A device and process method for preparing fine-grained magnesium alloy by multi-petal staggered torsion composite extrusion

Abstract

The application aims to provide a device and a process method for preparing fine-grain magnesium alloy through multi-petal staggered torsion composite extrusion, and belongs to the technical field of plastic processing of metal materials.The device comprises a die holder, and a lower concave die is arranged on the inner side of the die holder.The lower concave die is a fixed deformation concave die with a necked channel at the lower side, the top end of the necked channel is provided with an upper convex die, the outer side wall of the upper convex die is provided with a wedge-shaped block, and the wedge-shaped block is embedded in a wedge-shaped guide groove in the inner side wall of the lower concave die.The upper convex die is a funnel-shaped deformation die, which comprises eight fan-shaped convex die petals that are sequentially wedged, and the inner side wall of the fan-shaped convex die petal is a fan blade curved surface.The initial state of the fan-shaped convex die petal is high-low interval arrangement.Through the use of the application, the magnesium alloy blank can be subjected to continuous, uniform and significantly accumulated strain severe plastic deformation in a single extrusion process, the grain structure is significantly refined and the texture is weakened, the tensile strength, yield strength and elongation of the magnesium alloy at room temperature are greatly improved, and the preparation cost is reduced.

Description

Technical Field

[0001] This invention belongs to the field of metal material plastic processing technology, specifically relating to an apparatus and process for preparing fine-grained magnesium alloys by multi-lobed misaligned torsional composite extrusion. Background Technology

[0002] Magnesium alloys are the least dense metallic structural materials, possessing advantages such as high specific strength and specific stiffness, non-sparking during friction, good hot formability, and easy recyclability. Therefore, they occupy an important position in the automotive, 3C, aerospace, and military industries, and are even hailed as "green energy materials of the 21st century." However, due to their close-packed hexagonal crystal structure, magnesium alloys cannot meet the Von-Mises criterion for plastic deformation at room temperature, resulting in poor room-temperature mechanical properties and limiting their application in various fields. Grain refinement can significantly improve the mechanical properties of metallic materials, and this is also true for magnesium alloys. Intense plastic deformation techniques have been proven to effectively refine magnesium alloy grains. Techniques such as equal channel angle extrusion (ECAP) and high-pressure torsion (HPT) can achieve extremely fine grains, even ultrafine-grained structures. However, the plastic deformation process has a crucial impact on the crystal structure. Furthermore, the mold processing and equipment requirements are relatively high, requiring the material to withstand significant pressure, and measures must be taken to prevent insufficient material deformation and uneven process, which greatly limits the realization of large-scale continuous production. Therefore, it is essential to invent an apparatus and processing method for preparing fine-grained magnesium alloys by high-pressure plastic extrusion deformation. Summary of the Invention

[0003] The purpose of this invention is to provide an apparatus and process for preparing fine-grained magnesium alloys by multi-lobe misaligned torsional composite extrusion, which solves the technical defects of existing magnesium alloy severe plastic deformation technology, such as complex mold structure, high preparation cost, poor deformation uniformity, low production efficiency and difficulty in achieving continuous batch preparation.

[0004] The present invention adopts the following technical solution: An apparatus for preparing fine-grained magnesium alloy by multi-lobed misaligned torsional composite extrusion includes a mold frame, a lower die provided on the inner side of the mold frame, the lower die being a fixed deformation die with a necking channel at the bottom, an upper punch provided at the top of the necking channel, a wedge block provided on the outer side wall of the upper punch, the wedge block being embedded in a wedge guide groove on the inner side wall of the lower die, and an extruder provided at the top of the upper punch. The upper punch is a funnel-shaped deformation mold, comprising eight fan-shaped punch segments that are sequentially wedge-fitted together. The inner sidewalls of the fan-shaped punch segments are fan-blade curved surfaces. The initial state of the fan-shaped punch segments is that they are arranged at high and low intervals.

[0005] Furthermore, the formwork includes a left column, a right column, an upper crossbeam, and a lower crossbeam. The two ends of the left column are connected to the upper crossbeam and the lower crossbeam, respectively, and the two ends of the right column are connected to the upper crossbeam and the lower crossbeam, respectively. The surface of the lower crossbeam is provided with a positioning groove for the lower die to be inserted.

[0006] Furthermore, the extruder includes an extrusion cylinder located at the top of the upper crossbeam, with an extrusion rod connected to the bottom of the extrusion cylinder, the extrusion rod passing through a guide hole at the center of the upper crossbeam.

[0007] Furthermore, the axis of the lower die is aligned with the axis of the guide hole at the center of the upper beam.

[0008] Furthermore, the angle between the outer surface of the fan-shaped punch and the horizontal plane is 60°; the inner wall fan blade surface of the fan-shaped punch is a spiral twisted surface extending along the axial direction, with a spiral helix angle of 15°-45°.

[0009] Furthermore, the bottom of the fan-shaped punch located at the high position forms an axial gap with the upper end face of the necking channel, and the central angle of the gap is 45°; The inner wall of the constricted channel is a tapered surface that tapers uniformly from top to bottom, with a taper of 3°-8°.

[0010] Furthermore, a heating layer is provided around the lower die.

[0011] Furthermore, it also includes a control device connected to the extrusion cylinder and the heating layer.

[0012] A method for preparing fine-grained magnesium alloys by multi-lobe dislocation torsional composite extrusion includes the following steps: S1, Pretreated magnesium alloy bar stock; S2, preheated and pretreated magnesium alloy bars; S3, Lubrication, assembly and preheating of the device; S4. Preparation of fine-grained magnesium alloys by displaced torsion composite extrusion deformation: a. Feeding and pressing: The preheated magnesium alloy bar is placed into the cavity of the upper punch, and the extrusion rod applies pressure downward at a uniform speed. During the extrusion process, the temperature of the heating layer is 350-500℃. b. Multi-stage deformation: During the axial compression process, the magnesium alloy bar undergoes the following four deformation stages in sequence: In the first stage, the misaligned pre-opening deformation stage, after the magnesium alloy bar enters the cavity of the upper punch, it first contacts only the inner wall fan blade curved surface of the four fan-shaped punch petals located at the high position. In the second stage, the transition stage of the fan-shaped punch, as the extrusion rod continues to press down, the magnesium alloy bar applies axial pressure to the inner curved surface of the high-position fan-shaped punch. Driven by this axial pressure, the high-position fan-shaped punch gradually slides down along the wedge groove on the outer side. The high-position fan-shaped punch eventually moves down to be completely flush with the low-position fan-shaped punch in the axial direction. In the third stage, the uniform torsional composite extrusion deformation stage, after the eight fan-shaped punches are completely aligned, the upper punch forms an extrusion channel with a fixed cross section. When the magnesium alloy bar passes through this channel, it is subjected to the coupled effect of axial compression and circumferential shear. In the fourth stage, the necking and sizing stage, the magnesium alloy bar, after the eight fan-shaped punches are completely aligned, continues to descend and enters the necking channel of the lower die, where it undergoes further radial compression deformation to complete the sizing and shaping.

[0013] S5. Sampling and post-processing: After the extrusion process is completed, the fine-grained magnesium alloy obtained by extrusion is taken out from the lower end of the necking channel and subjected to post-processing.

[0014] The principle of this invention is as follows: When using the apparatus and processing method of this invention to prepare fine-grained magnesium alloys, the grain refinement effect of the resulting fine-grained magnesium alloys is more significant than that of magnesium alloy rods obtained by traditional processes due to the synergistic effect of the following two factors: 1. Initial Displacement Pre-deformation Effect: In the initial state of the device, there is an axial height difference between the two sets of alternating segments of the eight fan-shaped punch segments. The billet first contacts the four segments with higher positions, while the four axial gaps below provide stress release channels, causing the billet to bear asymmetric loads and produce local non-uniform plastic deformation in the initial contact stage. This initial deformation introduces a non-uniform shear strain field inside the billet, which is beneficial for breaking the original coarse grain structure and initiating the dynamic recrystallization process, laying the foundation for grain refinement in the subsequent uniform deformation stage. 2. Torsional Composite Extrusion Deformation Effect: After the four higher lobes move down along the wedge groove until they are completely flush with the four lower lobes, the eight lobes form a complete constant-section extrusion channel. The billet undergoes continuous torsional composite extrusion deformation under the circumferential shear force of the fan-shaped torsional surface. This coupled deformation mode simultaneously generates axial compressive strain and circumferential shear strain within the billet. Compared with single extrusion deformation, it can activate more slip systems and promote dislocation multiplication and dynamic recrystallization, thereby achieving a more significant grain refinement effect.

[0015] This invention organically combines a dynamic misalignment pre-deformation mechanism with a torsional composite extrusion deformation mechanism. Through the wedge-groove interlocking structure design of the eight-lobed punch in the device, the magnesium alloy billet sequentially undergoes four continuous deformation stages during a single axial extrusion stroke: "initial non-uniform pre-deformation → lobe-alignment transition → uniform torsional composite extrusion deformation → necking and sizing shaping," achieving a multi-stage, multi-mode synergistic grain refinement effect. Using this device and its processing method, continuous, uniform, and significantly cumulative strain-driven drastic plastic deformation can be induced in the magnesium alloy billet during a single extrusion process, significantly refining the grain structure and weakening the texture. This greatly improves the room temperature tensile strength, yield strength, and elongation of the magnesium alloy, while simultaneously reducing manufacturing costs and meeting the needs of continuous industrial production.

[0016] The beneficial effects of this invention are as follows: 1. The device has a simple structure, regular shapes of each part, simple manufacturing process, low manufacturing cost, and is easy to promote industrialization; 2. The multi-lobed misaligned structure causes non-uniform pre-deformation of the billet in the initial stage. Combined with subsequent torsional composite extrusion deformation, a multi-stage synergistic refining mechanism is realized. The resulting fine-grained magnesium alloy has a significant grain refinement effect and excellent microstructure uniformity. 3. Torsional compound extrusion deformation can be achieved by unidirectional (axial) extrusion, without the need for complex horizontal reciprocating motion mechanisms, resulting in simple equipment requirements and reliable operation; 4. The eight-lobed segmented structure design is flexible and can adjust parameters such as the initial misalignment height difference and wedge groove inclination angle according to process requirements to adapt to the preparation requirements of different alloy systems and different target grain sizes; 5. The sizing and shaping function of the necking die ensures the dimensional accuracy and surface quality of the final bar stock without the need for additional finishing processes. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the overall structure of the device of the present invention; Figure 2 This is a schematic diagram of the structure when the upper punch is initially misaligned and pre-opened. Figure 3 This is a partial cross-sectional view of the intermediate state when the upper punch begins to flatten. Figure 4 A partial cross-sectional view of the uniform extrusion-torsion deformation state after the upper punch is completely aligned; Figure 5 This is a top view after the upper punch is completely flush with the top. Figure 6 This is a schematic cross-sectional view of the lower die. Figure 7 This is a schematic diagram of the structure of a single fan-shaped punch lobe. Figure 8This is a schematic diagram of the structure after the upper punch is completely flush with the upper die. Wherein: 1-Left column; 2-Upper crossbeam; 3-Extrusion cylinder; 4-Extrusion rod; 5-Upper punch; 6-High position punch flap; 7-Low position punch flap; 8-Wedge guide groove; 9-Lower die; 10-Necking channel; 11-Right column; 12-Display screen; 13-Start button; 14-Pause button; 15-Stop button; 16-Central control console; 17-Lower crossbeam; 18-Magnesium alloy billet; 19-Heating layer controller switch; 20-Heating layer. Detailed Implementation

[0018] The present invention will be further described and explained in detail with reference to the accompanying drawings and specific embodiments.

[0019] In describing this invention, it should be noted that the terms "center," "middle," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., refer to the relative positions shown in the accompanying drawings. These terms are only used to simplify the description of the relevant structures and positional relationships of this invention, and do not specifically indicate that the devices or elements involved must have a specific orientation or operate in a specific orientation. Therefore, they should not be construed as limiting the scope of protection of this invention.

[0020] A multi-lobed misaligned torsional composite extrusion device for preparing fine-grained magnesium alloys includes an external rigid die frame, an upper punch 5, a lower die 9, and an extruder. The lower die 9 is a fixed deformation die with a necking channel 10 at the bottom.

[0021] The external rigid mold frame includes a left column 1, a right column 11, an upper crossbeam 2, and a lower crossbeam 17. The left column 1 and right column 11 are high-strength steel columns with cylindrical cross-sections, respectively fastened to the upper crossbeam 2 and lower crossbeam 17 by high-strength bolts, forming a closed rigid outer frame that provides rigid load-bearing capacity and precise positioning reference for the device. The upper crossbeam 2 has a guide hole at its center for the extrusion rod 4 to pass through. A guide bushing is provided on the inner wall of the guide hole to ensure that the extrusion rod 4 moves smoothly in the vertical direction during pressure application. A positioning groove for installing the lower die 9 is provided above the lower crossbeam 17. The shape of the positioning groove precisely matches the outer contour of the lower die 9, ensuring that the central axis of the lower die 9 is strictly collinear with the central axis of the guide hole of the upper crossbeam 2 after installation (coaxiality error ≤ 0.05 mm). The extrusion press includes an extrusion cylinder 3 installed above the upper crossbeam 2 and an extrusion rod 4 connected to it. The extrusion rod 4 passes through the central hole of the upper crossbeam 2 to transmit the extrusion load downwards.

[0022] The upper punch 5 is located in the central area between the upper crossbeam 2 and the lower crossbeam 17, and is the core functional component for realizing high-pressure misaligned torsional composite extrusion deformation of magnesium alloy billets. The upper punch 5 adopts a multi-lobed misaligned structure, which is composed of eight punch lobes sequentially assembled along the circumference. The cross-section of each punch lobe is fan-shaped (central angle 45°), and the eight punch lobes are assembled along the circumference to form a complete inverted truncated cone cavity. The structural features of each punch lobe are as follows: (i) Inner wall surface: The inner wall surface of the punch is a spirally twisted fan-shaped curved surface extending along the axial direction. There is a spatial angle between the normal direction of this curved surface and the axial direction of the blank (determined by the helix angle), so that when the blank moves downward along the axial direction, the inner wall surface applies a circumferential frictional shear force to the outer surface of the blank, thereby introducing circumferential shear deformation. The helix angle of the fan-shaped curved surface is determined according to the magnitude of the target circumferential shear strain, and is generally 15°-45°; (ii) Outer surface: The outer surface of the punch is an inclined micro-arc surface at a 60° angle to the horizontal reference plane. A wedge-shaped block is formed along the axial direction on the outer surface. The cross-sectional shape of the wedge-shaped block is trapezoidal, which precisely matches the wedge-shaped guide groove on the inner wall of the lower die 9. The mating surfaces are in line contact, and the contact line forms a 60° angle with the horizontal direction. The groove parameters (groove depth, groove width, groove inclination angle) of the wedge groove are precisely designed to make the axial sliding movement of the punch along the wedge groove direction flexible and smooth, while being strictly constrained in the radial and circumferential directions to prevent the punch from radially spreading or circumferentially misaligning during operation. (iii) Wedge groove between adjacent punches: Two adjacent punches are connected by a wedge groove opened in the radial direction. The cross section of the wedge groove is also trapezoidal. After the engagement, the adjacent punches can only slide up and down relative to each other in the direction of the wedge groove (i.e., the axial direction). They cannot separate in the radial direction or move in the circumferential direction, thus ensuring the geometric integrity of the cavity assembled from eight punches during the working process.

[0023] In its initial state, the upper punch 5 is arranged with eight punch segments at varying heights, divided into two groups: the higher segments (1#, 3#, 5#, 7#) are positioned higher axially, and the lower segments (2#, 4#, 6#, 8#) are positioned lower axially, with an axial height difference H between the two groups of punch segments. Four axial gaps are formed circumferentially between the lower end face of the higher segments and the upper end face of the lower die 9. The central angle corresponding to each gap is 45°. In this initial state, the effective flow section of the upper punch is not a complete inverted truncated cone annular section, but rather an irregular section defined by the contours of the lower end faces of the four higher segments. The blank can release stress in this region through the axial gaps. When the extrusion rod 4 applies downward pressure, the billet first enters the entrance cavity of the upper punch 5. At this time, the billet only contacts the inner fan-shaped curved surface of the four high-position petals and bears the compressive load. The area above the corresponding low-position petals is in a partially free state due to the axial clearance. The billet can release stress here. Thus, in the initial deformation stage of the billet, high-stress areas and low-stress areas are introduced along the circumferential direction, forming a non-uniform stress field and strain field, resulting in non-uniform initial deformation.

[0024] The lower die 9 is fixedly installed on the bottom plate positioning groove of the lower crossbeam 17, directly below the upper punch 5. It is a functional component for sizing and shaping the blank, and its outer contour is cylindrical. An annular heating groove is provided inside for installing the heating layer 20 (annular resistance heating element). The heating layer 20 is connected to the heating layer controller switch 19 via wires. The lower die 9 has a necking channel 10 whose cross-section gradually decreases from top to bottom. The upper entrance diameter D of the necking channel 10 is... in The extrusion orifice diameter D when the eight punches in the upper punch 5 are completely flush with each other out Same (D) in = D out =D), ensuring that the billet, after being extruded from the upper punch 5, can directly enter the necking channel 10 without steps or gaps. The lower outlet diameter d of the necking channel 10 is slightly smaller than the upper inlet diameter D. in The inner wall of the necking channel 10 is a tapered surface that contracts uniformly from top to bottom. The billet undergoes uniform radial compression deformation as it passes through the necking channel 10, and is finally extruded from the lower outlet in the form of a bar with precise cross-sectional dimensions and a smooth surface.

[0025] The internal deformation space formed by the upper punch 5 and the lower die 9 is divided into two parts along the axial direction from top to bottom: the misaligned pre-opening deformation zone (corresponding to the area where the punch petals are not completely aligned) and the uniform extrusion-torsion-necking deformation zone (corresponding to the continuous deformation area from the bottom of the necking channel after the punch petals are completely aligned).

[0026] All eight punches and the lower die 9 are made of hot-working die 4Cr5MoSiV1. This steel, after quenching and secondary tempering, exhibits excellent high-temperature yield strength (≥1500 MPa at room temperature, ≥800 MPa at 550℃), thermal fatigue resistance, and tempering softening resistance within a temperature range from room temperature to 600℃. This meets the requirements for long-term stable operation of this device under high temperature and high pressure conditions of 350-500℃. The surface roughness of all components reaches Ra 0.16μm, and the surface roughness of the mating working surfaces of the upper crossbeam 2 and lower crossbeam 17 also reaches Ra 0.16μm.

[0027] In this specific embodiment, the angle between the outer surface of the punch and the horizontal reference plane is 60°. This angle is chosen based on the following mechanical considerations: the 60° inclination angle ensures that when the punch bears the axial downward pressure of the blank, the normal contact force between its outer surface and the wedge-shaped surface of the lower die wall generates a significant radial constraint component in the radial direction, effectively preventing the punch from spreading outwards radially during operation. Simultaneously, the 60° inclination angle is not too large, preventing excessive axial sliding resistance along the wedge-shaped groove on the outer surface of the punch, thus achieving a balance between radial constraint capability and motion flexibility. The fit between the outer surface of the punch and the wedge-shaped surface of the lower die wall is a line contact, with the contact line forming a 60° angle with the horizontal direction. Compared to surface contact, line contact has the advantages of lower contact stress concentration and lower frictional resistance when the punch slides along the wedge-shaped groove, which helps ensure the smoothness and guiding accuracy of the punch's axial movement.

[0028] The above-mentioned apparatus is used in a process for preparing fine-grained magnesium alloys by torsional composite extrusion, which is implemented through the following steps: 1. Selected materials and chemical reagents: Magnesium alloy billet 18 (cylindrical or regular polygonal billet), material selected is AZ31 magnesium alloy (containing Al 2.5-3.5%, Zn 0.6-1.4%, Mn≥0.2%, balance Mg); sandpaper, solid abrasive material; graphite grease (oil-based colloidal graphite), viscous lubricant; anhydrous ethanol, liquid solvent, purity ≥99.5%; acetone, liquid solvent, purity ≥99%.

[0029] 2. Pre-treated magnesium alloy billet 18: ① Surface polishing: Use 600-grit silicon carbide sandpaper to polish the outer surface of the magnesium alloy bar along the axial direction to remove oxide scale, oil, and machining residue. Then, use 800-grit, 1000-grit, and 1200-grit silicon carbide sandpaper in sequence to polish the bar in alternating directions (e.g., 600-grit along the axial direction, 800-grit along the circumferential direction, 1000-grit along the axial direction, and 1200-grit along the circumferential direction) to eliminate directional scratches generated by the previous polishing stage. The final surface roughness of the bar should reach Ra≤0.8 μm, and the surface finish should meet the requirements for subsequent ultrasonic cleaning and lubricant application. ② Pour acetone and anhydrous ethanol into the cleaning tank at a volume ratio of 3:2, and stir and mix them for 5 minutes at a speed of 300 r / min using a magnetic stirrer to ensure that the two solvents are fully miscible and to prepare a uniform acetone + anhydrous ethanol mixed cleaning solution. ③ Completely immerse the polished magnesium alloy rod in an acetone + anhydrous ethanol cleaning solution. Place the cleaning tank in an ultrasonic cleaner (operating frequency 40 kHz) and perform ultrasonic cleaning for 30-60 minutes at room temperature. The ultrasonic cavitation effect can effectively remove residual grease, particulate contaminants, and other impurities from the surface and micropores of the rod. After cleaning, remove the magnesium alloy rod and immediately rinse the surface with anhydrous ethanol to remove any remaining cleaning solution. Then, use a clean, dry airflow (nitrogen or clean compressed air, pressure 0.2-0.4 MPa) to dry the surface of the rod. ④ Apply graphite grease evenly to the outer surface of the magnesium alloy billet using a brush, controlling the coating thickness to be 0.05-0.15 mm, forming a continuous solid lubricating film. This graphite lubricating film can effectively reduce the coefficient of friction between the billet and the die cavity during high-temperature extrusion (from μ≈0.3-0.5 without lubrication to μ≈0.08-0.15), reducing frictional heat generation and preventing defects such as sticking and tearing on the billet surface.

[0030] 3. Preheating magnesium alloy bars: Operate a vacuum atmosphere heating furnace and set the heating temperature to 350-500 ℃. After reaching the set temperature, put the magnesium alloy bars into the furnace and keep them at that temperature for 2-4 hours to make the temperature of the bars uniform and reach the temperature range suitable for hot extrusion deformation.

[0031] 4. Lubrication, assembly, and preheating: ① Before assembling the device, graphite grease should be evenly applied to all surfaces that come into contact with the blank or adjacent parts, including all outer surfaces, inner curved surfaces, inner cavity of the lower die, and mating working surfaces of the upper and lower crossbeams. ② During assembly, firstly, the left column 1 and right column 11 are fastened to the upper crossbeam 2 and lower crossbeam 17 respectively with high-strength bolts to form a rigid outer frame; the extrusion rod 4 is passed through the guide hole in the center of the upper crossbeam 2, and its upper end is connected to the piston rod of the extrusion cylinder 3 by threads or flanges. The lower die 9 is placed in the positioning groove of the bottom plate above the lower crossbeam 17, ensuring that its axis is collinear with the axis of the central through hole of the upper crossbeam 2 (coaxiality error ≤ 0.05 mm); the eight punch pieces are sequentially embedded into the wedge-shaped guide groove of the necking channel of the lower die along the circumferential direction. After each punch piece is embedded, it is necessary to confirm that the line contact between the wedge-shaped surface of its outer side and the wedge-shaped groove of the inner wall of the necking channel is correct, and that the axial sliding movement along the wedge-shaped groove direction is free and without jamming. After all eight punch segments are inserted, adjust each segment to the initial misaligned pre-opening state: operate the extrusion cylinder 3 controller to position the higher segments (1#, 3#, 5#, 7#) in a higher axial position and the lower segments (2#, 4#, 6#, 8#) in a lower axial position, confirming that the axial height difference Δh between the higher and lower segments meets the process setting value. After adjustment, finally tighten the upper crossbeam 2 and lower crossbeam 17 with high-strength bolts to ensure that the left column 1 and right column 11 are firmly fixed together to form a stable overall structure; ③ After the device is assembled, operate the heating layer controller to control the heating layer temperature of the device to 350-500 ℃. After reaching the set temperature, keep it at the temperature for 2-4 hours so that the mold as a whole reaches the hot extrusion working temperature.

[0032] 5. Preparation of fine-grained magnesium alloys by displaced torsion combined extrusion deformation: ① Feeding and Pressing: The preheated magnesium alloy bar is removed from the furnace (operation time should be controlled within 30 seconds to reduce temperature loss) and quickly placed into the inlet cavity above the upper punch 5. The extrusion cylinder controller is used to control the upper extrusion rod to apply pressure downwards at a uniform speed v, where v is 10-100 mm / min (preferably 60 mm / min). Throughout the extrusion process, the heating layer controller is used to maintain the heating layer temperature at 350-500 ℃ (preferably 450 ℃) to compensate for temperature fluctuations caused by plastic work conversion during billet deformation and maintain the stability of the temperature field in the deformation zone. ② Multi-stage deformation process: The billet undergoes the following four deformation stages sequentially during axial downward pressing: The first stage (misalignment pre-deformation stage): After the billet enters the cavity through the upper punch 5, it initially contacts only the fan-shaped curved surfaces of the inner walls of the four high-position lobes (1#, 3#, 5#, 7#). Because the inner curved surfaces of the high-position lobes extend further axially, while the low-position lobes are positioned lower axially, the billet experiences a coupling effect of axial compression and circumferential shear in the contact area with the high-position lobes. Simultaneously, in the region above the corresponding low-position lobes, due to the axial gap between the lower end face of the high-position lobes and the upper end face of the necking channel, the radial constraint on the billet is partially released, and the stress can be released through the axial gap, forming a circumferentially alternating high-pressure zone (the area in contact with the high-position lobes) and a low-pressure zone (the area corresponding to the axial gap). This non-uniform stress field generates non-uniform shear strain distributed circumferentially within the billet, causing the initial coarse grain structure of the billet to break down, inducing dislocation proliferation within the grains and initiating a dynamic recrystallization nucleation process, laying a favorable microstructure foundation for the grain refinement in the subsequent uniform deformation stage.

[0033] The second stage (petal flushing transition stage): As the extrusion rod 4 continues to press down, the billet applies axial pressure to the inner curved surface of the higher petal. Driven by this axial pressure, the higher petal gradually slides downward along the wedge-shaped groove on its outer side. Because the wedge-shaped groove exerts strict radial constraint on the punch petal (the punch petal can only slide axially along the wedge-shaped groove direction, and cannot undergo radial separation or circumferential misalignment), and the line contact between the outer surface of the punch petal and the wedge-shaped surface of the lower die inner wall provides precise motion guidance, the downward movement of the higher petal is smooth and controllable. During this process, the higher petal gradually approaches the position of the lower petal, the axial clearance gradually decreases, the radial constraint on the billet gradually strengthens, and the cross-section of the deformed area gradually becomes more complete. At this point, the lower end faces of the lower die petals (2#, 4#, 6#, 8#) abut against the axial stop step on the upper end face of the lower die 9, and are axially limited, preventing them from moving further downward. There is a preset axial height difference H between the lower end faces of the higher die petals (1#, 3#, 5#, 7#) and the stop step. Therefore, driven by the axial pressure of the blank, the higher die petals can continue to slide downward along the wedge groove until they are flush with the lower die petals. When the higher die petals finally move down to be completely flush with the lower die petals axially, the inner wall surfaces of the eight die petals form a continuous and complete spiral twisted fan-shaped surface. The cavity of the deformed die assembly reaches a complete inverted truncated cone shape, and the cross-section of the blank transitions from an irregular cross-section to a complete annular cross-section.

[0034] The third stage (uniform torsional compound extrusion deformation stage): After the eight punches are completely aligned, the lower section of the upper punch forms a "funnel-shaped extrusion channel with a gradually decreasing cross-section." The billet experiences a coupling effect of axial compression and circumferential shear as it passes through this channel. The billet experiences the following coupling effects as it passes through this channel: (a) Axial compression: Under the continuous pressure of the extrusion rod, the billet is forced out axially, and its cross-sectional area gradually decreases due to the constant volume, resulting in axial compressive strain; (b) Circumferential shear: Because the inner wall of the punch is a spirally twisted fan-shaped curved surface, there is a spatial angle between the normal direction of this surface and the axial movement direction of the billet. When the billet moves downwards axially, the fan-shaped curved surface applies a circumferential frictional shear force τθ to the outer surface of the billet. This shear force drives the outer metal layer of the billet to circumferentially shift relative to the inner metal layer, thereby introducing a circumferential shear strain γθz inside the billet. The superposition of the axial compressive strain εz and the circumferential shear strain γθz makes the equivalent strain εeff of the billet significantly higher than the equivalent strain of single axial extrusion deformation. According to the von Mises equivalent strain formula: εeff =√[(2 / 3)(εz² + εr² + εθ²)+(1 / 3)(γrz² + γθz² + γrθ²)] (under the axisymmetric extrusion deformation conditions of this invention, the main effective strain components are the axial compressive strain εz and the circumferential shear strain γθz, while the contributions of other strain components are relatively small). The introduction of the circumferential shear strain γθz increases εeff, which can activate more non-basal plane slip systems in the HCP crystal structure of magnesium alloy - including prism slip system {10-10} and conical slip system {10-11}. Although the critical shear stress of these slip systems has been reduced at high temperature, a higher equivalent strain is still required to fully activate them. The synergistic activation of multiple slip systems promotes the proliferation and interaction of dislocations on multiple slip surfaces (dislocation entanglement, dislocation cell formation). At the same time, under high temperature conditions of 350-500 ℃, the climb and rearrangement process of dislocations is accelerated, dynamic recrystallization continues and new fine equiaxed grains are constantly generated, thereby achieving extreme grain refinement.

[0035] The fourth stage is the necking and sizing stage; the billet is extruded from the lower end of the upper punch and enters the necking channel located directly below it. The necking channel is a tapered channel with a gradually decreasing cross-section from top to bottom. Its upper inlet diameter is the same as the extrusion port diameter when the upper punch is completely flush (ensuring that the billet can smoothly enter the necking channel after being extruded from the deforming punch assembly, without steps or gaps), and the lower outlet diameter is slightly smaller than the upper inlet diameter. The taper is generally 3°-8° (preferably 5°). As the billet passes through the necking channel, it undergoes further radial compressive deformation (i.e., necking deformation). This deformation serves to: (a) sizing and shaping the billet cross-section, eliminating fluctuations in cross-sectional dimensions caused by minute gaps or misalignments between the segments of the upper punch, ensuring the final bar's cross-sectional dimensions are within ±0.1 mm; (b) applying uniform radial compressive stress to the outer surface of the billet, eliminating potential microcracks, scratches, and other defects, and improving the surface finish of the bar; and (c) further increasing the cumulative plastic strain of the billet, further refining the already refined grain structure. The final product is a fine-grained magnesium alloy with precise cross-sectional dimensions, a smooth surface, and a uniform microstructure.

[0036] ③ Sampling and Post-processing: After the extrusion process, the fine-grained magnesium alloy obtained from the extrusion is removed from the lower end of the die. The outer surface of the rod is lightly polished with 400-grit silicon carbide sandpaper to remove any residual graphite lubricant coating and oxide film. The rod is then immersed in an acetone and anhydrous ethanol cleaning solution for ultrasonic cleaning for 10-15 minutes, rinsed with anhydrous ethanol, and finally dried with a drying airflow. The resulting rod can be used directly or subjected to subsequent microstructural analysis (EBSD, XRD, TEM, etc.) and mechanical property testing (tensile, hardness, impact, etc.).

[0037] The principle of obtaining fine-grained magnesium alloys through the above steps is described in detail below with reference to the accompanying drawings: The core innovation of this invention lies in the organic combination of initial misalignment pre-opening deformation and torsional composite extrusion deformation, which achieves efficient grain refinement of magnesium alloy through a multi-stage synergistic deformation mechanism.

[0038] In the initial misaligned pre-opening state of the device, four of the eight punches (1#, 3#, 5#, and 7#) are in a higher axial position, while four of them (2#, 4#, 6#, and 8#) are in a lower axial position, with a preset axial height difference H between the two sets of punches. When the extrusion rod 4 applies downward pressure, the billet first contacts the higher-position punches. Due to the axial gap between the bottom of the higher-position punch and the upper end face of the necking channel 10, the billet experiences an asymmetric load in this area—the area in contact with the punch is compressed, while the area corresponding to the gap has stress release space, thus generating uneven stress distribution and asymmetric plastic deformation on the cross-section of the billet. This initial non-uniform deformation introduces an initial shear strain field inside the billet, which can effectively break the original coarse grain structure and induce dynamic recrystallization nucleation, providing a favorable microstructure basis for grain refinement in the subsequent uniform deformation stage. As the extrusion rod 4 continues to press down, the higher-position punches gradually move downward along the wedge groove under the push of the billet. Because the wedge groove exerts a radial constraint on the movement of the punch segments, the punch segments can only slide axially along the direction of the wedge groove. At the same time, the line contact between the outer surface of the punch segment and the wedge-shaped surface of the inner wall of the lower die 9 further ensures the stability of the movement and the guiding accuracy. When the high-position punch segment moves down to be completely flush with the low-position punch segment in the axial direction, the eight punch segments form a complete inverted truncated cone cavity, and its inner wall forms a continuous spiral twisted fan-shaped curved surface.

[0039] Subsequently, the billet enters the uniform extrusion-torsional deformation stage. In this stage, while the billet is continuously compressed and extruded axially, it is subjected to circumferential shear force applied by a fan-shaped torsional surface, resulting in torsional composite extrusion deformation. This coupled deformation mode simultaneously generates axial compressive strain εz and circumferential shear strain γθz within the billet. Its equivalent strain is significantly higher than that of single extrusion deformation, activating more slip systems (including basal slip, cylindrical slip, and conical slip) in the magnesium alloy HCP crystal structure. This promotes the extensive multiplication of dislocations and the continuous dynamic recrystallization, thereby achieving extreme grain refinement.

[0040] Finally, the billet, after being deformed by torsion and compound extrusion, is extruded from the upper punch 5 and enters the necking channel 10 of the lower die 9. The cross-section of the necking channel 10 gradually decreases from top to bottom, applying further radial compression deformation to the billet, which plays a role in sizing and shaping, eliminating any residual dimensional deviations and microstructure inhomogeneities on the billet cross-section, and finally forming a uniform fine-grained bar with precise cross-sectional dimensions and a smooth surface.

[0041] The fine-grained magnesium alloys obtained by the apparatus and processing method of this invention have significantly reduced average grain size, significantly improved microstructure uniformity, and weakened texture compared to traditional processes. These improvements in microstructure characteristics directly correspond to a significant increase in the room temperature mechanical properties (tensile strength, yield strength, and elongation) of the magnesium alloys.

[0042] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. An apparatus for preparing fine-grained magnesium alloys by multi-lobe dislocation torsional composite extrusion, characterized in that: Includes a mold frame, the inner side of which is provided with a lower die (9), the lower die (9) is a fixed deformation die with a necking channel (10) at the bottom, the top of the necking channel (10) is provided with an upper punch (5), the outer side wall of the upper punch (5) is provided with a wedge block, the wedge block is embedded in the wedge guide groove (8) of the inner side wall of the lower die, and the top of the upper punch (5) is provided with an extruder; The upper punch (5) is a funnel-shaped deformation mold, including eight fan-shaped punch segments that are wedged together in sequence. The inner sidewall of the fan-shaped punch segments is a fan blade curved surface. The initial state of the fan-shaped punch segments is that they are arranged at high and low intervals.

2. The apparatus for preparing fine-grained magnesium alloys by multi-lobe misaligned torsional composite extrusion according to claim 1, characterized in that: The formwork includes a left column (1), a right column (11), an upper crossbeam (2) and a lower crossbeam (17). The two ends of the left column (1) are connected to the upper crossbeam (2) and the lower crossbeam (17) respectively, and the two ends of the right column (11) are connected to the upper crossbeam (2) and the lower crossbeam (17) respectively. The surface of the lower crossbeam (17) is provided with a positioning groove for the lower die (9) to be inserted.

3. The apparatus for preparing fine-grained magnesium alloys by multi-lobe misaligned torsional composite extrusion according to claim 2, characterized in that: The extruder includes an extrusion cylinder (3) located at the top of the upper crossbeam (2), and an extrusion rod (4) is connected to the bottom of the extrusion cylinder (3). The extrusion rod (4) passes through the guide hole in the center of the upper crossbeam (2).

4. The apparatus for preparing fine-grained magnesium alloys by multi-lobe misaligned torsional composite extrusion according to claim 3, characterized in that: The axis of the lower die (9) is on the same straight line as the axis of the guide hole at the center of the upper beam (2).

5. The apparatus for preparing fine-grained magnesium alloys by multi-lobe misaligned torsional composite extrusion according to claim 1, characterized in that: The angle between the outer side of the fan-shaped punch and the horizontal plane is 60°; the inner wall fan blade surface of the fan-shaped punch is a spiral twisted surface extending along the axial direction, with a spiral helix angle of 15°-45°.

6. The apparatus for preparing fine-grained magnesium alloys by multi-lobe misaligned torsional composite extrusion according to claim 1, characterized in that: The bottom of the fan-shaped punch located at the high position forms an axial gap with the upper end face of the necking channel (10), and the central angle of the gap is 45°; The inner wall of the constricted channel (10) is a tapered surface that contracts uniformly from top to bottom, with a taper of 3°-8°.

7. The apparatus for preparing fine-grained magnesium alloys by multi-lobe misaligned torsional composite extrusion according to claim 3, characterized in that: The lower die (9) is provided with a heating layer (20) around its perimeter.

8. The apparatus for preparing fine-grained magnesium alloys by multi-lobe misaligned torsional composite extrusion according to claim 7, characterized in that: It also includes a control device, which is connected to the extrusion cylinder (3) and the heating layer (20).

9. A method for preparing fine-grained magnesium alloys by multi-lobe misaligned torsional composite extrusion using the apparatus described in any one of claims 1-8, characterized in that: The steps include the following: S1, Pretreated magnesium alloy bars; S2, preheated and pretreated magnesium alloy bars; S3. Lubrication, assembly, and preheating of the device; S4. Preparation of fine-grained magnesium alloys by displaced torsion composite extrusion deformation: a. Feeding and pressing: The preheated magnesium alloy bar is placed into the cavity of the upper punch (5), and the extrusion rod (4) applies pressure downward at a uniform speed. During the extrusion process, the temperature of the heating layer is 350-500℃. b. Multi-stage deformation: During the axial compression process, the magnesium alloy bar undergoes the following four deformation stages in sequence: In the first stage, the misaligned pre-opening deformation stage, after the magnesium alloy bar enters the cavity of the upper punch (5), it first contacts only the inner wall fan blade curved surface of the four fan-shaped punch petals located at the high position. In the second stage, the transition stage of the fan-shaped punch, as the extrusion rod (4) continues to press down, the magnesium alloy bar applies axial pressure to the inner wall surface of the high-position fan-shaped punch. Driven by this axial pressure, the high-position fan-shaped punch gradually slides down along the wedge groove on the outer side. The high-position fan-shaped punch eventually moves down to be completely flush with the low-position fan-shaped punch in the axial direction. In the third stage, the uniform torsional composite extrusion deformation stage, after the eight fan-shaped punches are completely aligned, the upper punch forms an extrusion channel with a fixed cross section. When the magnesium alloy bar passes through this channel, it is subjected to the coupled effect of axial compression and circumferential shear. In the fourth stage, the necking and sizing stage, the magnesium alloy bar continues to descend after the eight fan-shaped punches are completely aligned and enter the necking channel (10) of the lower die (9), where it undergoes further radial compression deformation to complete the sizing and shaping. S5. Sampling and post-processing: After the extrusion process is completed, the fine-grained magnesium alloy obtained by extrusion is taken out from the lower end of the necking channel (10) and post-processed.

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