Laser welding method for improving performance of coating hot-formed ultrahigh-strength steel butt joint

Abstract

A laser welding method for improving the performance of a butt joint of coating hot-formed ultrahigh-strength steel comprises the steps that two pieces of Al-Si coating hot-formed ultrahigh-strength steel are placed on a welding platform in a butt joint mode after being cleaned and dried, and a welding clamp is used for clamping and fixing the Al-Si coating hot-formed ultrahigh-strength steel; alloy powder with the particle size of 150-400 meshes is coated at the joint of the two pieces of Al-Si coating hot forming ultrahigh strength steel, and the coating thickness is 50-200 microns; the alloy powder is FeN4 alloy powder; laser welding is carried out on the joint portion of the two pieces of Al-Si coating hot forming ultrahigh strength steel, during welding, the incident angle of laser ranges from 85 degrees to 90 degrees, the power of the laser ranges from 1 kW to 2.5 kW, the welding speed ranges from 1 m / min to 4 m / min, argon is adopted for protecting a molten pool, and the gas flow ranges from 10 L / min to 25 L / min. According to the method, the strength of the weld joint can be greatly improved, weld joint collapse can be effectively prevented, the forming quality of the weld joint can be remarkably improved, and the weld joint structure can be effectively optimized.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of laser processing of metal materials, and specifically relates to a laser welding method for improving the performance of a butt joint of a plated hot-formed ultra-high-strength steel. BACKGROUND

[0002] Automobile lightening is an important direction of the development of the automobile industry. Hot-formed steel, as a light-weight material, has the advantages of high strength, good forming performance, the ability to manufacture complex parts and resistance to large impact loads, and is currently widely used in automobile key components. Hot-formed steel (PHS) is an alloy steel containing a small amount of boron elements, which is heated in a furnace and then transferred to a closed mold for quenching and forming (the cooling speed must exceed 30℃ / s), and the maximum tensile strength can reach 1500 MPa or even higher.

[0003] During the hot forming process, the surface of the plate is prone to decarburization and oxidation when it is transferred at high temperature. This will have many negative effects. On the one hand, the surface structure of the steel after decarburization will decrease in hardness, and the ability to resist crack initiation will also decrease; on the other hand, metal oxides will form on the surface of the steel, which will fall off during the stamping process, which will further exacerbate the wear of the die, ultimately affecting the service life of the die. To solve the problem of surface metal oxides, early factories often use sandblasting or shot blasting processes for treatment, which effectively cleans the oxide layer but increases production procedures and costs. At the same time, this cannot solve the problem of surface decarburization. Therefore, new solutions (plating technology) are sought to effectively prevent surface metal oxidation and decarburization. The Al alloy series of plating is the most popular on the market, especially the Al-Si plating, which can maintain the form and performance of the plating at high temperatures. For example, the Al-Si plated hot-formed steel produced by ArcelorMittal has an annual output of more than 3.5 million tons, which has greatly promoted the development of hot-formed steel. During the hot forming process, Al-Si plating diffuses Al, Fe and Si atoms under high temperature conditions to form a high-temperature-resistant compound, thereby protecting the surface of the steel substrate from decarburization and oxidation.

[0004] In the application of hot formed ultra-high strength steel with coating, the welding problem will inevitably be encountered. At present, there are two major problems in the welding of hot formed steel for automobile enterprises. First, after welding of hot formed steel, softening zone is prone to occur at the joint. Second, in order to prevent decarburization and oxidation of steel during hot stamping, a coating is coated on the surface, which will melt into the weld during welding, which will cause the deterioration of the mechanical properties of the joint, and the strength will be much lower than that of the base material. The former problem has been well solved: first, directly weld the blank, and then hot stamp the part, which can avoid the occurrence of softening zone at the joint, and also meet the performance requirements. The latter problem has not yet appeared a perfect solution. Therefore, how to eliminate the negative effects of the coating and achieve the strength of the welded joint comparable to that of the base material is the key to improving the welding technology of hot formed steel.

[0005] Compared with resistance welding and arc welding, laser welding has the advantages of high precision, small heat input, small heat affected zone, and higher joint mechanical properties. In addition, laser welding is a non-contact welding method, which can completely avoid the electrode wear of resistance spot welding; there is no limitation on the size and size of the material. Therefore, in recent years, laser welding of Al-Si coated hot formed ultra-high strength steel has attracted much attention. However, during laser welding, the Al-Si coating enters the molten pool and forms non-equilibrium high-temperature δ-ferrite (a low-toughness low-strength phase), causing macrosegregation, which seriously endangers the mechanical properties of the joint. Therefore, various automobile companies and research institutions are trying to find ways to suppress the negative effects of the coating. The main methods include removing the coating before welding, special welding techniques, and filler alloys. The method of removing the coating before welding increases a cleaning process, which will complicate the entire welding process, and also increase the manufacturing cost; special welding techniques, such as vibration laser welding and pulse laser welding. By applying auxiliary energy to make the molten pool vibrate, the microstructure is uniform, however, these welding processes are complex to operate and have high implementation costs.

[0006] At present, the method of adding alloy elements to improve the structure is more and more valued by researchers and enterprises. The commonly used method of adding alloy elements to improve the structure is mainly adding austenite elements to improve the formation of ferrite. The mechanism is that by adding austenite elements, the interval of austenite formation in the transformation process is increased, thereby increasing the amount of austenite transformation (i.e. the amount of martensite after low temperature phase change) and reducing the formation of high temperature ferrite. Although this method can reduce the formation of high temperature ferrite, research has found that the amount of added austenite elements needs to reach a high proportion to limitedly improve the strength of the lap joint of laser welding. Even when the amount of addition is further increased, it will also produce an adverse phase that is not conducive to the strength. With the increase of the amount of added austenite alloy elements Ni or Cu, the size and content of ferrite in the weld will decrease, but after a critical value, the addition of alloy elements will generate a new phase that is not conducive to the strength of the joint, resulting in a decrease in the strength of the joint. Therefore, in order to effectively solve the problems of the prior art, it is urgent to provide a laser welding method for improving the performance of a plated hot-formed ultra-high strength steel butt joint. SUMMARY

[0007] In view of the problems existing in the prior art, the present application provides a laser welding method for improving the performance of a plated hot-formed ultra-high strength steel butt joint. The method has a simple implementation process and low implementation cost, can greatly improve the strength of the weld, effectively prevent the collapse of the weld, significantly improve the forming quality of the weld, and effectively optimize the weld structure.

[0008] In order to achieve the above-mentioned purpose, the present application provides a laser welding method for improving the performance of a plated hot-formed ultra-high strength steel butt joint, comprising the following steps: Step one: two pieces of Al-Si plated hot-formed ultra-high strength steel are placed on the welding platform in a butt joint manner after cleaning and drying treatment, and are clamped and fixed by using a welding fixture; Step two: the alloy powder with a particle size of 150-400 mesh is coated on the joint of the two pieces of Al-Si plated hot-formed ultra-high strength steel, and the coating thickness is controlled to be 50-200 μm; wherein the alloy powder is FeN4 alloy powder; Step three: laser welding is performed on the joint part of the two pieces of Al-Si plated hot-formed ultra-high strength steel. During welding, the incident angle of the laser is 85-90°, the power of the laser is 1-2.5 kW, the welding speed is 1-4 m / min, and argon is used to protect the molten pool, and the gas flow of argon is 10-25 L / min.

[0009] In order to ensure that the tensile strength of the joint after welding can meet the requirements, in step one, the thickness of the Al-Si plated hot-formed ultra-high strength steel is 1-3 mm, and the aluminum-silicon plating layer is provided on both the front and back surfaces, and the thickness of the aluminum-silicon plating layer is 20-40 μm.

[0010] In order to ensure the tensile strength of the joint, in step two, the particle size of the alloy powder is 300 mesh, the coating thickness is 150 microns, the thickness of the Al-Si plated hot forming ultra-high strength steel is 1 mm, the thickness of the aluminum silicon plating layer is 30 microns, the incident angle of the laser is 85 degrees, the power of the laser is 2 kW, the welding speed is 3 m / min, and the argon gas flow rate is 15 L / min.

[0011] In order to ensure the tensile strength of the joint, in step two, the particle size of the alloy powder is 300 mesh, the coating thickness is 150 microns, the thickness of the Al-Si plated hot forming ultra-high strength steel is 1 mm, the thickness of the aluminum silicon plating layer is 30 microns, the incident angle of the laser is 85 degrees, the power of the laser is 2 kW, the welding speed is 3 m / min, and the argon gas flow rate is 15 L / min.

[0012] In order to ensure the cleaning and drying quality, in step one, the cleaning and drying process is as follows: S11: acetone is used to clean the Al-Si plated hot forming ultra-high strength steel, and during the cleaning process, cotton balls are used to wipe the front and back surfaces of the Al-Si plated hot forming ultra-high strength steel to remove impurities on the surface of the Al-Si plated hot forming ultra-high strength steel; S12: rapid blow-drying treatment is performed, and then the Al-Si plated hot forming ultra-high strength steel is placed in a dry place for welding.

[0013] In order to ensure the coating quality of the alloy powder, in step two, the alloy powder is spread along the joint direction of the two Al-Si plated hot forming ultra-high strength steels by means of adhesion.

[0014] Compared with the prior art, the present application has the following technical advantages: according to the knowledge of materials science, C, N, Cu, Ni, Mn and Co are all austenite forming elements. In particular, the austenitizing effect of C and N elements is very obvious. However, in actual use, C powder is easily lost during laser welding, resulting in very scattered welding results. At the same time, it is difficult to add N element in the form of pure element. The present application innovatively finds that the commonly used alloy FeN4 on the market contains N, and the composition is uniform and stable, and the alloy element Fe is also the base element of the base material. Adding FeN4 alloy powder to the joint part can significantly expand the austenite phase region. Further, the present application uses phase transition simulation software and basic knowledge of materials science, and through a large number of experimental processes and design combined practical methods, adds austenite alloy elements, strictly controls the content of alloy elements, and combines the matching laser welding process parameters to weld the aluminum silicon plated hot forming ultra-high strength steel, which can make the weld form well and greatly improve the joint strength.

[0015] The method has simple implementation process and low implementation cost, the innovative FeN4 alloy powder is used as the coating material at the joint, the amount of the FeN4 alloy powder is accurately controlled, the content of ferrite with low toughness and low strength in the weld can be effectively reduced after the N element in the FeN4 alloy powder enters the weld, the strength of the weld is greatly improved, meanwhile, the metal is filled due to the addition of the alloy, the weld collapse is effectively prevented, the weld forming quality can be obviously improved, the weld structure can be effectively optimized, the tensile strength of the weld part is obviously improved compared with the tensile strength of the traditional weld part, and the problems of low strength of the plated hot-formed ultra-high strength steel butt joint and easy weld collapse in the traditional welding process are solved. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 is a flow chart of the present application; Figure 2 is a weld structure image of example 1 in the present application; Figure 3 is a weld structure image of example 2 in the present application; Figure 4 is a weld structure image of the comparative example in the present application. DETAILED DESCRIPTION

[0017] The present application will be further described below in combination with the drawings.

[0018] As shown in Figure 1 , the present application provides a laser welding method for improving the performance of plated hot-formed ultra-high strength steel butt joint, comprising the following steps: Step one: after being cleaned and dried, two pieces of Al-Si plated hot-formed ultra-high strength steel are placed on a welding platform in a butt joint manner, and are clamped and fixed by using a welding fixture; Step two: the alloy powder with a particle size of 150-400 mesh is coated on the joint of the two pieces of Al-Si plated hot-formed ultra-high strength steel, and the coating thickness is controlled to be 50-200 μm; wherein the alloy powder is FeN4 alloy powder; Step three: the joint parts of the two pieces of Al-Si plated hot-formed ultra-high strength steel are laser welded, when welding, the incident angle of the laser is 85-90°, the power of the laser is 1-2.5 kW, the welding speed is 1-4 m / min, and argon is used to protect the molten pool, and the gas flow of the argon is 10-25 L / min.

[0019] The tensile strength of the joint after welding can meet the requirements, in step one, the thickness of the Al-Si plated hot-formed ultra-high strength steel is 1-3 mm, and the aluminum-silicon plating layer is provided on both sides of the Al-Si plated hot-formed ultra-high strength steel, and the thickness of the aluminum-silicon plating layer is 20-40 μm.

[0020] In order to ensure the tensile strength of the joint, in step two, the particle size of the alloy powder is 300 mesh, the coating thickness is 150 μm, the thickness of the Al-Si coated hot formed ultra-high strength steel is 1 mm, the thickness of the aluminum silicon coating is 30 μm, the incident angle of the laser is 85°, the power of the laser is 2 kW, the welding speed is 3 m / min, and the argon gas flow rate is 15 L / min.

[0021] In order to ensure the tensile strength of the joint, in step two, the particle size of the alloy powder is 300 mesh, the coating thickness is 150 μm, the thickness of the Al-Si coated hot formed ultra-high strength steel is 1 mm, the thickness of the aluminum silicon coating is 30 μm, the incident angle of the laser is 85°, the power of the laser is 2 kW, the welding speed is 3 m / min, and the argon gas flow rate is 15 L / min.

[0022] In order to ensure the cleaning and drying quality, in step one, the cleaning and drying process is as follows: S11: acetone is used to clean the Al-Si coated hot formed ultra-high strength steel, and during the cleaning process, cotton balls are used to wipe the front and back surfaces of the Al-Si coated hot formed ultra-high strength steel to remove impurities on the surface of the Al-Si coated hot formed ultra-high strength steel; S12: rapid drying treatment is performed, and then the Al-Si coated hot formed ultra-high strength steel is placed in a dry place for welding.

[0023] In order to ensure the coating quality of the alloy powder, in step two, the alloy powder is spread along the joint direction of the two Al-Si coated hot formed ultra-high strength steels by means of adhesion.

[0024] Embodiment 1: The embodiment provides a laser welding method for butt joint performance of high-coated hot formed ultra-high strength steel. Two Al-Si coated hot formed ultra-high strength steels with a thickness of 1 mm are prepared. The front and back surfaces of the Al-Si coated hot formed ultra-high strength steels are provided with aluminum silicon coatings, and the thickness of the aluminum silicon coatings is 30 μm. Acetone is used to clean the front and back surfaces of the Al-Si coated hot formed ultra-high strength steels, and then rapid drying treatment is performed. The two Al-Si coated hot formed ultra-high strength steels after the cleaning and drying treatment are placed in a butt joint manner on a welding platform and clamped and fixed by using a welding fixture. FeN4 alloy powder with a particle size of 300 mesh is coated on the joints of the two Al-Si coated hot formed ultra-high strength steels, and the coating thickness is 150 μm. Laser technology is used for welding. During welding, the incident angle of the laser is 85°, the power of the laser is 2 kW, the welding speed is 3 m / min, and argon is used for protection during welding, and the argon gas flow rate is 15 L / min.

[0025] The joint image after welding is collected, and the tensile strength of the joint is calculated from the joint image. Figure 2As can be seen from the above process, the weld structure obtained by the above process has almost no ferrite, and almost all is transformed into martensite structure. The upper and lower surfaces of the joint part have no collapse phenomenon. The tensile strength experiment of the joint part shows that the tensile strength of the joint part reaches 1380MPa, which is significantly higher than the tensile strength of the joint part after welding without adding alloy powder (1020MPa).

[0026] Example 2: The embodiment provides a laser welding method for butt joint performance of high-coating hot-formed ultra-high-strength steel. Two pieces of Al-Si coating hot-formed ultra-high-strength steel with a thickness of 1mm are prepared. The front and back surfaces of the Al-Si coating hot-formed ultra-high-strength steel are provided with aluminum-silicon coatings, and the thickness of the aluminum-silicon coatings is 30μm. The front and back surfaces of the Al-Si coating hot-formed ultra-high-strength steel are cleaned with acetone, and then subjected to rapid blow-drying treatment. The two pieces of Al-Si coating hot-formed ultra-high-strength steel are placed on a welding platform in a butt joint manner after the cleaning and drying treatment, and clamped and fixed by using a welding fixture. FeN4 alloy powder with a particle size of 300 mesh is applied to the joint of the two pieces of Al-Si coating hot-formed ultra-high-strength steel, so that the application thickness is 50μm. Laser process is used for welding. During welding, the incident angle of the laser is 85°, the power of the laser is 2kW, and the welding speed is 3m / min. In the process of welding, argon is used for protection, and the gas flow of argon is 15L / min.

[0027] The joint image after welding is collected, and the microstructure of the joint is observed. Figure 3 As can be seen from the above process, the weld structure obtained by the above process has almost no ferrite, and almost all is transformed into martensite structure. The upper and lower surfaces of the joint part have no collapse phenomenon. The tensile strength experiment of the joint part shows that the tensile strength of the joint part reaches 1380MPa, which is significantly higher than the tensile strength of the joint part after welding without adding alloy powder (1020MPa).

[0028] Comparative example: The difference between the comparative example and example 1 is that FeN4 coating is not applied to the joint before welding by using laser process, and the rest of the conditions are the same as those of example 1.

[0029] The joint image after welding is collected, and the microstructure of the joint is observed. Figure 4 As can be seen from the above process, the weld structure obtained by the above process has almost no ferrite, and almost all is transformed into martensite structure. The upper and lower surfaces of the joint part have no collapse phenomenon. The tensile strength experiment of the joint part shows that the tensile strength of the joint part reaches 1380MPa, which is significantly higher than the tensile strength of the joint part after welding without adding alloy powder (1020MPa).

Claims

1. A laser welding method for improving the performance of butt joints of hot-formed ultra-high-strength steel with coating, characterized in that, Includes the following steps: Step 1: After cleaning and drying, place two pieces of Al-Si coated hot-formed ultra-high-strength steel on the welding platform in a butt joint manner, and use welding clamps to fix them. Step 2: Apply the pre-mixed alloy powder with a particle size of 150-400 mesh to the joint of two pieces of Al-Si coated hot-formed ultra-high strength steel, and control the coating thickness to be 50-200 μm; wherein, the alloy powder is FeN4 alloy powder. Step 3: Laser welding is performed on the joint of the two Al-Si coated hot-formed ultra-high strength steel pieces. During welding, the incident angle of the laser is 85-90°, the laser power is 1-2.5kW, the welding speed is 1-4m / min, and argon gas is used to protect the molten pool with a flow rate of 10-25L / min.

2. The laser welding method for improving the performance of butt joints of hot-formed ultra-high-strength steel with coating as described in claim 1, characterized in that, In step one, the thickness of the Al-Si coated hot-formed ultra-high strength steel is 1-3 mm, and both its front and back sides are coated with aluminum-silicon coatings with a thickness of 20-40 μm.

3. The laser welding method for improving the performance of butt joints of hot-formed ultra-high-strength steel with coating as described in claim 2, characterized in that, In step two, the alloy powder has a particle size of 300 mesh, a coating thickness of 150 μm, an Al-Si coating thickness of 1 mm, an aluminum-silicon coating thickness of 30 μm, a laser incident angle of 85°, a laser power of 2 kW, a welding speed of 3 m / min, and an argon gas flow rate of 15 L / min.

4. The laser welding method for improving the performance of butt joints of hot-formed ultra-high-strength steel with coating as described in claim 2, characterized in that, In step two, the alloy powder has a particle size of 300 mesh, a coating thickness of 50 μm, an Al-Si coating thickness of 1 mm, an aluminum-silicon coating thickness of 30 μm, a laser incident angle of 85°, a laser power of 2 kW, a welding speed of 3 m / min, and an argon gas flow rate of 15 L / min.

5. The laser welding method for improving the performance of butt joints of hot-formed ultra-high-strength steel with coating as described in claim 1, characterized in that, In step one, the cleaning and drying process is as follows: S11: Acetone is used to clean the Al-Si coated hot-formed ultra-high strength steel. During the cleaning process, cotton balls soaked in acetone are used to wipe the front and back surfaces of the Al-Si coated hot-formed ultra-high strength steel to remove impurities from the surface of the Al-Si coated hot-formed ultra-high strength steel. S12: Perform a rapid drying process, and then place it in a dry place to await soldering.

6. The laser welding method for improving the performance of butt joints of hot-formed ultra-high-strength steel with coating as described in claim 1, characterized in that, In step two, the alloy powder is spread along the joint direction of the two Al-Si coated hot-formed ultra-high-strength steel pieces by means of an adhesive.

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