A multi-scale composite structure and method for realizing a heterogeneous friction stir welding joint

Abstract

A multi-scale composite structure and method for realizing a heterogeneous friction stir welding joint. The present invention relates to a composite structure of a welding joint and a welding method. The present invention aims to solve the problem of low strength and toughness of the joint caused by the weak single interface bonding and the large thickness of intermetallic compounds in the aluminum / steel dissimilar welding joint. It includes a friction stir working part, a plate, and a special milling cutter. The friction stir working part is located above the plate. The friction stir working part includes a concave shoulder, a driving stirring pin, and a driven stirring pin. The special milling cutter is sleeved on the concave shoulder. The driving stirring pin and the driven stirring pin are installed at the bottom of the concave shoulder. The driving stirring pin is arranged coaxially with the concave shoulder, and the driven stirring pin is located outside the driving stirring pin. Through the macro-micro multi-scale structure design, the present invention enables the joint to take into account the dual modes of "mechanical interlocking + interfacial metallurgy", significantly enhancing the strength and toughness of the aluminum / steel dissimilar welding joint. The present invention belongs to the technical field of solid-phase welding and connection.

Description

Technical Field

[0001] The present invention relates to a welded joint composite structure and a welding method, and more particularly to a multi-scale composite structure and method for realizing the strengthening and toughening of dissimilar friction stir welded joints. The present invention belongs to the technical field of solid-phase welding and connection. Background Art

[0002] Facing the urgent demands for high performance and lightweight of structures in the fields of aerospace, new energy, etc., the aluminum / steel dissimilar composite structure is an effective way to meet the lightweight design and high service strength. Aluminum / steel dissimilar materials are often used as transition joints for connecting large structures, and have strict requirements for strength, toughness, and sealing performance. However, due to the huge differences in physical and chemical properties between aluminum and steel, the connection of aluminum / steel dissimilar metals has always been a difficult point in the welding field, and the main difficulties are as follows: (1) The linear expansion coefficient and thermal conductivity of aluminum are much larger than those of steel, which will affect the heat conduction and material expansion and contraction deformation during welding, resulting in large residual stresses and deformations, and aggravating the crack tendency; (2) The mutual solubility between Al and Fe is extremely low. During the post-weld cooling, Fe and Al are prone to form hard and brittle aluminum-rich intermetallic compounds such as Fe 2 Al 5 and FeAl 3 etc., which greatly affect the mechanical properties of the joint, especially the toughness is significantly reduced, mainly characterized by typical brittle fracture, and significantly reduces the service performance of the aluminum / steel dissimilar joint.

[0003] Currently, the welding of aluminum-steel dissimilar materials mainly adopts the methods of brazing, fusion brazing, and friction stir welding. When using brazing, it is difficult to control the intermetallic compounds at the brazed joint interface, the product quality is extremely unstable, and the yield rate is not more than 50%. Fusion brazing mainly improves the joint strength by increasing the weld reinforcement and contact area, but the intermetallic compounds at the interface grow severely at high temperatures and are difficult to control; friction stir welding can effectively control the growth of intermetallic compounds at the interface. Compared with brazing / fusion brazing, it can reduce the welding stress and the tendency of crack generation, but the joint still mainly has a thin layer of intermetallic compounds, which improves the joint strength to a certain extent, but the toughness is still low, greatly restricting the wide application of aluminum / steel dissimilar joints. Summary of the Invention

[0004] In order to solve the problems of low joint strength and toughness caused by weak single-interface bonding and large thickness of intermetallic compounds in aluminum / steel dissimilar welded joints, the present invention further provides a multi-scale composite structure and method for realizing dissimilar friction stir welded joints. The technical solutions adopted by the present invention to solve the above problems are as follows:

[0005] The present invention includes a friction stir working part, a sheet material, and a special milling cutter. The friction stir working part is located above the sheet material. The friction stir working part includes a concave-shaped shoulder, a driving stirring pin, and a driven stirring pin. The special milling cutter is sleeved on the concave-shaped shoulder. The driving stirring pin and the driven stirring pin are installed at the bottom of the concave-shaped shoulder. The driving stirring pin is arranged coaxially with the concave-shaped shoulder, and the driven stirring pin is located outside the driving stirring pin.

[0006] A welding method for realizing a multi-scale composite structure of a dissimilar friction stir welding joint is achieved through the following steps:

[0007] Step 1: According to the aluminum / steel material and the performance requirements of the welded joint, design and manufacture the shapes, sizes of the concave shoulder, the driving stirring pin, and the driven stirring pin of the multi-pin friction stir working part for enhancing interfacial metallurgy and driving strong material flow, as well as the spatial distribution of the two driven stirring pins.

[0008] Step 2: Select a steel plate with appropriate thickness and size. Mechanically process prefabricated millimeter-scale grooves on the front and back sides of the steel plate to be welded. In an environment of high-purity argon, use laser texturing to prepare micro-scale micron-level scratches on the bottom and side surfaces of the grooves on the steel plate and on the surface of the steel plate outside the grooves.

[0009] Step 3: Select an aluminum plate with appropriate thickness and size. Prepare a convex platform on the surface of the aluminum plate. At the same time, mechanically subtract material to process a concave structure in the middle position of the butt joint surface of the aluminum plate.

[0010] Step 4: During welding, adopt a lap joint configuration form. The convex structure of the steel plate with macro-micro multi-scales can be assembled and inserted into the concave structure of the aluminum plate, ensuring that the convex platform structure of the aluminum plate is located directly above the groove of the steel plate. Use a restraint tooling to clamp the aluminum plate and the steel plate to ensure the contact state and assembly accuracy of the welding interface.

[0011] Step 5: During welding, move the multi-pin friction stir working part to directly above the convex platform structure of the aluminum alloy. The multi-pin friction stir working part rotates at a high speed and penetrates into the interior of the aluminum plate / steel plate until the driving stirring pin penetrates into the middle of the butt joint surface of the aluminum plate / steel plate, and the driven stirring pin slightly contacts the aluminum / steel lap joint surface to perform the front welding of the aluminum / steel dissimilar joint.

[0012] Step 6: After completing the front welding of the aluminum / steel dissimilar joint, use a special milling cutter with surface refinement treatment to process the weld seam on the front.

[0013] Step 7: After the surface treatment is completed, flip and rigidly fix and clamp the aluminum plate / steel plate to prepare for back welding. Repeat Steps 5 and 6 to complete the back welding of the aluminum / steel joint.

[0014] The beneficial effects of the present invention are:

[0015] The present invention has the following advantages compared with the current mechanical connection or welding methods:

[0016] 1. Through the macro-micro multi-scale structure design of the macroscopic grooves and microscopic scratches on the steel plate, the joint combines the dual modes of "mechanical interlocking + interfacial metallurgy", significantly enhancing the strength and toughness of the aluminum / steel dissimilar welded joint;

[0017] 2. The design of the convex platform on the aluminum plate effectively supplements the materials required for filling the macroscopic grooves, avoiding the problem of weld thinning caused by the filling of the upper plate material, and realizing the design of equal-thickness and large-load-bearing joints;

[0018] 3. The macro-micro multi-scale structure of the macroscopic grooves and microscopic scratches symmetrically designed on the front and back sides of the steel plate is conducive to realizing the uniform load-bearing between dissimilar joints;

[0019] 4. The present invention proposes a multi-pin friction stir working part including an active stirring pin and a driven stirring pin, effectively increasing the interfacial bonding area of the dissimilar joint, promoting the formation of interfacial metallurgical connections of dissimilar materials in the horizontal and vertical directions, effectively ensuring the all-direction load-bearing of the dissimilar joint structure, and further ensuring the sealing performance of the circumferential weld structure;

[0020] 5. The multi-pin stirring head induces the thermoplastic aluminum alloy to effectively fill the macro-micro multi-scale groove and scratch structures, realizing the multi-scale load-bearing between aluminum and steel, which is significantly higher than the load-bearing of the traditional single welding metallurgical interface;

[0021] 6. The interface of the dissimilar joint prepared by the present invention is mainly a simple amorphous structure or an extremely thin intermetallic compound layer, avoiding the problem of poor corrosion resistance caused by other elements introduced by traditional brazing;

[0022] 7. The welding method of the present invention is applicable to the reliable connection between most "hard-soft" dissimilar materials, such as aluminum / titanium, magnesium / titanium, magnesium / steel, polymer / metal, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Schematic diagram of the macro-micro multi-scale structure in the steel plate designed by the present invention;

[0024] Figure 2 is Figure 1 Partial enlarged view of;

[0025] Figure 3 Schematic diagram of the convex platform and concave structure in the aluminum plate designed by the present invention;

[0026] Figure 4 Schematic diagram of the multi-pin friction stir working part structure designed by the present invention;

[0027] Figure 5 Schematic diagram of the special milling cutter for surface refinement treatment designed by the present invention;

[0028] Figure 6 Schematic diagram of the aluminum / steel welding process designed for the present invention;

[0029] 1 - Friction stir working part, 101 - Concave shoulder, 102 - Active stirring pin, 103 - Driven stirring pin;

[0030] 2 - Plate, 201 - Steel plate, 2011 - Protrusion structure, 2012 - Groove, 202 - Aluminum plate, 2021 - Concave structure, 2022 - Boss;

[0031] 3 - Special milling cutter. Specific implementation mode

[0032] Specific implementation mode 1: In combination with Figures 1 to 6 To illustrate this implementation mode, the multi-scale composite structure for realizing a heterogeneous friction stir welding joint in this implementation mode includes a friction stir working part 1, a plate 2, and a special milling cutter 3. The friction stir working part 1 is located above the plate 2. The friction stir working part 1 includes a concave shoulder 101, an active stirring pin 102, and a driven stirring pin 103. The active stirring pin 102 and the driven stirring pin 103 are installed at the bottom of the concave shoulder 101. The active stirring pin 102 is arranged coaxially with the concave shoulder 101, and the driven stirring pin 103 is located outside the active stirring pin 102. The length of the active stirring pin 102 is greater than the length of the driven stirring pin 103. Both the active stirring pin 102 and the driven stirring pin 103 are mainly in the form of threads and milled planes; the length of the active stirring pin 102 is greater than the length of the driven stirring pin 103, taking into account realizing the interfacial metallurgical connection of aluminum / steel at different depths and in the horizontal direction, increasing the interfacial bonding area and the load-bearing capacity in all directions.

[0033] Furthermore, the plate 2 includes a steel plate 201 and an aluminum plate 202. The right end of the steel plate 201 is inserted into the left end of the aluminum plate 202. The thickness of the aluminum alloy plate 202 should be greater than or equal to the thickness of the steel plate 201; the thickness of the aluminum alloy plate 202 can be between 2 - 100 mm;

[0034] Furthermore, the left end of the aluminum plate 202 is provided with a concave structure 2021, and bosses 2022 are provided on the upper and lower surfaces of the aluminum plate 202 opposite to each other. The bosses 2022 are located above the concave structure 2021. The right end of the steel plate 201 is provided with a protrusion structure 2011 that is fitted and installed with the concave structure 2021.

[0035] Furthermore, prefabricated millimeter-scale grooves 2012 are provided on the upper and lower surfaces of the steel plate 201 opposite to each other. Microscopic micron-scale scratches are provided on the bottom surface, side surface, and the steel plate surface outside the groove of each groove 2012. The microscopic micron-scale scratches are serrated scratches.

[0036] Furthermore, the steel plate 201 is mainly characterized by macroscopic millimeter-level grooves 2012 and microscopic micron-level laser marks; there are specially prepared bosses 2022 on the surface of the aluminum plate 202 to ensure that during the welding process, the macro-micro multi-scale structures are fully filled in the weld without thinning; ensuring that the heterogeneous structure has the dual advantages of "mechanical interlocking + interfacial metallurgy", and the macro-micro multi-scale structures are distributed on both sides of the steel plate 201 to be welded, promoting the uniform stress and load-bearing of the heterogeneous joint, and realizing the design and manufacture of strong and tough aluminum / steel heterogeneous joints.

[0037] Furthermore, the special milling cutter 3 with surface refinement treatment is mainly responsible for defects such as flash on the friction stir welding joint of aluminum / steel plates, ensuring the accurate measurement of the penetration amount and the positioning of the friction stir working part during double-sided welding, and ensuring high-quality welding formation. Through the mutual cooperation of the friction stir working part 1 and the macro-micro multi-scale structure (i.e., the plate 2), it can ensure that the heterogeneous structure welding has the characteristics of macro-micro multi-scale mechanical interlocking, large-area metallurgical bonding, and high-quality forming with equal thickness and large load-bearing, realizing the strong and tough manufacturing of aluminum / steel heterogeneous joints.

[0038] Specific Embodiment 2: Combined Figure 4 To illustrate this embodiment, the number of the driven stirring pins 103 in this embodiment can be one or more, and the eccentric positions can be axially symmetrically distributed or dispersedly distributed. It enhances the stirring effect of aluminum / steel, promotes interfacial metallurgical bonding, and is beneficial to breaking the intermetallic compounds generated at the interface.

[0039] The other components and connection relationships of this embodiment are the same as those of Specific Embodiment 1.

[0040] Specific Embodiment 3: Combined Figure 4 To illustrate this embodiment, the active stirring pin 102 and the driven stirring pin 103 in this embodiment can be designed as a split assembly with the shoulder 101, and their materials can be hard materials such as H13 tool steel, tungsten rhenium alloy, and PCBN, reducing problems such as wear caused by the contact between the stirring pin and the steel plate during the welding process.

[0041] The other components and connection relationships of this embodiment are the same as those of Specific Embodiment 1 or 2.

[0042] Specific Embodiment 4: Combined Figure 6 To illustrate this embodiment, the diameter of the concave shoulder 101 in this embodiment is 1 - 10 mm larger than the distance from the center of the driven stirring pin 103 to the center of the shoulder 101, and the distance from the center of the driven stirring pin 103 to the axis of the welding tool is 2 - 10 mm larger than the radius of the active stirring pin 102; the length of the active stirring pin 102 is 0.5 - 5 mm larger than that of the driven stirring pin 103; the diameter of the shoulder 101 is 3 - 5 times the thickness of the aluminum alloy plate 202, and the end diameter of the active stirring pin 102 is 0.0 - 1.0 mm larger than the width of the groove 2012;

[0043] The other components and connection relationships of this embodiment are the same as those in the first, second, or third specific embodiments.

[0044] Specific Embodiment Five: Figure 1 In this embodiment, the depth of the groove 2012 can be between 0.5 - 10 mm, the width of the groove 2012 can be between 2 - 20 mm, and the included angle between the bottom surface and the side surface of the groove is 30 - 150°; the depth of the micro laser engraving can be between 1 - 100 μm, the width of the engraving can be between 1 - 100 μm, and the spacing between the engravings can be between 1 - 5000 μm.

[0045] The other components and connection relationships of this embodiment are the same as those in the first, second, third, or fourth specific embodiments.

[0046] Specific Embodiment Six: Figure 3 In this embodiment, the height of the boss 2022 of the aluminum plate 202 can be between 0.5 - 10 mm, and the width of the boss 2022 should be greater than or equal to the width of the macroscopic groove 2012; the height of the concave structure 2021 in the aluminum plate should be consistent with the thickness of the butt joint area of the steel plate 201, and its depth should be consistent with the width of the surface processing area of the steel plate 201. Ensure high-precision fit between the two.

[0047] The other components and connection relationships of this embodiment are the same as those in the first, second, third, fourth, or fifth specific embodiments.

[0048] Specific Embodiment Seven: Figures 1 to 6 In this embodiment, a welding method for realizing a multi-scale composite structure of a dissimilar friction stir welding joint is described, which is achieved through the following steps:

[0049] Step 1: According to the aluminum / steel material and the performance requirements of the welded joint, design and manufacture the shapes and sizes of the concave shoulder 101, the active stirring pin 102, and the driven stirring pin 103 of the multi-pin friction stir working part 1 for enhancing interface metallurgy and driving strong material flow, as well as the spatial distribution of the two driven stirring pins 103, so as to drive the material to fill the macro-micro multi-scale structure under the thermo-mechanical coupling effect and enhance the interface metallurgical bonding in the horizontal and vertical directions;

[0050] Step 2: Select a steel plate 201 with appropriate thickness and size, and mechanically process prefabricated millimeter-scale grooves 2012 on the front and back surfaces of the steel plate 201 to be welded. The sum of the depths of the grooves 2012 is not greater than the thickness of the steel plate 201, and the width of the grooves 2012 is slightly smaller than the diameter of the active stirring pin 102; in an environment of high-purity argon, use laser texturing to prepare micro-micron-scale engravings on the bottom surface and side surface of the groove 2012 of the steel plate 201 and on the surface of the steel plate 201 outside the groove. The argon environment prevents surface oxidation from hindering the metallurgical reaction, and the micron-scale engravings enhance the micro-mechanical interlocking effect at the aluminum / steel interface;

[0051] Step 3: Select an aluminum plate 202 with appropriate thickness and size. Prepare a boss 2022 on the surface of the aluminum plate 202 by means of hot extrusion or mechanical material reduction. The width of the boss 2022 is greater than the diameter of the friction stir working part. The thickness of the boss 2022 is determined according to the size of the groove 2012 of the steel plate 201. Generally, the volume of the boss 2022 needs to be greater than the volume of the groove 2012 of the steel plate 201, so as to ensure that the groove 2012 of the steel plate can be fully filled with thermoplastic aluminum alloy and the weld seam has no thinning. At the same time, at the middle position of the butt joint surface of the aluminum plate 202, a concave structure 2021 is machined by mechanical material reduction to ensure that the structure 2021 of the steel plate 201 with macro-micro multi-scale can be assembled and inserted into the interior of the aluminum plate 202.

[0052] Step 4: During welding, adopt a butt-lap configuration form. The convex structure 2011 of the steel plate 201 with macro-micro multi-scale can be assembled and inserted into the concave structure 2021 of the aluminum plate 202, ensuring that the aluminum alloy boss 2022 structure is directly above the macro groove 2012 of the steel plate. Use a restraint tooling to clamp the aluminum plate 202 and the steel plate 201 to ensure the contact state and assembly accuracy of the welding interface.

[0053] Step 5: During welding, move the multi-pin friction stir working part 1 to directly above the boss 2022 structure of the aluminum plate. The multi-pin friction stir working part 1 rotates at a high speed and penetrates into the aluminum 202 / steel 201 material until the active stirring pin 102 penetrates into the middle of the butt joint surface of the aluminum 202 / steel 201, and the driven stirring pin 103 slightly contacts the aluminum / steel lap joint surface. Under the action of frictional heat and plastic flow, the active stirring pin promotes the thermoplastic aluminum alloy to fill the micro-micron-scale scratches on the steel side of the interface, and induces the formation of an amorphous layer or a thin-layer interfacial intermetallic compound on the aluminum / steel butt joint surface, forming a micro-mechanical interlocking riveting and interfacial metallurgical connection. Synchronously, the driven stirring pin 103 slightly contacts the aluminum / steel lap joint surface. Driven by the concave shoulder 101 and the driven stirring pin 103, the thermoplastic aluminum alloy is promoted to fill into the macro groove 2012 of the lap joint surface and the micro-scratches inside it, forming a macro-micro multi-scale mechanical riveting. At the same time, the driven stirring pin 103 interacts with the aluminum alloy on the outer edge of the macro groove 2012, promoting the thermoplastic aluminum alloy to fill into the internal micro-micron-scale scratches, forming a micro-mechanical interlocking riveting and interfacial metallurgy. Since the driven stirring pin 103 sweeps a larger area when rotating compared to the active stirring pin 102, a larger area of interfacial metallurgy is generated, improving the load-bearing capacity of the heterogeneous structure. The multi-pin friction stir working part 1 does not need to consume the aluminum alloy material of the upper plate to fill into the lower plate groove, mainly relying on the material of the additionally prepared boss to fill, which is beneficial to obtaining a heterogeneous joint without thickness reduction, realizing the equal-thickness design and manufacturing on the front side and single side.

[0054] Step 6: After completing the front-side welding of the aluminum / steel dissimilar joint, use a special milling cutter 3 with surface refinement treatment to process the weld seam on the front side, eliminate defects such as surface flash and arc patterns, make the thickness of the weld seam area on the front side equal to that of the aluminum alloy base material, improve the welding positioning accuracy during back-side welding, and ensure the forming quality;

[0055] Step 7: After the surface treatment is completed, flip and rigidly fix the aluminum 202 / steel plate 201 materials for clamping, and prepare for back-side welding; repeat Steps 5 and 6 to complete the back-side welding of the aluminum / steel joint, and obtain a joint with a dual strengthening mode of "mechanical interlock + interfacial metallurgy" on both the front and back sides, comprehensively improving the strength and toughness of the aluminum / steel joint. The combined action of the above factors enables the joint to have a dual structure of "mechanical interlock + interfacial metallurgy", meeting the requirements of high strength, high toughness, etc. of the dissimilar joint.

[0056] This embodiment can not only realize the welding of aluminum / steel dissimilar materials, but also be applicable to other "hard-soft" dissimilar combinations, such as aluminum / titanium, magnesium / titanium, magnesium / steel, polymer / metal, etc. During welding, a macro-micro multi-scale composite structure is prepared on the side of the high-hardness hard material, and a boss 2022 and a concave structure 2021 are prepared on the side of the low-hardness material to form a macro-micro multi-scale strengthened and toughened joint.

[0057] Specific Embodiment 8: In combination with Figures 1 to 5 This embodiment is described. The spindle rotation speed range of this embodiment is 10 - 10000 rpm, the welding speed range is 1 - 5000 mm / min, the welding tool inclination angle range is 0° - 5°, and the penetration depth of the active stirring pin 102 into the boss 2022 of the upper aluminum alloy plate is between 0 and 1 mm.

[0058] The above is only a preferred embodiment of the present invention, and it does not impose any form of limitation on the present invention. Although the present invention has been disclosed above with a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art can make some changes or modifications to it as equivalent embodiments within the scope of the technical solution of the present invention. However, as long as it does not depart from the content of the technical solution of the present invention and is based on the technical essence of the present invention, any simple modification, equivalent replacement, and improvement of the above embodiments still fall within the protection scope of the technical solution of the present invention.

Claims

1. A multi-scale composite structure for realizing heterogeneous friction stir welding joints, characterized in that: The multi-scale composite structure for realizing heterogeneous friction stir welding joints comprises a friction stir working part (1), a plate (2) and a special milling cutter (3), wherein the friction stir working part (1) is located on the upper part of the plate (2), the friction stir working part (1) comprises an inner concave shaft shoulder (101), an active stirring needle (102) and a driven stirring needle (103), the special milling cutter (3) is sleeved on the inner concave shaft shoulder (101), an active stirring needle (102) and a driven stirring needle (103) are installed at the bottom of the inner concave shaft shoulder (101), the active stirring needle (102) and the driven stirring needle (103) are arranged coaxially with the inner concave shaft shoulder (101), and the driven stirring needle (103) is located outside the active stirring needle (102); The plate (2) comprises a steel plate (201) and an aluminum plate (202), the left end of the aluminum plate (202) is provided with an inner concave structure (2021), and the upper and lower surfaces of the aluminum plate (202) are provided with bosses (2022) opposite to each other. The right end of the steel plate (201) is provided with a protruding structure (2011) which is mounted in cooperation with the concave structure (2021); prefabricated millimeter-level grooves (2012) are provided on the upper surface and the lower surface of the steel plate (201) respectively, and the bottom surface, side surface of each groove (2012) and the surface of the steel plate (201) outside the groove are provided with microscopic micrometer-level notches.

2. The multi-scale composite structure for realizing heterogeneous friction stir welding joints according to claim 1, characterized in that: The length of the active stirring needle (102) is greater than the length of the driven stirring needle (103).

3. A multi-scale composite structure for realizing heterogeneous friction stir welding joints according to claim 1 or 2, characterized in that: The number of the driven stirring needles (103) is one or more, and the eccentric positions are axially symmetrically distributed or dispersedly distributed.

4. The multi-scale composite structure for realizing heterogeneous friction stir welding joints according to claim 1, characterized in that: The height of the boss (2022) is between 0.5 and 10 mm, and the width of the boss (2022) is greater than or equal to the width of the groove (2012).

5. The multi-scale composite structure for realizing heterogeneous friction stir welding joints according to claim 1, characterized in that: The depth of the groove (2012) is between 0.5-10 mm, the width of the groove (2012) is between 2-20 mm, and the angle between the bottom surface and the side surface of the groove is 30-150°; the depth of the microscopic laser notch is between 1-100 μm, the width of the notch is between 1-100 μm, and the spacing between the notches is between 1-5000 μm.

6. A welding method for realizing a multi-scale composite structure of a heterogeneous friction stir welding joint according to any one of claims 1 to 5, characterized in that: The method is implemented by the following steps: Step 1: Based on the aluminum / steel material and the performance requirements of the welded joint, design and manufacture the concave shoulder (101) of the multi-needle friction stir working part (1) for enhancing interface metallurgy and driving material strong flow, the shape and size of the active stirring needle (102) and the driven stirring needle (103), and the spatial distribution of the two driven stirring needles (103); Step 2: Select a steel plate (201) of suitable thickness and size, and mechanically process a prefabricated millimeter-level groove (2012) on the front and back sides of the steel plate (201) to be welded, and use laser texturing in a high-purity argon environment to prepare microscopic micrometer-level notches on the bottom and side surfaces of the groove (2012) of the steel plate (201) and on the surface of the steel plate (201) outside the groove; Step 3: Select an aluminum plate (202) of suitable thickness and size, and prepare a boss (2022) on the surface of the aluminum plate (202); at the same time, mechanically reduce the material to form a concave structure (2021) at the middle of the butt joint surface of the aluminum plate (202); Step 4: During welding, the lap joint configuration is adopted, and the protruding structure (211) of the macro-micro multi-scale steel plate (201) can be assembled and inserted into the concave structure (221) of the aluminum plate (202), ensuring that the aluminum plate boss (222) structure is located directly above the steel plate groove (2012), and the aluminum plate (202) and the steel plate (201) are clamped using a restraining tool to ensure the contact state of the welding interface and the assembly accuracy; Step 5: During welding, the multi-needle friction stirring working part (1) is moved to the top of the aluminum alloy boss (2022) structure, and the multi-needle friction stirring working part (1) is rotated at high speed and penetrated into the interior of the aluminum plate (202) / steel plate (201) until the active stirring needle (102) penetrates into the middle of the aluminum plate (202) / steel plate (201) butt joint, and the driven stirring needle (103) slightly contacts the aluminum / steel overlap surface, so as to perform front welding of the aluminum / steel heterogeneous joint; Step 6: After the front welding of the aluminum / steel heterogeneous joint is completed, a special milling cutter (3) for surface refinement is used to process the front weld; Step 7: After the surface treatment is completed, the aluminum plate (202) / steel plate (201) is turned over and rigidly fixed and clamped to prepare for reverse side welding; steps 5 and 6 are repeated to complete the back side welding of the aluminum / steel joint.

7. A welding method for realizing a multi-scale composite structure of a heterogeneous friction stir welding joint according to claim 6, characterized in that: The main shaft rotation speed range is 10 to 10000 rpm, the welding speed range is 1 to 5000 mm / min, the welding tool inclination range is 0° to 5°, and the penetration amount of the active stirring needle (102) into the aluminum plate boss (2022) is between 0 and 1 mm.

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