Welding method for manufacturing three-dimensional structure reinforced steel / aluminum welding joint on steel surface

Abstract

The invention discloses a welding method for manufacturing a three-dimensional structure reinforced steel / aluminum welding joint on a steel surface, and belongs to the technical field of dissimilar metal welding. A steel three-dimensional geometric structure is manufactured on the surface of a steel substrate by adopting a selective laser melting method, and a surface with a high length-diameter ratio and a high specific surface area is formed; a brazing or melt-brazing method is adopted, the bonding area between the molten aluminum and the steel plate and the bonding area between the molten aluminum and the three-dimensional geometric structure are greatly increased, expansion of cracks is effectively restrained, and the strength and toughness of the steel / aluminum dissimilar metal joint are enhanced. By means of the method, the steel / aluminum connector interface specific surface area can be increased, the sliding resistance is improved when an external load is borne, and meanwhile the three-dimensional geometric structure bears part of stress. And meanwhile, the three-dimensional geometric interface structure can relieve the interface stress of the connector and improve the stress distribution state, and the welded connector with good use performance can be obtained.

Description

Technical Field

[0001] The invention belongs to the technical field of dissimilar metal welding, and specifically relates to a method for preparing a steel / aluminum transition joint with a three-dimensional geometric structure by laser selective melting on a steel surface to improve strength and toughness. Background Art

[0002] With the rapid consumption of limited global energy and the accompanying serious air pollution, improving fuel efficiency is of great significance to the sustainable development of world energy. Steel / aluminum composite structures are widely used in vehicle structures due to their advantages such as lightweight, low cost, and good balance between structure and performance. However, due to the huge differences in a series of thermophysical properties of steel / aluminum such as thermal conductivity, linear expansion coefficient, melting point, etc., a large number of brittle intermetallic compounds in the form of flat or nearly flat layers are produced during the steel / aluminum welding process, resulting in poor stability of the welded joint. Under harsh service conditions, the low fracture toughness of the interface will increase the risk of failure of the welded joint, seriously affecting the performance of the joint.

[0003] In recent years, researchers have applied porous interlayers to fundamentally change the morphology of intermetallic compounds. Porous interlayers have three main advantages in the process of steel / aluminum dissimilar welding: First, when molten aluminum infiltrates into the porous structure, the morphology of the intermetallic compound will completely change. When an external load is applied, the irregular morphology of the intermetallic compound will hinder the initiation and expansion of cracks, while increasing the specific surface area at the joint interface and improving the mechanical properties of the joint. Second, a large number of studies have shown that this porous structure can effectively alleviate the residual stress at the joint interface caused by the large difference in the thermophysical properties of the two materials, greatly improving the performance of the joint. Third, because this porous structure has a special three-dimensional framework structure, it has good deformation and absorption capabilities, good plasticity, and can absorb part of the fracture energy. Although porous interlayers can significantly improve the performance of the joint, the preparation of porous interlayers mostly relies on sintering processes. This process preparation process is relatively cumbersome, and the pores of the prepared porous interlayers are irregular, resulting in unstable joint performance. Moreover, it seems that porous structures are more suitable for overlapping types in welding, relying on the gravity of the molten metal to wet and spread into the gap. When the butt joint type is adopted, this complex porous structure will affect the wetting of the molten metal and increase the difficulty of welding. Summary of the invention

[0004] In view of the above problems, the present invention provides a method for preparing a steel / aluminum transition joint with a three-dimensional geometric structure strengthened and toughened by laser selective melting on the steel surface. The method can make the prepared joint have the advantages of double strength and toughness, and various workpiece sizes and joint types.

[0005] The present invention achieves the above object through the following technical solutions:

[0006] A method for preparing a steel / aluminum transition joint with a three-dimensional geometric structure reinforced and toughened by laser selective melting on a steel surface. First, a three-dimensional geometric structure is prepared on the steel surface by laser selective melting. Before welding, the steel surface is strictly polished and cleaned, and a suspension of brazing flux and alcohol is evenly coated on the steel surface and the surface of the three-dimensional geometric structure to enhance the wetting and spreading of liquid aluminum on the steel surface. A laser-arc hybrid welding method is used for steel / aluminum dissimilar metal welding.

[0007] The specific steps include:

[0008] 1) Before welding, clean the steel plate and aluminum plate surfaces to be welded;

[0009] 2) Laser selective melting is performed on the butt joint surface of the steel plates to prepare a three-dimensional steel geometric structure, so that the butt joint surface of the steel plates forms a regular three-dimensional geometric morphology;

[0010] 3) After the steel plate with the three-dimensional geometric structure prepared on the surface is cooled naturally, the surface of the steel plate and the surrounding of the three-dimensional geometric structure are cleaned and wetted and spread;

[0011] 4) Assembling and welding the aluminum plate and the steel plate having the steel three-dimensional geometric surface structure.

[0012] Furthermore, the steel plate described in step 1) is any one of stainless steel, galvanized steel and carbon steel.

[0013] Furthermore, the initial powder sample material of the steel three-dimensional geometric structure in step 2) is any one of various types of stainless steel and iron powder.

[0014] Furthermore, the three-dimensional geometric structure described in step 2) is in the form of any one of other complex lattice structures such as columnar, hexahedral, and polyhedral lattice structures.

[0015] Furthermore, the size parameters of the columnar three-dimensional structure are: height of 0.2-2mm, diameter of 0.1-0.7mm, and center spacing between adjacent cylinders of 0.5-2mm; the size parameters of the hexahedral three-dimensional structure are: height of 0.2-2mm, length and width of 0.1-0.7mm, and center spacing between adjacent hexahedrons of 0.5-2mm; the multi-sided pyramid lattice structure frame is composed of cylindrical or hexahedral pillars, wherein the size parameters of the cylinder are: length of 0.2-4mm, diameter of 0.1-0.7mm, inclination angle of the cylindrical pillar and the base of 30-75°, and center spacing between the multi-sided pyramids of 0.5-4mm. The size parameters of the hexahedron are: height of 0.2-4mm, length and width of 0.1-0.7mm, inclination angle of the hexahedral pillar and the base of 30-75°, and center spacing between the multi-sided pyramids of 0.5-4mm.

[0016] Furthermore, the regular three-dimensional geometric morphology described in step 2) is a geometric structure neatly arranged along the thickness direction.

[0017] Furthermore, the laser selective melting parameters described in step 2) are: laser power is 200-450W, spot size is 0.03-0.1mm, scanning speed is 500-2000mm / s, and powder layer thickness is 10-50μm.

[0018] Furthermore, the surface cleaning treatment described in step 3) is to rinse the surface of the steel plate and the surrounding of the three-dimensional geometric structure using alcohol, acetone, etc.

[0019] Furthermore, the method of the wetting and spreading treatment in step 3) is but not limited to galvanizing or applying flux on the surface of the steel plate to be welded, so as to improve the wetting and spreading of liquid aluminum on the solid steel surface and around the three-dimensional geometric structure during welding.

[0020] Furthermore, the welding method in step 4) is any one of arc brazing, laser brazing, laser-arc composite brazing and various types of brazing.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] 1. By using laser selective melting technology on the steel surface, three-dimensional geometric structures of different lengths, diameters, densities and shapes can be manufactured, forming a surface with a high aspect ratio and high specific surface area, which enhances the "mechanical interlocking" effect between the weld metal and the three-dimensional geometric structure. This structure makes the stress distribution more uniform, reduces stress concentration, reduces the stress level of the interface, and ultimately improves the mechanical properties of the joint. The introduction of the three-dimensional geometric structure increases the specific surface area of ​​the interface and improves the slip resistance during the tensile process. At the same time, under the action of bending load, the three-dimensional geometric structure can absorb part of the energy.

[0023] 2. The brazing or melt brazing method greatly increases the bonding area between liquid aluminum and steel plate and three-dimensional geometric structure, effectively inhibits the expansion of cracks, and realizes the dual enhancement of strength and toughness of steel / aluminum dissimilar metal joints.

[0024] 3. With the continuous development of laser selective melting technology, the laser selective melting manufacturing process is simple and efficient, and various complex structures can be manufactured on the surface of the substrate, improving the performance of steel / aluminum dissimilar metal welding joints. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of a steel / aluminum welded transition joint with three-dimensional columnar structure reinforced and toughened by selective laser melting on the steel surface.

[0026] Figure 2 Surface morphology of steel / aluminum dissimilar metal joints strengthened by preparing three-dimensional columnar structure by selective laser melting on steel surface.

[0027] Figure 3 Cross-sectional morphology of steel / aluminum dissimilar metal joints strengthened by three-dimensional columnar structure prepared by selective laser melting on the steel surface.

[0028] Figure 4 Schematic diagram of the tetrahedral lattice structure prepared by laser selective melting on the steel surface. DETAILED DESCRIPTION

[0029] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0030] The pre-welding treatment of the steel plate to be welded after the three-dimensional geometric structure has been prepared by laser selective melting is to grind the upper and lower surfaces of the steel plate mechanically, then clean the steel plate and the three-dimensional geometric structure surface with alcohol or acetone and dry it for use; before welding, the side of the aluminum plate to be welded is first grinded flat by mechanical grinding, and the oil on the surface of the aluminum plate is removed by alcohol or acetone, firstly alkali-washed with 10% to 15% NaOH solution for 3-5 minutes, then pickled with 10% to 15% HNO3 solution for 3-5 minutes, and finally rinsed under running water and dried for use. Galvanize or apply brazing flux on the surface of the steel plate to be welded and the surface of the prepared three-dimensional geometric structure.

[0031] Example 1

[0032] This implementation is achieved through the following steps:

[0033] 1. Mechanically polish and clean the 2mm thick 316L stainless steel plate, and perform laser selective melting on the butt surface of the steel plate to prepare a three-dimensional columnar structure. The powder material is 316L stainless steel spherical powder. The three-dimensional geometric structure is a columnar structure with the following size parameters: height 1.0mm, diameter 0.5mm, two rows printed along the thickness direction, and the center spacing between the three-dimensional geometric structures is 1.0mm. The laser selective melting parameters are: laser power 300W, spot size: 0.05mm, scanning speed: 800mm / s, powder layer thickness: 25μm.

[0034] 2. Pre-welding treatment was performed on the 6061-T6 aluminum alloy plate and the 316L stainless steel plate which had been prepared into a three-dimensional columnar structure by laser selective melting.

[0035] 3. The processed steel plate to be welded and the aluminum plate to be welded are butt-jointed and assembled, and clamped with a homemade fixture. Fiber laser and servo arc welding machine are used for brazing. The welding wire is AlSi5, the laser power is 3.0KW, the scanning speed is 30mm / s, the defocus is +30mm, the spot offset is +0.7mm (with the boundary of the aluminum plate butt surface as the origin, and the offset in the direction of the aluminum plate is set as the positive direction), the CMT wire feeding speed is 8.5m / min, the CMT voltage is 13V, the shielding gas is Ar gas, and the flow rate is 10-15L / min.

[0036] 4. Figure 1 Schematic diagram of a steel / aluminum welded transition joint with a three-dimensional columnar structure reinforced and toughened by laser selective melting of the steel surface, where 1 is the steel plate, 2 is the weld, 3 is the aluminum alloy plate, and 4 is the three-dimensional columnar structure. Figure 2 and Figure 3 It can be seen that the upper and lower surfaces of the joint are well formed, the weld surface is uniform and continuous, and no pores or microcracks are observed. The weld is completely melted through, there are no defects such as weld bumps on the back, and both the front and back sides have a silvery-white luster without oxidation. The three-dimensional columnar structure interface changes the distribution morphology of the intermetallic compound layer and greatly increases the specific surface area of ​​the interface. During the stretching process, the crack will change its original expansion direction after encountering short fibers. At the same time, the three-dimensional columnar structure and the weld metal produce a "mechanical interlocking" effect, which improves the interface bonding strength. When subjected to bending load, the three-dimensional columnar structure absorbs part of the energy, thereby improving the fracture toughness of the welded joint.

[0037] In the tensile test of this group of joints, the tensile strength of the joint can reach 205MPa, which is 41.4% (145MPa) higher than that of bare steel / aluminum welded joints. The bending test results show that when bending transversely (parallel to the weld direction), the joint with a three-dimensional columnar structure has no cracks at a bending angle of 46.2°, while the bare steel / aluminum joint has brittle fracture at a bending angle of 10.9°; when bending longitudinally (perpendicular to the weld direction), the joint with a three-dimensional columnar structure is still intact at a bending angle of 70.6°, while the bare steel / aluminum joint has transverse cracks at the weld when the bending angle reaches 36.8°. This fully demonstrates that the welded joint with a three-dimensional columnar structure has the effect of increasing both strength and toughness compared to the bare steel / aluminum joint.

[0038] Example 2

[0039] This implementation is achieved through the following steps:

[0040] 1. Mechanically polish and clean the 2mm thick 316L stainless steel plate, and perform laser selective melting on the butt surface of the steel plate to prepare a three-dimensional columnar structure. The powder material is 316L stainless steel spherical powder. The three-dimensional geometric structure is a columnar structure with the following size parameters: height 1.0mm, diameter 0.5mm, three rows printed along the thickness direction, and the center spacing between the three-dimensional geometric structures is 1.0mm. The laser selective melting parameters are: laser power 300W, spot size: 0.05mm, scanning speed: 800mm / s, powder layer thickness: 25μm.

[0041] 2. Pre-welding treatment was performed on the 6061-T6 aluminum alloy plate and the 316L stainless steel plate which had been prepared into a three-dimensional columnar structure by laser selective melting.

[0042] 3. The processed steel plate to be welded and the aluminum plate to be welded are butt-jointed and assembled, and clamped with a homemade fixture. Fiber laser and servo arc welding machine are used for brazing. The welding wire is AlSi5, the laser power is 3.0KW, the scanning speed is 30mm / s, the defocus is +30mm, the spot offset is +0.7mm (with the boundary of the aluminum plate butt surface as the origin, and the offset in the direction of the aluminum plate is set as the positive direction), the CMT wire feeding speed is 8m / min, the CMT voltage is 12.5V, the shielding gas is Ar gas, and the flow rate is 10-15L / min.

[0043] 4. In the tensile test of this group of joints, the tensile strength of the joint can reach 224.8MPa, which is 55% (145MPa) higher than that of bare steel / aluminum welded joints. The bending test results show that when bending transversely (parallel to the weld direction), the joint with a three-dimensional columnar structure has no cracks at a bending angle of 57.3°, while the bare steel / aluminum joint has brittle fracture at a bending angle of 10.9°; when bending longitudinally (perpendicular to the weld direction), the joint with a three-dimensional columnar structure is still intact at a bending angle of 68.3°, while the bare steel / aluminum joint has transverse cracks at the weld when the bending angle reaches 36.8°. This fully demonstrates that the welded joint with a three-dimensional columnar structure has the effect of increasing both strength and toughness compared to the bare steel / aluminum joint.

[0044] Example 3

[0045] This implementation is achieved through the following steps:

[0046] 1. Mechanically grind and clean the 2mm thick carbon steel plate, and perform laser selective melting on the butt surface of the steel plate to prepare a three-dimensional columnar structure. The powder material is 316L stainless steel spherical powder. The three-dimensional geometric structure is a columnar structure with the following size parameters: height 0.8mm, diameter 0.5mm, two rows printed along the thickness direction, and the center spacing between the three-dimensional geometric columns is 1.0mm. The laser selective melting parameters are: laser power 300W, spot size: 0.05mm, scanning speed: 800mm / s, powder layer thickness: 30μm.

[0047] 2. Pre-welding treatment was performed on 6061-T6 aluminum alloy plates and carbon steel plates with three-dimensional columnar structures prepared by laser selective melting.

[0048] 3. The processed steel plate to be welded and the aluminum plate to be welded are butt-jointed and assembled, and clamped with a homemade fixture. Fiber laser and servo arc welding machine are used for brazing. The welding wire is AlSi5, the laser power is 3.0KW, the scanning speed is 30mm / s, the defocus is +30mm, the spot offset is +0.7mm (with the boundary of the aluminum plate butt surface as the origin, and the offset in the direction of the aluminum plate is set as the positive direction), the CMT wire feeding speed is 7.5m / min, the CMT voltage is 11V, the shielding gas is Ar gas, and the flow rate is 10-15L / min.

[0049] 4. The tensile performance test of the joint showed that the tensile strength of the joint was 185MPa, which was 53.2% higher than that of the bare steel / aluminum joint (120.7MPa). The bending test results showed that when the joint with a three-dimensional columnar structure was bent transversely (parallel to the weld direction), the bending angle of the joint reached 50.8° without cracking, while the bare steel / aluminum joint had brittle fracture when the bending angle reached 14.2°; when the joint with a three-dimensional columnar structure was bent longitudinally (perpendicular to the weld direction), the bending angle of the joint reached 72.5° and the joint was intact, while the bare steel / aluminum joint had transverse cracks at the weld when the bending angle reached 30.6°. This shows that in addition to the joint strength, the joint toughness has also been greatly improved.

[0050] Example 4

[0051] This implementation is achieved through the following steps:

[0052] 1. Mechanically grind and clean the 2mm thick 316L stainless steel plate, and perform laser selective melting on the butt surface of the steel plate to prepare a hexahedral structure. The powder material is 316L stainless steel spherical powder. The size parameters of the hexahedron are: height 1.2mm, length and width 0.3mm, two rows are printed along the thickness direction, and the center spacing between the hexahedral structures is 1.0mm. The laser selective melting parameters are: laser power 200W, spot size: 0.03mm, scanning speed: 600mm / s, powder layer thickness: 35μm.

[0053] 2. Pre-welding treatment was performed on the 6061-T6 aluminum alloy plate and the 316L stainless steel plate with hexahedral structure prepared by laser selective melting.

[0054] 3. The processed steel plate to be welded and the aluminum plate to be welded are butt-jointed and assembled, and clamped with a homemade fixture. Fiber laser and servo arc welding machine are used for brazing. The welding wire is AlSi5, the laser power is 3.0KW, the scanning speed is 30mm / s, the defocus is +30mm, the spot offset is +0.7mm (with the boundary of the aluminum plate butt surface as the origin, and the offset in the direction of the aluminum plate is set as the positive direction), the CMT wire feeding speed is 9m / min, the CMT voltage is 14V, the shielding gas is Ar gas, and the flow rate is 10-15L / min.

[0055] 4. The tensile performance test of the joint showed that the tensile strength of the joint was 198MPa, which was 36.6% higher than that of the bare steel / aluminum joint (145MPa). The bending test results showed that when the joint with a hexahedral structure was bent transversely (parallel to the weld direction), the bending angle of the joint with a hexahedral structure reached 22.1° without cracks; when the joint was bent longitudinally (perpendicular to the weld direction), the bending angle of the joint with a hexahedral structure reached 48.5° and remained intact, and the toughness of the joint was significantly improved.

[0056] Example 5

[0057] This implementation is achieved through the following steps:

[0058] 1. Mechanically grind and clean the 2mm thick 316L stainless steel plate, and perform laser selective melting on the butt surface of the steel plate to prepare a tetrahedral lattice structure. Figure 4 Schematic diagram of the preparation of a tetrahedral lattice structure by laser selective melting on a steel surface, where 5 is a cylindrical pillar. The powder material is 316L stainless steel spherical powder. The tetrahedral lattice structure is composed of cylindrical pillars, and the cylindrical size parameters are: length 2mm, diameter 0.5mm, the inclination angle between the cylindrical pillar and the substrate is 60°, and the center distance between adjacent vertebral structures is 4mm. The laser selective melting parameters are: laser power 200W, spot size: 0.03mm, scanning speed: 600mm / s, powder layer thickness: 50μm.

[0059] 2. Pre-welding treatment was performed on the 6061-T6 aluminum alloy plate and the 316L stainless steel plate with a tetrahedral lattice structure prepared by laser selective melting.

[0060] 3. The processed steel plate to be welded and the aluminum plate to be welded are butt-jointed and assembled, and clamped with a homemade fixture. Fiber laser and servo arc welding machine are used for brazing. The welding wire is AlSi5, the laser power is 3.0KW, the scanning speed is 30mm / s, the defocus is +30mm, the spot offset is +0.7mm (with the boundary of the aluminum plate butt surface as the origin, and the offset in the direction of the aluminum plate is set as the positive direction), the CMT wire feeding speed is 9m / min, the CMT voltage is 14V, the shielding gas is Ar gas, and the flow rate is 10-15L / min.

[0061] 4. The tensile properties of the joint were tested, and the results showed that the tensile strength of the joint was 212MPa, which was 46.2% higher than that of the bare steel / aluminum joint (145MPa). The bending test results showed that when the joint with a tetrahedral lattice structure was bent transversely (parallel to the weld direction), the bending angle of the joint with a tetrahedral lattice structure reached 36.2° without cracking; when the joint was bent longitudinally (perpendicular to the weld direction), the bending angle of the joint with a tetrahedral lattice structure reached 56.5° and remained intact, and the toughness of the joint was significantly improved.

[0062] The above description is only a preferred embodiment of the present invention, but the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements and improvements based on the technical solutions and inventive concepts of the present invention within the technical scope disclosed by the present invention, and all of these should be covered by the protection scope of the present invention.

Claims

1. A method for preparing a steel / aluminum transition joint with a three-dimensional structure by laser selective melting of a steel surface, characterized in that: The following steps are involved: 1) Before welding, clean the steel plate and aluminum plate surfaces to be welded; 2) Laser selective melting is performed on the butt joint surface of the steel plates to prepare a three-dimensional steel geometric structure, so that the butt joint surface of the steel plates forms a regular three-dimensional geometric morphology; 3) After the steel plate with the three-dimensional geometric structure prepared on the surface is cooled naturally, the surface of the steel plate and the surrounding of the three-dimensional geometric structure are cleaned and wetted and spread; 4) Assembling and welding the aluminum plate and the steel plate having the steel three-dimensional geometric surface structure.

2. The preparation method according to claim 1, characterized in that: The steel plate described in step 1) is any one of stainless steel, galvanized steel and carbon steel.

3. The preparation method according to claim 1, characterized in that: The initial powder sample material of the steel three-dimensional geometric structure in step 2) is any one of stainless steel and iron powder.

4. The preparation method according to claim 1, characterized in that: The three-dimensional geometric structure described in step 2) is in the form of any one of a columnar, hexahedral, or multi-pyramid lattice structure.

5. The preparation method according to claim 4, characterized in that: The size parameters of the columnar three-dimensional structure are as follows: the height is 0.2-2mm, the diameter is 0.1-0.7mm, and the center spacing between adjacent cylinders is 0.5-2mm; the size parameters of the hexahedral three-dimensional structure are as follows: the height is 0.2-2mm, the length and width are 0.1-0.7mm, and the center spacing between adjacent hexahedrons is 0.5-2mm; the multi-sided pyramid lattice structure frame is composed of cylindrical or hexahedral pillars, wherein the size parameters of the cylinder are as follows: the length is 0.2-4mm, the diameter is 0.1-0.7mm, the inclination angle between the cylindrical pillar and the base is 30-75°, and the center spacing between the multi-sided pyramids is 0.5-4mm. The size parameters of the hexahedron are as follows: the height is 0.2-4mm, the length and width are 0.1-0.7mm, the inclination angle between the hexahedral pillar and the base is 30-75°, and the center spacing between the multi-sided pyramids is 0.5-4mm.

6. The preparation method according to claim 1, characterized in that: The laser selective melting parameters described in step 2) are: laser power of 200-450 W, spot size of 0.03-0.1 mm, scanning speed of 500-2000 mm / s, and powder layer thickness of 10-50 μm.

7. The preparation method according to claim 1, characterized in that: The surface cleaning treatment described in step 3) is to rinse the surface of the steel plate and the surrounding of the three-dimensional geometric structure using alcohol and acetone.

8. The preparation method according to claim 1, characterized in that: The method used in the wet spreading treatment in step 3) is to galvanize or apply brazing flux on the surface of the steel plate to be welded.

9. The preparation method according to claim 1, characterized in that: The welding method described in step 4) is any one of arc brazing, laser brazing, and laser-arc composite brazing.

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