Patent Publication Number: US-2021170527-A1

Title: Welding method and welding apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation of International Application No. PCT/JP2019/034848, filed on Sep. 4,2 2019 which claims the benefit of priority of the prior Japanese Patent Application No. 2018-165499, filed on Sep. 4, 2018, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     The present disclosure relates to a welding method and a welding apparatus. 
     Laser welding is known as one of methods of welding metal materials such as iron and copper. The laser welding is a welding method of irradiating laser beam on a welding area of a workpiece and melting the welding area with energy of the laser beam. In the welded area irradiated by the laser beam, a liquid pool of the melted metal material called molten pool is formed. Thereafter, the metal material in the molten pool solidifies, whereby welding is performed. 
     When the laser beam is irradiated on the workpiece, a profile of the laser beam is sometimes shaped according to a purpose of the laser beam irradiation. For example, there has been known a technology for shaping a profile of laser beam when the laser beam is used for cutting a workpiece (see, for example, Japanese Unexamined Patent Application No. 2010-508149). 
     Incidentally, according to investigations by an experiment and the like of the inventors, in the welded workpiece, a shape of a bottom surface of a weld trace, which is the solidified molten pool, is sometimes an unstable shape such as an irregular uneven shape. The unstable shape of the bottom surface of the weld trace is sometimes undesirable depending on a use of welding. 
     SUMMARY 
     The present disclosure has been devised in view of the above, and an object of the present disclosure is to provide a welding method and a welding apparatus that can stabilize a shape of a bottom surface of a weld trace. 
     According to an embodiment, a welding method includes a step of, while irradiating laser beam toward a workpiece, relatively moving the laser beam and the workpiece and, while sweeping the laser beam on the workpiece, melting the workpiece in an irradiated area to perform welding. Further, the laser beam is configured by a main power region and a sub-power region, at least a part of the sub-power region is present on a sweeping direction side of the main power region, a power density of the main power region is equal to or higher than a power density of the sub-power region, and the power density of the main power region is at least power density that can generate a keyhole. 
     According to an embodiment, a welding apparatus includes: a laser system; and an optical head that receives laser beam oscillated by the laser system to generate laser beam, irradiates the generated laser beam toward a workpiece, and melts the workpiece in an irradiated portion to perform welding. Further, the optical head is configured such that the laser beam and the workpiece are capable of relatively moving, the optical head performing the melting to perform welding while sweeping the laser beam on the workpiece, and the laser beam is configured by a main power region and a sub-power region, at least a part of the sub-power region is present on a sweeping direction side, and power density of a main power region is equal to or higher than power density of a sub-power region. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a schematic configuration of a welding apparatus according to a first embodiment; 
         FIG. 2  is a diagram for explaining a concept of a diffractive optical element; 
         FIG. 3  is a diagram illustrating an example of a sectional shape of laser beam; 
         FIG. 4A  is a sectional view illustrating a state in which the laser beam melts a workpiece; 
         FIG. 4B  is a sectional view perpendicular to  FIG. 4A  illustrating the state in which the laser beam melts the workpiece; 
         FIG. 5A  is a sectional view illustrating a state in which laser beam melts the workpiece; 
         FIG. 5B  is a sectional view perpendicular to  FIG. 5A  illustrating the state in which the laser beam melts the workpiece; 
         FIG. 6A  is a diagram illustrating an example of a sectional shape of laser beam; 
         FIG. 6B  is a diagram illustrating an example of a sectional shape of the laser beam; 
         FIG. 6C  is a diagram illustrating an example of a sectional shape of the laser beam; 
         FIG. 6D  is a diagram illustrating an example of a sectional shape of the laser beam; 
         FIG. 6E  is a diagram illustrating an example of a sectional shape of the laser beam; 
         FIG. 6F  is a diagram illustrating an example of a sectional shape of the laser beam; 
         FIG. 6G  is a diagram illustrating an example of a sectional shape of the laser beam; 
         FIG. 7  is a diagram illustrating a schematic configuration of a welding apparatus according to a second embodiment; 
         FIG. 8  is a diagram illustrating a schematic configuration of a welding apparatus according to a third embodiment. 
         FIG. 9  is a diagram illustrating a schematic configuration of a welding apparatus according to a fourth embodiment; 
         FIG. 10  is a diagram illustrating a schematic configuration of a welding apparatus according to a fifth embodiment; 
         FIG. 11  is a diagram illustrating a schematic configuration of a welding apparatus according to a sixth embodiment; 
         FIG. 12A  is a diagram illustrating a configuration example of an optical fiber; 
         FIG. 12B  is a diagram illustrating a configuration example of an optical fiber; 
         FIG. 13  is a diagram illustrating a sectional shape of laser beam used for an experiment; 
         FIG. 14A  is a sectional photograph of a workpiece in a comparative example of No. 1 in Table 1; 
         FIG. 14B  is a sectional photograph of a workpiece in an example of No. 2 in Table 1; 
         FIG. 15A  is a schematic diagram for explaining an example of a power distribution shape of laser beam; and 
         FIG. 15B  is a schematic diagram for explaining an example of a power distribution shape of laser beam. 
     
    
    
     DETAILED DESCRIPTION 
     Welding methods and welding apparatuses according to embodiments of the present disclosure are explained in detail below with reference to the accompanying drawings. Note that the present disclosure is not limited by the embodiments explained below. It is to be noted that the drawings are schematic and relations among dimensions of elements, ratios of the elements, and the like are sometimes different from real ones. Among the drawings, portions where relations and ratios of dimensions thereof are different are sometimes included. 
     First Embodiment 
       FIG. 1  is a diagram illustrating a schematic configuration of a welding apparatus according to a first embodiment. As illustrated in  FIG. 1 , a welding apparatus  100  according to a first embodiment is an example of a configuration of an apparatus that irradiates laser beam L on a workpiece W to melt the workpiece W. As illustrated in  FIG. 1 , a welding apparatus  100  includes a laser system  110  that oscillates laser beam, an optical head  120  that irradiates laser beam on the workpiece W, and an optical fiber  130  that guides the laser beam oscillated by the laser system  110  to the optical head  120 . The workpiece W is configured by at least two members that should be welded. 
     The laser system  110  is configured to be able to oscillate, for example, laser beam in a multi-mode having an output of several kW. For example, the laser system  110  may include a plurality of semiconductor laser elements on the inside and may be configured to be able to oscillate the laser beam in the multi-mode having an output of several kW, which is a total output of the plurality of semiconductor laser elements. Various lasers such as a fiber laser, a YAG laser, and a disk laser may be used. 
     The optical head  120  is an optical device for focusing the laser beam L guided from the laser system  110  to predetermined power density and irradiating the laser beam L on the workpiece W. Therefore, the optical head  120  includes a collimate lens  121  and a focusing lens  122  on the inside. The collimate lens  121  is an optical system for once collimating the laser beam L guided by the optical fiber  130 . The focusing lens  122  is an optical system for focusing the collimated laser beam L on the workpiece W. 
     The optical head  120  is provided to be capable of changing a relative position to the workpiece W in order to move (sweep) an irradiation position of the laser beam L in the workpiece W. A method of changing the relative position to the workpiece W includes moving the optical head  120  itself or moving the workpiece W. That is, the optical head  120  may be configured to be capable of sweeping the laser beam L on the fixed workpiece W. Alternatively, an irradiation position of the laser beam L from the optical head  120  may be fixed and the workpiece W may be held to be capable of moving with respect to the fixed laser beam L. In a process for disposing the workpiece W in a region where the laser beam L is irradiated, at least two members that should be welded are disposed to be placed one on top of the other, in contact with each other, or adjacent to each other. 
     The optical head  120  according to the first embodiment includes a diffractive optical element  123  functioning as a beam shaper between the collimate lens  121  and the focusing lens  122 . The diffractive optical element indicates an optical element  1502  obtained by integrating a plurality of diffraction gratings  1501  having different periods, a concept of which is as illustrated in  FIG. 2 . Laser beam passed through the diffractive optical element  123  are bent in a direction affected by the diffraction gratings and overlap. The laser beam can be formed in any shape. The diffractive optical element  123  may be configured to be rotatably provided. The diffractive optical element  123  may also be configured to be replaceably provided. 
     In this embodiment, the diffractive optical element  123  is for shaping the laser beam L such that a profile concerning a moving direction of power density of the laser beam L on the workpiece W has, further on a moving direction side than a main beam having high power density, a sub-beam having power density equal to or lower than power density of the main beam. 
     The laser beam L shaped by the diffractive optical element  123  is configured by a main beam B 1  having a peak P 1  and two sub-beams B 2  having a peak P 2  as indicated by an example of a sectional shape on a surface perpendicular to a traveling direction of the laser beam L in  FIG. 3 . An arrow v indicates a relative moving direction of the laser beam L with respect to the workpiece W and is equivalent to a sweeping direction. The two sub-beams B 2  are located on sides of the main beam B 1  when the sweeping direction is set as the front. The sides of the main beam B 1  mean regions A 1  and A 2  defined by broken lines passing positions of a beam diameter of the main beam B 1  and parallel to the sweeping direction as illustrated in  FIG. 3 . In an example illustrated in  FIG. 3 , the two sub-beams B 2  are arranged such that a line connecting the centers of beam diameters of the two sub-beams B 2  passes substantially the center of the beam diameter of the main beam B 1  and the line is substantially perpendicular to the sweeping direction. However, the positions of the sub-beams B 2  are not limited to this. If the sub-beams B 2  are located somewhere in the regions A 1  and A 2 , the sub-beams B 2  can be regarded as being located on the sides of the main beam B 1 . The rear of the main beam B 1  is a direction on the opposite side of the moving direction in a region sandwiched by the region A 1  and the region A 2 . 
     Note that the power density of the main beam or the sub-beam is power density in a region including a peak and having strength equal to or more than 1/e 2  of peak strength. A beam diameter of the main beam or the sub-beam is a diameter of the region including the peak and having the strength equal to or more than 1/e 2  of the peak strength. In the case of a beam that is not circular, in this specification, length of a region having the strength equal to or more than 1/e 2  of the peak strength of a longer axis (for example, a major axis) passing near the center of the beam or a shorter axis (for example, a minor axis) in a direction perpendicular to the longer axis (the major axis) is defined as a beam diameter. The beam diameter of the sub-beam may be substantially equal to or larger than the beam diameter of the main beam. Therefore, the area of the sub-beam may be substantially equal to or larger than the area of the main beam. 
     It is preferable that a power distribution of at least the main beam B 1  have a sharp shape to a certain degree. If the power distribution of the main beam B 1  has the sharp shape to a certain degree, penetration depth in melting the workpiece W can be increased. Therefore, welding strength can be secured. When the beam diameter is used as an indicator of sharpness of the main beam B 1 , the beam diameter of the main beam B 1  is preferably 600 μm or less and more preferably 400 μm or less. Note that, when the main beam B 1  has the sharp shape, power for realizing the same penetration depth can be reduced and machining speed can be increased. Accordingly, it is possible to realize a reduction of power consumption of the laser welding apparatus  100  and improvement of machining efficiency. The power distribution of the sub-beams B 2  may be sharp in the same degree as the main beam B 1 . 
     Note that the beam diameter can be adjusted by setting, as appropriate, characteristics of a laser device  110 , the optical head  120 , and the optical fiber  130  in use. For example, the beam diameter can be adjusted by setting of a beam diameter of laser beam input to the optical head  120  from the optical fiber  130  or setting of optical systems such as the diffractive optical element  123  and lenses  121  and  122 . 
     Action of a profile concerning the moving direction of power density of the laser beam L on the workpiece W having, further on the moving direction side than the main beam having high power density, the sub-beam having the power density equal to or lower than the power density of the main beam is not always clarified but is considered to be, for example, as explained below.  FIGS. 4A to 5B  are diagrams illustrating states in which laser beam melts a workpiece. 
       FIG. 4A  illustrates a cross section of the workpiece W along the moving direction (the arrow v) of the laser beam.  FIG. 4B  illustrates a cross section perpendicular to the cross section illustrated in  FIG. 4A . When the laser beam is configured by one beam B, a molten pool WP 1  obtained by melting the workpiece W is formed as a weld region to extend in the opposite direction of the moving direction from a position where the beam B of the laser beam is irradiated. The molten pool WP 1  solidifies to be a weld trace W 1 . A shape of a bottom surface BS 11  of the weld trace W 1  is sometimes an unstable shape such as an irregular uneven shape. 
     A reason for this is not always clarified but is considered that the bottom surface BS 11  of the molten pool WP 1  has an unstable shape and the molten pool WP 1  solidifies to be the weld trace W 1  while keeping reflecting the shape to a certain degree. In this case, the reason is also considered to be that, for example, a liquid surface of the molten pool WP 1  is unstably swayed by energy given by the beam B or a keyhole KH generated by the energy and the bottom surface BS 11  changes to the unstable shape according to the swaying of the liquid surface. 
     In contrast, as illustrated in  FIG. 5A  and  FIG. 5B , in the welding apparatus and the welding method using the welding apparatus according to the first embodiment, the profile concerning the moving direction of the power density of the laser beam L on the workpiece W has the main beam B 1  and the two sub-beams B 2 . The power density of the main beam B 1  is, for example, at least strength that can generate a keyhole. Both of the two sub-beams B 2  have power density lower than the power density of the main beam B 1  and are located on the moving direction sides of the laser beam L with respect to the main beam B 1 . The power density of the sub-beams B 2  is density that can melt the workpiece W under the presence of the main beam B 1  or independently. Therefore, as illustrated in  FIG. 5B , a molten pool WP 2  spreading to positions, where the sub-beams B 2  are irradiated, on sides of a position where the main beam B 1  is irradiated is formed as a weld region. When the power density of the sub-beams B 2  is lower than the power density of the main beam B 1 , a shallow region, which is a region shallower than depth melted by the main beam B 1 , is formed by the sub-beams B 2 . 
     Note that melting strength regions of the main beam B 1  and the sub-beams B 2  may overlap but do not need to always overlap. Molten pools formed by the beams only have to be connected. The melting strength region means a range of a beam of laser beam having power density that can melt the workpiece W around the main beam B 1  or the sub-beams B 2 . 
     In the welding apparatus and the welding method using the welding apparatus according to the first embodiment, a shape of a bottom surface BS 22  of a weld trace W 2 , which is the solidified molten pool WP 2 , is a stable shape having less unevenness and higher flatness than a bottom surface BS 12  by the beam B illustrated in  FIG. 4 . A reason for this is not always clarified but is considered that a bottom surface BS 21  of the molten pool WP 2  has a stabler shape. A reason for this is considered that, for example, since the molten pool WP 2  further spreading to the sides than in the case of  FIG. 4  is formed by the main beam B 1  and the sub-beams B 2 , energy is diffused, the swaying of the liquid surface of the molten pool WP 2  is suppressed or the unstable movement of the keyhole KH is suppressed and the bottom surface BS 21  has the stable shape according to the suppression of the swaying of the liquid surface of the molten pool WP 2  or the suppression of the unstable movement of the keyhole KH. 
     The welding method according to the first embodiment includes a step of disposing the workpiece W in a region where the laser beam L from the laser system  110 , which is a laser device, is irradiated, relatively moving the laser beam L and the workpiece W while irradiating the laser beam L from the laser system  110  toward the workpiece W, and melting the workpiece W in an irradiated portion and performing welding while sweeping the laser beam L on the workpiece W. At this time, the laser beam L is configured by the main beam B 1  and the sub-beams B 2 , at least a part of which is present on sweeping direction sides. Power density of the main beam B 1  is equal to or higher than power density of the sub-beams B 2 . The step of disposing the workpiece W in the region where the laser beam L is irradiated is a step of disposing at least two members to be placed one on top of the other, in contact with each other, or adjacent to each other. 
     Subsequently, an example of a sectional shape of laser beam for a profile concerning a moving direction of power density of the laser beam on a workpiece to have sub-beams on moving direction sides of the main beam is explained with reference to  FIGS. 6A to 6G . The example of the sectional shape of the laser beam illustrated in  FIG. 6  is not an essential configuration. However, a profile of laser beam suitable for carrying out the present invention is realized by designing the diffractive optical element  123  such that the sectional shape of the laser beam illustrated in  FIG. 6  is realized on the surface of the workpiece W. 
       FIG. 6A  is an example in which the sub-beam B 2  having the peak P 2  is arranged on the moving direction left side of the main beam B 1  having the peak P 1 . It is preferable that a region of power density that can melt the workpiece around the sub-beam B 2  is wider concerning the moving direction than a region of power density that can melt the workpiece around the main beam B 1 . Therefore, the sub-beam B 2  may be formed in a shape extending in a certain direction. Such a shape of the sub-beam B 2  may be realized by arranging a plurality of beams close to one another or may be realized by a single beam. 
       FIG. 6B  is an example in which the sub-beams B 2  are respectively arranged on the moving direction left and right sides of the main beam B 1 . 
       FIG. 6C  is an example in which the sub-beams B 2  are not only arranged on the moving direction left side of the main beam B 1  but the sub-beam B 2  having power density lower than the power density of the main beam B 1  is also arranged in the moving direction rear of the main beam B 1 . In this example, even when a moving direction of the main beam B 1  and the sub-beams B 2  changes, the sub-beam B 2  can be arranged on the moving direction side of the main beam B 1 . 
       FIG. 6D  is an example in which the sub-beams B 2  are not only arranged on the moving direction left and right sides of the main beam B 1  but the sub-beam B 2  having power density lower than the power density of the main beam B 1  is also arranged in the moving direction rear of the main beam B 1 . In this example as well, even when the moving direction of the main beam B 1  and the sub-beams B 2  changes, the sub-beams B 2  can be arranged on the moving direction sides of the main beam B 1 . 
       FIG. 6E  is an example in which the sub-beams B 2  having power density lower than the power density of the main beam B 1  are dispersed and arranged around the main beam B 1 . The sub-beams B 2  are arranged to form an arcuate shape, which is a part of a substantial ring shape surrounding the circumference of the main beam B 1 . In the examples illustrated in  FIGS. 6A to 6E , the sub-beams B 2  are formed in a linear shape. However, in  FIG. 6E , the sub-beams B 2  are arranged at close intervals to a certain degree and molten pools formed by the sub-beams B 2  are connected. 
       FIG. 6F  and  FIG. 6G  are examples in which a V-shaped sub-beam B 2  having power density lower than the power density of the main beam B 1  is arranged from the moving direction side to the rear of the main beam B 1 .  FIG. 6F  is an example in which the main beam B 1  and the sub-beam B 2  overlap.  FIG. 6G  is an example in which the main beam B 1  and the sub-beam B 2  do not overlap. 
       FIG. 6C  to  FIG. 6G  are examples in which at least a part of the sub-beam(s) B 2  is present on the side of the main beam B 1  and are examples in which the sub-beam B 2  is arranged only on the moving direction side and in the rear of the main beam B 1 .  FIG. 6A  and  FIG. 6B  are examples in which the sub-beam(s) B 2  is arranged only on the moving direction side of the main beam B 1 . 
     Note that a distance d (for example, illustrated in  FIG. 6A ) between the main beam B 1  and the sub-beam(s) B 2  is the shortest distance between the outer edge of the beam diameter of the main beam B 1  and the outer edge of the beam diameter of the sub-beam B 2 . The distance d only has to be a distance for enabling a molten pool formed by the sub-beam B 2  and a weld region formed by the main beam B 1  in the molten pool to come into contact and is preferably less than ten times, more preferably less than six times, still more preferably less than three times, and yet still more preferably less than one time of the beam diameter of the main beam. 
     The power density of the main beam B 1  and the power density of the sub-beam(s) B 2  may be equal. 
     In the examples in  FIG. 6A  to  FIG. 6G , at least a part of the main beam has a region not overlapping the respective sub-beams 
     Second Embodiment 
       FIG. 7  is a diagram illustrating a schematic configuration of a welding apparatus according to a second embodiment. As illustrated in  FIG. 7 , a welding apparatus  200  according to the second embodiment is an example of a configuration of an apparatus that irradiates the laser beam L on the workpiece W to melt the workpiece W. The welding apparatus  200  according to the second embodiment realizes a welding method according to the same action principle as the action principle of the welding apparatus according to the first embodiment. Therefore, in the following explanation, an apparatus configuration of the welding apparatus  200  is only explained. 
     As illustrated in  FIG. 7 , the welding apparatus  200  includes a laser system  210  that oscillates laser beam, an optical head  220  that irradiates the laser beam on the workpiece W, and an optical fiber  230  that guides the laser beam oscillated by the laser system  210  to the optical head  220 . 
     The laser system  210  is configured to be able to oscillate, for example, laser beam in a multi-mode having an output of several kW. For example, the laser system  210  may include a plurality of semiconductor laser elements on the inside and may be configured to be able to oscillate the laser beam in the multi-mode having an output of several kW, which is a total output of the plurality of semiconductor laser elements. Various lasers such as a fiber laser, a YAG laser, and a disk laser may be used. 
     The optical head  220  is an optical device for focusing the laser beam L guided from the laser system  210  to predetermined power density and irradiating the laser beam L on the workpiece W. Therefore, the optical head  220  includes a collimate lens  221  and a focusing lens  222  on the inside. The collimate lens  221  is an optical system for once collimating the laser beam guided by the optical fiber  230 . The focusing lens  222  is an optical system for focusing the collimated laser beam on the workpiece W. 
     The optical head  220  includes a Galvano scanner between the focusing lens  222  and the workpiece W. The Galvano scanner is a device that can move an irradiation position of the laser beam L without moving the optical head  220  by controlling angles of two mirrors  224   a  and  224   b . In an example illustrated in  FIG. 7 , the optical head  220  includes a mirror  226  in order to guide the laser beam L emitted from the focusing lens  222  to the Galvano scanner. The angles of the mirrors  224   a  and  224   b  of the Galvano scanner are respectively changed by motors  225   a  and  225   b.    
     The optical head  220  according to the second embodiment includes a diffractive optical element  223  between the collimate lens  221  and the focusing lens  222 . The diffractive optical element  223  is for shaping the laser beam L such that a profile concerning a moving direction of power density of the laser beam L on the workpiece W has, on a moving direction side of a main beam, a sub-beam having power density equal to or lower than power density of the main beam. Action of the diffractive optical element  223  is the same as the action in the first embodiment. That is, the diffractive optical element  223  is designed to realize a profile of laser beam suitable for carrying out the present invention like the sectional shape of the laser beam illustrated in  FIG. 6 . 
     Third Embodiment 
       FIG. 8  is a diagram illustrating a schematic configuration of a welding apparatus according to the third embodiment. As illustrated in  FIG. 8 , a welding apparatus  300  according to the third embodiment is an example of a configuration of an apparatus that irradiates the laser beam L on the workpiece W to melt the workpiece W. The welding apparatus  300  according to the third embodiment realizes a welding method according to the same action principle as the action principle of the welding apparatus according to the first embodiment. Components (a laser system  310  and an optical fiber  330 ) other than an optical head  320  are the same as the components in the second embodiment. Therefore, in the following explanation, an apparatus configuration of the optical head  320  is only explained. 
     The optical head  320  is an optical device for focusing the laser beam L guided from the laser system  310  to predetermined power density and irradiating the laser beam L on the workpiece W. Therefore, the optical head  320  includes a collimate lens  321  and a focusing lens  322  on the inside. The collimate lens  321  is an optical system for once collimating the laser beam guided by the optical fiber  330 . The focusing lens  322  is an optical system for focusing the collimated laser beam on the workpiece W. 
     The optical head  320  includes a Galvano scanner between the collimate lens  321  and the focusing lens  322 . Angles of mirrors  324   a  and  324   b  of the Galvano scanner are respectively changed by motors  325   a  and  325   b . In the optical head  320 , the Galvano scanner is provided in a position different from the position in the second embodiment. However, as in the second embodiment, the Galvano scanner can move an irradiation position of the laser beam L without moving the optical head  320  by controlling the angles of the two mirrors  324   a  and  324   b.    
     The optical head  320  according to the third embodiment includes a diffractive optical element  323  between the collimate lens  321  and the focusing lens  322 . The diffractive optical element  323  is for shaping the laser beam L such that a profile concerning a moving direction of power density of the laser beam L on the workpiece W has, on a moving direction side of a main beam, a sub-beam having power density equal to or lower than power density of the main beam. Action of the diffractive optical element  323  is the same as the action in the first embodiment. That is, the diffractive optical element  323  is designed to realize a profile of laser beam suitable for carrying out the present invention like the sectional shape of the laser beam illustrated in  FIG. 6 . 
     Fourth Embodiment 
       FIG. 9  is a diagram illustrating a schematic configuration of a welding apparatus according to a fourth embodiment. As illustrated in  FIG. 9 , a welding apparatus  400  according to the fourth embodiment is an example of a configuration of an apparatus that irradiates laser beams L 1  and L 2  on the workpiece W to melt the workpiece W. The welding apparatus  400  according to the fourth embodiment realizes a welding method according to the same action principle as the action principle of the welding apparatus according to the first embodiment. Therefore, in the following explanation, an apparatus configuration of the welding apparatus  400  is only explained. 
     As illustrated in  FIG. 9 , the welding apparatus  400  includes a plurality of laser systems  411  and  412  that oscillate laser beam, an optical head  420  that irradiates the laser beam on the workpiece W, and optical fibers  431  and  432  that guide the laser beam oscillated by the laser systems  411  and  412  to the optical head  420 . 
     The laser systems  411  and  412  are configured to be able to oscillate, for example, laser beam in a multi-mode having an output of several kW. For example, the laser systems  411  and  412  may include a plurality of semiconductor laser elements on the inside of each of the laser systems  411  and  412  and may be configured to be able to oscillate the laser beam in the multi-mode having an output of several kW, which is a total output of the plurality of semiconductor laser elements. Various lasers such as a fiber laser, a YAG laser, and a disk laser may be used. 
     The optical head  420  is an optical device for focusing the laser beam beams L 1  and L 2  guided from the laser systems  411  and  412  to predetermined power density and irradiating the laser beams L 1  and L 2  on the workpiece W. Therefore, the optical head  420  includes a collimate lens  421   a  and a focusing lens  422   a  for the laser beam L 1  and a collimate lens  421   b  and a focusing lens  422   b  for the laser beam L 2 . The collimate lenses  421   a  and  421   b  are respectively optical systems for once collimating the laser beam guided by the optical fibers  431  and  432 . The focusing lenses  422   a  and  422   b  are optical systems for focusing the collimated laser beam on the workpiece W. 
     The optical head  420  according to the fourth embodiment is also configured such that a profile concerning a moving direction of power density of the laser beams L 1  and L 2  on the workpiece W has, on a moving direction side of a main beam, a sub-beam having power density equal to or lower than power density of the main beam. That is, for example, of the laser beams L 1  and L 2  irradiated on the workpiece W by the optical head  420 , the laser beam L 1  only has to be used for main beam formation and the laser beam L 2  only has to be used for sub-beam formation. Note that, in an example illustrated in the figure, only the laser beams L 1  and L 2  are used. However, the number of laser beam may be increased. The optical head  420  only has to be configured to realize a profile of laser beam suitable for carrying out the present invention like the sectional shape of the laser beam illustrated in  FIG. 6 . Note that wavelengths of the laser beams L 1  and L 2  may be the same or may be different from each other. Similarly, when the number of laser beams is three or more, wavelengths of at least two laser beams among the laser beams may be different from each other or wavelengths of all the laser beams may be the same. A wavelength of laser beams forming at least the sub-beam of the main beam and the sub-beam may be a wavelength having reflectivity lower than reflectivity of an infrared region of the workpiece, for example, a wavelength of a visible region. 
     Fifth Embodiment 
       FIG. 10  is a diagram illustrating a schematic configuration of a welding apparatus according to a fifth embodiment. As illustrated in  FIG. 10 , a welding apparatus  500  according to the fifth embodiment is an example of a configuration of an apparatus that irradiates the laser beams L 1  and L 2  on the workpiece W to melt the workpiece W. The welding apparatus  500  according to the fifth embodiment realizes a welding method according to the same action as the action of the welding apparatus according to the first embodiment. Therefore, in the following explanation, an apparatus configuration of the welding apparatus  500  is only explained. 
     As illustrated in  FIG. 10 , the welding apparatus  500  includes a laser system  510  that oscillates laser beam, an optical head  520  that irradiates the laser beam on the workpiece W, and optical fibers  531 ,  533 , and  534  that guide the laser beam oscillated by the laser system  510  to the optical head  520 . 
     In the fifth embodiment, the laser system  510  is, for example, a fiber laser, a YAG laser, or a disk laser and is used to oscillate both of the laser beams L 1  and L 2  irradiated on the workpiece W. Therefore, a dividing unit  532  is provided between the optical fiber  531  and the optical fibers  533  and  534  that guide the laser beam oscillated by the laser system  510  to the optical head  520  and is configured to divide the laser beam oscillated by the laser system  510  and then guide the laser beam to the optical head  520 . 
     The optical head  520  is an optical device for focusing the laser beams L 1  and L 2  divided by the dividing unit  532  to predetermined power density and irradiating the laser beams L 1  and L 2  on the workpiece W. Therefore, the optical head  520  includes a collimate lens  521   a  and a focusing lens  522   a  for the laser beam L 1  and a collimate lens  521   b  and a focusing lens  522   b  for the laser beam L 2 . The collimate lenses  521   a  and  521   b  are respectively optical systems for once collimating the laser beam guided by the optical fibers  533  and  534 . The focusing lenses  522   a  and  522   b  are optical systems for focusing the collimated laser beam on the workpiece W. 
     The optical head  520  according to the fifth embodiment is also configured such that a profile concerning a moving direction of power density of the laser beams L 1  and L 2  on the workpiece W has, on a moving direction side of a main beam, a sub-beam having power density equal to or lower than power density of the main beam. That is, for example, of the laser beams L 1  and L 2  irradiated on the workpiece W by the optical head  520 , the laser beam L 1  only has to be used for main beam formation and the laser beam L 2  only has to be used for sub-beam formation. Note that, in an example illustrated in the figure, only the laser beams L 1  and L 2  are used. However, the number of laser beams may be increased. The optical head  420  only has to be configured to realize a profile of laser beam suitable for carrying out the present invention like the sectional shape of the laser beam illustrated in  FIG. 6 . 
     Sixth Embodiment 
       FIG. 11  is a diagram illustrating a schematic configuration of a welding apparatus according to a sixth embodiment. As illustrated in  FIG. 11 , a welding apparatus  600  according to the sixth embodiment is an example of a configuration of an apparatus that irradiates the laser beam L on the workpiece W to melt the workpiece W. The welding apparatus  600  according to the sixth embodiment realizes a welding method according to the same action principle as the action principle of the welding apparatus according to the first embodiment. Therefore, in the following explanation, an apparatus configuration of the welding apparatus  600  is only explained. 
     As illustrated in  FIG. 11 , the welding apparatus  600  includes a plurality of laser systems  611  and  612  that oscillate a laser beam, which is, for example, a fiber laser, a YAG laser, or a disk laser, an optical head  620  that irradiates the laser beam on the workpiece W, and optical fibers  631 ,  632 , and  635  that guide the laser beam oscillated by the laser systems  611  and  612  to the optical head  620 . 
     In the sixth embodiment, the laser beams oscillated by the laser systems  611  and  612  are combined before being guided to the optical head  620 . Therefore, a combining unit  634  is provided between the optical fibers  631  and  632  and the optical fiber  635  that guide the laser beam oscillated by the laser systems  611  and  612  to the optical head  620 . The laser beam oscillated by the laser systems  611  and  612  are guided in the optical fiber  635  in parallel. 
     Configuration examples of the optical fiber  631  (and  632 ) and the optical fiber  635  are explained with reference to  FIGS. 12A and 12B . As illustrated in  FIG. 12A , the optical fiber  631  (and  632 ) is a normal optical fiber. That is, the optical fiber  631  (and  632 ) is an optical fiber in which, around one core Co, a clad Cl having a refractive index lower than the refractive index of the core Co is formed. On the other hand, as illustrated in  FIG. 12B , the optical fiber  635  is an optical fiber of a so-called multi-core. That is, the optical fiber  635  includes two cores Co 1  and Co 2 . Around the two cores Co 1  and Co 2 , the clad Cl having a refractive index lower than the refractive index of the cores Co 1  and Co 2  is formed. In the combining unit  634 , the core Co of the optical fiber  631  and the core Co 1  of the optical fiber  635  are combined and the core Co of the optical fiber  632  and the core Co 2  of the optical fiber  635  are combined. 
     Referring back to  FIG. 11 , The optical head  620  is an optical device for focusing the laser beam L combined by the combining unit  634  to predetermined power density and irradiating the laser beam L on the workpiece W. Therefore, the optical head  620  includes a collimate lens  621  and a focusing lens  622  on the inside. 
     In this embodiment, the optical head  620  does not include a diffractive optical element and does not include an independent optical system for a plurality of laser beams either. However, since the laser beams oscillated by the laser systems  611  and  612  are combined before being guided to the optical head  620 , the optical head  620  is configured such that a profile concerning a moving direction of power density of the laser beam L on the workpiece W has, on a moving direction side of a main beam, a sub-beam having power density equal to or lower than power density of the main beam. 
     Note that, concerning all the embodiments in this specification, a welding form of the main beam may be keyhole-type welding or may be thermal conduction-type welding. The keyhole-type welding is a welding method using a keyhole. On the other hand, the thermal conduction-type welding is a welding method for melting the workpiece W using heat generated by laser beam being absorbed on the surface of a preform. 
     EXPERIMENT EXAMPLES 
     Subsequently, experiment examples are explained. In the experiment examples, an apparatus configuration of an example was the configuration of the welding apparatus  100  according to the first embodiment and, as an apparatus configuration of a comparative example, a configuration obtained by excluding the diffractive optical element  123  from the welding apparatus  100  was used. Note that, as common experiment conditions, an output of the laser system  110  was set to 3 kW and relative moving speed of the optical head  120  and the workpiece W was set to 5 m/minute. 
     The diffractive optical element  123  is configured such that, as illustrated in  FIG. 13 , laser beam is formed by the main beam B 1  having the peak P 1  and the sub-beam B 2  having the peak P 2  and a sectional shape of arcuate laser beam, which is a part of a ring shape of the sub-beam B 2  surrounding the circumference of the main beam B 1 , is irradiated on the workpiece W. An experiment was performed by moving the laser beam shaped by the diffractive optical element  123  in a moving direction indicated by an arrow v in the figure. In the apparatus configuration of the comparative example, laser beam obtained by deleting an arcuate portion from the sectional shape of the laser beam illustrated in  FIG. 13  is irradiated on the workpiece W. 
     Table 1 illustrates two experiment examples. A material of a workpiece is SUS304 having thickness of 10 mm. DOE is a diffractive optical element. A focal position is focal positions of a main beam and a sub-beam and is just-focus on a surface. A setting output is power of laser beam output from a laser system. Speed is sweeping speed. The inventors cut workpieces of the experiment examples and observed shapes of bottom surfaces of weld traces. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Material 
                   
                 Focal 
                 Setting 
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Experiment 
                   
                 position 
                 output 
                 Speed 
                 Bottom 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 No. 
                 Material 
                 DOE 
                 [mm] 
                 [W] 
                 mm/s 
                 m/min 
                 surface 
               
               
                   
               
               
                 1 
                 SUS304 
                 Absent 
                 Surface 
                 3000 
                 83.3 
                 5.0 
                 Unstable 
               
               
                   
                 10 mm 
                   
                 JF 
               
               
                 2 
                 SUS304 
                 DOE is 
                 Surface 
                 3000 
                 83.3 
                 5.0 
                 Stable 
               
               
                   
                 10 mm 
                 present 
                 JF 
               
               
                   
               
            
           
         
       
     
       FIG. 14A  is a photograph illustrating a cross section of a workpiece in a comparative example of an experiment No. 1 in Table 1.  FIG. 14B  is a photograph illustrating a cross section of a workpiece in an example of an experiment No. 2 in Table 1. As illustrated in  FIG. 14A , in the comparative example of the experiment No. 1, a bottom surface had an unstable shape having unevenness. In contrast, as illustrated in  FIG. 14B , in the example of the experiment No. 2, a bottom surface had a flat stable shape. If  FIG. 14A  and  FIG. 14B  are compared, it could be understood that flatness and stabilization of a shape can be achieved by the welding apparatus  100  according to the first embodiment. 
     In the embodiments, the profile (a power distribution shape) of the laser beam has a discrete power region configured by the main beam and the sub-beam. The power region is a region having power contributing to melting of the workpiece in a plane perpendicular to a laser-beam traveling direction of the laser beam. However, individual power regions do not always need to independently have power that can melt the workpiece. The power regions only have to be able to melt the workpiece with the influence of energy given to the workpiece by the other power regions. 
     In the example explained above, the sub-power region was configured by nine beams. When a ratio of power of the main power region and power of the sub-power region was changed from 6:4 to 1:9, the bottom surface had the flat stable shape at both the ratios. When the ratio is 6:4, a ratio of the power of the main beam and the power of one sub-beam is 6:4/9=27:2. When the ratio is 1:9, a ratio of the power of the main beam and the power of one sub-beam is 1:9/9=1:1. 
     Under the same conditions as the conditions in the example, the sub-power region was configured by twenty-one beams, a ratio of the power of the main power region and the power of the sub-power region was set to 10:21, and an experiment was performed. In this case as well, the bottom surface had the flat stable shape. A ratio of the power of the main beam and the power of one sub-beam is 10:21/21=10:1. 
     However, the power region is not limited to the discrete power region. A plurality of power regions may be continuous in a symmetrical or asymmetrical distribution. For example,  FIG. 15A  illustrates an example of a power distribution shape in a side direction of laser beam L 12  different from a power distribution shape of the laser beam L. In the power distribution shape of the laser beam L 12 , arranged two power regions PA 121  and PA 122  are continuous in the side direction. The power region PA 121  has a unimodal shape having a peak and is, for example, a main power region. The power region PA 122  has a shoulder-like shape and is, for example, a sub-power region. A boundary between the two power regions PA 121  and PA 122  in a curve illustrated in  FIG. 15A  can be specified as, for example, a position of an inflection point present between the power regions PA 121  and PA 122 . 
     On the other hand,  FIG. 15B  illustrates another example of a power distribution shape in a side direction of laser beam L 13  different from the power distribution shape of the laser beam L. In the power distribution shape of the laser beam L 13 , arranged two power regions PA 131  and PA 132  are continuous. Both of the power regions PA 131  and PA 132  have a unimodal shape having a peak and are respectively, for example, a main power region and a sub-power region. A boundary between the two power regions PA 131  and PA 132  in a curve illustrated in  FIG. 15B  can be specified as, for example, a position of a minimum point present between the power regions PA 131  and PA 132 . Both of the laser beams L 12  and L 13  can be applied as the laser beam configured by the main power region and the sub-power region in the present invention. The laser beams L 12  and L 13  can be realized by using, as a beam shaper, for example, an optical component such as a properly designed diffractive optical element or optical lens or an optical fiber that can control a power distribution. 
     Note that a welding technology that can stabilize a shape of a bottom surface of a weld trace as in the embodiments can be suitably applied to, for example, three-dimensional molding. That is, in the three-dimensional molding, when a material is melted, solidified, and deposited by laser welding to form a three-dimensional shape, if an interface equivalent to the bottom surface of the weld trace is stable, it is possible to obtain various suitable effects such as improvement of accuracy of the three-dimensional molding. 
     When laser beam is swept on a workpiece, the sweeping may be performed by publicly-known wobbling, weaving, output modulation, or the like to stabilize a molten pool. 
     The present invention are explained above based on the embodiments. However, the present invention is not limited by the embodiments. Components configured by combining, as appropriate, the components in the embodiments explained above are also included in the category of the present invention. Further effects and modifications can be easily derived by those skilled in the art. Accordingly, a wider aspect of the present invention is not limited by the embodiments and various changes are possible. 
     INDUSTRIAL APPLICABILITY 
     The present invention can be used for laser welding. The welding method and the welding apparatus according to the present disclosure achieves an effect that it is possible to stabilize a shape of a bottom surface of a weld trace. 
     Although the disclosure has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.