A special semi-continuous casting crystallizer hot top preform for aluminum-lithium alloy and a manufacturing method thereof

Abstract

This invention discloses a preform for the hot top of a crystallizer specifically for semi-continuous casting of aluminum-lithium alloys and its manufacturing method. Addressing the problems of severe interfacial reactions between existing refractory materials and molten aluminum-lithium alloys, resulting in poor forming quality of semi-continuous casting ingots, this invention employs a gradient composite structure design: lithium aluminate is introduced into the working layer to thermodynamically suppress interfacial reactions by increasing the intrinsic lithium chemical sites; the insulation layer contains β-lithium nepheline and lithium aluminate to improve the insulation performance and thermal shock resistance of the hot top. Furthermore, by using a gradient distribution of lithium compound concentrations and types, combined with a multi-stage sintering process, the interlayer bonding strength is improved. Results show that this invention not only significantly reduces the interfacial reaction between the molten aluminum-lithium alloy and the hot top during semi-continuous casting, improving the forming quality of aluminum-lithium alloy ingots, but also extends the service life of the preform.

Description

Technical Field

[0001] This invention relates to the field of aluminum alloy casting technology, and more specifically, to a hot top preform for a semi-continuous casting crystallizer for aluminum-lithium alloys and its manufacturing method. Background Technology

[0002] Compared to other aluminum alloys, aluminum-lithium alloys possess lower density, higher strength and elastic modulus, and excellent damage tolerance, earning them the reputation as a key strategic structural material for the next generation of aerospace applications. Research shows that adding 1% lithium by mass to each aluminum alloy can reduce its density by approximately 3% and increase its elastic modulus by approximately 6%. For this reason, major aerospace powers in Europe and America have already extensively used aluminum-lithium alloys in their mainstream aircraft and launch vehicles to reduce weight and improve maneuverability. In contrast, my country is still in the catching-up stage in the independent design, preparation, and application technologies of high-quality aluminum-lithium alloys. Breakthroughs in the key preparation technologies of aluminum-lithium alloys are crucial for enhancing the competitiveness of the nation's high-end equipment.

[0003] The manufacturing of high-end products such as high-quality aluminum-lithium alloy forgings, extruded profiles, and rolled plates all rely on large-size, high-performance ingot raw materials. In industrial practice, large-size aluminum-lithium alloy ingots are mainly formed using a semi-continuous casting process. Semi-continuous casting serves as the intermediate link between the initial aluminum-lithium alloy melt preparation and the subsequent plastic processing of the aluminum-lithium alloy. The semi-continuous casting process has a significant impact on the microstructure and compositional uniformity of the ingot, as well as the formation of internal defects. Therefore, the forming quality of the semi-continuously cast ingot plays a decisive role in the quality of subsequent wrought aluminum-lithium alloy products.

[0004] In semi-continuous casting systems, the hot top of the crystallizer must possess excellent heat preservation capabilities. The hot top significantly influences the temperature field, flow field, and solidification process of the molten aluminum alloy within the crystallizer, making it a crucial component for achieving sequential solidification and efficient feeding of the molten aluminum, and determining the quality of the semi-continuous casting ingot. Currently, the hot top preforms widely used in the aluminum casting industry are made of refractory materials, with common material series including: aluminum silicate-based, mullite-corundum-based, and alumina-based (corundum-based). These refractory materials mostly use Al2O3 and SiO2 as aggregates, and also contain small amounts of other oxides (such as TiO2, MgO, CaO, etc.), exhibiting sufficient stability and good heat preservation properties for conventional aluminum alloys. However, when exposed to highly chemically reactive aluminum-lithium alloy melt, the lithium in the melt undergoes a violent displacement reaction with the refractory material. For example, the reaction with silicon dioxide (SiO2): 4Li + SiO2 → 2Li2O + Si is extremely rapid and is the main cause of refractory material structural collapse, silicon contamination in the melt, and Li2O inclusions. The reaction with mullite (3Al2O3·2SiO2) can be seen as an intensification of the above reactions, while generating various oxide inclusions. These interfacial reactions have serious consequences: on the one hand, the working surface of the hot-top preform is rapidly eroded, becoming loose and porous, and its service life drops drastically from tens or even hundreds of castings for conventional aluminum alloys to only 1-2 castings, greatly increasing production costs and operational burden; on the other hand, the products generated by the reaction, such as Li2O and elemental Si, rapidly enter the melt as inclusions, severely deteriorating the purity of the melt, causing a large number of inclusions and hot cracks in the ingot, and in severe cases, it can also lead to excessively low strength of the solidified shell layer of the ingot and large amounts of leakage, posing serious safety hazards. Currently, there are no reported prefabricated refractory materials for the hot top of crystallizers suitable for aluminum-lithium alloys, which has become one of the key bottlenecks restricting the semi-continuous casting of high-quality aluminum-lithium alloy ingots.

[0005] To address this issue, both academia and industry have conducted in-depth research and attempted to shift towards non-oxide ceramics, such as low-reactivity silon ceramics. Patent CN201810105581.3 (A silon-bonded fused silica preform for aluminum refractory channels and its manufacturing method) discloses a novel aluminum industrial preform with high chemical stability and resistance to wetting by molten aluminum. However, while such materials mitigate interfacial reactions to some extent, their inherent high thermal conductivity results in poor insulation performance, making it difficult to effectively fulfill the feeding function that a hot top should possess. Furthermore, compared to commonly used aluminosilicate-based and mullite-corundum-based refractory preforms, silon ceramics still exhibit poor thermal shock resistance, making them prone to cracking under cyclical hot-cold casting conditions, thus failing to meet the requirements of practical applications.

[0006] In summary, the inventors believe there is an urgent need to develop a novel hot-top preform for aluminum-lithium alloy crystallizers. This preform must not only fundamentally suppress interfacial reactions with the molten aluminum-lithium alloy, but also ensure the overall thermal insulation and thermal shock resistance of the preform. Developing such a preform can help overcome the bottlenecks in the semi-continuous casting technology of aluminum-lithium alloys and is of great significance to achieving the urgent need for self-sufficiency in high-end materials for my country's aerospace industry. Summary of the Invention

[0007] To address the technical bottlenecks of existing hot-top preforms for semi-continuous casting of aluminum-lithium alloys, such as severe interfacial reactions, short service life, and poor thermal shock stability, this invention provides a refractory preform based on material functionalization gradient design and its manufacturing method. In existing technologies, mullite-corundum refractory materials commonly used in the aluminum industry undergo violent chemical reactions with active lithium in the aluminum-lithium melt (e.g., 4Li + SiO2 → 2Li2O + Si), leading to refractory material erosion and melt contamination. While non-oxide ceramics (such as silane) possess some erosion resistance, their high thermal conductivity, poor thermal shock stability, and high cost make them unsuitable for hot-top conditions.

[0008] Unlike existing technologies, this invention innovatively proposes the concept of "material functionalization gradient design": In the working layer, high-melting-point, highly stable lithium aluminate (LiAlO2) is used as the first lithium-containing compound. By pre-introducing lithium elements, the intrinsic lithium chemical sites in the working layer are increased, thermodynamically suppressing the interfacial reaction with the aluminum-lithium melt. In the insulation layer, low-thermal-expansion-coefficient, low-density β-lithium nepheline (LiAlSiO4) and lithium aluminate are used as the second lithium-containing compounds, achieving excellent insulation performance and thermal stress matching with the working layer. Through the composite gradient distribution of lithium-containing compound types and concentrations, the interfacial reaction problem between the hot-top preform and the aluminum-lithium alloy melt is solved, while ensuring the overall thermal shock stability and structural integrity of the preform.

[0009] The objective of this invention is achieved through the following technical solution:

[0010] In a first aspect, the present invention provides a hot top preform for a semi-continuous casting crystallizer for aluminum-lithium alloys, the hot top preform being composed of a working layer that is in direct contact with the molten aluminum-lithium alloy and a heat insulation layer that is not in contact with the molten aluminum-lithium alloy. The working layer contains 10-15% of a first lithium-containing compound by mass percentage, wherein the first lithium-containing compound is lithium aluminate (LiAlO2). The insulation layer contains 5-15% by mass of a second lithium-containing compound, which includes β-lithium nepheline (LiAlSiO4) and lithium aluminate (LiAlO2).

[0011] As some specific embodiments of the present invention, the thickness of the working layer accounts for 1 to 10% of the cross-sectional thickness of the hot top preform, and the thickness of the insulation layer accounts for 90 to 99% of the cross-sectional thickness of the hot top preform.

[0012] As some specific embodiments of the present invention, in the second lithium-containing compound, the mass ratio of β-lithium nepheline to lithium aluminate is 1:1-3:1.

[0013] The core innovation of this invention lies in breaking through the limitations of traditional single-material systems. Through a composite gradient design of the working layer and the insulation layer, precise control of material properties is achieved. The working layer uses lithium aluminate with a melting point higher than 1600℃. This compound is thermodynamically more stable and can effectively resist the erosion of aluminum-lithium melt, while not containing other impurity elements such as Si that are harmful to aluminum-lithium alloy melts. The insulation layer uses β-lithium nepheline and lithium aluminate. This is because β-lithium nepheline has negative expansion characteristics, and its combination with lithium aluminate with a positive expansion coefficient can effectively reduce the thermal expansion rate and heat storage capacity of the insulation layer. At the same time, this dual gradient design of lithium-containing compound types and concentrations ensures a smooth transition of performance from the working surface to the insulation surface, solving the problem of interlayer stress concentration and cracking caused by abrupt changes in material properties.

[0014] As some specific embodiments of the present invention, the working layer further comprises, by mass percentage: 5-20% SiO2, 1-5% TiO2, 1-5% CaO, and the remainder being Al2O3. This composition design ensures that the working layer has a sufficiently high Al2O3 content to guarantee refractoriness and strength. At the same time, by controlling the content of the first lithium-containing compound to 10-15%, the lithium chemical sites in the working layer are significantly increased to suppress interfacial reactions, while avoiding the deterioration of the material's high-temperature performance due to excessive content.

[0015] As some specific embodiments of the present invention, the insulation layer further comprises, by mass percentage: 10-25% SiO2, 1-5% TiO2, 1-5% CaO, and the remainder Al2O3. Controlling the content of the second lithium-containing compound in the insulation layer to 5-10% ensures a sufficiently low coefficient of thermal expansion and good insulation performance while maintaining the necessary structural strength of the material. A higher SiO2 content helps to form more glassy phase, further reducing the thermal conductivity and coefficient of thermal expansion, and improving the thermal shock resistance of the insulation layer.

[0016] As some specific embodiments of the present invention, the insulation layer further comprises 5-20 wt% alumina hollow spheres, and the overall density of the insulation layer is not greater than 2.2 g / cm³. 3Introducing hollow alumina spheres is key to achieving lightweight and efficient insulation. By controlling the amount added and the overall density, the thermal conductivity and heat storage value of the insulation layer can be greatly reduced without significantly sacrificing strength. This is crucial for controlling the temperature gradient within the hot top of the crystallizer and ensuring sequential solidification.

[0017] Secondly, the present invention provides a method for manufacturing a hot top preform for a semi-continuous casting crystallizer of aluminum-lithium alloy as described in any of the preceding claims, comprising the following steps: S1. Prepare the working layer powder and the insulation layer powder according to the mass percentage respectively; S2. The insulation layer powder and the working layer powder are sequentially laid into the mold and pressed into a single piece to obtain a green body; S3. Perform multi-stage sintering on the green blank, including: S31. Heat to 800-1000℃ and hold for 1-3 hours to remove organic matter and allow the powder to initially bind. S32. Heat to 1000-1150℃ and hold for 2-5 hours as a key phase change insulation zone to allow β-lithium nepheline to fully form and release phase change stress. S33. Heat to 1350-1500℃ and hold for 2-6 hours to complete the final densification sintering, and the product is obtained.

[0018] The core innovation of this method lies in its multi-stage sintering process, which is matched to the material gradient design. Specifically, it includes: first, holding at 800-1000℃ to remove organic matter and achieve initial bonding between powder particles; then, holding at 1000-1150℃ for a critical phase transformation, a stage within the critical temperature range for β-lithium nepheline formation and structural relaxation, where prolonged holding fully releases the internal stress generated by the crystal transformation; and finally, completing the final densification at 1350-1500℃. This non-linear sintering regime is specifically designed for lithium-containing compounds, particularly β-lithium nepheline, and effectively solves the problem of easy cracking of preforms during traditional single-stage sintering.

[0019] In step S32 above, the key phase transition insulation zone of 1000-1150℃ aims to promote the formation and structural relaxation of β-nepheline in the insulation layer powder, and through sustained insulation, allow elements at the interface between the working layer and the insulation layer to interdiffusion, forming a tightly bonded gradient transition zone. The 1000-1150℃ key phase transition insulation zone not only promotes the full formation of β-nepheline, but also utilizes the high atomic diffusion capacity of this temperature range to induce interdiffusion of elements (such as Li, Al, and Si) at the interface between the working layer and the insulation layer. This creates a gradient transition zone with continuously changing composition and properties between the two layers, rather than a clear interface with abrupt changes in composition and properties. This greatly enhances the interlayer bonding force and avoids interlayer delamination caused by thermal cycling during use.

[0020] As some specific embodiments of the present invention, in step S2, the pressing is carried out by isostatic pressing with a pressing pressure of 30-150 MPa; and / or, in step S32, when sintering is carried out in the critical phase change heat preservation zone, an axial mechanical pressure of 5-20 MPa is applied to the green blank.

[0021] The application of isostatic pressing in step S2 ensures uniform density in all parts of the gradient green body; while the application of auxiliary axial pressure in the phase transformation insulation zone is a key process innovation. It can actively suppress the initiation and propagation of microcracks in the most vulnerable phase transformation stage inside the material, while promoting particle rearrangement and interface healing. It is an important guarantee for achieving a highly dense and defect-free gradient structure.

[0022] As some specific embodiments of the present invention, in step S1, the median particle size of the working layer powder is 20-45 μm; the median particle size of the insulation layer powder is 5-20 μm. Titanium alloy wire mesh or titanium alloy fibers can be pre-embedded in the preform mold. The coarser working layer powder ensures the structural strength and corrosion resistance of the working layer, while the finer β-lithium nepheline facilitates its low-temperature synthesis and uniform distribution. In particular, pre-embedding titanium alloy wire mesh or fibers in the preform mold utilizes the characteristics of titanium, which has a closer coefficient of thermal expansion than the alumina matrix, does not contaminate the aluminum-lithium alloy melt, and possesses good toughness. This significantly improves the overall thermal shock resistance of the preform through crack deflection and stress transfer.

[0023] As some specific embodiments of the present invention, the ratio of the overall average thermal expansion coefficient of the working layer to the overall average thermal expansion coefficient of the insulation layer is between 1.0 and 1.5.

[0024] This invention adjusts the selection and ratio of the first and second lithium-containing compounds to ensure that the ratio of the overall average thermal expansion coefficient of the working layer to that of the insulation layer is between 1.0 and 1.5. This ensures that the working layer and the insulation layer can deform in coordination under drastic temperature changes, minimizing thermal stress and fundamentally avoiding internal delamination or cracking caused by thermal expansion mismatch, thus greatly improving the service life and reliability of the prefabricated component.

[0025] As some specific embodiments of the present invention, in step S3, the multi-stage sintering process is carried out under a protective atmosphere of nitrogen or argon. The protective atmosphere prevents the oxidation and volatilization of lithium-containing compounds at high temperatures, ensuring the accuracy of the composition.

[0026] As some specific embodiments of the present invention, in step S33, after the final densification sintering is completed, programmed cooling is adopted, and the cooling rate in the stage above 1000°C does not exceed 3°C / min. The controlled slow cooling regime is to avoid the generation of new thermal stress cracks at the interface due to the asynchronous shrinkage of the working layer and the insulation layer during the cooling process, which is a necessary measure to ensure the final integrity of the product.

[0027] Compared with the prior art, the present invention has the following beneficial effects: (1) By introducing a stable lithium-containing compound into the working layer of the hot top preform of the aluminum-lithium alloy semi-continuous casting crystallizer, the present invention significantly reduces the reaction driving force between the aluminum-lithium melt and the refractory material, significantly reduces the lithium burn-off and lithium oxide inclusions in the melt, and improves the purity of the aluminum-lithium alloy melt and the quality of the ingot.

[0028] (2) This invention optimizes the thermal stress accumulation between the working layer and the insulation layer of the hot top preform of the aluminum-lithium alloy semi-continuous casting crystallizer by functional gradient design of materials and thermal stress matching. This is beneficial to improving the thermal shock resistance and structural durability of the hot top preform of the crystallizer, and can extend the service life of the hot top preform of the crystallizer and reduce the production cost.

[0029] (3) The process of the present invention is simple and does not involve expensive and complex equipment. It also has important reference value for the selection of refractory materials in other melting and casting forming processes of aluminum-lithium alloys. It is very suitable for industrial promotion and application and has good engineering application prospects. Attached Figure Description

[0030] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 The reaction of the hot top preform of the crystallizer after use in Example 1; Figure 2 The reaction of the hot top preform of the crystallizer after use in Comparative Example 1 is shown. Detailed Implementation

[0031] The present invention will now be described in detail with reference to specific 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 various modifications and improvements without departing from the concept of the present invention. These all fall within the scope of protection of the present invention.

[0032] Example 1 This embodiment prepares a hot top preform for crystallizers used in semi-continuous casting of aluminum-lithium alloys.

[0033] The working layer materials, by weight percentage, are: 80% Al₂O₃, 10% lithium aluminate (D50=20μm), 5% SiO₂, 3% TiO₂, and 2% CaO. The insulation layer materials, by weight percentage, are: 70% Al₂O₃, 5% β-nepheline (D50=5μm), 5% lithium aluminate, 10% SiO₂, 3% TiO₂, 2% CaO, plus 5% hollow alumina spheres. The working layer to insulation layer thickness ratio is 1:99. Titanium alloy fibers are pre-embedded in the mold.

[0034] During preparation, the working layer and insulation layer powders are first sequentially laid into the mold and shaped under isostatic pressure of 30 MPa. The green body is sintered under nitrogen protection: first, the temperature is raised to 800℃ at 5℃ / min and held for 1 hour; then the temperature is raised to 1000℃ and held for 2 hours, during which an axial pressure of 5 MPa is applied; finally, the temperature is raised to 1350℃ and held for 2 hours. After sintering, the cooling rate is controlled to ≤3℃ / min above 1000℃ to obtain the hot-top preform.

[0035] The hot-top precast component was installed on a semi-continuous casting crystallizer for testing, casting 2050 aluminum-lithium alloy (Al-3.5 Cu-1.0 Li-0.4 Mg-0.4 Ag-0.4 Mn-0.1 Zr) with a casting rod diameter of 300 mm. After 5 casting cycles, the machine was stopped for inspection, and the cross-section is shown below. Figure 1 As shown, cross-sectional metallographic observation reveals that the reaction layer between the working layer and the aluminum-lithium melt has a thickness of 0.48 mm. EDS elemental analysis shows that Al, Si, and Li elements at the interface between the working layer and the insulation layer exhibit a continuous gradient distribution and are tightly bonded.

[0036] Example 2 This embodiment prepares a hot top preform for crystallizers used in semi-continuous casting of aluminum-lithium alloys.

[0037] The working layer materials, by weight percentage, are: 78% Al₂O₃, 12% lithium aluminate (D50=30μm), 5% SiO₂, 3% TiO₂, and 2% CaO. The insulation layer materials, by weight percentage, are: 63% Al₂O₃, 7% β-nepheline (D50=10μm), 7% lithium aluminate, 10% SiO₂, 3% TiO₂, 2% CaO, plus 8% alumina hollow spheres. The working layer to insulation layer thickness ratio is 3:97. Titanium alloy wire mesh is pre-embedded in the mold.

[0038] The preparation process is as follows: The powder is isostatically pressed at 80 MPa. Sintering is then performed under argon protection: holding at 800℃ for 1 hour; holding at 1050℃ for 3 hours with an axial pressure of 10 MPa; and holding at 1400℃ for 3 hours. After sintering, the cooling rate is controlled to be ≤2℃ / min above 1000℃.

[0039] The precast component was used in a semi-continuous casting test of 2050 aluminum-lithium alloy, with a casting rod diameter of 300 mm. After five casting cycles, the machine was stopped for inspection. Metallographic observation of the cross-section showed that the reaction layer between the working layer and the molten aluminum-lithium alloy was 0.41 mm thick. EDS elemental analysis showed that Al, Si, and Li elements at the interface between the working layer and the insulation layer exhibited a continuous gradient distribution and were tightly bonded.

[0040] Example 3 This embodiment prepares a hot top preform for crystallizers used in semi-continuous casting of aluminum-lithium alloys.

[0041] The working layer materials, by weight percentage, are: 75% Al₂O₃, 15% lithium aluminate (D50=35μm), 5% SiO₂, 3% TiO₂, and 2% CaO. The insulation layer materials, by weight percentage, are: 58% Al₂O₃, 10% β-nepheline (D50=15μm), 5% lithium aluminate, 10% SiO₂, 3% TiO₂, 2% CaO, plus 12% hollow alumina spheres. The working layer to insulation layer thickness ratio is 5:95. No metal reinforcement is pre-embedded.

[0042] The preparation process is as follows: The powder is isostatically pressed at 120 MPa. Sintering is carried out under nitrogen protection: 850℃ for 2 hours; 1100℃ for 4 hours with an axial pressure of 15 MPa; and 1450℃ for 4 hours. After sintering, the cooling rate is controlled to be ≤2℃ / min above 1000℃.

[0043] The precast component was used in a semi-continuous casting test of 2050 aluminum-lithium alloy, with a casting rod diameter of 300 mm. After five casting cycles, the machine was stopped for inspection. Metallographic observation of the cross-section showed that the reaction layer between the working layer and the molten aluminum-lithium alloy was 0.35 mm thick. EDS elemental analysis showed that Al, Si, and Li elements at the interface between the working layer and the insulation layer exhibited a continuous gradient distribution and were tightly bonded.

[0044] Example 4 This embodiment prepares a hot top preform for crystallizers used in semi-continuous casting of aluminum-lithium alloys.

[0045] The working layer materials, by weight percentage, are: 72% Al2O3, 15% lithium aluminate powder (1:1, D50=45μm), 8% SiO2, 3% TiO2, and 2% CaO. The insulation layer materials, by weight percentage, are: 56% Al2O3, 8% β-nepheline (D50=20μm), 4% lithium aluminate, 12% SiO2, 3% TiO2, 2% CaO, plus 15% hollow alumina spheres. The working layer to insulation layer thickness ratio is 8:92. Titanium alloy fibers are pre-embedded in the mold.

[0046] The preparation process is as follows: The powder is isostatically pressed at 150 MPa. Sintering is then performed under argon protection: holding at 900℃ for 2 hours; holding at 1150℃ for 5 hours with an axial pressure of 20 MPa; and holding at 1500℃ for 5 hours. After sintering, the cooling rate is controlled to be ≤1℃ / min above 1000℃.

[0047] The precast component was used in a semi-continuous casting test of 2050 aluminum-lithium alloy, with a casting rod diameter of 300 mm. After five casting cycles, the machine was stopped for inspection. Metallographic observation of the cross-section showed that the reaction layer between the working layer and the molten aluminum-lithium alloy was 0.29 mm thick. EDS elemental analysis showed that Al, Si, and Li elements at the interface between the working layer and the insulation layer exhibited a continuous gradient distribution and were tightly bonded.

[0048] Example 5 This embodiment prepares a hot top preform for crystallizers used in semi-continuous casting of aluminum-lithium alloys.

[0049] The working layer materials, by weight percentage, are: 70% Al₂O₃, 15% lithium aluminate (D50=25μm), 10% SiO₂, 3% TiO₂, and 2% CaO. The insulation layer materials, by weight percentage, are: 49% Al₂O₃, 10% β-nepheline (D50=8μm), 4% lithium aluminate, 12% SiO₂, 3% TiO₂, 2% CaO, plus 20% hollow alumina spheres. The working layer to insulation layer thickness ratio is 10:90. No metal reinforcement is embedded.

[0050] The preparation process is as follows: The powder is isostatically pressed at 100 MPa. Sintering is carried out under nitrogen protection: holding at 1000℃ for 3 hours; holding at 1150℃ for 4 hours with an axial pressure of 12 MPa; and holding at 1480℃ for 6 hours. After sintering, the cooling rate is controlled to be ≤1.5℃ / min above 1000℃.

[0051] The precast component was used in a semi-continuous casting test of 2050 aluminum-lithium alloy, with a casting rod diameter of 300 mm. After five casting cycles, the machine was stopped for inspection. Metallographic observation of the cross-section showed that the reaction layer between the working layer and the molten aluminum-lithium alloy was 0.22 mm thick. EDS elemental analysis showed that Al, Si, and Li elements at the interface between the working layer and the insulation layer exhibited a continuous gradient distribution and were tightly bonded.

[0052] Comparative Example 1 The preparation process of the hot top preform in this comparative example is basically the same as that in Example 1. The only difference is that the preform is made of conventional aluminum silicate-based refractory material used in the aluminum industry (composition: 85% Al2O3, 10% SiO2, 2% TiO2, 2% CaO, 1% other oxides), does not contain any lithium compounds, and is made of a single material without a gradient structure.

[0053] The preforms were used in a semi-continuous casting test of 2050 aluminum-lithium alloy, with a casting rod diameter of 300 mm. During the first casting cycle, a vigorous reaction was observed at the interface between the molten aluminum-lithium alloy and the preform, resulting in a large amount of slag. After the casting cycle, the machine was stopped for inspection. Figure 2 As shown, the working surface of the precast component was found to be severely corroded, with a loose and rotten surface. Metallographic observation of the cross-section revealed a reaction layer thickness exceeding 4.5 mm, indicating fundamental damage to the material structure. The results demonstrate that conventional aluminosilicate-based refractory materials are extremely incompatible with aluminum-lithium alloy melts, resulting in a violent interfacial displacement reaction that fails to meet the basic requirements for semi-continuous casting of aluminum-lithium alloys.

[0054] Comparative Example 2 The preparation process of the hot top preform in this comparative example is basically the same as that in Example 1. The only difference is that the preform is prepared using the working layer composition described in Example 1, without an insulation layer and without alumina hollow spheres.

[0055] The preforms were used in a semi-continuous casting test of 2050 aluminum-lithium alloy, with a casting rod diameter of 300 mm. After the first casting cycle, multiple macroscopic cracks appeared on the surface of the preforms, rendering them unusable. Cross-sectional metallographic observation showed that the reaction layer between the working layer and the molten aluminum-lithium alloy was 0.45 mm thick, but the preforms had completely cracked due to excessive thermal stress. The results indicate that although a single working layer composition has good resistance to interfacial reactions, its high coefficient of thermal expansion and lack of gradient buffer result in extremely poor thermal shock resistance, making it unsuitable for use under actual working conditions.

[0056] Comparative Example 3 The preparation process of the hot-top prefabricated component in this comparative example is basically the same as that in Example 1, except that the entire prefabricated component is prepared using the insulation layer composition described in Example 1, and no working layer is provided.

[0057] The preforms were used in a semi-continuous casting test of 2050 aluminum-lithium alloy, with a casting rod diameter of 300 mm. After one casting cycle, the machine was stopped for inspection, and it was found that the surface of the preform in contact with the molten metal was severely eroded, becoming rough and porous. Cross-sectional metallographic observation showed that the reaction layer thickness reached 2.5 mm. The results indicate that the single insulation layer composition, due to insufficient lithium compound content and poor corrosion resistance, could not effectively suppress the interfacial reaction with the aluminum-lithium molten metal, leading to rapid failure.

[0058] Comparative Example 4 The preparation process of the hot top preform in this comparative example is basically the same as that in Example 1, except that the types of lithium compounds are reversed, that is, the working layer uses a mixture of β-lithium nepheline and lithium aluminate, and the insulation layer uses lithium aluminate.

[0059] The preforms were used in a semi-continuous casting test of 2050 aluminum-lithium alloy with a casting rod diameter of 300 mm. In the second casting, interlayer spalling occurred in the preforms. Cross-sectional metallographic observation showed that the reaction layer between the working layer and the molten aluminum-lithium alloy was 1.8 mm thick, and there was a significant gap at the interface between the working layer and the insulation layer. The results indicate that the working layer used a mixture of β-lithium nepheline and lithium aluminate with a lower coefficient of thermal expansion, while the insulation layer used lithium aluminate with a higher coefficient of thermal expansion. This reverse gradient design resulted in significant thermal stress at the interface, causing interlayer spalling, and the working layer also exhibited insufficient resistance to erosion.

[0060] Comparative Example 5 The preparation process of the hot-top prefabricated component in this comparative example is basically the same as that in Example 1, except that the working layer thickness accounts for 15% of the total thickness of the prefabricated component and the insulation layer accounts for 85%.

[0061] The preform was used in a semi-continuous casting test of 2050 aluminum-lithium alloy, with a casting rod diameter of 300 mm. After three casting cycles, the preform cracked. Cross-sectional analysis showed that the crack originated at the interface between the working layer and the insulation layer and propagated outwards. The results indicate that the thickness ratio of the working layer to the insulation layer deviated from the required range of 1%~10% / 90%~99% of the present invention. The excessively thick working layer exacerbated the thermal expansion mismatch between it and the insulation layer, causing the thermal stress to exceed the interfacial bonding strength and triggering cracking.

[0062] Comparative Example 6 The comparative example is basically the same as the hot-top preform preparation process in Example 1, except that the multi-stage sintering process is cancelled and the green blank is directly heated from room temperature to 1350°C and held for 2 hours.

[0063] The preforms were used in a semi-continuous casting test of 2050 aluminum-lithium alloy, with a casting rod diameter of 300 mm. Severe cracking occurred in the first casting. Cross-sectional metallographic observation revealed numerous microcracks within the billet, and a clear interface between the working layer and the insulation layer, with no element diffusion transition zone observed. The results indicate that without multi-stage sintering through the critical phase transformation insulation zone, β-lithium nepheline cannot fully transform and release phase transformation stress. Simultaneously, interlayer elements cannot interdiffused to form a gradient bond, leading to stress concentration and weak bonding within the material, making it highly susceptible to cracking.

[0064] Comparative Example 7 The comparative example is basically the same as the hot-top preform preparation process in Example 1, except that no axial mechanical pressure is applied in the critical phase change insulation zone (1000°C).

[0065] The preforms were used in a semi-continuous casting test of 2050 aluminum-lithium alloy, with a casting rod diameter of 300 mm. After two casting cycles, the preforms showed signs of interlayer delamination. Cross-sectional metallographic observation revealed microcracks at the interface between the working layer and the insulation layer, with a reaction layer thickness of 0.50 mm. The results indicate that the lack of axial mechanical pressure in the phase transformation temperature range fails to effectively suppress phase transformation stress and promote interface healing, resulting in insufficient interlayer bonding strength and susceptibility to delamination during use.

[0066] Comparative Example 8 The preparation process of the hot top preform in this comparative example is basically the same as that in Example 1, except that the median particle size D50 of the lithium aluminate powder used in the working layer is 80 μm, and the median particle size D50 of the β-lithium nepheline powder used in the insulation layer is 50 μm.

[0067] The preforms were used in a semi-continuous casting test of 2050 aluminum-lithium alloy, with a casting rod diameter of 300 mm. After three casting cycles, the machine was stopped for inspection, and uneven erosion was found on the surface of the working layer. Cross-sectional metallographic observation showed that the reaction layer thickness was 0.90 mm, and the overall material density was poor. The results indicate that excessively coarse powder particle size reduces sintering activity, leading to decreased material density and weakened erosion resistance. At the same time, coarse particles generate greater internal stress during phase transformation, affecting the service life.

[0068] Comparative Example 9 The preparation process of the hot top preform in this comparative example is basically the same as that in Example 1. The only difference is that the raw material ratio of the working layer is adjusted to 87% Al2O3, 10% lithium aluminate (D50=20μm), 1% SiO2, 1% TiO2, and 1% CaO. Due to the high alumina content, the overall thermal expansion coefficient of the working layer is too large, so the ratio of the overall average thermal expansion coefficient of the working layer to the overall average thermal expansion coefficient of the insulation layer reaches 1.8.

[0069] The preform was used in a semi-continuous casting test of 2050 aluminum-lithium alloy with a casting rod diameter of 300 mm. After the first thermal cycle, a macroscopic annular crack appeared at the interface between the working layer and the insulation layer in the preform. The results show that when the ratio of the thermal expansion coefficients of the two layers exceeds the range of 1.0 to 1.5 required by this invention, the thermal stress generated during the thermal cycle exceeds the interfacial bonding strength, inevitably leading to interlayer cracking and causing the preform to fail.

[0070] 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, and 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 hot top preform for semi-continuous casting of an aluminum-lithium alloy characterized in that, The hot top prefabricated component is composed of a working layer that is in direct contact with the molten aluminum-lithium alloy and an insulation layer that is not in contact with the molten aluminum-lithium alloy. By mass percentage, the working layer contains 10-15% of a first lithium-containing compound, wherein the first lithium-containing compound is lithium aluminate; The insulation layer contains 5-15% by mass of a second lithium-containing compound, which includes β-lithium nepheline and lithium aluminate.

2. The crystallizer hot top preform of claim 1, wherein, The thickness of the working layer accounts for 1 to 10% of the cross-sectional thickness of the hot top prefabricated component, and the thickness of the insulation layer accounts for 90 to 99% of the cross-sectional thickness of the hot top prefabricated component.

3. The crystallizer hot top preform of claim 1, wherein, In the second lithium-containing compound, the mass ratio of β-lithium nepheline to lithium aluminate is 1:1 to 3:

1.

4. The mold hot top preform of claim 1, wherein, The working layer, by mass percentage, also includes: 5-20% SiO2, 1-5% TiO2, 1-5% CaO, and the remainder is Al2O3.

5. The prefabricated hot top component for a crystallizer according to claim 1, characterized in that, By mass percentage, the insulation layer further comprises: 10-25% SiO2, 1-5% TiO2, 1-5% CaO, and the remainder is Al2O3.

6. The prefabricated hot top component for a crystallizer according to claim 1, characterized in that, The heat preservation layer further comprises 5-20wt% of alumina hollow spheres, and the overall density of the heat preservation layer is not greater than 2.2 g / cm 3 .

7. A method for manufacturing a hot top preform for a semi-continuous casting crystallizer for aluminum-lithium alloys as described in any one of claims 1-6, characterized in that, Includes the following steps: S1. Prepare the working layer powder and the insulation layer powder according to the mass percentage; S2. The insulation layer powder and the working layer powder are sequentially laid into the mold and pressed into a single piece to obtain a green body; S3. Perform multi-stage sintering on the green blank, including: S31. Heat to 800-1000℃ and hold for 1-3 hours; S32, heat to 1000-1150℃ and hold for 2-5 hours; S33. Heat to 1350-1500℃ and hold for 2-6 hours to obtain the product.

8. The manufacturing method according to claim 7, characterized in that, In step S2, the pressing and molding process adopts isostatic pressing, and the molding pressure is 30-150 MPa. And / or, in step S32, during sintering in the critical phase transformation insulation zone, an axial mechanical pressure of 5-20 MPa is applied to the green compact.

9. The manufacturing method according to claim 7, characterized in that, In step S1, the median particle size of the working layer powder is 20-45 μm; the median particle size of the insulation layer powder is 5-20 μm.

10. The manufacturing method according to claim 7, characterized in that, In step S3, the multi-stage sintering process is carried out under a protective atmosphere of nitrogen or argon.

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