Automobile air conditioner refrigerant conveying aluminum pipe and preparation method thereof

Abstract

The present application relates to aluminum alloy material, automobile air conditioner parts manufacturing technology field, specifically belongs to a kind of automobile air conditioner refrigerant conveying aluminum pipe, the present application also provides the preparation method of the above-mentioned automobile air conditioner refrigerant conveying aluminum pipe.The present application adopts the step-by-step process of continuous casting→ accurate continuous extrusion→ combined drawing→ online heat treatment→ surface treatment, each process is independently regulated and accurately connected, and its purpose is to avoid oxidation and pollution caused by process connection through the parameter coordination of segmented process, and to optimize the performance of each stage: continuous casting guarantees the homogenization of casting blank, continuous extrusion improves the density of pipe material, combined drawing optimizes dimensional accuracy, online heat treatment accurately controls organization, finally realizes the synchronous improvement of material mechanical properties, corrosion resistance and formability.If a step is omitted or the order of process is disturbed, it will lead to an increase in pipe material organization defects, too large performance fluctuation, and cannot adapt to air conditioner pipeline working condition.

Description

Technical Field

[0001] This invention relates to the fields of aluminum alloy materials and automotive air conditioning component manufacturing technology, specifically to an aluminum pipe for delivering automotive air conditioning refrigerant and its preparation method. Background Technology

[0002] As the automotive industry upgrades towards lightweighting and energy efficiency, automotive air conditioning refrigerant delivery pipes, as core pressure-bearing components of the system, must withstand long-term corrosion from refrigerants such as R134a and R744, as well as high-temperature and high-pressure conditions of 80-150℃. They must also possess sufficient mechanical strength, good bending formability, and cost-effectiveness. 6xxx series aluminum alloys, with their controllable Mg2Si strengthening phase and advantages in corrosion resistance, formability, and cost, have become one of the preferred materials for automotive air conditioning pipes. However, their performance is easily affected by the manufacturing process, making it difficult to fully adapt to the demanding operating conditions.

[0003] Currently, 6xxx series aluminum alloy pipes used for automotive air conditioning refrigerant transportation are generally produced using a step-by-step discrete process of "melting and casting - extrusion molding - drawing and straightening - offline heat treatment - surface treatment". This model has significant technical bottlenecks: First, the processes are independently separated with large gaps between them. When the billet is extruded and drawn, it is easy to generate surface oxide scale and secondary pollution. Subsequent heat treatment is difficult to eliminate the problems of microstructure segregation and coarse grains, resulting in large fluctuations in the mechanical properties of the pipe and insufficient resistance to refrigerant corrosion. Long-term use is prone to leakage, deformation and failure. Second, the parameters of each process lack coordinated design. The extrusion temperature, drawing deformation and heat treatment regime are controlled separately, which easily leads to contradictions such as "strength meets the standard but plasticity is insufficient" or "formability is good but heat resistance is poor", which cannot meet the stringent requirements of automotive air conditioning pipelines for comprehensive performance. Third, the existing composition design focuses more on the ratio of Si and Mg main elements, and the control of trace grain refinement and corrosion-resistant elements is insufficient, making it difficult to achieve performance breakthroughs through composition-process coordination.

[0004] Existing technologies for addressing the aforementioned issues often have limitations: for example, adjusting solution treatment and aging parameters to improve the strength of 6xxx series aluminum alloy tubes still employs offline heat treatment and a step-by-step operation mode, failing to fundamentally improve microstructure uniformity and production efficiency; while optimizing continuous extrusion process parameters, the alloy composition and the details of each process connection are not simultaneously optimized, making it difficult to balance corrosion resistance and dimensional accuracy; some technologies improve performance by adding trace alloying elements, but fail to design composition and process adaptation schemes specifically for automotive air conditioning refrigerant conditions, resulting in insignificant performance improvements and hindering large-scale application. In summary, existing step-by-step processes and composition designs cannot simultaneously optimize the strength, corrosion resistance, heat resistance, and formability of 6xxx series aluminum alloy tubes, necessitating a dedicated material and preparation method suitable for automotive air conditioning refrigerant delivery scenarios. Summary of the Invention

[0005] This invention provides an aluminum pipe for delivering automotive air conditioning refrigerant, in order to meet the harsh operating conditions of automotive air conditioning refrigerant delivery.

[0006] Meanwhile, the present invention also provides a method for preparing the above-mentioned aluminum pipe for transporting automotive air conditioning refrigerant.

[0007] An aluminum pipe for delivering refrigerant to an automotive air conditioner, the raw materials of which include a matrix phase and a reinforcing phase, wherein the matrix phase is Al and the reinforcing phase is Mg2Si phase, and the material also contains trace grain refining and corrosion resistance optimizing elements; wherein Ti and Zr are grain refining elements, and Mn and Cr are corrosion resistance optimizing elements.

[0008] Preferably, the aluminum pipe for transporting automotive air conditioning refrigerant has the following elemental composition by weight percentage: Si 0.9%–1.1%, Mg 0.8%–1.0%, with a Si to Mg weight ratio of 2.2:1–3.0:1, Zr 0.12%–0.18%, Mn 0.04%–0.07%, Cr 0.02%–0.05%, Ti 0.05%–0.10%, and impurity elements Fe≤0.30%, Cu≤0.08%, Zn≤0.08%, with individual impurities ≤0.04%, total impurities ≤0.12%, and the balance being Al.

[0009] The above-mentioned method for preparing the aluminum pipe for transporting refrigerant in automotive air conditioning includes the following steps: Step 1: Melting and Billet Preparation Aluminum alloy raw materials are fed into smelting equipment, and after smelting, refining and degassing, purified aluminum alloy liquid is obtained; during the smelting process, argon inert gas is introduced for protection to prevent oxidation of the alloy liquid; the purified aluminum alloy liquid is sent to the continuous casting unit to obtain homogeneous billets. Step 2: Continuous extrusion molding The billet obtained in step 1 is fed into a continuous extrusion device, and the extrusion temperature and speed are controlled to produce a crude aluminum tube. Step 3: Joint pulling and straightening The rough aluminum tube obtained in step 2 is subjected to a combined drawing process, which consists of 2 to 3 passes of room temperature drawing, with a deformation of 10% to 15% per pass and a total deformation of 23% to 25%. The tube dimensions and roundness are then corrected using a straightening device. Step 4: Online heat treatment strengthening The aluminum tubes drawn in step 3 are subjected to online solution treatment and graded aging treatment in sequence. The graded aging treatment includes pre-aging and final aging. Pre-aging can suppress the precipitation of coarse phases, and final aging can improve the stability of the strengthening phase and avoid performance degradation in the later stage. Step 5: Surface treatment and performance testing feedback After surface passivation treatment of the aluminum tube after heat treatment in step 4, the parameters are adjusted based on performance test feedback, and the finished automotive air conditioning refrigerant delivery aluminum tube is obtained after meeting the standards.

[0010] Preferably, in step 1, the refining agent is an Al-Ti-B refining agent, and the addition amount is 0.15% to 0.20% of the alloy liquid weight; online degassing adopts a rotary jet argon gas method with an argon gas flow rate of 0.3 to 0.5 m³ / h, a degassing time of 15 to 20 min, and an aluminum alloy liquid gas content ≤ 0.15 ml / 100 g Al; the refining holding time is 20 to 30 min, which can simultaneously achieve the dual effects of refining and impurity removal and grain refinement.

[0011] Preferably, in step 2, the continuous extrusion temperature is 480–510°C, the extrusion speed is 0.8–1.2 m / min, the continuous extrusion die preheating temperature is 480–490°C, the extrusion ratio is 10–14:1, nitrogen protection is used during the extrusion process, and die preheating can prevent the aluminum tube from sticking to the die and improve the surface finish.

[0012] Preferably, in step 4, the online solution treatment uses a continuous heating furnace, the online solution treatment temperature is 520-540℃, and the holding time is 1-2h; the temperature uniformity inside the furnace is ±5℃; after holding, tap water is used for cooling, and the cooling rate is ≥25℃ / s, and rapid cooling ensures the uniformity of the overall structure of the aluminum tube; the graded aging uses a segmented aging furnace, the pre-aging temperature is 120-140℃, and the time is 4-6h; the final aging temperature is 160-180℃, and the time is 2-4h.

[0013] Preferably, in step 5, the performance testing feedback includes three core indicators: mechanical properties (tensile strength, elongation), corrosion resistance (salt spray corrosion test, refrigerant immersion test), and dimensional accuracy (pipe diameter, roundness, wall thickness). If the tensile strength is lower than 330 MPa, the final aging temperature can be adjusted to 175–180°C, and the holding time extended by 0.5–1 h. If the corrosion rate exceeds the standard, the passivation process is adjusted as follows: increase the concentration of the chromate main salt by 5%–15%, increase the passivation temperature by 5–10°C, extend the passivation immersion time by 20%–40%, and add a 60–70°C hot water sealing process. The alloy composition fine-tuning scheme is as follows: Mn is increased by 0.01%–0.02% (not exceeding 0.09%) based on the original ratio, and Cr is increased by 0.005%–0.010% (not exceeding 0.06%), maintaining the Mn to Cr mass ratio of approximately 2:1.

[0014] The reaction mechanism for Ti and Zr as grain refining elements, and Mn and Cr as corrosion resistance optimizing elements is as follows: The roles of Ti and Zr: Ti combines with Al-Ti-B to form TiB2 and Al3Ti nucleation cores, which solidify and refine the as-cast grains; Zr precipitates Al3Zr dispersed phases, which pin grain boundaries during extrusion and heat treatment, preventing grain growth.

[0015] The role of Mn and Cr: to generate dispersed intermetallic compounds, counteract the harmful effects of Fe and Cu impurities, block intergranular corrosion channels, and significantly improve the corrosion resistance of refrigerants and salt spray environments.

[0016] Beneficial effects of the present invention This invention employs a step-by-step process: continuous casting → precise continuous extrusion → combined drawing → online heat treatment → surface treatment. Each step is independently controlled yet precisely connected. The aim is to avoid oxidation and contamination caused by transitions between steps through parameter coordination within the segmented process, while also optimizing the performance at each stage: continuous casting ensures billet homogenization, continuous extrusion improves pipe density, combined drawing optimizes dimensional accuracy, and online heat treatment precisely controls the microstructure, ultimately achieving a simultaneous improvement in the material's mechanical properties, corrosion resistance, and formability. Omitting any step or disrupting the process sequence will lead to increased microstructural defects and excessive performance fluctuations in the pipe, making it unsuitable for air conditioning piping conditions.

[0017] This invention also improves the mechanical strength, corrosion resistance, and heat resistance of the material by precisely optimizing the alloy composition ratio, especially by controlling the ratio of Si to Mg and the content of trace elements. This solves the contradiction of "single performance optimization" in existing aluminum alloy tubes and makes it suitable for various refrigerant conditions. The invention adopts a step-by-step process and optimizes the parameters of each process to ensure the flexibility and scalability of the production process, while effectively reducing defects such as oxidation and impurity segregation. This results in a uniform internal structure and excellent surface quality of the aluminum alloy tube, while shortening the production cycle and reducing manufacturing costs. It has significant industrial application value. Attached Figure Description

[0018] Figure 1 This is a process flow diagram of the present invention. Detailed Implementation

[0019] The technical solution of the present invention will be further explained and illustrated below through specific embodiments. Example 1

[0020] An aluminum pipe for conveying refrigerant in automotive air conditioning has the following composition by weight percentage: Si 1.0%, Mg 0.9%, Zr 0.15%, Mn 0.06%, Cr 0.04%, Ti 0.08%, Fe 0.25%, Cu 0.06%, Zn 0.07%, with a total impurity content of 0.10% and the balance being Al.

[0021] The method for preparing the aluminum pipe for transporting refrigerant in automotive air conditioning in this embodiment includes the following steps: Step 1: Melting and Billet Preparation Raw material preparation and smelting: Weigh aluminum ingots, magnesium ingots, Al-Si master alloy (90wt% Al content), Al-Zr master alloy (92wt% Al content), Al-Mn master alloy (95wt% Al content), Al-Cr master alloy (95wt% Al content), and Al-Ti-B master alloy (95wt% Al content) according to the specified proportions. Preheat the aluminum ingots at 250℃ for 30 minutes, and preheat the remaining raw materials at 200℃ for 30 minutes to remove surface moisture. Put the preheated aluminum ingots into the smelting furnace and melt them at 720℃ until completely melted. Raise the temperature to 740℃, add the magnesium ingots and various master alloys, and hold for 40 minutes, stirring every 10 minutes. After stirring, skim off the slag to remove surface dross, take samples for spectral analysis to ensure the composition is within the set range, and obtain the aluminum alloy liquid.

[0022] In-furnace refining and billet preparation: The molten aluminum alloy is transferred to a holding furnace, and Al-Ti-B refining agent accounting for 0.18% of the weight of the molten alloy is added. After thorough stirring, the temperature is lowered to 730℃ and held for 20-30 minutes, which can simultaneously achieve the dual effects of refining and impurity removal and grain refinement. Argon gas is introduced by rotary jet blowing at a flow rate of 0.4 m³ / h for 18 minutes. Subsequently, impurities are removed by filtration with a ceramic filter plate. After standing for 25 minutes, the purified molten aluminum alloy is sent to the continuous casting unit, and the casting speed is controlled at 0.7 m / min and the cooling water temperature at 30℃ to obtain a homogeneous billet with a diameter of 90 mm. Step 2: Precision Continuous Extrusion Molding After cleaning the surface of the billet, it is fed into a continuous extrusion machine. The die is preheated to 485℃ to prevent the aluminum tube from sticking to the die and to improve the surface finish. The extrusion temperature is controlled at 495℃, the extrusion speed at 1.0m / min, and the extrusion ratio at 12:1. Nitrogen gas is introduced for protection during the extrusion process to produce a rough aluminum tube with a diameter of 27mm. Step 3: Joint pulling and straightening The coarse aluminum tube was subjected to two-pass room temperature combined drawing, with a deformation of 12.5% ​​per pass and a total deformation of 24%. After drawing, the dimensions were corrected by straightening equipment to obtain a finished aluminum tube with a diameter of 22mm and the diameter tolerance was controlled within ±0.04mm. Step 4: Online heat treatment strengthening The finished aluminum tube is fed into a continuous heating furnace and solution-treated at 532℃ for 1.5 hours. After holding at this temperature, it is cooled with tap water at a rate of 28℃ / s to ensure uniform microstructure. It is then placed in a segmented aging furnace for pre-aging at 132℃ for 5.5 hours, followed by final aging at 172℃ for 3.5 hours to complete the heat treatment. Pre-aging can suppress the precipitation of coarse phases, while final aging can improve the stability of the strengthening phase and prevent performance degradation in the later stages.

[0023] Step 5: Surface Treatment and Performance Testing The heat-treated aluminum tubes are subjected to chromate passivation to form a 0.8μm thick anti-corrosion film; after being scraped, ultrasonically cleaned, cleaned with distilled water, and dried, performance tests are performed.

[0024] The aluminum alloy tube prepared in this embodiment has a tensile strength of 345 MPa, a yield strength of 215 MPa, an elongation of 13%, a corrosion rate of 0.007 mm / a, and a bending radius of 2.5 times the tube diameter (without cracking). It is compatible with R744 refrigerant and shows no leakage or deformation after standing for 1000 hours at 150℃ and 2.5 MPa. Scanning electron microscopy of the tube reveals a uniformly distributed Mg2Si strengthening phase with a grain size of 10–12 μm and no obvious impurity agglomeration.

[0025] Example 2 An aluminum pipe for conveying refrigerant in automotive air conditioning has the following composition by weight percentage: Si 0.9%, Mg 0.8%, Zr 0.12%, Mn 0.04%, Cr 0.02%, Ti 0.05%, Fe 0.28%, Cu 0.07%, Zn 0.06%, with a total impurity content of 0.11% and the balance being Al.

[0026] The method for preparing the aluminum pipe for transporting refrigerant in automotive air conditioning in this embodiment includes the following steps: Step 1: Melting and Billet Preparation Raw material preparation and smelting: Weigh all raw materials according to the proportions. Preheat aluminum ingots to 250℃ for 30 minutes, and preheat the remaining raw materials to 200℃ for 30 minutes. Melt the aluminum ingots at 715℃, then raise the temperature to 735℃ and add the remaining raw materials. Hold the temperature for 35 minutes, stirring every 10 minutes. After removing the slag, perform spectral analysis to obtain the aluminum alloy liquid.

[0027] In-furnace refining and billet preparation: Add 0.15% Al-Ti-B refining agent, stir thoroughly, cool to 725℃, degas with argon gas (flow rate 0.35m³ / h, time 15min), filter and let stand for 20min before continuous casting at a casting speed of 0.65m / min and a cooling water temperature of 28℃ to obtain a billet with a diameter of 85mm. Step 2: Precision Continuous Extrusion Molding The mold was preheated to 480℃, the extrusion temperature was 490℃, the speed was 0.95m / min, the extrusion ratio was 11:1, and nitrogen protection was used to produce a rough aluminum tube with a diameter of 26mm. Step 3: Joint pulling and straightening The rough aluminum tube was drawn in two passes at room temperature, with a deformation of 12% per pass and a total deformation of 23%. After drawing, a finished aluminum tube with a diameter of 21 mm was obtained with a diameter tolerance of ±0.05 mm. Step 4: Online heat treatment strengthening The finished aluminum tubes are fed into a continuous heating furnace and solution-treated at 530℃ for 1.8 hours. After holding at this temperature, they are cooled with tap water (at a rate of 26℃ / s). They are then placed into a segmented aging furnace and pre-aged at 130℃ for 6 hours, followed by final aging at 170℃ for 4 hours to complete the heat treatment.

[0028] Step 5: Surface Treatment and Performance Testing The heat-treated aluminum tubes were passivated with chromate to form a 0.6 μm thick anti-corrosion film; after routine cleaning and drying, they were tested.

[0029] The aluminum alloy tube prepared in this embodiment has a tensile strength of 335 MPa, a yield strength of 205 MPa, an elongation of 12.5%, a corrosion rate of 0.008 mm / a, a bending radius of 2.8 times the tube diameter (without cracking), is compatible with R134a refrigerant, and exhibits good stability under operating conditions of 120℃ and 1.8 MPa.

[0030] Example 3 An aluminum pipe for conveying refrigerant in automotive air conditioning has the following composition by weight percentage: Si 1.1%, Mg 1.0%, Zr 0.18%, Mn 0.07%, Cr 0.05%, Ti 0.10%, Fe 0.22%, Cu 0.05%, Zn 0.05%, total impurity content 0.09%, and the balance being Al.

[0031] The steps for using the aluminum pipe for delivering refrigerant to the automotive air conditioning system in this embodiment are as follows: Step 1: Melting and Billet Preparation Raw material preparation and smelting: The raw material preheating parameters are the same as in Example 1. The aluminum ingot is smelted at 725°C. The temperature is raised to 745°C and the remaining raw materials are added. The temperature is held for 45 minutes. After stirring and removing the slag, the aluminum alloy liquid is obtained by spectral analysis.

[0032] In-furnace refining and billet preparation: Add 0.20% Al-Ti-B refining agent, stir thoroughly, cool to 735℃, degas with argon (flow rate 0.45m³ / h, time 20min), filter and let stand for 30min before continuous casting at a casting speed of 0.75m / min and a cooling water temperature of 32℃ to obtain a billet with a diameter of 95mm.

[0033] Step 2: Precision Continuous Extrusion Molding After cleaning the surface of the billet, it is fed into a continuous extrusion machine. The die is preheated to 490℃, the extrusion temperature is 500℃, the speed is 1.05m / min, the extrusion ratio is 13:1, and nitrogen protection is provided during the extrusion process to obtain a rough aluminum tube with a diameter of 28mm.

[0034] Step 3: Joint pulling and straightening The rough aluminum tube was drawn in two passes at room temperature, with a deformation of 13% per pass and a total deformation of 25%. After drawing, a finished aluminum tube with a diameter of 23 mm and a diameter tolerance of ±0.04 mm was obtained.

[0035] Step 4: Online heat treatment strengthening The finished aluminum tubes are fed into a continuous heating furnace and solution-treated at 535℃ for 1.2 hours. After holding at this temperature, they are cooled with tap water (at a rate of 30℃ / s). They are then placed into a segmented aging furnace and pre-aged at 135℃ for 5 hours, followed by final aging at 175℃ for 3 hours.

[0036] Step 5: Surface Treatment and Performance Testing The heat-treated aluminum tubes were passivated with chromate to form a 0.9μm thick anti-corrosion film, and then tested after routine cleaning and drying.

[0037] The aluminum alloy tube prepared in this embodiment has a tensile strength of 348 MPa, a yield strength of 218 MPa, an elongation of 13.5%, a corrosion rate of 0.006 mm / a, a bending radius of 2.4 times the tube diameter (without cracking), is compatible with R744 refrigerant, and has the best comprehensive performance under high temperature and high pressure conditions.

[0038] Example 4 An aluminum pipe for conveying refrigerant in automotive air conditioning has the following composition by weight percentage: Si 0.95%, Mg 0.85%, Zr 0.14%, Mn 0.05%, Cr 0.03%, Ti 0.07%, Fe 0.26%, Cu 0.05%, Zn 0.06%, with a total impurity content of 0.10% and the balance being Al.

[0039] The aluminum alloy tube in this embodiment is specifically designed for R134a air conditioning systems in commercial vehicles, and its manufacturing method is as follows: Step 1: Melting and Billet Preparation Aluminum ingots were preheated to 250℃ for 30 minutes, and other raw materials were preheated to 200℃ for 30 minutes. The aluminum ingots were melted at 720℃, and then magnesium ingots and various intermediate alloys were added at 738℃. The mixture was held at this temperature for 38 minutes, stirred every 10 minutes, and the slag was removed. The composition was analyzed by spectroscopy to ensure that the weight ratio of Si to Mg was 2.5:1, thus obtaining a liquid aluminum alloy.

[0040] In-furnace refining and billet preparation: The molten aluminum alloy was transferred to a holding furnace, and 0.17% Al-Ti-B refining agent was added. The temperature was lowered to 728℃, and argon gas was sprayed in a rotary jet for degassing (flow rate 0.38 m³ / h, time 16 min). After filtration through a ceramic filter plate, the mixture was allowed to stand for 22 min. The purified molten aluminum alloy was then sent to a continuous casting unit. The continuous casting parameters were controlled as follows: speed 0.68 m / min and cooling water temperature 29℃, to obtain a homogeneous billet with a diameter of 88 mm.

[0041] Step 2: Precision Continuous Extrusion Molding After the surface of the billet is cleaned, it is sent into a continuous extrusion machine. The die is preheated to 483℃, the extrusion temperature is 492℃, the speed is 0.98m / min, the extrusion ratio is 11.5:1, and a rough aluminum tube with a diameter of 26.5mm is prepared under nitrogen protection to avoid surface oxidation and scratches.

[0042] Step 3: Joint pulling and straightening The coarse aluminum tube is drawn in two passes at room temperature. The deformation is 12.2% in the first pass and 12.3% in the second pass, with a total deformation of 23.5%. After drawing, the tube is straightened to obtain a finished aluminum tube with a diameter of 21.5mm and a diameter tolerance of ±0.04mm, which is suitable for the pipe size requirements of commercial vehicles.

[0043] Step 4: Online heat treatment strengthening The finished aluminum tubes are fed into a continuous heating furnace and solution-treated at 531℃ for 1.6 hours. After holding at this temperature, they are cooled with tap water (at a rate of 27℃ / s). They are then placed into a segmented aging furnace and pre-aged at 131℃ for 5.8 hours, followed by final aging at 171℃ for 3.8 hours, balancing strength and toughness.

[0044] Step 5: Surface Treatment and Performance Testing The heat-treated aluminum tubes were subjected to chromate passivation to form a 0.7μm thick anti-corrosion film, which was then cleaned, dried, and tested.

[0045] In this embodiment, the aluminum alloy tube has a tensile strength of 340MPa, a yield strength of 210MPa, an elongation of 12.8%, a corrosion rate of 0.0075mm / a, and a bending radius of 2.6 times the tube diameter (without cracking). It is compatible with R134a refrigerant under operating conditions of 120℃ and 1.8MPa and exhibits excellent long-term stability.

[0046] Example 5 An aluminum pipe for conveying refrigerant in automotive air conditioning has the following composition by weight percentage: Si 1.05%, Mg 0.95%, Zr 0.16%, Mn 0.065%, Cr 0.045%, Ti 0.09%, Fe 0.23%, Cu 0.06%, Zn 0.05%, total impurity content 0.095%, and the balance being Al.

[0047] This embodiment is adapted to the R744 high-pressure air conditioning system for passenger vehicles, and the preparation method is as follows: Step 1: Melting and Billet Preparation The raw material preheating parameters are the same as in Example 1. The aluminum ingot is melted at 722°C, and the remaining raw materials are added at 742°C. The mixture is kept at this temperature for 42 minutes. After stirring and removing the slag, spectral analysis is performed to ensure that the composition is accurately up to standard, and an aluminum alloy liquid is obtained.

[0048] In-furnace refining and billet preparation: 0.19% Al-Ti-B refining agent was added, the temperature was lowered to 732℃, and argon gas was used for degassing (flow rate 0.42m³ / h, time 19min). After filtration and standing for 28min, continuous casting was carried out at a casting speed of 0.72m / min and a cooling water temperature of 31℃ to obtain a billet with a diameter of 92mm. The density of the billet was improved to adapt to high-pressure conditions.

[0049] Step 2: Precision Continuous Extrusion Molding After cleaning the surface of the billet, it is fed into a continuous extrusion machine. The die is preheated to 487℃, the extrusion temperature is 498℃, the speed is 1.02m / min, the extrusion ratio is 12.5:1, and nitrogen protection is used for extrusion to produce a rough aluminum tube with a diameter of 27.5mm. The extrusion parameters are optimized to improve the uniformity of the internal structure of the tube.

[0050] Step 3: Joint pulling and straightening The coarse aluminum tube was drawn in two passes at room temperature, with a deformation of 12.8% per pass and a total deformation of 24.5%. After drawing, the tube was straightened to obtain a finished aluminum tube with a diameter of 22.5 mm and a diameter tolerance of ±0.04 mm, which meets the lightweight requirements of passenger vehicle piping.

[0051] Step 4: Online heat treatment strengthening The finished aluminum tubes were fed into a continuous heating furnace and solution-treated at 533℃ for 1.4 hours. After holding at this temperature, they were cooled with tap water (at a rate of 29℃ / s). Subsequently, they were fed into a segmented aging furnace and pre-aged at 133℃ for 5.2 hours and then final-aged at 173℃ for 3.2 hours to enhance the precipitation of the Mg2Si phase.

[0052] Step 5: Surface Treatment and Performance Testing The heat-treated aluminum tubes were passivated with chromate to form a 0.85μm thick anti-corrosion film, and then tested after routine cleaning and drying.

[0053] In this embodiment, the aluminum alloy tube has a tensile strength of 346 MPa, a yield strength of 216 MPa, an elongation of 13.2%, a corrosion rate of 0.0065 mm / a, and a bending radius of 2.45 times the tube diameter (without cracking). It is compatible with R744 refrigerant under high pressure conditions of 150℃ and 2.5 MPa without leakage or deformation. Its overall performance is suitable for high-pressure air conditioning systems in passenger vehicles.

[0054] When Si is 1.0%, Mg is 0.9%, and Zr is 0.15%, the product has the best performance when used with R744 refrigerant: tensile strength of 340-350 MPa, yield strength of 210-220 MPa, elongation of 12%-14%, and no leakage or deformation under long-term use at 150℃ and 2.5 MPa.

[0055] Example 6 An aluminum pipe for conveying refrigerant in automotive air conditioning has the following composition by weight percentage: Si 0.85%, Mg 0.75%, Zr 0.13%, Mn 0.045%, Cr 0.025%, Ti 0.06%, Fe 0.27%, Cu 0.07%, Zn 0.065%, with a total impurity content of 0.11% and the balance being Al.

[0056] This embodiment focuses on the R134a air conditioning system for economy passenger vehicles, balancing performance and cost. The preparation method is as follows: Step 1: Melting and Billet Preparation Aluminum ingots were preheated at 250℃ for 30 minutes, and the remaining raw materials were preheated at 200℃ for 30 minutes. The aluminum ingots were melted at 718℃. The temperature was raised to 736℃ and the remaining raw materials were added. The mixture was kept at this temperature for 36 minutes and stirred every 10 minutes. After removing the slag, the composition was analyzed by spectroscopy to obtain aluminum alloy liquid.

[0057] In-furnace refining and billet preparation: 0.16% Al-Ti-B refining agent was added, and after thorough stirring, the temperature was lowered to 726℃. Argon gas was used for degassing (flow rate 0.36 m³ / h, time 17 min). After filtration and standing for 21 min, continuous casting was carried out at a casting speed of 0.66 m / min and a cooling water temperature of 28.5℃ to obtain a billet with a diameter of 86 mm. Casting parameters were optimized to control costs.

[0058] Step 2: Precision Continuous Extrusion Molding After cleaning the surface of the billet, it is fed into a continuous extrusion machine. The die is preheated to 482℃, the extrusion temperature is 491℃, the speed is 0.96m / min, the extrusion ratio is 11.2:1, and a rough aluminum tube with a diameter of 26.2mm is prepared under nitrogen protection.

[0059] Step 3: Joint pulling and straightening The rough aluminum tube was drawn in two passes at room temperature, with a deformation of 12.1% per pass and a total deformation of 23.2%. After drawing, the tube was straightened to obtain a finished aluminum tube with a diameter of 21.2 mm and a diameter tolerance of ±0.05 mm.

[0060] Step 4: Online heat treatment strengthening The finished aluminum tubes are fed into a continuous heating furnace and solution-treated at 530.5℃ for 1.7 hours. After holding at this temperature, they are cooled with tap water (at a rate of 26.5℃ / s). They are then placed into a segmented aging furnace and pre-aged at 130.5℃ for 5.9 hours, followed by final aging at 170.5℃ for 3.9 hours, ensuring performance within a controllable cost range.

[0061] Step 5: Surface Treatment and Performance Testing The heat-treated aluminum tube was passivated with chromate to form a 0.65μm thick anti-corrosion film, and then tested after cleaning and drying.

[0062] In this embodiment, the aluminum alloy tube has a tensile strength of 336 MPa, a yield strength of 206 MPa, an elongation of 12.4%, a corrosion rate of 0.0078 mm / a, and a bending radius of 2.7 times the tube diameter (without cracking). It is suitable for R134a air conditioning systems in economy passenger cars and has a significant cost-performance advantage.

[0063] Example 7 (3-pass room temperature combined drawing scheme) An aluminum pipe for conveying refrigerant in automotive air conditioning has the following composition by weight percentage: Si 1.0%, Mg 0.9%, Zr 0.15%, Mn 0.06%, Cr 0.04%, Ti 0.08%, Fe 0.24%, Cu 0.06%, Zn 0.06%, total impurity content 0.09%, and the balance being Al.

[0064] This embodiment is adapted for large-diameter, high-pressure R744 air conditioning pipes, and the preparation method is as follows: Step 1: Melting and Billet Preparation The parameters for raw material preheating, smelting, refining, and continuous casting were kept consistent with those in Example 1, resulting in a homogeneous billet with a diameter of 90 mm.

[0065] Step 2: Precision Continuous Extrusion Molding The die was preheated to 485℃, the extrusion temperature was 495℃, the extrusion speed was 1.0m / min, the extrusion ratio was 12:1, and nitrogen protection was used for extrusion to produce a rough aluminum tube with a diameter of 27mm.

[0066] Step 3: Joint pulling and straightening The process employs a three-pass room temperature combined drawing method, with deformation of 9% in the first pass, 8% in the second pass, and 7% in the third pass. The deformation of each pass is controlled within 10%, and the total deformation is 23.2%. Online micro-straightening is performed after each drawing pass, and a second overall straightening is performed after all drawing passes are completed. The final product is a 22mm diameter finished aluminum tube with a diameter tolerance of ±0.03mm, which is better than that of a two-pass drawing method.

[0067] Step 4: Online heat treatment strengthening Solution treatment at 532℃ for 1.5 hours, with a cooling rate of 28℃ / s; pre-aging at 132℃ for 5.5 hours, and final aging at 172℃ for 3.5 hours.

[0068] Step 5: Surface Treatment and Performance Testing Chromate passivation forms a 0.8μm anti-corrosion film, which is then cleaned, dried, and tested.

[0069] In this embodiment, the aluminum alloy tube has a tensile strength of 342 MPa, a yield strength of 213 MPa, an elongation of 13.8%, a corrosion rate of 0.0068 mm / a, and a crack-free bending radius of 2.3 times the tube diameter. It is drawn in three stages with small deformation, resulting in lower internal stress in the tube. It can be bent at ultra-large angles without micro-cracks, making it suitable for complex and multi-bend air conditioning pipelines.

[0070] Comparative Example 1 (without continuous extrusion process) An aluminum alloy tube, with the same composition as in Example 1, differs in its preparation method by omitting the continuous extrusion process, as detailed below: Step 1 is the same as in Example 1, to obtain a 90mm diameter billet; Step 2: Directly draw the billet to a diameter of 22mm in 4 passes, with a deformation of 15% per pass and a total deformation of 55%. Stress-relieving annealing (300℃, 2h) is performed every 2 passes during the drawing process. Repeat steps 4-5 in Example 1.

[0071] The aluminum alloy tube prepared in this comparative example has a tensile strength of 295 MPa, a yield strength of 175 MPa, an elongation of 9%, a corrosion rate of 0.015 mm / a, and microcracks appear when the bending radius is 4 times the tube diameter. Due to the lack of a continuous extrusion process, the tube has insufficient density and poor uniformity of structure, and its performance is significantly inferior to that of Example 1.

[0072] Comparative Example 2 (without pre-aging treatment) An aluminum alloy tube, with the same composition as in Example 1, differs in its preparation method in that the pre-aging process is omitted during heat treatment, as detailed below: Steps 1-3 are the same as in Example 1; Step 4: Online heat treatment: Solution treatment at 532℃ for 1.5 hours, followed by cooling with tap water and final aging at 172℃ for 3.5 hours; Step 5 is the same as in Example 1.

[0073] The aluminum alloy tube prepared in this comparative example has a tensile strength of 310 MPa, a yield strength of 190 MPa, an elongation of 10%, and a corrosion rate of 0.012 mm / a. Due to the lack of pre-aging, the precipitation of the Mg2Si strengthening phase is uneven, and some coarse phases appear, resulting in a decrease in strength and corrosion resistance.

[0074] Comparative Example 3 (without trace elements Zr and Cr) An aluminum alloy tube, with Zr and Cr elements removed from its composition, and the remaining components are the same as in Example 1 (Si 1.0%, Mg 0.9%, Mn 0.06%, Ti 0.08%, with the remainder being Al and impurities), is prepared using the same method as in Example 1.

[0075] The aluminum alloy tube prepared in this comparative example has a tensile strength of 305 MPa, a yield strength of 188 MPa, an elongation of 11%, and a corrosion rate of 0.014 mm / a. Due to the lack of Zr and Cr to refine the grains and enhance corrosion resistance, the grain size increases to 18-22 μm, and its resistance to refrigerant corrosion is significantly insufficient.

[0076] The aluminum alloy tubes prepared in Examples 1-7 and Comparative Examples 1-3 were subjected to uniform performance tests, and the test methods are as follows: tensile strength and yield strength were tested according to GB / T 228.1-2010 "Metallic Materials - Tensile Testing - Part 1: Test Method at Room Temperature", and elongation was measured as elongation after fracture (A50). Corrosion rate was determined by a combination of neutral salt spray test (GB / T 10125-2021, 500h) and R134a / R744 refrigerant immersion test (1000h under corresponding operating conditions). Bending formability was tested according to GB / T 232-2010 "Metallic Materials - Bending Test Method", and the minimum bending radius without cracking was recorded (expressed as a multiple of the tube diameter). The test results are shown in Table 1.

[0077] As shown in Table 1, the aluminum alloy tubes prepared in Examples 1-7 of this invention, through optimization of the aluminum alloy composition ratio (introduction of trace elements Zr, Mn, Cr, and Ti) and combined with a step-by-step process of continuous casting → precise continuous extrusion → combined drawing → online graded heat treatment, achieved simultaneous optimization of mechanical properties, corrosion resistance, and formability. The tensile strength of all qualified examples is ≥355MPa. Among them, Examples 3 and 5, which are suitable for R744 high-pressure conditions, have corrosion rates as low as 0.006~0.0065mm / a and a crack-free bending radius as low as 2.4 times the pipe diameter. The products suitable for conventional R134a conditions have corrosion rates of 0.0075~0.008mm / a and crack-free bending radii of 2.6~2.8 times the pipe diameter.

[0078] After optimizing the process and proportions, the general core performance range of the aluminum alloy pipe of this invention is as follows: tensile strength 330-350MPa, yield strength 200-220MPa, elongation after fracture ≥12%; the corrosion rate of the optimal formula product can be as low as 0.006mm / a, the corrosion rate of the conventional product is ≤0.008mm / a, the crack-free bending radius of the optimal product can be up to 2.3 times the pipe diameter, and the crack-free bending radius of the conventional product is no higher than 2.8 times the pipe diameter, all of which meet the requirements for use in automotive air conditioning pipes.

[0079] Comparing Example 1 with Comparative Examples 1-3, it is evident that the continuous extrusion process significantly improves the density and strength of the tube. Online graded heat treatment (pre-aging + final aging) optimizes the precipitation morphology of strengthening phases, and trace elements such as Zr and Cr refine grains and enhance corrosion resistance. The absence of any key process or component leads to a significant decline in the performance of the aluminum alloy tube. Therefore, the synergistic effect of the component design and step-by-step process of this invention effectively solves the performance shortcomings of existing 6xxx series aluminum alloy tubes when adapted to automotive air conditioning refrigerant conditions.

[0080] Table 1: Performance Test Results of Aluminum Alloy Tubes in Examples 1-7 and Comparative Examples 1-3

[0081] Note: 1. Compatible Refrigerant: Specify the target refrigerant type for each group of aluminum alloy pipes, with the compatible vehicle model and scenario indicated in parentheses. R744 is high-pressure carbon dioxide refrigerant, and R134a is conventional hydrofluorocarbon refrigerant; 2. Tensile Strength and Yield Strength: Tested according to GB / T 228.1-2010 "Metallic Materials - Tensile Testing - Part 1: Test Method at Room Temperature", with standard round specimens and a tensile rate of 2 mm / min; 3. Elongation: Elongation after fracture (A50) corresponding to a specimen with a gauge length of 50 mm; 4. Corrosion Rate: Calculated by combining neutral salt spray test (GB / T 10125-2021, 500 h, salt spray concentration 5% NaCl, temperature 35℃) and corresponding refrigerant immersion test (constant temperature and pressure immersion for 1000 h under suitable operating conditions), reflecting the material's comprehensive resistance to refrigerant corrosion and atmospheric corrosion; 5. Cracking-Free Bending Radius: According to GB / T According to GB / T 232-2010 "Metallic Materials Bending Test Method", the bending angle is 180°. The minimum bending radius when the sample has no cracks or breaks is recorded and expressed as a multiple of the outer diameter of the pipe. The smaller the value, the better the formability.

Claims

1. An aluminum pipe for delivering refrigerant to an automotive air conditioning system, characterized in that, The raw materials include a matrix phase and a reinforcing phase, wherein the matrix phase is Al and the reinforcing phase is Mg2Si phase, and the material also contains trace amounts of grain refinement and corrosion resistance optimization elements. Ti and Zr are grain-refining elements, while Mn and Cr are corrosion-resistant optimization elements.

2. The automotive air conditioning refrigerant delivery aluminum pipe according to claim 1, characterized in that, The aforementioned aluminum pipe for transporting automotive air conditioning refrigerant has the following elemental composition by weight percentage: Si 0.9%–1.1%, Mg 0.8%–1.0% (Si to Mg weight ratio 2.2:1–3.0:1), Zr 0.12%–0.18%, Mn 0.04%–0.07%, Cr 0.02%–0.05%, Ti 0.05%–0.10%, impurity elements Fe≤0.30%, Cu≤0.08%, Zn≤0.08% (single impurity ≤0.04%), total impurities ≤0.12%, and the balance being Al.

3. A method for preparing an aluminum pipe for transporting refrigerant in an automotive air conditioning system according to claim 1, characterized in that, Includes the following steps: Step 1: Melting and Billet Preparation Aluminum alloy raw materials are fed into smelting equipment, and after smelting, refining and degassing, purified aluminum alloy liquid is obtained; during the smelting process, argon inert gas is introduced for protection to prevent oxidation of the alloy liquid; the purified aluminum alloy liquid is sent to the continuous casting unit to obtain homogeneous billets. Step 2: Continuous extrusion molding The billet obtained in step 1 is fed into a continuous extrusion device, and the extrusion temperature and speed are controlled to produce a crude aluminum tube. Step 3: Joint pulling and straightening The rough aluminum tube obtained in step 2 is subjected to a combined drawing process, which consists of 2 to 3 passes of room temperature drawing, with a deformation of 10% to 15% per pass and a total deformation of 23% to 25%. The tube dimensions and roundness are then corrected using a straightening device. Step 4: Online heat treatment strengthening The aluminum tubes drawn in step 3 are subjected to online solution treatment and graded aging treatment in sequence. The graded aging treatment includes pre-aging and final aging. Pre-aging can suppress the precipitation of coarse phases, and final aging can improve the stability of the strengthening phase and avoid performance degradation in the later stage. Step 5: Surface treatment and performance testing feedback After surface passivation treatment of the aluminum tube after heat treatment in step 4, the parameters are adjusted based on performance test feedback, and the finished automotive air conditioning refrigerant delivery aluminum tube is obtained after meeting the standards.

4. The method for preparing the automotive air conditioning refrigerant delivery aluminum pipe according to claim 3, characterized in that, In step 1, the refining agent is Al-Ti-B, and the addition amount is 0.15% to 0.20% of the alloy liquid weight; online degassing is carried out by rotary jet argon gas, with an argon gas flow rate of 0.3 to 0.5 m³ / h and a degassing time of 15 to 20 min, and the gas content of the aluminum alloy liquid is ≤0.15 ml / 100 g Al; the refining holding time is 20 to 30 min, which can simultaneously achieve the dual effects of refining and impurity removal and grain refinement.

5. The method for preparing the automotive air conditioning refrigerant delivery aluminum pipe according to claim 3, characterized in that, In step 2, the continuous extrusion temperature is 480-510℃, the extrusion speed is 0.8-1.2m / min, the continuous extrusion die preheating temperature is 480-490℃, the extrusion ratio is 10-14:1, nitrogen protection is used during the extrusion process, and die preheating can prevent the aluminum tube from sticking to the die and improve the surface finish.

6. The method for preparing the automotive air conditioning refrigerant delivery aluminum pipe according to claim 3, characterized in that, In step 4, the online solution treatment uses a continuous heating furnace with a temperature of 520–540℃ and a holding time of 1–2 hours; the temperature uniformity inside the furnace is ±5℃; after holding, tap water is used for cooling with a cooling rate ≥25℃ / s to ensure the uniformity of the overall structure of the aluminum tube; the graded aging uses a segmented aging furnace with a pre-aging temperature of 120–140℃ and a time of 4–6 hours; the final aging temperature is 160–180℃ and the time is 2–4 hours.

7. The method for preparing the automotive air conditioning refrigerant delivery aluminum pipe according to claim 3, characterized in that, In step 5, the performance testing feedback includes three core indicators: mechanical properties, corrosion resistance, and dimensional accuracy. If the tensile strength is lower than 330 MPa, the final aging temperature can be adjusted to 175–180℃, and the holding time extended by 0.5–1 h. If the corrosion rate exceeds the standard, the passivation process can be adjusted as follows: increase the concentration of the chromate main salt by 5%–15%, increase the passivation temperature by 5–10℃, extend the passivation soaking time by 20%–40%, and add a 60–70℃ hot water sealing process. The alloy composition fine-tuning scheme is as follows: Mn is increased by 0.01%–0.02% (not exceeding 0.09%) based on the original ratio, and Cr is increased by 0.005%–0.010% (not exceeding 0.06%), maintaining the Mn to Cr mass ratio of approximately 2:1.

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