Treatment method for casting aluminum-lithium alloy foundry returns and obtained casting aluminum-lithium alloy casting

Abstract

The invention discloses a treatment method for casting aluminum-lithium alloy foundry returns and an obtained casting aluminum-lithium alloy casting, and belongs to the technical field of non-ferrous metal smelting. According to the technical scheme, the method comprises the following steps: mixing class I foundry returns and class II foundry returns according to the mass ratio of (70-80): (20-30) to obtain recycled materials; the class I foundry returns are casting residual materials and waste pieces of the crucible, and the class II foundry returns are risers and runners; the recycled material and the new material are put into a smelting furnace, and the surface is covered with a composite protection flux; the composite protective flux is a LiCl-KCl-Na2SiF6 composite flux, and the composite protective flux is a And after hexachloroethane is added into the fully-molten furnace charge to remove sodium, sampling is carried out for component analysis, elements needing to be supplemented are calculated according to target components, and the elements are added in the form of intermediate alloy. The method is applied to the aspect of recycling of the foundry returns of the cast aluminum-lithium alloy, solves the problem that high-proportion and high-quality recycling of the foundry returns of the cast aluminum-lithium alloy cannot be realized by an existing process, and can remarkably reduce lithium burning loss, effectively remove impurities of hydrogen and sodium and realize accurate component compensation, structure refinement and high-proportion safe recycling of the foundry returns.

Description

Technical Field

[0001] This invention belongs to the field of non-ferrous metal smelting technology, and particularly relates to a method for processing recycled aluminum-lithium alloy castings and the resulting aluminum-lithium alloy castings. Background Technology

[0002] Aluminum-lithium alloys possess advantages such as low density, high specific strength, and high specific stiffness, making them key materials for lightweight structural components in aerospace applications. Aluminum-lithium alloys include cast aluminum-lithium alloys and wrought aluminum-lithium alloys. Wrought aluminum-lithium alloys are primarily smelted under vacuum casting or atmosphere protection, and then semi-continuous or continuous casting is used to directly cast large-sized flat ingots or round bars, preparing them for subsequent hot deformation processing. Rods are then produced into plates through hot-rolling and extrusion processes, and further processed into finished products. Cast aluminum-lithium alloys, on the other hand, are smelted and then used in low-pressure / differential-pressure casting machines to produce casting blanks. Specifically, after smelting, the molten aluminum-lithium alloy is transferred to the differential-pressure casting machine station using a crucible, with smelting taking place at the smelting station. Because this process requires transfer and pouring, current casting aluminum-lithium alloy production methods employ local atmosphere protection and covering agents, making it impossible to completely isolate the alloy from air throughout the process.

[0003] The production of cast aluminum-lithium alloys generates a large amount of remelted material (including gatings, risers, scrap castings, and machining chips), accounting for over 50% of the total input. Therefore, the efficient recycling of this material is crucial for reducing costs and resource consumption. Current remelted material processing involves remelting under inert gas protection and covering with LiCl-KCl flux. However, the lithium burn-off rate remains high at 12-40%, and the remelted material addition ratio is less than 30%. Furthermore, the recycled alloys contain numerous oxide inclusions and high gas content, failing to meet the requirements for high-performance castings. Consequently, both domestically and internationally, the treatment of cast aluminum-lithium alloy remelted material primarily involves simple remelting followed by downgrading, which is unsuitable for producing castings requiring high quality and hinders the achievement of high-proportion, high-quality recycling of the remelted material. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the technical problem to be solved by this invention is to overcome the inability of existing processes to achieve high-proportion, high-quality recycling of cast aluminum-lithium alloy remelting materials. This invention proposes a method for processing cast aluminum-lithium alloy remelting materials that can significantly reduce lithium burn-off, effectively remove impurities such as hydrogen and sodium, achieve precise composition compensation and microstructure refinement, and enable high-proportion safe recycling of remelting materials, as well as the resulting cast aluminum-lithium alloy castings.

[0005] To solve the aforementioned technical problem, the technical solution adopted by the present invention is as follows: This invention provides a method for processing recycled aluminum-lithium alloy castings, comprising: The selection steps for recycled materials involve mixing Class I and Class II recycled materials at a mass ratio of 70-80:20-30 to obtain recyclable materials; Class I recycled materials consist of casting residues and scrap parts from crucibles, while Class II recycled materials consist of risers and gating systems. The charging and solvent covering steps involve loading recycled materials and new materials into the melting furnace and covering the surface with a composite protective flux; the composite protective flux is a LiCl-KCl-Na2SiF6 composite flux. The sodium removal and composition adjustment steps involve adding hexachloroethane to the fully melted furnace charge to remove sodium, then taking a sample for composition analysis, calculating the elements that need to be added according to the target composition, and adding them in the form of intermediate alloys. The vacuum degassing and final treatment steps involve covering the surface of the furnace charge with a composite protective flux after refining, adding lithium ingots at 730-740℃, applying vacuum treatment and maintaining the vacuum level at 10-100Pa.

[0006] In some embodiments, the LiCl-KCl-Na2SiF6 composite flux comprises 50-60% LiCl, 30-40% KCl, and 5-10% Na2SiF6 by mass fraction.

[0007] In some embodiments, during the charging and solvent covering steps, the amount of composite protective flux added is 1.5-2.5% of the total mass of the charge; during the sodium removal and composition adjustment steps, the amount of hexachloroethane added is 0.2-0.4% of the melt mass.

[0008] In some embodiments, a rotary jet refining step is further included between the charging and solvent covering step and the sodium removal and composition adjustment step. The rotary jet refining step includes: injecting a mixed gas of Ar and Cl2 into the melt at a rotation speed of 200-300 rpm, a gas flow rate of 1.5-2.5 L / min•tonne, and a processing time of 10-15 minutes.

[0009] In some embodiments, the rotary jet refining step further includes adding a refining agent, wherein the amount of the refining agent added is 0.3-0.4% of the melt mass.

[0010] In some embodiments, the proportion of Cl2 in the mixed gas is 1-3%.

[0011] In some embodiments, a recyclable material classification step is included before the recyclable material selection step. The recyclable material classification step includes: classifying the recyclable material into Class I recyclable material, Class II recyclable material and Class III recyclable material; Class I recyclable material is the casting residue and scrap parts of the crucible, Class II recyclable material is the riser and gating system, and Class III recyclable material is the horizontal gating system and the cleaned seams.

[0012] In some embodiments, a full melting and stirring step is included between the charging and solvent covering step and the sodium removal and composition adjustment step. The full melting and stirring step includes: heating to 700-720°C under argon protection to completely melt the furnace charge, and applying 5-15Hz low-frequency electromagnetic intermittent stirring during the melting process.

[0013] In some embodiments, the amount of recycled material used in the charging and solvent covering steps is not less than 30% of the total amount of recycled material and virgin material; for castings requiring Class II requirements, the amount of recycled material used in the charging and solvent covering steps is not less than 40% of the total amount of recycled material and virgin material.

[0014] In another aspect, the present invention provides a cast aluminum-lithium alloy casting obtained by the processing method of any of the above-mentioned technical solutions, wherein the lithium burn-off rate of the cast aluminum-lithium alloy casting is ≤5%, the hydrogen content is ≤0.10mL / 100gAl, and the sodium content is ≤5ppm. This cast aluminum-lithium alloy casting is produced by differential pressure casting.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention provides a method for processing recycled aluminum-lithium alloy castings. A LiCl-KCl-Na2SiF6 composite flux is used as a covering agent to form a stable covering layer on the melt surface. Na2SiF6 decomposes and releases SiF4 gas, forming a gas curtain that effectively blocks lithium from contacting air, controlling the lithium burn-off rate to below 5%. Hexachloroethane (C2Cl6) is used as a sodium removal agent to effectively reduce the content of impurities such as Na and K. By selecting specific recycled materials and using them in combination with virgin materials, the proportion of recycled materials can be increased to over 40%, and the mechanical properties of the recycled alloy can reach over 90% of the virgin alloy. This significantly reduces costs and meets the requirements of high-end castings, achieving a high-proportion, high-quality recycling of recycled aluminum-lithium alloy castings. Attached Figure Description

[0016] Figure 1 This is a schematic flowchart of the method for processing recycled aluminum-lithium alloy castings provided in an embodiment of the present invention. Figure 2 This is a schematic diagram of the rotary jet refining apparatus provided in an embodiment of the present invention; In the above figures: 1. Baffle; 2. Graphite rod; 3. Graphite rotor. Detailed Implementation

[0017] The technical solutions in specific embodiments of the present invention will now be described in detail and completely with reference to the accompanying drawings. Obviously, the described embodiments are merely some specific implementations of the overall technical solution of the present invention, and not all implementations. Based on the overall concept of the present invention, all other embodiments obtained by those skilled in the art fall within the protection scope of the present invention.

[0018] The smelting characteristics of cast aluminum-lithium alloys: After melting the alloy, it is poured into a sand mold or metal mold cavity for casting. Low-pressure or differential-pressure casting methods are used during casting, filling the mold under pressure to overcome the disadvantage of poor fluidity, thus ensuring the quality of the internal structure of the casting. However, the pouring process involves transfer, and the crucible and mold cannot be sealed, so only gas protection and covering agents can be used for smelting. Surface oxidation is more severe than in vacuum melting. The remaining liquid material after pouring must be immediately poured into ingots for recycling; otherwise, the crucible becomes unusable after cooling. Gravity pouring of recycled ingots results in more oxide slag and more severe burn-off in the recycled material.

[0019] Cast aluminum-lithium alloy materials contain elements such as lithium, copper, and magnesium. Because lithium is reactive and easily combustible, it suffers severe burn-off during the smelting process. Currently, it is used only once. The process of remelting the material will release heat and raise the temperature. At the same time, the furnace charge contains a lot of oxide scale and oxide slag, which will cause serious gas absorption and slag inclusions in the furnace charge. Composition control is difficult. The internal quality and mechanical properties of castings recast using recycled material do not meet the Class III requirements for castings and cannot be used.

[0020] The recycling of aluminum-lithium alloy scrap faces the following technical challenges: lithium is highly reactive and easily oxidizes and burns, generating impurities such as Li₂O and LiOH; the melt readily absorbs hydrogen, leading to high hydrogen embrittlement sensitivity; alkali metal impurities such as sodium and potassium easily cause hot brittleness; repeated reuse leads to increased oxide inclusions, severe component segregation, and decreased mechanical properties. Among these, the most difficult technical problem to solve is: 1) Lithium is highly oxidizable: Lithium is chemically extremely reactive and readily reacts with oxygen and water vapor in the air during the smelting process to generate impurities such as Li₂O, Li₃N, and LiOH. Once these oxides are mixed into the melt, they will severely deteriorate the mechanical properties of the alloy, especially its toughness and fatigue performance.

[0021] 2) Sodium embrittlement sensitivity: Aluminum-lithium alloys are highly sensitive to alkali metal impurities (such as sodium), and even trace amounts of sodium can cause severe intergranular embrittlement. Recycled materials, especially those contaminated with cutting fluid or from different batches, may introduce sodium contamination.

[0022] 3) Coarsening of structure and tendency of porosity: Repeated remelting will lead to the accumulation of hydrogen content and coarsening of grains, which will reduce the metallurgical quality of recycled materials and make it difficult to meet the requirements of high-performance castings.

[0023] Currently, the processing of aluminum-lithium alloy remelting materials both domestically and internationally mostly involves simple remelting followed by downgrading for reuse, or mixing them with large quantities of new metal materials. This approach fails to achieve a high proportion and high quality of recycling. Conventional flux covering methods (such as using a NaCl-KCl mixture) offer limited protection for Li and may introduce Na ions. While high-vacuum melting is effective, the equipment is expensive, the process is lengthy, and the costs are high, making it unsuitable for large-scale industrial production.

[0024] In summary, during the preparation of cast aluminum-lithium alloys, because lithium has a low melting point (only 170℃), the recycled material already contains various elements in the cast aluminum-lithium alloy. During melting, as the temperature rises, lithium combustion releases heat, accelerating lithium loss (for example, the melting point of Al2Li2Cu alloy is 640℃; after melting, lithium combustion is accompanied by heat release, causing a rapid temperature rise and severe lithium loss). Therefore, a covering agent is needed during the preparation process to reduce loss. However, existing conventional flux covering methods (such as using a NaCl-KCl mixture) have limited protective effects on Li and may introduce Na ions. Therefore, the main technical challenge in recycling recycled material is the loss and protection of lithium. Furthermore, because the loss is uncontrollable, new material and recycled material must be added together, and the amount of recycled material cannot exceed 30%, thus preventing the high-proportion, high-quality recycling of recycled material.

[0025] The impact of different proportions of Type I and Type II remelting materials on the product: Using a small crucible furnace with a melting capacity of 1 kg, and with argon protection throughout the process, the recycled material was placed in the crucible and then sealed with a lid. Argon gas was continuously introduced, the temperature was raised to 800℃, and then lowered to 750℃. The sealed lid was then opened and the sample was poured. To reduce the error in composition analysis caused by surface oxidation, only the middle area was selected for composition analysis and chemical composition analysis comparison. The initial composition was tested according to the same batch of material with Li 2.5% and Cu 2.5%.

[0026] 1) The proportion of Class I and Class II remelting materials is 50% and 50% respectively. Experimental results: Li 1.5%, Cu 2.5%, and a lot of oxide slag, with a Li element burn-off rate of over 40%.

[0027] 2) The proportions of Class I and Class II remelting materials are 60% and 40%, respectively. Experimental results: Li 1.8%, Cu 2.5%, Li element burn-off rate over 28%.

[0028] 3) The proportions of Class I and Class II remelting materials are 70% and 30%, respectively. Experimental results: Li 2%, Cu 2.5%, Li element burn-off rate is over 20%.

[0029] 4) The proportions of Class I and Class II remelting materials are 80% and 20%, respectively. Experimental results: Li 2.2%, Cu 2.5%, Li element burn-off rate over 12%.

[0030] Therefore, in order to ensure that the burn-off is below 20%, the amount of existing recycled material added should be controlled below 30%, and the Li element should be added during the smelting process to supplement the recycled material amount by 20%.

[0031] Using Li 2.5% and Cu 2.5% as the main components, and following the differential pressure sand casting process with a melting volume of over 200 kg, a specific shell was selected for casting. Experiments were conducted with addition ratios of 50%, 40%, 30%, and 20%, all following a standardized melting process. After melting the recycled material, a covering agent was continuously applied to the surface for protection. An intermediate alloy was added for refining, followed by further refining. Finally, lithium ingots were added, and after melting, a vacuum was applied for 30 minutes before pouring. The internal microstructure was compared with that of virgin material.

[0032] 1) 50% recycled material addition ratio The chemical composition is Li 1.3% and Cu 2.5%. The fracture test sample showed a large number of oxide inclusions, which were widely distributed inside the casting. The attached casting sample failed the test.

[0033] 2) 40% recycled material addition ratio The chemical composition is Li 1.3% and Cu 2.5%. A small amount of oxide inclusions are still present in the casting. The oxide inclusions are not obvious in the fracture surface test sample, and the attached casting sample is qualified.

[0034] 3) 30% recycled material addition ratio The chemical composition is Li 2.0% and Cu 2.5%. The oxide inclusions in the castings are greatly reduced, the fracture surface test samples have almost no oxide inclusions, and the attached casting samples pass the test.

[0035] 4) 20% recycled material addition ratio The chemical composition is Li 2.2% and Cu 2.5%. There are basically no oxide inclusions in the casting. The fracture surface test sample has basically no oxide inclusions. The attached casting sample passed the test.

[0036] The above results also show that for Class I castings, the proportion of recycled material added should be within 30% to ensure the chemical composition, internal quality, and mechanical properties of the test bars.

[0037] The addition ratio of recycled material to Class II castings is within 40%, which can ensure the chemical composition, internal quality requirements and mechanical property results of the test bars.

[0038] This invention provides, for example Figure 1 The method for processing recycled aluminum-lithium alloy castings shown below includes the following steps: 1. Classification and pretreatment of recycled materials: 1) The recycled materials are divided into three categories according to quality: Category I recycled materials (casting residue and scrap from crucibles), Category II recycled materials (risers and gating systems), and Category III recycled materials (horizontal gating systems and cleaned seams, which are discarded). 2) Type I remelting material and Type II remelting material are mixed at a mass ratio of (70-80):(20-30) and mechanically crushed to a particle size of <30×30mm; 3) Use ultrasonic cleaning to remove oil and cutting fluid, dry and preheat to 180-250℃ for later use; 2. Charging and Flux Covering: Preheated recycled material is loaded into the smelting furnace and covered with a composite protective flux with the following composition (mass fraction): LiCl: 50-60%, KCl: 30-40%, Na₂SiF₆: 5-10%. The amount of composite protective flux added is 1.5-2.5% of the total mass of the furnace charge. The use of the aforementioned composite protective flux can specifically include: pre-sprinkling a layer of covering agent (composite protective flux) on the bottom of the furnace, which is 0.1% of the total furnace charge; continuously raising the temperature; after complete melting, sprinkling the covering agent on the surface again; adding 0.3% of the total furnace charge; when liquid appears at the bottom of the furnace, covering the surface with a protective cover; introducing argon gas for protection; and controlling the temperature below 680℃ until it is completely melted.

[0039] 3. Staged melting and electromagnetic stirring: 1) Heat the furnace charge to 700-720℃ under argon protection to melt it completely; 2) Apply 5-15Hz low-frequency electromagnetic stirring during the melting process, using intermittent mode (on for 2 minutes / off for 1 minute). 4. Sodium removal and component adjustment: After the melt is cleared, add 0.2-0.4% hexachloroethane (C2Cl6) by mass of the melt and stir at low speed for 8-12 minutes. Samples were taken for rapid spectral analysis. The required addition of elements such as Li, Cu, and Mg was calculated based on the target composition and added in the form of an intermediate alloy. The intermediate alloy was added under gas protection (the composition of each element was controlled according to the intermediate value), and stirring and slag removal were performed to ensure that the alloying elements were fully melted.

[0040] Taking Al-Li(1-3%)-Cu(1-3%)-Zr(0.2-0.4%) as an example: Cu is added to the new material, supplemented with AlCu50 master alloy and Zr is supplemented with AlZr10 master alloy. Since these two elements are high melting point elements, these two master alloys can be added normally. However, the loss of Li elements needs to be controlled.

[0041] After the alloy is fully melted, the content of each element such as Li, Cu, and Zr is analyzed using a direct-reading spectrometer. Then, the elements are added according to the target values. However, the loss of lithium is relatively large, so more than 20% of the heat loss needs to be considered (the heat loss value varies depending on the melting amount) in order to ensure the elemental composition of the casting.

[0042] The calculation process is as follows, with Al-Li(2.5%)-Cu(3%)-Zr(0.4%) as the target value: The smelting volume is 500 kg, and the recycled material is added at 30%. The composition of the recycled material is Al-2Li-2Cu-Zr(0.2). The amount of intermediate alloy AlCu50 added = (500 × 3% - 500 × 30% × 0.02) ÷ 50% The amount of AlZr10 intermediate alloy added = (500 × 0.4% - 500 × 30% × 0.2%) ÷ 10% Lithium ingot addition amount = (500×2.5%-500×30%×2%)×(1+30%).

[0043] 5. Rotary jet refining: Using a mixture of Ar and Cl2 gas (Cl2 content 1-3%), through, as... Figure 2 The rotary jetting device shown sprays the melt at a speed of 200-300 rpm, with a gas flow rate of 1.5-2.5 L / min•tonne and a processing time of 10-15 minutes. The structure of the rotary jetting device is the same as that of the existing rotary jetting device, including a baffle 1, a graphite rod 2 and a graphite rotor 3.

[0044] Furthermore, the temperature is raised to 720℃, and the refining agent is injected. During the injection process, the crucible needs to be protected with a protective cover. The amount of refining agent added is 0.3%-0.4%. The main refining agents for aluminum liquid include potassium chloride, sodium chloride, and magnesium chloride.

[0045] The refining agent is mainly composed of various inorganic salts that have been dried and mixed in specific proportions. Its main components include chloride salts, fluoride salts, polyol ether derivatives, nonionic and anionic surfactant complexes, and special nonionic surfactants. These components decompose at high temperatures, removing harmful gases and impurities from the molten aluminum through physical and chemical changes. The decomposition of the aluminum refining agent at high temperatures produces primarily chlorine gas, which reacts with the molten aluminum to remove hydrogen and floating oxide inclusions, resulting in a purer molten aluminum. Some components of the refining agent possess strong adsorption capabilities, adsorbing oxides and suspended solids in the melt and carrying them to the surface of the molten aluminum as bubbles rise, thus achieving degassing and slag removal.

[0046] 6. Vacuum degassing and final treatment: After refining, sprinkle in the covering agent (composite protective flux), let it stand for 10-15 minutes, and then skim off the slag. During this process, the covering agent can be evenly sprinkled on the surface of the molten aluminum, and then quickly pressed into the molten aluminum, stirred thoroughly, and left to stand for a period of time before skimming off the slag.

[0047] Apply vacuum treatment at 730-740℃, maintain the vacuum degree at 10-100Pa, and treat for 10-15 minutes; Furthermore, lithium ingots are added at 730℃. After 5 minutes, a vacuum device is placed on the upper surface of the crucible and started. The vacuum device is activated for 10 minutes, and the vacuum degree can reach 10-100Pa, ensuring that a negative pressure environment of 100Pa-200Pa is formed during operation.

[0048] After treatment, apply a covering agent again and control the temperature to 720-730℃ before pouring.

[0049] It should be noted that Al-Li alloys are extremely sensitive to the harmful effects of hydrogen. Strict control of hydrogen content is essential during smelting and casting because Al-Li alloys are highly susceptible to hydrogen embrittlement. Hydrogen can segregate and accumulate at grain boundaries, forming hydrides, which weakens and embrittles the grain boundaries, leading to cracks and ultimately intergranular fracture – this is hydrogen embrittlement. Generally, the hydrogen content of Al-Li alloy materials should be less than 1 mL / (100 g Al). Conventional alloy refining and degassing methods cannot meet this requirement. Therefore, high-purity argon refining and vacuum degassing are necessary, with the mass fractions of H2O, O2, and H2 in the argon gas each less than 10 × 10⁻⁶. -6 10-100Pa, ensuring a negative pressure environment of 100Pa-200Pa during operation.

[0050] The method described in this invention is a treatment method for recycled materials of cast aluminum-lithium alloys. It is applicable to the melting and processing of recycled materials of Al-Li-Cu and Al-Li-Mg series cast aluminum-lithium alloys (particularly suitable for the comprehensive treatment of efficient purification, precise composition control, and microstructure refinement of recycled materials of cast aluminum-lithium alloys for aerospace applications). This method helps reduce resource waste, save production costs, and create good economic benefits. Specifically, this invention provides a process for efficient purification and composition control of recycled aluminum-lithium alloy materials. Through the synergistic effect of multiple processes, including graded pretreatment of recycled materials, composite flux covering, electromagnetic stirring-assisted melting, C2Cl6 sodium removal, Ar-Cl2 rotary blowing refining, and vacuum degassing, lithium burn-off and impurity content are significantly reduced, achieving a high proportion of safe recycling of recycled materials. The performance of the recycled alloy reaches more than 95% of that of the primary alloy, making it suitable for the production of high-end castings for aerospace applications.

[0051] The above-mentioned method for processing recycled aluminum-lithium alloy castings has the following characteristics: 1) Innovative composite flux system: Through LiCl-KCl-Na2SiF6 composite flux, a stable coating layer is formed on the surface of the melt. Na2SiF6 decomposes and releases SiF4 gas to form a gas curtain, which double blocks the contact between lithium and air, and controls the lithium burn-off rate to below 5%. 2) High-efficiency impurity removal technology: C2Cl6 is used as a sodium removal agent to effectively reduce the content of impurities such as Na and K; combined with Ar-Cl2 rotary jet refining, hydrogen and non-metallic inclusions are deeply removed, and the hydrogen content can be controlled below 0.10mL / 100gAl. The Cl2 and C2Cl4 produced by the decomposition of C2Cl6 can react with Na to form NaCl slag, which can then be removed without introducing Na ions as in conventional fluxes. Combined with subsequent Ar-Cl2 mixed gas rotary jet refining, hydrogen and non-metallic inclusions in the melt are deeply removed, achieving efficient and precise removal of harmful impurities.

[0052] 3) Electromagnetic stirring + vacuum treatment: enhances melt mass transfer and composition uniformity, and vacuum treatment further degasses and promotes the flotation of inclusions; 4) High recycling rate: This process can increase the proportion of recycled materials to over 40%, and the mechanical properties of recycled alloys can reach over 90% of those of primary alloys, significantly reducing costs and meeting the requirements of high-end castings; 5) Whole-process composition and microstructure control: This process integrates physical pretreatment, chemical protection, physical field assistance, chemical purification, and composition fine-tuning into a complete system solution. The recycled aluminum-lithium alloy castings have stable composition, fine grains, and low hydrogen content. Their mechanical properties (tensile strength, yield strength, elongation) can reach more than 95% of the original alloy standard, enabling direct recycling of more than 80% of the remelted material, breaking through the bottleneck of existing technologies that can only be downgraded for use.

[0053] To more clearly and in detail introduce the method for processing recycled aluminum-lithium alloy castings and the resulting aluminum-lithium alloy castings provided in the embodiments of the present invention, the following description will be based on specific embodiments.

[0054] Example 1 500 kg of Al-2.5Li-2.5Cu casting alloy remelting material was processed according to the processing method given in the specific implementation method.

[0055] The proportion of recycled materials is as follows: 75% of Class I recycled materials and 25% of Class II recycled materials. After crushing and cleaning, the materials are preheated to 200°C. Feed ratio: 30% recycled material and 70% virgin material; Flux composition: 55% LiCl + 40% KCl + 5% Na2SiF6, added at 2%; Melting temperature 710℃, electromagnetic stirring 10Hz; Add 1.5 kg of C2Cl6 to remove sodium, and after spectral analysis, add intermediate alloys such as Al-10Li and Al-50Cu. Rotary jet refining: Ar-2%Cl2, flow rate 2.0 L / min•tonne, 12 minutes; Vacuum treatment: 50 Pa vacuum, 12 minutes; The pouring temperature is 725℃.

[0056] Results: Lithium burn-off rate ≤4%, [H]≈0.09 mL / 100gAl, sodium content ≤3 ppm, the tensile strength of the recycled alloy in T6 state is 420MPa, reaching 96% of the performance of the original alloy.

[0057] The necessity of gas protection for the aforementioned aluminum-lithium alloys: Due to the alloy's high reactivity and flammability, it oxidizes severely under gravity casting. Therefore, gas protection is essential during alloy smelting to reduce lithium alloy burn-off. The oxide film on the surface of the Al-Li alloy melt is not fine, leading to severe oxidation. It can also react with moisture and oxygen in the furnace gas, significantly increasing slag production. Furthermore, the hydrogen generated from the reaction enters the melt, severely impacting the material's processing and performance. Therefore, Al-Li alloy smelting and casting should be carried out under strict inert gas or flux protection, ideally in a vacuum furnace. Strict control of Na, K, and Ca inclusions in the melt is crucial, as they are highly detrimental to the alloy's processing and performance. For example, a Na mass fraction of 15 × 10⁻⁶ should be avoided. -6 If it is too small, it will be difficult to hot roll and it will easily crack. Trace amounts of Ca are detrimental to the fracture toughness of the alloy. Therefore, preventing impurity elements from being mixed into the Al-Li melt is also one of the key technologies.

[0058] The remelting materials for cast aluminum-lithium alloys are classified into three categories: crucible bottom residue, gap gating, and horizontal gating. Among these, the crucible bottom residue has less oxide slag on the molded surface, but suffers from severe burn-off. The gap gating, formed under pressure, experiences less lithium burn-off, but its surface, in contact with the sand mold, suffers from severe sand adhesion and a large amount of impurities under high temperatures. The horizontal gating, being the source of feeding, is the earliest to fill under pressure and the last to cool, experiencing the longest sustained high-temperature effect, resulting in the most severe sand adhesion and the highest amount of impurities, making its recycling largely pointless. Therefore, based on the remelting material classification, crucible bottom residue is classified as Category I, gap gating as Category II, and horizontal gating as Category III. Only Category I and Category II remelting materials are used based on the classification of cast aluminum-lithium alloys.

[0059] Due to the complex internal structure and thin wall thickness, the large-diameter shell is mostly produced using differential pressure and low-pressure casting processes. Instead of using wrought aluminum-lithium alloys made from bars or plates for machining, the price of wrought aluminum-lithium alloys is currently 600 yuan / kg, while that of cast aluminum-lithium alloys is 200 yuan / kg. This reduces production costs and, most importantly, minimizes deformation and cracking during processing.

[0060] However, both differential pressure casting and low-pressure casting employ slit-type gating systems, and the riser pipe after pouring must submerge the lower end of the riser pipe by more than 200mm, resulting in the generation of a large amount of remelted material. For every 100kg of casting produced, 350-400kg of remelted material is generated. Currently, there is no research on the use of remelted material in the casting of aluminum-lithium alloys. Therefore, increasing the use of remelted material can significantly reduce casting costs and create good economic benefits.

[0061] Process for adding recycled materials: Because cast aluminum-lithium alloys contain lithium, and both LiO and LiH are exothermic reactions, the rapid increase in melt temperature causes the melt to expand dramatically. This exacerbates the loss of lithium. For coating applications, existing surface treatments primarily use LiCl and LiF composite fluxes. Fluoride salts have good wettability and can penetrate the loose oxide film for protection. Fluoride salts can dissolve impurities such as Na and Al2O3, and also purify gases adsorbed on impurities. Therefore, fluoride salts are commonly used as coating agents for cast aluminum-lithium alloys. However, excessively high temperatures can lead to the decomposition of fluoride salts, so the surface temperature of the alloy should be kept as low as possible. These factors result in severe lithium loss, reaching approximately 20%. Therefore, the reuse of cast aluminum-lithium alloy scrap differs significantly from that of ordinary aluminum alloy scrap.

[0062] The present invention discloses a method for processing recycled aluminum-lithium alloy castings, comprising the steps of gas protection, flux protection, refining, and adding alloying elements. The processing of the recycled material is carried out under an inert gas protective environment to prevent further oxidation and burn-off of the lithium-containing alloy. In the smelting stage: the furnace charge is first classified into Class I, Class II, and Class III recycled materials according to the usage environment and charge quality. Class I and Class II recycled materials are used as much as possible. A covering agent is applied before the recycled material is placed in the crucible. The molten surface is covered with the covering agent before pouring. Based on sampling and analysis during the smelting process, an intermediate alloy is added to ensure the composition requirements of the casting are met. Finally, a refining process is performed. After adding lithium ingots, a vacuum device is used to evacuate the alloy, allowing hydrogen to escape before immediate pouring.

Claims

1. A method for processing recycled aluminum-lithium alloy castings, characterized in that, include: The selection steps for recycled materials involve mixing Class I and Class II recycled materials at a mass ratio of 70-80:20-30 to obtain reusable materials; Class I recycled materials are casting residues and scrap parts from crucibles, and Class II recycled materials are risers and gating systems; The charging and solvent covering step involves loading the recycled material and the new material into the melting furnace and covering the surface with a composite protective flux; the composite protective flux is a LiCl-KCl-Na2SiF6 composite flux. The sodium removal and composition adjustment steps involve adding hexachloroethane to the fully melted furnace charge to remove sodium, then taking a sample for composition analysis, calculating the elements that need to be added according to the target composition, and adding them in the form of intermediate alloys. The vacuum degassing and final treatment steps involve covering the surface of the furnace charge with the composite protective flux after refining, adding lithium ingots at 730-740°C, applying vacuum treatment and maintaining the vacuum level at 10-100 Pa.

2. The processing method according to claim 1, characterized in that, The LiCl-KCl-Na2SiF6 composite flux comprises 50-60% LiCl, 30-40% KCl, and 5-10% Na2SiF6 by mass fraction.

3. The processing method according to claim 1, characterized in that, In the charging and solvent covering step, the amount of the composite protective flux added is 1.5-2.5% of the total mass of the furnace charge; in the sodium removal and composition adjustment step, the amount of hexachloroethane added is 0.2-0.4% of the melt mass.

4. The processing method according to claim 1, characterized in that, It also includes a rotary jet refining step between the charging and solvent covering step and the sodium removal and composition adjustment step, wherein the rotary jet refining step includes: using a mixed gas of Ar and Cl2, injecting it into the melt at a speed of 200-300 rpm, a gas flow rate of 1.5-2.5 L / min•tonne, and a processing time of 10-15 minutes.

5. The processing method according to claim 4, characterized in that, The rotary jet refining step further includes adding a refining agent, wherein the amount of the refining agent added is 0.3-0.4% of the melt mass.

6. The processing method according to claim 4, characterized in that, The Cl2 content in the mixed gas is 1-3%.

7. The processing method according to claim 1, characterized in that, It also includes a recyclable material classification step before the recyclable material selection step, wherein the recyclable material classification step includes: classifying the recyclable material into Class I recyclable material, Class II recyclable material and Class III recyclable material; Class I recyclable material is the casting residue and waste parts of the crucible, Class II recyclable material is the riser and gating system, and Class III recyclable material is the horizontal gating system and the cleaned seams.

8. The processing method according to claim 1, characterized in that, It also includes a full melting and stirring step between the charging and solvent covering step and the sodium removal and composition adjustment step. The full melting and stirring step includes: heating to 700-720°C under argon protection to completely melt the furnace charge, and applying 5-15Hz low-frequency electromagnetic intermittent stirring during the melting process.

9. The processing method according to claim 1, characterized in that, In the charging and solvent covering step, the amount of recycled material used shall not be less than 30% of the total amount of recycled material and new material used; for castings requiring Class II requirements, the amount of recycled material used in the charging and solvent covering step shall not be less than 40% of the total amount of recycled material and new material used.

10. The cast aluminum-lithium alloy casting obtained by the processing method according to any one of claims 1-9, characterized in that, The lithium burn-off rate of the cast aluminum-lithium alloy casting is ≤5%, the hydrogen content is ≤0.10mL / 100gAl, and the sodium content is ≤5ppm.

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