Chapter12MetalCasting:Design,MaterialsandEconomicsContent12.1IntroductionThischapterdescribesthegeneraldesignconsiderationsandguidelinesformetalcastingandpresentssuggestionsforavoidingdefects.Italsodescribesthecharacteristicsofthealloysthatarecommonlycast,togetherwiththeirtypicalapplications.Theeconomicsofcastingoperationsarejustasimportantasthetechnicalconsiderationsthathavealreadybeendescribed,sothischapteralsooutlinesthebasiceconomicfactorsrelevanttocastingoperations.本章讨论金属铸件的一般设计考虑和指导方针，并给出如何避免铸件缺陷的建议。还结合器典型应用来讨论常用铸造合金的特性。铸造的经济性与已讨论过的技术性问题一样重要，因此本章也简略讨论与铸造有关的基本经济因素。12.2DesignConsiderations12.2.1Designforexpendable-moldcasting(一次性模铸造的设计)Sectionchangesincastingsshouldbesmoothlyblendedintoeachother.Becausethecoolingrateinregionswithlargethicknessislower,theyarecalledhotspots(热节),whichcanbedevelopedtoshrinkagecavitiesandporosity(缩孔和疏松).Itisimportanttomaintain(保持),asmuchaspossible,uniformcross-sectionsandwallthicknessesthroughoutthecastingtoavoidshrinkagecavities.Fig.12.3Examplesofdesignmodificationstoavoidshrinkagecavities.2.Flatareas(平面面积).Largeflatareas(plainsurfaces)shouldbeavoided,becausetheymaywarp(翘曲)duringcoolingbecauseoftemperaturegradientsordeveloppoorsurfacefinish.Flatsurfacescanbebrokenupwithribs(肋条)andserrations.3.Shrinkage.Thereshouldbeallowancesforshrinkageduringsolidification.Stagger:错开排列,intersectiongeometry:横截面几何形状.Patterndimensionsshouldalsoprovideforshrinkageofthemetalduringsolidificationandcooling.Patternmaker’sshrinkageallowances:模型制作的收缩允许率,usuallyrangefromabout10mm/mto20mm/m.4.Partingline.Tobealongaflatplane,ratherthancontoured,locatedatthecornersoredgesofcastings.Itisimportantformolddesignandaffectsmanyfactors.5.Draft(减面率).Toenableremovalofthepatternwithoutdamagingthemold.Rangingfrom5mm/mmto15mm/m,angles:0.5oto2o,forinsideangles,1oto4o.6.Dimensionaltolerance(尺寸公差).Aswideaspossible,withinthelimitsofgoodpartperformance,usually±0.8to±6mmforsmallandlargecastings.7.MachiningAllowance.Allowancesusuallyrangefromabout2mmto5mmforsmallcastingstomor