CN118720424A - A method for lap welding of dissimilar metals of steel and aluminum assisted by steel mesh interlayer - Google Patents

Abstract

The present invention discloses a method for lap welding of steel-aluminum dissimilar metals assisted by a steel mesh interlayer, which belongs to the technical field of dissimilar metal welding, and includes: grinding and cleaning the surfaces of the aluminum alloy plate and the stainless steel plate to be welded; cutting the steel mesh and cleaning and drying; dissolving the flux in a solvent and evenly coating it on the surface of the treated stainless steel plate to be welded; fixing the treated steel mesh on the surface of the treated stainless steel plate to be welded, and evenly coating the flux on the surface of the steel mesh, lap-jointing the aluminum alloy plate directly above the stainless steel plate so that the steel mesh becomes an interlayer, placing a pad under the aluminum alloy plate, and clamping and fixing it with a fixture; and performing laser-MIG brazing on the clamped welded parts. The present invention effectively improves the spreading ability of the molten aluminum liquid on the steel surface during welding by adding a steel mesh interlayer, greatly increases the spreading width, and effectively refines the weld structure, and enhances the mechanical properties of the steel-aluminum dissimilar metal lap joint.

Description

A method for lap welding of dissimilar metals of steel and aluminum assisted by steel mesh interlayer

Technical Field

The invention relates to the technical field of dissimilar material welding, and in particular to a method for lap welding dissimilar metals of steel and aluminum assisted by a steel wire mesh interlayer.

Background Art

Lightweight structures have broad prospects in achieving higher speeds, energy conservation and emission reduction in transportation industries such as automobiles, rail transit vehicles, and aerospace. Stainless steel has high specific strength and corrosion resistance, while aluminum alloy has low density and its mechanical properties also meet the requirements of many structural or functional components. Therefore, aluminum/steel lightweight composite structures have received great attention. Welding is the best means to manufacture steel/aluminum composite structures. Nowadays, steel/aluminum lap joints are gradually being used in body components. However, the difference in physical and metallurgical properties between steel and aluminum alloys is still a difficulty in connection. The resulting joint performance is not high, which makes it impossible to apply steel/aluminum lap joints to components with certain load-bearing requirements, greatly limiting the development of steel/aluminum composite structures.

Due to the great difference in the thermophysical properties of the two metals, it is greatly difficult to achieve high-quality welding of aluminum/steel. For example, the difference in melting point easily leads to poor metallurgical compatibility, while the difference in thermal expansion coefficient easily brings large welding residual stress to the joint; secondly, aluminum is a highly active metal and is prone to produce a dense, high-melting point Al ₂ O ₃ oxide film on the surface, which reduces the strength of the joint. The most critical thing is that brittle Fe-Al intermetallic compounds (IMC) are inevitably present at the interface of steel/aluminum welding joints, which greatly limits the strength and ductility of the joint. After a lot of research, it was found that the plasticity and toughness of the Al-rich IMC phase are lower than those of the Fe-rich IMC phase; at the same time, the optimal thickness of the interface IMC phase is 4~8μm. Too thin IMC layer will weaken its own strength, while too thick will generate excessive stress inside the IMC layer, which is easy to initiate cracks. Therefore, it is necessary to avoid and inhibit the excessive formation and growth of intermetallic compounds at the interface.

At present, melting brazing is the main method for welding dissimilar materials with large differences in melting points. Laser-MIG (MeltInert-Gas) composite heat source deep melting brazing can not only use the preheating effect of the arc to promote the spread of the brazing material to the steel surface to achieve better bridging, but the laser can stabilize the arc; at the same time, when the plate thickness is large, the laser can increase the melting depth; and by changing the relative spatial positions of the two heat sources, the temperature field distribution at the interface can be changed, thereby achieving the regulation of the uniformity of the interface compound layer and realizing high-quality connection of aluminum and steel.

Precise control of welding heat input can effectively regulate the thickness and phase composition of the IMC layer. Usually, it is necessary to reduce the heat input or increase the cooling rate during welding to reduce or avoid the melting of steel, and reduce the peak temperature and residence time of the molten pool, so as to regulate the mass transfer process in the Fe/Al interface reaction zone and ultimately reduce the thickness of the IMC layer. However, while reducing the heat input and increasing the cooling rate, many factors that are unfavorable to the joint performance are also generated. On the one hand, the Fe element diffused into the molten pool is reduced, and the Fe-Al particles that can play a role in fine grain strengthening of the aluminum alloy weld are reduced, which is unfavorable to the joint; most importantly, for lap joints, under low welding heat input, the spreading and wetting ability of molten aluminum on the steel surface is insufficient, resulting in a significant reduction in the effective connection area, and the molten aluminum liquid cannot spread and accumulates, and the joint contour changes suddenly after cooling and solidification. After the joint is loaded, it is very easy to cause stress concentration, which is not conducive to the bearing performance of the joint.

Therefore, there is an urgent need for a new method to improve the spreading and wetting ability of aluminum liquid on the stainless steel surface under low welding heat input conditions and to improve the performance of aluminum/steel lap joints.

Summary of the invention

The present invention aims to solve the problems in the prior art of using fusion brazing for steel-aluminum dissimilar metal welding, such as poor spreading and wetting ability of aluminum liquid on the stainless steel surface under low welding heat input conditions, and proposes a method for steel-aluminum dissimilar metal lap welding assisted by a steel mesh interlayer. The method effectively improves the spreading ability of molten aluminum liquid on the steel surface during welding by adding a steel mesh interlayer, greatly increases the spreading width, effectively refines the weld structure, and enhances the mechanical properties of the steel-aluminum dissimilar metal lap joint.

In order to achieve the above-mentioned object of the invention, the technical solution of the present invention is as follows:

A method for lap welding of dissimilar metals of steel and aluminum assisted by a steel mesh interlayer comprises the following steps:

Step a, grinding and cleaning the surface of the aluminum alloy plate and stainless steel plate to be welded, cutting the steel wire mesh and cleaning and drying it;

Step b, dissolving the Nocolok flux composed of KAlF4 in a solvent, and then evenly coating it on the surface of the treated stainless steel plate in the area to be welded;

Step c, fixing the treated steel wire mesh on the surface of the treated stainless steel plate to be welded, and evenly coating the flux on the surface of the steel wire mesh; then overlapping the aluminum alloy plate just above the stainless steel plate so that the steel wire mesh is located between the two to form a sandwich, and then placing a pad under the aluminum alloy plate, and using a clamp to clamp and fix the weldment;

Step d: performing laser-MIG brazing on the clamped welded parts.

Furthermore, in step c, the overlapping length L ₂ of the stainless steel plate and the aluminum alloy plate is 10-20 mm.

Furthermore, in step c, the external width L ₃ of the wire mesh is 20-25 mm.

Furthermore, in step b, the Nocolok flux composed of KAlF4 is dissolved in anhydrous ethanol, and its spreading thickness on the surface of the stainless steel plate to be welded is 0.2-0.3 mm.

Furthermore, the mesh number of the wire mesh is 200-300.

Furthermore, in step d, the offset amount L ₁ of the welding laser beam and the MIG welding gun toward the aluminum alloy plate is 0.5-1 mm, the defocus amount is 0, and the filament spacing is 3 mm.

Furthermore, in step d, the laser-MIG brazing parameters are: laser power 700~1000 W; welding speed 7~9 mm/s; wire feeding speed 3~5 m/min; shielding gas is argon, which is delivered from the MIG welding gun; shielding gas flow rate 15~30 L/min.

Furthermore, in step d, the welding laser head is tilted at an angle of 5 to 10°.

Furthermore, in step a, the areas to be welded of the aluminum alloy plate and the stainless steel plate are grinded with an angle grinder to remove the surface oxide film; and acetone or industrial alcohol is used to wipe and remove the surface oil stains.

In summary, the present invention has the following advantages:

1. The present invention solves the problem of poor wetting and spreading of molten aluminum on steel. It can also significantly increase the wetting and spreading width under low welding heat input, with the spreading width increased by 123%; and effectively refines the weld structure and improves the mechanical properties of the weld;

2. The present invention reduces welding heat input, is conducive to regulating the mass transfer process in the Fe/Al interface reaction zone, realizes the regulation of the uniformity of the interface compound layer, and improves the bonding quality of the interface;

3. The present invention changes the fracture mode of steel-aluminum lap joints. The increase in the spreading width after adding the steel wire mesh, the improvement in the interface bonding quality and the refinement of the weld grains increase the mechanical properties of the weld, and transforms the brittle-tough combination fracture originally occurring in the weld into a tough fracture occurring in the heat-affected zone, greatly improving the mechanical properties of the joint, and the tensile strength is increased by 20%;

4. In the welding process of the present invention, the Nocolok flux and the steel wire mesh work synergistically. The flux reduces the surface tension of the stainless steel and preliminarily improves the wettability. The capillary action of the steel wire mesh provides an additional driving force for the spreading of the molten aluminum liquid, greatly improving its wetting and spreading ability on the stainless steel surface.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a technical principle diagram of the present invention;

Figure 2 is a comparison of the cross-section formation of aluminum-steel lap joints without and with steel mesh;

FIG3 is a comparison diagram of the Fe-Al interface of the aluminum-steel lap joint without and with steel mesh;

FIG4 is a comparison of the weld structures of aluminum-steel lap joints without and with steel mesh;

FIG5 is a comparison of the microhardness test results of the aluminum-steel lap joint weld without and with steel mesh;

FIG6 is a comparison of the tensile properties test results of aluminum-steel lap joints without and with steel mesh;

In Figure 1:

1. Aluminum alloy plate, 2. Backing plate, 3. Welding laser head, 4. MIG welding gun, 5. Steel wire mesh, 6. Stainless steel plate, 11. Welding seam, 31. Welding laser beam.

DETAILED DESCRIPTION

In order to explain the present invention more clearly, the present invention is further described below in conjunction with preferred embodiments and drawings. It should be understood by those skilled in the art that the content described below is illustrative rather than restrictive, and should not be used to limit the scope of protection of the present invention.

The present invention provides a method for lap welding of dissimilar metals of steel and aluminum assisted by a steel mesh interlayer, comprising the following steps:

Step (1), pretreatment: Before welding, use an angle grinder to grind the areas to be welded of the aluminum alloy plate 1 and the stainless steel plate 6 to remove the surface oxide film; and use acetone or industrial alcohol to wipe and remove the surface oil stains. Select the steel mesh 5. When the mesh number of the steel mesh is too low, its capillary effect becomes smaller, and the additional driving force provided for the spread of the molten aluminum liquid is too small; when the mesh number is too high, the additional driving force provided is too large, and the molten aluminum liquid spreads excessively, causing the central area of the weld 11 to collapse. Therefore, the mesh number of the steel mesh is preferably 200-300. The pretreatment method is: cut the steel mesh 5 into a suitable size, put it into anhydrous ethanol, and use an ultrasonic cleaner to clean it for 1-5 minutes to remove impurities in the steel mesh 5, and blow it dry after cleaning.

Step (2), flux coating: Nocolok flux composed of KAlF4 is dissolved in anhydrous ethanol to make it into a paste, and spread on the surface of the stainless steel plate 6 in the area to be welded to a thickness of 0.2-0.3 mm. Nocolok flux has good film removal and the ability to reduce the surface tension of molten metal, which can promote the wetting and spreading of liquid molten metal on the stainless steel surface and preliminarily improve the wettability.

Step (3), adding steel mesh interlayer and overlapping clamping: fix the steel mesh 5 treated in step (1) to the surface of the to-be-welded area of the stainless steel plate 6 treated in step (2); then evenly apply flux on the surface of the steel mesh 5, and then overlap the aluminum alloy plate 1 just above the stainless steel plate 6 so that the steel mesh 5 is located between the two to form an interlayer, and the stainless steel plate 6 and the aluminum alloy plate 1 overlap and overlap, and the overlap length _L2 = 10~20 mm. At the same time, in order to ensure that the molten aluminum liquid can be fully spread during the welding process, the external width of the steel mesh _L3 = 20~25 mm.

In this step, since the opposite side of the aluminum alloy plate 1 to be welded is suspended, in order to prevent the aluminum plate from being displaced in the z direction during the clamping process, a pad 2 made of stainless steel or aluminum alloy with a thickness of 2 mm is placed under the aluminum plate, as shown in Figure 1; and the plate to be welded is clamped by the clamp to fix the relative position of the plate to be welded.

Step (4), welding: determine the welding path and start welding. As shown in Figure 1, laser guidance is used for welding, and v represents the welding speed and direction. The key welding parameters are: laser spot offset, filament spacing, laser power, wire feeding speed, welding speed, defocusing amount, and shielding gas flow rate. Since aluminum alloy has a high laser reflectivity, in order to protect the equipment, the welding laser head 3 is tilted 5~10°. In order to avoid the laser and arc directly acting on the stainless steel plate 6, causing the stainless steel plate to melt, a large amount of Fe elements enter the molten pool, and reduce the mechanical properties of the joint, the offset _L1 of the welding laser beam 31 and the MIG welding gun 4 to the aluminum side is 0.5~1 mm, and the defocusing amount is set to 0 mm; the preferred filament spacing is 3 mm.

During the welding process, the amount of heat input will directly affect the thickness and phase composition of the interface IMC layer, and ultimately determine the quality of the joint. After a large number of experiments, it was found that the optimal laser power is 700~1000 W, the welding speed is 7~9 mm/s, and the wire feeding speed is 3~5m/min; the selected shielding gas is high-purity argon, which is delivered from the MIG welding gun 4, and the shielding gas flow rate is 15~30 L/min.

In the present invention, Nocolok flux reduces the surface tension of stainless steel and preliminarily improves wettability; the capillary action of the steel wire mesh provides an additional driving force for the spreading of molten aluminum liquid, greatly improving its wetting and spreading ability on the stainless steel surface; the increase in spreading width, the improvement in interface bonding quality and the refinement of weld grains after adding the steel wire mesh increase the mechanical properties of the weld, transforming the brittle-ductile bonding fracture originally occurring in the weld into a ductile fracture occurring in the heat-affected zone, greatly improving the mechanical properties of the joint.

Example 1

SUS304 stainless steel and A 6061-T6 aluminum alloy with a size of 100 mm × 50 mm × 2 mm were used as welding specimen plates, and ER 4043 aluminum alloy welding wire with a diameter of 1.2 mm was selected as the filler metal.

According to step (1), the SUS304 stainless steel plate and the A 6061-T6 aluminum plate were pretreated. In this embodiment, a 300-mesh steel wire mesh was cut into a long strip of mesh with a size of 100 mm × 40 mm, and then cleaned and dried. 1 g of Nocolok flux composed of KAlF4 was dissolved in 10 mL of anhydrous ethanol and evenly applied to the surface of the stainless steel plate to be welded with a bristle brush.

The treated stainless steel plate and aluminum plate are overlapped by 15 mm to ensure that the molten aluminum has enough space to penetrate backward during welding; the aluminum plate is located above the stainless steel plate, and the wire mesh is clamped between the stainless steel plate and the aluminum plate; the relative positions of the stainless steel plate and the aluminum plate are fixed by a clamp; during the clamping process, the wire mesh must not be warped and must be close to the surface of the stainless steel plate to be welded; the external width of the wire mesh is L ₃ = 25 mm, and the Nocolok flux composed of KAlF4 is evenly coated on the surface of the wire mesh. Since the opposite side of the aluminum plate to be welded is suspended, a stainless steel pad with a thickness of 2 mm is placed under the aluminum plate to prevent the aluminum plate from being displaced in the z direction during the clamping process.

After clamping, welding begins. The welding process parameters are as follows: the inclination angle of the welding laser head is 8°, the offset of the welding laser and welding gun to the aluminum side is L ₁ =1 mm, the defocus is set to 0 mm, the wire spacing is 3 mm, the laser power is 900 W, the welding speed is 7 mm/s, and the wire feeding speed is 4.5 m/min; the shielding gas is high-purity argon, and the shielding gas flow rate is 25 L/min.

During the welding process, Nocolok flux and wire mesh work synergistically. The flux reduces the surface tension of stainless steel and initially improves wettability. The closely arranged micropores of the wire mesh exert capillary force on the molten aluminum during welding. The generated force becomes an additional driving force for the molten aluminum to spread outward, ultimately greatly improving the wetting and spreading ability of the molten aluminum on the stainless steel surface.

Figure 2 shows the weld cross-section formation of the aluminum/steel lap joint without and with steel mesh. It can be found that the wettability of the molten aluminum liquid is extremely poor when there is no steel mesh, and the spreading width is only 6.93 mm. In addition, the molten metal cannot spread outward and accumulate, and there is a sudden change in the contours of the weld and the heat-affected zone. When the joint is loaded, stress concentration will occur, which becomes the weak point of the joint and greatly reduces the mechanical properties of the joint. When there is a steel mesh, it can be found that the spreading width of the molten aluminum liquid is extremely large, up to 15.45 mm, which is 123% higher than that without the steel mesh. At the same time, since the molten aluminum liquid spreads out smoothly, the molten metal accumulation phenomenon disappears, and the contours of the weld and the heat-affected zone transition smoothly, which avoids the stress concentration when the joint is loaded and can effectively improve the mechanical properties of the joint.

The bonding quality of the Fe-Al interface also determines the mechanical properties of the joint. As shown in Figure 3, after adding the steel mesh, the thickness of the IMCs layer at the interface is uniform, about 6 μm, which is within the layer thickness range with the best performance. When the steel mesh is added, the interface is well bonded and there are no cracks. When there is no steel mesh, there are obvious cracks at the interface, and the thickness of the IMCs layer is about 9 μm.

Figure 4 shows the weld microstructure of the aluminum/steel lap joint without and with steel mesh. It can be found that the weld microstructure is finer after adding the steel mesh. This is because the Fe element in the steel mesh enters the weld during welding and forms Fe-Al phase during solidification, providing more nucleation points for the solidification of the molten metal, thereby refining the grains. When the steel mesh is not added, the stainless steel plate is almost not melted, resulting in very little Fe element entering the weld, and the molten metal solidifies normally, so the grain size is not refined.

The microhardness of a material has a certain relationship with its mechanical properties. A higher hardness value usually indicates that the material also has better mechanical properties. Microhardness tests were conducted on aluminum/steel lap welds without and with steel mesh. As shown in Figure 5, the weld structure was refined after adding the steel mesh, and the hardness value was higher, reaching 88 HV, which was 20% higher than 73 HV without the steel mesh.

The tensile properties of aluminum/steel lap joints without and with steel mesh were tested. When there is no steel mesh, due to the extremely poor wettability of molten aluminum, the molten metal cannot spread outward and accumulate, and there is a sudden change in the contour of the weld and the heat-affected zone, resulting in stress concentration. The fracture occurs in the area where the weld contour changes suddenly, as shown in Figure 6. The tensile strength is only 156 MPa, and the fracture mainly shows a tough-brittle combination fracture mode; at the same time, due to the extremely poor wettability of molten aluminum when there is no steel mesh, the spreading length is small, and the interface becomes the weakest area of the joint, so some joint failures occur at the interface. After adding the steel mesh, the spreading of the molten aluminum is greatly improved, the interface of the effective connection becomes larger, the bearing capacity of the interface is enhanced, and there is no sudden change in the joint contour. There is no stress concentration when the joint is loaded. At the same time, the weld structure is refined and the performance is enhanced. Finally, the joint failure occurs in the heat-affected zone, as shown in Figure 6, and the tensile strength is as high as 185 MPa, which is about 20% higher than that without the steel mesh.

The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any simple modification or equivalent change made to the above embodiment based on the technical essence of the present invention shall fall within the protection scope of the present invention.

Claims (9)

1. A method for lap welding of dissimilar metals of steel and aluminum assisted by a steel mesh interlayer, characterized in that it comprises the following steps: Step a, grinding and cleaning the surface of the aluminum alloy plate and stainless steel plate to be welded, cutting the steel wire mesh and cleaning and drying it; Step b, dissolving the Nocolok flux composed of KAlF4 in a solvent, and then evenly coating it on the surface of the treated stainless steel plate in the area to be welded; Step c, fixing the treated steel wire mesh on the surface of the treated stainless steel plate to be welded, and evenly coating the flux on the surface of the steel wire mesh; then overlapping the aluminum alloy plate just above the stainless steel plate so that the steel wire mesh is located between the two to form a sandwich, and then placing a pad under the aluminum alloy plate, and using a clamp to clamp and fix the weldment; Step d: performing laser-MIG brazing on the clamped welded parts. 2. A method for steel-aluminum dissimilar metal lap welding assisted by a steel mesh interlayer as claimed in claim 1, characterized in that in step c, the overlapping length L ₂ of the stainless steel plate and the aluminum alloy plate is 10-20 mm. 3. A method for steel-aluminum dissimilar metal lap welding assisted by a steel mesh interlayer as claimed in claim 1, characterized in that in step c, the external width L ₃ of the steel mesh is 20-25 mm. 4. A method for steel-aluminum dissimilar metal lap welding assisted by a steel mesh interlayer as claimed in claim 1, characterized in that in step b, the Nocolok flux composed of KAlF4 is dissolved in anhydrous ethanol, and its spreading thickness on the surface of the stainless steel plate to be welded is 0.2-0.3 mm. 5. A method for lap welding of dissimilar steel and aluminum metals assisted by a steel mesh interlayer as described in claim 1, characterized in that the mesh number of the steel mesh is 200-300. 6. A wire mesh interlayer assisted steel-aluminum dissimilar metal lap welding method as claimed in claim 1, characterized in that in step d, the offset _L1 of the welding laser beam and the MIG welding gun to the aluminum alloy plate side is 0.5-1 mm, the defocus is 0, and the filament spacing is 3 mm. 7. A method for steel-aluminum dissimilar metal lap welding assisted by a wire mesh interlayer as described in claim 1, characterized in that in step d, the laser-MIG brazing parameters are: laser power 700~1000 W; welding speed 7~9 mm/s; wire feeding speed 3~5m/min; shielding gas is argon, which is delivered from a MIG welding gun; shielding gas flow rate 15~30 L/min. 8. A method for steel-aluminum dissimilar metal lap welding assisted by a steel mesh interlayer as described in claim 1, characterized in that in step d, the inclination angle of the welding laser head is 5 to 10°. 9. A method for steel-aluminum dissimilar metal lap welding assisted by a wire mesh interlayer as described in claim 1, characterized in that in step a, the areas to be welded of the aluminum alloy plate and the stainless steel plate are grinded by an angle grinder to remove the surface oxide film; and acetone or industrial alcohol is used to wipe and remove the surface oil stains.