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, at least two laser beams are symmetrically arranged in the welding advancing direction in the laser welding process, the quality factor M2 of the laser beams is smaller than 1.5, each laser beam irradiates a welding assembly, and the welding assembly is welded with the aluminum coating on the surface of the metal workpiece. More than two deep melting small holes and molten metal surrounding the small holes are formed; under the irradiation of laser beam energy, the welding zone of the assembly is melted to form an overall slender molten pool, and the front edge of the molten pool tends to the vertical advancing direction to form a nearly rectangular morphology front end; the multiple deep penetration small holes drive surrounding liquid metal to flow and enable an aluminum coating on the surface of the weldment assembly to be melted, and distribution of aluminum in a molten pool tends to be uniform. 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 present invention relates to the manufacture of welded components of high-strength steel coated plates, and particularly to a laser welding method for metal workpieces with an aluminum coating on the surface and the workpieces thereof. Background Art

[0002] In some fields, especially in the manufacture of components for automobiles and aerospace, in the use of materials, higher corrosion resistance is required. Therefore, more and more materials with coatings on the surface are being applied. Since the steel materials coated with aluminum or aluminum alloy on the surface manufactured by cold rolling or hot rolling methods not only have good corrosion resistance but also have high heat resistance, which is beneficial for the manufacture of some components by hot working methods, the steel plates coated with aluminum alloy on the surface are especially applied to the manufacture of automobile bodies, applied to parts of the automobile structure, such as door reinforcement parts, B-pillar structure parts, roof reinforcement parts, etc. Especially in the application of ultra-high-strength hot stamping plates, so as to achieve higher corrosion resistance, reduce the weight of the automobile body, and improve the energy absorption during collision.

[0003] At the same time, in the automobile manufacturing industry, in order to reduce the vehicle weight and achieve the goal of lightweight, more and more materials with the same thickness, different thicknesses, the same quality, and different qualities are first welded and then the welded parts are hot stamped. Thus, better weight reduction effects and cost reduction can be achieved. The laser welding method is the preferred method for this approach, with higher quality, higher efficiency, and better flexibility, which is called "laser tailor welded blanks". Usually, the flat workpieces are first subjected to laser butt welding and then the workpieces with welds are hot stamped and quenched to obtain high-strength martensite steel structures. When hot stamping, the weld structure is first heated to exceed the AC3 temperature of the steel workpiece for complete austenitization. Due to the high temperature, surface oxidation and decarburization will inevitably occur on the bare plate, resulting in a decline in the final structural performance. Therefore, a coating is also required on the surface, and aluminum or aluminum alloy coatings are widely used in this process. Documents represented by patent CN101583486B have described this type of material in detail.

[0004] However, there are great difficulties in manufacturing laser tailor welded blanks with aluminum or aluminum alloy coatings on the surface. When welding materials with aluminum or aluminum alloy coatings on the surface, the coatings on the initial surface will enter the melting zone, especially into the melting zone of the weld, which will cause the formation of ferrite structures at high temperatures during the cooling process. Even if the cooling passes through high-temperature austenitization, they will remain in the final weld, resulting in a decline in joint performance. Under the action of mechanical loads on the welded parts in the future, these large amounts of ferrite will become the initial positions of failure cracks, seriously weakening the bearing capacity of the welded joint and directly breaking at the weld when loaded. Therefore, it is very necessary to suppress the decline in joint performance caused by the entry of aluminum into the weld.

[0005] Patents such as CN101426612B and 106334875A remove the coating on the surface by pre-mechanical method, laser method or other methods 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 the condition of protective gas. This patent uses an oxidizing gas to combine with the aluminum element in the coating during welding to produce aluminum oxide, and aluminum oxide has no effect on the toughness of the weld bead. However, during the welding process of this solution, the time for the metal to melt to form a molten pool and then solidify into a weld bead is very short, and the reaction time of the oxidizing gas with the aluminum element is limited. Once the aluminum element that has not reacted with the oxidizing gas enters the molten pool, aluminum accumulation will occur inside the molten pool, reducing the toughness and strength of the weld bead, 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 filler wire is made of a stainless steel alloy with the following composition by weight percentage: 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 of iron and inevitable impurities. In this method, when laser and arc welding are mixed, the heat input increases, increasing the probability of thermal deformation of the sheet, which is not conducive to the splicing of thin sheets. Moreover, the welding speeds of laser and arc welding are limited when they are mixed, reducing the production efficiency. At the same time, the types of alloying elements added are relatively many, which is difficult for process control and the manufacturing cost is also relatively high.

[0008] It is disclosed in CN 111432975 A that a welding wire filled with components by the following weight percentages is used: 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 the filler material, Cr, as an austenite stabilizing element, a high content of Cr will lead to the risk of forming ferrite in the weld zone of the tailor-welded blank after hot stamping, resulting in a decline in the mechanical properties of the welded joint, being unable to guarantee the quality of the product, and there is a high risk of failure of the welded joint; at the same time, a very large number of alloying elements are added, with various types, complex process control, and high costs. It is disclosed in CN 112368105 A that a laser welding method for a coated steel billet using a filler wire is provided. The welding wire includes nickel, chromium, and carbon elements, with weight percentages of 1.68 - 10.48%, 0 - 2.70%, and 0.91 - 2.00% respectively. The high content of carbon in this welding wire will lead to too high a carbon content in the weld-bonding zone, reducing plasticity and impact properties, and the too 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 tailor-welded blank. And this welding wire has a high content of C element, and there are great difficulties in the wire drawing process of wire production, and it is extremely easy to work harden and cause the phenomenon of wire breakage.

[0009] It is disclosed in CN 108025400 A that a laser welding method for a quenchable steel semi-finished plate with an aluminum-based or aluminum-silicon-based coating is provided. In this method, a filler containing the sum of Cr and Mo with a weight percentage of 0.5 to 2.0 and Ni with a weight percentage of 1 to 4 is used. Adding 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] It is disclosed in CN 104994989 A that a welding wire filled with components by the following weight percentages is used: 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. This welding wire contains lower chromium and molybdenum elements. The hardenability and corrosion resistance of the welded joint obtained by using this welding wire as the filler wire are poor, and it is unable to completely suppress the influence of ferrite during the high-temperature heat treatment process, increasing the risk of failure of the welded joint in the weld and the heat-affected zone.

[0011] CN104023899B discloses a laser welding method using a filler wire containing carbon or manganese elements, enabling the welded part not to produce a ferrite structure at 900 - 950°C. A high content of carbon or manganese easily leads to brittleness, and the manganese element is prone to segregation, making the joint performance worse.

[0012] A wire for laser filler wire welding, a preparation method thereof and a tailor-welded blank process are disclosed in CN 112548395 A. By filling a wire with a relatively high carbon content, the carbon content in the wire is about 2.5 to 10 times that of the welded base material, resulting in too high a carbon content in the tailor-welded area, reducing the plasticity and impact properties. Moreover, the too 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 blank.

[0013] A welding method for coated steel plates is disclosed in US20210008665 A1. The weight percentage of the carbon content of the wire is 0.80 to 2.28 times that of the carbon content of the base material. The too high carbon content will make the weld prone to cracks and harden and become brittle, and will also reduce the corrosion resistance of the weld and the service life of the tailor-welded blank, resulting in poor mechanical properties of the welded joint, reducing the product quality and being not suitable for mass production.

[0014] It can be seen that for high-quality laser welding of steel billets coated with aluminum plating, it is usually necessary to add wires with a variety of complex elements during the welding process; the types of added wires are mainly composed of a variety of alloy elements, including multiple alloy elements such as C, Cr, Ni, Mo, Mn, Si, etc. On the one hand, it is difficult to fully ensure that the welded parts have stable and excellent mechanical properties and corrosion resistance after welding. At the same time, due to the addition of a variety of alloy elements, work hardening is extremely likely to occur during the wire drawing and forming stage, resulting in a low finished product rate of the wire and great manufacturing difficulty; at the same time, the complex ratio of a variety of alloy elements is difficult to effectively control during the process, and it is extremely easy to cause unqualified final weld performance. Moreover, more expensive elements such as Mo also make the manufacturing cost higher. And high contents of Mo and Cr elements are likely to form coarse carbides inside the weld, resulting in embrittlement of the weld performance, especially the impact performance. And due to the addition of a variety of alloy elements, galvanic corrosion will occur in the weld, and its corrosion performance will also deteriorate. The addition of extra wires will undoubtedly increase the manufacturing cost and bring complex process optimization. In addition, for laser welding with extremely high strength, due to the characteristics of fast heating and fast cooling of laser welding, hardened martensite structure is extremely likely to be generated in the weld and the heat affected zone, greatly increasing the cold cracking sensitivity and being extremely prone to brittle fracture after welding. At the same time, the currently adopted laser welding methods are mostly multi-mode laser beams with a single laser spot, which will undoubtedly cause great energy loss, low laser energy utilization rate and high cost. Therefore, there is an urgent need for a simple and low-cost method to achieve high-efficiency laser welding of surface-coated plates and prepare laser welding joints with higher strength, higher toughness and higher corrosion resistance. Summary of the Invention

[0015] In the present invention, a method for manufacturing a high-performance laser welded joint is provided. By controlling the common action range of multiple laser beams with high beam quality and multiple processes of heating and cooling during the laser welding process, especially by forming multiple keyholes and making the molten metal around them flow, the molten aluminum coating on the surface tends to be evenly distributed after entering the molten pool, thereby weakening the adverse effects of aluminum on the weld joint. The weld obtained has a weld structure mainly composed of martensite, and a welded joint with excellent comprehensive performance is obtained after subsequent quenching heat treatment. At the same time, the laser beam with high beam quality is beneficial to improving the energy utilization rate and reducing the manufacturing cost.

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

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

[0018] 1) Provide 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, and at least one workpiece surface has an aluminum coating; butt the first metal workpiece and the second metal workpiece to form a workpiece combination to be welded and a butt joint line.

[0019] 2) Control at least two laser beams to travel along the direction of the butt joint line to melt the butt joint position of the first metal workpiece and the second metal workpiece, and form a weld after cooling and solidification; wherein, the at least two laser beams are symmetrically arranged along the welding travel direction, and the beam quality factor M of each laser beam 2 is less than 1.5, and the beam quality of each laser beam approximates single-mode output; each laser beam irradiates the top surface of the butt joint line to form more than two keyholes and the molten metal surrounding the keyholes.

[0020] Under the action of the laser beam energy irradiation inside and outside the keyholes, the welding area of the workpiece combination is melted to form an overall slender molten pool, and the front edge of the molten pool tends to be perpendicular to the welding travel direction, forming a nearly rectangular front end; the keyholes drive the surrounding molten metal to flow, melting the aluminum coating on the surface of the workpiece combination and making the distribution of aluminum in the molten pool tend to be uniform.

[0021] The laser welded joint needs to be subjected to hot stamping and quenching treatment, including a heating and holding stage and a rapid cooling stage, to austenitize the weld area and the base metal area of the welded joint, and cool to undergo martensite transformation to form the final welded joint.

[0022] During the heating and heat preservation stage, the welded joint is kept at a temperature of about 850 - 960 °C for 3 - 10 minutes. In the rapid cooling stage, the cooling rate exceeds the critical cooling rate of martensite transformation, the cooling rate is not less than 25 °C / s, and the final temperature of the welded part does not exceed 250 °C.

[0023] A metal workpiece with an aluminum coating on its surface welded by the above laser welding method for metal workpieces with an aluminum coating on their surface.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0025] 1) By the synergistic effect of multiple laser beams, precise control of the welding range and melting amount can be achieved, thereby realizing precise control of the weld formation and dimensions. By adjusting the relative position and energy distribution of multiple laser beams, multiple keyholes can be formed in the molten pool. The coordinated movement of the liquid around multiple keyholes enables the molten metal to melt and weld the entire required range, driving the uniform distribution of the surface-melted aluminum coating in the molten pool and improving the quality and performance of the weld.

[0026] 2) Through the energy distribution and position distribution of multiple laser beams, the shape of the keyhole and the flow pattern of the melt around each keyhole can be controlled, enabling effective melting of the original base material for welding under different specifications (such as unequal thickness), making the shape of the overall molten pool customized, improving the versatility and adaptability of welding, and reducing defects such as welding spatter.

[0027] 3) By controlling the energy distribution of multiple laser beams in the thickness direction of the butt joint assembly and the change process of the molten pool temperature over time, a high-performance welded joint is obtained. The obtained welded joint has, on the one hand, high tensile strength and elongation, and at the same time has high hydrogen embrittlement sensitivity, high corrosion resistance, and high surface wear resistance.

[0028] 4) Combining the hot stamping quenching process, through the heating and heat preservation and rapid cooling processes, the weld zone and the base material zone are fully austenitized and martensite transformation occurs, thereby improving the hardness and strength of the welded joint.

[0029] 5) The welding method can be adapted to various environments with different gaps, having high adaptability. The whole process takes a short time, which can reduce internal residual stress, greatly reduce manufacturing costs, improve production efficiency, and obtain high-performance welded joints.

[0030] 6) The use of laser beams with high beam quality can greatly improve the welding efficiency. Even when adding filler wire, the amount of filler wire added can be reduced, realizing the reduction of filler wire usage on the basis of high welding quality and improving manufacturing efficiency. Description of the Drawings

[0031] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the accompanying drawings required in the description of the embodiments or the prior art. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other accompanying drawings can be obtained based on these drawings.

[0032] Figure 1 The following shows the welding schematic diagram involved in the present invention;

[0033] Figure 2 The following shows the cross-sectional schematic diagram of the welded workpiece involved in the present invention;

[0034] Figure 3 The following shows the assembly and splicing schematic diagram of two workpieces in the present invention.

[0035] Figure 4 The following shows the top view during the welding process involved in the present invention.

[0036] Figure 5 The following shows the position schematic diagram of the spots of two laser beams on the top surface of the workpiece in the present invention.

[0037] Figure 6 The following shows another relative position schematic diagram of the spots of two laser beams on the top surface of the workpiece in the present invention.

[0038] Figure 7 The following shows another relative position schematic diagram of the spots of two laser beams on the top surface of the workpiece in the present invention.

[0039] Figure 8 The following shows the relative position schematic diagram of the spots on the top surface of the workpiece when using four laser beams in the present invention.

[0040] Figure 9 The following shows the relative position schematic diagram of the spots on the top surface of the workpiece when using three laser beams in the present invention.

[0041] Figure 10 The following shows the relative position schematic diagram of the spots on the top surface of the workpiece when using five laser beams in the present invention.

[0042] Figure 11 The following shows another relative position schematic diagram of the spots on the top surface of the workpiece when using three laser beams in the present invention.

[0043] Figure 12 The following shows another relative position schematic diagram of the spots on the top surface of the workpiece when using three laser beams in the present invention.

[0044] Figure 13 The following shows the cross-sectional schematic diagram of the molten pool formed during the laser welding process in the present invention.

[0045] Figure 14 Shown is a top - view schematic diagram of another weld pool formed during the laser welding process in the present invention.

[0046] Figure 15 Shown is a top - view schematic diagram of another weld pool formed during the laser welding process in the present invention.

[0047] Figure 16 Shown is a cross - sectional schematic diagram of another weld pool formed during the laser welding process in the present invention.

[0048] Figure 17 Shown is a schematic diagram when adding welding wire during the welding process in the present invention.

[0049] Figure 18 Shown is a top - view schematic diagram of the weld pool formed when adding welding wire during the welding process in the present invention.

[0050] Figure 19 Shown is a side - view schematic diagram of the weld pool formed when adding welding wire during the welding process in the present invention.

[0051] Figure 20 This is a cross - sectional view of the raw materials involved in Example 1 of the present invention.

[0052] Figure 21 This is a high - speed camera photo of the weld pool formed in Example 1 of the present invention.

[0053] Figure 22 This is a photo of the final joint after tensile failure in Example 1 of the present invention.

[0054] Figure 23 This is a photo of the final joint after tensile failure in Comparative Example 1.

[0055] Figure 24 This is a cross - sectional view of the raw materials involved in Example 2 of the present invention.

[0056] Figure 25 This is a high - speed camera photo of the weld pool formed in Example 2 of the present invention.

[0057] Figure 26 This is Figure 25 a partial enlarged view of

[0058] Figure 27 This is a photo of the final joint after tensile failure in Example 1 of the present invention.

[0059] Figure 28 This is a photo of the final joint after tensile failure in Comparative Example 1.

[0060] 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; 30 - laser beam spot with additional centerline distribution; 31 - first laser beam; 32 - second laser beam; d: center distance between the spots of the first laser beam and the second laser beam; d1, d2, d3, d4 - diameters of the spots of the first, second, third, and fourth laser beams; d0 - diameter of the laser beam spot with additional centerline distribution; d10 - center distance between the laser beam spot with additional centerline distribution and the symmetrically distributed spot; 4 - welding wire; 5 - molten pool; 6 - weld seam; h1 - depth of the laser keyhole formed by the first laser beam; 51 - molten metal on the front side of the laser keyhole in the molten pool; 52 - molten metal on the rear side of the laser keyhole in the molten pool; 511 - front wall of the molten pool; 512 - rear wall of the molten pool; 71, 72, 73, 74, 75 - laser keyholes; 711 - wall of the laser keyhole; B - thickness of the assembly; 12a - top surface of the alloy butt joint position; 12b - bottom surface of the assembly butt joint position; 70 - second laser keyhole; h0 - depth of the second laser keyhole; 33 - third laser beam spot; 41 - projected area of the welding wire on the top surface of the alloy; h2 - height of the intersection point of the welding wire and the laser beam from the top surface; B12 - center distance between the first beam spot and the second beam spot; B23 - center distance between the second beam spot and the third beam spot; B34 - center distance between the third beam spot and the fourth beam spot. Detailed implementation manners

[0061] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the present invention will be further described below in combination with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, the drawings are schematic diagrams, so the devices and equipment of the present invention are not limited by the dimensions or ratios of the schematic diagrams.

[0062] It should be noted that in the claims and the specification of this patent, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, such that a process, method, article or device comprising a series of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "comprising one" does not exclude the presence of additional identical elements in the process, method, article or device comprising the element. In the present invention, the final weld seam refers to the laser welding joint.

[0063] This embodiment provides a laser welding method for a metal workpiece with an aluminum coating on its surface. It is divided into the following steps as a whole:

[0064] 1) Provide 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, and at least one workpiece surface has an aluminum coating; butt the first metal workpiece and the second metal workpiece to form a workpiece combination to be welded and a butting line.

[0065] 2) The laser beam travels along the direction of the butting line of the workpiece combination to be welded to melt the butting positions of the first metal workpiece and the second metal workpiece, and a weld seam is formed after cooling and solidification; wherein the laser beam includes at least 2 laser beams which are symmetrically arranged along the welding direction, the beam quality factor M2 of the symmetrically arranged laser beams is less than 1.5, and the beam quality of each laser beam approximates single-mode output; each laser beam irradiates the top surface of the butting line, and the laser beam irradiation forms more than two keyholes and molten metal surrounding the keyholes.

[0066] Under the irradiation action of the laser beam energy inside the keyhole and the laser beam energy outside the keyhole, the welding area of the workpiece combination is melted to form an overall slender molten pool, the front edge of the molten pool tends to be perpendicular to the traveling direction, forming a front end with a nearly rectangular morphology; a plurality of keyholes drive the surrounding liquid metal to flow and melt the aluminum coating on the surface of the workpiece combination, and the aluminum is evenly distributed in the molten pool.

[0067] Among them, the cross-sectional schematic diagram of the steel workpiece is as Figure 2As shown in the figure; in the present invention, the steel workpiece mentioned is a high-strength steel that can be transformed into hot-formed 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 contains about 0.002 - 0.006 of boron element to meet the hardenability of the subsequent quenching process, enabling a fully martensitic structure to be formed after quenching, having a relatively high strength, and the typical tensile strength of boron steel is not less than 500 Mpa. The typical steel plate base material is grade 22MnB5, and the mass ratio of the elements it contains is mainly: 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 inevitable impurities. As a material with a higher strength level, it generally adds more alloying elements such as C, for example, typical grades such as 34MnB5, 28MnB5, 37MnB4, etc., and its tensile strength can exceed 2000 Mpa. As a representative of higher strength, its mass fraction of carbon content is higher than 0.25%. In the present invention, the surface coating generally mainly consists of aluminum and silicon, with the aluminum content not less than 80%, preferably the aluminum content exceeding 85%, and the total thickness b of the coating is generally 5 - 50 μm, preferably 10 - 40 μm, and more narrowly 25 - 40 μm.

[0068] Figure 3 The figure shows a schematic diagram of workpiece assembly. Among them, the two workpieces 11 and 12 can be two completely identical materials, or two materials with any difference in thickness, coating type, and strength; the thickness of the two workpieces is generally 0.8 - 3.0 mm, and the two workpieces are butted to form an assembly, where the butting gap is B1, which is 0 - 0.5 mm. The laser beam 3 irradiates along the welding path on the butting line to melt the base material and the surface coating to form a molten pool 5, and after cooling and solidification, a weld 6 is formed, as Figure 4 shown in the figure.

[0069] Among them, the laser beam 3 contains at least 2 beams, and the laser beams are symmetrically arranged along the welding direction. When there are 2 beams, as Figure 5 shown in the figure, the two symmetrically arranged beam spots 31 and 32 generate heat simultaneously in the irradiation area to melt the base material, and after cooling and solidification, a weld is formed. Figure 5 The figure shows that the two beam spots are symmetrically distributed along the weld center line. The two beam spots have a spacing d, and the projection shapes of the two irradiation beam spots on the top surface can have various types. Figure 5The schematic diagram shows the case when it is circular. At this time, the spot diameters are d1 and d2 respectively. Generally, both d1 and d2 are ≤ 0.3 mm, and the distance d between the centers of the two spots satisfies: when the two spots have no intersection at all, at this time d > (d1 + d2) / 2, as shown in Figure 5 shown in; when the two spots have an overlapping area, (d1 - d2) / 2 ≤ d ≤ (d1 + d2) / 2, as shown in Figure 6 shown in. When the number of spots is an even number, the geometric shapes of the two spots can be shapes composed of broken lines and curves such as circles and polygons, and the spot shapes are exactly the same. In short, the shape of the irradiation area of the spots on the top surface of the assembly is a mirror-symmetric structure with respect to the direction of the weld center line (butt line), as shown in Figure 7 The schematic diagram shows the case when the shapes of the two spots are both annular morphologies. Each spot has a surface area, and the diameter of the equivalent circle of the surface area does not exceed 0.3 mm. In a preferred example, the diameter of the equivalent circle of the area irradiated by each laser beam spot does not exceed 0.2 mm, and more preferably does not exceed 0.1 mm.

[0070] When the number of symmetrically arranged spots formed by the laser beam 3 irradiating on the top surface of the assembly exceeds 2, it can be divided into even and odd cases. When it is an even number, the laser beam spots are symmetrically distributed along the weld center line, as shown in Figure 8 The schematic diagram shows four spots. The four spots are 31, 32, 33, and 34 respectively. The mutual relationship is: 31 and 34 are mirror-symmetric along the weld center line, and 32 and 33 are mirror-symmetric; at the same time, the energy distribution and power magnitude in the spots are: 31 and 34 are the same, 32 and 33 are the same, and 31 and 32 can be the same or different. The intervals between them are B12, B23, and B34 respectively; the diameter d1, d2, and d3 of the equivalent circle of each spot do not exceed 0.3 mm, preferably do not exceed 0.2 mm, and more preferably do not exceed 0.1 mm. And B12, B23, and B34 generally do not exceed 0.5 mm. If marked in the order from top to bottom, the center distance from the first spot to the last spot generally does not exceed 2 mm, preferably does not exceed 1.5 mm.

[0071] When the number of symmetrically arranged spots formed by the laser beam 3 irradiating on the top surface of the assembly is an odd number exceeding 2. As shown in Figure 9 The schematic diagram shows the case when the number is 3. The central spot 32 is arranged at the weld center line position, and the other two spots 31 and 33 are symmetrically distributed with respect to the center line, and the sizes and energy distributions of the spots 31 and 33 are the same; and the beam quality factor M of each 2 is less than 1.5, and the beam quality of each laser beam is approximately single-mode output; the spot diameters d1 and d2 do not exceed 0.3 mm. And the energy distribution and laser power of the laser beam 32 can be the same as or different from those of 31.Figure 10 The figure shows a schematic diagram when there are 5 symmetrically distributed laser beams. At this time, similar to the case of 3 beams, the laser beams 31 and 35, 32 and 34 are in mirror image distribution, with the same energy distribution and power magnitude; and the beam quality factor M of each one 2 is less than 1.5, and the beam quality of each laser beam approximates single-mode output; the spot diameters d1 and d2 do not exceed 0.3 mm, preferably not exceeding 0.2 mm. And the laser beam 33 can be the same as or different from 31 and 32. When the symmetrically distributed laser beams are in other odd-number cases, the distribution is similar, and will not be elaborated here.

[0072] In the present invention, at least one laser beam 30 can also be arranged along the welding progress direction, which is located in front of or behind the symmetrically arranged laser beams, and its irradiation spot diameter is not less than 0.3 mm. As Figure 11 shown in the figure when the laser beam 30 is located behind. At this time, the distance between the laser beam 30 and the centers of the laser beams 31 and 32 is d10, generally d10 ≤ 3 mm; the molten metal formed by the laser beams 31 and 32 reduces the flow range and cooling rate of the molten metal under the action of the laser beam 30, maintains the molten state of the liquid metal, and makes the distribution of aluminum elements in the coating more uniform. And the equivalent circular diameter d0 of the irradiation spot of the laser beam 30 can be the same as or different from d1, and the beam quality M0 can also exceed or not exceed the M0 corresponding to the laser beam 31. Generally at this time, the laser powers and energy distributions adopted by 31 and 32 are exactly the same.

[0073] The laser beam 30 can also be located at the front end of the laser beams 31 and 32, as Figure 12 shown in the figure. At this time, the laser beam 30 is generally a multi-mode laser beam, its beam quality M0 ≥ 1.5, and the equivalent circular diameter d0 of its irradiation spot ≥ 0.3 mm. Its spot shape can be rectangular, annular or other forms; by preheating and melting the substrate in front by the laser beam 30, it can better maintain the high-temperature duration of the molten metal and make the flow and diffusion of aluminum elements in the coating more uniform. At this time, the situations of the laser beams 31 and 32 are the same as those described above. In addition, the laser beam 30 can be paired with more than 2 laser beams 31, 32, 33, 34..., and the situations of each laser beam are the same as those described above.

[0074] Now refer to Figure 13, the core of the present invention is that at least one keyhole can be formed when multiple high beam quality laser beams 31, 32, 33, 34 irradiate the top surface 12a of the assembly. The keyhole has a boundary 711, and there is molten metal 51 around the keyhole 71. Under the thermal effect of the irradiation of another laser beam 32 in the molten metal 52 behind the keyhole when observed along the welding direction, the range of the molten metal covers the entire thickness range of the assembly to reach the bottom surface 12b of the assembly. At the same time, the molten metal contains the melted surface aluminum plating elements, forming an overall molten pool. The molten pool has a melting front wall 511 along the welding direction and a solidified edge 512, making the distribution of aluminum in the molten pool tend to be uniform. The multiple keyholes formed can be connected through or not connected through. Figure 14 The following is a top view schematic diagram of the present invention when four laser beams are used and the formed keyholes 71, 72, 73, 74 are not connected through. At this time, the overall shape of the through keyholes is similar to an 8 shape. Figure 15 The following is a schematic diagram when four keyholes are connected through. The molten metal flows backward along the front direction to form an elongated molten pool 5, and its front edge is close to a rectangular shape. The following laser keyholes 71 all refer to the common keyholes formed by multiple laser beams or the deepest keyholes formed by a single laser beam.

[0075] In Figure 13 As shown in the figure, the laser keyhole 71 formed by the laser beam does not completely penetrate the assembly to reach the bottom surface 12b, that is, the depth h1 of the keyhole is less than the plate thickness B. In fact, the laser keyhole 71 can penetrate the plate thickness. Figure 16 The following is a schematic diagram of the cross-section of the molten pool formed when at least another laser beam 30 is included and it is located at the rear. Among them, the laser keyhole 71 penetrates the bottom surface 12b of the assembly, and the laser beam 30 forms a second laser keyhole 70. Finally, under the action of the two laser beams, a molten pool with a front wall 511 and a solidified rear wall 512 is formed, making the distribution of aluminum elements in the molten pool tend to be uniform; and the center distance d0 between the two laser keyholes is the distance between the beam spot centers formed on the top surface 12a between the irradiations of the two plate combinations. Generally, the depth of the laser keyhole 71 is greater than the depth h0 of the laser keyhole 70. In addition, when the laser beam 30 is located in front of the welding, the front laser beam 30 can provide a preheating effect, making the temperature rise faster when the main laser beam acts on the molten metal, fully melting the workpiece in the thickness direction, and having a lower cooling rate when the molten pool cools and solidifies to form a weld. This can also reduce the hardenability of the formed weld and improve better fluidity. In addition, the two laser keyholes can be overlapping or completely independent. Overall, in the present invention, the shape of the molten pool formed by multi-laser beam irradiation can have various types. Especially when observed from the top surface, the front edge of the molten pool 5 is in a nearly rectangular shape, and the liquid metal at the front edge flows backward sufficiently to drive the uniform distribution of aluminum elements in the surface coating.

[0076] Generally, symmetrically arranged laser beams are laser light beams with the same or different wavelengths, which are emitted by infrared lasers with wavelengths of 0.3 - 10 μm, especially 0.5 - 3 μm. More commonly, they have wavelengths of 900 - 1100 nm, which is common in laser processing. The laser beams are generally emitted by lasers, and the lasers can include various types, such as solid-state laser beams or gas laser beams. Specifically, they can include fiber lasers, disk lasers, semiconductor diode lasers, and solid-state lasers of the Nd:YAG type, or gas lasers of CO2. Of course, other types can also be included as long as they can generate laser beams and form keyholes and molten weld pools. The beam quality M0 of multiple laser beams is less than 1.5, and in this field, it is more commonly referred to as single-mode laser; multiple laser beams can be emitted by the same laser or separately by multiple independent lasers. Multiple laser beams can be generated by the same laser and decomposed into multiple beams of laser through a beam splitter inside an integral laser processing head. The beam splitter achieves this purpose through any internal optical element or its combination, such as prisms, mirrors, 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 of light and their coordinated output during processing, such as power level, mutual position distance d, and defocus amount, etc. The generation of multiple laser beams is generally composed of optical elements or their combination inside the laser processing head. It can be composed of completely fixed optical elements to form laser beams with fixed beam characteristics, or it can include movable and deflectable optical elements inside, so that the irradiation area formed by the laser beam on the surface of the raw material is variable, and the beam can be scanned at high speed in the spatial position during processing through its movement, deflection, etc. The energy distribution of the laser beam can be in various forms such as Gaussian distribution, uniform distribution, or dot-ring distribution, etc. During the welding process, when the laser beam travels along the welding direction, it can also be fixed or move at high speed in various forms such as synchronous oscillation. At this time, the oscillation shape can include various shapes such as circular, broken line, figure-eight, ∞-shaped, etc. The oscillation frequency is generally 50 - 500 HZ, and the oscillation amplitude is between 0.2 - 1.5 mm. During the welding process, various single or mixed shielding gases can also be added, such as Ar gas, He gas, etc., or the welding can be carried out under the condition of no shielding gas. The power of each laser beam is generally 500 W - 5 KW, preferably 800 W - 3 KW; the traveling 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 during the laser welding process, filler metal can also be added synchronously. The filler metal can be in the form of powder, wire, or rod. When it is in wire form, it is the well-known laser wire filling welding in the field. As Figure 18The figure shows a schematic diagram of synchronously adding a welding wire during welding. At this time, when observing along the welding direction, the welding wire is generally placed on the front side, and its end is at the center position of the multi-laser beam spot. The height h2 from the intersection position of the welding wire and the laser beam to the top surface 12a of the assembly is 0 - 1 mm. During the laser welding process, the first beam spot heats and melts the welding wire through laser energy, making it integrate with the base material to form a molten pool, as Figure 18 The figure shows a top view schematic diagram of synchronously adding a welding wire, Figure 19 and the figure shows a side view schematic diagram; where 41 is the projected area formed by the bonding position of the welding wire 4 on the top surface of the assembly. The wire feeding device can be, for example, MAG (Metal Active Gas), MIG (Metal Inert Gas), or TIG (Tungsten Inert Gas) as a feeding device alternative. And the welding wire can be a solid welding wire or a flux-cored welding wire. The addition method of the solder can also be composed of two different devices combined, for example, through two filling devices, respectively conveying different solders, one conveying solder containing a certain component, and the other conveying another component of solder, and through the combination of the two solders to achieve that the solder finally conveyed into the molten pool is the solder with the required proportion of components. In addition, the solder 4 can also be added through 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 combination of the two solders meets the content described in the present invention. During the welding process, the wire feeding speed of the welding wire is generally 1 - 5 m / min, and the ratio between it and 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, C, etc.; during the laser welding process, the austenitizing elements are conveyed to achieve a greater degree of offset of the negative effect on the aluminum element melted into the weld, eliminate the influence of ferrite, and reduce defects such as undercut.

[0078] For those skilled in the art, this welded joint generally needs to be subjected to hot stamping quenching to obtain the final welded joint. Generally, the preliminary laser welded joint obtained above is subjected to quenching heat treatment, and this process includes a heating and holding stage and a rapid cooling stage. In the heating and holding stage, the welded joint is held in an environment with a temperature of about 850 - 960 °C for 3 - 10 min, so that the weld zone and the base material zone of the welded joint are fully and completely austenitized, and during this stage, elements such as C, Mn, and Al in the weld diffuse and tend to be evenly distributed; the heating method can be heating through a gas medium, putting the workpiece as a whole into an air furnace, a vacuum heat treatment furnace, or other gas atmosphere heating furnaces, or heating through a liquid medium, immersing the workpiece as a whole or the welded part in oil or salt within the heating temperature range, where the oil is the oil used for conventional heat treatment oil baths and the salt is the salt used for low-temperature salt baths.

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

[0080] Finally, the welded joint includes a base metal area and a weld fusion area; the average micro-Vickers hardness of the base metal area and the fusion area is HV BM 、HV FZ , and it satisfies that HV BM / HV FZ is between 0.8 - 1.2, preferably between 0.85 - 1.15, and more preferably between 0.97 - 1.06; HV BM ≥475 HV; and the carbon content by mass percentage in the base metal area is not lower than 0.2%, and the average aluminum content by mass percentage in the weld area is not higher than 1.8%, preferably not more than 1.4%. This structure can be used to manufacture various components for locomotives, such as automotive A and B pillars, and door ring structures. Its joint tensile strength is not lower than 1400 Mpa, even exceeding 1700 Mpa, the elongation rate exceeds 5%, and the tensile fracture fails in the base metal area, having excellent comprehensive performance.

[0081] Example 1

[0082] The original welded material has an aluminum-silicon coating with a thickness of about 10 μm on both the upper and lower surfaces, Figure 20 as shown in the cross-sectional morphology of the original coating; the substrate thickness is 1.4 mm, and the strength after hot stamping and quenching exceeds 1500 Mpa. In the laser welding process, a four-beam welding method is adopted. Each of the four beams is 700 w, the light spot is circular, with a diameter of 0.04 mm, the light spot spacing is 0.25 mm, the welding speed is 3 m / min, the four light spots are arranged perpendicular to the weld travel direction, and the beam quality of each laser beam is <1.5. Among them Figure 21The figure shows a high-speed camera photo of the molten pool formed during the welding process. It can be seen that a molten pool with an approximately right-angled front is formed, and there are 4 independent laser holes, which helps to make the diffusion flow of the molten aluminum coating in the molten pool more uniform. After welding, the initial weld is placed in a heating furnace at a heating temperature of 930 °C and held for about 300 s to austenitize the weld and the base metal. Subsequently, rapid pressure cooling is carried out in a mold with rapid cooling. After holding the pressure for about 10 s, martensitic transformation occurs in both the weld and the base metal to obtain the final welded joint. Figure 22 The figure shows that all the specimens failed by fracture in the base metal area. The tensile strength of the joint reached 1582 Mpa, and the elongation was 9.59.

[0083] Comparative Example 1

[0084] The comparative example uses the same welding raw materials as in Example 1. It adopts the ordinary single-beam welding method during the laser welding process. The light spot is circular with a diameter of 0.98 mm, the welding speed is 3 m / min, the laser power is 4300 W, and the beam quality M0 of the laser beam > 1.5. After welding, the same quenching process is carried out. Figure 23 The figure shows that all the specimens failed by fracture in the base metal area. The tensile strength of the joint was 1611 Mpa, and the elongation was only 5.21%.

[0085] Example 2

[0086] The welded raw material is an aluminum-silicon coating with a thickness of about 28 μm on both the upper and lower surfaces. Figure 24 The figure shows the cross-sectional morphology of the original coating; the thickness of the base material is 1.4 mm, and the strength can reach more than 1500 Mpa after hot stamping and quenching. It adopts the four-beam welding method during the laser welding process. Each of the four beams is 1000 w, the light spots are all circular with a diameter of 0.04 mm, the light spot spacing is 0.25 mm, the welding speed is 3 m / min, the four light spots are arranged perpendicular to the weld travel direction, the beam quality of each laser beam < 1.5, and a wire with austenitizing elements is synchronously added during the welding process. The composition of the wire is Ni / Cr > 1 calculated by mass percentage, and the wire feeding speed is 0.6 m / min. As Figure 25 The figure shows a high-speed camera photo of the molten pool formed during the welding process. Figure 26 It is a partial enlarged photo; it can be seen that a molten pool with an approximately right-angled front is formed, and the wire melts into the molten pool under the combined action of the laser beam, which helps to make the diffusion flow of the molten aluminum coating in the molten pool more uniform. After welding, the initial weld is placed in a heating furnace at a heating temperature of 930 °C and held for about 300 s to austenitize and perform rapid cooling to undergo martensitic transformation to obtain the final welded joint. Figure 21It is shown that all the specimens fractured and failed in the base metal area, the tensile strength of the joint reached over 1551 Mpa, and the elongation reached 9.21%.

[0087] Comparative example

[0088] The comparative example uses the same welding raw materials as in Example 1. It adopts the ordinary single-beam welding method during the laser welding process. The light spot is circular with a diameter of 0.98 mm, the welding speed is 3 m / min, the beam quality M0 of the laser beam is > 1.5, the same welding wire is used, and the wire feeding speed is 1.2 m / min. The same quenching process is carried out after welding is completed. Figure 23 It is shown that all the specimens fractured and failed in the base metal area, the tensile strength of the joint was only 1283 Mpa, and the elongation was only 1.73%.

[0089] Table 1 summarizes the tensile results of the examples and comparative examples of the present invention

[0090] Tensile strength (Mpa) Elongation rate (%) Example 1 1582 9.59 Comparative Example 1 1611 5.20 Example 2 1551 9.21 Comparative Example 2 1284 1.73 。

Claims

1. A laser welding method for metal workpieces with an aluminum coating on the surface, characterized in that, The method comprises the following steps: 1) Providing 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, and at least one workpiece surface has an aluminum coating; butt-joining the first metal workpiece and the second metal workpiece to form a workpiece assembly to be welded and a butt joint line; 2) Control at least two laser beams to travel along the direction of the butt joint line to melt the butt joint position of the first metal workpiece and the second metal workpiece, and form a weld seam after cooling and solidification; wherein, the at least two laser beams are symmetrically arranged along the welding travel direction, and the beam quality factor M of each laser beam 2 is less than 1.5, and the beam quality of each laser beam approximates single-mode output; each laser beam irradiates the top surface of the butt joint line to form more than two keyholes and molten metal surrounding the keyholes; Under the action of laser beam energy irradiation inside and outside the keyhole, the welding area of the assembly is melted to form an integral elongated molten pool, and the front edge of the molten pool tends to be perpendicular to the welding travel direction, forming a nearly rectangular front end; the keyhole drives the flow of the surrounding molten metal, melting the aluminum coating on the surface of the assembly and making the distribution of aluminum in the molten pool tend to be uniform.

2. The laser welding method for a metal workpiece with an aluminum coating on its surface as described in claim 1, characterized in that, The number of symmetrically arranged laser beams is not less than 3, and the beam quality factor M of the laser beams 2 is less than 1.5, and the equivalent circular diameter of the irradiation spot of any beam does not exceed 0.3 mm.

3. A laser welding method for a metal workpiece with an aluminum coating on its surface as claimed in claim 1, characterized in that, It further includes at least one additional laser beam arranged along the welding travel direction, and the additional laser is located in front of or behind the symmetrically arranged laser beams.

4. A laser welding method for a metal workpiece with an aluminum coating on its surface as described in claim 1, characterized in that, The keyholes formed by the symmetrically arranged multiple laser beams are interconnected.

5. A laser welding method for a metal workpiece with an aluminum coating on its surface as claimed in claim 1, characterized in that, The laser power of any one of the symmetrically arranged laser beams is not less than 500 w, and each laser power can be the same or different.

6. A laser welding method for a metal workpiece with an aluminum coating on its surface according to claim 1, characterized in that, The equivalent circle diameter of the projection area of the irradiation of any one of the symmetrically arranged laser beams on the top surface of the assembly does not exceed 0.2 mm.

7. A laser welding method for a metal workpiece with an aluminum coating on its surface according to claim 1, characterized in that, During the welding process, any one of the laser beams can simultaneously perform high-speed periodic swinging travel in a certain path pattern.

8. A laser welding method for a metal workpiece with an aluminum coating on its surface according to claim 1, characterized in that, 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, with the aluminum content not less than 80%, and the total coating thickness is 5-50 μm.

9. A laser welding method for a metal workpiece with an aluminum coating on its surface according to claim 1, characterized in that, During the laser welding process, a welding wire is also synchronously filled to melt and enter the molten pool, and the welding wire is an austenitizing element containing at least one of Ni or Mn.

10. A metal workpiece with an aluminum coating on its surface, which is welded by the laser welding method for metal workpieces with an aluminum coating on their surfaces according to any one of claims 1-9, characterized in that, The aluminum content in the weld area of the workpiece is not higher than 1.8%, the tensile strength is not less than 1400 MPa, and the tensile fracture failure occurs in the base material area.

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