Titanium alloy and niobium alloy dissimilar metal laser welding method

Abstract

The application discloses a titanium alloy and niobium alloy dissimilar metal laser welding method, and belongs to the technical field of dissimilar metal welding. The method comprises the following steps: treating the surfaces of titanium alloy and niobium alloy to be welded; assembling the treated titanium alloy and niobium alloy by adopting an asymmetric thinning lock bottom structure to form a closed lock bottom cavity; arranging a back protective gas path below the surface to be welded and arranging a front protective cover above the surface to be welded to form a double-face gas protection device; clamping the workpiece in a self-adaptive clamp and applying a pre-tightening force to position the workpiece; presetting a scanning path of a laser beam; adopting a continuous fiber laser to make the laser beam perform a welding operation along the preset scanning path to complete main welding and perform targeted remelting treatment along the surface of the weld; continuously inputting protective gas after welding to slowly cool to room temperature; and performing stress relief heat treatment on the welded joint. The application realizes high-quality laser welding of the titanium alloy and the niobium alloy, has high joint strength, excellent surface quality and no oxidation defects, and does not need a vacuum environment, and is flexible in process and controllable in cost.

Description

Technical Field

[0001] This invention relates to the field of dissimilar metal welding technology, and in particular to a laser welding method for titanium alloy and niobium alloy dissimilar metals. Background Technology

[0002] Titanium alloys are characterized by low density, high specific strength, and excellent corrosion resistance, while niobium alloys possess excellent high-temperature mechanical properties and thermal stability. The dissimilar connection structure of the two has irreplaceable application value in aerospace (such as rocket engine thrust chambers and nozzle extensions) and high-end equipment manufacturing, achieving the dual goals of "low cost and lightweight" and "high-temperature performance assurance", significantly reducing the overall structural weight and manufacturing cost.

[0003] However, titanium alloys and niobium alloys have vastly different thermophysical properties: titanium alloys have low thermal conductivity and high molten pool fluidity; niobium alloys have high thermal conductivity and low molten pool fluidity. The melting points of titanium alloys and niobium alloys differ by about 1000℃. This fundamental difference leads to three major challenges in dissimilar welding: First, it is difficult to balance the heat input, with the titanium alloy side prone to overmelting and propagation, and the niobium alloy side prone to insufficient penetration; second, both metals are highly reactive metals, easily reacting with elements such as oxygen, nitrogen, and hydrogen during welding to form brittle compounds (such as TiO2 and Nb2O5), leading to weld embrittlement and decreased mechanical properties; third, the weld surface is prone to undercut defects due to uneven molten pool flow, affecting structural integrity and corrosion resistance.

[0004] In the existing technology, the dissimilar joining of titanium alloy and niobium alloy is mainly achieved by electron beam welding, but there are significant limitations: 1) Dependence on vacuum environment: Electron beam welding needs to be carried out in a vacuum chamber, and the equipment purchase and operation costs are extremely high; 2) Poor process flexibility, it is difficult to adapt to the welding of large or complex structural parts for parts whose length exceeds the size of the vacuum chamber or have irregular shapes.

[0005] Therefore, developing a dissimilar metal welding method for titanium alloys and niobium alloys that does not require a vacuum environment, has precise heat input control, reliable anti-oxidation effect, and can effectively optimize surface defects has become an urgent technical problem to be solved in this field. Summary of the Invention

[0006] The purpose of this invention is to provide a laser welding method for dissimilar metals, namely titanium alloy and niobium alloy, to solve the problems of high equipment cost, complex process, uneven heat input, and prominent oxidation and surface defects in existing titanium alloy and niobium alloy dissimilar metal welding technologies.

[0007] To achieve the above objectives, the present invention provides a method for laser welding dissimilar metals of titanium alloy and niobium alloy, comprising the following steps:

[0008] S1. Grind and degrease the surfaces of the titanium alloy and niobium alloy to be welded. Use an asymmetric thinning and locking structure to assemble the treated titanium alloy and niobium alloy. The low-melting-point titanium alloy side is thinned more, and the high-melting-point niobium alloy side is thinned less. The thinned surfaces on both sides are fitted together to form a closed locking cavity.

[0009] S2. A back protective gas path is arranged directly below the area to be welded after assembly in S1, and a front protective cover is set above the area to be welded. The back protective gas path and the front protective cover constitute a double-sided gas protection device. The front protective cover integrates an annular gas path duct assembly, which introduces protective gas into the back protective gas path and the front protective cover respectively.

[0010] S3. Clamp the workpiece processed by S2 into the adaptive fixture and apply a preload force to the adaptive fixture for positioning.

[0011] S4. To address the thermophysical differences between titanium alloys and niobium alloys, a pre-defined scanning path for the laser beam is established. The scanning path can be a spiral trajectory or a sinusoidal trajectory.

[0012] S5. A continuous fiber laser is used to emit a laser beam, which is then used to perform welding operations on the area to be welded along the scanning path preset in S4, completing the main welding and forming a welded joint; then targeted remelting is performed along the surface of the weld formed after welding.

[0013] S6. After welding is completed, continue to supply protective gas to allow the workpiece to cool slowly to room temperature with the double-sided gas protection device.

[0014] S7. Perform stress-relieving heat treatment on the welded joint after slow cooling in S6.

[0015] Preferably, in step S1, the thinning amount on the titanium alloy side is 80-90% of the titanium alloy thickness, and the thinning amount on the niobium alloy side is 10-20% of the niobium alloy thickness; the width of the sealed bottom cavity is 0.4-0.6 times the preset weld width, and the butt joint gap between the titanium alloy and the niobium alloy is ≤0.1mm; the bevel edge of the sealed bottom cavity is rounded with a 0.1-0.2mm radius.

[0016] Preferably, in step S2, the back protective air passage is a copper U-shaped groove, and the inner wall of the back protective air passage has evenly distributed back air outlets, which are arranged at an upward angle.

[0017] Preferably, in step S2, the front protective cover covers the area to be welded and the surrounding area of ​​at least 10cm, the bottom of the front protective cover is provided with a flexible sealing edge, and the top of the front protective cover is reserved with a laser transmission hole; the annular gas duct assembly has an array of front air outlets along the circumference and axis, the front air outlets face the center of the area to be welded, and the angle between the axis of the front air outlet and the axis of the preset laser beam is 15-25°.

[0018] Preferably, in step S2, the protective gas is high-purity argon with a purity ≥ 99.999%; the back protective gas path and the front protective cover are supplied with independent gas, and the pressure and flow rate of the two gas supply paths are adjusted independently; the gas supply is started 30-60 seconds before laser welding begins, the gas supply continues during the welding process, and the gas supply continues for 30-60 seconds after the welding is completed before stopping.

[0019] Preferably, in step S4, the spiral scanning trajectory is a circular oscillation superimposed with linear feed; the sine wave scanning trajectory is a reciprocating oscillation superimposed with linear feed along the weld width direction.

[0020] Preferably, in step S4, in the spiral scanning trajectory or the sinusoidal scanning trajectory, the coverage time of the niobium alloy side accounts for 55-65%, and the coverage time of the titanium alloy side accounts for 35-45%; the coverage area of ​​the niobium alloy side is 15-25% larger than that of the titanium alloy side.

[0021] Preferably, in step S5, the process parameters for the main welding are: laser power 2200~2800W, welding speed 1.5~2.0m / min, and defocusing amount 0~+5mm; the process parameters for the targeted remelting are: laser power 1400~1800W, scanning speed 1.8~2.4m / min, and defocusing amount 0~+5mm.

[0022] Preferably, in step S7, the process parameters for stress-relieving heat treatment are: a vacuum degree of not less than 10... -3 Pa, heating temperature 500-800℃, hold for 30-60 minutes, then slowly cool with the furnace.

[0023] Therefore, the present invention employs the above-mentioned laser welding method for dissimilar metals, titanium alloy and niobium alloy, which has the following beneficial effects:

[0024] (1) This invention does not require a vacuum environment or high-voltage equipment, and the equipment purchase and operating costs are relatively low, which is conducive to large-scale application. This method can perform laser welding in an open environment, breaking through the limitations of vacuum chamber on workpiece size, and can be adapted to large and complex irregular structural parts, thus improving process flexibility.

[0025] (2) This invention adopts a dual-path precise zoned air supply system, with the back protective air path and the front protective cover supplying air independently and working together to achieve effective anti-oxidation protection for the front and back surfaces of the weld and the surrounding area. The weld surface has a low degree of oxidation, no obvious undercut defects, and good surface quality.

[0026] (3) The present invention effectively balances the heat input difference between the titanium alloy and the niobium alloy by using the synergistic design of the asymmetric thinning bottom lock structure and the energy-adaptive scanning path, promotes the full metallurgical bonding of the dissimilar metal interface, and the tensile strength of the welded joint can reach or exceed the strength of the niobium alloy base material. The mechanical properties are stable at high temperature, and a reliable connection of dissimilar metals is achieved.

[0027] (4) The present invention adopts a process that combines main welding and targeted remelting. On the basis of achieving reliable penetration connection in the main welding, the surface of the weld is melted and flowed again through targeted remelting, which effectively improves the slight undercut defects that may remain after the main welding. The weld surface is smooth and free of defects such as cracks, pores, and inclusions. In some cases, no further processing is required to meet the assembly requirements.

[0028] (5) The present invention effectively reduces the residual stress of the welded joint by post-weld stress relief heat treatment, reduces or avoids the generation of welding deformation and cracks, and improves the dimensional stability and service reliability of the welded structure.

[0029] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0030] Figure 1 This is a flowchart of a laser welding method for dissimilar metals, titanium alloy and niobium alloy, according to the present invention.

[0031] Figure 2 This is a schematic diagram of the asymmetric locking bottom connector structure of the present invention;

[0032] Figure 3 This is a schematic diagram of the double-sided gas protection device of the present invention;

[0033] Figure 4 This is a schematic diagram of the energy-adaptive scanning path morphology of the present invention, wherein (a) is a spiral trajectory diagram and (b) is a sine wave trajectory diagram;

[0034] Figure 5 This is a schematic diagram of the assembly state of the present invention;

[0035] Figure 6 This is a schematic diagram of the back U-shaped protective groove structure of the present invention.

[0036] Reference numerals: 1. Titanium alloy workpiece; 2. Niobium alloy workpiece; 3. Asymmetric thinning and locking structure; 31. Titanium alloy side thinning section; 32. Niobium alloy side thinning section; 33. Rounded corner; 4. Double-sided gas protection device; 41. Back copper U-shaped groove; 411. Back air outlet; 412. Back air inlet; 42. Front protective cover; 421. Laser transmission hole; 422. Annular gas duct assembly; 423. Front air outlet; 424. Front air inlet; 5. Adaptive fixture. Detailed Implementation

[0037] The following detailed description of embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely illustrates selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0038] It should be noted that the laser welding method for dissimilar metals of titanium alloy and niobium alloy described in this invention is applicable to welding workpieces of various shapes, including but not limited to butt joints of flat plates, circumferential welds, and curved surface connections. The following embodiments use flat plate welding as an example for illustration; however, those skilled in the art should understand that for non-flat structures such as circumferential welds, the clamping method of the adaptive fixture and the arrangement of the double-sided gas protection device can be adjusted according to the workpiece shape (e.g., changing the back U-shaped groove to a circumferential groove, or changing the front protective cover to a circumferential cover, etc.). These adaptive adjustments all fall within the scope of protection of this invention.

[0039] Figure 1 This is a process flow diagram of a laser welding method for dissimilar metals, titanium alloy and niobium alloy, according to the present invention. Figure 1 As shown, the method of the present invention includes seven steps, S1 to S7.

[0040] Example 1: Welding of 2mm thick titanium alloy (TC4) and 2mm thick niobium alloy (Nb-W-Mo-Zr).

[0041] S1. Grinding and degreasing the surfaces of titanium alloy and niobium alloy to be welded: Use 800-grit sandpaper to perform circumferential cross-grinding on the surfaces of TC4 titanium alloy and Nb-W-Mo-Zr niobium alloy to be welded, covering the edge of the welding area and the surrounding 20mm area to remove the surface oxide scale, passivation film and impurities; then wipe with industrial silk dipped in anhydrous ethanol to degrease, and blow dry with hot air.

[0042] like Figure 2As shown, an asymmetric thinning locking structure 3 is used to assemble the treated titanium alloy workpiece 1 and niobium alloy workpiece 2. The low-melting-point titanium alloy side undergoes multiple thinning treatments to form a titanium alloy side thinning section 31, with a thinning amount of 80% of the titanium alloy thickness, resulting in a thickness of 0.4 mm. The high-melting-point niobium alloy side undergoes less thinning treatment to form a niobium alloy side thinning section 32, with a thinning amount of 20% of the niobium alloy thickness, resulting in a thickness of 1.6 mm. The two thinning surfaces are precisely fitted to form a sealed locking cavity. The width of the locking cavity is 0.5 times the preset weld width (the preset weld width is 2 mm, and the locking cavity width is 1 mm). The butt joint gap between the titanium alloy workpiece 1 and the niobium alloy workpiece 2 is controlled at 0.1 mm. The beveled edge of the locking cavity is rounded 33 with a radius of 0.15 mm to guide the orderly flow of the molten pool and prevent molten pool accumulation or spread. Figure 5 This is a schematic diagram of the assembly state of the present invention, showing the overall structure after the asymmetric thinning lock bottom structure 3 is assembled.

[0043] S2, such as Figure 3 As shown, a U-shaped groove 41 made of copper material is arranged directly below the area to be welded after assembly by S1, and a front protective cover 42 is set above the area to be welded. The U-shaped groove 41 made of copper material and the front protective cover 42 constitute a double-sided gas protection device 4 to achieve dual-dimensional gas protection.

[0044] like Figure 6 As shown, the inner wall of the U-shaped groove 41 made of copper on the back has evenly distributed back vent holes 411, which are arranged at an upward angle. One end of the U-shaped groove 41 made of copper on the back has a back air inlet 412, through which the back protective gas is introduced. The back protective gas is high-purity argon gas with a purity of ≥99.999%.

[0045] The front protective cover 42 covers the area to be welded and its surrounding area of ​​at least 10cm. The bottom of the front protective cover 42 has a flexible sealing edge to ensure airtightness. The top of the front protective cover 42 has a pre-drilled laser transmission hole 421, the edge of which is coated with an anti-reflective coating to prevent laser reflection from damaging the equipment. The front protective cover 42 integrates an annular gas duct assembly 422 surrounding the area to be welded. The annular gas duct assembly 422 has an array of front exhaust holes 423 along its circumference and axial direction, enabling uniform diffusion of the protective gas in all directions and rapid filling of the cover space. The front exhaust holes 423 face the center of the area to be welded, and the angle between the axis of the front exhaust holes 423 and the axis of the laser beam preset in step S4 is 20°, preferably between 15-25°, to suppress plasma scattering generated during welding. The annular gas duct assembly 422 has a front air inlet 424 through which the front protective gas, which is high-purity argon gas with a purity ≥99.999%, is introduced.

[0046] The copper U-shaped groove 41 on the back and the protective cover 42 on the front are supplied with gas independently, forming a dual-path precise zoned gas supply system. The pressure and flow rate of the two gas supply paths are independently adjustable to achieve full-process anti-oxidation protection. The preferred gas supply pressure range for the back protective gas path is 15-25MPa, and the preferred gas flow rate range is 10-15L / min; the preferred gas supply pressure range for the front protective gas path is 20-30MPa, and the preferred gas flow rate range is 20-25L / min. The protection sequence is as follows: gas supply is started 30-60 seconds before laser welding begins, gas supply continues during welding, and gas supply continues for 30-60 seconds after welding ends before stopping, ensuring that the front and back of the weld and the surrounding area are in a high-purity argon protective atmosphere throughout the entire process, completely avoiding oxidation reactions.

[0047] In this embodiment, the back protective gas flow rate is 10L / min and the pressure is 20MPa; the front protective gas flow rate is 25L / min and the pressure is 25MPa; the gas supply is started 40s before laser welding begins and continues for 40s after welding ends before stopping.

[0048] S3. Clamp the workpiece processed in S2 into the adaptive fixture 5. The pressure head of the adaptive fixture 5 has a variable orientation structure, which can adapt to different planes and thicknesses of the workpiece. Apply a preload to the adaptive fixture 5 for positioning to avoid misalignment at the welding point, ensuring assembly accuracy and welding stability. The assembly state is as follows: Figure 5 As shown.

[0049] S4. To address the thermophysical differences between titanium and niobium alloys, a laser beam energy-adaptive scanning path is pre-set. The scanning path can be either a helical trajectory or a sinusoidal trajectory: the helical trajectory combines circular oscillation with linear feed motion in the welding direction, with a circular oscillation amplitude of 0.5-1 mm; the sinusoidal trajectory combines reciprocating linear oscillation along the weld width direction with linear feed motion in the welding direction, with a reciprocating linear oscillation amplitude of 0.5-1 mm. By optimizing the coverage time and area proportion of the trajectory on the titanium alloy side and the niobium alloy side, a foundation for balanced heat input is laid. The coverage time proportion on the niobium alloy side is 55-65%, and on the titanium alloy side, it is 35-45%, with the coverage area on the niobium alloy side being 15-25% larger than that on the titanium alloy side.

[0050] In this embodiment, as Figure 4 As shown, the scanning path is Figure 4 The sinusoidal trajectory shown in (b) is a reciprocating oscillation superimposed with linear feed along the weld width direction, with an oscillation amplitude of 0.7 mm. In the sinusoidal trajectory, the coverage time of the niobium alloy side accounts for 60%, the coverage time of the titanium alloy side accounts for 40%, and the coverage area of ​​the niobium alloy side is 20% larger than that of the titanium alloy side.

[0051] S5. A continuous fiber laser with a wavelength of 1064nm is used to emit a laser beam, which is then used to perform welding operations along the scanning path preset in S4. By adjusting the matching relationship between the oscillation amplitude, oscillation frequency, and welding speed, the energy distribution in the weld width direction is optimized, suppressing undercut defects caused by excessive molten pool fluidity on the titanium alloy side. The basic welding parameters are adjusted according to the base material thickness to complete the main welding and form a welded joint.

[0052] Due to the differences in the flow characteristics of dissimilar metal molten pools, slight undercut may remain on the weld surface after the main welding. Therefore, targeted remelting is performed along the weld surface formed after welding to further mitigate this slight undercut. The main welding and targeted remelting are a dual-sequence penetration-remelting synergistic process, deeply coupled and with precise functional zoning.

[0053] The first sequence is the main welding (penetration welding): laser power 2200-2800W, welding speed 1.5-2.0m / min, defocusing amount 0~+5mm, to achieve reliable penetration connection and interface metallurgical bonding of titanium and niobium dissimilar metals.

[0054] The second step is targeted remelting: laser power 1400-1800W, scanning speed 1.8-2.4m / min, defocusing amount 0~+5mm;

[0055] In this embodiment, the laser power for main welding is 2500W, the welding speed is 1.8m / min, and the defocusing amount is +3mm; the laser power for targeted remelting is 1500W, the scanning speed is 2.4m / min, the defocusing amount is +3mm, and the oscillation frequency is 120Hz.

[0056] S6. After welding, the protective gas is continuously introduced to allow the workpiece to cool slowly to room temperature with the double-sided gas protection device 4. High-purity argon gas is continuously introduced until the weld temperature drops to a safe range (≤150℃) to avoid secondary oxidation and reduce the accumulation of thermal stress.

[0057] S7. After slow cooling in S6, the welded joint undergoes stress-relieving heat treatment to stabilize its mechanical properties and reduce residual stress. The heat treatment process parameters are: vacuum degree not less than 10... -3 Pa, heating temperature 500-800℃, holding time 30-60 minutes, followed by slow cooling in the furnace. In this embodiment, the vacuum degree is 1×10⁻⁶. -3 Pa, heating temperature is 700℃, holding time is 60 minutes, and slow cooling is performed with the furnace.

[0058] Mechanical properties and surface quality tests were conducted on the welded joint of Example 1 above. The results showed that the room temperature tensile strength was 508 MPa, the high temperature tensile strength at 400℃ was 380 MPa, the fracture location was located in the niobium alloy base material, the weld surface roughness Ra was 2.8 μm, the oxide scale thickness was 3.2 μm, and the residual stress was 135 MPa.

[0059] Example 2: Welding of 2mm thick titanium alloy (TA15) and 2mm thick niobium alloy (Nb-W-Mo-Zr).

[0060] The basic operating steps of this embodiment are the same as those of Embodiment 1, the difference being the following process parameters:

[0061] In step S1, the thinning amount on the titanium alloy side is 75% of the titanium alloy thickness, resulting in a thickness of 0.5 mm; the thinning amount on the niobium alloy side is 25% of the niobium alloy thickness, resulting in a thickness of 1.5 mm; the width of the lock bottom cavity is 0.4 times the preset weld width (the preset weld width is 2 mm, and the lock bottom cavity width is 0.8 mm); the butt joint gap is controlled at 0.02 mm; and the radius of the fillet 33 is 0.2 mm.

[0062] In step S2, the back shielding gas flow rate is 10L / min and the pressure is 20MPa; the front shielding gas flow rate is 25L / min and the pressure is 25MPa; the gas supply is started 50s before laser welding begins and continues for 50s after welding ends before stopping; the angle between the axis of the front vent 423 and the axis of the preset laser beam is 25°.

[0063] In step S4, the following is adopted: Figure 4 The spiral trajectory shown in (a) is a circular oscillation superimposed with linear feed, with an oscillation amplitude of 1.0 mm. In the spiral trajectory, the niobium alloy side covers 65% of the time, the titanium alloy side covers 35% of the time, and the niobium alloy side covers 25% more than the titanium alloy side.

[0064] In step S5, the laser power for main welding is 2500W, the welding speed is 2.0m / min, and the defocusing amount is +3mm; the laser power for targeted remelting is 1600W, the scanning speed is 2.4m / min, the defocusing amount is +3mm, the oscillation frequency is 150Hz, and the oscillation amplitude is 1.2mm.

[0065] In step S7, the vacuum level is 2 × 10⁻⁶. -4 Pa, heating temperature is 730℃, holding time is 55 minutes, and slow cooling is performed with the furnace.

[0066] Mechanical properties and surface quality tests were conducted on the welded joint of Example 2 above. The results showed that the room temperature tensile strength was 495 MPa, the high temperature tensile strength at 400℃ was 378 MPa, the fracture location was located at the edge of the fusion zone, the weld surface roughness Ra was 3.0 μm, the oxide scale thickness was 4.5 μm, and the residual stress was 142 MPa.

[0067] Comparative example:

[0068] To verify the technical effectiveness of this invention, conventional laser welding methods (without gas shielding, targeted remelting, and energy-adaptive scanning path) were used to weld titanium alloy and niobium alloy of the same specifications. The results showed that the weld surface exhibited obvious oxidation discoloration and undercut defects, the room temperature tensile strength was only 320 MPa, and the fracture location was in the weld zone.

[0069] Therefore, the present invention employs the above-mentioned laser welding method for titanium alloy and niobium alloy dissimilar metals, which can stably obtain titanium-niobium dissimilar metal welded joints with high strength, high surface quality, low oxidation, and low residual stress. All performance indicators are superior to existing technologies, fully meeting the application requirements of high-end fields such as aerospace.

[0070] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method of laser welding a titanium alloy to a niobium alloy dissimilar metal, characterized by, Includes the following steps: S1. Grind and degrease the surfaces of the titanium alloy and niobium alloy to be welded. Use an asymmetric thinning and locking structure to assemble the treated titanium alloy and niobium alloy. The low-melting-point titanium alloy side is thinned more, and the high-melting-point niobium alloy side is thinned less. The thinned surfaces on both sides are fitted together to form a closed locking cavity. The thinning amount on the titanium alloy side is 80-90% of the titanium alloy thickness, and the thinning amount on the niobium alloy side is 10-20% of the niobium alloy thickness; the width of the sealed bottom cavity is 0.4-0.6 times the preset weld width, and the butt joint gap between the titanium alloy and the niobium alloy is ≤0.1mm; the bevel edge of the sealed bottom cavity is rounded with a 0.1-0.2mm radius. S2. A back protective gas path is arranged directly below the area to be welded after assembly in S1, and a front protective cover is set above the area to be welded. The back protective gas path and the front protective cover constitute a double-sided gas protection device. The front protective cover integrates an annular gas path duct assembly, which introduces protective gas into the back protective gas path and the front protective cover respectively. The back protection air passage is a copper U-shaped groove, and the inner wall of the back protection air passage has evenly distributed back air outlets, which are arranged at an upward angle. The front protective cover covers the area to be welded and the surrounding area of ​​at least 10cm. The bottom of the front protective cover is provided with a flexible sealing edge, and the top of the front protective cover is reserved with a laser transmission hole. The annular gas duct assembly has an array of front air outlets along the circumference and axis. The front air outlets face the center of the area to be welded, and the angle between the axis of the front air outlet and the axis of the preset laser beam is 15-25°. The protective gas is high-purity argon with a purity of ≥99.999%; the back protective gas path and the front protective cover are supplied with independent gas, and the pressure and flow rate of the two gas supply paths are adjusted independently; the gas supply is started 30-60 seconds before laser welding begins, the gas supply continues during the welding process, and the gas supply continues for 30-60 seconds after the welding is completed before stopping. The back protection air circuit has an air supply pressure range of 15-25MPa and an air supply flow range of 10-15L / min; the front protection air circuit has an air supply pressure range of 20-30MPa and an air supply flow range of 20-25L / min. S3. Clamp the workpiece processed by S2 into the adaptive fixture and apply a preload force to the adaptive fixture for positioning. S4. To address the thermophysical differences between titanium alloys and niobium alloys, a pre-defined scanning path for the laser beam is established. The scanning path can be a spiral trajectory or a sine wave trajectory. The spiral scanning trajectory is a circular oscillation superimposed with linear feed. The sine wave scanning trajectory is a reciprocating oscillation superimposed with linear feed along the weld width direction. In spiral or sinusoidal scanning trajectories, the niobium alloy side accounts for 55-65% of the coverage time, while the titanium alloy side accounts for 35-45%; the coverage area of ​​the niobium alloy side is 15-25% larger than that of the titanium alloy side. S5. A continuous fiber laser is used to emit a laser beam, which is then used to perform welding operations on the area to be welded along the scanning path preset in S4, completing the main welding and forming a welded joint; then targeted remelting is performed along the surface of the weld formed after welding. S6. After welding is completed, continue to supply protective gas to allow the workpiece to cool slowly to room temperature with the double-sided gas protection device. S7. Perform stress-relieving heat treatment on the welded joint after slow cooling in S6.

2. The laser welding method for dissimilar metals, titanium alloy and niobium alloy, according to claim 1, is characterized in that, In step S5, the process parameters for main welding are: laser power 2200~2800W, welding speed 1.5~2.0m / min, and defocusing amount 0~+5mm; the process parameters for targeted remelting are: laser power 1400~1800W, scanning speed 1.8~2.4m / min, and defocusing amount 0~+5mm.

3. The laser welding method for dissimilar metals, titanium alloy and niobium alloy, according to claim 1, is characterized in that, The process parameters of the stress relief heat treatment in step S7 are as follows: vacuum degree is not less than 10 -3 Pa, heating temperature is 500-800℃, holding time is 30-60 minutes, and slow cooling is performed with the furnace.

Images