A method for reducing hydrogen damage of laser welded joints of advanced high strength steel by in-situ post-weld heat treatment

Abstract

The application discloses a method for reducing hydrogen damage of an advanced high-strength steel laser welding joint by in-situ post-weld heat treatment, and the method comprises the following steps: constructing a database of the relationship among the plate thickness, the welding temperature, the laser spot diameter and the process parameters, and realizing efficient and low-cost in-situ post-weld heat treatment of the welding seam area and the heat affected zone by a single-beam laser, so that the carbonides are in-situ precipitated in the welding seam area and the heat affected zone of the advanced high-strength steel laser welding joint, the hydrogen atoms in the internal regions of the joint are pinned by the appropriate precipitation of the carbonides, the hydrogen diffusion capacity in each micro-zone of the high-strength steel laser welding joint is reduced, and the purpose of reducing the hydrogen damage of the advanced high-strength steel laser welding joint is achieved.

Description

Technical Field

[0001] This invention relates to the field of methods for reducing hydrogen damage in welded joints, and specifically to a method for reducing or mitigating hydrogen damage in laser-welded joints of advanced high-strength steel, and more specifically to a method for reducing hydrogen damage in laser-welded joints of advanced high-strength steel through in-situ post-weld heat treatment. Background Technology

[0002] Advanced high-strength steel is a high-strength, high-ductility material. Its application can reduce vehicle weight while ensuring passenger safety, leading to a growing number of automakers choosing to use it extensively in vehicle body manufacturing. With technological advancements, the strength levels of high-strength steel have gradually increased. However, hydrogen damage has become the primary threat restricting its application. During the manufacturing process of high-strength steel, hydrogen atoms are inevitably introduced into the steel structure, and their presence severely degrades its strength and toughness.

[0003] Laser welding technology, due to its advantages such as high energy density and fast welding speed, is now widely used in vehicle body manufacturing. Laser welding of high-strength steel results in an extremely narrow heat-affected zone (HAZ), but the microstructure and mechanical properties of different micro-regions within the HAZ vary significantly due to different thermal cycles experienced during welding. Both the coarse-grained and fine-grained regions within the HAZ exhibit martensitic structures, with local tensile strengths exceeding 1500 MPa. Since hydrogen damage typically intensifies with increasing strength of high-strength steel, the HAZ in laser-welded high-strength steel joints may be the area most severely affected by hydrogen damage, significantly impacting the application of laser-welded high-strength steel in vehicle body manufacturing. However, the carbides in high-strength steel can pin hydrogen, effectively reducing the hydrogen diffusion coefficient and mitigating hydrogen damage.

[0004] Currently, some scholars have conducted multi-beam laser welding to improve the mechanical properties and weldability of high-strength steel welded joints (application publication numbers CN 108356417 A, CN 110328448 A and CN 105689896). However, multi-beam laser welding has disadvantages such as high production equipment cost and low welding efficiency. Similarly, some scholars have conducted post-weld heat treatment and joint surface strengthening to improve the resistance of joints to hydrogen embrittlement (CN 113969343 A and CN 102127630 A). In actual production, high-strength steel structural components are relatively unsuitable for vacuum post-weld heat treatment and surface strengthening. Furthermore, vacuum post-weld heat treatment and surface strengthening methods have high production costs and low production efficiency, making them unsuitable for mass production in actual factories.

[0005] Therefore, there is an urgent need to develop a method that can effectively reduce hydrogen damage in laser-welded joints of high-strength steel while having low production equipment costs and high production efficiency. Summary of the Invention

[0006] In view of this, the present invention addresses the problem of severe hydrogen damage in high-strength steel laser-welded joints and the inability of existing technologies to meet the requirements of efficient production. It provides a method for reducing hydrogen damage in high-strength steel laser-welded joints. This method establishes a database of four parameters: plate thickness, welding temperature, laser spot diameter, and process parameters. During laser welding, rapid and efficient in-situ post-weld heat treatment with the laser beam causes appropriate carbide precipitation in each micro-region of the heat-affected zone. While ensuring that the mechanical properties of the joint do not decrease, the carbides pinning hydrogen reduces hydrogen movement within the joint, thereby reducing hydrogen damage in high-strength steel laser-welded joints.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A method for reducing hydrogen damage in laser-welded joints of advanced high-strength steel using in-situ post-weld heat treatment involves several steps. First, a database is established experimentally to determine the relationship between plate thickness, welding temperature, laser spot diameter, and process parameters. Then, after grinding and cleaning the advanced high-strength steel, it is joined using a laser beam. After the first weld pass, the defocusing amount of the laser beam is adjusted to cover the entire joint area. The plate thickness, laser spot diameter, and required heat treatment temperature are imported into the database to obtain the corresponding laser power. Subsequently, the defocused laser spot is used to perform in-situ post-weld heat treatment on the weld area and heat-affected zone. This causes appropriate amounts of carbides to precipitate in each micro-area of ​​the weld area and heat-affected zone. The pinning effect of the carbides on hydrogen reduces hydrogen movement within each micro-area of ​​the joint, thereby reducing hydrogen damage in the laser-welded joint of high-strength steel.

[0009] Optionally, the method specifically includes the following steps:

[0010] Step 1: After grinding the surface and back of the plate to be welded, thermocouples are placed on each side. Laser welding is performed on high-strength steel with different plate thicknesses (1mm-4mm). By adjusting process parameters such as laser spot diameter (0.6mm-5mm) and laser power (2kW-10kW), the temperature changes of the surface and back of the plate are collected under different plate thicknesses, spot diameters and process parameters. A database of the relationship between plate thickness, welding temperature, laser spot diameter and process parameters is constructed.

[0011] Step 2: After sanding and cleaning with acetone, place the high-strength steel to be welded on the workbench.

[0012] Step 3: Based on the parameters of the high-strength steel base material, such as plate thickness, set appropriate laser welding process parameters and spot diameter to perform laser welding on the base material. For example: laser power (2kW-10kW), laser spot diameter (0.6mm-1.2mm), welding speed (2m / min-8m / min), etc.

[0013] Step 4: According to the laser welding parameters and spot diameter, perform laser welding on the high-strength steel base material to be welded to form a high-strength steel welded plate with complete weld and no obvious defects.

[0014] Step 5: Adjust the laser spot according to the weld width and the heat-affected zone width so that the defocused laser spot can cover the weld area and the heat-affected zone.

[0015] It should be noted that the purpose of the above operation is to ensure that the entire weld area and heat-affected zone are subjected to post-weld heat treatment. After defocusing, the laser spot becomes larger, which can cover the entire weld area and heat-affected zone. In addition, after defocusing, the energy density concentration becomes smaller and more uniform, which facilitates post-weld heat treatment of large areas.

[0016] Step 6: Import the plate thickness, laser spot diameter, and required in-situ post-weld heat treatment temperature data into the database to obtain the required laser heat treatment process parameters, and use these parameters to perform in-situ post-weld heat treatment on the joint.

[0017] Step 7: Perform in-situ post-weld heat treatment on the weld and heat-affected zone using the spot diameter in Step 5 and the heat treatment process parameters in Step 6, so that carbides are precipitated in each micro-area of ​​the weld and heat-affected zone.

[0018] It should be noted that the purpose of the above operation is to precipitate carbides in the weld zone and heat-affected zone. Carbides have the effect of pinning hydrogen and hindering its movement, thereby improving the joint's resistance to hydrogen embrittlement. In addition, while precipitating carbides, the conventional mechanical properties of the joint are not weakened.

[0019] Step 8: Test the conventional mechanical properties of the in-situ welded heat-treated joint (uniaxial tensile test (tensile rate 0.5mm / min-5mm / min), cupping test (boob rate 0.5mm / min-20mm / min), etc.) to determine whether the conventional mechanical properties of the in-situ welded heat-treated joint can reach 90% of the original joint. If it can, proceed to the next step. If it cannot, readjust the heat treatment temperature so that the conventional mechanical properties of the in-situ welded heat-treated joint reach 90% of the original joint.

[0020] Step nine: Electrochemical hydrogen charging and slow strain rate tensile tests are used to evaluate the hydrogen damage of the welded plate after in-situ welding heat treatment.

[0021] Furthermore, K-type thermocouples were used to collect temperature change curves of each micro-region of the joint during welding and heat treatment. The temperature acquisition frequency was 20Hz-50Hz, and the acquisition temperature was 200℃-1250℃.

[0022] Furthermore, the current density of the electrochemical hydrogen charging is 5-30 mA / cm². 2The electrochemical hydrogen charging time is 5-60 min; the strain rate in the slow strain rate tensile test is 1×10⁻⁶ min. -6 s -1 -5×10 -6 s -1 .

[0023] The collected data was then used to construct a data structure relating plate thickness, welding temperature, laser spot diameter, and process parameters using software such as PDMan.

[0024] Compared with the prior art, the beneficial effects of the present invention are:

[0025] This invention enables welding and in-situ post-weld heat treatment of high-strength steel welded plates using a single laser beam. By defocusing the laser beam and performing in-situ heat treatment of the weld zone and heat-affected zone, carbide precipitation occurs in situ within these areas, thereby pinning hydrogen in the weld and heat-affected zone and reducing hydrogen damage to the laser-welded joints of high-strength steel. This method is simple to operate, requires no dual-beam or multi-beam configuration, and, based on a database of relationships between plate thickness, welding temperature, laser spot diameter, and process parameters, can meet the post-weld heat treatment needs of various materials and achieve efficient production. Attached Figure Description

[0026] 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 embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0027] Figure 1 The flowchart is for a method to reduce hydrogen damage in high-strength steel laser-welded joints provided in Embodiment 1 of the present invention.

[0028] Figure 2 This is a schematic diagram of laser welding and in-situ post-weld heat treatment provided in Embodiment 1 of the present invention.

[0029] Figure 3 The width of each micro-zone of the QP980 laser welding joint provided in Embodiment 1 of the present invention.

[0030] Figure 4 This is a diagram of carbide precipitation in the heat-affected zone of the QP980 laser-welded joint provided in Embodiment 1 of the present invention after in-situ welding and post-heat treatment. Detailed Implementation

[0031] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0032] The term "embodiment" used herein, as an example, is not necessarily to be construed as superior to or better than other embodiments. Performance testing in the embodiments of this application, unless otherwise specified, employs conventional testing methods in the art. It should be understood that the terminology used in this application is merely for describing particular implementations and is not intended to limit the scope of this disclosure.

[0033] Unless otherwise stated, the technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; other experimental methods and technical means not specifically mentioned herein refer to experimental methods and technical means commonly used by one of ordinary skill in the art.

[0034] In the description of this invention, it should be understood that the terms "middle", "upper", "lower", "rise", "fall", "vertical", "surface", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0035] To better illustrate the content of this application, numerous specific details are provided in the following detailed embodiments. Those skilled in the art should understand that this application can be implemented even without certain specific details. In the embodiments, some methods, means, instruments, and devices well-known to those skilled in the art are not described in detail in order to highlight the main points of this application.

[0036] Without conflict, the technical features disclosed in the embodiments of this application can be combined arbitrarily, and the resulting technical solution belongs to the content disclosed in the embodiments of this application.

[0037] This invention discloses a method for reducing hydrogen damage in laser-welded joints of high-strength steel. The method first constructs a database relating plate thickness, welding temperature, laser spot diameter, and process parameters. Then, after laser beam welding of advanced high-strength steel, the plate thickness, laser spot diameter, and required heat treatment temperature are imported into the database to obtain other laser process parameters. Next, a defocused laser spot is used to perform in-situ post-weld heat treatment on the weld zone and heat-affected zone, causing appropriate amounts of carbides to precipitate in each micro-region of the weld zone and heat-affected zone. The pinning effect of these carbides on hydrogen reduces the hydrogen diffusion coefficient in each micro-region of the heat-affected zone, thereby reducing hydrogen damage in the laser-welded joint of high-strength steel.

[0038] To better understand the present invention, the following embodiments are provided for further detailed description of the present invention, but they should not be construed as limiting the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above-described invention are also considered to fall within the protection scope of the present invention.

[0039] Example 1

[0040] This invention takes reducing hydrogen damage in high-strength steel laser-welded joints (taking third-generation automotive advanced high-strength steel QP980 laser welding as an example) as an example. It combines database construction methods, laser welding technology, thermocouple temperature acquisition, and in-situ post-weld heat treatment with laser beam defocusing to induce appropriate carbide precipitation in the weld zone and heat-affected zone, thereby reducing hydrogen damage in QP980 laser-welded joints. The specific steps are as follows:

[0041] Step 1: After grinding the surface and back of the 1.22mm thick QP980 sheet, place a K-type thermocouple, adjust the laser spot diameter (0.6mm-4mm), laser power (2kW-6kW), and welding speed (2m / min-6m / min), and collect the temperature changes of the QP980 surface and back under different process parameters through the K-type thermocouple to build a database of the relationship between sheet thickness, welding temperature, laser spot diameter, and process parameters.

[0042] Step 2: After grinding the QP980 steel area to be welded with 400-grit, 1500-grit, and 2500-grit sandpaper, place it in acetone for ultrasonic cleaning to remove oil and impurities from the surface of the area to be welded, and then place it on the laser welding worktable.

[0043] Step 3: Based on the thickness of the QP980 steel plate, set the laser welding parameters and spot diameter: laser power 2.5kW, welding speed 3m / min, spot diameter 0.6mm, defocusing amount 0, argon gas flow rate 15L / min;

[0044] Step 4: Start the laser welding equipment (fiber laser in conjunction with ABB robot) to complete the QP980 steel splicing connection and form a weld without obvious defects;

[0045] Step 5: Based on the width of the weld seam and heat-affected zone of the QP980 laser welding head (2.2mm), adjust the laser defocus state and adjust the laser spot diameter to 2.5mm so that the laser spot can completely cover the weld seam and heat-affected zone.

[0046] Step 6: Import the data parameters of 1.22mm plate thickness, 2.5mm laser spot diameter and required in-situ post-weld heat treatment temperature (400℃) into the database to obtain the required laser power (6.5kW), welding speed (3m / min) and other process parameters. Use these parameters to perform in-situ post-weld heat treatment on the joint to precipitate carbides in each micro-area of ​​the joint.

[0047] Step 7: Test the in-situ welded heat-treated QP980 joint and the original QP980 joint by performing uniaxial tensile test and cupping test. The conventional mechanical properties of the in-situ welded heat-treated QP980 joint reached 93% of those of the original joint. Detailed test data are shown in Table 1.

[0048] Step 8: Electrochemically charge the tensile specimen, including the joint, with hydrogen using a DC power supply (hydrogen charging current density 5 mA / cm²). 2 Hydrogen charging time 30 min), after hydrogen charging is complete, hydrogen damage assessment of the sample is performed using a slow strain rate tensile tester (strain rate 10 min). -5 s -1 The effect of in-situ post-weld heat treatment on hydrogen damage reduction of QP980 steel laser welded joints was analyzed. The slow strain rate tensile properties of QP980 steel laser welded joints after in-situ post-weld heat treatment and hydrogen purging are shown in Table 2.

[0049] Table 1

[0050]

[0051] Table 2

[0052]

[0053] As can be seen from Table 1, after in-situ post-weld heat treatment, the tensile strength of the joint decreased by only 7%, and the cupping value decreased by only 6%. Therefore, a suitable post-weld heat treatment process will not deteriorate the conventional mechanical properties of the joint.

[0054] To further demonstrate the beneficial effects of the present invention and to better understand it, the technical features disclosed in the present invention are further illustrated by the following comparative examples, but these should not be construed as limiting the present invention. Other improvements made by those skilled in the art based on the above description of the invention, without inventive effort, are also considered to fall within the protection scope of the present invention.

[0055] Comparative Example 1

[0056] Step 1: After grinding the surface and back of the 1.22mm thick QP980 sheet, place a K-type thermocouple, adjust the laser spot diameter (0.6mm-3mm), laser power (2kW-6kW), and welding speed (2m / min-6m / min), and collect the temperature changes of the QP980 surface and back under different process parameters through the K-type thermocouple to build a database of the relationship between sheet thickness, welding temperature, laser spot diameter, and process parameters.

[0057] Step 2: After grinding the QP980 steel area to be welded with 400-grit, 1500-grit, and 2500-grit sandpaper, place it in acetone for ultrasonic cleaning to remove oil and impurities from the surface of the area to be welded, and then place it on the laser welding worktable.

[0058] Step 3: Based on the thickness of the QP980 steel plate, set the laser welding parameters and spot diameter: laser power 2.5kW, welding speed 3m / min, spot diameter 0.6mm, defocusing amount 0, argon gas flow rate 15L / min;

[0059] Step 4: Start the laser welding equipment (fiber laser in conjunction with ABB robot) to complete the QP980 steel splicing connection and form a weld without obvious defects;

[0060] Step 5: Electrochemically charge the tensile specimen, including the joint, with hydrogen using a DC power supply (hydrogen charging current density 5 mA / cm²). 2 Hydrogen charging time 30 min), after hydrogen charging is complete, hydrogen damage assessment of the sample is performed using a slow strain rate tensile tester (strain rate 10 min). -5 s -1 The degree of hydrogen damage in the laser-welded joint of QP980 steel was analyzed. The slow strain rate tensile properties of the laser-welded joint of QP980 steel after hydrogen purging are shown in Table 3.

[0061] Table 3

[0062]

[0063] Table 3 shows that the QP980 laser-welded joint is particularly sensitive to hydrogen. After hydrogen purging, the joint strength decreases by 18%, while the elongation decreases by 66.7%. The presence of hydrogen in the joint severely deteriorates its mechanical properties. Combined with the data in Table 2, after in-situ post-weld heat treatment, the tensile strength and elongation of the joint only decrease by 3.2% and 28.6%, respectively. This demonstrates that in-situ post-weld heat treatment can significantly reduce hydrogen damage in advanced high-strength steel laser-welded joints.

[0064] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for reducing hydrogen damage of laser welded joints of advanced high strength steel by in-situ post-weld heat treatment, characterized in that, The method specifically comprises the following steps: 1) After polishing the surface and back of the to-be-welded plate, respectively place a thermocouple, laser weld advanced high-strength steel of different plate thicknesses, collect the temperature changes of the plate surface and back under different plate thicknesses and process parameters by adjusting the laser welding process parameters and laser spot diameter, and construct a database of the relationship between plate thickness, welding temperature, laser spot diameter, and process parameters; 2) Place the to-be-welded advanced high-strength steel in the workbench after polishing and acetone cleaning; and laser weld the base material according to the base material parameters of the high-strength steel; 3) Laser tailor-weld the to-be-welded advanced high-strength steel base material according to the laser welding process parameters set in step 2) to form a high-strength steel tailor-welded plate with a weld and no obvious defects; 4) Adjust the laser spot according to the weld width and heat-affected zone width of step 3) so that the defocused laser spot can cover the weld area and the heat-affected zone; 5) Import the plate thickness, laser spot diameter, and required in-situ post-weld heat treatment temperature data parameters into the database to obtain the required laser heat treatment process parameters, and use the laser heat treatment process parameters to perform in-situ post-weld heat treatment on the joint; 6) Perform in-situ post-weld heat treatment on the weld and heat-affected zone using the spot diameter in step 4) and the laser heat treatment process parameters in step 5) to make the weld area and heat-affected zone each microzone precipitate carbides; 7) Test the conventional mechanical properties of the in-situ post-weld heat treated joint, determine whether the conventional mechanical properties of the in-situ post-weld heat treated joint can reach 90% of the original joint, if yes, proceed to the next step, if not, adjust the post-weld heat treatment temperature again so that the conventional mechanical properties of the in-situ post-weld heat treated joint can reach 90% of the original joint; 8) Use electrochemical hydrogen charging and slow strain rate tensile test to evaluate the hydrogen damage of the tailor-welded plate after in-situ post-weld heat treatment.

2. The method for reducing hydrogen damage of laser welded joints of advanced high strength steel by in-situ post-weld heat treatment according to claim 1, characterized in that, The thickness of the advanced high-strength steel is 1mm-4mm, the laser spot diameter is 0.6mm-5mm, and the laser welding process parameters include: laser power is 2kW-10kW, welding speed is 2m / min-8m / min, and protective gas flow is 5-20L / min.

3. The method of claim 1, wherein the method is characterized in that, Use K-type thermocouples to collect the temperature change curves of each microzone of the joint during welding and heat treatment, the temperature collection frequency is 20Hz-50Hz, and the collected temperature is 200℃-1250℃.

4. The method of claim 1, wherein the method is characterized in that, The conventional mechanical property test includes conventional tensile test and cupping test; wherein, the tensile rate is 0.5mm / min-5mm / min, and the cupping test boss rate is 0.5mm / min-20mm / min.

5. The method of claim 1, wherein the method is characterized in that, The current density of the electrochemical hydrogen charging is 5-30 mA / cm 2 , and the electrochemical hydrogen charging time is 5-60 min; the strain rate in the slow strain rate test is 1 x 10 -6 s -1 -5 x 10 -6 s -1 .

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