Large trailing welding tool for friction stir lap welding

Abstract

The utility model belongs to the technical field of welding, and particularly relates to a large trailing welding tool for friction stir overlap welding, which comprises a stirring head and a stationary shaft shoulder, the stationary shaft shoulder and the stirring head are eccentrically arranged, and a centrifugal ring is formed in a gap between the stationary shaft shoulder and the stirring head. The length from the end face of the side, in the welding advancing direction, of the static shaft shoulder to the center of the stirring head is smaller than the length from the end face of the side, in the reverse direction of the welding advancing direction, of the static shaft shoulder to the center of the stirring head. The contact friction area between the stationary shaft shoulder and a material can be reduced, and the combination degree and the surface forming quality of the material are improved; meanwhile, the static shaft shoulder is provided with a centrifugal ring, so that abrasion and even breakage of the stirring needle caused by collision and friction between the stirring needle and the static shaft shoulder due to deformation of the stirring needle in the welding process can be effectively avoided.

Description

Technical Field

[0001] This utility model belongs to the field of welding technology, specifically relating to a large trailing welding tool for friction stir lap welding. Background Technology

[0002] Currently, the demand for homo / dissimilar metal composite structures is increasing in aerospace, automotive, and marine fields. However, the significant differences in physicochemical properties between the two materials lead to problems such as poor weldability, high tool wear, and difficulty in controlling intermetallic compounds during the welding of aluminum / steel composite structures. Friction stir lap welding (FSLW) has advantages in welding homo / dissimilar materials due to its low welding temperature and ease of controlling metal compounds. However, FSLW can experience insufficient upsetting and weak material flow during the friction stir process of homo / dissimilar alloys, which reduces the bonding strength between the homo / dissimilar materials and affects the quality of the weld joint.

[0003] Chinese patent CN 213289034 U discloses a welding tool and apparatus for friction stir welding. The designed stationary shoulder has a hollow structure, and the stationary shoulder, stirring pin, and main shaft are coaxial. Although this method can reduce the accumulation of welding material in the annular gap by reducing the outer diameter of the annular gap, the displacement of the stirring pin axis during welding can cause interference between the stirring pin and the stationary shoulder, easily leading to pin collision.

[0004] Chinese invention patent CN105772933A describes a combined stirring head for improving the surface quality of friction stir welded joints. The designed combined stirring head effectively eliminates defects such as arc marks, flash, and dents formed during friction stir welding. While the arc-shaped receiving groove designed in this method can effectively reduce scratches generated during the rolling process and improve the surface forming quality of the joint, the excessive assembly gap leads to increased material overflow during welding, affecting the internal forming of the joint.

[0005] Chinese invention patent CN111906432B discloses a stirring friction lap method based on impingement flow, which designs a concave, opposing threaded stirring pin, causing the hook-shaped structure to bend downwards and significantly shortening the cold lap length within the weld nugget. While this method can achieve a downwardly bent hook-shaped structure or significantly shorten the cold lap length, the stirring head is highly sensitive to changes in process parameters. Furthermore, the special concave stirring pin profile causes damage to the keyhole morphology when the stirring pin is pulled out, resulting in an unstable joint interface morphology in practical engineering applications. Utility Model Content

[0006] To address the shortcomings of existing technologies, this invention provides a large-tail welding tool for friction stir lap welding. The stationary shoulder features a large-tail design, which, while extending the upsetting time after mixing, reduces the contact friction area between the stationary shoulder and the material, improving the bonding strength and surface finish. Simultaneously, a centrifugal ring is incorporated into the stationary shoulder to effectively prevent wear or even breakage of the stirring pin caused by collision and friction between the stirring pin and the stationary shoulder due to deformation during welding. Furthermore, a semi-threaded design at the root of the stirring pin enhances material flowability, ultimately achieving high-quality forming of friction stir lap joints of the same / dissimilar alloys.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A large-tail welding tool for friction stir lap welding includes a stirring head and a stationary shoulder. The stirring head includes a moving shoulder and a stirring needle. The stationary shoulder is sleeved on the outer side of the stirring head and is fixedly connected to a tapered frame fixed on the spindle box. The stationary shoulder is eccentrically positioned with respect to the stirring head, and the gap between the stationary shoulder and the stirring head forms a centrifugal ring. The length of the end face of the stationary shoulder along the welding forward direction from the center of the stirring head is less than the length of the end face of the stationary shoulder along the reverse side of the welding forward direction from the center of the stirring head.

[0009] There is a longitudinal offset between the center of the inner ring of the bottom end face of the stationary shoulder and the center of the stirring needle. The gaps between the left and right sides 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 needle coincides with the center of the inner ring of the bottom end face of the stationary shoulder.

[0010] The inner ring diameter of the bottom end face of the stationary shoulder is D3 = D2 + 0.1 mm, and the 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 needle is b = 0.05 mm to 0.1 mm; D2 is the diameter of the root of the stirring needle, D2 = k × D1, 1.5 ≤ k ≤ 3, D1 is the diameter of the end face of the stirring needle, 2 mm ≤ D1 ≤ 5 mm.

[0011] The end face of the stationary shoulder along the welding forward direction and its opposite end face are both elliptical arcs. The center of the inner ring of the bottom end face of the stationary shoulder is the center of the circle, and the radius of the elliptical arc is R1, R1 = (0.75~1)×D3. The length of the parallel segment of the line connecting the endpoints of the elliptical arc along the welding forward direction and the opposite end face of the bottom end face of the stationary shoulder is a, a = 2×c; the width of the bottom end face of the stationary shoulder is c, c = (1.3~1.8)×D2.

[0012] The root of the stirring needle is provided with n semi-threaded grooves, n≥2; the thread width h of the semi-threaded groove is equal to the depth of 0.5mm~2mm, and the inclination angle θ between the two semi-threaded grooves is 15°~75°.

[0013] The thread direction of the semi-threaded groove at the root of the stirring needle needs to match the rotation direction of the stirring head. When the stirring head rotates counterclockwise, the thread direction of the semi-threaded groove at the root of the stirring needle is right-handed; when the stirring head rotates clockwise, the thread direction of the semi-threaded groove at the root of the stirring needle is left-handed.

[0014] The length L of the stirring needle satisfies D2≤L≤2×D2.

[0015] The beneficial effects of this utility model are:

[0016] 1. The stationary shoulder of this utility model adopts a centrifugal ring design, which allows for a small gap and equal spacing assembly with the stirring pin during the welding process. This reduces material overflow during welding and improves the forming quality of the weld joint. Simultaneously, it effectively avoids friction or collision between the stirring pin and the stationary shoulder caused by the displacement of the stirring pin's axis due to welding resistance, thus preventing wear or even breakage of the stirring pin and extending the service life of the stirring head, thereby saving manufacturing costs.

[0017] 2. The tail of the stationary shoulder of this utility model adopts a "meteor-shaped" large trailing tail design and the end face is elliptical. While extending the upsetting time of the material after stirring and mixing and ensuring the metallurgical bonding quality in the weld nugget, it can also reduce the weld width and make it easier to obtain a high-strength welded joint.

[0018] 3. The stirring pin of this utility model adopts a root semi-thread design, and the thread direction is used in conjunction with the stirring head rotation (right thread with counterclockwise or left thread with clockwise), so that the thread drives the material to be released at the end of the thread, forming a material accumulation area, and the end of the thread is located above the lap interface (in particular, for plates with aluminum cladding, such as 2xxx, 7xxx series aluminum alloys or aluminum-lithium alloy plates, in order to prevent the aluminum cladding from affecting the lap interface, the distance between the end of the thread and the upper surface of the aluminum cladding is required to be greater than 0). This allows the material concentration area to be mainly located above the lap interface, which helps to suppress the generation of hook-shaped defects at the lap interface. Attached Figure Description

[0019] Figure 1 A cross-sectional view of a large-tail welding tool used for friction stir lap welding;

[0020] Figure 2 for Figure 1 A bottom view of area A;

[0021] Figure 3 This is a schematic diagram showing the depth of the stirring pin during the welding process of this utility model.

[0022] Figure 4 This is a diagram showing the material flow during the welding process of this utility model;

[0023] Among them: 1. stationary shoulder; 2. stirring needle; 3. semi-threaded groove; 4. moving shoulder; 5. centrifugal ring; 6. upper plate to be welded; 7. lower plate to be welded. Detailed Implementation

[0024] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0025] like Figures 1-2 As shown, a large trailing welding tool for friction stir lap welding includes a stirring head and a stationary shoulder 1. The stirring head includes a moving shoulder 4 and a stirring pin 2. The stirring pin 2 is located below the moving shoulder 4. The stationary shoulder 1 is sleeved on the outside of the stirring head. The stationary shoulder 1 is fixedly connected to a tapered frame fixed on the spindle box. The stationary shoulder 1 is eccentrically positioned with respect to the stirring head, and the gap between the stationary shoulder 1 and the stirring head forms a centrifugal ring 5. The length of the end face of the stationary shoulder 1 along the welding forward direction from the center of the stirring head is less than the length of the end face of the stationary shoulder 1 along the reverse side of the welding forward direction from the center of the stirring head.

[0026] In this embodiment, when the welding tool is not in operation, the stationary shoulder 1 is located on the top outer side of the moving shoulder 4, and a centrifugal ring 5 is provided between them. During welding, the stirring needle 2 is inserted into the plate to be welded, and the spindle box moves along the welding direction at a certain welding speed, causing the stationary shoulder 1 to move at the same speed. The bottom end face of the stationary shoulder is always above the weld seam. At this time, the stirring needle 2 is located inside the stationary shoulder 1, and it and the moving shoulder 4 are coaxially connected with the spindle. The stirring needle 2 and the stationary shoulder 1 are in an eccentric clearance fit, and the centrifugal ring 5... The design prevents the stirring needle 2 from interfering with the stationary shoulder 1 during use. The length of the end face of the stationary shoulder 1 along the welding forward direction from the center of the stirring head is less than the length of the end face of the stationary shoulder 1 along the reverse side of the welding forward direction from the center of the stirring head. When the welding forward direction side is taken as the front end of the stationary shoulder 1, the length of the tail of the stationary shoulder 1 is greater than the length of the front end of the stationary shoulder. The length of the tail of the stationary shoulder 1 is greater than the length of the front end of the stationary shoulder, which makes it form a "meteor-shaped" large trailing tail design, providing frictional heat generation and upsetting effect for the lap joint.

[0027] There is a longitudinal offset between the center of the inner ring of the bottom end face of the stationary shoulder 1 and the center of the stirring needle 2. The gaps between the left and right sides of the inner ring of the bottom end face of the stationary shoulder 1 and the stirring head are the same. After the stirring needle 2 is deformed, the center of the stirring needle 2 coincides with the center of the inner ring of the bottom end face of the stationary shoulder 1. At this time, the gaps between the left and right sides of the inner ring of the bottom end face of the stationary shoulder 1 and the moving shoulder 4 are still the same. The diameter of the inner ring of the bottom end face of the stationary shoulder 1 is D3 = D2 + 0.1 mm. The longitudinal offset between the center of the inner ring of the bottom end face of the stationary shoulder 1 and the center of the stirring needle 2 is b = 0.05 mm ~ 0.1 mm. D2 is the diameter of the root of the stirring needle. D2 = k × D1, 1.5 ≤ k ≤ 3. D1 is the diameter of the end face of the stirring needle. 2 mm ≤ D1 ≤ 5 mm.

[0028] Considering the interference problem between the stirring pin 2 and the stationary shoulder 1 after deformation, this utility model is designed with an offset distance between the center of the inner ring of the bottom end face of the stationary shoulder 1 and the center of the stirring pin 2. During the welding process, after the stirring pin 2 is deformed and offset due to welding resistance, the center of the stirring pin 2 can coincide with the center of the inner ring of the bottom end face of the stationary shoulder 1. At this time, the stirring head is coaxial with the stationary shoulder, thus avoiding the wear of the stirring pin 2.

[0029] The end face of the stationary shoulder 1 along the welding direction and its reverse end face are both elliptical arcs. The center of the inner ring of the bottom end face of the stationary shoulder 1 is the center of the circle, and the radius of the elliptical arc is R1, R1 = (0.75~1)×D3. The length of the parallel segment connecting the endpoints of the elliptical arc along the welding direction and the reverse elliptical arc of the stationary shoulder 1 is a, a = 2×c. The width of the bottom end face of the stationary shoulder 1 is c, c = (1.3~1.8)×D2. The elliptical arc setting of the end face of the stationary shoulder 1 reduces the welding resistance during the welding process. By changing the length of the parallel segment a, the length of the tail of the stationary shoulder 1 can be changed, thereby changing the effect of frictional heat generation and upsetting provided by the lap joint.

[0030] The length L of the stirring needle 2 is D2≤L≤2×D2; the root of the stirring needle 2 is provided with n semi-threaded grooves 3, n≥2; the thread width h of the semi-threaded grooves 3 is equal to the depth of 0.5mm~2mm, and the inclination angle θ between two semi-threaded grooves 3 is 15°~75°; the thread direction of the semi-threaded grooves 3 at the root of the stirring needle 2 needs to match the rotation direction of the stirring head. When the rotation direction of the stirring head is counterclockwise, the thread direction of the semi-threaded grooves 3 at the root of the stirring needle is right-handed; when the rotation direction of the stirring head is clockwise, the thread direction of the semi-threaded grooves 3 at the root of the stirring needle is left-handed.

[0031] The semi-threaded groove 3 is only set on a part of the side of the root of the stirring needle 2. Through the design of the semi-threaded groove 3, the material concentration area is mainly located above the lap interface, which helps to suppress the generation of hook-shaped defects at the lap interface.

[0032] like Figure 3 As shown, a method for large tail welding in friction stir lap welding includes the following steps:

[0033] Step 1: Clamp the upper plate 6 and the lower plate 7 to be welded onto the fixture of the friction stir welding machine;

[0034] Step 2: Install the stirring head on the main shaft of the friction stir welding machine. Rotate the stirring head at a speed of 800rpm≤ω≤3000rpm and insert it into the upper plate 6 and the lower plate 7 to be welded at a speed of 0.2mm / min~5mm / min at the welding start position.

[0035] Step 3: When the stirring needle 2 is inserted to the end of the semi-threaded groove 3 at a distance e of 0.1mm to 0.5mm above the lap interface / above the aluminum cladding layer of the upper plate 6 and the lower plate 7 to be welded, and the stationary shoulder 1 penetrates 0.2mm to 0.5mm into the upper plate to be welded, the stirring head stops inserting and continues to rotate for 2min to 3min.

[0036] Step 4: The stirring head advances at a welding ratio of 10 to 150 to promote the vigorous downward and upward flow of the materials to be welded on the upper and lower plates, respectively, to form a full metallurgical bond;

[0037] Step 5: After reaching the end of the weld, pull back the stirring head, and the entire welding process is complete.

[0038] The upper plate 6 and the lower plate 7 to be welded are made of metal sheet or thermoplastic polymer sheet; the thickness of the upper plate 6 to be welded is 0.5mm to 2mm, and the thickness of the lower plate 7 to be welded is 1mm to 5mm.

[0039] like Figure 4 The diagram shows the material flow during the welding process of this utility model. As can be seen from the diagram, the material on the upper plate accelerates downward under the action of the counterclockwise rotating right-hand thread and is released at the end of the thread above the lap interface, forming a material accumulation area. This inhibits the upward migration of the lap interface, thereby increasing the effective connecting plate thickness of the joint and improving the joint performance.

[0040] Example 1

[0041] In this embodiment, both the upper plate 6 and the lower plate 7 to be welded are made of 2024 aluminum alloy and are flat plates with a thickness of 2mm.

[0042] In the welding tool, the diameters of the end face and root of the stirring pin 2 are D1 = 3mm and D2 = 6mm, respectively, and the length of the stirring pin 2 is L = 6mm. The root of the stirring pin 2 is provided with 4 semi-threaded grooves 3, the width of the semi-threaded grooves 3 is h = depth = 1.0mm, and the inclination angle between the two semi-threaded grooves is θ = 30°. The inner ring diameter of the bottom end face of the stationary shoulder 1 is D3 = 6.1mm, and the 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 pin is b = 0.1mm. The radius of the elliptical arc of the end face of the stationary shoulder 1 is R1 = 5.49mm, the width of the bottom end face is c = 7.8mm, and the length of the parallel section is a = 15.6mm. During the welding process, the distance between the end of the semi-threaded groove 3 of the stirring pin 2 and the top of the aluminum cladding layer is e = 0.2mm.

[0043] A method for large trailing welds in friction stir lap welding includes the following steps:

[0044] Step 1: Clamp the upper plate 6 and the lower plate 7 to be welded onto the fixture of the friction stir welding machine;

[0045] Step 2: Install the stirring head on the main shaft of the friction stir welding machine. The stirring head rotates at a speed of ω = 1800 rpm and penetrates into the upper plate 6 and the lower plate 7 to be welded at a speed of 2.0 mm / min at the welding start position.

[0046] Step 3: When the end of the semi-threaded groove 3 of the stirring needle 2 reaches a position 0.2mm above the aluminum cladding layer, and the stationary shoulder 1 penetrates 0.2mm into the upper plate to be welded, the stirring head stops penetrating and continues to rotate for 2.5 minutes.

[0047] Step 4: The stirring head advances at a 90-degree transfer ratio to promote the vigorous downward and upward flow of the upper and lower plate materials, respectively, to form a full metallurgical bond;

[0048] Step 5: After reaching the end of the weld, pull back the stirring head, and the entire welding process is complete.

[0049] In this embodiment, the stationary shoulder 1 adopts a centrifugal ring design, which can effectively avoid material overflow during the welding process and interference between tooling. Combined with the stirring pin with the root semi-threaded groove design, it can obtain a straight or even slightly bent interface morphology, thereby improving the load-bearing capacity of the lap weld joint.

[0050] Example 2

[0051] In this embodiment, the upper plate 6 to be welded is made of 2024 aluminum alloy, and the lower plate 7 to be welded is made of 7075 aluminum alloy, both being flat plates with a thickness of 2mm.

[0052] In the welding tool, the diameters of the end face and root of the stirring pin 2 are D1 = 2mm and D2 = 3mm, respectively, and the length of the stirring pin 2 is L = 5mm. The root of the stirring pin 2 is provided with 5 semi-threaded grooves 3, the width of the semi-threaded grooves 3 is h = depth = 1.5mm, and the inclination angle between the two semi-threaded grooves 3 is θ = 45°. The inner ring diameter of the bottom end face of the stationary shoulder 1 is D3 = 3.1mm, and the longitudinal distance between the center of the inner ring of the bottom end face of the stationary shoulder 1 and the center of the stirring pin is b = 0.1mm. The radius of the elliptical arc of the end face of the stationary shoulder 1 is R1 = 2.48mm, the width of the bottom end face is c = 4.5mm, and the length of the parallel section is a = 9mm. During the welding process, the distance between the end of the semi-threaded groove 3 of the stirring pin 2 and the top of the aluminum cladding layer is e = 0.3mm.

[0053] A method for large trailing welds in friction stir lap welding includes the following steps:

[0054] Step 1: Clamp the upper plate 6 and the lower plate 7 to be welded onto the fixture of the friction stir welding machine;

[0055] Step 2: Install the stirring head on the main shaft of the friction stir welding machine. The stirring head rotates at a speed of ω = 1200 rpm and penetrates into the upper plate 6 and the lower plate 7 to be welded at a speed of 1.5 mm / min at the welding start position.

[0056] Step 3: When the end of the semi-threaded groove 3 of the stirring needle 2 reaches a position 0.3mm above the aluminum cladding layer, and the stationary shoulder 1 penetrates 0.1mm into the upper plate to be welded, the stirring head stops penetrating and continues to rotate for 3.0min;

[0057] Step 4: The stirring head advances at a transfer ratio of 80 to promote the vigorous downward and upward flow of the upper and lower plate materials respectively, forming a full metallurgical bond;

[0058] Step 5: After reaching the end of the weld, pull back the stirring head, and the entire welding process is complete.

[0059] In this embodiment, since the material to be welded is 7075 aluminum alloy, the design of widening and deepening the semi-threaded groove 3 and increasing the inclination angle can effectively enhance the flow of material during the welding process. In conjunction with the large trailing static shoulder 1, a lap joint with good surface and internal forming is obtained, while achieving good bonding of the lap interface and obtaining a high load-bearing welded joint.

[0060] Example 3

[0061] In this embodiment, the upper plate 6 to be welded is made of PV polymer with a thickness of 1mm, and the lower plate 7 to be welded is made of 5083 aluminum alloy and is a flat plate with a thickness of 2mm.

[0062] In the welding tool, the diameters of the end face and root of the stirring pin 2 are D1 = 5mm and D2 = 10mm, respectively, and the length of the stirring pin 2 is L = 12mm. The root of the stirring pin 2 has six semi-threaded grooves 3, with a width h = depth = 1.5mm and an inclination angle θ = 60° between two semi-threaded grooves. The inner ring diameter of the bottom end face of the stationary shoulder 1 is D3 = 10.1mm, and the longitudinal offset distance b = 0.08mm between the center of the inner ring of the bottom end face of the stationary shoulder 1 and the center of the stirring pin. The radius R1 of the elliptical arc of the end face of the stationary shoulder 1 is 8.1mm, the width of the bottom end face is c = 15mm, and the length of the parallel section is a = 30mm. During welding, the distance e = 0.1mm between the end of the semi-threaded groove 3 of the stirring pin 2 and the top of the lap interface.

[0063] A method for large trailing welds in friction stir lap welding includes the following steps:

[0064] Step 1: Clamp the upper plate 6 and the lower plate 7 to be welded onto the fixture of the friction stir welding machine;

[0065] Step 2: Install the stirring head on the main shaft of the friction stir welding machine. The stirring head rotates at a speed of ω = 1000 rpm and penetrates into the upper plate 6 and the lower plate 7 to be welded at a speed of 1.8 mm / min at the welding start position.

[0066] Step 3: When the end of the semi-threaded groove 3 of the stirring needle 2 reaches a position 0.1mm above the lap interface and the stationary shoulder 1 penetrates 0.05mm into the upper plate to be welded, the stirring head stops penetrating and continues to rotate for 1.5min.

[0067] Step 4: The stirring head advances at a transfer ratio of 70 to promote the vigorous downward and upward flow of the upper and lower plate materials respectively, forming a full metallurgical bond;

[0068] Step 5: After reaching the end of the weld, pull back the stirring head, and the entire welding process is complete.

[0069] In this embodiment, since the material of the upper plate 6 to be welded is a low-melting-point PV polymer and the material of the lower plate 7 to be welded is a high-melting-point 5083 aluminum alloy, good joint surface forming can be obtained by combining with the large trailing static shoulder 1. At the same time, under the cooperation of the stirring pin 2 designed in the root semi-threaded groove 3, mechanical interlocking occurs at the interface, resulting in a welded joint with high load-bearing capacity.

Claims

1. A large trailing weld tool for friction stir lap welding, characterized by, The stirring head and the static shaft shoulder are provided with a dynamic shaft shoulder and a stirring needle, the stirring head is sleeved with the static shaft shoulder, the static shaft shoulder is fixedly connected with a conical frame fixed on the spindle box; the static shaft shoulder and the stirring head are eccentrically arranged, and the gap between the static shaft shoulder and the stirring head forms a centrifugal ring, the length of the end face of the static shaft shoulder on the side along the welding advancing direction from the center of the stirring head is less than the length of the end face on the reverse side along the welding advancing direction from the center of the stirring head.

2. A large trailing weld tool for friction stir lap welding as defined in claim 1, wherein, The longitudinal offset distance between the center of the inner ring of the bottom end face of the static shaft shoulder and the center of the stirring needle is provided, and the gap between the left and right sides of the inner ring of the bottom end face of the static shaft shoulder and the stirring head is the same; after the deformation of the stirring needle, the center of the stirring needle coincides with the center of the inner ring of the bottom end face of the static shaft shoulder.

3. A large trailing weld tool for friction stir lap welding according to claim 2, wherein, The diameter D3 of the inner ring of the bottom end face of the static shaft shoulder is D2+0.1mm, the longitudinal offset distance between the center of the inner ring of the bottom end face of the static shaft shoulder and the center of the stirring needle is b=0.05mm-0.1mm; D2 is the diameter of the root of the stirring needle, D2=k×D1, 1.5≤k≤3, D1 is the diameter of the end face of the stirring needle, 2mm≤D1≤5mm.

4. A large trailer tool for friction stir lap joining as defined in claim 3, wherein The end face of the static shaft shoulder on the side along the welding advancing direction and the reverse end face are both elliptical arcs, the center of the inner ring of the bottom end face of the static shaft shoulder is taken as the center, the radius of the elliptical arc is R1, R1=(0.75-1)×D3, the parallel segment length of the connecting line between the end points of the elliptical arcs on the side along the welding advancing direction and the reverse side of the bottom end face of the static shaft shoulder is a, a=2×c; the width of the bottom end face of the static shaft shoulder is c, c=(1.3-1.8)×D2.

5. A large trailer tool for friction stir lap joining as defined in claim 1, wherein The root of the stirring needle is provided with n half-thread grooves, n≥2; the thread width h of the half-thread groove is 0.5mm-2mm, and the inclination angle θ between the two half-thread grooves is 15°-75°.

6. A large trailing weld tool for friction stir lap welding according to claim 5, wherein, The thread rotation direction of the half-thread groove of the root of the stirring needle needs to be matched with the rotation direction of the stirring head, when the rotation direction of the stirring head is counterclockwise, the thread rotation direction of the half-thread groove of the root of the stirring needle is right-handed; when the rotation direction of the stirring head is clockwise, the thread rotation direction of the half-thread groove of the root of the stirring needle is left-handed.

7. A large trailing tool for friction stir lap joining as defined in claim 1, wherein The length L of the stirring needle satisfies D2≤L≤2×D2.

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