Method for reducing residual stress of thick-wall TC4 titanium alloy narrow-gap laser wire filling welding joint and application

Abstract

The invention discloses a method for reducing residual stress of a thick-wall TC4 titanium alloy narrow-gap laser wire filling welding joint and application. The invention belongs to the technical field of welding. The technical problem that an existing thick-wall TC4 titanium alloy narrow-gap laser wire filling welding joint is high in residual stress is solved. The method comprises the steps that firstly, a narrow-gap groove is machined, pre-welding vacuum heat treatment is conducted on a test plate to be welded, and then surface pretreatment is conducted; then putting the to-be-welded test plate subjected to surface pretreatment into a vacuum isothermal cabin for heat preservation; and finally, relative positions and welding parameters of a laser beam, a wire feeding pipe and a welding seam are set, the swing mode of the laser beam is linear swing, the scanning path of the laser beam is that the included angle between the scanning path of an initial layer and the welding direction is 0 degree, then each layer of scanning path rotates by 30 degrees relative to the previous layer, and a welding program is started to conduct layer-by-layer and pass-by-pass welding. The method is applied to narrow-gap laser wire filling welding of the thick-wall TC4 titanium alloy in the fields of naval vessels, aerospace and rail transit.

Description

Technical Field

[0001] The invention belongs to the technical field of welding, and in particular relates to a method for reducing residual stress of a narrow gap laser wire-filling welded joint of a thick-walled TC4 titanium alloy and an application thereof. Background Art

[0002] Titanium alloys have the characteristics of high specific strength, corrosion resistance and non-magnetism, and have been widely used in equipment manufacturing fields such as ships, aerospace and rail transportation. α-β dual-phase titanium alloys have both the good high-temperature deformation ability and hot workability of α-type titanium alloys and the heat treatment strengthening characteristics of β-type titanium alloys, thus having excellent comprehensive performance and are currently the most widely used titanium alloys. Among them, TC4 (Ti-6Al-4V) containing about 6% of the α-phase stabilizing element Al and about 4% of the β-phase stabilizing element V is a typical α-β dual-phase titanium alloy, which is the most representative application among titanium alloys.

[0003] In the field of deep-sea equipment manufacturing, the application demand for thick-walled titanium alloy structures is becoming more and more urgent, and the thickness of the titanium alloy materials used is gradually increasing, making high-quality and efficient welding technology one of the key technologies for the safe service of thick-walled titanium alloy welded structures. Narrow Gap Laser Wire Filler Welding (NG-LWFW) has the advantages of low welding heat input, narrow heat-affected zone, precise energy control, and high welding efficiency. At the same time, the addition of filler wire can also supplement the burnt elements of the weld, and can refine the weld structure by introducing nucleation particles, further improving the performance of the welded joint. In addition, the specifications and dimensions of thick-walled titanium alloy welded components are not restricted, and high-quality and efficient connections of large thick-walled titanium alloy structural parts can be achieved. Therefore, narrow gap laser wire filler welding technology will become a high-quality welding method for the industrial application of large titanium alloy thick-walled structures.

[0004] However, during the narrow gap laser wire welding process, a large amount of heat is accumulated as the number of welding passes increases. The welding area will repeatedly undergo a complex temperature rise and fall process, and residual stress will be generated in the weld joint. When the residual stress of the weld joint accumulates high enough, it will affect the mechanical properties and fatigue properties of the weld joint and threaten the service life. When the residual stress exceeds the local ultimate tensile strength of the material, warping or plastic deformation will occur. When the external load interacts with the residual stress, it is easy to cause the initiation of local cracks in the joint or even unstable fracture, leaving hidden dangers of failure for the fatigue and stability of thick-walled TC4 titanium alloy welded structures. Therefore, how to reduce the residual stress of titanium alloy welded structures has always been the focus of attention in the industry. Summary of the invention

[0005] The present invention aims to solve the technical problem of high residual stress in existing thick-walled TC4 titanium alloy narrow gap laser welding joints with wire filling, and provides a method and application for reducing the residual stress in thick-walled TC4 titanium alloy narrow gap laser welding joints with wire filling.

[0006] One of the purposes of the present invention is to provide a method for reducing residual stress in a narrow gap laser welding joint of thick-walled TC4 titanium alloy with wire filling, the method being carried out in the following steps:

[0007] Step 1: Process the narrow gap groove, and perform vacuum heat treatment on the test plate before welding, and then perform surface pretreatment;

[0008] Step 2: Place the surface pretreated test plate into a vacuum isothermal chamber for heat preservation;

[0009] Step 3: Set the relative position and welding parameters of the laser beam, wire feeding tube and weld. The laser beam swing mode is linear swing. The laser beam scanning path is that the angle between the initial layer scanning path and the welding direction is 0°. Then each layer scanning path rotates 30° relative to the previous layer. Start the welding program to perform layer-by-layer and pass-by-pass welding.

[0010] It is further defined that the thickness of the test plate to be welded in step 1 is ≥ 60 mm.

[0011] It is further defined that the narrow gap groove shape in step 1 is Y-shaped or U-shaped, and the groove angle is ≤2°.

[0012] It is further defined that the vacuum heat treatment procedure before welding in step 1 is: heating to 900-950°C and keeping warm for 1-2 hours, cooling to 450-500°C at a cooling rate of 510-530°C / s, then keeping warm at 450-500°C for 3-4 hours, and then cooling to room temperature at a rate of 230-250°C / s.

[0013] It is further defined that the insulation temperature in step 2 is 250-300° C. and the insulation time is ≥20 min.

[0014] It is further defined that in step 3, the laser beam is perpendicular to the weld surface, and the wire feeding tube is placed at the front end of the laser beam and tilted forward by 35° to 40° relative to the laser beam.

[0015] It is further defined that TC4 welding wire is used in step 3.

[0016] It is further defined that in step 3, the laser power is 3.0-4.0 kW, the laser beam defocus is +10 mm, the laser beam oscillation frequency is 150-180 Hz, the wire feeding speed is 0.6-0.8 m / min, the welding speed is 1.0-1.2 m / min, and the filament spacing is 0-2 mm.

[0017] A second object of the present invention is to provide a joint obtained by the above method.

[0018] The third object of the present invention is to provide an application of the above method in the narrow gap laser wire welding of thick-walled TC4 titanium alloy in the fields of ships, aerospace and rail transportation.

[0019] The advantages of the present invention compared to the prior art are:

[0020] The present invention aims at the problem of excessive residual stress in the narrow gap laser welding joint of thick-walled TC4 titanium alloy with wire filling. The present invention reduces the residual stress in the narrow gap laser welding joint of thick-walled TC4 titanium alloy with wire filling by placing the test plate in a vacuum heating chamber for heat preservation in advance, using the linear swing laser beam periodic rotation technology, and combining the pre-welding heat treatment of the test plate. The specific advantages are as follows:

[0021] (1) By placing the test plate in a vacuum heating isothermal chamber, the welding wire and the test plate can be preheated in the vacuum isothermal chamber, reducing the peak temperature of the molten pool during the welding process. At the same time, the vacuum heating isothermal chamber increases the base temperature of the parent material, reducing the thermal stress caused by the large local temperature difference. At the same time, the welding process can be well protected in a vacuum environment.

[0022] (2) By linearly oscillating the laser beam and periodically rotating it, the interlayer laser beam scanning vector direction is changed to reduce the unidirectional accumulated stress generated by single-layer multi-pass welding during the welding process.

[0023] (3) By heat treating the test plate before welding, the body-centered cubic lattice of the base material is transformed into a welded close-packed hexagonal lattice, avoiding the phase change stress caused by the change of the organizational structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 A schematic diagram of the laser beam scanning path in the narrow gap laser wire welding process of the present invention;

[0025] Figure 2 The microstructure of the welded test plate before and after heat treatment in Example 1; (a) before heat treatment, (b) after heat treatment;

[0026] Figure 3 The microstructure morphology of the cross section of the welded joint obtained in Example 1; wherein (a) is the overall cross section, (b), (e), and (h) are local cross sections of the upper, middle, and lower parts, respectively, (c), (f), and (i) are weld cross sections of the upper, middle, and lower parts, respectively, and (d), (g), and (j) are parent material cross sections of the upper, middle, and lower parts, respectively;

[0027] Figure 4 The stress distribution cloud diagram of the weld joint obtained in Example 1; (a) is after the 1st pass, (b) is after the 11th pass, (c) is after the 22nd pass, (d) is after the 33rd pass, and (e) is after the 44th pass. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0029] The experimental methods used in the following examples are conventional methods unless otherwise specified. The materials, reagents, methods and instruments used are conventional materials, reagents, methods and instruments in the art unless otherwise specified, and can be obtained through commercial channels by those skilled in the art.

[0030] Example 1 (combined Figure 1 ): The method of reducing the residual stress of the narrow gap laser welding joint of thick-walled TC4 titanium alloy in this embodiment is carried out by the following steps:

[0031] Step 1: Select a 96mm thick TC4 titanium alloy test plate and process the titanium alloy thick plate into a Y-shaped groove with a groove blunt edge of 4mm, a groove root gap of 3.2mm, and a single-side groove angle of 1°. Then, the processed test plate is subjected to vacuum heat treatment before welding. The heat treatment scheme is to heat at 930℃ for 1h and then cool rapidly to 480℃ at a cooling rate of 520℃ / s, then keep at 480℃ for 3h and then slowly cool to room temperature at a rate of 240℃ / s. The vacuum degree required for heat treatment is 1×10 -3 Pa, the effect diagram of the welded test plate before and after heat treatment is as follows Figure 2 Then, the surface of the test plate after vacuum heat treatment was pretreated by soaking it in 5% HF + 25% HNO3 + 7% HCl + 63% H2O cleaning solution for 30 seconds. After completion, the surface of the substrate was repeatedly wiped with alcohol to expose the metallic silver-white color, and the oil and oxide at the processed groove were removed. The pretreated test plate was placed in a welding fixture for assembly and preparation for welding.

[0032] Step 2: Place the pickled test plate in a vacuum isothermal chamber for clamping and assembly. After assembly, keep the temperature at 250℃ for 40 minutes in the vacuum isothermal chamber for welding. The vacuum degree of the vacuum isothermal chamber is ≤10 -3 Pa;

[0033] Step 3: Turn on the laser, water cooler, wire feeder and other welding equipment, use TC4 welding wire with a diameter of 1.2 mm, install the welding wire into the wire feeder and ensure the stability of wire feeding during welding, set the relative position of the laser beam, wire feed tube and weld and the welding parameters;

[0034] The laser is a fiber laser with a core diameter of 200 μm, and the laser welding head is an oscillating laser head. The laser beam is perpendicular to the surface of the weld to be welded, and the wire feeding tube is placed at the front end of the laser beam and tilted 40° relative to the laser beam. The laser power is 3.0 kW, the laser beam defocus is +10 mm, the laser beam is linearly oscillating, the laser beam swing amplitude is 5 mm, the laser beam swing frequency is 180 Hz, the filament spacing is 0 mm, the wire feeding speed is 0.6 m / min, and the welding speed is 1.2 m / min.

[0035] Among them, the laser beam scanning path is that the angle between the initial layer scanning path and the welding direction is 0°, and then each layer scanning path rotates 30° relative to the previous layer, specifically: when the first welding is performed, the angle between the light beam and the welding direction is 0°, when the second welding is performed, the light beam rotates 30° counterclockwise along the horizontal direction, when the third welding is performed, the light beam rotates 60° counterclockwise along the horizontal direction, when the fourth welding is performed, the light beam rotates 90° counterclockwise along the horizontal direction, when the fifth welding is performed, the light beam rotates 120° counterclockwise along the horizontal direction, when the sixth welding is performed, the light beam rotates 150° counterclockwise along the horizontal direction, when the seventh welding is performed, the light beam rotates 180° counterclockwise along the horizontal direction, when the eighth welding is performed, the light beam rotates 210° counterclockwise along the horizontal direction, and when the ninth welding is performed, the light beam rotates 100° counterclockwise along the horizontal direction. The beam rotates 240° counterclockwise in the horizontal direction. When the 10th pass is welded, the beam rotates 270° counterclockwise in the horizontal direction. When the 11th pass is welded, the beam rotates 300° counterclockwise in the horizontal direction. When the 12th pass is welded, the beam rotates 330° counterclockwise in the horizontal direction. When the 13th pass is welded, the beam rotates 0° (360°) counterclockwise in the horizontal direction. When the 14th pass is welded, the beam rotates 30° counterclockwise in the horizontal direction. When the 15th pass is welded, the beam rotates 60° counterclockwise in the horizontal direction. When the 16th pass is welded, the beam rotates 90° counterclockwise in the horizontal direction. When the 17th pass is welded, the beam rotates 120° counterclockwise in the horizontal direction. When the 18th pass is welded, the beam rotates 150° counterclockwise in the horizontal direction. When the 19th pass is welded, the beam rotates 0° (360°) counterclockwise in the horizontal direction. The beam rotates 180° counterclockwise along the horizontal direction. When the 20th pass is welded, the beam rotates 210° counterclockwise along the horizontal direction. When the 21st pass is welded, the beam rotates 240° counterclockwise along the horizontal direction. When the 22nd pass is welded, the beam rotates 270° counterclockwise along the horizontal direction. When the 23rd pass is welded, the beam rotates 300° counterclockwise along the horizontal direction. When the 24th pass is welded, the beam rotates 330° counterclockwise along the horizontal direction. When the 25th pass is welded, the beam rotates 0° counterclockwise along the horizontal direction (360°). When the 26th pass is welded, the beam rotates 30° counterclockwise along the horizontal direction. When the 27th pass is welded, the beam rotates 60° counterclockwise along the horizontal direction. When the 28th pass is welded, the beam rotates 90° counterclockwise along the horizontal direction. When the 29th pass is welded, the beam rotates 100° counterclockwise along the horizontal direction. When welding, the beam rotates 120° counterclockwise along the horizontal direction. When welding the 30th pass, the beam rotates 150° counterclockwise along the horizontal direction. When welding the 31st pass, the beam rotates 180° counterclockwise along the horizontal direction. When welding the 32nd pass, the beam rotates 210° counterclockwise along the horizontal direction. When welding the 33rd pass, the beam rotates 240° counterclockwise along the horizontal direction. When welding the 34th pass, the beam rotates 270° counterclockwise along the horizontal direction. When welding the 35th pass, the beam rotates 300° counterclockwise along the horizontal direction. When welding the 36th pass, the beam rotates 330° counterclockwise along the horizontal direction. When welding the 37th pass, the beam rotates 0° (360°) counterclockwise along the horizontal direction. When welding the 38th pass, the beam rotates 30° counterclockwise along the horizontal direction.During the 39th welding, the beam rotates 60° counterclockwise along the horizontal direction, during the 40th welding, the beam rotates 90° counterclockwise along the horizontal direction, during the 41st welding, the beam rotates 120° counterclockwise along the horizontal direction, during the 42nd welding, the beam rotates 150° counterclockwise along the horizontal direction, during the 43rd welding, the beam rotates 180° counterclockwise along the horizontal direction, during the 44th welding, the beam rotates 210° counterclockwise along the horizontal direction, and all welding is completed.

[0036] The morphology diagrams of the overall cross section, local cross section, weld cross section, and parent material cross section of the weld joint are as follows: Figure 3 shown. Figure 3 It can be seen that the macroscopic photos of the upper, middle and lower parts of the weld joint show that the microstructure distribution and morphological characteristics of the upper, middle and lower welds and the parent metal are consistent. The upper, middle and lower weld areas are cross-distributed by a number of parallel long needle-shaped α' martensites. Between the cross-distributed α' martensites, even finer secondary α' martensites will be formed. Once the martensite nucleates, it will quickly grow and grow into a scattered and staggered organization. Finally, the weld area presents a basket-like morphology; the upper, middle and lower parent metal areas are composed of needle-shaped α' martensite, a small amount of primary α p The Widmanstätten structure is arranged in parallel bundles, and the organizational size is very close to that of the weld area.

[0037] The stress-strain results of the welding process are shown in the figure Figure 4 As shown, (a) is the stress distribution cloud diagram after the 1st pass, (b) is the stress distribution cloud diagram after the 11th pass, (c) is the stress distribution cloud diagram after the 22nd pass, (d) is the stress distribution cloud diagram after the 33rd pass, and (e) is the stress distribution cloud diagram after the 44th pass, wherein the first column represents the overall stress, the second column represents the longitudinal residual stress, and the third column represents the transverse residual stress. Figure 4 (a) It can be seen that after the first welding pass is completed, the main stress in the center and vicinity of the weld is compressive stress; as the number of welding passes increases to 11, Figure 4 As shown in (b), the center of the weld changes from compressive stress to tensile stress, but the two sides and ends of the weld still maintain compressive stress. Compressive stress mainly exists in the center and vicinity of the weld. As the number of weld passes increases to 22, Figure 4 As shown in (c), the weld center and the surrounding area of ​​the weld joint are mainly tensile stress, and there is almost no compressive stress at both ends of the weld. As the number of weld passes increases to 33, Figure 4 As shown in (d), the stress in the center and nearby areas of the weld of the weld joint is still mainly tensile stress. At the same time, the stress in the center and nearby areas of the weld joint is mainly tensile stress. As the number of weld passes increases to 44, Figure 4As shown in (e), the main stress in the weld center and nearby areas of the weld joint is tensile stress, and the tensile stress gradually decreases as the weld moves toward the outer area. The residual stress of the joint does not increase significantly with the increase in the number of weld passes, and there is almost no residual stress in the weld joint.

[0038] Comparative Example 1: The method of the comparative example is carried out according to the following steps:

[0039] Step 1: Select a 96mm thick TC4 titanium alloy test plate, and process the titanium alloy thick plate into a Y-shaped groove with a groove blunt edge of 4mm, a groove root gap of 3.2mm, and a single-side groove angle of 1°. Then, pre-treat the surface of the test plate after vacuum heat treatment, and soak the test plate in a cleaning solution of 5% HF + 25% HNO3 + 7% HCl + 63% H2O by volume for 30s. After completion, use alcohol to repeatedly wipe the surface of the substrate to expose the metallic silver-white color, remove the oil and oxide at the processed groove, and place the pre-treated test plate in a welding fixture for assembly and preparation for welding.

[0040] Step 2: Place the pickled test plate in a vacuum isothermal chamber for clamping and assembly;

[0041] Step 3: Turn on the laser, water cooler, wire feeder and other welding equipment, use TC4 welding wire with a diameter of 1.2 mm, install the welding wire into the wire feeder and ensure the stability of wire feeding during welding, set the relative position of the laser beam, wire feed tube and weld and the welding parameters;

[0042] Among them, the laser is a fiber laser, the fiber core diameter is 200μm, and the laser welding head is a swinging laser head; the laser beam is perpendicular to the surface of the weld to be welded, and the wire feeding tube is placed at the front end of the laser beam and tilted 40° relative to the laser beam; the laser power is 3.0kW, the laser beam defocus is +10mm, the laser beam adopts a linear swing mode, and the laser beam scanning path does not change during the welding process. The swing amplitude of the laser beam is 5mm, the swing frequency of the laser beam is 180Hz, the filament spacing is 0mm, the wire feeding speed is 0.6m / min, and the welding speed is 1.2m / min; the welding process repeats the process of step 3 until all welding is completed.

[0043] The results of the residual stress test of the joint by X-ray diffraction method are shown in Table 1, and the results of the fatigue strength test of the welded joint are shown in Table 2. Through the optimization of the present invention, the residual stress of the welded joint is reduced by an average of 60.8%, the high cycle fatigue limit of the welded joint is increased by 50.5%, and the low cycle fatigue limit of the welded joint is increased by 10.6%.

[0044] Table 1 Comparison of residual stress test results of joints tested by X-ray diffraction method

[0045] Test location Upper residual stress / MPa Residual stress in the middle part / MPa Lower residual stress / MPa Comparative Example 1080 970 960 The present invention 430 360 390 Residual stress reduction percentage 60.2％ 62.9％ 59.4％

[0046] Table 2 Comparison of fatigue strength test results of welded joints

[0047]

[0048] The above are only preferred specific embodiments of the present invention, which are all different implementations based on the overall concept of the present invention, and the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A method for reducing residual stress in a narrow gap laser welding joint of thick-walled TC4 titanium alloy with wire filling, characterized in that: The method: Step 1: Process the narrow gap groove, and perform vacuum heat treatment on the test plate before welding, and then perform surface pretreatment; Step 2: Place the surface pretreated test plate into a vacuum isothermal chamber for heat preservation; Step 3: Set the relative position and welding parameters of the laser beam, wire feeding tube and weld. The laser beam swing mode is linear swing. The laser beam scanning path is that the angle between the initial layer scanning path and the welding direction is 0°. Then each layer scanning path rotates 30° relative to the previous layer. Start the welding program to perform layer-by-layer and pass-by-pass welding.

2. The method according to claim 1, characterized in that The thickness of the test plate to be welded in step 1 is ≥ 60mm.

3. The method according to claim 1, characterized in that In step 1, the narrow gap groove shape is Y-shaped or U-shaped, and the groove angle is ≤2°.

4. The method according to claim 1, characterized in that: The vacuum heat treatment procedure before welding in step 1 is: heating to 900-950°C and keeping it for 1-2 hours, then cooling to 450-500°C at a cooling rate of 510-530°C / s, keeping it at 450-500°C for 3-4 hours, and then cooling to room temperature at a rate of 230-250°C / s.

5. The method according to claim 1, characterized in that In step 2, the holding temperature is 250-300° C., and the holding time is ≥20 min.

6. The method according to claim 1, characterized in that In step 3, the laser beam is perpendicular to the weld surface, and the wire feeding tube is placed at the front end of the laser beam and tilted forward by 35° to 40° relative to the laser beam.

7. The method according to claim 1, characterized in that In step 3, TC4 welding wire is used.

8. The method according to claim 1, characterized in that In step 3, the laser power is 3.0-4.0 kW, the laser beam defocus is +10 mm, the laser beam oscillation frequency is 150-180 Hz, the wire feeding speed is 0.6-0.8 m / min, the welding speed is 1.0-1.2 m / min, and the filament spacing is 0-2 mm.

9. A joint obtained by the method according to any one of claims 1 to 8.

10. Application of the method according to any one of claims 1 to 8 in the narrow gap laser wire welding of thick-walled TC4 titanium alloy in the fields of ships, aerospace and rail transportation.

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