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Titaniumchemicallyreactswiththeoxideofthecoatingandlimitstool life. As a consequence the coatings AlCrN T andAl O areno longer consideredfor continuouscutof Ti–236Al–4V.ThecorrelationbetweenresidualstressesofthecoatingsIncombinationofinitialwearandwearatthetoollifeandflankwearincontinuouscutofTi–6Al–4Visshownincriterion t =20min the coating AlTiVXN exhibits acminimalwidthofflankwearland.Thiscanbeexplainedbythecompositionofthiscoating.VanadiumfromthecoatingFig.8. The measured wear after t =1min is correlatedcwith the residual stresses near the surface of the coatingsrsurface (Fig.8, I). Wear after t =20min is correlatedc123 Prod.Eng.Res.Devel.(2016)10:147–155153diffuses at the coatings surface with high temperatureswhicharesignificantforcuttingoftitaniumalloys.Atthecoating surface vanadium oxidizes, melts and thus func-tions self-lubricating and decreases friction between theabrasivetitaniumalloyandthetool[30].Theresultspointoutanincreasingwearresistanceincontinuouscuttingoftitanium with slightly compressive residual stresses nearthesurfaceandnearthesubstrate.However,theAlCrNCcoating with high residual stresses near substrate exhibitscutting time tc = 1.5 minAlTiVXNTiAlSiXNchipping500 µm500 µmslight flank wear after a cutting time of t =10min.cConsequentlythe subordinately consideredlayer structureofthecoatingsandtheirinfluenceonthewearbehaviorhastobeinvestigatedinfurtherstudies.process:tool:external turningCNMG 120412 MSworkpiece: 42CrMo4interruptedprocess parameters:vc = 200 m/minf = 0.2 mmap = 1 mmcut:4.2 InterruptedcutRi/79928 ©IFWToollifeininterruptedcutof42CrMo4islimitedbytoolbreakage due to dynamic thermal–mechanical load [31].Rapidly growing crater wear causes crater lips and thusweakensthetooledge.Figure9summarizesthetoollifeofthe used coatings. Regardless of residual stress state andcompositionthecoatedtoolsofgroupsA–CfailedduetoFig.10
Chippingininterruptedcutof42CrMo4residualstressesfromsurfacetosubstrateincreasetoollifeandtheresistanceagainsttoolbreakage.Figure10showsexemplarytherakefaceandchippingoftheAlTiVXNcoatingincomparisontotherakefaceofbreakageofthecraterlipatacuttingtimeoft =3minorctheTiAlSiXNcoatingafteracuttingtimeoft =1.5min.cless.ThecoatingsofgroupsDandEachieveasignificanthighercuttingtimeininterruptedcutof42CrMo4.Similarto the continuous cut the oxide Al O coating is moreCoatings with slightly compressive residual stresses likeAlTiVXN failed due to chipping in interrupted cut. TheTiAlSiXNcoatingwithhighcompressiveresidualstressesnearthesubstrateshowsnochippingafterthesamecutting23resistant to crater wear than nitride coatings and no toolbreakageduetoaloadedcraterliplimitstoollife.Reasonis the chemical resistance of the oxide coating againstcraterwear.timeoft =1.5min.cIninterruptedcutofTi–6Al–4Vcoatingsandsubstrateare loaded with tensile stresses because of the cuttingconditionsandchipformation.Thisresultsinashorttoollife because of a rapidly increasing flank wear or toolbreakage (Fig.11). Independent from residual stress stateand composition the used coatings failed due to toolbreakageininterruptedcutofTi–6Al–4VafteramaximumThe high compressive residual stresses of the TiAl-SiXN-coated tools increase stability of the crater lip.Contrary to the continuous cut increasing compressivecoating groupcuttingtimeoft =4min.NosignificantinfluenceofthecABCDE12min8coating properties on the cutting performance can beidentified.Furtherresearchoftoolandcoatingperformancein interrupted cut of Ti–6Al–4V is required. Due to thethermo-
mechanicalloadsinthisprocessavariationoftheprocess parameters and the substrate of the tool providesrudimentsforinvestigations.6420tool life critera:5 Conclusionstc = 10 min, tool failure due to crater wear or chipping (X)process:tool:workpiece: 42CrMo4cut: interruptedexternal turningCNMG 120412 MSprocess parameters:vc = 200 m/minf = 0.2 mmIn this study the influence of residual stress state andcomposition of nine different coatings on cutting perfor-manceincontinuous andinterrupted cutof42CrMo4 andTi–6Al–4V wasinvestigated. Thedepthresolved residualstress states of the coatings were measured by the scat-teringvectormethod.Theinteractionsofresidualstresses,ap = 1 mmRi/79925 ©IFWFig.9