Fastener material based on high-strength alloy and preparation process thereof

Abstract

The invention relates to a fastener material based on a high-strength alloy and a preparation process thereof, and belongs to the technical field of alloy materials. According to the material, 7075 aluminum alloy serves as a base body, and a novel composite refining agent is adopted for conducting purification treatment on a melt. According to the refining agent, boron nitride is used as a core, silane coupling and organic amine modification are carried out, a cerium element is chelated and loaded, and a stable composite structure of boron nitride / silicon oxide / carbon-cerium is formed through roasting. According to the invention, high-efficiency deep deoxidation can be realized, almost no residue exists, and the alloy cleanliness is greatly improved, so that the fastener material finally subjected to T73 treatment has excellent strength, toughness, fatigue resistance and stress corrosion resistance at the same time.

Description

Technical Field

[0001] This invention belongs to the field of alloy materials technology, specifically relating to a fastener material based on a high-strength alloy and its preparation process. Background Technology

[0002] Fasteners, such as bolts, nuts, and threaded rods, are a widely used type of mechanical parts used for fastening connections and are the foundation of the equipment manufacturing industry. With the development of high-end equipment such as aerospace, high-performance automobiles, and precision instruments towards lightweight, high reliability, and long service life, extremely stringent requirements are placed on the performance of fasteners: they not only need extremely high static strength but also excellent resistance to fatigue, stress corrosion, and vibration relaxation. Traditional steel fasteners, while strong, have high density, making it difficult to meet weight reduction demands. Against this backdrop, high-strength aluminum alloys, represented by the 7000 series (Al-Zn-Mg-Cu), have become ideal materials for manufacturing high-end fasteners due to their high specific strength, good machinability, and excellent corrosion resistance.

[0003] Among numerous high-strength aluminum alloys, 7075 aluminum alloy is widely recognized for its excellent comprehensive performance. Through solution hardening and aging heat treatment, 7075 aluminum alloy can achieve very high strength. In particular, the T73 over-aging treatment, by intentionally coarsening the main strengthening phase (η phase, MgZn2) to a certain extent, forms discontinuously distributed coarse η phase particles at the grain boundaries, effectively cutting off the path of stress corrosion crack propagation along the grain boundaries. This adjustment of the microstructure allows the material to achieve a significant improvement in toughness and resistance to stress corrosion cracking while sacrificing a small amount of strength, thus achieving an excellent balance between mechanical properties and corrosion resistance.

[0004] However, when applying this high-quality alloy to fasteners that are extremely sensitive to defects, a fundamental technical bottleneck emerges: oxide inclusions remaining from the smelting process. These oxides (mainly Al2O3, MgO, spinel, etc.) are formed during smelting and transfer due to the reaction of molten aluminum with air and the furnace lining. They have high melting points, high hardness, and brittleness, and exhibit extremely poor adhesion to the aluminum matrix. Unlike the η phase, which is controlled through heat treatment and has a coherent / semi-coherent relationship with the matrix, these oxides are completely independent external defects. They exist in irregular forms in the matrix or at grain boundaries, and under service stress (especially alternating loads), they are highly susceptible to becoming initiation sources of microcracks. More importantly, the harmful effects of these inclusions completely "bypass" the phase advantages brought by the T73 treatment. Even if the T73 treatment optimizes the grain boundary phase distribution, brittle oxide particles present within the grains or at the grain boundaries can still directly induce early fatigue fracture or stress corrosion cracking, causing the fasteners to fail to achieve the expected long service life and high reliability.

[0005] Therefore, obtaining aluminum alloy melts with extremely high purity is a prerequisite for manufacturing top-grade fasteners. Industrially, melt refining technology is widely used to remove oxides and dissolved hydrogen. Common refining agents include fluorine-based agents (such as cryolite, aluminum fluoride, and calcium fluoride) and chlorine-based agents (such as hexachloroethane and manganese chloride). They mainly remove oxides by physically adsorbing them, chemically decomposing them to generate gases, or forming low-melting-point double salts, thus carrying oxides to the melt surface to form scum. In recent years, rare earth metals (especially cerium) have attracted much attention due to their extremely strong affinity for oxygen. They can react with dissolved oxygen and some oxides in molten aluminum to generate denser rare earth oxides or oxysulfides that float to the surface and are removed, theoretically possessing excellent deoxidation potential.

[0006] However, directly adding rare earth elements to molten aluminum introduces a new and serious problem: the residual rare earth elements readily react with aluminum, zinc, magnesium, and other elements in the alloy to form coarse rare earth-rich intermetallic compounds. These phases are typically hard and brittle, acting as stress concentration points in the matrix and severely impairing the mechanical properties of the alloy, particularly fatigue performance and toughness, which are sensitive to defects. This contradicts the original purpose of refining. To mitigate this side effect, existing technologies have developed supported refining agents, which load rare earth elements onto porous supports (such as porous ceramics or molecular sieves). After refining, the support and reaction products are removed together by skimming. However, in practical applications and long-term research, the residue problem persists and has a substantial impact. The root cause lies mainly in two aspects: First, the thermophysical properties of the support and the loaded material are mismatched. Porous ceramic supports typically have a high coefficient of thermal expansion, generating enormous thermal stress between them and rare earth oxides (which increase in volume after oxygen absorption) at high refining temperatures (≥700℃). This leads to the pulverization or cracking of the loaded structure, causing rare earth elements to detach and remain in the melt. Secondly, the bonding strength of the load is insufficient. Traditional physical impregnation adsorption has weak bonding force, and rare earth active components are very easy to detach from the carrier surface under intense high-temperature melt stirring and scouring.

[0007] In summary, developing a composite refining agent that can both leverage the efficient deoxidation capabilities of rare earth elements and ensure their near-complete separation from the molten aluminum alloy after refining, while maintaining high stability and easy separation, is key to overcoming the current performance bottlenecks of 7075-T73 aluminum alloy fasteners and realizing their ultimate applications. This invention is proposed based on this objective. Summary of the Invention

[0008] In order to solve the technical problems mentioned in the background art, the purpose of this invention is to provide a fastener material based on high-strength alloy and its preparation process.

[0009] The objective of this invention can be achieved through the following technical solutions: The fastener material is based on a high-strength alloy and its process is adjusted based on the 7075-T73 aluminum alloy system. The core of the adjustment is the introduction of a newly developed composite refining agent into the refining process, which maximizes the purity of the aluminum melt and fully leverages the T73 treatment to enhance the strength and toughness of the alloy material. The specific technical solution includes the following two aspects: I. Preparation of Compound Refining Agent Step A1: N-benzylethylenediamine, 3-chloropropyltrimethoxysilane and anhydrous dioxane are premixed under a dry nitrogen atmosphere, potassium carbonate and zeolite desiccant are added and the mixture is heated to 70-85℃ and stirred for 4-6 hours. After the reaction is completed, the mixture is filtered and then evaporated under reduced pressure to obtain the modified coupling agent. Furthermore, the ratio of N-benzylethylenediamine, 3-chloropropyltrimethoxysilane, potassium carbonate, and anhydrous dioxane is 0.1 mol: 0.1 mol: 15-20 g: 300-400 mL.

[0010] Step A2: Disperse boron nitride micro powder and isopropanol aqueous solution by ultrasonication, add modified coupling agent and mix well, adjust pH value to 4-5, heat to 50-60℃ and stir for 1-1.5h, then neutralize and let stand to precipitate, centrifuge to wash precipitate and dry to obtain composite carrier; Furthermore, the ratio of boron nitride micro powder, modified coupling agent, and isopropanol aqueous solution is 50g: 6.2-8.5g: 450-520mL.

[0011] Furthermore, the average particle size of the boron nitride micro powder is 20-40 μm.

[0012] Step A3: The composite carrier and cerium nitrate aqueous solution are ultrasonically dispersed, and then stirred in a water bath at a temperature of 35-50℃ for 8-12 hours. The precipitate is centrifuged, dried to constant weight, and then calcined in an argon atmosphere in multiple stages without oxygen. The composite refining agent is obtained by cooling in the furnace. Furthermore, the concentration of the cerium nitrate aqueous solution is 0.1-0.12 mol / L, and the solid-liquid mass ratio is 1:12-15.

[0013] Furthermore, the multi-stage anaerobic calcination adopts a two-stage calcination process, specifically: in the first stage, the heating rate is 5-8℃ / min, the temperature is 350-400℃, and the holding time is 1-1.3h; in the second stage, the heating rate is 1-5℃ / min, the temperature is 720-780℃, and the holding time is 2.5-3h.

[0014] II. Preparation of Fastener Materials Process S1: Pure aluminum ingots, copper master alloy and chromium master alloy are loaded into the furnace and melted to 750±5℃ under argon atmosphere; Step S2: Control the melt temperature to 730±10℃, introduce argon gas for degassing for 15-18 minutes, add covering agent and rough refining to remove slag; Process S3: Control the melt temperature to 720±5℃, add pure zinc ingots until completely dissolved, then press the composite refining agent into the bottom of the aluminum melt, remove the slag and add pure magnesium ingots to dissolve; Step S4: Control the melt temperature to 710±5℃, add grain refiner and stir to mix, then cast the melt into shape; Process S5: The ingot is subjected to homogenization treatment, solution treatment and T73 two-stage over-aging treatment in sequence to obtain fastener material.

[0015] Furthermore, the amount of the composite refining agent is 0.24-0.32 wt% of the molten aluminum.

[0016] The beneficial effects of this invention are: This invention, through chemical design and process control, prepares a composite refining agent with a unique "boron nitride core / silicon oxide intermediate layer / carbon-cerium composite surface layer" structure. Its application in the preparation of 7075-T73 high-strength aluminum alloy fasteners produces a synergistic and beneficial effect, specifically: First, this invention abandons the traditional porous ceramic carrier and selects non-porous, dense boron nitride micropowder as the core. Boron nitride itself is chemically stable and hardly reacts or decomposes in aluminum melt, fundamentally avoiding the release of water vapor or gas adsorbed inside the porous material into the melt at high temperatures, thereby improving the initial environment of refining and making the subsequent deoxidation reaction purer and more efficient.

[0017] Secondly, the carbon-cerium composite surface layer formed by anaerobic calcination in this invention not only provides highly active deoxidation sites, but the carbides and amorphous carbon within it also act as an "armor," reducing the direct scouring and melting losses of active cerium by the high-temperature aluminum melt in the initial stage of adding the refining agent to the melt. More importantly, the construction of the silicon oxide intermediate layer plays a dual role as a "stress buffer" and a "bonding enhancer." The coefficient of thermal expansion of silicon oxide is between that of the low-expansion boron nitride core and the outer carbon-cerium composite layer. At the high temperature of refining, the difference in thermal expansion steps between the three layers is significantly reduced, greatly alleviating the internal stress caused by thermal mismatch and preventing structural collapse. At the same time, the chemical compatibility and wettability of silicon oxide with the upper and lower layers are better than that of boron nitride directly bonding with carbon-cerium, forming a strong chemical bond and ensuring the mechanical integrity of the overall structure during the refining process.

[0018] Third, the aforementioned stable structure ensures that the refining agent particles remain intact in the molten aluminum, allowing them to fully utilize their surface area and activity for efficient oxygen capture. When the reaction is complete and the refining agent particles rise to the surface due to density changes caused by adsorbed oxides and the formation of rare earth oxides, the intact structure ensures that the active components do not detach midway. Ultimately, the particles are completely removed as a whole by slag removal, achieving the stringent goals of deep deoxidation and extremely low rare earth residue. This directly leads to a revolutionary improvement in the internal cleanliness of the alloy, allowing the full release of the potential of T73 heat treatment to optimize grain boundaries, improve toughness, and enhance resistance to stress corrosion, without being offset by residual brittle oxides or rare earth phases.

[0019] In summary, this invention, through innovation in materials and processes, effectively resolves the long-standing contradiction between ultra-clean purification of high-strength aluminum alloy melts and control of harmful element residues. The prepared fastener material achieves excellent synergistic effects in strength, toughness, fatigue performance, and stress corrosion resistance, making it particularly suitable for aerospace, high-performance automotive, and other fields with extreme requirements for reliability and lifespan, and has significant industrial application value. Detailed Implementation

[0020] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0021] The aluminum alloy fastener materials used in the implementation process are based on the standard 7075 alloy composition system. The raw materials are industrial pure aluminum (≥99.7%), pure magnesium ingots, pure zinc ingots, Al-50Cu, Al-10Cr, Al-5Ti-B (used as a grain refiner) master alloys. All raw materials must be cleaned of surface oil and oxide scale before use.

[0022] Example 1: Fastener material prepared based on 7075 aluminum alloy. The specific implementation process is as follows: I. Preparation of Compound Refining Agent Step A1: Dry nitrogen gas is introduced into the reactor, and N-benzylethylenediamine, 3-chloropropyltrimethoxysilane and anhydrous dioxane are added and premixed with stirring. Then potassium carbonate and a small amount of zeolite desiccant (about 5 wt% of the reaction system) are added. The temperature is raised to 85℃ and the reaction is stirred for 4 hours. The ratio of N-benzylethylenediamine, 3-chloropropyltrimethoxysilane, potassium carbonate and anhydrous dioxane is 0.1 mol: 0.1 mol: 20 g: 400 mL. After the reaction is completed, the zeolite desiccant and insoluble salts are removed by filtration. Then, dioxane is removed by rotary evaporation under reduced pressure to obtain the modified coupling agent.

[0023] Step A2: Prepare a 55% (v / v) isopropanol aqueous solution, add boron nitride micro powder and ultrasonically disperse to form a uniform dispersion, add a modified coupling agent and stir to mix, and acidify with hydrochloric acid to pH 4, heat to 60℃ and stir for 1 hour. The boron nitride micro powder is a commercially available BN-S type micro powder raw material with an average particle size of about 25 μm. The ratio of boron nitride micro powder, modified coupling agent and isopropanol aqueous solution is 50 g: 8.5 g: 520 mL. Then neutralize with ammonia water and let stand for 6 hours to precipitate. Take the precipitate, wash with water, combine the precipitates and dry to obtain the composite carrier.

[0024] Step A3: Prepare a 0.1 mol / L cerium nitrate aqueous solution, add the composite carrier, and control the solid-liquid mass ratio of the two to be 1:15. Use ultrasonic dispersion to form a uniform dispersion, then control the temperature in a water bath at 50℃ and stir for 8 hours. After centrifugation, collect the precipitate, dry it to constant weight, and send the dried product into an atmosphere roasting furnace. Evacuate to 10 Pa, introduce argon gas to a slightly positive pressure, and use two-stage roasting. In the first stage, the heating rate is 8℃ / min, the temperature is 400℃, and the holding time is 1 hour. In the second stage, the heating rate is 5℃ / min, the temperature is 780℃, and the holding time is 2.5 hours. Finally, cool it in the furnace to 80℃ and discharge it to obtain the composite refining agent.

[0025] II. Preparation of Fastener Materials Step S1: The calculated pure aluminum ingots, copper master alloy and chromium master alloy are loaded into a preheated graphite-clay crucible and placed in a medium-frequency induction melting furnace. The furnace charge is heated to 750±5℃ under an argon atmosphere to completely melt the furnace charge.

[0026] Step S2: After all the metal has melted, adjust the melt temperature to 730±10℃, and use a rotary jetting device to introduce high-purity argon gas (flow rate 1.5L / min, rotation speed 300rpm) for degassing treatment for 15 minutes. At the same time, sprinkle 0.4wt% of the melt with a covering agent for rough refining to initially remove large pieces of slag.

[0027] Step S3: Control the melt temperature to 720±5℃, add pure zinc ingots until completely dissolved, then press 0.24wt% of composite refining agent into the bottom of the aluminum melt, maintain the temperature for 6 minutes after adding, then remove all slag from the surface of the melt, and then add pure magnesium ingots to dissolve.

[0028] Step S4: Control the melt temperature to 710±5℃, add grain refiner and stir for 5 minutes, then pour the melt into a preheated (300℃) low carbon steel mold to cast Φ80mm round ingots.

[0029] Process S5: The ingot is homogenized (465℃×24h), cooled in the furnace and the surface is removed by machining, then solution treated (475℃×2h, 60℃ water quenching), and finally subjected to T73 double-stage over-aging treatment (110℃×8h, 165℃×18h) to obtain fastener material in 7075-T73 condition.

[0030] Example 2: Fastener material prepared based on 7075 aluminum alloy. The specific implementation process is as follows: I. Preparation of Compound Refining Agent Step A1: Dry nitrogen gas is introduced into the reactor, and N-benzylethylenediamine, 3-chloropropyltrimethoxysilane and anhydrous dioxane are added and premixed with stirring. Then potassium carbonate and a small amount of zeolite desiccant (about 5 wt% of the reaction system) are added. The temperature is raised to 70°C and the reaction is stirred for 6 hours. The ratio of N-benzylethylenediamine, 3-chloropropyltrimethoxysilane, potassium carbonate and anhydrous dioxane is 0.1 mol: 0.1 mol: 15 g: 300 mL. After the reaction is completed, the zeolite desiccant and insoluble salts are removed by filtration. Then, dioxane is removed by rotary evaporation under reduced pressure to obtain the modified coupling agent.

[0031] Step A2: Prepare a 55% (v / v) isopropanol aqueous solution, add boron nitride micro powder and ultrasonically disperse to form a uniform dispersion, add a modified coupling agent, stir and mix, and acidify with hydrochloric acid to a pH of 5. Heat to 50℃ and stir for 1.5 h. The boron nitride micro powder is a commercially available BN-SS type micro powder raw material with an average particle size of about 40 μm. The ratio of boron nitride micro powder, modified coupling agent and isopropanol aqueous solution is 50 g: 6.2 g: 450 mL. Then neutralize with ammonia water and let stand for 6 h to precipitate. Take the precipitate, wash with water, combine the precipitates and dry to obtain the composite carrier.

[0032] Step A3: Prepare a 0.12 mol / L cerium nitrate aqueous solution, add the composite carrier, and control the solid-liquid mass ratio of the two to be 1:12. Use ultrasonic dispersion to form a uniform dispersion, then control the temperature in a water bath at 35℃ and stir for 12 hours. After centrifugation, collect the precipitate, dry it to constant weight, and send the dried product into an atmosphere roasting furnace. Evacuate to 10 Pa, introduce argon gas to a slightly positive pressure, and use two-stage roasting. In the first stage, the heating rate is 5℃ / min, the temperature is 350℃, and the holding time is 1.3 hours. In the second stage, the heating rate is 1℃ / min, the temperature is 720℃, and the holding time is 3 hours. Finally, cool it in the furnace to 80℃ and discharge it to obtain the composite refining agent.

[0033] II. Preparation of Fastener Materials Step S1: The calculated pure aluminum ingots, copper master alloy and chromium master alloy are loaded into a preheated graphite-clay crucible and placed in a medium-frequency induction melting furnace. The furnace charge is heated to 750±5℃ under an argon atmosphere to completely melt the furnace charge.

[0034] Step S2: After all the metal has melted, adjust the melt temperature to 730±10℃, and use a rotary jetting device to introduce high-purity argon gas (flow rate 1.2L / min, rotation speed 300rpm) for degassing treatment for 18 minutes. At the same time, sprinkle 0.4wt% of the melt with a covering agent for rough refining to initially remove large pieces of slag.

[0035] Step S3: Control the melt temperature to 720±5℃, add pure zinc ingots until completely dissolved, then press 0.28wt% of composite refining agent into the bottom of the aluminum melt, maintain the temperature for 7 minutes after adding, then remove all slag from the surface of the melt, and then add pure magnesium ingots to dissolve.

[0036] Step S4: Control the melt temperature to 710±5℃, add grain refiner and stir for 5 minutes, then pour the melt into a preheated (300℃) low carbon steel mold to cast Φ80mm round ingots.

[0037] Process S5: The ingot is homogenized (465℃×24h), cooled in the furnace and the surface is removed by machining, then solution treated (475℃×2h, 60℃ water quenching), and finally subjected to T73 double-stage over-aging treatment (110℃×8h, 165℃×18h) to obtain fastener material in 7075-T73 condition.

[0038] Example 3: Fastener material was prepared based on 7075 aluminum alloy. The specific implementation process is as follows: I. Preparation of Compound Refining Agent Step A1: Dry nitrogen gas is introduced into the reactor, and N-benzylethylenediamine, 3-chloropropyltrimethoxysilane and anhydrous dioxane are added and premixed with stirring. Then potassium carbonate and a small amount of zeolite desiccant (about 5 wt% of the reaction system) are added. The temperature is raised to 75℃ and the reaction is stirred for 5.5 h. The ratio of N-benzylethylenediamine, 3-chloropropyltrimethoxysilane, potassium carbonate and anhydrous dioxane is 0.1 mol: 0.1 mol: 17 g: 350 mL. After the reaction is completed, the zeolite desiccant and insoluble salts are removed by filtration. Then, dioxane is removed by rotary evaporation under reduced pressure to obtain the modified coupling agent.

[0039] Step A2: Prepare a 55% (v / v) isopropanol aqueous solution, add boron nitride micro powder and ultrasonically disperse to form a uniform dispersion, add a modified coupling agent, stir and mix, and acidify with hydrochloric acid to a pH of 4. Heat to 50℃ and stir for 1.3 h. The boron nitride micro powder is a commercially available BN-S type micro powder raw material with an average particle size of about 25 μm. The ratio of boron nitride micro powder, modified coupling agent and isopropanol aqueous solution is 50 g: 7.7 g: 480 mL. Then neutralize with ammonia water and let stand for 6 h to precipitate. Take the precipitate, wash with water, combine the precipitates and dry to obtain the composite carrier.

[0040] Step A3: Prepare a 0.1 mol / L cerium nitrate aqueous solution, add the composite carrier, and control the solid-liquid mass ratio of the two to be 1:13. Use ultrasonic dispersion to form a uniform dispersion, then control the temperature in a water bath at 45℃ and stir for 11 hours. After centrifugation, collect the precipitate, dry it to constant weight, and send the dried product into an atmosphere roasting furnace. Evacuate to 10 Pa, introduce argon gas to a slightly positive pressure, and use two-stage roasting. In the first stage, the heating rate is 7℃ / min, the temperature is 380℃, and the holding time is 1.3 hours. In the second stage, the heating rate is 2℃ / min, the temperature is 750℃, and the holding time is 2.8 hours. Finally, cool it to 80℃ in the furnace and discharge it to obtain the composite refining agent.

[0041] II. Preparation of Fastener Materials Step S1: The calculated pure aluminum ingots, copper master alloy and chromium master alloy are loaded into a preheated graphite-clay crucible and placed in a medium-frequency induction melting furnace. The furnace charge is heated to 750±5℃ under an argon atmosphere to completely melt the furnace charge.

[0042] Step S2: After all the metal has melted, adjust the melt temperature to 730±10℃, and use a rotary jetting device to introduce high-purity argon gas (flow rate 1.3L / min, rotation speed 300rpm) for degassing treatment for 18 minutes. At the same time, sprinkle 0.4wt% of the melt with a covering agent for rough refining to initially remove large pieces of slag.

[0043] Step S3: Control the melt temperature to 720±5℃, add pure zinc ingots until completely dissolved, then press 0.32wt% of composite refining agent into the bottom of the aluminum melt, maintain the temperature for 8 minutes after adding, then remove all slag from the surface of the melt, and then add pure magnesium ingots to dissolve.

[0044] Step S4: Control the melt temperature to 710±5℃, add grain refiner and stir for 5 minutes, then pour the melt into a preheated (300℃) low carbon steel mold to cast Φ80mm round ingots.

[0045] Process S5: The ingot is homogenized (465℃×24h), cooled in the furnace and the surface is removed by machining, then solution treated (475℃×2h, 60℃ water quenching), and finally subjected to T73 double-stage over-aging treatment (110℃×8h, 165℃×18h) to obtain fastener material in 7075-T73 condition.

[0046] Example 4: Fastener material prepared based on 7075 aluminum alloy. The specific implementation process is as follows: I. Preparation of Compound Refining Agent Step A1: Dry nitrogen gas is introduced into the reactor, and N-benzylethylenediamine, 3-chloropropyltrimethoxysilane and anhydrous dioxane are added and premixed with stirring. Then potassium carbonate and a small amount of zeolite desiccant (about 5 wt% of the reaction system) are added. The temperature is raised to 80℃ and the reaction is stirred for 5 h. The ratio of N-benzylethylenediamine, 3-chloropropyltrimethoxysilane, potassium carbonate and anhydrous dioxane is 0.1 mol: 0.1 mol: 20 g: 330 mL. After the reaction is completed, the zeolite desiccant and insoluble salts are removed by filtration. Then, dioxane is removed by rotary evaporation under reduced pressure to obtain the modified coupling agent.

[0047] Step A2: Prepare a 55% (v / v) isopropanol aqueous solution, add boron nitride micro powder and ultrasonically disperse to form a uniform dispersion, add a modified coupling agent, stir and mix, and acidify with hydrochloric acid to pH 5. Heat to 60℃ and stir for 1.5 h. The boron nitride micro powder is a commercially available BN-SS type micro powder raw material with an average particle size of about 40 μm. The ratio of boron nitride micro powder, modified coupling agent and isopropanol aqueous solution is 50 g: 8.0 g: 500 mL. Then neutralize with ammonia water and let stand for 6 h to precipitate. Take the precipitate, wash with water, combine the precipitates and dry to obtain the composite carrier.

[0048] Step A3: Prepare a 0.12 mol / L cerium nitrate aqueous solution, add the composite carrier, and control the solid-liquid mass ratio of the two to be 1:15. Use ultrasonic dispersion to form a uniform dispersion, then control the temperature in a water bath at 40℃ and stir for 10 hours. After centrifugation, collect the precipitate, dry it to constant weight, and send the dried product into an atmosphere roasting furnace. Evacuate to 10 Pa, introduce argon gas to a slightly positive pressure, and use two-stage roasting. In the first stage, the heating rate is 5℃ / min, the temperature is 370℃, and the holding time is 1.2 hours. In the second stage, the heating rate is 2℃ / min, the temperature is 750℃, and the holding time is 3 hours. Finally, cool it to 80℃ in the furnace and discharge it to obtain the composite refining agent.

[0049] II. Preparation of Fastener Materials Step S1: The calculated pure aluminum ingots, copper master alloy and chromium master alloy are loaded into a preheated graphite-clay crucible and placed in a medium-frequency induction melting furnace. The furnace charge is heated to 750±5℃ under an argon atmosphere to completely melt the furnace charge.

[0050] Step S2: After all the metal has melted, adjust the melt temperature to 730±10℃, and use a rotary jetting device to introduce high-purity argon gas (flow rate 1.5L / min, rotation speed 300rpm) for degassing treatment for 18 minutes. At the same time, sprinkle 0.4wt% of the melt with a covering agent for rough refining to initially remove large pieces of slag.

[0051] Step S3: Control the melt temperature to 720±5℃, add pure zinc ingots until completely dissolved, then press 0.30wt% of composite refining agent into the bottom of the aluminum melt, maintain the temperature for 8 minutes after adding, then remove all slag from the surface of the melt, and then add pure magnesium ingots to dissolve.

[0052] Step S4: Control the melt temperature to 710±5℃, add grain refiner and stir for 5 minutes, then pour the melt into a preheated (300℃) low carbon steel mold to cast Φ80mm round ingots.

[0053] Process S5: The ingot is homogenized (465℃×24h), cooled in the furnace and the surface is removed by machining, then solution treated (475℃×2h, 60℃ water quenching), and finally subjected to T73 double-stage over-aging treatment (110℃×8h, 165℃×18h) to obtain fastener material in 7075-T73 condition.

[0054] Comparative Example 1 follows the same implementation process as Example 4, using HY-WJ5 general-purpose refining agent for refining at a dosage of 0.5 wt%, with the rest of the implementation process being exactly the same.

[0055] Comparative Example 2 follows the same implementation process as Example 4, using SY-NXJ1 rare earth refining agent for refining at a dosage of 0.35 wt%, with the rest of the implementation process being exactly the same.

[0056] Samples were taken from the fastener materials prepared above, and the tensile strength (Rm), yield strength (Rp0.2), and elongation after fracture (ε) were tested according to GB / T 228.1-2021 standard; the shear strength (T) was tested according to GB / T 6400-2007 standard; slow strain rate tensile testing (SSRT) was performed in 3.5% NaCl aqueous solution according to GB / T 15970.6-2007 standard, and the stress corrosion susceptibility index (ISSRT) was used to evaluate the stress corrosion resistance of the samples; axial tensile-compression fatigue tests were performed according to GB / T 3075-2021 standard under the conditions of stress ratio R=0.1 and maximum stress of 350MPa, and the fatigue fracture cycles (Nf) were recorded; the rare earth element content in the alloy was tested according to GB / T 20975.24-2020 standard. Specific test results are shown in Table 1. Table 1 The test results show that, compared with the comparative example using commercially available refining agents, the comprehensive performance of each embodiment using the composite refining agent of this invention is significantly improved. Specifically, strength and plasticity are improved simultaneously, indicating that the material is stronger and tougher; shear strength and fatigue life are greatly increased, which is crucial for fastener applications; and stress corrosion resistance is significantly improved, with ISSRT values ​​much lower than the comparative example, indicating that the potential of T73 treatment has been fully realized.

[0057] In the description of this specification, the references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0058] The above description is merely an example and illustration of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described, or use similar methods to replace them, as long as they do not deviate from the invention or exceed the scope defined in the claims, all of which should fall within the protection scope of the present invention.

Claims

1. A fastener material based on a high-strength alloy, specifically based on 7075-T73 aluminum alloy, characterized in that... A composite refining agent is introduced into the refining process for refining treatment; the preparation method of the composite refining agent is as follows: Step A1: N-benzylethylenediamine, 3-chloropropyltrimethoxysilane and anhydrous dioxane are premixed under a dry nitrogen atmosphere, potassium carbonate and zeolite desiccant are added and the mixture is heated to 70-85℃ and stirred for 4-6 hours to prepare a modified coupling agent. Step A2: Disperse boron nitride micro powder and isopropanol aqueous solution by ultrasonication, add modified coupling agent and mix well, adjust pH value to 4-5, heat to 50-60℃ and stir for 1-1.5h, then neutralize and let stand to precipitate, centrifuge to wash precipitate and dry to obtain composite carrier; Step A3: The composite carrier and cerium nitrate aqueous solution are ultrasonically dispersed, and then stirred in a water bath at a temperature of 35-50℃ for 8-12 hours. The precipitate is centrifuged, dried to constant weight, and then calcined in an argon atmosphere in multiple stages without oxygen. The composite refining agent is obtained by cooling in the furnace.

2. The fastener material based on high-strength alloy according to claim 1, characterized in that, The ratio of N-benzylethylenediamine, 3-chloropropyltrimethoxysilane, potassium carbonate, and anhydrous dioxane is 0.1 mol: 0.1 mol: 15-20 g: 300-400 mL.

3. The fastener material based on high-strength alloy according to claim 2, characterized in that, The ratio of boron nitride micro powder, modified coupling agent and isopropanol aqueous solution is 50g: 6.2-8.5g: 450-520mL.

4. The fastener material based on high-strength alloy according to claim 3, characterized in that, The average particle size of boron nitride micro powder is 20-40 μm.

5. The fastener material based on high-strength alloy according to claim 4, characterized in that, The concentration of the cerium nitrate aqueous solution is 0.1-0.12 mol / L, and the solid-liquid mass ratio of the composite carrier and the cerium nitrate aqueous solution is 1:12-15.

6. The fastener material based on high-strength alloy according to claim 5, characterized in that, The multi-stage anaerobic calcination adopts a two-stage calcination process, specifically: in the first stage, the heating rate is 5-8℃ / min, the temperature is 350-400℃, and the holding time is 1-1.3h; in the second stage, the heating rate is 1-5℃ / min, the temperature is 720-780℃, and the holding time is 2.5-3h.

7. The preparation process of fastener material based on high-strength alloy according to any one of claims 1-6, characterized in that, The process includes the following steps: Process S1: Pure aluminum ingots, copper master alloy and chromium master alloy are loaded into the furnace and melted to 750±5℃ under argon atmosphere; Step S2: Control the melt temperature to 730±10℃, introduce argon gas for degassing for 15-18 minutes, add covering agent and rough refining to remove slag; Process S3: Control the melt temperature to 720±5℃, add pure zinc ingots until completely dissolved, then press the composite refining agent into the bottom of the aluminum melt, remove the slag and add pure magnesium ingots to dissolve; Step S4: Control the melt temperature to 710±5℃, add grain refiner and stir to mix, then cast the melt into shape; Process S5: The ingot is subjected to homogenization treatment, solution treatment and T73 two-stage over-aging treatment in sequence to obtain fastener material.

8. The preparation process of the fastener material based on high-strength alloy according to claim 7, characterized in that, The amount of compound refining agent used is 0.24-0.32 wt% of the aluminum liquid.

Images