Hot working method for improving strength of rare earth magnesium alloy

Abstract

The invention discloses a hot working method for improving the strength of a rare earth magnesium alloy in the technical field of hot working. The hot working method comprises the steps that a rare earth magnesium alloy cast ingot is buried in graphite powder and placed in a resistance furnace to be subjected to solid solution heat treatment, and a solid solution magnesium alloy is obtained; carrying out thermoplastic deformation on the solid solution magnesium alloy to obtain a deformed magnesium alloy, and carrying out air cooling on the deformed magnesium alloy; and performing variable stress aging treatment on the deformed magnesium alloy to obtain the fine-grain strengthened magnesium alloy, unloading the fine-grain strengthened magnesium alloy after the variable stress aging treatment is finished, and performing air cooling on the fine-grain strengthened magnesium alloy. According to the method, pre-deformation and variable stress aging treatment are combined, the synergistic regulation effect of the pre-deformation and the variable stress aging treatment on a matrix structure and a precipitation structure is fully exerted, and the strength of the rare earth magnesium alloy is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of hot processing, and particularly relates to a hot processing method for improving the strength of rare earth magnesium alloys. Background Art

[0002] Compared with metal structural materials such as steel and aluminum alloys that have been applied in engineering, magnesium alloys have a lower density and are suitable materials for manufacturing components with high requirements for lightweight. Among various magnesium alloy systems, precipitation-strengthened rare earth magnesium alloys have attracted much attention due to their good room temperature strength and heat resistance.

[0003] However, due to insufficient mechanical properties of rare earth magnesium alloys, the working environment adaptability of cast magnesium alloy components is limited. Aging treatment, as a key subsequent process after plastic deformation processing of precipitation-strengthened magnesium alloys, can improve the strength and toughness of materials.

[0004] During the aging treatment process, the solubility and precipitation behavior of different alloying elements in the magnesium matrix are restricted by many factors other than temperature. However, conventional aging only focuses on the temperature factor and it is difficult to achieve the best strengthening effect. Summary of the Invention

[0005] The purpose of the present invention is to provide a hot processing method for improving the strength of rare earth magnesium alloys, which combines pre-deformation and variable stress aging treatment to fully exert the synergistic regulation effect of both on the matrix structure and precipitation structure, and improve the strength of rare earth magnesium alloys.

[0006] To achieve the above purpose, the first aspect of the present invention provides a hot processing method for improving the strength of rare earth magnesium alloys, including: Burying a rare earth magnesium alloy ingot in graphite powder and placing it in a resistance furnace for solution heat treatment to obtain a solution-state magnesium alloy; Performing hot plastic deformation on the solution-state magnesium alloy to obtain a deformed-state magnesium alloy and air-cooling the deformed-state magnesium alloy; Performing variable stress aging treatment on the deformed-state magnesium alloy to obtain a fine-grained strengthened magnesium alloy. The process includes: putting the deformed-state magnesium alloy into a preheated variable stress aging furnace for heat preservation, then applying a single axial compressive stress to the deformed-state magnesium alloy, and the compressive stress applied to the deformed-state magnesium alloy increases gradually with time within the set variable stress aging time, and a fine-grained strengthened magnesium alloy is obtained after the variable stress aging time ends; After the variable stress aging treatment is completed, unloading the fine-grained strengthened magnesium alloy and air-cooling the fine-grained strengthened magnesium alloy.

[0007] Further, burying a rare earth magnesium alloy ingot in graphite powder and placing it in a resistance furnace for solution heat treatment to obtain a solution-state magnesium alloy specifically includes: The rare earth magnesium alloy ingot is buried in graphite powder and placed in a resistance furnace, and the furnace temperature of the resistance furnace is controlled to rise from room temperature to the solution temperature; the solution temperature is 490 - 510 °C, the heating rate is 15 °C / min, and after the furnace temperature of the resistance furnace reaches the solution temperature, it is held for 20 - 24 h.

[0008] Further, the rare earth magnesium alloy ingot includes Mg element, Gd element and Ag element; the mass percentage of Gd element is 11%; the mass percentage of Ag element is 2%.

[0009] Further, the solution-state magnesium alloy is subjected to thermoplastic deformation to obtain a deformed-state magnesium alloy, specifically including: The solution-state magnesium alloy is subjected to continuous cooling equal-channel angular pressing using an extrusion die. The initial temperature of the extrusion die is 340 - 350 °C, the temperature of the extrusion die at the end of thermoplastic deformation is 300 - 310 °C, the extrusion rate is 3 - 4 mm / s, and the deformed-state magnesium alloy is obtained by continuously processing 8 - 10 passes of continuous cooling equal-channel angular pressing.

[0010] Further, the variable stress aging time is 25 - 35 h, the initial compressive stress is set to 290 - 300 MPa, and the compressive stress increases by 0.5 - 1.5 MPa per hour during the variable stress aging time.

[0011] Further, the solution-state magnesium alloy is subjected to thermoplastic deformation to obtain a deformed-state magnesium alloy, specifically including: The solution-state magnesium alloy is subjected to continuous cooling equal-channel angular pressing using an extrusion die. The initial temperature of the extrusion die is 340 - 350 °C, the temperature of the extrusion die at the end of thermoplastic deformation is 300 - 310 °C, the extrusion rate is 3 - 4 mm / s, and the processed-state magnesium alloy is obtained by continuously processing 8 - 10 passes of continuous cooling equal-channel angular pressing; The processed-state magnesium alloy after continuous cooling equal-channel angular pressing is cut into magnesium alloy plates using a wire electrical discharge machine. The magnesium alloy plates are placed in a resistance furnace with a furnace temperature of 450 - 470 °C and held for 4 - 5 min; the magnesium alloy plates are taken out of the resistance furnace and rolled. The rolling direction is parallel to the extrusion direction of the extrusion die; the reduction per pass of rolling is 0.2 - 0.3 mm; the deformed-state magnesium alloy is obtained after 12 - 14 passes of rolling, and the magnesium alloy plates are placed in a resistance furnace with a furnace temperature of 450 - 470 °C for annealing for 2 - 3 min between adjacent rolling passes.

[0012] Further, the direction of the compressive stress is parallel to the extrusion direction of the extrusion die.

[0013] Further, the preheating temperature of the variable stress aging furnace for the deformed-state magnesium alloy is set to 190 - 200 °C, and the preheating holding time is 3 min.

[0014] Further, the time of variable stress aging is 45 - 55 h, the initial compressive stress is set to 190 - 200 MPa, and the compressive stress increases by 2.5 - 3.5 MPa per hour during the variable stress aging time.

[0015] Further, the value of the initial compressive stress for the variable stress aging treatment is 89% - 90% of the room temperature compressive yield strength of the deformed magnesium alloy; the value of the final compressive stress is 89% - 90% of the room temperature compressive yield strength of the deformed magnesium alloy after aging treatment; the time and temperature of the variable stress aging treatment are the same as those of the aging treatment.

[0016] Compared with the prior art, the beneficial effects achieved by the present invention are as follows: In the present invention, through solution heat treatment, alloying elements are fully dissolved to obtain a uniform solid solution structure; then thermoplastic deformation treatment helps to break the coarse grains in the as-cast structure, refine the grains and improve the structural uniformity, thereby enhancing the plasticity of the alloy. Finally, variable stress aging treatment further promotes grain refinement and structural optimization, and at the same time produces favorable element segregation / precipitation inside the alloy. Under the synergistic effect of multiple processes, the mechanical properties such as strength, hardness, and plasticity of the rare earth magnesium alloy are comprehensively and significantly improved, meeting the usage requirements under complex working conditions.

[0017] In the present invention, after the deformed magnesium alloy is placed in a preheated variable stress aging furnace and kept warm, a single axial compressive stress is applied to the deformed magnesium alloy. The compressive stress applied to the deformed magnesium alloy increases step by step with time within the set variable stress aging time, and a fine grain strengthened magnesium alloy is obtained after the variable stress aging time ends; variable stress aging can additionally introduce vacancies and dislocations to promote the recovery process, or activate twinning to introduce twin boundaries, both of which can cause grain refinement; and these crystal defects can reduce the nucleation energy of nano-precipitates, help to reduce the size of precipitates and increase the precipitation density. Fine grains can increase the grain boundary area and hinder the movement of dislocations, thereby effectively improving the strength and hardness of the alloy. Description of the Drawings

[0018] Figure 1 It is a flow chart of the hot working method for improving the strength of the rare earth magnesium alloy in Example 1; Figure 2 It is a broken line graph of the change of compressive stress during the variable stress aging treatment provided in Example 1; Figure 3 It is the metallographic diagram, scanning electron microscope diagram and transmission electron microscope diagram of the high-strength magnesium alloy prepared in Example 1; Figure 4 It is a broken line graph of the change of compressive stress during the variable stress aging treatment provided in Example 2; Figure 5Metallographic diagrams, scanning electron microscope diagrams and transmission electron microscope diagrams of the high-strength magnesium alloy prepared in Example 2; Figure 6 Room temperature compression stress and strain curves of the high-strength magnesium alloy prepared in Example 1 and Example 2. Specific implementation mode

[0019] The present invention will be further described below with reference to the accompanying drawings. The following examples are only used to more clearly illustrate the technical solutions of the present invention and cannot be used to limit the protection scope of the present invention.

[0020] Example 1

[0021] As Figure 1 shown, this example provides a hot working method for improving the strength of rare earth magnesium alloy, including burying the rare earth magnesium alloy ingot in graphite powder and placing it in a resistance furnace for solution heat treatment to obtain a solution-state magnesium alloy, specifically including: The selected rare earth magnesium alloy ingot in this example includes Mg element, Gd element and Ag element; the mass percentage of Gd element is 11%; the mass percentage of Ag element is 2%. Bury the rare earth magnesium alloy ingot in graphite powder and place it in a resistance furnace, and control the furnace temperature of the resistance furnace to rise from room temperature to the solution temperature; the solution temperature is 490-510 °C, and the heating rate is 15 °C / min. After the furnace temperature of the resistance furnace reaches the solution temperature, keep it warm for 20-24 h to obtain a solution-state magnesium alloy.

[0022] Perform hot plastic deformation on the solution-state magnesium alloy to obtain a deformed-state magnesium alloy, specifically including: Use an extrusion die to perform non-step cooling equal-channel angular extrusion on the solution-state magnesium alloy. The initial temperature of the extrusion die is 340-350 °C, and the temperature of the extrusion die at the end of hot plastic deformation is 300-310 °C. The extrusion rate is 3-4 mm / s. Continuously process 8-10 passes of non-step cooling equal-channel angular extrusion to obtain a deformed-state magnesium alloy.

[0023] Air-cool the deformed-state magnesium alloy; then perform variable stress aging treatment on the deformed-state magnesium alloy to obtain a fine-grained strengthened magnesium alloy. The process includes: Put the deformed-state magnesium alloy into a preheated variable stress aging furnace, where the preheating temperature is set to 190-200 °C; after preheating and holding for 3 min, apply a single axial compressive stress to the deformed-state magnesium alloy. The direction of the compressive stress is parallel to the extrusion direction of the extrusion die. During the set variable stress aging time, the compressive stress applied to the deformed-state magnesium alloy increases step by step with time. After the variable stress aging time ends, a fine-grained strengthened magnesium alloy is obtained.

[0024] The initial compressive stress value of the variable stress aging treatment is 89% - 90% of the room temperature compressive yield strength of the deformed magnesium alloy; the final compressive stress value is 89% - 90% of the room temperature compressive yield strength of the deformed magnesium alloy after aging treatment; the time and temperature of the variable stress aging treatment are the same as those of the aging treatment. In this embodiment, the variable stress aging time is 25 - 35 h, the initial compressive stress is set to 290 - 300 MPa, and the compressive stress increases by 0.5 - 1.5 MPa per hour during the variable stress aging time; the temperature of the variable stress aging treatment in this embodiment is set to 200 °C; the difference between the variable stress aging treatment and the aging treatment is that no compressive stress is applied during the aging treatment.

[0025] After the variable stress aging treatment is completed, unload the fine grain strengthened magnesium alloy and air-cool the fine grain strengthened magnesium alloy.

[0026] To verify the strengthening effect of this embodiment on the magnesium alloy, the specific implementation plan is as follows: Fill the rare earth magnesium alloy ingot with high-purity graphite powder and put it into an electric resistance furnace. The furnace temperature is raised from room temperature to 500 °C at a heating rate of 15 °C / min; after reaching the temperature, hold for 24 h to obtain the solution-treated magnesium alloy.

[0027] Use an extrusion die to perform equal-channel angular extrusion with stepless cooling on the solution-treated magnesium alloy. The initial temperature of the extrusion die is 350 °C, the temperature of the extrusion die at the end of the thermoplastic deformation is 310 °C, the extrusion rate is 3.5 mm / s, and the deformed magnesium alloy is obtained by continuously processing 8 passes of equal-channel angular extrusion with stepless cooling; Put the deformed magnesium alloy into a preheated variable stress aging furnace, where the preheating temperature is set to 200 °C; after preheating and holding for 3 min, apply a single axial compressive stress to the deformed magnesium alloy, as Figure 2 shown, the stress aging time is 30 h, the initial and final stress values are set to 300 MPa and 329 MPa respectively, the externally applied stress increases by 1 MPa per hour during the variable stress aging period, and after the variable stress aging treatment is completed, unload the fine grain strengthened magnesium alloy and air-cool the fine grain strengthened magnesium alloy.

[0028] As Figure 3 (a)-(c) shows that the fine grain strengthened magnesium alloy exhibits a uniform ultrafine grain microstructure feature, with an average grain size of 0.78 μm; there are numerous second-phase particles dispersed in the magnesium matrix, the average diameter of the second-phase particles is 250 nm, and the volume fraction is 16.9%; nano-solute clusters precipitate in the grains.

[0029] Example 2

[0030] This embodiment provides a hot working method for improving the strength of rare earth magnesium alloys, including The rare earth magnesium alloy ingot is buried in graphite powder and placed in a resistance furnace for solution heat treatment to obtain a solution-state magnesium alloy, specifically including: The selected rare earth magnesium alloy ingot in this embodiment includes Mg element, Gd element and Ag element; the mass percentage of Gd element is 11%; the mass percentage of Ag element is 2%. The rare earth magnesium alloy ingot is buried in graphite powder and placed in a resistance furnace, and the furnace temperature of the resistance furnace is controlled to rise from room temperature to the solution temperature; the solution temperature is 490 - 510 °C, the heating rate is 15 °C / min, and after the furnace temperature of the resistance furnace reaches the solution temperature, it is kept warm for 20 - 24 h.

[0031] The solution-state magnesium alloy is subjected to thermoplastic deformation to obtain a deformed-state magnesium alloy, specifically including: The solution-state magnesium alloy is subjected to continuous cooling equal-channel angular pressing using an extrusion die. The initial temperature of the extrusion die is 340 - 350 °C, the temperature of the extrusion die at the end of thermoplastic deformation is 300 - 310 °C, the extrusion rate is 3 - 4 mm / s, and the processed magnesium alloy is obtained after 8 - 10 passes of continuous cooling equal-channel angular pressing; The processed magnesium alloy after continuous cooling equal-channel angular pressing is cut into magnesium alloy sheets using a wire electrical discharge machine. The magnesium alloy sheets are placed in a resistance furnace with a furnace temperature of 450 - 470 °C and kept warm for 4 - 5 min; the magnesium alloy sheets are taken out of the resistance furnace and rolled. The rolling direction is parallel to the extrusion direction of the extrusion die; the reduction per pass of single-pass rolling is 0.2 - 0.3 mm; the deformed-state magnesium alloy is obtained after 12 - 14 passes of rolling. Between adjacent rolling passes, the magnesium alloy sheets are placed in a resistance furnace with a furnace temperature of 450 - 470 °C and annealed for 2 - 3 min.

[0032] The deformed-state magnesium alloy is air-cooled; then the deformed-state magnesium alloy is subjected to variable stress aging treatment to obtain a fine-grained strengthened magnesium alloy. The process includes: The deformed-state magnesium alloy is put into a preheated variable stress aging furnace, and the preheating temperature is set to 190 - 200 °C; after the preheating holding time of 3 min, a single axial compressive stress is applied to the deformed-state magnesium alloy. The direction of the compressive stress is parallel to the extrusion direction of the extrusion die. The compressive stress applied to the deformed-state magnesium alloy increases gradually with time within the set variable stress aging time, and the fine-grained strengthened magnesium alloy is obtained after the variable stress aging time ends.

[0033] The initial compressive stress value of the variable stress aging treatment is 89% - 90% of the room temperature compressive yield strength of the deformed magnesium alloy; the final compressive stress value is 89% - 90% of the room temperature compressive yield strength of the deformed magnesium alloy after aging treatment; the time and temperature of the variable stress aging treatment are the same as those of the aging treatment. In this embodiment, the variable stress aging time is 45 - 55 h, the initial compressive stress is set to 190 - 200 MPa, and the compressive stress increases by 2.5 - 3.5 MPa per hour during the variable stress aging time; the difference between the variable stress aging treatment and the aging treatment is that no compressive stress is applied during the aging treatment.

[0034] After the variable stress aging treatment is completed, unload the fine-grained strengthened magnesium alloy and air-cool the fine-grained strengthened magnesium alloy.

[0035] To verify the strengthening effect of this embodiment on magnesium alloy, the specific implementation plan is as follows: Bury the rare earth magnesium alloy ingot with high-purity graphite powder and put it into a resistance furnace. The furnace temperature is raised from room temperature to 500 °C at a heating rate of 15 °C / min; after reaching the temperature, hold for 24 h to obtain the solution-treated magnesium alloy.

[0036] Use an extrusion die to perform equal-channel angular extrusion with stepless cooling on the solution-treated magnesium alloy. The initial temperature of the extrusion die is 350 °C, the temperature of the extrusion die at the end of the thermoplastic deformation is 310 °C, the extrusion rate is 3.5 mm / s, and the processed magnesium alloy is obtained by continuously processing 8 passes of equal-channel angular extrusion with stepless cooling.

[0037] Use a wire electrical discharge machining machine to cut the processed magnesium alloy into magnesium alloy plates with a thickness of 6.0 mm. Place the magnesium alloy plates in a resistance furnace with a furnace temperature of 470 °C and hold for 5 min; take out the magnesium alloy plates from the resistance furnace and roll them; the reduction per pass during single-pass rolling is 0.25 mm; after 12 passes of rolling, the deformed magnesium alloy is obtained. Between adjacent rolling passes, the magnesium alloy plates are placed in a resistance furnace with a furnace temperature of 470 °C and annealed for 3 min. After the final rolling is completed, air-cool the deformed magnesium alloy.

[0038] Put the deformed magnesium alloy into a preheated variable stress aging furnace, where the preheating temperature is set to 200 °C; after preheating and holding for 3 min, apply a single axial compressive stress to the deformed magnesium alloy, as Figure 4 shown, the compressive stress aging time is 50 h, the initial and final stress values are set to 200 MPa and 347 MPa respectively, the externally applied stress increases by 3 MPa per hour during the variable stress aging period, and after the variable stress aging treatment is completed, unload the fine-grained strengthened magnesium alloy and air-cool the fine-grained strengthened magnesium alloy.

[0039] As Figure 5As shown in (a)-(c), the fine-grained strengthened magnesium alloy consists of fine grains of magnesium matrix, a small amount of submicron spherical second-phase particles and a high density of nano-precipitates. Specifically, the average grain size is 22 μm, the average diameter of the second-phase particles is 800 nm, and the volume fraction is 3.4%; the nano-precipitates include β'' and γ'' phases, as well as precipitate particles distributed at dislocations and grain boundaries.

[0040] As Figure 6 shown, the room-temperature compression stress and strain curves of the high-strength magnesium alloys prepared in Example 1 and Example 2. Through solution heat treatment, alloying elements are fully dissolved to obtain a uniform solid-solution structure; then thermoplastic deformation treatment helps to break the coarse grains in the as-cast structure, refine the grains and improve the uniformity of the structure, and improve the plasticity of the alloy. Finally, variable stress aging treatment further promotes grain refinement and tissue optimization, and at the same time produces favorable element segregation / precipitation inside the alloy. Under the synergistic action of multiple processes, the mechanical properties such as strength, hardness and plasticity of the rare-earth magnesium alloy are comprehensively and significantly improved, meeting the usage requirements under complex working conditions.

[0041] The following provides data of 2 pre-deformation combined conventional aging and 2 single conventional aging for preparing Mg-11Gd-2Ag magnesium alloy for comparison to illustrate that the method provided by the present invention shows better effects in strengthening magnesium alloys. Among them: Comparative Example 1: The Mg-11Gd-2Ag (wt.%) rare-earth magnesium alloy ingot was buried with high-purity graphite powder and placed in a resistance furnace. The furnace temperature was raised from room temperature to 500 °C at a heating rate of 15 °C / min; after reaching the temperature, it was held for 24 h to obtain a solution-treated magnesium alloy.

[0042] The solution-treated magnesium alloy was subjected to equal-channel angular extrusion with stepless cooling using an extrusion die. The initial temperature of the extrusion die was 350 °C, the temperature of the extrusion die at the end of thermoplastic deformation was 310 °C, the extrusion rate was 3.5 mm / s, and the solution-treated magnesium alloy was continuously processed for 8 passes of equal-channel angular extrusion with stepless cooling to obtain a deformed magnesium alloy; the deformed magnesium alloy was subjected to aging heat treatment, the aging temperature was 200 °C, and the aging time was 30 h; after aging, the average grain size of the alloy was 0.94 μm, the average diameter of the dynamically precipitated second-phase particles was 240 nm, and the volume fraction was 16.5%; nano-solute clusters were precipitated in the grains.

[0043] Comparative Example 2: The Mg-11Gd-2Ag (wt.%) rare-earth magnesium alloy ingot was buried with high-purity graphite powder and placed in a resistance furnace. The furnace temperature was raised from room temperature to 500 °C at a heating rate of 15 °C / min; after reaching the temperature, it was held for 24 h to obtain a solution-treated magnesium alloy. The solution-treated magnesium alloy was subjected to equal-channel angular pressing with stepless cooling using an extrusion die. The initial temperature of the extrusion die was 350 °C, and the temperature of the extrusion die at the end of the thermoplastic deformation was 310 °C. The extrusion rate was 3.5 mm / s. The processed magnesium alloy was obtained by continuously processing 8 passes of equal-channel angular pressing with stepless cooling.

[0044] The processed magnesium alloy was cut into magnesium alloy sheets with a thickness of 6.0 mm using a wire electrical discharge cutting machine. The magnesium alloy sheets were placed in an electric resistance furnace with a furnace temperature of 470 °C and held for 5 min; the magnesium alloy sheets were taken out of the electric resistance furnace and rolled; the reduction per pass of rolling was 0.25 mm; the deformed magnesium alloy was obtained after 12 passes of rolling. Between adjacent rolling passes, the magnesium alloy sheets were placed in an electric resistance furnace with a furnace temperature of 470 °C for annealing for 3 min. After the final rolling was completed, the deformed magnesium alloy was air-cooled.

[0045] The deformed magnesium alloy was subjected to aging heat treatment. The aging temperature was 200 °C and the aging time was 50 h; after aging, the average grain size of the alloy was 32 μm, the average diameter of the dynamically precipitated second-phase particles was 790 nm, and the volume fraction was 3.1%; a large number of nano-β', β'' and γ'' phases were generated in the grains, as well as precipitated particles distributed at dislocations and grain boundaries.

[0046] For Comparative Example 3, an Mg-11Gd-2Ag (wt.%) rare-earth magnesium alloy ingot was solution-treated at 500 °C for 24 h, and then aged at 200 °C for 30 h.

[0047] For Comparative Example 4, an Mg-11Gd-2Ag (wt.%) rare-earth magnesium alloy ingot was solution-treated at 500 °C for 24 h, and then aged at 200 °C for 50 h.

[0048] The room-temperature compressive mechanical properties of the magnesium alloys prepared in Examples 1 and 2 and Comparative Examples 1 to 4 were tested using a SUNS UTM4204X universal testing machine, as shown in Table 1.

[0049] Table 1 shows the compressive mechanical property data of Examples 1 and 2 and Comparative Examples 1 to 4.

[0050]

[0051] As can be seen from Table 1, compared with the traditional method of combining single conventional aging and pre-deformation with conventional aging treatment, the processing method of pre-deformation combined with variable stress aging provided by the present invention achieves a higher strength improvement in the Mg-11Gd-2Ag (wt.%) magnesium alloy, and the compressive yield strength of the prepared alloy exceeds 400 MPa. The number of processing passes of equal-channel angular pressing with stepless cooling is 8-10 passes, which avoids the too low temperature of the alloy block in the later stage of processing caused by too many processing passes, and increases the cracking tendency of the alloy block. The reason for the rolling temperature of 450-470 °C is that on the one hand, a higher rolling temperature can play the role of solution treatment to force the dynamic precipitation phases generated by the previous equal-channel angular pressing with stepless cooling to dissolve, and on the other hand, it can avoid the excessive growth of grains at high temperature and introduce special grain boundaries and substructures.

[0052] In the rare earth magnesium alloys prepared by Example 1 and Example ②, the grain size can be adjusted within a wide range from the micron scale to the submicron scale. The dynamically precipitated second-phase particles are small and dispersed in the matrix, and their number density can be controlled by changing the number of rolling passes. Compared with conventional aging treatment, variable stress aging can additionally introduce vacancies and dislocations to promote the recovery process, or activate twinning to introduce twin boundaries, both of which can cause grain refinement; and these crystal defects can reduce the nucleation energy of nano-precipitates, which helps to reduce the size of precipitates and increase the precipitation density. The adjustment of the grain and precipitation structure increases the contribution of the corresponding microstructure to strength, and the room-temperature compressive yield strength of the alloy after variable stress aging can exceed 400 MPa. Compared with the common constant stress aging, the stress mode of variable stress aging is formulated according to the conventional aging strengthening ability of the pre-deformed alloy, so its control over the aging precipitation process does not weaken with the progress of aging, which helps to improve the aging strengthening efficiency. The inventive process can be extended to other precipitation-strengthened magnesium alloys and aluminum alloys.

[0053] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the technical principle of the present invention, several improvements and deformations can be made, and these improvements and deformations should also be regarded as the protection scope of the present invention.

Claims

1. A hot working method for improving the strength of rare earth magnesium alloy, characterized in that, including burying a rare earth magnesium alloy ingot in graphite powder and placing it in a resistance furnace for solution heat treatment to obtain a solution-state magnesium alloy; performing thermoplastic deformation on the solution-state magnesium alloy to obtain a deformed-state magnesium alloy and air-cooling the deformed-state magnesium alloy; performing variable stress aging treatment on the deformed-state magnesium alloy to obtain a fine-grained strengthened magnesium alloy, the process including: putting the deformed-state magnesium alloy into a preheated variable stress aging furnace for heat preservation, then applying a compressive stress in a single axial direction to the deformed-state magnesium alloy, and the compressive stress applied to the deformed-state magnesium alloy increasing step by step with time within a set variable stress aging time, and obtaining a fine-grained strengthened magnesium alloy after the variable stress aging time ends; unloading the fine-grained strengthened magnesium alloy after the variable stress aging treatment ends and air-cooling the fine-grained strengthened magnesium alloy.

2. The hot working method for improving the strength of rare earth magnesium alloy according to claim 1, characterized in that, burying a rare earth magnesium alloy ingot in graphite powder and placing it in a resistance furnace for solution heat treatment to obtain a solution-state magnesium alloy, specifically including: burying a rare earth magnesium alloy ingot in graphite powder and placing it in a resistance furnace, controlling the furnace temperature of the resistance furnace to rise from room temperature to the solution temperature; the solution temperature is 490 - 510 °C, the heating rate is 15 °C / min, and heat preservation is carried out for 20 - 24 h after the furnace temperature of the resistance furnace reaches the solution temperature.

3. The hot working method for improving the strength of rare earth magnesium alloy according to claim 1 or 2, characterized in that, The rare earth magnesium alloy ingot includes Mg element, Gd element and Ag element; the mass percentage of Gd element is 11%; the mass percentage of Ag element is 2%.

4. The hot working method for improving the strength of rare earth magnesium alloy according to claim 1, characterized in that, performing thermoplastic deformation on the solution-state magnesium alloy to obtain a deformed-state magnesium alloy, specifically including: performing non-step cooling equal channel angular pressing on the solution-state magnesium alloy using an extrusion die, the initial temperature of the extrusion die is 340 - 350 °C, the temperature of the extrusion die at the end of thermoplastic deformation is 300 - 310 °C, the extrusion rate is 3 - 4 mm / s, and a deformed-state magnesium alloy is obtained by continuously processing 8 - 10 passes of non-step cooling equal channel angular pressing.

5. The hot working method for improving the strength of rare earth magnesium alloy according to claim 4, characterized in that, The variable stress aging time is 25 - 35 h, the initial compressive stress is set to 290 - 300 MPa, and the compressive stress increases by 0.5 - 1.5 MPa per hour during the variable stress aging time.

6. The hot working method for improving the strength of rare earth magnesium alloy according to claim 1, characterized in that, performing thermoplastic deformation on the solution-state magnesium alloy to obtain a deformed-state magnesium alloy, specifically including: performing non-step cooling equal channel angular pressing on the solution-state magnesium alloy using an extrusion die, the initial temperature of the extrusion die is 340 - 350 °C, the temperature of the extrusion die at the end of thermoplastic deformation is 300 - 310 °C, the extrusion rate is 3 - 4 mm / s, and a processed-state magnesium alloy is obtained by continuously processing 8 - 10 passes of non-step cooling equal channel angular pressing; using a wire electrical discharge machine to cut the processed-state magnesium alloy after non-step cooling equal channel angular pressing into magnesium alloy plates, placing the magnesium alloy plates in a resistance furnace with a furnace temperature of 450 - 470 °C and heat preserving for 4 - 5 min; taking out the magnesium alloy plates from the resistance furnace and rolling them, the rolling direction being parallel to the extrusion direction of the extrusion die; the reduction per single pass of rolling is 0.2 - 0.3 mm; a deformed-state magnesium alloy is obtained after 12 - 14 passes of rolling, and the magnesium alloy plates are annealed in a resistance furnace with a furnace temperature of 450 - 470 °C for 2 - 3 min between adjacent rolling passes.

7. The hot working method for improving the strength of rare earth magnesium alloy according to claim 4 or 6, characterized in that, The direction of the compressive stress is parallel to the extrusion direction of the extrusion die.

8. The hot working method for improving the strength of rare earth magnesium alloy according to claim 4 or 6, characterized in that, The preheating temperature of the variable stress aging furnace for the deformed magnesium alloy is set to 190 - 200 °C, and the preheating holding time is 3 min.

9. The hot working method for improving the strength of rare earth magnesium alloy according to claim 6, characterized in that, The variable stress aging time is 45 - 55 h, the initial compressive stress is set to 190 - 200 MPa, and the compressive stress increases by 2.5 - 3.5 MPa per hour during the variable stress aging time.

10. The hot working method for improving the strength of rare earth magnesium alloy according to claim 1, characterized in that, The initial compressive stress value of the variable stress aging treatment is 89% - 90% of the room temperature compressive yield strength of the deformed magnesium alloy; the final compressive stress value is 89% - 90% of the room temperature compressive yield strength of the deformed magnesium alloy after aging treatment; the time and temperature of the variable stress aging treatment and the aging treatment are the same.

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