Solder, and laser welded joint and manufacturing method therefor

Abstract

Disclosed is a laser welded joint (2). When two steel plate base materials are butt-jointed for laser welding, a laser welding solder is added to a molten pool, thereby forming an initial weld seam, and the initial weld seam is quenched to form the laser welded joint (2), wherein at least one steel plate is a steel plate having an aluminum coating or an aluminum alloy coating; the volume fraction of lath martensite (21) in the laser welded joint (2) is not less than 90%, the average mass fraction of Mn is not less than 3.2%, the average mass fraction of Al is not higher than 1.4%, and the average mass fraction of C is lower than the C content of the steel plate base materials; and the height h1 of a reinforcement on the front face of a final weld seam formed by the laser welded joint (2) and the height h2 of a reinforcement on the back face of the final weld seam do not exceed 0.4 mm.

Description

A solder, laser welded joint and method of manufacturing the same TECHNICAL FIELD

[0001] The present invention relates to the manufacture of high strength steel plated parts, in particular to a solder, laser welded joint and method of manufacturing the same. BACKGROUND

[0002] In certain fields, especially in the manufacture of parts for automobiles and aerospace, it is required that the materials used must have high corrosion resistance, so more and more materials with surface plating are applied. The steel material with surface plating of aluminum or aluminum alloy manufactured by cold rolling or hot rolling not only has good corrosion resistance, but also has high high-temperature resistance, which is beneficial to the manufacture of certain parts by hot working method, so the steel plate with surface plating of aluminum alloy is particularly applied in the manufacture of automobile body, applied in the parts of automobile structure, such as door reinforcement, B-pillar structure, roof reinforcement, etc., especially in the application of super-high-strength hot stamping plate parts, so as to achieve high corrosion resistance, reduce the weight of automobile body and improve the collision energy absorption.

[0003] In the automobile manufacturing industry, in order to reduce the weight of the car and achieve the goal of light weight, more and more materials of equal thickness, unequal thickness, homogeneity and heterogeneity are first welded and then hot stamped to manufacture the welded parts, so as to achieve better weight reduction effect and cost reduction. The laser welding method is the preferred method of this approach, which has higher quality, higher efficiency and better flexibility, and is called "laser tailor-welded blank". Often, the flat workpiece is first laser butt-welded and then hot stamped and quenched to obtain a high-strength full martensitic steel structure. When hot stamping, the weld structure is first heated to above the AC3 temperature of the steel workpiece to complete austenitization. Due to the high temperature, surface oxidation and decarburization of the bare plate are inevitable, which causes the final structure performance to decrease; therefore, it is also necessary to coat the surface, and aluminum or aluminum alloy plating is widely used in this process; the document represented by patent CN101583486B describes such materials in detail.

[0004] However, there are great difficulties in manufacturing laser tailor-welded blanks with aluminum or aluminum alloy coated on the surface. When welding the material with aluminum or aluminum alloy coated on the surface, the coating layer on the original surface will enter the molten zone, especially the molten zone of the weld, which will lead to the formation of ferrite structure at high temperature during cooling, even if the cooling is through high temperature austenitization, the ferrite will remain in the final weld to cause the joint to form a drop. Under the action of subsequent mechanical load, these large amounts of ferrite will become the initial position of failure cracks, which will seriously weaken the load capacity of the welded joint, and directly break in the weld under load, so it is necessary to inhibit the performance drop of the joint caused by the aluminum entering the weld.

[0005] Patents CN101426612B, 106334875A and the like remove the coating layer on the surface by mechanical, laser or other methods in advance to prevent it from entering the weld, but such methods have problems of 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. The patent uses an oxidizing gas to combine with aluminum elements in the coating layer to produce aluminum oxide during welding, which has no effect on the toughness of the weld. However, in the welding process, the time for the metal to melt and form a molten pool and then solidify into a weld is very short, and the reaction time of the oxidizing gas with the aluminum elements is limited. Once the aluminum elements that have not reacted with the oxidizing gas enter the molten pool, aluminum accumulation will occur in the molten pool, reducing the toughness and strength of the weld, and there is a risk of cracking of the hot stamping part.

[0007] Patent CN106488824B discloses a method for joining two blanks by filling welding wire, the welding wire is made of stainless steel alloy including the following components by weight percentage: 0%-0.3% of carbon, 0%-1.3% of silicon, 0.5%-7% of manganese, 5%-22% of chromium, 6%-20% of nickel, 0%-0.4% of molybdenum, 0%-0.7% of niobium and the balance of iron and inevitable impurities. In this method, when laser and arc welding are used, the heat input increases, increasing the probability of thermal deformation of the plate, which is not conducive to the splicing of thin plates. Moreover, the welding speed of laser and arc welding is limited, which reduces the production efficiency.

[0008] In CN 111432975 A, a welding wire is filled with the following components by weight percentage: 0.03% carbon, 0.5% silicon, 1.8% manganese, 20.5% chromium, 25% nickel, 4.7% molybdenum, less than 0.05% sulfur, less than 0.05% phosphorus, and 1.6% copper. When using this welding wire as a filler material, the high content of Cr can cause the risk of forming ferrite in the weld zone of the tailor-welded blank after hot stamping, resulting in a decrease in the mechanical properties of the welded joint, unable to guarantee the quality of the product, and there is a high risk of failure of the welded joint. In CN 112368105 A, a laser welding of a coated steel blank is performed using a filler wire, which 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 can cause the carbon content in the tailor-welded zone to be too high, reducing plasticity and impact performance, and the high carbon content can also cause the weld to be prone to cracking, and can also reduce the corrosion resistance of the weld and the service life of the tailor-welded blank. Furthermore, this welding wire has a high content of C element, which makes it difficult to produce and draw during the welding wire production process, and it is prone to work hardening and breaking.

[0009] In CN 108025400 A, a laser welding method is disclosed for a quenchable steel semi-finished plate with an aluminum-based or aluminum-silicon-based coating layer. Before the base material is melted, the welding rod is first heated using another heat source and then delivered into the molten weld to control the performance of the weld. This process requires an additional second heat source, and the weld width needs to be controlled to at least 0.15mm, and the welding rod needs to be melted into more than 20% of the material. This requires a large amount of welding material, and the multiple control processes are complex. The additional heat source also increases the overall process cost. In this method, the weight percentage of the sum of Cr and Mo is 0.5 to 2.0, and the weight percentage of Ni is 1 to 4. The addition of Mo and Cr can cause the precipitation of carbides in the weld and increase the brittleness of the joint, making the performance unstable.

[0010] In CN 104994989 A, a welding wire is filled with the following components by weight percentage: 0.05-0.15% carbon, 0.5-2.0% silicon, 1.0-2.5% manganese, 0.5-2.0% chromium and molybdenum, 1.0-4.0% nickel, and the balance of iron and unavoidable impurities. This welding wire contains low chromium and molybdenum elements, and the use of this welding wire as a filler wire results in poor hardenability and corrosion resistance of the welded joint. Furthermore, it cannot completely suppress the influence of ferrite during high-temperature heat treatment, increasing the risk of failure of the welded joint in the weld and heat-affected zone.

[0011] CN104023899B discloses a laser welding method for unequal thickness or unequal strength plate filling welding wire, 900-950℃ does not produce ferrite structure, which is mainly composed of carbon or manganese, the carbon content of the filling welding wire is 0.1wt% to 0.8wt% higher than that of the base material of the coated steel plate, and the manganese content of the filling welding wire is 1.5wt% to 7.0wt% higher than that of the base material of the coated steel plate. It is known to those skilled in the art that the Mn content of hot formed steel is between 1.0-2.5, i.e. the Mn content of the welding wire is between 2.5% and 9.5%; when relying solely on carbon or manganese elements to achieve austenitization of the weld, Mn segregation or high C content brittleness is easily caused, and the low density of C element requires the addition of a large volume of welding wire to achieve the final weld C element mass compared to the base material; and the austenitization ability of Mn element is poor compared to Ni element, so a large amount of welding wire needs to be added, which will cause the welding speed to decrease and the wire feeding speed to increase; welding efficiency and welding material consumption increase, and manufacturing cost increases.

[0012] CN112548395A discloses a laser filling welding welding wire and preparation method and tailor-welded plate process, which fills a welding wire with a higher carbon content, the carbon content in the welding wire is about 2.5-10 times that of the welding base material, resulting in a too high carbon content in the tailor-welded area, reducing plasticity and impact performance, and the too high carbon content will make the weld prone to cracking, also reducing the corrosion resistance of the weld and the service life of the tailor-welded plate.

[0013] US20210008665A1 discloses a welding method for coated steel plates, the carbon content of the welding wire is 0.80 to 2.28 times the carbon content of the base material, the too high carbon content will make the weld prone to cracking and quenching and become brittle, also reducing the corrosion resistance of the weld and the service life of the tailor-welded plate, causing the mechanical properties of the welded joint to be inconsistent, reducing product quality, and not suitable for mass production.

[0014] At present, the welding wire for laser welding of coated steel blanks usually needs to be matched with special laser process or equipment to improve the steel plate welding quality. In addition, the added welding wire is mainly composed of multiple alloy elements, including C, Cr, Ni, Mo, Mn, Si and other alloy elements. On the one hand, it is difficult to fully ensure that the welded part has stable and excellent mechanical properties and corrosion resistance after welding. At the same time, due to the addition of multiple alloy elements, work hardening is easy to occur during the drawing forming stage of the welding wire, resulting in low yield of the welding wire and high manufacturing difficulty. At the same time, the complex ratio of multiple alloy elements is difficult to effectively control in the process, which is easy to cause the final weld performance to be unqualified. The more expensive Mo element also makes the manufacturing cost higher. In addition, the applicant's another research uses Ni element to realize the expansion of the austenite interval of the weld and inhibits the formation of ferrite. However, Ni element is an expensive strategic metal, and the use cost is high. Therefore, a relatively lower cost way is needed to realize high-efficiency laser welding of surface coated plates, while ensuring high-strength and high-toughness laser welded joints. SUMMARY

[0015] The present application provides a solder, a high-performance laser welding joint and a manufacturing method. In the present application, a high-performance laser welding joint is obtained by adding a solder composed of Fe, Mn and Ni to the molten pool during welding without removing the surface aluminum plating layer. The internal platelet martensite volume fraction of the laser welding joint is not less than 90%, the average mass fraction of Mn element is not less than 3.2%, the average mass fraction of Al element is not higher than 1.4%, and the average mass fraction of C element is lower than the C content of the steel plate base material. The final weld front face excess height h1 and the final weld back face excess height h2 are not more than 0.4 mm, which greatly improves the mechanical properties and stability of the welded part. The present method can greatly reduce the addition of other alloy elements by adding Fe, Mn and Ni to the molten pool during welding. In the manufacturing of the solder, Mn can be infinitely dissolved in austenite instead of Ni element, which expands the austenitizing interval, suppresses the formation of high-temperature ferrite, and ensures that the welding joint has excellent performance. Therefore, on the one hand, the use cost of the solder can be reduced; on the other hand, the complexity of the solder ratio and the process control difficulty in the manufacturing process are also reduced. The present application mainly adds Fe, Mn and Ni elements without additional alloy elements, which can reduce the formation of coarse carbides in the final weld, and the carbon-manganese compounds formed can be greatly dissolved in the subsequent austenitizing process, reducing the adverse effects of Mn segregation and improving the mechanical properties of the joint. At the same time, the addition of high-Mn content solder can ensure the removal of ferrite in the final weld, retain the weld structure with more than 90% martensite, and ensure the weld performance. However, in the actual welding process, there is often an uncertain assembly gap. When adding other alloy elements to the solder, it is often difficult to completely match, and the solder adaptability is poor. The solder and method of the present application can ensure that a high-reliability and stable weld is formed under different assembly gaps, with excellent fracture strength and elongation. The use of Mn instead of Ni greatly reduces the manufacturing cost. The overall idea is to rely on the preferential mutual solubility of Mn and Ni to suppress the formation of brittle phases (including ferrite and high-brittle carbides) in the weld by using high-Mn and low-Ni welding wire, and to synergistically improve the joint strength and toughness by solid solution strengthening of Mn and Ni to greatly improve the overall joint performance.

[0016] The following solutions are specifically provided:

[0017] In a first aspect, the present application provides a laser welding filler for forming a laser welding joint, the laser welding filler being composed of Fe, Mn, Ni, trace alloying elements and inevitable impurities, wherein the following conditions are met in terms of percentage by weight: Mn≥15%, 1.5%≤Ni≤5%, 0-0.36% of trace alloying elements, and ≤2% of inevitable impurities; the trace alloying elements are at least one of Nb, V and Ti, and the content of each of Nb, V and Ti is not more than 0.12%.

[0018] In a second aspect, the present application provides a laser welding joint, which is formed by adding the laser welding filler of the first aspect to a molten pool when two steel base materials are butt-jointed and laser welded, and then quenching the initial weld to form the laser welding joint, wherein at least one of the two steel base materials is a steel plate with an aluminum or aluminum alloy coating, the volume fraction of the lath martensite in the laser welding joint is not less than 90%, the average mass fraction of Mn is not less than 3.2%, the average mass fraction of Al is not more than 1.4%, and the average mass fraction of C is lower than the C content of the steel base material, and the front face reinforcement h1 and the back face reinforcement h2 of the final weld formed by the laser welding joint are not more than 0.4 mm. Preferably, the average mass fraction of Mn is 3.2%-5.2%.

[0019] Preferably, the volume fraction of the lath martensite is not less than 92%.

[0020] Further, the specific quenching step is that the initial weld is kept at 880-980℃ for 90-500s, and then rapidly cooled by a mold with rapid cooling or a liquid medium, with a cooling speed of ≥25℃ / s.

[0021] Further, the rapid cooling mode is flat mold stamping cooling, and the front face reinforcement h1 and the back face reinforcement h2 of the final weld formed by the laser welding joint meet the following conditions: wherein t1 is the thickness of the first steel plate, and t2 is the thickness of the second steel plate.

[0022] Further, the rapid cooling mode is water quenching, and the front face reinforcement h10 and the back face reinforcement h11 of the initial weld meet the following conditions: wherein t1 is the thickness of the first steel plate, t2 is the thickness of the second steel plate, B10 is the width of the upper surface of the initial weld, and B11 is the width of the lower surface of the initial weld.

[0023] In a third aspect, the present application provides a manufacturing method of a laser welding joint, which comprises the following steps:

[0024] Step 1: providing two steel base materials, at least one of which is a steel plate with an aluminum or aluminum alloy coating, and butt-jointing the two steel base materials;

[0025] Step two: laser welding on the butt joint of the steel plates, wherein the filler metal of the first aspect is added to the molten pool during the welding process to form an initial weld;

[0026] Step three: quenching cooling on the initial weld to obtain the laser welded joint of the second aspect;

[0027] During the laser welding process, the molten volume of the two steel plates is V1, and the volume of the filler metal in the molten pool is V2, V1 and V2 satisfy

[0028] In the fourth aspect, the present application provides a high-strength steel workpiece, which is composed of two steel plate base metals and a laser welded joint for connecting the base metals; when the high-strength steel workpiece is subjected to a tensile test, the fracture failure is in the base metal region.

[0029] In the fifth aspect, the present application provides a component containing the above laser welded joint, which is applied to a structural component or a safety component of a mechanized land motor vehicle. BRIEF DESCRIPTION OF DRAWINGS

[0030] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or the prior art description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0031] Fig. 1 shows a schematic diagram of a typical laser welded joint cross section of the present application;

[0032] Fig. 2 shows a schematic diagram of a cross section of the steel plate base metal involved in the present application;

[0033] Fig. 3 shows a schematic diagram of the main steps of the laser welded joint manufacturing process involved in the present application;

[0034] Fig. 4 shows a schematic diagram of the butt joint of the steel plates involved in the present application;

[0035] Fig. 5 shows a schematic diagram of the laser welding process involved in the present application;

[0036] Fig. 6 shows a schematic diagram of the laser welding process involved in the present application from the top;

[0037] Fig. 7 shows a schematic diagram of a cross section of the obtained initial weld involved in the present application;

[0038] Fig. 8 shows another schematic diagram of a cross section of the obtained initial weld involved in the present application;

[0039] Fig. 9 shows another cross-sectional view of an initial weld obtained in accordance with the present application;

[0040] Fig. 10 shows another cross-sectional view of an initial weld obtained in accordance with the present application;

[0041] Fig. 11 shows a metallographic cross-section of an initial weld obtained in accordance with one embodiment of the present application;

[0042] Fig. 12 shows a cross-sectional view of the final weld of the embodiment of Fig. 11 after quenching and cooling in accordance with the present application;

[0043] Fig. 13 shows the state of the welded joint of the embodiment of Fig. 12 after tensile failure in accordance with the present application;

[0044] Fig. 14 shows a metallographic cross-section of a final weld obtained in accordance with one embodiment of the present application;

[0045] Fig. 15 shows the state of the welded joint of the embodiment of Fig. 14 after tensile failure in accordance with the present application;

[0046] Fig. 16 shows a metallographic cross-section of a final weld obtained in accordance with one embodiment of the present application;

[0047] Fig. 17 shows the state of the welded joint of the embodiment of Fig. 16 after tensile failure in accordance with the present application;

[0048] Fig. 18 shows a metallographic cross-section of a final weld obtained in accordance with one embodiment of the present application;

[0049] Fig. 19 shows the state of the welded joint of the embodiment of Fig. 18 after tensile failure in accordance with the present application. DETAILED DESCRIPTION

[0050] In order to more clearly illustrate the technical solutions of the embodiments of the present application or the prior art, the present application will be further described below with reference to the specific embodiments. It should be understood that these embodiments are only used to illustrate the present application and not used to limit the scope of the present application. In addition, the drawings are schematic drawings, therefore the present application devices and apparatuses are not limited by the size or proportion of the schematic drawings.

[0051] It is to be understood that the phrases such as first and second, and the like, used in the specification are used merely for identifying purposes and do not necessarily imply or require a specially order or sequence among the referenced items. Also, the terms "comprising," "including," 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 can include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by "comprises a" does not, without more constraints, exclude the existence of additional identical elements in the process, method, article, or apparatus that comprises the recited element. The final weld in the present invention refers to a laser welded joint. The present invention provides a welding material for laser welding of steel sheets with aluminum or aluminum alloy plating, which is composed of Fe, Mn, Ni, trace alloying elements and inevitable impurities, wherein the following conditions are met: Mn≥15%, 1.5%≤Ni≤5%, trace alloying elements 0-0.36%, inevitable impurities≤2%, in terms of percentage by weight; the trace alloying elements are at least one of Nb, V and Ti, and the content of Nb, V and Ti is not more than 0.12% respectively. Except Fe, Mn and Ni, other elements in the welding material are not necessary. Higher total content of Mn can ensure the minimum ferrite content of the welded joint and the performance of the joint. The welding material can be in various forms such as wire, powder or sheet, for example, solid wire or flux-cored wire, and when in the form of wire, the diameter is generally 0.6-2.0mm, preferably 0.8-1.2mm.

[0052] Referring to Fig. 1, a laser welded joint 2 is formed by butt joining two steel base materials (a first steel plate 11 and a second steel plate 12) and laser welding. At least one of the first steel plate 11 and the second steel plate 12 has an aluminum plating layer on one surface thereof. The joint has the following characteristics when viewed along a cross section of a weld: the joint 2 is composed of lath martensite 21 with a volume fraction of not less than 90%, and contains austenite 22 in an amount of not more than 10%; preferably, the lath martensite accounts for not less than 92%; in one preferred example, the martensite accounts for more than 95% in the weld, and more preferably, the weld is composed of lath martensite. In addition, the final weld can contain a trace amount of carbide components other than lath martensite and austenite. The residual austenite can have various forms and sizes in the weld, and the presence of a trace amount of austenite in the weld ensures that TRIP effect occurs during load bearing of the weld to transform the austenite into martensite, provides additional plasticity of the weld, and ensures sufficient strength. The average mass fraction of Mn in the joint 2 is not less than 3.2%, and the Mn and Ni elements mainly exist in the weld in a solid solution form. The average mass fraction of Al is not higher than 1.4%, and the average mass fraction of C is lower than the C content of the original steel base material. The weld can further contain Si, Cr, Mo and other elements from the steel base material, and the average mass fraction of Si is 0.2-1.0%. It is worth mentioning that the weld can further contain trace amounts of Nb, V and Ti alloy elements for precipitation strengthening and fine-grain strengthening, each of which has a content of generally not higher than 0.12%, and can be from the steel base material or welding material to further improve the tensile properties of the joint. The steel base material described above generally has a coating layer on at least one surface, and the first steel plate 11 has a first surface coating layer 111 and a second surface coating layer 112 as shown in Fig. 2. Generally, the coating layer is composed of aluminum and silicon, and the aluminum content is not less than 85%, and preferably, the aluminum content is more than 90%. The total thickness of the coating layer is generally 10-50 μm. The coating layer of the coated steel plate is generally composed of a metal alloy layer and an intermetallic compound alloy layer. The steel base material is generally composed of ferrite and pearlite, and has a carbon content of not less than 0.15% and a boron content of about 0.002-0.006 to satisfy the hardenability of the subsequent quenching process, so that full martensite structure is formed after quenching, and the boron steel has a tensile strength of not less than 1000 Mpa.Typical steel sheet base material grade 22MnB5, the mass fraction of the elements contained mainly for: 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 is Fe and inevitable impurities. The steel sheet base material involved in the present application is various steel grades known in the art as hot stamping forming steel, boron steel, martensitic steel, etc. with different carbon content or alloying elements, but it is the raw material before quenching (stamping forming), and its thickness is generally 1.0-3.0mm, which is well known in the art and will not be repeated here. In addition, the welded joint can be laser welded by the first steel sheet 11 and the second steel sheet 12 with different thicknesses or different plating states, including the strength level can also have differences, i.e. the combination of different thicknesses or different strengths, for example, the tensile strength of the steel sheet base material after hot stamping forming can exceed 2000Mpa or more.

[0053] The manufacturing method of the laser welded joint 2 described above mainly includes three steps: steel sheet butt joint, laser welding and quenching treatment, as shown in Figure 3.

[0054] In step 1, a set of first steel sheets 11 and second steel sheets 12 with aluminum plating to be welded are first provided, and then the first steel sheets 11 and the second steel sheets 12 are butt jointed for welding, and the first steel sheets 11 and the second steel sheets 12 can have a gap or no gap. As shown in Figure 4, the assembly gap width B is generally between 0-0.5mm.

[0055] In step 2, laser welding is performed on the assembled steel sheets, as shown in Figure 5. A laser beam 3 and a welding material 4 are provided and delivered to the butt joint position to travel along the welding path at a certain welding speed, and the first steel sheets 11, the second steel sheets 12 and the welding material are melted by the laser beam to form an initial weld 6 on the travel path. During laser welding, the volume of the two steel sheets melted is V1, and the volume of the welding material in the molten pool is V2, V1 and V2 satisfy Solder 4 is the laser welding solder of this invention. The relative position between the solder and the laser beam can include various types, including coaxial or off-axis transport, which is well known in the art. During laser welding, the laser beam is generally emitted by a laser, which can include various types, such as solid-state laser beams or gas laser beams. Specifically, it can include fiber lasers, disk lasers, semiconductor diode lasers, and Nd:YAG type solid-state lasers, or CO2 gas lasers. Other types are also possible, as long as they can generate a laser beam and create a keyhole and a molten weld pool. Figure 6 shows a top view of the welding process, where 5 represents the weld pool formed by the first steel plate 11, the second steel plate 12, and the solder 4 under the action of the laser beam, and 6 represents the initial weld formed after cooling and solidification. During welding, the laser beam can also consist of one or more beams, and the beam energy distribution can be Gaussian, average, or dot-ring distribution, among other forms. During the welding process, the laser beam can travel along the welding direction in a fixed or synchronous high-speed motion, including oscillations. The oscillation shape can include various forms such as circles, broken lines, figure-eights, and infinity symbols. The oscillation frequency is typically 50-500 Hz, and the oscillation amplitude is between 0.2-1.5 mm. Multiple single or mixed shielding gases, such as Ar and He, can be added during the welding process, or the welding can be performed without a shielding gas.

[0056] Solder can be added in various ways during welding, including by feeding powder or welding wire. When welding wire is used, it is called laser filler wire welding, which is well known in the art. The wire feeding device can be, for example, MAG (metal active gas), MIG (metal inert gas), or TIG (tungsten inert gas). The welding wire can be solid or flux-cored. Adding flux-cored wire or powder makes it easier to control the composition ratio, which is also well known in the art. Solder addition can also be achieved by combining two different devices. For example, two filler devices can be used to feed different solders; one feeds solder with a certain composition, and the other feeds solder with a different composition. By combining the two solders, the final solder delivered to the molten pool has the required proportion of composition. Alternatively, solder 4 can be synthesized and added through multiple steps or components; for example, solder 4 consists of three independent welding wires, and the average mass composition ratio of the final three solder combinations meets the requirements described in this invention; the three solders can be added simultaneously, one primarily composed of Fe, one primarily composed of Ni, and one primarily composed of Mn. Furthermore, the addition of solder can be done in a multi-step manner, such as sequentially, to ensure that the microstructure within the final weld joint meets the requirement of over 90% lath martensite. Using multiple welding wire combinations will be more beneficial for welding wire manufacturing and reduce the difficulty of welding wire production.

[0057] In this invention, the amount of melting is controlled by matching the laser welding speed with the wire feeding speed and controlling the wire composition. The welding speed and wire feeding speed are controlled to ensure that the reinforcement heights on both sides of the initial weld (h10 on the front and h11 on the back) are not lower than the reinforcement heights on the front and back of the final weld (h1 and h2 respectively). This can be estimated based on the wire diameter, welding speed, and wire feeding speed. Generally, the welding speed S1 is not less than 3 m / min, and the wire diameter is d, typically 0.6-1.6 mm, more commonly 1.0-1.2 mm, which is generally more than twice the assembly gap width B. In this invention, assuming the average width of the upper and lower surfaces of the formed weld is B1, the wire feeding speed is S0, and the average thickness of the two steel plates is t, then the following conditions must be met: Generally, S0 is 0.5-4.0 m / min; thus ensuring that the initial weld reinforcement height h10 on the front and h11 on the back of the initial weld do not exceed 0.4 mm. Generally, the reinforcement height on both sides of the resulting initial weld cross-section has an overall dome-shaped morphology higher than the surface of the steel plate base. The volume of the weld melted during the entire process is V2, including cases with or without gaps; while the total volume melted by the steel plate base material is V1; satisfying 0.02≤V2 / V1≤0.8, or more narrowly 0.02≤V2 / V1≤0.5, the entire weld volume consists of V1 and V2. The average width B1 of the upper and lower surfaces of the weld is generally between 0.5 mm and 4.0 mm, B10 is the width of the upper surface of the initial weld, and B11 is the width of the lower surface of the initial weld. Figure 7 shows a schematic diagram of the cross-section of the initial weld obtained after laser welding. The cross-sectional morphology of the initial weld can include various types; Figure 7 shows an overall X-shaped morphology; it can also include Y-shaped and U-shaped morphologies; Figures 8 and 9 show schematic diagrams of Y-shaped and U-shaped welds, respectively, and their specific morphologies can be adjusted by matching the process parameters of the laser welding process. Figure 10 shows a schematic diagram when the thicknesses t1 of the first steel plate and t2 of the second steel plate are different. Generally, the internal structure of the obtained initial weld is mainly martensite and retained austenite, with a higher martensite content. Since the addition of Ni-containing solder into the weld will inevitably cause the complete austenitization temperature of the initial weld to differ from that of the base metal, it is ensured that the austenitization temperature Ac31 of the final obtained initial weld joint (weld) deviates from the austenitization temperature Ac30 of the base metal by no more than 100°; and the martensitic transformation start temperature Ms1 deviates from the martensitic transformation start temperature Ms0 of the base metal by no more than 100°. To ensure that the Mn content in the weld after hot stamping and quenching is not less than 2.0% and not more than 8%, and that the volume fraction of retained austenite in the weld does not exceed 10%, or more precisely, not more than 5%. The retention of some retained austenite can improve the joint performance by causing the TRIP effect during subsequent stress.

[0058] During the hot stamping (quenching) process, the initial weld is ensured to achieve complete austenitization under heating and holding temperatures, typically 880-980℃, with a holding time of 90-500s. After holding, rapid cooling is performed, which can be achieved using a mold with rapid cooling or a liquid medium, with a cooling rate generally not less than 25℃ / s. When using a mold for stamping cooling, various formed parts, including flat plates and part shapes, can be formed, i.e., using a flat plate mold or a forming mold. When using a flat plate mold for stamping cooling, the morphology of the initial weld reinforcement becomes flattened under pressure. At this time, the dimensions of the final weld reinforcement h1 on the front and h2 on the back are smaller than the initial weld reinforcement h10 on the front and h11 on the back. Approximating the surface reinforcement as a rectangular cross-section, the following mathematical formula is satisfied: 0.03≤(h1+h2) / (t1+t2)≤0.2. When water-cooled quenching is used, the shape of the initial weld reinforcement is not significantly different from that of the final weld reinforcement, which also satisfies the following condition: The invention, method, solder, and weld joint can be used to manufacture components that incorporate hot-stamped steel applications, such as various vehicles.

[0059] The technical solution of the present invention will be described in detail and completely below through specific embodiments.

[0060] Example 1

[0061] Figure 11 shows a cross-sectional view of the initial weld obtained by laser welding using the method of the present invention. Figure 12 shows a cross-sectional view of the final weld after quenching and cooling. The original material used for welding is hot-formed steel 22MnB5 with an aluminum-silicon coating of approximately 30 μm thickness on both the upper and lower surfaces, and a strength exceeding 1500 MPa after quenching, with a thickness of 1.5 mm. The final weld is generally X-shaped, with a lath martensite content of approximately 96%, a Mn element mass fraction of approximately 4.0%, an average aluminum element mass fraction of approximately 1.4%, a Ni element content of approximately 0.6%, and a carbon element content lower than that of the original material. The widths of the two sides of the final weld are approximately 2.3 mm and 2.5 mm, respectively, obtained by stamping with a flat die during quenching. The reinforcement height on the front and back of the final weld is approximately 0.08 mm and 0.14 mm, respectively. In the welding wire added during the welding process, Mn accounts for 22% of the welding wire mass fraction, and Ni content is 3.2%. The welding wire used has a diameter of 1.2 mm, and laser-assisted wire feeding is employed to manufacture the welded plates. The assembly gap width B is 0 mm, the welding speed S1 is 5.0 m / min, the wire feed speed S0 is 2.5 m / min, and the average melting volume V1 of the two steel plates per minute is approximately 18000 mm². 3 Based on the wire feed speed, the average melting volume V2 of the solder in the molten pool is estimated to be approximately 2827 mm² per minute.3 V2 / V1≈0.157, satisfying 0.02≤V2 / V1≤0.8. After welding, the initial weld was placed in a heating furnace at 930℃ and held for approximately 300s. Subsequently, it was hot-stamped and cooled under a flat die at a cooling rate of approximately 40℃ / s to obtain the laser-welded joint. Figure 13 shows the mechanical property test results of the obtained laser-welded joint after standard tensile test specimen preparation. The results show that all three specimens fractured and failed in the base material region, with a fracture elongation of 8.62%.

[0062] Example 2

[0063] Figure 14 shows a cross-sectional view of another final weld obtained using the method of the present invention. The original material used for welding is hot-formed steel 22MnB5 with an aluminum-silicon coating of approximately 30 μm thickness on both the upper and lower surfaces, and a strength exceeding 1500 MPa after quenching, with a thickness of 2.0 mm. The final weld is generally X-shaped, with a lath martensite content of approximately 96%, a Mn element mass fraction of approximately 3.85%, an average aluminum element mass fraction of approximately 1.3%, a Ni element content of approximately 0.50%, and a carbon element content lower than that of the original material. The widths of the two sides of the final weld are approximately 1.6 mm and 1.8 mm, respectively, and it was obtained by stamping with a flat die during quenching. The reinforcement height on the front and back sides of the final weld are approximately 0.1 mm and 0.07 mm, respectively. In the welding wire added during the welding process, Mn accounts for 15% of the welding wire mass fraction, and the Ni content is 3.2%. The welding wire used has a diameter of 1.2 mm, and laser wire feeding is employed to manufacture the welded plates. The assembly gap width B is 0.1 mm, the welding speed S1 is 5.0 m / min, the wire feed speed S0 is 2.8 m / min, and the average melting volume V1 of the two steel plates per minute is approximately 17000 mm². 3 Based on the wire feed speed, the average melting volume V2 of the solder in the molten pool is estimated to be approximately 3167 mm² per minute. 3 V2 / V1≈0.186, satisfying 0.02≤V2 / V1≤0.8. After welding, the initial weld was placed in a heating furnace at 930℃ and held for approximately 300s. Subsequently, it was hot-stamped and cooled under a flat mold at a cooling rate of approximately 40℃ / s to obtain the laser-welded joint. Figure 15 shows the mechanical property test results of the obtained laser-welded joint. The results show that all three samples failed by fracture in the base material region, with a fracture elongation of approximately 8.52%.

[0064] Example 3

[0065] Figure 16 shows a cross-sectional view of a final weld obtained using the method of this invention. The original materials used for welding are hot-formed steels with varying strengths after quenching: aluminum-silicon coatings with a surface thickness of approximately 30 μm, and strengths of approximately 1500 MPa and 2000 MPa respectively, both with a thickness of 1.4 mm. The final weld is generally X-shaped, with approximately 92% martensite content in the laths. The mass fraction of Mn in the weld is approximately 4.5%, the average mass fraction of aluminum is approximately 1.2%, the Ni content is approximately 0.45%, and the carbon content is lower than that of the original materials. The widths of the two sides of the final weld are approximately 1.8 mm and 1.6 mm, respectively, obtained by water quenching during quenching. The reinforcement height on the front and back sides of the final weld are approximately 0.18 mm and 0.24 mm, respectively. In the welding wire added during the welding process, Mn accounts for 30% of the welding wire mass fraction, and Ni content is 2.5%. The welding wire used has a diameter of 1.2 mm. Laser wire feeding is employed to manufacture the welded plates. The assembly gap width B is approximately 0.2 mm. The welding speed S1 is 5.0 m / min, and the wire feed speed S0 is 2.0 m / min. The average melting volume V1 of the two steel plates per minute is approximately 11900 mm³. Based on the wire feed speed, the estimated average melting volume V2 of the weld metal in the molten pool per minute is approximately 2262 mm³. 3 V2 / V1≈0.084, satisfying 0.02≤V2 / V1≤0.8. After welding, the initial weld was placed in a heating furnace at 930℃ and held for approximately 300s. Subsequently, it was hot-stamped and cooled under a flat die at a cooling rate of approximately 40℃ / s to obtain the laser-welded joint. Figure 17 shows the mechanical property test results of the laser-welded joint after standard tensile test specimen preparation. The results show that all three specimens fractured and failed in the lower strength base material region, with an elongation after fracture of approximately 8.17%.

[0066] Example 4

[0067] Figure 18 shows a cross-sectional view of a final weld obtained using the method of this invention. The original materials used for welding are aluminum-silicon coated steel with a surface thickness of approximately 20 μm, and hot-formed steel with a strength of approximately 1500 MPa after quenching, with thicknesses of 1.4 mm and 1.6 mm, respectively. The final weld is generally X-shaped, with a lath martensite content of over 97%. The weld contains approximately 4.8% Mn by mass, approximately 1.0% aluminum by mass, approximately 0.68% Ni by mass, and a lower carbon content than the original materials. The initial weld widths B10 and B11 on the upper and lower surfaces are approximately 1.4 mm and 1.7 mm, respectively, obtained through water quenching. The initial weld reinforcement heights h10 and h11 on the front and back surfaces are approximately 0.2 mm and 0.3 mm, respectively. The final weld reinforcement heights h1 and h2 on the front and back surfaces are approximately 0.2 mm and 0.3 mm, respectively. The welding wire used in the welding process contains 22% Mn and 3.2% Ni by mass. The welding wire diameter is 1.2 mm, and laser-filled wire welding is used to manufacture the welded plates. The assembly gap width B is approximately 0 mm, the welding speed S1 is 5.0 m / min, the wire feed speed S0 is 2.0 m / min, and the average melting volume V1 of the two steel plates per minute is approximately 11625 mm². 3 Based on the wire feed speed, the average melting volume V2 of the solder in the molten pool is estimated to be approximately 2262 mm² per minute. 3 V2 / V1≈0.19, satisfying 0.02≤V2 / V1≤0.8. After welding, the initial weld was placed in a heating furnace at 930℃ and held for approximately 300s. Subsequently, it was water-cooled at a cooling rate exceeding 40℃ / s to obtain the laser-welded joint. Figure 19 shows the mechanical property test results of the obtained laser-welded joint after standard tensile test specimen preparation. The results show that all three specimens fractured and failed in the thin-plate side of the base material region, with an elongation after fracture reaching 7.83%.

[0068] Example 5

[0069] The original material used for welding was 22MnB5 hot-formed steel with an aluminum-silicon coating of approximately 30 μm thickness on both the upper and lower surfaces, and a strength exceeding 1500 MPa after quenching, with a thickness of 1.5 mm. The final weld was X-shaped overall, with approximately 94% martensite content in the laths, approximately 5.2% Mn by mass, approximately 1.3% aluminum by mass, approximately 0.4% Ni by mass, and a carbon content lower than that of the original material. The widths of the final weld on both sides were approximately 2.4 mm and 2.5 mm, respectively, obtained by stamping with a flat die during quenching. The reinforcement height on the front and back of the final weld was approximately 0.09 mm and 0.16 mm, respectively. In the welding wire added during the welding process, Mn accounted for 30% of the welding wire by mass, and Ni accounted for 1.5%. The welding wire used has a diameter of 1.2 mm, and laser-guided wire feeding is employed to manufacture the welded plates. The assembly gap width B is 0 mm, the welding speed S1 is 5.0 m / min, the wire feed speed S0 is 2.8 m / min, and the average melting volume V1 of the two steel plates per minute is approximately 18375 mm². 3 Based on the wire feed speed, the average melting volume V2 of the solder in the molten pool is estimated to be approximately 3167 mm² per minute. 3 V2 / V1≈0.172, satisfying 0.02≤V2 / V1≤0.8. After welding, the initial weld was placed in a heating furnace at 930℃ and held for approximately 300s. Subsequently, it was hot-stamped and cooled under a flat mold at a cooling rate of approximately 40℃ / s to obtain the laser-welded joint. All three joint samples fractured and failed in the base material region, with a fracture elongation of 7.88%.

[0070] Example 6

[0071] The original material used for welding was 22MnB5 hot-formed steel with an aluminum-silicon coating of approximately 30μm thickness on both the upper and lower surfaces, and a strength exceeding 1500MPa after quenching, with a thickness of 1.5mm. The final weld was X-shaped overall, with approximately 94% martensite content in the laths, approximately 3.2% Mn by mass, approximately 1.3% aluminum by mass, approximately 0.8% Ni by mass, and a carbon content lower than that of the original material. The widths of the final weld on both sides were approximately 2.4mm and 2.5mm, respectively, obtained by stamping with a flat die during quenching. The reinforcement height on the front and back of the final weld was approximately 0.09mm and 0.16mm, respectively. In the welding wire added during the welding process, Mn accounted for 15% of the welding wire by mass, and Ni content was 5.0%. The welding wire used has a diameter of 1.2 mm, and laser-assisted wire feeding is employed to manufacture the welded plates. The assembly gap width B is 0 mm, the welding speed S1 is 5.0 m / min, the wire feed speed S0 is 2.4 m / min, and the average melting volume V1 of the two steel plates per minute is approximately 16875 mm². 3Based on the wire feed speed, the average melting volume V2 of the solder in the molten pool is estimated to be approximately 2714 mm² per minute. 3 V2 / V1≈0.161, satisfying 0.02≤V2 / V1≤0.8. After welding, the initial weld was placed in a heating furnace at 930℃ and held for approximately 300s. Subsequently, it was hot-stamped and cooled under a flat mold at a cooling rate of approximately 40℃ / s to obtain the laser-welded joint. All three joint samples fractured and failed in the base material region, with a fracture elongation of 8.05%.

[0072] Table 1 lists the performance and related results of the welded joints obtained in the above embodiments.

[0073] Table 1 Results data for different embodiments

[0074] All samples of the laser-welded joints in Examples 1-6 fractured and failed in the base material region, with a fracture strength greater than 1560 MPa and an elongation greater than 7.5, exhibiting very high mechanical properties that can meet the requirements for the production and manufacturing of related components.

[0075] Comparative Example

[0076] Using the same raw welding materials as in Example 1, and without filler wire, when different surface treatments were applied, all welded joints in the comparative example failed at the weld seam location. The relevant mechanical properties are as follows:

[0077] Table 2

[0078] All documents mentioned in this invention are incorporated herein by reference as if each document were individually incorporated by reference. Furthermore, it should be understood that after reading the foregoing teachings of this invention, those skilled in the art can make various alterations or modifications to this invention, and these equivalent forms also fall within the scope defined by the appended claims.

Claims

1. A laser-welded joint, wherein during laser welding of two steel plate base materials, laser welding filler is added to the molten pool to form an initial weld, and the initial weld is quenched to form the laser-welded joint, wherein at least one of the steel plates is a steel plate with an aluminum coating or an aluminum alloy coating, characterized in that... The internal lath martensite volume fraction of the laser-welded joint is not less than 90%, the average mass fraction of Mn element is 3.2% to 5.2%, the average mass fraction of Al element is not higher than 1.4%, and the average mass fraction of C element is lower than the C content of the steel plate base material. The final weld reinforcement height h1 on the front side and the final weld reinforcement height h2 on the back side formed by the laser-welded joint do not exceed 0.4 mm. The solder is composed of Fe, Mn, Ni, trace alloying elements, and other unavoidable elements, wherein, by weight percentage, the following conditions are met: Mn ≥ 15%, 1.5% ≤ Ni ≤ 5%, trace alloying elements 0–0.36%, and other unavoidable elements ≤ 2%; the trace alloying element is at least one of Nb, V, and Ti, and the content of each of Nb, V, and Ti does not exceed 0.12%; The solder is welding wire, and the welding speed S1 and the wire feeding speed S0 satisfy the following relationship during the welding process: Where d is the diameter of the welding wire, the value of d ranges from 0.6 to 1.6 mm and exceeds twice the assembly gap width B, the value of B ranges from 0 to 0.5 mm, S1 is not less than 3 m / min, B1 is the average width of the upper and lower surfaces of the formed weld, S0 ranges from 0.5 to 4.0 m / min, and t is the average thickness of the two steel plates. The specific steps of the quenching are as follows: the initial weld is held at 880-980℃ for 90-500s, and then rapidly cooled by a mold or liquid medium with rapid cooling, with a cooling rate ≥25℃ / s. The rapid cooling method is flat die stamping cooling, and the final weld reinforcement height h1 on the front side and h2 on the back side of the laser welded joint meet the following requirements. Where t1 is the thickness of the first steel plate and t2 is the thickness of the second steel plate; Alternatively, the rapid cooling method may be water quenching, and the initial weld reinforcement height h10 on the front side and h11 on the back side satisfy the following conditions: Where t1 is the thickness of the first steel plate, t2 is the thickness of the second steel plate, B10 is the width of the initial weld upper surface, and B11 is the width of the initial weld lower surface.

2. The laser-welded joint as described in claim 1, characterized in that, The volume fraction of martensite in the lath is not less than 92%.

3. The laser-welded joint as described in claim 2, characterized in that, The volume fraction of lath martensite exceeds 95%.

4. The laser-welded joint as described in claim 1, characterized in that, The laser-welded joint also contains Si, Cr, and Mo elements derived from the steel plate base material.

5. The laser-welded joint as described in claim 3, characterized in that, The average mass fraction of Si in the laser-welded joint is 0.2-1.0%.

6. The laser-welded joint as described in claim 1, characterized in that, The steel plate with aluminum coating or aluminum alloy coating has a coating on at least one side, the coating being composed of aluminum and silicon, wherein the aluminum content is not less than 85%.

7. The laser-welded joint as described in claim 1, characterized in that, Its features are, The solder is either solid welding wire or flux-cored welding wire.

8. The laser-welded joint as described in claim 7, characterized in that, The diameter of the welding wire is 0.8-1.2 mm.

9. A method for manufacturing a laser-welded joint, characterized in that, The method includes the following steps: Step 1: Provide two steel plates as base materials, at least one of which is a steel plate with an aluminum coating or an aluminum alloy coating, and then join the two steel plates together. Step 2: Perform laser welding on the butt joint of the steel plates, during which flux is gradually added into the molten pool to form the initial weld. Step 3: Quenching and cooling the initial weld seam to obtain the laser welded joint as described in any one of claims 1 to 8; In step two, during laser welding, the molten volume of the two steel plates is V1, and the volume of the weld in the molten pool is V2. V1 and V2 satisfy... In step two, the solder is a welding wire, which is composed of Fe, Mn, Ni, trace alloying elements, and other unavoidable elements. The composition, by weight percentage, satisfies the following: Mn ≥ 15%, 1.5% ≤ Ni ≤ 5%, trace alloying elements 0–0.36%, and other unavoidable elements ≤ 2%. The trace alloying element is at least one of Nb, V, and Ti, and the content of each of Nb, V, and Ti does not exceed 0.12%. During the welding process, the welding speed S1 and the wire feeding speed S0 satisfy the following relationship: Where d is the diameter of the welding wire, the value of d ranges from 0.6 to 1.6 mm and exceeds twice the assembly gap width B, the value of B ranges from 0 to 0.5 mm, S1 is not less than 3 m / min, B1 is the average width of the upper and lower surfaces of the formed weld, S0 ranges from 0.5 to 4.0 m / min, and t is the average thickness of the two steel plates. The specific steps of the quenching are as follows: the initial weld is held at 880-980℃ for 90-500s, and then rapidly cooled by a mold or liquid medium with rapid cooling, with a cooling rate ≥25℃ / s. The rapid cooling method is flat die stamping cooling, and the final weld reinforcement height h1 on the front side and h2 on the back side of the laser welded joint meet the following requirements. Where t1 is the thickness of the first steel plate and t2 is the thickness of the second steel plate; Alternatively, the rapid cooling method may be water quenching, and the initial weld reinforcement height h10 on the front side and h11 on the back side satisfy the following conditions: Where t1 is the thickness of the first steel plate, t2 is the thickness of the second steel plate, B10 is the width of the initial weld upper surface, and B11 is the width of the initial weld lower surface.

10. A high-strength steel workpiece, characterized in that, The high-strength steel workpiece consists of two steel plate base materials and a laser-welded joint as described in any one of claims 1-8 for connecting the base materials; when the high-strength steel workpiece is subjected to tensile testing, the fracture failure occurs in the base material region and the fracture elongation is not less than 5%.

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