A semi-continuous casting forming method of large-size high-strength high-modulus aluminum-lithium alloy ingot

Abstract

The application provides a semi-continuous casting forming method of large-specification high-strength high-modulus aluminum-lithium alloy ingot blank, improves intrinsic strength of primary condensed shell by optimizing components of the aluminum-lithium alloy, and combines two-stage negative pressure refining process, composite covering agent and application of low-interface reaction refractory to improve purification and protection effect of aluminum-lithium alloy melt in preparation and transmission process; further, by optimizing a hot top structure of a crystallizer and a brand-new process of'solidifying while annealing', residual stress and cracking tendency in the semi-continuous casting process of the aluminum-lithium alloy are effectively reduced. The method solves the problems that in the prior art, purity of the aluminum-lithium alloy melt is difficult to control, there are more low-melting-point eutectics, and therefore, in the semi-continuous casting of the large-specification ingot blank, the primary condensed shell has low strength, high cracking sensitivity, great risk of liquid leakage and poor internal quality, and has important significance for promoting large-scale stable production of high-end aluminum-lithium alloy large-specification ingot blank.

Description

Technical Field

[0001] This invention belongs to the field of aluminum alloy material technology, and relates to a forming method of aluminum-lithium alloy, specifically, a semi-continuous casting forming method for large-size, high-strength, high-modulus aluminum-lithium alloy ingots. Background Technology

[0002] As a new generation of lightweight, high-strength, and high-modulus strategic structural materials, aluminum-lithium alloys have shown broad application prospects in aerospace, defense, low-altitude economy, and heavy-duty robots due to their excellent specific strength, specific stiffness, and damage tolerance. Taking the C919 large passenger aircraft as an example, the proportion of aluminum-lithium alloys used in a single aircraft has reached nearly 8%, which is of great significance for achieving "weight reduction and stiffness increase" and improving the overall performance of the aircraft. Currently, the production of various aluminum-lithium alloy profiles, plates, and forgings all rely on large-size, high-quality semi-continuous casting ingots as raw materials. Therefore, the quality of large-size aluminum-lithium alloy semi-continuous casting ingots directly determines the final performance and service reliability of subsequent precision and deep-processed products.

[0003] However, the semi-continuous casting of large-size, high-performance aluminum-lithium alloy ingots faces significant technical challenges, primarily in three areas. First, the preparation of high-quality melt is difficult. Aluminum-lithium alloy melts are extremely chemically reactive, with an oxidation and gas absorption tendency dozens of times greater than that of ordinary aluminum alloys, making it difficult to effectively control the hydrogen content, oxide inclusions, and impurity elements within the melt. Second, the transport of high-quality melt is challenging. During the transport of the prepared clean melt to the crystallizer, it undergoes violent interfacial reactions with conventional oxide refractory materials such as the lining of the runner, degassing box / filter box, or foam ceramic filter (e.g., 4Li + SiO2 → 2Li2O + Si), generating new secondary inclusions. Third, the semi-continuous casting process is difficult to control. Because aluminum-lithium alloys contain elements such as Li and Mg, a large number of low-melting-point eutectic phases easily form at grain boundaries during solidification, significantly weakening the strength of the solidified shell formed during primary cooling. Under the combined effects of casting tensile stress, hot-top melt static pressure, and thermal stress, the solidified shell is highly prone to cracking, leading to melt leakage (pull-out), which not only prevents successful forming but also poses significant safety hazards.

[0004] To address the aforementioned issues, the industry has conducted numerous studies. Regarding melt treatment, patent application CN202510938454.1 (A method for preparing large-volume melts to reduce oxidation and gas absorption in aluminum-lithium alloys) discloses a method using rare earth elements to form a surface film to protect the melt, but this still makes it difficult to completely avoid oxidation during subsequent processing. Regarding refractory materials, patent CN201810105581.3 (A sialon-bonded fused silica preform for aluminum water flow channels and its manufacturing method) discloses a method using non-oxide ceramics such as sialon to reduce reactivity with aluminum-lithium alloy melts, but its thermal shock stability is poor, its cost is high, and its heat insulation performance is difficult to meet requirements. Regarding casting processes, conventional methods involve optimizing three parameters: cooling water flow rate, casting speed, and casting temperature, but the effect is very limited for aluminum-lithium alloys with low intrinsic strength of the solidified shell. Importantly, most of these existing technologies address problems in isolation, failing to take a systematic approach from "composition design - melt purification - interface reaction control - continuous casting process matching," thus making it difficult to achieve stable, efficient, and safe continuous casting of large-size aluminum-lithium alloy ingots.

[0005] In summary, given the difficulties in semi-continuous casting of large-size aluminum-lithium alloy ingots, there is an urgent need to develop a systematic method for semi-continuous casting of large-size aluminum-lithium alloys that can synergistically solve the three major challenges mentioned above. This has become a pressing need to promote the large-scale industrial application of this high-end material. This invention aims to provide such an innovative systematic solution. Summary of the Invention

[0006] To address the aforementioned shortcomings of existing technologies, this invention provides a semi-continuous casting method for large-size, high-strength, and high-modulus aluminum-lithium alloy ingots. By controlling key aspects such as melt treatment, melt transport, and semi-continuous casting, it is expected to significantly improve the quality and stability of high-performance aluminum-lithium alloy semi-continuous casting. To achieve the above technical effects, this invention proposes a systematic solution, which is fundamentally different from the isolated optimization of single aspects in existing technologies. Existing technologies either focus only on melt purification (such as using covering agents or inert gases), but struggle to avoid the entrapment or transport of contaminants by the covering agent; or attempt to use expensive and thermally unstable non-oxide refractories to mitigate the reaction, but fail to solve problems such as refractory insulation and lifespan. In contrast, this invention innovatively starts from the alloy composition design source, through a full-chain innovation of melt purification, transport protection, and forming process. Specifically, it strengthens the solidified shell through strict composition control, ensures melt purity through unique low-reactivity material design, and releases stress through an innovative "simultaneous solidification and annealing" process, thus providing a reliable path for the stable forming of large-size aluminum-lithium alloy ingots.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] In a first aspect, the present invention provides a semi-continuous casting forming method for large-size, high-strength, high-modulus aluminum-lithium alloy ingots, comprising the following steps: S1. Alloy smelting and composition control: For the target aluminum-lithium alloy, the raw materials are batched and smelted to control the sum of the mass percentage of lithium and magnesium in the melt to ≤1.3%, and the total mass percentage of potassium, calcium and sodium to ≤0.15%; S2. Melt deep purification treatment: The aluminum-lithium alloy melt is subjected to two-stage vacuum treatment. The first stage is refining by blowing argon gas while drawing negative pressure, and the second stage is static negative pressure degassing. S3. Melt Cover Protection: Add a composite covering agent containing a foaming agent to the surface of the melt; S4. Low-reactivity melt transport and filtration: The melt flows sequentially through a flow channel with a low-reactivity refractory lining, a degassing box, and a filtration box under a protective atmosphere. S5. Semi-continuous casting: The molten metal is injected into a crystallizer with a low hot top height for casting, and the temperature of the secondary cooling water is controlled. During the continuous casting process, the upper edge solidifies, and the lower solidified part is removed by the scraper in the continuous casting well and then enters the online annealing zone for slow cooling.

[0009] This invention, through strict composition control in step S1, reduces the amount of low-melting-point eutectic phase at grain boundaries from the source. This is a fundamental measure to improve the high-temperature strength of the primary cooling shell and reduce its tendency to thermal cracking. Step S2 employs a unique two-stage treatment mode of "argon blowing + medium negative pressure" and "high negative pressure static". First, bubbles are used to efficiently remove inclusions and hydrogen, and then deep negative pressure is used to achieve an ultra-low hydrogen content in the melt, providing "clean" raw materials for subsequent forming. Step S3 introduces a foaming agent to form a lightweight covering layer, preventing the covering agent from being entrained. Step S4 constructs a fully protected "inert" transport channel, which together ensures the purity of the melt before entering the crystallizer. Step S5, through a low-heat top design to reduce static pressure, control water temperature, and an innovative synchronous online annealing process, creates a "low-stress, slow-cooling" forming condition for the fragile aluminum-lithium alloy shell.

[0010] As some specific embodiments of the present invention, in step S1, the melting temperature is 700-800°C.

[0011] As some specific embodiments of the present invention, in step S2, during the first-stage refining process of blowing argon gas while drawing negative pressure, the system pressure is 1000~5000 Pa, and the refining time is 30~40 min. In the second stage of static negative pressure degassing, the system pressure is 200~1000 Pa, and the refining time is 20~30 min.

[0012] As some specific embodiments of the present invention, in step S3, the composite covering agent comprises a foaming agent, lithium chloride, and lithium fluoride, wherein the foaming agent is lithium carbonate, the lithium carbonate content is 5-15 wt%, and the mass ratio of lithium chloride to lithium fluoride is 1:1-2:1. Its core innovation lies in utilizing the thermal decomposition characteristics of lithium carbonate. When the covering agent is heated (600-800℃, the melting temperature range of aluminum-lithium alloys), lithium carbonate decomposes to produce CO2 gas, causing the covering agent layer to foam and expand in situ, forming a low-density, thick, and loosely structured physical barrier layer. This not only significantly improves the thermal insulation effect of the covering agent and reduces the amount of covering agent used, but more importantly, this lightweight porous structure can effectively prevent the covering agent from being entangled, greatly reducing the risk of contamination of the melt during subsequent transport. Simultaneously, the decomposition product, lithium oxide, can adjust the viscosity and surface tension of the chlorofluorocarbon melt, giving it both good spreadability for rapid melt coverage and appropriate viscosity to improve the physical protection effect, while reducing the difficulty of subsequent separation from the melt.

[0013] In some specific embodiments of the present invention, in step S4, the protective atmosphere is argon; the filter box is equipped with a foam ceramic filter with alumina and lithium oxide as the main material components, wherein the mass percentage content of lithium oxide is 3-10%. The reason for introducing lithium oxide into the traditional alumina filter matrix is ​​to change the equilibrium of the interfacial reaction from a thermodynamic perspective. The introduction of lithium oxide increases the local lithium chemical sites in the filter skeleton, making it closer to the aluminum-lithium alloy melt, thereby significantly reducing the driving force for the displacement reaction between active lithium and alumina in the melt. This allows the filter to efficiently intercept non-metallic inclusions while significantly enhancing its resistance to melt erosion, effectively avoiding the secondary pollution problem of "new inclusions generated during the filtration process".

[0014] As some specific embodiments of the present invention, in step S4, the preparation method of the low-reactivity refractory lining includes: mixing lithium oxide and silicon dioxide powder, and ball milling to uniformly coat the surface of silicon dioxide with lithium oxide; and then sintering after hot pressing or casting molding.

[0015] Furthermore, the median particle size D50 of the silica powder is 50~200 μm, and the median particle size D50 of the lithium oxide powder is 1~10 μm; and / or, the ball milling speed is 50~100 r / min, and the time is 0.5~10 h. This is a key innovation in material design. Using powders with significantly different particle sizes and ball milling to achieve a "fine-envelope-coarse" encapsulation structure aims to allow the fine lithium oxide powder to adhere tightly to the surface of the coarse silica particles, forming a "protective shell." This lithium oxide shell provides a high lithium chemical potential environment before contacting the molten aluminum-lithium alloy, thereby preferentially stabilizing the interface and inhibiting the reduction and erosion reaction of lithium in the melt on the core silica. Simultaneously, using silica, with its extremely low coefficient of thermal expansion, as the main structural component ensures excellent thermal shock stability of the refractory lining, enabling it to withstand thermal shock during melt transport. Hot pressing or casting sintering ensures that this microscopic protective structure is maintained and functions effectively in the macroscopic component.

[0016] As some specific embodiments of the present invention, in step S5, the height (H) of the hot top of the crystallizer does not exceed 50% of the target diameter (D, for round ingots) or thickness (T, for flat ingots) of the ingot, i.e., H≤0.5D or H≤0.5T; the temperature of the secondary cooling water is controlled at 20~40℃. Limiting the hot top height is an original structural design addressing the low strength of the solidified shell of aluminum-lithium alloys. The enormous static pressure of the melt generated by a conventional high hot top (proportional to its height) is the main mechanical cause of cracking in the weak solidified shell. Limiting the hot top height to less than half the ingot size essentially actively and significantly reduces this destructive load, opening a safe casting mechanical window for aluminum-lithium alloys. Simultaneously, the solidified aluminum-lithium alloy melt at the bottom releases a large amount of hydrogen; reducing the hot top height will reduce the escape distance of hydrogen bubbles, which is beneficial for the removal of hydrogen from the molten aluminum. Using relatively mild cooling water at 20~40℃ avoids the surge in thermal stress caused by excessive temperature difference between the inside and outside of the condensation shell and excessive cooling rate due to excessively low water temperature (such as cold water in winter), thus achieving "soft cooling" and further reducing the tendency for thermal cracking.

[0017] As some specific embodiments of the present invention, in step S5, a negative pressure environment of 500-3000 Pa is maintained above the hot top of the crystallizer throughout the semi-continuous casting process. This negative pressure environment, in conjunction with the low-hot top design, enhances the kinetic conditions for hydrogen removal. Under negative pressure, the supersaturation of hydrogen in the melt increases, making it easier for bubbles to nucleate and grow; simultaneously, the external pressure that the bubbles need to overcome to rise and escape decreases, accelerating their rising speed. This systematically solves the problem of internal porosity in ingots caused by residual hydrogen in the aluminum-lithium alloy melt after refining, which is particularly beneficial for improving the core quality of large-size ingots.

[0018] In some specific embodiments of the present invention, in step S5, the scraper is positioned immediately below the secondary cooling water outlet, and the vertical distance between its lower edge and the lower edge of the graphite ring in the crystallizer is 1 to 3 times the target diameter or thickness of the ingot; the vertical distance between the top starting point of the online annealing zone and the lower edge of the scraper is no more than 0.5 meters; the temperature of the annealing zone is controlled at 300-500°C; and the ingot transit time is no less than 30 minutes. This is key to realizing the core process concept of "simultaneous solidification and stress-relieving annealing." By precisely defining the relative positions of the scraper, the annealing zone, and the crystallizer, it is ensured that the ingot can enter the temperature-controlled annealing zone without delay, even when it has just finished secondary cooling, the surface moisture has been immediately scraped off, the internal temperature is still high, and stress is concentrated. This "online synchronous" annealing can effectively and promptly relieve casting thermal stress, especially significantly suppressing the central longitudinal cracks (through-the-core cracks) that are easily generated in aluminum-lithium alloys due to their high elastic modulus and large thermal stress. Strict control of distance and temperature is the core to ensuring the reproducibility and effectiveness of this process.

[0019] Secondly, the present invention provides a large-size, high-strength, high-modulus aluminum-lithium alloy ingot, which is prepared by the forming method described in any of the above-mentioned methods.

[0020] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention achieves high-quality, stable, safe and efficient production through systematic component control, melt purification, interface reaction control and casting forming process innovation; (2) This invention creatively proposes a number of specific solutions, such as low-heat top design, in-situ foaming of covering agent, lithium oxide modified refractory and filter, and synchronous online annealing, which comprehensively improve the adaptability and reliability of aluminum-lithium alloy semi-continuous casting process from multiple perspectives such as thermodynamics, kinetics and structural mechanics. (3) The method of the present invention has clear steps and parameters, does not rely on extremely expensive special equipment, has excellent engineering application prospects and economic benefits, and is of great significance to promoting the independent large-scale production of high-end aluminum-lithium alloy materials in my country.

[0021] (4) The method of the present invention solves the problems of difficulty in controlling the purity of aluminum-lithium alloy melt and the large amount of low melting point eutectic in the prior art, resulting in low primary solidification strength, high cracking sensitivity, high risk of leakage and poor internal quality of large-size ingots during semi-continuous casting. It is of great significance to promote the large-scale and stable production of high-end aluminum-lithium alloy large-size ingots. Attached Figure Description

[0022] 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 1This is a cross-sectional view of the aluminum-lithium alloy semi-continuous casting ingot prepared in Comparative Example 2; Figure 2 This is a magnified micrograph of the cross-section of the aluminum-lithium alloy semi-continuous casting ingot prepared in Comparative Example 3. Detailed Implementation

[0023] 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.

[0024] Example 1 This embodiment prepares an aluminum-lithium alloy round ingot with a diameter of 300 mm. The target alloy is a low-magnesium type 2050 aluminum-lithium alloy (Al-3.5Cu-1.1Li-0.2Mg-0.4Ag-0.4Mn-0.1Zr). Pure aluminum, aluminum-copper master alloy, pure lithium, pure magnesium, pure silver, aluminum-manganese master alloy, and aluminum-titanium-boron master alloy are used as raw materials to achieve a total mass percentage content of 1.3% for lithium (Li) and magnesium (Mg) in the melt, and a total mass percentage content of 0.15% for potassium, calcium, and sodium.

[0025] First, pure aluminum is melted, and other raw materials are sequentially added to the molten aluminum at 700°C to prepare an aluminum-lithium alloy melt. The aluminum-lithium alloy melt is then subjected to a two-stage vacuum treatment. The first stage is refining by blowing argon gas while simultaneously drawing negative pressure, with the system pressure maintained at 1000 Pa for 40 minutes; the second stage is static negative pressure degassing, with the system pressure maintained at 200 Pa for 30 minutes.

[0026] Subsequently, a composite covering agent is added to the surface of the melt. This composite covering agent contains blowing agents lithium carbonate, lithium chloride, and lithium fluoride, wherein the content of lithium carbonate is 5 wt%, and the mass ratio of lithium chloride to lithium fluoride is approximately 2:1.

[0027] Then, under argon protection, the melt flows sequentially through a flow channel with a low-reactivity refractory lining, a degassing box, and a filter box. The raw material for the low-reactivity refractory lining comprises silica powder and lithium oxide powder, wherein the median particle size D50 of the silica powder is 50 μm, and the median particle size D50 of the lithium oxide powder is 10 μm. The ball milling speed is 50 r / min, and the ball milling time is 10 h, so that the lithium oxide is uniformly coated on the surface of the silica. The filter box is equipped with a foam ceramic filter, which is mainly composed of alumina and lithium oxide, wherein the mass percentage content of lithium oxide is 3%.

[0028] Finally, semi-continuous casting is performed. The molten metal is injected into the crystallizer, and the height (H) of the hot top of the crystallizer is set to 150 mm, satisfying H = 0.5D (D = 300 mm). A negative pressure environment of 500 Pa is maintained in the space above the hot top. The secondary cooling water temperature is controlled at 20℃. In the continuous casting well, the scraper is set immediately below the secondary cooling water outlet, and the vertical distance between its lower edge and the lower edge of the graphite ring of the crystallizer is 300 mm (1.0 times the ingot diameter). The vertical distance between the top starting point of the online annealing zone and the lower edge of the scraper is 0.3 meters, and the temperature in this zone is controlled at 300℃. The ingot transit time is approximately 35 minutes.

[0029] Testing revealed that the surface quality of the obtained ingot was good, with no cracks or aluminum leakage. The internal hydrogen content of the ingot was 0.18 mlH2 / 100g Al. Ultrasonic testing showed no obvious internal defects, and low-magnification microstructure examination revealed no central cracks.

[0030] Example 2 This embodiment prepares a 400 mm thick aluminum-lithium alloy flat ingot, with the target alloy being 2195 aluminum-lithium alloy (Al-4.0Cu-1.0Li-0.3Mg-0.4Ag-0.1Zr). Pure aluminum, aluminum-copper master alloy, pure lithium, pure magnesium, pure silver, and aluminum-titanium-boron master alloy are used as raw materials to ensure that the sum of the mass percentages of lithium (Li) and magnesium (Mg) in the melt is 1.3%, and the total mass percentage of potassium, calcium, and sodium is 0.10%.

[0031] First, pure aluminum is melted, and then other furnace materials are sequentially added to the molten pure aluminum at 750°C for melting, thus preparing an aluminum-lithium alloy melt. The aluminum-lithium alloy melt is then subjected to a two-stage vacuum treatment. The first stage is refining by blowing argon gas while simultaneously drawing negative pressure, with the system pressure maintained at 3000 Pa, for 35 minutes; the second stage is static negative pressure degassing, with the system pressure maintained at 600 Pa, for 25 minutes.

[0032] Subsequently, a composite covering agent is added to the surface of the melt. This composite covering agent contains the foaming agents lithium carbonate, lithium chloride, and lithium fluoride, wherein the content of lithium carbonate is 10 wt%, and the mass ratio of lithium chloride to lithium fluoride is approximately 2:1.

[0033] Then, under argon protection, the melt flows sequentially through a flow channel with a low-reactivity refractory lining, a degassing box, and a filter box. The raw material for the low-reactivity refractory lining comprises silica powder and lithium oxide powder, wherein the median particle size D50 of the silica powder is 125 μm, and the median particle size D50 of the lithium oxide powder is 5 μm. The ball milling speed is 75 r / min, and the ball milling time is 5 h, so that the lithium oxide is uniformly coated on the surface of the silica. The filter box is equipped with a foam ceramic filter, which is mainly composed of alumina and lithium oxide, wherein the mass percentage content of lithium oxide is 6%.

[0034] Finally, semi-continuous casting is performed. The molten metal is injected into the crystallizer, and the height (H) of the hot top of the crystallizer is set to 160 mm, satisfying H = 0.4T (T = 400 mm). A negative pressure environment of 1500 Pa is maintained in the space above the hot top. The secondary cooling water temperature is controlled at 30℃. In the continuous casting well, the scraper is set immediately below the secondary cooling water outlet, and the vertical distance between its lower edge and the lower edge of the graphite ring of the crystallizer is 800 mm (2.0 times the ingot thickness). The vertical distance between the top starting point of the online annealing zone and the lower edge of the scraper is 0.4 meters. The temperature in this zone is controlled at 400℃, and the ingot transit time is approximately 40 minutes.

[0035] Testing revealed that the surface quality of the obtained ingot was good, with no cracks or aluminum leakage. The internal hydrogen content of the ingot was 0.2 mlH2 / 100g Al. Ultrasonic testing showed no obvious internal defects, and low-magnification microstructure examination revealed no central cracks.

[0036] Example 3 This embodiment prepares an aluminum-lithium alloy round ingot with a diameter of 500 mm. The target alloy is 2060 aluminum-lithium alloy (Al-4.0Cu-0.9Li-0.4Mg-0.4Mn-0.4Ag-0.1Zr). Pure aluminum, aluminum-copper master alloy, pure lithium, pure magnesium, pure silver, and aluminum-titanium-boron master alloy are used as raw materials to ensure that the sum of the mass percentages of lithium (Li) and magnesium (Mg) in the melt is 1.3%, and the total mass percentage of potassium, calcium, and sodium is 0.05%.

[0037] First, pure aluminum is melted, and then other furnace materials are sequentially added to the molten pure aluminum at 800℃ to prepare an aluminum-lithium alloy melt. The aluminum-lithium alloy melt is then subjected to a two-stage vacuum treatment. The first stage is refining by blowing argon gas while drawing negative pressure, with the system pressure maintained at 5000 Pa, for 30 minutes; the second stage is static negative pressure degassing, with the system pressure maintained at 1000 Pa, for 20 minutes.

[0038] Subsequently, a composite covering agent is added to the surface of the melt. This composite covering agent contains blowing agents lithium carbonate, lithium chloride, and lithium fluoride, wherein the content of lithium carbonate is 15 wt%, and the mass ratio of lithium chloride to lithium fluoride is approximately 2:1.

[0039] Then, under argon protection, the melt flows sequentially through a flow channel with a low-reactivity refractory lining, a degassing box, and a filter box. The raw material for the low-reactivity refractory lining comprises silica powder and lithium oxide powder, wherein the median particle size D50 of the silica powder is 200 μm, and the median particle size D50 of the lithium oxide powder is 1 μm. The ball milling speed is 100 r / min, and the ball milling time is 0.5 h, so that the lithium oxide is uniformly coated on the surface of the silica. The filter box is equipped with a foam ceramic filter, which is mainly composed of alumina and lithium oxide, wherein the mass percentage content of lithium oxide is 10%.

[0040] Finally, semi-continuous casting is performed. The molten metal is injected into the crystallizer, and the height (H) of the hot top of the crystallizer is set to 200 mm, satisfying H = 0.4D (D = 500 mm). A negative pressure environment of 3000 Pa is maintained in the space above the hot top. The secondary cooling water temperature is controlled at 40℃. In the continuous casting well, the scraper is set immediately below the secondary cooling water outlet, and the vertical distance between its lower edge and the lower edge of the graphite ring of the crystallizer is 1500 mm (3.0 times the ingot diameter). The vertical distance between the top starting point of the online annealing zone and the lower edge of the scraper is 0.5 meters, and the temperature in this zone is controlled at 500℃. The ingot transit time is approximately 45 minutes.

[0041] Testing revealed that the surface quality of the obtained ingot was good, with no cracks or aluminum leakage. The internal hydrogen content of the ingot was 0.18 mlH2 / 100g Al. Ultrasonic testing showed no obvious internal defects, and low-magnification microstructure examination revealed no central cracks.

[0042] Comparative Example 1 The chemical composition and process of this comparative example are basically the same as those of Example 1. The only difference is that the mass percentage content of lithium (Li) and magnesium (Mg) in the alloy melt is changed to 1.2% and 0.3% respectively, that is, the sum of the contents of the two elements is 1.5%, and the total mass percentage content of potassium, calcium and sodium is 0.25%.

[0043] The process described in Example 1 was followed, and the results showed that the solidified shell strength was significantly insufficient at the start of continuous casting, quickly leading to cracks and severe melt leakage (pull-out), resulting in casting failure. This indicates that without strict control of the Li+Mg and alkaline earth metal content, it is impossible to obtain a solidified shell with sufficient high-temperature strength, thus making safe continuous casting impossible from the outset.

[0044] Comparative Example 2 The process of this comparative example is basically the same as that of Example 1, except that the melt purification process only uses conventional single atmospheric pressure rotary argon refining (system pressure is one atmosphere), and does not use the two-stage vacuum process in Example 1.

[0045] Figure 1The image shows a cross-sectional view of the aluminum-lithium alloy semi-continuous casting ingot prepared in Comparative Example 2. The results indicate that although the ingot is formed, the internal porosity defects are significantly increased. Sampling tests show that the average hydrogen content of the ingot is as high as 0.44 ml H2 / 100gAl. This demonstrates that for aluminum-lithium alloy melts that readily absorb hydrogen, the two-stage stepped vacuum treatment of this invention (especially deep static degassing under low negative pressure) is crucial for obtaining a pure melt with ultra-low hydrogen content, an effect that conventional methods cannot achieve.

[0046] Comparative Example 3 The process of this comparative example is basically the same as that of Example 1, except that a conventional lithium carbonate-free LiF-LiCl covering agent (a mixture of chloride and fluoride salts) is used, and the mass ratio of lithium chloride to lithium fluoride remains unchanged.

[0047] The results are as follows Figure 2 The image shown is a magnified micrograph of the cross-section of the semi-continuous casting aluminum-lithium alloy ingot obtained in this comparative example, revealing inclusions in the ingot. During the process, it was observed that part of the covering agent was submerged in the melt. In the low-magnification microstructure of the subsequent ingot, blocky molten salt inclusions with a size of 2-3 mm were found, and metallographic analysis confirmed that they were chloride inclusions. This demonstrates that the lithium carbonate-containing (5 wt%) foaming covering agent used in this invention can loosen and expand the covering agent layer by decomposing to generate CO2 gas, thereby significantly reducing its density and the risk of entrapment, which is impossible with traditional covering agents.

[0048] Comparative Example 4 The process of this comparative example is basically the same as that of Example 1, except that the flow channel uses a conventional mullite-corundum refractory lining, and the filter box uses a common high-purity alumina foam ceramic filter (without lithium oxide).

[0049] The results showed that the silicon (Si) content was abnormally high after ingot processing and spectral analysis, increasing by more than 120% compared to the sample in Example 1. Metallographic observation revealed numerous composite non-metallic inclusions containing Si, O, and Al. This demonstrates that conventional oxide refractory materials undergo violent interfacial reactions with molten aluminum and lithium (e.g., 4Li + SiO2 → 2Li2O + Si), causing severe secondary pollution. The low-reactivity liner (lithium oxide coating on silicon dioxide) and modified filter (with added lithium oxide) of this invention can fundamentally inhibit such reactions, ensuring the purity of the transport process.

[0050] Comparative Example 5 The process of this comparative example is basically the same as that of Example 1, except that: a conventional high-heat top design is adopted, the height of the heat top is 240 mm (0.8D), and no negative pressure is applied above the heat top.

[0051] The results showed that transverse cracks appeared on the ingot surface shortly after continuous casting began. As casting progressed, the solidified shell ruptured and aluminum leaked out. Simultaneously, the porosity of the ingot was significantly higher than in Example 1, and testing revealed an internal hydrogen content of 0.92 ml H2 / 100g Al. This demonstrates that the enormous molten static pressure from the high-heat top is the primary cause of cracking and leakage in the thin solidified shell of the aluminum-lithium alloy; the lack of a negative pressure environment also hinders hydrogen escape. The low hot top height (≤0.5D) and negative pressure environment (500-3000 Pa) specified in this invention are key synergistic measures to prevent leakage and improve quality.

[0052] Comparative Example 6 The process of this comparative example is basically the same as that of Example 1, except that: no online annealing zone is set up, and the ingot is directly cooled naturally in the room temperature environment of the continuous casting well after the water is removed by the scraper.

[0053] The results showed that the external macroscopic inspection of the ingot was intact, but after sawing, a significant, penetrating longitudinal central crack (through-the-core crack) was found in its core. The ingot of Example 1, after the same inspection, did not exhibit this defect. This demonstrates that for large-sized aluminum-lithium alloy ingots prone to high casting thermal stress, an online annealing process (300-500℃) adjacent to the cooling zone is an essential and crucial step for timely stress release and suppression of internal cracking, a problem that traditional continuous casting processes cannot solve.

[0054] 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 semi-continuous casting forming method for large-size, high-strength, high-modulus aluminum-lithium alloy ingots, characterized in that, Includes the following steps: S1. Alloy smelting and composition control: For the target aluminum-lithium alloy, the raw materials are batched and smelted to control the sum of the mass percentage of lithium and magnesium in the melt to ≤1.3%, and the total mass percentage of potassium, calcium and sodium to ≤0.15%; S2. Melt Deep Purification Treatment: The aluminum-lithium alloy melt undergoes a two-stage vacuum treatment. The first stage is refining by blowing argon gas while drawing negative pressure; the second stage is static negative pressure degassing. S3. Melt Cover Protection: Add a composite covering agent containing a foaming agent to the surface of the melt; S4. Low-reactivity melt transport and filtration: The melt flows sequentially through a flow channel with a low-reactivity refractory lining, a degassing box, and a filtration box under a protective atmosphere. S5. Semi-continuous casting: The molten metal is injected into a crystallizer with a low hot top height for casting, and the temperature of the secondary cooling water is controlled. During the continuous casting process, the upper edge solidifies, and the lower solidified part is removed by the scraper in the continuous casting well and then enters the online annealing zone for slow cooling.

2. The method according to claim 1, characterized in that, In step S1, the melting temperature is 700-800 ℃.

3. The method according to claim 1, characterized in that, In step S2, during the first-stage refining process of blowing argon gas while drawing negative pressure, the system pressure is 1000~5000 Pa, and the refining time is 30~40 min. In the second stage of static negative pressure degassing, the system pressure is 200~1000 Pa, and the refining time is 20~30 min.

4. The method according to claim 1, characterized in that, In step S3, the composite covering agent comprises a foaming agent, lithium chloride, and lithium fluoride, wherein the foaming agent is lithium carbonate, the lithium carbonate content is 5~15wt%, and the mass ratio of lithium chloride to lithium fluoride is 1:1-2:

1.

5. The method according to claim 1, characterized in that, In step S4, the protective atmosphere is argon; the filter box is equipped with a foam ceramic filter with alumina and lithium oxide as the main material components, wherein the mass percentage content of lithium oxide is 3~10%.

6. The method according to claim 1, characterized in that, In step S4, the preparation method of the low-reactivity refractory lining includes: mixing lithium oxide and silicon dioxide powders and ball milling them to uniformly coat the surface of silicon dioxide with lithium oxide; and then sintering them after hot pressing or casting.

7. The method according to claim 6, characterized in that, The median particle size D50 of the silica powder is 50~200 μm, and the median particle size D50 of the lithium oxide powder is 1~10 μm; And / or, the ball milling speed is 50~100 r / min, and the time is 0.5~10 h.

8. The method according to claim 1, characterized in that, In step S5, the height of the hot top of the crystallizer does not exceed 50% of the target diameter or thickness of the ingot, satisfying H ≤ 0.5D or H ≤ 0.5T; the temperature of the secondary cooling water is controlled at 20~40℃.

9. The crystallizer according to claim 1, characterized in that, In step S5, the space above the hot top of the crystallizer is maintained at a negative pressure environment of 500~3000 Pa during the semi-continuous casting process. And / or, in step S5, the scraper is positioned immediately below the secondary cooling water outlet, and the vertical distance between its lower edge and the lower edge of the graphite ring in the crystallizer is 1 to 3 times the target diameter or thickness of the ingot; the vertical distance between the top starting point of the online annealing zone and the lower edge of the scraper is no more than 0.5 meters, the temperature in this zone is controlled at 300 to 500°C, and the ingot transit time is no less than 30 minutes.

10. A large-size, high-strength, high-modulus aluminum-lithium alloy ingot prepared by the method according to any one of claims 1-9.

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