1、Chapter 12 Metal Casting: Design, Materials and Economics,12章 金属铸件： 设计、材料和经济性,Content,12.1 Introduction 简介12.2 Design Considerations 设计考虑12.3 Casting Alloys 铸造合金12.4 The Economics of Casting 铸件经济性,12.1 Introduction,In the preceding two chapters, it was noted that successful casting practice requires
2、 carefully control of al large number of variables. These variables pertain (属于) to the particular characteristics of the metals and alloys cast, the method of casting, the mold and die materials, the mold design and various process parameters. The flow of the molten metal in the mold cavities, the
3、gating systems, the rate of cooling, and the gases evolved all influence the quality of a casting.从前两章中的介绍中，我们知道了只有对大量的变量进行仔细的控制才能获得成功的铸造生产。这些变量与金属和合金的铸造特性、铸造方法、铸型设计以及各种工艺参数相关。金属液在型腔中的流动、浇注系统、冷却率和卷入气体等都影响铸件质量。,This chapter describes the general design considerations and guidelines for metal casting
4、and presents suggestions for avoiding defects. It also describes the characteristics of the alloys that are commonly cast, together with their typical applications. The economics of casting operations are just as important as the technical considerations that have already been described, so this cha
5、pter also outlines the basic economic factors relevant to casting operations.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。,12.2 Design Considerations,These principles（原则） were established primarily through practical experience, but new analytic
6、al methods, process modeling, and computer-aided design and manufacturing techniques are now coming into wider use, improving productivity (生产力) and the quality of castings and resulting in significant cost savings.,12.2.1 Design for expendable-mold casting (一次性模铸造的设计),1. Corners, angles and section
7、 thickness. Sharp corners (方角), angles (倒角) and fillets (肩角) should be avoided (Fig. 12.1), because they act as stress raisers and may cause cracking and tearing of the metals (as well as of the dies) during solidification. Fillet radii should be selected to reduce stress concentrations and to ensur
8、e proper liquid-metal flow during the pouring process. Fillet radii usually range from 3mm to 25mm, although smaller radii may be permissible (容许的) in small castings and in limited applications. On the other hand, if the fillet radii are too large, the volume of the materials in those regions is als
9、o large and, consequently, the rate of cooling is lower.,The most significant considerations are identified (标示) and addressed (强调).,Fig. 12.1 Suggested design modifications to avoid defects in castings. Note that sharp corners are avoided to reduce stress concentrations.,Section changes in castings
10、 should be smoothly blended into each other. Because the cooling rate in regions with large thickness is lower, they are called hot spots (热节), which can be developed to shrinkage cavities and porosity (缩孔和疏松). It is important to maintain (保持), as much as possible, uniform cross-sections and wall th
11、icknesses throughout the casting to avoid shrinkage cavities.,Fig. 12.2 Examples of designs showing the importance of maintaining uniform cross-sections in castings to avoid hot spots and shrinkage cavities,Fig. 12.3 Examples of design modifications to avoid shrinkage cavities.,Fig. 12.4 The use of
12、metal padding (chills) to increase the rate of cooling in thick regions in a casting to avoid shrinkage cavities.,2. Flat areas (平面面积). Large flat areas (plain surfaces) should be avoided, because they may warp (翘曲) during cooling because of temperature gradients or develop poor surface finish. Flat
13、 surfaces can be broken up with ribs (肋条) and serrations.3. Shrinkage. There should be allowances for shrinkage during solidification. Stagger: 错开排列, intersection geometry: 横截面几何形状. Pattern dimensions should also provide for shrinkage of the metal during solidification and cooling. Patternmakers shr
14、inkage allowances: 模型制作的收缩允许率, usually range from about 10mm/m to 20mm/m.4. Parting line. To be along a flat plane, rather than contoured, located at the corners or edges of castings. It is important for mold design and affects many factors.,Fig. 12.5 Redesign of a casting by making the parting line
15、 straight to avoid defects,5. Draft (减面率). To enable removal of the pattern without damaging the mold. Ranging from 5mm/mm to 15mm/m, angles: 0.5o to 2o, for inside angles, 1o to 4o. 6. Dimensional tolerance (尺寸公差). As wide as possible, within the limits of good part performance, usually 0.8 to 6mm
16、for small and large castings.7. Machining Allowance. Allowances usually range from about 2mm to 5mm for small castings to more than 25mm for large castings.8. Residual stress (残余应力). The different cooling rates with the body of a casting cause residual stress. Stress relieving may thus be necessary
17、to avoid distortions in critical applications.,12.2.3. Design for Permanent-Mold Casting: 永久型模铸设计,It is similar with expendable-mold casting, except draft angles reduce to 1/2 to 1/4o. galling: 咬住,Fig. 12.7 Examples of undesirable and desirable design practices for die-cast parts. Note that section-
18、thickness uniformity is maintained throughout the part.,12.3 Casting Alloys: 铸造合金,Their properties and casting manufacturing characteristics are summarized in Fig. 12.8 and Tables 12.2 to 12.5.Piston: 活塞，clutch housing: 离合器壳，intake manifold: 进气管， pump：泵，gear blank：齿轮坯料， propeller：螺旋桨， heavy-duty： 重载
19、的，block：缸体， drum：鼓，crankcase：曲轴箱，malleable iron: 韧性铁，bearing：支座，railroad rolling stock: 铁路机车，blade: 叶片，die block：模具坯料，undercarriage：起落架，rock crusher jaw：碎石机卡抓， mill line：磨机衬里， shot blasting nozzle：喷丸清理喷口，brake shoe：制动块，pulverize：研磨机，radiator grill：散热器护栅，Table 12.3: sanitary ware: 卫生洁具，hardware: 五金器件
20、，coupling：管接头，propeller hub： 螺旋桨毂,Fig. 12.8 Mechanical properties for various groups of cast alloys. Note that gray iron has very little ductility and toughness. Compared with most other cast alloys ,some of which undergo considerable elongation and reduction of area in tension. Note also that even
21、within the same group, the properties of cast alloys vary over wide range, particularly for cast steels.,12.3.1. Nonferrous casting alloys: 非铁磁性铸造合金,1. Aluminum-based alloys (铝合金) have high electrical conductivity (电导率) and generally good atmospheric corrosion resistance. Nontoxic (非毒性) and good mac
22、h inability. Application: architectural and decorative use, automobile components. Fig. 12.8 Mechanical properties for various groups of cast alloys. Note that gray iron has very little ductility and toughness. Compared with most other cast alloys, some of which undergo considerable elongation and r
23、eduction of area in tension. Note also that even within the same group, the properties of cast alloys vary over wide range, particularly for cast steels.Parts made of Al- and Mg-based alloys are known as light-metal casting.,2. Magnesium-based alloys with lowest density of all commercial casting all
24、oys, good corrosion resistance and moderate strength.3. Copper-based alloys, good electrical and thermal conductivity, corrosion resistance and nontoxicity, wear properties, suitable for bearing materials.4. Zinc-based alloys, good fluidity and sufficient strength for structural applications, common
25、 use in die casting.5. High-temperature alloys, Titanium alloys and superalloys, melting temperature up to 1650oC; special techniques are used to cast these alloys into jet- and rocket-engine components.,12.3.3 Ferrous Casting Alloys 铁磁性铸造合金,Cast irons, largest amount of all metals cast, easily are
26、cast into intricate shapes, good wear resistance, hardness, and good mechanability. Including gray cast iron (gray iron:灰铸铁), ductile (nodular or spheroidal: 粒状，球状的) iron （球磨铸铁）, white iron （白口铁）, malleable iron （ 可锻铸铁） , and compacted graphite iron (蠕墨铸铁).a. Gray cast iron , few shrinkage cavities
27、and little porosity, termed ferric, pearlitic, and martensitic gray cast irons. Typical uses are in engine blocks, machine bases, electric-moter housings, pipes and wear surfaces for machines. Specified in many classes, as shown in Table 12.4: good damping ( 阻尼，消声) and thermal conductivity but low d
28、uctility.,Commonly cast ferrous alloys are as follows:,Ductile (nodular) cast iron, typically used for machine parts, and crankshafts, specified by two-digit numbers, poor damping and thermal conductivity, high tensile strength and fatigue resistance.White cast iron used mainly in liners for machine
29、ry to process abrasive materials rolls for rolling mills, and railroad-car brakes because of its extreme hardness and wear resistance.Malleable iron. Used for railroad equipment and various types of hardware. Specified by a five-digit designation (命名).Compacted graphite iron. First produced commerci
30、ally in 1976, with properties that fall between those of gray and ductile irons.,2. Cast steels. The high temperatures involved present difficulties in the selection of mold materials, particularly in view of the high reactivity of steels with oxygen, in melting and pouring the metal. It possesses m
31、ore uniform (isotropic) than those made by mechanical working possessing. Be welded, but affecting the microstructure and mechanical properties. Subsequent heat treatment needed to restore the mechanical properties. Cast Stainless Steels. Casting of stainless steels involves considerations similar t
32、o those for steels. They generally have long freezing ranges and high melting temperatures, developing several structures depending on their composition and processing parameters. Cast stainless steels are available in various compositions and can be heat treated and welded. High heat and corrosion resistance. Nickel-based casting alloys are used for severely corrosive environments and very high-temperature service.,12.4 The Economics of Casting 铸造的经济性,The cost of a product includes the costs of materials, labor, tooling, and equipment. The sequence of casting cost is listed in Table 12.6.,
