Forging and Stamping Industry
Forging and Stamping Industry
a branch of heavy machine building that produces various metal products (ranging from machine parts to household items) by forging, stamping, and pressing. The processes are based on the deformability of materials, that is, on their ability to change their shape without destruction under the influence of external forces. Conditions favorable for plastic deformation are selected according to the fundamental principles of the theory of metalworking by pressure. The value of forging and stamping methods is that the shape of the stock changes as a result of redistribution of metal rather than removal of excess metal, as in machining, which makes possible a sharp reduction in waste and a simultaneous increase in the strength of the material. Therefore, metalworking by pressure is used for the manufacture of very critical machine parts: 80-90 percent of the parts in aircraft and up to 85 percent of the parts in automobiles (in terms of total weight) are made by stamping and pressing. Forging and stamping machines are more efficient than metal-cutting machines; for example, the capacity of cold-heading machines is 5-6 times greater than that of automatic turning lathes, and metal waste is reduced by a factor of 2-3. For each million tons of rolled metal processed, up to 250, 000 tons of metal can be saved with the use of mechanical stamping.
In pressure working, heating is often used to increase ductility, making possible a reduction of forces by a factor of 10-15 in comparison with cold forging of stock, the elimination of cracks, and a reduction in working time. Batch and continuous reverberatory furnaces and electric furnaces (induction furnaces and furnaces with heating elements) are used in forging and stamping.
The main metalworking methods used in forging and stamping are characterized by the form of the initial material (rolled metal, ingots, sheets, and so on), equipment (press or hammer), tooling, and production processes. According to these features a distinction is made among forging (primarily with heating), hot and cold closed impression die forging and sheet-metal stamping, and pressing (mainly with heating).
Forging. Forging is a method of metalworking characterized by the fact that lateral flow of material is not restricted by a tool (die). Forging is used for products weighing up to 200 tons. It was used even before the introduction of smelting iron from ore; forgings made of meteoritic iron were made by hand in the fourth to third millennia B.C. Modern forging is performed by hand or on machines: hammers with dropping parts weighing 1 to 5, 000 kg and hydraulic forging presses with forces of 2-200 meganewtons (MN), or 200-20, 000 tons-force (tf). Forging stock is formed directly by the upper face of a stamp or by very simple accessories and forging tools. Bridge and jib cranes, turners, and manipulators are used for moving heavy forging stock (up to 350 tons) and tools. Forging is used for parts made from ingots or rolled metal.
Stamping. Stamping is a method of pressure working of metals in which the flow of metal is restricted by the surfaces of the recesses and projections of a die. The upper and lower parts of the die form a closed cavity in the shape of the part being made. A distinction is made between closed impression die forging and sheet-metal stamping, depending on the form of the forging stock (sheets, rolled metal, and so on), the type of equipment, and the production processes. In addition, stamping may be done with or without heating.
The minting of coins may be regarded as the prototype of stamping. Large-scale use of the stamping process began in the late 18th and early 19th centuries in connection with the transition to the industrial manufacture of products; the first patent for the manufacture of brass cases (cartridges) by stamping was granted in Germany in 1796. The appearance of steam engines on stamping presses was followed by the introduction of various punching and cutting operations, such as piercing holes for rivets. The greatest developments in stamping came in the mid-20th century with the growth of series and mass production in the instrument and machine-tool industries and in radio electronics, since the productivity of stamping is dozens of times greater than that of forging. The machining allowances of stamped parts are 2-3 times smaller than those of forged parts—that is, the percentage of metal use is higher. Stamping is done on hammers with dropping parts weighing 0.5 to 30 tons, hot-stamping crank presses with forces of 6-100 MN (600-10, 000 tf), hydraulic presses with forces up to 750 MN (75, 000 tf), horizontal forging machines, upsetters (automatic forging and stamping machines), and hydraulic-screw press-hammers. Flashless die forging is used to reduce metal waste and subsequent working. During hot stamping (drop forging), scale on the hammers is removed in the intervals between the first few strokes.On presses the stamping is done in one stroke. Nonoxidizing heating of the forging stock (usually rolled metal)—for example, in induction furnaces—is used in this case. This ensures the production of almost scaleless products. Materials that are difficult to form are usually stamped in hydraulic presses in dies heated to the temperature of the material to be worked (up to 800°C for titanium alloys); this is called isothermal stamping.
To provide a better surface finish and greater precision of the dimensions of the product, cold stamping is used. The cold stamping process is similar to hot stamping (drop forging) but is performed without heating.
Sheet-metal stamping is a method for producing flat or three-dimensional thin-walled products. Thin sheet material (up to 4 mm) is stamped without heating, and sheet material more than 4 mm thick is stamped with heating. Parts made by this method have precise dimensions and usually do not require further machining. Universal machines and equipment are used in small-series and series production; multioperation dies are used in large-series and mass production. Stamping is performed on crank presses with forces of 63 to 50, 000 kN (6.3 to 5, 000 tf) at 5-15 strokes per minute; on sheet-stamping machines with forces of 50 to 40, 000 kN (5 to 4, 000 tf) at up to 120 strokes per minute; and on hydraulic drawing presses with forces of 8-200 MN (800-20, 000 tf). In mass production, special use is made of combination dies, mechanization and automation of entire sections, and automatic transfer lines.
For the production of standard parts made of sheet metal (sheet aluminum 1.5-2.0 mm thick, sheet steel 0.5-0.6 mm thick), particularly in the aircraft industry, much use is made of rubber-pad forming, which reduces tooling costs and simplifies press construction. In a single-acting hydraulic press, a cushion made of rubber or other elastic material is installed in a metal box mounted on the movable parts of the press. The bottom face of the die has the form of the part to be stamped. While parts are being stamped, new forging stock is prepared for stamping.
In hydraulic drawing, which is also a type of sheet-metal stamping, the forging stock is formed directly by a fluid.
Pressing. Pressing is a method of producing products made from various types of profiles, sections, bars, and pipes in which the forging stock is placed in a special container, from which it is extruded by a punch (press plunger) through the opening of a die that has the shape of the intended product. Pressing is done on hydraulic presses with forces up to 200 MN (20, 000 tf). Pressing was widely used in the 1860’s, although the first patent for a hydraulic press was granted in 1797 (in Great Britain). Pressed products have a high degree of precision of dimensions and, in most cases, require almost no subsequent mechanical working. Mounting of the tool is simple, which makes possible rapid resetting of the equipment for the manufacture of different types of products.
Modern metalworking methods. In the 1950’s the forging and stamping industry saw the development and successful introduction of fundamentally new production processes that made possible the working of almost undeformable materials (heat-resistant steels; titanium, molybdenum, tungsten, and other alloys).
In explosive forming, a shock wave created by an explosive charge deforms the forging stock, giving it the required shape. Because of the short duration of the process (from milliseconds to microseconds), the dies can be made not only from inexpensive low-carbon steels but also from such materials as concrete, wood, and plastic.
High-speed stamping is a method of working almost unformable materials with high precision on hammers whose ram reaches an impact speed of 40-60 m/sec. In conventional hammers the speed does not exceed 8 m/sec. The weight of the impacting parts of high-speed hammers is many times less than that of conventional hammers, but the impact energy is the same.
Various products are produced by forming forging stock by means of forces created during the interaction of electromagnetic fields formed during the discharge of a high-capacity capacitor battery. This method, known as electromagnetic forming, makes possible the production of up to 600 items per hour. It is used for the manufacture of various products, such as reflectors made of polished metal sheets, without subsequent treatment, and to join parts, including those made of dissimilar materials (for example, metal and ceramics). A variety of this method is electrohydraulic forming, which uses the energy of a shock wave formed as the result of an electric discharge in a fluid.
Hydrostatic pressing, or hydroextrusion, is based on the use of a fluid to transmit high pressures—from 10 to 3, 000 MN/m2 (100-30, 000 kgf/cm2). This method is used for the extrusion of bars and profiled objects, and for the production of parts having the shape of solids of revolution, primarily from low-plasticity alloys and alloys that are difficult to form.
Products made from powders are manufactured by the method of hydrostatic (or gas-pressure) pressing. Materials in elastic sheathing made of rubber, polymers, lead, or thin steel are placed in a container into which fluid is fed under a pressure of up to 800 MN/m2(8, 000 kgf/cm2). In this way, uniform compacting of the forging stock takes place, and the shape of the objects being produced duplicates the shape of the initial forging stock. Hydrostatic compacting is performed at temperatures of 400°-500°C, and gas-pressure compacting is done at temperatures of 2000°-2500°C. In gas-pressure compacting the working medium is an inert gas (usually argon). The gas pressure reaches 200-500 MN/m2(2, 000-5, 000 kgf/cm2).
Forged and stamped products are used in the automotive and aviation industries, tractor construction, instrument-making, and light industry, as well as in the manufacture of consumer goods. Annual forging machine production in the USSR was 4, 700 units in 1940 and 43, 900 units in 1972; by 1975 it will reach 60, 000-65, 000 units.
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B. V. ROZANOV and V. P. LINTS