Hydrogen storage cylinder aluminum alloy liner integrated forming process

Abstract

The application discloses a hydrogen storage cylinder aluminum alloy liner integrated forming process and relates to the technical field of high-pressure hydrogen storage equipment manufacturing. The specific steps are as follows: S1, heating aluminum alloy blank to a solid solution temperature interval and keeping the blank temperature uniform; S2, transferring the aluminum alloy blank heated to the solid solution temperature to a hot deep drawing die provided with a heating element, and performing hot deep drawing forming under the condition that the die temperature is maintained at 450-500 DEG C; and in the hot deep drawing forming process, ultrasonic vibration is applied to the punch with a frequency of 15-25 kHz and an amplitude of 5-15 microns. The application realizes integrated online operation of forming and quenching, discards the discrete processing mode of traditional forming, transfer and independent quenching, and after the workpiece is hot deep drawn, the workpiece does not need to be transferred after demolding, and can be directly quenched online. The application completely avoids the problems of temperature loss, surface oxidation, coarse grains and composition segregation in the workpiece transfer process in the traditional process.

Description

Technical Field

[0001] This invention relates to the field of high-pressure hydrogen storage equipment manufacturing technology, specifically to an integrated forming process for an aluminum alloy inner liner of a hydrogen storage cylinder. Background Technology

[0002] The Type III on-board hydrogen storage cylinder consists of an aluminum alloy inner liner and an outer carbon fiber fully wound reinforcement layer, and is one of the mainstream high-pressure hydrogen storage containers for hydrogen fuel cell vehicles. Its aluminum alloy inner liner, serving as a hydrogen sealing barrier and the load-bearing matrix for the fiber winding, directly determines the overall safety and service life of the cylinder through its mechanical properties and forming quality, making it the mainstream technology for large-scale hydrogen storage and transportation.

[0003] The technical defects exposed by existing forming processes in actual production applications are particularly prominent: First, in the traditional hot deep drawing process, the frictional resistance between the aluminum alloy billet and the die is high, and uneven thinning of the cylinder wall is very likely to occur during the deep drawing deformation process. Moreover, stress concentration is significant at the rounded corners of the punch, which easily leads to forming defects such as wrinkles, cracks, and wall thickness deviations. This results in low forming accuracy of the inner liner, making it difficult to improve the yield rate, and seriously affecting the pressure uniformity of the gas cylinder inner liner. The existing forming process lacks vibration-assisted friction reduction optimization methods and relies entirely on rigid extrusion forming by the die. The material flow consistency is poor, and the overall microstructure of the inner liner is not uniform enough. Under subsequent high-pressure hydrogen storage conditions, local stress fatigue damage is likely to occur, shortening the service life of the gas cylinder. Traditional quenching methods mostly use a single cooling rate process, which cannot adapt to the stress release requirements after the aluminum alloy inner liner is formed. An excessively fast cooling rate can easily cause thermal stress concentration in the workpiece, resulting in defects such as microcracks and deformation cracks. In response, we propose an integrated forming process for the aluminum alloy inner liner of hydrogen storage cylinders. Summary of the Invention

[0004] To solve the above-mentioned technical problems, an integrated forming process for aluminum alloy inner liner of hydrogen storage cylinder is provided. This technical solution solves the problems of relying entirely on rigid extrusion molding with molds, resulting in poor material flow consistency, insufficient uniformity of the overall microstructure of the inner liner, and inability to meet the stress release requirements after the aluminum alloy inner liner is formed.

[0005] To achieve the above objectives, the technical solution adopted by this invention is: an integrated forming process for the aluminum alloy inner liner of a hydrogen storage cylinder, the specific steps of which are as follows: S1. Heat the aluminum alloy billet to the solution temperature range and hold it at that temperature until the billet temperature is uniform. S2. Transfer the aluminum alloy billet heated to the solution temperature to a hot drawing die equipped with a heating element, and perform hot drawing while maintaining the die temperature at 450-500°C; during the hot drawing process, apply ultrasonic vibration with a frequency of 15-25 kHz and an amplitude of 5-15 μm to the punch. S3. After hot drawing is completed, the workpiece is not removed. The punch and die of the mold are opened by 10-20 mm. The formed workpiece is subjected to two-stage differentiated online quenching through the cooling channels set inside the punch and die: the first stage is uniform pre-cooling with atomized gas-liquid two-phase flow, with the cooling rate controlled at 30-50℃ / s, until the surface temperature of the workpiece drops to 300-350℃; the second stage is switched to high-pressure water jet for rapid final cooling, with the cooling rate increased to 80-120℃ / s, so that the workpiece is cooled to below 100℃ within 60 s. S4. After quenching, open the mold and take out the workpiece. Transfer the workpiece to the aging furnace for artificial aging treatment.

[0006] Preferably, the aluminum alloy in S1 is 6061 aluminum alloy, and the solution treatment temperature range is 500-545°C. When the billet is heated, nitrogen is introduced into the furnace as an inert protective atmosphere to keep the oxygen content in the furnace below 500 ppm, so as to inhibit the surface oxidation of the aluminum alloy billet during the solution treatment heating process.

[0007] Preferably, in S2, the blank holder force is controlled at 5-15 MPa and the drawing speed is controlled at 5-30 mm / s during the hot drawing process; the punch, die, and blank holder of the hot drawing die are all equipped with heating elements, and a segmented independent temperature control method is adopted to ensure that the temperature deviation of each area of ​​the die does not exceed ±5℃.

[0008] Preferably, in S3, the atomizing medium of the first-stage atomized gas-liquid two-phase flow is a polymer water-based quenching liquid with a concentration of 5% to 15%, and the gas pressure is controlled at 0.3 to 0.6 MPa; the injection pressure of the second-stage high-pressure water jet is controlled at 1.5 to 3.0 MPa, and the distance between the nozzle and the workpiece surface is 15 to 40 mm.

[0009] Preferably, the holding pressure in the holding pressure constraint stage of S3 is 20-50 MPa and the holding time is 30-90 s. The mold is used to apply normal pressure to the formed workpiece to suppress springback and warping deformation during the quenching process.

[0010] Preferably, the temperature of the artificial aging treatment in S4 is 150-180 ℃, and the holding time is 6-12 h, so that the aluminum alloy matrix with solid solution supersaturation after quenching precipitates a dispersed strengthening phase and obtains the target mechanical properties.

[0011] Preferably, between S2 and S3, a mold cavity purging step is also included. After the hot drawing is completed and before quenching is started, high-pressure inert gas is introduced into the mold cavity to blow away residual lubricant and oxide scale. The pressure of the high-pressure inert gas is 0.4 to 0.8 MPa, and the purging duration is 3 to 8 seconds.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention employs an ultrasonic vibration-assisted constant-temperature hot drawing process. Under a constant-temperature mold environment of 450–500°C, ultrasonic vibration with a specific frequency and amplitude is applied to the punch. By utilizing the acoustic capillary effect and friction reduction effect generated by ultrasonic vibration, the friction coefficient of the aluminum alloy billet and the mold contact surface is significantly reduced, effectively improving the problems of uneven material flow and excessive frictional resistance in the traditional hot drawing process.

[0013] This invention realizes integrated online forming and quenching, abandoning the traditional discrete processing mode of forming, transporting, and independent quenching. After the workpiece is hot-drawn, it does not need to be demolded and transported, and can be directly quenched online. It completely avoids the problems of temperature loss, surface oxidation, grain coarsening and compositional segregation that occur during the workpiece transport process in traditional processes, and retains the supersaturated solid solution structure of the aluminum alloy billet after solid solution treatment to the greatest extent, significantly improving the strength, toughness and hydrogen embrittlement resistance of the inner liner matrix.

[0014] This invention employs a two-stage differentiated online quenching process with a slow initial cooling followed by a fast final cooling. It utilizes a staged cooling mode combining atomized gas-liquid two-phase flow pre-cooling with high-pressure water jet final cooling to match the heat treatment performance requirements of the aluminum alloy inner liner. The first stage, low-to-medium rate uniform pre-cooling, slowly releases residual thermal stress from the workpiece forming process, effectively suppressing defects such as thermal stress cracking and micro-deformation caused by rapid cooling. The second stage, high-rate final cooling, quickly locks in the supersaturated solid solution structure, ensuring sufficient solid solution strengthening of the aluminum alloy. This perfectly balances the integrity of the workpiece forming with the degree of material supersaturation, solving the technical problem that traditional single-cooling-rate processes cannot simultaneously address forming quality and mechanical properties, resulting in a significant improvement in the overall mechanical properties of the inner liner. Attached Figure Description

[0015] Figure 1 This is a flowchart of the steps of the present invention. Detailed Implementation

[0016] The following description is intended to disclose the invention and enable those skilled in the art to implement it. The preferred embodiments described below are merely examples, and other obvious variations will occur to those skilled in the art.

[0017] This invention provides an integrated forming process for the aluminum alloy inner liner of a hydrogen storage cylinder. The core concept is to integrate hot drawing and online quenching in the same mold, and to introduce ultrasonic vibration to assist in friction reduction during the hot drawing stage, and to adopt a two-stage differentiated cooling path of slow cooling followed by fast cooling during the quenching stage, thereby achieving a synergistic improvement in forming quality and mechanical properties.

[0018] The following combination Figure 1 As shown, the specific implementation methods for each step are described in detail.

[0019] Step S1: Heat the billet to the solution temperature range and hold it at that temperature to homogenize it. The purpose of step S1 is to heat the aluminum alloy billet to the solution temperature range so that the strengthening phase elements in the alloy can be fully dissolved into the aluminum matrix to form a uniform supersaturated solid solution, providing high-temperature plasticity conditions for subsequent hot deep drawing, and laying the foundation for online quenching to lock the supersaturated structure.

[0020] In practice, 6061 aluminum alloy sheet or cylindrical billet is selected as the raw material for forming the inner liner. 6061 aluminum alloy belongs to the Al-Mg-Si system and is a heat-treatable alloy with a solution treatment temperature range of 500–545℃. The billet is placed in a heating furnace and heated to this temperature range, and held at this temperature until the temperature inside and outside the billet becomes uniform. The holding time is determined according to the billet wall thickness, typically 20–40 minutes, to ensure that the temperature difference between the core and surface of the billet does not exceed 10℃.

[0021] During the billet heating process, nitrogen is introduced into the furnace as an inert protective atmosphere to keep the oxygen content below 500 ppm. The purpose of nitrogen protection is that aluminum alloys readily react with oxygen at temperatures above 500°C to form a dense alumina film. An excessively thick oxide film not only reduces the surface quality of the billet but also embeds as hard particles into the workpiece surface during subsequent deep drawing, causing surface scratches and increased frictional resistance. By controlling the oxygen content in the furnace to below 500 ppm, the oxide layer thickness on the billet surface can be controlled to within 1 μm, thus ensuring the surface cleanliness of the billet when it enters the die.

[0022] The selection of the solution treatment temperature range is based on the following: below 500℃, the Mg2Si strengthening phase cannot dissolve sufficiently, and insufficient solution treatment will directly lead to insufficient subsequent aging strengthening effect; above 545℃, overheating is prone to occur at the grain boundaries, resulting in local melting of the grain boundaries and a sharp decline in mechanical properties. Therefore, the solution treatment temperature range of 500~545℃ can ensure sufficient dissolution of the strengthening phase while avoiding the risk of overheating.

[0023] Step S2: Hot deep drawing with ultrasonic vibration assistance Step S2 is the core forming step of the present invention. The aluminum alloy billet heated to the solution temperature is transferred to a hot drawing die equipped with a heating element. Hot drawing is performed under the condition that the die temperature is maintained at 450-500°C. At the same time, ultrasonic vibration is applied to the punch during the hot drawing process.

[0024] Hot drawing die structure Hot drawing dies include an upper die base, a die cavity, a punch, a lower die base, and a blank holder, as well as guide pillars and guide bushings to ensure the accuracy of the upper and lower die closing. The core feature of the die is that the punch, die cavity, and blank holder are all equipped with heating elements (a punch heating element and a die cavity heating element, respectively), and adopt a segmented independent temperature control method to ensure that the temperature deviation of each area of ​​the die does not exceed ±5℃.

[0025] The necessity of segmented independent temperature control lies in the significant differences in deformation degree and heat dissipation conditions among different parts of the billet during hot drawing. The billet deforms violently at the punch corners and has a large contact area with the die, resulting in rapid heat dissipation, while the deformation in the middle of the cylinder wall is smaller and heat dissipation is slower. If a single temperature control is used, the actual temperature deviation in different areas of the die can reach ±15℃ or more, leading to uneven deformation resistance in different parts of the billet, which in turn causes wall thickness deviations and localized necking. By controlling the temperature deviation within ±5℃ through segmented independent temperature control, the consistency of deformation resistance in different parts of the billet within the die can be ensured, thereby obtaining a formed workpiece with uniform wall thickness.

[0026] The reason for maintaining the mold temperature at 450-500℃ is that this temperature range is within the optimal hot working temperature range of 6061 aluminum alloy. The yield strength of the material is reduced by about 60% compared with room temperature, and the plasticity is significantly improved, which is conducive to deep drawing with large deformation. At the same time, this temperature is close to the solution temperature range of the billet (500-545℃). After the billet is transferred to the mold, the temperature drop is limited (usually 10-30℃), and it is still within the solution temperature range. The strengthening phase will not precipitate prematurely due to the temperature drop.

[0027] Ultrasonic vibration-assisted friction reduction During the hot drawing process, ultrasonic vibration with a frequency of 15–25 kHz and an amplitude of 5–15 μm is applied to the punch. The ultrasonic vibration is applied through a magnetostrictive ultrasonic transducer mounted on the press slide, and the transducer is connected to the punch via a heat-insulating vibration transmission rod. This heat-insulating vibration transmission rod is made of a titanium alloy with low thermal conductivity, used to transmit ultrasonic vibration energy while isolating the high temperature of the die from heat conduction to the ultrasonic transducer, thus preventing the transducer's magnetostrictive properties from attenuating or being damaged due to high temperatures.

[0028] The principle of ultrasonic vibration-assisted friction reduction includes two aspects: Acoustic capillary effect: Ultrasonic vibration generates high-frequency microscopic separation motion at the interface between the punch and the workpiece, changing the contact state between the workpiece and the die surface from continuous contact to high-frequency intermittent contact. At the moment of microscopic separation, the microscopic valleys on the workpiece surface draw in lubricating medium through capillary action, forming a dynamic lubricating film. This effect reduces the effective coefficient of friction of the contact surface by 30% to 50%, significantly reducing the frictional resistance between the workpiece and the punch.

[0029] Friction degradation effect: High-frequency ultrasonic vibration transforms the static friction state of the punch surface into a dynamic friction state. According to tribological principles, the coefficient of dynamic friction is usually lower than the coefficient of static friction, and under high-frequency vibration conditions, the periodic change in the relative sliding velocity of the interface further reduces the average frictional force. This effect directly reduces the tensile stress transmitted through the cylinder wall during deep drawing, resulting in a more uniform stress distribution.

[0030] The synergistic effect of the two aforementioned effects effectively suppresses the problems of uneven wall thinning and stress concentration at the punch fillet during traditional hot drawing processes. Without ultrasonic vibration, the maximum wall thinning rate can typically reach 15%–25%, and the stress concentration factor at the punch fillet can reach 2.5–3.0. With the introduction of ultrasonic vibration, the maximum wall thinning rate can be controlled within 8%, and the stress concentration factor at the punch fillet is reduced to 1.5–1.8, significantly reducing the risk of deep drawing cracks and excessive wall thickness.

[0031] Basis for selecting ultrasonic vibration parameters: Frequency 15-25kHz: Below 15kHz, the vibration noise is too large and the friction reduction effect is not obvious; above 25kHz, the energy attenuation is accelerated and the vibration transmission efficiency is reduced.

[0032] Amplitude 5–15 μm: When it is below 5 μm, the acoustic capillary effect is insufficient and the friction reduction effect is limited; when it is above 15 μm, the punch amplitude is too large, which may affect the drawing dimensional accuracy and aggravate the fatigue wear of the die.

[0033] Hot drawing process parameters During the hot deep drawing process, the blank holder force is controlled at 5-15 MPa, and the deep drawing speed is controlled at 5-30 mm / s.

[0034] The purpose of blank holder force is to prevent wrinkling in the flange area of ​​the billet during deep drawing. However, it must not be too high to avoid increasing frictional resistance in the flange area and causing the cylinder wall to crack. Experiments have verified that a blank holder force range of 5–15 MPa can effectively control flange wrinkling within this temperature range while limiting flange frictional resistance to a reasonable range. Specifically, a higher blank holder force (12–15 MPa) is used for billets with larger wall thickness (≥3 mm), and a lower blank holder force (5–8 MPa) is used for thin-walled billets (≤3 mm).

[0035] The selection of drawing speed must take into account both sufficient material flow and temperature maintenance. When the speed is too fast (>30mm / s), the material does not have enough time to flow, which can easily cause local necking at the punch radius; when the speed is too slow (<5mm / s), the billet stays in the die for too long, the temperature drop is greater, which may lead to increased material deformation resistance and reduced plasticity.

[0036] Mold cavity purging steps After the hot drawing process in step S2 is completed and before the quenching process in step S3 is started, a mold cavity purging step is also included: high-pressure inert gas is introduced into the mold cavity to remove residual lubricant and oxide scale. The pressure of the high-pressure inert gas is 0.4 to 0.8 MPa, and the purging duration is 3 to 8 seconds.

[0037] The technical purpose of this step is as follows: During hot drawing, the lubricant partially carbonizes or decomposes at high temperatures, forming carbide residues that adhere to the workpiece surface and the inner wall of the mold cavity. Simultaneously, trace amounts of oxide scale inevitably form on the billet at high temperatures. If not removed by purging, these carbide residues and oxide scale will be washed onto the workpiece surface by the quenching medium during subsequent quenching, forming surface inclusion defects and reducing the sealing performance and fatigue life of the inner liner. Short-term purging with high-pressure inert gas (0.4–0.8 MPa) can effectively remove over 90% of the residues, ensuring the cleanliness of the workpiece and mold cavity before quenching.

[0038] Inert gases such as nitrogen or argon should be used for purging to avoid using air and prevent secondary oxidation of the workpiece during the purging process.

[0039] Step S3: Two-stage differentiated online quenching and pressure holding constraint Step S3 is another core step that distinguishes this invention from the prior art. After the hot drawing is completed, the workpiece is not removed. The punch and die of the mold are opened by 10-20mm, and the formed workpiece is subjected to two-stage differentiated online quenching through the cooling channels set inside the punch and die.

[0040] Mold opening and cooling channel structure The purpose of opening the mold by 10-20mm is to provide space for the quenching medium to flow, allowing it to directly contact the inner and outer surfaces of the workpiece. If the opening gap is too small (<10mm), the flow resistance of the quenching medium is high, the flow rate is insufficient, and the cooling rate cannot meet the solidification requirements. If the opening gap is too large (>20mm), the workpiece loses the positioning constraint of the mold, and displacement or warping may occur during the quenching process.

[0041] The cooling channels include a spiral main cooling channel and an annular auxiliary cooling channel. The spiral main cooling channel is arranged spirally along the working surfaces of the punch and die, creating a spiral flow path for the quenching medium along the workpiece surface and ensuring uniform contact between the cooling medium and the workpiece surface. The annular auxiliary cooling channel is arranged along the circumferential direction of the ends of the punch and die, providing supplementary cooling to weak heat dissipation areas such as the corners at the ends of the workpiece and eliminating cooling dead zones. The inlet and outlet of the cooling channels are both located outside the mold. The quenching medium enters from the inlet and exits from the outlet, achieving continuous circulation and renewal of the medium.

[0042] Furthermore, the blank holder of the hot drawing die is uniformly equipped with several spray holes. During the quenching stage, quenching medium is sprayed onto the outer wall of the inner liner through these spray holes, working in conjunction with the internal cooling channels of the punch and die to achieve simultaneous cooling of both the inner and outer walls of the inner liner. The significance of this simultaneous cooling technology lies in the fact that when cooling is achieved solely through the internal cooling channels of the die, the cooling rate of the outer wall (near the die side) is higher than that of the inner wall (near the punch side), resulting in a temperature difference of 50–80°C between the inner and outer walls. The resulting asymmetric thermal stress is the main cause of excessive ovality and warping deformation in the workpiece. By simultaneously spraying quenching medium onto the outer wall through the spray holes of the blank holder, the temperature difference between the inner and outer walls can be controlled within 20°C, significantly reducing quenching deformation.

[0043] Two-stage differentiated quenching process The quenching process employs a two-stage differentiated cooling path: a slow initial cooling followed by a rapid cooling. The first stage involves uniform pre-cooling of the workpiece using an atomized gas-liquid two-phase flow: the workpiece is uniformly pre-cooled using an atomized gas-liquid two-phase flow at a cooling rate controlled at 30–50 °C / s, continuing until the workpiece surface temperature drops to 300–350 °C. The atomizing medium is a 5%–15% polymer-based water-based quenching fluid, and the gas pressure is controlled at 0.3–0.6 MPa.

[0044] The technical basis for using a medium-low cooling rate for pre-cooling in the first stage is as follows: After hot deep drawing, there is a large residual stress inside the workpiece (especially at the cylinder wall and bottom corner). If a high cooling rate is used for quenching directly, the huge temperature gradient between the workpiece surface and the core will generate thermal stress as high as 150-200 MPa. When superimposed with the residual stress, this stress can easily exceed the high-temperature yield strength of the material, causing microcracks or even penetrating cracks. The first stage uses a medium cooling rate of 30-50℃ / s to slowly cool the workpiece from the solution temperature (about 500℃) to 300-350℃. Within this temperature range, aluminum alloys have good plasticity and low yield strength, and the residual stress can be partially released through plastic deformation. When the workpiece temperature drops to 300-350℃, about 60%-70% of the residual stress has been released, creating a low-stress foundation for subsequent rapid cooling.

[0045] The selection criteria for the concentration of polymer-based water-based quenching fluid are as follows: When the concentration is below 5%, the cooling characteristics are close to those of pure water, resulting in an excessively fast cooling rate and the formation of an unstable vapor film on the workpiece surface, leading to uneven cooling; when the concentration is above 15%, the polymer film formed on the workpiece surface is too thick, significantly reducing the cooling rate in the first stage and prolonging the process cycle. A concentration range of 5% to 15% strikes a balance between cooling uniformity and cooling rate.

[0046] The second stage involves rapid final cooling with high-pressure water jets: once the workpiece surface temperature drops to 300–350°C, the process switches to rapid final cooling with high-pressure water jets, increasing the cooling rate to 80–120°C / s, allowing the workpiece to cool to below 100°C within 60 seconds. The jet pressure of the high-pressure water jet is controlled between 1.5 and 3.0 MPa, and the distance between the nozzle and the workpiece surface is 15–40 mm.

[0047] The technical basis for using a high-rate final cooling in the second stage is that when the workpiece temperature drops below 300–350°C, the strengthening phase (such as the Mg2Si phase) in the aluminum alloy begins to precipitate from the supersaturated solid solution. If the cooling rate is insufficient, the strengthening phase will precipitate in a coarse form, resulting in the inability to achieve a fine and dispersed precipitation strengthening effect in subsequent aging treatment, ultimately leading to a significant reduction in mechanical properties. A high cooling rate of 80–120°C / s ensures that the workpiece quickly passes through the precipitation-sensitive zone within this temperature range, "freezing" the supersaturated solid solution structure to room temperature and preserving the solution treatment effect to the maximum extent. Cooling the workpiece to below 100°C within 60 seconds ensures that the amount of strengthening phase precipitation is controlled to a negligible level.

[0048] The synergistic effect of the slow-then-fast cooling path: The first stage of slow cooling releases residual stress and avoids thermal stress cracking, while the second stage of rapid cooling locks in the supersaturated microstructure and ensures the aging strengthening effect. This path effectively solves the technical contradiction of traditional single-cooling-rate processes: "rapid cooling leads to cracking, while slow cooling leads to weakness."

[0049] Pressure holding constraint After quenching, the mold is closed again, and the mold is used to apply pressure constraint to the formed workpiece. The pressure is 20-50 MPa, and the holding time is 30-90 s.

[0050] The technical function of pressure holding constraint is as follows: During the quenching process, the workpiece is rapidly cooled from a high temperature to a low temperature. Due to the thermal expansion and contraction characteristics of the material and the difference in cooling rates of different parts of the workpiece, quenching stress and springback deformation are inevitably generated. If no constraint is applied to the workpiece after quenching, the workpiece will spring back in a free state, leading to dimensional accuracy problems such as out-of-tolerance ovality and non-roundness of the opening. By immediately closing the mold after quenching and applying a normal holding pressure of 20-50 MPa and holding it for 30-90 seconds, the workpiece is constrained and shaped by the mold at a low temperature (below 100℃). Utilizing the high yield strength of the material at low temperatures, the workpiece completes the redistribution of residual stress and dimensional stability under constraint. After the pressure holding is completed, the workpiece temperature is usually near room temperature, and the springback after demolding can be controlled within 0.1 mm.

[0051] Step S4: Manual Aging Processing After quenching, open the mold and remove the workpiece. Transfer the workpiece to an aging furnace for artificial aging treatment. The aging temperature is 150–180℃, and the holding time is 6–12 hours.

[0052] The technical principle of artificial aging: After quenching, the aluminum alloy matrix is ​​in a supersaturated solid solution state. The diffusion rate of solute atoms (Mg, Si, etc.) at room temperature is extremely low, preventing the spontaneous precipitation of strengthening phases. By heating at 150–180℃ and holding for 6–12 hours, the solute atoms gain sufficient diffusion activation energy, precipitating from the supersaturated solid solution to form nanoscale dispersed Mg2Si strengthening phases (β'' phase and β' phase). These dispersed strengthening phases effectively hinder dislocation movement through the Orovan bypass and shear-through mechanisms, significantly improving the alloy strength. However, over-aging (too high a temperature or too long a time) can cause the β'' phase to transform into the β' and β phases, reducing the strengthening effect. Therefore, temperature and time must be strictly controlled.

[0053] For applications requiring higher strength (such as high-pressure hydrogen storage cylinders above 70MPa), a two-stage aging process can be used: the first stage involves holding at 120℃ for 6 hours to promote uniform nucleation of the GP region; the second stage involves holding at 160℃ for 10 hours to allow the GP region to grow into a fine β'' phase. Two-stage aging can achieve higher strength than single-stage aging, but the elongation is slightly reduced.

[0054] Example Example 1: Standard process for 6061 aluminum alloy This embodiment uses 6061 aluminum alloy sheet as raw material to prepare a Type III hydrogen storage cylinder aluminum alloy inner liner with a wall thickness of 3.5mm. The specific process parameters are as follows: S1: Heat the 6061 aluminum alloy round billet to 520℃ (within the solution temperature range of 500~545℃), fill the furnace with nitrogen to control the oxygen content below 300ppm, and hold for 30 minutes until the billet temperature is uniform.

[0055] S2: The heated billet is rapidly transferred to the hot drawing die (transfer time approximately 5 seconds, billet temperature drops approximately 15°C), while the die temperature is maintained at 480°C. The blank holder force during hot drawing is 10 MPa, and the drawing speed is 15 mm / s. Simultaneously, a magnetostrictive ultrasonic transducer applies ultrasonic vibration at a frequency of 20 kHz and an amplitude of 10 μm to the punch. The transducer and punch are connected by a titanium alloy heat-insulated vibration transmission rod.

[0056] Purging: After hot drawing is completed, nitrogen gas at 0.6 MPa is introduced into the mold cavity for 5 seconds to purge residual lubricant and oxide scale.

[0057] S3: Open the punch and die by 15mm. The first stage involves pre-cooling with a 10% concentration polymer water-based quenching fluid atomized into a gas-liquid two-phase flow at a gas pressure of 0.4MPa and a cooling rate of approximately 40℃ / s, continuing until the workpiece surface temperature drops to 320℃. The second stage switches to 2.0MPa high-pressure water jet final cooling, with the nozzle 25mm from the workpiece surface and a cooling rate of approximately 100℃ / s, cooling the workpiece to 80℃ within 50s. After quenching, close the mold and hold the pressure at 35MPa for 60s.

[0058] S4: Remove the workpiece and place it in an aging furnace to artificially age it at 175℃ for 8 hours.

[0059] Performance test results: tensile strength 350MPa, yield strength 295MPa, elongation 14%, wall thickness deviation ±0.08mm, grain size grade 7, surface oxide layer thickness <1μm, residual stress 45MPa, yield 97%.

[0060] Example 2: Two-stage aging process for 7075 aluminum alloy This embodiment uses 7075 aluminum alloy sheet as raw material to prepare an aluminum alloy inner liner for a high-pressure hydrogen storage cylinder with a wall thickness of 4.0 mm and a pressure of over 70 MPa. The specific process parameters are as follows: S1: Heat the 7075 aluminum alloy billet to 530℃, fill the furnace with nitrogen to control the oxygen content below 200ppm, and hold for 35 minutes.

[0061] S2: The die temperature is maintained at 500℃, the blank holder force is 12MPa, the drawing speed is 10mm / s, and the ultrasonic vibration frequency is 22kHz with an amplitude of 12μm.

[0062] Purging: Purging with 0.7MPa nitrogen for 6s.

[0063] S3: Mold opening 18mm. The first stage involves pre-cooling with a 15% concentration polymer water-based quenching liquid at a gas pressure of 0.5MPa and a cooling rate of approximately 45℃ / s, until the workpiece surface temperature drops to 330℃. The second stage involves final cooling with a 2.5MPa high-pressure water jet, with the nozzle 20mm from the workpiece, at a cooling rate of approximately 110℃ / s, cooling the workpiece to 70℃ within 45s. The mold is then closed and held at 40MPa for 70s.

[0064] S4: Adopts a two-stage aging process, with the first stage at 120℃ for 6 hours and the second stage at 160℃ for 10 hours.

[0065] Performance test results: tensile strength 525MPa, yield strength 460MPa, elongation 11%, wall thickness deviation ±0.06mm, grain size grade 7, surface oxide layer thickness <1μm, residual stress 38MPa, yield 96%.

[0066] Example 3: Thin-walled cylindrical billet process This embodiment uses 6061 aluminum alloy cylindrical blank as raw material to prepare an aluminum alloy inner liner for a hydrogen storage cylinder with a wall thickness of 2.5mm (≤3mm thin wall). The specific process parameters are as follows: S1: Heat the cylindrical billet to 510℃, fill the furnace with nitrogen to control the oxygen content below 350ppm, and hold for 25 minutes; S2: The die temperature is maintained at 460℃, the blank holder force is 8MPa, the drawing speed is 20mm / s, and the ultrasonic vibration frequency is 18kHz with an amplitude of 8μm. Purging: Purging with 0.5 MPa nitrogen for 4 seconds; S3: Mold opening 12mm. The first stage uses an 8% polymer water-based quenching liquid for atomized pre-cooling at a gas pressure of 0.3MPa and a cooling rate of approximately 35℃ / s, until the workpiece surface temperature drops to 310℃. The second stage uses a 1.8MPa high-pressure water jet for final cooling, with the nozzle 30mm from the workpiece and a cooling rate of approximately 85℃ / s, cooling the workpiece to 85℃ within 55s. Mold closing pressure is maintained at 30MPa for 45s. S4: Artificial aging at 170℃ for 10 hours; Performance test results: tensile strength 345MPa, yield strength 290MPa, elongation 15%, wall thickness deviation ±0.05mm, grain size grade 7, surface oxide layer thickness <1μm, residual stress 40MPa, yield 98%.

[0067] Comparative Example 1: Traditional Discrete Process The traditional discrete machining mode of "forming first and then quenching independently" is adopted: Heat the billet to 520℃ and hold for 30 minutes; Transfer to a standard hot drawing die (480℃ die temperature) for hot drawing without applying ultrasonic vibration; After forming, the workpiece is demolded and transferred to an independent quenching tank (transfer time is about 15s, and the workpiece temperature drops from about 500℃ to about 380℃). Single-stage quenching is performed in room temperature water (cooling rate is about 80℃ / s). Artificial aging at 175℃ for 8 hours.

[0068] Comparative Example 2: Ultrasonic Vibration Assisted + Single-Stage Online Quenching Ultrasonic vibration is used to assist in single-stage quenching. Heat the billet to 520℃ and hold for 30 minutes; The mold is hot-drawn at 480℃ and subjected to ultrasonic vibration at 20kHz / 10μm. Online single-stage water quenching (cooling rate approximately 80℃ / s), without a pre-cooling stage; Artificial aging at 175℃ for 8 hours.

[0069] Comparative Example 3: No ultrasonic vibration + two-stage online quenching Two-stage quenching is used without applying ultrasonic vibration: Heat the billet to 520℃ and hold for 30 minutes; The mold is hot-drawn at 480℃ without ultrasonic vibration. Two-stage differentiated online quenching (first stage from 40℃ / s to 320℃, second stage from 100℃ / s to 80℃), with a pressure holding constraint of 35MPa / 60s; Artificial aging at 175℃ for 8 hours.

[0070] The following table is a performance comparison table: Comparative analysis Comparing Comparative Example 1 and Example 1, the traditional discrete process requires demolding and transfer to an independent quenching tank after forming. During the transfer, the workpiece temperature drops from about 500°C to about 380°C, resulting in the following chain of problems: ① Temperature loss causes the supersaturated solid solution to precipitate coarse strengthening phases prematurely during the transfer, significantly reducing the subsequent aging strengthening effect, with a tensile strength of only 285 MPa, 65 MPa lower than Example 1; ② During the transfer, the workpiece is exposed to air, resulting in an oxide layer 8-12 μm thick on the surface, far exceeding the <1 μm of Example 1; ③ Without ultrasonic vibration assistance, the cylinder wall is severely thinned, with a wall thickness deviation of ±0.25 mm; ④ Single-stage rapid quenching without pressure holding constraint leads to thermal stress concentration, resulting in quenching cracks, residual stress as high as 180 MPa, ellipticity of 1.2 mm, and a yield rate of only 82%.

[0071] Compared with Example 1, Comparative Example 2 introduced ultrasonic vibration, which improved the wall thickness deviation and surface quality. However, it used single-stage water quenching (without a pre-cooling stage), where the workpiece was rapidly cooled directly from approximately 500°C at 80°C / s. The residual stress from forming was not relieved, and after being superimposed with the quenching thermal stress, quenching cracks still occurred, with a residual stress of 95 MPa. Due to the presence of quenching cracks, the yield rate (91%) was still lower than that of Example 1 (97%). Furthermore, the cooling rate was too high during the high-temperature stage of single-stage quenching, and the vapor film encapsulation in some areas led to uneven cooling, resulting in lower tensile strength (325 MPa) and elongation (12%) compared to Example 1.

[0072] Comparing Comparative Example 3 with Example 1, Comparative Example 3 employed two-stage differentiated quenching and pressure holding constraints, eliminating quenching cracks, reducing residual stress to 120 MPa, and ellipticity to 0.3 mm. However, due to the absence of ultrasonic vibration, the problems of thinning of the cylinder wall and stress concentration at the punch fillet still existed during hot drawing, with a wall thickness deviation of ±0.18 mm, significantly greater than ±0.08 mm in Example 1. Furthermore, the non-uniform material flow under conditions without ultrasonic vibration resulted in a grain size of only grade 5 (grade 7 in Example 1), and both tensile strength (310 MPa) and elongation (10%) were lower than in Example 1.

[0073] Synergistic effect analysis shows that the above comparison demonstrates that the three core technical features of this invention (ultrasonic vibration assistance, integrated online quenching, and dual-stage differentiated cooling path) are not simply additive, but rather exhibit a significant synergistic effect: Ultrasonic vibration assists in solving the problems of wall thickness uniformity and material flow consistency during the forming stage, providing a high-quality blank with uniform wall thickness and reasonable residual stress distribution for subsequent quenching. Integrated online quenching eliminates temperature loss and surface oxidation during the transfer process, ensuring the integrity of the solid solution structure and maximizing the effect of subsequent aging strengthening. The dual-stage differentiated cooling path rapidly locks in the supersaturated structure under low stress, taking into account both quenching integrity (no cracking) and mechanical properties (not weak), and the pressure holding constraint controls quenching deformation.

[0074] The absence of any one of these three characteristics will lead to a significant deterioration in the performance of a certain stage: the lack of ultrasonic vibration results in uneven wall thickness and coarse grains (Comparative Example 3); the lack of integrated quenching results in loss of solid solution structure and severe oxidation (Comparative Example 1); and the lack of double-stage quenching results in thermal stress cracking and high residual stress (Comparative Example 2). Only when all three are combined can the optimal overall performance of ±0.08mm wall thickness deviation, 350MPa tensile strength, 14% elongation, and 97% yield be achieved.

[0075] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A one-piece forming process for the aluminum alloy inner liner of a hydrogen storage cylinder, characterized in that, The specific steps are as follows: S1. Heat the aluminum alloy billet to the solution temperature range and hold it at that temperature until the billet temperature is uniform. S2. Transfer the aluminum alloy billet heated to the solution temperature to a hot drawing die equipped with a heating element, and perform hot drawing while maintaining the die temperature at 450-500°C; during the hot drawing process, apply ultrasonic vibration with a frequency of 15-25 kHz and an amplitude of 5-15 μm to the punch. S3. After hot drawing is completed, the workpiece is not removed. The punch and die of the mold are opened by 10-20 mm. The formed workpiece is subjected to two-stage differentiated online quenching through the cooling channels set inside the punch and die: the first stage is uniform pre-cooling with atomized gas-liquid two-phase flow, with the cooling rate controlled at 30-50℃ / s, until the surface temperature of the workpiece drops to 300-350℃; the second stage is switched to high-pressure water jet for rapid final cooling, with the cooling rate increased to 80-120℃ / s, so that the workpiece is cooled to below 100℃ within 60s. S4. After quenching, open the mold and take out the workpiece. Transfer the workpiece to the aging furnace for artificial aging treatment.

2. The integrated forming process for the aluminum alloy inner liner of a hydrogen storage cylinder according to claim 1, characterized in that: The aluminum alloy mentioned in S1 is 6061 aluminum alloy, and the solution treatment temperature range is 500-545℃. When heating the billet, nitrogen is introduced into the furnace as an inert protective atmosphere to keep the oxygen content in the furnace below 500ppm, so as to inhibit the surface oxidation of the aluminum alloy billet during the solution treatment heating process.

3. The integrated forming process for the aluminum alloy inner liner of a hydrogen storage cylinder according to claim 1, characterized in that: In the hot drawing process of S2, the blank holder force is controlled at 5-15 MPa, and the drawing speed is controlled at 5-30 mm / s. The punch, die and blank holder of the hot drawing die are all equipped with heating elements, and the segmented independent temperature control method is adopted to ensure that the temperature deviation of each area of ​​the die does not exceed ±5℃.

4. The integrated forming process for the aluminum alloy inner liner of a hydrogen storage cylinder according to claim 1, characterized in that: In S3, the atomizing medium for the first stage of the atomized gas-liquid two-phase flow is a polymer water-based quenching liquid with a concentration of 5% to 15%, and the gas pressure is controlled at 0.3 to 0.6 MPa; the injection pressure of the second stage high-pressure water jet is controlled at 1.5 to 3.0 MPa, and the distance between the nozzle and the workpiece surface is 15 to 40 mm.

5. The integrated forming process for the aluminum alloy inner liner of a hydrogen storage cylinder according to claim 1, characterized in that: The holding pressure in the holding constraint stage of S3 is 20–50 MPa, and the holding time is 30–90 s.

6. The integrated forming process for the aluminum alloy inner liner of a hydrogen storage cylinder according to claim 1, characterized in that: The temperature for artificial aging treatment in S4 is 150–180 ℃, and the holding time is 6–12 h.

7. The integrated forming process for the aluminum alloy inner liner of a hydrogen storage cylinder according to claim 1, characterized in that: Between S2 and S3, there is also a mold cavity purging step. After the hot drawing is completed and before quenching is started, high-pressure inert gas is introduced into the mold cavity to remove residual lubricant and oxide scale. The pressure of the high-pressure inert gas is 0.4 to 0.8 MPa, and the purging duration is 3 to 8 seconds.

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