Aluminum alloy surface functionalized passivation layer construction method based on electrochemical mechanical polishing (ECMP) and application of aluminum alloy surface functionalized passivation layer construction method in diffusion welding

Abstract

The invention discloses an aluminum alloy surface functionalized passivation layer construction method based on anhydrous ECMP and application of the aluminum alloy surface functionalized passivation layer construction method in diffusion welding, and belongs to the technical field of metal material surface treatment and connection. According to the method, a non-aqueous electrolyte system with terpilenol as a matrix is adopted, and a phosphorus-doped composite passivation layer with double functions of polishing and welding is constructed on the surface of the aluminum alloy in situ through an electrochemical mechanical polishing (ECMP) process. The water-free electrolyte comprises a supporting electrolyte, a film-forming agent triethyl phosphate, a corrosion inhibitor benzotriazole, a nanometer grinding material and a viscosity modifier. The functional passivation layer can be decomposed in a controlled mode in the diffusion welding process, interface diffusion energy barriers are effectively reduced, aluminum atom mutual diffusion is promoted, and high-quality metallurgical bonding is achieved. According to the method, the technical problem that a natural oxidation film hinders atomic diffusion in aluminum alloy diffusion welding is fundamentally solved, the strength of a diffusion welding joint can reach 90% or above of that of base metal, meanwhile, the diffusion welding temperature is reduced by 20-40 DEG C, the process time is shortened by 30% or above, and the method has remarkable engineering application value.

Description

Technical Field

[0001] This invention relates to the field of surface treatment and joining technology of metal materials, specifically to a method for constructing a functionalized passivation layer on an aluminum alloy surface based on anhydrous ECMP and its application in diffusion welding. Background Technology

[0002] With the rapid development of fields such as artificial intelligence, high-performance computing, 5G communication, aerospace, and new energy vehicles, the power density and heat flux density of electronic devices are increasing exponentially, creating unprecedented demands for efficient and reliable thermal management solutions. Aluminum alloy liquid cooling plates, as a core active heat dissipation component, have become one of the key technologies for solving the "thermal bottleneck" problem in these fields due to their excellent thermal conductivity, lightweight design, and designability.

[0003] In applications with extreme requirements for reliability, lightweight design, and thermal management efficiency, such as aerospace, high-power lasers, high-end server computing units (CPU / GPU), and new energy vehicle electronic control systems, aluminum alloy liquid cooling plates are the preferred solution. In these fields, traditional vacuum brazing processes, due to the need to introduce third-party brazing filler metals such as silicon-aluminum or aluminum-silicon-magnesium base metals, create regions at the weld with different chemical compositions and potentials than the base material. When liquid cooling plates use cooling media such as water or ethylene glycol aqueous solutions, galvanic corrosion is highly likely to occur in the weld area. Long-term operation may lead to corrosion perforation and coolant leakage, posing a serious threat to system safety. Therefore, current high-end applications generally require the use of solid-state diffusion welding technology to manufacture liquid cooling plates. This technology uses high temperature and pressure to cause atomic diffusion at the contact interface to form a metallurgical bond, without introducing any dissimilar materials. This ensures that the weld composition is consistent with the base material, fundamentally eliminating the risk of galvanic corrosion and ensuring the long-term sealing and reliability of components under harsh environments.

[0004] However, achieving high-quality, high-efficiency diffusion welding of aluminum alloys faces a fundamental and almost unavoidable technical challenge—the obstruction caused by the dense oxide film (Al2O3) on the aluminum alloy surface. In the atmosphere, aluminum alloys spontaneously and instantaneously form a dense alumina film approximately 2-10 nanometers thick. This alumina layer is chemically extremely stable, with a melting point (approximately 2050℃) far exceeding that of the aluminum alloy substrate (approximately 660℃), and it does not decompose at the diffusion welding process temperature. It acts like a layer of "armor," tightly sealing the metal surface and severely hindering direct contact and interdiffusion between aluminum atoms. This is the primary reason for the low strength, large performance dispersion, and low yield of diffusion-welded joints.

[0005] Even more challenging is the exceptional regeneration capacity of this oxide film. Studies have confirmed that even after precise and rigorous surface pretreatment (such as mechanical scraping and chemical cleaning), the oxide film on newly exposed pure aluminum surfaces rapidly regenerates and reaches a certain thickness within milliseconds to minutes after contact with an oxygen-containing environment (even a high vacuum environment). For example, at 10⁻ 4 Under vacuum conditions of Pa, the oxide layer on a clean aluminum surface can grow to 1-2 nanometers in a short time. This "inevitability" and "instantaneousness" of surface oxidation makes the window of traditional diffusion welding process extremely narrow. It imposes stringent requirements on the control of equipment (such as vacuum level better than 10⁻³ Pa or even higher), process parameters (temperature, pressure, time), and the state of the surface to be welded. This greatly increases production costs and technical complexity, becoming a common bottleneck restricting the large-scale application of this technology in a wider range of high-end aluminum-based products.

[0006] To address this challenge, existing technologies mainly focus on how to "remove" or "bypass" this existing oxide film, and can be summarized into the following categories: (1) Physical / mechanical removal method: Under vacuum or protective atmosphere, the weld surface is scraped, ground or ultrasonically treated in situ before welding to mechanically remove the oxide film. This method is simple and direct, but it is easy to introduce pollution, has poor adaptability to complex surfaces, and the surface will quickly re-oxidize after treatment, so the effect is unstable; (2) Chemical / electrochemical activation method: Before welding, the oxide layer is removed by pickling, alkaline washing or water-based electrochemical polishing. Although this method can effectively clean the surface, strict anti-oxidation protection is still required after treatment, the process window is narrow, and the waste liquid treatment brings environmental pressure. More importantly, after treatment with traditional water-based electrolyte, the fresh aluminum surface will immediately react with water or air to form a new and thicker oxide layer, and may even lead to surface pitting or hydrogen embrittlement risks due to the presence of water; (3) Intermediate layer / activator method: A low-melting-point metal foil or an intermediate layer containing active elements is added between the surfaces to be soldered. The active elements preferentially react with Al2O3 at high temperatures, disrupting its continuity and promoting metal contact. This is currently the most mainstream method in industrial applications, but its disadvantages are that it introduces heterogeneous materials, which may change the joint composition, form brittle intermetallic compounds, and increase process complexity and cost; (4) Ultra-high pressure / high temperature method: Applying extremely high pressure and / or temperature to physically crush the oxide film. This method has stringent requirements for equipment, is prone to component deformation, and has irreversible adverse effects on the microstructure of certain heat-treated aluminum alloys.

[0007] A thorough analysis of the aforementioned solutions reveals a common, yet unresolved, fatal flaw: all these methods essentially attempt to passively break through, penetrate, or bypass the oxide layer after it has already formed and become an obstacle. Neither the instantaneous exposure after mechanical scraping nor the brief cleanliness after chemical cleaning can sustainably maintain a metal active surface state free of (or easily handled) harmful oxides within the heating cycle required for diffusion welding. In other words, existing technologies have failed to innovate at the source: preventing the formation of harmful oxides and proactively constructing interfaces conducive to bonding.

[0008] Therefore, developing a new, efficient, and reliable diffusion bonding technology that can effectively break through or penetrate this stubborn natural oxide barrier to achieve aluminum-aluminum intrinsic metal bonding is not only the key to improving the performance and yield of existing high-end aluminum alloy liquid cooling plates, but also the core to unlocking its full potential in the multi-billion dollar high-end heat dissipation and lightweight structure market. Summary of the Invention

[0009] The purpose of this invention is to overcome the technical defects of existing aluminum alloy diffusion welding caused by the obstruction of natural oxide films, and to provide a method for constructing a functionalized passivation layer on the surface of aluminum alloys based on anhydrous ECMP and its application in diffusion welding. This method, through a strategy of source intervention and proactive design, simultaneously obtains an ultra-precise surface and constructs a functionalized "artificial passivation layer" in situ to replace the harmful natural alumina film, thereby achieving high-quality and high-efficiency aluminum alloy diffusion welding.

[0010] To achieve the above objectives, the present invention adopts the following technical solution: a method for constructing a functionalized passivation layer on an aluminum alloy surface based on anhydrous ECMP, comprising the following steps: (1) Preparation of anhydrous electrolyte: using terpineol as solvent, add supporting electrolyte tetrabutylammonium hexafluorophosphate (concentration 0.1-0.5 mol / L), film-forming agent triethyl phosphate (volume fraction 5-10%), corrosion inhibitor benzotriazole (concentration 0.01-0.1 mol / L), nano silica abrasive (particle size 50-100 nm, mass fraction 5-10%) and viscosity modifier glycerol (volume fraction 10-20%). (2) The aluminum alloy workpiece is used as the anode and placed in the anhydrous electrolyte. An anode potential of 1.0-3.0 V vs. Ag / AgCl is applied, and electrochemical mechanical polishing is performed at a speed of 60-300 rpm under a mechanical pressure of 10-50 kPa. (3) During the electrochemical mechanical polishing process, a functionalized passivation layer with a thickness of 5-50 nanometers is grown in situ on the surface of the aluminum alloy through the synergistic effect of electrochemical action and mechanical action; the functionalized passivation layer is a phosphorus-doped organic-inorganic hybrid composite structure with a gradient composition distribution, wherein phosphorus exists in the passivation layer in the form of Al-OP bonds, etc.

[0011] A diffusion welding method for aluminum alloys, wherein the aluminum alloy workpiece pretreated by the above method is subjected to diffusion welding, the diffusion welding is carried out under vacuum or protective atmosphere, the welding temperature is 540-650°C, the welding pressure is 5-20 MPa, and the holding time is 10-60 minutes.

[0012] The beneficial effects of this invention are as follows: (1) Paradigm shift from “passive treatment” to “active prevention”: By using terpineol (a high-boiling-point, aprotic organic solvent) to completely replace the water-based electrolyte in traditional CMP, the main pathway for water molecules to participate in the formation of stable α-Al2O3 is fundamentally cut off from a thermodynamic and kinetic perspective, creating the possibility of obtaining a “clean” metal surface. (2) Synergistic engineering of surface morphology and interface chemistry: In the anhydrous ECMP process, a specific passivation layer with controllable composition and structure is actively induced on the aluminum surface through precise control of electrolyte additives and applied potential. This passivation layer has both "polishing advantages" (facilitating efficient and uniform removal to achieve nanoscale surface roughness Ra<1 nm) and "welding advantages" (it can be more effectively dissolved, diffused through, or undergo beneficial phase transformations during subsequent diffusion welding). (3) Significant performance improvement: The aluminum alloy diffusion welded joint pretreated by the method of the present invention can achieve a shear strength of more than 90% of the strength of the base material, while reducing the diffusion welding temperature by 20-40°C and shortening the process time by more than 30%, thus realizing a high-performance, low-energy-consumption and high-efficiency connection. (4) Environmental friendliness and engineering potential: Terpineol has low toxicity and moderate volatility, which is more in line with the development trend of green manufacturing and has the potential for future engineering applications. Attached Figure Description

[0013] Figure 1 This is a schematic diagram comparing the functionalized passivation layer constructed by the anhydrous ECMP method of the present invention with that of the traditional process.

[0014] Figure 2 This is a schematic diagram illustrating the expected evolution of the interface structure in this invention. Detailed Implementation

[0015] The present invention will be further described in detail below with reference to specific embodiments.

[0016] Example 1: This example uses 7075 aluminum alloy as an example, adjusting the ECMP process parameters and diffusion welding parameters.

[0017] (1) Preparation of anhydrous electrolyte: pine oleyl alcohol solvent, TBAPF6 concentration 0.4 mol / L, TEP volume fraction 6%, BTA concentration 0.08 mol / L, SiO2 abrasive (particle size 60 nm) mass fraction 6%, glycerol volume fraction 12%.

[0018] (2) Anhydrous ECMP treatment: 7075 aluminum alloy sample, anodic potential 2.5 V vs. Ag / AgCl, mechanical pressure 40 kPa, rotation speed 200 rpm, treatment time 12 minutes.

[0019] (3) Diffusion welding experiment: welding temperature 550°C, welding pressure 15 MPa, holding time 40 minutes, vacuum degree 5×10⁻ 4 Pa.

[0020] (4) Joint performance test: The average shear strength of the joint is 485 MPa, which is 91.5% of the strength of the 7075-T6 aluminum alloy base material (530 MPa). The width of the interface diffusion zone is about 4-6 μm, and the joint performance is excellent.

[0021] Example 2 (Comparative Experiment): This example uses traditional water-based electrochemical polishing pretreatment followed by diffusion welding as a comparison.

[0022] (1) Preparation of water-based electrolyte: Deionized water is used as solvent, phosphoric acid concentration is 10 wt%, sulfuric acid concentration is 5 wt%, and oxalic acid concentration is 2 wt%.

[0023] (2) Water-based electrochemical polishing treatment: 6061 aluminum alloy sample, anodic current density 0.5 A / cm², treatment time 5 minutes, temperature 40°C.

[0024] (3) Diffusion welding experiment: Two 7075 aluminum alloy samples that have undergone the above-mentioned anhydrous ECMP treatment were butt-jointed and placed in a vacuum diffusion welding furnace. Diffusion welding was carried out under a vacuum of 1×10⁻³ Pa, with a welding temperature of 560°C, a welding pressure of 10 MPa, and a holding time of 30 minutes. After welding, the samples were cooled to room temperature and removed.

[0025] (4) Joint performance test: The average shear strength of the joint is only 125 MPa, which is about 61% of the strength of the base material. Metallographic observation revealed obvious unbonded areas and oxide residues at the interface, and the diffusion zone width was less than 1 μm.

[0026] Comparative experiments show that the aluminum alloy diffusion weld joint pretreated by the anhydrous ECMP method of this invention has significantly better performance than the traditional water-based electrochemical polishing pretreatment method, verifying the superiority of this invention.

[0027] Taking into account both joint strength and microstructure, 560-570°C is the optimal diffusion welding temperature range, which is 30-40°C lower than the traditional process (which usually requires above 600°C).

[0028] As can be seen from the above embodiments, the method for constructing a functionalized passivation layer on the surface of aluminum alloys based on anhydrous ECMP provided by this invention can effectively form a phosphorus-doped composite passivation layer on the surface of aluminum alloys that has dual functions of "enhancing polishing" and "enhancing welding". This passivation layer undergoes controlled decomposition during subsequent diffusion welding, releasing active aluminum surface material. Furthermore, the phosphorus doping effect lowers the interfacial diffusion barrier, significantly promoting the interdiffusion of aluminum atoms, thereby achieving high-quality metallurgical bonding. This invention fundamentally solves the technical problem of natural oxide films hindering atomic diffusion in aluminum alloy diffusion welding, and has significant engineering application value and broad market prospects.

[0029] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for constructing a functionalized passivation layer on an aluminum alloy surface based on anhydrous ECMP, characterized in that, Includes the following steps: (1) Prepare an anhydrous electrolyte, wherein the anhydrous electrolyte uses terpineol as a solvent and contains a supporting electrolyte, a film-forming agent, a corrosion inhibitor, nano-abrasives and a viscosity modifier; (2) The aluminum alloy workpiece is used as the anode and placed in the anhydrous electrolyte. An external potential is applied to perform electrochemical mechanical polishing. (3) During the electrochemical mechanical polishing process, a functionalized passivation layer is grown in situ on the surface of the aluminum alloy through the synergistic effect of electrochemical action and mechanical action; the functionalized passivation layer is a phosphorus-doped organic-inorganic hybrid composite structure with a thickness of 5-50 nanometers and a gradient composition distribution.

2. The method for constructing a functionalized passivation layer on an aluminum alloy surface based on anhydrous ECMP according to claim 1, characterized in that, The supporting electrolyte is tetrabutylammonium hexafluorophosphate with a concentration of 0.1-0.5 mol / L.

3. The method for constructing a functionalized passivation layer on an aluminum alloy surface based on anhydrous ECMP according to claim 1, characterized in that, The film-forming agent is triethyl phosphate, with a volume fraction of 5-10%.

4. The method for constructing a functionalized passivation layer on an aluminum alloy surface based on anhydrous ECMP according to claim 1, characterized in that, The corrosion inhibitor is benzotriazole, with a concentration of 0.01-0.1 mol / L.

5. The method for constructing a functionalized passivation layer on an aluminum alloy surface based on anhydrous ECMP according to claim 1, characterized in that, The nano-abrasive is silica nanoparticles with a particle size of 50-100 nanometers and a mass fraction of 5-10%.

6. The method for constructing a functionalized passivation layer on an aluminum alloy surface based on anhydrous ECMP according to claim 1, characterized in that, The viscosity modifier is glycerol, with a volume fraction of 10-20%.

7. The method for constructing a functionalized passivation layer on an aluminum alloy surface based on anhydrous ECMP according to claim 1, characterized in that, The applied potential is the anode potential, ranging from 1.0 to 3.0 V vs. Ag / AgCl.

8. The method for constructing a functionalized passivation layer on an aluminum alloy surface based on anhydrous ECMP according to claim 1, characterized in that, The mechanical pressure of the electrochemical mechanical polishing treatment is 10-50 kPa, and the polishing pad rotation speed is 60-300 rpm.

9. A diffusion welding method for aluminum alloys, characterized in that, Aluminum alloy workpieces pretreated by the method described in any one of claims 1-8 are subjected to diffusion welding.

10. The aluminum alloy diffusion welding method according to claim 9, characterized in that, The diffusion welding is performed under vacuum or a protective atmosphere, with a welding temperature of 540-650°C, a welding pressure of 5-20 MPa, and a holding time of 10-60 minutes.

11. The aluminum alloy diffusion welding method according to claim 9, characterized in that, After pretreatment by the method and diffusion welding, the shear strength of the joint reaches more than 90% of the strength of the base material.

12. The application of a functionalized passivation layer constructed by the method described in claim 1 in diffusion welding of aluminum alloys, characterized in that, The functionalized passivation layer undergoes controlled decomposition during diffusion welding heating, releasing active aluminum surface and promoting inter-atomic interdiffusion at the interface.

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