成形--外文文獻

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英語翻譯FormingForming can be defined as a process in which the desired size and shape are obtained through the plastic deformations 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 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.The forming processes can be grouped under two broad categories, namely, cold forming, and hot forming. If the working temperature is higher than the recrystallization 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 entirely different above and below its recrystallization temperature. During hot working, a large amount of plastic deformation can be imparted without significant strain hardening. This is important because a large amount of strain hardening renders the material brittle. The frictional characteristics 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 capacity can be used even for a product having large dimensions.The typical forming processes are rolling, forging, drawing, deep drawing, bending, and extrusion. For a better understanding of the mechanics of various forming operations, we shall briefly discuss each of these processes.RollingIn this process, the job is drawn by means of friction through a regulated opening between two power-driven roll. 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 production of sheet metal and various common sections, e.g., rail, channel, angle, and round.ForgingIn 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.DrawingIn this process, the cross-section of a wire or that of a bar or tube is reduced by pulling the workpiece through the conical orifice of a die. When high reduction is required, it may be necessary to perform the operation in several passes.Deep DrawingIn deep drawing, a cup-shaped product is obtained from a flat sheet metal with the help of 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.BendingAs 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. ExtrusionThis is a process basically similar to the closed die forming. But in this operation, the workpiece 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-section) can be produced.Advantages and Disadvantages of Hot and Cold FormingNow 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 the proper working conditions 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-consuming intermediate annealing. Of course, strain hardening is advisable during some operations (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 (e.g., in deep drawing, strain hardening prevents the rupture of the material around the bottom circumference where the stress is maximum). Large products and high strength materials can be worked upon under hot conditions since the elevated temperature lowers down the strength and, consequently, the work load. Moreover, for most materials, the ductility increases with temperature and, as a result, brittle can also be worked upon by the hot working operation. It should, however, be remembered that there are certain materials (viz., steels containing sulphur ) which become more brittle at elevated temperatures. When a very accurate 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, 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 the product. However, apart from other limitations of cold working (viz., 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.