A laser welding method for 1500MPa aluminum-silicon hot-formed steel

Abstract

This invention provides a method for laser welding 1500MPa aluminum-silicon hot-formed steel. The method includes removing surface contaminants from two hot-formed steel plates, placing them in a welding fixture and fixing the plates, and then performing laser welding under inert gas protection. During the welding process, welding wire is added to the weld seam to obtain a laser-welded plate. The chemical composition and mass percentage of the welding wire used in the laser welding process are as follows: C: 0.04–0.15; Si: 0.20–0.50; Mn: 1.00–1.60; Cr: 0.20–0.50; Mo: 0.40–0.80; Ni: 0.30–5.0; W: 0.02–2.0; Cu: 0.1–0.40; P: ≤0.020; S: ≤0.015. In this invention, the welding wire and laser welding process work together to produce a martensitic weld metal structure. After quenching in a hot-forming mold, the tensile strength of the weld seam is ≥1500MPa.

Description

Technical Field

[0001] This invention relates to the field of metal material processing technology, specifically to a welding method for 1500MPa aluminum-silicon hot-formed steel, and more particularly to a laser welding method for 1500MPa aluminum-silicon hot-formed steel. Background Technology

[0002] Aluminum-silicon hot-formed steel has been applied to automobile bodies. Before being stamped into automotive parts, the hot-formed steel sheets need to be laser-welded to form the required stamping shape. Then, they are heated and held in a furnace. Afterward, the heated steel sheets are placed in a hot-forming mold for stamping and in-mold quenching, resulting in high-strength automotive parts. Hot-formed steel sheets are classified according to their surface coating: uncoated hot-formed steel, Al-Si coated hot-formed steel, and Zn coated hot-formed steel. During laser welding of Al-Si coated hot-formed steel, alloying elements in the coating enter the weld, promoting the formation of δ-ferrite. This δ-ferrite does not undergo a phase transformation during subsequent weld cooling and remains in the weld, severely weakening its strength and affecting the production and application of Al-Si coated hot-formed steel.

[0003] To address the challenges of laser welding of Al-Si coated hot-formed steel, adding alloying elements to the weld seam during the laser welding process to improve or suppress the formation of δ-ferrite is a crucial solution. Existing patent CN106475683 describes a laser welding method for Al-Si coated hot-formed steel sheets. This patent employs pre-placed nickel or chromium foil of a certain thickness within the weld seam, resulting in a martensitic weld microstructure after laser welding. The biggest drawback of this patent is its impracticality for industrial mass production. Furthermore, the type of nickel or chromium foil significantly impacts the weld seam formability and penetration rate, and different thicknesses of Al-Si coated hot-formed steel sheets require different types of nickel or chromium foil. Therefore, this patent lacks practical industrial applicability.

[0004] Existing patent CN111390425B discloses welding wire and welding method for laser welding of hot-stamped Al-Si coated plates. This patent achieves direct welding of Al-Si coated hot-formed steel plates without removing the coating by controlling the gap between the welded plates and adding welding wire of appropriate diameter to the weld seam. However, the welding wire in this patent contains a high proportion of precious metals, resulting in high production costs for the laser-welded plates. Furthermore, this patent requires selecting welding wires of different diameters for Al-Si coated hot-formed steel plates of varying thicknesses, necessitating the stockpiling of a series of welding wires of different diameters in industrial production, leading to high operating costs. Changing the welding wire when welding different thickness plates also disrupts production schedules.

[0005] Existing patent CN110666275A discloses a method for welding hot-formed steel with aluminum or aluminum alloy coating. This patent involves removing the aluminum or aluminum alloy coating and part of the substrate from the surface of the hot-formed steel plate, and then welding it using laser filler wire welding. The process of removing the coating and part of the substrate from the steel plate requires high precision, making industrial production difficult. The composition of the welding wire and the composition of the steel plate being welded are strictly limited, requiring the manufacture of different welding wire compositions for welding steel plates with different compositions. Therefore, this method cannot be widely applied.

[0006] Therefore, a low-cost laser welding method for aluminum and aluminum alloy coated hot-formed steel is needed to eliminate the problem of low weld joint strength after laser welding of aluminum and aluminum alloy coated hot-formed steel. Summary of the Invention

[0007] To address the aforementioned technical issues, a laser welding method for 1500MPa aluminum-silicon hot-formed steel is provided. This method solves the problem of low weld joint strength in aluminum and aluminum alloy coated hot-formed steel after laser welding and hot forming. Furthermore, by utilizing a new welding wire in conjunction with the laser welding process, the weld joint strength of aluminum and aluminum alloy coated hot-formed steel after laser welding and hot forming is higher than that of the hot-formed steel plate.

[0008] The specific solution of this invention is as follows:

[0009] A method for laser welding 1500MPa aluminum-silicon hot-formed steel includes: removing surface contaminants from two hot-formed steel plates, placing them in a welding fixture and fixing the plates; adjusting the position of the laser welding wire to the center of the weld seam at an angle of 5°–35° to the steel plates, with the tip of the welding wire positioned 0–1mm from the intersection of the laser beam and the upper surface of the steel plate, and the wire feeding direction opposite to the laser welding direction; performing laser welding of the steel plates under inert gas protection; adding welding wire to the weld seam during the welding process to obtain a laser-welded plate; the welding wire, after being melted by the laser beam, enters the molten pool generated by the laser beam on the steel plate in liquid form, and the molten metal of the welding wire and the molten metal of the steel plate fuse together to form the weld metal; the steel plate is aluminum or aluminum alloy coated hot-formed steel, and the single-sided coating weight of the aluminum or aluminum alloy coating is 10–40 g / m². 2 .

[0010] The chemical composition and mass percentage of the welding wire used in the laser welding process are as follows: C: 0.04–0.15%; Si: 0.20–0.50%; Mn: 1.00–1.60%; Cr: 0.20–0.50%; Mo: 0.40–0.80%; Ni: 0.30–5.0%; W: 0.02–2.0%; Cu: 0.1–0.40%; P: ≤0.020%; S: ≤0.015%. The welding wire may also include one or more of V: ​​0–0.15%, Nb: 0.01–0.20%, and B: 0.002–0.005%, with the balance being Fe and unavoidable impurity elements.

[0011] After welding, the excess weld height is removed from the steel plate surface, ensuring it is controlled within 0–0.2 mm of the plate surface. If the weld surface is lower than the plate surface, the weld cross-sectional area is small, resulting in insufficient weld strength after hot forming. If the weld surface is too high, it causes severe wear on the hot forming mold and can lead to insufficient cooling during hot forming, reducing strength. Therefore, the excess weld height is controlled within 0–0.2 mm. Removal methods include milling, scraping, and grinding.

[0012] After the hot forming process, the weld has a tensile strength ≥1500MPa, a martensitic microstructure, and the fracture location is in the steel plate. The heating temperature in the hot forming process is 950-970℃, the holding time is 5-8 minutes, and the subsequent cooling rate is not less than 30℃ / s.

[0013] The functions and purposes of each component in the welding wire are as follows:

[0014] Carbon (C) is an important element for improving weld strength and increasing weld metal hardenability. It is also an austenite-enlarging element, and increasing the C content can effectively prevent the formation of δ-ferrite in the high-temperature region of the weld. However, increasing the C content is detrimental to the smelting and drawing of the welding wire. Too low a C content results in low weld strength and is not effective in controlling high-temperature ferrite in the weld. Considering all factors, the C content of the welding wire is designed to be 0.04–0.15%.

[0015] Si is a deoxidizing element and also plays a role in solid solution strengthening in weld metal. Low Si content in welding wire leads to insufficient deoxidation, increasing oxide levels and affecting the wire's drawability and strength. Excessive Si content increases non-metallic inclusions in the weld and weld slag on the weld surface. Considering all factors, the Si content of the welding wire in this invention is controlled between 0.20% and 0.50%.

[0016] Mn can both improve weld strength and act as a deoxidizer. Mn can expand the austenite phase region, which is beneficial for the formation of more austenite structure during welding, resulting in martensite structure after cooling. As the Mn content increases, the strength of the steel plate increases; however, excessively high Mn content can easily lead to Mn segregation in the steel, causing an uneven composition distribution. Considering all factors, the Mn content in the welding wire of this invention is controlled between 1.00% and 1.60%.

[0017] P and S are harmful elements, and the lower their content in welding wire, the better. However, deep P and S removal smelting costs are high. Considering all factors, the welding wire of this invention has P≤0.020% and S≤0.015%.

[0018] Cr (Cr) increases the hardenability of steel, improving the hardness, strength, and corrosion resistance of weld metal. Appropriately increasing Cr content positively impacts the hardenability of weld metal, ensuring the formation of martensite during hot forming. However, Cr promotes ferrite formation, and high Cr content hinders the control of high-temperature ferrite in the weld. Therefore, considering all factors, the Cr content is controlled between 0.20% and 0.50%.

[0019] Mo, dissolved in ferrite and austenite, significantly improves the hardenability of steel and enhances its tempering stability. It also exhibits high strength and creep resistance at high temperatures and reduces temper brittleness. However, excessive Mo content can lead to the formation of twinned martensite in the weld, resulting in high weld hardness and low plasticity, making the weld metal highly susceptible to cracking. The Mo content in the welding wire of this invention is 0.40–0.80%.

[0020] Ni is a strong austenitizing element that can significantly expand the austenite phase region. Maintaining a certain content of Ni, along with C and W, has a positive effect on controlling the formation of high-temperature ferrite in the weld. However, Ni is a precious metal element, and adding too much increases the cost of the welding wire. Therefore, this invention limits the content to Ni: 0.3% to 5.0%.

[0021] Cu (Cu) expands the austenite region, promoting the entry of weld metal into the austenite region during the liquid-to-solid transformation, thus avoiding the high-temperature ferrite region. Cu can increase the strength and toughness of steel plates and improve the fatigue performance of welds. It also has a certain degree of corrosion resistance, improving the corrosion resistance of welds. However, the amount of copper added should not be too high, as its solubility in steel is low; a high content will lead to supersaturation, which is detrimental to hot deformation processing and easily causes copper embrittlement. This invention limits the range to Cu: 0.1–0.4%.

[0022] Tungsten (W) alters the microstructure of the weld metal during cooling, preventing the formation of ferrite. This effectively reduces stress concentration caused by the weld microstructure transformation, improving weld strength. Simultaneously, tungsten effectively inhibits grain growth during heating, minimizing the performance degradation caused by coarse grains. However, W is a precious metal, and excessively high content increases welding wire costs. Therefore, a W content of 0.02%–2.0% is recommended.

[0023] V in welding wire has a grain-refining effect, improving strength without reducing weld toughness. In steel, it can form precipitates with C and N, inhibiting austenite grain growth, while reducing failure susceptibility and cold brittleness, thus improving weldability. However, a large amount of precipitates directly affects the drawability of the welding wire, causing wire breakage during pull-out. Therefore, appropriate addition is necessary.

[0024] Nitrogen (Nb) can refine grain size and reduce the overheating sensitivity and temper brittleness of steel, effectively improving the high-temperature and low-temperature strength of welds.

[0025] Boron (B) is a potent element that significantly improves the hardenability of steel; even trace amounts of B can noticeably enhance the hardenability of steel plates. B has a strong affinity for oxygen (O) and nitrogen (N), making it prone to generating non-metallic inclusions.

[0026] In this invention, the angle between the welding wire and the steel plate is set to 5°–35°. This angle ensures that the weld has a certain surface reinforcement and weld formation. If the angle is too large, the surface reinforcement of the weld is small, while the back reinforcement is large; if the angle is too small, the surface reinforcement is large, while the back reinforcement is small. Simultaneously, this angle results in a uniform weld surface formation, free from welding defects such as undercut and bulges. Testing led to the determination of a welding wire angle between 5° and 35°.

[0027] The distance between the welding wire tip and the intersection of the laser beam and the upper surface of the steel plate is used to control welding spatter and weld formation. When the welding wire tip is less than 0 mm, welding spatter is greater and the weld formation is uneven. When the welding wire tip is greater than 1 mm, the welding wire enters the laser welding zone after contacting the steel plate, resulting in low wire stability and poor weld formation.

[0028] The method disclosed in this invention does not change the existing hot forming process. After the steel plates are welded together, they are heated in a heating furnace, stamped and formed by hot forming molds, and then quenched to finally obtain hot-formed parts.

[0029] Compared with the prior art, the present invention has the following advantages:

[0030] 1. This method allows for laser welding of steel plates without removing the aluminum or aluminum alloy coating. After quenching with a hot forming mold, the weld strength is greater than that of the base material.

[0031] 2. The welding wire for laser welding of the present invention has a simple composition and low cost, and can be manufactured under the existing welding wire production process.

[0032] 3. The welding wire and laser welding process of this invention are combined to suppress the adverse effects of high-temperature ferrite in the weld on the weld. After quenching by hot forming mold, the weld structure is martensitic, resulting in a high-quality welded joint.

[0033] Based on the above reasons, this invention can be widely promoted in fields such as 1500MPa aluminum-silicon hot-forming steel welding methods. Detailed Implementation

[0034] It should be noted that, unless otherwise specified, the embodiments and features described in this invention can be combined with each other. The described embodiments are merely some, not all, of the embodiments of this invention. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the invention or its application or use. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0035] A method for laser welding 1500MPa aluminum-silicon hot-formed steel includes removing surface contaminants from two hot-formed steel plates, placing them in a welding fixture and fixing the plates. The welding wire is positioned at the center of the weld seam at an angle of 5°–35° to the steel plate, with the tip of the wire at a distance of 0–1mm from the intersection of the laser beam and the upper surface of the steel plate. The wire feed direction is opposite to the laser welding direction. Laser welding of the steel plates is performed under argon gas protection with a purity of not less than 99.9%. During the welding process, welding wire is added to the weld seam to obtain a laser-welded plate. After the welding wire is melted by the laser beam, it enters the molten pool generated by the laser beam on the steel plate in liquid form. The molten metal of the welding wire and the molten metal of the steel plate fuse together to form the weld metal. A post-weld scraper is used to remove the excess weld surface height, controlling the excess weld surface height to 0–0.2mm on the steel plate surface. The steel plate is hot-formed steel with an aluminum or aluminum alloy coating, and the single-sided coating weight of the aluminum or aluminum alloy coating is 10-40 g / m². 2 .

[0036] The chemical composition and mass percentage of the welding wire used in the laser welding process are as follows: C: 0.04-0.15%; Si: 0.20-0.50%; Mn: 1.00-1.60%; Cr: 0.20-0.50%; Mo: 0.40-0.80%; Ni: 0.30-5.0%; W: 0.02-2.0%; Cu: 0.1-0.40%; P: ≤0.020%; S: ≤0.015%. The welding wire may also include one or more of V: ​​0-0.15%, Nb: 0.002-0.20%, and B: 0.002-0.005%, with the balance being Fe and unavoidable impurity elements.

[0037] After the hot forming process, the weld has a tensile strength ≥1500MPa, and the fracture occurs in the steel plate. The heating temperature in the hot forming process is 950-970℃, held for 5-8 minutes, and the subsequent cooling rate is not less than 30℃ / s.

[0038] This specific embodiment uses laser wire-filling technology to weld 1500MPa grade 22MnB5 hot-formed steel plates. The surface of the welding test plate was cleaned with industrial alcohol. The test welding wire diameter was 1.2mm, using a Fonnius 7000 wire feeder and a fiber laser. The welding test was conducted under industrial argon gas protection at a gas flow rate of 20L / min. The composition of the test welding wire is shown in Table 1. Parameters such as the welding wire angle and wire feed speed during welding are shown in Table 2.

[0039] Table 1. Chemical composition of welding wire and comparative example (wt%)

[0040] C Si Mn P S Cr Mo Cu Ni W B V Nb Example 1 0.04 0.25 1.54 0.014 0.007 0.20 0.80 0.14 4.05 0.17 0.002 0.05 Example 2 0.15 0.35 1.00 0.015 0.006 0.36 0.65 0.22 0.30 0.02 0.08 Example 3 0.07 0.30 1.32 0.009 0.008 0.26 0.74 0.30 2.50 0.30 0.10 0.13 Example 4 0.12 0.26 1.46 0.016 0.010 0.47 0.53 0.10 3.75 1.80 0.04 Example 5 0.06 0.28 1.30 0.017 0.009 0.28 0.47 0.40 1.20 1.05 0.003 0.15 Example 6 0.12 0.50 1.25 0.014 0.010 0.50 0.40 0.22 5.00 0.98 0.19 Example 7 0.14 0.45 1.60 0.010 0.006 0.29 0.55 0.31 0.60 0.10 0.002 Comparative Example 1 0.07 0.50 1.85 0.015 0.006 0.30 0.35 0.07 2.00 Comparative Example 2 0.12 0.45 1.78 0.012 0.007 0.21 0.60 0.07 1..45 Comparative Example 3 0.05 0.35 1.60 0.011 0.008 0.19 0.86 0.11 Comparative Example 4 0.07 0.38 1.18 0.012 0.010 0 0.77 0 3.30

[0041] Table 2 Laser Welding Parameters

[0042]

[0043] After laser welding, the steel plates are heated in a furnace to 950℃~970℃ for 5~8 minutes, then formed and quenched in a hot forming mold (cooling rate of 30℃ / s). The properties of the welded joint after quenching are shown in Table 3. Table 3 clearly shows that the tensile strength of the steel plate formed using this method and welding wire is ≥1500MPa, and the fracture location is in the base metal, with the weld microstructure being martensite.

[0044] Table 3 Performance of Welded Joints

[0045] Tensile strength (MPa) fracture location weld structure Example 1 1510 parent material Martensite Example 2 1560 parent material Martensite Example 3 1525 parent material Martensite Example 4 1540 parent material Martensite Example 5 1532 parent material Martensite Example 6 1550 parent material Martensite Example 7 1545 parent material Martensite Comparative Example 1 1020 weld Ferrite + Martensite Comparative Example 2 985 weld Ferrite + Martensite Comparative Example 3 978 weld Ferrite + Martensite Comparative Example 4 1015 weld Ferrite + Martensite

[0046] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions 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 method of laser welding 1500 MPa aluminum-silicon hot-formed steel, characterized in that, The process includes removing surface contaminants from steel plates, placing them in a welding fixture and fixing them in place; performing laser welding of the steel plates under a protective gas atmosphere; adding welding wire to the weld seam during the welding process to obtain a laser-welded plate; adjusting the position of the welding wire to the center of the weld seam and at an angle of 5° to 35° to the steel plate during the laser welding process; positioning the tip of the welding wire at a distance of 0 to 1 mm from the intersection of the laser beam and the upper surface of the steel plate; melting the welding wire with the laser beam and then entering the molten pool generated by the laser beam on the steel plate in liquid form; the molten metal of the welding wire and the molten metal of the steel plate are fused together to form the weld metal; After welding, the excess weld height is removed from the steel plate surface to control the excess weld height to 0~0.2mm on the steel plate surface. The steel sheet is an aluminum or aluminum alloy coated hot-formed steel, the aluminum or aluminum alloy coating having a single-sided coating weight of 10 to 40 g / m 2 ; The chemical composition and mass percentage of the welding wire used in laser welding are as follows: C: 0.04~0.15%; Si: 0.20~0.50%; Mn: 1.00~1.60%; Cr: 0.20~0.50%. Mo: 0.40~0.80%; Ni: 0.30~5.0%; W: 0.02~2.0%; Cu: 0.1~0.40%; P: ≤0.020%; S: ≤0.015%, balance is Fe and unavoidable impurity elements.

2. The method of laser welding a 1500 MPa Al-Si hot formed steel strip according to claim 1, wherein, The welding wire also includes one or more of the following: V: 0~0.15%, Nb: 0.01~0.20%, and B: 0.002~0.005%.

3. The method of laser welding a 1500 MPa Al-Si hot formed steel of claim 1, wherein, The weld metal has a martensitic structure.

4. The laser welding method for 1500MPa aluminum-silicon hot-formed steel according to claim 1, characterized in that, After the welding plate undergoes a thermoforming process, the weld tensile strength is ≥1500MPa, and the fracture location is in the base material.

5. The laser welding method for 1500MPa aluminum-silicon hot-formed steel according to claim 4, characterized in that, The heating temperature in the thermoforming process is 950-970℃, held for 5-8 minutes, and then the cooling rate is not less than 30℃ / s.

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