Method for preparing submicron ultra-fine grain magnesium alloy through low-pressure torsion

Abstract

The invention discloses a method for preparing a submicron ultra-fine grain magnesium alloy through low-pressure torsion, which comprises the following steps: respectively mounting a convex die and a concave die on a linear feeding mechanism and a torsion mechanism, then driving the convex die to feed along the axial direction by the linear feeding mechanism, placing the convex die into a material placing barrel, and enabling a cross-shaped groove of the convex die to be in contact with a blank; the linear feeding mechanism drives the male die to continuously move, and the blank is subjected to upsetting deformation until the rear section of the upsetting cavity and the flow blocking belt are filled with the blank; the female die is driven by the twisting mechanism to rotate, the fixed base plate also rotates along with the female die, and an arc-shaped boss surface in the twisting and extruding cavity drives one end, in contact with the arc-shaped boss surface, of the blank to generate torsional deformation; and after one-time torsional deformation is completed, the female die is detached, the deformed blank is taken out of the female die and rotated by 180 degrees, the steps are repeated, and torsional deformation is applied to the other undeformed end of the blank through the female die. According to the method, reinforced shear stress is continuously applied to the magnesium alloy blank to achieve high-strain accumulation, the technological process is shortened, and preparation of the weak-anisotropy uniform ultra-fine grain magnesium alloy ingot blank is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of metal plastic deformation, and in particular refers to a method for preparing a submicron-level ultrafine-grained magnesium alloy by low-pressure torsion. Background Art

[0002] With the increasing requirements for lightweight high-end equipment in aerospace and other fields, the demand for magnesium alloy components with high strength and plasticity has become more urgent. In this context, ultrafine-grained magnesium alloys, with their excellent strength and ductility, have become the key to solving the difficult problem of balancing the strength and ductility of magnesium alloys. Compared with coarse-grained alloys, ultrafine-grained magnesium alloys have achieved a simultaneous improvement in strength and ductility through grain refinement technology.

[0003] During the thermal deformation process, due to the limited strain and the growth of recrystallized grains, it is difficult for traditional large plastic deformation processes to obtain ultrafine-grained structures to improve strength and toughness. The severe plastic deformation (SPD) method can produce ultrafine-grained materials through the synergistic effect of high strain accumulation and high hydrostatic pressure without almost changing the shape change.

[0004] Although SPD methods such as large strain rolling and equal-diameter angular extrusion can reduce the grain size and significantly improve the mechanical properties of alloys, their preparation process is cumbersome and costly, which largely limits their widespread application in the industrial field. On the other hand, although high-pressure torsion deformation can produce submicron-level ultrafine-grained alloys in a shorter process, due to the requirement of large-scale equipment forming force, this method is limited to the preparation of micron or submicron-level ultrafine-grained billets for laboratory research and cannot be used in industry. In addition, the size of the ultrafine-grained alloys produced is limited. Summary of the invention

[0005] The purpose of the present invention is to provide a method for preparing submicron ultrafine-grained magnesium alloy by low-pressure torsion, that is, by continuously applying strong shear stress to the magnesium alloy billet under appropriate pressure conditions to achieve high strain accumulation, shorten the process flow, and realize the preparation of weakly anisotropic and uniform ultrafine-grained magnesium alloy ingots.

[0006] To achieve the above object, the solution of the present invention is: a method for preparing a submicron ultrafine-grained magnesium alloy by low-pressure torsion, comprising the following steps:

[0007] S1 blanking: taking a section of magnesium alloy cylindrical billet;

[0008] S2 mold installation: assemble the mold, the mold includes a punch, a die, a discharge barrel and a fixed pad, the die has a cavity at one end close to the punch, the discharge barrel and the fixed pad are fixedly installed in the cavity, the discharge barrel has a hollow upsetting cavity along the axial direction for the blank and the punch to be placed, the inner diameter of the front section of the upsetting cavity is smaller than the inner diameter of the rear section to form a flow blocking zone to prevent the blank from overflowing, the rear section of the upsetting cavity is an upsetting working zone, the fixed pad has a hollow twisting and extrusion cavity, the front section of the twisting and extrusion cavity is provided with a gradually inward arc-shaped boss surface at the inner periphery of the connection with the rear section of the upsetting cavity, for the blank to flow in and contact from the upsetting working section, and the end of the punch is provided with a cross-shaped groove in contact with the blank;

[0009] S3 Preparation before forming: The tail ends of the punch and the die are respectively mounted on the linear feed mechanism and the twisting mechanism, and then the linear feed mechanism drives the punch to feed axially, and the punch with a cross-shaped groove is placed in the discharge barrel and moved until the cross-shaped groove of the punch contacts the blank;

[0010] S4 slight upsetting deformation: the linear feed mechanism drives the punch to continue to move, and the blank undergoes slight upsetting deformation until the blank fills the rear section of the upsetting cavity and the choke zone;

[0011] S5 primary torsion deformation: the die rotates under the drive of the torsion mechanism, and the fixed pad also rotates accordingly, and the arc-shaped boss surface in the torsion cavity drives the end of the blank in contact with it to undergo torsion deformation;

[0012] S6 secondary torsional deformation: after the primary torsional deformation is completed, the die is disassembled, the deformed blank is taken out of the die and rotated 180°, and steps S2 and S4 are repeated to apply torsional deformation to the other undeformed end of the blank through the die;

[0013] S7 demolding: After the secondary torsion deformation is completed, the die is disassembled and the deformed blank is taken out, thus completing the forming of the ultrafine-grained magnesium alloy.

[0014] As a further improvement, the rear section of the twisting and extrusion cavity is a square hole.

[0015] As a further improvement, the die is designed as two symmetrical split die, and after the two symmetrical split die are closed, a nest is placed on the outer periphery, and the inner wall of the nest matches the outer wall of the die, and the discharge barrel is designed as two symmetrical split discharge barrels, and the outer walls of the two symmetrical split discharge barrels and the outer wall of the fixed pad are both provided with a straight groove along the axial direction, and the inner wall of the cavity is provided with a straight rib at the position corresponding to the straight groove, and the straight groove and the straight rib are tightly matched; when assembling the mold, firstly place the fixed pad in cooperation with one of the discharge barrels in the upsetting cavity of one of the die, place the blank wrapped with graphite paper on the discharge barrel, and then place the other discharge barrel and the other die above the assembled mold to close the split discharge barrel and the split die, and finally place the nest axially from the end of the split die to complete the assembly of the mold.

[0016] As a further improvement, the nested inner wall and the outer wall of the die are provided with the same slope, and the nested inner wall and the outer wall of the die are embedded.

[0017] As a further improvement, the inclination of one side of the groove wall of the cross-shaped groove is greater than the inclination of the other side.

[0018] As a further improvement, except for the end portion of the punch that contacts the blank, the rest of the interior of the punch is a hollow structure.

[0019] As a further improvement, the mold cavity is provided with a clearance cavity on the side of the fixed pad away from the discharge barrel, and the clearance cavity is located on the same axis as the upsetting cavity and the twisting cavity.

[0020] After adopting the above scheme, the gain effect of the present invention is:

[0021] During the forming process of the present invention, the punch applies appropriate axial force, and the rare earth magnesium alloy billet placed by the discharge barrel first undergoes upsetting deformation in the upsetting cavity of the discharge barrel and slight reverse extrusion deformation in the choke zone until the metal fills the upsetting cavity and its choke zone, and then the die rotates under the drive of the torsion unit to cause the magnesium alloy billet to undergo torsion deformation in the torsion cavity of the fixed pad, thereby obtaining a submicron-level ultrafine-grained magnesium alloy ingot. The present invention effectively improves the degree of refinement of the deformed alloy under isostrain conditions by increasing strong shear stress under high hydrostatic pressure conditions, thereby forming a submicron-level ultrafine-grained bulk magnesium alloy billet with good uniformity and weak anisotropy; in addition, the present invention can also simplify the preparation process and reduce the forming force requirements, laying a solid foundation for the engineering preparation of ultrafine-grained magnesium alloy billets. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the structure of the present invention before the male die enters the female die;

[0023] Figure 2 It is a schematic diagram of the structure when the male die of the present invention enters the female die (without upsetting);

[0024] Figure 3 It is a schematic diagram of the structure of the punch of the present invention when twisting and extruding the blank;

[0025] Figure 4 It is a schematic structural diagram of the male mold of the present invention;

[0026] Figure 5 It is a schematic diagram of a partial structure of a cross-shaped groove in the front view of the present invention;

[0027] Figure 6 It is a schematic diagram of a partial structure of a cross-shaped groove in a side view of the present invention;

[0028] Figure 7 It is a structural schematic diagram of the concave mold of the present invention;

[0029] Figure 8 yes Figure 7 Schematic diagram of the structure of the middle die in the A direction;

[0030] Fig. 9 It is a schematic diagram of the nested structure of the present invention;

[0031] Fig.10 It is a structural schematic diagram of the discharge barrel of the present invention;

[0032] Fig.11 yes Fig.10 Schematic diagram of the structure of the middle discharge barrel B direction;

[0033] Fig.12 It is a structural schematic diagram of the fixed cushion block of the present invention;

[0034] Fig.13 yes Fig.10 Schematic diagram of the structure of the middle fixed pad in the C direction;

[0035] Fig.14 This is the microstructure of the low-pressure torsion-formed alloy of the present invention.

[0036] Description of labels:

[0037] 1. Punch; 11. Hollow structure; 12. Cross-shaped groove; 13. Annular flange; 2. Die; 21. Cavity; 22. I-shaped rib; 23. Give way cavity; 3. Discharge barrel; 31. I-shaped groove; 32. Upsetting cavity; 33. Flow-blocking strip; 4. Fixed pad; 41. Twisting cavity; 42. Arc boss surface; 43. Square hole; 5. Nesting; 6. Blank. DETAILED DESCRIPTION

[0038] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0039] The present invention provides a method for preparing a submicron-grade ultrafine-grained magnesium alloy mold by low-pressure torsion, such as Figures 1 to 13 As shown, it includes a punch 1, a die 2, a discharge barrel 3 and a fixed pad 4.

[0040] Figure 1 The overall structure of the mold of the present invention is shown in FIG. Figure 1 The annular flange shown in the figure is fixedly mounted on a linear feed mechanism (specifically, a hydraulic press) and is limited by bolts to ensure the tightness of the connection between the mold and the linear feed mechanism. Figure 2 As shown, the other end of the punch 1 is placed in the die 2 to contact the blank 6, and the linear feed mechanism drives the punch 1 to make axial linear motion. The die 2 has a cavity 21 at one end close to the punch 1, and the die 2 is fixedly mounted on the torsion mechanism (specifically, it can also be an annular flange 13 that is the same as one end of the punch 1) at the end away from the punch 1. Except for the end of the punch 1 that contacts the blank 6, the interior of the other parts is a hollow structure 11. On the one hand, it is convenient to ensure that the punch 1 maintains a certain temperature during heating and insulation, and on the other hand, it can also save production costs.

[0041] The discharge barrel 3 and the fixed pad 4 are fixedly installed in the cavity 21. In one embodiment, the discharge barrel 3 and the fixed pad 4 are both cylindrical rotating bodies; the discharge barrel 3 and the fixed pad 4 can be designed as two symmetrical petal structures, which can be helpful for mold installation, feeding, unloading, etc. Fig.10 and Fig.11 As shown, the outer walls of the two symmetrical petal discharge barrels 3 are provided with a straight groove 31 along the axial direction. Figure 7 and Figure 8 As shown, the inner wall of the cavity 21 is provided with a straight-shaped rib 22 at a position corresponding to the straight-shaped groove 31. Figure 1 It is a schematic diagram of the structure in which the discharge barrel 3 and the fixed pad 4 are fixedly installed in the cavity 21. The straight groove 31 and the straight rib 22 are tightly matched to prevent the circumferential movement of the discharge barrel 3 during torsional deformation.

[0042] Similarly, the die 2 can also be designed as two symmetrical split die 2, and after the two symmetrical split die 2 are molded together, a nest 5 is placed on the outer periphery, such as Figure 1 and Fig. 9 As shown, the inner wall of the nest 5 matches the outer wall of the die 2, so that the nest 5 can clamp the split die 2. Further, the inner wall of the nest 5 and the outer wall of the die 2 are provided with the same slope, and the inner wall of the nest 5 and the outer wall of the die 2 are fitted together, so as to help the nest 5 to tightly fix the split die 2 and facilitate the loading and unloading of the mold after twisting forming.

[0043] like Figure 1 and Figure 2As shown, the discharge barrel 3 is provided with a hollow upsetting cavity 32 along the axial direction for placing the blank 6 and the punch 1. The inner diameter R1 of the front section of the upsetting cavity 32 is smaller than the inner diameter R1 of the rear section to form a flow blocking zone 33 to prevent the blank 6 from overflowing. The rear section of the upsetting cavity 32 is an upsetting working zone. Figure 3 As shown, the flow-blocking strip 33 can prevent the blank 6 from flowing toward the feeding direction of the punch 1 due to the pressure during torsion deformation, that is, prevent the blank 6 from overflowing, which is conducive to regulating the orderly flow of the metal. By simply replacing the discharge barrel 3 with different thicknesses, the ultrafine-grained magnesium alloy ingots of different sizes can be formed, and the ultrafine-grained blocks prepared have good uniformity; Figure 1 , Fig.12 and Fig.13 As shown, the fixed pad 4 is provided with a hollow extrusion cavity 41. The front section of the extrusion cavity 41 is provided with a gradually inwardly contracted arc-shaped boss surface 42 at the inner periphery of the connection with the rear section of the upsetting cavity 32, for the blank 6 to flow in and contact from the upsetting working section, so that the blank 6 is twisted and deformed during operation. The rear section of the extrusion cavity 41 is a square hole 43. The transition from the arc-shaped boss surface 42 to the square hole 43 is more significant in grain refinement. The mold cavity 21 is provided with a clearance cavity 23 on the side of the fixed pad 4 away from the discharge barrel 3. The clearance cavity 23 is located on the same axis as the upsetting cavity 32 and the extrusion cavity 41. When the volume of the blank 6 is large, the required forming time is also long. The clearance cavity 23 can be provided for the blank 6 from the extrusion cavity 41 to enter, so as to extend the overall forming process and make the forming more complete.

[0044] Before the torsional deformation, by controlling the magnitude of the axial compressive stress applied by the linear feed mechanism, the blank 6 can be fully upset into the desired shape before entering the port of the arc-shaped boss surface 42. When the deformation temperature and other parameters are consistent, the forming force required for the upset of the blank 6 is relatively small, approximately 0.5 tons; and the extrusion of the blank 6 into the port of the arc-shaped boss surface 42 is a typical positive extrusion, and because the rear section of the torsional cavity 41 is a square hole 43, the forming force required for positive extrusion is greater, generally about 2 tons (the specific value is for a rod-shaped blank 6 with a diameter of 9m).

[0045] The final extrusion to the torsional extrusion cavity belongs to the deformation of the extrusion ratio. Its deformation is more difficult than upsetting. This only requires the equal volume method or simulation to know the required extrusion distance to ensure that the billet is upset and deformed to a fixed shape without being squeezed into the torsional extrusion cavity.

[0046] like Figure 5 and Figure 6As shown, the end of the punch 1 is provided with a cross-shaped groove 12 in contact with the blank 6. Furthermore, the slope of one side of the groove wall of the cross-shaped groove 12 is greater than the slope of the other side, so that the metal flows slowly from the larger slope of the cross-shaped groove 12 into the cross-shaped groove 12, thereby preventing the contact end of the punch 1 of the blank 6 from being constantly scraped and causing the fixation to weaken, providing a basis for large-circle torsional deformation, effectively solving the problem of traditional severe plastic deformation cumulative strain limitation, and creating conditions for the blank 6 not to slip during the torsion process after the torque is applied.

[0047] The present invention also provides a method for preparing a submicron-grade ultrafine-grained magnesium alloy by low-pressure torsion, comprising the following steps:

[0048] S1 blanking: taking a section of magnesium alloy cylindrical billet 6;

[0049] S2 mold installation: Figure 1 As shown, the mold is assembled as described above. When assembling the mold, firstly, the fixed pad 4 is matched with one of the discharging cylinders 3 and placed in the upsetting cavity 32 of one of the die 2, and the blank 6 wrapped with graphite paper is placed on the discharging cylinder 3, and then the other discharging cylinder 3 and the other die 2 are placed above the assembled mold to close the discharging cylinder 3 and the die 2, and finally, the nest 5 is placed axially from the end of the die 2 to complete the assembly of the mold;

[0050] S3 preparation before forming: Figure 2 As shown, the tail ends of the punch 1 and the die 2 are respectively mounted on the linear feed mechanism and the twisting mechanism, and then the linear feed mechanism drives the punch 1 to feed axially, and the punch 1 with the cross-shaped groove is placed in the discharge barrel 3, and moves until the cross-shaped groove 12 of the punch 1 contacts the blank 6;

[0051] S4 slight upsetting deformation: the linear feed mechanism drives the punch 1 to continue to move, and the blank 6 undergoes slight upsetting deformation until the blank 6 fills the rear section of the upsetting cavity 32 and the flow-blocking zone 33;

[0052] S5: primary torsion deformation: the die 2 rotates under the drive of the torsion mechanism, and the fixed pad 4 also rotates accordingly, and the arc-shaped boss surface 42 in the torsion cavity 41 drives the end of the blank 6 in contact with it to undergo torsion deformation;

[0053] S6 secondary torsional deformation: after the primary torsional deformation is completed, the die 2 is disassembled, the deformed blank 6 is taken out of the die 2 and rotated 180°, and steps S2 and S4 are repeated to apply torsional deformation to the other undeformed end of the blank 6 through the die 2;

[0054] S7 demoulding: After the secondary torsion deformation is completed, the die 2 is disassembled and the deformed blank 6 is taken out, thus completing the forming of the ultrafine-grained magnesium alloy.

[0055] In step S7, the end of the punch 1 is knocked so that the punch 1 drives the die 2, the petal discharge barrel 3, the fixed pad 4 and the deformed blank 6 to be separated from the nest 5 as a whole; then the unseparated parts are lightly knocked to separate the petal die 2 and the petal discharge barrel 3 to facilitate the removal of the deformed blank 6.

[0056] The mold and blank 6 of the present invention can be heated simultaneously before and during forming. The specific operation is: use Gleeble-3500 (thermal simulation test machine) to heat the blank 6 and the mold synchronously, and at the same time weld a thermocouple wire at the working end of the punch 1 close to the die 2 to detect and control the deformation temperature in real time.

[0057] The present invention optimizes the low-pressure torsion process parameters based on the torsion unit of the Gleeble-3500 thermal simulation test machine, and can lay a foundation for the preparation of large-scale ultrafine crystal ingots for industrial applications. Fig.14 It is the microstructure of the low-pressure torsion-formed alloy of the present invention. Part (a) in the figure represents the optical microscope (OM) image, and part (b) represents the transmission electron microscope (TEM) image. Both reflect that the magnesium alloy billet 6 achieves high strain accumulation under continuous application of strong shear stress, resulting in the effect of obtaining a weakly anisotropic, uniform ultrafine-grained magnesium alloy ingot.

[0058] During the plastic deformation process, the unique rotational dynamic recrystallization behavior of magnesium alloys under strong compressive shear stress conditions promotes the grain refinement process, and the continuous formation and rotation of subgrains can improve the refinement of the grains and the second phase of the billet 6. The thickness of the prepared ultrafine grains depends on the orderly transmission of strong shear stress along the axial direction, that is, the preparation of ultrafine grain billets 6 of different thicknesses depends on the port diameter of the arc-shaped boss surface 42 with different areas. This technology can realize the preparation of ultrafine-grained magnesium alloy ingots and break through the current size dilemma of ultrafine-grained bulk alloys. The introduction of strong shear stress promotes the activation of cone slip, promotes the random orientation of dynamically recrystallized grains, significantly reduces the basal texture, and effectively solves the problems of strong anisotropy and poor uniformity of traditional severe plastic deformation alloys.

[0059] In addition, when the rare earth magnesium alloy is deformed on a Gleeble-3500 thermal simulation test machine, while ensuring that the size, deformation temperature, etc. of the billet 6 are consistent, different deformation modes require different forming forces, among which pure extrusion deformation is about 2 tons, rotary extrusion deformation is about 1.2 tons, and low-pressure torsion forming only requires 0.45 tons (the above is the forming force value when the deformation temperature is 420°C). Compared with large plastic deformations such as equal-diameter angular extrusion and high-pressure torsion, the preparation method proposed in the present invention is simple to implement and can reduce the forming load and equipment tonnage, i.e., low pressure, laying the foundation for the industrial mass production of large-scale ultrafine-grained ingots.

[0060] The above description is only a preferred embodiment of the present invention and is not a limitation on the design of this case. Any equivalent changes made based on the design key of this case shall fall within the protection scope of this case.

Claims

1. A method for preparing submicron ultrafine-grained magnesium alloy by low-pressure torsion, characterized in that: The following steps are involved: S1 blanking: taking a section of magnesium alloy cylindrical billet; S2 mold installation: assemble the mold, the mold includes a punch, a die, a discharge barrel and a fixed pad, the die has a cavity at one end close to the punch, the discharge barrel and the fixed pad are fixedly installed in the cavity, the discharge barrel has a hollow upsetting cavity along the axial direction for the blank and the punch to be placed, the inner diameter of the front section of the upsetting cavity is smaller than the inner diameter of the rear section to form a flow blocking zone to prevent the blank from overflowing, the rear section of the upsetting cavity is an upsetting working zone, the fixed pad has a hollow twisting and extrusion cavity, the front section of the twisting and extrusion cavity is provided with a gradually inward arc-shaped boss surface at the inner periphery of the connection with the rear section of the upsetting cavity, for the blank to flow in and contact from the upsetting working section, and the end of the punch is provided with a cross-shaped groove in contact with the blank; S3 Preparation before forming: The tail ends of the punch and the die are respectively mounted on the linear feed mechanism and the twisting mechanism, and then the linear feed mechanism drives the punch to feed axially, and the punch with a cross-shaped groove is placed in the discharge barrel and moved until the cross-shaped groove of the punch contacts the blank; S4 slight upsetting deformation: the linear feed mechanism drives the punch to continue to move, and the blank undergoes slight upsetting deformation until the blank fills the rear section of the upsetting cavity and the choke zone; S5 primary torsion deformation: the die rotates under the drive of the torsion mechanism, and the fixed pad also rotates accordingly, and the arc-shaped boss surface in the torsion cavity drives the end of the blank in contact with it to undergo torsion deformation; S6 secondary torsional deformation: after the primary torsional deformation is completed, the die is disassembled, the deformed blank is taken out of the die and rotated 180°, and steps S2 and S4 are repeated to apply torsional deformation to the other undeformed end of the blank through the die; S7 demolding: After the secondary torsion deformation is completed, the die is disassembled and the deformed blank is taken out, thus completing the forming of the ultrafine-grained magnesium alloy.

2. The method for preparing submicron ultrafine-grained magnesium alloy by low-pressure torsion as claimed in claim 1, characterized in that: The rear section of the twisting and extruding cavity is a square hole.

3. The method for preparing submicron ultrafine-grained magnesium alloy by low-pressure torsion as claimed in claim 1, characterized in that: The die is designed as two symmetrical split die. After the two symmetrical split die are closed, a nest is placed on the outer periphery. The inner wall of the nest matches the outer wall of the die. The discharge barrel is designed as two symmetrical split discharge barrels. The outer walls of the two symmetrical split discharge barrels and the outer wall of the fixed pad are both provided with a straight groove along the axial direction. The inner wall of the cavity is provided with a straight rib at the position corresponding to the straight groove. The straight groove and the straight rib are tightly matched. When assembling the mold, firstly place the fixed pad in cooperation with one of the discharge barrels in the upsetting cavity of one of the die, place the blank wrapped with graphite paper on the discharge barrel, and then place the other discharge barrel and the other die above the assembled mold to close the split discharge barrel and the split die. Finally, place the nest axially from the end of the split die to complete the assembly of the mold.

4. The method for preparing submicron ultrafine-grained magnesium alloy by low-pressure torsion as claimed in claim 1, characterized in that: The nested inner wall and the outer wall of the die are provided with the same slope, and the nested inner wall and the outer wall of the die are embedded.

5. The method for preparing submicron ultrafine-grained magnesium alloy by low-pressure torsion as claimed in claim 1, characterized in that: The inclination of one side of the groove wall of the cross-shaped groove is greater than the inclination of the other side.

6. The method for preparing submicron ultrafine-grained magnesium alloy by low-pressure torsion as claimed in claim 1, characterized in that: Except for the portion at the end of the punch that contacts the blank, the interior of the rest of the punch is a hollow structure.

7. The method for preparing submicron ultrafine-grained magnesium alloy by low-pressure torsion as claimed in claim 1, characterized in that: The mold cavity is provided with a clearance cavity on one side of the fixed pad away from the discharge barrel, and the clearance cavity, the upsetting cavity and the twisting and extruding cavity are located on the same axis.

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