A process for preparing a superplastic magnesium alloy

Abstract

The application discloses a preparation process of superplastic magnesium alloy, which comprises the following steps: magnesium alloy waste is treated by low-temperature ball milling to prepare nanoscale powder, the low-temperature ball milling temperature is less than or equal to 200 DEG C, the powder particle size after ball milling is less than or equal to 100 nm, and argon gas is introduced to protect the ball milling process to inhibit oxidation; the nanoscale powder is mixed with 5-8% mass ratio of Mg-Zn-Ca alloy powder, densification is carried out by adopting spark plasma sintering (SPS) at 350-400 DEG C, the sintering pressure is 20 MPa, the holding time is 10 min, and the density after sintering is greater than or equal to 98%; the sintered blank is subjected to equal channel angular pressing (ECAP) treatment, the extrusion temperature is 300 DEG C, the extrusion pass is 4 times, the mold channel included angle is 90 DEG, and the superfine-grained magnesium alloy with an average grain size of 2.1 mu m is obtained; the application realizes grain superfine of 1.5-2.1 mu m through a multi-field coupling process, activates a non-basal slip system at room temperature, the strain rate sensitivity index m is greater than or equal to 0.35, the elongation at 250 DEG C reaches 280%, and the superplastic magnesium alloy has high strength and superplastic deformation capacity.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of magnesium alloy preparation, and particularly relates to a preparation process of superplastic magnesium alloy. BACKGROUND

[0002] As a lightweight engineering material, the existing preparation process of magnesium alloy mainly includes remelting casting, hot extrusion, conventional annealing and deep plastic deformation (such as ECAP). When the traditional remelting casting recycles waste materials, it needs to be treated at a high temperature above 700 DEG C, which leads to serious grain coarsening (such as the grain size of AZ61 magnesium alloy reaching 18.5 μm) and obvious performance degradation. Although the hot extrusion and conventional annealing process can realize a certain degree of grain refinement, the annealing temperature usually needs to be above 400 DEG C, the energy consumption is as high as 2500 kWh / ton, and only 2 basal slip systems can be started at room temperature, the strain rate sensitivity index m is only 0.21, and the superplastic activation effect is limited. Although the deep plastic deformation process can refine the grains, it is difficult to meet the requirements of green preparation and high performance without combining with waste material recycling and multi-field coupling technology.

[0003] From the existing technology, the core deficiency of the prior art is that:

[0004] 1. The grain refinement effect is limited, and the conventional process is difficult to control the grain size below 5 μm, such as the metallographic graph shown in FIG. 1, the grain size is about 20 μm or even larger, which leads to poor room temperature plasticity (elongation is only 15-17%), and cannot meet the requirements of superplastic forming; Figure 3

[0005] 2. The energy consumption of waste material recycling is high and the performance degradation is significant, the recycling rate of remelting casting is only 75%, and the elongation of the recycled material is less than 20%;

[0006] 3. The superplastic activation temperature is high and the energy consumption is large, the traditional process needs to be annealed at above 400 DEG C, and it is difficult to realize low-temperature and high-efficiency forming.

[0007] Therefore, we propose a preparation process of superplastic magnesium alloy. SUMMARY

[0008] In order to overcome the above-mentioned defects of the prior art, the application provides a preparation process of superplastic magnesium alloy, which solves the problems of grain coarsening, poor performance of waste material recycling and high energy consumption of superplastic activation in the traditional process.

[0009] In order to achieve the above-mentioned purpose, the application provides the following technical scheme:

[0010] A preparation process of superplastic magnesium alloy, comprising the following steps:

[0011] ​S1: the magnesium alloy waste is treated by low-temperature ball milling to form a nano-sized powder, the low-temperature ball milling temperature is ≤200℃, the ball-to-material ratio is 10:1, the rotating speed is 200 rpm, the powder particle size after ball milling is ≤100 nm, and argon gas is introduced during the ball milling process to inhibit oxidation.

[0012] The low-temperature ball milling introduces high-density dislocations (1.5eV / atom) and lattice distortion (1.5eV / atom), which provides a nucleation driving force for subsequent sintering, and reduces the oxidation inclusion rate by 80% compared with traditional ball milling (300℃).

[0013] S2: the nano-sized powder of step S1 is mixed with 5-8% mass ratio of Mg-Zn-Ca alloy powder, and is densified by spark plasma sintering (SPS) at 350-400℃, the sintering pressure is 20 MPa, and the holding time is 10 min, to obtain a sintered blank with a sintering density of ≥98%.

[0014] Zn forms MgZn strengthening phase (lattice mismatch degree 12%) to pin the grain boundary, and Ca generates phase (melting point 720℃) to inhibit grain growth, and the sintering density reaches 98.5%, so that the grain size is controlled within 1μm.

[0015] S3: the sintered blank is subjected to equal channel angular pressing (ECAP) treatment, the extrusion temperature is 300℃, the extrusion pass is 4 times, the die channel angle is 90°, and an ultra-fine-grained magnesium alloy with an average grain size of 2.1μm is obtained, a pulse current is applied during the ECAP extrusion process, the current density is , the pulse frequency is 5 kHz, and the duty cycle is 1:1.

[0016] 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 the grain refinement is promoted by alloying.

[0017] The Zn content of AZ61 is insufficient to form an effective strengthening phase, and the Mg-Zn-Ca alloy is added to increase the Zn content to a critical value (more than 2%) and introduce Ca to inhibit grain growth, and both are indispensable, for example, if only Zn powder is added without Ca, the grain size still reaches 2.5μm, and the elongation is 200%.

[0018] Preferably, the CEC process is performed after the ECAP treatment in step S3, the strain rate is , and the grain size is further refined to 1.5μm through dynamic recrystallization.

[0019] Preferably, it further comprises:

[0020] ​S4: pulse electric field assisted annealing step, annealing temperature 250 DEG C, holding time 20 min, electric field intensity 15V / mm, making grain homogenization and energy consumption reducing 45% than traditional annealing, grain size deviation after annealing ≤8%, preparing homogenized ultrafine grain blank.

[0021] Preferably, the strain rate sensitivity index m of the superplastic magnesium alloy is 0.35, the elongation at 250 DEG C reaches 280%, and the tensile strength is 220 MPa.

[0022] Preferably, the SPS sintering energy consumption in step S2 is 800±20 kWh / ton.

[0023] Preferably, the method further comprises:

[0024] S5: based on the homogenized ultrafine grain blank after step S4, semi-solid superplastic forming is carried out, the blank is heated to a semi-solid temperature interval of 580 DEG C, a friction stir assisted plastic forming process is adopted, friction heat generated by rotation of a stirring head is used to maintain a forming temperature of 280 DEG C, and mechanical stirring is used to promote uniform flow of the semi-solid slurry, and the forming strain rate is ; cracks and pores in the forming process are inhibited by relying on stirring, and a magnesium alloy plate member with a wall thickness of 0.3 mm is obtained, and the surface roughness Ra is ≤1.6 mu m.

[0025] Preferably, ultrasonic vibration with an amplitude of 8 mu m is applied in the semi-solid superplastic forming process, and pores are eliminated, so that the density of the member reaches 99%.

[0026] Preferably, the total process energy consumption is 1200±50 kWh / ton, and no harmful gas such as fluoride is discharged.

[0027] Preferably, the method further comprises:

[0028] The gradient temperature field annealing step is implemented after step S4, and is locally processed on a joint area formed when the magnesium alloy plate member is connected, a temperature gradient of 50-200 DEG C / mm is formed on the joint area of the magnesium alloy plate member by using a combination of an induction heating coil and a magnetic conductor, the center temperature is 300-400 DEG C, the edge temperature is 200-300 DEG C, and the holding time is 5-30 min; a pulse current with a density of is synchronously applied, the homogenized ultrafine grain structure formed in S4 is used as a matrix, solute atoms are directionally migrated along the temperature gradient direction, a nano-crystal-ultrafine grain gradient structure is formed, and the tensile strength of the joint is increased to more than 28 MPa.

[0029] The technical effects and advantages of the preparation process of the superplastic magnesium alloy are as follows:

[0030] 1. The application significantly improves material performance, realizes grain ultra-fining (1.5-2.1 μm) through multi-field coupling process, breaks through the bottleneck of traditional process grain coarsening (20 μm or more), activates non-basal slip system at room temperature, strain rate sensitivity index m≥0.35, elongation at 250℃ reaches 280%, and has high strength and superplastic deformation capacity.

[0031] 2. The application realizes green preparation and high-efficiency recovery: low-temperature process chain (≤400℃) reduces energy consumption of the whole process by more than 40% compared with traditional process, waste recovery rate reaches 96% (increases by 21%), simultaneously solves the performance degradation problem caused by high-temperature remelting, and realizes green recycling of magnesium alloy.

[0032] 3. The application can prepare 0.3 mm thin-walled parts by combining semi-solid forming with ultrasonic vibration, and the surface precision reaches Ra≤1.6 μm; gradient temperature field annealing makes the joint shear strength increase to 28 MPa, and the heat affected zone width is reduced by 30%. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 is a system flowchart of the preparation process of the superplastic magnesium alloy proposed by the application;

[0034] Figure 2 is a metallographic diagram of the magnesium alloy component prepared by the preparation process of the application;

[0035] Figure 3 is a metallographic diagram of the magnesium alloy component prepared by the conventional process. DETAILED DESCRIPTION

[0036] The technical solutions in the embodiments of the application will be described clearly and completely below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments of the application. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the application,

[0037] It should be noted that, in this article, the relationship terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between the entities or operations, and the terms "include", "contain" or any other variants thereof are intended to cover non-exclusive inclusion, so that the process, method, article or equipment including a series of elements not only includes those elements, but also includes other elements not explicitly listed or inherent to such process, method, article or equipment, and the elements defined by the statement "include" without more limitation do not exclude the presence of other identical elements in the process, method, article or equipment including the elements.

[0038] Embodiment 1

[0039] Reference Figure 1 This embodiment provides a preparation process of superplastic magnesium alloy, for the implementation of AZ61 waste superplastic regeneration process, the specific implementation includes:

[0040] The purpose of the implementation is to verify the superplastic regeneration effect of the "low-temperature ball milling-SPS-ECAP-electroplasticity assisted" whole process on waste.

[0041] Implementation steps:

[0042] S1: processing, taking 10 kg of AZ61 magnesium alloy cutting chips, ball milling in QM-3SP2 planetary ball mill with ball-to-material ratio of 10:1, 200 rpm rotation speed, temperature control 180℃, argon gas flow 5L / min, obtaining powder with average particle size of 80nm.

[0043] S2: sintering, adding 0.6 kg of Mg-3Zn-1Ca alloy powder mixture, sintering in SPS-330MKII equipment at 380℃, 20MPa pressure for 10min, the density after sintering is 98.5%, and the energy consumption is 780kWh / ton.

[0044] S3: extrusion, extruding in ECAP die (channel angle 90°) at 300℃ temperature for 4 passes, extrusion speed 5mm / min, obtaining billet with average grain size of about 2.1μm as shown in Figure 2 Figure 2 is a microstructure diagram under the scale of 20μm, i.e. enlarged by 1000 times.

[0045] Electroplasticity assistance: applying pulse current during extrusion, frequency 5kHz, the measured deformation resistance is reduced by 28%.

[0046] Implementation effect: ​

[0047] 250℃ tensile test elongation of 275%, strain rate sensitivity index m = 0.34, meet the definition of superplasticity.

[0048] The whole process energy consumption is 1180 kWh / ton, which is reduced by 52.8% compared with the traditional process (700℃ remelting casting as the core of magnesium alloy recycling method), waste recovery rate is 96%, and no oxidation slag is produced.

[0049] Example 2

[0050] The embodiment provides a preparation process of superplastic magnesium alloy, and is used for implementation of a reciprocating extrusion (CEC) grain refinement process, and the implementation content includes:

[0051] Implementation purpose: verify the further refinement effect of CEC on the grain after ECAP.

[0052] Implementation steps:

[0053] The ECAP blank (grain size 2.1 μm) is prepared according to steps S1-S3 of the embodiment 1.

[0054] The CEC die (pass angle 90°) is used, and the 2-pass reciprocating extrusion is carried out at 300℃ temperature and The argon gas is introduced during the extrusion process.

[0055] Implementation effect:

[0056] The grain size is further refined to 1.5 μm, and the phase at the grain boundary is uniformly and dispersedly distributed, and the grain uniformity is improved by 30% compared with that after ECAP treatment. The outer circumferential elongation reaches 290% in the 250℃ compression test, and the strain rate sensitivity index m = 0.38, which proves the synergistic refinement effect of CEC and ECAP.

[0057] Example 3

[0058] The embodiment provides a preparation process of superplastic magnesium alloy, and is used for implementation of a pulse electric field assisted annealing energy-saving process, and the specific implementation content includes:

[0059] Implementation purpose: verify the energy-saving effect and grain uniformization effect of the pulse electric field annealing.

[0060] Implementation steps:

[0061] The ECAP blank in the embodiment 1 is taken, and is cut into 10mm*10mm*10mm samples.

[0062]

[0063] ​The 15V / mm electric field intensity is applied in the Gleeble-1500 thermal simulation machine, and the pulse frequency is 10 kHz, and the traditional annealing process (300 DEG C * 60 min, no electric field) is used.

[0064] Implementation effect:

[0065] The actual energy consumption of annealing is 850 kWh / ton, which is reduced by 45.2% compared with the traditional annealing (1550 kWh / ton of magnesium alloy recycling method based on 700 DEG C remelting casting), which meets the energy saving demand.

[0066] The grain size deviation is reduced from 22% of the traditional annealing (1550 kWh / ton of magnesium alloy recycling method based on 700 DEG C remelting casting) to 8%, and the average grain size is 1.8 μm, and the microstructure uniformity is significantly improved.

[0067] Example 4

[0068] The embodiment provides a preparation process of superplastic magnesium alloy, which is used for the implementation of a semi-solid forming+ultrasonic vibration composite process, and the specific implementation content comprises:

[0069] Implementation purpose: verify the effect of ultrasonic vibration on eliminating pores in semi-solid forming.

[0070] Implementation steps:

[0071] The blank prepared in example 1 is heated to 580 DEG C (solid phase rate 60%), and is put into a friction stir forming die.

[0072] 30 kHz ultrasonic vibration (amplitude 10 μm) is applied in the forming process, the strain rate , the forming temperature is 280 DEG C, and the forming part without applying ultrasonic vibration is compared.

[0073] Implementation effect:

[0074] The compactness of the forming part is 99.1%, and the porosity is less than or equal to 0.5%, which is significantly improved compared with the process without ultrasonic wave (porosity 8%), and the pore defect problem is solved.

[0075] The surface roughness Ra is 1.2 μm, which meets the appearance requirement of 3C product plate (wall thickness 0.3 mm).

[0076] Example 5

[0077] The embodiment provides a preparation process of superplastic magnesium alloy, which is used for the implementation of a semi-solid forming+ultrasonic vibration composite process, and the specific implementation content comprises:

[0078] Implementation purpose: based on the ultra-fine grain magnesium alloy blank of example 1, verify the promotion effect of gradient temperature field on the shear strength of the joint position of magnesium alloy plate component.

[0079] Implementation steps:

[0080] Take the homogenized ultra-fine grain blank (50mmx20mmx10mm) treated by S3+S4 in Example 1, and polish the welding surface to a roughness Ra≤3.2μm with 2000# sandpaper.

[0081] A segmented heating mold is used to make the welding interface temperature gradient rise from 300℃ (edge) to 380℃ (center), and a 12MPa compressive stress annealing is applied for 18min, and the shear strength is tested.

[0082] Implementation effect:

[0083] The shear strength of the magnesium alloy plate component joint position is 28.5MPa, which is 12.3% higher than that of the uniform temperature annealing (25.37MPa), and breaks through the upper limit of the strength of the prior art.

[0084] The grain size deviation is 5%, and the heat-affected zone width is 100μm, which is 30% less than that of the traditional annealing, proving the synergistic strengthening effect of temperature field and compressive stress.

[0085] Comparative Example 1

[0086] This comparative example provides a traditional remelting casting recycling process.

[0087] Comparison purpose: to highlight the performance and energy consumption advantages of the process of the present application in waste recycling.

[0088] Implementation steps:

[0089] The AZ61 waste material is remelted at 700℃ in a resistance furnace, and 0.5% hexachloroethane is added for refining and degassing.

[0090] Pouring into a Φ50mm ingot, and after furnace cooling, annealing at 400℃x2h.

[0091] Comparison results:

[0092] The grain size is 18.5μm, which is 9 times larger than that of Example 1 of the present application (2.1μm), verifying the problem of grain coarsening in traditional processes.

[0093] The elongation at 250℃ is 18%, and the tensile strength is 180MPa, which is significantly lower than that of the process of the present application (elongation 280%, strength 220MPa).

[0094] The energy consumption is 2200kWh / ton, which is 83% higher than that of the present application (1200kWh / ton), and generates oxidized slag waste, highlighting the green advantage of the present application.

[0095] Invention principle:

[0096] Grain refinement synergistic mechanism

[0097] Low-temperature ball milling activation: ≤200℃ ball milling introduces high-density dislocations and lattice distortion, providing nucleation driving force for SPS sintering. The specific surface area of the nano-powder increases after ball milling, promoting sintering densification.

[0098] SPS dynamic recrystallization: During the low-temperature sintering process at 350-400℃, the pulsed current accelerates atomic diffusion, and Mg-Zn-Ca alloying inhibits grain growth, realizing simultaneous "sintering-fining".

[0099] ECAP / CEC severe plastic deformation: Through 4 passes of ECAP and CEC shear deformation, the grains are broken into submicron along the extrusion direction, and dynamic recrystallization makes the grains equiaxed.

[0100] Electroplastic effect assistance: Pulsed current reduces the resistance of dislocation movement, enabling non-basal slip systems to start, further refining the grains to 1.5-2.1μm in cooperation with the deformation energy storage of ECAP.

[0101] Superplastic activation mechanism

[0102] Fine-grain strengthening: The ultra-fine grain structure of 1.5-2.1μm provides more grain boundary slip paths, and the proportion of grain boundary slip increases to more than 60%.

[0103] Temperature-rate synergy: At a deformation temperature of 300℃, the critical shear stress of non-basal slip systems is reduced to close to that of basal slip systems, and the strain rate The strain rate sensitivity index m≥0.3, realizing superplastic activation.

[0104] Green energy-saving principle

[0105] Low-temperature process chain: ≤200℃ ball milling, 350-400℃ SPS sintering, and 300℃ ECAP extrusion form a low-temperature process chain, which significantly reduces energy consumption compared to traditional high-temperature processes (700℃ remelting, 400℃ annealing).

[0106] Electroplastic energy consumption optimization: Pulsed current reduces the deformation resistance by 30% during ECAP, reducing mechanical work consumption, synchronously activating superplasticity, and avoiding additional heat treatment energy consumption.

[0107] Compared with Examples 1-5 and Comparative Example 1, the present application realizes significant grain refinement through multi-field coupling process:

[0108] Example 1 uses "low-temperature ball milling (≤200℃) + spark plasma sintering (SPS) + equal channel angular pressing (ECAP) + electroplasticity assistance", which refines the AZ61 scrap grain from the original 20.67μm to 2.1μm. The refinement mechanism includes mechanical activation, sintering densification, plastic deformation, and electro-activation.

[0109] Example 2 increased the reciprocating extrusion (CEC) after ECAP, used severe shear deformation to initiate dynamic recrystallization, and the grains were further refined to 1.5 microns, and the strengthening phase at the grain boundary was evenly divided.

[0110] Example 3 promotes grain boundary migration by pulse electric field assisted annealing (250℃, 15V / mm), makes the grain uniform to 1.8 microns, and the size deviation is ≤8%.

[0111] Comparative Example 1 uses traditional remelting casting, high temperature (700℃) leads to grain coarsening to 18.5 microns, and verifies the grain coarsening defect of traditional process.

[0112] The process of the application significantly improves the superplasticity of magnesium alloy:

[0113] Example 1-2 has an elongation of 275%-290% at 250℃, and the strain rate sensitivity index m=0.34-0.38, which meets the definition of superplasticity; Comparative Example 1 has an elongation of only 18%, and has no superplasticity.

[0114] Example 4 has an elongation of 205% at 400℃, and still maintains an elongation of 205% at a high strain rate, m=0.32, which proves the stability of high strain rate superplasticity.

[0115] Example 5 connects the head position of the magnesium alloy component by gradient temperature field annealing (300-380℃ gradient + 12MPa compressive stress), and the shear strength reaches 28.5MPa.

[0116] The application realizes green preparation:

[0117] The energy consumption of the whole process is 1180-1200kWh / ton, which is reduced by 45% compared with Comparative Example 1 (2200kWh / ton), the waste recovery rate reaches 96% (Comparative Example only 75%), and there is no harmful emission such as oxidation slag.

[0118] The energy consumption of pulse electric field annealing in Example 3 is reduced by 45.2% compared with traditional annealing.

[0119] Waste recycling innovation: low temperature ball milling (≤200℃) avoids magnesium oxidation, and cooperates with SPS sintering (350-400℃) to realize solid regeneration and retain superplasticity; traditional remelting casting leads to performance degradation due to high temperature.

[0120] Multi-field coupling synergy: electric plasticity assistance ( Pulse current) reduces the deformation resistance by 30%, combined with gradient temperature field regulation, realizes low temperature superplasticity activation at 300℃, and reduces the temperature by more than 100℃ compared with traditional process.

[0121] Improved forming accuracy: Semi-solid forming + ultrasonic vibration (30kHz) eliminates pores, achieving a density of 99%, enabling the fabrication of 0.3mm thin-walled parts to meet the needs of 3C products.

[0122] The above embodiments can be implemented in whole or in part by software, hardware, firmware or other arbitrary combinations. When implemented by software, the above embodiments can be implemented in whole or in part in the form of a computer program product.

[0123] Those skilled in the art will recognize that the modules and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application.

[0124] In addition, the functional modules in the various embodiments of this application can be integrated into one processing module, or each module can exist physically separately, or two or more modules can be integrated into one module.

[0125] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of protection of the claims.

[0126] In conclusion, 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 within the protection scope of the present invention.

Claims

1. A process for preparing a superplastic magnesium alloy, characterized by, The method comprises the following steps: S1: magnesium alloy waste is treated by low-temperature ball milling to form nanoscale powder, the low-temperature ball milling temperature is ≤200℃, the powder particle size after ball milling is ≤100nm, and argon gas is introduced for protection during the ball milling process to inhibit oxidation; S2: the nanoscale powder of step S1 is mixed with 5-8% mass ratio of Mg-Zn-Ca alloy powder, and is densified at 350-400℃ by using spark plasma sintering (SPS), and then is kept warm to obtain a sintered blank with a density ≥98% after sintering; S3: the sintered blank is subjected to equal channel angular pressing (ECAP) treatment, and the extrusion pass is 4 times to obtain an ultrafine-grained magnesium alloy, and a pulse current is applied during the ECAP extrusion process; S4: pulse electric field assisted annealing step, annealing temperature 250℃, holding time 20min, electric field strength 15V / mm, grain homogenization and energy consumption reduced by 45% compared with traditional annealing, grain size deviation ≤8% after annealing, and a homogenized ultrafine-grained blank is obtained.

2. The process for preparing a superplastic magnesium alloy according to claim 1, wherein The magnesium alloy waste 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 superplastic magnesium alloy as claimed in claim 1 wherein, The CEC process of reciprocating extrusion after the ECAP process in step S3 is performed at a strain rate The grain size is further refined from the average grain size of 2.1 pm to 1.5 pm by dynamic recrystallization.

4. The process for preparing superplastic magnesium alloy according to claim 1, wherein The strain rate sensitivity index m of the superplastic magnesium alloy is 0.35, the elongation at 250℃ reaches 280%, and the tensile strength is 220MPa.

5. The process for preparing superplastic magnesium alloy as claimed in claim 1 wherein, The energy consumption of SPS sintering in step S2 is 800±20kWh / ton.

6. The process for preparing superplastic magnesium alloy according to claim 1, wherein Further comprising: S5: based on the homogenized ultra-fine grain blank processed in step S4, semi-solid superplastic forming is performed, the blank is heated to a semi-solid temperature interval of 580℃, a friction stir assisted plastic forming process is adopted, friction heat generated by rotation of a stirring head is used to maintain a forming temperature of 280℃, and mechanical stirring is used to promote uniform flow of the semi-solid slurry, a forming strain rate of 1.0×10-3s-1, a forming pressure of 50MPa, and a forming time of 30min are adopted, and a magnesium alloy plate member with a wall thickness of 0.3mm is obtained. Cracks and pores in the forming process are inhibited by relying on stirring, and a magnesium alloy plate member with a wall thickness of 0.3mm and a surface roughness Ra≤1.6μm is obtained.

7. The process for preparing superplastic magnesium alloy as claimed in claim 6 wherein, The ultrasonic vibration with an amplitude of 8μm is applied during the semi-solid superplastic forming process to eliminate the pore defects, so that the density of the magnesium alloy plate member reaches 99%.

8. The process for preparing superplastic magnesium alloy as claimed in claim 1 wherein, The total process energy consumption is 1200±50kWh / ton, and no fluoride harmful gas is discharged.

9. The process for preparing superplastic magnesium alloy as claimed in claim 1 wherein, Further comprising: The gradient temperature field annealing step, implemented after step S4, is locally processed for the joint area formed when the magnesium alloy plate member is connected, uses an induction heating coil combined with a magnetic conductor to form a temperature gradient of 50-200℃ / mm in the joint area of the magnesium alloy plate member, with a center temperature of 300-400℃ and an edge temperature of 200-300℃, and a holding time of 5-30min; a pulse current with a density of is synchronously applied, the homogeneous ultra-fine grain structure formed in S4 is used as a matrix to make solute atoms migrate along the direction of the temperature gradient, forming a nano-crystal-ultra-fine grain gradient structure, and the joint tensile strength is increased to above 28MPa.

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