Laser welding method and method for manufacturing laser welded joint

Abstract

The purpose of the present invention is to provide a laser welding method that enables manufacturing of a high-quality laser welded joint and a tailored blank. In this laser welding method, two steel sheets to be butt welded are respectively cut by using a laser beam and the steel sheets cut by the laser beam are butt welded. The cutting is performed such that an angle of a cut plane is θ(deg) in one of the steel sheets, and is 180 - θ(deg) in the other of the steel sheets, and the θ satisfies formula (1). (1): 55.0 ≤ θ < 70.0

Description

[Technical field]

[0001] The present invention relates to a laser welding method and a method for producing a laser welded joint. [Background technology]

[0002] In the manufacture of automobile bodies, tailored blanks, in which steel plates of different thicknesses and strengths are pre-welded, are sometimes used as press materials. Tailored blanks are a type of welded joint that can partially change the properties of a single piece of material while also reducing the number of parts, so they are often used for the purpose of highly efficient part manufacturing, especially in the manufacture of automobile bodies.

[0003] However, tailored blank manufacturing can require butt welding of more than 1m in some cases, and variations in the butt gap when the steel plates are butted together, i.e. the gap between the butt surfaces of the plates, can adversely affect the performance of the parts after welding.

[0004] Therefore, in order to ensure the accuracy of the butt joint between the steel plates to be welded, the end faces of the steel plates are sheared just before the tailored blank is manufactured. This shearing is mainly performed using a mechanical shearing machine.

[0005] This shearing process reduces the butt gap, making it possible to produce parts with a certain level of weld quality. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2012-135796 A Summary of the Invention [Problem to be solved by the invention]

[0007] However, when shearing using a mechanical shearing machine, if the warp of the steel plate is not uniform in the coil width direction, dimensional errors occur in the plate width direction, and the butt gap during welding may vary in the coil width direction.In addition, when the blades of the shearing machine wear out, sagging or burrs may occur, causing the butt gap in that area to vary and adversely affecting part performance.

[0008] Patent Document 1 proposes a method for improving welding quality by welding with an angle on the cut surfaces of steel plates to be butted together using a cutting tool. However, due to dimensional processing limitations of the cutting tool, the cutting angle is 20° or less, and at angles of 20° or less, the overlap when butting together is insufficient, making it difficult to obtain a sufficient effect of suppressing the variation in the butt gap.

[0009] In view of the above problems, an object of the present invention is to provide a laser welding method that can obtain a more sufficient effect of suppressing the variation in the butt gap by using a laser cutting method that has no processing limits. [Means for solving the problem]

[0010] In the present invention, we first focused on the shearing process before welding, that is, the cutting process of the plate. Conventionally, plate shearing has been performed by a mechanical shearing machine. However, when making an oblique cut end surface by a contact processing method, the tool slips off the steel plate and escapes, so only an end surface with an angle of 20° or less could be made. Therefore, in order to solve the above problem, in the present invention, cutting by non-contact thermal processing with a laser beam is applied to the end surface before laser welding, instead of contact mechanical processing. This makes it possible to perform cutting more obliquely in the plate thickness direction compared to mechanical processing, and makes it possible to reduce the butt gap fluctuation in the direction perpendicular to both the direction of the welding laser beam and the welding direction during welding. By reducing the butt gap fluctuation, it becomes possible to manufacture higher quality laser welded joints and tailored blanks. In addition, since it is a non-contact processing, there is no wear of the tool, and it becomes possible to perform cutting with less fluctuation due to continuous operation and less variation in the welding direction for each part in one laser cutting. This makes it possible to manufacture high-quality tailored blanks. The present invention has been completed based on the above findings, and has the following gist. [1] A laser welding method in which two steel plates to be butt-welded are cut using a laser beam, and the two steel plates cut by the laser beam are butt-welded, The laser welding method includes cutting one steel plate so that the angle of the cut surface is θ (deg) and the angle of the cut surface of the other steel plate is 180-θ (deg), and the angle θ satisfies formula (1). 55.0≦θ<70.0 (1) However, if the thicknesses of the two steel plates to be butt-welded are different, the angle of the cut surface of the thinner steel plate shall be θ (deg), and if the two steel plates have the same thickness, the angle of one of the cut surfaces shall be θ (deg). [2] A laser welding method as described in [1], in which the thickness t (mm) of the thinner of the two steel plates to be butt-welded, or the thicknesses t (mm) of both steel plates if the thicknesses are the same, satisfies formula (2). 0.6≦t≦3.2 (2) [3] The laser welding method according to [1], wherein the welded portion of the laser weld is welded so as to satisfy the following formula (3): 1.2w≧t / tanθ (3) Here, w is the fusion width (mm) and t is the thickness (mm) of the thinner of the two steel plates to be butt welded, or the thickness (mm) of both steel plates if they are the same thickness. [4] The laser welding method according to [2], wherein the welded portion of the laser weld is welded so as to satisfy the following formula (3): 1.2w≧t / tanθ (3) where w is the melt width (mm). [5] A laser welding method as described in [1], in which, when the thicknesses of the two steel plates to be butt-welded are different, the acute angle part of the cross section of the thicker steel plate and the obtuse angle part of the cross section of the thinner steel plate are placed on the bottom, the heights of their bottom surfaces are aligned and they are butted together horizontally, and a laser beam is irradiated from above in the vertical direction, aiming at the center of the thickness of the thinner steel plate and the position of the butt surface, to weld them. [6] A laser welding method as described in [2], in which, when the thicknesses of the two steel plates to be butt-welded are different, the acute angle part of the cross section of the thicker steel plate and the obtuse angle part of the cross section of the thinner steel plate are placed on the bottom, the heights of their bottom surfaces are aligned and they are butted together horizontally, and a laser beam is irradiated from above in the vertical direction, aiming at the center of the thickness of the thinner steel plate and the position of the butt surface, to weld them. [7] A method for producing a laser welded joint, comprising the steps of: producing a laser welded joint using the laser welding method according to any one of [1] to [6]. [8] A method for manufacturing a laser welded joint described in [7], wherein the laser welded joint is a tailored blank. Effect of the Invention

[0011] The present invention makes it possible to suppress butt gap fluctuations in directions perpendicular to both the direction of the welding laser beam and the welding direction (directions perpendicular to the cut surface when the cutting angle is 90°), thereby producing high-quality laser welded joints, especially tailored blanks. [Brief description of the drawings]

[0012] [Figure 1] FIG. 1 is a perspective view showing an example of a laser cutting process for a steel plate according to the present invention. [Diagram 2] FIG. 2 is a cross-sectional view showing an example of a laser cutting process for a steel plate according to the present invention. [Diagram 3] FIG. 3 is a perspective view showing an example of a laser welding process for a steel plate according to the present invention. [Figure 4] FIG. 4 is a cross-sectional view showing an example of a laser welding process for a steel plate according to the present invention. [Diagram 5] FIG. 5 is a diagram showing the laser welding positions of the present invention. [Figure 6] FIG. 6 is a diagram showing the details of the butt cross section before welding. [Figure 7] FIG. 7 is a schematic diagram of a state in which a welded portion remains on the butt joint cross section after welding. [Figure 8] FIG. 8 is a diagram showing the details of the butt cross section after welding. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, a laser welding method and a method for manufacturing a laser welded joint according to the present invention will be described with reference to the drawings. Note that the present invention is not limited to this embodiment.

[0014] The laser welding method of the present invention is a laser welding method in which two steel plates to be butt-welded are cut using a laser beam by a laser beam cutting machine described below, and then butt-welded by a laser beam welder described below, to produce a laser welded joint described below.

[0015] <Laser beam cutting machine> As shown in Figures 1 and 2, a laser beam cutting machine for carrying out the laser welding method of the present invention has two laser oscillators 1a, 1b that oscillate laser beams, transmission systems 2a, 2b that transmit the laser beams from the two oscillators, and cutting processing heads 3a, 3b that are connected to the two transmission systems and use the beams to process metal plates. The laser beam cutting machine cuts steel plates 4 and 5 facing each other with a gap by irradiating them with cutting laser beams 6a, 6b from the cutting processing heads 3a, 3b, respectively, to form cut portions 7a, 7b. Furthermore, the laser beam cutting machine that can be used in the present invention cuts steel plates 4 and 5 at an angle θ by irradiating the cutting laser beams 6a, 6b obliquely to the steel plates 4 and 5, respectively. a , θ b During cutting, assist gases 8a and 8b are sprayed from the cutting heads 3a and 3b. In FIG. 1, the two cutting heads 3a and 3b are aligned in the cutting direction d. a , d b The laser beam is irradiated while moving to cut the steel plate 4 and the steel plate 5. In order to simplify the device, the laser beam cutting machine may be configured with a single oscillator and the laser beam may be divided into two systems, or the same head may be used to cut both steel plates. For example, a fiber laser may be used as the laser beam cutting machine. When the laser beam is divided into multiple systems, the output of the laser beam may be, for example, 2.0 to 8.0 kW for each system.

[0016] <Laser beam welding machine> As shown in Figures 3 and 4, a laser beam welder for carrying out the laser welding method of the present invention has a laser oscillator 1c for oscillating a laser beam other than the laser beam for cutting, a transmission system 2c for transmitting the laser beam, and a welding processing head 3c for processing a metal plate using the laser beam. The laser beam welder irradiates a welding laser beam 6c from the welding processing head 3c between the steel plates 4 and 5 butted together after the cutting process in a direction perpendicular to the surfaces of the steel plates 4 and 5, and welds the steel plates 4 and 5 in a welding direction d. cThe laser beam welding machine has a function of forming a welded portion 9 by moving the laser beam 6c from the laser beam welding machine to the laser beam cutting machine. At this time, when the thicknesses of the two steel plates to be butt-welded are different, it is preferable to place the acute angle part of the cross section of the thick steel plate and the obtuse angle part of the cross section of the thin steel plate on the lower side, align the height of the lower surface, and butt them in a horizontal direction. Furthermore, the irradiation position of the welding laser beam 6c in the laser beam welding machine that can be used in the present invention is the oblique butt surface and the center of the thickness of the thinner steel plate 5 of the steel plates 4 and 5, as shown in FIG. 5, and the laser beam is irradiated from above in the vertical direction aiming at this position. In order to simplify the device, the oscillator of this laser beam welding machine may be the same as the oscillator of the laser beam cutting machine, and in that case, a configuration in which the laser beam is divided into a total of three systems, two systems for cutting and one system for welding, or a total of two systems, one system for cutting and one system for welding, is also conceivable. As the laser beam welding machine, for example, a fiber laser can be used. When the laser beam is divided into a plurality of systems, the output of the laser beam can be, for example, 2.0 to 8.0 kW for each system.

[0017] <Laser welding method> In an embodiment of the laser welding method of the present invention, the steel plates before welding are cut so that the angle of the butt cross section shown in Fig. 6 satisfies the range of θ in the following formula (1). In addition, in an embodiment of the laser welding method, it is preferable that the plate thickness t satisfies the range of the following formula (2). 55.0≦θ<70.0 (1) 0.6≦t≦3.2 (2) However, if the thicknesses of the two steel plates to be butt-welded are different, the angle of the cut surface of the thinner steel plate is taken as θ (deg), and if the thicknesses of the two steel plates are the same, the angle of one of the cut surfaces is taken as θ (deg). Also, t is the thickness (mm) of the thinner of the two steel plates to be butt-welded, or the thicknesses (mm) of both steel plates when the thicknesses are the same.

[0018] In the present invention, the angle of the cut surface refers to the angle of the butt surface with respect to the upper surface of the steel plate to be butt-welded. bThe angle of the cut surface can be from 10 deg, which is possible for laser cutting, to the conventional cutting angle of 90 deg for the thinner steel sheet of the two steel sheets to be butt welded. However, if the angle is too small, a large amount of energy is required for cutting, which is inefficient. Furthermore, if the plate thickness is thick, the width of the butt surface during subsequent laser welding will be wide, and there is a possibility that unmelted cut surfaces 10a and 10b will be generated on the front and back surfaces of the weld after welding, as shown in Figure 7. This unmelted cut surface is likely to become the starting point of weld fracture, so it is preferable that it does not exist. On the other hand, if the angle is large, there is a concern that a gap will occur when the plates are butted together as in the past, which makes it easy to cause underfill. Like unmelted cut surfaces, underfill is likely to become the starting point of weld fracture, so it is preferable that it does not exist. Therefore, in the laser welding method of the present invention, the angle θ (deg) of the cut surface of the thinner of the two steel plates to be butt-welded is cut by the laser beam cutter so that it is 55.0 (deg) or more and less than 70.0 (deg). The angle θ is preferably 60.0 (deg) or more, and is preferably 65.0 (deg) or less. When flat steel plates are butted together and laser-welded, the angle of the cut surface of the other (the thicker of the two steel plates to be butt-welded) is preferably 180-θ (deg), so the other steel plate is also cut by the laser beam cutter so that it has this angle. As a geometric method, the cut steel plate can be turned over to make the angle θ (deg) of the cut surface 180-θ (deg). When the thicknesses of the two steel plates to be butt-welded are the same, the angle of either cut surface may be θ.

[0019] The laser welding method of the present invention is applicable to the range of thickness of the thinner steel plate of two steel plates to be butt-welded to produce a tailored blank, which is a laser welded joint, from 0.6 mm, which is the thinnest thickness, to 3.2 mm, which is the thickest thickness. This is because the welded portion 9 is formed at the position shown in FIG. 4, so it is preferable that the thickness t of the thinner steel plate of the two steel plates to be butt-welded satisfies formula (2). In addition, when the plate thicknesses are the same, it is preferable that the thicknesses t of both steel plates satisfy formula (2). In laser welding, if the plate thickness is thin, there is a concern that accidental burn-through (hole formation) troubles will occur. Therefore, the thickness t of the thinner steel plate of the two steel plates to be butt-welded, or the thicknesses t of both steel plates when the plate thicknesses are the same, is preferably 1.2 mm or more. In addition, if the plate thickness is thick, it is possible to obtain an end face with relatively good dimensional accuracy even with the conventional method, so it is difficult to obtain the advantages of the present invention. Therefore, the thickness t of the thinner of the two steel plates to be butt-welded, or the thickness t of both steel plates when they are the same, is preferably 2.9 mm or less, more preferably 2.3 mm or less. The thickness of the thicker of the two steel plates may be determined according to the specifications of the tailored blank, but is preferably 3.2 mm or less, and more preferably 2.9 mm or less.

[0020] In order to prevent the occurrence of unmelted residues 10a and 10b, it is preferable that the fusion width w at the center of the thickness of the steel sheet 5 at the laser welded portion 9 shown in FIG. 8 satisfies the following formula (3). 1.2w≧t / tanθ (3) Here, t is the thickness (mm) of the thinner of the two steel plates to be butt-welded, or the thickness (mm) of both steel plates when the plate thicknesses are the same, and w is the weld width (mm) at the center of the plate thickness of the thinner of the two steel plates, or of both steel plates when the plate thicknesses are the same. In addition, when the thicknesses of the two steel plates to be butt-welded are different, the angle of the cut surface of the thinner steel plate is taken as θ (deg), and when the thicknesses of the two steel plates are the same, the angle of the cut surface of one of the steel plates is taken as θ (deg). Therefore, it is preferable that the laser welded joint produced by the laser welding method of the present invention is produced by laser welding with the laser beam welder so as to satisfy formula (3).

[0021] <Target cases> A tailored blank for an A-pillar is one example of a suitable example of a laser welded joint manufactured using the laser welding method of the present invention. In the case of a tailored blank for an A-pillar, the present invention is used because the weld length is long and gaps are likely to occur between the butted steel sheets. In other words, it is preferable that the laser welded joint manufactured using the laser welding method of the present invention is a tailored blank. EXAMPLES

[0022] The operation and effects of the present invention will be described below using examples, but the present invention is not limited to the following examples.

[0023] The combination of thickness of the steel plates in this embodiment is 0.6mm-0.8mm, 1.2mm-1.6mm, 2.0mm-2.3mm, or 2.9mm-3.2mm, and the width of the plate is 1500mm. In this embodiment, among the steel plates to be butt-welded, the thick steel plate is steel plate 4 (thickness: t1), and the thin steel plate is steel plate 5 (thickness: t2). Table 1 shows the composition of the steel type used for each steel plate. Using these steel plates, laser cutting and butt laser welding were performed as shown in Figures 1 to 4. A 10 kW fiber laser was used for the laser cutting and laser welding. The output was 2.0 to 8.0 kW.

[0024] [Table 1]

[0025] The quality of the welds was judged according to the following procedure. (1) Cross-sectional macrostructure observations were carried out at three locations of the weld start, the center of the plate width, and the end of the weld of the prepared joint, and the fusion width w at the center of the plate thickness of the steel plate 5 at the butt laser weld 9 shown in Figure 8 and the underfill amounts h1 and h2 on the front and back sides of the weld were measured. In addition, it was confirmed whether there was any unmelted residue on the cut surface. The amount of underfill was recorded as 0 when the front and back sides were convex with respect to the steel plate 5. (2) The Erichsen test was performed at three locations: the start of the weld, the center of the plate width, and the end of the weld. The joint was rated as passing (○) only if the weld metal did not fracture along the welding direction at any of the locations. (3) Three tensile test pieces according to JIS Z 2241 were taken from the joint and subjected to a tensile test. All of the pieces that broke in the base metal were judged to pass (○).

[0026] The results are shown in Table 2. The cut front and back surface dimensional difference a in Table 2 is the length (mm) by which the intersection point between the cut surface and the front surface of steel plate 5, which is a thin steel plate, is shifted in the direction perpendicular to the plate thickness direction from the intersection point between the cut surface and the back surface, and can be geometrically calculated by t2 / tan θ.

[0027] [Table 2]

[0028] As shown in Table 2, the examples of the present invention, i.e., the laser welded joints manufactured using the laser welding method of the present invention, did not suffer from underfill, did not show weld metal fracture along the welding direction in the Erichsen test, and showed base metal fracture in the tensile test, resulting in high-quality laser welded joints with the effect of suppressing butt gap variation.

[0029] On the other hand, among the comparative examples, No. 1, No. 6, No. 11, and No. 16 had a large angle on the cut surface, which resulted in a gap when the joint was joined, causing underfill. In the Erichsen test, weld metal fracture occurred along the welding direction, and weld metal fracture also occurred in the tensile test.

[0030] In addition, among the comparative examples, Nos. 5, 10, 15, and 20 had a small angle of the cut surface, and unmelted portions were present on the cut surface. In the Erichsen test, weld metal fracture occurred along the welding direction, and weld metal fracture also occurred in the tensile test.

[0031] 1a: Laser oscillator 1b: Laser oscillator 1c: Laser oscillator 2a: Transmission system 2b: Transmission system 2c: Transmission system 3a: Cutting head 3b: Cutting head 3c: Welding head 4: Steel plate 5: Steel plate 6a: Cutting laser beam 6b: Cutting laser beam 6c: Welding laser beam 7a: Cutting section 7b: Cutting section 8a: Assist gas 8b: Assist gas 9: Welded parts 10a: Residual 10b: Residual d a :Cutting direction d b :Cutting direction d c : Welding direction θ a : Angle of the cutting surface θ b : Angle of the cutting surface θ: Angle of the cutting surface t: plate thickness w: Melting width at the center of the plate thickness h1: Surface underfill amount h2: Amount of underfill on the back side

Claims

1. A laser welding method in which two steel plates to be butt-welded are cut using a laser beam, and the two steel plates cut by the laser beam are butt-welded, The cutting is performed so that the angle of the cut surface is θ (deg) for one steel plate and 180-θ (deg) for the other steel plate, and the θ satisfies formula (1). 55.0≦θ<70.0 (1) However, the two steel plates to be butt-welded have different thicknesses, the angle of the cut surface of the thinner steel plate is θ (deg), the acute angle of the cross section of the thicker steel plate and the obtuse angle of the cross section of the thinner steel plate are placed on the bottom, and they are butted together horizontally with their bottom surfaces aligned, and the laser beam is irradiated from above in the vertical direction aiming at the center of the thickness of the thinner steel plate and the position of the butt surface to perform welding.

2. 2. The laser welding method according to claim 1, wherein the thinner of the two steel plates to be butt-welded has a thickness t (mm) that satisfies formula (2). 0.6≦t≦3.2 (2)

3. The laser welding method according to claim 1 , wherein the welded portion is welded so as to satisfy the following formula (3): 1.2w≧t / tanθ (3) Here, w is the fusion width (mm), and t is the thinner plate thickness (mm) of the two steel plates to be butt-welded.

4. The laser welding method according to claim 2 , wherein the welded portion is welded so as to satisfy the following formula (3): 1.2w≧t / tanθ (3) where w is the weld width (mm).

5. A method for producing a laser welded joint, comprising the steps of: producing a laser welded joint by using the laser welding method according to any one of claims 1 to 4.

6. The method for manufacturing a laser welded joint according to claim 5, wherein the laser welded joint is a tailored blank.

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