外文翻译---模具成型轧制.doc

英语翻译FormingFormingcanbedefinedasaprocessinwhichthedesiredsizeandshapeareobtainedthroughtheplasticdeformationsofamaterial.Thestressesinducedduringtheprocessaregreaterthantheyieldstrength,butlessthanthefracturestrength,ofthematerial.Thetypeofloadingmaybetensile,compressive,bending,orshearing,oracombinationofthese.Thisisaveryeconomicalprocessasthedesiredshape,size,andfinishcanbeobtainedwithoutanysignificantlossofmaterial.Moreover,apartoftheinputenergyisfruitfullyutilizedinimprovingthestrengthoftheproductthroughstrainhardening.Theformingprocessescanbegroupedundertwobroadcategories,namely,coldforming,andhotforming.Iftheworkingtemperatureishigherthantherecrystallizationtemperatureofthematerial,thentheprocessiscalledhotforming.Otherwisetheprocessistermedascoldforming.Theflowstressbehaviorofamaterialisentirelydifferentaboveandbelowitsrecrystallizationtemperature.Duringhotworking,alargeamountofplasticdeformationcanbeimpartedwithoutsignificantstrainhardening.Thisisimportantbecausealargeamountofstrainhardeningrendersthematerialbrittle.Thefrictionalcharacteristicsofthetwoformingprocessesarealsoentirelydifferent.Forexample,thecoefficientoffrictionincoldformingisgenerallyoftheorderof0.1,whereasthatinhotformingcanbeashighas0.6.Further,hotforminglowersdownthematerialstrengthsothatamachinewithareasonablecapacitycanbeusedevenforaproducthavinglargedimensions.Thetypicalformingprocessesarerolling,forging,drawing,deepdrawing,bending,andextrusion.Forabetterunderstandingofthemechanicsofvariousformingoperations,weshallbrieflydiscusseachoftheseprocesses.RollingInthisprocess,thejobisdrawnbymeansoffrictionthrougharegulatedopeningbetweentwopower-drivenroll.Theshapeandsizeoftheproductaredecidedbythegapbetweentherollsandtheircontours.Thisisaveryusefulprocessfortheproductionofsheetmetalandvariouscommonsections,e.g.,rail,channel,angle,andround.ForgingInforging,thematerialissqueezedbetweentwoormorediestoalteritsshapeandsize.Dependingonthesituation,thediesmaybeopenorclosed.DrawingInthisprocess,thecross-sectionofawireorthatofabarortubeisreducedbypullingtheworkpiecethroughtheconicalorificeofadie.Whenhighreductionisrequired,itmaybenecessarytoperformtheoperationinseveralpasses.DeepDrawingIndeepdrawing,acup-shapedproductisobtainedfromaflatsheetmetalwiththehelpofapunchandadie.Thesheetmetalisheldoverthediebymeansofablankholdertoavoiddefectsintheproduct.BendingAsthenameimplies,thisisaprocessofbendingametalsheetplasticallytoobtainthedesiredshape.Thisisachievedbyasetofsuitablydesignedpunchanddie.ExtrusionThisisaprocessbasicallysimilartothecloseddieforming.Butinthisoperation,theworkpieceiscompressedinaclosedspace,forcingthematerialtoflowoutthroughasuitableopening,calledadie.Inthisprocess,onlytheshapeswithconstantcross-sections(dieoutletcross-section)canbeproduced.AdvantagesandDisadvantagesofHotandColdFormingNowthatwehavecoveredthevarioustypesofmetalworkingoperations,itwouldonlybeappropriatethatweprovideanoverallevaluationofthehotandcoldworkingprocesses.Suchadiscussionwillhelpinchoosingtheproperworkingconditionsforagivensituation.Duringhotworking,apropercontrolofthegrainsizeispossiblesinceactivegraingrowthtakesplaceintherangeoftheworkingtemperature.Asaresult,thereisnostrainhardening,andthereforethereisnoneedofexpensiveandtime-consumingintermediateannealing.Ofcourse,strainhardeningisadvisableduringsomeoperations(viz.,drawing)toachieveanimprovedstrength；insuchcases,hotworkingislessadvantageous.Apartfromthis,strainhardeningmaybeessentialforasuccessfulcompletionofsomeprocesses(e.g.,indeepdrawing,strainhardeningpreventstheruptureofthematerialaroundthebottomcircumferencewherethestressismaximum).Largeproductsandhighstrengthmaterialscanbeworkeduponunderhotconditionssincetheelevatedtemperaturelowersdownthestrengthand,consequently,theworkload.Moreover,formostmaterials,theductilityincreaseswithtemperatureand,asaresult,brittlecanalsobeworkeduponbythehotworkingoperation.Itshould,however,berememberedthattherearecertainmaterials(viz.,steelscontainingsulphur)whichbecomemorebrittleatelevatedtemperatures.Whenaveryaccuratedimensionalcontrolisrequired,hotworkingisnotadvisedbecauseofshrinkageandlossofsurfacemetalduetoscaling.Moreover,surfacefinishispoorduetooxideformationandscaling.Themajoradvantagesofcoldworkingarethatitiseconomical,quicker,andeasiertohandlebecauseherenoextraarrangementsforheatingandhandlingarenecessary.Further,themechanicalpropertiesnormallygetimprovedduringtheprocessduetostrainhardening.Whatismore,thecontrolofgrainflowdirectionsaddstothestrengthcharacteristicsoftheproduct.However,apartfromotherlimitationsofcoldworking(viz.,difficultywithhighstrengthandbrittlematerialsandlargeproductsizes),theinabilityoftheprocesstopreventthesignificantreductionbroughtaboutincorrosionresistanceisanundesirablefeature.成形成形可以定义为一种通过材料的塑性变形获得所需尺寸和形状的工艺。在此工艺中引起的应力大于材料的屈服强度，但小于材料的断裂强度。加载的类型可以是拉应力、压应力、弯曲应力或剪应力，或者是这些类型的组合。这是个很经济的方法，因为可以获得所需的形状、尺寸和光洁度而无需使材料有任何大的损失。此外，一部分输入的能量在通过应变硬化提高产品的强度上得到了卓有成效的利用。成形工艺可以分为以下两个大类，即冷成形和热成形。如果加工温度高于材料的再结晶温度，那么这一过程就叫热变形，否则，这一个过程就被称为冷变形。材料的流动应力在再结晶温度之上或之下全然不同。在热加工过程中，可以产生大的塑性变形而无大的冷作硬化。这一点很重要，因为大的冷作硬化会使材料变脆。两种成形方法的摩擦特性也完全不同。例如，冷成形的摩擦系数一般为0.1左右，而热变形的摩擦系数可以高达0.6。此外，热变形降低了材料的强度，结果甚至可以使用具有一定容量的机器加工很大尺寸的产品。典型的成形方法有轧制、锻造、拉延、拉深、弯曲和挤压。为了更好地理解各种成形操作地机械学原理，我们将简要讨论每一种方法。轧制在这一工艺中，通过一个调整过的位于两个动力驱动的轧辊之间的孔利用摩擦来拉伸工件。产品的形状和尺寸由轧辊及其轮廓之间的间隙来决定。这是一种很有用途的工艺，用于生产金属薄板和各种常用端面，如铁轨、槽钢、角钢和圆钢。锻造锻造时，材料在两个和多个磨具间受到挤压以改变其形状和尺寸。根据情况不同，磨具可以是开式或闭式。拉延在这一工艺中，金属丝的截面或者是条钢或钢管的截面由于工件被拉过磨具的锥形孔而减小。当截面需要减小很多时，也许有必要通过几个阶段来完成此操作。拉深在拉深中，杯形产品是在一个凸模和一个凹模的帮助下由一块金属板获得的。金属薄板被放在磨具上利用一个坯料压板来避免产品缺陷。弯曲如其名所示，这是一道塑性弯曲一块金属薄板以获得所需形状的工艺。这道工艺由一套设计适当的凸模和凹模来完成。挤压这是一道基本上类似于闭式磨具锻造的工艺。但是在该工序中，工件被压进一个封闭空间迫使材料通过一个被称为磨具的适当的开口处流出。在这一工艺中，只有具有固定截面（磨具出口截面）的形状可以制造。热、冷成形的优点与缺点既然我们已经谈及了各类金属加工工序，现在我们应该给热加工和冷加工工艺一个总体评价了。这一讨论将有助于为给定的情况选择适合的加工条件。在热加工过程中，因为活跃晶粒在加工温度范围内会生长，就有可能适当控制晶粒尺寸。于是没有冷作硬化，因此无须昂贵耗时的中间退火。当然，在一些操作（如拉延）中应变硬化是可取的，可以提高强度；在这些情况下，热加工几乎没有优势。除此之外，要成功地完成一些工艺应变硬化可能是必不可少的（如在拉深中，冷作硬化可防止应力最大处的深处圆周周围的材料断裂）。只要在热的状态下，就可以加工大件产品和高强度材料，因为提升的温度降低了强度，进而降低了载荷。此外，对于大多数材料来说，延展性随温度而增大，因此易碎的材料也可采用热加工工序来加工。但是应该记住某些材料（如含硫磺的钢）在提高温度时会变得更脆。当需要非常精确地控制尺寸时，热加工就不太适合，因为金属的表面会生成氧化皮而收缩或损失；再者，由于氧化物的形成和起氧化皮表面光洁度很差。冷加工的主要优点是经济、操作更为迅速、容易，因为无须安排额外的加热和处理。另外，在加工过程中由于冷作硬化，机械特性通常得以提高。此外，晶粒流动方向的控制增加了产品的强度特性。然而，冷加工除了其他局限性以外（如难以加工高强度和脆性材料以及大的产品尺寸），该工艺还有一个不好的特性，即无法防止防腐蚀性能明显减小。