Process optimization method for preventing multi-pass closing-in spinning instability of thin-wall pipe fitting

Abstract

The invention belongs to the technical field of material plastic processing, and particularly relates to a process optimization method for preventing multi-pass closing-in spinning instability of a thin-wall pipe fitting, which comprises the following steps: S1, determining the maximum radial feed rate of each pass according to a thin-wall shell instability mechanical model; s2, the total pass number needed by multi-pass closing-in spinning of the thin-wall pipe fitting is calculated according to the radial feeding amount, and the total pass number and the feeding amount of each pass are recorded; s3, according to the total number of passes and the feeding amount of each pass, the motion trail of a spinning roller in the multi-pass closing-in spinning process is generated; s4, a thin-wall pipe fitting closing-in spinning finite element model is established, closing-in spinning process simulation is conducted according to the motion trail of the spinning roller, and spinning process parameters are obtained based on the simulation result; according to the method, the instability phenomenon in the thin-wall pipe fitting closing-in spinning process can be effectively avoided, the optimized closing-in numerical value of each pass is obtained, efficient planning of the spinning roller track is achieved, the research and development cost and the time cost of enterprises are greatly reduced, and obvious economic benefits are achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of material plastic processing, and particularly relates to a process optimization method for preventing instability in multi-pass necking spinning of thin-walled pipe fittings. Background Art

[0002] The fuel tanks of launch vehicles, the hulls of deep-sea manned submersibles, the heads of nuclear power plant containments, and most gas cylinders are all thin-walled shell components with complex curved surface shapes and high forming accuracy requirements. At present, the relatively advanced manufacturing process is to use multi-pass necking spinning forming technology. Due to the weak overall stiffness of thin-walled shell components and poor self-anti-instability ability, during the multi-pass necking spinning forming process, one end in an unconstrained state is prone to instability defects, which will cause waste of costs in the manufacturing process of enterprises. Due to the particularity of the materials of thin-walled shell components, it will cause great economic losses.

[0003] In the prior art, for example, the Chinese patent application with the publication number CN113695449A discloses a method for hot spinning processing of gas cylinders based on finite element modeling. A three-dimensional thermo-mechanical coupling model is used to calculate the hot spinning necking process, and the heat transfer coefficients on the inner and outer surfaces of the tube blank are optimized according to the difference between the calculated temperature value and the actual value to obtain the hot spinning necking process parameters and guide practice. The material constitutive equation adopted in this technical solution is relatively generalized, the influencing factors considered are too few, and the accuracy of the model can only be feedback through the surface temperature of the spun tube blank, and it cannot guide the prevention of instability phenomena in thin-walled pipe fittings.

[0004] In addition, the Chinese patent application with the publication number CN108838265A discloses a method for constructing the multi-pass spinning process trajectory of curved surface components. First, wrinkle prediction is carried out on the flange to obtain the limit spinning-out angle of each pass of the curved surface component spinning, then the number of passes of the multi-pass spinning forming is determined, and finally the involute trajectory of the spinning wheel for each pass of the spinning forming is drawn. This technical solution can only avoid the instability wrinkling phenomenon of thin plate parts during the spinning process, cannot guide the necking spinning process of thin-walled pipe fittings, and cannot guide the selection of spinning process parameters.

[0005] Therefore, there is an urgent need for a method to prevent instability in the necking spinning of thin-walled pipe fittings to solve the above technical problems. Summary of the Invention

[0006] The purpose of the present invention is to provide a process optimization method for preventing instability in multi-pass necking spinning of thin-walled pipe fittings, which can effectively avoid the occurrence of instability phenomena during the necking spinning process of thin-walled pipe fittings, obtain optimized process parameters, and achieve efficient planning of the spinning wheel trajectory.

[0007] The present invention proposes a process optimization method for preventing instability in multi-pass necking spinning of thin-walled pipe fittings, including the following steps: S1. Determine the maximum radial feed per pass according to the mechanical model of thin-walled shell instability; the mechanical model of thin-walled shell instability is constructed based on formula (14) and formula (19): (14), (19), where, is the minimum bending strain energy required for deformation; is the neutral surface radius of the shell before necking deformation; is the neutral surface radius of the shell after necking deformation; θ is the circumferential angular coordinate; is the shell wall thickness; E is the Young's modulus; is the Poisson's ratio; is the axial strain; is the circumferential strain; is the shear strain; is the radial coordinate variable; ΔT is the work done by the external force; is the differential force; ΔL is the differential displacement; L is the necking length, in meters; l is the axial coordinate variable, with a value range of 0 to L, in meters; is the absolute value of the circumferential stress; is the absolute value of the radial stress; S2. Calculate the total number of passes required for multi-pass necking spinning of the thin-walled pipe according to the radial feed, and record the total number of passes and the feed per pass; S3. Generate the motion trajectory of the spinning wheel during multi-pass necking spinning according to the total number of passes and the feed per pass; S4. Establish a finite element model for necking spinning of the thin-walled pipe, simulate the necking spinning process according to the motion trajectory of the spinning wheel, and obtain the spinning process parameters based on the simulation results.

[0008] Specifically, step S1 specifically includes the following steps: S11. Initialize the radial feed per pass, with a range between the wall thickness and ; S12. Obtain the shell shape parameters before and after necking deformation, and the shell shape parameters include the neutral surface radius of the shell before necking deformation, the neutral surface radius of the shell after necking deformation, the necking length L, and the shell wall thickness ; S13. Obtain the basic material parameters, and the material parameters include the Young's modulus E, the Poisson's ratio ν, and the equivalent true stress-true strain curve; S14. Based on the shell shape parameters and the basic material parameters, calculate the work With the minimum bending strain energy required for deformation ; S15. According to the work done by the external force With the minimum bending strain energy required for deformation Judge whether the predetermined instability criterion condition is satisfied. If it is satisfied, determine the radial feed per pass.

[0009] Specifically, the predetermined instability criterion condition described in step S15 is that the work done by the external force is less than or equal to the minimum bending strain energy required for deformation and the radial feed per pass does not need to be increased.

[0010] Specifically, the total number of passes required for multi-pass necking spinning of the thin-walled pipe fitting described in step S2 is calculated according to formula (20): , where N is the total number of passes required for multi-pass necking spinning of the thin-walled pipe fitting, is the total radial feed of multi-pass necking spinning of the thin-walled pipe fitting, is the radial feed per pass.

[0011] Specifically, step S3 specifically includes: In the spinning forming of each pass, the feed trajectory of the spinning wheel adopts a straight line trajectory, and a multi-pass spinning forming trajectory planning using a pull-press reciprocating spinning method is used as the movement trajectory of the spinning wheel, and the coordinates of the reference point of the spinning wheel relative to the geometric center of the blank are calculated.

[0012] Specifically, step S4 specifically includes the following steps: S41. Establish a three-dimensional model of the thin-walled pipe fitting, the spinning wheel and the fixture based on finite element software, set the thin-walled pipe fitting as a shell, and set the mesh type as hexahedron SC8R element; S42. Set the working angle α between the spinning wheel and the thin-walled pipe fitting, and set penalty contact; S43. Couple and constrain the thin-walled pipe fitting and the fixture through the reference point, and apply a rotational boundary condition to the fixture to drive the thin-walled pipe fitting to rotate self; S44. Assign the material parameters required for thermo-mechanical coupling calculation to the thin-walled pipe fitting; S45: Assign the spinning process parameters to the finite element model and perform a simulation calculation of the necking spinning process according to the movement trajectory of the spinning wheel, and optimize the spinning process parameters according to the simulation results.

[0013] Specifically, the method further includes: S5. Perform actual spinning on the thin-walled pipe fitting according to the spinning process parameters and the movement trajectory of the spinning wheel.

[0014] Specifically, the working angle α described in step S42 is calculated by formula (21): , Wherein, L is the closing length, with the unit of millimeter; is the radial feed per pass, with the unit of millimeter.

[0015] Specifically, the material parameters required for the thermo-mechanical coupling calculation described in step S44 include Young's modulus, Poisson's ratio, constitutive equation, initial temperature, specific heat capacity, thermal conductivity, coefficient of thermal expansion, heat transfer coefficient, and friction heat generation coefficient.

[0016] Specifically, the spinning process parameters described in step S44 include spindle speed, feed ratio, and roller fillet radius.

[0017] Specifically, the thin-walled pipe fitting refers to a metal pipe fitting with a wall thickness of 0.5 mm to 15 mm for tube blank that can be used for spinning processing, and the outer diameter of the tube blank is greater than or equal to 50 times the wall thickness.

[0018] The present invention provides a process optimization method for preventing instability in multi-pass closing spinning of thin-walled pipe fittings, including: S1. Determining the maximum radial feed per pass according to the instability mechanical model of the thin-walled shell; S2. Calculating the total number of passes required for multi-pass closing spinning of the thin-walled pipe fitting according to the radial feed; S3. Generating the motion trajectory of the roller during multi-pass closing spinning according to the total number of passes; S4. Establishing a finite element model for closing spinning of the thin-walled pipe fitting, simulating the closing spinning process according to the motion trajectory of the roller, and obtaining the spinning process parameters based on the simulation results; The present invention realizes the optimized design of the closing spinning process of the thin-walled pipe fitting by establishing an instability mechanical model of the thin-walled shell and combining thermo-mechanical coupling finite element analysis, and has the following beneficial effects: 1. The process optimization method for preventing instability in multi-pass closing spinning of thin-walled pipe fittings provided by the present invention can effectively avoid the occurrence of instability during the closing spinning of thin-walled pipe fittings, obtain optimized closing values for each pass, and realize the efficient planning of the roller trajectory.

[0019] 2. The present invention uses a method combining finite element technology and instability mechanical criteria to optimize the design of the closing spinning process parameters of thin-walled pipe fittings, and realizes the accurate planning of the multi-pass closing spinning process of thin-walled pipe fittings.

[0020] 3. The present invention establishes an instability criterion based on energy balance by calculating the relationship between the work done by external forces and the minimum bending strain energy required for deformation, provides an optimization method for spinning process parameters combining theory and numerical simulation, and significantly reduces the R & D cost and time cost of enterprises compared with the traditional empirical method or a large number of experimental methods, and has obvious economic benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a flowchart of the process optimization method for preventing instability in multi-pass closing spinning of thin-walled pipe fittings in an embodiment of the present invention; Figure 2 It is the flow chart for optimizing the necking value for each pass in the embodiment of the present invention; Figure 3 It is the schematic diagram of the basic mechanical model of instability in the cylindrical shell coordinates in the embodiment of the present invention; Figure 4 It is the curve of the roller path during the multi-pass necking spinning process in the embodiment of the present invention; Figure 5 It is the setting diagram of the geometric constraints of the finite element model in the embodiment of the present invention; Figure 6 It is the simulation result diagram obtained by the process optimization method (single-pass roller feed is 8 mm) for preventing the instability of multi-pass necking spinning of thin-walled pipe fittings in the embodiment of the present invention; Figure 7 It is the simulation result diagram obtained when the roller feed is increased to 10 mm and the multi-pass necking spinning is unstable in the embodiment of the present invention. Specific embodiments

[0022] Please refer to the appendix Figures 1-5 Next, the present invention will be described in detail in conjunction with the accompanying drawings and specific embodiments.

[0023] As Figure 1 shown, a process optimization method for preventing the instability of multi-pass necking spinning of thin-walled pipe fittings provided by the present invention includes the following steps: S1. Determine the radial feed for each pass according to the instability mechanical model of the thin-walled shell; As Figure 2 shown, the specific steps of S1 include the following: S11. Initialize the radial feed for each pass, and the range is between the wall thickness and ; In an embodiment of the present invention, the setting of the initial feed is very crucial. Usually, it can start from a small value and gradually increase. For example, for a thin-walled pipe fitting with a wall thickness of 2 mm, the feed can be initially set to 3 mm for the first calculation, which can not only ensure the processing efficiency but also prevent the feed from being too large at the beginning and directly showing instability in the calculation results.

[0024] S12. Obtain the shell shape parameters before and after the necking deformation. The shell shape parameters include the neutral surface radius of the shell before the necking deformation, the neutral surface radius of the shell after the necking deformation, the necking length L, and the shell wall thickness ; These shape parameters describe the geometric characteristics of the thin-walled pipe fittings before and after necking spin forming. The neutral surface of the shell refers to the ideal surface in the middle of the wall thickness, where the stress is zero during the deformation process. The necking length L represents the axial distance from the starting position to the ending position of the necking. These parameters can be obtained from design drawings or initial measurements, and they directly affect the stress distribution during the necking process.

[0025] S13. Obtain the basic material parameters, where the material parameters include Young's modulus E, Poisson's ratio ν, and the equivalent true stress-true strain curve; The material parameters are crucial for accurately calculating the instability condition. Young's modulus E represents the stiffness of the material in the elastic deformation stage, and its unit is usually GPa. Poisson's ratio ν describes the ratio of the transverse deformation to the axial deformation of the material, which is usually a dimensionless number. For example, the Poisson's ratio of aluminum alloy is about 0.33. The equivalent true stress-true strain curve describes the mechanical behavior of the material in the plastic deformation stage, which is usually obtained through the tensile test of the material and input into the calculation model in the form of a table or a curve.

[0026] S14. Based on the shell shape parameters and the basic material parameters, calculate the work done by the external force during the deformation process of the shell through the mechanical model of thin-walled shell instability and the minimum bending strain energy required for deformation ; In this embodiment, the mechanical model of thin-walled shell instability is established based on the principle of energy balance. Calculate the work done by the external force during the deformation process of the shell through the mechanical model of thin-walled shell instability and the minimum bending strain energy required for deformation to determine the radial feed per pass; The energy method proposed by Senior is an important theoretical analysis method for predicting wrinkling in sheet metal plastic forming. References: B.W. Senior, Flange wrinkling in deep-drawing operations, Journal of the Mechanics and Physics of Solids, 1956, 4(4); This method calculates the bending strain energy ΔU using the energy method and calculates the external force work ΔT using the analytical method or numerical method. If ΔT < ΔU, the blank is stable; otherwise, the blank is in an unstable state. Wang et al. established a thin plate bending instability model using this method, which has since been gradually improved and applied to many shell instability problems. References: X. Wang, J. Cao, On the prediction of side-wall wrinkling in sheet metal forming processes[J]. International journal of mechanical sciences, 2000. 42: 2369-2394.

[0027] The spinning process will cause wrinkles to appear. In this embodiment, the appearance of wrinkles is simplified to a problem of instability of a cylindrical shell under the action of a normal concentrated force.

[0028] This embodiment draws on the establishment process of the thin plate bending instability criterion and can still establish a numerical optimization model using the expressions of the deformation energy U containing displacement and the external force work T, that is, the instability mechanical model of the thin-walled shell described in this embodiment, and then solve it using a computer.

[0029] The following is the derivation and establishment process of the instability mechanical model of the thin-walled shell described in this embodiment: Establish a Figure 3 shown orthogonal curvilinear coordinate system of the shell, which is established on the curvature line and the normal line of the middle surface. In the orthogonal curvilinear coordinate system of the microelement body in the figure, the two surface directions of α and β, that is, the axial direction and the circumferential direction, α and β are the two curvature directions, R1 is the axial curvature radius, used to label a general bending cylindrical shell, R2 is the circumferential curvature radius, γ points to the convex direction of the middle surface, MM1 and MM2 are the neutral axes of the axial section and the radial section respectively, and PP1 is the bending layer at a certain distance from the neutral surface.

[0030] During the spinning process, the wrinkling displacement on the blank is linearly distributed along the axial direction and cosine distributed along the circumferential direction. Thus, a two-dimensional deflection equation is established as shown in Equation (1): , where, w is the deflection, in radians; s is the wrinkling height, in meters;m is the number of folds; θ is the circumferential angle, with a value range of 0 to 2π, and the unit is radian (rad); l is the closing length, with the unit of millimeter; L is the axial length, with the unit of millimeter; During the closing process, the circumferential change amount ∆ l is as shown in Equation (2): , where, represents the deflection at with respect to the circumferential angle w partial derivative; θ is the differential calculation symbol; d is the partial derivative calculation symbol; According to the geometric relationship, it can be known that: , where R0 is the radius of the neutral surface of the shell before closing deformation, with the unit of millimeter; R2 is the radius of the neutral surface of the shell after closing deformation, with the unit of millimeter; It should be noted that R2 as the circumferential curvature radius has the same meaning as R2 as the radius of the neutral surface of the shell after closing deformation, and R2 as the circumferential curvature radius is the description during general derivation; Therefore, the fold height s can be deduced as: , that is , which is equivalent to the equivalent radius; w Substituting Equation (4) into (1), the deflection is: , The neutral surface geometric equation is established as shown in Equation (6): <00002l0> , u where, v , w , α corresponds to the displacements in the γ, β , three coordinate axes, with the unit of meter; , , α respectively represent the γ, , β positive strains in the three coordinate axes directions, dimensionless; , α , γ, β respectively represent the A, B , shearing strains in the three coordinate axes directions, dimensionless; A, BThey are all structure constants. Here, for the cylindrical shell, A = B = 1 can be taken; Therefore, the geometric equations of the entire cylindrical shell are shown in Equation (7): , where, e 1, e 2, e 12 respectively represent α , γ, β the principal strains in the three coordinate axis directions, dimensionless; The bending strain energy of the shell per unit area is established U 0 as shown in Equation (8): , where, v is the Poisson's ratio, dimensionless; E is the elastic modulus, is the thickness of the shell, with the unit of meter; It should be noted that in metallic materials, the elastic modulus refers to Young's modulus; Then, integrating over the entire shell area gives the total bending strain energy U as shown in Equation (9): , Considering: , A1, A2, and A3 in Equation (9) are as shown in Equation (11): , A 1 , A 2 , A 3 are the coefficients of each term after integrating Equation (8). R is the expression of the pipe diameter along the axial direction. The pipe diameters at different axial positions are different. The maximum end is R 0 , and the minimum end is R 2 ; Equation (9) can be regarded as a double-hook function (ax + b / x). This function reaches its minimum value at ax = b / x, that is . In Equation (9), when taking it has the minimum value. Name the m at this time as the critical buckling number m c .

[0031] Therefore, from the mathematical relationship, it can be known that there exists a critical buckling number m c such that the total bending strain energy U is the minimum. The critical buckling number mc As shown in Equation (12): , then the theoretical minimum bending strain energy U min As shown in Equation (13): , Substitute Equations (7), (9), and (10) into Equation (8) and take the differential element to obtain the minimum bending strain energy required for deformation , and its expression is as shown in Equation (14): (14), where is the minimum bending strain energy required for deformation, with the unit of joule (J); θ is the circumferential angular coordinate, with the value range from 0 to 2π, and the unit is radian (rad); is the wall thickness of the shell, with the unit of millimeter (mm); E is the Young's modulus, with the unit of megapascal (MPa); is the Poisson's ratio, dimensionless; is the axial strain, dimensionless; is the circumferential strain, dimensionless; is the shear strain, dimensionless; is the radial coordinate variable, with the value range from - / 2 to / 2, and the unit is millimeter (mm); The first integral of this formula represents the integral along the axial direction, the second integral represents the integral along the circumferential direction, and the third integral represents the integral along the radial direction. The result of the triple integral is the bending strain energy of the entire deformation region.

[0032] Establish the external force work expression as shown in Equation (15): , where T is the theoretical total external force work, is the average stress; is the material yield stress, which is the vector sum of the stress components in the three coordinate axis directions ( , , ); R is the expression of the pipe diameter along the axial direction. The pipe diameters at different axial positions are different, with the maximum end being R 2 , and the minimum end being R 0 ; The stress components in each coordinate axis direction are as shown in Equation (17): , where is the axial stress, representing α the stress component in the axial direction; is the radial stress, representing γ the stress component in the radial direction; is the circumferential stress, representing β the stress component in the circumferential direction; K and n are material constants;

[0033] Substitute Equation (10) into Equation (15). Considering that no work is done by the axial force, the work done by the external force is calculated according to Formula (19): , where ΔT is the work done by the internal and external forces in each pass, with the unit of joule (J); is the differential element of force, with the unit of newton (N); ΔL is the differential displacement, with the unit of millimeter (mm); L is the necking length, with the unit of millimeter (mm); π is a mathematical constant, approximately equal to 3.14159; l is the axial coordinate variable, with the value range from 0 to L , with the unit of millimeter (mm); is the absolute value of the circumferential stress, with the unit of megapascal (MPa); is the absolute value of the radial stress, with the unit of megapascal (MPa); is the radius of the neutral plane of the shell before necking deformation, with the unit of millimeter (mm); is the radius of the neutral plane of the shell after necking deformation, with the unit of millimeter (mm); For example, the average stress of 7075 aluminum alloy is , , where is the Mises equivalent strain; The full name of the Mises equivalent strain is the von Mises equivalent strain, also known as the equivalent strain or equivalent total strain; the Mises equivalent strain can be obtained from the component strains, and its formula can be found in books. This basic concept is prior art and will not be elaborated here.

[0034] Then the expression of the external force work is as shown in Equation (18): , In summary, the condition for non - buckling is .

[0035] Therefore, when the work done by the external force in each pass of the necking spinning process Greater than the minimum bending strain energy required for deformation When this occurs, the system energy will not be balanced, resulting in instability and wrinkling.

[0036] The instability mechanical model of the thin-walled shell in this embodiment ultimately depends on the construction of Formula (14) and Formula (19). The comparison of the energy values of these two formulas constitutes the core of the instability criterion. Formula (14) calculates the minimum bending strain energy required for the elastic deformation of the thin-walled pipe fitting, which is the energy necessary to prevent instability; Formula (19) calculates the work done by the external force on the thin-walled pipe fitting during the spinning process, considering the circumferential stress and the radial stress contributions; S15. Determine whether the predetermined instability criterion condition is satisfied. If satisfied, determine the radial feed per pass .

[0037] The predetermined instability criterion condition is that the work done by the external force is less than or equal to the minimum bending strain energy required for deformation and the radial feed per pass is the maximum value. This is an embodiment of energy balance, that is, the deformation energy provided by the system is sufficient to resist the work done by the external force, thereby maintaining the structural stability, and the radial feed per pass is the maximum value without instability; Calculate the work done by the external force according to the initialized radial feed per pass and the minimum bending strain energy required for deformation , and judge whether it is less than or equal to ; if so, increase the radial feed per pass according to the preset feed increment and recalculate the work done by the external force and the minimum bending strain energy required for deformation , and judge again whether it is less than or equal to ; if not, increase the radial feed per pass according to the preset feed reduction amount and recalculate the work done by the external force and the minimum bending strain energy required for deformation , and judge again whether it is less than or equal to ; iterate in turn until the predetermined instability criterion condition is satisfied, that is, the work done by the external force is less than or equal to the minimum bending strain energy required for deformation and the radial feed per pass is the maximum value.

[0038] Specifically, the preset feed increment is [5%, 10%] of the radial feed per pass in the previous time. Preferably, in this example, it is taken as 10% of the radial feed per pass in the previous time; the preset feed reduction is half of the preset feed increment; thus, the maximum value of the radial feed per pass can be quickly determined. The value-taking methods of the preset increment and the preset reduction can be adjusted according to different processing stages to meet the processing requirements of different stages.

[0039] S2. Calculate the total number of passes required for multi-pass necking spinning of the thin-walled pipe fitting according to the radial feed, and record the total number of passes and the feed per pass. The total number of passes required for multi-pass necking spinning of the thin-walled pipe fitting is calculated according to formula (20): , where N is the total number of passes required for multi-pass necking spinning of the thin-walled pipe fitting, a dimensionless integer; is the total radial feed for multi-pass necking spinning of the thin-walled pipe fitting, with the unit of millimeter (mm); is the radial feed per pass, with the unit of millimeter (mm).

[0040] It can be known from the calculation of the feed by the instability mechanics model of the thin-walled shell that the limit radial feed to avoid instability in subsequent passes (such as the second, third, etc. passes) is smaller than the feed in the previous passes. That is to say, the smaller the blank diameter, the larger the limit value of the radial feed; in order to efficiently estimate the total number of feed passes and consider the requirements of the surface quality of the part in actual forming, the same feed per pass as the first pass is adopted to estimate the total number of passes. This calculation method is intuitive and clear, and the total necking amount is divided by the feed per pass to obtain the required number of passes.

[0041] For example, if the total radial amount that a thin-walled pipe fitting needs to neck is 60 millimeters, and the safe feed per pass is 8 millimeters, then the total number of passes is 60 / 8 = 7.5, and rounding up gives 8 passes. In actual operation, the feed of the last pass may be appropriately adjusted to make the total number of passes an integer. For example, the feed of the first 7 passes is 8 mm each, and the feed of the 8th pass is 4 mm, and record the total number of passes and the feed per pass.

[0042] In another possible implementation manner, in step S2, after the spinning is completed according to the radial feed calculated by the instability mechanics model of the thin-walled shell, the radial feed of the next pass of the current structure of the thin-walled pipe fitting can be recalculated according to the instability mechanics model of the thin-walled shell, record the feed per pass and the corresponding number of passes, repeat back and forth and perform superposition calculation on the radial feed of each time until the total radial feed obtained by the superposition calculation is greater than or equal to If it is greater than Then, appropriately adjust the feed rate of the last pass and record the total number of passes and the feed rate of each pass.

[0043] S3. Generate the motion trajectory of the spinning wheel during multi-pass necking spinning according to the total number of passes and the feed rate of each pass; Preferably, as Figure 4 shown, according to the total number of passes and the feed rate of each pass, the feed trajectory of the spinning wheel in each pass of spinning forming adopts a straight-line trajectory, and a drawing and pressing reciprocating spinning method is used for multi-pass spinning forming trajectory planning as the motion trajectory of the spinning wheel, and the coordinates of the reference point of the spinning wheel relative to the geometric center of the blank are calculated according to geometric coordinate conversion.

[0044] The straight-line trajectory is easier to control and implement compared to the curved trajectory, reducing the complexity of programming. At the same time, the drawing and pressing reciprocating spinning method can make full use of the reciprocating motion of the spinning wheel to improve the processing efficiency. In actual operation, the coordinate calculation of the reference point of the spinning wheel usually takes into account the geometric characteristics of the spinning wheel and the establishment method of the workpiece coordinate system to ensure that the spinning wheel can move precisely along the predetermined trajectory. For example, for a thin-walled pipe fitting with a diameter of 200 mm, in each pass of processing, the starting position of the spinning wheel may be set at a certain position outside the central axis of the workpiece, and then it moves along the radial direction according to the predetermined feed rate.

[0045] S4. Establish a finite element model for necking spinning of thin-walled pipe fittings, simulate the necking spinning process according to the motion trajectory of the spinning wheel, and obtain the spinning process parameters based on the simulation results; Preferably, the thin-walled pipe fittings described in the present invention refer to metal pipe fittings with a wall thickness of 0.5 mm to 15 mm that can be used for spinning processing, and the outer diameter of the pipe blank is greater than or equal to 50 times the wall thickness. This definition clarifies the applicable object range of the present invention and is particularly suitable for thin-walled structural parts commonly used in fields such as aerospace and deep-sea exploration. As Figure 5 shown, the steps of establishing a finite element model for necking spinning of thin-walled pipe fittings in S4 specifically include: S41. Establish a three-dimensional model of the thin-walled pipe fitting, the spinning wheel and the fixture based on finite element software, set the thin-walled pipe fitting as a shell, and set the mesh type as hexahedron SC8R element; In actual modeling, commercial finite element software such as ABAQUS or ANSYS can be used to establish the model. The hexahedron SC8R element is a continuous shell element with reduced integration function, which can effectively reduce the calculation amount while maintaining high calculation accuracy. For thin-walled components, shell elements are more suitable than solid elements because they can significantly reduce the calculation cost while ensuring the calculation accuracy.

[0046] S42. Set the working angle α between the spinning wheel and the thin-walled pipe fitting and set penalty contact; The working angle α refers to the angle between the axis of the wheel and the axis of the thin-walled tube, which is calculated by formula (21): , in, L is the closing length, in millimeters (mm); The radial feed of each pass is in millimeters (mm); Penalty contact is a commonly used contact algorithm that applies a penalty factor to the contact surfaces to prevent excessive penetration without overly constraining their relative motion. For complex contact problems like spinning, the penalty contact method ensures computational stability while effectively simulating actual contact behavior.

[0047] In this example, the penalty factor is 0.05; S43, coupling constraining the thin-walled tube and the fixture through a reference point, and applying a rotation boundary condition to the fixture to drive the thin-walled tube to rotate; The reference point will be the geometric center of the bottom of the pipe diameter, that is, the clamping center of the fixture; Reference point coupling is a common technique in finite element modeling. It links the motion of a reference point with the motion of a set of nodes or surfaces, simplifying the application of boundary conditions. In this invention, by coupling a fixture to a reference point and then applying rotational boundary conditions to the reference point, the self-rotation of a thin-walled tube can be easily achieved, which is consistent with the rotational motion of the workpiece during actual machining. For example, the fixture can be set to rotate around its axis at 300 rpm, which will cause the thin-walled tube to rotate at the same speed.

[0048] S44. Assign the material parameters required for thermal-mechanical coupling calculations to thin-walled pipe fittings, including Young's modulus, Poisson's ratio, constitutive equation, initial temperature, specific heat capacity, thermal conductivity, thermal expansion coefficient, heat transfer coefficient, and frictional heat generation coefficient; Thermal coupling analysis takes into account the mutual influence of temperature and deformation fields, which is crucial for accurately simulating the spinning process. The initial temperature is usually set to the ambient temperature, such as 20°C. The specific heat capacity describes the amount of heat required to increase the unit temperature of a material per unit mass, and the unit is J / (kg·K). The thermal conductivity coefficient indicates the ability of a material to conduct heat, and the unit is W / (m·K). The thermal expansion coefficient describes the degree to which a material expands with temperature changes, and the unit is K⁻¹. The heat transfer coefficient describes the rate of heat exchange between the material and the environment, and the unit is W / (m²·K). The frictional heat generation coefficient describes the proportion of frictional work converted into heat, and is usually a dimensionless number.

[0049] S45. Assign the spinning process parameters to the finite element model and perform simulation calculations of the necking spinning process according to the movement trajectory of the spinning wheel, and obtain the spinning process parameters based on the simulation results; the spinning process parameters include the spindle speed, feed ratio, and spinning wheel fillet radius.

[0050] S5. Perform actual spinning on the thin-walled pipe fitting according to the spinning process parameters and the movement trajectory of the spinning wheel.

[0051] In a typical embodiment, the spindle speed can be set in the range of 300 - 600 r / min, the feed ratio can be set in the range of 0.2 - 1.0 mm / r, and the spinning wheel fillet radius can be selected according to the wall thickness of the workpiece, usually 5 - 10 times the wall thickness. The selection of these parameters needs to comprehensively consider factors such as material properties, workpiece geometry, and equipment capabilities. Through finite element simulation, the stress distribution, deformation conditions, and possible instability risks under different parameter combinations can be predicted, so as to select the optimal parameter combination.

[0052] The following uses a specific embodiment to illustrate the application of the present invention: In this embodiment, a 7075 - O aluminum alloy thin-walled pipe fitting with an outer diameter of 72 mm and a wall thickness of 1.2 mm is used for necking spinning simulation calculation and processing, which specifically includes the following steps: Initialization stage: Initialize the radial feed per pass δd = 7 mm.

[0053] Extract the shape parameters of the necking deformed shell, and calculate the work done by the external force during the deformation process of the shell under the current radial feed δd according to the instability mechanical model of the thin-walled shell using MATLAB software = 1.1359×10 6 J and the minimum bending strain energy required for deformation = 1.6228×10 6 J, that is .

[0054] First iteration: Since , it shows that instability will not occur under the current conditions.

[0055] Feed adjustment: Increase the radial feed per pass according to the preset feed increment. The preset feed increment is 10% of the radial feed per pass in the previous (i.e., initial) pass, so the new radial feed δd1 = δd + δd×10% = 7 + 7×0.1 = 7.7 mm.

[0056] Recalculate and judge: Use MATLAB software to calculate the work done by the external force under the new radial feed δd1 = 1.5327×10 6 J and the minimum bending strain energy required for deformation =1.9451×10 6 J, that is ; Second iteration: Since , it shows that instability will not occur under the current conditions.

[0057] Feed adjustment: Increase the radial feed again according to the preset feed increment (10% of the previous radial feed δd1). The new radial feed δd1 == δd1 + δd1×10% = 7.7 + 7.7×0.1 = 8.47 mm.

[0058] Recalculate and judge: Use MATLAB software to calculate the work done by the external force under δd2 =1.9236×10 6 J and the minimum bending strain energy required for deformation =1.8215×10^6 J, that is ; Third iteration: Since , it shows that instability will occur under the current conditions.

[0059] Feed adjustment: Reduce the radial feed according to the preset feed reduction (half of the preset feed increment, that is, 5% of the previous radial feed δd1). The new radial feed δd3 = δd2 - δd1×5% = 8.47 - 7.7×0.05 = 8.47 - 0.385 = 8.085 mm.

[0060] Recalculate and judge: Use MATLAB software to calculate the work done by the external force under δd3 =1.9752×10 6 J and the minimum bending strain energy required for deformation =1.8764×10 6 J, continue to judge the size relationship between and ; Continuously repeat the above process of feed adjustment, recalculation and judgment. If in a certain judgment, it is found that , and when the feed is adjusted according to the increment, will occur, then gradually fine-tune the feed to make it approach the condition of meeting the predetermined instability criterion (the work done by the external force is less than or equal to the minimum bending strain energy required for deformation and the radial feed for each pass is the maximum value).

[0061] Finally, after multiple iterations, the maximum value of the radial feed for each pass is determined to be 8 mm. At this time, the requirement that the work done by the external force is less than or equal to the minimum bending strain energy required for deformation is met. Therefore, the radial feed for each pass can be set as =8 mm; Step 2: The total necking numerical value of the thin-walled pipe fitting = 56 mm, then the number of necking spinning passes N = ΔD / Δd = 7; Step 3: Select a linear trajectory and adopt a pulling and pressing reciprocating spinning method for multi-pass spinning forming process planning. Draw the roller trajectory as Figure 4 shown, and then convert it into the coordinates of the roller reference point relative to the geometric center of the blank; Step 4: Establish a simplified 3D model of the thin-walled pipe fitting, roller and fixture. Set the thin-walled pipe fitting as a shell, and set the mesh type as hexahedron SC8R element; Set the roller fillet radius r = 10 mm, and the necking length L = 60 mm, then the working angle α between the roller and the thin-walled pipe fitting is 52.6°, and set the penalty contact friction coefficient to 0.05; Couple and constrain the thin-walled pipe fitting and the fixture through reference points, and apply a rotational boundary condition to the fixture to drive the thin-walled pipe fitting to rotate; Assign the required material constants to the finite element model, including Young's modulus, Poisson's ratio, constitutive equation, initial temperature, specific heat capacity, thermal conductivity, coefficient of thermal expansion, heat transfer coefficient, friction heat generation coefficient; Conduct a necking spinning process simulation calculation based on the motion trajectory of the roller through the finite element model, specifically including: (1) Determine the roller motion trajectory according to the roller trajectory obtained in Step 3: In the finite element simulation, it is necessary to convert the motion trajectory of the roller into a mathematical expression or discrete motion points in order to apply the corresponding displacement load during the simulation process; (2) Apply motion loads: In the finite element software, apply the motion trajectory of the roller as a displacement load to the roller model; For the axial feeding motion, define the feeding amount of the roller along the axis of the mandrel; For the radial motion, define the feeding speed of the roller in the direction perpendicular to the axis of the mandrel to ensure that the application of the motion load is consistent with the actual roller motion during processing to accurately simulate the spinning process; (3) Conduct simulation calculations: Select a suitable solver (such as an explicit solver or an implicit solver), and set the solver parameters according to the characteristics of the spinning process and the calculation scale; Start the finite element simulation calculation. The software will calculate the distribution and variation laws of physical fields such as stress, strain, displacement, and temperature of the blank during the spinning process based on the established model, the given spinning process parameters (including spindle speed, feed ratio, roller fillet radius) and the applied motion loads; During the calculation process, it is necessary to monitor the stability and convergence of the calculation. If there are any abnormal situations, adjust the spinning process parameters in time to finally obtain the simulation calculation results; Observe the simulation calculation results. If it is confirmed that the blank deforms uniformly and there is no instability and wrinkling during the spinning process, the adjusted spinning process parameters and the roller motion trajectory can be selected for actual spinning.

[0062] As Figure 7As shown, this embodiment also provides a simulation result diagram obtained from the instability of multi-pass necking spinning when the roller feed is increased to 10 mm. It can be seen that there are many wrinkles at the top. The more wrinkles there are, the more serious the instability during the necking spinning process of the thin-walled pipe fitting; while the result of necking spinning the thin-walled pipe fitting by the necking spinning process optimization method of the thin-walled pipe fitting (the single-pass roller feed is 8 mm), the final simulation result is as Figure 6 shown. It can be seen that the surface is smooth and continuous, indicating that there is no instability during the necking spinning process of the thin-walled pipe fitting, and the spinning forming effect is very good.

[0063] Through the above steps, the optimization of the necking spinning process of the thin-walled pipe fitting is successfully achieved, effectively avoiding the occurrence of instability, and at the same time obtaining the optimal process parameters. In addition, compared with the traditional empirical method or the large number of experimental methods, the method of the present invention greatly reduces the development cost and time, and has obvious economic benefits.

[0064] The above are only the preferred embodiments of the present invention, and do not limit the patent protection scope of the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A process optimization method for preventing instability in multi-pass necking spinning of thin-walled pipe fittings, characterized in that The method includes the following steps: S1. Determine the maximum radial feed per pass according to the mechanical model of thin-walled shell instability; the mechanical model of thin-walled shell instability is constructed based on formula (14) and formula (19): （14）， （19）， Among them, is the minimum bending strain energy required for deformation; is the neutral surface radius of the shell before necking deformation; is the neutral surface radius of the shell after necking deformation; θ is the circumferential angular coordinate; is the wall thickness of the shell; E is Young's modulus; is the Poisson's ratio; is the axial strain; is the circumferential strain; is the shear strain; is the radial coordinate variable; ΔT is the work done by the external force; is the differential force; ΔL is the differential displacement; L is the necking length, in meters; l is the axial coordinate variable, with a value range of 0 to L, in meters; is the absolute value of the circumferential stress; is the absolute value of the radial stress; S2. Calculate the total number of passes required for multi-pass necking spinning of the thin-walled pipe fitting according to the radial feed, and record the total number of passes and the feed per pass; S3. Generate the motion trajectory of the spinning wheel during multi-pass necking spinning according to the total number of passes and the feed per pass; S4. Establish a finite element model for necking spinning of the thin-walled pipe fitting, simulate the necking spinning process according to the motion trajectory of the spinning wheel, and obtain the spinning process parameters based on the simulation results.

2. The process optimization method for preventing instability in multi-pass necking spin forming of thin-walled pipe fittings according to claim 1, characterized in that Step S1 specifically includes the following steps: S11. Initialize the radial feed per pass, with the range between the wall thickness and . S12. Obtain the shell shape parameters before and after necking deformation, where the shell shape parameters include the neutral plane radius of the shell before necking deformation , the neutral plane radius of the shell after necking deformation , the necking length L, and the shell wall thickness ; S13. Obtain the basic material parameters, which include Young's modulus E, Poisson's ratio ν, and the equivalent true stress-true strain curve; S14. Based on the shape parameters and basic material parameters of the housing, calculate the work done by the external force during the deformation process of the housing through the thin-walled housing instability mechanical model and the minimum bending strain energy required for deformation ; S15. Work done by external force and the minimum bending strain energy required for deformation Determine whether the predetermined buckling criterion condition is satisfied. If it is satisfied, determine the radial feed per pass.

3. The process optimization method for preventing instability in multi-pass necking spinning of thin-walled pipe fittings according to claim 2, wherein, The predetermined instability criterion condition described in step S15 is that the work done by the external force is less than or equal to the minimum bending strain energy required for deformation and the radial feed per pass does not need to be increased.

4. The process optimization method for preventing instability in multi-pass necking spinning of thin-walled pipe fittings according to claim 1, characterized in that The total number of passes required for multi-pass necking spinning of the thin-walled pipe fitting in step S2 is calculated according to formula (20): ， where N is the total number of passes required for multi-pass necking spinning of the thin-walled pipe fitting, is the total radial feed of multi-pass necking spinning of the thin-walled pipe fitting, is the radial feed per pass.

5. The process optimization method for preventing instability in multi-pass necking spin forming of thin-walled pipe fittings according to claim 1, wherein Step S3 specifically includes: the feed trajectory of the spinning wheel in each pass of spinning forming adopts a straight-line trajectory, and the multi-pass spinning forming trajectory is planned by using the pull-press reciprocating spinning method as the motion trajectory of the spinning wheel, and the coordinates of the reference point of the spinning wheel relative to the geometric center of the blank are calculated.

6. The process optimization method for preventing instability in multi-pass necking spinning of thin-walled pipe fittings according to claim 1, characterized in that, Step S4 specifically includes the following steps: S41. Establish a three-dimensional model of the thin-walled pipe fitting, the spinning wheel and the fixture based on finite element software, set the thin-walled pipe fitting as a shell, and set the mesh type as hexahedron SC8R element; S42. Set the working angle α between the spinning wheel and the thin-walled pipe fitting, and set penalty contact; S43. Couple and constrain the thin-walled pipe fitting and the fixture through the reference point, and apply a rotational boundary condition to the fixture to drive the thin-walled pipe fitting to rotate; S44. Assign the material parameters required for thermo-mechanical coupling calculation to the thin-walled pipe fitting; S45: Assign the spinning process parameters to the finite element model, simulate and calculate the necking spinning process according to the motion trajectory of the spinning wheel, and optimize the spinning process parameters according to the simulation results.

7. The process optimization method for preventing instability in multi-pass necking spinning of thin-walled pipe fittings according to claim 1, wherein The method further includes: S5. Perform actual spinning on the thin-walled pipe fitting according to the spinning process parameters and the motion trajectory of the spinning wheel.

8. The process optimization method for preventing instability in multi-pass necking spin forming of thin-walled pipe fittings according to claim 6, characterized in that, The working angle α in step S42 is calculated by formula (21): ， Among them, L is the closing length, with the unit of millimeter; is the radial feed per pass, with the unit of millimeter.

9. The process optimization method for preventing instability in multi-pass necking spin forming of thin-walled pipe fittings according to claim 6, characterized in that, The material parameters required for the thermo-mechanical coupling calculation in step S44 include Young's modulus, Poisson's ratio, constitutive equation, initial temperature, specific heat capacity, heat conduction coefficient, thermal expansion coefficient, heat transfer coefficient, and friction heat generation coefficient.

10. The process optimization method for preventing instability in multi-pass necking spinning of thin-walled pipe fittings according to claim 1, characterized in that, The thin-walled pipe fitting refers to a metal pipe fitting with a wall thickness of 0.5 mm to 15 mm that can be used for spinning processing, and the outer diameter of the pipe blank is greater than or equal to 50 times the wall thickness.

Images