.When the cutting temperatureroseabovethecriticalvalue,itwasnotedthat thesurface qualityofthepolydimethylsiloxane was signif-icantly degraded due to increased adhesion and reduced elasticmodulus.Dhokiaetal.[10]predictedby compensat-ingtheshrinkageofmidsolefoamedpolymerand ethylene vinylacetate which are widely used inthe manufacturing ofshoe,undercryogeniccondition.Theauthorsemphasized thatthecryogenicshrinkagefactorinthecryogenic process-ing was less than 1 % of the original CAD model. Dhokia et al.[11], inanother study,investigatedthe machinability ofEVA and neoprene elastomermaterialsunder cryogenic conditions. In the experimentalstudy, the glass transition temperatureataspecifictemperature andelasticity values ofthetwoselectedmaterialswerecharacterized.Kokluand Morkavuk[12]experimentallyinvestigateddrilling machin-abilityofcarbon fiber-reinforced compositematerialunder cryogeniccondition. Drillingtests werecarried out using a speciallydesignedthermallyisolatedmold.Theeffectsof dif-ferentcuttingconditionsonthrustforce,delamination,tool wear,andsurfaceroughnesswereinvestigated.Theauthors emphasizedthatthecryogenicmethodsignificantlyreduced toolwearandimprovedsurfacequality;however,itcausedto increasethethrustforce.

Therearemanyacademicpapersonthemachinabilityof magnesiumalloysinliterature.Themajorityofthese stud-ies arefocusedin theturning process.Onthe other hand, therearemanystudiesoncryogenicturningofmagnesium alloys [1,2,13–21]. But, limited study has been carried out ondrilling processesofmagnesium alloys undercryogenic condition.Kheireddineetal.[22]examinedtheinfluenceof cryogenic applicationon the surfaceintegrity ofmachined holesinAZ31BMgalloy.Intheexperimentalstudyconducted bythe authors,thrust force,torque, surfacehardness, and grainstructureweremeasured.Itwasreportedthatcryogenic applicationduringmachining resultedinimprovedsurface hardness of machined holes as compared with machined underdrycondition.Kheireddineetal.[23]investigatedthe effectof usingliquid nitrogenin drilling ofAZ31b magne-siumalloyontheholesurfaceintegrityusingaindexabledrill. Thrust force, torque,and surface hardnesswere examined bothexperimentallyandnumerically.Itwasstatedthatthe hardnessvaluewashigherinthetestsperformedunder cryo-genic condition. Finiteelement analysis withexperimental justificationhasbeendiscussed.Bhowmicketal.[24] investi-gateddryandminimumquantitylubricationdrillingofAM60 magnesiumalloy.Thrustforce,torque,cuttingtemperature, toollifetests, surfacetopography,chip,microhardnessand

toolwear were measured.Itisemphasized bythe authors thatuniformtorqueandthrustforce,smalland discontinu-ouschipsand smoothholesurfaceareobtainedbydrilling themagnesiumalloyunderMQLcondition.Wangetal.[25] studiedwearofHSStoolsduringdrillingofmagnesiumalloy. SEManalysisshowedthreetypesofwearmechanismsinHSS tools.Theseweartypesareadhesivewear,abrasivewearand diffusion wear. This wear mechanism map is indicated to be a good reference for selecting suitable drilling parame-tersfordrillingofcastmagnesiumalloys.Berzosaetal.[26] focused on cuttingtoolselection indrilling of magnesium workpieceunderdryandminimumquantitylubrication envi-ronments basedonsurfaceroughness.Intheexperimental study,twodifferentpointangles,cuttingspeeds,feedrates andMQLflowvariableswereselected.Foraeronautical sec-tor, the importance ofdeterminingthe tooland operation accordingtotherequirementofsurfaceroughnessvaluesof 0.8–1.6␮misemphasized.Gariboldi[27]investigateddrilling machinabilityofamagnesiumalloyusingPVDcoatedtwist drills. Thedrills werecoatedwithTiN,CrNand two differ-ent ZrN by PVD method. Tool life, tool wear and surface roughness wereinvestigated. Adhesivewearforms,cutting parameters, thepresenceand typeofcoatingare statedto berelated.KaracaandAksakal[28]studiedeffectoftheTiBN coating onHSSdrillindrillingMA8MMgalloy.The perfor-manceofHSSandTiBNcoateddrillbitsweredeterminedby performingtestsatvariousspindlespeedsandfeeds.Surface roughness,topographyandchipformationwereinvestigated. TheTiBN-coateddrillexhibitedpoorsurfacequality. Balamu-ruganetal.[29]investigateddrillingofMg/SiCcompositefor defenseapplications.Theeffectsofmachiningtemperature onchipmorphology,toolwearandsurfaceprofilewere inves-tigated.Ithasbeenobservedthatthemostimportanteffectin machiningtemperatureformationiscausedbyspindlespeed andalsobothabrasiveandadhesiveweartypeoccur.Sunil etal.[30]researchedinfluenceofaluminumcontentondrilling characteristicsofAZ31andAZ91magnesiumalloys.Drilling testswereperformedusingdifferentcuttingparameters. Cut-tingforcesandformedchipswereanalyzed.Itisstatedthat the presenceofsecondary phase(Mg17Al12) has a signifi-canteffectonthecuttingforcesandanincreaseincutting speed reducesthe resultingcuttingforce andload fluctua-tions.

As theliteraturereviewshows thatin thefield of cryo-genicmachiningofmagnesiumalloys,machinabilitystudies aregenerallyfocusedonturningprocess,andtherearevery fewstudiesondrilling.Inthemachiningundercryogenic con-dition,itisgenerallymadebysprayingthecryogenicliquid intothemachiningzonethroughanozzle.Inthisstudy,the magnesium alloyisdrilledinafixturewhichiscompletely filled withliquid nitrogen.By usingthis approach(dipping the workpiece into liquid nitrogen), drilling performance ofAZ31 magnesium alloywas experimentallyinvestigated. Two different cutting speeds (40 and 120m/min) and four different feed rates (0.1, 0.15, 0.2 and 0.25mm/rev) were chosenasdrillingparameters.Also,toolweartestswere per-formedunderbothdryandcryogenicconditionsatconstant 80m/min cutting speed and 0.08mm/rev and 0.16mm/rev feedrates.Theresultsoftheexperimentsdemonstratedthat cryogenic machiningtechnique proposed inthis study can

be applied in order to obtain less tool wear and smaller chips.

2.

Material

and

methods

TheworkpieceusedintheexperimentalstudywasanAZ31 magnesiumalloyplate.Themechanicalpropertiesand chem-icalcompositionoftheAZ31magnesiumalloyareshownin Tables1and2,respectively.ThedimensionsoftheAZ31 mag-nesiumalloyworkpiecewas150×100×10mm.

Inthe experimentalstudy,twoflutehelical PVD(TiAlN) coateddrillswithadiameterof4mmwereusedascutting tools(Fig.1).Thepointangleofthedrillwas140◦.Anewdrill

Table1–MechanicalpropertiesofAZ31magnesium alloy[31]. Tensile strength (MPa) Yield strength (MPa) Elongation % Hardness HB Machinability % 290 220 15 73 100

wasusedforeachseriesofexperiments.Thefirstseriesof experimentswereconductedtodeterminetheeffectofthe cuttingparametersontheresults.Theexperimentswere car-riedoutindryandcryogenicconditionsat40and120m/min cuttingspeedsand0.1,0.15,0.2and0.25mm/revfeedrates.In thesecondseriesoftheexperiments,toolweartestswere

per-Fig.1–Drillusedintheexperimentalstudy.

Table2–ChemicalcompositionofAZ31magnesiumalloy[31].

Element Mg Al Zn Mn Si Ca Cu Fe Ni

Wt% 96 2.5–3.5 0.6–1.4 ≥0.2 ≤0.1 ≤0.04 ≤0.05 ≤0.005 ≤0.005

formedunderbothdryandcryogenicconditionsat80m/min cuttingspeedand0.08mm/revand0.16mm/revfeedrates.

Alldrilling testswere performed on3axisCNC vertical machining center (Quaser MV154C CNC). At the moment of experiments, thrust forces were measured by a force dynamometer(Kistler9257Btypeanddataacquisition equip-ment).Measurement oftemperature inmachining arevery difficultduetoclosedspace,chipobstacles,andthenatureof thecontactphenomenabetweentoolandchip[32].Therefore, thethermalcameraispositionedtoseethecuttingprocess inthe bestpossibleway. Temperaturemeasurementswere madebyathermalcamera(Flirsystem).Manyimageswere capturedwiththethermalcameraduringthedrillingprocess andthemaximumtemperatureobtainedfromtheseimages wasdetermined.Thethermal camera featuredobject tem-peratureranges from −25◦_{C to 380}◦_{C withan accuracyof} ± 1.5 % or 1.5◦C, a field of view 50◦×38.6◦, IR resolution of80×60pixelsandathermalsensitivity/NETDof<150mK. Theemissivityvaluewasselectedas0.6inthe experimen-talstudy.Thewearonthedrillsweremonitoredbyadigital microscope (Keyence VHX-900F). Thechips formed during experimentswere visualizedwithadigitalmicroscope.The experimentalsetupandmeasurementinstrumentsareshown inFig.2.AfixturewasmanufacturedtodrilltheAZ31 mag-nesiumalloyincryogenicenvironmentwithoutdamagingthe machineandcuttingforcemeasurementdevices.Thethermal insulatorfixturewaspositionedontheforcedynamometer. Polytetrafluoroethylenewasemployedasathermalinsulation materialtoavoidtheeffectofhigh-levelcoldcryogeniccoolant onthedynamometer.Asthecryogeniccoolant,liquidnitrogen wasemployedat−196◦_{C.Moredetailedinformationaboutthe} experimentalset-upandthermalinsulationdiecanbefound inreference[12,33].Significantimprovementsin machinabil-ityandcuttingparameterscanbeachievedusingcryogenic processing[34].

3.

Results

and

discussion

Machinabilityisthefacilityordifficultyinmachininga mate-rialunderacertainsetofoperatingconditionsthatinclude cuttingdepth,feedrateand cuttingspeed.Thegeneral cri-teriacommonlyadoptedforassessingmachinabilityaretool life,powerconsumption,chipshape,surfacefinishand com-ponentforcesduringacuttingoperation[35,36].Inthisstudy thrustforce,temperature,toolwearandchipswere consid-ered.

3.1. Thrustforcecomparison

TheforcesintheXandYdirectionduringthedrilling opera-tionwereclosetozero.Ontheotherhand,themeasurement ofthrustforceisveryimportanttoanalyzemoreeffectively the machinability factors of AZ31 magnesium alloy under cryogenic and dry conditions. Each experiment series was

Fig.3–Theeffectoffeedratevariationonthrustforce.

madewith3replicates,andthearithmeticaveragewastaken. Fig. 3shows a samplechartof the overall thrustforce for theexperimentscarriedoutatfourfeedrates(0.1,0.15,0.2 and 0.25mm/rev)and 40m/mincuttingspeed.Thrust force increasesasthefeed rateincreases.Theeffectsofdry and cryogeniccuttingconditionsonthethrustforcesoccurredat differentcuttingspeeds(40m/minand120m/min)and con-stantfeedrate(0.1mm/rev)areshowninFig.4.

ThethrustforcerecordedduringdrillingoftheAZ31 mag-nesiumalloyispresentedinFig.5.Thrustforce indry and cryogenicenvironmentshowsdecreasingtrendwhencutting speed is increased. As cutting speed increases, the thrust force decreases byabout 16 %–27 %. This behaviorcan be ascribed to the reduction of the contact area at the drill-Mgalloyinterface andthe reductionofthespecificcutting energy.Furthermore,withanincreaseincuttingspeed,the cuttingtemperatureincreasedandsubsequentlyreducedthe materialhardness.Increasingthefeedrateinbothdry and cryogenicconditionsincreases(about34%–54%)thethrust force.Thisphenomenonstemmedfromthehigherfeedrates, whichcausedanincreaseintheamountofuncutchipand theenergyrequiredforcutting.Inaddition,drillingtests per-formed inthe cryogenic environment increased the thrust forcesby32%–39%comparedtodrycutting.Inthepresent study,dippedcryogenic drillingledto higherthrustforces, which was associatedwithincreasing Youngmodulus and tensile strength of AZ31magnesium alloy incaseexposed tocryogenicenvironment;andtherefore,higherthrustforces areobtainedincryogenicdrilling[33].Inpreviouslypublished studies, it was highlighted that tensile strength, hardness andyoungmodulusofthematerialsincreaseastemperature decrease[4,12,37–42].

Fig.4–Thrustforcevariationswithcuttingspeedunderdryandcryogenicconditions.

Fig.5–Variationofthrustforce(a)40m/mincuttingspeed(b)120m/mincuttingspeed.

Toolweartestswere conductedtoexaminetheeffectof thesubsequentholenumberonthethrustforce.Atdifferent feedrates(0.08and0.16mm/rev)andaconstantcuttingspeed (80m/min),360holes weredrilledon theAZ31magnesium plate underdry andcryogenic conditions. Inthe toolwear experiments,thethrustforcewasmeasuredperiodicallyafter drilling20consecutiveholes.Thethrustforcegraphobtained fromthetoolwearexperimentsisgiveninFig.6.Inbothdry andcryogenicconditions,thethrustforcetendstoincrease

withincreasingnumberofholes.Thefundamentalreasonfor thissituationwassimplytoolwear.Indrydrillingcondition, lowerthrustforcewasgeneratedthanthoseinthecryogenic condition.

3.2. Temperaturecomparison

Surfacetemperaturesofthecuttingtoolatthetimeofdrilling weremeasuredbyathermalcamera.Inthedrillingtests

per-Fig.6–Comparisonofthethrustforceobtainedatdryandcryogenicconditions.

Fig.7–Temperaturesoccurredindrycuttingconditions.

formedundercryogeniccondition,thetemperaturewasnot measured since liquid nitrogenwas supplied continuously thereforethetemperaturesoftooland workpiecewas sup-posed−196◦_{C.Temperaturesgeneratedat40and120m/min} cuttingspeedsanddifferentfeedratesindrycutting condi-tionaregiveninFig.7.Temperaturesinthedrillingprocess increasedlinearlywithincreasingbothcuttingspeedandfeed rate.Thetemperatureincreasedby20%withincreasingfeed rateatlowcuttingspeed,whileitincreasedby40%athigh cuttingspeed.Cuttingspeedhasadominanteffectonheat

for-mation.Sparkandchipignitionwasnotobservedevenathigh cuttingspeedsduringdrycutting.Thisisofvitalimportance formachiningsafety.

Inthetoolweartestscarriedoutwithaconstantcutting speed (80m/min)andtwo differentfeedrates(0.08mm/rev and0.16mm/rev)underdrydrillingcondition,temperatures measuredforeach60holes.Imagesfromthethermalcamera areshowninFig.8.Ascanbeshown,anincreaseinthe num-berofholesincreasesthetemperature.Atbothfeedrates,as thenumberofholesincreases,thetemperatureincreasesas

Fig.8–Temperaturechangewithnumberofholes.

well.At0.08mm/revfeedrate,thetemperaturemeasuredas 40.2◦Cinthe360thhole,whileatthefeedrateof0.16mm/rev, thetemperaturewas56.7◦Cinthe360thhole.

3.3. Toolwear

Becauseofrapidwearandfailureofcuttingtool,itis possi-bletoencountersomeproblemslikeshortlifecycleoftool, poorhole quality,low cuttingefficacy,and high machining costs[37].Inthetoolweartestsperformedatdryand cryo-genicconditionsataconstant80m/mincuttingspeedand0.08 and0.16mm/revfeedrates,thewearonthedrillisvisualized withadigitalmicroscope.Animagewastakenfrom adrill inevery120holes(Fig.9).Indrydrillingcondition,excessive adhesionofAZ31magnesiumalloyonthedrillwasobserved. Intestsperformedundercryogeniccondition,theadhesionis

verylow,whileathighfeedratestheadhesionisalmost negli-gible.Toolweartestsconductedunderdryconditionresulted ingreater wearthan thoseincryogenic condition.In addi-tion,athigherfeedratesinbothcuttingconditions,morewear occurredcomparedtothetestsconductedatlowfeedrate.

3.4. Chipmorphology

Chipshapeisthemostsignificantfactoraffectingthe smooth-nessofametalcuttingprocess.Theprocesswillbesmoothas long aschipsare brokenand fragmentedinto smallpieces. However,asthechipsgetlarger,theycannotmovewellvia theflutesofthedrill,andthisincreasestorquerequirements. Moreover,itmaycausethedrillbittobreak.Yet,many duc-tilematerialsdonotbreakbutformcontinuouschipsduring drilling.Inordertoshowthechipshapedependingoncutting

Fig.9–Comparisonofthetoolwearobservedatdryandcryogeniccuttingprocess.

parameters(cuttingspeedandfeedrate),numberof subse-quentholes,dryandcryogenicconditions[43],thechipswere collectedandvisualizedinadigitalmicroscope(Keyence VHX-900F)aftereachexperiment.Specimenswererepresentedby digitalmicroscope by selecting samplesfrom the collected chips.Chipsformedat40and120m/mincuttingspeedsand 0.1,0.15,0.2and0.25mm/revfeedrateindryandcryogenic conditionsare giveninFig.10.Bothcuttingspeedandfeed rateshaveadominanteffectonchipformation.Asthe cut-tingspeedincreases,chipsareformedlongerinbothdryand

cryogenic cuttingconditions. Withincreasingfeedrate, the chipsbecomeshorterinbothcuttingconditions.Thechips formedincryogenicconditionareshorterthanthoseinthe drycuttingcondition.Inthetestscarriedoutunderdrycutting condition, withthe cuttingspeed increasedfrom 40m/min to120m/min, moretemperature wasgeneratedduring the cuttingprocess (Fig.7).Inthe testsperformedathigh cut-tingspeed,theincreasedtemperatureduringcuttingcaused thechiptoemergeinalongerform.Thefactthatthechips are longer shows that the chips is in ductile form.

Previ-Fig.10–Photographsofchipsobtainedatdifferentdrillingconditions.

ously,similarfindings arereportedinthe literature[30,43]. Becausethecuttingtemperaturedidnotgreatlyincreaseinthe testsperformedundercryogenicconditionatcuttingspeedof 120m/min,thechipsweregeneratedinamuchshorterform comparedtothoseformedinthedrycutting.Althoughitis mentionedintheliteraturethatmagnesiumalloyhasa ten-dencytoigniteathighercuttingspeed,nosuchsituationhas beenobservedinthisstudy.

360holeswasdrilledonAZ31magnesiumplateata con-stantcuttingspeedof80m/minand0.08and0.16mm/revfeed rate.Chips were collectedineach 120holes. Thisseriesof experimentswereperformedunderbothdryandcryogenic cuttingconditions. Thechipsformedafterthe experiments arecategorizedandgiveninFig.11.Withanincreaseinthe numberofholes,shapeofthechipsalsochanges.Thechip shapeforthefirst240holesindrycuttingconditionwasin theformofaspiral conewhichwas moreeasilyremoved. Afterthe240thhole,dependingonanincreaseinthethrust forceandwear,thechipthicknessdecreasedasthechippitch increased,thusribbonchips wereformed. Inthetests per-formedincryogenicenvironment,shorterchipsare formed becausethematerialbecomesbrittle.Toolwearwasthe fun-damentalreasonforthevariationofchipshapebasedonthe increaseinthenumberofholes[43].Undercryogenic condi-tion,asmallamountofdiscolorationwasobservedinthechips formedat0.16mm/rev feedrateforthe 360thhole.By per-formingdrillingtestsinacryogenicenvironment,thematerial changesfromductilemode tobrittlemode.Brittlematerial becomesmorerigidandharder.Muchmorepowerisneededto drillthehardermaterial.InSection3.1itwasmentionedthat testscarriedout undercryogenicconditiongeneratedmore thrustforcethanunderdrycutting.Moreplasticdeformation

occurredduringdrillingofthehardenedmaterialunderthe cryogenicprocess.Thishighplasticdeformationwasreflected intheformofthechip.

4.

Conclusion

Theeffect ofdippedcryogenic approachanddry condition on thrustforce,temperature, toolwearand chipformation indrillingofAZ31magnesiumalloywasinvestigated. • Inthetestsperformedunderbothdryandcryogenic

con-ditions,thethrustforcedecreases(about16%–27%)with increasing cutting speed; and the thrust force increases (about34%–54%)withanincreaseofthefeedrate.Drilling testsperformedinthecryogenicenvironmentincreasethe thrustforcesby32%–39%comparedtodrycutting.Intests performedunderdryandcryogenicconditions,inaddition, thethrustforcetendstoincreasewithincreasingnumber ofholes.

• Temperaturesinthedrillingprosesincreasedlinearlywith increasingbothcuttingspeedandfeedrate.Cuttingspeed hasadominanteffectonheatformation.Sparkandchip ignitionwerenotobservedevenathighcuttingspeeds dur-ingdrycutting.Anincreaseinthenumberofholesincreases thetemperature.

• Indrydrillingcondition, theAZ31magnesium alloyisin anexcessiveamountofadhesionstothedrill.Intests per-formedundercryogeniccondition,theadhesionisverylow, whileathighfeedratesitisalmostnegligible.Toolwear testsconductedunderdryconditionresultedingreatertool

Fig.11–Chipchangewithnumberofholes.

wear.Inaddition,athigherfeedratesinbothcutting con-ditions,morewearoccurred.

• Itisobservedthatthecuttingspeedisdominantonthechip formationandthechipsformedincryogenicconditionare shorterthanthechipsformedindrycuttingcondition.

Conflict

of

interest

Theauthorsdeclarenoconflictofinterest.

Acknowledgments

This work was supported by Commission of Scientific Research Projects of Karamanoglu Mehmetbey University, Karaman-Turkey(ProjectNo.04-YL-17).

Appendix

A.

Supplementary

data

Supplementary material related to this article can be found,intheonlineversion,atdoi:https://doi.org/10.1016/j. jmrt.2020.01.038.

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