Pipe liquid-filling forming method based on gradient friction condition distribution

Abstract

The application discloses a kind of tube liquid filling forming methods based on gradient friction condition distribution, comprising: according to the preliminary determination of friction characteristic distribution of tube geometry characteristics and material attribute, significant friction parameter is screened by finite element simulation in combination with test design, and the optimal gradient distribution of friction coefficient is determined by adopting central composite test design response surface method in combination with entropy weight method for multi-objective optimization;Outer film is covered as base layer on the outer surface of the tube, and the bionic hexagonal texture or the structure of the bionic nepenthes peristome area is prepared based on the principle of bionics using femtosecond laser micro-nano processing technology, to realize the high-precision regulation and control of friction coefficient gradient;Design includes upper and lower die, left and right punch and balance punch liquid filling forming die;Calculate initial yield pressure, shaping pressure and clamping force, set the amount of advance and pressure value to carry out trial forming and correction.The application realizes material uniform flow by gradient friction regulation, effectively reduces wall thickness thinning, and is suitable for aluminum alloy and other ductile metal tube liquid filling forming.

Description

Technical Field

[0001] This invention relates to the field of pipe forming, and more specifically to a pipe forming method based on gradient friction condition distribution. Background Technology

[0002] In the aerospace field, aluminum alloy conduits are widely used due to their lightweight and high ductility. However, some complex structures, such as tees and reducers, are difficult to achieve using traditional CNC bending processes due to geometric limitations. These require the use of drop forming technology to prepare the semi-tubes, followed by manual TIG welding for subsequent connection to complete the component manufacturing. While this method meets basic production and manufacturing requirements to some extent, its limitations are also significant: First, thin-walled aluminum alloy semi-tubes are prone to deformation during welding, making precise control of their dimensional stability difficult and often requiring additional process compensation; second, the inconsistency of manual TIG welding operations further exacerbates the dispersion of welding deformation, leading to a significant reduction in the final assembly accuracy. This two-step process not only increases the complexity of the manufacturing process but also places higher technical demands on subsequent assembly stages. Summary of the Invention

[0003] The purpose of this invention is to provide a pipe filling and forming method based on gradient friction condition distribution, which aims to solve the problems of high manufacturing complexity, low dimensional accuracy and large dispersion of welding deformation in the traditional two-step process.

[0004] To achieve the above objectives, the present invention employs the following technical solution: A pipe filling and forming method based on gradient friction condition distribution includes: Determination of variable gradient friction force distribution in pipe: Based on the geometric characteristics and material properties of the pipe, the distribution law of friction characteristics in different regions of the pipe is initially determined; on this basis, the friction conditions are optimized through finite element simulation analysis, and then significant process factors are screened and multi-objective optimization is carried out to determine the optimal friction coefficient gradient distribution. Friction gradient arrangement based on outer film and laser micro-nano processing: An outer film is covered on the outer surface of the pipe as a base layer. Based on the principle of bionics, micro-nano scale structures with specific bionic morphology are prepared on the surface of the outer film using femtosecond laser micro-nano processing technology to achieve high-precision control of the friction coefficient gradient of the pipe surface. After determining the optimal friction coefficient distribution, the pipe is formed by filling it with liquid using a liquid forming mold.

[0005] Furthermore, the finite element simulation analysis includes: Create a three-dimensional geometric model of the pipe and the liquid filling forming mold, and import it into the finite element software; Performance testing methods were used to obtain the specific material parameters of the pipe. These material properties were then imported into the finite element software, and a rigid body assumption was made for the liquid filling forming mold. A dynamic explicit analysis step was set as the main solution method for the liquid filling forming process. Based on the different friction gradients required during the liquid filling forming process, the outer surface of the pipe was divided into multiple subsets, and the contact behavior between each subset and the liquid filling forming mold was defined, i.e., the corresponding friction coefficient was set in the contact properties of each subset. In surface-to-surface contact, the liquid filling forming mold was designated as the master surface. Completely fixed constraints were applied to the liquid filling forming mold, and process parameters were set, with parameter values ​​determined based on previous experimental data. Mesh generation was performed on both the pipe and the liquid filling forming mold, with the mesh density of the liquid filling forming mold being higher than that of the pipe. After completing the above settings, the solution calculation was performed.

[0006] Furthermore, by keeping other forming parameters constant and only adjusting the friction coefficient values ​​corresponding to each subset, the friction condition distribution of the pipe is optimized. The forming parameters include internal bulging pressure and axial feed speed. Using the Plackett-Burman experimental design method, the friction coefficients corresponding to each subset of the outer surface of the pipe are used as variables to be screened, and the forming quality index obtained by finite element simulation is used as the response. Key friction coefficients that have a significant impact on performance are screened from the friction coefficients of different regions. For the screened key friction coefficients, they are used as process parameters to be optimized. The optimal friction coefficient gradient distribution is determined by combining the central composite experimental design method with the response surface model method and the entropy weight method, combined with the finite element simulation method. The weight of the optimization objective is determined based on the simulation results combined with the entropy weight method.

[0007] Furthermore, the outer coating is a polyester film material that is tightly adhered to the outer surface of the pipe, serving as a base layer for subsequent friction coefficient adjustment. At locations where an increased friction coefficient is required, a biomimetic hexagonal texture 3D morphology is fabricated on the surface of the outer coating using femtosecond laser micro-nano processing methods. Multiple hexagonal morphologies are combined to form the outer coating texture of the area where friction needs to be increased. The size of the hexagons, the groove depth between the hexagons, the texture area, and the density can be adjusted as needed. If further friction is required, femtosecond laser processing is performed on the sidewalls of the already fabricated hexagonal texture to create hexagonal grooves on the sidewalls.

[0008] Furthermore, at locations where a reduction in the coefficient of friction is required, femtosecond laser micro-nano processing methods are used to fabricate a pitcher plant-like structure along the flow direction of the liquid filling medium on the surface of the outer coating. The width, area, groove depth, and groove density of this structure can be adjusted as needed. Alternatively, the low-friction characteristics of the outer coating itself can be directly utilized to reduce the coefficient of friction. By conducting high-precision friction coefficient calibration experiments, the variation law of the friction coefficient on the material surface under different processing parameters is determined, and a quantitative mapping relationship between processing parameters and the coefficient of friction is established.

[0009] Furthermore, the liquid-filling forming mold includes an upper mold, a lower mold, a left punch, a right extrusion punch, and a balancing punch; the tube to be formed is placed in the forming cavity formed by the upper and lower molds; the upper mold is fixed on a slider that can move up and down, and the lower mold is fixed on the plane of the liquid-filling forming equipment; the left punch and the right extrusion punch are respectively set at both ends of the forming cavity, and the diameter of the end of the punch that extends into the mold gradually decreases; for the T-shaped pipe, the balancing punch is used to maintain the supporting pressure during forming, and its end extends into the tube cavity on the side of the forming cavity.

[0010] Furthermore, a through hole is provided axially in the middle position of the left punch or the right extrusion punch, and a hose is arranged in the hole. One end of the hose is connected to an external hydraulic system, and the other end is connected to the forming cavity to transport liquid medium into the forming cavity.

[0011] Furthermore, the process of liquid filling and forming the pipe includes: Inspect and clean the pipes, cut the ends, and then inspect the liquid filling mold. The pipe surface is coated with a lubricant and covered with an outer plastic film for lubrication; at locations where friction needs to be adjusted, the outer film surface is processed using femtosecond laser micro-nano processing to adjust the friction. The pipe is placed in the forming cavity of the liquid-filled forming mold and fixed; the initial expansion pressure is set according to the calculated initial yield pressure to make the pipe begin to undergo plastic deformation; the forming pressure is set according to the calculated forming pressure to ensure the transition radius and dimensional accuracy of the cross-section in the later stage of forming; the clamping force of the hydraulic press is set according to the calculated clamping force to ensure that the mold closes without gaps during the forming process.

[0012] Furthermore, the initial yield pressure The calculation formula is ,in For the yield strength of the pipe, For pipe wall thickness, The diameter of the pipe; the forming pressure The calculation formula is ,in For the tensile strength of the pipe, The minimum transition radius of the pipe cross-section; the clamping force The calculation formula is ,in This represents the projected area of ​​the pipe on a horizontal plane.

[0013] Furthermore, after setting the feed rate and pressure value, a trial forming is performed using a test piece; the actual pressure of the part and the feed rate of the feed cylinder are corrected according to the results of the first batch of test pieces forming; no pressure drop is allowed during the forming process, and if it occurs, the feed rate of the left punch and the right extrusion punch should be changed.

[0014] Compared with the prior art, the present invention has the following technical features: To meet the diverse friction conditions required during the forming process of different types of pipes, this invention proposes a pipe fluid-filling forming method based on gradient friction condition distribution. This method designs a friction optimization scheme suitable for aluminum alloy pipes based on factors such as pipe material properties, geometry, and application scenarios. Specifically, when arranging friction functional zones on the pipe surface, the friction force distribution pattern (such as gradient friction or uniform friction) is adjusted according to the specific needs of the pipe by combining biomimetic principles and coating technology. Simultaneously, through finite element simulation and experimental verification, the stress distribution and material flow problems that different types of pipes may encounter during the forming process are systematically analyzed, and optimization strategies are proposed for each type. Based on the actual requirements of the pipes to be formed, an external fluid-filling forming mold for conduit fluid-filling forming is designed, enabling high-quality forming processes for different pipes. This method not only improves the adaptability and reliability of the pipe forming process but also provides flexible technical solutions for various industrial application scenarios, possessing significant engineering application value. Attached Figure Description

[0015] Figure 1 It has a hexagonal texture. Figure 2 It imitates the texture morphology of the mouth region of pitcher plants; Figure 3 A flowchart of a pipe filling and forming method; Figure 4 Schematic diagram of the overall structure for liquid filling and forming of pipes; Figure 5 A schematic diagram of the overall structure of the pipe after the upper mold has been removed and liquid filling has been formed; Figure 6 An extrusion punch with an opening on one side.

[0016] Explanation of reference numerals in the attached diagram: 1-Left punch, 2-Right extrusion punch, 3-Balancing punch, 4-Upper die, 5-Lower die, 6-Tube. Detailed Implementation

[0017] Against the backdrop of the aforementioned conduit processing, liquid-filled forming exhibits significant advantages. Firstly, it enables the one-step forming of tees and reducers, eliminating the need for traditional two-step processes, greatly simplifying the production process and reducing manufacturing costs. Secondly, compared to manual argon arc welding, liquid-filled forming provides more even material stress, effectively controlling the deformation risk of thin-walled aluminum parts during manufacturing, reducing the need for process compensation, and improving the overall consistency of components. By precisely controlling the distribution of liquid pressure and friction, liquid-filled forming promotes uniform material flow, significantly improving the uniformity of conduit wall thickness and dimensional accuracy, thereby reducing errors in subsequent assembly. These advantages not only simplify the production process but also significantly improve the precision of aerospace conduit forming.

[0018] While liquid-filled forming technology offers significant advantages, precise control of the forming process is still necessary in practical applications. Key factors include the coefficient of friction, liquid pressure, and feed rate. Crucially, the on-demand gradient friction setting in different regions during liquid-filled forming is one of the core technologies for achieving high-quality forming. By rationally adjusting the friction distribution in various parts of the conduit, the elongation and thinning rates of the material in different areas can be effectively controlled, thereby ensuring the uniformity and stability of the pipe forming process.

[0019] To address the issues of gradient friction condition distribution requirements and poor formability in the aforementioned aerospace duct filling and forming process, a tube filling and forming method based on gradient friction condition distribution is proposed. This method introduces a biomimetic gradient friction design concept into the filling and forming process. Through systematic analysis and design, precise control of the friction force on the outer surface of the tube is achieved. Combined with laser micro-nano processing and tube coating methods, uniform material flow and precise deformation are realized, thereby significantly improving the uniformity of material flow and forming quality during the filling and forming process, ensuring uniformity of duct wall thickness, dimensional accuracy, and overall consistency.

[0020] This embodiment uses aerospace-grade 5B2-O aluminum alloy tubing as an example for illustration. The overall structure of the forming device is as follows: Figure 4 and Figure 5 As shown, the method flow is as follows: Figure 3 As shown.

[0021] Part 1: Method for determining the distribution of variable gradient friction force in pipes.

[0022] Based on the geometric characteristics and material properties of pipe 6, the distribution law of friction characteristics in different regions of pipe 6 is first determined to provide a basis for subsequent design. The determination of the friction conditions of the pipe needs to take into account its initial length and final shape characteristics: a larger friction coefficient is set for the straight section area that does not require deformation, while the friction coefficient is reduced for the curved area that requires deformation, so as to avoid excessive friction leading to shear force concentration, thereby preventing local thinning of the pipe, which could lead to pipe failure in severe cases.

[0023] After initially determining the friction conditions of the pipe, in order to obtain the optimal friction condition distribution, a friction condition optimization based on finite element simulation analysis was carried out on the pipe filling and forming process; the friction condition optimization process specifically includes the following steps: The first step is to create a three-dimensional geometric model of the pipe 6 and the liquid filling forming mold, and import it into the finite element software to ensure that the geometric relationship between the pipe and the liquid filling forming mold is accurate. The liquid filling forming mold includes an upper mold 4, a lower mold 5, a left punch 1, a right extrusion punch 2, and a balancing punch 3.

[0024] The second step involves using performance testing methods to obtain the specific material parameters of pipe 6, including but not limited to density, elastic modulus, yield strength, and constitutive model parameters related to plastic deformation; importing these material properties into the finite element software and making a rigid body assumption on the liquid filling forming mold.

[0025] The third step involves assembling pipe 6 and the liquid filling forming mold, and then setting up a dynamic explicit analysis step as the main solution method for the liquid filling forming process. This analysis step is suitable for handling nonlinear problems such as large deformation.

[0026] The fourth step involves dividing the outer surface of pipe 6 into multiple subsets based on the different friction gradients required during the pipe filling and forming process. The contact behavior between each subset and the filling and forming mold is defined, i.e., the corresponding friction coefficient is set in the contact attributes of each subset. In surface-to-surface contact, the filling and forming mold is designated as the master surface to ensure the efficiency and stability of the contact algorithm.

[0027] The fifth step involves applying a complete and fixed constraint to the liquid-filled forming mold to prevent displacement or rotation. Simultaneously, process parameters such as internal bulging pressure and axial feed speed are set; these parameter values ​​are determined based on previous experimental data.

[0028] The sixth step involves meshing the pipe 6 and the liquid-filling forming mold. The mesh density of the liquid-filling forming mold (rigid body) is higher than that of the pipe 6 to improve calculation accuracy. It is also recommended to use tetrahedral or hexahedral elements, and adjust the element size according to actual needs.

[0029] Step 7: After completing the above settings, perform the calculation.

[0030] Step 8: After the simulation is completed, extract key data through the post-processor, including but not limited to the maximum thinning rate, maximum stress value and geometric feature parameters such as the branch pipe height of pipe 6; where the branch pipe refers to the pipe part that protrudes perpendicular to the axis of the main pipe when forming the tee pipe.

[0031] Based on the above finite element simulation method, by keeping other forming parameters (including but not limited to internal bulging pressure, axial feed speed and other set process parameters) unchanged, the friction coefficient values ​​corresponding to each subset of pipe surfaces divided in step four are adjusted to achieve the friction condition distribution optimization target of pipe 6.

[0032] For complex pipe structures, multiple regions may require different friction coefficients. Optimizing all regions would significantly increase the simulation workload and fail to obtain the optimal friction coefficient setting range. Therefore, this invention employs the Plackett-Burman (PB) experimental design method to screen for significant process factors. The PB experimental design method can screen for factors that significantly affect performance with a relatively small number of experiments. Through N experiments, a maximum of N-1 factors with significance levels can be screened, where N is a multiple of 4.

[0033] The first-order model of each response can be derived from the experimental matrix: ; in, The predicted response values ​​are for the forming quality indicators that need to be optimized (such as maximum thinning rate, maximum stress, branch height, etc.). It is a constant; For the first The coefficients of each variable; For the first One variable is the coefficient of friction.

[0034] The significance test is used to calculate the statistic. The probability density function of the distribution is used to analyze the significance level of the influence of each experimental factor on the response. ; in, This represents the number of sample points, which is the total number of trials in the PB experimental design. For the first The response value of the shape quality index obtained from the simulation calculation of a sample point (such as the maximum thinning rate value obtained from a certain simulation); The average value of the sample points ( (average of the results); The first-order model is calculated based on the first-order model. Predicted response values ​​for each sample point; The number of variables. For the statistic... By performing hypothesis testing, we can calculate Value. Generally. A value less than 0.5 is considered significant. A value less than 0.1 is considered highly significant.

[0035] Using the PB experimental design method described above, key friction coefficients that significantly affect performance are selected from the numerous friction coefficients corresponding to each subset of the pipe's outer surface defined in the finite element simulation model; that is, the friction coefficients corresponding to each subset of the pipe's outer surface are used as variables to be screened. The forming quality index obtained through finite element simulation calculation will be used as the actual response. .

[0036] For the selected key friction coefficients, these are used as process parameters to be optimized. A central composite experimental design method combined with response surface methodology and entropy weighting method, along with finite element simulation, is employed to determine the optimal friction coefficient gradient distribution. Generally, 3 to 4 process parameters are selected for optimization, with optimization objectives set as the maximum thinning rate of the part, the maximum stress of the part, and the degree to which the branch pipe height meets expectations. The weights of the optimization objectives can be determined based on simulation results combined with the entropy weighting method.

[0037] After determining the optimal friction coefficient gradient distribution for each region of the pipe through the above optimization process, the implementation stage of the friction gradient distribution begins. Using the optimal friction coefficient for each region as the target friction coefficient value, a biomimetic hexagonal texture three-dimensional morphology is fabricated on the outer coating surface using femtosecond laser micro-nano processing methods in areas where increased friction is needed; and a pitcher plant-like edge region structure is fabricated in areas where reduced friction is needed. The quantitative mapping relationship between processing parameters and friction coefficient established through high-precision friction coefficient calibration experiments allows for precise control of laser processing parameters (such as hexagonal size, groove depth, and texture density), ensuring that the friction coefficient of the processed outer coating surface is consistent with the optimal friction coefficient distribution designed in the first part.

[0038] Part Two: A method for arranging frictional gradients based on outer coating and laser micro / nano fabrication.

[0039] This invention proposes a gradient friction distribution method that creatively combines tube coating technology and laser micro / nano processing technology. Specifically, firstly, a tube coating process is used to cover the corresponding area on the outer surface of the tube where friction needs to be controlled with an outer film. Subsequently, based on biomimetic principles, high-precision laser micro / nano processing technology is used to prepare nanoscale structures with specific biomimetic morphologies on the surface of the thin film, thereby achieving high-precision control of the friction coefficient gradient on the tube surface.

[0040] To ensure a high-quality pipe surface, surface processing should be minimized. Therefore, regardless of whether the friction coefficient needs to be increased or decreased, a uniform outer film should first be applied to the outer surface of the pipe as a base layer for subsequent friction coefficient adjustment. Polyester film is the preferred material for the outer film, as it offers a wide range of thicknesses, excellent tensile and tear resistance, good corrosion resistance to most acids, alkalis, and organic solvents, and is inexpensive. This material, while adhering tightly to the outer surface of the pipe, effectively reduces the basic friction coefficient between the pipe's outer surface and the liquid-filling molding die. Subsequent modifications to the friction coefficient of the pipe's outer surface are also carried out on the film layer, avoiding damage to the pipe surface.

[0041] To create a frictional gradient between different areas of the tube's outer surface and the liquid-filling molding die during the liquid-filling process, a femtosecond laser micro-nano fabrication method is used to process the outer coating surface. Femtosecond lasers, with their ultrashort pulse width and extremely high peak power, can precisely remove material within a very small heat-affected zone, thereby fabricating micro- and nano-scale biomimetic structures on the coating surface.

[0042] In terms of strategies to enhance and reduce the coefficient of friction, biomimetic principles are applied to the design. In nature, various organisms have evolved unique surface microstructures to adapt to complex environments: frogs and insects have polygonal microstructures on their toes, which helps improve adhesion between their feet and the contact surface during climbing; springtails achieve stable habitation in complex soil environments through a specific arrangement of scales; pitcher plants rely on the moist properties of their oral margin surface to continuously reduce friction, facilitating insect predation. The micro- and nano-scale structural features of the surfaces of these organisms all significantly alter frictional properties. Inspired by these natural phenomena, a biomimetic method for designing microstructures on the outer surface of ducts is developed to effectively modify the frictional properties of duct surfaces.

[0043] Based on the aforementioned biomimetic principles, femtosecond laser micro / nano fabrication methods are used to process the outer coating surface at locations where an increased coefficient of friction is required. The resulting morphology is a biomimetic hexagonal textured three-dimensional morphology, such as... Figure 1 As shown, multiple hexagonal shapes are combined to form the outer texture of the area where friction needs to be increased. The groove depth between each hexagon can be processed as needed, and the hexagonal size, groove depth, texture area, and density can all be adjusted as required. If further friction is needed, laser processing can be performed on the sidewalls of the processed hexagonal texture. For example, further processing of hexagonal grooves on the sidewalls can achieve even better friction performance.

[0044] At locations where a reduced friction coefficient is required, femtosecond laser micro / nano fabrication methods are used to process the outer coating surface. The resulting morphology resembles the pitcher plant's mouth edge structure along the flow direction of the filling medium, such as... Figure 2As shown. Similarly, the width, area, trench depth, and trench density of the micro / nano structure can all be adjusted as needed; or they can be adjusted by utilizing the low-friction properties of the outer coating itself.

[0045] To clarify the friction gradient distribution law of femtosecond laser micro-nano fabrication method, it is necessary to carry out high-precision friction coefficient calibration test, determine the variation law of friction coefficient on material surface under different processing parameters (including laser power, scanning speed, processing spacing, etc.), establish a quantitative mapping relationship between processing parameters and friction coefficient, and provide a basis for the design of friction gradient in different regions.

[0046] Part Three: Design of tubular fluid filling forming molds considering the actual shape of the parts.

[0047] Based on the actual needs of the pipe filling and forming process, the filling and forming mold includes an upper mold 4, a lower mold 5, a left punch 1, a right extrusion punch 2, and a balancing punch 3. The pipe to be formed 6 is placed between the upper mold 4 and the lower mold 5.

[0048] The upper mold 4 and lower mold 5 are designed according to the final shape of the actual pipe. The upper mold 4 and lower mold 5 are assembled to form a forming cavity; for example... Figure 4 As shown, the forming cavity can be a tubular structure or a T-shaped structure (for forming a tee pipe); the upper mold 4 is fixed to the slide block that can move up and down by bolt connection, and the hydraulic press drives the slide block to move up and down by hydraulic pressure; the lower mold 5 is directly fixed to the plane of the liquid filling forming equipment by bolt connection.

[0049] The left punch 1 and the right extrusion punch 2 can be configured as long straight rods, or as a long straight rod on one side and a handle-like shape on the other side for easy pushing. The diameter of the ends of the left punch 1 and the right extrusion punch 2 that extend into the liquid forming mold gradually decreases to ensure the stability of the punches during axial feeding. To allow the liquid medium to smoothly enter the forming cavity of the liquid forming mold, a through hole is provided axially at the middle position of the left punch 1 or the right extrusion punch 2. A flexible hose is arranged in the hole, one end of which is connected to an external hydraulic system, and the other end is connected to the forming cavity of the liquid forming mold, for conveying the liquid medium into the forming cavity. Figure 6 As shown.

[0050] For a T-shaped pipe, the balancing punch 3 is used to maintain supporting pressure during the forming of the T-shaped pipe; that is, as... Figure 4 In the example, the end of the balancing punch 3 extends into the cavity on the side of the forming cavity. For ordinary forming tubes, the balancing punch 3 is not required.

[0051] Part Four: Pipe Fluid Filling Forming Process Based on Gradient Friction Condition Distribution.

[0052] Based on the above friction condition distribution method and forming liquid-filling mold, the pipe liquid-filling forming process based on gradient friction condition distribution is completed, specifically including the following steps: Step 1: Feeding and degreasing.

[0053] The surface of the pipe should be free of mechanical damage, indentations, pits, cracks, and other defects. The pipe should be cleaned and degreased.

[0054] Step 2: Cut the flat end face.

[0055] Cut the ends of the pipe to make the end faces flush and ensure perpendicularity.

[0056] Step 3: Install the liquid filling molding mold.

[0057] Check that the liquid filling forming mold is installed correctly, that there is no interference, and that its fixation on the machine tool is safe and reliable. Without installing the tubing, test the machine tool's operation; only after everything is normal can liquid filling forming proceed.

[0058] Step 4: Perform a final inspection before shaping.

[0059] The surfaces of the pipes and the liquid-filling forming molds should be clean and free of cracks and deformation.

[0060] Step 5: Lubrication preparation and friction gradient setting.

[0061] To lubricate the pipe surface and make the material flow more easily, lubricant can be applied to the pipe surface and an outer plastic film can be used for lubrication. At the location where friction needs to be adjusted, femtosecond laser micro-nano processing is performed on the outer film surface to adjust the friction. In areas where the friction coefficient needs to be increased, a biomimetic hexagonal texture three-dimensional morphology is processed. In areas where the friction coefficient needs to be reduced, a pitcher plant-like mouth edge structure is processed.

[0062] Step 6: Calculate the initial yield pressure.

[0063] Initial yield pressure (Unit: MPa) refers to the internal pressure required for the pipe to begin plastic deformation. The calculation formula is: ; in, The yield strength of the pipe is expressed in MPa. The pipe wall thickness is in mm. The diameter of the pipe is in mm.

[0064] Step 7: Calculate the shaping pressure.

[0065] In the later stages of liquid filling and forming, most of the pipe has been formed. At this point, higher pressure is needed to form the transition fillets of the cross-section and ensure dimensional accuracy. This stage is called shaping, and the pressure required for shaping is called shaping pressure. (Unit: MPa), the calculation formula is: ; in, The tensile strength of the pipe is expressed in MPa. This is the minimum transition fillet radius for the pipe cross-section, in mm.

[0066] Step 8: Calculate the clamping force.

[0067] Clamping force (Unit: kN) refers to the force required to close the liquid-filled molding die without gaps during the molding process. The calculation formula is: ; in, This is the projected area of ​​the pipe on a horizontal plane, in mm². For pipes with a curved axial direction, the projected area is the product of the width and the projected length of the axis on the horizontal plane.

[0068] Step nine: Trial forming and correction.

[0069] Place the tube 6 into the forming cavity between the upper mold 4 and the lower mold 5 and fix it, based on the initial yield pressure calculated in step six. Set the initial bulging pressure to induce plastic deformation in the pipe; then apply the shaping pressure calculated in step seven. Set the pressure during the shaping stage to ensure the transition fillet radius and dimensional accuracy of the cross-section in the later stages of forming; based on the clamping force calculated in step eight. Set the clamping force of the hydraulic press to ensure that the mold closes without gaps during the forming process. Simultaneously, based on simulation results and the above calculation formulas, set the feed rate and selected pressure value, and perform trial forming using a specimen. The actual pressure of the part and the feed rate of the feed cylinder should be corrected according to the results of the first batch of specimens. Pressure drop is not allowed during the forming process; if it occurs, the feed rate of the left punch 1 and the right extrusion punch 2 should be adjusted. Combining the above steps, the entire process of pipe filling and forming is completed.

[0070] In summary, this invention optimizes material flow and stress distribution through high-precision control of friction via micro / nano structures. The functional surface structure, designed using biomimetic principles, significantly improves the quality and efficiency of tubular fluid filling forming, effectively reduces wall thinning, and promotes uniform wall thickness distribution, thereby significantly reducing forming defects. This method is applicable not only to aluminum alloys but also to the fluid filling forming process of other ductile metals.

[0071] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application.

Claims

1. A pipe filling and forming method based on gradient friction condition distribution, characterized in that, include: Determination of variable gradient friction force distribution in pipe: Based on the geometric characteristics and material properties of the pipe, the distribution law of friction characteristics in different regions of the pipe is initially determined; on this basis, the friction conditions are optimized through finite element simulation analysis, and then significant process factors are screened and multi-objective optimization is carried out to determine the optimal friction coefficient gradient distribution. Friction gradient arrangement based on outer film and laser micro-nano processing: An outer film is covered on the outer surface of the pipe as a base layer. Based on the principle of bionics, micro-nano scale structures with specific bionic morphology are prepared on the surface of the outer film using femtosecond laser micro-nano processing technology to achieve high-precision control of the friction coefficient gradient of the pipe surface. After determining the optimal friction coefficient distribution, the pipe is formed by filling it with liquid using a liquid forming mold.

2. The pipe filling and forming method based on gradient friction condition distribution according to claim 1, characterized in that, The finite element simulation analysis includes: Create a three-dimensional geometric model of the pipe and the liquid filling forming mold, and import it into the finite element software; Performance testing methods were used to obtain the specific material parameters of the pipe. These material properties were then imported into the finite element software, and a rigid body assumption was made for the liquid filling forming mold. A dynamic explicit analysis step was set as the main solution method for the liquid filling forming process. Based on the different friction gradients required during the liquid filling forming process, the outer surface of the pipe was divided into multiple subsets, and the contact behavior between each subset and the liquid filling forming mold was defined, i.e., the corresponding friction coefficient was set in the contact properties of each subset. In surface-to-surface contact, the liquid filling forming mold was designated as the master surface. Completely fixed constraints were applied to the liquid filling forming mold, and process parameters were set, with parameter values ​​determined based on previous experimental data. Mesh generation was performed on both the pipe and the liquid filling forming mold, with the mesh density of the liquid filling forming mold being higher than that of the pipe. After completing the above settings, the solution calculation was performed.

3. The pipe filling and forming method based on gradient friction condition distribution according to claim 1, characterized in that, By keeping other forming parameters constant and adjusting only the friction coefficient values ​​corresponding to each subset, the friction condition distribution of the pipe is optimized. The forming parameters include internal bulging pressure and axial feed speed. Using the Plackett-Burman experimental design method, the friction coefficients corresponding to each subset of the pipe's outer surface are used as variables to be screened. The forming quality index obtained through finite element simulation is used as the response. Key friction coefficients that significantly affect performance are screened from the friction coefficients of different regions. For the screened key friction coefficients, they are used as process parameters to be optimized. The optimal friction coefficient gradient distribution is determined using a central composite experimental design method combined with response surface methodology and entropy weighting, along with finite element simulation. The weights of the optimization objectives are determined based on simulation results combined with the entropy weighting method.

4. The pipe filling and forming method based on gradient friction condition distribution according to claim 1, characterized in that, The outer coating is a polyester film material that is tightly adhered to the outer surface of the pipe, serving as a base layer for subsequent friction coefficient adjustment. At locations where an increased friction coefficient is required, a biomimetic hexagonal texture 3D morphology is fabricated on the surface of the outer coating using femtosecond laser micro-nano processing methods. Multiple hexagonal morphologies are combined to form the outer coating texture of the area where friction needs to be increased. The size of the hexagons, the groove depth between the hexagons, the texture area, and the density can be adjusted as needed. If further friction is required, femtosecond laser processing is performed on the sidewalls of the already fabricated hexagonal texture to create hexagonal grooves on the sidewalls.

5. The pipe filling and forming method based on gradient friction condition distribution according to claim 1, characterized in that, At locations where a reduction in the coefficient of friction is required, femtosecond laser micro-nano processing methods are used to fabricate a pitcher plant-like structure along the flow direction of the liquid filling medium on the surface of the outer coating. The width, area, groove depth, and groove density can be adjusted as needed. Alternatively, the low-friction properties of the outer coating itself can be used to reduce the coefficient of friction. By conducting high-precision friction coefficient calibration experiments, the variation law of the friction coefficient on the material surface under different processing parameters is determined, and a quantitative mapping relationship between processing parameters and the friction coefficient is established.

6. The pipe filling and forming method based on gradient friction condition distribution according to claim 1, characterized in that, The liquid-filling forming mold includes an upper mold, a lower mold, a left punch, a right extrusion punch, and a balancing punch. The tube to be formed is placed in the forming cavity formed by the upper and lower molds. The upper mold is fixed on a slider that can move up and down, and the lower mold is fixed on the plane of the liquid-filling forming equipment. The left punch and the right extrusion punch are respectively set at both ends of the forming cavity, and the diameter of the end of the punch that extends into the mold gradually decreases. For a T-shaped pipe, the balancing punch is used to maintain the supporting pressure during forming, and its end extends into the tube cavity on the side of the forming cavity.

7. The pipe filling and forming method based on gradient friction condition distribution according to claim 6, characterized in that, A through hole is provided axially in the middle position of the left punch or the right extrusion punch. A hose is arranged in the hole. One end of the hose is connected to the external hydraulic system, and the other end is connected to the forming cavity to transport liquid medium into the forming cavity.

8. The pipe filling and forming method based on gradient friction condition distribution according to claim 1, characterized in that, The process of filling and forming the pipe with liquid includes: Inspect and clean the pipes, cut the ends, and then inspect the liquid filling mold. The pipe surface is coated with a lubricant and covered with an outer plastic film for lubrication; at locations where friction needs to be adjusted, the outer film surface is processed using femtosecond laser micro-nano processing to adjust the friction. The pipe is placed in the forming cavity of the liquid-filled forming mold and fixed; the initial expansion pressure is set according to the calculated initial yield pressure to make the pipe begin to undergo plastic deformation; the forming pressure is set according to the calculated forming pressure to ensure the transition radius and dimensional accuracy of the cross-section in the later stage of forming; the clamping force of the hydraulic press is set according to the calculated clamping force to ensure that the mold closes without gaps during the forming process.

9. The pipe filling and forming method based on gradient friction condition distribution according to claim 8, characterized in that, The initial yield pressure The calculation formula is ,in For the yield strength of the pipe, For pipe wall thickness, The diameter of the pipe; the forming pressure The calculation formula is ,in For the tensile strength of the pipe, The minimum transition radius of the pipe cross-section; the clamping force The calculation formula is ,in This represents the projected area of ​​the pipe on a horizontal plane.

10. The pipe filling and forming method based on gradient friction condition distribution according to claim 8, characterized in that, After setting the feed rate and pressure value, test pieces are used for trial forming; the actual pressure of the part and the feed rate of the feed cylinder are corrected according to the results of the first batch of test pieces forming; no pressure drop is allowed during the forming process, if it occurs, the feed rate of the left punch and the right extrusion punch should be changed.

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