A method and device for deep cryo-nearly isothermal forging zoning forming of large-size aluminum alloy optical mirror blanks

Abstract

The present application relates to microcrystalline aluminum material preparation forming technical field, specifically relates to a kind of large specification aluminum alloy optical mirror blank deep cold near isothermal forging zoning forming method and device.The above-mentioned method includes: the casting aluminum alloy blank is treated by homogenization;After the aluminum alloy blank treated by homogenization is high-temperature multi-directional forging breaking down;Immediately water quenching;Room temperature unidirectional compression;Solid solution treatment is carried out;Initial blank is pre-cooled to set temperature and keeps temperature;Start forging press, set forging parameter, blank is placed on flat anvil core mould, the surface area of upper punch bottom is less than the surface area of blank, the lower surface area of blank is less than the surface area of flat anvil core mould, and the total assembly of deep cold near isothermal mould is kept refrigerated by liquid nitrogen refrigeration system to maintain at set temperature, maintain blank to forge in near isothermal state;Drive upper punch to carry out zoning compression to the upper surface of blank;Blank is turned over, and repeated operation is obtained to form workpiece.The forming product obtained by the above-mentioned method and device is not easy to crack, and the forming effect is good.

Description

Technical Field

[0001] This invention relates to the field of microcrystalline aluminum material preparation and forming technology, specifically to a method and apparatus for deep cryogenic near-isothermal forging and partitioning of large-size aluminum alloy optical mirror blanks. Background Technology

[0002] Large-format aluminum alloy optical mirrors are core components of high-end optical systems such as space exploration, remote sensing imaging, and laser communication. The uniformity of the microstructure, the degree of grain refinement, and the level of residual stress in the mirror blank directly determine the mirror surface processing accuracy and optical performance. With the continuous improvement of the resolution requirements for aerospace exploration, higher requirements are being placed on the aperture, grain size, and surface quality of the mirror blank.

[0003] Currently, the preparation of high-performance aluminum alloy mirror blanks mainly employs processes such as rapid solidification powder metallurgy, hot isostatic pressing, or multi-directional forging. Among these, multi-directional forging is widely used due to its strong equipment versatility, ability to eliminate as-cast structures, and ability to break down coarse phases. However, cryogenic multi-directional forging suffers from the following problems: high forming load and easy cracking of surface bulges.

[0004] For example, Chinese patent application CN118847901A discloses a device and method for in-mold forming of large-size microcrystalline aluminum alloy cryogenic near-isothermal forging, including a liquid nitrogen cooling system and a cryogenic near-isothermal forging die assembly. The cryogenic near-isothermal forging die assembly includes an upper punch and a lower die. The upper punch includes a forging head, and the lower die has a forging cavity that cooperates with the forging head. The forging head has an upper punch liquid nitrogen cooling channel inside, and the lower die has a lower die liquid nitrogen cooling channel near the forging cavity. The lower die liquid nitrogen cooling channel and the upper punch liquid nitrogen cooling channel are connected to the liquid nitrogen pipeline of the liquid nitrogen cooling system. The core process involves using a forging press to initiate ultra-low temperature multi-directional compression. The billet is ejected by driving the pallet and the loading shaft of the billet is adjusted before being placed into the forging chamber. The loading shaft of the billet is then cyclically adjusted to perform multiple compressions until the predetermined process is completed. This process has the advantages of large single deformation and fine average grain size. However, when preparing large-size aluminum alloy optical mirror blanks, there is a problem of surface bulging and easy cracking. Summary of the Invention

[0005] The present invention aims to solve the problems in the prior art and provide a method and apparatus for deep cryogenic near-isothermal forging of large-size aluminum alloy optical mirror blanks that is not prone to cracking and has good forming effect.

[0006] To achieve the above objectives, the first aspect of this application provides a method for deep cryogenic near-isothermal forging and partitioning of large-size aluminum alloy optical mirror blanks, including:

[0007] S1: Homogenize the cast aluminum alloy billet to eliminate as-cast segregation; S2: The homogenized aluminum alloy billet is subjected to high-temperature multi-directional forging to break up the as-cast structure and induce dynamic recrystallization. S3: After hot forging, the aluminum alloy billet is immediately water-quenched to prevent grain growth; S4: The water-quenched billet is subjected to room temperature unidirectional compression to introduce pre-strain energy storage, which provides driving force for subsequent recrystallization; S5: The billet after unidirectional compression at room temperature is subjected to solution treatment to form an initial billet with a fine and uniform structure; S6: Pre-cool the initial billet to the set temperature and hold it at that temperature; S7: Assemble the upper punch and lower anvil of the cryogenic near-isothermal forging partition forming die device onto the universal die holder of the forging press, and turn on the liquid nitrogen cooling system to supply liquid nitrogen to the cryogenic near-isothermal partition forming die assembly to pre-cool it to the set temperature. S8: Start the forging press, set the forging parameters, place the billet on the flat anvil core die, the surface area of ​​the bottom of the upper punch is smaller than the upper surface area of ​​the billet, the lower surface area of ​​the billet is smaller than the surface area of ​​the flat anvil core die, and maintain the cryogenic near-isothermal die assembly at the set temperature through the liquid nitrogen cooling system to keep the billet forged in a near-isothermal state. S9: Drive the upper punch to compress the upper surface of the blank in sections: Divide the upper surface of the blank into multiple areas to be compressed. After each compression, rotate the blank to the next area to be compressed, and then press down again. Repeat the operation until the entire upper surface is flat, completing one compression cycle. The blank is flipped over, and the above-mentioned zone compression process is repeated for the next compression pass; the preset total deformation is achieved through multiple passes to obtain the formed workpiece. S10: Turn off the liquid nitrogen cooling system, reset the forging press, drive the operating trolley to remove the formed workpiece, and turn off the forging press.

[0008] In one embodiment, in S4, the compression deformation is 20% to 40%, and the compression method is unidirectional compression.

[0009] In one embodiment, in S9, the deformation amount of a single compression is 1% to 3%, the total deformation amount in the height direction of the blank is 20% to 40%, and the blank forging termination temperature is ≤ -40°C.

[0010] In one embodiment, the surface of the formed workpiece is free from folds, twists, and cracks, with an average grain size ≤30μm and an average size of coarse phases ≤1μm.

[0011] In one embodiment, during operation S9, frost or lubricant residue is scraped off around the blank to obtain a cylindrical shaped workpiece.

[0012] In one embodiment, the diameter of the formed workpiece is ≥400mm.

[0013] In one embodiment, the homogenization treatment temperature is 540℃~560℃, and the holding time is 24~48 hours.

[0014] In one embodiment, the high-temperature multi-directional forging blanking temperature is 530℃~550℃, the upsetting and drawing number is 7 upsetting and 6 drawing, and the single deformation amount is 58%~62%.

[0015] In one embodiment, the solution treatment is performed at a temperature of 540°C to 560°C for a holding time of 4 to 6 hours.

[0016] According to a second aspect of the present invention, a cryogenic near-isothermal forging partitioned forming die apparatus is provided for the above-described method, comprising: a liquid nitrogen cooling system and a cryogenic near-isothermal forging partitioned forming die assembly, the cryogenic near-isothermal forging partitioned forming die assembly comprising an upper punch and a lower anvil, the upper punch comprising a forging head and a detachable upper die base connecting plate, the forging head having an upper punch liquid nitrogen cooling channel inside, the lower anvil comprising an anvil core mold, the lower anvil having a lower anvil liquid nitrogen cooling channel inside, and both the upper punch liquid nitrogen cooling channel and the lower anvil liquid nitrogen cooling channel being connected to the liquid nitrogen pipeline of the liquid nitrogen cooling system.

[0017] Compared with the prior art, this application has the following beneficial effects: In the deep cryogenic near-isothermal forging partitioned forming method and apparatus for large-size aluminum alloy optical mirror blanks of the present invention, homogenization treatment, high-temperature multi-directional forging blanking, post-forging water quenching, room temperature unidirectional compression and solution treatment are performed before partitioned compression. This effectively eliminates as-cast segregation, breaks up coarse as-cast structures, and especially the water quenching after hot forging and room temperature unidirectional compression, and the introduction of pre-strain energy storage, refines the grain structure and forms a uniform and fine initial blank. This provides a stable microstructure basis for subsequent deep cryogenic deformation and reduces the total strain required for deep cryogenic forming, thereby reducing the degree of surface protrusion and reducing the risk of surface cracking. Most importantly, an open mold structure with an upper punch and a lower anvil is adopted, and the bottom surface area of ​​the upper punch is smaller than the upper surface of the blank, and the lower surface of the blank is smaller than the surface area of ​​the anvil core mold. The blank is clamped by an operating carriage for partitioned compression. Because pressure is applied only to a localized area of ​​the billet each time, the small area of ​​pressure per pass significantly reduces the required forming load, allowing large-diameter mirror blanks to be processed on ordinary tonnage forging presses. This overcomes the problem of excessively high loads required for integral forging within a closed die. Furthermore, in the zoned compression process, the upper surface of the billet is divided into multiple compression zones. After each compression, the billet is rotated to the next zone, and this cycle is repeated until the entire upper surface is flat, completing one compression pass. Then, the billet is flipped over, and the process is repeated. This "small deformation, multi-pass" compression method controls the deformation per pass to 1%~3%, keeping the billet in a low strain rate state, ensuring uniform material flow, and allowing for free lateral extension. This avoids surface bulging and additional tensile stress and crack initiation caused by die wall constraints or large overall deformation, achieving crack-free forming. Simultaneously, by accumulating the preset total deformation through multiple passes, the flatness and roundness of the entire billet end face are ensured, with no folding or twisting, resulting in excellent forming quality. The partitioned compression process utilizes liquid nitrogen cooling to maintain a near-isothermal state, effectively suppressing dynamic recovery. Multiple small-deformation compressions in partitioned areas promote dislocation accumulation, achieving grain refinement effects close to those of cryogenic multi-directional forging with a total deformation of 180% (9 multi-directional compressions, 20% deformation per compression). This provides a new approach for large-size fine-grained mirror blanks. The final formed workpiece has an average grain size ≤30μm and an average coarse phase size ≤1μm. This achieves crack-resistant and highly uniform forming of large-diameter (workpiece diameter ≥400mm) aluminum alloy optical mirror blanks, meeting the requirements of nanoscale optical processing for mirror blank microstructure and properties while significantly reducing equipment tonnage and process risks, demonstrating promising industrial application prospects. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of the structure of a cryogenic near-isothermal forging partition forming device according to one embodiment; Figure 2 This is an isometric assembly schematic diagram of a cryogenic near-isothermal forging partitioned forming die assembly disclosed in one embodiment; Figure 3 This is an exploded schematic diagram of the upper punch of a cryogenic near-isothermal forging partitioned forming die assembly disclosed in one embodiment; Figure 4 This is an exploded schematic diagram of the lower anvil of a cryogenic near-isothermal forging partition forming die assembly disclosed in one embodiment; Figure 5 This is a schematic diagram of the internal liquid nitrogen cooling channel of a cryogenic near-isothermal forging partitioned forming die assembly disclosed in one embodiment; Figure 6 This is a schematic cross-sectional view of the internal liquid nitrogen cooling channel of a cryogenic near-isothermal forging partitioned forming die assembly disclosed in one embodiment; Figure 7 The image shows the EBSD diagram of 6061 aluminum alloy after hot forging of aluminum alloy in Example 1, followed by immediate water quenching, room temperature 20% uniaxial compression and direct solution treatment, with an average grain size of 96 μm. Figure 8 EBSD image of 6061 aluminum alloy with an average grain size of 844 μm, which was hot-forged and then air-cooled without room temperature uniaxial compression and directly solution-treated in Comparative Example 1. Figure 9 This is the EBSD image of the 6061 aluminum alloy prepared in Example 1; Figure 10 This is a photograph of the large-diameter 6061 aluminum alloy reflective mirror blank prepared in Example 1; Figure 11 The image shows a 6061 aluminum alloy block with cracks after undergoing cryogenic compression deformation as a comparative example 1.

[0020] Figure 12 The image shows the surface roughness of the large-diameter 6061 aluminum alloy mirror blank prepared in Example 1 after single-point diamond processing, with Ra=2.3 nm.

[0021] Figure label: 1. Liquid nitrogen cooling system; 11. Liquid nitrogen pipeline; 12. Self-pressurized liquid nitrogen storage tank; 2. Universal upper die base for forging press; 3. Deep cryogenic near-isothermal forging zone forming die assembly; 31. Upper punch; 311. Forging head; 3111. Upper flange; 312. Upper die base connecting plate; 313. Liquid nitrogen cooling channel for upper punch; 3121. Stepped hole in connecting plate; 3122. Groove in connecting plate; 314. Connecting screw; 315. Upper punch die base; 32. Lower anvil; 321. Anvil core die; 322. Removable heat insulation plate; 323. Liquid nitrogen cooling channel for lower anvil; 325. Lower anvil die base; 35. Liquid nitrogen quick connector; 3231. Keyway; 3132. Grooved universal lower die base for forging press; 4. Universal lower die base for forging press. Detailed Implementation

[0022] To facilitate understanding of this application, the following description will be more comprehensive and detailed in conjunction with the accompanying drawings and preferred embodiments, but the scope of protection of this application is not limited to the following specific embodiments.

[0023] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of this application.

[0024] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0025] Please see Figures 1-12 One embodiment of the cryogenic near-isothermal forging partition forming die device includes a liquid nitrogen cooling system 1 and a cryogenic near-isothermal forging die assembly 3. The cryogenic near-isothermal forging partition forming die assembly 3 can be detachably connected to the universal upper die holder 2 and the universal lower die holder 4 of the forging press. The liquid nitrogen cooling system 1 includes a self-pressurized liquid nitrogen storage tank 12 and a liquid nitrogen pipeline 11. The liquid nitrogen pipeline 11 and the cryogenic near-isothermal forging partition forming die assembly 3 can be externally insulated with heat insulation cotton to reduce liquid nitrogen loss.

[0026] Specifically, the cryogenic near-isothermal forging sectional forming die assembly 3 includes an upper punch 31 and a lower anvil 32. The upper punch 31 includes a forging head 311. Specifically, in one embodiment, the forging head 311 has a rectangular parallelepiped structure and a certain rounded corner at the end in contact with the blank. The interior of the forging head 311 is provided with an upper punch liquid nitrogen cooling channel 313. The upper punch 31 also includes an upper die holder connecting plate 312 for mounting the forging head 311 and connecting to the upper platform of the universal upper die holder 2 of the forging press. The upper die holder connecting plate 312 is provided with a connecting plate stepped hole 3121. The upper flange 3111 of the forging head 311 is engaged in the connecting plate stepped hole 3121, thereby achieving the lower limit of the forging head 311. The upper die holder connecting plate 312 is provided with multiple connecting plate grooves 3122 for connecting to the universal upper die holder 2 of the forging press in the circumferential direction. The connecting plate grooves 3122 are T-shaped grooves and are fixed by connecting bolts 314.

[0027] The lower anvil 32 includes a flat anvil core mold 321. A removable heat insulation plate 322 is provided on the outside of the flat anvil core mold 321, and a lower flat anvil liquid nitrogen cooling channel 323 is provided inside the flat anvil core mold 321. Both the lower flat anvil liquid nitrogen cooling channel 323 and the upper punch liquid nitrogen cooling channel 313 are connected to the liquid nitrogen pipeline 11 of the liquid nitrogen refrigeration system 1. A liquid nitrogen quick connector 35 is provided at the connection point for easy and quick connection. Liquid nitrogen can then pass through the upper punch liquid nitrogen cooling channel 313 and the lower flat anvil liquid nitrogen cooling channel 323, thereby cooling the forging head 311 and the flat anvil core mold 321.

[0028] By controlling the liquid nitrogen flow rate, near-isothermal cryogenic multi-directional forging in the range of -50 to -160℃ can be achieved, suppressing dynamic recovery, realizing multi-pass large deformation, accumulating large material dislocations, promoting uniform grain refinement, and meeting the nano / sub-nanometer precision machining requirements of large-diameter mirrors.

[0029] The aforementioned device can smoothly implement multi-pass cyclic compression, is suitable for continuous and stable production, and ensures the stability of the mold, billet temperature field, and billet strain field, thereby improving production efficiency and reducing production costs. On the other hand, when linked with a forging robot, it can achieve high-efficiency, low-cost automated production of large-size, high-quality mirror blanks. It can also quickly change the forging direction of the billet, accumulate billet strain over multiple passes, or introduce new billets for continuous forging production.

[0030] One embodiment of the method for deep cryogenic near-isothermal forging of large-size aluminum alloy optical mirror blanks includes: S1: Homogenize the cast aluminum alloy billet to eliminate as-cast segregation; Specifically, the cast aluminum alloy billet is placed in an electric resistance furnace and heated to 540℃~560℃ at a rate of 5℃ / min, held at that temperature for 24~48 hours, and then cooled to room temperature with the furnace to eliminate as-cast segregation.

[0031] S2: The homogenized aluminum alloy billet is subjected to high-temperature multi-directional forging to break up the as-cast structure and induce dynamic recrystallization. Specifically, the homogenized aluminum alloy billet is heated to 530℃~550℃ and subjected to multi-directional forging using a hydraulic press. The forging process consists of 7 upsetting and 6 drawing operations, with each upsetting deformation amounting to 58%~62%. After each upsetting, the billet is drawn back to its original size, with the upsetting and drawing directions alternating (X→Y→Z direction cycle). The final forging temperature is not lower than 440℃. Multi-directional forging breaks down the coarse as-cast structure and induces dynamic recrystallization.

[0032] S3: Immediately quench the billet after high-temperature multi-directional forging in water to prevent grain growth; Specifically, the water temperature for water quenching is ≤30℃, and the transfer time from the furnace to the water is ≤15 seconds, in order to prevent grain growth and preserve the high-temperature deformation structure and dislocation structure.

[0033] S4: The water-quenched billet is subjected to room temperature unidirectional compression to introduce pre-strain energy storage, which provides driving force for subsequent recrystallization; Specifically, the water-quenched billet is unidirectionally compressed along the axial direction at room temperature. The deformation during room temperature compression is 20% to 40%, and the compression speed is 0.5 mm / s to 1 mm / s. High-density dislocations are introduced through room temperature compression, providing energy storage driving force for recrystallization in subsequent solution treatment.

[0034] S5: The billet after unidirectional compression at room temperature is subjected to solution treatment to form an initial billet with a fine and uniform structure; Specifically, the billet compressed at room temperature is heated to 540℃~560℃ and held at that temperature for 4~6 hours. After solution treatment, the billet undergoes sufficient recrystallization.

[0035] S6: Pre-cool the initial billet to the set temperature and hold it at that temperature; Specifically, the solution-treated billet is placed in an ultra-low temperature furnace for cryogenic treatment at -196℃ and held for ≥30 minutes; S7: Assemble the upper punch and lower anvil of the cryogenic near-isothermal forging partition forming die device onto the universal die holder of the forging press, and turn on the liquid nitrogen cooling system to supply liquid nitrogen to the cryogenic near-isothermal partition forming die assembly to pre-cool it to the set temperature. Specifically, the liquid nitrogen cooling system is activated to supply liquid nitrogen to the cryogenic near-isothermal partitioned forming mold assembly to pre-cool it to a temperature ≤-60℃; S8: Start the forging press, set the forging parameters, place the billet on the flat anvil core die, the surface area of ​​the bottom of the upper punch is smaller than the upper surface area of ​​the billet, the lower surface area of ​​the billet is smaller than the surface area of ​​the flat anvil core die, and maintain the cryogenic near-isothermal die assembly at the set temperature through the liquid nitrogen cooling system to keep the billet forged in a near-isothermal state. Specifically, the cryogenically treated billet is taken out and quickly placed on the worktable of the cryogenic near-isothermal forging partition forming die device, with the billet located between the rectangular upper die and the planar lower die.

[0036] S9: Drive the upper punch to compress the upper surface of the blank in sections: Divide the upper surface of the blank into multiple areas to be compressed. After each compression, rotate the blank to the next area to be compressed, and then press down again. Repeat the operation until the entire upper surface is flat, completing one compression cycle. The blank is flipped over, and the above-mentioned zone compression process is repeated for the next compression pass; the preset total deformation is achieved through multiple passes to obtain the formed workpiece. Preferably, during the partitioned compression operation, the frost or lubricant residue around the blank is scraped off simultaneously to obtain a cylindrical shaped workpiece.

[0037] Specifically, the upper die's downward pressing speed is 0.5 mm / s-1 mm / s, the compression deformation per pass is 1%~3%, and the total deformation is 20%~40%. Liquid nitrogen is continuously used during the forging process to maintain the billet temperature ≤-40℃. The entire local continuous forging process takes approximately 30 minutes.

[0038] S10: Turn off the liquid nitrogen cooling system, reset the forging press, drive the operating trolley to remove the formed workpiece, and turn off the forging press.

[0039] The surface of the formed workpiece is free from folds, twists, and cracks, with an average grain size ≤30μm and an average coarse phase size ≤1μm. The large-size aluminum alloy optical mirror blank in this application refers to a formed workpiece with a diameter ≥400mm.

[0040] In the aforementioned partitioned compression process, only a small portion of the billet is forged and deformed during each compression. As the upper punch reciprocates and moves the billet, the entire end face of the billet is gradually flattened. The direction of free deformation gradually shifts to a larger diameter as the size increases, further reducing the deformation per compression. This prevents cracking while the billet is being forged. Simultaneously, operators use a long scraper to remove frost, lubricant, and other residues around the billet in real time, preventing uneven forging surfaces and ensuring roundness during deformation. This also improves the smoothness and flatness of the billet's surfaces. Furthermore, the multi-pass compression process suppresses the problem of uneven edge and core structure in conventional integral forging processes, significantly reducing forming load and equipment requirements. This provides a new approach for preparing large-sized (forming workpiece diameter ≥ 400mm) fine-grained mirror blanks. The device is currently in use on a 4000-ton forging press, achieving significant technical and economic benefits. The billet structure and quality are stable with no adverse effects, and the grain size of the large-diameter mirror billet is uniform at ≤30μm.

[0041] Example 1 S1: Homogenize the 6061 aluminum alloy billet by holding it at 560℃ for 48 hours to eliminate as-cast segregation. S2: The homogenized billet is subjected to high-temperature multi-directional forging at 550℃, with 7 upsetting and 6 drawing cycles, and a single deformation of 62%, which breaks the as-cast structure and induces dynamic recrystallization. S3: After the high-temperature multi-directional forging is completed, the billet is transferred to water at 25°C within 10 seconds for water quenching to freeze the high-temperature fine-grain structure and prevent grain growth. S4: The water-quenched billet is compressed at room temperature at a compression speed of 0.5 mm / s and a deformation of 20%, introducing pre-strain energy storage to provide driving force for subsequent recrystallization; S5: The billet compressed at room temperature is subjected to solution treatment at 560℃ for 4 hours to obtain uniform and fine recrystallized grains and form a billet with a fine structure. S6: Place the billet obtained in S5 in an ultra-low temperature furnace at -196℃ for deep cryogenic treatment and hold for 30 minutes; S7: Assemble the upper punch and lower anvil of the cryogenic near-isothermal forging partition forming die device onto the universal die base of the forging press, and turn on the liquid nitrogen cooling system to supply liquid nitrogen to the cryogenic near-isothermal partition forming die assembly to pre-cool it to -60℃. S8: Start the forging press, set the forging parameters, place the billet on the flat anvil core die, the surface area of ​​the bottom of the upper punch is smaller than the upper surface area of ​​the billet, the lower surface area of ​​the billet is smaller than the surface area of ​​the flat anvil core die, and maintain the cryogenic near-isothermal die assembly at -60℃ through the liquid nitrogen cooling system to keep the billet forged in a near-isothermal state. S9: Drive the upper punch to compress the upper surface of the billet in sections at a pressing speed of 0.5 mm / s: Divide the upper surface of the billet into multiple areas to be compressed. After each pressing, rotate the billet to the next area to be compressed, and press again. Repeat the operation until the entire upper surface is flat, completing one compression pass. The deformation amount of each compression pass is 2%. Turn the billet over and repeat the above section compression process for the next compression pass. The total deformation amount is 30%. By accumulating multiple passes, the preset total deformation amount is reached to obtain the formed workpiece. The final forging temperature of the billet is -50℃. S10: Turn off the liquid nitrogen cooling system, reset the forging press, drive the operating trolley to remove the formed workpiece, and turn off the forging press.

[0042] See Figures 9-12As shown in the actual image of the formed 6061 aluminum alloy mirror blank from Example 1, the size of the formed 6061 aluminum alloy mirror blank is 530mm. The surface of the formed 6061 aluminum alloy mirror blank is smooth and flat, without cracks or twisting, providing a good foundation for subsequent ultra-precision machining. According to the EBSD image of the formed 6061 aluminum alloy mirror blank, the results show that after the preceding forging and immediate quenching and room temperature deformation processes, the grain structure of the material has been refined, the grains have basically completed the recrystallization process, exhibiting typical equiaxed characteristics, and the average grain size is controlled within a relatively ideal range. The surface roughness Ra of the formed 6061 aluminum alloy mirror blank after single-point diamond machining is 2.3nm, meeting the manufacturing requirements of high-end optical mirror surface nano-machining.

[0043] Comparative Example 1: The main differences between Comparative Example 1 and Example 1 are as follows: 1. No water quenching or room temperature compression treatment was performed; the homogenized billet was directly subjected to solution treatment after air cooling. The average grain size of the solution-treated 6061 aluminum alloy billet was 844 μm, and its EBSD diagram is shown below. Figure 8 As shown.

[0044] 2. After solution treatment, deep cryogenic multi-directional forging is performed. Both the billet and the die undergo deep cryogenic treatment, with the billet temperature at -196℃ and the die temperature at -60℃. The billet is compressed sequentially along the X, Y, and Z orthogonal directions, one compression per pass, with a single deformation of 20%. The forging termination temperature is -40℃. During compression, the bulging on the sides of the billet gradually intensifies, and the increased tensile stress leads to cracking. At the time of cracking, the billet's length, width, and height are 380mm, 300mm, and 260mm, respectively. See also... Figure 11 The actual image of the 6061 aluminum alloy mirror blank after forming in Comparative Example 1 shows obvious cracking. It is impossible to obtain the large-size aluminum alloy optical mirror blank of this application by using the method of Comparative Example 1. This application refines the microstructure in advance by performing operations such as room temperature compression on the water-quenched blank, introduces energy storage, and adopts a zoned small deformation accumulation strategy under near isothermal conditions to avoid cracking of aluminum alloy during deep cryogenic forging and obtain ultra-fine grain high-precision optical blank.

[0045] The above are merely preferred embodiments of this application. It should be noted that this application is not limited to the above embodiments. For those skilled in the art, several improvements and modifications can be made without departing from the principles of this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should also be considered within the scope of protection of this application.

Claims

1. A method for deep cryogenic near-isothermal forging and partitioning of large-size aluminum alloy optical mirror blanks, characterized in that, include: S1: Homogenize the cast aluminum alloy billet to eliminate as-cast segregation; S2: The homogenized aluminum alloy billet is subjected to high-temperature multi-directional forging to break up the as-cast structure and induce dynamic recrystallization. S3: After hot forging, the aluminum alloy billet is immediately water-quenched to prevent grain growth; S4: The water-quenched billet is subjected to room temperature unidirectional compression to introduce pre-strain energy storage, which provides driving force for subsequent recrystallization; S5: The billet after unidirectional compression at room temperature is subjected to solution treatment to form an initial billet with a fine and uniform structure; S6: Pre-cool the initial billet to the set temperature and hold it at that temperature; S7: Assemble the upper punch and lower anvil of the cryogenic near-isothermal forging partition forming die device onto the universal die holder of the forging press, and turn on the liquid nitrogen cooling system to supply liquid nitrogen to the cryogenic near-isothermal partition forming die assembly to pre-cool it to the set temperature. S8: Start the forging press, set the forging parameters, place the billet on the flat anvil core die, the surface area of ​​the bottom of the upper punch is smaller than the upper surface area of ​​the billet, the lower surface area of ​​the billet is smaller than the surface area of ​​the flat anvil core die, and maintain the cryogenic near-isothermal die assembly at the set temperature through the liquid nitrogen cooling system to keep the billet forged in a near-isothermal state. S9: Drive the upper punch to compress the upper surface of the blank in sections: Divide the upper surface of the blank into multiple areas to be compressed. After each compression, rotate the blank to the next area to be compressed, and then press down again. Repeat the operation until the entire upper surface is flat, completing one compression cycle. The blank is flipped over, and the above-mentioned zone compression process is repeated for the next compression pass; the preset total deformation is achieved through multiple passes to obtain the formed workpiece. S10: Turn off the liquid nitrogen cooling system, reset the forging press, drive the operating trolley to remove the formed workpiece, and turn off the forging press.

2. The method for deep cryogenic near-isothermal forging and partitioning of large-size aluminum alloy optical mirror blanks according to claim 1, characterized in that, In S4, the compression deformation is 20%~40%, and the compression method is unidirectional compression.

3. The method for deep cryogenic near-isothermal forging and partitioning of large-size aluminum alloy optical mirror blanks according to claim 1, characterized in that, In S9, the deformation per compression is 1%~3%, the total deformation in the height direction of the blank is 20%~40%, and the blank forging termination temperature is ≤-40℃.

4. The method for deep cryogenic near-isothermal forging and partitioning of large-size aluminum alloy optical mirror blanks according to claim 1, characterized in that, The surface of the formed workpiece is free from folds, twists, and cracks, with an average grain size ≤30μm and an average size of coarse phases ≤1μm.

5. The method for deep cryogenic near-isothermal forging and partitioning of large-size aluminum alloy optical mirror blanks according to any one of claims 1-4, characterized in that, During the S9 operation, frost or lubricant residue is scraped off around the blank to obtain a cylindrical shaped workpiece.

6. The method for deep cryogenic near-isothermal forging and partitioning of large-size aluminum alloy optical mirror blanks according to claim 1, characterized in that, The diameter of the formed workpiece is ≥400mm.

7. The method for deep cryogenic near-isothermal forging and partitioning of large-size aluminum alloy optical mirror blanks according to claim 1, characterized in that, The homogenization treatment temperature is 540℃~560℃, and the holding time is 24~48 hours.

8. The method for deep cryogenic near-isothermal forging and partitioning of large-size aluminum alloy optical mirror blanks according to claim 1, characterized in that, The high-temperature multi-directional forging blanking temperature is 530℃~550℃, the upsetting and drawing number is 7 upsetting and 6 drawing, and the deformation amount per step is 58%~62%.

9. The method for deep cryogenic near-isothermal forging and partitioning of large-size aluminum alloy optical mirror blanks according to claim 1, characterized in that, The solution treatment is performed at a temperature of 540℃~560℃ for a holding time of 4~6 hours.

10. A cryogenic near-isothermal forging partitioned forming die apparatus, used in the method according to any one of claims 1-9, comprising: The device includes a liquid nitrogen cooling system and a cryogenic near-isothermal forging sectional forming die assembly. The cryogenic near-isothermal forging sectional forming die assembly includes an upper punch and a lower anvil. The upper punch includes a forging head and a detachable upper die base connecting plate. The forging head has an upper punch liquid nitrogen cooling channel inside. The lower anvil includes a flat anvil core mold and has a lower anvil liquid nitrogen cooling channel inside. Both the upper punch liquid nitrogen cooling channel and the lower anvil liquid nitrogen cooling channel are connected to the liquid nitrogen pipeline of the liquid nitrogen cooling system.

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