Preparation process of superplastic magnesium alloy

Abstract

The preparation technology comprises the following steps that magnesium alloy waste is subjected to low-temperature ball milling treatment to be made into nanoscale powder, the low-temperature ball milling temperature is smaller than or equal to 200 DEG C, the particle size of the powder obtained after ball milling is smaller than or equal to 100 nm, and argon is introduced in the ball milling process for protection so as to inhibit oxidation; the nano powder is mixed with Mg-Zn-Ca alloy powder with the mass ratio of 5-8%, spark plasma sintering SPS is adopted for densifying at 350-400 DEG C, the sintering pressure is 20 MPa, the heat preservation time is 10 min, and the density after sintering is larger than or equal to 98%; and performing equal channel angular pressing (ECAP) treatment on the sintered blank, wherein the extrusion temperature is 300 DEG C, the extrusion pass is four times, the included angle of a mold channel is 90 degrees, and the ultra-fine grain magnesium alloy with the average grain size of 2.1 microns is obtained. According to the preparation method disclosed by the invention, grain ultra-fining of 1.5-2.1 microns is realized through a multi-field coupling process, a non-basal plane slip system is activated at room temperature, the strain rate sensitive index m is greater than or equal to 0.35, the elongation at 250 DEG C reaches 280%, and the tensile strength of the magnesium alloy is greater than or equal to 250MPa. And high strength and superplastic deformation capacity are achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of magnesium alloy preparation, and in particular relates to a preparation process of a superplastic magnesium alloy. Background Art

[0002] As a lightweight engineering material, magnesium alloys are currently manufactured using processes primarily including remelting casting, hot extrusion, conventional annealing, and deep plastic deformation (e.g., ECAP). Traditional remelting casting requires high-temperature treatment above 700°C to recycle scrap, resulting in severely coarsened grains (e.g., the grain size of AZ61 magnesium alloy reaches 18.5μm) and significant performance degradation. While hot extrusion and conventional annealing processes can achieve a certain degree of grain refinement, annealing temperatures typically exceed 400°C, consuming up to 2500kWh / ton. Furthermore, only two basal slip systems can be activated at room temperature, resulting in a strain rate sensitivity index m of only 0.21, and limited superplastic activation. While deep plastic deformation processes can refine grains, they are not integrated with scrap recycling and multi-field coupling technologies, making it difficult to balance green manufacturing with high performance requirements.

[0003] From the perspective of existing technologies, the core deficiencies of existing technologies are: 1. The grain refinement effect is limited. Conventional processes are difficult to control the grain size below 5μm. Figure 3 The metallographic images shown in the figure show that the grain size is around 20 μm or even larger, resulting in poor room temperature plasticity (elongation of only 15-17%) and failure to meet superplastic forming requirements; 2. Scrap recycling consumes a lot of energy and has significant performance degradation. The remelting and casting recovery rate is only 75%, and the elongation of the recycled material is less than 20%; 3. Superplasticity activation temperature is high and energy consumption is high. Traditional processes require annealing above 400°C, making it difficult to achieve low-temperature and efficient forming.

[0004] Therefore, we propose a preparation process for superplastic magnesium alloy. Summary of the Invention

[0005] In order to overcome the above-mentioned defects of the prior art, the present invention provides a preparation process of superplastic magnesium alloy to solve the problems of coarse grains, poor waste recycling performance and high superplastic activation energy consumption in the traditional process.

[0006] To achieve the above object, the present invention provides the following technical solutions: A process for preparing a superplastic magnesium alloy comprises the following steps: S1: Magnesium alloy scrap is subjected to cryogenic ball milling to produce nano-scale powder. The cryogenic ball milling temperature is ≤200°C, the ball-to-material ratio is 10:1, and the rotation speed is 200 rpm. The powder particle size after ball milling is ≤100 nm. Argon gas is introduced during the ball milling process to inhibit oxidation.

[0007] Low temperature ball milling introduces high density dislocations ( ) and lattice distortion (distortion energy 1.5eV / atom), providing nucleation driving force for subsequent sintering, and reducing the oxidation inclusion rate by 80% compared with traditional ball milling (300℃).

[0008] S2: The nano-scale powder of step S1 is mixed with 5-8% by mass of Mg-Zn-Ca alloy powder, and densified by spark plasma sintering (SPS) at 350-400°C, a sintering pressure of 20 MPa, and a holding time of 10 minutes to obtain a sintered blank with a density of ≥98% after sintering.

[0009] Zn forms MgZn strengthening phase (lattice mismatch 12%) to pin grain boundaries, and Ca generates phase (melting point 720℃) inhibits grain growth, and the sintering density reaches 98.5%, so that the grain size is controlled within 1μm.

[0010] S3: The sintered billet was subjected to equal channel angular extrusion (ECAP) treatment at an extrusion temperature of 300°C, four extrusion passes, and a die channel angle of 90° to obtain an ultrafine-grained magnesium alloy with an average grain size of 2.1 μm. A pulse current was applied during the ECAP extrusion process, and the current density was 2.1 μm. , pulse frequency 5kHz, duty cycle 1:1.

[0011] Preferably, the magnesium alloy waste is AZ61 magnesium alloy cutting chips, and the waste recycling rate reaches 96%. The Mg-Zn-Ca alloy is Mg-3Zn-1Ca, and the balance is Mg, and grain refinement is promoted by alloying.

[0012] Because the Zn content of AZ61 is insufficient to form an effective strengthening phase, Mg-Zn-Ca alloy is added to increase the Zn content to a critical value (above 2%) and introduce Ca to inhibit grain growth. Both are indispensable. If only Zn powder is added without Ca, the grain size still reaches 2.5μm and the elongation is 200%.

[0013] Preferably, the ECAP treatment in step S3 is followed by a reciprocating extrusion CEC process, with a strain rate of , and the grain size was further refined to 1.5 μm through dynamic recrystallization.

[0014] Preferably, it also includes: S4: Pulsed electric field assisted annealing step, annealing temperature 250 ° C, holding time 20 minutes, electric field strength 15V / mm, which makes the grains uniform and reduces energy consumption by 45% compared with traditional annealing. After annealing, the grain size deviation is ≤8%, and a uniform ultrafine-grained billet is produced.

[0015] Preferably, the superplastic magnesium alloy has a strain rate sensitivity index m=0.35, an elongation of 280% at 250°C, and a tensile strength of 220 MPa.

[0016] Preferably, the SPS sintering energy consumption in step S2 is 800±20 kWh / ton.

[0017] Preferably, it also includes: S5: Based on the homogenized ultrafine grain billet after the treatment in step S4, semi-solid superplastic forming is performed. The billet is heated to a semi-solid temperature range of 580°C. A stir friction assisted plastic forming process is used. The friction heat generated by the rotation of the stirring head is used to maintain the forming temperature of 280°C. At the same time, mechanical stirring is used to promote the uniform flow of the semi-solid slurry. The forming strain rate is ; Relying on the stirring effect to suppress cracks and pores during the forming process, a magnesium alloy sheet component with a wall thickness of 0.3mm and a surface roughness of Ra≤1.6μm was obtained.

[0018] Preferably, during the semi-solid superplastic forming process, ultrasonic vibration of 30kHz and an amplitude of 8μm are applied to eliminate pore defects and make the density of the component reach 99%.

[0019] Preferably, the energy consumption of the entire process is 1200±50kWh / ton, and no harmful gases such as fluoride are emitted.

[0020] Preferably, it also includes: The gradient temperature field annealing step is implemented after step S4, and performs local treatment on the joint area formed when the magnesium alloy plate components are connected. A combination of an induction heating coil and a magnetic conductor is used to form a temperature gradient of 50-200°C / mm in the joint area of ​​the magnesium alloy plate components, with a center temperature of 300-400°C and an edge temperature of 200-300°C, and the temperature is kept for 5-30 minutes; a density of The pulse current relies on the uniform ultrafine grain structure formed by S4 as the matrix, solute atoms migrate directionally along the temperature gradient direction, forming a nanocrystalline-ultrafine grain gradient structure, and the tensile strength of the joint is increased to more than 28MPa.

[0021] The technical effects and advantages of the preparation process of a superplastic magnesium alloy of the present invention are as follows: 1. This invention significantly improves the material performance. It achieves ultra-fine grains (1.5-2.1μm) through a multi-field coupling process, breaking through the bottleneck of coarse grains (above 20μm) in traditional processes. It activates the non-basal slip system at room temperature, has a strain rate sensitivity index m≥0.35, and an elongation of 280% at 250℃, combining high strength and superplastic deformation capabilities.

[0022] 2. This invention, green preparation and efficient recycling: The low-temperature process chain (≤400℃) reduces the energy consumption of the entire process by more than 40% compared with the traditional process, and the waste recovery rate reaches 96% (an increase of 21%). It also solves the performance degradation problem caused by high-temperature remelting and realizes the green recycling of magnesium alloys.

[0023] 3. This invention uses semi-solid forming combined with ultrasonic vibration to produce 0.3mm thin-walled parts with a surface accuracy of Ra≤1.6μm; gradient temperature field annealing increases the shear strength of the joint to 28MPa and reduces the width of the heat-affected zone by 30%. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a system flow diagram of a preparation process of a superplastic magnesium alloy proposed by the present invention; Figure 2 Schematic diagram of the metallographic structure of the magnesium alloy component produced by the preparation process of the present invention; Figure 3 This is a metallographic diagram of a magnesium alloy component produced by conventional technology. DETAILED DESCRIPTION

[0025] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention. It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or apparatus that includes a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or apparatus. In the absence of further restrictions, the elements defined by the sentence "includes..." do not exclude the presence of other identical elements in the process, method, article or apparatus that includes the elements.

[0026] Example 1

[0027] refer to Figure 1 This embodiment provides a preparation process of a superplastic magnesium alloy for implementing a superplastic recycling process of AZ61 scrap. The specific implementation contents include: Implementation purpose: To verify the superplastic regeneration effect of the whole process of "low temperature ball milling-SPS-ECAP-electroplastically assisted" on waste materials.

[0028] Implementation steps: S1: Treatment: 10 kg of AZ61 magnesium alloy chips were ball-milled in a QM-3SP2 planetary ball mill at a ball-to-material ratio of 10:1 and a rotation speed of 200 rpm for 12 h. The temperature was controlled at 180 °C and an argon flow rate of 5 L / min to obtain a powder with an average particle size of 80 nm.

[0029] S2: Sintering: add 0.6kg Mg-3Zn-1Ca alloy powder and mix, sinter in SPS-330MKII equipment at 380℃ and 20MPa pressure for 10min. The density after sintering is 98.5%, and the energy consumption is 780kWh / ton.

[0030] S3: Extrusion, extrusion in ECAP die (channel angle 90°) at 300℃ for 4 times, extrusion speed 5mm / min, to obtain Figure 2 The average grain size of the blank shown is about 2.1μm. Figure 2 This is a microstructure diagram at a scale of 20 μm, which is 1000 times magnified.

[0031] Electroplastic assist: applied during extrusion Pulse current, frequency 5kHz, measured deformation resistance reduced by 28%.

[0032] Implementation effect:

[0033] The elongation in the tensile test at 250℃ reached 275%, and the strain rate sensitivity index m=0.34, which meets the definition of superplasticity.

[0034] The energy consumption of the entire process is 1180kWh / ton, which is 52.8% lower than the traditional process (a magnesium alloy recycling method centered on 700°C remelting casting). The waste recovery rate is 96%, and no oxide slag is produced.

[0035] Example 2

[0036] This embodiment provides a preparation process for a superplastic magnesium alloy, which is used for implementing a CEC grain refinement process. The implementation includes: Implementation purpose: To verify the effect of CEC on further grain refinement after ECAP.

[0037] Implementation steps: An ECAP blank (grain size 2.1 μm) was prepared according to steps S1-S3 of Example 1.

[0038] Using CEC mold (channel angle 90°), at 300℃ The sample was extruded in two passes at a high strain rate, and argon gas was introduced for protection during the extrusion process.

[0039] Implementation effect:

[0040] The grain size is further refined to 1.5μm, and the grain boundaries The phase is evenly dispersed, and the grain uniformity is improved by 30% compared with that after ECAP treatment.

[0041] In the 250 °C compression test, the circumferential elongation of the outer edge reached 290%, and the strain rate sensitivity index m = 0.38, which proved the synergistic refinement effect of CEC and ECAP.

[0042] Example 3

[0043] This embodiment provides a preparation process for a superplastic magnesium alloy, which is used for implementing a pulsed electric field assisted annealing energy-saving process. The specific implementation contents include: Implementation purpose: To verify the energy-saving effect and grain homogenization effect of pulse electric field annealing.

[0044] Implementation steps: The ECAP blank in Example 1 was taken and cut into 10 mm × 10 mm × 10 mm specimens.

[0045] An electric field strength of 15 V / mm was applied in a Gleeble-1500 thermal simulator, the temperature was kept at 250°C for 20 minutes, and the pulse frequency was 10 kHz, compared with the traditional annealing process (300°C×60 minutes, no electric field).

[0046] Implementation effect:

[0047] The actual annealing energy consumption is 850kWh / ton, which is 45.2% lower than the traditional annealing (1550kWh / ton for the magnesium alloy recycling method centered on 700°C remelting casting), meeting energy-saving requirements.

[0048] The grain size deviation was reduced from 22% in traditional annealing (a magnesium alloy recycling method centered on 700°C remelting casting) to 8%, the average grain size was 1.8μm, and the structural uniformity was significantly improved.

[0049] Example 4

[0050] This embodiment provides a preparation process for a superplastic magnesium alloy, which is used for implementing a semi-solid forming + ultrasonic vibration composite process. The specific implementation content includes: Implementation purpose: To verify the effect of ultrasonic vibration on eliminating pores in semi-solid forming.

[0051] Implementation steps: The blank prepared in Example 1 was heated to 580° C. (solid fraction 60%) and placed in a friction stir forming mold.

[0052] During the forming process, 30kHz ultrasonic vibration (amplitude 10μm) was applied, and the strain rate , forming temperature 280℃, compared with the formed parts without applying ultrasonic waves.

[0053] Implementation effect:

[0054] The density of the formed parts is 99.1%, and the porosity is ≤0.5%, which is significantly improved compared with the non-ultrasonic process (porosity 8%), solving the problem of porosity defects.

[0055] The surface roughness Ra=1.2μm, meeting the appearance requirements of 3C product plates (wall thickness 0.3mm).

[0056] Example 5

[0057] This embodiment provides a preparation process for a superplastic magnesium alloy, which is used for implementing a gradient temperature field annealing joint strengthening process. The specific implementation content includes: Implementation purpose: Based on the ultrafine-grained magnesium alloy billet of Example 1, verify the effect of the gradient temperature field on improving the shear strength of the joint position of the magnesium alloy sheet component.

[0058] Implementation steps: The homogenized ultrafine-grained billet (50 mm×20 mm×10 mm) treated with S3+S4 in Example 1 was taken, and the surface to be welded was polished with 2000# sandpaper to a roughness of Ra≤3.2 μm.

[0059] A segmented heating mold was used to increase the welding interface temperature from 300°C (edge) to 380°C (center). A compressive stress of 12 MPa was applied for annealing for 18 minutes, and the shear strength was tested.

[0060] Implementation effect:

[0061] The shear strength of the magnesium alloy sheet component joint is 28.5MPa, which is 12.3% higher than that of uniform temperature annealing (25.37MPa), breaking through the existing technical strength limit.

[0062] The grain size deviation is 5%, and the width of the heat-affected zone is 100 μm, which is 30% less than that of conventional annealing, proving the synergistic strengthening effect of the temperature field and compressive stress.

[0063] Comparative Example 1

[0064] This comparative example provides a traditional remelting casting recycling process.

[0065] Purpose of comparison: To highlight the performance and energy consumption advantages of the process of the present invention in waste recycling.

[0066] Implementation steps: The AZ61 scrap was remelted at 700℃ in a resistance furnace and refined and degassed by adding 0.5% hexachloroethane.

[0067] The casting was carried out into Φ50mm ingots, which were then annealed at 400℃ for 2h after cooling in the furnace.

[0068] Comparison results: The grain size is 18.5 μm, which is 9 times larger than that of Example 1 (2.1 μm) of the present invention, verifying the coarse grain problem of the traditional process.

[0069] The elongation at 250°C is 18%, and the tensile strength is 180 MPa, which are significantly lower than those of the process of the present invention (elongation 280%, strength 220 MPa).

[0070] The energy consumption is 2200 kWh / ton, which is 83% higher than that of the present invention (1200 kWh / ton), and no oxide slag waste is produced, highlighting the green advantage of the present invention.

[0071] Principle of the invention: Grain refinement synergistic mechanism Low-temperature ball milling activation: Ball milling at ≤200℃ introduces high-density dislocations and lattice distortion, which provides nucleation driving force for SPS sintering. After ball milling, the specific surface area of ​​the nanopowder increases, promoting sintering densification.

[0072] SPS dynamic recrystallization: During the low-temperature sintering process at 350-400℃, the pulse current accelerates atomic diffusion, and the Mg-Zn-Ca alloying inhibits grain growth, achieving simultaneous "sintering-refinement".

[0073] Severe plastic deformation of ECAP / CEC: Through four passes of ECAP and CEC shear deformation, the grains are broken into submicron size along the extrusion direction, and dynamic recrystallization makes the grains equiaxed.

[0074] Electroplastic effect assistance: The pulse current reduces the resistance to dislocation movement, activates the non-basal slip system, and cooperates with the deformation energy storage of ECAP to further refine the grain size to 1.5-2.1μm.

[0075] Superplasticity activation mechanism Grain refinement: Ultrafine grains of 1.5-2.1 μm provide more grain boundary sliding paths, and the proportion of grain boundary sliding increases to more than 60%.

[0076] Temperature-rate synergy: At a deformation temperature of 300°C, the critical shear stress of the non-basal slip system decreases to a value close to that of the basal slip system, and the strain rate When the strain rate sensitivity index m≥0.3, superplastic activation is achieved.

[0077] Green energy-saving principle Low-temperature process chain: ≤200℃ ball milling, 350-400℃ SPS sintering, and 300℃ ECAP extrusion constitute a low-temperature process chain, which greatly reduces energy consumption compared with traditional high-temperature processes (700℃ remelting, 400℃ annealing).

[0078] Optimization of electroplastic energy consumption: Pulse current reduces deformation resistance by 30% during the ECAP process, reduces mechanical work consumption, simultaneously activates superplasticity, and avoids additional heat treatment energy consumption.

[0079] Compared with Examples 1-5 and Comparative Example 1, the present invention achieves significant grain refinement through the multi-field coupling process: In Example 1, “low-temperature ball milling (≤200°C) + spark plasma sintering (SPS) + equal channel angular pressing (ECAP) + electroplastic assistance” was used to refine the AZ61 scrap grains from the original 20.67 μm to 2.1 μm. The refinement mechanism included mechanical activation, sintering densification, plastic deformation and electrical activation.

[0080] In Example 2, reciprocating extrusion (CEC) is added after ECAP, and dynamic recrystallization is induced by severe shear deformation. The grains are further refined to 1.5 μm, and the strengthening phase is evenly distributed at the grain boundaries.

[0081] In Example 3, pulsed electric field assisted annealing (250° C., 15 V / mm) was used to promote grain boundary migration, so that the grains were homogenized to 1.8 μm, with a size deviation of ≤8%.

[0082] Comparative Example 1 uses traditional remelting casting, and the high temperature (700°C) causes the grains to coarsen to 18.5μm, verifying the coarse grain defect of the traditional process.

[0083] The process of the present invention significantly improves the superplasticity of magnesium alloys: The elongation of Examples 1-2 at 250° C. is 275%-290%, and the strain rate sensitivity index m is 0.34-0.38, which meets the definition of superplasticity; the elongation of Comparative Example 1 is only 18%, and there is no superplasticity.

[0084] Example 4: At 400°C, The elongation is still maintained at 205% at high strain rate, m=0.32, which proves the superplastic stability at high strain rate.

[0085] In Example 5, the shear strength of the joint of the magnesium alloy component reaches 28.5 MPa through gradient temperature field annealing (300-380°C gradient + 12 MPa compressive stress).

[0086] The present invention realizes green preparation: The energy consumption of the entire process is 1180-1200kWh / ton, which is 45% lower than that of comparative example 1 (2200kWh / ton). The waste recovery rate reaches 96% (only 75% in the comparative example), and there is no harmful emission such as oxidized slag.

[0087] The energy consumption of pulse electric field annealing in Example 3 is reduced by 45.2% compared with traditional annealing.

[0088] Innovation in waste recycling: Low-temperature ball milling (≤200°C) prevents magnesium oxidation, combined with SPS sintering (350-400°C) to achieve solid-state regeneration and retain superplasticity; traditional remelting casting causes performance degradation due to high temperatures.

[0089] Multi-field coupling synergy: electroplasticity assistance ( Pulse current) reduces deformation resistance by 30%, and combined with gradient temperature field control, it achieves low-temperature superplastic activation at 300℃, which is more than 100℃ lower than the traditional process temperature.

[0090] Improved forming accuracy: Semi-solid forming + ultrasonic vibration (30kHz) eliminates pores, achieving a density of 99%, and capable of producing 0.3mm thin-walled parts to meet the needs of 3C products.

[0091] The above embodiments may be implemented in whole or in part through software, hardware, firmware or any other combination. When implemented using software, the above embodiments may be implemented in whole or in part in the form of a computer program product.

[0092] Those skilled in the art will appreciate that the modules and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented with electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of this application.

[0093] In addition, each functional module in each embodiment of the present application may be integrated into one processing module, or each module may exist physically separately, or two or more modules may be integrated into one module.

[0094] The above is only a specific implementation method of the present application, but the scope of protection of the present application is not limited to this. Any technician familiar with this technical field can easily think of changes or replacements within the technical scope disclosed in this application, which should be included in the scope of protection of the present application. Therefore, the scope of protection of the present application should be based on the scope of protection of the claims.

[0095] Finally: The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A process for preparing a superplastic magnesium alloy, characterized in that: The following steps are involved: S1: Processing magnesium alloy scrap into nano-scale powder by cryogenic ball milling, wherein the cryogenic ball milling temperature is ≤200°C, the powder particle size after ball milling is ≤100nm, and argon gas is introduced during the ball milling process to inhibit oxidation; S2: mixing the nano-scale powder of step S1 with 5-8% by mass of Mg-Zn-Ca alloy powder, densifying at 350-400°C by spark plasma sintering (SPS), and then keeping the temperature to obtain a sintered blank with a density of ≥98% after sintering; S3: The sintered billet is subjected to equal channel angular extrusion (ECAP) treatment with four extrusion passes to obtain an ultrafine-grained magnesium alloy. A pulse current is applied during the ECAP extrusion process.

2. A process for preparing a superplastic magnesium alloy according to claim 1, characterized in that: The magnesium alloy scrap is AZ61 magnesium alloy cutting chips, the Mg-Zn-Ca alloy is Mg-3Zn-1Ca, and the balance is Mg, and grain refinement is promoted by alloying.

3. The process for preparing a superplastic magnesium alloy according to claim 1, wherein: After the ECAP treatment in step S3, a reciprocating extrusion CEC process is performed, and the strain rate Through dynamic recrystallization, the grain size is further refined from the average grain size of 2.1μm to 1.5μm.

4. The process for preparing a superplastic magnesium alloy according to claim 1, wherein: Also includes: S4: Pulsed electric field assisted annealing step, annealing temperature 250 ° C, holding time 20 minutes, electric field strength 15V / mm, which makes the grains uniform and reduces energy consumption by 45% compared with traditional annealing. After annealing, the grain size deviation is ≤8%, and a uniform ultrafine-grained billet is produced.

5. The process for preparing a superplastic magnesium alloy according to claim 1, wherein: The superplastic magnesium alloy has a strain rate sensitivity index m=0.35, an elongation of 280% at 250° C., and a tensile strength of 220 MPa.

6. The process for preparing a superplastic magnesium alloy according to claim 1, wherein: The energy consumption of SPS sintering in step S2 is 800±20 kWh / ton.

7. A process for preparing a superplastic magnesium alloy according to claim 4, characterized in that: Also includes: S5: Based on the homogenized ultrafine grain billet after the treatment in step S4, semi-solid superplastic forming is performed. The billet is heated to a semi-solid temperature range of 580°C. A stir friction assisted plastic forming process is used. The friction heat generated by the rotation of the stirring head is used to maintain the forming temperature of 280°C. At the same time, mechanical stirring is used to promote the uniform flow of the semi-solid slurry. The forming strain rate is ; Relying on the stirring effect to suppress cracks and pores during the forming process, a magnesium alloy sheet component with a wall thickness of 0.3mm and a surface roughness of Ra≤1.6μm was obtained.

8. A process for preparing a superplastic magnesium alloy according to claim 7, characterized in that: During the semi-solid superplastic forming process, 30kHz ultrasonic vibration with an amplitude of 8μm is applied to eliminate pore defects and make the density of the magnesium alloy plate component reach 99%.

9. The process for preparing a superplastic magnesium alloy according to claim 1, wherein: The energy consumption of the entire process is 1200±50kWh / ton, and there is no emission of harmful gases such as fluoride.

10. The process for preparing a superplastic magnesium alloy according to claim 4, wherein: Also includes: The gradient temperature field annealing step is implemented after step S4, and performs local treatment on the joint area formed when the magnesium alloy plate components are connected. A combination of an induction heating coil and a magnetic conductor is used to form a temperature gradient of 50-200°C / mm in the joint area of ​​the magnesium alloy plate components, with a center temperature of 300-400°C and an edge temperature of 200-300°C, and the temperature is kept for 5-30 minutes; a density of The pulse current relies on the uniform ultrafine grain structure formed by S4 as the matrix, solute atoms migrate directionally along the temperature gradient direction, forming a nanocrystalline-ultrafine grain gradient structure, and the tensile strength of the joint is increased to more than 28MPa.

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