Preparation method of high-toughness magnesium-lithium alloy plate

Abstract

The application relates to a preparation method of a high-toughness magnesium-lithium alloy plate and relates to a preparation method of a magnesium-lithium alloy plate. The application aims to solve the technical problem that the Mg2Si phase in the existing magnesium-lithium alloy has a large grain size, the formability is poor, and the magnesium-lithium alloy plate prepared by a conventional rolling process is difficult to achieve high strength and toughness. In the smelting process, an Al-27Si eutectic alloy formed by spray deposition is added, the small-size Mg2Si phase is introduced to facilitate the acquisition of fine crystal organization and better formability; the rolling process of first temperature rise and then temperature drop is adopted to introduce high-density dislocations, adjust the microstructure of the magnesium-lithium alloy, refine the second-phase particles, and generate a large number of small recrystallized grains; and high-temperature annealing is further adopted to improve the comprehensive mechanical properties of the magnesium-lithium alloy; the high-toughness dual-phase magnesium-lithium alloy plate prepared by the application has a tensile strength of 280MPa to 305MPa and an elongation rate of 8% to 16%.

Description

TECHNICAL FIELD

[0001] The present application relates to a preparation method of a magnesium-lithium alloy plate. BACKGROUND

[0002] Magnesium-lithium alloy shows great application potential in transportation and aerospace field due to its low density, excellent specific strength and outstanding electromagnetic interference shielding capability. When the Li content is in the content range of 5.7wt% to 10.3wt%, a dual-phase (α-Mg+β-Li) microstructure appears, which can effectively balance the strength and ductility. Although magnesium-lithium alloy performs well in lightweight and processability, its strength is usually not as good as that of traditional magnesium alloy, which to some extent limits its application potential in various fields.

[0003] Alloying is one of the important means to strengthen magnesium-lithium alloy. Rare earth elements such as Y and Ce or non-rare earth elements such as Zn, Al and Si are commonly used to strengthen the matrix in magnesium-lithium alloy. Although the addition of rare earth elements has obvious strengthening effect, it further increases the cost. Among non-rare earth elements, Zn will form unstable phases, and the strengthening effect is not stable, while Al has excellent solid solution strengthening and second phase strengthening effect in the alloy, and Si can form high-temperature stable Mg2Si phase with Mg, which has a positive effect on the improvement of alloy performance. However, the Mg2Si phase formed by adding traditional smelting aluminum-silicon eutectic alloy to introduce Si element has a large grain size, which is not conducive to the improvement of its performance. The coarse Mg2Si phase leads to ineffective grain refinement, and poor formability during deformation.

[0004] Roll forming is one of the key means to prepare magnesium-lithium alloy plate, which can further strengthen the alloy and plays a decisive role in its microstructure and mechanical properties. In general research, magnesium-lithium dual-phase alloy usually adopts cooling rolling to improve the mechanical properties, but cooling rolling has the effect of softening caused by natural aging, which makes the improvement of its mechanical properties lower, and it is difficult to meet the high strength and toughness. SUMMARY

[0005] The present application is to solve the technical problem that the Mg2Si phase in the existing magnesium-lithium alloy has a large grain size and poor formability, and the magnesium-lithium alloy plate prepared by conventional rolling process is difficult to achieve high strength and toughness, and to provide a preparation method of high strength and toughness magnesium-lithium alloy plate.

[0006] The preparation method of high strength and toughness magnesium-lithium alloy plate of the present application is carried out according to the following steps:

[0007] I. Raw material preparation: raw materials are prepared according to the following mass percentage of each element: Li: 7.8-8.2wt.%, Al: 2.7-3.2wt.%, Si: 0.3-0.4wt.%, and the rest is Mg; the raw materials are magnesium blocks, aluminum blocks, lithium particles and Al-27Si eutectic alloy, the Al-27Si eutectic alloy is prepared by spray deposition; the magnesium blocks, aluminum blocks and Al-27Si eutectic alloy are polished;

[0008] II. Melting alloy ingot: put the raw materials prepared in step I into a vacuum furnace, vacuumize, then introduce a mixed gas of SF6 and Ar to a pressure of 0.013-0.015 MPa, heat to 450-455°C, the paraffin attached to the surface of lithium will emit white smoke, stop heating after volatilizing for 10-11 min, start vacuumizing, when the furnace is clear, raise the temperature to 680-690°C, after the alloy is completely melted, lower the stirring paddle below the alloy liquid surface, stir at a speed of 250-300 rpm for 3-5 min, stop stirring, keep the temperature at 680-690°C for 10-12 min, raise the stirring paddle above the liquid surface, pour the alloy liquid into the mold below, then cool to room temperature with the furnace, to obtain an alloy ingot of 60mm×60mm×200mm; the mold is also located in the vacuum furnace;

[0009] III. Preparation before rolling: cut the alloy ingot obtained in step II into a thin plate of 60mm×60mm×5mm using an electric spark wire cutting machine, then polish the surface using coarse sandpaper, then heat the cut alloy thin plate in a muffle furnace at 200-210°C for 20-25 min;

[0010] IV. Rolling and annealing: control the temperature of the roller to be kept at 120-130°C, and the linear speed of the roller to be kept at 5-8 m / min; quickly transfer the magnesium-lithium alloy plate sample after the heat preservation treatment in step III to a double-roller rolling mill for rolling:

[0011] ①, first pass at 200-210°C, deformation of 10%; then heat preservation at 200-210°C for 10-12 min, then roll, deformation of 20%;

[0012] ②, then heat preservation at 250-260°C for 10 min, then roll again, deformation of 20%;

[0013] ③, repeat operation ② once more;

[0014] ④, heat preservation at 300-310°C for 10 min, then roll again, deformation of 20%;

[0015] ⑤, repeat operation ④ once more;

[0016] ⑥ The final six passes are rolled at room temperature with a deformation of 30%;

[0017] ⑦ Finally, anneal at 240℃~320℃ for 1h~1.5h to obtain magnesium-lithium alloy sheet.

[0018] The Al-27Si eutectic alloy mentioned in step one of this invention was purchased from Jiangsu Haoran Spray Forming Alloy Co., Ltd., and its grade is Al-27Si.

[0019] The present invention has the following advantages:

[0020] (1) In the process of smelting, the present invention adds Al-27Si eutectic alloy formed by spray deposition. The fine primary Si crystals in the Al-27Si eutectic alloy prepared by spray deposition are uniformly and diffusely distributed in the Al matrix, which ensures that the Si size introduced during the smelting process is not very large, so that the generated Mg2Si phase is fine (about 5μm) and uniformly distributed. The introduction of the fine Mg2Si phase is conducive to obtaining fine-grained structure and better formability. Moreover, the Mg2Si phase has the characteristics of high melting point, high hardness and high temperature stability, which improves the application potential of the alloy in high temperature and high strength environments.

[0021] (2) In step four of this invention, a high-density dislocation is introduced by a rolling process of heating first and then cooling, which adjusts the microstructure of the magnesium-lithium alloy, refines the second phase particles, and generates a large number of fine recrystallized grains, thereby refining the grains and improving the mechanical properties of the magnesium-lithium alloy.

[0022] (3) By adopting high-temperature annealing after cold rolling, the dislocation density is reduced, the recrystallization volume fraction and grain size are increased, and the volume fraction of precipitated phase is increased, which provides sufficient second phase strengthening effect for magnesium-lithium alloy and significantly improves the comprehensive mechanical properties of magnesium-lithium alloy.

[0023] (4) The high-strength and tough dual-phase magnesium-lithium alloy plate prepared by the present invention has a tensile strength of 280MPa to 305MPa and an elongation of 8% to 16%. Attached Figure Description

[0024] Figure 1 A photograph of the magnesium-lithium alloy sheet obtained in Experiment 2;

[0025] Figure 2 SEM images of the products from steps ⑥ and ⑦ in step four of Experiment 2;

[0026] Figure 3 This is a comparative graph showing the effect of different annealing temperatures on the mechanical properties of the sheet metal in Experiments 1 through 3. Detailed Implementation

[0027] Specific implementation one: the preparation method of the high-toughness magnesium-lithium alloy plate, specifically is carried out according to the following steps:

[0028] I. Raw material preparation: prepare raw materials according to the following mass percentage of each element: Li: 7.8-8.2wt.%, Al: 2.7-3.2wt.%, Si: 0.3-0.4wt.%, and the rest is Mg; the raw materials are magnesium blocks, aluminum blocks, lithium particles and Al-27Si eutectic alloy, the Al-27Si eutectic alloy is prepared by spray deposition; the magnesium blocks, aluminum blocks and Al-27Si eutectic alloy are polished;

[0029] II. Melting alloy ingot: put the raw materials prepared in step I into a vacuum furnace, vacuumize, then introduce a mixed gas of SF6 and Ar to a pressure of 0.013-0.015 MPa, heat to 450-455°C, the paraffin attached to the surface of lithium will volatilize white smoke, stop heating after volatilizing for 10-11 min, start vacuumizing, when the furnace is clear, raise the temperature to 680-690°C, after the alloy is completely melted, lower the stirring paddle below the alloy liquid surface, stir at a speed of 250-300 rpm for 3-5 min, stop stirring, keep the temperature at 680-690°C for 10-12 min, raise the stirring paddle above the liquid surface, pour the alloy liquid into the mold below, then cool to room temperature with the furnace, obtain an alloy ingot with a size of 60mm×60mm×200mm; the mold is also located in the vacuum furnace;

[0030] III. Preparation before rolling: cut the alloy ingot obtained in step II into a thin plate with a size of 60mm×60mm×5mm using an electric spark wire cutting machine, then polish the surface using coarse sandpaper, then heat the cut alloy thin plate in a muffle furnace at 200-210°C for 20-25 min;

[0031] IV. Rolling and annealing: control the temperature of the roller to be kept at 120-130°C, and the linear speed of the roller to be kept at 5-8 m / min; quickly transfer the magnesium-lithium alloy plate sample after the heat preservation treatment in step III to a double-roller rolling mill for rolling:

[0032] ①. First pass at 200-210°C, deformation of 10%; then heat preservation at 200-210°C for 10-12 min, then roll, deformation of 20%;

[0033] ②. Then heat preservation at 250-260°C for 10 min, then roll again, deformation of 20%;

[0034] ③. Repeat the operation of ② once again;

[0035] ④, after 10 min of heat preservation at 300-310℃, rolling again, deformation 20%;

[0036] ⑤, repeat the operation of ④ once again;

[0037] ⑥, the last pass is rolled at room temperature, deformation 30%;

[0038] ⑦, finally annealing at 240-320℃ for 1-1.5 h to obtain magnesium-lithium alloy plate.

[0039] Specific embodiment two: the difference between this embodiment and the first embodiment is that the purity of the magnesium block, aluminum block and lithium particles in step one is 99.9%. The others are the same as the first embodiment.

[0040] Specific embodiment three: the difference between this embodiment and the first or second embodiment is that the volume ratio of SF6 and Ar in step two is (4-5):(45-46). The others are the same as the first or second embodiment.

[0041] Specific embodiment four: the difference between this embodiment and any one of the first to third embodiments is that the cut alloy sheet is heat preserved at 200℃ for 20 min in a muffle furnace in step three. The others are the same as any one of the first to third embodiments.

[0042] Specific embodiment five: the difference between this embodiment and the fourth embodiment is that the temperature of the roller is controlled to be kept at 120℃ and the linear speed of the roller is kept at 5 m / min in step four. The others are the same as the fourth embodiment.

[0043] Specific embodiment six: the difference between this embodiment and the fifth embodiment is that in step four, ①, the first pass is rolled at 200℃, deformation 10%; then after 10 min of heat preservation at 200℃, rolling, deformation 20%;

[0044] ②, then after 10 min of heat preservation at 250℃, rolling again, deformation 20%;

[0045] ③, repeat the operation of ② once again;

[0046] ④, after 10 min of heat preservation at 300℃, rolling again, deformation 20%.

[0047] The others are the same as the fifth embodiment.

[0048] Specific embodiment seven: the difference between this embodiment and the sixth embodiment is that in step four, ⑦, finally annealing at 240-320℃ for 1 h to obtain magnesium-lithium alloy plate. The others are the same as the sixth embodiment.

[0049] The present application is verified by the following tests:

[0050] Test one: the present application is a preparation method of a high-toughness magnesium-lithium alloy plate, which is carried out according to the following steps:

[0051] I. Preparation of raw materials: prepare raw materials according to the following mass percentages of each element: Li: 8wt.%, Al: 3wt.%, Si: 0.3wt.%, and the rest is Mg; the raw materials are magnesium blocks, aluminum blocks, lithium particles and Al-27Si eutectic alloy, the Al-27Si eutectic alloy is prepared by spray deposition and purchased from Jiangsu Haoran Spray Forming Alloy Co., Ltd., and the brand is Al-27Si; the magnesium blocks, aluminum blocks and Al-27Si eutectic alloy are polished;

[0052] The purity of the magnesium blocks, aluminum blocks and lithium particles all reaches 99.9%;

[0053] II. Melting and alloying ingot: put the raw materials prepared in step I into a vacuum furnace, vacuumize, then introduce a mixed gas of SF6 and Ar to a pressure of 0.013 MPa, heat to 450℃, the paraffin attached to the surface of lithium will volatilize white smoke, stop heating after volatilizing for 10 min, start vacuumizing, when the furnace is clear, raise the temperature to 680℃, after the alloy is completely melted, lower the stirring paddle below the alloy liquid surface, stir at a speed of 250 rpm for 3 min, stop stirring, keep the temperature at 680℃ for 10 min, raise the stirring paddle above the liquid surface, pour the alloy liquid into the mold below, then cool to room temperature with the furnace, and obtain an alloy ingot with a size of 60mm×60mm×200mm; the mold is also located in the vacuum furnace; the volume ratio of SF6 and Ar is 1:9;

[0054] III. Preparation before rolling: cut the alloy ingot obtained in step II into a thin plate with a size of 60mm×60mm×5mm using an electric spark wire cutting machine, then polish the surface using coarse sandpaper, and then heat the cut alloy thin plate in a muffle furnace at 200℃ for 20 min;

[0055] IV. Rolling and annealing: control the temperature of the roller to keep at 120℃, and keep the linear speed of the roller at 5m / min; quickly transfer the magnesium-lithium alloy plate sample after the heat preservation treatment in step III to a double-roller rolling mill for rolling:

[0056] ①. First pass at 200℃, deformation of 10%; then heat preservation at 200℃ for 10 min, then roll, deformation of 20%;

[0057] ②. Then heat preservation at 250℃ for 10 min, then roll again, deformation of 20%;

[0058] ③. Repeat operation ② once more;

[0059] (4) After holding at 300 °C for 10 min, rolling again with a deformation of 20%;

[0060] (5) Repeating the operation of (4) once again;

[0061] (6) The last pass is rolled at room temperature with a deformation of 30%;

[0062] (7) Finally, annealing at 320 °C for 1 h, a magnesium-lithium alloy plate with a thickness of 1 mm is obtained, which has a tensile strength of 303 MPa and an elongation of 8% at room temperature.

[0063] Test Two: The difference between this test and Test One is that in step four (7), the final annealing is performed at 280 °C for 1 h, and a magnesium-lithium alloy plate with a thickness of 1 mm is obtained, which has a tensile strength of 292 MPa and an elongation of 14% at room temperature.

[0064] Figure 1 The physical photograph of the magnesium-lithium alloy plate obtained in Test Two can be seen that the surface quality of the alloy material after rolling is good, indicating that the rolling process is suitable.

[0065] Figure 2 The SEM images of the products of steps four (6) and (7) in Test Two, i.e., the comparison images before and after annealing, can be seen from the figure that after cold rolling, the α-Mg phase and β-Li phase are elongated along the rolling direction, and shear deformation bands appear in the α-Mg phase, while no shear bands are observed in the β-Li phase. The appearance of shear deformation bands is because the non-basal plane slip of magnesium is difficult to activate at low temperature deformation, and the dislocation migration between adjacent grains is difficult, therefore, the larger shear stress promotes the occurrence of shear deformation, and the β-Li phase has more slip systems, which is easy to start during plastic deformation, and the deformation is easier. After annealing, due to the effect of phase interface tension, the α / β phase boundary changes from a flat interface to a corrugated interface, and the α / β phase boundary is relatively smooth at this time. Fine β-Li phase grains are observed inside the α-Mg phase, indicating that annealing promotes recrystallization and increases the grain size. Fine blocky Mg2Si phases (about 5 μm) are uniformly distributed in the α-Mg phase, further enhancing the mechanical properties of the alloy and improving the application potential of the alloy in high temperature and high strength environments. Recrystallization occurs in the original grain boundaries and near the phase boundaries of the plate, and the internal stress is released, which promotes the decrease of strength, but under high temperature annealing conditions, the AlLi phase particles originally dispersed in the β-Li phase dissolve into the matrix, forming a supersaturated β-Li phase, as shown in Figure 2 After annealing, the supersaturated β-Li phase improves the strength, resulting in a significant decrease in overall strength and an increase in plasticity.

[0066] Test three: the difference between the test and test one is that: in step four 7, the final annealing is carried out at 240 DEG C for 1h, and the magnesium-lithium alloy plate with a thickness of 1mm is obtained, the tensile strength at room temperature is 282MPa, and the elongation is 16%.

[0067] The plate obtained in test one to test three is respectively subjected to tensile test mechanical property analysis, Figure 3 In order to analyze the influence of each annealing temperature on the mechanical property, it can be seen from the figure that after cold rolling and annealing at 240 DEG C for 1h, the tensile strength is 282MPa, and the elongation is 16%; with the increase of the annealing temperature, the strength gradually increases, and the plasticity gradually decreases; when the annealing temperature is increased to 320 DEG C, the tensile strength is 303MPa, and the elongation is 8%, and the strength is close to the strength after cold rolling.

[0068] The reasonable rolling process in the application ensures that the magnesium-lithium alloy will not produce microcracks and other defects in the hot rolling process, at the same time, the grains are further refined after large plastic deformation, which plays a key role in the improvement of the alloy performance, and then the magnesium-lithium alloy plate in the application shows good performance.

Claims

1. A method for preparing a high-strength and high-toughness magnesium-lithium alloy plate, characterized in that... The preparation method of high-strength and high-toughness magnesium-lithium alloy plates is carried out according to the following steps: I. Rolled Plate Preparation: ① Prepare raw materials according to the following mass percentages of each element: Li: 7.8-8.2 wt.%, Al: 2.7-3.2 wt.%, Si: 0.3-0.4 wt.%, with the remainder being Mg; the raw materials are magnesium blocks, aluminum blocks, lithium granules, and Al-27Si eutectic alloy, wherein the Al-27Si eutectic alloy is prepared by spray deposition; ② Place the raw materials prepared in step one into a vacuum furnace, evacuate the vacuum, and then introduce a mixed gas of SF6 and Ar to a pressure of 0.013MPa~0.015MPa. Heat the furnace to 680℃~690℃. After the alloy has completely melted, stir the furnace with a stirring paddle at a speed of 250rpm~300rpm for 3min~5min. Then hold the furnace at 680℃~690℃ for 10min~12min. Finally, cast the alloy ingot to obtain a size of 60mm×60mm×200mm. ③ Cut the alloy ingot obtained in step ② into a thin plate of 60mm×60mm×5mm, then polish the surface with coarse sandpaper, and then heat the alloy plate in a muffle furnace at 200℃~210℃ for 20min~25min. II. Rolling and Annealing: The temperature of the rolls is controlled at 120℃~130℃, and the linear speed of the rolls is maintained at 5m / min~8m / min; the magnesium-lithium alloy plate sample after the heat preservation treatment in step one is quickly transferred to a twin-roll mill for rolling: ① The first pass is rolled at 200℃~210℃ with a deformation of 10%; then it is rolled after holding at 200℃~210℃ for 10min~12min with a deformation of 20%. ② Then, after holding at 250℃~260℃ for 10 minutes, roll it again, with a deformation of 20%; ③ Repeat step ② one more time; ④ After holding at 300℃~310℃ for 10 minutes, roll it again, and the deformation amount is 20%; ⑤ Repeat step ④ one more time; ⑥ The final rolling pass is performed at room temperature, with a deformation of 30%; ⑦ Finally, anneal at 240℃~320℃ for 1h~1.5h to obtain magnesium-lithium alloy sheet.

2. The method for preparing a high-strength and high-toughness magnesium-lithium alloy plate according to claim 1, characterized in that... The purity of the magnesium block, aluminum block, and lithium granules mentioned in step 1① all reached 99.9%.

3. The method for preparing a high-strength and high-toughness magnesium-lithium alloy plate according to claim 1, characterized in that... The volume ratio of SF6 and Ar mentioned in step 1② is (4~5):(45~46).

4. The method for preparing a high-strength and high-toughness magnesium-lithium alloy plate according to claim 1, characterized in that... In step 1, the cut alloy sheet is held at 200°C for 20 minutes in a muffle furnace.

5. The method for preparing a high-strength and high-toughness magnesium-lithium alloy plate according to claim 1, characterized in that... In step two, the temperature of the rolls is controlled at 120℃ and the linear speed of the rolls is maintained at 5m / min.

6. The method for preparing a high-strength and high-toughness magnesium-lithium alloy plate according to claim 1, characterized in that... In step two, ① the first pass is rolled at 200℃ with a deformation of 10%; then it is rolled after holding at 200℃ for 10 minutes with a deformation of 20%. ② Then, after holding at 250℃ for 10 minutes, it is rolled again, with a deformation of 20%; ③ Repeat step ② one more time; ④ After holding at 300℃ for 10 minutes, roll again; the deformation amount is 20%. ⑤ Repeat step ④ one more time; ⑥ The final rolling pass is performed at room temperature with a deformation of 30%.

7. The method for preparing a high-strength and high-toughness magnesium-lithium alloy plate according to claim 1, characterized in that... Step 2.7: Finally, anneal at 240℃~320℃ for 1 hour to obtain magnesium-lithium alloy sheet.

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