Thermodynamic calculation assisted design of preparation process of high-strength and high-corrosion-resistant copper alloy material

Abstract

The application belongs to the technical field of copper alloy material preparation, and discloses a preparation process of high-strength and high-corrosion-resistance copper alloy material assisted by thermodynamic calculation, which comprises the following steps: alloy composition design based on the CALPHAD method; vacuum smelting and ingot casting; homogenization heat treatment; hot plastic forming and online quenching and solid solution; precise cold drawing and straightening; multi-stage aging treatment; low-temperature stress relief annealing; and surface treatment. The content of nickel, silicon and tin is optimized through thermodynamic calculation to control the precipitation of Ni2Si phase, and the deformation aging sequence and online solid solution integrated process are adopted to obtain intracrystalline dispersed nanometer precipitated phase organization. The process realizes efficient preparation of large-size high-precision copper alloy pipe material, and cooperatively improves the strength and corrosion resistance of the material.

Description

Technical Field

[0001] This invention relates to the field of copper alloy material preparation technology, and more specifically, to a thermodynamic calculation-aided design process for preparing high-strength, high-corrosion-resistant copper alloy materials. Background Technology

[0002] Copper alloys are widely used in marine engineering, petrochemicals, shipbuilding, nuclear power equipment, and high-end heat exchangers due to their excellent electrical and thermal conductivity, machinability, and corrosion resistance. As modern industry moves towards higher parameters and extreme operating conditions, the performance requirements for copper alloy pipes are increasing. These requirements not only demand high strength to withstand high-pressure conditions but also excellent corrosion resistance to resist the erosion of harsh environments such as seawater, acidic media, and stress corrosion cracking.

[0003] Currently, research on high-performance copper alloys both domestically and internationally mainly focuses on alloy systems such as copper-nickel-silicon, copper-chromium-zirconium, and copper-tin-phosphorus. Among these, copper-nickel-silicon alloys (such as C70250 and C19010) have become a key research focus in the field of high-strength, high-conductivity copper alloys due to their combination of high strength, good electrical conductivity, and excellent resistance to stress relaxation. The strengthening mechanism of these alloys mainly relies on the nanoscale Ni2Si phase precipitated during aging treatment. By controlling the size, distribution, and volume fraction of the precipitated phase, the material strength can be significantly improved.

[0004] However, existing technologies still have the following technical shortcomings in the preparation of high-strength and high-corrosion-resistant copper alloy pipes:

[0005] First, alloy composition design relies on the traditional "trial and error" method, which is inefficient and makes it difficult to achieve synergistic optimization of multiple performance indicators. Traditional copper alloy composition design mainly relies on experience accumulation and extensive experimental screening, constantly adjusting the proportions of alloying elements and repeatedly smelting and testing performance to find the optimal composition range. This "trial and error" method is not only time-consuming and costly, but also makes it difficult to accurately predict the impact of the interaction between different alloying elements on phase transformation behavior, precipitation kinetics, and final performance. Especially when pursuing the goal of synergistic improvement of high strength and high corrosion resistance, it is difficult to achieve precise control of the proportions of elements such as Ni, Si, and Sn, so that the Ni2Si phase can be fully precipitated while avoiding the formation of continuous network precipitates at grain boundaries (which would deteriorate corrosion resistance). Traditional methods are difficult to achieve precise control.

[0006] Secondly, existing manufacturing processes suffer from redundant steps or improper sequence, affecting microstructure uniformity and performance stability. Typical copper alloy tubing manufacturing processes usually include melting and casting, homogenization annealing, hot extrusion molding, solution treatment, cold working, and aging treatment. However, many existing processes treat hot extrusion molding and solution treatment as two separate heating steps. That is, after homogenization annealing, the ingot is first heated for hot extrusion, and then the extruded billet is reheated to the solution temperature for solution treatment. This repeated heating not only increases energy consumption and production cycle but may also lead to abnormal grain growth, deteriorating the material's ductility, toughness, and corrosion resistance. Furthermore, some processes place aging treatment before cold drawing, causing the age-precipitated nanoscale Ni2Si strengthening phase to be cut or broken by dislocations during subsequent large-deformation cold drawing, weakening the precipitation strengthening effect and failing to fully realize the potential of synergistic strengthening through deformation aging.

[0007] Third, the forming and dimensional control technologies for large-size, high-precision tubes are still immature. For large-diameter copper alloy tubes with an outer diameter ≥80mm and a wall thickness of 5-15mm, achieving high-precision dimensional control (tolerance ≤0.25mm, eccentricity ≤6%) while ensuring uniform microstructure and properties is a technological bottleneck restricting their industrial application. Existing processes lack systematic optimization in cold drawing pass design, die matching, and straightening process control, often resulting in uneven tube wall thickness, excessive straightness, or excessive residual stress, affecting subsequent processing and service life.

[0008] Fourth, there is a contradiction in the matching and control of high strength and high corrosion resistance. The strength improvement of copper alloys usually relies on increasing the number of precipitates or refining the grains, but these microstructure control measures sometimes sacrifice the material's corrosion resistance. For example, the continuous distribution of precipitates at grain boundaries can induce localized galvanic corrosion and exacerbate the tendency for dezincification corrosion; while excessively high dislocation density can also increase corrosion-sensitive points. How to achieve a synergistic improvement in strength and corrosion resistance while obtaining high strength (tensile strength ≥ 520 MPa, yield strength ≥ 450 MPa) and controlling the maximum intergranular corrosion depth within the range of 500-750 μm is a current technical challenge.

[0009] Fifth, the application of thermodynamic calculation-aided design methods in the manufacturing process of copper alloy tubes is still lacking. Although the CALPHAD (phase diagram calculation) method has been widely used in the steel and high-temperature alloy fields, in the industrial manufacturing of copper alloy tubes, there is no technical solution that systematically couples thermodynamic calculations with all process parameters (homogenization temperature, solution temperature, aging regime, cold working deformation, etc.). Existing process parameters mostly rely on empirical settings and lack quantitative guidance based on phase change thermodynamics and precipitation kinetics, resulting in a narrow process window and insufficient stability.

[0010] In summary, developing a copper alloy material preparation process capable of precise composition design, optimized process flow, precise dimensional control, and synergistic improvement in strength and corrosion resistance is of great significance for meeting the urgent demand in the high-end equipment sector for ultra-large size, high precision, and high performance copper alloy tubing. To address the aforementioned technical problems, this invention provides a thermodynamic calculation-aided design process for preparing high-strength, high-corrosion-resistant copper alloy materials. The aim is to achieve efficient preparation and synergistic performance improvement of large-size copper alloy tubing through thermodynamic-guided composition design, optimized process step sequence, and precise control of processing and heat treatment regimes.

[0011] The above background information is provided only to aid in understanding the inventive concept and technical solution of this invention. It does not necessarily belong to the prior art of this patent application. In the absence of clear evidence that the above information was disclosed on the filing date of this patent application, the above background information should not be used to evaluate the novelty and inventiveness of this application. Summary of the Invention

[0012] The main objective of this invention is to propose a thermodynamic calculation-aided design process for the preparation of high-strength and high-corrosion-resistant copper alloy materials, in order to solve the technical problems of low R&D efficiency caused by the reliance on trial and error in alloy composition design, coarsening of microstructure and unstable performance due to redundant process flow, and difficulty in synergistically matching high strength and high corrosion resistance in the existing technologies.

[0013] To achieve the above technical objectives, the present invention adopts the following technical solution:

[0014] A thermodynamic calculation-aided design process for preparing high-strength, high-corrosion-resistant copper alloy materials includes the following steps:

[0015] S1. Thermodynamic phase diagram calculation and composition design: A thermodynamic database of copper alloy system is established based on the CALPHAD method. The effects of alloy element addition on solid solubility product, precipitation temperature and phase transformation driving force are calculated to determine the alloy composition range.

[0016] S2. Vacuum melting and refining: According to the alloy composition range determined in S1, copper, nickel, silicon and tin are placed in a vacuum induction melting furnace and melted under a vacuum degree ≤0.1Pa, and then cast to obtain alloy ingots.

[0017] S3. Homogenization heat treatment: The alloy ingot is heated to 30-80°C above the solution temperature for homogenization heat treatment.

[0018] S4. Thermoplastic forming and solution treatment: The homogenized ingot is heated to the hot working temperature range and a tube blank is prepared by extrusion or hot rolling process. The hot working deformation is controlled to be ≥70%. After forming, online quenching is performed immediately with a cooling rate of ≥80℃ / s to obtain a supersaturated solid solution structure.

[0019] S5. Precision drawing and straightening: The quenched tube is subjected to multiple precision cold drawing processes, combined with online straightening, to obtain the finished tube.

[0020] S6. Multi-stage aging treatment: The finished pipes are subjected to multi-stage aging treatment, including the first stage of low-temperature aging and the second stage of high-temperature aging.

[0021] S7. Stress-relief annealing: Low-temperature stress-relief annealing is performed on the aged pipe, with the annealing temperature lower than the aging start temperature of the alloy.

[0022] Preferably, in S1, the alloy composition by mass percentage includes: Ni: 4.0-6.0%, Si: 0.8-1.5%, Sn: 0.5-1.2%, with the balance being Cu and unavoidable impurities.

[0023] Preferably, in step S3, the heating temperature for homogenization heat treatment is 850-950℃, and the holding time is 8-16h; in step S4, the heating temperature for thermoplastic molding is 800-900℃, the extrusion ratio is 10-25, and the extrusion speed is controlled at 10-50mm / s.

[0024] Preferably, in step S4, the solution temperature is 750-850℃ and the holding time is 1-4h.

[0025] Preferably, in step S6, the multi-stage aging process specifically involves: a first-stage low-temperature aging process where the temperature is maintained at 300-400℃ for 1-4 hours, and a second-stage high-temperature aging process where the temperature is maintained at 400-500℃ for 2-8 hours.

[0026] Preferably, in step S5, the precision cold drawing is performed in 2-4 passes, with the elongation coefficient of each pass controlled at 1.2-1.5 and the total processing rate controlled at 40-70%. By controlling the drawing die and straightening process, the outer diameter of the finished tube is ≥80mm with a tolerance ≤0.25mm, the wall thickness is 5-15mm with a tolerance ≤0.25mm, and the eccentricity is ≤6% when the wall thickness is ≤6mm.

[0027] Preferably, in step S7, the stress-relief annealing temperature is 200-250℃, the holding time is 1-3 hours, and a protective atmosphere is used.

[0028] Preferably, the thermodynamic phase diagram calculation in S1 is specifically used to control the precipitation driving force of the Ni2Si phase during the aging process.

[0029] Preferably, in step S4, the wall thickness of the pipe blank is reserved for subsequent processing; in step S6, the average particle size of the precipitated phase after multi-stage aging treatment is ≤15nm.

[0030] Preferably, the preparation process further includes S8, surface treatment: pickling and passivation or surface polishing of the stress-relief annealed pipe to obtain a finished pipe with a surface roughness Ra≤1.6μm.

[0031] The beneficial effects of this invention compared to the prior art include:

[0032] I. Achieve precise design of alloy composition, overcoming the blindness of traditional trial and error.

[0033] Traditional methods rely on experience-based screening, which is time-consuming, costly, and makes it difficult to predict element interactions. This invention introduces CALPHAD thermodynamic phase diagram calculations, establishes a thermodynamic database for copper alloys, calculates the solid solubility product, precipitation temperature, and phase transformation driving force, and predetermines the type, volume fraction, and precipitation window of the target precipitate, enabling scientific composition prediction and improving design accuracy and R&D efficiency.

[0034] Second, optimize the process flow to avoid repeated heating that could lead to coarsening of the microstructure.

[0035] Traditional processes involve separate steps for thermoplastic forming and solution treatment, with two heating cycles leading to abnormal grain growth and reduced strength and corrosion resistance. This invention combines these two processes, performing online quenching immediately after thermoforming and utilizing residual heat to complete the solution treatment. This eliminates the need for secondary heating, avoids grain coarsening, preserves the supersaturated solid solution and fine substructure, simplifies the process, improves efficiency, and yields uniform, fine grains.

[0036] Third, rationally allocate the aging time and cold processing sequence to give full play to the synergistic effect of deformation and precipitation.

[0037] Traditional processes involve aging followed by cold drawing, which results in the strengthening phase being cut and broken by dislocations during subsequent deformation, weakening the strengthening effect. This invention places multi-stage aging after cold drawing. Cold drawing first introduces high-density dislocations, forming deformation energy storage. Then, aging is performed, and the dislocations provide non-uniform nucleation sites, making the precipitated phase finer and more uniform. This achieves a superposition of deformation strengthening and precipitation strengthening, resulting in a high strength and good plasticity balance.

[0038] IV. Achieve high-precision dimensional control for large-size pipes.

[0039] Controlling the dimensional accuracy of large-diameter pipes (outer diameter ≥ 80mm) is challenging. This invention employs multi-pass cold drawing, rationally matching the elongation coefficient and total processing rate to control material flow uniformity. Combined with online straightening, this effectively reduces wall thickness deviation and eccentricity. Ultimately, it achieves outer diameter tolerance and wall thickness tolerance ≤ 0.25mm, and eccentricity ≤ 6% when wall thickness ≤ 6mm, meeting the stringent requirements of high-end equipment.

[0040] V. Synergistic regulation of strength and corrosion resistance to solve the matching problem.

[0041] High strength and high corrosion resistance are often contradictory; continuous distribution of precipitates at grain boundaries accelerates intergranular corrosion. This invention controls the type and distribution of precipitates through thermodynamic calculations to avoid continuous network precipitates at grain boundaries; two-stage aging ensures the dispersion of strengthening phases within the grains, maintaining grain boundary purity; low-temperature stress-relief annealing after cold drawing eliminates residual tensile stress and reduces corrosion susceptibility. This achieves excellent corrosion resistance at high strength.

[0042] VI. Integrate thermodynamic calculations throughout the entire process to improve the robustness of the process window.

[0043] Existing technologies only use thermodynamic calculations in the composition design stage, and subsequent process parameters rely on experience, resulting in a narrow process window and large fluctuations. This invention integrates thermodynamic calculations throughout the homogenization, solution treatment, and aging processes. Based on the solution line temperature, re-dissolution conditions, nucleation driving force, and growth kinetics, process parameters are determined, making each process parameter based on physical mechanisms to form an organic whole. This results in a wide process window, strong adaptability, and facilitates stable control and large-scale production. Attached Figure Description

[0044] Figure 1 This is a metallographic microstructure of the copper alloy tube prepared in Example 1 of the present invention.

[0045] Figure 2 The image shows a uniform and dense microstructure obtained by immersing the copper alloy tubing prepared in Example 1 in a 3.5 wt.% NaCl solution for 30 days.

[0046] Figure 3 Curves showing the effect of heat treatment temperature on the tensile strength and maximum intergranular corrosion depth of the material to achieve homogenization;

[0047] Figure 4 This is a graph showing the effect of solution treatment temperature on the tensile strength and maximum intergranular corrosion depth of the material.

[0048] Figure 5 The graph shows the effect of the first-stage aging temperature on the tensile strength and maximum intergranular corrosion depth of the material.

[0049] Figure 6 The graph shows the effect of the second-stage aging temperature on the tensile strength and maximum intergranular corrosion depth of the material.

[0050] Figure 7 A graph showing the effect of the total machining rate of precision cold drawing on the tensile strength and elongation of the material;

[0051] Figure 8 A graph showing the effect of stress-relief annealing temperature on the tensile strength and maximum intergranular corrosion depth of the material;

[0052] Figure 9Curves showing the effect of heat treatment holding time on the tensile strength and maximum intergranular corrosion depth of the material in order to homogenize the heat treatment;

[0053] Figure 10 The graph shows the comparison of tensile strength and elongation between Examples 1-6 and Comparative Examples 1-6. Detailed Implementation

[0054] The present invention will be further described in detail below with reference to specific embodiments. It should be emphasized that the following description is merely exemplary and is not intended to limit the scope and application of the present invention.

[0055] This invention provides a thermodynamic calculation-aided design process for preparing high-strength, high-corrosion-resistant copper alloy materials, comprising the following steps:

[0056] 1. Thermodynamic phase diagram calculation and composition design

[0057] A thermodynamic database for the Cu-Ni-Si-Sn quaternary copper alloy system was established based on the CALPHAD phase diagram calculation method. This database includes thermodynamic interaction parameters, mixing enthalpy, excess free energy, solubility curves, and key thermodynamic parameters such as Gibbs free energy, formation enthalpy, phase transformation temperature, and precipitation driving force for various precipitated phases, including Ni2Si, Ni3Si, and Cu5Sn. Using thermodynamic calculation software such as Thermo-Calc and Pandat, the effects of different Ni, Si, and Sn element additions on the matrix solid solubility product, precipitate precipitation temperature, phase transformation driving force, precipitate volume fraction, and precipitation sequence were systematically calculated. This accurately predicted the precipitation window and precipitation behavior of the target strengthening phase Ni2Si, thereby scientifically determining the optimal alloy composition range that balances strength and corrosion resistance. Specifically, with the goal of controlling the precipitation driving force of the Ni2Si phase during the aging process, thermodynamic calculations ensured that the mass fraction of the Ni2Si phase in the room temperature microstructure was 1.5%–3.5%. Based on the calculation results, the alloy composition is determined by mass percentage as follows: nickel content is 4.0%-6.0%, silicon content is 0.8%-1.5%, tin content is 0.5%-1.2%, and the balance is copper and unavoidable impurities.

[0058] 2. Vacuum melting and refining

[0059] Based on the alloy composition range determined in step 1, high-purity copper, nickel, silicon, and tin are selected as raw materials and placed in a vacuum induction melting furnace. During the melting process, the vacuum level is controlled to not exceed 0.1 Pa to ensure that gases and low-melting-point impurities in the melt are fully removed. Refining is performed in the later stages of melting to further purify the melt, which is then poured into a mold to obtain an alloy ingot.

[0060] 3. Homogenization heat treatment

[0061] The alloy ingot obtained in step 2 is placed in a heat treatment furnace and heated to 30°C-80°C above the solution temperature for homogenization heat treatment. Specifically, the heating temperature is set to 850°C-950°C, and the holding time is set to 8-16 hours depending on the ingot size. The purpose of homogenization heat treatment is to eliminate dendritic segregation formed during the solidification process, promote the uniform distribution of solute elements in the matrix, and eliminate casting internal stress. After holding, the ingot is cooled in the furnace or rapidly water-cooled to room temperature.

[0062] 4. Thermoplastic molding and solution treatment

[0063] The homogenized ingot is heated to the hot working temperature range, specifically 800℃-900℃. The ingot is then processed into a tube blank using extrusion or hot rolling. When using extrusion, the extrusion ratio is controlled at 10-25, and the extrusion speed is controlled between 10mm / s and 50mm / s. The hot working deformation is controlled to be no less than 70% to ensure sufficient fragmentation and recrystallization of the as-cast structure. After thermoplastic forming, the tube blank is still at a high temperature and is immediately subjected to online quenching using high-pressure water quenching or gas quenching at a cooling rate of no less than 80℃ / s to obtain a supersaturated solid solution structure in the tube. The specific process parameters for solution treatment are: solution temperature 750℃-850℃, holding time 1h-4h. The wall thickness of the tube blank is reserved to allow for subsequent precision drawing.

[0064] 5. Precision drawing and straightening

[0065] The quenched tubes undergo multi-pass precision cold drawing. Precision cold drawing employs 2-4 passes, with the elongation coefficient of each pass controlled between 1.2-1.5, and the total processing rate controlled between 40%-70%. During the drawing process, an online straightening device is used to adjust the straightness of the tubes in real time. By controlling the geometric accuracy of the drawing dies and the straightening process parameters, the finished tubes meet the following dimensional requirements: outer diameter not less than 80mm with a tolerance not exceeding 0.25mm, wall thickness 5mm-15mm with a tolerance not exceeding 0.25mm, and when the tube wall thickness is not greater than 6mm, the eccentricity not exceeding 6%.

[0066] 6. Multi-level timeliness processing

[0067] The finished pipe from step 5 is placed in a heat treatment furnace for multi-stage aging treatment. The multi-stage aging treatment includes two stages: a first-stage low-temperature aging and a second-stage high-temperature aging. The first-stage low-temperature aging is carried out at 300℃-400℃ for 1-4 hours to promote the uniform nucleation of the precipitated phase. The second-stage high-temperature aging is carried out at 400℃-500℃ for 2-8 hours to allow the nucleated precipitated phase to grow to the target size. Through the synergistic effect of the two-stage aging, the co-precipitation of the precipitated phase within the grains and at grain boundaries is promoted, controlling the average particle size of the precipitated phase to not exceed 15nm, forming a diffusely distributed nanoscale Ni2Si strengthening phase.

[0068] 7. Stress-relief annealing

[0069] The precision-drawn tubes undergo low-temperature stress-relief annealing. The annealing temperature is set at 200℃-250℃, and the holding time is 1h-3h, using a protective atmosphere to prevent surface oxidation. The stress-relief annealing temperature is selected below the alloy's aging initiation temperature to avoid uncontrollable secondary precipitation during stress relief. This step eliminates residual stress introduced during cold drawing, stabilizing the tube's microstructure and dimensions.

[0070] 8. Surface treatment

[0071] After stress-relief annealing, the pipes undergo pickling and passivation or surface polishing to remove surface oxide scale and microcracks. The surface roughness Ra of the finished pipes after surface treatment should not exceed 1.6 μm to obtain good surface quality and corrosion resistance.

[0072] Technical principles of the invention:

[0073] The technical principle of this invention is based on the cross-integration of multiple disciplines such as alloy design theory, phase transformation thermodynamics, deformation strengthening and precipitation strengthening. The following explanation is from two aspects: raw material system design and process parameter control.

[0074] I. Mechanism of Action of the Raw Material System

[0075] 1. The role of the copper matrix: Copper accounts for more than 90% of the total alloy content and has excellent electrical and thermal conductivity as well as workability, making it an ideal matrix material. However, pure copper has relatively low strength (200-250 MPa) and needs to be strengthened through alloying elements. The face-centered cubic structure of copper is conducive to the formation of coherent or semi-coherent interfaces of precipitates, resulting in a good strengthening effect.

[0076] 2. Synergistic strengthening of nickel and silicon: Nickel and silicon form the intermetallic compound Ni₂Si, which has a strong thermodynamic driving force for precipitation. The synergistic effect is manifested in the following ways: solid solution strengthens the matrix by causing lattice distortion during solid solution treatment; during aging, an orthorhombic Ni₂Si phase is formed, which is coherent or semi-coherent with the matrix; Ni₂Si has high hardness, and dislocation movement needs to overcome the resistance of cutting or bypassing, thus improving strength; precipitation consumes nickel and silicon, purifying the matrix and helping to maintain conductivity.

[0077] 3. Synergistic effect of tin's solid solution strengthening and corrosion resistance: Tin has a larger atomic radius than copper. Solid solution causes lattice distortion, resulting in significant solid solution strengthening, which complements the strengthening effect of Ni₂Si precipitation. Tin forms a dense oxide film or passivation film in corrosive media, inhibiting dezincification corrosion and changing the corrosion mode from selective dissolution to uniform corrosion, controlling the dezincification depth to 500-750 μm. Simultaneously, tin inhibits continuous network precipitates at grain boundaries, preventing intergranular corrosion.

[0078] 4. Synergistic effect of nickel, silicon, and tin: Ni₂Si formed by nickel and silicon is the main strengthening phase, and the solid solution strengthening of tin is superimposed on it, resulting in strength far exceeding that of a single mechanism. In terms of corrosion resistance, nickel increases the electrode potential of the substrate, silicon forms a silicon oxide film to enhance passivation, and tin changes the structure of corrosion products to inhibit zinc dezincification. Thermodynamic calculations (CALPHAD) precisely determine the ratio of the three components: when nickel is 4.0-6.0%, silicon is 0.8-1.5%, and tin is 0.5-1.2%, the amount of Ni₂Si precipitation is ideal, and tin is fully dissolved without forming harmful tin compounds, achieving the best match between strength and corrosion resistance.

[0079] (1) Homogenization heat treatment: Select a temperature 30-80℃ (850-950℃) above the solution line. If the temperature is too low, dendrite segregation will not be eliminated, and subsequent hot working will easily result in defects; if the temperature is too high, the grains will grow. Hold for 8-16 hours to ensure sufficient diffusion of the solute.

[0080] (2) Thermoplastic molding and solution treatment integration: The molding temperature is 800-900℃, which is higher than the Ni2Si remelting temperature and lower than the copper matrix overheating temperature. The material is completely dissolved and has low deformation resistance. The hot deformation amount is ≥70%, which breaks the as-cast structure and obtains a fine recrystallized structure. The online quenching cooling rate is ≥80℃ / s, which quickly inhibits the re-precipitation of Ni2Si and retains the supersaturated solid solution to room temperature. If the cooling is slow, the precipitated phase will precipitate prematurely, which will weaken the age hardening.

[0081] (3) Precision cold drawing: 2-4 passes are used, with an elongation coefficient of 1.2-1.5 per pass. The upper limit of 1.5 avoids excessive deformation in a single pass, which may lead to cracking, while the lower limit of 1.2 ensures sufficient dislocation accumulation. The total machining rate is 40%-70%: below 40%, the deformation strengthening is insufficient and the dimensional accuracy is difficult to guarantee; above 70%, the work hardening is excessive, the plasticity decreases, and the residual stress is too high. This range is matched with stress-relief annealing.

[0082] (4) Multi-stage aging treatment: The first stage is low-temperature aging at 300-400℃ for 1-4 hours, which provides a strong nucleation driving force but weak diffusion, thus facilitating the acquisition of high-density, fine nuclei. The second stage is high-temperature aging at 400-500℃ for 2-8 hours, which promotes atomic diffusion and allows the precipitated phase to grow to the target size, eliminating lattice distortion. The two-stage aging avoids the problem of single-stage aging failing to simultaneously address nucleation and growth, controlling the average particle size of the precipitated phase to within 15 nm. When the precipitated phase size is less than the critical value, dislocation over-strengthening occurs; when it exceeds the critical value, it switches to bypass strengthening, with 15 nm being the optimal strengthening range.

[0083] (5) Stress-relief annealing: The temperature is 200-250℃, which is lower than the aging start temperature of the alloy. If the temperature is higher than this, uncontrollable secondary precipitation will be triggered, which will change the microstructure and properties; if the temperature is too low, the residual stress will not be fully eliminated, which will affect the dimensional stability and resistance to stress corrosion. Hold at this temperature for 1-3 hours to allow dislocations to recover, eliminate internal stress, and prevent recrystallization or precipitation.

[0084] (6) Deformation Aging Sequence Design: This invention arranges multi-stage aging after cold drawing, which is the opposite of the traditional sequence. The principle is: Cold drawing in a supersaturated solid solution state greatly increases the dislocation density, forming deformation energy storage and dislocation entanglement. These dislocation lines provide a large number of non-uniform nucleation sites for Ni2Si precipitation, resulting in a lower nucleation energy barrier and a higher nucleation rate, thus obtaining a finer and more uniform precipitate. At the same time, dislocations recover during aging to form subgrain boundaries, which themselves generate strengthening. The superposition of deformation strengthening and precipitation strengthening results in higher comprehensive mechanical properties.

[0085] (7) Precision dimensional control: outer diameter and wall thickness tolerance ≤ 0.25 mm, eccentricity ≤ 6%. Through multi-pass drawing dimensional accumulation control, the deformation amount of each pass is reasonably distributed to maintain uniform metal flow and avoid wall thickness deviation. Online straightening adjusts the straightening roller reduction and angle to eliminate bending deformation and ensure straightness.

[0086] In summary, this invention is based on clear scientific principles in all aspects, including raw material selection, component design, process flow, and parameter control. The various technical features are interconnected and work synergistically to achieve the stable preparation of high-strength and high-corrosion-resistant copper alloy pipes.

[0087] To make the present invention more fully disclosed, more specific embodiments are described below.

[0088] Example 1:

[0089] Thermodynamic phase diagram calculation and composition design: A thermodynamic database for copper alloys was established based on the CALPHAD method. The effects of alloying elements on solid solubility product, precipitation temperature, and phase transformation driving force were calculated to determine the alloy composition. By mass percentage, nickel 5.2%, silicon 1.1%, tin 0.8%, with the balance being copper and unavoidable impurities.

[0090] Vacuum melting and refining: High-purity copper, nickel, silicon and tin are placed in a vacuum induction melting furnace with a vacuum degree of 0.08 Pa. The alloy ingots are then cast after refining in the later stage of melting.

[0091] Homogenization heat treatment: The ingot is heated to 900℃ and held for 12 hours to fully eliminate dendritic segregation, and then rapidly water-cooled to room temperature.

[0092] Thermoplastic forming and solution treatment: The ingot is heated to 850℃, and a tube blank is prepared by extrusion process with an extrusion ratio of 18, an extrusion speed of 30 mm / s, and a deformation of 75%. After forming, it is immediately water-quenched online at a cooling rate of 100℃ / s to obtain a supersaturated solid solution structure. The solution treatment temperature is 800℃ and held for 2.5 hours, with the blank wall thickness leaving a machining allowance.

[0093] Precision drawing and straightening: Three-pass cold drawing is employed, with an elongation coefficient of 1.35 per pass and a total processing rate of 55%, combined with online straightening. The finished pipe has an outer diameter of 85mm (tolerance 0.18mm), a wall thickness of 10mm (tolerance 0.20mm), and an eccentricity of 4.5%.

[0094] Multi-stage aging treatment: The first stage of low-temperature aging is held at 350℃ for 2.5 hours, and the second stage of high-temperature aging is held at 450℃ for 5 hours, so that the Ni2Si phase is dispersed and precipitated, and the average particle size is controlled within 15nm.

[0095] Stress-relief annealing: Hold at 230℃ for 2 hours under nitrogen protection to eliminate residual stress.

[0096] Surface treatment: pickling and passivation to remove oxide scale, surface roughness Ra is 1.2μm.

[0097] The metallographic microstructure of the copper alloy tubing prepared in this embodiment is as follows: Figure 1 As shown, the microstructure of the corroded surface of the copper alloy pipe prepared in Example 1 after immersion in 3.5 wt.% NaCl solution for 30 days is as follows. Figure 2 As shown.

[0098] Example 2:

[0099] Thermodynamic phase diagram calculation and composition design: A thermodynamic database for copper alloys was established based on the CALPHAD method. The effects of alloying elements on solid solubility product, precipitation temperature, and phase transformation driving force were calculated to determine the alloy composition. By mass percentage, nickel 4.5%, silicon 0.9%, tin 0.6%, with the balance being copper and unavoidable impurities.

[0100] Vacuum melting and refining: High-purity copper, nickel, silicon and tin are placed in a vacuum induction melting furnace with a vacuum degree of 0.05 Pa. The materials are then refined in the later stage of melting and then cast to obtain alloy ingots.

[0101] Homogenization heat treatment: The ingot is heated to 870℃ and held for 16 hours to fully eliminate dendrite segregation, and then rapidly water-cooled to room temperature.

[0102] Thermoplastic forming and solution treatment: The ingot is heated to 820℃, and a tube blank is prepared by extrusion process with an extrusion ratio of 15, an extrusion speed of 25 mm / s, and a deformation of 72%. Immediately after forming, the blank is water-quenched online at a cooling rate of 90℃ / s to obtain a supersaturated solid solution structure. The solution treatment temperature is 780℃ and held for 3 hours, with the blank wall thickness pre-machining allowance.

[0103] Precision drawing and straightening: Four-pass cold drawing is employed, with an elongation coefficient of 1.25 per pass and a total processing rate of 48%, combined with online straightening. The finished pipe has an outer diameter of 92mm (tolerance 0.22mm), a wall thickness of 12mm (tolerance 0.18mm), and an eccentricity of 5.0%.

[0104] Multi-stage aging treatment: The first stage of low-temperature aging is held at 320℃ for 3.5 hours, and the second stage of high-temperature aging is held at 430℃ for 7 hours, so that the Ni2Si phase is dispersed and precipitated, and the average particle size is controlled within 15nm.

[0105] Stress-relief annealing: Hold at 210℃ for 3 hours under nitrogen protection to eliminate residual stress.

[0106] Surface treatment: Surface polishing removes oxide scale and microcracks, and the surface roughness Ra is 1.1μm.

[0107] Example 3:

[0108] Thermodynamic phase diagram calculation and composition design: A thermodynamic database for copper alloys was established based on the CALPHAD method. The effects of alloying elements on solid solubility product, precipitation temperature, and phase transformation driving force were calculated to determine the alloy composition. By mass percentage, nickel 5.8%, silicon 1.4%, tin 1.1%, with the balance being copper and unavoidable impurities.

[0109] Vacuum melting and refining: High-purity copper, nickel, silicon and tin are placed in a vacuum induction melting furnace with a vacuum degree of 0.10 Pa. The alloy ingots are then cast after refining in the later stage of melting.

[0110] Homogenization heat treatment: The ingot is heated to 940℃ and held for 8 hours to fully eliminate dendrite segregation, and then rapidly water-cooled to room temperature.

[0111] Thermoplastic forming and solution treatment: The ingot is heated to 890℃ and hot-rolled to prepare the pipe billet, with a hot rolling deformation of 78%. After forming, it is immediately gas quenched online at a cooling rate of 110℃ / s to obtain a supersaturated solid solution structure. The solution treatment temperature is 830℃ and held for 1.5 hours, with the billet wall thickness leaving machining allowance.

[0112] Precision drawing and straightening: Two-pass cold drawing is used, with an elongation coefficient of 1.45 per pass and a total processing rate of 65%, combined with online straightening. The finished pipe has an outer diameter of 105mm (tolerance 0.24mm), a wall thickness of 8mm (tolerance 0.22mm), and an eccentricity of 5.5%.

[0113] Multi-stage aging treatment: The first stage of low-temperature aging is held at 380℃ for 1.5h, and the second stage of high-temperature aging is held at 480℃ for 3h, so that the Ni2Si phase is dispersed and precipitated, and the average particle size is controlled within 15nm.

[0114] Stress-relief annealing: Hold at 245℃ for 1.5 hours under nitrogen protection to eliminate residual stress.

[0115] Surface treatment: pickling and passivation remove oxide scale and microcracks, with a surface roughness Ra of 1.4 μm.

[0116] Example 4:

[0117] Thermodynamic phase diagram calculation and composition design: A thermodynamic database for copper alloys was established based on the CALPHAD method. The effects of alloying elements on solid solubility product, precipitation temperature, and phase transformation driving force were calculated to determine the alloy composition. By mass percentage, nickel 5.0%, silicon 1.0%, tin 0.9%, with the balance being copper and unavoidable impurities.

[0118] Vacuum melting and refining: High-purity copper, nickel, silicon and tin are placed in a vacuum induction melting furnace with a vacuum degree of 0.06 Pa. The alloy ingots are then cast after refining in the later stage of melting.

[0119] Homogenization heat treatment: The ingot is heated to 920℃ and held for 10 hours to fully eliminate dendrite segregation, and then rapidly water-cooled to room temperature.

[0120] Thermoplastic forming and solution treatment: The ingot is heated to 860℃, and a tube blank is prepared by extrusion process with an extrusion ratio of 22, an extrusion speed of 40 mm / s, and a deformation of 80%. Immediately after forming, the blank is water-quenched online at a cooling rate of 95℃ / s to obtain a supersaturated solid solution structure. The solution treatment temperature is 810℃ and held for 2 hours, with the blank wall thickness leaving machining allowance.

[0121] Precision drawing and straightening: Three-pass cold drawing is employed, with an elongation coefficient of 1.30 per pass and a total processing rate of 52%, combined with online straightening. The finished pipe has an outer diameter of 88mm (tolerance 0.19mm), a wall thickness of 9mm (tolerance 0.21mm), and an eccentricity of 4.8%.

[0122] Multi-stage aging treatment: The first stage of low-temperature aging is held at 340℃ for 3 hours, and the second stage of high-temperature aging is held at 460℃ for 6 hours, so that the Ni2Si phase is dispersed and precipitated, and the average particle size is controlled within 15nm.

[0123] Stress-relief annealing: Hold at 220℃ for 2.5 hours under nitrogen protection to eliminate residual stress.

[0124] Surface treatment: Surface polishing removes oxide scale and microcracks, and the surface roughness Ra is 1.3μm.

[0125] Example 5:

[0126] Thermodynamic phase diagram calculation and composition design: A thermodynamic database for copper alloys was established based on the CALPHAD method. The effects of alloying elements on solid solubility product, precipitation temperature, and phase transformation driving force were calculated to determine the alloy composition. By mass percentage, nickel 4.2%, silicon 0.85%, tin 0.7%, with the balance being copper and unavoidable impurities.

[0127] Vacuum melting and refining: High-purity copper, nickel, silicon and tin are placed in a vacuum induction melting furnace with a vacuum degree of 0.09 Pa. The alloy ingots are then cast after refining in the later stage of melting.

[0128] Homogenization heat treatment: The ingot is heated to 860℃ and held for 14 hours to fully eliminate dendrite segregation, and then rapidly water-cooled to room temperature.

[0129] Thermoplastic forming and solution treatment: The ingot is heated to 830℃ and hot-rolled to prepare the pipe billet, with a hot rolling deformation of 73%. After forming, it is immediately gas quenched online at a cooling rate of 85℃ / s to obtain a supersaturated solid solution structure. The solution treatment temperature is 790℃ and held for 3.5 hours, with the billet wall thickness leaving machining allowance.

[0130] Precision drawing and straightening: Four-pass cold drawing is employed, with an elongation coefficient of 1.22 per pass and a total processing rate of 45%, combined with online straightening. The finished pipe has an outer diameter of 82mm (tolerance 0.17mm), a wall thickness of 14mm (tolerance 0.23mm), and an eccentricity of 4.2%.

[0131] Multi-stage aging treatment: The first stage of low-temperature aging is held at 310℃ for 4 hours, and the second stage of high-temperature aging is held at 420℃ for 8 hours, so that the Ni2Si phase is dispersed and precipitated, and the average particle size is controlled within 15nm.

[0132] Stress-relief annealing: Hold at 205℃ for 3 hours under nitrogen protection to eliminate residual stress.

[0133] Surface treatment: pickling and passivation remove oxide scale and microcracks, with a surface roughness Ra of 1.5 μm.

[0134] Example 6:

[0135] Thermodynamic phase diagram calculation and composition design: A thermodynamic database for copper alloys was established based on the CALPHAD method. The effects of alloying elements on solid solubility product, precipitation temperature, and phase transformation driving force were calculated to determine the alloy composition. By mass percentage, nickel 5.5%, silicon 1.2%, tin 1.0%, with the balance being copper and unavoidable impurities.

[0136] Vacuum melting and refining: High-purity copper, nickel, silicon and tin are placed in a vacuum induction melting furnace with a vacuum degree of 0.07 Pa. The alloy ingots are then cast after refining in the later stage of melting.

[0137] Homogenization heat treatment: The ingot is heated to 880℃ and held for 13 hours to fully eliminate dendrite segregation, and then rapidly water-cooled to room temperature.

[0138] Thermoplastic forming and solution treatment: The ingot is heated to 840℃, and a tube blank is prepared by extrusion process with an extrusion ratio of 20, an extrusion speed of 35 mm / s, and a deformation of 76%. Immediately after forming, the blank is water-quenched online at a cooling rate of 105℃ / s to obtain a supersaturated solid solution structure. The solution treatment temperature is 820℃ and held for 2 hours, with the blank wall thickness leaving machining allowance.

[0139] Precision drawing and straightening: Three-pass cold drawing is employed, with an elongation coefficient of 1.40 per pass and a total processing rate of 60%, combined with online straightening. The finished pipe has an outer diameter of 95mm (tolerance 0.20mm), a wall thickness of 11mm (tolerance 0.19mm), and an eccentricity of 5.2%.

[0140] Multi-stage aging treatment: The first stage of low-temperature aging is held at 360℃ for 2 hours, and the second stage of high-temperature aging is held at 440℃ for 4 hours, so that the Ni2Si phase is dispersed and precipitated, and the average particle size is controlled within 15nm.

[0141] Stress-relief annealing: Hold at 240℃ for 1.5 hours under nitrogen protection to eliminate residual stress.

[0142] Surface treatment: Surface polishing removes oxide scale and microcracks, with a surface roughness Ra of 1.2 μm.

[0143] Comparative Example 1:

[0144] The difference between this comparative example and Example 1 is that the S1 thermodynamic phase diagram calculation and composition design steps are omitted, and the alloy composition is determined by traditional empirical proportions, specifically 5.2% nickel, 1.1% silicon, 0.8% tin, with the balance being copper. All other steps and parameters remain the same as in Example 1.

[0145] Comparative Example 2:

[0146] The difference between this comparative example and Example 1 is that the order of the S6 multi-stage aging treatment and the S5 precision drawing has been changed. The S6 multi-stage aging treatment is performed before the S5 precision drawing, that is, the multi-stage aging treatment is performed first, followed by the precision cold drawing. All other steps and parameters are consistent with those of Example 1.

[0147] Comparative Example 3:

[0148] The difference between this comparative example and Example 1 is that the homogenization heat treatment parameters in S3 exceed the range defined by this invention, and a low-temperature long-time treatment is used, specifically a heating temperature of 820°C and a holding time of 24 hours. All other steps and parameters remain consistent with Example 1.

[0149] Comparative Example 4:

[0150] The difference between this comparative example and Example 1 is that in the S4 thermoplastic molding and solution treatment step, online quenching is omitted. Instead, the thermoplastic molded pipe is allowed to cool naturally to room temperature before being reheated to the solution temperature for independent solution treatment. The solution treatment parameters remain unchanged. All other steps and parameters are consistent with those in Example 1.

[0151] Comparative Example 5:

[0152] The difference between this comparative example and Example 1 is that the S6 multi-stage aging treatment uses a single-stage aging process instead of a two-stage aging process, and the specific process parameters are 400℃ for 7 hours. All other steps and parameters are consistent with Example 1.

[0153] Comparative Example 6:

[0154] The difference between this comparative example and Example 1 is that the tin content in the S1 composition design exceeds the range defined by this invention, specifically 2.0% tin, 5.2% nickel, 1.1% silicon, with the balance being copper. All other steps and parameters remain consistent with Example 1.

[0155] Single-factor experiment:

[0156] To determine the optimal process parameters for each key step in the preparation process of this invention, single-factor experiments were conducted on homogenization heat treatment temperature, solution treatment temperature, first-stage aging temperature, second-stage aging temperature, total machining rate of precision cold drawing, stress-relief annealing temperature, and homogenization heat treatment holding time. Each experiment used the process parameters of Example 1 as a baseline, changing only the target parameters while keeping the other parameters constant. The copper alloy tubing obtained from the experiments was subjected to performance testing according to the methods described later, with tensile strength and maximum intergranular corrosion depth as the main evaluation indicators.

[0157] Experiment 1: Effect of Homogenization Heat Treatment Temperature on Material Properties

[0158] This experiment aimed to determine the optimal temperature for homogenization heat treatment. The homogenization holding time was fixed at 12 hours, and other process parameters were consistent with those in Example 1. Five homogenization temperatures were selected: 850℃, 875℃, 900℃, 925℃, and 950℃, corresponding to groups A1-A5 respectively. The experimental results are shown in Table 1.

[0159] Table 1. Effect of homogenization heat treatment temperature on material properties

[0160]

[0161] Experimental results show that the effect of homogenization heat treatment temperature on the tensile strength and maximum intergranular corrosion depth of the material is illustrated in Figure 3. When the homogenization temperature is 900℃, the material exhibits the best overall performance, with a tensile strength of 545.3 MPa and a maximum intergranular corrosion depth of 82 μm. Below 900℃, the homogenization temperature is insufficient, dendritic segregation in the ingot is not fully eliminated, and the distribution of solute elements is uneven. This results in insufficient and unevenly distributed nucleation sites for precipitates during subsequent aging treatment, leading to poor strength and resistance to intergranular corrosion. Above 900℃, abnormal grain growth occurs. The coarse grains not only reduce the yield strength but also cause the precipitates to concentrate at the grain boundaries due to the reduced grain boundary area, increasing the susceptibility to intergranular corrosion and thus decreasing corrosion resistance. Therefore, the optimal homogenization heat treatment temperature is 900℃.

[0162] Experiment 2: Effect of solution treatment temperature on material properties

[0163] This experiment aimed to determine the optimal temperature for solution treatment. The solution treatment holding time was fixed at 2.5 h, and the online quenching cooling rate was 100 °C / s. Other process parameters were consistent with those in Example 1. Five solution treatment temperatures were selected: 750 °C, 775 °C, 800 °C, 825 °C, and 850 °C, corresponding to groups B1-B5 respectively. The experimental results are shown in Table 2.

[0164] Table 2 Effect of solution treatment temperature on material properties

[0165]

[0166] Experimental results show that the effect of solution treatment temperature on the tensile strength and maximum intergranular corrosion depth of the material is as follows: Figure 4As shown, the material exhibits optimal overall performance at a solution treatment temperature of 800℃, with a tensile strength of 545.3 MPa and a maximum intergranular corrosion depth of 82 μm. Below 800℃, the Ni₂Si phase fails to fully dissolve, and the remaining undissolved phase cannot effectively strengthen the material during subsequent aging. Furthermore, insufficient concentrations of dissolved nickel and silicon in the matrix lead to a decrease in the driving force for aging precipitation, resulting in unsatisfactory strength and resistance to intergranular corrosion. Above 800℃, grain coarsening occurs, and excessively high temperatures may cause tin to segregate at grain boundaries, forming localized low-melting-point regions. These regions generate microcracks during subsequent cooling, reducing the material's ductility, toughness, and resistance to intergranular corrosion. Therefore, the optimal solution treatment temperature is 800℃.

[0167] Experiment 3: Effect of the first-stage aging temperature on material properties

[0168] This experiment aimed to determine the optimal temperature for the first-stage aging treatment. The first-stage aging holding time was fixed at 2.5 hours, and the second-stage aging temperature was 450℃ with a holding time of 5 hours. Other process parameters were consistent with Example 1. Five levels of the first-stage aging temperature were selected: 300℃, 325℃, 350℃, 375℃, and 400℃, corresponding to groups C1-C5 respectively. The experimental results are shown in Table 3.

[0169] Table 3. Effect of first-stage aging temperature on material properties

[0170]

[0171] Experimental results show that the influence of the first-stage aging temperature on the tensile strength and maximum intergranular corrosion depth of the material is as follows: Figure 5 As shown, the material exhibits optimal overall performance at a first-stage aging temperature of 350℃, with a tensile strength reaching 545.3 MPa and a maximum intergranular corrosion depth of 82 μm. Below 350℃, atomic diffusion is weak, resulting in a low nucleation rate of the Ni₂Si phase and insufficient precipitate nuclei. This leads to larger and more unevenly distributed precipitates during subsequent high-temperature aging, limiting the strengthening effect and reducing corrosion resistance. Above 350℃, while the nucleation rate is higher, the precipitates begin to grow rapidly at low temperatures, exceeding the optimal strengthening range. Simultaneously, the proportion of precipitates at grain boundaries increases, reducing resistance to intergranular corrosion. Therefore, the optimal temperature for the first-stage aging treatment is 350℃.

[0172] Experiment 4: Effect of the second-stage aging temperature on material properties

[0173] This experiment aimed to determine the optimal temperature for the second-stage aging treatment. The first-stage aging temperature was fixed at 350℃ with a holding time of 2.5 hours, and the second-stage aging holding time was fixed at 5 hours. Other process parameters were consistent with Example 1. Five levels of the second-stage aging temperature were selected: 400℃, 425℃, 450℃, 475℃, and 500℃, corresponding to groups D1-D5 respectively. The experimental results are shown in Table 4.

[0174] Table 4. Effect of the second-stage aging temperature on material properties

[0175]

[0176] Experimental results show that the influence of the second-stage aging temperature on the tensile strength and maximum intergranular corrosion depth of the material is as follows: Figure 6 As shown, the material exhibits optimal overall performance at a second-stage aging temperature of 450℃, with a tensile strength of 545.3 MPa and a maximum intergranular corrosion depth of 82 μm. Below 450℃, atomic diffusion is insufficient, precipitated phase growth is inadequate, and some nucleation sites fail to grow to the effective strengthening size, resulting in incomplete strengthening and lower tensile strength. However, due to the small overall size of the precipitated phase and the low proportion of grain boundary precipitation, the intergranular corrosion depth is relatively smaller. Above 450℃, the precipitated phase coarsens, exceeding the critical size, and the dislocation strengthening mechanism changes from a cutting-through mechanism to a bypass mechanism, reducing the strengthening effect. Simultaneously, the coherent relationship between the coarse precipitated phase and the matrix interface is disrupted, leading to a continuous distribution of precipitated phases at grain boundaries, which become preferential corrosion channels, resulting in a significant deterioration in intergranular corrosion resistance. Therefore, 450℃ represents the optimal balance between strength and intergranular corrosion resistance, and is the optimal temperature for the second-stage aging treatment.

[0177] Experiment 5: The Influence of Total Machining Rate of Precision Cold Drawing on Material Properties

[0178] This experiment aimed to determine the optimal total machining rate for precision cold drawing. The drawing process was fixed at three passes, with the elongation coefficient evenly distributed across each pass. Other process parameters remained consistent with Example 1. Five levels of total machining rate were selected: 40%, 47.5%, 55%, 62.5%, and 70%, corresponding to groups E1-E5, respectively. The experimental results are shown in Table 5.

[0179] Table 5. Influence of Total Machining Rate in Precision Cold Drawing on Material Properties

[0180]

[0181] Experimental results show that the influence of the total machining rate of precision cold drawing on the tensile strength and elongation of the material is as follows: Figure 7As shown in the accompanying intergranular corrosion performance test, the material exhibits optimal overall performance when the total machining rate is 55%, with a tensile strength of 545.3 MPa, an elongation of 11.5%, and a maximum intergranular corrosion depth of 82 μm. When the total machining rate is below 55%, the strain strengthening effect is insufficient, the dislocation density is low, and the promoting effect on subsequent aging precipitation is limited, while dimensional accuracy is difficult to guarantee; although the plasticity is better, the strength and corrosion resistance do not reach the optimal level. When the total machining rate is above 55%, although the strain strengthening effect is enhanced and the tensile strength still increases slightly, the elongation decreases significantly. When the total machining rate reaches 70%, the elongation drops to 9.7%, which is lower than the conventional requirements for engineering applications; at the same time, the excessively high machining rate leads to excessive residual stress inside the material, increasing the risk of stress corrosion and intergranular corrosion, and the maximum intergranular corrosion depth increases. Therefore, the optimal total machining rate for precision cold drawing is 55%.

[0182] Experiment 6: Effect of stress-relief annealing temperature on material properties

[0183] This experiment aimed to determine the optimal temperature for stress-relief annealing. The annealing holding time was fixed at 2 hours, and other process parameters were consistent with those in Example 1. Five annealing temperatures were selected: 200℃, 212.5℃, 225℃, 237.5℃, and 250℃, corresponding to groups F1-F5 respectively. The experimental results are shown in Table 6.

[0184] Table 6. Effect of stress-relief annealing temperature on material properties

[0185]

[0186] Experimental results show that the influence of stress-relief annealing temperature on the tensile strength and maximum intergranular corrosion depth of the material is as follows: Figure 8 As shown, the material exhibits optimal overall performance at an annealing temperature of 225℃, with a tensile strength of 545.3 MPa and a maximum intergranular corrosion depth of 82 μm. Below 225℃, dislocation recovery is insufficient, residual stress is not completely eliminated, and significant internal stress exists within the material. This not only affects dimensional stability but also increases the susceptibility to stress corrosion cracking, leading to a greater intergranular corrosion depth. Above 225℃, the annealing temperature approaches the aging initiation temperature of this alloy system, potentially triggering uncontrollable secondary precipitation, altering the material's microstructure, and causing a decrease in strength. Furthermore, excessively high temperatures can cause large-scale dislocation annihilation, resulting in excessive loss of strain strengthening effect, increased grain boundary precipitates, and deterioration of corrosion resistance. Therefore, the optimal stress-relief annealing temperature is 225℃.

[0187] Experiment 7: Effect of Homogenization Heat Treatment Holding Time on Material Properties

[0188] This experiment aimed to determine the optimal holding time for homogenization heat treatment. The homogenization temperature was fixed at 900℃, and other process parameters were consistent with those in Example 1. Five holding times were selected: 8h, 10h, 12h, 14h, and 16h, corresponding to groups G1-G5, respectively. The experimental results are shown in Table 7.

[0189] Table 7. Effect of Homogenization Heat Treatment Holding Time on Material Properties

[0190]

[0191] Experimental results show that the effect of homogenization heat treatment holding time on the tensile strength and maximum intergranular corrosion depth of the material is as follows: Figure 9 As shown, the material exhibits optimal overall performance with a holding time of 12 hours, achieving a tensile strength of 545.3 MPa and a maximum intergranular corrosion depth of 82 μm. When the holding time is less than 12 hours, solute atom diffusion is insufficient, dendritic segregation is not completely eliminated, and compositional inhomogeneity leads to uneven distribution of aging precipitates, resulting in unsatisfactory strength and resistance to intergranular corrosion. When the holding time exceeds 12 hours, grain coarsening occurs to some extent, and prolonged high-temperature exposure may cause some low-melting-point impurities to accumulate at grain boundaries, weakening grain boundary bonding and causing a slight decrease in strength and corrosion resistance. Therefore, the optimal holding time for homogenization heat treatment is 12 hours.

[0192] Performance testing:

[0193] The copper alloy tubing prepared according to this invention was subjected to performance testing according to the following testing methods:

[0194] Tensile strength and yield strength testing: The test shall be conducted in accordance with GB / T228.1-2021 "Metallic materials - Tensile testing - Part 1: Test at room temperature", using standard tensile specimens, with a tensile speed of 5 mm per minute, and the tensile strength Rm and the specified plastic elongation strength Rp0.2 shall be determined.

[0195] Hardness testing: The test was conducted in accordance with GB / T231.1-2018 "Metallic materials - Brinell hardness test - Part 1: Test method". A cemented carbide ball with a diameter of 5 mm was used, the test force was 750 kgf, and the holding time was 15 s. The Brinell hardness value was measured and converted to HRB.

[0196] Elongation test: conducted in accordance with GB / T228.1-2021, using a specimen with a gauge length of 50 mm, and measuring the elongation after fracture A50 mm.

[0197] Maximum intergranular corrosion depth detection: The test was conducted according to the sulfuric acid-copper sulfate boiling method specified in GB / T4334-2020 "Corrosion of metals and alloys - Test method for intergranular corrosion of stainless steel". The sample was mounted and sealed with cold-mounted resin, exposing only the specified test working surface, and placed in the copper sulfate-sulfuric acid test solution. It was immersed in the solution at a constant temperature for 16 hours under slight boiling reflux. After immersion, the sample was removed, rinsed with deionized water and dried with cold air. The sample was then ground and polished along the cross-section perpendicular to the corrosion surface. The maximum intergranular corrosion depth of the sample cross-section was measured under a metallographic optical microscope.

[0198] The performance test results of Examples 1-6 and Comparative Examples 1-6 are shown in Table 8.

[0199] Table 8 Performance test results of Examples 1-6 and Comparative Examples 1-6

[0200]

[0201] Performance test results comparison and analysis:

[0202] Comparison of tensile strength and elongation properties of various embodiments of the present invention with comparative examples: Figure 10 As shown.

[0203] 1. Comparison of tensile strength

[0204] Example 1 achieved a strength of 545.3 MPa, Example 6 540.5 MPa, Example 3 538.1 MPa, Example 4 535.6 MPa, Example 2 528.7 MPa, and Example 5 523.8 MPa. Examples 1, 6, and 3 are in the higher range, while Example 5 is relatively lower. Comparative Examples 1 to 4 achieved strengths of 496.3, 513.2, 506.5, and 519.4 MPa, respectively, all lower than all examples. Comparative Example 5 achieved a strength of 521.1 MPa, slightly lower than Example 5. Comparative Example 6 achieved a strength of 536.2 MPa, between that of Examples 4 and 3, but its elongation was significantly lower.

[0205] 2. Comparison of yield strength

[0206] The trend is completely consistent with the tensile strength. Example 1 is 475.6 MPa, Example 6 is 471.2 MPa, Example 3 is 468.4 MPa, Example 4 is 465.3 MPa, Example 2 is 458.2 MPa, and Example 5 is 453.5 MPa. Comparative Examples 1-4 are all below 455 MPa, Comparative Example 5 is 451.0 MPa, and Comparative Example 6 is 466.5 MPa.

[0207] 3. Hardness Comparison

[0208] Example 1 was 89.8 HRB, Example 6 was 89.1 HRB, Example 3 was 88.1 HRB, Example 4 was 87.6 HRB, Example 2 was 86.5 HRB, and Example 5 was 85.8 HRB. Comparative Examples 1 to 3 were all below 85 HRB, Comparative Example 4 was 86.0 HRB, Comparative Example 5 was 86.7 HRB, and Comparative Example 6 was 88.0 HRB.

[0209] 4. Comparison of elongation rates

[0210] Example 5 had a yield of 12.1%, Example 2 had a yield of 11.9%, Example 1 had a yield of 11.5%, Example 4 had a yield of 11.3%, Example 6 had a yield of 10.9%, and Example 3 had a yield of 10.6%. Comparative Examples 1, 3, and 6 had yields of 9.6%, 9.9%, and 8.6%, respectively, which were significantly lower than all examples. Comparative Examples 2, 4, and 5 had yields of 10.2%, 10.4%, and 10.5%, respectively, which were slightly lower than Example 3.

[0211] 5. Comparison of maximum intergranular corrosion depth

[0212] Example 1, with a diameter of 82 μm, exhibited the best resistance to intergranular corrosion. Example 3, with 89 μm, Example 6, with 93 μm, and Example 4, with 97 μm, were all at excellent levels. Example 2, with 118 μm, and Example 5, with 132 μm, showed a decrease in corrosion resistance with adjustments to composition and process. Comparative Example 1, with a diameter of 215 μm, was significantly higher than all examples, exhibiting the worst resistance to intergranular corrosion. Comparative Example 3, with 186 μm, remained at a relatively high level. Comparative Examples 2, 4, and 5, with diameters of 116 μm, 142 μm, and 105 μm respectively, were generally inferior to the best performance of the example groups. Comparative Example 6, with a diameter of 88 μm, was close to that of Example 1, but this was due to excessive tin enrichment at grain boundaries altering the corrosion behavior. This resulted in a sharp decrease in plasticity and a risk of grain boundary brittleness, which could actually reduce the long-term reliability of the material.

[0213] 6. Performance Correlation and Process Difference Analysis

[0214] Example 1 achieved the best performance among all samples in all four core indicators: tensile strength, yield strength, hardness, and resistance to intergranular corrosion. Its elongation was also in the upper-middle range. This was attributed to its balanced combination of process parameters, which achieved synergistic optimization of precipitation strengthening, deformation strengthening, and corrosion resistance. Example 5 had the highest elongation but the lowest strength. Its process employed low-temperature long-term homogenization, low-temperature long-term aging, and a low total drawing rate, resulting in insufficient precipitation strengthening and deformation strengthening, and neither strength nor corrosion resistance reached its optimal level. Example 3 had high strength but a low elongation. Its process employed high-temperature short-term homogenization, high-temperature short-term aging, and a high total processing rate, resulting in sufficient precipitation of the strengthening phase but a decrease in plasticity reserve.

[0215] Comparative Examples 1-6, by altering or omitting key process steps, all exhibited varying degrees of performance degradation. Comparative Example 1 omitted thermodynamically-assisted composition design, resulting in elemental mismatch, a tendency for continuous grain boundary precipitates, and overall performance degradation. Comparative Example 2 placed multi-stage aging before cold drawing; the strengthening phases precipitated during aging were fragmented by dislocations during subsequent large deformations, significantly weakening the strengthening effect, resulting in noticeably lower strength, and uneven precipitate distribution also led to decreased corrosion resistance. Comparative Example 3 had an excessively low homogenization temperature, leaving residual dendrite segregation in the ingot, uneven solute element distribution, and relatively concentrated grain boundary precipitates during subsequent aging, resulting in poor strength and resistance to intergranular corrosion. Comparative Example 4 eliminated the online quenching process, reheating after hot forming for independent solution treatment; this secondary heating caused grain coarsening and increased grain boundary precipitation, leading to overall performance degradation. Comparative Example 5 used single-stage aging instead of double-stage aging. The nucleation and growth of precipitated phases were not controlled in stages, resulting in poor size uniformity, increased grain boundary precipitation, limited strengthening effect, and lower corrosion resistance compared to the double-stage aging process. Comparative Example 6 had an excessively high tin content. While this reduced the intergranular corrosion depth to some extent, the excessive tin triggered the precipitation of brittle grain boundary phases, leading to a sharp decrease in plasticity and a significant reduction in the material's service reliability.

[0216] 7. Overall Conclusion

[0217] The performance differences among the embodiments reflect the control of strength, plasticity, and corrosion resistance by different process parameters, with Embodiment 1 showing the most balanced overall performance. The performance of each comparative example is inferior to all embodiments, demonstrating the necessity and synergistic effect of the various technical features of this invention, and indicating substantial progress in the technical solution.

[0218] The above description, in conjunction with specific embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various substitutions or modifications can be made to these described embodiments without departing from the inventive concept, and all such substitutions or modifications should be considered within the scope of protection of the present invention.

[0219] Although the invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and modifications can be made without departing from the spirit and scope of the invention. Furthermore, the scope of the invention is not limited to the specific embodiments of the processes, methods, and steps described in the specification. From the disclosure of this invention, those skilled in the art will readily utilize existing or future processes, methods, steps that substantially perform the same function or achieve the same results as the corresponding embodiments described herein. Therefore, the appended claims are intended to cover such processes, methods, steps.

Claims

1. A thermodynamic calculation-aided design process for preparing high-strength, high-corrosion-resistant copper alloy materials, characterized in that, Includes the following steps: S1. Thermodynamic phase diagram calculation and composition design: A thermodynamic database of copper alloy system is established based on the CALPHAD method. The effects of alloy element addition on solid solubility product, precipitation temperature and phase transformation driving force are calculated to determine the alloy composition range. S2. Vacuum melting and refining: According to the alloy composition range determined in S1, copper, nickel, silicon and tin are placed in a vacuum induction melting furnace and melted under a vacuum degree ≤0.1Pa, and then cast to obtain alloy ingots. S3. Homogenization heat treatment: The alloy ingot is heated to 30-80°C above the solution temperature for homogenization heat treatment. S4. Thermoplastic forming and solution treatment: The homogenized ingot is heated to the hot working temperature range and a tube blank is prepared by extrusion or hot rolling process. The hot working deformation is controlled to be ≥70%. After forming, online quenching is performed immediately with a cooling rate of ≥80℃ / s to obtain a supersaturated solid solution structure. S5. Precision drawing and straightening: The quenched tube is subjected to multiple precision cold drawing processes, combined with online straightening, to obtain the finished tube. S6. Multi-stage aging treatment: The finished pipes are subjected to multi-stage aging treatment, including the first stage of low-temperature aging and the second stage of high-temperature aging. S7. Stress-relief annealing: Low-temperature stress-relief annealing is performed on the aged pipe, with the annealing temperature lower than the aging start temperature of the alloy.

2. The thermodynamic calculation-aided design process for preparing high-strength, high-corrosion-resistant copper alloy materials according to claim 1, characterized in that, In S1, the alloy composition by mass percentage includes: Ni: 4.0-6.0%, Si: 0.8-1.5%, Sn: 0.5-1.2%, with the balance being Cu and unavoidable impurities.

3. The thermodynamic calculation-aided design process for preparing high-strength, high-corrosion-resistant copper alloy materials according to claim 1, characterized in that, In step S3, the heating temperature for homogenization heat treatment is 850-950℃, and the holding time is 8-16h; in step S4, the heating temperature for thermoplastic molding is 800-900℃, the extrusion ratio is 10-25, and the extrusion speed is controlled at 10-50mm / s.

4. The thermodynamic calculation-aided design process for preparing high-strength, high-corrosion-resistant copper alloy materials according to claim 1, characterized in that, In S4, the solution temperature is 750-850℃ and the holding time is 1-4h.

5. The thermodynamic calculation-aided design process for preparing high-strength, high-corrosion-resistant copper alloy materials according to claim 1, characterized in that, In S6, the multi-stage aging process is as follows: the first stage of low-temperature aging is maintained at 300-400℃ for 1-4 hours, and the second stage of high-temperature aging is maintained at 400-500℃ for 2-8 hours.

6. The thermodynamic calculation-aided design process for preparing high-strength, high-corrosion-resistant copper alloy materials according to claim 1, characterized in that, In S5, precision cold drawing is performed in 2-4 passes, with the elongation coefficient of each pass controlled at 1.2-1.5 and the total processing rate controlled at 40-70%. By controlling the drawing die and straightening process, the outer diameter of the finished tube is ≥80mm with a tolerance ≤0.25mm, the wall thickness is 5-15mm with a tolerance ≤0.25mm, and the eccentricity is ≤6% when the wall thickness is ≤6mm.

7. The thermodynamic calculation-aided design process for preparing high-strength, high-corrosion-resistant copper alloy materials according to claim 1, characterized in that, In step S7, the stress-relief annealing temperature is 200-250℃, the holding time is 1-3h, and a protective atmosphere is used.

8. The thermodynamic calculation-aided design process for preparing high-strength, high-corrosion-resistant copper alloy materials according to claim 1, characterized in that, The thermodynamic phase diagram calculation in S1 is specifically used to control the precipitation driving force of the Ni2Si phase during the aging process.

9. The thermodynamic calculation-aided design process for preparing high-strength, high-corrosion-resistant copper alloy materials according to claim 1, characterized in that, In S4, the wall thickness of the pipe blank is reserved for subsequent processing; in S6, the average particle size of the precipitated phase after multi-stage aging treatment is ≤15nm.

10. The thermodynamic calculation-aided design process for preparing high-strength, high-corrosion-resistant copper alloy materials according to claim 1, characterized in that, The preparation process also includes S8, surface treatment: pickling and passivation or surface polishing of the stress-relief annealed pipe to obtain a finished pipe with a surface roughness Ra≤1.6μm.

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