Method for laser welding of a metal workpiece having an aluminum plated surface and workpiece

Abstract

The present application provides a kind of laser welding method for the surface of aluminum plating metal workpiece, comprising the following steps: 1) providing a pair of first metal workpiece and second metal workpiece for welding, and laser beam for laser welding;The first metal workpiece and second metal workpiece have top surface and bottom surface respectively, wherein at least one workpiece surface has aluminum plating;The first metal workpiece and second metal workpiece are butted to form a welded assembly and a butt line;2) laser beam travels along the direction of the butt line of the welded assembly, to melt the butt position of the first metal workpiece and second metal workpiece, and forms a weld after cooling and solidification.This method provides multiple laser beams acting on the laser welding of aluminum plated steel butt joint, which makes the distribution of aluminum element in the weld uniform, weakens the influence of aluminum on the welded joint, and the method is low in cost, simple in operation and can obtain a laser welded joint with excellent performance.

Description

Technical Field

[0001] This invention relates to the manufacture of welded components from high-strength steel coated plates, and particularly to a laser welding method for a metal workpiece having an aluminum coating on its surface, and the workpiece thereof. Background Technology

[0002] In certain fields, particularly in the manufacturing of automotive and aerospace components, high corrosion resistance is required for the materials used. Therefore, increasingly more materials with surface coatings are being applied. Steel coated with aluminum or aluminum alloys, manufactured through cold or hot rolling, not only possesses good corrosion resistance but also high high-temperature resistance, facilitating the hot-working of certain components. Therefore, aluminum alloy-coated steel sheets are especially used in automotive body manufacturing, particularly in structural components such as door reinforcements, B-pillar structures, and roof reinforcements. This is especially true in the application of ultra-high-strength hot-stamped sheet metal, achieving high corrosion resistance, reducing vehicle weight, and improving collision energy absorption.

[0003] Meanwhile, in the automotive manufacturing industry, to reduce vehicle weight and achieve lightweighting goals, an increasing number of materials of equal or unequal thickness, homogeneous or heterogeneous materials are first welded together, and then the welded parts are hot-stamped to achieve better weight reduction and cost reduction. Laser welding is the preferred method for this approach, offering higher quality, higher efficiency, and better flexibility, and is referred to as "laser-welded plates." This process often involves first laser-buttoning flat workpieces, and then hot-stamping and quenching the workpiece with the weld seam to obtain high-strength martensitic steel structural parts. During hot stamping, the weld seam structure is first heated to above AC3 temperature of the steel workpiece for complete austenitization. Due to the high temperature, surface oxidation and decarburization of the bare plate are inevitable, resulting in a decrease in the final structural performance. Therefore, a coating is required on the surface, and aluminum or aluminum alloy coatings are widely used in this process. Patent CN101583486B, for example, describes this type of material in detail.

[0004] However, manufacturing laser-welded plates with aluminum or aluminum alloy surface coatings presents significant challenges. When welding materials coated with aluminum or aluminum alloys, the initial surface coating can penetrate into the molten zone, particularly the weld seam. This leads to the formation of ferrite structures at high temperatures during cooling, which, even after austenitization, remain in the final weld, resulting in decreased joint performance. Under subsequent mechanical loads, this abundant ferrite becomes the initial site of failure cracks, severely weakening the weld joint's load-bearing capacity and causing direct fracture at the weld seam under load. Therefore, preventing the degradation of joint performance caused by aluminum infiltration into the weld is crucial.

[0005] Patents CN101426612B and 106334875A remove the coating layer from the surface in advance by mechanical methods, laser methods or other methods to prevent it from entering the weld. However, this method has problems such as high cost and complex process.

[0006] Patent CN106392328B discloses a method for welding aluminum-silicon coated hot-formed steel under a protective gas environment. This patent utilizes an oxidizing gas that combines with aluminum in the coating during welding to produce aluminum oxide (aluminum oxide), which does not affect weld toughness. However, in this method, the time between metal melting to form a molten pool and then solidifying into a weld bead is very short. The reaction time between the oxidizing gas and aluminum is limited. Aluminum that has not reacted with the oxidizing gas will accumulate inside the molten pool, reducing weld toughness and strength, and posing a risk of cracking in the hot-stamped parts.

[0007] Patent CN 106488824 B discloses a method for joining two blanks by welding with filler wire. The filler wire is made of a stainless steel alloy comprising the following weight percentages: 0%-0.3% carbon, 0%-1.3% silicon, 0.5%-7% manganese, 5%-22% chromium, 6%-20% nickel, 0%-0.4% molybdenum, 0%-0.7% niobium, and the balance being iron and unavoidable impurities. In this method, the increased heat input from the combined laser and arc welding increases the likelihood of thermal deformation of the sheet metal, which is detrimental to the joining of thin plates. Furthermore, the welding speed of both laser and arc welding is limited, reducing production efficiency. Additionally, the large number of alloying elements involved makes process control more difficult and increases manufacturing costs.

[0008] CN 111432975 A discloses a welding wire filled with the following components by weight percentage: 0.03% carbon, 0.5% silicon, 1.8% manganese, 20.5% chromium, 25% nickel, 4.7% molybdenum, less than 0.05% sulfur, less than 0.05% phosphorus, and 1.6% copper. When using this welding wire as filler material, chromium (Cr), as an austenite stabilizing element, can lead to the formation of ferrite in the weld zone of the welded plate after hot stamping, resulting in decreased mechanical properties of the welded joint, compromised product quality, and a high risk of welded joint failure. Furthermore, the addition of numerous and complex alloying elements complicates process control and increases costs. CN 112368105 A discloses a method for laser welding coated steel billets using filler wire. The welding wire comprises nickel, chromium, and carbon, with weight percentages of 1.68-10.48%, 0-2.70%, and 0.91-2.00%, respectively. The high carbon content of this welding wire leads to excessive carbon content in the weld zone, reducing plasticity and impact resistance. Furthermore, the excessive carbon content makes the weld prone to cracking, reduces corrosion resistance, and shortens the service life of the welded plate. Additionally, the high carbon content of this welding wire presents significant challenges during the wire drawing process, making it highly susceptible to work hardening and wire breakage.

[0009] CN 108025400 A discloses a laser welding method for quenchable steel semi-finished plates with aluminum-based or aluminum-silicon-based coatings. In this method, a filler containing 0.5 to 2.0% by weight of the sum of Cr and Mo and 1 to 4% by weight of Ni is added. The addition of Mo and Cr can easily lead to the precipitation of carbides in the weld and increase the brittleness of the joint, resulting in unstable performance.

[0010] CN 104994989 A discloses a filler wire containing the following components by weight percentage: 0.05-0.15% carbon, 0.5-2.0% silicon, 1.0-2.5% manganese, 0.5-2.0% chromium and molybdenum, 1.0-4.0% nickel, and the balance being iron and unavoidable impurities. This filler wire contains low levels of chromium and molybdenum, resulting in weld joints with poor hardenability and corrosion resistance. Furthermore, it cannot guarantee complete suppression of ferrite influence during high-temperature heat treatment, increasing the risk of weld joint failure in the weld and heat-affected zone.

[0011] CN104023899B discloses a laser welding method using filler wire containing carbon or manganese elements, which prevents the formation of ferrite structure in the weldment at 900-950℃. High carbon or manganese content can easily lead to brittleness, and manganese element can easily segregate, resulting in poor joint performance.

[0012] CN 112548395 A discloses a welding wire for laser filler welding, its preparation method, and a welding plate process. This method uses a welding wire with a high carbon content, which is about 2.5 to 10 times that of the base material. This results in an excessively high carbon content in the welding zone, which reduces the plasticity and impact resistance. Furthermore, the excessively high carbon content makes the weld prone to cracking and also reduces the corrosion resistance of the weld and the service life of the welding plate.

[0013] US20210008665 A1 discloses a welding method for coated steel plates, wherein the carbon content of the welding wire is 0.80 to 2.28 times the carbon content of the base material by weight. Excessive carbon content will make the weld prone to cracking and hardening and brittleness, and will also reduce the corrosion resistance of the weld and the service life of the welded plate, resulting in poor consistency of mechanical properties of the welded joint, reducing product quality, and making it unsuitable for mass production.

[0014] As can be seen, high-quality laser welding of aluminum-coated steel billets typically requires the addition of welding wires with multiple complex elements during the welding process. These welding wires are mainly composed of various alloying elements, including C, Cr, Ni, Mo, Mn, and Si. On the one hand, such welding wires struggle to guarantee stable and excellent mechanical and corrosion resistance of the welded parts. On the other hand, the addition of multiple alloying elements easily leads to work hardening during the wire drawing and forming stage, resulting in low yield and significant manufacturing difficulties. Furthermore, the complex proportions of these alloying elements are difficult to control effectively during the process, easily causing substandard weld performance. The more expensive elements, such as Mo, further increase manufacturing costs. Moreover, high levels of Mo and Cr can easily form large carbides within the weld, leading to brittle weld properties, particularly impact resistance. The addition of multiple alloying elements also causes galvanic corrosion in the weld, further degrading its corrosion resistance. The addition of extra welding wire undoubtedly increases manufacturing costs and necessitates complex process optimization. Furthermore, for ultra-high strength laser welding, due to the rapid heating and cooling characteristics of laser welding, hardened martensite structures are easily generated in the weld and heat-affected zone, greatly increasing the susceptibility to cold cracking and making brittle fracture very likely after welding. Therefore, there is an urgent need for a simple and low-cost method to achieve high-efficiency laser welding of surface-coated plates to prepare laser-welded joints with higher strength, high toughness, and high corrosion resistance. Summary of the Invention

[0015] This invention provides a high-performance laser-welded joint manufacturing method. By controlling the effective range of multiple laser beams and various heating and cooling processes during laser welding, particularly by creating multiple temperature rise and fall processes around the deep-penetrating small hole, the surface aluminum coating tends to be uniformly distributed after melting and entering the molten pool, thus weakening the adverse effects of aluminum on the weld joint. The resulting weld has a martensitic microstructure, and after subsequent quenching heat treatment, a welded joint with excellent overall performance is obtained.

[0016] The technical solution of the present invention is as follows:

[0017] A laser welding method for metal workpieces with an aluminum coating on their surface, characterized in that the method comprises the following steps:

[0018] 1) Provide a pair of first and second metal workpieces for welding, and a laser beam for laser welding; the first and second metal workpieces respectively have a top surface and a bottom surface, wherein at least one workpiece surface has an aluminum coating; the first and second metal workpieces are butt-jointed to form a weld assembly and a butt joint;

[0019] 2) Using at least two laser beams, the laser travels along the welding line direction of the assembly to be welded to melt the joint between the first metal workpiece and the second metal workpiece, and forms a weld after cooling and solidification.

[0020] In this process, each laser beam irradiates the top surface of the mating line, with at least one laser beam serving as the main laser beam to form a deep-melting small hole and molten metal surrounding the small hole. While the molten metal around the deep-melting small hole is still in a molten liquid state, at least another laser beam irradiates the outside of the small hole. Under the irradiation of the laser beam energy inside and outside the small hole, the welding area of ​​the assembly melts to form an integral, elongated molten pool. The aluminum coating on the surface of the assembly melts, and the aluminum distribution in the molten pool tends to become more uniform.

[0021] Furthermore, the first and second metal workpieces are high-strength steels that can be transformed into hot-formable steel after hot stamping. The substrate contains ferrite and pearlite in its microstructure before forming, has a carbon content of not less than 0.1%, and has boron content of about 0.002-0.006%.

[0022] Furthermore, the high-strength steel has a tensile strength of not less than 500 MPa, and the surface coating is mainly composed of aluminum and silicon, with an aluminum content of not less than 80% and a total coating thickness of 5-50 μm.

[0023] Furthermore, at least a portion of the molten metal around the deep-melting aperture underwent a thermal cycle process involving at least two stages of temperature rise and fall.

[0024] Furthermore, the deep-penetrating aperture formed by the first laser beam may or may not penetrate the metal assembly.

[0025] Furthermore, the second laser beam also forms a second deep-melting aperture, and the deep-melting aperture formed by the second laser beam does not penetrate the metal assembly.

[0026] Furthermore, the second deep-melting aperture formed by the second laser beam is interconnected with the deep-melting aperture formed by the first laser beam.

[0027] Furthermore, the width of the weld top surface formed by the solidification of the molten pool is not less than 0.8 times the thickness of the workpiece;

[0028] Furthermore, the area irradiated by the laser beam on the top surface of the assembly includes at least two beam spots, one formed by the main laser beam and the other by the second laser beam. Both beam spots simultaneously generate heat in the irradiated area, melting the substrate, which then cools and solidifies to form a weld. The relative position and energy distribution of the two beam spots can be varied, including arranging them sequentially along the direction of travel or side-by-side perpendicular to the path of travel. The geometric shapes of the two beam spots can be circles, polygons, or other shapes composed of broken lines or curves.

[0029] Furthermore, the ratio of the energy of the main laser beam to the energy of the second laser beam is not less than 1.

[0030] Furthermore, the equivalent circle diameter of the projected area of ​​the first main laser beam and the second laser beam irradiating the top surface of the assembly is not less than 0.6 mm.

[0031] Furthermore, the laser welding process also includes a filler wire, which is melted and introduced into the molten pool.

[0032] The laser-welded joint needs to undergo hot stamping and quenching treatment, including heating and holding and rapid cooling stages, so that the weld zone and base material zone of the weld joint are fully austenitized and undergo martensitic transformation to form the final weld joint.

[0033] During the heating and holding stage, the welded joint is held at an environment of approximately 850-960℃ for 3-10 minutes. During the rapid cooling stage, the cooling rate exceeds the critical cooling rate for martensitic transformation, and the cooling rate is not less than 25℃ / s. The final temperature of the weldment does not exceed 250℃.

[0034] A metal workpiece with an aluminum coating on its surface, which is welded using the laser welding method described above for metal workpieces with an aluminum coating on their surface.

[0035] Compared with the prior art, the beneficial effects of the present invention are:

[0036] 1) Employing at least two laser beams working in synergy to form a molten pool at the workpiece joint not only improves welding efficiency but also allows for precise control of the molten pool's shape and size through accurate control of the energy distribution and irradiation area of ​​each laser beam. By adjusting the relative position and energy distribution of the two laser beams, deep-penetrating small holes can be formed in the molten pool, ensuring that the molten metal around them covers the entire thickness of the workpiece plate. This effectively promotes the uniform distribution of the aluminum coating in the molten pool, improving the quality and strength of the weld.

[0037] 2) The irradiation area of ​​the laser beam can have various shapes and configurations, such as circles and polygons, and the two beams can be arranged one after the other along the welding direction or side by side perpendicularly, making the welding process adaptable to workpieces of different shapes and sizes, thus improving the versatility and adaptability of welding. By optimizing the geometry and relative position of the laser beam, a specific molten pool shape, such as an ellipsoid, can be formed in the molten pool. This shape helps to homogenize the aluminum coating in the molten pool and reduces the generation of welding defects.

[0038] 3) By controlling the energy distribution of multiple laser beams on the mating assembly in the thickness direction and the change of molten pool temperature over time, a high-performance welded joint is obtained. The obtained welded joint has high tensile strength and elongation, as well as high resistance to hydrogen embrittlement, high corrosion resistance, and surface wear resistance.

[0039] 4) It combines hot stamping and quenching process, and through heating, holding and rapid cooling process, the weld zone and the base material zone are fully austenitized and martensitic transformation occurs, thereby improving the hardness and strength of the welded joint.

[0040] 5) The welding method is highly adaptable to various environments with different gaps. The entire process is short, which reduces internal residual stress. The welding method is simple and does not require additional welding wire, which can significantly reduce manufacturing costs, improve production efficiency, and obtain high-performance welded joints. Attached Figure Description

[0041] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0042] Figure 1 The diagram shown is a schematic diagram of the welding process involved in this invention;

[0043] Figure 2 The figure shown is a cross-sectional schematic diagram of the welded workpiece involved in this invention;

[0044] Figure 3 The diagram shown is a schematic diagram of the assembly and splicing of two workpieces in this invention.

[0045] Figure 4 The image shown is a top view of the welding process involved in this invention.

[0046] Figure 5 The diagram shows the positions of the two laser beam spots on the top surface of the workpiece in this invention.

[0047] Figure 6 The diagram shows another relative position of the two laser beam spots on the top surface of the workpiece in this invention.

[0048] Figure 7 The diagram shows another relative position of the two laser beam spots on the top surface of the workpiece in this invention.

[0049] Figure 8 The diagram shows another relative position of the two laser beam spots on the top surface of the workpiece in this invention.

[0050] Figure 9 The diagram shown is a schematic cross-section of the molten pool formed during the laser welding process in this invention.

[0051] Figure 10 The diagram shown is a cross-sectional view of another molten pool formed during the laser welding process in this invention.

[0052] Figure 11 The diagram shown is a cross-sectional view of another molten pool formed during the laser welding process in this invention.

[0053] Figure 12 The diagram shown is a cross-sectional view of another molten pool formed during the laser welding process in this invention.

[0054] Figure 13 The diagram shown is a top view of another weld pool formed during the laser welding process in this invention.

[0055] Figure 14 The diagram shown is a schematic diagram of the two beams being mirror-symmetrical with respect to the connecting line in this invention.

[0056] Figure 15 The diagram shows another relative position of the three laser beam spots on the top surface of the workpiece in this invention.

[0057] Figure 16 The diagram shown is a schematic of adding welding wire during the welding process in this invention.

[0058] Figure 17 The diagram shown is a top view of another weld pool formed during the laser welding process in this invention.

[0059] Figure 18 The diagram shown is a cross-sectional view of another molten pool formed during the laser welding process in this invention.

[0060] Figure 19 The figure shown is a schematic diagram of the workpiece cross-section in one embodiment of the present invention.

[0061] Figure 20 The figure shows an embodiment of the tensile mechanical properties of the weld joint obtained by the present invention.

[0062] Figure 21 The figure shows another embodiment of the tensile mechanical properties of the weld joint obtained by the present invention.

[0063] Figure 22 The figure shows another embodiment of the tensile mechanical properties of the weld joint obtained by the present invention.

[0064] Figure 23 The results shown are from the comparative example.

[0065] Reference numerals: 11-First steel workpiece; 12-Second steel workpiece; 111-Coating on the upper surface of the first steel workpiece; 112-Coating on the lower surface of the first steel workpiece; 12-Second steel workpiece; 11a-Top surface of the first steel workpiece; 11b-Bottom surface of the first steel workpiece; B1-Butt gap; b-Coating thickness; 3-Laser beam; 31-First laser beam; 32-Second laser beam; d: Center distance between the beam spots of the first and second laser beams; d1-Diameter of the beam spot of the first laser beam; d2-Diameter of the beam spot of the second laser beam; 4-Welding wire; 5-Molten pool; 6-Weld; h1-First... 51 - Depth of the laser aperture formed by the laser beam; 52 - Molten metal in the molten pool at the leading edge of the laser aperture; 511 - Molten pool wall at the leading edge; 512 - Molten pool wall at the trailing edge; 71 - Laser aperture; 711 - Laser aperture wall; B - Assembly thickness; 12a - Top surface of the assembly joint; 12b - Bottom surface of the assembly joint; 72 - Second laser aperture; h2 - Depth of the second laser aperture; 33 - Third laser beam spot; 41 - Projected area of ​​the welding wire on the top surface of the assembly; h2 - Height of the intersection point of the welding wire and the laser beam from the top surface. Detailed Implementation

[0066] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the present invention is further described below in conjunction with specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, the accompanying drawings are schematic diagrams; therefore, the apparatus and device of the present invention are not limited by the size or scale of the schematic diagrams.

[0067] It should be noted that in the claims and specification of this patent, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one" does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element. In this invention, the final weld refers to the laser-welded joint.

[0068] This embodiment provides a laser welding method for metal workpieces with an aluminum coating on their surface. The method consists of the following steps:

[0069] 1) Now for reference Figure 1 Provides a pair of first metal workpieces 11 and second metal workpieces 12 for welding, and a laser beam 3 for laser welding; the first metal workpiece and the second metal workpiece have a top surface 11a and a bottom surface 11b respectively, wherein at least one workpiece surface has an aluminum coating 111 or 112; the first metal workpiece and the second metal workpiece are butt-jointed to form a welding assembly and a butt joint.

[0070] 2) The laser beam travels along the welding line direction of the assembly to be welded to melt the joint between the first and second metal workpieces, forming a weld after cooling and solidification. The laser beam comprises at least two laser beams, each irradiating the top surface of the welding line. At least one main laser beam forms a deep-penetrating aperture and molten metal surrounding the aperture. While the molten metal around the aperture is still in a molten liquid state, at least one second laser beam irradiates the outside of the aperture. Under the irradiation of the laser beam energy inside and outside the aperture, the welding area of ​​the assembly melts to form a uniform, elongated molten pool. The aluminum plating on the surface of the assembly melts, and the aluminum distribution within the molten pool tends to become more uniform.

[0071] The schematic diagram of the cross-section of the steel workpiece is shown below. Figure 2As shown; the steel workpiece mentioned in this invention is a high-strength steel that can be transformed into hot-formable steel after hot stamping, which is well known in the art; its base material generally consists of ferrite and pearlite before forming, with a carbon content of not less than 0.1% and a boron content of about 0.002-0.006 to meet the hardenability of the subsequent quenching process, so that a full martensitic structure can be formed after quenching, which has high strength. The tensile strength of typical boron steel is not less than 500 MPa. A typical base material for steel plates is grade 22MnB5, whose main elemental composition by mass is: 0.15% ≤ C ≤ 0.45%; 0.5% ≤ Mn ≤ 2.5%; 0.08% ≤ Si ≤ 0.4%; Al ≤ 0.45%; 0.01% ≤ Cr ≤ 0.5%; Ti ≤ 0.1%; Nb ≤ 0.1%; V ≤ 0.1%; S ≤ 0.05%; P ≤ 0.05%; 0.002% ≤ B ≤ 0.006%; the remainder being Fe and unavoidable impurities. Higher strength grades generally incorporate more alloying elements such as C, such as the typical grades 34MnB5, 28MnB5, and 37MnB4, whose tensile strength can exceed 2000 MPa. These even higher strength grades typically have a carbon content exceeding 0.25% by mass. In this invention, the surface coating is generally composed mainly of aluminum and silicon, wherein the aluminum content is not less than 80%, preferably more than 85%, and the total coating thickness b is generally 5-50 μm, preferably 7-40 μm, and more narrowly 8-20 μm.

[0072] Figure 3 The diagram shows a workpiece assembly. Workpieces 11 and 12 can be made of identical materials, or they can be made of materials with different thicknesses, coating types, or strengths. The thickness of the two workpieces is generally 0.8-3.0 mm. The two workpieces are joined together to form an assembly, with a joint gap of B1 of 0-0.5 mm. A laser beam 3 irradiates the joint line, melting the substrate and surface coating along the welding path to form a molten pool 5. After cooling and solidification, a weld 6 is formed. Figure 4 As shown.

[0073] The area irradiated by the laser beam 3 on the top surface of the assembly includes at least two beam spots, including a beam spot 31 formed by a first main beam and a beam spot 32 formed by a second beam, such as... Figure 5 As shown, two beams simultaneously generate heat in the irradiated area, melting the substrate, which then cools and solidifies to form a weld. It is worth noting that the relative positions and energy distribution of the two beams can have various types. Figure 5 The diagram shows two beams arranged one after the other along their direction of travel, with a distance d between them. The projection shapes of the two irradiated beams onto the top surface can have various types. Figure 5The diagram shows the circular beam pattern, where the beam diameters are d1 and d2. Generally, d1 and d2 are both ≥ 0.5 mm, and the center-to-center distance d between the two beams satisfies the following condition: when the two beams have no intersection, d > (d1 + d2) / 2. Figure 5 As shown in the figure; when the two beams have overlapping regions, (d1-d2) / 2≤d≤(d1+d2) / 2, as Figure 6 As shown. The geometric shapes of the two beams can be circles, polygons, or other shapes formed by intersecting broken lines or curves; they can be the same or different, such as... Figure 7 The diagram shows the two beam spots as circular and quadrilateral, respectively. In summary, the shape of the beam spot in the irradiated area on the top surface of the assembly is mirror-symmetrical with respect to the weld centerline (butt joint line), as shown below. Figure 8 The diagram shows the second beam spot when it has an annular shape. Each beam spot has a surface area, and the diameter of the equivalent circle of the surface area is not less than 0.5 mm. The relative positions of the two beam spots can be changed; that is, when viewed along the welding direction, the main beam spot can be located on the front or the back side. The term "main beam spot" is only used to distinguish the different names of the multiple beam spots.

[0074] Now for reference Figure 9 The core of this invention is that when multiple laser beams 31 and 32 irradiate the top surface 12a of the assembly, at least one deep-melting small hole 71 can be formed. The small hole has a boundary 711, and molten metal 51 surrounds it. Under the irradiation heat of another laser beam 32, the molten metal 52 behind the small hole, viewed along the welding direction, covers the entire thickness of the assembly plate, reaching the lower surface 12b. Simultaneously, the molten metal includes molten surface aluminum plating elements, forming a unified molten pool. The molten pool has a melting leading edge wall 511 along the welding direction and a solidified edge 512, making the aluminum distribution in the molten pool tend to be uniform. Furthermore, at least a portion of the molten metal in the rear edge portion 52 of the molten pool undergoes two stages of temperature rise and fall. Figure 9 As shown, the laser aperture 71 formed by the main laser beam 31 does not completely penetrate the assembly to reach the bottom surface 12b, while the second laser beam 32 does not form a laser aperture; that is, the aperture depth h1 is less than the plate thickness B. In reality, the laser aperture 71 can penetrate the plate thickness, and the laser beam 32 could also form a second laser aperture. Figure 10The diagram shows a laser aperture 71 formed by the first laser beam penetrating the bottom surface 12b of the assembly, and a second laser beam 32 forming a second laser aperture 72. Finally, under the action of the two laser beams, a molten pool is formed with a leading edge wall 511 and a solidified edge wall 512, causing the aluminum element to be distributed more uniformly within the molten pool. The center-to-center distance d between the two laser apertures is the distance between the centers of the beam spots formed by the two beams irradiating the top surface 12a of the assembly. Furthermore, the depth of the laser apertures formed by the two laser beams can be interchanged along the welding direction, for example... Figure 11 The diagram shows that the laser aperture formed at the front end has a shallower depth h2, while the laser aperture formed at the rear end has a deeper depth h1. This is in contrast to... Figure 10 Conversely, the front-end laser beam provides preheating, allowing the main laser beam to heat the molten metal more quickly, fully melting the workpiece in the thickness direction. It also results in a lower cooling rate during the solidification of the molten pool to form the weld, which reduces the hardenability of the weld. Generally, the aperture depth h1 formed by the main laser beam is greater than 0, while the aperture formed by the second laser beam satisfies: 0 ≤ h2 < B; preferably, 0 ≤ h2 < B / 2. The two laser apertures can overlap or be completely independent. Figure 10 , 11 The image shows two laser pinholes with an overlapping portion, where this overlapping portion experienced a higher temperature rise and fall. Figure 12 The laser apertures formed by the two laser beams shown are completely independent and separate. This will cause the area without an aperture in the middle of the two laser apertures to undergo more pronounced temperature rise and fall processes, thus ensuring the aluminum is fully homogenized. Overall, in this invention, the shape of the molten pool formed by multi-laser irradiation can have various types. In particular, when viewed from the top surface, the molten pool 5 has at least one laser aperture 71 that is ellipsoidal, such as... Figure 13 The diagram shown illustrates the formation of two deep-melting micropores.

[0075] It is worth noting that, when observed along the laser beam's path, the multiple laser beams can be arranged in a front-to-back pattern along the docking centerline, or they can be arranged side-by-side perpendicular to the path, such as... Figure 14 The diagram shows two beams 31 and 32 that are mirror-symmetrical with respect to the alignment line. Furthermore, the multiple laser beams mentioned in this invention refer to at least two or more; the aforementioned diagrams use two beams as examples, but in practice, there can be more, such as... Figure 15 The diagram shows the configuration of three laser beams on the top surface. At this time, two laser beams 31 and 32 are mirror-symmetrical along the dotted line, and the third beam 33 is located at the center of the dotted line, thus forming a molten pool. The number of laser apertures formed is not less than one. There are two cases: both symmetrical beams form laser apertures or neither forms a laser aperture.

[0076] Generally, laser beams 31 and 32 are laser beams with the same or different wavelengths, typically 0.3-10 μm, particularly emitted by infrared lasers with wavelengths of 0.5-3 μm, which is common in laser processing. The laser beams are generally emitted by a laser, which can be of various types, such as solid-state laser beams or gas laser beams. Specifically, they can include fiber lasers, disk lasers, semiconductor diode lasers, and Nd:YAG type solid-state lasers, or CO2 gas lasers. Other types are also possible, as long as they can generate a laser beam and create a keyhole and a molten weld pool. The first and second laser beams can be emitted by the same laser or by two independent lasers. Two laser beams can be generated by the same laser and split into two laser beams 31 and 32 by a beam splitter within an integrated laser processing head. The beam splitter achieves this purpose through any internal optical element or combination thereof, such as a prism or mirror. The laser system controller can be connected to the beam splitter to control the energy distribution and irradiation area size of the two beams, as well as the coordinated output during processing, such as power level, relative position distance d, and defocusing amount. The generation of multiple laser beams is generally achieved through optical elements or combinations thereof within the laser processing head. These can be composed of completely fixed optical elements to form laser beams with fixed beam characteristics, or they can include internal movable and deflectable optical elements, allowing the irradiation areas 31 and 32 formed by the laser beams on the raw material surface to be variable. During processing, high-speed scanning of the beam in space can be achieved through its movement and deflection. The energy distribution of the laser beams can be Gaussian, average, or a point-ring distribution, among other forms. During the welding process, the laser beam can be fixed or synchronously oscillating at high speed along the welding direction. The oscillation shape can include various forms such as circles, broken lines, figure-eights, and infinity symbols. The oscillation frequency is generally 50-500 Hz, and the oscillation amplitude is between 0.2-1.5 mm. Multiple single or mixed shielding gases, such as Ar and He, can be added during the welding process, or the welding can be performed without a shielding gas. The power of the laser beams 31 and 32 is generally 3-15 kW, preferably 4-10 kW; and the travel speed of the laser beam relative to the workpiece assembly is generally 2-10 m / min, preferably 2-8 m / min.

[0077] It is worth mentioning that solder can also be added simultaneously during the laser welding process. The solder can be in the form of powder, wire, or rod. When it is wire, it is the laser wire filler welding well-known in the field. For example... Figure 16The diagram illustrates the simultaneous addition of welding wire during welding. Observing along the welding direction, the welding wire is typically positioned at the front, with its tip at the location of the first laser beam spot. The distance h2 from the point where the welding wire intersects with the laser beam to the top surface 12a of the assembly is 0-1 mm. During laser welding, the first laser beam spot heats and melts the welding wire using laser energy, causing it to fuse with the molten substrate of the assembly melted by the first and second laser beams to form a molten pool. Figure 17 The diagram shown is a top view of the simultaneous addition of welding wire. Figure 18 The diagram shown is a side view; where 41 represents the projected area formed by the welding wire 4 at the bonding position on the top surface of the assembly. Generally, the equivalent diameter d1 of the laser beam spot is not less than 0.6 times the welding wire diameter, preferably not less than 0.8 times the welding wire diameter. The wire feeding device can be replaced by, for example, MAG (metal active gas), MIG (metal inert gas), or TIG (tungsten inert gas). The welding wire can be a solid welding wire or a flux-cored welding wire. The solder addition method can also be a combination of two different devices, for example, using two filling devices to deliver different solders, one delivering solder with a certain composition and the other delivering solder with a different composition. The combination of the two solders achieves the desired proportion of solder composition in the final solder delivered into the molten pool. Alternatively, the solder 4 can be synthesized and added through multiple steps or components; for example, the solder 4 is composed of two independent welding wires, and the average mass composition ratio of the final two solder combinations satisfies the content described in this invention. During the welding process, the wire feed speed is generally 1-5 m / min, and its ratio to the welding speed is between 0.3 and 1.5. The welding wire is iron-based and contains one or more austenitizing elements such as Ni, Mn, and C. By delivering austenitizing elements during laser welding, the negative effects of aluminum elements molten into the weld are offset to a greater extent, eliminating the influence of ferrite.

[0078] For those skilled in the art, the welded joint generally requires hot stamping and quenching to obtain the final welded joint. Typically, the preliminary laser-welded joint obtained above undergoes quenching heat treatment, which includes heating and holding stages followed by rapid cooling. During the heating and holding stage, the welded joint is held at approximately 850-960°C for 3-10 minutes, thereby ensuring complete austenitization of the weld zone and base metal zone. During this stage, elements such as C, Mn, and Al diffuse and tend to achieve a uniform distribution in the weld. The heating method can be gaseous, with the workpiece placed entirely in an air furnace, vacuum heat treatment furnace, or other gas atmosphere heating furnace; or liquid, with the workpiece or weldment immersed in oil or salt within the heating temperature range. The oil used is conventional heat treatment oil bath oil, and the salt is low-temperature salt bath salt.

[0079] After heating and holding, the weldment is rapidly transferred to the next cooling device for rapid cooling. This transfer process typically takes no more than 15 seconds (t1). The cooling rate exceeds the critical cooling rate for martensitic transformation, being no less than 25°C / s, preferably greater than 30°C / s. The final temperature of the weldment typically does not exceed 250°C, preferably no more than 200°C. Generally, this process can be completed in a solid mold or liquid medium at a certain temperature. For example, a hot stamping process can be performed using a solid mold with a cooling medium, followed by rapid stamping cooling. The applied pressure during this process is generally no less than 3 MPa, and the holding time is no less than 3 seconds and no more than 12 seconds. Under rapid stamping cooling, the internal austenite of the weldment transforms into martensite. However, this quenching process can also be carried out in a liquid medium, such as a container filled with oil or salt. The weldment is rapidly cooled in the liquid medium, causing a martensitic transformation to form the final weld joint.

[0080] Ultimately, the welded joint comprises a base metal region and a weld fusion zone; wherein the average micro Vickers hardness of the base metal region and the fusion zone are HV, respectively. BM HV FZ And satisfy HV BM / HV FZ The HV value is between 0.8 and 1.2, preferably between 0.85 and 1.1, and more preferably between 0.95 and 1.05. BM The weld strength is ≥470 HV; and the carbon content (by mass percentage) in the base material area is not less than 0.2%, while the average aluminum content (by mass percentage) in the weld area is not higher than 1.8%, preferably not exceeding 1.5%. This structure can be used to manufacture various components for automobiles, such as A and B pillars and door ring structures. Its joint tensile strength is not less than 1400 MPa, and even exceeds 1700 MPa, with an elongation exceeding 5%. Tensile fracture failure occurs in the base material area, exhibiting excellent comprehensive performance.

[0081] Example 1

[0082] The original material used for welding was an aluminum-silicon coating with a thickness of approximately 10 μm on both the upper and lower surfaces. Figure 19The image shows the cross-sectional morphology of the original coating; the substrate thickness is 1.4 mm. A dual-beam welding method was used in the laser welding process, with a total laser power of 4800 W, an energy ratio of 20:80 (lower in the front, higher in the back), two circular beams with a diameter of 0.98 mm, a beam spacing of 0.25 mm, and a welding speed of 3 m / min. After welding, the initial weld was placed in a heating furnace at 930℃ and held for approximately 300 s, causing both the weld and the base material to transform into austenite. Subsequently, rapid pressure cooling was performed in a mold with rapid cooling, holding the pressure for approximately 10 s to induce martensitic transformation in both the weld and the base material, resulting in the final welded joint. Figure 20 The image shows the mechanical property test results of a final weld obtained using the method of this invention after preparation of standard tensile specimens. The results show that both specimens fractured and failed in the base material region, with the joint tensile strength exceeding 1500 MPa and elongation exceeding 8%.

[0083] Example 2

[0084] The raw material used for welding consisted of an aluminum-silicon coating with a thickness of approximately 10 μm on both the upper and lower surfaces, with a substrate thickness of 1.4 mm. A dual-beam welding method was employed during the laser welding process. The total laser power of the two beams was 4800 W, with an energy ratio of 40:60 (higher in the front, lower in the back). Both beams were circular with a diameter of 0.98 mm and a beam spacing of 0.25 mm. The welding speed was 3 m / min. After welding, the initial weld was placed in a heating furnace at 930°C and held for approximately 300 seconds, causing both the weld and the base material to transform into austenite. Subsequently, rapid pressure cooling was performed in a mold with rapid cooling capabilities, holding the pressure for approximately 10 seconds to induce a martensitic transformation in both the weld and the base material, resulting in the final welded joint. Figure 21 The image shows the mechanical property test results of a final weld obtained using the method of this invention after preparation of standard tensile specimens. The results show that both specimens fractured and failed in the base material region, with the joint tensile strength exceeding 1500 MPa and elongation exceeding 8%.

[0085] Example 3

[0086] The raw material used for welding consisted of an aluminum-silicon coating with a thickness of approximately 10 μm on both the upper and lower surfaces, with a substrate thickness of 1.4 mm. A dual-beam welding method was employed during the laser welding process, with a total laser power of 5300 W, an energy ratio of 50:50, two circular beams with a diameter of 0.98 mm, a beam spacing of 0.49 mm, and a welding speed of 3 m / min. After welding, the initial weld was placed in a heating furnace at 930°C and held for approximately 300 seconds, causing both the weld and the base material to transform into austenite. Subsequently, rapid pressure cooling was performed in a mold with rapid cooling properties, holding the pressure for approximately 10 seconds to induce a martensitic transformation in both the weld and the base material, resulting in the final welded joint. Figure 22 The image shows the mechanical property test results of a final weld obtained using the method of this invention after preparation of standard tensile specimens. The results show that both specimens fractured and failed in the base material region, with the joint tensile strength exceeding 1500 MPa and elongation exceeding 8%.

[0087] Comparative Example

[0088] The raw material used for welding consisted of an aluminum-silicon coating with a thickness of approximately 10 μm on both the upper and lower surfaces, with a substrate thickness of 1.4 mm. A single-laser beam welding method was used during the laser welding process, with a laser power of 4500 W, a circular spot shape, a diameter of 0.98 mm, and a welding speed of 3 m / min. After welding, the initial weld was placed in a heating furnace and heated to 930°C for approximately 300 seconds, causing both the weld and the base material to transform into austenite. Subsequently, rapid pressurization and cooling were performed in a mold with rapid cooling, and after holding the pressure for approximately 10 seconds, the weld and the base material underwent a martensitic transformation to obtain the final welded joint. Figure 23 The image shows the mechanical property test results of a final weld obtained using the method of this invention after preparation of standard tensile specimens. The results show that all three specimens fractured and failed in the weld area; the tensile strength of the joint was less than 1500 MPa, and the elongation was less than 5%, failing to meet the usage requirements.

Claims

1. A laser welding method for metal workpieces with an aluminum coating on their surface, characterized in that, The method includes the following steps: 1) Provide a pair of first and second metal workpieces for welding, and a laser beam for laser welding; the first and second metal workpieces have a top surface and a bottom surface, respectively, wherein at least one workpiece surface has an aluminum coating; the first and second metal workpieces are butt-jointed to form a weld assembly and a butt joint. 2) Using two laser beams, the laser travels along the welding line direction of the assembly to be welded to melt the joint between the first and second metal workpieces, and after cooling and solidification, a weld is formed. In this process, each laser beam irradiates the top surface of the mating line. The first laser beam, as the main laser beam, forms a deep-melting aperture and molten metal surrounding the aperture. While the molten metal around the deep-melting aperture is still in a molten liquid state, the second laser beam irradiates the outside of the aperture. The second deep-melting aperture formed by the second laser beam is interconnected with the deep-melting aperture formed by the first laser beam. Under the irradiation of the laser beam energy inside and outside the aperture, the welding area of ​​the assembly melts to form an integral, elongated molten pool. The aluminum coating on the surface of the assembly melts, and the aluminum distribution in the molten pool tends to become uniform. This welding method does not require the addition of welding wire.

2. The laser welding method for a metal workpiece with an aluminum coating as described in claim 1, characterized in that, At least a portion of the molten metal around the deep-melting orifice underwent a thermal cycle process involving at least two phases of temperature rise and fall.

3. The laser welding method for a metal workpiece with an aluminum coating as described in claim 1, characterized in that, The deep-penetrating aperture formed by the first laser beam may or may not penetrate the assembly.

4. The laser welding method for a metal workpiece with an aluminum coating as described in claim 1, characterized in that, The second laser beam also forms a second deep-melting aperture, and the deep-melting aperture formed by the second laser beam does not penetrate the assembly.

5. The laser welding method for a metal workpiece with an aluminum coating as described in claim 1, characterized in that, The width of the weld top surface formed by the solidification of the molten pool is not less than 0.8 times the thickness of the workpiece.

6. The laser welding method for a metal workpiece with an aluminum coating as described in claim 1, characterized in that, The area formed by the laser beam irradiating the top surface of the assembly includes two beam spots, one formed by the main laser beam and the other by the second laser beam. The two beam spots simultaneously generate heat in the irradiated area to melt the substrate, which then cools and solidifies to form a weld.

7. The laser welding method for a metal workpiece with an aluminum coating as described in claim 6, characterized in that, The ratio of the energy of the main laser beam to the energy of the second laser beam is not less than 1.

8. The laser welding method for a metal workpiece with an aluminum coating as described in claim 1, characterized in that, The equivalent circle diameter of the projected area of ​​the first laser beam and the second laser beam irradiating the top surface of the assembly is not less than 0.6 mm.

9. A metal workpiece with an aluminum coating surface welded by the laser welding method for a metal workpiece with an aluminum coating surface as described in any one of claims 1-8.

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