As-cast high strength and toughness fine-grained magnesium-lithium dual-phase alloy and resistance heating preparation method thereof

Abstract

The application discloses a cast high-strength and high-toughness fine-grain Mg-Li dual-phase alloy and a preparation method thereof, belongs to the technical field of light-weight high-strength magnesium alloy materials, and solves the technical problems of low strength, high preparation cost and high energy consumption of a cast Mg-Li alloy.The solution is that the Mg-Li dual-phase alloy is an Mg-Li-Al-Si Mg-Li alloy, the composition of alloy elements and the mass percentage content are as follows: Li is 8wt.%, Al is 3wt.%, Si is 0.3wt.%, and the rest is Mg.The application adopts a resistance furnace heating mode, a smelting atmosphere is high-purity argon and a small amount of SF6, the protective film is thin, and the integrity of the surface film effectively prevents the oxidation of Mg and Li;after a small amount of SF6 is introduced into the argon, the surface morphology of the rolled plate is obviously better than that of the rolled plate into which only argon is introduced.The Mg-Li dual-phase alloy prepared by the application has a high strength of 210-220MPa, low production cost and low loss, and the surface quality of the rolled plate is improved.

Description

Technical Field

[0001] This invention belongs to the technical field of lightweight high-strength magnesium alloy materials, specifically relating to a cast high-strength, tough, fine-grained magnesium-lithium dual-phase alloy and its resistance heating preparation method. Background Technology

[0002] Existing reports indicate that magnesium-lithium alloys are typically smelted in an argon atmosphere using medium / high frequency induction furnaces for rapid heating. Mg has a melting point of 933 K, while Li has a melting point of only 453 K. Rapid heating allows magnesium ingots to melt quickly, and the molten magnesium encapsulates the lithium, preventing its volatilization. However, medium / high frequency induction heating places extremely high demands on equipment and consumes a large amount of energy, hindering sustainable development and resulting in cast alloys with low strength. The mechanical properties of induction-melted Mg-Li alloys with lithium contents of 1%, 7%, and 12.5% ​​are 90.3 MPa and 6.4%, 113 MPa and 15.8%, and 93.7 MPa and 10.8%, respectively. There are also reports of alloys with added alloying elements; Mg-8Li-3(Al-Si) alloys produced by magnetic levitation vacuum high-frequency induction heating followed by copper mold casting exhibit mechanical properties of 210 MPa and 18%.

[0003] Besides using medium / high frequency induction heating to melt alloys, some researchers have also tried air melting, which involves melting the alloy using resistance wire heating. The alloy is melted in a resistance furnace with a graphite crucible under a protective atmosphere of SF6 gas. A covering agent is used to separate the melt from the air. A Mg-8Li-5Al-2Zn-0.5Y alloy was prepared by a semi-continuous casting process, with mechanical properties of 196 MPa and 12.7%. Researchers also cast LAZ832-0.5Y ingots under a mixed atmosphere of SF6 (1% by volume) and CO2 (99% by volume), with LiCl and LiF covering agents protecting the melt, with mechanical properties of 197 MPa and 13.1%. Because Mg and Li have different melting temperatures, researchers melted magnesium ingots and other materials at 983K in an argon atmosphere. After removing the crucible from the furnace and cooling it to 933K, Li strips were pressed into the melt and melted. The melt was then heated again to 983K. After removing the surface flux from the liquid metal, the copper mold was cooled, producing a Mg-6.4Li-3.6Zn-0.37Al-0.36Y ingot. This alloy has a strength of 110MPa and a plasticity of 9%. Heating in an atmospheric resistance furnace leads to unstable product quality, low production efficiency, and, due to the low ignition point of magnesium, it is easily combustible, posing a safety hazard to workers.

[0004] Existing technologies employ vacuum melting to prepare Mg-Li alloys, avoiding the harm to humans caused by the combustion of magnesium. Furthermore, vacuum melting of magnesium-lithium alloys results in better metal microstructure because the O2 and H2O content is significantly lower under vacuum than under air melting conditions. Therefore, vacuum melting offers better protection for the melt, and the quality of ingots obtained under vacuum is significantly superior to those obtained through air melting. Simultaneously, impurities in the ingot primarily originate from the raw materials and equipment itself, with no new impurities introduced during the melting process. Thus, vacuum melting is currently the most stable and protective process for ingot composition among all melting techniques. However, it must be noted that due to the special and precise nature of the equipment, the melting capacity of vacuum melting is limited, making refining, purification, and slag removal inconvenient. It also cannot be directly applied to the production of magnesium-lithium alloy castings, resulting in excessively high costs for its industrial application.

[0005] Besides using heating methods such as induction melting, air melting, and vacuum melting to strengthen magnesium-lithium alloys, alloying is also an important means of strengthening them. Al has excellent solid solution strengthening and second-phase strengthening effects in alloys, and Si can form a high-temperature stable Mg2Si phase with Mg, both of which have a positive effect on improving alloy performance. However, the strengthening phase grains generated in Si-containing magnesium alloys prepared by ordinary casting processes are coarse, which is not conducive to improving the alloy performance. Researchers directly added aluminum ingots and silicon blocks to prepare Mg-Li-3 (Al-Si) alloys, with as-cast mechanical properties of 182 MPa and 12.1%, of which the average grain size of the Mg2Si phase was 8.26 μm. Other researchers first melted aluminum ingots and silicon blocks to obtain an aluminum-silicon eutectic, and then added it to magnesium ingots and lithium granules to obtain Mg-Li-3 (Al-Si) alloys, with an average grain size of 10.68 μm for the Mg2Si phase, and the rolled properties of 210 MPa and 11.6%. The coarse Mg2Si phase prevents effective grain refinement in the as-cast state, and large cracks easily form in the Mg2Si phase during deformation, resulting in poor formability. The addition of Si to the matrix to generate fine strengthening phases is a pressing research challenge. Summary of the Invention

[0006] The main objective of this invention is to overcome the shortcomings of the prior art and solve the technical problems of low strength, high preparation cost, and high energy consumption of as-cast Mg-Li alloys. This invention provides an as-cast high-strength, high-toughness, fine-grained magnesium-lithium dual-phase alloy and its resistance heating preparation method.

[0007] The design concept of this invention is as follows:

[0008] On the one hand, the use of resistance furnace heating greatly reduces energy consumption and cost; to improve the high strength and toughness of magnesium-lithium alloy, it is necessary to ensure the quality of raw materials and the vacuum degree of the furnace cavity. Before feeding, the furnace wall is wiped wet once with an industrial lint-free cloth and dry once, for a total of two times. Before feeding, the lithium particles are vacuumed and coated with industrial paraffin wax to solve the oxidation problem of lithium particles; the melting atmosphere is high-purity argon (99% by volume) and a small amount of SF6 (1% by volume), and the gas is charged to positive pressure (0.013MPa). SF6 reacts with Mg to form a MgF2 surface film, which not only has a thin protective film thickness, but also effectively prevents the oxidation of Mg and Li by maintaining the integrity of the surface film; a small amount of SF6 affects the surface morphology of the rolled plate. Experiments have shown that the surface morphology of the rolled plate after introducing a small amount of SF6 into the argon gas is significantly better than that of the rolled plate after introducing only argon gas.

[0009] On the other hand, using spray-formed aluminum-silicon alloy as raw material, the Si particles introduced into the matrix are small in size, which is conducive to obtaining fine-grained structure and better formability, and has become an important means of obtaining fine-grained structure.

[0010] In summary, this invention provides a solution for the resistance heating preparation method of high-strength, high-toughness, fine-grained cast magnesium-lithium dual-phase alloys. While ensuring experimental safety, it proposes detailed improvements to the melting process, providing new ideas and methods for magnesium-lithium alloy melting.

[0011] This invention is achieved through the following technical solution:

[0012] A cast high-strength and high-toughness fine-grained magnesium-lithium dual-phase alloy, wherein the magnesium-lithium dual-phase alloy is a Mg-Li-Al-Si magnesium-lithium alloy, and the composition and mass percentage content of its alloying elements are: Li: 8wt.%, Al: 3wt.%, Si: 0.3wt.%, with the remainder being Mg.

[0013] Grain refinement can effectively improve the overall strength and plasticity of materials. The addition of Si allows the alloy liquid to form a high-strength, high-modulus Mg2Si reinforcing phase during solidification. The precipitation of Mg2Si improves the strength and stability of the magnesium-lithium matrix. The reinforcing phase spheroidizes and is uniformly distributed in the matrix, which can pin dislocations and hinder grain boundary migration, resulting in grain refinement. This is an effective means to improve the strength and toughness of the magnesium-lithium matrix.

[0014] A resistance heating method for preparing a cast high-strength, high-toughness, fine-grained magnesium-lithium dual-phase alloy includes the following steps:

[0015] S1. To improve the vacuum level, wipe the furnace wall with an industrial lint-free cloth once with a wet cloth, and then wipe it with a dry cloth once to ensure that the furnace body is clean, dust-free, and free of water stains.

[0016] S2. Weigh the lithium granules from the paraffin according to the composition and mass percentage of the alloying elements, wrap them immediately with aluminum foil, and place them at the bottom of the crucible; weigh the magnesium ingots, aluminum ingots, and aluminum-silicon master alloy according to the composition and mass percentage of the alloying elements, place them in the crucible, and close the furnace door.

[0017] S3. First, the furnace is evacuated for 1 hour until the furnace pressure reaches 19 Pa. Second, the furnace is evacuated for 30 minutes until the furnace pressure reaches 13 Pa. Third, the furnace is evacuated for 30 minutes until the furnace pressure reaches 11 Pa. Finally, SF6 is introduced into the furnace until the furnace pressure reaches 0.003 MPa, followed by argon gas until the furnace pressure reaches 0.013 MPa. Introducing a small amount of SF6 into a high-purity argon atmosphere can suppress Mg volatilization and improve the surface quality of the rolled plate. The rolled alloy plate has a smooth surface without edge cracks and good formability.

[0018] S4. Resistance heating furnace (reduces energy consumption and lowers costs): After heating the raw material to 450℃, observe whether the paraffin wax adhering to the surface of the lithium particles begins to volatilize. When the paraffin wax volatilizes to the point where the crucible can no longer be clearly seen from the observation port, the melting furnace stops heating and begins to evacuate. When the view inside the furnace becomes clear again, stop evacuating and introduce argon gas until the pressure inside the furnace is 0.013MPa. Industrial paraffin wax is used to protect the surface of the lithium particles. The white smoke emitted by the paraffin wax at 450℃ will not be adsorbed on the observation port. Through intermediate gas washing, excess SF6 and white smoke volatilized inside the furnace during gas filling are removed, purifying the furnace environment. Excessive SF6 will not reduce the alloy's deformation ability and will not affect the experimenter's observation of the situation inside the furnace.

[0019] S5. The melting furnace is heated to 680℃. After the solid alloy in the crucible is completely melted, the stirring paddle is slowly lowered below the surface of the alloy liquid. The speed is set to 250 rpm and the stirring time is 3 minutes. After stirring is completed, the stirring paddle is kept in place. The melting furnace is kept at the temperature for 10 minutes and then the paddle is removed. The transmission rod is slowly swung to complete the casting and obtain the cast high-strength and tough fine-grained magnesium-lithium dual-phase alloy Mg-8Li-3Al-0.3Si (i.e., LA83-0.3Si).

[0020] Further, in step S2, the aluminum-silicon master alloy is a hypereutectic Al-27%Si alloy. The preparation of the hypereutectic Al-27%Si alloy includes the following steps: The preparation process of the hypereutectic Al-27%Si alloy is carried out in a vacuum environment. First, industrial pure aluminum ingots and silicon wafers are melted under a nitrogen protective atmosphere to obtain a molten alloy liquid with a superheat of 200K. Then, the pressure inside the melting furnace is adjusted to 0.6MPa, and the molten alloy liquid is sprayed out from the pouring nozzle with a diameter of 3mm. A nitrogen scanning free-fall atomizer is used to atomize the molten alloy liquid droplets sprayed from the pouring nozzle. Finally, the atomized metal liquid is sprayed onto a rotating substrate with an inclination angle of 25°. During the spraying and forming process, the extraction speed of the substrate is adjusted to 12.8mm / min to obtain the hypereutectic Al-27%Si alloy.

[0021] Furthermore, in step S5, a filter iron plate is installed at the casting port during casting to skim off some of the slag floating on the surface of the molten metal, ensuring the cleanliness of the melt.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] 1. Excellent performance and formability: This invention uses spray-formed Al-27wt.%Si master alloy as raw material. The Si particles introduced into the matrix are small and dispersed in α-Mg and the two-phase grain boundaries. The average size of the Mg2Si phase is 3.5μm, which has a finer microstructure and higher mechanical properties compared with ordinary cast aluminum-silicon alloys. The Mg-8Li-3Al-0.3Si (LA83-0.3Si) prepared by this invention has high strength, reaching 210MPa-220MPa, and plasticity between 18.6% and 20%. Under a microscope, the α and β microstructures are fine, exhibiting high strength and deformability. After rolling, the surface is smooth and crack-free, and the maximum final rolling reduction can reach 60%.

[0024] 2. Low production cost and low loss: The power of medium and high frequency induction heating is about 15KW, while the power of resistance wire heating is about 3KW. Compared with medium and high frequency induction heating, resistance wire heating greatly saves costs and reduces losses.

[0025] 3. Improved experimental environment and enhanced alloy quality: A small amount of SF6 inhibits metal volatilization while improving the surface quality of the rolled sheet. The selection of the highest quality lithium particles and a high-purity argon atmosphere has a positive impact on the strength and plasticity of the alloy. Attached Figure Description

[0026] Figure 1 Comparison of metallographic morphology of as-cast LA83-0.3Si under the same field of view and different magnifications;

[0027] Figure 2The tensile stress-strain curve of as-cast LA83-0.3Si;

[0028] Figure 3 The images show a comparison of the surface morphology of LA83-0.3Si hot-rolled plates. Figure (a) shows the morphology of the hot-rolled plates obtained by melting in an argon atmosphere, where the final rolling reduction of the two plates on the left is 30%, and the final rolling reduction of the two plates on the right is 40%. Figure (b) shows the morphology of the hot-rolled plates obtained by melting in an argon atmosphere with a small amount of SF6, where the final rolling reduction of the plates from left to right is 30%, 40%, 50%, and 60%, respectively.

[0029] Figure 4 for Figure 3 (b) Stress-strain curves corresponding to the rolled plate. Detailed Implementation

[0030] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0031] A cast high-strength and high-toughness fine-grained magnesium-lithium dual-phase alloy, wherein the magnesium-lithium dual-phase alloy is a Mg-Li-Al-Si magnesium-lithium alloy, and the composition and mass percentage content of its alloying elements are: Li: 8wt.%, Al: 3wt.%, Si: 0.3wt.%, with the remainder being Mg.

[0032] A resistance heating method for preparing a cast high-strength, high-toughness, fine-grained magnesium-lithium dual-phase alloy includes the following steps:

[0033] S1. Wipe the furnace wall with an industrial lint-free cloth once, and then wipe it dry once to ensure that the furnace body is clean, dust-free, and free of water stains.

[0034] S2. Weigh the lithium granules from the paraffin according to the composition and mass percentage of the alloying elements, then immediately wrap them in aluminum foil and place them at the bottom of the crucible. Weigh the magnesium ingots, aluminum ingots, and aluminum-silicon master alloy according to the composition and mass percentage of the alloying elements and place them in the crucible. Close the furnace door. The aluminum-silicon master alloy is a hypereutectic Al-27%Si alloy. The preparation of the hypereutectic Al-27%Si alloy includes the following steps: The preparation process of the hypereutectic Al-27%Si alloy is carried out in a vacuum environment. First, industrial pure aluminum ingots and silicon wafers are melted under a nitrogen protective atmosphere to obtain a superheat of 20°C. The molten alloy liquid was prepared at 0K; then, the pressure inside the melting furnace was adjusted to 0.6MPa, and the molten alloy liquid was sprayed out from the pouring nozzle with a diameter of 3mm. Nitrogen gas was used to scan the free fall atomizer to atomize the molten alloy liquid droplets sprayed from the pouring nozzle, and the atomized metal liquid obtained a high flow rate; finally, the atomized metal liquid was sprayed onto a rotating substrate with an inclination angle of 25°. During the spraying process, the extraction speed of the substrate was adjusted to 12.8mm / min to obtain a uniform deposit diameter and maintain a constant spraying distance, thus obtaining a hypereutectic Al-27%Si alloy;

[0035] S3. First, the furnace is evacuated for 1 hour until the furnace pressure reaches 19 Pa. Second, the furnace is evacuated for 30 minutes until the furnace pressure reaches 13 Pa. Third, the furnace is evacuated for 30 minutes until the furnace pressure reaches 11 Pa. Finally, SF6 is introduced into the furnace until the furnace pressure reaches 0.003 MPa, and then argon is introduced until the furnace pressure reaches 0.013 MPa. The difficulty of this invention lies in controlling the SF6 content.

[0036] S4. After the resistance heating furnace is heated until the actual temperature of the raw material reaches 450℃, observe whether the paraffin attached to the surface of the lithium particles begins to volatilize. When the paraffin volatilizes to the point that the crucible can no longer be clearly seen from the observation port, the melting furnace stops heating and starts to draw a vacuum. When the view inside the furnace is clear again, stop drawing a vacuum and introduce argon gas until the pressure inside the furnace is 0.013MPa.

[0037] S5. The melting furnace is heated to 680℃. After the solid alloy in the crucible is completely melted, the stirring paddle is slowly lowered below the surface of the alloy liquid. The speed is set to 250 rpm and the stirring time is 3 minutes. After stirring, the stirring paddle is kept in place. The melting furnace is kept at the temperature for 10 minutes and then the paddle is removed. The transmission rod is slowly swung. A filter iron plate is installed at the casting port. During casting, some of the slag floating on the metal liquid can be skimmed off to ensure the cleanliness of the melt. Casting is completed to obtain a cast high-strength, tough, fine-grained magnesium-lithium dual-phase alloy.

[0038] The as-cast LA83-0.3Si alloy prepared in this specific embodiment exhibits high mechanical properties, the reason for which is as follows:

[0039] I. Fine-grained structure in the as-cast state, such as Figure 1 As shown, under a metallographic microscope, the β-Li phase structure is fine and exhibits a continuous network distribution, which divides the α-Mg matrix into fine and irregular phases. The β phase has good deformation ability, and the dispersed and continuous distribution of the β phase gives the alloy a high elongation at break.

[0040] Second, the fine black granular phase distributed in the matrix, namely the Mg2Si hard phase, the Mg2Si phase spheroidizes and is uniformly distributed in the matrix, which can pin dislocations and hinder grain boundary migration, resulting in grain refinement.

[0041] Depend on Figure 2 The stress-strain curves show that the mechanical properties of the as-cast alloy exhibit good repeatability.

[0042] In addition, the present invention introduces a small amount of SF6 into a high-purity argon atmosphere to generate a MgF2 surface film, which can suppress the volatilization of Mg and Li, without affecting the plasticity of the alloy and improving the surface quality of the rolled plate.

[0043] To further verify the surface quality of the rolled plate, the initial thickness was 5mm, and the final thickness was 1mm. Rolling was performed at a roll temperature of 250℃. The first pass was held in a muffle furnace at 250℃ for 30 minutes with a reduction of 10%; the intermediate passes were held for 10 minutes with a reduction of 20%. Figure 3 As shown in Figure (b), the final rolling reduction of the plates from left to right is 30%, 40%, 50%, and 60%, corresponding to 7, 6, 5, and 4 rolling passes, respectively. The surface of the rolled alloy plates is smooth and free of edge cracks. The stress-strain curves of the rolled plates in Figure (b) are shown below. Figure 4 As shown, the as-cast high-strength and tough fine-grained magnesium-lithium dual-phase alloy provided in this application has good formability.

[0044] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A resistance heating method for preparing a cast high-strength, high-toughness, fine-grained magnesium-lithium dual-phase alloy, characterized in that: Includes the following steps: S1. Wipe the furnace wall with an industrial lint-free cloth once, and then wipe it dry once to ensure that the furnace body is clean, dust-free, and free of water stains. S2. The magnesium-lithium dual-phase alloy is a Mg-Li-Al-Si magnesium-lithium alloy, and its alloy element composition and mass percentage content are: Li: 8wt.%, Al: 3wt.%, Si: 0.3wt.%, with the remainder being Mg; after weighing the lithium particles from the paraffin according to the alloy element composition and mass percentage, immediately wrap them in aluminum foil and place them at the bottom of the crucible; weigh the magnesium ingots, aluminum ingots, and aluminum-silicon master alloy according to the alloy element composition and mass percentage and place them in the crucible, then close the furnace door; The aluminum-silicon master alloy is a hypereutectic Al-27%Si alloy. The preparation of the hypereutectic Al-27%Si alloy includes the following steps: The preparation process of the hypereutectic Al-27%Si alloy is carried out in a vacuum environment. First, industrial pure aluminum ingots and silicon wafers are melted under a nitrogen protective atmosphere to obtain a molten alloy liquid with a superheat of 200K. Then, the pressure inside the melting furnace is adjusted to 0.6MPa, and the molten alloy liquid is sprayed out from a pouring nozzle with a diameter of 3mm. A nitrogen scanning free-fall atomizer is used to atomize the molten alloy liquid droplets sprayed from the pouring nozzle. Finally, the atomized metal liquid is sprayed onto a rotating substrate with an inclination angle of 25°. During the spraying and forming process, the extraction speed of the substrate is adjusted to 12.8 mm / min to obtain the hypereutectic Al-27%Si alloy. S3. First, the furnace is evacuated for 1 hour until the furnace pressure reaches 19 Pa. Second, the furnace is evacuated for 30 minutes until the furnace pressure reaches 13 Pa. Third, the furnace is evacuated for 30 minutes until the furnace pressure reaches 11 Pa. Finally, SF6 is introduced into the furnace until the furnace pressure reaches 0.003 MPa, and then argon is introduced until the furnace pressure reaches 0.013 MPa. S4. After the resistance heating furnace is heated until the actual temperature of the raw material reaches 450℃, observe whether the paraffin attached to the surface of the lithium particles begins to volatilize. When the paraffin volatilizes to the point that the crucible can no longer be clearly seen from the observation port, the melting furnace stops heating and starts to draw a vacuum. When the view inside the furnace is clear again, stop drawing a vacuum and introduce argon gas until the pressure inside the furnace is 0.013MPa. S5. The melting furnace is heated to 680℃. After the solid alloy in the crucible is completely melted, the stirring paddle is slowly lowered below the surface of the alloy liquid. The speed is set to 250 rpm and the stirring time is 3 minutes. After stirring is completed, the stirring paddle is kept in place. The melting furnace is kept at the temperature for 10 minutes and then the paddle is removed. The transmission rod is slowly swung to complete the casting and obtain a cast high-strength and tough fine-grained magnesium-lithium dual-phase alloy.

2. The method for preparing a cast high-strength, high-toughness, fine-grained magnesium-lithium dual-phase alloy by resistance heating according to claim 1, characterized in that: In step S5, a filter iron sheet is installed at the casting port during casting.

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