A method for laser welding a 2gpa grade hot stamped steel to a low carbon steel

Abstract

The present application relates to the field of automobile lightweight material welding technology, in particular to a laser welding method of 2GPa grade hot forming steel and low carbon steel. The method first butts and assembles the ultrahigh strength hot forming steel and the soft low carbon steel, and uses laser beam to weld, wherein the laser beam spot center is deviated to the low carbon steel side by 0.5-2mm; after welding, immediately use laser to perform single pass tempering treatment on the weld area. Through laser offset, the thermal physical mismatch of heterogeneous materials is effectively coordinated, and the formation of brittle Fe-Al-Si intermetallic compound of Al and Si elements in the molten pool is inhibited; the subsequent laser tempering optimizes the performance of the double side heat affected zone, and relieves the softening of the high strength steel side and the hardening of the low carbon steel side. The weld forming of the present application is excellent, the joint shear strength recovery rate reaches more than 90% of the soft base material, the fatigue life is significantly prolonged, and it is especially suitable for batch manufacturing of automobile lightweight safety structural parts.

Description

Technical Field

[0001] This invention relates to the field of welding technology for lightweight automotive materials, specifically a laser welding method for 2GPa grade hot-formed steel and low-carbon steel. Background Technology

[0002] Against the backdrop of the coordinated development of lightweighting and safety in the automotive industry, the design of vehicle body structures increasingly favors a "multi-material hybrid" strategy. Among these, ultra-high-strength hot-formed steels with tensile strengths exceeding 2 GPa (such as PHS2000 and its derivatives) have become core materials for manufacturing critical safety structural components such as A-pillars, B-pillars, and anti-collision beams due to their excellent collision energy absorption capacity and high strength thinning potential. Meanwhile, low-carbon steels with excellent formability and low cost (such as the DC04 and DX51D series) are widely used in body panels and non-primary load-bearing areas. Reliably connecting these two materials with vastly different properties is essential for achieving a lightweight vehicle body design that is "strong where it needs to be strong, and flexible where it needs to be flexible," while simultaneously controlling manufacturing costs.

[0003] However, joining dissimilar materials between hot-formed steel (taking the typical grade PHS2000 as an example) and soft low-carbon steel faces inherent, mutually coupled technical challenges, making it difficult to obtain stable and reliable joints using traditional welding methods. The current technological bottlenecks are mainly reflected in the following three aspects:

[0004] First, there is the fundamental problem of poor metallurgical compatibility. Hot-formed steel often contains a certain amount of alloying elements such as aluminum (Al) and silicon (Si) to improve its hardenability and oxidation resistance. At the high temperatures of welding, these elements readily react with the iron in the low-carbon steel matrix, forming hard and brittle Fe-Al-Si ternary intermetallic compounds (with a hardness exceeding 700 HV) at the weld interface. This brittle phase acts as a microcrack initiation point, severely impairing the toughness and ductility of the weld, leading to low-stress brittle fracture of the joint under stress, posing a significant threat to structural safety.

[0005] Secondly, there is a significant mismatch in thermophysical properties. The melting point of low-carbon steel (approximately 1530°C) is typically higher than that of hot-formed steel (approximately 1400-1450°C), a difference exceeding 80°C. Simultaneously, their thermal conductivity differs by nearly double. This difference makes precise coordination of heat input difficult when using conventional resistance spot welding or arc welding. If energy is applied based on the higher melting point side, the lower melting point hot-formed steel side is prone to overheating and burn-through; conversely, if the lower melting point side is applied, the higher melting point low-carbon steel side may not fully melt, resulting in incomplete fusion defects. The stability and control of the molten pool become exceptionally difficult.

[0006] Finally, there is the synergistic degradation of the weld heat-affected zone (HAZ). The thermal cycling during welding has an irreversible impact on the microstructure of the base material. For hot-formed steel (martensitic microstructure), its HAZ softens under heat, potentially causing a strength reduction of over 40% in this area, forming a "softened zone" that becomes the weakest link in the entire joint. For low-carbon steel (ferrite-pearlite microstructure), its HAZ hardens due to abnormal grain growth, resulting in a sharp drop in ductility. This "soft and hard" failure mode on both sides of the HAZ severely weakens the joint's synergistic load-bearing capacity and fatigue life under complex loads.

[0007] Currently, the industry has explored various methods for joining dissimilar steels, including medium-frequency spot welding and cold metal transfer (CMT) arc welding, and has improved welding quality by optimizing parameters. However, these methods struggle to achieve an ideal balance among several key indicators: ensuring joint strength (typically requiring no less than 80% of the strength of the softer base material), controlling the impact of coatings (such as zinc or aluminum-silicon coatings) on the weld, and considering interface toughness and long-term process stability. These inherent contradictions severely restrict the large-scale, high-reliability application of ultra-high-strength hot-formed steel and low-carbon steel dissimilar material integration technology in vehicle body manufacturing. Summary of the Invention

[0008] To overcome the shortcomings of existing technologies, this invention provides a laser welding method for hot-formed steel and low-carbon steel with a 2GPa rating, which improves the metallurgical compatibility, structural integrity and service reliability of the joint between hot-formed steel and low-carbon steel.

[0009] To achieve the above objectives, the present invention employs the following technical solution:

[0010] A laser welding method for 2GPa grade hot-formed steel and low-carbon steel, wherein the tensile strength of the 2GPa grade hot-formed steel is ≥2GPa and the tensile strength of the low-carbon steel is <300MPa, specifically including the following steps:

[0011] 1) Butt-fit the 2GPa hot-formed steel plate to be welded with the low-carbon steel plate;

[0012] 2) The joint between the 2GPa grade hot-formed steel plate and the low-carbon steel plate is welded using a laser beam. The center of the laser beam spot is offset to one side of the low-carbon steel plate, and the offset distance is 0.5~2mm.

[0013] The process parameters for laser welding are: laser power 4~8kW, welding speed 220~240cm / min, and defocusing amount +4~+6mm;

[0014] 3) After completing the welding in step 2), the weld area formed in step 2) is subjected to single-pass laser tempering using a laser beam;

[0015] The process parameters for laser tempering are: laser power 1~2kW, processing speed 40~60cm / min.

[0016] Furthermore, in step 2), a protective gas is used to protect the molten pool and heat-affected zone during the welding process. The protective gas is argon with a purity of ≥99.97% and a gas flow rate of 20~25L / min.

[0017] Further, in step 2), a transverse pressure of 100~200N is applied to the weld.

[0018] Furthermore, in step 2), the laser used is a ruby ​​fiber laser with a wavelength in the range of 1090±10nm and a spot diameter of 0.3mm.

[0019] Furthermore, after completing the welding in step 2), the cooling rate of the weld is controlled at 500~800℃ / s by using a protective gas. The distance between the nozzle of the protective gas and the weld surface is 3~5mm, the gas blowing pressure is 150~180mbar, and the gas temperature is controlled at 15~25℃.

[0020] Furthermore, in step 3), the wavelength of the annealing laser beam used is 1200±40nm, and the spot diameter is 0.8mm.

[0021] Furthermore, the low-carbon steel plate is a DX51D steel plate, whose chemical composition by weight percentage includes: C: 0.01%~0.08%, Si≤0.03%, Mn: 0.15%~0.26%, P: 0.015%~0.026%, S≤0.01%, Als: 0.02%~0.06%, Ti≤0.30%, with the balance being Fe and unavoidable impurities.

[0022] The 2GPa grade hot-formed steel plate is a PHS2000 steel plate, whose chemical composition by weight percentage includes: C: 0.3%~0.4%, Si: 0.1%~0.2%, Mn: 1.4%~1.8%, Al: 0.15%~0.26%, V: 0.02%~0.06%, S≤0.010%, Ti≤0.003%, with the balance being Fe and unavoidable impurities.

[0023] Compared with the prior art, the beneficial effects of the present invention are:

[0024] 1. Significantly improves the metallurgical quality of welded joints and effectively inhibits the formation of brittle intermetallic compounds.

[0025] Addressing the core defect in existing technologies where the reaction of Al and Si elements in hot-formed steel with the low-carbon steel matrix forms a hard and brittle Fe-Al-Si phase (>700HV), leading to a sharp decrease in weld toughness, this invention actively regulates the migration path and solidification behavior of elements within the molten pool by shifting the laser beam spot center 0.5~2mm towards the low-carbon steel side and strictly controlling the welding heat input (power 4~8kW, speed 220~240cm / min). This ensures preferential and complete melting of the high-melting-point low-carbon steel while limiting excessive melting and violent vaporization of Al and Si elements on the hot-formed steel side, thus suppressing the enrichment and precipitation of banded brittle phases at the source. After this process, the enrichment of Al and Si elements at the weld joint surface is effectively controlled, ensuring high toughness of the weld body and laying the foundation for high joint strength.

[0026] 2. Precisely coordinate the thermophysical response of heterogeneous materials to obtain a stable and symmetrical molten pool morphology.

[0027] To address the significant melting point difference (>80℃) and thermal conductivity difference between low-carbon steel and hot-formed steel, which leads to molten pool instability, weld misalignment, and undercut defects, this invention employs a directional laser beam deflection strategy towards the high-melting-point low-carbon steel side, coupled with a specific defocusing amount (+4~+6mm), reshaping the energy distribution across the joint cross-section. This balances the melting and solidification processes of both base materials, promoting the formation of a symmetrical and contour-stable molten pool about the mating surface. It completely eliminates inherent welding defects caused by thermophysical property mismatch, achieving stable full penetration with single-sided welding and double-sided forming, resulting in excellent weld formation.

[0028] 3. Strong process adaptability, providing a reliable solution for the manufacturing of lightweight automotive components.

[0029] This invention provides a complete and parameterized process solution that allows direct welding of steel plates with substrates without relying on expensive specialized welding wires or complex pretreatment for coating removal. Stable implementation is achieved through the use of general-purpose laser equipment and customized fixtures. It is particularly suitable for butt welding of hot-formed steel with a tensile strength ≥2GPa (such as PHS2000) to low-carbon steel (such as DX51D) with lower strength but better formability, meeting the requirements for "high strength, high toughness, and high efficiency" manufacturing of lightweight safety structural components such as automotive B-pillars, sill beams, and crash beams, which integrate dissimilar materials. Attached Figure Description

[0030] Figure 1 This is a topographic image of the fusion zone of hot-formed steel and low-carbon steel laser welding in Embodiment 1 of the present invention.

[0031] Figure 2 This is a topographic image of the laser welding fusion zone in Embodiment 1 of the present invention, obtained by corresponding line scanning at the scanning position.

[0032] Figure 3This is an AL element distribution diagram of the weld interface between hot-formed steel and low-carbon steel in Embodiment 1 of the present invention.

[0033] Figure 4 This is a Si element distribution diagram at the interface between the hot-formed steel and low-carbon steel weld seam in Embodiment 1 of the present invention.

[0034] Figure 5 This is a topographic image of the fusion zone of hot-formed steel and low-carbon steel laser welding in Embodiment 2 of the present invention.

[0035] Figure 6 This is a topographic image of the laser welding fusion zone in Embodiment 2 of the present invention, obtained by scanning the corresponding line.

[0036] Figure 7 This is an AL element distribution diagram of the weld interface between hot-formed steel and low-carbon steel in Embodiment 2 of the present invention.

[0037] Figure 8 This is a Si element distribution diagram of the weld interface between hot-formed steel and low-carbon steel in Embodiment 2 of the present invention.

[0038] Figure 9 This is a tensile specimen diagram from Embodiment 1 of the present invention. Detailed Implementation

[0039] The technical solution of the present invention will be clearly and completely described below with reference to examples. Obviously, the described embodiments are merely one example of the present invention, and those skilled in the art can refer to the content herein to appropriately improve the process parameters. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in the present invention. The methods and applications of the present invention have been described through preferred embodiments, and those skilled in the art can obviously make modifications or appropriate changes and combinations to the methods and applications described herein without departing from the content, spirit, and scope of the present invention to realize and apply the technology of the present invention.

[0040] General experimental conditions and testing methods:

[0041] 1. Experimental materials:

[0042] The low-carbon steel uses commercial DX51D steel plate, and its chemical composition (mass percentage) is as follows: C: 0.01%~0.08%, Si≤0.030%, Mn: 0.15%~0.26%, P: 0.015%~0.026%, S≤0.010%, Als: 0.02%~0.06%, Ti≤0.300%, with the balance being Fe and unavoidable impurities.

[0043] The 2GPa grade hot-formed steel uses commercially available PHS2000 steel plate, and its chemical composition (mass percentage) is as follows: C: 0.3%~0.4%, Si: 0.1%~0.2%, Mn: 1.4%~1.8%, Al: 0.15%~0.26%, V: 0.02%~0.06%, S≤0.010%, Ti≤0.003%, with the balance being Fe and unavoidable impurities.

[0044] Both types of steel plates have dimensions of 150mm × 50mm × 1.5mm.

[0045] 2. Main equipment:

[0046] Laser welding and tempering equipment: Utilizing an IPGYLS-8000-S2T ytterbium-doped fiber laser with a rated power of 8kW and a beam product (BPP) of <4mm*mrad. The welding process employs a KUKAKR60HA six-axis industrial robot for precise trajectory and attitude control. The shielding gas is 99.99% pure argon.

[0047] 3. Connector type and assembly:

[0048] The DX51D steel plate and the PHS2000 steel plate are butt-welded together along the 150mm long side with zero gap. A special pneumatic clamp is used to apply uniform clamping force to both sides of the weld to ensure that the interface to be welded remains tightly fitted throughout the welding process, without warping or gaps.

[0049] 4. Testing and characterization methods:

[0050] (1) Macro / micro morphology observation: After welding, samples were cut along the direction perpendicular to the weld, and after mounting, grinding, and polishing, they were etched with 4% nitric acid alcohol solution. The macroscopic formation and fusion line of the weld were observed using an optical microscope. The microstructure of the weld, fusion line and heat-affected zone were observed using a field emission scanning electron microscope (SEM).

[0051] (2) Elemental distribution analysis: The surface distribution (Mapping) analysis of Al and Si elements and the line scan analysis along a specific path were performed using an energy dispersive spectrometer (EDS) to evaluate the elemental segregation behavior.

[0052] (3) Mechanical property testing:

[0053] A. Microhardness Test: A microhardness tester using a Vickers indenter was used. The test load was 500 gf, and the holding time was 15 s. Hardness tests were performed along the center of the weld cross-section, from the DX51D base material through the weld center to the PHS2000 base material, with a test point spacing of 0.1 mm.

[0054] B. Shear Strength Test: Standard shear specimens were prepared according to the specifications for shear specimens in ISO 14273:2016, "Specimen dimensions and procedures for resistance spot welding, projection welding and seam welding" (see schematic diagram). Figure 9 Quasi-static tensile shear tests were performed on an Instron 5982 universal testing machine at a loading rate of 2 mm / min. The formula for calculating the joint shear strength recovery rate is: (joint shear strength / shear strength of the soft low-carbon steel base material) × 100%.

[0055] C. Fatigue Life Test: According to ISO 14324:2003 "Resistance spot welding – Fatigue testing method", tensile fatigue tests were conducted under conditions of 100 MPa stress amplitude, stress ratio R = 0.1, and loading frequency of 50 Hz. The number of cycles at which the specimen fractured was recorded. If the number of cycles reached 2.1 × 10⁻⁶, the test was considered successful. 6 If the fault does not break, it is recorded as "run-out".

[0056] II. Example:

[0057] Example 1:

[0058] The low-carbon steel uses commercial DX51D steel plate, and its chemical composition (mass percentage) is: C: 0.06%, Si: 0.020%, Mn: 0.18%, P: 0.017%, S≤0.006%, Als: 0.03%, Ti: 0.3%, with the balance being Fe and unavoidable impurities.

[0059] The 2GPa grade hot-formed steel uses commercially available PHS2000 steel sheet, and its chemical composition (mass percentage) is: C: 0.4%, Si: 0.2%, Mn: 1.5%, Al: 0.19%, S: 0.007%, V: 0.04%, Ti: 0.003%, with the balance being Fe and unavoidable impurities.

[0060] (1) Welding and tempering process parameters:

[0061] Prepare the PHS2000 and DX51D mating joint according to the general method described above. Welding is performed using a ruby ​​fiber laser, with the following key process parameters: laser power 7kW, wavelength 1096nm, spot diameter 0.3mm, welding speed 220cm / min, spot diameter 0.3mm, and defocusing amount +4mm. Crucially, a straight path is used, with the laser beam spot center offset 1mm towards the DX51D (low-carbon steel). 99.99% pure argon gas is used as the shielding gas at a flow rate of 22L / min. Immediately after welding, the weld area is subjected to single-pass laser tempering using the same laser system (with adjusted parameters). The tempering process parameters are: laser power 1.2kW, wavelength 1220nm, processing speed 40cm / min, and spot diameter 0.8mm. High-temperature tempering is achieved by irradiating the weld area along the mating surface.

[0062] The welding process is secured by a custom-made fixture. A transverse pressure of 180N is applied to the weld to promote a tight bond at the interface. The cooling rate after welding is controlled at 650℃ / s. The shielding gas nozzle is 5mm away from the weld surface, the blowing pressure is 150mbar, and the gas temperature is controlled at 22℃. No clamping force is applied during the tempering process.

[0063] (2) Characterization results and performance data:

[0064] The welded joint obtained in Example 1 was analyzed according to the general test methods described above. The macroscopic morphology is as follows: Figure 1 As shown, the weld formation is continuous, uniform, and aesthetically pleasing, without defects such as undercut, collapse, or depression, successfully achieving single-sided welding with double-sided forming.

[0065] Microstructure analysis such as Figure 1 , 2 As shown (SEM photo), Figure 2 The specific morphology of the location where the line scan was performed is shown. The weld center consists of a mixed microstructure of fine lath martensite and a small amount of bainite. No continuous or discontinuous banded anomalous structures were observed near the fusion line on the PHS2000 side. EDS surface distribution analysis results are as follows... Figure 3 and Figure 4 As shown, Al and Si elements are dispersed and uniformly distributed throughout the weld area, without forming obvious local enrichment zones in key areas such as grain boundaries, fusion lines, or weld centers.

[0066] The hardness of the weld center region remained stable between 380 and 420 HV. The lowest hardness of the softened zone in the heat-affected zone (HAZ) of PHS2000 steel was 410 HV, representing a softening rate of approximately 18% compared to the hardness of the PHS2000 base material (approximately 500 HV). The hardness of the HAZ on the DX51D low-carbon steel side transitioned smoothly, with a maximum hardness of approximately 165 HV, and no abnormal hardening peaks due to abnormal grain growth were observed.

[0067] Mechanical property test results: The average shear strength of the joint was 15.2 kN. Based on the shear strength calculation of the DX51D base material, the shear strength recovery rate of the joint reached 92%. Fatigue tests were conducted under a stress amplitude of 100 MPa, and all specimens showed a recovery rate of 2.1 × 10⁻⁶ kN. 6 No fracture occurred during the second cycle (run-out), demonstrating excellent fatigue resistance.

[0068] Example 2: Implementation method for changing the offset:

[0069] The low-carbon steel uses commercial DX51D steel plate, and its chemical composition (mass percentage) is: C: 0.08%, Si: 0.030%, Mn: 0.26%, P: 0.015%, S: 0.010%, Als: 0.06%, Ti: 0.2%, with the balance being Fe and unavoidable impurities.

[0070] The 2GPa grade hot-formed steel uses commercially available PHS2000 steel sheet, and its chemical composition (mass percentage) is: C: 0.4%, Si: 0.20%, Mn: 1.8%, Al: 0.23%, S: 0.010%, V: 0.04%, Ti: 0.002%, with the balance being Fe and unavoidable impurities.

[0071] (1) Welding and tempering process parameters:

[0072] The main difference between this embodiment and Embodiment 1 lies in the adjustment of welding parameters to verify the effects of different offsets. Specific parameters are: laser power 7kW, wavelength 1100nm, spot diameter 0.3mm, welding speed 240cm / min, and a straight path (c). The laser beam spot center is offset 2mm towards the DX51D side, with a defocusing amount of +5mm. Argon gas with a purity of 99.99% is used as the protective gas, with a flow rate of 25L / min. Laser tempering parameters are adjusted to: power 1.5kW, wavelength 1230nm, spot diameter 0.8mm, and processing speed 45cm / min. High-temperature tempering is achieved by irradiating the weld area along the bonding surface. The welding process is secured using a custom-made fixture, with 100N of lateral pressure applied to the weld to promote tight bonding at the interface. Post-weld cooling rate is controlled at 700℃ / s, the protective gas nozzle is 5mm from the weld surface, the blowing pressure is 180mbar, and the gas temperature is controlled at 25℃. No clamping force is applied during the tempering process.

[0073] (2) Characterization results and performance data:

[0074] The macroscopic morphology of the weld obtained in this embodiment, tested using the same general method, is as follows: Figure 5 As shown, the shape is good and the melt width is uniform. Figure 6The corresponding line scan morphology is shown. Microscopic observation indicates that the weld and heat-affected zone have normal microstructure, without defects such as cracks or porosity. EDS analysis shows that, as Figure 7 and Figure 8 As shown, Al and Si elements are also dispersed, with no harmful enrichment. The joint shear strength recovery rate reaches over 90%, and the fatigue performance is comparable to that of Example 1, meeting the requirements for high-strength, lightweight structural components. This example demonstrates that the method of the present invention remains effective even with large offsets.

[0075] Example 3: Boundary implementation method with lower power and smaller offset:

[0076] (1) Welding and tempering process parameters:

[0077] This embodiment aims to verify the applicability of the method of the present invention within the parameter range boundaries described in the claims (lower power, smaller offset). Specific parameters are: laser power 4kW (close to the lower limit of the claim), welding speed 230cm / min, defocusing amount +4.5mm. The laser beam offset towards the DX51D side is 0.5mm (close to the lower limit of the claim). The shielding gas flow rate is 20L / min. A lateral pressure of 150N is applied during welding. Laser tempering parameters are: power 1.0kW, processing speed 50cm / min.

[0078] (2) Characterization results and performance data:

[0079] Despite the relatively low heat input, the weld achieved full penetration and passed macroscopic shaping. Microstructural analysis showed that the above parameter combination, especially the 0.5 mm offset, effectively prevented the concentrated formation of brittle Fe-Al-Si phases. The joint's mechanical properties passed testing, with a shear strength recovery rate exceeding 85%.

[0080] Comparative Example 1: Comparison of laser beams without deviation:

[0081] (1) Process parameters:

[0082] This comparative example is used for comparison to illustrate the impact of omitting the core feature of this invention, "the laser beam is offset towards the low-carbon steel side." Except for the laser beam spot center being precisely aligned with the center of the butt joint of the two plates (i.e., the offset is 0), all other process parameters, base materials, equipment, and testing methods are exactly the same as in Example 1.

[0083] (2) Characterization results and performance data:

[0084] The joint obtained in Comparative Example 1 was analyzed using the same method. SEM observation revealed continuous or discontinuous bright white bands in the center region of the weld. EDS line scanning and area distribution analysis confirmed that these bands were brittle phases rich in Al and Si, primarily identified as Fe-Al-Si intermetallic compounds. Microhardness testing showed that the microhardness of this brittle phase region was abnormally high, exceeding 720 HV. Due to the presence of this hard and brittle phase, the average shear strength of the joint significantly decreased to 9.8 kN during tensile shear testing, with a recovery rate of only about 60%. Furthermore, during a simple bending test, cracks preferentially initiated and propagated from the brittle phase in the weld center, leading to brittle fracture of the joint. These comparative results strongly demonstrate that "laser beam deflection towards the low-carbon steel side" plays an indispensable and decisive role in controlling the flow of the molten pool, diluting and disrupting the distribution of Al and Si elements, thereby inhibiting the continuous formation of harmful brittle phases in critical areas.

[0085] III. Summary of Results:

[0086] The above embodiments and comparative examples fully demonstrate that the process of "laser beam deflection towards the low-carbon steel side" combined with "single-pass laser tempering" of this invention can systematically solve the complex technical problems of metallurgical phase compatibility, thermophysical mismatch, and performance degradation of the heat-affected zone in dissimilar welding of 2GPa-grade hot-formed steel and low-carbon steel. Specifically, it effectively eliminates brittle intermetallic compounds, resulting in a reliable welded joint with a smooth hardness distribution, high shear strength (recovery rate ≥90%), and excellent fatigue performance (no fracture after ultra-high cycles). Conversely, as shown in Comparative Example 1, if the core step of laser deflection is omitted, even under the same conditions, brittle phase precipitation will occur, leading to a sharp deterioration in joint performance.

[0087] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A laser welding method for 2GPa grade hot-formed steel and low-carbon steel, characterized in that, The tensile strength of the 2GPa grade hot-formed steel is ≥2GPa, and the tensile strength of the low-carbon steel is <300MPa; The low-carbon steel plate is DX51D steel plate, and its chemical composition by weight percentage includes: C: 0.01%~0.08%, Si≤0.03%, Mn: 0.15%~0.26%, P: 0.015%~0.026%, S≤0.01%, Als: 0.02%~0.06%, Ti≤0.30%, with the balance being Fe and unavoidable impurities; The 2GPa grade hot-formed steel plate is PHS2000 steel plate, and its chemical composition by weight percentage includes: C: 0.3%~0.4%, Si: 0.1%~0.2%, Mn: 1.4%~1.8%, Al: 0.15%~0.26%, V: 0.02%~0.06%, S≤0.010%, Ti≤0.003%, with the balance being Fe and unavoidable impurities; The method specifically includes the following steps: 1) Butt-fit the 2GPa hot-formed steel plate to be welded with the low-carbon steel plate; 2) The joint between the 2GPa grade hot-formed steel plate and the low-carbon steel plate is welded using a laser beam. The center of the laser beam spot is offset to one side of the low-carbon steel plate, and the offset distance is 0.5~2mm. A transverse pressure of 100~200N is applied to the weld. The process parameters for laser welding are: laser power 4~8kW, welding speed 220~240cm / min, and defocusing amount +4~+6mm; 3) After completing the welding in step 2), the cooling rate of the weld is controlled at 500~800℃ / s by the shielding gas, and a laser beam is used to perform single-pass laser tempering on the weld area formed in step 2). The process parameters for laser tempering are: laser power 1~2kW, processing speed 40~60cm / min.

2. The laser welding method for 2GPa grade hot-formed steel and low-carbon steel according to claim 1, characterized in that, In step 2), a protective gas is used to protect the molten pool and heat-affected zone during the welding process. The protective gas is argon with a purity of ≥99.97% and a gas flow rate of 20~25L / min.

3. The laser welding method for 2GPa grade hot-formed steel and low-carbon steel according to claim 1, characterized in that, In step 2), the laser used is a ruby ​​fiber laser with a wavelength in the range of 1090±10nm.

4. The laser welding method for 2GPa grade hot-formed steel and low-carbon steel according to claim 1, characterized in that, In step 3), the nozzle of the protective gas is 3-5 mm away from the weld surface, the gas blowing pressure is 150-180 mbar, and the gas temperature is controlled at 15-25℃.

5. The laser welding method for 2GPa grade hot-formed steel and low-carbon steel according to claim 1, characterized in that, In step 3), the wavelength of the annealing laser beam used is 1200±40nm and the spot diameter is 0.8mm.

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