A method for steel / aluminum butt fusion brazing joint based on grid regulation

Abstract

This invention provides a method for steel / aluminum butt brazing joints based on grid control, belonging to the field of dissimilar material welding. The method includes: spot welding a steel grid to the back of the area to be welded on a steel plate; applying flux and then clamping the aluminum alloy plate butt-welded to the steel plate; performing brazing using a laser-MIG composite heat source; feeding welding wire into the molten pool and adding an additional cold wire. During welding, the back grid partially dissolves to provide Fe element and utilizes capillary action to promote the spread of molten aluminum to the back side. The cold wire absorbs heat to compensate for the metal on the front side. Together, these two elements create a uniform intermetallic compound layer on the front, sides, and back of the joint, achieving a large back weld width and a full front weld height. Using this method, the joint's positive and negative bending angles both reach 180° without failure, significantly improving the bending performance of the steel / aluminum butt brazing joint.

Description

Technical Field

[0001] This application relates to the field of dissimilar material welding technology, and more specifically, to a method for steel / aluminum butt brazing joints based on grid control. Background Technology

[0002] In recent years, lightweight structures have become an important development direction in fields such as automobile manufacturing, rail transportation, and aerospace, and are an effective way to achieve energy conservation and emission reduction. Aluminum / steel composite structures retain the high strength and rigidity advantages of steel while leveraging the low density and good overall performance of aluminum alloys, thus attracting widespread attention. As the application of aluminum / steel welded structures in these fields increases, the performance requirements are also constantly rising. In particular, the stamping and bending forming of sheet metal places extremely high demands on the bending resistance of dissimilar joints, which is precisely the weak point of existing aluminum / steel welded joints.

[0003] The main challenge in steel / aluminum welding lies in the low solubility of Fe-Al, which inevitably leads to the formation of a brittle Fe-Al intermetallic compound (IMC) at the interface during welding. The aluminum / steel joint relies on this IMC layer for connection; however, an excessively thick IMC layer introduces significant residual stress, while an excessively thin layer results in insufficient interfacial bonding strength. Studies indicate that the optimal IMC layer thickness is approximately 4–8 μm. Furthermore, the uniformity of the IMC layer directly affects the interfacial bonding strength. Therefore, to obtain a high-performance joint, precise control of the welding heat input is crucial: it is necessary to suppress excessive Fe diffusion leading to an overly thick IMC layer, while also avoiding insufficient heat input resulting in inadequate interfacial reaction. Simultaneously, a uniform temperature field distribution is essential to achieve a homogeneous IMC layer. This places extremely high demands on the welding process.

[0004] Currently, fusion brazing is the primary method for joining dissimilar materials with significantly different melting points, especially laser-MIG composite heat source fusion brazing. This method utilizes the heating effect of an electric arc to promote the spread of molten aluminum on the steel surface, achieving good metal bridging. Simultaneously, the laser stabilizes the arc, improving the stability of the welding process. For aluminum / steel laser-MIG fusion brazed butt joints, relatively good tensile properties can currently be achieved. However, because the back side of the joint is far from the heat source, the welding heat is lower, resulting in insufficient interfacial reaction and an extremely thin IMC layer in the back interface region. This problem is particularly prominent when the plate thickness is large. When the joint is subjected to bending moment, the back interface region is highly susceptible to failure first, making the back side a weak point and significantly weakening the bending resistance of the aluminum / steel joint, severely limiting the application of aluminum / steel composite structures. Improving the bending performance of aluminum / steel welded joints is a pressing challenge in the welding field. Increasing heat input can raise the peak temperature of the molten aluminum and promote the back-side interface reaction. However, excessive heat input can lead to over-melting of the front-side steel substrate, resulting in a thickened interfacial IMC layer with highly uneven thickness and morphology, increased residual stress, and a sharp decline in bending resistance. Another approach is to control the thickness of the back-side IMC layer through back-side welding, but this adds extra steps and costs, and further exacerbates the softening of the aluminum alloy's heat-affected zone, reducing the overall strength of the joint. Therefore, effectively controlling the interfacial reaction of the steel / aluminum joint to obtain an IMC layer of moderate thickness and uniform distribution is crucial for improving the bending resistance of aluminum / steel joints.

[0005] In summary, there is an urgent need for a welding method that can effectively control the interface reaction on the back side of steel / aluminum welded joints in order to improve the bending performance of aluminum / steel butt brazed joints. Summary of the Invention

[0006] To address the aforementioned issues, this invention provides a method for improving the bending performance of steel / aluminum butt brazing joints based on grid control. A steel grid is spot-welded to the back of the steel plate to be welded, and additional cold wire is added as filler metal. Butt brazing is performed using a laser-MIG composite heat source. During welding, the back-side grid promotes the spread of molten aluminum to the back of the steel plate through capillary action, increasing the back-side weld width. Simultaneously, the steel grid partially dissolves in the molten pool, providing additional Fe elements to the interface, compensating for insufficient interfacial reaction due to insufficient heat input on the back side. This results in a moderately thick (approximately 4-5 μm) and uniform intermetallic compound layer on the front, sides, and back of the joint. Simultaneously, the added cold wire compensates for the metal loss on the front side caused by the aluminum flowing to the back side without increasing heat input, ensuring a full front-side weld height. The synergistic effect of these three factors shifts the weak area of ​​the joint from the brittle interface to the well-ductile heat-affected zone, achieving excellent bending performance without failure during both positive and negative bends of 180°.

[0007] This invention provides a method for steel / aluminum butt brazing joints based on grid control, comprising the following steps: The steel grid is spot-welded to the back of the area to be welded on the steel plate. Flux is evenly applied to the front and back of the area to be welded on the steel plate. Then the aluminum alloy plate is butted and clamped to the steel plate. Laser-MIG composite heat source is used for fusion brazing. During the welding process, welding wire is fed into the molten pool and additional cold wire is added.

[0008] The steel grid and cold wire exhibit a significant synergistic effect in laser-MIG composite brazing. On one hand, the steel grid, spot-welded to the back of the steel plate, partially dissolves during welding, providing additional Fe elements to the interface. This compensates for the lack of Fe atoms on the back due to insufficient heat input, thereby promoting interfacial reaction and forming a moderately thick, continuous, and uniform intermetallic compound layer on the back of the joint. The capillary action of the grid actively attracts molten aluminum to the back of the steel plate, significantly increasing the back spread width and bonding area. On the other hand, the additional cold wire is not connected to the welding circuit and melts solely through residual heat from the molten pool and thermal radiation from the rear of the composite heat source. It absorbs excess heat from the molten pool, reducing the peak temperature on the front side and suppressing excessive growth and inhomogeneity of the IMC layer on the front side. Furthermore, it compensates for the front metal loss caused by the aluminum flowing to the back side, ensuring a full weld reinforcement on the front side. The two work together to achieve a moderately thick and evenly distributed IMC layer in all areas of the joint, including the front, side, and back sides. At the same time, it achieves a large back weld width and a full front weld height, ultimately transferring the weak area of ​​the joint from the brittle interface to the heat-affected zone with good plasticity. The bending angles of both the positive and back bends reach 180° without failure, significantly improving the bending performance of the steel / aluminum butt joint.

[0009] Preferably, the steel mesh is stainless steel wire mesh with a mesh count of 200 to 400.

[0010] Preferably, the gap between the steel grid and the edge of the aluminum alloy plate is 0 to 1 mm.

[0011] Preferably, the cold wire is an aluminum alloy welding wire with a diameter of 1.2 to 1.6 mm and a wire feeding speed of 0.5 to 0.8 m / min.

[0012] Preferably, the cold wire is fed in the following manner: the cold wire is straightened and fixed to the edge of the steel plate to be welded; or a wire feeding mechanism is used to feed the wire synchronously, with the angle between the wire feeding nozzle and the plane of the plate to be welded being 15°, and the front end of the cold wire being 2 mm away from the rear end of the welding wire.

[0013] Preferably, the flux is KAlF4.

[0014] Preferably, the parameters of the laser-MIG composite heat source include: laser power 700-1200 W, welding speed 7-12 mm / s, MIG welding wire feed speed 3-5.5 m / min, and shielding gas is high-purity argon with a flow rate of 15-30 L / min.

[0015] Preferably, during the welding process, the laser-MIG composite heat source is offset towards the aluminum side by 0.5 to 1.0 mm, the filament spacing is 3 to 4 mm, the defocusing amount is 0 to ±0.5 mm, the laser head tilt angle is 8 to 12°, and the MIG welding torch tilt angle is 70°.

[0016] Preferably, a backing is provided on the back of the steel plate during welding. The backing has a forming groove with a radius of 13 mm, a depth of 1 mm, and a width of 10 mm.

[0017] Preferably, the aluminum alloy plate is 6061-T6 aluminum alloy, and the steel plate is SUS304 stainless steel.

[0018] In summary, the present invention has at least one of the following beneficial technical effects: 1. This invention fixes a steel grid to the back of a steel plate by spot welding. The grid promotes the spread of molten aluminum to the back side through capillary action. At the same time, the partial dissolution of the grid replenishes Fe elements to the interface. Combined with the heat absorption and temperature control of the cold wire, the thickness of the IMC layer on the front, middle and back of the joint is controlled at 4.66 μm, 5.12 μm and 4.61 μm respectively. The thickness difference is reduced from about 8 μm in conventional methods to less than 0.5 μm. A moderately thick (4~5 μm) continuous and uniform intermetallic compound layer is obtained, and the interfacial bonding strength is greatly improved.

[0019] 2. In conventional processes, increasing the back-side spread of the weld pool easily leads to collapse of the front-side reinforcement, making it difficult to achieve coordinated control of front and back-side forming. This invention introduces a non-conductive cold wire, utilizing the residual heat of the weld pool to melt and fill it, independently compensating for the amount of metal on the front side of the weld without increasing the welding heat input. Implementation results show that while the back-side spread width of the weld increases from 1.5 mm to 6.5 mm (an increase of 333%), the front-side reinforcement can be stably maintained above 0.8 mm, effectively solving the long-standing technical bottleneck of mutual constraint between front and back-side forming in steel / aluminum butt brazing.

[0020] 3. The increased bonding area at the back brazing interface significantly enhances the interface's shear strength, while the full weld reinforcement on the front effectively improves the weld body's stiffness. Through this synergistic effect, the weak point of failure in the joint shifts from the brittle steel / aluminum interface to the more ductile heat-affected zone on the aluminum side. Bending performance tests show that under conventional processes, the joint exhibits brittle interface cracking at a 16° positive bend and a 34° back bend. The joint prepared using the method of this invention achieves both positive and back bend angles of 180° without cracking, with bending deformation occurring entirely within the heat-affected zone. This meets the stringent requirements for high bending performance in steel / aluminum composite structures in fields such as rail transportation and automotive manufacturing. Attached Figure Description

[0021] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the description of the embodiments of this application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 A model diagram of the welding apparatus provided in an embodiment of this application is shown; Figure 2 This invention provides a schematic top view illustrating the spatial positioning of the laser head and welding torch according to an embodiment of the present application. Figure 3 This application provides comparative diagrams showing the IMC layer morphology in different regions of the steel / aluminum brazed joint under different process conditions; among which... Figure 3 a is a topographic image of the upper IMC layer of the joint under normal conditions; Figure 3 b is a topographic image of the IMC layer in the middle of the joint under normal conditions; Figure 3 c is a topographic image of the lower IMC layer of the joint under normal conditions; Figure 3 d is a topographic image of the IMC layer on the upper part of the steel / aluminum brazing head under the method of the present invention; Figure 3 e is a topographic image of the IMC layer in the middle of the steel / aluminum brazing joint under the method of the present invention; Figure 3 f is a topographic image of the IMC layer at the bottom of the steel / aluminum brazing joint under the method of the present invention; Figure 4 The comparative diagrams provided in this application show weld cross-section formation under different process conditions. Figure 5 The accompanying comparative diagrams of this application show bending test results of aluminum / steel joints under conventional conditions; Figure 6 The diagram shows a bending test of an aluminum / steel joint using the method of the present invention, as provided in the comparative example of this application. Detailed Implementation

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the present invention or its application or use. Based on the embodiments of the present invention, any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention. Furthermore, all other embodiments obtained by those skilled in the art without inventive effort are within the protection scope of the present invention.

[0024] Specific experimental steps or conditions are not specified in the embodiments; they can be performed according to the conventional experimental steps or conditions described in the prior art. Reagents and other instruments used, unless otherwise specified, are all commercially available conventional reagent products. Furthermore, the accompanying drawings are merely illustrative diagrams of the embodiments of the present invention and are not necessarily drawn to scale. The same reference numerals in the drawings denote the same or similar parts, and therefore, repeated descriptions of them will be omitted. Some block diagrams shown in the drawings are functional entities and do not necessarily correspond to physically or logically independent entities.

[0025] Techniques, methods, and devices known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and devices should be considered part of this specification.

[0026] In the description of this invention, it should be understood that the use of terms such as "first" and "second" to define components is merely for the purpose of distinguishing the corresponding components. Unless otherwise stated, the above terms have no special meaning and therefore should not be construed as limiting the scope of protection of this invention.

[0027] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0028] To enable those skilled in the art to better understand this application, the following embodiments will be used to provide a detailed description of a method for steel / aluminum butt brazing joints based on grid control.

[0029] Example Example 1 The laser-MIG hybrid welding device used in this invention is a conventional device in the field, such as... Figure 1 As shown, it mainly includes: a laser welding head (5), a MIG welding torch (6), a wire feeding mechanism, a shielding gas system, and welding fixtures. Among them, the laser welding head (5) and the MIG welding torch (6) are fixed by a connecting bracket, and the relative spatial position between the two is adjustable.

[0030] The tilt angle α1 of the laser welding head (5) is adjustable (set to 8~12° in this embodiment), the tilt angle α2 of the MIG welding gun (6) is fixed at 70°, and the wire spacing L1 (the distance between the front end of the welding wire 61 and the laser spot 51) is 3~4 mm. During welding, the laser beam is in front and the MIG arc is behind, and the composite heat source can be shifted towards the aluminum side by 0.5~1.0 mm. A pad (3) with a forming groove (31) is placed on the back of the steel plate (2). The forming groove has a radius R=13 mm, a depth h=1 mm, and a width L2=10 mm to stabilize the back weld formation. In addition, a cold wire (4) for additional filling and its fixing or feeding mechanism are provided.

[0031] The specific steps are as follows: Commercial steel mesh (7) is cut into 100 mm × 10 mm pieces with a mesh size of 300. It is then placed in anhydrous ethanol and cleaned with an ultrasonic cleaner for 3-5 minutes to remove oil and dust from the mesh and prevent impurities from being introduced during welding. After cleaning, the mesh (7) is fixed to the back of the welding area of ​​the steel plate (2) by spot welding at both ends.

[0032] After the grid (7) is fixed, flux is uniformly coated on the front and back surfaces of the steel plate (2) to be welded. The main component of the flux is KAlF4, which has good film removal and surface tension reduction capabilities, can promote the wetting and spreading of liquid molten metal on the stainless steel surface, initially improve wettability, and improve the bonding quality of the interface.

[0033] The pretreated aluminum plate (1) and steel plate (2) are clamped together with a butt joint and a gap of 0 mm between the plates. The aluminum plate (1) is made of 6061-T6 aluminum alloy with a diameter of 100 mm × 50 mm × 2 mm, and the steel plate (2) is made of SUS304 stainless steel with a diameter of 100 mm × 50 mm × 2 mm.

[0034] like Figure 2 During the welding process shown, the heat source is laser-guided, and the welding speed and direction are denoted as v. To avoid the heat source acting directly on the steel plate (2) and causing a large amount of Fe to enter the molten pool, the laser / MIG composite heat source is offset towards the aluminum side by L3=0.5mm. The wire spacing L1 is set to 3 mm, and the defocusing amount is set to 0 mm. The laser power is 900 W, the welding speed is 10 mm / s, the wire feed speed is 4.5 m / min, and the welding current is 97 A. The selected shielding gas is high-purity argon. The shielding gas and welding wire (61) are both fed from the MIG welding torch (6). The shielding gas flow rate is 25 L / min, the tilt angle of the MIG welding torch (6) is α2=70°, and the tilt angle of the laser head (5) is set to α1=8°.

[0035] During the welding process, an auxiliary cold wire (4) is added as an additional filler metal to ensure the formation of both sides of the weld. The cold wire (4) is an ER 4043 aluminum alloy welding wire with a diameter of 1.6 mm. After being cut to an appropriate length and straightened, it is directly fixed to the edge of the area to be welded on the steel plate (2). During the welding process, the cold wire (4) is melted by the energy of the rear of the composite heat source and the heat of the molten pool.

[0036] During the welding process, the steel grid (7) promotes the spread of molten aluminum to the back of the steel plate through capillary action, increasing the back weld width. On the other hand, the grid (7) partially dissolves to provide additional Fe elements to the interface, compensating for the insufficient interfacial reaction caused by insufficient heat input on the back. As a result, a moderately thick and uniform IMC layer is obtained on the front, side and back of the joint. At the same time, the addition of the cold wire (4) compensates for the loss of front metal due to the flow of molten aluminum to the back, ensuring a full front weld height. Finally, the weak area of ​​the joint is transferred from the brittle interface to the heat-affected zone with good plasticity, achieving excellent bending performance.

[0037] Example 2 This embodiment uses the same laser-MIG hybrid welding device and pretreatment process as in Embodiment 1. Specific process parameters are adjusted as follows: Commercial steel mesh (7) is cut into 100 mm × 10 mm pieces with a mesh size of 200. It is then placed in anhydrous ethanol and cleaned with an ultrasonic cleaner for 3-5 minutes to remove oil and dust. After cleaning, the mesh (7) is fixed to the back of the welding area of ​​the steel plate (2) by spot welding at both ends.

[0038] After the grid (7) is fixed, flux is uniformly coated on the front and back surfaces of the steel plate (2) to be welded. The main component of the flux is KAlF4, and the coating thickness is 0.2~0.3 mm.

[0039] The aluminum plate (1) is made of 6061-T6 aluminum alloy with a diameter of 100 mm × 50 mm × 2 mm, and the steel plate (2) is made of SUS304 stainless steel with a diameter of 100 mm × 50 mm × 2 mm. The joint type is butt joint, and the gap between the plates is 0 mm.

[0040] The heat source is laser-guided. To avoid the heat source directly acting on the steel plate (2) and causing a large amount of Fe to enter the molten pool, the laser / MIG composite heat source is offset towards the aluminum side by L3=1.0 mm. The wire spacing is L1=4 mm, and the defocusing amount is set to +0.5 mm. The laser power is 700 W, the welding speed is 7 mm / s, the wire feed speed is 3.0 m / min, and the welding current is adjusted accordingly to about 80 A. The selected shielding gas is high-purity argon. The shielding gas and welding wire (61) are both fed from the MIG welding torch (6), and the shielding gas flow rate is 15 L / min. The tilt angle of the MIG welding torch (6) is α2=70°, and the tilt angle of the laser head (5) is set to α1=12°.

[0041] An auxiliary cold wire (4) is added as additional filler metal during the welding process. The cold wire (4) is an ER 4043 aluminum alloy welding wire with a diameter of 1.6 mm. Unlike the method of cutting and fixing to the edge of the steel plate to be welded in Example 1, this example uses a wire feeding mechanism to feed the wire synchronously. The angle between the wire feeding nozzle and the plane of the plate to be welded is set to 15°, the distance between the front end of the cold wire and the rear end of the welding wire (61) is 2 mm, and the cold wire feeding speed is 0.6 m / min. During the welding process, the cold wire (4) is melted by the energy of the rear of the composite heat source and the heat of the molten pool.

[0042] A pad (3) with a forming groove (31) is placed on the back of the steel plate (2). The dimensions of the forming groove are the same as those in Example 1: radius R = 13 mm, depth h = 1 mm, and width L2 = 10 mm.

[0043] In this embodiment, the 200-mesh steel grid provides moderate capillary force, which, combined with the synchronous wire feeding of the cold wire, still enables effective spreading of molten aluminum on the back side of the steel plate. Partial dissolution of the grid provides additional Fe elements to the interface, compensating for the lack of Fe atoms on the back side due to insufficient heat input; the cold wire absorbs some heat and compensates for the lack of metal on the front side. Ultimately, a moderately thick and uniformly distributed IMC layer is obtained on the front, sides, and back of the joint, with a full excess height on the front and a significantly increased melt width on the back. Bending tests show that both forward and backward bending angles reach 180° without failure, achieving excellent bending performance.

[0044] Comparative Example Comparative Example 1 This comparative example uses the same base materials as Example 1: aluminum plate (1) is 6061-T6 aluminum alloy with a diameter of 100 mm × 50 mm × 2 mm, and steel plate (2) is SUS304 stainless steel with a diameter of 100 mm × 50 mm × 2 mm. The joint type is butt joint, and the gap between the plates is 0 mm.

[0045] Before welding, the areas to be welded on the aluminum and steel plates are ground to remove the oxide film and wiped with acetone to remove oil. The steel plate (2) is uniformly coated with KAlF4 flux (same as in Example 1) on both sides of the area to be welded, but no steel grid or wire mesh is added. No cold wire is added during the welding process.

[0046] The welding equipment and parameters were consistent with those in Example 1: laser-MIG composite heat source, laser leader, laser head tilt angle α1 = 8°, MIG welding torch tilt angle α2 = 70°, wire spacing L1 = 3 mm, composite heat source offset towards aluminum L3 = 0.5 mm, defocusing amount 0 mm. Laser power 900 W, welding speed 10 mm / s, wire feed speed 4.5 m / min, welding current 97 A. The shielding gas was high-purity argon, with a flow rate of 25 L / min, delivered from the MIG welding torch. A liner with a forming groove (dimensions same as in Example 1) was placed on the back of the steel plate.

[0047] After conventional welding, samples were taken from the weld cross-section. Following grinding, polishing, and etching, the morphology of the IMC layer in the interface regions of the upper (front), middle, and lower (back) parts of the joint was observed using a scanning electron microscope (SEM), and the average thickness was measured. The results are as follows: Figure 3 As shown in a~3c.

[0048] The upper front area of ​​the connector is as follows Figure 3 As shown in Figure a, this area is closest to the laser-MIG composite heat source, resulting in high peak temperatures and long high-temperature dwell times during welding. A large amount of Fe diffuses from the stainless steel base material to the interface, undergoing a violent metallurgical reaction with molten aluminum, leading to excessive growth of the IMC layer. The thickness of the front-side IMC layer is extremely uneven, reaching over 12 μm at its thickest point, with an average thickness of approximately 9.5 μm. Furthermore, the IMC layer grows towards the steel side in a coarse, serrated or tongue-like pattern. This coarse and uneven IMC layer generates significant residual interfacial stress during cooling, and the IMC layer itself is significantly brittle, making it highly susceptible to microcrack initiation under minor loads.

[0049] In the middle area of ​​the weld, such as Figure 3 As shown in b, this region is moderately far from the heat source, with a relatively uniform temperature distribution and a moderate degree of interfacial reaction. The IMC layer has a relatively smooth and continuous morphology, with an average thickness of approximately 6.21 μm, which is within the optimal thickness range (4–8 μm) reported in the literature. Therefore, the interfacial bonding quality in this region is relatively good, and it is the main contributing area to the mechanical properties of the joint under normal conditions.

[0050] In contrast, after applying the method of this invention, namely applying a steel grid and adding cold wire, the morphology and thickness uniformity of the IMC layer in the upper, middle, and lower parts of the joint are significantly improved. Figure 3 As shown in d~3f. During the welding process, the cold wire added to the front side absorbs a large amount of heat and prevents the electric arc from directly acting on the front side of the steel plate; at the same time, the back grid promotes the molten aluminum liquid to spread to the back side through capillary action, carrying away a large amount of heat, thereby effectively reducing the front side temperature and suppressing excessive interfacial reaction.

[0051] Therefore, the average thickness of the front IMC layer was significantly reduced from approximately 9.5 μm under conventional conditions to 4.66 μm, and the morphology was more uniform. Figure 3 d). The temperature in the central region of the weld remains relatively moderate and uniform. After applying the method of this invention, the average thickness of the IMC layer in this region is 5.12 μm, and the morphology is smooth and continuous. Figure 3 e), slightly reduced compared to 6.21 μm under conventional conditions, but still within the optimal range of 4–8 μm. For the back side of the joint, the back grid partially dissolved in the molten aluminum, significantly increasing the Fe content in the interface region. This compensated for the lack of Fe atoms due to insufficient temperature, promoting a more complete interface reaction and allowing the IMC layer to grow fully. The average thickness significantly increased from 1.62 μm under conventional conditions to approximately 4.61 μm. Figure 3 f). This thickness falls within the optimal range of 4–8 μm, ensuring sufficient interfacial bonding strength while avoiding excessive residual stress. Therefore, the method of this invention effectively solves the inherent problems of the IMC layer being thicker at the top and thinner at the bottom, and having uneven distribution, in conventional processes.

[0052] Comparative Example 2 This comparative example shows a comparison of the joint cross-sectional forming and bending performance obtained under the same welding parameters, using conventional conditions (i.e., without back mesh and without cold wire) and using the method of the present invention (with back mesh and cold wire assistance).

[0053] Comparison of weld cross-section formation: Figure 4 This is a comparison diagram of the joint cross-section formation under conventional conditions and under the method of the present invention, with the same welding parameters.

[0054] like Figure 4 As shown in Figure a, under normal conditions, the back side of the weld is poorly spread, with a back weld width of only 3.6 mm, and the spread width of the molten aluminum on the back side of the steel plate (2) is only 1.5 mm. This indicates that the molten aluminum can hardly effectively wet and spread on the back side of the steel plate, resulting in a serious lack of interface bonding area on the back side. At the same time, the weld reinforcement on the front side is relatively full, reaching 1.3 mm. This indicates that under a fixed filler metal amount, the molten aluminum mainly accumulates on the front side and fails to flow effectively to the back side, forming an asymmetric and uneven state of excessive reinforcement on the front side and insufficient penetration on the back side.

[0055] After adopting the method of the present invention, such as Figure 4 As shown in b, the spreading of the molten aluminum on the back side is significantly improved, with the spreading width of the molten aluminum on the back side of the steel plate reaching 6.5 mm, an increase of approximately 333%. At the same time, due to the additional filling of the cold wire, the weld reinforcement height on the front side remains at 0.8 mm. Although this is slightly lower than the 1.3 mm under normal conditions, it is still full and effectively avoids the problem of front collapse caused by the improved back side spreading.

[0056] Bending performance comparison Figure 5 The image shows the bending test results of the aluminum / steel joint under normal conditions. The positive bending test results are as follows: Figure 5 As shown in Figure a, when the bending angle reaches 16°, the joint cracks at the interface; the back bending test results are as follows. Figure 5 As shown in Figure b, interfacial cracking also occurs when the bending angle reaches 34°. Both failure modes are brittle interfacial fracture, and the cracks occur in the back-side interface region. This indicates that the shear resistance of the back-side interface of the joint is extremely low under normal conditions, making it unable to withstand bending loads and thus the weakest link in the entire joint.

[0057] Figure 6 These are the bending test results of the welded joint under the method of this invention (synergistic effect of wire mesh and cold wire). In both the positive and negative bending tests, the joint did not fail when the bending angle reached 180°, the weld and interface remained intact, and the bending deformation area was located in the heat-affected zone. The bending performance of the joint has been significantly improved.

[0058] In summary, this application demonstrates through embodiments that in steel / aluminum butt brazing, spot welding a steel grid to the back of the steel plate and adding cold wire significantly improves the back-side spreading of molten aluminum within the preferred parameter range, and obtains a moderately thick (approximately 4.6~5.2 μm) uniform and continuous intermetallic compound layer on the front, middle, and back of the joint. Comparative Example 1 shows an excessively thick (average approximately 9.5 μm) and uneven IMC layer on the front, and an excessively thin (average only 1.62 μm) IMC layer on the back, resulting in extremely poor interfacial bonding quality. Comparative Example 2 shows insufficient back-side spreading of the weld, a back-side weld width of only 3.6 mm, and failure angles of only 16° and 34° for positive and negative bends, respectively, both exhibiting brittle cracking at the interface. The results of the embodiments show that the method of the present invention can transfer the weak area from the brittle interface to the plastic heat-affected zone, achieving excellent bending performance without failure during positive and negative bends of 180°, effectively solving the problems of poor back-side wetting, uneven IMC layer distribution, and insufficient bending resistance in existing steel / aluminum butt joints.

[0059] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0060] Although preferred embodiments of the present application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of the embodiments of the present application.

[0061] Finally, it should be noted that in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or terminal device that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or terminal device. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or terminal device that includes said element.

[0062] The above provides a detailed description of a method for steel / aluminum butt brazing joint based on grid control provided in this application. Specific examples have been used to illustrate the principle and implementation of this application. The description of the above embodiments is only for the purpose of helping to understand the method and core idea of ​​this application. At the same time, for those skilled in the art, there will be changes in the specific implementation and application scope based on the idea of ​​this application. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A method for steel / aluminum butt brazing joints based on grid control, characterized in that, Includes the following steps: The steel grid is spot-welded to the back of the area to be welded on the steel plate. Flux is evenly applied to the front and back of the area to be welded on the steel plate. Then the aluminum alloy plate is butted and clamped to the steel plate. Laser-MIG composite heat source is used for fusion brazing. During the welding process, welding wire is fed into the molten pool and additional cold wire is added.

2. The method according to claim 1, characterized in that, The steel mesh is made of stainless steel wire mesh with a mesh count of 200 to 400.

3. The method according to claim 1, characterized in that, The gap between the steel grid and the edge of the aluminum alloy plate is 0 to 1 mm.

4. The method according to claim 1, characterized in that, The cold wire is an aluminum alloy welding wire with a diameter of 1.2 to 1.6 mm, and the wire feeding speed is 0.5 to 0.8 m / min.

5. The method according to claim 4, characterized in that, The cold wire is fed in the following ways: the cold wire is straightened and fixed to the edge of the steel plate to be welded; or a wire feeding mechanism is used to feed the wire synchronously, with the angle between the wire feeding nozzle and the plane of the plate to be welded being 15°, and the front end of the cold wire being 2 mm away from the rear end of the welding wire.

6. The method according to claim 1, characterized in that, The flux is KAlF4.

7. The method according to claim 1, characterized in that, The parameters of the laser-MIG composite heat source include: laser power 700-1200 W, welding speed 7-12 mm / s, MIG welding wire feed speed 3-5.5 m / min, shielding gas is high-purity argon, flow rate 15-30 L / min.

8. The method according to claim 1, characterized in that, During the welding process, the laser-MIG composite heat source is offset towards the aluminum side by 0.5 to 1.0 mm, the filament spacing is 3 to 4 mm, the defocusing amount is 0 to ±0.5 mm, the laser head tilt angle is 8 to 12°, and the MIG welding torch tilt angle is 70°.

9. The method according to claim 1, characterized in that, During welding, a backing is placed on the back of the steel plate. The backing has a forming groove with a radius of 13 mm, a depth of 1 mm, and a width of 10 mm.

10. The method according to claim 1, characterized in that, The aluminum alloy plate is 6061-T6 aluminum alloy, and the steel plate is SUS304 stainless steel.

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