ORIGINAL ARTICLEEffect of cutting speed on surface integrity and chipmorphology in high-speed machining of PM nickel-basedsuperalloy FGH95Du Jin & Zhanqiang LiuReceived: 21 March 2011 / Accepted: 26 September 2011 / Published online: 14 October 2011# Springer-Verlag London Limited 2011Abstract High-speed machining is being recognized asone of the key manufacturing technologies for gettinghigher productivity and better surface integrity. FGH95powder metallurgy superalloy is a kind of nickel-basedsuperalloy which is produced by near-net-shape technology.With increasing demands for high precision and highperformanceofFGH95componentsinaerospaceindustry,it is essential to recognize that the machined surfaceintegrity may determine machined part service performanceandreliability. Then, littleis knownabout themachinedsurface integrity of this superalloy. Thus, the surfaceintegrity in high-speed machining of FGH95 is investigatedin this paper. Experiments are conducted on a CNC millingcenter with coated carbide tools under dry cutting con-ditions. The surface integrity is evaluated in terms ofsurface roughness, microhardness, and white layer. Theinfluence of cutting speed on chip morphology is alsoinvestigated. Experiment results show that surface integrityandchipmorphologyof FGH95areverysensitivetothecutting speed. When cutting speeds are below 2,400 m/min,thevaluesofsurfaceroughnesshavelittlevariation,whilewhen cutting speeds are in the range of 2,8003,600 m/min,the values of surface roughness are higher than that of othercutting speeds. Severe work hardening is observed resultingfromhigh-speed machining of FGH95 superalloy. Thehigher thecuttingspeed, thehigher thesurfacehardness.When cutting speeds are in the range of 2,8003,600 m/min,the white layer thickness is slightly higher than that of othercutting speeds. In high-speed machining of FGH95, the chipis segmented and has a typical sawtooth shape. The degree ofserratedchipincreases withthe cuttingspeed. Whenthecutting speeds exceed 2,400 m/min, serrated chips change intofragment chips.KeywordsCutting speed.Powder metallurgy.Surfaceintegrity.High-speed machining.Chip morphology1 IntroductionHigh-speedmachiningiscurrentlyattractingconsiderableworldwideinterests for moldanddiemanufacturers as ameans of directly machining components in a range ofhardenedtoolsteels[1,2].Thesemachiningprocessesarecharacterized by their high productivity, good surfacequality, and higher dimensional tolerances [3]. Additionally,high-speed machining technology makes it possible toexecute final machining operations without consequentgrinding or similar finishing operations.Superalloys are heat-resistant alloys of nickel, nickeliron,orcobaltthatexhibitacombinationofmechanicalstrengthand resistance to surface degradation generally unmatched byother metallic compounds, which represents a significantmetal portion of the aircraft structural and engine components[4]. However, the physical material properties and machiningcharacteristicsofthisclassofmaterialshaveledtoclassifythemas difficult-to-machine [5]. Anumber of factorscontribute to this classification including high shear strength,workhardeningtendency, highlyabrasivecarbideparticlesinthemicrostructure, strongtendencytoweldandformabuild-up tool edge, and lowthermal conductivity [6].D. Jin:Z. Liu (*)School of Mechanical Engineering, Shandong University,Jinan,Shandong 250061, Chinae-mail: melius@sdu.edu.cnInt J Adv Manuf Technol (2012) 60:893899DOI 10.1007/s00170-011-3679-6Among the commercially available superalloys used in theaeronautical industry, FGH95 is a relatively advanced kindof turbine disks material, whichis one kindof powdermetallurgy (PM) nickel-based superalloy. It plays anincreasingly important role in the development andmanufactureofturbinedisksowingtoitsuniqueproper-ties such as high oxidation resistance and corrosionresistanceevenat veryhightemperatures. Whencompo-nents in aerospace industry are manufactured with FGH95superalloy to reach high reliability levels, surface integrityis one of the most relevant parameters used for evaluatingthe quality of finish machined surfaces [7]. The machinedsurface integritysuchas surface roughness andsurfacealteration (work hardening and white layer) has significanteffects on the surface properties such as fatigue, stresscorrosion resistance, and creep strength, which in turnaffects the service life of components. Therefore, highdegreeofFGH95surfaceintegrityisanessentialrequire-ment for better performance, reliability, andlongevityofthe machinedparts during service[8, 9].Extensive past researches have been concerned withsurfaceintegrityandchipformationespeciallyforInconel718 induced during machining [1015]. Although knowledgeof the surface integrity of PMnickel-based superalloyproducedinhigh-speedmachiningiscritical toitsperfor-mance, there is a lack of research done in this area. Cuttingforce and cutting temperature decrease with the cutting speedincreasesinhigh-speedmachining. Itisthereforeimportanttoknowhowtheadjustment of cuttingspeedaffects thesurfaceintegrityinhigh-speedmachining. Bearingthesein mind, it is critical to further study surface integrityandchipmorphologyinhigh-speedmachiningofFGH95PMnickel-basedsuperalloy. Thus, theaimof this workistofillthegapintheliteraturebydescribinganin-depthinvestigation of the evolution of FGH95 surface integrity andchipmorphologyinhigh-speedmachiningofFGH95. Inthis paper, the effects of cutting speed on the surfaceintegrity of PM nickel-based superalloy FGH95 includingsurface roughness, microhardness, and white layer inhigh-speedmillingarepresented. The effects of cuttingspeedonchipmorphologyarealsodescribed.2 Experimental works2.1 Work material and cutting toolsThe material in the cutting experiments was PM nickel-basedsuperalloy FGH95. The chemical composition of theFGH95 includes: Ni 62.63%, Cr 12.98%, Co 8.00%,Nb3.50%, Al 3.48%, W3.40%, Mo3.40%, Ti 2.55%,andC0.06%[16].The sheet specimens of FGH95 with size 60402.5mmwereusedforthecuttingexperiments, whichareshowninFig. 1a. InFig. 1b, thecuttinginsert geometryapplied in this experiment is also shown. Its full descriptionis SNHX12L5PZTNGP KC725Mwith TiNand AlTiNadvancedPVDcoatings. KC725Misahigh-performancegrade for milling steel, stainless steel, and superalloy. Goodthermal shockresistanceofthesubstratemakesthisgradeideal for dry high-speed machining. Before each experimentalcutting, theinsert waschangedtoafreshoneinorder toeliminate the influence of tool wear on the machined surfaceintegrity.2.2 Orthogonal milling testsHigh-speed machining tests were carried out on a three-axisCNCmachiningcenter withamaximumspindlerotationspeedof 12,000rpm. Orthogonal millingoperations, i.e.,feeding the tool fromthe cylinder surface towards thecenter of thecylinder, wereperformedunder drycuttingconditions. The experimental setup is shown in Fig. 2. Thediameter of the tool used in this experiment is 160 mm. Inthis paper, the surface integrity in terms of surfaceroughness, workhardening, andwhitelayerofhigh-speedmillingFGH95is presentedwhenthecuttingspeeds arechanged. Thecuttingspeedsemployedinthisexperimentwere800, 1,200,1,600,2,000,2,400,2,800,3,200,3,600,and4,000m/min. The axial depthof cut andfeedwasmaintainedconstantat2mmand0.02mm/r,respectively.Whenmachiningprocesses were finished, the workpiecesheetswereuninstalledandremovedfor measurement oftheir machined surface roughness.60mm40mm2mm5.4mm12.7mm12.7mm(a) Workpiece sheet (b) Cutting insertFig. 1 Workpiece and cuttinginsert employed in theexperiments894 Int J Adv Manuf Technol (2012) 60:893899After the measurement of surface roughness, themachinedworkpiecesheets were cut into square specimens(1010mm) inorder tobe mountedinthe bakelite asshown in Fig. 3. Then, thesamples werepolishedandetched with 2.5%copper chloride, 48.8%hydrochloricacid,and48.7%ethanoltoanalyzepossiblemetallurgicalchanges beneaththemachinedsurfaces usinganopticalmicroscope. The same samples were usedfor assessingmicrohardnessdistributionbeneaththemachinedsurface.Chipswerealsocollectedduringmachiningtestsfor theinvestigations of the effects of cutting speeds on chipmorphology.2.3 Surfaceintegrity measurementsIn this investigation, machined surface integrity wassystematicallycharacterizedbysurfaceroughness, micro-hardness, and microstructure changes (white layer). Theroughness of machined surface after each test was measuredusing WYKONT9300 white light interferometer. Themeasuredsurfaceareawas 94.2125.7m. Thecontourarithmetic mean deviation of the profile, i.e., surfaceroughness Ra, was measuredandrepeatedthreetimes ateachpoint onthefaceof themachinedsurface, andtheaveragevalueswere adopted.Vickers hardness tester was employed with a load of 50 gf(0.49N) tomeasurethemachinedsurfacemicrohardness.Microhardness of 495 HV was recorded for bulk material ofFGH95. The indentation for microhardness measurementwas penetrated at every 20 m depth beneath the machinedsurface until the hardness was achievedat 495HVforbulk material. The microhardness measurements wererepeated three times for each inspection point, and theaveragevalueswerefinallyadopted. A digital microscope(Keyence VHX600E) andscanningelectronmicroscope(SEM) were used for the analysis of the chip morphologyandmicrostructure changeof themachinedsurface.3 Results and discussionsTheresults anddiscussions arefocusedontheworkpiecesurface integrity and chip morphology aspects on high-speedmachining of PM nickel-based superalloy FGH95.3.1 SurfaceroughnessSurface roughness is a widely used index of machinedsurface quality; in mostcases, it is a technical requirementformechanical productswhichplaysanimportant roleinparts of accuracy and service life. In this paper, the effect ofcuttingspeedonsurfaceroughnesshasbeeninvestigatedduring high-speed machining FGH95. The workpiecesurface roughness measurements (Ra across the feeddirection, sampling length is 94 m, sampling width is126 m) are presented as shown in Fig. 4.It canbeseenthat theeffect of cuttingspeedonthesurface roughness is significant. In Fig. 4, surface roughnessWorkpiece specimenBakeliteMachined surfaceMachined surfaceFig. 3 Workpiece specimen mountingFig. 2 Schematic of orthogonal milling00.10.20.30.40.50 800 12001600200024002800320036004000Cutting speed(m/min)Surface roughness(m)Fig. 4 Variation of surface roughness with cutting speedInt J Adv Manuf Technol (2012) 60:893899 895values are in the range of 0.160.46 m which is generallyrequiredfor components suchas turbinedisks. Whenthecutting speeds are below 2,400 m/min, the values of surfaceroughness have little change. Surface roughness valuesincrease with the increase of cutting speed. Figure 5aexhibits the surface topography obtained by WYKONT9300 white light interferometer at cutting speed of800m/min. Themaximumvalueof surfaceroughness isachievedwhencuttingspeedis 3,200m/min; its surfacetopography is presented in Fig. 5b. With further increase ofcutting speed, the values of surface roughness decrease. Thereason for this tendency is PMnickel-based superalloyFGH95 is a kind of plastic material, and lower cutting speedcaninduce smaller plastic deformationandlower cuttingtemperature; thus, the values of surface roughness are lower.When cutting speeds are in the range of 2,8003,600 m/min,larger plastic deformation, build-up tool edge, and burrscomeintoformationowingtotheexistenceoffrictionandextrusion. Thus, the value of surface roughness increaseshigher. However, with the further increase of cuttingspeed,the build-up tool edge and burrs are decreased or evendisappeared, andat thesametime, plasticdeformationofworkpieceis alsoreduced. Thesurfaceroughness is thendecreased. It can be seen that among the nine cutting speedvalues, except for cutting speed in the range of 2,8003,600 m/min, good surface roughness values can beachieved.3.2 MicrohardnessWork hardening of the deformed layer beneath themachinedsurface upto160mcausedhigher hardnessthan the average hardness of the bulk material. Thus,subsurfaceof machinedsamplescanbedividedintotworegions asexhibited inFig.6, that is, hardened region andbulkmaterial region. Microhardnesswasmeasuredunderdifferent depths beneath the machined surface. The measure-ments of microhardness were performed three times at every20 m depth up to 160 m beneath the machined surface, andtheaverageresultforeachdepthwasrecorded andplotted.The results of microhardness versus the indent positionbeneath the machined surface for machined surface samplesproducedat variouscuttingspeedsareshowninFig. 7.Eachmicrohardnessprofilerepresentstheaveragedthreesets of microhardness data. The microhardness profiles arecharacterized by the higher surface hardness (520590 HV)and stable bulk hardness (475 HV). It means the machinedsurface materials experienced significant strain hardening intherangeof 9.4724.2%inducedbysurfacedeformation.Furthermore, higher cutting speeds generate larger deforma-tiononthemachinedsurfacewhichinturninduceshighersurface hardness. The higher the cutting speeds, the higher thesurface hardness.3.3 White layerThe termwhite layer originates fromthe fact that this surfaceappears white under an optical microscope or featureless in ascanning electron microscope (SEM) [17, 18]. Thus, inliterature, thetermwhitelayer isusedasagenericphrasereferring to very hard surface layers formed under a variety ofconditions, which appear white under the microscope.Superficial white layers formed during high-speed machininghavenegativeeffectsonfatiguestrengthofmachinedparts.Whitelayer is oftenassociatedwithresidual tensilestressleading to reduced fatigue strength and poor wear resistance[1922].Superficial white layers fall into three main areas assuggested by Griffiths [23], namely, those at the surface ofengineering components, those resulting fromlaboratoryexperiments, and those formed as a result of manufacturingprocesses. The three main mechanisms of white layerformationare plastic flow, rapidheatingandquenching,and surface reaction. In practice, it is conceivable that thesemechanisms cannot be separated, and white layer formationresultsfromacombinationof mechanismswithlesser ora 800m/min b 3200m/min896 Int J Adv Manuf Technol (2012) 60:893899Fig. 5 WYKO images of surfacetopography. a v=800 m/min, b v=3,200 m/mingreater degree of activation. Figure 8shows the typicalcharacteristic of the white layer generatedinhigh-speedmachining of FGH95 at a cutting speed of 800 m/min.It can be seen from Fig. 8 that the typical thickness of awhite layer zone is approximately10m. However, thethickness of white layer can vary depending on the thermal,mechanical, or chemical properties present. Belowthewhitelayer, thereisaplasticdeformationregionwhichislargely affected by the machining process.Cutting speed has a significant influence on thethickness of white layer. Figure 9shows that the whitelayer thickness reduces as the cutting speed increases. It canbeseenfromFig.9thatthewhitelayerthicknessremainsapproximately constant when cutting speeds are in therange of 8002,400 m/min. When the cutting speeds reachaparticular rangeof 2,8003,600m/min, thewhitelayerthickness is slightly higher than that of other cutting speeds.At the cutting speed range of 2,8003,600 m/min, thethermalinfluenceis very essential as the machined surfaceisheatedandcooleddownrapidlybytheunheatedmassandenvironment. Therapidheatingupandcoolingdownprocess results in the machined surface microstructuretransformation which is known as a modified layer.Anotherprimaryreasontogeneratethewhitelayercanbeattributedtothesevereplasticdeformationat thecuttingspeed range of 2,8003,600 m/min. When the cuttingspeeds exceed3,600m/min, thewhitelayer thickness issimilar to that of cutting speeds at the range of 8002,400 m/min.3.4 Chip morphologyChipshapes, i.e., continuousanddiscontinuous(sawtoothor serratedtype), inmachiningof FGH95dependonthecombinedeffectsofworkpieceandinsert material proper-4004505005506006500 20 40 60 80 100 120 140 160800m/min 1600m/min2400m/min 3200m/min4000m/minIndent position beneath the machined surface(m)Microhardness(HV)Fig. 7 The microhardness values measured on the subsurface ofmachined surface samplesFig. 6 Themicrohardness regionbeneaththemachinedsurfaceofFGH95White layerPlastic deformation regionWhite layerPlastic deformation regionFig. 8 Whitelayer inFGH95machinedsurfaceformedat cuttingspeed of 800 m/min5791113150 800 12001600200024002800320036004000Cutting speed(m/min)White layer thickness(m)Fig. 9 Typical variation of white layer thickness with cutting speedInt J Adv Manuf Technol (2012) 60:893899 897ties, cutting speed, feed rate, and cutting tool geometry[24]. The comprehension of chip formation plays animportant role in hard milling process optimization andsurface integrity, and thus part performance. Sawtoothchips are usually formed in hard machining at highspeeds and represent an essential feature of the chipmorphology[25].Sawtoothchipformationisatwo-stageprocess in which workpiece material is plastically deformedahead of the tool causing it to bulge. When a criticalstrainlevel is reached, catastrophicfailureoccurs andashear bandisformedextendingfromthetool tiptotheworkpiecesurface[26].As is generallyknown, thechipsegmentationis verydistinct in a nickel-based superalloy. The chip formation forPMnickel-based superalloy FGH95 is described in thecutting speed range of 800 and 4,000 m/min. In high-speedmachiningof PMnickel-basedsuperalloy, serratedchipswere observed even at relative lower cutting speeds asshowninFig. 10. It canbeseenfromFig. 10that whenmachiningwithcuttingspeedof 800m/min, thechipissegmented and has a typical sawtooth shape. The influencesof cutting speeds on the chip formation are shown inFig. 11. Figure11showsthedegreeof serratedincreaseswiththeincreaseincuttingspeed. Whencuttingspeedis2,000m/min, thechipshapeisheavilysegmented. Whencutting speeds exceed 2,400 m/min, serrated chips changedinto fragment chips.4 ConclusionsTheexperimental investigationof cuttingspeedinfluenceonsurfaceintegrityandchipmorphologywithhigh-speedmachiningof powder metallurgynickel-basedsuperalloyFGH95 is presentedinthis paper. The mainresults aresummarized in the following:1. The surface integrity of FGH95 resulting fromtheorthogonalmillingprocessathighmachiningspeedissignificantlyaffectedbythevariationincuttingspeedand in the range of experimental cutting speedsobserved in this work.EnlargedFig. 10 SEM images of chip morphology in high-speed machining of FGH95 formed at cutting speed of 800 m/min(a)V=800m/min (b)V=2200m/min (c)V=2400m/minFig. 11 Chip morphology inhigh-speed machining ofFGH95 (100)898 Int J Adv Manuf Technol (2012) 60:8938992. Values of surface roughness showlittle variation atcutting speeds below 2,000 m/min. Maximum value ofsurfaceroughnesswasobservedat acuttingspeedof3,200m/min, and surfaceroughnessdecreased atevenhigher cutting speeds.3. There exists severe workhardeningwhenhigh-speedmachining FGH95. Observed values of surface hardnessincrease with cutting speed.4. Except for cutting speeds in particular range of2,8003,600 m/min, the white layer thickness is slightlyhigher. 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Annal CIRP ManufTechnol 57:9396Int J Adv Manuf Technol (2012) 60:893899 899