Abstract:
Provided is a method of laser-welding that can achieve uniform welding of a member with large laser reflectance on a surface thereof A laser welding step (S 1 ) includes a surface roughening step (S 11 ) in which a first laser processing device irradiates laser light to a surface (each welding portion ( 31 )) of a welding section ( 30, 30, 30, 30 ) between a negative electrode terminal ( 20 ) (a first member) and a negative electrode lead ( 21 ) (a second member) and carries out roughening, thereby forming a laser marker ( 32 ) on the surface; and a welding step (S 12 ) in which a second laser processing device irradiates laser light to each welding portion ( 31 ) roughened in the surface roughening step (S 11 ) and melts each welding portion ( 31 ), thereby carrying out the laser-welding of the negative electrode terminal ( 20 ) to the negative electrode lead ( 21 ).

Description:
TECHNICAL FIELD  
       [0001]    The present invention relates to a method of welding using laser beam and a method of manufacturing a battery including the same, especially to a technique of laser welding a member, such as a copper member, having high laser reflectance on the surface thereof 
       BACKGROUND ART  
       [0002]    There is a technique of bonding two metal members by using the laser welding, and that is broadly used in the industrial field. 
         [0003]    The laser welding using YAG laser beam is easy to be used under air atmosphere, and therefore it is really advantageous in the respect of costs and control, compared to other welding technique, e.g. the electron beam welding (EBW) which is performed under vacuum atmosphere. The laser welding using YAG laser tends to be applied to the mass production line. 
         [0004]    When laser welding the member with high laser reflectivity on the surface such as the copper member, most of the laser light reflects on the surface and it is difficult to obtain enough heat input. Now, the laser welding to such member is not put to practical use. 
         [0005]    There is a laser machining apparatus using green laser that has good absorption to the copper member. Unfortunately, the laser machining apparatus using the green laser generally has low output, and hence the apparatus is used only for welding or machining of the thin member. 
         [0006]    As to the manufacturing process for a battery such as the lithium ion secondary battery, in the light of the efficiency in the assembling process, two copper members are welded (e.g., the lead of the negative electrode and the negative electrode terminal). 
         [0007]    However, as mentioned above, if the normal laser machining apparatus is used, the enough heat input will not be obtained because the copper members reflect the laser beam or the laser output is low, and thereby, it is hard to provide enough welding depth. 
         [0008]    One of the methods to solve the above problem is to use a laser machining apparatus with high laser output. Unfortunately, spatter and soot may occur or the welding point may be penetrated, whereby there is a problem with the weldability. 
         [0009]    Furthermore, the laser machining apparatus with high laser output gives heat input to the surrounding members more than necessary, so that the members disposed around the welding point may cause thermal failure. 
         [0010]    JP 2003-263977 A discloses a technique of lowering the laser reflectance on the surface by roughening the surface to be welded in advance by using a sand paper, a grind agent, a blast shot, or a chemical etching. 
         [0011]    The roughening method of JP 2003-263977 A is applicable to roughen broadly, but that is hardly applicable to roughen the minute point evenly that has the shape variation such as step surface, so that the variation in the surface condition easily appears. Moreover, the roughening methods need masking during the roughening process or cleaning after roughening, and the process may be complex so that it is troublesome to use in the production line. 
         [0012]    Especially in using the high-output laser machining apparatus, it is easy to be influenced on the variation in the surface condition or the product condition (for instance, the shape thereof or the assemble accuracy). So, the heat input during the laser welding becomes unstable and the welding failure easily happens. 
         [0013]    Thus, the conventional laser welding method may fail to weld evenly the members of high laser reflectance on the surface. 
       CITATION LIST  
       [0014]    Patent Literature
       PTL 1: JP 2003-263977 A       
 
       SUMMARY OF INVENTION  
     Technical Problem 
       [0016]    The objective of the present invention is to provide methods of laser welding and of manufacturing a battery enabled to weld evenly a member having high laser reflectance on the surface thereof. 
       Technical Solution 
       [0017]    The first aspect of the present invention is a method of laser welding a first member and a second member. The method includes a roughening step for roughening a surface of the welding portions of the first and second members using a laser beam applied from a first laser machining apparatus and a welding step for melting the roughened surface using laser beam applied from a second laser machining apparatus and welding the first and second members. 
         [0018]    In the first aspect of the present invention, the first and second members are members of high laser reflectance on the surfaces thereof with respect to the laser beam applied from the second laser machining apparatus. 
         [0019]    In the preferable embodiment of the first aspect of the present invention, the welding using the second laser machining apparatus is performed under oxygen atmosphere. 
         [0020]    The second aspect of the present invention is a method of manufacturing a battery including the first and second members as a configuration, in which the first and second members are bonded using the method according to the first aspect of the present invention. 
         [0021]    In the method of manufacturing the battery, it is advantageously applicable to the case where the first and second members include a portion formed by caulking a rivet portion in the welding portion. That is, the second aspect of the present invention is applicable to the member to be welded has high laser reflectance and has the deformed minute portion. 
       ADVANTAGEOUS EFFECTS OF INVENTION  
       [0022]    According to the embodiment of the present invention, the even welding for the members of high laser reflectance on the surface thereof is achieved. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0023]      FIG. 1  schematically illustrates a battery. 
           [0024]      FIG. 2  is a section view of welding portions in the battery that is an object to be laser welded. 
           [0025]      FIG. 3  is a plain view of the welding portions. 
           [0026]      FIG. 4  is a flowchart of a laser welding step. 
           [0027]      FIG. 5  depicts the laser welding step. 
           [0028]      FIG. 6  is an enlarged view showing the welding portions. 
       
    
    
     REFERENCE SIGNS LIST  
       [0000]    
       
           10 : battery 
           20 : negative electrode terminal (first member) 
           21 : negative electrode lead (second member) 
           22 : rivet portion 
           30 : welding portion 
           31 : welding point 
           32 : laser mark 
       
     
       DESCRIPTION OF EMBODIMENTS  
       [0036]    Referring to drawings, a laser welding step S 1  as an embodiment of the present invention is described. In the laser welding step S 1 , a negative electrode terminal  20  and a negative electrode lead  21  composing the negative electrode of a battery  10  is welded by using laser beam. 
         [0037]    Hereinafter, the structure of the battery  10  to be welded in the laser welding step S 1  is explained. 
         [0038]    The battery  10  is a lithium ion secondary battery, and as shown in  FIG. 1 , includes a casing  11  housing an electric power generating element  12 . The casing  11  has a box  13  and a lid  14  covering the opening of the box  13 . The lid  14  is formed with two holes  14   a  through which a positive electrode terminal  15  and the negative electrode terminal  20  are projected outward. 
         [0039]    The negative electrode terminal  20  is an outer electrode terminal made of copper, and electrically connected to the element  12  via the negative electrode lead  21 . 
         [0040]    In detail, as depicted in  FIGS. 2 and 3 , the negative electrode terminal  20  and the negative electrode lead  21  are welded to the hole  14   a  of the lid  14  where a rivet portion  22  formed at the head of the lead  21  is caulked through a sealing member  23  and an insulating member  24 . The terminal  20  and the lead  21  are connected by four welded portions  30 . 
         [0041]    In the viewpoint of quality control, the welded portions  30  are four in number, but the number thereof may be determined selectively. 
         [0042]    The negative electrode lead  21  is a collective electrode terminal made of copper as same as the terminal  20 , and connected to the negative electrode side of the element  12 . 
         [0043]    The rivet portion  22  is formed at the end of the lead  21  having the shape of rivet. The head of the rivet portion  22  has the larger diameter than the inner diameter of the hole  14   a  of the lid  14 . As shown in  FIG. 2 , in the welded portion  30 , the head of the rivet portion  22  is above the top (highest position) of the terminal  20 . 
         [0044]    The sealing member  23  is made of resin, and is disposed between the lead  21  and the lid  14  to seal the inside of the casing  11 . 
         [0045]    The insulating member  24  is made of resin, and insulates between the lead  21  and the lid  14 , thereby preventing the electric conduction from the lead  21  to the lid  14 . 
         [0046]    As described above, the laser welding step S 1  includes a step for laser welding the negative electrode terminal  20  as a copper member and the rivet portion  22  of the negative electrode lead  21  as a copper member, both of which are contained in the battery  10 . 
         [0047]    As shown in  FIG. 2 , the welding portions  30  between the terminal  20  and the rivet portion  22  of the lead  21  are formed in an uneven shape and has a step, and thereby the shape of the welding portion is complex. Further, the head of the rivet portion  22  is caulked, so that the surface of the welding portion is rough. Due to these features, the laser welding requires high accuracy, for example, it requires stable heat input and enough laser penetration. 
         [0048]    In the vicinity of the welding portions  30  between the terminal  20  and the lead  21 , the resin members are disposed that have weaker heat resistance than the metal members, so that the heat influence on these members around the welding portions  30  should be considered, e.g., the laser output should be lowered. 
         [0049]    As described above, the laser welding step S 1  provides the laser welding with high accuracy and takes into consideration of the heat influence. 
         [0050]    The laser welding step S 1  is described below. 
         [0051]    As depicted in  FIGS. 4 and 5 , the laser welding step S 1  includes a roughening step S 11  and a welding step S 12 . 
         [0052]    The roughening step S 11  includes applying a laser beam to the surface of the portions to be formed as the welding portions  30  using a first laser machining apparatus and roughening the surface where the laser beam is applied. In the roughening S 11 , the first laser machining apparatus emits a laser beam having a wavelength with good absorption to such members that have high laser reflectance on the surface (the member of high reflectance, such as copper). 
         [0053]    The welding step S 12  includes applying a laser beam to the surface roughened in the roughening step S 11  to be formed as each welding portion  30  using the second laser machining apparatus and melting the portion to which the laser beam is applied for laser welding. The second laser machining apparatus is used for the general laser welding and applies the YAG laser beam. 
         [0054]    In the roughening step S 11 , the green laser beam with 532 nm wavelength is applied to the surface of the portions to be formed as the welding portions  30  (in this embodiment, four welding points  31  shown in  FIG. 5(   a )) to form laser marks  32  at these points, thereby roughening the welding points  31 . 
         [0055]    The welding points  31  are set so as to provide the required welding portions  30 , and as shown in  FIG. 5 , the welding points are set in the parts of the periphery of the rivet portion  22  of the lead  21  and the parts of the terminal  20  overlapped by the periphery of the rivet portion  22 . 
         [0056]    It should be noted that the arrangement of the welding points  31  set in the roughening step S 11  is not limited in accordance with the positions or the areas of the welding portions  30 . That is, the welding points  31  may be set in any position where the required welding portions  30  are formed. 
         [0057]    More specifically, as shown in  FIG. 5(   b ), in the roughening step S 11 , the first laser machining apparatus applies the green laser beam to the welding points  31  that are set in the outer periphery of the rivet portion  22  caulked in circular shape and in the part of the terminal  20  where the outer periphery of the rivet portion  22  overlaps. Thereby, the laser marks  32  having rectangular shapes are created and the surface where the laser beam is emitted is evenly roughened. The laser marks  32  have grooves with a predetermined depth (e.g., 0.3 μm to 0.4 μm) and are formed as the step surface. 
         [0058]    Thus, the surface of the portions where the laser marks  32  are formed are tarnished, so that it is possible to use the second laser machining apparatus using the YAG laser beam that has the high laser reflectance on the surface of the copper member. 
         [0059]    When the YAG laser beam is applied to the members of high laser reflectivity such as the copper member without surface processing, the most of the laser beam reflects on the surface and the absorption rate is low, and therefore it is difficult to obtain good weldability. However, due to the laser marks  32  formed on the laser points  31 , the surface loses gloss and the YAG laser beam emitted from the second laser machining apparatus is efficiently absorbed in the laser points  31 , thereby obtaining good weldability. 
         [0060]    In the embodiment, “member of high laser reflectance/reflectivity” means the member as follows: when the YAG laser beam is applied to the surface thereof without surface processing, the most of the laser beam is reflected on the surface and the absorption into the member is low, and therefore it is difficult to obtain good weldability. 
         [0061]    The first laser machining apparatus used in the roughening step S 11  is controlled with position and timing of laser applying by a controller so that the laser marks  32  have required areas with regard to the welding points  31  (for example, larger than the welding point  31  shown in  FIG. 5(   b )) and required depths of laser penetration. 
         [0062]    As described above, the green laser beam that is controllable with high accuracy provides the laser marks  32  on the welding points  31 , and hence the roughening step is easily set in the mass production line such as manufacturing line of the batteries  10 . 
         [0063]    In the welding step S 12 , the YAG laser beam with 1064 nm wavelength is applied to the welding points  31  formed with the laser marks  32  to melt the surfaces of the terminal  20  and the rivet portion  22  of the lead  21  so as to weld the terminal  20  and the lead  21 , thereby forming the welding portions  30 . 
         [0064]    More specifically, as shown in  FIG. 5(   c ), in the welding step S 12 , the second laser machining apparatus applies the YAG laser beam to the laser marks  32  formed in the rivet portion  22  and the terminal  20  to melt the welding points  31  so as to laser weld the terminal  20  and the rivet portion  22 . 
         [0065]    As described above, the surface of the welding points  31  formed with the laser marks  32  lose the gloss which the copper member originally has and the area of the surface is increased due to the rough surface formed by the laser mark  32 . Thus, at the welding points  31 , the YAG laser beam emitted from the second laser machining apparatus is efficiently absorbed. Therefore, even in the welding points  31  made by copper, the laser penetration due to the heat input is deepen, so that the sufficient depth of laser penetration and welding areas are obtained. 
         [0066]    Moreover, each welding points  31  has the laser mark  32  which roughens the surface evenly, and hence the surface condition is stable and the heat input to the welding points  31  from the laser beam is stable. 
         [0067]    According to the laser welding step S 1 , the negative electrode terminal  20  made of copper having high laser reflectance on the surface thereof (especially high reflectivity with respect to the second laser machining apparatus) and the rivet portion  22  of the negative electrode lead  21  also made of copper are evenly welded at the welding points  31 , thereby preventing the welding failures such as blow holes or cracks. 
         [0068]    Furthermore, the same effects enhance the robustness against the variation in caulked portion of the rivet portion  22  (the shape of the surface or the caulking gap) and in surface condition. 
         [0069]    The laser output of the second laser machining apparatus used in the welding step S 12  is kept low, and thereby preventing the influences on the members disposed around the welding points  31 . 
         [0070]    In the welding step S 12 , the second laser machining apparatus applying the YAG laser beam sprays an oxygen gas as an assist gas, and the laser welding is performed under oxygen atmosphere. 
         [0071]    Hence, the radical oxidation occurs when the member is melted, which leads exothermic reaction, thereby accelerating the laser penetration. 
         [0072]    Due to the above feature, the welding step S 12  provides improved weldability. 
         [0073]    When creating the laser marks  32  are formed in the roughening step S 11 , the surface and the inside of the grooves of the laser marks  32  are covered with minute dusts. 
         [0074]    As described above, the welding step S 12  is performed under oxygen atmosphere, and thus the minute dusts remained on the laser marks  32  burn (a dust explosion occurs), the burning in the laser welding is activated, thereby accelerating the welding. As a result, in the welding points  31 , the laser penetration and welding areas are obtained sufficiently. 
         [0075]    The rivet portion  22  of the negative electrode lead  21  that is the one member to be welded in the laser welding step S 1  is the member of high laser reflectivity and the minute portion which is deformed in caulking. So, the conventional method of laser welding cannot be applied to the welding in the case that the member to be welded has high reflectivity on the surface thereof and unstable surface condition due to the caulking and is formed as the minute portion. 
         [0076]    On the other hand, the laser welding step S 1  includes a first welding step for roughening the surface in the roughening step S 11  and a second welding step for welding the surface roughened in the roughening step S 11  in the welding step S 12 , and thereby providing the welding that cannot be provided by the conventional welding method. Moreover, the laser welding step of the embodiment is preferably applicable to the manufacturing process for the battery  10  containing the negative electrode terminal  20  and the negative electrode lead  21  both of which include the above-mentioned welding spots. 
         [0077]    As depicted in  FIG. 6 , the rivet portion  22  is formed with the edge portion  25  that is a thin portion involved in the welding points  31 , and in the welding step S 12 , the laser beam is preferably applied to the edge portion with 30° to 45° inclined from the inside to the outside. 
         [0078]    Thus, when laser welding, the edge portion  25  absorbs the laser beam efficiently, so that the laser output can be decreased and the welding stability is improved. 
         [0079]    In the roughening step S 11 , the laser marks  32  are formed in square and each area thereof is larger than the welding point  31 , but the configuration of the laser mark is not limited to this embodiment. For instance, the area of the laser mark  32  may be smaller than that of the welding point  31 , and in this case the laser marks  32  is sufficiently melted and welded, and there is enough heat input in the vicinity of the laser marks due to the heat conduction. 
         [0080]    The embodiment is applied to the laser welding for the negative side of the battery  10 , the lithium ion secondary battery, and may be used in the welding for the member of high laser reflectance on the surface thereof in the same way. For instance, the laser welding step S 1  is applicable to the bonding of copper wires broadly installed in electric devices, in this case, the laser welding step substitutes for the soldering. 
       INDUSTRIAL APPLICABILITY  
       [0081]    The present invention is applicable to a laser welding process in which a laser beam is applied to a rough surface, especially to a technique of laser welding a member of high laser reflectance on the surface thereof.