Evaluation of tool wear, surface roughness/topography and chip morphology when machining of Ni-based alloy 625 under MQL, cryogenic cooling and CryoMQL

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

Availableonlineatwww.sciencedirect.com

Original

Article

Evaluation

of

tool

wear,

surface

roughness/topography

and

chip

morphology

when

machining

of

Ni-based

alloy

625

under

MQL,

cryogenic

cooling

and

CryoMQL

C¸

a ˘grı

Vakkas

Yıldırım

_,

_Turgay

_Kıvak

_,

_Murat

_Sarıkaya

_,

_S¸

_enol

_S¸

_irin

a_{DepartmentofAirframesandPowerplants,ErciyesUniversity,Kayseri,Turkey}

b_{DepartmentofMechanicalandManufacturingEngineering,FacultyofTechnology,DuzceUniversity,Duzce,Turkey} c_{DepartmentofMechanicalEngineering,SinopUniversity,Sinop,Turkey}

d_{DepartmentofMachineandMetalTechnologies,GumusovaVocationalSchool,DuzceUniversity,Duzce,Turkey}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received2December2019 Accepted22December2019 Availableonline3January2020

Keywords:

Hybridcooling/lubrication Toolwear

Surfacetopography Chipmorphology Ni-basedaerospacealloy

a

b

s

t

r

a

c

t

Although nickel-basedaerospacesuperalloyssuchasalloy625havesuperiorproperties includinghigh-tensileandfatiguestrength,corrosionresistanceandgoodweldability,etc., itsmachinabilityisadifficulttaskwhichcanbesolvedwithalternativecooling/lubrication strategies. Itisalsoimportantthatthesesolutionmethodsare sustainable.In orderto facilitatethemachinabilityofalloy625withsustainabletechniques,weinvestigatedthe effect of minimum quantity lubrication (MQL), cryogenic cooling with liquid nitrogen (LN2)andhybrid-CryoMQLmethodsontoolwearbehavior,cuttingtemperature,surface

roughness/topographyandchipmorphologyinaturningoperation.Theexperimentswere performed atthreecutting speeds(50, 75and100m/min),fixedcuttingdepth(0.5mm) andfeedrate(0.12mm/rev).Asaresult,CryoMQLimprovedsurfaceroughness(1.42␮m) by24.82%comparedtocryogeniccooling.Themediumlevelofcuttingspeed(75m/min) canbepreferredforthelowestroughnessvalueandlowestpeak-to-valleyheightwhen turningofalloy625.Further,toolwearisdecreasedby50.67%and79.60%bytheuseofMQL andCryoMQLcomparedwithcryogenicmachining.AninterestingresultthatMQLismore effectivethancryogenicmachininginreducingcuttingtoolwear.

©2019TheAuthors.PublishedbyElsevierB.V.Thisisanopenaccessarticleunderthe CCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1.

Introduction

Nickel-based superalloys are preferred in critical tasks because oftheir superior propertiesat very high and very

∗ _{Correspondingauthor.}

E-mail:msarikaya@sinop.edu.tr(M.Sarıkaya).

lowtemperatures.Inconel625isoneofthesealloysandhas beenusedformanyyears.Itisusedinoiland gas produc-tioncomponents,marinevehicles,varioussurfacesincontact with acids, biomedical applications, automotive industry, aerospace industry and nuclearreactors, etc.However, the behaviors of the material such as good mechanical char-acteristicsunder stress,poorheatconductivity, high strain hardeningandhighchemicalclosenesstotoolmaterialcause https://doi.org/10.1016/j.jmrt.2019.12.069

2238-7854/©2019 The Authors. Publishedby Elsevier B.V. This isan open access articleunder the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

EDX Energydispersivex-rayanalysis CR Coolingregime

PVD Physicalvapordeposition AMS AerospaceMaterialSpecification Ra Averagesurfaceroughness

ISO Internationalorganizationforstandardization

VBmaxq Flankwear

BUE Builtupedge BUL Builtuplayer

␯ Kinematicviscosity(mm2_/s)

VI Viscosityindex

someconcernswhicharehightemperatureincuttingzone, quicktoolwearandpoorsurfaceintegrityinthemachining ofthesealloys[1,2].Oneofthemethodsusedtocontribute tothemachinabilitycharacteristicsofthesematerialsisthe employmentofcuttingfluid.

Cuttingfluids havean importantplace in chip removal operations.They performbasic taskssuchasreducing the frictionandpowerconsumption,chipevacuationaswellas cooling/lubricationofthecuttingzonethatdirectlyaffectthe efficiency of chip removal operations. Moreover, there are alsobenefitssuchasprotectingthecuttingtoolandmachine toolfromoxidation[3].Duetoallthesepositiveeffects,the use ofcuttingfluids isvital especiallyin turningofnickel basedalloy-Inconel625.However,therearesomedamagesto theenvironmentandworkerhealthwhenusingconventional (flood)cuttingfluids[4].Inadditiontothese,itisknownthat theuseofconventionalcuttingfluidincreasestheproduction costs[5]. Asaresult,the amount ofcoolantused inmetal removaloperationsshould bereduced.Currently,thereare variousalternative techniquesavailable.For example, min-imumquantitylubrication(MQL) andcryogeniccoolingare someofthemwhichare quitepopular[6].IntheMQL sys-tem,thecuttingfluidatanaverageflowrateof10−100ml/his mixedwithairandsenttothetool-workregionasanaerosol [7].Inthissystem,thelubricationfunctioniscarriedoutby meansofcuttingoil,whilethecoolingfunctionisachieved byusingcompressedairathighpressure.ThankstotheMQL system,verysmallamountofcuttingfluidisusedandsothe negativeimpactofconventionaltechniqueonenvironment, workerhealthandproductioncostsisminimized[8–10].Some researchesinthe literatureavailable[11–16] haveindicated thattheMQLtechniquecanbeanoptiontotheconventional techniques.Anotheralternativecoolingmethodiscryogenic cooling.Cryogeniccoolingisusedespeciallyinheavy machin-ingconditions,suchasnickelalloys[17],titaniumalloys[18]

2

themostpreferredgasincryogeniccooling.TheeffectofLN2

onmachinabilitywasinvestigatedinseveralstudies[23–25]. Inthesestudies,thepositivecontributionofcryogeniccooling withLN2tomachinabilityhasbeenreportedbyresearches.

In the above studies, it is seen that both the MQL and the cryogeniccoolingcanbeanalternativetowet machin-ing(conventionalcooling).However,althoughthesemethods are efficient in light and medium cutting conditions, they are ineffectiveunderheavymachiningconditions.Inheavy machining conditions, the MQL system shows deficiencies with regard to the cooling, while cryogenic cooling also exhibitsdeficienciesrelatedtolubrication[26].Therefore,in ordertobenefitfromfurthercoolingandlubrication,several studieshavebeenconductedespeciallyunderheavy machin-ingconditions,wherecryogeniccoolingisusedtogetherwith theMQL[27].

Accordingtotheliteraturereview,itisseenthatstudies have been made byresearches toexplore the influenceof differentcooling/lubricationmethodsonmachinability indi-cators.Inaddition,someexperimentalstudiesfocusedonthe hybridcoolingmethodsuchasCryoMQL[25].Asaresultof the authors’research, no studieswere found onthe effect of the hybrid cooling/lubrication method as well as MQL and cryogenic cooling on investigating the surface rough-ness/topography, cutting insert wear, wear mechanisms, cuttingtemperaturesandchipmorphologyinmachining Ni-based alloy Inconel 625. In present work, to fill this gap, weaimedtoinvestigatetheeffectivenessofMQL,cryogenic coolingandCryoMQLcooling/lubricationregimesinturning Ni-based alloy 625. In addition, the performance of these regimesunderdifferentvariationsofthecuttingspeedwas alsostudied.

2.

Material

and

methods

2.1. Material-alloy625,machineandcuttingtool

Inconel 625 (alternatively known as alloy 625)was chosen as workpiece material withthe specification ofAMS 5666. Thismaterialisanickel-basedalloywithexcellent thermo-mechanicalproperties.Thankstotheseproperties,theyfind numerousapplicationsincriticalsectorssuchasaerospace, nuclearchemistryandpetrochemicalindustry.Theworkpiece materialproperties arelisted inTables 1and2. ACCUWAY JT-150-8CNClathe(maximumspindlespeed:4000rpm) man-ufacturedbyTaiwanwasemployedduringtheexperiments. Intheturningexperiments,atypeofPVD-TiAlN/TiNcoated

Table1–Chemicalcompositionofalloy625(%weight).

Ni Cr Fe Mo Nb C Mn Si Al Ti Co

58 20–23 5 8–10 3.15–4.15 <0.1 ≤0.5 ≤0.5 ≤0.4 ≤0.4 ≤1

Table2–Mechanicalandphysicalpropertiesofalloy625. Ultimatetensilestrength[MPa] Modulusof

elasticity[MPa]

Density[g/cm3_{] Meltingrange} [◦C] Thermal conductivity [W/mK] Elongation(%) 880 209 8.47 1290–1350 9.8 35

Table3–Cuttingfluidproperties. Kinematic viscosity,at 20◦C(mm2_/s) Kinematic viscosity,at 40◦C(mm2_/s) Viscosityindex, VI

Density(kg/m3_{) Flashpoint(}◦_{C) Thermal}

conductivity (W/mK)

18 10 192.02 860 205 0.1684

Table4–Turningparameters.

Cuttingparameters Unit Level1 Level2 Level3

Coolingregime,CR – MQL Cryogenic(LN2) CryoMQL(MQL+LN2)

Cuttingspeed,Vc m/min 50 75 100

Feedrate,f mm/rev 0.12 – –

Cuttingdepth,ap mm 0.5 – –

carbidetool(ISOdesignation:CNGG120404(S05-S25)) man-ufactured by Taegutec, Korea (manufacturer’s insert code: TT5080)wasutilized withspecificationsofrakeangle: −6◦, clearanceangle:0◦,majoredgecuttingangle:75◦ and nose radius:0.4mmbecausethePVDAlTiNcoatedqualityonthe ultra-thinsubstrategivesanexcellentsurfacefinishfor turn-ingofhightemperaturealloys.Thecuttinginsertwasrigidly mounted to a tool holder having ISO designation: PCLNR 2020M12-TB.

2.2. Cooling/lubricationconditionsandcutting parameters

Intheexperiments,threedifferentcooling/lubrication strate-gies were used. These are MQL, cryogenic cooling and CryoMQL(MQL+LN2).ForMQLexperiments,VariomodelMQL devicemanufacturedbySKFwasoperated.Water-soluble cut-tingoilformulatedwithvegetableestersandspecialadditives wasemployedduringtheexperiments.Thephysical proper-tiesofcuttingfluidaregiveninTable3.TheMQLsystemwas configuredfor8barpressureand50ml/hflowrate.Thecutting oilwassprayedatadistanceof15mmwithnozzlediameterof 2mmandsprayangleof30◦.Thesprayingprocesswasdone ontherakeface.InordertodetermineMQLparameters,itwas utilizedfrom preliminaryexperiments andliterature[7,27]. Theoperatingparametersusedinthisstudy areillustrated inTable4.Here,inordertoseetheimpactcooling/lubrication regimesatdifferentcuttingspeeds,thefeedandcuttingdepth werekeptconstant.

Theliquidnitrogen(LN2)at−196◦Cwasemployedfor

cryo-geniccoolinginexperiments.TheLN2 wasstoredinTaylor

WhartonXL-45HPmodeltank.Itwasdeliveredtothecutting areawiththehelpofaflexiblevacuuminsulatedhose.Inthis

way,heatlosseshavebeentriedtobeminimized.Thenozzle witha3mmoutletdiameterwasusedtobesimilartotheMQL systemduringthesprayingoftheLN2.Theliquidnitrogenwas

passedtothecuttingpointovertherakefaceatadistanceof 15mmwithasprayangleof30◦.Aschematicoverviewofthe experimentalsetupisseeninFig.1.

2.3. Measurements

In order to collect the data of the cutting temperatures, Infrared Optris PI 450 camera with optical resolution of 382×288 pixels, framerate of80Hz and real-time thermo-graphicmonitoring wasemployed.Asoftware wasusedto evaluatethetemperaturedatadeterminedfromthecamera. Inthismeasurement,theemissivityvalueisveryimportant indeterminingthe temperaturecorrectlydependingonthe material.Inthisstudy,theemissivityvalueforInconel625was chosenas0.5.Forsurfaceroughnessmeasurement,Mahr Mar-surfPS10devicemanufacturedbyGermanywasemployed. Inmeasurements,Ra(averagesurfaceroughnessvalue)was considered.ThismeasurementwasconductedbasedonISO 4287standard[28].Measuringdevicehasbeensetto0.8mm, 4mmand4.8mmforsamplinglength,evaluationlengthand travellength,respectively.Theworkpiecewasrotatedinthe cuttingdirectionandthemaindataoftheRawascalculatedby measuringatdifferentregions.Moreover,aPhaseViewoptical profilometerdevicewasemployedfor3Dsurfacetopography image.Intoolwearexperiments,AM4113ZT(Dino-Lite) polar-ized digitalmicroscopewas usedtodeterminethe amount ofwear.WeartypesweredeterminedaccordingtoISO3685 standard.Afterthewearvaluewas determined,toanalyze thewearmechanismoncuttinginsertandchipmorphology, ascanningelectronmicroscope(SEM;ZeissLEO440)wasused.

Fig.1–Experimentalsetupandworkflow.

Moreover,it has previouslybeen reported that superalloys tend to adhere to the cutting tool [29]. Thus, work mate-rial adheredtothe cuttingtoolwasinvestigatedbymeans ofEnergy-dispersive X-rayspectroscopy (EDX)andso built-up-edge(BUE), built-up-layer (BUL)and their damagewere analyzed.

3.

Results

and

discussion

3.1. Cuttingtemperature

Themajorityofthemechanicalenergyusedduringthecutting processisconvertedtoheatenergy.Theresultingtemperature hasadirectimpactonfactorssuchasdimensionalaccuracy, geometricaccuracy,andsurfaceintegrityandparticularlytool wear/lifethatareofgreatimportanceformachinability[30]. Therefore,controllingthe cuttingtemperatureisimportant formachiningefficiency.Inthispartofthestudy,itisaimed tofindtheoptimumparametergroupforminimizingthe cut-tingtemperatures.WhenFig.2isexamined,itisseenthatthe mosteffectivecooling/lubricationenvironmentonthecutting temperatureisCryoMQL.However,itshouldbenotedthatthe resultsobtainedfromcryogeniccoolingandCryoMQLcooling areclosetoeachother.Thehighestcuttingtemperaturewas obtainedfromMQL.Thecuttingtemperaturesthatoccurred inLN2andCryoMQLwerereducedby21.7%and24.9%,

respec-tivelycomparedtotheMQL.TheuseofLN2providedafurther

reduction in cutting temperatures compared to MQL. It is clearly possible to say that cooling performance with LN2

Fig.2–Theeffectofcoolingregimesontemperature.

Fig.4–Imagesofmachinedsurfacesandtheir3Dtopographiesunderdifferentcuttingregimesa)MQL,Vc=50m/minb) MQL,Vc=75m/minc)MQL,Vc=100m/mind)Cryogenic,Vc=50m/mine)Cryogenic,Vc=75m/minf)Cryogenic,

Vc=100m/ming)CryoMQL,Vc=50m/minh)CryoMQL,Vc=75m/mini)CryoMQL,Vc=100m/min.

usageismoreeffectivethanMQL.Asitisknownfrom previ-ousstudies,thecryogeniccoolingenvironmentexhibitsbetter coolingperformanceunderheavymachiningconditions[31]. However,itisnotpossibletosaythatithasthesame perfor-manceintermsoflubrication.Inotherwords,inthecryogenic coolingprocess,thelubricationprocessisparticularlyweak forheavymachiningconditionsand thereforeitsefficiency decreases.Therefore,boththelubricationandcoolingfeatures wereeffectivelyusedintheCryoMQLsystem.Intermsof

cut-tingspeed,therewasalinearrelationshipbetweencutting speedand temperatures(Fig.2).Intheexperimentscarried outatcuttingspeedsof75m/minand100m/min,thecutting temperatureincreasedby7.2%and15.5%respectively, com-paredtothecuttingspeedof50m/min.Furthermore,withthe cuttingspeedrisingto100m/min,therateofincreaseinthe cuttingtemperaturewasmuchhigher.Thisisassociatedwith reducedcuttingabilityaswellasfriction.Asamatteroffact, theresultsconfirmingthissituationaregiveninsection3.3.

Fig.5–Animagefromthemachiningwithcryogenic cooling.

3.2. Surfaceroughness

Surfaceroughness Raonmaterialsiscommonly expressed toidentify the changing inthe heightofthe surface rela-tivetoabasicline. Theroughness ofsolidsurfacesisvery importantforsurfaceinteractionsincethesurfacefeatures impressthe actual contactarea,friction, wear, lubrication, fatiguestrength,etc.Furthermore,surfaceroughnessisalso prominentinsomeconditionsincludingoptical,electricaland thermalability,coloringandvisual,etc.[32].Therefore,itis veryimportanttodetermineandminimizesurfaceroughness. Therearemanyparametersaffectingthesurfaceroughness, suchascuttingspeed,feedrate,cuttingdepth,cuttingtool materialand toolcoating,cooling/lubricating environment, etc.Inthisstudy,otherparameterswerekeptconstantinorder tomoreclearlyanalyzetheeffectofthe cooling/lubrication regimetogetherwiththecuttingspeed.

Fig. 3 represents the average surface roughness. Here, it is seen that CryoMQL gives the lowest surface rough-ness(Ra=1.42␮m)under CryoMQL coolingenvironment at 75m/minofcuttingspeed;ontheotherhand,thehighest sur-faceroughness(Ra=2.438␮m)wasobtainedundercryogenic coolingat50mm/min ofcuttingspeed.Comparedto cryo-genicmachining, RavaluesobtainedbyMQLand CryoMQL decreasedby13.8%and24.82%,respectively.Fromthisresult, itissaidthatbothcoolingandlubricationwithCryoMQLhave a positiveeffect on the surfacequality. When CryoMQL is used,itismoreeffectivebecauseofcombinedwiththecooling processthatreducesthecuttingtemperatureandthe lubrica-tionthatreducesfrictionandthereforetheroughnessvalue onthemachinedsurfaceislower.Ontheotherhand,when

Fig.7–Variationintoolweardependingon cooling/lubricationregimeandcuttingspeed.

theeffectofcryogeniccoolingwithLN2iscomparedtothe

effectoflubricationwiththeMQLprocess,itcanbesaidthat vegetable-basedcuttingoilinMQLhasagoodlubricating abil-ityduetotheprinciplestructureofvegetable-basedlubricant moleculesandelementcomponent[33].Vegetable-based cut-tingoilmoleculescancreateafilm layeronthe workpiece surface,andthefattyacidinvegetableoilcaninteractwiththe worksurface,creatingamonofilmofmetallicsoap.Theycan reducefrictionandwearandthuscanimprovesurfacequality [34].Inaddition,thetoollifeisincreasedbyusingthe appropri-atecoolingmethod.Thisisdirectlyrelatedtosurfacequality. Becausehomogeneityoftoolflankweardirectlyaffects sur-facequality[17].Toreachthelowestsurfaceroughnessvalue, itisdeterminedthatthemediumcuttingspeedselectionis suitable.Thishasbeenassociatedwithaslightincrementin speedtosoftenthecuttingzoneandmakecuttingeasier. How-ever,withthecontinuedincrementinspeed,thecuttingtool enteredthewearprocessandstartedtoloseitseffective cut-tingability.Thus,thesurfacequalitydeteriorated.Itcanbe clearlystatedherethatwhenthecuttingspeed100m/minis used, thecuttingtoolentersthe wearprocessearlierinall threecoolingenvironments.

Inadditiontotheevaluationoftheaveragesurface rough-ness, the texture and topographyof the machinedsurface arealsoimportantforfinalproductsbecauseabettersurface topographycanmakemanypositivecontributionstothe prod-uctbyimprovingthetribologicalpropertiesofasurface[35]. Inthispartofthepaper,theinfluenceofcuttingspeedsand cuttingregimesonsurfacetopographyisexaminedandthe resultsaregiveninFig.4.When evaluatedintermsof cut-tingspeed,itwasobservedthatthepeak-to-valleyheightwas higherat50m/mincuttingspeed,butthisdistancedecreased withincreasingspeed.Inother words,the surface

Fig.8–SEMimagesoftheworncuttinginsertunderMQLcuttingconditiona)Vc=50m/min,b)Vc=75m/min,c) Vc=100m/min.

phyimprovedanincrementinspeedto75m/min.However, asthecuttingspeedcontinuedtoincrease,itwasobserved thattherehasbeensomeclimbinginthedistancebetweenthe peaksandthevalleys.Thehighcuttingspeedcauseshighheat generation,whichacceleratesthewearandadverselyaffects the surfaceroughness. Furthermore, the workpiece rotates fasteraroundthecuttingtoolasthecuttingspeedincreases. Thiscontributestosurfacedeteriorationandtheformation ofirregularsurfacetexture[36].Anotherfactoraffectingthe surfacetopographyisthetribologicalinteractionsatthe tool-chipinterface.WhenFig.4isanalyzedfromthisview,itisseen thatthelowestpeak-to-valleyheightisobtainedbyCryoMQL. Thehighestpeak-to-valleyheightisformedinthecryogenic coolingresults.Animagefromtheexperimentwithcryogenic coolingisgiveninFig.5,thechipwasnotregularlybrokenand thereforecouldnotbeevacuatedfromthecuttingarea.Asa resultoftheongoingcuttingoperation,itwasobservedthat chipwasplasteredontotheworkpiece.Althoughthecooling

abilityofcryogeniccoolingissuperior,thelackoflubrication duringcryogenicmachiningmakeschipremovaldifficultand thereforeadverselyaffectsthesurfacequality.The peak-to-valley heightwas lowerthan theother twomethodswhen cryogenic coolingand theMQLsystemwere usedtogether. Moreover,irregularfeedlinesanddebrishavebeenreduced withtheCryoMQLsystem.Thiscanbeexplainedbythe effi-cientoperationofbothlubricationandcooling[37].

3.3. Toolwearandwearmechanisms

Machinabilitycriteriasuchassurfaceroughness,cutting tem-perature,surfaceintegrity,etc.are oftendependentontool wearandaredirectlyinfluencedbyit.Itisundertheeffect ofmanyparameterssuchastoolmaterial,coating applica-tion,cuttingspeed,feed,cuttingdepthandcoolingcondition, etc.[38].Inthissectionofthestudy,toevaluatetheimpactof diversecooling/lubricationregimesandcuttingspeedontool

Fig.9–SEMimagesoftheworncuttinginsertundercryogeniccuttingconditiona)Vc=50m/min,b)Vc=75m/min,c) Vc=100m/min.

wear,thefeedandcuttingdepthwere fixedas0.12mm/rev and0.5mm.Ineachexperiment,afixedvolumeofthechip (40,000mm3_{)wastakenfromtheworkpieceandthestateof}

thewearwasobserved.Asaresultofthepreliminarytests, itisseenthattheeffectivewearkindoncuttinginsertwas notchwearasseeninFig.6.

Therefore, it can be said that notch wear is primarily responsibleforcompletingthelifeofthecuttingtools.Ithas also been reported inprevious studies that notch wear is mostlyobservedinthemachiningofNi-basedalloys[27,29].In manystudiesofmachiningofNi-basedalloys,althoughnotch wearhasbeenobserved,thereisnocommonconsensusabout thecauseofit.Inaddition,thecauseofthenotchhasbeen associatedwithmorethanonefactor.Theycanbecountedas hightemperature,stress,work-hardeningandabrasivechips [39].

Fig.7indicates the variationintoolweardependingon cooling/lubricationregimesandcuttingspeed.Itwasfound

thatwhilethemaximumtoolwear(2.602mm)occurredunder cryogenic cooling, the minimum toolwear (0.211mm) was reachedfromtheCryoMQLcuttingenvironmentinturningof Ni-basedInconel625.Accordingtothis,toolwearisreduced by50.67%and79.60%withtheuseofMQLandCryoMQL com-paredwithcryogenicmachining.Aninterestingfindingthat MQLismoreeffectivethancryogenicmachininginreducing toolwear.Thecryogeniccoolinghelpsreducethe tempera-tureinthecuttingzoneonlythroughforcedconvection,while theMQLmethodcontributesmorethanone.Firstly,the lubri-cation inMQLwrapsthe cutting regionwith alayerofoil andthishelpstoreducefriction.Secondly,MQLcontributes totheheattransferduetotheevaporationofdroplets. More-over,theimprovementintoolwearwasmoreobviouswith the useofCryoMQL.Similarresultsontheeffective perfor-manceoftheCryoMQLhavebeenreportedbyBagherzadeh and Budak[40]and Gupta[6].Bagherzadeh andBudak[40] claimedthatmorethanonemechanismonthisperformance

Fig.10–SEMimagesoftheworncuttinginsertunderCryo-MQLcuttingconditiona)Vc=50m/min,b)Vc=75m/min,c) Vc=100m/min.

couldbeeffectivewiththesimultaneousapplicationofMQL andcryogeniccooling.Thesearehigherpenetrationofoilinto thechiptoolinterface,higherheattransferbecauseofvery low-temperatureoil,sprayingtheoilintothepre-cooledzone andreducingoilburningwithcryogeniccoolingand increas-ingtheefficiencyofMQL.AsseeninFigs.8,9and10,SEM imagesindicate thecharacterizationofwearmechanismof thecuttingtoolunderdifferent cooling/lubricationregimes (MQL,cryogeniccoolingandCryoMQLandatcuttingspeeds of50,75,and100m/min). Itisobservedfrom thesefigures thattheactivewearmechanismwasfoundtobeadhesion inallcuttinginserts.Thepresenceoftheadhesion mecha-nismhasbeenprovenwiththebuilt-upedge(BUE)andlayer (BUL)formsoccuronthe cuttingtool.Moreover,asseen in Fig. 11, EDX analysis made on the rake surface of cutting toolsclearly demonstrates that adhesionis effectivein all cooling/lubricating regimes since the element composition ofthe workmaterialisobtainedintheseanalyses.Habeeb

et al.[41] statedthat thesephenomena (BUEand BUL)are verycommonduringcuttingNi-basedsuperalloys.Ithasbeen reportedbyEzugwuetal.[42]thatwelding/adhesionof Ni-based superalloys onto the cutting insert often observe in cuttingprocess,whichcausesseriousdamagetothecutter. Itcanbesaidthatoncethetemperatureofthetool-chip inter-facereachesacriticallevel,thetendencyofworkpiecetoweld increasesduetothechemicalclosenessbetweenthecutting toolmaterialandtheworkmaterial.WhenFigs.8,9and10 wereanalyzed,itwasseenthatthecooling/lubrication meth-odsusedinthisstudywereinsufficienttoeliminatetheBUE andBULforms.Moreover,anothercommontypeofdamage that occurredinallconditionswas chipping.Cantero etal. [43]emphasizedthattoolwearmechanismsarenottreatedas separatesubjects,butareallinterrelated.BUEandBUL result-ing from the adhesion mechanism tend to encouragetool chippingwhichisformedbytheseparationoftoolmaterials togetherwiththeworkpiecematerialadheredontool,because

Fig.11–EDXanalysisforworncuttinginsertundera)MQL,b)Cryogenic,c)CryoMQL.

the BUE and BUL are notcompletely stableon the cutting insert.

InFigs.8,9and10,itcanbeseenthatanothereffectivewear mechanismoncuttingtoolwasabrasivewearwhenturning ofNi-basedalloy625.Groovesparalleltodirectionofthechip flowintheflank-surfaceprovedthepresenceofabrasivewear. Asstatedinpreviousresearches,abrasiveweariscommon duringthecuttingofnickel-basedalloys[39].Hard-abrasive carbidesinworkpiecematerialenterthetool-workpiece inter-face,whichproducesasimilareffecttothegrindingprocess, whichcausesabrasivewear.Fig.9showsthatabrasivewear in machining under cryogenic cooling conditions is quite noticeablecomparedtoothercooling/lubricationconditions. Moreover,anincreaseincuttingspeedacceleratedthis situa-tion.Thereasonforthisisthoughttobethepoorperformance ofcryogenic coolinginlubricating, asthe friction between thetoolandtheworkpieceincreasesinunittime.WhenSEM

photographsareexamined,anotherfindingisnotching. How-ever, asmentionedabove,thereisnogeneralconsensusin thecategoryoffactorscausingnotching.Whenanevaluation wasmadeaccordingtocooling/lubricationconditionsand cut-tingparameters,notchingwasobservedinalmostallcutting speedsandcuttingconditions.However,itcannotbesaidthat thereisasignificant changeinparallelwiththe changein coolingconditionsandcuttingparameters.

3.4. Chipmorphology

Chip morphology provides important clues about the cut-tingmechanicsandiscloselyrelatedtosurfaceintegrityof thefinishedproduct(surfaceroughness,surfacetopography, etc.)andmachiningefficiency.Therearemanyvariablessuch as cutting tool and workpiece material properties, operat-ingparameters(feed,cuttingspeed,cuttingdepth,etc.)and

Fig.12–Chipmorphologiesoffrontandbacksideundercuttingspeed75m/minandfeedrate0.1mm/revwhenusinga) MQL,b)Cryoandc)CryoMQL.

cooling/lubricationconditionsaffectingchipmorphologyand itsshape [44].Inpresent work,the impactofdiverse cool-ing/lubricationstrategiesonchipformationmorphologyhas beenanalyzedwhilekeepingothervariablesconstant.Forthis purpose,SEMphotographsofthechipsproducedunder cryo-geniccooling,MQL,CryoMQLcoolingregimesat75m/minof speedand0.1mm/revoffeedwereconsidered.Fig.12shows themicrographsofthefrontandbacksurfacesofthechips producedunderdifferentcuttingconditions.Whenthe back-sideofthechipisexamined,it isseenthatlargescratches occurowingtotheseverefrictionbetweenthetoolrake sur-faceandthechip,especiallyincryogeniccoolingcondition. Thisisanindicationthatthelubricityofthecryogenic cool-ingatthetool-chipinterfaceisinsufficient.Thefactthatthe highestRavalueand poor surfacetopography(see section 3.2)obtainedundercryogeniccoolingconditionconfirmsthis situation.MQLandCryoMQLcoolingregimeswerefoundto significantlyreducescratches on the backface ofthe chip (Fig. 12(a) and(c)).Here,it canbesaid thattheMQLhas a decisiverole inreducing thefriction atthe tool-chip

inter-facethankstoitssuperiorlubricity.Bycombiningthesuperior coolingperformance ofcryogenic coolingand the superior lubricationpropertiesofMQL,thereductionofscratcheson the backface ofthechip, betterRavaluesand highertool lifehasbeenachieved.Lookingatthechipfrontsurfaces, ser-ratedchipformationisclearlyseenforallcuttingconditions (Fig.12).

When thefrontsurfacesofthechipsareexamined, ser-ratedchipformationisclearlyvisibleforallcuttingconditions (Fig. 12). Large serration occurred inthe cryogenic cutting condition while small serration occurred in the MQL and CryoMQLcuttingconditions.In addition,chip cross-section photographsgiveninFig.13showsthattheformationof ser-rationundercryogeniccoolingconditionhassharpandlinear linescomparedtoMQLandCryoMQL.Here,itcanbesaidthat thereisarelationshipbetweentheformationofserrationon thefrontfaceofthechipandthescratchesonthebacksurface ofthechip.Therefore,itcanbestatedthathighdeformation duetofrictionmaybeeffectiveinclarifyingserration forma-tion.

Fig.13–SEMimagesofchipcrosssectionproducedundera)MQL,b)Cryoandc)CryoMQL.

4.

Conclusions

Inthis experimentalwork,Ni-based superalloy-Inconel 625 was handled in order to determine the consequences of thevariouscooling/lubricatingcuttingconditionsandcutting speedontoolwearand mechanisms,cuttingtemperatures, surfaceroughness,surfacetopographyandchipmorphology inturningprocess.Thefindingsfromthisworkwere summa-rizedasfollows:

1) Hybridcooling/lubricationstrategy(CryoMQL)hasrevealed bettersurfaceroughnessRa(1.42␮m)andsurface topogra-phy(thelowestpeak-to-valleyheight),ontheotherhand, thehighestsurfaceroughness(2.438␮m)valueisobtained undercryogeniccooling.Accordingtothecalculations, Cry-oMQLimprovedtheRaby24.82%comparedtocryogenic cooling.Themediumlevelofcuttingspeed(75m/min)can bepreferred for the lowestroughness valueand lowest peak-to-valleyheightinthemachiningofInconel625. Fur-ther,MQLalsocontributedtotheimprovementofsurface roughnesswith%13.8.

2) The cutting temperature was highest (310◦C) in MQL cooling/lubricationregimeat100m/minofcuttingspeed while minimum cutting temperature (200◦C) obtained from CryoMQLat 50m/minof cuttingspeed.Thanks to cryogeniccoolingandCryoMQL,thecuttingtemperatures werereducedby21.7%and24.9%,respectivelycompared to the MQL. In the experiments carried out at cutting speedsof75m/minand100m/min,thecutting tempera-tureincreasedby7.2%and15.5%.Withthecuttingspeed rising to 100m/min, the rate of increase inthe cutting temperatureswasmuchhigher.Thiswasexplainedwith

thereducedcuttingabilityofthecuttingtoolaswellas friction.

3) Itwasobtainedthatthenotchwearisprimarilyresponsible forcompletingthelifeofthecuttingtoolsasseenclearly inallcuttingconditions.Itwasfoundthatmaximumtool wear(VB=2.602mm)isemerged from cryogeniccooling at100m/minofcuttingspeed,whileminimumtoolwear (VB=0.211mm)isinCryoMQLat75m/minofcuttingspeed. Moreover,toolwearisdecreasedby50.67%and79.60%by theuse ofMQL and CryoMQLcomparedwith cryogenic machining.AninterestingresultthatMQLismore effec-tivethancryogenicmachininginreducingtoolwearwhen turningNi-basedalloy625.

4) From SEM photographs, it was found that the effective mechanismforwearisadhesioninallcooling/lubrication regimesfollowedbyabrasivewearmechanism.Thesewear mechanismshavecauseddamagetothecuttingtoolsuch aschipping,BUEandBULformations,fracture,flankwear andnotchwear.Asaresultoftheadhesionmechanism, the BUE and BUL forms are very active on the cutting tool.Moreover,thepresenceoftheadhesionmechanism has been proven with EDX analysis. It is said that the cooling/lubricationmethodssuchasMQL,cryogenicand CryoMQLare insufficientto eliminate the BUE and BUL formsandtheirdamagessuchaschipping.

5) SEMphotographsofthebacksideofchipsproducedunder different cooling regimesindicated that large scratches formduetotheseverefrictionbetweenthetoolrake sur-faceandthechip,especiallyincryogeniccoolingcondition. ItwasfoundthatMQLandCryoMQLcoolingregimesreduce significantlythescratchesonthebacksurfaceofthechip. Furthermore,serratedchipformationatthefrontsurfaces ofthechipsisexistedforallcuttingconditions.Large

ser-rationoccurred inthecryogenic cuttingconditionwhile small serration occurred in the MQLand CryoMQL cut-tingconditions.Theformationofserrationundercryogenic coolingconditionhassharpandlinearlinescomparedto MQLandCryoMQL.

Conflict

of

interest

Theauthorsdeclarenoconflictsofinterest.

Acknowledgment

TheauthorsthanktheErciyesUniversityResearchFundfor financialsupport(ProjectNumber:FBA/2018/8074).

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