Patent Publication Number: US-2021178514-A1

Title: Welding method and welding apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation of International Application No. PCT/JP2019/034801, filed on Sep. 4,2 2019 which claims the benefit of priority of the prior Japanese Patent Application No. 2018-165082, filed on Sep. 4, 2018, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     The present disclosure relates to welding methods and welding apparatuses. 
     Background 
     Laser welding has been known as one of methods of welding workpieces made of metallic materials. Laser welding is a welding method in which area to be welded in a workpiece is irradiated with laser beam and the area is melted by the energy of the laser beam. A liquid pool of the metallic material melted, the liquid pool being called a molten pool, is formed at the area irradiated with the laser beam, and welding is thereafter done by solidification of the molten pool. 
     In irradiation of a workpiece with laser beam, a profile of the laser beam may be shaped depending on the purpose. For example, a technique for shaping a profile of laser beam when the laser beam is used to cut a workpiece has been known (see, for example, Japanese National Publication of International Patent Application No. 2010-508149). 
     During such welding, scattered matter called sputter is known to be generated from molten pools. This sputter is molten metal that has been scattered and it is important to reduce generation of sputter to prevent processing defects. Since sputter is molten metal that has been scattered, when sputter is generated, some of the metallic material at the welded spot is lost. That is, if generation of sputter is increased, the amount of the metallic material at the welded spot will become insufficient and problems, such as insufficient strength, may thus be caused. Furthermore, the generated sputter will adhere to the surroundings of the welded spot, and if the adhered sputter is peeled off later and adheres to a piece of equipment, such as an electric circuit, the electric circuit may not function properly. Therefore, performing welding of parts for electric circuits is sometimes difficult. 
     SUMMARY 
     There is a need for providing a welding method and a welding apparatus that enable reduction in generation of sputter. 
     According to an embodiment, in a welding method, laser beam and a workpiece including a metal are moved relatively to each other while the laser beam is being emitted to the workpiece to sweep the workpiece with the laser beam and melt and weld an area of the workpiece, the area being where the laser beam has been emitted to; the laser beam is formed of a main power region and at least one auxiliary power region having at least a part that is in front, in a sweep direction, of the main power region; a power of the main power region is larger than a power of each of the at least one auxiliary power region; and a ratio between the power of the main power region and the total of powers of the at least one auxiliary power region is in a range of 144:1 to 1:9. 
     According to an embodiment, a welding apparatus includes: a laser oscillator; and an optical head that receives light emitted from the laser oscillator to generate laser beam, and emits the generated laser beam to a workpiece to melt and weld an area of the workpiece, the area being where the laser beam has been emitted to. Further, the laser beam includes a main power region and at least one auxiliary power region, a power of the main power region is larger than a power of each of the at least one auxiliary power region, and a ratio between the power of the main power region and the total of powers of the at least one auxiliary power region is in a range of 144:1 to 1:9. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram illustrating a schematic configuration of a laser welding apparatus according to a first embodiment; 
         FIG. 2  is a schematic diagram for explanation of a diffractive optical element; 
         FIG. 3  is a diagram illustrating an example of a cross-sectional shape of laser beam; 
         FIG. 4  is a diagram illustrating a situation where laser beam melts a workpiece; 
         FIG. 5A  is a schematic diagram illustrating an example of beam arrangement; 
         FIG. 5B  is a schematic diagram illustrating an example of beam arrangement; 
         FIG. 5C  is a schematic diagram illustrating an example of beam arrangement; 
         FIG. 6  is a schematic diagram illustrating arrangement of plural beams of laser beam in a case where a DOE is used; 
         FIG. 7  is a diagram illustrating representative conditions under which states of irradiated spots in experiments were satisfactory; 
         FIG. 8  is a diagram illustrating relations between the sweep speed and the numbers of sputters for a comparative example, a first example, and a second example; 
         FIG. 9  is a schematic diagram for explanation of another example of beam arrangement; 
         FIG. 10  is a schematic diagram for explanation of yet another example of beam arrangement; 
         FIG. 11A  is a schematic diagram for explanation of still another example of beam arrangement; 
         FIG. 11B  is a schematic diagram for explanation of yet another example of beam arrangement; 
         FIG. 11C  is a schematic diagram for explanation of still another example of beam arrangement; 
         FIG. 11D  is a schematic diagram for explanation of yet another example of beam arrangement; 
         FIG. 11E  is a schematic diagram for explanation of still another example of beam arrangement; 
         FIG. 11F  is a schematic diagram for explanation of yet another example of beam arrangement; 
         FIG. 11G  is a schematic diagram for explanation of still another example of beam arrangement; 
         FIG. 11H  is a schematic diagram for explanation of yet another example of beam arrangement; 
         FIG. 11I  is a schematic diagram for explanation of still another example of beam arrangement; 
         FIG. 12  is a diagram illustrating a schematic configuration of a laser welding apparatus according to a second embodiment; 
         FIG. 13  is a diagram illustrating a schematic configuration of a laser welding apparatus according to a third embodiment; 
         FIG. 14  is a diagram illustrating a schematic configuration of a laser welding apparatus according to a fourth embodiment; 
         FIG. 15A  is a schematic diagram for explanation of an example of a power distribution profile of laser beam; and 
         FIG. 15B  is a schematic diagram for explanation of an example of a power distribution profile of laser beam. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will be described in detail below while reference is made to the appended drawings. The present disclosure is not limited by the embodiments described below. The same reference sign will be assigned to elements that are the same or corresponding to each other, as appropriate, throughout the drawings. 
     First Embodiment 
       FIG. 1  is a diagram illustrating a schematic configuration of a laser welding apparatus according to a first embodiment. A laser welding apparatus  100  includes a laser device  110 , an optical head  120 , and an optical fiber  130  that connects the laser device  110  and the optical head  120  to each other. A workpiece W includes a metal, such as iron or aluminum. The workpiece W is made of, for example: a ferroalloy, such as stainless steel; or an aluminum alloy, and is plate-shaped, with a thickness in a range of, for example, about 1 mm to 10 mm. 
     The laser device  110  includes a laser oscillator and is configured to output laser beam having a power of, for example, a few kilowatts (kW). For example, the laser device  110  may include plural semiconductor laser elements inside the laser device  110  and be configured to be capable of outputting multi-mode laser beam having a power of a few kilowatts (kW) as the total output of the plural semiconductor laser elements. Furthermore, the laser device  110  may include any of various laser beam sources, such as fiber lasers, YAG lasers, and disk lasers. The optical fiber  130  guides laser beam output from the laser device  110  to input the laser beam to the optical head  120 . 
     The optical head  120  is an optical device for emitting the laser beam input from the laser device  110 , to the workpiece W. The optical head  120  includes a collimator lens  121  and a condenser lens  122 . The collimator lens  121  is an optical system for making input laser beam into collimated light. The condenser lens  122  is an optical system for condensing the collimated laser beam and emitting the condensed collimated laser beam as laser beam L, to the workpiece W. 
     To sweep the workpiece W with the laser beam L while irradiating the workpiece W with the laser beam L, the optical head  120  is configured such that position of the optical head  120  is able to be changed relatively to the workpiece W. Examples of a method of changing the relative position to the workpiece W include: moving the optical head  120  itself; and moving the workpiece W. That is, the optical head  120  may be configured to be capable of sweeping, with the laser beam L, the workpiece W that has been fixed. Or, a position irradiated with the laser beam L from the optical head  120  may be fixed and the workpiece W may be held to be movable relatively to the laser beam L that has been fixed. 
     The optical head  120  includes a diffractive optical element  123  that is placed between the collimator lens  121  and the condenser lens  122 , the diffractive optical element  123  serving as a beam shaper. The diffractive optical element  123  referred to herein is also called a DOE (diffractive optical element) and is integrally formed of plural diffraction gratings  123   a  having different periods, as conceptually illustrated in  FIG. 2 . The diffractive optical element  123  is able to form a beam shape by doing one or both of: bending input laser beams in directions influenced by the respective diffraction gratings; and superimposing them. 
     The diffractive optical element  123  splits laser beam input from the collimator lens  121  into plural beams. Specifically, the diffractive optical element  123  splits laser beam to generate a main beam and at least one auxiliary beam. The diffractive optical element  123  generates the main beam and the auxiliary beam or beams such that at least a part of the at least one auxiliary beam is positioned in front, in a sweep direction, of the main beam. The laser beam L is thereby formed of the main beam and the at least one auxiliary beam. 
     The laser beam L shaped by the diffractive optical element  123  is formed of a main beam B 1  and an auxiliary beam B 2 , like an example of a cross-sectional shape of the laser beam L illustrated in  FIG. 3 , the cross-sectional shape being on a plane perpendicular to the direction in which the laser beam L travels. A sweep direction SD is the direction of movement of the laser beam L, the movement being relative to the workpiece W. The auxiliary beam B 2  is positioned in front, in the sweep direction, of the main beam B 1 . Being in front of the main beam B 1  means, as illustrated in  FIG. 3 , being in a region A defined by a broken line perpendicular to the sweep direction SD, the broken line passing a position at a beam diameter of the main beam B 1 , the position being forward in the sweep direction. The position of the auxiliary beam B 2  is not limited to this example, and as long as the auxiliary beam B 2  is positioned somewhere in the region A, the auxiliary beam B 2  is able to be regarded as being positioned in front of the main beam B 1 . 
     Furthermore, each of the main beam B 1  and the auxiliary beam B 2  has, in a radial direction of its beam cross-section, a power distribution having a Gaussian form, for example. In  FIG. 3 , the diameters of circles depicting the main beam B 1  and the auxiliary beam B 2  are beam diameters of these beams. A beam diameter of each beam is defined as a diameter of a region including a peak of that beam and having intensity that is 1/e 2  or more of the intensity of that peak. If the beam is not circular, the beam diameter is defined in this specification as a length of a region having intensity that is 1/e 2  or more of the peak intensity, the length being along a longer axis (for example, a major axis) passing the vicinity of the center of the beam or along a shorter axis (for example, a minor axis) perpendicular to the longer axis (major axis). In addition, the power of each beam is power in the region including the peak of that beam and having intensity that is 1/e 2  or more of the peak intensity. The power of the main beam B 1  is larger than the power of the auxiliary beam B 2 . 
     The power distribution profile of at least the main beam B 1  is preferably sharp to some degree. When the power distribution profile of the main beam B 1  is sharp to some degree, the depth melted is able to be increased in welding of the workpiece W, and welding strength is thus able to be attained and occurrence of poor welds is thus able to be lessened more ideally. If a beam diameter is used as an index of sharpness of the main beam B 1 , the main beam B 1  preferably has a beam diameter of 600 μm or less and more preferably 400 μm or less. When the main beam B 1  is sharp in form, the power for attaining the same melted depth is able to be reduced and the processing speed is able to be increased. Therefore, the electric power consumption by the laser welding apparatus  100  is able to be reduced and the processing efficiency is able to be improved. The power distribution of the auxiliary beam B 2  may be as sharp as the main beam B 1 . 
     Beam diameters may be designed by appropriately setting specifics of the laser device  110 , optical head  120 , and optical fiber  130  that are used. For example, beam diameters may be set by setting beam diameters of laser beam input to the optical head  120  from the optical fiber  130 , or setting the optical systems, such as the diffractive optical element  123  and lenses  121  and  122 . 
     When welding is performed using the laser welding apparatus  100 , firstly, the workpiece W is placed in a region, to which laser beam L is emitted. Subsequently, the laser beam L and the workpiece W are moved relatively to each other while the workpiece W is irradiated with the laser beam L including the main beam B 1  and the auxiliary beam B 2  that have been split by the diffractive optical element  123 , to perform sweeping with the laser beam L, and welding is performed by melting an area of the workpiece W, the area having been irradiated with the laser beam L. In  FIG. 1 , the sweep direction is, for example, the direction pointing out from the plane of the page of the figure or the direction pointing into the page of the figure. Welding of the workpiece W is thereby done. 
     In this welding, positioning the auxiliary beam B 2  in front, in the sweep direction SD, of the main beam B 1  in the laser beam L enables reduction in generation of sputter. 
       FIG. 4  is a diagram illustrating a situation where laser beam melts a workpiece. As illustrated in  FIG. 4 , in the laser welding apparatus  100  according to the first embodiment and a welding method using the laser welding apparatus  100 , the laser beam L includes the main beam B 1  and the auxiliary beam B 2 . A power density of the main beam B 1  is, for example, at least a power density that enables a key hole to be generated. A key hole is a depression or a hole that is generated by pressure of metallic vapor generated when the workpiece W is melted by the high power density of a laser beam. The auxiliary beam B 2  having a power smaller than that of the main beam B 1  is positioned in front, in the sweep direction SD, of the main beam B 1  having a large power. Furthermore, the power density of the auxiliary beam B 2  is a power density that enables the workpiece W to be melted: in the presence of the main beam B 1 ; or by the auxiliary beam B 2  itself alone. Therefore, a molten pool WP is formed as a molten region, the molten pool WP having a region that is in front of a position irradiated with the main beam B 1 , the region being shallower than the depth melted by the main beam B 1 . This region will be called a shallow region S for the sake of convenience. 
     Melt intensity regions of the main beam B 1  and the auxiliary beam B 2  may overlap each other but do not necessarily overlap each other as long as their molten pools overlap each other. The melt intensity region formed by the main beam B 1  is preferably able to reach the molten pool formed by the auxiliary beam B 2  before the molten pool solidifies. As described above, the power densities of the main beam B 1  and the auxiliary beam B 2  are power densities enabling the workpiece W to be melted, and the melt intensity region refers to the range of the laser beam having the power density enabling the workpiece W to be melted, the range being around the main beam B 1  or the auxiliary beam B 2 . 
     In the laser welding apparatus  100  according to the first embodiment and the welding method using the laser welding apparatus  100 , the presence of the shallow region S in front of the position irradiated with the main beam B 1  stabilizes the molten pool WP in the vicinity of the position irradiated with the main beam B 1 . As described above, sputter is molten metal that has been scattered and thus stabilization of the molten pool WP in the vicinity of the position irradiated with the main beam B 1  is considered to lead to reduction in generation of sputter. 
     In  FIG. 3 , the ratio between the power of the main beam B 1  and the power of the auxiliary beam B 2  is preferably in the range of 144:1 to 1:9. 
       FIG. 5A  to  FIG. 5C  are schematic diagrams illustrating examples of beam arrangement.  FIG. 5A  to  FIG. 5C  each illustrate arrangement of plural beams on a surface of the workpiece W, the surface being irradiated with laser beam L. In an example illustrated in  FIG. 5A , laser beam L includes a main beam B 1  and plural auxiliary beams B 2  that have been split by the diffractive optical element  123 . In the example illustrated in  FIG. 5A , the number of the auxiliary beams B 2  is 16. The 16 auxiliary beams B 2  are positioned to surround the periphery of the main beam B 1 . Specifically, the 16 auxiliary beams B 2  are positioned to form an approximate ring shape having a radius R between their peaks, the main beam B 1  being in the center of the approximate ring shape. Furthermore, the 16 auxiliary beams B 2  may be said to be positioned to form an approximate regular hexadodecagon shape having a distance R between its center and vertex, the main beam B 1  being in the center of the approximate regular hexadodecagon shape. 
     Like laser beam L′ illustrated in  FIG. 5B , plural auxiliary beams B 2  may continuously overlap each other to form an approximate ring shape surrounding the periphery of the main beam B 1 . Furthermore, like laser beam L″ illustrated in  FIG. 5C , an arc shape that is a part of an approximate ring shape surrounding the periphery of the main beam B 1  may be formed. 
     In the example illustrated in  FIG. 5A , seven auxiliary beams B 2  are positioned in front, in the sweep direction SD, of the main beam B 1 . Two auxiliary beams B 2  are positioned laterally, in the direction perpendicular to the sweep direction SD, with respect to the main beam B 1 . Seven auxiliary beams B 2  are positioned in back, in the sweep direction SD, of the main beam B 1 . 
     Furthermore, the ratio between the power of the main beam B 1  and the total of powers of the 16 auxiliary beams B 2  is in a range of 9:1 to 1:9. Therefore, if this ratio is 9:1, the ratio between the power of the main beam B 1  and the power of one of the auxiliary beams B 2  is 9:1/16 =144:1. In addition, if this ratio is 1:9, the ratio between the power of the main beam B 1  and the power of one of the auxiliary beams B 2  is 1:9/16 =16:9. 
     Because the laser beam L illustrated in  FIG. 5A  has the seven auxiliary beams B 2  that are a part of the 16 auxiliary beams B 2  and are positioned in front, in the sweep direction SD, of the main beam B 1 , and the ratio between the power of the main beam B 1  and the total of the powers of the 16 auxiliary beams B 2  is in the range of 9:1 to 1:9; generation of sputter is able to be reduced. Similarly, in each of the laser beam L′ and laser beam L″ illustrated in  FIG. 5B  and  FIG. 5C , a part of the auxiliary beams B 2  is positioned in front, in the sweep direction SD, of the main beam B 1 , and the ratio between the power of the main beam B 1  and the total of powers of the auxiliary beams B 2  is in the range of 9:1 to 1:9; and the generation of sputter is thereby able to be reduced. 
     Furthermore, in the examples illustrated in  FIG. 5A  and  FIG. 5B , since the auxiliary beams B 2  are positioned to form an approximate ring shape with the main beam B 1  in the center of the approximate ring shape, even if the sweep direction is changed from the sweep direction SD to any sweep direction, a part of the auxiliary beams B 2  is positioned in front, in the sweep direction changed, of the main beam B 1 . Therefore, the effect of reducing generation of sputter is able to be obtained for any sweep direction. In addition, in the example illustrated in  FIG. 5C  also, even if the sweep direction is changed from the sweep direction SD to some degree, the effect of reducing generation of sputter is able to be obtained. 
     Next, as an experimental example, experiments were conducted. In these experiments, plates serving as workpieces, made of SUS  304 , which is stainless steel, and having a thickness of 10 mm were irradiated with laser beam using a laser welding apparatus having the configuration illustrated in  FIG. 1 . The wavelength of laser beam output from the laser device was 1070 nm and the power of the laser beam was 5 kW. The experiments were performed using and not using a diffractive optical element (DOE). 
     For the use of a DOE, as illustrated in  FIG. 6 , plural DOEs that had been designed to split laser beam into a main beam and 16 auxiliary beams were prepared, the 16 auxiliary beams being positioned to form an approximate ring shape with the main beam in the center of the approximate ring shape. A diameter  2 R of this ring shape was set to be 600 μm on a surface of the workpiece. The sweep direction was upward in  FIG. 6 . The DOEs were designed such that ratios between the power of the main beam and the total of powers of the 16 auxiliary beams were respectively 9:1, 7:3, 5:5, 3:7, and 1:9 and the 16 auxiliary beams were equal in power. The ratios between the power of the main beam and the total of powers of the 16 auxiliary beams were in the range of 9:1 to 1:9. Therefore, when the ratio was 9:1, the ratio between the power of the main beam and the power of one of the auxiliary beams was 9:1/16 =144:1. When the ratio was 1:9, the ratio between the power of the main beam and the power of one of the auxiliary beams was 1:9/16 =16:9. 
     Furthermore, sweep speed for the workpiece with the laser beam was 0.5 m/min, 1 m/min, 2 m/min, 5 m/min, 10 m/min, 20 m/min, or 30 m/min. 
     Results of the experiments are shown in Table 1. In Table 1, power ratio (center:periphery) refers to the ratio between the power of the main beam and the total of powers of the auxiliary beams. Furthermore, “good” and “okay” indicate results of determination of the degree of scatter of sputter in a time period during which welding of a predetermined length was performed. Specifically, “good” and “okay” indicate by how much the volume of sputter was reduced as compared with a case where a DOE was not used (corresponding to a case where the power ratio was 10:0), when the volume of sputter scattered in that case is defined as 100%. “Good” means that 20% or more of the volume of sputter scattered was reduced as compared with the case where a DOE was not used. “Okay” means that the volume of sputter scattered was reduced as compared to the case where a DOE was not used but the reduction was less than 20%. 
     As shown in Table 1, when the power ratios were in the range of 9:1 to 1:9, the scattered amounts of sputter were reduced at all sweep speeds. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Power ratio 
                 Sweep speed (m/min) 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 (center:periphery) 
                 30 
                 20 
                 10 
                 5 
                 2 
                 1 
                 0.5 
               
               
                   
               
               
                 9:1 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Okay 
                 Okay 
               
               
                 7:3 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
               
               
                 5:5 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
               
               
                 3:7 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
               
               
                 1:9 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
                 Good 
               
               
                   
               
            
           
         
       
     
     Subsequently, irradiation experiments similar to the above were conducted using DOEs designed such that the power ratio was 7:3 and diameters  2 R of circular approximations of the approximate ring shapes each formed by 16 auxiliary beams on a surface of a workpiece were 450 μm, 600 μm, 800 μm, 1000 μm, 1400 μm, and 1800 μm. The sweep speed was set at 5 m/min. When these DOEs were used, distances were about 225 μm, about 300 μm, about 400 μm, about 500 μm, about 700 μm, and about 900 μm, each of the distances being that between the center of each auxiliary beam and the center of the main beam. As a result of these experiments, scattered amounts of sputter were reduced for all of the diameters  2 R.  FIG. 7  is a diagram illustrating representative conditions under which the scattered amounts of sputter were reduced in the above described two series of experiments. As evident from these results, the distance between the center of each auxiliary beam and the center of the main beam (corresponding to the radius R) is preferably in the range of 225 μm to 900 μm. The distance between the center of each auxiliary beam positioned forward in the sweep direction and the center of the main beam may be in the range of 225 μm to 900 μm, and distances between the centers of the plural auxiliary beams positioned laterally or rearward in the sweep direction and the center of the main beam may have values outside this range. 
     Next,  FIG. 8  is a diagram illustrating relations between sweep speeds and the numbers of scattered sputters, for a comparative example, a first example, and a second example. The horizontal axis represents the sweep speed, and the vertical axis represents the number of sputters scattered per weld length of 1 mm, the number being counted by analyzing a video captured by a high-speed camera. The comparative example corresponds to the cases where no DOEs are used in the above described experimental example. The first example corresponds to cases where experiments were performed similarly to the above described experimental example. In these experiments, plates were irradiated with laser beam like that illustrated in  FIG. 5C  using a laser welding apparatus having the configuration illustrated in  FIG. 1 . The laser beam was shaped using DOEs and the plates were made of SUS  304 , which is stainless steel, and 10 mm in thickness. The wavelength of laser beam output from the laser device was 1070 nm and the power of the laser beam was 5 kW. The distance (corresponding to the radius R) between the center of the main beam and a center of the arc-shaped auxiliary beams was set at 300 μm and the ratio between the power of the main beam and the total of powers of the arc-shaped auxiliary beams was set at 1:2. The arc-shaped auxiliary beams are shaped as a single arc-shaped auxiliary beam that includes plural auxiliary beams approximately equal in power and arranged in an arc shape, the single arc-shaped auxiliary beam having continuously distributed power. The second example corresponds to cases where the diameter  2 R is set at 600 μm and the power ratio is set at 3:7 in the above described experimental example. 
     As illustrated in  FIG. 8 , in the first example and the second example, the numbers of sputters were decreased significantly as compared to the comparative example, at all sweep speeds. 
     The cases where the laser beam sweeps the workpieces have been described above. However, forming laser beam with a main beam and plural auxiliary beams and setting the ratio between the power of the main beam and the total of powers of the plural auxiliary beams at 144:1 to 1:9 are also effective for welding, such as spot welding, for example, which does not include sweep of a workpiece with laser beam. The distance between the center of each of the plural auxiliary beams adjacent to the main beam and the center of the main beam is preferably 225 μm to 900 μm. 
     Other Examples of Beam Arrangement 
     Other examples of beam arrangement will be described below. For example, in an example illustrated in  FIG. 9 , laser beam L 1  emitted to a workpiece is split into a main beam B 1  and three auxiliary beams B 2 . All of the three auxiliary beams B 2  are positioned in front, in the sweep direction SD, of the main beam B 1 . The power of the main beam B 1  is larger than the power of each the auxiliary beams B 2 . Furthermore, the ratio between the power of the main beam B 1  and the total of powers of the three auxiliary beams B 2  is in the range of 9:1 to 1:9. This arrangement also enables reduction in generation of sputter, similarly to the above described first embodiment, experimental example, and first and second examples. 
     In the forward beam arrangement like the one illustrated in  FIG. 9 , an angle θ formed between lines joining the center of the main beam B 1  and the centers of two adjacent auxiliary beams B 2  is preferably 90° or less, more preferably 60° or less, and more preferably 45° or less. 
     Furthermore, the shape of the molten pool preferably is nearly line-symmetrical about the sweep direction SD, and thus the three auxiliary beams B 2  are also preferably arranged to be line-symmetrical about the sweep direction SD. 
     Furthermore, as described above by reference to  FIG. 6 , when laser beam is split into a main beam and 16 auxiliary beams using a DOE, the ratio between the power of the main beam and the power of one of the auxiliary beams may be 144:1. This ratio is applied to a case where laser beam is split into a main beam and two auxiliary beams using a DOE and the two auxiliary beams are positioned in front, in the sweep direction, of the main beam. The ratio between the power of the main beam and the total of powers of the two auxiliary beams then becomes 144:2 =72:1. Therefore, the ratio between the power of the main beam and the total of powers of the two auxiliary beams may be 72:1. In addition, in a case where laser beam is split into a main beam and three auxiliary beams using a DOE and the three auxiliary beams are positioned in front, in the sweep direction, of the main beam, like in  FIG. 9 , for example, the ratio may be 144:3 =48:1. Accordingly, the ratio may have a value of 144:1 or more. 
     Furthermore, in an example illustrated in  FIG. 10 , for example, laser beam L 2  emitted to a workpiece is split into a main beam B 1 , 16 auxiliary beams B 2 , and eight auxiliary beams B 3 . The 16 auxiliary beams B 2  form an auxiliary beam group G 2  and are positioned to form an approximate ring shape with the main beam B 1  in the center. The eight auxiliary beams B 3  form an auxiliary beam group G 3  and are positioned to form an approximate ring shape with the main beam B 1  in the center and a diameter smaller than that of the ring shape formed by the auxiliary beams B 2 . The ratio between the power of the main beam B 1  and the total of powers of the auxiliary beams B 2  and auxiliary beams B 3  is in the range of 9:1 to 1:9. This arrangement also enables reduction in generation of sputter, similarly to the above described first embodiment, experimental example, and first and second examples. 
       FIG. 11A  to  FIG. 11I  are schematic diagrams for explanation of yet other examples of beam arrangement. In an example illustrated in  FIG. 11A , laser beam L 31  includes a main beam B 1  and twelve auxiliary beams B 2 . The twelve auxiliary beams B 2  are positioned to form an approximate ring shape or an approximate regular hexagonal shape, with the main beam B 1  in the center. In an example illustrated in  FIG. 11B , laser beam L 32  includes a main beam B 1  and six auxiliary beams B 2 . The six auxiliary beams B 2  are positioned to form an approximate ring shape or hexagonal shape, with the main beam B 1  in the center. In an example illustrated in  FIG. 11C , laser beam L 33  includes a main beam B 1  and ten auxiliary beams B 2 . The ten auxiliary beams B 2  are positioned to form an approximate ring shape or pentagonal shape, with the main beam B 1  in the center. In an example illustrated in  FIG. 11D , laser beam L 34  includes a main beam B 1  and five auxiliary beams B 2 . The five auxiliary beams B 2  are positioned to form an approximate ring shape or pentagonal shape, with the main beam B 1  in the center. In an example illustrated in  FIG. 11E , laser beam L 35  includes a main beam B 1  and an auxiliary beam B 2 . The auxiliary beam B 2  includes plural auxiliary beams that overlap each other such that their power is continuously distributed, the plural auxiliary beams being positioned to form an approximate ring shape or hexagonal shape, with the main beam B 1  in the center. In an example illustrated in  FIG. 11F , laser beam L 36  includes a main beam B 1  and an auxiliary beam B 2 . The auxiliary beam B 2  includes plural auxiliary beams that overlap each other such that their power is continuously distributed, the plural auxiliary beams being positioned to form an approximate ring shape or pentagonal shape, with the main beam B 1  in the center. In an example illustrated in  FIG. 11G , laser beam L 37  includes a main beam B 1  and 16 auxiliary beams B 2 . The 16 auxiliary beams B 2  are positioned to form an approximate ring shape or an approximate regular octagonal shape, with the main beam B 1  in the center. In an example illustrated in  FIG. 11H , laser beam L 38  includes a main beam B 1  and an auxiliary beam B 2 . The auxiliary beam B 2  includes plural auxiliary beams that overlap each other such that their power is continuously distributed, the plural auxiliary beams being positioned to form an approximate ring shape or octagonal shape, with the main beam B 1  in the center. In the examples illustrated in  FIG. 11A  to  FIG. 11H , the sweep direction of the laser beam L 31  to L 38  may be in any direction, and may be in a direction not heading to a corner of the pentagon, hexagon, or octagon. Furthermore, the number of auxiliary beams that are not in the front in the sweep direction may be thinned out a little if the auxiliary beams are closely arranged as illustrated in  FIG. 11A ,  FIG. 11C , or  FIG. 11G . In addition, in an example illustrated in  FIG. 11I , laser beam L 39  includes a main beam B 1  and 16 auxiliary beams B 2 . The 16 auxiliary beams B 2  are positioned to form an approximate ring shape or an approximate octagonal shape, with the main beam B 1  in the center. The main beam B 1  and the auxiliary beams B 2  are arranged to fill rectangles of a grid defined by a matrix M. 
     Second Embodiment 
       FIG. 12  is a diagram illustrating a schematic configuration of a laser welding apparatus according to a second embodiment. A laser welding apparatus  200  irradiates a workpiece W 1  with laser beam L to perform welding of the workpiece Wl. The workpiece W 1  is formed of two plate-like metallic members W 11  and W 12  superimposed on each other. The laser welding apparatus  200  implements welding by principles that are similar to those of the laser welding apparatus  100 . Therefore, only a device configuration of the laser welding apparatus  200  will be described below. 
     The laser welding apparatus  200  includes a laser device  210 , an optical head  220 , and an optical fiber  230 . 
     The laser device  210  includes a laser oscillator, is configured similarly to the laser device  110 , and is configured to be able to output laser beam having a power of, for example, a few kilowatts (kW). The optical fiber  230  guides the laser beam output from the laser device  210  to input the laser beam to the optical head  220 . 
     Similarly to the optical head  120 , the optical head  220  is an optical device for emitting the laser beam input from the laser device  210 , to the workpiece W 1 . The optical head  220  includes a collimator lens  221  and a condenser lens  222 . 
     Furthermore, the optical head  220  has a galvanoscanner placed between the condenser lens  222  and the workpiece W 1 . The galvanoscanner is a device that moves the position irradiated with the laser beam L to enable sweeping with the laser beam L without moving the optical head  220 , by controlling the angles of two mirrors  224   a  and  224   b.  The laser welding apparatus  200  includes a mirror  226  for guiding the laser beam L emitted from the condenser lens  222 , to the galvanoscanner. The angles of the mirrors  224   a  and  224   b  of the galvanoscanner are respectively changed by motors  225   a  and  225   b.    
     The optical head  220  includes a diffractive optical element  223  placed between the collimator lens  221  and the condenser lens  222  and serving as a beam shaper. Similarly to the diffractive optical element  123 , the diffractive optical element  223  splits laser beam input from the collimator lens  221 , into a main beam and at least one auxiliary beam. At least a part of the at least one auxiliary beam is positioned in front, in a sweep direction, of the main beam. The power of the main beam is larger than the power of each auxiliary beam, and a ratio between the power of the main beam and the total of powers of the at least one auxiliary beam is in the range of 9:1 to 1:9. The laser welding apparatus  200  is thereby able to reduce generation of sputter in welding of the workpiece W 1 . The ratio may be in the range of 144:1 to 1:9, depending on how the splitting and arrangement of the auxiliary beams are done. 
     Third Embodiment 
       FIG. 13  is a diagram illustrating a schematic configuration of a laser welding apparatus according to a third embodiment. A laser welding apparatus  300  irradiates a workpiece W 2  with laser beam L to perform welding of the workpiece W 2 . The workpiece W 2  is formed of two plate-like metallic members W 21  and W 22  adjacently laid to butt against each other. The laser welding apparatus  300  includes a laser oscillator and implements welding by principles that are similar to those of the laser welding apparatuses  100  and  200 . The configurations of elements (a laser device  310  and an optical fiber  330 ) other than an optical head  320  are similar to the corresponding elements of the laser welding apparatuses  100  and  200 . Therefore, only a device configuration of the optical head  320  will be described below. 
     Similarly to the optical heads  120  and  220 , the optical head  320  is an optical device for emitting laser beam input from the laser device  310 , to the workpiece W 2 . The optical head  320  includes a collimator lens  321  and a condenser lens  322 . 
     Furthermore, the optical head  320  has a galvanoscanner placed between the collimator lens  321  and the condenser lens  322 . Angles of mirrors  324   a  and  324   b  of the galvanoscanner are respectively changed by motors  325   a  and  325   b.  In the optical head  320 , the galvanoscanner is provided at a position different from that in the optical head  220 . However, similarly to the optical head  220 , by controlling the angles of the two mirrors  324   a  and  324   b,  the position irradiated with laser beam L is moved to enable sweeping with the laser beam L, without moving the optical head  320 . 
     The optical head  320  includes a diffractive optical element  323  placed between the collimator lens  321  and the condenser lens  322  and serving as a beam shaper. Similarly to the diffractive optical elements  123  and  223 , the diffractive optical element  323  splits the laser beam input from the collimator lens  321  to generate a main beam and at least one auxiliary beam. At least a part of the at least one auxiliary beam is positioned in front, in a sweep direction, of the main beam. The power of the main beam is larger than the power of each auxiliary beam, and a ratio between the power of the main beam and the total of powers of the at least one auxiliary beam is in the range of 9:1 to 1:9. The laser welding apparatus  300  is thereby able to reduce generation of sputter in welding of the workpiece W 2 . The ratio may be in the range of 144:1 to 1:9, depending on how the splitting and arrangement of the auxiliary beams are done. 
     Fourth Embodiment 
       FIG. 14  is a diagram illustrating a schematic configuration of a laser welding apparatus according to a fourth embodiment. As illustrated in  FIG. 14 , a laser welding apparatus  400  according to the fourth embodiment is an example of a configuration of a device that melts a workpiece W by irradiating the workpiece W with laser beam L 4  and laser beam L 5 . The laser welding apparatus  400  implements a welding method by principles that are similar to those of the welding apparatus according to the first embodiment. Therefore, only a device configuration of the laser welding apparatus  400  will be described below. 
     The laser welding apparatus  400  includes plural oscillators  411  and  412  that output laser beams, an optical head  420  that emits laser beams to the workpiece W, and optical fibers  431  and  432  that guide the laser beam output by the oscillators  411  and  412  to the optical head  420 . 
     The oscillators  411  and  412  are configured to be capable of outputting, for example, multi-mode laser beam at a power of a few kilowatts (kW). For example, each of the oscillators  411  and  412  may have plural semiconductor laser devices inside and be configured to output multi-mode laser beam at a power of a few kilowatts (kW) as the total output of the plural semiconductor laser devices, or any of various lasers including fiber lasers, YAG lasers, and disk lasers may be used instead. 
     The optical head  420  is an optical device for condensing the laser beam L 4  and laser beam L 5  respectively guided from the oscillators  411  and  412  to power densities of strengths enabling the workpiece W to be melted and for irradiating the workpiece W with the condensed laser beam L 4  and laser beam L 5 . Therefore, the optical head  420  includes a collimator lens  421   a  and a condenser lens  422   a  for the laser beam L 4 , and a collimator lens  421   b  and a condenser lens  422   b  for the laser beam L 5 . The collimator lenses  421   a  and  421   b  are optical systems for making laser beam guided by the optical fibers  431  and  432  into collimated light once and the condenser lenses  422   a  and  422   b  are optical systems for condensing the collimated laser beam onto the workpiece W. 
     This optical head  420  also has a function to cause the laser beam L 4  and laser beam L 5  on the workpiece W to include a main beam and at least one auxiliary beam, at least a part of the at least one auxiliary beam being in front, in a sweep direction, of the main beam. That is, of the laser beam L 4  and laser beam L 5  emitted to the workpiece W by the optical head  420 , the laser beam L 4  may be used for formation of the main beam and the laser beam L 5  may be used for formation of the auxiliary beam. The power of the main beam is larger than the power of each auxiliary beam, and a ratio between the power of the main beam and the total of powers of the at least one auxiliary beam is in the range of 9:1 to 1:9. This ratio may be in the range of 144:1 to 1:9, depending on how the splitting and arrangement of the auxiliary beams are done. Furthermore, only two sets of laser beam, the laser beam L 4  and laser beam L 5 , are used in the example illustrated in  FIG. 14 , but the number of sets of laser beam may be increased as appropriate, as long as they are formed to provide laser beam suitable for embodying the present disclosure, like the laser beam having the cross-sectional shapes exemplified by  FIG. 3 ,  FIG. 5 , and  FIG. 9  to  FIG. 11 . 
     In this fourth embodiment, the laser beam L 4  and laser beam L 5  are generated by using the two oscillators  411  and  412 . However, laser beam output from a single oscillator may be split into two by an optical divider to generate the laser beam L 4  and laser beam L 5 . Furthermore, in this fourth embodiment, the optical fibers  431  and  432 , the collimator lens  421   a  and condenser lens  422   a,  and the collimator lens  421   b  and condenser lens  422   b  are individually included for the two oscillators  411  and  412 , respectively. However, by using a multi-core fiber including two or more cores instead of the optical fibers  431  and  432 , sets of laser beam output respectively from the two oscillators  411  and  412  may be guided to an optical head through separate cores, and the optical head may emit the two sets of laser beam serving as the laser beam suitable for embodying the present disclosure, to a workpiece, the optical head using a collimator lens and a condenser lens common to the two sets of laser beam. 
     Furthermore, in the above described embodiments, the profile of the laser beam (power distribution profile) has discrete power regions formed of the main beam and auxiliary beams. A power region is a region having power contributing to melting of a workpiece, the region being in a plane perpendicular to the direction in which laser beam travels. However, having power enabling melting of a workpiece is not necessarily required for an individual power region alone, and each power region may just be capable of melting the workpiece by influence of energy given to the workpiece by the other power regions. 
     However, the power regions are not necessarily discrete, and plural power regions may be continuous with a line-symmetrical or asymmetrical distribution. For example,  FIG. 15A  illustrates a power distribution profile of an area of laser beam L 12 , the area being in the front. The laser beam L 12  is an example of laser beam having a power distribution profile different from that of the laser beam L. In this power distribution profile of the laser beam L 12 , two power regions PA 121  and PA 122  that are arranged in the front are continuous. The power region PA 121  has a single-peaked pattern with a peak, and is, for example, a main power region. The power region PA 122  has a shoulder pattern, and is, for example, an auxiliary power region. A boundary between the two power regions PA 121  and PA 122  in the curve of  FIG. 15A  may be defined, for example, as the position of an inflection point present between the two power regions PA 121  and PA 122 . 
       FIG. 15B  on the other hand illustrates a power distribution profile of an area of laser beam L 13 , the area being in the front. The laser beam L 13  is another example of laser beam having a power distribution profile different from that of the laser beam L. In this power distribution profile of the laser beam L 13 , two power regions PA 131  and PA 132  that are arranged are continuous. The power regions PA 131  and PA 132  each have a single-peaked pattern with a peak, and are for example, a main power region and an auxiliary power region, respectively. A boundary between the two power regions PA 131  and PA 132  in the curve of  FIG. 15B  may be defined, for example, as the position of the local minimum point present between the two power regions PA 131  and PA 132 . Each of the laser beam L 12  and laser beam L 13  may be used as laser beam formed of a main power region and an auxiliary power region according to the present disclosure. The laser beam L 12  and laser beam L 13  may be obtained by using optical components, such as, for example: diffractive optical elements and optical lenses serving as beam shapers and designed appropriately; and optical fibers that are able to control power distributions. Similarly, power distribution profiles of other forms of laser beam each formed of a main power region and an auxiliary power region according to the present disclosure, such as the laser beam L, laser beam L′, laser beam L″, laser beam Ll, laser beam L 2 , and laser beam L 31  to laser beam L 38 , may also be achieved by using optical components, such as, for example: diffractive optical elements and optical lenses serving as a beam shaper and designed appropriately; and optical fibers that are able to control power distributions. 
     The form of welding with the main beam (main power region) in each of the embodiments in this specification may be keyhole welding or heat conduction welding. Keyhole welding referred to herein is a welding method using keyholes. Heat conduction welding on the hand is a welding method in which heat generated by absorption of laser beam at a surface of a workpiece is used to melt the workpiece. 
     Furthermore, all of the auxiliary beams may have the same power, or one or more of the auxiliary beams may be higher in power than the other auxiliary beams. In addition, plural auxiliary beams may be classified into plural groups, auxiliary beams in the same group may substantially have the same power, and auxiliary beams in different groups may differ in power. In this case, when auxiliary beams classified into different groups are compared with each other, they differ in power in a stepwise manner. The number of auxiliary beams included in a group is not necessarily plural and may be singular. In any case, the ratio between the power of the main beam and the total of powers of the plural auxiliary beams is preferably 144:1 to 1:9. 
     Furthermore, workpieces are not limited to plates and forms of welding are not limited to lap welding and butt welding. Therefore, a workpiece may be formed by superimposing at least two members to be welded on each other, bringing them into contact with each other, or laying them adjacently to each other. 
     Furthermore, when sweeping a workpiece with laser beam, sweeping may be performed by a known technique, such as wobbling, weaving, or output modulation, to adjust the surface area of the molten pool. 
     Furthermore, like a plated metallic plate, a workpiece may have, on its metallic surface, a thin layer of another metal. In addition, workpieces having thicknesses of about 1 mm to 10 mm have been described as examples, but the workpieces may be thinner at about 0.01 mm or may be thicker. 
     The present disclosure is not limited by the above described embodiments. The present disclosure also includes those configured by combination of any of the above described components of the embodiments as appropriate. Furthermore, further effects and modifications can be easily derived by those skilled in the art. Therefore, wider aspects of the present disclosure are not limited to the above described embodiments and various modifications may be made. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure may be utilized in laser welding. 
     The present disclosure has an effect of enabling reduction in generation of sputter. 
     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.