Metal heat pipe fitting and vacuum laser welding method thereof

Abstract

The present application relates to the technical field of metal welding, in particular to a metal heat pipe fitting and a vacuum laser welding method thereof, the metal heat pipe fitting comprises coaxially assembled first pipe fitting, second pipe fitting, solid plug and open hole plug, the open hole plug is provided with annular boss for laser arc striking, the first pipe fitting is inserted into the open hole plug and the end face is attached to the boss, and the second pipe fitting is provided with annular intermediate layer between the plug; the welding method places the pipe fitting in a vacuum chamber, evacuates the vacuum chamber, preheats each girth weld and then welds, makes laser arc striking act on the annular boss when welding the inserted weld, makes the intermediate layer melt to form transition layer when welding the butt joint, and keeps vacuum cooling after welding and then takes out; the present application solves the problems of inserted pipe fitting collapse and molybdenum pipe homogenous butt joint embrittlement and cracking.

Description

Technical Field

[0001] This invention relates to the field of metal welding technology, specifically to a metal heat pipe fitting and its vacuum laser welding method. Background Technology

[0002] Molybdenum and its alloys have a melting point of approximately 2620℃ and possess high-temperature strength, a low neutron absorption cross-section, and good corrosion resistance to alkali metals such as sodium and potassium. They are frequently chosen as sealing materials for heat pipe structures in nuclear energy, aerospace, and electronic heat dissipation fields. These heat pipes typically consist of a molybdenum tube as the main body, sealed at both ends with molybdenum plugs. In engineering applications, they need to be integrated with other components such as niobium tubes. The connection between the main tube, plugs, and external pipe fittings relies on welding to achieve sealing and load-bearing capacity.

[0003] Molybdenum has poor weldability, which makes sealing the aforementioned structural components difficult. At high temperatures, molybdenum readily reacts with oxygen and nitrogen to form brittle compounds such as molybdenum dioxide and molybdenum nitride. Furthermore, its low room temperature plasticity and the concentration of thermal stress during welding make the joint prone to hot and cold cracking. When welding molybdenum using methods such as tungsten inert gas welding (TIG) and gas metal arc welding (GMAW), the shielding gas cannot completely isolate atmospheric oxygen and nitrogen, resulting in severe weld contamination. The joint strength is significantly lower than that of the base metal, and brittle fracture frequently occurs.

[0004] The problem becomes more pronounced when molybdenum tubes need to be connected to tubes with relatively low melting points, such as niobium tubes. Niobium has a lower melting point than molybdenum. If the laser's arc initiation and termination are directly applied to the surface of the niobium tube, the tube is prone to collapse and deformation under instantaneous high temperatures, which in turn affects the flatness, sealing performance, and even the overall mechanical properties of the joint, potentially leading to the scrapping of the workpiece. Summary of the Invention

[0005] In order to solve the above-mentioned technical problems, the present invention provides a metal heat pipe fitting and its vacuum laser welding method, which realizes stable welding of molybdenum-based heat pipe structural components and ensures their engineering application under high temperature and strong radiation conditions.

[0006] This invention is achieved through the following technical solution:

[0007] A metal heat pipe fitting includes: a first fitting, a second fitting, a solid plug, and a perforated plug. The first fitting, the second fitting, the solid plug, and the perforated plug are coaxially assembled to form a circumferential weld. The perforated plug is provided with an annular boss. The first fitting is inserted into the perforated plug, and the end face of the first fitting is in contact with the annular boss. An annular intermediate layer is provided between the second fitting and the welding surface of the solid plug.

[0008] Optionally, the first pipe fitting is a niobium pipe, the second pipe fitting is a molybdenum pipe, the perforated plug and the solid plug are both molybdenum plugs and are respectively assembled at both ends of the molybdenum pipe, the perforated plug has a through hole, the annular boss is located in the circumference of the through hole, the niobium pipe is inserted into the through hole, and an annular intermediate layer is provided between the molybdenum pipe and the welding surface of the perforated plug.

[0009] Optionally, the annular boss is integrally formed along the circumference of the through hole, and the end face of the annular boss is flush with the end face of the niobium tube; the niobium tube is inserted into the through hole with a clearance fit.

[0010] Optionally, the annular intermediate layer is a titanium ring, the outer diameter of which is the same as the outer diameter of the molybdenum tube, and the inner diameter of which is adapted to the inner diameter of the perforated plug and the solid plug.

[0011] Optionally, the perforated plug and the solid plug each have a small end that can be inserted into the molybdenum tube, and the outer diameter of the small end is adapted to the inner diameter of the molybdenum tube.

[0012] A vacuum laser welding method for metal heat pipe fittings, comprising the following steps:

[0013] S1. The first pipe fitting, the second pipe fitting, the solid plug and the perforated plug to be welded are coaxially assembled to form a circumferential weld, and each of the circumferential welds is tack welded.

[0014] S2. Place the assembled workpiece in the vacuum chamber, evacuate to the preset vacuum level and maintain it;

[0015] S3. The circumferential weld is preheated and then welded in a vacuum environment. When welding the circumferential weld between the first pipe and the opening plug, the arc initiation and termination of the laser are applied to the annular boss. When welding the circumferential weld between the second pipe and the solid plug, the annular intermediate layer is melted and a transition layer is formed.

[0016] S4. After welding, maintain a vacuum environment to allow the workpiece to cool before removing it.

[0017] Optionally, the preheating includes: continuously and uniformly scanning the laser beam along the circumferential weld, with the scanning area covering both sides of the weld, and the single-sided scanning width being 1.5 to 2 times the thickness of the base material, and vaporizing and removing the surface oxides of the surface to be welded during the scanning process.

[0018] Optionally, the preheating before welding includes: preheating and reinforcing the circumferential weld with a first heat input, and then continuously and uniformly welding the circumferential weld with a second heat input greater than the first heat input.

[0019] Optionally, the method may include the following steps before step S1:

[0020] The first pipe fitting, the second pipe fitting, the opening plug, the solid plug, and the area to be welded in the annular intermediate layer are ultrasonically cleaned; after welding the circumferential weld between the first pipe fitting and the opening plug, the workpiece is turned around, clamped and aligned, and then the circumferential welds at both ends of the first pipe fitting are preheated and welded in sequence.

[0021] Optionally, after the workpiece is removed in step S4, X-ray flaw detection and airtightness testing are performed on each of the circumferential welds.

[0022] This invention solves the problems of pipe fitting collapse and molybdenum tube embrittlement and cracking in the same process by setting an annular boss on the perforated plug for laser arc initiation and termination, setting an annular intermediate layer that can be melted into a transition layer between the molybdenum tube and the plug, and placing the entire welding process in a vacuum environment for preheating before welding. Compared with the prior art, it has the following advantages:

[0023] When welding the circumferential weld between the niobium tube and the opening plug, the laser's arc initiation and termination act on the molybdenum-made annular boss rather than the niobium tube surface. Taking advantage of the high melting point and good high-temperature stability of molybdenum, a local thermal buffer is formed before the niobium tube melts, preventing the niobium tube from collapsing or deforming due to local overheating.

[0024] The annular intermediate layer between the molybdenum tube and the plug melts first under the action of laser to form a transition layer, which avoids the problem of grain boundary segregation and generation of brittle phases in the weld when molybdenum is directly butted together, thereby reducing incomplete fusion and crack defects;

[0025] The circumferential weld was first preheated and reinforced with a lower initial heat input, and then welded continuously and at a constant speed with a higher second heat input. This reduced the temperature gradient in the welding zone and improved the welding quality. Attached Figure Description

[0026] The accompanying drawings illustrate exemplary embodiments of the present invention and, together with the description thereof, serve to explain the principles of the invention. These drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, but do not constitute a limitation on the embodiments of the present invention.

[0027] Figure 1 A schematic flowchart of a vacuum laser welding method for pipe fittings provided in an embodiment of the present invention;

[0028] Figure 2 This is a schematic diagram of the overall structure of the niobium-molybdenum heat pipe structure provided in an embodiment of the present invention;

[0029] Figure 3 This is a partially enlarged schematic diagram of the niobium tube and the perforated plug insertion structure provided in an embodiment of the present invention;

[0030] Reference numerals in the attached drawings: 1-First fitting; 2-Second fitting; 3-Solid plug; 4-Opening plug; 5-Annular intermediate layer; 6-Annular boss. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the invention.

[0032] It should also be noted that, for ease of description, only the parts relevant to the present invention are shown in the accompanying drawings.

[0033] Where there is no conflict, the embodiments and features described in this invention can be combined with each other. The invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0034] Example 1

[0035] This embodiment corresponds to the overall structure of the metal heat pipe fitting of the present invention, such as... Figure 2 , Figure 3 As shown, this embodiment provides a metal heat pipe fitting, including a first fitting 1, a second fitting 2, a solid plug 3, and a perforated plug 4. The first fitting 1, the second fitting 2, the solid plug 3, and the perforated plug 4 are coaxially assembled to form a circumferential weld. The perforated plug 4 is provided with an annular boss 6. The first fitting 1 is inserted into the perforated plug 4, and the end face of the first fitting 1 is in contact with the annular boss 6. An annular intermediate layer 5 is provided between the second fitting 2 and the surface of the solid plug 3 to be welded.

[0036] The annular boss 6 refers to the circumferentially raised annular structure at the end face to be welded of the opening plug 4. Its function is to provide a platform made of high melting point material for the laser arc initiation and termination during welding, so that the instantaneous high temperature of the laser acts on the boss first rather than the first pipe fitting 1 with a lower melting point.

[0037] An annular intermediate layer 5 is disposed between the second pipe fitting 2 and the welding surface of the plug. It melts during welding and forms a transition layer connecting the two, avoiding weld embrittlement caused by direct butt welding of the same material.

[0038] like Figure 2As shown, the first fitting 1 is a niobium tube, the second fitting 2 is a molybdenum tube, and both the perforated plug 4 and the solid plug 3 are molybdenum plugs, respectively assembled at both ends of the molybdenum tube. The perforated plug 4 has a through hole, and an annular boss 6 is located circumferentially in the through hole, into which the niobium tube is inserted. An annular intermediate layer 5 is provided between the welding surfaces of the molybdenum tube and the perforated plug 4, and between the welding surfaces of the molybdenum tube and the solid plug 3. Thus, three circumferential welds are formed on the entire metal heat pipe fitting: an insertion weld between the niobium tube and the perforated plug 4, and two butt welds between the molybdenum tube and the two plugs.

[0039] In a specific example, the niobium tube is of grade Nb1, with an outer diameter of 6 mm, a wall thickness of 1.5 mm, and a length of 50 mm. The molybdenum tube is of grade Mo1, with an outer diameter of 16 mm, a wall thickness of 1.0 mm, and a length of 60 mm. The plug specifications are compatible with the niobium tube and the molybdenum tube.

[0040] like Figure 3 As shown, the annular boss 6 is integrally machined along the circumference of the through hole, and its end face is flush with the end face of the niobium tube. This allows the laser heat source to fall on the boss during arc initiation and termination of welding. The end face of the niobium tube and the end face of the boss are flush and together form the surface to be welded. The cross-sectional dimensions of the annular boss 6 should not be too large or too small: if it is too large, it will increase unnecessary heat input; if it is too small, the buffering effect of arc initiation and termination will be insufficient. The cross-sectional height of the annular boss 6 is 0.3mm to 0.8mm (for example, 0.3mm, 0.5mm, 0.8mm can be used), and the cross-sectional width is 0.3mm to 0.8mm (for example, 0.3mm, 0.5mm, 0.8mm can be used).

[0041] In a specific example, the annular boss 6 has a cross-sectional height of 0.5 mm and a cross-sectional width of 0.5 mm. The niobium tube is inserted into the through hole with a clearance fit. The clearance between the outer diameter of the niobium tube and the inner diameter of the through hole is 0.01 mm to 0.02 mm (e.g., 0.01 mm, 0.015 mm, 0.02 mm). This clearance range ensures that the niobium tube fits tightly against the inner wall of the through hole after insertion, suppressing axial displacement and radial wobble, and also facilitates assembly.

[0042] The annular intermediate layer 5 is a titanium ring. Titanium has a strong affinity for oxygen at high temperatures, which can absorb and fix oxygen, nitrogen, and other impurities in the weld zone, reducing their segregation at the molybdenum weld grain boundaries. The outer diameter of the titanium ring is consistent with the outer diameter of the molybdenum tube, and the inner diameter is adapted to the inner diameter of the perforated plug 4 and the solid plug 3. The thickness of the titanium ring is 0.1mm to 0.2mm (e.g., 0.1mm, 0.15mm, 0.2mm), and the fitting clearance between the titanium ring and the surface to be welded is 0.01mm to 0.03mm (e.g., 0.01mm, 0.02mm, 0.03mm).

[0043] In a specific example, the titanium ring is made of industrial pure titanium of grade TA1 with a purity of not less than 99.9% and a thickness of 0.2 mm. The perforated plug 4 and the solid plug 3 each have a small end that inserts into the molybdenum tube. The outer diameter of the small end matches the inner diameter of the molybdenum tube, and the fitting clearance between the small end and the molybdenum tube is 0.01 mm to 0.02 mm (e.g., 0.015 mm, 0.02 mm) to ensure coaxiality and stability after assembly.

[0044] Example 2

[0045] This embodiment corresponds to the vacuum laser welding method for metal heat pipe fittings of the present invention, and is used to weld the metal heat pipe fittings of Embodiment 1, combined with... Figure 1 The welding method is described, including the following steps:

[0046] Step S1: The first pipe fitting 1, the second pipe fitting 2, the solid plug 3, and the perforated plug 4 are coaxially assembled to form a circumferential weld, and laser tack welding is performed on each circumferential weld. Pre-tack welding is used to fix the relative positions of each component and suppress axial displacement and radial wobble of the assembly during vacuuming and formal welding.

[0047] Step S2: Place the assembled workpiece in the vacuum chamber for clamping and alignment. After closing the chamber door, start the vacuum system to evacuate to the preset vacuum level and maintain it. Welding in a vacuum environment isolates the welding area from the atmosphere, reducing contamination of the weld by reactive gases such as oxygen, nitrogen, and hydrogen.

[0048] Step S3: Preheat the circumferential weld in a vacuum environment before welding. When welding the circumferential weld between the first pipe fitting 1 and the opening plug 4, the laser's arc initiation and termination are applied to the annular boss 6. Since the annular boss 6 is made of high-melting-point molybdenum, the instantaneous high temperature of the laser first acts on the boss rather than the first pipe fitting 1, which has a lower melting point, thus forming a local thermal buffer zone before the first pipe fitting 1 melts, thereby preventing the first pipe fitting 1 from collapsing and deforming. When welding the circumferential weld between the second pipe fitting 2 and the solid plug 3, the annular intermediate layer 5 is melted to form a transition layer connecting the two. After the intermediate layer melts, it fills the space between the two surfaces to be welded, avoiding the problem of grain boundary segregation and the formation of brittle phases in welds when the same type of molybdenum material is directly butted.

[0049] Step S4: After welding, maintain a vacuum environment to allow the workpiece to cool. Remove the workpiece after it has cooled down to prevent it from coming into contact with the atmosphere and oxidizing during the cooling process.

[0050] Example 3

[0051] This embodiment, based on Embodiment 2, illustrates the construction of the vacuum environment, the process parameters for preheating before welding, and cleaning and testing.

[0052] Before welding, the areas to be welded, including the niobium tube, molybdenum tube, perforated plug 4, solid plug 3, and annular intermediate layer 5, are ultrasonically cleaned for 10 to 15 minutes (e.g., 10 minutes, 12 minutes, or 15 minutes). The cleaning medium used is anhydrous ethanol to remove surface oil and other impurities.

[0053] During vacuuming, the workpiece is placed inside the vacuum chamber, and the mechanical pump and molecular pump are activated sequentially to evacuate the chamber. The preset vacuum level is better than... Pa (e.g., can be taken) Pa、 Pa、 To further reduce weld oxidation and porosity, the preset vacuum level is preferably not higher than (Pa). Pa. After evacuating to the preset vacuum level, maintain the vacuum for 5 to 60 minutes (e.g., 5 minutes, 30 minutes, or 60 minutes) to allow the residual gas in the cavity to be fully desorbed and discharged by the pump unit.

[0054] In a specific example, vacuuming is performed until... Hold for 5 minutes after applying pressure before starting welding.

[0055] During the preheating stage, the laser beam is continuously and uniformly scanned along the circumferential weld, with the scanning area covering both sides of the weld. The scanning width on one side is 1.5 to 2 times the thickness of the base material (for example, 1.5, 1.75, or 2 times the thickness of the base material can be used), so that the thermal stress is evenly distributed along both sides of the weld. During the scanning process, the high energy density of the laser is used to vaporize and remove the surface oxides of the surface to be welded, thus completing the interface purification.

[0056] Preheating and welding use different heat inputs: the circumferential weld is preheated and reinforced with a first heat input, and then the circumferential weld is continuously and uniformly welded with a second heat input greater than the first heat input.

[0057] The laser power corresponding to the first heat input is 800W to 1000W (e.g., 800W, 900W, 1000W), and the laser power corresponding to the second heat input is 1000W to 2500W (e.g., 1000W, 2000W, 2500W); the welding speed is 0.5m / min to 1.5m / min (e.g., 0.5m / min, 1.0m / min, 1.5m / min), and the defocusing amount is -3mm to 0mm (e.g., -3mm, -2mm, 0mm).

[0058] In a specific example, the preheating laser power is 1000W, the welding laser power is 2500W, the welding speed is 1.0m / min, and the defocusing amount is -3mm.

[0059] During welding, each circumferential weld should be processed in sequence: first, weld the circumferential weld between the niobium tube and the opening plug 4. After welding is completed and cooled, turn the workpiece around, clamp and align it. Then, preheat and weld the circumferential welds at both ends of the molybdenum tube in sequence.

[0060] After removing the workpiece in step S4, X-ray flaw detection and airtightness testing are performed on each circumferential weld.

[0061] In one specific example, visual inspection showed that the weld was well-formed, without undercut or lack of fusion; X-ray inspection revealed no cracks, porosity, or slag inclusions; and helium mass spectrometry leak detector analysis showed that the weld's airtightness was better than [previous standard]. Pa·m³ / s.

[0062] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0063] Those skilled in the art should understand that the above embodiments are merely for illustrating the present invention and are not intended to limit the scope of the invention. Those skilled in the art can make other changes or modifications based on the above invention, and these changes or modifications still fall within the scope of the present invention.

Claims

1. A metal heat pipe fitting, characterized in that, include: The first pipe fitting (1), the second pipe fitting (2), the solid plug (3), and the perforated plug (4) are coaxially assembled to form a circumferential weld. The perforated plug (4) is provided with an annular boss (6). The first pipe fitting (1) is inserted into the perforated plug (4), and the end face of the first pipe fitting (1) is in contact with the annular boss (6). An annular intermediate layer (5) is provided between the second pipe fitting (2) and the surface to be welded of the solid plug (3).

2. The metal heat pipe fitting according to claim 1, characterized in that, The first pipe fitting (1) is a niobium pipe, the second pipe fitting (2) is a molybdenum pipe, the perforated plug (4) and the solid plug (3) are both molybdenum plugs and are respectively assembled at both ends of the molybdenum pipe. The perforated plug (4) is provided with a through hole, the annular boss (6) is located in the circumference of the through hole, the niobium pipe is inserted into the through hole, and an annular intermediate layer (5) is provided between the molybdenum pipe and the welding surface of the perforated plug (4).

3. The metal heat pipe fitting according to claim 2, characterized in that, The annular boss (6) is integrally formed along the circumference of the through hole, and the end face of the annular boss (6) is flush with the end face of the niobium tube; the niobium tube is inserted into the through hole in a clearance fit manner.

4. The metal heat pipe fitting according to claim 2, characterized in that, The annular intermediate layer (5) is a titanium ring, the outer diameter of which is consistent with the outer diameter of the molybdenum tube, and the inner diameter of which is adapted to the inner diameter of the opening plug (4) and the solid plug (3).

5. The metal heat pipe fitting according to claim 2, characterized in that, The perforated plug (4) and the solid plug (3) each have a small end that can be inserted into the molybdenum tube, and the outer diameter of the small end is adapted to the inner diameter of the molybdenum tube.

6. A vacuum laser welding method for metal heat pipe fittings, characterized in that, Welding the metal heat pipe fitting as described in any one of claims 1-5 includes the following steps: S1. The first pipe fitting (1), the second pipe fitting (2), the solid plug (3) and the perforated plug (4) to be welded are coaxially assembled to form a circumferential weld, and the circumferential welds are tack welded. S2. Place the assembled workpiece in the vacuum chamber, evacuate to the preset vacuum level and maintain it; S3. The circumferential weld is preheated and then welded in a vacuum environment. When welding the circumferential weld between the first pipe (1) and the opening plug (4), the arc starting and ending of the laser is applied to the annular boss (6). When welding the circumferential weld between the second pipe (2) and the solid plug (3), the annular intermediate layer (5) is melted and a transition layer is formed. S4. After welding, maintain a vacuum environment to allow the workpiece to cool before removing it.

7. The vacuum laser welding method for metal heat pipe fittings according to claim 6, characterized in that, The preheating includes: continuously and uniformly scanning the laser beam along the circumferential weld, with the scanning area covering both sides of the weld, and the scanning width on one side being 1.5 to 2 times the thickness of the base material, and vaporizing and removing the surface oxides of the surface to be welded during the scanning process.

8. The vacuum laser welding method for metal heat pipe fittings according to claim 6, characterized in that, The preheating before welding includes: preheating and reinforcing the circumferential weld with a first heat input, and then continuously and uniformly welding the circumferential weld with a second heat input greater than the first heat input.

9. The vacuum laser welding method for metal heat pipe fittings according to claim 6, characterized in that, Step S1 is preceded by: Ultrasonic cleaning is performed on the areas to be welded of the first pipe fitting (1), the second pipe fitting (2), the opening plug (4), the solid plug (3), and the annular intermediate layer (5); after welding the circumferential weld between the first pipe fitting (1) and the opening plug (4), the workpiece is turned around, clamped and aligned, and then the circumferential welds at both ends of the first pipe fitting (1) are preheated and welded in sequence.

10. The vacuum laser welding method for metal heat pipe fittings according to claim 6, characterized in that, After removing the workpiece in step S4, X-ray flaw detection and airtightness testing are performed on each of the circumferential welds.

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