Laser welding method for inhibiting softening of heat affected zone of high-strength steel

Abstract

The invention relates to a laser welding method for inhibiting softening of a heat affected zone of high-strength steel, in particular to high-strength steel welding of an automobile safety structural part, and relates to the technical field of deep processing of high-strength automobile steel. The method comprises the following steps: performing surface cleaning operation on a to-be-welded area of the high-strength steel; the high-strength steel is fixed, active cooling is carried out through supercooled gas purging, and the temperature of the to-be-welded area is reduced to a preset low-temperature range; configuring a laser welding process parameter combination; and the cooled steel plate is subjected to laser welding through a laser welding process parameter combination, and laser welding is completed in a low-temperature environment. According to the technical scheme, by optimizing the welding parameters and the cooling conditions, the mechanical property of the welded joint is remarkably improved, the fracture position is located on the base metal side, and therefore the tensile strength of the welded joint is improved. In addition, the manufacturing cost and process steps are reduced, complex cooling devices and post-treatment are not needed, and the method is suitable for large-scale production.

Description

Technical Field

[0001] The present invention relates to the technical field of deep processing of high-strength automotive steel, and in particular to a laser welding method for suppressing softening in the heat-affected zone of high-strength steel. Background Art

[0002] In the field of automotive manufacturing, high-strength steel has gradually become an important material for body structural components due to its excellent mechanical properties and lightweight characteristics. The rapid development of new energy vehicles has further promoted the application of high-strength steel. Especially in body safety structural components, the usage ratio of high-strength steel has increased significantly. High-strength steel can not only improve the impact resistance and safety of the vehicle body, but also effectively reduce the vehicle body weight, thereby improving the endurance and energy efficiency of new energy vehicles.

[0003] Laser tailor welding is one of the main welding methods for the application of high-strength steel in automotive manufacturing, and the welding quality directly determines the safety of the vehicle body. Due to the high content of martensite in the microstructure of high-strength steel, during the welding process under the action of the welding thermal cycle, softening will occur in the heat-affected zone of laser welding. This softening phenomenon will lead to a reduction in the mechanical properties of the welded joint, and thus affect the safety performance of the whole vehicle.

[0004] The existing technology mainly reduces the softening phenomenon by applying a cooling plate or cooling block on the back of the steel plate to be welded, thereby reducing the cooling rate in the heat-affected zone of welding. In addition, ultrasonic treatment and reasonable welding parameters are adopted to further suppress the occurrence of softening in the heat-affected zone. For example, Patent CN108637437A discloses a method for reducing softening of aluminum alloy arc welding joints. This method uses a forced cooling device for local cooling, and by directly contacting the circulating water with the welded plate, the cooling rate of the softened zone of the joint is accelerated, thereby reducing the degree of softening of the joint and improving the hardness value and tensile strength of the joint. However, the cooling effect of the strong cooling device using water cooling in this patent is poor, and the cooling rate is not sufficient to suppress the softening of the heat-affected zone of martensitic steel when welding steel materials. Patent CN109604782A discloses a method for controlling softening in the heat-affected zone of quenched and tempered steel during welding by rotating impact. This method impacts the area exceeding the tempering temperature range during quenching and tempering treatment, so that this area reaches a large amount of plastic deformation, refines the grains in this area, and enhances the strength and toughness of this area to achieve the purpose of suppressing softening. This method requires a rotating impact device, which has a complex structure and limited effect of suppressing softening through impact.

[0005] In addition, although some existing technologies can alleviate the softening problem of the heat-affected zone of high-strength steel welding to a certain extent, there are still defects. For example, Patent CN112458243B discloses a method for preventing the softening of the heat-affected zone of QP steel arc welding. This method first determines the width and position of the softening zone, and then performs laser quenching on this area to improve the hardness of the softening zone and significantly reduce or even eliminate the softening zone. However, when laser quenching is performed on the softening zone, new softening zones will be generated around this area due to the effect of laser quenching, and the purpose of completely eliminating the softening zone cannot be achieved.

[0006] Therefore, there is an urgent need for a welding method that can effectively inhibit the softening of the heat-affected zone of high-strength steel welding to improve the mechanical properties of the welded joint and the safety of the whole vehicle. Summary of the Invention

[0007] In view of this, the present invention proposes a laser welding method for suppressing the softening of the heat-affected zone of high-strength steel to inhibit the softening phenomenon of the heat-affected zone of high-strength steel welding.

[0008] For this purpose, the present invention provides the following technical solutions:

[0009] The present invention provides a laser welding method for suppressing the softening of the heat-affected zone of high-strength steel, including the following steps:

[0010] S1: Perform a surface cleaning operation on the area to be welded of the high-strength steel;

[0011] S2: Fix the high-strength steel and perform active cooling by purging with supercooled gas to reduce the temperature of the area to be welded to between -40°C and 20°C;

[0012] S3: Configure a combination of laser welding process parameters; the welding process parameters include: laser welding heat input, laser welding output power, and laser welding speed;

[0013] S4: Perform laser welding on the steel plate cooled in step S2 using the process of step S3 to complete laser welding in a low-temperature environment.

[0014] Further, the supercooled gas includes inert gas, liquid nitrogen atomized gas, or compressed low-temperature gas.

[0015] Further, the surface cleaning operation includes chemical solvent cleaning or mechanical grinding treatment.

[0016] Further, when performing laser welding, the cooled steel plate is fixed by an adjustable fixture.

[0017] Further, configuring a combination of laser welding process parameters includes: selecting welding parameters according to the thickness of the steel plate to be welded.

[0018] Further, select welding parameters according to the thickness of the steel plate to be welded, including: according to Select laser welding process parameters, where Q is the heat input of laser welding, W is the output power of laser welding, v is the laser welding speed, and t is the thickness of the steel plate to be welded;

[0019] For the steel plate to be welded with a thickness of less than 1.2 mm, the output power W of laser welding is not less than 3000 W;

[0020] For the steel plate to be welded with a thickness of 1.2 mm - 2 mm, the output power W of laser welding is not less than 4500 W;

[0021] For the steel plate to be welded with a thickness greater than 2 mm, the output power W of laser welding is not less than 6000 W;

[0022] The heat input Q of the laser welding should be controlled between 15 - 30 J / mm 2 between.

[0023] Further, an auxiliary light source is introduced during the laser welding process to monitor the molten pool morphology in real time and feedback to adjust the welding parameters.

[0024] Advantages and positive effects of the present invention:

[0025] Before welding, the present invention pre-cools the area to be welded of the plate to be welded, and selects a suitable combination of welding process parameters according to the thickness of the steel plate to be welded. After welding, the supercooled plate in the unwelded area can cool the heat-affected zone of welding through heat conduction. The peak temperature and the residence time of the peak temperature in the heat-affected zone of high-strength steel welding are reduced, and to a certain extent, the tempering of martensite and the precipitation of carbides in high-strength steel are reduced, so as to inhibit the softening phenomenon of the heat-affected zone of high-strength steel welding, make its tensile strength reach the designed strength of the base material, and the tensile fracture position is on the base material side.

[0026] The technical solution of the present invention does not require a cooling device to be placed on the back of the plate to be welded. The steel plate can be cooled by blowing supercooled gas before welding to achieve the purpose of inhibiting the softening of the heat-affected zone, reducing the tooling difficulty and the manufacturing cost of high-strength steel laser tailor-welded blanks.

[0027] This technical solution does not require post-treatment such as ultrasonic treatment and laser quenching of the steel plate after welding, reduces the steps in the process, and further reduces the manufacturing cost of high-strength steel laser tailor-welded blanks. Description of the Drawings

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the accompanying drawings required for the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other accompanying drawings can also be obtained based on these drawings.

[0029] Figure 1 It is a flowchart of a laser welding method for suppressing softening in the heat affected zone of high-strength steel in an embodiment of the present invention;

[0030] Figure 2 It is a schematic diagram of the fracture position of a tensile specimen with a pre-cooling process in an embodiment of the present invention;

[0031] Figure 3 It is a schematic diagram of the fracture position of a tensile specimen without a pre-cooling process in an embodiment of the present invention;

[0032] Figure 4 It is a schematic diagram of the hardness comparison of laser welded joints under two processes in an embodiment of the present invention. Detailed implementation manners

[0033] In order to enable those skilled in the art to better understand the solution of the present invention, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0034] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above accompanying drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that such used data can be interchanged under appropriate circumstances so that the embodiments of the present invention described here can be implemented in an order different from those illustrated or described here. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or device that includes a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but may include other steps or units not clearly listed or inherent to these process, method, product, or device.

[0035] Embodiment 1:

[0036] As Figure 1 shown, a laser welding method for suppressing softening in the heat affected zone of high-strength steel includes the following steps:

[0037] S1: Perform a surface cleaning operation on the area to be welded of high-strength steel;

[0038] Among them, the high-strength steel can be any one of duplex steel, martensitic steel or high-strength low-alloy steel. The surface cleaning operation includes chemical solvent cleaning or mechanical grinding treatment. In this embodiment, two 1.2-mm-thick 780 MPa grade duplex steel plates are taken, and the original structure of the steel plates before welding is ferrite plus martensite. An appropriate amount of alcohol is taken to clean and wipe the side to be welded of the steel plates, removing the oil stains and impurities on the surface of the steel plates.

[0039] S2: Fix the high-strength steel and perform active cooling through gas purging to reduce the temperature of the area to be welded to a predetermined low temperature range;

[0040] Among them, the active cooling uses at least one medium of inert gas, liquid nitrogen atomization gas or compressed low-temperature gas. In this embodiment, the steel plates cleaned in step S1 are placed in a welding fixture, and an overcooled nitrogen gas is sprayed on the steel plates within 5 mm from the edge of the side to be welded by using a gas purging device, and the gas purging device is moved uniformly to cool the side to be welded of the steel plates evenly to 10°C. Among them, the high-strength steel is fixed by an adjustable fixture, and a cooling channel is integrated in the fixture.

[0041] S3: Configure the laser welding process parameter combination;

[0042] Among them, the laser welding process parameter combination includes the laser power W, the welding speed v, and the laser welding heat input Q. Configuring the laser welding process parameter combination includes: selecting the welding parameters according to the thickness of the steel plate to be welded.

[0043] Specifically, selecting the welding parameters according to the thickness of the steel plate to be welded includes: according to Select the laser welding process parameters, where Q is the laser welding heat input, W is the laser welding output power, v is the laser welding speed, and t is the thickness of the steel plate to be welded;

[0044] For the steel plate to be welded with a thickness of less than 1.2 mm, the laser welding output power W is not less than 3000 W;

[0045] For the steel plate to be welded with a thickness of 1.2 mm - 2 mm, the laser welding output power W is not less than 4500 W;

[0046] For the steel plate to be welded with a thickness greater than 2 mm, the laser welding output power W is not less than 6000 W;

[0047] The laser welding heat input Q should be controlled between 15 - 30 J / mm 2 between.

[0048] In this embodiment, an IPG6000W fiber laser is used to weld the plates to be welded. The laser power W of the welding process is 4500W, and the welding speed v is 140mm / s. The calculated welding heat input Q in this embodiment is 26.8J / mm 2 , and the shielding gas is argon with a gas flow rate of 19L / min.

[0049] S4: Laser weld the steel plate cooled in step S2 using the process of step S3, and complete the laser welding in a low-temperature environment;

[0050] After welding, observe the surface of the weld seam. The weld seam has good formation without spatter and a high weld penetration rate. Take tensile specimens from the laser tailor-welded blanks without macroscopic defects. Through the mechanical property testing of the mechanical specimens, it is found that for the 780MPa grade dual-phase steel welded with the pre-cooling process, the maximum tensile strength of its welded joint is 848MPa, the average tensile strength of the three tensile specimens is 836MPa, and the fracture positions are all on the base metal side.

[0051] Example 2:

[0052] The difference between this embodiment and Example 1 is that the steel plate to be welded is a 980MPa grade dual-phase steel plate with a thickness of 1.4mm, the pre-cooling temperature is 0°C, the laser power W is 4700W, the welding speed v is 170mm / s, and the welding heat input Q is 19.7J / mm 2 . Other processes are the same as those in Example 1. Similarly, conduct mechanical property testing on the laser tailor-welded blanks after welding. The average tensile strength is 1011MPa, the fracture position is on the base metal side, and the fracture position is as Figure 2 shown.

[0053] Example 3:

[0054] The difference between this embodiment and Example 1 is that the steel plate to be welded is a 1180MPa grade dual-phase steel with a thickness of 1.6mm. The cooling temperature is -30°C, the laser power W is 4700W, the welding speed v is 120mm / s, and the welding heat input Q is 24.5J / mm 2 . Other processes are the same as those in Example 1. Similarly, conduct mechanical property testing on the laser tailor-welded blanks after welding. The average tensile strength is 1202MPa, and the fracture position is on the base metal side.

[0055] Comparative Example 1:

[0056] The difference between this comparative example and Example 2 is that the 1.4mm thick 980MPa grade dual-phase steel is not pre-cooled before welding. The laser power W is 4700W, the welding speed v is 90mm / s, and the welding heat input Q is 37.3J / mm 2。The mechanical properties of the laser welded blanks after butt welding were tested. The tensile strength of the weld was 940 MPa, and the fracture location was in the heat affected zone, as shown in Figure 3 the figure.

[0057] Comparative Example 2:

[0058] The difference between this comparative example and Comparative Example 1 is that the 1.4 mm thick 980 MPa grade dual-phase steel was not pre-cooled. The welding process was the same as that of Example 2. The mechanical properties of the laser welded blanks after butt welding were tested. The tensile strength of the weld was 967 MPa, and the fracture location was in the heat affected zone.

[0059] Comparative Example 3

[0060] The difference between this comparative example and Comparative Example 1 is that the 1.4 mm thick 980 MPa grade dual-phase steel was pre-cooled, and the pre-cooling temperature was 0 °C. The welding process was the same as that of Comparative Example 1. The mechanical properties of the laser welded blanks after butt welding were tested. The tensile strength of the weld was 975 MPa, and the fracture location was in the heat affected zone.

[0061] The tensile strength of the laser welded blanks after welding in the three examples could exceed the design strength of the base metal, and the fracture location was in the base metal. In the three comparative examples, the tensile strength of the laser welded blanks was 940 - 975 MPa, lower than the design strength of the 980 MPa base metal, and the fracture location was in the heat affected zone. Through the three comparative examples, it was found that during the laser welding of high-strength steel, it was necessary to control the laser welding heat input Q between 15 - 30 J / mm 2 while applying pre-cooling treatment to achieve the purpose of effectively suppressing the softening of the heat affected zone in the laser welding of high-strength steel. Figure 4 Fig. is the hardness comparison diagram between Example 1 and the comparative examples. It can be seen that the pre-cooling process before welding can effectively reduce the width of the heat affected zone and increase the hardness of the heat affected zone. It can be seen that the pre-cooling process before welding has an obvious inhibitory effect on the softening phenomenon of the heat affected zone in the laser welding of high-strength steel, can effectively reduce the influence of the welding softening phenomenon of high-strength steel on its welding performance, and make the tensile fracture location of the high-strength steel laser welded blank located in the base metal.

[0062] In the laser welding method in the above examples, pre-cooling treatment was performed on the steel plate before welding, which could effectively inhibit the softening phenomenon of the heat affected zone in the laser welding of high-strength steel. This method achieved the purpose of suppressing or eliminating the adverse effects of the softening of the heat affected zone, significantly improved the mechanical properties of the laser welded blanks. There was no need to increase the back cooling device, nor was it necessary to reprocess the steel plate after welding, greatly improving the production efficiency and reducing the production cost.

[0063] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A laser welding method for suppressing softening in the heat-affected zone of high-strength steel, characterized in that, It includes the following steps: S1: Perform a surface cleaning operation on the area to be welded of the high-strength steel; S2: Fix the high-strength steel and actively cool it by purging with supercooled gas to reduce the temperature of the area to be welded to between -40°C and 20°C; S3: Configure the laser welding process parameter combination; the welding process parameters include: laser welding heat input, laser welding output power, and laser welding speed; S4: Perform laser welding on the steel plate cooled in step S2 using the process in step S3 to complete laser welding in a low-temperature environment.

2. The method according to claim 1, characterized in that The supercooled gas includes inert gas, liquid nitrogen atomized gas, or compressed cryogenic gas.

3. The method according to claim 1, wherein The surface cleaning operation includes chemical solvent cleaning or mechanical grinding treatment.

4. The method according to claim 1, wherein During laser welding, the cooled steel plate is fixed by an adjustable fixture.

5. The method according to claim 1, wherein Configuring the laser welding process parameter combination includes: selecting welding parameters according to the thickness of the steel plate to be welded.

6. The method according to claim 5, characterized in that Select welding parameters according to the thickness of the steel plate to be welded, including: according to Select laser welding process parameters, where Q is the heat input of laser welding, W is the output power of laser welding, v is the laser welding speed, and t is the thickness of the steel plate to be welded; For a steel plate to be welded with a thickness of less than 1.2 mm, the laser welding output power W is not less than 3000 W; For a steel plate to be welded with a thickness of 1.2 mm - 2 mm, the laser welding output power W is not less than 4500 W; For a steel plate to be welded with a thickness greater than 2 mm, the laser welding output power W is not less than 6000 W; The laser welding heat input Q should be controlled between 15 - 30 J / mm 2 in between.

7. The method according to claim 1, wherein An auxiliary light source is introduced during the laser welding process to monitor the molten pool morphology in real time and feedback to adjust the welding parameters.

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