Method for preparing bulk ultra-fine grained magnesium alloy by severe plastic deformation

Abstract

The application discloses a preparation method of large bulk ultra-fine grain magnesium alloy by severe plastic deformation, and comprises the following steps: S1, assembling a mold; S2, placing a convex die into a feeding channel through linear motion of an axial feeding unit; S3, the axial feeding unit goes down, and a torsion unit is started at the same time; due to the block of an extrusion cone angle area, a cylindrical blank is extruded in the extrusion cone angle area, gradually transversely filled and upset, and the extrusion cone angle area is filled; S4, the blank flows from the extrusion cone angle area to a secondary back pressure area; and S5, composite upsetting deformation: the blank enters an extrusion forming area, in the extrusion forming area, the blank is subjected to the continuous action of axial pressure on one hand and the constraint of the inner wall of an inner convex rib insert on the other hand, and under the combined action, the blank is subjected to composite upsetting deformation, the grains are fully refined and uniformly distributed, and finally, large bulk magnesium alloy with an ultra-fine grain structure is formed. The application optimizes stress distribution, realizes uniform grain refinement and efficient forming of the magnesium alloy in the process of rotary forward extrusion.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metal plastic forming, in particular to a method for preparing large bulk ultra-fine grain magnesium alloy by severe plastic deformation. BACKGROUND

[0002] Magnesium alloy is continuously expanding its application in high-end manufacturing fields such as aerospace, automobile manufacturing and electronic products due to its low density, high specific strength, excellent thermal conductivity and corrosion resistance. However, when traditional magnesium alloy is prepared by conventional processing technology, coarse grain structure is easily formed, resulting in low material strength and plasticity, which seriously restricts its application in a wider range. To break through this limitation, researchers have successfully prepared ultra-fine grain materials by grain refinement treatment of magnesium alloy through various severe plastic deformation (SPD) techniques, which not only significantly improves the overall mechanical properties of the material, but also effectively improves the ductility and plasticity of magnesium alloy at room temperature, fundamentally solving the problem of low ductility in traditional processing technology.

[0003] Rotary extrusion, as a new type of metal plastic processing technology, has significant advantages in improving the uniformity of material microstructure and enhancing the mechanical properties compared with traditional extrusion process. The core competitiveness of this technology lies in the design of the die structure - the form of the die structure directly determines the grain refinement effect and the stability of the forming quality of the material. Although rotary extrusion technology has been proven to achieve significant grain refinement and form ultra-fine grain structure in local areas, existing research data shows that the grain refinement effect often presents uneven spatial distribution problems, especially in the overall forming process of large bulk ultra-fine grain materials. Under the condition of complex stress field distribution in rotary forward extrusion, how to optimize the uniform distribution of stress and solve the problem of uneven grain refinement and reduce the anisotropy of the material, and ultimately achieve the overall improvement of the performance of magnesium alloy, has become a key technical problem that needs to be solved in this field. SUMMARY

[0004] The purpose of the present application is to provide a method for preparing large bulk ultra-fine grain magnesium alloy by severe plastic deformation, which optimizes the stress distribution and realizes uniform grain refinement and efficient forming of magnesium alloy during rotary forward extrusion.

[0005] To achieve the above purpose, the present application provides the following technical solution: a method for preparing large bulk ultra-fine grain magnesium alloy by severe plastic deformation, comprising the following steps:

[0006] S1 Assembling the mold: the mold includes a punch, a die, a feeding cylinder, a bearing, a tapered insert and an inner convex rib insert; one end of the punch is fixed in the axial feeding unit, and the other end is the insertion end into the feeding passage; one end of the die is fixed with the twisting unit, and the other end is provided with an upper cavity and a lower cavity, which can be sequentially divided into three regions along the extrusion direction of the blank: the feeding region is composed of the upper section of the upper cavity, and is internally provided with coaxially placed bearings and a feeding cylinder, the inner side of the bearing is tightly fitted with the outer wall of the feeding cylinder, and the outer side contacts the inner wall of the upper cavity, the feeding cylinder is in the shape of a cylinder, the middle is a feeding passage for placing the cylindrical blank; the extrusion taper angle region is composed of the lower section of the upper cavity, and is internally provided with a tapered insert and a bearing, the inner side of the bearing is tightly fitted with the outer wall of the tapered insert, and the inner side of the tapered insert is provided with a downward inclined taper; the extrusion forming region is composed of the lower cavity, which axially places the inner convex rib insert, the outer side of the inner convex rib insert contacts the inner wall of the lower cavity, the inner side of the inner convex rib insert is a hollow structure with an open upper part and a closed bottom part, and the upper part communicates with the extrusion taper angle region, the inner side of the inner convex rib insert forms a first inner convex rib and a second inner convex rib in a ring shape from top to bottom along the axial direction, a secondary back pressure region is formed between the first inner convex rib and the second inner convex rib, and an extrusion forming region is formed between the second inner convex rib and the bottom of the inner convex rib insert;

[0007] S2 Feeding and pre-preparation for forming: the pretreated magnesium alloy cylindrical blank is slowly placed into the feeding passage from the upper end opening of the feeding cylinder until the lower end of the cylindrical blank contacts the extrusion taper angle region, the insertion end of the punch is placed into the feeding passage through the linear motion of the axial feeding unit, so that the insertion end of the punch contacts the cylindrical blank, at this time, the blank is in the state of being ready to be extruded;

[0008] S3 Upsetting taper angle extrusion deformation: the axial feeding unit goes down, and at the same time, the twisting unit is also started, due to the blockage of the extrusion taper angle region, the cylindrical blank is extruded in the extrusion taper angle region and gradually fills transversely to be upset, the special tapered structure of the extrusion taper angle region causes the blank to be subjected to uneven pressure distribution in the cavity, thereby producing a back pressure effect; the cylindrical blank is slightly upset to fill the extrusion taper angle region;

[0009] S4 Secondary back pressure deformation: as the punch continues to press down, the pressure on the blank increases, when the pressure is sufficient to overcome the resistance of the extrusion taper angle region, the blank flows from the extrusion taper angle region to the secondary back pressure region, in the secondary back pressure region, the blank further deforms, the internal grain structure is further refined and homogenized under the joint action of pressure and shear force, thereby realizing the secondary back pressure effect;

[0010] S5 composite upsetting deformation: after the deformation of the blank in the secondary back pressure zone, the blank continues to move downward into the extrusion forming zone between the second inner rib and the bottom of the inner rib insert block. In the extrusion forming zone, the blank is subjected to the continuous action of axial pressure on one hand and the constraint of the inner wall of the inner rib insert block on the other hand. Under the combined action, the blank undergoes composite upsetting deformation, its volume is further compressed, the grains are fully refined and uniformly distributed, and finally a bulk magnesium alloy with ultra-fine grain structure is formed. During steps S3-S5, the concave die is rotated at a certain angular velocity by the torsion unit. Due to the contact and friction force transmission between the concave die and the bearing and the discharge cylinder, the bearing and the discharge cylinder also rotate, thereby making the blank subjected to axial pressure deformation and shear force generated by the rotation of the concave die at the same time.

[0011] Preferably, the inner diameter of the first inner rib is smaller than the inner diameter of the discharge channel, and the inner side of the upper surface of the first inner rib is matched and connected with the tapered insert block, and the outer side of the upper surface of the first inner rib is partially provided for placing the bearing, and the inner diameter of the second inner rib is smaller than the inner diameter of the first inner rib, so that the blank flows from the extrusion cone angle zone to the secondary back pressure zone to realize secondary back pressure.

[0012] Preferably, a sleeve is further included, the concave die is a split structure symmetrical on both sides, and the outer wall of the combined concave die is fixed by nesting the sleeve.

[0013] Preferably, the outer wall of the sleeve is provided with the same slope as the outer wall of the concave die in contact therewith.

[0014] Preferably, the outer wall of the insert block is provided with a rib, and the inner wall of the concave die in contact with the outer wall of the insert block is provided with a groove corresponding to the rib.

[0015] Preferably, the upper surface of the second inner rib is a plane or has a taper different from that of the tapered insert block.

[0016] Preferably, the bearing is provided with multiple groups and is stacked together along the same axis.

[0017] Preferably, the inner side of the tapered insert block and the inner rib insert block is provided with a round corner at the corner.

[0018] Preferably, the end face of the placement end of the convex die is provided with a one-letter type groove for embedding the top of the cylindrical blank.

[0019] Preferably, the outer contour of the tapered insert block and the first inner rib is fitted together to form a complete outer contour.

[0020] Compared with the prior art, the beneficial effects of the present application are that the present application divides the blank into three independent areas, adds bearings outside the feeding cylinder, minimizes the friction between the blank and the mold, improves the stress transmission effect, and the high hydrostatic pressure and shear stress applied on the blank are more evenly distributed. In addition, aiming at the problem of anisotropy of the mechanical properties of the blank after rotary extrusion forming, the extrusion cone angle area is set as an independent area with bearings added, the back pressure effect is introduced at the extrusion opening, and a secondary back pressure area containing a back pressure cavity is designed in the extrusion forming area. In the process of multiple extrusion deformation and upsetting, the metal flow is improved, the mechanical property difference between the core and the edge of the blank is reduced, the mechanical properties of the material are effectively improved, and the forming of large-volume bulk ultra-fine grain magnesium alloy is promoted. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 It is a schematic diagram of the overall structure of the mold of the present application;

[0022] Figure 2 It is a schematic diagram of the structure of the female die of the present application;

[0023] Figure 3 It is a schematic diagram of the split structure of the insert block of the present application;

[0024] Figure 4 It is a schematic diagram of the structure of the feeding state of the present application;

[0025] Figure 5 It is a schematic diagram of the structure of the upsetting back pressure deformation state of the present application;

[0026] Figure 6 It is a schematic diagram of the structure of the secondary back pressure deformation state of the present application;

[0027] Figure 7 It is a schematic diagram of the structure of the composite upsetting deformation process of the present application;

[0028] Figure 8 It is a schematic diagram of the structure of the completed composite upsetting deformation of the present application.

[0029] In the figure: 1, male die; 2, female die; 3, feeding cylinder; 4, bearing; 5, inner rib insert block; 6, upper cavity; 7, lower cavity; 8, feeding channel; 9, cylindrical blank; 10, inserted end; 11, first inner rib; 12, second inner rib; 13, extrusion cone angle area; 14, secondary back pressure area; 15, extrusion forming area; 16, sleeve; 17, independent bearing; 18, conical insert block. DETAILED DESCRIPTION

[0030] With reference to the drawings of the embodiments of the present application, the technical solutions in the embodiments of the present application will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by a person of ordinary skill in the art without creative effort belong to the scope of protection of the present application.

[0031] Please refer to Figures 1-8 The present application provides a large bulk ultra-fine grain magnesium alloy preparation mold based on efficient transmission of shear strain, comprising a punch 1, a die 2, a feeding cylinder 3, a bearing 4, an inner convex rib insert 5, a tapered insert 18 and a sleeve 16.

[0032] The punch 1 is fixed at one end of an axial feeding unit (not shown in the figure), and the other end is a placement end 10 for entering the feeding passage 8 in the axial direction;

[0033] The die 2 is fixed at one end with a torsion unit (not shown in the figure), and the other end is provided with an upper cavity 6 and a lower cavity 7,

[0034] Along the direction of extrusion of the blank, it can be sequentially divided into a feeding area, an extrusion cone angle area and an extrusion forming area:

[0035] The feeding area is composed of the upper section of the upper cavity 6, and is internally provided with the coaxially placed bearing 4 and feeding cylinder 3. The inner side of the bearing 4 is tightly fitted with the outer wall of the feeding cylinder 3, and the outer side of the bearing 4 contacts the inner wall of the upper cavity 6. The feeding cylinder 3 is in the shape of a cylinder, and the middle part is a feeding passage 8 for placing the cylindrical blank 9,

[0036] The extrusion cone angle area is composed of the lower section of the upper cavity 6, and is internally provided with the tapered insert 18 and the bearing 4. The inner side of the bearing 4 is tightly fitted with the outer wall of the tapered insert 18, and the inner side of the tapered insert 18 is provided with a downwardly inclined taper;

[0037] The extrusion forming area is composed of the lower cavity 7, which is axially placed with the inner convex rib insert 5. The outer side of the inner convex rib insert 5 contacts the inner wall of the lower cavity 7, and the inner side of the inner convex rib insert 5 is a hollow structure with an open upper part and a closed bottom part, and the upper part is in communication with the extrusion cone angle area. The inner side of the inner convex rib insert 5 forms a first inner convex rib 11 and a second inner convex rib 12 in a ring shape from top to bottom in the axial direction. The first inner convex rib 11 and the second inner convex rib 12 form a secondary back pressure area 14 therebetween, and the second inner convex rib 12 and the bottom part of the inner convex rib insert 5 form an extrusion forming area 15 therebetween.

[0038] The inner diameter of the first inner convex rib 11 is smaller than the inner diameter of the feeding channel 8, and the inner side of the upper surface of the first inner convex rib 11 is matched with the conical insert 18; the outer contour of the conical insert 18 and the first inner convex rib 11 are combined to form a complete outer contour, which helps the flow of the blank and avoids the formation of dead zones and the like; the outer side of the upper surface of the first inner convex rib 11 is used to place the bearing 4, and the inner diameter of the second inner convex rib 12 is smaller than the inner diameter of the first inner convex rib 11, so that the blank flows from the extrusion cone angle area 13 to the secondary back pressure area 14 to realize secondary back pressure.

[0039] The die 2 is designed as a split structure symmetrical on both sides, and the two halves of the die 2 are accurately aligned and combined during assembly. To ensure the stability of the combined die 2 structure, a sleeve 16 is sleeved on the outer wall of the die 2. The inner wall of the sleeve 16 and the outer wall of the die 2 are both provided with the same slope, and when the sleeve 16 is sleeved on the die 2, the slope cooperation makes the sleeve 16 tightly fit with the die 2, which plays a good fixing role and prevents the die 2 from shifting or deforming in the subsequent preparation process. The slope is a self-locking slope of 2°~5°.

[0040] The outer wall of the insert 5 is provided with a convex rib, and the inner wall of the die 2 corresponding to the convex rib is provided with a groove. During installation, the convex rib of the insert 5 is accurately embedded in the groove of the inner wall of the die 2. This matching mode not only realizes the accurate positioning of the insert 5 in the die 2, but also enhances the connection stability between the insert 5 and the die 2, preventing the insert 5 from rotating or shifting during the extrusion of the blank. The inner side of the insert 5 is a specific hollow structure, which is open at the upper part and communicates with the feeding channel 8, and is closed at the bottom. This structure provides a specific space for the deformation of the blank.

[0041] The upper surface of the second inner convex rib 12 is a plane or has a taper different from that of the conical insert 18. When uniform deformation of the blank in the secondary back pressure area 14 is required, the upper surface of the second inner convex rib 12 can be designed as a plane, so that the pressure received by the blank when passing through the second inner convex rib 12 is relatively uniform, which is conducive to realizing uniform grain refinement. If it is desired to specially control the deformation process of the blank, for example, to guide the metal flow direction inside the blank to obtain a specific microstructure, the upper surface of the second inner convex rib 12 can be designed to have a specific taper. By adjusting the size and direction of the taper, the stress state and metal flow path of the blank in the secondary back pressure area 14 can be changed to meet different preparation process requirements.

[0042] A plurality of sets of bearings 4 are radially arranged in the upper cavity 6, and the sets of bearings 4 are stacked in the coaxial direction. The arrangement of the plurality of sets of bearings 4 can uniformly support the feeding cylinder 3, reduce the shaking of the feeding cylinder 3 during rotation, and facilitate the uniform transmission of shear strain. After the feeding cylinder 3 is placed in the upper cavity 6, the outer side of the feeding cylinder 3 is ensured to be in close contact with the inner side of the bearing 4, and the outer side of the bearing 4 is in contact with the inner wall of the upper cavity 6. The feeding cylinder 3 is in a cylindrical shape, and the feeding passage 8 in the middle is used to place the cylindrical blank 9. The structure design of the feeding cylinder 3 ensures the stability and guidance of the blank when the blank enters the mold.

[0043] A fillet is arranged at the corner between the inner rib insert 5 and the inner side of the conical insert 18. The arrangement of the fillet can effectively reduce the stress concentration phenomenon of the blank during deformation. When the blank flows and deforms in the insert 5, if the corner is a sharp angle, the blank will bear a large local stress at the corner, which can easily cause cracks and other defects on the surface of the blank, and can also easily cause dead zones. The presence of the fillet makes the flow of the blank at the corner smoother, and the stress distribution is more uniform, thereby avoiding the generation of cracks and dead zones, and also facilitating the smooth movement of the blank in the inner rib insert 5, improving the quality and production efficiency of the product.

[0044] A magnesium alloy cylindrical blank 9 with a suitable size is selected. The size of the blank needs to be accurately determined according to the size of the feeding passage 8 of the mold and the requirements of the final product. For example, the diameter of the blank should be slightly smaller than the inner diameter of the feeding passage 8, so as to ensure that the blank can be smoothly placed into the feeding passage 8, and also to ensure that the blank has good contact and interaction with the inner wall of the mold during extrusion.

[0045] A method for preparing a bulk ultra-fine grain magnesium alloy by severe plastic deformation, comprising the following steps:

[0046] S1 Assemble the mold: one end of the male die 1 is fixed to the axial feeding unit of the Gleeble-3500 thermal simulator, the other end is in contact with the feeding port of the female die 2, one end of the female die 2 is fixed to the torsion unit of the thermal simulator, the bearings 4 and the feeding cylinder 3 are placed in the upper cavity 6 of the female die 2, then the split insert 5 is placed, and finally the two parts of the split female die 2 are closed and fastened by the sleeve 16;

[0047] S2 Feeding and preparation before forming:

[0048] For example, Figure 4As shown, the pre-processed magnesium alloy cylindrical blank 9 is slowly put into the discharging channel 8 from the upper end opening of the discharging cylinder 3, and due to the guidance of the discharging channel 8 and the cooperation of the discharging cylinder 3 and the bearing 4, the cylindrical blank 9 can smoothly and stably move downward along the discharging channel 8 until the lower end of the cylindrical blank 9 contacts the extrusion cone angle area 13, the insertion end 10 of the punch 1 is placed into the discharging channel 8 through the linear motion of the hot simulator axial feeding unit, so that the insertion end 10 of the punch 1 contacts the cylindrical blank 9, and the pre-formation preparation is completed by adjusting the pre-tightening force of the hot simulator. At this time, the blank is in a state of being ready to be extruded;

[0049] S3 Upsetting cone angle extrusion deformation: as shown in Figure 5 The axial feeding unit goes down, and at the same time, the torsion unit is started. Due to the blockage of the extrusion cone angle area 13, the cylindrical blank 9 is extruded in the extrusion cone angle area 13 and gradually filled transversely to be upset. The special tapered structure of the extrusion cone angle area 13 causes the blank to be subjected to uneven pressure distribution in the cavity, thereby producing a back pressure effect. This back pressure can cause the blank to deform initially, and the internal structure begins to adjust, laying the foundation for further deformation, and the cylindrical blank 9 is slightly upset to fill the extrusion cone angle area 13;

[0050] S4 Secondary back pressure deformation: as shown in Figure 6 With the continuous pressing of the punch 1, the pressure on the blank increases, and when the pressure is sufficient to overcome the resistance of the extrusion cone angle area 13, the blank flows from the extrusion cone angle area 13 to the secondary back pressure area 14. Since the inner diameter of the second inner rib 12 is smaller than that of the first inner rib 11, the flow area of the blank suddenly decreases after entering the secondary back pressure area 14, gradually filling transversely to be upset and the blank is again subjected to strong back pressure. In the secondary back pressure area 14, the blank further deforms, and the internal grain structure is further refined and homogenized under the combined action of pressure and shear force, realizing the secondary back pressure effect and effectively improving the performance of the magnesium alloy;

[0051] S5 Compound upsetting deformation: as shown in Figure 7 and Figure 8As shown, after the deformation of the billet through the secondary back pressure zone 14, the billet continues to move downward into the extrusion forming zone 15 between the second inner rib 12 and the bottom of the inner rib insert 5. In the extrusion forming zone 15, the billet is subjected to the continuous action of axial pressure on one hand and the constraint of the inner wall of the inner rib insert 5 on the other hand. Under the combined action, the billet undergoes complex upsetting deformation, its volume is further compressed, and the grains are fully refined and uniformly distributed, finally forming a bulk magnesium alloy with ultra-fine grain structure; during steps S3-S5, the concave die 2 is rotated at a certain angular velocity by the torsion unit, and due to the contact and friction force transmission between the concave die 2 and the bearing 4 and the feeding cylinder 3, the bearing 4 and the feeding cylinder 3 also rotate, thereby causing the billet to be deformed by axial pressure while also being subjected to shear force generated by the rotation of the concave die 2. The shear force and the axial pressure act synergistically to greatly promote the efficient transmission of shear strain in the billet, helping the billet to obtain finer and more uniform grain structure and significantly improving its mechanical properties such as strength and toughness.

[0052] The taper structure provides a smooth transition path for the cylindrical billet 9 to enter the extrusion cone angle zone 13 from the feeding channel 8. When the cylindrical billet 9 moves downward under the axial pressure of the convex die 1, the tapered surface will form a "progressive constraint" on the bottom of the billet. The edge of the billet first contacts the tapered slope, and as the pressure increases, it gradually shrinks to the center, avoiding the sudden stress surge caused by the "sudden blockage" of traditional flat transition, reducing die wear and prolonging the service life of the part. This smooth transition can reduce the flow resistance of the billet and reduce surface cracks or internal porosity caused by violent impact, especially suitable for materials such as magnesium alloy which has poor plasticity. In addition, the back pressure experienced by the billet during the flow process increases in a gradient manner. This gradient pressure can promote "layered deformation" in the billet: the outer layer of metal first undergoes shearing and extrusion, and the inner layer of metal gradually participates in the deformation as the pressure is transmitted, achieving progressive grain crushing from the surface to the core. Compared with the straight-wall back pressure cavity without taper, the taper design can improve the uniformity of the shear strain of the billet; furthermore, the contact between the tapered surface and the bottom of the billet is a gradual process from line contact to surface contact, and as the billet deforms, the contact area between the two gradually increases, which can effectively reduce the outflow of metal ("run-out" phenomenon); moreover, the taper of the extrusion cone angle zone 13 and the size constraint of the second inner rib 12 form a "stepped pressure field": the gradient pressure of the extrusion cone angle zone 13 makes the billet complete the initial plastic deformation and grain refinement, and then when it enters the secondary back pressure zone 14 with a smaller inner diameter, the pre-deformation state formed by the taper (the billet has a certain shrinkage tendency) can make the stress concentration of the secondary back pressure more uniform, avoiding uneven deformation caused by "sudden shrinkage". This synergistic effect improves the strain transmission efficiency of the billet from the extrusion cone angle zone 13 to the secondary back pressure zone 14, laying a uniform organizational foundation for the densification forming of the final extrusion forming zone 15.

[0053] The above merely describes preferred embodiments of the present application, and is not intended to limit the design of the present application. Any equivalent changes made according to the design of the present application shall fall within the protection scope of the present application.

Claims

1. A method for preparing bulk ultra-fine grained magnesium alloy by severe plastic deformation, characterized in that: The method comprises the following steps: S1, assembling a mold: the mold comprises a punch, a die, a feeding cylinder, a bearing, a tapered insert and an inner convex rib insert; one end of the punch is fixed on an axial feeding unit, and the other end is an insertion end into a feeding passage; one end of the die is fixed with a twisting unit, and the other end is provided with an upper cavity and a lower cavity, which can be sequentially divided into a feeding area, an extrusion taper angle area and an extrusion forming area along the extrusion direction of the blank; the feeding area is composed of an upper section of the upper cavity, and is internally provided with a coaxially placed bearing and a feeding cylinder; the inner side of the bearing is tightly fitted with the outer wall of the feeding cylinder, and the outer side contacts the inner wall of the upper cavity; the feeding cylinder is in a cylindrical shape, and the middle part is a feeding passage for placing a cylindrical blank; the extrusion taper angle area is composed of a lower section of the upper cavity, and is internally provided with a tapered insert and a bearing; the inner side of the bearing is tightly fitted with the outer wall of the tapered insert, and the inner side of the tapered insert is provided with a downwardly inclined taper; the extrusion forming area is composed of the lower cavity, and the inner convex rib insert is placed in the axial direction; the outer side of the inner convex rib insert contacts the inner wall of the lower cavity; the inner side of the inner convex rib insert is a hollow structure with an open upper part and a closed bottom part, and the upper part is in communication with the extrusion taper angle area; the inner side of the inner convex rib insert forms a first inner convex rib and a second inner convex rib in the axial direction from top to bottom; the first inner convex rib and the second inner convex rib form a secondary back pressure area; the second inner convex rib and the bottom part of the inner convex rib insert form an extrusion forming area; S2, feeding and pre-preparation before forming: the pretreated magnesium alloy cylindrical blank is slowly placed into the feeding passage from the upper end opening of the feeding cylinder until the lower end of the cylindrical blank contacts the extrusion taper angle area; the insertion end of the punch is placed into the feeding passage through the linear motion of the axial feeding unit, so that the insertion end of the punch contacts the cylindrical blank; at this time, the blank is in a state of being ready to be extruded; S3, upsetting taper angle extrusion deformation: the axial feeding unit goes down, and the twisting unit is also started; due to the blockage of the extrusion taper angle area, the cylindrical blank is extruded in the extrusion taper angle area and gradually filled transversely to be upset; the special tapered structure of the extrusion taper angle area causes the blank to be subjected to uneven pressure distribution in the cavity, thereby generating a back pressure effect; the cylindrical blank is slightly upset to fill the extrusion taper angle area; S4, secondary back pressure deformation: as the punch continues to press down, the pressure on the blank continuously increases; when the pressure is sufficient to overcome the resistance of the extrusion taper angle area, the blank flows from the extrusion taper angle area to the secondary back pressure area; in the secondary back pressure area, the blank further deforms, the internal grain structure is further refined and homogenized under the joint action of pressure and shear force, thereby realizing a secondary back pressure effect. S5 composite upsetting deformation: after the deformation of the blank through the secondary back pressure zone, the blank continues to move downward into the extrusion forming zone between the second inner convex rib and the bottom of the inner convex rib insert block. In the extrusion forming zone, the blank is subjected to the continuous action of axial pressure on one hand and the constraint of the inner wall of the inner convex rib insert block on the other hand. Under the combined action, the blank undergoes composite upsetting deformation, its volume is further compressed, the grains are fully refined and uniformly distributed, and finally a bulk magnesium alloy with ultra-fine grain structure is formed; during steps S3-S5, the concave die is rotated at a certain angular velocity by the torsion unit, and the bearing and the discharge cylinder are also rotated due to the contact and friction force transmission between the concave die and the bearing and the discharge cylinder, thereby making the blank subjected to axial pressure deformation while also subjected to shear force from the concave die torsion.

2. The method of claim 1, wherein the bulk ultra-fine grained magnesium alloy is subjected to severe plastic deformation. The inner diameter of the first inner convex rib is smaller than the inner diameter of the discharge channel, and the inner side of the upper surface of the first inner convex rib is connected with the tapered insert block, and the outer side of the upper surface of the first inner convex rib is provided for placing the bearing, and the inner diameter of the second inner convex rib is smaller than the inner diameter of the first inner convex rib, so that the blank flows from the extrusion cone angle zone to the secondary back pressure zone to realize secondary back pressure.

3. The method of claim 1, wherein the bulk ultra-fine grained magnesium alloy is subjected to severe plastic deformation. The concave die is a split structure symmetrical on both sides, and the outer wall of the combined concave die is fixed by nesting the sleeve.

4. The method of claim 3, wherein the bulk ultra-fine grained magnesium alloy is subjected to severe plastic deformation. The outer wall of the concave die in contact with the inner wall of the sleeve is provided with the same slope.

5. The method of claim 1, wherein the bulk ultra-fine grained magnesium alloy is subjected to severe plastic deformation. The outer wall of the insert block is provided with a convex rib, and the inner wall of the concave die in contact with the outer wall of the insert block is provided with a groove corresponding to the convex rib.

6. The method of claim 1, wherein the bulk ultra-fine grained magnesium alloy is subjected to severe plastic deformation. The upper surface of the second inner convex rib is a flat surface or has a taper different from that of the tapered insert block.

7. The method of claim 1, wherein the bulk ultra-fine grained magnesium alloy is subjected to severe plastic deformation. The bearing is provided with multiple groups and is stacked together along the same axis.

8. The method of claim 1, wherein the bulk ultra-fine grained magnesium alloy is subjected to severe plastic deformation. The inner side of the tapered insert block and the inner convex rib insert block is provided with a round corner at the corner.

9. The method of claim 1, wherein the bulk ultra-fine grained magnesium alloy is subjected to severe plastic deformation. The end face of the male die is provided with a one-shaped groove for embedding the top of the cylindrical blank.

10. The method of claim 1, wherein the bulk ultra-fine grained magnesium alloy is subjected to severe plastic deformation. The outer contour of the tapered insert block and the first inner convex rib is spliced together to form a complete outer contour. The end face of the male die is provided with a one-shaped groove for embedding the top of the cylindrical blank.

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