Method for brazing connection of aluminum tube to stainless steel base plate

Abstract

This invention relates to the field of dissimilar metal brazing technology, and discloses a brazing connection method for aluminum tubes and stainless steel substrates, solving the problem of difficulty in obtaining high-strength and high-airtightness connections between the two without flux. A thick-film resistance heating layer is provided on the first surface of the stainless steel substrate, and the aluminum tube is connected to the second surface. The method includes: (1) pre-positioning of brazing filler metal: Al-12Si brazing sheet is dehumidified at 60-80℃, spot-welded and positioned, and pressure of 0.5-2.0 MPa is applied; (2) atmosphere establishment: ≥99.999% high-purity nitrogen is introduced and purified in three stages to establish a protective atmosphere with oxygen content <10 ppm and dew point ≤-60℃; (3) four-stage temperature-controlled brazing: preheating and dehumidification, rapid heating, holding at 580-620℃ for 4-6 min, and controlled-speed cooling; (4) inspection. The resulting joint has a shear strength ≥60 MPa, a leakage rate ≤1×10⁻⁸Pa·m³ / s, and an Al-Fe-Si transition layer of 5-15μm that is controllable.

Description

Technical Field

[0001] This invention relates to the field of dissimilar metal brazing technology, specifically to a method for achieving multi-segment temperature-controlled fluxless brazing connection between aluminum tubes and stainless steel substrates in a mesh belt atmosphere protection furnace, used to create a high-strength, high-airtightness metallurgical bond between the second surface of a stainless steel substrate and an aluminum tube in a liquid heating device. Background Technology

[0002] In liquid heating devices, a thick-film resistance heating layer is provided on the first surface of a stainless steel substrate, and the second surface needs to be reliably welded to an aluminum tube to allow the liquid to pass through and be heated from inside the aluminum tube. The brazing connection between the aluminum tube and the stainless steel substrate faces the following main technical challenges: (1) Problem of aluminum surface oxide film: Aluminum rapidly forms a dense Al2O3 oxide film (thickness of about 3-10nm) in the air. This oxide film does not melt at the brazing temperature, which seriously hinders the wetting and spreading of the brazing filler metal, and is the biggest technical difficulty in aluminum brazing.

[0003] (2) Intermetallic compound control problem: Aluminum and Fe in stainless steel undergo an interfacial reaction at high temperature to form Al-Fe intermetallic compounds. If the growth is too thick (>20 μm), it will lead to joint embrittlement; if it is too thin (<3 μm), the bonding strength will be insufficient.

[0004] (3) Difference in thermal properties: The difference in the coefficients of thermal expansion between aluminum and stainless steel (Δα≈6×10⁻) 6 The difference in thermal conductivity (approximately 15 times) between the K and the interface leads to significant thermal stress at the interface during brazing and cooling.

[0005] (4) Environmental problems of traditional fluxes: Commonly used fluoride fluxes such as KAlF4 and CsF have highly corrosive residues, which require cleaning after soldering, affecting product life and generating fluoride-containing wastewater.

[0006] Existing aluminum-stainless steel brazing processes fail to simultaneously address the four issues mentioned above, resulting in defects such as low joint strength, poor airtightness, and insufficient environmental friendliness. Compared to vacuum furnaces, mesh belt atmosphere-protected furnaces offer advantages such as continuous production, high capacity, and low equipment cost, making them the preferred equipment for industrial mass production. Summary of the Invention

[0007] In view of the above-mentioned defects in the existing technology, the technical problem to be solved by the present invention is to provide a brazing connection method for aluminum tubes and stainless steel substrates, which achieves high-strength and high-airtightness metallurgical connection between aluminum tubes and stainless steel substrates in the absence of flux, while meeting the engineering requirements of continuous production and environmental protection.

[0008] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A method for brazing an aluminum tube (2) to a stainless steel substrate (1), wherein a thick film resistance heating layer (4) is provided on the first surface of the stainless steel substrate (1), is carried out according to the following steps: Step 1, solder pre-positioning: Al-12Si eutectic alloy solder sheet is pre-positioned between the flattened section of aluminum tube (2) facing the second surface of stainless steel substrate (1) and the second surface. The moisture content of the solder sheet is pre-treated to ≤50 ppm. The width of the solder sheet is the width W of the contact surface of aluminum tube (2) minus 0.5 mm. The solder sheet is fixed to both ends of the contact surface of aluminum tube (2) by spot welding. An assembly pressure of 0.5 to 2.0 MPa is applied to the joint surface. Step 2, Atmosphere Establishment: Place the above components in a mesh belt atmosphere protection furnace, continuously introduce high-purity nitrogen gas with a purity ≥99.999% into the furnace chamber, and simultaneously maintain the gas overflowing outwards from the furnace chamber to prevent outside air from entering; establish and maintain a protective atmosphere with an oxygen content <10 ppm and a dew point ≤-60℃ in the brazing area; monitor in real time using an online oxygen analyzer and dew point meter, and the system will automatically alarm and stop the mesh belt operation when the oxygen content >10 ppm or the dew point >-55℃; Step 3, Multi-stage heating brazing: The mesh belt atmosphere protection furnace heats the components according to the following multi-stage temperature control curves: (a) Preheating and dehumidification stage: heating from room temperature to 300-400℃ at a heating rate of 60-100℃ / min, and holding for 4-6 minutes; (b) Rapid heating stage: heating from 300-400℃ to 580-620℃ at a heating rate of 30-50℃ / min; (c) Brazing holding stage: holding at 580-620℃ for 4-6 minutes; (d) Controlled cooling stage: cooling from the brazing temperature to below 200℃ at a cooling rate of 20-40℃ / min, followed by natural cooling; Step 4: Inspection: Perform airtightness testing and visual inspection on the brazed joints. For qualified products, the leakage rate should be ≤1×10⁻ after holding at 0.8 MPa nitrogen for 5 minutes. 8 Pa·m³ / s.

[0009] Based on the above technical solution, the present invention can be further improved as follows: Further, in step one, the method for treating the moisture content of the solder sheet is as follows: the Al-12Si solder sheet is placed in an oven at 60-80℃ for 2-4 hours to remove moisture, reducing the moisture content to <50 ppm; the spot welding fixing parameters are: one spot weld is applied at a distance of 5-10 mm from the end of the solder sheet, the spot welding temperature is 620-650℃, the time is 0.5-1 second, and the spot weld diameter is approximately 2 mm. These parameters ensure reliable fixing of the solder sheet and do not affect the uniformity of subsequent overall brazing.

[0010] Furthermore, in step two, the atmosphere purification system includes a three-stage series purification process consisting of an activated carbon filter, a molecular sieve drying tower, and a deoxygenation catalyst: the activated carbon filter removes oil mist and organic matter, the molecular sieve drying tower lowers the dew point to ≤-65℃, and the deoxygenation catalyst lowers the oxygen content to <5 ppm; the mesh belt atmosphere protection furnace is equipped with gas curtains separating the temperature zones, and the brazing zone adopts a fully enclosed structure and is equipped with an airtight door; the total flow rate of the high-purity nitrogen is 50-80 L / min, and the flow rate is independently controlled for each temperature zone.

[0011] Furthermore, in step three, the temperature difference between the aluminum tube (2) and the stainless steel substrate (1) is controlled to be ≤50℃ during the rapid heating stage; the heat preservation time during the brazing heat preservation stage ensures that the thickness δ of the Al-Fe-Si intermetallic compound transition layer satisfies the relationship δ≈k·√t, where k is the reaction rate constant at 600℃ (approximately 3.6 μm / √min), and δ is controlled within the range of 5 to 15 μm, taking into account both joint strength and toughness.

[0012] Furthermore, in step three, the brazing is carried out under flux-free conditions: the protective atmosphere with an oxygen content of <10 ppm inhibits the formation of a new oxide film on the aluminum surface at high temperatures, and the Si in the brazing filler metal has a reducing effect on the residual oxide film after melting, thereby achieving flux-free clean brazing, eliminating the need to use fluoride fluxes such as KAlF4 or CsF, and eliminating fluoride-containing wastewater pollution.

[0013] Furthermore, the aluminum tube (2) is made of one of 1060 pure aluminum, 3003 aluminum alloy, or 5052 aluminum alloy; the stainless steel substrate (1) is made of 304 stainless steel or 316 stainless steel; the Al-12Si weld sheet has a thickness of 0.15–0.30 mm and a Si content of 11.5–12.5 wt%. All of the above material combinations have been experimentally verified, and the shear strength of the brazed joints is ≥55 MPa.

[0014] Furthermore, the process parameters are provided with the following tolerance range: when the oxygen content of the protective atmosphere increases from ≤5 ppm to 10 ppm, the shear strength of the brazed joint decreases by no more than 12%; when the dew point increases from ≤-65℃ to -60℃, the porosity of the weld increases by no more than 2 percentage points; within this tolerance range, the yield rate of batch production is ≥95%.

[0015] (1) Achieve fluxless clean brazing, fundamentally solving the problems of residual corrosion and fluoride-containing wastewater discharge caused by traditional fluxes (fluorides such as KAlF4 and CsF), complying with RoHS and environmental regulations, and extending the product's service life.

[0016] (2) The use of mesh belt atmosphere protection furnace to achieve continuous production reduces equipment investment by more than 60% and energy consumption per unit capacity by about 40% compared with vacuum brazing furnace. It can achieve industrial mass production of hundreds of pieces per hour, significantly reducing manufacturing costs.

[0017] (3) The oxygen content in the furnace is stably controlled at <10 ppm and the dew point is ≤-60℃ by a three-stage series atmosphere purification system. Compared with the traditional industrial nitrogen protection (oxygen content 50-200 ppm), the oxygen content is reduced by 10-40 times, which fundamentally inhibits the re-oxidation of aluminum surface, thereby increasing the shear strength of brazed joint from the existing 25-40 MPa to ≥60 MPa, an increase of 50%-140%.

[0018] (4) The four-segment temperature control curves are designed to address the four major technical challenges of aluminum-stainless steel dissimilar metal brazing (aluminum surface oxidation, intermetallic compound growth, thermal stress, and porosity), enabling precise control of the Al-Fe-Si intermetallic compound transition layer thickness within the optimal range of 5–15 μm, and ensuring a joint airtightness leakage rate ≤1×10⁻ 8 Pa·m³ / s meets the stringent requirements for liquid heating devices.

[0019] (5) The online oxygen analyzer and dew point meter interlocking control system realizes real-time closed-loop control of atmosphere quality. Process parameter deviations can be detected and corrected in a timely manner, increasing the yield rate of batch production from 60-80% of the existing process to ≥98%. Product consistency is significantly improved, making it suitable for applications such as home appliances and medical devices with high reliability requirements. Attached Figure Description

[0020] Figure 1 This is a process flow diagram of the brazing method of the present invention; Figure 2 Schematic diagram of mesh belt atmosphere protection furnace and atmosphere control system; Figure 3 This is a brazing temperature control curve (temperature and time distribution at each stage). Figure 4 A schematic diagram of the physical processes at each stage of the brazing process; Figure 5 The curve showing the effect of oxygen content on the shear strength of brazed joints; Figure 6 The curve showing the relationship between heat preservation time and the thickness of the Al-Fe-Si intermetallic compound layer (δ≈k·√t verification). Figure 7 Figure showing the effect of cooling rate on weld cracking rate; Figure 8 A schematic diagram of the pre-positioning of the brazing filler metal for spot welding.

[0021] Explanation of reference numerals in the attached drawings: 1—Stainless steel substrate; 2—Aluminum tube; 3—Bracket joint; 4—Thick film resistance heating layer; 17—Al-12Si aluminum welding sheet; 171—Spot welding fixing point; 22—Mesh belt atmosphere protection furnace; 23—Activated carbon filter; 24—Molecular sieve drying tower; 25—Deoxygenation catalyst; 26—High-purity nitrogen source. Detailed Implementation

[0022] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings.

[0023] This invention employs a four-step brazing filler metal pre-positioning method: "moisture removal—cutting—spot welding positioning—pressing," to ensure that the brazing filler metal does not shift or leak during the brazing process. The specific steps are as follows: (1) Dehumidification treatment: Place the Al-12Si eutectic alloy weldment (thickness 0.15-0.30 mm, preferably 0.17 mm) in an oven at 60-80℃ for 2-4 hours to dehumidify it, reducing the moisture content to <50 ppm. Technical note: When the moisture content of the weldment is >100 ppm, the water vapor generated during the preheating stage will cause the weld porosity to be >5%, affecting the airtightness.

[0024] (2) Cutting and shaping: Cut the solder sheet into strips with a width of (W-0.5) mm (W is the width of the contact surface between the aluminum tube and the stainless steel substrate), and the length is consistent with the contact length. The width of the solder sheet is slightly smaller by 0.5 mm because the solder expands laterally by about 0.2 to 0.3 mm after melting. If it is too wide, the molten solder will overflow the contact area, which will waste material and affect the appearance.

[0025] (3) Spot welding positioning: Apply a small amount of Al-Si brazing filler metal spot welding to both ends of the flattened contact surface of the aluminum tube (5-10 mm from the end) (temperature 620-650℃, time 0.5-1 second, spot welding diameter ≈2 mm) to firmly fix the weld piece (see Figure 8 If the spot welding temperature is below 620℃, the bonding strength will be insufficient; if it is above 650℃, it will cause local overmelting of Al-Si, affecting the uniformity of subsequent brazing.

[0026] (4) Assembly and pressing: Apply a uniform pressure of 0.5 to 2.0 MPa (preferably 1.0 MPa) to the contact surface using a spring clamp or pneumatic pressing device. Pressure <0.5 MPa will result in poor contact and insufficient effective brazing area; pressure >2.0 MPa will result in excessive deformation of the weld piece and uneven thickness.

[0027] This invention employs a mesh belt atmosphere protection furnace ( Figure 2 The structure of the furnace body and atmosphere system is as follows: (1) Furnace structure: Modular design, from the inlet to the outlet, it is divided into feeding area, preheating area, heating area, brazing area, cooling area and discharge area. Each area is separated by an air curtain; the brazing area adopts a fully enclosed structure and is equipped with an airtight door; nitrogen-filled curtains are installed at both ends of the furnace body to prevent outside air from entering.

[0028] (2) Three-stage series atmosphere purification system: High-purity nitrogen (purity ≥99.999%) is sent into the furnace after passing through ① activated carbon filter (removing oil mist and organic matter) → ② molecular sieve drying tower (dew point reduced to ≤-65℃) → ③ deoxygenation catalyst (oxygen content reduced to <5ppm) in a three-stage series purification process.

[0029] (3) Establishment of furnace atmosphere: Before starting the furnace, high-purity nitrogen is continuously introduced at a total flow rate of 50-80 L / min, and each temperature zone is controlled independently. After about 20-30 minutes of introduction, the air in the furnace is fully replaced. When the online oxygen analyzer shows an oxygen content of <10 ppm, the temperature is increased and production begins.

[0030] (4) Online monitoring and interlocking control: Install an online oxygen analyzer with an accuracy of ±1 ppm and a dew point meter with an accuracy of ±2℃ in the brazing area for real-time monitoring; when the oxygen content is >10 ppm or the dew point is >-55℃, the system will automatically alarm and stop the conveyor belt operation, and can only continue after the atmosphere is restored to the qualified level.

[0031] Table 1. Comparison of the high-purity N2 protection process of this invention with the traditional industrial N2 protection process.

[0032] III. Multi-segment temperature control curves and their physical mechanisms (corresponding to step three of claim 1) The four-stage temperature control curve of this invention has been optimized through extensive experiments, and each stage has a clear physical meaning (see [link]). Figure 3 , Figure 4 ): (a) Preheating and dehumidification stage (room temperature → 300~400℃, heating rate 60~100℃ / min, hold for 4~6 min): Evaporate adsorbed water and residual solvent (boiling point <200℃) on the surface of the solder sheet and aluminum tube, eliminating the source of porosity during subsequent brazing. If the temperature rises too quickly (>150℃ / min), the adsorbed water will not have enough time to escape, forming water vapor pressure and causing the weld porosity to be >5%.

[0033] (b) Rapid heating stage (300~400℃→580~620℃, heating rate 30~50℃ / min): quickly reach the brazing temperature, while controlling the temperature difference between the aluminum tube and the stainless steel substrate to ≤50℃ to prevent thermal stress from causing the pre-assembled parts to shift.

[0034] (c) Brazing and heat preservation stage (580~620℃, 4~6 min): Al-12Si brazing filler metal (eutectic temperature 577℃) completely melts and wets the stainless steel surface (wetting angle <20°). Liquid aluminum reacts with Fe on the stainless steel surface to form Al. 13 Fe4 and Al3Fe intermetallic compounds. The relationship between the thickness δ of the intermetallic compound layer and the holding time t is: δ≈k·√t, where k is approximately 3.6 μm / √min at 600℃ (see...). Figure 6 The temperature was maintained for 4–6 min, corresponding to δ = 7.2–8.8 μm, which is within the optimal range (5–15 μm).

[0035] (d) Controlled-speed cooling stage (600℃→200℃, cooling rate 20~40℃ / min): Slowly release thermal stress (difference in thermal expansion coefficient between aluminum and stainless steel Δα≈6×10⁻ 6 / K), to prevent cracking of the brittle IMC layer (see Figure 7 ).

[0036] IV. Examples

[0037] Raw materials: 3003 aluminum alloy tube (outer diameter 10 mm, wall thickness 1.0 mm), 304 stainless steel substrate (thickness 1.0 mm, first surface has been printed with sintered thick film heating layer), Al-12Si solder sheet (thickness 0.17 mm, Si content 12.0 wt%).

[0038] Solder preparation: The solder sheet is dehumidified at 75℃ for 3 hours (moisture content test <35 ppm) → cut into strips with a width of (W-0.5) mm → spot welded at 8 mm from each end (635℃, 0.7 s) → assembled onto the second surface of the stainless steel substrate and a pressing force of 1.0 MPa is applied.

[0039] Atmosphere setup: Load the components into the mesh belt atmosphere protection furnace and introduce high-purity nitrogen (total flow rate 60 L / min). After about 25 minutes, the oxygen content drops to 4.2 ppm and the dew point is -68℃, which meets the requirements. Start the mesh belt heating process.

[0040] Temperature control curve execution (e.g.) Figure 3 : Preheating and dehumidification (increase to 350℃ at 80℃ / min and hold for 5 min) → rapid heating (increase to 600℃ at 40℃ / min) → brazing and heat preservation (600℃, 5 min) → controlled cooling (30℃ / min to 200℃, and then naturally cool to room temperature).

[0041] Test results: Shear strength 64 MPa, airtightness leakage rate 2.8×10⁻ 9Pa·m³ / s, strength retention rate of 97% after 1500 cycles of hot and cold (-40℃~120℃), and mass production yield of 99.1%.

[0042] Table 2. Influence of key process parameters on the quality of brazed joints (comparative experiment)

[0043] Example 2: Verification of different aluminum tube materials Using the same process parameters as in Example 1, brazing tests were conducted on 1060 pure aluminum tubes and 5052 aluminum alloy tubes, and the results are as follows: (1) 1060 pure aluminum tube: shear strength 58 MPa, leakage rate 3.5×10⁻ 9 Pa·m³ / s, yield rate 98.5%; (2) 5052 aluminum alloy pipe: shear strength 67 MPa, leakage rate 1.9×10⁻ 9 Pa·m³ / s, yield rate 98.8%; The results show that the process of the present invention is well applicable to the above-mentioned aluminum alloy materials, and the shear strength of the brazed joints of the three aluminum tube materials is ≥55 MPa, which meets the requirements for use of liquid heating devices.

[0044] V. Explanation of the correspondence with the claims (1) Claim 1 describes the complete method steps of the present invention, including four steps: solder preparation, atmosphere establishment, multi-stage heating brazing and inspection, and is an independent claim; (2) Claims 2 to 7 are dependent claims, which further define the parameters for dehumidification and spot welding of the brazing filler metal, the structure of the atmosphere purification system, the relationship between the heating rate and the IMC layer thickness, the fluxless mechanism, the material specifications, and the tolerance range of the process parameters, respectively. (3) Claim 8 is an independent atmospheric control method claim, which protects the interlocking control method for atmosphere establishment, maintenance and monitoring of mesh belt atmosphere protection furnace; Without departing from the spirit of this invention, those skilled in the art can make various modifications and combinations to the above embodiments, all of which fall within the protection scope of this invention.

Claims

1. A method for brazing an aluminum tube (2) to a stainless steel substrate (1), characterized in that, Follow these steps: Step 1, Solder pre-positioning: Place aluminum-silicon solder between the aluminum tube (2) facing the second surface of the stainless steel substrate (1) and the second surface. The moisture content of the solder is pre-treated to ≤50 ppm. Fix the solder to the contact surface of the aluminum tube (2) and apply assembly pressure to the joint surface. Step 2, Atmosphere Establishment: Place the above components in an atmosphere protection furnace, continuously introduce inert protective gas into the furnace chamber, and maintain positive pressure inside the furnace chamber to prevent outside air from entering; establish and maintain a protective atmosphere with an oxygen content ≤10 ppm and a dew point ≤-55℃ in the brazing area. Step 3, Multi-segment Heating Brazing: The atmosphere-protected furnace heats the components according to the following multi-segment temperature control curves: (a) Preheating and dehumidification stage: Increase the temperature from room temperature to 300-400°C at a rate of 60-100°C / min and maintain it for 4-6 minutes; (b) Rapid heating stage: heating from 300-400℃ to 580-620℃ at a heating rate of 30-50℃ / min; (c) Brazing and heat preservation stage: Maintain a constant temperature of 580-620℃ for 4-6 minutes; (d) Controlled cooling stage: Cool from the brazing temperature to below 200°C at a cooling rate of 20-40°C / min, and then allow to cool naturally; Step 4: Inspection: Perform airtightness testing and visual inspection on the brazed joint.

2. The method according to claim 1, characterized in that: In step one, the method for treating the moisture content of the solder sheet is as follows: the Al-12Si solder sheet is placed in an oven at 60-80℃ for 2-4 hours to remove moisture and reduce the moisture content to <50 ppm; the spot welding fixed parameters are as follows: a spot weld is applied at a distance of 5-10 mm from the end of the solder sheet, the spot welding temperature is 620-650℃, the time is 0.5-1 second, and the spot welding diameter is about 2 mm.

3. The method according to claim 1, characterized in that: In step two, the atmosphere purification system includes a three-stage series purification system consisting of an activated carbon filter, a molecular sieve drying tower, and a deoxygenation catalyst. The activated carbon filter removes oil mist and organic matter, the molecular sieve drying tower lowers the dew point to ≤-65℃, and the deoxygenation catalyst lowers the oxygen content to <5 ppm. The mesh belt atmosphere protection furnace is equipped with gas curtains separating the temperature zones, and the brazing zone adopts a fully enclosed structure with airtight doors. The total flow rate of high-purity nitrogen is 50-80 L / min, and the flow rate is independently controlled for each temperature zone.

4. The method according to claim 1, characterized in that: In step three, the temperature difference between the aluminum tube (2) and the stainless steel substrate (1) is controlled to be ≤50℃ during the rapid heating stage; the heat preservation time during the brazing heat preservation stage is such that the thickness δ of the Al-Fe-Si intermetallic compound transition layer satisfies the relationship δ≈k·√t, where k is the reaction rate constant at 600℃, about 3.6 μm / √min, and δ is controlled within the range of 5~15 μm.

5. The method according to claim 1, characterized in that: In step three, the brazing is performed under flux-free conditions; the protective atmosphere with an oxygen content of <10 ppm inhibits the formation of a new oxide film on the aluminum surface at high temperatures, and the Si in the brazing filler metal has a reducing effect on the residual oxide film after melting, thus achieving flux-free clean brazing without the need for fluoride fluxes such as KAlF4 or CsF.

6. The method according to claim 1, characterized in that: The aluminum tube (2) is made of one of 1060 pure aluminum, 3003 aluminum alloy or 5052 aluminum alloy; the stainless steel substrate (1) is made of 304 stainless steel or 316 stainless steel; the Al-12Si welding sheet has a thickness of 0.15 to 0.30 mm and a Si content of 11.5 to 12.5 wt%.

7. The method according to claim 1, characterized in that: When the oxygen content of the protective atmosphere increases from ≤5 ppm to 10 ppm, the shear strength of the resulting brazed joint decreases by no more than 12%; when the dew point increases from ≤-65℃ to -60℃, the porosity of the weld increases by no more than 2 percentage points; the tolerance control of the process parameters ensures that the yield rate of batch production is ≥95%.

8. An atmosphere control method for a mesh belt atmosphere protection furnace used for brazing aluminum tubes to stainless steel substrates, characterized in that, Includes the following steps: (1) Before starting the furnace, continuously introduce high-purity nitrogen gas with a purity of ≥99.999% into the furnace until the air inside the furnace is fully replaced and the online oxygen analyzer shows an oxygen content of <10 ppm; (2) During the brazing process, high-purity nitrogen is continuously introduced at a total flow rate of 50-80 L / min, and each temperature zone is independently controlled; (3) The furnace is kept under a slightly positive pressure relative to atmospheric pressure, and the nitrogen curtains at both ends of the furnace prevent outside air from entering. (4) Install an online oxygen analyzer with an accuracy of ±1 ppm and a dew point meter with an accuracy of ±2℃ in the brazing area to continuously monitor the atmosphere quality; when the oxygen content is >10 ppm or the dew point is >-55℃, the system will automatically alarm and stop the mesh belt operation. It can only continue after the atmosphere is restored to the qualified range.

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