Steel-aluminum dissimilar metal laser welding method utilizing laser surface nitridation

Abstract

The invention relates to the technical field of metal material welding, in particular to a steel-aluminum dissimilar metal laser welding method utilizing laser surface nitridation. The nitrogen plasma and the first laser are used for treating the to-be-welded area of the stainless steel plate at the same time, the nitride layer is formed in the to-be-welded area, and the nitrided stainless steel plate is obtained; and the nitrided stainless steel plate and the aluminum alloy plate are subjected to second laser welding. By the adoption of the method, the problems that the welding process is complex and the efficiency is low due to the fact that other materials are added to the surface of the Fe-Al welding area to serve as the middle layer can be solved, and the method has the advantages of being high in automation degree and high in efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of metal material welding, and particularly to a laser welding method for dissimilar steel-aluminum metals using laser surface nitridation. Background Art

[0002] In modern manufacturing, both aluminum and steel are widely used materials. Aluminum alloys have advantages such as low density, good electrical conductivity, thermal conductivity, and corrosion resistance, and are commonly used in fields such as aerospace, automotive manufacturing, and electronic equipment; steel, with its high strength, good toughness, and mechanical properties, plays a key role in many industries such as construction, machinery manufacturing, and energy, and is the most common and widely used heavy metal. As product design develops towards multi-functionality and lightweight, the demand for using aluminum and steel simultaneously in the same structure to play their respective advantages is increasing. For example, in the automotive body structure, aluminum is used to reduce weight while ensuring that certain key parts are strengthened with steel to meet functional and structural requirements. Therefore, the dissimilar aluminum-steel metal welding technology is crucial.

[0003] Compared with other welding processes, laser welding has obvious advantages in the welding of dissimilar aluminum-steel metals due to its high energy density, fast welding speed, precise control of heat input, and high degree of automation, and can adapt to different weld joint forms, having great application prospects and value. However, due to the different physical and chemical properties between aluminum and steel, their mutual solubility and metallurgical compatibility are poor, and brittle Fe-Al intermetallic compounds (IMCs) are likely to be generated at the interface of the steel / aluminum welded joint. On the one hand, because the relatively thick IMC layer has a low stress intensity factor and a high crack propagation rate, it will significantly reduce the strength of the welded joint; on the other hand, the significant differences in their physical, chemical, and metallurgical characteristics result in poor wettability during welding, and the molten metal is difficult to effectively spread and combine on the surface of dissimilar materials, which will also lead to a reduction in the strength of the welded joint. Therefore, controlling the formation, growth of Fe-Al IMCs at the interface of the welded joint and improving the welding wettability are the current challenges in the welding of aluminum and steel.

[0004] To address the control of the formation and growth of IMCS at the interface of Fe-Al welded joints, a related technology discloses a laser welding method for dissimilar metals of aluminum and steel with a vanadium-nickel composite interlayer. The stainless steel material and the aluminum alloy material to be welded are overlapped, and the vanadium-nickel composite interlayer is embedded in the overlapping area, and then the overlapping area is laser welded. The vanadium-nickel composite interlayer serves as a buffer zone. During the laser welding process, the interlayer completely melts, hindering the mixing and mutual diffusion of iron and aluminum elements, effectively reducing the formation of brittle Fe-Al intermetallic compounds at the joint interface; at the same time, adding vanadium and nickel elements to the aluminum-steel weld will also generate mixed ductile phases with strong ductility such as Al5FeNi, AlV, AlNi3, AlV3, FeNi3, FeV, etc., further improving the mechanical properties of the joint. In addition, another related technology discloses a method for assisted lap welding of dissimilar metals of steel and aluminum with a wire mesh interlayer. This method mainly effectively improves the spreading ability of molten aluminum on the steel surface during the welding process, that is, improves the welding wettability, greatly increases the spreading width, and effectively refines the weld microstructure, enhancing the mechanical properties of the Fe-Al dissimilar metal lap joint.

[0005] The above methods all require adding other materials as interlayers on the contact surface of the Fe-Al welding area, and it is necessary to precisely control the composition, thickness, and morphology of the interlayer material, etc. The process of the process method is relatively complex and cumbersome, and the welding efficiency is relatively low. Summary of the Invention

[0006] In view of this, the purpose of the present invention is to provide a laser welding method for dissimilar metals of aluminum and steel using laser surface nitriding; using the method of the present invention can avoid the problems of complex welding process and low efficiency caused by adding other materials as interlayers on the surface of the Fe-Al welding area, and has the advantages of high automation and high efficiency.

[0007] In order to achieve the above invention purpose, the present invention provides the following technical solutions:

[0008] The present invention provides a laser welding method for dissimilar metals of aluminum and steel using laser surface nitriding, including the following steps:

[0009] Using nitrogen plasma and the first laser to simultaneously process the area to be welded of the stainless steel plate, forming a nitride layer in the area to be welded to obtain a nitrided stainless steel plate;

[0010] Performing second laser welding on the nitrided stainless steel plate and the aluminum alloy plate.

[0011] Preferably, the first laser is an infrared nanosecond pulsed laser with a wavelength of 1064 nm.

[0012] Preferably, the average power of the first laser is 100 - 200 W.

[0013] Preferably, the pulse width of the first laser is 1 - 100 ns, and the repetition frequency is 1 - 10 MHz.

[0014] Preferably, the scanning speed of the first laser is 60 - 100 mm / s.

[0015] Preferably, the thickness of the nitride layer is 100 - 400 nm.

[0016] Preferably, the distance between the nozzle of the nitrogen plasma and the stainless steel plate is 10 - 20 mm.

[0017] Preferably, the nitrogen plasma is generated by a plasma device; the power supply voltage of the plasma device is 220V ± 20%, the rated power is 1000W, and nitrogen at 0.2 - 0.3 MPa is supplied.

[0018] Preferably, the thicknesses of the stainless steel plate and the aluminum alloy plate are independently 0.5 - 5 mm.

[0019] Preferably, the second laser welding is performed in a lap joint or a corner joint manner.

[0020] The present invention provides a laser welding method for dissimilar steel-aluminum metals using laser surface nitriding, comprising the following steps: simultaneously treating the area to be welded of the stainless steel plate with nitrogen plasma and the first laser to form a nitride layer in the area to be welded, obtaining a nitrided stainless steel plate; performing second laser welding on the nitrided stainless steel plate and the aluminum alloy plate. The nitrogen plasma on the surface of the stainless steel plate to be welded is irradiated by the first laser to generate active nitrogen atoms, ions, and excited-state nitrogen substances. The chemical activity of these active particles is much higher than that of ordinary nitrogen, and can break the chemical inertness of the iron surface; in addition, due to the input of laser energy, the ionization of the nitrogen plasma covering the surface of the steel plate to be welded is further maintained and enhanced, so that it is easier to form a dense nitride layer (iron nitride) on the surface of the stainless steel plate to be welded. The presence of the nitride layer can effectively inhibit the diffusion of iron atoms into the molten aluminum liquid during the second laser welding process, reduce the generation of hard and brittle intermetallic compounds IMCS at the Fe-Al welded joint, and at the same time improve the spreading and wettability of the molten Al liquid on the steel plate during the welding process, thereby effectively improving the mechanical properties of the welded joint. Moreover, the whole process is simple and controllable, with high efficiency, and can achieve high automation. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a welding schematic diagram of Example 1, where 1-backing plate, 2-aluminum alloy plate, 3-laser head, 4-laser beam, 5-weld seam, 6-nozzle, 7-laser scanning trace, 8-nitriding layer, 9-stainless steel plate;

[0022] Figure 2Schematic flow chart of the laser welding method for dissimilar steel-aluminum metals in a specific embodiment. Detailed implementation manner

[0023] The present invention provides a laser welding method for dissimilar steel-aluminum metals using laser surface nitriding, comprising the following steps:

[0024] Simultaneously process the area to be welded of the stainless steel plate using a nitrogen plasma and a first laser to form a nitride layer in the area to be welded, obtaining a nitrided stainless steel plate;

[0025] Perform second laser welding on the nitrided stainless steel plate and the aluminum alloy plate.

[0026] In the present invention, the thicknesses of the stainless steel plate and the aluminum alloy plate are independently preferably 0.5 - 5 mm, and in specific embodiments, they can be 0.5 mm, 1 mm, 1.5 mm, 2 mm, 2.5 mm, 3 mm, 3.5 mm, 4 mm, 4.5 mm, or 5 mm. In the present invention, the aluminum alloy plate can be a 5000 series (Al-Mg alloy) or 6000 series (Al-Mg-Si alloy) aluminum alloy plate, and in a specific embodiment, it is a 6061 aluminum alloy plate; the stainless steel plate can be a 304 stainless steel plate or a 316L stainless steel plate, and in a specific embodiment, it is a SUS304 stainless steel plate.

[0027] The present invention preferably pre-treats the areas to be welded of the stainless steel plate and the aluminum alloy plate first. In the present invention, the pre-treatment preferably includes grinding, polishing, cleaning, and drying in sequence. In the present invention, the grinding is preferably performed using a grinding wheel; the polishing is preferably mechanical polishing using silicon carbide sandpaper and diamond grinding paste with a particle size of 1 μm; the present invention removes the surface oxide film by grinding and polishing. In the present invention, the cleaning is preferably wiping and cleaning with acetone or alcohol; the present invention removes the oil stains on the surface to be welded by cleaning. The drying is preferably drying with a hair dryer.

[0028] After completing the surface treatment, the present invention simultaneously processes the area to be welded of the stainless steel plate using a nitrogen plasma and a first laser to form a nitride layer in the area to be welded, obtaining a nitrided stainless steel plate.

[0029] The present invention has no special requirements for the generation conditions of the nitrogen plasma, and it is only necessary to be able to generate the nitrogen plasma. In the present invention, the nitrogen plasma is preferably generated by a plasma device; in the embodiments of the present invention, the plasma device is specifically the horizontal single-gun plasma surface treatment machine CSM-SC1H of Dongguan Yaotian Electric Technology Co., Ltd. for experimental use; in a specific embodiment, the power supply voltage of the plasma device is 220V±20%, the rated power is 1000W, and nitrogen with a pressure of 0.2 to 0.3 MPa is supplied. The purity of the nitrogen is preferably ≥99.99%. The present invention adopts a relatively high air pressure, which increases the density of nitrogen gas molecules and reduces the breakdown threshold, facilitating the formation of nitrogen plasma.

[0030] In the present invention, the distance between the nozzle of the nitrogen plasma and the stainless steel plate is preferably 10 to 20 mm, and in a specific embodiment, it can be 10 mm, 12 mm, 15 mm, 18 mm or 20 mm. The present invention controls the distance between the nozzle and the stainless steel plate within the above range, and in combination with the power of the first laser, it can prevent the density of the nitrogen plasma from being too high due to the nozzle being too close to the steel plate, which may cause surface ablation or local overheating and damage the uniformity of the nitrided layer; and it can also prevent the distance from being too far, resulting in a decrease in the energy density due to the diffusion of the nitrogen plasma and a reduction in the nitrogen penetration efficiency.

[0031] In the present invention, the nozzle of the nitrogen plasma preferably swings back and forth in a direction perpendicular to the weld, and the swing length is preferably 3 to 5 mm. In the present invention, since the width of the nozzle of the nitrogen plasma is relatively narrow (the nozzle width used in the embodiment is only 10 mm), the present invention can increase the width of the nitrided area by swinging back and forth, thereby increasing the nitrided area of the area to be welded and effectively avoiding the weld being in a non-nitrided area during welding.

[0032] In the present invention, the first laser is preferably an infrared nanosecond pulsed laser, and the wavelength is preferably 1064 nm; the average power of the first laser is preferably 100 to 200 W, and in a specific embodiment, it can be 100 W, 120 W, 140 W, 160 W, 180 W or 200 W; the pulse width of the first laser is preferably 1 to 100 ns, and in a specific embodiment, it can be 1 ns, 20 ns, 40 ns, 60 ns, 80 ns or 100 ns; the repetition frequency of the first laser is preferably 1 to 10 MHz, and in a specific embodiment, it can be 1 MHz, 3 MHz, 5 MHz, 8 MHz or 10 MHz. Compared with using other types of lasers, the duration of the nanosecond laser pulse is very short (10 -9(in seconds), the energy is deposited in a short time, and the heat diffusion is significantly inhibited, mainly confined to the action area. At an appropriate pulse interval, the residual heat deposited by the previous pulse does not have time to diffuse outside the area to be welded on the stainless steel plate before the arrival of the next pulse, thereby improving the utilization rate of laser energy and forming a smaller heat-affected zone on the surface of the stainless steel plate. This characteristic can reduce the oxides generated by the oxidation reaction on the metal surface and significantly inhibit the formation of brittle intermetallic compounds IMC between Fe-Al dissimilar materials, ultimately improving the strength of the welded joint after welding.

[0033] In the present invention, the scanning speed of the first laser is preferably 60 - 100 mm / s, and in specific embodiments, it can be 60 mm / s, 70 mm / s, 80 mm / s, 90 mm / s, or 100 mm / s; the scanning interval of the first laser is preferably 1 - 2 mm (referring to the interval between adjacent laser scanning traces), and in specific embodiments, it can be 1 mm, 1.5 mm, or 2 mm. By controlling the scanning speed and scanning interval of the first laser within the above ranges, the present invention can not only ensure the scanning efficiency but also prevent insufficient nitridation on the surface of the stainless steel plate.

[0034] In the present invention, the first laser irradiation is performed during the nitrogen plasma treatment. Since there are active particles (electrons, ions, high-energy neutral atoms, etc.) in the nitrogen plasma, and the chemical activity of these active particles is much higher than that of ordinary nitrogen, it can break the chemical inertness of the iron surface; in addition, due to the input of laser energy, the ionization of the nitrogen plasma covering the surface to be welded on the steel plate is further maintained and enhanced, so that it is easier to form a dense nitride layer on the surface to be welded of the stainless steel plate. The main iron nitride phases contained in the nitride layer are γ-Fe(N) (nitrogen austenite), ε-Fe2N 1-x (with a close-packed hexagonal structure), and a small amount of α'-Fe(N) (martensite nitride). Different phases during the nitriding process are composed of different crystal structures as the matrix.

[0035] In the present invention, the thickness of the nitride layer can be 100 - 400 nm. The present invention forms a dense nitride layer in the area to be welded. The presence of the nitride layer can effectively inhibit the diffusion of iron atoms into the molten aluminum liquid during the second laser welding process, reduce the generation of hard and brittle intermetallic compounds at the Fe-Al welded joint, and at the same time improve the spreading and wettability of the molten Al liquid on the steel plate during the welding process, thereby effectively improving the mechanical properties of the welded joint.

[0036] In the present invention, the nozzle of the nitrogen plasma and the first laser beam can move synchronously or asynchronously. When the weld is short, synchronous movement is not required, and the nitrogen plasma atmosphere can cover the weld area; when the weld is long, synchronous movement is required to ensure sufficient nitridation of the weld area. In the embodiments of the present invention, synchronous movement is not performed.

[0037] After obtaining the nitrided stainless steel plate, the present invention performs second laser welding on the nitrided stainless steel plate and the aluminum alloy plate.

[0038] In the present invention, the manner of the second laser welding is preferably lap joint or corner joint; the second laser welding is preferably laser brazing welding. In the present invention, the laser brazing welding refers to making use of the difference in melting points of two base materials, locally melting the low melting point base material, i.e., the aluminum alloy plate, by laser heating, while the high melting point base material, the stainless steel plate, basically remains solid state. The melted low melting point base material infiltrates the interface of the high melting point base material and undergoes a metallurgical reaction to achieve connection. That is to say, it is fusion welding on the side of the aluminum alloy plate and brazing on the side of the stainless steel plate. Using the liquid aluminum alloy to wet the nitrided stainless steel base material on the surface, under the capillary action of the gap, the liquid aluminum alloy automatically fills into the gap of the base material and diffuses in the base material to form a connection without using a filler metal.

[0039] The present invention does not make special limitations on the specific conditions of the second laser welding. According to the thickness of the aluminum alloy plate, the corresponding laser power is selected for laser brazing welding. In the present invention, the second laser welding is preferably carried out under the protection of an inert gas, and the inert gas preferably includes argon. The present invention performs the second laser welding under the protection of an inert gas, which can avoid the oxidation reaction of the metal at the weld joint during the welding process, resulting in a reduction in the mechanical properties of the welded joint.

[0040] In the embodiment of the present invention, when using a 2 mm thick SUS304 stainless steel plate and a 1.5 mm thick 6061 aluminum alloy plate, the second laser welding uses a nanosecond fiber laser, and the second laser welding parameters are selected as a laser wavelength of 1064 nm, a pulse width of 100 ns, a repetition frequency of 20000 Hz, a welding speed of 60 mm / s, and a maximum average power of 3000 W.

[0041] Figure 2 It is a schematic flow chart of the laser welding method for steel-aluminum dissimilar metals in a specific embodiment, as Figure 2 shown. After the surface pretreatment of the stainless steel plate and the aluminum alloy plate, the nitrogen plasma and the first laser are used to process the area to be welded of the stainless steel plate (i.e., nitriding treatment) simultaneously, forming a nitride layer in the area to be welded to obtain a nitrided stainless steel plate; then the nitrided stainless steel plate and the aluminum alloy plate are subjected to laser brazing welding (i.e., the second laser welding) in a corner joint or lap joint manner.

[0042] The following combines the embodiments to detail the laser welding method for steel-aluminum dissimilar metals using laser surface nitriding provided by the present invention, but they cannot be understood as limiting the protection scope of the present invention.

[0043] The plasma equipment used in the following examples is the horizontal single-gun plasma surface treatment machine CSM-SC1H of Dongguan Yaotian Electric Technology Co., Ltd. for experimental use; the nanosecond fiber laser used is the IPGYLPN-S series laser, and the maximum average power can reach 3KW.

[0044] Example 1

[0045] Figure 1 It is a welding schematic diagram of Example 1. As Figure 1 shown, an SUS304 stainless steel plate 9 with dimensions of 80mm×40mm×2mm and a 6061 aluminum alloy plate 2 with dimensions of 80mm×40mm×1.5mm are used as welding test samples.

[0046] Before welding, use a grinding wheel to polish the areas to be welded on the aluminum alloy plate and the stainless steel plate, and use silicon carbide sandpaper and 1μm diamond grinding paste for mechanical polishing to remove the surface oxide film. Wipe the surface with alcohol to remove surface oil stains, and then use a dry hair dryer to dry the material surface.

[0047] First, use a fixture to clamp and fix the SUS304 stainless steel plate to be welded. Use the plasma equipment to ionize nitrogen to generate nitrogen plasma. In the experiment, the plasma surface treatment machine is selected as the plasma equipment. Its main parameters are a power supply voltage of AC 220V (+ / -20%), a rated power of 1000W, and pure nitrogen (purity 99.99%) at 0.2 - 0.3MPa (about 2 - 3bar) is supplied. Nitrogen plasma is sprayed on the area to be welded on the stainless steel plate through the nozzle 6. The distance between the nozzle and the steel plate surface is 10mm. At the same time, use the laser head 3 to emit a laser beam 4 (power set to 100W) to scan the stainless steel plate, generating laser scanning traces 7 on the stainless steel plate surface. By controlling the input of low-power laser energy, the nitriding effect on the stainless steel surface is optimized. As Figure 1 shown, during nitriding treatment, the laser beam scanning speed is selected as 100mm / s, and the scanning interval is selected as 1mm. At the same time, the nozzle swings back and forth along the direction perpendicular to the weld. In the plasma surface treatment equipment selected in this example, the width of the nozzle treatment is only 10mm. Swinging back and forth along the direction perpendicular to the weld can increase the width of the nitriding area, thereby increasing the nitriding area of the area to be welded, effectively avoiding the weld being in a non-nitrided area during welding. The length of the swing is between 3 - 5mm, forming a nitriding layer 8 as Figure 1 shown. The surface of the SUS304 stainless steel weld area is completed with surface nitriding treatment, generating a nitriding layer with a thickness of 100 - 400nm.

[0048] After the surface nitriding treatment of the stainless steel test plate is completed, the 6061 aluminum alloy plate is lapped on the surface of the nitrided layer of the stainless steel test plate. The aluminum alloy test plate is supported by the backing plate 1, and the aluminum alloy and stainless steel plates are clamped and fixed with a fixture.

[0049] In this embodiment, when welding, laser brazing is used. The selected nanosecond fiber laser has a wavelength of 1064 nm, a pulse width of 100 ns, a repetition frequency of 20000 Hz, and the maximum average power is set to 3000 W. Argon is used as the shielding gas during the welding process. The laser head starts emitting light for welding from the starting position of the weld zone, and a welding speed of 60 mm / s is selected, and the light emission ends after moving along the weld 5 to the termination position of the weld to complete the welding.

[0050] The results show that the welded joints obtained in Example 1 treated by the nitriding process exhibit good mechanical properties. The welded joints show good plastic deformation ability in tensile and shear tests, obvious macroscopic plastic deformation occurs before fracture, the fracture surface shows an obvious dimple morphology, and has the characteristics of ductile fracture. It shows that during the welding process, the nitrided layer effectively inhibits the formation of brittle intermetallic compounds IMCs at the Fe-Al interface of the welded joint, making the crack propagation require more energy consumption, and thus forming dimples through the plastic deformation of the matrix. In the non-nitrided specimens, due to the weak bonding caused by the difference in thermal expansion coefficient and poor wettability at the interface between IMCs and the matrix, high stress concentration is generated under tensile and shear loads, resulting in the preferential initiation and rapid propagation of microcracks along the IMC / matrix interface, thus resulting in brittle fracture.

[0051] The above is only the preferred embodiment of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and retouches can be made, and these improvements and retouches should also be regarded as the protection scope of the present invention.

Claims

1. A laser welding method for dissimilar steel-aluminum metals using laser surface nitridation, comprising the following steps: Simultaneously treating the area to be welded of the stainless steel plate with nitrogen plasma and a first laser to form a nitride layer in the area to be welded, obtaining a nitrided stainless steel plate; Performing second laser welding on the nitrided stainless steel plate and the aluminum alloy plate.

2. The laser welding method for dissimilar steel-aluminum metals according to claim 1, wherein The first laser is an infrared nanosecond pulsed laser with a wavelength of 1064 nm.

3. The laser welding method for dissimilar steel and aluminum metals according to claim 2, wherein The average power of the first laser is 100 - 200 W.

4. The laser welding method for dissimilar steel and aluminum metals according to claim 2, characterized in that The pulse width of the first laser is 1 - 100 ns, and the repetition frequency is 1 - 10 MHz.

5. The method for laser welding of dissimilar steel and aluminum metals according to any one of claims 1 to 4, characterized in that, The scanning speed of the first laser is 60 - 100 mm / s.

6. The laser welding method for dissimilar steel-aluminum metals according to any one of claims 1 to 4, characterized in that, The thickness of the nitride layer is 100 - 400 nm.

7. The laser welding method for dissimilar steel and aluminum metals according to claim 1, characterized in that, The distance between the nozzle of the nitrogen plasma and the stainless steel plate is 10 - 20 mm.

8. The laser welding method for dissimilar steel-aluminum metals according to claim 1, characterized in that The nitrogen plasma is generated by a plasma device; the power supply voltage of the plasma device is 220V ± 20%, the rated power is 1000W, and nitrogen of 0.2 - 0.3 MPa is supplied.

9. The method for laser welding of dissimilar steel and aluminum metals according to claim 1, characterized in that, The thicknesses of the stainless steel plate and the aluminum alloy plate are independently 0.5 - 5 mm.

10. The laser welding method for dissimilar steel and aluminum metals according to claim 1, wherein The second laser welding is performed in an overlapping or corner joint manner.

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