A semi-solid die-cast ultralight magnesium-lithium alloy suitable for thin-walled parts and its preparation method

Abstract

This invention belongs to the field of magnesium-lithium alloy material preparation technology, specifically relating to an ultralight magnesium-lithium alloy suitable for semi-solid die casting of thin-walled parts and its preparation method. This invention proposes an economical and effective magnesium-lithium alloy suitable for the semi-solid die casting process of ultra-thin-walled parts by adjusting the alloy composition and process method. The semi-solid die casting ultralight magnesium-lithium alloy for thin-walled parts described in this invention is composed of the following components by mass percentage: Li: 4.0-7.0%, Al: 2.5-5.5%, Ca: 0-1.0%, Zn: 0-1.0%, Mn: 0-0.3%, RE: 0-1.0%, with the balance being Mg and unavoidable impurity elements; RE is one or more of Ce, Y, Nd, Sm, and Er, and the sum of the contents of all elements except magnesium is <13%. The method of this invention can realize the low-temperature semi-solid molding and manufacturing of ultra-thin wall parts of 0.3~0.8mm, with a semi-solid die casting yield of over 98%; it has good molding performance, high efficiency, good product rigidity, and excellent performance; and it is easy to realize mass industrial production.

Description

Technical Field

[0001] This invention belongs to the field of magnesium-lithium alloy material preparation technology, specifically relating to an ultralight magnesium-lithium alloy suitable for semi-solid die casting of thin-walled parts and its preparation method. Background Technology

[0002] Magnesium-lithium alloys are alloys made by adding lithium and small amounts of other elements to magnesium metal, and their density ranges from 1.4 to 1.6 g / cm³. 3 Between these two extremes, magnesium-lithium alloys have the lowest density among metallic structural materials. Furthermore, magnesium-lithium alloys possess advantages such as high specific strength, high specific stiffness, easy machining, good plasticity, good low-temperature toughness, good magnetic shielding, shock resistance, and recyclability, meeting the urgent needs for lightweight materials in aerospace, military industry, and consumer electronics, thus attracting widespread attention and high importance. However, due to the difficulty in smelting magnesium-lithium alloys, their relatively low mechanical properties, and the relatively high price of raw materials such as lithium, their widespread application is limited.

[0003] Die casting is the most important casting process for magnesium alloys. Semi-solid die casting technology, in particular, can form and process parts with complex shapes, improve material utilization, and greatly leverage the performance advantages of semi-solid materials, thereby increasing the strength of semi-solid die castings. Therefore, semi-solid die casting offers advantages such as high efficiency and energy saving, excellent performance, and near-net-shape forming.

[0004] Currently, there is considerable research on semi-solid forming technology for magnesium-lithium alloys in existing technologies. However, most studies have found that magnesium-lithium alloys used for semi-solid die casting have a high degree of alloying and poor alloy fluidity. For example, Chinese patent CN112593132B discloses a high-strength semi-solid dual-phase die-casting magnesium-lithium alloy and its preparation method. Due to the increased content of Li and Al elements, the alloy structure is a dual-phase structure. The presence of low-melting-point β-Li phase and a large amount of high-melting-point phases such as Al2Li leads to an increase in the viscosity of the alloy semi-solid slurry. When processing thin-walled parts, the alloy described in this patent is prone to insufficient filling, flash, overflow, and sticking during die casting. After cooling, the thin-walled die-cast parts are prone to deformation. Therefore, it is only suitable for use on plates with a thickness of 0.8-2mm.

[0005] For example, Chinese patent CN105838950A discloses a magnesium alloy in which the Li element ranges from 6 to 12%. The magnesium-lithium alloy with a dual-phase structure in this patent is prone to significant cooling shrinkage after die casting. At the same time, during the cooling process of a large number of softer β phase and α phase, the difference in shrinkage coefficient leads to a large stress concentration, and even shrinkage cracks occur at locations with large differences in wall thickness.

[0006] Based on this, the present invention addresses the problems existing in the preparation of semi-solid die-cast thin plates of magnesium-lithium alloys in the prior art. By adjusting the content of alloying elements and reducing the total amount and proportion of alloying elements, the fluidity of the alloy is improved, and a magnesium-lithium alloy with low linear shrinkage, fine grains, good temperature resistance, and suitable for semi-solid die-cast ultra-thin wall parts is successfully prepared.

[0007] Meanwhile, the semi-solid die casting process of the magnesium-lithium alloy described in this invention enables the mass production of thin-walled parts, significantly improving material utilization and die casting yield, reducing production costs, and making it suitable for high-end thin and light products such as 3C products, optics, and precision electronics. This effectively promotes the mass production and application of semi-solid die casting of thin-walled magnesium-lithium alloy parts, meeting the lightweight requirements of alloy components. Summary of the Invention

[0008] This invention addresses the problems of insufficient filling, flash, overflow, and sticking in existing magnesium-lithium alloys used for semi-solid die casting of ultra-thin-walled parts during the die casting process. By adjusting the alloy composition and process method, this invention proposes an economical and effective magnesium-lithium alloy suitable for the forming process of semi-solid die casting of ultra-thin-walled parts.

[0009] The present invention also provides a method for preparing thin-walled parts using the above-mentioned semi-solid die-cast ultralight magnesium-lithium alloy.

[0010] To achieve the above objectives, the present invention adopts the following technical solution:

[0011] A semi-solid die-cast ultralight magnesium-lithium alloy suitable for thin-walled parts is composed of the following components by mass percentage: Li: 4.0-7.0%, Al: 2.5-5.5%, Ca: 0-1.0%, Zn: 0-1.0%, Mn: 0-0.3%, RE: 0-1.0%, with the balance being Mg and unavoidable impurity elements, each impurity element having a content of less than 0.05%, and the total amount of impurity elements being less than 0.3%; RE is one or more of Ce, Y, Nd, Sm and Er, wherein the mass ratio of Li to Al is controlled at (0.7-2.8):1, and the sum of the contents of all elements except magnesium is <13%.

[0012] Furthermore, the density of the semi-solid die-cast ultralight magnesium-lithium alloy is 1.55–1.60 g / cm³. 3 .

[0013] Furthermore, the solid-liquid phase temperature difference of the semi-solid die-cast ultralight magnesium-lithium alloy is controlled at 30-50℃. During semi-solid die casting, the solid phase volume fraction is 25-40%, the slurry has low apparent viscosity and good fluidity, and can achieve precision forming of thin-walled complex parts with a wall thickness of 0.3-0.8mm.

[0014] The method for preparing thin-walled parts using the magnesium-lithium alloy includes the following steps:

[0015] (1) Ingot casting: The alloy is batched according to the mass percentage of each component. After batching, the alloy is melted in a vacuum heating furnace to obtain molten metal. The molten metal is then cast to obtain ingots.

[0016] (2) Extrusion: The billet is heated to 360-390℃ and extruded to form a bar with a specification of φ90mm;

[0017] (3) Sawing and turning: The extruded bar is sawn into small segments with a specification of φ90*310mm, and then turned into cylindrical bars with a specification of φ90±0.1mm*300±1mm;

[0018] (4) Semi-solid die casting: The magnesium-lithium alloy rods are die-cast using a DAK450-54RC semi-solid die casting machine. The semi-solid die casting machine includes a heating cylinder, ejector pin, induction heating device, die casting mold, etc. The semi-solid die casting method of the present invention combines the slurry preparation and die casting processes, with a high degree of integration. The specific process method is as follows:

[0019] The cylindrical bars obtained in step (3) are heated to semi-solid in stages to obtain a melt. The temperature of the melt is controlled to be maintained at 550-620℃. Then, the semi-solid melt is die-cast by injection to obtain a semi-solid die-cast magnesium-lithium alloy thin-walled part with a thickness of 0.3-0.8mm.

[0020] (5) Aging treatment: The magnesium-lithium alloy thin-walled parts obtained by semi-solid die casting are placed in a heating furnace and aged at 160-180℃ for 2-6 hours. Then, they are cooled to room temperature with the furnace to obtain the final product.

[0021] Specifically, in step (1), the vacuum melting and casting steps of the magnesium-lithium alloy are as follows: first, the vacuum degree in the vacuum heating furnace is evacuated to 0.1-2 Pa, then 500-2000 Pa of argon gas is introduced, and then heating is carried out. After the metal elements are completely melted, the temperature is raised to 735-755℃ for refining for 5-10 minutes to obtain molten metal. After refining, the molten metal is left to stand for 5-20 minutes, and then the molten metal is cast to obtain ingots with a specification of φ300-500mm.

[0022] Specifically, in step (2) during the extrusion process, the heated billet is first placed in the extrusion die and then placed in the extrusion cylinder for extrusion. During the extrusion process, the extrusion cylinder is heated to a temperature of 360-390°C. Due to the fact that the die is located in the center of the extrusion cylinder and the heat accumulates, the temperature of the extrusion die is 380-400°C.

[0023] Specifically, in step (3), the cross-section of the machined bar stock is rounded with an R3 fillet.

[0024] Specifically, in step (4), the semi-solid die-casting machine includes a heating cylinder, a push rod, an induction heating device, a die-casting mold, etc. The left end of the heating cylinder has a loading port, the push rod's pushing end is inserted into the heating cylinder from the loading port, and the outlet end of the heating cylinder has an injection nozzle. The die-casting mold includes two modules, left and right, with a gap between them for forming thin-walled magnesium-lithium alloy parts. The left module has an injection port at its left end, which is in contact with the injection nozzle. The induction heating device is wrapped around the heating cylinder. The semi-solid die-casting device uses commonly used equipment in the prior art, and its structure is not the inventive point of this invention, so it will not be described in detail.

[0025] Specifically, in step (4), the semi-solid die casting process involves first preheating the semi-solid die casting machine to 180-200°C, then placing several cylindrical bars obtained in step (3) into the heating cylinder of the semi-solid die casting machine in batches of 3-8 bars per batch, and using an induction heating device for staged heating. The bars are heated to semi-solid state according to their placement order, i.e., the bars placed first are heated to semi-solid state and maintained at 550-620°C, and the bars placed later are heated subsequently. At this time, the unheated bars and the semi-solid melt are in a solid-liquid mixed state. Then, the semi-solid melt is die-cast using an injection method.

[0026] Specifically, in step (4), the volume fraction of the solid phase in the semi-solid melt is 25-40%.

[0027] Specifically, in step (4), the die casting process parameters are: injection speed 3-5 m / s, injection specific pressure 25-32 MPa, and boosting pressure 38-47 MPa.

[0028] Specifically, in step (4), after die casting, the thin-walled part is held under pressure and cooled for 6 to 12 seconds before being removed.

[0029] Specifically, in the process of preparing thin-walled parts, the semi-solid die-cast ultralight magnesium-lithium alloy described in this application has a tensile strength of 230-270 MPa, a yield strength of 140-180 MPa, and an elongation of 10-20% at room temperature after the extrusion process.

[0030] The above method involves heat-treating a magnesium-lithium alloy at high temperature to form a melt, followed by die casting to obtain a semi-solid die-cast ultralight magnesium-lithium alloy thin-walled part. At room temperature, the wall thickness of the thin-walled part is 0.3-0.8 mm, the tensile strength is ≥240 MPa, the yield strength is ≥190 MPa, and the elongation is 10-15%.

[0031] Specifically, the thin-walled part obtained by semi-solid die casting of the magnesium-lithium alloy has a two-phase structure with an average grain size of <10μm.

[0032] Specifically, thin-walled parts obtained through semi-solid die casting have a smaller linear shrinkage rate (0.2% to 0.5%) and lower sensitivity to hot cracking, thus improving the dimensional accuracy of the thin-walled parts after forming.

[0033] Specifically, the method for preparing thin-walled parts from magnesium-lithium alloys has a die-casting yield of over 98%, demonstrating good die-casting efficiency.

[0034] Furthermore, the present invention also provides the application of the semi-solid die-cast ultralight magnesium-lithium alloy in the manufacture of ultra-thin 3C products (thickness of 0.3-0.8 mm), optical instruments, or precision electronic instruments.

[0035] Compared with existing technologies, this invention addresses the problems of poor fluidity, insufficient filling, flash, overflow, sticking, shrinkage cracking, and high linear shrinkage that often occur when using magnesium-lithium alloys for semi-solid die casting to prepare ultra-lightweight thin-walled parts. By optimizing the types and contents of alloying elements, this invention obtains magnesium-lithium alloy semi-solid die-cast ultra-lightweight thin-walled parts that possess both excellent die-casting performance and mechanical properties. This invention has the following beneficial effects:

[0036] 1. This invention controls the Li element content between 4.0% and 7.0%, enabling the alloy microstructure to transform from a single-phase α microstructure to an α+β microstructure. By reducing the Li and Al element contents (controlled between 2.5% and 5.5%), and the Li to Al mass ratio (controlled between 0.7 and 2.8), the proportion of high-melting-point, high-viscosity MgLi2Al and Al2Li phases in the alloy is reduced, while the proportion of low-viscosity, low-melting-point eutectic phase Mg is increased. 17 Al 12 The ratio improves the low-temperature fluidity of the alloy and optimizes the filling performance of the alloy semi-solid die-cast ultra-light thin-walled parts.

[0037] 2. In addition, adding trace amounts of Ca and Zn elements to the alloy can form high-melting-point spherical Al2Ca phase and Mg2Ca6Zn3 phase, which also has a certain effect on improving the solid phase content of the alloy, and Ca element can reduce the alloy density. Adding trace amounts of RE element to the alloy can, on the one hand, act as heterogeneous nucleation particles as the die casting temperature decreases, rapidly refining the grains; on the other hand, due to the low solid solubility of RE element, it can also be squeezed to the solid-liquid interface front during solidification, enriched on the phase interface, hindering grain growth, and effectively enhancing the performance of semi-solid die-cast magnesium-lithium alloy.

[0038] 3. During the filling process, since the solid volume fraction of the magnesium-lithium alloy described in this invention is 20-40%, the solidification rate of semi-solid die casting is fast, which greatly reduces the linear shrinkage rate of the material (from 4.1% in conventional molding to 0.2%-0.5%). While improving the rigidity of the product, it significantly improves the deformation and warping phenomena after cooling of conventional thin-walled parts semi-solid die casting, and ensures the dimensional stability of the semi-solid die-cast ultra-thin-walled parts.

[0039] 4. This invention improves the apparent viscosity of the alloy semi-solid die casting slurry by adjusting the content of alloying elements and controlling the solid-liquid phase temperature difference between 30 and 50°C, and further controlling the volume fraction of the alloy solid phase (20%-40%). This enhances the alloy fluidity and reduces the frequency of phenomena such as overflow, shrinkage, flash, and sticking in the alloy semi-solid die casting, resulting in a product yield of over 98%.

[0040] 5. This invention reduces the content of all elements except magnesium, and limits the sum of the contents of all elements except magnesium to <13%. It further optimizes the semi-solid die casting process parameters, reducing the semi-solid die casting temperature from 610-640℃ to 550-620℃, a temperature reduction of 10-80℃. While reducing production energy consumption and production costs, it significantly improves production efficiency from 15-20 pieces / h to 100-150 pieces / h.

[0041] 6. The semi-solid die-cast ultralight magnesium-lithium alloy thin-walled parts prepared by this invention have high strength and good plasticity. The tensile strength can reach more than 240MPa, the yield strength can reach more than 190MPa, and the elongation can reach 10-15%. This is conducive to the mass production and application of magnesium-lithium alloy semi-solid die-cast parts and expands the application range of magnesium-lithium alloy. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of the semi-solid die casting process of the present invention;

[0043] Figure 2 This is a metallographic micrograph of the magnesium-lithium alloy of the semi-solid die-cast ultrathin wall part of Embodiment 1 of the present invention. Detailed Implementation

[0044] To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the invention is further described in detail below. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. The raw materials used in the following embodiments are all commercially available products.

[0045] Example 1

[0046] A semi-solid die-cast ultralight magnesium-lithium alloy suitable for thin-walled parts is composed of the following components by mass percentage: Li: 4.5%, Al: 5.0%, Ca: 0.7%, Zn: 0.8%, Mn: 0.2%, RE: 0.5%, wherein the mass ratio of Li to Al is controlled at 0.9, RE consists of three elements: Ce, Y, and Nd, each with a content of 0.1%, and the balance is Mg and unavoidable impurity elements; the content of each impurity element is less than 0.05%, and the total amount of impurity elements is less than 0.3%.

[0047] The method for preparing thin-walled parts using the aforementioned magnesium-lithium alloy includes processes such as melting and casting, extrusion, cutting, and semi-solid die casting. The specific steps are as follows:

[0048] (1) Ingot casting: The alloy is batched according to the mass percentage of each component. After batching, the alloy is smelted in a vacuum heating furnace. Specifically, the vacuum degree in the vacuum heating furnace is first evacuated to 0.1 Pa, and then 2000 Pa of argon gas is introduced. Then, it is heated. When the metal elements are completely melted, the temperature is raised to 745℃ and refined for 10 min to obtain molten metal. After refining, it is allowed to stand for 20 min. The molten metal is then cast to obtain an ingot with a specification of φ500mm.

[0049] (2) Extrusion: The billet is heated to 380°C and placed in an extrusion die, and then placed in an extrusion cylinder for extrusion to form a bar with a specification of φ90mm. During the extrusion process, the extrusion cylinder is heated to 380°C. Due to the fact that the die is located in the center of the extrusion cylinder and the heat is concentrated, the temperature of the extrusion die is 390°C.

[0050] (3) Sawing and turning: The extruded bar is sawn into small sections with a specification of φ90*310mm, and then turned into cylindrical bars with a specification of φ90±0.1mm*300±1mm, and the cross-section of the bar is rounded with R3.

[0051] (4) Semi-solid die casting: A DAK450-54RC semi-solid die casting machine is used for die casting magnesium-lithium alloy rods. The semi-solid die casting machine includes a heating cylinder, ejector rod, induction heating device, and die casting mold. The connection relationships between the heating cylinder, ejector rod, induction heating device, and die casting mold are as follows: Figure 1 As shown, the heating cylinder has a loading port at its left end, and the ejector rod is inserted into the heating cylinder from the loading port at the left end. The heating cylinder has an injection nozzle at its right end. The die-casting mold includes two modules, left and right, with a gap between them for forming thin-walled magnesium-lithium alloy parts. The left module has an injection port at its left end, which is in contact with the injection nozzle. The induction heating device is wrapped around the heating cylinder. The semi-solid die-casting device uses commonly used equipment in the prior art, and its structure is not the inventive point of this invention, so it will not be described in detail.

[0052] The specific process method is as follows:

[0053] First, the mold is preheated to 190°C. Then, the cylindrical bars obtained in step (3) are loaded into the heating cylinder from the left end of the loading port in order of 4 bars / time. Then, the ejector rod is inserted and the heating cylinder is heated in stages using an induction heating device. The cylindrical bars on the right end are first heated to a semi-solid state to obtain a melt. The temperature of the melt is controlled to be maintained at 570°C before injection. The unheated bars on the left end are in a solid state. At this time, the magnesium-lithium alloy bars and the semi-solid melt are in a solid-liquid mixed state, wherein the solid volume fraction of the melt is 30%.

[0054] Then, the die casting process parameters are set as follows: injection speed is 5m / s, injection specific pressure is 31MPa, and boosting pressure is 42MPa. Under the action of the ejector rod, the semi-solid melt is injected from the injection nozzle into the gap inside the die casting mold, thereby obtaining a semi-solid die-cast magnesium-lithium alloy thin-walled part with a thickness of 0.5mm. After holding the pressure and cooling for 10s, the die-cast thin-walled part is taken out.

[0055] During the test, no overflow occurred in the die casting process of Example 1, the mold was completely filled, and 138 pieces could be die cast in one hour, during which the mold could be cleaned normally.

[0056] (5) Aging treatment: The magnesium-lithium alloy thin-walled part obtained by semi-solid die casting is placed in a heating furnace and aged at 160°C for 6 hours. Then it is cooled to room temperature with the furnace. Finally, a magnesium-lithium alloy thin-walled part with a tensile strength of 247MPa, a yield strength of 193MPa, and an elongation of 10.5% is obtained. The linear shrinkage rate is 0.22% after testing.

[0057] Example 2

[0058] A semi-solid die-cast ultralight magnesium-lithium alloy suitable for thin-walled parts is composed of the following components by mass percentage: Li: 5.0%, Al: 2.5%, Ca: 0.9%, Zn: 0.8%, Mn: 0.1%, RE: 0.7%, wherein the mass ratio of Li to Al is controlled at 2, RE consists of three types of Y, Nd, and Sm, with contents of 0.2%, 0.4%, and 0.1%, respectively, and the balance is Mg and unavoidable impurity elements; the content of each impurity element is less than 0.05%, and the total amount of impurity elements is less than 0.3%.

[0059] The method for preparing thin-walled parts using the aforementioned magnesium-lithium alloy includes processes such as melting and casting, extrusion, cutting, and semi-solid die casting. The specific steps are as follows:

[0060] (1) Ingot casting: The alloy is batched according to the mass percentage of each component. After batching, the alloy is smelted in a vacuum heating furnace. Specifically, the vacuum degree in the vacuum heating furnace is first reduced to 0.4 Pa, and then 1800 Pa argon gas is introduced. Then, it is heated. When the metal elements are completely melted, the temperature is raised to 740℃ and refined for 8 minutes to obtain molten metal. After refining, it is allowed to stand for 17 minutes. The molten metal is then cast to obtain an ingot with a specification of φ400mm.

[0061] (2) Extrusion: The billet is heated to 370°C and placed in an extrusion die, and then placed in an extrusion cylinder for extrusion to form a bar with a specification of φ90mm. During the extrusion process, the extrusion cylinder is heated to 370°C. Due to the fact that the die is located in the center of the extrusion cylinder and the heat is concentrated, the temperature of the extrusion die is 380°C.

[0062] (3) Sawing and turning: The extruded bar is sawn into small sections with a specification of φ90*310mm, and then turned into cylindrical bars with a specification of φ90±0.1mm*300±1mm, and the cross-section of the bar is rounded with R3.

[0063] (4) Semi-solid die casting: The apparatus used for semi-solid die casting is the same as in Example 1;

[0064] The specific process method is as follows:

[0065] First, the mold is preheated to 185°C. Then, the cylindrical bars obtained in step (3) are loaded into the heating cylinder from the left end of the loading port in order of 4 bars / time. Then, the ejector rod is inserted and the heating cylinder is heated in stages using an induction heating device. The cylindrical bars on the right end are first heated to a semi-solid state to obtain a melt. The temperature of the melt is controlled to be maintained at 580°C before injection. The unheated bars on the left end are in a solid state. At this time, the magnesium-lithium alloy bars and the semi-solid melt are in a solid-liquid mixed state, wherein the solid volume fraction of the melt is 25%.

[0066] Then, the die casting process parameters are set as follows: injection speed is 4m / s, injection specific pressure is 28MPa, and boosting pressure is 42MPa. Under the action of the ejector rod, the semi-solid melt is injected from the injection nozzle into the gap inside the die casting mold, thereby obtaining a semi-solid die-cast magnesium-lithium alloy thin-walled part with a thickness of 0.4mm. After holding the pressure and cooling for 8s, the die-cast thin-walled part is taken out.

[0067] During the test, no overflow occurred in the die casting process of Example 1, the mold was completely filled, and 145 pieces could be die cast in one hour, during which the mold could be cleaned normally.

[0068] (5) Aging treatment: The magnesium-lithium alloy thin-walled parts obtained by semi-solid die casting are placed in a heating furnace and aged at 175°C for 4 hours. Then, they are cooled to room temperature with the furnace to finally obtain magnesium-lithium alloy thin-walled parts with tensile strength of 252MPa, yield strength of 197MPa and elongation of 11.5%. The linear shrinkage rate is 0.27% after testing.

[0069] Example 3

[0070] A semi-solid die-cast ultralight magnesium-lithium alloy suitable for thin-walled parts is composed of the following components by mass percentage: Li: 7.0%, Al: 5.0%, Ca: 0.8%, Zn: 0.8%, Mn: 0.3%, RE: 0.9%, wherein the mass ratio of Li to Al is controlled at 2.33, RE consists of three types: Ce, Y, and Er, each with a content of 0.3%, and the balance is Mg and unavoidable impurity elements; wherein the content of impurity element Na is <0.001%, the content of each of the other impurity elements is less than 0.05%, and the total amount of impurity elements is less than 0.3%.

[0071] The method for preparing thin-walled parts using the aforementioned magnesium-lithium alloy includes processes such as melting and casting, extrusion, cutting, and semi-solid die casting. The specific steps are as follows:

[0072] (1) Ingot casting: The alloy is batched according to the mass percentage of each component. After batching, the alloy is smelted in a vacuum heating furnace. Specifically, the vacuum degree in the vacuum heating furnace is first reduced to 0.5 Pa, and then 1500 Pa of argon gas is introduced. Then, it is heated. When the metal elements are completely melted, the temperature is raised to 735℃ and refined for 8 minutes to obtain molten metal. After refining, it is allowed to stand for 13 minutes. The molten metal is then cast to obtain an ingot with a specification of φ300mm.

[0073] (2) Extrusion: The billet is heated to 360°C and placed in the extrusion die, and then placed in the extrusion cylinder for extrusion to form a bar with a specification of φ90mm. During the extrusion process, the extrusion cylinder is heated to 360°C. Due to the fact that the die is located in the center of the extrusion cylinder and the heat is concentrated, the temperature of the extrusion die is 380°C.

[0074] (3) Sawing and turning: The extruded bar is sawn into small sections with a specification of φ90*310mm, and then turned into cylindrical bars with a specification of φ90±0.1mm*300±1mm, and the cross-section of the bar is rounded with R3.

[0075] (4) Semi-solid die casting: The apparatus used for semi-solid die casting is the same as in Example 1;

[0076] The specific process method is as follows:

[0077] First, the mold is preheated to 200°C. Then, the cylindrical bars obtained in step (3) are loaded into the heating cylinder from the left end of the loading port in order of 4 bars / time. Then, the ejector rod is inserted and the heating cylinder is heated in stages using an induction heating device. The cylindrical bars on the right end are first heated to semi-solid to obtain melt. The temperature of the melt is controlled to be maintained at 610°C before injection. The unheated bars on the left end are in a solid state. At this time, the magnesium-lithium alloy bars and the semi-solid melt are in a solid-liquid mixed state, wherein the solid volume fraction of the melt is 20%.

[0078] Then, the die casting process parameters are set as follows: injection speed is 3.5 m / s, injection specific pressure is 28 MPa, and boosting pressure is 47 MPa. Under the action of the ejector rod, the semi-solid melt is injected from the injection nozzle into the gap inside the die casting mold, thereby obtaining a semi-solid die-cast magnesium-lithium alloy thin-walled part with a thickness of 0.5 mm. After holding the pressure and cooling for 12 seconds, the die-cast thin-walled part is taken out.

[0079] During the test, no overflow occurred in the die casting process of Example 1, the mold was completely filled, and 127 pieces could be die cast in one hour, during which the mold could be cleaned normally.

[0080] (5) Aging treatment: The magnesium-lithium alloy thin-walled parts obtained by semi-solid die casting are placed in a heating furnace and aged at 170°C for 6 hours. Then, they are cooled to room temperature with the furnace to finally obtain magnesium-lithium alloy thin-walled parts with tensile strength of 243MPa, yield strength of 192MPa, and elongation of 11.0%. The linear shrinkage rate is 0.4% after testing.

[0081] Comparative Example 1

[0082] The semi-solid die-cast magnesium-lithium alloy described in Comparative Example 1 uses a different composition ratio than that of Examples 1-3. Specifically, it consists of the following components by mass percentage: Li: 12.0%, Al: 1.0%, Ca: 1.2%, Zn: 0.8%, Mn: 0.2%, RE: 0.7%. The mass ratio of Li to Al in the alloy is 1:2. RE consists of three types: Ce, Y, and Nd, with contents of 0.2%, 0.25%, and 0.25%, respectively. The balance is Mg and other unavoidable impurity elements. Among them, the content of impurity element Na is <0.001%, the content of each of the other impurity elements is less than 0.05%, and the total amount of impurity elements is less than 0.3%.

[0083] The preparation method described in Comparative Example 1 is the same as that in Example 1, except that:

[0084] In Comparative Example 1, the solid volume fraction of the melt was 80%. During the semi-solid die casting process, the magnesium-lithium alloy thin-walled part failed to fill the mold completely, resulting in localized missing parts and failure to form a finished product.

[0085] The alloy properties in Comparative Example 1 were tested, and the tensile strength was 121 MPa, the yield strength was 96 MPa, the elongation was 18.5%, and the linear reduction of area was 2.2%.

[0086] Comparative Example 2

[0087] The semi-solid die-cast magnesium-lithium alloy described in Comparative Example 2 has a different composition ratio than that of Examples 1-3. Specifically, it consists of the following components by mass percentage: Li: 2.0%, Al: 5.5%, Ca: 0.3%, Zn: 0.8%, Mn: 0.2%, RE: 0.3%, with a Li to Al mass ratio of 0.36. RE consists of three types: Ce, Y, and Er, each with a content of 0.1%. The balance is Mg and other unavoidable impurity elements. Among them, the impurity element Na content is <0.001%, the content of each of the other impurity elements is less than 0.05%, and the total amount of impurity elements is less than 0.3%.

[0088] The preparation method described in Comparative Example 2 is the same as that in Example 2, except that:

[0089] In Comparative Example 2, the solid volume fraction of the melt was 18%. During the semi-solid die casting process, a large amount of liquid overflow and combustion occurred. The die casting nozzle was cleaned 4 times, and the mold was cleaned an additional 3 times during the die casting process.

[0090] The die-casting method in Comparative Example 2 can die-cast 14 pieces per hour. The alloy properties in Comparative Example 2 were tested and found to be 263 MPa tensile strength, 204 MPa yield strength, 4.5% elongation, and 2.7% linear shrinkage. The product exhibited shrinkage cracking.

[0091] Figure 2 The image shows a metallographic micrograph of a semi-solid die-cast ultralight magnesium-lithium alloy suitable for thin-walled parts according to Embodiment 1 of the present invention. As can be seen from the image, the grain size after semi-solid die casting is small and uniform, with a grain size of 8.8 μm.

[0092] In Examples 1-3 and Comparative Examples 1 and 2, the room temperature mechanical properties of the alloys were tested according to the methods in GB / T 228.1-2021 "Metallic Materials - Tensile Specimens - Part 1: Test at Room Temperature", and the linear shrinkage of the alloys was tested according to the methods in JB / T 4022.1-1999 "Methods for Determining Casting Properties of Alloys - Method for Determining Free Linear Shrinkage".

[0093] Performance testing

[0094] The mechanical properties of the semi-solid die-cast magnesium-lithium alloy thin-walled parts of Examples 1-3 and Comparative Examples 1-2 were tested using a SUNS10 electronic universal testing machine. The average value of three parallel samples was taken as the test result. The metallographic structure of the semi-solid die-cast magnesium-lithium alloy was observed using a DMI 2000mm microscope, and the average grain size was calculated. The test results of the samples are shown in Table 1.

[0095] Table 1. Test results for examples and comparative examples.

[0096]

[0097] As shown in Table 1, the tensile strength of Examples 1-3 of the present invention all exceeded 240 MPa, the yield strength all exceeded 190 MPa, and the elongation also reached more than 10%, indicating a significant improvement in performance. Furthermore, the grain size of Examples 1-3 was generally smaller than that of Comparative Examples 1-2. Compared with Examples 1-3, Comparative Example 1 showed a significant decrease in strength and die-casting efficiency, and a substantial increase in linear shrinkage. This was due to the increased content of low-melting-point Li in the alloy, which led to increased viscosity of the alloy melt, poor flowability, and localized incomplete filling of thin-walled parts during filling. Additionally, the significantly reduced Al content in the alloy increased the linear shrinkage to 2.2%. Compared with Example 2, Comparative Example 2 reduced the content of elements such as Li, resulting in a significant decrease in the elongation of the alloy under the same semi-solid die-casting conditions.

[0098] The thin-walled parts prepared by the methods in Examples 1-3 of this invention maintain good mechanical properties while significantly reducing the linear shrinkage rate of the material. The molding process is simple, enabling short-process, large-scale mass production, reducing production costs, and showing broad application prospects in precision instruments, military industry, and high-end civilian fields.

[0099] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above. Those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A semi-solid die cast ultra-light magnesium-lithium alloy suitable for thin-walled components, characterized in that, Composed of the following components in percentage by mass: Li: 4.0-7.0 %, Al: 2.5-5.5 %, Ca: 0-1.0 %, Zn: 0-1.0 %, Mn: 0-0.3 %, RE: 0-1.0 %, the balance being Mg and inevitable impurity elements; RE is one or more of Ce, Y, Nd, Sm and Er, wherein the mass ratio of Li to Al is controlled to be (0.7-2.8):1, and the total content of the elements other than Mg is <13 %; The RE is Ce, Y and Nd, each in an amount of 0.1 %, or the RE is Y, Nd and Sm, each in an amount of 0.2 %, 0.4 % and 0.1 %, respectively, or the RE is Ce, Y and Er, each in an amount of 0.3 %; The semi-solid state die-casting ultra-light magnesium-lithium alloy has a solid-liquid phase line temperature difference controlled within 30-50 DEG C, and when semi-solid state die-casting is performed, the solid phase volume fraction is 25-40 %, the slurry has low apparent viscosity and good flowability, and the precise forming processing of thin-walled complex parts with a wall thickness of 0.3-0.8 mm can be realized; The magnesium-lithium alloy is prepared by the following steps: (1) Ingot casting: ingredients are prepared according to the mass percentage of each component in the alloy, and after preparation, vacuum melting is performed to obtain a metal liquid, and an ingot blank is obtained by casting; (2) Extrusion: the ingot blank is heated to 360-390 DEG C for extrusion to form a rod with a specification of φ90 mm; (3) Cutting and turning: the rod obtained by extrusion is cut and then turned into a rod with a specification of φ90±0.1 mm*300±1 mm; (4) Semi-solid state die-casting: a plurality of rods obtained in step (3) are sequentially heated to a semi-solid state in stages to obtain a melt, the temperature of the melt is controlled to be maintained at 550-620 DEG C, and then the melt in the semi-solid state is die-cast in an injection manner to obtain a semi-solid state die-casting magnesium-lithium alloy thin-walled part with a thickness of 0.3-0.8 mm; (5) Aging treatment: the magnesium-lithium alloy thin-walled part obtained by semi-solid state die-casting is subjected to aging treatment at a temperature of 160-180 DEG C for 2-6 h, and then cooled to room temperature to obtain a final product; In the extrusion process of step (2), the heated ingot blank is first placed in an extrusion die, and then in an extrusion cylinder for extrusion, and the extrusion cylinder is heated to 360-390 DEG C during the extrusion process, and due to the heat accumulation factor that the die is in the center of the extrusion cylinder, the temperature of the extrusion die is 380-400 DEG C; In step (4), the semi-solid state die-casting step is as follows: the die of the semi-solid state die-casting machine is preheated to 180-200 DEG C, and then a plurality of cylindrical rods obtained in step (3) are put into the heating cylinder of the semi-solid state die-casting machine at a rate of 3-8 rods per time, and are sequentially heated to a semi-solid state by using an induction heating device according to the order of placement, i.e. the rods placed first are heated to a semi-solid state first and the temperature is maintained at 550-620 DEG C, and the rods placed later are heated subsequently, and at this time, the rods not heated and the melt in the semi-solid state are in a solid-liquid mixed state; and then the melt in the semi-solid state is die-cast in an injection manner. In step (4), the die casting process parameters are: injection speed 3-5 m / s, injection specific pressure 25-32 MPa, and boost pressure 38-47 MPa. In step (4), after die casting, the thin-walled part is kept pressure and cooled for 6-12 s before being taken out.

2. The semi-solid die cast ultra-light Mg-Li alloy according to claim 1, characterized in that, The alloy has a density of 1.55 to 1.60 g / cm 3 .

3. A method of producing thin-walled parts from the magnesium-lithium alloy according to claim 1 or 2, characterized in that, The method comprises the following steps: (1) Ingot casting: ingredients are prepared according to the mass percentage of each component in the alloy, and vacuum melting is performed after the ingredients are prepared to obtain a molten metal, and an ingot blank is obtained by casting; (2) Extrusion: the ingot blank is heated to 360-390 DEG C for extrusion to form a rod with a specification of φ90mm; (3) Cutting and turning: the rod obtained by extrusion is cut and then turned into a rod with a specification of φ90±0.1mm*300±1mm; (4) Semi-solid die casting: a plurality of rods obtained in step (3) are heated to semi-solid state in stages in sequence to obtain a melt, the temperature of the melt is controlled to be maintained at 550-620 DEG C, and then the melt in semi-solid state is die cast by injection to obtain a semi-solid die cast magnesium-lithium alloy thin-walled part with a thickness of 0.3-0.8mm; (5) Aging treatment: the magnesium-lithium alloy thin-walled part obtained by semi-solid die casting is aged at a temperature of 160-180 DEG C for 2-6 h, and then cooled to room temperature to obtain a final product.

4. The method of claim 3, wherein, In step (1), the vacuum melting and casting steps of the magnesium-lithium alloy are as follows: vacuum is first extracted to 0.1-2 Pa, then 500-2000 Pa of argon gas is introduced, then heating is performed, the temperature is raised to 735-755 DEG C for 5-10 min of refining after the metal elements are completely melted, the molten metal is obtained, the molten metal is kept still for 5-20 min after the refining is completed, and the ingot blank is obtained by casting the molten metal, and the specification of the ingot blank is φ300-500mm.

5. The method of claim 3, wherein, In step (4), the semi-solid die casting step is as follows: the mold of the semi-solid die casting machine is preheated to 180-200 DEG C, then a plurality of rods obtained in step (3) are placed in the semi-solid die casting machine in a manner of 3-8 rods per time for staged heating, the rods placed each time are heated to semi-solid state in sequence according to the order of placement, and then the melt in semi-solid state is die cast by injection.

6. The method of claim 3, wherein, In step (4), the solid volume fraction in the semi-solid melt is 25-40%.

7. The method of claim 3, wherein, In step (4), the die casting process parameters are: injection speed 3-5 m / s, injection specific pressure 25-32 MPa, and boost pressure 38-47 MPa.

8. The method of claim 3, wherein, In step (4), after die casting, the thin-walled part is kept pressure and cooled for 6-12 s before being taken out.

9. Application of the semi-solid die cast ultra-light magnesium-lithium alloy in claim 1 or 2 in manufacturing ultra-thin 3C products, optical instruments or precision electronic instruments.

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