A method for improving the forming quality of large curved thin-walled parts by liquid filling deep drawing

Abstract

The present invention relates to the field of liquid-filled deep drawing of thin-walled curved surface parts. An auxiliary plate is added to assist the forming of the lower sheet material, and radial stress is applied to the upper auxiliary plate to promote its preferential deformation, thereby improving the forming quality of thin-walled parts. The preformed sheet material and the auxiliary forming sheet material are placed on the die in turn, the punch is lowered to a position flush with the upper plate and kept stationary, hydraulic pressure is applied to perform the pre-expansion process of the sheet material, and a reasonable liquid chamber pressure is loaded after the pre-expansion pressure value. At the same time, the punch is lowered to perform the liquid-filled deep drawing process, and a fixed radial pressure is applied to the upper sheet material through a special-shaped stepped push rod during these two processes. Therefore, the risk of instability and wrinkling in the suspended area of ​​the sheet material can be significantly reduced through the auxiliary support of the upper sheet material and the suppression of the tangential stress of the sheet material by the soft draw ribs formed in the pre-expansion stage. In addition, the auxiliary sheet material is deformed in advance under radial pressure, which can better drive the lower sheet material to be uniformly formed and improve the forming quality of the sheet material.

Description

Technical Field

[0001] The invention relates to the field of metal sheet processing and is a liquid filling deep drawing process method suitable for improving the forming quality of large curved surface thin-walled parts. Background Art

[0002] In the aerospace and automotive fields, some high-end equipment parts have high precision and high lightness requirements, such as the curved thin-walled parts at the bottom of rocket tanks and the acquisition of automotive covers, which require high-performance aluminum alloys, titanium alloys, etc. to be precisely formed. However, they are often accompanied by disadvantages such as difficulty in deformation and poor plasticity. The liquid deep drawing technology replaces the traditional rigid die with a soft die of flexible medium, thereby improving the forming properties of these sheets such as mold adhesion and drawing ratio.

[0003] During the liquid-filled deep drawing process of large curved thin-walled parts, the instability cracking defects of the formed parts are suppressed to a certain extent. However, due to the relatively certain taper of the punch, the area of ​​the suspended area of ​​the sheet metal is increased. During the deformation process, the sheet metal in the suspended area is not bound by the thickness stress, and the tangential stress generated during the deformation process cannot be suppressed, resulting in obvious wrinkling defects. By introducing the upper auxiliary sheet, a certain thickness stress and beneficial friction between the plates can be provided for the suspended area of ​​the lower forming sheet metal, and with the larger liquid chamber pressure, the wrinkling of the suspended area can be further suppressed. On this basis, in combination with the pre-expansion process, the tangential stress of the sheet metal can be reduced by forming soft draw ribs, which can significantly improve the wrinkling trend of the sheet metal in the suspended area, but at the same time, the radial tension of the sheet metal is also increased, resulting in a certain degree of overall thinning of the sheet metal.

[0004] It can be seen that while ensuring the forming accuracy of thin-walled curved parts, it is particularly important to further increase its wall thickness. Summary of the invention

[0005] In view of the above defects of the prior art, the present invention proposes a liquid filling deep drawing process method for improving the forming quality of large curved thin-walled parts, by promoting the preferential deformation of the upper auxiliary sheet material, thereby better driving the deformation of the lower sheet material.

[0006] To achieve the above object, the present invention proposes a liquid filling deep drawing process method for improving the forming quality of large curved thin-walled parts, which comprises the following steps:

[0007] S1 places the preformed sheet and the auxiliary formed sheet on the die in turn and keeps them coaxial with the die. Then, the liquid in the liquid chamber is added to the position flush with the lower sheet through the channel. Then, the mold is closed and the punch is lowered to the position flush with the upper sheet and kept stationary.

[0008] S2 uses a hydraulic pump to pressurize the liquid chamber to pre-expand the sheet. At the same time, the special-shaped stepped push rod continuously applies a fixed radial pressure to the upper sheet to fully fit the mold, thereby improving the mold fit of the lower sheet.

[0009] While S3 loads reasonable die hydraulic pressure, the punch moves downward to perform liquid filling and deep drawing. At this stage, under the action of a large liquid chamber pressure, the two layers of sheet metal are tightly attached together, and the upper layer of sheet metal has a certain supporting effect on the lower layer of sheet metal, thereby reducing the degree of wrinkling in the suspended area of ​​the curved surface parts. At the same time, the special-shaped stepped push rod continuously applies a fixed radial pressure to the upper layer of sheet metal until the deformation ends, which promotes the full deformation of the upper layer of sheet metal, thereby better driving the uniform deformation of the lower layer of sheet metal, and further increasing the wall thickness value of the bottom and suspended area of ​​the thin-walled parts.

[0010] S4 The liquid filling and deep drawing process is completed, and the pressure is released, the mold is opened and the parts are taken out.

[0011] The step S2 is specifically as follows: after the pressure of the liquid chamber is increased to a certain pressure by an external hydraulic pump, the pressure is maintained for a period of time, so that the double-layer sheet material expands and fits the mold. At the same time, the hydraulic cylinder applies thrust to the special-shaped ejector rod, thereby generating radial pressure on the upper sheet material, causing the upper sheet material to deform in advance relative to the lower sheet material and fit the mold fully. The early deformation of the upper sheet material drives the deformation of the lower sheet material through the friction between the sheets, so that the formed sheet material is fully deformed in the pre-expansion stage.

[0012] The step S3 is specifically as follows: after the pre-expansion liquid chamber pressure, a monotonically increasing liquid chamber loading path is set in the liquid filling and drawing stage until the forming is completed, and the radial pressure in this stage is the same as that in the pre-expansion stage. By setting different radial pressures on the upper plate, the mold adhesion and wall thickness distribution of the final formed part are compared, and the optimal radial pressure is selected to form the sheet.

[0013] In general, the above technical solution provided by the present invention has the following technical advantages compared with the prior art:

[0014] 1. Due to the existence of double-layer sheet, the relative thickness of the sheet is increased, and a higher liquid chamber pressure can be used to improve the mold adhesion of the parts. At the same time, the upper sheet is thicker than the lower sheet, which can provide a certain support force to the suspended area of ​​the formed sheet, thereby greatly reducing the risk of wrinkling of the lower sheet.

[0015] 2. Before the double-layer plate is filled with liquid and drawn, a pre-expansion treatment is performed. The soft draw ribs formed can reduce the tangential stress in the suspended area of ​​the plate and further alleviate the wrinkling problem in the suspended area. At the same time, the increased radial tensile stress can further drive the plate in the flange area to flow toward the center.

[0016] 3. Use stepped special-shaped ejector pins to radially pressurize the upper sheet alone to promote its early deformation relative to the lower sheet. While the upper sheet is deformed in advance, the lower sheet is fully deformed under the beneficial friction of the upper sheet, further increasing the wall thickness of the formed part at the bottom and the suspended area. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a liquid-filled deep drawing forming device with a special-shaped stepped push rod to radially pressurize the upper sheet material;

[0018] Figure 2 The size and shape of the parts in Example 1 of the present invention;

[0019] Figure 3 is an optimal liquid chamber pressure loading curve diagram of Example 1 of the present invention;

[0020] Figure 4 This is a profile comparison diagram of the formed part of Example 1 of the present invention;

[0021] Figure 5 This is a wall thickness distribution diagram of a formed part according to Example 1 of the present invention; DETAILED DESCRIPTION

[0022] In order to make the technical solutions and advantages more clearly understood, the present invention is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

[0023] Figure 1 The device comprises a male die (1), a blank holder (2), a female die (7), a liquid chamber (8), a rubber sealing ring (5), a special-shaped step push rod (3) and a hydraulic cylinder (4). One end of the special-shaped step push rod (3) is connected to the hydraulic cylinder (4), and the other end is stepped, wherein the upper layer protrudes a portion more than the lower layer, so as to better contact the upper layer of the sheet material. Figure 1 As shown, they are arranged circumferentially around the sheet material, and the hydraulic cylinder (4) applies pressure to the push rod through the piston.

[0024] S1 places the preformed sheet and the auxiliary formed sheet on the die in turn and keeps them coaxial with the die, then adds the liquid in the liquid chamber to a position flush with the lower sheet through the channel (6), then closes the mold, and moves the punch down to a position flush with the upper sheet and keeps it stationary.

[0025] S2 pressurizes the liquid chamber through a hydraulic pump to perform the process of pre-expanding the sheet material. At the same time, a fixed radial pressure is continuously applied to the upper auxiliary plate through a special-shaped stepped push rod (3) to make it fully fit the mold, thereby improving the mold fit of the lower plate.

[0026] While S3 loads reasonable die hydraulic pressure, the punch moves downward to perform liquid filling and deep drawing. At this stage, under the action of a large liquid chamber pressure, the two layers of sheet metal are tightly attached together, and the upper layer of sheet metal has a certain supporting effect on the lower layer of sheet metal, thereby reducing the degree of wrinkling in the suspended area of ​​the curved surface part. At the same time, the special-shaped stepped push rod (3) continuously applies a fixed radial pressure to the upper layer of sheet metal until the deformation is completed. By promoting the full deformation of the upper layer of sheet metal, the lower layer of sheet metal is better driven to deform evenly, the rupture is controlled, and the wall thickness value of the bottom and suspended area of ​​the thin-walled part is further increased.

[0027] S4 The liquid filling and deep drawing process is completed, and the pressure is released, the mold is opened and the parts are taken out.

[0028] Example 1

[0029] This embodiment applies this method to the research of double-layer sheet metal liquid drawing of thin-walled hemispherical parts. The specific part size is as follows: Figure 2 As shown. The material of the forming sheet is AA6061-T6 aluminum alloy. According to the size of the part, a 170mm round billet is selected, and the sheet thickness is 0.5mm. The upper sheet is 1Cr18Ni9Ti stainless steel with a thickness of 1.5mm. The friction coefficient between the two layers is 0.1 by default. The meshing tool of the finite element software is used to divide the surface mesh, and the forming process of this method is simulated and analyzed to verify the reliability of the process.

[0030] The implementation steps are as follows

[0031] 1. The model of the thin-walled hemispherical part is established by 3D modeling software and imported into the finite element software. Multiple groups of liquid chamber pressure loading curves are selected for deep drawing simulation. The forming condition of the lower sheet is analyzed and the optimal pre-expansion pressure of the formed sheet is determined to be 6Mpa, the pre-expansion time is 0.1 seconds, and the optimal liquid chamber pressure loading curve is as follows: Figure 3 .

[0032] 2. On this basis, a fixed radial pressure is continuously applied to the upper sheet during the forming process in the finite element software, and three radial pressures of 0Mpa, 7Mpa, and 15Mpa are set for simulation analysis. Figure 4 , Figure 5 They are part contour diagrams and wall thickness distribution diagrams of the parts in a row. By comparison, it can be found that with the increase of radial pressure, the mold adhesion of the parts has no obvious change, but the sheet thickness of the ball bottom and the suspended area shows a steady upward trend, and the forming quality of the parts has been improved.

Claims

1. A method for liquid drawing to improve the forming quality of large curved thin-walled parts. The implementation process of the method is as follows: The device comprises a male die (1), a blank holder (2), a female die (7), a liquid chamber (8), a rubber sealing ring (5), a special-shaped step push rod (3) and a hydraulic cylinder (4); wherein one end of the special-shaped step push rod (3) is connected to the hydraulic cylinder (4), and the other end is stepped, wherein the upper layer protrudes a portion more than the lower layer so as to better contact the upper layer of sheet material, the special-shaped step rod is arranged circumferentially around the sheet material, and the hydraulic cylinder (4) applies pressure to the special-shaped step push rod through a piston; Step S1: The preformed sheet and the auxiliary formed sheet are placed on the die in sequence and kept coaxial with the die. Then, the liquid in the liquid chamber is added to a position flush with the lower sheet through the channel (6). Then, the die is closed and the convex die is lowered to a position flush with the upper sheet and kept stationary. Step S2: pressurizing the liquid chamber by a hydraulic pump to perform a process of pre-expanding the sheet material, and at the same time, applying a fixed radial pressure to the upper sheet material by a special-shaped stepped push rod (3) to make it fully adhere to the mold, thereby improving the mold adhesion of the lower sheet material; Step S3: While loading a reasonable die hydraulic pressure, the punch moves downward to perform liquid filling and deep drawing. At this stage, under the action of the liquid chamber pressure, the two layers of sheet metal are tightly attached together, and the upper layer of sheet metal has a certain supporting effect on the lower layer of sheet metal, thereby reducing the degree of wrinkling in the suspended area of ​​the curved surface part. At the same time, the special-shaped stepped push rod (3) continuously applies a fixed radial pressure to the upper layer of sheet metal until the deformation is completed, thereby promoting the full deformation of the upper layer of sheet metal, thereby better driving the uniform deformation of the lower layer of sheet metal, controlling the rupture, and further increasing the wall thickness value of the bottom and suspended area of ​​the thin-walled part; Step S4: the liquid filling and deep drawing process is completed, and the pressure is released, the mold is opened, and the part is taken out.

2. A method for liquid drawing to improve the forming quality of large curved thin-walled parts according to claim 1, characterized in that: The step S1 specifically comprises: fixing the sheet material with a fixed gap, that is, the distance between the pressure ring and the die is fixed to 1.1 times the sum of the thickness of the upper sheet material and the thickness of the lower sheet material, wherein a rubber sealing ring is arranged between the lower sheet material and the die to prevent the liquid from flowing out of the liquid chamber (8).

3. The method for liquid drawing for improving the forming quality of large curved thin-walled parts according to claim 1, characterized in that The special-shaped step push rod causes the upper auxiliary plate to deform first. The step S2 specifically comprises: after the pressure of the liquid chamber is increased to a certain pressure by an external hydraulic pump, the pressure is maintained for a period of time to make the double-layer plate expand and fit the mold. At the same time, the hydraulic cylinder (4) applies a thrust to the special-shaped step push rod, thereby applying a constant radial pressure to the upper auxiliary plate, causing the upper plate to deform in advance relative to the lower plate and fit the mold fully. The early deformation of the upper plate drives the deformation of the lower plate through the friction between the plates, so that the formed plate is fully deformed in the expansion stage.

4. The method for liquid drawing for improving the forming quality of large curved thin-walled parts according to claim 1, characterized in that: The step S3 is specifically as follows: in this stage, a monotonically increasing liquid chamber loading path is set after the pre-expansion liquid chamber pressure until the forming is completed, and the radial pressure in this stage is the same as that during the pre-expansion; by setting different constant radial pressures on the upper plate, the mold adhesion and wall thickness distribution of the final formed part are compared, and the optimal radial pressure is selected to form the sheet.

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