A bellows forming springback control method and die based on stress regulation

Abstract

The present invention provides a bellows forming springback control method and mold based on stress regulation, relating to the field of metal processing technology. The method comprises obtaining an expression for the circumferential strain increment based on the principle of material bending deformation; simultaneously establishing an expression for the plastic strain increment, a plastic stress differential equation, and an expression for the circumferential strain increment to obtain an expression for the relationship between the circumferential stress and the axial stress; and, based on the expression for the relationship between the circumferential stress and the axial stress, obtaining a relationship curve with the circumferential compression strain as the independent variable and the ratio of the axial stress to the stress before compression as the dependent variable. After the tube is bent and deformed, the trough can be moved radially toward the axis of the tube along the radial direction of the tube to achieve radial contraction of the trough, thereby increasing the circumferential stress of the trough and reducing the axial stress difference between the inner and outer surfaces, thereby reducing the axial stress of the bellows and effectively controlling the axial springback of the bellows.

Description

Technical Field

[0001] The present invention relates to the technical field of metal processing, and in particular to a bellows forming springback control method and die based on stress regulation. Background Art

[0002] Due to their excellent flexibility, energy absorption, and motion compensation properties, bellows are often used in pipe connections and engine damping structures, enjoying widespread application in the aerospace, automotive, chemical, and oil and gas industries. Bellows are typically made from materials such as stainless steel, aluminum alloys, and high-temperature alloys. Titanium alloy, with its high specific strength, corrosion resistance, high service temperature, fatigue resistance, and excellent low-temperature toughness, is an ideal material for bellows under extreme service conditions.

[0003] The hydraulic forming of corrugated pipes is the most mainstream method of forming corrugated pipes. Its advantages lie in its low economic and time costs. The general process is as follows: the two ends of the linear tube blank in the axial direction are fixed by the movable die and the fixed die of the forming equipment respectively. At the same time, multiple intermediate dies are set between the movable die and the fixed die. The multiple intermediate dies are provided with trough forming protrusions and peak forming arc surfaces symmetrical about the trough forming protrusions. The movable die and the fixed die are respectively provided with the same peak forming arc surfaces on one end facing the intermediate die. During preparation, the trough forming protrusions on the intermediate die are pressed against the outer wall of the tube blank. Then, the tube blank is pressurized from the inside of the tube blank, and the movement of the movable die toward the fixed die makes the adjacent peak forming arc surfaces form the peaks of the corrugated pipe, and the trough forming protrusions form the troughs of the corrugated pipe.

[0004] However, during the forming process of the troughs and peaks, the outer wall of the tube blank undergoes elastic bending deformation. After the bellows are prepared, the troughs and peaks will elastically rebound along the axial direction of the tube blank, resulting in a relatively large forming error of the bellows, affecting the forming quality of the bellows. Summary of the Invention

[0005] The problem solved by the present invention is how to reduce the springback amount of the corrugated pipe forming so as to improve the forming quality of the corrugated pipe.

[0006] To solve the above problems, the present invention provides a bellows forming springback control method based on stress regulation, comprising:

[0007] According to the bending deformation process of the tube blank, a mold profile is constructed, and the curvature of the mold profile is set to be constant so that the trough of the mold profile moves radially toward the axis of the tube blank;

[0008] According to the material bending deformation principle, the difference between the axial strain of the inner surface of the trough of the mold surface and the axial strain of the neutral layer, and the difference between the hoop strain of the inner surface and the hoop strain of the neutral layer during radial movement are analyzed to obtain the plastic strain increment expression;

[0009] According to the material plastic deformation yield condition expression, the plastic stress differential equation is obtained;

[0010] According to the material bending deformation principle, the expression of hoop strain increment is obtained;

[0011] By combining the expression of plastic strain increment, plastic stress differential equation and hoop strain increment, the expression of the relationship between hoop stress and axial stress is obtained. Based on the expression of the relationship between hoop stress and axial stress, a relationship curve is obtained with the hoop compression strain as the independent variable and the ratio of axial stress to the value before compression as the dependent variable.

[0012] Optionally, the step of causing the trough of the mold profile to move radially toward the axis of the tube blank comprises:

[0013] The peak diameter of the desired bellows is used as the peak diameter of the intermediate mold; and the difference between the peak diameter of the bellows and the inner diameter of the trough of the bellows is used as the first difference;

[0014] Taking the difference between the diameter of the peak forming structure of the intermediate mold and the diameter of the trough forming structure of the intermediate mold as a second difference, such that the second difference is smaller than the first difference;

[0015] Fixing one axial end of the tube blank by a fixed die; fixing the other axial end of the tube blank by a movable die; and sleeve the intermediate die on the portion of the tube blank located between the fixed die and the movable die;

[0016] The tube blank is subjected to bulging and drum wave operations, and the tube blank is separated from the trough forming structure of the intermediate die by the push wave movement of the movable die toward the fixed die.

[0017] Optionally, it is characterized in that the bellows forming springback control method based on stress regulation further includes:

[0018] The formed tube blank is subjected to pressure relief and unloading operations.

[0019] Optionally, the fixed mold is fixed on a frame of the forming equipment, and the movable mold moves toward the fixed mold through a hydraulic drive system.

[0020] Optionally, the bellows forming springback control method based on stress regulation further includes:

[0021] Check the axial springback of the tube blank after forming. If the axial springback of the tube blank after forming is greater than the actual required springback error range, reduce the second difference.

[0022] Compared with the prior art, the bellows forming springback control method based on stress regulation of the present invention constructs a mold surface according to the bending deformation process of the tube blank, sets the curvature of the mold surface to be constant, and makes the trough of the mold surface move radially toward the axis of the tube blank; according to the bending deformation principle of the material, analyzes the difference between the axial strain of the inner surface of the trough of the mold surface and the axial strain of the neutral layer, and the difference between the circumferential strain of the inner surface and the circumferential strain of the neutral layer during the radial movement to obtain the plastic strain increment expression; according to the plastic deformation yield condition expression of the material, obtains the plastic stress differential equation; according to the bending deformation principle of the material, the plastic stress differential equation is obtained. , obtain the expression of the circumferential strain increment; jointly combine the expression of the plastic strain increment, the plastic stress differential equation and the circumferential strain increment expression to obtain the expression of the relationship between the circumferential stress and the axial stress, and according to the expression of the relationship between the circumferential stress and the axial stress, obtain a relationship curve with the circumferential compression strain as the independent variable and the ratio of the axial stress to the stress before compression as the dependent variable. After the tube billet is bent and deformed, the trough can be moved along the radial direction of the tube billet toward the axis of the tube billet to achieve radial contraction of the trough, thereby increasing the circumferential stress of the trough, reducing the axial stress difference between the inner and outer surfaces, and then reducing the axial stress of the bellows, thereby effectively controlling the axial rebound of the bellows.

[0023] On the other hand, the present invention also provides a mold, based on the corrugated tube forming rebound control method based on stress regulation as described above, including a movable mold, an intermediate mold and a fixed mold, all of which are arranged in a ring shape, the intermediate mold is located between the movable mold and the fixed mold, the movable mold and the fixed mold are respectively used to be fixedly connected to the two ends of the tube blank in the axial direction, the movable mold is used to be moved toward the fixed mold, and the movable mold and the fixed mold are both provided with a first arc surface structure at one end facing the intermediate mold, the intermediate mold is provided with an arc-shaped protrusion and a second arc surface structure located on both sides of the arc-shaped protrusion, the first arc surface structure and the second arc surface structure cooperate to form a wave peak forming groove, or, two adjacent second arc surface structures on different intermediate molds cooperate to form a wave peak forming groove; the arc-shaped protrusion is used to form the trough of the corrugated tube; the inner diameter of the intermediate mold is larger than the inner diameter of the movable mold and the fixed mold.

[0024] Optionally, the intermediate mold is configured to be movable along the radial direction of the tube blank.

[0025] Optionally, there are multiple intermediate molds, and the multiple intermediate molds are used to move synchronously along the radial direction of the tube blank.

[0026] Optionally, the number of the intermediate molds is N-1, where N is the number of waves of the bellows actually required.

[0027] Optionally, the movable mold is configured to move toward the fixed mold via a hydraulic drive structure.

[0028] Compared with the prior art, the mold of the present invention utilizes the setting that the diameter of the trough on the intermediate mold is larger than the diameter of the aperture surrounded by the movable mold. After the peak is formed, the material in the trough area can shrink and deform, so that the material enters the plastic deformation state again. In this way, after the bellows is prepared, the axial rebound of the trough can be reduced, and the axial rebound of the bellows can be effectively controlled to improve the forming quality of the bellows. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 1 is a flowchart of a method for forming a titanium alloy bellows in an embodiment of the present invention;

[0030] Figure 2 Schematic diagram of the structure in which the movable die and the fixed die are respectively connected to the two ends of the tube blank in the axial direction in an embodiment of the present invention;

[0031] Figure 3 Schematic diagram of the structure of the tube blank during the bulging process in an embodiment of the present invention;

[0032] Figure 4 Schematic diagram of the structure of the tube blank during the drum wave process in an embodiment of the present invention;

[0033] Figure 5 Schematic diagram of the structure of the tube blank during the wave pushing process in an embodiment of the present invention;

[0034] Figure 6 Schematic diagram of the structure of the tube blank during the diameter reduction process in an embodiment of the present invention;

[0035] Figure 7 Schematic diagram of the structure of the tube blank during the pressure relief and unloading operations in an embodiment of the present invention;

[0036] Figure 8 Schematic diagram of displacement of a movable mold in an embodiment of the present invention;

[0037] Figure 9 is a line graph showing changes in internal pressure of a tube blank in an embodiment of the present invention;

[0038] Figure 10 is a line graph of the springback change of the tube blank in the embodiment of the present invention;

[0039] Figure 11 This is a principle diagram of the stress of the tube blank after processing in an embodiment of the present invention;

[0040] Figure 12 A schematic diagram illustrating the spacing between adjacent floating molds in an embodiment of the present invention;

[0041] Figure 13 Schematic diagram of the structure of the floating mold in an embodiment of the present invention;

[0042] Figure 14 3 is a curve diagram showing the change of the annular shrinkage strain and the ratio of the axial stress to the stress before compression in an embodiment of the present invention.

[0043] Description of reference numerals:

[0044] 1-movable mold; 2-floating mold; 21-arc-shaped protrusion; 3-fixed mold; 4-tube blank. DETAILED DESCRIPTION

[0045] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0046] In the accompanying drawings, the Z-axis represents a vertical position, and the positive direction of the Z-axis (i.e., the direction indicated by the arrow on the Z-axis) represents the upper side, and the negative direction of the Z-axis (i.e., the direction opposite to the positive direction of the Z-axis) represents the lower side. In the accompanying drawings, the X-axis represents a horizontal position, and the positive direction of the X-axis (i.e., the direction indicated by the arrow on the X-axis) represents the right side, and the negative direction of the X-axis (i.e., the direction opposite to the positive direction of the X-axis) represents the left side. It should also be noted that the aforementioned Z-axis and X-axis are merely for the purpose of facilitating the description of the present invention and simplifying the description, and are not intended to indicate or imply that the device or component referred to must have a specific orientation, be constructed, or operate in a specific orientation. Therefore, they should not be construed as limiting the present invention.

[0047] It should be noted that the terms "first," "second," and the like in the description and claims of the present invention and the accompanying drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or precedence. It should be understood that the terms used in this manner are interchangeable where appropriate, such that the embodiments of the present invention described herein can be practiced in an order other than that illustrated or described herein.

[0048] Due to their excellent flexibility, energy absorption, and motion compensation properties, bellows are often used in pipe connections and engine damping structures, enjoying widespread application in the aerospace, automotive, chemical, and oil and gas industries. Bellows are typically made from materials such as stainless steel, aluminum alloys, and high-temperature alloys. Titanium alloy, with its high specific strength, corrosion resistance, high service temperature, fatigue resistance, and excellent low-temperature toughness, is an ideal material for bellows under extreme service conditions.

[0049] The hydraulic forming of corrugated pipes is the most mainstream method of forming corrugated pipes. Its advantages lie in its low economic and time costs. The general process is as follows: the two ends of the linear tube blank in the axial direction are fixed by the movable die and the fixed die of the forming equipment respectively. At the same time, multiple intermediate dies are set between the movable die and the fixed die. The multiple intermediate dies are provided with trough forming protrusions and peak forming arc surfaces symmetrical about the trough forming protrusions. The movable die and the fixed die are respectively provided with the same peak forming arc surfaces on one end facing the intermediate die. During preparation, the trough forming protrusions on the intermediate die are pressed against the outer wall of the tube blank. Then, the tube blank is pressurized from the inside of the tube blank, and the adjacent peak forming arc surfaces form the peaks of the corrugated pipe and the trough forming protrusions form the troughs of the corrugated pipe by relying on the movement of the movable die toward the fixed die.

[0050] However, during the formation of the troughs and crests, the outer wall of the tube blank undergoes elastic bending deformation. After the bellows are manufactured, the troughs and crests will elastically rebound along the axial direction of the tube blank, resulting in relatively large forming errors and affecting the forming quality of the bellows. To address the above problems, the present invention provides a bellows forming method.

[0051] Combine Figure 1 and Figure 14 As shown, the present invention provides a bellows forming springback control method based on stress regulation, comprising the following steps:

[0052] Step S100: constructing a mold profile according to the bending deformation process of the tube blank 4, setting the curvature of the mold profile constant, and making the trough of the mold profile move radially toward the axis of the tube blank 4;

[0053] Step S200: Based on the material bending deformation principle, the difference between the axial strain of the inner surface of the trough of the mold surface and the axial strain of the neutral layer, and the difference between the hoop strain of the inner surface and the hoop strain of the neutral layer during radial movement are analyzed to obtain an expression for the plastic strain increment;

[0054] Step S300: Obtaining a plastic stress differential equation based on the material plastic deformation yield condition expression;

[0055] Step S400: obtaining an expression for the hoop strain increment based on the material bending deformation principle;

[0056] Step S500: The plastic strain increment expression, the plastic stress differential equation, and the hoop strain increment expression are combined to obtain the relationship expression between the hoop stress and the axial stress. Based on the relationship expression between the hoop stress and the axial stress, a relationship curve is obtained with the hoop compressive strain as the independent variable and the ratio of the axial stress to the value before compression as the dependent variable.

[0057] Specifically, the inner surface of the trough refers to the surface of the trough facing the bending direction, the outer surface of the trough refers to the surface of the trough away from the bending direction, and the neutral layer refers to the position between the inner surface of the trough and the outer surface of the trough. The process of the tube bending deformation process can be referred to the attached Figure 2 To the attached Figure 6 The radial movement of the trough of the bellows model toward the axis of the tube blank is referred to as the trough shrinkage. During the trough shrinkage process, since the bending curvature remains unchanged, the increment of the difference in axial strain between the inner surface and the neutral layer during the compression process remains unchanged, that is,

[0058] dε l -dε l,mid =0#(1)

[0059] During the compression process, the deformation in the compression direction is uniform, and the increment of the difference between the hoop strain of the inner surface and the neutral layer remains unchanged, that is,

[0060] dε θ -dε θ,mid =0#(2)

[0061] Expanding the above formula for elastic and plastic deformation, we can get

[0062]

[0063] After combining and sorting

[0064]

[0065] During the plastic deformation of the material, the stress meets the yield condition

[0066]

[0067] Ignoring material work hardening, differentiating the above formula yields

[0068] 2σ θ dσ θ +2σ l dσ l -σ θ dσ l -σ l dσ θ =0#(7)

[0069] Hoop strain increment dε b Perform elastic and plastic expansion for

[0070]

[0071] Combining equations (9), (11) and (12), we get

[0072]

[0073] Solving the above differential equation we can get Figure 14 The results shown in the figure show that when the circumferential shrinkage exceeds 4%, the axial stress is reduced to 1.5%; when it exceeds 5%, the axial stress can be controlled within 5‰. At this time, the stress on the outer and inner surfaces of the trough area is basically the same. Therefore, when the bellows is circumferentially compressed, the stress on both the inner and outer surfaces of the tube billet is reduced, and the springback of the bellows is effectively suppressed.

[0074] Therefore, in this embodiment, according to the bending deformation process of the tube blank 4, a mold surface is constructed, and the curvature of the mold surface is set to be constant, so that the trough of the mold surface moves radially toward the axis of the tube blank 4; according to the principle of material bending deformation, the difference between the axial strain of the inner surface of the trough of the mold surface and the axial strain of the neutral layer, and the difference between the circumferential strain of the inner surface and the circumferential strain of the neutral layer during the radial movement are analyzed to obtain the expression of the plastic strain increment; according to the expression of the plastic deformation yield condition of the material, the plastic stress differential equation is obtained; according to the principle of material bending deformation, the circumferential strain increment expression is obtained; and the plastic strain increment is obtained by combining the plastic strain increment equations. The expression of the quantity, the differential equation of plastic stress and the expression of the hoop strain increment are obtained to obtain the expression of the relationship between the hoop stress and the axial stress. According to the expression of the relationship between the hoop stress and the axial stress, a relationship curve is obtained with the hoop compression strain as the independent variable and the ratio of the axial stress to the stress before compression as the dependent variable. After the tube blank 4 is bent and deformed, the radial contraction of the trough can be achieved by moving the trough along the radial direction of the tube blank 4 toward the axis of the tube blank 4, so as to increase the hoop stress of the trough, reduce the axial stress difference between the inner and outer surfaces, and then reduce the axial stress of the bellows, thereby effectively controlling the axial rebound of the bellows.

[0075] Optionally, combined Figures 2 to 6 、 Figure 13 As shown, the trough model is moved radially toward the axis of the tube blank 4, including:

[0076] The peak diameter of the desired bellows is used as the peak diameter of the intermediate mold 2; and the difference between the peak diameter of the bellows and the inner diameter of the trough of the bellows is used as the first difference;

[0077] The difference between the diameter of the peak forming structure of the intermediate mold 2 and the diameter of the trough forming structure of the intermediate mold 2 is set as a second difference, and the second difference is smaller than the first difference;

[0078] One end of the tube blank 4 in the axial direction is fixed by the fixed die 3; the other end of the tube blank 4 in the axial direction is fixed by the movable die 1; and the intermediate die 2 is placed on the portion of the tube blank 4 between the fixed die 3 and the movable die 1;

[0079] The tube blank 4 is subjected to bulging and drum wave operations, and the movable die 1 is moved toward the fixed die 3 by pushing waves, so that the tube blank 4 is separated from the trough forming structure of the intermediate die 2.

[0080] Specifically, bulging involves applying internal pressure to the tube 4, forcing its outer wall to contact the lower end of the intermediate die 2. Drumming involves maintaining pressure on the tube 4 after bulging, causing the outer wall of the tube 4 to further bulge outward, forming a corrugated shape. During the rumping process, the tube 4 is blocked by the lower end of the intermediate die 2, gradually forming troughs. During forming, the tube 4 is inserted into the intermediate die 2; its axial ends are connected to the movable die 1 and the fixed die 3, respectively, with a gap set between the intermediate die 2 and the outer wall of the tube 4. Pressurize the tube blank 4 so that it expands and deforms. Then, the outer wall of the tube blank 4 abuts against the lower end of the intermediate mold 2. Maintaining the current pressure on the tube blank 4 causes the outer wall of the tube blank 4 to bulge toward the gap between the intermediate mold 2, the movable mold 1, and the fixed mold 3 to form a preliminary waveform. The portion where the outer wall of the tube blank 4 contacts the intermediate mold 2 forms a trough. The movable mold 1 is driven to move so that the movable mold 1 moves toward the fixed mold 3 to eliminate the gap and form the peak of the corrugated pipe. As the peak is formed, the trough is gradually moved away from the lower end of the intermediate mold 2 by the obstruction of the intermediate mold 2. Figure 10 According to the Mises yield criterion, the part between the crest and the trough is stretched, generating hoop stress to achieve plastic deformation, thereby reducing the inner diameter of the corrugated section of the tube blank 4.

[0081] In this way, by taking the peak diameter of the required corrugated pipe as the peak diameter of the intermediate die 2; taking the difference between the peak diameter of the corrugated pipe and the inner diameter of the trough of the corrugated pipe as the first difference; taking the difference between the diameter of the peak forming structure of the intermediate die 2 and the diameter of the trough forming structure of the intermediate die 2 as the second difference, and making the second difference smaller than the first difference, which is equivalent to enlarging the trough diameter of the intermediate die 2, a certain gap can be created between the tube blank 4 and the intermediate die 2 before forming, and fixing one end of the tube blank 4 in the axial direction by the fixed die 3; fixing the other end of the tube blank 4 in the axial direction by the movable die 1; and The intermediate die 2 is sleeved on the part where the tube blank 4 is located between the fixed die 3 and the movable die 1; the tube blank 4 is subjected to bulging and drum wave operations, and the movable die 1 is moved toward the fixed die 3 by pushing the wave to separate the tube blank 4 from the trough forming structure of the intermediate die 2. Therefore, combined with the expression of the relationship between the hoop stress and the axial stress described above, when the trough diameter of the intermediate die 2 is expanded, a certain gap can be created between the tube blank 4 and the intermediate die 2 before forming, so that radial diameter reduction can occur in the final stage of the bellows forming, so as to effectively control the axial rebound of the bellows.

[0082] Optionally, the bellows forming springback control method based on stress regulation further includes:

[0083] The formed tube blank is subjected to pressure relief and unloading operations.

[0084] Specifically, pressure relief refers to the release of pressure inside the tube blank, and unloading refers to the removal of the formed bellows from the mold.

[0085] In this way, the pressure relief and unloading operations are performed on the formed tube blank to prevent the pressure inside the formed bellows from affecting the disassembly of the bellows.

[0086] Optionally, the fixed die is fixed to the frame of the forming equipment, and the movable die is moved toward the fixed die by a hydraulic drive system. This not only improves the stability of the fixed die, but also improves the movement accuracy of the movable die, thereby ensuring the forming quality of the corrugated pipe.

[0087] Optionally, the bellows forming springback control method based on stress regulation further includes:

[0088] Check the axial springback of the formed tube blank. If the axial springback of the formed tube blank is greater than the actual required springback error range, reduce the second difference.

[0089] In this way, in the process of controlling springback, no additional equipment or additional processing steps are required. It is only necessary to reduce the difference between the diameter of the peak forming structure of the intermediate mold 2 and the diameter of the trough forming structure of the intermediate mold 2, that is, to increase the size of the gap between the intermediate mold 2 and the outer wall of the tube blank 4, thereby improving the applicability of corrugated tube forming.

[0090] Based on the above-mentioned bellows forming method, the embodiments of the present invention provide several different bellows forming cases.

[0091] Example 1

[0092] This example uses the forming method of a TA34 bellows component with a yield strength σs of 500 MPa, with a wave number N of 10, a thickness t of 1 mm, a wave pitch L0 of 15 mm, a peak width D0 of 10 mm, a trough diameter D1 of 100 mm, a peak diameter D2 of 120 mm, and a maximum axial dimensional accuracy error ΔL of 10 mm as an example. The upper dimension R1 of intermediate die 2 is 1 mm, and the die surface adjustment dimension ΔD is 5 mm. The initial billet length L1 is 28.5 mm, and the distance L1 - L0 between the intermediate dies is 13.5 mm. The internal pressure P1 at the end of bulging is 12 MPa, the internal pressure P2 at the end of drum wave is 15 MPa, the displacement d1 of movable die 1 at the end of push wave is 85 mm, and the punch displacement d2 at the end of compression is 135 mm.

[0093] Example 2

[0094] This example uses the forming method of a TA34 bellows component with a yield strength σs of 500 MPa, with a wave number N of 10, a thickness t of 1 mm, a wave pitch L0 of 15 mm, a peak width D0 of 10 mm, a trough diameter D1 of 100 mm, a peak diameter D2 of 120 mm, and a maximum axial dimensional accuracy error ΔL of 10 mm as an example. The upper dimension R1 of intermediate die 2 is 1 mm, and the die surface adjustment dimension ΔD is 5 mm. The initial billet length L1 is 28.5 mm, and the distance L1 - L0 between the intermediate dies is 13.5 mm. The internal pressure P1 at the end of bulging is 24 MPa, the internal pressure P2 at the end of drum wave is 30 MPa, the displacement d1 of movable die 1 at the end of push wave is 85 mm, and the punch displacement d2 at the end of compression is 135 mm.

[0095] Example 3

[0096] This example uses the forming method of a TA34 bellows component with a yield strength σs of 500 MPa, with a wave number N of 10, a thickness t of 1 mm, a wave pitch L0 of 15 mm, a peak width D0 of 10 mm, a trough diameter D1 of 100 mm, a peak diameter D2 of 120 mm, and a maximum axial dimensional accuracy error ΔL of 10 mm as an example. The upper dimension R1 of intermediate die 2 is 1 mm, and the die surface adjustment dimension ΔD is 5 mm. The initial billet length L1 is 28.5 mm, and the distance L1 - L0 between the intermediate dies is 13.5 mm. The internal pressure P1 at the end of bulging is 3 MPa, the internal pressure P2 at the end of drum wave is 30 MPa, the displacement d1 of movable die 1 at the end of push wave is 85 mm, and the punch displacement d2 at the end of compression is 135 mm.

[0097] Example 4

[0098] This example uses the forming method of a TA34 bellows component with a yield strength σs of 500 MPa, with a wave number N of 10, a thickness t of 1 mm, a wave pitch L0 of 15 mm, a peak width D0 of 10 mm, a trough diameter D1 of 100 mm, a peak diameter D2 of 120 mm, and a maximum axial dimensional accuracy error ΔL of 10 mm as an example. The upper dimension R1 of intermediate die 2 is 1 mm, and the die surface adjustment dimension ΔD is 10 mm. The initial billet length L1 is 28.5 mm, and the distance L1 - L0 between the intermediate dies is 13.5 mm. The internal pressure P1 at the end of bulging is 12 MPa, the internal pressure P2 at the end of drum wave is 15 MPa, the displacement d1 of movable die 1 at the end of push wave is 85 mm, and the displacement d2 of movable die 1 at the end of compression wave is 135 mm.

[0099] Example 5

[0100] This example uses the forming method of a 2219-O aluminum alloy bellows component with a wave number N of 10, a thickness t of 1mm, a wave pitch L0 of 15mm, a peak width D0 of 10mm, a trough diameter D1 of 100mm, a peak diameter D2 of 120mm, and a maximum axial dimensional accuracy error ΔL of 10mm, with a yield strength σs of 500MPa. The upper dimension R1 of intermediate die 2 is 1mm, and the die surface adjustment dimension ΔD is 5mm. The initial billet length L1 is 28.5mm, and the distance L1-L0 between the intermediate dies is 13.5mm. The internal pressure P1 at the end of bulging is 1.8MPa, and the internal pressure P2 at the end of drum wave is 2.25MPa. The displacement d1 of movable die 1 at the end of push wave is 85mm, and the displacement d2 of movable die 1 at the end of compression wave is 135mm.

[0101] Combine Figures 2 to 11 As shown, another embodiment of the present invention also provides a mold, based on the bellows forming rebound control method based on stress regulation as described above, including a movable mold 1, an intermediate mold 2 and a fixed mold 3, all of which are arranged in a ring shape, the intermediate mold 2 is located between the movable mold 1 and the fixed mold 3, the movable mold 1 and the fixed mold 3 are respectively used to be fixedly connected to the two ends of the tube blank 4 in the axial direction, the movable mold 1 is used to be moved toward the fixed mold 3, and the movable mold 1 and the fixed mold 3 are both provided with a first arc surface structure on one end facing the intermediate mold 2, the intermediate mold 2 is provided with an arc-shaped protrusion 21 and a second arc surface structure located on both sides of the arc-shaped protrusion 21, the first arc surface structure and the second arc surface structure cooperate to form a peak forming groove, or, two adjacent second arc surface structures on different intermediate molds 2 cooperate to form a peak forming groove; the arc-shaped protrusion 21 is used to form a trough of the corrugated tube; the inner diameter of the intermediate mold 2 is larger than the inner diameter of the movable mold 1 and the fixed mold 2.

[0102] It can be understood that the movable mold 1, the intermediate mold 2 and the fixed mold 3 are all annular structures for being sleeved on the tube blank 4. Since the two axial ends of the tube blank 4 can be fixed by the movable mold 1 and the fixed mold 3 respectively, the apertures of the movable mold 1 and the fixed mold 3 are the same and are consistent with the outer diameter of the tube blank 4, while the inner diameter of the intermediate mold 2 (that is, the diameter of the hole surrounded by the arc-shaped protrusion 21) is larger than the inner diameters of the movable mold 1 and the fixed mold 3 (the diameter of the hole surrounded by the movable mold 1 and the fixed mold 3), that is, the inner diameter of the intermediate mold 2 is larger than the outer diameter of the tube blank 4, or in other words, before the tube blank 4 is formed, the arc-shaped protrusion 21 of the intermediate mold 2 has a certain distance from the outer wall of the tube blank 4.

[0103] Specifically, when preparing the bellows, Figure 2As shown, the two ends of the straight tube 4 in the axial direction are fixed by the movable die 1 and the fixed die 3 respectively. Since the aperture enclosed by the arc-shaped protrusion 21 on the intermediate die 2 is larger than the aperture enclosed by the movable die 1, a certain initial distance ΔD is separated between the arc-shaped protrusion 21 on the intermediate die 2 and the outer wall of the tube 4. After the tube 4 is fixed, the tube 4 is pressurized from the inside. Figure 3 As shown, after the tube 4 is pressurized, the outer wall (corrugated section) of the tube 4 between the movable die 1 and the fixed die 3 will expand outwards until it abuts against the arc-shaped protrusion 21 on the middle die 2. Figure 4 As shown, the outer wall of the tube blank 4 will continue to expand due to the obstruction of the arc-shaped protrusion 21 (i.e., the trough-forming structure described above), thereby bulging outwards between the two adjacent second arc-shaped structures to form a preliminary corrugated shape. Figure 5 As shown, the movable die 1 moves toward the fixed die 3, reducing the linear distance between the two adjacent second curved surface structures (i.e., the peak forming structure mentioned above), so that the outer wall of the tube blank 4 between the two adjacent second curved surface structures undergoes a larger bending deformation. At the same time, the arc-shaped protrusion 21 completes the formation of the trough of the corrugated tube. Keep the tube blank 4 pressurized, and as the movable die 1 continues to move, as shown in FIG. Figure 6 As shown, the movement of the movable mold 1 makes the two adjacent second arc surface structures spliced ​​together, so that the outer wall of the tube blank 4 forms a wave peak under the approach of the two adjacent second arc surface structures, and in the process of wave peak formation, the two arc segments forming the wave peak approach each other under the two adjacent second arc surface structures, and the outer wall of the tube blank 4 is attached to the second arc surface structure. Therefore, due to the obstruction of the second arc surface structure, in the process of the two arc segments forming the wave peak gradually approaching, the wave trough will gradually move away from the arc-shaped protrusion 21, that is, the wave trough moves toward the central axis of the tube blank 4, thereby increasing the distance between the wave trough and the wave peak, and realizing that the outer wall between the wave trough and the wave peak is stretched to produce plastic deformation, and in the process of stretching, since the wave trough is separated from the support of the arc-shaped protrusion 21, the two arc segments of the wave trough will be further approached. In this way, when the pressure is released, as shown in FIG. Figure 7 As shown, the overall rebound amount of the bellows is very small, and the rebound amount of the bellows can be effectively controlled.

[0104] Corresponding to the above, Figure 11As shown in the figure, from a stress perspective, the forming process of the corrugated pipe's peaks and troughs is a process of overcoming the material's yield strength. The elliptical curve corresponds to the Mises yield criterion, where σ11 represents the axial stress, σ22 represents the hoop stress, and X represents the thickness direction of the tube blank. The elliptical curve on the left is a stress analysis coordinate diagram before stress regulation (traditional bellows forming). During the forming process, taking the forming at the trough as an example, the outer wall at the trough is compressed, while the inner wall at the trough is stretched. The two stresses are equal in magnitude and opposite in direction, located in the first and third quadrants, respectively. In the late forming stage, the stress causes the material to undergo microplastic deformation, meaning that the material maintains the formed shape and cannot rebound to its initial shape. The corresponding elliptical curve changes as the stress gradually increases during bending, then reverses after reaching the elliptical curve. The lateral distance between the reversed stress and the coordinate origin is the elastic rebound. After stress regulation (the corrugated tube formed by the present invention), the trough separates from the arc-shaped protrusion 21 in the later stage, as shown in the elliptical curve coordinate system on the right, the stress gradually increases, and turns back after reaching the elliptical curve. After turning back, the downward circumferential stress is increased to reach the elliptical curve, and moves toward the coordinate origin along the elliptical curve. The lateral distance between the stress on the elliptical curve and the coordinate origin is the rebound amount after stress regulation. It can be seen that the rebound amount after stress regulation is smaller than the rebound amount before stress regulation. The deformation process corresponding to the circumferential stress is that as the peak is formed, the trough and the peak gradually move away, the aperture surrounded by the trough gradually decreases, and the aperture surrounded by the peak remains unchanged. The material between the trough and the peak is further stretched under the action of the circumferential stress to undergo plastic deformation. Therefore, after the bellows is formed, the rebound amount at the trough is smaller, so as to reduce the overall axial rebound amount of the bellows, thereby effectively controlling the axial rebound of the bellows.

[0105] Therefore, in this embodiment, the diameter of the aperture surrounded by the arc-shaped protrusion 21 on the intermediate mold 2 is larger than the diameter of the aperture surrounded by the movable mold 1 and the fixed mold 3. After the peak is formed, the material in the trough area can shrink and deform, so that the material enters the plastic deformation state again. In this way, after the preparation of the bellows is completed, the axial rebound of the trough can be reduced, thereby achieving effective control of the axial rebound of the bellows to improve the forming quality of the bellows.

[0106] In some embodiments, if the aperture surrounded by the arc-shaped protrusion 21 is 50 mm and the aperture surrounded by the movable mold 1 is 40 mm, and it is found after preparation that the axial rebound of the corrugated tube is greater than the actual requirement, the aperture surrounded by the arc-shaped protrusion 21 is adjusted to 55 mm, so that the distance between the arc-shaped protrusion 21 and the outer wall of the tube blank 4 is further increased.

[0107] Based on the above embodiments, combined with Figures 8 to 10, 12 and 13, the embodiment of the present invention provides a forming process of a corrugated tube. For the forming of a corrugated tube with wave number N, thickness t, wave pitch L0, peak width D0, trough diameter D1, peak diameter D2, and maximum axial dimensional accuracy error ΔL, the size of the intermediate mold 2 is combined with Figure 12 As shown, R1 is the fillet at the die wave peak, which is generally 1mm; ΔD is the die surface adjustment size, which is the main factor affecting the springback of the final formed part. It is determined according to the actual springback of the formed part and is generally 0.05D1. Before forming, multiple intermediate dies 2 can be fixed equidistantly between the moving die 1 and the fixed die 3, where the distance between the intermediate dies 2 is L1-L0, L1 is the initial billet length, which is expressed as km[D2-D1+(π / 2-1)L0], and km is the material influence coefficient, which is usually in the range of 1 to 1.1. During the forming process, Figure 9 As shown, the punch is fed into the displacement dPunch, and the pressure P inside the tube is used to form the tube. Figure 8 As shown. The internal pressure P during the forming process is as follows Figure 10 As shown in Figure 2. P1 is the internal pressure at the end of bulging, expressed as kP1σst / D1; P2 is the internal pressure at the end of drum wave, expressed as kP2σst / D1, kP1 and kP2 are the internal pressure influence coefficients, ranging from 2.3 to 2.5 and 2.8 to 3.0 respectively. The punch feed displacement dPunch during the forming process is as follows: Figure 10 As shown in the figure, d1 is the displacement of the movable die 1 at the end of the push wave, expressed as kdN(L1-L0-ΔD), kd is the displacement influence coefficient, typically ranging from 0.8 to 1.2; d2 is the punch displacement at the end of compression, expressed as N(L1-L0). After forming, the component will experience axial springback of Δd. If the axial springback Δd of the formed component is greater than the maximum axial dimensional accuracy error ΔL, the die surface adjustment dimension ΔD must be increased to further reduce the springback.

[0108] For example, for the forming of TA34 corrugated pipe with a wave number N of 10, thickness t of 1mm, wave pitch L0 of 15mm, wave peak width D0 of 10mm, wave valley diameter D1 of 100mm, wave peak diameter D2 of 120mm, and maximum axial dimension accuracy error ΔL of 10mm, the yield strength σs is 500MPa. The size R1 of the middle mold 2 is 1mm, the mold surface adjustment size ΔD is 5mm, and the other dimensions are based on Figure 13 The initial billet length L1 is 28.5 mm, and the distance between the floating die pieces L1-L0 is 13.5 mm. During the forming process, the matching relationship between the displacement dPunch of the mobile die 1 and the internal pressure P of the tube blank is given by Figure 9 、 Figure 10The internal pressure P1 at the end of bulging is 12 MPa, the internal pressure P2 at the end of drum wave is 15 MPa, the punch displacement d1 at the end of push wave is 85 mm, and the punch displacement d2 at the end of compression is 135 mm. After the punch is unloaded at the end of forming, the component will produce axial springback Δd. If the axial springback Δd of the formed component is greater than the maximum error of axial dimensional accuracy of 10 mm, it is necessary to increase the die surface adjustment dimension ΔD to further reduce the springback.

[0109] In some embodiments, for forming high-precision corrugated tubes with a maximum axial dimensional accuracy error of 1 mm, ΔD is 10 mm. This is to generate a larger strain during the diameter reduction stage of the tube blank 4 to further control springback.

[0110] In some embodiments, for forming a corrugated tube with a trough diameter D1 of 1000 mm and a peak diameter D2 of 1100 mm, the die surface adjustment dimension ΔD is 20 mm. This is to prevent wrinkling of the tube blank 4 by using a relatively smaller compression amount for structures with a large diameter-to-thickness ratio.

[0111] In some embodiments, for forming a multi-layer corrugated tube with a total thickness of 1 mm, the die surface adjustment dimension ΔD is 10 mm. This is to generate a larger strain during the compression stage of component forming to further control springback.

[0112] In some embodiments, for forming a thicker corrugated tube with a thickness of 3 mm, the upper dimension R1 of the floating die is 3 mm. The purpose is to increase the contact area between the intermediate die 2 and the tube 4 during the corrugation process to reduce wear.

[0113] Optionally, the middle mold 2 is configured to be movable along the radial direction of the tube blank 4 .

[0114] Specifically, the intermediate die 2 can be hydraulically driven to move radially along the tube 4. That is, the distance between the intermediate die 2 and the outer wall of the tube 4 is adjustable. Thus, by allowing the intermediate die 2 to move radially along the tube 4, the distance between the intermediate die 2 and the outer wall of the tube 4 can be adjusted, thereby increasing the flexibility of the intermediate die 2.

[0115] Optionally, there are multiple intermediate molds 2, and the multiple intermediate molds 2 are used to move synchronously along the radial direction of the tube blank 4.

[0116] Specifically, the multiple intermediate dies 2 are positioned between the movable die 1 and the fixed die 3. The leftmost intermediate die 2 is spaced a certain distance from the movable die 1, and the leftmost intermediate die 2 is spaced a certain distance from the fixed die 3. Adjacent intermediate dies 2 are evenly spaced, and the multiple intermediate dies 2 can move synchronously along the radial direction of the tube 4. This synchronized movement of the multiple intermediate dies 2 along the radial direction of the tube 4 improves the movement accuracy of the multiple intermediate dies 2 while achieving multiple trough formation, thereby enhancing the forming quality of the corrugated tube.

[0117] Optionally, the number of intermediate dies 2 is N-1, where N is the number of waves of the bellows required. Specifically, when the number of waves N of the bellows is 5, the number of intermediate dies 2 is 5-1=4. This allows the intermediate dies 2 to form multiple troughs at once, improving forming efficiency.

[0118] Optionally, the movable mold 1 is configured to move toward the fixed mold 3 via a hydraulic drive structure. Specifically, the hydraulic drive structure is a hydraulic cylinder, the output end of which is drivingly connected to the movable mold 1 to control the movement of the movable mold 1 through the telescopic motion of the cylinder rod. Thus, through the driving connection between the hydraulic drive structure and the movable mold 1, the movement precision of the movable mold 1 can be effectively controlled, thereby improving the movement stability of the movable mold 1 during the wave pushing process.

[0119] Optionally, the fixed die 3 is fixed on a frame of a forming device, so as to improve the stability of the fixed die 3 during the tube blank 4 forming process.

[0120] Although the present disclosure is disclosed as above, the protection scope of the present disclosure is not limited thereto. Those skilled in the art may make various changes and modifications without departing from the spirit and scope of the present disclosure, and these changes and modifications will fall within the protection scope of the present invention.

Claims

1. A bellows forming springback control method based on stress regulation, characterized in that: include: According to the bending deformation process of the tube blank, a mold profile is constructed, and the curvature of the mold profile is set to be constant so that the trough of the mold profile moves radially toward the axis of the tube blank; According to the material bending deformation principle, the difference between the axial strain of the inner surface of the trough of the mold surface and the axial strain of the neutral layer, and the difference between the hoop strain of the inner surface and the hoop strain of the neutral layer during radial movement are analyzed to obtain the plastic strain increment expression; According to the material plastic deformation yield condition expression, the plastic stress differential equation is obtained; According to the material bending deformation principle, the expression of hoop strain increment is obtained; By combining the expression of plastic strain increment, plastic stress differential equation and hoop strain increment, the expression of the relationship between hoop stress and axial stress is obtained. Based on the expression of the relationship between hoop stress and axial stress, a relationship curve is obtained with the hoop compression strain as the independent variable and the ratio of axial stress to the value before compression as the dependent variable.

2. The bellows forming springback control method based on stress regulation according to claim 1, characterized in that: The step of causing the trough of the mold surface to move radially toward the axis of the tube blank comprises: The peak diameter of the desired bellows is used as the peak diameter of the intermediate mold; and the difference between the peak diameter of the bellows and the inner diameter of the trough of the bellows is used as the first difference; Taking the difference between the diameter of the peak forming structure of the intermediate mold and the diameter of the trough forming structure of the intermediate mold as a second difference, such that the second difference is smaller than the first difference; Fixing one axial end of the tube blank by a fixed die; fixing the other axial end of the tube blank by a movable die; and sleeve the intermediate die on the portion of the tube blank located between the fixed die and the movable die; The tube blank is subjected to bulging and drum wave operations, and the tube blank is separated from the trough forming structure of the intermediate die by the push wave movement of the movable die toward the fixed die.

3. The bellows forming springback control method based on stress regulation according to claim 2, characterized in that: Also includes: The formed tube blank is subjected to pressure relief and unloading operations.

4. The bellows forming springback control method based on stress regulation according to claim 2, characterized in that: The fixed mold is fixed on a frame of the forming equipment, and the movable mold moves toward the fixed mold through a hydraulic drive system.

5. The bellows forming springback control method based on stress regulation according to claim 2, characterized in that: Also includes: Check the axial springback of the tube blank after forming. If the axial springback of the tube blank after forming is greater than the actual required springback error range, reduce the second difference.

6. A mold, based on the bellows forming springback control method based on stress regulation according to any one of claims 1 to 5, characterized in that: It includes a movable mold, an intermediate mold and a fixed mold, all of which are arranged in a ring shape. The intermediate mold is located between the movable mold and the fixed mold. The movable mold and the fixed mold are respectively used to be fixedly connected to the two ends of the tube blank in the axial direction. The movable mold is used to be moved toward the fixed mold. The movable mold and the fixed mold are both provided with a first arc surface structure on one end facing the intermediate mold. The intermediate mold is provided with an arc-shaped protrusion and a second arc surface structure located on both sides of the arc-shaped protrusion. The first arc surface structure cooperates with the second arc surface structure to form a peak forming groove, or two adjacent second arc surface structures on different intermediate molds cooperate to form a peak forming groove; the arc-shaped protrusion is used to form the trough of the corrugated pipe; the inner diameter of the intermediate mold is larger than the inner diameter of the movable mold and the fixed mold.

7. The mold according to claim 6, characterized in that The intermediate mold is used to be movably arranged along the radial direction of the tube blank.

8. The mold according to claim 6, characterized in that There are multiple intermediate molds, and the multiple intermediate molds are used to move synchronously along the radial direction of the tube blank.

9. The mold according to claim 8, characterized in that The number of the intermediate molds is N-1, where N is the number of waves of the bellows actually required.

10. The mold according to claim 6, characterized in that The movable mold is used to move toward the fixed mold through a hydraulic driving structure.

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