Thin-wall metal bulging forming method based on liquid phase change expansion

Abstract

The invention relates to the technical field of thin-wall metal part plastic forming, in particular to a thin-wall metal bulging forming method based on liquid phase change expansion, which comprises three continuous steps of pretreatment, freezing bulging and post-treatment, and specifically comprises the following steps: pretreatment: selecting a thin-wall metal pipe, and carrying out surface friction optimization treatment on the inner wall of the thin-wall metal pipe; an elastic buffer layer is arranged between the inner wall of the thin-wall metal pipe and the phase change medium; a phase change medium is injected into the thin-wall metal pipe, the filling rate of the phase change medium is 100%, freezing bulging is conducted, specifically, the pretreated thin-wall metal pipe serves as a forming part, and the thin-wall metal pipe is loaded into a precision forming tool and sealed; and post-treatment is conducted, specifically, stress relief treatment is conducted on the bulged thin-wall metal pipe, and the stress relief treatment is a 300-DEG C vacuum annealing process. According to the method, the defects of an existing bulging technology are remarkably overcome, and breakthrough is achieved in the aspects of technical performance, cost efficiency, environmental protection, safety and application expansion.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of plastic forming of thin-walled metal parts, in particular to a local bulging forming method for thin-walled metal pipes based on liquid phase change expansion (specifically, the volume expansion of water-based medium frozen into ice), which is especially suitable for local asymmetric bulging processing of thin-walled metal pipes (such as stainless steel pipes, titanium alloy pipes, and nickel-titanium alloy pipes) with high requirements for forming precision, surface quality, and low-temperature resistance in the fields of aerospace, biomedical treatment, and micro-channel heat exchange. BACKGROUND

[0002] Local bulging forming of thin-walled metal pipes (wall thickness usually ≤0.5 mm) is a key process for preparing complex structure metal components (such as micro-channel heat exchanger protrusions, blood vessel stents, and aviation hydraulic pipeline branches). The mainstream bulging technology in the industry has significant defects, which is difficult to meet the industrialization needs of high precision, low damage, and low cost. The specific problems are as follows: Traditional hydraulic bulging technology relies on a high-pressure pump station to provide a hydraulic driving force of ≥100 MPa, and requires a complex sealing structure to prevent high-pressure oil leakage. On the one hand, the high-pressure system has very high energy consumption (about 12 kWh per piece of processing energy consumption), and the equipment purchase and maintenance cost is high. On the other hand, sealing failure can easily lead to oil leakage, which not only pollutes the workpiece and the environment, but also reduces the forming precision due to pressure fluctuations (forming tolerance ±0.3 mm). In addition, the hydraulic bulging has poor adaptability for local bulging of asymmetric and multi-curvature, and can only process axisymmetric structures, which cannot meet the forming needs of complex components; Traditional mechanical tube expanding technology applies mechanical force to the inner wall of the metal pipe through a rigid die or a ball to achieve expansion, which can easily cause scratches on the inner wall of the pipe, resulting in a surface roughness Ra>3.2 μm, and additional polishing treatment is required, which increases the process cost. At the same time, the rigid transmission of mechanical force can easily cause uneven wall thickness (wall thickness reduction rate 12%~18%), and the local deformation area cannot be precisely controlled, especially for thin-walled pipes (wall thickness ≤0.5 mm), which can easily cause pipe rupture; Traditional explosive forming technology uses the shock wave generated by the explosion of explosives to drive metal deformation, which can achieve complex shape forming, but has high safety risks and requires a special explosion-proof site. In addition, the explosion energy is difficult to control accurately, the forming precision is poor (tolerance ±0.5 mm), and the workpiece consistency is low, which cannot be used for mass production; Existing liquid phase change bulging (such as water ice expansion-based forming technology) has been initially studied due to its cleanliness and low energy consumption, but the existing technology is only limited to overall expansion scenarios such as pipe anti-cracking, and cannot meet the local bulging forming needs of precision components, with the following core bottlenecks: The prior art adopts a whole freezing mode, and cannot realize the precise control of "expansion of a target region and no deformation of a non-target region", leading to a blurred boundary of the expanded region and failing to meet the local forming requirements of multiple curvatures and asymmetry (such as the local convex structure of a micro-channel heat exchanger); The thin-walled metal pipe (especially 304 stainless steel, titanium alloy) has significantly enhanced brittleness at low temperature, for example, the impact energy of 304 stainless steel is less than 5 J at -196 ℃, and direct freezing is easy to cause the pipe body to crack. The prior art does not take effective measures against brittle cracking, which limits the application of the technology to thin-walled metal pipes; When water ice is naturally frozen, the growth direction of ice crystals is random, the expansion force is isotropically distributed, the radial expansion efficiency is low, the pipe diameter expansion rate is less than 5%, and the wall thickness reduction rate of the formed workpiece is high and the surface roughness is poor, which cannot meet the technical requirements of precision components (such as the surface roughness Ra of a biomedical implant should be less than 0.4 μm); In summary, the existing expansion technology has multiple defects in local control, low-temperature safety, precision, energy consumption and cost, and there is an urgent need for a thin-walled metal forming method that can realize precise local expansion, avoid low-temperature brittle cracking, and balance high precision and low cost. SUMMARY

[0003] The present application aims to overcome the defects of the existing expansion technology and solve the following core technical problems: How to realize precise local expansion of a thin-walled metal pipe, control the boundary of the deformed region and meet the asymmetric and multiple curvature forming requirements; How to avoid brittle cracking of a thin-walled metal pipe during low-temperature freezing and improve the safety of the forming; How to improve the radial efficiency of liquid phase change expansion, reduce the wall thickness reduction rate and improve the forming precision and surface quality; How to simplify the forming equipment, reduce energy consumption and production cost and realize environmentally friendly and sustainable production.

[0004] TECHNICAL SCHEME To solve the above technical problems, the present application provides a thin-walled metal expansion forming method based on liquid phase change expansion, which realizes precise local expansion of a thin-walled metal pipe through the coordinated design of three continuous steps of "pretreatment-freezing expansion-post-treatment" and three core innovations of local gradient freezing, ice crystal directional growth and elastic stress buffering. The specific technical scheme is as follows: Pretreatment step: The core purpose of pretreatment is to optimize the inner wall state of the metal pipe, build a stress buffer layer and inject a high-performance phase change medium, laying a foundation for subsequent precise expansion. The specific operation is as follows: Selection and inner wall treatment of thin-walled metal pipe: select 304 stainless steel pipe, 316L stainless steel pipe, titanium alloy pipe, composite material pipe or nickel-titanium alloy pipe as the workpiece to be formed: If it is a stainless steel tube (304 / 316L): the outer diameter is Φ10~50mm, and the wall thickness is 0.3mm; the inner wall is electrolytically polished to make the surface roughness Ra≤0.2μm, reducing the frictional resistance between the phase change medium and the tube wall, and avoiding surface scratches during the bulging process.

[0005] If it is a nickel-titanium alloy tube: the inner wall surface roughness Ra after bulging should be less than 0.4μm, meeting the ISO13485 medical device implant surface standard, and suitable for the biomedical field.

[0006] If it is a titanium alloy tube: low-temperature resistant grades are selected, which can withstand the freezing bulging of liquid nitrogen medium at-196℃, and are suitable for the aerospace field.

[0007] Elastic buffer layer setting: an elastic buffer layer is set between the inner wall of the thin-walled metal tube and the phase change medium, which is used to absorb the local stress peak value during the freezing expansion process, and to avoid low-temperature brittle fracture of the tube wall caused by stress concentration. The design parameters of the elastic buffer layer are as follows: Material selection: at least one of silica gel microspheres, polyurethane porous membrane, aerogel, or organic-inorganic composite modified body of the above-mentioned materials (such as composite particles of silica gel microspheres and aerogel) is selected; the composite modified body can have elasticity and thermal insulation, further improving the brittle fracture resistance.

[0008] Form and parameters: If it is a granular material (such as silica gel microspheres): the particle size is 50~500μm, and the uniformity of particle size distribution is ≥90% (detected by a laser particle size analyzer), ensuring uniform thickness of the buffer layer and avoiding local stress unevenness.

[0009] If it is a film-like material (such as polyurethane porous membrane): the thickness is 20~100μm, and the porosity is 40%~60%; the pore structure can accommodate the small deformation of ice crystals during expansion, enhancing the buffering effect.

[0010] Volume ratio and arrangement: the volume ratio of the elastic buffer layer between the tube wall and the phase change medium is 10%~30% (calibrated by weighing and volume method); the arrangement can be pre-coated on the inner wall (by spraying or dipping process), or injected as a suspension with the phase change medium (dispersing the granular buffer material in the phase change medium, and then injecting after uniform stirring).

[0011] Phase change medium injection: the phase change medium is injected into the thin-walled metal tube, and the filling rate of the phase change medium is 100% (no air bubbles are left, and the process is realized by vacuum pumping and injection), ensuring that the expansion force is uniformly transmitted to the tube wall. The formula and performance parameters of the phase change medium are as follows: Ingredients: 2wt% nano-Al2O3 solution in deionized water; particle size of nano-Al2O3 ≤50nm (detected by transmission electron microscope), modified by silane coupling agent (such as KH-550) to improve its dispersion stability in water (no obvious sedimentation after 72h standing, dispersion concentration change rate ≤5% detected by ultraviolet spectrophotometer).

[0012] Key performance: freezing point is -5℃ (measured by differential scanning calorimeter DSC), can freeze and expand in the range of -5℃~-30℃; volume expansion rate during freezing process is 8.9%~9.2% (measured by specific gravity bottle method), and the bonding strength with ice crystals is ≥1.2MPa (interface bonding force measured by tensile testing machine), which ensures that the expansion force is effectively transmitted to the pipe wall.

[0013] Freezing expansion step: freezing expansion is the core step of the present application, which realizes local precise expansion through the process of "precision tooling fixation-directional gradient cooling-pulse magnetic field crystal control-expansion forming-melting cleaning", and the specific operation is as follows: Precision forming tool assembly and sealing: the pretreated thin-walled metal pipe is used as the forming part, which is assembled and sealed in the precision forming tool, the tool is used for positioning the part, isolating the non-expanding area, and transferring the cooling and magnetic field action, and its structure and parameters are as follows: Tool composition: including 2 end face sealing plates, positioning assembly and heat insulation sleeve; End face sealing plate: made of polytetrafluoroethylene material (low temperature resistant, good sealing), with built-in O-shaped sealing ring (fluorine rubber, low temperature resistant-40℃), sealing pressure resistance ≥200MPa, to ensure that the phase change medium does not leak during expansion.

[0014] Positioning assembly: interference fit with the non-expanding area of the thin-walled metal pipe, with a gap ≤0.02mm (detected by three-coordinate measuring instrument), made of 45 steel after quenching treatment (hardness HRC50~55), to avoid the influence of tool deformation on positioning accuracy.

[0015] Heat insulation sleeve: wrapping the non-cooling area of the tool, made of polyurethane foam, thickness 10~15mm, thermal conductivity coefficient ≤0.03W / (m・K), used to block the transfer of cooling energy to the non-expanding area and maintain the temperature gradient.

[0016] Assembly process: insert the thin-walled metal pipe into the center hole of the positioning assembly, and press the two ends with the end face sealing plate to ensure that the non-expanding area is completely wrapped by the positioning assembly and the heat insulation sleeve, and only the target expansion area is exposed to the cooling action range.

[0017] Chambered freezing room and directional cooling: The assembled "tooling-part" assembly is placed in a chambered freezing room, and a directional cooling device is used to apply a low temperature of -5℃ to -30℃ to the target bulging area of the part while maintaining the temperature of the non-bulging area at >0℃, forming an axial and / or circumferential temperature gradient to achieve local frozen expansion. The specific design is as follows: Directional cooling device type: Type I: Semiconductor cooling sheet (model TEC1-12706) is placed close to the outside of the target bulging area of the precision forming tooling (heat transfer is enhanced by thermal silicone grease), with a refrigeration power of ≥50W, which can achieve stable temperature control of -5℃ to -30℃ (adjusted by a PID temperature controller).

[0018] Type II: Liquid nitrogen spraying device, which uses a programmable nozzle (accuracy ±0.1mm) to achieve temperature adjustment of -196℃ to -30℃, with a spraying flow control accuracy of ±0.5L / min (controlled by a mass flow meter), suitable for titanium alloy pipe bulging that requires lower temperatures.

[0019] Temperature gradient control: The temperature control range of the non-bulging area is 0℃ to 10℃ (monitored in real time by the temperature sensor built into the tooling), the temperature difference of the axial temperature gradient is ≥35℃ (e.g. -25℃ for the bulging area and 10℃ for the non-bulging area, with a temperature difference of 35℃), and the temperature difference of the circumferential temperature gradient is ≥30℃. Formation of temperature gradient: The temperature gradient is formed through the cooperation of the heat insulation layer (material: aerogel, thickness: 2-5mm, thermal conductivity: ≤0.02W / (m・K)) built into the tooling and the directional cooling device - the heat insulation layer blocks the spread of cooling energy to the non-bulging area, and the directional cooling device only cools the target area, thus forming a stable temperature gradient.

[0020] Pulsed magnetic field application and ice crystal directional growth: During the freezing process, a pulsed magnetic field application device is used to continuously apply a pulsed magnetic field to the part, causing the ice crystals formed by the phase change medium to grow directionally along the radial direction of the thin-walled metal pipe, improving the radial expansion efficiency and avoiding waste of expansion force. The specific parameters are as follows: Pulsed magnetic field parameters: frequency of 500Hz to 20kHz (adjusted by a signal generator), magnetic field strength of 0.1 to 1T (detected by a gauss meter); the magnetic field direction is arranged along the radial direction of the metal pipe to ensure that the ice crystals grow in the radial direction.

[0021] Pulsed magnetic field application device type: Type I: Electromagnetic coil, number of turns 50-200, winding density 5 turns / cm, radial distance from thin-walled metal pipe ≤10mm (to ensure magnetic field strength), coil material copper enameled wire (high electrical conductivity, low heating), powered by a high-frequency power supply to generate a pulsed magnetic field.

[0022] Type two: permanent magnet array, composed of neodymium iron boron magnetic block (grade N52), magnetic block spacing 3-8 mm, magnetic field uniformity error ≤5% (through the magnetic field distribution instrument detection), suitable for batch production scene without adjusting the magnetic field strength.

[0023] Ice crystal directional growth principle: pulsed magnetic field can change the arrangement direction of water molecules, so that ice crystals grow along the magnetic field direction (i.e. the radial direction of the metal tube), reducing the ineffective expansion in the circumferential or axial direction. Experimental data show that this design can increase the radial expansion efficiency by more than 40% (compared with the radial expansion rate without magnetic field).

[0024] The expansion force generated by the phase change expansion drives the target area tube diameter to expand by 8%-10% (the change in tube diameter is monitored in real time by a laser diameter measuring instrument), and at this time the peak expansion force reaches 150 MPa±5 MPa (detected by a pressure sensor built into the tool); stop freezing, take the assembly out of the sealed freezing chamber, and warm up to 20°C to completely melt the ice crystals; open the end face sealing plate, discharge the melted liquid, and use high-pressure nitrogen gas (pressure 0.5 MPa) to blow the inner wall of the metal tube to remove the residual melted liquid and buffer layer debris, ensuring the cleanliness of the inner wall.

[0025] Post-processing steps: The purpose of post-processing is to eliminate residual stress generated during the bulging process and improve the mechanical properties and dimensional stability of the metal tube. The specific operation is as follows: Put the cleaned bulging thin-walled metal tube into a vacuum annealing furnace, use a 300°C vacuum annealing process: vacuum degree ≥1×10 -3 Pa (to avoid oxidation), heating rate 5°C / min (to prevent deformation caused by sudden temperature rise), and holding at 300°C for 2h (to ensure that residual stress is fully released), and then cooling to room temperature at a rate of 3°C / min (to avoid rapid cooling and generate new stress).

[0026] After the process, the residual stress elimination rate of the metal tube is ≥90% (detected by an X-ray stress analyzer), the dimensional stability is improved, and there is no obvious deformation during subsequent use.

[0027] Recycling of phase change medium: To achieve environmental protection and cost optimization, the invention also designs a recycling process for the phase change medium: Collect the discharged melted liquid into a recovery tank, and sequentially go through gravity sedimentation (remove large particle impurities, sedimentation time 30 min), precision filtration (use a microporous filter membrane with a pore size of 0.22μm to remove small impurities and buffer layer debris), and nano-particle supplementation, which can be reused as a phase change medium.

[0028] Experiments show that the recycling process can make the phase change medium reuse ≥50 times, the volume expansion rate of the medium after reuse is ≤3% (compared with the new medium), effectively reducing the medium cost and environmental pollution.

[0029] Advantages The present application overcomes the defects of the existing bulging technology by the synergistic design of "pretreatment-freezing bulging-post-treatment", combined with three innovations of local gradient freezing, ice crystal directional growth and elastic stress buffering, and achieves breakthroughs in technical performance, cost efficiency, environmental safety and application expansion. The local gradient freezing controls the deformation area, the pulse magnetic field directionally transmits the expansion force, and the precise tool positioning makes the forming precision of the bulged thin-walled metal pipe reach ±0.08mm (compared with ±0.3mm of traditional hydraulic bulging, the precision is increased by 73%), and the curvature radius deviation of asymmetric bulging is ≤0.1mm, which can accurately realize local forming of multiple curvature and asymmetry (such as local protrusion of a micro-channel heat exchanger), breaking through the limitation of traditional process to axisymmetric shape. The pretreated inner wall electrochemical polishing (Ra≤0.2μm), elastic buffer layer to avoid scratching and no high-pressure oil pollution make the inner wall surface roughness of the bulged metal pipe Ra≤0.4μm (wherein the nickel-titanium alloy pipe Ra<0.4μm, meeting the ISO13485 implant standard), and the finish is improved by 80% compared with the traditional mechanical pipe expansion Ra>3.2μm; at the same time, the synergistic effect of ice crystal directional growth and elastic buffer layer makes the wall thickness thinning rate ≤5% (compared with 12%~18% of traditional hydraulic bulging, the uniformity is improved by 60%), avoiding the strength reduction caused by local thinning of the pipe wall. The combination of elastic buffer layer (absorbing stress peak) and gradient freezing (non-bulging area temperature >0℃, avoiding overall low-temperature embrittlement) makes the thin-walled metal pipe have no cracking risk in-30℃ environment; for titanium alloy pipe, it can withstand-196℃ liquid nitrogen medium freezing bulging, solving the core bottleneck of low-temperature brittle cracking of thin-walled metal pipe in existing technology. The present application does not need high-pressure pump station (≥100MPa) and complex sealing device of traditional hydraulic bulging, only needs airtight freezing chamber, directional cooling device, pulse magnetic field device and simple precision tooling, reducing equipment investment; at the same time, it only needs positioning fixture without high-precision metal bulging die, reducing die cost. The combination of directional cooling (semiconductor refrigeration or liquid nitrogen spraying) and pulse magnetic field has very low energy consumption, single piece processing energy consumption ≈0.8kWh (compared with ≈12kWh of traditional hydraulic bulging, the energy consumption is greatly reduced); freezing bulging (10min±1min) + melting cleaning (3min) + post-treatment (vacuum annealing about 2.5h, which can be batch processed), the total production cycle of single piece is ≤15 minutes (compared with more than 30 minutes of hydraulic bulging, the efficiency is greatly improved), which is suitable for batch industrialization.

[0030] The phase change medium is a deionized water solution containing nanomaterials, has no risk of oil leakage, and avoids oil pollution of traditional hydraulic bulging; the elastic buffer layer material (silica gel, polyurethane, etc.) can be removed by recycling and filtering of the medium, and has no residual pollution. The phase change medium can be recycled and reused for more than 50 times, and the utilization rate is greatly improved; the thin-walled metal pipe (such as stainless steel, titanium alloy) after bulging can be directly smelted and regenerated, without surface coating pollution, and meets the green manufacturing concept. The technical scheme of the present application can be adapted to different types of thin-walled metal pipes, and meets the precise forming requirements in multiple fields: the titanium alloy pipe or composite material pipe is frozen and expanded by using liquid nitrogen medium at -196℃, the specific strength of the formed pipe is improved (due to the elimination of residual stress, the mechanical properties are stable), and can be used for light-weight components such as hydraulic pipes and fuel pipes of aircrafts. The inner wall of the nickel-titanium alloy pipe after bulging has Ra<0.4μm, meets the ISO13485 implant standard, and the ice expansion force is gentle and has no scratches, so it can be used for preparing medical devices such as vascular stents and orthopedic implants, and avoids stimulating human tissues. Local protrusions (protrusion height 0.8-1.2mm) of 0.5mm micro-pipes can be realized, the protrusion structure enhances fluid disturbance, the heat transfer coefficient of the micro-pipe after bulging is improved by 30%±2%, and is suitable for high-efficiency heat exchange components such as new energy automobile radiators and electronic equipment heat dissipation modules. DETAILED DESCRIPTION

[0031] In order to make the technical scheme of the present application clearer and easier to understand, the following will be described in detail in combination with specific embodiments, it should be noted that the protection scope of the present application is not limited to the following embodiments, any modification or equivalent replacement based on the technical scheme of the present application should be included in the protection scope.

[0032] Example 1: Local bulging of 304 stainless steel pipe (for micro-channel heat exchanger) Pre-treatment: Thin-walled metal pipe: 304 stainless steel pipe with an outer diameter of Φ20mm and a wall thickness of 0.3mm; the inner wall is subjected to electrolytic polishing treatment, and the surface roughness Ra=0.15μm is detected.

[0033] Elastic buffer layer: silica gel microspheres (particle size 200μm, particle size distribution uniformity 92%) are injected in the form of a suspension (the silica gel microspheres are dispersed in the phase change medium, and the volume ratio is 20%).

[0034] Phase change medium: deionized water solution containing 2wt% nanometer Al2O3; the nanometer Al2O3 has a particle size of 30nm, is modified by KH-550 silane coupling agent, and has a dispersion stability of 72h without sedimentation; the filling rate is 100% (vacuum pumping, no air bubbles).

[0035] Freezing and bulging Precision forming tooling: end face sealing plate (polytetrafluoroethylene + fluororubber O ring), positioning assembly (45 steel quenching, clearance 0.01 mm), heat insulation sleeve (polyurethane foam, thickness 12 mm); after assembly, only the target bulging area (length 50 mm) is exposed.

[0036] Directional cooling device: semiconductor refrigeration sheet (TEC1-12706, refrigeration power 60 W), closely attached to the outside of the tooling target area, temperature control -25℃±2℃.

[0037] Pulsed magnetic field application device: electromagnetic coil (100 turns, winding density 5 turns / cm, radial spacing 8 mm), pulsed magnetic field with frequency 5 kHz and intensity 0.5 T.

[0038] Forming process: frozen for 10 min, detected expansion force peak value 152 MPa, target area pipe diameter expansion 9% (from Φ20 mm to Φ21.8 mm); melted at 20℃, nitrogen purging the inner wall Vacuum annealing: 300℃, vacuum degree 5×10 -4 Pa, heating rate 5℃ / min, holding time 2h, cooling rate 3℃ / min.

[0039] Forming effect detection: Forming precision: ±0.07mm (three-coordinate measuring instrument detection); Surface roughness: Ra=0.18μm (surface roughness meter detection); Wall thickness reduction rate: 3.5% (ultrasonic thickness gauge detection); Residual stress: elimination rate 92% (X-ray stress analyzer detection); Heat transfer performance: after used in micro-channel heat exchanger, heat transfer coefficient increased by 31% (hot state experiment table detection).

[0040] Medium recycling: The melted liquid is subjected to gravity sedimentation (30 min), 0.22μm filtration, and addition of nano-Al2O3 to 2wt%, and after 10 times of reuse, the volume expansion rate is still 9.0% (only attenuated by 0.2%).

[0041] Example 2: Nickel-titanium alloy tube bulging (for blood vessel stent) Preprocessing: Thin-walled metal tube: nickel-titanium alloy tube with outer diameter Φ6mm and wall thickness 0.3mm; after electrolytic polishing of the inner wall, Ra=0.35μm.

[0042] Elastic buffer layer: polyurethane porous membrane (thickness 50μm, porosity 50%) is selected and pre-coated on the inner wall (immersion coating process, volume fraction 15%).

[0043] Phase change medium: deionized water solution containing 2wt% nano-Al2O3; nano-Al2O3 particle size 30nm, modified by KH-550 silane coupling agent, dispersion stability 72h without sedimentation; filling rate 100% (vacuum injection, no bubbles).

[0044] Freeze expansion: Precision forming tooling: positioning assembly fitting gap 0.02mm, heat insulation sleeve thickness 10mm; target expansion area 3 symmetrical protrusions (curvature radius 5mm) in circumference.

[0045] Directional cooling device: semiconductor refrigeration sheet, temperature control -15℃±2℃.

[0046] Pulsed magnetic field application device: permanent magnet array (neodymium iron boron N52, magnet block spacing 5mm, magnetic field strength 0.3T, uniformity error 3%).

[0047] Forming process: freezing 8min, bulging force peak 148MPa, protrusion height 1.0mm; nitrogen blowing after melting.

[0048] Post-processing: Vacuum annealing: 300℃, vacuum degree 5×10 -4 Pa, heating rate 5℃ / min, holding time 2h, cooling rate 3℃ / min.

[0049] Forming effect detection: Forming precision: protrusion curvature radius deviation 0.08mm; Surface roughness: Ra=0.38μm (in line with ISO13485 standard); Wall thickness thinning rate: 4.2%; Superelasticity: original superelasticity of nickel-titanium alloy is retained (tensile testing machine detection, elastic deformation rate ≥8%).

[0050] Example 3: titanium alloy tube bulging (for aviation hydraulic pipeline) Preprocessing: Thin-walled metal tube: TC4 titanium alloy tube with outer diameter Φ30mm and wall thickness 0.3mm is selected; after polishing the inner wall, Ra=0.2μm.

[0051] Elastic buffer layer: composite particles of aerogel and silica gel microspheres (silica gel microsphere particle size 300μm, aerogel proportion 30%) are selected, and the suspension is injected with a volume proportion of 25%.

[0052] Phase change medium: deionized water solution containing 2wt% nano-Al2O3; nano-Al2O3 particle size 30nm, modified by KH-550 silane coupling agent, dispersion stability 72h without sedimentation; filling rate 100% (vacuum injection, no bubbles).

[0053] Freeze expansion: Precision forming tool: heat insulation sleeve thickness 15mm, positioning assembly fit clearance 0.01mm.

[0054] Directional cooling device: liquid nitrogen spraying device, spraying flow 2L / min, temperature control-196℃ (adapt to titanium alloy low temperature requirement).

[0055] Pulsed magnetic field application device: electromagnetic coil (turns 200 turns, magnetic field strength 1.0T, frequency 20kHz).

[0056] Forming process: freezing 12min, expansion force peak 155MPa, pipe diameter expansion 8.5%; clean after melting.

[0057] Post-processing: Vacuum annealing: 300℃, vacuum degree 5×10 -4 Pa, heating rate 5℃ / min, holding time 2h, cooling rate 3℃ / min.

[0058] Forming effect detection: Low temperature resistance: no cracking after-196℃ freezing (no crack observed by metallographic microscope); Forming precision: ±0.08mm; Wall thickness thinning rate: 4.8%; Mechanical properties: tensile strength ≥900MPa (tensile testing machine detection, in line with aviation standards).

[0059] The above shows and describes the basic principles, main features and advantages of the present application. Those skilled in the art should understand that the present application is not limited to the above examples, and the above embodiments and descriptions in the specification are only to illustrate the principles of the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. A thin-walled metal bulging forming method based on liquid phase change expansion, characterized by, The method comprises three continuous steps of pretreatment, freeze expansion and post-treatment, and specifically comprises the following steps. The pretreatment comprises the following steps: selecting a thin-walled metal pipe, performing surface friction optimization treatment on the inner wall of the thin-walled metal pipe, arranging an elastic buffer layer between the inner wall of the thin-walled metal pipe and the phase change medium, and injecting the phase change medium into the thin-walled metal pipe, and the filling rate of the phase change medium is 100%. The freeze expansion comprises the following steps: taking the pretreated thin-walled metal pipe as a forming part, assembling the forming part into a precision forming tool and sealing the forming part, placing the assembly into a closed freezing chamber, applying a low temperature of-5℃ to-30℃ to the target expansion area of the part through a directional cooling device while maintaining the temperature of the non-expansion area to be greater than 0℃, forming an axial and / or circumferential temperature gradient, continuously applying a pulsed magnetic field to the part through a pulsed magnetic field application device during the freezing process, so that the ice crystals formed by the phase change medium grow in the radial direction of the thin-walled metal pipe, and stopping freezing and heating to 20℃ after the expansion force generated by the phase change expansion drives the target area to expand by 8% to 10% of the pipe diameter, melting the ice crystals, discharging the melted liquid, and then blowing nitrogen to the inner wall. The post-treatment comprises the following step: performing stress relief treatment on the expanded thin-walled metal pipe, and the stress relief treatment is a 300℃ vacuum annealing process. The above steps solve the technical problems of poor local control, low-temperature brittle fracture and insufficient precision in the existing expansion technology through the synergistic effect of gradient temperature control, ice crystal directional growth and stress buffering. The combination of the elastic buffer layer and gradient freezing makes the thin-walled metal pipe have no risk of cracking in a-30℃ environment, and the pulsed magnetic field greatly improves the radial expansion efficiency.

2. A thin-walled metal bulge forming method based on liquid phase change expansion according to claim 1, characterized in that, The directional cooling device is selected from a semiconductor refrigeration sheet or a liquid nitrogen spraying device. When the semiconductor refrigeration sheet is used, it is arranged outside the target expansion area of the precision forming tool, and the refrigeration power is greater than or equal to 50W. When the liquid nitrogen spraying device is used, the temperature is adjusted to-196℃ to-30℃ through a programmable spray head, and the spraying flow control precision is ±0.5L / min.

3. The thin-walled metal bulge forming method based on liquid phase change expansion according to claim 1, characterized in that, The temperature control range of the non-expansion area is 0℃ to 10℃, the temperature difference of the axial temperature gradient is greater than or equal to 35℃, and the temperature difference of the circumferential temperature gradient is greater than or equal to 30℃. The temperature gradient is formed through the cooperation of the heat insulation layer built in the tool and the directional cooling device. The heat insulation layer is made of aerogel, and the thickness is 2 to 5mm and the thermal conductivity is less than or equal to 0.02W / (m·K).

4. The thin-walled metal bulge forming method based on liquid phase change expansion according to claim 1, characterized in that, The frequency of the pulsed magnetic field is 500Hz to 20kHz, and the magnetic field strength is 0.1 to 1T. The pulsed magnetic field application device is an electromagnetic coil or a permanent magnet array. The number of turns of the electromagnetic coil is 50 to 200 turns, the winding density is 5 turns / cm, and the radial distance from the thin-walled metal pipe is less than or equal to 10mm. The permanent magnet array is composed of neodymium iron boron blocks, the block spacing is 3 to 8mm, and the magnetic field uniformity error is less than or equal to 5%.

5. The thin-walled metal bulge forming method based on liquid phase change expansion according to claim 1, characterized in that, The material of the elastic buffer layer is selected from at least one of silica gel microspheres, polyurethane porous membranes or aerogels, or an organic-inorganic composite modified body of the above materials; when being a particulate material, the particle size is 50-500 μm, and the uniformity of the particle size distribution is ≥ 90%; when being a membrane material, the thickness is 20-100 μm, and the porosity is 40%-60%; the volume ratio of the elastic buffer layer between the tube wall and the phase change medium is 10%-30%, and the arrangement mode is pre-coated on the inner wall or injected in the form of a suspension with the phase change medium.

6. The thin-walled metal bulge forming method based on liquid phase change expansion according to claim 1, characterized in that, The thin-walled metal tube is a 304 stainless steel tube, a 316L stainless steel tube, a titanium alloy tube, a composite material tube or a nickel-titanium alloy tube; when being a stainless steel tube, the outer diameter is Φ10-50 mm, the wall thickness is 0.3 mm, and the inner wall is treated by electrolytic polishing, and the surface roughness Ra is ≤0.2 μm; when being a nickel-titanium alloy tube, the surface roughness Ra of the inner wall after bulging is <0.4 μm, which meets the ISO13485 implant surface standard; when being a titanium alloy tube, it can withstand the freezing bulging of a liquid nitrogen medium at -196 ℃.

7. The method of claim 1, wherein the method is a liquid phase transformation based thin-walled metal bulging forming method. The phase change medium is a deionized water solution containing 2wt% nano-Al2O3, the particle size of the nano-Al2O3 is ≤50 nm, and the dispersion stability in water after modification by a silane coupling agent is ≥72 h; the freezing point of the phase change medium is -5 ℃, the volume expansion rate during freezing is 8.9%-9.2%, and the bonding strength with ice crystals is ≥1.2 MPa.

8. The thin-walled metal bulge forming method based on liquid phase change expansion according to claim 1, characterized in that, The process parameters of the freezing bulging are as follows: the freezing temperature is -25 ℃±2 ℃, the freezing time is 10 min±1 min, and the peak bulging force is 150 MPa±5 MPa; the forming precision of the thin-walled metal tube after bulging is ±0.08 mm, the wall thickness thinning rate is ≤5%, and the curvature radius deviation of asymmetric bulging is ≤0.1 mm.

9. The method of claim 1, wherein the method is a liquid phase transformation based thin-walled metal bulging forming method. The precision forming tooling includes two end face sealing plates, a positioning assembly and a heat insulation sleeve; the end face sealing plate is made of polytetrafluoroethylene, has an O-shaped sealing ring built-in, and can resist a sealing pressure of ≥200 MPa; the positioning assembly is in interference fit with the non-bulging area of the thin-walled metal tube, the fit clearance is ≤0.02 mm, and the material is 45 steel treated by quenching; the heat insulation sleeve wraps the non-cooling area of the tooling and is made of polyurethane foam with a thickness of 10-15 mm.

10. The method of claim 1, wherein the method is a liquid phase transformation based thin-walled metal bulging forming method. The phase change medium can be recycled, filtered and reused, the recycling process includes gravity sedimentation, precision filtration (filter hole diameter 0.22 μm) and nano-particle supplement, the reuse times are ≥50, and the volume expansion rate attenuation of the medium after reuse is ≤3%; when used for bulging a 0.5 mm micro tube of a micro-channel heat exchanger, a local protrusion height of 0.8-1.2 mm can be achieved, and the heat transfer coefficient of the bulged micro tube is improved by 30%±2%.