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1、精品文档,仅供学习与交流,如有侵权请联系网站删除编号: 毕业设计(论文)外文翻译(原文)学 院: 机电工程学院 专 业:机械设计制造及其制动化学生姓名: 学 号: 指导教师单位: 机电工程学院 姓 名: 周定礼 职 称: 工程师 2012年 5 月 15 日【精品文档】第 18 页Forming Processes Charles W. Beardsley Abstract:Forming can be defined as a process in which the desired size and shape are ob-tained through the plastic defor
2、mations of a material. The stresses induced during the process are greater than the yield strength, but less than the fracture strength, of the material. The type of loading may be tensile, compressive, bending, or shearing, or a combination of these. This is a very economical process as the desired
3、 shape, size, and finish can be obtained without any significant loss of material. Moreover, a part of the input energy is fruitfully utilized in improving the strength of the product through strain hardening. Keywords: Forming Rolling Forging Extrusion 1.1 Forming The forming processes can be group
4、ed under two broad categories, namely, cold forming, and hot forming. If the working temperature is higher than the recrystalliza-tion temperature of the material, then the process is called hot forming. Otherwise the process is termed as cold forming. The flow stress behavior of a material is entir
5、ely different above and below its recrystallization temperature. During hot working, a large amount of plastic deformation can be imparted without significant strain hard-ening. This is important because a large amount of strain hardening renders the mate-rial brittle. The frictional characteristics
6、 of the two forming processes are also entirely different. For example, the coefficient of friction in cold forming is generally of the order of 0.1, whereas that in hot forming can be as high as 0. 6. Further, hot forming lowers down the material strength so that a machine with a reasonable capacit
7、y can be used even for a product having large dimensions. The typical forming processes are rolling, forging, drawing, deep draining, bending, and extrusion. For a better understanding of the mechanics of various form-ing operations, we shall briefly discuss each of these processes. 1.2 Rolling In t
8、his process, the job is drawn by means of friction through a regulated opening between two power-driven rolls. The shape and size of the product are decided by the gap between the rolls and their contours. This is a very useful process for the produc-tion of sheet metal and various common sections,
9、e. g., rail, channel, angle, and round. 1.3 Forging In forging, the material is squeezed between two or more dies to alter its shape and size. Depending on the situation, the dies may be open or closed.1.4 Drawing In this process, the cross-section of a wire or that of a bar or tube is reduced by pu
10、lling the workpiece through the conical orifice of a die represents the operation schematically. When high reduction is required, it may be necessary to perform the operation in several passes. 1.5 Deep Drawing In deep drawing, a cup-shaped product is obtained from a flat sheet metal with the help o
11、f a punch and a die. The sheet metal is held over the die by means of a blank holder to avoid defects in the product. 1.6 Bending As the name implies, this is a process of bending a metal sheet plastically to obtain the desired shape. This is achieved by a set of suitably designed punch and die. 1.7
12、 Extrusion This is a process basically similar to the closed die forging. But in this operation, the workplace is compressed in a closed space, forcing the material to flow out through a suitable opening, called a die. In this process, only the shapes with constant cross-sections (die outlet cross-s
13、ection) can be produced. 1.8 Advantages and Disadvantages of Hot and Cold Forming Now that we have covered the various types of metal working operations, it would only be appropriate that we provide an overall evaluation of the hot and cold working processes. Such a discussion will help in choosing
14、the proper working condi-tions for a given situation. During hot working, a proper control of the grain size is possible since active grain growth takes place in the range of the working temperature. As a result, there is no strain hardening, and therefore there is no need of expensive and time-cons
15、uming intermediate annealing. Of course, strain hardening is advisable during some opera-tions (viz., drawing) to achieve an improved strength; in such cases, hot working is less advantageous. Apart from this, strain hardening may be essential for a successful completion of some processes 4 (e. g.,
16、in deep drawing, strain hardening prevents the rupture of the material around the bottom circumference where the stress is maxi-mum). Large products and high strength materials can be worked upon under hot con-ditions since the elevated temperature lowers down the strength and, consequently, the wor
17、k load. Moreover, for most materials, the ductility increases with temperature and, as a result, brittle materials can also be worked upon by the hot working operation. It should, however, be remembered that there are certain materials (viz., steels contain-ing sulphur) which become more brittle at
18、elevated temperatures. When a very accu-rate dimensional control is required, hot working is not advised because of shrinkage and loss of surface metal due to scaling. Moreover, surface finish is poor due to oxide formation and scaling. The major advantages of cold working are that it is economical,
19、 quicker, and easier to handle because here no extra arrangements for heating and handling are necessary. Further, the mechanical properties normally get improved during the process due to strain hardening. What is more, the control of grain flow directions adds to the strength characteristics of th
20、e product. However, apart from other limitations of cold working (viz., I difficulty with high strength and brittle materials and large product sizes), the inability of the process to prevent the significant reduction brought about in corrosion resistance is an undesirable feature. 1.9 Roll Forming
21、Roll forming is a continuous process for forming metal from sheet, strip, or coiled stock into shapes of essentially uniform cross section. The material is fed be-tween successive pairs of rolls, which progressively shape it until the desired cross section is produced. During the process, only bendi
22、ng takes place; the material thick-ness is not changed except for a slight thinning at bend radii. 2.0 Roll Forming Methods The two methods used when shaped parts are roll formed are the precut or cut-to- length method and the post-cut method. Method selection is based on the complexity of the cross
23、 section and the production length specification. Precut Method In precut operations, the material is cut to length prior to en-tering the roll forming machine. This process usually incorporates a stacking and feeding system to move the blanks into the roll forming machine, a roll forming ma-chine r
24、unning at a fixed speed of about 15-76m /min, and an exit conveyor and stack-ing system. The cut-to-length process is used primarily for lower volume parts and whenever notching cannot be easily accomplished in a post-cut line; for example, mi-ter cuts in vertical legs. Many times, the material is r
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