A device and method for preparing magnesium alloy plate by two-way step differential extrusion

Abstract

The application aims to provide a device and method for preparing magnesium alloy plate by bidirectional stepped differential extrusion, and belongs to the technical field of light metal plastic forming. The device comprises an external die frame, an internal bidirectional stepped differential extrusion torsional shear die and a bidirectional stepped differential extrusion device. The bidirectional stepped differential extrusion process comprises four processes: first stage pre-extrusion forming, second stage asymmetric torsion induced deformation, third stage shear strengthening and fourth stage split shaping. Through the device and method, three times of texture weakening effect and four times of grain refinement process are realized in a single extrusion process. Compared with the traditional equal angle extrusion process, the energy consumption is reduced while the production efficiency is improved, and an innovative solution is provided for preparing fine-grained weak face texture magnesium alloy.

Description

Technical Field

[0001] This invention belongs to the field of light metal plastic forming technology, specifically relating to an apparatus and method for preparing magnesium alloy sheets by bidirectional stepped differential extrusion. Background Technology

[0002] Magnesium alloys, as the lightest metallic structural materials currently available, have attracted much attention due to their unique close-packed hexagonal crystal structure and excellent comprehensive properties. This material system not only possesses ultra-low density but also exhibits superior specific strength, outstanding damping performance, and excellent biocompatibility, showing broad application prospects in aerospace lightweighting, new energy vehicles, 3C electronic products, and biomedical implants. However, the inherent close-packed hexagonal crystal structure of magnesium alloys limits their room-temperature plastic deformation capacity, making it difficult to meet the five independent slip systems required by the Von-Mises criterion. This results in poor room-temperature mechanical properties, limiting their applications in various fields. Research shows that grain refinement can effectively improve the strength and plasticity of magnesium alloys. Intense plastic deformation technology has been proven to be an effective way to achieve ultra-fine grain structure. Intense plastic deformation, represented by high-pressure torsion, can refine grain size while weakening basal texture. However, torsional deformation is generally complex, requiring sophisticated mold processing and equipment, withstanding high pressure, and exhibiting low material torsion, which greatly limits the realization of large-scale continuous production. Magnesium alloy technology has broad development prospects and important strategic significance. In the future, with the continuous advancement of technology and the continuous expansion of application fields, magnesium alloys will play a role in more fields.

[0003] Therefore, inventing an effective device and method for weakening the texture and refining the grains of magnesium alloy sheets is of great importance for expanding the application range of magnesium alloys. Summary of the Invention

[0004] To address the aforementioned problems, the present invention aims to provide an apparatus and method for preparing magnesium alloy sheets by bidirectional stepped differential extrusion. Through this apparatus and processing method, the magnesium alloy billet undergoes continuous and severe torsional shear deformation during processing, causing the c-axis in the magnesium alloy grains to deflect, thereby refining the grains and weakening the texture, improving the room temperature mechanical properties of the magnesium alloy, and expanding the application range of the magnesium alloy.

[0005] The present invention adopts the following technical solution:

[0006] An apparatus for preparing magnesium alloy sheets by bidirectional stepped differential extrusion includes an outer mold frame, an inner bidirectional stepped differential extrusion torsional shearing mold, and a bidirectional stepped differential extrusion device.

[0007] The external mold frame includes an upper crossbeam and a lower crossbeam. The two ends of the upper crossbeam and the lower crossbeam are respectively connected to columns. The two ends of the upper crossbeam and the lower crossbeam are respectively connected to left and right punch mold fixing frames through positioning pins. Left and right punch molds are respectively provided between the left and right punch mold fixing frames.

[0008] The bidirectional stepped differential extrusion equipment includes an upper extrusion cylinder and a lower extrusion cylinder that are respectively connected to the lower side of the upper crossbeam and the lower crossbeam, and an upper extrusion rod and a lower extrusion rod are respectively connected to one end of the upper extrusion cylinder and the lower extrusion cylinder.

[0009] The internal bidirectional stepped differential extrusion torsion shearing die includes an asymmetrically arranged upper die and a lower die, which are respectively connected to the upper extrusion rod and the lower extrusion rod.

[0010] Furthermore, heating layers are provided on the outer sides of the left and right punches respectively.

[0011] Furthermore, both the upper and lower dies include a circular arc platform with radius R1, a circular arc platform with radius R2, and an irregular platform connected in sequence; the heights of the irregular platforms are h1, h2, h3, and the widths are W1 and W2, respectively; wherein, R1 > R2, h2 > h3 > h1;

[0012] Furthermore, a bidirectional three-step shearing channel is formed between the upper and lower dies; the bidirectional three-step shearing channel includes a first bidirectional stepped differential extrusion torsion region, a second bidirectional stepped differential extrusion torsion region, and a third bidirectional stepped differential extrusion torsion region.

[0013] Furthermore, a π-shaped flow channel is provided between the left punch and the right punch. The π-shaped flow channel is the torsional shear deformation region of the extrusion section. The two ends of this region are the extrusion section channel inlet and the extrusion section channel outlet, respectively. The length of the extrusion section channel inlet is greater than the length of the extrusion section channel outlet. Both the upper and lower ends of the extrusion section channel inlet are rounded chamfers with the same radius.

[0014] The left punch, right punch, upper die, and lower die are all made of hot-work die steel 4Cr5MoSiV1. This material possesses excellent heat resistance, wear resistance, and toughness, meeting the performance requirements of dies under high-temperature and high-pressure working environments. The surface roughness of the dies all reaches Ra0.16-0.4μm, ensuring smooth flow of the billet within the die and reducing the generation of surface defects. The surface roughness of the upper and lower crossbeams also reaches Ra0.16-0.4μm, ensuring the assembly accuracy and stability of the entire device.

[0015] Furthermore, the device also includes a control device, which is electrically connected to the heating layer, the upper extrusion cylinder, and the lower extrusion cylinder.

[0016] A method for preparing magnesium alloy sheets by bidirectional stepped differential extrusion includes the following steps:

[0017] S1. Pretreated magnesium alloy billet;

[0018] S1-1. Use 600-grit sandpaper to polish the surface of the magnesium alloy billet to remove oil stains, and then use 800, 1000 and 1200-grit sandpaper in sequence to polish it to ensure that its surface is smooth.

[0019] S1-2. Mix acetone and anhydrous ethanol in a cleaning tank at a volume ratio of 3:2 to prepare an acetone + anhydrous ethanol cleaning solution.

[0020] S1-3. Immerse the magnesium alloy billet in acetone + anhydrous ethanol cleaning solution. Place the cleaning tank on an ultrasonic cleaner and ultrasonically clean the magnesium alloy billet for 30-60 minutes. Then take out the magnesium alloy billet and clean it with anhydrous ethanol, and then dry it with a hair dryer.

[0021] S1-4. Apply graphite oil solution to the surface of magnesium alloy billet;

[0022] S2, Preheated magnesium alloy billet;

[0023] Operate the vacuum atmosphere heating furnace and set the temperature between 380-450℃. After the heating furnace reaches the set temperature, put the magnesium alloy billet into the furnace and keep it at that temperature for 2-4 hours.

[0024] S3, Lubrication, Assembly and Preheating;

[0025] Apply graphite oil solution to the outer surfaces of the left and right punches and the inner cavities of the upper and lower dies; after assembly, heat to 450℃ through the heating layer, and keep warm for 3 hours after reaching the set temperature;

[0026] S4, bidirectional stepped differential extrusion torsion shearing of magnesium alloy billets;

[0027] S4-1. Withdraw the upper die and place the magnesium alloy billet in. Control the upper and lower extrusion cylinders to move at different speeds through the control device. As the upper and lower dies move toward the middle position, the magnesium alloy billet undergoes torsional shearing extrusion deformation and the billet rotates continuously. The magnesium alloy billet in the torsional shearing deformation area is never the same billet. The heating layer temperature is controlled at 380-450℃.

[0028] S4-2. After the magnesium alloy billet is extruded from the upper and lower dies, it immediately enters the extrusion channel. In the extrusion channel, as the upper and lower dies move closer to the middle deformation area, the billet undergoes a flow splitting phenomenon and flows to both sides, eventually being extruded from the left and right extrusion channels. During this extrusion process, the magnesium alloy sheet undergoes further torsional shear deformation, which weakens the texture of the magnesium alloy sheet and continuously refines the grains. Once the billet is extruded, an extrusion process ends.

[0029] S5. Take out the fine-grained weak-surface textured magnesium alloy sheet obtained by bidirectional stepped differential extrusion torsion shearing, polish its surface with sandpaper, clean it with the above-mentioned acetone + anhydrous ethanol cleaning solution, clean it again with anhydrous ethanol, and finally dry it with a hair dryer before use.

[0030] A composite die assembly, consisting of upper and lower concave dies, left and right convex dies (flow-diverting constraints), and variable cross-section torsion channels, forms a four-stage deformation space: pre-extrusion, torsion, shearing, and flow diversion. The first stage, pre-extrusion and initial shearing, involves the magnesium alloy billet being propelled forward by the axial pressure of the upper and lower concave dies. At this stage, the intervention of the extrusion dies causes the material to undergo its first shearing in the three-dimensional stress field. The second stage involves the die system pressing down and up at rates V1 and V2, respectively, causing the billet to enter a spiral streamlined region. The speed difference between the upper and lower dies generates torque, forcing the material to twist at a large angle (45-48°) along the spiral trajectory. This torsional shearing causes a small-angle random deflection of the grain c-axis. The third stage involves entering the wedge-shaped gap region, where the left and right convex dies and the moving upper and lower concave dies form a dynamic shearing band. The fourth stage is flow diversion, where the material is diverted at the fixed die, and the variable cross-section torsion channels of the left and right convex dies create reverse shearing, undergoing reverse torsion and radial expansion. Finally, when the double sheet material undergoes gradient deformation and is extruded through the through-hole, the extrusion channel area of ​​the die smooths the surface of the billet, reducing the surface roughness of the sheet. Three asymmetric differential extrusion processes reduce the strength of the basal texture, and the four-stage deformation refines the grains, breaking through the room temperature processing limits of magnesium alloys.

[0031] When processing bidirectional stepped differential extrusion magnesium alloy billets using the apparatus and processing method of this invention, due to:

[0032] a. Reliable connection between the upper crossbeam, upper extruder, upper and lower dies, lower extruder, and lower crossbeam;

[0033] b. Internal bidirectional asymmetric torsional shearing and extrusion motion;

[0034] c. The extrusion channels formed by the left and right punches undergo further torsional shear deformation.

[0035] Under the synergistic effect of the above three factors, the resulting magnesium alloy sheet exhibits more significant effects in terms of weakened surface texture and refined grains compared to magnesium alloy sheets obtained through traditional processes.

[0036] The beneficial effects of this invention are as follows:

[0037] 1. The components of the device of the present invention have simple shapes, simple manufacturing processes, and low manufacturing costs;

[0038] 2. Innovative composite mold design: The design integrates the flow divider mold, shearing mold and torsion mold, and achieves excellent texture weakening effect through asymmetric velocity field, spiral-wedge composite flow channel and three-stage reverse shearing;

[0039] 3. By controlling the strain path in a time-space manner, cumulative strain and grain refinement are achieved in a single-pass machining process, and the strength of the basal texture is reduced;

[0040] 4. Modular structure, allowing users to modify modules to prepare weak-textured magnesium alloy sheets with different cross-sections according to their needs.

[0041] This device, through innovative structural design and process integration, successfully solved the technical bottleneck of strong surface texture and poor formability of magnesium alloy plates. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of the main structure of the device of the present invention;

[0043] Figure 2 This is a schematic diagram of the front view structure of the concave mold of the device of the present invention;

[0044] Figure 3 This is a detailed view of the concave mold of the device of the present invention;

[0045] Figure 4 This is a schematic diagram of the front view structure of the punch of the device of the present invention;

[0046] Figure 5 This is a schematic diagram of the left-hand structure of the punch of the device of the present invention;

[0047] Figure 6 A schematic diagram of the deformation process of magnesium alloy billet used to prepare magnesium alloy sheets according to the present invention;

[0048] Wherein: 1-Upper crossbeam; 2-Upper extrusion cylinder; 3-Left column; 4-Positioning pin; 5-Upper extrusion rod; 6-Upper die; 7-Right punch; 8-Heating layer; 9-Left punch; 10-Left punch fixing frame; 11-Lower extrusion rod; 12-Lower die; 13-Right punch fixing frame; 14-Lower extrusion cylinder; 15-Right column; 16-Magnesium alloy billet; 17-Lower crossbeam; 18-Wire; 19-Base; 20-Display screen; 21-Start button; 22-Control device; 23-Pause button; 24-Stop button; 25-Upper extrusion cylinder controller; 26-Lower extrusion cylinder controller; 27-Indicator light; 28-Heating layer controller.

[0049] Ⅰ-First stage bidirectional stepped differential extrusion torsion zone; Ⅱ-Second stage bidirectional stepped differential extrusion torsion zone; Ⅲ-Third stage bidirectional stepped differential extrusion torsion zone; Ⅳ-Extrusion stage torsion shear deformation zone. Detailed Implementation

[0050] The invention will be further described with reference to the accompanying drawings.

[0051] As shown in the figure, an apparatus for preparing magnesium alloy sheets by bidirectional stepped differential extrusion includes an outer mold frame, an inner bidirectional stepped differential extrusion torsion shearing mold, and a bidirectional stepped differential extrusion device.

[0052] The external mold frame includes an upper crossbeam 1 and a lower crossbeam 17. The two ends of the upper crossbeam 1 and the lower crossbeam 17 are respectively connected to a left column 3 and a right column 15. The two ends of the upper crossbeam 1 and the lower crossbeam 17 are respectively connected to a left punch mold fixing frame 10 and a right punch mold fixing frame 13 through positioning pins 4. A left punch mold 9 and a right punch mold 7 are respectively provided between the left punch mold fixing frame 10 and the right punch mold fixing frame 13.

[0053] The left column 3 and the right column 15 form a vertical load-bearing unit, which, together with the upper beam 1 and the lower beam 17, constitutes a closed prestressed frame structure. The overall stiffness is optimized by finite element design and can withstand multi-directional asymmetric compressive loads.

[0054] The bidirectional stepped differential extrusion equipment includes an upper extrusion cylinder 2 and a lower extrusion cylinder 14 that are respectively connected to the lower side of the upper crossbeam 1 and the lower crossbeam 17. One end of the upper extrusion cylinder 2 and the lower extrusion cylinder 14 is respectively connected to an upper extrusion rod 5 and a lower extrusion rod 11.

[0055] The internal bidirectional stepped differential extrusion torsional shearing die includes an asymmetrically arranged upper die 6 and a lower die 12, which are respectively connected to the upper extrusion rod 5 and the lower extrusion rod 11. A relatively movable space is reserved between the upper die 6 and the lower die 12, creating conditions for the asymmetrical differential torsional shearing deformation of the billet.

[0056] A dynamic fit clearance of 0.25±0.02mm is reserved between the upper and lower dies to achieve multi-directional metal flow control of magnesium alloy billet 16.

[0057] Furthermore, heating layers 8 are respectively provided on the outer sides of the left punch mold 9 and the right punch mold 7.

[0058] Furthermore, both the upper die 6 and the lower die 12 include a circular arc platform with radius R1, a circular arc platform with radius R2, and an irregular platform connected in sequence; the heights of the irregular platforms are h1, h2, h3, and the widths are W1 and W2, respectively; wherein, R1 > R2, h2 > h3 > h1.

[0059] Furthermore, a bidirectional three-step shearing channel is formed between the upper die 6 and the lower die 12; the bidirectional three-step shearing channel includes a first bidirectional stepped differential extrusion torsion region, a second bidirectional stepped differential extrusion torsion region, and a third bidirectional stepped differential extrusion torsion region.

[0060] Furthermore, a π-shaped flow channel is provided between the left punch mold 9 and the right punch mold 7. The π-shaped flow channel is the torsional shear deformation area of ​​the extrusion section. The two ends of this area are the extrusion section channel inlet and the extrusion section channel outlet, respectively. The length of the extrusion section channel inlet is greater than the length of the extrusion section channel outlet. The upper and lower ends of the extrusion section channel inlet are rounded chamfers with the same radius.

[0061] Within the bidirectional stepped differential torsional shearing extrusion space, constructed by the left punch die 9, right punch die 7, upper die 6, lower die 12, left punch die holder 10, and right punch die holder 13, a magnesium alloy billet is filled. This torsional shearing extrusion space consists of the left punch die, the right punch die, and the bidirectional stepped differential extrusion torsional shearing deformation zone. The left and right extrusion channels are used to shape the magnesium alloy billet in the extrusion torsional zone to achieve the desired shape and dimensional accuracy. During device operation, the magnesium alloy billet continuously enters the torsional shear zone, subjecting it to severe plastic deformation, thereby achieving grain refinement.

[0062] Furthermore, it also includes a control device 22, which is electrically connected to the heating layer 8, the upper extrusion cylinder 2, and the lower extrusion cylinder 14.

[0063] Example

[0064] A method for preparing magnesium alloy sheets by bidirectional stepped differential extrusion includes the following steps:

[0065] 1. Carefully selected materials and chemical reagents: Magnesium alloy billet, material selected is AZ31, containing 96% magnesium, 3% aluminum and 1% zinc; sandpaper, solid; graphite oil solution, viscous liquid; anhydrous ethanol, liquid, purity 99.5%; acetone, liquid, purity 99%.

[0066] 2. Pre-treatment of magnesium alloy billets:

[0067] ① Use 600-grit sandpaper to polish the surface of magnesium alloy blank 16 to remove oil stains, and then use 800, 1000 and 1200-grit sandpaper in sequence to polish it to ensure that its surface is smooth.

[0068] ② Mix acetone and anhydrous ethanol in a cleaning tank at a volume ratio of 3:2 to prepare an acetone + anhydrous ethanol cleaning solution.

[0069] ③ Immerse the magnesium alloy billet 16 in acetone + anhydrous ethanol cleaning solution, place the cleaning tank on an ultrasonic cleaner to ultrasonically clean the magnesium alloy billet 16 for 60 minutes, take out the magnesium alloy billet 16 and clean it with anhydrous ethanol, and then blow it dry with a hair dryer.

[0070] ④ Apply graphite oil solution to the surface of magnesium alloy blank 16.

[0071] 3. Preheating magnesium alloy billet 16: Operate a vacuum atmosphere heating furnace, set the temperature to 450 ℃, and after reaching the set temperature, put the magnesium alloy billet 16 into the furnace and keep it at that temperature for 3 h.

[0072] 4. Lubrication, assembly, and preheating:

[0073] ① Before assembling the device, graphite oil solution should be applied to all surfaces of the outer surface of the left punch mold 9, the outer surface of the right punch mold 7, the inner cavity of the upper die 6, and the inner cavity of the lower die 12.

[0074] ② During assembly, the left punch mold 9 is installed on the left punch mold fixing frame 10, the right punch mold 7 is installed on the right punch mold fixing frame 13, and the upper crossbeam 1 and lower crossbeam 17 are installed in the corresponding holes of the left column 3 and right column 15. The upper extrusion rod 5 is threaded to the upper die 6, and the lower extrusion rod 11 is threaded to the lower die 12. The upper extrusion rod 5 and the lower extrusion rod 11 are respectively connected to the upper extrusion cylinder 2 and the lower extrusion cylinder 14. The distance between the upper die 6 and the lower die 12 is adjusted by operating the upper extrusion cylinder controller 25 and the lower extrusion cylinder controller 26 so that the magnesium alloy billet 16 can be placed between them. The left punch mold holder 10 is in close contact with the upper crossbeam 1 and the lower crossbeam 17, and the right punch mold holder 13 is in close contact with the upper crossbeam 1 and the lower crossbeam 17. The upper extrusion cylinder controller 25 is operated to push the upper extrusion rod 5 to move the upper die 6 downward, and the lower extrusion cylinder controller 26 is operated to push the lower extrusion rod 11 to move the lower die 12 upward, so that the magnesium alloy billet in the torsional shear deformation area is never the same billet, and the resulting weak surface texture magnesium alloy sheet has a uniform structure and weakens the texture. The upper extrusion cylinder controller 25 and the lower extrusion cylinder controller 26 are operated to make the internal bidirectional stepped differential extrusion torsion shearing die move asynchronously at different speeds. Observe whether there is blank extrusion in the extrusion channel. a. If not, the distance between the upper die 6 and the lower die 12 should be adjusted. b. If so, control the upper extrusion cylinder controller 25 and the lower extrusion cylinder controller 26 to move the internal bidirectional three-step torsion shearing die to the middle position and tighten the upper crossbeam 1 and the lower crossbeam 17 to ensure that the left column 3 and the right column 15 are tightened as one unit.

[0075] ③ After assembling the device, operate the heating layer controller 28 to control the temperature of the heating layer 8 to 450 ℃, and keep it at the set temperature for 3 hours.

[0076] 5. Two-way stepped differential extrusion torsional shearing of magnesium alloy billet 16:

[0077] ① First, the upper die 6 is withdrawn from the left punch die 9 and the right punch die 7, and then the magnesium alloy billet 16 is placed in. After the billet is placed, the upper die 6 and the upper extrusion rod 5 are tightly connected by threads. Next, the upper extrusion cylinder controller 25 and the lower extrusion cylinder controller 26 are operated to precisely control the moving speeds V1 and V2 (mm / min) of the internal bidirectional stepped differential speed extrusion die. Throughout the entire bidirectional stepped differential speed extrusion torsional shearing process, the heating layer controller 28 is continuously operated to stably control the temperature of the heating layer 8 at 450℃. By precisely controlling the heating layer temperature, the deformation resistance of the magnesium alloy billet can be effectively reduced, the plasticity can be improved, and good hot working conditions can be created for the billet to undergo complex torsional shearing deformation in the die.

[0078] ② After the magnesium alloy billet is extruded from the die, it immediately enters the extrusion channel. Within the extrusion channel, as the upper and lower dies move closer to the central deformation area, the billet undergoes a flow splitting phenomenon, flowing to both sides and ultimately being extruded from the left and right extrusion channels. During this extrusion process, the magnesium alloy sheet undergoes further torsional shear deformation. This torsional shear deformation causes the c-axis grains to tilt again, further weakening the basal texture and intensifying the deformation of the magnesium alloy. After this series of deformation processes, two high-performance magnesium alloy sheets can be obtained in a single extrusion. The stop button 24 can be pressed only after the internal torsional die has reached the center position, thus ending the bidirectional stepped differential extrusion torsional shear deformation process. In this process, precise die motion control and deformation process design ensure that the billet experiences suitable deformation conditions at different stages, thereby achieving the dual goals of grain refinement and texture weakening, and producing high-performance magnesium alloy sheets.

[0079] ③ After removing the weak-textured magnesium alloy sheet obtained from the bidirectional stepped differential extrusion, it needs to undergo further processing. First, the surface of the sheet is sanded to remove any possible oxide scale, burrs, or other defects, improving its smoothness and gloss. After sanding, the sheet is cleaned with an acetone + anhydrous ethanol cleaning solution prepared in a specific ratio to remove residual oil, debris, and other impurities. After cleaning, it is cleaned again with anhydrous ethanol to further ensure the cleanliness of the sheet surface. Finally, the sheet is dried with a hairdryer. After this series of treatments, the sheet is ready for use.

[0080] The principle of obtaining weak-surface textured magnesium alloy sheets in this invention is as follows:

[0081] 1) Dimensional parameters of the extrusion channel: such as Figure 2 and Figure 3As shown, the extrusion cross sections of the three extrusion sections are an arc with radius R1, an arc with radius R2, heights h1, h2, h3, and irregular platforms with widths W1 and W2 respectively, where R1 > R2; h2 > h3 > h1. Furthermore, the length of the extrusion section channel inlet is greater than the length of the extrusion section channel outlet, i.e., L1 > L2, L3 > L4. The radii of the chamfers are R3, R4, R5, and R6, where R3 = R4 = R5 = R6. In addition, the torsional extrusion of the two side punches has a torsional angle of θ1, which makes the cross-sectional area of ​​the extrusion channel gradually smaller, causing the c-axis grains of the magnesium alloy billet to deflect, weakening the basal texture of the magnesium alloy billet and refining the grains.

[0082] 2) Bidirectional Three-Stage Differential Extrusion Process: First Stage (Pre-extrusion Forming): Under the initial extrusion action of the upper and lower dies, the magnesium alloy billet undergoes axial flow. When it enters shear deformation zone I, the synergistic effect of the extrusion dies induces the material to undergo initial plastic deformation in a three-dimensional compressive stress field. Second Stage (Asymmetric Torsional Induced Deformation): As the upper and lower dies continue to move, the billet enters zone II, which has helical streamline characteristics. In this stage, the asymmetric movement of the dies generates torque, inducing the billet to undergo large angles (45-50°). During this deformation process, the c-axis of the grains randomly deflects, effectively disrupting the initial basal texture. Third Stage (Shear Strengthening): When the material advances into zone III, the wedge-shaped gap formed by the fixed punch and the moving die generates a high shear stress field. This multi-directional shear coupling further deflects the c-axis of the grains, reducing the strength of the basal texture. Fourth Stage (Flow Splitting and Shaping): In zone IV, the material, after multiple deformation passes, undergoes bidirectional flow at the flow splitter. The geometric constraints of the variable cross-section torsional channel ultimately shape the material, resulting in two magnesium alloy sheets obtained in a single extrusion. This process, through the structural design of the die system, achieves a three-stage texture weakening effect and a four-stage grain refinement process during a single extrusion. Compared with the traditional constant diameter angular extrusion process, energy consumption is reduced while production efficiency is improved, providing an innovative solution for preparing fine-grained magnesium alloys with weak surface texture.

[0083] During operation, the upper crossbeam 1, upper extrusion rod 5, and upper die 6 are in close contact, as are the lower crossbeam 17, lower extrusion rod 11, and lower die 12. This close contact ensures effective force transmission and precise control of the die movement. Therefore, when the upper extrusion rod 5 and lower extrusion rod 11 begin to work, the upper die 6 and lower die 12 begin to move towards the center position at different speeds. When the upper die 6 moves downward and the lower die 12 moves upward, due to their different speeds, the upper die 6 and lower die 12 act on the billet within the first stage of bidirectional stepped differential extrusion die, causing the billet to undergo torsional extrusion deformation. As the upper extrusion rod 5 and lower extrusion rod 11 move further towards the center position, the second stage of bidirectional stepped differential extrusion shear deformation occurs, and the billet again undergoes torsional extrusion deformation. When the upper extrusion rod 5 and lower extrusion rod 11 continue to move towards the center position, the third stage of bidirectional three-step differential extrusion shear deformation occurs. During the third stage of deformation, not only torsional deformation but also shear deformation occurs. The torsional shear deformation provided by the bidirectional stepped differential extrusion process significantly refines the grain size of the magnesium alloy billet. Furthermore, due to the special design of the die, the magnesium alloy billet in the torsional shear deformation region is never from the same location, thus ensuring a uniform microstructure in the resulting weak-surface textured magnesium alloy sheet and greatly reducing the basal surface texture. As the magnesium alloy billet is extruded from the die and enters the next extrusion stage, it continues to undergo torsional shear deformation within the extrusion channel, further optimizing the microstructure and properties of the sheet.

[0084] Furthermore, during the operation of the device, when the internal bidirectional stepped differential extrusion die moves to its upper (lower) limit position, that is, when the asymmetric torsional shearing extrusion section of the upper die 6 and the asymmetric torsional shearing extrusion section of the lower die 12 come into contact with each other, the upper extrusion cylinder controller and the lower extrusion cylinder controller must be immediately controlled to control the upper extrusion rod 5 and the lower extrusion rod 11, so that the internal bidirectional stepped differential extrusion die no longer moves up (down). Since the left punch die 9 and the right punch die 7 are fixed on the upper crossbeam 1 and the lower crossbeam 17 respectively by the left punch die fixing frame 10 and the right punch die fixing frame 13 during the operation of the device, and the distance between the left column 3 and the right column 15 is fixed, when the die moves to the limit position, timely stopping the movement of the die can avoid excessive extrusion and damage to the die, ensure the safe and stable operation of the entire device, and also ensure the processing quality and dimensional accuracy of the billet.

[0085] Based on the above principles, a magnesium alloy sheet with refined grains and weak surface texture is finally obtained.

[0086] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. An apparatus for preparing magnesium alloy sheets by bidirectional stepped differential extrusion, characterized in that: Includes an external mold frame, an internal two-way stepped differential extrusion torsion shearing die, and a two-way stepped differential extrusion equipment; The external mold frame includes an upper crossbeam (1) and a lower crossbeam (17). The two ends of the upper crossbeam (1) and the lower crossbeam (17) are respectively connected to columns. The two ends of the upper crossbeam (1) and the lower crossbeam (17) are respectively connected to a left punch mold fixing frame (10) and a right punch mold fixing frame (13) through positioning pins (4). A left punch mold (9) and a right punch mold (7) are respectively provided between the left punch mold fixing frame (10) and the right punch mold fixing frame (13). The bidirectional stepped differential extrusion equipment includes an upper extrusion cylinder (2) and a lower extrusion cylinder (14) connected to the lower side of the upper crossbeam (1) and the lower crossbeam (17) respectively. One end of the upper extrusion cylinder (2) and the lower extrusion cylinder (14) is connected to an upper extrusion rod (5) and a lower extrusion rod (11) respectively. The internal bidirectional stepped differential extrusion torsion shearing die includes an asymmetrically arranged upper die (6) and a lower die (12), which are respectively connected (11) to the upper extrusion rod (5) and the lower extrusion rod; both the upper die (6) and the lower die (12) include a circular arc platform with radius R1, a circular arc platform with radius R2, and an irregular platform connected in sequence; the heights of the irregular platforms are h1, h2, h3, and the widths are W1 and W2, respectively; Among them, R1>R2, h2>h3>h1.

2. The apparatus for preparing magnesium alloy sheets by bidirectional stepped differential extrusion according to claim 1, characterized in that: Heating layers (8) are provided on the outer sides of the left punch mold (9) and the right punch mold (7).

3. The apparatus for preparing magnesium alloy sheets by bidirectional stepped differential extrusion according to claim 1, characterized in that: A bidirectional three-step shearing channel is formed between the upper die (6) and the lower die (12); the bidirectional three-step shearing channel includes a first bidirectional stepped differential extrusion torsion region, a second bidirectional stepped differential extrusion torsion region and a third bidirectional stepped differential extrusion torsion region.

4. The apparatus for preparing magnesium alloy sheets by bidirectional stepped differential extrusion according to claim 1, characterized in that: A π-shaped flow channel is provided between the left punch mold (9) and the right punch mold (7). The π-shaped flow channel is the torsional shear deformation area of ​​the extrusion section. The two ends of this area are the extrusion section channel inlet and the extrusion section channel outlet, respectively. The length of the extrusion section channel inlet is greater than the length of the extrusion section channel outlet. The upper and lower ends of the extrusion section channel inlet are rounded chamfers with the same radius.

5. The apparatus for preparing magnesium alloy sheets by bidirectional stepped differential extrusion according to claim 2, characterized in that: It also includes a control device (22), which is electrically connected to the heating layer (8), the upper extrusion cylinder (2), and the lower extrusion cylinder (14).

6. A method for preparing magnesium alloy sheets using the bidirectional stepped differential extrusion of the apparatus described in claim 4, characterized in that: Includes the following steps: S1. Pretreated magnesium alloy billet; S1-1. Use 600-grit sandpaper to polish the surface of the magnesium alloy billet to remove oil stains, and then use 800, 1000 and 1200-grit sandpaper in sequence to polish it to ensure that its surface is smooth. S1-2. Mix acetone and anhydrous ethanol in a cleaning tank at a volume ratio of 3:2 to prepare an acetone + anhydrous ethanol cleaning solution. S1-3. Immerse the magnesium alloy billet in acetone + anhydrous ethanol cleaning solution. Place the cleaning tank on an ultrasonic cleaner and ultrasonically clean the magnesium alloy billet for 30-60 minutes. Then take out the magnesium alloy billet and clean it with anhydrous ethanol, and then dry it with a hair dryer. S1-4. Apply graphite oil solution to the surface of magnesium alloy billet; S2, Preheated magnesium alloy billet; Operate the vacuum atmosphere heating furnace and set the temperature between 380-450℃. After the heating furnace reaches the set temperature, put the magnesium alloy billet into the furnace and keep it at that temperature for 2-4 hours. S3, Lubrication, Assembly and Preheating; Apply graphite oil solution to the outer surfaces of the left and right punches and the inner cavities of the upper and lower dies; after assembly, heat to 450℃ through the heating layer, and keep warm for 3 hours after reaching the set temperature; S4, bidirectional stepped differential extrusion torsion shearing of magnesium alloy billets; S4-1. Withdraw the upper die and place the magnesium alloy billet in. Control the upper and lower extrusion cylinders to move at different speeds through the control device. As the upper and lower dies move toward the middle position, the magnesium alloy billet undergoes torsional shearing extrusion deformation and the billet rotates continuously. The magnesium alloy billet in the torsional shearing deformation area is never the same billet. The heating layer temperature is controlled at 380-450℃. S4-2. After the magnesium alloy billet is extruded from the upper and lower dies, it immediately enters the extrusion section channel. In the extrusion section channel, as the upper and lower dies move closer to the middle deformation area, the billet undergoes a flow splitting phenomenon and flows to both sides, eventually being extruded from the left and right extrusion section channels. During this extrusion process, the magnesium alloy sheet undergoes further torsional shear deformation, which weakens the texture of the magnesium alloy sheet and continuously refines the grains. Once the billet is extruded, one extrusion process ends. S5. Take out the fine-grained weak-surface textured magnesium alloy sheet obtained by bidirectional stepped differential extrusion torsion shearing, polish its surface with sandpaper, clean it with the above-mentioned acetone + anhydrous ethanol cleaning solution, clean it again with anhydrous ethanol, and finally dry it with a hair dryer before use.

Images