论文翻译《材料的选择》.docx

1、论文翻译材料的选择Selecting MaterialsAn ever-increasing variety of materials is now available, each having its own characteristics, applications, advantages, limitations, and costs. The following are the general types of materials used in manufacturing, either individually or in combination with other materi

2、als: Ferrous metals: carbon, alloy, stainless, and tool and die steels. Nonferrous metals: aluminum, magnesium, copper, nickel, titanium, superalloys, refractory metals, beryllium, zirconium, low-melting alloys, and precious metals. Plastics (polymers): thermoplastics, thermosets, and elastomers. Ce

3、ramics, glasses, glass ceramics, graphite, diamond, and diamond-like materials. Composite materials: reinforced plastics, metal-matrix and ceramic-matrix composites, also known as engineered materials. Nanomaterials, shape-memory alloys, amorphous alloys, semiconductor, superconductors, and various

4、other advanced materials with unique properties.As new materials are developed, the selection of appropriate materials becomes even more challenging. Aerospace structures, automotive applications, military equipment, and sporting goods have been at the forefront of new-material usage. There are cont

5、inually shifting trends in the use of materials in all products, trends that are driven principally by economic considerations, but also by other considerations, as will be described below.Properties of materialsWhen selecting materials for products, we first consider their mechanical properties: st

6、rength, toughness, ductility, hardness, elasticity, fatigue, and creep. The mechanical properties specified for a product and its components should, of course, be appropriate to the conditions under which the product is expected to function. Next the physical properties of materials are considered:

7、density, specific heat, thermal expansion and conductivity, melting point, and electrical and magnetic properties. A combination of mechanical and physical properties is the strength-to-weight and stiffness-to-weight ratios of materials, particularly important for aerospace and automotive applicatio

8、ns, as well as for sports equipment. Aluminum, titanium, and reinforced plastics, for example, generally have higher such ratios than steels and cast irons.TABLE 3.1.1 General Manufacturing Characteristics of Various AlloysAlloyCastabilityWeldabilityMachinabilityAluminumEFE-GCopperG-FFG-FGray cast i

9、ronEDGWhite cast ironGVPVPNickelFFFSteelsFEFZincEDENote: E, excellent; G, good; F, fair; D, difficult; VP, very poor.Chemical properties also play a significant role, both in hostile and in normal environments. Oxidation, corrosion, general degradation of properties, toxicity, and flammability of ma

10、terials are among the important factors to be considered. In some commercial airline accidents, for example, many deaths have been caused by toxic fumes from burning nonmetallic materials in the aircraft cabin.The manufacturing properties of materials determine whether they can be cast, formed, mach

11、ined, joined, and heat-treated with relative ease (Table 3.1.1). The method or methods used to process materials to the desired final shapes can affect the products performance, service life, and cost.Cost and availabilityCost and availability of raw as well as processed materials and of manufacture

12、d components are major concerns in manufacturing. The economic aspects of material selection are as important as technological considerations of the properties and characteristics of materials. If raw or processed materials or manufactured components are not available in the desired shapes, dimensio

13、ns, and quantities, substitutes and/or additional processing will be required, and these can contribute significantly to product cost. For example, if we need a round bar of a certain diameter and it is not available in standard form, then we have to purchase a larger rod and reduce its diameter by

14、some means (such as machining, drawing through a die, or grinding). However, a product design can often be modified to take advantage of standard dimensions of raw materials and, thus, avoid extra manufacturing costs.Reliability of supply, as well as demand for the material, affects cost. Most count

15、ries import numerous raw materials that are essential for manufacturing. For manufacturing purposes, the United States, for example, imports most of the following materials: natural rubber, diamond, cobalt, titanium, chromium, aluminum, and nickel. The broad geopolitical implications of such relianc

16、e on other countries are self-evident. Reliability of supply is also important in industries, especially automotive, when it is crucial for materials and components to arrive at the plant at the proper time intervals.Different costs are involved in processing materials by different methods. Some met

17、hods require expensive machinery, others require extensive labor, and still others require personnel with special skills, a high level of education, or specialized training.Appearance, service life, and recyclingThe appearance of materials, after they have been manufactured into products, influences

18、 their appeal to the consumer. Color, feel, and surface texture are characteristics that we all consider when making a decision about purchasing a particular product.Time- and service-dependent phenomena, such as wear, fatigue, creep, and dimensional stability, also are important. These phenomena ca

19、n significantly affect a products performance and, if not controlled, can lead to malfunction or failure of the product. Similarly, compatibility of materials used in a product is important. Friction and wear, corrosion (including galvanic corrosion between mating parts made of dissimilar metals), a

20、nd other phenomena can shorten a products life or cause it to fail prematurely.As we have become increasingly conscious of the need for conserving resources and for maintaining a clean and healthy environment, recycling or proper disposal of component materials at the end of a products useful servic

21、e life has become a major consideration. Note, for example, the use of biodegradable packaging materials and of recyclable plastic bottles and aluminum beverage cans. The proper treatment and disposal of toxic wastes and materials is also a major consideration.EXAMPLE 3.1.1 Material selection for U.

22、S. penniesBillions of pennies are minted each year (FiG.3.1.1). The materials used by the U.S. Mint to make pennies have undergone significant changes throughout history, mainly because of material shortages and the fluctuating cost of raw materials. The following table shows a chronological develop

23、ment of material substitutions in pennies. These materials, or their combinations, need to impart appropriate properties to the pennies during their circulation and use.1793-1837100% copper1837-185795% copper, 5% tin and zinc (bronze) 88%1857-1863copper, 12% nickel (nickel-bronze) 95%1864-1962copper

24、, 5% tin and zinc (bronze) Steel1943 (WW II years)(plated with zinc) 95% copper, 5% zinc1962-1982(bronze) 97.5% zinc, plated with 2.5%1982-presentcopperFIG.3.1.1 U.S. pennies38.3 Computer-Integrated ManufacturingThe various levels of automation in manufacturing operations (described in Chapter 37) h

25、ave been extended further by including information processing functions and by utilizing an extensive network of interactive computers. This has led to computer- integrated manufacturing (CIM), which describes the computerized integration of all aspects of product design, process planning, productio

26、n, and distribution, as well as the management and operation of the whole manufacturing organization.Computer-integrated manufacturing is a methodology rather than an assemblage of machines, equipment, and computers. The effectiveness of CIM critically depends on the use of a large-scale integrated

27、communications system involving computers, machines, and their controls (described in Section 39.7). Because CIM ideally should involve the total operation of an organization, it must have an extensive database concerning the technical and business aspects of the operation. Consequently, if planned

28、all at once, CIM can be prohibitively expensive, particularly for small- and medium-sized companies.Implementation of CIM in existing plants may begin with modules in various phases of the total operation. For new manufacturing plants, on the other hand, comprehensive and long-range strategic planni

29、ng covering all phases of the operation is essential in order to fully benefit from CIM. Such planning must take into account the following considerations: The mission, goals, and culture of the organization. Availability of financial, technical, and human resources. Existing as well as emerging tec

30、hnologies in the areas of the products to be manufactured. The level of integration required.Subsystems of CIM. Computer-integrated manufacturing systems consist of subsystems that are integrated into a whole. These subsystems consist of the following (see Fig. 38.1): Business planning and support P

31、roduct designFIGURE 38.1 A schematic illustration of a computer-integrated manufacturing system. The manufacturing cells and their controls shown at the lower left are described in Section 39.2. Source: After U. Rembold. Manufacturing process planning Process automation and control Production monito

32、ring systemsThe subsystems are designed, developed, and implemented in such a manner that the output of one subsystem serves as the input of another subsystem. Organizationally, these subsystems usually are divided into two functions: Business planning functions: These include activities such as forecasting, scheduling, material-requirements planning, invoicing, and accounting. Business execution functions: Includes production and process control, material handling, testing,