A device and method for preparing a strengthened and toughened heterogeneous composite joint by friction stir riveting welding

Abstract

A device and method for preparing a strengthened and toughened heterogeneous composite joint by friction stir riveting, it relates to a device and method for preparing a strengthened and toughened heterogeneous composite joint, the present invention is to solve the problems of poor toughness and sealing caused by weak single interface bonding in welding of dissimilar materials such as aluminum / steel and aluminum / titanium, and difficult filling of traditional welding and riveting structure design. The device described in the present invention includes a forming composite device, a macro-micro multi-scale structure and a special welding material; the forming composite device is composed of a working part and a cold metal transition welder, the macro-micro multi-scale structure is composed of a steel plate and an aluminum plate, the special welding material is arranged between the steel plate and the aluminum plate, and the working part and the welding gun of the cold metal transition welder are arranged above the special welding material. The present invention belongs to the field of solid phase welding and connection technology.

Description

Technical Field

[0001] The invention relates to a device and a method for preparing a strengthened and toughened heterogeneous composite joint, belonging to the technical field of solid phase welding and connection. Background Art

[0002] In order to reduce the weight of vehicles, fuel consumption and greenhouse gas emissions, the transportation, aerospace, shipbuilding and other industries are increasingly using dissimilar metal composite load-bearing structures, such as aluminum / steel, aluminum / titanium, aluminum / copper, etc., and the demand for dissimilar metal welding technology is becoming more and more vigorous. Taking aluminum / steel as an example, if aluminum-steel composite materials are used on ships, they can play a role in firmly connecting the deck steel material and the cabin aluminum alloy, thereby ensuring the safe operation of the ship. However, the physical and chemical properties of aluminum alloy and steel are quite different, and the traditional fusion welding cannot achieve effective connection of the two materials. The main reason is that the solubility of iron in aluminum at room temperature is extremely low, and it is easy to react to form a variety of high-hardness intermetallic compounds, which has always been a major problem in the field of welding. Controlling the generation and size of IMCs at the joint interface is the key to successful heterogeneous welding. Heterogeneous material connections can be divided into mechanical connection, fusion welding, brazing and solid phase welding. Among them, although the mechanical connection method will not produce IMCs at the interface, it will bring additional weight, which is not conducive to structural weight reduction; fusion welding is mainly based on high heat input at the interface, which can easily lead to the thickening of the interface compound; brazing is difficult to obtain high-strength welded joints and the presence of brazing material often leads to poor corrosion resistance; during solid phase welding, the base material does not melt, which can well control the generation of interface IMCs and is suitable for the connection of heterogeneous materials.

[0003] At present, for the solid-phase connection of heterogeneous materials, the main way to improve the performance is to regulate the morphology of interface compounds and change the interface structure of heterogeneous materials. One way is to improve the morphology of interface compounds by adding a transition layer to the interface of heterogeneous materials, such as adding a titanium alloy transition layer during the manufacturing process of aluminum / steel composite plates to avoid the production of aluminum-iron compounds, but two heterogeneous interfaces will appear inside the material, indirectly increasing the difficulty of regulating the interface organization of the aluminum / steel composite structure and its failure risk. Another main way is to make holes or grooves on the harder material side of the heterogeneous material to enhance the mechanical-solid-phase composite connection effect between heterogeneous materials, but it is difficult to achieve complete filling of thermoplastic materials and interface metallurgical bonding of macroscopic pre-set holes or grooves, resulting in poor interface bonding strength and sealing. It can be seen that although the strength of heterogeneous joints has been improved to a certain extent through the above means, it is still difficult to simultaneously meet the requirements of high strength, high toughness, ultra-low temperature, corrosion resistance and sealing required by the extreme service environment of heterogeneous joints through a single method, which limits the further engineering application and development of heterogeneous materials.

[0004] Therefore, it is urgent to develop new welding technologies to achieve the requirements of strengthening and sealing of heterogeneous material welded joints, and to provide technical support and guidance for the reliable manufacturing of heterogeneous materials. Summary of the invention

[0005] The present invention aims to solve the problems of poor toughness and sealing caused by weak single interface bonding in welding of dissimilar materials such as aluminum / steel and aluminum / titanium, and difficulty in filling in traditional welding and riveting structure design, and further proposes a device and method for preparing toughened heterogeneous composite joints by stir friction riveting.

[0006] The technical solution adopted by the present invention to solve the above problems is: the device for preparing a strengthened and toughened heterogeneous composite joint by stir friction riveting welding of the present invention comprises a forming composite device, a macro-micro multi-scale structure and a special welding material;

[0007] The forming composite device consists of a working part and a cold metal transition welder, the macro-micro multi-scale structure consists of steel plates and aluminum plates, special welding materials are arranged between the steel plates and the aluminum plates, and the welding guns of the working part and the cold metal transition welder are arranged above the special welding materials.

[0008] Further, the working part includes a synchronously rotating surface finishing milling cutter, an inner concave shoulder, a coaxial active stirring pin and a plurality of eccentric driven stirring pins;

[0009] The concave shoulder is fixed on the lower surface of the synchronously rotating surface finishing milling cutter, the coaxial active stirring pin is arranged in the middle of the lower surface of the concave shoulder, and a plurality of eccentric driven stirring pins are arranged on the lower surface of the concave shoulder along the circumferential direction.

[0010] The method for preparing a strengthened and toughened heterogeneous composite joint by friction stir riveting welding according to the present invention specifically comprises the following steps:

[0011] Step 1: Select a suitable cold metal transfer welder based on the aluminum / steel material and the performance requirements of the welded joint, and design and select welding materials to achieve the design and regulation of the interface structure;

[0012] Step 2, design and manufacture a working part that enhances interface metallurgy and drives strong flow of materials, wherein the working part comprises an external synchronously rotating surface finishing milling cutter, an inner concave shoulder, a coaxial active stirring needle and an eccentric driven stirring needle. According to the size and performance of the plate, the shape and size of the working part and the spatial distribution of the two driven stirring needles are designed to drive the material to fill the macro-micro multi-scale structure under the action of thermo-mechanical coupling, and enhance the interface metallurgical bonding in the horizontal and vertical directions;

[0013] Step 3: Prefabricate millimeter-level grooves on the surface of the steel plate to be welded, the depth of the grooves is not greater than the thickness of the steel plate, and prepare grooves on the butt joint surface to enhance the contact area of ​​the aluminum / steel butt joint surface; in a high-purity argon environment, use laser texturing to prepare microscopic micrometer-level notches on the bottom and side surfaces of the macroscopic grooves of the steel plate, the surface of the steel plate outside the grooves, and the surface of the grooves. The argon environment prevents surface oxidation from hindering metallurgical reactions, and the micrometer-level notches enhance the micromechanical interlocking effect of the aluminum / steel interface;

[0014] Step 4: Before welding, the steel plate and the aluminum alloy plate with macro-micro multi-scale structure are placed flat on the workbench in a butt-jointed configuration, and a certain gap is set between the steel plate and the aluminum alloy plate to ensure that the cold metal transition deposition welding wire can fully fill the groove area. The aluminum plate and the steel plate are supported by a restraining tool to ensure the contact state and assembly accuracy of the welding interface;

[0015] Step 5: During welding, place the cold metal transition deposition welding gun just above the aluminum / steel interface, feed the special welding wire gradually, and melt the welding wire by gradually swinging the welding gun, deposit and fill it into the gap of the butt groove and the inside of the macro and micro grooves until a certain excess height is formed on the weld surface. The welding gun continues to move forward to complete the gradual filling of the butt groove;

[0016] Step 6: After the cold metal transition deposition and stir friction treatment are completed, a strengthened and toughened heterogeneous composite joint is obtained.

[0017] Furthermore, the cold metal transfer deposition welding gun has an output power of 1000-3000W, a welding voltage of 20-40V, and a welding current of 10-100A.

[0018] Furthermore, the diameter of the concave shoulder of the working part is 1-10mm larger than the distance from the center of the eccentric driven stirring needle to the center of the concave shoulder, and the distance from the center of the eccentric driven stirring needle to the center of the concave shoulder is 2-10mm larger than the radius of the active stirring needle; the length of the coaxial active stirring needle is 0.5-5mm larger than the eccentric driven stirring needle; the diameter of the concave shoulder is 3 to 5 times the thickness of the aluminum alloy plate.

[0019] Furthermore, the macro groove depth is 0.5-10mm, the groove width is 2-20mm, the angle between the groove bottom and the side is 30-150°, and when multiple macro grooves are welded, the groove width is 1-200mm; the micro laser notch depth is 1-100μm, the notch width is 1-100μm, and the notch spacing is 1-5000μm; the steel side groove angle is 5-90°.

[0020] Furthermore, the spindle rotation speed of the working part is 10-10000rpm, the welding speed is 1-5000mm / min, the welding tool inclination angle range is 0°-5°, the penetration amount of the concave shoulder of the stir friction high-precision forming working part into the aluminum alloy plate is 0-1mm; the distance between the stir friction high-precision forming working part and the cold metal transition welding machine is 5-500mm.

[0021] The beneficial effects of the present invention are:

[0022] 1. The present invention takes into account the dual modes of "mechanical interlocking + interface metallurgy". By designing the macro-micro multi-scale structure and groove on the steel side, the macro-micro mechanical interlocking and interface reaction between heterogeneous materials are enhanced, and the strength and toughness of the joint are significantly improved.

[0023] 2. Cold metal transition deposition effectively supplements the materials needed to fill macro-micro grooves and notches, increases the interface bonding area of ​​heterogeneous joints, and promotes the formation of interface metallurgical connections in the horizontal and vertical directions, which is significantly higher than the traditional single welding metallurgical interface load, effectively ensuring the directional load of heterogeneous joint structures; further avoids the problem of weld thinning caused by upper plate material filling, and realizes the equal thickness and large load design of the joint; at the same time, the thermal effect of the cold metal transition deposition process can soften the steel side material, reduce the wear of the stir friction high-precision forming working part, and extend its service life;

[0024] 3. The design of special welding materials includes alloy elements that can react violently at heterogeneous interfaces, effectively inhibit the growth of heterogeneous interface reaction layers, and strengthen the aluminum / steel butt weld gap; the interface of the prepared heterogeneous joint is mainly a simple amorphous structure or an extremely thin intermetallic compound layer, avoiding the problem of poor corrosion caused by other elements introduced by traditional brazing;

[0025] 4. The design of coaxial active stirring needle and eccentric driven stirring needle is not only conducive to breaking the vertical and horizontal interface reaction layers of aluminum / steel and making them dispersed, but also can transform the internal structure of the weld into forging structure and remove internal defects such as pores and microcracks, thus strengthening the weld structure;

[0026] 5. The external synchronous rotating surface finishing milling cutter design in the stir friction high-precision forming working part can effectively remove the excess height caused by cold metal transition deposition and avoid welding forming problems caused by inconsistent excess height; at the same time, it can further remove defects such as flash and arc marks after the stir friction treatment, and simultaneously realize the surface treatment before and after welding, which significantly improves the welding efficiency and forming quality;

[0027] 6. This method is suitable for reliable connection between most "soft and hard" heterogeneous materials, such as aluminum / titanium, magnesium / titanium, magnesium / steel, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic diagram of the macro-micro multi-scale structure and groove of the steel plate designed by the present invention;

[0029] Figure 2 It is a schematic diagram of the friction stir high-precision forming working part designed by the present invention;

[0030] Figure 3 It is a schematic diagram of the aluminum / steel welding configuration assembly designed by the present invention;

[0031] Figure 4It is a schematic diagram of the aluminum / steel welding process designed by the present invention;

[0032] Figures 1 to 4 Among them, 1-forming composite device, 101-working part, 10101-synchronous rotating surface finishing milling cutter, 10102-concave shoulder, 10103-coaxial active stirring needle, 10104-eccentric driven stirring needle, 102-cold metal transfer welder, 10201-welding gun, 2-macro-micro multi-scale structure, 201-steel plate, 202-aluminum plate, 3-special welding materials. DETAILED DESCRIPTION

[0033] Specific implementation method 1: Figures 1 to 4 As shown, a device for preparing a strengthened and toughened heterogeneous composite joint by stir friction riveting welding comprises a forming composite device 1, a macro-micro multi-scale structure 2 and a special welding material 3;

[0034] The forming composite device 1 is composed of a working part 101 and a cold metal transition welder 102, the macro-micro multi-scale structure 2 is composed of a steel plate 201 and an aluminum plate 202, the special welding material 3 is arranged between the steel plate 201 and the aluminum plate 202, and the working part 101 and the welding gun 10201 of the cold metal transition welder 102 are arranged above the special welding material 3.

[0035] Among them, the cold metal transfer welder 102, on the one hand, provides deposition materials in the multi-scale grooves to promote the filling and interface of the materials; on the other hand, it can play the role of preheating the steel plate, avoiding the wear of the stirring needle during the thickness stirring friction high-precision forming process, and improving the welding quality and the life of the welding tool.

[0036] The macro-micro multi-scale structure 2 is mainly characterized by macro millimeter-level grooves and micro micrometer-level laser scratches. The macro millimeter-level grooves are distributed on the surface of the steel plate 201 and may be one or more. The micro micrometer-level laser scratches are distributed not only inside the macro grooves but also outside the grooves, so as to ensure that the heterogeneous structure has a macro-micro multi-scale strengthening effect, enhance the interface bonding effect and delay the crack propagation effect, thereby improving the toughness.

[0037] The special welding material 3 can be wire or rod, and its main components include Al, Si, Fe, Cr, Ni, Co and other components. It mainly plays the role of delaying the mutual diffusion and reaction between aluminum and iron elements and inhibiting the thickening of the interface reaction layer. At the same time, the welding wire is a high-component welding wire, which plays the role of strengthening the weld while ensuring the controllable interface structure.

[0038] With the mutual cooperation of the forming composite device 1, the macro-micro multi-scale structure 2, and the special welding material 3, it can ensure that the heterogeneous structure welding has the characteristics of macro-micro multi-scale mechanical interlocking, large-area metallurgical bonding, and equal thickness and large load-bearing high-quality forming, thereby realizing the strengthening and toughening manufacturing of aluminum / steel heterogeneous joints.

[0039] Specific implementation method 2: Figures 1 to 4 As shown, based on the specific implementation mode 1, the working part 1 includes a synchronously rotating surface finishing milling cutter 10101, an inner concave shoulder 10102, a coaxial active stirring pin 10103 and a plurality of eccentric driven stirring pins 10104;

[0040] The concave shoulder 10102 is fixed on the lower surface of the synchronously rotating surface finishing milling cutter 10101, the coaxial active stirring needle 10103 is arranged in the middle of the lower surface of the concave shoulder 10102, and a plurality of eccentric driven stirring needles 10104 are arranged on the lower surface of the concave shoulder 10102 along the circumferential direction.

[0041] Among them, the synchronous rotating surface finishing milling cutter 10101 in the working part 101 mainly solves the problems of surface excess height and weld unevenness caused by cold metal transition deposition, and can also solve the characteristics of flash and arc lines after stir friction treatment, so as to realize the fine processing of the weld surface;

[0042] The concave shoulder 10102 can prevent material from overflowing, reduce material loss, and thus avoid weld thinning and internal defects;

[0043] The coaxial active stirring needle 10103 is mainly responsible for stirring the materials at the interface of the heterogeneous joint, achieving mechanical interlocking in the depth direction and breaking and redistributing the reaction layer;

[0044] The eccentric driven stirring needle 10104 is mainly responsible for mixing and stirring the interface of heterogeneous materials in the horizontal direction, and improving the distribution state of the interface reaction layer;

[0045] Furthermore, the coaxial active stirring needle 10103 and the eccentric driven stirring needle 10104 can also improve the internal structure of the weld, transform the casting structure formed by cold metal transition deposition into forging induced by large plastic deformation of stirring friction, and further eliminate internal defects such as pores and cracks, thereby realizing controllable manufacturing of the internal structure of the weld.

[0046] The synchronously rotating surface finishing milling cutter 10101 of the working part 101 may be one or more, distributed on the edge of the inner concave shoulder 10102, and the material may be cemented carbide or other materials;

[0047] The number of the eccentric driven stirring needles 10104 can be one or more, and can be axially symmetrically distributed or dispersedly distributed, so as to enhance the stirring effect of aluminum / steel, promote the metallurgical bonding of the interface and facilitate the breaking of the intermetallic compounds generated at the interface;

[0048] The active stirring needle 10103 and the driven stirring needle 10104 can be designed as a split assembly with the shaft shoulder. The material can be hard materials such as H13 tool steel, tungsten-rhenium alloy, PCBN, etc., to reduce the wear and other problems caused by the contact between the stirring needle and the steel plate during welding.

[0049] Specific implementation method three: Figures 1 to 4 As shown, a method for preparing a strengthened and toughened heterogeneous composite joint by friction stir riveting welding comprises the following specific steps:

[0050] Step 1: Select a suitable cold metal transfer welding machine 102 according to the aluminum / steel material and the performance requirements of the welded joint, and design and select welding materials to achieve the design and regulation of the interface structure;

[0051] Step 2, design and manufacture a working part 101 for enhancing interface metallurgy and driving material strong flow, wherein the working part 101 comprises an external synchronous rotating surface finishing milling cutter 10101, an inner concave shoulder 10102, a coaxial active stirring needle 10103 and an eccentric driven stirring needle 10104, and according to the size and performance of the plate, the shape and size of the working part 101 and the spatial distribution of the two driven stirring needles 10104 are designed, and the material is driven to fill the macro-micro multi-scale structure under the action of thermo-mechanical coupling, and the interface metallurgical bonding in the horizontal and vertical directions is enhanced;

[0052] Step 3, machining a millimeter-level groove on the surface of the steel plate 201 to be welded, the depth of the groove is not greater than the thickness of the steel plate 201, and a groove is prepared on the butt joint surface to enhance the contact area of ​​the aluminum / steel butt joint surface; in a high-purity argon environment, laser texturing is used to prepare micro-micrometer-level notches on the bottom and side surfaces of the macro-grooves of the steel plate 201, the surface of the steel plate outside the grooves, and the surface of the grooves. The argon environment prevents surface oxidation from hindering metallurgical reactions, and the micrometer-level notches enhance the micro-mechanical interlocking effect of the aluminum / steel interface;

[0053] Step 4: Before welding, the steel plate 201 with the macro-micro multi-scale structure 2 and the aluminum alloy plate 202 are placed flat on the workbench in a butt-jointed configuration, and a certain gap is set between the steel plate 201 and the aluminum alloy plate 202 to ensure that the cold metal transition deposition welding wire can fully fill the groove area. The aluminum plate 202 and the steel plate 201 are supported by a restraining tool to ensure the contact state and assembly accuracy of the welding interface;

[0054] Step 5: During welding, place the cold metal transition deposition welding gun 10201 just above the aluminum / steel interface, feed the special welding wire gradually, and melt the welding wire by gradually swinging the welding gun 10201, deposit and fill it into the gap of the butt groove and the inside of the macro and micro grooves until a certain excess height is formed on the weld surface. The welding gun 10201 continues to move forward to complete the gradual filling of the butt groove;

[0055] Step 6: After the cold metal transition deposition and stir friction treatment are completed, a strengthened and toughened heterogeneous composite joint is obtained.

[0056] Among them, in step 5, when the cold metal transition deposition welding gun 10201 moves forward and the weld is cooled and solidified, the stir friction high-precision forming working part 101 intervenes, and the stir friction high-precision forming working part 101 rotates at a high speed and gradually penetrates into the aluminum / steel weld, and the shoulder surface is pressed into the weld until it is flush with the surface of the aluminum alloy plate 202 or the steel plate 201, then stops penetrating and moves at a certain speed to modify the internal structure of the weld; synchronously, the surface finishing milling cutter 10101 rotates synchronously at a high speed to remove the excess height of the weld in front of the stir friction high-precision forming working part, so as to ensure the stable forming of the modified area and prevent the weld from being damaged due to uneven weld. The problem of inconsistent joint forming and internal defects caused by flattening can be solved, and defects such as flash and arc lines formed behind the high-precision forming working part 101 of stir friction can be further removed to ensure beautiful forming; synchronously, the coaxial active stirring needle 10103 and the eccentric driven stirring needle 10104 stir the vertical butt joint of aluminum / steel and the horizontal overlap joint of aluminum / steel respectively, breaking the intermetallic compounds of the aluminum / steel interface and making them dispersed, and at the same time, the casting structure formed by cold metal transition deposition can be transformed into forging induced by large plastic deformation of stir friction, and internal defects such as pores and cracks can be further eliminated to realize the controllable manufacturing of the internal structure of the weld. The above factors work together to make the joint take into account the dual structure of "mechanical interlocking + interface metallurgy".

[0057] Specific implementation method four: Figures 1 to 4 As shown, the output power of the cold metal transfer deposition welding gun 10201 is 1000-3000W, the welding voltage is 20-40V, and the welding current is 10-100A.

[0058] Specific implementation method five: Figures 1 to 4 As shown, the diameter of the concave shoulder 10102 of the working part 101 is 1-10mm larger than the distance from the center of the eccentric driven stirring needle 10104 to the center of the concave shoulder 10102; the center of the eccentric driven stirring needle 10104 is 2-10mm larger than the radius of the active stirring needle 10103 from the center of the concave shoulder 10102; the length of the coaxial active stirring needle 10103 is 0.5-5mm larger than the eccentric driven stirring needle 10104; the diameter of the concave shoulder 10102 is 3 to 5 times the thickness of the aluminum alloy plate.

[0059] Specific implementation method six: Figures 1 to 4 As shown, the macro groove depth is 0.5-10mm, the groove width is 2-20mm, the angle between the groove bottom and the side is 30-150°, and when multiple macro grooves are welded, the groove width is 1-200mm; the micro laser notch depth is 1-100μm, the notch width is 1-100μm, and the notch spacing is 1-5000μm; the steel side groove angle is 5-90°.

[0060] Specific implementation method seven: Figures 1 to 4 As shown, the spindle rotation speed of the working part 101 is 10-10000rpm, the welding speed is 1-5000mm / min, the welding tool inclination angle range is 0°-5°, the penetration amount of the concave shoulder 10102 of the stir friction high precision forming working part 101 into the aluminum alloy plate 202 is 0-1mm; the distance between the stir friction high precision forming working part 101 and the cold metal transition welder 102 is 5-500mm.

[0061] The above is only a preferred embodiment of the present invention and does not limit the present invention in any form. Although the present invention has been disclosed as a preferred embodiment as above, it is not used to limit the present invention. Any technician familiar with this profession can make some changes or modify the technical contents disclosed above into equivalent embodiments without departing from the scope of the technical solution of the present invention. However, any simple modification, equivalent replacement and improvement made to the above embodiments without departing from the content of the technical solution of the present invention, based on the technical essence of the present invention, within the spirit and principles of the present invention, still fall within the protection scope of the technical solution of the present invention.

Claims

1. A device for preparing a strengthened and toughened heterogeneous composite joint by stir friction riveting welding, characterized in that: It comprises a forming composite device (1), a macro-micro multi-scale structure (2) and a special welding material (3); The forming composite device (1) is composed of a working part (101) and a cold metal transition welder (102); the macro-micro multi-scale structure (2) is composed of a steel plate (201) and an aluminum plate (202); a special welding material (3) is arranged between the steel plate (201) and the aluminum plate (202); and a welding gun (10201) of the working part (101) and the cold metal transition welder (102) is arranged above the special welding material (3); The macro-micro multi-scale structure (2) is mainly characterized by macro millimeter-scale grooves and micro micrometer-scale laser scratches. The macro millimeter-scale grooves are distributed on the surface of the steel plate (201) and can be one or more. The micro micrometer-scale laser scratches are distributed not only inside the macro grooves but also outside the grooves, thereby ensuring that the heterogeneous structure has a macro-micro multi-scale strengthening effect, enhancing the interface bonding effect and delaying the crack propagation effect, thereby achieving an improvement in toughness. The special welding material (3) can be a wire or a rod, and its main components include Al, Si, Fe, Cr, Ni, Co and the like. It mainly plays the role of delaying the mutual diffusion and reaction between aluminum and iron elements and inhibiting the thickening of the interface reaction layer. At the same time, the welding wire is a high-component welding wire, which plays the role of strengthening the weld while ensuring that the interface structure is controllable.

2. The device for preparing a strengthened and toughened heterogeneous composite joint by friction stir riveting welding according to claim 1, characterized in that: The working part (1) comprises a synchronously rotating surface finishing milling cutter (10101), an inner concave shaft shoulder (10102), a coaxial active stirring needle (10103) and a plurality of eccentric driven stirring needles (10104); The concave shoulder (10102) is fixed on the lower surface of the synchronously rotating surface finishing milling cutter (10101), the coaxial active stirring needle (10103) is arranged in the middle of the lower surface of the concave shoulder (10102), and a plurality of eccentric driven stirring needles (10104) are arranged on the lower surface of the concave shoulder (10102) along the circumferential direction.

3. A method for preparing a strengthened and toughened heterogeneous composite joint by stir friction riveting welding, characterized in that: The specific steps include: Step 1: Select a suitable cold metal transfer welding machine (102) according to the aluminum / steel material and the performance requirements of the welded joint, and design and select welding materials to achieve the design and regulation of the interface structure; Step 2, designing and manufacturing a working part (101) for enhancing interface metallurgy and driving material strong flow, wherein the working part (101) comprises an external synchronously rotating surface finishing milling cutter (10101), an inner concave shoulder (10102), a coaxial active stirring needle (10103) and an eccentric driven stirring needle (10104), and according to the size and performance of the plate, the shape and size of the working part (101) and the spatial distribution of the two driven stirring needles (10104) are designed, and the material is driven to fill the macro-micro multi-scale structure under the action of thermo-mechanical coupling, and the interface metallurgical bonding in the horizontal and vertical directions is enhanced; Step 3, machining a prefabricated millimeter-scale groove at the surface of the steel plate (201) to be welded, wherein the depth of the groove is not greater than the thickness of the steel plate (201), and preparing a groove on the butt joint surface to enhance the contact area of ​​the aluminum / steel butt joint surface; in a high-purity argon environment, laser texturing is used to prepare microscopic micrometer-scale notches on the bottom and side surfaces of the macroscopic groove of the steel plate (201), the surface of the steel plate outside the groove, and the surface of the groove, wherein the argon environment prevents surface oxidation from hindering metallurgical reactions, and the micrometer-scale notches enhance the microscopic mechanical interlocking effect of the aluminum / steel interface; Step 4: Before welding, the steel plate (201) having the macro-micro multi-scale structure (2) and the aluminum alloy plate (202) are placed flat on a workbench in a butt-jointed configuration, and a certain gap is set between the steel plate (201) and the aluminum alloy plate (202) to ensure that the cold metal transition deposition welding wire can fully fill the groove area, and a restraining tool is used to support the aluminum plate (202) and the steel plate (201) to ensure the contact state of the welding interface and the assembly accuracy; Step 5: During welding, the cold metal transition deposition welding gun (10201) is placed just above the aluminum / steel interface, and the special welding wire is gradually fed in. The welding gun (10201) is gradually swung to melt the welding wire, deposit and fill it into the gap between the butt grooves and inside the macro and micro grooves until a certain excess height is formed on the weld surface. The welding gun (10201) continues to move forward to complete the gradual filling of the butt groove. Step 6: After the cold metal transition deposition and the friction stir treatment are completed, a toughened heterogeneous composite joint is obtained; The special welding material (3) can be a wire or a rod, and its main components include Al, Si, Fe, Cr, Ni, Co and the like. It mainly plays the role of delaying the mutual diffusion and reaction between aluminum and iron elements and inhibiting the thickening of the interface reaction layer. At the same time, the welding wire is a high-component welding wire, which plays the role of strengthening the weld while ensuring that the interface structure is controllable.

4. The method for preparing a strengthened and toughened heterogeneous composite joint by friction stir riveting welding according to claim 3, characterized in that: The cold metal transfer deposition welding gun (10201) has an output power of 1000-3000W, a welding voltage of 20-40V, and a welding current of 10-100A.

5. The method for preparing a strengthened and toughened heterogeneous composite joint by friction stir riveting welding according to claim 3, characterized in that: The diameter of the concave shoulder (10102) of the working part (101) is 1-10 mm larger than the distance from the center of the eccentric driven stirring needle (10104) to the center of the concave shoulder (10102); the distance from the center of the eccentric driven stirring needle (10104) to the center of the concave shoulder (10102) is 2-10 mm larger than the radius of the active stirring needle (10103); the length of the coaxial active stirring needle (10103) is 0.5-5 mm larger than that of the eccentric driven stirring needle (10104); and the diameter of the concave shoulder (10102) is 3 to 5 times the thickness of the aluminum alloy plate.

6. The method for preparing a strengthened and toughened heterogeneous composite joint by friction stir riveting welding according to claim 3, characterized in that: The depth of the macro groove is 0.5-10mm, the width of the groove is 2-20mm, the angle between the bottom and the side of the groove is 30-150°, and when welding multiple macro grooves, the groove width is 1-200mm; the depth of the micro laser notch is 1-100μm, the notch width is 1-100μm, and the notch spacing is 1-5000μm; the steel side groove angle is 5-90°.

7. The method for preparing a strengthened and toughened heterogeneous composite joint by friction stir riveting welding according to claim 3, characterized in that: The main shaft rotation speed of the working part (101) is 10 to 10000 rpm, the welding speed is 1 to 5000 mm / min, the welding tool inclination angle ranges from 0° to 5°, the pressing amount of the concave shoulder (10102) of the stir friction high-precision forming working part (101) penetrating into the aluminum alloy plate (202) is 0 to 1 mm; and the distance between the stir friction high-precision forming working part (101) and the cold metal transition welding machine (102) is 5 to 500 mm.

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