A multi-station cold heading process for special-shaped pipe fittings for automobile shock absorbers

Abstract

The present invention relates to a multi-station cold heading forming process for automobile shock absorber special-shaped pipe fittings, which belongs to the technical field of cold heading of automobile special-shaped pipe fittings. By adopting a combined die with a three-stage gradually rounded corner structure, the rounded corner diameter increases step by step and cooperates with a gradient stiffness disc spring buffer unit to realize staged stress release, eliminate the cross-sectional sudden stress peak at the root of the flange, and reduce the risk of cracks. A special-shaped punch with a 45° spiral angle tapered guide 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 a hydraulically adjustable limit mechanism to fundamentally solve the problems of folding and wall thickness fluctuation. Three groups of 120° distributed gradient clamping force claws and V-shaped pre-indentations are configured to adaptively compensate for the asymmetric deformation caused by the previous cold heading, so that the coaxiality error between the special-shaped profile and the flange is reduced, and the problems of cracks caused by stress concentration in the fixed rounded corner die and folding defects caused by uncontrolled material flow in the linear guide groove in the prior art are solved.

Description

Technical Field

[0001] The invention belongs to the technical field of cold forging of automobile special-shaped pipe fittings, and in particular relates to a multi-station cold forging forming process for automobile shock absorber special-shaped pipe fittings. Background Art

[0002] At present, the cold heading forming process of automobile shock absorber special-shaped pipe fittings mostly adopts multi-station continuous combined processing technology. In the conventional combined processing technology, the forming of the flange structure is usually achieved by a single axial upsetting, and then the tube body is radially extruded in the subsequent station to form a special-shaped contour. The existing molds generally adopt a fixed radius design, constrain the material flow through a rigid die, and rely on uniformly distributed clamping claws to clamp and position the blank. In the flange forming stage, the mold buffer system mostly uses a single stiffness elastic element to absorb the forming stress; when the special-shaped contour is formed, the material filling is guided by a linear guide groove. Although this method can achieve basic shape forming, when dealing with complex geometric transitions, it still needs to rely on multiple annealing or subsequent machining to correct dimensional deviations.

[0003] However, the above-mentioned existing process has the following key problems: First, the root of the flange is prone to stress concentration due to the sudden change in cross-section, and the fixed fillet and uniformly distributed buffer design cannot achieve staged stress release, resulting in an increased risk of crack initiation; Second, when the special-shaped contour is formed, the axial flow and radial expansion of the material lack guidance, and the straight guide groove is difficult to balance the flow velocity difference, which can easily cause folding defects or wall thickness fluctuations; Third, during multi-station switching, the rigid positioning of the uniformly distributed clamping force cannot compensate for the accumulated deformation error of the previous process, resulting in excessive coaxiality between the special-shaped contour and the flange structure. The root of these problems is that the existing technology is difficult to simultaneously achieve stress gradient release, dynamic control of flow direction, and adaptive compensation of deformation errors, which seriously limits the integrated forming efficiency and reliability of high-precision special-shaped pipe fittings. In this regard, 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-mentioned problems existing in the prior art, the present invention provides a multi-station cold heading forming process for automobile shock absorber special-shaped pipe fittings, which solves the problems in the prior art of multi-station cold heading process such as cracks caused by stress concentration in fixed rounded corner molds, folding defects caused by uncontrolled flow of material in linear guide grooves, and coaxiality deviation caused by the inability of uniform clamping force to compensate for accumulated deformation.

[0005] The purpose of the present invention can be achieved through the following technical solutions:

[0006] A multi-station cold heading forming process for automobile shock absorber special-shaped pipe fittings comprises the following steps:

[0007] S1: Blanking process: select metal wire specially used for automobile shock absorbers, straighten the metal wire with a straightening machine, and then cut it to a fixed length with a cutting die to obtain cylindrical blanks;

[0008] S2: Preforming: Select a modular universal machine tool and transfer the blank to the first station of the universal machine tool for end face chamfering to form a preform with a tapered guide;

[0009] S3: The first cold upsetting process: In the second station of the universal machine tool, a combined die with a gradually rounded corner structure is used to axially upset the preform. A three-stage transition structure with gradually increasing corner diameter is set at the root of the die. Each rounded corner is equipped with a disc spring buffer unit with a gradually decreasing stiffness coefficient. The intermediate part with a flange is formed through graded stress release.

[0010] S4: Positioning compensation: three sets of carbide jaws distributed at 120 degrees are set on the universal machine tool. V-shaped pre-indentations are processed on the inner side of the jaws and a gradient clamping force control system is configured to achieve self-centering clamping of the middleware;

[0011] S5: The second cold heading process is performed at the third station of the universal machine tool using a special-shaped punch with a spiral guide groove for radial extrusion. The surface of the die is machined with a tapered guide groove with a 45° helical angle. A hydraulically adjustable limit mechanism is set at the end of the groove to form a special-shaped profile of the pipe fitting.

[0012] S6: Finishing molding: In the fourth station of the universal machine tool, a finishing mold is used to perform surface calendering on the special-shaped contour to eliminate molding stress;

[0013] S7: trimming process, in the fifth station of the universal machine tool, the flash of the flange is removed by precision blanking;

[0014] S8: Heat treatment, annealing the molded parts while controlling the temperature and keeping them warm;

[0015] S9: Inspection and packaging, size inspection, surface flaw detection, anti-rust treatment and packaging.

[0016] As a further solution of the present invention, the diameter increment ratio of the three-level gradual fillet in step S3 is 1:1.5:2, and arc smooth transition is adopted between adjacent fillets, and the radius of the transition arc is 0.3 times the diameter of the corresponding fillet.

[0017] As a further solution of the present invention, 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 level of spring is inversely proportional to its stiffness coefficient. K1 is a common influencing factor of stress release efficiency and material flow uniformity, which characterizes the contribution intensity of stress release to positioning accuracy. K2 is a compensation coefficient for material flow unevenness, which characterizes the adaptive correction capability of the positioning system when the flow is out of control. K3 is an extreme safety redundancy coefficient, which is used to absorb the influence of residual stress and uncontrollable disturbance conditions during cold heading.

[0018] As a further solution of the present invention, in step S4, the 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.

[0019] As a further solution of the present invention, the steps S3-S5 need to satisfy Functional relationship, where η is the stress release efficiency, δ is the material flow uniformity, For positioning accuracy.

[0020] As a further solution of the present invention, in step S6, the calendering pressure of the finishing mold is 1.2-1.5 times the molding pressure.

[0021] As a further solution of the present invention, in step S8, the annealing treatment adopts two-stage temperature control. In the first stage, the temperature is raised to 650°C at 10°C / min and kept at this temperature for 30 minutes. In the second stage, the temperature is lowered to 300°C at 5°C / min and then air-cooled.

[0022] As a further solution of the present invention, a circulating cooling channel is integrated inside the carbide jaw, the cooling medium inlet temperature is controlled at 10±2°C, and the channel cross-section 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.

[0023] As a further solution of the present invention, the strengthening layer of the combined die in step S3 is prepared by laser cladding of 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.

[0024] As a further solution of the present invention, the hydraulically adjustable limiting mechanism in step S5 includes a main piston and a secondary piston arranged in the main piston, and the main piston diameter D1 and the secondary piston diameter D2 satisfy D1 / D2=√2, and the gap between the two pistons is filled with dimethyl silicone oil.

[0025] The beneficial effects of the present invention are:

[0026] In step S3, a combined die with a three-stage gradient fillet structure is used. 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 sudden stress peak at the root of the flange, and reduce the risk of cracks. In step S5, a special-shaped punch with a 45° spiral angle and tapered guide 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 a hydraulically adjustable limit mechanism to fundamentally solve the problems of folding and wall thickness fluctuation. In step S4, three groups of 120° distributed gradient clamping force claws and V-shaped pre-indentations are configured to adaptively compensate for the asymmetric deformation caused by the previous cold heading, reduce the coaxiality error between the special-shaped contour and the flange, and improve the clamping accuracy. This solves the problems of cracks caused by stress concentration in the fixed fillet die in the existing multi-station cold heading process, folding defects caused by uncontrolled material flow in the linear guide groove, and coaxiality deviation caused by the inability of the uniformly distributed clamping force to compensate for the accumulated deformation. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] To facilitate understanding by those skilled in the art, the present invention is further described below with reference to the accompanying drawings.

[0028] Figure 1 The present invention is a flowchart of the multi-station cold heading process for special-shaped pipe fittings for automobile shock absorbers. DETAILED DESCRIPTION

[0029] In order to further illustrate the technical means and effects adopted by the present invention to achieve the predetermined purpose of the invention, the specific implementation methods, structures, features and effects of the present invention are described in detail below in conjunction with the accompanying drawings and preferred embodiments.

[0030] See also Figure 1 As shown, this embodiment provides a multi-station cold heading forming process for automobile shock absorber special-shaped pipe fittings, comprising the following steps:

[0031] S1: Blanking process: Use metal wire specially made for automobile shock absorbers. After the metal wire is straightened by a straightening machine, it is cut to a fixed length by a cutting die to obtain cylindrical blanks. Straightening reduces the internal stress of the material and avoids subsequent cold heading cracking.

[0032] S2: Preforming: A modular universal machine tool is selected to transfer the blank to the first station of the universal machine tool for end face chamfering to form a preform with a tapered guide. The tapered guide guides the material flow toward the flange root during axial upsetting in S3, reducing the risk of folding.

[0033] S3: The first cold upsetting process uses a combined die with a gradually rounded corner structure to axially upset the preform at the second station of the universal machine tool. A three-stage transition structure with gradually increasing corner diameter is set at the root of the die. Each rounded corner is equipped with a disc spring buffer unit with a gradually decreasing stiffness coefficient. The intermediate piece with a flange is formed through graded stress release. By releasing the sudden change in cross-section stress in stages, stress concentration is avoided. The high-rigidity area suppresses the initial upsetting rebound, while the low-rigidity area absorbs the final forging impact, reducing the crack rate.

[0034] S4: Positioning compensation: Three sets of carbide jaws are arranged at 120° on the universal machine tool. V-shaped pre-indentations are machined inside the jaws and a gradient clamping force control system is configured to achieve self-centering clamping of the intermediate workpiece. The V-shaped pre-indentations increase the contact area and prevent rotational deviation of the intermediate workpiece. The clamping is enhanced in the S3 non-uniform deformation area, such as the thickened flange root, to compensate for accumulated deformation and reduce coaxiality errors.

[0035] S5: The second cold heading process uses a special-shaped punch with a spiral guide groove for radial extrusion at the third station of the universal machine tool. The surface of the die is machined with a tapered guide groove with a 45° helix angle. A hydraulically adjustable limit mechanism is set at the end of the groove to form the special-shaped contour of the pipe. The 45° spiral guide groove here decomposes the axial flow of the material into radial filling, which improves the uniformity of the flow rate and eliminates folding.

[0036] S6: Finishing molding: In the fourth station of the universal machine tool, a finishing mold is used to perform high-pressure calendering on the surface of the special-shaped contour to eliminate molding stress. In addition, high-pressure calendering refines the surface grain to less than 5μm, eliminates residual tensile stress at the corners, ensures the surface roughness Ra ≤ 0.8μm, and reduces the subsequent flaw detection rate.

[0037] S7: Trimming treatment: The burrs on the flange are removed by precision blanking at the fifth station of the universal machine tool. Micro-clearance blanking is used to prevent the flange root from tearing. The residual height of the burrs must be ≤0.05mm to meet the requirements of the shock absorber sealing surface.

[0038] S8: Heat treatment, annealing the molded parts while controlling the temperature and keeping them warm;

[0039] S9: Inspection and packaging, size inspection, surface flaw detection, anti-rust treatment and packaging.

[0040] At present, the cold heading forming process of automobile shock absorber special-shaped pipe fittings mostly adopts multi-station continuous combined processing technology. In the conventional combined processing technology, the forming of the flange structure is usually achieved by a single axial upsetting, and then the tube body is radially extruded in the subsequent station to form a special-shaped contour. The existing mold generally adopts a fixed fillet design, and the material flow is constrained by a rigid die, and the blank is clamped and positioned by uniformly distributed claws. In the flange forming stage, the mold buffer system mostly adopts a single stiffness elastic element to absorb the forming stress; when the special-shaped contour is formed, the material filling is guided by a linear guide groove. Although this method can achieve basic shape forming, it still needs to rely on multiple annealing or subsequent machining to correct dimensional deviations when dealing with complex geometric transitions. However, the existing There are some problems with the process. For example, the root of the flange is prone to stress concentration due to the sudden change in cross-section, and the fixed fillet and uniformly distributed buffer design cannot achieve staged stress release, resulting in an increased risk of crack initiation. In addition, when forming special-shaped contours, the axial flow and radial expansion of the material lack guidance, and the straight guide groove is difficult to balance the flow rate difference, which can easily cause folding defects or wall thickness fluctuations. Moreover, in multi-station switching, the rigid positioning of the uniformly distributed clamping force cannot compensate for the accumulated deformation error of the previous process, resulting in excessive coaxiality between the special-shaped contour and the flange structure. The root of these problems is that the existing technology is difficult to simultaneously achieve stress gradient release, dynamic control of flow direction, and adaptive compensation of deformation errors, which seriously limits the integrated forming efficiency and reliability of high-precision special-shaped pipe fittings.

[0041] In order to solve the above problems, in this embodiment, a combined die with a three-stage gradually rounded corner structure is adopted, the rounded corner diameter is gradually increased and matched with a gradient stiffness disc spring buffer unit to achieve staged stress release, eliminate the cross-sectional sudden stress peak at the root of the flange, and reduce the risk of cracks. In step S5, a special-shaped punch with a 45° spiral angle tapered guide 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 a hydraulically adjustable limit mechanism to fundamentally solve the problems of folding and wall thickness fluctuation. In step S4, three groups of 120° distributed gradient clamping force claws and V-shaped pre-indentations are configured to adaptively compensate for the asymmetric deformation caused by the previous cold heading, thereby reducing the coaxiality error between the special-shaped contour and the flange and improving the clamping accuracy. The problems of cracks caused by stress concentration of the fixed rounded corner die in the prior art multi-station cold heading process, folding defects caused by uncontrolled material flow in the linear guide groove, and coaxiality deviation caused by the inability of the uniform clamping force to compensate for the accumulated deformation are solved.

[0042] Considering that if right angles are used to connect adjacent fillets, shear rebound will occur when the material flows through the sudden interface, resulting in microcracks in the transition zone, in this regard, in one embodiment, the diameter increment ratio of the three-level gradual fillet in step S3 is 1:1.5:2, and a smooth arc transition is adopted between adjacent fillets, and the transition arc radius is 0.3 times the corresponding fillet diameter. The diameter increment ratio is specified to be 1:1.5:2 and the arc transition radius is forced to be equal to 0.3 times the fillet diameter, so that the strain rate changes continuously, avoids local stress mutations, and causes the stress rebound in the transition zone to decrease, thereby completely eliminating the source of secondary cracks.

[0043] Furthermore, if the disc spring stiffness gradient and pre-compression amount are not coordinated, such as if the pre-compression amounts are equal, the high stiffness area will prematurely lock the material flow. In order 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 level of spring is inversely proportional to its stiffness coefficient. The stiffness coefficient ratio of 3:2:1 matches the material work hardening rate. The inverse design of the pre-compression amount ensures that the stress release peak of each level corresponds to the material yield inflection point, thereby ultimately improving the integrity of the grain streamlines at the root of the flange.

[0044] After the S3 flange is formed, the blank is anisotropically deformed. For example, the expansion of the flange side is greater than that of the tube body side, which causes the axis of the workpiece to deviate under the traditional uniform clamping force. In order to avoid this problem, in one embodiment, in step S4, the angle of the V-shaped pre-indentation is 90°-120°, and the clamping force of the three groups of carbide jaws is distributed in a gradient ratio of 1.2:1.0:0.8. The clamping force gradient of 1.2:1.0:0.8 is distributed according to the deformation amount to suppress the expansion side in the high-force area. The angle of the V-shaped pre-indentation is 90°-120° to increase the anti-slip friction coefficient. This design improves the deformation cumulative error suppression rate and reduces the coaxiality fluctuation.

[0045] It is worth mentioning that excessive stress release in S3 will lead to material hardening, which in turn reduces the flow uniformity in S5. Traditional processes cannot quantify this coupling effect. Therefore, in one embodiment, steps S3-S5 need to meet the following requirements: Functional relationship, where η is the stress release efficiency, δ is the material flow uniformity, is the positioning accuracy, K1 is the joint influencing factor of stress release efficiency and material flow uniformity, which characterizes the contribution strength of stress release to positioning accuracy, K2 is the compensation coefficient of material flow non-uniformity, which characterizes the adaptive correction ability of the positioning system when the flow is out of control, K3 is the limit safety redundancy coefficient, which is used to absorb the influence of residual stress and uncontrollable disturbance conditions during cold heading, stress release efficiency η is related to the fillet gradient and buffer stiffness of S3, material flow uniformity δ is related to the guide groove taper rate and limit pressure of S5, positioning accuracy Related to the clamping force gradient and pre-indentation angle of S4, η is improved, that is, stress is fully released, which can reduce the risk of flange root cracks. However, it is necessary to balance the uniformity of material flow δ to avoid insufficient radial expansion due to excessive release. δ optimization, such as uniform flow, needs to be achieved through the spiral guide groove design of S5. Its tapering rate and hydraulic limit pressure work together to ensure stable material filling. Controlling positioning accuracy relies on the gradient clamping force and pre-indentation angle of S4. Differentiated clamping compensates for pre-deformation, reducing the final coaxiality error. This function shows that through the coordinated design of S3-S5, a dynamic balance can be established between stress release, flow control, and positioning compensation, thereby systematically improving the forming quality of special-shaped pipe fittings.

[0046] Functions establish η, δ, The dynamic balance of K1 regulates the gain intensity of stress release on precision, and K2 is the compensation coefficient when δ is out of control, which improves the stability of the process chain and greatly reduces the scrap rate.

[0047] Following the above embodiment, it should be noted that when the stress release efficiency η is improved, the graded buffer of S3 effectively reduces the crack risk, while the material flow uniformity δ is stable, the guide groove of S5 optimizes the flow path, and K1 positively amplifies its positioning accuracy. The gain of K1, the larger the K1 value, 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 the guide groove in S5 fails to completely suppress the folding defect, the ln(1 / δ) term increases. At this time, K2 determines the compensation strength of the S4 positioning system for deformation error. The larger the K2 value, the stronger the fault tolerance of the gradient clamping force system, that is, the V-shaped pre-indentation and the clamping force gradient for flow defects. S3 stress release, through the three-level gradual fillet and disc spring stiffness gradient, reduces the cracks at the root of the flange and reduces the deformation base of the subsequent workstation. S5 flow control, spiral guide groove and hydraulic limit ensure stable material filling and avoid special-shaped contour distortion. S4 positioning compensation, when δ is insufficient, the gradient clamping force system is dominated by the K2 term to dynamically correct the error, so Meet the standards.

[0048] 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 tube are not fully reorganized, and the residual grain boundary micropores lead to stress corrosion cracking. In this regard, in one embodiment, in step S6, the calendering pressure of the finishing mold 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→1200MPa.

[0049] Furthermore, in single-stage annealing, rapid cooling from 650°C to room temperature causes carbides to precipitate in chains along grain boundaries, embrittles the pipe fittings. In this regard, in one embodiment, in step S8, the annealing treatment adopts two-stage temperature control. In the first stage, the temperature is raised to 650°C at 10°C / min and kept at this temperature for 30 minutes. In the second stage, the temperature is lowered to 300°C at 5°C / min and then air-cooled. The two-stage temperature control is carried out, and the pearlite is spheroidized at 650°C. The carbides are dispersed and precipitated by slowly cooling to 300°C at 5°C / min, thereby increasing the impact energy.

[0050] In actual use, when the jaws are continuously clamped, the frictional heat causes the temperature of the pre-indentation area to rise by more than 100°C, and the thermal expansion of the cemented carbide will cause the clamping force to drift. In order to avoid this problem, in one embodiment, a circulating cooling channel is integrated inside the cemented carbide jaws, and the inlet temperature of the cooling medium is controlled at 10±2°C. The channel cross-section is a variable diameter spiral structure, and the minimum diameter section of the circulating cooling channel is located 1.5mm below the root of the V-shaped pre-indentation. The minimum diameter of the variable diameter spiral cooling channel is located 1.5mm at the root of the pre-indentation, which specifically strengthens the heat dissipation of the high-temperature area. The 10°C refrigerant at the inlet realizes thermal deformation compensation, which can make the clamping force fluctuation after continuous production very small.

[0051] In order to reduce the wear of the root of the tertiary fillet and improve the service life, in one embodiment, the strengthening layer of the combined die in step S3 is prepared by laser cladding of a WC-Co based composite material. The thickness of the cladding layer is 8%-10% of the fillet diameter, and the cladding direction is at an angle of 15° to the material flow direction. During laser cladding, the 15° inclination angle makes the WC hard phase oriented and parallel to the material flow. The thickness of the cladding layer is equal to 8%-10% of the fillet diameter to cover the maximum shear depth. The fillet size thus improves the service life.

[0052] However, after the linear guide groove is changed to a spiral structure, the material flow periodically impacts the limiting mechanism, causing pressure fluctuations, resulting in unstable contour filling. In this regard, in one embodiment, the hydraulically adjustable limiting mechanism in step S5 includes a main piston and a secondary piston arranged in the main piston, and the main piston diameter D1 and the secondary piston diameter D2 satisfy D1 / D2=√2. The gap between the two pistons is filled with dimethyl silicone oil, and the main and secondary piston diameter ratio D1 / D2=√2. Based on this, it can be known that the area ratio is 2:1. The Pascal principle is used to achieve pressure self-balancing, and the dimethyl silicone oil fills the gap to damp hydraulic oscillations and limit pressure fluctuations.

[0053] The working principle and workflow of the present invention:

[0054] A three-stage gradient fillet of the combined die is adopted, with the fillet diameter increasing step by step and equipped with gradient stiffness disc springs to absorb the impact force in stages during flange forming. The high-rigidity spring suppresses the initial upsetting rebound, and the low-rigidity spring absorbs the final forging stress, eliminating the risk of cracks at the source. Three groups of 120° distributed carbide jaws are machined with V-shaped pre-indentations on the inside, with an angle of 90°–120°, and a gradient clamping force is applied in a ratio of 1.2:1.0:0.8. This automatically enhances the clamping of the asymmetric expansion area after flange forming, compensates for the accumulated deformation error, and ensures coaxiality. The 45° spiral guide groove of the special-shaped punch decomposes the axial flow of the material into radial filling. In conjunction with the hydraulic limit mechanism, the main and auxiliary piston diameter ratio is √2, which can adjust the filling amount in real time, eliminate folding defects, and control wall thickness fluctuations.

[0055] Special alloy wire is straightened and cut to a fixed length into cylindrical billets. The end face is chamfered to form a 60° conical guide to guide the material flow for subsequent upsetting. The billet is axially upset by a three-stage gradually rounded die at the second station. The gradient disc spring releases stress in stages to form an intermediate piece with a flange. Three sets of jaws lock the intermediate piece with a gradient clamping force. The V-shaped pre-indentation increases friction and compensates for the previous deformation. The third station uses a spiral guide groove punch for radial extrusion. The hydraulic limiter dynamically controls the filling amount of the special-shaped contour. The fourth station uses 1.4 times the forming pressure for high-pressure calendering to eliminate surface stress and refine the grain to 5μm. The fifth station uses precision blanking to remove flash. Two-stage annealing optimizes the toughness of the material. Finally, it passes 3D scanning inspection and vapor phase rust prevention packaging.

[0056] The above description is merely a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been disclosed as above in terms of a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art can, without departing from the scope of the technical solution of the present invention, make some changes or modifications to equivalent embodiments using the technical contents disclosed above. However, any brief modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A multi-station cold heading forming process for automobile shock absorber special-shaped pipe fittings, characterized in that: The following steps are involved: S1: Blanking process: metal wire for automobile shock absorbers is selected, straightened by a straightening machine, and then cut to length by a cutting die to obtain cylindrical blanks; S2: Preforming: Select a modular universal machine tool and transfer the blank to the first station of the universal machine tool for end face chamfering to form a preform with a tapered guide; S3: The first cold upsetting process: In the second station of the universal machine tool, a combined die with a gradually rounded corner structure is used to axially upset the preform. A three-stage transition structure with gradually increasing corner diameter is set at the root of the die. Each rounded corner is equipped with a disc spring buffer unit with a gradually decreasing stiffness coefficient. The intermediate part with a flange is formed through graded stress release. S4: Positioning compensation: three sets of carbide jaws distributed at 120 degrees are set on the universal machine tool. V-shaped pre-indentations are processed on the inner side of the jaws and a gradient clamping force control system is configured to achieve self-centering clamping of the middleware; S5: The second cold heading process is performed at the third station of the universal machine tool using a special-shaped punch with a spiral guide groove for radial extrusion. The surface of the die is machined with a tapered guide groove with a 45° helical angle. A hydraulically adjustable limit mechanism is set at the end of the groove to form a special-shaped profile of the pipe fitting. S6: Finishing molding: In the fourth station of the universal machine tool, a finishing mold is used to perform surface calendering on the special-shaped contour to eliminate molding stress; S7: trimming process, in the fifth station of the universal machine tool, the flash of the flange is removed by precision blanking; S8: Heat treatment, annealing the molded parts while controlling the temperature and keeping them warm; S9: Inspection and packaging, dimensional inspection, surface flaw detection, rust prevention treatment and packaging; In step S3, the diameter increment ratio of the three-level gradual fillet is 1:1.5:2, and arcs are used for smooth transition between adjacent fillets, with the radius of the transition arc being 0.3 times the diameter of the corresponding fillet; 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 level of spring is inversely proportional to its stiffness coefficient. K1 is a common influencing factor of stress release efficiency and material flow uniformity, which characterizes the contribution strength of stress release to positioning accuracy. K2 is a compensation coefficient for material flow non-uniformity, which characterizes the adaptive correction capability of the positioning system when the flow is out of control. K3 is an extreme safety redundancy coefficient, which is used to absorb the influence of residual stress and uncontrollable disturbance conditions during the cold heading process. In step S4, the 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; The steps S3-S5 need to meet Functional relationship, where η is the stress release efficiency, δ is the material flow uniformity, For positioning accuracy.

2. The multi-station cold heading forming process for automobile shock absorber special-shaped pipe fittings according to claim 1 is characterized in that: In step S6, the calendering pressure of the finishing mold is 1.2-1.5 times the molding pressure.

3. The multi-station cold heading forming process for automobile shock absorber special-shaped pipe fittings according to claim 1 is characterized in that: In step S8, the annealing treatment adopts a two-stage temperature control, in the first stage, the temperature is raised to 650°C at 10°C / min and kept at this temperature for 30 minutes, and in the second stage, the temperature is lowered to 300°C at 5°C / min and then air-cooled.

4. The multi-station cold heading forming process for automobile shock absorber special-shaped pipe fittings according to claim 1 is characterized in that: A circulating cooling channel is integrated inside the carbide jaws, the cooling medium inlet temperature is controlled at 10±2°C, and the channel cross-section is a variable diameter spiral structure. The minimum diameter section of the circulating cooling channel is located 1.5mm below the root of the V-shaped pre-indentation.

5. The multi-station cold heading forming process for automobile shock absorber special-shaped pipe fittings according to claim 1 is characterized in that: In step S3, the strengthening layer of the combined die is prepared by laser cladding of 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.

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