Laser welding method

Abstract

To provide a laser welding method that can suppress the occurrence of poor welding, in welding a plurality of laminated metal plates while erecting the plates.SOLUTION: A laser welding method, which welds a plurality of laminated metal plates, by irradiating the plurality of metal plates with laser beams, while erecting the plurality of metal plates, comprises a step of irradiating the plurality of metal plates with laser beams to form circular molten pools and a step of scanning the outer peripheries of the molten pools with laser beams in one circle to enlarge the molten pools. In the step of enlarging the molten pools, a starting point for scanning with a laser beam is set in a range from 135 degrees to 315 degrees in a scanning direction of a laser beam with a portion just above the molten pool in a vertical direction set as zero degree.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present disclosure relates to laser welding methods. [Background technology]

[0002] Patent Document 1 discloses welding a plurality of stacked metal plates by irradiating the metal plates with a laser beam while scanning the laser beam in a circular pattern. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-228715 Summary of the Invention [Problem to be solved by the invention]

[0004] The inventors have found the following problems with a laser welding method in which a laser beam is irradiated onto a plurality of stacked metal plates while being scanned in a circular pattern to weld the plurality of metal plates together. When welding multiple stacked metal plates in an upright position, if the molten pool becomes large enough and the amount of molten metal increases, gravity acts on the molten pool, causing the thickness of the upper part of the molten pool in the thickness direction to become thinner. As a result, the keyhole formed in the molten pool by the laser beam irradiation becomes too large, which can lead to welding defects such as holes.

[0005] The present disclosure has been made in consideration of the above circumstances, and provides a laser welding method that can suppress the occurrence of welding defects when welding multiple stacked metal plates in an upright state. [Means for solving the problem]

[0006] A laser welding method according to one aspect of the present disclosure includes: A laser welding method for welding a plurality of stacked metal plates by irradiating the metal plates with a laser beam while the metal plates are in an upright position, comprising: irradiating the plurality of metal plates with the laser beam to form circular molten pools; and scanning the laser beam once around the periphery of the molten pool to expand the molten pool. In the step of expanding the molten pool, The scanning start point of the laser beam is set to 0 degrees directly above the molten pool in the vertical direction, and is set to a range of 135 degrees to 315 degrees in the scanning direction of the laser beam.

[0007] In a laser welding method according to one embodiment of the present disclosure, in the step of scanning a laser beam once around the outer periphery of the molten pool to expand the molten pool, the starting point of the laser beam scanning is set to 0 degrees directly above the molten pool in the vertical direction, and is set to a range of 135 degrees to 315 degrees in the scanning direction of the laser beam. In this configuration, when the laser beam is irradiated to the vertically upper side of the molten pool, the molten pool does not expand sufficiently, and the amount of molten metal in the molten pool is small. As a result, the constriction that occurs in the vertically upper side of the molten pool is small, and the keyhole formed in the vertically upper side of the molten pool is not too large. Therefore, the occurrence of welding defects such as holes can be suppressed.

[0008] In the step of expanding the molten pool, the scanning start point of the laser beam may be set to a range of 180 degrees to 270 degrees in the scanning direction of the laser beam. With this configuration, the occurrence of welding defects such as holes can be further suppressed.

[0009] In the step of expanding the molten pool, a vertical direction may be detected by a sensor, and the scanning start point of the laser beam may be determined based on the vertical direction detected by the sensor. With this configuration, the laser welding system can automatically determine the scanning start point of the laser beam.

[0010] In the step of expanding the molten pool, the irradiation energy density of the laser beam may be smaller at a vertically upper side of the molten pool than at a vertically lower side of the molten pool. With this configuration, the occurrence of welding defects such as holes can be further suppressed.

[0011] A laser welding method according to one aspect of the present disclosure includes: A laser welding method for welding a plurality of stacked metal plates by irradiating the metal plates with a laser beam while the metal plates are in an upright position, comprising: irradiating the plurality of metal plates with the laser beam to form circular molten pools; and scanning the laser beam once around the periphery of the molten pool to expand the molten pool. In the step of expanding the molten pool, The irradiation energy density of the laser beam is made smaller on the vertically upper side of the molten pool than on the vertically lower side.

[0012] In a laser welding method according to one aspect of the present disclosure, in the step of expanding the molten pool, the irradiation energy density of the laser beam is made smaller on the vertically upper side of the molten pool than on the vertically lower side. Therefore, the keyhole formed on the vertically upper side of the molten pool does not become too large, and the occurrence of welding defects such as holes can be suppressed.

[0013] In the step of expanding the molten pool, the irradiation energy density of the laser beam may be gradually decreased from the vertical lower side of the molten pool to the vertical upper side, and gradually increased from the vertical upper side of the molten pool to the vertical lower side. [Effects of the Invention]

[0014] The present disclosure provides a laser welding method that can suppress the occurrence of welding defects when welding multiple stacked metal plates in an upright position. [Brief explanation of the drawings]

[0015] [Figure 1]1 is a block diagram showing a laser welding system according to a first embodiment. [Figure 2] This is a cross-sectional view along the xz plane showing how the laser beam LB is scanned around the circumference of the circular molten pool MP, thereby expanding the molten pool MP. [Figure 3] This is a cross-sectional view along the xz plane showing how the laser beam LB is scanned around the circumference of the circular molten pool MP, thereby expanding the molten pool MP. [Figure 4] This is a yz plane view schematically showing the scanning trajectory when the laser beam LB is scanned once around the outer periphery of the circular molten pool MP to expand the molten pool MP in the laser welding method of the first embodiment. [Figure 5] 10 is a macrophotograph showing a keyhole KH formed directly above in the vertical direction in a molten pool MP in a laser welding method according to an example of the first embodiment. [Figure 6] 10 is a macrophotograph showing a keyhole KH formed directly above in the vertical direction in a molten pool MP in a laser welding method according to a comparative example of the first embodiment. [Figure 7] This is a yz plane view schematically showing the scanning trajectory when the laser beam LB is scanned once around the outer periphery of the circular molten pool MP to expand the molten pool MP in the laser welding method according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, specific embodiments of the present disclosure will be described in detail with reference to the drawings. However, the present disclosure is not limited to the following embodiments. In addition, the following description and drawings have been simplified appropriately for clarity of explanation.

[0017] (First embodiment) <Laser welding system configuration> First, a laser welding system according to a first embodiment will be described with reference to FIG. Fig. 1 is a block diagram showing a laser welding system according to the first embodiment. As shown in Fig. 1, in the laser welding system according to the present embodiment, two stacked metal sheets M1 and M2 are held upright and irradiated with a laser beam LB to form a molten pool MP that penetrates the two metal sheets M1 and M2, thereby welding the two metal sheets M1 and M2. The laser welding system shown in FIG. 1 is mounted on, for example, a manipulator (robot arm).

[0018] As shown in FIG. 1, the laser welding system according to this embodiment includes a laser oscillator 101, a galvanometer scanner 102, a gravity sensor 103, and a laser control unit 104. Naturally, the right-handed xyz Cartesian coordinate system shown in Figure 1 and other drawings is for the convenience of explaining the positional relationships of the components. In Figure 1 and other drawings, the positive direction of the z axis is vertically upward, and the xy plane is the horizontal plane, which is common to all drawings.

[0019] The metal plates M1 and M2, which are the objects to be welded, are not limited in any way and may be, for example, steel plates or aluminum alloy plates that constitute a vehicle. The metal plates M1 and M2 are made of, for example, the same metal, but may also be made of different metals. The number of metal plates to be welded may be three or more.

[0020] The laser oscillator 101 oscillates a laser beam LB with a laser output based on a control signal output from a laser control unit 104. The laser beam LB output from the laser oscillator 101 is input to a galvano scanner . The laser oscillator 101 and the galvano scanner 102 constitute a laser irradiation unit that irradiates the metal plates M1 and M2, which are the objects to be welded, with a laser beam LB.

[0021] The galvanometer scanner 102 irradiates the metal plates M1 and M2, which are the welding objects, with a laser beam LB while scanning the metal plates M1 and M2 based on a control signal output from the laser control unit 104. For example, the spot diameter d of the laser beam LB irradiated from the galvanometer scanner 102 is set based on the control signal output from the laser control unit 104. s , scanning speed v s , trajectory, scanning start point, etc. are determined.

[0022] As shown in FIG. 1, a laser beam LB emitted from a galvano scanner 102 is scanned, for example, circularly over stacked metal plates M1 and M2, to form a molten pool MP having a circular shape when viewed in the yz plane. Furthermore, the laser beam LB is scanned once around the periphery of the circular molten pool MP that has been formed, thereby enlarging the molten pool MP.

[0023] The gravity sensor 103 is a sensor that detects the direction of gravity, that is, the vertical direction. Note that the gravity sensor 103 is not essential. The laser control unit 104 controls the irradiation conditions of the laser beam LB. Specifically, the laser control unit 104 controls the output (laser output) P of the laser beam LB oscillated by the laser oscillator 101. The laser control unit 104 also controls the spot diameter d of the laser beam LB irradiated from the galvano scanner 102. s , scanning speed v s , to control the trajectory.

[0024] Furthermore, the laser control unit 104 determines the scanning start point of the laser beam LB when enlarging the molten pool MP by scanning the laser beam LB once, based on the vertical direction detected by the gravity sensor 103. The method of determining the scanning start point by the laser control unit 104 will be described in detail later.

[0025] Although not shown, the laser control unit 104 includes a calculation unit such as a CPU (Central Processing Unit) and a storage unit such as a RAM (Random Access Memory) or a ROM (Read Only Memory) that stores various control programs, data, etc. In other words, the laser control unit 104 has the functions of a computer and performs various control processes based on the control programs.

[0026] 1 can be configured in hardware by the CPU, storage unit, other circuits, etc., and can be realized in software by various control programs stored in the storage unit, etc. In other words, the laser control unit 104 can be realized in various forms by hardware, software, or a combination of both.

[0027] <Laser welding method> Next, a laser welding method according to a first embodiment will be described with reference to Figures 2 to 4. Figures 2 and 3 are cross-sectional views in the xz plane showing the state in which a laser beam LB is scanned once around the periphery of a circular molten pool MP to expand the molten pool MP. Figure 4 is a yz plane view schematically showing the scanning trajectory when a laser beam LB is scanned once around the periphery of a circular molten pool MP to expand the molten pool MP in the laser welding method according to the first embodiment.

[0028] First, as described with reference to Fig. 1, two stacked metal plates M1 and M2 are held upright and irradiated with a laser beam LB emitted from a galvanometer scanner 102. Here, as shown in Fig. 1, the stacked metal plates M1 and M2 are scanned with the laser beam LB in a circular pattern, for example, to form a molten pool MP that is circular in the yz plane.

[0029] The state in which the two stacked metal plates M1 and M2 are upright is not limited to a state in which the main surfaces of the metal plates M1 and M2 are parallel to the vertical direction. For example, the state in which the two stacked metal plates M1 and M2 are upright also includes a state in which the main surfaces of the metal plates M1 and M2 are inclined to some extent with respect to the vertical direction.

[0030] Next, as shown in FIGS. 2 and 3, the laser beam LB is scanned once around the periphery of the circular molten pool MP, thereby expanding the molten pool MP. 2 and 3, the molten pool MP becomes larger, and the thickness of the molten pool MP at the upper vertical side becomes thinner due to gravity acting on the molten pool MP. In other words, a constriction occurs at the upper vertical side of the molten pool MP in the thickness direction. Furthermore, the larger the gap between the metal sheets M1 and M2, the larger the constriction at the upper vertical side of the molten pool MP.

[0031] 2 and 3, the irradiation of the laser beam LB forms, for example, a keyhole KH penetrating the molten pool MP in the molten pool MP. As described above, the thickness of the molten pool MP on the vertically upper side becomes thinner. Therefore, if the irradiation energy density of the laser beam LB is constant, the keyhole KH formed on the vertically upper side of the molten pool MP shown in FIG. 3 will be larger than the keyhole KH formed on the vertically lower side of the molten pool MP shown in FIG. 2. The irradiation energy density of the laser beam LB will be described in detail in the second embodiment.

[0032] Therefore, as shown in Figure 3, if the molten pool MP is expanded and then the laser beam LB is irradiated vertically upward in the molten pool MP, the keyhole KH becomes too large, which may result in poor welding such as holes.

[0033] Therefore, in the laser welding method according to this embodiment, the laser beam LB is scanned once around the outer periphery of the circular molten pool MP, and when the molten pool MP is enlarged, the starting point of the scanning of the laser beam LB is set to a predetermined range as shown in Figure 4. The radius of the circular scanning locus shown in FIG. 4 is, for example, 1.0 mm or more, and preferably 2.0 mm or more.

[0034] Specifically, as shown by the thin double-headed arrow in Fig. 4, the scanning start point of the laser beam LB is set to 0 degrees directly above the molten pool MP in the vertical direction, and is set to a range of 135 degrees to 315 degrees in the scanning direction of the laser beam LB. Preferably, the scanning start point of the laser beam LB is set to a range of 180 degrees to 270 degrees in the scanning direction of the laser beam LB, as shown by the thick double-headed arrow in Fig. 4.

[0035] In this configuration, when the laser beam LB is irradiated to the vertically upper side of the molten pool MP, the molten pool MP is not expanded sufficiently, and the amount of molten metal in the molten pool MP is small. As a result, the constriction that occurs in the vertically upper side of the molten pool MP shown in Figures 2 and 3 is small, and the keyhole KH formed in the vertically upper side of the molten pool MP is not too large, which prevents the occurrence of welding defects such as holes.

[0036] Although FIG. 4 is a plan view, the molten pool MP is shown as dots to facilitate understanding. In addition, although the laser beam LB is scanned counterclockwise in FIG. 4, the laser beam LB may be scanned clockwise.

[0037] Here, Fig. 5 is a macrophotograph showing a keyhole KH formed directly above the molten pool MP in the vertical direction in a laser welding method according to an example of the first embodiment, and Fig. 6 is a macrophotograph showing a keyhole KH formed directly above the molten pool MP in the vertical direction in a laser welding method according to a comparative example of the first embodiment.

[0038] In the example shown in Fig. 5, the scanning start point of the laser beam LB when expanding the molten pool MP was set at 225 degrees in the scanning direction of the laser beam LB, with 0 degrees being directly above the molten pool MP in the vertical direction. On the other hand, in the comparative example shown in Fig. 6, the scanning start point of the laser beam LB when expanding the molten pool MP was set at 45 degrees in the scanning direction of the laser beam LB, with 0 degrees being directly above the molten pool MP in the vertical direction.

[0039] The keyhole KH in the comparative example shown in FIG. 6 is larger than the keyhole KH in the example shown in FIG. In the comparative example shown in Figure 6, the laser beam LB scans 315 degrees out of a total of 360 degrees before reaching the vertically upper part of the molten pool MP (0 degrees in Figure 4). As a result, the molten pool MP is sufficiently enlarged, and the amount of molten metal in the molten pool MP increases. This is thought to result in a larger necking on the vertically upper side of the molten pool MP shown in Figures 2 and 3, and a larger keyhole KH.

[0040] In contrast, in the example shown in Figure 5, the laser beam LB scans 135 degrees out of a total of 360 degrees before reaching the vertically upper part of the molten pool MP (0 degrees in Figure 4). As a result, the molten pool MP is not sufficiently expanded, and the amount of molten metal in the molten pool MP is small. Therefore, it is thought that the constriction that occurs vertically above the molten pool MP shown in Figures 2 and 3 is small, and the keyhole KH is also small. As such, the laser welding method according to the example can suppress the occurrence of welding defects such as holes compared to the laser welding method according to the comparative example. The details of the examples and comparative examples will be described later.

[0041] As described above, in the laser welding method according to this embodiment, the laser beam LB is scanned once around the periphery of the circular molten pool MP, and when the molten pool MP is enlarged, the scanning start point of the laser beam LB is set within a predetermined range. Specifically, the scanning start point of the laser beam LB is set within a range from 135 degrees to 315 degrees in the scanning direction of the laser beam LB, with 0 degrees being directly above the molten pool MP in the vertical direction.

[0042] In this configuration, when the laser beam LB is irradiated to the vertically upper side of the molten pool MP, the molten pool MP is not sufficiently expanded, and the amount of molten metal in the molten pool MP is small. Therefore, the constriction that occurs in the vertically upper side of the molten pool MP shown in Figures 2 and 3 is small, and the keyhole KH formed in the vertically upper side of the molten pool MP is not too large. Therefore, the laser welding method according to this embodiment can suppress the occurrence of welding defects such as holes.

[0043] (Second embodiment) Next, a laser welding method according to a second embodiment will be described with reference to Fig. 7. Fig. 7 is a yz plan view schematically showing the scanning trajectory when the laser beam LB is scanned once around the outer periphery of the circular molten pool MP to expand the molten pool MP in the laser welding method according to the second embodiment.

[0044] Although Fig. 7 is a plan view, the molten pool MP is displayed as a dot for ease of understanding, as in Fig. 4. Also, although the laser beam LB is scanned counterclockwise in Fig. 7, the laser beam LB may be scanned clockwise.

[0045] In the laser welding method according to this embodiment, the laser beam LB is scanned once around the periphery of the circular molten pool MP, and when expanding the molten pool MP, the irradiation energy density E of the laser beam LB is made smaller on the vertically upper side of the molten pool MP than on the vertically lower side. In the laser welding method according to this embodiment, the scanning start point of the laser beam LB is arbitrary.

[0046] Here, the irradiation energy density E [J / mm 2 ] is the laser power P [W], the scanning speed v s [mm / s], spot diameter d s Using [mm], it can be expressed by the following formula (1). E=P / (v s ×d s )...Equation (1)

[0047] Therefore, it is necessary to reduce the laser output P or the scanning speed v s or the spot diameter d s By increasing the irradiation energy density E, the irradiation energy density E of the laser beam LB on the vertically upper side of the molten pool MP is reduced. As an example, the irradiation energy density E of the laser beam LB on the vertically lower side of the molten pool MP is set to about 50% to 90% of the irradiation energy density E of the laser beam LB on the vertically lower side of the molten pool MP.

[0048] 7, the irradiation energy density of the laser beam LB is set to a maximum value directly below the molten pool MP in the vertical direction (180 degrees in FIG. 7) and a minimum value directly above the molten pool MP in the vertical direction (0 degrees in FIG. 7).The irradiation energy density is then gradually decreased from directly below the molten pool MP in the vertical direction toward the vertical top, and gradually increased from directly above the molten pool MP in the vertical direction toward the vertical bottom.

[0049] The irradiation energy density of the laser beam LB may be constant at a maximum value within a predetermined range including the area directly below the molten pool MP in the vertical direction (for example, from 135 degrees to 225 degrees in FIG. 7), or may be constant at a minimum value within a predetermined range including the area directly above the molten pool MP in the vertical direction (for example, from 0 degrees to 45 degrees and from 315 degrees to 360 degrees in FIG. 7).

[0050] As described above, in the laser welding method according to this embodiment, the laser beam LB is scanned once around the periphery of the circular molten pool MP, and when the molten pool MP is enlarged, the irradiation energy density of the laser beam LB is made smaller on the vertically upper side of the molten pool MP than on the vertically lower side. Therefore, regardless of the scanning start point of the laser beam LB, the keyhole KH formed on the vertically upper side of the molten pool MP does not become too large, and the occurrence of welding defects such as holes can be suppressed.

[0051] The other configurations are the same as those in the first embodiment, and therefore the description will be omitted. This embodiment can be combined with the first embodiment. That is, when expanding the molten pool MP, the scanning start point of the laser beam LB may be set in the range of 135° to 315° in the scanning direction of the laser beam LB, and the irradiation energy density of the laser beam LB may be made smaller on the vertically upper side of the molten pool MP than on the vertically lower side. [Example]

[0052] The laser welding method according to the first embodiment will be described in detail below with reference to examples and comparative examples. However, the laser welding method according to the first embodiment is not limited to the following examples.

[0053] <Test conditions> First, the test conditions for the laser welding methods according to the examples and comparative examples will be described. The three stacked steel plates (top, middle, and bottom plates) were welded by irradiating a laser beam LB from the top plate side while they were standing upright. Hot-dip galvanized steel sheets SCGA440 were used for the top, middle, and bottom plates. The thickness of the top plate was 1.4 mm, that of the middle plate was 2.0 mm, and that of the bottom plate was 1.8 mm.

[0054] The gap between the upper and middle plates was set equal to the gap between the middle and lower plates. In each of the examples and comparative examples, three-piece steel plate assemblies were used in which both gaps were varied in three stages: 0.1 mm, 0.3 mm, and 0.6 mm. Welding was performed at 1,000 locations for each of the three types of three-piece steel plate assemblies in the examples and comparative examples.

[0055] A 20 kW ring laser oscillator manufactured by IPG was used as the laser oscillator 101. Furthermore, a 3D galvano head unit YD-3000ML (optical magnification 7.4 times) manufactured by Yaskawa Electric Corporation was used as the galvano scanner 102.

[0056] The irradiation conditions of the laser beam LB are: laser power P = 15 kW, scanning speed v s = 108 mm / s, spot diameter d s First, a circular molten pool MP with a radius of about 1.5 mm was formed, and then a laser beam LB was scanned in a circular pattern with a radius of 2.5 mm along the periphery of the molten pool MP. In addition, compressed air was supplied at a flow rate of 3.0 m / s to the area irradiated with the laser beam LB, that is, the welding area.

[0057] 5 and 6, the scanning start point of the laser beam LB when expanding the molten pool MP (when the laser beam LB is scanned once around the periphery of the circular molten pool MP) was changed between the example and the comparative example. In the example, as shown in Fig. 5, the scanning start point of the laser beam LB was set at 225 degrees in the scanning direction of the laser beam LB, with 0 degrees directly above the molten pool MP in the vertical direction. On the other hand, in the comparative example, as shown in Fig. 6, the scanning start point of the laser beam LB when expanding the molten pool MP was set at 45 degrees in the scanning direction of the laser beam LB, with 0 degrees directly above the molten pool MP in the vertical direction.

[0058] <Test Results> In the laser welding method according to the example, no welding defects occurred when the gap between the three steel plates was 0.1 mm, 0.3 mm, or 0.6 mm. Even with the laser welding method according to the comparative example, no welding defects occurred when the gap between the three steel plates was 0.1 mm or 0.3 mm. On the other hand, with the laser welding method according to the comparative example, when the gap between the three steel plates was 0.6 mm, 12% of welding defects occurred.

[0059] The keyhole KH in the comparative example shown in FIG. 6 is larger than the keyhole KH in the example shown in FIG. In the comparative example shown in Figure 6, the laser beam LB scans 315 degrees out of a total of 360 degrees before reaching the vertically upper part of the molten pool MP (0 degrees in Figure 4). As a result, the molten pool MP is sufficiently enlarged, and the amount of molten metal in the molten pool MP increases. This is thought to result in a larger necking on the vertically upper side of the molten pool MP shown in Figures 2 and 3, and a larger keyhole KH.

[0060] In contrast, in the embodiment shown in Figure 5, the laser beam LB scans 135 degrees out of a total of 360 degrees before reaching the vertically upper part of the molten pool MP (0 degrees in Figure 4). As a result, the molten pool MP is not sufficiently enlarged, and the amount of molten metal in the molten pool MP is small. Therefore, the constriction that occurs vertically above the molten pool MP shown in Figures 2 and 3 is small, and the keyhole KH is also small.

[0061] As described above, the laser welding method according to the example was able to suppress the occurrence of welding defects such as holes compared to the laser welding method according to the comparative example. In other words, it was found that the laser welding method according to the first embodiment can suppress the occurrence of welding defects such as holes. It was also found that the laser welding method according to the first embodiment can more effectively suppress the occurrence of welding defects as the gap between the metal plates becomes larger.

[0062] The present disclosure is not limited to the above-described embodiments, and can be modified as appropriate within the scope of the present disclosure. [Explanation of symbols]

[0063] 101 Laser oscillator 102 Galvanometer Scanner 103 Gravity Sensor 104 Laser control unit KH Keyhole LB laser beam M1, M2 metal plate MP molten pool

Claims

1. A laser welding method for welding a plurality of stacked metal plates by irradiating the metal plates with a laser beam while the metal plates are in an upright position, comprising: irradiating the plurality of metal plates with the laser beam to form circular molten pools; and scanning the laser beam once around the periphery of the molten pool to expand the molten pool. In the step of expanding the molten pool, The scanning start point of the laser beam is set to 0 degrees directly above the molten pool in the vertical direction, and the scanning start point of the laser beam is set to a range of 135 degrees to 315 degrees in the scanning direction of the laser beam. Laser welding method.

2. In the step of expanding the molten pool, The scanning start point of the laser beam is set in a range of 180 degrees to 270 degrees in the scanning direction of the laser beam. The laser welding method according to claim 1 .

3. In the step of expanding the molten pool, The sensor detects the vertical direction, determining a scanning start point of the laser beam based on the vertical direction detected by the sensor; The laser welding method according to claim 1 or 2.

4. In the step of expanding the molten pool, The irradiation energy density of the laser beam is made smaller on a vertically upper side of the molten pool than on a vertically lower side of the molten pool. The laser welding method according to claim 1 or 2.

5. A laser welding method for welding a plurality of stacked metal plates by irradiating the metal plates with a laser beam while the metal plates are in an upright position, comprising: irradiating the plurality of metal plates with the laser beam to form circular molten pools; and scanning the laser beam once around the periphery of the molten pool to expand the molten pool. In the step of expanding the molten pool, The irradiation energy density of the laser beam is made smaller on a vertically upper side of the molten pool than on a vertically lower side of the molten pool. Laser welding method.

6. In the step of expanding the molten pool, The irradiation energy density of the laser beam is gradually decreased from the vertical lower side to the vertical upper side of the molten pool, and is gradually increased from the vertical upper side to the vertical lower side of the molten pool. The laser welding method according to claim 5 .

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