Blanking and flanging compound forming die and method based on critical stress regulation

Abstract

A kind of plate precision coining flanging compound forming die and method based on critical stress regulation, comprising: oppositely arranged upper die assembly and lower die assembly and press driving unit, the upper die assembly includes: upper die, fixedly linked with the male die with ejector block and the pressure plate set to the male die outside;Lower die assembly includes: lower die, fixedly arranged on the female die protection sleeve, the female die, the reverse top device and the punch in turn set to its inside, wherein: blank is located between ejector block and punch, pressure plate is connected with female die protection sleeve, punch is fixedly connected with lower die and is opposite to male die to carry out blanking.The present application realizes high-quality flanging feature forming by the phased critical regulation of thick stress;According to the difference of sheet metal plastic deformation capacity, warm precision coining or cold precision coining route can be flexibly selected.

Description

Technical Field

[0001] This invention relates to a technology in the field of sheet metal plastic processing, specifically a composite forming mold and method for sheet metal fine blanking and flanging based on critical stress control. Background Technology

[0002] Existing blanking and flanging composite dies have the following problems: the blanking process uses a conventional gap, resulting in many micro-cracks at the cut edge, which expand under the tensile stress of subsequent flanging, leading to cracking of the hole edge; the flanging stage relies solely on the geometry of the die to complete the deformation, lacking a means of actively controlling the material stress state in stages; the existing improved fine blanking forming technology does not involve active control of thickness stress during the forming process, and there is a risk of edge cracking due to micro-cracks during punching. Summary of the Invention

[0003] To address the aforementioned shortcomings of existing technologies, this invention proposes a composite forming mold and method for sheet metal fine blanking and flanging based on critical stress control. High-quality flanging features are formed through staged critical control of thickness stress. The method allows for flexible selection of warm or cold fine blanking routes based on the differences in the plastic deformation capacity of the sheet metal.

[0004] This invention is achieved through the following technical solution:

[0005] This invention relates to a sheet metal precision blanking and flanging composite forming die, comprising: an upper die assembly and a lower die assembly arranged opposite to each other, and a press drive unit. The upper die assembly includes: an upper template, a punch with an ejector block fixedly connected thereto, and a pressure plate sleeved outside the punch. The lower die assembly includes: a lower template, a die protection sleeve fixedly mounted thereto, a die, an anti-ejection device, and a punch sequentially sleeved inside thereto. The blank is located between the ejector block and the punch, the pressure plate is connected to the die protection sleeve, and the punch is fixedly connected to the lower template and faces the punch for blanking.

[0006] The end of the punch is provided with a groove, which is used as a punching die during the punching stage, and a punching gap is formed between the punch and the inner wall of the groove at the end of the punch.

[0007] The blanking gap δ satisfies the smaller value of δ ≤ 0.02 mm or δ ≤ 0.01 t, where t is the thickness of the blank. Under this small gap condition, the blanking cross-section is mainly bright band, suppressing the generation of microcracks from the source.

[0008] This invention relates to a composite forming method for sheet metal fine blanking and flanging based on the critical stress control of the above-mentioned device, which can realize a cold fine blanking route with an elongation of ≥10% at room temperature or a warm fine blanking route with an elongation of <10% at 150~350℃, comprising:

[0009] Phase 1 (Pre-deformation phase): As the punch descends, the sheet metal first bends and bulges in the punch's rounded corner area, gradually forming a cup-shaped profile. During this phase, the ejector device maintains a slight contact force (approximately 3%–5% of the material's yield strength) or applies no force, allowing the material to flow freely and form.

[0010] Phase Two (Fine Blanking Back Pressure Phase): The punch continues to descend, and the lower surface of the sheet metal contacts the tip of the punch. Under the fine blanking conditions with a small clearance formed by the punch groove (blank die) and the punch (blank punch), the central area of ​​the sheet metal is precisely blanked away. During this phase, the anti-ejection device applies a fine blanking back pressure P to the sheet metal. b The function of back pressure is to establish a triaxial compressive stress state in the punching and shearing zone, suppressing the initiation of microcracks during the punching process and increasing the proportion of bright bands on the cut surface. b The value range is 10%–40% of the material's yield strength, according to P. b = k b · σ y (T) is determined, where: σ y (T) represents the yield strength of the sheet metal at forming temperature T, k b This is the back pressure coefficient.

[0011] Phase Three (Flanging Critical Stress Assistance and Release Phase): After fine blanking, the punch continues to descend, driving the sheet metal into the flanging process, specifically including:

[0012] Sub-stage IIIa (stress-assisted flanging period): The anti-topping device transfers the thickness stress from P b Increase to the critical stress P of the flange c or above, i.e., P ≥ P c (T), where: P c (T) represents the critical thickness stress threshold related to forming temperature and material type, P c It decreases monotonically as T increases. During this stage, the thickness stress is used to suppress the propagation of microcracks and the growth of cavities at the flange edge. The thickness stress is maintained until the punch stroke reaches the critical stroke S. c .

[0013] Sub-stage IIIb (Free Flanging Period): When the punch stroke reaches S... c At this time, the anti-ejection device relieves the thickness stress, and the punch continues to descend to the preset termination position, completing the remaining flanging stroke in a free state.

[0014] This invention selectively activates the heating system based on the plastic deformation capacity of the sheet metal at a target temperature: when the elongation after fracture of the sheet metal at room temperature is less than 10% (such as magnesium alloys, titanium alloys, high-strength aluminum alloys, and other difficult-to-deform materials), the heating system is activated, and the forming temperature T is selected within the range of 150°C–350°C, which is the warm fine blanking route. Heating improves the plasticity of the material, which on the one hand, combined with small-gap fine blanking, results in better cut quality (warm fine blanking), and on the other hand, lowers the critical threshold P. c (T) reduces the output requirements of the reverse-top device. When the elongation after fracture of the sheet metal at room temperature is not less than 10% (such as annealed aluminum alloys, low-carbon steel, copper alloys, and other materials with good plasticity), the heating system is not activated, and the entire forming process is completed at room temperature (15°C–35°C), which is the cold precision stamping route. At this time, P c Taking the calibration value at room temperature, the anti-jacking device satisfies P ≥ P with a higher output force. c (RT).

[0015] Technical effect

[0016] This invention relates to a composite forming die for sheet metal precision blanking and flanging based on critical stress control, and a method for achieving high-quality blanking and flanging feature forming by controlling the critical thickness stress at each forming stage. Attached Figure Description

[0017] Figure 1 This is a flowchart of the present invention;

[0018] Figure 2 This is a schematic diagram of a three-stage stress control strategy;

[0019] Figure 3 The curves show the variation of thickness stress with the punch stroke in three segments;

[0020] Figure 4 The critical threshold P c (T) and back pressure P b Calibration and temperature / cold route selection flowchart;

[0021] Figure 5 This is a schematic diagram of the overall assembly structure of the mold of the present invention;

[0022] In the diagram: 1 Upper template, 2 Punch, 3 High-temperature spring, 4 Thermocouple, 5 Pressure plate, 6 Ejector block, 7 Blank, 8 Punch (stamping punch), 9 Flanging anti-ejector block, 10 Die, 11 Die protective sleeve, 12 Heating coil, 13 Anti-ejector rod, 14 Anti-ejector screw, 15 Anti-ejector spring, 16 Heat insulation plate, 17 Rubber, 18 Lower template, 19 Thermocouple controller;

[0023] Figure 6 This is a partially enlarged schematic diagram of the working components of the mold of the present invention. Detailed Implementation

[0024] like Figure 5 As shown, this embodiment relates to a sheet metal precision blanking and flanging composite forming die, which includes: an upper die assembly and a lower die assembly arranged opposite to each other, and a press drive unit. The upper die assembly includes: an upper template 1, a punch 2 with a top material block 6 fixedly connected thereto, and a pressure plate 5 sleeved outside the punch 2; the lower die assembly includes: a lower template 18, a die protection sleeve 11 fixedly installed thereon, a die 10 sequentially sleeved inside thereon, a back-ejection device, and a punch 8, wherein: the blank 7 is located between the top material block 6 and the punch 8, the pressure plate 5 is connected to the die protection sleeve 11, and the punch 8 is fixedly connected to the lower template 18 through the back-ejection device and faces the punch 2 for blanking.

[0025] The end of the punch 2 is provided with a groove, which is used as a punching die during the punching stage, and a punching gap is formed between the punch 8 and the inner wall of the groove at the end of the punch.

[0026] The punch 8 can adjust the initial distance between its top end and the lower surface of the plate within a certain range through a thread or shim mechanism, thereby controlling the amount of pre-deformation.

[0027] like Figure 6 As shown, a top material block 6 is provided in the groove, and the top material block 6 is supported by a high-temperature resistant spring 3, which plays the role of unloading material after the punching is completed.

[0028] The die 10 is provided with a flange forming cavity for installing a reverse ejector device.

[0029] The aforementioned anti-top device includes: a flange anti-top block 9, an anti-top screw 14, and an anti-top spring 15 arranged sequentially, as well as an anti-top rod 13 and a rubber 17 located in the flange anti-top block 9 and connected to the punches 8 sequentially. The lower end of the flange anti-top block 9 is connected to the lower template through the anti-top screw 14. The anti-top spring 15 is arranged around the screw between the anti-top block 9 and the lower template to fix the position of the anti-top block under normal conditions. The top surface of the flange anti-top block does not contact the blank during the punching stage, but it contacts the lower surface of the blank during the flange stage. The anti-top screw and the anti-top spring can apply a segmented adjustable upward thrust to the flange anti-top block, providing controllable compressive stress, i.e., thickness stress, in the thickness direction of the sheet material.

[0030] The die protective sleeve 11 is equipped with a heating coil 12 and a matching temperature controller, which monitors the die temperature in real time via thermocouple 4. For plates with poor room temperature plasticity (such as magnesium alloys, titanium alloys, and high-strength aluminum alloys), the heating system is activated to achieve warm precision punching and warm flanging; for plates with good room temperature plasticity (such as annealed aluminum alloys, low-carbon steel, and copper alloys), the heating system is not activated, and the entire process is completed at room temperature. The temperature control range is from room temperature to 400°C.

[0031] A heat insulation plate 16 is provided between the cavity mold 10 and the cavity mold protective sleeve 11 and the lower template 18 to prevent heat from spreading outward.

[0032] like Figure 1 and Figure 2 As shown in this embodiment, a composite forming method for sheet metal fine blanking and flanging based on the above-mentioned device with critical stress control is involved. This method can achieve a cold fine blanking route with an elongation ≥10% at room temperature or a warm fine blanking route with an elongation <10% at 150~350℃, including:

[0033] Step 1, Pre-deformation stage: The punch 2 moves downward, the sheet metal 7 forms a cup-shaped contour, the anti-ejection device makes slight contact, and the material flows freely;

[0034] Step 2, Fine Blanking and Back Pressure Stage: Based on the preset blanking gap, the center area of ​​the sheet is removed by fine blanking using punch 8, and back pressure P is applied by the anti-top device. b , where: P b =k b σ y , σ y k is the yield strength of the sheet metal at forming temperature T. b The back pressure coefficient is used to suppress microcrack initiation and increase the proportion of bright bands in the cut surface through triaxial compressive stress in the shear zone.

[0035] The fine stamping back pressure coefficient k b The value is determined as follows: taking the larger value to maximize the proportion of bright band while ensuring smooth punching. For materials with lower elongation, the value is closer to the upper limit (0.3–0.4).

[0036] Step 3, Critical Stress-Assisted Flanging Stage: The anti-jacking device increases the pressure to P≥P c (T), where: P c (T) Critical thickness stress threshold related to forming temperature and material type, which decreases with increasing temperature; thickness stress inhibits the propagation of microcracks at the hole edge and the growth of pores, and the flange height increases nonlinearly;

[0037] The critical threshold P c (T) is determined as follows: At the target temperature T, perform flanging or flat-bottom bulging tests with different thickness stresses, record the curve of the ultimate forming height versus thickness stress, and determine the thickness stress value corresponding to the inflection point on the curve where the ultimate forming height exhibits a non-linear jump (increment within 20 MPa stress increment ≥ 50%) as P. c (T); or take the condition that the stress triaxiality changes from a positive value to a negative value in the numerical simulation as P. c The criteria are calculated.

[0038] Step 4, Free Flanging Stage: When the punch stroke reaches the critical stroke Sc At this time, the thickness stress is relieved to avoid excessive reduction of the contact area, which would lead to local stress concentration, and the flanging is completed in a free state.

[0039] The critical stroke S c The following method is used to determine the contact area between the anti-flanging block and the sheet metal during the flanging process: by calculating or simulating the change in the contact area between the anti-flanging block and the sheet metal as a function of the punch stroke, the punch stroke corresponding to the point where the contact area decreases to the point that the compressive stress per unit area exceeds 80% of the sheet metal's compressive strength is defined as S. c The calibration can be completed by combining one or more of the following methods: flat-bottom bulging test, numerical simulation, or small-batch process test.

[0040] After the process is completed, the punch is reset and the part is removed.

[0041] Based on practical application experiments, using a high-strength aluminum alloy sheet with a thickness of t = 2mm (elongation at room temperature of approximately 6%) as the object, a warm precision punching route was adopted, with a forming temperature of T = 225°C, a punch diameter of d0 = 33mm, and an initial distance h from the tip of the punch to the lower surface of the sheet. p =5mm.

[0042] Step 1: Route Determination and Mold Preheating. The room temperature elongation after fracture of the sheet metal is measured to be approximately 6% (<10%), indicating a warm precision punching route. Assemble the mold and adjust the initial distance h between the tip of punch 8 and the lower surface of the blank 7. p =5mm. Turn on the heating coil 12, heat the concave mold 10 to 225°C and keep it at that temperature for 10–15 minutes until the temperature is uniform.

[0043] Step 2: Loading. After applying high-temperature lubricant to the surface of the 100mm outer diameter sheet, place it on the preheated die. The sheet is centered using the elastic positioning pins. The edge clamping plate 5 presses the sheet around its perimeter.

[0044] Step 3: Pre-deformation stage. Punch 2 descends at a rate of 5 mm / min. The punch end face contacts the sheet metal, and the sheet metal bends and bulges in the punch's rounded corner area, forming a cup-shaped profile. During this stage, the ejector block applies only a slight contact force (approximately 5–10 MPa), corresponding to... Figure 3 The low-stress platform of the middle stage I.

[0045] Step 4: Fine Blanking + Back Pressure Stage. The punch continues to descend, and the lower surface of the sheet metal contacts the punch pin. The punch groove (inner diameter ≈ 33.02 mm) and the punch pin (diameter = 33 mm) form a blanking clearance of δ ≈ 0.01 mm. Simultaneously, the anti-ejection device applies fine blanking back pressure P to the lower surface of the sheet metal through the anti-ejection block. b σ was found. y (225°C)≈180MPa, take k b =0.2, then P b=36MPa. This stage corresponds to... Figure 3 The stress plateau in stage II. Under back pressure, the fine blanking shear zone is in a triaxial compressive stress state, effectively suppressing the initiation of microcracks. After fine blanking, the central disc is pressed into the punch groove.

[0046] Step 5a: Critical stress assistance period for flanging. After fine blanking, the punch continues to descend. The ejector device transfers the thickness stress from P. b =36MPa increased to P =100MPa. According to the pre-calibration, P c (225°C)≈80MPa, therefore P =100MPa ≥ P c This stage corresponds to Figure 3 The stress jump and peak plateau in the middle stage IIIa.

[0047] Step 5b: Free Flanging Period. As the flanging process progresses, the contact area between the ejector block and the sheet metal gradually decreases. When the punch stroke reaches S... c When the thickness reaches 10mm (judgment criterion: the unit compressive stress in the contact area is close to the compressive strength of the sheet metal), the anti-ejection device relieves the thickness stress to near zero through the elastic force application mechanism. The punch continues to descend to a total stroke of 25mm, completing the remaining flanging in a free state. This stage corresponds to... Figure 3 The stress reduction region in the middle stage IIIb.

[0048] Step 6: Part Removal. The punch resets, and the ejector block 6 pushes out the blanking waste under the action of the high-temperature spring 3. The formed part is removed and air-cooled to room temperature. Testing shows that the flanged hole edge has no cracks or necking, the proportion of bright band on the cut surface is ≥90%, and the effective flange height H... f ≥16mm.

[0049] Further, using annealed aluminum alloy sheets with a thickness of t = 2 mm (elongation at room temperature of about 22%) as the target, a cold precision stamping route was adopted, with a forming temperature of T = 25°C (room temperature), i.e., the heating system was not activated.

[0050] During the fine blanking + back pressure stage, the yield strength σ of the sheet metal at room temperature y (25°C)≈120MPa, take k b =0.3, then P b =36MPa. The back pressure P applied by the anti-jacking device. b Small gap fine blanking is completed at room temperature.

[0051] During the critical stress assistance and release stage of flanging, according to pre-calibration, P at room temperature c (25°C)≈140MPa. The anti-roofing device increases the thickness stress to P =160MPa (≥ P c ), assisted in flanging and forming at room temperature. Achieving Sc Remove the flange when the diameter reaches 10mm, completing the remaining flanging. Inspection revealed no cracks at the edge of the flanged hole in this experiment; the proportion of a bright band on the cut surface is ≥85%; and the effective flanging height H... f ≥13mm.

[0052] Further, using AZ31 magnesium alloy sheet with a thickness of t = 1.5 mm (room temperature elongation of approximately 8%) as the subject, a warm precision stamping route was adopted, with a forming temperature T = 250°C and h... p =5mm, d0=25mm. Magnesium alloys have low slip coefficient and poor plasticity at room temperature, so they must be processed using warm fine forming. σ at 250°C y ≈80MPa, take k b =0.4, P b =32MPa. Calibration P c (250°C)≈50MPa, P =70MPa, after testing, no cracks were found on the edge of the flanged hole in this experiment, and the effective flange height H f ≥10mm. Compared with the scheme under the same conditions at room temperature without applying thickness stress, the sheet metal fractures brittlely during the punching stage and cannot be formed.

[0053] In addition, an experimental process with no back pressure, no thickness stress, and conventional blanking clearance was used as a comparative example. Compared with Example 1, the blanking clearance was set to δ = 0.2 mm (0.10 t, conventional blanking), and no back pressure was applied during the fine blanking stage (P). b =0), no thickness stress is applied during the flanging stage (P =0). Testing revealed multiple cracks at the edge of the flanged hole in this experiment, making it impossible to obtain a complete component.

[0054] Additionally, by setting a back pressure but with the flange stress below a critical threshold as a comparative example, compared to scenario 1: during the fine blanking stage, P is applied... b =36MPa, but only maintain P during the flanging stage. b Horizontal pressure not increased (P = 36MPa < P) c =80MPa), not in S c The defect was removed. Testing showed that the cut quality in this experiment was good (bright band ≥88%), but localized necking and microcracks appeared at the edge of the flanged hole. This indicates that fine blanking back pressure alone cannot solve the flange cracking problem; the critical stress jump during the flange stage is a necessary step (verified). Figure 3 The necessity of the stress jump in the intermediate stage II→IIIa).

[0055] Furthermore, by setting the flange stress to be maintained throughout the entire process without release as a comparative example, compared with scenario 1: during the flange stage, P=100MPa is maintained throughout, without release at S cThe plate was removed at a point of 10mm. Testing revealed that during this experiment, when the flange was being rolled to approximately 12mm from the punch travel, localized cracks appeared in the center contact area of ​​the sheet metal due to excessive unit compressive stress. This indicates that releasing the plate at the critical stroke is a necessary measure to prevent overload in the center area (verified). Figure 3 The necessity of stress reduction in the intermediate stage IIIa→IIIb).

[0056] Table 1 Summary of Implementation Results

[0057] As shown in Table 1, the present invention enables the successful processing of fine blanking flanging parts with multiple cracks in the original warm forming route through the phased critical control of the thickness stress, as shown in Example 1 and Comparative Example 1; parts with local micro-shrinkage cracks in the original warm forming route can be successfully processed without cracks and the flanging height is increased from 12.0 mm to 13.2 mm, as shown in Example 2 and Comparative Example 2.

[0058] In summary, the technical effects of this invention include:

[0059] 1) Precise stress control in three stages. The anti-jacking device operates according to the following steps throughout the entire process: "pre-deformation contact → fine punching back pressure P". b → Critical stress for flanged edge P ≥ P c → Critical stroke release sequentially outputs different levels of thickness stress. The stress value and purpose of each stage are precisely matched according to the deformation characteristics of that stage, such as... Figure 3 As shown.

[0060] 2) Fine blanking back pressure improves cut quality. The back pressure P applied during the fine blanking stage... b By establishing a triaxial compressive stress state in the shear zone, the initiation of microcracks is effectively suppressed. The proportion of bright bands on the cut surface is significantly higher than that of fine blanking without back pressure and conventional blanking, thus reducing the crack initiation point at the flanging from the source.

[0061] 3) Prevent cracking by applying critical stress during flanging. Increase the thickness stress to P during the initial flanging stage. c The above methods utilize thickness stress exceeding the critical threshold to suppress microcrack propagation and pore growth; timely release in the later stages avoids local stress concentration due to excessive reduction in contact area.

[0062] 4) Flexible selection of warm / cold processing routes. For materials that are difficult to deform, a warm finishing route can be used, utilizing temperature to reduce P... c It also improves the quality of the cut; for materials with good plasticity, the cold precision blanking route is used, which requires no heating, saves energy, and has no heat-affected zone. The two routes share the same mold and the same three-stage stress strategy framework, only changing the temperature setting and stress parameters.

[0063] 5) Highly integrated processes. Pre-deformation → fine blanking (+back pressure) → flanging (+critical stress → release) are completed continuously within one die and one stroke, eliminating the pre-hole process and any heat treatment process.

[0064] 6) Wide range of applicable materials. Through flexible selection of warm / cold routes, it covers various thin sheet materials such as magnesium alloys, titanium alloys, aluminum alloys (including high-strength and annealed states), high-strength steel, low-carbon steel, and copper alloys, with a thickness range of 0.5–6 mm.

[0065] The above-described specific implementations can be partially adjusted by those skilled in the art in different ways without departing from the principles and purpose of the present invention. The scope of protection of the present invention is defined by the claims and is not limited to the above-described specific implementations. All implementation schemes within the scope of the claims are bound by the present invention.

Claims

1. A composite forming die for sheet metal precision blanking and flanging, characterized in that, include: The upper die assembly and lower die assembly, along with the press drive unit, are arranged opposite to each other. The upper die assembly includes: an upper template, a punch with an ejector block fixedly connected thereto, and a pressure plate sleeved outside the punch; the lower die assembly includes: a lower template, a die protection sleeve fixedly mounted thereto, a die, an anti-ejection device, and a punch sequentially sleeved inside thereto, wherein: the blank is located between the ejector block and the punch, the pressure plate is connected to the die protection sleeve, and the punch is fixedly connected to the lower template and faces the punch for punching.

2. The sheet metal precision blanking and flanging composite forming die according to claim 1, characterized in that, The end of the punch is provided with a groove, which is used as a punching die during the punching stage, and a punching gap is formed between the punch and the inner wall of the groove at the end of the punch. The blanking gap δ satisfies the smaller value of δ ≤ 0.02 mm or δ ≤ 0.01 t, where t is the thickness of the blank. Under this small gap condition, the blanking cross-section is mainly bright band, which suppresses the generation of microcracks from the source.

3. The sheet metal precision blanking and flanging composite forming die according to claim 1, characterized in that, The groove is equipped with a top material block, which is supported by a high-temperature resistant spring and serves to unload the material after the punching is completed.

4. The sheet metal precision blanking and flanging composite forming die according to claim 1, characterized in that, The cavity of the die is provided with a flange forming cavity for installing the anti-roll device.

5. The sheet metal precision blanking and flanging composite forming die according to claim 1, characterized in that, The aforementioned anti-top device includes: a flanged anti-top block, an anti-top screw, and an anti-top spring arranged in sequence, as well as an anti-top rod and rubber located in the flanged anti-top block and connected to punches in sequence.

6. The sheet metal precision blanking and flanging composite forming die according to claim 5, characterized in that, The lower end of the flange counter-top block is connected to the lower template via a counter-top screw. A counter-top spring is arranged around the screw between the counter-top block and the lower template to fix the position of the counter-top block under normal conditions. The top surface of the flange counter-top block does not contact the blank during the punching stage, but it will contact the lower surface of the blank during the flange stage. The counter-top screw and counter-top spring can apply an adjustable upward thrust to the flange counter-top block, providing controllable compressive stress, i.e., thickness stress, in the thickness direction of the sheet.

7. The sheet metal precision blanking and flanging composite forming die according to claim 1, characterized in that, The outer casing of the concave mold is equipped with a heating coil and a matching temperature controller for heating, and the mold temperature is monitored in real time by a thermocouple; A heat insulation plate is provided between the cavity mold and the cavity mold protective sleeve and the lower template to prevent heat from spreading outward.

8. A method for composite forming of sheet metal fine blanking and flanging based on critical stress control using the device described in any one of claims 1-7, characterized in that, Capable of achieving cold finishing lines with elongation ≥10% at room temperature or warm finishing lines with elongation <10% at 150~350℃, including: Stage 1, Pre-deformation stage: As the punch descends, the sheet metal first bends and bulges in the rounded corner area of ​​the punch, gradually forming a cup-shaped profile. During this stage, the anti-ejection device maintains a slight contact force or does not apply any force, so that the material can flow freely and form. Phase Two, Fine Blanking Back Pressure Phase: The punch continues to descend, and the lower surface of the sheet metal contacts the tip of the punch. Under the fine blanking conditions with a small gap formed by the punch groove and the punch, the central area of ​​the sheet metal is precisely blanked away. In this phase, the anti-ejection device applies fine blanking back pressure P to the sheet metal. b Press P b = k b · σ y (T) is determined, where: σ y (T) represents the yield strength of the sheet metal at forming temperature T, k b This is the back pressure coefficient; Phase Three: Flanging Critical Stress Assistance and Release Phase: After fine blanking, the punch continues to descend, driving the sheet metal into the flanging process, specifically including: Sub-stage IIIa, stress-assisted flanging period: The anti-topping device transfers the thickness stress from P b Increase to the critical stress P of the flange c or above, i.e., P ≥ P c (T), where: P c (T) represents the critical thickness stress threshold related to forming temperature and material type, P c The thickness stress decreases monotonically as T increases. During this stage, the thickness stress is used to suppress the propagation of microcracks and the growth of pores at the edge of the flange hole. The thickness stress is maintained until the punch stroke reaches the critical stroke S. c ; Sub-stage IIIb, Free Flanging Period: When the punch stroke reaches S... c At this time, the anti-ejection device relieves the thickness stress, and the punch continues to descend to the preset termination position, completing the remaining flanging stroke in a free state. c The determination is based on the following: during the flanging process, the contact area between the anti-flanging block and the board gradually decreases. When the unit compressive stress in the contact area approaches the compressive strength of the board at that temperature, it reaches S. c .

Images