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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 requi
2、res 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, t
3、he 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 cast
4、ing 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
5、 chapter 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 ana
6、lytical 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 se
7、ction 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
8、ensure 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 i
9、s also 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 cas
10、tings 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 wa
11、ll thicknesses 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 us
12、e of 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.
13、 Flat 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. Patternmake
14、rs shrinkage 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 parti
15、ng line 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
16、to 6mm 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 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. gal
