Laser welding method for metal workpiece with aluminum coating on surface and workpiece thereof

Abstract

The invention provides a laser welding method for a metal workpiece with an aluminum coating on the surface. The laser welding method comprises the following steps that (1) a first metal workpiece and a second metal workpiece which are used for welding and a laser beam used for laser welding are provided; the first metal workpiece and the second metal workpiece are respectively provided with a top surface and a bottom surface, and at least one workpiece surface is provided with an aluminum coating; the first metal workpiece and the second metal workpiece are in butt joint to form a to-be-welded assembly and a butt joint line; and (2) a laser beam advances in the direction of the butt joint line of the to-be-welded assembly so as to melt the butt joint position of the first metal workpiece and the second metal workpiece, and a welding seam is formed after cooling and solidification. According to the method, multiple laser beams are provided to act on laser welding of butt joint of the steel with the aluminum coating, so that the aluminum element is uniformly distributed in a welding seam, the influence of aluminum on a welding joint is weakened, the method is low in cost and simple to operate, and the laser welding joint with excellent performance can be obtained.

Description

Technical Field

[0001] The invention relates to the manufacture of high-strength steel coated plate welding parts, in particular to a laser welding method for a metal workpiece with an aluminum coating on the surface and the workpiece. Background Art

[0002] In some fields, especially in the manufacture of automobile and aerospace parts, the use of materials requires high corrosion resistance, so more and more materials with coatings on the surface are used. Since the steel with aluminum or aluminum alloy coated on the surface manufactured by cold rolling or hot rolling not only has good corrosion resistance, but also has high high temperature resistance, which is conducive to the manufacture of certain parts by hot processing, the steel plate with aluminum alloy coated on the surface is especially used in the manufacture of automobile bodies, and is used in automobile structural parts, such as door reinforcements, B-pillar structural parts, roof reinforcements and other parts, especially in the application of ultra-high strength hot stamping plates, so as to achieve high corrosion resistance, reduce the weight of the automobile body, and improve collision absorption energy.

[0003] At the same time, in the automotive manufacturing industry, in order to reduce vehicle weight and achieve lightweight goals, more and more materials of equal thickness, unequal thickness, homogeneous and heterogeneous quality are first welded, and then the welded parts are hot stamped to achieve better weight reduction and cost reduction. The laser welding method is the preferred method of this approach, with higher quality, higher efficiency and better flexibility, and is called "laser welded blanks". High-strength martensitic steel structural parts are often obtained by first laser butt welding the flat workpiece and then hot stamping and quenching the workpiece with the weld. When hot stamping, the weld structure is first heated to a temperature exceeding the AC3 temperature of the steel workpiece for complete austenitization. Due to the high temperature, the bare plate will inevitably oxidize and decarburize the surface, resulting in a decrease in the final structural performance; so it is also necessary to coat the surface, and aluminum or aluminum alloy coatings are widely used in this process; documents represented by patent CN101583486B describe this type of material in detail.

[0004] However, there are great difficulties in manufacturing laser welded plates with aluminum or aluminum alloy coating on the surface. When the welding surface is coated with aluminum or aluminum alloy, the coating on the surface will enter the melting area, especially the melting area of ​​the weld, which will cause the formation of high-temperature ferrite structure during cooling. Even after cooling and high-temperature austenitization, it will remain in the final weld and cause the joint performance to deteriorate. Under the action of subsequent mechanical loads, these large amounts of ferrite will become the initial position of failure cracks, seriously weakening the bearing capacity of the welded joint and directly breaking in the weld when loaded. Therefore, it is very necessary to suppress the degradation of joint performance caused by aluminum entering the weld.

[0005] Patents CN101426612B, 106334875A, etc. use mechanical methods, laser methods or other methods to remove the surface coating in advance to prevent it from entering the weld, but 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 protective gas conditions. This patent uses oxidizing gas to combine with the aluminum element in the coating during the welding process to produce aluminum oxide, which has no effect on the toughness of the weld. However, in this solution, the time for the metal to melt to form a molten pool and then solidify into a weld is very short during the welding process. The reaction time of the oxidizing gas and the aluminum element is limited. Once the aluminum element that has not reacted with the oxidizing gas enters the molten pool, aluminum accumulation will be generated inside the molten pool, reducing the toughness and strength of the weld, and there is a risk of cracking of the hot stamping parts.

[0007] Patent CN 106488824 B discloses a method for joining two blanks by welding with a filler wire, the welding wire being 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 remainder of iron and unavoidable impurities. In this method, when laser and arc welding are mixed, the heat input increases, the probability of thermal deformation of the plate increases, and it is not conducive to the splicing of thin plates. Moreover, the welding speed of laser and arc welding mixed welding is limited, which reduces production efficiency. At the same time, the types of alloy elements added are more, which makes process control more difficult and the manufacturing cost is also higher.

[0008] CN 111432975 A discloses a welding wire filled with the following weight percentages: 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 the welding wire is used as a filling material, Cr is an austenite stabilizing element. A high content of Cr will cause the formation of ferrite in the weld zone of the tailor-welded plate after hot stamping, resulting in a decrease in the mechanical properties of the welded joint, the quality of the product cannot be guaranteed, and there is a high risk of failure of the welded joint; at the same time, many alloy elements are added, the types are complicated, the process control is complex, and the cost is high. CN 112368105 A discloses a method for laser welding a coated steel blank using a filler wire, wherein the welding wire includes nickel, chromium, and carbon elements, which are 1.68-10.48%, 0-2.70%, and 0.91-2.00% by weight, respectively. The high carbon content of the welding wire will lead to an excessively high carbon content in the tailored weld zone, which will reduce the plasticity and impact properties. The excessively high carbon content will make the weld prone to cracking, and will also reduce the corrosion resistance of the weld and the service life of the tailored weld. In addition, the welding wire has a high content of C element, which is very difficult to produce during the drawing process of the welding wire, and is very easy to be hardened and break the welding wire.

[0009] CN 108025400 A discloses a laser welding method for semi-finished plates of hardenable steel having an aluminum-based or aluminum-silicon-based coating, in which a filler containing a sum of Cr and Mo in a weight percentage of 0.5 to 2.0 and Ni in a weight percentage of 1 to 4 is filled. The addition of Mo and Cr easily leads to the precipitation of carbides in the weld and increases the brittleness of the joint, making the performance unstable.

[0010] CN 104994989 A discloses a welding wire filled with the following weight percentages: 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 of iron and inevitable impurities. The welding wire contains relatively low chromium and molybdenum elements, and the hardenability and corrosion resistance of the welded joint obtained by using the welding wire as a filler wire are poor, and it is impossible to ensure that the influence of ferrite is completely suppressed during high-temperature heat treatment, which increases the failure risk of the welded joint in the weld and heat-affected zone.

[0011] CN104023899B discloses a laser welding method using a welding wire filled with carbon or manganese elements, so that the weldment does not produce a ferrite structure at 900-950°C. High content of carbon or manganese easily leads to brittleness, and manganese elements easily segregate to deteriorate joint performance.

[0012] CN 112548395 A discloses a welding wire for laser wire-filling welding, a preparation method and a tailor-welded plate process. This method fills a welding wire with a high carbon content. The carbon content in the welding wire is about 2.5 to 10 times that of the welding base material, resulting in an excessively high carbon content in the tailor-welded area, which reduces the plasticity and impact properties. In addition, an excessively high carbon content will make the weld prone to cracks, and will also reduce the corrosion resistance of the weld and the service life of the tailor-welded plate.

[0013] A welding method for coated steel plates is disclosed in US20210008665 A1. The weight percentage of the carbon content of the welding wire is 0.80 to 2.28 times the carbon content of the base material. Too high a 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 mechanical properties of the welded joint, reducing product quality, and is not suitable for mass production.

[0014] It can be seen that at present, high-quality laser welding of steel billets coated with aluminum coating usually requires the addition of welding wires with multiple complex elements during the welding process; the types of added welding wires are mainly composed of multiple alloy elements, including C, Cr, Ni, Mo, Mn, Si and other alloy elements. On the one hand, this type of welding wire is difficult to fully ensure that the weldment has stable and excellent mechanical properties and corrosion properties after welding. At the same time, due to the addition of multiple alloy elements, it is very easy to produce work hardening during the wire drawing and forming stage, which makes the welding wire yield low and the manufacturing difficulty is relatively large; at the same time, the complex ratio of multiple alloy elements is difficult to effectively control during the process, which is very likely to cause the final weld performance to be unqualified. More expensive elements such as Mo also make the manufacturing cost higher. Moreover, high content of Mo and Cr elements are easy to form coarse carbides inside the weld, resulting in brittle weld performance, especially impact performance. Due to the addition of multiple alloy elements, the weld will produce galvanic corrosion and its corrosion performance will also deteriorate. The addition of additional welding wire will undoubtedly increase the manufacturing cost and bring about complex process optimization. In addition, for ultra-high strength laser welding, due to the rapid heating and cooling characteristics of laser welding, hardened martensite structure is easily generated in the weld and heat-affected zone, which greatly increases the sensitivity of cold cracking and easily causes brittle fracture after welding. Therefore, a simple and low-cost method is urgently needed to achieve high-efficiency laser welding of surface-coated plates to achieve laser welding joint preparation with higher strength, high toughness and high corrosion resistance. Summary of the invention

[0015] The present invention provides a high-performance laser welding joint manufacturing method, which controls the range of action of multiple laser beams and multiple heating and cooling processes during the laser welding process, especially realizes multiple temperature rise and fall processes around the deep melting hole, so as to realize that the surface aluminum coating melts into the molten pool and tends to be evenly distributed, thereby weakening the adverse effects of aluminum on the weld joint. The weld obtained is a weld structure with martensite as the main body, and a weld joint with excellent comprehensive performance is obtained after the subsequent quenching heat treatment process.

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

[0017] A laser welding method for a metal workpiece having an aluminum coating on its surface, characterized in that the method comprises the following steps:

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

[0019] 2) using at least two laser beams to travel along the butt joint direction of the assembly to be welded to melt the butt joint position of the first metal workpiece and the second metal workpiece, and forming a weld after cooling and solidification;

[0020] Among them, each laser beam irradiates the top surface of the butt line, at least one laser beam is used as the main laser beam to form a deep-melting small hole and molten metal surrounding the small hole, and when the molten metal around the deep-melting small hole is still in a molten liquid state, at least another laser beam is irradiated on the outside of the small hole; under the irradiation of the laser beam energy inside the small hole and the laser beam energy outside the small hole, the welding area of ​​the assembly is melted to form an overall slender molten pool, the aluminum coating on the surface of the assembly is melted, and the distribution of aluminum in the molten pool tends to be uniform.

[0021] Furthermore, the first metal workpiece and the second metal workpiece are high-strength steel that can be transformed into hot-formed steel after hot stamping, and the substrate thereof comprises ferrite and pearlite before forming, has a carbon content of not less than 0.1%, and has a boron content of about 0.002-0.006%.

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

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

[0024] Furthermore, the deep melting hole formed by the first laser beam penetrates or does not penetrate the metal assembly.

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

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

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

[0028] Furthermore, the laser beam comprises at least two beam spots in the area formed by irradiation of the top surface of the assembly, including a beam spot formed by the main laser beam and a beam spot formed by the second laser beam, and the two beam spots simultaneously generate heat in the irradiation area to melt the base material and form a weld after cooling and solidification. There are many types of relative positions and energy distributions of the two beam spots, including two beam spots arranged front and back along the travel direction or arranged side by side perpendicular to the travel path, and the geometric shapes of the two beam spots can be shapes composed of broken lines and curves such as circles and polygons.

[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 circular diameter of the projection area irradiated by the first main laser beam and the second laser beam on the top surface of the assembly is not less than 0.6 mm.

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

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

[0033] During the heating and holding stage, the weld joint is kept in an environment of about 850-960°C for 3-10 minutes. During the rapid cooling stage, the cooling rate exceeds the critical cooling rate of martensitic transformation, the cooling rate is not less than 25°C / s, and the final temperature of the weldment does not exceed 250°C.

[0034] A metal workpiece with an aluminum coating on its surface welded by the laser welding method for the metal workpiece with an aluminum coating on its surface.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] 1) Using at least two laser beams to work together to form a molten pool at the workpiece joint, not only improves welding efficiency, but also achieves fine control of the shape and size of the molten pool by precisely controlling the energy distribution and irradiation area of ​​each laser beam. By adjusting the relative position and energy distribution of the two laser beams, a deep melting hole can be formed in the molten pool, and the range of the surrounding molten metal covers the entire workpiece plate thickness, effectively promoting the uniform distribution of the aluminum coating in the molten pool and improving the quality and strength of the weld.

[0037] 2) The irradiation area of ​​the laser beam can have a variety of shapes and configurations, such as circular, polygonal, etc., and the two beam spots can be arranged front and back or side by side vertically along the welding direction, so that the welding process can adapt to workpieces of different shapes and sizes, improving the versatility and adaptability of welding. By optimizing the geometric shape and relative position of the laser beam, a specific molten pool shape can be formed in the molten pool, such as an ellipsoid, etc. This shape helps to homogenize the aluminum coating in the molten pool and reduces the occurrence of welding defects.

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

[0039] 4) Combining the hot stamping quenching process, through the heating and heat preservation and rapid cooling process, the weld area and the base material area are fully austenitized and martensitic transformation occurs, thereby improving the hardness and strength of the weld joint.

[0040] 5) The welding method can be applied to various environments with different gaps and has high adaptability. The whole process is interrupted, which can reduce the internal residual stress. The welding method is simple and does not require the addition of additional welding wire, which can greatly reduce the manufacturing cost, improve production efficiency, and obtain high-performance welding joints. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0042] Figure 1 Shown is a schematic diagram of welding involved in the present invention;

[0043] Figure 2 Shown is a schematic cross-sectional view of a welding workpiece involved in the present invention;

[0044] Figure 3 Shown is a schematic diagram of the assembly and splicing of two workpieces in the present invention.

[0045] Figure 4 Shown is a top view of the welding process involved in the present invention.

[0046] Figure 5 Shown is a schematic diagram of the positions of the two laser beam spots on the top surface of the workpiece in the present invention.

[0047] Figure 6 Shown is another schematic diagram of the relative positions of the two laser beam spots on the top surface of the workpiece in the present invention.

[0048] Figure 7 Shown is another schematic diagram of the relative positions of the two laser beam spots on the top surface of the workpiece in the present invention.

[0049] Figure 8 Shown is another schematic diagram of the relative positions of the two laser beam spots on the top surface of the workpiece in the present invention.

[0050] Fig. 9 Shown is a schematic cross-sectional view of a molten pool formed during the laser welding process of the present invention.

[0051] Fig.10 Shown is another schematic cross-sectional view of a molten pool formed during the laser welding process of the present invention.

[0052] Fig.11 Shown is another schematic cross-sectional view of a molten pool formed during the laser welding process of the present invention.

[0053] Fig.12 Shown is another schematic cross-sectional view of a molten pool formed during the laser welding process of the present invention.

[0054] Fig.13 Shown is another schematic top view of a weld pool formed during the laser welding process of the present invention.

[0055] Fig.14 It is a schematic diagram showing the case where two beam spots are mirror-symmetrical with respect to the butt joint line in the present invention.

[0056] Fig.15 Shown is another schematic diagram of the relative positions of the three laser beam spots on the top surface of the workpiece in the present invention.

[0057] Fig.16 Shown is a schematic diagram of adding welding wire during the welding process of the present invention.

[0058] Fig.17 Shown is another schematic top view of a weld pool formed during the laser welding process of the present invention.

[0059] Fig.18 Shown is another schematic cross-sectional view of a molten pool formed during the laser welding process of the present invention.

[0060] Fig.19 Shown is a schematic diagram of a cross section of a workpiece in one embodiment of the present invention.

[0061] Fig. 20 Shown is an example result of the tensile mechanical properties of the weld joint obtained by the present invention.

[0062] Fig.21 Another example result of the tensile mechanical properties of the weld joint obtained by the present invention is shown.

[0063] Fig. 22 Another example result of the tensile mechanical properties of the weld joint obtained by the present invention is shown.

[0064] Fig.23 Shown are the results of a comparative example.

[0065] Figure numerals, 11-first steel workpiece, 12-second steel workpiece; 111-upper surface coating of the first steel workpiece; 112-lower surface coating 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: distance between the center of the first laser beam and the second laser beam spot; d1-beam spot diameter of the first laser beam; d2-beam spot diameter of the second laser beam; 4-welding wire; 5-molten pool; 6-weld; h1-second The depth of the laser pinhole formed by a laser beam; 51-molten metal in the molten pool on the front side of the laser pinhole; 52-molten metal in the molten pool on the rear side of the laser pinhole; 511-front wall of the molten pool; 512-rear wall of the molten pool; 71-laser pinhole; 711-laser pinhole wall; B-assembly thickness; 12a-top surface of the assembly alloy joint position; 12b-bottom surface of the assembly joint position; 72-second laser pinhole; h2-depth of the second laser pinhole; 33-third laser beam spot; 41-projection area of ​​the welding wire on the top surface of the assembly alloy; h2-height of the intersection of the welding wire and the laser beam from the top surface. DETAILED DESCRIPTION

[0066] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the present invention is further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, the accompanying drawings are schematic diagrams, so the device and equipment of the present invention are not limited by the size or ratio of the schematic diagrams.

[0067] It should be noted that in the claims and description of this patent, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, the elements defined by the sentence "including one" do not exclude the existence of other identical elements in the process, method, article or device including the elements. The final weld in the present invention refers to the laser welded joint.

[0068] This embodiment provides a laser welding method for a metal workpiece having an aluminum coating on its surface. The overall method is divided into the following steps:

[0069] 1) Now refer to Figure 1 , providing 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 respectively have a top surface 11a and a bottom surface 11b, wherein at least one workpiece surface has an aluminum coating 111 or 112; butting the first metal workpiece and the second metal workpiece to form a to-be-welded assembly and a butt joint;

[0070] 2) The laser beam travels along the butt joint direction of the assembly to be welded to melt the butt joint position of the first metal workpiece and the second metal workpiece, and forms a weld after cooling and solidification. The laser beam comprises at least two laser beams, wherein each laser beam irradiates the top surface of the butt joint line, wherein at least one main laser beam forms a deep melting small hole and molten metal surrounding the small hole, and when the molten metal around the deep melting is still in a molten liquid state, at least another second laser beam is irradiated on the outside of the small hole; under the irradiation of the laser beam energy inside the small hole and the laser beam energy outside the small hole, the welding area of ​​the assembly is melted to form an overall elongated molten pool, the aluminum coating on the surface of the assembly is melted, and the distribution of aluminum in the molten pool tends to be uniform.

[0071] The cross-sectional schematic diagram of the steel workpiece is as follows: Figure 2As shown; the steel workpiece mentioned in the present invention is a high-strength steel that can be transformed into hot-formed steel after hot stamping, which is well known in the art; its substrate generally has a ferrite and pearlite structure before forming, wherein the carbon content is not less than 0.1%, and has a boron content of about 0.002-0.006 to meet the hardenability of the subsequent quenching process, and can form a full martensitic structure after quenching, and has a higher strength. The tensile strength of a typical boron steel is not less than 500Mpa. The typical steel plate base material is grade 22MnB5, and the elements contained are mainly as follows: 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 rest are Fe and unavoidable impurities. As a material with a higher strength level, it is generally added with more alloy elements such as C, such as 34MnB5, 28MnB5, 37MnB4 and other typical grades, whose tensile strength can exceed 2000Mpa. As a representative of higher strength, the mass fraction of its carbon content is higher than 0.25%. In the present invention, the surface coating is generally mainly composed of aluminum and silicon, wherein the aluminum content is not less than 80%, preferably the aluminum content exceeds 85%, wherein the total coating thickness b is generally 5-50μm, preferably 7-40μm, more narrowly 8-20μm.

[0072] Figure 3 The figure shows a schematic diagram of workpiece assembly. The two workpieces 11 and 12 can be made of exactly the same material or can be made of materials with different thickness, coating type, or strength. The thickness of the two workpieces is generally 0.8-3.0 mm. The two workpieces are butted together to form a component, and the butt gap is B1, which is 0-0.5 mm. The laser beam 3 irradiates the butt joint line and moves along the welding path to melt the substrate and the surface coating to form a molten pool 5, which is cooled and solidified to form a weld 6. Figure 4 shown.

[0073] The area formed by the laser beam 3 irradiating the top surface of the assembly includes at least two beam spots, including a beam spot 31 formed by the first main beam and a beam spot 32 formed by the second beam. Figure 5 As shown, the two beams simultaneously generate heat in the irradiated area to melt the base material and form a weld after cooling and solidification. It is worth mentioning that the relative position and energy distribution of the two beams can have various types. Figure 5 As shown, two beam spots are arranged front and back along the traveling direction and have a spacing d. The projection shapes of the two irradiation beam spots on the top surface can have various types. Figure 5The figure shows a circular diagram, where the beam spot diameters are d1 and d2 respectively. Generally, d1 and d2 are both ≥ 0.5 mm, and the center distance d between the two beam spots satisfies: when the two beam spots have no intersection at all, d>(d1+d2) / 2, as Figure 5 As shown in ; when the two beam spots have an overlapping area, (d1-d2) / 2≤d≤(d1+d2) / 2, as Figure 6 The geometric shapes of the two beam spots can be shapes composed of circular, polygonal, or other broken lines or curves, and can be the same or different, such as Figure 7 The figure shows the schematic diagram of the two beam spots being circular and quadrilateral respectively. In summary, the shape of the beam spot in the irradiated area on the top surface of the assembly is a mirror-symmetrical structure relative to the direction of the weld centerline (butt line), such as Figure 8 The figure shows a schematic diagram of the second beam spot having a ring shape. Each beam spot has a surface area, and the diameter of the surface area equivalent circle is not less than 0.5 mm. The front and rear relative positions of the two beam spots can be switched, that is, when observed along the welding forward direction, the main beam spot can be located at the front side or the rear side. The so-called main beam spot is only used to distinguish the difference in the names of multiple beam spots.

[0074] Reference now Fig. 9 The core of the present 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, wherein the small hole has a boundary 711, and there is molten metal 51 around the small hole 71. When observed along the welding direction, the molten metal 52 behind the small hole is irradiated with the heat of another laser beam 32, so that the molten metal range covers the entire thickness range of the assembly plate and reaches the lower surface 12b of the assembly. At the same time, the molten metal contains molten surface aluminum coating elements to form an overall molten pool. The molten pool has a melting front wall 511 along the welding direction and a rear solidification edge 512, so that the aluminum is evenly distributed in the molten pool, and at least a part of the molten metal at the rear edge 52 of the molten pool undergoes two stages of temperature rise and fall. Fig. 9 In the figure, the laser pinhole 71 formed by the main laser beam 31 does not completely penetrate the assembly to reach the bottom surface 12b, and the second laser beam 32 does not form a laser pinhole, that is, the pinhole depth h1 is less than the plate thickness B. In fact, the laser pinhole 71 can penetrate the plate thickness, and the laser beam 32 can also form a second laser pinhole. Fig.10As shown, the laser pinhole 71 formed by the first laser beam penetrates the bottom surface 12b of the assembly, and the second laser beam 32 forms the second laser pinhole 72. Finally, under the action of the two laser beams, a molten pool having a front wall 511 and a rear wall 512 after solidification is formed, so that the distribution of aluminum elements in the molten pool tends to be uniform; and the center distance d of the two laser pinholes is the distance between the centers of the beam spots formed on the top surface 12a of the two-beam plate irradiation assembly. In addition, the depth of the laser pinholes formed by the two laser beams can be replaced along the welding direction, for example Fig.11 As shown, the depth h2 of the laser hole formed at the front end is shallower, while the depth h1 of the laser hole formed at the rear end is deeper. Fig.10 The opposite front laser beam can provide preheating effect, so that the temperature of the main laser beam rises faster when it acts on the molten metal, fully melts the workpiece in the thickness direction, and has a lower cooling rate when the molten pool cools and solidifies to form a weld, which can also reduce the hardenability of the formed weld. In general, the depth of the keyhole formed by the main laser beam is h1>0, and the laser keyhole formed by the second laser beam satisfies: 0≤h2<B; preferably 0≤h2<B / 2. The other two laser keyholes can be overlapping or completely independent. Fig.10 , 11 The two laser apertures have an overlapping portion, which experiences a higher temperature rise and fall. Fig.12 The laser pinholes formed by the two laser beams are completely independent and separated, which will make the position without pinholes between the two laser pinholes experience more obvious multiple temperature rise and fall processes, so that the aluminum is fully homogenized. In general, in the present invention, the shape of the molten pool formed by multiple laser beam irradiation can have various types. In particular, when observed from the top surface, the molten pool 5 has at least one laser pinhole 71 in an ellipsoidal shape, such as Fig.13 Shown is a schematic diagram of the formation of two deep melting holes.

[0075] It is worth noting that, when observing along the path of the laser beam, the irradiation spots of the multiple laser beams can be arranged in a front-to-back manner and move along the docking center line, or they can be arranged side by side and perpendicular to the path of travel, such as Fig.14 The figure shows a schematic diagram of two beam spots 31 and 32 being mirror-symmetrical relative to the butt joint line. In addition, the multiple laser beams described in the present invention are at least more than 2, and the above schematic diagrams are all introduced with two as an example. In fact, there can be more, such as Fig.15 The figure shows a schematic diagram of the configuration of three laser beam spots on the top surface during irradiation. At this time, the two laser beams 31 and 32 are mirror-symmetrical along the butt joint line, and the third beam spot 33 is located at the center of the butt joint line, thereby forming a molten pool, and the number of laser pinholes formed is not less than one. At this time, there are two situations: both symmetrical beam spots form laser pinholes or neither forms laser pinholes.

[0076] Generally, the laser beams 31 and 32 are laser beams with the same wavelength or different wavelengths, and the wavelength is 0.3-10 μm, especially 0.5-3 μm infrared laser emission, which is common in laser processing. The laser beam is generally emitted by a laser, wherein the laser may include various types, such as a solid-state laser beam or a gas laser beam, and may specifically include a fiber laser, a disk laser, a semiconductor diode laser, and a Nd:YAG-type solid-state laser, or a CO2 gas laser. Of course, other types may also be included, as long as they can generate a laser beam and generate a keyhole and a molten weld pool. The first laser beam and the second laser beam may be emitted by the same laser, or by two independent lasers. The two laser beams can be generated by the same laser and decomposed into two laser beams 31 and 32 through a beam splitter in an integral laser processing head. The beam splitter achieves this purpose through any internal optical element or combination thereof, such as a prism, a reflector, etc. The laser system controller can be connected to the beam splitter to control the energy distribution and irradiation area size of the two beams and the coordinated output during processing, such as power level, mutual position distance d, and defocus amount. The generation of multiple laser beams is generally formed by having optical elements or combinations thereof inside the laser processing head. It can be formed by completely fixed optical elements to form a laser beam with fixed beam characteristics, or it can be formed by including movable and deflectable optical elements inside, so that the irradiation areas 31 and 32 formed by the formed laser beam on the surface of the raw material are variable, and the high-speed scanning of the beam in the spatial position can be achieved through its movement, deflection, etc. during the processing process. The energy distribution of the laser beam can be in various forms such as Gaussian distribution, average distribution, or point ring distribution. During the welding process, the laser beam can be fixed or synchronously move in various forms such as swinging in the welding direction. The swinging shape can include various shapes such as circle, broken line, 8-shaped, ∞-shaped, etc. The swing frequency is generally 50-500HZ, and the swing amplitude is between 0.2-1.5mm. During the welding process, a variety of single or mixed protective gases, such as Ar gas, He gas, etc., can also be added, or it can be carried out without protective gas. The power of the laser beams 31 and 32 is generally 3-15KW, preferably 4-10KW; and the speed of the laser beam relative to the workpiece assembly is generally 2-10m / min, preferably 2-8m / min.

[0077] It is worth mentioning that solder can also be added synchronously during the laser welding process, wherein the solder can be in the form of powder, wire or rod. When it is wire, it is laser wire welding well known in the art. Fig.16The figure shows a schematic diagram of adding welding wire simultaneously during welding. At this time, when observed along the welding direction, the welding wire is generally placed at the front side, and its end is at the first laser beam spot position. The height h2 of the intersection position with the laser beam from the top surface 12a of the assembly is 0-1mm. During the laser welding process, the first beam spot heats and melts the welding wire through laser energy, so that it merges with the assembly substrate melted by the first and second laser beams to form a molten pool. Fig.17 The figure shows a top view of the simultaneous addition of welding wire. Fig.18 It is a side view schematic diagram; wherein 41 is the projection area formed by the welding wire 4 at the joint position of the top surface of the assembly. Generally, the equivalent diameter d1 of the laser beam spot is not less than 0.6 times the diameter of the welding wire, and preferably not less than 0.8 times the diameter of the welding wire. The wire feeding device can be replaced by a feeding device such as 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 addition method of the solder can also be a composite of two different devices, for example, two filler devices are used to deliver different solders respectively, one delivering a solder containing a certain component, and the other delivering a solder of another component, and the combination of the two solders is used to achieve the final delivery of the solder into the molten pool as a solder of the required proportion of components. In addition, the solder 4 can also be added by multiple steps or multiple 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 meets the content described in the present invention. The wire feeding speed of the welding wire during welding is generally 1-5 m / min, and its ratio to the welding speed is between 0.3-1.5. The welding wire is iron-based and contains one or more austenitizing elements such as Ni, Mn, and C; by delivering the austenitizing elements during laser welding, the negative effects of the aluminum element melted into the weld are offset to a greater extent, eliminating the influence of ferrite.

[0078] For those in this field, the weld joint generally needs to be hot stamped and quenched to obtain the final weld joint. Generally, the preliminary laser weld joint obtained above is subjected to quenching heat treatment, wherein the process includes heating and heat preservation and rapid cooling stages. In the heating and heat preservation stage, the weld joint is kept warm for 3-10 minutes in an environment with a temperature of about 850-960°C, so that the weld area and the parent material area of ​​the weld joint are fully and completely austenitized, and in this stage, the elements such as C, Mn, and Al in the weld tend to be uniformly distributed by diffusion; wherein the heating method can be heated by a gas medium, the workpiece as a whole is placed in an air furnace or a vacuum heat treatment furnace and other gas atmosphere heating furnaces, or heated by a liquid medium, the workpiece as a whole or the weld is immersed in oil or salt in the heating temperature range, the oil is the oil for conventional heat treatment oil bath, and the salt is the salt for low temperature salt bath.

[0079] After the heating and heat preservation, the weldment is quickly transferred to the next cooling device and rapidly cooled. The time t1 of the transfer process is generally no more than 15s. The cooling rate of the process exceeds the critical cooling rate of martensitic transformation, the cooling rate is not less than 25℃ / s, preferably greater than 30℃ / s, and the final temperature of the weldment is generally not more than 250℃, preferably not more than 200℃. Generally, the process can be completed in a solid mold with a certain temperature or a liquid medium, for example, a hot stamping process is performed through a solid mold with a cooling medium, and then rapid stamping cooling is performed. The pressure applied in the process is generally not less than 3Mpa, and the holding time is not less than 3s and not more than 12s. Under rapid stamping cooling, the internal austenite of the weldment is transformed into a martensitic structure. However, the quenching process can also be carried out in a liquid medium, such as a container filled with oil or salt, and the weldment is rapidly cooled in a liquid medium to cause martensitic transformation to form the final welded joint.

[0080] Finally, the weld joint includes a base metal zone and a weld molten zone; wherein the average micro-Vickers hardness of the base metal zone and the molten zone is HV BM 、HV FZ , and satisfies HV BM / HV FZ Between 0.8-1.2, preferably between 0.85-1.1, more preferably between 0.95-1.05; HV BM ≥470HV; and the carbon content of the parent metal area is not less than 0.2% by mass, and the average aluminum content of the weld area is not higher than 1.8% by mass, preferably not more than 1.5%. The structure can be used to manufacture various parts for locomotives, such as automobile A and B pillars, door ring structures. The tensile strength of the joint is not less than 1400Mpa, even more than 1700Mpa, the elongation exceeds 5%, and the tensile fracture failure occurs in the parent metal area, with excellent comprehensive performance.

[0081] Example 1

[0082] The original material to be welded is an aluminum-silicon coating with a thickness of about 10 μm on both the upper and lower surfaces. Fig.19The figure shows the cross-sectional morphology of the original coating; the thickness of the substrate is 1.4mm. The dual-beam welding method adopted in the laser welding process, in which the total laser power of the dual beams is 4800W, the energy ratio is 20:80, the front is low and the back is high, the two spots are circular, the diameter is 0.98mm, the spot spacing is 0.25mm, and the welding speed is 3m / min. After welding, the initial weld is placed in a heating furnace with a heating temperature of 930°C and a holding time of about 300s, so that the weld and the base material are transformed into austenite. Subsequently, rapid pressurized cooling is performed in a mold with rapid cooling. After holding the pressure for about 10s, the weld and the base material undergo martensitic transformation to obtain the final weld joint. As shown Fig. 20 The figure shows the mechanical property test results of a final weld obtained by the method of the present invention after being prepared by a standard tensile specimen. The results show that both specimens broke and failed in the parent material area, the tensile strength of the joint reached more than 1500Mpa, and the elongation exceeded 8%.

[0083] Example 2

[0084] The original material to be welded is an aluminum-silicon coating with a thickness of about 10μm on both the upper and lower surfaces, and the thickness of the substrate is 1.4mm. The dual-beam welding method adopted in the laser welding process has a total laser power of 4800W, an energy ratio of 40:60, high in the front and low in the back, and both spots are circular with a diameter of 0.98mm. The spot spacing is 0.25mm, and the welding speed is 3m / min. After welding, the initial weld is placed in a heating furnace with a heating temperature of 930°C and a holding time of about 300s, so that the weld and the base material are transformed into austenite. Subsequently, rapid pressurized cooling is performed in a mold with rapid cooling. After holding the pressure for about 10s, the weld and the base material undergo martensitic transformation to obtain the final weld joint. Fig.21 The figure shows the mechanical property test results of a final weld obtained by the method of the present invention after being prepared by a standard tensile specimen. The results show that both specimens broke and failed in the parent material area, the tensile strength of the joint reached more than 1500Mpa, and the elongation exceeded 8%.

[0085] Example 3

[0086] The original material to be welded is an aluminum-silicon coating with a thickness of about 10μm on both the upper and lower surfaces, and the thickness of the substrate is 1.4mm. The dual-beam welding method adopted in the laser welding process, in which the total laser power of the dual beams is 5300W, the energy ratio is 50:50, the two spots are circular, with a diameter of 0.98mm, a spot spacing of 0.49mm, and a welding speed of 3m / min. After welding, the initial weld is placed in a heating furnace with a heating temperature of 930°C and a holding time of about 300s, so that the weld and the base material are transformed into austenite. Subsequently, rapid pressurized cooling is performed in a mold with rapid cooling, and after holding the pressure for about 10s, the weld and the base material undergo a martensitic transformation to obtain the final weld joint. Fig. 22 The figure shows the mechanical property test results of a final weld obtained by the method of the present invention after being prepared by a standard tensile specimen. The results show that both specimens broke and failed in the parent material area, the tensile strength of the joint reached more than 1500Mpa, and the elongation exceeded 8%.

[0087] Comparative Example

[0088] The original material to be welded is an aluminum silicon coating with a thickness of about 10μm on both the upper and lower surfaces, and the thickness of the substrate is 1.4mm. The single laser beam welding method adopted in the laser welding process has a laser power of 4500W, a circular spot shape with a diameter of 0.98mm, and a welding speed of 3m / min. After welding, the initial weld is placed in a heating furnace with a heating temperature of 930°C and a holding time of about 300s, so that the weld and the base material are transformed into austenite. Subsequently, rapid pressurized cooling is performed in a mold with rapid cooling, and after holding the pressure for about 10s, the weld and the base material undergo a martensitic transformation to obtain the final weld joint. Fig.23 The figure shows the mechanical property test results of a final weld obtained by the method of the present invention after being prepared by a standard tensile specimen. The results show that all three specimens broke and failed in the weld area, the tensile strength of the joint was less than 1500Mpa, and the elongation was less than 5%, which could not meet the use requirements.

Claims

1. A laser welding method for a metal workpiece having an aluminum coating on its surface, characterized in that: The method comprises the following steps: 1) providing a pair of a first metal workpiece and a second metal workpiece for welding, and a laser beam for laser welding; the first metal workpiece and the second metal workpiece respectively have a top surface and a bottom surface, wherein at least one workpiece surface has an aluminum coating; butting the first metal workpiece and the second metal workpiece to form a to-be-welded assembly and a butt joint; 2) using at least two laser beams to travel along the butt joint direction of the assembly to be welded to melt the butt joint position of the first metal workpiece and the second metal workpiece, and forming a weld after cooling and solidification; Among them, each laser beam irradiates the top surface of the butt line, at least one laser beam is used as the main laser beam to form a deep-melting small hole and molten metal surrounding the small hole, and when the molten metal around the deep-melting small hole is still in a molten liquid state, at least another laser beam is irradiated on the outside of the small hole; under the irradiation of the laser beam energy inside the small hole and the laser beam energy outside the small hole, the welding area of ​​the assembly is melted to form an overall slender molten pool, the aluminum coating on the surface of the assembly is melted, and the distribution of aluminum in the molten pool tends to be uniform.

2. A laser welding method for a metal workpiece having an aluminum coating on its surface as claimed in claim 1, characterized in that: At least a portion of the molten metal around the deep melting hole undergoes a thermal cycle process of at least two stages of temperature rise and fall.

3. A laser welding method for a metal workpiece having an aluminum coating on its surface as claimed in claim 1, characterized in that: The deep melting hole formed by the first laser beam penetrates or does not penetrate the metal assembly.

4. A laser welding method for a metal workpiece having an aluminum coating on its surface as claimed in claim 1 or 4, characterized in that: The second laser beam also forms a second deep-penetrating hole, and the deep-penetrating hole formed by the second laser beam does not penetrate the metal assembly.

5. A laser welding method for a metal workpiece having an aluminum coating on its surface as claimed in claim 1, characterized in that: The second deep melting hole formed by the second laser beam is connected to the deep melting hole formed by the first laser beam.

6. A laser welding method for a metal workpiece having an aluminum coating on its surface as claimed in claim 1, characterized in that: The width of the top surface of the weld formed by the solidification of the molten pool is not less than 0.8 times the thickness of the workpiece.

7. A laser welding method for a metal workpiece having an aluminum coating on its surface as claimed in claim 1, characterized in that: The laser beam irradiates the top surface of the assembly in an area including at least two beam spots, including a beam spot formed by a main laser beam and a beam spot formed by a second laser beam. The two beam spots simultaneously generate heat in the irradiated area to melt the substrate and form a weld after cooling and solidification.

8. A laser welding method for a metal workpiece having an aluminum coating on its surface as claimed in claim 7, 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.

9. A laser welding method for a metal workpiece having an aluminum coating on its surface as claimed in claim 1, characterized in that: The projection area of ​​the first main laser beam and the second laser beam irradiated on the top surface of the assembly has an equivalent circular diameter of not less than 0.6 mm.

10. A laser welding method for a metal workpiece having an aluminum coating on its surface as claimed in claim 1, characterized in that: The laser welding process also includes a welding wire for filling and melting it into the molten pool.

11. A metal workpiece with an aluminum coating on its surface welded by the laser welding method for a metal workpiece with an aluminum coating on its surface as claimed in any one of claims 1 to 9.

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