Welding method for multilayer plate structure of high-strength stainless steel automobile fuel tank

Abstract

The present application relates to the field of welding technology, especially to a welding method for high-strength stainless steel automobile fuel tank multi-layer plate structure. The present application first adopts resistance seam welding by adding triangular transition wedge for the multi-layer plate structure of the fuel tank shell and end cover, and then carries out circular polarization mode laser remelting welding along the newly formed weld, which can effectively reduce the welding defects of high-strength stainless steel automobile fuel tank multi-layer plate structure. The present application can effectively reduce the resistance welding defects caused by the sudden reduction of material due to the special overhanging structure of the transition area of high-strength stainless steel automobile fuel tank multi-layer plate structure by adding right-angled triangular wedge, and the second resistance welding circular polarization mode laser remelting can release the welding residual stress caused by high-strength steel resistance seam welding, so that the fine grain structure is formed in the weld area, the performance of the fusion zone is improved, the oil tightness of the automobile fuel tank during long-term use is ensured, and the high-frequency vibration requirement of high-strength stainless steel automobile fuel tank multi-layer plate structure during automobile driving is met.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of welding technology, and particularly relates to a welding method for a multi-layer plate structure of a high-strength stainless steel automobile fuel tank. BACKGROUND

[0002] With the improvement of the light weight requirement of energy saving and emission reduction vehicle body, the material selection of the fuel tank of the extended range new energy vehicle is mainly based on medium and high strength steel sheet material, which can reduce the weight of the vehicle body by 5-10%. High-strength steel can achieve the effect of light weight of the vehicle while ensuring the performance of the structural part. The high-strength stainless steel plate is selected as the material of the fuel tank, which can meet the requirements of the use environment of the fuel tank and achieve the purpose of light weight of the vehicle.

[0003] The fuel tank is an important safety part of the vehicle. The metal fuel tank is a storage device for flammable and explosive fuel of the vehicle. It has high requirements for corrosion resistance, sealing and strength. The fuel tank is composed of a shell and upper and lower end covers. The fuel tank shell adopts lap welding instead of butt welding, mainly based on the following reasons: butt welding is prone to welding redundancy, and the fuel in the fuel tank will have a significant impact on its performance. When the fuel in the fuel tank shakes, the maximum stress value borne by the fuel tank changes constantly, and the maximum stress value is concentrated at the butt welding joint of the fuel tank. The surge of fuel continuously impacts the joint part, which makes it difficult to ensure the oil tightness of the fuel tank and cannot meet the requirements of high-frequency vibration of the fuel tank during vehicle driving. The fuel tank shell adopts lap welding, and the multi-layer plate structure will appear when the fuel tank shell and the end cover are welded. Specifically, the welded part of the shell is two overlapping thin plates, and the rest is a single thin plate. The shell and the end cover are welded, and the end cover is also a single thin plate punched out, so the combination of the single plate of the end cover and the shell is a double plate, and the combination of the welded overlapping part of the end cover and the shell is a three-layer plate. Therefore, when the end cover and the shell of the fuel tank are welded, there will be a plate thickness gap (usually 0.8mm) when the double plate transitions to the three-layer plate, and the multi-layer plate structure will increase the welding difficulty of the fuel tank shell and the end cover.

[0004] Laser welding has certain requirements for the gap between the plates. In actual operation, the welding gap is usually controlled within 15% of the plate thickness. When the gap between the plates is too large, lap welding connection is more difficult, which is easy to cause weld light leakage and is not easy to get effective weld, affecting the sealing of the weld, so the traditional welding method for the automobile fuel tank is resistance seam welding. However, the high-strength stainless steel base material has high hardness, and resistance seam welding needs to overcome the reverse action of the gap between the plates and warping during the welding process. Using the conventional material process window will lead to low effective welding pressure, causing local stress concentration, cracks, spatter, shrinkage, loose structure and other defects, which is difficult to meet the requirements of high-frequency vibration of the fuel tank during vehicle driving, and has the risk of cracking. SUMMARY

[0005] In view of this, the purpose of this invention is to provide a welding method for a multi-layer plate structure of a high-strength stainless steel automotive fuel tank. The welding method of this invention can effectively reduce welding defects in the multi-layer plate structure of the high-strength stainless steel automotive fuel tank, meet the requirements of high-frequency vibration during vehicle operation, and prevent fuel tank cracking.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] This invention provides a welding method for a multi-layer plate structure of a high-strength stainless steel automotive fuel tank, comprising the following steps:

[0008] A right-angled triangular wedge is added to the transition position from the double-layer plate formed by the overlap of the fuel tank shell and the end cover to the triple-layer plate to form a transition slope. Resistance seam welding is performed on the fuel tank shell and the end cover, and then a second laser welding is performed along the newly formed weld. The second laser welding is circular polarization mode laser remelting.

[0009] The outer shell of the fuel tank has an overlapping structure;

[0010] The thickness of the right-angled triangular wedge is the same as the thickness of the single-layer plate of the fuel tank outer shell;

[0011] The outer shell of the fuel tank, the end cap, and the right-angled triangular wedge are all made of the same type of high-strength stainless steel.

[0012] Preferably, the slope angle of the transition slope is 30~45°; the length of the right-angled triangular wedge is 340~510mm.

[0013] Preferably, the outer shell of the fuel tank is formed by sequentially positioning the shaped shell through a first spot weld and a first laser weld.

[0014] Preferably, the conditions for resistance seam welding include an on / off ratio of 2:5;

[0015] Straight section of double-layer plate: welding pressure is 5kN, welding current is 6kA, and welding speed is 850mm / min;

[0016] Double-layer plate rounded corner section: welding pressure is 5kN, welding current is 6kA, and welding speed is 800mm / min;

[0017] Three-layer plate: welding pressure is 6.5kN, welding current is 7.4kA, and welding speed is 500mm / min.

[0018] Preferably, the conditions for the second laser welding include: laser power of 3400W, welding speed of 2000mm / min, and defocusing amount of -15mm.

[0019] Preferably, the first laser welding is a continuous fiber laser welding; the conditions for the first laser welding include: laser power of 3400W, welding speed of 2000mm / min, and defocusing amount of -15mm.

[0020] Preferably, the conditions for the first spot weld include: welding current of 4.8~5.2kA, welding pressure of 3.2~3.8kN, and welding time of 4 cycles.

[0021] Preferably, before adding the right-angled triangular wedge, the tank shell and end cap are positioned by a second spot weld.

[0022] Preferably, the conditions for the second spot welding include: welding current of 4.8~5.2kA, welding pressure of 3.2~3.8kN, and welding time of 4 cycles.

[0023] Preferably, the high-strength stainless steel includes BFS400 high-strength stainless steel, BFS500 high-strength stainless steel, QN1803 high-strength stainless steel, or QD2001 high-strength stainless steel.

[0024] Preferably, the composition of the BFS400 high-strength stainless steel is: Si 0.46wt.%, Cr 16.2wt.%, Mn 6.1wt.%, Cu 1.7wt.%, Ni 4wt.%, N 0.097wt.%, C 0.058wt.%, Fe balance.

[0025] This invention provides a welding method for a multi-layer plate structure of a high-strength stainless steel automotive fuel tank, comprising the following steps: adding a right-angled triangular wedge to form a transition slope at the transition position from the double-layer plate formed by the overlap of the fuel tank shell and the end cap to the triple-layer plate; performing resistance seam welding on the fuel tank shell and the end cap; and then performing a second laser welding along the newly formed weld; the second laser welding is circular polarization mode laser remelting; the fuel tank shell is an overlap structure; the thickness of the right-angled triangular wedge is the same as the thickness of the single-layer plate of the fuel tank shell; and the fuel tank shell, the end cap, and the right-angled triangular wedge are all made of the same type of high-strength stainless steel. This invention employs a triangular transition wedge block for resistance seam welding of the multi-layered plate structure of the fuel tank shell and end cap. This allows the welding rollers to smoothly pass over the wedge block, which fills the gap in the transition zone from three layers to two layers. This effectively reduces weld defects caused by the sudden reduction of material due to the special suspended structure in the transition area of ​​the high-strength stainless steel automotive fuel tank multi-layered plate structure. Then, a circularly polarized laser remelting is performed along the newly formed weld. This not only effectively reduces welding defects in the high-strength stainless steel automotive fuel tank multi-layered plate structure but also releases some of the residual stress from the resistance seam welding, resulting in a fine-grained structure with uniform grain distribution in the weld area. This increases the hardness of the fusion zone, achieving a dense weld structure free of cracks, shrinkage cavities, and other defects, ensuring the fuel tank's oil tightness. Furthermore, the circularly polarized laser can remelt and refine the weld surface after resistance welding, eliminating surface defects. This not only makes the weld surface smoother and flatter, improving the appearance quality of the welded part but also makes the weld tighter, improving its sealing performance and ensuring the sealing performance of the welded structure. Attached Figure Description

[0026] Figure 1 This is a flowchart of the welding method for a high-strength stainless steel fuel tank according to Embodiment 1 of the present invention;

[0027] Figure 2 This is a schematic diagram of a high-strength stainless steel fuel tank.

[0028] Figure 3 This is a cross-sectional view of the multi-layer plate structure of a high-strength stainless steel fuel tank.

[0029] Figure 4 This is a schematic diagram of resistance seam welding of a multi-layer plate high-strength stainless steel fuel tank according to the present invention.

[0030] Figure 5 This is another schematic diagram of resistance seam welding of multi-layer plates for high-strength stainless steel fuel tanks according to the present invention.

[0031] Figure 6The images show a comparison of the macroscopic morphology of welds using conventional welding methods and the welding method of the present invention. In the images, (a) shows the macroscopic morphology of the weld seam after resistance welding in Example 1, (b) shows the macroscopic morphology of the weld seam after welding with circularly polarized laser, and (c) shows the macroscopic morphology of the weld seam after resistance welding using conventional methods in Comparative Example 1.

[0032] Figure 7 The figures show a comparison of the microstructure of welded joints using conventional welding methods and the welding method of the present invention. (a) and (b) show the microstructure of the welded joint using resistance seam welding in Comparative Example 1 using conventional methods, and (c) shows the microstructure of the welded joint in Example 1.

[0033] Figure 8 This is a schematic diagram of the right-angled triangular wedge block in Example 1;

[0034] Figure 9 This is a physical image of the right-angled triangular wedge block in Example 1;

[0035] Figure 10 The image shows the tensile test results of the specimen prepared in Example 1.

[0036] Figure 11 The tensile test results of the specimen prepared in Comparative Example 2 are shown in the figure.

[0037] Figure 12 The figure shows the tensile test results of the specimen prepared in Comparative Example 3. Detailed Implementation

[0038] This invention provides a welding method for a multi-layer plate structure of a high-strength stainless steel automotive fuel tank, comprising the following steps:

[0039] A right-angled triangular wedge is added to the transition position from the double-layer plate formed by the overlap of the fuel tank shell and the end cover to the triple-layer plate to form a transition slope. Resistance seam welding is performed on the fuel tank shell and the end cover, and then a second laser welding is performed along the newly formed weld. The second laser welding is circular polarization mode laser remelting.

[0040] The outer shell of the fuel tank has an overlapping structure;

[0041] The thickness of the right-angled triangular wedge is the same as the thickness of the single-layer plate of the fuel tank outer shell;

[0042] The outer shell of the fuel tank, the end cap, and the right-angled triangular wedge are all made of the same type of high-strength stainless steel.

[0043] In this invention, the fuel tank is preferably a car fuel tank, specifically such as the fuel tank of a range-extended electric vehicle.

[0044] In this invention, the high-strength stainless steel preferably includes BFS400, BFS500, QN1803, or QD2001 high-strength stainless steel. This invention does not impose any special requirements on the specific composition of each high-strength stainless steel; compositions well-known in the art are acceptable. In a specific embodiment, the composition of the BFS400 high-strength stainless steel is: Si 0.46 wt.%, Cr 16.2 wt.%, Mn 6.1 wt.%, Cu 1.7 wt.%, Ni 4 wt.%, N 0.097 wt.%, C 0.058 wt.%, Fe balance.

[0045] The present invention does not have any special requirements on the thickness of the outer shell and end cap of the fuel tank; any thickness known in the art is acceptable. In the specific embodiment, they are all stamped from a thin plate with a thickness of 0.8 mm.

[0046] In this invention, the outer shell of the fuel tank is preferably formed by sequentially positioning the shaped shell through a first spot weld and a first laser weld.

[0047] This invention does not impose special requirements on the forming process; a forming process well-known in the art can be used, such as stamping. In this invention, the first spot weld is preferably resistance spot welding; the conditions for the first spot weld include: a welding current preferably of 4.8~5.2kA, a welding pressure preferably of 3.2~3.8kN, and a welding time preferably of 4 cycles. In specific embodiments, the welding current for the first spot weld can be 4.8kA, 5.0kA, or 5.2kA, and the welding pressure can be 3.2kN, 3.4kN, 3.6kN, or 3.8kN. In this invention, the electrode material used for the first spot weld is preferably made of chromium-zirconium copper, and the electrode end face is preferably a frustum-shaped end face with a diameter preferably of 5mm.

[0048] After positioning is completed, the present invention preferably performs a first laser welding along the positioning point to complete the welding of the fuel tank shell.

[0049] In this invention, the first laser welding is preferably continuous fiber laser welding; the conditions for the first laser welding include: a laser power preferably of 3400W, a welding speed preferably of 2000mm / min, and a defocusing amount preferably of -15mm. In this invention, lap welding is used during the first spot welding positioning and the first laser welding to ensure oil tightness. This invention does not have special requirements for the lap amount; lap amounts well known in the art can be used.

[0050] In related technologies, resistance seam welding is often used when welding the outer shell of a fuel tank. However, since the material used in this invention is high-strength steel, warping between plates and residual welding stress can occur under high welding pressure. This invention uses laser welding, which avoids localized stress concentration and ensures the strength and sealing of the weld. It is worth noting that when welding the outer shell of the fuel tank, the overlapping portion is a double-layer plate, and the overlap is positioned by spot welding, which also reduces the gap between the plates. Therefore, this invention allows for direct laser welding when welding the outer shell of the fuel tank.

[0051] After obtaining the fuel tank shell, the present invention adds a right-angled triangular wedge to the transition position from the double-layer plate formed by the overlap of the fuel tank shell and the end cover to the triple-layer plate to form a transition slope, and performs resistance seam welding on the fuel tank shell and the end cover.

[0052] In this invention, the thickness (i.e., cross-sectional height) of the right-angled triangular wedge is the same as the thickness of the single-layer plate of the fuel tank outer shell, which is 0.8 mm in a specific embodiment. In this invention, the length of the right-angled triangular wedge is preferably 340-510 mm, and in a specific embodiment it can be 340 mm, 400 mm, 450 mm, 480 mm, or 510 mm. When the thickness of the right-angled triangular wedge is 0.8 mm and the length is 510 mm, its structural schematic diagram is as follows. Figure 8 As shown in the figure. In this invention, the slope angle of the transition slope is preferably 30~45°, and in specific embodiments it can be 30° or 45°. A schematic diagram of resistance welding after adding a right-angled triangular wedge block in this invention is shown in the figure. Figure 4 or Figure 5 As shown. Adding right-angled triangular wedges during resistance seam welding effectively reduces the gap between plates. By controlling the angle of the transition slope, the resistance seam welding roller transitions smoothly, further reducing welding defects in multi-layer plate structures. This invention performs resistance seam welding on the outer shell and end cap of an oil tank.

[0053] Before adding the right-angled triangular wedge, the present invention preferably performs a second spot welding positioning on the fuel tank shell and end cap.

[0054] In this invention, the second spot weld is preferably resistance spot welded; the conditions for the second spot weld include: a welding current preferably of 4.8~5.2kA, a welding pressure preferably of 3.2~3.8kN, and a welding time preferably of 4 cycles; in specific embodiments, the welding current for the second spot weld can be 4.8kA, 5.0kA, or 5.2kA, and the welding pressure can be 3.2kN, 3.4kN, 3.6kN, or 3.8kN. The electrode material used for the second spot weld is the same as that used for the first spot weld, and will not be described again here.

[0055] In this invention, the resistance seam weld is a lap weld. This invention does not have special requirements for the lap length of the lap weld; any lap length well-known in the art can be used.

[0056] In this invention, the overlapping area of ​​the fuel tank shell consists of two overlapping thin plates, while the remaining areas are single-layer thin plates. The shell and end cap are welded together. The end cap is also stamped from a single-layer thin plate. Therefore, the overlapping area between the end cap and the single-layer shell forms a double-layer plate, and the overlapping portion of the welded end cap and shell forms a triple-layer plate. Figure 2 and Figure 3 As shown. Due to the multi-layer plate structure involved in welding the end cap to the outer shell, two right-angled triangular wedges of the same material were added to the multi-layer plate structure to achieve better weld seam closure during resistance welding. This effectively fills the gaps in the weld seam, resulting in better welding performance for subsequent circularly polarized laser welding (e.g., Figure 4 or Figure 5 (As shown).

[0057] In this invention, the conditions for resistance seam welding include: the preferred energizing / resting ratio is 2:5;

[0058] For straight sections of double-layer plates: the preferred welding pressure is 5kN, the preferred welding current is 6kA, and the preferred welding speed is 850mm / min;

[0059] For the rounded corner section of the double-layer plate: the preferred welding pressure is 5kN, the preferred welding current is 6kA, and the preferred welding speed is 800mm / min;

[0060] For three-layer plates: the preferred welding pressure is 6.5kN, the preferred welding current is 7.4kA, and the preferred welding speed is 500mm / min.

[0061] In this invention, the preferred dimensions of the roller electrodes used for resistance seam welding are: a large roller of 280mm and a small roller of 170mm. Performing resistance seam welding first in this invention can effectively reduce the gap between plates, thereby meeting the requirements of subsequent laser welding.

[0062] After completing the resistance seam welding, the present invention performs a second laser welding along the newly formed weld seam.

[0063] In this invention, the second laser welding is circularly polarized mode laser remelting.

[0064] Polarized lasers are classified into linearly polarized lasers, circularly polarized lasers, and elliptically polarized lasers. This invention uses circularly polarized lasers to weld high-strength stainless steel thin plates, which can reduce porosity defects, refine grains, and obtain high-quality welds. During the welding process, the heat input to the thin plate is more uniform, which can effectively reduce welding deformation.

[0065] The conditions for the second laser welding include: a power preferably of 3400W, a welding speed preferably of 2000mm / min, and a defocusing amount preferably of -15mm.

[0066] This invention innovatively employs a combined welding process of resistance seam welding and laser welding. By adding right-angled triangular wedges, the defects in resistance welds caused by the sudden reduction of material due to the special suspended structure in the transition area of ​​the multi-layer plate structure of high-strength stainless steel automotive fuel tanks can be effectively reduced. Resistance seam welding is used to achieve pre-connection of the substrate to ensure structural stability. Then, laser welding, specifically through circular polarization mode laser remelting, completes the precision stitching. The synergistic effect of the two processes effectively suppresses the welding deformation gradient, while realizing the gradient release of residual stress and the optimized control of the weld metallurgical structure. This allows the composite weld to have both high-strength connection performance and long-term sealing guarantee.

[0067] The following detailed description of the welding method for the multi-layer plate structure of high-strength stainless steel automotive fuel tank provided by the present invention, with reference to the embodiments, should not be construed as limiting the scope of protection of the present invention.

[0068] Example 1

[0069] like Figure 1 As shown, the composition of BFS400 high-strength stainless steel is as follows: Si is 0.46 wt.%, Cr is 16.2 wt.%, Mn is 6.1 wt.%, Cu is 1.7 wt.%, Ni is 4 wt.%, N is 0.097 wt.%, C is 0.058 wt.%, and Fe is the balance.

[0070] After the fuel tank outer shell is stamped (made from a 0.8mm thin sheet), it is positioned using resistance spot welding. The electrode material is made of chromium zirconium copper, and the electrode end face is tapered with a diameter of 5mm. The resistance spot welding process parameters are as follows: welding current 4.8kA, welding pressure 3.2kN, and welding time 4 cycles. After positioning, the fuel tank outer shell is welded along the positioning points using laser welding. The laser welding process parameters are as follows: laser power 3400W, welding speed 2000mm / min, and defocusing amount -15mm.

[0071] After the fuel tank outer shell is welded, resistance spot welding is used to position the fuel tank outer shell and the fuel tank end cap (made of 0.8mm thin plate). The resistance spot welding process parameters are as follows: welding current 4.8kA, welding pressure 3.2kN, and welding time 4 cycles. After positioning, two right-angled triangular wedges of the same material BFS400 stainless steel (angles of 30°, 60°, and 90° respectively, length 510mm, placed as shown in the figure) are added to the multilayer plate structure. Figure 4 As shown, the structural diagram of the right-angled triangular wedge block with a slope angle of 30° is as follows. Figure 8 As shown in the picture, the actual product is as follows. Figure 9As shown, resistance seam welding was used to weld the fuel tank shell and fuel tank end cap to eliminate the gap between the plates. The resistance seam welding process parameters are as follows: roller electrode size: large roller 280mm, small roller 170mm; current / rest ratio: 2:5; straight section: welding pressure 5kN, welding current 6kA, welding speed 850mm / min; rounded section: welding pressure 5kN, welding current 6kA, welding speed 800mm / min; three-layer plate: welding pressure 6.5kN, welding current 7.4kA, welding speed 500mm / min. After the gap elimination treatment of the fuel tank end cap and fuel tank shell is completed, a circular polarization mode laser remelting is performed along the newly formed weld to complete the welding of the fuel tank shell and fuel tank end cap. The circular polarization mode laser welding process parameters are as follows: laser power 3400W, welding speed 2000mm / min, defocusing amount -15mm.

[0072] Tensile tests were performed on the specimens prepared in Example 1, and the results are shown in the figure. Figure 10 .Depend on Figure 10 It can be seen that the joint of the sample prepared in Example 1 has a tensile strength of 8.35kN and an elongation of 45% under the optimal process specifications. The weld structure is dense and free of defects such as cracks and shrinkage cavities, ensuring the oil tightness of the fuel tank and meeting the requirements of high-frequency vibration of the fuel tank during vehicle operation.

[0073] The tensile strength was obtained with reference to GB / T 2651, and the elongation was obtained with reference to GB / T 228.1.

[0074] Comparative Example 1

[0075] The difference from Example 1 is that the fuel tank and end cap are welded using conventional methods, and the specific steps are as follows:

[0076] After stamping and welding, the fuel tank shell is positioned using resistance spot welding, and then directly welded using resistance seam welding. The resistance spot welding process parameters are as follows: welding current 4.8kA, welding pressure 3.2kN, welding time 4 cycles. The resistance seam welding process parameters are as follows: roller electrode size: large roller 240mm, small roller 140mm; current / rest ratio: 2:5; welding pressure 5kN, welding current 7.7kA, welding speed 850mm / min.

[0077] When welding the outer shell and end cap, resistance spot welding is used for positioning, and then resistance seam welding is used to weld the outer shell and end cap together. The resistance spot welding process parameters are as follows: welding current is 4.8kA, welding pressure is 3.2kN, and welding time is 4 cycles. The resistance seam welding process parameters are as follows: roller electrode size: large roller 240mm, small roller 140mm; current / rest ratio: 2:5; straight section: welding pressure is 5kN, welding current is 7.7kA, and welding speed is 850mm / min; rounded section: welding pressure is 5kN, welding current is 7.4kA, and welding speed is 800mm / min; three-layer plate: welding pressure is 6.5kN, welding current is 12kA, and welding speed is 500mm / min.

[0078] Figure 6 Image (a) shows the macroscopic morphology of the resistance seam weld in Example 1. Figure 5 Image (b) shows the macroscopic morphology of the weld after circularly polarized mode laser welding. Figure 6 (c) shows the macroscopic morphology of the weld seam welded using the conventional method in Comparative Example 1. It can be seen that using the conventional method for resistance seam welding under high current and high pressure results in a deeper weld indentation, while the combined weld seam is smoother and of higher quality.

[0079] Figure 7 In Comparative Example 1, (a) and (b) show the microstructure of the resistance seam welded joint using the conventional method. It can be seen that under the high current and high pressure of the conventional method, the gap between the plates is large, the joint line is deeply penetrated, and the connection strength and sealing performance of the joint are compromised. Figure 7 (c) shows the microstructure of the welded joint in Example 1. It can be seen that using combined welding to weld high-strength steel can effectively reduce the gap between plates and warping, and obtain a weld with dense microstructure, no cracks or shrinkage cavities, good sealing performance and high connection strength.

[0080] Comparative Example 2

[0081] The difference from Embodiment 1 is that the right-angled triangular wedge block at the fuel tank outer shell and end cap is omitted. The specific steps are as follows:

[0082] After the fuel tank outer shell is stamped (made from a 0.8mm thin sheet), it is positioned using resistance spot welding. The electrode material is made of chromium zirconium copper, and the electrode end face is tapered with a diameter of 5mm. The resistance spot welding process parameters are as follows: welding current 4.8kA, welding pressure 3.2kN, and welding time 4 cycles. After positioning, the fuel tank outer shell is welded along the positioning points using laser welding. The laser welding process parameters are as follows: laser power 3400W, welding speed 2000mm / min, and defocusing amount -15mm.

[0083] After the fuel tank outer shell is welded, resistance spot welding is used to position the fuel tank outer shell and the fuel tank end cap (made of 0.8mm thin plate). The resistance spot welding process parameters are as follows: welding current 4.8kA, welding pressure 3.2kN, welding time 4 cycles. After positioning, resistance seam welding is used to weld the fuel tank outer shell and the fuel tank end cap to eliminate the gap between the plates. The resistance seam welding process parameters are as follows: roller electrode size: large roller 280mm, small roller 170mm; current / rest ratio: 2:5; straight section: welding pressure 5kN, welding current 6kA, welding speed 850mm / min; rounded section: welding pressure 5kN, welding current 6kA, welding speed 800mm / min; three-layer plate: welding pressure 6.5kN, welding current 7.4kA, welding speed 500mm / min. After the seam filling treatment of the fuel tank end cap and fuel tank shell is completed, a circular polarization mode laser remelting is performed along the newly formed weld to complete the welding of the fuel tank shell and fuel tank end cap. The circular polarization mode laser welding process parameters are as follows: laser power is 3400W, welding speed is 2000mm / min, and defocusing amount is -15mm.

[0084] Tensile tests were performed on the specimens prepared in Comparative Example 2, and the results are shown in the figure. Figure 11 .Depend on Figure 11 It can be seen that the joint of the sample prepared in Comparative Example 2 achieved a tensile strength of 7.55 kN and an elongation of 41% under the optimal process specifications. The results of Example 1 and Comparative Example 2 show that, by adding right-angled triangular wedges, the present invention, compared to not adding right-angled triangular wedges, not only enhances the low-speed passage of the roller electrode at the transition between two / three layers of plates, but also avoids inter-plate splashing defects caused by a sharp increase in contact resistance, thereby improving tensile strength and comprehensively enhancing the sealing performance and safety level of the oil tank.

[0085] Comparative Example 3

[0086] The difference from Example 1 is that the circularly polarized laser is replaced with a dot-ring laser, and the specific steps are as follows:

[0087] After the fuel tank outer shell is stamped (made from a 0.8mm thin sheet), it is positioned using resistance spot welding. The electrode material is made of chromium zirconium copper, and the electrode end face is tapered with a diameter of 5mm. The resistance spot welding process parameters are as follows: welding current 4.8kA, welding pressure 3.2kN, and welding time 4 cycles. After positioning, the fuel tank outer shell is welded along the positioning points using laser welding. The laser welding process parameters are as follows: laser power 3400W, welding speed 2000mm / min, and defocusing amount -15mm.

[0088] After the fuel tank outer shell is welded, resistance spot welding is used to position the fuel tank outer shell and the fuel tank end cap (made of 0.8mm thin plate). The resistance spot welding process parameters are as follows: welding current 4.8kA, welding pressure 3.2kN, and welding time 4 cycles. After positioning, two right-angled triangular wedges of the same material BFS400 stainless steel (angles of 30°, 60°, and 90°, length 510mm) are added to the multi-layer plate structure. Resistance seam welding is then used to weld the fuel tank outer shell and the fuel tank end cap to eliminate the gap between the plates. The resistance seam welding process parameters are as follows: roller electrode size: large roller 280mm, small roller 170mm; current / rest ratio: 2:5; straight section: welding pressure 5kN, welding current 6kA, welding speed 850mm / min; rounded section: welding pressure 5kN, welding current 6kA, welding speed 800mm / min; three-layer plate: welding pressure 6.5kN, welding current 7.4kA, welding speed 500mm / min. After the seam filling treatment of the fuel tank end cover and fuel tank shell is completed, a spot-ring laser remelting is performed along the newly formed weld to complete the welding of the fuel tank shell and fuel tank end cover. The spot-ring laser welding process parameters are as follows: laser center power 3400W, ring power 1500W, welding speed 2000mm / min, defocusing amount -15mm.

[0089] Tensile tests were performed on the specimens prepared in Comparative Example 3, and the results are shown in the figure. Figure 12 .Depend on Figure 12 It can be seen that the joint of the sample prepared in Comparative Example 3 achieved a tensile strength of 7.86 kN and an elongation of 42% under the optimal process specifications. The results of Example 1 and Comparative Example 3 show that the present invention, by employing circularly polarized laser remelting, can improve weld quality and significantly enhance the tensile strength and elongation of the joint compared to spot-ring laser welding.

[0090] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A welding method for a multi-layer plate structure of a high-strength stainless steel automotive fuel tank, characterized in that, Includes the following steps: A right-angled triangular wedge is added to the transition position from the double-layer plate formed by the overlap of the fuel tank shell and the end cover to the triple-layer plate to form a transition slope. Resistance seam welding is performed on the fuel tank shell and the end cover, and then a second laser welding is performed along the newly formed weld. The second laser welding is circular polarization mode laser remelting. The outer shell of the fuel tank has an overlapping structure; The thickness of the right-angled triangular wedge is the same as the thickness of the single-layer plate of the fuel tank outer shell; The outer shell of the fuel tank, the end cap, and the right-angled triangular wedge are all made of the same type of high-strength stainless steel.

2. The method according to claim 1, characterized in that, The slope angle of the transition slope is 30~45°; the length of the right-angled triangular wedge is 340~510mm.

3. The method according to claim 1, characterized in that, The outer shell of the fuel tank is formed by sequentially positioning the pre-formed shell through a first spot weld and a first laser weld.

4. The method according to claim 1, characterized in that, The conditions for resistance seam welding include: an on / off ratio of 2:5; Straight section of double-layer plate: welding pressure is 5kN, welding current is 6kA, and welding speed is 850mm / min; Double-layer plate rounded corner section: welding pressure is 5kN, welding current is 6kA, and welding speed is 800mm / min; Three-layer plate: welding pressure is 6.5kN, welding current is 7.4kA, and welding speed is 500mm / min.

5. The method according to claim 1, characterized in that, The conditions for the second laser welding include: laser power of 3400W, welding speed of 2000mm / min, and defocusing amount of -15mm.

6. The method according to claim 3, characterized in that, The first laser welding is a continuous fiber laser welding; the conditions for the first laser welding include: laser power of 3400W, welding speed of 2000mm / min, and defocusing amount of -15mm.

7. The method according to claim 3, characterized in that, The conditions for the first spot weld include: welding current of 4.8~5.2kA, welding pressure of 3.2~3.8kN, and welding time of 4cyc.

8. The method according to claim 1, characterized in that, Before adding the right-angled triangular wedge, a second spot welding positioning is also performed on the tank shell and end cap.

9. The method according to claim 8, characterized in that, The conditions for the second spot welding include: welding current of 4.8~5.2kA, welding pressure of 3.2~3.8kN, and welding time of 4 cycles.

10. The method according to claim 1, characterized in that, The high-strength stainless steel includes BFS400 high-strength stainless steel, BFS500 high-strength stainless steel, QN1803 high-strength stainless steel, or QD2001 high-strength stainless steel.

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