特种铸造

§3.6Specialcastingprocesses特种铸造1）INVESTMENTCASTING熔模铸造2）PERMANENTMOLDCASTING金属型铸造3）DIECASTING压力铸造4）CENTRIFUGALCASTING离心铸造5）EVAPORATIVECASTINGPROCESS实型铸造3-6-1INVESTMENTCASTING熔模铸造（失腊铸造）Oneoftheoldestmethodsofcastingmetalsistheinvestmentcasting(i.e.,lostwax)process.熔模铸造又称失蜡铸造：1.工艺过程压制蜡模→蜡模上涂覆耐火材料→耐火材料层固化→熔化蜡模去除制成型壳→高温焰烧型壳→浇铸→铸件用熔模铸造制造的铸件具有较高的尺寸精度和较好的表面质量。熔模铸造工艺过程2.熔模铸造铸件的结构工艺性熔模铸造铸件的结构的要求具有特殊性:对铸件的结构要求：铸孔一般应大于2mm，铸孔不能太小和太深。铸件壁厚不可太薄，一般为2～8mm。铸件的壁厚应尽量均匀。3.熔模铸造的特点和应用①熔模铸造的铸件精度高、表面质量好。②可制造形状复杂铸件。③可铸造各种合金。④生产批量基本不受限制。既可成批、大批量生产，又可单件、小批量生产。Mostpartsmadebythisprocessarelimitedtoasmallsizeandthereareotherlimitations,suchasintheuseofholesandcavities.Theprocessismoreinvolvedthansomeothercastingprocesses,suchaspermanentmoldinganddiecasting,andisthereforemoreexpensivepercasting.缺点：①熔模铸造工序繁杂，生产周期长，原、辅材料费用高，生产成本较高。②熔模铸造铸件一般不宜太大、太长。应用：主要用于生产汽轮机及燃气轮机的叶片、泵的叶轮、切削刀具以及飞机、汽车、拖拉机、风动工具和机床上的小型零件。Thismethodisusedfordentistry,artsandcrafts,andforanytypeofveryaccuratenearnet-shapecasting.Whenitisusedforindustrialpurposes,significantproductionratesarepossible.Thismethodcanbeautomatedtosomeextentandiswidelyusedintheautomotiveandaerospaceindustries.Someoftheadvantagesofinvestmentcastingare:①Unusualandnonsymmetricalshapesthatwouldnotallowwithdrawalofanordinarypatternareeasilyproduced.②Smoothsurfacesandhighaccuracycanbeobtained.Machiningcanbereducedoreliminated.③Nonmanhinablealloyscanbecastwithhighprecision.3-6-2PERMANENTMOLDCASTING金属型铸造Themoldhalvesmaybehingedormountedonacastingmachinesotheycanbeopenedandclosedquicklyandaccurately.Thegreatestdisadvantageofsandcastingisthatanewmoldmustbemadeforeachcasting.砂型铸造最大的缺点就是“一型一用”。Inaddition,someinherentdimensionalinaccuraciesarepresentinthesandcasting.Thesedisadvantagesgaverisetothedevelopmentofapermanentmold.金属型铸造金属型铸造是将液态金属在重力作用下浇入金属铸型以获得铸件的一种方法。铸型用金属制成，可以反复使用几百次到几千次，故又称硬模铸造。Despitethename,permanentmoldscanbereusedatmostforseveralthousandpours,afterwhichtheylosetheirtrueshapeandmustbescrapped.(1)金属型的结构与材料金属型可分为垂直分型式、水平分型式和复合分型式三种结构。垂直分型式金属型开设浇注系统和取出铸件比较方便，易实现机械化，应用较广。金属铸型的材料：灰铸铁、合金铸铁或钢。金属型用的芯子：砂芯和金属芯。Mostpermanentmoldsaremadeofgraycastironorsteel.Graphitemoldsareoftenusedforcastinghighertemperaturemetals.Themoldsaremadebymachiningprocessesandarehandfinishedorpolished.Arefractorywashisappliedtothemoldpriortocastinginordertoprolongitslife.词汇：inherentdimensionalinaccuracies（固有的尺寸精度差）gaveriseto（产生）scrap（丢弃）graycastiron（灰口铸铁）graphite(石墨）handfinishedorpolished（手工加工或抛光）refractorywash（耐火液）prolong（延长）Inpermanentmoldcasting,themetalispouredfromaladle,usinggravitytofillthemold.Castironandnonferrousmetalsarecastinthismanner.Themoldsareheatedpriortoarunandthetemperatureismaintainedbythemoltenmetal.Thisisnecessarytoavoidchillingthemetaltooquickly,whichcanproducelapsorcold-shutsinthecasting(crackswheretwoadjacentportionsofmetaldonotsolidifytogether).(2)金属型铸件的结构工艺性①铸件结构斜度比砂型铸件大。②铸件壁厚要均匀，铸件的壁厚不能过薄。③铸孔的孔径不能过小、过深。(3)金属型铸造工艺措施①加强金属型的排气。②在型腔表面喷刷涂料。③金属型浇注前需预热，预热温度一般为200～350℃。④开型要快，铸铁件及时从铸型中取出。(4)金属型铸造的特点及应用范围①尺寸精度高，机械加工余量小。②铸件的晶粒较细，力学性能好。③一型多铸，劳动生产率高。④节省造型材料，环境污染小，劳动条件好。缺点：①金属型制造成本高；②不宜生产大型、形状复杂和薄壁的铸件；③受金属型材料熔点的限制，熔点高的合金不适宜用金属型铸造。Usuallyonlysimpleshapesarecastbythisprocessbecauseoftherigidityofthemoldandshrinkingofthemetalwhenitsolidifies.Sincethesemoldsarenotpermeable,theymustbeventedattheplanewherethetwomoldhalvescometogether.Inadditiontogravitypour,low-pressureandvacuum-pouringtechniquesareusedinconjunctionwiththepermanentmoldprocess.Goodsurfacefinish,gooddimensionalaccuracy,andlowporositycanbeobtainedwithpermanentmolds.Repeateduseofmoldsandarapidproductionratewithlowscraplossmakethiscastingprocessidealformoderateproductionrunsofafewthousandpieces.Somedisadvantagesarehighmoldcost,limitedintricacyofcastingshape,andunsuitabilityforhigh-temperaturemetalssuchassteel.3-6-3DIECASTING压力铸造压力铸造(简称压铸)：将熔融合金在高压(5～150MPa)条件下高速充型，并冷却凝固成型的精密铸造方法。压力铸造设备：压铸机压力铸造铸型：金属铸型。压力铸造重要特点：高压和高速。压铸所用的压射比压为30～70MPa，金属液充满铸型的时间为0.01～0.2s；(1)压铸生产设备和压铸工艺过程压铸生产的基本设备:压铸机，分为冷室压铸机和热室压铸机两种类型低压铸造(2)压铸件的结构工艺性①压铸件上应无内侧凹。②压力铸造可铸出细小的螺纹、孔、齿和文字等。③压铸件为薄壁、壁厚要均匀。(3)压力铸造的特点及其应用范围①可以铸出形状复杂、轮廓清晰的薄壁铸件。②铸件的尺寸精度高，表面质量好。一般不需机械加工可直接使用；而且组织细密，铸件强度高。③压铸件中可嵌铸其他材料(如钢、铁、铜合金、金刚石等)的零件，以节省贵重材料和机械加工工时。有时嵌铸还可以代替部件的装配过程。④生产率高，劳动条件好，压力铸造是所有铸造方法中生产率最高的。压力铸造缺点：①压铸机造价高、投资大，铸型结构复杂、成本费用高、生产周期长。②压铸件内部易产生气孔，因此压铸件机械加工的余量不能过大，以免气孔暴露于表面，影响铸件的使用性能。③压铸件一般也不能进行热处理。压力铸造适用范围：主要适用于大批量生产非铁合金(铝合金、镁合金、锌合金等)的中小型铸件，如汽缸盖、箱体、发动机汽缸体、化油器、发动机罩、管接头、仪表和照相机的壳体与支架、齿轮等，在汽车、拖拉机、仪表、电器、航空、医疗器械等行业获得广泛的应用。Diecastingissimilartopermanentmoldinginthatametalmoldmadeintwohalvesisused.Thedifferenceisthatthemetalisnotgravitypouredintothemold(die),butinsteadthemetalisinjectedunderhighpressuresrangingfrom1,000to100,000psi.Thisrequiresmassivemachinesthataregenerallyoperatedhydraulicallytoexertthehundredsoftonsofforcenecessarytoholdthetwohalvesofthedietogetherwhenthemoltenmetalisbeinginjected.词汇：gravitypour（重力浇铸）inject（注入）psi（poundpersquareinch磅/平方英寸）massivemassivemachines（大吨位设备）hydraulically（以液压方式）exert（施加）Thediesareusuallymadeofalloyortoolsteelandarequiteexpensivetomake.Itiseasytoseethatthesecomplexdiesarequiteexpensive