文献翻译原文在新材料精密加工的材料去除机制.docx

文献翻译原文在新材料精密加工的材料去除机制.docx

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文献翻译原文在新材料精密加工的材料去除机制

Materialremovalmechanismsinprecisionmachiningofnewmaterials

Abstract

Modern-dayproductsarecharacterisedbyhigh-precisioncomponents.Awiderangeofmaterials,includingmetalsandtheiralloys,ceramics,glassesandsemiconductors,arefinishedtoagivengeometry,finish,accuracyandsurfaceintegritytomeettheservicerequirements.Foradvancedtechnologysystems,demandsforhigherfabricationprecisionarecomplicatedbytheuseofbrittlematerials.Forefficientandeconomicalmachiningofthesematerials,anunderstandingofthematerialremovalmechanismisessential.Thispaperfocusesonthedifferentmaterialremovalmechanismsinvolvedinmachiningofbrittlematerials.2001PublishedbyElsevierScienceLtd.

Keywords:

Brittle;Defects;Ductility;Materialremoval;Precisionmachining

1.Introduction

Ultra-precisionmachiningtechnologyhasbeendevelopedoverrecentyearsforthemanufactureofcost-effectiveandquality-assuredprecisionpartsforseveralindustrialapplicationssuchaslasers,optics,semiconductors,aerospaceandautomobileapplications.Precisionmanufacturingdealswiththerealisationofproductswithhighshapeaccuracyandsurfacequality.Theaccuracymaybeatthenanometriclevel.Severalmachiningtechniquescanbementionedherelikediamondturning,grinding,lapping,polishing,honing,ionandelectron-beammachining,lasermachining,etc.EfficientoverviewsoftheprocessesaregiveninRefs.[1–3].

Ultra-precisionmachiningtechnologyhasbeenhighlydevelopedsincethe1980smainlybecauseofitshighaccuracyandhighproductivityinthemanufacturingofoptical,mechanicalandelectroniccomponentsforindustrialuse.Formanyadvancedtechnologysystems,higherfabricationprecisioniscomplicatedbytheuseofbrittlematerials.Thepastdecadehasseenatremendousresurgenceintheuseofceramicsinstructuralapplications.Theexcellentthermal,chemicalandwearresistanceofthesematerialscanberealisedbecauseofrecentimprovementsintheoverallstrengthanduniformityofadvancedceramics[4].

Ceramicmaterialshavebeenwidelyadaptedasfunctionalmaterialsaswellasstructuralmaterialsinvariousindustrialfieldsandtheirapplicationtoprecisionpartsisalsoincreasing[5].

However,thehighdimensionalaccuracyandgoodsurfacequalityrequiredforprecisionpartsarenotnecessarilyobtainedbytheconventionalformingandsinteringprocessofceramicpowders.Thusprecisionfinishingoftheceramicsafterformingandsinteringisrecognisedasakeytechnologytoprecisionceramicparts[6].

Thequantityofceramicmaterialtoberemovedbythefinishingprocessmustbeverysmall,sothatmicrocracksdonotremainonthefinishedsurface.Abrasiveprocessessuchasgrindingorlappingwithdiamondabrasiveshavegenerallybeenadoptedforprecisionfinishingofceramics[7–9].

However,itisexpectedthatbettersurfaceintegrityandhigherproductionratescanberealisedbycuttingprocesses.Comparedwithotherprocesses,cuttingisalsoadvantageousinmachiningcomplexshapes.Brittlematerialscanbedividedintothreegroups:

amorphousglasses,hardcrystalsandadvancedceramics.Advancedceramicsareamoderndevelopment.Theyaremadefromfineporousparticlesthatareformed,consolidatedandthermallytreatedunderpreciselycontrolledconditions.Useofthesematerialsenablesdevelopmentofhigh-technologydevicesandsystemsthatsimplycouldnotbeproducedotherwise[10].

Thesamestatementcouldbemadeabouttheuseofcertaincrystallinematerials（e.g.,semiconductors）andadvancedhigh-temperatureglasses.

2.Ductileregimemachining

Improvementsinmachiningtoleranceshaveenabledresearcherstoexposetheductilematerialremovalofbrittlematerials.Undercertaincontrolledconditions,itispossibletomachinebrittlematerialslikeceramicsusingsingle-ormulti-pointdiamondtoolssothatmaterialisremovedbyplasticflow,leavingacrack-freesurface（Fig.4）.Thisprocessiscalledductileregimemachining.

Ductileregimemachiningfollowsfromthefactthatallmaterialswilldeformplasticallyifthescaleofdeformationisverysmall.AnotherwayofviewingtheductileregimemachiningproblemisthatdescribedbyMiyashita[17],asshowninFig.5.Thematerialremovalratesforgrindingandpolishingarecomparedandthereisagapinwhichneithertechniquehasbeenutilised.Thisregioncanbetermedthemicro-grindinggapsincetheregionliesinbetweengrindingandpolishing.Thisgapisimportantbecauseitrepresentsthethresholdbetweenductileandbrittlegrindingregimesforawiderangeofmaterialslikeceramics,glassesandsemiconductors.

2.1.Principleofductileregimemachining

Thetransitionfrombrittletoductilemodeduringmachiningofbrittlematerialsisdescribedintermsoftheenergybalancebetweenstrainenergyandsurfaceenergy[18].Localisedfracturesproducedduringapplicationofloadareofinterestinmachiningofbrittlematerials.Machiningisanindentationprocessduringwhichindentationcracksaregenerated,andthesecracksplayanimportantroleinductileregimemachining[19].

Acriticalpenetrationdepthdcforfractureinitiationisdescribedasfollows[20]

whereKcisthefracturetoughness,Histhehardness,Eistheelasticmodulusandbisaconstantwhichdependsontoolgeometry.Fig.6showsaprojectionofthetoolperpendiculartothecuttingdirection.Accordingtotheenergybalanceconcept,fracturedamagewillinitiateattheeffectivecuttingdepthandwillpropagatetoanaveragedepthyc.Ifthedamagedoesnotcontinuebelowthecutsurfaceplane,ductileregimeconditionsareachieved.Thecross-feedfdeterminesthepositionofdcalongthetoolnose.Largervaluesoffmovedcclosertothetoolcentreline.Anotherinterpretationofductiletransitionphenomenaisbasedoncleavagefractureduetothepresenceofdefects[21].Thecriticalvaluesofacleavageandplasticdeformationareaffectedbythedensityofdefects/dislocationsintheworkmaterial.Sincethedensityofdefectsisnotsolargeinbrittlematerials,thecriticalvalueoffracturedependsonthesizeofthestressfield.Fig

7showsamodelofchipremovalwithsizeeffects.Whentheuncutchipthicknessissmall,thesizeofthecriticalstressfieldissmalltoavoidcleavage.Consequentlyatransitioninthechip

2.2.Materialremovalmechanismsinductileregimemachining

Machininggeneratesausefulsurfacebyintimatecontactoftwomatingsurfaces,namelytheworkpieceandabrasivetool.However,themicromechanismsofmaterialremovaldifferfrommaterialtomaterialdependinguponthemicrostructureofbothworkpieceandtoolmaterial.

Generally,duringhigh-precisionmachiningofbrittlematerials,toolshavinglargenegativerakeanglesareused（ashighas-30︒）.Thenegativerakeangleprovidestherequiredhydrostaticpressureforenablingplasticdeformationoftheworkmaterialbeneaththetoolradius.Duringconventionalmachiningwithasingle-pointtool,therakeanglewillbepositiveorcloseto0︒.Withpositiverakeangle,thecuttingforcewillgenerallybetwicethethrustforce.Hencethedeformationaheadofthetoolwillbeinaconcentratedshearplaneorinanarrowplaneasshown

inFig.8.Duringthegrindingprocess,itisgenerallyagreedthatthetoolwillhavealargenegativerakeangleandalsothatthecuttingforceisabouthalfofthethrustforce[Fig.8（b）].Inultraprecisionmachiningofbrittlematerialsatdepthsofcutsmallerthanthetooledgeradius,thetoolpresentsalargenegativerakeangleandtheradiusofthetooledgeactsasanindenterasshowninFig.8（c）.Thisrepresentsindentationslidingofabluntindenteracrosstheworkpiecesurface.ThisissimilartoasituationwherethetoolisrigidlysupportedandcutstheworkpieceunderastresssuchthatnomedianventsaregeneratedbutthematerialbelowthetoolisplasticallydeformedduetolargehydrostaticpressureasinFig.8（d）.

3.Materialremovalinglassandceramics

Theductilegrindingofopticalglassisconsideredasthemostperfectadaptationofamachiningmethodtothematerial[22].Glassisaninorganicmaterialsupercooledfromthemoltenstatetothesolidstatewithoutcrystallising.Glassesarenon-crystalline（oramorphous）andrespondintermediatebetweenaliquidandasolid;i.e.,atroomtemperaturetheybehaveinabrittlemanner

1838P.S.Sreejith,B.K.A.Ngoi/InternationalJournalofMachineTools&Manufacture41（2001）1831–1843

butabovetheglasstransitiontemperatureinaviscousmanner.Thehighbrittlenessofglassisduetotheirregulararrangementofatoms.Incrystallinematerialslikemetals,theatomshaveafixedarrangementandregularitydescribedbyMillerindices,whereasglassstructuredoesnotshowanydefiniteorientation[23].

Theuniquephysicalandmechanicalpropertiesofceramicssuchashardnessandstrength,chemicalinertnessandhighwearresistancehavecontributedtotheirincreasedapplicationinmechanicalandelectricalcomponents.Theadvancedceramicsforstructuralandwearapplicationsincludealumina（Al2O3）,siliconnitride（Si3N4）,siliconcarbide（SiC）,zirconia（ZrO2）andSiAlON.ThenatureofatomicbondingdeterminesthehardnessofthematerialaswellastheYoung’smodulus.Forductilemetallic-bondedmaterialstheratioE/Hisabout250,whileforcovalentbondedbrittlematerialstheratioisabout20.Theratiowilllieinbetweenthesevaluesforionicbondedmaterials.Lowdensityandlowmobilityofdislocationsarethereasonsforthehighhardnessofsomeofbrittlematerials.

4.Gentlegrinding

Thereisanalternativehypothesiscalled“gentle”machiningwhereinitisbelievedthatplasticdeformationisnotinvolvedexclusi