A method for improving softening of a friction stir welded joint of dissimilar aluminum alloys

Abstract

The application discloses a method for improving softening of a friction stir welded joint of dissimilar aluminum alloys, comprising the following steps: S1. preparation before welding; S2. interlayer placement: selecting a pure zinc foil with a purity of not less than 99.9%, and a thickness of 0.06mm to 0.1mm, butting 6061 aluminum alloy plate and 7075 aluminum alloy plate, and then placing the zinc foil at the center of the butt joint interface of the 6061 aluminum alloy plate and the 7075 aluminum alloy plate; S3. clamping and fixing; S4. friction stir welding: the stir head makes the pure zinc foil diffuse to the weld zone under the action of heat and mechanical force in a solid state; and S5. post-welding treatment. The method for improving softening of the friction stir welded joint of the dissimilar aluminum alloys adjusts the composition gradient of the weld zone through a solid-state diffusion alloying mechanism, optimizes the types and distribution of precipitated strengthening phases, and thus effectively relieves the joint softening, and improves the tensile strength and hardness uniformity of the joint.

Description

Technical Field

[0001] This invention relates to a method for improving the softening of dissimilar aluminum alloy friction stir welding joints by introducing a pure zinc foil interlayer, particularly for 6061 aluminum alloy and 7075 aluminum alloy, and belongs to the field of aluminum alloy welding technology. Background Technology

[0002] Friction stir welding (FSW), as a solid-state joining technology, is widely used in the welding of dissimilar aluminum alloys. 6061 aluminum alloy is a Mg-Si strengthened, heat-treatable aluminum alloy with good corrosion resistance and moderate strength; 7075 aluminum alloy is a Zn-Mg-Cu strengthened high-strength aluminum alloy, widely used in aerospace structural components. The joining of these two dissimilar materials has significant application value in lightweight structural design.

[0003] However, due to the significant differences in thermodynamic properties between 6061 and 7075 aluminum alloys, the thermo-mechanical cycle during friction stir welding causes the precipitated strengthening phases (such as MgZn2 and Mg2Si) in the weld zone and heat-affected zone to coarsen or dissolve, resulting in severe joint softening. Simultaneously, the uneven distribution of the two alloying elements and the abrupt compositional gradient in the weld zone further reduce the mechanical properties of the joint, hindering the widespread application of this technology in practical engineering.

[0004] Several invention patents have been issued regarding the improvement of the performance of friction stir welded joints of dissimilar aluminum alloys. For example: CN108296624A discloses a method for improving the strength of aluminum alloy friction stir spot welded joints. This method involves placing aluminum alloy plates in an lap joint configuration, with an intermediate layer (pure silver, pure zinc, zinc-aluminum alloy, or pure copper) of 0.01–1 mm thickness placed between the two alloy plates. Ultrasonic vibration is then used to assist friction stir spot welding, promoting diffusion bonding and eliminating hook defects. However, this method targets lap joint spot welded joints and employs an ultrasonic-assisted process, which differs fundamentally from butt joint continuous friction stir welding in terms of joint type, welding method, and mechanism of action.

[0005] CN119525689A discloses a method for improving the flowability of weld metal in friction stir welding of dissimilar aluminum alloys. This method involves butt-welding 7075 and 6061 aluminum alloy plates, and influencing the flowability and uniformity of hardness distribution of the weld joint by adjusting the ratio of the stirring pin length to the aluminum alloy plate thickness to 0.675~0.725. However, this method only improves joint performance by optimizing the stirring pin geometry and does not involve the use of any intermediate layer material, thus its effectiveness in addressing the joint softening problem is limited.

[0006] CN113369667A discloses a welded joint of dissimilar high-strength aluminum alloys and its preparation method, including abnormally high-temperature preheating before welding (200~450℃, preheating for 0.5~2 hours), high-heat-input friction stir welding (stirring head speed 500~1000rpm, welding speed 60~120mm / min), and solution aging treatment. This method achieves long-range diffusion of alloying elements through preheating and high heat input, forming a new intermediate alloy in the weld fusion zone, and overcomes the softening phenomenon caused by high heat input through solution aging treatment. However, this method does not use any intermediate layer material, relies on high-temperature preheating and high heat input processes, approaches a semi-molten state, and is complex and energy-intensive.

[0007] The existing technologies have the following drawbacks: 1. When butt welding dissimilar aluminum alloys (6061 and 7075), existing friction stir welding technologies cause coarsening or dissolution of the strengthening phases precipitated in the weld zone and heat-affected zone, leading to severe joint softening and a significant decrease in mechanical properties, making it difficult to meet the engineering requirements for high-strength connections. 2. Existing intermediate-layer assisted friction stir welding technologies are mostly designed for lap joints or spot welds, employing complex processes such as ultrasonic assistance, and are not suitable for continuous butt welding, thus limiting their application scope. 3. Existing methods for improving joint performance by optimizing welding process parameters (such as the geometry of the stirring pin) fail to fundamentally solve the problems of uneven element distribution and abrupt changes in composition gradient among dissimilar aluminum alloys, resulting in limited improvement in joint softening. 4. Existing methods that promote element diffusion through high-temperature preheating and high heat input before welding are complex, energy-intensive, and approach a semi-molten state, easily introducing new defects and hindering industrial-scale application. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a method for improving the softening of dissimilar aluminum alloy friction stir welded joints. By regulating the composition gradient of the weld zone through a solid-state diffusion alloying mechanism, the type and distribution of precipitated strengthening phases are optimized, thereby effectively alleviating joint softening, improving the tensile strength and hardness uniformity of the joint, and improving the comprehensive mechanical properties of 6061 / 7075 dissimilar aluminum alloy friction stir welded joints.

[0009] To address the aforementioned technical problems, this invention proposes a method for improving the softening of dissimilar aluminum alloy friction stir welded joints, comprising the following steps: S1. Pre-welding preparation: Mechanically grind and chemically clean the mating surfaces of 6061 aluminum alloy plate (2) and 7075 aluminum alloy plate (3) to remove oxide film and oil stains; S2. Intermediate layer placement: Select pure zinc foil with a purity of not less than 99.9% and a thickness of 0.06mm to 0.1mm, butt the 6061 aluminum alloy plate (2) and the 7075 aluminum alloy plate (3), and then place the zinc foil at the center of the butt interface between the 6061 aluminum alloy plate (2) and the 7075 aluminum alloy plate (3). S3. Clamping and fixing: Fix and press the plate with zinc foil placed on it on the special fixture for friction stir welding to ensure that the mating surfaces are tightly fitted; S4. Friction stir welding: 7075 aluminum alloy plate (3) is placed on the back side and 6061 aluminum alloy plate (2) is placed on the forward side; the stirring head contacts the zinc foil, the welding speed is 50-150 mm / min, and the pure zinc foil diffuses to the weld area under solid conditions under thermo-mechanical action; S5. Post-weld treatment: Allow to cool naturally to room temperature to obtain the welded joint.

[0010] The welding speed for friction stir welding is 60–80 mm / min.

[0011] The rotational speed for friction stir welding is 800~1500 rpm.

[0012] The shoulder pressing amount of the friction stir welding is 0.15 to 0.25 mm.

[0013] The stirring head (1) of the friction stir welding adopts a threaded conical stirring needle.

[0014] The shoulder diameter of the stirring needle is 2.5 to 3.5 times the diameter of the root of the stirring needle.

[0015] During welding, the angle of the stirring head (1) relative to the normal of the plate is 2.5°.

[0016] The beneficial effects of this invention are as follows: By introducing a pure zinc foil interlayer at the center of the interface between 6061 and 7075 aluminum alloys, the thermo-mechanical action during friction stir welding promotes the diffusion of zinc into the weld zone under solid conditions. The stirring head directly contacts the zinc foil, and the compositional gradient of the weld zone is controlled through a solid-state diffusion alloying mechanism, optimizing the type and distribution of precipitated strengthening phases. This effectively alleviates joint softening, improves the tensile strength and hardness uniformity of the joint, and enhances the overall mechanical properties of the 6061 / 7075 dissimilar aluminum alloy friction stir welded joint. Compared with existing technologies, this invention has a simple process, requires no preheating or post-weld heat treatment, and the zinc foil diffuses under solid conditions, maintaining the solid-state bonding characteristics of friction stir welding. It is suitable for continuous welding in butt joint configurations and has good prospects for industrial application. The pure zinc foil thickness range of 0.06 mm to 0.1 mm ensures that zinc fully diffuses into the weld metal, playing an effective role in alloying and microstructure control, while avoiding residue or the formation of a continuous brittle layer due to excessive zinc layer thickness. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the friction stir welding of dissimilar aluminum alloys according to the present invention.

[0018] Figure 2 This is a line graph showing the tensile strength properties of the embodiments and comparative examples of the present invention.

[0019] The above figures are labeled as follows: 1. Stirring head; 2. 6061 aluminum alloy sheet; 3. 7075 aluminum alloy sheet; 4. Zinc foil. Detailed Implementation

[0020] Example 1 See Figure 1 The method for improving the softening of dissimilar aluminum alloy friction stir welded joints in this embodiment includes the following steps: S1. Pre-welding preparation: Select 5mm thick 6061-T6 aluminum alloy sheet and 7075-T6 aluminum alloy sheet, both with dimensions of 200mm×100mm×5mm. Use sandpaper to mechanically grind the mating surfaces of the two sheets to remove the surface oxide film, then use acetone for chemical cleaning to remove oil stains, and finally wipe them clean with anhydrous ethanol and let them air dry naturally.

[0021] S2. Intermediate Layer Placement: Select pure zinc foil with a purity of 99.95%, a thickness of 0.10 mm, a width of 5 mm, and a length of 100 mm. Place the pure zinc foil at the center of the interface between the 6061 aluminum alloy sheet 2 and the 7075 aluminum alloy sheet 3, ensuring that the zinc foil 4 is tightly bonded to the mating surfaces of the two sheets.

[0022] S3. Clamping and fixing: Place the 6061 aluminum alloy plate 2 and the 7075 aluminum alloy plate 3 on the special fixture for friction stir welding, with the 7075 aluminum alloy plate on the backward side and the 6061 aluminum alloy plate on the forward side. Fix and press the plates with the fixture to ensure that they do not move during the welding process.

[0023] S4. Friction Stir Welding: A cylindrical stirring head 1 with a stirring pin diameter of 5mm and a shoulder diameter of 18mm is used. The material of stirring head 1 is H13 tool steel. Welding process parameters are set as follows: rotation speed 1000rpm, welding speed 80mm / min, shoulder depressurization 0.2mm, and tilt angle 2.5°. The friction stir welding equipment is started, stirring head 1 rotates and depresses to the set position, contacting zinc foil 4, and continuous welding is performed along the butt joint interface. During the welding process, zinc foil 4 diffuses into the weld zone under solid conditions under thermo-mechanical action, undergoing a solid-state diffusion alloying reaction with 6061 aluminum alloy sheet 2 and 7075 aluminum alloy sheet 3.

[0024] S5. Post-weld treatment: After welding is completed, remove the fixture and allow the welded joint to cool naturally to room temperature.

[0025] Example 2 The method for improving the softening of dissimilar aluminum alloy friction stir welded joints in this embodiment includes the following steps: S1. Pre-welding preparation: Select 6061-T6 aluminum alloy sheet and 7075-T6 aluminum alloy sheet with a thickness of 6mm, both with dimensions of 250mm×120mm×6mm. Use sandpaper to mechanically grind the mating surfaces of the two sheets to remove the surface oxide film, then use acetone for chemical cleaning to remove oil stains, and finally wipe them clean with anhydrous ethanol and let them air dry naturally.

[0026] S2. Intermediate Layer Placement: Select pure zinc foil with a purity of 99.99%, a thickness of 0.08mm, a width of 6mm, and a length of 250mm. Place the pure zinc foil at the center of the interface between the 6061 aluminum alloy sheet 2 and the 7075 aluminum alloy sheet 3, ensuring that the zinc foil 4 is tightly bonded to the mating surfaces of the two sheets.

[0027] S3. Clamping and fixing: Place the 6061 aluminum alloy plate 2 and the 7075 aluminum alloy plate 3 on the special fixture for friction stir welding, with the 7075 aluminum alloy plate 3 on the backward side and the 6061 aluminum alloy plate 2 on the forward side. Fix and press the fixture to ensure that the plates do not move during the welding process.

[0028] S4. Friction Stir Welding: A cylindrical stirring head 1 with a stirring pin diameter of 6mm and a shoulder diameter of 20mm is used. The material of stirring head 1 is H13 tool steel. Welding process parameters are set as follows: rotation speed 1200rpm, welding speed 60mm / min, shoulder pressure 0.18mm, and tilt angle 2.5°. The friction stir welding equipment is started, and stirring head 1 rotates and presses down to the set position, performing continuous welding along the butt joint interface. During the welding process, zinc foil 4 diffuses into the weld zone under solid-state conditions under thermo-mechanical action, undergoing a solid-state diffusion alloying reaction with the 6061 aluminum alloy sheet 2 and the 7075 aluminum alloy sheet 3.

[0029] S5. Post-weld treatment: After welding is completed, remove the fixture and allow the welded joint to cool naturally to room temperature.

[0030] Example 3 The method for improving the softening of dissimilar aluminum alloy friction stir welded joints in this embodiment includes the following steps: S1. Pre-welding preparation: Select 4mm thick 6061-T6 aluminum alloy sheet and 7075-T6 aluminum alloy sheet, both with dimensions of 180mm×90mm×4mm. Use sandpaper to mechanically grind the mating surfaces of the two sheets to remove the surface oxide film, then use acetone for chemical cleaning to remove oil stains, and finally wipe them clean with anhydrous ethanol and let them air dry naturally.

[0031] S2. Intermediate Layer Placement: Select pure zinc foil with a purity of 99.95%, a thickness of 0.06mm, a width of 4mm, and a length of 180mm. Place the pure zinc foil at the center of the interface between the 6061 aluminum alloy and the 7075 aluminum alloy, ensuring that the zinc foil is tightly bonded to the mating surfaces of the two plates.

[0032] S3. Clamping and fixing: Place the 6061 aluminum alloy sheet and the 7075 aluminum alloy sheet on the special fixture for friction stir welding, with the 7075 aluminum alloy on the backward side and the 6061 aluminum alloy on the forward side. Fix and press the fixture to ensure that the sheet does not move during the welding process.

[0033] S4. Friction Stir Welding: A cylindrical stirring head with a stirring pin diameter of 4mm and a shoulder diameter of 16mm is used. The stirring head material is H13 tool steel. Welding process parameters are set as follows: rotation speed 800rpm, welding speed 100mm / min, shoulder depressurization 0.15mm, and tilt angle 2.5°. The friction stir welding equipment is started, the stirring head rotates and depresses to the set position, and continuous welding is performed along the butt joint interface. During the welding process, pure zinc foil diffuses into the weld zone under solid-state conditions under thermo-mechanical action, undergoing a solid-state diffusion alloying reaction with 6061 aluminum alloy and 7075 aluminum alloy.

[0034] S5. Post-weld treatment: After welding is completed, remove the fixture and allow the welded joint to cool naturally to room temperature.

[0035] Example 4 The method for improving the softening of dissimilar aluminum alloy friction stir welding joints in this embodiment is the same as in Embodiment 1, except that the zinc foil thickness is 0.75 mm and the welding speed is 60 mm / min.

[0036] Comparative Example 1 The aluminum alloy friction stir welding method in this comparative example is the same as that in Example 1, except that no intermediate layer is added.

[0037] Comparative Example 2 The aluminum alloy friction stir welding method in this comparative example is the same as that in Example 1, except that the thickness of the zinc foil is 0.05 mm.

[0038] Comparative Example 3 The aluminum alloy friction stir welding method in this comparative example is the same as that in Example 1, except that the thickness of the zinc foil is 0.12 mm.

[0039] Tensile strength tests were performed on the welded samples from Examples 1 to 4 and Comparative Examples 1 and 2, using the national standard GB / T 228.1-2021. The test results are as follows: Table 1 Tensile Strength Properties Example 1 292 Example 2 298 Example 3 293 Example 4 302 Comparative Example 1 268 Comparative Example 2 275 Comparative Example 3 277 As can be seen from the table above, such as Figure 2 As shown, the tensile strength is highest when the zinc foil thickness is 0.75 mm and 0.08 mm, indicating that this zinc foil thickness ensures sufficient diffusion of zinc into the weld metal, effectively alloying and regulating the microstructure, improving the mechanical properties of the joint, and mitigating softening. In Comparative Example 2, when the zinc foil thickness is 0.05 mm, the zinc content is insufficient, resulting in inadequate mechanical properties of the joint. In Comparative Example 3, when the zinc foil thickness is 0.12 mm, the excessive zinc layer leads to residue, forming a continuous brittle layer.

[0040] Obviously, the above embodiments are merely examples to clearly illustrate the embodiments of the present invention, and are not intended to limit the embodiments of the present invention. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all embodiments here. However, these obvious variations or modifications derived from the spirit of the present invention are still within the protection scope of the present invention.

Claims

1. A method for improving the softening of dissimilar aluminum alloy friction stir welded joints, comprising the following steps: S1. Pre-welding preparation: Mechanically grind and chemically clean the mating surfaces of 6061 aluminum alloy plate (2) and 7075 aluminum alloy plate (3) to remove oxide film and oil stains; S2. Intermediate layer placement: Select pure zinc foil with a purity of not less than 99.9% and a thickness of 0.06mm to 0.1mm, butt the 6061 aluminum alloy plate (2) and the 7075 aluminum alloy plate (3), and then place the zinc foil at the center of the butt interface between the 6061 aluminum alloy plate (2) and the 7075 aluminum alloy plate (3). S3. Clamping and fixing: Fix and press the plate with zinc foil placed on it on the special fixture for friction stir welding to ensure that the mating surfaces are tightly fitted; S4. Friction stir welding: 7075 aluminum alloy plate (3) is placed on the back side and 6061 aluminum alloy plate (2) is placed on the forward side; the stirring head (1) contacts the zinc foil (4), the welding speed is 50-150 mm / min, and the pure zinc foil diffuses to the weld area under solid conditions under thermo-mechanical action; S5. Post-weld treatment: Allow to cool naturally to room temperature to obtain the welded joint.

2. The method of claim 1, wherein: The welding speed for friction stir welding is 60–80 mm / min.

3. The method of claim 2, wherein: The rotational speed for friction stir welding is 800~1500 rpm.

4. The method of improving softening of a friction stir welded joint of dissimilar aluminum alloys of claim 3, wherein: The shoulder pressing amount in the friction stir welding is 0.15 to 0.25 mm.

5. The method of improving softening of a friction stir welded joint of dissimilar aluminum alloys of any one of claims 1-4, wherein: The stirring head (1) of the friction stir welding adopts a threaded conical stirring needle.

6. The method of improving softening of a friction stir welded joint of dissimilar aluminum alloys of claim 5, wherein: The shoulder diameter of the stirring needle is 2.5 to 3.5 times the diameter of the root of the stirring needle.

7. The method of improving softening of a friction stir welded joint of dissimilar aluminum alloys of claim 6, wherein: During welding, the angle of the stirring head (1) relative to the normal of the plate is 2.5°.

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