Shape control method of multi-step roll bending skin

Abstract

The invention belongs to the technical field of aircraft manufacturing, and discloses a shape control method for a multi-step roll bending skin. According to the shape control method for the multi-step roll bending skin, through the combination of pre-roll bending, machining and regional roll bending, the situation that the shape size of a roll bending formed plate is out of tolerance due to deformation after machining is effectively reduced, the machining precision is improved, shape correction after machining is reduced, the machining difficulty is lowered, and the machining efficiency is improved.

Description

Technical Field

[0001] The invention relates to the technical field of skin processing, and in particular to a method for controlling the shape of a multi-step rolled skin. Background Art

[0002] The side skin of the fuselage in an aircraft is a large, multi-step, thin-walled part with equal curvature. This part is made of aluminum-lithium alloy. Since lithium is one of the most active elements and has greater chemical activity than aluminum, lithium corrodes faster than aluminum during chemical milling, so it is easy to produce uneven phenomena such as pitting and small pits, which increases the surface roughness of the parts in the chemical milling area, which will have a great impact on the fatigue strength and other properties of the structure. In order to ensure the service performance of the parts, the parts need to be processed by mechanical milling instead of chemical milling. The parts are manufactured by flat milling and pad gate pressure forming. The plastic deformation and residual stress state inside the workpiece are not good under this process route.

[0003] The main forming methods of curved skin are roll forming and stretch forming. There is a large residual stress inside the rolled skin. After forming, machining will cause large thickness deformation and higher technical difficulty. The mirror milling of large-sized multi-step rolled skin is difficult. Even after conventional five-axis skin milling, the workload of correction caused by large deformation of parts is also very large.

[0004] Therefore, there is an urgent need for a shape control method for a multi-step rolled skin to solve the problems in the prior art. Summary of the invention

[0005] The purpose of the present invention is to provide a method for controlling the shape of a multi-step rolled skin, which effectively reduces the size deviation of the rolled sheet due to deformation after machining, and improves the machining accuracy through the combination of pre-rolling, machining and regional rolling.

[0006] To achieve this object, the present invention adopts the following technical solutions:

[0007] The shape control method of multi-step rolled skin includes:

[0008] S100, performing a pre-rolling and bending process on the plate to make the plate have a certain curvature;

[0009] S200, machining the pre-bent sheet material so that the pre-bent sheet material has a desired thickness characteristic;

[0010] S300, performing a rolling bending process on the machined plate in different regions so that the machined plate has a desired curvature characteristic.

[0011] Optionally, the pre-bending process includes:

[0012] S110, presetting a plurality of thickness regions on the plate, and obtaining a target curvature radius of the minimum thickness region of the plate, selecting a test strip having the same thickness value as the plate, and performing finite element simulation and test verification on the test strip according to the target curvature radius of the minimum thickness region to obtain a minimum thickness pre-rolling curvature radius value;

[0013] S120, pre-rolling the plate according to the minimum thickness pre-rolling bending radius value.

[0014] Optionally, the pre-bending process further includes:

[0015] S130, if a plurality of the thickness regions are arranged side by side, the pre-rolling curvature radius value of the plate is equal to the pre-rolling curvature radius value of the minimum thickness;

[0016] S140: If a plurality of the thickness regions are distributed in a nested manner, the pre-rolling curvature radius value of the plate is smaller than the minimum thickness pre-rolling curvature value.

[0017] Optionally, the machining process includes:

[0018] S210, machining the plate by five-axis skin milling or mirror milling, so that the pre-rolled plate has a desired thickness characteristic.

[0019] Optionally, step S210 includes:

[0020] S211. In the process of processing the plate by five-axis skin milling or mirror milling, the plate is processed in a sequence from thick to thin according to the thickness values ​​of the plurality of thickness regions.

[0021] Optionally, step S210 further includes:

[0022] S212: If there is a region with a thickness of less than 1.5 mm on the plate, the region with a thickness of less than 1.5 mm is first processed in layers, and other regions are processed in a sequence from thick to thin.

[0023] Optionally, the regional rolling process includes:

[0024] S310, when the curvature change direction is consistent with the thickness change direction, the area rolling is performed on other areas outside the minimum thickness area of ​​the plate, so that the other areas reach their respective target curvature radius values.

[0025] Optionally, the regional rolling process further includes:

[0026] S310, when the curvature change direction is perpendicular to the thickness change direction, performing regional rolling on the maximum thickness area of ​​the plate so that the maximum thickness area reaches the maximum thickness target curvature radius value.

[0027] Beneficial effects:

[0028] The shape control method of the multi-step rolled skin provided by the present invention effectively reduces the sheet material to be rolled and formed, and the sheet material shape dimensional tolerance caused by deformation after machining, improves the machining accuracy, reduces the shape correction after machining, reduces the machining difficulty, and improves the machining efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a flow chart of the shape control method of the multi-step rolled skin provided by the present invention;

[0030] Figure 2 It is a schematic diagram of the direction of change of curvature and thickness of the partitioned roll provided by the present invention;

[0031] Figure 3 It is a simplified geometric relationship diagram in the shape control method of the multi-step rolled skin of the present invention;

[0032] Figure 4 It is a stress distribution diagram in the shape control method of the multi-step rolled skin of the present invention.

[0033] In the figure:

[0034] 1100, first thickness characteristic; 1200, second thickness characteristic; 1300, third thickness characteristic. DETAILED DESCRIPTION

[0035] The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It is to be understood that the specific embodiments described herein are only used to explain the present invention, rather than to limit the present invention. It should also be noted that, for ease of description, only parts related to the present invention, rather than all structures, are shown in the accompanying drawings.

[0036] In the description of the present invention, unless otherwise clearly specified and limited, the terms "connected", "connected", and "fixed" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0037] In the present invention, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may include that the first and second features are in direct contact, or may include that the first and second features are not in direct contact but are in contact through another feature between them. Moreover, a first feature being "above", "above" and "above" a second feature includes that the first feature is directly above and obliquely above the second feature, or simply indicates that the first feature is higher in level than the second feature. A first feature being "below", "below" and "below" a second feature includes that the first feature is directly below and obliquely below the second feature, or simply indicates that the first feature is lower in level than the second feature.

[0038] In the description of this embodiment, the terms "upper", "lower", "left", "right" and other directions or positional relationships are based on the directions or positional relationships shown in the drawings, and are only for the convenience of description and simplification of operation, rather than indicating or implying that the device or element referred to must have a specific direction, be constructed and operate in a specific direction, and therefore cannot be understood as limiting the present invention. In addition, the terms "first" and "second" are only used to distinguish in the description and have no special meaning.

[0039] This embodiment provides a method for controlling the shape of a multi-step rolled skin. Figure 1 As shown, the shape control method of the multi-step rolled skin includes:

[0040] S100, pre-rolling and bending the plate to give the plate a certain curvature;

[0041] S200, machining the pre-bent sheet material so that the pre-bent sheet material has a desired thickness characteristic;

[0042] S300, performing a roll bending process on the machined plate in different regions so that the machined plate has a desired curvature characteristic.

[0043] The shape control method of the multi-step rolled skin effectively reduces the sheet material that is rolled and formed, and the sheet material shape dimensional tolerance caused by deformation after machining, by combining pre-rolling, machining and regional rolling, thereby improving machining accuracy, reducing the shape correction after machining, reducing machining difficulty and improving machining efficiency.

[0044] Optionally, the pre-bending process includes:

[0045] S110. Preset multiple thickness areas on the plate, and obtain the target curvature radius of the minimum thickness area of ​​the plate, select a test strip with the same thickness value as the plate, and perform finite element simulation and experimental verification on the test strip according to the target curvature radius of the minimum thickness area to obtain the minimum thickness pre-rolling curvature radius value; through finite element simulation and experimental verification, a more accurate minimum thickness pre-rolling curvature radius value can be obtained to ensure that the design requirements are met.

[0046] S120, pre-bending the plate according to the minimum thickness pre-bending radius value. Pre-bending according to the target curvature radius value of the minimum thickness area can more accurately control the curvature characteristics of the plate, thereby improving the control accuracy of the overall shape.

[0047] Optionally, the pre-bending process further includes:

[0048] S130. If multiple thickness regions are arranged side by side, the pre-rolling curvature radius value of the plate is equal to the pre-rolling curvature radius value of the minimum thickness, which can ensure that the plate after rolling has the same curvature characteristics in these thickness regions arranged side by side. Since the thickness regions arranged side by side have the same curvature radius value, the shape and appearance characteristics of the plate after rolling will be consistent in these regions, which helps to ensure the uniformity and consistency of the overall appearance. And using the same rolling process parameters to process these regions without additional adjustment or control can simplify the processing process, reduce the risk of deformation, and further improve processing efficiency and accuracy.

[0049] S140. If multiple thickness regions are nested, and the pre-rolling curvature radius of the plate is less than the minimum thickness pre-rolling curvature value, the shape of the nested distribution region can be controlled, the shape characteristics can be optimized, stress concentration can be reduced, and the appearance quality and structural strength after processing can be improved. Such a processing method helps to ensure that the plate meets the design requirements and improves the overall processing efficiency and accuracy.

[0050] Optionally, the machining process includes:

[0051] S210, use five-axis skin milling or mirror milling to machine the plate so that the pre-rolled plate has the required thickness characteristics. Five-axis skin milling and mirror milling are commonly used plate machining technologies. By adjusting the position and angle of the tool, complex curved surfaces can be processed and can flexibly cope with plates of different shapes and sizes, thereby meeting the thickness characteristics of the pre-rolled plate.

[0052] Specifically, the mirror milling technology uses a direct control method for skin thickness. It only needs to control the distance between the support head and the processing head to achieve precise control of the remaining thickness of the skin. Mirror milling can be used to mill large curved skins without special tooling. The support head can offset the axial milling force of the processing head, improve local rigidity, reduce workpiece deformation, suppress processing chatter, and ensure processing accuracy. In addition, mirror milling also integrates functions such as trimming, notching, hole making, real-time thickness monitoring and error compensation. In this way, all processing after the skin blank is formed can be completed in one clamping, thereby eliminating the repeated positioning errors caused by multiple clamping and ensuring the processing accuracy of skin parts.

[0053] Optionally, step S210 includes:

[0054] S211. When processing plates using five-axis skin milling or mirror milling, the sequential processing from thick to thin can be used according to the thickness values ​​of multiple thickness areas to optimize the processing process, reduce processing deformation, protect the integrity of thinner areas, and improve processing efficiency and accuracy. Such a processing sequence can better control cutting force and deformation risk, ensuring that the final product meets the design requirements.

[0055] Optionally, step S210 further includes:

[0056] S212. If there is a thickness area less than 1.5 mm on the plate, first perform layered processing on the thickness area less than 1.5 mm, and process the other areas in a sequence from thick to thin. Layered processing of the thickness area less than 1.5 mm can effectively control the cutting force and processing quality. Since the small thickness area is easily affected by vibration and deformation, layered processing can reduce the cutting depth of each processing, reduce the cutting force, thereby reducing the risk of deformation and improving the processing quality and precision. Other areas are processed in a sequence from thick to thin. This processing sequence can optimize the processing process and improve processing efficiency and precision.

[0057] Therefore, if there are areas with a thickness of less than 1.5 mm on the plate, these areas should be processed in layers first, and the other areas should be processed in order from thick to thin. This can optimize the processing process, reduce the risk of deformation in small thickness areas, protect their integrity, and improve processing efficiency and accuracy, ensuring that the plate meets the design requirements and reducing the risk of dimensional deviations.

[0058] Alternatively, if Figure 2 As shown, during the regional rolling bending process, when there are a first thickness feature 1100, a second thickness feature 1200 and a third thickness feature 1300 on the plate, the regional rolling bending process includes:

[0059] S310, when the curvature change direction is consistent with the thickness feature change direction, the area rolling is performed on other areas outside the minimum thickness area of ​​the plate to make the other areas reach their respective target curvature radius values. By correcting the curvature features of the non-minimum thickness area, the curvature deviation caused by machining or rolling can be corrected, the shape size deviation can be reduced, and the accuracy and consistency of the overall shape can be improved.

[0060] Optionally, the regional rolling process further includes:

[0061] S310, when the curvature change direction is perpendicular to the thickness feature change direction, the maximum thickness area of ​​the plate is rolled in different regions to make the maximum thickness area reach the maximum thickness target curvature radius value. After the maximum thickness area is deformed, the deformation of other areas can be driven by the characteristics of the material, thereby optimizing the processing process, improving the shape control accuracy and appearance quality, reducing the risk of shape size deviation, and optimizing the shape and processing quality of the plate. It should be noted that this embodiment only lists the situation where there are three thickness features on the plate. In the actual processing process, the number of thickness features on the plate can be changed accordingly according to the actual working conditions, and the number of thickness features on the plate is not limited.

[0062] The specific theoretical derivation and experimental verification process of the shape control method of multi-step rolled skin is as follows:

[0063] 1. Forward Theory Derivation

[0064] Since it is very difficult to accurately calculate the bending moment before unloading, some basic assumptions need to be made for approximate calculations. The assumptions are as follows:

[0065] (1) The bending process is pure bending;

[0066] (2) The position of the neutral layer in the cross section remains unchanged before and after unloading and passes through the center of gravity of the cross section;

[0067] (3) The stress-strain state during bending is linear, and only the stress σ generated in the tangential direction is considered θ and strain ε θ ;

[0068] (4) The relationship between stress and strain, in the elastic deformation zone (ε θ ≤ε s ) obeys Hooke's law σ θ =Eε θ ; In the plastic deformation zone (ε θ >ε s ) obeys the simplified straight line law D is the plastic modulus.

[0069] Based on the above assumptions, the applied bending moment M can be determined by the following formula:

[0070]

[0071] Considering the existence of elastic deformation zone in the cross section, equation (1) can be rewritten as

[0072]

[0073] In the formula, y s is the distance between the elastic-plastic interface and the neutral layer, ρ is the radius of curvature of the neutral layer, and B is the width of the plate.

[0074] Under the action of the external bending moment M, the tangential strain of the fiber at a distance y from the neutral layer can be expressed as When y≤y s When the tangential stress is When y ≥ y s When , the tangential stress can be expressed as

[0075] Substituting the first and second terms in formula (2) respectively, we get:

[0076]

[0077] Integrate and sort, we get

[0078]

[0079] Where ρ is determined by the radius r of the roller, the spacing L between the lower rollers and the amount of pressure H; s The value of stress-strain relationship is used in y = y s Continuously solve at, we have:

[0080]

[0081]

[0082] The simplified geometric relationship in the rolling process is as follows: Figure 3 shown.

[0083] According to the geometric relationship in the rolling process, it can be obtained:

[0084]

[0085]

[0086] The stress distribution on the plate section during loading is as follows: Figure 4 As shown in (a), the internal bending moment disappears when unloading occurs. Since unloading is an elastic deformation, the following will be caused in the plate section: Figure 4 (b) shows the change of recovery stress. The algebraic sum of the above two stresses is Figure 4 (c) shows the residual stress. The internal bending moments of the residual stress are balanced with each other.

[0087] Assuming that elastic rebound occurs during unloading and the stress is distributed linearly, let the recovery stress expression at any point y be σ'=ky, then

[0088]

[0089] After finishing, we can get:

[0090]

[0091] After unloading, the bending moment expression after removing the material of thickness y from the inner area to the outer area is:

[0092]

[0093] Expand to get:

[0094]

[0095]

[0096] Where, the expressions of ρ,k are shown in equations (8) and (10).

[0097] Plate ρ 卸 is the radius of the neutral layer after unloading.

[0098] The above formula can be rearranged into:

[0099]

[0100] At this point, the theoretical analysis of the material removal process of rolled parts is completed.

[0101] 2. Experimental data verification

[0102] The thickness of the plate is t = 6.5 mm, the elastic modulus is E = 71980 MPa, and the yield strength is σ s =453MPa, plastic modulus D=919.2MPa, initial rebound curvature radius ρ0=1980mm, sheet width B=2000mm. Roller radius r=50mm, spacing between lower rollers L=250mm, downward pressure H=12.15mm. (Note: H conflicts with ρ0 at this time, ρ0 data shall prevail, and H=21.06mm shall be selected).

[0103] 3. Reverse calculation analysis

[0104] For this system, the input is the pressed thickness H, the measured quantity is the removed thickness y, and the output is the calculated springback radius ρ after removing the thickness y. 卸 , since the remaining non-calculated values ​​are given constants.

[0105] Therefore, for the pressing thickness H, the removal thickness y and the springback radius ρ 卸 , knowing two of them we can find the third one.

[0106] Given the removal thickness y and the springback radius ρ 卸 , calculate the initial pressing thickness H.

[0107] Let M be the external bending moment during the pressing process, Mx is the unloading bending moment generated during the unloading process, J1 is the J value when the plate thickness is 6.5 mm, J2 is the J value when the plate thickness is 2 mm, up = t / 2-2, indicating the position of the inner surface relative to the neutral plane after unloading, and down = -t / 2 indicating the position of the initial position of unloading relative to the neutral plane. 卸 Use ρ x express.

[0108] From the previous analysis, it is easy to get:

[0109]

[0110] Right now:

[0111]

[0112] Combining equations (4) and (6) we can get:

[0113]

[0114]

[0115] From formula (11), we can get:

[0116]

[0117]

[0118] According to the actual data, it is easy to obtain So there is y s >u.

[0119]

[0120] By combining equations (14), (15), (17), and (18), we can get ρ, and then use equation (8) to solve for H.

[0121] 4. Positive and negative data verification

[0122] For the selected data to be verified, x The value of =1980mm is input into the inverse solution program and the result is H=18.6519.

[0123] In the forward solution program, enter H = 18.6519 and calculate ρ x =1995.07mm. The error between the two is 0.76%, which is a calculation accuracy error and is within the allowable error of the calculation.

[0124] Thinning rule of the inner side of a plate:

[0125] 1) When the reduced thickness / total thickness is about 2 / 5 (2.75 / 6.5 in this experiment), the radius of the sheet is basically the same before and after the sheet is rolled to remove material;

[0126] 2) When the reduced thickness / total thickness is about 1 / 5 (1.25 / 6.5 in this experiment), the radius of the sheet reaches the maximum after the sheet is rolled and material is removed;

[0127] 3) When the milling depth reaches 3mm or above, there is a certain linear relationship between the milling depth and the plate radius, and the plate radius / part remaining thickness is about 400.

[0128] Table 1 Verification data table

[0129]

[0130] According to the above stress and thinning laws, the influence of material removal on stress redistribution is analyzed, the deformation trend is predicted, and the initial rolling radius and material removal thickness are reasonably planned so that the final curvature meets the part curvature requirements or the additional torque generated after stress redistribution approaches zero.

[0131] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those skilled in the art, various obvious changes, readjustments and substitutions can be made without departing from the protection scope of the present invention. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A method for controlling the shape of a multi-step rolled skin, characterized in that: include: S100, performing a pre-rolling and bending process on the plate to make the plate have a certain curvature; S200, machining the pre-bent sheet material so that the pre-bent sheet material has a desired thickness characteristic; S300, performing a rolling bending process on the machined plate in different regions so that the machined plate has a desired curvature characteristic.

2. The method for controlling the shape of a multi-step rolled skin according to claim 1, characterized in that: The pre-bending process includes: S110, presetting a plurality of thickness regions on the plate, and obtaining a target curvature radius of the minimum thickness region of the plate, selecting a test strip having the same thickness value as the plate, and performing finite element simulation and test verification on the test strip according to the target curvature radius of the minimum thickness region to obtain a minimum thickness pre-rolling curvature radius value; S120, pre-rolling the plate according to the minimum thickness pre-rolling bending radius value.

3. The method for controlling the shape of a multi-step rolled skin according to claim 2, characterized in that: The pre-bending process also includes: S130, if a plurality of the thickness regions are arranged side by side, the pre-rolling curvature radius value of the plate is equal to the pre-rolling curvature radius value of the minimum thickness; S140: If a plurality of the thickness regions are distributed in a nested manner, the pre-rolling curvature radius value of the plate is smaller than the minimum thickness pre-rolling curvature value.

4. The method for controlling the shape of a multi-step rolled skin according to claim 2, characterized in that: The machining process includes: S210, machining the plate by five-axis skin milling or mirror milling, so that the pre-rolled plate has a desired thickness characteristic.

5. The method for controlling the shape of a multi-step rolled skin according to claim 4, characterized in that: Step S210 includes: S211. In the process of processing the plate by five-axis skin milling or mirror milling, the plate is processed in a sequence from thick to thin according to the thickness values ​​of the plurality of thickness regions.

6. The method for controlling the shape of a multi-step rolled skin according to claim 4, characterized in that: Step S210 also includes: S212: If there is a region with a thickness of less than 1.5 mm on the plate, the region with a thickness of less than 1.5 mm is first processed in layers, and other regions are processed in a sequence from thick to thin.

7. The method for controlling the shape of a multi-step rolled skin according to claim 2, characterized in that: The regional rolling process includes: S310, when the curvature change direction is consistent with the thickness change direction, the area rolling is performed on other areas outside the minimum thickness area of ​​the plate, so that the other areas reach their respective target curvature radius values.

8. The method for controlling the shape of a multi-step rolled skin according to claim 2, characterized in that: The area rolling process also includes: S310, when the curvature change direction is perpendicular to the thickness change direction, performing regional rolling on the maximum thickness area of ​​the plate so that the maximum thickness area reaches the maximum thickness target curvature radius value.

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