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
a,
Turgay
Kıvak
b,
Murat
Sarıkaya
c,∗,
S¸
enol
S¸
irin
daDepartmentofAirframesandPowerplants,ErciyesUniversity,Kayseri,Turkey
bDepartmentofMechanicalandManufacturingEngineering,FacultyofTechnology,DuzceUniversity,Duzce,Turkey cDepartmentofMechanicalEngineering,SinopUniversity,Sinop,Turkey
dDepartmentofMachineandMetalTechnologies,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(1.42m) 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=1.42m)under CryoMQL coolingenvironment at 75m/minofcuttingspeed;ontheotherhand,thehighest sur-faceroughness(Ra=2.438m)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(1.42m)andsurface topogra-phy(thelowestpeak-to-valleyheight),ontheotherhand, thehighestsurfaceroughness(2.438m)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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