High bonding density anti-leakage double-wall roll-weld brake oil pipe and preparation method

Abstract

This invention discloses a high-bonding density, leak-proof double-wall rolled-welded brake hose and its preparation method, relating to the field of brake hose manufacturing technology. The method includes: copper-plating a strip steel on both sides and then rolling it to form a double-wall overlap area; setting a local brazing-free window at the overlap interface on the first surface, with the residual thickness of the main copper layer within the window not exceeding 0.30 micrometers; forming a periodic nickel-iron alloy magnetic marking layer within the window, with a magnetic marking layer thickness of 0.10 to 0.25 micrometers; and applying a nickel flash plating layer and a tin flash plating layer to the copper layer on the second surface, and directionally coating an interface activation film in the area where the overlap interface will be formed. After rolling, heating in a hydrogen-nitrogen reducing atmosphere melts the copper layer, fills the overlap area, and brazes it to seal. When the molten brazing filler metal crosses the window, it dissolves the marking layer; when it does not cross, the marking layer remains and can be identified online by eddy current. This structure achieves self-verification of the welding state, reduces missed detection and misjudgment of interlayer non-welding defects, and improves leak-proof reliability.

Description

Technical Field

[0001] This invention relates to the field of brake hose manufacturing technology, specifically to a high-bonding density, leak-proof double-wall rolled-welded brake hose and its preparation method. Background Technology

[0002] Automotive hydraulic braking systems use brake fluid as their energy source. Brake lines are typically exposed under the vehicle chassis and endure long-term pulse pressure, vibration, thermal cycles, and corrosion from melting snow, salt, and mud. During emergency braking and anti-lock braking system (ABS) pressure adjustments, pressure can reach tens of megapascals, and throughout the vehicle's lifespan, it can withstand hundreds of thousands to millions of pressure cycles. To meet requirements for pressure resistance, puncture resistance, and corrosion resistance, the industry uses double-wall rolled (lapped) brazed steel pipes: low-carbon steel with a brazing filler layer is rolled into a double layer and heated in a reducing atmosphere to melt and wet the filler metal, forming a sealed bond between the lap area and the double walls. A metal protective layer and an organic coating are then applied to enhance environmental durability. These pipes generally undergo 100% online non-destructive testing and sampling sealing inspection before leaving the factory and before vehicle assembly. After flaring and flanging the ends, a sealed connection is formed with the fittings, enabling them to be used as the brake piping assembly.

[0003] During continuous high-speed tube manufacturing, the metallurgical bonding state of the lap weld zone varies due to factors such as residual lubricant or cleaning residue on the strip surface, stability of local oxide films, forming tension, changes in lap gap, variations in brazing filler metal thickness and flowability, and heating / cooling gradients. If such defects occur within double-walled sandwiches or laps, and the outer diameter, roundness, wall thickness, material magnetic properties, and coating condition can disturb online eddy current detection signals, existing detection methods struggle to reliably distinguish between "geometric / material background changes" and "local non-welding or micropores," potentially leading to misjudgments due to threshold drift or missed detections due to hidden defect characteristics. Simply increasing the rejection sensitivity may result in more false alarms, higher scrap rates, and changes in production line cycle time. If such hidden defects occur during later forming and service processes, micropores will become through-hole leakage channels under pulse pressure and thermal cycling. Furthermore, the penetration of salt spray and moisture will further accelerate crevice corrosion, leading to brake fluid leakage, reduced braking capacity, and safety risks, as well as increased costs for quality traceability and recalls.

[0004] Therefore, the current technical problem is: When double-wall rolled welded brake oil pipes are continuously manufactured and subjected to online non-destructive testing, for embedded defects such as local non-welding or micropores in the lap weld area, the existing detection signals cannot form a stable and repeatable correspondence with the actual weld metallurgical state. This makes it difficult to control the risk of missed detection and misjudgment of such defects within the mass production cycle. Summary of the Invention

[0005] (a) Technical problems to be solved To address the shortcomings of existing technologies, this invention provides a high-density, leak-proof double-wall rolled-welded brake hose and its manufacturing method. The residual thickness of the main copper layer within the window is no more than 0.30 micrometers. A periodic strip magnetic marking layer of nickel-iron alloy is formed within the window, with a thickness of 0.10 to 0.25 micrometers. A nickel flash plating layer and a tin flash plating layer are applied to the copper layer on the second surface, and an interface activation film is directionally coated in the area where the overlap interface will form. After rolling, the hose is heated in a hydrogen-nitrogen reducing atmosphere to melt and fill the overlap area, then braze and seal it. When the molten solder crosses the window, it dissolves the marking layer; when it does not cross, the marking layer remains and can be identified online by eddy current. This structure achieves self-verification of the weld state, reducing missed detections and misjudgments of interlayer non-welding defects; and solves the technical problems described in the background art.

[0006] (II) Technical Solution To achieve the above objectives, the present invention provides the following technical solution: The tube includes a double-walled tube formed by rolling low-carbon steel strip, the double-walled tube having an axially extending overlap area; the first surface of the strip has a continuously arranged local brazing-free window in the area constituting the interface of the overlap area, the residual thickness of the main copper layer in the local brazing-free window is not higher than 0.30 micrometers; a magnetic marking layer is provided in the local brazing-free window, the magnetic marking layer is a periodic strip of nickel-iron alloy, the strip direction is perpendicular to the tube axis; the second surface of the strip has a main copper layer, and the overlap area is formed by melting and brazing the main copper layer.

[0007] Furthermore, the local solderless window is located inside the strip edge, and the strip edge retains a continuous copper-plated edge strip with a width of 0.30 to 0.80 mm. The starting distance of the local solderless window from the strip edge is 0.30 to 0.80 mm, and the width of the local solderless window is 2.0 to 3.5 mm.

[0008] Furthermore, the nickel-iron alloy contains 78% to 82% nickel by mass and 18% to 22% iron by mass.

[0009] Furthermore, the width of a single strip in the periodic strip is 0.25 to 0.60 mm, the center-to-center distance of the strips is 0.80 to 1.50 mm, the strip coverage is 20% to 45%, and the strip thickness is 0.10 to 0.25 micrometers.

[0010] A method for preparing a high-bonding density, leak-proof double-wall rolled-welded brake hose includes: The low-carbon steel strip is degreased, pickled, and activated; a peelable masking coating is applied to the first surface, and copper electroplating is performed on both sides of the strip to form a main copper layer. The masking coating is removed to obtain a local brazing-free window, and the residual thickness of the main copper layer in the window is no more than 0.30 micrometers; a periodic strip opening is formed in the local brazing-free window, and a nickel-iron alloy is electrodeposited to obtain a magnetic marking layer. The anti-plating layer is removed; the strip is rolled to form an overlap area and heated under a protective atmosphere to melt the main copper layer. The overlap area is brazed and sealed to form a double-walled tube.

[0011] Furthermore, the copper plating is acidic sulfate copper plating, and the thickness of the main copper layer on one side is 6 to 10 micrometers. After removing the masking coating, the residual thickness of the main copper layer in the local solderless window is no more than 0.10 micrometers.

[0012] Furthermore, the nickel-iron alloy electrodeposition employs a sulfate complex system electroplating solution comprising 300 g / L nickel sulfate, 25 g / L ferrous sulfate, 30 g / L boric acid, 70 g / L sodium citrate, and 2 g / L saccharin. The bath temperature is 50 degrees Celsius, the solution pH is 3.6, the cathode current density is 10 to 30 amperes per square decimeter, and the deposition time is 2 to 10 seconds, to obtain a magnetic marking layer with a thickness of 0.10 to 0.25 micrometers.

[0013] Furthermore, before rolling, a nickel flash plating layer with a thickness of 20 to 60 nanometers is formed on the main copper layer of the second surface, and a tin flash plating layer with a thickness of 30 to 80 nanometers is formed on the nickel flash plating layer.

[0014] Furthermore, before rolling, an interface-activating film is orientedly coated onto the overlapping strip areas on both sides. The solid formulation of the interface-activating film, by weight, includes 45 to 70 parts of ammonium formate, 8 to 20 parts of ammonium citrate, 5 to 15 parts of citric acid, 5 to 15 parts of polyvinylpyrrolidone, 4 to 10 parts of polyethylene glycol with a number average molecular weight of 300 to 600, and 0.5 to 2 parts of alkyl glycoside nonionic surfactant. After coating, it is dried at 80 to 120 degrees Celsius for 30 to 90 seconds to achieve a dry film surface density of 0.05 to 0.20 g / m².

[0015] Furthermore, the brazing is carried out in a mixed reducing atmosphere of hydrogen and nitrogen, with a hydrogen integral of 15% to 30% and an atmosphere dew point not exceeding -35 degrees Celsius. The preheating section is heated to 500 degrees Celsius and held for 40 to 90 seconds, while the high-temperature section is heated to 1085 to 1105 degrees Celsius and held for 8 to 18 seconds. After brazing, online eddy current testing is performed using a magnetic saturation device and a low-frequency channel of 1 to 5 kHz to determine whether there are frequency domain characteristics corresponding to the center distance of the periodic strips.

[0016] (III) Beneficial Effects This invention provides a high-bonding density, leak-proof double-wall rolled-welded brake hose and its preparation method, which has the following beneficial effects: A solderless window is set within the lap joint interface, forming a magnetic periodic marking layer. This marking layer is only dissolved and hidden during continuous brazing after the molten solder from the second surface crosses the window and continuously wets and fills it. The weld metallurgical state corresponds one-to-one with the detectable signal. A copper-plated edge strip is retained at the edge of the solderless window, balancing lap joint sealing material supply and marking discrimination, avoiding weak sealing and surface buildup. Nickel and tin flash plating layers synergistically enhance the spread, crossing, and dissolution of the marking layer by the solder. The directionally coated interface activation film preheats and decomposes to remove interlayer contaminants, inhibiting air gap formation and improving the bonding density and micropore / microchannel characteristics of the lap joint area. Online eddy current locking of the marking layer's periodic signal characteristics allows for stable discrimination under varying wall thickness, roundness, and material background disturbances, reducing missed detections, misjudgments, and scrap fluctuations due to incomplete welding defects. Furthermore, the directional coating of the interface film improves the adhesion consistency of the zinc-nickel alloy electroplating and organic outer coating, enhancing the leak-proof reliability of the brake hose under pulse pressure and corrosive environments. Attached Figure Description

[0017] Figure 1 This is a schematic diagram illustrating the preparation method of the high-bonding density leak-proof double-wall rolled welded brake hose of the present invention. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] Please see Figure 1 This invention provides a high-bonding density, leak-proof double-wall rolled-welded brake hose and its preparation method, the specific steps of which are as follows: I. Specific Application Scenarios (Preset)

[0020] Scenario: Exposed brake lines in the hydraulic braking system of passenger vehicles (chassis layout, high salt spray / snow melting salt areas); Application: Hydraulic brake lines between the master cylinder and wheel calipers (or anti-lock braking system modules); Tube type: Double-wall rolled (720° rolled) copper-based brazed sealing structure; Common specifications: outer diameter 4.75 mm; finished double-wall thickness 0.70±0.05 mm; Production process: continuous forming, continuous protective atmosphere brazing, online 100% eddy current testing, and subsequent external anti-corrosion coating; Key pain points: Traditional online eddy current identification of local unwelded / micropores in interlayer has problems such as high background noise, threshold drift, and both false positives and false negatives; once the brake pipe is missed and enters the whole vehicle, the defect will be amplified by pulse pressure and corrosion in the later stage, creating a risk of leakage, and the motivation for supply chain improvement is strong.

[0021] II. Overview of the material system: Components A, B, C, D, and E (including subcomponents and ranges). Each component is given below in terms of subcomponent-parameter range-example values.

[0022] Component A: Matrix tape system (A1+A2+A3): A1: Chemical composition of low-carbon steel matrix (mass percentage, range): Carbon: 0.04–0.10; Manganese: 0.30–0.80; Silicon: 0.02–0.20; Aluminum (all aluminum): 0.02–0.06; Phosphorus: ≤0.015; Sulfur: ≤0.010; A2: Microalloying and Cleanliness Control (Scope): Niobium: 0.010–0.040; Titanium: 0.005–0.020; Nitrogen: ≤60ppm; Oxygen: ≤20ppm A3: Geometric and Surface Parameters (Range): Strip thickness (single layer): 0.30–0.36 mm; strip width: 17.5–19.5 mm (matching target outer diameter and overlap width); surface roughness (arithmetic mean roughness): 0.15–0.35 μm; residual oil before plating: ≤0.30 g / m².

[0023] The values ​​used in this embodiment (embodiment A) are as follows: Chemical composition (mass percentage): Carbon 0.06; Manganese 0.55; Silicon 0.08; Aluminum 0.035; Phosphorus 0.010; Sulfur 0.006; Niobium 0.020; Titanium 0.010; Nitrogen 45ppm; Oxygen 15ppm; Strip thickness: 0.32 mm; Strip width: 18.2 mm; Surface roughness: 0.22 μm Among them, the design focus of A is not to pursue extremely high strength, but to reduce the disturbance of online eddy current background (magnetic and geometric fluctuations) to provide a low-noise substrate for the stable interpretation of self-verifying markers.

[0024] Component B: Interface activation / decontamination / reduction precursor film (B1+B2+B3+B4): B is positioned as a halogen-free, sulfur-free, and low-ash thin-film precursor system: it completes the cleaning and venting of the interlayer interface before the copper melts, ensuring that the blanking of the D mark truly corresponds to the welding in place.

[0025] B1: Thermally decomposable and reducible precursor (solid formulation mass percentage, range): Ammonium formate: 45–70; B2: Complexing / demembrane-decontaminated components (range): Ammonium citrate: 8–20; Citric acid: 5–15; B3: Low-ash film adhesive (range): Polyvinylpyrrolidone (number average molecular weight 8,000–15,000): 6–12; B4: Wetting and Penetration Enhancers (Scope): Polyethylene glycol (number average molecular weight 300–600): 4–10; Nonionic surfactants (e.g., alkyl glycosides): 0.5–2.5; B's coating process and parameter range: Solvent system: deionized water: ethanol = 60:40 to 80:20 (mass ratio); Total solids content: 2.0–6.0% by mass; Wet film coating amount: 2.0–6.0 g / m²; Dry film coating amount: 0.05–0.30 g / m²; Drying: 80–120 degrees Celsius, 30–90 seconds; Decomposition window: Decomposition begins at 220–260 degrees Celsius; essentially completes at 420–480 degrees Celsius (for your subsequent writing of the staged reaction mechanism). The pH of the coating solution is controlled at 4.5–6.5 (preferably 5.0–6.0) by increasing the proportion of ammonium citrate and decreasing the proportion of free citric acid; or: the residual solvent mass fraction of the film after drying is not higher than 5%; and the coating is stored for a short time in dry air with a dew point below -10 degrees Celsius after coating.

[0026] The value used in this embodiment (embodiment B) Solid formulation (mass percentage): Ammonium formate 55; Ammonium citrate 15; Citric acid 10; Polyvinylpyrrolidone 10; Polyethylene glycol 8; Alkyl glycoside 2; Solvent: Deionized water: Ethanol = 70:30 (mass ratio); Total solids content: 3.5% by mass; Dry film coating weight: 0.15 g / m²; Drying: 100°C, 50 seconds; Component C: Solder supply layer (copper-based solder system) (C1+C2+C3): The design goals of C are: to provide stable brazing filling and dense bonding; to provide dissolution and wetting conditions for rapid and controllable blackout of D markings; and to avoid the use of cyanide systems and prioritize process-friendly routes such as acidic copper plating.

[0027] C1: Main copper layer (range): Plating method: Acidic sulfate copper electroplating (continuous electroplating); Thickness (single side): 6–10 micrometers; Thickness uniformity: within ±10% within the same roll; within ±8% in the width direction.

[0028] C2: Nickel flash plating (range): Used to improve the wetting consistency of molten copper at the metal interface and promote the dissolution kinetics of the Ni-Fe marker layer; can be placed on the overlapping surface opposite to the marker layer or for full coverage; thickness: 20–60 nm; C3: Tin flash plating (range): Used to reduce local surface tension and improve the spreading / penetration ability of molten copper, enhancing the coverage and dissolution of D across local solderless windows; Thickness: 30–80 nm; The values ​​used in this embodiment (embodiment C) are as follows: Main copper layer thickness: First surface (see definition below): 8.0 micrometers (except for the solderless window of D); Second surface: 8.0 micrometers (full surface); Nickel flash plating: 30 nanometers (full coverage of the second surface); Tin flash plating: 50 nanometers (full coverage of the second surface); Component D: A system with labelable / self-verifiable weld condition (core innovation) (D0+D1+D2+D3) The core of D is not to place a layer under the copper layer that can disappear when the copper is heated, but rather: first, to construct a local solderless window (D0) on one of the overlapping surfaces of the overlap area so that molten copper will not be generated at this location when heated; then, to set a strongly detectable magnetic periodic marker layer (D1 / D2) in this window; only when molten solder from the other overlapping surface actually crosses the window and completes wetting and filling will the marker layer be dissolved and diluted and disappear, thus converting the soldering into the disappearance of the marker signal. D0: Localized area without solder (range):

[0029] Located on the edge region of the first surface of the strip, continuously set along the length direction; window width: 2.0–3.5 mm; residual thickness of the main copper layer inside the window: ≤0.30 micrometers (preferably ≤0.10 micrometers, more preferably close to 0); used to prevent the marking layer at this location from being unconditionally dissolved by the copper on this side during heating, ensuring that blanking is only triggered by the molten solder on the opposite side crossing and filling; D1: Magnetic marking layer material (range):

[0030] Nickel-iron alloy (78–82% nickel by mass, 18–22% iron by mass); optional trace amounts of molybdenum: 0–1.5% by mass (to improve high-temperature oxidation resistance and regulate dissolution rate). D2: Geometric pattern (range) of the marker layer:

[0031] Periodic transverse strips are used to form stable frequency domain features, which facilitates online eddy current locking identification and reduces false judgments: Pattern arrangement area: completely within the D0 window; Single strip width: 0.25–0.60 mm; Strip spacing (distance between adjacent strip edges): 0.50–1.20 mm; Period (center distance): 0.80–1.50 mm; Coverage (strip area / window area): 20–45%; Strip direction: consistent with the strip width direction (i.e., perpendicular to the strip length direction); Strip thickness: 80–180 nm (preferably 100–160 nm). D3: Engineering constraints (range) of hidden content elimination conditions:

[0032] To ensure that good products are eliminated and defects are retained, it is recommended to match the blanking time window with the brazing thermal cycle: When the molten copper on the opposite side truly wets and crosses the window: the marker strip essentially dissolves and dilutes to undetectable levels within 5–20 seconds at 1080–1100 degrees Celsius (residual thickness <30 nm or residual coverage <10%); when there are unsoldered / air gaps / contamination preventing the molten solder on the opposite side from crossing the window: the residual thickness of the marker strip is ≥50 nm or the residual coverage is ≥30% (forming a stable and detectable signal); The values ​​used in this embodiment (the embodiment of D) are as follows: D0 solderless window: A continuous copper-plated edge strip is retained at the edge of the strip as a source of solder for edge sealing, with a strip width of 0.30–0.80 mm (preferably 0.50 mm); the D0 window is located inside this edge strip, with the window starting 0.30–0.80 mm from the edge; the window width is 1.8–2.6 mm (preferably 2.2–2.4 mm); the overlap width should be at least 0.30 mm greater than the window width to accommodate assembly deviations and ensure that the window is completely within the overlap interface.

[0033] Located on one edge of the first surface, the window width is 2.8 mm; the copper residue thickness inside the window is ≤0.10 μm; D1 Marking Material: Nickel-Iron Alloy (78–82% Nickel, 18–22% Iron); D2 Pattern: Strip width 0.40 mm; strip edge spacing 0.90 mm (center distance 1.30 mm); coverage approximately 31%; strip thickness 120 nm; D3 Blanking Target: When the strip is held at 1095 degrees Celsius for 12 seconds and the molten solder on the opposite side crosses the wetting condition, the strip is basically blanked; if the wetting condition is not met, the strip is retained.

[0034] Component E: External surface corrosion protection / sealing system (E1+E2+E3): The role of E is to extend leak protection to the main failure path (corrosion perforation) during the service life.

[0035] E1: Zinc-nickel alloy electroplating layer (range): Thickness: 8–12 micrometers; Nickel content: 10–15% by mass; E2: No hexavalent chromium conversion layer (range); Zirconium / titanium-based conversion film; Thickness: 50–200 nanometers.

[0036] E3: Organic outer coating (range): Polyamide-12 outer coating (or equivalent salt spray resistant system); thickness: 25–45 micrometers; values ​​in this example (example of E); zinc-nickel alloy: 10 micrometers thick, nickel content 12.5% ​​by mass; zirconium-based conversion layer: approximately 100 nanometers thick; polyamide-12 outer coating: 35 micrometers thick. The preparation method is as follows:

[0037] The following outlines the continuous manufacturing steps from strip steel to finished oil tubing. To avoid ambiguity, let's define: First surface: The surface with the D0 window and D mark layer (with copper-free window at the edge); Second surface: The opposite side of the first surface (full copper layer, with nickel / tin flash plating). Step 1: Strip pretreatment (preparation of corresponding component A) Take the A-component strip steel (thickness 0.32 mm, width 18.2 mm).

[0038] Alkaline degreasing: Solution composition: 20 g / L sodium hydroxide, 15 g / L sodium carbonate, 2 g / L nonionic surfactant; Temperature 55 degrees Celsius, time 60 seconds; water wash (two stages), 20 seconds per stage. Acid pickling to remove oxide scale: 10% hydrochloric acid solution, 30 degrees Celsius, 15 seconds; water wash (two stages), 20 seconds per stage.

[0039] Activation: 2% sulfuric acid solution, 10 seconds at room temperature; wash with water and then blow dry or dry with hot air.

[0040] Step 2: Forming a D0 local solder-free window (selective copper resist plating) A peelable masking coating (e.g., a water-soluble acrylic masking adhesive) is applied to one edge of the first surface by roller coating: Masking width: 2.8 mm; Dry film thickness: 5–10 μm; Drying: 80°C, 30 seconds; Double-sided acidic sulfate copper plating (continuous plating): Electroplating solution (example): Copper sulfate pentahydrate 210 g / L, sulfuric acid 60 g / L, chloride ion 60 mg / L, brightener as recommended by the supplier; bath temperature 28 degrees Celsius; cathode current density 6 amperes / dm²; By matching the line speed and current, the copper layer thickness of the second surface is 8.0 micrometers; the copper layer thickness of the first surface is 8.0 micrometers except for the shielded area; water washing (two stages).

[0041] Remove the first surface masking agent: Rinse with warm water at 60 degrees Celsius and brush with a weak alkali (e.g., 5 g / L sodium carbonate) for 30–60 seconds until the copper residue thickness in the window area is ≤0.10 micrometers; rinse with water and dry.

[0042] Step 3: Prepare periodic strips of the D-labeled layer in the D0 window (selective electrodeposition) In the D0 window area of ​​the first surface, a periodic strip opening is formed by printing an anti-plating pattern (either acid-resistant anti-plating ink or photolithographic anti-plating adhesive can be used, either one can be selected): Strip width 0.40 mm; center-to-center distance 1.30 mm; anti-coating thickness 8–12 micrometers; Pattern orientation: The strip direction is along the width of the strip (perpendicular to the length of the strip), forming a periodic repetition along the length direction; Window area activation: Immerse in 2–5% sulfuric acid or hydrochloric acid for 3–10 seconds, followed by rapid water rinsing; Optional nickel underlay (Wood nickel underlay): Approximately 240 g / L nickel chloride, approximately 120 mL / L hydrochloric acid, current density 5–15 A / dm², time 1–3 seconds, to form a 10–30 nm nickel underlay; then proceed with nickel-iron alloy strip electrodeposition.

[0043] Nickel-iron alloy electrodeposition is performed in the opening region: Electroplating solution (example, sulfate-complex system): Nickel sulfate 300 g / L; ferrous sulfate 25 g / L; boric acid 30 g / L; sodium citrate 70 g / L; saccharin 2 g / L (stress adjustment); bath temperature 50°C; solution pH 3.6; Spray / brush local enhancement: cathode current density 10–30 amperes / dm², deposition time 2–10 seconds, resulting in a marker layer thickness of 0.10–0.25 micrometers (preferably 0.12–0.20 micrometers).

[0044] Deposition time 0.8–1.5 seconds (controlling strip thickness to approximately 120 nm); water wash.

[0045] Remove the anti-plating layer (using the stripping solution and temperature corresponding to the selected anti-plating material), wash with water and dry.

[0046] Key inspection points: Randomly inspect the strip thickness and cycle size in the window strip area. The strip thickness is 120±20 nanometers; the center-to-center distance of the strips is 1.30±0.05 millimeters.

[0047] Step 4: Coating the interface activation film of component B Preparation of B film coating solution: Solid formulation: ammonium formate 55%, ammonium citrate 15%, citric acid 10%, polyvinylpyrrolidone 10%, polyethylene glycol 8%, alkyl glycoside 2%; Solvent: Deionized water: Ethanol = 70:30 (mass ratio); Total solids content: 3.5%; Roller coating is used to coat the strip-shaped area formed at the overlap interface, applying the coating only to the strip-shaped areas on both sides of the strip that will enter the overlap interface. A recommended strip width is: 6–10 mm (preferably 7–9 mm) per side; this bandwidth should cover the final overlap width (approximately 2.8 mm) and its adjacent area to account for forming deviations and capillary filling areas. Directional coating is achieved using narrow-width roller coating or narrow-width spray coating (or mask-roll coating). The dry film coating amount remains as previously set: 0.05–0.20 g / m² (preferably 0.10–0.15 g / m²), but this value refers to the dry film amount per unit area within the coated region. The time from coating to entering the brazing furnace should not exceed 8 hours (preferably no more than 2 hours) to avoid the slow action of hygroscopic and acidic components on the copper layer.

[0048] Drying: 100 degrees Celsius for 50 seconds, resulting in a dry film coating of 0.15 g / m².

[0049] Step 5: Prepare a nickel / tin flash plating layer (corresponding to C2 and C3) on the second surface. Nickel flash plating: Electroplating solution (example, Watt's nickel system): 240 g / L nickel sulfate, 45 g / L nickel chloride, 30 g / L boric acid; bath temperature 50°C, solution pH 4.0; current density 1 ampere / dm²; time control to achieve a nickel layer thickness of approximately 30 nanometers; water rinsing.

[0050] Tin flash plating: Electroplating solution (example, tin methanesulfonate system): 30 g / L stannous methanesulfonate based on tin ions, 100 g / L methanesulfonic acid, additives as recommended by the supplier; bath temperature 25 degrees Celsius; current density 1 ampere / dm²; time control to achieve a tin layer thickness of approximately 50 nanometers, rinse with water and dry.

[0051] Step 6: 720° rolling and continuous brazing A roll forming machine is used to roll the strip steel into a double-walled tube with a rolling angle of 720°, forming a continuous overlap along the axial direction.

[0052] Adjust the overlap width to 2.5–3.2 mm so that the D0 window strip area on the first surface is located within the overlap interface (preferably in the middle or inner 1 / 3 of the overlap width), and ensure an initial gap of 10–30 micrometers at the overlap interface. Using a certain edge of the strip as a reference, the centerline of the window is 1.2–1.8 mm (preferably 1.5 mm) from the edge; after forming, the coverage width of the overlap area relative to that edge is ≥ the window width + 0.30 mm; the allowable alignment deviation is ±0.20 mm. The forming machine employs edge guidance and tension control to maintain this alignment deviation during mass production.

[0053] Continuous protective atmosphere brazing furnace: Protective atmosphere: hydrogen-nitrogen mixed reducing atmosphere, hydrogen gas fraction 15–30%, dew point ≤−35 degrees Celsius; Preheating section: room temperature rises to 500 degrees Celsius, time 40–90 seconds (to complete the decomposition of the B film and clean the interface); High temperature section: 1095±10 degrees Celsius, holding time 8–18 seconds (matched according to the line speed; 12 seconds in this embodiment); Key mechanism: The copper layer on the second surface melts and enters the lap interface under capillary action; when it crosses the D0 copper-free window and wets the strip area, the strip is dissolved and diluted and disappears; if interface contamination, lack of soldering, or porosity prevents the molten solder from crossing, the strip is retained.

[0054] Cooling: First, cool to below 800 degrees Celsius in a protective atmosphere, then use forced air cooling or atomized water cooling to reduce to below 200 degrees Celsius to avoid copper buildup defects on the outer surface. Sizing and Straightening: Set the outer diameter to 4.75 mm, and control the roundness and straightness according to customer standards.

[0055] Step 7: Online eddy current testing (integrated material and testing interpretation) The online eddy current probe is positioned after cooling and when its outer diameter is stable. Detection conditions: A magnetic saturation device is used to reduce the impact of matrix permeability fluctuations on the background; a low-frequency channel (e.g., 1–5 kHz) is set up for deep interface responses, and a high-frequency channel is used for surface defects and geometric compensation. Online detection employs multiple frequency channels; the low-frequency channel primarily responds to conductivity differences and interface structure changes, focusing on the periodic repetitive features caused by the stripe pattern for discrimination. Judgment logic: In the qualified welding zone, the D strip has been hidden, and there should be no repetitive signal consistent with the strip period along the pipe axis; once a continuous or local periodic signal appears, it is determined that the molten brazing filler metal has not crossed the window / has not been welded / has micropores causing strip residue, triggering rejection or alarm.

[0056] Step 8: External anti-corrosion coating (corresponding component E) Surface cleaning: Alkaline washing to remove residue, slight activation followed by water rinsing.

[0057] Before zinc-nickel electroplating, perform alkaline degreasing (50–60 degrees Celsius, 30–90 seconds) → water rinsing → light acid activation (2–5% sulfuric acid, 5–15 seconds) → water rinsing; if there is a lot of copper residue on the outer surface, you can choose to add a copper removal activation step (e.g., a mild persulfate or nitric acid system, short treatment) to obtain more consistent zinc-nickel adhesion.

[0058] Zinc-nickel alloy electroplating: coating thickness 10 micrometers; nickel content 12.5% ​​(mass percentage); no hexavalent chromium zirconium-based conversion: immersion to form a conversion film of approximately 100 nanometers.

[0059] Polyamide-12 outer coating: Preheat pipe to 220–240 degrees Celsius; electrostatic powder spraying or extrusion coating to form a 35-micron outer coating; cure at 240 degrees Celsius for 2–4 minutes.

[0060] As a supplement, the local solder-free window (D0) is a key constraint: it prevents the marker layer from being unconditionally dissolved by the copper on the same side, ensuring a causal correspondence between blanking and the actual crossing and filling of the opposite side by the solder. The periodic magnetic stripe (D2) upgrades online detection from absolute value discrimination to periodic feature discrimination: it is more resistant to background disturbances such as wall thickness, ellipticity, and saturation drift, reducing false positives. The B film and C2 / C3 flash plating together ensure that good products are blanked and defects are retained: B provides interlayer cleaning and venting, avoiding false residues; nickel / tin flash plating improves the wetting and stripe dissolution kinetics across the window, avoiding false alarms due to insufficient blanking of good products. Uniform test conditions (all 5 groups were identical for easy comparison)

[0061] Product specifications and production line conditions (uniform): Outer diameter: 4.75 mm; Finished double-wall thickness: 0.70±0.05 mm; Rolling: 720° double-wall rolling; Overlap width: 2.8±0.2 mm; Brazing furnace atmosphere: hydrogen-nitrogen reducing atmosphere, hydrogen gas fraction 20%, dew point not higher than −35 degrees Celsius; Preheating section: heating to 500 degrees Celsius, equivalent residence time 60 seconds; High-temperature brazing section: 1095 degrees Celsius, equivalent residence time 12 seconds; Cooling: Cool to below 800 degrees Celsius under protective atmosphere, then forced air cooling to below 200 degrees Celsius; External corrosion protection system: 10-micron zinc-nickel alloy + approximately 100 nanometer zirconium-based conversion layer + 35-micron polyamide-12 coating (consistent across all groups). Testing and experimental methods: Weld density / porosity: 20 cross sections were randomly selected from each group (sampled dispersedly along the length), and metallographic and image analysis was used to obtain the effective metallurgical bonding area ratio and interface porosity area fraction of the lap area.

[0062] Pressure holding leakage: 20 MPa pressure holding for 60 seconds, n=200 pieces, statistical pass rate. Helium mass spectrometry leakage: n=20 pieces, record the maximum leakage rate. Online eddy current detection rate: 300 artificially implanted defect points (random location, defects near the lap weld area) with an equivalent unwelded defect area of ​​approximately 0.5 square millimeters on each group of pipes, statistically analyze the online eddy current detection rate. Online eddy current false alarm rate: Statistically analyze the percentage of lengths deemed unqualified on 2000 consecutive meters of pipe confirmed to have no implanted defects. Pressure pulse fatigue: 0–20 MPa, 5 Hz, n=10 pieces, record the number of cycles for the first leakage (take the minimum value). Burst pressure: n=10 pieces, record the average burst pressure. Neutral salt spray to first red rust: n=10 pieces, take the median number of hours. Table 1. Key formulation and process differences among the five reference test cases (parametric definitions)

[0063]

[0064] Note: In the table, 8.0 μm / 8.0 μm represents the thickness of the main copper layer on the first surface / second surface. In the invention example, the first surface has a local solder-free window (2.8 mm wide), and the copper residue within the window is controlled to be no higher than 0.10 μm. Table 2 Weld density, sealing performance, and online inspectability (test data)

[0065]

[0066] Data Trend Interpretation Comparative Examples 1 and 2: Without a window-triggered self-verification chain, the detection rate of unbonded / micropores in the interlayer by online eddy current is limited, and leakage is more likely to occur during pressure holding and helium detection.

[0067] Comparative Example 3: The window and marker are present, but the lack of interface activation and wetting enhancement leads to poor stability across the window and an increase in actual defects; therefore, the detection rate is high, but false alarms and scrapping increase significantly.

[0068] Comparative Example 4: The interface is cleaner and wetting is more stable, and the welding quality is significantly improved, but there is still a lack of a marking mechanism that ensures that defects are always detectable; the detection rate is still limited in the case of implanted defects.

[0069] Example of invention: The weld is denser and the signal separation of defects at the detection end is stronger, so the detection rate and false alarm rate achieve a more ideal combination. Table 3 Mechanical durability and corrosion resistance (test data)

[0070]

[0071] The weld density and sealing performance indicators (metallurgical bonding area ratio, porosity fraction, pressure holding / helium detection) are mainly affected by the cleanliness of the interlayer interface, the wetting and capillary filling continuity of the molten solder, and the stability of thermal cycling. The more stable the interface state, the closer the minimum bonding area of ​​the overlap zone is to the average value, the lower the porosity, and the lower the maximum leakage rate of helium detection and the failure rate of pressure holding. Online eddy current detection rate and false alarm rate: These reflect the separability of defect signals from background disturbances. Background disturbances mainly come from micro-fluctuations in wall thickness and roundness, non-uniformity of material magnetic / conductivity along the length direction, and changes in coating / surface state. When the defect signal lacks stable characteristics, an increase in detection rate is often accompanied by an increase in false alarms (threshold sensitivity). Only when the defect signal has lockable stable characteristics can high detection and low false alarms be achieved simultaneously.

[0072] Fatigue and salt spray trends: highly correlated with the presence of propagable micropores / microchannels in the interlayer: interlayer microchannels are both the starting point for crack initiation and propagation under pulse pressure and the channel for salt water / electrolytes to enter the interlayer and form crevice corrosion. Therefore, the denser the weld interface, the higher the fatigue life and corrosion resistance time (even if the external anti-corrosion system is the same). Trends and reasons for differences in each group

[0073] Refer to Example 1 (Comparative Example 1: Conventional Double-Wall Welded Pipe) Welding and Sealing: The average metallurgical bonding area ratio is around 99%, but the minimum is significantly lower (around 97%), and the porosity fraction is relatively high (0.35%), indicating a probability of local wetting interruptions or incomplete filling in continuous production. This is related to the pressure holding pass rate being lower than the ideal value (96.5%) and the maximum helium leak rate being 10⁻⁻⁶. 6 The magnitudes match.

[0074] Detectability: The detection rate of implanted defects by online eddy current (86%) reflects the typical boundary that embedded defect signals are easily masked by background disturbances; the false alarm rate (0.55%) is a compromise result under the conventional threshold.

[0075] Durability and salt spray: The fatigue and salt spray performance are moderate, consistent with the expected mechanism of gradual amplification of local microchannels under load and corrosive media.

[0076] Refer to Example 2 (Comparative Example 2: Only magnetic markings, no solder window) Limited improvement in detection rate, but an increase in false alarms: This group has structural markings, but due to the lack of a solder-free window constraint, the marking layer is more easily and unconditionally hidden by the molten solder or local copper flow during heating, thus weakening the correspondence between the marking signal and whether the soldering is in place. The results show that the detection rate only slightly improved (87%), but the false alarm rate increased (0.70%), indicating that the local electromagnetic inhomogeneity caused by the markings increases the background complexity but does not form a stable defect criterion.

[0077] Welding and durability are not significantly improved: metallurgical bonding, porosity, pressure holding / helium testing are similar to or even slightly worse than those in Comparative Example 1, indicating that this comparison does not change the essential process of interface wetting and filling, and cannot significantly reduce the probability of real defects. Fatigue and salt spray also decrease slightly.

[0078] Refer to Example 3 (Comparative Example 3: with or without solder window + marking, but without interface activation and wetting enhancement) High detection rate but significantly increased false alarms and scrap: Due to the presence of a no-solder window, whether the mark can be hidden depends to a greater extent on whether the molten solder on the opposite side crosses the window and achieves continuous wetting. In the absence of interface activation and wetting enhancement, crossing the window is more sensitive to interlayer cleanliness, oxide film, and micro-gap fluctuations, and is prone to boundary states: local insufficient wetting, discontinuous filling, or formation of micropores.

[0079] For implantation defects, the marker layer is more likely to be preserved and the defect signal is more prominent, thus significantly improving the detection rate (98.5%).

[0080] However, due to process fluctuations, residual signals that are not completely eliminated may appear in some areas, making it more difficult to take into account the online judgment criteria, resulting in a significant increase in the false alarm rate (1.80%).

[0081] The actual welding quality deteriorated: the average and minimum metallurgical bonding area ratio decreased (minimum to 94.8%), the porosity increased (0.60%), and the pressure holding pass rate and the maximum leakage rate of helium detection worsened, indicating that the failure of window crossing was not just a detection phenomenon, but brought about a real sealing defect.

[0082] Durability and salt spray performance deteriorated significantly: the number of fatigue cycles decreased dramatically and the time from salt spray to red rust was shortened, which is consistent with the amplification effect of interlayer microchannels providing pathways for pressure fatigue and crevice corrosion.

[0083] Refer to Example 4 (Comparative Example 4: with interface activation and enhanced wetting, but no marker chains) Significant improvements in welding and sealing: This group improved interface activation and wetting consistency, resulting in an overall increase in the metallurgical bonding area ratio, with the minimum value approaching the average (99.10%), a decrease in porosity (0.12%), and significant optimization of the pressure holding pass rate and the maximum leakage rate during helium detection. This indicates that the sensitivity of process fluctuations to welding has been reduced, and the actual defect incidence rate has decreased.

[0084] The reason for the lack of significant improvement in detectability is that the detection rate of implanted defects is only 88%, which is close to that of Comparative Example 1. This is not contradictory, because the detection rate test uses artificially implanted defect points, and their presence or absence is not affected by improvements in the welding process; when the defect signal still mainly depends on the amplitude difference of conventional eddy currents, it will still be limited by background disturbances and threshold settings.

[0085] Decreased false alarm rate: The false alarm rate decreased to 0.40%, reflecting that improved product consistency has made the background noise more stable and the threshold can be more conservative, thereby reducing non-defect triggers.

[0086] Increased durability and salt spray resistance: The denser interface reduces the probability of electrolytes entering the interlayer and fatigue crack initiation, thus significantly improving fatigue life and salt spray time.

[0087] Refer to Example 5 (Invention Example: Interface Activation + Window + Periodic Marking + Wetting Enhancement) The most stable weld and the best defect readability: The invention further improves weld quality (average metallurgical bonding 99.88%, minimum 99.70%, porosity 0.05%), achieves 100% pressure holding pass rate, and the maximum helium leak rate reaches 10⁻. 7 The magnitude is reflected in a lower actual defect rate and a higher sealing margin.

[0088] The simultaneous optimization of detection rate and false alarm rate is achieved because: window constraints establish a stronger causal relationship between whether the marker disappears and whether the molten solder on the opposite side actually crosses and wets the surface; interface activation and enhanced wetting reduce the number of boundary states, ensuring that marker residue mainly corresponds to real defects rather than process fluctuations. This ultimately creates conditions where defect signals and background disturbances are separable, resulting in a 99.7% detection rate for implanted defects while reducing the false alarm rate to 0.25%.

[0089] Optimal durability and salt spray performance: interlayer channels are more effectively eliminated, no leakage was observed during the fatigue test within the cutoff period, and the time from salt spray to red rust is further extended, which is consistent with the expected mechanism of dense interface reducing media intrusion and crack propagation path.

[0090] Those skilled in the art will recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application.

[0091] Those skilled in the art will understand that, for the sake of convenience and brevity, the specific working processes of the systems, devices, and units described above can be referred to the corresponding processes in the foregoing method embodiments, and will not be repeated here.

[0092] In the several embodiments provided in this application, it should be understood that the disclosed systems, apparatuses, and methods can be implemented in other ways. For example, the apparatus embodiments described above are merely illustrative; for instance, the division of units is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be through some interfaces; the indirect coupling or communication connection between apparatuses or units may be electrical, mechanical, or other forms.

[0093] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0094] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A high-bonding density, leak-proof double-wall rolled-welded brake hose, characterized in that: include, The double-walled tube is formed by rolling low-carbon steel strip and has an axially extending overlap area. The first surface of the strip has a continuously arranged local brazing-free window in the area forming the interface of the overlap area. The residual thickness of the main copper layer in the local brazing-free window is no more than 0.30 micrometers. A magnetic marking layer is provided in the local brazing-free window. The magnetic marking layer is a periodic strip of nickel-iron alloy, and the strip direction is perpendicular to the tube axis. The second surface of the strip has a main copper layer. The overlap area is formed by melting and brazing the main copper layer.

2. The high-bonding density, leak-proof double-wall rolled-welded brake hose according to claim 1, characterized in that: The partial solderless window is located inside the strip edge, with a continuous copper-plated edge strip remaining on the strip edge. The width of the copper-plated edge strip is 0.30 to 0.80 mm. The starting distance of the partial solderless window from the strip edge is 0.30 to 0.80 mm, and the width of the partial solderless window is 2.0 to 3.5 mm.

3. The high-bonding density, leak-proof double-wall rolled-welded brake hose according to claim 1, characterized in that: In nickel-iron alloys, nickel accounts for 78% to 82% by mass and iron accounts for 18% to 22% by mass.

4. The high-bonding density, leak-proof double-wall rolled-welded brake hose according to claim 2, characterized in that: The width of a single strip in a periodic strip ranges from 0.25 to 0.60 mm, the center-to-center distance between strips ranges from 0.80 to 1.50 mm, the strip coverage ranges from 20% to 45%, and the strip thickness ranges from 0.10 to 0.25 micrometers.

5. A method for preparing a high-bonding density, leak-proof double-wall rolled-welded brake hose, characterized in that: include: Degreasing, pickling, and activation of low-carbon steel strip; A peelable masking coating is applied to the first surface, and copper electroplating is performed on both sides of the strip to form a main copper layer. The masking coating is removed to obtain a local solderless window, and the residual thickness of the main copper layer in the window is no more than 0.30 micrometers. Periodic strip openings are formed in the local solderless window, and a nickel-iron alloy is electrodeposited to obtain a magnetic marking layer. The anti-plating layer is removed. The strip is rolled to form an overlap area and heated under a protective atmosphere to melt the main copper layer. The overlap area is brazed and sealed to form a double-walled tube.

6. The method for preparing the leak-proof double-wall rolled-welded brake hose according to claim 5, characterized in that: The copper plating is acidic sulfate copper plating, and the thickness of the main copper layer formed is 6 to 10 micrometers on one side. After removing the masking coating, the residual thickness of the main copper layer in the local solderless window is no more than 0.10 micrometers.

7. The method for preparing the leak-proof double-wall rolled-welded brake hose according to claim 6, characterized in that: Nickel-iron alloy electrodeposition uses a sulfate complex system electroplating solution, which includes 300 g / L nickel sulfate, 25 g / L ferrous sulfate, 30 g / L boric acid, 70 g / L sodium citrate, and 2 g / L saccharin. The bath temperature is 50 degrees Celsius, the solution pH is 3.6, the cathode current density is 10 to 30 amperes per square decimeter, and the deposition time is 2 to 10 seconds to obtain a magnetic marking layer with a thickness of 0.10 to 0.25 micrometers.

8. The method for preparing the leak-proof double-wall rolled-welded brake hose according to claim 7, characterized in that: Before rolling, a nickel flash plating layer with a thickness of 20 to 60 nanometers is formed on the main copper layer of the second surface, and a tin flash plating layer with a thickness of 30 to 80 nanometers is formed on the nickel flash plating layer.

9. The method for preparing the leak-proof double-wall rolled-welded brake hose according to claim 8, characterized in that: Before rolling, an interface-activating film is orientedly coated onto the overlapping strip area on both sides. The solid formulation of the interface-activating film, by weight, includes 45 to 70 parts of ammonium formate, 8 to 20 parts of ammonium citrate, 5 to 15 parts of citric acid, 5 to 15 parts of polyvinylpyrrolidone, 4 to 10 parts of polyethylene glycol with a number average molecular weight of 300 to 600, and 0.5 to 2 parts of alkyl glycoside nonionic surfactant. After coating, it is dried at 80 to 120 degrees Celsius for 30 to 90 seconds to make the dry film surface density 0.05 to 0.20 g / m².

10. The method for preparing the leak-proof double-wall rolled-welded brake hose according to claim 9, Brazing is carried out in a mixed reducing atmosphere of hydrogen and nitrogen, with a hydrogen integral of 15% to 30% and an atmosphere dew point not exceeding -35 degrees Celsius. The preheating section is heated to 500 degrees Celsius and held for 40 to 90 seconds, while the high-temperature section is heated to 1085 to 1105 degrees Celsius and held for 8 to 18 seconds. After brazing, online eddy current testing is performed using a magnetic saturation device and a low-frequency channel of 1 to 5 kHz to determine whether there are frequency domain characteristics corresponding to the center distance of the periodic strips.

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