1 Introduction High-voltage switch products have many types of parts and are frequently modified. The tie rod is a key component in the LW8-35SF6 type outdoor circuit breaker. It requires high electrical and thermal conductivity and good mechanical properties to reduce energy consumption and improve product reliability. Aluminum alloy materials not only have good electrical and thermal conductivity, excellent mechanical properties, but also have high specific strength and low density. Therefore, aluminum alloys are widely used in the manufacture of high-voltage electrical parts except copper and its alloys. Studies have shown that for a class of functional parts with higher overall performance requirements, such as tie rods, joints, conductors, contact seats, etc., aluminum alloy extruded rods (tubes) are generally made by machining. The 2A50 alloy is one of the commonly used materials. one. 2A50 alloy has good plasticity in the hot state, can improve the organization through the casting, extrusion and other deformation process to improve performance, and can be strengthened by heat treatment, good processability, and thus become the preferred material of high-voltage switch parts.
The rod extrusions are shown in Fig. 1. Traditionally, rods are used for direct cutting. The utilization of materials is generally 16% to 40%, and the waste is severe and the efficiency is low. The new process adopts the compound hot extrusion method of rod extrusion of the back extrusion head, which can greatly improve the dimensional accuracy of the blank, reduce the weight of the blank by more than 72%, and the electrical conductivity, hardness and strength of the product fully meet the design standards.
2Tie rod hot extrusion process analysis The rod part material is 2A50 (LD5) alloy, belongs to A1-Mg-Si-Cu system, has good forging performance, easy to deform in hot state, and corrosion resistance, welding performance and cutting performance Good, medium strength, good plasticity. In the production process, the surface of the cylindrical blank is coated with water-soluble graphite, and then heated to 490° C., and placed in a mold of a combination die to be extruded. Before the work, the mold is preheated to about 250°C. Before each extrusion, the mold cavity is sprayed with lubricant. After the extrusion deformation can be solution heat treatment aging to increase its hardness, solution temperature (515 ± 5) °C, time is 3h, aging temperature (160 ± 5) °C, time is 5h.
Pull rod extrusion can use the positive extrusion or back extrusion method to form the stem. Since the deformation of the tie rod is large and the length ratio of the stem to the stem is greater than 7, the direction of flow of the metal is the same as the movement of the punch when it is being squeezed. There is friction between the blank and the die, and not only the deformation force in the extrusion force, but also This friction is also included. When the blank and die temperature is too high and the lubrication is poor, the relative movement between the blank and the die will further increase the extrusion force. Because the rod part of the part is long, it is easy to destabilize and bend when directly ejecting. If the structure of the indirect ejector die is complicated, the operation is difficult to add.
The above problems can be solved by adopting a composite extrusion forming process, that is, a composite extrusion forming process in which the back extrusion of the stem portion is positively extruded. The process flow is shown in FIG. 2 . Due to the back extrusion of the stem, there is no frictional force between the blank and the die, which reduces the extrusion force. The program mold structure is simple, high production efficiency YA23-315 four-column hydraulic multifunction hydraulic beam to the work surface distance of 1250mm, long stroke, punch design is a hollow structure, the molding cavity of the stem on the punch, you can complete the off mold. The production process of the rod hot extrusion process is: blanking - heating - extrusion - heat treatment - finishing.
3 tie rod hot extrusion process design 3.1 mold structure and working process Hot extrusion process design is the hot extrusion die design, directly affect the quality of parts, production efficiency, mold life, production costs and so on. According to the shape of the extrusion, the punch is designed to be hollow, and a two-layer composite die structure is used. The compound hot extrusion die structure is shown in Fig. 3. During extrusion, the blank is placed in the cavity of the cavity. As the punch 4 descends, the blank is extruded in the composite die and the rod is back extruded. Forming, as the compressive deformation force increases, as the metal flows positively to the top piece, the head shape ends and the metal continues to flow in the opposite direction. When the squeezing is completed, the upper die returns and the workpiece is left in the die 7, the lower cylinder of the press moves, and the large diameter portion of the head is pushed out of the die 7 by the ejector pin 11, and the demolding can be completed. There should be a certain degree of inclination between the inner shape of the workpiece head and the mouth of the top piece, so as to ensure that the workpiece and the top piece do not lock up. The top bar 1 also doubles as the female die that the head is pressing.
3.2 Calculation of blank size According to the requirements of tie rod parts, taking into account the 2A50 part size after the heat treatment and the remaining machining allowance, the unilateral machining allowance of 2mm is left inside and outside the extrusion. According to the availability of raw materials, it was decided to use Φ90mm bar stock in the production of blanks with a height of 85mm.
3.3 Calculation of the allowable deformation degree Using the hot extrusion forming process, the allowable deformation degree of the material needs to be verified. The allowable deformation degree is expressed by the reduction of area ε to indicate the degree of deformation of the blank in the extrusion process:
3.4 Calculation of extrusion force In this composite extrusion, the punch downwards, the extrusion force overcomes the metal's deformation resistance and the friction between the blank and the mold, the metal begins to flow into the human cavity, the rod head is preformed, and the metal flow is preformed. After the corner of the bar, the counter-squeezing; the punch continues downwards. When the bar forming is finished, the pressing force is larger, and the composite extrusion force is P=P=P.
4 mold structure characteristics and work process should pay attention to the problem This process uses a single extrusion molding, using a common mold frame, the mold design is a two-tier combination structure. The actual production proves that the mold has a simple structure and is easy to use. By changing the punch and the top piece, parts with different head shapes and stem diameters and lengths can be extruded.
Because the punch is a hollow structure, the cross-sectional area is small, the unit extrusion force is high, and it works at a high temperature for a long time, and it is easy to deform. Therefore, a 3Cr2W8V material with higher heat strength should be used, and the heat treatment hardness should be 50-55HRc. The single-layer pre-tightening structure is adopted for the female die, 5CrNiMo is selected for the female die material, and the heat treatment hardness is 44-84HRC. Die pre-tightening ring requirements are not high, material selection 40Cr on it, heat treatment hardness 24-46HRC.
The rational design of the angle of the mannequin and the width of the working belt facilitates metal flow to minimize the friction between the metal and the mold and reduce the extrusion force. Die size and top piece should have a slope, work to maintain a certain degree of friction between the die and the workpiece, but does not affect the mold release after opening the mold, and should pay attention to the preheating of the mold. Ensure that the cone surface is evenly rubbed to avoid misalignment of the rod head during extrusion. In the process of back extrusion, the cleanliness and gap size of the blank and the mold must be ensured, and the layering and the air bubbles should be reduced. It is worthwhile to produce the high-voltage switch parts LW8-35SF6 aluminum alloy rod by using the composite extrusion process of extruding the head and extruding the head. The promotion of new technology is not only reasonable but also easy to operate. This process makes full use of the equipment capacity of the 3150k hydraulic press. It can be ejected in one time. The mold structure is simple, versatility is strong, and the extrusion force is small. It is especially suitable for hot extrusion of long rods with large deformation. The adoption of new technology will greatly increase production efficiency, and it will be of great reference value for other similar long rod parts with large forming deformation on small equipment.

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