Diffusion welding method for aluminum alloy complex cavity

Abstract

The invention discloses an aluminum alloy complex cavity diffusion welding method, which is characterized in that a uniform and compact organic molecule adsorption protection layer is formed on the surface of aluminum by completely immersing a cleaned blank in a diethylene glycol dimethyl ether solution for ultrasonic-assisted treatment. The protective film can effectively isolate air at normal temperature and prevent the oxidation film from regenerating; and in the initial stage of diffusion welding temperature rise, the aluminum alloy can be completely decomposed and volatilized, no impurity is left, and the high-activity clean aluminum surface is exposed. According to the method, diffusion welding is carried out under the conditions that the vacuum degree is high, the temperature is 500-560 DEG C and the pressure is 5-15 MPa in cooperation with a positioning tool system matched with the thermal expansion coefficient, aluminum atoms are fully diffused under the condition that no oxidation film hinders exist, and high-quality metallurgical bonding is formed. By means of the technical scheme, the problem that the welding rate is low due to an oxidation film on the surface of the aluminum alloy is successfully solved, meanwhile, the defects of ultrahigh vacuum equipment input and middle layer material introduction are overcome, and the manufacturing quality and production efficiency of complex aluminum alloy components are greatly improved.

Description

Technical Field

[0001] This invention relates to the field of aluminum alloy technology, specifically to a diffusion welding method for complex cavities in aluminum alloys. Background Technology

[0002] The manufacturing of complex aluminum alloy components has always been a technical challenge in the field of high-end equipment, especially for parts with complex internal flow channels and cavity structures, which are difficult to achieve using traditional casting or machining methods. Diffusion welding technology is considered an ideal process for manufacturing high-performance aluminum alloy components with complex internal cavities due to its advantages such as achieving atomic-level metallurgical bonding, minimal deformation, and the ability to weld complex shapes.

[0003] However, in practical applications, diffusion welding of 6061 aluminum alloy faces a prominent challenge: its surface rapidly forms a dense layer in the air. The oxide film, although only a few nanometers thick, is chemically extremely stable and severely hinders the interdiffusion between aluminum atoms, resulting in low welding rate and insufficient joint strength.

[0004] Existing technologies mainly utilize ultra-high vacuum environments ( Alternatively, adding intermediate layer materials such as copper or nickel can overcome this problem, but ultra-high vacuum equipment is expensive and has low processing efficiency; adding an intermediate layer introduces a heterogeneous material interface, reducing the joint's corrosion resistance and overall mechanical properties, while also increasing process complexity. These methods struggle to balance product quality with production efficiency and economy. Summary of the Invention

[0005] This invention aims to provide a diffusion welding method for complex cavities of aluminum alloys, which solves the problem of low welding rate caused by oxide film on the surface of aluminum alloys, while avoiding the drawbacks of investing in ultra-high vacuum equipment and introducing intermediate layer materials, thus greatly improving the manufacturing quality and production efficiency of complex aluminum alloy components.

[0006] To achieve the above objectives, the technical solution adopted by this invention is: a diffusion welding method for complex cavities in aluminum alloys, comprising: The aluminum alloy blanks are subjected to alkaline washing and acid washing to remove surface oil, oxide layer and residue; The cleaned billet is completely immersed in a diethylene glycol dimethyl ether solution for ultrasonic-assisted treatment, forming an organic molecule adsorption protective layer on the aluminum surface. The blank that has undergone anti-oxidation treatment is loaded into the positioning fixture system. The fixture uses a base material with a matching coefficient of thermal expansion and ceramic positioning pins to ensure accurate positioning in three-dimensional space. The blank that has been positioned is under vacuum. Diffusion welding is performed under conditions of 500-560℃ and 5-15MPa pressure for 60-180 minutes. The joints that have completed diffusion welding are subjected to ultrasonic testing and hydraulic testing to verify structural integrity. Tooling marks and heat treatment oxidation color are removed from the surface of the workpiece after diffusion welding. The reference surface is milled and the surface to be welded is rough machined. Components that have completed preliminary machining are then joined using friction stir welding or electron beam welding. Components that have completed all connections undergo final finishing and are delivered after helium mass spectrometry leak detection to verify their airtightness.

[0007] Preferably, the alkaline washing and acid washing treatment includes: soaking in a 5-10% NaOH solution at 50-70°C for 1-3 minutes to remove surface oil and part of the oxide layer, followed by soaking in a 10-20% NaOH solution. The solution is neutralized and brightened, and finally rinsed thoroughly with deionized water and dried with clean air.

[0008] Preferably, the ultrasonic-assisted treatment includes: performing a diethylene glycol dimethyl ether solution at a temperature of 40-60°C for 10-30 minutes to form a uniform organic molecule adsorption protective layer on the cleaned aluminum surface, and loading the billet that has completed the anti-oxidation treatment into the positioning fixture system within 30 minutes.

[0009] Preferably, the ceramic positioning pins in the positioning fixture system are inserted into positioning holes pre-machined on the blank, with the fitting clearance controlled at 0.01-0.02mm. A laser tracker is used to measure the spatial coordinates of the assembled module to ensure that the gap between the mating surfaces is ≤0.03mm and the overall cavity contour deviation is ≤0.05mm.

[0010] Preferably, the diffusion welding includes: raising the temperature to 300°C at a rate of 10°C / min and holding it for 10 minutes to remove adsorbed gas, then raising the temperature to 550°C at a rate of 5°C / min and holding it for 120 minutes, while simultaneously applying an axial pressure of 10 MPa, with the pressure fluctuation controlled within the range of ±0.2 MPa.

[0011] Preferably, the ultrasonic testing and hydraulic test include: using a 5MHz longitudinal wave probe for A-mode scanning, using a 64-element phased array probe for fan-shaped scanning of the suspicious area, applying 1.5 times the design working pressure and holding the pressure for 30 minutes, and determining that the sealing performance is qualified if the pressure drop is ≤0.5%.

[0012] Preferably, the removal of tooling marks and heat treatment oxidation color on the surface of the workpiece after diffusion welding is carried out by low-pressure sandblasting with non-metallic abrasive, pressure ≤0.3MPa, sandblasting angle 30°-45°, and surface roughness Ra≤3.2μm after treatment. The reference surface is milled to ensure flatness ≤0.02mm and surface roughness Ra≤1.6μm.

[0013] Preferably, the friction stir welding uses a WC-Co carbide stirring head with a shoulder diameter of Φ18mm and a needle length of 3.2mm, with a set rotation speed of 800rpm, a travel speed of 150mm / min, and a downward pressure of 0.2mm. Within 24 hours after welding, the excess height of the friction stir weld is milled, and the milling direction is consistent with the weld direction.

[0014] Preferably, the electron beam welding is performed in a vacuum degree ≤1×10⁻⁶. -3 The welding was carried out under Pa conditions, using a focused electron beam with an accelerating voltage of 60kV, a beam current of 8mA, a beam spot diameter of 0.3mm, a welding speed of 300mm / min, and a penetration depth of 2.5mm. The welds after electron beam welding were inspected by digital radiographic imaging with an image resolution ≤20μm.

[0015] Preferably, the final finishing is performed on a five-axis machining center using a solid carbide ball end mill, with a spindle speed of 12000 rpm, a feed rate of 600 mm / min, and a depth of cut of 0.1 mm, ensuring that the final dimensional tolerance is ≤ ±0.02 mm, the form and position tolerance is ≤ 0.03 mm, and the helium mass spectrometry leak detection sensitivity reaches [value missing]. Leakage rate exceeds The seal was deemed unqualified.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention involves ultrasonically assisted immersion of cleaned billets in a diethylene glycol dimethyl ether solution to form a uniform and dense organic molecular adsorption protective layer on the aluminum surface. This protective film effectively isolates air at room temperature, preventing oxide film regeneration; however, during the initial heating phase of diffusion welding, it completely decomposes and volatilizes, leaving no impurities and exposing a highly active, clean aluminum surface. Combined with a positioning fixture system with a matched coefficient of thermal expansion, this process is achieved under vacuum conditions. Diffusion welding is performed at temperatures of 500-560℃ and pressures of 5-15MPa, allowing aluminum atoms to diffuse fully without the obstruction of an oxide film, resulting in a high-quality metallurgical bond. This technical solution successfully solves the problem of low weldability caused by oxide films on aluminum alloy surfaces, while avoiding the drawbacks of investing in ultra-high vacuum equipment and introducing intermediate layer materials, significantly improving the manufacturing quality and production efficiency of complex aluminum alloy components. Attached Figure Description

[0017] Figure 1 This is a flowchart of the diffusion welding method for complex cavities in aluminum alloys according to the present invention. Detailed Implementation

[0018] 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.

[0019] Example 1 This embodiment proposes a diffusion welding method for complex cavities in aluminum alloys, such as... Figure 1 As shown, it includes: The aluminum alloy billet undergoes alkaline washing and acid pickling to remove surface oil, oxide layer, and residues. Specifically, it is immersed in a 5-10% NaOH solution at 50-70℃ for 1-3 minutes to remove surface oil and part of the oxide layer, followed by a 10-20%... The solution is neutralized and brightened, and finally rinsed thoroughly with deionized water and dried with clean air.

[0020] Alkaline washing and acid washing thoroughly remove organic contaminants, fine particles, and irregular oxide layers from the aluminum alloy surface through chemical action, achieving deep surface activation. After selective dissolution with alkaline solution, the surface micro-roughness is moderately improved, increasing the effective contact area for subsequent treatments; while nitric acid neutralization treatment not only eliminates the potential corrosion risk caused by alkaline residues, but also forms a uniform micro-nano morphology structure on the aluminum alloy surface through oxidation-dissolution balance, significantly improving surface energy.

[0021] The cleaned billet is completely immersed in a diethylene glycol dimethyl ether solution for ultrasonic-assisted treatment, forming an organic molecule adsorption protective layer on the aluminum surface. Specifically, the process is carried out at a temperature of 40-60℃ for 10-30 minutes, during which the diethylene glycol dimethyl ether solution forms a uniform organic molecule adsorption protective layer on the cleaned aluminum surface. The billet that has completed the anti-oxidation treatment is then loaded into the positioning fixture system within 30 minutes.

[0022] This ultrasonic-assisted organic molecular adsorption treatment constructs a dense, continuous, and thermally stable temporary protective barrier on the aluminum alloy surface, effectively preventing oxygen and water vapor from contacting the active aluminum surface and fundamentally inhibiting the formation of the alumina film that regenerates within seconds after cleaning. Because diethylene glycol dimethyl ether molecules contain multiple ether-bonded oxygen atoms, they can form stable coordination adsorption with unsaturated coordination sites on the aluminum surface. Their flexible carbon chain structure further enhances the integrity of the molecular layer coverage, allowing the surface to maintain high reactivity in room temperature air for more than 30 minutes. This protective layer can be completely pyrolyzed and volatilized in the initial stage of subsequent diffusion welding, leaving no residual carbon or ash, thus avoiding interference with metallurgical bonding caused by traditional physical coatings or passivation films.

[0023] The blanks that have undergone anti-oxidation treatment are loaded into the positioning fixture system. The fixture uses a base material with a matching coefficient of thermal expansion and ceramic positioning pins to ensure accurate positioning in three-dimensional space. The ceramic positioning pins in the positioning fixture system are inserted into positioning holes pre-machined on the blanks, with the fit clearance controlled at 0.01-0.02mm. A laser tracker is used to measure the spatial coordinates of the assembled modules to ensure that the gap between the mating surfaces is ≤0.03mm and the overall cavity contour deviation is ≤0.05mm.

[0024] This positioning fixture system, through synergistic design of thermal expansion behavior and micron-level mechanical coordination, effectively suppresses relative displacement and cavity geometric distortion caused by thermal deformation differences in aluminum alloy components during high-temperature diffusion welding. The precisely controlled fit clearance between the ceramic positioning pins and the aluminum alloy blank provides sufficient guiding accuracy during room-temperature assembly, while allowing the material to expand along a preset path during the heating phase, avoiding additional stress or jamming. Laser-guided spatial coordinate calibration ensures sub-micron-level alignment of the multi-module splicing interface on a three-dimensional scale, ensuring that the contour continuity and flow channel permeability of the complex internal cavity meet high-precision fluid dynamics requirements. The resulting stable constraint system not only guarantees uniform contact pressure in the weld area throughout the entire thermo-mechanical cycle but also significantly reduces localized incomplete welding defects caused by misalignment or uneven gaps.

[0025] The blank that has been positioned is under vacuum. Diffusion welding is performed under the conditions of 500-560℃ and 5-15MPa for 60-180 minutes. During diffusion welding, the temperature is increased to 300℃ at a rate of 10℃ / min and held for 10 minutes to remove adsorbed gas. Then, the temperature is increased to 550℃ at a rate of 5℃ / min and held for 120 minutes, while applying an axial pressure of 10MPa. The pressure fluctuation is controlled within ±0.2MPa.

[0026] This diffusion welding process, through the synergistic effect of segmented temperature control and highly stable pressure, effectively activates the interfacial atomic migration capacity while preventing excessive softening of the aluminum alloy matrix. The initial stepped temperature increase effectively desorbs residual gases and pyrolysis products of the organic protective layer, preventing their aggregation and formation of interfacial pores at high temperatures. Subsequently, stable axial pressure is applied within a window where the aluminum matrix is ​​close to its recrystallization temperature but without significant grain growth, inducing plastic flow at the micro-contact points and achieving rapid expansion of the oxide film breakage and the actual metal contact area. Strictly limited pressure fluctuations ensure uniform stress distribution across the complex cavity, avoiding geometric distortion caused by localized overpressure or insufficient bonding due to underpressure. Under these thermo-mechanical coupling conditions, aluminum atoms diffuse efficiently along grain boundaries and bulk paths, while interfacial pores continuously contract and close due to surface tension, ultimately forming a metallurgical interface free of macroscopic defects, with continuous composition and a dense microstructure.

[0027] The joints that have completed diffusion welding are subjected to ultrasonic testing and hydraulic testing to verify structural integrity. Tooling marks and heat treatment oxidation color are removed from the surface of the workpiece after diffusion welding. The reference surface is milled and the surface to be welded is rough machined. During ultrasonic testing and hydraulic testing, a 5MHz longitudinal wave probe is used for A-mode scanning, and a 64-element phased array probe is used for sector scanning of suspicious areas. During hydraulic testing, 1.5 times the design working pressure is applied and the pressure is maintained for 30 minutes. A pressure drop of ≤0.5% is considered to be qualified for sealing.

[0028] In addition, to remove tooling marks and heat treatment oxidation color from the workpiece surface after diffusion welding, low-pressure sandblasting with non-metallic abrasive is used, with a pressure ≤0.3MPa and a sandblasting angle of 30°-45°. After treatment, the surface roughness Ra≤3.2μm, and the reference surface is milled to ensure flatness ≤0.02mm and surface roughness Ra≤1.6μm.

[0029] This post-weld inspection and surface treatment combination process achieves precise dual control over the internal quality and external condition of the diffusion weld joint. Ultrasonic inspection combined with A-scan and phased array imaging can non-destructively identify hidden defects such as micron-level incomplete welds, interface pores, or inclusions, ensuring the continuity of the internal metallurgical bond. Hydraulic testing verifies the structural sealing reliability of the component under overload conditions from a macroscopic functional perspective. The two complement each other to form a multi-dimensional quality criterion, eliminating the risk of missing potential leakage channels. On this basis, low-pressure non-metallic sandblasting selectively removes tooling indentations and high-temperature oxidation discoloration layers without damaging the weld body, avoiding surface microcracks or residual stress concentrations that may be introduced by traditional mechanical grinding.

[0030] Components that have completed preliminary machining are then joined using friction stir welding or electron beam welding. Components that have completed all connections undergo final finishing and are delivered after helium mass spectrometry leak detection to verify their airtightness.

[0031] Preferably, the friction stir welding uses a WC-Co carbide stirring head with a shoulder diameter of Φ18mm and a needle length of 3.2mm, with a set rotation speed of 800rpm, a travel speed of 150mm / min, and a downward pressure of 0.2mm. Within 24 hours after welding, the excess height of the friction stir weld is milled, and the milling direction is consistent with the weld direction.

[0032] This friction stir welding parameter combination achieves an optimal balance between heat input and mechanical stirring in the secondary joining of complex aluminum alloy components. The frictional heat generated by the carbide stirring head at specific rotation and travel speeds is sufficient to bring the material into a stable thermoplastic state without excessive softening that could lead to cavity collapse. Precisely controlled pressure ensures full forging at the weld root while avoiding excessive extrusion that could cause wall thinning. The resulting weld microstructure is fine, free of porosity and cracks, and the width of the heat-affected zone is effectively limited, maximizing the preservation of the metallurgical bond integrity already formed in the diffusion weld area. Excess height milling is completed within 24 hours post-weld, utilizing the window period before the material is fully age-hardened, significantly reducing cutting forces and tool wear. Simultaneously, consistent milling along the weld direction avoids disturbance of the heat-affected zone by transverse shear stress, ensuring weld surface smoothness and a smooth transition to the base material.

[0033] Preferably, electron beam welding is performed in a vacuum environment. The welding was carried out under the following conditions: a focused electron beam was used, with an accelerating voltage of 60kV, a beam current of 8mA, a beam spot diameter of 0.3mm, a welding speed of 300mm / min, and a penetration depth of 2.5mm. The weld seam after electron beam welding was inspected by digital radiographic imaging, with an image resolution of ≤20μm.

[0034] This electron beam welding process, through a high-vacuum environment and precise beam current control, achieves a fusion joint with a moderate aspect ratio and minimal heat-affected zone in a tiny, precision interface region. The focused electron beam, under high accelerating voltage, possesses extremely strong penetration and energy density. Combined with optimized beam current and welding speed, this allows the molten pool to form and solidify in a very short time, effectively suppressing the volatilization of aluminum alloy elements and the tendency for hot cracking. The 0.3mm beam spot diameter ensures that energy is concentrated on the narrow weld area, avoiding thermal interference to adjacent diffusion welding or friction stir welding areas and maintaining the overall structural stability of the component. The resulting weld has uniform penetration depth, a clear outline, and is free of typical defects such as undercut and lack of fusion; the joint strength is close to that of the base material. Subsequent digital radiographic imaging provides non-destructive visualization of the weld interior at micron-level resolution, accurately identifying sub-millimeter-level porosity, shrinkage cavities, and other volumetric defects, ensuring the structural reliability of the tiny interface.

[0035] Specifically, the final finishing is performed on a five-axis machining center using a solid carbide ball end mill, with a spindle speed of 12,000 rpm, a feed rate of 600 mm / min, and a depth of cut of 0.1 mm, ensuring a final dimensional tolerance of ≤ ±0.02 mm, a form and position tolerance of ≤ 0.03 mm, and a helium mass spectrometry leak detection sensitivity of 100%. Leakage rate exceeds The seal was deemed unqualified.

[0036] The synergistic effect of this final finishing process and high-sensitivity leak detection ensures that complex cavity components simultaneously meet high-end application requirements in both geometric accuracy and functional sealing. Five-axis simultaneous machining, under conditions of micro-depth cutting and high speed, achieves efficient material removal and surface integrity control. This not only stably controls the dimensional and positional errors of key functional surfaces within the micrometer range but also avoids deformation of thin-walled areas caused by cutting heat or vibration, fully preserving the internal metallurgical bonding quality formed by the previous multi-stage welding process. The high-precision geometry provides structural assurance for the stable flow of fluid media within the complex internal cavity, effectively suppressing problems such as turbulence and abnormal pressure drop caused by local abrupt changes or misalignments. Based on this, helium mass spectrometry leak detection verifies the sealing of the entire cavity system with near-molecular-level detection sensitivity, capable of identifying micrometer-level penetrating defects or interfacial micro-leakage channels, ensuring the reliability of the product during long-term service under extreme conditions.

[0037] Example 2 This embodiment further refines the diffusion welding method for complex cavities in aluminum alloys proposed in Embodiment 1. Taking the manufacturing process of a neutron knife target assembly as an example, this embodiment details the complete process flow of a diffusion welding method for complex cavities in aluminum alloys. Using 6061 aluminum alloy as the base material, this method achieves high-quality metallurgical bonding of complex internal cavity structures under high temperature and high pressure conditions through systematic and phased process steps, including the following steps:

[0038] Step 1: Blank preparation and surface pretreatment This stage aims to provide a clean, contamination-free aluminum alloy billet surface free from the risk of oxide film regeneration for subsequent diffusion welding, and is the starting point for the entire welding quality control process. The surface condition directly determines the ability to form atomic diffusion interfaces; therefore, it is necessary to thoroughly remove original surface contaminants and inhibit the rapid formation of new oxide films through multi-stage cleaning and activation methods.

[0039] Step 1.1: Billet Receipt and Initial Inspection Obtain 6061-T6 aluminum alloy billets conforming to GB / T3190 standards from the raw material warehouse. The billet dimensions are based on the design drawings, allowing for subsequent machining allowances. Perform a visual inspection on each batch of billets to confirm the absence of macroscopic defects such as scratches, dents, and inclusions. Verify their chemical composition report to ensure that the magnesium and silicon contents are within the ranges of 0.8%-1.2% and 0.4%-0.8%, respectively. This step provides the physical and chemical basis for all subsequent processing. Compositional deviations or surface damage in the billets will directly affect the subsequent cleaning effect and the quality of the diffusion interface.

[0040] Step 1.2: Alkaline washing to remove oil and initial oxide layer peeling The inspected and qualified billets are immersed in a 5%-10% (mass fraction) sodium hydroxide (NaOH) aqueous solution at a constant temperature of 60℃ for 2 minutes. This process utilizes the alkaline solution to dissolve grease, organic contaminants, and some loose alumina (Al2O3) on the aluminum alloy surface, achieving preliminary surface activation. The reaction formula is as follows:

[0041] ; ; Under controlled conditions of time and concentration, the above reaction can effectively remove the surface contaminant layer while avoiding excessive corrosion of the substrate. This step follows the physical state of the billet in 1.1, providing it with a chemically active surface, which is a prerequisite for subsequent pickling and anti-oxidation treatment.

[0042] Step 1.3: Nitric acid neutralization and surface brightening The alkali-washed billet was immediately transferred to a 15% (mass fraction) nitric acid solution. In the solution, the mixture is treated at room temperature for 1 minute. Nitric acid plays a dual role in this process: firstly, it neutralizes residual NaOH, preventing the alkaline substances from forming salt crystals during subsequent drying; secondly, it creates a uniform, dense, but extremely thin new oxide layer on the aluminum alloy surface through oxidation-dissolution equilibrium, while simultaneously imparting a metallic luster to the surface. This process is represented as follows:

[0043] ; Although aluminum nitrate is produced by the reaction, the extremely short processing time results in only a slight reaction on the surface, causing no significant material loss. This step directly relies on the active aluminum surface exposed after the alkaline wash in step 1.2. If the alkaline wash is skipped, the nitric acid treatment will not be able to effectively remove oil stains, leading to uneven surface activation.

[0044] Step 1.4: Rinse with deionized water and dry thoroughly. After pickling, the billet is continuously rinsed with flowing deionized water (resistivity ≥ 18.2 MΩ·cm) for at least 3 minutes to ensure that no acid or alkali residue ions remain on the surface. Then, clean compressed air (dew point ≤ -40℃, oil content...) is used. Blow the workpiece surface from multiple angles until completely dry. The drying process must be completed in a cleanroom (ISO Class 8) to prevent moisture and dust in the air from recontaminating the surface. This step follows the wet state after the nitric acid treatment in 1.3, and through physical rinsing and airflow drying, provides a clean substrate free of ions and liquid films for the next step of anti-oxidation treatment.

[0045] Step 2: Construction of Anti-oxidation Interface and Formation of Protective Layer After cleaning, the exposed aluminum atoms readily react with oxygen in the air, regenerating a dense structure within seconds. The film severely hinders atomic migration during subsequent diffusion bonding. Therefore, a temporary, removable protective layer that does not affect the metallurgical bond must be constructed immediately after cleaning to maintain surface cleanliness.

[0046] Step 2.1: Preparation of diethylene glycol dimethyl ether solution and preparation of ultrasonic system Prepare a diethylene glycol dimethyl ether (DME) solution with a purity ≥99.5%, without dilution, and use it directly as the treatment medium. Inject the solution into an ultrasonic cleaning tank made of polytetrafluoroethylene (PTFE) to avoid metal ion contamination. Set the ultrasonic frequency to 40kHz, the power density to 0.5W / cm², and preheat the solution to 50°C. This step is based on the dried blank obtained in step 1.4, and solution preparation must be completed within 10 minutes of drying to ensure timely treatment.

[0047] Step 2.2: Ultrasonic-assisted adsorption treatment of organic molecules The dried billet is completely immersed in a preheated DME solution, and the ultrasonic system is activated for 20 minutes. Under the effect of ultrasonic cavitation, DME molecules are forcibly adsorbed onto the aluminum alloy surface, forming an organic molecular monolayer with a thickness of approximately 2-5 nm. This monolayer is bonded to the aluminum surface through coordination bonds between oxygen atoms, resulting in a dense and hydrophobic structure that effectively isolates oxygen and water vapor. The adsorption process can be approximated using the Langmuir isotherm adsorption model. ;

[0048] in, For surface coverage, This refers to the concentration of DME (in this case, a pure liquid). ), This represents the adsorption equilibrium constant. Because DME molecules have two ether bonds and a flexible carbon chain, their adsorption energy on aluminum surfaces is relatively high (approximately -45 kJ / mol), allowing them to remain stable at room temperature for several hours. This step directly follows the clean and dry surface condition described in section 1.4. The presence of a water film or residual ions on the surface will significantly reduce the adsorption efficiency of DME.

[0049] Step 2.3: Verification of Protective Layer Stability and Preparation for Transfer After processing, the billet is slowly lifted from the DME solution to avoid splashing and causing localized dry spots. Under a nitrogen protective atmosphere (oxygen content <10ppm), the surface is visually inspected for uniform wetting, absence of watermarks or frost. The billet is then placed in a clean PTFE tray, covered with a nitrogen purge hood, and prepared for transfer to the diffusion welding fixture assembly area. This step relies on the organic protective layer formed in 2.2; uneven adsorption will lead to localized oxidation at subsequent high temperatures, resulting in a decreased weld success rate.

[0050] Step 2.4: Anti-oxidation aging control and process window locking The time from the end of DME treatment to diffusion welding in the furnace is strictly controlled within 30 minutes. During this period, the ambient humidity is maintained at ≤30%RH and the temperature at 23±2℃. This time window is determined by the thermodynamic stability of the protective layer: the DME adsorption layer can maintain its structural integrity below 50℃, but as time goes on, trace amounts of moisture in the air will slowly penetrate, leading to localized desorption. Therefore, the furnace loading must be completed before the protective layer fails.

[0051] Step 3: Assembly of high-precision positioning fixtures and setting of diffusion welding parameters Complex cavities are typically assembled from multiple aluminum alloy modules, with irregular internal geometry and extremely high alignment precision requirements. Misalignment at high temperatures can lead to cavity deformation or weld displacement. Therefore, a tooling system with thermal expansion matching is required to achieve precise three-dimensional positioning before furnace loading.

[0052] Step 3.1: Install ceramic locating pins and matching bases Zirconia-toughened alumina (ZTA) ceramic locating pins are selected, with a coefficient of thermal expansion of [missing information]. , with 6061 aluminum alloy ( Although not entirely identical, relative displacement at high temperatures can be minimized through structural compensation design (such as flexible slots). The locating pin is inserted into a pre-machined Φ5H7 locating hole on the blank, with the clearance controlled at 0.01-0.02mm. The base is made of Invar alloy (Fe-36%Ni), with a thermal expansion coefficient of... This is used to secure the bottom of the module. This step is based on the blank that has undergone anti-oxidation treatment in section 2.4, and mechanical assembly must be completed without damaging the surface protective layer.

[0053] Step 3.2: Three-dimensional alignment calibration and gap control The assembled module is measured using a laser tracker to ensure that the gap between each mating surface is ≤0.03mm and the overall cavity contour deviation is ≤0.05mm. If deviations are found, they are corrected by fine-tuning the position of the ceramic pins or replacing them with compensation shims of different diameters. This process must be performed in a nitrogen atmosphere to avoid surface oxidation caused by repeated disassembly and assembly. This step directly uses the positioning system already installed in section 3.1; if the positioning pins are not correctly embedded, subsequent high-precision alignment will not be possible.

[0054] Step 3.3: Presetting Vacuum Diffusion Welding Furnace Parameters The assembled fixture is placed into the vacuum diffusion welding furnace, the furnace door is closed, and a vacuum is drawn until... The heating program is set as follows: Increment to 300℃ at a rate of 10℃ / min, hold for 10 minutes to remove adsorbed gases; then increase to 550℃ at a rate of 5℃ / min, hold for 120 minutes, and simultaneously apply an axial pressure of 10MPa. The pressure is applied via a hydraulic servo system, with pressure fluctuations ≤ ±0.2MPa. This step follows the tooling alignment completed in section 3.2. Poor alignment will cause the module to slip under high temperature and pressure, compromising the geometric accuracy of the cavity.

[0055] Step 3.4: Monitoring and Abnormal Response of Thermo-Mechanical Coupling Process During the welding process, the furnace temperature, pressure, and vacuum level are monitored in real time. If the temperature deviation exceeds ±5℃ or the pressure fluctuation exceeds ±0.5MPa, the system automatically pauses heating and issues an alarm. This monitoring logic is based on the process parameters set in section 3.3, ensuring that the thermal-mechanical field acts on the workpiece within the predetermined range, avoiding insufficient metallurgical bonding or tooling deformation due to parameter drift.

[0056] Step 4: Diffusion welding metallurgical bonding and interfacial atomic migration Under high temperature, high pressure, and high vacuum conditions, atoms at the aluminum alloy contact surfaces gain sufficient energy to overcome the interfacial barrier, undergo interdiffusion, and ultimately form a seamless metallurgical bond. This process is the core of the entire method, and its quality depends on the interface state and positioning accuracy provided by the preceding steps.

[0057] Step 4.1: Interface atomic activation and oxide film rupture When the temperature rises above 500℃, the DME protective layer begins to pyrolyze and volatilize, exposing a clean aluminum surface. Under a pressure of 10MPa, the microscopic protrusions first come into contact and undergo plastic deformation, and localized stress concentration leads to the rupture of the residual Al2O3 film. According to Hertz contact theory, the actual contact area at the contact point... Represented as: ;

[0058] in, For normal force, The equivalent radius of curvature, This is the equivalent elastic modulus. As pressure continues to act, The process continues to expand, providing a pathway for atomic diffusion. This step directly depends on the stable thermo-mechanical environment described in 3.4; if the pressure is insufficient, the oxide film cannot be effectively broken, and diffusion is difficult to initiate.

[0059] Step 4.2: Bulk diffusion and grain boundary diffusion proceed in tandem. During the 550℃ holding period, aluminum atoms migrate through two pathways: bulk diffusion (the vacancy mechanism in the body-centered cubic structure) and grain boundary diffusion. Bulk diffusion coefficient. With grain boundary diffusion coefficient They are represented as follows:

[0060] ; in, This indicates that grain boundary diffusion dominates at low temperatures. As the holding time increases, the pores gradually close, and the interface disappears. This process follows the initial contact region formed in 4.1. If contaminants are present at the interface, they will significantly increase the diffusion activation energy and inhibit atomic migration.

[0061] Step 4.3: Pore shrinkage and interface healing dynamics According to the Kuczynski pore shrinkage model, the interface pore radius... Over time The changes satisfy: ; in, This is a constant related to the diffusion coefficient and surface tension. During the 120-minute heat preservation period, the initial pore size ( The structure can be completely closed, achieving a theoretical density of >99%. This step is a macroscopic manifestation of the atomic diffusion results in section 4.2. If the holding time is insufficient, residual pores will lead to a decrease in the welding rate.

[0062] Step 4.4: Stress relief and microstructure stabilization during the cooling stage After the heat treatment, the alloy is cooled to 300°C at a rate of 3°C / min, and then cooled to room temperature in the furnace. Slow cooling reduces thermal stress accumulation and prevents microcracks from forming in the weld area. Meanwhile, in 6061 aluminum alloy... The uniform distribution of precipitated phases during cooling helps improve joint strength. This step follows the healed interface state described in 4.3. If cooling is too rapid, tensile stress will be generated at the interface due to differences in thermal expansion, compromising the integrity of the metallurgical bond.

[0063] Step 5: Post-weld non-destructive testing and structural integrity verification After diffusion welding is completed, a comprehensive non-destructive test must be performed on the weld joint to confirm the metallurgical bonding quality, internal defect status, and structural integrity. This stage directly inherits the weld microstructure formed in the fourth step; if systematic testing is not performed, potential defects may lead to failure during subsequent processing or service.

[0064] Step 5.1: Initial ultrasonic A-scan inspection and defect location The component, cooled to room temperature, is placed on an ultrasonic testing platform. A 5MHz longitudinal wave probe is used, with deionized water as the coupling agent, to perform a full-coverage A-mode scan over the weld area. The echo amplitude and time delay are used to determine the presence of defects such as incomplete welding, voids, or inclusions. According to NB / T47013.3-2015 standard, any signal with an echo amplitude exceeding the equivalent of a Φ2mm flat-bottomed hole is considered unacceptable. This step directly references the component's condition after cooling as described in section 4.4. If microcracks occur during cooling, they will appear as sharp echoes in the A-mode scan.

[0065] Step 5.2: Phased array ultrasound imaging and 3D defect reconstruction For suspicious areas identified in the A-scan, a 64-element phased array probe is used for a sector scan (S-scan) to generate a C-scan image. Synthetic Aperture Focusing Technique (SAFT) is then used to fuse the multi-angle echo data, reconstructing the three-dimensional spatial morphology and depth distribution of the defect. This process can identify linear or volumetric defects with a major diameter greater than 1 mm and accurately mark their position within the cavity wall thickness. This step relies on the initial inspection results in section 5.1; if no abnormalities are found in the initial inspection, the phased array detection is skipped to improve efficiency.

[0066] Step 5.3: Hydraulic test to verify overall sealing performance and pressure bearing capacity Connect the component sealing interface to the hydraulic test system, inject deionized water as the medium, slowly increase the pressure to 1.5 times the design working pressure (e.g., 15 MPa), and hold the pressure for 30 minutes. During this period, monitor the change in the pressure gauge reading and the leakage situation on the outer surface. If the pressure drop ≤ 0.5% or there is no visible leakage, the sealing is judged to be qualified. This step follows the components that have been confirmed to have no internal defects in 5.2. If there are undetected through-holes, it will show continuous pressure relief during the hydraulic test.

[0067] Step 5.4: Detection Data Archiving and Traceability Establishment Store data such as ultrasonic A-scan waveforms, phased array images, and hydraulic test curves in the Manufacturing Execution System (MES) according to the component number, generating a unique quality file. This file includes the detection time, operator, equipment parameters, and judgment conclusions, ensuring traceability throughout the product's life cycle. This step is the information processing of the test results in 5.3, providing a decision-making basis for subsequent machining and final inspection.

[0068] Sixth Step: Post-Weld Surface Cleaning and Preliminary Machining During the diffusion welding process, indentations may remain in the areas where the tooling contacts, and the high-temperature environment may also cause a light-colored oxide film to form on the surface. To meet the geometric and surface requirements of subsequent joining processes, cleaning and preliminary machining are required, and this stage is directly based on the components confirmed to be qualified in the fifth step.

[0069] Step 6.1: Removal of Tooling Indentations and Cleaning of Thermal Discoloration Use non-metallic abrasives (such as alumina ceramic beads with a particle size of 80 - 120 μm) to perform low-pressure sandblasting (pressure ≤ 0.3 MPa) on the tooling contact area to remove surface indentations and heat treatment oxidation color. Control the sandblasting angle within 30° - 45° to avoid damaging the weld area. The surface roughness Ra after treatment is controlled within 3.2 μm. This step follows the qualified components archived in 5.4. If cleaning is skipped, the remaining indentations will affect the establishment of the subsequent machining reference surface.

[0070] Step 6.2: Milling of Reference Surfaces and Reconstruction of Positioning Features [[ID=第十九]] On a five-axis machining center, use the outer contour of the cavity as the rough reference to mill two mutually perpendicular process reference surfaces with a flatness ≤ 0.02 mm and a surface roughness Ra ≤ 1.6 μm. At the same time, process a Φ10H7 process hole in the non-functional area for the fixture positioning of subsequent friction stir welding. This step depends on the surface state after cleaning in 6.1. If there is an oxide film or stain on the surface, it will cause increased wear of the milling tool and affect the accuracy of the reference surface.

[0071] Step 6.3: Rough Machining of the Welding Surface and Remainder Control According to the subsequent friction stir welding requirements, the joint surfaces are rough milled, leaving a finishing allowance of 0.8-1.2mm. High-pressure internal cooling tools are used during machining, and the cutting fluid is a low-chlorine synthetic ester-based fluid to avoid chloride ion corrosion of the aluminum alloy. The cutting parameters are set as follows: spindle speed 3000 rpm, feed rate 800 mm / min, depth of cut 1.5 mm. This step follows the process reference established in 6.2. If the reference surface is not flat, the flatness of the surfaces to be welded will exceed the tolerance, affecting the lap quality of the friction stir welding.

[0072] Step 6.4: Initial Dimensional Inspection and Feedback on Machining Errors A coordinate measuring machine (CMM) is used to inspect the flatness of the datum surface, the position of the process holes, and the contour of the surface to be welded. The measured values ​​are recorded and compared with the theoretical model. If any critical dimension deviates from the tolerance by ≥0.05mm, the process returns to step 6.2 for remachining of the datum surface. The inspection data is simultaneously uploaded to the MES system to optimize the machining path for subsequent batches. This step is a closed-loop verification of the machining results in step 6.3, ensuring that the geometry is under control before proceeding to the next connection process.

[0073] Step 7: Multi-process composite connection and weld finishing For complex components, diffusion welding alone cannot complete all connection tasks. It is necessary to combine friction stir welding and electron beam welding to handle large cross-section lap joints and precise micro-interfaces, respectively. This stage achieves seamless integration of multiple processes based on the welding surfaces provided in step six.

[0074] Step 7.1: Control of lap joints and heat-affected zone in friction stir welding The components to be joined are clamped in a dedicated friction stir welding fixture according to the dimensions confirmed in section 6.4, using a WC-Co carbide stirring head with a shoulder diameter of Φ18mm and a needle length of 3.2mm. The welding parameters are set as follows: rotation speed 800rpm, travel speed 150mm / min, and pressure 0.2mm. During welding, the frictional heat generated by the rotating stirring head puts the material in a thermoplastic state, achieving solid-phase bonding through mechanical stirring. The width of the heat-affected zone is controlled within 4-6mm. This step directly references the surface to be welded processed in section 6.3. If the surface roughness exceeds the tolerance, it will lead to accelerated wear of the stirring head and poor weld formation.

[0075] Step 7.2: Milling and surface smoothing of the weld reinforcement. Within 24 hours of welding, the excess height of the friction stir weld should be milled. A diamond-coated end mill should be used, with a depth of cut of 0.3 mm and a feed rate of 500 mm / min, to ensure the weld surface is flush with the base material, with a flatness ≤0.1 mm. The milling direction should be consistent with the weld direction to avoid surface tearing caused by transverse cutting. This step follows the weld morphology established in 7.1; if processing is delayed, the weld area will become difficult to cut due to age hardening.

[0076] Step 7.3: Electron beam welding precision interface connection For narrow joints where friction stir welding is not feasible (such as annular joints with a diameter <8mm), the process is transferred to electron beam welding equipment. Under vacuum conditions... Under these conditions, focused electron beam welding (accelerating voltage 60kV, beam current 8mA) is used. The beam spot diameter is controlled at 0.3mm, the welding speed is 300mm / min, and the penetration depth is 2.5mm. The concentrated electron beam energy and low heat input can avoid microstructural degradation in the peripheral diffusion welding area. This step depends on the precision interface geometry processed in 6.4. If the interface gap is >0.1mm, it will lead to insufficient electron beam penetration and incomplete fusion.

[0077] Step 7.4: X-ray inspection of internal defects in electron beam welds Digital radiography (DR) inspection of electron beam welds was performed using an X-ray tube voltage of 120 kV, a current of 1.0 mA, and an exposure time of 2 seconds. The image resolution was ≤20 μm, capable of identifying volumetric defects such as porosity and shrinkage cavities. According to ISO 17636-2, single pores or continuous chains of pores with a diameter ≥0.3 mm were not permitted. This step directly verifies the weld quality described in section 7.3; any undetected defects will be exposed during the final leak detection.

[0078] Step 8: Final finishing, sealing verification, and product delivery After multiple rounds of connection and testing, the components enter the final molding stage. This stage requires the completion of all dimensional adjustments, high-sensitivity sealing verification, and comprehensive final inspection to ensure that the product fully meets the design drawings and usage requirements.

[0079] Step 8.1: Five-axis linkage final finishing On a high-precision five-axis machining center, all functional surfaces, mounting holes, and flow channel cavities are precision-machined based on the original 3D model. A solid carbide ball end mill is used, with a spindle speed of 12000 rpm, a feed rate of 600 mm / min, and a depth of cut of 0.1 mm. Thermal deformation is compensated for in real-time during machining to ensure that the final dimensional tolerance is ≤ ±0.02 mm and the form and position tolerances (such as coaxiality and position) are ≤ 0.03 mm. This step follows the confirmation in section 7.4 that the component has no internal defects. If there is deformation in the weld seam in the early stage, it will increase the difficulty of precision machining and may even lead to insufficient wall thickness.

[0080] Step 8.2: High-sensitivity leak detection using helium mass spectrometry The component was filled with 0.5 MPa helium gas and placed in a vacuum leak detection chamber. The helium mass spectrometer was then activated. The detection sensitivity reached [value missing]. It can identify micron-level leak channels. If the leak rate exceeds... If the weld is damaged during finishing, it will be deemed unqualified and requires rework or scrapping. This step directly refers to the final geometric state completed in 8.1. If the weld is damaged during finishing, it will appear as a significant leak during leak detection.

[0081] Step 8.3: Final ultrasound re-examination and document preparation All weld areas are subjected to ultrasonic testing again (standard as in 5.1) to confirm that no new defects were introduced during finishing and handling. Simultaneously, the process flow chart, test reports, material certificates, and other documents are checked for completeness and consistency. This step completes the final loop of the entire manufacturing process; if any documents are missing, the product, even if qualified, must not be delivered.

[0082] Step 8.4: Rust-proof packaging and warehousing delivery The components are wrapped with VCI vapor phase corrosion inhibitor film, and then further reinforced with EPE pearl cotton and a wooden crate to prevent damage during transportation. A humidity indicator card is placed inside the packaging box to ensure that the relative humidity is ≤40%. After packaging, the QR code is scanned and entered into the ERP system to generate an outbound slip, which is then delivered to the user or transferred to the finished goods warehouse.

[0083] In summary, by immersing the cleaned billet completely in a diethylene glycol dimethyl ether solution for ultrasonic-assisted treatment, a uniform and dense organic molecular adsorption protective layer is formed on the aluminum surface. This protective film effectively isolates air at room temperature, preventing oxide film regeneration; and in the initial stage of diffusion welding, it completely decomposes and volatilizes, leaving no impurities and exposing a highly active, clean aluminum surface.

[0084] With a positioning tooling system that matches the coefficient of thermal expansion, under vacuum conditions Diffusion welding is performed at temperatures of 500-560℃ and pressures of 5-15MPa, allowing aluminum atoms to diffuse fully without the obstruction of an oxide film, thus forming a high-quality metallurgical bond.

[0085] This technical solution successfully solved the problem of low welding rate caused by oxide film on aluminum alloy surface, while avoiding the drawbacks of investing in ultra-high vacuum equipment and introducing intermediate layer materials, thus significantly improving the manufacturing quality and production efficiency of complex aluminum alloy components.

[0086] 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. The scope of protection claimed by the appended claims and their equivalents is defined.

Claims

1. A method of diffusion bonding of an aluminum alloy complex cavity, characterized by, The method comprises the following steps: Aluminum alloy blanks are subjected to alkaline cleaning and acid cleaning to remove surface oil stains, oxide layers and residues; The cleaned blanks are completely immersed in a diethylene glycol dimethyl ether solution for ultrasonic-assisted treatment to form an organic molecular adsorption protective layer on the aluminum surface; The blanks subjected to the anti-oxidation treatment are loaded into a positioning tool system, and the tool system uses a thermal expansion coefficient matching the base material and ceramic positioning pins to ensure accurate positioning in three-dimensional space; The diffusion welding of the completed positioning blank is carried out under the conditions of vacuum degree 500-560 ℃, pressure 5-15 MPa, holding for 60-180 minutes. The joints subjected to diffusion welding are subjected to ultrasonic testing and hydraulic testing to verify the structural integrity, remove tool marks and heat treatment oxidation color on the surface of the workpiece after diffusion welding, and perform reference surface milling and rough machining of the welding surface; The components subjected to preliminary machining are connected by friction stir welding or electron beam welding, and the components subjected to all connections are subjected to final machining and delivered after verifying the sealing property by helium mass spectrometry.

2. The method of claim 1, wherein, The alkaline cleaning and acid cleaning comprise: The surface oil and partial oxide layer are removed by immersing in 5-10% NaOH solution at 50-70°C for 1-3 minutes, followed by neutralization and brightening treatment using 10-20% HNO3 solution, and finally rinsing thoroughly with deionized water and blowing dry with clean air. The surface oil and partial oxide layer are removed by immersing in 5-10% NaOH solution at 50-70°C for 1-3 minutes, followed by neutralization and brightening treatment using 10-20% HNO3 solution, and finally rinsing thoroughly with deionized water and blowing dry with clean air.

3. The method of claim 1, wherein The ultrasonic-assisted treatment comprises: The diethylene glycol dimethyl ether solution forms a uniform organic molecular adsorption protective layer on the cleaned aluminum surface at a temperature of 40-60°C for 10-30 minutes, and the blanks subjected to the anti-oxidation treatment are loaded into the positioning tool system within 30 minutes.

4. The method of claim 1, wherein, The ceramic positioning pins in the positioning tool system are inserted into the positioning holes previously machined on the blanks, the cooperation gap is controlled within 0.01-0.02mm, a laser tracker is used to measure the spatial coordinates of the assembled module to ensure that the gap between the spliced surfaces is ≤0.03mm and the overall cavity profile deviation is ≤0.05mm.

5. The method of claim 1, wherein, The diffusion welding comprises: The temperature is raised to 300°C at a rate of 10°C / min for 10 minutes to drive out the adsorbed gas, and then raised to 550°C at a rate of 5°C / min for 120 minutes while applying an axial pressure of 10MPa, and the pressure holding fluctuation is controlled within ±0.2MPa.

6. The method of claim 1, wherein, The ultrasonic testing and hydraulic testing comprise: A 5MHz longitudinal wave probe is used for A-scan, a 64-element phased array probe is used for sector scan on suspicious areas, the hydraulic test applies a pressure of 1.5 times the design working pressure and holds for 30 minutes, and the sealing property is qualified when the pressure drop is ≤0.5%.

7. The method of claim 1, wherein the method further comprises: Non-metallic abrasives are used for low-pressure sandblasting to remove tool marks and heat treatment oxidation color on the surface of the workpiece after diffusion welding, the pressure is ≤0.3MPa, the sandblasting angle is 30°-45°, the surface roughness Ra after treatment is ≤3.2μm, the reference surface milling ensures that the flatness is ≤0.02mm and the surface roughness Ra is ≤1.6μm.

8. The method of claim 1, wherein the aluminum alloy complex cavity diffusion bonding method is characterized by, A WC-Co hard alloy stir pin with a shoulder diameter of Φ18mm and a needle length of 3.2mm is used for friction stir welding, the rotation speed is set to 800rpm, the travel speed is 150mm / min, the downward pressure is 0.2mm, and the excess height of the friction stir welding seam is milled within 24 hours after welding, and the milling direction is consistent with the welding direction.

9. The method of claim 1, wherein, The electron beam welding is performed under vacuum. The welding was carried out under the following conditions: a focused electron beam was used, with an accelerating voltage of 60kV, a beam current of 8mA, a beam spot diameter of 0.3mm, a welding speed of 300mm / min, and a penetration depth of 2.5mm. The weld seam after electron beam welding was inspected by digital radiographic imaging, with an image resolution of ≤20μm.

10. The method of claim 1, wherein, The final finishing is carried out on a five-axis machining center, using a whole carbide ball head milling cutter, the spindle speed is 12000 rpm, the feed speed is 600 mm / min, the cutting depth is 0.1 mm, the final size tolerance is ensured to be ≤±0.02 mm, the form and position tolerance is ≤0.03 mm, and the helium mass spectrum leak detection sensitivity reaches , the leakage rate exceeds , and the sealing performance is determined to be unqualified.

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