Cold Forging and Sheet-Metal Forming
Cold Forging and Sheet-Metal Forming
the pressure-shaping processes used to work billets and sheet metal, usually accomplished without heating the stock. The production processes are divided into operations and transfers on specialized dies. Hardening occurs during the processes (that is, there is an increase in the strength of the metal and a decrease in ductility), which makes the metal difficult to deform in subsequent operations. In order to avoid the undesirable effect of hardening, a heat treatment—recrystallization annealing—is used between operations. Cold forging and sheet-metal forming make it possible to produce very accurate parts with high-quality surfaces, so that almost no machining is required in the manufacturing process. The absence of heating during manufacture facilitates the mechanization and automation of the production process, thus increasing productivity and improving working conditions.
In cold sheet-metal forming (see alsoSHEET-METAL STAMPING), material failure will occur during the separation process for a smaller intrusion of the tool’s cutting edges than is the case for hot stamping, and the shear strength is approximately 0.8 of the ultimate strength. During the forming operations, the phenomenon of hardening has a substantial effect on the maximum permissible degree of deformation. The maximum deformation can be increased by ensuring optimum strain conditions—force application, die design, efficient tool configuration, rate of deformation, and lubrication. During manufacture, the stock acquires different strains in different sections, and, correspondingly, different hardenings. The combination of an efficient strain distribution (which depends on the dimensions and shape of the workpiece, together with the type of operations used and the conditions under which the operations are performed) with heat treatment (both for the whole workpiece and for individual sections) ensures that the item manufactured will have the best possible service characteristics (rigidity, strength, wear resistance) with the lowest weight.
Cold forging (see alsoCLOSED IMPRESSION DIE FORGING) is divided into open die forging, cold extrusion, and cold upsetting. In impression die forging, similar dies are used regardless of whether the workpiece is heated. Through a succession of processes, the workpiece acquires the shape of the finished product. Because hardening occurs, cold forging is usually divided into a larger number of operations and transfers than in hot forging, and annealings are used between operations to increase plasticity and reduce the resistance to deformation. When open dies are used, the flash is trimmed between operations in order to reduce the force required for deformation and increase the dimensional accuracy of the forged items. The unit forces for deformation reach 3,000 meganewtons per m2; the process is therefore suitable only for the fabrication of small parts. In order to reduce unit forces, a lubricant may be used that resists expulsion from the contacting surfaces under high unit forces; examples include mineral oils with graphite, talc, and molybdenum disulfide fillers.
Deformation in cold extrusion is similar to that in pressworking. Forward, backward, radial, and combination extrusions are used; they differ in the direction of metal flow compared with the direction of the displacement of the punch relative to the die. In combination extrusion the tooling is equipped with several channels, through which the metal flows from the die cavity so that forward, backward, and radial extrusion processes can occur simultaneously. Cold extrusion is suitable for the manufacture of solid and hollow parts with fairly complicated configurations. The uniform compression pattern characteristic of cold extrusion ensures an increase in the plasticity of the metal and permits a relatively large change in the shape of the workpiece without material failure. The hardening that occurs during cold extrusion limits the maximum allowable change in shape and in many cases requires annealing between operations; the maximum change in shape is also usually limited by the strength of the tooling because of the large unit forces of deformation. In order to reduce the latter, efficient shapes and dimensions are chosen for the tooling, and various lubricants are used. Tooling strength may be improved through the use of high-strength tool steels and effective heat treatments for punches and dies, as well as by reinforcing dies.
Cold forging and sheet-metal forming of ductile metals and alloys can produce hollow parts with a wall thickness of tenths and even hundredths of a millimeter. In addition to conventional methods, increasing use is being made of nonpress techniques, including explosive forming, electrohydraulic forming, and magnetic pulse forming.
REFERENCERomanovskii, V. P. Spravochnik po kholodnoi shtampovke, 5th ed. Leningrad, 1971.
E. A. POPOV