A friction stir welding method for steel-aluminum dissimilar metal joint

Abstract

The application discloses a steel-aluminum dissimilar metal joint friction stir welding method, which comprises the following steps: dividing steel plates and aluminum alloy plates with the same thickness into upper, middle and lower regions along the thickness direction; pretreating the upper and lower regions of the steel plates and the aluminum alloy plates; arranging material plates in the middle regions of the steel plates and the aluminum alloy plates, and welding the middle regions of the butt surfaces by using a double-shoulder friction stir welding tool; arranging filler plug plates in the upper and lower gaps of the upper and lower regions of the steel plates and the aluminum alloy plates; and welding the upper regions of the steel plates and the aluminum alloy plates and the lower regions of the steel plates and the aluminum alloy plates by using a conventional friction stir welding tool. The application solves the problem that the load capacity and rigidity of equipment are too high when the friction stir welding is used to weld large-thickness steel-aluminum joints, thereby limiting the application of the technology in large-thickness structures.

Description

Technical Field

[0001] This invention relates to the field of thick plate joining technology, specifically to a friction stir welding method for steel-aluminum dissimilar metal joints. Background Technology

[0002] Steel is currently the most widely used metal, while aluminum alloys are the most widely used non-ferrous metals and important materials for lightweighting. Steel-aluminum joints have a very broad application prospect in various fields such as automobiles, ships, and cryogenic engineering. However, due to the significant differences in their physical and chemical properties, joining steel and aluminum has become a global challenge. Fusion welding, represented by laser welding, easily generates porosity and a large number of intermetallic compounds, resulting in poor overall performance, especially sealing performance. Cold joining methods such as riveting have low connection strength and cannot meet the requirements of modern industrial production.

[0003] Friction stir welding (FSW) is a novel solid-state welding method. A high-speed rotating stirring tool is inserted into the materials being welded. Friction generates heat, plasticizes the materials, and forms a dense joint structure under the action of the FSW tool. It has advantages such as being environmentally friendly and exhibiting minimal post-weld deformation, making it suitable for joining dissimilar metal materials.

[0004] Friction stir welding requires a large force to be applied to the materials being welded, which places high demands on the rigidity of the equipment. When welding 10mm aluminum alloy, the force can reach 50KN, and the required force increases linearly with the increase in thickness. This severely limits the application of friction stir welding in thick steel-aluminum joints. Summary of the Invention

[0005] The purpose of this invention is to provide a friction stir welding method for steel-aluminum dissimilar metal joints, in order to solve the problem that friction stir welding requires excessively high load capacity and rigidity of equipment when welding thick steel-aluminum joints, thus limiting the application of this technology in thick structures.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for friction stir welding of steel-aluminum dissimilar metal joints includes the following steps: S1: Divide steel plates and aluminum alloy plates of the same thickness into upper, middle and lower regions along the thickness direction, and divide the two plates in the same way; pre-treat the upper and lower regions of the steel plate and aluminum alloy plate to form upper and lower gaps at the butt joint for subsequent welding. S2: A material plate sandwiched between the steel plate and aluminum alloy plate is set in the middle area of ​​the mating surface as the first intermediate layer, and the middle area of ​​the mating surface is connected by friction stir welding using a double-shoulder friction stir welding tool. S3: Filler plates are respectively installed in the upper and lower gaps of the upper and lower regions of the steel plate and the aluminum alloy plate. A material plate sandwiched between the upper filler plate and the upper region of the steel plate is installed as a second intermediate layer. A material plate sandwiched between the lower filler plate and the lower region of the steel plate is installed as a third intermediate layer. The upper region of the steel plate and the upper filler plate, the upper region of the aluminum alloy plate and the upper filler plate, the lower region of the steel plate and the lower filler plate, and the lower region of the aluminum alloy plate and the lower filler plate are respectively connected by friction stir welding using conventional friction stir welding tools.

[0007] To optimize the above technical solution, the specific limitations also include: In step S1, the pretreatment of the upper and lower regions of the steel plate and aluminum alloy plate specifically involves machining along the opposite direction of the mating surface of the steel plate and aluminum alloy plate to remove the plate material from the upper and lower regions of the steel plate and aluminum alloy plate to accommodate the passage of the double-shoulder friction stir welding tool.

[0008] Preferably, the width of the upper and lower gaps on the steel plate side and the aluminum alloy plate side is not less than the shoulder radius of the double-shoulder friction stir welding tool.

[0009] Preferably, the filling plug is an aluminum alloy plug, the shape of which corresponds to the upper and lower gaps between the upper and lower regions of the steel plate and the aluminum alloy plate; a gap is reserved between the filling plug and the steel plate to accommodate the second / third intermediate layer; a butt joint is formed between the filling plug and the aluminum alloy plate to be welded.

[0010] Preferably, the upper and lower shoulders of the dual-shoulder friction stir welding tool are the same size, and the stirring pin of the dual-shoulder friction stir welding tool is cylindrical with serrated threads machined on its side, and the threads of its upper half are right-hand threads and the threads of its lower half are left-hand threads; the stirring pin of the conventional friction stir welding tool has a trapezoidal cross-section and threads machined on its side.

[0011] Furthermore, the shapes of the two sides of the first intermediate layer are the same as the mating surfaces of the steel plate and the aluminum alloy plate in the middle region, and the surface is machined with a shape that matches the thread of the stirring pin of the double-shoulder friction stir welding tool; the shapes of the two sides of the second intermediate layer and the third intermediate layer are the same as the mating surfaces of the steel plate and the filler plate, and the surface is machined with a shape that matches the thread of the stirring pin of the conventional friction stir welding tool.

[0012] Furthermore, the upper and lower regions of the steel plate are respectively machined with a first inclined surface and a third inclined surface on the side near the mating surface. The upper filling plug plate is machined with a second inclined surface matching the first inclined surface on the corresponding side, and the lower filling plug plate is machined with a fourth inclined surface matching the third inclined surface on the corresponding side. All inclined surfaces are machined with thread morphology, which matches the thread of the stirring pin of a conventional friction stir welding tool.

[0013] Preferably, when welding the middle region of the steel plate and aluminum alloy plate, the dual-shoulder friction stir welding tool is offset towards the aluminum alloy plate, so that the side of its stirring pin is tangent to the mating surface of the steel plate; when welding the upper and lower regions of the steel plate and aluminum alloy plate, the conventional friction stir welding tool is offset towards the filler plate, so that the side of its stirring pin is tangent to the first / third inclined surface of the steel plate.

[0014] Preferably, the thickness of the upper and lower regions is one-quarter of the plate thickness, and the thickness of the middle region is one-half of the plate thickness.

[0015] Preferably, the materials of the first intermediate layer, the second intermediate layer, and the third intermediate layer are selected from zinc, titanium, or copper; and the thickness of the first intermediate layer, the second intermediate layer, and the third intermediate layer is 0.2 mm to 0.3 mm.

[0016] Compared with the prior art, the beneficial effects of the present invention are: This invention provides a friction stir welding method for steel-aluminum dissimilar metal joints. The method involves dividing the steel plate and aluminum alloy plate into upper, middle, and lower regions along their thickness. First, a biaxial shoulder friction stir welding tool is used to weld the middle region. Then, conventional friction stir welding tools are used to weld the upper and lower regions. Utilizing the near-zero upsetting force characteristic of biaxial shoulder friction stir welding when welding the middle region, this method solves the problem of the enormous axial force required for traditional single-pass welding of thick materials. This allows conventional friction stir welding equipment with relatively low load-bearing capacity to be used for welding ultra-thick steel-aluminum joints, reducing reliance on heavy equipment and expanding the engineering application scope of this technology.

[0017] This invention pre-machines serrations or thread shapes on the mating surfaces of the steel plate, aluminum plate, and intermediate layer to match the threads of the stirring pin in a friction stir welding tool. Through mechanical engagement with the rotating stirring pin, it actively controls the flow behavior and thermal circulation of the interface material, optimizing the morphology and distribution of brittle intermetallic compounds. Simultaneously, by adding an intermediate layer, it effectively wets the steel-aluminum interface, inhibiting the continuous formation and growth of brittle intermetallic compounds between the steel and aluminum, thereby improving the toughness and connection strength of the joint.

[0018] When welding the upper and lower regions, this invention matches the angle of the inclined surface of the steel plate with the angle of the stirring pin of a conventional friction stir welding tool, and combines this with the precise offset of the tool towards the aluminum plate. This achieves that the upper part of the stirring pin mainly stirs in the aluminum plate, while the lower part stirs more in the steel plate. This optimizes the heat distribution from the shoulder to the root, ensures that the interface region receives suitable heat input for welding without overheating, and further controls the formation of harmful phases. Attached Figure Description

[0019] Figure 1 : A schematic flowchart of the friction stir welding method for steel-aluminum dissimilar metal joints of the present invention.

[0020] Figure 2 : A schematic diagram of the friction stir welding method for steel-aluminum dissimilar metal joints of the present invention.

[0021] Figure 3 : A schematic diagram of the welding of the middle region in the friction stir welding method for steel-aluminum dissimilar metal joints of the present invention.

[0022] Figure 4 : A schematic diagram of the upper region welding of the friction stir welding method for steel-aluminum dissimilar metal joints of the present invention.

[0023] Figure 5 : A schematic diagram of the biaxial shoulder friction stir welding tool structure for the friction stir welding method of steel-aluminum dissimilar metal joints of the present invention.

[0024] In the diagram: 1-Steel plate, 101-Middle area of ​​steel plate, 102-Upper area of ​​steel plate, 103-Lower area of ​​steel plate, 2-Aluminum alloy plate, 201-Middle area of ​​aluminum alloy plate, 202-Upper area of ​​aluminum alloy plate, 203-Lower area of ​​aluminum alloy plate, 3-First intermediate layer, 4-Second intermediate layer, 5-Upper filler plate, 6-Third intermediate layer, 7-Lower filler plate, 801-Upper shoulder of double-shoulder friction stir welding tool, 802-Stirring pin of double-shoulder friction stir welding tool, 803-Lower shoulder of double-shoulder friction stir welding tool, 901-Shoulder of conventional friction stir welding tool, 902-Stirring pin of conventional friction stir welding tool. Detailed Implementation

[0025] The present invention will be further described in detail below through specific embodiments, but it should not be construed as limiting the scope of the subject matter of the present invention to the following embodiments. All technologies implemented based on the above content of the present invention fall within the scope of the present invention.

[0026] This invention provides a method for friction stir welding of steel-aluminum dissimilar metal joints, the flowchart of which is shown below. Figure 1 and Figure 2 As shown, the entire method includes the following steps: S1: Divide the steel plate 1 and aluminum alloy plate 2 of the same thickness into an upper region, a middle region and a lower region along the thickness direction, and the division method of the two plates is the same; pre-treat the upper region and lower region of the steel plate 1 and aluminum alloy plate 2 to form upper and lower gaps at the butt joint for subsequent welding. S2: A material plate sandwiched between the steel plate 1 and the aluminum alloy plate 2 is set in the middle area of ​​the mating surface as the first intermediate layer 3. A double-shoulder friction stir welding tool is used to perform friction stir welding on the middle area of ​​the mating surface. S3: Filler plates are respectively installed in the upper and lower gaps of the upper and lower regions of the steel plate 1 and the aluminum alloy plate 2. A material plate sandwiched between the upper filler plate 5 and the upper region 102 of the steel plate is installed as the second intermediate layer 4. A material plate sandwiched between the lower filler plate 7 and the lower region 103 of the steel plate is installed as the third intermediate layer 6. The upper region 102 of the steel plate and the upper filler plate 5, the upper region 202 of the aluminum alloy plate and the upper filler plate 5, the lower region 103 of the steel plate and the lower filler plate 7, and the lower region 203 of the aluminum alloy plate and the lower filler plate 7 are respectively connected by friction stir welding using conventional friction stir welding tools.

[0027] After welding, grind off the burrs on the top and bottom surfaces of the joint to ensure a smooth weld joint.

[0028] The pretreatment of the upper and lower regions of steel plate 1 and aluminum alloy plate 2 specifically involves machining along the opposite direction of the mating surfaces of steel plate 1 and aluminum alloy plate 2 to remove the plate material of the upper and lower regions of steel plate 1 and aluminum alloy plate 2, which is the width of the plate material to accommodate the passage of the double-shoulder friction stir welding tool.

[0029] The width of the upper and lower gaps on the steel plate 1 side and the aluminum alloy plate 2 side is not less than the shoulder radius of the double-shoulder friction stir welding tool, so that the double-shoulder friction stir welding tool can pass smoothly in the middle area of ​​the steel plate 1 and the aluminum alloy plate 2 during friction stir welding.

[0030] like Figure 5As shown, the upper shoulder 801 and lower shoulder 803 of the dual-shoulder friction stir welding tool have the same dimensions. The stirring pin 802 of the dual-shoulder friction stir welding tool is cylindrical with serrated threads machined on its side. The upper half of the thread is a right-hand thread with a pitch of 0.3 mm and a depth of 0.2 mm; the lower half of the thread is a left-hand thread with a pitch of 0.3 mm and a depth of 0.2 mm. When the dual-shoulder friction stir welding tool rotates, the upper half of the thread drives the surrounding material, creating a downward flow tendency; simultaneously, the lower half of the thread drives the surrounding material, creating an upward flow tendency. These two opposing trends meet, mix, and compress at the steel-aluminum interface near the center line of the stirring pin height, forcing the two materials with significantly different physical properties to undergo more intense and complete plastic deformation and mechanical mixing at the interface.

[0031] The upper and lower shoulders of the dual-shoulder friction stir welding tool form a closed constraint on the upper and lower surfaces of the weld. Combined with the reverse threads of the intermediate stirring pin, an internal material circulation zone is formed in the middle of the plate thickness, further ensuring the density and uniformity of the weld and reducing defects such as tunnel holes and voids.

[0032] The stirring pin 902 of a conventional friction stir welding tool has a trapezoidal cross-section with threads machined on its side. The threads are right-hand threads with a pitch of 0.3 mm and a depth of 0.2 mm. The working surface of the shoulder 901 of the conventional friction stir welding tool has an involute structure, and the angle φ of the hypotenuse near the root of the trapezoid is between 70° and 80°, preferably 75°. The stirring pins and shoulders of both the double-shoulder friction stir welding tool and the conventional friction stir welding tool are made of tungsten-rhenium alloy, high-temperature alloy, or cubic boron nitride, etc.

[0033] The filling plug is an aluminum alloy plug, and its shape corresponds to the upper and lower gaps of the upper and lower regions of the steel plate 1 and the aluminum alloy plate 2. A gap is reserved between the upper filling plug 5 and the upper region 102 of the steel plate to accommodate the second intermediate layer 4. A gap is reserved between the lower filling plug 7 and the lower region 103 of the steel plate to accommodate the third intermediate layer 6. A butt joint is formed between the filling plug and the aluminum alloy plate 2 to be welded.

[0034] The shapes of the two sides of the first intermediate layer 3 are the same as the mating surfaces of the central regions of the steel plate 1 and the aluminum alloy plate 2, and the surfaces are machined with a morphology that matches the thread of the stirring pin 802 of the dual-shoulder friction stir welding tool. The shapes of the two sides of the second intermediate layer 4 and the third intermediate layer 6 are the same as the mating surfaces of the steel plate 1 and the filler plate, and the surfaces are machined with a morphology that matches the thread of the stirring pin 902 of the conventional friction stir welding tool. By correspondingly machining and threading the morphologies on the two sides of the first intermediate layer 3, the two sides of the second intermediate layer 4, and the two sides of the third intermediate layer 6, the temperature and flow behavior of the steel-aluminum mating surface are controlled, the amount of intermetallic compounds is optimized, and thus the overall mechanical properties of the joint are improved.

[0035] The upper region 102 and lower region 103 of the steel plate are respectively machined with a first bevel and a third bevel on the side near the mating surface. The upper filler plate 5 is machined with a second bevel matching the first bevel on the corresponding side, and the lower filler plate 7 is machined with a fourth bevel matching the third bevel on the corresponding side. The angle of the bevel is φ-4°. All bevels are machined with thread morphology, which matches the thread of the stirring pin 902 of the conventional friction stir welding tool. By pre-machined the bevels in the area to be friction stir welded, a precise cutting guide surface and contact position are provided for the bevel of the stirring pin 902 of the conventional friction stir welding tool, ensuring that the stirring pin can enter the steel-aluminum interface area at the designed depth and angle.

[0036] When welding the middle region 101 of the steel plate and the middle region 201 of the aluminum alloy plate, the dual-shoulder friction stir welding tool is offset to one side of the aluminum alloy plate 2, so that the side of its stirring pin is tangent to the mating surface of the steel plate 1.

[0037] When welding the upper region 102 of the steel plate and the upper region 202 of the aluminum alloy plate, the conventional friction stir welding tool is offset to the side of the upper filler plate 5, so that the side of its stirring pin is tangent to the first inclined surface of the steel plate 1. When welding the lower region 103 of the steel plate and the lower region 203 of the aluminum alloy plate, the conventional friction stir welding tool is offset to the side of the lower filler plate 7, so that the side of its stirring pin is tangent to the third inclined surface of the steel plate 1. The tangent of the stirring pin and the inclined surface of the steel plate 1 intersect at the center of the welding depth. The upper part of the stirring pin mainly stirs the zinc and aluminum alloy parts, while the lower part stirs the steel, increasing the heat generation in the lower part, thereby allowing the material to flow fully and ensuring the connection quality of the joint.

[0038] The thickness of the upper and lower regions is one-quarter of the plate thickness, and the thickness of the middle region is one-half of the plate thickness.

[0039] The materials of the first intermediate layer 3, the second intermediate layer 4, and the third intermediate layer 6 are selected from zinc, titanium, or copper, with zinc being preferred as an additive element. This achieves wetting of the steel-aluminum interface structure, inhibits the formation and growth of brittle compounds between steel and aluminum metals, and improves the overall mechanical properties of the joint. The thickness of the first intermediate layer 3, the second intermediate layer 4, and the third intermediate layer 6 is 0.2mm~0.3mm.

[0040] To further understand the technical solution of the present invention, a detailed description is provided in conjunction with specific embodiments: Example Welding of joints between 40mm thick 304 stainless steel plate and 40mm thick 6061-T6 aluminum alloy plate.

[0041] Select a suitable double-shoulder friction stir welding tool, with both the upper and lower shoulders having the same diameter of 50mm. (D) 上轴肩 =D 下轴肩 =50mm), the stirring pin is cylindrical with serrated threads machined on its sides, and the pin length is... L 双轴肩针长 It is 20mm.

[0042] Select appropriate conventional friction stir welding tools with a shoulder diameter D. 常规轴肩 The diameter is 25mm, and its stirring needle has a trapezoidal cross-section with a root diameter D. 搅拌针根部 It is 11mm, and its end diameter D 搅拌针端部 The diameter is 5mm, the needle length is 11mm, the trapezoidal angle φ=75°, and its side is also machined into a sawtooth thread shape. The thread depth is 0.2mm, the thread width is 0.3mm, it is a right-hand thread, and the thread rotation angle is 30°.

[0043] Within a 10mm thickness range from the upper surface of steel plate 1, a 25mm section of steel is milled away along the opposite direction of the mating surface. Using CNC machining, the upper region of steel plate 1, near the mating surface, is milled into a 71° (φ-4°) bevel. Simultaneously, a thread shape identical to the stirring pin 902 of a conventional friction stir welding tool (thread depth 0.2mm, thread width 0.3mm) is milled onto this bevel. Similarly, within a 10mm thickness range from the lower surface of steel plate 1, a 25mm section of steel is milled away along the opposite direction of the mating surface. Using CNC machining, the lower region of steel plate 1, near the mating surface, is milled into a 71° bevel. Simultaneously, a thread shape identical to the stirring pin 902 of a conventional friction stir welding tool (thread depth 0.2mm, thread width 0.3mm) is milled onto this bevel.

[0044] The mating surface of the middle region of steel plate 1, with a thickness of 20 mm, is CNC milled into a thread shape that matches the shape of the stirring pin 802 of the dual-shoulder friction stir welding tool used.

[0045] On the upper surface of aluminum alloy plate 2, within a thickness range of 10mm, a 25mm section of aluminum is milled away along the opposite direction of the mating surface. Similarly, on the lower surface of the aluminum plate, within a thickness range of 10mm, a 25mm section of aluminum is milled away along the opposite direction of the mating surface. The middle 20mm thickness of the mating surface of aluminum alloy plate 2 is CNC milled into a thread shape that matches the shape of the stirring pin 802 of the dual-shoulder friction stir welding tool used.

[0046] The first intermediate layer 3 is shaped like the mating surface of steel plate 1 and aluminum alloy plate 2. Zinc is selected as its material, and its thickness is 0.2~0.3mm. The two sides of it that are mating with steel plate 1 and aluminum alloy plate 2 are CNC milled to match the thread of the stirring pin 802 of the double-shoulder friction stir welding tool.

[0047] like Figure 3 As shown, the processed steel plate 1, the first intermediate layer 3 and the aluminum alloy plate 2 are butted together and fastened so that the thread shape of the middle region 101 of the steel plate matches the butt side of the first intermediate layer 3, and the thread shape of the other side of the first intermediate layer 3 matches the thread shape of the middle region 201 of the aluminum alloy plate.

[0048] Welding is performed by entering from the side using a dual-shoulder friction stir welding tool. The tool is offset to one side of the aluminum alloy plate 2, so that the serrated edge of its stirring pin is tangent to the mating surface of the steel plate 1. After dual-shoulder friction stir welding, the central region 101 of the steel plate and the central region 201 of the aluminum alloy plate are welded into a whole, and then the flash on the upper and lower surfaces is removed.

[0049] A 10mm thick aluminum alloy plate with a width of 49.7mm~49.8mm and made of 6061-T6 aluminum alloy is cut as a filler plate. The side of the filler plate that contacts the upper region 102 of the steel plate is milled into a 71° bevel. At the same time, a thread shape with the same shape as the stirring pin 902 of a conventional friction stir welding tool is milled on the bevel by CNC. The other side of the filler plate is a vertical surface corresponding to the upper region 202 of the aluminum alloy plate. The filler plate is divided into an upper filler plate 5 and a lower filler plate 7, both of which have the same specifications.

[0050] The second intermediate layer 4 and the third intermediate layer 6 are prepared using zinc as the material. Their shape matches the mating surface of the upper region 102 of the steel plate and the filling plug plate. Their cross-section is a parallelogram with a slope angle of 71° and a thickness of 0.2mm to 0.3mm. Both sides of the second intermediate layer 4 and the third intermediate layer 6 are processed into threaded shapes. The threaded shape on the side that connects to the upper region 102 of the steel plate matches the thread on the steel plate 1, and the threaded shape on the side that connects to the filling plug plate matches the thread on the filling plug plate.

[0051] The upper filling plug plate 5 and the second intermediate layer 4 are placed into the gap formed by the upper region 202 of the steel plate 1 and the aluminum alloy plate, and then tightened. Using conventional friction stir welding tools, the upper region 102 of the steel plate and the upper filling plug plate 5, and the upper filling plug plate 5 and the upper region 202 of the aluminum alloy plate are welded sequentially. When welding the upper region 102 of the steel plate and the upper filling plug plate 5, the center of the stirring pin is offset to one side of the upper filling plug plate 5, and the inclined edge of the stirring pin is tangent to the inclined surface of the upper region 102 of the steel plate. Figure 4 As shown, complete the friction stir welding. Flip the entire joint over and repeat the steps to weld the lower region 103 of the steel plate and the lower filler plate 7, and the lower filler plate 7 and the lower region 203 of the aluminum alloy plate.

[0052] After welding, grind the burrs on the upper and lower surfaces of the joint to ensure a smooth welded joint.

[0053] Comparative Example 1 Comparative Example 1 uses the same plate material as the embodiments of the present invention above, except that: step-by-step friction stir welding is not used; instead, an extra-large friction stir welding tool is selected for welding. Specifically: Both steel plate 1 and aluminum alloy plate 2 are flat, 40mm thick rectangular plates without zoning or milling. The mating surfaces of steel plate 1 and aluminum alloy plate 2 are cleaned and kept perpendicular. A 0.25mm thick flat zinc plate is placed between the mating surfaces of steel plate 1 and aluminum alloy plate 2 as the first intermediate layer 3. Then, steel plate 1, the first intermediate layer 3, and aluminum alloy plate 2 are sequentially butt-jointed and rigidly clamped to form a simple planar butt joint. An extra-large standard friction stir welding tool is selected, with a shoulder diameter of 60mm, a tapered stirring pin, and a pin length of 38mm.

[0054] Because the plate thickness is 40mm, the upsetting force required by the friction stir welding tool exceeds the equipment's capacity. During the welding process, the equipment spindle vibrates under a huge load, forcing a reduction in welding speed. The welding process is interrupted when the welding stroke reaches about 50mm due to excessive torque and equipment overload.

[0055] Compared to the embodiments of the present invention, this comparative example shows significant disadvantages in the interface bonding state and joint mechanical properties of the upper and lower regions. Analysis of the welded specimens revealed that the joint failed to achieve effective connection in most areas. Within the initial 20mm length, partial plastic flow occurred on both sides of the aluminum alloy plate, while the steel plate side remained largely unchanged. Some tunnel-like voids and gaps were observed in the weld due to insufficient material flow. At the few steel-aluminum interfaces where contact occurred, due to uncontrolled heat input and uneven material mixing, a brittle, coarse Fe-Al intermetallic compound layer of uneven thickness and continuous distribution was formed. Simultaneously, the threaded portion of the stirring pin experienced severe wear and adhesion due to intense dry friction with the steel and localized overheating.

[0056] Comparative Example 2 Comparative Example 2 uses the same process steps, plate partitioning, welding tools and process parameters as the above embodiments of the present invention, except that the addition of the first intermediate layer 3, the second intermediate layer 4 and the third intermediate layer 6 is cancelled.

[0057] Microstructural analysis and mechanical property testing of the welded joint revealed that, compared with the embodiment of the present invention, the steel-aluminum connection interface in the upper, middle and lower regions all contained a layer of brittle intermetallic compound with uneven thickness, mainly composed of FeAl3 and Fe2Al5. This layer had a weak bond with the base materials on both sides, resulting in cracks.

[0058] Tensile testing of the joints showed that their tensile strength and elongation after fracture were far lower than those of the embodiments of the present invention. All samples underwent low-stress brittle fracture along the brittle compound layer at the steel-aluminum interface, with smooth fracture surfaces exhibiting a distinct candy-like morphology. The joints exhibited extremely poor toughness and reliability. The continuous brittle layer and potential microcrack network posed serious risks to the corrosion resistance and airtightness of the joints, failing to meet reliability requirements.

[0059] Comparative Example 3 Comparative Example 3 uses the same process steps, plate partitioning, welding tools and process parameters as the above embodiments of the present invention, except that the thread morphology processing on all mating surfaces of steel plate 1, aluminum alloy plate 2, first intermediate layer 3, second intermediate layer 4 and third intermediate layer 6 is cancelled.

[0060] Microstructural analysis and mechanical property testing of the welded joints revealed that, compared to the embodiments of the present invention, the interface between steel plate 1 and aluminum alloy plate 2 in Comparative Example 3 had more unbonded micro-gaps, and the intermetallic compound layer, although continuous, was more brittle and hard. The tensile strength and toughness of the joints between steel plate 1 and aluminum alloy plate 2 were both lower than those in the embodiments. Tensile specimens all fractured brittlely along the smooth steel-zinc or zinc-aluminum interface, with smooth fracture surfaces, exhibiting weak bonding characteristics.

[0061] For joining thick plates, friction stir welding places high demands on the strength of the welding tools. The method provided in this invention employs a step-by-step strategy: first, welding the middle section with a double-shoulder weld, then welding the upper and lower sections conventionally. This decomposes the enormous axial force required for a single weld. In particular, by utilizing the near-zero upsetting force during welding the middle region with a double-shoulder friction stir weld, it avoids reliance on heavy equipment, enabling conventional equipment with lower load capacity to weld ultra-thickness (≥40mm) joints, thus overcoming the bottleneck in the application of this technology in thick structures. Furthermore, steel and aluminum alloys have significant differences in physical properties. Steel has high hardness, poor plasticity and fluidity, and low thermal conductivity. Direct contact with aluminum easily generates continuous, brittle, and hard Fe-Al intermetallic compounds, while aluminum alloys have good plasticity and excellent fluidity, making them easier to weld. By setting a first intermediate layer 3, a second intermediate layer 4, and a third intermediate layer 6 between the steel plate 1 and the aluminum alloy plate 2, the core problems of poor wetting, easy formation of embrittled phases, and poor fluidity at the steel plate interface can be directly addressed, inhibiting the formation of brittle compounds from the source and improving the interfacial plastic bonding effect. Furthermore, the pre-formed thread morphology on the mating surfaces of steel plate 1, aluminum alloy plate 2, first intermediate layer 3, second intermediate layer 4, and third intermediate layer 6 forms a threaded mechanical engagement with the rotating stirring needle. This increases the contact area and enhances the stirring needle's ability to grasp and drive the interface material, promoting the mixing of dissimilar materials and precisely controlling the plastic deformation and thermal cycling in the interface area. Together with step-by-step welding and the addition of intermediate layers, this optimizes the temperature field of the weld surface, enabling controllable interface structure and improving joint performance and quality.

[0062] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, equivalent substitutions, and improvements made by those skilled in the art to the above embodiments without departing from the scope of the technical solution of the present invention, based on the technical essence of the present invention, shall still fall within the protection scope of the technical solution of the present invention.

Claims

1. A method for friction stir welding of steel-aluminum dissimilar metal joints, characterized in that, Includes the following steps: S1: Divide steel plates and aluminum alloy plates of the same thickness into upper, middle and lower regions along the thickness direction, and divide the two plates in the same way; pre-treat the upper and lower regions of the steel plate and aluminum alloy plate to form upper and lower gaps at the butt joint for subsequent welding. S2: A material plate sandwiched between the steel plate and aluminum alloy plate is set in the middle area of ​​the mating surface as the first intermediate layer, and the middle area of ​​the mating surface is connected by friction stir welding using a double-shoulder friction stir welding tool. S3: Filler plates are respectively installed in the upper and lower gaps of the upper and lower regions of the steel plate and the aluminum alloy plate. A material plate sandwiched between the upper filler plate and the upper region of the steel plate is installed as a second intermediate layer. A material plate sandwiched between the lower filler plate and the lower region of the steel plate is installed as a third intermediate layer. The upper region of the steel plate and the upper filler plate, the upper region of the aluminum alloy plate and the upper filler plate, the lower region of the steel plate and the lower filler plate, and the lower region of the aluminum alloy plate and the lower filler plate are respectively connected by friction stir welding using conventional friction stir welding tools. The stirring pin of the dual-shoulder friction stir welding tool is cylindrical with serrated threads machined on its side; the stirring pin of the conventional friction stir welding tool has a trapezoidal cross-section with threads machined on its side. The shapes of the two sides of the first intermediate layer are the same as the mating surfaces of the steel plate and the aluminum alloy plate in the middle area, and the surface is machined with a shape that matches the thread of the stirring pin of the double-shoulder friction stir welding tool; the shapes of the two sides of the second intermediate layer and the third intermediate layer are the same as the mating surfaces of the steel plate and the filler plate, and the surface is machined with a shape that matches the thread of the stirring pin of the conventional friction stir welding tool. The upper and lower regions of the steel plate are respectively machined with a first inclined surface and a third inclined surface on the side near the mating surface. The upper filling plug is machined with a second inclined surface matching the first inclined surface on the corresponding side, and the lower filling plug is machined with a fourth inclined surface matching the third inclined surface on the corresponding side. All inclined surfaces are machined with thread morphology, which matches the thread of the stirring pin of a conventional friction stir welding tool. When welding the middle region of the steel plate and aluminum alloy plate, the dual-shoulder friction stir welding tool is offset towards the aluminum alloy plate, so that the side of its stirring pin is tangent to the mating surface of the steel plate; when welding the upper and lower regions of the steel plate and aluminum alloy plate, the conventional friction stir welding tool is offset towards the filler plate, so that the side of its stirring pin is tangent to the first / third inclined surface of the steel plate. The thickness of the upper and lower regions is one-quarter of the plate thickness, and the thickness of the middle region is one-half of the plate thickness.

2. The friction stir welding method for steel-aluminum dissimilar metal joints according to claim 1, characterized in that: In step S1, the pretreatment of the upper and lower regions of the steel plate and aluminum alloy plate specifically involves machining along the opposite direction of the mating surface of the steel plate and aluminum alloy plate to remove the plate material of the upper and lower regions of the steel plate and aluminum alloy plate, which is the width of the plate material to accommodate the passage of the double-shoulder friction stir welding tool.

3. The friction stir welding method for steel-aluminum dissimilar metal joints according to claim 1, characterized in that: The width of the upper and lower gaps on the steel plate side and the aluminum alloy plate side shall not be less than the shoulder radius of the double-shoulder friction stir welding tool.

4. The friction stir welding method for steel-aluminum dissimilar metal joints according to claim 1, characterized in that: The filling plug is an aluminum alloy plug, and its shape corresponds to the upper and lower gaps of the upper and lower regions of the steel plate and the aluminum alloy plate; a gap is reserved between the filling plug and the steel plate to accommodate the second / third intermediate layer; a butt joint is formed between the filling plug and the aluminum alloy plate to be welded.

5. The friction stir welding method for steel-aluminum dissimilar metal joints according to claim 1, characterized in that: The upper and lower shoulders of the dual-shoulder friction stir welding tool are the same size, and the upper half of the thread is a right-hand thread, while the lower half of the thread is a left-hand thread.

6. The friction stir welding method for steel-aluminum dissimilar metal joints according to claim 1, characterized in that: The first, second, and third intermediate layers are made of zinc, titanium, or copper; the thickness of the first, second, and third intermediate layers is 0.2 mm to 0.3 mm.

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