Patent Publication Number: US-2021162539-A1

Title: Welding method and welding apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation of International Application No. PCT/JP2019/035075, filed on Sep. 5, 2019, which claims the benefit of priority of the prior Japanese Patent Application No. 2018-166399, filed on Sep. 5, 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 the methods for welding a workpiece made of metal material such as iron or copper. Laser welding is a welding method in which a welding area of the workpiece is irradiated with a laser beam and the welding area is melted by the energy of the laser beam. A pool of molten metal material called a molten pool is formed in the welding area irradiated with the laser beam, and then welding is performed by solidifying the molten pool. 
     In some cases, two plate materials are layered to form a workpiece, and the plate materials are joined by welding to perform lap welding. In this case, when the plate material is a plating plate material in which a plating layer is formed on the surface of the preform, for example, a galvanized steel plate, the plating layer evaporates to become a gas when the steel material melts. This occurs when the boiling point of the plating layer is lower than the melting point of the preform. The gas generated in this way may disturb the molten pool and deteriorate the flatness of the surface of the molten pool. Such deterioration of the flatness of the surface of the molten pool causes welding defect. In addition, since part of the molten liquid constituting the molten pool is blown off by the pressure of this gas, an underfill may occur in the workpiece after the molten pool is solidified. 
     To solve the problem of melting defect as described above, a technique is disclosed in which a protrusion is formed on a first plated steel plate, and the top of the protrusion is brought into contact with the surface of a second plated steel plate when the first and second plated steel plates are layered, and the first plated steel plate is irradiated with a laser beam from the surface opposite the top of the protrusion to weld the first and second plated steel plates (see, for example, Japanese Laid-open Patent Publication No. H07-155974). 
     However, the above-mentioned technique has a problem that an additional process of forming a protrusion on one of the plated steel plates is required. 
     SUMMARY 
     There is a need for providing a welding method and a welding apparatus capable of suppressing the occurrence of the welding defect during lap welding of plating plate materials. 
     According to an embodiment, a welding method includes: layering two or more plate materials each including a plating plate material having a preform on a surface of which a plating layer is formed to form a workpiece; disposing the workpiece in a region to be irradiated with a processing laser beam; generating the processing laser beam having a power distribution shape in which two or more power regions are disposed along a predetermined direction in a plane perpendicular to a light traveling direction; irradiating a surface of the workpiece with the processing laser beam; and moving the processing laser beam and the workpiece relatively while performing the irradiation, and melting an irradiated area of the workpiece to perform welding while sweeping the processing laser beam in the predetermined direction on the workpiece during a swing of the processing laser beam. 
     According to an embodiment, a welding apparatus includes: a laser system; and an optical head that irradiates a workpiece with a processing laser beam generated so as to have a power distribution shape in which two or more power regions are disposed along a predetermined direction in a plane perpendicular to a light traveling direction from a laser beam output from the laser system to melt an irradiated area of the workpiece to perform welding. Further, the workpiece has a configuration in which two or more plate materials each including a plating plate material having a preform on a surface of which a plating layer is formed are layered, and the optical head has a configuration in which the processing laser beam and the workpiece are capable of moving relatively to perform the melting to perform the welding while sweeping the processing laser beam on the workpiece in the predetermined direction while swinging the processing laser beam. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a schematic configuration of a laser welding apparatus according to a first embodiment; 
         FIG. 2  is a schematic diagram illustrating a diffractive optical element; 
         FIG. 3A  is a schematic diagram illustrating a processing laser beam and swing; 
         FIG. 3B  is a diagram illustrating an example of power distribution in the radial direction of the cross section of each beam included in the processing laser beam; 
         FIG. 4A  is a diagram illustrating a cross-sectional photograph of a comparative example of a welded workpiece. 
         FIG. 4B  is a diagram illustrating a cross-sectional photograph of an example of the welded workpiece; 
         FIG. 5A  is a schematic diagram illustrating an example of the power distribution shape of the processing laser beam; 
         FIG. 5B  is a schematic diagram illustrating an example of the power distribution shape of the processing laser beam; 
         FIG. 5C  is a schematic diagram illustrating an example of the power distribution shape of the processing laser beam; 
         FIG. 5D  is a schematic diagram illustrating an example of the power distribution shape of the processing laser beam; 
         FIG. 5E  is a schematic diagram illustrating an example of the power distribution shape of the processing laser beam; 
         FIG. 5F  is a schematic diagram illustrating an example of the power distribution shape of the processing laser beam; 
         FIG. 6A  is a schematic diagram illustrating an example of the power distribution shape of the processing laser beam; 
         FIG. 6B  is a schematic diagram illustrating an example of the power distribution shape of the processing laser beam; 
         FIG. 6C  is a schematic diagram illustrating an example of the power distribution shape of the processing laser beam; 
         FIG. 6D  is a schematic diagram illustrating an example of the power distribution shape of the processing laser beam; 
         FIG. 6E  is a schematic diagram illustrating an example of the power distribution shape of the processing laser beam; 
         FIG. 7  is a schematic diagram illustrating a schematic configuration of a laser welding apparatus according to a second embodiment; 
         FIG. 8A  is a schematic diagram illustrating an example of a power distribution shape of a processing laser beam. 
         FIG. 8B  is a schematic diagram illustrating an example of the power distribution shape of the processing laser beam; 
         FIG. 9A  is a schematic diagram illustrating an example of a swing of the processing laser beam; and 
         FIG. 9B  is a schematic diagram illustrating an example of the swing of the processing laser beam. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The present disclosure is not limited to the embodiments described below. Further, in the description of the drawings, the same or corresponding elements are appropriately assigned the same reference numerals. 
     First Embodiment 
       FIG. 1  is a schematic diagram illustrating a schematic configuration of the laser welding apparatus according to a first embodiment. A laser welding apparatus  100  includes a laser system  110 , an optical head  120 , and an optical fiber  130  that connects the laser system  110  and the optical head  120 . Further, a workpiece W has a configuration in which two galvanized steel plates W 1  and W 2 , which are plating plate materials, are layered. The galvanized steel plate W 1  has galvanized layers formed on both surfaces of a steel plate, which is a preform. The galvanized steel plate W 2  also has galvanized layers formed on both surfaces of a steel plate, which is a preform. The galvanized layer has a boiling point lower than the melting point of the steel plate. 
     The laser system  110  is configured to be capable of outputting, for example, a laser beam having a power of several kW. For example, the laser system  110  may include a plurality of semiconductor laser elements inside, and may be configured to be able to output a multi-mode laser beam having a power of several kW as the total output of the plurality of semiconductor laser elements. Further, the laser system  110  may include various laser beam sources such as a fiber laser, a YAG laser, and a disk laser. The optical fiber  130  guides the laser beam output from the laser system  110  and inputs the laser beam to the optical head  120 . 
     The optical head  120  is an optical device that radiates the laser beam input from the laser system  110  toward the workpiece W. The optical head  120  includes a collimating lens  121 , a condensing lens  122 , and a diffractive optical element  123 . The collimating lens  121  is an optical system that converts the input laser beam into collimated light. The diffractive optical element  123  is an optical element that forms a beam shape of laser beams that have been collimated as described later to generate a processing laser beam L, and is an aspect of a beam shaper. The condensing lens  122  is an optical system that condenses the processing laser beam L, with which the workpiece W is irradiated. 
     The optical head  120  has a galvano scanner mounted between the condensing lens  122  and the workpiece W in order to sweep the processing laser beam L while irradiating the workpiece W with the processing laser beam L. The galvano scanner is a device that can move the position which is irradiated with the processing laser beam L on the workpiece W without moving the optical head  120  by controlling the angles of two mirrors  124   a  and  124   b  to sweep the processing laser beam L. The optical head  120  includes a mirror  126  that guides the processing laser beam L emitted from the condensing lens  122  to the galvano scanner. The angles of the mirrors  124   a  and  124   b  of the galvano scanner are changed by motors  125   a  and  125   b , respectively. The motors  125   a  and  125   b  are driven by a driver (not illustrated). 
     As illustrated conceptually in  FIG. 2 , the diffractive optical element  123  integrally includes a plurality of diffraction gratings  123   a  having different periods. The diffractive optical element  123  can form a beam shape by bending or superimposing the input laser beam in the direction affected by each diffraction grating. 
     The diffractive optical element  123  divides the laser beam input from the collimating lens  121  into a plurality of beams (three in the first embodiment) to generate the processing laser beam L. The processing laser beam L has a power distribution shape in which three beams are disposed along a predetermined direction in a plane perpendicular to the light traveling direction. These beams constitute a power region in the processing laser beam L. The power region is a region having power that contributes to melting of the workpiece W in a plane perpendicular to the light traveling direction of the processing laser beam L. However, it is not always necessary for each power region to have the power to melt the workpiece W independently. It is sufficient that each power region can melt the workpiece W by the influence of the energy given to the workpiece W by the other power regions. 
     As illustrated in  FIG. 3A , the processing laser beam L includes three beams B 1 , B 2 , and B 3  whose light traveling direction is perpendicular to the paper surface. Each of the beams B 1 , B 2 , and B 3  has a Gaussian-shaped power distribution in the radial direction of the beam cross section, as illustrated in  FIG. 3B . The beams B 1 , B 2 , and B 3  are disposed linearly along a welding direction WD, which is a predetermined direction. The line connecting the beams B 1 , B 2 , and B 3  extends in the welding direction WD. Here, the welding direction WD is a direction in which the workpiece W is welded, and corresponds to the direction in which the welding bead is formed. Further, the Gaussian shape in the present description is not limited to a shape that is exactly a Gaussian shape, but includes a shape that can be approximated by a Gaussian shape. 
     The beam diameter of each of the beams B 1 , B 2 , and B 3  is defined as a diameter of a region having a peak and an intensity of 1/e 2  or more of the peak intensity. In the case of a non-circular beam, the length of a region having an intensity of 1/e 2  or more of the peak intensity in a direction perpendicular to the sweep direction is defined as a beam diameter in this description. Beam power is defined as the sum of the power in the region where the intensity is 1/e 2  or more of the peak intensity. 
     Further, it is preferable that the power distribution of the beams B 1 , B 2 , and B 3  have a sharp shape to some extent. Since the melting depth when melting the workpiece W can be deepened when the beams B 1 , B 2 , and B 3  have a sharp power distribution to some extent, welding strength can be secured, so that the occurrence of the welding defect can be more preferably suppressed. When the beam diameter is used as an index of the sharpness of the beams B 1 , B 2 , and B 3 , the beam diameters of the beams B 1 , B 2 , and B 3  are preferably 600 μm or less, more preferably 400 μm or less. When the beams B 1 , B 2 , and B 3  have a sharp shape, the power for achieving the same melting depth can be reduced and the processing speed can be increased. Therefore, it is possible to reduce the power consumption of the laser welding apparatus  100  and improve the processing efficiency. 
     Note that the beam diameter can be adjusted by appropriately setting the characteristics of the laser system  110 , the optical head  120 , and the optical fiber  130  to be used. For example, the beam diameter can be adjusted by setting the beam diameter of the laser beam input from the optical fiber  130  to the optical head  120 , and setting the optical system such as the diffractive optical element  123  and the lenses  121  and  122 . 
     When welding is performed by using the laser welding apparatus  100 , first, the workpiece W having a configuration in which the galvanized steel plates W 1  and W 2  are layered is disposed in the region irradiated with the processing laser beam L. Subsequently, while irradiating the workpiece W with the processing laser beam L including the beams B 1 , B 2 , and B 3  divided by the diffractive optical element  123 , the processing laser beam L and the workpiece W are relatively moved to sweep the processing laser beam L, and a portion, of the workpiece W, irradiated with the processing laser beam L is melted and welded. 
     In the first embodiment, while the processing laser beam L is swept in the welding direction WD on the workpiece W, while being swung. Specifically, as illustrated in  FIG. 3A , the processing laser beam L is swept in the welding direction WD while drawing a circle at a predetermined rotation frequency on the workpiece W. At this time, in the first embodiment, the positional relationship between the beams B 1 , B 2 , and B 3  and the arrangement of them with respect to the welding direction WD are not changed. As a result, a locus T drawn by the processing laser beam L on the workpiece W is trochoidal. In the present description, when the processing laser beam L draws a trochoidal locus T on the workpiece W in this way, the processing laser beam L is said to wobble. A radius in the direction (hereinafter, appropriately referred to as the welding width direction) perpendicular to the welding direction WD of the circular portion in the locus T is represented by R 1 , and a radius in the welding direction WD is represented by R 2 . 
     In this way, when the processing laser beam L in which the three beams B 1 , B 2 , and B 3  are disposed along the welding direction WD is swept while being swung in a manner of wobbling, the beam B 1 , the beam B 2 , and the beam B 3  generate a molten pool that is gradually deepened in the workpiece W. That is, first, the beam B 1  generates a molten pool with a certain depth, the beam B 2  deepens the molten pool, and the beam B 3  further deepens the molten pool. At the same time, the molten pool expands in the welding width direction due to the swing of the processing laser beam L. 
     Therefore, when the processing laser beam L is radiated, the molten pool is formed so as to be deepened relatively slowly in a relatively wide region on the surface of the workpiece W. Therefore, the galvanized layers formed on the galvanized steel plates W 1  and W 2  gradually are evaporated and gasified rather than rapidly. As a result, the generated gas does not disturb the molten pool, and the deterioration of flatness of the surface of the molten pool is also reduced. Also, melting occurs slowly in a relatively large region. For this reason, a gas discharge route is secured so that the gradually generated gas is sufficiently discharged from the surface of the molten pool, which has a relatively large surface area, and bubbles are less likely to remain after the molten pool has been solidified. As a result, the occurrence of the welding defect can be suppressed. 
     Specifically, since the layered surface of the galvanized steel plate W 1  and the galvanized steel plate W 2  located inside the workpiece W forms two layers with the galvanized layers formed on each surface being layered to, the amount of gas generated is large when the galvanized layers are evaporated. However, when the processing laser beam L is swept while being swung in a manner of wobbling, the gas is gradually generated and its discharge route is secured, so that it is possible to suppress the occurrence of the welding defect. 
     Further, when the molten pool reaches the back side of the workpiece W (the exposed surface of the galvanized steel plate W 2 ) by the processing laser beam L, the gas can be discharged from the surface of the molten pool toward the back face, so that the occurrence of the welding defect can be further suppressed. 
     Note that, when, instead of the processing laser beam L as illustrated in  FIG. 3A , the processing laser beam consisting of a single Gaussian beam having a peak with a relatively sharp power distribution is swept while being swung, the temperature rises sharply in a relatively narrow region on the surface of the workpiece W, and melting occurs to form the molten pool which is sharply deepened. Therefore, the galvanized layer in which the two layers overlap evaporates rapidly and sometimes explosively in a short time and gasifies. The generated gas may disturb the molten pool and deteriorate the flatness of the surface of the molten pool. Such deterioration of the flatness of the surface of the molten pool causes the welding defect such as abnormal shape of the welding bead. Further, since melting occurs rapidly in a relatively narrow region, the surface area of the molten pool is relatively small with respect to the amount of gas generated rapidly. Therefore, the gas may not be sufficiently discharged from the surface of the molten pool to the outside, and the welding defect such as bubbles remaining after the molten pool is solidified is likely to occur. Further, when the processing laser beam L as illustrated in  FIG. 3A  is swept linearly without being swung, the melting defect is likely to occur, in the same way. 
     Therefore, in the first embodiment, the surface area of the molten pool formed by melting the workpiece W can be increased by swinging the processing laser beam L having a predetermined power distribution shape, compared with the surface area of the molten pool when no swing is performed. As a result, the gas generated when the galvanized layer located inside the workpiece W is evaporated can be suitably discharged from the surface of the molten pool. 
     To suppress the welding defect even more effectively, for the beams B 1 , B 2 , B 3 , it is preferable to disperse the position which is irradiated with the beams B 1 , B 2 , and B 3  so that the gas is gradually and sufficiently discharged from the surface of the molten pool. Specifically, it is preferable that the power distribution shape of the processing laser beam L be set so that the welding defect generated due to the gas has a level equal to or less than an allowable level to disperse the position which is irradiated with the beams B 1 , B 2 , and B 3  so as to have the power distribution shape. That is, the power distribution shape of the processing laser beam L can be set by adjusting the power and the arrangement interval of the beams B 1 , B 2 , and B 3 . Here, the allowable level means a level that is allowable according to, for example, the required specifications for welding. Further, it is preferable to set the mode of the swing of the processing laser beam L so that the welding defect has a level equal to or less than the allowable level. In the case of wobbling, the mode of the swing is determined by the radii R 1 , R 2 , rotation frequency and the like. Further, it is preferable to set at least one of the power distribution shape and the swing mode. 
     To suppress the welding defect even more effectively, it is preferable to set at least one of the power distribution shape and the swing mode of the processing laser beam L depending on the characteristics of the workpiece W (material, base material thickness, plating layer thickness, etc.). 
     Examples 1 to 5, Comparative Example 
     As an Example 1, a laser welding apparatus having the configuration illustrated in  FIG. 1  was used to perform lap welding on a workpiece having a configuration in which two 1 mm thick electrogalvanized steel plates are layered. The laser beam output from the laser system has a Gaussian distribution, and its wavelength is 1070 nm. The power of the laser beam was set to 3 kW. Then, this laser beam was divided into three beams disposed linearly as illustrated in  FIG. 3A  by a diffractive optical element to generate a processing laser beam, and the workpiece was irradiated with the processing laser beam. Here, the rotation frequency when drawing a circle with the laser beam was 64 Hz, the radius R 1  in the welding width direction was 0.5 mm, and the radius R 2  in the welding direction was 0.5 mm. The sweeping speed of the laser beam for the workpiece in the welding direction was set to 20 mm/s. Further, a spacing X between the three beams was adjusted so that they were evenly spaced on the surface of the workpiece. The beam spacing X is the peak intensity spacing (see  FIG. 3A ). 
     Also, as a comparative example, a laser welding apparatus having a configuration in which the diffractive optical element is removed from the configuration illustrated in  FIG. 1  was used to perform lap welding on a workpiece having a configuration in which two 1 mm thick electrogalvanized steel plates are layered. The laser beam output from the laser system has a Gaussian distribution, and its wavelength is 1070 nm. The power of the laser beam was set to 3 kW. Then, this laser beam was radiated to the workpiece via the optical head without wobbling. Here, the sweeping speed of the laser beam for the workpiece in the welding direction was set to 20 mm/s. 
     A hole was formed in the welding bead on the back face of the workpiece in comparative example. The workpiece was cut along the welding bead so as to include the hole, and part thereof was photographed. As depicted in the photograph in  FIG. 4A , it was confirmed that the shape of the workpiece was deformed at the place where the hole was formed, and the shape of the hole was irregular and deep, compared with the thickness of the workpiece. It is conceivable that such holes and deformations were formed by the rapid evaporation of the galvanized layer. 
     On the other hand, no hole was formed in the workpiece in Example 1. The workpiece was cut along the welding bead, and part of it was photographed. As depicted in the photograph in  FIG. 4B , it was confirmed that the welding area of the workpiece was in good condition with no holes or deformations. 
     Next, as Examples 2, 3, and 4, a laser welding apparatus having the configuration illustrated in  FIG. 1  was used to perform lap welding on a workpiece having a configuration in which two 1 mm thick alloyed hot-dip galvanized steel plates (GA material) are layered. The laser beam output from the laser system has a Gaussian distribution, and its wavelength is 1070 nm. The power of the laser beam was set to 3 kW. Then, this laser beam was divided into three beams disposed linearly as illustrated in  FIG. 3A  by a diffractive optical element to generate a processing laser beam, and the workpiece was irradiated with the processing laser beam. Here, the rotation frequency when drawing a circle with the processing laser beam was set to 64 Hz, the radius R 1  in the welding width direction is 0.5 mm (Example 2), 1 mm (Example 3) or 1.5 mm (Example 4), and the radius R 2  in the welding direction was 0.5 mm. The sweeping speed of the processing laser beam for the workpiece in the welding direction was set to 20 mm/s. 
     When the welding beads of the workpiece in Examples 2, 3 and 4 were observed, it was confirmed that the appearance was good and the condition was good with no holes or deformation. 
     Next, as Example 5, under the same conditions as in Example 1, for two types with the radius R 1 =R 2  set to 0.5 mm and 0.25 mm, welding was performed with the beam-to-beam distance X being changed to 0.05 mm, 0.1 mm, 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm, 1.0 mm, 1.5 mm, 2.0 mm, or 2.5 mm. When the welding bead of each workpiece was observed, it was confirmed that the appearance was good and that the welding bead was in good condition with no holes or deformation in each case. Therefore, it was found that at least R 1 =R 2  should be set between ⅕ and 5 times X. 
     Subsequently, as Example 6, the laser beam was set to two beams as illustrated in  FIG. 5A  under the same conditions as in Example 1. The beam spacing X is the interval between the peak intensities of the beam B 11  and the beam B 12 . The power ratio of the preceding beam B 11  and beam B 12  was 10:0, 9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, 1:9, and 0:10, and for each power ratio, welding was performed with the beam-to-beam distance X being changed to 0.025 mm, 0.05 mm, 0.1 mm, 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm, or 1.0 mm. When observing the welding bead of the workpiece, defects such as holes and deformation were found in a case where the power ratio was 10:0 and 0:10, but good appearance was observed in any of the other power ratios, and the good condition with no holes or deformation were confirmed. 
     Next, as Example 7, under the same conditions as in Example 1, lap welding was performed on the workpiece having a configuration in which an electrogalvanized steel plate having a thickness of 1 mm and a high-tensile steel plate having a thickness of 1 mm without a plating layer or a cold-rolled steel plate having a thickness of 1 mm without a plating layer were layered. Welding was performed on each workpiece when the beam was irradiated from the electrogalvanized steel plate side and when the beam was irradiated from the opposite side. In any case, when the welding bead of the workpiece was observed, it was confirmed that the appearance was good and the condition was good with no holes or deformation. 
     Example of Power Distribution Shape 
     The power distribution shape of the processing laser beam is not limited to that illustrated in  FIG. 3A .  FIGS. 5A to 5F and 6A to 6E  are schematic diagrams illustrating an example of the power distribution shape of the processing laser beam. In the examples illustrated in  FIGS. 5A to 5F and 6A to 6E , the diffractive optical element  123  divides the laser beam into two or more beams to generate a processing laser beam. Note that, it is assumed that the welding direction WD is directed to the right side of the paper. 
     In the example illustrated in  FIG. 5A , a processing laser beam L 1  includes two beams B 11  and B 12 , each of which has a Gaussian shape. The beams B 11  and B 12  are disposed along the welding direction WD in a plane perpendicular to the light traveling direction of the processing laser beam L 1 . The line connecting the beams B 11  and B 12  extends in the welding direction WD. 
     In the examples illustrated in  FIGS. 5B to 5F , the processing laser includes four beams, each of which has a Gaussian shape. In the example illustrated in  FIG. 5B , a processing laser beam L 2  includes four beams B 21 , B 22 , B 23 , and B 24 . The beams B 21 , B 22 , B 23 , and B 24  are disposed linearly along the welding direction WD in a plane perpendicular to the light traveling direction of the processing laser beam L 2 . The line connecting the beams B 21 , B 22 , B 23 , and B 24  extends in the welding direction WD. 
     In the example illustrated in  FIG. 5C , a processing laser beam L 3  includes four beams B 31 , B 32 , B 33 , and B 34 . The beams B 31  and B 32  are disposed along the welding direction WD in a plane perpendicular to the light traveling direction of the processing laser beam L 3 . The line connecting the beams B 31  and B 32  extends in the welding direction WD. The beams B 33  and B 34  are disposed along the welding width direction in a plane perpendicular to the light traveling direction of the processing laser beam L 3 . The line connecting the beams B 33  and B 34  extends in the welding width direction. In  FIGS. 5C, 5D to 5F, and 6A to 6E , which will be described later, the processing laser beam has a power distribution shape in which two or more power regions are disposed in the welding direction and, further, along the welding width direction. As a result, it is possible to preferably form a molten pool that is gradually deepened not only in the welding direction but also in the welding width direction. 
     Note that, in the processing laser beam L 3 , the beams B 31  and B 33  can be said to be disposed along the welding direction WD, or can be said to be disposed along the welding width direction. The reason is that the line connecting the beams B 31  and B 33  is neither perpendicular to the welding direction WD nor perpendicular to the welding width direction. In this way, when the line connecting the two beams is not perpendicular to a certain direction and forms an angle of 45 degrees or less in particular, it can be said that the two beams are disposed along that direction. 
     In the example illustrated in  FIG. 5D , a processing laser beam L 4  includes four beams B 41 , B 42 , B 43 , and B 44 . The beams B 41  and B 42  are disposed along the welding direction WD in a plane perpendicular to the light traveling direction of the processing laser beam L 4 . The line connecting the beams B 41  and B 42  extends in the welding direction WD. The beams B 43  and B 44  are disposed along the welding width direction in a plane perpendicular to the light traveling direction of the processing laser beam L 4 . The line connecting the beams B 43  and B 44  extends in the welding width direction. In addition, a region A 4  is an elliptical representation of the approximate region where the beams B 41  to B 44  are disposed and the molten pool is formed. 
     In the example illustrated in  FIG. 5E , a processing laser beam L 5  includes four beams B 51 , B 52 , B 53 , and B 54 . The beams B 51  and B 52  are disposed along the welding direction WD in a plane perpendicular to the light traveling direction of the processing laser beam L 5 . The line connecting the beams B 51  and B 52  extends in the welding direction WD. The beams B 53 , B 52 , and B 54  are disposed linearly along the welding width direction in a plane perpendicular to the light traveling direction of the processing laser beam L 5 . The line connecting the beams B 53 , B 52 , and B 54  extends in the welding width direction. In addition, a region A 5  is an elliptical representation of the approximate region where the beams B 51  to B 54  are disposed and the molten pool is formed. 
     In the example illustrated in  FIG. 5F , a processing laser beam L 6  includes four beams B 61 , B 62 , B 63 , and B 64 . The beams B 61  and B 62  are disposed along the welding direction WD in a plane perpendicular to the light traveling direction of the processing laser beam L 6 . The beams B 63  and B 64  are disposed along the welding width direction in a plane perpendicular to the light traveling direction of the processing laser beam L 6 . In addition, a region A 6  is an elliptical representation of the approximate region where the beams B 61  to B 64  are disposed and the molten pool is formed. 
     In the examples illustrated in  FIGS. 6A to 6E , the processing laser contains five beams, each of which has a Gaussian shape. In the example illustrated in  FIG. 6A , a processing laser beam L 7  includes five beams B 71 , B 72 , B 73 , B 74 , and B 75 . The beams B 71 , B 72 , and B 73  are disposed linearly along the welding direction WD in a plane perpendicular to the light traveling direction of the processing laser beam L 7 . The line connecting the beams B 71 , B 72  and B 73  extends in the welding direction WD. The beams B 74 , B 72 , and B 75  are disposed linearly along the welding width direction in a plane perpendicular to the light traveling direction of the processing laser beam L 3 . The line connecting the beams B 74 , B 72 , and B 75  extends in the welding width direction. 
     In the example illustrated in  FIG. 6B , a processing laser beam L 8  includes five beams B 81 , B 82 , B 83 , B 84 , and B 85 . The beams B 81 , B 82 , and B 83  are disposed linearly along the welding direction WD in a plane perpendicular to the light traveling direction of the processing laser beam L 8 . The line connecting the beams B 81 , B 82  and B 83  extends in the welding direction WD. The beams B 84 , B 81 , and B 85  are disposed linearly along the welding width direction in a plane perpendicular to the light traveling direction of the processing laser beam L 8 . The line connecting the beams B 84 , B 81 , and B 85  extends in the welding width direction. In addition, a region A 8  is an elliptical representation of the approximate region where the beams B 81  to B 85  are disposed and the molten pool is formed. 
     In the example illustrated in  FIG. 6C , a processing laser beam L 9  includes five beams B 91 , B 92 , B 93 , B 94 , and B 95 . The beams B 91 , B 92 , and B 93  are disposed linearly along the welding direction WD in a plane perpendicular to the light traveling direction of the processing laser beam L 9 . The line connecting the beams B 91 , B 92  and B 93  extends in the welding direction WD. The beams B 94 , B 91 , and B 95  are disposed linearly along the welding width direction in a plane perpendicular to the light traveling direction of the processing laser beam L 9 . The line connecting the beams B 94 , B 91 , and B 95  extends in the welding width direction. In addition, a region A 9  is an elliptical representation of the approximate region where the beams B 91  to B 95  are disposed and the molten pool is formed. 
     In the example illustrated in  FIG. 6D , a processing laser beam L 10  includes five beams B 101 , B 102 , B 103 , B 104 , and B 105 . The beams B 101 , B 102 , and B 103  are disposed linearly along the welding direction WD in a plane perpendicular to the light traveling direction of the processing laser beam L 10 . The line connecting the beams B 101 , B 102  and B 103  extends in the welding direction WD. The beams B 104 , B 102 , and B 105  are disposed linearly along the welding width direction in a plane perpendicular to the light traveling direction of the processing laser beam L 10 . The line connecting the beams B 104 , B 102 , and B 105  extends in the welding width direction. In addition, a region A 10  is an elliptical representation of the approximate region where the beams B 101  to B 105  are disposed and the molten pool is formed. 
     In the example illustrated in  FIG. 6E , a processing laser beam L 11  includes five beams B 111 , B 112 , B 113 , B 114 , and B 115 . The beams B 111 , B 112 , and B 113  are disposed linearly along the welding direction WD in a plane perpendicular to the light traveling direction of the processing laser beam L 11 . The line connecting the beams B 111 , B 112  and B 113  extends in the welding direction WD. The beams B 114 , B 112 , and B 115  are disposed linearly along the welding width direction in a plane perpendicular to the light traveling direction of the processing laser beam L 11 . The line connecting the beams B 114 , B 112 , and B 115  extends in the welding width direction. In addition, a region A 11  is an elliptical representation of the approximate region where the beams B 111  to B 115  are disposed and the molten pool is formed. 
     Note that, in all of  FIGS. 3A, 5A to 5F, and 6A to 6E , it is important to set the peak power of each beam so that the workpiece does not melt suddenly immediately after the irradiation. 
     Further, in  FIGS. 5C and 6A , since a plurality of beams are disposed isotropically, there is an effect that the melting characteristics for the workpiece do not change even when the welding direction is changed in any direction. 
     The examples illustrated in  FIGS. 3A, 5A to 5F, and 6A to 6E  can be realized by appropriately designing the characteristics of the diffraction grating constituting the diffractive optical element  123 . 
     Second Embodiment 
       FIG. 7  is a schematic diagram illustrating a schematic configuration of the laser welding apparatus according to a second embodiment. A laser welding apparatus  200  irradiates the workpiece W with the processing laser beam L to weld the workpiece W. The laser welding apparatus  200  performs welding by the same operating principle as the laser welding apparatus  100 . The laser welding apparatus  200  includes the laser system  110 , an optical head  220 , and the optical fiber  130 , and has a configuration in which the optical head  120  is replaced with the optical head  220  in the laser welding apparatus  100 . Therefore, in the following, only the device configuration of the optical head  220  will be described. 
     As in the optical head  120 , the optical head  220  is an optical device that radiates the laser beam input from the laser system  110  toward the workpiece W. The optical head  220  includes a collimating lens  221 , a condensing lens  222 , and a diffractive optical element  223 . 
     Further, the optical head  220  has a galvano scanner disposed between the collimating lens  221  and the condensing lens  222 . The angles of mirrors  224   a  and  224   b  of the galvano scanner are changed by motors  225   a  and  225   b , respectively. The motors  225   a  and  225   b  are driven by a driver (not illustrated). As in the optical head  120 , the optical head  220  can move the position which is irradiated with the processing laser beam L without moving the optical head  220  by controlling the angles of the two mirrors  224   a  and  224   b  to sweep the processing laser beam L. 
     The diffractive optical element  223  is disposed between the collimating lens  221  and the condensing lens  222 . As in the diffractive optical element  123 , the diffractive optical element  223  forms the beam shape of the laser beam input from the collimating lens  221  to generate the processing laser beam L. As illustrated in  FIG. 3A , the processing laser beam L generated by the diffractive optical element  223  has a power distribution shape in which three beams are disposed along the welding direction WD in a plane perpendicular to the light traveling direction. Note that, the diffractive optical element  223  may be designed to generate any of the processing laser beams L 1  to LL 11  illustrated in  FIGS. 5A to 5F and 6A to 6E . As a result, the laser welding apparatus  200  can suppress the occurrence of the welding defect when welding the workpiece W. 
     Note that, in the above embodiment, the diffractive optical element divides the laser beam into a plurality of beams having the same peak power. However, the peak powers of a plurality of beams do not have to be exactly equal. Further, the power distribution of each beam is not limited to have the Gaussian shape, and may have another single-peak shape. 
     Further, the distance between the centers of adjacent beams among the divided beams is, for example, 20 times or less the beam diameter. Further, it is preferable that the molten regions of adjacent beams overlap. Here, the melting region by the beam is a region in which the workpiece has a temperature higher than the melting point due to the energy given by the beam, and is melt, and the area may be larger than the beam diameter depending on the thermal conductivity of the workpiece or the like. In this case, the peak powers of the beams may be equal, substantially equal, or different. 
     Further, in the above embodiment, the power distribution shape of the processing laser beam has a discrete power region due to the beams, but the plurality of power regions may be continuous with axisymmetric or asymmetric distribution. For example,  FIG. 8A  illustrates the power distribution shape of a processing laser beam L 12  in the welding direction. In the power distribution shape of the processing laser beam L 12 , two power regions PA 121  and PA 122  disposed along the welding direction are continuous. The power region PA 121  has a single-peak shape with a peak. Further, the power region PA 122  has a shoulder shape. The boundary between the two power regions PA 121  and PA 122  in the curve of  FIG. 8A  can be defined as, for example, the position of an inflection point existing between them. 
     On the other hand,  FIG. 8B  illustrates the power distribution shape of a processing laser beam L 13  in the welding direction. In the power distribution shape of the processing laser beam L 13 , two power regions PA 131  and PA 132  disposed along the welding direction are continuous. Each of the power regions PA 131  and PA 132  has a single-peak shape with a peak. The boundary between the two power regions PA 131  and PA 132  in the curve of  FIG. 8B  can be defined as, for example, the position of the minimum point existing between them. Both the processing laser beam L 12  and L 13  can be applied as the processing laser beam of the present disclosure. The processing laser beams L 12  and L 13  can be realized by using, for example, a combination of two or more lenses as a beam shaper. In addition, the processing lasers L 12  and L 13  illustrated in  FIG. 8  and the processing lasers L and L 1  to L 11  illustrated in  FIGS. 3, 5 , and  6  can have a configuration in which two or more laser light beams output from two or more laser systems are combined in an optical head or the laser beam output from one laser systems is divided into two or more beams, and these beams are combined in the optical head. It should be noted that each of the two or more laser beams may be transmitted to the optical head by separate cores of one multi-core fiber. In these cases, the optical head and the multi-core fibers generates a processing laser beam so as to have a power distribution shape in which two or more power regions are disposed along a predetermined direction in a plane perpendicular to the light traveling direction from the laser light, and, for example, functions as a beam shaper. 
     Further, when the processing laser beam is swept in the welding direction on the workpiece while being swung, the locus drawn by the processing laser beam on the workpiece is not limited to be trochoidal. For example, as illustrated in  FIG. 9A , a locus T 1  drawn when the processing laser beam L is swept in the welding direction WD while being swung may be triangular wavy. Further, as illustrated in  FIG. 9B , a locus T 2  drawn when the processing laser beam L is swept in the welding direction WD while being swung may be sinusoidal. Although not illustrated, when the processing laser beam L is swept in the welding direction WD while being swung, the laser beam L may be swept while the locus is drawn in a figure eight when viewed from vertically or a figure eight when viewed from horizontally with respect to the welding direction WD. When the processing laser beam L draws a wavy locus in this way, the processing laser beam L may weave. 
     Further, in the above embodiment, the workpiece W has a configuration in which two galvanized steel plates are layered without a gap, but the present disclosure can be applied to even the workpiece having a configuration in which two or more galvanized steel plates are layered with a gap. Further, the plating plate material constituting the workpiece W is not limited to the galvanized steel plate, and the present disclosure can be applied to the plating plate material on which lap welding is to be performed. 
     Further, in the above embodiment, the optical heads  120  and  220  have a galvano scanner as a mechanism for sweeping the processing laser beam L while irradiating the workpiece W with the processing laser beam L, but they may have another known scanner that has a different mechanism than the galvano scanner. Further, a mechanism for swinging and sweeping the laser beam while irradiating the workpiece W with the laser beam may include a mechanism that changes the relative position between the optical head and the workpiece W. Examples of such a mechanism include a mechanism that moves the optical head itself and a mechanism that moves the workpiece. That is, the optical head may be configured so that the processing laser beam can be moved with respect to a fixed workpiece. Alternatively, the position which is irradiated with the processing laser beam from the optical head may be fixed, and the workpiece may be held movably with respect to the fixed processing laser beam. 
     Further, in order to control the optimum thermal energy to be given to the workpiece by the processing laser beam, the power of the processing laser may be controlled so as to change with time. The time waveform of the power of the processing laser may be controlled so as to be a rectangular wave, a triangular wave, a sine wave, or the like. By controlling the time waveform like this, the optimum thermal energy by which a suitable melting depth (depth of the molten pool) can be obtained without excessive evaporation of zinc vapor may be input by the processing laser beam, and a good welding bead may be obtained. 
     Further, the present disclosure is not limited to the above embodiments. The present disclosure also includes those configured by appropriately combining the constituent elements of the above-described embodiments. Further, further effects and modifications can be easily derived by those skilled in the art. Therefore, the broader aspect of the present disclosure is not limited to the above-described embodiments, and various modifications can be made. 
     The present disclosure can be used for laser welding. 
     According to the present disclosure, it is possible to suppress the occurrence of the welding defect during lap welding of plating plate materials. 
     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.