Hollow welding tool and method for friction stir butt welding

Abstract

The invention belongs to the technical field of welding, and particularly relates to a hollow welding tool and method for friction stir butt welding.The hollow welding tool comprises a stirring head and a static shaft shoulder, the static shaft shoulder is fixedly connected with a conical frame fixed to a spindle box, the periphery of the stirring head is sleeved with the static shaft shoulder, and the stirring head and the static shaft shoulder are eccentrically arranged; the center of the bottom end face of a stirring needle of the stirring head is provided with a slow flow groove, the stirring needle of the stirring head is of a hollow design, the material fluidity of the area near the tip of the stirring needle in the welding process can be effectively enhanced, and the indirect action range of the stirring needle is enlarged. The defect of incomplete penetration of the root of the butt joint is effectively avoided; meanwhile, defect-free butt welding can be achieved under the condition that the length of the stirring needle is small, contact between the stirring needle and the base plate is avoided, the stirring needle is prevented from being broken, and the service life of the stirring needle is prolonged.

Description

Technical Field

[0001] The present invention belongs to the field of welding technology, and particularly relates to a hollow welding tool and method for friction stir butt welding. Background Art

[0002] Friction stir welding (FSW), as a solid-phase joining technology, has the advantages of low welding temperature, high joint quality, and less material consumption. It can not only achieve high-quality joining of high-strength aluminum alloys, but also conform to the energy-saving and emission-reduction trend that has attracted much attention internationally, and is widely used in the fields of equipment manufacturing such as aviation, aerospace, shipbuilding, and vehicles. During the conventional FSW process, the arc pattern defects on the weld surface are determined by the movement characteristics of the stirring head. The arc pattern defects are inevitable during the welding process and are not conducive to the surface quality and joint performance. At the same time, in the final stage of welding, the keyhole defect caused by the withdrawal of the stirring head that composes the welding tool system from the test piece will cause serious stress concentration and deteriorate the mechanical properties of the joint. Scholars have proposed improvement methods and measures for the keyhole problem after friction stir welding.

[0003] Chinese Patent CN201820615823.9 discloses a stationary shoulder for friction stir welding used for welding non-planar plates. The bottom end of the designed stationary shoulder is a non-planar structure that matches the surface of the non-planar plate to be welded. Although this method can achieve the welding of non-planar structures, during the welding process, the right-angle design at the front edge of the stationary shoulder will increase the thinning amount of the plate to be welded, and the design of coaxially assembling the stirring head and the stationary shoulder will increase the probability of interference in the actual welding process.

[0004] Chinese Invention Patent CN111906432B discloses a friction stir lap welding method based on impinging streams, and designs an inwardly concave opposing-thread stirring pin, making the hook-shaped structure bend downward and greatly shortening the cold lap in the weld nugget. Although this method can obtain a downward-bent hook-shaped structure or greatly shorten the cold lap length, the stirring head is very sensitive to changes in process parameters, and due to the special inwardly concave stirring pin profile, the keyhole morphology will be damaged when the stirring pin is pulled out. When used in engineering practice, the joint interface morphology is not stable enough.

[0005] Chinese Invention Patent CN 111438433 A discloses a method and a welding tool for measuring the peak temperature in the central region of a FSW weld, relating to a welding tool for measuring the peak temperature in the central region of a FSW weld, including a stirring tool, a shoulder, a stirring pin, and filler metals with different melting points. The stirring pin is provided in the stirring tool, and part of the stirring pin is a hollow structure, and a filler metal ball is placed in the cavity of the stirring pin. Although this method adopts a similar hollow stirring pin morphology design, it only prevents the filler metal ball to accurately measure the center temperature of the weld, and does not consider the influence of the change of the stirring pin morphology on the material flow behavior and weld formation during the welding process. Summary of the Invention

[0006] In view of the problems that the existing welding processes and tools cannot effectively alleviate the arc pattern and keyhole defects at the same time, and the welding tool system has problems such as high manufacturing cost of components, complex coordination between components, and easy damage of structures such as stirring pins / sleeves, the present invention proposes a hollow welding tool and method for friction stir butt welding.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] A hollow welding tool for friction stir butt welding includes a stirring head and a stationary shoulder. The stationary shoulder is fixedly connected to a conical frame fixed on a main spindle box. A stationary shoulder is sleeved outside the stirring head. The stirring head and the stationary shoulder are eccentrically arranged, and a centrifugal ring is formed by the gap between the stationary shoulder and the stirring head. A flow retardation groove is opened at the center of the bottom end face of the stirring pin of the stirring head.

[0009] The diameter D1 of the flow retardation groove is D2 / 2, where D2 is the outer diameter of the stirring pin, and 4 mm ≤ D2 ≤ 20 mm; the depth b of the flow retardation groove satisfies 2D1 ≤ b ≤ 5D1.

[0010] There is a longitudinal offset distance between the center of the inner ring at the bottom end face of the stationary shoulder and the center of the stirring head. The gaps between the left and right ends of the inner ring at the bottom end face of the stationary shoulder and the stirring head are the same; after the stirring pin is deformed, the center of the stirring head coincides with the center of the inner ring at the bottom end face of the stationary shoulder.

[0011] The longitudinal offset distance c between the center of the inner ring at the bottom end face of the stationary shoulder and the center of the stirring head satisfies c = 0.05 - 0.1 mm; the diameter D3 of the inner ring at the bottom end face of the stationary shoulder satisfies D3 = D2 + 0.2 mm.

[0012] Taking the end face of the stationary shoulder in the welding forward direction as the front side, through grooves are symmetrically opened along the middle line at the left and right outer edges of the bottom end face of the stationary shoulder, so that the bottom end face of the stationary shoulder forms a convex platform structure, and the left and right side lines of the raised end face of the convex platform structure are parallel to the center line of the stationary shoulder.

[0013] The distance between the left and right side lines of the raised end face of the raised platform structure is e, satisfying 1.2D3 ≤ e ≤ 0.9D4, and the raised height of the raised platform structure is h, satisfying 0.5 mm ≤ h ≤ 3 mm.

[0014] At the connection between the edge of the bottom end face and the edge of its front end face of the stationary shoulder in the welding advancing direction, a drag reduction angle is provided, and the radius R1 of the drag reduction angle is 1 mm to 3 mm.

[0015] The outer ring diameter D4 of the bottom end face of the stationary shoulder satisfies D4 = k × D3; k is any value between 1.5 and 4.

[0016] A hollow welding method for friction stir butt welding includes the following steps:

[0017] Step 1: Select a stirring head with a suitable stirring pin length according to the thicknesses of the first plate and the second plate.

[0018] Step 2: Clamp the first plate and the second plate to be welded on the fixture of the friction stir welding machine, wherein the first plate is butted against the second plate.

[0019] Step 3: Install the stirring head on the main shaft of the friction stir welding machine. The stirring head rotates at a speed of 800 rpm ≤ ω ≤ 2100 rpm, and penetrates into the first plate and the second plate to be welded at a speed of 0.2 mm / min ≤ ν ≤ 2 mm / min at the welding starting position. When the stirring pin of the stirring head reaches a predetermined penetration depth of 1.0 mm to 4.0 mm and the stationary shoulder penetrates into the thickness of the first plate by 0.2 mm to 0.5 mm, the stirring head stops penetrating and continues to rotate for 2 min to 3 min.

[0020] Step 4: The stirring head advances at a rotation-to-welding ratio of 10 to 100 to promote the material flow between the first plate and the second plate and form a sufficient metallurgical bond.

[0021] Step 5: After reaching the weld end point, retract the stirring head, and the entire welding process is completed.

[0022] The first plate and the second plate have the same thickness, which is 1.0 mm to 5.0 mm; the first plate and the second plate are of the same aluminum alloy material or different aluminum alloy materials, and the first plate and the second plate are flat plates or curved plates.

[0023] The beneficial effects of the present invention:

[0024] 1. The bottom end face of the stationary shoulder adopts a boss structure in the direction parallel to the weld. The left and right side lines of the raised end face of the boss structure are parallel to the center line of the stationary shoulder, which can reduce the weld width and improve the welding quality. In addition, if the welded plate is curved, the bottom end face of the stationary shoulder can be designed with the same curvature radius as the welded plate, which also applies to the entire stirring tool.

[0025] 2. The stationary shoulder and the stirring head adopt an eccentric design with a centrifugal ring (the initial assembly gap is not equidistant), which can effectively prevent the stirring head from rubbing or colliding with the stationary shoulder caused by the displacement of the stirring head during the welding process due to the welding resistance, avoid the wear and even breakage of the stirring head, thereby increasing the service life of the stirring head and saving costs. At the same time, it can ensure that the stirring head and the stationary shoulder are in a small-gap and equidistant fit during the welding process, effectively alleviating the outflow of materials from the assembly gap during the welding process. In addition, a drag reduction angle is provided at the connection between the bottom end face and the front end face of the stationary shoulder, and this design helps to reduce the traveling resistance during the welding process and avoid the stationary shoulder from being pushed into the base material during the traveling process, affecting the forming quality.

[0026] 3. The stirring pin of the stirring head adopts a hollow design, which can effectively enhance the material fluidity in the area near the tip of the stirring pin during the welding process, increase the indirect action range of the stirring pin, and effectively avoid the generation of incomplete penetration defects at the root of the butt joint. At the same time, since defect-free butt welding can be achieved under the condition of a smaller stirring pin length, the contact between the stirring pin and the backing plate is avoided, the stirring pin is prevented from breaking, and the service life of the stirring pin is extended, which has great practical significance in engineering. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 is the front view sectional view of the hollow welding tool of the present invention;

[0028] Figure 2 is Figure 1 the bottom view of area A;

[0029] Figure 3 is the material flow diagram of the welding process using the welding tool of the present invention;

[0030] Figure 4 is the comparison diagram of the test result effects of welding using the welding tool with a circular bottom end face of the stationary shoulder and the welding tool of the present invention;

[0031] Among them, (a) is the test result effect diagram of welding using the welding tool with a circular bottom end face of the stationary shoulder;

[0032] (b) is the test result effect diagram of welding using the welding tool of the present invention;

[0033] In the attached drawings: 1. Static shoulder; 2. Stirring head; 3. Flow retardation groove; 4. Centrifugal ring; 5. Groove; 6. Drag reduction angle. Detailed implementation manners

[0034] The present invention will be further described in detail below with reference to the attached drawings and embodiments.

[0035] As Figures 1-2 shown, a hollow welding tool for friction stir butt welding includes a stirring head 2 and a static shoulder 1. The static shoulder 1 is fixedly connected to a conical frame fixed on the main spindle box. The periphery of the stirring head 2 is sleeved with the static shoulder 1. The stirring head 2 and the static shoulder 1 are eccentrically arranged, and a centrifugal ring 4 is formed by the gap between the static shoulder and the stirring head. A flow retardation groove 3 is opened at the center of the bottom end face of the stirring pin of the stirring head 2; the diameter D1 of the flow retardation groove 3 is D2 / 2, D2 is the outer diameter of the stirring pin, and 4 mm ≤ D2 ≤ 20 mm; the depth b of the flow retardation groove 3 satisfies 2D1 ≤ b ≤ 5D1;

[0036] During welding, the stirring head 2 is coaxially and cooperatively connected with the main spindle. The stirring head 2 and the static shoulder 1 are eccentrically arranged to form a centrifugal ring 4 to prevent interference between the stirring pin and the static shoulder 1 during the use of the stirring pin; the static shoulder 1 and the conical frame fixed on the main spindle box are connected by bolts. During the welding process, the main spindle box moves along the welding direction at a certain welding speed, driving the static shoulder 1 to move at the same speed and providing sealing and upsetting effects for the welding joint. At the same time, the stirring pin of the stirring head 2 adopts a hollow design, which can effectively enhance the material fluidity in the area near the tip of the stirring pin during the welding process, increase the indirect action range of the stirring pin, and effectively avoid the generation of incomplete penetration defects at the root of the butt joint;

[0037] There is a longitudinal offset distance between the center of the inner ring at the bottom end face of the static shoulder 1 and the center of the stirring head 2. The gaps between the left and right ends of the inner ring at the bottom end face of the static shoulder 1 and the stirring head 2 are the same; after the stirring pin is deformed, the center of the stirring head 2 coincides with the center of the inner ring at the bottom end face of the static shoulder; the longitudinal offset distance c between the center of the inner ring at the bottom end face of the static shoulder and the center of the stirring head 2 satisfies c = 0.05 mm to 0.1 mm; the diameter D3 of the inner ring at the bottom end face of the static shoulder satisfies D3 = D2 + 0.2 mm;

[0038] Through the design of the centrifugal ring 4, it can effectively prevent the stirring pin of the stirring head 2 from rubbing or colliding with the static shoulder 1 due to the displacement of the stirring pin of the stirring head 2 caused by the welding resistance during the welding process, avoid the wear and even breakage of the stirring pin of the stirring head 2, thereby increasing the service life of the stirring head 2 and saving costs.

[0039] Taking the end face of the stationary shoulder 1 in the welding advancing direction as the front side, through grooves 5 are symmetrically arranged along the left and right outer edges of the bottom end face of the stationary shoulder 1 along the center line, so that the bottom end face of the stationary shoulder 1 forms a boss structure, and the left and right side lines of the raised end face of the boss structure are parallel to the center line of the stationary shoulder 1; the distance between the left and right side lines of the raised end face of the boss structure is e, satisfying 1.2D3 ≤ e ≤ 0.9D4, and the raised height of the boss structure is h, satisfying 0.5mm ≤ h ≤ 3mm.

[0040] The design of the boss structure of the stationary shoulder 1, and the left and right side lines of the raised end face of the boss structure are parallel to the center line of the stationary shoulder 1, which can reduce the weld width and improve the welding quality.

[0041] At the connection between the bottom end face edge and the front side end face edge of the stationary shoulder 1 in the welding advancing direction, a drag reduction angle 6 is provided, and the radius R1 of the drag reduction angle 6 is 1mm to 3mm. The outer ring diameter D4 of the bottom end face of the stationary shoulder 1 satisfies D4 = k × D3; k is any value between 1.5 and 4.

[0042] The design of the drag reduction angle 6 helps to reduce the advancing resistance during welding and avoid the stationary shoulder 1 being pushed into the base material during the advancing process, affecting the forming quality.

[0043] A hollow welding method for friction stir butt welding includes the following steps:

[0044] Step 1: Select a stirring head 2 with a suitable stirring pin length according to the thicknesses of the first plate and the second plate. The thicknesses of the first plate and the second plate are the same, and the thickness is 1.0mm to 5.0mm; the first plate and the second plate are of the same aluminum alloy material or different aluminum alloy materials, and the first plate and the second plate are flat plates or curved plates.

[0045] Step 2: Clamp the first plate and the second plate to be welded on the fixture of the friction stir welding machine, where the first plate is butt-jointed with the second plate.

[0046] Step 3: Install the stirring head 2 on the main shaft of the friction stir welding machine. The stirring head 2 rotates at a speed of 800rpm ≤ ω ≤ 2100rpm and penetrates into the first plate and the second plate to be welded at a speed of 0.2mm / min ≤ ν ≤ 2mm / min at the welding starting position. When the stirring pin of the stirring head 2 reaches a predetermined penetration depth of 1.0mm to 4.0mm and the stationary shoulder 1 penetrates 0.2mm to 0.5mm into the thickness of the first plate, the stirring head 2 stops penetrating and continues to rotate for 2min to 3min.

[0047] Step 4: The stirring head 2 advances at a rotation-welding ratio of 10 to 100 to promote the material flow between the first plate and the second plate and form a sufficient metallurgical bond.

[0048] Step 5: After reaching the weld end point, retract the stirring head 2, and the entire welding process is completed.

[0049] By Figure 3 It can be seen that when the hollow welding tool of the present invention is used, it effectively enhances the material fluidity in the area near the tip of the stirring pin during the welding process, increases the indirect action range of the stirring pin, and effectively avoids the generation of incomplete penetration defects at the root of the butt joint.

[0050] Embodiment 1

[0051] For the hollow welding tool of this embodiment, the outer diameter D2 of the stirring pin is 6 mm, the diameter D1 of the flow retardation groove 3 is 3 mm, the depth b of the flow retardation groove 3 is 12 mm, the diameter D3 of the inner ring at the bottom end face of the stationary shoulder 1 is 6.2 mm, and the longitudinal offset distance c between the center of the inner ring at the bottom end face of the stationary shoulder 1 and the center of the stirring head 2 is 0.1 mm; the outer diameter D4 of the bottom end face of the stationary shoulder 1 is 12.2 mm, the protruding height h of the convex structure at the bottom end face of the stationary shoulder 1 is 1.5 mm, and the distance e between the side lines of the protruding end of the convex structure at the bottom end face of the stationary shoulder 1 is 10 mm; the radius R1 of the drag reduction angle 6 of the stationary shoulder 1 is 2 mm. The first plate and the second plate are flat plates, both made of 2024 aluminum alloy and both with a thickness of 2 mm.

[0052] A hollow welding method for friction stir butt welding includes the following steps:

[0053] Step 1: Select a stirring head 2 with a suitable stirring pin length according to the thickness of the first plate and the second plate;

[0054] Step 2: Clamp the first plate and the second plate to be welded on the fixture of the friction stir welding machine, where the first plate is butt-jointed with the second plate;

[0055] Step 3: Install the stirring head 2 on the main shaft of the friction stir welding machine. The stirring head 2 rotates at a speed of ω = 1200 rpm and penetrates into the first plate and the second plate to be welded at a speed of ν = 1.0 mm / min at the welding starting position. When the stirring pin of the stirring head 2 reaches a predetermined penetration depth of 1.5 mm and the stationary shoulder 1 penetrates 0.2 mm into the thickness of the first plate, the stirring head 2 stops penetrating and continues to rotate for 3 min;

[0056] Step 4: The stirring head 2 advances at a rotation-to-welding ratio of 80 to promote the material flow between the first plate and the second plate and form a sufficient metallurgical bond;

[0057] Step 5: After reaching the weld end point, retract the stirring head 2, and the entire welding process is completed.

[0058] The welding result is shown in Figure 4As shown, good surface morphology can be obtained both when using a welding tool with a circular bottom end face of a stationary shoulder and when using the welding tool of the present invention for welding. The sealing effect of the stationary shoulder 1 makes no obvious flash on the joint surface, and the "scraping" effect of the bushing avoids the generation of arc lines. Compared with welding using a welding tool with a circular bottom end face of an existing stationary shoulder, the weld width on the joint surface obtained by the welding tool of the present invention is significantly reduced due to the smaller scraping area.

[0059] Example 2

[0060] For the hollow welding tool of this embodiment, the outer diameter D2 of the stirring pin is 8 mm, the diameter D1 of the flow retardation groove 3 is 4 mm, and the diameter D3 of the inner ring at the bottom end face of the stationary shoulder 1 is 8.2 mm; the longitudinal offset distance c between the center of the inner ring at the bottom end face of the stationary shoulder 1 and the center of the stirring head 2 is 0.1 mm; the protruding height h of the convex structure at the bottom end face of the stationary shoulder 1 is 1.5 mm, and the spacing e between the side lines of the protruding end of the convex structure at the bottom end face of the stationary shoulder 1 is 10.5 mm; the outer diameter D4 of the bottom end face of the stationary shoulder 1 is 12.5 mm; the radius R1 of the drag reduction angle 6 of the stationary shoulder 1 is 2 mm; the first plate and the second plate are flat plates, the material of the first plate is 2024 aluminum alloy, the material of the second plate is 7050 aluminum alloy, and the thickness of both is 3 mm.

[0061] A hollow welding method for friction stir butt welding includes the following steps:

[0062] Step 1: Select a stirring head 2 with a suitable length dimension of the stirring pin according to the thickness of the first plate and the second plate;

[0063] Step 2: Clamp the first plate and the second plate to be welded on the fixture of the friction stir welding machine, wherein the first plate is butted against the second plate;

[0064] Step 3: Install the stirring head 2 on the main shaft of the friction stir welding machine. The stirring head 2 rotates at a speed of ω = 1500 rpm and penetrates into the plate to be welded at a speed of ν = 1.5 mm / min at the welding starting position. When the stirring pin of the stirring head 2 reaches a predetermined penetration depth of 4.0 mm and the stationary shoulder 1 penetrates 0.3 mm into the thickness of the first plate, the stirring head 2 stops penetrating and continues to rotate for 2 minutes;

[0065] Step 4: The stirring head 2 advances at a rotation-to-welding ratio of 100 to promote the material flow between the first plate and the second plate and form a sufficient metallurgical bond;

[0066] Step 5: After reaching the weld end point, retract the stirring head 2, and the entire welding process is completed.

[0067] The use of a "hollow" stirring head 2 can avoid the generation of incomplete penetration defects at the root of the joint, thereby achieving good connection of dissimilar material butt joints. At the same time, it can increase the distance between the tip of the stirring pin and the bottom of the plate, avoid interference between the stirring pin and the backing plate, and effectively prevent the stirring pin from breaking.

[0068] Example 3

[0069] For the hollow welding tool of this embodiment, the outer diameter D2 of the stirring pin is 4 mm, the diameter D1 of the buffer groove 3 is 2 mm, the depth b of the buffer groove 3 is 4 mm, the inner ring diameter D3 of the bottom end face of the stationary shoulder 1 is 4.2 mm, and the longitudinal offset distance c between the center of the inner ring of the bottom end face of the stationary shoulder 1 and the center of the stirring head 2 is 0.1 mm; the outer diameter D4 of the bottom end face of the stationary shoulder 1 is 10 mm, the protruding height h of the convex structure on the bottom end face of the stationary shoulder 1 is 2 mm, and the distance e between the side lines of the protruding end of the convex structure on the bottom end face of the stationary shoulder 1 is 7 mm; the radius R1 of the drag reduction angle 6 of the stationary shoulder 1 is 3 mm; the first plate and the second plate are curved plates with a bending radius of 900 mm, made of 2024 aluminum alloy, and the thickness of both is 2 mm.

[0070] A hollow welding method for friction stir butt welding includes the following steps:

[0071] Step 1: Select a stirring head 2 with a suitable stirring pin length according to the thickness of the first plate and the second plate;

[0072] Step 2: Clamp the first plate and the second plate to be welded on the fixture of the friction stir welding machine, where the first plate is butted against the second plate;

[0073] Step 3: Install the stirring head 2 on the main shaft of the friction stir welding machine. The stirring head 2 rotates at a speed of ω = 1600 rpm and penetrates into the plate to be welded at a speed of ν = 1.2 mm / min at the welding starting position. When the stirring pin of the stirring head 2 reaches a predetermined penetration depth of 1.5 mm and the stationary shoulder 1 penetrates 0.2 mm into the thickness of the first plate, the stirring head 2 stops penetrating and continues to rotate for 3 minutes;

[0074] Step 4: The stirring head 2 advances at a rotation-to-welding ratio of 40 to promote the material flow between the first plate and the second plate and form sufficient metallurgical bonding;

[0075] Step 5: After reaching the weld end point, retract the stirring head 2 to complete the entire welding process.

[0076] The stationary shoulder 1 with a drag reduction angle can effectively avoid the scraping effect of the bushing without a drag reduction angle on the material at the front end of the weld during the welding of the curved panel, and at the same time reduce the welding resistance to a certain extent, avoiding the interference between the stirring head and the stationary bushing and the occurrence of needle breakage. The hollow design adopted by the stirring head can effectively increase the indirect action range of the stirring pin, reducing the probability of the stirring pin breaking while effectively avoiding the root lack of penetration defect.

Claims

1. A hollow welding tool for friction stir butt welding, characterized in that: It includes a stirring head and a stationary shoulder. The stationary shoulder is fixedly connected to a conical frame fixed on the main shaft box. The outer sleeve of the stirring head is provided with a stationary shoulder. The stirring head and the stationary shoulder are eccentrically arranged, and the gap between the stationary shoulder and the stirring head forms a centrifugal ring. A slow flow groove is opened in the center of the bottom end face of the stirring needle of the stirring head.

2. A hollow welding tool for friction stir butt welding according to claim 1, characterized in that: The diameter D1 of the slow flow groove is D2 / 2, D2 is the outer diameter of the stirring needle, 4mm≤D2≤20mm; the depth b of the slow flow groove satisfies 2D1≤b≤5D1.

3. A hollow welding tool for friction stir butt welding according to claim 1, characterized in that: There is a longitudinal offset distance between the center of the inner ring of the bottom end face of the stationary shoulder and the center of the stirring head, and the gaps between the left and right ends of the inner ring of the bottom end face of the stationary shoulder and the stirring head are the same; after the stirring needle is deformed, the center of the stirring head coincides with the center of the inner ring of the bottom end face of the stationary shoulder.

4. A hollow welding tool for friction stir butt welding according to claim 3, characterized in that: The longitudinal offset distance c between the center of the inner ring of the bottom end face of the static shoulder and the center of the stirring head satisfies c=0.05-0.1 mm; the diameter D3 of the inner ring of the bottom end face of the static shoulder satisfies D3=D2+0.2 mm.

5. A hollow welding tool for friction stir butt welding according to claim 1, characterized in that: With the end face of the stationary shoulder in the welding forward direction as the front side, the left and right outer edges of the bottom end face of the stationary shoulder are symmetrically provided with penetrating grooves along the center line, so that the bottom end face of the stationary shoulder forms a boss structure, and the left and right side lines of the raised end face of the boss structure are parallel to the center line of the stationary shoulder.

6. A hollow welding tool for friction stir butt welding according to claim 5, characterized in that: The spacing between the left and right side lines of the raised end surface of the boss structure is e, satisfying 1.2D3≤e≤0.9D4, and the raised height of the boss structure is h, satisfying 0.5mm≤h≤3mm.

7. A hollow welding tool for friction stir butt welding according to claim 1, characterized in that: A drag reduction angle is provided at the connection between the edge of the bottom end face of the stationary shoulder in the welding forward direction and the edge of its front end face, and the drag reduction angle radius R1=1mm~3mm.

8. A hollow welding tool for friction stir butt welding according to claim 1, characterized in that: The bottom end face diameter D4 of the stationary shoulder satisfies D4=k×D3; k is any value between 1.5 and 4.

9. A hollow welding method for friction stir butt welding, according to the hollow welding tool of claim 1, characterized in that: The following steps are involved: Step 1: Select a stirring head with a suitable stirring needle length according to the thickness of the first plate and the second plate; Step 2: Clamp the first plate and the second plate to be welded on a tooling of the friction stir welding machine, wherein the first plate is butted against the second plate; Step 3: The stirring head is mounted on the main shaft of the friction stir welding machine, the stirring head rotates at a speed of 800rpm≤ω≤2100rpm, and penetrates into the first plate and the second plate to be welded at a speed of 0.2mm / min≤ν≤2mm / min at the welding starting position. When the stirring needle of the stirring head reaches a predetermined penetration depth of 1.0mm-4.0mm and the stationary shoulder penetrates into the thickness of the first plate by 0.2mm-0.5mm, the stirring head stops penetrating and continues to rotate for 2min-3min; Step 4: The stirring head is advanced at a transfer welding ratio of 10 to 100 to promote material flow between the first plate and the second plate to form a sufficient metallurgical bond; Step 5: After reaching the end point of the weld, retract the stirring head and the entire welding process is completed.

10. A hollow welding method for stir friction butt welding according to claim 9, characterized in that: The first plate and the second plate have the same thickness, which is 1.0 mm to 5.0 mm. The first plate and the second plate are made of the same aluminum alloy material or different aluminum alloy materials, and the first plate and the second plate are flat plates or curved plates.

Images