Multi-station cold heading forming process for special-shaped pipe fitting of automobile shock absorber

Abstract

The invention relates to a multi-station cold heading forming process for a special-shaped pipe fitting of an automobile shock absorber, and belongs to the technical field of cold heading of special-shaped pipe fittings of automobiles. A combined female die with a three-stage gradual-change fillet structure is adopted, and the diameters of fillets are gradually increased to be matched with a gradient stiffness belleville spring buffer unit, so that staging release of stress is realized; a cross section sudden change stress peak value of the root of the flange is eliminated, the crack risk is reduced, a special-shaped punch of a 45-degree helical angle gradually-shrinking type flow guide groove is designed, axial and radial flow of materials is dynamically guided, meanwhile, the filling amount is controlled in real time through a hydraulic adjustable limiting mechanism, and the problems of folding and wall thickness fluctuation are radically solved; three sets of gradient clamping force clamping jaws distributed at 120 degrees and V-shaped pre-indentations are arranged, asymmetric deformation generated by preorder cold heading is compensated in a self-adaptive mode, and the coaxiality error of a special-shaped contour and a flange is reduced. The problems that in the multi-station cold heading process in the prior art, cracks are caused due to stress concentration of a fixed fillet die, and folding defects are caused due to out-of-control flowing of a linear flow guide groove material are solved.

Description

Technical Field

[0001] The invention belongs to the technical field of cold heading of special-shaped pipe fittings for automobiles, and particularly relates to a multi-station cold heading forming process for special-shaped pipe fittings of automobile shock absorbers. Background Art

[0002] At present, the cold heading forming process of special-shaped pipe fittings for automobile shock absorbers mostly adopts multi-station continuous combined processing and forming technology. In the conventional combined processing technology, the forming of the flange structure is usually realized by single axial upsetting, and then the tube body is radially extruded in subsequent stations to form a special-shaped profile. The existing molds generally adopt a fixed fillet design, restrict the material flow through a rigid die cavity, and rely on uniformly distributed clamping claws for blank clamping and positioning. In the flange forming stage, the mold buffer system mostly adopts elastic elements with a single stiffness to absorb the forming stress; when forming the special-shaped profile, the material is guided to fill through a straight flow guide groove. Although such methods can realize the forming of the basic shape, when dealing with complex geometric transitions, it is still necessary to rely on multiple annealing or subsequent machining to correct the dimensional deviation.

[0003] However, the above existing processes have the following key problems: First, due to the sudden change of the cross-section at the flange root, stress concentration is likely to occur, and the fixed fillet and uniformly distributed buffer design cannot achieve the staged release of stress, resulting in an increased risk of crack initiation; Second, when forming the special-shaped profile, the axial flow and radial expansion of the material lack guidance, and the straight flow guide groove is difficult to balance the flow velocity difference, easily causing folding defects or wall thickness fluctuations; Third, during the multi-station switching, the rigid positioning of the uniformly distributed clamping force cannot compensate for the cumulative deformation error of the previous process, resulting in an out-of-tolerance coaxiality between the special-shaped profile and the flange structure. The root cause of these problems is that it is difficult for the existing technology to synchronously achieve stress gradient release, dynamic regulation of the flow direction, and adaptive compensation of deformation errors, which severely limits the integrated forming efficiency and reliability of high-precision special-shaped pipe fittings. In view of this, a multi-station cold heading forming process for special-shaped pipe fittings of automobile shock absorbers is proposed. Summary of the Invention

[0004] In order to solve the above problems existing in the prior art, the invention provides a multi-station cold heading forming process for special-shaped pipe fittings of automobile shock absorbers, which solves the problems of cracks caused by stress concentration in the multi-station cold heading process due to fixed fillet molds, folding defects caused by out-of-control material flow in the straight flow guide groove, and out-of-tolerance coaxiality caused by the inability of uniformly distributed clamping force to compensate for cumulative deformation.

[0005] The object of the invention can be achieved by the following technical solutions: A multi-station cold heading forming process for special-shaped pipe fittings of automobile shock absorbers includes the following steps: S1: Blanking treatment. Select special metal wire for automobile shock absorbers. After straightening the metal wire by a straightening machine, perform fixed-length cutting through a cutting die to obtain a cylindrical blank. S2: Pre - forming, select a modular universal machine tool, transfer the blank to the first station of the universal machine tool for end chamfering to form a pre - formed part with a conical guiding portion; S3: First cold - heading forming, in the second station of the universal machine tool, use a combined die with a gradually changing fillet structure to axially upset the pre - formed part. A three - stage transition structure with gradually increasing fillet diameters is set at the root of the die, and each stage of the fillet is equipped with a disc spring buffer unit with a gradually decreasing stiffness coefficient. A middle part with a flange is formed through graded stress release; S4: Positioning compensation, set three groups of cemented carbide jaws distributed at 120° in the universal machine tool. V - shaped pre - indentations are machined on the inner side of the jaws and a gradient clamping force control system is configured to complete the self - centering clamping of the middle part; S5: Second cold - heading forming, in the third station of the universal machine tool, use a special - shaped punch with a spiral diversion groove to perform radial extrusion. A tapered diversion groove with a 45° spiral angle is machined on the surface of the die, and a hydraulic adjustable limit mechanism is set at the end of the groove to form the special - shaped contour of the pipe fitting; S6: Finishing forming, in the fourth station of the universal machine tool, use a finishing die to perform surface burnishing on the special - shaped contour to eliminate the forming stress; S7: Trimming, in the fifth station of the universal machine tool, remove the flash at the flange through precision blanking; S8: Heat treatment, anneal the formed part while controlling the temperature and holding time; S9: Inspection and packaging, perform dimensional inspection, surface flaw detection, then implement rust prevention treatment and packaging.

[0006] As a further scheme of the present invention, in the step S3, the diameter increment ratio of the three - stage gradually changing fillets is 1:1.5:2, and an arc is used for smooth transition between adjacent fillets, and the radius of the transition arc is 0.3 times the diameter of the corresponding fillet.

[0007] As a further scheme of the present invention, in the step S3, the stiffness coefficient gradient of the disc spring buffer unit is K1:K2:K3 = 3:2:1, and the pre - compression amount of each stage of the spring is inversely proportional to its stiffness coefficient. k1 is a common influencing factor of stress release efficiency and material flow uniformity, representing the contribution intensity of stress release to positioning accuracy. k2 is a compensation coefficient for material flow non - uniformity, representing the adaptive correction ability of the positioning system when the flow gets out of control. K3 is an ultimate safety redundancy coefficient, used to absorb the influence of residual stress and uncontrollable disturbance conditions during the cold - heading forming process.

[0008] As a further scheme of the present invention, in the step S4, the included angle of the V - shaped pre - indentation is 90° - 120°, and the clamping forces of the three groups of cemented carbide jaws are distributed in a gradient ratio of 1.2:1.0:0.8.

[0009] As a further solution of the present invention, between steps S3 - S5, it is necessary to satisfy the functional relationship of, where η is the stress release efficiency, δ is the material flow uniformity, and is the positioning accuracy.

[0010] As a further solution of the present invention, in step S6, the burnishing pressure of the finishing die is 1.2 - 1.5 times the forming pressure.

[0011] As a further solution of the present invention, in step S8, the annealing treatment adopts two - stage temperature control. In the first stage, it is heated to 650°C at a rate of 10°C / min and held for 30 min, and in the second stage, it is cooled to 300°C at a rate of 5°C / min and then air - cooled.

[0012] As a further solution of the present invention, the internal of the cemented carbide jaw is integrated with a circulating cooling channel. The inlet temperature of the cooling medium is controlled at 10 ± 2°C, and the cross - section of the channel is a variable - diameter spiral structure. The minimum - diameter section of the circulating cooling channel is located 1.5 mm below the root of the V - shaped pre - indentation.

[0013] As a further solution of the present invention, in step S3, the strengthening layer of the combined die is prepared by laser cladding with a WC - Co - based composite material. The thickness of the cladding layer is 8% - 10% of the fillet diameter, and the cladding direction forms a 15° angle with the material flow direction.

[0014] As a further solution of the present invention, in step S5, the hydraulically adjustable limit mechanism includes a main piston and a sub - piston arranged inside the main piston, and the diameter D1 of the main piston and the diameter D2 of the sub - piston satisfy D1 / D2 = √2. The gap between the two pistons is filled with dimethyl silicone oil.

[0015] The beneficial effects of the present invention are as follows: In step S3, a combined die with a three - stage gradually changing fillet structure is adopted. The fillet diameter increases step by step and is matched with a gradient - stiffness disc spring buffer unit to achieve staged stress release, eliminate the cross - section mutation stress peak at the flange root, and reduce the crack risk. In step S5, a special - shaped punch with a 45° spiral - angle gradually shrinking flow - guiding groove is designed to dynamically guide the axial and radial flow of the material. At the same time, the filling amount is controlled in real time through the hydraulically adjustable limit mechanism to completely solve the problems of folding and wall - thickness fluctuation. In step S4, three groups of gradient clamping force jaws and V - shaped pre - indentations distributed at 120° are configured to adaptively compensate for the asymmetric deformation generated by the previous cold heading, reduce the coaxiality error between the special - shaped profile and the flange, and at the same time improve the clamping accuracy. It solves the problems in the prior art of multi - station cold heading process, such as cracks caused by stress concentration in the fixed - fillet die, folding defects caused by out - of - control material flow in the straight flow - guiding groove, and coaxiality out - tolerance caused by the inability of uniformly distributed clamping force to compensate for the cumulative deformation. Description of the Drawings

[0016] For the convenience of those skilled in the art to understand, the present invention will be further described below in conjunction with the accompanying drawings.

[0017] Figure 1 This is the process flow chart of the multi-station cold heading forming of the special-shaped pipe fittings for automobile shock absorbers of the present invention. Specific embodiments

[0018] To further elaborate on the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the following will describe in detail the specific embodiments, structures, features and their effects of the present invention in conjunction with the accompanying drawings and preferred embodiments.

[0019] Please refer to Figure 1 As shown, this embodiment provides a multi-station cold heading forming process for special-shaped pipe fittings of automobile shock absorbers, including the following steps: S1: Blanking treatment. Select the special metal wire for automobile shock absorbers. After straightening the metal wire by a straightening machine, perform fixed-length cutting through a cutting die to obtain a cylindrical blank. Straightening reduces the internal stress of the material and avoids cracking during subsequent cold heading. S2: Pre-forming. Select a modular universal machine tool, transfer the blank to the first station of the universal machine tool for end chamfering treatment to form a pre-form with a tapered guiding part. The tapered guiding part guides the material flow to the flange root during the axial upsetting in S3, reducing the risk of folding. S3: First cold heading forming. At the second station of the universal machine tool, use a combined die with a gradually changing fillet structure to axially upset the pre-form. The root of the die is provided with a three-stage transition structure with gradually increasing fillet diameters, and each stage of fillet is configured with a disc spring buffer unit with a gradually decreasing stiffness coefficient. A middle part with a flange flange is formed through staged stress release. By releasing the cross-section mutation stress in stages, it avoids the inhibition of the initial upsetting springback in the high-stiffness stress concentration area, and the low-stiffness area absorbs the final forging impact, resulting in a decrease in the crack rate. S4: Positioning compensation. Set three groups of cemented carbide jaws distributed at 120° on the universal machine tool. The inner side of the jaws is machined with a V-shaped pre-indentation and configured with a gradient clamping force control system to complete the self-centering clamping of the middle part. Through the V-shaped pre-indentation, the contact area is increased to prevent the rotation and offset of the middle part. For the non-uniform deformation area in S3, such as the thickened part at the flange root, the clamping is enhanced to compensate for the cumulative deformation, reducing the coaxiality error. S5: Second cold heading forming. At the third station of the universal machine tool, use a special-shaped punch with a spiral guide groove to perform radial extrusion. The surface of the die is machined with a tapered guide groove with a 45° spiral angle, and a hydraulically adjustable limit mechanism is set at the end of the groove to form the special-shaped contour of the pipe fitting. The 45° spiral guide groove here decomposes the axial material flow into radial filling, improving the flow velocity uniformity and eliminating folding. S6: Finishing forming. At the fourth station of the universal machine tool, a finishing die is used to perform surface high-pressure burnishing on the special-shaped contour to eliminate the forming stress. In addition, high-pressure burnishing makes the surface grains refined to less than 5 μm, eliminates the residual tensile stress at the corners, ensures that the surface roughness Ra ≤ 0.8 μm, and reduces the subsequent flaw detection miss rate. S7: Trimming. At the fifth station of the universal machine tool, the flash at the flange edge is removed by precision blanking. Micro-gap blanking avoids tearing at the root of the flange. It is necessary to ensure that the residual height of the flash ≤ 0.05 mm to meet the requirements of the shock absorber sealing surface. S8: Heat treatment. The formed parts are annealed while controlling the temperature and holding time. S9: Inspection and packaging. After dimensional inspection and surface flaw detection, rust prevention treatment and packaging are carried out.

[0020] At present, the cold heading forming process of automotive shock absorber special-shaped pipe fittings mostly adopts multi-station continuous combined processing and forming technology. In the conventional combined processing technology, the forming of the flange structure is usually achieved by single axial upsetting, and then the tube body is radially extruded in subsequent stations to form a special-shaped contour. Existing dies generally adopt a fixed fillet design, restrict the material flow through a rigid die cavity, and rely on evenly distributed claws for blank clamping and positioning. In the flange forming stage, the die buffer system mostly uses elastic elements with a single stiffness to absorb the forming stress; when forming the special-shaped contour, the material is guided to fill through a straight-shaped flow channel. Although such methods can achieve the forming of the basic shape, when dealing with complex geometric transitions, it still relies on multiple annealing or subsequent machining to correct the dimensional deviation. However, there are some problems in the existing process. For example, due to the sudden change of the cross-section at the root of the flange, stress concentration is likely to occur, and the fixed fillet and evenly distributed buffer design cannot achieve the staged release of stress, resulting in an increased risk of crack initiation; in addition, when forming the special-shaped contour, there is a lack of guidance for the axial flow and radial expansion of the material, and the straight flow channel is difficult to balance the flow velocity difference, easily causing folding defects or wall thickness fluctuations; and during the multi-station switching, the rigid positioning of the evenly distributed clamping force cannot compensate for the cumulative deformation error of the previous process, resulting in an out-of-tolerance coaxiality between the special-shaped contour and the flange structure. The root cause of these problems is that it is difficult for the existing technology to simultaneously achieve stress gradient release, dynamic regulation of the flow direction, and adaptive compensation of deformation errors, which severely limits the integrated forming efficiency and reliability of high-precision special-shaped pipe fittings.

[0021] To solve the above problems, in this embodiment, a combined die with a three-stage gradually changing fillet structure is adopted. The fillet diameter increases step by step and is matched with a gradient stiffness disc spring buffer unit to achieve staged stress release, eliminate the cross-sectional mutation stress peak at the flange root, reduce the crack risk, and design a special-shaped punch with a 45° spiral angle tapered flow guide groove in step S5 to dynamically guide the axial and radial flow of the material. At the same time, the filling amount is controlled in real time through a hydraulically adjustable limit mechanism to completely solve the problems of folding and wall thickness fluctuation. In step S4, three groups of gradient clamping force jaws distributed at 120° and V-shaped pre-indentations are configured to adaptively compensate for the asymmetric deformation generated by the previous cold heading, reduce the coaxiality error between the special-shaped profile and the flange, and improve the clamping accuracy. This solves the problems in the prior art of multi-station cold heading process, such as cracks caused by stress concentration in the fixed fillet die, folding defects caused by out-of-control material flow in the straight flow guide groove, and out-of-tolerance coaxiality caused by the inability of the uniform clamping force to compensate for the cumulative deformation.

[0022] Considering that if a right-angle connection is adopted between adjacent fillets, shear springback will occur when the material flows through the mutation interface, resulting in microcracks in the transition zone. To this end, in one embodiment, the diameter increment ratio of the three-stage gradually changing fillets in step S3 is 1:1.5:2, and a circular arc is used for smooth transition between adjacent fillets. The radius of the transition arc is 0.3 times the diameter of the corresponding fillet. Specifying the diameter increment ratio of 1:1.5:2 and forcing the radius of the arc transition to be equal to 0.3 times the fillet diameter makes the strain rate change continuously, avoids local stress mutation, reduces the stress springback amount in the transition zone, and completely eliminates the secondary crack source.

[0023] Furthermore, if the stiffness gradient of the disc spring and the pre-compression amount are not coordinated, such as equal pre-compression amounts, the high-stiffness area will lock the material flow prematurely. To avoid this problem, in one embodiment, the stiffness coefficient gradient of the disc spring buffer unit in step S3 is K1:K2:K3 = 3:2:1, and the pre-compression amount of each stage of the spring is inversely proportional to its stiffness coefficient. The stiffness coefficient ratio of 3:2:1 matches the material work hardening rate, and the inverse design of the pre-compression amount ensures that the stress release peak of each stage corresponds to the material yield inflection point, ultimately improving the integrity of the grain flow line at the flange root.

[0024] Since after the flange is formed in S3, the blank has anisotropic deformation, such as the flange side expanding more than the tube side, causing the workpiece axis to shift under the traditional uniform clamping force. To avoid this problem, in one embodiment, in step S4, the included angle of the V-shaped pre-indentation is 90°–120°, and the clamping forces of the three groups of cemented carbide jaws are distributed in a gradient ratio of 1.2:1.0:0.8. The clamping force gradient of 1.2:1.0:0.8 distributes the high-force area for suppressing the expanding side according to the deformation amount. The included angle of the V-shaped pre-indentation of 90°–120° increases the anti-slip friction coefficient. This design improves the suppression rate of the cumulative deformation error and reduces the coaxiality fluctuation.

[0025] It is worth mentioning that excessive stress release in S3 will cause material hardening, which will instead reduce the flow uniformity of S5. The traditional process cannot quantify this coupling effect. Therefore, in one embodiment, the following functional relationship needs to be satisfied between steps S3 and S5, where η is the stress release efficiency, δ is the material flow uniformity, is the positioning accuracy, k1 is the common influence factor of the stress release efficiency and the material flow uniformity, representing the contribution intensity of the stress release to the positioning accuracy, k2 is the compensation coefficient for the material flow non-uniformity, representing the adaptive correction ability of the positioning system when the flow gets out of control, K3 is the limit safety redundancy coefficient, used to absorb the influence of residual stress and uncontrollable disturbance conditions during the cold heading forming process. The stress release efficiency η is related to the fillet gradient and buffer stiffness of S3, the material flow uniformity δ is related to the taper rate of the diversion groove and the limit pressure of S5, and the positioning accuracy is related to the clamping force gradient and pre-indentation angle of S4. When η increases, that is, the stress release is sufficient, the risk of cracks at the flange root can be reduced, but the material flow uniformity δ needs to be balanced to avoid insufficient radial expansion caused by excessive release. When δ is optimized, such as uniform flow, it needs to be achieved through the spiral diversion groove design of S5. The combined action of its taper rate and the hydraulic limit pressure ensures stable material filling. Controlling the positioning accuracy depends on the gradient clamping force and pre-indentation angle of S4. By compensating for the previous deformation through differential clamping, the final coaxiality error can be reduced. This function shows that through the coordinated design of S3 - S5, a dynamic balance can be established among stress release, flow control, and positioning compensation, thereby systematically improving the forming quality of special-shaped pipe fittings. The function establishes a dynamic balance among η, δ, . k1 regulates the gain intensity of the stress release on the accuracy, and k2 is the compensation coefficient when δ gets out of control, which improves the stability of the process chain and greatly reduces the scrap rate.

[0026] Continuing from the above embodiment, it should be noted that when the stress release efficiency η increases, the hierarchical buffering of S3 effectively reduces the crack risk, and at the same time, the material flow uniformity δ is stable. The diversion groove of S5 optimizes the flow path, and k1 amplifies its positive effect on the positioning accuracy The gain. The larger the value of k1, the more significant the contribution of the synergistic effect of stress release and flow control to the final coaxiality accuracy. When the material flow uniformity δ decreases, such as when the flow guide groove in S5 fails to completely suppress the folding defect, the term ln(1 / δ) increases. At this time, k2 determines the compensation intensity of the S4 positioning system for the deformation error. The larger the value of k2, the stronger the fault tolerance ability of the gradient clamping force system, that is, the cooperation of the V-shaped pre-indentation and the clamping force gradient for flow defects. S3 stress release, through the three-stage gradual fillet and the disc spring stiffness gradient, reduces the crack at the flange root and reduces the deformation base number in the subsequent workstations. S5 flow control, the spiral flow guide groove and the hydraulic limit ensure stable material filling and avoid the distortion of the special-shaped contour. S4 positioning compensation, when δ is insufficient, the gradient clamping force system is dominated by the k2 term to dynamically correct the error, so that meet the standard.

[0027] In addition, when the conventional calendering pressure is less than or equal to the forming pressure, the grains at the corners of the special-shaped pipe are not fully reorganized, and the residual grain boundary micropores cause stress corrosion cracking. For this reason, in one embodiment, in step S6, the calendering pressure of the finishing die is 1.2 - 1.5 times the forming pressure, and the grain boundaries are forced to close below the recrystallization temperature of the material, so that the fatigue strength at the corners is improved, where the calendering pressure = 1.2 - 1.5 times the forming pressure, about 800 → 1200 MPa.

[0028] Furthermore, during single-stage annealing, rapid cooling from 650°C to room temperature causes carbides to precipitate in a chain along the grain boundaries, embrittling the pipe fittings. For this reason, in one embodiment, in step S8, the annealing treatment uses two-stage temperature control. The first stage is heated to 650°C at a rate of 10°C / min and held for 30 min, and the second stage is cooled to 300°C at a rate of 5°C / min and then air-cooled. Two-stage temperature control, holding at 650°C completes the spheroidization of pearlite, and slow cooling at 5°C / min to 300°C allows carbides to precipitate dispersedly, resulting in an increase in the impact energy.

[0029] During actual use, the frictional heat during continuous clamping of the jaws causes the temperature rise in the pre-indentation area to be >100°C, and the thermal expansion of the cemented carbide will cause the clamping force to drift. To avoid this problem, in one embodiment, a circulating cooling channel is integrated inside the cemented carbide jaws, the inlet temperature of the cooling medium is controlled at 10 ± 2°C, and the cross-section of the channel is a variable-diameter spiral structure. The minimum diameter section of the circulating cooling channel is located 1.5 mm below the root of the V-shaped pre-indentation. The variable-diameter spiral cooling channel, with the minimum diameter located 1.5 mm from the root of the pre-indentation, specifically strengthens the heat dissipation in the high-temperature area, and the 10°C refrigerant at the inlet realizes thermal deformation compensation, which can make the clamping force fluctuation very small after continuous production.

[0030] In order to reduce the wear at the root of the three-stage rounded corner and improve the service life, in one embodiment, the strengthening layer of the combined die in step S3 is prepared by laser cladding with a WC-Co-based composite material. The thickness of the cladding layer is 8%-10% of the rounded corner diameter, and the cladding direction forms a 15° angle with the material flow direction. The 15° inclination angle during laser cladding makes the WC hard phase orientedly arranged, parallel to the material flow. The thickness of the cladding layer is equal to 8%–10% of the rounded corner diameter to cover the maximum shear depth, thereby improving the service life of the rounded corner size.

[0031] However, after changing the straight flow guide groove to a spiral structure, the material flow periodically impacts the limit mechanism, causing pressure fluctuations and resulting in unstable profile filling. In one embodiment, the hydraulic adjustable limit mechanism in step S5 includes a main piston and a sub-piston arranged inside the main piston. The diameter D1 of the main piston and the diameter D2 of the sub-piston satisfy D1 / D2 = √2. The gap between the two pistons is filled with dimethyl silicone oil. The diameter ratio of the main and sub-pistons D1 / D2 = √2. From this, it can be known that the area ratio is 2:1. The Pascal's principle is used to achieve pressure self-equilibrium, and the dimethyl silicone oil filled in the gap damps the hydraulic oscillation and limits the pressure fluctuation.

[0032] The working principle and working process of the present invention: The three-stage gradually changing rounded corners of the combined die are adopted, and the rounded corner diameters gradually increase and are matched with the gradient stiffness disc springs. During flange forming, the impact force is absorbed in stages - the high-stiffness springs suppress the initial upsetting springback, and the low-stiffness springs absorb the final forging stress, eliminating the crack risk from the source. V-shaped pre-indentations are machined on the inner sides of the three groups of carbide claws distributed at 120°, with an included angle of 90°–120°, and a gradient clamping force is applied, with a ratio of 1.2:1.0:0.8. It automatically enhances the clamping of the asymmetric expansion area after flange forming, compensates for the accumulated deformation error, and ensures coaxiality. The 45° spiral flow guide groove of the special-shaped punch decomposes the axial material flow into radial filling, and cooperates with the hydraulic limit mechanism, with a main and sub-piston diameter ratio of √2, to real-time control the filling amount, cure the folding defect, and control the wall thickness fluctuation.

[0033] The special alloy wire is straightened and cut into cylindrical blanks of a fixed length, and the end faces are chamfered to form a 60° conical guiding part to guide the material flow for subsequent upsetting. The blanks are axially upset in the second station by the three-stage gradually changing rounded corner die, and the gradient disc springs release stress in stages to form an intermediate part with a flange. The three groups of claws lock the intermediate part with a gradient clamping force, and the V-shaped pre-indentation increases the friction force and compensates for the deformation in the previous process. In the third station, the spiral flow guide groove punch is used for radial extrusion, and the hydraulic limit dynamically controls the filling amount of the special-shaped profile. In the fourth station, it is high-pressure polished with a forming pressure of 1.4 times to eliminate the surface stress, and the grain size is refined to 5μm. In the fifth station, the flash is removed by precision blanking, and the two-stage annealing optimizes the material toughness. Finally, it is detected by three-dimensional scanning and packaged with vapor phase rust prevention.

[0034] As described above, it is only the preferred embodiment of the present invention and does not impose any form of limitation on the present invention. Although the present invention has been disclosed above in the preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art can make some changes or modifications to equivalent embodiments of equivalent changes within the scope of the technical solution of the present invention by using the technical content disclosed above. However, as long as it does not depart from the content of the technical solution of the present invention, any brief modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A multi-station cold heading forming process for special-shaped pipe fittings of an automotive shock absorber, characterized in that, It includes the following steps: S1: Blanking process. Select special metal wire for automotive shock absorbers. After straightening the metal wire by a straightening machine, perform fixed-length cutting through a cutting die to obtain cylindrical blanks. S2: Preforming. Select a modular universal machine tool. Transfer the blank to the first station of the universal machine tool for chamfering the end face to form a preform with a tapered guiding part. S3: First cold heading forming. At the second station of the universal machine tool, use a combined concave die with a gradually changing fillet structure to perform axial upsetting on the preform. The root of the concave die is provided with a three-stage transition structure with gradually increasing fillet diameters. Each stage of the fillet is equipped with a disc spring buffer unit with a gradually decreasing stiffness coefficient. A middle part with a flange is formed through hierarchical stress release. S4: Positioning compensation. Set three groups of carbide jaws distributed at 120° on the universal machine tool. Process a V-shaped pre-indentation on the inner side of the jaws and configure a gradient clamping force control system to complete the self-centering clamping of the middle part. S5: Second cold heading forming. At the third station of the universal machine tool, use a special-shaped punch with a spiral flow guide groove to perform radial extrusion. Process a tapered flow guide groove with a 45° spiral angle on the surface of the concave die, and set a hydraulically adjustable limit mechanism at the end of the groove to form a special-shaped profile of the pipe fitting. S6: Finishing forming. At the fourth station of the universal machine tool, use a finishing die to perform surface burnishing on the special-shaped profile to eliminate the forming stress. S7: Trimming process. At the fifth station of the universal machine tool, remove the flash at the flange through precision blanking. S8: Heat treatment. Anneal the formed part while controlling the temperature and holding time. S9: Inspection and packaging. Perform dimensional inspection and surface flaw detection, then implement rust prevention treatment and packaging.

2. The multi-station cold heading forming process for special-shaped pipe fittings of an automotive shock absorber according to claim 1, characterized in that, In step S3, the diameter increment ratio of the three-stage gradually changing fillets is 1:1.5:2, and a circular arc is used for smooth transition between adjacent fillets. The radius of the transition circular arc is 0.3 times the diameter of the corresponding fillet.

3. A multi-station cold heading forming process for special-shaped pipe fittings of an automotive shock absorber according to claim 1, characterized in that, In step S3, the stiffness coefficient gradient of the disc spring buffer unit is K1:K2:K3 = 3:2:1, and the pre-compression amount of each stage of the spring is inversely proportional to its stiffness coefficient. k1 is a common influencing factor of stress release efficiency and material flow uniformity, representing the contribution intensity of stress release to positioning accuracy. k2 is a compensation coefficient for material flow non-uniformity, representing the adaptive correction ability of the positioning system when the flow gets out of control. K3 is the limit safety redundancy coefficient, used to absorb the influence of residual stress and uncontrollable disturbance conditions during the cold heading forming process.

4. A multi-station cold heading forming process for special-shaped pipe fittings of an automotive shock absorber according to claim 1, characterized in that, In step S4, the included angle of the V-shaped pre-indentation is 90°–120°, and the clamping forces of the three groups of carbide jaws are distributed in a gradient ratio of 1.2:1.0:0.

8.

5. A multi-station cold heading forming process for special-shaped pipe fittings of an automotive shock absorber according to claim 3, characterized in that, The following functional relationship needs to be satisfied between steps S3 - S5 where η is the stress release efficiency, δ is the material flow uniformity, and is the positioning accuracy.

6. A multi-station cold heading forming process for special-shaped pipe fittings of an automotive shock absorber, characterized in that, In step S6, the burnishing pressure of the finishing die is 1.2 - 1.5 times the forming pressure.

7. A multi-station cold heading forming process for special-shaped pipe fittings of an automotive shock absorber according to claim 1, characterized in that, In step S8, the annealing treatment uses two-stage temperature control. In the first stage, heat up to 650°C at a rate of 10°C / min and hold for 30 min. In the second stage, cool down to 300°C at a rate of 5°C / min and then air cool.

8. A multi-station cold heading forming process for special-shaped pipe fittings of an automotive shock absorber according to claim 4, characterized in that, The internal of the cemented carbide jaw is integrated with a circulating cooling channel, the inlet temperature of the cooling medium is controlled at 10±2°C, and the cross-section of the channel is a variable-diameter spiral structure. The minimum diameter section of the circulating cooling channel is located 1.5 mm below the root of the V-shaped pre-indentation.

9. A multi-station cold heading forming process for special-shaped pipe fittings of an automotive shock absorber, characterized in that, In the step S3, the strengthening layer of the combined die is prepared by laser cladding with a WC-Co-based composite material. The thickness of the cladding layer is 8%-10% of the fillet diameter, and the cladding direction forms an angle of 15° with the material flow direction.

10. A multi-station cold heading forming process for special-shaped pipe fittings of an automotive shock absorber, characterized in that, In the step S5, the hydraulically adjustable limit mechanism includes a main piston and a sub-piston arranged inside the main piston, and the diameter D1 of the main piston and the diameter D2 of the sub-piston satisfy D1 / D2 = √2. The gap between the two pistons is filled with dimethyl silicone oil.

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