Efficient postweld-heat-treatment-free laser filler welding method for ultrahigh-strength steel

Abstract

The invention discloses an efficient postweld heat treatment-free ultrahigh strength steel laser filler welding method, which belongs to the technical field of laser welding, and adopts an iron-nickel alloy filler (containing 25-45% of Ni, 3-5% of Mn, trace Co, C and the like) with specific components to cooperate with a high-nitrogen protective atmosphere (N2 accounts for 75-90%) to promote the formation of austenite, so that the welding quality of the ultrahigh strength steel is improved. And after-welding ultrafast cooling (ethylene glycol-water mixed cooling medium, the cooling rate is about 100 DEG C / s) is combined, so that a welding seam directly obtains a tough structure which takes martensite as a main part and contains a small amount of retained austenite under the condition that after-welding heat treatment is not needed. According to the method, air holes are effectively inhibited (the porosity is smaller than 0.1%), the performance of a connector is remarkably improved, the tensile strength is larger than 1000 MPa, the yield strength is larger than 800 MPa, the hardness is larger than 450 HV, meanwhile, strength loss caused by softening or embrittlement in traditional welding is avoided, and high-quality, high-efficiency and heat-treatment-free laser welding of the ultrahigh-strength steel is achieved.

Description

Technical Field

[0001] This invention belongs to the field of laser welding technology, and particularly relates to a highly efficient laser filler welding method for ultra-high strength steel without post-weld heat treatment. Background Technology

[0002] Aerospace-grade ultra-high-strength steel, possessing both extremely high strength and sufficient ductility, is preferentially used in components with stringent requirements for weight reduction and load-bearing capacity, such as solid rocket engine casings, manned modules, and aircraft landing gear. The manufacture of these structures relies heavily on welding, with current processes primarily using tungsten inert gas (GTAW) and vacuum electron beam welding. GTAW suffers from high heat input, leading to significant residual stress and deformation; furthermore, its low arc energy density and shallow penetration mean that for wall thicknesses exceeding 3mm, beveling and multi-layer, multi-pass welding are necessary, requiring multiple steps including preheating, post-heating, dehydrogenation, and stress relief, resulting in extremely low efficiency.

[0003] Laser welding, with its advantages of concentrated energy, deep penetration in a single pass, and narrow heat-affected zone, holds immense potential in critical aerospace equipment. However, ultra-high-strength steel, with its high carbon equivalent, complex alloy system, viscous molten pool, extremely rapid cooling rate, large depth-to-width ratio, difficulty in bubble bubbling, and challenges in controlling bottom compositional uniformity, results in decreased weld toughness and plasticity. Porosity and cold cracking have become core bottlenecks hindering its engineering application. A key challenge in laser-welded joints of ultra-high-strength steel is that tensile tests show fracture along the heat-affected zone, with a significantly reduced elongation. To improve weld quality, post-weld heat treatment and laser shock blasting are typically used to reduce alloy element segregation and refine grain size. While these methods can improve the strength and toughness of welded joints, they are costly and cumbersome.

[0004] The vacuum laser welding method disclosed in patent CN120901474A effectively suppresses the plasma shielding effect by performing laser welding operations in a vacuum negative pressure environment, thereby significantly improving the utilization efficiency of laser energy and the welding penetration. However, this technical solution has certain limitations: its welding process relies on a specific vacuum environment and special equipment, and the requirements for the working environment are quite stringent, making it difficult for this method to achieve large-scale and universal engineering applications.

[0005] Patent CN114434005A discloses a laser welding and heat treatment method for ultra-high strength steel. This technical solution uses a weld metal with austenite stabilizing elements as filler material for laser welding, and utilizes post-weld heat treatment to obtain a welded joint with excellent mechanical properties. Although this process improves the welding quality, the method is complex to operate. The welded steel plate must be held at 850-1100℃ for 2-30 minutes, and the temperature before quenching cannot be lower than 500℃, with a cooling rate greater than 35℃ / s. This not only increases production steps and costs but also reduces production efficiency.

[0006] Patent CN115026429A discloses a method for improving the mechanical properties of lap welds in hot-formed ultra-high-strength steel with aluminum-silicon coatings by adding carbon-iron alloy powder and using laser welding. The core idea is to suppress the formation of high-temperature ferrite and promote the transformation of austenite to martensite, thereby increasing weld strength. However, without effective and rapid cooling of the weld, it is impossible to completely transform the austenite into martensite below the Ms point. Due to the low proportion of martensite in the weld microstructure, the tensile strength of the weld joint is only 701 MPa, which cannot meet the tensile strength requirements of weld joints for aerospace steel. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention proposes a highly efficient laser filler welding method for ultra-high strength steel without post-weld heat treatment. The method involves pre-treating the ultra-high strength steel plate welding sample, then mounting the pre-treated sample on a laser welding experimental platform using a welding fixture, and joining the weldable parts together. A uniformly mixed iron-nickel alloy is selected as the filler, and the laser welding system is activated. The laser melts the filler for welding. Simultaneously, a special gas that promotes the formation of more austenite in the weld microstructure is output to the weld pool through a gas vent. This special gas is a mixture of Ar and N2, which regulates and protects the weld pool microstructure. A cooling device is added 10 cm to the left of the welding torch to rapidly cool the weld pool after welding, forming the weld seam upon cooling. This method promotes the formation of more austenite in the molten pool through a vent pipe, and uses a cooling device to force rapid cooling of the molten pool area. By controlling the cooling rate of the weld, the influence on the microstructure of the heat-affected zone of the joint is reduced, the morphology of the solidification microstructure of the weld metal is changed, the grains are refined, the transformation of austenite to ferrite or pearlite is inhibited, and the transformation of austenite to martensite below the Ms point is promoted. This method can significantly improve the weld microstructure, increase the strength of butt welded joints, provide anti-brittle fracture welding technology for major projects such as deep-sea equipment and spacecraft, extend the service life of key components, simplify the welding process of ultra-high strength steel (eliminating the need for post-weld heat treatment), reduce manufacturing costs, and expand into the field of rapid solidification processing of other alloy systems.

[0008] To achieve the above objectives, the present invention provides the following technical solution: A highly efficient laser filler welding method for ultra-high strength steel without post-weld heat treatment includes the following steps: (1) Fill the spaces between the pretreated ultra-high strength steel samples with iron-nickel alloy filler; (2) Laser welding is performed under Ar+N2 mixed protective gas conditions; (3) Use a cooling device to cool the weld so that the austenite is completely transformed into martensite.

[0009] Optionally, the pretreatment includes the following steps: operating in a fume hood, thoroughly wiping both sides of the sample and all edges with acetone-soaked medical degreased cotton balls or special non-woven fabric, and quickly drying the wiped sample.

[0010] Optionally, by weight percentage, the iron-nickel alloy filler comprises: Mn: 3~5%, Ni: 25~45%, trace elements (C: 0.07~0.15%, Si: 0~0.04%, Co: 2~4%), with the balance being iron.

[0011] Furthermore, the ratio of the cross-sectional area of ​​the iron-nickel alloy filler to the maximum cross-sectional area of ​​the weld is 1.2:1.

[0012] Furthermore, the amount of the iron-nickel alloy filler accounts for 5-15% of the weld volume.

[0013] Optionally, in the Ar+N2 mixed protective gas, the volume percentage content of N2 is 75%~90%, and the remainder is Ar; The flow rate of the Ar+N2 mixed protective gas is 30L / min, and the angle between the vent pipe and the weld is 30~45°.

[0014] Optionally, the laser welding conditions are as follows: adjust the welding torch defocusing amount to 0mm~+20mm, the welding torch welding angle to 70~85°, set the root pass laser power to 5000~10000W, and the welding speed to 0.25~0.75mm / s.

[0015] Optionally, the cooling medium in the cooling device is ethylene glycol and an aqueous solution, wherein the ethylene glycol content is 30-50 wt.% and the purified water content is 50-70 wt.%.

[0016] Furthermore, the water flow rate of the cooling medium is 100L / h, the water gun nozzle radius is 0.5mm, and the angle between the water gun and the weld is 30~45°.

[0017] Furthermore, the cooling device cools the weld seam after welding at a cooling rate of 100℃ / s.

[0018] A laser-welded joint for ultra-high strength steel is prepared by the above-described method.

[0019] Optionally, the ultra-high strength steel laser-welded joint has a tensile strength of 1021~1163MPa, a yield strength of 809~897MPa, a hardness of 450~520HV, and a porosity of 0.02~0.1%.

[0020] Compared with the prior art, the present invention has the following advantages and technical effects: This invention employs an innovative gas metallurgical strategy, incorporating the highly austenitizing element N2 into the base protective gas Ar. This mixed gas is blown into the molten pool in a non-contact manner, achieving a dual effect: firstly, highly reactive nitrogen atoms directly dissolve into the high-temperature molten pool, undergoing in-situ alloying, effectively compensating for nitrogen loss during welding and ensuring the nitrogen content of the weld; secondly, the dissolved nitrogen acts as a powerful austenite stabilizer, significantly promoting the formation and retention of austenite in the molten pool. This precise metallurgical control achieved through a gas medium fundamentally optimizes the weld microstructure, thereby simultaneously improving its overall mechanical properties.

[0021] This invention employs a rapid cooling device with water as the cooling medium, added to the left side of the laser welding torch, to achieve active and efficient cooling control of the weld seam after welding. This innovative process works synergistically in two ways to fundamentally improve the quality of the welded joint. First, it significantly increases the supercooling of the liquid weld metal by greatly increasing the cooling rate of the weld seam and heat-affected zone. This greatly promotes the nucleation rate during solidification, effectively reduces the microsegregation of alloying elements, and thus inhibits grain coarsening and growth. The result is a uniform and fine microstructure, and by changing the solidification morphology, the grains are significantly refined, thereby improving the microstructure uniformity of the heterogeneous joint and ultimately enhancing the overall strength, plasticity, and formability of the welded joint. Second, this accelerated cooling strategy directly intervenes in the solid-state phase transformation process of the weld metal. It inhibits the transformation of austenite to soft phases such as ferrite or pearlite, forcing it to directly transform into high-strength martensite at a lower temperature (below the Ms point). Therefore, the final microstructure of the weld joint consists mostly of martensite supplemented by trace amounts of retained austenite. This combination of high strength and good toughness significantly improves the overall mechanical properties of the weld joint, ultimately resulting in a high-quality weld joint with strength and toughness matching that of the base metal.

[0022] In summary, the core advantage of this invention lies in achieving welding without post-weld processing, completely eliminating the post-weld treatment steps such as ultrasonic impact and laser quenching required in traditional processes. This fundamentally simplifies the welding process for ultra-high-strength steel and significantly reduces the overall manufacturing cost of laser-welded plates. This technological breakthrough is particularly suitable for fields with stringent requirements for structural reliability, lightweighting, and cost control, such as the welding and manufacturing of critical equipment like deep-sea pressure hulls and spacecraft load-bearing frames. By directly obtaining high-strength, high-toughness, and low-defect welded joints, the fatigue resistance and brittle fracture resistance of key structures can be effectively improved, extending their service life in extreme environments. Furthermore, the improved welding process parameters and rapid post-weld cooling principles adopted in this invention to improve the weld microstructure have good process versatility and can be further extended to the precision welding and rapid solidification forming of other high-performance metal materials such as titanium alloys and high-entropy alloys. Attached Figure Description

[0023] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is a process flow diagram of an efficient laser filler welding process for ultra-high strength steel without post-weld heat treatment, according to the present invention. Figure 2 This is a schematic diagram of a three-dimensional model of an efficient laser filler welding method for ultra-high strength steel without post-weld heat treatment, according to the present invention. Detailed Implementation

[0024] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0025] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0026] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0027] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0028] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0029] This invention discloses a highly efficient laser welding method for ultra-high strength steel without post-weld heat treatment, comprising the following steps: 1. Pretreatment of Ultra-High Strength Steel Specimens: The ultra-high strength steel substrate is processed into a 200×100×3mm welding test plate with a surface roughness of 3.2μm. The operation is carried out in a fume hood. Wearing nitrile gloves, use a dust-free, low-lint medical degreased cotton ball or special non-woven fabric, thoroughly soaked in acetone. Wipe both sides of the specimen and all edges thoroughly in one direction with moderate pressure, replacing the cotton ball after wiping each area to avoid re-applying contaminants. After removing surface impurities, the wiped ultra-high strength steel plate specimen is quickly dried and then placed in a dry place for welding. The pretreated ultra-high strength steel specimen plate is mounted on the laser welding experimental platform using a welding fixture, employing a butt welding placement method.

[0030] 2. Filler: By weight percentage, the iron-nickel alloy filler comprises: Mn: 3~5%, Ni: 25~45%, with the balance being iron and other trace elements: C: 0.07~0.15%, Si: 0~0.04%, Co: 2~4%. This filler is used to fill the 1mm gap between two ultra-high strength steel welded specimens. The weld volume percentage is 5~15%, and the ratio of the cross-sectional area of ​​the iron-nickel alloy filler to the maximum cross-sectional area of ​​the weld is 1.2:1.

[0031] 3. Delivery of protective gas: A uniform mixture of Ar and N2 gas, which promotes the formation of more austenite in the weld structure, is delivered to the welding site through a gas pipe. The volume percentage of N2 in the gas mixture is 75%~90%, and the remainder is Ar. The gas flow rate is 30L / min, and the angle between the gas pipe and the weld is 30~45°.

[0032] 4. Laser welding process parameter combination settings: Adjust the welding torch defocusing amount from 0mm to +20mm, the welding torch welding angle from 70 to 85°, set the root pass laser power from 5000 to 10000W, and the welding speed from 0.25 to 0.75mm / s.

[0033] 5. Cooling Device Setup: The cooling medium in the cooling device is a uniformly mixed ethylene glycol and aqueous solution. The ethylene glycol content in the cooling medium is 30~50 wt.%, and the pure water content is 50 wt.%~70 wt.%. The water gun nozzle radius is 0.5 mm, and the water flow rate is 100 L / h. The angle between the water gun and the weld seam is 30~45° at a distance of 10 cm to the left of the welding gun. The cooling device contains a water gun baffle with a length of 80 mm, a width of 60 mm, and a thickness of 3 mm, which moves at the same speed as the water gun to prevent coolant splashing and affecting the welding. This cooling device can rapidly cool the weld seam after welding at a cooling rate of 100℃ / s, so that the austenite completely transforms into martensite below the Ms point.

[0034] 6. Weld formation: Turn on the laser welding gun and cooling device to form the weld. The microstructure of the weld is: martensite + a small amount of retained austenite. The tensile strength of the weld after welding is >1000MPa, the yield strength is >800MPa, the weld hardness is >450HV, and the porosity is <0.1%.

[0035] All raw materials used in this invention were purchased from the market.

[0036] The technical solution of the present invention will be further illustrated by the following embodiments.

[0037] Example 1 A highly efficient laser welding method for ultra-high strength steel without post-weld heat treatment includes the following steps: 1. Pre-treat the welding test specimens of ultra-high strength steel plates (the standard for preparing welding test specimens of ultra-high strength steel plates is: the ultra-high strength steel substrate is processed into a 200×100×3mm welding test plate with a surface roughness of 3.2μm). The specific steps are as follows: Perform the operation inside a fume hood. Wear nitrile gloves and thoroughly saturate acetone-free, low-lint medical cotton balls or special non-woven fabric with acetone. Wipe both sides of the sample and all edges thoroughly in one direction with moderate pressure, replacing the cotton ball after each area is wiped to avoid re-applying contaminants. Remove surface impurities, then quickly dry the wiped ultra-high strength steel plate sample and place it in a dry place to await welding. The pretreated ultra-high strength steel sample plate was clamped onto the laser welding experimental platform using a welding fixture and placed using a butt welding method. 2. Apply the uniformly mixed iron-nickel alloy filler to the gaps between ultra-high strength steel plates; The iron-nickel alloy filler, by weight percentage, comprises the following components: Mn: 5%, Ni: 45%, trace elements (C: 0.15%, Si: 0.04%, Co: 4%), with the balance being iron. Its weld volume percentage is 14% (i.e., the iron-nickel alloy filler accounts for 14% of the weld volume). The ratio of the cross-sectional area of ​​the iron-nickel alloy filler to the maximum cross-sectional area of ​​the weld is 1.2:1. 3. A special gas (composed of Ar and N2, with N2 accounting for 80% by volume and the remainder being Ar; with a gas flow rate of 30 L / min) that promotes the formation of more austenite in the weld is delivered to the welding site through a vent pipe. The angle between the vent pipe and the weld is 45°. 4. Set the laser welding process parameter combination; the welding process parameters include: adjusting the welding torch defocusing amount to 5mm, the welding torch welding angle to 70°, setting the root pass laser power to 5000W, and the welding speed to 0.25mm / s; 5. Install a weld cooling device to rapidly cool the molten pool after welding. The cooling medium in the cooling device is a uniformly mixed ethylene glycol and aqueous solution, wherein the ethylene glycol is 30 wt.% and the pure water is 70 wt.%. The water gun nozzle radius is 0.5 mm, the water flow rate is 100 L / h, the cooling medium temperature is -20℃, and the water gun is 10 cm to the left of the welding gun. The angle between the water gun and the weld is 45°. 6. Turn on the laser welding device and start welding to form a weld. The weld microstructure is: martensite + a small amount of retained austenite.

[0038] After welding, the weld surface was observed, and the weld formation was good with no spatter and the weld penetration rate was high. The laser filler welded sample obtained by the method of Example 1 of this invention has a weld joint yield strength of 897 MPa and a maximum tensile strength of 1163 MPa; the weld penetration rate is 97.3%, the weld hardness is 520 HV, and the weld porosity was detected to be 0.02% after ultrasonic testing (UT).

[0039] Example 2 A highly efficient laser welding method for ultra-high strength steel without post-weld heat treatment includes the following steps: 1. Sample Preparation and Clamping: The welded samples of ultra-high strength steel plates (the standard for preparing welded sample plates of ultra-high strength steel plates is: the ultra-high strength steel substrate is processed into a 200×100×3mm welded test plate with a surface roughness of 3.0μm) are pretreated. The specific steps are as follows: Perform the procedure inside a fume hood. Wear nitrile gloves and thoroughly saturate a clean, low-lint medical cotton ball or a specialized non-woven fabric with acetone. Wipe both sides of the sample and all edges thoroughly in one direction with moderate pressure, replacing the cotton ball after each area is wiped to prevent re-application of contaminants. Remove surface impurities, then quickly dry the wiped ultra-high-strength steel plate sample and place it in a dry place before welding.

[0040] The pretreated ultra-high strength steel sample plate was clamped onto the laser welding experimental platform using a welding fixture and placed using a butt welding method. 2. Alloy filler filling: The uniformly mixed iron-nickel alloy filler is used to fill the gaps between ultra-high strength steel plates; The iron-nickel alloy filler, by weight percentage, comprises the following components: Mn: 4%, Ni: 35%, C: 0.15%, Si: 0.02%, Co: 2%, with the balance being Fe. It is filled into the butt joint of the test plate. The ratio of the cross-sectional area of ​​the iron-nickel alloy filler to the maximum cross-sectional area of ​​the weld is 1.2:1, and the total filler volume accounts for 5% of the weld volume. 3. Protective and Activating Gas Delivery: A specially selected mixed gas is delivered to the welding area through a vent pipe. The gas composition is 80% N2 and 20% Ar (volume percentage), the delivery flow rate is 30 L / min, and the vent pipe is at a 45° angle to the weld plane; 4. Welding parameter settings: Set the combination of laser welding process parameters; the welding process parameters include: adjusting the welding torch defocusing amount to 15mm, the welding torch welding angle to 75°, setting the root pass laser power to 7500W, and the welding speed to 0.5mm / s; 5. Post-weld rapid cooling setup: A cooling device is installed on the left side of the welding torch's movement direction to spray the molten pool for rapid cooling. The cooling medium in the cooling device is a uniformly mixed solution of ethylene glycol and water, with ethylene glycol at 40 wt.% and pure water at 60 wt.%. The nozzle radius of the water gun is 0.5 mm, the water flow rate is 100 L / h, the temperature of the cooling medium is -25℃, and the angle between the water gun and the weld is 45° at a distance of 10 cm to the left of the welding torch. 6. Welding Implementation: Turn on the laser welding device and complete the welding according to the above settings to form a weld. The microstructure of the weld is: martensite + a small amount of retained austenite.

[0041] After welding, the weld surface was observed, and the weld formation was good with no spatter and the weld penetration rate was high. The laser filler welded sample prepared by the method of Example 2 of this invention had a weld joint yield strength of 863 MPa and a maximum tensile strength of 1089 MPa; the weld penetration rate was 98.6% and the weld hardness was 480 HV. Ultrasonic testing (UT) showed that the weld porosity after welding was 0.04%.

[0042] Example 3 A highly efficient laser welding method for ultra-high strength steel without post-weld heat treatment includes the following steps: 1. Standardized pretreatment and precise clamping of welding specimens: The selected ultra-high strength steel substrate is milled into a standard butt welding test plate with dimensions of 200mm×100mm×3mm, ensuring that the surface roughness (Ra value) of the end face to be welded is no greater than 2.8μm. The welding specimens of the ultra-high strength steel plate are pretreated to ensure uniform laser absorption and reduce assembly gaps. The pretreatment operation method for the welding specimens of ultra-high strength steel plates is as follows: Perform the procedure inside a fume hood. Wear nitrile gloves and thoroughly saturate a clean, low-lint medical cotton ball or a specialized non-woven fabric with acetone. Wipe both sides of the sample and all edges thoroughly in one direction with moderate pressure, replacing the cotton ball after each area is wiped to prevent re-application of contaminants. Remove surface impurities, then quickly dry the wiped ultra-high-strength steel plate sample and place it in a dry place before welding.

[0043] 2. Filling with iron-nickel alloy based on alloying design: A uniformly mixed iron-nickel alloy is used to fill the gaps between ultra-high strength steel sample plates. The iron-nickel alloy filler, by weight percentage, comprises the following components: Mn: 3%, Ni: 25%, trace elements (C: 0.15%, Si: 0.02%, Co: 2%), with the balance being iron. Its weld volume percentage is 5% (i.e., the iron-nickel alloy filler accounts for 5% of the weld volume). The ratio of the cross-sectional area of ​​the iron-nickel alloy filler to the maximum cross-sectional area of ​​the weld is 1.2:1. 3. Optimized Activation Shielding Gas Delivery for Austenite Structure: A specially selected mixed gas is delivered to the laser welding molten pool area via a vent pipe. This mixed gas consists of high-purity Ar and high-purity N2, with N2 comprising 80% by volume and the remainder being Ar. This high-nitrogen atmosphere aims to strongly promote and stabilize the austenite structure in the weld through nitrogen solid solution. The gas flow rate is precisely controlled at 30 L / min, and the outlet of the vent pipe is at a 45° angle to the weld axis, blowing in from behind the welding direction to form a stable laminar shielding and alloying atmosphere.

[0044] 4. Precise matching and setting of high-energy beam welding process parameters: Set the numerical combination of laser welding process parameters; the welding process parameters include: adjusting the defocusing amount of the laser welding torch to +20mm to appropriately expand the spot area; setting the angle between the welding torch and the workpiece normal direction to 85°; setting the laser output power for root pass welding to 10000W; and setting the welding speed to 0.75mm / s. This parameter combination aims to achieve deep penetration welding while providing suitable line energy to ensure sufficient melting of the filler and metallurgical reaction. 5. Active temperature control system for ultra-fast cooling: To suppress high-temperature phase transformation and obtain the target microstructure, a dedicated active cooling device for the weld seam is installed to force rapid cooling of the molten pool and heat-affected zone after welding. The cooling medium in the cooling device is a uniformly mixed aqueous solution of ethylene glycol and deionized water, with ethylene glycol at 50 wt.% and purified water at 50 wt.%. The temperature of the cooling medium needs to be pre-cooled and maintained at -30℃. Cooling of the red-hot weld seam is achieved through a 0.5mm radius conical water jet nozzle at a high flow rate of 100L / h, at a 45° angle to the weld seam surface, starting 10cm to the left of the welding torch's direction of movement.

[0045] 6. Welding Implementation and Joint Microstructure Formation: The laser welding device is activated, and the process is executed sequentially according to the conditions set above, ultimately forming a continuous and uniform weld. After welding begins, under the alloying effect of the high-nitrogen atmosphere and the phase transformation regulation of ultra-fast cooling, the solidification and solid-state phase transformation of the weld metal are precisely controlled, ultimately forming a multiphase microstructure with lath martensite as the matrix and a small amount of stable retained austenite dispersed within it.

[0046] After welding, the weld surface was observed, and the weld formation was good with no spatter and the weld penetration rate was high. The laser filler welded sample prepared by the method of Example 3 of this invention had a weld joint yield strength of 809 MPa and a maximum tensile strength of 1021 MPa; the weld penetration rate was 96.1% and the weld hardness was 450 HV. Ultrasonic testing (UT) showed that the weld porosity after welding was 0.07%.

[0047] Comparative Example 1 The difference between this comparative example and Example 1 is as follows: Step 3: Ar protective gas is introduced through the vent pipe, but N2 gas, which promotes the formation of more austenite in the molten pool, is not introduced.

[0048] Mechanical properties of the weld surface were tested after welding: the maximum yield strength of the welded joint was 638 MPa, the maximum tensile strength was 887 MPa, the weld penetration rate was 89.1%, and the weld hardness was 302 HV; ultrasonic testing (UT) showed that the weld porosity was 0.08%.

[0049] Compared with Example 1, it was found that due to the lack of nitrogen austenitizing effect, the austenite formation in the weld of Comparative Example 1 was insufficient, resulting in incomplete martensite transformation. This may lead to the formation of more soft phase structures such as ferrite or pearlite, reducing the martensite content, coarsening the microstructure, and increasing porosity in the weld. Consequently, the mechanical properties decreased significantly, and the porosity increased dramatically: the tensile strength decreased from 1163 MPa in Example 1 to 887 MPa, the yield strength decreased from 897 MPa to 638 MPa, and the penetration rate decreased from 97.3% to 89.1%. The hardness decreased from 520 HV to 302 HV, and the porosity also increased from 0.02% to 0.08%.

[0050] This demonstrates that the Ar and N2 mixed gas used in Example 1 plays a crucial role in improving weld performance and is an important process step in achieving high-strength welds without post-weld heat treatment.

[0051] Comparative Example 2 The difference between this comparative example and Example 2 is that no cooling device is set up, and it is directly cooled by natural cooling. The remaining process steps and parameters are completely the same as those in Example 2.

[0052] Mechanical properties of the weld surface were tested after welding: the maximum yield strength of the welded joint was 721 MPa, the maximum tensile strength was 883 MPa, the weld penetration rate was 86.2%, and the weld hardness was 367 HV; ultrasonic testing (UT) showed that the weld porosity was 0.09%.

[0053] Compared with Example 2, it was found that Comparative Example 2, lacking a rapid cooling device, relied on natural cooling of the weld after welding, resulting in a significantly reduced cooling rate. This caused austenite to preferentially transform into soft phases such as ferrite and pearlite at higher temperatures, failing to fully transform into martensite below the Ms point, leading to a coarser weld structure and insufficient martensite content. Simultaneously, slow cooling prolonged the molten pool's existence time, exacerbating porosity defects. Consequently, its mechanical properties deteriorated significantly: tensile strength decreased from 1089 MPa in Example 2 to 883 MPa, yield strength decreased from 863 MPa to 721 MPa, and penetration decreased from 98.6% to 86.2%. Hardness decreased from 480 HV to 367 HV, and porosity increased from 0.04% to 0.09%.

[0054] The results show that the lack of an active rapid cooling process not only fails to achieve high-strength martensitization of the weld microstructure, but also loses the core technological advantage of the present invention of no post-weld heat treatment, making it difficult to meet the high-performance requirements of ultra-high strength steel welded joints.

[0055] Comparative Example 3 The difference between this comparative example and Example 3 is that the type of filler element is changed in step 2, and conventional carbon steel filler (by weight percentage, C: 0.2%, Mn: 0.8%, Si: 0.3%, with the balance being iron) is used. The key austenitizing elements Ni and Co in the iron-nickel alloy filler are removed. The remaining process steps and parameters are completely consistent with Example 3.

[0056] Mechanical properties of the weld surface were tested after welding: the maximum yield strength of the welded joint was 698 MPa, the maximum tensile strength was 863 MPa, the weld penetration rate was 81.2%, and the weld hardness was 347 HV; ultrasonic testing (UT) showed that the weld porosity was 0.12%.

[0057] Compared with Example 3, it was found that Comparative Example 3, due to the replacement of the iron-nickel alloy filler with conventional carbon steel filler, lacked the key elements Ni and Co that promote austenitization. The high-nitrogen protective atmosphere alone could not achieve sufficient austenite formation and retention in the molten pool. Without the solid solution strengthening and austenitizing effect of Ni, the weld metal could not form fine lath martensite during ultra-rapid cooling; instead, a large amount of coarse dendritic ferrite and pearlite mixed structure appeared. Simultaneously, the wettability of the molten pool decreased, leading to a significant reduction in penetration and the appearance of incomplete fusion defects. The absence of Co further reduced the strength-toughness matching of the weld, resulting in a significant deterioration in mechanical properties such as hardness and tensile strength: tensile strength decreased from 1021 MPa in Example 3 to 863 MPa, yield strength decreased from 809 MPa to 698 MPa, and penetration decreased from 96.1% to 81.2%. Hardness decreased from 450 HV to 347 HV, and porosity increased from 0.07% to 0.12%.

[0058] The results clearly show that Ni and Co in the iron-nickel alloy filler are the core elements for achieving austenitization of the weld and promoting subsequent martensitic transformation in this invention. They are indispensable together with the high-nitrogen shielding gas to promote austenitization. At the same time, the precise ratio of Mn and trace amounts of C also works in combination with Ni and Co to ensure the penetration, microstructure uniformity and mechanical properties of the ultra-high strength steel weld.

[0059] In summary, the method of the present invention can effectively improve the weld microstructure, refine the grains, and increase the strength of the butt weld joint. It eliminates the need for post-weld treatments such as ultrasonic treatment and laser quenching of the steel plate, reducing the number of steps and further lowering the manufacturing cost of ultra-high strength steel laser-welded plates.

[0060] Table 1. Laser filler welding parameters for Examples 1-3 and Comparative Examples 1-3 Table 2. Results of mechanical property testing of welds after welding in Examples 1-3 and Comparative Examples 1-3. The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A highly efficient laser filler welding method for ultra-high strength steel without post-weld heat treatment, characterized in that, Includes the following steps: (1) Fill the spaces between the pretreated ultra-high strength steel samples with iron-nickel alloy filler; (2) Laser welding was performed under a mixed protective gas of Ar and N2; (3) Use a cooling device to cool down the weld.

2. The method for high-efficiency laser filler welding of ultra-high strength steel without post-weld heat treatment according to claim 1, characterized in that, The iron-nickel alloy filler comprises, by weight percentage: Mn: 3~5%, Ni: 25~45%, C: 0.07~0.15%, Si: 0~0.04%, Co: 2~4%, with the balance being iron.

3. The method for high-efficiency laser filler welding of ultra-high strength steel without post-weld heat treatment according to claim 2, characterized in that, The ratio of the cross-sectional area of ​​the iron-nickel alloy filler to the maximum cross-sectional area of ​​the weld is 1.2:1; The amount of the iron-nickel alloy filler is 5-15% of the weld volume.

4. The method for high-efficiency laser filler welding of ultra-high strength steel without post-weld heat treatment according to claim 1, characterized in that, In the Ar+N2 mixed protective gas, the volume percentage of N2 is 75%~90%, and the remainder is Ar; The flow rate of the Ar+N2 mixed protective gas is 30L / min, and the angle between the vent pipe and the weld is 30~45°.

5. The method for high-efficiency laser filler welding of ultra-high strength steel without post-weld heat treatment according to claim 1, characterized in that, The laser welding parameters are as follows: the defocusing amount of the welding torch is adjusted to 0mm~+20mm, the welding angle of the welding torch is 70~85°, the laser power for the initial pass is 5000~10000W, and the welding speed is 0.25~0.75mm / s.

6. The method for high-efficiency laser filler welding of ultra-high strength steel without post-weld heat treatment according to claim 1, characterized in that, The cooling medium in the cooling device is ethylene glycol and water, wherein the content of ethylene glycol is 30~50 wt.% and the balance is water.

7. The method for high-efficiency laser filler welding of ultra-high strength steel without post-weld heat treatment according to claim 6, characterized in that, The cooling medium has a water flow rate of 100 L / h, a water gun nozzle radius of 0.5 mm, and an angle of 30~45° between the water gun and the weld.

8. The method for high-efficiency laser filler welding of ultra-high strength steel without post-weld heat treatment according to claim 6, characterized in that, The cooling device cools the weld seam after welding at a cooling rate of 100℃ / s.

9. A laser-welded joint for ultra-high strength steel, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 8.

10. A laser-welded joint for ultra-high strength steel according to claim 9, characterized in that, The ultra-high strength steel laser-welded joint has a tensile strength of 1021~1163MPa, a yield strength of 809~897MPa, a hardness of 450~520HV, and a porosity of 0.02~0.10%.

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