Manufacturing process of carbon fiber clad aluminum tube for aircraft and composite tube fitting thereof

Abstract

The application discloses a manufacturing process of a carbon fiber coated aluminum pipe for an aircraft and a composite pipe fitting, and relates to the technical field of aircraft structural member manufacturing. The process comprises the following steps: first rolling: wrapping carbon fiber prepreg on the outer wall of an aluminum alloy inner pipe; first solidification: heating and solidifying into an integrated whole; centerless grinding: grinding the outer surface after solidification to remove a resin-rich layer and form a grinding surface; coating a glue layer on the grinding surface; second rolling: wrapping and rolling the outer appearance carbon fiber cloth; and second solidification: heating and solidifying to obtain the composite pipe fitting. The composite pipe fitting comprises an aluminum alloy inner pipe, a carbon fiber composite layer, a glue layer and an outer appearance carbon fiber layer, and the interface between the carbon fiber composite layer and the glue layer is a rough grinding surface. The application solves the problems of poor dynamic balance, large wind resistance noise of the traditional carbon aluminum composite pipe, and the pipe fitting is light in weight, high in rigidity, excellent in dynamic balance, low in wind resistance, low in noise and low in vibration, and high in interface bonding strength, and is suitable for an aircraft arm support pipe.

Description

Technical Field

[0001] This invention relates to the field of aircraft structural component manufacturing technology, specifically to a manufacturing process for carbon fiber-coated aluminum tubes for aircraft and its composite tubes, which are particularly suitable for structural components such as main support tubes for the arms and fuselage support tubes of eVTOL (electric vertical takeoff and landing) aircraft that have stringent requirements for lightweight, high rigidity, dynamic balance and vibration reduction and noise reduction performance. Background Technology

[0002] With the rapid development of the low-altitude economy, eVTOL (electric vertical takeoff and landing) aircraft have become an important carrier of urban air transportation. Core load-bearing structural components of eVTOL aircraft, such as the arm support tubes and fuselage support tubes, need to simultaneously meet multiple performance requirements, including lightweight, high strength, high rigidity, excellent dynamic balance, and vibration and noise reduction. As a key force-transmitting component connecting the rotor system and the fuselage, the performance of the support tube directly affects the aircraft's endurance, flight stability, passenger comfort, and structural fatigue life.

[0003] Currently, the support tubes of eVTOL aircraft on the market mainly adopt the following three technical routes: The first option is pure aluminum alloy tubing. Aluminum alloy tubing is readily available and relatively inexpensive to process, but its high density (approximately 2.7 g / cm³) results in a heavier tubing, directly reducing the aircraft's range and payload. Furthermore, aluminum alloys have limited specific strength and specific stiffness, often requiring increased wall thickness to meet the high strength requirements of eVTOL aircraft, further exacerbating the weight issue.

[0004] The second type is all-carbon fiber tubing. Carbon fiber composites have extremely high specific strength and specific stiffness, with a density only about 60% that of aluminum alloys, offering significant lightweight advantages. However, all-carbon fiber tubing has poor impact toughness and high brittleness. Under the turbulence of eVTOL aircraft takeoff and landing, low-level airflow disturbances, and high-frequency vibrations and impacts generated by the high-speed rotation of multi-rotors, it is highly susceptible to brittle fractures such as cracking and edge chipping, resulting in insufficient safety. At the same time, all-carbon fiber tubing lacks the plastic deformation energy absorption capacity of a metal matrix, and once damage occurs, it often results in catastrophic fracture with little warning.

[0005] The third type is the traditional carbon-aluminum composite pipe fitting. In order to balance the toughness of aluminum alloy and the lightweight and high strength of carbon fiber, some technical solutions adopt a composite structure in which a carbon fiber layer is coated on the outer wall of the aluminum alloy pipe.

[0006] For example, Chinese patent document CN105799196A discloses a method for preparing aluminum alloy composite carbon fiber square tube.

[0007] However, existing carbon-aluminum composite pipe fittings have the following prominent problems: (1) Unreasonable layup design and poor dynamic balance performance. The carbon fiber winding layup of traditional carbon-aluminum composite pipes often adopts a single direction or asymmetrical arrangement, which leads to uneven distribution of resin shrinkage stress during curing, uneven pipe wall thickness, large mass eccentricity after molding, and poor inherent dynamic balance of the rod. Under the high-speed rotation of eVTOL multi-rotor, poor dynamic balance will cause severe vibration, which seriously affects flight stability and ride comfort.

[0008] (2) Rough manufacturing process and poor surface quality. Most existing carbon-aluminum composite pipe fittings adopt a one-time winding and curing molding process. During the curing process, the resin in the carbon fiber prepreg fabric fluctuates due to factors such as resin content fluctuation, uneven winding tension, and indentation from the release tape (such as BOPP tape), resulting in a microscopic unevenness on the outer surface of the pipe fitting with alternating resin-rich and resin-poor areas. This microscopic surface deformation not only affects the appearance of the pipe fitting, but also increases aerodynamic drag and generates additional aerodynamic noise during high-speed flight of eVTOL. Furthermore, the uneven surface areas are prone to becoming stress concentration points, accelerating fatigue damage.

[0009] (3) One-time molding process cannot simultaneously ensure structural performance and surface quality. If surface grinding or secondary coating is performed directly on the uncured prepreg, the mechanical strength of the prepreg resin is extremely low due to its semi-fluid viscoelastic state. The grinding pressure can easily cause the pipe wall to flatten and become out of round, the fibers to slip and misalign, and the layers to delaminate, which seriously damages the bending and torsional strength of the pipe. In addition, the heat generated by grinding will cause the uncured resin to gel prematurely in some areas, resulting in uneven curing and huge internal stress. During subsequent baking, the pipe will bend, twist, or bulge.

[0010] Therefore, there is an urgent need for a carbon-aluminum composite pipe fitting and its manufacturing process that can ensure a high-strength bond between the carbon fiber composite layer and the aluminum alloy tube, achieve excellent dynamic balance, low wind resistance and noise reduction surface, and have good process controllability. Summary of the Invention

[0011] In order to overcome the shortcomings of the prior art, the present invention aims to provide a technical solution that can solve the above problems.

[0012] This invention provides a manufacturing process for carbon fiber coated aluminum tubes for aircraft, comprising the following steps: S1. First winding: The carbon fiber prepreg is wrapped around the outer wall of the aluminum alloy inner tube according to the preset layup angle. S2, First Curing: The tube after the first roll is heated and cured to make the carbon fiber prepreg fabric and the aluminum alloy inner tube a single unit; S3, Centerless Grinding: Centerless grinding is performed on the outer surface of the pipe after the first curing to remove the resin-rich layer on the surface and form a grinding surface with a preset roughness. S4. Applying adhesive: Applying an adhesive layer to the grinding surface; S5. Second rolling: The outer carbon fiber cloth is attached to the outer surface of the tube after the adhesive is applied and then rolled. S6. Second Curing: The pipe after the second rolling is heated and cured to make the outer carbon fiber cloth and the cured layer of the carbon fiber prepreg composite together.

[0013] Further: In step S1, the carbon fiber prepreg includes an inner prepreg and an outer prepreg; the inner prepreg is wrapped around the outer wall of the aluminum alloy inner tube after being folded together by UD prepregs in the 0° and 90° directions; the outer prepreg is attached to the outer wall of the inner prepreg after being folded together by UD prepregs in the 0° and ±45° directions; in step S2, the temperature for heat curing is 140℃~160℃, and the curing time is 2.5h~3.5h; a release tape is wrapped around the outer surface of the tube before curing, and the release tape is removed after curing.

[0014] Further: In step S3, the grinding allowance of the centerless grinding is 0.4mm to 0.6mm, the outer diameter tolerance of the pipe after grinding is controlled within ±0.05mm, and the surface roughness Ra of the grinding surface is 1.6μm to 3.2μm; the centerless grinding only removes the surface resin-rich layer and the indentation of the demolding zone, without damaging the main carbon fiber structure.

[0015] Further: In step S4, the adhesive layer is an epoxy resin adhesive layer with a thickness of 0.05mm to 0.15mm, and the curing temperature of the epoxy resin adhesive is thermally matched with the resin system of the carbon fiber prepreg; before applying the adhesive, the steps of cleaning and baking the polished pipes are also included; in step S5, the outer carbon fiber cloth is 3k twill carbon fiber cloth.

[0016] Further: In step S6, the heating and curing temperature is 140℃~160℃, and the curing time is 2.5h~3.5h; before curing, a release tape is wrapped around the outer surface of the pipe fitting, and the release tape is removed after curing; after the second curing, surface coating and CNC cutting are performed in sequence.

[0017] The present invention also provides a composite tubing for aircraft made of carbon fiber coated aluminum tube, manufactured using the manufacturing process described in any of the preceding claims, wherein the composite tubing comprises: Aluminum alloy inner tube; A carbon fiber composite layer is wrapped around the outer wall of the aluminum alloy inner tube and cured into one piece. The carbon fiber composite layer is composed of multiple layers of carbon fiber prepreg fabric arranged symmetrically and evenly according to the layup angle. An adhesive layer is applied to the outer surface of the carbon fiber composite layer; The outer carbon fiber layer is bonded to the outer surface of the carbon fiber composite layer by the adhesive layer and cured into a single unit. The interface between the carbon fiber composite layer and the adhesive layer is a ground rough surface.

[0018] Further: the carbon fiber composite layer includes an inner prepreg layer and an outer prepreg layer; the inner prepreg layer is formed by stacking and curing UD prepregs in the 0° and 90° directions; the outer prepreg layer is formed by stacking and curing UD prepregs in the 0° and ±45° directions.

[0019] Furthermore: the surface roughness Ra of the ground roughened surface is 1.6μm to 3.2μm; the adhesive layer is an epoxy resin adhesive layer with a thickness of 0.05mm to 0.15mm.

[0020] Furthermore: the outer carbon fiber layer is a 3k twill carbon fiber cloth layer; the outer surface of the outer carbon fiber layer is also provided with a coating layer, which is a semi-matte paint finish.

[0021] Furthermore, the aluminum alloy inner tube has a wall thickness of 1.0mm to 2.0mm and an outer diameter of 17mm to 30mm.

[0022] Compared with the prior art, the beneficial effects of the present invention are: (1) Combining lightweight and high rigidity. This invention uses an aluminum alloy inner tube as the matrix and a carbon fiber composite layer on the outer wall. It makes full use of the high specific strength and high specific stiffness of carbon fiber and the plasticity and toughness of aluminum alloy to significantly reduce the self-weight of the tube while ensuring structural strength and rigidity. Compared with pure aluminum tubes of the same specifications, the composite tube of this invention has a 146.68% higher bending strength, an 86.62% higher bending elastic modulus, an 88.50% higher strength, and a more than 35% lower deformation, meeting the stringent load-bearing requirements of the main support of the eVTOL aircraft arm.

[0023] (2) Excellent dynamic balance performance. The carbon fiber composite layer of this invention adopts a symmetrical and balanced arrangement of layup angles. The inner prepreg is formed by overlapping UD prepregs in the 0° and 90° directions, and the outer prepreg is formed by overlapping UD prepregs in the 0° and ±45° directions. This symmetrical layup design makes the curing shrinkage stress cancel each other out in the circumferential and axial directions, avoids the warping deformation of the pipe, ensures uniform wall thickness and symmetrical mass distribution after molding, small mass eccentricity, and excellent inherent dynamic balance of the rod, effectively suppressing the vibration transmission under the high-speed rotation condition of the eVTOL multi-rotor.

[0024] (3) Low wind resistance and noise reduction surface with excellent aerodynamic performance. This invention effectively solves the problem of microscopic unevenness of the surface after curing of prepreg in traditional one-time molding process by using a two-stage composite molding process of "first curing → centerless grinding → adhesive coating → second roll-up of outer fabric → second curing". Centerless grinding precisely removes the resin-rich layer and demolding marks on the surface to obtain a uniform rough surface with Ra 1.6μm~3.2μm; then a thin adhesive layer of 0.05mm~0.15mm is applied and bonded with 3k twill carbon fiber outer fabric for secondary curing. The adhesive layer is thin and uniform, and the surface deformation is not easily generated during the curing process, making the outer surface of the pipe smooth and flat, which significantly reduces aerodynamic resistance and aerodynamic noise during flight.

[0025] (4) High interfacial bonding strength and excellent anti-delamination performance. After the first curing, the outer surface of the pipe is centerlessly ground to remove only the surface resin-rich layer without damaging the main carbon fiber structure, thus moderately exposing the carbon fiber fabric texture and providing good micro-mechanical interlocking conditions for secondary coating. The epoxy resin adhesive layer coated after grinding is thermally matched with the prepreg resin system (both have a curing temperature of 150℃). The coefficient of thermal expansion is consistent during the second curing, and no additional thermal stress is generated. The interfacial bonding strength is high, effectively preventing interlayer delamination and peeling under the high-frequency vibration conditions of eVTOL.

[0026] (5) Good process controllability and high product consistency. This invention adopts a two-stage independent curing process. The first curing process forms a rigid whole between the carbon fiber prepreg and the aluminum alloy tube, providing a stable mechanical basis for subsequent centerless grinding and avoiding defects such as tube wall flattening, fiber slippage, and premature resin gelation caused by grinding in the green state. The centerless grinding step strictly controls the grinding allowance (0.4mm~0.6mm) and outer diameter tolerance (±0.05mm) to ensure high dimensional accuracy and good consistency of the tubes, making them suitable for mass production.

[0027] (6) Vibration and noise reduction, extending service life. The high damping characteristics of the carbon fiber composite layer combined with the plastic energy absorption capacity of the aluminum alloy inner tube form a natural vibration and noise reduction composite structure. The carbon fiber layer effectively attenuates the transmission of high-frequency vibrations from the multi-rotor to the fuselage, reducing structural noise; the aluminum alloy inner tube provides tough support, preventing brittle fracture. The synergistic effect of the two significantly reduces vibration and noise, alleviates structural fatigue, bolt loosening, and component aging caused by long-term vibration, extends the service life of the aircraft, and improves flight safety.

[0028] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a schematic diagram of the overall structure of the composite pipe fitting of the present invention; Figure 2 This is a schematic diagram of the peeling of the layered structure of the composite pipe fitting of the present invention; Figure 3 This is a flowchart of the manufacturing process of the present invention; Figure 4 This is a diagram showing the layered structure and interface of the composite pipe fitting of the present invention.

[0031] The reference numerals and names in the figure are as follows: 1-Aluminum alloy inner tube; 2-Carbon fiber composite layer; 21-Inner prepreg layer; 22-Outer prepreg layer; 3-Adhesive layer; 4-Outer carbon fiber layer; 5-Coating layer. Detailed Implementation

[0032] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] Please see Figures 1 to 4 The present invention provides the following embodiments: Definitions and Explanations: In this invention, the 0° direction refers to the axial direction of the pipe fitting, the 90° direction refers to the circumferential (circumferential) direction of the pipe fitting, and the ±45° direction refers to the spiral direction forming an angle of +45° and -45° with the axis of the pipe fitting; "overlapping" refers to stacking two layers of UD prepreg fabric at different angles as a group for wrapping or attaching. The release tape is a biaxially oriented polypropylene film tape (BOPP tape). Unless otherwise specified, the epoxy resin system used in this invention is a 150°C curing system. UD can be made of unidirectional fabric with T300 carbon fiber as the base material.

[0034] Example 1: Manufacturing process of 25mm×30mm carbon-coated aluminum composite pipe fittings This embodiment provides a manufacturing process for a composite tube fitting made of carbon fiber coated aluminum tubing for aircraft. The composite tube fitting has an inner diameter of 25mm and an outer diameter of 30mm, and the aluminum alloy inner tube 1 has a wall thickness of 1.5mm (i.e., the outer diameter of the aluminum alloy inner tube 1 is 28mm). Specifically, it includes the following steps: S0. Pretreatment of aluminum alloy inner tube: Before step S1, the aluminum alloy inner tube 1 is pretreated, specifically including: (1) Pre-treatment of tube: Cut the aluminum alloy inner tube 1, remove the burrs at both ends and straighten it, and remove the extrusion oil and surface release agent from its outer wall; (2) Knurling: The aluminum alloy inner tube 1 is centrally positioned and supported. The knurling wheel is radially fed by servo drive. The tube rotates freely and is continuously rolled to form a knurled texture (knurled concave and convex texture) on the outer wall of the aluminum alloy inner tube 1. (3) Rounding correction: The inner tube 1 of the rolled aluminum alloy is rounded to ensure the outer diameter tolerance; (4) Degreasing and cleaning and surface activation: The knurled aluminum alloy inner tube 1 is degreased and ultrasonically cleaned to remove residual aluminum shavings and oil stains in the grooves and grooves, so as to avoid residual impurities in the dead corners of the grooves causing delamination of the interface between the carbon fiber composite layer 2 and the aluminum alloy inner tube 1; then the surface activation treatment is performed, which includes weak alkali degreasing, acid washing to remove the natural oxide film and purification film treatment in sequence; in this embodiment, the outer surface of the aluminum alloy inner tube 1 is not subjected to heavy anodizing, so as to avoid the oxide film being too smooth and reducing the interlocking bonding strength between the carbon fiber composite layer 2 and the aluminum alloy inner tube 1.

[0035] After the above pretreatment, the carbon fiber composite layer 2 is cured and formed in the knurled texture, forming a mechanical interlock at the carbon-aluminum interface, which further improves the interfacial bonding strength between the aluminum alloy inner tube 1 and the carbon fiber composite layer 2, and prevents interface delamination under long-term high-frequency vibration conditions; the rounding correction ensures the outer diameter tolerance of the aluminum alloy inner tube 1, which is beneficial to improving the dynamic balance performance after forming.

[0036] S1. Fabric cutting: Use a fabric cutting machine to cut the UD prepreg at 0° and 90°, and 0° and ±45°, and cut 3k twill outer fabric for later use.

[0037] S2. Overlapping: Fold the cut fabric at 0°+90° to form the inner prepreg fabric, and fold it at 0°+±45° to form the outer prepreg fabric.

[0038] S3. First winding: The stacked inner prepreg fabric (0°+90°) is first wrapped around the outer wall of the aluminum alloy inner tube 1. Then, one end of the outer prepreg fabric (0°+±45°) is attached to the inner prepreg fabric and placed into the winding machine for winding, so that the carbon fiber prepreg fabric covers the outer wall of the aluminum alloy inner tube 1 according to the preset layup angle. This layup is symmetrically and evenly arranged. The 0° direction provides axial tensile and bending strength, the 90° direction provides circumferential restraint, and the ±45° direction provides torsional and shear resistance. The curing shrinkage stress cancels each other out in the circumferential and axial directions, ensuring that the tube has good dynamic balance performance after molding.

[0039] S4. First wrapping: Place the rolled tube on a biaxial horizontal wrapping machine and wrap a layer of release tape (biaxially oriented polypropylene film tape, BOPP tape) on the surface to apply uniform radial pressure during the curing process and remove excess resin.

[0040] S5. First curing: Fix the tube in the composite curing oven and bake it at 150℃ for 3 hours to allow the epoxy resin to cross-link and harden, and the carbon fiber prepreg cloth and aluminum alloy inner tube 1 to be combined into one, forming a rigid composite material component; remove it and peel off the release strip on the surface.

[0041] S6. Centerless Grinding: After the UD fabric has dried and cured, the outer diameter is ground using centerless grinding technology. In this step, the grinding allowance is controlled at 0.42mm~0.5mm. After the first curing, the outer diameter of the pipe is approximately 29.92mm~30.08mm. After grinding, the outer diameter is controlled at 29.5mm~29.58mm, and the outer diameter tolerance is controlled within ±0.05mm. The surface roughness Ra after grinding is 1.6μm~3.2μm. The purpose of this process is to remove the resin-rich layer, demolding marks, and outer diameter contour defects from the surface of the cured pipe, and to appropriately expose the carbon fiber fabric texture, providing good interfacial engagement conditions for secondary coating. The process strictly controls the grinding allowance to ensure that only the surface resin is removed without damaging the main carbon fiber structure.

[0042] The reason why curing is necessary before grinding is that the uncured preform has extremely low mechanical strength. After the layers are laid, the resin in the tube that has not yet dried and cured is in a semi-fluid viscoelastic state. Centerless grinding relies on the extrusion and friction of the grinding wheel to drive the tube to rotate. The preform is easily flattened, out of round, wrinkled, and the fibers slip and misalign under pressure. Delamination easily occurs between the carbon fiber cloth layers. After slippage, the fiber angle is destroyed, and the bending and torsional strength of the finished tube is greatly reduced. After curing, the epoxy resin cross-links and hardens, and the carbon tube becomes a rigid composite material component that can withstand the grinding pressure without deformation or fiber displacement. In addition, the heat generated by grinding will damage the uncured resin. The preform resin will gel and cure prematurely in some areas due to heat, resulting in uneven resin curing and huge internal stress. During the subsequent overall baking, the shrinkage of the locally cured areas is inconsistent with that of the rest of the area, causing the tube to bend, twist, and bulge.

[0043] S7. Cleaning and Baking, Applying Adhesive: After cleaning the surface of the ground pipe fitting with clean water, bake it for 30 minutes. Remove it, wipe the surface clean, and then apply a layer of adhesive. The adhesive is epoxy resin adhesive, and the thickness of the adhesive layer 3 is 0.1mm. Its curing temperature (150℃) is thermally matched with the prepreg resin system of the first curing to avoid internal stress caused by thermal expansion differences during the second curing, which would lead to thermal bending of the pipe.

[0044] S8. Second Rolling: After applying the adhesive, attach one end of the 3K twill outer fabric to the carbon-coated aluminum tube semi-finished product and place it into the rolling machine for rolling. Because the adhesive layer is thin and uniform, it is less likely to cause deformation of the outer fabric surface during the curing process, resulting in a smoother overall appearance and reduced wind resistance and noise.

[0045] S9. Second wrapping: Place the rolled tube on a twin-shaft horizontal wrapping machine and wrap a layer of release tape (BOPP tape) on the surface.

[0046] S10. Second curing: Fix the tube in the composite curing oven, bake it at 150℃ for 3 hours, then remove the release tape from the surface, so that the outer carbon fiber layer 4 is bonded to the outer surface of the carbon fiber composite layer 2 through the adhesive layer 3 and cured into one piece.

[0047] S11. Coating and painting: The surface is coated and painted. After coating, the surface is a 3k twill semi-matte material, forming a coating layer 5.

[0048] S12, CNC cutting: The painted product is fed into the CNC lathe by the feeder and cut to the required length.

[0049] S13, QC carbon inspection: Inspect the straightness, outer diameter, appearance and carbon layer integrity of the finished product.

[0050] II. Example 2: Alternative Implementation Methods for Ply Lay Variations and Process Parameter Windows This embodiment is basically the same as embodiment 1, except that: (1) The inner diameter of the aluminum alloy inner tube 1 is 22mm and the wall thickness is 1.5mm, and the outer diameter of the composite pipe fitting is 25mm; (2) Another variation of the layup sequence is adopted: the cut fabric is folded at 0°+0° and 0°+90°; when rolling, the folded 0°+0° is first wrapped around the aluminum tube, and then the 0°+90° end is attached to the aluminum tube before being placed into the tube rolling machine for rolling. This variation also achieves a symmetrical and balanced layup arrangement, ensuring the dynamic balance performance of the pipe fitting; (3) The temperature for the first curing and the second curing are both 140℃~160℃, and the curing time is 2.5h~3.5h (preferably 160℃ and 2.5h in this embodiment); the grinding allowance for centerless grinding is 0.4mm~0.6mm; the thickness of adhesive layer 3 is 0.05mm~0.15mm (preferably 0.08mm in this embodiment).

[0051] Any combination of the parameters in the above parameter window can achieve the purpose of this invention, indicating that the process of this invention has good robustness and adaptability to mass production.

[0052] III. Example 3: Product Structure of Composite Pipe Fittings like Figures 1-2As shown, the composite pipe fitting prepared using the process of Example 1 or Example 2 includes: an aluminum alloy inner tube 1; a carbon fiber composite layer 2, which covers the outer wall of the aluminum alloy inner tube 1 and is cured into one piece. The carbon fiber composite layer 2 is composed of multiple layers of carbon fiber prepreg fabric arranged symmetrically and evenly according to the layup angle, including an inner prepreg fabric layer 21 and an outer prepreg fabric layer 22; an adhesive layer 3, which is coated on the outer surface of the carbon fiber composite layer 2; an outer carbon fiber layer 4, which is bonded to the outer surface of the carbon fiber composite layer 2 through the adhesive layer 3 and cured into one piece; and a coating layer 5, which is disposed on the outer surface of the outer carbon fiber layer 4. The interface between the carbon fiber composite layer 2 and the adhesive layer 3 is a ground rough surface with a surface roughness Ra of 1.6μm~3.2μm. This ground rough surface moderately exposes the carbon fiber fabric texture, forming a microscopic mechanical interlock with the adhesive layer 3, resulting in high interfacial bonding strength.

[0053] In some embodiments, the outer wall of the aluminum alloy inner tube 1 has a knurled texture, and the carbon fiber composite layer 2 enters the knurled texture to form a mechanical engagement.

[0054] In this embodiment, the outer diameter of the aluminum alloy inner tube 1 is 28mm, the total radial thickness of the carbon fiber composite layer 2, the adhesive layer 3, and the outer carbon fiber layer 4 is approximately 1mm, and the outer diameter of the composite tube is 30mm. The wall thickness of the aluminum alloy inner tube 1 can be 1.0mm~2.0mm, and the outer diameter of the composite tube can be 17mm~30mm.

[0055] IV. Example 4: Performance Testing and Comparative Verification (1) Three-point bending resistance comparison test Three-point bending tests were conducted on the carbon-coated aluminum composite pipe fitting prepared in Example 1 (sample name: carbon-coated aluminum composite pipe, specification 30×25, outer diameter 30.00mm, inner diameter 25.00mm) and a control sample of the same specification pure carbon pipe (sample name: pure carbon pipe, outer diameter 30.00mm, inner diameter 25.00mm). The tests were performed according to GB / T 13096-2008, with a span of 150.00mm and a test speed of 3mm / min. The test results are shown in Table 1.

[0056] Table 1. Results of three-point bending tests (maximum force and displacement at maximum force) As shown in Table 1, the average maximum force of the carbon-coated aluminum composite tube in this design is 626.386 Kgf, which is about 11.9% higher than the 559.719 Kgf of the pure carbon tube control sample; the average maximum force displacement is 14.19 mm, which is about 21.2% higher than the 11.711 mm of the pure carbon tube control sample. This indicates that the tube fittings in this design not only have a higher load-bearing capacity than pure carbon tubes, but also exhibit greater deformation and better toughness before failure, overcoming the defects of poor impact toughness, high brittleness, and easy cracking and chipping of all-carbon fiber tubes, thus significantly improving safety.

[0057] (2) Comparison test of grinding process A comparison was made between "un-ground direct secondary wrapping of the tube" and "cured centerless grinding followed by secondary wrapping of the tube" (i.e., the composite pipe fitting of the present invention), and the results are shown in Table 2.

[0058] Table 2. Comparison Results of Grinding Processes As shown in Table 2, the present invention, through the process of "first curing → centerless grinding → applying adhesive → secondary wrapping", reduces the dynamic balance eccentricity by more than 60%, increases the lower limit of the interface peel strength from about 2.2 N / mm to more than 3.0 N / mm and maintains a stable range, reduces the vibration amplitude of the installed equipment by about 25%~40%, and reduces the risk of resonance. It effectively solves the problems of poor dynamic balance, lack of vibration reduction and noise reduction structure, and easy structural fatigue and loosening of screws caused by long-term vibration in traditional carbon-aluminum composite pipe fittings.

[0059] (3) Comparison of pure aluminum tubes of the same specification Under the same three-point bending test conditions, the carbon-clad aluminum composite tube prepared in this embodiment was compared with a pure aluminum tube support rod of the same specification. The results show that compared with pure aluminum tubes, the composite tube of this embodiment exhibits a 146.68% increase in bending strength, an 86.62% increase in bending elastic modulus, an 88.50% increase in breaking strength (maximum force), and a reduction of over 35% in deformation under the same load. This indicates that while maintaining lightweight design, the present invention achieves a significant leap in bending stiffness and load-bearing capacity compared to the traditional pure aluminum tube approach, meeting the standards for main support of eVTOL aircraft arms.

[0060] (4) Overall effect This solution integrates carbon fiber coating onto the outer surface of an aluminum alloy inner tube and then curing it with epoxy resin. This achieves lightweight construction, high strength, excellent dynamic balance, high rigidity, vibration and noise reduction, and ease of processing and assembly while maintaining strength and rigidity. The overall weight of the tube is light, and its bending stiffness is significantly improved, reducing deformation by more than 35% compared to pure aluminum tubes. It also features minimal mass eccentricity, excellent dynamic balance, significant vibration and noise reduction, and increased strength that meets the standards for main arm supports in eVTOL aircraft.

[0061] V. Example 5: Applicable Specification Series This invention's process is applicable to various support tube specifications for eVTOL aircraft, including but not limited to: 17mm×20mm, 22mm×25mm, 25mm×28mm, and 25mm×30mm. The wall thickness of the aluminum alloy inner tube is 1.5mm (or 1.0mm~2.0mm). For different specifications, the number of carbon fiber layers and grinding allowance are adaptively adjusted according to the outer diameter, achieving lightweight, high rigidity, excellent dynamic balance, and low wind resistance and noise reduction.

[0062] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

Claims

1. A manufacturing process for carbon fiber-coated aluminum tubes for aircraft, characterized in that, Includes the following steps: S1. First winding: The carbon fiber prepreg is wrapped around the outer wall of the aluminum alloy inner tube according to the preset layup angle. S2, First Curing: The tube after the first roll is heated and cured to make the carbon fiber prepreg fabric and the aluminum alloy inner tube a single unit; S3, Centerless Grinding: Centerless grinding is performed on the outer surface of the pipe after the first curing to remove the resin-rich layer on the surface and form a grinding surface with a preset roughness. S4. Applying adhesive: Applying an adhesive layer to the grinding surface; S5. Second rolling: The outer carbon fiber cloth is attached to the outer surface of the tube after the adhesive is applied and then rolled. S6. Second Curing: The pipe after the second rolling is heated and cured to make the outer carbon fiber cloth and the cured layer of the carbon fiber prepreg composite together.

2. The manufacturing process according to claim 1, characterized in that, In step S1, the carbon fiber prepreg includes an inner prepreg and an outer prepreg; the inner prepreg is wrapped around the outer wall of the aluminum alloy inner tube after being folded together by UD prepregs in the 0° and 90° directions; the outer prepreg is attached to the outer wall of the inner prepreg after being folded together by UD prepregs in the 0° and ±45° directions; in step S2, the temperature for heat curing is 140℃~160℃, and the curing time is 2.5h~3.5h; a release tape is wrapped around the outer surface of the tube before curing, and the release tape is removed after curing.

3. The manufacturing process according to claim 1, characterized in that, In step S3, the grinding allowance of the centerless grinding is 0.4mm to 0.6mm, the outer diameter tolerance of the pipe after grinding is controlled within ±0.05mm, and the surface roughness Ra of the grinding surface is 1.6μm to 3.2μm. The centerless grinding only removes the surface resin-rich layer and the indentation of the demolding zone, without damaging the main carbon fiber structure.

4. The manufacturing process according to claim 1, characterized in that, In step S4, the adhesive layer is an epoxy resin adhesive layer with a thickness of 0.05mm to 0.15mm. The curing temperature of the epoxy resin adhesive is thermally matched with the resin system of the carbon fiber prepreg. Before applying the adhesive, the steps of cleaning and baking the polished pipes are also included. In step S5, the outer carbon fiber cloth is 3k twill carbon fiber cloth.

5. The manufacturing process according to claim 1, characterized in that, In step S6, the heating and curing temperature is 140℃~160℃, and the curing time is 2.5h~3.5h; before curing, release tape is wrapped around the outer surface of the pipe fitting, and the release tape is removed after curing; after the second curing, surface coating and CNC cutting are performed in sequence.

6. A composite tubing fitting for aircraft, comprising carbon fiber coated aluminum tubing, characterized in that, The composite pipe fitting is manufactured using the manufacturing process described in any one of claims 1 to 5, and comprises: Aluminum alloy inner tube; A carbon fiber composite layer is wrapped around the outer wall of the aluminum alloy inner tube and cured into one piece. The carbon fiber composite layer is composed of multiple layers of carbon fiber prepreg fabric arranged symmetrically and evenly according to the layup angle. An adhesive layer is applied to the outer surface of the carbon fiber composite layer; The outer carbon fiber layer is bonded to the outer surface of the carbon fiber composite layer by the adhesive layer and cured into a single unit. The interface between the carbon fiber composite layer and the adhesive layer is a ground rough surface.

7. The composite pipe fitting according to claim 6, characterized in that, The carbon fiber composite layer includes an inner prepreg layer and an outer prepreg layer; the inner prepreg layer is formed by stacking and curing UD prepregs in the 0° and 90° directions; the outer prepreg layer is formed by stacking and curing UD prepregs in the 0° and ±45° directions.

8. The composite pipe fitting according to claim 6, characterized in that, The surface roughness Ra of the ground roughened surface is 1.6μm to 3.2μm; the adhesive layer is an epoxy resin adhesive layer with a thickness of 0.05mm to 0.15mm.

9. The composite pipe fitting according to claim 6, characterized in that, The outer carbon fiber layer is a 3k twill carbon fiber cloth layer; the outer surface of the outer carbon fiber layer is also provided with a coating layer, which is a semi-matte paint finish.

10. The composite pipe fitting according to claim 6, characterized in that, The aluminum alloy inner tube has a wall thickness of 1.0mm to 2.0mm and an outer diameter of 17mm to 30mm.

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