A method and device for sheet metal forming assisted by magnetorheological fluid with controllable back pressure

Abstract

The present invention discloses a method and device for sheet metal forming assisted by magnetorheological fluid with controllable back pressure, belonging to the technical field of sheet metal forming. According to the geometric shape characteristics and complexity of the parts to be formed, the rheological properties of the magnetorheological fluid in the cavity can be changed by adjusting the current of the coil, and further, by adjusting the flow resistance of the magnetorheological fluid, the back pressure generated by the magnetorheological fluid can be controlled to provide appropriate back pressure for different stages of sheet metal forming. The regulation method is fast and simple, and the regulation structure is simple. The present invention can effectively alleviate the serious wall thickness reduction and springback phenomenon caused by the mismatch of back pressure during the sheet metal forming process. In addition, when using magnetorheological fluid to provide the back pressure for sheet metal forming, the mechanical properties of the force transmission medium can be changed at any time to meet the stress requirements for forming different materials at different stages, reduce the sealing requirements, make the forming process more flexible, and improve the forming accuracy.

Description

Technical Field

[0001] The present invention belongs to the technical field of sheet metal forming, and particularly relates to a method and device for back-pressure controllable magnetorheological fluid assisted sheet metal forming. Background Art

[0002] With the increasing demand for lightweight in the manufacturing field, the manufacturing of sheet metal curved surface parts with complex shapes is becoming more and more widespread. However, the local deformation of complex-shaped parts is relatively large, the stress is relatively concentrated, and it is easy to occur local excessive thinning or even rupture of the sheet metal during the forming process, which poses a challenge to traditional rigid die forming. At present, complex-shaped parts are often manufactured by hot forming or multi-step forming methods, but it also brings disadvantages such as increased processing costs and increased energy consumption.

[0003] The sheet hydroforming process has the advantages of high drawing limit, high surface quality of parts, reduced processing costs, fewer forming passes, and no need for annealing to eliminate work hardening. To a certain extent, it solves the problems of unqualified strength, poor surface quality, and high processing costs of forming such complex parts. When the filled liquid is used to provide back pressure, the stress state of the sheet metal formed by the traditional rigid die can be changed, which is beneficial to the full play of the ductility of the sheet metal, thus avoiding the damage of parts caused by serious local thinning.

[0004] However, hydroforming also has disadvantages such as high requirements for sealing performance, high price of hydraulic forming equipment, and great control difficulty. Moreover, once the filled liquid is determined, its mechanical properties cannot be adjusted. Most of the current back-pressure control methods use springs, pistons, servo hydraulic control systems, etc. When using simple springs and pistons, the adjustment accuracy is low, the adjustment range is small, and it requires operation experience; when using a servo hydraulic control system for pressure regulation, an additional hydraulic system needs to be equipped, which occupies a large area, the equipment price is high, the maintenance cost increases, and there is a certain reaction period for adjusting the pressure. Summary of the Invention

[0005] In order to solve the above problems, the present invention provides a method and device for back-pressure controllable magnetorheological fluid assisted sheet metal forming, which can quickly adjust the magnitude of the back pressure generated by the magnetorheological fluid by regulating the magnetic field for sheets with different shapes and structures, and can adjust the magnitude of the back pressure in real time according to the deformation degree and deformation stage of the sheet during the forming process, effectively promoting the sheet to fit the die and realizing high-quality forming of parts.

[0006] The present invention is realized by the following technical solutions:

[0007] The present invention discloses a method for back-pressure controllable magnetorheological fluid assisted sheet metal forming, including the following steps:

[0008] S1: According to the shape characteristics of the part to be formed, process a punch with a lower end face of a matching shape; fix the upper part of the back pressure adjusting die in the lower part of the central hole of the medium chamber, inject magnetorheological fluid into the cavity formed by the medium chamber and the back pressure adjusting die, and the cavity communicates with the outside through the extrusion holes on the back pressure adjusting die; place the sheet between the medium chamber and the blank holder, the lower end face of the punch contacts the upper surface of the sheet, and the magnetorheological fluid contacts the lower surface of the sheet; the first coil is sleeved outside the medium chamber, and the second coil is sleeved on the lower part of the back pressure adjusting die;

[0009] S2: Apply a blank holding force to the blank holder to clamp the sheet, and energize the first coil to generate a magnetic field B 1 , making the magnetorheological fluid become a soft mold with specific mechanical properties; energize the second coil to generate a magnetic field B 2 , change the mechanical properties of the magnetorheological fluid in the extrusion holes, adjust the resistance of the magnetorheological fluid flowing out of the back pressure adjusting die, and form a certain pressure in the cavity;

[0010] S3: The punch moves downward and extrudes the sheet, causing the sheet to deform and flow into the cavity of the medium chamber. The magnetorheological fluid is extruded and flows out of the extrusion holes of the back pressure adjusting die. A certain pressure is formed in the cavity, and the magnetorheological fluid exerts a pressure P 1 on the sheet, causing the sheet to deform and be initially fitted with the punch;

[0011] S4: Adjust the current input into the first coil and the second coil so that the pressure exerted by the magnetorheological fluid on the sheet in the cavity becomes P 2 , and the sheet further flows and deforms; then adjust the current input into the first coil and the second coil so that the back pressure exerted by the magnetorheological fluid on the sheet becomes P 3 , making the sheet completely fit with the punch;

[0012] S5: After meeting the forming requirements, the first coil and the second coil are powered off in sequence, the punch retracts to the starting position, the blank holder is removed, and the formed part is taken out.

[0013] Preferably, the magnetorheological fluid includes methyl silicone oil, hydroxyl iron powder and a stabilizer; among them, the volume fraction of the hydroxyl iron powder is 30% - 50%.

[0014] Preferably, in S3, the downward movement speed of the punch is 0.1 - 0.5 mm / s.

[0015] Preferably, in S2, the magnetic field intensity generated by the first coil is 0.05 - 0.6 T, and the magnetic field intensity generated by the second coil is 0.05 - 1 T.

[0016] Preferably, in S4, the magnetic field intensity generated by the first coil is 0.05 - 0.6 T, and the magnetic field intensity generated by the second coil is 0.05 - 1 T.

[0017] Preferably, in S4, adjusting the currents in the first coil and the second coil is divided into two stages. When there is no curvature mutation point on the part to be formed, the pressure P in the cavity in the second stage in S4 3 > the pressure P in the cavity in S3 1 > the pressure P in the cavity in the first stage in S4 2 ; when there are multiple curvature mutation points on the part to be formed, the pressure P in the cavity in S3 1 > the pressure P in the cavity in the second stage in S4 3 > the pressure P in the cavity in the first stage in S4 2 .

[0018] The device for implementing the above-mentioned backpressure controllable magnetorheological fluid assisted sheet metal forming method disclosed by the present invention includes a punch, a blank holder, a medium chamber, a first coil, a backpressure adjusting die and a second coil; the blank holder is arranged above the medium chamber, the upper part of the backpressure adjusting die is fixed at the lower part in the central hole of the medium chamber, the cavity formed by the medium chamber and the backpressure adjusting die is filled with magnetorheological fluid, and the cavity communicates with the outside through an extrusion hole on the backpressure adjusting die; the shape of the lower end face of the punch matches the shape of the part to be formed; the first coil is sleeved outside the medium chamber, and the second coil is sleeved on the lower part of the backpressure adjusting die.

[0019] Preferably, a conical port with a gradually shrinking flow-through surface is arranged in front of the inlet of the extrusion hole on the backpressure adjusting die. The diameter range of the straight section of the extrusion hole is 0.5-5 mm, and the length is 5-10 mm. A magnetorheological fluid collecting device is arranged below the outlet of the extrusion hole.

[0020] Preferably, a sealing ring is arranged between the backpressure adjusting die and the medium chamber.

[0021] Preferably, the materials of the punch, the blank holder, the medium chamber and the backpressure adjusting die are non-magnetic steel.

[0022] Compared with the prior art, the present invention has the following beneficial technical effects:

[0023] The backpressure controllable magnetorheological fluid assisted sheet metal forming method disclosed by the present invention can, according to the geometric shape characteristics and complexity of the parts to be formed, adjust the rheological properties such as the viscosity of the magnetorheological fluid in the cavity by changing the current of the coil, and further control the flow resistance of the magnetorheological fluid in the backpressure regulating die area by adjusting the backpressure, so as to control the backpressure generated by the magnetorheological fluid and provide appropriate backpressure for different stages of sheet metal forming. The regulation method is fast and simple, and the regulation structure is simple. The present invention solves the problems of low regulation accuracy and small regulation range of the backpressure in the current sheet hydroforming, or complex auxiliary equipment and difficult maintenance, etc., and can effectively alleviate the serious wall thickness reduction and springback phenomenon caused by the mismatch of the backpressure during the sheet metal forming process. In addition, when using the magnetorheological fluid to provide the backpressure for sheet metal forming, the mechanical properties of the force transmission medium can be changed at any time to meet the stress requirements of different materials forming at different stages; at the same time, the requirement for sealing is also greatly reduced, the forming process is more flexible, and the forming accuracy is higher.

[0024] Further, the magnetorheological fluid includes methyl silicone oil, hydroxyl iron powder and a stabilizer; wherein, the volume fraction of the hydroxyl iron powder is 30% - 50%, and the magnetorheological fluid with this composition has good mechanical properties and can generate a suitable pressure regulation range.

[0025] Further, in S3, the downward movement speed of the punch is 0.1 - 0.5 mm / s, and this speed can keep the sheet metal at an appropriate deformation rate and prevent cracking.

[0026] Further, in S2, the magnetic field intensity generated by the first coil is 0.05 - 0.6 T, and the magnetic field intensity generated by the second coil is 0.05 - 1 T. At this stage, the magnetic field range of the two coils can not only ensure the full regulation of the properties of the magnetorheological fluid and generate a suitable pressure range, but also reduce the volume of the coil and increase the portability of the device.

[0027] Further, in S4, the magnetic field intensity generated by the first coil is 0.05 - 0.6 T, and the magnetic field intensity generated by the second coil is 0.05 - 1 T. At this stage, the magnetic field range of the two coils can not only ensure the full regulation of the properties of the magnetorheological fluid and generate a suitable pressure range, but also reduce the volume of the coil and increase the portability of the device.

[0028] Further, in S4, adjusting the currents in the first coil and the second coil is divided into two stages. When (the bottom cross-section of the specimen is hemispherical, linear, etc., without curvature mutation points), the pressure in the cavity in the second stage in S4 > the pressure in the cavity in S3 > the pressure in the cavity in the first stage in S4; when processing this type of part, the back pressure in the initial stage is relatively large to eliminate the suspended area, relying on the reverse bulging effect of the sheet to improve the forming performance of the sheet; during the forming process, a moderate back pressure is maintained to meet the requirements of sheet flow; at the end of forming, a relatively high back pressure is appropriately maintained for a certain period of time to prevent the sheet from springing back. When (the bottom cross-section of the specimen is W-shaped, corrugated, etc., with multiple curvature mutation points), the pressure in the cavity in S3 > the pressure in the cavity in the second stage in S4 > the pressure in the cavity in the first stage in S4; when processing this type of part, the overall back pressure adopts a form of being large in the early stage, moderate in the middle, and large in the later stage. At the beginning of forming, a relatively large back pressure is adopted, taking advantage of the characteristic of uniform initial wall thickness to pre-form the complex shape at the bottom first; then a certain back pressure is maintained to promote the flow of the sheet; finally, the back pressure is increased to promote the sheet to conform to the mold, and the complex shape is completely formed.

[0029] The device for realizing the method of connecting a pipe and a sheet by plastic deformation of the pipe disclosed by the present invention has a simple structure, reasonable design, convenient installation, good compatibility with conventional equipment, and is suitable for mass production.

[0030] Further, a tapered port with a gradually shrinking flow surface is provided in front of the inlet of the extrusion hole on the back pressure adjusting die, which is convenient for the flow and extrusion of the magnetorheological fluid; a magnetorheological fluid collecting device is provided below the outlet of the extrusion hole, which is convenient for collecting the magnetorheological fluid.

[0031] Further, a sealing ring is provided between the back pressure adjusting die and the medium chamber, which can improve the sealing performance and prevent the magnetorheological fluid from flowing out.

[0032] Further, the materials of the punch, the blank holder, the medium chamber, and the back pressure adjusting die are non-magnetic steel, which ensures that the magnetic field circuit can pass through the magnetorheological fluid, thereby changing its rheological properties. Description of the Drawings

[0033] Figure 1 is the overall structural schematic diagram of the present invention;

[0034] Figure 2 is the front view when the forming device of the hemispherical bottom cylindrical part in Embodiment 1 is assembled;

[0035] Figure 3 is the front view of the intermediate state of the forming device of the hemispherical bottom cylindrical part in Embodiment 1;

[0036] Figure 4 is the front view of the device at the moment when the forming of the hemispherical bottom cylindrical part in Embodiment 1 is completed;

[0037] Figure 5 Front view when the forming device for the W-shaped bottom cylindrical part in Example 2 is assembled

[0038] Figure 6 Front view of the intermediate state of the forming device for the W-shaped bottom cylindrical part in Example 2

[0039] Figure 7 Front view of the device when the forming of the W-shaped bottom cylindrical part in Example 2 is completed

[0040] In the figure: 1 is the punch, 2 is the blank holder, 3 is the sheet metal, 4 is the magnetorheological fluid, 5 is the medium chamber, 6 is the first coil, 7 is the sealing ring, 8 is the back pressure adjusting die, 9 is the second coil, and 10 is the magnetorheological fluid collecting device Detailed implementation mode

[0041] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments, which are explanations of the present invention rather than limitations

[0042] As Figure 1 , a set of specific devices for implementing the present invention is taken as an example to specifically explain the back pressure controllable magnetorheological fluid assisted sheet metal forming method of the present invention, which does not constitute a limitation to the present invention. The method of the present invention can be implemented by using any existing devices that can realize its step-by-step functions

[0043] The device includes a punch 1, a blank holder 2, a medium chamber 5, a first coil 6, a back pressure adjusting die 8, and a second coil 9; the blank holder 2 is arranged above the medium chamber 5, the upper part of the back pressure adjusting die 8 is fixed at the lower part of the central hole of the medium chamber 5, the cavity formed by the medium chamber 5 and the back pressure adjusting die 8 is filled with the magnetorheological fluid 4, and the cavity is communicated with the outside through the extrusion hole on the back pressure adjusting die 8; the shape of the lower end surface of the punch 1 matches the shape of the formed part; the first coil 6 is sleeved outside the medium chamber 5, and the second coil 9 is sleeved at the lower part of the back pressure adjusting die 8

[0044] In a preferred embodiment of the present invention, a tapered port with a gradually shrinking flow surface is provided in front of the inlet of the extrusion hole on the back pressure adjusting die 8, and a magnetorheological fluid collecting device 10 is provided below the outlet of the extrusion hole

[0045] In a preferred embodiment of the present invention, a sealing ring 7 is provided between the back pressure adjusting die 8 and the medium chamber 5. Preferably, a sealing ring accommodating groove is provided on the upper surface of the medium chamber 5, and a sealing ring is provided inside to seal the gap between the sheet metal 3 and the medium chamber 5. The sealing ring can be an O-ring

[0046] In a preferred embodiment of the present invention, the materials of the punch 1, the blank holder 2, the medium chamber 5, the back pressure adjusting die 8, and the magnetorheological fluid collecting device 10 are non-magnetic steel

[0047] The following further explains the present invention by taking the processing processes of two plate components with different shapes as examples:

[0048] Embodiment 1

[0049] As shown in Figure 2 , 3 and 4, in this embodiment, a hemispherical-bottom cylindrical part needs to be processed. This part has a large arc bulge. Since during the drawing process, the material at the top of the arc bulge thins too severely and is prone to cracking. In view of the geometric characteristics and processing requirements of this part, a stage-varying magnetic field is used during the forming process of the part to generate a stepped-back pressure. The back pressure in the initial stage is large to eliminate the suspended area. Relying on the anti-bulging effect of the plate, the forming performance of the plate is improved; during the forming process, a moderate back pressure is maintained to meet the requirements of the plate flow; at the end of the forming, a high back pressure is appropriately maintained for a certain period of time to prevent the springback of the plate.

[0050] The specific processing steps are as follows:

[0051] S1: According to the shape characteristics of the part to be formed, design a matching bottom structure of the punch 1; screw the back pressure adjusting die 8 into the central hole of the medium chamber 5, inject the magnetorheological fluid 4 into the cavity formed by the medium chamber 5 and the back pressure adjusting die 8, place the plate 3 between the medium chamber 5 and the blank holder 2, the magnetorheological fluid 4 contacts the lower surface of the plate 3, the punch 1 contacts the upper surface of the plate 3, and the first coil 6 and the second coil 9 are respectively placed outside the medium chamber 5 and the back pressure adjusting die 8;

[0052] S2: Apply a blank holding force F to clamp the plate 3, and pass a current I into the first coil 6 1 , generating a magnetic field B 1 , making the magnetorheological fluid 4 become a soft die with specific mechanical properties; pass a current I into the second coil 9 2 , generating a magnetic field B 2 , changing the mechanical properties of the magnetorheological fluid 4 in the extrusion hole, adjusting the resistance of the magnetorheological fluid flowing out of the back pressure adjusting die 8, and forming a certain pressure in the cavity;

[0053] S3: The punch 1 moves downward at a speed V and extrudes the plate 3, causing the plate 3 to deform and flow into the cavity of the medium chamber 5. The magnetorheological fluid 4 is extruded and flows out from the lower part of the back pressure adjusting die 8, forming a certain pressure in the cavity. The magnetorheological fluid exerts a pressure P 1 on the plate 3, causing the plate 3 to deform and be initially fitted with the punch 1;

[0054] S4: Adjust the current input into the first coil 6 and the second coil 9 so that the pressure exerted by the magnetorheological fluid on the plate in the cavity becomes P 2 , and the plate 3 further flows and deforms; then adjust the coil input current so that the back pressure of the magnetorheological fluid 4 acting on the plate 3 becomes P3 , the sheet metal 3 is completely fitted with the punch 1; where P 3 > P 1 > P 2 ;

[0055] S5: After the shape requirements of the formed part are met, the first coil 6 is powered off first, and then the second coil 9 is powered off. The punch 1 retracts to the starting position, the blank holder 2 is removed, and the formed part is taken out.

[0056] Embodiment 2

[0057] As shown in Figure 5 , 6 and 7, in this embodiment, a W-shaped bottom cylindrical part needs to be processed. The difficulty in forming this part lies in that there are multiple curvature mutation points at the bottom section. When the concavity of the material reaches a certain degree, the stress concentration points are severely thinned. According to the geometric shape and processing requirements of this part, the overall back pressure adopts a form of large in the early stage, moderate in the middle, and large in the later stage. At the beginning of forming, a large back pressure is adopted. Utilizing the characteristic of uniform initial wall thickness, the bottom W shape is preformed first; then a certain back pressure is maintained to promote the flow of the sheet metal; finally, the back pressure is increased to promote the sheet metal to conform to the mold and the W shape is completely formed. As described above, different back pressure controls are adopted during the forming process to achieve the expected processing effect.

[0058] The specific processing steps are as follows:

[0059] S1: According to the shape characteristics of the part to be formed, design a matching bottom structure of the punch 1; screw the back pressure adjusting die 8 into the central hole of the medium chamber 5, inject the magnetorheological fluid 4 into the cavity formed by the medium chamber 5 and the back pressure adjusting die 8, place the sheet metal 3 between the medium chamber 5 and the blank holder 2, the magnetorheological fluid 4 contacts the lower surface of the sheet metal 3, the punch 1 contacts the upper surface of the sheet metal 3, and the first coil 6 and the second coil 9 are respectively placed outside the medium chamber 5 and the back pressure adjusting die 8;

[0060] S2: Apply a blank holding force F to clamp the sheet metal 3, and pass a current I into the first coil 6 1 , generating a magnetic field B 1 , making the magnetorheological fluid 4 become a soft mold with specific mechanical properties; pass a current I into the second coil 9 2 , generating a magnetic field B 2 , changing the mechanical properties of the magnetorheological fluid 4 in the extrusion hole, adjusting the resistance of the magnetorheological fluid 4 flowing out of the back pressure adjusting die 8, and forming a certain pressure in the cavity;

[0061] S3: The punch 1 moves downward at a speed V and extrudes the sheet metal 3, deforming and flowing into the cavity of the medium chamber 5. The magnetorheological fluid 4 is extruded and flows out from the lower part of the back pressure adjusting die 8, and a certain pressure is formed in the cavity. The magnetorheological fluid exerts a pressure P on the sheet metal 1, so that the bottom section of the sheet 3 becomes W-shaped, completing the pre-forming of the bottom surface;

[0062] S4: Adjust the currents input into the first coil 6 and the second coil 9 so that the pressure exerted by the magnetorheological fluid 4 in the cavity on the sheet 3 becomes P 2 , and the sheet 3 flows and deforms under an appropriate back pressure to avoid excessive thinning at the bottom fillet; Finally, adjust the coil input current so that the back pressure exerted by the magnetorheological fluid 4 on the sheet 3 becomes P 3 , the sheet 3 is fully attached to the punch 1 and maintained for a period of time to suppress springback; where P 1 > P 3 > P 2 ;

[0063] S5: After meeting the shape requirements of the formed part, the first coil 6 is powered off first, followed by the second coil 9 being powered off. The punch 1 retracts to the starting position, the blank holder 2 is removed, and the formed part is taken out.

[0064] It should be noted that the present invention is not limited to the above embodiments. Any obvious improvement or change made by those skilled in the art to the above embodiments will not exceed the scope of the concept of the present invention and the protection scope of the appended claims.

Claims

1. A method for forming a sheet assisted by magnetorheological fluid with controllable back pressure, characterized in that, it includes the following steps: S1: According to the shape characteristics of the part to be formed, a punch (1) with a lower end surface of a matching shape is machined; the upper part of the back pressure adjusting die (8) is fixed at the lower part of the central hole of the medium chamber (5), and the magnetorheological fluid (4) is injected into the cavity formed by the medium chamber (5) and the back pressure adjusting die (8), and the cavity communicates with the outside through the extrusion holes on the back pressure adjusting die (8); the sheet (3) is placed between the medium chamber (5) and the blank holder (2), the lower end surface of the punch (1) contacts the upper surface of the sheet (3), and the magnetorheological fluid (4) contacts the lower surface of the sheet (3); the first coil (6) is sleeved outside the medium chamber (5), and the second coil (9) is sleeved at the lower part of the back pressure adjusting die (8); S2: Apply a blank-holding force to the blank-holder (2) to clamp the sheet metal (3), and energize the first coil (6) to generate a magnetic field B 1 , making the magnetorheological fluid (4) become a soft mold; energize the second coil (9) to generate a magnetic field B 2 , change the mechanical properties of the magnetorheological fluid (4) in the extrusion hole, adjust the resistance of the magnetorheological fluid (4) flowing out of the back pressure regulating die (8), and form a certain pressure in the cavity; S3: The punch (1) moves downward and presses the sheet (3), deforming and flowing the sheet (3) into the cavity of the medium chamber (5). The magnetorheological fluid (4) is pressed and flows out of the extrusion holes of the back pressure adjusting die (8), forming a certain pressure in the cavity. The magnetorheological fluid (4) exerts a pressure P on the sheet (3). 1 , causing the sheet (3) to deform and initially fit with the punch (1); S4: Adjust the currents input into the first coil (6) and the second coil (9) so that the pressure exerted by the magnetorheological fluid (4) on the sheet material (3) in the cavity becomes P 2 , and the sheet material (3) further flows and deforms; then adjust the currents input into the first coil (6) and the second coil (9) so that the back pressure exerted by the magnetorheological fluid (4) on the sheet material (3) becomes P 3 , so that the sheet material (3) is fully fitted to the punch (1); Among them, when there is no curvature mutation point on the part to be formed, P 3 > P 1 > P 2 ; when there are multiple curvature mutation points on the part to be formed, P 1 > P 3 > P 2 ; S5: After meeting the forming requirements, the first coil (6) and the second coil (9) are powered off in sequence, the punch (1) returns to the starting position, the blank holder (2) is removed, and the formed part is taken out.

2. The method for forming a sheet assisted by magnetorheological fluid with controllable back pressure according to claim 1, characterized in that, the magnetorheological fluid (4) includes methyl silicone oil, hydroxyl iron powder and a stabilizer; wherein, the volume fraction of the hydroxyl iron powder is 30% - 50%.

3. The method for forming a sheet assisted by magnetorheological fluid with controllable back pressure according to claim 1, characterized in that, in S3, the downward movement speed of the punch (1) is 0.1 - 0.5 mm / s.

4. The method for forming a sheet assisted by magnetorheological fluid with controllable back pressure according to claim 1, characterized in that, in S2, the magnetic field intensity generated by the first coil (6) is 0.05 - 0.6 T, and the magnetic field intensity generated by the second coil (9) is 0.05 - 1 T.

5. The method for forming a sheet assisted by magnetorheological fluid with controllable back pressure according to claim 1, characterized in that, in S4, the magnetic field intensity generated by the first coil (6) is 0.05 - 0.6 T, and the magnetic field intensity generated by the second coil (9) is 0.05 - 1 T.

6. A device for realizing the method for forming a sheet assisted by magnetorheological fluid with controllable back pressure according to any one of claims 1 - 5, characterized in that, it includes a punch (1), a blank holder (2), a medium chamber (5), a first coil (6), a back pressure adjusting die (8) and a second coil (9); the blank holder (2) is arranged above the medium chamber (5), the upper part of the back pressure adjusting die (8) is fixed at the lower part of the central hole of the medium chamber (5), the cavity formed by the medium chamber (5) and the back pressure adjusting die (8) is filled with the magnetorheological fluid (4), and the cavity communicates with the outside through the extrusion holes on the back pressure adjusting die (8); the shape of the lower end surface of the punch (1) matches the shape of the formed part; the first coil (6) is sleeved outside the medium chamber (5), the second coil (9) is sleeved at the lower part of the back pressure adjusting die (8), and when the second coil (9) is energized, it can generate a magnetic field to change the mechanical properties of the magnetorheological fluid (4) in the extrusion holes and adjust the resistance of the magnetorheological fluid (4) flowing out of the back pressure adjusting die (8).

7. The device for forming a sheet assisted by magnetorheological fluid with controllable back pressure according to claim 6, It is characterized in that Before the inlet of the extrusion hole on the back pressure adjusting die (8), there is a conical opening with a gradually shrinking flow-through surface. The diameter range of the straight section of the extrusion hole is 0.5-5 mm, and the length is 5-10 mm. Below the outlet of the extrusion hole, there is a magnetorheological fluid collecting device (10).

8. The device for back pressure controllable magnetorheological fluid assisted sheet metal forming according to claim 6, It is characterized in that A sealing ring (7) is provided between the back pressure adjusting die (8) and the medium chamber (5).

9. The device for back pressure controllable magnetorheological fluid assisted sheet metal forming according to claim 6, It is characterized in that The materials of the punch (1), the blank holder (2), the medium chamber (5) and the back pressure adjusting die (8) are non-magnetic steel.

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