High-toughness rare earth-containing magnesium alloy and preparation method thereof

Abstract

The invention relates to the technical field of magnesium alloys, in particular to a high-toughness rare earth-containing magnesium alloy and a preparation method thereof.The high-toughness rare earth-containing magnesium alloy is prepared from, by weight, 3-8 parts of Zn, 1-5 parts of RE, 0.5-3 parts of Al, 0.1-1 part of Mn, 0.05-0.3 part of Zr and the balance Mg; rE is a combination of Y, Nd and La in a mass ratio of (2-3): 1: (0.5-1). Zinc is used as a secondary main element to replace part of aluminum, efficient solid solution strengthening is achieved through high solid solubility of zinc in magnesium, meanwhile, the morphology and distribution of a precipitated phase are regulated through rare earth, a beta-MgZn2 phase is formed, the high-temperature stability is better, balance optimization of strength, toughness and corrosion resistance is achieved, meanwhile, the welding performance of the alloy is improved through combination of Y, Nd, La and Zr, and the welding performance of the alloy is improved. The hot crack tendency is reduced, the brittle phase problem of a traditional high aluminum alloy is avoided, and the corrosion resistance and the machining performance are synergistically improved.

Description

Technical Field

[0001] The present invention relates to the technical field of magnesium alloys, and in particular to a high-strength and toughness rare earth-containing magnesium alloy and a preparation method thereof. Background Art

[0002] As the lightest structural metal, magnesium alloy has a density of only 2 / 3 of aluminum and 1 / 4 of steel, and has irreplaceable advantages in the field of lightweighting. In order to improve the performance of magnesium alloy, it is currently mainly achieved by adding alloying elements and optimizing the preparation process. However, traditional Mg-Al-Zn (AZ) magnesium alloys have significant defects: insufficient room temperature strength: conventional AZ31 alloy has a tensile strength of only about 250 MPa and an elongation of about 15%, which is difficult to meet the requirements of load-bearing structural parts; high-temperature performance declines rapidly: when the temperature exceeds 150°C, the strength drops sharply due to the intensification of grain boundary sliding.

[0003] Currently, rare earth magnesium alloys can be formed by adding rare earth elements (such as Y, Nd, Gd, etc.). The addition of rare earth elements can effectively refine the grains and improve the structure and properties of the alloy. However, the existing rare earth magnesium alloys still have alloy design limitations in composition design and preparation process. Currently, most rare earth magnesium alloys contain aluminum as the main alloying element, resulting in insufficient room temperature toughness. In addition, aluminum and certain rare earths (such as Y) easily form brittle phases, making it difficult to fully exert the synergistic effect of rare earth elements and other alloying elements, and unable to meet the growing demand for high-strength and toughness magnesium alloys. In addition, traditional preparation requires multiple homogenization annealing and aging treatments, which has high energy consumption and long cycles. There is a problem of lengthy preparation process routes, and high-purity rare earth metals are expensive. In addition, the rare earth yield rate of existing processes is only 60-70%, and the preparation cost is relatively high.

[0004] In summary, the development of a high-strength and toughness rare earth-containing magnesium alloy and its preparation method is still a key issue that needs to be urgently addressed in the field of magnesium alloy technology. Summary of the Invention

[0005] In view of the problems existing in the prior art, the object of the present invention is to provide a high-strength and toughness rare earth-containing magnesium alloy and a preparation method thereof.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A high-strength and tough rare-earth magnesium alloy is prepared from the following raw materials in parts by weight: 3-8 parts of Zn, 1-5 parts of RE, 0.5-3 parts of Al, 0.1-1 parts of Mn, 0.05-0.3 parts of Zr, and the balance is Mg; RE is a combination of Y, Nd, and La, and the mass ratio is 2-3:1:0.5-1.

[0008] The present invention is further configured such that the mass ratio of Y, Nd and La in the rare earth elements is 2:1:1.

[0009] A method for preparing a high-strength and tough rare earth-containing magnesium alloy comprises the following steps:

[0010] Ⅰ. Raw material pretreatment: accurately weigh the raw materials according to the weight ratio, including magnesium ingot (99.95% pure magnesium), zinc ingot (99.99% pure zinc), aluminum ingot (99.98% pure aluminum), rare earth alloy (Y:Nd:La, purity ≥99.9%), manganese ingot (99.95% pure manganese) and zirconium salt (k2ZrF6, purity ≥98%), and pretreat the raw materials;

[0011] II. Ultrasonic assisted melting: The raw materials are melted in a vacuum environment, and ultrasonic vibration is used to promote the uniform dispersion of rare earth elements to obtain a mixed melt;

[0012] III. Differential temperature casting: Using gradient temperature mold combined with ultrasonic excitation, the liquid melt is cast and solidified, and after cooling, a magnesium alloy ingot is obtained, with a grain size controlled to be ≤20μm;

[0013] IV. Composite aging treatment: The magnesium alloy ingot is subjected to supersaturated solid solution treatment and double-temperature step aging treatment in sequence to precipitate nano-scale strengthening phase;

[0014] V. A rare earth-containing magnesium alloy is obtained by hot extrusion treatment.

[0015] The present invention is further configured as follows: in step I, the following steps are included:

[0016] A1. Cut the magnesium ingot into 5-10cm pieces 3 The surface of the magnesium block was ultrasonically cleaned with anhydrous ethanol for 10 min;

[0017] A2. Processing zinc ingots and aluminum ingots into zinc flakes and aluminum flakes with a thickness of ≤2mm;

[0018] A3. Crush the rare earth alloy into particles of 5-10 mm and dry them in a vacuum oven at 150°C for 2 h.

[0019] A4. Cut the manganese ingot into thin strips with a diameter of 5 mm. To compensate for the 20% burn-out rate during the smelting process, weigh the zirconium salt at 1.2 times the theoretical zirconium content and grind the zirconium salt to below 200 mesh.

[0020] The present invention is further configured as follows: in step II, the following steps are included:

[0021] B1. Melting of magnesium matrix: Add magnesium block to crucible and heat to 700°C at a heating rate of 10°C / min. Keep warm for 15-30 min until completely melted to obtain liquid pure magnesium melt. Turn on argon protection at a flow rate of 5 L / min and stir with a graphite stirring rod for three times with an interval of 5 min between each stirring.

[0022] B2. Adding the main alloying elements: Add zinc flakes and aluminum flakes to the crucible in sequence, using a progressive heating method. After each addition, increase the temperature by 10°C to 720°C. Use a mechanical stirrer to stir at 150 rpm for 5-10 minutes until the alloying elements are completely dissolved.

[0023] B3. Ultrasonic dispersion of rare earth alloy: Rare earth alloy preheated to 300-400°C was added to the crucible in three batches, with an interval of 2 minutes between each batch. The ultrasonic vibration system was simultaneously started with a power of 8kW and a duty cycle of 90%. The micro-jets generated by the ultrasonic cavitation effect at a speed of ≥100m / s were used to break up rare earth agglomerates. At the same time, the ultrasonic vibration caused the melt to produce an acoustic streaming effect, which forced convection of rare earth particles in the melt and uniformly distributed the elements.

[0024] B4. Trace element addition and refining: Add manganese bars and zirconium salts to the crucible, reduce the power of the ultrasonic vibration system to 6kW, continue ultrasonic treatment for 10-20 minutes, control the melt temperature at 720±10℃, then add 0.3% of the melt mass of a fluorine-free refining agent, spray the fluorine-free refining agent evenly into the melt through a pneumatic powder spraying device, and mechanically stir for 20-30 minutes. Utilize the wettability difference between the flux and inclusions to achieve impurity adsorption. After standing for 15-25 minutes, remove the surface slag layer with a thickness of ≤5mm to obtain a mixed melt.

[0025] The present invention is further configured as follows: in step B2, when the melt temperature fluctuation is monitored by an infrared thermometer and is ≤±5°C, the alloy elements are considered to be completely dissolved; in step B3, the melt conductivity is monitored in real time, and when the conductivity fluctuation is <2%, it is considered to be uniformly dispersed; in step B4, the fluorine-free refining agent component is a KCl-MgCl2-NaCl composite salt with a particle size of ≤1 mm.

[0026] The present invention is further configured as follows: in step III, the following steps are included:

[0027] C1. Mold design: A three-layer metal mold with an inner cavity, a middle gradient heating layer, and an outer circulating water cooling layer.

[0028] C2. Melt treatment: The refined mixed melt is filtered through a ceramic filter plate to remove inclusions ≥5μm. The temperature of the mixed melt after filtration is controlled at 700±5℃;

[0029] C3. Casting: Using a bottom pouring runner with a diameter of 50 mm and a casting speed of 50 cm / s, the filtered mixed melt is cast into a metal mold preheated to 200-250° C. to obtain a magnesium alloy ingot;

[0030] C4. Solidification: Immediately after casting is completed, the ultrasonic vibrator at the bottom of the mold is started and maintained for 30 seconds. Ultrasonic vibration is used to break up the primary α-Mg dendrites and promote the formation of equiaxed crystals. At the same time, the temperature gradient of 50-80℃ / cm is used to guide the melt to solidify sequentially from the mold wall to the center of the cavity.

[0031] The present invention is further configured as follows: in step IV, the following steps are included:

[0032] D1. Homogenization treatment: Heat the magnesium alloy ingot to 400-450℃, keep it warm for 8-12 hours, and then cool it to room temperature to eliminate the low-melting-point eutectic phase between crystals;

[0033] D2. Primary aging of supersaturated solid solution treatment: The magnesium alloy ingot after homogenization treatment was heated to 420°C in a box-type resistance furnace and kept at this temperature for 12 hours. It was then quenched in circulating water at a water temperature of 25°C and a quenching transfer time of less than 10 seconds to obtain a supersaturated α-Mg solid solution.

[0034] D3, low-temperature stage of secondary aging: the supersaturated α-Mg solid solution is placed in a low-temperature aging furnace, heated to 160°C and kept warm for 6 hours. The zinc atoms in the supersaturated solid solution begin to segregate, resulting in lattice distortion strengthening;

[0035] D4. High temperature section of secondary aging: The high temperature section is heated to 200°C and kept at this temperature for 6 hours, and the strengthening phase is precipitated through aging treatment.

[0036] The present invention is further configured as follows: in step V, the magnesium alloy ingot after solution treatment is heated to 350-400°C and hot extruded, with an extrusion ratio of 15-30:1 and an extrusion speed of 5-10 mm / s, the grains are further refined to 10-15 μm through dynamic recrystallization, the coarse intergranular precipitation phase is broken to form a fibrous structure, and the rare earth-containing magnesium alloy is air-cooled to room temperature after extrusion to obtain the rare earth-containing magnesium alloy.

[0037] Beneficial effects

[0038] Compared with the known public technology, the technical solution provided by the present invention has the following advantages:

[0039] Beneficial effects:

[0040] (1) In the present invention, zinc is used as a secondary main element to replace part of the aluminum, and the high solid solubility of zinc in magnesium is utilized to achieve efficient solid solution strengthening. At the same time, rare earth is used to regulate the morphology and distribution of the precipitated phase to form a β-MgZn2 phase, which has better high-temperature stability. Zinc and rare earth elements form a ternary compound, which is discontinuously distributed at the grain boundary, inhibiting grain boundary sliding, so that the magnesium alloy can achieve a balanced optimization of strength-toughness-corrosion resistance while maintaining a low density. At the same time, through the combination of Y, Nd, La, and Zr, Y strongly inhibits grain growth and reduces the average grain size, and Nd promotes the uniform precipitation of nano-scale precipitated phases. At the same time, trace zirconium is used as a grain refiner to further reduce the grain size through heterogeneous nucleation, thereby improving the alloy welding performance, reducing the tendency of hot cracking, avoiding the brittle phase problem of traditional high-aluminum alloys, and synergistically improving corrosion resistance and processing performance;

[0041] (2) In the present invention, during the preparation process, the short process of "ultrasonic assisted melting-differential temperature casting-composite aging" is adopted, the cavitation effect generated by ultrasonic vibration is utilized to break up rare earth agglomerates, the acoustic flow effect promotes forced convection of the melt, the diffusion coefficient of rare earth elements and the composition uniformity index are improved, and the rare earth yield rate is improved. By combining differential temperature casting with ultrasonic excitation, the grain size of the ingot is reduced, and the problems of uneven dispersion of rare earth elements and coarse grains are solved. Under the premise of maintaining low density, the room temperature tensile strength, elongation and high temperature tensile strength of the magnesium alloy are improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 The present invention is a flow chart of a method for preparing a high-strength and tough rare earth-containing magnesium alloy. DETAILED DESCRIPTION

[0043] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only a portion of the embodiments of the present invention, not all of them. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0044] The present invention will be further described below with reference to the embodiments.

[0045] Example 1: Please refer to Figure 1 As shown, this embodiment provides a high-strength and tough rare earth-containing magnesium alloy, 8 parts of Zn, 5 parts of RE, 0.5 parts of Al, 0.1 parts of Mn, 0.3 parts of Zr, and the balance is Mg; RE is a combination of Y, Nd, and La, and the mass ratio is 2:1:1.

[0046] A method for preparing a high-strength and tough rare earth-containing magnesium alloy comprises the following steps:

[0047] I. Raw material pretreatment: Raw materials are accurately weighed according to the weight ratio, including magnesium ingot (99.95% pure magnesium), zinc ingot (99.99% pure zinc), aluminum ingot (99.98% pure aluminum), rare earth alloy (Y:Nd:La, purity ≥99.9%), manganese ingot (99.95% pure manganese) and zirconium salt (k2ZrF6, purity ≥98%), and the raw materials are pretreated, including the following steps:

[0048] A1. Cut 500kg magnesium ingot into 5cm 3 The surface of the magnesium block was ultrasonically cleaned with anhydrous ethanol for 10 min to remove oil and oxide film;

[0049] A2. Process zinc ingots and aluminum ingots into zinc flakes and aluminum flakes with a thickness of ≤2mm to facilitate rapid melting;

[0050] A3. Crushing the rare earth alloy into particles of 10 mm in size and drying them in a vacuum oven at 150°C for 2 h to remove adsorbed water vapor;

[0051] A4. Cut the manganese ingot into thin strips with a diameter of 5 mm. To compensate for the 20% burn-out rate during the smelting process, weigh the zirconium salt at 1.2 times the theoretical zirconium content and grind the zirconium salt to below 200 mesh.

[0052] II. Ultrasonic assisted melting: The raw materials are melted in a vacuum environment, and ultrasonic vibration is used to promote the uniform dispersion of rare earth elements to obtain a mixed melt, including the following steps:

[0053] B1. Magnesium matrix melting: A magnesium block was placed in a crucible with an effective volume of 500 L and heated to 700°C at a heating rate of 10°C / min. The mixture was kept at this temperature for 30 min until it was completely melted to obtain a liquid pure magnesium melt. Argon protection was turned on at a flow rate of 5 L / min and stirred with a graphite stirring rod for three times with an interval of 5 min each time to break the surface oxide film and uniform the melt temperature.

[0054] B2. Adding the main alloying elements: Add zinc flakes and aluminum flakes to the crucible in sequence, using a progressive heating method. After each addition, increase the temperature by 10°C to 720°C. Use a mechanical stirrer to stir at 150 rpm for 10 minutes until the alloying elements are completely dissolved. When the melt temperature fluctuation is ≤±5°C as monitored by an infrared thermometer, the alloying elements are considered completely dissolved.

[0055] B3. Ultrasonic dispersion of rare earth alloys: Rare earth alloys preheated to 350°C were added to the crucible in three batches, with an interval of 2 minutes between each batch. The ultrasonic vibration system was simultaneously activated with a power of 8kW and a duty cycle of 90%. Microjets generated by ultrasonic cavitation at a speed ≥ 100m / s were used to break up rare earth agglomerates. Simultaneously, ultrasonic vibrations caused acoustic streaming in the melt, resulting in forced convection of rare earth particles in the melt and uniform distribution of the elements. The melt conductivity was monitored in real time, and uniform dispersion was considered when the conductivity fluctuation was <2%.

[0056] B4. Trace element addition and refining: Add manganese bars and zirconium salts to the crucible, reduce the power of the ultrasonic vibration system to 6kW, continue ultrasonic treatment for 20 minutes, control the melt temperature at 720±10℃, then add 0.3% of the melt mass of a fluorine-free refining agent, the composition of the fluorine-free refining agent is KCl-MgCl2-NaCl composite salt with a particle size of ≤1mm, and evenly spray the fluorine-free refining agent into the melt through a pneumatic powder spraying device. Mechanically stir for 30 minutes, and use the wettability difference between the flux and inclusions to achieve impurity adsorption. After standing for 25 minutes, remove the slag on the surface with a thickness of ≤5mm to obtain a mixed melt.

[0057] III. Differential temperature casting: Using a gradient temperature mold combined with ultrasonic excitation, the liquid melt is cast and solidified, and after cooling, a magnesium alloy ingot is obtained, with a grain size controlled to be ≤20μm. The process includes the following steps:

[0058] C1. Mold Design: A three-layer metal mold structure is used, consisting of an inner cavity, a middle gradient heating layer, and an outer circulating water-cooling layer. The surface of the inner cavity is coated with a TiN coating with a thickness of 5μm and a hardness of 2000HV. The middle layer is a gradient heating layer with multiple groups of independently temperature-controlled resistance heating rods. The outer layer is a circulating water-cooling layer. A low-frequency ultrasonic exciter is integrated at the bottom of the mold. The exciter amplitude is controlled at 50μm, and the excitation direction is perpendicular to the melt flow direction.

[0059] C2. Melt treatment: The refined mixed melt is filtered through a ceramic filter plate (pore size 40ppi) to remove inclusions ≥5μm. After filtration, the temperature of the mixed melt is controlled at 700±5℃;

[0060] C3. Casting: A bottom pouring runner with a diameter of 50 mm was used at a casting speed of 50 cm / s to avoid air entrainment during the casting process. The filtered mixed melt was cast into a metal mold preheated to 230°C to obtain a magnesium alloy ingot.

[0061] C4. Solidification: Immediately after casting is completed, the ultrasonic vibrator at the bottom of the mold is started and maintained for 30 seconds. Ultrasonic vibration is used to break up the primary α-Mg dendrites and promote the formation of equiaxed crystals (equiaxed crystal ratio ≥ 90%). At the same time, a temperature gradient of 50°C / cm is used to guide the melt to solidify sequentially from the mold wall to the center of the cavity to reduce shrinkage defects.

[0062] IV. Composite aging treatment: The magnesium alloy ingot is subjected to supersaturated solution treatment and dual-temperature step aging treatment in sequence to precipitate nano-scale strengthening phases, including the following steps:

[0063] D1. Homogenization treatment: Heat the magnesium alloy ingot to 450°C, keep it warm for 12 hours, and then cool it to room temperature to eliminate the low-melting-point eutectic phase between crystals, providing a uniform microstructure for subsequent hot working.

[0064] D2. Primary aging of supersaturated solid solution treatment: The magnesium alloy ingot after homogenization treatment is heated to 420°C (higher than the Mg-Zn binary eutectic temperature of 347°C and lower than the dissolution temperature of rare earth compounds of 435°C) in a box-type resistance furnace, kept at this temperature for 12 hours, and then quenched in circulating water at a water temperature of 25°C and a quenching transfer time of less than 10 seconds to obtain a supersaturated α-Mg solid solution. At this time, the alloy hardness is HV85-90, which is in the optimal toughness state;

[0065] D3, low-temperature stage of secondary aging: The supersaturated α-Mg solid solution is placed in a low-temperature aging furnace, heated to 160°C and kept warm for 6 hours. The zinc atoms in the supersaturated solid solution begin to segregate, forming GP (Guinier-Preston) zones with a diameter of 2-3 nm on the {1010} crystal plane. The GP zones maintain a coherent relationship with the matrix, resulting in lattice distortion strengthening. At this time, the alloy hardness is increased to HV105-110 while maintaining good plasticity.

[0066] D4. High-temperature stage of secondary aging: The high-temperature stage is heated to 200°C and held for 6 hours (lower than the melting point of β-MgZn2 phase 595°C). Aging treatment precipitates strengthening phases. The GP zone acts as a nucleation core, promoting the precipitation of β-MgZn2 phase (body-centered cubic structure, lattice constant a=0.632nm) around it, forming a core-shell structure of "GP zone core-β phase shell" (average size 8nm×15nm). This structure is semi-coherent with the matrix and has low interfacial energy, which can effectively hinder dislocation movement (dislocation bypass mechanism) and avoid the reduction in toughness caused by coarse precipitates (the precipitate phase density increases by 30% compared with single-stage aging, and the distribution uniformity index increases from 0.6 to 0.85).

[0067] V. A rare earth-containing magnesium alloy is obtained by hot extrusion treatment. The solution-treated magnesium alloy ingot is heated to 380°C and hot extruded with an extrusion ratio of 15:1 and an extrusion speed of 10 mm / s. The grains are further refined to 15 μm by dynamic recrystallization, and the coarse intergranular precipitated phases are broken to form a fibrous structure (the streamline direction is consistent with the extrusion direction to enhance the longitudinal mechanical properties). After extrusion, the alloy is air-cooled to room temperature to obtain a rare earth-containing magnesium alloy.

[0068] Example 2: Please refer to Figure 1 As shown, this embodiment provides a high-strength and tough rare earth-containing magnesium alloy, which comprises 5 parts of Zn, 2 parts of RE, 3 parts of Al, 1 part of Mn, 0.2 parts of Zr, and the balance is Mg; RE is a combination of Y, Nd, and La, and the mass ratio is 2:1:0.5;

[0069] A method for preparing a high-strength and tough rare earth-containing magnesium alloy comprises the following steps:

[0070] I. Raw material pretreatment: Raw materials were accurately weighed according to the weight ratio, including magnesium ingot (99.95% pure magnesium), zinc ingot (99.99% pure zinc), aluminum ingot (99.98% pure aluminum), rare earth alloy (Y:Nd:La, purity ≥99.9%), manganese ingot (99.95% pure manganese) and zirconium salt (k2ZrF6, purity ≥98%), and the raw materials were pretreated. The specific steps are the same as those in Example 1;

[0071] II. Ultrasonic-assisted melting: melting the raw materials in a vacuum environment, using ultrasonic vibration to promote uniform dispersion of the rare earth elements to obtain a mixed melt. The specific steps are the same as those in Example 1;

[0072] III. Differential temperature casting: using a gradient temperature mold combined with ultrasonic excitation to cast and solidify the liquid melt, and obtaining a magnesium alloy ingot after cooling, with a grain size controlled to be ≤20 μm. The specific steps are the same as those in Example 1;

[0073] IV. Composite aging treatment: The magnesium alloy ingot is subjected to supersaturated solution treatment and dual-temperature step aging treatment in sequence to precipitate a nano-scale strengthening phase. The specific steps are the same as those in Example 1;

[0074] V. A rare earth-containing magnesium alloy is obtained by hot extrusion treatment. The specific steps are basically the same as those in Example 1. An isothermal forging process is used in the hot extrusion step. The alloy is heated to 350°C and closed die forging is performed under the condition that the mold temperature is also 350°C to accurately control the size of the forging.

[0075] Example 3: Please refer to Figure 1As shown, this embodiment provides a high-strength and tough rare earth-containing magnesium alloy, which comprises 3 parts of Zn, 5 parts of RE, 3 parts of Al, 0.5 parts of Mn, 0.2 parts of Zr, and the balance is Mg; RE is a combination of Y, Nd, and La, and the mass ratio is 3:1:1;

[0076] A method for preparing a high-strength and tough rare earth-containing magnesium alloy comprises the following steps:

[0077] I. Raw material pretreatment: Raw materials were accurately weighed according to the weight ratio, including magnesium ingot (99.95% pure magnesium), zinc ingot (99.99% pure zinc), aluminum ingot (99.98% pure aluminum), rare earth alloy (Y:Nd:La, purity ≥99.9%), manganese ingot (99.95% pure manganese) and zirconium salt (k2ZrF6, purity ≥98%), and the raw materials were pretreated. The specific steps are the same as those in Example 1;

[0078] II. Ultrasonic-assisted melting: melting the raw materials in a vacuum environment, using ultrasonic vibration to promote uniform dispersion of the rare earth elements to obtain a mixed melt. The specific steps are the same as those in Example 1;

[0079] III. Differential temperature casting: using a gradient temperature mold combined with ultrasonic excitation to cast and solidify the liquid melt, and obtaining a magnesium alloy ingot after cooling, with a grain size controlled to be ≤20 μm. The specific steps are the same as those in Example 1;

[0080] IV. Composite aging treatment: The magnesium alloy ingot is subjected to supersaturated solution treatment and dual-temperature step aging treatment in sequence to precipitate a nano-scale strengthening phase. The specific steps are the same as those in Example 1;

[0081] V. Obtain a rare earth-containing magnesium alloy through hot extrusion treatment. The specific steps are the same as those in Example 1.

[0082] Comparative Example 1: Please refer to Figure 1 As shown, this embodiment provides a rare earth-containing magnesium alloy and a preparation method thereof. The preparation method is substantially the same as that of Example 1, with the main difference being that in step II, ultrasonic-assisted melting is not used when melting the raw materials.

[0083] Comparative Example 2: Please refer to Figure 1 As shown, this embodiment provides a rare earth-containing magnesium alloy and a preparation method thereof. The preparation method is roughly the same as that of Example 1, with the main difference being that in step III, when casting the mixed melt, a gradient temperature mold combined with ultrasonic excitation is not used.

[0084] Comparative Example 3: This embodiment provides a rare earth-containing magnesium alloy and a preparation method thereof. The traditional ZEK100 alloy composition, by mass percentage, is as follows: Zn6%, Nd2.5%, Zr 0.5%, and the balance is Mg. The preparation process is as follows: a traditional smelting process is adopted, and smelting is carried out in an ordinary resistance furnace without ultrasonic assistance. Conventional casting is performed after smelting, and single-stage aging (200°C×12h) is directly performed after homogenization treatment.

[0085] Test: The rare earth-containing magnesium alloys prepared by Examples 1 to 3 were recorded as Example Groups 1 to 3, the rare earth-containing magnesium alloys prepared by Comparative Examples 1 to 2 were recorded as Comparative Groups 1 to 2, and Comparative Example 3 was used as a traditional control group. According to standards such as GB / T228.1-202, the mechanical properties of the rare earth-containing magnesium alloys in Experimental Groups 1 to 3, Comparative Groups 1 to 2, and the control group were tested, and the relevant data are recorded in Table 1.

[0086] The specific test methods are as follows:

[0087] (1) Microstructure analysis

[0088] Metallographic microscope observation: Samples were taken from magnesium alloy ingots and hot extrusions, ground and polished, and then etched with a picric acid-acetic acid-alcohol solution. The grain morphology and size were observed under a metallographic microscope. Five fields of view were randomly selected for each sample, and the average grain size was calculated.

[0089] Scanning electron microscopy (SEM) analysis: The microstructure and precipitate distribution of the magnesium alloy were observed using SEM, and energy dispersive spectroscopy (EDS) was performed on typical areas to determine the precipitate composition;

[0090] Transmission electron microscopy (TEM) analysis: Prepare thin film samples, observe the morphology, size and distribution of nanoscale precipitates under TEM, and calculate the density and size distribution of precipitates.

[0091] (2) Mechanical properties test

[0092] Room temperature tensile test: According to GB / T228.1-2021 standard, the rare earth-containing magnesium alloy was processed into a standard tensile specimen and the tensile test was carried out on a universal material testing machine at a tensile speed of 2 mm / min. Three specimens were tested for each group of samples, and the average value was taken as the test result;

[0093] High temperature tensile test: Heat the sample to 150℃, keep it warm for 30 minutes, and then perform a tensile test at a speed of 2mm / min on a high temperature tensile testing machine. Similarly, three samples are tested in each group.

[0094] Hardness test: Use a Vickers hardness tester to measure the hardness of 5 points at different positions on the sample surface and take the average value. The load is 5kg and the holding time is 10s.

[0095] Table 1: Test data record table

[0096]

[0097] The above results show that the experimental data show that the average grain size of the ZEK100 alloy in the traditional comparative example 3 is 28.5μm, while the average grain size of the alloys in Examples 1 to 3 of the present invention is reduced to 12.8μm and 14.9μm, respectively. Grain refinement can significantly improve the strength and toughness of metal materials. The present invention enhances the heterogeneous nucleation effect and inhibits grain growth through ultrasonic assisted melting and differential temperature casting technology. At the same time, the average size of the precipitated phase of the embodiment alloy is smaller and the density is higher, forming a nano-scale "GP zone core-β phase shell" structure. Compared with the coarse and unevenly distributed precipitated phases of traditional alloys, it can more effectively hinder dislocation movement and achieve organizational strengthening.

[0098] In the room temperature tensile test, the tensile strength of the conventional alloy in comparative example 3 is 320 MPa, and the elongation is 12%. The tensile strengths of the rare earth-containing magnesium alloys in Examples 1 to 3 of the present invention reach 389 MPa, 377 MPa, and 368 MPa, respectively. Due to the synergistic effect of zinc and rare earth elements in the alloy, zinc provides strength support through solid solution strengthening and precipitation of β-MgZn2 phase, and rare earth elements refine grains and optimize the distribution of precipitated phases, thereby improving the plastic deformation ability of the alloy and achieving a balanced improvement in strength and toughness. In the high temperature tensile test at 150°C, the tensile strength of the conventional alloy dropped to 180 MPa, while the alloys in Examples 1 to 3 of the present invention can still maintain 26 The high strength of 5MPa, 250MPa and 246MPa is obvious. The intermetallic compounds with high thermal stability formed by rare earth in the alloy can effectively pin the grain boundaries at high temperatures and inhibit grain boundary sliding. Compared with traditional alloys, the strength drops sharply due to grain boundary weakening at high temperatures. At the same time, the Vickers hardness test shows that the hardness of the traditional alloy is HV105, and the hardness of the alloys of Examples 1 to 3 of the present invention is respectively increased to HV132, HV127 and HV128. The large amount of uniform distribution of nano-scale precipitated phases and the refined grain structure jointly increase the resistance to dislocation movement and significantly improve the hardness of the alloy, making it more suitable for bearing high loads and wear conditions.

[0099] In summary, the rare earth-containing magnesium alloys prepared by Example Groups 1 to 3 are significantly superior to the traditional ZEK100 alloy in terms of room temperature tensile strength, elongation, high temperature tensile strength and hardness. At the same time, by comparing Example 1 with Example Groups 1 to 3, it can be seen that ultrasonic vibration can improve the average diffusion coefficient of rare earth elements, reduce the size of melt inclusions to below 2um, and reduce the number of inclusions. By comparing Example 2 with Example Groups 1 to 3, it can be seen that the radial temperature gradient causes the melt to form a three-layer structure of "mold wall fine grain zone-columnar crystal zone-central equiaxed crystal zone", and the cavitation bubble collapse effect generated by ultrasonic excitation forms a large number of heterogeneous nucleation particles in the melt, significantly increases the nucleation rate, and inhibits grain growth.

[0100] In the preparation method of the invention, Zn, RE (a combination of Y, Nd, and La), Al, Mn, Zr, and Mg are used as raw materials. The raw materials are smelted in a vacuum environment, and ultrasonic vibration is used to promote the uniform dispersion of rare earth elements to obtain a mixed melt. A gradient temperature mold combined with ultrasonic excitation is then used to cast and solidify the liquid melt. After cooling, a magnesium alloy ingot is obtained, and the grain size is controlled to be ≤20μm. The magnesium alloy ingot is subjected to supersaturated solid solution treatment and double-temperature step aging treatment in sequence to precipitate nano-scale strengthening phases. Finally, a rare earth-containing magnesium alloy is obtained by hot extrusion.

[0101] The present invention replaces part of the aluminum with zinc as the secondary main element and utilizes the high solid solubility of Zn in magnesium to achieve efficient solid solution strengthening. At the same time, rare earths are used to regulate the morphology and distribution of the precipitated phase to form a β-MgZn2 phase with better high-temperature stability. Zn and rare earth elements form a ternary compound, which is discontinuously distributed at the grain boundaries, inhibiting grain boundary sliding, thereby achieving a balanced optimization of strength-toughness-corrosion resistance of the magnesium alloy while maintaining a low density. At the same time, through the combination of Y (yttrium), Nd (neodymium), La (lanthanum), and Zr (zirconium), Y strongly inhibits grain growth, making the average grain size ≤20um, Nd promotes the uniform precipitation of nano-scale precipitated phases, La acts as a purifier to form a high-melting point compound, which floats to the slag layer for removal, and a trace amount of zirconium acts as a grain refiner to further reduce the grain size through heterogeneous nucleation, thereby improving the alloy welding performance, reducing the tendency of hot cracking, avoiding the brittle phase problem of traditional high-aluminum alloys, and synergistically improving the corrosion resistance and processing performance.

[0102] During the preparation process, through the short process of "ultrasonic assisted melting-differential temperature casting-composite aging", ultrasonic assisted melting technology uses the cavitation effect generated by ultrasonic vibration to break up rare earth agglomerates, and the acoustic streaming effect promotes forced convection of the melt, thereby improving the diffusion coefficient and composition uniformity index of rare earth elements, solving the industry problem of easy segregation of rare earth elements, thereby improving the rare earth yield; through differential temperature casting combined with ultrasonic excitation, the grain size of the ingot is reduced, and composite aging treatment is used to increase the density of the precipitated phase, solving the problems of uneven dispersion of rare earth elements and coarse grains, and achieving precise control of the organization. While maintaining low density, the room temperature tensile strength, elongation and high temperature tensile strength of the magnesium alloy are improved; at the same time, fluorine-free refining agents are used to reduce the amount of hexachloroethane, and vacuum melting is combined to reduce waste gas emissions and reduce overall production costs.

[0103] Throughout this specification, references to terms such as "one embodiment," "example," or "specific example" indicate that the specific features, structures, materials, or characteristics described in conjunction with that embodiment or example are included in at least one embodiment or example of the present invention. In this specification, schematic representations of these terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.

[0104] The preferred embodiments of the present invention disclosed above are intended only to help illustrate the present invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the present invention to the specific embodiments described. Obviously, many modifications and variations are possible based on the content of this specification. These embodiments are selected and described in detail in this specification to better explain the principles and practical applications of the present invention, thereby enabling those skilled in the art to better understand and utilize the present invention. The present invention is limited only by the claims and their full scope and equivalents.

Claims

1. A high-strength and tough rare earth-containing magnesium alloy, characterized in that: The rare earth-containing magnesium alloy is prepared from the following raw materials in parts by weight: 3 to 8 parts of Zn, 1 to 5 parts of RE, 0.5 to 3 parts of Al, 0.1 to 1 part of Mn, 0.05 to 0.3 parts of Zr, and the balance is Mg; RE is a combination of Y, Nd, and La, and the mass ratio is 2 to 3:1:0.5 to 1.

2. The high-strength and tough rare earth-containing magnesium alloy according to claim 1, characterized in that: The mass ratio of Y, Nd and La in the rare earth elements is 2:1:

1.

3. A method for preparing a high-strength and tough rare earth-containing magnesium alloy, characterized in that: The high-strength and tough rare earth-containing magnesium alloy according to any one of claims 1 to 2 is used, characterized in that it includes the following steps: Ⅰ. Raw material pretreatment: accurately weigh the raw materials according to the weight ratio, including magnesium ingot (99.95% pure magnesium), zinc ingot (99.99% pure zinc), aluminum ingot (99.98% pure aluminum), rare earth alloy (Y:Nd:La, purity ≥99.9%), manganese ingot (99.95% pure manganese) and zirconium salt (k2ZrF6, purity ≥98%), and pretreat the raw materials; II. Ultrasonic assisted melting: The raw materials are melted in a vacuum environment, and ultrasonic vibration is used to promote the uniform dispersion of rare earth elements to obtain a mixed melt; III. Differential temperature casting: Using gradient temperature mold combined with ultrasonic excitation, the liquid melt is cast and solidified, and after cooling, a magnesium alloy ingot is obtained, with a grain size controlled to be ≤20μm; IV. Composite aging treatment: The magnesium alloy ingot is subjected to supersaturated solid solution treatment and double-temperature step aging treatment in sequence to precipitate nano-scale strengthening phase; V. A rare earth-containing magnesium alloy is obtained by hot extrusion treatment.

4. The method for preparing a high-strength and tough rare earth-containing magnesium alloy according to claim 3, characterized in that: In step I, the following steps are included: A1. Cut the magnesium ingot into 5-10cm pieces 3 The surface of the magnesium block was ultrasonically cleaned with anhydrous ethanol for 10 min; A2. Processing zinc ingots and aluminum ingots into zinc flakes and aluminum flakes with a thickness of ≤2mm; A3. Crush the rare earth alloy into particles of 5-10 mm and dry them in a vacuum oven at 150°C for 2 h. A4. Cut the manganese ingot into thin strips with a diameter of 5 mm. To compensate for the 20% burn-out rate during the smelting process, weigh the zirconium salt at 1.2 times the theoretical zirconium content and grind the zirconium salt to below 200 mesh.

5. The method for preparing a high-strength and tough rare earth-containing magnesium alloy according to claim 3, wherein: In step II, the following steps are included: B1. Melting of magnesium matrix: Add magnesium block to crucible and heat to 700°C at a heating rate of 10°C / min. Keep warm for 15-30 min until completely melted to obtain liquid pure magnesium melt. Turn on argon protection at a flow rate of 5 L / min and stir with a graphite stirring rod for three times with an interval of 5 min between each stirring. B2. Adding the main alloying elements: Add zinc flakes and aluminum flakes to the crucible in sequence, using a progressive heating method. After each addition, increase the temperature by 10°C to 720°C. Use a mechanical stirrer to stir at 150 rpm for 5-10 minutes until the alloying elements are completely dissolved. B3. Ultrasonic dispersion of rare earth alloy: Rare earth alloy preheated to 300-400°C was added to the crucible in three batches, with an interval of 2 minutes between each batch. The ultrasonic vibration system was simultaneously started with a power of 8kW and a duty cycle of 90%. The micro-jets generated by the ultrasonic cavitation effect at a speed of ≥100m / s were used to break up rare earth agglomerates. At the same time, the ultrasonic vibration caused the melt to produce an acoustic streaming effect, which forced convection of rare earth particles in the melt and uniformly distributed the elements. B4. Trace element addition and refining: Add manganese bars and zirconium salts to the crucible, reduce the power of the ultrasonic vibration system to 6kW, continue ultrasonic treatment for 10-20 minutes, control the melt temperature at 720±10℃, then add 0.3% of the melt mass of a fluorine-free refining agent, spray the fluorine-free refining agent evenly into the melt through a pneumatic powder spraying device, and mechanically stir for 20-30 minutes. Utilize the wettability difference between the flux and inclusions to achieve impurity adsorption. After standing for 15-25 minutes, remove the surface slag layer with a thickness of ≤5mm to obtain a mixed melt.

6. The method for preparing a high-strength and tough rare earth-containing magnesium alloy according to claim 5, characterized in that: In step B2, when the melt temperature fluctuation is monitored by an infrared thermometer and is ≤±5°C, the alloy elements are considered to be completely dissolved. In step B3, the melt conductivity is monitored in real time and is considered to be uniformly dispersed when the conductivity fluctuation is <2%. In step B4, the fluorine-free refining agent component is a KCl-MgCl2-NaCl composite salt with a particle size of ≤1 mm.

7. The method for preparing a high-strength and tough rare earth-containing magnesium alloy according to claim 3, characterized in that: In step III, the following steps are included: C1. Mold design: A three-layer metal mold with an inner cavity, a middle gradient heating layer, and an outer circulating water cooling layer. C2. Melt treatment: The refined mixed melt is filtered through a ceramic filter plate to remove inclusions ≥5μm. The temperature of the mixed melt after filtration is controlled at 700±5℃; C3. Casting: Using a bottom pouring runner with a diameter of 50 mm and a casting speed of 50 cm / s, the filtered mixed melt is cast into a metal mold preheated to 200-250° C. to obtain a magnesium alloy ingot; C4. Solidification: Immediately after casting is completed, the ultrasonic vibrator at the bottom of the mold is started and maintained for 30 seconds. Ultrasonic vibration is used to break up the primary α-Mg dendrites and promote the formation of equiaxed crystals. At the same time, the temperature gradient of 50-80℃ / cm is used to guide the melt to solidify sequentially from the mold wall to the center of the cavity.

8. The method for preparing a high-strength and tough rare earth-containing magnesium alloy according to claim 3, characterized in that: In step IV, the following steps are included: D1. Homogenization treatment: Heat the magnesium alloy ingot to 400-450℃, keep it warm for 8-12 hours, and then cool it to room temperature to eliminate the low-melting-point eutectic phase between crystals; D2. Primary aging of supersaturated solid solution treatment: The magnesium alloy ingot after homogenization treatment was heated to 420°C in a box-type resistance furnace and kept at this temperature for 12 hours. It was then quenched in circulating water at a water temperature of 25°C and a quenching transfer time of less than 10 seconds to obtain a supersaturated α-Mg solid solution. D3, low-temperature stage of secondary aging: the supersaturated α-Mg solid solution is placed in a low-temperature aging furnace, heated to 160°C and kept warm for 6 hours. The zinc atoms in the supersaturated solid solution begin to segregate, resulting in lattice distortion strengthening; D4. High temperature section of secondary aging: The high temperature section is heated to 200°C and kept at this temperature for 6 hours, and the strengthening phase is precipitated through aging treatment.

9. The method for preparing a high-strength and tough rare earth-containing magnesium alloy according to claim 3, characterized in that: In step V, the magnesium alloy ingot after solution treatment is heated to 350-400° C. and hot extruded with an extrusion ratio of 15-30:1 and an extrusion speed of 5-10 mm / s. The grains are further refined to 10-15 μm by dynamic recrystallization, and the coarse intergranular precipitation phase is broken to form a fibrous structure. After extrusion, the magnesium alloy is air-cooled to room temperature to obtain a rare earth-containing magnesium alloy.

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