Liquid heating device

Abstract

The application discloses a liquid heating device, comprising a stainless steel base plate (1), a first surface of the stainless steel base plate (1) is provided with a thick film resistance heating layer (4), and a second surface of the stainless steel base plate (1) is fixedly connected with an aluminum pipe (2). At least one section of the aluminum pipe (2) is flattened into a flat section, and the flat section is metallurgically combined with the second surface of the stainless steel base plate (1) through an Al-Si brazing layer (3). The microstructure of the brazing layer (3) sequentially comprises an Al-Si eutectic layer (thickness 80-200 microns, lambda 2=15-35 microns) and an Al-Fe-Si intermetallic compound transition layer (thickness 5-15 microns, main phase Al 13 Fe4+secondary phase Al3Fe, and Fe content is continuously gradient distributed from 2wt% on the aluminum pipe side to 80wt% on the stainless steel side). The application forms a metallurgical combined layer with high strength (shear strength >=60MPa), high airtightness (leakage rate <=1*10 ‑8 Pa·m 3 / s) and excellent thermal stability (>=1000 times of 10-95 DEG C cold-hot cycle without cracking) between the aluminum pipe and the stainless steel base plate in the liquid heating device by controlling the double-layer microstructure of the brazing layer, solves the technical problems of low connection strength and poor reliability of dissimilar metals, and can be mass produced (good product rate >=98%).

Description

Technical Field

[0001] This invention relates to the field of liquid heating equipment technology, specifically to a liquid heating device that uses aluminum tubes and stainless steel substrates to be brazed together, which can be widely used in household water heaters, water dispensers, industrial fluid heating and other applications. Background Technology

[0002] Liquid heating devices typically require a reliable connection between the liquid-carrying pipe and the heating substrate to achieve efficient heat transfer. A typical structure currently is as follows: a thick-film resistance heating layer is provided on the first surface (back liquid surface) of the stainless steel substrate as an electric heating element, and an aluminum tube is fixed on the second surface (towards the liquid surface) as a liquid flow channel; during heating, electrical energy is converted into heat energy by the thick-film layer, which is conducted through the stainless steel substrate to the liquid flowing inside the aluminum tube, achieving efficient heating.

[0003] However, the dissimilar metal connection between aluminum tubes and stainless steel substrates has always been an engineering challenge, and existing solutions have the following drawbacks: (1) Mechanical connection (bolts, clips): The aluminum tube and the stainless steel substrate only transfer heat through contact, resulting in high thermal resistance and low heat transfer efficiency; and under long-term alternating hot and cold loads, due to the aluminum (thermal expansion coefficient 23.6×10 -6 / K) and stainless steel (coefficient of thermal expansion 17.2×10) -6 The difference in expansion ( / K) causes the contact interface to gradually loosen, which affects heat transfer and also poses a risk of liquid leakage.

[0004] (2) Organic thermally conductive adhesive bonding: After curing, the thermally conductive adhesive forms an organic polymer layer with a shear strength usually lower than 20 MPa and a temperature resistance limit of about 150°C. Under long-term high temperature and cold and heat cycles, it ages and delaminates, and the thermal conductivity drops significantly from the initial about 3 W / (m·K). Furthermore, microcracks are generated after aging, leading to liquid leakage and poor reliability.

[0005] (3) Laser welding: The thermal properties of aluminum and stainless steel are vastly different—aluminum has a thermal conductivity of about 237 W / (m·K), while stainless steel has only about 16 W / (m·K); the coefficient of thermal expansion of aluminum is about 6 × 10⁻⁶ higher than that of stainless steel. -6 / K; The melting point of aluminum (660℃) is much lower than that of stainless steel (approximately 1400℃). These differences make it very easy for a brittle Fe-Al intermetallic compound layer with uncontrolled thickness to form at the interface during laser welding (the shear strength drops sharply when the thickness is >30μm), resulting in large variations in joint strength. The industrial yield is usually only 30%~50%, making it unsuitable for mass production.

[0006] (4) Insufficient control of existing brazing process: The aluminum surface oxide film (Al2O3, melting point 2050℃) cannot dissolve on its own at the brazing temperature. If the oxygen content in the protective atmosphere is too high (>50ppm), the re-oxidation rate of the aluminum tube surface exceeds the brazing filler metal spreading rate, resulting in poor wetting, brazing filler metal accumulation or false soldering. If the brazing temperature is too high or the holding time is too long, the Fe-Al interface reaction is excessive, the thickness of the intermetallic compound layer exceeds 20μm and forms a continuous brittle layer rich in Fe ends, and the fatigue resistance of the joint is significantly deteriorated.

[0007] In summary, the common drawback of existing technologies is that it is difficult to simultaneously achieve high-strength metallurgical bonding, excellent airtightness, and long-term reliability in thermal cycling between aluminum tubes and stainless steel substrates, which limits the service life and safety of liquid heating devices. Summary of the Invention

[0008] The technical problem to be solved by this invention, addressing the deficiencies of the prior art, is: how to form an Al-Si-Fe metallurgical bonding layer with controlled microstructure at the interface between aluminum tubes and stainless steel substrates, such that the bonding layer simultaneously satisfies: ① shear strength >= 60 MPa (metallurgical bonding strength, superior to existing adhesive and mechanical bonding methods); ② leakage rate <= 1 × 10⁻⁶ MPa. -8 Pa·m 3 / s (0.8MPa nitrogen, pressure held for 5min); ③ It can withstand >=1000 cycles of hot and cold air from 10℃ to 95℃ without fatigue cracks, fundamentally solving the technical problems of low strength, poor airtightness and insufficient thermal stability of dissimilar metal connection between aluminum tube and stainless steel substrate.

[0009] Technical Solution: To solve the above-mentioned technical problems, the present invention adopts the following technical solution: 1. Overall scheme of liquid heating device A liquid heating device includes a stainless steel substrate (1); a thick film resistance heating layer (4) is provided on the first surface (back liquid surface) of the stainless steel substrate (1), and an aluminum tube (2) is fixedly connected to the second surface (towards the liquid surface) of the stainless steel substrate (1), forming a double-sided functional arrangement structure.

[0010] At least one section of the aluminum tube (2) is flattened into a flat cross section, and the flat cross section is directed toward the second surface of the stainless steel substrate (1) to form a metallurgical bond with the second surface of the stainless steel substrate (1) by Al-Si brazing filler metal under controlled atmosphere protection conditions, forming an Al-Si brazing layer (3).

[0011] The brazing layer (3) has a controlled bilayer microstructure, which consists of the following layers from the aluminum tube (2) side to the stainless steel substrate (1) side: (a) Al-Si eutectic layer: The thickness is 80-200 μm, the spacing λ2 of the secondary dendrite arms of the primary α-Al dendrites is 15-35 μm, and the spacing between the Al-Si eutectic layers is 1.5-3.5 μm. This layer is the solidification zone of the solder body and provides the main toughness reserve of the bonding layer.

[0012] (b) Al-Fe-Si intermetallic compound transition layer: thickness 5-15 μm, main phase Al 13 Fe4 (monoclinic system), with Al3Fe as the secondary phase; the Fe content increases continuously from 2wt% on the aluminum tube (2) side to 80wt% on the stainless steel substrate (1) side. The thickness and composition gradient of this layer are precisely controlled by the brazing temperature and holding time. While providing metallurgical bonding at the aluminum-steel interface, it avoids the formation of a continuous brittle layer with a thickness >20μm, which is a key microstructure element for achieving high strength and high thermal stability.

[0013] The overall performance indicators of the brazed layer (3) are: shear strength >= 60 MPa; airtightness meets the requirement of leakage rate <= 1×10 when nitrogen pressure is maintained at 0.8 MPa for 5 minutes. -8 Pa·m 3 / s; capable of withstanding >=1000 cycles of hot and cold air from 10℃ to 95℃ without fatigue cracking.

[0014] 2. Material and Dimensional Parameter Scheme The aluminum tube (2) is preferably made of 1060 pure aluminum, 3003 aluminum alloy, or 5052 aluminum alloy, with an outer diameter of 6-16 mm and a wall thickness of 0.5-4.0 mm. The limitation on the outer diameter and wall thickness of the aluminum tube ensures, on the one hand, that the tube wall does not crack during cold pressing and flattening (wall thickness >= 0.5 mm), and on the other hand, that the contact area after flattening meets the heat transfer requirements (outer diameter >= 6 mm). The stainless steel substrate (1) has a thickness of 0.3-5.0 mm; the lower limit of the thickness ensures the structural rigidity of the substrate, while the upper limit takes into account the control of thermal resistance.

[0015] 3. Scheme for geometric parameters of the flattened section The height h of the flattened section of the aluminum tube (2) is 0.3 to 0.8 times the outer diameter D of the aluminum tube (2); the contact width W between the flattened section and the second surface of the stainless steel substrate (1) is 0.3 to 1.1 times the outer diameter D; and the contact length between the flattened section and the stainless steel substrate (1) along the axial direction of the aluminum tube (2) is not less than 3 times the outer diameter D. The above geometric parameters limit the effective brazing contact area between the aluminum tube and the stainless steel substrate: the contact width W and the contact length together determine the bearing cross-sectional area of ​​the joint, thereby ensuring that the overall shear bearing capacity and heat transfer area meet the design requirements. If h / D is too large (>0.8), the contact width will be insufficient and the joint strength will decrease; if h / D is too small (<0.3), the cross-sectional area of ​​the flow channel inside the aluminum tube will be too small and the flow resistance will increase significantly.

[0016] 4. Fine microstructure scheme of brazing layer The Al-Si eutectic layer has a microhardness (HV) of 40-60, reflecting its sufficient toughness to absorb thermal stress during thermal cycling; the Al-Fe-Si intermetallic compound transition layer has a microhardness (HV) of 250-400. 13 The volume ratio of Fe₄ to Al₃Fe is (3-5):1. Under this ratio, Al 13 The Fe4 main phase provides interfacial bonding strength, and the dispersed distribution of the Al3Fe secondary phase helps to suppress the coarsening of the main phase grains. The stainless steel substrate (1) has a Fe-poor layer at a depth of 3~5μm below the interface adjacent to the Al-Fe-Si intermetallic compound transition layer. The presence of this Fe-poor layer is microscopic evidence of the diffusion of Fe from the interface to the aluminum side to participate in the reaction, and at the same time indicates that the interfacial diffusion depth is effectively controlled and the stainless steel matrix is ​​not damaged in depth.

[0017] 5. Aluminum Tube Layout Scheme The aluminum tube (2) is arranged in a serpentine or spiral pattern on the second surface of the stainless steel substrate (1), with its two ends connected to the liquid inlet and the liquid outlet, respectively. The serpentine or spiral arrangement allows the liquid to form a meandering flow path on the substrate surface, significantly increasing the heat exchange length and residence time between the liquid and the heated substrate, thereby improving the heat exchange efficiency; at the same time, it ensures that the entire substrate area is uniformly heated and covered, avoiding local overheating.

[0018] 6. Fabrication scheme for thick film resistance heating layer The thick-film resistance heating layer (4) is formed on the first surface of the stainless steel substrate (1) through screen printing and sintering processes, with a surface resistivity of 0.05 to 5 Ω / □. The screen printing process can precisely control the printing pattern and thickness of the resistance paste. After sintering, a thick-film heating layer that is firmly bonded to the substrate is formed on the stainless steel surface. The limited range of surface resistivity ensures that the rated heating power is output under the normal power supply voltage (220V). Compared with traditional tubular electric heating elements, the thick-film heating layer is directly bonded to the stainless steel substrate, resulting in extremely low thermal resistance at the heat transfer interface and higher thermal efficiency.

[0019] 7. Thermal Insulation Layer Solution The liquid heating device further includes a heat insulation layer (5) disposed on the first surface side of the stainless steel substrate (1). The heat insulation layer (5) has a thermal conductivity of <=0.05W / (m·K) and is fixedly connected to the stainless steel substrate (1) by clips or bolts. The heat insulation layer allows the heat generated by the thick film heating layer to be preferentially conducted to the aluminum tube (second surface) side, suppressing heat loss to the external environment, which can significantly improve the overall thermal efficiency (reduce back heat loss >=30%). The clip or bolt fixing method facilitates disassembly and maintenance and does not affect the electrical connection of the heating layer.

[0020] 8. Sealing clamp solution The liquid heating device also includes a sealing clamp (6), which seals both ends of the aluminum tube (2) and forms an inlet and an outlet respectively. The sealing clamp (6) is connected to the end of the aluminum tube (2) by an O-ring seal or by welding. The sealing clamp converts the open end of the aluminum tube into a standard pipe interface. The O-ring seal is suitable for removable maintenance applications (pressure <= 1.0 MPa), while the welding connection is suitable for high-pressure or high-reliability applications (pressure >= 1.6 MPa). Both methods solve the problem of liquid sealing at both ends of the flattened aluminum tube.

[0021] 9. Grounding Protection Layer Scheme The first surface of the stainless steel substrate (1) is also provided with a grounding protection layer, which is located outside the thick film resistance heating layer (4) and is used for leakage current protection. The grounding protection layer is electrically connected to the stainless steel substrate (1) and connected to the external grounding terminal (PE). When the thick film heating layer leaks current due to aging or damage of the insulating dielectric layer (41), the leakage current is conducted to the ground through the grounding protection layer and the PE line, triggering the leakage current protection device to operate, preventing electric shock accidents, and meeting the grounding protection requirements of safety standards such as GB4706.1.

[0022] The present invention can also be a liquid heating system including the above-mentioned liquid heating device, further comprising at least one of a temperature controller (7), a temperature sensor (8), and an overheat protection element (9), which work together to achieve safe control of the liquid heating device. The temperature controller (7) is connected to the electrode terminals of the thick film resistance heating layer (4) and automatically switches the heating circuit on and off according to the set temperature; the temperature sensor (8) monitors the heating temperature in real time and feeds it back to the control circuit (13) to achieve closed-loop temperature regulation; the overheat protection element (9) (such as a thermal fuse or a PTC element) irreversibly cuts off or self-limits the heating circuit when the temperature abnormally exceeds the set threshold, providing final safety assurance. The combination of the above-mentioned control elements and the liquid heating device constitutes a liquid heating system with a complete safety control chain.

[0023] (III) Beneficial Effects Compared with the prior art, the present invention has the following beneficial technical effects: 1. It solves the contradiction between the metallurgical bonding strength and thermal stability of dissimilar metals. The present invention controls the thickness of the Al-Fe-Si intermetallic compound transition layer to 5-15 μm (claim 1), so that the main phase Al of this layer... 13 Fe4 (HV 250~400) provides sufficient interfacial bonding strength (shear strength >= 60MPa) while avoiding continuous brittle layers and crack-sensitive zones that occur when the thickness is >20μm. The continuous gradient distribution of Fe content from 2wt% on the aluminum tube side to 80wt% on the stainless steel side allows for a gradual transition in the interfacial thermal expansion coefficient (approximately 23×10⁻⁶ on the aluminum side). -6 / K gradually transitions to the steel side at approximately 17×10 -6 / K), fundamentally alleviating the thermal stress concentration generated at dissimilar metal interfaces during thermal cycling, achieving >=1000 cycles without cracking, surpassing existing thermal conductive adhesives (<500 cycles) and laser welding solutions (<1000 cycles, low yield). 2. Air tightness reaches instrument-grade level; a surface contact brazing metallurgical bond is formed between the flattened aluminum tube section and the stainless steel substrate (claims 1 and 3). The continuous and dense Al-Si eutectic layer (thickness 80-200μm) eliminates the inherent micropores and microcracks at the mechanical connection and adhesive interface, reducing the joint leakage rate to <=1×10 -8 Pa·m 3 / s (0.8MPa, 5min), superior to thermally conductive adhesive bonding (>10 -5 Pa·m 3The thick film resistance heating layer (4) is located on the first surface of the stainless steel substrate (1), and the aluminum tube (2) is located on the second surface (claim 1). The two functional areas do not interfere with each other. The heat transfer path is: thick film layer → stainless steel substrate → brazed metallurgical layer → aluminum tube wall (the thermal conductivity of aluminum is about 220W / (m·K), which is much better than stainless steel 16W / (m·K)) → liquid inside the tube. All interfaces are metallurgical contacts with no contact thermal resistance. Combined with the back insulation effect of the heat insulation layer (5) (claim 7), the effective utilization rate of heating heat is significantly improved. 4. The flattened aluminum tube structure increases the heat exchange area and reduces manufacturing difficulty. At least one section of the aluminum tube is flattened into a flat cross-section (claims 1 and 3), forming a surface contact rather than a line contact with the stainless steel substrate. The effective brazing contact area is proportional to the product of the contact width W (0.3 to 1.1 times the outer diameter D) and the contact length. Compared with direct connection of round tubes, the flattened structure increases the effective heat exchange area per unit tube length by 30% to 110%, while reducing the overall thickness of the device, which is beneficial for product miniaturization. The flattening process uses a cold pressing process, which does not require special molds and is suitable for aluminum tubes of various outer diameters. 5. It can be mass-produced. The brazing connection process of this invention (Al-12Si eutectic alloy brazing filler, mesh belt atmosphere-protected brazing furnace, high-purity nitrogen protection, oxygen content <10ppm, brazing temperature 580~620℃) has the characteristics of wide process parameter window and high degree of automation. The batch production yield is >=98%, which is significantly better than laser welding (about 30% to 50%). The serpentine or spiral aluminum tubes can be pre-bent and then brazed as a whole in the furnace. This allows for high single-furnace throughput and low process costs, making it suitable for the mass production needs of household water heaters, water dispensers, and other similar products. 6. Safety meets national mandatory standards. The grounding protection layer (claim 9) and overheat protection element (claim 10) ensure that the liquid heating system meets the mandatory requirements for grounding and thermal protection in GB 4706.1 "Safety of Household and Similar Electrical Appliances". The combination of the sealing clamp (claim 8) and the brazed joint provides a leak-proof safety requirement for pressurized liquid heating equipment. Attached Figure Description

[0024] Figure 1 This is a three-dimensional structural diagram of the liquid heating device of the present invention, showing the thick film resistance heating layer on the first surface of the stainless steel substrate and the serpentine aluminum tube on the second surface. Figure 2 This is a schematic diagram of the cross-sectional structure of a liquid heating device, showing the relative positional relationship of the stainless steel substrate (1), aluminum tube (2), brazing layer (3), and thick film resistance heating layer (4), as well as the dimensions (h, W, D) of the flattened aluminum tube cross-section. Figure 3The image shows a scanning electron microscope (SEM) diagram of the microstructure of the brazed joint, indicating the position and thickness of the Al-Si eutectic layer (11) and the Al-Fe-Si intermetallic compound transition layer (12); Figure 4 The EDS line scan composition distribution map of the brazing layer shows the concentration gradient distribution of Al, Fe, and Si from the aluminum tube side to the stainless steel side, confirming the continuous gradient characteristic of Fe content. Figure 5 Comparison curves showing the relationship between shear strength and number of thermal cycles for brazed joints with different connection methods; Figure 6 A top view of the serpentine arrangement of aluminum tubes; Figure 7 A side cross-sectional view of the serpentine arrangement of aluminum tubes; Figure 8 This is an enlarged cross-sectional schematic diagram of the welded aluminum tube structure. Figure 9 A top view of the spiral arrangement of aluminum tubes; Figure 10 A schematic side view of the spiral arrangement of aluminum tubes; Figure 11 This is a schematic diagram of an airtightness testing device, showing the test circuit for nitrogen pressurization, pressure holding, and leak detection; Figure 12 The diagram shows the connection between the temperature controller (7), temperature sensor (8), overheat protection element (9) and the liquid heating device in the liquid heating system.

[0025] Reference numerals: 1-Stainless steel substrate; 2-Fluid pipe; 3-Bratter layer; 4-Thick film resistance heating layer; 41-Insulating dielectric layer; 42-Resistant layer; 43-Covering layer; 5-Heat insulation layer; 6-Sealing clamp; 7-Thermostat; 8-Temperature sensor; 9-Overheat protection element; 10-Waterproof sealed shell; 11-Al-Si eutectic layer; 12-Al-Fe-Si intermetallic compound transition layer; 13-Control circuit; 14-Water inlet connector; 15-Water outlet connector; 16-Wire assembly; W-Contact surface width; D-Original outer diameter of aluminum pipe; H-Flattening height; L-Live wire; N-Neutral wire; PE-Grounding wire. Detailed Implementation

[0026] The present invention will be further described below with reference to the accompanying drawings and embodiments. The following embodiments are for illustrative purposes only and do not constitute a limitation on the scope of protection of the present invention. Without departing from the spirit and scope of the present invention, those skilled in the art can make appropriate changes and modifications to the embodiments, and all such changes and modifications should fall within the scope of protection of the claims of the present invention.

[0027] like Figure 1 , Figure 2As shown, the liquid heating device in this embodiment has the following structure: Stainless steel substrate 1: 304 stainless steel, 1.0 mm thick, 150 × 100 mm in size, surface roughness Ra <= 1.6 μm (which is beneficial for solder wetting and spreading).

[0028] Thick-film resistance heating layer 4: The following layers are sequentially screen-printed and sintered on the first surface of the stainless steel substrate 1: insulating dielectric layer 41 (lead borosilicate glass, approximately 50 μm thick, insulation resistance >= 100 MΩ / 500 V), resistance layer 42 (RuO2-based conductive paste, resistivity 1.2 Ω / □ after sintering), and cover layer 43 (protective glaze, temperature resistance >= 300℃). Each layer is sequentially sintered in a furnace at a peak temperature of 850℃. After cooling, the bonding strength between the resistance layer and the substrate is >= 20 MPa.

[0029] Aluminum Tube 2: 3003 aluminum alloy (Al-Mn series, corrosion resistance superior to pure aluminum), outer diameter D=10mm, wall thickness 1.0mm. Aluminum Tube 2 is formed by hydraulic cold pressing, flattened along the axial direction. After flattening, the cross-sectional height H=4mm (H / D=0.4), contact width W≈8.1mm (W / D≈0.8), net height of the internal flow channel is approximately 2mm, and the flow cross-sectional area is approximately 24mm². 2 This meets the pressure drop requirements at rated flow. The flattened aluminum tubes are arranged in a serpentine pattern. Figure 6 The tubes are arranged on the second surface of the stainless steel substrate 1, with a tube spacing of 15mm and a bending radius of 15mm. The two ends of the aluminum tubes extend 30mm beyond the edge of the substrate as inlet and outlet connectors.

[0030] Solder pre-placement: An Al-12Si (mass fraction) eutectic alloy solder foil, 0.17 mm thick, is pre-placed between the flattened section of the aluminum tube 2 and the second surface of the stainless steel substrate 1. The foil width matches the contact width of the flattened section. Solder is then applied by spot coating with flux (potassium fluoroaluminate based, coating amount approximately 5 g / m). 2 The positioning is fixed.

[0031] Brazing process: The assembled workpiece is placed in a mesh belt-type atmosphere-protected brazing furnace for continuous brazing: high-purity nitrogen protection, oxygen content in the furnace <10ppm, dew point <=-60℃; the heating zone (200~560℃, about 8min) thoroughly dries the flux and activates the aluminum surface; the brazing zone temperature is 580~620℃, holding time is 4~6min; the cooling zone is cooled to <150℃ under nitrogen protection before removal from the furnace. After brazing, residual flux is cleaned with deionized water. For details of the brazing process, please refer to the accompanying method patent application APP-002.

[0032] Detected by SEM-EDS and XRD ( Figure 3 , Figure 4 In Example 1, the brazing joints are presented sequentially from the aluminum tube 2 side to the stainless steel substrate 1 side: (a) Al-Si eutectic layer (layer 11): thickness 120-160 μm, λ2=20-28 μm, interlayer spacing 2.0-3.0 μm, HV45-55. EDS quantitative analysis: Al 87-89 wt%, Si 11-13 wt%, Fe<1.5 wt%, homogeneous composition, no component segregation.

[0033] (b) Al-Fe-Si intermetallic compound transition layer (layer 12): thickness 8–12 μm. XRD diffraction: main phase Al 13 Fe4 (monoclinic system), secondary phase Al3Fe (orthorhombic system), with a volume ratio of approximately 4:1, HV 280–360. EDS line scan ( Figure 4 The results show that the Fe content smoothly increases from about 2 wt% on the aluminum tube side to about 78 wt% on the stainless steel side, and the elemental concentration curve shows no abrupt steps, confirming the gradient distribution characteristics. Within 3-4 μm below the interface of the stainless steel substrate 1, the Fe content is slightly lower than that of the substrate (Fe-depleted layer), and the Cr content does not change significantly, indicating that the interfacial reaction is mainly Fe diffusion, and the substrate itself is not damaged.

[0034] Shear strength (performed according to GB / T 11363, universal testing machine, loading rate 1mm / min): measured 62~68MPa (n=10, mean 65MPa, standard deviation 2.1MPa). All specimens failed within the aluminum tube base material or Al-Si eutectic layer, and no debonding occurred at the interface, proving that the interfacial bonding strength is higher than the strength of the aluminum tube body (the tensile strength of 3003 aluminum alloy is about 145MPa, and the shear yield strength is about 55MPa).

[0035] Air tightness (according to GB / T 15823 helium mass spectrometry and 0.8MPa nitrogen pressure holding method): the leakage rate of all 10 samples is <=3×10 -9 Pa·m 3 / s, which is superior to the 1×10 specified in the claims. -8 Pa·m 3 / s.

[0036] Thermal cycling life (10~95℃, heating and cooling rate approximately 10℃ / min, 4 cycles / h, for a total of 1500 cycles): shear strength retention rate after cycling >=95%, no cracks were found during visual inspection and fluorescence penetrant testing. Figure 5 Air tightness maintained at <= 5×10 -9 Pa·m 3 / s.

[0037] Hydraulic burst test (pressure increase rate 0.5MPa / min): When the water pressure increased to 2.5MPa, the aluminum tube body (unflattened section) bulged and deformed. The brazing interface between the flattened section and the stainless steel substrate was intact and did not detach, proving that the joint strength was higher than the pressure bearing strength of the aluminum tube body and that the joint was not a weak link.

[0038] The only difference between this embodiment and Embodiment 1 is the material of the aluminum tube 2. The rest of the structure, dimensions and brazing process parameters are exactly the same, in order to verify the applicability of different aluminum alloy materials in claim 3.

[0039] 1060 pure aluminum tube (aluminum content >= 99.60%): Pure aluminum has a thermal conductivity of approximately 220 W / (m·K) and a yield strength of only about 35 MPa. It exhibits minimal work hardening and uniform forming during flattening, making it suitable for applications where the liquid heating temperature is <= 80℃ (pure aluminum has a relatively low softening temperature). The brazed joint shear strength is 55–62 MPa, and the airtightness meets requirements. After 500 cycles of thermal cycling, the strength retention rate is >= 95%.

[0040] 5052 aluminum alloy tube (Al-2.5Mg series): Its corrosion resistance is superior to 3003, making it especially suitable for materials containing minerals (Ca). 2+ Mg 2+ Suitable for heating in weakly acidic liquids; yield strength approximately 90 MPa; the springback after flattening is slightly large, requiring an appropriate increase in the flattening stroke; shear strength of brazed joints 60~67 MPa; airtightness meets requirements; strength retention rate >= 93% after 1500 cycles of hot and cold.

[0041] In this embodiment, the material of the stainless steel substrate 1 is replaced with 316L stainless steel and 430 stainless steel, respectively, while the other parameters are the same as in embodiment 1, in order to verify the range of stainless steel materials covered by claim 3.

[0042] 316L stainless steel substrate (ultra-low carbon austenitic type, containing 2-3 wt% Mo): The presence of Mo element improves the pitting corrosion resistance of stainless steel, making it suitable for seawater or high chloride ion environments; the Fe-Al interface reaction rate is similar to that of 304 stainless steel (due to the similar Cr and Ni content); the brazed joint shear strength is 63-70 MPa, and the airtightness meets the requirements; the strength retention rate after 1500 cycles of thermal cycling is >= 96%.

[0043] 430 stainless steel substrate (ferritic, containing 16-18 wt% Cr, excluding Ni): Ferritic stainless steel has lower cost and higher magnetic permeability, making it suitable for general household electromagnetic heating applications; its Fe-Al interfacial diffusion coefficient is similar to that of austenitic stainless steel; the Al-Fe-Si intermetallic compound transition layer thickness of the brazed joint is 9-13 μm, the shear strength is 58-65 MPa, and the airtightness meets requirements; the coefficient of thermal expansion (approximately 10.4 × 10⁻⁶) is [not specified in the original text]. -6The coefficient of thermal expansion (C / K) is slightly lower than that of austenitic aluminum, and the difference in expansion coefficient between C and aluminum is slightly larger. After 1500 cycles of thermal cycling, the strength retention rate is >=90%, which still meets the performance index of claim 1.

[0044] Table 1. Performance comparison of the brazing connection of the present invention with existing main connection methods (test substrate: 3003 aluminum tube + 304 stainless steel substrate, 150×100mm)

[0045] 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 liquid heating device, characterized in that, The device includes a metal substrate (1), a heating element (4) is provided on the first surface of the metal substrate (1), and a fluid tube (2) is fixedly connected to the second surface of the metal substrate (1); at least one section of the fluid tube (2) is flattened into a flat cross section, and the side of the flat cross section facing the second surface of the metal substrate (1) is metallurgically bonded to the second surface of the metal substrate (1) through a brazing layer (3).

2. The liquid heating device according to claim 1, characterized in that: The metal substrate (1) is a stainless steel substrate, the fluid tube (2) is an aluminum tube, and the brazing layer (3) is an Al-Si brazing layer. The Al-Si brazing layer includes, from the aluminum tube (2) side to the stainless steel substrate (1) side, the following layers in sequence: an Al-Si eutectic layer with a thickness of 80 to 200 μm; an Al-Fe-Si intermetallic compound transition layer with a thickness of 5 to 15 μm, wherein the Fe content increases continuously from 2 wt% on the aluminum tube (2) side to 80 wt% on the stainless steel substrate (1) side.

3. The liquid heating device according to claim 1, characterized in that: The fluid tube (2) is made of aluminum or aluminum alloy, with an outer diameter of 6 to 16 mm and a wall thickness of 0.5 to 4.0 mm; the metal substrate (1) is made of stainless steel and has a thickness of 0.3 to 5.0 mm.

4. The liquid heating device according to claim 1, characterized in that: The height h of the flattened section of the fluid tube (2) is 0.3 to 0.8 times the outer diameter of the fluid tube (2), the contact width between the flattened section and the second surface of the metal substrate (1) is 0.3 to 1.1 times the outer diameter of the fluid tube (2), and the contact length between the flattened section and the metal substrate (1) along the axial direction of the fluid tube (2) is not less than 3 times the outer diameter of the fluid tube (2).

5. The liquid heating device according to claim 2, characterized in that: The spacing λ2 of the secondary dendrite arms of the primary α-Al dendrites in the Al-Si eutectic layer is 15-35 μm, and the spacing between the Al-Si eutectic layers is 1.5-3.5 μm; the main phase of the Al-Fe-Si intermetallic compound transition layer is Al13Fe4, the secondary phase is Al3Fe, and the volume ratio of Al13Fe4 to Al3Fe is (3-5):1; the stainless steel substrate (1) has a Fe-depleted layer at a depth of 3-5 μm below the interface adjacent to the Al-Fe-Si intermetallic compound transition layer.

6. The liquid heating device according to claim 1, characterized in that: The fluid tube (2) is arranged in a serpentine or spiral shape on the second surface of the metal substrate (1), and the two ends of the fluid tube (2) are connected to the inlet and outlet of the liquid respectively.

7. The liquid heating device according to claim 1, characterized in that: The heating element (4) is a thick film resistance heating layer, which is formed on the first surface of the metal substrate (1) by screen printing and sintering process, and has a surface resistivity of 0.05 to 5 Ω / □.

8. The liquid heating device according to claim 1, characterized in that: The liquid heating device further includes a heat insulation layer (5) disposed on the first surface side of the metal substrate (1), the heat insulation layer (5) having a thermal conductivity of <=0.05W / (m·K), and being fixedly connected to the metal substrate (1) by a snap or bolt.

9. The liquid heating device according to claim 1, characterized in that: The liquid heating device also includes a sealing clamp (6), which seals both ends of the fluid pipe (2) and forms an inlet and an outlet respectively. The sealing clamp (6) is connected to the end of the fluid pipe (2) by an O-ring seal or by welding.

10. The liquid heating device according to claim 7, characterized in that: The first surface of the metal substrate (1) is also provided with a grounding protection layer, which is located outside the thick film resistance heating layer (4) and is used for leakage current protection.

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