A multi-layer, multi-pass laser welding method for L-PBF-formed Ti6Al4V titanium alloy filled with TAO powder (LMD).

Abstract

A multi-layer, multi-pass laser welding method for L-PBF-formed Ti6Al4V titanium alloy filled with TAO powder is disclosed, belonging to the field of welding. This method utilizes laser melting deposition equipment and employs multi-layer, multi-pass welding technology, filling the weld with TAO metal powder during the welding process. This method not only achieves excellent weld formation but also significantly improves the elongation while maintaining a certain joint strength, giving the joint excellent comprehensive mechanical properties. This opens up new possibilities for the widespread application of L-PBF-formed TC4 titanium alloy structural components in practical engineering fields.

Description

Technical Field

[0001] This invention belongs to the field of titanium alloy laser welding, specifically relating to a new laser welding method for forming TC4 titanium alloy filled with TAO powder using laser powder bed melting (L-PBF). Background Technology

[0002] In recent years, with the significant increase in the use of titanium alloys and the continuous improvement of application levels in related industries, the performance requirements for structural components have also been increasing, making the design and manufacturing of integral titanium alloy structures increasingly important. However, the processing difficulty of some titanium alloy structures with complex internal flow channels and porous lattices far exceeds the limits of traditional titanium alloy processing techniques. These titanium alloy structural components are not only difficult to process due to the material itself, but their structural complexity also presents additional manufacturing challenges. Laser Powder Bed Fusion (L-PBF), as a mainstream direct additive manufacturing technology for titanium and titanium alloy parts, has demonstrated significant advantages in the forming and manufacturing of complex and precision titanium alloy structural components due to its unique ability to freely manufacture complex parts.

[0003] TC4 titanium alloy is a typical α+β dual-phase titanium alloy, renowned for its high specific strength, strong heat resistance, and excellent corrosion resistance. TC4 titanium alloy formed using the L-PBF technique exhibits superior mechanical properties compared to forged and cast TC4 titanium alloys, thus finding wide application in aerospace, defense, marine engineering, and many other fields.

[0004] However, due to the limited size of the forming chamber in L-PBF equipment, this technology is difficult to meet the integrated forming requirements of large-sized, complex TC4 titanium alloy structural parts in actual production. In subsequent production and application stages, once parts are damaged or worn, their mechanical properties will be significantly weakened, and their service life will be correspondingly shortened. Directly discarding or replacing damaged parts would greatly increase manufacturing costs. Therefore, in order to efficiently, repairably, and cost-effectively produce large-sized L-PBF formed TC4 titanium alloy structural parts, the use of welding technology to connect L-PBF formed parts is particularly important and necessary.

[0005] However, the microstructure of the L-PBF-formed TC4 titanium alloy welded joint contains coarse β columnar crystals and hard and brittle acicular α′ martensite, which seriously reduces the joint's plasticity and toughness and cannot meet the requirements of practical engineering applications.

[0006] Therefore, obtaining L-PBF formed TC4 titanium alloy welded joints with good plasticity has become an extremely challenging task. Summary of the Invention

[0007] This invention proposes an improved laser welding method. First, the workpieces to be welded undergo pre-welding treatment, including mechanical grinding, chemical cleaning, and vacuum drying, to ensure the cleanliness of the weld surface. Then, a multi-layer, multi-pass laser powder-filled welding technique is employed, using TAO powder as the filler material, to weld L-PBF-formed TC4 titanium alloy. This method yields laser-welded joints with good formability and excellent mechanical properties, achieving high-quality joining of L-PBF-formed TC4 alloy.

[0008] The TAO titanium alloy raw material formed by L-PBF used in this invention has a particle size of 75-105μm, wherein the mass fraction of each element is as follows: Al: 0.38%~0.45%, V: 0.27%~0.33%, Fe: 0.17%~0.23%, Ni: ≤0.03%, and the balance is Ti.

[0009] To achieve the above objectives, the technical solution of the present invention is as follows:

[0010] (1) A bevel is made at the joint of the L-PBF-formed TC4 titanium alloy sheet;

[0011] (2) After the bevel is made, the base material is first mechanically ground, and then chemically cleaned.

[0012] (3) Place the cleaned base material to be welded into a vacuum drying oven to dry;

[0013] (4) Fix the dried material to be welded on the welding fixture and put it into the protective atmosphere box and pass a protective gas through it;

[0014] (5) Start the fiber laser and feed the metal powder into the weld seam using the coaxial powder feeding method. Weld the material to be welded layer by layer through the multi-layer and multi-pass laser welding process.

[0015] Preferably, the bevel in step (1) is a "V" shaped bevel with a single-sided angle of 30° and no blunt edge.

[0016] Preferably, the mechanical polishing in step (2) uses 80-400 grit sandpaper to polish the bevel of the workpiece to be welded to a metallic luster.

[0017] Preferably, the chemical cleaning in step (2) involves cleaning in a 20% (mass percentage) nitric acid + 1-2% hydrofluoric acid aqueous solution for 6-8 minutes until the silvery-white metal is exposed on the surface of the plate. The plate is then removed and rinsed under running water. Next, it is cleaned with a 20%-25% (mass fraction) sodium hydroxide aqueous solution for 4-6 minutes, followed by rinsing under running water, and finally cleaned with acetone.

[0018] The vacuum drying process described in step (3) is carried out at 80-100°C for 1-2 hours. The time from cleaning to the start of welding must not exceed 4 hours. If it exceeds 4 hours, the cleaning process of the two base materials to be welded needs to be repeated.

[0019] Preferably, the protective gas in step (4) is argon with a purity of 99.99%, and the gas flow rate is controlled at 20-25 L / min.

[0020] Preferably, the filling metal powder mentioned in step (5) is TAO powder prepared by gas atomization, with a particle size of 75-105 μm, wherein the mass fraction of each element is as follows: Al: 0.38%~0.45%, V: 0.27%~0.33%, Fe: 0.17%~0.23%, Ni: ≤0.03%, and the balance is Ti.

[0021] Preferably, the multi-layer, multi-pass laser welding process parameters in step (5) are as follows: the first pass is the root pass, with the following parameters: laser power 300-400W, spot diameter 2mm, welding speed 0.3-0.5m / min, and powder feed rate 5.87-7.05g / min; the second to fourth passes are the fill pass, with the following parameters: laser power 1300-1500W, spot diameter 3mm, welding speed 0.3-0.5m / min, and powder feed rate 7.05-9.40g / min; the fifth pass is the cover pass, with the following parameters: laser power 1300-1500W, spot diameter 3mm, welding speed 0.3-0.5m / min, and powder feed rate 7.05g / min.

[0022] The key to achieving welded joints with excellent mechanical properties in this invention lies in the use of multi-layer, multi-pass laser powder-filling welding technology. This technology uses a laser beam as a heat source to melt and deposit TAO powder layer by layer between the base materials to be welded, until the groove is completely filled and the weld is completed. On one hand, the multi-layer, multi-pass, layer-by-layer accumulation method means that the heat-affected zone of each pass is equivalent to heat-treating the lower weld pass, which helps to control the microstructure and thus improve the joint's plasticity. On the other hand, because the martensitic transformation temperature of TAO material is much higher than that of TC4 alloy, no β→α′ martensitic phase transformation occurs in the weld during welding, effectively avoiding the formation of the hard and brittle α′ martensite phase. Furthermore, due to the differences in elemental content and microstructure between the weld and the base material, the overheated β grains in the heat-affected zone cannot epitaxially grow into the weld and form coarse β columnar crystals. Instead, the microstructure in the weld is a softer, blocky α phase, which does not exhibit significant anisotropy and therefore has better deformation compatibility. These factors combined result in a significant improvement in the joint's plasticity.

[0023] The present invention provides a novel method for multi-layer, multi-pass laser powder-filled welding of TC4 titanium alloy using L-PBF forming, which has the following beneficial effects:

[0024] (1) The present invention adopts a multi-layer, multi-pass laser powder filling welding method. After optimization of process parameters, a joint with good forming and excellent mechanical properties was successfully obtained.

[0025] (2) The TAO powder filled in this invention is transformed into a blocky α phase during the welding process. Compared with the acicular α′ martensite phase in TC4 alloy, the blocky α phase is softer and less anisotropic, and has better plastic deformation coordination ability. Therefore, while maintaining a certain strength of the joint, the elongation of the joint is significantly improved.

[0026] Therefore, the multi-layer, multi-pass laser welding method using LMD filled with TA0 powder proposed in this invention is expected to solve the welding problem of L-PBF formed TC4 titanium alloy and promote the widespread application of L-PBF formed titanium alloy components in practical engineering fields. Attached Figure Description

[0027] Figure 1 A schematic diagram of the multi-layer, multi-pass laser welding process for forming TC4 titanium alloy using LPBF.

[0028] Figure 2 The cross-sectional morphology of the LPBF-formed TC4 titanium alloy multilayer multi-pass laser welded joint is shown in (a) for the joint of Example 1, (b) for the joint of Example 2, and (c) for the joint of Comparative Example 1.

[0029] Figure 3 The stress-strain curves are for Example 1, Example 2, and Comparative Example 1. Detailed Implementation

[0030] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0031] Unless otherwise specified, the experimental materials and related equipment used in the following examples and comparative examples are commercially available.

[0032] Example 1

[0033] This embodiment is a multi-layer, multi-pass laser welding method for L-PBF formed TC4 titanium alloy. The material involved is 50mm*25mm*5mm LPBF formed TC4 titanium alloy, and TAO powder is used as filler during the welding process.

[0034] The specific steps are as follows:

[0035] 1. Make a V-shaped bevel at the joint of the L-PBF formed TC4 titanium alloy base material to be welded, with an angle of 30° on one side and no blunt edge left;

[0036] 2. Mechanically grind the base material after beveling: Use 80 or 400 grit sandpaper to grind the oxide film on all surfaces of the base material to be welded until the metallic luster of the aluminum alloy is revealed;

[0037] 3. After mechanical grinding, chemical cleaning is required. The chemical cleaning involves washing in a 20% (by mass) nitric acid + 1-2% hydrofluoric acid aqueous solution for 8 minutes, until the silvery-white metal is exposed on the surface of the sheet. Then, remove the sheet and rinse it under running water. Next, wash it in a 20%-25% (by mass) sodium hydroxide aqueous solution for 5 minutes, followed by rinsing under running water. Finally, clean it with acetone.

[0038] 4. After completing the chemical cleaning, place the plates to be welded into a vacuum drying oven and dry them at 80°C for 2 hours;

[0039] 5. Fix the cleaned and dried workpieces to be welded onto the welding fixture, place the two workpieces together without leaving any assembly gap, and then put the entire fixture into the protective atmosphere chamber. During this process, it is essential to ensure that no oil contamination comes into contact with the workpieces to be welded.

[0040] 6. Set the laser welding parameters: Pass 1: Laser power 400W, spot diameter 2mm, welding speed 0.5m / min, powder feed rate 5.87g / min. Passes 2-4: Laser power 1500W, spot diameter 3.0mm, welding speed 0.5m / min, powder feed rate 9.40g / min. Pass 5: Laser power 1500W, spot diameter 3.0mm, welding speed 0.5m / min, powder feed rate 7.05g / min.

[0041] 7. The laser and powder feeding equipment are coaxially integrated, with 99.99% pure argon gas used as a protective gas, and the gas flow rate controlled at 25L / min. The laser emits a laser beam, simultaneously activating the powder feeding device to deliver powder. Finally, the robot is controlled to follow the specified path... Figure 1 The path planning shown illustrates the layer-by-layer melting and deposition process, ultimately completing the welding process. The resulting weld joint's weld cross-section is shown below. Figure 2 As shown in (a).

[0042] Example 2

[0043] This embodiment describes a multi-layer, multi-pass laser welding method for L-PBF formed TC4 titanium alloy. The material involved is 50mm*25mm*5mm LPBF formed TC4 titanium alloy, and TC4 powder is used as filler during the welding process (TC4 titanium alloy raw material powder has a particle size of 15-53μm, and the content of the main alloying elements is: Al: 5.77%-6.65%, V: 4.12%-4.56%, Fe: 0.255%-0.351%, Cr: 0.194%-0.380%, Ni: 0.136%-0.184%, with the balance being Ti).

[0044] The specific steps are as follows:

[0045] 1. Make a V-shaped bevel at the joint of the L-PBF formed TC4 titanium alloy base material to be welded, with an angle of 30° on one side and no blunt edge left;

[0046] 2. Mechanically grind the base material after beveling: Use 80 or 400 grit sandpaper to grind the oxide film on all surfaces of the base material to be welded until the metallic luster of the aluminum alloy is revealed;

[0047] 3. After mechanical grinding, chemical cleaning is required. The chemical cleaning involves washing in a 20% (mass percentage) nitric acid + 1-2% hydrofluoric acid aqueous solution for 8 minutes, until the silvery-white metal is exposed on the surface of the sheet. Then, remove the sheet and rinse it under running water. Next, wash it in a 20%-25% (volume fraction) sodium hydroxide aqueous solution for 5 minutes, followed by rinsing under running water. Finally, clean it with acetone.

[0048] 4. After completing the chemical cleaning, place the plates to be welded into a vacuum drying oven and dry them at 80°C for 2 hours;

[0049] 5. Fix the cleaned and dried workpieces to be welded onto the welding fixture, place the two workpieces together without leaving any assembly gap, and then put the entire fixture into the protective atmosphere chamber. During this process, it is essential to ensure that no oil contamination comes into contact with the workpieces to be welded.

[0050] 6. Set the laser welding parameters: Pass 1: Laser power 400W, spot diameter 2mm, welding speed 0.5m / min, powder feed rate 5.87g / min. Passes 2-4: Laser power 1500W, spot diameter 3.0mm, welding speed 0.5m / min, powder feed rate 9.40g / min. Pass 5: Laser power 1500W, spot diameter 3.0mm, welding speed 0.5m / min, powder feed rate 7.05g / min.

[0051] 7. The laser and powder feeding equipment are coaxially integrated, with 99.99% pure argon gas used as a protective gas, and the gas flow rate controlled at 25L / min. The laser emits a laser beam, simultaneously activating the powder feeding device to deliver powder. Finally, the robot is controlled to follow the specified path... Figure 1 The path planning shown illustrates the layer-by-layer melting and deposition process, ultimately completing the welding process. The resulting weld joint's weld cross-section is shown below. Figure 2 As shown in (b).

[0052] Comparative Example 1

[0053] This comparative example is a multi-layer, multi-pass laser welding method for L-PBF formed TC4 titanium alloy. The material involved is 50mm*25mm*5mm LPBF formed TC4 titanium alloy, and TC4 powder is used as filler during the welding process.

[0054] The specific steps are as follows:

[0055] 1. Make a V-shaped bevel at the joint of the L-PBF formed TC4 titanium alloy base material to be welded, with an angle of 30° on one side and no blunt edge left;

[0056] 2. Mechanically grind the base material after beveling: Use 80 or 400 grit sandpaper to grind the oxide film on all surfaces of the base material to be welded until the metallic luster of the aluminum alloy is revealed;

[0057] 3. After mechanical grinding, chemical cleaning is required. The chemical cleaning involves washing in a 20% (mass percentage) nitric acid + 1-2% hydrofluoric acid aqueous solution for 8 minutes, until the silvery-white metal is exposed on the surface of the sheet. Then, remove the sheet and rinse it under running water. Next, wash it in a 20%-25% (volume fraction) sodium hydroxide aqueous solution for 5 minutes, followed by rinsing under running water. Finally, clean it with acetone.

[0058] 4. After completing the chemical cleaning, place the plates to be welded into a vacuum drying oven and dry them at 80°C for 2 hours;

[0059] 5. Fix the cleaned and dried workpieces to be welded onto the welding fixture, place the two workpieces together without leaving any assembly gap, and then put the entire fixture into the protective atmosphere chamber. During this process, it is essential to ensure that no oil contamination comes into contact with the workpieces to be welded.

[0060] 6. Set the laser welding parameters: Pass 1: Laser power 300W, spot diameter 2mm, welding speed 0.3m / min, powder feed rate 7.05g / min. Passes 2-4: Laser power 1300W, spot diameter 3.0mm, welding speed 0.3m / min, powder feed rate 7.05g / min. Pass 5: Laser power 1300W, spot diameter 3.0mm, welding speed 0.3m / min, powder feed rate 7.05g / min.

[0061] 7. The laser and powder feeding equipment are coaxially integrated, with 99.99% pure argon gas used as a protective gas, and the gas flow rate controlled at 25L / min. The laser emits a laser beam, simultaneously activating the powder feeding device to deliver powder. Finally, the robot is controlled to follow the specified path... Figure 1 The path planning shown illustrates the layer-by-layer melting and deposition process, ultimately completing the welding process. The resulting weld joint's weld cross-section is shown below. Figure 2 As shown in (c).

[0062] Using the parameters in Table 1 and Figure 2 A comparison of the cross-sectional morphologies of each joint shows that, after optimization of the welding process parameters, the joint strength increased from 872 MPa to 1030 MPa, and the elongation increased from 2.1% to 3.2%. Using the optimized welding process parameters, the plasticity of the welded joint filled with TAO powder was significantly improved to 8.5%. It is evident that filling with TAO powder improved the weld microstructure, thereby achieving a significant improvement in the plasticity of the L-PBF formed TC4 titanium alloy welded joint.

[0063] Table 1 shows the mechanical properties obtained in Examples 1 and 2 and Comparative Example 1 using the tensile testing method for welded joints in GB / T 2651-2008.

[0064] Table 1

[0065] name Tensile strength (MPa) Elongation after fracture (%) Example 1 593 8.5 Example 2 1030 3.2 Comparative Example 1 872 2.1 .

Claims

1. A method for multi-layer, multi-pass laser welding of L-PBF-formed Ti6Al4V titanium alloy filled with TAO powder, characterized in that, Includes the following steps: (1) A bevel is made at the joint of the L-PBF-formed TC4 titanium alloy sheet; (2) After the bevel is made, the base material is first mechanically ground, and then chemically cleaned. (3) Place the cleaned base material to be welded into a vacuum drying oven to dry; (4) Fix the dried material to be welded onto the welding fixture and place it in the protective atmosphere chamber and pass a protective gas through it; (5) Start the fiber laser and feed the metal powder into the weld seam using a coaxial powder feeding method. Weld the material to be welded layer by layer through a multi-layer, multi-pass laser welding process. The metal powder mentioned in step (5) is TAO powder prepared by gas atomization, with a particle size of 75-105 μm. The mass fraction of each element is as follows: Al: 0.38%~0.45%, V: 0.27%~0.33%, Fe: 0.17%~0.23%, Ni: ≤0.03%, and the balance is Ti. The multi-layer, multi-pass laser welding process parameters described in step (5) are as follows: The first pass is the root pass, with the following parameters: laser power 300-400W, spot diameter 2mm, welding speed 0.3-0.5m / min, and powder feed rate 5.87-7.05g / min; the second to fourth passes are the fill pass, with the following parameters: laser power 1300-1500W, spot diameter 3mm, welding speed 0.3-0.5m / min, and powder feed rate 7.05-9.40g / min; the fifth pass is the cover pass, with the following parameters: laser power 1300-1500W, spot diameter 3mm, welding speed 0.3-0.5m / min, and powder feed rate 7.05g / min.

2. The method according to claim 1, characterized in that, The bevel mentioned in step (1) is a "V" shaped bevel with a single-sided angle of 30° and no blunt edge.

3. The method according to claim 1, characterized in that, The mechanical grinding described in step (2) uses 80-400 grit sandpaper to grind the bevel of the workpiece to be welded to a metallic luster.

4. The method according to claim 1, characterized in that, The chemical cleaning described in step (2) involves cleaning in a 20% nitric acid + 1~2% hydrofluoric acid aqueous solution for 6~8 minutes until the silvery-white metal is exposed on the surface of the plate. The plate is then removed and rinsed under running water. Next, it is cleaned in a 20%~25% sodium hydroxide aqueous solution for 4~6 minutes. After that, the plate is rinsed under running water and finally cleaned with acetone.

5. The method according to claim 1, characterized in that, The vacuum drying process described in step (3) is carried out at 80-100°C for 1-2 hours. The time from cleaning to the start of welding must not exceed 4 hours. If it exceeds 4 hours, the cleaning process of the two base materials to be welded needs to be repeated.

6. The method according to claim 1, characterized in that, The protective gas mentioned in step (4) is argon with a purity of 99.99%, and the gas flow rate is controlled at 20-25 L / min.

7. The welded part prepared according to any one of claims 1-6.

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