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外文翻译--切削加工性.doc

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外文翻译--切削加工性.doc

英文原文Themachinabilityofamaterial1.Themachinabilityofamaterialusuallydefinedintermsoffourfactors1、Surfacefinishandintegrityofthemachinedpart2、Toollifeobtained3、Forceandpowerrequirements4、Chipcontrol.Thus,goodmachinabilitygoodsurfacefinishandintegrity,longtoollife,andlowforceandpowerrequirements.Asforchipcontrol,longandthinstringycuredchips,ifnotbrokenup,canseverelyinterferewiththecuttingoperationbybecomingentangledinthecuttingzone.Becauseofthecomplexnatureofcuttingoperations,itisdifficulttoestablishrelationshipsthatquantitativelydefinethemachinabilityofamaterial.Inmanufacturingplants,toollifeandsurfaceroughnessaregenerallyconsideredtobethemostimportantfactorsinmachinability.Althoughnotusedmuchanymore,approximatemachinabilityratingsareavailableintheexamplebelow.2.MachinabilityOfSteelsBecausesteelsareamongthemostimportantengineeringmaterialsasnotedinChapter5,theirmachinabilityhasbeenstudiedextensively.Themachinabilityofsteelshasbeenmainlyimprovedbyaddingleadandsulfurtoobtainsocalledfreemachiningsteels.ResulfurizedandRephosphorizedsteels.Sulfurinsteelsformsmanganesesulfideinclusionssecondphaseparticles,whichactasstressraisersintheprimaryshearzone.Asaresult,thechipsproducedbreakupeasilyandaresmallthisimprovesmachinability.Thesize,shape,distribution,andconcentrationoftheseinclusionssignificantlyinfluencemachinability.Elementssuchastelluriumandselenium,whicharebothchemicallysimilartosulfur,actasinclusionmodifiersinresulfurizedsteels.Phosphorusinsteelshastwomajoreffects.Itstrengthenstheferrite,causingincreasedhardness.Hardersteelsresultinbetterchipformationandsurfacefinish.Notethatsoftsteelscanbedifficulttomachine,withbuiltupedgeformationandpoorsurfacefinish.Thesecondeffectisthatincreasedhardnesscausestheformationofshortchipsinsteadofcontinuousstringyones,therebyimprovingmachinability.LeadedSteels.Ahighpercentageofleadinsteelssolidifiesatthetipofmanganesesulfideinclusions.Innonresulfurizedgradesofsteel,leadtakestheformofdispersedfineparticles.Leadisinsolubleiniron,copper,andaluminumandtheiralloys.Becauseofitslowshearstrength,therefore,leadactsasasolidlubricantandissmearedoverthetoolchipinterfaceduringcutting.Thisbehaviorhasbeenverifiedbythepresenceofhighconcentrationsofleadonthetoolsidefaceofchipswhenmachiningleadedsteels.Whenthetemperatureissufficientlyhighforinstance,athighcuttingspeedsandfeedstheleadmeltsdirectlyinfrontofthetool,actingasaliquidlubricant.Inadditiontothiseffect,leadlowerstheshearstressintheprimaryshearzone,reducingcuttingforcesandpowerconsumption.Leadcanbeusedineverygradeofsteel,suchas10xx,11xx,12xx,41xx,etc.LeadedsteelsareidentifiedbytheletterLbetweenthesecondandthirdnumeralsforexample,10L45.Notethatinstainlesssteels,similaruseoftheletterLmeanslowcarbon,aconditionthatimprovestheircorrosionresistance.However,becauseleadisawellknowntoxinandapollutant,thereareseriousenvironmentalconcernsaboutitsuseinsteelsestimatedat4500tonsofleadconsumptioneveryyearintheproductionofsteels.Consequently,thereisacontinuingtrendtowardeliminatingtheuseofleadinsteelsleadfreesteels.Bismuthandtinarenowbeinginvestigatedaspossiblesubstitutesforleadinsteels.CalciumDeoxidizedSteels.Animportantdevelopmentiscalciumdeoxidizedsteels,inwhichoxideflakesofcalciumsilicatesCaSoareformed.Theseflakes,inturn,reducethestrengthofthesecondaryshearzone,decreasingtoolchipinterfaceandwear.Temperatureiscorrespondinglyreduced.Consequently,thesesteelsproducelesscraterwear,especiallyathighcuttingspeeds.StainlessSteels.Austenitic300seriessteelsaregenerallydifficulttomachine.Chattercanbeaproblem,necessitatingmachinetoolswithhighstiffness.However,ferriticstainlesssteelsalso300serieshavegoodmachinability.Martensitic400seriessteelsareabrasive,tendtoformabuiltupedge,andrequiretoolmaterialswithhighhothardnessandcraterwearresistance.Precipitationhardeningstainlesssteelsarestrongandabrasive,requiringhardandabrasionresistanttoolmaterials.TheEffectsofOtherElementsinSteelsonMachinability.Thepresenceofaluminumandsiliconinsteelsisalwaysharmfulbecausetheseelementscombinewithoxygentoformaluminumoxideandsilicates,whicharehardandabrasive.Thesecompoundsincreasetoolwearandreducemachinability.Itisessentialtoproduceandusecleansteels.Carbonandmanganesehavevariouseffectsonthemachinabilityofsteels,dependingontheircomposition.Plainlowcarbonsteelslessthan0.15Ccanproducepoorsurfacefinishbyformingabuiltupedge.Caststeelsaremoreabrasive,althoughtheirmachinabilityissimilartothatofwroughtsteels.Toolanddiesteelsareverydifficulttomachineandusuallyrequireannealingpriortomachining.Machinabilityofmoststeelsisimprovedbycoldworking,whichhardensthematerialandreducesthetendencyforbuiltupedgeformation.Otheralloyingelements,suchasnickel,chromium,molybdenum,andvanadium,whichimprovethepropertiesofsteels,generallyreducemachinability.Theeffectofboronisnegligible.Gaseouselementssuchashydrogenandnitrogencanhaveparticularlydetrimentaleffectsonthepropertiesofsteel.Oxygenhasbeenshowntohaveastrongeffectontheaspectratioofthemanganesesulfideinclusionsthehighertheoxygencontent,thelowertheaspectratioandthehigherthemachinability.Inselectingvariouselementstoimprovemachinability,weshouldconsiderthepossibledetrimentaleffectsoftheseelementsonthepropertiesandstrengthofthemachinedpartinservice.Atelevatedtemperatures,forexample,leadcausesembrittlementofsteelsliquidmetalembrittlement,hotshortnessseeSection1.4.3,althoughatroomtemperatureithasnoeffectonmechanicalproperties.Sulfurcanseverelyreducethehotworkabilityofsteels,becauseoftheformationofironsulfide,unlesssufficientmanganeseispresenttopreventsuchformation.Atroomtemperature,themechanicalpropertiesofresulfurizedsteelsdependontheorientationofthedeformedmanganesesulfideinclusionsanisotropy.Rephosphorizedsteelsaresignificantlylessductile,andareproducedsolelytoimprovemachinability.3.MachinabilityofVariousOtherMetalsAluminumisgenerallyveryeasytomachine,althoughthesoftergradestendtoformabuiltupedge,resultinginpoorsurfacefinish.Highcuttingspeeds,highrakeangles,andhighreliefanglesarerecommended.Wroughtaluminumalloyswithhighsiliconcontentandcastaluminumalloysmaybeabrasivetheyrequirehardertoolmaterials.Dimensionaltolerancecontrolmaybeaprobleminmachiningaluminum,sinceithasahighthermalcoefficientofexpansionandarelativelylowelasticmodulus.Berylliumissimilartocastirons.Becauseitismoreabrasiveandtoxic,though,itrequiresmachininginacontrolledenvironment.Castgrayironsaregenerallymachinablebutare.Freecarbidesincastingsreducetheirmachinabilityandcausetoolchippingorfracture,necessitatingtoolswithhightoughness.Nodularandmalleableironsaremachinablewithhardtoolmaterials.Cobaltbasedalloysareabrasiveandhighlyworkhardening.Theyrequiresharp,abrasionresistanttoolmaterialsandlowfeedsandspeeds.Wroughtcoppercanbedifficulttomachinebecauseofbuiltupedgeformation,althoughcastcopperalloysareeasytomachine.Brassesareeasytomachine,especiallywiththeadditionleadleadedfreemachiningbrass.Bronzesaremoredifficulttomachinethanbrass.Magnesiumisveryeasytomachine,withgoodsurfacefinishandprolongedtoollife.Howevercareshouldbeexercisedbecauseofitshighrateofoxidationandthedangeroffiretheelementispyrophoric.Molybdenumisductileandworkhardening,soitcanproducepoorsurfacefinish.Sharptoolsarenecessary.Nickelbasedalloysareworkhardening,abrasive,andstrongathightemperatures.Theirmachinabilityissimilartothatofstainlesssteels.Tantalumisveryworkhardening,ductile,andsoft.Itproducesapoorsurfacefinishtoolwearishigh.Titaniumanditsalloyshavepoorthermalconductivityindeed,thelowestofallmetals,causingsignificanttemperatureriseandbuiltupedgetheycanbedifficulttomachine.Tungstenisbrittle,strong,andveryabrasive,soitsmachinabilityislow,althoughitgreatlyimprovesatelevatedtemperatures.Zirconiumhasgoodmachinability.Itrequiresacoolanttypecuttingfluid,however,becauseoftheexplosionandfire.4.MachinabilityofVariousMaterialsGraphiteisabrasiveitrequireshard,abrasionresistant,sharptools.Thermoplasticsgenerallyhavelowthermalconductivity,lowelasticmodulus,andlowsofteningtemperature.Consequently,machiningthemrequirestoolswithpositiverakeanglestoreducecuttingforces,largereliefangles,smalldepthsofcutandfeed,relativelyhighspeeds,andpropersupportoftheworkpiece.Toolsshouldbesharp.Externalcoolingofthecuttingzonemaybenecessarytokeepthechipsfrombecominggummyandstickingtothetools.Coolingcanusuallybeachievedwithajetofair,vapormist,orwatersolubleoils.Residualstressesmaydevelopduringmachining.Torelievethesestresses,machinedpartscanbeannealedforaperiodoftimeattemperaturesrangingfromtoto,andthencooledslowlyanduniformlytoroomtemperature.Thermosettingplasticsarebrittleandsensitivetothermalgradientsduringcutting.Theirmachinabilityisgenerallysimilartothatofthermoplastics.Becauseofthefiberspresent,reinforcedplasticsareveryabrasiveandaredifficulttomachine.Fibertearing,pulling,andedgedelaminationaresignificantproblemstheycanleadtoseverereductionintheloadcarryingcapacityofthecomponent.Furthermore,machiningofthesematerialsrequirescarefulremovalofmachiningdebristoavoidcontactwithandinhalingofthefibers.ThemachinabilityofceramicshasimprovedsteadilywiththedevelopmentofNanoceramicsandwiththeselectionofappropriateprocessingparameters,suchasductileregimecutting.Metalmatrixandceramicmatrixcompositescanbedifficulttomachine,dependingonthepropertiesoftheindividualcomponents,i.e.,reinforcingorwhiskers,aswellasthematrixmaterial.5.ThermallyAssistedMachiningMetalsandalloysthataredifficulttomachineatroomtemperaturecanbemachinedmoreeasilyatelevatedtemperatures.Inthermallyassistedmachininghotmachining,thesourceofheatatorch,inductioncoil,highenergybeamsuchaslaserorelectronbeam,orplasmaarcisforces,bincreasedtoollife,cuseofinexpensivecuttingtoolmaterials,dhighermaterialremovalrates,andereducedtendencyforvibrationandchatter.Itmaybedifficulttoheatandmaintainauniformtemperaturedistributionwithintheworkpiece.Also,theoriginalmicrostructureoftheworkpiecemaybeadverselyaffectedbyelevatedtemperatures.Mostapplicationsofhotmachiningareintheturningofhighstrengthmetalsandalloys,althoughexperimentsareinprogresstomachineceramicssuchassiliconnitride.6.SUMMARYMachinabilityisusuallydefinedintermsofsurfacefinish,toollife,forceandpowerrequirements,andchipcontrol.Machinabilityofmaterialsdependsnotonlyontheirintrinsicpropertiesandmicrostructure,butalsoonproperselectionandcontrolofprocessvariables.

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