Classification and development trend of forging Forging can be classified by the following methods: 1) Classification by the placement of tools and dies used for forging. 2) Classification by forging forming temperature. 3) Classification by the relative movement of die forging tools and workpieces. 1
Classification of forging Forging can be divided into the following categories according to the placement of tools and dies used, see Table 1-1-1.
Die forging can be divided into the following categories according to the forming temperature, see Table 1-1-2.
Die forging is classified by the relative movement of tools and workpieces, see Table 1 -1-3.
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Development trend of forging 1. Develop labor-saving forming process The advantage of forging is that the forging is dense inside and the structure is relatively uniform, and the performance is higher than that of castings and welded parts, but the disadvantage is that it requires a larger deformation force. For many years, people have been exploring labor-saving forging processes and designing labor-saving tooling. The main factors that determine the deformation force F and the ways to save effort can be seen from the following formula: F = KReLA Where K is the stress state coefficient, also known as the constraint coefficient. For stress states of opposite signs, K < 1; for triaxial compressive stress states, K > 1, which may reach K = 6 or even higher; ReL is the flow stress, which characterizes the ability of the material to resist plastic deformation under specific conditions and depends on the composition, structure, deformation temperature, deformation degree, deformation rate, etc. of the deformed material; A is the projection of the contact area between the workpiece and the die in the direction of the main force. From the above analysis, it can be seen that there are three main ways to save effort: (1) Reduce the constraint coefficient K. In fact, in production, the diversion method is often used to reduce the deformation force. For example, ring-shaped blanks are often used for gear precision forging. During forging, the metal fills the tooth shape outward. At the same time, because part of the metal flows inward, the peak stress in the middle of the solid blank is avoided during compression, which reduces the deformation force, as shown in Figure 1-1-1. When back-extruding a cylindrical part, a storage rod is added to the middle of the workpiece to partially extrude a storage rod (see Figure 1-1-2), and then removed. In this way, the deformation force can be greatly reduced. Figure 1-1-3 shows the comparison of the deformation force distribution during compression with and without a storage rod. (2) Reduce flow stress. The forming methods belonging to this category include superplastic forming and liquid die forging (i.e., semi-solid forming or near-melting point forming). The former is a forming method with a lower strain rate, and the latter is a forming method at an extremely high temperature. (3) Reduce the contact area. 2. Develop precision forming technology. In recent years, there is a term called net shape forging, which means that forgings are no longer processed. At present, the tolerance of precision forgings can be controlled within 0.01 to 0.05 mm. Germany has achieved net shape forging of cross shafts (see Figure 1-1-4) and internal and external arc gears (see Figure 1-1-5) for automobile transmissions. In some cases, it is difficult to fully achieve the "net shape", and there is a corresponding term "near net shape", so there is "near net shape forming", near net shape forging (Near net shape forging). Obviously, there are strict requirements for the mold to achieve precision forming. Figure 1-1-6 is the mold device and product parts diagram for arc gear extrusion. The characteristics of this device are: 1) The spherical surface of the punch is self-supported to avoid lateral force. 2) The lower die has an adjustment device to ensure the concentricity of the upper and lower dies. 3) The lower die has a hydraulic clamping device to maintain centering clamping. Forming is divided into two steps, namely warm pre-forming of the cup-shaped billet with external teeth, and then cold finishing forming (see Figure 1-1-7). Finite element analysis shows that only the tooth shape of the preform is trapezoidal, which is the most suitable. The tooth shape of the extruded bar material is not processed and is only cut into gears. 3. Use a composite process. The billet for forging can be a powder sintered part or a billet made by injection molding. Figure 1-1-8 shows the forging of the billet formed by injection molding.
In recent years, semi-solid forming combines casting and forging to save energy and obtain relatively precise and high-performance workpieces. In addition, semi-solid forming is also a good method for forming low-fiber composite materials and particle-reinforced composite materials. Precision bending and precision welding process for large ring parts. Due to the difficulty in transporting large flange parts with a diameter of more than 8m, Wang Zhongren and others developed a precision bending and precision welding process for large ring parts. Its biggest advantage is that it can avoid the use of vertical lathe processes. The main processes of this method are shown in Figure 1-1-9: Figure a is a forged square billet, whose length should be greater than the length of each sector, and the processing amount of the heads at both ends should be reserved; Figure b is a special-shaped section processed by a gantry planer, including a sealing groove and a welding groove connected to the cylinder; Figure c is precision bending; Figure d is a welding groove for butt welding between the end heads and the butt heads accurately processed according to the arc length; Figure e is assembled into a ring; Figure f is a flange and a cylinder welding, and after welding into a flanged cylinder, the sealing surface is fine-machined using a simple machine tool at the construction site.
Figure 1-1-10 is a photo of the precision bending of a large flange. Considering that the cross section will change during the actual bending process, the numerical simulation method can be used for prediction, and then the cross-sectional shape can be corrected according to the prediction results to determine the processing size that should be guaranteed on the planer. The finite element numerical simulation results of the dimensional change of the bent part are shown in Figure 1-1-11.
4. Expand the application scope of forging process simulation. As software becomes more mature and computer prices continue to fall, CAD/CAM has been used more and more widely. It is worth emphasizing that forging process simulation has been able to successfully optimize the die structure design, predict defects such as folding and insufficiency that may occur during the forming process, optimize forming parameters, predict stress distribution in the die cavity, and avoid local cracking or excessive wear. Numerical simulation has moved from pure academic research to practical use. At present, the distribution of stress and strain rate in the workpiece can be predicted, and the organization and performance after deformation can be predicted when necessary. Figure 1-1-12 shows an example of eliminating folds generated during the forging process by optimizing the die shape through numerical simulation. As shown in Figure 1-1-12, the reason for the folding of the forging is the unreasonable design of the die shape. After modifying the die, the top of the workpiece is compressed under the clamping of the upper die, which can completely eliminate the folding. 5. Microforming Microforming in plastic processing is caused by the large demand for micro parts. The large demand for these micro parts is not only caused by the miniaturization of electrical appliances. With the development of medical devices, sensors and optoelectronic devices, the demand for micro parts has also increased rapidly. From the perspective of production cost and production efficiency, the plastic processing method is superior to the three-dimensional ultra-fine processing technology (LIGA process) that integrates deep X-ray lithography, electroforming molding, and micro-plastic casting. The so-called microforming usually means that at least one dimension of the formed part is less than 0.5mm. Since the grain size of the raw materials used has not changed much, that is, the ratio of the scale of micro parts to the grain size is much smaller than the ratio of the scale of conventional parts to the grain size, so the two do not follow the similar law. By the same token, the ratio of the surface area to the volume of micro parts is also much larger than the corresponding value of conventional parts. Correspondingly, the contact area has a much greater impact on microforming than the forming of conventional parts. Figure 1-1-13 vividly shows the change in the number of surface grains relative to the number of overall grains due to the reduction in size. In the figure, λx is the size reduction multiple.
Figure 1-1-14 shows that the convexity on the workpiece surface is easy to form a closed groove for storing lubricant after flattening. If the surface size is very small, such as for micro forming, it is not easy to form a groove for storing lubricant. Therefore, for the double cup extrusion shown in Figure 1-1-15, when the workpiece diameter is reduced from 4mm to 0.5mm, the test results show that under the condition of using extrusion oil as a lubricant, the friction force increases significantly with the reduction of the test piece size, and the increase can reach 20 times. Figure 1-1-16 shows a part forged with wire with a diameter of less than 0.3mm. For comparison, a match is placed on the right side of the figure. 6. Multi-point flexible forming Multi-point flexible forming is a new forming method for manufacturing large curvature shell workpieces, as shown in Figure 1-1-17. Its essence is to discretize the lower die into multiple adjustable small dies. In order to avoid the top of the small die from causing indentations on the workpiece surface, a steel plate is placed on the discrete die to produce a continuous flexible surface. The upper mold is composed of polyurethane blocks, and both sides of the workpiece are covered with polyurethane plates. Multi-point flexible forming can mainly manufacture the required workpiece by adjusting the shape of the lower mold. In order to consider the influence of the workpiece rebound on the forming accuracy, the mold surface can be corrected by adjusting the height of the small mold. This type of mold has been successfully used to manufacture the arc plate of the shrinkage body of a large wind tunnel. 7. Composite material forming Composite material forming has developed rapidly in recent years. For long fiber composite materials, semi-solid methods are mostly used to manufacture them. K. Sigert has developed AlMg alloy carbon fiber reinforced composite forming parts. As shown in Figure 1-1-18, the temperature of semi-solid forming is between the solidus and the liquidus, which is between 577 and 638℃. Its preform is shown in Figure 1-1-19. The fibers and the plates are laid alternately and wrapped with aluminum foil on the outside. For the forming of short fiber composite materials, the short fibers need to be pressed into a blank in advance and then the liquid metal is cast into the gaps between the fibers under pressure, cooled to a semi-solid state and then extruded. Hu Lianxi and others have done research in this regard. Zhang Libin once studied the preparation of PM-SiCp /2A12 composite materials. The process flow is shown in Figure 1-1-20. The hot pressing of the encapsulated mold, closed upsetting and isothermal hot reverse extrusion are all carried out on a domestic general-purpose four-column hydraulic press. The PM-SiCp /2A12 composite material processed by isothermal hot reverse extrusion has good mechanical properties. Compared with the room temperature tensile properties of the same state of ingot metallurgical 2A12, the conditional yield strength σ0.2 of the PM-SiCp /2A12 composite material containing SiCp15% (mass fraction) and 20% (mass fraction) is increased by 17.3% and 24.6%, respectively, and the tensile strength Rm is increased by 2.5% and 10.2%, respectively.
