A method and die for bending a double-layer thin-walled pipe under cryogenic conditions

Abstract

The application discloses a double-layer thin-wall pipe bending method and die under cryogenic condition and belongs to the field of pipe bending die design, which comprises the following steps: according to the assembly condition, filling medium, inner pipe and outer pipe are configured as a pipe blank; the pipe blank is subjected to bending treatment; the outer pipe clamping die and the bending die rotate synchronously around the bending center at a set angular velocity, drive the inner and outer pipes and the filling medium to bend together, and the pressure die moves horizontally along the bending feeding direction at a set speed to help the outer pipe deform; at this time, the inner pipe clamping die, the core die and the anti-wrinkle die remain stationary during the bending forming process, and after the bending is completed, the core die moves in the opposite direction of the bending by a set distance to complete the core pulling. The application solves the problems that the elongation of the double-layer thin-wall pipe is not high under room temperature condition, wrinkles, wall thickness thinning and cross-section deformation are easily generated during the bending deformation process and the like.

Description

Technical Field

[0001] This invention relates to the field of pipe bending die design technology, specifically to a method and die for bending double-layer thin-walled pipes under cryogenic conditions. Background Technology

[0002] The unique double-wall gap structure of 6061-T4 aluminum alloy double-layer thin-walled tubes endows them with heat insulation, leak prevention, and high efficiency, making them a key pipeline component in high-end equipment fuel environmental control systems. Currently, the bending and forming of 6061-T4 aluminum alloy double-layer thin-walled tubes is generally carried out at room temperature. However, at room temperature, there is a problem with low elongation, leading to defects such as wrinkling, wall thinning, and cross-sectional deformation during bending deformation, severely restricting the large-scale application of aluminum alloy double-layer thin-walled tubes in high-end equipment. For example, in the prior art, Chinese invention patent CN108620466A discloses a CNC bending mold and forming method for double-layer tubes, which involves a structure composed of a pressure mold, anti-wrinkle mold, clamping mold, core mold, and bending mold, and is a forming scheme at room temperature. Thin-walled tubes are complex structures that are difficult to deform. During large plastic deformation, the inner side of the outer tube is subjected to compressive stress, which can easily lead to instability and wrinkling; the outer side of the outer tube is subjected to tensile stress, which can easily lead to excessive thinning and fracture. Current technologies lack methods for bending and forming double-layer thin-walled tubes under cryogenic conditions, making it impossible to simultaneously control the quality of forming during the bending process. Therefore, improving the bending and forming quality of aluminum alloy double-layer thin-walled tubes and the yield of bent pipe parts has become a critical issue that urgently needs to be addressed. Summary of the Invention

[0003] To address the shortcomings of processing double-layer thin-walled aluminum alloy tubes at room temperature, this invention proposes a bending method and mold for double-layer thin-walled tubes under cryogenic conditions, which can improve the bending forming limit of double-layer thin-walled tubes, enhance their forming quality, and increase the yield of bent products.

[0004] To achieve the above-mentioned objectives, the technical solution of the present invention is as follows: A method for bending a double-layer thin-walled tube under cryogenic conditions includes: According to the assembly conditions, the filling medium is configured with the inner and outer tubes to form a tube blank; The tube blank is bent, so that the outer tube clamping mold and the bending mold rotate synchronously around the bending center at a set angular velocity, driving the inner and outer tubes and the filling medium to bend together. The pressure mold moves horizontally along the bending feed direction at a set speed to help deform the outer tube. At this time, the inner tube clamping mold, the core mold and the anti-wrinkle mold remain stationary during the bending process. After the bending is completed, the core mold moves a set distance in the opposite direction of the bending to complete the core pulling.

[0005] Further, the process of configuring the filling medium with the inner and outer tubes into a tube blank according to the assembly conditions includes: S1. Determine the dimensions of the filling medium at low temperature based on the interference fit between the filling medium and the inner and outer tubes at the determined low temperature bending temperature. S2. Calculate the machining dimensions of the filling medium at room temperature based on the thermal expansion coefficients of polymer and aluminum alloy materials; S3. Calculate the heating temperature during the assembly of the filling medium and the inner tube, and the cooling temperature after the filling medium and the inner tube are assembled. S4. Complete the assembly of the filling medium with the inner tube after heating and expansion, and the assembly of the filling medium and the inner tube with the outer tube after cooling and shrinking, so as to achieve interference fit between the filling medium and the inner and outer tubes at low temperature.

[0006] Furthermore, in step S1, the process of determining the initial dimensions at low temperature before assembling the filling medium is as follows: Based on the interference fit between the filling medium and the inner and outer pipe dimensions When machining the inner and outer diameters of the filling medium, the inner diameter of the filling medium should be smaller than the interference fit when there is zero clearance. The outer diameter is larger than the interference when there is zero clearance. Wherein, the zero gap is the state in which the filling medium just fills the entire gap.

[0007] Furthermore, in step S2, when the filling medium is processed at room temperature, the expansion amount of the filling medium as it recovers to room temperature needs to be considered. The calculation formula is as follows: ; In the formula, The expansion of the filling medium as it recovers from the low-temperature bending temperature to room temperature; The coefficient of thermal expansion of the filling medium material; To take into account the interference The inner diameter of the filling medium; This refers to the low-temperature bending temperature. This refers to room temperature.

[0008] Furthermore, the specific process of step S4 includes: S401. The filling medium is heated to a higher temperature so that the inner diameter of the expanded filling medium can be assembled with the inner tube without scratches. The expansion amount needs to meet the calculation formula as follows: ; In the formula, This is the minimum clearance required during assembly to prevent the inner and outer tubes from rubbing against each other with the filling medium. This refers to the low-temperature bending temperature. The coefficient of thermal expansion of the filling medium material; To take into account the interference The inner diameter of the filling medium; The minimum temperature to which heating is required; S402. Place the assembled filling medium and inner tube together into a low-temperature environment control box for cooling and shrinking, so that the shrunken filling medium can be assembled with the outer tube without scratches. When the filling medium contracts upon cooling, the inner diameter is supported by the inner tube, causing its contraction to differ from that when unconstrained. Furthermore, the inner tube contracts along with the inner diameter, resulting in a total contraction of the combined outer diameter. and The calculation formula is as follows:

[0009]

[0010]

[0011]

[0012] In the formula, The outer diameter of the filling medium. The coefficient of thermal expansion of the material. The outer diameter of the inner tube.

[0013] The present invention also proposes a double-layer thin-walled tube bending die under cryogenic conditions for performing the method described above, comprising: a tube blank and a tube mold disposed in contact with the tube blank; the tube blank comprises: an inner tube, an outer tube, and a filling medium disposed between the inner and outer tubes; the tube mold comprises: a core mold disposed in the inner tube, an inner tube clamping mold disposed in contact with the inner tube, an outer tube clamping mold disposed in contact with the outer tube, a pressure mold disposed in contact with the end of the tube blank, and an anti-wrinkle mold and a bending mold disposed corresponding to the pressure mold.

[0014] Preferably, the bending mold, the anti-wrinkle mold, the inner tube clamping mold, the outer tube clamping mold, the pressure mold, and the core mold are all provided with cooling holes.

[0015] Preferably, the core mold, the inner wall of the inner tube, and the anti-wrinkle mold are all coated with a lubricant; the lubricant is a liquid PTFE lubricant with a minimum operating temperature of -180℃.

[0016] Preferably, the filling medium is a PE polymer filling medium.

[0017] In summary, the present invention has the following advantages: 1. This invention significantly improves the forming quality of double-layer thin-walled tubes during bending by introducing a cryogenic (low temperature) forming environment (such as -120℃), thus improving problems such as wrinkling, thinning, and cross-sectional deformation.

[0018] 2. This invention proposes to use high-molecular PE material as the filling medium. Based on its thermal expansion and contraction characteristics, an innovative method of "heating up to assemble the inner tube and cooling down to assemble the outer tube" is designed to ensure that the filling medium provides necessary support for the gap between the two walls at low temperatures. 3. All components of the mold of the present invention are provided with cooling holes to ensure the stable maintenance of the cryogenic environment; at the same time, the core mold, inner wall and anti-wrinkle mold are coated with low-temperature applicable PTFE lubricant to further optimize friction conditions and forming consistency. Attached Figure Description

[0019] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments, wherein: Figure 1 This is a schematic diagram of the design method steps of the present invention; Figure 2 This is a schematic diagram illustrating the assembly principle of the filling medium with the inner tube after heating and expansion in Example 1. Figure 3 This is a schematic diagram of the assembly principle of the filling medium after cooling and shrinkage with the outer tube in Example 1; Figure 4 This is a graph showing the true stress-strain curves of the PE material at different temperatures in Example 1; Figure 5 This is a schematic diagram of the tube mold and tube blank composition in Example 2; Figure 6 This is a schematic diagram of the forming effect of a double-layer thin-walled tube under experimental conditions; Figure 7 This is a schematic diagram illustrating the forming effect of a double-layer thin-walled tube under simulated conditions. Figure 8 This is a comparison chart of the inner tube wall thickness reduction rate under experimental and simulated conditions in Comparative Example 1. Figure 9 This is a comparison chart of the outer tube wall thickness reduction rate under experimental and simulated conditions in Comparative Example 1. Figure 10 This is a comparison chart of the deformation rate of the inner tube wall thickness section under experimental and simulated conditions in Example 1. Figure 11 This is a comparison chart of the deformation rate of the inner tube wall thickness section under experimental and simulated conditions in Example 1. Figure 12 This is a schematic diagram of the wrinkling distribution of the inner tube at normal and low temperatures in Comparative Example 2. Figure 13 This is a schematic diagram of the wrinkling distribution of the outer tube at normal and low temperatures in Comparative Example 2; Figure 14 This is a schematic diagram of the inner tube wall thickness reduction rate at normal and low temperatures in Comparative Example 2. Figure 15This is a schematic diagram of the thinning rate of the outer tube wall at normal and low temperatures in Comparative Example 2. Figure 16 This is a schematic diagram of the deformation rate of the inner tube cross-section at normal and low temperatures in Comparative Example 2. Figure 17 This is a schematic diagram of the deformation rate of the outer tube cross section at normal and low temperatures in Comparative Example 2.

[0020] In the picture: 1. Filling medium; 2. Inner tube; 3. Outer tube; 4. Inner tube clamping mold; 5. Outer tube clamping mold; 6. Pressure mold; 7. Double-layer thin-walled tube; 8. Anti-wrinkle mold; 9. Core mold; 10. Bending mold. Detailed Implementation

[0021] To more clearly illustrate the present invention, the following description, in conjunction with preferred embodiments and accompanying drawings, further clarifies the invention. Those skilled in the art should understand that the specific description below is illustrative rather than restrictive and should not be construed as limiting the scope of protection of the present invention.

[0022] Example 1 This embodiment provides a method for bending double-layer thin-walled tubes under cryogenic conditions, such as... Figures 1-17 As shown, it includes: Step 1: According to the assembly conditions, assemble the inner tube 2, outer tube 3 and filling medium 1 into a whole. Heat the filling medium 1 to complete the assembly of the filling medium 1 and the inner tube 2. Cool the assembled filling medium 1 together with the inner tube 2 to complete the assembly with the outer tube 3, and obtain the configured tube blank. Among them, the filling medium 1 is a PE polymer filling medium. The coefficient of thermal expansion of the polymer filling medium is larger than that of metal. The principle of thermal expansion and contraction is used to assemble the inner and outer tubes 3 and the filling medium 1.

[0023] Step 2: The tube blank is bent, so that the outer tube clamping mold 5 and the bending mold 10 rotate synchronously around the bending center at a set angular velocity, driving the inner and outer tubes and the filling medium 1 to bend together. The pressure mold 6 moves horizontally along the bending feed direction at a set speed to help the outer tube 3 deform. At this time, the inner tube clamping mold 4, the core mold 9 and the anti-wrinkle mold 8 remain stationary during the bending process. After the bending is completed, the core mold 9 moves a set distance in the opposite direction of the bending to complete the core pulling.

[0024] Furthermore, the specific process of configuring the filling medium with the inner and outer tubes into a tube blank according to the assembly conditions is as follows: S1. Determine the dimensions of the filling medium 1 at low temperature based on the interference fit between the filling medium 1 and the inner and outer tubes 3 at the determined low temperature bending temperature. S2. Calculate the machining dimensions of filling medium 1 at room temperature based on the thermal expansion coefficients of polymer and aluminum alloy materials; S3. Calculate the heating temperature when filling medium 1 is assembled with inner tube 2 and the cooling temperature after filling medium 1 is assembled with inner tube 2. S4. Complete the assembly of the filling medium 1 with the inner tube 2 after heating and expansion, and the assembly of the filling medium 1 and the inner tube 2 with the outer tube 3 after cooling and shrinking, so as to achieve interference fit assembly of the filling medium 1 and the inner and outer tubes 3 at low temperature.

[0025] Furthermore, in step S1, the process of determining the initial dimensions of the filling medium 1 at low temperature before assembly is as follows: Based on the interference fit between the filling medium 1 and the dimensions of the inner and outer tubes When machining the inner and outer diameters of the filling medium, the inner diameter of the filling medium should be smaller than the interference fit when there is zero clearance. The outer diameter is larger than the interference when there is zero clearance. ; where zero gap is the state when the filling medium just fills the entire gap.

[0026] Furthermore, in step S2, the machining of the filling medium 1 is performed at room temperature, and the expansion of the filling medium 1 as it recovers to room temperature needs to be considered. The calculation formula is as follows: ; In the formula, The expansion of filling medium 1 as it recovers from the low-temperature bending temperature to room temperature; The coefficient of thermal expansion of the filling medium 1 material; To take into account the interference The inner diameter of the filling medium 1; This refers to the low-temperature bending temperature. This refers to room temperature.

[0027] As one specific implementation method, the specific process in step S4 is as follows: S401. The filling medium 1 is heated to a higher temperature so that the inner diameter of the expanded filling medium 1 can be assembled with the inner tube 2 without scratches. The expansion amount needs to meet the calculation formula as follows: ; In the formula, To avoid mutual rubbing between the inner and outer tubes 3 and the filling medium 1 during assembly, a clearance fit with the same nominal diameter is usually used. 7 / 6. Maximum gap.

[0028] The minimum temperature to which heating is required The calculation formula is as follows: ; After the inner tube 2 and the filling medium 1 are assembled, they are placed in a room temperature environment to allow the inner tube 2 and the filling medium 1 to gradually return to room temperature, thus completing the interference fit between the filling medium 1 and the inner tube 2.

[0029] S402. Place the assembled filling medium 1 and inner tube 2 together into a low-temperature environment control box for cooling and shrinking, so that the shrunken filling medium 1 can be assembled with the outer tube 3 without scratches. When filling medium 1 contracts, its inner diameter is supported by inner tube 2, causing its contraction amount to differ from that when unrestrained. Furthermore, inner tube 2 contracts along with the filling medium. The contraction of the combined outer diameter consists of two parts, calculated as follows:

[0030]

[0031]

[0032]

[0033] In the formula, The outer diameter of the filling medium. The coefficient of thermal expansion of the material. The outer diameter of the inner tube.

[0034] Figure 4 This is a true stress-strain curve of PE material at different temperatures in this embodiment. Polyethylene (PE) exhibits a significant temperature dependence. Although its plasticity decreases at a low temperature of -120℃, it can still achieve the elongation required for the double-walled gap tube to bend to 90°. Therefore, polyethylene (PE) is considered a qualified low-temperature filling medium for double-walled gap tubes.

[0035] Example 2 This embodiment proposes a double-layer thin-walled tube bending die under cryogenic conditions, applicable to the bending method in Embodiment 1. For example... Figure 5 As shown, the bending die includes a tube blank and a die that contacts the tube blank. The tube blank includes an inner tube 2, an outer tube 3, and a filling medium 1 disposed between the inner and outer tubes. The die includes a core mold 9, an inner tube clamping mold 4, a pressure mold 6, an anti-wrinkle mold 8, and a bending mold 10. The core mold 9 is disposed in the inner tube 2. The inner tube clamping mold 4 contacts the inner tube 2, the outer tube clamping mold 5 contacts the outer tube 3, and the pressure mold 6 contacts the end of the tube blank. The anti-wrinkle mold 8 is located between the bending mold 10 and the tube blank and is disposed corresponding to the pressure mold 6.

[0036] Preferably, the bending mold 10, the anti-wrinkle mold 8, the inner tube clamping mold 4, the outer tube clamping mold 5, the pressure mold 6, and the core mold 9 are all provided with cooling holes.

[0037] The core mold 9, the inner wall of the inner tube 2, and the anti-wrinkle mold 8 are all coated with lubricant. Preferably, the lubricant is liquid PTFE lubricant with a minimum operating temperature of -180℃.

[0038] Preferably, the filling medium is a PE polymer filling medium.

[0039] Comparative Example 1: like Figure 6 The diagram shows the forming effect of a double-layer thin-walled tube under experimental conditions. Under these conditions, the mold material was 35CrMo alloy structural steel, known for its high strength, good toughness, and low-temperature resistance; the low-temperature bending temperature was -120 ℃; the interference fit between the inner tube 2 and the filling medium 1 was 0.10 mm, and the interference fit between the outer tube 3 and the filling medium 1 was 0.05 mm; the bending angle was 90°; the bending angular velocity was 45.8 rad / s; the bending radius was 95.25 mm; the boosting speed was 76.8 mm / s; the gap between the inner tube 2 and the core mold 9 was 0.3 mm; the gap between the outer tube 3 and the anti-wrinkle mold 8 was 0.2 mm; the gap between the inner / outer tube 3 and other molds was 0 mm; the friction condition between the inner / outer tube 3 and the clamping mold was increased by adding sandpaper; the friction condition between the outer tube 3 and the pressure mold 6 was dry friction; the friction condition between the outer tube 3 and the bending mold 10 was dry friction; the friction condition between the inner / outer tube 3 and other molds was achieved using PTFE lubricant; and the friction between the inner / outer tube 3 and the filling medium 1 was dry friction.

[0040] like Figure 7 The diagram shows the forming effect of a double-layer thin-walled tube under simulated conditions. Under these conditions, the mold material is 35CrMo alloy structural steel, which is high-strength, tough, and resistant to low temperatures; the low-temperature bending temperature is -120 ℃; the interference fit between the inner tube 2 and the filling medium 1 is 0.10 mm, and the interference fit between the outer tube 3 and the filling medium 1 is 0.05 mm; the bending angle is 90°; the bending angular velocity is 45.8 rad / s; the bending radius is 95.25 mm; the boosting speed is 76.8 mm / s; the gap between the inner tube 2 and the core mold 9 is 0.3 mm; the gap between the outer tube 3 and the anti-wrinkle mold 8 is 0.2 mm; the gap between the inner / outer tubes and other molds is 0 mm; the friction coefficient between the inner / outer tubes 3 and the clamping mold is selected as rough; the friction coefficient between the outer tube 3 and the pressure mold 6 is 0.25; the friction coefficient between the outer tube 3 and the bending mold 10 is 0.1; the friction coefficient between the inner / outer tubes and other molds is 0.05; and the friction coefficient between the inner / outer tubes and the filling medium 1 is related to the normal pressure.

[0041] The results showed that both the experimental and simulated bent pipe parts were well formed, and no instability or wrinkling occurred in the entire bending deformation section.

[0042] The wall thickness reduction rates of inner tube 2 and outer tube 3 under experimental and simulated conditions are as follows: Figure 8 and Figure 9As shown. For inner tube 2, the difference in wall thickness reduction rate between experimental and simulated conditions is greatest at 80°, with a maximum relative error of 19.6% and an average relative error of 9.4%; for outer tube 3, the difference in wall thickness reduction rate between experimental and simulated conditions is greatest at 70°, with a maximum relative error of 10.7% and an average relative error of 7.1%.

[0043] The deformation rates of the inner tube 2 and outer tube 3 sections under experimental and simulation conditions are respectively as follows: Figure 10 and Figure 11 As shown, the distribution patterns of cross-sectional deformation rates of inner tube 2 under simulated and experimental conditions are very similar. In the bending deformation section, the relative error between the experiment and simulation is the largest at the 90° section, with a maximum error of 12.6% and an average error of 6.5%. The distribution patterns of cross-sectional deformation rates of outer tube 3 under simulated and experimental conditions are also roughly the same, with the maximum relative error occurring at the 60° section, but the maximum relative error does not exceed 13%, and the average error does not exceed 7%.

[0044] By comparing the simulation results and experimental results of the wall thickness reduction rate and cross-sectional deformation rate of the double-layer thin-walled tube, the finite element model of the double-layer thin-walled tube bending is accurate.

[0045] Comparative Example 2: like Figure 12 and Figure 13 The figures show the wrinkling distribution of the inner and outer tubes of the double-walled thin-walled tube under normal and low temperature conditions, respectively. For the inner tube 2 of the double-walled thin-walled tube, the wrinkling height at normal temperature is much greater than that at low temperature. For the outer tube 3 of the double-walled thin-walled tube, the wrinkling height distribution patterns at normal and low temperatures are very similar, but the wrinkling height at normal temperature is slightly higher than that at low temperature.

[0046] Therefore, it can be seen that for the wrinkling analysis of double-layer thin-walled tubes, the forming quality is better at low temperatures.

[0047] like Figure 14 and Figure 15 The diagrams shown illustrate the wall thickness reduction rates of the inner and outer tubes of the double-layer thin-walled tube under bending conditions at room temperature and low temperature. In the main bending deformation section, the wall thickness reduction rate of the inner tube 2 is slightly lower than that at room temperature. However, at the beginning and end of the bending deformation, the wall thickness reduction rate of the inner tube 2 is higher than that at room temperature. For the outer tube 3, the overall distribution pattern of the wall thickness reduction rate at room temperature is similar to that at low temperature, but it is much higher than that at low temperature.

[0048] Therefore, it can be seen that for the wall thickness reduction analysis of double-layer thin-walled tubes, the forming quality is better at low temperatures.

[0049] like Figure 16 and Figure 17 The figures show schematic diagrams illustrating the cross-sectional deformation rates of the inner and outer tubes of a double-layer, double-walled thin-walled tube formed by bending at room temperature and low temperature. For both the inner and outer tubes of the double-walled thin-walled tube, the overall distribution of cross-sectional deformation rates at room temperature is very similar to that at low temperature, but it is higher than that at low temperatures.

[0050] Therefore, it can be seen that for the cross-sectional deformation rate analysis of double-layer thin-walled tubes, the forming quality is better at low temperatures.

[0051] The methods and molds proposed in the embodiments and comparative examples of this invention have the following beneficial effects: This invention solves the problems of low elongation of double-layer thin-walled tubes at room temperature and easy wrinkling, wall thickness reduction and cross-sectional deformation during bending deformation by introducing low temperature conditions and selecting a suitable filling medium under low temperature conditions. It improves the bending forming limit of double-layer thin-walled tubes, as well as their forming quality and bending yield.

[0052] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications or equivalent changes made to the above embodiments based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims

1. A method for bending a double-layer thin-walled tube under cryogenic conditions, characterized in that, include: According to the assembly conditions, the filling medium is configured with the inner and outer tubes to form a tube blank; The tube blank is bent, so that the outer tube clamping mold and the bending mold rotate synchronously around the bending center at a set angular velocity, driving the inner and outer tubes and the filling medium to bend together. The pressure mold moves horizontally along the bending feed direction at a set speed to help deform the outer tube. At this time, the inner tube clamping mold, the core mold and the anti-wrinkle mold remain stationary during the bending process. After the bending is completed, the core mold moves a set distance in the opposite direction of the bending to complete the core pulling.

2. The method for bending a double-layer thin-walled tube under cryogenic conditions as described in claim 1, characterized in that, The process of configuring the filling medium with the inner and outer tubes into a tube blank according to the assembly conditions includes: S1. Determine the dimensions of the filling medium at low temperature based on the interference fit between the filling medium and the inner and outer tubes at the determined low temperature bending temperature. S2. Calculate the machining dimensions of the filling medium at room temperature based on the thermal expansion coefficients of polymer and aluminum alloy materials; S3. Calculate the heating temperature during the assembly of the filling medium and the inner tube, and the cooling temperature after the filling medium and the inner tube are assembled. S4. Complete the assembly of the filling medium with the inner tube after heating and expansion, and the assembly of the filling medium and the inner tube with the outer tube after cooling and shrinking, so as to achieve interference fit between the filling medium and the inner and outer tubes at low temperature.

3. The method for bending a double-layer thin-walled tube under cryogenic conditions as described in claim 2, characterized in that, In step S1, the process of determining the initial dimensions at low temperature before assembling the filling medium is as follows: Based on the interference fit between the filling medium and the inner and outer pipe dimensions When machining the inner and outer diameters of the filling medium, the inner diameter of the filling medium should be smaller than the interference fit when there is zero clearance. The outer diameter is larger than the interference when there is zero clearance. Wherein, the zero gap is the state in which the filling medium just fills the entire gap.

4. The method for bending a double-layer thin-walled tube under cryogenic conditions as described in claim 2, characterized in that, In step S2, when the filling medium is processed at room temperature, the expansion amount of the filling medium as it recovers to room temperature needs to be considered. The calculation formula is as follows: ； In the formula, The expansion of the filling medium as it recovers from the low-temperature bending temperature to room temperature; The coefficient of thermal expansion of the filling medium material; To take into account the interference The inner diameter of the filling medium; This refers to the low-temperature bending temperature. This refers to room temperature.

5. The method for bending a double-layer thin-walled tube under cryogenic conditions as described in claim 2, characterized in that, The specific process of step S4 includes: S401. The filling medium is heated to a higher temperature so that the inner diameter of the expanded filling medium can be assembled with the inner tube without scratches. The expansion amount needs to meet the calculation formula as follows: ； In the formula, This is the minimum clearance required during assembly to prevent the inner and outer tubes from rubbing against each other with the filling medium. This refers to the low-temperature bending temperature. The coefficient of thermal expansion of the filling medium material; To take into account the interference The inner diameter of the filling medium; The minimum temperature to which heating is required; S402. Place the assembled filling medium and inner tube together into a low-temperature environment control box for cooling and shrinking, so that the shrunken filling medium can be assembled with the outer tube without scratches. When the filling medium contracts upon cooling, the inner diameter is supported by the inner tube, causing its contraction to differ from that when unconstrained. Furthermore, the inner tube contracts along with the inner diameter, resulting in a total contraction of the combined outer diameter. and The calculation formula is as follows: In the formula, The outer diameter of the filling medium. The coefficient of thermal expansion of the material. The outer diameter of the inner tube.

6. A double-layer thin-walled tube bending die under cryogenic conditions, used to perform the method as described in any one of claims 1-5, characterized in that, include: A tube blank and a tube mold that contacts the tube blank; the tube blank includes: an inner tube, an outer tube, and a filling medium disposed between the inner and outer tubes; the tube mold includes: a core mold disposed in the inner tube, an inner tube clamping mold disposed in contact with the inner tube, an outer tube clamping mold disposed in contact with the outer tube, a pressure mold disposed in contact with the end of the tube blank, and an anti-wrinkle mold and a bending mold disposed corresponding to the pressure mold.

7. A double-layer thin-walled tube bending die under cryogenic conditions according to claim 6, characterized in that, The bending mold, the anti-wrinkle mold, the inner tube clamping mold, the outer tube clamping mold, the pressure mold, and the core mold are all provided with cooling holes.

8. A double-layer thin-walled tube bending die under cryogenic conditions according to claim 6, characterized in that, The core mold, the inner wall of the inner tube, and the anti-wrinkle mold are all coated with lubricant; the lubricant is liquid PTFE lubricant with a minimum operating temperature of -180℃.

9. A double-layer thin-walled tube bending die under cryogenic conditions according to claim 6, characterized in that, The filling medium is a PE polymer filling medium.

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