Oil cylinder cold heading forming method for pre-upsetting and then gradually extending stepped gas spring

Abstract

The application discloses a cold upsetting forming method for an oil cylinder for a gas spring, and belongs to the technical field of cold upsetting forming of the oil cylinder for the gas spring.The method comprises the following steps: 1) raw material preparation and shaping; 2) upsetting; 3) concave pressing; 4) reverse extrusion stretching; 5) one-time forming of thin-wall reduced-diameter extension; and 6) two-time forming of thin-wall reduced-diameter extension.Combining the small overall size, the large length-diameter ratio and the thin-wall characteristics of the oil cylinder for the gas spring and the characteristics of batch production, the method adopts the method of first upsetting and then gradual extension, divides the complex structure part into multiple simple stations, adopts the process of upsetting + composite extrusion + multi-step reducing-diameter stretching, and introduces the reduced-diameter distribution formula based on the strain hardening index and the flow stress model, so that the gradual reduction of the inner and outer diameters of the cylinder body and the uniformization control of the wall thickness are realized.On the one hand, the method improves the wall thickness uniformity of the product and avoids the abnormal local wall thickness; on the other hand, due to the adoption of the five-station progressive one-time forming process, the auxiliary clamping time is greatly shortened, so that the forming efficiency is improved.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of mechanical processing, in particular to a cold-upsetting forming method for an oil cylinder of a gas spring. BACKGROUND

[0002] The gas spring is an industrial accessory with functions of supporting, buffering, braking, height adjustment and angle adjustment, and the application focuses on the forming method of the oil cylinder part. The traditional manufacturing process of the oil cylinder for the gas spring is generally a split forming by welding, that is, the plug and the cylinder body are processed and then welded. This method has low pipe material utilization, long production cycle, low product quality and short service life. In view of the above problems, through literature retrieval, there are the following improved thin-walled pipe part processing technologies: warm forging process, warm forging and cold finishing process, and cold upsetting process.

[0003] For example, in 2020, Xu Yingquan and others of Jiangsu Enno Shaft Research Technology Co., Ltd. proposed a bearing roller warm forging production process (authorized publication number: CN 111531116B). The bar material is heated at low temperature, and then subjected to multi-station step forging, which greatly reduces the cost and improves the production efficiency. However, the temperature of the blank is difficult to control by using the warm forging method, and it is easy to be unevenly heated, especially the mold cavity cooling is difficult, which is easy to damage the mold.

[0004] For example, in 2020, Xu Xiangqian and others of Jiangsu Senwei Precision Forging Co., Ltd. proposed a forging method for a precision deep hole thin-walled pipe (authorized publication number: CN 107414416B). The warm forging and cold finishing process improves the material utilization and product strength by warm forging and cold finishing after warm forging and cold finishing. However, the warm and cold upsetting combination method greatly increases the production cost and reduces the production efficiency.

[0005] For example, in 2021, Weng Xuanzhang and others of Ningbo Huahong Machinery Co., Ltd. proposed a production process for a shock absorber bushing blank (authorized publication number: CN 112247053B). The multi-station cold upsetting forming process improves the production efficiency of the shock absorber bushing blank, reduces the loss of the head and tail, greatly saves the production cost, and effectively proves that the multi-station cold upsetting process has great significance in the batch production of small parts. However, for large-length-diameter-ratio thin-walled pipe parts, the wall thickness is uneven, the cross section is irregular, and even the hole wall cracks and other defects are caused by the forming of the deep hole and the thin wall.

[0006] Therefore, the cold heading forming method of the relevant thin-walled deep hole part is searched. In 2020, Li T et al. of Guizhou University proposed a multi-stage cold heading process for thin-walled A-286 superalloy tube in Analyzing Forged Quality of Thin-Walled A-286 Superalloy Tube under Multi-Stage Cold Forging Processes (collected in the journal Materials, Vol. 16), compared two forging sequences of the forming process by means of finite element analysis software, and determined the method of first heading and then backward extrusion forming, which is more suitable for actual production application.

[0007] In 2023, Jiang Yulan et al. of Guizhou University proposed A286 Superalloy Thin-Walled Tube Heading Process Optimization (collected in the journal Forging and Pressing Technology, pages 71-80) for the problems of folding and incomplete filling of thin-walled parts during heading forming, compared the forming methods of the inner hole by using Deform simulation, and obtained that backward extrusion is more conducive to the axial flow of metal and the distribution of stress state, and improves the stability of thin-walled feature forming.

[0008] The above two methods are to form a thin-walled deep hole by multiple backward extrusions, but as the number of backward extrusions increases, the thin-walled cavity becomes more and more narrow, the metal flow space is extremely limited, the friction with the cavity wall increases significantly, and the material flow is uneven, causing local thin-walled and low-quality hole wall problems. SUMMARY

[0009] The purpose of the present application is to overcome the defects of the prior art and provide a cold heading forming method for gas spring oil cylinder, which is based on the progressive five-station cold heading near-net forming process and the "first heading and then gradual extension" idea. The mold applies external force to the metal blank, causing repeated extrusion and stretching of the metal material. The overall processing scheme is first coarse and then fine. The coarse metal blank is first headed to improve the forging ratio and transverse mechanical properties of the backward extrusion stretching, and to reduce anisotropy. Then, the concave die gradually reduces the wall thickness of the blind hole of the gas spring oil cylinder, and the convex die diameter is also reduced to fine the inner diameter size of the gas spring oil cylinder, so as to gradually reduce the inner and outer diameter sizes of the cylinder body, improve the product size precision and material utilization rate, and avoid the low-quality situation of local thin-walled.

[0010] The purpose of the present application is achieved by a cold heading forming method for gas spring oil cylinder, which comprises the following steps:

[0011] S1: raw material preparation and shaping, according to the shape and volume of the final product, selecting the appropriate size of long cylindrical blank, cutting the blank into short rods of equal length with a bar cutting machine; the cut blank is shaped; the blank is pickled and phosphated;

[0012] S2: upsetting, the short rod after shaping is sent to the first station of the multi-station cold header through the feeder, and the short rod is fixed through the compression nut; the first station of the multi-station cold header is provided with an upsetting compression rod on the male die part, and the metal blank is upset by the upsetting compression rod; the first station upsetting female die is in the form of a boss, and when the upsetting compression rod is pressed down, the lower end of the blank is extruded to form a boss, and after the upsetting process is completed, the short rod is sent to the second station of the multi-station cold header through the clamping wheel rotating mechanism;

[0013] S3: concave pressing, the second station of the multi-station cold header is provided with a short and thick punch on the male die part, and a center top hole is punched on the upper end of the short rod by the short and thick punch; after the concave pressing process is completed, the short rod is sent to the third station of the multi-station cold header through the clamping wheel rotating mechanism;

[0014] S4: reverse extrusion stretching, the third station of the multi-station cold header is provided with a short punch needle on the male die part, the short punch needle is used to punch the center top hole, the short rod is preliminarily stretched, and the transition area of the inner and outer walls of the cylinder body is processed, and after the preliminary stretching is completed, the short rod is sent to the fourth station of the multi-station cold header through the clamping wheel rotating mechanism;

[0015] S5: thin-walled and reduced-diameter extension one-time forming, the fourth station of the multi-station cold header is provided with a medium-length punch needle on the male die part, the medium-length punch needle is pressed down to perform the first punching on the depth of the cylinder body, further extend the depth of the cylinder body, and at the same time, the thin-walled and reduced-diameter extension one-time forming female die is used to reduce the wall thickness of the cylinder body, and after the thin-walled and reduced-diameter extension one-time forming, the short rod is sent to the fifth station of the multi-station cold header through the clamping wheel rotating mechanism;

[0016] S6: thin-walled and reduced-diameter extension two-time forming, the fifth station of the multi-station cold header is provided with a long punch needle on the male die part, the long punch needle is pressed down to perform the second punching on the inside of the cylinder body, the short rod is stretched into a cylinder body with a standard inner diameter, and at the same time, the thin-walled and reduced-diameter extension two-time forming female die is used to reduce the wall thickness of the cylinder body, so as to meet the standard wall thickness requirement.

[0017] As a further limitation of the application, the ratio of the diameter of the short punch needle to the diameter of the short and thick punch in step 4) is in the range of 1.03-1.05.

[0018] As a further limitation of the application, the ratio of the diameter of the medium-length punch needle to the diameter of the short punch needle in step 5) is in the range of 0.98-0.99, and the first reduction of the outer diameter of the oil gas cylinder is Δd1, which is specifically shown in formulas (1) to (3):

[0019]

[0020] In the formula: Δd1 is the first diameter reduction, D3 is the outer diameter after reverse extrusion, α is the strain distribution weight coefficient, ε total is the total equivalent plastic strain of two times of diameter reduction, β is the process correction factor, A represents the yield strength of the material, B is the hardening coefficient of the material, is the equivalent strain of the material, n is the strain hardening index of the material, C is the strain rate sensitivity coefficient of the material, and D5 is the final target outer diameter.

[0021] As a further limitation of the application, the ratio of the diameter of the long punch in step 6) to the diameter of the medium-long punch is in the range of 0.99-0.995, which is consistent with the inner diameter of the finished gas spring oil cylinder, and the outer diameter of the oil cylinder is reduced by using a thin-walled diameter extension die, and the outer diameter reduction Δd2 of the second diameter reduction of the oil cylinder is specifically shown in formula (4):

[0022]

[0023] The application adopts the above technical scheme, compared with the prior art, has the beneficial effects that: 1) the application proposes a cold heading forming method for gas spring oil cylinders, which is based on a progressive five-station cold heading near-net forming process, and through the process of upsetting + composite extrusion + multi-step diameter reduction stretching, the wall thickness can be accurately controlled, the inner and outer diameters of the cylinder body are gradually reduced, the phenomenon of local metal accumulation is avoided, the product size precision is improved, and the problem of difficult forming of thin-walled features and easy occurrence of local thin-walled and metal accumulation in the cold heading forming process of the gas spring oil cylinder is solved; compared with the multi-step forming process, the five-station progressive one-step forming process can save auxiliary clamping time, thereby improving the forming efficiency.

[0024] 2) Based on the idea of "upsetting first and then gradually extending", the synergistic effect of the concave die gradual diameter reduction and the convex die synchronous extension of the inner hole is utilized to accurately control the wall thickness. Combined with the material strain hardening index n and the flow stress model σ=A+Bε n , an empirical formula for distributing the diameter reduction amount is introduced, which scientifically distributes the diameter reduction amount of two times, ensures that the single deformation amount is within the allowable deformation range of the material, avoids the uneven wall thickness or cracks caused by local stress exceeding the limit, and provides a theoretical guidance for cold heading forming of the same type of thin-walled pipes. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 The flowchart of the application.

[0026] Figure 2 The structure of the upsetting forming die of the application is simple.

[0027] Figure 3This is a simplified schematic diagram of the compression molding die structure of the present invention.

[0028] Figure 4 This is a simplified schematic diagram of the anti-extrusion stretching die structure of the present invention.

[0029] Figure 5 This is a simplified schematic diagram of the thin-walled diameter reduction and extension one-time forming mold structure of the present invention.

[0030] Figure 6 This is a simplified schematic diagram of the thin-walled diameter reduction and extension secondary forming mold structure of the present invention.

[0031] Figure 7 This is a simulation diagram of the damage values ​​at each station of the cold upsetting process for the hydraulic cylinder of the progressive gas spring of the present invention, which is formed by first upsetting and then gradually extending.

[0032] Among them, 1 is a buffer pad, 2 is a fastening nut, 3 is a punch sleeve, 4 is a positioning fastening ring, 5A is an upsetting pressure bar, 6 is a prestress ring, 7A is an upsetting die, 8 is a discharge ejector, 5B is a short and thick punch, 7B is a pressing die, 5C is a short punch, 7C is a reverse extrusion stretching die, 5D is a medium and long punch, 7D is a thin-walled diameter reduction and extension one-time forming die, 5E is a long punch, and 7E is a thin-walled diameter reduction and extension two-time forming die. Detailed Implementation

[0033] like Figure 1 The method for cold-forging a progressive gas spring cylinder with pre-upsetting and then gradually extending includes the following steps:

[0034] S1: Raw material preparation and shaping. Based on the final product shape and volume, select a suitable-sized long cylindrical 45# steel billet and cut it evenly into short bars of equal length using a bar cutter. Shape the cut billets to ensure a neat appearance, no burrs, and that the billet diameter meets the specified requirements. Pickle and phosphate the billets. Billet selection... The constitutive equations for No. 45 steel are shown in Formula 5 and Table 1.

[0035]

[0036] In the formula: σ is the material flow stress; ε is the plastic strain; A, B, C, n, and m are all material constants; The plastic strain rate is taken as 10s. -1 ; For dimensionless temperatures, T and T0 r T m These represent the material temperature, ambient temperature, and material melting point temperature, respectively.

[0037] Table 1 Constitutive model parameters for 45# steel

[0038]

[0039] like Figure 2 As shown, the upsetting forming die includes a buffer pad 1, a fastening nut 2, a punch sleeve 3, a positioning fastening ring 4, an upsetting pressure bar 5A, a prestressing ring 6, an upsetting die 7A, and an ejector 8. For the punch section: the buffer pad 1 is installed above the upsetting pressure bar 5A; the fastening nut 2 is installed on the outside of the punch sleeve 3; the punch sleeve 3 is fixed to the outside of the positioning fastening ring 4 by the fastening nut 2; the positioning fastening ring 4 is located between the punch sleeve 3 and the upsetting pressure bar 5A; the upsetting pressure bar 5A is fixed inside the punch sleeve 3 by the cooperation of the fastening nut 2 and the positioning fastening ring 4. For the die section: it adopts an integral structure, with the prestressing ring 6 sleeved on the outside of the upsetting die 7A to clamp it; the ejector 8 is installed below the upsetting die 7A.

[0040] S2: Upsetting. The shaped short bar is fed to the first station of the multi-station cold upsetting machine via a feeder and fixed by clamping nut 2. The punch of the first station of the multi-station cold upsetting machine is equipped with an upsetting pressure bar 5A. The metal billet is upset by pressing down the upsetting pressure bar 5A. The upsetting die 7A of the first station is in the shape of a boss. When the upsetting pressure bar 5A is pressed down, the lower end of the billet is extruded into a boss. After upsetting, the short bar is fed into the second station of the multi-station cold upsetting machine by a clamping and rotating mechanism. The length of the short bar is reduced and the diameter is increased. This increases the cross-section and flattens the end face of the billet before punching, which is beneficial to improving the forging ratio during reverse extrusion and stretching. The diameter after upsetting is 22.00 mm and the length is 17.44 mm.

[0041] S3: Indentation, such as Figure 3 As shown, the punch part of the second station of the multi-station cold heading machine is equipped with a short and thick punch 5B. By pressing down with the short and thick punch 5B, a large-diameter center top hole is punched out at the upper end of the short bar. After the die-cutting process is completed, the short bar is fed into the third station of the multi-station cold heading machine through the clamping and rotating mechanism for die-cutting. The pre-pressed indentation is for the purpose of pre-distributing the material for reverse extrusion and stretching, and at the same time to avoid the punch from breaking. The diameter after die-cutting is 22.20mm, the diameter of the hole is 17.20mm, and the hole depth is 1.00mm.

[0042] S4: Reverse compression stretching, such as Figure 4 As shown, the third station punch of the multi-station cold heading machine is equipped with a short punch 5C. The short punch 5C is used to punch the center top hole, and the short bar is initially stretched to process the transition area between the inner and outer walls of the cylinder. After the initial stretching is completed, the short bar is fed into the fourth station of the multi-station cold heading machine through the clamping and rotating mechanism. The diameter of the short punch is 0.60mm larger than the diameter of the short coarse punch. After reverse extrusion stretching, the diameter is 22.60mm, the inner diameter of the cylinder is 17.80mm, and the cylinder depth is 30.16mm.

[0043] S5: Thin-walled diameter reduction elongation one-time molding, such asFigure 5 As shown, the fourth station punch of the multi-station cold heading machine is equipped with a medium-length punch 5D. By pressing down the medium-length punch 5D, the medium-length punch performs the first punching of the cylinder depth. At the same time, the thin-walled diameter reduction and one-time forming die 7D is used to reduce the diameter, so that the wall thickness of the cylinder is reduced and the depth is further extended. After the thin-walled diameter reduction and one-time forming, the short bar is fed into the fifth station of the multi-station cold heading machine through the clamping and rotating mechanism. The diameter of the medium-length punch is 0.20mm shorter than the diameter of the short punch. The ε is calculated by combining the diameter reduction formula (1) with the constitutive model formula (5) of 45 steel material and Table 1. total =0.148, α=0.79, Δd1=2.00, wall thickness reduction of about 1.00mm, where the process correction factor β is 0.80. After thin-walled diameter reduction and one-time forming, the diameter is 20.60mm, the cylinder inner diameter is 17.60mm, and the cylinder depth is 54.87mm.

[0044] S6: Thin-walled diameter reduction elongation secondary forming, such as Figure 6 As shown, the fifth station punch of the multi-station cold heading machine is equipped with a long punch 5E. By pressing down with the long punch 5E, the cylinder body is stamped a second time, stretching the short bar into a cylinder body with a standard inner diameter. At the same time, the thin-walled diameter reduction extension secondary forming die 7E is used to reduce the diameter, so that the wall thickness of the cylinder body is reduced to meet the standard wall thickness requirements. The diameter of the long punch is 0.10 mm shorter than the diameter of the medium-length punch. The outer diameter reduction Δd2 of the second diameter reduction of the oil cylinder is calculated to be 1.1 mm by the diameter reduction formula (4), and the wall thickness is reduced by about 0.55 mm. After the thin-walled diameter reduction extension secondary forming, the diameter is 19.50 mm, the inner diameter of the cylinder body is 17.50 mm, and the cylinder body depth is 97.45 mm.

[0045] like Figure 7 As shown, finite element simulation of the cold heading process was performed to obtain simulation diagrams of damage values ​​at each station in the cold heading forming of the gas spring cylinder. The material is 45# steel (critical damage value 0.6232). During the upsetting process, deformation is uniform and the damage value is low; during the denting process, deformation is concentrated near the dent, resulting in a low damage value; during the reverse extrusion, the material flows violently, and stress concentration easily occurs in areas such as the punch fillet, increasing the damage value; during thin-wall extension, the thin wall thickness makes it prone to instability and deformation, increasing the damage value; during the second thin-wall extension, the internal stress becomes more complex due to the previous thin-wall extension, resulting in an even higher damage value. The maximum damage value at each station is less than the critical value, indicating a reasonable distribution of process deformation, which ensures forming quality.

[0046] This invention, based on cold heading forming, proposes a progressive cold heading method for gas spring cylinders, involving roughing followed by gradual extension. This method addresses the characteristics of gas spring cylinder parts: small overall size, numerous forming steps, and the need for mass production. Based on the concept of "roughing followed by gradual extension + cold heading," it utilizes the synergistic effect of progressive die diameter reduction and synchronous inner hole extension by the punch to precisely control the wall thickness. Furthermore, it incorporates the material strain hardening exponent n and the flow stress model σ=A+Bε. n This invention introduces an empirical formula for allocating the reduction amount, scientifically distributing the reduction amount in two stages to ensure that the deformation amount in a single step is within the allowable deformation range of the material, avoiding uneven wall thickness or cracks caused by excessive local stress. This provides theoretical guidance for the cold heading of similar thin-walled tubes. The forming method proposed in this invention reduces the number of forming steps compared to the original production process, adopting a progressive forming method that reduces the load on each forming step. It not only possesses the advantages of high dimensional accuracy, good surface quality, continuous metal flow lines, good microstructure, strong load-bearing capacity, and high material utilization of cold-headed parts, but also features high-speed forming, which helps improve efficiency and reduce production costs, making it suitable for the mass production of small parts.

[0047] This invention is not limited to the above embodiments. Based on the technical solutions disclosed in this invention, those skilled in the art can make some substitutions and modifications to some of the technical features without creative effort, and all such substitutions and modifications are within the protection scope of this invention.

Claims

1. A method for cold-forging a progressively extending gas spring cylinder using a pre-upsetting and then gradually extending process, characterized in that, A progressive cold heading method consisting of five stations includes the following steps: S1: Raw material preparation and shaping. Based on the shape and volume of the final product, select a suitable size long cylindrical blank and cut it into short bars of equal length. Shape, pickle, and phosphate the cut blanks. S2: Upsetting. The shaped short bar is fed to the first station of the multi-station cold heading machine via a feeder, and the short bar is fixed by a clamping nut. The punch part of the first station of the multi-station cold heading machine is equipped with an upsetting pressure bar. The metal billet is upset by pressing down the upsetting pressure bar. The upsetting die of the first station is in the shape of a boss. When the upsetting pressure bar is pressed down, the lower end of the billet is squeezed into a boss. After the upsetting process is completed, the short bar is fed into the second station of the multi-station cold heading machine through a clamping and rotating mechanism. S3: Concave cutting. The second station punch of the multi-station cold heading machine is equipped with a short and thick punch. The short and thick punch presses down to punch out a central top hole at the upper end of the short bar. After the concave cutting is completed, the short bar is fed into the third station of the multi-station cold heading machine through the clamping and rotating mechanism. S4: Reverse extrusion stretching. The third station punch of the multi-station cold heading machine is equipped with a short punch. The short punch is used to punch the center top hole to initially stretch the short bar material and process the transition area between the inner and outer walls of the cylinder. After the initial stretching is completed, the short bar material is fed into the fourth station of the multi-station cold heading machine through the clamping and rotating mechanism. S5: Thin-walled diameter reduction and one-time forming. The fourth station punch of the multi-station cold heading machine is equipped with a medium-length punch. By pressing down the medium-length punch, the medium-length punch performs the first punching on the cylinder depth, further extending the depth of the cylinder. At the same time, the thin-walled diameter reduction and one-time forming die is used to reduce the diameter, so that the wall thickness of the cylinder is reduced. After the thin-walled diameter reduction and one-time forming, the short bar is fed into the fifth station of the multi-station cold heading machine through the clamping and rotating mechanism. S6: Thin-walled diameter reduction extension secondary forming. The fifth station punch of the multi-station cold heading machine is equipped with a long punch. By pressing down with the long punch, the inside of the cylinder is stamped a second time, stretching the short bar into a cylinder with a standard inner diameter. At the same time, the diameter is reduced by using the thin-walled diameter reduction extension secondary forming die to reduce the wall thickness of the cylinder and achieve the standard wall thickness requirement.

2. The cold forging method for a progressively extending gas spring cylinder according to claim 1, characterized in that, The ratio of the diameter of the short punch to the diameter of the short, thick punch in step 4) is in the range of 1.03 to 1.

05.

3. The cold forging method for a progressively extending gas spring cylinder according to claim 1, characterized in that, The ratio of the diameter of the medium-length punch to the diameter of the short punch in step 5) is in the range of 0.98 to 0.

99. The outer diameter reduction Δd1 of the first reduction in diameter of the oil cylinder is specifically shown in formulas (1) to (3): In the formula: Δd1 is the outer diameter reduction during the first shrinkage, D3 is the outer diameter after reverse extrusion, α is the strain distribution weighting coefficient, and ε total It represents the total equivalent plastic strain from the two diameter reductions, β is the process correction factor, A represents the initial yield strength of the material, and B is the hardening coefficient of the material. is the equivalent strain of the material, n is the strain hardening index of the material, C is the strain rate sensitivity coefficient of the material, and D5 is the final target outer diameter.

4. The cold forging method for a progressively extending gas spring cylinder according to claim 3, characterized in that, In step 6), the ratio of the diameter of the long punch to the diameter of the medium-length punch is in the range of 0.99 to 0.

995. The outer diameter of the oil cylinder is reduced by the thin-walled diameter reduction extension secondary forming die. The outer diameter reduction amount Δd2 of the second diameter reduction of the oil cylinder is as shown in formula (4): Δd2=D3-D5-Δd1 (4).

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