CN119347112A - Laser welding method for metal workpiece with aluminum coating on the surface and workpiece - Google Patents

Abstract

The invention provides a laser welding method for a metal workpiece with an aluminum coating on the surface, which comprises the following steps of 1) providing a pair of a first metal workpiece and a second metal workpiece which are used for welding, and a laser beam used for laser welding, wherein the first metal workpiece and the second metal workpiece respectively have a top surface and a bottom surface, at least one workpiece surface is provided with the aluminum coating, butting the first metal workpiece and the second metal workpiece to form a to-be-welded assembly and a butting line, and 2) advancing the laser beam along the butting line direction of the to-be-welded assembly so as to melt the butting position of the first metal workpiece and the second metal workpiece, and forming a welding line after cooling and solidification. According to the method, multiple laser beams are provided to act in laser welding of butt joint of the aluminum-plated steel, so that the distribution of aluminum elements in a welding line is homogenized, the influence of aluminum on a welding joint is weakened, the method is low in cost, the operation is simple, and the laser welding joint with excellent performance can be obtained.

Description

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

Technical Field

The present invention relates to the manufacture of welded parts of high strength steel coated plates, and more particularly to a laser welding method for a metal workpiece having an aluminum coating on the surface thereof and the workpiece thereof.

Background

In some fields, especially in the manufacture of parts for automobiles and aerospace, materials having a high corrosion resistance are required for use, so that more and more materials having a plating layer on the surface are used. The steel product with aluminum alloy coated on the surface has better corrosion resistance and higher high temperature resistance, which is beneficial to the manufacture of certain parts by using a hot working method, so the steel plate with aluminum alloy coated on the surface is particularly applied to the manufacture of automobile bodies, such as door reinforcing parts, B-pillar structural parts, roof reinforcing parts and the like, and particularly applied to hot stamping plate parts with ultrahigh strength, thereby achieving higher corrosion resistance, reducing the weight of automobile bodies and improving the collision absorption energy.

Meanwhile, in the automobile manufacturing industry, in order to reduce the weight of the automobile and realize the aim of light weight, more and more materials with equal thickness, different thickness, same quality and different quality are manufactured by welding and then hot stamping the welding parts, so that better weight reduction effect and cost reduction can be achieved. The laser welding method is the preferred method of this approach, with higher quality, higher efficiency and better flexibility, and is called "laser tailor welded blank". The high-strength martensitic steel structural component is usually obtained by performing laser butt welding on a flat workpiece and then performing hot stamping forming quenching on the workpiece with the welding seam. During hot stamping, the welded seam structure is heated to a temperature exceeding the AC3 temperature of the steel workpiece to be fully austenitized, surface oxidation and decarburization phenomena are inevitably generated on the bare plate due to the higher temperature, the final structural performance is reduced, a coating layer is also required to be formed on the surface, aluminum or aluminum alloy coating layers are widely applied to the process, and the document represented by a patent CN101583486B describes the materials in detail.

But has great difficulty in manufacturing a laser tailor-welded blank coated with aluminum or an aluminum alloy on the surface. When the weld surface is coated with a material of aluminum or aluminum alloy, the coating on the original surface may enter into the molten area, particularly into the molten area of the weld, which may lead to the formation of ferrite structures at high temperatures during cooling, which may remain in the final weld even after cooling through high temperature austenitization, resulting in reduced joint performance. Under the subsequent mechanical load of the weldment, a great amount of ferrite becomes an initial position of failure cracks, the bearing capacity of the welded joint is seriously weakened, and the welded joint is directly broken at the welding seam when being loaded, so that how to inhibit the performance degradation of the joint caused by the entering of aluminum into the welding seam is necessary.

The CN101426612B, 106334875a, etc. remove the coating on the surface by a pre-mechanical method, a laser method or other methods to prevent it from entering the weld, but this method has problems of high cost, complex process, etc.

Patent CN106392328B discloses a method of welding aluminum silicon plated hot formed steel under a protective atmosphere. The patent adopts the combination of oxidizing gas and aluminum element in the coating to generate aluminum oxide in the welding process, and the aluminum oxide has no influence on the toughness of welding tracks. However, in the scheme, the time for melting metal to form a molten pool and resolidifying the molten pool into a welding bead is very short, the reaction time of oxidizing gas and aluminum element is limited, aluminum element which does not react with the oxidizing gas can generate aluminum accumulation in the molten pool once entering the inner part of the molten pool, the toughness and strength of the welding bead are reduced, and the risk of cracking of a hot stamping part exists.

In CN 106488824B, a method of joining two blanks is disclosed, welded by a filler wire, the wire being made of a stainless steel alloy comprising, by weight, 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 iron and unavoidable impurities. In the method, when the laser and arc welding are adopted for mixed welding, the heat input is increased, the probability of thermal deformation of the sheet is increased, and the splicing of the sheets is not facilitated. And the welding speed of the laser and the arc welding mixed welding is limited, the production efficiency is reduced, and meanwhile, the added alloy elements are more in variety, the difficulty in process control is higher, and the manufacturing cost is higher.

In CN 111432975A a welding wire is disclosed that is made up by filling a composition consisting of, by weight, 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, 1.6% copper. When the welding wire is used as a filling material, cr is used as an austenite stabilizing element, high content of Cr can lead to the risk of ferrite formation in a welding line area of a splice welding plate after hot stamping, so that the mechanical property of a welding joint is reduced, the quality of a product cannot be ensured, and the high failure risk of the welding joint exists. In CN 112368105A, a laser welding is disclosed for a coated steel blank using a filler wire, where the wire includes nickel, chromium, and carbon elements in an amount of 1.68-10.48%,0-2.70%, and 0.91-2.00% by weight, respectively, and the high carbon content of the wire results in an excessively high carbon content in the tailor-welded zone, which reduces plasticity and impact properties, and the excessively high carbon content can cause cracking of the weld, and also reduces corrosion resistance of the weld and the service life of the tailor-welded blank. The welding wire has higher content of C element, has higher difficulty in the production and drawing process of the welding wire, and is extremely easy to work and harden to cause the phenomenon of welding wire fracture.

In CN 108025400A, a laser welding method for a sheet of a hardenable steel semi-finished product with an aluminum-based or aluminum-silicon-based coating is disclosed, in which a composition containing 0.5 to 2.0 weight percent of the sum of Cr and Mo and 1 to 4 weight percent of ni is filled, and the addition of Mo and Cr tends to cause precipitation of carbides in the weld and increase joint brittleness, making the properties unstable.

In CN 104994989A, a welding wire is disclosed which is filled with a composition consisting of, by weight, 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 iron and unavoidable impurities. The welding wire contains lower chromium and molybdenum elements, the hardenability and the corrosion resistance of a welding joint obtained by using the welding wire as a filling welding wire are poor, the influence of ferrite cannot be completely restrained in the high-temperature heat treatment process, and the failure risk of the welding joint in a welding line and a heat affected zone is increased.

CN104023899B discloses a laser welding method using a filler wire containing carbon or manganese, which makes the weldment not generate ferrite structure at 900-950 ℃, high content of carbon or manganese easily causes brittleness, and manganese element easily segregates to deteriorate joint performance.

In CN 112548395A, a welding wire for laser filler wire welding, a preparation method and a welding plate splicing process are disclosed, and the method is characterized in that the welding wire with higher carbon content is filled, the carbon content in the welding wire is about 2.5-10 times of that in a welding parent metal, so that the carbon content in a welding area is too high, the plasticity and the impact property are reduced, the welding line is easy to crack due to the too high carbon content, and the corrosion resistance of the welding line and the service life of the welding plate are also reduced.

In US20210008665 A1, a welding method for a coated steel plate is disclosed, wherein the weight percentage of the carbon content of the welding wire is 0.80 to 2.28 times of the carbon content of a base material, and the excessive carbon content can cause the weld joint to be easy to crack and harden and become brittle, and can reduce the corrosion resistance of the weld joint and the service life of a splice welding plate, so that the consistency of the mechanical properties of the welding joint is poor, the product quality is reduced, and the welding method is not suitable for mass production.

It can be seen that the high-quality laser welding is carried out on the steel blank coated with the aluminum coating at present, a welding wire with various complex elements is generally required to be added in the welding process, the added welding wire is mainly composed of various alloy elements and comprises C, cr, ni, mo, mn, si and other various alloy elements, on one hand, the welding wire is difficult to fully ensure that the welded part has stable and excellent mechanical property and corrosion property, meanwhile, the addition of various alloy elements is extremely easy to produce work hardening in the drawing forming stage of the welding wire, so that the yield of the welding wire is low, the manufacturing difficulty is high, meanwhile, the complex proportion of various alloy elements is difficult to effectively control in the process, and the final weld performance is extremely easy to be disqualified. And more expensive Mo and other elements also make the manufacturing cost higher. Moreover, the high content of Mo and Cr elements is easy to form coarse carbide in the weld joint, so that the weld joint performance becomes brittle, in particular to impact performance. And due to the addition of various alloy elements, the weld joint can generate galvanic corrosion, and the corrosion performance of the weld joint can be also deteriorated. The addition of additional welding wire clearly increases manufacturing costs and introduces complex process optimizations. In addition, for the laser welding with ultrahigh strength and high speed, because of the characteristics of quick heating and quick cooling of the laser welding, a hardened martensitic structure is extremely easy to generate in a welding line and a heat affected zone, the sensitivity of cold cracking is greatly increased, and brittle fracture is extremely easy to generate after welding. There is a need for a simple, low cost method to achieve laser weld joint preparation for high efficiency laser welding of surface coated plates with higher strength, high toughness, high corrosion resistance.

Disclosure of Invention

The invention provides a high-performance laser welding joint manufacturing method, which is characterized in that the adverse effect of aluminum on a welding joint is weakened by controlling the action range of a plurality of laser beams and heating and cooling processes in the laser welding process, particularly realizing the formation of a plurality of temperature rising and falling processes around a deep-melting small hole, so that the uniform distribution of the molten aluminum coating on the surface tends to be realized after the molten aluminum coating enters a molten pool. The obtained welding seam is a welding seam structure taking martensite as a main body, and a welding joint with excellent comprehensive performance is obtained after a subsequent quenching heat treatment process.

The technical scheme of the invention is as follows:

A laser welding method for a metal workpiece having an aluminum plating layer on a surface thereof, the method comprising the steps of:

1) Providing a pair of first and second metal workpieces for welding, and a laser beam for laser welding, wherein the first and second metal workpieces respectively have a top surface and a bottom surface, and at least one of the workpiece surfaces has an aluminum plating;

2) Using at least two laser beams to travel along the butt joint line direction of the assembly to be welded so as to melt the butt joint positions of the first metal workpiece and the second metal workpiece, and forming a welding line after cooling and solidifying;

Wherein each laser beam irradiates the top surface of the opposite connection line, at least one laser beam is used as a main laser beam to form a deep-melting small hole and molten metal surrounding the small hole, at least one other laser beam is irradiated outside the small hole when the molten metal surrounding the deep-melting small hole is still in a molten liquid state, and the welding area of the assembly is melted to form an integral slender molten pool under the irradiation of the laser beam energy in the small hole and the laser beam energy outside the small hole, the aluminum plating layer on the surface of the assembly is melted and the aluminum distribution tends to be uniform in the molten pool.

Further, the first metal workpiece and the second metal workpiece are high-strength steel which can be converted into hot forming steel after hot stamping forming, the structure of the base material before forming comprises ferrite and pearlite, the carbon content is not less than 0.1%, and the base material has boron element with the content of about 0.002-0.006%.

Further, the tensile strength of the high-strength steel is not lower than 500Mpa, the surface coating is mainly composed of aluminum and silicon, the aluminum content is not lower than 80%, and the total thickness of the coating is 5-50 mu m.

Further, at least a portion of the molten metal surrounding the deep-melt aperture undergoes a thermal cycling process of at least two stages of temperature rise and fall.

Further, the first laser beam forms a deep-melt aperture that penetrates or does not penetrate the metal assembly.

Further, the second laser beam also forms a second deep-melt aperture, and the second laser beam forms a deep-melt aperture that does not penetrate the metal assembly.

Further, the second deep-melting small hole formed by the second laser beam is communicated with the deep-melting small hole formed by the first laser beam.

Further, the width of the top surface of the welding seam formed by solidification of the molten pool is not less than 0.8 times of the thickness of the workpiece;

Further, the area formed by irradiating the laser beam on the top surface of the assembly at least comprises two beam spots, wherein the two beam spots comprise a beam spot formed by a main laser beam and a beam spot formed by a second laser beam, and heat is generated in the irradiation area at the same time to melt the substrate, and the substrate is cooled and solidified to form a welding seam. The relative positions and energy distribution of the two beams of spots are of various types, including that the two beams of spots are arranged back and forth along the travelling direction or are arranged side by side perpendicular to the travelling path, and the geometric shapes of the two beams of spots can be the shapes formed by fold lines, curves such as circles, polygons and the like.

Further, the ratio of the energy of the main laser beam to the energy of the second laser beam is not lower than 1.

Further, the projected area equivalent circle diameter of the first main laser beam and the second laser beam irradiated on the top surface of the assembly is not less than 0.6mm.

Further, the laser welding process also includes a welding wire for filling, and the welding wire is melted into a molten pool.

The laser welding joint is subjected to hot stamping quenching treatment, which comprises a heating and heat preservation stage and a rapid cooling stage, so that a welding joint weld zone and a base material zone are fully and completely austenitized, and martensitic transformation occurs to form a final welding joint.

The heating and heat preserving stage keeps the welded joint at the temperature of about 850-960 ℃ for 3-10min, the cooling rate of the rapid cooling stage exceeds the critical cooling rate of martensitic transformation, the cooling rate is not lower than 25 ℃ per second, and the final temperature of the weldment is not higher than 250 ℃.

A metal workpiece with an aluminum coating on the surface, which is welded by the laser welding method for the metal workpiece with the aluminum coating on the surface.

Compared with the prior art, the invention has the beneficial effects that:

1) The welding efficiency is improved, and the shape and the size of the molten pool are finely regulated and controlled by precisely controlling the energy distribution and the irradiation area of each laser beam. By adjusting the relative positions and energy distribution of the two laser beams, deep-melting small holes can be formed in the molten pool, the range of molten metal around the deep-melting small holes can cover the thickness of the whole workpiece plate, the uniform distribution of an aluminum coating in the molten pool is effectively promoted, and the quality and strength of welding seams are improved.

2) The irradiation area of the laser beam can have various shapes and configurations, such as a circle, a polygon and the like, and the two beams of spots can be arranged back and forth or vertically side by side along the welding direction, so that the welding process can adapt to workpieces with different shapes and sizes, and the universality and the adaptability of welding are improved. By optimizing the geometry and relative position of the laser beam, a specific bath shape, such as an ellipsoid, can be formed in the bath, which shape aids in homogenizing the aluminum coating in the bath while reducing the occurrence of welding defects.

3) By controlling the distribution of the energy of multiple laser beams on the butt joint assembly in the plate thickness direction and the change process of the temperature change of a molten pool along with time, the high-performance welding joint is obtained, and the obtained welding joint has high tensile strength and elongation on one hand, and also has high hydrogen embrittlement resistance sensitivity, high corrosion resistance and high surface wear resistance.

4) By combining the hot stamping quenching process and the heating, heat preservation and rapid cooling processes, the weld zone and the base material zone are fully austenitized and martensitic transformation occurs, so that the hardness and strength of the welded joint are improved.

5) The welding method can be used for various environments with different gaps and has high adaptability. The whole process is interrupted, so that the internal residual stress can be reduced, the welding method is simple, no additional welding wire is needed, the manufacturing cost can be greatly reduced, the production efficiency is improved, and the high-performance welding joint is obtained.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings that are required in the embodiments or the description of the prior art will be briefly described, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.

FIG. 1 is a schematic diagram of a weld according to the present invention;

FIG. 2 is a schematic cross-sectional view of a welded workpiece in accordance with the present invention;

FIG. 3 is a schematic view of the assembly and splicing of two workpieces according to the present invention.

Fig. 4 shows a top view of the welding process according to the invention.

FIG. 5 is a schematic diagram of the location of two laser beam spots on the top surface of a workpiece according to the present invention.

FIG. 6 is a schematic diagram showing another relative position of two laser beam spots on the top surface of a workpiece according to the present invention.

FIG. 7 is a schematic diagram showing another relative position of two laser beam spots on the top surface of a workpiece according to the present invention.

FIG. 8 is a schematic diagram showing another relative position of two laser beam spots on the top surface of a workpiece according to the present invention.

FIG. 9 is a schematic cross-sectional view of a molten pool formed during laser welding in accordance with the present invention.

FIG. 10 is a schematic cross-sectional view of another melt pool formed during laser welding in accordance with the present invention.

FIG. 11 is a schematic cross-sectional view of another melt pool formed during laser welding in accordance with the present invention.

FIG. 12 is a schematic cross-sectional view of another melt pool formed during laser welding in accordance with the present invention.

FIG. 13 is a schematic top view of another weld puddle formed during laser welding in accordance with the present invention.

Fig. 14 is a schematic diagram of the invention when the two beams are mirror symmetric with respect to the opposite line.

FIG. 15 is a schematic view of another relative position of three laser beam spots on the top surface of a workpiece in accordance with the present invention.

FIG. 16 is a schematic illustration of the present invention with the addition of welding wire during the welding process.

FIG. 17 is a schematic top view of another weld puddle formed during laser welding in accordance with the present invention.

FIG. 18 is a schematic cross-sectional view of another melt pool formed during laser welding in accordance with the present invention.

Figure 19 is a schematic diagram illustrating a cross-section of a workpiece in one embodiment of the invention.

FIG. 20 shows the results of one example of tensile mechanical properties of a weld joint obtained according to the present invention.

FIG. 21 shows the results of another example of tensile mechanical properties of a weld joint obtained according to the present invention.

FIG. 22 shows the results of another example of tensile mechanical properties of a weld joint obtained according to the present invention.

Fig. 23 shows the results of the comparative example.

Reference numerals 11-first steel workpiece, 12-second steel workpiece, 111-upper surface coating of first steel workpiece, 112-lower surface coating of first steel workpiece, 12-second steel workpiece, 11 a-top surface of first steel workpiece, 11B-bottom surface of first steel workpiece, B1-butt gap, B-coating thickness, 3-laser beam, 31-first laser beam, 32-second laser beam, d: first laser beam spot center distance from second laser beam spot, d 1-first laser beam spot diameter, d 2-second laser beam spot diameter, 4-welding wire, 5-molten pool, 6-welding seam, h 1-laser spot depth formed by first laser beam, 51-molten pool front side molten metal, 52-molten pool rear side molten metal, 511-front wall, 512-rear wall of molten pool, 71-laser spot 711-laser spot wall, B-assembly thickness, 12 a-assembly gold butt position top surface, 12B-assembly butt position bottom surface, 72-second laser spot, 2-second laser spot depth and 3-second laser spot top surface projection distance from top surface of second laser spot to top surface of 2-welding wire, 2 h 1-first laser spot depth and 2-second laser spot depth.

Detailed Description

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 is to be understood that these examples are illustrative of the present invention and are not intended to limit the scope of the present invention. Furthermore, the drawings are schematic representations, and thus the apparatus and device of the present invention are not limited by the dimensions or proportions of the schematic representations.

It should be noted that in the claims and the description of this patent, relational terms such as first and second, and the like are used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Moreover, the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising" does not exclude the presence of other like elements in a process, method, article, or apparatus that comprises the element. The final weld joint is referred to herein as a laser welded joint.

The present embodiment provides a laser welding method for a metal workpiece having an aluminum plating layer on a surface thereof. The whole method comprises the following steps:

1) Referring now to fig. 1, a pair of first and second metal workpieces 11, 12 for welding, having top and bottom surfaces 11a, 11b, respectively, with at least one of the workpiece surfaces having an aluminum plating 111 or 112, are provided, together with a laser beam 3 for laser welding;

2) The laser beam advances along the butt joint line direction of the assembly to be welded to melt the butt joint positions of the first metal workpiece and the second metal workpiece, and the weld is formed after cooling and solidifying. Wherein the laser beams comprise at least two laser beams, wherein each laser beam irradiates the top surface of the butt wire, wherein at least one main laser beam forms a deep-melting small hole and molten metal surrounding the small hole, and at least one other second laser beam is irradiated outside the small hole when the molten metal surrounding the deep-melting is still in a molten liquid state, and the welding area of the assembly is melted to form an integral slender molten pool under the irradiation of the laser beam energy in the small hole and the laser beam energy outside the small hole, the aluminum plating layer on the surface of the assembly is melted and the aluminum distribution tends to be uniform in the molten pool.

The steel workpiece is high-strength steel which can be converted into hot forming steel after hot stamping forming, is well known in the art, the base material of the steel workpiece is generally composed of ferrite and pearlite before forming, wherein the carbon content is not less than 0.1%, and the base material is provided with boron elements with the content of about 0.002-0.006 so as to meet the hardenability of the subsequent quenching process, so that a full-martensitic structure can be formed after quenching, the strength is higher, and the typical tensile strength of the boron steel is not less than 500Mpa. The typical steel plate base material is 22MnB5, and the mass ratio of the contained elements is ：0.15％≤C≤0.45％;0.5％≤Mn≤2.5％;0.08％≤Si≤0.4％;Al≤0.45％;0.01％≤Cr≤0.5％;Ti≤0.1％;Nb≤0.1％;V≤0.1％;S≤0.05％;P≤0.05％;0.002％≤B≤0.006％;, and the balance is Fe and unavoidable impurities. As a material with higher strength level, more C and other alloy elements are added, for example, 34MnB5,28MnB5,37MnB4 and other typical marks, and the tensile strength can exceed 2000 Mpa. As a representative of the higher strength, the mass fraction of carbon content is higher than 0.25%. The surface coating in the present invention is generally composed mainly of aluminum and silicon, wherein the aluminum content is not less than 80%, preferably the aluminum content exceeds 85%, and wherein the total thickness b of the coating is generally 5 to 50 μm, preferably 7 to 40 μm, more narrowly 8 to 20 μm.

Fig. 3 shows a schematic view of the assembly of a workpiece. The two workpieces 11 and 12 can be made of two identical materials or two materials with any difference of different thickness, plating type and strength, the thickness of the two workpieces is generally 0.8-3.0mm, and the two workpieces are butted to form an assembly, wherein the butting gap is B1, and the butting gap is 0-0.5mm. The laser beam 3 irradiates the butt line to travel along a welding path to melt the substrate and the surface coating to form a molten pool 5, and after cooling and solidification, a weld 6 is formed, as shown in fig. 4.

Wherein the area of the assembly formed by irradiation of the laser beam 3 on the top surface comprises 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, as shown in fig. 5, the two beam spots simultaneously generate heat in the irradiated area to melt the substrate and solidify to form a weld joint after cooling. It is worth mentioning that the relative positions and energy distribution of the two beam spots can be of various types, the two beam spots are arranged back and forth along the traveling direction and have a spacing d, the projection shape of the two irradiation beam spots on the top surface can be of various types, the two beam spots are shown in a circular shape in the schematic view in the figure 5, the beam spot diameters are d1 and d2 respectively, in general, d1 and d2 are equal to or larger than 0.5mm, and the center spacing d of the two beam spots is satisfied when the two beam spots are completely out of intersection, d > (d1+d2)/2 is shown in the figure 5, and d1-d 2)/2 is equal to or smaller than d (d1+d2)/2 is shown in the figure 6 when the two beam spots have an overlapping area. The geometry of the two beams of spots can be the shape formed by fold lines, such as circles, polygons and the like, and curves, and can be the same or different, and the two beams of spots are respectively circular and quadrilateral as shown in fig. 7. In summary, the shape of the beam spot in the irradiated region of the top surface of the assembly is a mirror image of the weld centerline (butt line) as shown in FIG. 8, which is a schematic diagram of the second beam spot shape in the form of a ring. Each beam spot has a surface area, and the diameter of the surface area equivalent circle is not less than 0.5mm. The relative positions of the two beam spots can be switched, i.e. the main beam spot can be located on the front side or on the rear side as seen in the welding direction, the so-called main beam spot being merely the name difference for distinguishing the beam spots.

Referring now to fig. 9, the core of the present invention is that at least one deep-melting keyhole 71 is formed when a plurality of laser beams 31, 32 irradiate the top surface 12a of the assembly, wherein the keyhole has a boundary 711, molten metal 51 is provided around the keyhole 71, and the molten metal in the keyhole rear Fang Rongrong metal 52 as seen in the welding direction is allowed to spread over the entire thickness of the assembly to the bottom surface 12b of the assembly by the heat of irradiation of another laser beam 32, while the molten metal contains molten surface aluminum plating elements therein, forming an integral molten pool having a melting front wall 511 in the welding direction and a solidification rear edge 512, the aluminum distribution in the pool being made to be uniform, and the molten metal in at least a portion of the trailing portion 52 of the pool being subjected to two stages of temperature rise and fall. In fig. 9, the laser aperture 71 formed by the primary laser beam 31 is not fully penetrated through the assembly to the bottom surface 12B, while the secondary laser beam 32 is not laser aperture formed, i.e., the aperture depth h1 is less than the plate thickness B. In practice, the laser aperture 71 may penetrate the plate thickness, and the laser beam 32 may also be formed as a second laser aperture. As shown in fig. 10, the laser apertures 71 formed by the first laser beam penetrate the bottom surface 12b of the assembly, and the second laser beam 32 forms the second laser apertures 72, and finally, under the action of the two laser beams, a molten pool having a front wall 511 and a solidification rear wall 512 is formed, so that the distribution of aluminum element in the molten pool tends to be uniform, and the center-to-center distance d of the two laser apertures is the distance between the centers of the beam spots formed by the irradiation of the top surface 12a of the combination of two plates. In addition, the depth of the laser pinholes formed by the two laser beams may be changed along the welding traveling direction, for example, fig. 11 shows that the depth h2 of the laser pinholes formed by the front end is shallower and the depth h1 of the laser pinholes formed by the rear end is deeper, and the front end laser beam opposite to fig. 10 can provide preheating effect, so that the main laser beam can quickly raise the temperature when applied to the molten metal, sufficiently melt the workpiece in the thickness direction, and have lower cooling speed when the molten pool is cooled and solidified to form the weld joint, which can also reduce the hardenability of the formed weld joint. In general, the primary laser beam forms a hole depth h1>0, while the secondary laser beam forms a laser hole satisfying 0.ltoreq.h2 < B, with 0.ltoreq.h2 < B/2 being preferred. The other two laser orifices may be overlapping or completely independent, with overlapping portions of the two laser orifices being shown in fig. 10 and 11, where the overlapping portions experience higher temperature increases and decreases. Whereas the laser apertures formed by the two laser beams shown in fig. 12 are completely separate, this would cause the absence of apertures in the middle of the two laser apertures to undergo a more pronounced process of multiple temperature increases and decreases, making the aluminum substantially uniform. In the present invention, as a whole, the shape of a molten pool formed by irradiation of multiple laser beams may be of various types. In particular, the melt pool 5 has at least one laser aperture 71 which is ellipsoidal when viewed from the top surface, as shown in FIG. 13, which is a schematic view of the formation of two deep-melt apertures.

It is noted that the multiple laser beam irradiation spots may be generally disposed in tandem along the butt center line, or may be disposed side by side perpendicular to the travel path, as seen in fig. 14, which is a schematic view of the two spots 31, 32 mirror-symmetrical with respect to the butt line. In addition, the number of the plurality of laser beams is at least more than 2, the two foregoing schematic views are taken as examples, and in practice, more than two laser beams can be provided, as shown in fig. 15, which is a schematic view of the arrangement of three laser beam spots on the top surface when the two laser beams 31, 32 are in mirror symmetry along the butt line, the third beam spot 33 is located at the center of the butt line, so as to form a molten pool, and the number of laser pinholes is not less than 1, and in this case, the two symmetrical laser beam spots are formed or not formed.

In general, the laser beams 31, 32 are laser beams having the same wavelength or different wavelengths, with wavelengths of 0.3-10 μm, in particular 0.5-3 μm, emitted by infrared lasers, as is common in laser processing. The laser beam is typically emitted by a laser, wherein the laser may comprise a plurality of 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 CO ₂ gas laser. Of course, other types may be included as long as they are capable of generating a laser beam and generating a keyhole and a molten weld pool. Wherein the first laser beam and the second laser beam may be emitted by the same laser or by separate lasers. The two laser beams may be split into two laser beams 31, 32 by the same laser through a beam splitter in an integral laser processing head, the beam splitter being achieved by any optical element or combination thereof, such as a prism, a mirror, etc., and a laser system controller may be coupled to the beam splitter to control the energy distribution and irradiation area size of the two beams and the coordinated output of the processing, such as power level, mutual position distance d, defocus, etc. The generation of multiple laser beams is generally performed by an optical element or a combination thereof in the laser processing head, and the laser beams with fixed beam characteristics can be formed by completely fixed optical elements, or the laser beams can be formed by an optical element with movable and deflection in the laser processing head, so that irradiation areas 31 and 32 formed on the surface of the raw material by the formed laser beams can be changed, and the high-speed scanning of the laser beams in space positions can be realized by the movement, the deflection and the like during the processing. The laser beam energy distribution may be in various forms such as gaussian distribution, average distribution, or dot-loop distribution. The laser beam can also be fixed or synchronously move at high speed in various forms such as swinging and the like when moving along the welding direction in the welding process, and the swinging shape can comprise various shapes such as a circle, a fold line, a 8 shape, a infinity shape and the like, the swinging frequency is generally 50-500HZ, and the swinging amplitude is 0.2-1.5 mm. Various single or mixed shielding gases, such as Ar gas, he gas, etc., may be added or performed without shielding gas during the welding process. The power of the laser beams 31, 32 is typically 3-15KW, preferably 4-10KW, while the laser beam travel speed relative to the workpiece assembly is typically 2-10m/min, preferably 2-8m/min.

It should be noted that the solder may be added simultaneously during the laser welding process, wherein the solder may be in the form of powder or wire or rod, and is known in the art as a wire-filled laser welding. Fig. 16 is a schematic diagram showing the simultaneous addition of welding wire during welding. The welding wire is generally placed on the front side when viewed in the welding direction, the end of the welding wire is positioned at a first laser beam spot, the height h2 between the welding wire and the top surface 12a of the assembly is 0-1mm, the first laser beam spot heats and melts the welding wire through laser energy during the laser welding process to enable the welding wire and the base materials of the assembly melted by the first laser beam and the second laser beam to be integrated into a molten pool, as shown in a schematic top view when the welding wire is synchronously added in fig. 17, and a schematic side view in fig. 18, wherein 41 is the projection area formed by the welding wire 4 at the combining position of the top surface of the assembly, and generally the equivalent diameter d1 of the laser beam spot is not less than 0.6 times the diameter of the welding wire, preferably not less than 0.8 times the diameter of the welding wire. The wire feeder can be replaced by a feeding device such as MAG (metal active gas), MIG (metal inert gas) or TIG (tungsten inert gas). And the welding wire may be a solid wire or a flux-cored wire. The solder can be added by compounding two different devices, for example, two filling devices are used for respectively conveying different solders, one of the solders is used for conveying the solders containing a certain component, the other is used for conveying the solders containing another component, and the solders finally conveyed into the molten pool are used as solders with the components in the required proportion through the matching of the two solders. Alternatively, the solder 4 may be added synthetically by multiple steps or multiple parts, for example, the solder 4 is formed of two separate wires, and the average mass fraction of the final two solder combinations is such that the present invention is satisfied. The wire feed speed of the welding wire during the welding process is generally 1-5m/min, and the ratio of the wire feed speed to the welding speed is 0.3-1.5. The welding wire is iron-based and contains one or more austenitizing elements such as Ni, mn, C and the like, and the negative effects of the austenitizing elements on aluminum elements which are fused into a welding line are counteracted to a greater extent by conveying the austenitizing elements in the laser welding process, so that the influence of ferrite is eliminated.

It is generally necessary for those skilled in the art to hot stamping and quenching the welded joint to obtain the final welded joint. Typically, the preliminary laser welded joint obtained as described above is subjected to a quenching heat treatment, wherein the process includes a heating and holding stage and a rapid cooling stage. In the heating and heat preserving stage, the welding joint is kept in an environment with the temperature of about 850-960 ℃ for 3-10min, so that the welding joint weld zone and the base metal zone of the welding joint are fully austenitized, and elements such as C, mn, al and the like in the welding joint are diffused and tend to be uniformly distributed in the stage, wherein the heating mode can be that the whole workpiece is placed into an air furnace or a vacuum heat treatment furnace and other gas atmosphere heating furnaces through heating of a gas medium, or the whole workpiece or a welding piece is immersed into oil or salt in a heating temperature range through heating of a liquid medium, and the oil is conventional heat treatment oil bath oil and the salt is low-temperature salt bath salt.

After the heat soak is performed, the weldment is rapidly transferred to the next cooling device and rapidly cooled, with t1 typically not exceeding 15 seconds, at a cooling rate exceeding the martensitic transformation critical cooling rate, not less than 25 ℃, preferably greater than 30 ℃ per second, and the weldment finish temperature typically not exceeding 250 ℃, preferably not exceeding 200 ℃. In general, the process may be performed in a solid mold or a liquid medium having a certain temperature, for example, by performing a hot stamping process through a solid mold with a cooling medium, and then performing rapid press cooling, in which a pressure of not lower than 3Mpa is applied, a dwell time of not lower than 3s, and not higher than 12s is applied, and the weld is transformed from internal austenite to martensitic structure under rapid press cooling. However, the quenching process may also be carried out in a liquid medium, such as a vessel containing oil or salt, and the weld is rapidly cooled in the liquid medium to cause martensitic transformation to form the final weld joint.

Finally, the welded joint comprises a parent material zone and a weld fusion zone, wherein the average micro Vickers hardness of the parent material zone and the fusion zone is HV _BM、HV_FZ respectively, and HV _BM/HV_FZ is 0.8-1.2, preferably 0.85-1.1, more preferably 0.95-1.05, HV _BM is more than or equal to 470HV, the carbon content of the parent material zone is not less than 0.2% by mass, and the average aluminum content of the weld zone is not more than 1.8% by mass, preferably not more than 1.5% by mass. The structure can be used to make various components for locomotives, such as automobile A, B columns, door ring structures. The tensile strength of the joint is not lower than 1400Mpa, even more than 1700Mpa, the elongation is more than 5%, and the joint is failed in a base material area by stretching fracture, so that the joint has excellent comprehensive performance.

Example 1

Wherein the welding raw material is an aluminum silicon plating layer with the thickness of about 10 mu m on the upper surface and the lower surface, the section appearance of the original plating layer is shown in fig. 19, and the thickness of the base material is 1.4mm. The double-beam welding mode is adopted in the laser welding process, wherein the total laser power of the double beams is 4800W, the energy ratio is 20:80, the front part is low, the rear part is high, the two light spots are circular, the diameter is 0.98mm, the light spot distance is 0.25mm, and the welding speed is 3m/min. And after welding, placing the initial welding seam in a heating furnace, wherein the heating temperature is 930 ℃, and the heat preservation time is about 300s, so that the welding seam and the base metal are both converted into austenite. And then performing quick pressurized cooling in a die with quick cooling, and keeping the pressure for about 10 seconds to enable the welding line and the parent metal to generate martensitic transformation so as to obtain the final welding joint. The results of mechanical property testing after a final weld obtained using the method of the present invention was prepared using standard tensile test specimens are shown in FIG. 20. As a result, it was found that all of the two samples failed to fracture in the base material region, and the tensile strength of the joint reached 1500MPa or more and the elongation exceeded 8% or more.

Example 2

Wherein the welding raw material is an aluminum silicon plating layer with the thickness of about 10 mu m on the upper surface and the lower surface, and the thickness of the base material is 1.4mm. The double-beam welding mode is adopted in the laser welding process, wherein the total laser power of the double beams is 4800W, the energy ratio is 40:60, the front part is high, the rear part is low, the two light spots are circular, the diameter is 0.98mm, the light spot distance is 0.25mm, and the welding speed is 3m/min. And after welding, placing the initial welding seam in a heating furnace, wherein the heating temperature is 930 ℃, and the heat preservation time is about 300s, so that the welding seam and the base metal are both converted into austenite. And then performing quick pressurized cooling in a die with quick cooling, and keeping the pressure for about 10 seconds to enable the welding line and the parent metal to generate martensitic transformation so as to obtain the final welding joint. The results of mechanical property testing after a final weld obtained using the method of the present invention was prepared with standard tensile test specimens are shown in FIG. 21. As a result, it was found that all of the two samples failed to fracture in the base material region, and the tensile strength of the joint reached 1500MPa or more and the elongation exceeded 8% or more.

Example 3

Wherein the welding raw material is an aluminum silicon plating layer with the thickness of about 10 mu m on the upper surface and the lower surface, and the thickness of the base material is 1.4mm. The double-beam welding mode is adopted in the laser welding process, wherein the total laser power of the double beams is 5300W, the energy ratio is 50:50, two light spots are circular, the diameter is 0.98mm, the light spot distance is 0.49mm, and the welding speed is 3m/min. And after welding, placing the initial welding seam in a heating furnace, wherein the heating temperature is 930 ℃, and the heat preservation time is about 300s, so that the welding seam and the base metal are both converted into austenite. And then performing quick pressurized cooling in a die with quick cooling, and keeping the pressure for about 10 seconds to enable the welding line and the parent metal to generate martensitic transformation so as to obtain the final welding joint. The results of mechanical property testing after a final weld obtained using the method of the present invention was prepared with standard tensile test specimens are shown in FIG. 22. As a result, it was found that all of the two samples failed to fracture in the base material region, and the tensile strength of the joint reached 1500MPa or more and the elongation exceeded 8% or more.

Comparative example

Wherein the welding raw material is an aluminum silicon plating layer with the thickness of about 10 mu m on the upper surface and the lower surface, and the thickness of the base material is 1.4mm. The single laser beam welding mode is adopted in the laser welding process, wherein the laser power is 4500W, the shape of a light spot is round, the diameter is 0.98mm, and the welding speed is 3m/min. And after welding, placing the initial welding seam in a heating furnace, wherein the heating temperature is 930 ℃, and the heat preservation time is about 300s, so that the welding seam and the base metal are both converted into austenite. And then performing quick pressurized cooling in a die with quick cooling, and keeping the pressure for about 10 seconds to enable the welding line and the parent metal to generate martensitic transformation so as to obtain the final welding joint. FIG. 23 shows the results of mechanical property testing after a final weld obtained using the method of the present invention was prepared using standard tensile test specimens. The results show that all three samples are broken and fail in the welding line area, the tensile strength of the joint is less than 1500Mpa, the elongation is less than 5%, and the use requirement cannot be met.

Claims (11)

1. A laser welding method for a metal workpiece having an aluminum plating layer on a surface thereof, the method comprising the steps of:

1) Providing a pair of first and second metal workpieces for welding, and a laser beam for laser welding, wherein the first and second metal workpieces respectively have a top surface and a bottom surface, and at least one of the workpiece surfaces has an aluminum plating;

2) Using at least two laser beams to travel along the butt joint line direction of the assembly to be welded so as to melt the butt joint positions of the first metal workpiece and the second metal workpiece, and forming a welding line after cooling and solidifying;

Wherein each laser beam irradiates the top surface of the opposite connection line, at least one laser beam is used as a main laser beam to form a deep-melting small hole and molten metal surrounding the small hole, at least one other laser beam is irradiated outside the small hole when the molten metal surrounding the deep-melting small hole is still in a molten liquid state, and the welding area of the assembly is melted to form an integral slender molten pool under the irradiation of the laser beam energy in the small hole and the laser beam energy outside the small hole, the aluminum plating layer on the surface of the assembly is melted and the aluminum distribution tends to be uniform in the molten pool.

2. A laser welding method for a metal workpiece having an aluminum plating layer on a surface thereof as claimed in claim 1, wherein at least a part of the molten metal around the deep-melting hole is subjected to a thermal cycle process of temperature rise and fall in at least two stages.

3. A laser welding method for a metal workpiece having an aluminum plating layer on a surface thereof as recited in claim 1, wherein said first laser beam forms a deep-melting keyhole penetrating or not penetrating said metal assembly.

4. A laser welding method for a metal workpiece having an aluminum plating layer on a surface thereof as recited in claim 1 or 4, wherein said second laser beam also forms a second deep-melting keyhole, and said second laser beam forms a deep-melting keyhole which does not penetrate said metal assembly.

5. A laser welding method for a metal workpiece having an aluminum plating layer on a surface thereof as claimed in claim 1, wherein the second deep-melting small hole formed by the second laser beam is in communication with the deep-melting small hole formed by the first laser beam.

6. A laser welding method for a metal workpiece having an aluminum plating layer on a surface thereof as recited in claim 1, wherein a top surface width of a weld formed by solidification of said molten pool is not less than 0.8 times a thickness of the workpiece.

7. A laser welding method for a metal workpiece having an aluminum plating layer on a surface thereof as claimed in claim 1, wherein the laser beam is irradiated to form a region on the top surface of the assembly comprising 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, and the two beam spots simultaneously generate heat in the irradiated region to melt the substrate and solidify to form a weld joint upon cooling.

8. A laser welding method for a metal workpiece having an aluminum plating layer on a surface thereof as recited in claim 7, wherein a ratio of energy of the main laser beam to energy of the second laser beam is not lower than 1.

9. A laser welding method for a metal workpiece having an aluminum plating layer on a surface thereof as recited in claim 1, wherein a projected area equivalent circular diameter of the first main laser beam and the second laser beam irradiated on the top surface of the assembly is not less than 0.6mm.

10. A laser welding method for a metal workpiece having an aluminum coating on a surface thereof as recited in claim 1, further comprising a filler wire for filling and melting into the molten pool.

11. A metal work piece with an aluminum plating layer on a surface welded by the laser welding method for a metal work piece with an aluminum plating layer on a surface according to any one of claims 1 to 9.