Patent Publication Number: US-2023158606-A1

Title: Laser welding method and laser welding device

Description:
This application is a continuation application of the PCT International Application No. PCT/JP2021/036379 filed on Oct. 1, 2021, which claim the benefit of foreign priority of Japanese patent application No. 2020-168501 filed on Oct. 5, 2020, the contents all of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a laser welding method and a laser welding device. 
     BACKGROUND ART 
     Laser welding enables performing welding at high speed and with high quality because a workpiece of an object to be welded is irradiated with a laser beam having high power density. In particular, scanning welding for performing welding while a surface of a workpiece is scanned with a laser beam at high speed enables the laser beam to be moved to a next welding point at high speed during a period in which welding is not performed, and thus enabling total welding time to be shortened (e.g., see PTL 1). Conventionally proposed scanning methods with a laser beam include a method of scanning with a laser beam while drawing a Lissajous pattern on a surface of a workpiece (e.g., see PTL 2 and PTL 3). 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Unexamined Japanese Patent Publication No. 2005-095934 
         PTL 2: Unexamined Japanese Patent Publication No. S60-177983 
         PTL 3: Unexamined Japanese Patent Publication No. H11-104877 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     Unfortunately, when the Lissajous pattern is drawn on the surface of the workpiece, a conventional method as disclosed in PTLs 2, 3 may cause drawing speed in each part of the pattern, or welding speed to be inconstant. In particular, a large speed difference may occur between a part where the pattern is linearly drawn and a part where the pattern is changed in direction. 
     As described above, when a large speed difference occurs during drawing of the Lissajous pattern, the amount of heat input to the workpiece, specifically, the amount of heat input per unit drawing length varies depending on the pattern, or a region of a molten pool. That is, the amount of heat input to the workpiece becomes non-uniform in the Lissajous pattern (molten pool), so that a weld bead may not be formed in a favorable shape during welding. Such a problem also occurs when the scanning pattern of the laser beam is not a Lissajous pattern, but a continuous pattern in which two circular patterns are in contact with each other at one point, for example. 
     The present disclosure is made in view of such a point, and it is an object of the present disclosure to provide a laser welding method and a laser welding device capable of obtaining a weld bead in a favorable shape by making the amount of heat input in a scanning pattern of a laser beam uniform. 
     Solution to Problem 
     To achieve the above object, a laser welding method according to the present disclosure includes a welding step of welding a workpiece by irradiating a surface of the workpiece with a laser beam by two-dimensionally sweeping the laser beam while causing the laser beam to travel in a first direction. In the welding step, the laser beam is swept to draw a predetermined pattern on the surface of the workpiece, and drawing speed and output of the laser beam are controlled to have an equal amount of heat input per unit drawing length in the predetermined pattern over an entire length of the predetermined pattern. The predetermined pattern is a continuous pattern in which two annular patterns are in contact with each other at one point. 
     A laser welding device according to the present disclosure at least includes a laser oscillator that generates a laser beam, a laser head that receives the laser beam and irradiates a workpiece with the laser beam, and a controller that controls operation of the laser head. The laser head includes a laser scanner that sweeps the laser beam in each of a first direction and a second direction intersecting the first direction. The controller drives and controls the laser scanner to cause the laser beam to draw a predetermined pattern on a surface of the workpiece. The controller controls drawing speed and output of the laser beam to have an equal amount of heat input per unit drawing length in the predetermined pattern over an entire length of the predetermined pattern. The predetermined pattern is a continuous pattern in which two annular patterns are in contact with each other at one point. 
     Advantageous Effect of Invention 
     The present disclosure enables a weld bead in a favorable shape to be obtained by making the amount of heat input in a scanning pattern of a laser beam uniform. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic configuration view of a laser welding device according to a first exemplary embodiment. 
         FIG.  2    is a schematic configuration view of a laser scanner. 
         FIG.  3    is a schematic diagram illustrating a drawing distance in a Lissajous pattern. 
         FIG.  4    is a diagram illustrating a scanning pattern of a laser beam according to the present exemplary embodiment. 
         FIG.  5    is a diagram illustrating a relationship between drawing speed and output of a laser beam at a drawing position of the laser beam. 
         FIG.  6 A  is a diagram illustrating a first scanning pattern of a laser beam according to a first modification. 
         FIG.  6 B  is a diagram illustrating a second scanning pattern of a laser beam according to the first modification. 
         FIG.  7 A  is a diagram illustrating a third scanning pattern of a laser beam according to the first modification. 
         FIG.  7 B  is a diagram illustrating a fourth scanning pattern of a laser beam according to the first modification. 
         FIG.  7 C  is a diagram illustrating a fifth scanning pattern of a laser beam according to the first modification. 
         FIG.  8    is a diagram illustrating an example of a combination of parameters when a Lissajous pattern is drawn. 
         FIG.  9    is a diagram illustrating a relationship between a drawing position of a laser beam and drawing speed of the laser beam according to a second exemplary embodiment. 
         FIG.  10 A  is a diagram illustrating a first scanning pattern of a laser beam according to a second modification. 
         FIG.  10 B  is a diagram illustrating a second scanning pattern of a laser beam according to the second modification. 
         FIG.  10 C  is a diagram illustrating a third scanning pattern of a laser beam according to the second modification. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Hereinafter, exemplary embodiments of the present disclosure will be described with reference to the drawings. The following description of preferable exemplary embodiments is merely illustrative in nature, and is not intended to limit the present disclosure, its application, or its use. 
     First Exemplary Embodiment 
     [Configurations of Laser Welding Device and Laser Scanner] 
       FIG.  1    illustrates a schematic view of a configuration of a laser welding device according to the present exemplary embodiment, and  FIG.  2    illustrates a schematic configuration view of a laser scanner. 
     In the following description, a direction parallel to a traveling direction of laser beam LB from reflection mirror  33  toward laser scanner  40  may be referred to as an X-direction, a direction parallel to an optical axis of laser beam LB emitted from laser head  30  may be referred to as a Z-direction, and a direction orthogonal to each of the X-direction and the Z-direction may be referred to as a Y-direction. When a surface of workpiece  200  is formed as a flat surface, an XY plane including the X-direction and the Y-direction therein may be substantially parallel to the surface of workpiece  200 , or may form a certain angle with respect to the surface of workpiece  200 . 
     As illustrated in  FIG.  1   , laser welding device  100  includes laser oscillator  10 , optical fiber  20 , laser head  30 , controller  50 , and manipulator  60 . 
     Laser oscillator  10  is a laser beam source that is supplied with power from a power supply (not illustrated), and that generates laser beam LB. Laser oscillator  10  may include a single a laser beam source, or may include multiple laser modules. In the latter case, laser beams emitted from respective multiple laser modules are combined into and then emitted as laser beam LB. The laser beam source or the laser modules used in laser oscillator  10  are appropriately selected in accordance with a material, a form of a weld, or the like of workpiece  200 . 
     For example, a fiber laser or a disk laser, or an yttrium aluminum garnet (YAG) laser can be used as the laser beam source. In this case, laser beam LB has a wavelength set in a range from 1000 nm to 1100 nm, inclusive. A semiconductor laser may also be used as the laser beam source or the laser module. In this case, laser beam LB has a wavelength set to a range from 800 nm to 1000 nm, inclusive. A visible-light laser may be also used as the laser beam source or the laser module. In this case, laser beam LB ha a wavelength set in a range from 400 nm to 600 nm, inclusive. 
     Optical fiber  20  is optically coupled to laser oscillator  10 , and laser beam LB generated in laser oscillator  10  is incident to optical fiber  20  and then transmitted through the inside of optical fiber  20  toward laser head  30 . 
     Laser head  30  is attached to an end part of optical fiber  20  to emit laser beam LB toward workpiece  200 , laser beam LB being transmitted through optical fiber  20 . 
     Laser head  30  includes collimation lens  32 , reflection mirror  33 , condenser lens  34 , and laser scanner  40 , which serve as optical components, and housing  31  houses inside these optical components while maintaining a predetermined placement relationship among them. 
     Collimation lens  32  receives laser beam LB emitted from optical fiber  20 . Collimation lens  32  converts laser beam LB into collimated light, and causes the collimated light to be incident on reflection mirror  33 . Collimation lens  32  is connected to a driver (not illustrated), and is configured to be able to displace in the Z-direction in response to a control signal from controller  50 . Displacing collimation lens  32  in the Z-direction causes laser beam LB to be changed in focal position, and thus enabling laser beam LB to be appropriately emitted in accordance with the form of workpiece  200 . That is, collimation lens  32  in combination with the driver (not illustrated) also functions as a focal position adjustment mechanism for laser beam LB. The driver may displace condenser lens  34  to change the focal position of laser beam LB. 
     Reflection mirror  33  reflects laser beam LB transmitted through collimation lens  32  to cause laser beam LB to be incident to laser scanner  40 . Reflection mirror  33  is provided with a surface forming an angle of about 45 degrees with respect to the optical axis of laser beam LB transmitted through collimation lens  32 . 
     Condenser lens  34  condenses laser beam LB on the surface of workpiece  200 , laser beam LB being reflected by reflection mirror  33  and swept by laser scanner  40 . 
     As illustrated in  FIG.  2   , laser scanner  40  is a known galvano-scanner, which includes first galvano-mirror  41  and second galvano-mirror  42 . First galvano-mirror  41  includes first mirror  41   a , first rotation shaft  41   b , and first driver  41   c . Second galvano-mirror  42  includes second mirror  42   a , second rotation shaft  42   b , and second driver  42   c . Laser beam LB transmitted through condenser lens  34  is reflected at first mirror  41   a , and is further reflected at second mirror  42   a . Then, the surface of workpiece  200  is irradiated with laser beam LB. 
     For example, first driver  41   c  and second driver  42   c  are each a galvano motor, and first rotation shaft  41   b  and second rotation shaft  42   b  are each an output shafts of the motor. Although not illustrated, when first driver  41   c  is rotationally driven by a driver that operates in response to a control signal from controller  50 , first mirror  41   a  attached to first rotation shaft  41   b  is rotated about the axis of first rotation shaft  41   b . Similarly, when second driver  42   c  is rotationally driven by a driver that operates in response to a control signal from controller  50 , second mirror  42   a  attached to second rotation shaft  42   b  is rotated about the axis of second rotation shaft  42   b.    
     When first mirror  41   a  is rotationally moved about the axis of first rotation shaft  41   b  to a predetermined angle, laser beam LB is swept in the X-direction. When second mirror  42   a  is rotationally moved about the axis of second rotation shaft  42   b  to a predetermined angle, laser beam LB is swept in the Y-direction. That is, laser scanner  40  is configured to sweep laser beam LB two-dimensionally within the XY plane to emit laser beam LB toward workpiece  200 . 
     Controller  50  controls laser oscillation of laser oscillator  10 . Specifically, controller  50  performs the laser oscillation control by providing control signals for an output current, on/off time, and the like to a power supply (not illustrated) connected to laser oscillator  10 . 
     Controller  50  also controls operation of laser head  30  in accordance with content of a selected laser welding program. Specifically, controller  50  performs drive control on laser scanner  40  and the driver (not illustrated) of collimation lens  32  that are provided in laser head  30 . Controller  50  further controls operation of manipulator  60 . The laser welding program is stored in a storage (not illustrated) provided inside controller  50  or at another place, and is invoked in controller  50  by a command from controller  50 . 
     Controller  50  includes an integrated circuit (not illustrated) such as a large-scale integration (LSI) or a microcomputer. When the laser welding program, which is software, is performed on the integrated circuit, the above-described functions of controller  50  are implemented. Controller  50  that controls the operation of laser head  30  and controller  50  that controls the output of laser beam LB may be provided separately. 
     Manipulator  60  is an articulated robot, and is attached to housing  31  of laser head  30 . Manipulator  60  is connected to controller  50  to allow transmission and reception of a signal therebetween, and moves laser head  30  while causing a predetermined trajectory to be drawn in accordance with the laser welding program described above. Another controller (not illustrated) may be provided for controlling the operation of manipulator  60 . 
     [Drawing Speed of Lissajous Pattern] 
       FIG.  3    is a schematic diagram illustrating a drawing distance in a Lissajous pattern. As illustrated in  FIG.  3   , laser beam LB is swept to draw a Lissajous pattern (hereinafter, it is also referred to as a Lissajous figure) in the XY plane, in this case, on the surface of workpiece  200 . 
     The Lissajous pattern illustrated in  FIG.  3    has a width in the X direction that is equal to a width in the Y direction, and the width in the Y direction is substantially equal to width W in the Y direction of a weld bead (not illustrated) when welding speed is very high. In contrast, when the welding speed is low, the width of the weld bead increases under influence of heat conduction. Thus, the width of the Lissajous pattern in the Y direction is slightly narrower than the width of the weld bead in the Y direction. In the specification of the present application, “substantially equal” or “substantially identical” means that control results of respective control targets are identical or identical with an error of a control system, and does not require both the control targets to be strictly equal or identical. Additionally, “substantially equal” or “substantially identical” is also used as a meaning of equality or identification including manufacturing tolerances and assembly tolerances of each component and the like. 
     The Lissajous pattern illustrated in  FIG.  3    is obtained by vibrating laser beam LB at a predetermined frequency in the X direction in a sinusoidal wave shape and vibrating laser beam LB at a frequency different from that in the X direction (½ of the frequency in the X direction) in the Y direction in a sinusoidal wave shape. As described above, scanning figures in the X direction and the Y direction of laser beam LB are determined based on rotational movements of first mirror  41   a  and second mirror  42   a . When the Lissajous pattern illustrated in  FIG.  3    and obtained by driving first mirror  41   a  has a position coordinate designated as X1, and the Lissajous pattern illustrated in  FIG.  3    and obtained by driving second mirror  42   a  has a position coordinate designated as Y1, position coordinates X1, Y1 are generally expressed by Expressions (1) and (2) below, respectively. 
         X 1= a ×sin( nt )  (1)
 
         Y 1= b ×sin( mt +φ)  (2)
 
     where a is an amplitude of Lissajous pattern illustrated in  FIG.  3    in the X direction, b is an amplitude of the Lissajous pattern illustrated in  FIG.  3    in the Y direction, n is a frequency of first mirror  41   a, m  is a frequency of second mirror  42   a, t  is time, and y is a phase difference when first mirror  41   a  or second mirror  42   a  is driven, specifically, an angle difference provided between first mirror  41   a  and second mirror  42   a  at the time of rotational movement. 
     Position coordinates X1, Y1 indicated in Expressions (1), (2) are expressed by a stationary coordinate system of a Lissajous waveform in a state where laser head  30  is fixed at a position. 
     Frequency n and frequency m correspond to driving frequencies of first mirror  41   a  and second mirror  42   a , respectively. 
     The Lissajous pattern illustrated in  FIG.  3    is an 8-shaped Lissajous pattern corresponding to a case where a=1, b=1, n=2, m=1, and φ=0 in Expressions (1), (2). Amplitudes a and b are normalized by 1. Phase difference φ in Expressions (1), (2) may be any one of 0 degrees and 180 degrees. 
     Here, when a drawing distance of the Lissajous pattern in the X direction at a predetermined time variation Δt is denoted by ΔX, a drawing distance of the Lissajous pattern in the Y direction is denoted by ΔY, and a drawing distance of the Lissajous pattern at the time variation Δt is denoted by ΔL, as illustrated in  FIG.  3   , ΔX, ΔY, ΔL are expressed by Expressions (3) to (5), respectively. 
       Δ X=a×n ×cos( nt )×Δ t   (3)
 
       Δ Y=b×m ×cos( mt +φ)×Δ t   (4)
 
       Δ L=Δt ×{(Δ X ) 2 +(Δ Y ) 2 } 1/2   (5)
 
     Thus, drawing speed V of the Lissajous pattern is expressed by Expression (6) below. 
         V=ΔL/Δt   (6)
 
     [Laser Welding Method] 
       FIG.  4    illustrates a scanning pattern of a laser beam according to the present exemplary embodiment, and  FIG.  5    illustrates a relationship between a drawing speed and an output of the laser beam at a drawing position of the laser beam. The scanning pattern illustrated in  FIG.  4    is identical in shape to the Lissajous pattern illustrated in  FIG.  3   . That is, the scanning pattern illustrated in  FIG.  4    is an 8-shaped Lissajous pattern corresponding to a case where a=1, b=1, n=2, m=1, and φ=0 in Expressions (1), (2) described above. The Lissajous pattern has an actual size, or an amplitude in each of the X direction and the Y direction, being about 1 mm to 10 mm that however may vary depending on the workpiece to be welded. 
       FIG.  5    illustrates drawing speed V of laser beam LB that has a value calculated based on Expression (6) described above. Drawing speed V is normalized with drawing speed of laser beam LB when passing through original point O as 1. Similarly, output P of laser beam LB is normalized with output of laser beam LB when passing through original point O as 1. 
     The present exemplary embodiment allows the surface of workpiece  200  to be irradiated with laser beam LB while laser head  30  is moved at a predetermined speed in the X direction by manipulator  60 . There is described an example in which workpiece  200  is welded by laser welding by further sweeping laser beam LB two-dimensionally using laser scanner  40  to draw the Lissajous pattern illustrated in  FIG.  4    on the surface of workpiece  200 . The pattern illustrated in  FIG.  4    is obtained by sweeping laser beam LB from original point O to pass through drawing positions A, B, C, O, D, E, F, and O in this order during one cycle. 
     When output of laser beam LB is denoted by P in the present exemplary embodiment, output P and drawing speed V of laser beam LB are controlled to allow a relationship between output P and drawing speed V when the Lissajous pattern illustrated in  FIG.  4    is drawn by laser beam LB to satisfy a relationship represented by Expression (7) below. 
         PV =const  (7)
 
     where const. is a constant, and is a value corresponding to a shape of a weld of workpiece  200 , which is an object to be welded, a penetration shape in the weld, or the like. 
     When the Lissajous pattern illustrated in  FIG.  4    is drawn on the surface of workpiece  200  by sweeping laser beam LB, drawing speed V usually decreases in a region where the Lissajous pattern is changed in direction, or in a region near each of drawing positions A, C, D, F, as illustrated in  FIG.  5   . 
     Then, when laser beam LB has output P with an equal value at each drawing position as indicated by a broken line in  FIG.  5   , the amount of heat input per unit drawing length of the Lissajous pattern illustrated in  FIG.  4    differs at each drawing position. For example, the amount of heat input per unit drawing length in a region near drawing position B is smaller than the amount of heat input per unit drawing length in a region near original point O. 
     In this case, a weld bead may not be formed in a favorable shape because the amount of heat input to each irradiated region of laser beam LB during welding is non-uniform as described above to hinder a key hole from having a stable depth to keep a penetration depth constant. This case also may cause a narrow appropriate condition range of drawing speed V and output P of laser beam LB in laser welding. 
     Thus, the present exemplary embodiment allows laser beam LB to be controlled to have drawing speed V and output P satisfying the relationship shown in Expression (7) above. In other words, drawing speed V and output P of a laser beam are controlled to have an equal amount of heat input per unit drawing length in the Lissajous pattern over the entire length of the Lissajous pattern. 
     Specifically, laser beam LB is controlled to have output P increasing as laser beam LB approaches each of drawing positions A, C, D, F where drawing speed V decreases, as indicated by a solid line in  FIG.  5   . Laser beam LB is also controlled to have output P decreasing as laser beam LB separates from each of drawing positions A, C, D, F. 
     Laser beam LB is also controlled to have output P decreasing as laser beam LB approaches each of drawing positions O, B, E where drawing speed V increases. Laser beam LB is also controlled to have output P increasing as laser beam LB separates from each of drawing positions O, B, E. Both drawing speed V and output P continuously change with respect to each drawing position. 
     [Effects and the Like] 
     As described above, the laser welding method according to the present exemplary embodiment includes the welding step of welding workpiece  200  by irradiating the surface of workpiece  200  with laser beam LB that is swept two-dimensionally while being advanced in the X direction (first direction). 
     The welding step is configured to vibrate laser beam LB not only at a first frequency corresponding to frequency n along the X direction in a sinusoidal wave shape, but also at a second frequency corresponding to frequency m along the Y direction in a sinusoidal wave shape. As a result, laser beam LB is swept to draw the Lissajous pattern on the surface of workpiece  200 . 
     Additionally, drawing speed V and output P of laser beam LB are controlled to have an equal amount of heat input per unit drawing length in the Lissajous pattern over the entire length of the Lissajous pattern. 
     The laser welding method of the present exemplary embodiment enables an equal amount of heat input per unit drawing length in the Lissajous pattern to be supplied over the entire length of the Lissajous pattern, so that a penetration depth in a weld can be kept constant by stabilizing a depth of a key hole (not illustrated) in the weld. Additionally, a weld bead can be formed in a favorable shape. Then, a process margin of laser welding can be secured without narrowing an appropriate condition range of drawing speed V and output P of laser beam LB. 
     Laser welding device  100  according to the present exemplary embodiment at least includes laser oscillator  10  that generates laser beam LB, laser head  30  that receives laser beam LB and that applies laser beam LB to workpiece  200 , and controller  50  that controls operation of laser head  30 . 
     Laser head  30  includes laser scanner  40  that sweeps laser beam LB in each of the X-direction (the first direction) and the Y-direction (the second direction) intersecting the X-direction. 
     Controller  50  vibrates laser beam LB not only at the first frequency along the X direction in a sinusoidal wave shape, but also at the second frequency along the Y direction in a sinusoidal wave shape. As a result, controller  50  drives and controls laser scanner  40  to cause laser beam LB to draw a Lissajous pattern on the surface of workpiece  200 . 
     Additionally, controller  50  controls drawing speed V and output P of laser beam LB to have an equal amount of heat input per unit drawing length in the Lissajous pattern over the entire length of the Lissajous pattern. 
     Laser welding device  100  of the present exemplary embodiment enables a penetration depth to be kept constant by stabilizing a depth of a key hole. Additionally, a weld bead can be formed in a favorable shape. Then, a process margin of laser welding can be secured without narrowing an appropriate condition range of drawing speed V and output P of laser beam LB. 
     Laser welding device  100  further includes manipulator  60  to which laser head  30  is attached, and controller  50  controls the operation of manipulator  60 . Manipulator  60  causes laser head  30  to move in a predetermined direction with respect to the surface of workpiece  200 . 
     Providing manipulator  60  in this manner enables changing a welding direction of laser beam LB. Additionally, laser welding can be easily performed on workpiece  200  having a complex shape such as a three-dimensional shape. 
     Laser oscillator  10  and laser head  30  are connected by optical fiber  20 , and laser beam LB is transmitted from laser oscillator  10  to laser head  30  through optical fiber  20 . 
     Providing optical fiber  20  in this manner enables performing laser welding on workpiece  200  disposed at a position away from laser oscillator  10 . As a result, a degree of freedom in placement of each component of laser welding device  100  can be enhanced. 
     Laser scanner  40  includes first galvano-mirror  41  that sweeps laser beam LB in the X-direction, and second galvano-mirror  42  that sweeps laser beam LB in the Y-direction. 
     Laser scanner  40  configured as described above enables sweeping laser beam LB two-dimensionally. The known galvano-scanner is used for laser scanner  40 , and thus increase in cost of laser welding device  100  can be suppressed. 
     Laser head  30  further includes collimation lens  32 , and collimation lens  32  is configured to change a focal position of laser beam LB along the Z direction intersecting each of the X direction and the Y direction. That is, collimation lens  32  in combination with the driver (not illustrated) also functions as a focal position adjustment mechanism for laser beam LB. 
     This configuration enables the focal position of laser beam LB to be easily changed, so that laser beam LB can be appropriately emitted in accordance with a shape of workpiece  200 . 
     Although in the present exemplary embodiment, laser head  30  is moved in the X direction to advance laser beam LB in the X direction, the present invention is not particularly limited thereto. Laser head  30  may be moved in the Y direction to advance laser beam LB in the Y direction. 
     Additionally, a drawing direction of the Lissajous pattern is also not particularly limited to the description above. For example, the Lissajous pattern may be drawn by sweeping laser beam LB from original point O to pass through drawing positions C, B, A, O, F, E, D, and O in this order during one cycle. Alternatively, the Lissajous pattern may be drawn by sweeping laser beam LB from original point O to pass through drawing positions D, E, F, O, A, B, C, and O in this order during one cycle. Additionally, the Lissajous pattern may be drawn by sweeping laser beam LB from original point O to pass through drawing positions F, E, D, O, C, B, A, and O in this order during one cycle. 
     &lt;First Modification&gt; 
       FIG.  6 A  illustrates a first scanning pattern of a laser beam according to the present modification, and  FIG.  6 B  illustrates a second scanning pattern.  FIG.  7 A  illustrates a third scanning pattern of a laser beam according to the present modification,  FIG.  7 B  illustrates a fourth scanning pattern, and  FIG.  7 B  illustrates a fifth scanning pattern.  FIG.  8    illustrates an example of a combination of parameters when a Lissajous pattern is drawn.  FIG.  6 A  and sequent drawings denote the same parts as those of the first exemplary embodiment with the same reference numerals, and details thereof will not be described. 
     Parameters a, b, n, and m shown in Expressions (1) and (2) in actual laser welding can be appropriately changed depending on a material, a joint shape, a required bead shape width, and the like of workpiece  200 . Thus, the scanning pattern of laser beam LB is not particularly limited to the pattern illustrated in  FIG.  4   . 
     For example, parameter a may be decreased to decrease an amplitude of the Lissajous pattern in the X direction, as illustrated in  FIGS.  6 A,  6 B . Frequency n may be set to 1 and frequency m may be set to 2 to form a scanning pattern obtained by rotating the Lissajous pattern illustrated in  FIG.  4    by 90 degrees as illustrated in  FIG.  7 A . Parameter b may be decreased to decrease an amplitude in the Y direction of the Lissajous pattern illustrated in  FIG.  7 A , as illustrated in  FIGS.  7 B,  7 C . 
     Parameters a, b illustrated in Expressions (1), (2), respectively, have values that are not particularly limited to the examples illustrated in  FIGS.  6 A,  6 B  and  FIGS.  7 A to  7 C , and that can be appropriate values within a range illustrated in  FIG.  8   , for example.  FIG.  8    shows a pattern group  1  that is the Lissajous patterns illustrated in  FIG.  4    and  FIGS.  6 A,  6 B , and a pattern group  2  that the Lissajous patterns illustrated in  FIGS.  7 A to  7 C . 
     When a ratio of frequency n of first mirror  41   a  to frequency m of second mirror  42   a , i.e., a ratio of the first frequency being a vibration frequency in the X direction of laser beam LB to the second frequency being a vibration frequency in the Y direction, is set to 2:1 or 1:2, an 8-shaped Lissajous pattern can be obtained. First mirror  41   a  and second mirror  42   a  each have a drive frequency that may be changed depending on a shape of workpiece  200  or a required bead shape as long as the ratio of frequencies is maintained. 
     Second Exemplary Embodiment 
       FIG.  9    illustrates a relationship between a drawing position of a laser beam and drawing speed of the laser beam. 
     As is evident from  FIG.  9   , laser beam LB is controlled in the present exemplary embodiment to have constant drawing speed regardless of a drawing position of laser beam LB. That is, the laser welding method according to the present exemplary embodiment includes the welding step in which drawing speed V of laser beam LB is controlled to be constant over the entire length of the Lissajous pattern. Laser welding device  100  according to the present exemplary embodiment includes controller  50  that controls drawing speed of laser beam LB to be constant over the entire length of the Lissajous pattern. 
     As with the first exemplary embodiment, the present exemplary embodiment allows output P and drawing speed V of laser beam LB to be controlled to satisfy the relationship shown in Expression (7). Thus, the present exemplary embodiment allows drawing speed V and output P of laser beam LB to be each controlled to be constant over the entire length of the Lissajous pattern. However, drawing speed V in this case does not satisfy the relationships shown in Expressions (3) to (6). 
     This control described above enables achieving effects similar to those achieved by the configuration shown in the first exemplary embodiment. That is, the amount of heat input per unit drawing length in the Lissajous pattern can be made equal over the entire length of the Lissajous pattern, so that a penetration depth can be kept constant by stabilizing a depth of the key hole. Additionally, a weld bead can be formed in a favorable shape. Scanning control of laser beam LB is simplified by making drawing speed V and output P of laser beam LB constant over the entire length of the Lissajous pattern. Control of the amount of heat input to workpiece  200  is also facilitated. 
     &lt;Second Modification&gt; 
       FIGS.  10 A to  10 C  illustrate first to third scanning patterns of a laser beam according to the present modification, respectively.  FIGS.  10 A to  10 C  each show arrows that indicate drawing directions of a laser beam LB. 
     The scanning pattern of laser beam LB of the present disclosure is not limited to the Lissajous pattern described in the first exemplary embodiment or the first modification. For example, the scanning pattern may be a composite pattern of two circular patterns disposed symmetrically with respect to the X axis while being in contact with each other at original point O as illustrated in  FIG.  10 A . Alternatively, the scanning pattern may be a composite pattern of two elliptical patterns disposed symmetrically with respect to the X axis while being in contact with each other at original point O as illustrated in  FIG.  10 B . Although  FIG.  10 B  illustrates an example in which each of the two elliptical patterns has a major axis in the Y direction and a minor axis in the X direction, the major axis may be in the X direction and the minor axis may be in the Y direction. As illustrated in  FIG.  10 C , the scanning pattern may be a composite pattern of two rhombus patterns disposed symmetrically with respect to the X axis while being in contact with each other at original point O. Although not illustrated, each of the scanning patterns illustrated in  FIGS.  10 A to  10 C  may be a combined pattern of two annular patterns disposed symmetrically with respect to the Y axis. In this case, each of the two annular patterns may be a pattern rotated by 90 degrees from the examples illustrated in  FIGS.  10 A to  10 C . Each of the two annular patterns can be further appropriately changed in size. 
     That is, the scanning pattern of laser beam LB in the present specification may be a pattern in which two annular patterns are continuous and in contact with each other at one point, and is not limited to the examples illustrated in  FIGS.  10 A to  10 C  and the modifications thereof. These patterns are obtained by driving first mirror  41   a  and second mirror  42   a  according to a predetermined drive pattern. 
     Thus, the laser welding method of the present disclosure includes the welding step in which laser beam LB is swept to draw a predetermined pattern on the surface of workpiece  200 . 
     Additionally, drawing speed V and output P of laser beam LB are controlled to have an equal amount of heat input per unit drawing length in the predetermined pattern over the entire length of the predetermined pattern. 
     Controller  50  in laser welding device  100  of the present disclosure drives and controls laser scanner  40  such that laser beam LB draws a predetermined pattern on the surface of workpiece  200 . 
     Additionally, controller  50  controls drawing speed V and output P of laser beam LB to have an equal amount of heat input per unit drawing length in the predetermined pattern over the entire length of the predetermined pattern. 
     The “predetermined pattern” is the scanning pattern of laser beam LB in which two annular patterns are continuous and in contact with each other at one point, in this case, at original point O. More specifically, the two annular patterns are identical to each other. It is needless to say that the “predetermined pattern” includes the Lissajous pattern disclosed in the present specification. 
     The laser welding method and laser welding device  100  configured as described above enables achieving effects similar to those achieved by the configurations described in the first and second exemplary embodiments and the first modification. 
     Other Exemplary Embodiments 
     Another exemplary embodiment can be formed by appropriately combining components described in the first and second exemplary embodiments and the first and second modifications. 
     For example, when each scanning pattern illustrated in the first and second modifications is drawn, drawing speed V and output P of laser beam LB can be controlled to be constant over the entire length of the predetermined pattern as shown in the second exemplary embodiment. 
     In the first and second modifications, and the second exemplary embodiment, a predetermined pattern may be drawn by sweeping laser beam LB from original point O to pass through drawing positions C, B, A, O, F, E, D, and O in this order during one cycle, for example. Alternatively, the predetermined pattern may be drawn by sweeping laser beam LB from original point O to pass through drawing positions D, E, F, O, A, B, C, and O in this order during one cycle. Additionally, the predetermined pattern may be drawn by sweeping laser beam LB from original point O to pass through drawing positions F, E, D, O, C, B, A, and O in this order during one cycle. 
     Although  FIG.  1    illustrates the example in which condenser lens  34  is disposed at a stage prior to laser scanner  40 , condenser lens  34  may be disposed at a stage subsequent to laser scanner  40 , or a position between laser scanner  40  and a beam emission port of laser head  30 . 
     Laser beam LB may has a scanning pattern of the Lissajous pattern by vibrating laser beam LB not only at a first frequency along the X direction in a cosine wave shape but also at a second frequency along the Y direction in a cosine wave shape. It is needless to say that amplitudes a, b of first mirror  41   a  and second mirror  42   a , frequencies n, m of first mirror  41   a  and second mirror  42   a , and phase φ are appropriately changed in this case. 
     INDUSTRIAL APPLICABILITY 
     The laser welding method and the laser welding method according to the present disclosure are useful because a weld bead can be formed in a favorable shape. 
     REFERENCE MARKS IN THE DRAWINGS 
     
         
         
           
               10  laser oscillator 
               20  optical fiber 
               30  laser head 
               31  housing 
               32  collimation lens 
               33  reflection mirror 
               34  condenser lens 
               40  laser scanner 
               41  first galvano-mirror 
               41   a  first mirror 
               41   b  first rotation shaft 
               41   c  first driver 
               42  second galvano-mirror 
               42   a  second mirror 
               42   b  second rotation shaft 
               42   c  second driver 
               50  controller 
               60  manipulator 
               200  workpiece