Method for improving strength of narrow-gap laser welding joint

Abstract

The invention discloses a method for improving the strength of a narrow-gap laser welding joint. The method comprises the specific steps that S1, firstly, laser welding is used for completing narrow-gap laser welding; s2, electric pulse treatment is carried out on a welding seam formed after welding; and S3, finally, the weld joint is subjected to hot rolling treatment, and the weld joint in the step S3 is subjected to hot rolling treatment for multiple times. After welding, electric pulse treatment is firstly carried out, then hot rolling treatment is carried out, and the strength of a sample can be improved through work hardening generated in the rolling process. Through rolling treatment, the grain size of the sample is changed, the dislocation density is increased, the texture strength is improved, and the Schmidt factor is reduced, so that the strength of the sample is improved; equipment used in the method is simple in structure, easy and convenient to operate and short in consumed time, and the whole treatment process is efficient and rapid.

Description

Technical Field

[0001] This invention relates to the field of metal processing technology, and more specifically to a method for providing strength in narrow-gap laser-welded joints. Background Technology

[0002] Welding technology, as one of the core processes in modern manufacturing, is closely related to industrial development and is currently widely used in various industries such as aerospace, automotive, shipbuilding, and bridge construction. To date, many mature welding methods exist, such as arc welding, laser welding, plasma welding, and friction stir welding. The post-weld performance is the most critical factor determining the quality of the weld. The performance of the welded specimen is our primary concern, and post-treatment is often required to regulate its properties, such as heat treatment, laser hardening, and carburizing.

[0003] Taking laser welding as an example, as an advanced welding technology with high efficiency and precision, it is currently mainly used in fields with high requirements for processing efficiency and quality. It has advantages such as low heat input, small thermal deformation, large weldable thickness in a single pass, and single-sided welding with double-sided forming, which can greatly improve welding quality and production efficiency. Laser wire-filled welding has a fast welding speed, good forming quality, and is easy to automate. It can achieve high-quality and efficient connections of medium-thick plates (medium plate thickness greater than or equal to 12mm, thick plate thickness greater than 20mm). Laser welding uses a high-energy-density laser beam as a heat source. In the early stages of laser equipment development, due to the low power and large size of lasers, it was mainly used for welding thin-walled materials. Laser welding is a non-contact welding process, and the influence of welding limits is relatively small. The main welding parameters include laser power, defocusing amount, welding speed, and wire feed speed.

[0004] In recent years, narrow-gap laser wire-filled welding technology has combined the advantages of both narrow-gap welding and laser welding, effectively meeting the requirements for low-deformation, high-quality welding of thick plates and showing great potential in thick plate welding. Narrow-gap welding is a welding method that involves creating a small bevel at the weld joint, significantly reducing the cross-sectional area of ​​the weld joint and the amount of welding material used. It is a simple, efficient, and economical welding method, with U-shaped and I-shaped bevels being commonly used. Compared to traditional laser autofusion welding, narrow-gap laser wire-filled welding effectively overcomes the limitation of single-pass penetration depth in laser autofusion welding, which is easily restricted by the laser power of each pass. However, the strength of the samples after narrow-gap laser welding often does not meet the requirements for engineering applications, thus requiring post-treatment to improve the mechanical properties of the samples.

[0005] Chinese patent document CN103769746A discloses a pulsed high magnetic field assisted laser welding method and equipment, which can be used for laser spot welding and seam welding. The method involves first irradiating the workpiece with a laser beam to form a weld joint, then applying a pulsed high magnetic field to the freshly solidified weld joint and its surrounding area. The pulsed high magnetic field generates compressive stress on the workpiece surface, causing plastic deformation in the joint area, releasing residual stress, thereby reducing stress concentration and structural deformation of the weld joint, and improving the fatigue strength of the weld joint. The equipment includes a laser, a pulsed high magnetic field generator, a CNC system, an optical transmission system, and a laser-pulse-high magnetic field composite processing head. The processing head integrates the laser beam and the pulsed high magnetic field, adjusting the vertical distance between the electromagnetic conversion device and the workpiece surface, as well as the horizontal distance between the electromagnetic conversion device and the laser beam. This technical solution does not perform rolling treatment on the weld seam after welding.

[0006] Chinese patent document CN118385800A discloses a method for rapidly and synchronously improving the strength and plasticity of welds online. The method involves placing electrical pulse generators on both sides of the welding torch's travel direction. During welding, the welding torch performs filler wire welding in a circular oscillating manner, and the electrical pulse generators move synchronously with the welding torch, simultaneously supplying shielding gas to the weld area. This technical solution mainly improves the strength and plasticity of the weld by synchronously adding instantaneous electrical pulses during welding, with the welding torch and electrical pulse generators moving in sync. However, this technical solution does not significantly improve tensile strength, is not suitable for narrow-gap laser welding, and does not perform post-weld rolling treatment on the weld.

[0007] Therefore, it is necessary to develop a method to improve the strength of narrow-gap laser welded joints, to treat the weld after narrow-gap laser welding, and to improve the strength of the specimens so that the strength of the specimens meets the requirements of engineering applications. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a method for improving the strength of narrow-gap laser welded joints, thereby increasing the strength of the specimen and enabling the specimen to meet the requirements of engineering applications.

[0009] To solve the above-mentioned technical problems, the technical solution of the present invention is: a method for improving the strength of narrow-gap laser-welded joints, the specific steps of which are as follows:

[0010] S1: First, use laser welding to complete narrow gap laser welding;

[0011] S2: Apply electrical pulse treatment to the weld formed after welding;

[0012] S3: Finally, the weld is subjected to hot rolling treatment.

[0013] Using the above technical solution, since the strength of the weld seam after narrow-gap laser welding often fails to meet the application requirements, the sample is first subjected to electrical pulse treatment after welding, followed by hot rolling. The work hardening that occurs during rolling can improve the sample's strength. During rolling, downward pressure is applied to induce forced plastic deformation in the sample. After rolling, the sample's thickness, dimensions, and cross-sectional area are all deformed. Not only does the physical appearance of the sample change, but the internal microstructure also changes, with the grains becoming elongated strips along the rolling direction. The intense plastic deformation hinders dislocation slip within the crystal, leading to a sharp increase in dislocation density. Dislocation entanglement and interaction form a local stress field, hindering subsequent dislocation movement (work hardening effect), directly improving the material's resistance to deformation, resulting in a significant increase in sample strength, but at the expense of plasticity. Furthermore, since the rolling process is unidirectional, the grain orientation will preferentially align along the rolling direction, resulting in a single grain orientation and a significant increase in texture strength. Simultaneously, the regions where the Schmidt factor accumulates within the sample also change, gradually transforming from "soft grains" to "hard grains." These changes in grain size, dislocation density, texture, and Schmidt factor all contribute to a significant increase in the sample's strength.

[0014] Preferably, the weld is subjected to multiple hot rolling processes in step S3.

[0015] Preferably, in step S2, an electric pulse generator is installed on both sides of the weld to perform electric pulse treatment on the weld. The current density range of the electric pulse generator is 43 A / mm². 2 ~44A / mm 2 The pulse width range is 4900μs~5100μs, the duty cycle range is 0.45~0.53, the energizing time range is 40s~80s, and the roller speed is 40 cm / min~45cm / min.

[0016] Preferably, the hot rolling process involves rolling 6 to 13 times at 440°C to 480°C, with the single-pass reduction gradually decreasing from 25% to 34% to 4% to 10%. In metal rolling processes, traditional methods typically employ a constant reduction rate (e.g., 25% to 34%), which can lead to stress concentration within the material, affecting the mechanical properties of the finished product. This invention optimizes the material deformation process by dynamically adjusting the single-pass reduction rate, gradually reducing it from 25% to 34% in the initial stage to 4% to 10% in the final rolling stage, thereby alleviating stress concentration and improving material uniformity.

[0017] Preferably, the hot rolling temperature is 440℃~460℃, the number of rolling passes is 8~11, and the single-pass reduction rate gradually decreases from 28%~32% to 6%~8%.

[0018] Preferably, the hot rolling temperature is 430℃~450℃, the number of rolling passes is 6~9, and the single-pass reduction rate gradually decreases from 25%~29% to 4%~6%.

[0019] Preferably, the hot rolling temperature is 440℃~480℃, the number of rolling passes is 10~13, and the single-pass reduction rate gradually decreases from 30%~34% to 8%~10%.

[0020] Preferably, the specific steps of the narrow-gap laser welding in step S1 are as follows:

[0021] S11: A bevel with a blunt edge is machined at the welding position of the workpiece to be welded;

[0022] S12: Fix the workpiece to be welded and set the welding parameters; the specific parameters are set as follows: when the fill pass is 1, the laser power is 3.4~3.7kW, the welding speed is 7~10mm / s, the oscillation amplitude is 1.3~1.7mm, and the oscillation frequency is 50~70Hz; when the fill pass is 2~7, the laser power is 3.6~3.9kW, the welding speed is 7~10mm / s, the wire feed speed is 410~470mm / min, the defocusing amount is 14~18mm, the oscillation amplitude is 1.3~1.7mm, and the oscillation frequency is 50~70Hz.

[0023] S13: Perform laser autofusion welding. Start the welding machine and process each pass with the same electrical pulse parameters. After each pass is completed, clean the surface oxide film until the welding is finished.

[0024] Preferably, the shielding gas used in the narrow gap laser welding in step S1 is pure Ar with a purity of 99.999%.

[0025] Preferably, the positive and negative rollers of the electrical pulse generator are placed on both sides of the weld. That is, the weld must be located in the exact center of the positive and negative rollers of the electrical pulse generator.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] (1) The strength of the sample is improved by changing the grain size, increasing the dislocation density, increasing the texture strength and decreasing the Schmidt factor through rolling treatment;

[0028] (2) By dynamically adjusting the single reduction rate, the material deformation process is optimized, gradually reducing it from 25% to 34% in the initial stage to 4% to 10% in the final rolling stage, thereby alleviating stress concentration and improving material uniformity; after adopting this scheme, the uniformity of material grain size is improved, and the tensile strength is increased by 62% to 83%;

[0029] (3) The equipment used in this method has a simple structure, is easy to operate, takes little time, and the whole process is efficient and fast. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1 This is a schematic diagram of the narrow-gap laser welding weld seam rolling process in the method for improving the strength of narrow-gap laser welded joints according to the present invention; wherein: 1-plate workpiece, 2-weld seam, 31-upper rolling roller, 32-lower rolling roller;

[0032] Figure 2 This is a comparison of stress-strain curves at narrow gap laser welded joints with and without rolling treatment, as part of the method for improving the strength of narrow gap laser welded joints according to the present invention.

[0033] Figure 3 The images show a comparison of the tensile fracture microstructures of welded joints with and without rolling treatment in the method for improving the strength of narrow-gap laser welded joints according to the present invention. (a) is Comparative Example 1; (b) is Specific Example 1; (c) is Specific Example 2; and (d) is Specific Example 3. Detailed Implementation

[0034] The following detailed description of some embodiments of the present invention is provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0035] like Figure 1 As shown, in the technical solution provided in the embodiment, the laser is a YLS-4000 fiber laser, the welding robot is an M-20iA model robot manufactured by FANUC Corporation of Japan, and the electrical pulse generator is a SOYI-368000M model. After completing the narrow-gap laser welding, the positive and negative rollers of the electrical pulse generator are fixed on both sides of the weld, ensuring that the weld is located in the exact center of the positive and negative rollers. After electrical pulse treatment, the sample is heated and then placed in a rolling mill for rolling processing.

[0036] Example: The method for improving the strength of narrow-gap laser-welded joints comprises the following steps:

[0037] S1: First, use laser welding to complete narrow gap laser welding;

[0038] S11: A bevel with a blunt edge is machined at the welding position of the workpiece to be welded. Specifically, two 20mm thick AZ31 magnesium alloy pieces are machined with bevels with blunt edges. The blunt edge thickness is 1-2mm, the bevel root width is 1-2mm, and the bevel angle is 3° on one side. After the workpieces to be welded are joined together, a 6° bevel is formed.

[0039] S12: Fix the workpiece to be welded, that is, use existing tooling fixtures to fix the workpiece to be welded and then perform laser welding on it;

[0040] Next, set the welding parameters as follows: when there is 1 fill pass, the laser power is 3.4-3.7kW, the welding speed is 7-10mm / s, the oscillation amplitude is 1.3-1.7mm, and the oscillation frequency is 50-70Hz; when there are 2-7 fill passes, the laser power is 3.6-3.9kW, the welding speed is 7-10mm / s, the wire feed speed is 410-470mm / min, the defocusing amount is 14-18mm, the oscillation amplitude is 1.3-1.7mm, and the oscillation frequency is 50-70Hz.

[0041] In this embodiment, the preferred parameters are set as follows: when the fill pass is 1, the laser power is 3.5kW, the welding speed is 8mm / s, the oscillation amplitude is 1.5mm, and the oscillation frequency is 60Hz; when the fill pass is 7, the laser power is 3.8kW, the welding speed is 8mm / s, the wire feed speed is 450mm / min, the defocusing amount is 15mm, the oscillation amplitude is 1.6mm, and the oscillation frequency is 65Hz; the shielding gas used is pure Ar with a purity of 99.999% and a shielding gas flow rate of 15L / min.

[0042] S13: Perform laser autofusion welding. Start the welding machine and process each pass with the same electrical pulse parameters. After each pass is completed, clean the surface oxide film until the welding is finished.

[0043] S2: Perform electrical pulse treatment on the weld 2 formed after welding; specifically: after welding, perform electrical pulse treatment on the weld 2. Electrical pulse generating devices are installed on both sides of the weld 2. The positive and negative rollers of the electrical pulse generating devices are fixed on both sides of the weld 2, ensuring that the weld 2 is located in the exact center of the rollers. The electrical pulse generating devices are then activated, with a current density range of 43 A / mm². 2 ~44A / mm 2 The pulse width range is 4900μs~5100μs, the duty cycle range is 0.45~0.53, the energizing time range is 40s~80s, and the roller speed is 40 cm / min~45cm / min;

[0044] In this embodiment, the preferred current density range in the electrical pulse parameters is 43.9 A / mm². 2 The pulse width range is 5000μs, the duty cycle range is 0.5, the energizing time range is 60s, and the roller speed is 45cm / min.

[0045] S3: Finally, the weld 2 is subjected to hot rolling treatment; the weld is subjected to multiple hot rolling treatments; specifically: the weld 2 of the plate workpiece 1 after electrical pulse treatment is placed between the upper rolling roller 31 and the lower rolling roller 32 of the hot rolling device, the rolling parameters are set, and the weld 2 is subjected to multiple hot rolling treatments. The hot rolling treatment is performed at 440℃~480℃ for 6~13 times, and the single reduction rate needs to be gradually reduced from 25%~34% to 4%~10%.

[0046] Specific Implementation Example 1: The method used is the same as that in the above embodiments, except that the hot rolling process is performed. The hot rolling temperature is 440℃~460℃, the number of rolling passes is 8~11, the reduction rate of the first 1-3 passes is 28%~32%, the reduction rate of the 4th-6th passes is 19%~24%, the reduction rate of the 7th-9th passes is 11%~13%, and the reduction rate of the last 1-2 passes is 6%~8%.

[0047] Specific embodiment 2: The difference from specific embodiment 1 is that the hot rolling temperature is 430℃~450℃, the number of rolling passes is 6~9, the reduction rate of the first 1-2 passes is 25%~29%, the reduction rate of the third to sixth passes is 15%~18%, and the reduction rate of the last 1-2 passes is 4%~6%.

[0048] Specific embodiment 3: The difference from specific embodiment 1 is that the hot rolling temperature is 440℃~480℃, the number of rolling passes is 10~13, the reduction rate of the first 1-4 passes is 30%~34%, the reduction rate of the 5th-8th passes is 20%~24%, the reduction rate of the 9th-11th passes is 12%~15%, and the reduction rate of the last 1-2 passes is 8%~10%.

[0049] Comparative Example 1: The difference from the example is that no hot rolling treatment is applied after the electrical pulse treatment. The specific steps are as follows:

[0050] S1: First, use laser welding to complete narrow gap laser welding;

[0051] S11: A bevel with a blunt edge is machined at the welding position of the workpiece to be welded. Specifically, two 20mm thick AZ31 magnesium alloy pieces are machined with bevels with blunt edges. The blunt edge thickness is 1-2mm, the bevel root width is 1-2mm, and the bevel angle is 3° on one side. After the workpieces to be welded are joined together, a 6° bevel is formed.

[0052] S12: Fix the workpiece to be welded, that is, use existing tooling fixtures to fix the workpiece to be welded and then perform laser welding on it;

[0053] Next, the welding parameters were set as follows: when there was one fill pass, the laser power was 3.5kW, the welding speed was 8mm / s, the oscillation amplitude was 1.5mm, and the oscillation frequency was 60Hz; when there were seven fill passes, the laser power was 3.8kW, the welding speed was 8mm / s, the wire feed speed was 450mm / min, the defocusing amount was 15mm, the oscillation amplitude was 1.6mm, and the oscillation frequency was 65Hz; the shielding gas used was pure Ar with a purity of 99.999% and a shielding gas flow rate of 15L / min.

[0054] S13: Perform laser autofusion welding. Start the welding machine and process each pass with the same electrical pulse parameters. After each pass is completed, clean the surface oxide film until the welding is finished.

[0055] S2: Perform electrical pulse treatment on the weld 2 formed after welding; specifically: after welding, perform electrical pulse treatment on weld 2. Electrical pulse generating devices are installed on both sides of weld 2. The positive and negative rollers of the electrical pulse generating devices are fixed on both sides of weld 2, ensuring that weld 2 is located in the exact center of the rollers. The electrical pulse generating devices are then activated, with the current density range in the electrical pulse parameters being 43.9 A / mm². 2 The pulse width range is 5000μs, the duty cycle range is 0.5, the energizing time range is 60s, and the roller speed is 45cm / min.

[0056] Comparative Example 2: This is a sample in the prior art that undergoes pulse treatment simultaneously during laser welding. The parameters of laser welding and pulse treatment are the same as those in Comparative Example 1. No hot rolling treatment is applied after welding.

[0057] Figure 2 The stress-strain curves are obtained from tensile specimens tested in four control groups: Comparative Example 1 (without rolling treatment), and Specific Example 1, Specific Example 2, and Specific Example 3 (with rolling treatment). Figure 2 As can be seen, the tensile strength of the electrically pulsed samples corresponding to Specific Examples 1, 2, and 3 was significantly improved, while the plasticity decreased substantially. Compared with the samples without rolling treatment, the tensile strength of the electrically pulsed samples corresponding to Specific Examples 1, 2, and 3 increased by 82.6%, 75.2%, and 62.2%, respectively. Furthermore, the tensile strength of these samples was compared with that of Comparative Example 2, which underwent electrical pulsed treatment during welding but did not undergo hot rolling after welding. The specific data on the improvement in tensile strength are shown in Table 1.

[0058] Table 1. Comparison of tensile strength improvement between Comparative Example 2 and Specific Examples 1-3

[0059] Group Comparative Example 2 Specific Implementation Example 1 Specific Implementation Example 2 Specific Implementation Example 3 tensile strength 13.4% 82.6% 75.2% 62.2%

[0060] Figure 3 (a), (b), (c), and (d) are microscopic fracture surfaces of tensile specimens from the weld region of four control groups (Specific Examples 1, 2, and 3) without rolling treatment, respectively. The figures show that the tensile specimens without rolling treatment exhibit tearing ridges and a few dimples in their microscopic fracture surfaces, indicating a ductile fracture mode and low strength. In contrast, the rolled specimens (Specific Examples 1, 2, and 3) all show numerous cleavage plateaus in their tensile fracture surfaces, indicating a brittle fracture mode and high strength. This demonstrates that rolling treatment significantly improves the strength of narrow-gap laser-welded welds.

[0061] The above comparison shows that this invention, by rolling the weld after narrow-gap laser welding, utilizes the work hardening generated during the rolling process to significantly improve the strength of the treated sample. In particular, by dynamically adjusting the single-pass reduction rate and optimizing the material deformation process, gradually reducing it from 25%–34% in the initial stage to 4%–10% in the final rolling stage, stress concentration is alleviated, material uniformity is improved, and the strength of the weld area is effectively increased, raising the tensile strength by 62.2%–82.6%.

[0062] This embodiment uses laser welding as an example to illustrate the technical solution. Experiments have shown that other methods, such as CMT welding, can also achieve the purpose of this invention.

[0063] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements can be made without departing from the principle of the present invention, such as changing the parameters of laser welding or hot rolling. These improvements should also be considered within the scope of protection of the present invention.

Claims

1. A method for improving the strength of narrow-gap laser-welded joints, characterized in that, The specific steps are as follows: S1: First, use laser welding to complete narrow gap laser welding; S2: Apply electrical pulse treatment to the weld formed after welding; S3: Finally, the weld is subjected to hot rolling treatment.

2. The method for improving the strength of narrow-gap laser-welded joints according to claim 1, characterized in that, In step S3, the weld is subjected to multiple hot rolling processes.

3. The method for improving the strength of narrow-gap laser-welded joints according to claim 1, characterized in that, In step S2, an electric pulse generator is installed on both sides of the weld to perform electric pulse treatment on the weld. The current density range of the electric pulse generator is 43 A / mm². 2 ~44A / mm 2 The pulse width range is 4900μs~5100μs, the duty cycle range is 0.45~0.53, the energizing time range is 40s~80s, and the roller speed is 40 cm / min~45cm / min.

4. The method for improving the strength of narrow-gap laser-welded joints according to claim 2, characterized in that, The hot rolling process involves rolling 6 to 13 times at 440°C to 480°C, with the single reduction rate gradually decreasing from 25% to 34% to 4% to 10%.

5. The method for improving the strength of narrow-gap laser-welded joints according to claim 4, characterized in that, The hot rolling temperature is 440℃~460℃, the number of rolling passes is 8~11, and the single reduction rate gradually decreases from 28%~32% to 6%~8%.

6. The method for improving the strength of narrow-gap laser-welded joints according to claim 4, characterized in that, The hot rolling temperature is 430℃~450℃, the number of rolling passes is 6~9, and the single-pass reduction rate gradually decreases from 25%~29% to 4%~6%.

7. The method for improving the strength of narrow-gap laser-welded joints according to claim 4, characterized in that, The hot rolling temperature is 440℃~480℃, the number of rolling passes is 10~13, and the single reduction rate gradually decreases from 30%~34% to 8%~10%.

8. The method for improving the strength of narrow-gap laser-welded joints according to claim 1, characterized in that, The specific steps of the narrow-gap laser welding in step S1 are as follows: S11: A bevel with a blunt edge is machined at the welding position of the workpiece to be welded; S12: Fix the workpiece to be welded and set the welding parameters; the specific parameters are set as follows: when the fill pass is 1, the laser power is 3.4~3.7kW, the welding speed is 7~10mm / s, the oscillation amplitude is 1.3~1.7mm, and the oscillation frequency is 50~70Hz; when the fill pass is 2~7, the laser power is 3.6~3.9kW, the welding speed is 7~10mm / s, the wire feed speed is 410~470mm / min, the defocusing amount is 14~18mm, the oscillation amplitude is 1.3~1.7mm, and the oscillation frequency is 50~70Hz. S13: Perform laser autofusion welding. Start the welding machine and process each pass with the same electrical pulse parameters. After each pass is completed, clean the surface oxide film until the welding is finished.

9. The method for improving the strength of narrow-gap laser-welded joints according to claim 5, characterized in that, The shielding gas used in the narrow-gap laser welding in step S1 is pure Ar with a purity of 99.999%.

10. The method for improving the strength of narrow-gap laser-welded joints according to claim 3, characterized in that, The positive and negative rollers of the electrical pulse generator are placed on both sides of the weld.

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