Laser welding method of twip steel based on 8-shaped beam oscillation

Abstract

The application relates to the technical field of metal material welding processing, and discloses a TWIP steel laser welding method based on 8-shaped light beam oscillation, which comprises the following steps: preparing a TWIP steel butt joint sample to be welded and controlling a butt joint gap, applying a mechanical pressing force to rigidly fix; introducing a protective gas into a welding area; adopting a fiber laser to cooperate with a two-dimensional galvanometer scanning system to perform welding, setting a specific laser beam reverse inclination angle; driving a laser spot to move in real time according to a 8-shaped track, and forcibly stirring a molten pool by controlling an oscillation frequency and an amplitude. The application induces a convection eddy current in the molten pool by using the track movement of the spot, improves a shearing rate to inhibit alloy element segregation, blocks a hard and brittle carbide precipitation condition, overcomes a heat affected zone softening problem, and obtains a double-scale structure joint with edge columnar crystals and center ultra-fine equiaxed crystals after solidification, so that the comprehensive mechanical properties of a TWIP steel welded joint are improved.

Description

Technical Field

[0001] This invention relates to the field of metal material welding and processing technology, specifically to a TWIP steel laser welding method based on figure-eight beam oscillation. Background Technology

[0002] TWIP steel possesses high tensile strength and good plastic deformation capacity. Its unique energy absorption characteristics make it highly valuable for engineering applications in high-end equipment operating in extreme low-temperature and high-impact load environments, such as spacecraft soft landing systems, deep space exploration, liquefied natural gas cryogenic storage tanks, and anti-collision structures for new energy vehicles. Laser welding is commonly used for welding TWIP steel plates. However, in conventional laser welding processes, there are certain metallurgical incompatibilities between the compositional characteristics of TWIP steel and the application mode of conventional laser heat sources.

[0003] TWIP steel base metal contains high mass fractions of manganese and carbon. During conventional single-beam laser welding, the fluid within the molten pool is primarily transported via surface tension and natural convection, lacking forced stirring. This relatively singular heat and mass transfer state leads to solute enrichment at the solid-liquid interface, inducing elemental segregation. Localized high concentrations of elemental segregation provide the thermodynamic and kinetic conditions for the precipitation of hard, brittle carbides. These brittle phases directly disrupt the continuity of the metal matrix, resulting in decreased joint plasticity and toughness under extreme low-temperature conditions.

[0004] Furthermore, the thermal cycle of conventional laser welding adversely alters the microstructure around the joint. On one hand, the large temperature gradient promotes the formation of coarse, columnar crystals that grow through the weld; on the other hand, the heat-affected zone adjacent to the weld undergoes grain growth and loses its original strengthening effect under heat input, leading to significant local softening. The combined effect of these defects results in the overall load-bearing capacity of conventional TWIP steel laser-welded joints being only 80% of that of the base material. Under stress, the joint is highly susceptible to fracture from the softened zone or brittle phase-rich areas, failing to fully utilize the excellent intrinsic mechanical properties of TWIP steel and limiting its application in structural components with extremely high reliability requirements. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a laser welding method for TWIP steel based on figure-eight beam oscillation, which solves the problems of severe elemental segregation, easy precipitation of hard and brittle carbides, and local softening of the heat-affected zone in conventional TWIP steel laser welding, thereby reducing the overall mechanical properties of the welded joint.

[0006] To achieve the above objectives, the present invention provides the following technical solution: The first aspect of this invention provides a laser welding method for TWIP steel based on figure-eight beam oscillation, comprising the following steps: Prepare two TWIP steel butt joint samples to be welded. Place the two TWIP steel butt joint samples flat on the welding fixture, control the butt joint gap between the two TWIP steel butt joint samples, apply mechanical clamping force to rigidly fix the two TWIP steel butt joint samples to be welded, and complete the preparation of the parts to be welded. A protective gas is introduced into the welding area of ​​the workpiece to be welded; A fiber laser, in conjunction with a two-dimensional galvanometer scanning system, is used to perform welding on a workpiece that has been introduced with a protective gas. The reverse tilt angle of the laser beam is set, and the two-dimensional galvanometer scanning system is set to drive the laser spot to move in real time along an 8-shaped trajectory. The oscillation frequency and amplitude of the two-dimensional galvanometer scanning system are controlled, and the molten pool is forcibly stirred by the laser spot moving in real time along the 8-shaped trajectory. After cooling and solidification, a dual-scale welded joint is obtained.

[0007] By employing the aforementioned technical solution, the figure-eight beam trajectory substantially alters the hydrodynamic state and heat transfer process of the molten pool. Specifically, the periodic movement of the laser spot within the liquid-phase molten pool induces convective eddies. This eddy effect directly increases the shear rate within the molten pool, and the higher shear rate disrupts the solute-rich layer at the solid-liquid interface, resulting in a more uniform distribution of alloying elements such as carbon and manganese in the melt. Furthermore, segregation is suppressed, thereby blocking the thermodynamic and kinetic conditions required for the precipitation of hard and brittle carbides.

[0008] Besides improving compositional segregation, the continuous oscillation of the laser spot also makes the temperature field of the molten pool more uniform. The fluctuation of the local thermal gradient decreases accordingly, which changes the ratio of the temperature gradient at the solidification front to the solidification rate. When this ratio decreases, the compositionally undercooled zone expands accordingly, which provides a greater driving force for non-spontaneous nucleation.

[0009] It is worth noting that as the heat source moves forward, the heat-affected zone (HAZ) generates a thermo-mechanical coupling effect, inducing the accumulation of deformation energy within the material. With the help of this deformation energy, the grains in the HAZ are more prone to dynamic recrystallization, which not only refines the grain size but also overcomes the problem of softening in this region under conventional welding conditions.

[0010] Ultimately, during the solidification and cooling stage of the molten pool, different regions exhibit varying crystal morphologies. Near the edges, due to close contact with the base material and a faster heat dissipation rate, the grains tend to grow in a columnar morphology along the temperature gradient. In the central region of the molten pool, the continuous agitation by the aforementioned convective eddies increases the number of free crystal nuclei within the liquid metal. These nuclei grow independently and gradually evolve into equiaxed crystals. Through the synergy of these mechanisms, a dual-scale microstructure of equiaxed columnar crystals plus centrally ultrafine equiaxed crystals is formed within the welded joint, which plays a crucial role in improving the joint's mechanical strength and plasticity.

[0011] Preferably, the TWIP steel butt joint samples to be welded are prepared from Fe-Mn-C series TWIP steel plates with a thickness of 1.5-5.0 mm. The chemical composition of the Fe-Mn-C series TWIP steel plates, by mass percentage, is as follows: Mn: 15.0%-25.0%, C: 0.4%-0.8%, balance being Fe and unavoidable impurities.

[0012] By employing the above technical solution and limiting the content of manganese and carbon elements within a specific range, it is possible to ensure that the base material itself can maintain the core mechanism of deformation-induced twinning, thereby providing basic mechanical property support. Simultaneously, combined with a plate thickness setting of 1.5-5.0 mm, it makes it easier to achieve the penetration requirement within the conventional laser power adjustment range, thus reducing the incidence of underlying defects such as incomplete penetration.

[0013] Preferably, the steps for preparing two butt joint test specimens of TWIP steel to be welded include: Fe-Mn-C TWIP steel plates are processed into rectangular specimens, and the sides of the rectangular specimens to be joined are precision machined. The oxide scale on the weldable end face and both sides of the weld of the rectangular specimen is removed by mechanical grinding. The ground rectangular specimen is then placed in an ultrasonic cleaning tank and cleaned with acetone or anhydrous ethanol. After being dried with cold air, the TWIP steel butt joint specimen is obtained. The butt joint gap ranges from 0.05 to 0.15 mm.

[0014] By employing the aforementioned technical solution, through machining, mechanical grinding, and subsequent ultrasonic chemical cleaning, the main purpose is to remove the oxide layer and residual grease and other impurities from the surface of the area to be welded. This pretreatment cuts off potential hydrogen and oxygen sources, significantly reducing the probability of porosity formation in the subsequent weld. Furthermore, strictly controlling the butt joint gap between 0.05 and 0.15 mm allows the liquid metal to bridge more easily under surface tension, resulting in a relatively smooth weld surface and preventing collapse due to excessively large gaps or incomplete fusion due to excessively small gaps.

[0015] Preferably, the protective gas is high-purity argon with a purity ≥ 99.99%, and the flow rate of the protective gas is set to 12-20 L / min.

[0016] By employing the above technical solution, argon gas with a purity of over 99.99% is used to replace the air around the welding area, isolating the high-temperature molten metal from the external environment and preventing the loss of alloying elements due to the intrusion of oxygen and nitrogen. Controlling the flow rate within the range of 12 to 20 L / min aims to construct a stable protective gas shield with sufficient coverage, which plays a certain auxiliary role in maintaining the morphology of the laser deep-penetration aperture and the dynamic stability of the molten pool surface.

[0017] Preferably, the laser power of the fiber laser is set to 1800-2100W, the welding speed of the fiber laser is set to 0.8-1.2m / min, the defocusing amount of the fiber laser is set to -1mm to +1mm, and the laser beam back tilt angle is set to 5°-15°.

[0018] By adopting the above technical solution, the coordinated matching of laser power and welding speed provides thermal input line energy adapted to the corresponding plate thickness, while the defocusing amount is used to fine-tune the energy density of the laser spot acting on the material surface. To address the instability during the welding process, a reverse tilt angle of 5° to 15° is introduced. The significance of this setting is to guide the metal vapor generated inside the laser aperture to be ejected behind the weld, thereby suppressing molten spatter by changing the airflow direction and ensuring the continuity of the entire welding process.

[0019] Preferably, the oscillation frequency of the two-dimensional galvanometer scanning system is set to 150-250Hz, and the oscillation amplitude of the two-dimensional galvanometer scanning system is set to 0.3-0.7mm.

[0020] By adopting the above technical solution, the speed and coverage of the light spot in space are actually determined by the combination of oscillation frequency and amplitude. These two parameters are directly related to the shear rate of the fluid in the molten pool. By limiting the frequency and amplitude to the ranges of 150-250Hz and 0.3-0.7mm, respectively, the shear rate of the molten pool can be made to cross the critical threshold affecting segregation, maintaining the required convective vortex state without excessively increasing the equipment load.

[0021] Preferably, the oscillation frequency of the two-dimensional galvanometer scanning system is set to 200Hz and the oscillation amplitude of the two-dimensional galvanometer scanning system is set to 0.5mm; Alternatively, the oscillation frequency of the two-dimensional galvanometer scanning system can be set to 150 Hz and the oscillation amplitude of the two-dimensional galvanometer scanning system can be set to 0.3 mm.

[0022] By adopting the above technical solutions, different parameter combinations are designed to adapt to base materials of different specifications. For example, a 200Hz frequency combined with a 0.5mm amplitude center setting provides a relatively balanced energy distribution for medium-thickness samples; while when processing thin plate samples, using a low-frequency, small-amplitude combination of 150Hz and 0.3mm can control the overall disturbance of the molten pool while achieving stirring, reducing heat input fluctuations, and thus avoiding significant warping or thermal deformation of the thin plate.

[0023] Preferably, the oscillation frequency of the two-dimensional galvanometer scanning system is set to 250Hz and the oscillation amplitude of the two-dimensional galvanometer scanning system is set to 0.7mm; Alternatively, the oscillation frequency of the two-dimensional galvanometer scanning system can be set to 150 Hz and the oscillation amplitude of the two-dimensional galvanometer scanning system can be set to 0.7 mm.

[0024] By employing the above technical solutions, for thick plate samples, the deep molten pool often suffers from slow flow. In this case, the combination of 250Hz and 0.7mm high-frequency wavelengths can transfer the stirring effect to the bottom of the molten pool, breaking the solute concentration gradient formed along the thickness direction. Another approach, using 150Hz combined with 0.7mm slow, wide-amplitude oscillation, focuses on laterally expanding the melt width. This helps provide more cooling time and space for the center of the molten pool, thereby increasing the nucleation ratio of equiaxed crystals to some extent.

[0025] Preferably, the dual-scale welded joint does not contain hard and brittle carbides and has equiaxed columnar crystals and central ultrafine equiaxed crystals.

[0026] By employing the above technical solution, the elimination of hard and brittle phase precipitation, combined with a dual-scale microstructure exhibiting different grain morphologies at the edges and center, improves the slip resistance at grain boundaries from a material micromechanical perspective. This microstructure allows the final welded joint to better coordinate strain under load, thereby achieving comprehensive mechanical properties that better meet engineering requirements.

[0027] This invention provides a laser welding method for TWIP steel based on figure-eight beam oscillation, which has the following beneficial effects: 1. This invention induces continuous convective eddies within the molten pool by driving a laser spot to move in a figure-eight trajectory in real time. This stirring effect increases the shear rate of the fluid within the molten pool. When the shear rate exceeds a specific critical threshold, it disrupts the solute-rich layer at the solid-liquid interface, promoting the full and uniform distribution of alloying elements in the molten metal. This fluid dynamic process blocks the thermodynamic and kinetic conditions for the precipitation of γ-(Fe,Mn)3C-type hard and brittle carbides, fundamentally eliminating the embrittlement phenomenon easily caused by conventional welding and ensuring the low-temperature toughness of the joint under cryogenic extreme service environments.

[0028] 2. The oscillating beam of this invention alters the heat transfer state during the welding process, regulating the ratio of the temperature gradient at the solidification front to the solidification rate. Combined with the thermo-mechanical coupling effect generated by the moving heat source, corresponding deformation energy is accumulated within the heat-affected zone, inducing dynamic recrystallization. This change not only overcomes the defect of softening in the heat-affected zone during conventional TWIP steel welding, resulting in a smaller grain size in the heat-affected zone than the base material, but also promotes the formation of a dual-scale microstructure after joint solidification, consisting of equiaxed columnar crystals at the edges and ultrafine equiaxed crystals at the center. This resolves the material contradiction that strength improvement in traditional fine-grained strengthening often accompanies a decrease in plasticity.

[0029] 3. The dual-scale synergistic microstructure and grain-refined heat-affected zone of this invention reshape the mechanical load-bearing path of the welded joint. The optimized microstructure eliminates stress concentration effects and delays the yield response of the specimen, resulting in the overall yield strength and tensile strength of the joint exceeding those of the base material. The structural load-bearing capacity is enhanced by actively shifting the tensile failure location from the traditional heat-affected softening zone to the weld center, completely changing the premature fracture failure mode of TWIP steel welded joints and fully meeting the reliability assembly requirements of aerospace and extreme transportation equipment for high-strength, high-toughness, and high-energy-absorbing structural components. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the welding process and spatial orientation of the present invention; Figure 2 This is a schematic diagram showing the sampling scheme and dimensions of the test specimens for this invention. Figure 3 This is a diagram showing the spatial motion trajectory of the laser spot in this invention. Figure 4 Microscopic image of the welded joint of the present invention; Figure 5 This is a comparison diagram of stress-strain curves in the tensile test of this invention; Figure 6 This is a cross-regional microhardness distribution curve of the joint of the present invention; Figure 7 This is a Charpy impact absorption energy distribution diagram of the connector of the present invention at -196℃. Figure 8 This is a distribution diagram of the quasi-static tensile properties of the joint at room temperature according to the present invention, wherein (a) is a distribution diagram of the tensile strength and yield strength of the joint at room temperature, and (b) is a distribution diagram of the elongation after fracture of the joint at room temperature. Figure 9 The diagram shows the dynamic mechanical response and energy absorption distribution of the joint of the present invention under high strain rate loading, wherein (a) is the dynamic true stress-strain response diagram of the joint under high strain rate, and (b) is the scatter plot and mean distribution of the high-frequency impact energy absorption of the joint; Figure 10 The distribution diagram of non-destructive testing results of the internal quality of the joint of the present invention is shown in (a) and (b) is the distribution diagram of the area ratio of pore defects inside the joint. Detailed Implementation

[0031] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] Preparation Examples 1-3: Preparation Example 1: This preparation example provides a method for preparing butt weld specimens of TWIP steel, including the following steps: Material preparation: Select 2.5mm thick Fe-Mn-C series TWIP steel plate, whose chemical composition by mass percentage is as follows: Mn: 20.0%, C: 0.6%, balance Fe and unavoidable impurities; Cutting and shaping: The above steel plate is processed into a rectangular sample of standard size (such as 100mm×50mm) by wire cutting or laser cutting, and the side to be joined is precision machined to ensure the flatness and perpendicularity of the joint end face; Surface cleaning: Use an angle grinder or sandpaper (e.g., 400 to 800 grit) to grind the upper and lower surfaces of the sample to be welded and the weld seam within a range of at least 10 mm to completely remove the oxide scale on the surface; then immerse the ground sample in an ultrasonic cleaning tank containing acetone or anhydrous ethanol for 10-15 minutes to remove processing oil and dust, and dry it with cold air for later use. Clamping and positioning: Place the two processed butt joint samples flat in the fixture of the high-precision welding workbench, use feeler gauges for calibration, strictly control the butt joint gap to 0.10mm, and then apply mechanical clamping force for rigid fixation to complete the preparation of the parts to be welded.

[0033] Preparation Example 2: This preparation example provides a method for preparing butt weld specimens of TWIP steel, including the following steps: Materials preparation: A 1.5mm thick Fe-Mn-C series TWIP steel plate was selected, with the following chemical composition by mass percentage: Mn: 15.0%, C: 0.4%, and the balance being Fe and unavoidable impurities; Cutting and shaping: The above steel plates are processed into rectangular samples of standard size, and the sides to be joined are precision machined to ensure that the joint end faces are flat; Surface cleaning: Remove oxide scale from the end face to be welded and both sides of the weld by mechanical grinding, then place in an ultrasonic cleaner and clean with acetone for 15 minutes to remove oil stains, and dry with cold air for later use. Clamping and positioning: Place the two prepared thin plate samples flat on the welding fixture, use a precision feeler gauge for calibration, strictly control the butt gap to the lower limit of 0.05mm, apply clamping force to fix them to prevent the thin plates from deforming during welding, and complete the preparation of the parts to be welded.

[0034] Preparation Example 3: This preparation example provides a method for preparing butt weld specimens of TWIP steel, including the following steps: Material preparation: Select 5.0mm thick medium-thick Fe-Mn-C series TWIP steel plate, whose chemical composition by mass percentage is as follows: Mn: 25.0%, C: 0.8%, balance being Fe and unavoidable impurities; Cutting and shaping: The above steel plate is processed into a rectangular sample of standard size, and the sides to be joined are precision machined (for 5.0mm thick plates, they can be processed into I-type or micro Y-type bevels according to actual needs; in this embodiment, I-type joint is maintained) to ensure that the joint end face is straight; Surface cleaning: Use mechanical grinding to deeply clean the oxide layer on the end face and both sides of the surface to be welded, put it into the ultrasonic cleaning tank and clean it with acetone for 15 minutes to remove oil stains, take it out and blow it dry for later use. Clamping and positioning: Place the prepared sample in the welding fixture, use a feeler gauge for calibration, strictly control the butt gap to the upper limit of 0.15mm, apply mechanical clamping force for rigid constraint, and complete the preparation of the part to be welded.

[0035] Examples 1-5: Example 1: This embodiment provides a TWIP steel laser welding method based on figure-eight beam oscillation, including the following steps: Sample preparation and gas protection: Take the 2.5 mm thick butt weld sample prepared in Example 1 above and fix it on the automated welding platform. High-purity argon gas with a purity ≥99.99% is continuously introduced as a protective gas above and behind the welding area, and the argon gas flow rate is controlled at 15 L / min; Welding equipment and parameters configuration: A fiber laser with a two-dimensional galvanometer scanning system was used. The welding parameters were set as follows: laser power 1950W, welding speed 1.0m / min, defocusing amount 0mm, and laser beam reverse tilt angle (tilted in the opposite direction of welding) 10°. During the welding process, the laser power was matched and set according to the thickness of the TWIP steel sample to be welded, and the laser energy density satisfied the following relationship: ; In the formula: The energy density of the laser volume. This refers to the output power of the fiber laser. For welding speed, The effective diameter of the laser spot. The thickness of the TWIP steel sample.

[0036] By adjusting the laser output power, the laser energy density (EV) is maintained within a certain range (20-50 J / mm²). 3 This ensures consistent weld penetration and uniform weld structure for TWIP steels of different thicknesses.

[0037] Set oscillation parameters and perform welding: Start the galvanometer system and set the oscillation frequency. =200Hz, oscillation amplitude =0.5mm. At this point, the product of frequency and amplitude... =100Hz·mm (falling within the 45-175Hz·mm range). The laser spot moves in real time according to an 8-shaped trajectory equation: ; ; Physical state control and forming: The molten pool is forcibly stirred by the above-mentioned figure-eight high-frequency oscillation, and the shear rate of the molten pool is controlled to reach approximately 1500 s. -1 (≥1200s) -1 This improves elemental segregation and simultaneously increases the ratio of the temperature gradient G to the solidification rate R at the weld solidification front to approximately 300 K·s / mm. 2 As the heat source moves, the weld cools and solidifies, ultimately yielding a high-strength TWIP steel joint with a dual-scale microstructure consisting of equiaxed columnar crystals and central ultrafine equiaxed crystals, free of hard and brittle carbides.

[0038] Example 2: This embodiment provides a TWIP steel laser welding method based on figure-eight beam oscillation, including the following steps: Sample preparation and gas protection: Take the 1.5 mm thick butt weld sample prepared in Example 2 above. Turn on high-purity argon gas for all-around protection, with a flow rate set to 12 L / min; Configure welding equipment and parameters: Set basic laser welding parameters: Laser power 1800W, welding speed 1.1m / min, defocusing amount 1mm, laser beam reverse tilt angle 5°; Set the oscillation parameters and perform the welding: Set the lower limit parameter for oscillation: oscillation frequency =150Hz, oscillation amplitude =0.3mm, at this time the product =45Hz mm. The driving galvanometer system moves in space according to the figure-eight trajectory equation claimed in the claim; Physical state control and forming: Under this combination of low power and low amplitude, the melt pool shear rate is controlled above the critical point (1250s). 1 The G / R ratio is approximately 150K. s / mm 2 After the weld solidifies, dynamic recrystallization occurs in the heat-affected zone without softening, resulting in a TWIP steel sheet welded joint with a good overall strength and toughness match.

[0039] Example 3: This embodiment provides a TWIP steel laser welding method based on figure-eight beam oscillation, including the following steps: Sample preparation and gas protection: Take the 5.0 mm thick plate butt welding sample prepared in Example 3 above. Turn on high-purity argon gas for high-flow protection, with the flow rate set to 20 L / min; Configure welding equipment and parameters: Set the basic parameters for laser welding within the upper limit range: laser power 2100W to ensure sufficient penetration, welding speed 0.8m / min, defocusing amount +1mm, laser beam reverse tilt angle 15°; Set the oscillation parameters and perform the welding: Set oscillation parameters: oscillation frequency =250Hz, oscillation amplitude =0.7mm, at this time the product =175Hz mm. The laser spot is controlled to move according to an 8-shaped Lissajous trajectory equation for powerful stir welding; Physical state control and forming: Under the action of this high-power strong oscillation, extremely strong convective vortices are formed inside the molten pool, and the shear rate surges to about 1800s. 1 And the G / R ratio is approximately 480K. s / mm 2 The component segregation in the deep melt pool is completely broken, and after cooling, a high-quality dual-scale microstructure joint with ultra-deep penetration and no carbide precipitation is obtained.

[0040] Example 4: This embodiment provides a TWIP steel laser welding method based on figure-eight beam oscillation, including the following steps: Sample preparation and gas protection: Take the 2.5 mm thick butt weld sample prepared in Example 1 above. Introduce high-purity argon gas for protection. Configure welding equipment and parameters: Set the basic parameters for laser welding: laser power 1900W, welding speed 0.9m / min, defocusing amount 0mm, laser beam reverse tilt angle 12°; Set oscillation parameters and perform welding: Employ a crossover strategy combining high frequency and small amplitude: Set the oscillation frequency =250Hz, oscillation amplitude =0.3mm, product =75Hz mm. High-speed micro-amplitude oscillation welding is performed strictly according to the above figure-eight trajectory equation; Physical state control and forming: In this state, high-frequency micro-oscillation reduces the fluctuation of heat input to the molten pool and maintains the shear rate at approximately 1350 s. 1 The G / R ratio is approximately 280K. s / mm 2 This induces deformation and energy storage accumulation in the heat-affected zone, resulting in grain refinement in the HAZ region.

[0041] Example 5: This embodiment provides a TWIP steel laser welding method based on figure-eight beam oscillation, including the following steps: Sample preparation and gas protection: Take the 2.5 mm thick butt weld sample prepared in Example 1 above. Introduce high-purity argon gas for protection. Configure welding equipment and parameters: Set basic laser welding parameters: laser power 2000W, welding speed 1.2m / min, defocusing amount 0.5mm, laser beam reverse tilt angle 8°; Set oscillation parameters and perform welding: Employ a low-frequency combined with a large-amplitude crossover strategy: Set the oscillation frequency. =150Hz, oscillation amplitude =0.7mm, product =105Hz mm. The laser beam oscillates slowly over a wide range in a figure-eight pattern; Physical state control and forming: Large-amplitude, wide-frequency oscillations extended the molten pool width, while the shear rate remained at approximately 1400 s. 1 The G / R ratio is approximately 350K. s / mm 2 The welding process was smooth, with no spatter or undercut, resulting in a dual-scale welded joint with a wide weld and a very high proportion of equiaxed grains in the center.

[0042] Comparative Examples 1-4: Comparative Example 1: Compared with Example 1, the difference is that the oscillation function of the laser galvanometer is turned off, and direct welding is performed using a conventional straight-line trajectory; otherwise, they are the same.

[0043] Comparative Example 2: Compared with Example 1, the difference is that the "figure-eight" oscillation trajectory of the laser spot is changed to a "circular" oscillation trajectory, while the oscillation frequency and amplitude parameters remain the same, and all other parameters are the same.

[0044] Comparative Example 3: Compared to Example 1, the difference lies in that: the oscillation frequency is set to 100Hz and the oscillation amplitude is 0.3mm, so that the product... =30Hz·mm (far below the lower limit of 45Hz·mm in the claims), all other values ​​are the same.

[0045] Comparative Example 4: Compared to Example 1, the difference lies in that: the oscillation frequency is set to 300Hz and the oscillation amplitude is 0.8mm, so that the product... =240Hz·mm (far exceeding the upper limit of 175Hz·mm in the claims), all other values ​​are the same.

[0046] Test Examples 1-5: Test Example 1: Test steps: Samples from Examples 1, 3, Comparative Example 1, and Comparative Example 3 that have been welded were selected as test objects. A cross-sectional sample with a complete joint morphology was cut from the middle of the sample along the direction perpendicular to the weld.

[0047] The heat-affected zone and weld samples were placed in a hot mounting machine for shaping. They were then unidirectionally ground with 400-2000 grit silicon carbide wet sandpaper until the surface scratches were uniform. Subsequently, they were mechanically polished with diamond polishing liquid until the surface was mirror-like. Residual particles on the surface were cleaned and dried.

[0048] A Vickers microhardness tester was used to perform continuous spot tests across the thickness centerline of the cross-section. The test load was set to 200 gf, and the holding time was set to 15 s. The test path started from the left base metal area, passed through the left heat-affected zone, the weld center, the right heat-affected zone, and finally extended to the right base metal area.

[0049] Using the weld center as the zero point, distances in the left and right directions are marked with positive and negative values, and the spacing between adjacent hardness test points is set to 0.4 mm. The diagonal indentation size at each test point is read and recorded, calculated and converted into the corresponding Vickers hardness value (HV), and the hardness distribution profile of the area is summarized and drawn.

[0050] Test data: Table 1. Distribution of microhardness test data in the joint area of ​​each sample in conclusion: Figure 6 This is a cross-regional microhardness distribution curve of the joint of the present invention. The figure includes the transverse hardness test data change trajectory of Comparative Example 1, which uses conventional linear welding, Comparative Example 3, whose oscillation parameters are outside the lower limit range, and Examples 1 and 3, which are welded according to the parameters within the process limits of the present invention.

[0051] Based on the data distribution trend in Table 1 and Figure 6The evolution of the feedback curve profile shows that different conditions introduced into the molten pool cause different mechanical responses to the interference effect on the microstructure of adjacent areas of the joint. In the test paths of Comparative Example 1 and Comparative Example 3, when the hardness tester probe enters the core position of the heat-affected zone at a distance of ±1.6 mm from the center line, the reading drops below 200 HV.

[0052] At this point, heat conduction dominates the changes in this region, and the lack of fluid shear force leads to static grain growth within the austenitic phase transformation temperature range. The decreased deformation resistance of the coarsened structure makes it easier for crack initiation to occur at the joint in this region. This weakened deformation resistance of the coarsened structure becomes a structural weakness in the assembly level of engineering structures, and crack initiation at the joint is common in conventional tests. In contrast, the data distribution trajectories of Examples 1 and 3 show that this softening collapse was not only contained, but the local hardness even showed a reverse increase to a hardening band of 280 HV in the heat-affected zone. This reversal of mechanical properties confirms the hypothesis that the shear process is conducted to the solid periphery.

[0053] The interlaced, figure-eight-shaped high-frequency beams oscillated and stirred the central liquid phase, while the boundary fluid exerted intense shear stress friction on the adjacent base material. High-density deformation dislocation entanglement provided the intrinsic driving force for dynamic recrystallization. The large number of equiaxed fine grains generated in situ offset the growth tendency induced by simple thermal cycling, completing a metallurgical phase transformation compensation dependent on mechanical energy intervention in microscopic evolution. In comparison, the specific vibration frequency threshold did not act in isolation within the weld pool; the deformation energy storage caused by its boundary disturbance altered the traditional annealing degradation pattern of the heat-affected zone, improving the original load-bearing weak areas of the joint.

[0054] Test Example 2: Test steps: Welded joints from Examples 1, 2, 1, and 2 were selected as test objects. Samples were taken along a plane perpendicular to the weld by wire cutting and processed into Charpy V-notch standard impact specimens with dimensions of 10mm × 10mm × 55mm. The notch depth was set to 2mm and the bottom radius of curvature was 0.25mm. During processing, the root of the notch was aligned with the central axis of the weld cross-section.

[0055] Prepare an ultra-low temperature liquid nitrogen immersion cooling bath and completely immerse each group of impact test samples in the liquid nitrogen bath. The test samples are kept at a constant temperature of -196℃ in the liquid environment for 40 minutes to ensure that the internal temperature field of the material reaches a completely uniform state.

[0056] Dynamic fracture testing was performed using a pendulum impact testing machine. A special fixture with an insulation layer was used to remove the specimen from the liquid nitrogen and quickly transfer it to the anvil of the testing machine for centering. The contact time between the specimen leaving the liquid nitrogen surface and the release of the pendulum was controlled within 4 seconds.

[0057] Record the impact absorption energy fed back by the dial or sensor at the moment of fracture, and collect the fractured sample. Repeat the test on three parallel samples under each parameter system and read the test results.

[0058] Test data: Table 2. Charpy impact energy distribution of each group of samples at -196℃ in conclusion: Figure 7 This is a Charpy impact absorption energy distribution diagram of the connector of the present invention at -196℃. The diagram shows the actual test values ​​of each parallel sample in Example 1, Example 2, and Comparative Examples 1 and 2 using discrete solid dots, and connects the average absorption energy trends of each group of test parameters with solid lines containing hollow squares.

[0059] According to the data in Table 2, Comparative Example 1, which was conventionally welded directly, showed a significant decrease in toughness at -196℃, with the average impact absorption energy falling back to the range of 31.2 J. During the traditional solidification process of high-manganese steel, the solute partition constant deviates from equilibrium, leading to the enrichment of carbon and manganese atoms at the solid-liquid interface. This microscopic non-uniform elemental distribution easily triggers phase transformation during the cooling stage, generating a network of manganese-containing carbide particles.

[0060] Static tensile testing at room temperature can sometimes mask some structural defects by utilizing the coordinated deformation of residual austenite. When the service temperature enters the cryogenic range, the thermally activated slip system inside the matrix is ​​suppressed, and the tiny carbide particles are transformed into stress concentration sources, thereby inducing rapid crack propagation.

[0061] Example 1 and Example 2 exhibit different load-bearing performances. Even at the lowest energy input process boundary, the impact energy remains stably above 90 J. The dynamic shear force generated by high-frequency mechanical oscillation exceeds the critical threshold, and the solute layer originally intended to accumulate at the solidification front is dispersed by the alternating flow field and drawn into the high-temperature liquid region. The homogenized composition field cuts off the thermodynamic channels for brittle phase nucleation, and the joint does not undergo brittle transition behavior under cryogenic conditions.

[0062] The effect of fluid disturbance in a single dimension is limited. Comparative Example 2 uses a circular trajectory scan, which, while partially improving fracture dissipation energy, increases numerical dispersion. Conventional rotating flow fields tend to stagnate in the central region of the molten pool, and the edge interface layer lacks sufficient shear rate gradient to scour the solute. The spatial asymmetric convection established by the specific figure-eight trajectory repeatedly intervenes in atomic diffusion within the region, and the coupling setting of process parameters and trajectory equations provides an organizational basis for suppressing low-temperature embrittlement.

[0063] Test Example 3: Test steps: Welded joints prepared in Examples 1 to 5 and Comparative Examples 1 to 4 were selected as experimental subjects. The welded templates were machined into non-proportional flat tensile specimens in the shape of a dog bone using a slow wire EDM machine along a direction perpendicular to the weld seam, ensuring that the weld seam was located at the exact center of the parallel section of the specimen.

[0064] The side cut surfaces of the tensile specimens were longitudinally polished with sandpaper to eliminate microcracks and surface remelting layers remaining from the wire cutting process. The actual width and thickness of the parallel segments of each specimen were measured with a micrometer.

[0065] A quasi-static tensile test at room temperature was performed on a computer-controlled electronic universal testing machine. The upper and lower clamps were fixed at both ends of the specimen using hydraulic wedge clamps. A contact extensometer with a gauge length of 50 mm was installed on the parallel section of the specimen to record the actual deformation.

[0066] The crossbeam movement speed of the testing machine was set to 2.0 mm / min, and an axial tensile load was applied until the specimen completely fractured. The testing machine automatically recorded the load-displacement data and converted it into an engineering stress-strain curve.

[0067] Collect the fractured samples, record the location of the fracture, and measure the gauge length after fracture by splicing the fracture surface. Calculate the tensile strength, yield strength, and elongation after fracture for each group.

[0068] Test data: Table 3. Quasi-static tensile properties and failure location distribution of the specimens at room temperature in conclusion: Based on the measured data in Table 3 and Figure 8 As shown in the distribution in (a), Comparative Example 1, which uses conventional straight-line welding, exhibits a mechanically weak zone, with its tensile strength dropping to 645.3 MPa. Furthermore, all tensile failure sites are located within the heat-affected zone (HAZ). The room-temperature plastic deformation of TWIP steel depends on the formation of deformation twins during loading. The austenite grain coarsening induced by the welding thermal cycle in the HAZ weakens the work hardening capacity of the matrix, leading to a concentration of plastic deformation and early fracture at this location. This is reflected in… Figure 8 In (b), the elongation after fracture of Comparative Example 1 showed a trough.

[0069] The yield strength of Examples 1 to 5 all reached over 450 MPa. High-frequency figure-eight oscillations transferred mechanical energy to the liquid phase, and the melt in the central region altered the crystallization process under shear rate intervention. The temperature gradient to solidification rate ratio established at the solid-liquid front blocked the path of epitaxial columnar crystals extending towards the center, forming a distributed equiaxed crystal network at the weld axis. The grains in the surrounding HAZ region were repaired under this intervention, and the overall load-bearing path of the joint was redistributed. This resulted in the elongation of the example group remaining stably above 43%, and the fracture location shifting from the heat-affected zone to the base metal region or the weld center.

[0070] Increasing external energy input cannot continuously improve the load-bearing capacity of materials. Figure 8 The decrease in data at the corresponding position in Comparative Example 4 reflects the impact of parameter exceeding limits. When the product of frequency and amplitude climbed to 240 Hz·mm, the extremely turbulent eddies entrained the protective gas into the liquid metal. The rapid interface propagation speed during the cooling stage hindered bubble overflow, leaving pores inside. Even with stirring, the structural discontinuity still caused the tensile specimen to fracture brittlely at 592.1 MPa, with the elongation dropping to 12.8%. This indicates that process parameters need to be maintained within a specific range; excessive deviation from the set thermo-coupling intervention cannot break the segregation barrier of solute elements, but instead increases the structural defect rate.

[0071] Test Example 4: Test steps: Welded joints from Example 1, Comparative Example 1, and Comparative Example 2 were selected as test objects. Miniature dynamic tensile specimens suitable for the Split Hopkinson Tension Bar (SHTB) test were machined by wire cutting along a direction perpendicular to the weld direction, ensuring that the center of the gauge length was completely coincident with the weld axis. The surface of the specimen was ground until there were no obvious machining scratches, and the initial cross-sectional area and length of the gauge length were recorded using vernier calipers.

[0072] Apply an appropriate amount of high-frequency vacuum grease to the threads at both ends of the sample to reduce acoustic impedance mismatch during stress wave transmission, and then assemble it between the incident rod and the transmission rod of the SHTB system.

[0073] Adjust the pressure in the high-pressure chamber to the preset value, release the impact rod to generate a high-frequency dynamic loading stress wave, and control the nominal loading strain rate of the specimen at approximately 1500 s by controlling the air pressure. 1 Order of magnitude.

[0074] The voltage signals of incident, reflected, and transmitted waves are collected in real time using semiconductor strain gauges attached to the surfaces of the incident and transmitted rods, and the voltage signals are synchronously converted into digital strain signals using an ultra-dynamic strain gauge.

[0075] Based on the one-dimensional elastic stress wave theory, the collected signals are decoupled and processed to calculate the dynamic true stress-strain curves of each group of samples. The area under the curve is numerically integrated to obtain the dynamic energy absorbed per unit volume of the sample before fracture, i.e., the specific energy absorption index.

[0076] Test data: Table 4. Connectors with different processes at approximately 1500s 1 Dynamic mechanical response and energy absorption data under strain rate in conclusion: Figure 9 This is a diagram showing the dynamic mechanical response and energy absorption distribution of the joint of the present invention under high strain rate loading. Among them, Figure 9 (a) Demonstrates Example 1, Comparative Example 1, and Comparative Example 2 at approximately 1500 s 1 The dynamic real stress-strain curve under strain rate is marked with a gray shaded area to indicate the energy absorption area corresponding to the curve integral, and the fracture instability location of the specimen is marked by a cross mark. Figure 9 (b) shows the dynamic specific energy absorption measured scatter distribution and mean evolution trend of the parallel tensile specimens corresponding to the above three groups of objects.

[0077] According to the data in Table 4, aerospace or automotive crash protection components are subjected to high strain rate loads during service. The energy absorption index under dynamic conditions reflects the material's ability to resist crack propagation. Comparative Example 1, with conventional straight welding, fractured earlier under this loading environment, with an average dynamic specific energy absorption of only 99.4 MJ / m. 3 The high-frequency impact process does not have the dislocation multiplication time of room temperature tension. The heat-affected zone of TWIP steel becomes a stress wave acoustic impedance distortion zone under dynamic load. Unrefined grains cannot block plastic deformation, reducing the energy dissipation capacity of the structure.

[0078] The mechanical feedback exhibited in Example 1 under impact verifies the role of fluid dynamics in microforming. (See Table 4 and...) Figure 9 As can be seen, the dynamic specific energy absorption of Example 1 is increased to 450 MJ / m. 3 The dynamic peak stress reached 1200 MPa. The figure-eight oscillations at the center of the melt pool, forming equiaxed crystals, and the reshaped columnar crystals at the edges created a grain boundary network. These dense grain boundaries act as obstacles to dislocation slip at high strain rates, stimulating the matrix to activate the TWIP effect and generate deformation twins. The formation of twin boundaries increased the dynamic work hardening rate of the material, delayed necking, and allowed the joint to absorb more impact energy through uniform plastic deformation before fracture. While Comparative Example 2, using a circular oscillation trajectory, showed some improvement, its performance lagged behind the example, and the data exhibited a discrete state.

[0079] A single circular flow field generates a relatively uniform shear strain rate gradient in the liquid phase, failing to alter the solute distribution at the solidification front. Local microstructural inhomogeneities evolve into micropore nucleation points during high strain rate testing, hindering stress transfer. The leap in dynamic mechanical properties depends not only on the scale of heat input, but also on the alternating eddies generated by the figure-eight trajectory, which play a crucial role in reshaping the solidification structure and enhancing the material's energy absorption potential.

[0080] Test Example 5: Test steps: Examples 1 to 5, and the welded sample of Comparative Example 4 with corresponding out-of-tolerance parameter settings, were selected as test objects for non-destructive testing. Mechanical grinding equipment was used to remove excess weld thickness on both sides of each sample to ensure that the base material and weld thickness in the test area remained consistent. Subsequently, anhydrous ethanol was used to clean the surface to eliminate interference from external deposits on the imaging.

[0081] X-ray digital imaging (DR) non-destructive testing system was used to perform full-range quality inspection of the sample interior. Based on the sample thickness, the X-ray tube voltage was set to 120kV, the tube current to 3.5mA, the focal length to 600mm, and the detector exposure time to 2 seconds to obtain a high signal-to-noise ratio two-dimensional projection digital image.

[0082] The acquired grayscale X-ray images are imported into industrial defect recognition software. A local adaptive threshold segmentation algorithm is used to extract the contours of discontinuous low-density grayscale abnormal regions (such as pores and cracks) in the images.

[0083] The software automatically calculates and outputs the equivalent diameter data of defects in each sample evaluation area, compares the total pixel area of ​​all extracted defects with the total pixel area of ​​the weld area, and finally calculates the proportion of porosity defect area that reflects the overall tightness of the joint.

[0084] Test data: Table 5. Distribution of Non-destructive Testing Identification Data for Internal Defects in Each Group of Samples in conclusion: Figure 10 This is a distribution diagram of the non-destructive testing results of the internal quality of the joint of the present invention. Among them, Figure 10 (a) The total area percentage of pores inside each joint is shown by combining a grayscale bar chart with numerical labels. Figure 10 (b) The trajectory of the change in the equivalent diameter of the largest defect identified inside was traced by a line graph marked with solid diamonds.

[0085] Based on the detection indicators presented in Table 5 and Figure 10The resulting surge in magnitude reflects a strict nonlinear boundary between the joint forming quality and the externally input mechanical intervention parameters. Within the parameter closed loop defined in Examples 1 to 5, despite the high-frequency oscillations applying intense dynamic agitation to the molten pool, the defect rate inside the weld remains firmly suppressed to an extremely low level below 0.2%. At this point, the thermo-coupling field is in dynamic equilibrium; the figure-eight flow field accelerates the convective heat transfer of the molten metal, and the uniformly distributed temperature gradient prolongs the escape time window of bubbles at the solidification interface front. The equivalent diameter of the tiny residual pores does not exceed 0.3 mm. This level of micro-discontinuity is far below the flaw detection and acceptance threshold for conventional engineering applications. It not only does not substantially reduce the load-bearing area of ​​the joint but also indirectly confirms that a reasonably set fluid shear boundary can accommodate or even optimize the gas overflow mechanism during solidification.

[0086] The consequences of disrupting this dynamic equilibrium are quantified in the test data of Comparative Example 4. When the product of oscillation frequency and amplitude exceeds the limit and rises to 240 Hz·mm, the tightness of the joint fundamentally collapses. A porosity of up to 4.82% and a pore structure with an equivalent diameter of 2.56 mm indicate that the hydrodynamic morphology inside the molten pool has evolved from controlled convection to extremely turbulent vortices. Violent interfacial tumbling, carrying external protective gas and metal vapor, forcibly entrains them into the high-temperature liquid center. The shear strain rate exceeding the critical value destroys the liquid feeding channels during solidification contraction, and the excessively rapid local interfacial advance speed locks the path for large bubbles to overflow to the surface.

[0087] This data intuitively explains the mechanical root cause of the low-stress brittle fracture in Comparative Example 4 during the aforementioned tensile test. Process intervention is not a blind accumulation of energy; the kinetic output deviating from a specific range will undermine the theoretical benefits of microstructural refinement. Data feedback from non-destructive testing directly confirms the findings in this invention. ≤175Hz The upper limit of the mm parameter is necessary to ensure the engineering rigidity and density level.

[0088] Combination Figure 1 In the implementation environment shown, a high-energy laser beam acts on the surface of a steel plate along a specified welding direction, and the spot does not maintain a conventional straight-line movement within the spatial coordinate system formed by the normal (ND), transverse (TD), and rolling direction (RD).

[0089] like Figure 3 As shown, the light spot performed a specific '8' shaped oscillation trajectory on the two-dimensional plane. It is this controlled flow field perturbation that guides the benign evolution of the internal microstructure.

[0090] To obtain accurate performance data, such as Figure 2 As shown, specialized microstructure specimens and tensile specimens were prepared in specific areas of the formed weld.

[0091] pass Figure 4 Microscopic morphological observation confirms that a typical dual-scale microstructure distribution is formed inside the weld within the parameter closed loop. The edge region exhibits dense columnar dendrites, while the central region of the molten pool shows dense equiaxed dendrites. This reshaping of the microstructure is directly projected onto the macroscopic mechanical properties.

[0092] Figure 5 The engineering stress-strain curves clearly reflect this gain effect. Compared with conventional joints obtained in a non-oscillation state, the tensile stress and strain test indicators of the specimens using the figure-eight oscillation process are both increased, even highly approaching the intrinsic mechanical properties of the base material (BM). This fully confirms the advanced nature of the above-mentioned fluid shear boundary in practical engineering applications.

Claims

1. A TWIP steel laser welding method based on figure-eight beam oscillation, characterized in that, Includes the following steps: Two TWIP steel butt joint samples to be welded are prepared. The two TWIP steel butt joint samples are placed flat on the welding fixture. The butt joint gap between the two TWIP steel butt joint samples is controlled. Mechanical clamping force is applied to rigidly fix the two TWIP steel butt joint samples to be welded, thus completing the preparation of the parts to be welded. A protective gas is introduced into the welding area of ​​the workpiece to be welded; A fiber laser is used in conjunction with a two-dimensional galvanometer scanning system to perform welding on the workpiece under a protective gas atmosphere. The reverse tilt angle of the laser beam is set, and the two-dimensional galvanometer scanning system is set to drive the laser spot to move in real time along an 8-shaped trajectory. The oscillation frequency and amplitude of the two-dimensional galvanometer scanning system are controlled. The molten pool is forcibly stirred by the laser spot moving in real time along the 8-shaped trajectory. After cooling and solidification, a dual-scale welded joint is obtained.

2. The TWIP steel laser welding method based on figure-eight beam oscillation according to claim 1, characterized in that, The TWIP steel butt weld samples were prepared from Fe-Mn-C series TWIP steel plates with a thickness of 1.5-5.0 mm. The chemical composition of the Fe-Mn-C series TWIP steel plates, by mass percentage, was as follows: Mn: 15.0%-25.0%, C: 0.4%-0.8%, balance being Fe and unavoidable impurities.

3. The TWIP steel laser welding method based on figure-eight beam oscillation according to claim 2, characterized in that, The steps for preparing two butt joint test specimens of TWIP steel to be welded include: The Fe-Mn-C TWIP steel plate was processed into a rectangular sample, and the side of the rectangular sample to be joined was precision machined. The oxide scale on the weldable end face and both sides of the weld of the rectangular sample is removed by mechanical grinding. The ground rectangular sample is then placed in an ultrasonic cleaning tank and cleaned with acetone or anhydrous ethanol. After being dried with cold air, the TWIP steel butt sample to be welded is obtained.

4. The TWIP steel laser welding method based on figure-eight beam oscillation according to claim 1, characterized in that, The numerical range of the docking gap is 0.05-0.15mm.

5. The TWIP steel laser welding method based on figure-eight beam oscillation according to claim 1, characterized in that, The protective gas is high-purity argon, with a purity ≥ 99.99%, and the flow rate of the protective gas is set to 12-20 L / min.

6. The TWIP steel laser welding method based on figure-eight beam oscillation according to claim 1, characterized in that, The welding speed of the fiber laser is set to 0.8-1.2 m / min, the defocusing amount of the fiber laser is set to -1 mm to +1 mm, and the laser beam back tilt angle is set to 5°-15°. The laser power during welding is set according to the thickness of the TWIP steel sample to be welded, and the laser energy density satisfies the following relationship: ； In the formula: The energy density of the laser volume. This refers to the output power of the fiber laser. For welding speed, The effective diameter of the laser spot. The thickness of the TWIP steel sample; By adjusting the laser output power, the laser energy density (EV) is maintained within a certain range (20-50 J / mm²). 3 This ensures consistent weld penetration and uniform weld structure for TWIP steels of different thicknesses.

7. The TWIP steel laser welding method based on figure-eight beam oscillation according to claim 1, characterized in that, By adjusting the product of the amplitude and frequency of the figure-eight beam oscillation ( The oscillation frequency of the two-dimensional galvanometer scanning system is set to 150-250Hz, and the oscillation amplitude of the two-dimensional galvanometer scanning system is set to 0.3-0.7mm.

8. The TWIP steel laser welding method based on figure-eight beam oscillation according to claim 7, characterized in that, The oscillation frequency of the two-dimensional galvanometer scanning system is set to 200Hz and the oscillation amplitude of the two-dimensional galvanometer scanning system is set to 0.5mm; Alternatively, the oscillation frequency of the two-dimensional galvanometer scanning system is set to 150Hz and the oscillation amplitude of the two-dimensional galvanometer scanning system is set to 0.3mm.

9. The TWIP steel laser welding method based on figure-eight beam oscillation according to claim 7, characterized in that, The oscillation frequency of the two-dimensional galvanometer scanning system is set to 250Hz and the oscillation amplitude of the two-dimensional galvanometer scanning system is set to 0.7mm; Alternatively, the oscillation frequency of the two-dimensional galvanometer scanning system is set to 150Hz and the oscillation amplitude of the two-dimensional galvanometer scanning system is set to 0.7mm.

10. The TWIP steel laser welding method based on figure-eight beam oscillation according to claim 1, characterized in that, The dual-scale welded joint contains no hard and brittle carbides and has equiaxed columnar crystals and central ultrafine equiaxed crystals.

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