Method for controlling rebound state of formed part through rebound bending moment self-adjusting balance method

Abstract

The invention relates to the technical field of punch forming, in particular to a method for controlling the rebound state of a formed part through a rebound bending moment self-adjusting balance method. The stress distribution of the bottom of the U-shaped piece in the thickness direction of the plate is offset from the rebound bending moment generated by the inner and outer layer pressing-pulling stress to the rebound bending moment generated by the inner and outer layer pulling-pressing stress, the stress difference in the thickness direction of the plate is weakened by means of the positive reaction force distribution characteristic, and then the rebound stress state is actively eliminated or weakened. The internal stress of the U-shaped formed part is kept in self-balance after the U-shaped formed part is formed, the springback stress distribution is fundamentally eliminated, the internal stress left by a traditional springback compensation method is eliminated, the formed part is in a stable size state, the process is in a flexible and adjustable state, and the molded surface of the mold does not need to be modified.

Description

Technical Field

[0001] This invention relates to the field of stamping technology, and in particular to a method for controlling the rebound state of formed parts using a self-adjusting balance method for rebound moment. Background Technology

[0002] In the stamping process of parts, springback is a major factor affecting the accuracy of the formed parts and is also the most difficult process problem to solve. Springback is caused by the uneven distribution of internal stress along the thickness direction of the sheet metal after the die is unloaded. This is especially true for bent U-shaped parts, where the springback problem is more pronounced due to the lack of stretching assistance from the drawing process. Currently, the accuracy problem caused by springback in bent parts is mainly solved by the surface compensation method. This involves changing the geometry of the die, such as over-bending at an angle, which may lead to negative angle forming problems (e.g., the roll forming method for U-shaped cross-section parts of high-strength steel plates for automobiles disclosed in Chinese patent application number 201910327656.7), or modifying the radius and radius dimensions (e.g., the springback control method for cold stamping of high-strength steel disclosed in Chinese patent application number 201410190077.X), to achieve compensation by changing the amount of springback. However, due to various unstable factors such as fluctuations in material properties and process conditions, the surface compensation is often unstable, requiring repeated mold repairs and trial molding adjustments, resulting in excessive cost and time waste.

[0003] In addition, the principle of surface compensation is to increase the forming amount, thereby compensating for the dimensional changes caused by springback. However, since surface compensation does not eliminate the springback stress state, but changes the springback amount to meet the dimensional change requirements, the biggest drawback of the surface compensation method is the instability of the dimensional of the molded part. The instability of the surface compensation method includes not only the compensation instability caused by material property fluctuations and process fluctuations, but also the residual stress after springback, which makes the molded part always in a state of possible re-deformation.

[0004] Therefore, the defects are very obvious, and a solution is urgently needed. Summary of the Invention

[0005] To address the aforementioned technical problems, the present invention aims to provide a method for controlling the rebound state of molded parts using a self-adjusting balance method for rebound bending moment.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: The method for controlling the rebound state of formed parts using the self-adjusting balance method of rebound moment includes the following steps: S1 uses the simulation analysis system of simulation software to perform process simulation analysis on the forming of sheet metal, and determines the concave arc and convex arc suitable for the current U-shaped forming part. S11 obtains relevant data on the thickness, yield strength, tensile strength, thickness anisotropy index, hardening index, Young's modulus, Poisson's ratio, and stress-strain curve of the selected plate through tensile tests or material library data, and constructs a simulation constitutive model adapted to the selected plate. S12 sets the radius of curvature R1 of the concave arc in the first stage of the sheet forming process and the radius of curvature R2 of the convex arc in the second stage of the forming process, with a target springback error range of (-a, a). S13 simulates the forming of the sheet metal according to the set R1 and R2, and obtains the maximum springback of the U-shaped part as B; S14 determines whether the value of B falls within the range of (-1.5a, 1.5a). If it does not fall within the range, adjust the value of R1 and repeat S13 and S14 until the range of B falls within (-1.5a, 1.5a). S15 checks if the value of B falls within the range of (-a, a). If it does not fall within the range, adjust the value of R2 and repeat S13 and S15 until the range of B falls within the range of (-a, a). S16 exports the process simulation data from the simulation software. At this time, the values ​​of R1 and R2 are the concave arc and convex arc applicable to the current U-shaped part. The upper forming head of the S2 stamping die stamps the sheet metal into a U-shaped part, and the pressure plate with a curvature radius R1 of the concave arc presses down on the bottom center of the U-shaped part, causing the bottom center of the U-shaped part to be concave downward. S3 Keeping the U-shaped part under continuous pressure in step S2, the floating material plate with the curvature radius R2 of the upward convex arc is used to extrude the bottom center of the U-shaped part upward, so that the bottom center of the U-shaped part convexes upward. After the S4 stamping process is completed, the stamping die is opened and the U-shaped part is removed from the mold. The bottom of the S5 U-shaped part is springed back within the springback error range to form a U-shaped part, and the U-shaped part is subjected to quality inspection. S6 produces qualified U-shaped parts.

[0007] Furthermore, in step S14, the numerical adjustment process of R1 is as follows: when the value of B does not fall within the range of (-1.5a, 1.5a), first determine whether B < -1.5a or B > 1.5a; if B < -1.5a, it indicates that the side plate of the U-shaped part is recessed inward, and R1 should be increased in this case; if B > 1.5a, it indicates that the side plate of the U-shaped part is extended outward, and R1 should be decreased in this case.

[0008] Furthermore, in step S15, the numerical adjustment process of R2 is as follows: when the value of B does not fall within the range of (-a, a), first determine whether B < -a or B > a; if B < -a, it means that the side plate of the U-shaped part is bent inward, and R2 should be increased at this time; if B > a, it means that the side plate of the U-shaped part is stretched outward, and R2 should be decreased at this time.

[0009] Furthermore, the upper forming head of the stamping die is a hollow stamping head with rounded chamfers on both bottom corners. The pressure plate with a concave radius of curvature R1 and the floating plate with a convex radius of curvature R2 are set vertically and vertically respectively, and the pressure plate and the floating plate are located in the area corresponding to the hollow space of the stamping head.

[0010] The beneficial effects of this invention are as follows: By performing bidirectional bending forming in both positive and negative directions at the part between the two rounded corners of the U-shaped part, the stress distribution at the bottom of the U-shaped part along the thickness direction of the sheet metal is such that the rebound bending moment generated by the compressive-tensile stress of the inner and outer layers cancels out the rebound bending moment generated by the tensile-compressive stress of the inner and outer layers. By utilizing the positive reaction force distribution characteristics, the stress difference along the thickness direction is weakened, thereby actively eliminating or weakening the rebound stress state. This allows the internal stress of the U-shaped part to remain in self-balance after forming, fundamentally eliminating the springback stress distribution and eliminating the internal stress left over from traditional springback compensation methods. This results in the formed part being in a stable dimensional state, and the process is flexible and adjustable, requiring no modification to the mold surface. Attached Figure Description

[0011] Figure 1 This is a schematic diagram of the U-shaped component with its bottom recessed downwards in this embodiment.

[0012] Figure 2 This is a schematic diagram of the U-shaped component in this embodiment, showing that the bottom protrudes upwards.

[0013] Figure 3 This is a schematic diagram of the bottom of the U-shaped molded part in this embodiment springing back within the springback error range.

[0014] Figure 4 The springback cloud of the molded part illustrated in this embodiment Figure 1 .

[0015] Figure 5 The springback cloud of the molded part illustrated in this embodiment Figure 2 .

[0016] Figure 6 The springback cloud of the molded part illustrated in this embodiment Figure 3 .

[0017] Figure 7 This is a schematic diagram illustrating the use of the stamping die in this embodiment to stamp and form sheet metal. Figure 1.

[0018] Figure 8 This is a schematic diagram illustrating the usage of the stamping die in this embodiment for stamping sheet metal (with the bottom of the U-shaped part recessed downwards). Figure 2 .

[0019] Figure 9 This is a schematic diagram illustrating the usage of the stamping die in this embodiment for stamping sheet metal (with the bottom of the U-shaped part protruding upwards). Figure 3 .

[0020] Figure 10 This is a schematic diagram of the U-shaped molded part in this embodiment within the target springback error range.

[0021] Figure 11 This is a hardening curve diagram of the board material used in this embodiment.

[0022] Figure 12 This is a yield surface diagram of the plate material used in this embodiment.

[0023] Figure 13 This is a molding limit curve diagram of the sheet material used in this embodiment. Detailed Implementation

[0024] To facilitate understanding by those skilled in the art, the present invention will be further described below with reference to embodiments and accompanying drawings. The content mentioned in the embodiments is not intended to limit the present invention.

[0025] like Figures 1 to 13 As shown, the method for controlling the rebound state of a molded part using the self-adjusting balance method of rebound moment provided by the present invention includes the following steps: S1 uses the simulation analysis system of simulation software to perform process simulation analysis on the forming of sheet metal, and determines the concave arc and convex arc suitable for the current U-shaped forming part. S11 obtains relevant data on the thickness, yield strength, tensile strength, thickness anisotropy index, hardening index, Young's modulus, Poisson's ratio, and stress-strain curve of the selected plate through tensile tests or material library data, and constructs a simulation constitutive model adapted to the selected plate. S12 sets the radius of curvature R1 of the concave arc in the first stage of the sheet forming process and the radius of curvature R2 of the convex arc in the second stage of the forming process, with a target springback error range of (-a, a). S13 simulates the forming of the sheet metal according to the set R1 and R2, and obtains the maximum springback of the U-shaped part as B; S14 determines whether the value of B falls within the range of (-1.5a, 1.5a). If it does not fall within the range, adjust the value of R1 and repeat S13 and S14 until the range of B falls within (-1.5a, 1.5a). S15 checks if the value of B falls within the range of (-a, a). If it does not fall within the range, adjust the value of R2 and repeat S13 and S15 until the range of B falls within the range of (-a, a). S16 exports the process simulation data from the simulation software. At this time, the values ​​of R1 and R2 are the concave arc and convex arc applicable to the current U-shaped part. The upper forming head of the S2 stamping die stamps the sheet metal into a U-shaped part, and the pressure plate with a curvature radius R1 of the concave arc presses down on the bottom center of the U-shaped part, causing the bottom center of the U-shaped part to be concave downward. S3 Keeping the U-shaped part under continuous pressure in step S2, the floating material plate with the curvature radius R2 of the upward convex arc is used to extrude the bottom center of the U-shaped part upward, so that the bottom center of the U-shaped part convexes upward. After the S4 stamping process is completed, the stamping die is opened and the U-shaped part is removed from the mold. The bottom of the S5 U-shaped part is springed back within the springback error range to form a U-shaped part, and the U-shaped part is subjected to quality inspection. S6 produces qualified U-shaped parts.

[0026] This invention involves bidirectional bending in both directions at the point between two rounded corners in a U-shaped part. This causes the stress distribution along the thickness of the sheet metal at the bottom of the U-shaped part to cancel out the rebound bending moments generated by the compressive-tensile stress of the inner and outer layers. By utilizing the positive reactive force distribution characteristics, the stress difference along the thickness of the sheet metal is weakened, thereby actively eliminating or weakening the rebound stress state. This allows the internal stress of the U-shaped part to remain in self-balance after forming, fundamentally eliminating the springback stress distribution and the internal stress left over from traditional springback compensation methods. This results in the formed part being in a stable dimensional state, and the process is flexible and adjustable, requiring no modification to the mold surface.

[0027] This method eliminates the need for flattening the bottom surface of the molded part, saving development costs. However, the values ​​of the concave curvature radius R1 and the convex curvature radius R2 must be within a suitable range. If the values ​​are too large, the plastic deformation of the sheet will be small, and the springback control effect will be insignificant; if the values ​​are too small, the deformation of the sheet will be too large, and the bottom surface will not be able to return to a flat state. The criterion for judging whether the values ​​are suitable is: the horizontal distance d between the two side plates of the U-shaped molded part and the height h of the arc apex of the arc bottom surface of the U-shaped molded part should satisfy d / h = (20~30):1. Figure 10 As shown.

[0028] It should be noted that the curvature radius R1 of the concave arc and the curvature radius R2 of the convex arc can be flexibly adjusted according to different sheet materials and workpiece shapes; the larger the curvature of R1 and R2, the greater the outward expansion of the side plate (side wall) of the U-shaped part; the smaller the curvature of R1 and R2, the greater the inward contraction of the side plate (side wall) of the U-shaped part.

[0029] In this embodiment, the numerical adjustment process of R1 in step S14 is as follows: when the value of B does not fall within the range of (-1.5a, 1.5a), first determine whether B < -1.5a or B > 1.5a; if B < -1.5a, it indicates that the side plate of the U-shaped part has an inward buckling problem, and R1 should be increased appropriately; if B > 1.5a, it indicates that the side plate of the U-shaped part has an outward bulging problem, and R1 should be decreased appropriately.

[0030] In this embodiment, the numerical adjustment process of R2 in step S15 is as follows: when the value of B does not fall within the range of (-a, a), first determine whether B < -a or B > a; if B < -a, it indicates that the side plate of the U-shaped part has an inward buckling problem, and R2 should be increased appropriately; if B > a, it indicates that the side plate of the U-shaped part has an outward flaring problem, and R2 should be decreased appropriately.

[0031] Since R1 and R2 work together to affect the forming result of the sheet metal, we first adjust R1 to make the forming result conform to a larger threshold range, and then adjust R2 to make the springback of the U-shaped part conform to the target range. At the same time, we combine the positive and negative values ​​of the springback B to represent the outward expansion and inward contraction of the U-shaped part, so as to quickly determine the values ​​that meet the processing requirements.

[0032] In this embodiment, the upper forming head of the stamping die is a hollow stamping head with rounded corners on both bottom edges. The pressure plate with a concave radius of curvature R1 and the floating plate with a convex radius of curvature R2 are arranged vertically and vertically, and the pressure plate and the floating plate are located in the area corresponding to the hollow space of the stamping head.

[0033] Specifically, during the first stage of forming, the upper layer of material on the bottom surface of the sheet is compressed, exhibiting compressive stress, while the lower layer is stretched, exhibiting tensile stress. In this stage, the sheet undergoes a permanent downward (plastic) bend, simultaneously storing elastic strain corresponding to the deformation direction of the first stage. Its stress distribution is similar to traditional unidirectional bending, but the direction is towards the "concave" shape. During the second stage of forming, the bottom material of the sheet is pushed upward, effectively applying a reverse bending moment to the "concave" shape. This reverse bending moment first cancels out and covers the elastic strain generated in the first stage. In other words, the elastic deformation stored in the first stage, attempting to restore the part to a flat surface, is canceled out. After canceling out the elastic strain of the first stage, the continued upward convex deformation begins to force the material to undergo a second plastic deformation. This time, the deformation direction is reversed; the lower layer of the sheet, which was originally stretched, is now compressed, generating compressive stress and compressive plastic strain, while the upper layer of the sheet is now stretched, generating tensile stress and tensile plastic strain. After two reverse deformations, the upper and lower layers between the two rounded corners undergo a process of "compression followed by tension" or "tension followed by compression," resulting in a more symmetrical distribution of residual stress along the thickness of the sheet metal. After the external force of the mold is unloaded, the stress is released, the plastic deformation that occurred during the first and second stages of forming is retained, and the elastic deformation cancels each other out. The stress distribution is symmetrical, and the resultant torque approaches zero. The springback is controlled within the springback error range, ultimately yielding a U-shaped forming part that meets dimensional accuracy requirements. In contrast, the bottom layer of a unidirectional stamping forming workpiece is in a compressed state while the other layer is in a stretched state, resulting in a larger stress difference and making the workpiece more prone to springback. Using this method, both the upper and lower layers of the bottom surface of the workpiece undergo plastic deformation, resulting in a smaller stress difference, less springback, and easier control.

[0034] for example: The sheet material used is ultra-high strength steel with a thickness of 2mm and a strength of 1470MPa.

[0035] S1 uses the simulation analysis system of simulation software to perform process simulation analysis on the forming of sheet metal, and determines the concave arc and convex arc suitable for the current U-shaped forming part. The material of a certain U-shaped part (S11) is 1470DPCR ultra-high strength steel plate. The part thickness is 2 mm. Simulation material data was obtained through tensile testing. Figure 11-13 As shown, (a) hardening curve, (b) yield surface diagram, and (c) forming limit curve.

[0036] S12 sets the radius of curvature R1 of the concave arc in the first stage of the sheet forming process to be 200mm and the radius of curvature R2 of the convex arc in the second stage of the forming process to be 100mm. The target springback error range is (-a, a), where a is 0.5. S13 performs a molding simulation on the sheet metal according to the set R1 and R2, such as Figure 4-6 As shown, the springback contour map of the molded part was obtained, and the maximum springback of the U-shaped molded part was found to be B, which is 1.357 mm. Figure 4 As shown; The maximum springback value B in S14 did not fall within the range of (-1.5a, 1.5a), causing the side plate of the U-shaped part to spring back outwards, resulting in an excessively large radius of curvature R1. R1 was adjusted to a smaller value in increments of 10mm, and S13 and S14 were repeated until the maximum springback value B fell within the range of (-0.75, 0.75). Figure 5 As shown; S15, using S14 to control the maximum springback amount B to 0.785mm, still does not meet the product tolerance requirements. R2 is then adjusted in 5mm increments, repeating S13 and S15 until B falls within the range of (-0.5, 0.5). Figure 6 As shown; S16 exports the process simulation data from the simulation software. At this time, R1 is 170mm and R2 is 85mm, which are the concave and convex arcs applicable to the current U-shaped part. The upper forming head of the S2 stamping die stamps the sheet metal into a U-shaped part, and the pressure plate with a curvature radius of 170mm presses down on the bottom center of the U-shaped part, so that the bottom center of the U-shaped part is concave downward. S3 Keeping the U-shaped part under continuous pressure in step S2, the bottom center of the U-shaped part is extruded upwards by a floating material plate with a curvature radius of 85mm, so that the bottom center of the U-shaped part bulges upwards. After the S4 stamping is completed, the stamping die is opened and the U-shaped part is demolded. The product inspection of the U-shaped part is carried out, and its maximum springback error value is 0.129 mm. The bottom of the S5 U-shaped part is springed back within the springback error range to form a U-shaped part, and the U-shaped part is subjected to quality inspection. S6 produces qualified U-shaped parts.

[0037] The above embodiments are preferred implementations of the present invention. In addition, the present invention can be implemented in other ways. Any obvious substitutions without departing from the concept of the present technical solution are within the protection scope of the present invention.

Claims

1. A method for controlling the rebound state of a formed part using a self-adjusting equilibrium method for rebound moment, characterized in that: The following methods and steps are included: S1 uses the simulation analysis system of simulation software to perform process simulation analysis on the forming of sheet metal, and determines the concave arc and convex arc suitable for the current U-shaped forming part. S11 obtains relevant data on the thickness, yield strength, tensile strength, thickness anisotropy index, hardening index, Young's modulus, Poisson's ratio, and stress-strain curve of the selected plate through tensile tests or material library data, and constructs a simulation constitutive model adapted to the selected plate. S12 sets the radius of curvature R1 of the concave arc in the first stage of the sheet forming process and the radius of curvature R2 of the convex arc in the second stage of the forming process, with a target springback error range of (-a, a). S13 simulates the forming of the sheet metal according to the set R1 and R2, and obtains the maximum springback of the U-shaped part as B; S14 determines whether the value of B falls within the range of (-1.5a, 1.5a). If it does not fall within the range, adjust the value of R1 and repeat S13 and S14 until the range of B falls within (-1.5a, 1.5a). S15 checks if the value of B falls within the range of (-a, a). If it does not fall within the range, adjust the value of R2 and repeat S13 and S15 until the range of B falls within the range of (-a, a). S16 exports the process simulation data from the simulation software. At this time, the values ​​of R1 and R2 are the concave arc and convex arc applicable to the current U-shaped part. The upper forming head of the S2 stamping die stamps the sheet metal into a U-shaped part, and the pressure plate with a curvature radius R1 of the concave arc presses down on the bottom center of the U-shaped part, causing the bottom center of the U-shaped part to be concave downward. S3 Keeping the U-shaped part under continuous pressure in step S2, the floating material plate with the curvature radius R2 of the upward convex arc is used to extrude the bottom center of the U-shaped part upward, so that the bottom center of the U-shaped part convexes upward. After the S4 stamping process is completed, the stamping die is opened and the U-shaped part is removed from the mold. The bottom of the S5 U-shaped part is springed back within the springback error range to form a U-shaped part, and the U-shaped part is subjected to quality inspection. S6 produces qualified U-shaped parts.

2. The method for controlling the rebound state of a formed part using the self-adjusting balance method of rebound moment according to claim 1, characterized in that: In step S14, the adjustment process for R1 is as follows: when the value of B does not fall within the range of (-1.5a, 1.5a), first determine whether B < -1.5a or B > 1.5a; if B < -1.5a, it means that the side plate of the U-shaped part is recessed inward, and R1 should be increased in this case; if B > 1.5a, it means that the side plate of the U-shaped part is extended outward, and R1 should be decreased in this case.

3. The method for controlling the rebound state of a formed part using the self-adjusting balance method of rebound moment according to claim 1, characterized in that: In step S15, the adjustment process for R2 is as follows: when the value of B does not fall within the range of (-a, a), first determine whether B < -a or B > a; if B < -a, it means that the side plate of the U-shaped part is recessed inward, and R2 should be increased; if B > a, it means that the side plate of the U-shaped part is extended outward, and R2 should be decreased.

4. The method for controlling the rebound state of a formed part using the self-adjusting balance method of rebound moment according to claim 1, characterized in that: The upper forming head of the stamping die is a hollow stamping head with rounded corners on both bottom edges. The pressure plate with a concave radius of curvature R1 and the floating plate with a convex radius of curvature R2 are set vertically and vertically, and the pressure plate and the floating plate are located in the area corresponding to the hollow space of the stamping head.

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