Magnesium-lithium alloy and semi-solid thixotropic die-casting preparation method of magnesium-lithium alloy

Abstract

The invention provides a magnesium-lithium alloy and a semi-solid thixotropic die-casting preparation method of the magnesium-lithium alloy. The magnesium-lithium alloy comprises the following components in percentage by mass: 5-14% of Li, 2-9% of Al, 1-3% of Zn, 1-5% of Ca, 0.3-3% of RE and the balance of Mg. The semi-solid thixotropic die-casting preparation method of the magnesium-lithium alloy comprises the steps that S1, burdening and drying are conducted according to the preset mass percent, vacuum melting is conducted in the protective gas atmosphere, and molten metal is obtained and cast into a magnesium-lithium alloy cast ingot; performing extrusion deformation on the magnesium-lithium alloy ingot, and cutting to prepare millimeter-scale long rod-shaped magnesium-lithium alloy particles; s2, the magnesium-lithium alloy particles obtained in the S1 are heated to the solidus temperature Ts or above, electromagnetic stirring is conducted, and semi-solid slurry is prepared; and S3, the semi-solid slurry is injected into a die-casting die, a die cavity is filled with the slurry by applying pressure, die opening and part taking are conducted after pressure maintaining, and the ultra-light high-strength semi-solid thixotropic die-casting magnesium-lithium alloy casting is obtained.

Description

Technical Field

[0001] This invention relates to a metal forming method, a magnesium-lithium alloy and a semi-solid thixotropic die casting preparation method for the magnesium-lithium alloy, and particularly to an ultra-lightweight, high-strength semi-solid thixotropic die casting magnesium-lithium alloy and its preparation method. It is particularly suitable for the semi-solid thixotropic die casting forming process of ultra-lightweight, high-strength magnesium-lithium alloy and belongs to the field of semi-solid forming technology of metal materials. Background Technology

[0002] Magnesium-lithium alloys have the lowest known density (1.30~1.65 g / cm³). 3 Magnesium-lithium alloys, with their superior specific strength, specific stiffness, excellent elongation, and electromagnetic shielding properties, have promising development prospects in fields with urgent lightweighting needs, such as aerospace, new energy vehicles, and 3C electronics. However, the high chemical reactivity of Mg and Li elements in magnesium-lithium alloy systems makes them prone to oxidation reactions in traditional liquid forming processes (stir casting, squeeze casting, etc.), leading to a decline in alloy purity, performance, and productivity. Furthermore, traditional liquid forming processes struggle to avoid metallurgical defects such as inclusions and porosity, severely hindering the industrialization of magnesium-lithium alloys.

[0003] Semi-solid forming technology, as a novel metal processing method, focuses on controlling the metal material within a specific temperature range in the liquid-solid two-phase region for forming. Compared to traditional liquid forming technology, semi-solid forming offers many advantages, such as lower forming temperature, longer mold life, higher dimensional accuracy of castings, denser microstructure, and fewer defects. Depending on the process flow, semi-solid forming technology can be divided into rheological casting and thixotropic casting. The forming characteristic of rheological casting is the external disturbance of the solid-liquid coexisting melt within the semi-solid temperature range, disrupting the dendritic structure. When the melt reaches a certain solid fraction, it is finally shaped using extrusion or die casting. Huang Yuchuan et al. published "An Ultra-Light High-Strength Semi-Solid Rheological Die Casting Magnesium-Lithium Alloy and Its Preparation Method" (Publication No. CN1171187643A). Through alloy composition design, they prepared an ultra-light high-strength semi-solid rheological die casting magnesium-lithium alloy using "atmospheric melting + electromagnetic stirring + rheological die casting." By refining the matrix and second phase, they obtained a semi-solid rheological die casting magnesium-lithium alloy with excellent mechanical properties and strong process applicability. However, this technology faces technical challenges such as poor stability of semi-solid slurry storage and large temperature fluctuations during transportation, making it difficult to meet the requirements of industrial production of magnesium-lithium alloys. Thixotropic forming, on the other hand, employs a two-step process of "slurry solidification-billet remelting": first, a semi-solid billet with thixotropic properties is prepared; then, the billet is precisely remelted into a liquid-solid two-phase region through heating; and finally, it is die-cast. This technology has significant advantages: firstly, the billet can be stored for a long time; secondly, die casting can effectively ensure the density of the casting.

[0004] The use of semi-solid thixotropic die casting technology to prepare magnesium-lithium alloys has the following advantages: First, the magnesium-lithium alloy has a uniform, dense microstructure, fine grains, and fewer defects, which improves its mechanical properties. Second, the preparation of intermediate billets is also easier for the preparation and transportation of the semi-solid magnesium-lithium alloy slurry. Finally, the semi-solid slurry is injected under certain pressure, resulting in high production efficiency, dimensional accuracy, and surface quality.

[0005] Due to the high chemical reactivity of Mg and Li elements, there is limited research on the semi-solid forming of magnesium-lithium alloys. In addition, the semi-solid range of conventional magnesium-lithium alloys is small, and the semi-solid temperature is difficult to control precisely. Therefore, it is necessary to optimize the composition of magnesium-lithium alloys and develop ultra-lightweight and high-strength magnesium-lithium alloys suitable for thixotropic die casting to meet the demand for lightweight materials in the aerospace, new energy vehicle, and 3C electronic product fields. Summary of the Invention

[0006] Therefore, this invention provides a magnesium-lithium alloy and a method for preparing the magnesium-lithium alloy using semi-solid thixotropic die casting. This invention improves the strength and thermal stability of the magnesium-lithium alloy by adding rare earth elements; the addition of calcium (Ca) effectively broadens the semi-solid range of the magnesium-lithium alloy, which is more conducive to semi-solid thixotropic die casting. This invention obtains a high-quality, ultra-lightweight, high-strength semi-solid thixotropic die-cast magnesium-lithium alloy through the preparation of magnesium-lithium alloy particles, the preparation of semi-solid slurry, and semi-solid thixotropic die casting.

[0007] The objective of this invention is achieved through the following technical solution: First aspect: an ultra-lightweight, high-strength semi-solid thixotropic die-cast magnesium-lithium alloy, wherein, by mass percentage, the composition of the magnesium-lithium alloy includes: Li 5~14%, Al 2~9%, Zn 1~3%, Ca 1~5%, RE 0.3~3%, with the balance being Mg.

[0008] The RE includes one or more of Yb, Er, Gd, and Y.

[0009] The second aspect: a semi-solid thixotropic die casting method for preparing magnesium-lithium alloys as described above, comprising the following steps: S1. Preparation of magnesium-lithium alloy particles: The materials are prepared according to the preset mass percentage, dried, and vacuum melted under a protective gas atmosphere to obtain molten metal and cast into magnesium-lithium alloy ingots; the magnesium-lithium alloy ingots are cut after extrusion deformation to produce millimeter-sized long rod-shaped magnesium-lithium alloy particles. S2. Preparation of semi-solid slurry: The magnesium-lithium alloy particles obtained in step S1 are heated to above the solidus temperature Ts and electromagnetically stirred to obtain a semi-solid slurry. S3. Semi-solid thixotropic die casting: The semi-solid slurry is injected into the die casting mold, and pressure is applied to fill the mold cavity with the slurry. After holding the pressure, the mold is opened and the part is removed to obtain an ultra-lightweight, high-strength semi-solid thixotropic die casting magnesium-lithium alloy casting.

[0010] In S1, the vacuum melting process includes the following steps: (1) Dry the magnesium-based raw materials, aluminum-based raw materials, zinc-based raw materials, rare earth element raw materials and calcium-based raw materials; (2) Prepare the raw materials according to the mass percentage of each component in the alloy, melt the raw materials except lithium, heat them under a protective atmosphere until they are completely melted, and then stir them at 700-750℃ for 5-10 minutes. (3) Cool down to 680-700℃, add lithium element encapsulated in inert material, heat up to 700-720℃ and stir for 3-8 minutes; (4) Casting and molding; The vacuum degree of vacuum melting is ≤10Pa, and the protective gas is high-purity argon or helium.

[0011] In step S1, the extrusion deformation temperature is 250-350℃, the extrusion speed is 1.0-3.0m / min, and the diameter of the extruded rod-shaped sample is 3-8mm.

[0012] In step S2, the magnesium-lithium alloy particles are heated to 550℃~620℃ and electromagnetically stirred to obtain a semi-solid slurry.

[0013] The electromagnetic stirring frequency is 30-50Hz, and the stirring time is 10-30 min.

[0014] In step S3, the preheating temperature of the die-casting mold is 200-300℃, the injection speed is 30-50m / s, the injection specific pressure is 40-80MPa, and the holding time is 20-40s.

[0015] As one embodiment of the present invention, step S1 specifically includes the following steps: S11. Take pure Mg, pure Al, pure Zn, Mg-RE master alloy, and pure Ca respectively, and dry the above raw materials at 100-135 ℃ for 0.5-1 h; S12. Prepare the raw materials according to the mass percentage of each component in the alloy, and melt the alloy using a vacuum melting furnace. Place the raw materials except for Li into a crucible, evacuate to 5~10 Pa, and then introduce 3 Pa. 10 4 ~8 10 4Pa of high-purity argon gas was used for heating. After the alloy was completely melted, the temperature was raised to 720 ℃~750 ℃ ​​and mechanically stirred for 8~10 min. The temperature was then lowered to 680 ℃~700 ℃ and pure Li wrapped in aluminum foil was added. The temperature was then raised to 700 ℃~720 ℃ and held. The temperature was mechanically stirred for 4~8 min at a stirring rate of 100-200 r / min. The mixture was then allowed to stand for 5~15 min and cooled to 680 ℃~700 ℃ before casting into an ingot with a diameter of Φ50. S13. Place the magnesium-lithium alloy ingot in a vacuum heat treatment furnace and keep it at 300-350 ℃ for 1-4 h to homogenize the microstructure. Then remove it and air cool it. S14. Preheat the mold and ingot to 250-300℃, and immediately place them into the extruder for forming. The extrusion speed is 1.0-2.0 m / min to obtain a rod-shaped extrusion sample with a diameter of Φ 5. Then, cut the rod-shaped extrusion sample into segments, each segment with a length of about 10-12 mm, to make millimeter-sized long rod-shaped magnesium-lithium alloy particles.

[0016] In step S3: The temperature of the semi-solid slurry is 550℃~620℃; The preheating temperature of the die-casting mold is 250℃~300℃; The injection velocity is 40 m / s ~ 50 m / s; The injection pressure is 50 MPa ~ 65 MPa; The pressure holding time is 20 s ~ 30 s.

[0017] The key point of this invention is that: existing technologies such as CN 117187643 A rheoforming are suitable for simple structural parts (such as bars and plates), but due to the high difficulty in controlling the melt flowability, cold shuts and air entrapment are easy to occur when filling complex cavities; while the granular billet of this application has more stable flowability after secondary heating, and can meet the die casting requirements of thin-walled and irregularly shaped components.

[0018] Furthermore, existing technologies such as CN 117187643 A rely on CO2+SF6 protective gas in atmospheric melting, but high-Li alloys still face the risk of oxidation; and direct melt stirring easily leads to the loss of rare earth elements (Y, Gd), weakening the precipitation effect of strengthening phases (Al-RE, Mg-Zn-RE) and affecting the alloy strength. This application, by combining Ca element addition with vacuum melting, can further reduce the risk of combustion.

[0019] This application addresses the challenges of high-activity magnesium-lithium alloys, such as easy oxidation and a narrow semi-solid range during rheological forming, through a step-by-step process of "vacuum melting → extrusion billet preparation → granulation → secondary heating activation". Furthermore, billet pretreatment (including vacuum heat treatment and extrusion) significantly improves microstructure uniformity (refining grains and reducing segregation).

[0020] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention designs an ultralight, high-strength magnesium-lithium alloy suitable for semi-solid thixotropic die casting. The addition of rare earth elements (RE) forms strengthening rare earth phases such as Al-RE and Mg-Zn-RE phases. Furthermore, RE reacts with impurity elements to form slag, purifying the melt and improving alloy purity. Secondly, RE provides nucleation sites during melting, thus refining the grain size. The addition of calcium (Ca) effectively broadens the semi-solid range of the magnesium-lithium alloy, making it more suitable for semi-solid thixotropic die casting. The addition of Al and Zn provides strong solid solution strengthening, improving the mechanical properties of the magnesium-lithium alloy. The addition of 5-14% Li (by mass) ensures the alloy's ultralight characteristics, with a density of less than 1.6 g / cm³. 3 This gives the alloy high specific strength and specific stiffness.

[0021] 2. This invention employs a process of "vacuum melting + electromagnetic stirring + semi-solid thixotropic die casting" to prepare an ultra-lightweight, high-strength semi-solid thixotropic die-cast magnesium-lithium alloy. The introduction of a protective gas under vacuum conditions effectively prevents the loss of Mg and Li elements and the formation of oxide inclusions, resulting in high-quality magnesium-lithium alloy ingots. The semi-solid thixotropic die casting process effectively reduces defects such as porosity and shrinkage cavities, and also improves the microstructure uniformity of the magnesium-lithium alloy and the surface quality of the die-cast parts.

[0022] 3. This invention uses "vacuum melting + electromagnetic stirring + semi-solid thixotropic die casting" to prepare ultra-lightweight and high-strength semi-solid thixotropic die casting magnesium-lithium alloy. Within the die casting temperature range of 550 ℃ to 620 ℃, the alloy achieves complete filling of 1550 mm in a single spiral sample, and no cracks appear within the range of 5-40 mm in the hot crack ring width.

[0023] 4. The preparation method of the ultralight high-strength semi-solid thixotropic die-cast magnesium-lithium alloy described in this invention is a near-net-shape forming technology with a simple and controllable process flow, suitable for mass production, and has good development prospects in the fields of aerospace, new energy vehicles, and 3C electronic products.

[0024] This invention utilizes a process of "vacuum melting → extrusion molding → granulation → secondary heating activation" to prepare an ultralight, high-strength semi-solid thixotropic die-cast magnesium-lithium alloy. This process addresses the low strength issue of magnesium-lithium alloys and enables the semi-solid thixotropic die-casting of ultralight, high-strength magnesium-lithium alloys. The technology of this invention can replace traditional stir casting processes to produce various magnesium-lithium alloy products, achieving near-net-shape forming of ultralight, high-strength semi-solid thixotropic die-cast magnesium-lithium alloys, and has broad development prospects. Detailed Implementation

[0025] The present invention will be described in detail below with reference to embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several adjustments and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0026] Example 1 This embodiment relates to a semi-solid thixotropic die casting method for a magnesium-lithium alloy, wherein the mass percentage of each component in the magnesium-lithium alloy is Li 8%, Al 5%, Zn 2%, Ca 1%, Y 1%, Gd 1%, and the balance is Mg.

[0027] The preparation process includes three stages: preparation of magnesium-lithium alloy particles, preparation of semi-solid slurry, and semi-solid thixotropic die casting. The specific preparation steps of ultra-lightweight, high-strength semi-solid thixotropic die casting magnesium-lithium alloy are as follows: (1) Preparation of magnesium-lithium alloy particles: Materials were batched according to the mass percentage of each component in the alloy. After batching, pure Mg, pure Al, pure Zn, Mg-Y master alloy, Mg-Gd master alloy, and pure Ca were taken separately. These raw materials were dried at 135 ℃ for 0.5 h. Then, all raw materials except Li were placed in a crucible. Pure Li was wrapped in aluminum foil and placed into the feeding port. A vacuum of 5 Pa was applied, followed by the introduction of 3.5 kPa. 10 4 High-purity argon gas (Pa) was used for heating. After the alloy was completely melted, the temperature was raised to 730 °C and mechanically stirred for 8 min. The temperature was then lowered to 690 °C, and pure Li wrapped in aluminum foil was added. The mixture was heated to 710 °C and held for 5 min, with mechanical stirring at a stirring rate of 200 r / min. After standing for 10 min, the temperature was lowered to 690 °C and cast into an ingot to obtain a magnesium-lithium alloy ingot with a diameter of Φ 50. The ingot was then placed in a vacuum heat treatment furnace and held at 350 °C for 4 h to homogenize the microstructure. After that, it was removed and air-cooled. The mold and ingot were preheated to 300 °C and immediately placed in an extruder for forming. The extrusion speed was 2.0 m / min to obtain a rod-shaped extrusion sample with a diameter of Φ 5. The rod-shaped extrusion sample was then cut into segments, each about 10 mm in length, to produce millimeter-long rod-shaped magnesium-lithium alloy particles. (2) Preparation of semi-solid slurry: millimeter-sized long rod-shaped magnesium-lithium alloy particles are put into a semi-solid injection molding machine through a hopper and heated to prepare melt. The melt temperature is controlled at 600 ℃ and electromagnetic stirring is performed. The electromagnetic stirring parameters are set as follows: stirring frequency is 40 Hz and stirring time is 10 min. (3) Semi-solid thixotropic die casting: The semi-solid melt at 590 ℃ is injected into the die casting mold through a semi-solid injection molding machine. The mold preheating temperature is 250 ℃, the injection speed is 40 m / s, the injection pressure is 60 MPa, and the holding time is 30 s. After holding the pressure, the mold is opened and the part is removed to obtain a high-strength, ultra-lightweight, and high-lightweight semi-solid thixotropic die casting magnesium-lithium alloy casting.

[0028] Example 2 This embodiment relates to a semi-solid thixotropic die casting method for a magnesium-lithium alloy, wherein the mass percentage of each component in the magnesium-lithium alloy is 14% Li, 9% Al, 3% Zn, 3% Ca, 1% Y, 1% Gd, 1% Er, and the balance is Mg.

[0029] The preparation process includes three stages: preparation of magnesium-lithium alloy particles, preparation of semi-solid slurry, and semi-solid thixotropic die casting. The specific steps for preparing ultra-lightweight, high-strength semi-solid thixotropic die-cast magnesium-lithium alloy are as follows: (1) Preparation of magnesium-lithium alloy particles: The raw materials were batched according to the mass percentage of each component in the alloy. After batching, pure Mg, pure Al, pure Zn, Mg-Y master alloy, Mg-Gd master alloy, Mg-Er master alloy, and pure Ca were taken respectively. The above raw materials were dried at 135℃ for 0.5 h. Then, the raw materials except Li were placed in the crucible. Pure Li was wrapped in aluminum foil and placed into the feeding port. The vacuum was drawn to 10 Pa, and then 8 Pa was introduced. 10 4 High-purity argon gas (Pa) was used for heating. After the alloy was completely melted, the temperature was raised to 720 °C and mechanically stirred for 10 min. The temperature was then lowered to 680 °C, and pure Li wrapped in aluminum foil was added. The mixture was heated to 700 °C and held for 5 min, with mechanical stirring at a stirring rate of 200 r / min. After standing for 5 min, the temperature was lowered to 690 °C and cast into an ingot to obtain a magnesium-lithium alloy ingot with a diameter of Φ 50. The ingot was then placed in a vacuum heat treatment furnace and held at 350 °C for 4 h to homogenize the microstructure. After that, it was removed and air-cooled. The mold and ingot were preheated to 300 °C and immediately placed in an extruder for forming. The extrusion speed was 2.0 m / min to obtain a rod-shaped extrusion sample with a diameter of Φ 5. The rod-shaped extrusion sample was then cut into segments, each approximately 10 mm in length, to produce millimeter-long rod-shaped magnesium-lithium alloy particles. (2) Preparation of semi-solid slurry: millimeter-sized long rod-shaped magnesium-lithium alloy particles are put into a semi-solid injection molding machine through a hopper and heated to prepare melt. The melt temperature is controlled at 600 ℃ and electromagnetic stirring is performed. The electromagnetic stirring parameters are set as follows: stirring frequency is 50 Hz and stirring time is 20 min. (3) Semi-solid thixotropic die casting: The semi-solid melt at 590 ℃ is injected into the die casting mold through a semi-solid injection molding machine. The mold preheating temperature is 250 ℃, the injection speed is 40 m / s, the injection pressure is 60 MPa, and the holding time is 30 s. After holding the pressure, the mold is opened and the part is removed to obtain a high-strength, ultra-lightweight, and high-lightweight semi-solid thixotropic die casting magnesium-lithium alloy casting.

[0030] Example 3 This embodiment relates to a semi-solid thixotropic die casting method for a magnesium-lithium alloy, wherein the mass percentage of each component in the magnesium-lithium alloy is 6% Li, 4% Al, 1% Zn, 2% Ca, 0.5% Y, 0.5% Gd, and the balance is Mg.

[0031] The preparation process includes three stages: preparation of magnesium-lithium alloy particles, preparation of semi-solid slurry, and semi-solid thixotropic die casting. The specific steps for preparing ultra-lightweight, high-strength semi-solid thixotropic die-cast magnesium-lithium alloy are as follows: (1) Preparation of magnesium-lithium alloy particles: The raw materials were batched according to the mass percentage of each component in the alloy. After batching, pure Mg, pure Al, pure Zn, Mg-Y master alloy, Mg-Gd master alloy, and pure Ca were taken separately. The above raw materials were dried at 135 ℃ for 0.5 h. Then, the raw materials except Li were placed in the crucible. Pure Li was wrapped in aluminum foil and placed into the feeding port. The vacuum was drawn to 5 Pa, and then 3 Pa was introduced. 10 4 High-purity argon gas (Pa) was used for heating. After the alloy was completely melted, the temperature was raised to 740 °C and mechanically stirred for 10 min. The temperature was then lowered to 700 °C, and pure Li wrapped in aluminum foil was added. The mixture was heated to 710 °C and held at that temperature, with mechanical stirring for 8 min at a stirring rate of 200 r / min. After standing for 10 min, the temperature was lowered to 680 °C and cast into an ingot to obtain a magnesium-lithium alloy ingot with a diameter of Φ 50. The ingot was then placed in a vacuum heat treatment furnace and held at 350 °C for 4 h to homogenize the microstructure. After that, it was removed and air-cooled. The mold and ingot were preheated to 300 °C and immediately placed in an extruder for forming. The extrusion speed was 2.0 m / min to obtain a rod-shaped extrusion sample with a diameter of Φ 5. The rod-shaped extrusion sample was then cut into segments, each approximately 10 mm in length, to produce millimeter-long rod-shaped magnesium-lithium alloy particles. (2) Preparation of semi-solid slurry: Millimeter-sized long rod-shaped magnesium-lithium alloy particles are put into a semi-solid injection molding machine through a hopper and heated to prepare melt. The melt temperature is controlled at 600 ℃ and electromagnetic stirring is performed. The electromagnetic stirring parameters are set as follows: stirring frequency is 40 Hz and stirring time is 30 min. (3) Semi-solid thixotropic die casting: The semi-solid melt at 590 ℃ is injected into the die casting mold through a semi-solid injection molding machine. The mold preheating temperature is 250 ℃, the injection speed is 40 m / s, the injection pressure is 60 MPa, and the holding time is 30 s. After holding the pressure, the mold is opened and the part is removed to obtain a high-strength, ultra-lightweight, and high-lightweight semi-solid thixotropic die casting magnesium-lithium alloy casting.

[0032] Comparative Example 1 The magnesium-lithium alloy described in Comparative Example 1 was prepared by vacuum melting and stirring casting. The specific steps are as follows: the same component ratio as in Example 1 was used for melting under a protective atmosphere. After the metal melted, the temperature was raised to 700 °C and mechanically stirred (stirring speed 200 rpm, time 20 min). After stirring, the temperature was raised to 720 °C and then the heating was stopped. After standing for 10 min, the temperature was lowered to 700 °C and the molten metal was poured into a metal mold to obtain an ingot with qualified composition.

[0033] Comparative Example 2 The magnesium-lithium alloy and its preparation method described in Comparative Example 2 are specifically composed of the following components by mass percentage: Li 8%, Al 5%, Zn 2%, Ca 1%, with the balance being Mg. Compared with Example 1, it does not contain RE elements.

[0034] The preparation method of the magnesium-lithium alloy is the same as that in Example 1.

[0035] Comparative Example 3 Comparative Example 3 describes a magnesium-lithium alloy and its preparation method, wherein the alloy uses the same component ratio as in Example 1. The difference from Example 1 is that Comparative Example 3 uses a semi-solid slurry prepared at 660°C.

[0036] Comparative Example 4 Comparative Example 4 describes a magnesium-lithium alloy and its preparation method, wherein the alloy uses the same component ratio as in Example 1. The difference from Example 1 is that Comparative Example 4 employs a different electromagnetic stirring process for preparing the semi-solid slurry; specifically, the stirring frequency is 20 Hz and the stirring time is 30 min.

[0037] Comparative Example 5 Comparative Example 5 describes a magnesium-lithium alloy and its preparation method, wherein the alloy uses the same component ratio as in Example 1. The difference from Example 1 is that Comparative Example 5 employs a different semi-solid thixotropic die casting process: injection speed 20 m / s, injection pressure 60 MPa, and holding time 30 s.

[0038] Comparative Example 6 The difference between this comparative example and Example 1 is that the step of preparing millimeter-sized long rod-shaped magnesium-lithium alloy particles is omitted in step (1). Instead, a semi-solid slurry is prepared directly from an air-cooled magnesium-lithium alloy ingot. The alloy composition and dosage are the same as in Example 1. The specific preparation method is as follows: (1) Preparation of magnesium-lithium alloy particles: Materials were batched according to the mass percentage of each component in the alloy. After batching, pure Mg, pure Al, pure Zn, Mg-Y master alloy, Mg-Gd master alloy, and pure Ca were taken separately. These raw materials were dried at 135 ℃ for 0.5 h. Then, all raw materials except Li were placed in a crucible. Pure Li was wrapped in aluminum foil and placed into the feeding port. A vacuum of 5 Pa was applied, followed by the introduction of 3.5 kPa. 10 4 High-purity argon gas (Pa) was used for heating. After the alloy was completely melted, the temperature was raised to 730 °C and mechanically stirred for 8 min. The temperature was then lowered to 690 °C, and pure Li wrapped in aluminum foil was added. The mixture was heated to 710 °C and held, with mechanical stirring for 5 min at a stirring rate of 200 r / min. After standing for 10 min, the temperature was lowered to 690 °C and cast into an ingot to obtain a magnesium-lithium alloy ingot with a diameter of Φ 50. The ingot was then placed in a vacuum heat treatment furnace and held at 350 °C for 4 h to homogenize the microstructure. Finally, it was removed and air-cooled.

[0039] (2) Preparation of semi-solid slurry: Same as in Example 1; (3) Semi-solid thixotropic die casting: Same as Example 1.

[0040] Performance testing The mechanical properties of the magnesium-lithium alloys in Examples 1-3 and Comparative Examples 1-6 were tested according to GB / T 228.1-2010. The density of the magnesium-lithium alloys was measured using the water displacement method based on Archimedes' principle. Each sample underwent at least three valid tests, and the average value was taken.

[0041] In addition, regarding the process performance testing of the magnesium-lithium alloys in Examples 1-3 and Comparative Examples 1-6, the main focus was on testing for fluidity and hot cracking tendency, specifically as follows: Flowability test: The melt from the preparation of semi-solid slurry in Examples 1 to Comparative Examples 6 was cast into a single-spiral alloy flowability tester preheated to 200°C. The melt flowed under the action of gravity and capillary action and eventually solidified. The longer the filling length, the better the alloy flowability. Hot cracking tendency test: The melt from the preparation of semi-solid slurry in Examples 1 to Comparative Examples 6 above is cast into a cross-shaped hot cracking test device. After the melt is poured in, the cross arm generates tensile stress on the central node when it solidifies and shrinks. The central part is prone to hot cracking because it is the last to solidify and has a large temperature gradient. The hot cracking tendency is judged by observing the cracking of the central node and the arm.

[0042] Flaw detection: Non-destructive testing techniques were used to detect and evaluate the presence of shrinkage porosity and other defects within the alloy. The test results of the obtained samples are shown in Table 1.

[0043] Table 1

[0044] As shown in Table 1, the magnesium-lithium alloy prepared in Comparative Example 1 uses the same composition as in Example 1, but the stirring casting process is different from that in Example 1. Compared with the stirring casting process, the magnesium-lithium alloy in Example 1, which is formed by semi-solid thixotropic die casting, has a denser structure, finer grains, and fewer metallurgical defects such as pores. Therefore, the mechanical properties of the magnesium-lithium alloy prepared in Example 1 are significantly improved compared with those in Comparative Example 1, with tensile strength increased by 33 MPa and elongation increased by 221%. It also has good fluidity and a low tendency to hot crack.

[0045] The magnesium-lithium alloy prepared in Comparative Example 2 was prepared using the same method and composition as in Example 1. However, it did not contain RE elements compared to Example 1. Due to the lack of the strengthening effect of the high-strength and stable RE phase, the magnesium-lithium alloy in Comparative Example 2 had lower mechanical properties, with a tensile strength of only 231 MPa. At the same time, it had good fluidity and a low tendency to hot crack.

[0046] Comparative Example 3 used the same composition as Example 1 and prepared a semi-solid slurry at 660°C. The excessively high temperature caused the magnesium-lithium alloy to completely melt into molten metal at 660°C, resulting in spontaneous overflow of the molten metal from the die-casting nozzle. Furthermore, since the semi-solid thixotropic die casting process involves molten metal, it affects the fluidity and thermal cracking tendency of the magnesium-lithium alloy, reducing its casting performance and causing shrinkage porosity, ultimately impacting the final properties of the magnesium-lithium alloy.

[0047] Comparative Example 4 used the same composition as Example 1 and employed different electromagnetic stirring processes to prepare semi-solid slurry. The stirring frequency was 20 Hz and the stirring time was 30 min. If the stirring frequency was too low, it would affect the refinement effect of the primary α-Mg phase, affect the fluidity and hot cracking tendency of the magnesium-lithium alloy, reduce the casting performance of the magnesium-lithium alloy, and cause cracking, shrinkage porosity and shrinkage cavities, thus affecting the final performance of the magnesium-lithium alloy.

[0048] Comparative Example 5 uses the same composition as Example 1, but employs a different semi-solid thixotropic die casting process. The injection speed is 20 m / s, the injection pressure is 60 MPa, and the holding time is 30 s. If the injection speed is too low, it will affect the fluidity and hot cracking tendency of the magnesium-lithium alloy, thereby reducing the casting performance of the magnesium-lithium alloy and causing cracking, shrinkage porosity, and other phenomena. This will affect the formability of the magnesium-lithium alloy and the final performance of the magnesium-lithium alloy die casting.

[0049] Comparative Example 6 uses the same composition as Example 1, and prepares a semi-solid slurry directly from an air-cooled magnesium-lithium alloy ingot. Since the magnesium-lithium alloy ingot has a larger volume and slower heat conduction compared to particles, when heated to the semi-solid temperature range (550–620°C), the temperature difference between the inside and outside is large, which can easily lead to surface overheating and incomplete semi-solidification of the inside, resulting in uneven structure. In addition, the large volume and irregular shape of the ingot can also make electromagnetic stirring difficult to work effectively, affecting the fluidity and hot cracking tendency of the magnesium-lithium alloy, thus reducing the casting performance of the magnesium-lithium alloy, causing shrinkage porosity and affecting the formability of the magnesium-lithium alloy, and ultimately affecting the final performance of the magnesium-lithium alloy die casting.

[0050] The ultralight, high-strength semi-solid thixotropic die-cast magnesium-lithium alloys obtained in Examples 1-3 of this invention possess excellent mechanical properties. They exhibit a tensile strength greater than 250 MPa, an elongation greater than 13%, and a density less than 1.6 g / cm³. 3 It also possesses excellent semi-solid thixotropic die casting properties.

[0051] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that these are merely illustrative examples, and any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A lightweight, high-strength semi-solid thixotropic die-cast magnesium-lithium alloy, characterized in that, The magnesium-lithium alloy comprises, by weight percentage: Li 5-14%, Al 2-9%, Zn 1-3%, Ca 1-5%, RE 0.3-3%, with the balance being Mg.

2. The ultralight, high-strength semi-solid thixotropic die-cast magnesium-lithium alloy as described in claim 1, characterized in that, The RE includes one or more of Yb, Er, Gd, and Y.

3. A semi-solid thixotropic die casting method for preparing magnesium-lithium alloy as described in claim 1 or 2, characterized in that, Includes the following steps: S1. Preparation of magnesium-lithium alloy particles: The materials are prepared according to the preset mass percentage, dried, and vacuum melted under a protective atmosphere to obtain molten metal and cast into magnesium-lithium alloy ingots; the magnesium-lithium alloy ingots are cut after extrusion deformation to produce millimeter-sized long rod-shaped magnesium-lithium alloy particles. S2. Preparation of semi-solid slurry: The magnesium-lithium alloy particles are heated to above the solidus temperature Ts and electromagnetically stirred to obtain a semi-solid slurry. S3. Semi-solid thixotropic die casting: The semi-solid slurry is injected into the die casting mold, pressure is applied to fill the mold cavity with the slurry, and after holding the pressure, the mold is opened and the part is removed to obtain an ultra-lightweight and high-strength semi-solid thixotropic die casting magnesium-lithium alloy casting.

4. The preparation method according to claim 3, characterized in that, In S1, the vacuum melting process includes the following steps: (1) Dry the magnesium-based raw materials, aluminum-based raw materials, zinc-based raw materials, rare earth element raw materials and calcium-based raw materials; (2) Prepare the raw materials according to the mass percentage of each component in the alloy, melt the raw materials except lithium, heat them under a protective atmosphere until they are completely melted, and then stir them at 700-750℃ for 5-10 minutes. (3) Cool down to 680-700℃, add lithium element encapsulated in inert material, heat up to 700-720℃ and stir for 3-8 minutes; (4) Casting and molding; The vacuum degree of vacuum melting is ≤10Pa, and the protective gas is high-purity argon or helium.

5. The preparation method according to claim 3, characterized in that, The extrusion deformation temperature in step S1 is 250-350℃, the extrusion speed is 1.0-3.0m / min, and the diameter of the rod-shaped sample after extrusion is 3-8mm.

6. The preparation method according to claim 3, characterized in that, In step S2, the magnesium-lithium alloy particles are heated to 550℃~620℃ and electromagnetically stirred to obtain a semi-solid slurry.

7. The preparation method according to claim 6, characterized in that, The electromagnetic stirring frequency is 30-50Hz, and the stirring time is 10-30 min.

8. The preparation method according to claim 3, characterized in that, In step S3, the preheating temperature of the die-casting mold is 200-300℃, the injection speed is 30-50m / s, the injection specific pressure is 40-80MPa, and the holding time is 20-40s.

9. The preparation method according to claim 3, characterized in that, Step S1 specifically includes the following steps: S11. Take pure Mg, pure Al, pure Zn, Mg-RE master alloy, and pure Ca respectively, and dry the above raw materials at 100-135 ℃ for 0.5-1 h; S12. Prepare the raw materials according to the mass percentage of each component in the alloy, and melt the alloy using a vacuum melting furnace. Place the raw materials except for Li into a crucible, evacuate to 5~10 Pa, and then introduce 3 Pa. 10 4 ~8 10 4 Pa of high-purity argon gas was used for heating. After the alloy was completely melted, the temperature was raised to 720 ℃~750 ℃ ​​and mechanically stirred for 8~10 min. The temperature was then lowered to 680 ℃~700 ℃, and pure Li wrapped in aluminum foil was added. The temperature was then raised to 700 ℃~720 ℃ and held. The temperature was mechanically stirred for 4~8 min at a stirring rate of 100-200 r / min. The mixture was then allowed to stand for 5~15 min and cooled to 680 ℃~700 ℃ before casting into an ingot with a diameter of Φ50. S13. Place the magnesium-lithium alloy ingot in a vacuum heat treatment furnace and keep it at 300-350 ℃ for 1-4 h to homogenize the microstructure. Then remove it and air cool it. S14. Preheat the mold and ingot to 250-300℃, and immediately place them into the extruder for forming. The extrusion speed is 1.0-2.0 m / min to obtain a rod-shaped extrusion sample with a diameter of Φ 5. Then, cut the rod-shaped extrusion sample into segments, each segment with a length of about 10-12 mm, to make millimeter-sized long rod-shaped magnesium-lithium alloy particles.

10. The preparation method according to claim 3, characterized in that, In step S3: The temperature of the semi-solid slurry is 550℃~620℃; The preheating temperature of the die-casting mold is 250℃~300℃; The injection velocity is 40 m / s ~ 50 m / s; The injection pressure is 50 MPa ~ 65 MPa; The pressure holding time is 20 s ~ 30 s.

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