Laser welding method and method for manufacturing laser welded joint

Abstract

The purpose of the present application is to provide a laser welding method capable of producing a high-quality laser welded joint and a tailor-welded blank. In the laser welding method, two steel plates to be butt-welded are each cut using a laser beam, and the steel plates cut by the laser beam are butt-welded, in the cutting, the cutting is performed such that the angle of the cut surface of one steel plate is [theta] (deg) and the angle of the cut surface of the other steel plate is 180-[theta] (deg), and the angle of the cut surface of the other steel plate is 180-[theta] (deg). Cutting is performed in a manner that theta satisfies formula (1): 55.0 < = theta lt; 70.0 (1).

Description

Technical Field

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

[0002] In the manufacture of automobile bodies, as a stamping material, a tailor-welded blank in which steel plates having different thicknesses and strengths are pre-welded is sometimes used. A tailor-welded blank is a type of welded joint, and it can partially change the characteristics of a single raw material, and it is also possible to expect a reduction in the number of parts. Therefore, it is often used for the purpose of manufacturing parts with high efficiency, especially in the manufacture of automobile bodies.

[0003] However, in the manufacture of this tailor-welded blank, depending on the situation, butt welding of more than 1 m may be required, and the variation in the butt gap, that is, the gap between the butting surfaces of the plates, when the steel plates are butted against each other may sometimes have an adverse effect on the performance of the welded parts.

[0004] Therefore, in order to ensure the butt accuracy of the welded steel plates, the end faces of the steel plates are subjected to shearing before the manufacture of the tailor-welded blank. This shearing mainly uses a mechanical shearing machine.

[0005] By this shearing, the butt gap becomes smaller, and thus it is possible to manufacture parts with a certain welding quality or higher.

[0006] Prior Art Documents

[0007] Patent Documents

[0008] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2012-135796 Summary of the Invention

[0009] Problems to be Solved by the Invention

[0010] However, in the shearing based on a mechanical shearing machine, when the warping of the steel plate is uneven in the coil width direction, dimensional errors sometimes occur in the plate width direction, and the butt gap at the time of welding varies in the coil width direction. In addition, "collapse" and "burrs" sometimes occur when the blades of the shearing machine are worn, and the butt gap at this part varies, which has an adverse effect on the performance of the parts.

[0011] In Patent Document 1, a method of improving the welding quality by performing welding by giving an angle to the cut surface of the steel plate butt-jointed by a cutting tool is proposed. However, due to the machining limit in the size of the cutting tool, the cutting angle is 20° or less, and when butting at an angle of 20° or less, the overlap is insufficient, and it is difficult to obtain a sufficient effect of suppressing the variation in the butt gap.

[0012] In view of the above problems, an object of the present invention is to provide a laser welding method capable of obtaining a more sufficient effect of suppressing the variation of the butt gap by a laser cutting method without machining limits.

[0013] Means for Solving the Problem

[0014] In the present invention, first, attention is paid to the shearing process before welding, that is, the cutting process of the plate. Conventionally, the shearing process of the plate has been carried out by a mechanical shearing machine. However, in the case of producing an inclined cutting end face by a contact processing method, the tool slides and retracts from the steel plate, so that an end face with an angle of 20° or less can only be produced. Therefore, in the present invention, in order to solve the above problems, instead of based on contact mechanical processing, the cutting of non-contact thermal processing based on a laser beam is applied to the end face before laser welding. Thus, compared with mechanical processing, cutting can be carried out more inclined in the plate thickness direction, and the variation of the butt gap in the direction perpendicular to both the direction of the laser beam for welding and the welding direction during welding can be reduced. By reducing the variation of the butt gap, a higher-quality laser welded joint and tailor welded blank can be manufactured. In addition, since it is non-contact processing, there is no tool wear, and cutting can be carried out to reduce the variation caused by continuous operation and the variation of each part in the welding direction during one laser cutting. Therefore, a high-quality tailor welded blank can be produced.

[0015] The present invention has been completed based on the above insights, and its gist is as follows.

[0016] [1] A laser welding method, which uses a laser beam to cut each of two steel plates to be butt-welded, and butt-welds the two steel plates cut by the laser beam, wherein in the cutting, the angle of the cutting surface of one steel plate is θ (deg), the angle of the cutting surface of the other steel plate is 180 - θ (deg), and the cutting is carried out in such a manner that θ satisfies the formula (1),

[0017] 55.0 ≤ θ < 70.0 (1)

[0018] Wherein, when the plate thicknesses of the two steel plates to be butt-welded are different, the angle of the cutting surface of the thinner steel plate is set as θ (deg), and when the plate thicknesses of the two steel plates are the same, the angle of the cutting surface of either one is set as θ (deg).

[0019] [2] The laser welding method according to [1], wherein in the two steel plates to be butt-welded, the plate thickness t (mm) of the thinner one or the plate thicknesses t (mm) of both when the plate thicknesses are the same satisfy the formula (2),

[0020] 0.6 ≤ t ≤ 3.2 (2).

[0021] [3] According to the laser welding method described in [1], wherein the welded portion of the laser welding is welded in such a manner as to satisfy the following formula (3),

[0022] 1.2w ≥ t / tanθ (3)

[0023] Herein, w is the melting width (mm), and t is the thickness of the thinner one of the two steel plates to be butt-welded (mm) or the thickness of both steel plates when the thicknesses are the same (mm).

[0024] [4] According to the laser welding method described in [2], wherein the welded portion of the laser welding is welded in such a manner as to satisfy the following formula (3),

[0025] 1.2w ≥ t / tanθ (3)

[0026] Herein, w is the melting width (mm).

[0027] [5] According to the laser welding method described in [1], wherein in the case where the thicknesses of the two steel plates to be butt-welded are different, the acute-angle portion of the cross-section of the thicker steel plate and the obtuse-angle portion of the cross-section of the thinner steel plate are respectively located on the lower side, and they are arranged in a horizontal direction with the same height of the lower surface for butt-joint, and the laser beam is irradiated from the upper side in the vertical direction while aligning with the center of the thickness of the thinner steel plate and the position of the butt-joint surface for welding.

[0028] [6] According to the laser welding method described in [2], wherein in the case where the thicknesses of the two steel plates to be butt-welded are different, the acute-angle portion of the cross-section of the thicker steel plate and the obtuse-angle portion of the cross-section of the thinner steel plate are respectively located on the lower side, and they are arranged in a horizontal direction with the same height of the lower surface for butt-joint, and the laser beam is irradiated from the upper side in the vertical direction while aligning with the center of the thickness of the thinner steel plate and the position of the butt-joint surface for welding.

[0029] [7] A method for manufacturing a laser welded joint, which uses the laser welding method described in any one of [1] to [6] to manufacture a laser welded joint.

[0030] [8] According to the method for manufacturing a laser welded joint described in [7], wherein the laser welded joint is a tailor-welded blank.

[0031] Advantages of the Invention

[0032] According to the present invention, it is possible to suppress the variation of the butt-joint gap in the direction perpendicular to both the direction of the laser beam for welding and the welding direction (the direction perpendicular to the cutting surface when the cutting angle is 90°), and it is possible to manufacture a high-quality laser welded joint, especially a tailor-welded blank. Brief Description of the Drawings

[0033] Figure 1 Figure 1 is a perspective view showing an example of the laser cutting process of the steel plate of the present invention.

[0034] Figure 2 Figure 2 is a cross-sectional view showing an example of the laser cutting process of the steel plate of the present invention.

[0035] Figure 3 Figure 3 is a perspective view showing an example of the laser welding process of the steel plate of the present invention.

[0036] Figure 4 Figure 4 is a cross-sectional view showing an example of the laser welding process of the steel plate of the present invention.

[0037] Figure 5 Figure 5 is a view showing the laser welding position of the present invention.

[0038] Figure 6 Figure 6 is a view showing the details of the butt joint section before welding.

[0039] Figure 7 Figure 7 is a schematic view of the state where melting residue is generated in the butt joint section after welding.

[0040] Figure 8 Figure 8 is a view showing the details of the butt joint section after welding. DETAILED DESCRIPTION OF THE INVENTION

[0041] Hereinafter, with reference to the respective drawings, the laser welding method and the method for manufacturing a laser welded joint of the present invention will be described. In addition, the present invention is not limited to this embodiment.

[0042] The laser welding method of the present invention is a laser welding method for manufacturing a laser welded joint described below by using a laser beam cutting machine described later to cut two steel plates to be butt-welded with a laser beam and performing butt welding with a laser beam welding machine described later.

[0043] <Laser beam cutting machine>

[0044] As Figure 1 and Figure 2 ​​​​​​​​​​​​​​​​As shown in the figure, the laser beam cutting machine for implementing the laser welding method of the present invention has: two laser oscillators 1a and 1b that oscillate laser beams; transmission systems 2a and 2b that transmit the laser beams from the two oscillators; and cutting heads 3a and 3b that are connected to the two transmission systems and process a metal plate using the light beams. The laser beam cutting machine cuts by irradiating cutting laser beams 6a and 6b from the cutting heads 3a and 3b onto steel plates 4 and 5 that face each other with a gap therebetween, forming cutting parts 7a and 7b. Moreover, the laser beam cutting machine that can be used for the present invention can form cutting parts 7a and 7b with angles θ a , θ b by irradiating the cutting laser beams 6a and 6b obliquely with respect to the steel plates 4 and 5 respectively. During cutting, auxiliary gases 8a and 8b are ejected from each of the cutting heads 3a and 3b. In Figure 1 , the two cutting heads 3a and 3b irradiate laser beams while moving in the cutting directions d a , d b to cut the steel plates 4 and 5. It should be noted that, for the sake of simplifying the device, it is also considered that the laser beam cutting machine has a structure in which one oscillator is used and the laser beam is divided into two systems, or a situation where the same head is used to cut both steel plates. As the laser beam cutting machine, for example, a fiber laser can be used. When the output of the laser beam is divided into multiple systems, in each system, it can be set to 2.0 - 8.0 kW, for example.

[0045] <Laser Beam Welder>

[0046] As Figure 3 and Figure 4 shown, the laser beam welder for implementing the laser welding method of the present invention has: a laser oscillator 1c that oscillates other laser beams different from the cutting laser beam; a transmission system 2c that transmits the laser beam; and a welding head 3c that processes a metal plate using the laser beam. The laser beam welder has the following function: between the steel plates 4 and 5 butt - joined after the cutting process, a welding laser beam 6c is irradiated from the welding head 3c in a direction perpendicular to the surfaces of the steel plates 4 and 5, and it moves in the welding direction d c to form a welded part 9. At this time, when the plate thicknesses of the two butt - welded steel plates are different, it is preferable to arrange the acute - angled part of the cross - section of the thicker steel plate and the obtuse - angled part of the cross - section of the thinner steel plate on the lower side respectively, and arrange them horizontally with the same height of the lower surface for butt - joining. And, as Figure 5As shown, the irradiation position of the laser beam 6c for welding in the laser beam welding machine that can be used in the present invention is the inclined butt joint surface and the center of the thickness of the thinner steel plate 5 among the steel plates 4 and 5, and the laser beam is irradiated from the upper side in the vertical direction aiming at this position. In addition, in order to simplify the device, the oscillator of this laser beam welding machine can be the same as that of the laser beam cutting machine. In this case, a structure with a total of three systems, namely two systems for cutting and one system for welding, or a total of two systems, namely one system for cutting and one system for welding, can also be considered. As the laser beam welding machine, for example, a fiber laser can be used. When the output of the laser beam is divided into multiple systems, it can be set to 2.0 to 8.0 kW in each system, for example.

[0047] <Laser welding method>

[0048] In an embodiment of the laser welding method of the present invention, as Figure 6 shown, cutting is performed in such a way that the angle θ of the butt joint section of the steel plate before welding 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):

[0049] 55.0 ≤ θ < 70.0 (1)

[0050] 0.6 ≤ t ≤ 3.2 (2).

[0051] However, when the plate thicknesses of the two steel plates for butt welding are different, the angle of the cutting surface of the thinner steel plate is set as θ (deg), and when the plate thicknesses of the two steel plates are the same, the angle of the cutting surface of either one is set as θ (deg). In addition, t is set as the plate thickness (mm) of the thinner one of the two steel plates for butt welding, or the plate thickness (mm) of both steel plates when the plate thicknesses are the same.

[0052] In addition, in the present invention, the angle of the cutting surface refers to the angle of the butt joint surface with respect to the upper surface of the steel plate for butt welding. That is, it refers to Figure 4 the θ b angle. For the thinner steel plate among the two steel plates for butt welding, the angle of the cutting surface can cope with the angle from 10 deg that can be processed by laser cutting to the conventional cutting angle of 90 deg. However, when the angle is too small, a large amount of energy is required for cutting, and the efficiency is not high. In addition, when the plate thickness is relatively thick, the width of the butt joint surface during subsequent laser welding becomes wider, so as Figure 7As shown, there may be melting residues 10a and 10b of the cut surface on the back of the welding table after welding. Such melting residues are likely to become the starting point of fracture of the welded part, so it is preferably absent. On the other hand, if the angle is large, there may be a gap during butt welding as in the past, and bottom filling is likely to occur. Bottom filling, like melting residues, is also likely to become the starting point of fracture of the welded part, so it is preferably absent. Therefore, in the laser welding method of the present invention, the laser beam cutting machine is used to cut the cut surface of the thinner one of the two steel plates for butt welding at an angle θ (deg) of 55.0 (deg) or more and less than 70.0 (deg). The angle θ is preferably 60.0 (deg) or more, and preferably 65.0 (deg) or less. It should be noted that in the case of laser welding by butt-jointing flat steel plates to each other, the angle of the cut surface of the other party (the thicker one of the two steel plates for butt welding) is preferably 180 - θ (deg), so the other steel plate is also cut by the laser beam cutting machine so as to have this angle. In addition, as a geometric method, by flipping the cut steel plate, the angle θ (deg) of the cut surface can be set to 180 - θ (deg). In addition, when the thicknesses of the two steel plates for butt welding are the same, the angle of any cut surface can be set to θ.

[0053] The laser welding method of the present invention can be applied to the range of the thickness of the thinner steel plate among the two steel plates for butt welding to manufacture a tailor-welded blank as a laser welding joint, from the thinnest plate thickness of 0.6 mm to the thickest plate thickness of 3.2 mm. This is because, since the welded part 9 is formed at Figure 4 the position shown, it is preferable that the thickness t of the thinner one of the two steel plates for butt welding satisfies formula (2). In addition, when the 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 may accidentally occur a burn-through (perforation) failure. Therefore, the thickness t of the thinner one of the two steel plates for butt welding, or the thicknesses t of both steel plates when the thicknesses are the same, is preferably 1.2 mm or more. In addition, if the plate thickness is thick, even with the existing method, an end face with relatively good dimensional accuracy can be obtained, so it is difficult to obtain the advantages of the present invention. Therefore, the thickness t of the thinner one of the two steel plates for butt welding, or the thicknesses t of both steel plates when the thicknesses are the same, is preferably 2.9 mm or less, more preferably 2.3 mm or less. The thickness of the thicker one of the two steel plates can be determined according to the specifications of the tailor-welded blank, but it is preferably 3.2 mm or less. More preferably, it is 2.9 mm or less.

[0054] In addition, in order to prevent the generation of melting residues 10a and 10b, Figure 8The melting width w at the center of the plate thickness of the steel plate 5 at the welded part 9 of the laser welding shown preferably satisfies the following formula (3):

[0055] 1.2w ≥ t / tanθ (3).

[0056] Here, t is the plate thickness (mm) of the thinner one of the two steel plates for butt welding or the plate thickness (mm) of both steel plates when the plate thicknesses are the same, and w is the welding width (mm) at the center of the plate thickness of the thinner one of the two steel plates or both steel plates when the plate thicknesses are the same. In addition, when the plate thicknesses of the two steel plates for butt welding are different, the angle of the cut surface of the thinner steel plate is set as θ (deg), and when the plate thicknesses of the two steel plates are the same, the angle of the cut surface of either one is set as θ (deg). Therefore, the laser welded joint manufactured by the laser welding method of the present invention is preferably manufactured by laser welding using the laser beam welding machine in a manner that satisfies formula (3).

[0057] <Object example>

[0058] As an example of a preferred example of the laser welded joint manufactured by using the laser welding method of the present invention, there is a tailor welded blank facing the A-pillar. In the case of the tailor welded blank facing the A-pillar, gaps are likely to occur between the steel plates that are butt-welded with a long welding length, so the present invention is used. That is, the laser welded joint manufactured by using the laser welding method of the present invention is preferably a tailor welded blank.

[0059] Examples

[0060] Hereinafter, the functions and effects of the present invention will be described using examples. In addition, the present invention is not limited to the following examples.

[0061] The combination of the plate thicknesses of the steel plates in this example is any one of 0.6 mm - 0.8 mm, 1.2 mm - 1.6 mm, 2.0 mm - 2.3 mm, and 2.9 mm - 3.2 mm, and the width of the plate is 1500 mm. In this example, the thicker steel plate among the steel plates for butt welding is used as steel plate 4 (plate thickness: t1), and the thinner steel plate is used as steel plate 5 (plate thickness: t2). In addition, Table 1 shows the component compositions of the steel grades used in each steel plate. Using these steel plates, as Figures 1 to 4 shown, laser cutting and butt laser welding are performed. Laser cutting and laser welding use a 10 kW fiber laser. The output is 2.0 - 8.0 kW.

[0062] [Table 1]

[0063]

[0064] In addition, the determination of whether the welding state is good is performed according to the following steps.

[0065] (1) Conduct macroscopic structure observation of the cross-sections at three locations: the welding start section, the center section of the plate width, and the welding end section of the fabricated joint, and measure Figure 8 the melting width w at the center of the plate thickness of the steel plate 5 at 9 butt laser welding parts shown, and the bottom filling amounts h1 and h2 on the front and back surfaces of the welding part. Additionally, confirm whether there is no unmelted residue on the cut surface. The bottom filling amount is set to 0 when it protrudes from the back surface of the steel plate 5.

[0066] (2) Conduct Erichsen cupping tests at three locations: the welding start section, the center section of the plate width, and the welding end section of the joint. It is considered qualified (○) only when there is no fracture of the welded metal along the welding direction at any of the locations.

[0067] (3) Collect three tensile test pieces specified in JIS Z2241 from the above joint and conduct tensile tests. The tensile test pieces with all base metals fractured are considered qualified (○).

[0068] The obtained results are shown in Table 2. It should be noted that the cutting front-back surface dimension difference a in Table 2 is the length (mm) by which the intersection point of the cut surface and the front surface of the steel plate 5 with a thinner plate thickness deviates from the intersection point of the cut surface and the back surface in the direction perpendicular to the plate thickness direction, and can be calculated geometrically by t2 / tanθ.

[0069] [Table 2]

[0070]

[0071] As shown in Table 2, the laser welded joints manufactured using the example of the present invention, i.e., the laser welding method of the present invention, do not have bottom filling, do not have fracture of the welded metal along the welding direction in the Erichsen cupping test, and have fracture of the base metal in the tensile test, resulting in high-quality laser welded joints capable of obtaining the effect of suppressing the change in the butt gap.

[0072] On the other hand, in the comparative examples, the cutting surface angles of No.1, No.6, No.11, and No.16 are large, gaps are generated during butt jointing, so bottom filling occurs, fracture of the welded metal along the welding direction occurs in the Erichsen cupping test, and fracture of the welded metal also occurs in the tensile test.

[0073] Additionally, in the comparative examples, the cutting surface angles of No.5, No.10, No.15, and No.20 are small, melting residue on the cutting surface occurs, fracture of the welded metal along the welding direction occurs in the Erichsen cupping test, and fracture of the welded metal also occurs in the tensile test.

[0074] 1a: Laser oscillator

[0075] 1b: Laser oscillator

[0076] 1c: Laser oscillator

[0077] 2a: Transmission system

[0078] 2b: Transmission system

[0079] 2c: Transmission system

[0080] 3a: Cutting head

[0081] 3b: Cutting head

[0082] 3c: Welding processing head

[0083] 4: Steel plate

[0084] 5: Steel plate

[0085] 6a: Laser beam for cutting

[0086] 6b: Laser beam for cutting

[0087] 6c: Laser beam for welding

[0088] 7a: Cutting part

[0089] 7b: Cutting part

[0090] 8a: Auxiliary gas

[0091] 8b: Auxiliary gas

[0092] 9: Welding Department

[0093] 10a: Melting residue

[0094] 10b: Melting residue

[0095] d a : Cutting direction

[0096] d b : Cutting direction

[0097] d c :Welding direction

[0098] θ a : Angle of the cutting surface

[0099] θ b : Angle of the cutting surface

[0100] θ: Angle of the cutting surface

[0101] t: plate thickness

[0102] w: Melting width at the center of the plate thickness

[0103] h1: Bottom fill amount of the surface

[0104] h2: Bottom fill amount of the back surface

Claims

1. A laser welding method, which uses a laser beam to cut each of two steel plates to be butt-welded, and butt-welds the two steel plates cut by the laser beam, wherein, in the cutting, the cutting is performed such that the angle of the cutting surface of one steel plate is θ (deg) and the angle of the cutting surface of the other steel plate is 180 - θ (deg), and the cutting is performed such that θ satisfies formula (1), wherein, when the plate thicknesses of the two steel plates to be butt-welded are different, the angle of the cutting surface of the steel plate with the thinner plate thickness is set as θ (deg), and when the plate thicknesses of the two steel plates are the same, the angle of the cutting surface of either one is set as θ (deg).

2. The laser welding method according to claim 1, wherein, in the two steel plates to be butt-welded, the plate thickness t (mm) of the thinner one or the plate thicknesses t (mm) of both steel plates when the plate thicknesses are the same satisfy formula (2), 。 3. The laser welding method according to claim 1, wherein, the welded portion of the laser welding is welded in such a manner as to satisfy the following formula (3), wherein, w is the melting width (mm), and t is the plate thickness (mm) of the thinner one of the two steel plates to be butt-welded or the plate thicknesses (mm) of both steel plates when the plate thicknesses are the same.

4. The laser welding method according to claim 2, wherein, the welded portion of the laser welding is welded in such a manner as to satisfy the following formula (3), wherein, w is the melting width (mm).

5. The laser welding method according to claim 1, wherein, when the plate thicknesses of the two steel plates to be butt-welded are different, the acute-angle portion of the cross-section of the thicker steel plate and the obtuse-angle portion of the cross-section of the thinner steel plate are respectively positioned on the lower side, and they are arranged horizontally with the same height of the lower surface for butt-joint, and the laser beam is irradiated from the upper side in the vertical direction while aligning with the center of the plate thickness of the thinner steel plate and the position of the butt-joint surface for welding.

6. The laser welding method according to claim 2, wherein, when the plate thicknesses of the two steel plates to be butt-welded are different, the acute-angle portion of the cross-section of the thicker steel plate and the obtuse-angle portion of the cross-section of the thinner steel plate are respectively positioned on the lower side, and they are arranged horizontally with the same height of the lower surface for butt-joint, and the laser beam is irradiated from the upper side in the vertical direction while aligning with the center of the plate thickness of the thinner steel plate and the position of the butt-joint surface for welding.

7. A method for manufacturing a laser welded joint, which uses the laser welding method according to any one of claims 1 to 6 to manufacture a laser welded joint.

8. The method for manufacturing a laser welded joint according to claim 7, wherein, the laser welded joint is a tailor-welded blank.

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