Device and method for preparing magnesium alloy plate through bidirectional stepped differential extrusion

Abstract

The invention aims to provide a device and method for preparing a magnesium alloy plate through bidirectional stepped differential extrusion, and belongs to the technical field of light metal plastic forming, the device comprises an external mold frame, an internal bidirectional stepped differential extrusion torsion shearing mold and bidirectional stepped differential extrusion equipment; the bidirectional stepped differential extrusion process comprises four processes: a first stage of pre-extrusion forming, a second stage of asymmetric torsion induced deformation, a third stage of shear strengthening and a fourth stage of split-flow shaping, and through the device and the method provided by the invention, three times of texture weakening effect and four times of grain refinement process in a single extrusion process are realized. And compared with a traditional equal channel angular extrusion process, the energy consumption is reduced, meanwhile, the production efficiency is improved, and an innovative solution is provided for preparing the fine-grain weak-plane texture magnesium alloy.

Description

Technical Field

[0001] The invention belongs to the technical field of light metal plastic forming, and in particular relates to a device and a method for preparing a magnesium alloy plate by bidirectional step differential extrusion. Background Art

[0002] As the lightest metal structural material currently, magnesium alloy has attracted much attention for its unique close-packed hexagonal crystal structure and excellent comprehensive performance. This material system not only has ultra-low density, but also has excellent specific strength, outstanding damping performance, excellent biocompatibility and other characteristics. It shows broad application prospects in the fields of aerospace lightweight, new energy vehicles, 3C electronic products and biomedical implants. However, the inherent close-packed hexagonal crystal structure of magnesium alloy leads to its limited room temperature plastic deformation ability, which is difficult to meet the five independent slip systems required by the Von-Mises criterion. The macroscopic performance is poor room temperature mechanical properties, which limits its application in various fields. Studies have shown that the strength and plasticity of magnesium alloys can be effectively improved by grain refinement. Among them, severe plastic deformation technology has been proven to be an effective way to achieve ultrafine grain structure. Severe plastic deformation represented by high-pressure torsion can refine the grain size and weaken the basal texture. However, torsional deformation is generally more complicated, with high mold processing requirements and equipment requirements, and needs to withstand greater pressure. The material has a low degree of torsion, which has great limitations on 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, it is very important to invent an effective device and method for weakening the texture of magnesium alloy sheets and refining the grains to expand the application range of magnesium alloys. Summary of the invention

[0004] In view of the above problems, the purpose of the present invention is to provide a device and method for preparing magnesium alloy plates by bidirectional step differential extrusion. Through the device and its processing method, the magnesium alloy billet undergoes continuous and severe torsional shear deformation during the processing, causing the c-axis in the magnesium alloy grains to deflect, thereby achieving grain refinement and weakening of 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: A device for preparing a magnesium alloy plate by bidirectional step differential extrusion, comprising an external die frame, an internal bidirectional step differential extrusion torsion shearing die and a bidirectional step differential extrusion device; The external mold frame includes an upper beam and a lower beam, both ends of the upper beam and the lower beam are respectively connected to columns, both ends of the upper beam and the lower beam are respectively connected to left and right punch mold fixing frames through positioning pins, and left and right punch molds are respectively arranged between the left and right punch mold fixing frames; The bidirectional step differential extrusion equipment comprises an upper extrusion oil cylinder and a lower extrusion oil cylinder respectively connected to the lower sides of the upper crossbeam and the lower crossbeam, and one end of the upper extrusion oil cylinder and the lower extrusion oil cylinder are respectively connected to an upper extrusion rod and a lower extrusion rod; The internal bidirectional stepped differential extrusion torsion shearing die comprises an upper concave die and a lower concave die which are arranged asymmetrically, and the upper concave die and the lower concave die are connected to an upper extrusion rod and a lower extrusion rod respectively.

[0006] Furthermore, heating layers are provided on the outer sides of the left and right male molds respectively.

[0007] Furthermore, the upper and lower concave dies each include a radius R connected in sequence. 1 The arc table has a radius of R 2 The arc table 2 and the irregular table top; the heights of the irregular table tops are h 1 、h 2 、h 2 、h 3 , the widths are W 1 , W 2 ; Among them, R 1 >R 2 ,h 2 >h 3 >h 1 ; Furthermore, a bidirectional three-step shear flow channel is formed between the upper and lower dies; the bidirectional three-step shear flow channel includes a first bidirectional step differential extrusion torsion area, a second bidirectional step differential extrusion torsion area and a third bidirectional step differential extrusion torsion area.

[0008] Furthermore, a π-shaped flow channel is provided between the left punch mold and the right punch mold, and the π-shaped flow channel is a torsional shear deformation area of ​​the extrusion section, and the two ends of the area are respectively an extrusion section channel inlet and an extrusion section channel outlet, the length of the extrusion section channel inlet is greater than the length of the extrusion section channel outlet, and the upper and lower ends of the extrusion section channel inlet are circular arc chamfers with the same radius.

[0009] The left punch mold, right punch mold, upper die and lower die mold are all made of hot working die steel 4Cr 5 MoSv 1 This material has good heat resistance, wear resistance and toughness, and can meet the performance requirements of the mold in high temperature and high pressure working environment. The surface roughness of the mold reaches Ra0.16-0.4μm, ensuring the smooth flow of the blank in the mold and reducing the occurrence of surface defects. The surface roughness of the upper and lower beams also reaches Ra0.16-0.4μm, ensuring the assembly accuracy and stability of the entire device.

[0010] 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.

[0011] A method for preparing a magnesium alloy sheet by bidirectional step differential extrusion comprises the following steps: S1. Pretreating magnesium alloy billet; S1-1. Use 600-grit sandpaper to polish the surface of the magnesium alloy billet to remove oil stains, and then polish it with 800, 1000, and 1200-grit sandpaper in sequence to ensure that the surface is smooth; S1-2, stir and 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, immersing the magnesium alloy blank in an acetone + anhydrous ethanol cleaning solution, placing the cleaning tank on an ultrasonic cleaning machine to ultrasonically clean the magnesium alloy blank for 30 to 60 minutes, taking out the magnesium alloy blank and cleaning it with anhydrous ethanol, and then drying it with a hair dryer; S1-4, applying graphite oil solution on the surface of the magnesium alloy billet; S2, preheating the magnesium alloy billet; Operate the vacuum atmosphere heating furnace and set the temperature between 380-450°C. After the heating furnace reaches the set temperature, put the magnesium alloy billet into the furnace and keep it warm for 2-4 hours; S3, lubrication, assembly and preheating; Apply graphite oil solution to the outer surface of the left and right male molds and the inner cavity of the upper and lower female molds; after assembly, heat to 450°C through the heating layer, and keep warm for 3 hours after reaching the set temperature; S4, bidirectional step differential extrusion torsion shearing magnesium alloy billet; S4-1. The upper die is withdrawn and the magnesium alloy billet is placed in the die. The upper and lower extrusion cylinders are controlled to move at different speeds by the control device. As the upper die and the lower die move toward the middle position, the magnesium alloy billet undergoes torsional shearing and extrusion deformation, and the billet rotates continuously. The magnesium alloy billet in the torsional shearing deformation area is not always in the same place. The temperature of the heating layer is controlled to be 380-450°C. 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 continue to approach the middle deformation area, the billet produces a diversion phenomenon and flows to both sides, and is finally extruded from the left and right extrusion channels. During this extrusion process, the magnesium alloy sheet further undergoes torsional shear deformation, which weakens the texture of the magnesium alloy sheet and continuously refines the grains. After the billet is extruded, one extrusion process ends; S5. Take out the fine-grained weak-surface texture magnesium alloy sheet obtained by bidirectional step differential extrusion torsional shearing, polish its surface with sandpaper, clean it with the above-mentioned acetone + anhydrous ethanol cleaning solution, then clean it again with anhydrous ethanol, and finally dry it with a hair dryer before putting it into use.

[0012] The composite die set consisting of the upper and lower concave dies, the left and right punch dies (diversion constraints) and the variable cross-section torsion channel forms a four-stage deformation space of "pre-extrusion-twist-shear-diversion". The first stage: pre-extrusion and initial shearing, the magnesium alloy billet is pushed forward under the axial pressure of the upper and lower concave dies. At this time, the intervention of the extrusion concave die causes the material to be sheared for the first time in the three-dimensional stress field. In the second stage, when the concave die system is V 1 、V 2 When the material is pressed down and up at a rate of 1000, the blank enters the spiral streamlined area. At this time, 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. The torsional shear causes the c-axis of the grain to deflect randomly at a small angle. In the third stage, after entering the wedge-shaped gap area, the left and right punch dies and the moving upper and lower dies form a dynamic shear zone. The fourth stage is diversion. The material is diverted at the fixed die, and the variable-section torsion channels of the left and right punches form reverse shear, undergoing reverse torsion and radial expansion. Finally, when the double sheet after gradient deformation is extruded from the through hole, the extrusion channel area of ​​the die smoothes the surface of the blank to reduce the surface roughness of the sheet. The base texture strength is reduced by three asymmetric differential extrusions, and the four-stage deformation refines the grains, breaking through the room temperature processing limit of magnesium alloys.

[0013] When the device and processing method of the present invention are used to process bidirectional step differential extrusion magnesium alloy billets, due to: a. Reliable connection between the upper crossbeam, upper extruder, upper and lower dies, lower extruder and lower crossbeam; b. Internal bidirectional asymmetric torsional shearing and extrusion motion; c. The extrusion channel formed by the left and right punch dies undergoes further torsional shear deformation.

[0014] Under the synergistic effect of the above three factors, the base surface texture weakening and grain refinement effects of the obtained magnesium alloy sheet are more obvious than those of the magnesium alloy sheet obtained by traditional process methods.

[0015] The beneficial effects of the present invention are as follows: 1. The parts of the device of the present invention are simple in shape, simple in manufacturing process and low in manufacturing cost; 2. Innovative design of composite mold: The splitter mold, shear mold and torsion mold are integrated to achieve excellent texture weakening effect through asymmetric velocity field, spiral-wedge composite flow channel and three-level reverse shearing; 3. Through the time-space control of the strain path, the accumulated strain and grain refinement are achieved in a single-pass processing, and the base surface texture strength is reduced; 4. Block structure, users can change modules according to needs to prepare weak surface textured magnesium alloy sheets with different cross-sections.

[0016] This device successfully solved the technical bottleneck of strong base surface texture and poor formability of magnesium alloy sheets through innovative structural design and process combination. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic diagram of the main structure of the device of the present invention; Figure 2 It is a schematic diagram of the main structure of the concave mold of the device of the present invention; Figure 3 A detailed view of the concave mold of the device of the present invention; Figure 4 It is a schematic diagram of the main structure of the male mold of the device of the present invention; Figure 5 It is a left-side structural schematic diagram of the male mold of the device of the present invention; Figure 6 A schematic diagram of the deformation process of a magnesium alloy blank for preparing a magnesium alloy sheet by the present invention; Among them: 1-upper crossbeam; 2-upper extrusion cylinder; 3-left column; 4-locating pin; 5-upper extrusion rod; 6-upper concave die; 7-right punch mold; 8-heating layer; 9-left punch mold; 10-left punch mold fixing frame; 11-lower extrusion rod; 12-lower concave die; 13-right punch mold 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.

[0018] Ⅰ-the first section of bidirectional step differential extrusion torsion zone; Ⅱ-the second section of bidirectional step differential extrusion torsion zone; Ⅲ-the third section of bidirectional step differential extrusion torsion zone; Ⅳ-the extrusion section torsion shear deformation zone. DETAILED DESCRIPTION

[0019] The present invention will be further described with reference to the accompanying drawings.

[0020] As shown in the figure, a device for preparing magnesium alloy plates by bidirectional step differential extrusion includes an external die frame, an internal bidirectional step differential extrusion torsion shearing die and a bidirectional step differential extrusion device; 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 a positioning pin 4, and a left punch mold 9 and a right punch mold 7 are respectively arranged between the left punch mold fixing frame 10 and the right punch mold fixing frame 13; The left column 3 and the right column 15 form a vertical load-bearing unit, which cooperates with the upper crossbeam 1 and the lower crossbeam 17 to form a closed prestressed frame structure. The overall stiffness is optimized by finite element design and can withstand multi-directional asymmetric extrusion loads.

[0021] The bidirectional step differential extrusion device comprises an upper extrusion cylinder 2 and a lower extrusion cylinder 14 respectively connected to the lower sides of the upper crossbeam 1 and the lower crossbeam 17, and one end of the upper extrusion cylinder 2 and the lower extrusion cylinder 14 are respectively connected to an upper extrusion rod 5 and a lower extrusion rod 11; The internal bidirectional step differential extrusion torsion shearing die comprises an asymmetrically arranged upper die 6 and a lower die 12, and the upper die 6 and the lower die 12 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, which creates conditions for the asymmetrical differential torsion shearing deformation of the blank.

[0022] A dynamic fit clearance of 0.25±0.02 mm is reserved between the upper and lower concave dies to achieve multi-directional metal flow control of the magnesium alloy blank 16 .

[0023] Furthermore, a heating layer 8 is provided on the outer sides of the left punch mold 9 and the right punch mold 7 respectively.

[0024] Furthermore, the upper die 6 and the lower die 12 each include a plurality of dies having a radius R connected in sequence. 1 The arc table has a radius of R 2 The arc table 2 and the irregular table top; the heights of the irregular table tops are h 1 、h 2 、h 2 、h 3 , the widths are W 1 , W 2 ; Among them, R 1 >R 2 ,h 2 >h 3 >h 1 .

[0025] Furthermore, a bidirectional three-step shear flow channel is formed between the upper die 6 and the lower die 12; the bidirectional three-step shear flow channel includes a first bidirectional step differential extrusion torsion area, a second bidirectional step differential extrusion torsion area and a third bidirectional step differential extrusion torsion area.

[0026] 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 a torsional shear deformation area of ​​the extrusion section. The two ends of the area are respectively an extrusion section channel inlet and an extrusion section channel outlet. 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 both circular arc chamfers with the same radius.

[0027] The bidirectional step differential torsional shear extrusion space constructed by the left punch mold 9, the right punch mold 7, the upper die 6, the lower die 12, the left punch mold fixing frame 10 and the right punch mold fixing frame 13 is filled with magnesium alloy billet. The torsional shear extrusion space is composed of the left punch mold, the right punch mold and the bidirectional step differential extrusion torsional shear deformation area. The left extrusion channel and the right extrusion channel are used to shape the magnesium alloy billet in the extrusion torsional area to achieve the expected shape and size accuracy. During the operation of the device, the magnesium alloy billet continuously enters the torsional shear area, causing it to undergo severe plastic deformation, thereby achieving the purpose of grain refinement.

[0028] 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 .

[0029] Example A method for preparing a magnesium alloy sheet by bidirectional step differential extrusion comprises the following steps: 1. Select materials and chemical reagents: magnesium alloy billet, material selected is AZ31, containing 96% magnesium, 3% aluminum, and 1% zinc; sandpaper, solid solid; graphite oil solution, viscous liquid; anhydrous ethanol, liquid liquid, purity 99.5%; acetone, liquid liquid, purity 99%.

[0030] 2. Pretreatment of magnesium alloy billets: ① Use 600-grit sandpaper to polish the surface of the magnesium alloy blank 16 to remove oil stains, and then polish it with 800, 1000, and 1200-grit sandpaper in sequence to ensure that the surface is smooth; ② Mix acetone and anhydrous ethanol in a cleaning tank at a volume ratio of 3:2 and stir until evenly mixed to prepare an acetone + anhydrous ethanol cleaning solution; ③ Immerse the magnesium alloy blank 16 in the acetone + anhydrous ethanol cleaning solution, place the cleaning tank on an ultrasonic cleaning machine to ultrasonically clean the magnesium alloy blank 16 for 60 minutes, take out the magnesium alloy blank 16 and clean it with anhydrous ethanol, and then blow dry it with a hair dryer; ④ Apply graphite oil solution on the surface of the magnesium alloy blank 16.

[0031] 3. Preheating the magnesium alloy billet 16: operate a vacuum atmosphere heating furnace, set the temperature to 450°C, and after reaching the set temperature, place the magnesium alloy billet 16 into the furnace and keep it warm for 3 hours.

[0032] 4. Lubrication, assembly and preheating: ① Before assembling the device, 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 all the surfaces of the inner cavity of the lower die 12 should be coated with graphite oil solution; ② When assembling the device, install the left punch mold 9 on the left punch mold fixing frame 10, install the right punch mold 7 on the right punch mold fixing frame 13, install the upper crossbeam 1 and the lower crossbeam 17 in the corresponding holes of the left column 3 and the right column 15, connect the upper extrusion rod 5 with the upper die 6 by thread, connect the lower extrusion rod 11 with the lower die 12 by thread, and connect the upper extrusion rod 5 and the lower extrusion rod 11 with the upper extrusion cylinder 2 and the lower extrusion cylinder 14 respectively. Operate the upper extrusion cylinder controller 25 and the lower extrusion cylinder controller 26 to adjust the distance between the upper die 6 and the lower die 12 so that the magnesium alloy billet 16 can be placed between the two. The left punch mold fixing frame 10 is in close contact with the upper beam 1 and the lower beam 17, and the right punch mold fixing frame 13 is in close contact with the upper beam 1 and the lower beam 17. The upper extrusion cylinder controller 25 is operated to make the upper extrusion rod 5 push the upper die 6 to move downward, and the lower extrusion cylinder controller 26 makes the lower extrusion rod 11 push the lower die 12 to move upward, so that the magnesium alloy billet in the torsional shear deformation zone is not always the same billet, and the obtained weak surface texture magnesium alloy sheet has uniform organization and weakened texture. Operate the upper extrusion cylinder controller 25 and the lower extrusion cylinder controller 26 to make the internal two-way step differential extrusion torsion shearing die move asynchronously and differentially, and observe whether there is billet extrusion in the extrusion channel. a. If not, continue to adjust the distance between the upper concave die 6 and the lower concave die 12. b. If yes, control the upper extrusion cylinder controller 25 and the lower extrusion cylinder controller 26 to move the internal two-way 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 a whole; 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 warm for 3 hours after reaching the set temperature.

[0033] 5. Bidirectional step differential extrusion torsion shearing of magnesium alloy billet 16: ① First, withdraw the upper die 6 from the left punch die 9 and the right punch die 7, and then put the magnesium alloy billet 16. After the billet is placed, the upper die 6 and the upper extrusion rod 5 are tightly connected through threads. Then, operate the upper extrusion cylinder controller 25 and the lower extrusion cylinder controller 26 to accurately control the moving speed V of the internal bidirectional step differential extrusion die. 1 、V 2(mm / min). During the entire bidirectional step differential extrusion torsion shearing process, the heating layer controller 28 is continuously operated to stably control the temperature of the heating layer 8 at 450°C. By accurately controlling the temperature of the heating layer, the deformation resistance of the magnesium alloy billet can be effectively reduced, the plasticity can be improved, and good thermal processing conditions can be created for the billet to undergo complex torsion shearing deformation in the die.

[0034] ② After the magnesium alloy billet is extruded from the die, it immediately enters the extrusion channel. In the extrusion channel, as the upper and lower die dies continue to approach the middle deformation area, the billet will produce a diversion phenomenon and flow to both sides, and finally be extruded from the left and right extrusion channels. During this extrusion process, the magnesium alloy sheet further undergoes torsional shear deformation. Torsional shear deformation can cause the c-axis grains to tilt again, thereby further weakening the basal texture and aggravating the deformation degree of the magnesium alloy. After this series of deformation processes, two high-performance magnesium alloy sheets can be obtained in one extrusion. When the internal torsion die stops at the middle position, the stop button 24 can be pressed, and the bidirectional step differential extrusion torsional shear deformation process ends. In this process, precise die motion control and deformation process design ensure that the billet can experience suitable deformation conditions at different stages, thereby achieving the dual goals of grain refinement and texture weakening, and preparing high-performance magnesium alloy sheets.

[0035] ③ After taking out the weak surface textured magnesium alloy sheet obtained by bidirectional step differential extrusion, it needs to be processed later. First, the surface of the sheet is polished with sandpaper to remove possible defects such as oxide scale and burrs on the surface, and to improve the flatness and finish of the sheet surface. After polishing, the sheet is cleaned with the acetone + anhydrous ethanol cleaning solution prepared in a specific proportion to remove impurities such as oil, debris, etc. remaining on the surface. 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 hair dryer. After this series of treatments, the sheet can be put into use.

[0036] The principle of obtaining the weak surface texture magnesium alloy sheet of the present invention is as follows: 1) Dimensional parameters of the extrusion channel: Figure 2 and Figure 3As shown, the extrusion cross-sections of the three extrusion sections are respectively an arc with a radius of R1 and an arc with a radius of R2, and have heights of h1, h2, h2, h3 and irregular tables with widths of W1 and W2, wherein R1>R2; h2>h3>h1, and the length of the inlet of the extrusion section channel is greater than the length of the outlet of the extrusion section channel, i.e., L1>L2, L3>L4, and the radii of the chamfers are R3, R4, R5, R6, wherein R3=R4=R5=R6, and secondly, the torsion angle of the torsional extrusion performed by the punch dies on both sides is θ1, which gradually reduces the cross-sectional area of ​​the extrusion channel, deflects the c-axis grains of the magnesium alloy billet, weakens the basal texture of the magnesium alloy billet and refines the grains.

[0037] 2) Bidirectional three-step differential extrusion process: In the first stage (pre-extrusion forming), the magnesium alloy billet generates axial flow under the initial extrusion of the upper and lower concave dies. When entering the shear deformation area I, the synergistic effect of the extrusion concave die causes the material to undergo initial plastic deformation in the three-dimensional compressive stress field. In the second stage (asymmetric torsion induced deformation), as the upper and lower concave dies continue to move, the billet enters the area II with spiral streamline characteristics. In this stage, the asymmetric movement of the die generates torque, inducing the billet to undergo a large angle (45-50°). During this deformation process, the c-axis of the grain is randomly deflected, effectively destroying the initial basal surface texture. In the third stage (shear strengthening), when the material advances to area III, the wedge-shaped gap formed by the fixed punch die and the moving concave die generates a higher shear stress field. This multi-directional shear coupling effect further deflects the c-axis of the grain and reduces the strength of the basal surface texture. In the fourth stage (diversion shaping), in area IV, the material after multiple deformations generates bidirectional flow at the diversion die. The geometric constraints of the variable cross-section torsion channel finally shape the material, and two magnesium alloy sheets are obtained in one extrusion. This process achieves three texture weakening effects and four grain refinement processes in a single extrusion process through the structural design of the die system. Compared with the traditional equal-channel angular extrusion process, the energy consumption is reduced while the production efficiency is improved, providing an innovative solution for the preparation of fine-grained weak-surface textured magnesium alloys.

[0038] During the operation of the device, the upper crossbeam 1, the upper extrusion rod 5 and the upper die 6 are in close contact, and the lower crossbeam 17, the lower extrusion rod 11 and the lower die 12 are in close contact. This close contact relationship ensures the effective transmission of force and the precise control of the mold movement. Therefore, when the upper extrusion rod 5 and the lower extrusion rod 11 start to work, the upper die 6 and the lower die 12 start to move to the middle position at different travel speeds. When the upper die 6 moves downward and the lower die 12 moves upward, due to the different moving speeds of the two, the upper die 6 and the lower die 12 act on the blank in the first section of the two-way step differential extrusion die, causing the blank to be torsionally extruded and deformed. As the upper extrusion rod 5 and the lower extrusion rod 11 move further to the middle position, the second section of the two-way step differential extrusion shear deformation occurs, and the blank is torsionally extruded and deformed again. When the upper extrusion rod 5 and the lower extrusion rod 11 continue to move to the middle position, the third section of the two-way three-step differential extrusion shear deformation occurs. In the third section of deformation, not only torsion deformation occurs, but also shear deformation occurs. The torsional shear deformation provided by the bidirectional step differential extrusion process makes the grain refinement effect of the magnesium alloy billet more significant. In addition, due to the special design of the mold, the magnesium alloy billet in the torsional shear deformation area is not always the same billet, thereby ensuring that the obtained weak surface texture magnesium alloy sheet has uniform organization and greatly weakens the base surface texture. When the magnesium alloy billet is extruded from the die and enters the next stage of extrusion, when the billet enters the extrusion channel, the magnesium alloy billet continues to undergo torsional shear deformation in the extrusion channel, further optimizing the organizational structure and performance of the sheet.

[0039] Furthermore, during the operation of the device, when the internal bidirectional step differential extrusion die moves to the upper (lower) limit position, that is, when the asymmetric torsional shear extrusion section of the upper die 6 contacts the asymmetric torsional shear extrusion section of the lower die 12, the upper extrusion cylinder controller and the lower extrusion cylinder controller need to be immediately controlled to control the upper extrusion rod 5 and the lower extrusion rod 11, so that the internal bidirectional step differential extrusion die no longer moves up (down). Since the left punch die 9 and the right punch die 7 are respectively fixed on the upper beam 1 and the lower beam 17 by the left punch die fixing frame 10 and the right punch die fixing frame 13 during the operation of the device, and the spacing between the left column 3 and the right column 15 is fixed, therefore, when the die moves to the limit position, the movement of the die is stopped in time, which can avoid excessive extrusion and damage of the die, ensure the safe and stable operation of the entire device, and also ensure the processing quality and dimensional accuracy of the blank.

[0040] Through the above principle, a magnesium alloy sheet with refined grains and weak surface texture is finally obtained.

[0041] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope of the claims.

Claims

1. A device for preparing magnesium alloy sheets by bidirectional step differential extrusion, characterized in that: It includes an external die frame, an internal bidirectional step differential extrusion torsion shearing die and a bidirectional step differential extrusion device; The external mold frame comprises 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 upright posts, 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) via positioning pins (4), and 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 step differential extrusion device comprises an upper extrusion cylinder (2) and a lower extrusion cylinder (14) respectively connected to the lower sides of an upper crossbeam (1) and a lower crossbeam (17); one end of the upper extrusion cylinder (2) and the lower extrusion cylinder (14) are respectively connected to an upper extrusion rod (5) and a lower extrusion rod (11); The internal bidirectional stepped differential extrusion torsion shearing die comprises an asymmetrically arranged upper die (6) and a lower die (12), wherein the upper die (6) and the lower die (12) are respectively connected to an upper extrusion rod (5) and a lower extrusion rod (11).

2. The device for preparing magnesium alloy sheet by bidirectional step differential extrusion according to claim 1, characterized in that: The outer sides of the left punch mold (9) and the right punch mold (7) are respectively provided with heating layers (8).

3. The device for preparing magnesium alloy sheet by bidirectional step differential extrusion according to claim 1, characterized in that: The upper concave die (6) and the lower concave die (12) both comprise a circular arc table 1 with a radius of R1, a circular arc table 2 with a radius of R2, and an irregular table top, which are connected in sequence; the heights of the irregular table tops are h1, h2, h2, and h3, respectively, and the widths are W1 and W2, respectively; Among them, R1>R2, h2>h3>h1.

4. The device for preparing magnesium alloy sheet by bidirectional step differential extrusion according to claim 1, characterized in that: A bidirectional three-step shear flow channel is formed between the upper die (6) and the lower die (12); the bidirectional three-step shear flow channel comprises a first bidirectional step differential extrusion and twisting area, a second bidirectional step differential extrusion and twisting area, and a third bidirectional step differential extrusion and twisting area.

5. The device for preparing magnesium alloy sheet by bidirectional step 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 a torsional shear deformation area of ​​the extrusion section. The two ends of the area are respectively an extrusion section channel inlet and an extrusion section channel outlet. 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 both circular arc chamfers with the same radius.

6. The device for preparing magnesium alloy sheet by bidirectional step differential extrusion according to claim 1, characterized in that: It also includes a control device (22), wherein the control device (22) is electrically connected to the heating layer (8), the upper extrusion cylinder (2), and the lower extrusion cylinder (14).

7. A method for preparing a magnesium alloy sheet by bidirectional step differential extrusion using the device according to claim 1, characterized in that: The steps include: S1. Pretreating magnesium alloy billet; S1-1. Use 600-grit sandpaper to polish the surface of the magnesium alloy billet to remove oil stains, and then polish it with 800, 1000, and 1200-grit sandpaper in sequence to ensure that the surface is smooth; S1-2, stir and 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, immersing the magnesium alloy blank in an acetone + anhydrous ethanol cleaning solution, placing the cleaning tank on an ultrasonic cleaning machine to ultrasonically clean the magnesium alloy blank for 30 to 60 minutes, taking out the magnesium alloy blank and cleaning it with anhydrous ethanol, and then drying it with a hair dryer; S1-4, applying graphite oil solution on the surface of the magnesium alloy billet; S2, preheating the magnesium alloy billet; Operate the vacuum atmosphere heating furnace and set the temperature between 380-450°C. After the heating furnace reaches the set temperature, put the magnesium alloy billet into the furnace and keep it warm for 2-4 hours; S3, lubrication, assembly and preheating; Apply graphite oil solution to the outer surface of the left and right male molds and the inner cavity of the upper and lower female molds; after assembly, heat to 450°C through the heating layer, and keep warm for 3 hours after reaching the set temperature; S4, bidirectional step differential extrusion torsion shearing magnesium alloy billet; S4-1. The upper die is withdrawn and the magnesium alloy billet is placed in the die. The upper and lower extrusion cylinders are controlled to move at different speeds by the control device. As the upper die and the lower die move toward the middle position, the magnesium alloy billet undergoes torsional shearing and extrusion deformation, and the billet rotates continuously. The magnesium alloy billet in the torsional shearing deformation area is not always in the same place. The temperature of the heating layer is controlled to be 380-450°C. 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 continue to approach the middle deformation area, the billet produces a diversion phenomenon and flows to both sides, and is finally extruded from the left and right extrusion channels. During this extrusion process, the magnesium alloy sheet further undergoes torsional shear deformation, which weakens the texture of the magnesium alloy sheet and continuously refines the grains. After the billet is extruded, one extrusion process ends; S5. Take out the fine-grained weak-surface texture magnesium alloy sheet obtained by bidirectional step differential extrusion torsional shearing, polish its surface with sandpaper, clean it with the above-mentioned acetone + anhydrous ethanol cleaning solution, then clean it again with anhydrous ethanol, and finally dry it with a hair dryer before putting it into use.

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