A laser welding method for a high-strength steel workpiece with a plated layer, a laser welding joint and application thereof

Abstract

The application discloses a laser welding method for a high-strength steel workpiece with a plated layer, and comprises the following steps: selecting a medium-C low-Mn welding wire with a C content of 0.4% to 0.7% and a Mn content of 0.3% to 2.4%; taking two hot forming steel plates with aluminum-silicon plated layers to perform filling welding; and performing hot stamping after welding to complete quenching, so as to obtain a high-performance welded joint. The application does not need to remove the plated layer before welding, and can realize effective connection of the 2 GPa aluminum-silicon plated layer hot forming steel only by filling the welding wire. After the hot stamping, the joint strength and elongation are consistent with those of the base material, so that the production process can be reduced and the cost can be lowered.

Description

Technical Field

[0001] This invention relates to a laser welding method for high-strength steel metal workpieces, specifically to a laser welding method for coated high-strength steel workpieces, a laser welding joint and its application, applicable to the welding of 2 GPa grade hot-formed steel with aluminum-silicon coating, so that its strength and elongation after hot stamping are comparable to the base material. Background Technology

[0002] With the rapid development of the automotive industry, the number of cars on the road and their usage frequency have increased dramatically, leading to increasingly prominent problems such as energy shortages, environmental pollution, and the greenhouse effect. Studies show that fuel efficiency is directly related to vehicle weight; a 10% reduction in weight can improve fuel efficiency by approximately 8%. Therefore, lightweight design has become a crucial goal for the automotive industry. The new generation of commercially available aluminum-silicon coated hot-formed steel has an ultimate tensile strength exceeding 1.5 GPa, further increasing the lightweighting of vehicle bodies.

[0003] Laser welding offers minimal deformation and a high degree of automation, making it the preferred welding method for this type of material. To prevent oxidation and decarburization of the sheet metal surface during hot stamping, an aluminum-silicon coating is typically applied beforehand. This causes the aluminum from the coating to diffuse into the weld during welding. Under the pressure of the steam inside the keyhole, this aluminum readily agglomerates at the weld edge, leading to the formation of ferrite with a hardness significantly lower than martensite, severely impairing joint performance.

[0004] Patents CN 101426612 B and CN 106334875 A both employ a technique of locally removing the coating. By using mechanical grinding and laser cleaning, the coating in the weld area is removed before welding, preventing ferrite formation caused by Al segregation. The welded joint exhibits good mechanical properties, but this requires additional processing steps and involves expensive equipment, significantly increasing production costs.

[0005] Patent CN 202211536691 A discloses a device that utilizes both an external electric field and a magnetic field, along with alloy welding wire. By applying a large external current and using an electromagnetic induction coil to generate an alternating magnetic field, the molten metal in the weld pool is driven to flow, mitigating the negative impact of elemental segregation on strength. However, this device requires the simultaneous configuration of multiple mechanisms, resulting in poor process stability. Furthermore, this technology still requires a large amount of alloy welding wire, further increasing manufacturing costs.

[0006] Patent CN 111050980 A discloses a process for welding manganese-boron steel using an oscillating laser filler wire. By oscillating the laser beam, the base material and welding wire are melted simultaneously, promoting the flow of the molten pool to obtain a plate with excellent mechanical properties. However, this is achieved through an optical lens assembly, making it difficult to guarantee the stability of the molten pool and welding wire in actual production, resulting in poor process stability.

[0007] CN 106488824 B discloses a method for joining two blanks. It employs a laser-arc hybrid welding process, with stainless steel alloy filler used to improve the joint's mechanical properties. This process involves a large heat input, easily causing significant deformation of the sheet metal, limiting its use with thin plates. Furthermore, compared to single-heat-source methods, the precise coordination of laser and arc is difficult to achieve, resulting in greater process control challenges. Patent CN 112368105 A discloses a welding material for laser welding of coated steel billets, which mainly includes elements such as nickel (1.68%~10.48%), chromium (0%~2.70%), and carbon (0.91%~2.00%). However, excessive carbon content will inevitably lead to greater brittleness in the joint area, a significant decrease in indicators such as bending toughness and impact toughness, and a relatively high risk of joint failure.

[0008] Both patents CN 112548395 A and US 20210008665 disclose a method for preparing welding wire and a welding process. To avoid the decrease in plasticity caused by excessively high carbon content, high proportions of alloying elements such as Ni, Cr, and Mo are added, with Ni accounting for 8.0%~15.0%, Cr for 3.0%~6.0%, and Mo for 0.5%~1.5%. This leads to the introduction of expensive alloying elements, and the multi-alloying element system significantly increases the difficulty of welding wire manufacturing, increases production costs, and increases the risk of alloying element segregation.

[0009] Patent CN 104994989 A discloses a welding wire with the following chemical composition by mass fraction: carbon 0.05%~0.15%, silicon 0.5%~2.0%, manganese 1.0%~2.5%, total chromium and molybdenum 0.5%~2.0%, nickel 1.0%~4.0%, with the remainder being base iron and unavoidable impurities. This welding wire has a low proportion of chromium and molybdenum alloying elements, resulting in joints with poor hardenability and corrosion resistance; furthermore, it can only reduce the presence of ferrite to a certain extent, but cannot effectively and completely eliminate it.

[0010] Patent CN 120551642 B discloses a solder with a carbon content 120% to 400% higher than that of the base material. However, its excessively high carbon content poses a high risk of delayed cracking when welding 2 GPa hot-formed steel, which has greater hardenability. Patent CN104023899 B reduces the amount of carbon and adds a large amount of manganese (Mn), with the Mn content in the solder being 1.5% to 7.0% higher than that in the base material. However, the addition of a large amount of Mn leads to higher raw material costs and greater production difficulty.

[0011] Compared to 1.5 GPa grade hot-formed steel, 2 GPa grade materials have a further increased carbon content, significantly increasing the welding difficulty. Currently, most solutions require complex laser processes and welding wires rich in expensive alloying elements such as Ni, Mo, and Mn to improve joint performance. However, these welding wires are expensive, and work hardening easily occurs during the wire drawing process, making production difficult. Therefore, there is an urgent need to develop a simple, low-cost, and stable process to achieve effective joining of aluminum-silicon coated hot-formed steel without removing the coating. Summary of the Invention

[0012] The purpose of this invention is to provide a laser welding method suitable for hot-formed steel with aluminum-silicon coating. No coating removal is required before welding. By using medium-carbon, low-manganese welding wire during the welding process, the dilution of the base metal's carbon concentration by the added welding wire is avoided, and a solid solution strengthening effect is achieved, significantly improving the joint strength. After quenching, the volume fraction of lath martensite inside the joint is ≥95%, with an average carbon content of 0.36%~0.45%, an average manganese content of 0.9%~1.2%, and an average al content of 0.3%~1.2%.

[0013] This method involves adding Fe and C-based solder to the molten pool during welding, without adding large amounts of alloying elements. This results in low solder production costs and easier process control, while also preventing the formation of large carbides. The low Mn content in the solder avoids the formation of large amounts of carbon-manganese compounds that could degrade performance. Simultaneously, it ensures a martensite volume fraction of ≥95% in the joint, guaranteeing joint performance. The overall approach, through the addition of medium-C, low-Mn solder, aims to reduce solder costs and improve process stability while maintaining joint hardenability and excellent mechanical properties.

[0014] To achieve the above-mentioned objectives, this invention provides a laser welding method for high-strength steel workpieces with coatings, comprising the following process steps: S1. Welding wire preparation process: Medium carbon low manganese welding wire is selected as the filler solder. The chemical composition of the welding wire, by mass percentage, includes: C element content of 0.4%~0.7%, Mn element content of 0.3%~2.4%, and the balance being Fe and unavoidable impurities.

[0015] The carbon (C) content in the welding wire is configured to be no less than the C content of the base material (preferably slightly higher than the C content of the base material) to compensate for the dilution of the C concentration in the molten pool caused by the mixing of Al elements in the aluminum-silicon coating into the molten pool, ensuring that the fusion zone obtains a reinforced lath martensite structure with high dislocation density after quenching, thereby inhibiting the formation of harmful δ-ferrite; the manganese (Mn) content in the welding wire is configured to be less than 1.5 times the Mn content of the base material to avoid the precipitation of large carbon manganese compounds due to excessive Mn addition, which would worsen the plasticity and toughness of the joint, while reducing the cost of welding wire raw materials and the difficulty of drawing and manufacturing.

[0016] S2. Workpiece preparation process: Provide at least two metal plates to be welded, wherein at least one of the metal plates has an aluminum-silicon coating on its surface (preferably both are hot-formed steel plates with an aluminum-silicon coating, such as 38MnB5). Join the two metal plates edge-to-edge to form an I-type butt joint, controlling the assembly gap to be 0 mm to 0.2 mm (preferably 0 mm).

[0017] Before welding, only organic solvents (such as anhydrous ethanol or acetone) are used to perform routine cleaning treatment on the surface of the steel plate and the edges to be welded to remove surface impurities such as oil and iron filings. No form of removal treatment is performed on the aluminum-silicon coating in the weld area, including but not limited to mechanical grinding, laser cleaning or chemical etching.

[0018] S3, Laser filler wire welding process: A laser beam is used as a heat source to heat and melt the area to be welded to form a molten pool. At the same time, a wire feeding device continuously fills the molten pool with the medium carbon low manganese welding wire from step S1 at a certain angle and speed to perform wire-filled laser welding.

[0019] During the welding process, the laser welding process parameters must be controlled to meet the following conditions: The laser power is 4000 W to 6000 W (preferably 5200 W), the laser scanning rate is 2.0 m / min to 4.0 m / min (preferably 3.0 m / min), the laser spot diameter is 0.3 mm to 1.5 mm (preferably 0.8 mm), the wire feeding rate is 1.5 m / min to 2.5 m / min (preferably 1.9 m / min to 2.2 m / min), and the wire feeding angle (i.e., the angle between the welding wire axis and the surface of the metal plate) is 35° to 45° (preferably 45°).

[0020] During welding, either no shielding gas can be used, or Ar gas or a mixture of Ar and CO2 gas can be used for shielding.

[0021] The laser beam's trajectory can be selected to be either non-oscillating or oscillating; the oscillation modes include, but are not limited to, clockwise oscillation, counterclockwise oscillation, Z-shaped oscillation, or ∞-shaped (infinite shape) oscillation. As a preferred embodiment, the laser beam's oscillation frequency is 50 Hz to 200 Hz, and the oscillation amplitude is 0.5 mm to 2.0 mm, to further promote metal flow within the molten pool, resulting in a more uniform Al element distribution and improved weld sidewall fusion.

[0022] By adjusting the matching relationship between laser heat input and wire feed, the longitudinal section of the weld formed after the molten pool solidifies satisfies the following: the ratio of the weld width d1 on the upper surface to the weld width d2 on the lower surface is d1 / d2≤1.18, that is, the weld widths on the upper and lower surfaces are basically the same, thus avoiding stress concentration caused by excessive difference in weld width.

[0023] The wire feeding device can be a cold wire feeding device, or it can adopt an arc-assisted wire feeding method such as MIG (gas inert gas welding), MAG (gas active gas welding), or TIG (non-gas inert gas welding). In this case, the laser beam is used as the main heat source, and the electric arc is used as an auxiliary heat source to assist in melting the welding wire, so as to further improve the welding wire cladding efficiency and adaptability to assembly gaps.

[0024] The laser beam can be a single laser beam or a combination of multiple laser beams. When using a combination of multiple laser beams (multiple spots), the multiple spots can be arranged in series or parallel, with a spot spacing of 0 mm to 2 mm. The multi-spot method can improve the temperature field distribution of the molten pool, reduce the cooling rate gradient of the molten pool, and help reduce the inhomogeneity of the microstructure after quenching.

[0025] S4. Hot stamping and quenching process: The welded plates are heated to a temperature of 900℃~950℃ (preferably 930℃) and held for 180 s~600 s (preferably 270 s). After holding, the plates are quickly transferred to a flat die or a pre-shaped hot stamping die for hot stamping and quenching, followed by a cooling rate of ≥50℃ / s (preferably ≥70℃ / s) to induce martensitic transformation in the weld and base material.

[0026] S5. Tempering process (preferred option): The quenched hot-formed parts are placed back into the heating furnace and tempered (dried) at 120℃~180℃ for 600 s~1800 s to remove the internal stress generated during quenching, further stabilize the retained austenite, and further improve the overall plasticity and fatigue performance of the base material and joint.

[0027] The aluminum-silicon coated hot-formed steel laser-welded joint prepared by the above method, after hot stamping and quenching, exhibits a weld zone microstructure of ≥95% lath martensite by volume, with no δ-ferrite or coarse carbide precipitation. The average C content in the weld zone is 0.36%~0.45%, the average Mn content is 0.9%~1.2%, and the average Al content is 0.3%~1.2%. The resulting joint has a tensile strength ≥2000 MPa, an A50 gauge length elongation ≥6%, and an average hardness of 595 HV~613 HV in the weld zone, comparable to the base metal hardness (approximately 600 HV). The tensile fracture location is in the base metal region, achieving equal strength matching between the joint and the base metal.

[0028] After being formed by hot stamping dies, the upper and lower excess height of the weld area is significantly reduced, and the cross-section is approximately rectangular, which is beneficial for subsequent painting and assembly processes.

[0029] Compared with the welding process solutions mentioned in the prior art, the advantages of the present invention are: 1) No need to remove the aluminum-silicon coating before welding, reducing production steps and improving manufacturing efficiency.

[0030] 2) The mechanical properties of the laser-welded joint after stamping are good and consistent with the tensile properties and elongation of the base material.

[0031] 3) The equipment is simple and the process window is wide, avoiding the problems of complex manufacturing and high environmental pollution of high C and high Mn welding wire.

[0032] 4) Although Al elements in the coating will inevitably mix into the molten pool during the welding process, this invention effectively compensates for the dilution effect of Al elements on C concentration by precisely designing the medium carbon (0.4%~0.7%) composition in the welding wire and using a C content no less than that of the base material. Furthermore, it promotes the uniform distribution of Al elements in the molten pool through the coordinated control of process parameters, thereby inhibiting the nucleation and growth of δ ferrite.

[0033] 5) Unlike existing technologies that use expensive alloy systems with high Ni, high Mo, and high Cr (such as CN 112548395 A) or excessively high C content (such as CN 112368105 A), which lead to high costs and increased brittleness, this invention uses a low-alloy system based on Fe-C, without relying on any rare or precious metal elements. Simultaneously, the Mn content (0.3%~2.4%) is strictly controlled to be less than 1.5 times the Mn content of the base material, avoiding the increased raw material costs, difficulties in smelting and drawing, and damage to toughness caused by the precipitation of large carbon manganese compounds, as in CN 104023899 B, due to excessive Mn addition (1.5%~7.0% higher than the base material). This welding wire not only has low raw material costs but also excellent drawing performance, facilitating industrial mass production.

[0034] 6) The physical metallurgical mechanism of “C compensation + low Mn control” synergistic inhibition of ferrite in 2 GPa grade aluminum-silicon coated hot-formed steel ensures that a high proportion of lath martensite structure of ≥95% is obtained after quenching, providing an organizational guarantee for the high strength and high plasticity of the joint. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the welding process of the laser welding method for hot-formed steel with aluminum-silicon coating according to the present invention; Figure 2 This is a metallographic cross-sectional view of the weld joint involved in this invention after hot stamping and quenching; Figure 3 This is a hardness cloud map of the weld area of ​​the welded joint after hot stamping and quenching, according to Embodiment 1 of the present invention. Figure 4 These are comparative diagrams of the engineering stress-strain tensile curves of the welded joints involved in Embodiment 1, Embodiment 2 and Comparative Examples 1 and 2 of the present invention after hot stamping and quenching. Figure 5 This is a diagram showing the fracture location of the joint of the C-welding wire used in the filling process of this invention; Figure 6 This is a hardness cloud diagram of the welded joint after hot stamping according to Embodiment 2 of the present invention; Figure 7 This is a diagram showing the location of the fracture at another joint of the C-welding wire used in the filling process of this invention; Figure 8 This is a diagram showing the fracture location of a laser autofusion welded joint, as shown in Comparative Example 1. Figure 9 This is a diagram showing the fracture location of the welded joint in Comparative Example 2, which is filled with ER120G welding wire. Figure 1 In the middle: 1—First plate (base material), 2—Second plate (base material); 3—Area to be welded; 4—First aluminum-silicon coating, 5—Second aluminum-silicon coating; 6—Weld (after solidification); 7—Laser beam; 8—Laser head; 9—Molten pool; 10—Wire feeding angle; 11—Welding wire; 12—Wire feeding nozzle; RD—Rolling direction. Detailed Implementation

[0036] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific implementation examples. Obviously, the described embodiments are only a part of the embodiments of the present invention, and the present invention is not limited to the specific implementation methods described below. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative effort are all within the scope of protection of the present invention.

[0037] Figure 1This diagram illustrates the welding process of the present invention. A first plate 1 and a second plate 2 serve as the base materials for welding, and are joined to form a welding area 3. The first plate 1 has a first aluminum-silicon coating 4, and the second plate 2 has a second aluminum-silicon coating 5. A laser head 8 emits a laser beam 7 to irradiate the welding area 3 to form a molten pool 9, while a wire feed nozzle 12 continuously fills the molten pool 9 with welding wire 11 at a wire feed angle 10. As the laser beam 7 and the wire feed nozzle 12 move along the welding direction, the molten pool 9 cools and solidifies to form a weld 6. Figure 1 As can be seen, both aluminum-silicon coatings 4 and 5 remain on the surface of the sheet during welding and do not need to be removed beforehand. The weld is perpendicular to the rolling direction RD.

[0038] The aluminum-silicon coating on the weld surface does not require grinding and is allowed to melt and enter the molten pool. The welding wire has a slightly higher carbon content than the base metal, which can significantly suppress ferrite and ensure a fully martensitic joint with good mechanical properties.

[0039] The laser beam can travel in two modes: non-oscillating and oscillating. Oscillating trajectories include clockwise oscillation, counterclockwise oscillation, Z-shaped oscillation, and infinite shape oscillation. The upper and lower weld pool widths should be kept as consistent as possible, and the ratio of the upper and lower weld pool widths d1 and d2 should be d1 / d2 ≤ 1.18.

[0040] The wire feeding device can be replaced by processes such as MIG (gas inert gas welding), MAG (gas active gas welding), and TIG (non-gas inert gas welding).

[0041] The laser beam can be a single laser or a combination of multiple laser beams, with a spot diameter of 0.3~1.5 mm. In the case of multiple spots, they can be used in a serial or parallel manner, with a spacing of 0~2 mm.

[0042] The relative position of the wire feeding nozzle and the laser beam is such that the laser beam lags behind the wire feeding nozzle. The relative position of the two needs to be fixed during welding and moves synchronously with the robotic arm.

[0043] The welded joint may further include: hot stamping followed by reheating in a furnace and drying at 120℃~180℃ for 600s~1800s to remove internal stress generated during quenching, which can improve the overall plasticity of the base material and the joint. All of the above methods can be used to join two steel plates. In this case, effective connection of the plates can be achieved without removing the coating, reducing processes and significantly improving production efficiency.

[0044] Figure 2 The metallographic cross-section of the laser-welded joint obtained after hot stamping is shown. There are no defects such as porosity or lack of fusion; the cross-section is well-formed and highly similar to the microstructure of the base material. After hot stamping, the upper and lower excess heights of the joint are significantly reduced, and the entire cross-section can be approximated as rectangular.

[0045] The joint prepared by this invention, after hot stamping, achieves a joint strength of up to 2000 MPa and an A50 elongation of ≥6%. By using different stamping dies, various shaped parts can be obtained. The method of this invention can be used to manufacture various parts for hot-formed steel applications, including vehicles.

[0046] Example 1: A laser welding method for hot-formed steel with an aluminum-silicon coating, comprising the following steps: S1. The base material used is a 38MnB5 metal plate with an aluminum-silicon coating. The coating thickness before stamping is approximately 26 μm. The tensile strength of the base material before quenching is 670 MPa, and the strength after quenching reaches 2000 MPa. The C content is approximately 0.38%, the Mn content is approximately 1.04%, and the Si content is approximately 0.28%.

[0047] S2. Two 1.8 mm thick metal plates with aluminum-silicon coatings are cut using a shearing machine. Their microstructure consists of ferrite and pearlite. Before welding, the surfaces of the steel plates to be welded are cleaned with alcohol to remove impurities and oil, and the assembly gap is 0 mm.

[0048] S3. The laser used is a fiber laser, and the welding parameters are: laser power 5200 W, laser scanning speed 3.0 m / min, laser spot diameter 0.8 mm, wire feed speed 1.9 m / min, and wire feed angle 45°. No shielding gas is used during the welding process.

[0049] S4. The laser beam generated by the laser irradiates the joint of the metal plates. At the same time, the welding wire is fed into the molten pool through the wire feed nozzle, and the welding wire must be at the center of the indicator beam. The molten welding wire and the base material melt and mix, and after solidification, a weld is formed.

[0050] S5. After welding, the plate is placed in a muffle furnace and heated to 930 ℃ and held for 270 s. Then it is placed in a flat die for hot stamping, with a cooling rate of >70 ℃ / s.

[0051] S6. After quenching, the plate is reheated and dried at 120℃~180℃ for 600s~1800s to remove the internal stress generated during quenching, which can improve the plasticity of the base material and joints as a whole.

[0052] Figure 2 The image shows a laser-welded joint after hot stamping. Figure 3 The figure shows the hardness of the joint after hot stamping. With the filling of medium-C welding wire, the average hardness of the joint after quenching is about 595 HV, which is comparable to the hardness of the base material (about 600 HV). Figure 4 , Figure 5 As shown, the sample eventually fractured at the location of the parent material, with a tensile strength exceeding 2000 MPa and an elongation exceeding 6%, demonstrating excellent mechanical properties.

[0053] Example 2: A laser welding method for hot-formed steel with an aluminum-silicon coating, comprising the following steps: S1. The base material used is a 38MnB5 metal plate with an aluminum-silicon coating. The coating thickness before stamping is approximately 26 μm. The tensile strength of the base material before quenching is 670 MPa, and the strength after quenching reaches 2000 MPa. The C content is approximately 0.38%, the Mn content is approximately 1.04%, and the Si content is approximately 0.28%.

[0054] The key technical means employed in this invention is a medium-carbon, low-manganese welding wire filled with a specific composition. This welding wire, by mass percentage, contains the following chemical composition: 0.4%~0.7% carbon (C), 0.3%~2.4% manganese (Mn), with the balance being fe (Fe) and unavoidable impurities. The C content in this welding wire is designed to be no less than (preferably slightly higher than) the C content of the base material to compensate for the dilution effect of Al in the coating on the C concentration after mixing into the molten pool, ensuring that the fusion zone obtains reinforced lath martensite with high dislocation density after quenching. Simultaneously, the Mn content is strictly controlled to be less than 1.5 times the Mn content of the base material to avoid the precipitation of large carbon-manganese compounds due to excessive Mn addition, which would deteriorate the joint's plasticity and toughness, while also reducing the cost of welding wire raw materials and the difficulty of drawing and manufacturing.

[0055] S2. Two 1.8 mm thick metal plates with aluminum-silicon coatings are cut using a shearing machine. Their microstructure consists of ferrite and pearlite. Before welding, the surfaces of the steel plates to be welded are cleaned with alcohol to remove impurities and oil, and the assembly gap is 0 mm.

[0056] S3. The laser used is a fiber laser, and the welding parameters are: laser power 5200 W, laser scanning speed 3.0 m / min, laser spot diameter 0.8 mm, wire feed speed 2.2 m / min, and wire feed angle 45°. No shielding gas is used during the welding process.

[0057] S4. The laser beam generated by the laser irradiates the joint of the metal plates. Simultaneously, wire is fed into the molten pool through a wire feed nozzle, ensuring the welding wire is positioned at the center of the indicator light. The welding wire used meets the requirements of claims 1-2. The molten welding wire mixes with the base material and solidifies to form a weld.

[0058] S5. After welding, the plate is placed in a muffle furnace and heated to 930 ℃ and held for 270 s. Then it is placed in a flat die for hot stamping, with a cooling rate of >70 ℃ / s.

[0059] S6. After quenching, the plate is reheated and dried at 120℃~180℃ for 600s~1800s to remove the internal stress generated during quenching, which can improve the plasticity of the base material and joints as a whole.

[0060] Figure 2 The image shows a laser-welded joint after hot stamping. Figure 3 The figure shows the hardness of the joint after hot stamping. With the filling of medium-C welding wire, the average hardness of the joint after quenching is about 613 HV, which is comparable to the hardness of the base material (about 600 HV). Figure 4 , Figure 5 As shown, the sample eventually fractured at the location of the parent material, with a tensile strength exceeding 2000 MPa and an elongation exceeding 6%, demonstrating excellent mechanical properties.

[0061] Comparative Example 1: As a comparative implementation, the same sheet material and pretreatment method as in the embodiment were selected as the object of the welding board.

[0062] The self-fusion welding method (i.e., without any filler wire) was selected, with the following parameters: laser power 3800 W, laser scanning speed 3.0 m / min, and laser spot diameter 0.8 mm. No shielding gas was used during the welding process. The hot stamping process was also consistent with the example. After stamping, the microstructure of the fusion zone was a mixed structure of δ-ferrite and lath martensite. The tensile strength was only 1685 MPa, the elongation was only 2.23%, and the fracture occurred at the weld seam. Figure 4 , Figure 8 As shown in the figure, this comparative example demonstrates that without filler wire of a specific composition, the damage to the weld caused by Al elements in the aluminum-silicon coating cannot be suppressed.

[0063] Comparative Example 2: As a comparative implementation, the same sheet material and pretreatment method as in the embodiment were selected as the object of the welding board.

[0064] The laser used was a fiber laser, and the welding parameters were: laser power 5200 W, laser scanning speed 3.0 m / min, laser spot diameter 0.8 mm, filler wire ER120G, wire feed speed 2.4 m / min, and wire feed angle 45°. No shielding gas was used during welding. After stamping, the microstructure of the fusion zone was a mixed structure of δ-ferrite and lath martensite. The tensile strength was 1947 MPa, the elongation was only 2.34%, and the fracture occurred in the fusion zone. Figure 4 , Figure 9 As shown in the figure, this comparative example demonstrates that even with commercially available welding wires filled with high alloy content, the welding weakening problem of 2 GPa grade aluminum-silicon coated steel cannot be effectively overcome due to the fundamental difference in composition system compared to this invention (lack of precise enhancement of C content and strict low control of Mn content).

[0065] This invention solves the problems of brittle δ-ferrite phase and low hardness that are prone to occur during the welding of 2 GPa grade high-strength steel by synergistic matching of the specific welding wire composition and welding process parameters. After hot stamping and quenching, the martensite volume fraction in the weld zone is stabilized above 95%, and the average hardness is 595 HV~613 HV, which is comparable to the hardness of the base metal (approximately 600 HV). Tensile test results show that the tensile strength of the joint exceeds 2000 MPa, the A50 elongation remains above 6%, and the fracture location occurs 100% of the time in the base metal region far from the weld (see examples and...). Figure 5 , Figure 7 This completely solves the bottleneck problem of insufficient joint strength and plasticity in traditional self-fusion welding (Comparative Example 1, strength only 1685 MPa, elongation 2.23%) and conventional high-strength welding wire welding (Comparative Example 2, strength 1947 MPa, elongation 2.34%).

[0066] While some examples have been shown, other alternatives, modifications, and uses are possible. Therefore, it should be understood that any alterations or modifications made by those skilled in the art after reading the foregoing description of this invention are equivalent to those forms that fall within the scope defined by the appended claims.

Claims

1. A laser welding method for high-strength steel workpieces with coating, characterized in that, Includes the following steps: S1, Welding wire preparation: Medium carbon low manganese welding wire is selected as the filler filler. The chemical composition of the welding wire, by mass percentage, is: C: 0.4%~0.7%, Mn: 0.3%~2.4%, with the balance being Fe and unavoidable impurities. The C content in the welding wire is not lower than the C content of the base material to compensate for the dilution of C concentration caused by the mixing of Al element in the aluminum-silicon coating into the molten pool. The Mn content in the welding wire is less than 1.5 times the Mn content of the base material to avoid excessive precipitation of carbon and manganese compounds. S2. Workpiece Preparation: Provide at least two metal plates to be welded, at least one of the metal plates having an aluminum-silicon coating on its surface. The edges of the two metal plates to be welded are joined together to form a welding area. The aluminum-silicon coating on the welding area is not removed before welding. The thickness L of the aluminum-silicon coating and the plate thickness D satisfy the following condition: 0.5% ≤ L / D ≤ ​​3%. S3. Laser filler wire welding: A laser beam is used as a heat source to irradiate the area to be welded, and medium carbon low manganese welding wire is continuously filled into the molten pool to perform filler wire laser welding. During the welding process, the laser power is controlled at 4000 W~6000 W, the laser scanning rate is 2.0 m / min~4.0 m / min, the laser spot diameter is 0.3 mm~1.5 mm, the wire feeding rate is 1.5 m / min~2.5 m / min, the wire feeding angle is 35°~45°, and the ratio of the upper weld width d1 to the lower weld width d2 of the longitudinal section of the weld is controlled to satisfy d1 / d2≤1.

18. S4. Hot stamping and quenching: The welded plates after welding are heated to 900℃~950℃ and held for 180 s~600 s, then hot stamped and quenched at a cooling rate of ≥50℃ / s to obtain the laser welded joint of aluminum-silicon coated hot-formed steel.

2. The laser welding method for high-strength steel workpieces with coatings according to claim 1, characterized in that, Step S4 is followed by a tempering process: the hot-stamped and quenched aluminum-silicon coated hot-formed steel laser welded joint is dried at 120 ℃~180 ℃ for 600 s~1800 s to remove internal stress.

3. The laser welding method for high-strength steel workpieces with coatings according to claim 1, characterized in that, The average C content of the laser-welded hot-formed steel joint with aluminum-silicon coating is 105%~130% of the C content of the plate, and the average Mn content is 90%~120% of the Mn content of the base material. The absolute value of the difference between the Mn content of the joint and the plate does not exceed 1%.

4. The laser welding method for high-strength steel workpieces with coatings according to claim 1, characterized in that, In step S3, the trajectory of the laser beam is either a non-oscillating mode or an oscillating mode, and the oscillating mode includes clockwise oscillation, counterclockwise oscillation, Z-shaped oscillation or ∞-shaped oscillation.

5. The laser welding method for high-strength steel workpieces with coatings according to claim 1, characterized in that, In step S3, the laser beam is in the form of a single laser beam or a combination of multiple laser beams; when multiple laser beams are combined, the multiple light spots are arranged in a serial or parallel manner, and the distance between the light spots is 0 mm to 2 mm.

6. The laser welding method for high-strength steel workpieces with coatings according to claim 1, characterized in that, In step S2, the assembly gap between the two metal plates is 0 mm to 0.2 mm.

7. The laser welding method for high-strength steel workpieces with coatings according to claim 1, characterized in that, In step S3, a shielding gas may or may not be used during the welding process; the shielding gas is Ar gas or a mixture of Ar and CO2.

8. The laser welding method for high-strength steel workpieces with coatings according to claim 1, characterized in that, In step S3, the medium-carbon low-manganese welding wire is filled into the molten pool through a wire feeding device, which can be a cold wire feeding device, a MIG wire feeding device, a MAG wire feeding device, or a TIG wire feeding device.

9. The laser welding method for high-strength steel workpieces with coatings according to claim 1, characterized in that, In step S3, the melting volume of the base material during welding is V1, and the total volume of the molten pool is V. The two satisfy the relationship: 0.70≤V1 / V≤0.

85.

10. A laser-welded joint for hot-formed steel with an aluminum-silicon coating, characterized in that, It is prepared by laser welding method for high-strength steel workpieces with coating as described in any one of claims 1 to 9.

11. The aluminum-silicon coated hot-formed steel laser-welded joint according to claim 10, characterized in that, The weld zone microstructure of the joint is ≥95% lath martensite by volume, with an average C content of 0.36%~0.45%, an average Mn content of 0.9%~1.2%, and an average Al content of 0.3%~1.2%. The joint has a tensile strength ≥2000 MPa, an A50 elongation ≥6%, and the tensile fracture location is in the base material region.

12. The application of the laser welding method for coated high-strength steel workpieces according to any one of claims 1 to 9 in the manufacture of hot-stamped structural parts for vehicles, characterized in that, The hot-stamped structural components of the vehicle include A-pillars, B-pillars, anti-collision beams, or sill beams.

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