Aluminum / steel dissimilar metal laser melting-brazing method

Abstract

The present invention provides an aluminum / steel dissimilar metal laser melting-brazing method, which belongs to the technical field of dissimilar metal precision connection. The melting-brazing method adopted by the present invention is between fusion welding and brazing, and combines the high-strength structure of fusion welding and the good wettability of brazing, so as to obtain a high-strength welded joint. The present invention first processes the butt sides of the aluminum plate and the steel plate to be welded into sloped grooves respectively before welding. Compared with vertical surface butt welding, the mechanical properties of the joint are improved, and the porosity defects are effectively suppressed; the synergistic effect of the sloped semi-lap groove and laser melting-brazing organically unifies the welding structure and metallurgical regulation, improves the geometric and mechanical structure, interface metallurgical reaction, micro-component segregation and welding defects of aluminum / steel plate butt welding, and obtains a melting-brazing joint with excellent microstructure and mechanical properties.

Description

Technical Field

[0001] The invention belongs to the technical field of precision connection of dissimilar metals, and in particular relates to an aluminum / steel dissimilar metal laser melting-brazing method. Background Art

[0002] Aluminum and steel are two typical metal engineering materials. Due to their unique physical and chemical properties, they play an indispensable role in the manufacture of electronic equipment and structural parts. However, energy conservation and emission reduction, lightweight, and cost reduction and efficiency improvement have become important development trends in the engineering field. The application of a single material is often difficult to meet the comprehensive performance requirements under complex working conditions. Therefore, aluminum / steel dissimilar material welding technology came into being. It combines the lightweight, high strength, high electrical / thermal conductivity, and corrosion resistance of aluminum with the high load-bearing capacity and low cost of steel, which not only achieves the optimal configuration of material properties, but also significantly reduces the overall structural weight, improves energy efficiency and environmental performance. Therefore, it has a wide range of applications in engineering fields such as electrical equipment and its heat dissipation components, lightweight automobiles, aerospace structural parts, rail transportation, and oil pipelines.

[0003] However, there are still many problems in aluminum / steel welding. Due to the significant differences in the melting point, thermal / electrical conductivity, expansion coefficient and other properties of aluminum / steel, high residual stress and crack tendency are generated during welding. The solid solubility between aluminum and iron is extremely low, and chemical reactions are easy to occur at high temperatures, causing the solid solubility of aluminum and iron atoms in the liquid metal state to be almost zero to form brittle intermetallic compounds (IMCs), such as FeAl3, Fe2Al5, Fe2Al, etc. Aluminum generates Al2O3 at high temperatures. When the thickness of the intermetallic compound layer is greater than 10μm, the excessive brittle intermetallic compounds in the weld will seriously reduce the mechanical properties represented by toughness.

[0004] In order to improve the quality of aluminum / steel welding, researchers have made technical improvements to typical welding methods: traditional melting welding is combined with experimental design methods such as Taguchi method and response surface method to optimize parameters such as average power, peak power, pulse width, frequency, welding speed, etc., to find the best parameter combination to reduce the generation of intermetallic compounds and cracks and improve welding quality. However, it is difficult to completely avoid the generation of intermetallic compounds that increase the brittleness of the joint by optimizing process parameters. At the same time, melting welding usually requires higher energy input and longer welding time, which is prone to high heat input, causing welding defects and excessive residual stress; brazing is particularly suitable for welding between dissimilar metals, and the melting point of the brazing material is lower than the melting point of the connected metal, so the connected metal will not be melted during the connection process, so the thermal stress and deformation are very small, and the joint surface is smooth and flat, which is suitable for the welding of high-precision parts such as electronic component packaging, electronic equipment heat sinks, and metal crafts. However, the welding time of brazing is relatively long, and the weld basically does not generate an intermetallic compound layer, resulting in no metallurgical bonding of the weld metal, and the weld strength is relatively low, which is not suitable for welding parts under high load conditions and high temperature environments. Therefore, there is an urgent need for a welding method that can obtain better mechanical properties. Summary of the invention

[0005] The purpose of the present invention is to provide a laser melting-brazing method for aluminum / steel dissimilar metals. The method provided by the present invention can obtain a welded joint with no obvious defects and excellent mechanical properties.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0007] The present invention provides an aluminum / steel dissimilar metal laser melting-brazing method, comprising the following steps:

[0008] (1) The butt joint sides of the aluminum plate and the steel plate to be welded are respectively processed into a sloped groove to obtain an aluminum plate with a groove and a steel plate with a groove; the angle between the groove slope of the steel plate with a groove and the bottom edge of the steel plate is less than 90°; the sum of the angle between the groove slope of the aluminum plate with a groove and the bottom edge of the aluminum plate and the angle between the groove slope of the steel plate with a groove and the bottom edge of the steel plate is 180°;

[0009] (2) coating a mixed slurry on the groove surface of the steel plate containing the groove obtained in step (1) to obtain a steel plate containing a coating layer; the mixed slurry comprises Nocolok powder and acetone;

[0010] (3) Butting the aluminum plate with the groove obtained in step (1) and the steel plate with the coating layer with the groove obtained in step (2), and then laser welding the butt joint to obtain a composite plate.

[0011] Preferably, in step (1), the angle between the groove slope of the steel plate having the groove and the bottom edge of the steel plate is 20-70°.

[0012] Preferably, the thickness of the coating layer in step (2) is 20-40 μm.

[0013] Preferably, the thickness of the coating layer in step (2) is 30 μm.

[0014] Preferably, the mass ratio of Nocolok powder to acetone in the mixed slurry in step (2) is 1:(2-3).

[0015] Preferably, the steel plate with grooves in step (2) is also pretreated before surface coating.

[0016] Preferably, during docking in step (3), the aluminum plate at the interface is above the steel plate.

[0017] Preferably, during laser welding in step (3), the laser beam is aimed at one side of the aluminum plate.

[0018] Preferably, in the step (3), the offset distance between the center of the laser spot and the joint during laser welding is calculated according to formula I:

[0019] Formula I;

[0020] In formula I, d is the offset distance, b is the thickness of the steel plate to be welded, and θ is the angle between the groove slope of the steel plate with the groove and the bottom edge of the steel plate.

[0021] Preferably, in the step (3), the laser power of the laser welding is 1-4 kW, the welding speed is 0.4-1.2 m / min, the defocus is +1-30 mm, and the beam inclination is 5-20°.

[0022] The present invention provides an aluminum / steel dissimilar metal laser melting-brazing method, comprising the following steps: (1) processing the butt-joining sides of an aluminum plate and a steel plate to be welded into a sloped groove respectively, so as to obtain an aluminum plate with a groove and a steel plate with a groove; the angle between the groove slope of the steel plate with a groove and the bottom edge of the steel plate is less than 90°; the sum of the angle between the groove slope of the aluminum plate with a groove and the bottom edge of the aluminum plate and the angle between the groove slope of the steel plate with a groove and the bottom edge of the steel plate is 180°; (2) coating the groove surface of the steel plate with a groove obtained in step (1) with a mixed slurry, so as to obtain a steel plate with a coating layer; the mixed slurry comprises Nocolok powder and acetone; (3) butting the grooves of the aluminum plate with a groove obtained in step (1) and the steel plate with a coating layer obtained in step (2), and then laser welding the butt joint to obtain a composite plate. The laser melting-brazing method adopted by the present invention is between fusion welding and brazing, combining the high-strength structure of fusion welding and the good wettability of brazing, and can obtain high-strength welded joints. First, the butt-jointed sides of the aluminum plate and the steel plate to be welded are respectively processed into sloped grooves before welding. Compared with vertical face butt welding, the welding interface bonding area is increased, and the weld stress area is increased without changing the thickness of the parent material, thereby improving the mechanical properties of the joint. The aluminum side does not need to be welded through a large area, and the average melting depth of the parent material in the molten welding area is reduced. Under the condition of the same heat input, the energy of the brazing interface for the solid-liquid diffusion reaction of aluminum / steel is increased, the thickness of the intermetallic compound layer of the structural organization is increased, and the uniformity of the thickness of the intermetallic compound layer near the joint interface is improved, thereby improving the strength of the joint due to the cross-sectional structure when subjected to external force. The stress concentration caused by the segregation of the fabric, in addition, the liquid metal is not easy to collapse during welding, which improves the stability of the keyhole. At the same time, due to the existence of the sloped groove, the maximum depth of the keyhole is limited, and the distance required for the bubbles in the molten pool to escape the liquid metal is shortened. The Marangoni convection caused by the difference in surface tension in the liquid metal stirs the molten pool, causing the molten pools on the left and right sides of the laser beam to flow up and down along the slope respectively, providing a greater driving force for the movement of bubbles in the molten pool, and the pore defects are effectively suppressed; the synergistic effect of the sloped semi-lap groove and laser melting-brazing welding organically unifies the welding structure and metallurgical regulation, improves the geometric and mechanical structure, interface metallurgical reaction, microscopic component segregation and welding defects of aluminum / steel plate butt welding, and obtains a melting-brazing joint with excellent microstructure and mechanical properties. The results of the embodiment show that the average tensile strength of the joint obtained by the method of the present invention is 125.42MPa, while the average tensile strength of the joint obtained by vertical butt welding is 81.61MPa. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 A schematic diagram of the welding structure of the aluminum / steel dissimilar metal laser melting-brazing method provided by the present invention;

[0024] Figure 2 This is a macroscopic morphology of the aluminum / steel dissimilar metal welded joint of Example 1 of the present invention;

[0025] Figure 3 This is a macroscopic morphology of the aluminum / steel dissimilar metal welded joint of Example 2 of the present invention;

[0026] Figure 4 This is a macroscopic morphology of the aluminum / steel dissimilar metal welded joint of Comparative Example 1;

[0027] Figure 5 This is a SEM image of the aluminum / steel dissimilar metal welded joint interface of Example 1 of the present invention;

[0028] Figure 6 This is the SEM image of the aluminum / steel dissimilar metal welded joint interface of Comparative Example 1;

[0029] Figure 7 It is a comparison chart of the mechanical properties of the aluminum / steel dissimilar metal welded joints of Example 1, Example 2 and Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0030] The present invention provides an aluminum / steel dissimilar metal laser melting-brazing method, comprising the following steps:

[0031] (1) The butt joint sides of the aluminum plate and the steel plate to be welded are respectively processed into a sloped groove to obtain an aluminum plate with a groove and a steel plate with a groove;

[0032] (2) coating the mixed slurry on the groove surface of the steel plate containing the groove obtained in step (1) to obtain a steel plate containing a coating layer;

[0033] (3) Butting the aluminum plate with the groove obtained in step (1) and the steel plate with the coating layer with the groove obtained in step (2), and then laser welding the butt joint to obtain a composite plate.

[0034] The invention processes the butt joint sides of the aluminum plate and the steel plate to be welded into inclined grooves respectively, so as to obtain the aluminum plate with grooves and the steel plate with grooves.

[0035] In the present invention, the angle between the groove slope of the grooved steel plate and the bottom edge of the steel plate is less than 90°, preferably 20-70°, more preferably 30-60°, and further preferably 40-45°.

[0036] In the present invention, the sum of the angle between the bevel surface of the aluminum plate with a bevel and the bottom edge of the aluminum plate and the angle between the bevel surface of the steel plate with a bevel and the bottom edge of the steel plate is 180°, that is, the angle between the bevel surface of the aluminum plate with a bevel and the bottom edge of the aluminum plate and the angle between the bevel surface of the steel plate with a bevel and the bottom edge of the steel plate are complementary angles, which is beneficial to the subsequent docking of the two.

[0037] The present invention has no special limitation on the method of processing the butt joint sides of the aluminum plate and the steel plate to be welded into a sloped groove. A processing method familiar to those skilled in the art can be used to ensure that the angle between the groove slope and the bottom edge of the plate meets the above requirements.

[0038] The present invention has no particular limitation on the specific types and models of the steel plate and the aluminum plate, and any steel plate and aluminum plate familiar to those skilled in the art may be used.

[0039] In the present invention, the method of processing the butt joint sides of the aluminum plate and the steel plate to be welded into a sloped groove preferably includes electric spark wire cutting. The present invention preferably processes the plate by clamping with an adjustable angle clamp. The present invention has no special limitation on the specific structure of the adjustable angle clamp, and the adjustable angle clamp known to those skilled in the art can be used.

[0040] After obtaining the steel plate with the groove, the present invention coats the mixed slurry on the groove surface of the steel plate with the groove to obtain the steel plate with the coating layer.

[0041] In the present invention, the steel plate having the groove is preferably pretreated before surface coating.

[0042] In the present invention, the pretreatment preferably includes, in sequence, mechanically grinding the groove and the position near the groove, soaking the steel plate containing the groove in acetone, and wiping the groove with alcohol and drying.

[0043] The present invention has no special limitation on the specific operation of mechanical grinding of the groove and the position near the groove, and it is sufficient to ensure that the processing traces during groove processing are eliminated. The present invention can improve the flatness of the surface of the plate to be welded through mechanical grinding, make the parent material closely combined, and avoid hole defects.

[0044] The present invention has no special limitation on the specific operations of acetone soaking and alcohol wiping, and the oil stains and oxides on the surface of the plate to be welded can be removed.

[0045] The present invention has no special limitation on the drying operation, and it is sufficient to ensure that the groove is fully dried.

[0046] In the present invention, the mixed slurry includes Nocolok powder and acetone.

[0047] In the present invention, the particle size of the Nocolok powder is preferably 0.1-1 μm.

[0048] In the present invention, the mass ratio of the Nocolok powder to acetone is preferably 1:(2-3).

[0049] In the present invention, the thickness of the coating layer is preferably 20-40 μm, more preferably 25-35 μm, and further preferably 30 μm. In the present invention, coating is performed on the butt joint of the groove of the plate to be welded on the steel side, the purpose of which is to improve the wettability of molten aluminum on the solid steel surface and remove the oxide film on the surface of the aluminum alloy. Since the aluminum alloy part is directly heated and melted into a liquid state during welding, while the steel part is indirectly heated and does not melt and remains solid, coating on the steel side is beneficial to reduce the loss of brazing material caused by the melting of the aluminum alloy and the stirring of the molten pool when the brazing material is coated on the aluminum alloy side, and is also beneficial to improve the wettability of molten aluminum on the solid steel surface. The present invention limits the thickness of the coating layer within the above range, which can further improve the mechanical properties of the joint.

[0050] The present invention preferably uses a feeler gauge of corresponding thickness as a reference during coating, and gradually withdraws the feeler gauge along the welding direction to perform multiple coatings, while scraping off excess slurry with a cover glass to ensure uniform coating thickness.

[0051] After obtaining the aluminum plate with a groove and the steel plate with a coating layer, the present invention butts the grooves of the aluminum plate with a groove and the steel plate with a coating layer, and then performs laser welding on the butt joint to obtain a composite plate.

[0052] In the present invention, the aluminum plate with the groove is preferably pretreated before butt-jointing.

[0053] In the present invention, the pretreatment method of the aluminum plate with a groove is preferably the same as the pretreatment method of the steel plate with a groove, which will not be described in detail here.

[0054] The present invention preferably clamps the pre-treated aluminum plate with a groove and the steel plate with a coating layer onto a welding butt fixture, and then performs laser welding.

[0055] The present invention has no special limitation on the structure of the welding butt clamp, and any welding butt clamp commonly used by those skilled in the art may be used.

[0056] The clamping of the present invention is butt-jointed through the semi-lap groove of the aluminum / steel parent material, so that the angles of the groove inclined surface and the bottom surface are complementary to each other, forming a butt joint with a semi-lap groove structure. The groove of the aluminum side parent material is at the top, and the groove of the steel side parent material is at the bottom. The laser beam focus is located above the joint to irradiate the parent material below it.

[0057] In the present invention, during the docking, the aluminum plate is preferably above the steel plate at the interface.

[0058] In the present invention, the laser beam is preferably directed toward one side of the aluminum plate during laser welding. The present invention controls the aluminum plate at the interface to be above the steel plate and directs the laser beam toward one side of the aluminum plate during laser welding, in order to melt the low-melting-point aluminum base material and prevent the steel base material from being heated and melted in large quantities. In addition, by adjusting the laser beam irradiation position, the heat transfer at each position of the aluminum / steel interface can be balanced, so that the interface reaction process at each position is consistent.

[0059] In the present invention, the offset distance between the center of the laser spot and the joint during laser welding is preferably calculated according to Formula I:

[0060] Formula I;

[0061] In the formula I, d is the offset distance, b is the thickness of the steel plate to be welded, and θ is the angle between the groove slope of the steel plate having the groove and the bottom edge of the steel plate.

[0062] The present invention offsets the center of the laser spot from the joint by a certain distance during laser welding, the purpose of which is to ensure that the heat input acting on the aluminum alloy base material on the left and right sides of the laser beam is the same, so that the sloped groove on the steel side transfers heat evenly during the welding process. The present invention limits the offset distance to the above range, which can further improve the mechanical properties of the joint.

[0063] In the present invention, the laser power of the laser welding is preferably 1-4 kW, more preferably 2-3 kW; the welding speed is preferably 0.4-1.2 m / min, more preferably 0.5-0.8 m / min; the defocus is preferably +1-30 mm, more preferably +2-20 mm; the beam inclination is preferably 5-20°, more preferably 5-10°. The present invention limits the parameters of laser welding within the above range, which can increase the heated area of ​​the plate to be welded and further improve the mechanical properties of the joint.

[0064] The melting-brazing method used in the present invention is between fusion welding and brazing, combining the high-strength joint of fusion welding and the good wettability of brazing, and can obtain a high-strength welded joint. Compared with traditional brazing, only part of the parent material is melted during welding, which can improve the mechanical properties of the joint; and the use of laser as a heat source can achieve rapid heating and cooling, thereby improving production efficiency. Compared with traditional fusion welding, it can prevent the direct contact and mutual reaction of aluminum / steel liquid metals, effectively inhibit the generation of a large number of intermetallic compounds, prevent excessively thick intermetallic compound layers from causing brittle joints, and generate microcracks, thereby reducing the structural strength of the joint. The above-mentioned beneficial effects are mainly because a certain amount of aluminum-iron intermetallic compounds must be generated to achieve effective connection of aluminum / steel dissimilar metals, and when the thickness of the intermetallic compound layer is less than 10μm, the amount generated is proportional to the strength of the joint. When the thickness of the intermetallic compound layer is greater than 10μm, the strength of the joint usually decreases.

[0065] Compared with the direct vertical surface connection, the sloped half-lap groove processed by the present invention has an increased melting-brazing interface bonding area, so the weld stress area is increased without changing the thickness of the parent material, which directly improves the mechanical properties of the joint; the machining method of the sloped groove is simple, flexible and low-cost, and can be prepared by low-cost machining processes such as wire cutting machine tools, chamfering machines, and groove machines; since the aluminum side does not need a large area of ​​penetration, the average melting depth of the parent material in the melting zone is reduced. Therefore, under the same heat input, the energy of the brazing interface used for the solid-liquid diffusion reaction of aluminum / iron is increased, which can increase the thickness of the intermetallic compound layer of the joint organization; due to the use of a sloped half-lap groove and the adjustment of the laser offset, the heat transfer at different positions of the melting-brazing interface can be uniform, thereby greatly improving the uniformity of the thickness of the intermetallic compound layer near the interface of the joint, and improving the stress concentration caused by the segregation of the interface organization when subjected to external force. Due to the uniform heat transfer at the interface, it is also convenient to accurately control the thickness of the intermetallic compound layer by adjusting the laser parameters to keep it within the range of the optimal organization and performance.

[0066] In the technical solution of the present invention, the laser beam divides the aluminum fusion welding area into two left and right areas: the triangular area close to the steel side has less aluminum alloy parent material, slow heat dissipation, and low energy required for melting; while the area close to the aluminum side has more aluminum alloy parent material, fast heat dissipation, and high energy required for melting this area. Therefore, limiting the laser offset distance within the above-mentioned aluminum side range can balance the thermal radiation absorbed by the parent materials on the left and right sides of the laser during welding, thereby achieving uniform radiation heat transfer in heat transfer. Thanks to the sloped semi-lap groove processed by the present invention, the keyhole is located above the sloped groove during the advancement of the laser beam, and the sloped groove on the steel side can support the keyhole. Compared with the butt joint of the vertical section, the liquid metal is not easy to collapse, which improves the stability of the keyhole; in addition, due to the existence of the sloped groove, the maximum depth of the keyhole is limited, and the distance required for the bubbles in the molten pool to escape the liquid metal is shortened. At the same time, due to the stirring effect of the Marangoni convection on the molten pool caused by the surface tension difference in the liquid metal, the molten pools on the left and right sides of the laser beam flow up and down along the slope respectively, providing a greater driving force for the movement of bubbles in the molten pool, so that welding defects such as pores are effectively suppressed. The synergistic effect of the sloped semi-lap groove and laser melting-brazing welding of the present invention organically unifies the welding structure and metallurgical regulation, improves the geometric and mechanical structure, interface metallurgical reaction, micro-component segregation and welding defects of aluminum / steel plate butt welding, and obtains a melting-brazing joint with excellent microstructure and mechanical properties.

[0067] The schematic diagram of the welding structure of the aluminum / steel dissimilar metal laser melting-brazing method provided by the present invention is as follows: Figure 1 shown. Figure 1 The groove angle is the angle between the groove slope of the steel plate with groove and the bottom edge of the steel plate.

[0068] The technical solutions in the present invention will be described clearly and completely below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0069] Example 1

[0070] A laser melting-brazing method for aluminum / steel dissimilar metals, the specific steps are:

[0071] (1) Using an electric spark wire cutting machine, a tiltable guide rod clamp is used for clamping. The clamp can be tilted in the range of 0 to 90 degrees. After adjusting to the required angle, it is fixed by an angle locking pin and a quick locking device. Then, the butt sides of the aluminum plate to be welded and the steel plate to be welded are processed to form a sloped groove, respectively, to obtain an aluminum plate with a groove and a steel plate with a groove. The angle between the groove slope of the steel plate and the bottom edge of the steel plate (i.e., the groove angle) is 45 degrees, and the angle between the groove slope of the aluminum plate and the bottom edge of the aluminum plate is 135 degrees. The aluminum plate used is a 6061-T6 aluminum alloy plate, and the steel plate used is a 316 stainless steel plate. The plate thickness is 1.5 mm;

[0072] (2) The grooves and the 20 mm range on both sides of the processed aluminum plates with grooves and steel plates with grooves are mechanically polished until the wire cutting processing marks are completely eliminated, and then soaked in acetone for 20 minutes, and then the surface to be welded is wiped with alcohol, and then blown dry with a hair dryer to remove the oil and oxide on the surface of the aluminum / steel plates to be welded; a mixed slurry made by mixing Nocolok powder (particle size 0.1~1μm) and acetone in a mass ratio of 1:2 is applied to the groove joint of the cleaned steel plates to be welded, and a 0.03mm (30μm) feeler gauge is used for covering and comparison, and the slurry is applied with a length of 15mm. After drying, the excess slurry is scraped off with a cover piece, and then the feeler gauge is withdrawn about 15mm in the uncoated direction and the coating steps are repeated until the entire weld is evenly coated and the coating thickness is ensured to be 30μm, thereby obtaining a steel plate with a coating layer;

[0073] (3) The pre-treated aluminum plate with groove and the steel plate with coating layer are butted together to form a semi-lap weldment with the groove inclined surface and bottom surface complementary to each other, and then clamped to a universal butt welding fixture. Laser welding is performed using a straight welding route: laser power 2kW, welding speed 0.5m / min, defocusing amount +2mm, laser spot center offset distance from the seam 0.9mm during laser welding, beam inclination angle 5°,

[0074] Among them, the offset distance between the center of the laser spot and the joint during laser welding is calculated according to formula I:

[0075] Formula I;

[0076] In formula I, d is the offset distance, b is the thickness of the aluminum plate to be welded, and θ is the angle between the groove slope of the steel plate with the groove and the bottom edge of the steel plate.

[0077] Example 2

[0078] The only difference from Example 1 is that the welding speed during laser welding in step (3) is 0.6 m / min, and the other parameters are the same as those in Example 1.

[0079] Comparative Example 1

[0080] The difference from Example 1 is that: no sloped semi-lap groove is processed on the butt side of the aluminum / steel plate to be welded, and the plate cross-section butt joint is adopted, that is, vertical butt joint, and other parameters are the same as those in Example 1.

[0081] The weld after welding in Example 1 is characterized by using an electric spark wire cutting machine to cut a cross-sectional sample from the welded aluminum / steel dissimilar metal joint. The sampling position should ensure that the full width of the weld and part of the parent material are covered. From the observation direction, the steel is on the left and the aluminum alloy is on the right. The cross-sectional sample is metallographically prepared, and the microstructure of the weld cross section is observed using an optical microscope (OM) and a scanning electron microscope (SEM). According to GB / T 228.1-2010 standard, a tensile specimen suitable for the welded structure is prepared, and a tensile test is carried out in accordance with the standard, and the maximum load and fracture position are recorded to evaluate the tensile strength and elongation of the joint.

[0082] The same method was used to characterize the welds after welding in Example 2 and Comparative Example 1. In order to better distinguish the micrographs of Example 1 and Example 2, the sampling direction of the metallographic cross section of Example 2 is opposite to that of Example 1, that is, in the micrograph of Example 2, the aluminum alloy is on the left and the steel is on the right. In order to better distinguish the micrographs of Example 1 and Comparative Example 1, the sampling direction of the metallographic cross section of Comparative Example 1 is opposite to that of Example 1, that is, in the micrograph of Comparative Example 1, the aluminum alloy is on the left and the steel is on the right.

[0083] The results are as follows Figure 2-Figure 7 shown. Figure 2 This is a macroscopic morphology of the aluminum / steel dissimilar metal welded joint of Example 1; Figure 3 This is a macroscopic morphology of the aluminum / steel dissimilar metal welded joint of Example 2; Figure 4 This is a macroscopic morphology of the aluminum / steel dissimilar metal welded joint of Comparative Example 1; Figure 5 This is a SEM image of the aluminum / steel dissimilar metal welded joint interface of Example 1; Figure 6 This is the SEM of the aluminum / steel dissimilar metal welded joint interface of Comparative Example 1; Figure 7 It is a comparison diagram of the mechanical properties of aluminum / steel dissimilar metal welded joints of Example 1, Example 2 and Comparative Example 1; Figure 5-Figure 6 The elemental compositions and possible phases at different positions of the joint are shown in Tables 1 and 2, respectively.

[0084] Table 1 Figure 5 Element composition and possible phase formation at different positions of the joint

[0085]

[0086] Table 2 Figure 6 Element composition and possible phase formation at different positions of the joint

[0087]

[0088] Depend on Figure 2-Figure 4 It can be seen that after processing the sloped half-lap groove auxiliary structure, both Example 1 and Example 2 can obtain welded joints with no obvious defects. Only a small number of small-sized circular pores appeared in the upper part of the aluminum side weld of Example 2, while a certain number of pore defects appeared in the aluminum side joint of Comparative Example 1. The overall number of pore defects at the joints in Example 1 and Example 2 is significantly reduced, and the generation of irregular large process pores caused by the instantaneous instability and collapse of the laser keyhole during the welding process is completely suppressed. And because the molten pool and the keyhole are shallower than those in Comparative Example 1, the number of small-sized circular pores formed by the incomplete escape of gas in the molten pool is significantly reduced. The small-sized circular pores that appear in the joint of Example 2 are due to the increase in welding speed, the shortening of the solidification time of the molten pool, and the small amount of gas remaining in the molten pool that cannot completely escape and remains on the top of the molten pool to form pores. This shows that the processed sloped half-lap groove can improve the stability of the keyhole during the welding process and suppress the generation of welding defects such as pores. Figure 5-Figure 6 It can be seen that an intermetallic compound layer is generated near the aluminum / steel bonding surface of Example 1, and its average thickness is 7.93 μm, while the average thickness of the intermetallic compound layer of Comparative Example 1 is 1.82 μm, indicating that the metallurgical bonding effect of the weld metal of Example 1 is better than that of Comparative Example 1, and the average thickness of the intermetallic compound layer of Example 1 does not exceed 10 μm, which is within the optimal microstructure range. Figure 7 It can be seen that the average tensile strength of the joint in Example 1 is 125.42MPa, the average tensile strength of the joint in Example 2 is 104.53MPa, and the average tensile strength of the joint in Comparative Example 1 is 81.61MPa. Compared with Example 2, the tensile strength of Example 1 is increased by 19.98%, and compared with Comparative Example 1, the tensile strength of Example 1 is increased by 53.6%. The fracture form of Example 1 is mixed fracture, and Comparative Example 1 is brittle fracture, indicating that the toughness of the joint in Example 1 is improved.

[0089] In summary, compared with the prior art, the present invention transforms the butt joint form of aluminum / steel plates into a semi-lap joint form. A butt joint with excellent performance can be obtained without adding welding wire during welding, and the sloped groove can be prepared by simple machining equipment, effectively reducing welding materials and machining costs; the contact area of ​​the melt-brazing interface is increased by the sloped semi-lap groove, effectively improving the mechanical properties of the joint; and by adjusting and optimizing the laser offset distance, it plays a role in balancing the temperature field of the molten pool during welding, so that the metallurgical reaction at the interface is uniform and the segregation of the joint structure is reduced. Compared with the vertical cross-section docking, the sloped groove reduces the average melting depth of the aluminum side, and the heat input is more used for the interface reaction, increasing the amount of intermetallic compounds generated by about 335%. At the same time, it is convenient to regulate the laser heat input, inhibit the excessive contact between iron and aluminum atoms caused by excessive local heat input, and control the average thickness of the intermetallic compound layer below 10μm, which also reduces the residual stress and crack tendency of the joint; in addition, the sloped semi-lap groove on the steel side can support the molten pool, improve the keyhole stability and avoid collapse. Since the melting range of the base material is reduced, the distance for bubbles to float up and escape is reduced, which provides favorable conditions for reducing welding defects such as weld porosity. In short, the synergistic effect of the sloped semi-lap groove and laser melting-brazing welding organically unifies the welding structure and metallurgical regulation, greatly improving the microstructure and mechanical properties of aluminum / steel dissimilar metal butt joints.

[0090] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A laser melting-brazing method for aluminum / steel dissimilar metals, characterized in that: The following steps are involved: (1) The butt joint sides of the aluminum plate and the steel plate to be welded are respectively processed into a sloped groove to obtain an aluminum plate with a groove and a steel plate with a groove; the angle between the groove slope of the steel plate with a groove and the bottom edge of the steel plate is less than 90°; the sum of the angle between the groove slope of the aluminum plate with a groove and the bottom edge of the aluminum plate and the angle between the groove slope of the steel plate with a groove and the bottom edge of the steel plate is 180°; (2) coating a mixed slurry on the groove surface of the steel plate containing the groove obtained in step (1) to obtain a steel plate containing a coating layer; the mixed slurry comprises Nocolok powder and acetone; (3) butting the aluminum plate with the groove obtained in step (1) and the steel plate with the coating layer obtained in step (2), and then laser welding the joint to obtain a composite plate; when butting, the aluminum plate at the interface is above the steel plate, and the laser beam is aimed at one side of the aluminum plate during laser welding; during laser welding, the offset distance of the center of the laser spot from the joint is calculated according to formula I: Formula I; In formula I, d is the offset distance, b is the thickness of the steel plate to be welded, and θ is the angle between the groove slope of the steel plate with the groove and the bottom edge of the steel plate.

2. The aluminum / steel dissimilar metal laser melting-brazing method according to claim 1, characterized in that: In the step (1), the angle between the groove slope of the steel plate with the groove and the bottom edge of the steel plate is 20-70°.

3. The aluminum / steel dissimilar metal laser melting-brazing method according to claim 1, characterized in that: The thickness of the coating layer in step (2) is 20-40 μm.

4. The aluminum / steel dissimilar metal laser melting-brazing method according to claim 3, characterized in that: The thickness of the coating layer in step (2) is 30 μm.

5. The aluminum / steel dissimilar metal laser melting-brazing method according to claim 1, characterized in that: The mass ratio of Nocolok powder to acetone in the mixed slurry of step (2) is 1:(2-3).

6. The aluminum / steel dissimilar metal laser melting-brazing method according to claim 1, characterized in that: In the step (2), the steel plate with grooves is also pre-treated before surface coating.

7. The aluminum / steel dissimilar metal laser melting-brazing method according to claim 1, characterized in that: In the step (3), the laser power of the laser welding is 1-4 kW, the welding speed is 0.4-1.2 m / min, the defocusing amount is +1-30 mm, and the beam inclination angle is 5-20°.

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