Sheet-Metal Forming and Forging

(in Russian, shtampovka), in metalworking, pressure-shaping processes in which articles are formed by a special tool—a press tool or die; such production processes constitute a branch of the forging and stamping industry. A distinction is made between die forging and sheet-metal stamping on the basis of the type of stock and between the analogous cold and hot pressure-shaping processes on the basis of the temperature of the process. Compared with classical (smith) forging, these processes provides greater efficiency, since the entire workpiece or a significant part of it is subjected to simultaneous plastic deformation.

Die forging is capable of producing articles with more complex shapes than is possible with smith forging, and the articles require significantly less machining to complete production. Metal flow is limited by the walls of the die, which increases the resistance to deformation with increasing complexity of the shape of the article. Heating of the workpiece reduces the resistance to deformation by a factor of 10–15 and increases the ductility of the metal. Cold die forging is used for the production of small articles weighing less than 1 kg; hot die forging is used for articles weighing up to 1.5–2 tons. Heavier items are produced by smith forging. The boundary between the two processes is changing with the improvement of forging equipment and the increasing deformation forces generated by such equipment. Since the cost of dies and stock metal is a major component of the cost of the finished products, the use of die forging is economically advantageous in series production.

Die forging is accompanied by metal loss when the flash is removed. The use of closed dies lessens waste, but the unit loads are greater than in open dies, which reduces the durability of the dies. When open dies are used, waste can be reduced by preliminary processing of the stock in preforming die impressions and forging rolls, by the use of stock blanks that approximate the shape of the final product, and by selecting the proper shape for the flash gutter. In hot working processes the surface of the metal stock is oxidized, forming a layer of scale, which also leads to metal losses. Such losses can be reduced by nonoxidizing heating in fuel-fired furnaces (in a protective atmosphere) or by rapid heating in induction heating units. The use of high-speed processes (with hammer velocities of 10–25 m/sec) lessens the cooling of the stock during deformation and produces articles with thin edges. Isothermic pressure-shaping processes, mainly for nonferrous metals, are also used to lessen cooling of the stock; the technique calls for the die to be heated to temperatures close to the forging temperature. The combined use of casting and hot pressure-shaping processes is being introduced. In this method, the stock blank is cast in a shape close to that of the final product; after the metal hardens and cools to forging temperatures, the blank is die-forged in open or closed dies. The die forging improves the mechanical properties of the cast blank; the waste formed is immediately remelted.

In sheet-metal forming, the metal stock consists of sheet, strip, or ribbon metal. The process is used in the production of flat (sheet) and folded articles (including those with complex shapes), in which the thickness is considerably less than the other dimensions—usually less than 10 mm. Stock of greater thickness is usually hot-stamped to forging temperature. Sheet-metal forming (especialy cold stamping) does not usually require metal cutting to finish the article. The technique is suitable for production runs with various output quantities.

Special techniques are used in small-series production: pressure shaping with elastic media (a fluid, rubber, or polyurethane) and techniques that use the energy of a shock wave in a fluid (explosive and electrohydraulic forming) or the action of powerful, rapidly varying magnetic fields (magnetic pulse forming). Processes with differential heating are sometimes used to increase the maximum permissible deformation of the metal stock. The section of the metal blank to be deformed is heated by contact with a heated tool or the passage of an electric current. The correct distribution of temperatures and of the corresponding metal properties in the stock significantly increases the maximum possible deformation.

Various types of equipment are used in sheet-metal forming and forging, including hammers, crank presses, horizontal forging machines, hydraulic presses, and automatic forging and stamping machines. Large-series production uses automatic machines and automated production lines as well as all types of loading and manipulating devices (including industrial robots), which contribute to a significant increase in labor productivity.