Closed Impression Die Forging

a process of the forging and stamping industry that involves changing simple solid blanks of cylindrical, prismatic, and other shapes into more complex articles, whose shapes correspond to cavities in special tools called dies. Closed impression die forging as a process of redistribution of metal in the blank takes place as a result of plastic deformation.

The main operations in closed impression die forging are swaging, upsetting, broaching, pressing, bending, spreading, sizing, and formation of projections, cambers, and recesses. These operations are performed on forging and pressing machines, such as hammers, presses, and special-purpose machines. Machining and heat treatment are used to produce a variety of parts from forgings, such as connecting rods, crankshafts, levers, gears, turbine blades, fasteners, and balls, rollers, and rings of bearings.

A distinction is made between cold and hot closed impression die forging. Cold forging is performed without heating. The initial material consists of calibrated rods, which are cut into uniform (individual) blanks, or coiled wire. The weight of the resulting articles ranges from several grams to several kilograms; the grade of fit is of class 3–2; and the surface finish, of grade 7–10. Cold forging is used to produce critical parts with good mechanical properties that remain stable; such quality results from work hardening and the absence of recrystallization of the metal. Since the blanks are not heated, processes of scale formation, decarburization, and dezincing do not take place on the surface. This improves the quality of the forgings as a whole and reduces allowances for subsequent processing. In many cases the forgings do not require further machining and therefore are finished parts (the coefficient of metal use is equal to 1). However, cold closed impression die forging requires application of considerable forces (up to 2,500 meganewtons per sq m (MN/m²; 1 MN = 100 tons-force), which has an adverse effect on the stability of dies. Heating of the blanks—that is, hot forging—makes possible a considerable reduction in the required forces (by a factor of 10–15).

Hot forging is performed after heating to 200°–300°C; the temperature depends on the composition of the alloy and the processing conditions. The initial material consists of rolled rods divided into uniform blanks that are equal in volume to the future forgings (taking into account unavoidable waste). The weight of the resulting articles ranges from several grams to 6–8 tons; the dimensional precision of the forgings depends on their weight and configuration and may be increased by subsequent cold sizing; and the surface finish corresponds to grade 3–7. The process of hot forging is analogous to free forging in its physical nature, but it is performed in dies. Hot forging is used to produce forgings of uniform structure, relatively high precision, and complex configurations, which cannot be achieved by free forging. However, the average coefficient of metal use in hot forging is 0.5–0.6 (up to 40–50 percent of metal is wasted). This coefficient is considerably higher in cold forging.

Dies for closed impression die forging are most frequently of the two-part type, consisting of top and bottom halves (Figure 1, left), or a punch and die (Figure 1, right). In drop forging and maxirolling, the bottom half of the die is usually fixed, and the top half is movable. Closed impression die forging is performed with open dies, in which the parting line is perpendicular to the direction of stamping (Figure 1, left), or with closed dies, in which the parting lines pass around the perimeter of the forging (Figure 1, right). The open die is distinguished by simplicity of design and versatility of application, but hot forging in this type of die leads to the formation of flash, which fills in the complex relief of the die cavities. A special channel is provided to arrange the flash within the die. The flash is cut off in the die by means of a cutting press. In this case, the metal waste is 5–20 percent, but it may be as high as 50–80 percent.

Figure 1. Diagram of a die for closed impression die forging: (1) bottom half, (2) top half, (3) plunger, (4) matrix, (5) knockout

In closed dies, which are used for hot and cold forging, the flash is very small (not more than l percent), or is absent altogether, since the forging is formed from the entire piece of metal. However, such dies are less versatile, since they cannot be used, for example, to produce spherical forgings. In cases when it is necessary to produce blanks of high volumetric precision, closed dies with volume equalizers are used. Volume equalizers are additional cavities into which flows excess metal from the blank. The equalizers are located in parts of the die into which the metal flows last, to avoid premature flow of metal into them. However, this method is uneconomical, because the metal entering the equalizer becomes waste.

Another technique used in hot forging is the use of drafts, which are made in the die cavities to facilitate the knockout of the finished articles. The forging is produced in a distorted form—for example, a truncated cone is produced instead of a cylinder. The usual drafts in drop forging dies are 5°–7°. The excess metal on the forging (lap) is also waste. Knockouts are used in dies for forging with smaller drafts (1°-2°); only lower knockouts are used in drop forging, and upper and lower knockouts are used in press forging.

An efficient method is hot closed impression die forging in split dies (that is, dies with two or more parting lines); it is most frequently done on horizontal forging machines. The dies in such machines do not have drafts, and they may be used for the production of forgings that widen toward the die bottom. Split dies may also be used for forging in hydraulic screw presses and crank presses. Forgings for the same part may be produced by methods of hot forging using drop hammers or presses. The blanks produced by these two methods will differ in external appearance and will have different allowances (Figure 2).

Figure 2. A forging of which the left-hand part was produced by drop forging and the right-hand part was produced on a press: (1) finished article after processing, (2) allowance for processing on forging produced on a press, (3) allowance on forging produced by drop forging

Closed impression die forging is used as a single-pass process for the production of very simple forgings and as a multiple-pass process for parts with complex shapes. Multiple-pass forging includes preparatory operations (shaping of blanks), which is followed by the final stamping. Multiple-pass forging is performed on mechanized or automatic equipment, in hydraulic presses capable of developing a force of up to 750 MN, with hammers whose dropping components develop 20–25 tons-force, on hot-stamping crank presses capable of developing a force of up to 80 MN, on automatic machines for single-position and multiposition stamping, on cold-pressing machines and rolling-out machines, on forging mills, and on other specialized equipment. Stamping in hydraulic screw presses and with high-speed drop hammers makes possible the production of thin-walled forgings. Multiposition automatic cold and hot stamping machines are used to produce articles with the largest diameters: up to 50 mm by cold stamping, with a productivity of up to 500 articles per minute, and up to 120 mm by hot stamping, with a productivity of up to 70 articles per minute.

Dies are precise, complex, and expensive tools; therefore, the use of closed impression die forging is appropriate for long-run and mass production.

Possibilities for future development of closed impression die forging include broader use of dies in low-waste hot stamping and design of high-powered equipment for cold stamping and the adoption of new processes for the deformation of metals involving the use of phenomena of superplasticity, as well as hydrostatic methods.