A negative pressure laser brazing method for single-sided sealing of ceramic tube / metal plate

Abstract

The application belongs to the technical field of material welding, and particularly relates to a negative pressure laser brazing method for single-sided sealing of a ceramic tube / metal plate. Through the synergistic regulation and design among the surface micro-texture, negative pressure environment and laser heat field regulation during laser brazing, high-reliability connection is realized, the temperature gradient of the connection interface is effectively reduced, and through accurate regulation of parameters, the interface wettability, thermal stress and uniformity of the structure are synergistically improved. In addition, the ceramic tube / metal plate composite obtained by the laser brazing method is suitable for high-temperature and high-pressure environments, and can be applied to the manufacturing of sealing structures of nuclear power equipment, chemical reactors and solid oxide fuel cells.

Description

Technical Field

[0001] This invention belongs to the technical field of material welding, specifically relating to a negative pressure laser brazing method for single-sided sealing of ceramic tubes / metal plates. Background Technology

[0002] Ceramic / metal composite structures, combining the high hardness and high-temperature stability of ceramics with the high toughness and thermal conductivity of metals, have been widely used in high-end fields such as aerospace, energy equipment, and electronic packaging. Among them, the ceramic tube-metal plate overlap structure is a typical heterogeneous sealing form and a key component of high-temperature and high-pressure sealing assemblies (such as heat exchangers, microreactors, and nuclear reactor core shells).

[0003] However, due to the differences in thermal properties of materials and the complexity of the structure, this type of lap joint structure faces the following core challenges in the traditional brazing process: (1) Conventional brazing uses a ring-shaped filler metal filling method. Due to the limitations of gravity direction and wetting path, the filler metal is prone to generating discontinuous filling areas at the bottom of the interface, resulting in local non-welding defects. Under high pressure or vacuum service environment, leakage channels are easily formed, making it difficult to meet the airtightness requirements.

[0004] (2) The thermal expansion coefficients of ceramics and metals differ significantly. Especially in tube-plate lap joint structures, the stress superposition effect is formed in the cylindrical insertion area, which makes the thermal stress concentration coefficient at the interface higher, which easily induces crack initiation and propagation, seriously threatening the reliability of the connection.

[0005] (3) The tube-plate structure of ceramic tube / metal plate has a three-dimensional complex morphology. Traditional laser brazing heating method is difficult to achieve precise control of local thermal field, which easily forms uncontrollable temperature gradient and non-uniform melting area, further aggravating wetting barrier, non-uniform growth of IMC and structural deformation risk.

[0006] To address the aforementioned bottlenecks, there is an urgent need to develop a novel laser brazing method suitable for complex lap joint structures, which takes into account the continuity of brazing filler metal wetting, the ability to relieve interfacial stress, and the adaptability of thermal field control, so as to achieve a high-strength, high-airtightness, and reliable connection of ceramic tube-metal plate structures under extreme working conditions. Summary of the Invention

[0007] The purpose of this invention is to provide a negative pressure laser brazing method for single-sided sealing of ceramic tubes / metal plates, addressing bottleneck problems such as poor wettability, discontinuous interface filling, high and concentrated thermal stress leading to cracks, difficulty in microstructure control, and insufficient adaptability of the method structure during the connection process of ceramic tubes / metal plates.

[0008] By introducing a synergistic control design involving surface microtexture, negative pressure environment, and laser thermal field modulation during laser brazing, a highly reliable connection is achieved, effectively reducing the temperature gradient at the connection interface. Furthermore, precise parameter control enables a synergistic improvement in interface wettability, thermal stress, and microstructure uniformity. In addition, the ceramic tube / metal plate composite obtained by this laser brazing method is suitable for high-temperature and high-pressure environments and can be applied to the manufacture of sealing structures for nuclear energy equipment, chemical reactors, and solid oxide fuel cells.

[0009] The negative pressure environment effectively eliminates interfacial gases, enhancing capillary action and process stability; the gradient microtexture significantly increases the specific surface area and constructs adaptive capillary channels, optimizing solder wettability, filling continuity, and airtightness; the optimized laser scanning strategy, combined with local heating characteristics, enables precise and uniform control of the complex thermal field of the tube sheet joint. These three factors synergistically reduce the temperature gradient, promote uniform growth of the interfacial IMC layer, effectively suppress component deformation, and significantly alleviate thermal stress concentration and interfacial cracking tendency, thereby greatly improving the strength, airtightness, and service reliability of heterogeneous material joints.

[0010] The specific technical solution is as follows:

[0011] A negative pressure laser brazing method for single-sided sealing of ceramic tubes / metal plates includes the following steps:

[0012] (1) A gradient density microtexture consisting of concentric rings and radial straight lines is prepared on the surface to be joined at the end of the ceramic tube. The pattern of the surface microtexture is a special stress relief pattern design.

[0013] The fabrication of the aforementioned surface microtextures is primarily achieved through precision laser processing, using any type of laser, such as a nanosecond laser, picosecond laser, or femtosecond laser. Building upon traditional laser processing, the gaseous environment can be controlled, such as a negative pressure environment, an argon environment, or a nitrogen environment, to regulate the physicochemical properties of the processed surface. Alternatively, based on traditional laser processing, the liquid environment can be controlled, such as deionized water, organic solvents, acidic solutions, or alkaline solutions, to achieve synergistic regulation of surface morphology and surface function.

[0014] (2) The ceramic tube and the metal plate are assembled by an inverted overlapping method with the metal plate on top and the ceramic tube on the bottom to overcome the poor wetting behavior of the ceramic side caused by gravity; and a matching brazing filler metal of the same thickness is pre-placed in the 30~100μm overlap gap.

[0015] (3) Place the assembly under a negative pressure of 10 -2 Welding is performed in a chamber with a pressure of ~100Pa using a laser at a power of 2000~4000W and a welding rate of 1.5~3mm / min.

[0016] The laser can be any type of fiber laser, semiconductor laser, or CO2 laser. The core diameter of the fiber is 400 μm. The beam energy characteristics of the laser can be any type of Gaussian laser, rectangular laser, multi-beam laser, or tunable ring-core mode laser.

[0017] The concentric circle scanning path (number of channels, order, start and end point offset) and positive defocusing amount (+10~+30mm) are optimized based on the matching of tube wall thickness and spot size. The ratio of the laser head's collimation distance to its focusing distance is not less than 3, and the spot diameter can be estimated by multiplying the fiber core diameter by the ratio of the collimation distance to the focusing distance. Spot diameter ≈ fiber core diameter × (focal length of focusing lens / focal length of collimating lens)

[0018] The laser welding path described uses a concentric circle scanning path, and the number of concentric circle scanning paths is adjusted according to the thickness of the ceramic tube wall.

[0019] When the pipe wall thickness is less than the diameter of the laser spot, a circular path scanning welding method is used;

[0020] When the pipe wall thickness exceeds the spot diameter but is less than or equal to twice the spot diameter, two concentric circular paths are used for welding. The outer concentric circle is welded first, followed by the inner concentric circle, and the starting points of the two welds are offset by 180°.

[0021] When the pipe wall thickness is greater than or equal to three times the spot diameter, welding is performed using three or more scanning paths. The number of paths, 'a', is the ratio of the pipe wall thickness to the spot diameter, rounded up. Welding is performed in the order of first the middle, then the outside, and finally the inside. The starting point of adjacent paths is rotated 360 / a° clockwise.

[0022] The parameters are set as follows: laser power of 2000~4000W, welding speed of 1.5~3mm / min, positive defocus welding, and defocusing amount of +10~+30mm. The values ​​of power, welding speed, and defocusing amount are adjusted according to the type and thickness of the selected metal material and brazing filler metal to ensure that the brazing filler metal is fully melted and the metal plate is not laser-welded through.

[0023] The aforementioned negative pressure environment is primarily achieved through a negative pressure chamber. By using a vacuum pump to extract some of the gas inside the chamber, the pressure drops below standard atmospheric pressure, creating a negative pressure environment. A vacuum gauge is installed to display the ambient pressure inside the chamber in real time. The top of the negative pressure chamber is equipped with a high-transmittance optical window, with a transmittance of no less than 99.9%. The optical window is circular with a diameter of 50-100 mm and a thickness of no less than 3 mm.

[0024] In the present invention, in the negative pressure laser brazing method for single-sided sealing of ceramic tube / metal plate, the ceramic tube in step (1) has a wall thickness of 1-5mm and a maximum diameter of 5-30mm.

[0025] In the present invention, in the negative pressure laser brazing method for single-sided sealing of ceramic tube / metal plate, the number of radially distributed straight lines of gradient density microtexture in step (1) remains unchanged from the outer wall to the inner wall when 1mm≤ceramic tube wall thickness<2mm.

[0026] When 2mm≤ceramic tube wall thickness≤5mm, the number of radially distributed straight lines of gradient density microtexture gradually decreases from the outer wall to the center of the tube wall thickness, while the number of radially distributed straight lines gradually increases from the center of the tube wall thickness to the inner wall.

[0027] In the present invention, in the negative pressure laser brazing method for single-sided sealing of ceramic tube / metal plate, in step (1), when 1mm≤ceramic tube wall thickness≤2mm, the concentric rings of gradient density microtexture are arranged at equal intervals with a spacing of 0.1~0.3mm, a groove depth of 30~80μm, and a groove width of 30~60μm.

[0028] Its radially distributed straight lines are evenly spaced between any adjacent rings, and the straight lines on the inner side of the rings are not continuous with the straight lines on the outer side. The minimum distance between the straight lines is 0.06~0.2mm, the groove depth is 30~50μm, and the groove width is 30~50μm.

[0029] In the present invention, in the negative pressure laser brazing method for single-sided sealing of ceramic tube / metal plate, in step (1), when 2mm < ceramic tube wall thickness ≤ 5mm, the concentric rings of gradient density microtexture are arranged with non-equal spacing. Near the outer 1 / 3 and inner 1 / 3 of the thickness, the spacing is 0.1~0.2mm, the groove depth is 30~100μm, and the groove width is 30~60μm; the spacing of the lines at the middle position is 0.2~0.3mm, the groove depth is 30~80μm, and the groove width is 30~60μm.

[0030] The radially distributed straight lines are evenly spaced between any adjacent rings, and the straight lines on the inner side of the rings are discontinuous with the straight lines on the outer side. Near the outer 1 / 3 and inner 1 / 3 of the thickness, the minimum distance between the straight lines is 0.06~0.1mm, the groove depth is 30~80μm, and the groove width is 30~50μm; in the middle position, the minimum distance between the straight lines is 0.1~0.2mm, the groove depth is 30~50μm, and the groove width is 30~50μm.

[0031] In this invention, the negative pressure laser brazing method for single-sided sealing of ceramic tube / metal plate is described. The ceramic tube in step (1) is made of any one of oxide ceramics (such as Al2O3, ZrO2), carbide ceramics (such as SiC), or nitride ceramics (such as Si3N4, AlN), including transparent or non-transparent ceramics.

[0032] In this invention, the negative pressure laser brazing method for single-sided sealing of ceramic tubes / metal plates can be any one of the following in step (2): carbon steel, stainless steel, titanium alloy, Kovar alloy, etc., and the thickness of the area to be connected is 2-5mm. The shape of the metal plate can be circular, square, rectangular, or irregularly shaped with a special structural design.

[0033] When the thickness of the metal plate to be joined is less than 3mm and 2mm, the negative pressure of welding is 1~100Pa, the laser power is 3000~4000W, and the welding speed is 1.5~2mm / min.

[0034] When the thickness of the metal sheet to be joined is 3mm or less and the thickness of the area to be joined is 5mm or less, the negative pressure of welding is 10. -2 ~1Pa, laser power of 2000~4000W, welding speed of 2~3mm / min.

[0035] In the present invention, in the negative pressure laser brazing method for single-sided sealing of ceramic tube / metal plate, the melting point of the brazing filler metal in step (1) is at least 150°C lower than the melting point of the metal.

[0036] Preferably, the solder can be any one of silver-based solder, copper-based solder, or tin-based solder.

[0037] Preferably, the solder can be in powder, foil, or ring form.

[0038] The beneficial effects of this invention are as follows: The core innovation of this invention lies in the synergistic integration of three major technical elements: negative pressure environment regulation, ceramic tube surface microtexture design, and precise thermal field control of laser brazing.

[0039] First, by using laser irradiation to melt the brazing filler metal through heat conduction on the metal side, the local overheating and violent interfacial reaction caused by direct laser irradiation on the brazing filler metal are effectively avoided. On this basis, the negative pressure environment actively and effectively eliminates the interfacial gap gas, significantly reduces the flow resistance of the brazing filler metal, enhances capillary action, strengthens capillary driving force, and ensures the high stability of the energy transfer process and the uniform and controllable morphology of the interfacial compound during the welding process.

[0040] Secondly, the innovative gradient microtexture (a combination of concentric rings and radial straight lines) constructs an adaptive capillary channel, which significantly increases the specific surface area, comprehensively optimizes and improves the wettability and filling continuity of the solder, and ensures uniform, dense, and near-defect-free high airtightness filling of the annular interface, meeting the high airtightness requirements.

[0041] Finally, based on the matching of tube wall thickness and laser spot characteristics, the laser scanning path (number of passes, sequence, and start / endpoint offset) was optimized, achieving precise and uniform control of the local thermal field in complex tube-to-plate joints. This effectively induced the formation of a stable, continuous, and moderately fluid laser-grown molten pool. Through precise control of the molten pool morphology, flow characteristics, and solidification process, this technical approach ensured a high degree of controllability in the brazing filler metal melting process, simultaneously reducing the temperature gradient, promoting uniform growth of the intermetallic compound (IMC) layer, and effectively suppressing component deformation. The stable maintenance and controllable solidification of the molten pool further significantly alleviated thermal stress concentration and cracking tendency at the ceramic / metal interface, thereby comprehensively improving connection reliability and enhancing the overall service performance and hermetic integrity of the welded structure. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of a negative pressure laser brazing method for single-sided sealing of ceramic-metal tube sheets according to the present invention.

[0043] Figure 2 This is a cross-sectional schematic diagram of the connection area of ​​a negative pressure laser brazing method for single-sided sealing of ceramic-metal tube sheets, as described in this invention.

[0044] Figure 3 This is an effect diagram of the microtextured pattern on the end surface of a ceramic tube according to Embodiment 1 of the present invention.

[0045] Figure 4 This is an illustration of the effect of microtextured pattern processing on the end surface of a ceramic tube according to Embodiment 2 of the present invention.

[0046] Figure 5 This is an effect diagram of the microtextured pattern on the end surface of the ceramic tube in Embodiment 3 of the present invention.

[0047] In the figure, 1 is a ceramic tube, 2 is a metal plate, 3 is a molten pool on the metal side, 4 is a laser, 5 is a negative pressure chamber, 6 is brazing filler metal, 101 is the microtexture on the ceramic side surface, 102 is the untextured ceramic part, 301 is the start point of the first weld, 302 is the end point of the first weld, 303 is the direction of the first weld, 304 is the start point of the second weld, 305 is the end point of the second weld, 306 is the direction of the second weld, 307 is the start point of the third weld, 308 is the end point of the third weld, and 309 is the direction of the third weld. Detailed Implementation

[0048] The technical solution of the present invention will now be described in detail with reference to the accompanying drawings.

[0049] The specific welding process is as follows: after processing a special patterned ceramic side surface microtexture 101 at the end of the ceramic tube 1, the metal plate 2 and the ceramic tube 1 are assembled with the metal plate 2 on top and the ceramic tube 1 on the bottom.

[0050] The brazing filler metal 6 is placed in the overlap gap between the ceramic tube 1 and the metal plate 2 to complete the assembly.

[0051] Place the assembly into the negative pressure chamber 5, turn on the air pump to evacuate the chamber, and monitor the gas pressure inside the chamber using a vacuum gauge until the required negative pressure environment is reached, then stop evacuating.

[0052] Adjust the laser head of laser 4 to be directly above the assembly, complete the matching welding path programming design according to the thickness of ceramic tube 1, and set reasonable welding parameters.

[0053] Then, the laser 4 is turned on, and the laser shines through the optical window at the top of the negative pressure chamber 5 to irradiate the surface of the workpiece to be welded, thus completing the welding. A metal side molten pool 3 is formed on the side of the metal plate 2, and the brazing filler metal 6 is melted through heat conduction to achieve the connection between the metal plate 2 and the ceramic tube 1.

[0054] Example 1

[0055] This embodiment provides a negative pressure laser brazing method for single-sided sealing of ceramic-metal tube sheets. Based on actual working requirements, the metal sheets to be connected are stainless steel circular plates with a diameter of 5mm and a thickness of 2mm.

[0056] The ceramic tube to be connected is an alumina ceramic tube with an outer diameter of 5mm, an inner diameter of 3mm, and a wall thickness of 1mm.

[0057] The brazing filler metal used is silver-based AgCu28Ti2 alloy foil. The aim is to achieve a single-sided sealing connection between a stainless steel circular plate and an alumina ceramic tube.

[0058] Specifically, the following steps are included:

[0059] (1) Prepare surface microtexture on the surface to be connected at the end of the ceramic tube.

[0060] The laser selected can be a femtosecond laser.

[0061] The surface microtexture pattern is designed as a special stress relief pattern, mainly composed of concentric rings and radially distributed straight lines.

[0062] The number of radially distributed straight lines remains constant from the outer wall to the inner wall.

[0063] Its concentric circles are arranged at equal intervals of 0.1 mm, the groove depth is 30 μm, and the groove width is 30 μm.

[0064] Its radially distributed straight lines are equally spaced between any adjacent rings, and the straight lines on the inner side of the rings are not continuous with the straight lines on the outer side. The minimum distance between the straight lines is 0.1 mm, the groove depth is 30 μm, and the groove width is 30 μm.

[0065] (2) Assemble the ceramic tube and the metal plate by overlapping, and place the brazing filler metal in the overlap gap.

[0066] An inverted overlapping structure with a metal sheet on top and a ceramic tube on the bottom is used for assembly to overcome poor ceramic side wetting behavior caused by gravity.

[0067] A solder ring with a thickness of 50μm and the same size as the end face of the metal tube is used and placed in the overlap gap between the metal and ceramic.

[0068] (3) Place the assembly in the negative pressure environment of the construction, adjust the laser to the appropriate position, complete the welding path and parameter settings, and perform welding.

[0069] The negative pressure environment is mainly achieved through a negative pressure chamber. An air pump is activated to evacuate the chamber, lowering the internal gas pressure below standard atmospheric pressure to create a negative pressure environment. During this process, a vacuum gauge monitors the internal pressure in real time, stopping the evacuation once the pressure reaches 100 Pa.

[0070] The top of its negative pressure chamber is equipped with a high-transmittance optical window with a transmittance of not less than 99.9%. The optical window is a circle with a diameter of 50 mm and a thickness of 3 mm.

[0071] The laser selected is a fiber laser with a fiber core diameter of 400μm. The ratio of the laser head collimation distance to the focusing distance is 3, and its spot energy characteristics are Gaussian. Its spot diameter is 1.2mm.

[0072] The welding path is set as a concentric circle scanning path. Welding is performed using a single circular path scan, with the laser power set to 2350W, the welding speed set to 2mm / min, and positive defocus welding used with a defocusing amount of +10mm.

[0073] A helium leak test was performed on the sealed tube sheet structure, and its gas leakage rate was 0.72 × 10⁻⁶. -6 Pa·m 3 / s, meeting the airtightness requirements, thus demonstrating that the application of multi-layered concentric micro-texture and negative pressure environment effectively ensures the bonding rate of the interface.

[0074] Furthermore, destructive cross-sectional analysis showed no obvious cracks at the interface, indicating that the interface stress was effectively alleviated by using precise control of laser energy and synergistic regulation of microtexture pattern design during negative pressure laser brazing.

[0075] Example 2

[0076] This embodiment provides a negative pressure laser brazing method for single-sided sealing of ceramic metal tube sheets. According to actual working conditions, the metal to be connected is a stainless steel round plate with a diameter of 40mm and a thickness of 3mm.

[0077] The ceramic to be connected is an alumina ceramic tube with an outer diameter of 15mm, an inner diameter of 6mm, and a wall thickness of 4.8mm.

[0078] The aim is to achieve single-sided sealing between stainless steel circular plates and alumina ceramic tubes.

[0079] Specifically, the following steps are included:

[0080] (1) Prepare surface microtexture on the surface to be connected at the end of the ceramic tube.

[0081] The laser selected can be a femtosecond laser.

[0082] The surface microtexture pattern is designed as a special stress-relieving pattern, mainly composed of concentric rings and radially distributed straight lines. The number of radially distributed straight lines gradually decreases from the outer wall to the center of the pipe wall thickness, while the number of radially distributed straight lines gradually increases from the center of the pipe wall thickness to the inner wall.

[0083] Its concentric rings are arranged at non-equal intervals. The spacing is 0.1 mm near the outer 1.5 mm and inner 1.5 mm thickness, while the spacing is 0.3 mm at other positions. The groove depth is 50 μm and the groove width is 30 μm.

[0084] The radially distributed straight lines are evenly spaced between any adjacent rings, and the straight lines on the inner side of the rings are discontinuous with those on the outer side. The minimum distance between the straight lines is 0.1 mm near the outer 1.6 mm and inner 1.6 mm thickness, and the minimum distance between the straight lines is 0.2 mm in the middle position. The groove depth is 30 μm, and the groove width is 30 μm.

[0085] (2) Assemble the ceramic tube and the metal plate by overlapping, and place the brazing filler metal in the overlap gap.

[0086] An inverted overlapping structure with a metal sheet on top and a ceramic tube on the bottom is used for assembly to overcome poor ceramic side wetting behavior caused by gravity.

[0087] A paste-like brazing filler metal of the same size as the end face of the ceramic tube and a thickness of 60μm is placed in the overlap gap between the metal plate and the ceramic tube.

[0088] (3) Place the assembled sample to be welded in the constructed negative pressure environment, adjust the laser to the appropriate position, complete the welding path and parameter settings, and perform welding.

[0089] The negative pressure environment is mainly achieved through a negative pressure chamber. An air pump is activated to evacuate the chamber, lowering the internal gas pressure below standard atmospheric pressure to create a negative pressure environment. During this process, a vacuum gauge monitors the internal pressure in real time, stopping the evacuation once the pressure reaches 0.1 Pa.

[0090] The top of its negative pressure chamber is equipped with a high-transmittance optical window with a transmittance of not less than 99.9%. The optical window is a circle with a diameter of 50 mm and a thickness of 5 mm.

[0091] The laser selected is a fiber laser with a fiber core diameter of 400μm. The ratio of the laser head collimation distance to the focusing distance is 4, and its spot energy characteristics are Gaussian. Its spot diameter is 1.6mm.

[0092] The welding path is set as a concentric circle scanning path. Three circular paths are used for welding, in the order of first the middle, then the outside, and finally the inside. The starting point of adjacent passes is rotated 120° clockwise. The laser power is set to 2750W, the welding speed is 2.5mm / min, and positive defocus welding is used with a defocusing amount of +15mm.

[0093] A helium leak test was performed on the sealed tube sheet structure, and its gas leakage rate was 0.62 × 10⁻⁶. -6 Pa·m 3 / s, meeting the airtightness requirements, and destructive cross-sectional analysis showed no obvious cracks at the interface.

[0094] The following insights can be gained from comparing with Example 1: as the wall thickness of the ceramic tube increases, the pattern design of the microtexture needs to be adjusted. The pattern at the edge of the weld should be densified, and the pattern density in the middle of the weld should be reduced, so that the welding stress shifts from the edge to the middle, thereby reducing the greater risk of stress cracking due to the increased thickness.

[0095] In addition, due to the increase in the thickness of the metal sheet, the parameters of negative pressure, welding path, laser power, and defocusing amount during the welding process also need to be adjusted. Through more reasonable energy distribution control, the stress of the joint can be further relieved.

[0096] As the thickness of metal and ceramic materials increases, higher requirements are placed on the precise control of laser energy and the coordinated regulation of microtexture pattern design during the negative pressure laser brazing process. However, this method can still produce high-quality welds.

[0097] Example 3

[0098] This embodiment provides a negative pressure laser brazing method for single-sided sealing of ceramic metal tube sheets. According to actual working conditions, the metal to be connected is a stainless steel round plate with a diameter of 40mm and a thickness of 3mm.

[0099] The ceramic to be connected is a silicon carbide ceramic tube with an outer diameter of 15mm, an inner diameter of 6mm, and a wall thickness of 3mm.

[0100] The aim is to achieve a single-sided sealing between a stainless steel circular plate and a silicon carbide ceramic tube. Borosilicate glass is selected as the solder and is pre-placed at the interface between the ceramic tube and the metal plate.

[0101] Specifically, the following steps are included:

[0102] (1) Prepare surface microtexture on the surface to be connected at the end of the ceramic tube.

[0103] The laser selected can be a femtosecond laser.

[0104] The surface microtexture pattern is designed as a special stress-relieving pattern, mainly composed of concentric rings and radially distributed straight lines. The number of radially distributed straight lines gradually decreases from the outer wall to the center of the pipe wall thickness, while the number of radially distributed straight lines gradually increases from the center of the pipe wall thickness to the inner wall.

[0105] Its concentric rings are arranged at non-equal intervals. The spacing is 0.1 mm near the outer 1.0 mm and inner 1.0 mm thickness, while the spacing at other positions is 0.3 mm. The groove depth is 50 μm and the groove width is 30 μm.

[0106] The radially distributed straight lines are evenly spaced between any adjacent rings, and the straight lines on the inner side of the rings are discontinuous with those on the outer side. The minimum distance between the straight lines is 0.1 mm near the outer 1.0 mm and inner 1.0 mm thickness, and the minimum distance between the straight lines is 0.2 mm in the middle position. The groove depth is 30 μm, and the groove width is 30 μm.

[0107] (2) Assemble the ceramic tube and the metal plate by overlapping, and place the brazing filler metal in the overlap gap.

[0108] An inverted overlapping structure with a metal sheet on top and a ceramic tube on the bottom is used for assembly to overcome poor ceramic side wetting behavior caused by gravity.

[0109] A paste-like brazing filler metal of the same size as the end face of the ceramic tube and a thickness of 60μm is placed in the overlap gap between the metal and the ceramic.

[0110] (3) Place the assembled sample to be welded in the constructed negative pressure environment, adjust the laser to the appropriate position, complete the welding path and parameter settings, and perform welding.

[0111] The negative pressure environment is mainly achieved through a negative pressure chamber. An air pump is activated to evacuate the chamber, lowering the internal gas pressure below standard atmospheric pressure to create a negative pressure environment. During this process, a vacuum gauge monitors the internal pressure in real time, stopping the evacuation once the pressure reaches 0.1 Pa.

[0112] The top of its negative pressure chamber is equipped with a high-transmittance optical window with a transmittance of not less than 99.9%. The optical window is a circle with a diameter of 50 mm and a thickness of 5 mm.

[0113] The laser selected is a fiber laser with a fiber core diameter of 400μm. The ratio of the laser head collimation distance to the focusing distance is 4, and its spot energy characteristics are Gaussian. Its spot diameter is 1.6mm.

[0114] The welding path is set as a concentric circle scanning path. Two circular paths are used for scanning and welding, with the outer edge first and the inner edge last. The starting point of adjacent passes is rotated 180° clockwise. The laser power is set to 2950W, the welding speed is 2.5mm / min, and positive defocus welding is used with a defocusing amount of +15mm.

[0115] A helium leak test was performed on the sealed tube sheet structure, and the gas leakage rate was 0.67 × 10⁻⁶. -6 Pa·m 3 / s, meeting the airtightness requirements, and destructive cross-sectional analysis showed no obvious cracks at the interface.

[0116] Compared to Examples 1 and 2, Example 3 offers the following insights: the relative relationship between the ceramic tube wall thickness and the laser spot diameter is a key factor in determining the optimal welding path strategy. When the wall thickness is less than the spot diameter (as in Example 1), a single concentric circle path is sufficient to achieve complete sealing; when the wall thickness exceeds twice the spot diameter (as in Example 2), a three-path approach is required to ensure complete weld fusion and stress dispersion; and when the wall thickness is between these two (as in Example 3), the optimized two-path approach achieves the best balance between efficiency and stress while ensuring sealing quality. This reflects the core idea of ​​this method: optimizing the process by precisely matching the welding path with geometric features. Furthermore, as the ceramic wall thickness and material type change, the required laser power needs to be adjusted accordingly (increasing from 2350W in Example 1 to 2950W in Example 3), while maintaining consistency with core measures such as the negative pressure environment and inverted assembly. This demonstrates that this method, by stabilizing the core process framework and adapting key parameters, possesses strong adaptability and reliability in handling different sizes and material combinations, providing a systematic solution for the precision joining of heterogeneous materials.

[0117] Comparative Example 1

[0118] This comparative example provides a traditional laser brazing method for single-sided sealing of ceramic-metal tube sheets. The difference from Example 1 is that this comparative example does not provide a negative pressure environment, but rather a conventional atmospheric pressure environment under argon protection. Other welding materials, structures, and process parameters are the same.

[0119] Based on actual working conditions, the metal plate to be connected is a stainless steel round plate with a diameter of 5mm and a thickness of 2mm.

[0120] The ceramic tube to be connected is an alumina ceramic tube with an outer diameter of 5mm, an inner diameter of 3mm, and a wall thickness of 1mm.

[0121] The brazing filler metal used is silver-based AgCu28Ti2 alloy foil. The aim is to achieve a single-sided sealing connection between a stainless steel circular plate and an alumina ceramic tube.

[0122] Specifically, the following steps are included:

[0123] (1) Prepare surface microtexture on the surface to be connected at the end of the ceramic tube.

[0124] The laser selected can be a femtosecond laser.

[0125] The surface microtexture pattern is designed as a special stress relief pattern, mainly composed of concentric rings and radially distributed straight lines.

[0126] The number of radially distributed straight lines remains constant from the outer wall to the inner wall.

[0127] Its concentric circles are arranged at equal intervals of 0.1 mm, the groove depth is 30 μm, and the groove width is 30 μm.

[0128] Its radially distributed straight lines are equally spaced between any adjacent rings, and the straight lines on the inner side of the rings are not continuous with the straight lines on the outer side. The minimum distance between the straight lines is 0.1 mm, the groove depth is 30 μm, and the groove width is 30 μm.

[0129] (2) Assemble the ceramic tube and the metal plate by overlapping, and place the brazing filler metal in the overlap gap.

[0130] An inverted overlapping structure with a metal sheet on top and a ceramic tube on the bottom is used for assembly to overcome poor ceramic side wetting behavior caused by gravity.

[0131] A solder ring with a thickness of 50μm and the same size as the end face of the metal tube is used and placed in the overlap gap between the metal and ceramic.

[0132] (3) The assembled sample to be welded was placed in the same chamber as in Example 1, and argon gas was continuously introduced as a protective gas. During the process, the environmental pressure inside the chamber was monitored in real time by a vacuum gauge to keep the pressure inside the chamber at 1.01 × 10⁻⁶. 5Pa, adjust the laser to the appropriate position, complete the welding path and parameter settings, and then perform welding.

[0133] The top of its negative pressure chamber is equipped with a high-transmittance optical window with a transmittance of not less than 99.9%. The optical window is a circle with a diameter of 50 mm and a thickness of 5 mm.

[0134] A fiber laser is selected, equipped with a fiber core diameter of 400μm. The ratio of the laser head collimation distance to the focusing distance is 3, and its spot energy characteristics are Gaussian. Its spot diameter is approximately 1.2mm.

[0135] The welding path is set as a concentric circle scanning path. Welding is performed using a single circular path scan, with the laser power set to 2350W, the welding speed set to 2mm / min, and positive defocus welding used with a defocusing amount of +10mm.

[0136] Compared to Example 1, this comparative example uses a conventional laser brazing process under atmospheric argon protection, and the results show a significant difference in joint quality. Helium detection testing was performed on the sealed tube sheet structure, and its gas leakage rate was 3.41 × 10⁻⁶. -3 Pa·m 3 / s, which does not meet the airtightness requirements. Metallographic observation shows that there are localized unwelded areas and voids in the brazing seam area of ​​the joint. The brazing filler metal near the ceramic interface is discontinuous, and in some places the brazing filler metal is not melted or not fully filled into the microtextured grooves.

[0137] The above phenomena indicate that traditional laser brazing technology cannot create a uniform and stable thermal field at the interface, thus failing to achieve a reliable connection.

[0138] Comparative Example 2

[0139] The difference from Example 1 is that this comparative example provides a negative pressure laser brazing method for single-sided sealing of ceramic metal tube sheets. According to the actual working conditions, the metal plates to be connected are stainless steel round plates with a diameter of 5mm and a thickness of 2mm.

[0140] The ceramic tube to be connected is an alumina ceramic tube with an outer diameter of 5mm, an inner diameter of 3mm, and a wall thickness of 1mm. The surfaces to be connected are not treated with surface microtexturing.

[0141] The others are the same as in Example 1.

[0142] Compared to Example 1, this comparative example, without microtexturing the ceramic surfaces to be joined, used the same negative pressure laser brazing process, and the results showed that the joint interface structure and performance were significantly degraded.

[0143] Because the ceramic surface lacks the microtexture formed by concentric rings and radial straight lines designed in Example 1, the interface cannot achieve effective mechanical interlocking and stress relief. At the same time, the smooth ceramic surface reduces the wetting and spreading area and adhesion points of the solder, weakening the interfacial bonding strength.

[0144] Metallographic observation revealed that the brazing filler metal spread discontinuously and unevenly on the ceramic interface. The interface bonding line was smooth but contained localized unbonded areas. Radial microcrack initiation and propagation were more readily observed near the ceramic interface. Helium detection of the sealed tube sheet structure showed a gas leakage rate of 5.2 × 10⁻⁶. -5 Pa·m 3 / s, which does not meet the airtightness requirements.

[0145] The shear strength test result of the joint was only 29.9 MPa, which is significantly lower than that of Example 1. This fully demonstrates that, under negative pressure, prefabricating specific surface microtextures on the ceramic connection surface is an indispensable key process for increasing the bonding area, achieving mechanical locking, relieving welding stress, and ultimately obtaining a dense, high-strength ceramic-metal sealing joint.

[0146] Comparative Example 3

[0147] The difference from Example 2 is that the concentric circle scanning path in step (3) of this comparative example is set as follows: three circular paths are used for welding, and the welding is performed in the order of first the middle, then the outside, and finally the inside. The starting point of adjacent passes is at the same position on the joint, and there is no clockwise 120° deflection.

[0148] The others are the same as in Example 2.

[0149] Compared to Example 2, this comparative example did not deflect the starting points of adjacent passes in the welding path clockwise, resulting in severe heat accumulation in the overlapping areas. This caused excessive local thermal stress during the welding process and created a significant temperature gradient.

[0150] Test results show that the airtightness of the joint has deteriorated, with a helium leakage rate of 0.82 × 10⁻⁶. -5 Pa·m 3 Cross-sectional analysis further revealed that the intermetallic compound at the interface was significantly thickened near the overlapping region of the starting point, and defects such as microcracks and uneven solder distribution appeared.

[0151] This confirms that implementing a path strategy with the starting point deflected clockwise is crucial for uniform heat input, dispersing and mitigating welding stress, thereby obtaining a complete, reliable, and high-quality weld joint.

[0152] Comparative Example 4

[0153] The difference from Example 2 is that the concentric circle scanning path in step (3) is set as follows: three circular paths are used for scanning and welding, and the welding is carried out in the order of first the outer part, then the middle part, and finally the inner part. The starting point of adjacent passes is deflected 120° clockwise.

[0154] The others are the same as in Example 2.

[0155] Compared to Example 2, this comparative example only changed the order of the laser concentric circle scanning path, adopting a welding sequence of first the outer part, then the middle part, and finally the inner part. This adjustment led to an imbalance in heat distribution and stress evolution during the welding process. The outer region solidified first, constraining the subsequent shrinkage of the inner region, and introducing higher residual tensile stress at the interface. As a result, the joint airtightness decreased significantly, and the helium leak detection rate increased to 0.74 × 10⁻⁶. -5 Pa·m 3 / s, destructive cross-sectional analysis revealed microcracks at the interface, resulting in reduced bond strength. This confirms that a proper scanning sequence plays a crucial role in controlling the temperature gradient, promoting uniform stress release, and obtaining a dense and reliable sealing joint.

[0156] 1. The airtightness of ceramic tube / metal plate joints is tested using helium mass spectrometry leak detection. First, the component under test is connected to the leak detection system and evacuated. Then, helium gas is injected outside the joint. If a leak is found, helium gas enters the system and is detected by the mass spectrometer. The leak rate is calculated based on the signal intensity to assess whether its sealing performance meets the requirements.

[0157] 2. The joint strength was tested using a universal testing machine. The ceramic tube / metal plate joint specimen was clamped in the testing machine fixture, ensuring alignment. An axial tensile load was then applied at a constant rate until the joint failed.

[0158] Record the load-displacement curves throughout the process, and calculate the tensile strength of the weld based on the maximum load recorded during the test and the original cross-sectional area of ​​the weld.

[0159] The test results are shown in Table 1.

[0160] Table 1. Relevant indicators of the connection interface obtained from each embodiment and comparative example.

[0161]

Claims

1. A negative pressure laser brazing method for single-sided sealing of ceramic tubes / metal plates, characterized in that, Includes the following steps: (1) A gradient density microtexture consisting of concentric rings and radial straight lines is prepared on the surface to be connected at the end of the ceramic tube; (2) The ceramic tube and the metal plate are assembled by an inverted overlapping method with the metal plate on top and the ceramic tube on the bottom; and a matching brazing filler metal of the same thickness is pre-placed in the 30~100μm overlap gap; (3) Place the assembly in a chamber with a negative pressure of 10⁻² to 100 Pa and use a laser to weld it under the conditions of laser power of 2000 to 4000 W and welding rate of 1.5 to 3 mm / min. The laser welding path described herein employs a concentric circle scanning path, and the number of concentric circle scanning paths is adjusted according to the thickness of the ceramic tube wall. When the pipe wall thickness is less than the diameter of the laser spot, a circular path scanning welding method is used; When the pipe wall thickness exceeds the spot diameter but is less than or equal to twice the spot diameter, two concentric circular paths are used for welding. The outer concentric circle is welded first, followed by the inner concentric circle, and the starting points of the two welds are offset by 180°. When the pipe wall thickness is greater than or equal to three times the spot diameter, welding is performed using three or more scanning paths. The number of paths, 'a', is the ratio of the pipe wall thickness to the spot diameter, rounded up. Welding is performed in the order of first the middle, then the outside, and finally the inside. The starting point of adjacent paths is rotated 360 / a° clockwise.

2. The negative pressure laser brazing method for single-sided sealing of ceramic tubes / metal plates according to claim 1, characterized in that, In step (1), the wall thickness of the ceramic tube is 1-5mm and the maximum diameter is 5-30mm.

3. The negative pressure laser brazing method for single-sided sealing of ceramic tubes / metal plates according to claim 1, characterized in that, The number of radially distributed straight lines of the gradient density microtexture in step (1) remains unchanged from the outer wall to the inner wall when 1 mm ≤ ceramic tube wall thickness < 2 mm. When 2mm≤ceramic tube wall thickness≤5mm, the number of radially distributed straight lines of gradient density microtexture gradually decreases from the outer wall to the center of the tube wall thickness, while the number of radially distributed straight lines gradually increases from the center of the tube wall thickness to the inner wall.

4. The negative pressure laser brazing method for single-sided sealing of ceramic tubes / metal plates according to claim 1, characterized in that, In step (1), when 1mm ≤ ceramic tube wall thickness ≤ 2mm, the concentric rings of the gradient density microtexture are arranged at equal intervals with a spacing of 0.1~0.3mm, a groove depth of 30~80μm, and a groove width of 30~60μm. Its radially distributed straight lines are evenly spaced between any adjacent rings, and the straight lines on the inner side of the rings are not continuous with the straight lines on the outer side. The minimum distance between the straight lines is 0.06~0.2mm, the groove depth is 30~50μm, and the groove width is 30~50μm.

5. The negative pressure laser brazing method for single-sided sealing of ceramic tubes / metal plates according to claim 1, characterized in that, In step (1), when 2mm < ceramic tube wall thickness ≤ 5mm, the concentric rings of the gradient density microtexture are arranged with non-equidistant spacing. Near the outer 1 / 3 and inner 1 / 3 of the thickness, the spacing is 0.1~0.2mm, the groove depth is 30~100μm, and the groove width is 30~60μm; the spacing of the lines at the middle position is 0.2~0.3mm, the groove depth is 30~80μm, and the groove width is 30~60μm. The radially distributed straight lines are evenly spaced between any adjacent rings, and the straight lines on the inner side of the rings are discontinuous with the straight lines on the outer side. Near the outer 1 / 3 and inner 1 / 3 of the thickness, the minimum distance between the straight lines is 0.06~0.1mm, the groove depth is 30~80μm, and the groove width is 30~50μm; in the middle position, the minimum distance between the straight lines is 0.1~0.2mm, the groove depth is 30~50μm, and the groove width is 30~50μm.

6. The negative pressure laser brazing method for single-sided sealing of ceramic tubes / metal plates according to claim 1, characterized in that, The ceramic tube in step (1) is made of any one of oxide ceramics, carbide ceramics or nitride ceramics.

7. The negative pressure laser brazing method for single-sided sealing of ceramic tubes / metal plates according to claim 1, characterized in that, The metal sheet in step (2) is any one of carbon steel, stainless steel, titanium alloy or Kovar alloy.

8. The negative pressure laser brazing method for single-sided sealing of ceramic tubes / metal plates according to claim 1, characterized in that, The thickness of the area to be connected in the metal plate in step (2) is 2-5mm; When the thickness of the metal plate to be joined is less than 3mm and 2mm, the negative pressure of welding is 1~100Pa, the laser power is 3000~4000W, and the welding speed is 1.5~2mm / min. When the thickness of the metal plate to be joined is 3mm or less and the thickness of the area to be joined is 5mm or less, the negative pressure of welding is 10-2 to 1Pa, the laser power is 2000 to 4000W, and the welding speed is 2 to 3mm / min.

9. The negative pressure laser brazing method for single-sided sealing of ceramic tubes / metal plates according to claim 1, characterized in that, The melting point of the brazing filler metal in step (1) is at least 150°C lower than the melting point of the metal.

10. The negative pressure laser brazing method for single-sided sealing of ceramic tubes / metal plates according to claim 1, characterized in that, The solder in step (1) is any one of powder, foil or ring solder.

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