Thin-walled tube narrow gap flattening forming method

Abstract

The invention relates to a thin-walled tube narrow gap flattening forming method which comprises the following steps: designing an intermediate blank and a bilateral radial circumferential constraint mold by adopting a hinge support model in structural mechanics and combining a reverse solution method; the pipe fitting is horizontally placed between the radial circumferential restraining molds on the two sides; an upper die of the double-side radial circumferential constraint die vertically moves downwards by a certain displacement and then stops, so that an intermediate blank in a specific shape is formed; a forming mold matched with the target molded surface is replaced, and narrow-gap flattening is conducted; unloading, resetting the forming die, and taking out the workpiece; and a small amount of middle sunken areas at the ends of the workpiece are cut off, and the narrow-gap thin-wall hollow structure with the ideal section is obtained. The double-side radial and circumferential constraint die is used for improving the overall structural rigidity, providing double-side radial compression and circumferential shear stress, optimizing a strain path and improving the forming limit, the problems that crushing instability and section distortion are prone to being generated during flattening are solved, a specific deformation form can be achieved through combination of multiple basic types of dies, and the machining precision is improved. And high flexibility and high precision are realized.

Description

Technical Field

[0001] The method of the present invention belongs to the field of material processing, and specifically relates to a method for forming a thin-walled tube with a narrow gap by flattening. Background Art

[0002] The statements in this section only provide background technical information related to the present disclosure, and these statements may constitute prior art. Thin-walled hollow shells, which can be used in lightweight structures, heat exchangers, protective shells, etc., have a wide range of applications in fields such as aerospace, automotive industry, high-end equipment manufacturing, and building structures. In the process of implementing the present invention, the inventors found that there are at least the following problems in the prior art.

[0003] As products gradually evolve towards features such as being slender, thin, and having an ultra-narrow gap, the existing thin-plate welded structure is prone to cracking at the weld, making it difficult to meet the increasingly stringent requirements. There is an urgent need for an integral structure; flattening of thin-walled tubes, as a performance testing method, is often used for impact energy absorption and has the potential to form an integral narrow-gap hollow structure.

[0004] However, the thin-walled tube structure has low stiffness, and local stress concentration occurs during flattening, easily resulting in buckling instability and cross-sectional distortion, reducing product quality and even causing material waste. For example:

[0005] (1) When flattening a thin-walled tube, the tube locally depresses under the contact load with the flat die line, forming a plastic hinge and gradually buckling, resulting in the inner concavity of the tube wall;

[0006] (2) When flattening a thin-walled tube, the cross-section can change from circular to dumbbell-shaped;

[0007] (3) Strain localization occurs, and the side wall is prone to cracking when the compression amount is large.

[0008] Therefore, there is an urgent need for a method that can suppress buckling instability and cross-sectional distortion during the flattening process of thin-walled tubes to obtain a high-quality integral narrow-gap hollow shell.

[0009] The prior art mainly adopts the following several solutions to solve the problems existing in the above-mentioned narrow-spacing flattening:

[0010] (1) Using media such as foam metal and rubber for filling, or winding with composite materials to form a metal matrix composite tube, which can improve the structural stiffness and the energy absorption effect during the flattening process;

[0011] (2) Using an inclined flat plate for flattening, which can obtain a unilateral pure shear, compression, and compression-shear composite deformation mode;

[0012] (3) Optimizing its impact resistance through gradient thickness design, which is generally used for SLM additive manufacturing;

[0013] (4) Using a nested design containing metal tubes, such as built-in elliptical supports, movable supports along the tube axis, and variable cross-section supports, can achieve better flattening quality;

[0014] (5) A fixed end face positioning device is used to limit the movement of the pipe along the axial direction and the rotation along the circumferential direction, so that the pipe can be accurately positioned. The clamping device drives the pressing device to always press the weld position of the pipe during the flattening process to avoid bulges or depressions on the pipe.

[0015] However, the above method also has the following disadvantages:

[0016] (1) When the thin-walled tube is flattened on a flat plate or an inclined plate, there is no mold limit, the deformation process is easy to deviate, and the deformation mode is similar, and it is still easy to collapse and cross-sectional distortion;

[0017] (2) The radial pressure is too large or unevenly distributed, and the product dimensional accuracy decreases or cannot meet the requirements;

[0018] (3) Dielectric filling or outer wall auxiliary material winding not only has complex processes and high production costs, but also leads to inharmonious deformation.

[0019] (4) The gradient thickness design requires high precision, which is difficult to achieve, and a slight deviation may cause its performance to deviate;

[0020] (5) When an elliptical tube is nested with a circular tube or a multi-layer elliptical tube, the geometric shapes of the inner and outer tubes must be strictly matched, and the nested tubes are consumed once, which will cause a waste of resources; the movable variable cross-section support along the axial direction requires relatively complex mold design and process, and has high requirements for mold accuracy and forming process, otherwise it is easy to cause uneven assembly gaps or contact stress concentration.

[0021] Among them, in order to solve the problem of excessive radial pressure or uneven distribution, such as the patent application number 202222206554.X, whose name is "A flattening forming mold", the segmented support of the pipe wall deformation process is achieved through the forming inner core and the inner core driving cylinder to ensure that the flattened pipe will not collapse or have defects such as white marks and cracks on the side walls. This method mainly relies on internal support to avoid collapse and side wall fracture, but when the product gap is small and the required compression deformation is large, the side wall will still break. Moreover, although flat molds have been used, the multi-pass segmented support process during the flattening deformation process is relatively cumbersome.

[0022] How to avoid crushing instability and cross-sectional distortion during flattening at narrow intervals while ensuring high precision is the technical problem to be solved by the present invention. Summary of the invention

[0023] In view of the above problems, the object of the present invention is to solve part of the problems in the prior art, or at least alleviate these problems.

[0024] A method for narrow-gap flattening forming of thin-walled tubes, comprising the following steps:

[0025] Place the pipe fitting between the bilateral diameter circumferential constraint molds;

[0026] The upper mold of the bilateral diameter circumferential constraint mold moves vertically downward by a specific displacement and then stops to form an intermediate blank with a specific shape;

[0027] Replace the forming mold matching the target profile and perform narrow-gap flattening;

[0028] Unload, reset the forming mold, and take out the workpiece;

[0029] Cut off a small amount of the intermediate concave area at the end of the workpiece to obtain a narrow-gap thin-walled hollow structure with an ideal cross-section.

[0030] The method for narrow-gap flattening forming of thin-walled tubes further includes adopting the "hinged support" model in structural mechanics, combining the inverse solution method to design the intermediate blank and the bilateral diameter circumferential constraint mold, and then using numerical simulation to correct the shape and size of the bilateral diameter circumferential constraint mold and the intermediate blank.

[0031] Further, adopting the "hinged support" model in structural mechanics, combining the inverse solution method to design the intermediate blank and the bilateral diameter circumferential constraint mold, and then using numerical simulation to correct the shape and size of the bilateral diameter circumferential constraint mold and the intermediate blank, including the following steps:

[0032] Divide the target workpiece to be formed along the vertical symmetry center line;

[0033] Rotate each straight-line segment part around the hinge point by a specific angle;

[0034] Keep the rotation result, and rotate the upper and lower two sections of the side arc around the arc center hinge point so that they are tangent to the straight-line segment part;

[0035] On the premise of ensuring that the cross-sectional perimeter remains unchanged, close the contour and make the transition zone smooth;

[0036] Design the pressing plate mold according to the rotated and closed profile;

[0037] Using the designed pressing plate mold, calculate the size of the pipe blank according to the principle of constant cross-sectional perimeter and carry out simulation analysis;

[0038] Combined with the simulation results, correct the rotation angle and the design of the transition zone of the closed contour to obtain the bilateral diameter circumferential constraint mold and the intermediate blank.

[0039] Further, the bilateral diameter circumferential constraint mold adopts one-step or multi-step forming to obtain an ideal intermediate blank.

[0040] Preferably, the dimensional ratio of the diameter d to the length l of the pipe fitting satisfies d / l ≤ 1, and the ratio of the wall thickness t to the diameter d is 0.01 ≤ t / d ≤ 0.1.

[0041] Preferably, the ratio of the clearance value of the workpiece to the original wall thickness of the pipe fitting is between 0 and 5.

[0042] Furthermore, the double-sided circumferential constraint die is a V-shaped cross-section die or a V-shaped cross-section arc die.

[0043] Optionally, when the workpiece is symmetrically distributed, the contact points of the double-sided circumferential constraint die with the horizontally placed pipe fitting are located on both sides of the vertical center line of the cross-section and are symmetrically distributed.

[0044] Optionally, when the workpiece is asymmetrically distributed, one side of the double-sided circumferential constraint die first contacts the wall surface of the horizontally placed pipe fitting, and the die is used to limit the position of the pipe fitting.

[0045] The present invention has the following beneficial effects:

[0046] (1) By using the method of the present invention, a limiting function can be provided for the thin-walled pipe, and the deformation process is stable;

[0047] (2) By using the method of the present invention, the double-sided circumferential constraint of the die can improve the structural stiffness of the thin-walled pipe, regulate the stress state of the pipe wall, optimize the strain path of the flattening process, and can effectively prevent the thin-walled circular pipe from suffering from crushing instability and cross-section distortion during the radial stamping process, and improve the forming limit;

[0048] (3) By using the method of the present invention, the die is simple, the production cost is low, the operation is convenient, and it has high flexibility and high precision for narrow-gap thin-walled hollow structures of different materials, shapes, and sizes;

[0049] (4) By using the method of the present invention, a narrow-gap integral thin-walled hollow structure with a regular cross-sectional shape can be obtained, which saves materials and is efficient. Description of the Drawings

[0050] Figure 1 is the flow chart of the present invention;

[0051] Figure 2 is the schematic diagram of Embodiment 1 of the present invention;

[0052] Figure 3 is the schematic diagram of Embodiment 2 of the present invention;

[0053] Figure 4 is the design flow chart of the intermediate blank and the die;

[0054] Figure 5 is the schematic diagram of the equivalent plastic strain of the intermediate blank in Embodiment 2;

[0055] Figure 6 Schematic diagram of the equivalent plastic strain of the workpiece in the second embodiment;

[0056] Figure 7 Schematic diagram of other narrow-gap thin-walled hollow shells that can be formed using this method;

[0057] Figure 8 Schematic diagram of the bilateral radial and circumferential constraint die when the workpiece of the present invention is asymmetrically distributed.

[0058] Wherein: 1 - bilateral radial and circumferential constraint die; 2 - circular tube; 3 - intermediate blank; 4 - forming die; 5 - workpiece; 6 - hinge point; 7 - arc center hinge point. Detailed implementation manners

[0059] The following further describes the present invention in conjunction with the accompanying drawings. The embodiments of the present invention are only used to illustrate the present invention rather than limit the present invention. Without departing from the technical idea of the present invention, various substitutions and changes made according to ordinary technical knowledge and conventional means in the art should be included within the scope of the present invention.

[0060] The present invention changes the traditional flat plate flattening method and adopts a special die through a two / multi-pass forming method to solve problems such as easy occurrence of buckling instability and cross-section distortion during the flattening of existing thin-walled tubes, and expands the application of the performance test method of "tube flattening" to the forming and manufacturing field, and a regular-shaped integral narrow-gap thin-walled hollow workpiece can be obtained.

[0061] As Figure 1 shown, a narrow-gap flattening forming method for a thin-walled tube includes the following steps:

[0062] Place the pipe fitting 2 between the bilateral radial and circumferential constraint dies 1;

[0063] The upper die of the bilateral radial and circumferential constraint die 1 moves vertically downward by a specific displacement and then stops to form an intermediate blank 3 with a specific shape;

[0064] Replace the forming die 4 that matches the target profile and perform narrow-gap flattening;

[0065] Unload, the forming die 4 returns to its original position, and the workpiece 5 is taken out;

[0066] Cut off a small amount of the intermediate concave area at the end of the workpiece 5 to obtain a narrow-gap thin-walled hollow structure with an ideal cross-section.

[0067] The present invention adjusts the contact points between the double-sided radial circumferential constraint die 1 and the pipe wall of the pipe fitting 2, while improving the structural stiffness of the thin-walled pipe, changing its stress distribution, and optimizing the strain path during the flattening process, thereby effectively preventing the pipe fitting 2 from experiencing crushing instability and cross-sectional distortion during the radial stamping process. By adopting this method, a narrow-gap integral thin-walled hollow structure with a regular cross-sectional shape can be obtained, saving materials and being highly efficient. And it can provide a limiting function for the pipe fitting 2, ensuring a stable deformation process.

[0068] The method for forming a thin-walled pipe with a narrow gap further includes designing the intermediate blank 3 and the double-sided radial circumferential constraint die 1 by using the "hinged support" model in structural mechanics and combining the inverse solution method, and then correcting the shape and size of the double-sided radial circumferential constraint die 1 and the intermediate blank 3 by using numerical simulation. As Figure 4 shown, it includes the following steps:

[0069] Dividing the narrow-gap thin-walled hollow shell from the vertical symmetry center line;

[0070] Rotating the four straight-line segments around the hinge point 6 by a specific angle; the specific angle can be angles such as 10, 15, 30, 45, 60, etc.;

[0071] Keeping the rotation result, rotating the upper and lower two segments of the side arc around the arc center hinge point 7 to make it tangent to the straight-line segment part;

[0072] Closing the contour while ensuring that the cross-sectional perimeter remains unchanged, with a smooth transition zone;

[0073] Designing the pressing plate die according to the rotated and closed profile;

[0074] Using the designed pressing plate die, calculating the size of the pipe blank according to the principle of unchanged cross-sectional perimeter, and carrying out simulation analysis;

[0075] Combining the simulation results, correcting the rotation angle and the design of the transition zone of the closed contour to obtain the double-sided radial circumferential constraint die 1 and the intermediate blank 3.

[0076] In the present invention, under the constraint of the double-sided radial circumferential constraint die 1, a specific intermediate blank 3 is first formed, and then flattened according to the target shape. However, the applicant found in practice that if the double-sided radial circumferential constraint die 1 is not reasonably designed, it may still lead to shape distortion or rupture. Therefore, the double-sided radial circumferential constraint die 1 needs to reasonably design its shape and size, and combine one-step or multi-step forming strategies to obtain a reasonable stress state and strain path, so as to obtain an ideal intermediate blank 3.

[0077] The stress and other numerical values of pipe fittings 2 with different sizes are different. In order to improve the accuracy while preventing the problems of crushing instability and cross-sectional distortion, the applicant has made the following restrictions on the numerical values in the method of the present invention through in-depth research and multiple experiments:

[0078] The dimensional ratio d / l of the diameter d and the length l of the pipe fitting 2 is d / l ≤ 1, such as 0.01, 0.05, 0.1, 0.5, 1, etc. The ratio of the wall thickness t to the diameter d is 0.01 ≤ t / d ≤ 0.1.

[0079] The ratio of the clearance value of the workpiece 5 to the original wall thickness of the pipe fitting 2 is between 0 and 5, such as 0, 0.05, 0.1, 1, 5, etc.

[0080] By adopting the above numerical limitations, not only the crushing instability and cross-section distortion during the narrow-spacing flattening can be avoided, but also the accuracy can be improved, making the finished product more in line with the requirements.

[0081] The shape of the bilateral diameter circumferential constraint die 1 can be adjusted according to the actual situation, such as: (inverted) serrated, arc-shaped, V-shaped or other geometric shapes, which can be symmetric or asymmetric with respect to the vertical center line of the thin-walled pipe cross-section. Such as Figure 2 As shown in FIG. 3, the bilateral diameter circumferential constraint die 1 is a V-shaped cross-section die or a V-shaped cross-section arc die.

[0082] The method of the present invention is applicable to circular pipes of different sizes, shapes and materials. Such as: thin-walled circular pipes, thick-walled circular pipes, metal circular pipes, composite material circular pipes, etc., stainless steel (SUS304), high-strength steel (30Cr13), titanium (TA1 / TA2), aluminum alloy (Al6063), etc. In addition to circular pipes, this solution can also be used to form thin-walled pipes with elliptical, rectangular, hexagonal and other cross-sections into Figure 7 The integral narrow-gap thin-walled hollow shell shown in the shape.

[0083] When the workpiece 5 is symmetrically distributed, the contact points of the bilateral diameter circumferential constraint die 1 and the horizontally placed pipe fitting 2 are located on both sides of the vertical center line of the cross-section and are symmetrically distributed.

[0084] When the workpiece 5 is asymmetrically distributed, as Figure 7 Shown in the non-axisymmetric model in FIG., the contact points of the bilateral diameter circumferential constraint die 1 and the horizontally placed pipe fitting 2 are asymmetrically distributed with respect to the vertical center line of its cross-section to ensure that the pipe fitting does not slide during the deformation process, as Figure 8 Shown.

[0085] The specific embodiments are as follows.

[0086] Embodiment 1: V-shaped cross-section die, as Figure 2 Shown

[0087] The following takes 304 stainless steel (target size φ41mm×φ40mm×60mm) as an example to illustrate the specific implementation manner of the method of the present invention.

[0088] (1) Step 1: Place the pipe fitting 2 between the V-shaped dies;

[0089] (2) Step 2: The V-shaped upper die moves downward at a speed of 0.1 mm / s and stops after moving for 80 s;

[0090] (3) Step 3: Reset the punch, take out the pipe fitting 2, replace the V-shaped die with a flat die, and place the pipe fitting 2 in the middle of the flat plate;

[0091] (4) Step 4: The flat upper die presses down vertically at a speed of 0.1 mm / s and stops after moving for 310 s. At this time, the gap value of the thin-walled hollow structure is 1 mm;

[0092] (5) Step 5: After the deformation is completed, the die is reset and the pipe fitting 2 is taken out;

[0093] (6) Step 6: Cut off the middle concave area about 3 mm wide at the end of the pipe to obtain a narrow-gap thin-walled hollow structure with an ideal "racetrack" cross-section.

[0094] Example 2: A V-shaped cross-section arc die, as Figure 3 shown

[0095] The following takes 304 stainless steel (target size φ41 mm × φ40 mm × 60 mm) as an example to illustrate the specific implementation manner of the method of the present invention.

[0096] (1) Step 1: Place the pipe fitting 2 between the V-shaped cross-section arc dies;

[0097] (2) Step 2: The V-shaped upper die moves at a speed of 0.1 mm / s and stops after moving for 100 s to obtain an intermediate blank 3; The stress of the intermediate blank 3 is as Figure 5 shown;

[0098] (3) Step 3: Reset the punch, take out the pipe fitting 2, replace the V-shaped cross-section arc die with a flat die, and place the pipe fitting 2 in the middle of the flat plate;

[0099] (4) Step 4: The flat upper die presses down vertically at a speed of 0.1 mm / s and stops after moving for 290 s. At this time, the gap value of the thin-walled hollow structure is 1 mm; The stress at this time is as Figure 6 shown;

[0100] (5) Step 5: After the deformation is completed, the die is reset and the pipe fitting 2 is taken out;

[0101] (6) Step 6: Cut off the middle concave area about 3 mm wide at the end of the pipe to obtain a narrow-gap thin-walled hollow structure with an ideal "racetrack" cross-section.

[0102] From Figure 5 、 6From the schematic diagram of equivalent plastic strain, it can be seen that during the flattening process, the deformation of the side wall of the pipe fitting 2 is relatively small. There are not only no crushing instability and cross-section distortion defects, but also the side wall does not rupture.

[0103] The present invention provides a method for flattening a thin-walled pipe with a narrow gap. By using a double-sided radial circumferential constraint die, the overall structural stiffness is increased, double-sided radial compression and circumferential shear coupling stresses are provided, and the strain path is optimized to improve the forming limit. The problems of easy crushing instability, cross-section distortion and side wall rupture during flattening are solved, and the combination configuration of various basic types of dies can achieve specific deformation forms, with high flexibility and high precision.

[0104] The above embodiments do not limit the method of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the method of the present invention, such as changing the shape of the fixed die, using other methods to disperse the concentrated stress and change the position of the plastic hinge, etc., shall be included in the protection scope of the method of the present invention.

Claims

1. A method for forming a thin-walled tube by narrow gap flattening, characterized in that: The following steps are involved: Placing the pipe fitting (2) between the double-sided radial and circumferential constraint dies (1); The upper die of the double-sided radial circumferential constraint die (1) moves vertically downward for a specific displacement and then stops, so as to form an intermediate blank (3) of a specific shape; Replacing the forming die (4) that matches the target profile to perform narrow gap flattening; Unloading, resetting the forming mold (4), and taking out the workpiece (5); A small amount of the middle concave area at the end of the component (5) is cut off to obtain a narrow gap thin-wall hollow structure with an ideal cross-section.

2. The method for forming a thin-walled tube with narrow gap flattening according to claim 1, characterized in that: The invention also includes adopting a "hinge" model in structural mechanics and combining it with an inverse method to design an intermediate blank (3) and a double-sided radial circumferential constraint mold (1), and then using numerical simulation to correct the shape and size of the double-sided radial circumferential constraint mold (1) and the intermediate blank (3).

3. The method for forming a thin-walled tube by narrow gap flattening according to claim 2, characterized in that: The "hinge" model in structural mechanics is used in combination with the inverse method to design the intermediate blank (3) and the double-sided radial circumferential constraint die (1), and then the shape and size of the double-sided radial circumferential constraint die (1) and the intermediate blank (3) are corrected by numerical simulation, including the following steps: Splitting the target part (5) to be formed from the vertical symmetry center line; Rotate each straight line segment around the hinge point (6) by a specific angle; Keeping the rotation result, the upper and lower segments of the side arc are rotated around the central hinge point (7) of the arc so that they are tangent to the straight line segment; Under the premise of ensuring that the perimeter of the cross section remains unchanged, the contour is closed and the transition zone is smooth; Design the pressing plate mold according to the rotating and closed profile; Using the designed pressing plate die, the tube size is calculated according to the principle of constant cross-section perimeter, and simulation analysis is carried out; Combined with the simulation results, the rotation angle and the transition zone design of the closed contour are modified to obtain a double-sided radial circumferential constraint die (1) and an intermediate blank (3).

4. The method for forming a thin-walled tube by narrow gap flattening according to claim 1, characterized in that: The double-sided radial and circumferential constraint die (1) is formed in one step or multiple steps to obtain an ideal intermediate blank (3).

5. The method for narrow gap flattening of thin-walled tubes according to claim 1, characterized in that: The dimension ratio of the diameter d and the length l of the pipe (2) is d / l≤1, and the ratio of the wall thickness t and the diameter d is 0.01≤t / d≤0.

1.

6. The method for forming a thin-walled tube by narrow gap flattening according to claim 1 or 5, characterized in that: The ratio of the gap value of the product (5) to the original wall thickness of the pipe (2) is between 0 and 5.

7. The method for forming a thin-walled tube by narrow gap flattening according to claim 1, characterized in that: The double-sided radial circumferential constraint mold (1) is a V-shaped cross-section mold or a V-shaped cross-section circular arc mold.

8. The method for forming a thin-walled tube by narrow gap flattening according to claim 1, characterized in that: When the workpiece (5) is symmetrically distributed, the contact points between the double-sided radial circumferential constraint mold (1) and the horizontally placed pipe (2) are located on both sides of the vertical center line of the cross section and are symmetrically distributed.

9. The method for narrow gap flattening of thin-walled tubes according to claim 1, characterized in that: When the workpiece (5) is asymmetrically distributed, the contact points between the double-sided radial circumferential constraint mold (1) and the horizontally placed pipe (2) are asymmetrically distributed, and the mold is used to limit the position of the pipe (2).

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