Method for heating and bending an aluminum crosslinked polyethylene copper composite fishtail tube

Abstract

This invention discloses a method for heating and bending an aluminum cross-linked polyethylene copper composite figure-eight tube, belonging to the field of pipe forming and processing technology. The method includes: preparing and inspecting the composite tube blank; establishing a three-dimensional centerline model based on the target trajectory and setting parameters such as curvature in segments; heating the bending area to 80°C to 120°C to soften the cross-linked polyethylene layer; using a six-axis free bending device for spatial linkage forming; monitoring the cross-sectional distortion rate and interlayer slip in real time during the forming process and dynamically adjusting the process parameters; and compensating and correcting the set angle based on the measured springback after unloading. This invention effectively reduces springback, cross-sectional distortion, and interlayer slip defects through coordinated control of heating temperature, bending curvature, spatial posture, and feed speed, improving the forming accuracy and process stability of the layered composite figure-eight tube. It is suitable for bending and forming irregularly shaped cross-section composite pipes in new energy equipment and complex pipeline systems.

Description

Technical Field

[0001] This invention relates to the field of pipe forming and processing technology, specifically to a method for heating and bending aluminum cross-linked polyethylene copper composite figure-eight pipes. Background Technology

[0002] Aluminum / cross-linked polyethylene / copper layered composite materials combine the structural support properties of metallic materials, the insulation properties of polymer materials, and the comprehensive functions of composite structures, showing promising application prospects in complex piping systems, new energy equipment, and special functional components. Compared with single-material pipe fittings, this type of material has synergistic advantages in conductivity, insulation, corrosion resistance, and a certain degree of impact resistance, making it particularly suitable for scenarios with high requirements for spatial layout, functional zoning, and service safety. As a typical spatial component of this type of composite material, the figure-eight tube must not only meet the requirements of spatial orientation and installation path but also ensure synchronous deformation of the dual-cavity contour, smooth transition section connection, and local wall thickness stability, thus placing higher demands on the forming process.

[0003] Due to differences in elastic modulus, thermal response characteristics, and interfacial bonding behavior among the aluminum, cross-linked polyethylene, and copper layers, traditional single-material tube bending methods cannot simultaneously meet the forming accuracy and interlayer integrity requirements of the herringbone tube. The aluminum layer tends to bear the main bending load under stress, the cross-linked polyethylene layer exhibits significant softening and viscoelastic response upon temperature increase, while the copper layer is more sensitive to local strain and interfacial slippage. The coordination among these three materials during bending is poor. If conventional cold bending or uniaxial pressing processes are still used, excessive stretching on the outer arc side, instability and wrinkling on the inner arc side, increased relative displacement between layers, and excessive springback after unloading can easily occur, further causing the final spatial trajectory of the herringbone tube to deviate from the design value.

[0004] In the prior art, patent document CN106862330B discloses a six-axis free bending forming equipment and process analysis method for irregularly shaped cross-section hollow metal components, including an X-axis motion system, a Y-axis motion system, a Z-axis feeding system, a bending die rotation system around the Y-axis, a bending die rotation system around the Z-axis, and a bending die pitching and oscillating system around its own axis. However, this technology mainly targets single metal material components and does not address the interlayer coordinated deformation control problem of layered composite materials during bending, nor does it establish a corresponding control model for the changes in the mechanical properties of composite materials under heating conditions. In addition, existing figure-eight tube bending forming technologies mostly use forming molds in conjunction with clamping mechanisms for forming, which is difficult to meet the high requirements of forming accuracy and interlayer integrity for layered composite material figure-eight tubes.

[0005] Therefore, it is necessary to propose a heating and bending forming method for aluminum cross-linked polyethylene copper composite figure-eight tubes to improve forming stability and dimensional consistency. Summary of the Invention

[0006] The present invention aims to provide a method for controlling the heating and bending forming of figure-eight tubes made of aluminum / cross-linked polyethylene / copper layered composite materials, so as to solve the problems of large springback, obvious cross-sectional distortion, severe interlayer slippage and insufficient forming accuracy in the prior art.

[0007] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: Composite tube blank preparation and pre-forming inspection.

[0008] Aluminum / crosslinked polyethylene / copper layered composite tube blanks were prepared, and the external dimensions, layer thickness distribution, initial ellipticity, end flatness, and interlayer bonding state of the tube blanks were inspected. Tube blanks with warping, local delamination, interface defects, or out-of-tolerance dimensions were repaired or rejected to ensure stability and consistency in the subsequent bending forming process.

[0009] Target trajectory modeling and segmentation parameter setting.

[0010] A three-dimensional centerline trajectory model is established based on the spatial structure of the target figure-eight tube. The entire bending path is divided into several control segments, and corresponding curvature, torsion, bending radius, transition length, attitude angle, and feed parameters are set for each control segment. The target trajectory must satisfy the requirement of continuous and smooth transition to avoid local stress concentration and cross-sectional distortion caused by abrupt changes in curvature. The bending angle of the i-th control segment is expressed as:

[0011] in, Let be the bending angle of the i-th control segment. Let be the curvature of the i-th control segment. Let be the arc length of the i-th control segment.

[0012] Heating state adjustment and material softening control.

[0013] The target bending area is heated either entirely or locally to bring the aluminum, cross-linked polyethylene, and copper layers to a suitable temperature window for bending, ranging from 80°C to 120°C. Heating methods include one or more combinations of hot air heating, radiation heating, induction heating, and resistance heating. By adjusting the heating conditions, the cross-linked polyethylene layer achieves better flexibility and adaptability to deformation, while reducing the risk of interlayer slippage. The temperature-corrected equivalent bending stiffness is expressed as:

[0014] in, This represents the equivalent bending stiffness at heating temperature T. This is the temperature correction factor. This is the equivalent bending stiffness at the reference temperature.

[0015] Six-axis free bending linkage forming.

[0016] The heated composite tube blank is clamped in a six-axis free bending forming device, which includes a three-dimensional translation unit that moves along the x, y, and z directions, and a rotational unit. , , The three-dimensional attitude adjustment unit with axis rotation applies spatial attitude constraints and feed control to the tube blank through six-axis linkage control, so that it completes segmented bending and forming according to the preset target trajectory.

[0017] Forming process monitoring and closed-loop parameter control.

[0018] During the bending process, the bending angle, cross-sectional distortion, wall thickness variation, interlayer slip, and springback trend are monitored in real time. Based on the monitoring results, at least one of the following parameters is dynamically adjusted: bending speed, feed speed, support force, constraint force, rotation angle, and attitude angle. The cross-sectional distortion rate is expressed as:

[0019] in, The cross-sectional distortion rate, This is the maximum characteristic dimension of the cross-section after forming. This is the minimum characteristic dimension of the cross-section after forming. This represents the original feature size. When the cross-sectional distortion rate exceeds a set threshold, the feed rate in that region is reduced and the support force is increased.

[0020] The relative slip between layers is expressed as:

[0021] Where λ is the relative slip between layers, and Δu is the relative displacement between layers. The thickness represents the interface characteristics. When the interlayer slip tendency increases, adjusting the heating uniformity or reducing the local bending loading rate can achieve coordinated deformation of the composite tube blank.

[0022] Springback compensation and shaping correction.

[0023] After reaching the predetermined bending state, the formed part undergoes unloading, cooling, and shaping. The target set angle is then compensated and corrected based on the springback amount obtained from the first piece or online inspection to improve the angular accuracy and trajectory consistency of the final product. The springback compensation angle is expressed as:

[0024] in, The set angle after compensation, For the target bending angle, The measured rebound angle after uninstallation.

[0025] The present invention has the following beneficial effects: This invention reduces cross-sectional ellipticization and local collapse. By coordinating the adjustment of bending radius, support force, and attitude angle, the cross-sectional profile can be maintained during the forming process, avoiding flattening, shrinkage, or local out-of-roundness in the dual-cavity region, thereby improving cross-sectional integrity.

[0026] This invention suppresses interlayer slip between the aluminum layer, the cross-linked polyethylene layer, and the copper layer. By controlling the heating temperature window and the loading rate, the peak value of the interlayer shear stress can be reduced, the relative displacement between different material layers can be decreased, and interface peeling, void expansion, and local delamination can be avoided.

[0027] This invention improves spatial bending accuracy and forming stability. By implementing closed-loop control of trajectory modeling, segmented forming, and springback compensation, the spatial curvature changes of the figure-eight tube can be made smoother, ultimately achieving higher consistency and repeatability. Attached Figure Description

[0028] Figure 1 This is a flowchart of the method of the present invention.

[0029] Figure 2 This is a schematic diagram of the cross-sectional structure of the figure-eight tube of the present invention.

[0030] Figure 3 This is a schematic diagram of the three-dimensional centerline trajectory of the figure-eight tube of the present invention.

[0031] Figure 4 This is a schematic diagram of interlayer slippage according to the present invention. Detailed Implementation

[0032] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0033] like Figure 2 As shown, the figure-eight tube is a three-layer composite structure of aluminum / cross-linked polyethylene / copper. Its cross-sectional shape has a total width of 32.70 mm, a total height of 15.41 mm, an outer arc radius of 7.1 mm, and a middle transition fillet radius of 2.2 mm. This cross-sectional configuration ensures independent flow function in the internal dual cavities while also meeting the integrated functional requirements of load-bearing, insulation, and conductivity.

[0034] like Figure 3 As shown, the six-axis free bending forming device used in this invention includes a translation unit that moves along the x, y, and z directions, and a circumferential unit. , , The attitude adjustment unit for shaft rotation. Through six-axis linkage control, the spatial position and attitude of the tube blank can be continuously corrected in real time during the bending process, thereby adapting to the bending requirements of any spatial trajectory.

[0035] Trajectory Modeling and Curvature Assignment The spatial trajectory of the figure-eight tube target is described by a three-dimensional parametric curve:

[0036] Where s is the trajectory arc length parameter, , , These represent the coordinate components of the spatial trajectory in three orthogonal directions. The entire trajectory is fitted with a cubic B-spline curve to ensure that the curvature changes continuously along the arc length direction, avoiding local stress concentration caused by abrupt changes in curvature.

[0037] curvature The relationship between the local bending radius R and the bending radius is as follows:

[0038] In practice, the entire trajectory is divided into multiple control segments based on the points of change in curvature direction and the extreme points of curvature. Within each control segment, the curvature remains constant or changes gradually according to a linear pattern. The curvature angle of the i-th control segment is... From the curvature of this segment With arc length The product is determined as follows:

[0039] The length of the transition section between each control section is set to be no less than 1.5 times the outer diameter of the pipe to ensure a smooth connection of the bending shape.

[0040] Heating temperature selection and stiffness control mechanism This invention employs localized heating to control the temperature of the bending section. The heating temperature window is selected as 80°C to 120°C, based on the following criteria: When the temperature is below 80°C, the molecular chain segments of the cross-linked polyethylene layer lack mobility, resulting in limited improvement in flexibility and a still high peak interlaminar shear stress, making it difficult to effectively suppress interlaminar slip. When the temperature is above 120°C, the cross-linked polyethylene layer softens excessively, significantly reducing its load-bearing capacity. This may lead to insufficient interlayer support between the aluminum and copper layers during bending, exacerbating interface instability and cross-sectional distortion. Therefore, controlling the heating temperature within the range of 80°C to 120°C allows the cross-linked polyethylene layer to reach a suitable flexible-viscosity state, minimizing interlaminar shear stress without sacrificing overall structural stiffness.

[0041] The heating method is selected from one or more combinations of hot air heating, radiation heating, infrared heating, induction heating, or resistance heating, depending on the geometric characteristics of the bending section and the production cycle requirements. For single-piece and small-batch production, hot air heating or radiation heating is preferred because it has low equipment cost and the temperature field uniformity is easy to control. For batch production, induction heating or resistance heating is preferred because it has a fast heating rate and a narrow heat-affected zone, which helps to shorten the single-piece forming cycle.

[0042] Equivalent bending stiffness at heating temperature T Correct it as follows:

[0043] Among them, temperature correction coefficient The expression is:

[0044] In the formula, Reference temperature (Equivalent bending stiffness at room temperature of 25°C) The temperature sensitivity coefficient, determined in advance through uniaxial bending tests, reflects the sensitivity of the composite material's bending stiffness to temperature changes. For the aluminum / crosslinked polyethylene / copper layered composite material of this invention, The typical range of values ​​is to .

[0045] Neutral axis position and strain distribution control During the bending deformation of the composite tube blank, the position of the neutral axis is determined by the elastic modulus, cross-sectional area, and centroid position of each material layer. The distance of the neutral axis from the reference axis... Calculate using the following formula:

[0046] Where i=1,2,3 correspond to the aluminum layer, cross-linked polyethylene layer, and copper layer, respectively. Let be the elastic modulus of the i-th layer. This is the cross-sectional area of ​​the layer. This is the distance from the centroid of this layer to the reference axis.

[0047] Under pure bending conditions, the longitudinal strain of any i-th layer along the bending direction for:

[0048] The maximum tensile strain occurs on the outermost arc-shaped surface layer furthest from the neutral axis:

[0049] In the formula, This is the distance from the outermost edge of the tube blank section to the reference axis. In actual operation, the curvature of each control segment is appropriately allocated. To ensure that the maximum tensile strain does not exceed the allowable fracture strain of the copper layer (for this composite material, control is recommended). (not exceeding 8%), thereby preventing cracks from appearing on the outer arc side.

[0050] Cross-section distortion control strategy The cross-sectional distortion rate δ is used to quantitatively evaluate the cross-sectional shape retention capability of a figure-eight tube after bending and forming, and is defined as follows:

[0051] In the formula, This refers to the maximum characteristic dimension of the cross-section perpendicular to the bending plane after forming. The smallest characteristic dimension of the same cross section. The original feature dimensions before forming (for the cross-section of the figure-eight tube, the total width of 32.70 mm is taken as the reference dimension).

[0052] In the operation of this invention, the cross-sectional distortion rate control threshold is set to 5%. When the cross-sectional distortion rate δ monitored in real time approaches or exceeds this threshold, the control system automatically performs the following adjustment actions: increasing the support constraint force in this area (the recommended increase in support force is 10% to 20% of the current value), while reducing the bending loading rate (the recommended rate reduction is 20% to 30%), to reduce the compressive instability tendency of the cross-section in the bending direction. Furthermore, for the figure-eight tube double-cavity structure, a flexible mandrel conforming to the contour of the double cavities is provided in the bending die, feeding synchronously with the tube blank during bending to provide internal support to the double-cavity region, further suppressing cross-sectional collapse.

[0053] Interlayer slip monitoring and suppression Interlayer relative slip The definition is as follows:

[0054] In the formula, Δu represents the interlayer relative displacement, which is measured in real time by a laser displacement sensor or a high-speed camera system installed on the outside of the curved section. The interfacial characteristic thickness (for this composite material, the interfacial characteristic thickness is the average thickness of the transition zone between the aluminum layer and the cross-linked polyethylene layer, typically ranging from 0.05 mm to 0.15 mm).

[0055] Interfacial shear stress relative displacement between floors The relationship is:

[0056] in, The equivalent shear modulus of the interface is determined beforehand by an interface peel test. When the interface shear stress... When the interfacial bond strength is exceeded, interfacial debonding will occur. Therefore, in practice, when the relative interlayer slip λ exceeds a set threshold (recommended threshold is 0.5) or the interfacial shear stress... When the interfacial bond strength exceeds 80%, the control system shall implement at least one of the following control measures: increase the heating temperature (by 5°C to 10°C within the range of 80°C to 120°C) to further reduce the shear modulus of the cross-linked polyethylene layer; reduce the loading rate of the bending section (the recommended reduction is 15% to 25% of the current value); or increase the heating and holding time (extend it by 5 s to 15 s) to make the temperature field distribution more uniform and avoid interfacial shear concentration caused by local temperature differences.

[0057] Springback compensation and precision control After unloading, the tube blank springs back due to the release of elastic internal stress. The springback angle Δθ is defined as:

[0058] in, For the target bending angle, The actual bending angle was measured after unloading. When the value is positive, it indicates that the actual bending angle is less than the target value (positive springback); when... When the value is negative, it indicates that the actual bending angle is greater than the target value (negative springback, which is rare).

[0059] To eliminate trajectory deviation caused by rebound, this invention employs a feedforward compensation strategy, with a set angle after compensation. Determine using the following formula:

[0060] The specific operation process is as follows: First, the bending and forming of the first tube blank is completed. After unloading and cooling, the actual bending angle of the key control points is obtained by three-dimensional optical scanning or coordinate measuring machine. and the target angle Compare and calculate the rebound angle. The rebound amount is then added in reverse to the setting parameters of subsequent fittings in the same batch, i.e., according to... Set a compensation value. For materials from the same batch, since the material properties and initial state are consistent, this compensation value applies to all fittings in that batch, and the springback angle only needs to be re-measured when the batch is changed.

[0061] Complete Operation Process Example Example 1 One aluminum / crosslinked polyethylene / copper layered composite tube blank was selected. The outer diameter of the tube blank was 32.70 mm, the wall thickness was 2.5 mm (of which the aluminum layer was 0.8 mm thick, the crosslinked polyethylene layer was 1.0 mm thick, and the copper layer was 0.7 mm thick), and the initial straight section length was 800 mm. First, the tube blank was inspected before forming. Industrial CT scanning was used to confirm that the thickness distribution of each layer was uniform, the interface bonding was good, and the end flatness deviation was less than 0.1 mm, which met the forming requirements.

[0062] The tube blank is clamped in a six-axis free bending forming device. After clamping, the distance between the protruding end of the tube blank and the front end face of the bending die is 120 mm. The heating system is turned on to locally heat the target bending area with hot air. The temperature is set to 95°C and the holding time is 30 s. After the temperature field distribution of the bending section stabilizes, the bending program is started.

[0063] Based on the three-dimensional trajectory of the target figure-eight pipe, the bending path is divided into three control segments: the first segment is the straight introductory segment (arc length 100 mm), the second segment is the main bending segment (arc length 150 mm, curvature...). The third section is the transition straightening section (arc length 80 mm, curvature from...). Linear transition to 0). Apply displacement in the x, y, and z directions segment by segment and , , The attitude was adjusted, the bending loading rate was 5 mm / s, and the feed rate was 8 mm / s.

[0064] During the forming process, the real-time monitoring system collects data on bending angle, cross-sectional profile, and interlayer displacement at a sampling frequency of 50 Hz. The maximum measured cross-sectional distortion rate is 3.2%, and the maximum interlayer relative slip is 0.28, both within the control threshold range, requiring no additional adjustment.

[0065] After forming, the tube blank is unloaded and allowed to cool naturally to room temperature before undergoing a shaping process (shaping time is 60 seconds). A coordinate measuring machine is used to inspect the formed part. The measured bending angle of the main bending section is 88.5°, the target angle is 90°, and the springback angle is... After correction according to the compensation formula, the set bending angle of the next tube blank is corrected to 91.5°. The deviation between the overall trajectory of the final formed part and the design trajectory is controlled within ±0.5 mm, the double cavity section remains intact, and no cracking, wrinkling, or delamination occurs in the bending area.

[0066] Example 2 Based on Example 1, a batch trial production was conducted on 10 tube blanks from the same batch. The first piece was formed according to the process of Example 1, and the springback angle was measured. Directly substitute into the compensation formula The setting parameters for the subsequent 9 tube blanks were corrected.

[0067] During the subsequent forming process, the heating temperature of each tube blank is uniformly set to 95°C. However, based on the slight differences in the clamping state of each tube blank, the support force (adjustment range is 90% to 110% of the initial value) and attitude angle (adjustment range is ±0.5°) are dynamically adjusted through the online monitoring system to ensure the stability of each forming process.

[0068] Batch verification showed that the angular errors of key control points after forming of all nine tube blanks were controlled within ±0.5°, the cross-sectional distortion rate was less than 4%, the trajectory deviation was less than ±0.6 mm, and no interlayer debonding defects were observed. This example demonstrates that, within a fixed batch, the strategy of compensation and correction based on the springback measurement results of the first piece has excellent repeatability and process stability. For different batches of materials or different layer thickness combinations, only the springback angle needs to be re-measured. and temperature sensitivity coefficient By using the same control framework, rapid process migration and parameter adaptation can be achieved.

[0069] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for heating and bending an aluminum cross-linked polyethylene copper composite figure-eight tube, characterized in that, Includes the following steps: S1 tube blank preparation steps: Prepare aluminum / crosslinked polyethylene / copper layered composite tube blanks, and perform pre-forming inspection on the tube blanks to remove tube blanks with defects or out-of-tolerance dimensions. The S2 trajectory modeling and segmentation setting steps are as follows: a three-dimensional centerline trajectory model is established based on the spatial structure of the target figure-eight tube. The bending path is divided into multiple control segments, and corresponding curvature, torsion, bending radius, transition length, attitude angle and feed parameters are set for each control segment to make the curvature of the target trajectory change continuously. The S3 heating and softening step heats the target bending area to bring the aluminum layer, cross-linked polyethylene layer, and copper layer to a preset temperature window suitable for bending. In the S4 six-axis free bending forming step, the heated composite tube blank is clamped in the six-axis free bending forming device. The spatial posture constraint and feed control are applied to the tube blank through six-axis linkage control, so that the tube blank completes segment bending forming according to the preset target trajectory. The S5 process monitoring and closed-loop control steps monitor the billet state in real time during the bending forming process and dynamically adjust at least one of the following based on the monitoring results: bending speed, feed speed, support force, constraint force, rotation angle, and attitude angle. The S6 springback compensation and shaping process involves unloading, cooling, and shaping the formed part after it reaches the predetermined bending state, and compensating and correcting the target set angle based on the detected springback amount.

2. The method according to claim 1, characterized in that, In the heating and softening step, the heating method is one or more combinations of hot air heating, radiation heating, induction heating, and resistance heating.

3. The method according to claim 1, characterized in that, The preset temperature window is 80°C to 120°C.

4. The method according to claim 1, characterized in that, In the trajectory modeling and segmentation setting steps, the curvature angle of the i-th control segment... The following relationship must be satisfied: ； in, Let be the bending angle of the i-th control segment. Let be the curvature of the i-th control segment. Let be the arc length of the i-th control segment.

5. The method according to claim 1, characterized in that, In the heating and softening step, the equivalent bending stiffness under heating state satisfies the following relationship: ； in, This represents the equivalent bending stiffness at heating temperature T. This is a temperature correction factor. This is the equivalent bending stiffness at the reference temperature.

6. The method according to claim 1, characterized in that, The six-axis free bending forming device includes a three-dimensional translation unit that moves along the x, y, and z directions, and a rotational unit that... , , A three-dimensional attitude adjustment unit for axis rotation.

7. The method according to claim 1, characterized in that, In the process monitoring and closed-loop control steps, the monitored billet state includes at least one of the following: bending angle, cross-sectional distortion, wall thickness change, interlayer slip, and springback trend.

8. The method according to claim 7, characterized in that, The cross-sectional distortion rate δ is calculated using the following formula: ； in, The cross-sectional distortion rate, This is the maximum characteristic dimension of the cross-section after forming. This is the minimum characteristic dimension of the cross-section after forming. Original feature size; When the cross-sectional distortion rate exceeds a set threshold, the feed speed in that region is reduced and the support force is increased.

9. The method according to claim 7, characterized in that, The interlayer relative slip λ is calculated using the following formula: ； Where λ is the relative slip between layers, and Δu is the relative displacement between layers. For interface feature thickness; When the relative slip between layers increases, adjust the heating uniformity or reduce the local bending loading rate.

10. The method according to claim 1, characterized in that, In the springback compensation and shaping steps, the compensation setting angle is... Determine using the following formula: ； in, The set angle after compensation, For the target bending angle, The measured rebound angle after unloading; Based on the measured springback angle obtained from the first prototype The setting angle of subsequent pipe fittings in the same batch is compensated and corrected.

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