铸造合金性能

Theengineermustchooseproductionmethodsmachining机加工weldingfabrication焊接powdermetallurgy粉末冶金pressingandcoldforming压力加工及冷成型hotforging热锻casting铸造第三章液态金属成形工艺（铸造Casting）ObjectivesThischapterwillenableyouto:概述液态金属成形工艺的概念、铸造工艺过程、铸造产品、铸造工艺方法分类、铸造工艺的重要性、铸造工艺特点合金的铸造性能2）Describesandcastingprocess砂型铸造3）Describespecialcastingprocesses.特种铸造4）Showhowgoodcastingdesigncanpreventmanycastingproblems.铸件设计Historyofcasting铸造的历史Castingisoneoftheoldestmethodsofmanufacturingmetals.Prehistorichumansmadetoolsbypouringmoltenmetalintoopenmoldsmadeofstoneorbakedclay.Castobjectsover4000yearsoldhavebeenfounddatingfromancientAssyrian,Egyptian,andChinesecultures.词汇：pouring（浇铸）bakedclay（烧硬的粘土）Assyrian（亚述人）whatisthecastingprocess?Theprocessofcastingmetalsisdonebypouringorforcingmoltenmetalintoamoldcavityhavingadesiredshape.Whenthemetalhassolidified,thecastingisremovedfromthemold.Virtuallyanyshapecanbeproducedbythismethod,insomecaseswithsuchprecisionthatsubsequentmachiningisnotrequired.词汇：moldcavity（铸型型腔）precision（精度）subsequentmachining（后续机加工）Mainadvantages铸造的优点1.partswithintricateinternalcavities/passages(complexgeometricfeature)复杂内腔2.partswithintricateshapes复杂外形3.partswithsmooth,flowingdesigns(withtheeye-pleasingappearance)可获得平滑、流畅的铸件4.theeconomyofusinglessmetal节省金属5.partsize:rangefromthetiniestprecisionpartstohugecastingsformachinerysectionsweighingmanytons铸件大小、重量不限§3-1概述1.什么是铸造工艺方法？液态金属成形或金属浇注成形，又称为铸造，它是指把熔炼好的、符合一定化学成分要求的金属液体浇注到预制的铸型中，使之在重力场或外力场的作用下冷却、凝固而形成铸件(零件)的一种工艺方法。属于热加工成形方法。2.铸造工艺过程：金属的熔炼、造型、浇注和冷却凝固。3.铸造产品：主要为毛坯件4.铸造工艺方法分类：①重力铸造(液态金属靠自身重力填充型腔)、低压铸造、挤压铸造、压力铸造(液态金属在一定的压力下填充型腔)等。②一次型(如砂型铸造、陶瓷型铸造、壳型铸造)和永久型(如金属型铸造)。5.铸造工艺的重要性国民经济中占有极其重要的地位，广泛应用于机械制造、矿山冶金、能源与运输设备、航天航海、轻工纺织等各个领域。6.铸造工艺特点(1)适用范围广(2)尺寸精度高(3)成本低廉、易实现机械化、半自动化生产、尺寸精度高，加工余量少，加工工时较小，故生产成本低。缺点：铸件组织的内部晶粒粗大，常有缩孔、缩松、气孔、砂眼、成分偏析等铸造缺陷，故力学性能不如锻件高；工艺过程难以精确控制，且工序繁多，有时导致废品率高；铸造生产的工作条件差。§3-2合金的铸造性能合金的铸造性能：铸造合金的流动性、凝固与收缩特性、以及偏析与裂纹倾向。1.合金的流动性(1)定义合金的流动性是指熔融液态铸造合金本身的流动能力。(2)影响合金的流动性的因素：合金的物理性质、化学成分、温度、杂质含量有关。(3)合金的流动性的重要性流动性好，易于充满薄而复杂的型腔，可避免出现冷隔、浇不足等缺陷，易于获得形状完整、轮廓清晰的铸件；流动性好，有利于液态合金中气体、夹杂物及时浮出，从而减少气孔和夹渣缺陷的产生；流动性好，有利于填充和弥合铸件在凝固期间产生的缩孔或因收缩受阻产生的裂纹缺陷。(4)铸造合金流动性的测试方法浇注“流动性试样”的方法来测试---螺旋线形试样根据浇注后金属所形成的螺旋线长度确定某种合金流动性的好坏，螺旋线长度越长，流动性就越好，浇口杯直浇道螺旋线长度2.铸造合金的凝固与收缩特性(1)铸造合金的凝固三种凝固方式：逐层凝固方式中间凝固方式体积凝固方式。SOLIDIFICATIONOFMETALSAftermoltenmetalispouredintoamold,aseriesofeventstakesplaceduringthesolidificationofthecastinganditscoolingtoambienttemperature.Theseeventsgreatlyinfluencethesize,shape,uniformity,andchemicalcompositionofthegrainsformedthroughoutthecasting,whichinturninfluenceitsoverallproperties.Thesignificantfactorsaffectingtheseeventsarethetypeofmetal;thethermalpropertiesofboththemetalandthemold;thegeometricrelationshipbetweenvolumeandsurfaceareaofthecasting;andtheshapeofthemold.纯金属的凝固：Atthemoldwalls,whichareatambienttemperature,themetalcoolsrapidly.Rapidcoolingproducesasolidifiedskin,orshell,offineequiaxedgrains.caststructuresofpuremetalssolidifiedinasquaremoldThegrainsgrowinadirectionoppositetothatoftheheattransferoutthroughthemold.Thosegrainsthathavefavorableorientationwillgrowpreferentiallyandarecalledcolumnargrains.FIGURE10.2Developmentofapreferredtextureatacoolmoldwall.Notethatonlyfavorablyorientedgrainsgrowawayfromthesurfaceofthemold.FIGURE10.1Schematicillustrationofthreecaststructuresofmetalssolidifiedinasquaremold:(a)puremetals;(b)solidsolutionalloys;and(c)structureobtainedbyusingnucleatingagents.puremetalssolidsolutionalloysusingnucleatingagentsAsthedrivingforceoftheheattransferisreducedawayfromthemoldwalls,thegrainsbecomeequiaxedandcoarse.Thosegrainsthathavesubstantiallydifferentorientationsareblockedfromfurthergrowth.Suchgraindevelopmentisshownashomogenousnucleation,meaningthatthegrains(crystals)growuponthemselves,startingatthemoldwall.EffectsofCoolingRatesSlowcoolingrates(ontheorderof102K/s)orlonglocalsolidificationtimesresultincoarsedendriticstructureswithlargespacingbetweenthedendritearms.ColumnardendriticForfastercoolingrates(ontheorderof104K/s)orshortlocalsolidificationtimes,thestructurebecomesfinerwithsmallerdendritearmspacing.Forstillhighercoolingrates(ontheorderoffrom106to108K/s)thestructuresdevelopedareamorphous.EquiaxeddendriticEquiaxednondendriticThestructuresdevelopedandtheresultinggrainsizeinfluencethepropertiesofthecasting.Asgrainsizedecreases:thestrengthandtheductilityofthecastalloyincrease;microporosity(interdendriticshrinkagevoids)inthecastingdecreases;thetendencyforthecastingtocrack(hottearing)duringsolidificationdecreases.Lackofuniformityingrainsizeandgraindistributionresultsincastingswithanisotropicproperties.(2)铸造合金的收缩特性铸造合金的收缩特性是指将具有一定过热度的铸造合金液体浇入铸型，合金从高温液态冷却到固态的某一温度时所发生的体积和尺寸减小的现象。收缩是铸造合金的物理本性，但会造成铸造缺陷。产生的铸造缺陷：在铸件中产生：缩孔、缩松、热应力、变形及裂纹等铸造缺陷。产生的铸造缺陷：在铸件中产生：缩孔、缩松、热应力、变形及裂纹等铸造缺陷。热撕裂直浇道横浇道浇口杯收缩的种类：体收缩、线收缩体收缩：铸造合金由液态到常温的收缩若用体积改变量来表示，称为体收缩。线收缩：合金在固态时的收缩，若用线尺寸改变量来表示，称为线收缩。铸造合金由液态冷却到常温的三个阶段：液态收缩阶段、凝固收缩阶段、固态收缩阶段。收缩的三个阶段所产生的危害：液态收缩和疑固收缩是铸件产生缩孔、缩松缺陷的基本原因；固态收缩因收缩受阻而引起较大的铸造应力，既而产生变形和裂纹缺陷，影响铸件的尺寸精度。①液态收缩：合金从浇注温度冷却到开始凝固的液相线温度时所产生的收缩称为液态收缩。②凝固收缩：合金从液相线温度(开始凝固的温度)冷却到固相线温度(凝固