Preparation method of reinforced and toughened magnesium-lithium alloy

Abstract

The invention belongs to the technical field of metal material preparation, and particularly relates to a preparation method of a reinforced and toughened magnesium-lithium alloy, which comprises the following steps: weighing magnesium, lithium, TiB2 particles, graphene nanosheets, sodium hexametaphosphate and a lithium-boron alloy, and carrying out chemical pickling, vacuum drying and low-temperature plasma activation on a magnesium block and a lithium block to obtain the reinforced and toughened magnesium-lithium alloy. Carrying out three-dimensional mixing and dispersion on the TiB2 particles and the graphene nanosheets; magnesium and lithium are heated to be molten, the dispersed TiB2 particles, graphene nanosheets, sodium hexametaphosphate and lithium-boron alloy are added, electromagnetic stirring is carried out, the obtained melt is poured into a preheated metal mold, a 0.5-1.5 T stable magnetic field is applied, then low-stress deformation is carried out, dynamic recrystallization is initiated, and finally stress relief annealing is carried out on a formed part. The tensile strength, fracture toughness and elongation of the magnesium-lithium alloy can be effectively improved, the production period is shortened, and the percent of pass is increased.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal material preparation, and in particular relates to a method for preparing a reinforced and toughened magnesium-lithium alloy. Background Art

[0002] Magnesium-lithium alloy has low density (1.3-1.6g / cm 3 ), high specific strength, and has broad application prospects in aerospace, automotive lightweighting and other fields. However, traditional magnesium-lithium alloys have problems such as coarse grains, insufficient room temperature mechanical properties (tensile strength <150MPa, elongation <8%), and complex forming processes (requiring multiple extrusion / rolling passes), which limit their engineering applications.

[0003] While existing technologies can strengthen alloys to a certain extent by adding ceramic particles (such as SiC and Al2O3) or carbon materials (such as carbon nanotubes), they suffer from defects such as uneven dispersion of the reinforcing phase and weak interfacial bonding. Furthermore, traditional solidification processes struggle to achieve effective grain refinement. Furthermore, traditional casting-extrusion processes have long production cycles (typically >24 hours), high energy consumption, and a yield rate of only 60%-70%. There is an urgent need to develop a more efficient, low-energy process for strengthening and toughening alloys. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for preparing a reinforced and toughened magnesium-lithium alloy, which can effectively improve the tensile strength, fracture toughness and elongation of the magnesium-lithium alloy, shorten the production cycle and improve the qualified rate.

[0005] The technical solutions adopted by the present invention are as follows:

[0006] A method for preparing a reinforced and toughened magnesium-lithium alloy comprises the following steps:

[0007] Step 1, raw material preparation: weigh 55%-65% magnesium, 25%-35% lithium, 0.5%-1.5% TiB2 particles, 0.1%-0.3% graphene nanosheets, 0.005%-0.03% sodium hexametaphosphate, and 0.2%-0.5% lithium boron alloy by mass percentage;

[0008] Step 2: Raw material pretreatment: chemical pickling, vacuum drying and low-temperature plasma activation of magnesium and lithium blocks, and three-dimensional mixing and dispersion of TiB2 particles and graphene nanosheets;

[0009] Step 3: Vacuum induction melting: at a vacuum degree of ≤1×10 -3 Under Pa conditions, magnesium and lithium are heated to 700-800°C to melt, and dispersed TiB2 particles, graphene nanosheets, sodium hexametaphosphate and lithium boron alloy are added and electromagnetically stirred;

[0010] Step 4, magnetic field assisted solidification: pour the melt prepared in step 3 into a preheated metal mold, apply a 0.5-1.5T constant magnetic field, and control the cooling rate to form a fine-grained structure;

[0011] Step 5: Superplastic forming: at 250-350℃ and strain rate 10 -4 -10 -3 s -1 Low stress deformation is carried out under conditions to induce dynamic recrystallization;

[0012] Step 6: Subsequent processing: Stress relief annealing of the molded parts.

[0013] Furthermore, the step 2 includes the following steps:

[0014] Step 201, chemical pickling: immerse magnesium and lithium in a 1:5 dilute hydrochloric acid solution for 5-10 minutes, and rinse with deionized water until neutral;

[0015] Step 202: vacuum drying and dehydration: the magnesium and lithium after impurities removal are dried at 120°C and vacuum degree ≤ 1×10 -2 Drying under Pa conditions for 4h;

[0016] Step 203, low-temperature plasma activation: The dehydrated magnesium and lithium are treated in an environment with a pressure of 10-100 Pa using a mixture of argon and hydrogen with a volume ratio of 9:1 and a power of 100-200 W for 5-10 minutes, and the high-energy particles in the plasma bombard the surfaces of the magnesium and lithium;

[0017] Step 204, three-dimensional mixed dispersion of the reinforcement phase: the TiB2 particles and graphene nanosheets are first dispersed by high-speed shearing at 8000-10000 rpm for 30 minutes, then ball-milled at 400 rpm for 3 hours, and finally subjected to 60 kHz ultrasonic vibration for 1 hour.

[0018] Furthermore, the step 3 includes the following steps:

[0019] Step 301: When the vacuum degree is ≤1×10 -3 Melt magnesium and lithium in a melting furnace at 700-800°C at a rate of 10°C / min and keep the temperature for 10 minutes.

[0020] Step 302: Add the dispersed TiB2 particles, start electromagnetic stirring simultaneously, continue stirring for 15 minutes, and let it stand for 5 minutes.

[0021] Furthermore, the step 4 includes the following steps:

[0022] Step 401: Pour the melt into a water-cooled metal mold preheated to 200-300°C, and immediately apply a 0.5-1.5T constant magnetic field;

[0023] Step 402: Control the cooling rate: if the alloy has a high lithium content (Li≥30%), the cooling rate is 10-20°C / min; if the alloy has a low lithium content (Li<30%), the cooling rate is 30-50°C / min.

[0024] Furthermore, the step 5 includes the following steps:

[0025] Step 501: heating the blank to 250-350°C and keeping the temperature for 30-60 minutes to fully soften the grains;

[0026] Step 502: At a strain rate of 10 -4 -10 -3 s -1 Low stress deformation is applied within a certain range to induce dynamic recrystallization.

[0027] Furthermore, in step 501, the lithium content is taken as a starting point of 25%, and the temperature is reduced by 5-10° C. for every 1% increase in the lithium content.

[0028] Furthermore, the low stress deformation in step 502 is tensile deformation or bulging forming.

[0029] Furthermore, the step 6 is to anneal the molded part at 150-250° C. for 1-3 hours.

[0030] The technical effects achieved by the present invention are:

[0031] The present invention provides a method for preparing a reinforced and toughened magnesium-lithium alloy. By increasing the nucleation rate through magnetic field control and combining it with dynamic recrystallization, the method realizes multi-level refinement of grains from the solidified state to the formed state, effectively improving the tensile strength, fracture toughness and elongation of the magnesium-lithium alloy. Compared with the traditional casting-extrusion process, the production cycle is greatly shortened, energy consumption is effectively reduced, and the qualified rate is greatly improved. Complex structural parts such as thin-walled pipes and ribbed plates can be prepared. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a flow chart of the steps of the present invention. DETAILED DESCRIPTION

[0033] In order to make the purpose and advantages of the present invention more clearly understood, the present invention is described in detail below with reference to the following examples. It should be understood that the following text is only used to describe one or more specific embodiments of the present invention and does not strictly limit the scope of protection of the present invention.

[0034] like Figure 1 As shown, a method for preparing a reinforced and toughened magnesium-lithium alloy includes the following steps.

[0035] Step 1, raw material preparation: weigh 55%-65% magnesium, 25%-35% lithium, 0.5%-1.5% TiB2 particles, 0.1%-0.3% graphene nanosheets, 0.005%-0.03% sodium hexametaphosphate, and 0.2%-0.5% lithium boron alloy by mass percentage;

[0036] Among them, magnesium and lithium form the matrix of magnesium-lithium alloy. In the magnesium-lithium alloy matrix, magnesium provides strength support, and lithium reduces the alloy density and improves superplasticity.

[0037] Among them, TiB2 particles and graphene nanosheets are the reinforcement phase;

[0038] TiB2 particles act as heterogeneous nucleating agents to promote grain refinement and at the same time act as hard points to hinder dislocation slip and improve strength;

[0039] Graphene nanosheets utilize their high elastic modulus (1TPa) and large aspect ratio (>100) to hinder crack propagation through a "bridging effect" and simultaneously serve as a second nucleation site to synergistically refine grains.

[0040] Among them, sodium hexametaphosphate and lithium boron alloy are process additives;

[0041] Sodium hexametaphosphate acts as a degassing agent and refining agent, which absorbs H2 and oxide inclusions in the melt by forming a low-melting-point composite salt (such as Na3PO4-MgO-Li2O), thereby reducing the porosity.

[0042] Lithium-boron alloy (Li-B binary alloy, B content 10%-15%) acts as a deoxidizer and lithium content regulator, reacting with MgO in the melt to form Li2O·B2O3·MgO composite slag, while compensating for the volatilization loss of lithium during the smelting process.

[0043] Step 2: Raw material pretreatment: chemical pickling, vacuum drying and low-temperature plasma activation of magnesium and lithium blocks, and three-dimensional mixing and dispersion of TiB2 particles and graphene nanosheets;

[0044] Specifically, step 2 includes the following steps:

[0045] Step 201, chemical pickling: Soak magnesium and lithium in a 1:5 dilute hydrochloric acid solution for 5-10 minutes to remove oxides (such as MgO, Li2O), oil stains and adsorbed impurities on the surfaces of magnesium and lithium, to avoid the formation of inclusions or reduction of alloy purity during smelting.

[0046] After pickling, rinse thoroughly with deionized water until it is neutral to prevent residual acid from introducing harmful ions such as Cl-, which can prevent pores or corrosion problems during subsequent smelting.

[0047] Step 202: vacuum drying and dehydration: the magnesium and lithium after impurities removal are dried at 120°C and vacuum degree ≤ 1×10-2 Drying for 4 hours under Pa conditions can completely remove moisture from the surface and pores of magnesium and lithium to avoid melt splashing or formation of pore defects due to water vaporization during smelting (magnesium-lithium alloys react violently when exposed to water), ensuring the safety of the smelting process and the density of the ingot.

[0048] Step 203, low-temperature plasma activation: The dehydrated magnesium and lithium are treated in a low-pressure environment of 10-100 Pa (0.1-1 mbar) using an argon-hydrogen mixture (Ar-H2) with a volume ratio of 9:1 and a power of 100-200 W for 5-10 minutes. The high-energy particles in the plasma are used to bombard the surfaces of the magnesium and lithium to achieve surface cleaning and surface activation. Surface cleaning can further remove residual organic matter or very thin oxide films, exposing fresh metal surfaces. Surface activation can increase surface atomic active sites, improve the wettability of the magnesium-lithium melt and the reinforcement phase (TiB2 particles), promote heterogeneous nucleation and interfacial bonding during solidification, and reduce particle agglomeration or segregation.

[0049] The "low-temperature environment" in step 203 is a non-thermal equilibrium plasma environment generated by RF power within a low-pressure vacuum chamber and an argon-hydrogen mixed atmosphere. Its key parameters are: pressure 10–100 Pa, gas temperature near room temperature, electron energy 5–10 eV, and treatment time 5–10 minutes. This environment achieves efficient cleaning and nanoscale active modification of the magnesium-lithium alloy surface through precisely controlled synergistic effects of physical sputtering and chemical activation, while avoiding thermal damage to the substrate.

[0050] Step 204, three-dimensional mixed dispersion of the reinforcement phase: TiB2 particles and graphene nanosheets are first dispersed by high-speed shearing at 8000-10000 rpm for 30 minutes, then ball milled at 400 rpm for 3 hours (ball-to-material ratio 10:1, agate grinding balls), and finally subjected to 60 kHz ultrasonic vibration for 1 hour (anhydrous ethanol medium).

[0051] Specifically, step 204 is to first perform high-speed shear dispersion (8000-10000 rpm, 30 minutes) on the TiB2 particles, using strong shear force to break up the initial agglomerates of the TiB2 particles, so that they are initially dispersed into submicron-sized small particle groups, reducing the van der Waals force between the particles.

[0052] The suspension after shearing and dispersion of TiB2 particles is then freeze-dried. Through the low-temperature solidification-vacuum sublimation process, the particles are prevented from re-agglomerating due to solvent evaporation in conventional drying, and the particles are kept in a dispersed state.

[0053] The material is then ball-milled at 400 rpm for three hours. This further breaks down the particles (to the nanoscale) through ball collisions, while also introducing lattice distortion, increasing the surface energy of the TiB2 particles and strengthening the mechanical engagement and chemical interaction between the TiB2 particles and the magnesium-lithium matrix. The increased surface defects during ball milling serve as nucleation sites during solidification, promoting grain refinement.

[0054] After ball milling, the TiB2 particles were ultrasonically treated at 60kHz for 1 hour. The microjets generated by the cavitation effect were used to impact the TiB2 particle agglomerates, achieving nanoscale dispersion and promoting uniform distribution of the particles in the melt to avoid gravity sedimentation or local enrichment.

[0055] Step 3: Vacuum induction melting: at a vacuum degree of ≤1×10 -3 Under Pa conditions, magnesium and lithium are heated to 700-800°C to melt, and dispersed TiB2 particles, graphene nanosheets, sodium hexametaphosphate and lithium boron alloy are added and electromagnetically stirred;

[0056] Specifically, step 3 includes the following steps:

[0057] Step 301: When the vacuum degree is ≤1×10 -3 Melt magnesium and lithium in a melting furnace at 700-800°C at a rate of 10°C / min and keep the temperature for 10 minutes.

[0058] Step 302: Slowly add dispersed TiB2 particles (accounting for 0.5%-1.5%) and simultaneously start electromagnetic stirring (speed 200-300 rpm) for 15 minutes to evenly distribute the particles. Let it stand for 5 minutes to remove scum and prevent lithium volatilization and oxidation.

[0059] Step 4, magnetic field assisted solidification: pour the melt prepared in step 3 into a preheated metal mold, apply a 0.5-1.5T constant magnetic field, and control the cooling rate to form a fine-grained structure;

[0060] Specifically, step 4 includes the following steps:

[0061] Step 401: Pour the melt into a water-cooled metal mold preheated to 200-300°C, and immediately apply a 0.5-1.5T constant magnetic field (direction perpendicular to the mold axis);

[0062] Step 402: Control the cooling rate using a water cooling system: 10-20°C / min for high lithium content alloys (e.g., Li ≥ 30%) to avoid precipitation of low-temperature brittle phases; 30-50°C / min for low lithium content alloys (e.g., Li < 30%) to promote nucleation;

[0063] The magnetic field induces forced convection in the melt, inhibiting the growth of coarse dendrites and forming an equiaxed crystal structure. The grain size is smaller than that of traditional processes, and the uniformity index of TiB2 particle distribution is improved.

[0064] Step 5: Superplastic forming: at 250-350℃ and strain rate 10 -4 -10 -3 s -1 Low stress deformation is carried out under conditions to induce dynamic recrystallization;

[0065] Specifically, step 5 includes the following steps:

[0066] Step 501: heating the blank to 250-350° C. (starting from 25% lithium content, the temperature decreases by 5-10° C. for every 1% increase in lithium content), and keeping the temperature for 30-60 minutes to fully soften the grains;

[0067] Step 502: At a strain rate of 10 -4 -10 -3 s -1 Low stress deformation (such as stretching and bulging) is applied within a certain range to induce dynamic recrystallization, further refine the grains, form high-density substructures at the grain boundaries, and improve the elongation.

[0068] Here, the low stress deformation in step 502 may be a tensile deformation or a bulging forming;

[0069] Among them, tensile deformation is to apply axial force to the blank through a hydraulic press or servo press, which is suitable for upsetting or stretching of bars and pipes (such as preparing bolts and shaft parts).

[0070] Among them, bulging forming is to use gas pressure (such as argon) or mechanical punch to apply radial bulging force to the tubular blank, which is suitable for thin-walled pipes, complex curved surface structures (such as automobile mufflers, aerospace special-shaped parts).

[0071] Step 6: Subsequent processing: Stress relief annealing of the molded parts.

[0072] Anneal the molded parts at 150-250℃ for 1-3h to reduce residual stress;

[0073] Sand blasting or electrolytic polishing is optional, with surface roughness Ra≤0.8μm.

[0074] In summary, this technical solution improves the nucleation rate through magnetic field regulation, and combines dynamic recrystallization to achieve multi-level refinement of grains from the solidified state to the formed state, effectively improving the tensile strength, fracture toughness and elongation of magnesium-lithium alloys. The production cycle is greatly shortened compared to the traditional casting-extrusion process, energy consumption is effectively reduced, and the qualified rate is greatly improved. It can be used to prepare complex structural parts such as thin-walled pipes and ribbed plates.

[0075] The foregoing is merely a preferred embodiment of the present invention. It should be noted that those skilled in the art may make various improvements and modifications without departing from the principles of the present invention, and such improvements and modifications are also within the scope of protection of the present invention. Structures, devices, and operating methods not specifically described or explained herein shall, unless otherwise specified or limited, be implemented in accordance with conventional means in the art.

Claims

1. A method for preparing a reinforced and toughened magnesium-lithium alloy, characterized in that: The following steps are involved: Step 1, raw material preparation: weigh 55%-65% magnesium, 25%-35% lithium, 0.5%-1.5% TiB2 particles, 0.1%-0.3% graphene nanosheets, 0.005%-0.03% sodium hexametaphosphate, and 0.2%-0.5% lithium boron alloy by mass percentage; Step 2: Raw material pretreatment: chemical pickling, vacuum drying and low-temperature plasma activation of magnesium and lithium blocks, and three-dimensional mixing and dispersion of TiB2 particles and graphene nanosheets; Step 3: Vacuum induction melting: at a vacuum degree of ≤1×10 - Under 3Pa conditions, magnesium and lithium are heated to 700-800℃ to melt, and dispersed TiB2 particles, graphene nanosheets, sodium hexametaphosphate and lithium boron alloy are added and electromagnetically stirred; Step 4, magnetic field assisted solidification: pour the melt prepared in step 3 into a preheated metal mold, apply a 0.5-1.5T constant magnetic field, and control the cooling rate to form a fine-grained structure; Step 5: Superplastic forming: at 250-350℃ and strain rate 10 -4 -10 -3 s -1 Low stress deformation is carried out under conditions to induce dynamic recrystallization; Step 6: Subsequent processing: Stress relief annealing of the molded parts.

2. The method for preparing a reinforced and toughened magnesium-lithium alloy according to claim 1, characterized in that: The step 2 comprises the following steps: Step 201, chemical pickling: immerse magnesium and lithium in a 1:5 dilute hydrochloric acid solution for 5-10 minutes, and rinse with deionized water until neutral; Step 202: vacuum drying and dehydration: the magnesium and lithium after impurities removal are dried at 120°C and vacuum degree ≤ 1×10 -2 Drying under Pa conditions for 4h; Step 203, low-temperature plasma activation: The dehydrated magnesium and lithium are treated in an environment with a pressure of 10-100 Pa using a mixture of argon and hydrogen with a volume ratio of 9:1 and a power of 100-200 W for 5-10 minutes, and the high-energy particles in the plasma bombard the surfaces of the magnesium and lithium; Step 204, three-dimensional mixed dispersion of the reinforcement phase: the TiB2 particles and graphene nanosheets are first dispersed by high-speed shearing at 8000-10000 rpm for 30 minutes, then ball-milled at 400 rpm for 3 hours, and finally subjected to 60 kHz ultrasonic vibration for 1 hour.

3. The method for preparing a reinforced and toughened magnesium-lithium alloy according to claim 1, wherein: The step 3 comprises the following steps: Step 301: When the vacuum degree is ≤1×10 -3 Melt magnesium and lithium in a melting furnace at 700-800°C at a rate of 10°C / min and keep the temperature for 10 minutes. Step 302: Add the dispersed TiB2 particles, start electromagnetic stirring simultaneously, continue stirring for 15 minutes, and let it stand for 5 minutes.

4. The method for preparing a reinforced and toughened magnesium-lithium alloy according to claim 1, wherein: The step 4 comprises the following steps: Step 401: Pour the melt into a water-cooled metal mold preheated to 200-300°C, and immediately apply a 0.5-1.5T constant magnetic field; Step 402: Control the cooling rate: if the alloy has a high lithium content (Li≥30%), the cooling rate is 10-20°C / min; if the alloy has a low lithium content (Li<30%), the cooling rate is 30-50°C / min.

5. The method for preparing a reinforced and toughened magnesium-lithium alloy according to claim 1, wherein: The step 5 comprises the following steps: Step 501: heating the blank to 250-350°C and keeping the temperature for 30-60 minutes to fully soften the grains; Step 502: At a strain rate of 10 -4 -10 -3 s -1 Low stress deformation is applied within a certain range to induce dynamic recrystallization.

6. The method for preparing a reinforced and toughened magnesium-lithium alloy according to claim 5, characterized in that: In step 501 , the starting point is a lithium content of 25%. For every 1% increase in lithium content, the temperature is reduced by 5-10° C.

7. The method for preparing a reinforced and toughened magnesium-lithium alloy according to claim 5, characterized in that: The low stress deformation in step 502 is tensile deformation or bulging forming.

8. The method for preparing a reinforced and toughened magnesium-lithium alloy according to claim 1, characterized in that: The step 6 is to anneal the molded part at 150-250° C. for 1-3 hours.

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