WO2015114445A2

WO2015114445A2 - Welding method and welding structure

Info

Publication number: WO2015114445A2
Application number: PCT/IB2015/000087
Authority: WO
Inventors: Hiroya Umeyama; Yasushi Hirakawa; Yukio Harima
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2014-01-30
Filing date: 2015-01-29
Publication date: 2015-08-06
Also published as: JP6071010B2; WO2015114445A3; JP2015163412A

Abstract

A welding method disclosed herein is a method for laser-welding a plate-shaped first member and a plate-shaped second member. This method includes emitting first laser light having a strength at which a keyhole can be generated to the wide surface of the first member along the first bonding surface and forming a first molten pool formed by being melted by the first laser light, the first molten pool being formed over the second member; and emitting second laser light having a strength lower than a strength at which a keyhole can be generated to the side surface portion of the second member along the second bonding surface and forming a second molten pool formed by being melted by the second laser light, the second molten pool being formed over the first member. The first molten pool and the second molten pool are integrated with each other and form a molten pool and the first member and the second member are welded by a welding portion formed when the molten pool is solidified.

Description

WELDING METHOD AND WELDING STRUCTURE

BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] The invention relates to a welding method and a welding structure. More particularly, the invention relates to a welding method and a welding structure, which allow appropriate welding while suppressing void generation. 2. Description of Related Art

[0002] According to the related art, an electric power storage element such as a lithium-ion secondary battery, a sodium battery, an electric double layer capacitor, and a lithium ion capacitor is produced by accommodating an electric power generating element performing electric power generation in a case and performing sealing. Typically, a case main body that has an opening in one surface and a lid member that is shaped to correspond to the opening constitute the case, and the opening portion is sealed by the lid member by fitting the lid member into the case main body and bonding the case main body and the lid member to each other by a method such as welding. In general, laser welding (also referred to as laser beam welding) is adopted for the sealing of the case. Japanese Patent Application Publication No. 2011-092944 (JP 2011-092944 A) and Japanese Patent Application Publication No. 2011-204396 (JP 2011-204396 A) are examples of the related art relating to the laser welding.

[0003] According to JP 2011-092944 A, for example, superimposed laser light in which one low-brightness laser and two high-brightness lasers are superimposed on each other is used when a first member and a second member are allowed to abut against each other and are bonded to each other. The low-brightness laser is emitted for a gap between the first member and the second member to be an optical axis and for a laser irradiation area to be large. In addition, regarding the high-brightness laser light, the two high-brightness lasers smaller in irradiation area than the low-brightness laser light are emitted to the first member and the second member, respectively. The first member and the second member are bonded as the gap is filled with a molten material of the first member and a molten material of the second member that are melted by the superimposed laser light.

SUMMARY OF THE IN VENTION

[0004] Plate-shaped members constituting the case have been made thinner, in the interest of weight reduction, cost reduction, and the like, for the case of the electric power storage element described above and the like. For example, the lid member is fitted into an inner side of the opening portion of the case main body and the laser light is emitted from above the case during the welding of the case main body and the lid member. In this case, a side surface portion of a plate material constituting the case main body corresponds to a laser irradiation surface irradiated with the laser light during the welding when it comes to the case main body. In other words, a dimension (hereinafter, the dimension will be simply referred to as a "width" of the laser irradiation surface in some cases) of a direction orthogonal to an interface (the interface can be bonding surfaces) of the case main body and the lid member, which is a plane direction in which the laser light is emitted, corresponds to a thickness of the plate material in the case main body.

[0005] When the welding method disclosed in JP 2011-092944 A is applied to the welding of the case main body having the small laser irradiation surface width and the lid member, a void (also referred to as a blowhole, porosity, a pore, or the like) is likely to be formed in welding depth portions of the case main body and the lid member. The void that is formed in the case main body having the small dimension (width) considerably affects strength of the plate material constituting the case main body to be decreased and a sufficient welding strength cannot be obtained. In contrast, when the welding is performed by emitting the low-brightness laser light without forming the void, a sufficient melting depth cannot be ensured and, eventually, a sufficient bonding surface depth cannot be ensured, and thus a sufficient welding strength cannot be obtained. The invention provides a welding method and a welding structure that are capable of suppressing a decrease in welding strength attributable to a void by suppressing void generation and stably realizing a high welding strength.

[0006] According to a first aspect of the invention, there is provided a welding method for laser-welding a first member having a plate shape and a second member having a plate shape. The welding method includes the followings:

allowing the first member and the second member to abut against each other in a direction in which wide surfaces of the first member and the second member are orthogonal to each other and allowing a side surface portion of the first member and a wide surface end portion of the second member to abut against each other such that a side surface portion of the second member is substantially flush with the wide surface of the first member, a dimension of the first member in a direction orthogonal to a first bonding surface being greater than a dimension of the second member in a direction orthogonal to a second bonding surface, the side surface portion of the first member that faces the second member being the first bonding surface, and the wide surface end portion of the second member that faces the first member being the second bonding surface;

emitting first laser light having a strength at which a keyhole can be generated to the wide surface of the first member along the first bonding surface and forming a first molten pool formed by being melted by the first laser light, the first molten pool being formed over the second member; and

emitting second laser light having a strength lower than a strength at which a keyhole can be generated to the side surface portion of the second member along the second bonding surface and forming a second molten pool formed by being melted by the second laser light, the second molten pool being formed over the first member.

The first molten pool and the second molten pool are integrated with each other and form a molten pool and the first member and the second member are welded by a welding portion formed when the molten pool is solidified.

[0007] According to this configuration, the keyhole first molten pool having a sufficient penetration depth can be formed in the first member by the first laser light having a relatively high strength (high brightness) and a sufficient amount of a molten metal can be ensured. Regarding the thinner second member, the heat conduction second molten pool can be formed by the second laser light having a relatively low strength (low brightness) and generation of a void, which can decrease welding strength, in a thin portion can be suppressed. In this case, the second molten pool is superimposed on the first molten pool. Then, a ratio of the welding portion to the bonding surface which is a boundary between the first member and the second member can be easily and stably increased. In other words, the bonding surfaces of the first member and the second member as a whole can be connected with the welding portion having a sufficient area and the welding strength can be stably ensured even in a case where the keyhole first molten pool is misaligned from a predetermined position due to, for example, a trajectory misalignment of the first laser light.

[0008] In the first aspect of the welding method disclosed herein, an output density of the first laser light may be at least 5.6xl0⁶ W/cm² and the output density of the first laser light may be less than 1.1x10^s W/cm². When the output density of the first laser light is approximately within the above-described range in this manner, the keyhole molten pool can be reliably formed and poor welding attributable to laser having a surplus strength can be suppressed. Accordingly, the welding strength can be further reliably increased to allow high-quality welding.

[0009] In the first aspect of the welding method disclosed herein, an output density of the second laser light may be at least 2.8x10 W/cm and the output density of the second laser light may be less than 5.6xl0⁶ W/cm². According to this configuration, a heat conduction molten pool having a sufficient penetration depth can be reliably formed in the second member by adjusting the output density of the second laser light. Accordingly, the generation of the void in the second member can be reliably prevented.

[0010] In the first aspect of the welding method disclosed herein, the first laser light and the second laser light may have a scanning speed of at least 20 m per minute when the output density of the second laser light is at least 3.8x10 W/cm and is less than 5.6xl0⁶ W/cm². In this case, welding of high bonding strength and higher quality can be realized at a relatively high speed. [0011] In the first aspect of the welding method disclosed herein, the first laser light and the second laser light may have a scanning speed of less than 20 m per minute when the output density of the second laser light is at least 2.8x10 W/cm and is less than 3.8x10° W/cm^. In this case, a sufficient amount of welding heat can be input and welding of high bonding strength and higher quality can be realized even in a case where the low-brightness laser is used as the second laser light.

[0012] In the first aspect of the welding method disclosed herein, the output density of the first laser light and the output density of the second laser light may satisfy the following expression in a case where the output density of the first laser light is defined as li and the output density of the second laser light is defined as I₂:

Ii≥5xl₂. According to this configuration, the above-described effects of the welding method can be achieved more efficiently.

[0013] In the first aspect of the welding method disclosed herein, an irradiation diameter of the first laser light and an irradiation diameter of the second laser light may satisfy the following expression in a case where the irradiation diameter of the first laser light is defined as di and the irradiation diameter of the second laser light is defined as d₂: di<d₂. According to this configuration, the welding described above can be carried out more efficiently.

[0014] In the first aspect of the welding method disclosed herein, a depth of the welding portion of the bonding surface in a cross section orthogonal to the first bonding surface or the second bonding surface may be equal to or greater than the dimension of the second member in the direction orthogonal to the second bonding surface. According to this configuration, the welding strength can be increased with greater reliability.

[0015] A second aspect of the invention relates to a welding structure. The welding structure includes a first member having a plate shape, a second member having a plate shape, and a welding portion where the first member and the second member are bonded. A dimension of the first member in a direction orthogonal to bonding surfaces of the first member and the second member is greater than a dimension of the second member in the direction. The welding portion has a first welding portion and a second welding portion divided by a surface including the bonding surfaces. A keyhole welding portion formed by laser with which the first member is irradiated mainly constitutes the first welding portion and the first welding portion has a tip part of the keyhole welding portion. A heat conduction welding portion formed by laser with which the second member is irradiated mainly constitutes the second welding portion and the second welding portion includes at least a part of a site of the keyhole welding portion other than the tip part and at least a part of the heat conduction welding portion.

[0016] According to this configuration, two types of laser light having different characteristics are used in combination and are appropriately applied to a welding target member such that a shape of the welding portion and defect occurrence can be appropriately controlled. Accordingly, a great welding depth can be stably ensured while, for example, the formation of a blowhole (which can be the void) is being suppressed. As a result, the welding structure can be realized with the welding strength stably ensured.

[0017] In the welding structure according to the second aspect disclosed herein, a depth of the welding portion of the bonding surface in a cross section orthogonal to the bonding surface may be equal to or greater than a dimension of the second member in a direction of the cross section. According to this configuration, the welding strength can be ensured with greater reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG 1 is a plan view for showing an embodiment of a welding method disclosed herein;

FIG 2A is a schematic cross-sectional view illustrating an example of a welding structure that is formed by the welding method illustrated in FIG 1;

FIG. 2B is a schematic cross-sectional view illustrating an example of a welding structure that is formed in a case where a first member is irradiated only with first laser light;

FIG. 3A is a view illustrating an appearance of a cross section in a welding direction of a welding portion that is formed by a welding method 1 according to an example;

FIG 3B is a view illustrating an appearance of a cross section in a welding direction of a welding portion that is formed by a welding method 2 according to an example;

FIG. 4 is a cross-sectional view illustrating an appearance in which a void is formed, in a second member by a welding method according to the related art by the welding method 2 according to the example;

FIG 5 is a cross-sectional cut-away view showing a configuration of a lithium-ion battery to which the welding method according to the invention is applied; and

FIG 6 is a top view of FIG 5.

DETAILED DESCRIPTION OF EMBODIMENTS

[0019] Hereinafter^ an electrode that is proposed herein will be described in detail with reference to accompanying drawings based on a preferred embodiment. Matters required for implementing the invention but not particularly mentioned in this specification (for example, general information such as a configuration of and a method for operating a welder) are to be understood as design matters of those skilled in the art based on the related art pertaining to the relevant field. Each of the drawings is depicted schematically. Dimensional relationships (length, width, thickness, and the like) in the respective drawings do not reflect actual dimensional relationships. The same reference numerals will be used to refer to members and sites achieving the same effect such that duplicate description is omitted or simplified.

[0020] FIG 1 is a plan view for showing the embodiment of a welding method according to the invention. FIG. 2 A is a view illustrating an example of a welding structure that is formed by the welding method disclosed herein and a view illustrating an example of a welding structure from a cross section which is orthogonal to a welding progress direction G. According to the welding method of this embodiment, first laser light 1 and second laser light 2 are used as illustrated in FIG. 1 such that a plate-shaped first member 10 and a plate-shaped second member 20 abut against and are bonded to each other. In this specification, the expression of "plate-shaped" relating to the shapes of the first member 10 and the second member 20 means th¾t the cross section of each member orthogonal to the welding progress direction G has a plate shape, and a shape of each member not in the vicinity of a welding portion affecting the welding method (for example, the welding portion including a thermal effect portion) is not limited at all.

[0021] According to the welding method, the first member 10 and the second member 20 are allowed to abut against each other in a direction in which wide surfaces of the first member 10 and the second member 20 are orthogonal to each other and a side surface portion of the first member 10 and a wide surface end portion of the second member 20 are allowed to abut against each other such that a side surface portion of the second member 20 is substantially flush with the wide surface of the first member 10. Herein, the side surface portion of the first member 10 facing the second member 20 is a first bonding surface 12 and the wide surface end portion of the second member 20 facing the first member 10 is a second bonding surface 22. Hereinafter, a dimension of the second member 20 in a direction orthogonal to the bonding surface (that is, a width of the second member 20 on a laser irradiation surface) will be described as L2. Herein, a dimension of the first member 10 in a direction orthogonal to the first bonding surface 12 is greater than the dimension of the second member 20 in the direction orthogonal to the second bonding surface 22. Although not particularly limited, the second member 20 can be typically thinner than the first member 10 regarding the first member 10 and the second member 20 that have the above-described shapes.

[0022] When the abutting is performed, the first member 10 and the second member 20 may be configured to be fitted into each other such that the abutting is carried out depending on the shapes of the first member 10 and the second member 20. Depending on the shapes of the first member 10 and the second member 20, a gap can be disposed between the first bonding surface 12 and the second bonding surface 22 so as to, for example, facilitate the fitting of the first member 10 and the second member 20 into each. other. Alternatively, the gap can be generated due to various errors or the like. The first member 10 and the second member 20 are pressed as the case may be, from outside the bonding surfaces 12, 22 toward the inside where the first member 10 and the second member 20 abut against each other, such that the gap between the first member 10 and the second member 20 can be narrowed or removed. In these drawings, an example in which no gap is present between the first member 10 and the second member 20 is illustrated. However, the welding method disclosed herein can be implemented similarly even when the gap is present. In a case where the gap is generated between the first member 10 and the second member 20, the example of the invention that is described below may be carried out on the assumption that the center of the gap is a bonding surface.

[0023] The first laser light 1 is high-brightness (high output density) laser light with which the wide surface of the first member 10 is irradiated and is used mainly to melt the first member 10. The first laser light 1 has a laser strength (output density) at which a keyhole can be generated in the first member 10. In a case where a welding target member is melted by laser light having a high output density, the welding target member starts to be evaporated in a molten pool at the same time as the laser light irradiation, a repulsive force is generated in a material surface due to steam resulting therefrom, and a depression is generated in the molten pool. Diffused reflection of the laser light is repeated in the depression, and the depression is deepened rapidly and a cavity is formed. This cavity is referred to as the keyhole. It is important that laser light having an output density that allows the formation of the keyhole constitutes the first laser light 1. The first laser light 1 is emitted to the laser irradiation surface (wide surface) of the first member 10 along the first bonding surface 12 and travels in the welding progress direction G. Lwi in FIG. 1 illustrates an example of a trajectory of the first laser light 1.

[0024] Laser strength of the first laser light 1 depends on a material of the first member 10 and the like, and thus cannot be said to be strictly fixed. As an approximate guide, the. first laser light 1 can be laser light having an output density Ii of 5.6xl0⁶ W/cm²≤Ii<l.lxl0⁸ W/cm². Preferably, the output density is at least lOxlO⁶ W/cm². More preferably, the output density Ii is at least 5.6xl0⁷ W/cm². An upper limit of the output density Ιχ is not particularly limited, but more-than-necessary laser energy input is r v ,

10

not desirable because the more-than-necessary laser energy input is likely to result in sputtering and can cause poor welding. Accordingly, the output density Ιχ can be, for example, an output density at which the first member 10 is not penetrated. Preferably, the output density Ii is equal to or less than lOxlO⁷ W/cm². More preferably, the output density 1_\ is equal to or less than 8xl0⁷ W/cm². Even more preferably, the output density Ii can be determined by using an output density at which a predetermined bonding surface depth Dw illustrated in FIG 2A can be stably ensured as a guide. The bonding surface depth Dw is a dimension of a welding portion 30 in a depth direction (vertical direction in the drawings) on the bonding surfaces 12, 22 formed when the first bonding surface 12 and the second bonding surface 22 abut against each other. For example, in the cross section orthogonal to the welding progress direction G illustrated in FIG, 2 A, the bonding surface depth Dw can be understood as the dimension of the welding portion 30 on a surface including an interface (bonding surface) between the first member 10 and the second member 20. In a case where the entire welding portion 30 is taken into account, the bonding surface depth Dw can be an index with which a welding area of the welding portion 30 can be reflected on the surface including the interface (bonding surface) between the first member 10 and the second member 20.

[0025] Referring to FIG. 2B for example, a shape of a molten pool that is formed only by the first laser light 1 forms a so-called keyhole. Accordingly, a bead width and the thermal effect portion in the cross section orthogonal to the welding progress direction G are narrow and a welding portion 16' with a deep penetration is obtained. For example, assuming that dimensions of a first molten pool 14' and the first welding portion 16' on a surface of the first member 10 (also referred to as, for example, melting widths) is al and melting depths of the first molten pool 14' and the welding portion 16' in the first member 10 is bl in the cross section, an aspect ratio between the first molten pool 14' and the first welding portion 16' (referred to as a first aspect ratio) that is expressed as (bl/al) can be a value exceeding one. Typically, the first aspect ratio is at least 1.5. More preferably, the first aspect ratio is at least 2. In actuality, not only is the first member 10 irradiated with the first laser light 1 but also a thermal effect from the second laser light 2 is added to the first member 10. Accordingly, it may be difficult to check the first aspect ratio during actual welding. In this case, the same material as the first member 10 and the second member 20 is used in advance as illustrated in the example of FIG. 2B for example such that the first aspect ratio can be obtained regarding the first molten pool 14' and the welding portion 16' formed by the irradiation with the first laser light 1 alone.

[0026] The second laser light 2 is low-brightness (low output density) laser light with which the side surface portion of the second member 20 is irradiated and is used mainly to melt the second member 20 and assist in the melting by the first laser light 1. The second laser light 2 has a laser strength that is lower than a laser strength at which a keyhole can be generated in the second member 20. When the welding target member is irradiated with the laser light having the output density, the laser light is absorbed by a surface of the welding target member and the absorbed light is converted into heat. Then, the welding target member is melted in an area where the heat energy is conducted and a molten pool is formed. In this form of heat conduction melting, no keyhole is formed. The second laser light 2 is emitted to the laser irradiation surface (side surface portion) of the second member 20 along the second bonding surface 22 and travels in the welding progress direction G. Lw2 in FIG. 1 illustrates an example of a trajectory of the second laser light 2.

:. [0027] Laser strength of the second laser light 2 depends on a material of the second member 20 and the like, and thus cannot be said to be strictly fixed. As an approximate guide, the second laser light 2 can be laser light having an output density I₂ of 2.8xl0⁶ W/cm²≤l2<5.6xl0⁶ W/cm². Preferably, the output density I₂ can be equal to or less than 5.5xl0⁶ W/cm². More preferably, the output density I₂ can be equal to or less than 4x10 W/cm . A lower limit of the output density I₂ is not particularly limited. Preferably, laser energy that allows a penetration depth at which the melting by the first laser light 1 can be assisted is input. Accordingly, the laser strength I₂ of the second laser light 2 can be, for example, determined by using an output density at which the predetermined bonding surface depth Dw can be stably ensured as a guide. Preferably, the output density I₂ is at least 3x10 W/cm . More preferably, the output density I₂ is at least 3.2xl0⁶ W/cm². Even more preferably, the output density I₂ can be an output density at which, for example, a shape of a molten pool 24 that is formed in the second member 20 forms a convex portion (refer to a right end portion of a second welding portion 26 in the drawing) in the depth direction in the laser irradiation direction in the cross section orthogonal to the welding progress direction G as illustrated in FIG. 2A.

[0028] The output density Ii of the first laser light 1 and the output density I₂ of the second laser are not particularly limited. Preferably, a relationship of Ιχ≥5 Ϊ2 is satisfied. When the output densities have this relationship, a shape of the welding portion 30 (described later) can be appropriately arranged. More preferably, the output density Ii of the first laser light 1 and the output density I₂ of the second laser satisfy Ii≥8xl₂. Even more preferably, 1^10 1₂, for example, I₁≥12xl₂ is satisfied.

[0029] A shape of a molten pool that is formed only by the second laser light 2 forms a so-called heat conduction. Accordingly, assuming that a dimension of a second molten pool on a surface of the second member 20 (also referred to as, for example, melting width) is a2 and a depth of the second molten pool in the second member 20 is b2, an aspect ratio of the second molten pool (referred to as a second aspect ratio) that is expressed as (b2/a2) can be a value equal to or less than one. Typically, the second aspect ratio is equal to or less than 0.7. More preferably, the second aspect ratio is equal to or less than 0.5. In actuality, not only is the second member 20 irradiated with the second laser light 2 but also the molten pool by the first laser light 1 is formed in the second member 20. Accordingly, it may be difficult to check the second aspect ratio during actual welding. Although not illustrated, the same material as the first member 10 and the second member 20 is used in advance in this case for example, as is the case with the first aspect ratio, such that the second aspect ratio can be obtained regarding the second molten pool and a second molten section formed by the irradiation with the second laser light 2 alone.

[0030] Irradiation diameters of the first laser light 1 and the second laser light 2 are not particularly limited. For example* the irradiation diameter of the first laser light 1 can be adjusted, in accordance with an output of a laser welding device (laser oscillation device) that is used, precision of an optical system, and the like, to allow keyhole welding. Likewise, the irradiation diameter of the second laser light 2 can be adjusted, in accordance with the output of the laser welding device (laser oscillation device) that is used, the precision of the optical system, and the like, to allow heat conduction welding and be capable of assisting the melting by the first laser light 1. Accordingly, with the first laser light 1 realizing the high output density and the second laser light 2 realizing the low output density, the irradiation diameter di of the first laser light 1 and the irradiation diameter d₂ of the second laser light 2 can typically have a relationship of di<d₂. For example, the irradiation diameter d₁ of the first laser light 1 and the irradiation diameter d₂ of the second laser light 2 being adjusted to have a relationship of 2di≤d₂ is shown as an example in which a balance between both of the lasers is appropriate. Preferably, the irradiation diameter di of the first laser light 1 and the irradiation diameter d₂ of the second laser light 2 have a relationship of 3di≤d₂. More preferably, the irradiation diameter dj of the first laser light 1 and the irradiation diameter d₂ of the second laser light 2 have a relationship of 5d₁≤d₂. An upper limit of the irradiation diameter d₂ of the second laser light 2 is not particularly limited but can be determined in view of a thickness L2 of the second member irradiated with the second laser light 2. For example, it is desirable that the irradiation diameter d₂ of the second laser light 2 is less than the thickness of the second member (d₂<L₂). More preferably, the irradiation diameter d₂ of the second laser light 2 is set by using approximately d₂≤0.8xL₂ (for example, 0.7xL₂≤d₂≤0.9xL₂) as a guide in view of a trajectory misalignment of the second laser light 2. Areas of the thermal effect portions (irradiation areas) of the first laser light 1 and the second laser light 2 may be superimposed, but the laser light itself does not necessarily have to be superimposed. Preferably, the first laser light 1 and the second laser light 2 can be emitted individually without being superimposed.

[0031] The first laser light 1 is emitted to, for example, a position on the wide surface (surface) of the first member 10 that is at a predetermined distance apart from the bonding surface, and is moved in the welding progress direction G along the first bonding surface 12 of the first member 10. In other words, an optical axis of the first laser light 1 is moved along the bonding surface, as illustrated by the L_W1 in FIG. 1, at the position on the surface of the first member 10 that is at a predetermined distance from the bonding surface. The second laser light 2 is emitted to, for example, a position in the side surface portion (surface) of the second member 20 that is at a predetermined distance apart from 5 the bonding surface, and is moved in the welding progress direction G along the second bonding surface 22 of the second member 20. In other words, an optical axis of the second laser light 2 is moved along the bonding surface, as illustrated by the Lw₂ in FIG. 1, at the position on the surface of the second member 20 that is at a predetermined distance from the bonding surface.

0 [0032] The melting of the first member 10 by the first laser light 1 mainly constitutes a first molten pool 14. The molten pool is formed over the second member. In other words, the second member 20 is also melted by the first laser light 1. The melting of the second member 20 by the second laser light 2 mainly constitutes the second molten pool 24. The molten pool is formed over the first member. In other words, the5 first member 10 is also melted by the second laser light 2. The first molten pool 14 and the second molten pool 24 are integrated with each other and form a single molten pool, and the welding portion 30 can be formed when the single molten pool is solidified. In this manner, the first member 10 and the second member 20 can be bonded by the welding , portion 30. Even in a case where the gap is generated between the first member 10 and0 the second member 20, the first member 10 and the second member 20 can be bonded by the welding portion 30 while the gap is being filled with the first molten pool 14 and the second molten pool 24. Preferably, the optical axes of the first laser light 1 and the second laser light 2 are allowed to travel in parallel or substantially together, such that the first molten pool 14 and the second molten pool 24 can be integrated with each other, so as5 to appropriately form the welding portion 30.

[0033] In essence, the first laser light 1 forms the keyhole first molten pool 14' as illustrated in, for example, FIG. 2B. Accordingly, the cross section of the first molten pool 14' orthogonal to the bonding surface (welding progress direction) can include a tip portion T that has an acute angle and a shape similar to a fan shape. When a position closer to the first bonding surface 12 is irradiated with the first laser light 1, a bonding surface depth Dw' by the first laser light 1 can be increased. The second molten pool 24 by the second laser light 2 is superimposed on the first molten pool 14', and thus symmetry of the welding portion of the keyhole is disturbed and the bonding surface depth Dw can be increased with convenience as illustrated in, for example, FIG. 2A.

[0034] In this case, the melting depth becomes rapidly shallow away from the tip portion T regarding a shape of a cross section of the first molten pool 14' by the first laser light 1. In other words, the melting depth rapidly decreases. Also, in an area of the second member 20, the depth rapidly decreases away from the second bonding surface 22. Accordingly, the trajectory of the first laser light 1 is misaligned toward a center side (left side in the drawing) of the first member 10 for any error, and even the first molten pool 14' as a whole is formed to be misaligned toward the center side of the first member 10 from the first bonding surface 12. In this case, the bonding surface depth Dw' formed by the first laser light 1 can be significantly decreased for a shape of the first molten pool 14'.

[0035] According to the welding method disclosed herein, the second molten pool by the second laser light 2 is superimposed on the first molten pool 14' by the first laser light 1. Accordingly, as illustrated in FIG. 2A for example, the degree of decrease in the melting depth of the second molten pool 24 of the second member 20 during separation ^• from the tip portion T can be relaxed. Accordingly, the sufficient bonding surface depth Dw can be stably ensured even in a case where the trajectory of the first laser light 1 is misaligned toward the center side of the first member 10. In addition, the heat conduction second molten pool 24 is formed by the second laser light 2, and thus a welding defect such as a void is unlikely to be formed in the second member 20. Accordingly, the strength of the welding portion 30 in the second member 20 can be maintained at a high level.

[0036] In a case where the dimension L₂ of the second member 20 in a width direction orthogonal to the second bonding surface 22 is relatively small, that is, thin, or the like, heat input by the second laser light 2 propagates to the second member 20 and can reach the wide surface on the side opposite to the second bonding surface 22. For example, even a corner portion of the second member 20 that is an end portion on the side opposite to the second bonding surface 22 in the side surface portion of the second member 20 may be melted as the case may be. In this case, further heat input cannot propagate to a width-direction outer side of the second member 20 (side opposite to the second bonding surface 22), and thus propagate in the depth direction along the wide surface on the opposite side. The heat input increases the bonding surface depth Dw in the vicinity of the bonding surface 22 and is transmitted in the depth direction in the vicinity of the opposite-side surface and can change the melting depth of the second molten pool 24. As a result, the shape of the welding portion 30 in the cross section of the second member 20 orthogonal to the second bonding surface 22 can be a characteristic one as illustrated in, for example, FIG 2A. For example, the welding portion 30 is divided by the surface including the bonding surfaces 12, 22 as illustrated in FIG. 2 A such that the welding portion positioned on the first member 10 side is a first welding portion 16 and the welding portion positioned on the second member 20 side is the second welding portion 26. Then, in other words, the melting depth of the second welding portion 26 gently decreases temporarily away from the second bonding surface 22 but increases again (inclination on a lower side in the drawing) for further separation and approach to the opposite-side surface and a part convex in the depth direction (convex downward in the drawing) can be included. The specific shape of the second welding portion 26 is realized^ and the: shape of the cross section of the entire welding portion 30 has the keyhole in the first member 10 and the convex portion in the depth direction in the second member 20. In other words, the welding portion 30 pinches the bonding surface by the two convex portions in the direction parallel to the bonding surface and suppresses separation between the first member 10 and the second member 20. Accordingly, bonding strength in the width direction orthogonal to the bonding surface can be further increased. In addition, the bonding surface depth Dw of the welding portion 30 can be further stably ensured even in a case where the trajectory of the first laser light 1 is misaligned toward the center side of the first member. .

[0037] The characteristic cross-sectional shape of the second welding portion 26 described above can be appropriately formed by appropriately adjusting the amount of heat input to the second member 20 by the second laser light. For example, input of a sufficient amount of welding heat within a range in which the heat conduction second molten pool is formed in the second member 20 and transmission of the welding heat along 5 the surface of the second member 20 are described as a preferred example. Preparation of the amount of heat input depends on a material and a width (dimension in the direction orthogonal to the bonding surface) of the second member 20 and the like, and thus cannot be said to be strictly fixed. As an example, however, the welding can be carried out by using the following conditions as a guide in a case where the second member 20 is a thin

10 member with a width of 1 mm or less (for example, 0.5 mm or less).

[0038] In other words, specifically, and for example, (1) a sufficient amount of heat can be instantly supplied from the laser when the output density I₂ of the second laser light has a low brightness of at least 3.8xl0⁶ W/cm² and less than 5.6xl0⁶ W/cm². Accordingly, the welding can be performed at a relatively high scanning speed of the

15 second laser light of at least approximately 20 m per minute. (2) When the output density I₂ of the second laser light has a lower brightness of at least 2.8xl0⁶ W/cm² and less than

6 2

3.8x10 W/cm , a sufficient amount of heat is unlikely to be instantly supplied from the laser. Accordingly, it is preferable that the scanning speed of the second laser light is a ^{! ;} relatively low speed of less than 20 m per minute (preferably, equal to or less than 15 m 0 per minute, for example, equal to or less than 10 m per minute) so as to ensure the formation of the characteristic cross-sectional shape of the second welding portion 26. In this manner, formation of the characteristic cross-sectional shape of the second welding portion 26 can be ensured even in a case where the second laser light has a low output.

[0039] The dimension of the second member 20, that is, the thickness of the 5 second member 20, is not particularly limited because the effect of the welding method disclosed herein is achieved with ease insofar as the dimension of the second member 20, that is, the thickness of the second member 20, is smaller than the dimension of the first member 10 in the direction orthogonal to the bonding surface, that is, the dimension of the wide surface of the first member 10. However, the void having a diameter of at least 0.1 mm, which is significantly generated in the keyhole welding portion, can affect the strength of the welding portion 30 to decrease the strength of the welding portion 30. Particularly, a significant strength decrease can occur in a welding portion in which the dimension L2 is small (that is, thin) and it is difficult to ensure welding strength. However, the welding method disclosed herein is preferable because the effect can be significantly achieved in a case where the second member 20 in which the dimension L₂ is low is welded. From the viewpoint of the significant welding strength decrease due to the presence of the void, the welding method disclosed herein can employ the thin second member 20, whose dimension L2 is at least 0.1 mm, for example, 1 mm or less, and more restrictively, 0.5 mm or less, for example, 0.4 mm or less, as the welding target member.

[0040] As described above, the first member 10 and the second member 20 can also achieve firm mechanical (structural) fixing by using the first welding portion 16 that includes the keyhole-shaped welding portion formed in the first member 10 and the second welding portion 26 that includes the welding portion convex in the depth direction and formed in the second member 20. In other words, the first member 10 and the second member 20 can be firmly coupled mechanically because of the shape of the welding portion 30 as well as because of chemical bonding at the interface resulting from the solidification of the welding portion 30. Although a welding defect such as a void may : significantly affect the welding strength in the thin second member 20, the void is unlikely to be formed in the second member 20. Accordingly, the welding between the first member 10 and the second member 20 can be performed with high strength and high reliability, and a welding structure with high strength and high reliability can be provided.

[0041] Preferably, the first laser light 1 and the second laser light 2 are controlled such that the bonding surfaces 12, 22 or the gap between the first member 10 and the second member 20 is not irradiated therewith. In other words, the surface of the first member 10 and a side surface of the second member 20 are irradiated, as described above, with the first laser light 1 and the second laser light 2, respectively. Particularly, the surface of the first member 10 is irradiated with the first laser light 1 if possible. In other words, when the first high output strength laser light 1 penetrates between the first member 10 and the second member 20, the first laser light 1 is reflected by the gap and can reach (penetrate) back surface sides of the first member 10 and the second member 20 with ease. Then, constituting members that are arranged on the back surface sides of the first member 10 and the second member 20 may be impaired by the irradiation by the first laser light 1. Accordingly, it is preferable that particularly the first laser light 1 is controlled such that the first laser light 1 is emitted to the center side apart at a predetermined distance from the end portion of the first member 10. The predetermined distance can be appropriately determined in view of spot diameters of the first laser light 1 and the second laser, a mechanical error of the laser welding device which is used or the like, a dimension error of a welding target material, and the like.

[0042] The bonding surface depth Dw described above is preferable because the welding strength increases as the bonding surface depth Dw increases. A target value of the bonding surface depth Dw can be determined in accordance with welding strength required for a welding material for the welding as well as materials and dimensions of the first member 10 and the second member 20. An appropriate value of the bonding surface depth Dw is not strictly determined, but it is preferable that the appropriate value of the bonding surface depth Dw is equal to the dimension of the second member 20 in the direction orthogonal to the bonding surface. Alternatively, it is more preferable that the appropriate value of the bonding surface depth Dw is greater than the L^. The bonding surface depth Dw and the dimension 1Q of the second member 20 in the direction) orthogonal to the bonding surface may have a relationship of, for example, approximately Dw≥0.8xL₂, preferably Dw≥L2, and more preferably Dwal^xl^.

[0043] As described above, the welding method disclosed herein allows a sufficient penetration depth to be ensured by the first laser light 1 having the high output density. Also, a shape of a welding metal is controlled such that a predetermined bonding surface depth Dw is stably obtained by the second laser light 2 having the low output density. Accordingly, the bonding surface depth Dw can be ensured sufficiently and stably even in a case where a slight misalignment occurs in the welding trajectory or in a case where the gap is generated between the first member 10 and the second member 20. In addition, since the heat conduction melting is carried out, the generation of the void can be suppressed and a welding strength decrease attributable to the void can be prevented for the second member 20.

[0044] The welding method disclosed herein can particularly demonstrate the effect thereof in a case where a member with an even smaller dimension L₂ in the direction perpendicular to the welding surface 22 (for example, the member can be a thin member) is adopted as the second member 20. The member that has the small dimension L₂ can be, for example, sufficiently melted even by the second laser having a low output density and an irradiation diameter over a wide range as in the related art does not necessarily have to be ensured. Accordingly, the laser output can be decreased and facility costs can be maintained at a low level. For example, the inventors of the invention confirmed that the welding was appropriately performed, with the formation of the void suppressed, by using the welding method disclosed herein and using a thin plate material whose dimension L2 in the direction orthogonal to the bonding surface is 0.5 mm or less (for example, approximately 0.4 mm) as the second member 20.

[0045] In the welding structure that is formed by the welding method, for example, the shape of the welding portion in the cross section perpendicular to the bonding surface has the following characteristics. In other words, (1) In the first member, the welding portion of the so-called keyhole, having a high aspect ratio and an acute angle, is included. (2) In addition, the keyhole first molten pool and the heat conduction second molten pool are superimposed, and thus the symmetry of the welding portion of the keyhole is disturbed in the welding portion formed thereby and the greater bonding surface depth Dw can be stably realized in the bonding surface. In addition, (3) the second welding portion that is formed in the second member can be shaped to be inclined downward (shaped to be convex downward) away from the bonding surface. In this,, manner, the bonding surface depth Dw can be ensured with even more stability even in a case where the misalignment of the trajectory of the first laser light 1 occurs.

[0046] The laser light that can be used in the welding method disclosed herein includes various types depending on purposes, without having to depend on laser generating mechanisms thereof. For example, various types of laser light capable of realizing the keyhole melting and the heat conduction melting described above can be adopted depending on the dimension of the welding target member and the like. The types of the laser light may include, as specific examples, C0₂ laser, YAG laser, semiconductor laser (laser diode; also referred to as LD), LD excitation solid-state laser, and fiber laser.

[0047] According to the welding method disclosed herein, the first member and the second member that are objects of the welding are not particularly limited, and members formed of a wide variety of of materials provided in general for welding can be taken into account. Examples of the materials include metallic materials, ceramic materials, and plastic materials. Particularly preferably, the welding method can be appropriately applied to welding of a member formed of a metallic material. The metallic materials are not particularly limited, and representative examples thereof include aluminum, aluminum alloys, iron, iron alloys (including various types of stainless steel), copper, and copper alloys. The first member and the second member may be configured to be formed of the same material or may be configured to be formed of different materials.

[0048] According to the welding method disclosed herein as described above, the first member that is formed of, for example, a relatively thicker plate and the second member that is formed of,, for example, a relatively thinner plate can be appropriately welded while the welding strength is maintained at a high level. The welding method can be particularly appropriately applied to, for example, applications including welding of a thin plate member in which reliable and airtight sealing are required. The welding of the thin plate member from which the reliability and airtightness are achieved includes, as a representative example, can-sealing welding of a case of an electric power storage element. Hereinafter, the invention will be further described based on an example of a case where the welding method disclosed is applied to the can-sealing welding of the case of the electric power storage element.

[0049] Hereinafter, an example relating to the invention will be described. However, the invention is not limited to the example. Welding of a case main body will be described as Example 1. In the procedure of the following description, sealing processing was performed on a case 130 of a lithium ion battery 100. FIG 5 is a partial cut-away front view showing a configuration of the lithium ion battery, and FIG 6 is a top view thereof. The case 130, as illustrated in the drawings, is a rectangular case that has a rectangular shape and a small depth dimension. A case main body 132 that has an open upper surface and a substantially rectangular lid member 134 that is long in a lateral direction to correspond to the opening constitute the case 130. The case main body 132 is formed of an aluminum alloy that has a thickness of 0.4 mm and a JlS-defined alloy number of A3003. The lid member 134 is formed of an aluminum alloy that has a thickness of 1.4 mm and a JlS-defined alloy number of A1050. As illustrated in FIG. 6, positive and negative external terminals 142, 144, an injection hole 146, a safety valve 148, and the like are disposed in the lid member 134. An external force can be applied to these sites during manufacturing or use, and thus the lid member 134 is formed of a thicker plate material. The case main body 132 is formed of a plate material that is slightly higher in strength than the lid member 134 but is thin and allows weight reduction.

[0050] Although not illustrated, supporting portions that are capable of holding the lid member 134 to be flush with an upper end of the case main body 132 are disposed in two respective short side portions of the opening of the case main body 132. An electrode body 120 that is an electric power generating element of the lithium ion battery 100 is mounted on the lid member 134 in a predetermined procedure, and then the electrode body 120 is inserted into the case main body 132 and the lid member 134 is fitted into the opening portion of the case main body 132 to form a lid. Then, a welding method 1 or a welding method 2 was carried out such that the battery case 130 was sealed by welding of the lid member 134 as the first member and the case main body 132 as the second member.

[0051] First, a welding method 1 will be described. A high output fiber laser welder was used in the welding, and laser light oscillating from the welder was branched by using a diffraction optical element (DOE) lens such that high output density laser light (first laser light) and low output density laser light (second laser light) were generated. Oscillation conditions for each of the high output density laser light and the low output density laser light of this embodiment are the five conditions that are shown as follows in Table 1. After combining the oscillation conditions for the first laser light and the second laser light with each other, the welding was performed under a total of 25 welding conditions.

Table 1

face each other as described above. Then, the center side 0.15 mm apart from the end portion (bonding surface) on the surface of the lid member was set as a welding line Lw_b and continuous (CW) irradiation with the high output density laser light described above ¾ was performed along the welding line L i. At the same time, the upper surface of the case main body, that is, an upper end surface of a case main body side surface member was set as the laser irradiation surface, an outer side of the case main body 0.15 mm apart from the bonding surface on the surface (that is, inner surface of the case main body) was set as a welding line Lw2, and continuous (CW) welding by the low output density laser light described above was performed along the welding line Lw₂. Each laser light was allowed to travel with each other at the same time at a speed of 24 m per minute. In this manner, the case main body and the lid member were sealed by round welding (approximately 240 mm) of circumferential edge portions thereof on the upper surface of the case of the lithium ion secondary battery.

[0053] Next, a welding method 2 will be described. A high output fiber laser welder was used in the welding, and laser light oscillating from the welder was branched by using a diffraction optical element (DOE) lens such that first high output density laser light (first laser light), second high output density laser light (second laser light), and low output density laser light (third laser light) were generated. Oscillation conditions for the laser, light according to this embodiment are as follows.

[0054] The oscillation conditions for the first laser light are as follows.

Spot diameter: φθ.04 mm, laser output: 800 W, laser output density: 6.4xl0⁷ W/cm²

[0055] The oscillation conditions for the second laser light are as follows.

Spot diameter: φ0.04 mm, laser output: 540 W, laser output density: 4.3xl0⁷ W/cm²

[0056] The oscillation conditions for the third laser light are as follows.

Spot diameter: φθ.5 mm, laser output: 1,000 W, laser output density: 5.1xl0⁵ W/cm²

[0057] Firstly, the lid member as the first member 10 was fitted into the case main body as the second member 20 and the respective bonding surfaces 12, 22 thereof were allowed to face each other as described above. Then, the center side 0.15 mm apart from the end portion (bonding surface 12) on the surface of the lid member was set as a welding line Lwi, and continuous (CW) irradiation with the first laser light (first high output density laser light) described above was performed along the welding line Lwi- The upper surface of the case main body, that is, an upper end surface of a case main body side surface member was set as the laser irradiation surface, an outer side of the case main body 0.15 mm apart from the bonding surface on the surface (that is, inner surface of the case main body) was set as a welding line L 2, and continuous (CW) welding by the second laser light (second high output density laser light) described above was performed along the welding line Lw₂. In addition, the bonding surface between the case main body and the lid member was set as a welding line, and continuous (CW) welding by the third laser light (low output density laser light) described above was performed along the welding line. In a case where the gap was generated between the case main body and the lid member, the middle between the case main body and the lid member was set as the welding line. Each laser light was allowed to travel with each other, at a speed of 24 m per minute, with the optical axis aligned in a direction orthogonal to the traveling direction. In this manner, the case main body and the lid member were sealed by round welding (approximately 240 mrri) of circumferential edge portions thereof on the upper surface of the case of the lithium ion secondary battery.

[0058] Next, welding quality evaluation will be described. The welding portions of the cases sealed by the welding method 1 (25 conditions) and the welding method 2 (one condition) were evaluated with the following evaluations items of [1] to [6]. Results of the evaluation ere used to evaluate the quality of the welding portions as a whole.

[0059] Welding Defect

A non-destructive X-ray inspection device (micro focus X-ray transmission device SMX-225CT-F manufactured by Shimadzu Corporation) was used to inspect whether or not the welding defect (a welding crack and the void) occurs in the welding portions of the cases sealed by the welding method 1 and the welding method 2. FIG 3A illustrates an appearance of the welding portion that is formed in the case main body by the welding method 1 (welding condition; lcx2c) and illustrates a cross-sectional image in the welding progress direction. FIG. 3B illustrates an appearance of the welding portion that is formed in the case main body by the welding method 2 and illustrates a cross-sectional image in the welding progress direction. Regarding the welding method 2, FIG 4 illustrates an observation view of the cross section orthogonal to the welding progress direction which shows the relatively large void observed in the case main body. The part surrounded by the circle in FIG. 4 is the void. . The presence or absence of the welding crack and a situation of void generation in the case main body part were researched by performing image processing on the X-ray fluoroscopy image of the cross section of the welding portion. Regarding the presence or absence of the welding crack, the result is shown as follows in the column of Evaluation Item [1] in Table 2. The number of the voids was counted in a case where the void at least 0.1 mm in diameter, which is considered to be capable of causing a strength decrease, was found. The number of the voids per welding distance of 1 mm was calculated from the result and is shown in the column of Evaluation Item [2] in Table 2 and the number of the voids per battery (case) was calculated from the result and is shown in the column of Evaluation Item [3] in Table 2.

[0060] Shape of Welding Portion

Places of the welding portion of the case sealed as described above were cut and penetration shapes of the lid member as the first member and the case main body as the second member were observed based on the cross-sectional images of the welding portions of the respective examples acquired by cross-sectional observation. As a result, a case where the penetration depth of the lid member was shallow (that is, no keyhole was formed) was marked as "shallow", a case where the lid member was penetrated was marked as "penetrated", and a case where the penetration shape was appropriate was marked as "A", the marks being shown in the column of Evaluation Item [4] in Table 2. Regarding the penetration shape of the case main body, a case where the penetration depth was deep and the void was generated was marked as "void", a case where the penetration shape was appropriate and the convex portion was not formed in the depth direction in the vicinity of the surface on the side opposite to the bonding surface was marked as "no convexity", and a case where the penetration shape was appropriate and the convex portion was formed was marked as "A", the marks being shown in the column of Evaluation Item [5] in Table 2.

[0061] Laser Output Density Ratio

The ratio of the output densities (Ii/I₂) of the first laser light and the second laser light used in the welding methods 1, 2 was calculated and is shown in the column of Evaluation Item [6] in Table 2.

[Summary] Comprehensive Evaluation

Appropriateness of the quality of the welding portions formed under the respective welding conditions was comprehensively evaluated by using the results of Evaluation Items [1] to [6] described above. A case of determination as appropriate was marked as "A", a case of determination as particularly appropriate was marked as "A⁺", and a case of determination as inappropriate was marked as "C", the marks being shown in the column of Evaluation Item [Summary] in Table 2. In the comprehensive evaluation, marks A and A⁺ mean that requirements for the technique disclosed herein are satisfied. Table 2

[0062] As illustrated in Evaluation Item [1] in Table 2, no superficial welding defect such as the welding crack was found in the welding portion by each of the welding method 1 and the welding method 2. Accordingly, it could be confirmed that welding appropriate to some extent can be performed by any one of the welding method 1 and the welding method 2.

[0063] According to a further detailed inspection, the laser output density weakened in this embodiment in a case where the irradiation condition of the first laser with which the lid member as the first member is irradiated was la regarding the welding method 1, and it was found that the keyhole welding portion was not formed. It was found that the bonding surface depth Dw extremely decreases and a sufficient welding strength was not obtained when the keyhole welding portion was not formed in the first member. In contrast, it was found that the laser output density was excessively high and the keyhole penetrated the lid member in a case where the irradiation condition of the first laser was le. It was confirmed that welding was possible with an appropriate keyhole formed in a case where the irradiation condition of the first laser was lb to Id. In this case, it was possible to ensure a sufficient bonding surface depth Dw.

[0064] It was confirmed that the laser output density was sufficient and the heat conduction welding portion was formed in a case where the irradiation condition of the second laser with which the case main body as the second member was irradiated was 2a. However, no welding shape convex downward was formed outside the cross section of the case main body. It was found that the characteristic welding shape convex downward was formed outside the cross section of the case main body in a case where the irradiation condition of the second laser was 2b to 2d. Regarding this point, the comprehensive evaluation was marked as "A" in a case where the irradiation condition of the first laser was lb to Id and the irradiation condition of the second laser was 2a and the comprehensive evaluation was marked as "A⁺" in a case where the irradiation condition of the second laser was 2b to 2d. However, it was found that the laser output density was excessively high, the welding portion such as the keyhole was formed, and a number of the voids were generated in a case where the irradiation condition of the second laser was 2e.

[0065] Accordingly, it was found that the irradiation condition of the first laser was better at an output density of approximately lb to Id, higher than la and lower than le, in this embodiment. Also, it was found that the irradiation condition of the second laser was better at an output density of approximately 2a to 2d, lower than 2e, and more preferably, approximately 2b to 2d. In this manner, it could be confirmed that the welding could be performed, without generating any void at least 0.1 mm in diameter in the thinner case main body, when the welding was performed by using the welding method 1 disclosed herein. In contrast, it was confirmed that the void at least 0.1 mm in diameter was frequently generated in the thinner case main body and the conditions for the second laser and the third laser were not appropriate, even when the irradiation condition of the first laser was lc that was appropriate in the welding method 1, when the welding was performed by using the welding method 2 of the related art. Generation of the large void in the thinner member means that the case main body may be impaired by the void in a case where an unpredicted external force is applied to the welding portion. This results in

c a decrease in welding strength and, thereby deteriorating welding quality and welding portion reliability.

[0066] Next, Example 2 will be described. Oscillation conditions for each of the first laser light and the second laser light are the three conditions shown as follows in Table 3. A total of nine welding conditions were prepared by combining the oscillation conditions for the first laser light and the second laser light with each other. In performing the sealing welding of the case of the lithium ion battery, the first laser light and the second laser light were allowed to travel together at a speed of 9 m per minute with the other conditions being identical to those of the welding method 1 of Example 1 described above. In addition, as in Example 1 described above, the welding portion was evaluated regarding Evaluation Items [1] to [6] and quality evaluation was performed on the entire welding portion. The result thereof is shown in Table 4.

Table 3

Second Laser Light

Condition 2f 2g 2h d₂ [mm] 0.15

Table 4

[0067] As illustrated in Table 3, the output densities of the first laser light and the second laser light of this embodiment are set in ranges relatively lower than in Example 1. However, even in the case of these relatively lower output densities, it was confirmed that the appropriate welding portion convex downward could be formed in the case main body by lowering the laser scanning speed and ensuring sufficient heat input for the welding target member. In other words, the keyhole welding portion is formed by chain reflection of the laser light on a molten section wall surface. Accordingly, it can be understood that the output density of the first laser light may be low regarding the lid member within a range in which a predetermined laser output density allowing the formation of the keyhole welding portion can be ensured. Regarding the heat conduction welding portion, a general heat conduction welding portion is formed when the output density of the second laser light is low. It was confirmed that the appropriate welding portion convex downward could be formed, even in a case where the output density of the second laser light was low, by inputting a sufficiently large amount of heat in accordance with the material and the shape of the case main body (dimension in the direction orthogonal to the bonding surface, plate thickness) and the like. Specifically, in Example 1 described above for example, it was impossible to input a sufficient amount of heat for forming the welding portion convex downward into the case main body, despite the output condition of 2a (I₂: 3.5x10 W/cm ), because the laser scanning speed was as high as 24 m per minute. In contrast, Example 2 shows that the laser scanning speed is lowered to 9 m per minute and thus a sufficient amount of heat can be input into the case main body and the welding portion convex downward can be formed under any one of the output conditions 2f to 2h (I₂: 2.8 to 3.8xl0⁶ W/cm²).

[0068] It was found that the output conditions for the first laser light and the second laser light could be changed in various ways according to the welding method of this embodiment such that welding of higher quality could be performed. For example, the shape of the welding portion can be appropriately adjusted within the range of the output density described above by changing the scanning speed in accordance with the form of welding of the first member and the second member, the shape of the welding line, and the like. It could be confirmed that the appropriate keyhole welding portion was formed in the first member, the appropriate heat conduction welding portion convex downward was formed in the second member, and the welding of high strength and higher quality could be realized, even for the thin member, by superimposing the first member and the second member.

[0069] In the embodiment described above, an example in which the case main body and the lid member of the electric power storage element are adopted as the first welding member and the second welding member and the first welding member and the second welding member are welded has been described. However, the application of the welding method proposed herein is not limited to the welding of the case of the electric power storage element. Instead, the welding method proposed herein can be used for welding of various types of welding members. In the embodiment described above, the members formed of the aluminum alloys are used as the first welding member and the second welding member. However, the welding method disclosed herein can also be used for welding of various non-aluminum alloy materials. Materials that constitute the welding members include, for example, iron alloys represented by an SUS material, metallic materials including other types of pure metals and alloys, inorganic materials such as ceramic, and organic materials such as plastic. Accordingly, the welding method disclosed herein can be suitably used for abutting welding of welding members formed of various materials. The welding method disclosed herein can be particularly suitable for welding of thinner welding members such as the case main body of the case of the electric power storage element described above.

Claims

CLAIMS:

1. A welding method for laser- welding a first member having a plate shape and a second member having a plate shape, the welding method comprising:

allowing the first member and the second member to abut against each other in a direction in which wide surfaces of the first member and the second member are orthogonal to each other and allowing a side surface portion of the first member and a wide surface end portion of the second member to abut against each other such that a side surface portion of the second member is substantially flush with the wide surface of the first member, a dimension of the first member iii a direction orthogonal to a first bonding surface being greater than a dimension of the second member in a direction orthogonal to a second bonding surface, the side surface portion of the first member that faces the second member being the first bonding surface, and the wide surface end portion of the second member that faces the first member being the second bonding surface;

emitting second laser light having a strength lower than a strength at which a keyhole can be generated to the side surface portion of the second member along the second bonding surface and forming a second molten pool formed by being melted by the second laser light, the second molten pool being formed over the first member,

wherein the first molten pool and the second molten pool are integrated with each other and form a molten pool and the first member and the second member are welded by a welding portion formed when the molten pool is solidified.

2. The welding method according to claim 1, wherein

6 2

an output density of the first laser light is at least 5.6 10 W/cm , and

the output density of the first laser light is less than 1.1x10^s W/cm².

3. The welding method according to claim 1 or 2, wherein

an output density of the second laser light is at least 2.8xl0⁶ W/cm², and

the output density of the second laser light is less than 5.6xl0⁶ W/cm².

4. The welding method according to claim 3, wherein

the first laser light and the second laser light have a scanning speed of at least 20 m per minute when the output density of the second laser light is at least 3.8xl0⁶ W/cm² and is less than 5.6xl0⁶ W/cm².

5. The welding method according to claim 3, wherein

the first laser light and the second laser light have a scanning speed of less than 20 m per minute when the output density of the second laser light is at least 2.8x10 W/cm and is less than 3.8xl0⁶ W/cm².

6. The welding method according to any one of claims 1 to 5, wherein

the output density of the first laser light and the output density of the second laser light satisfy the following expression in a case where the output density of the first laser light is defined as \_\ and the output density of the second laser light is defined as I₂:

7. The welding method according to any one of claims 1 to 6, wherein

an irradiation diameter of the first laser light and an irradiation diameter of the second laser light satisfy the following expression in a case where the irradiation diameter of the first laser light is defined as di and the irradiation diameter of the second laser light is defined as d₂:

8. The welding method according to any one of claims 1 to 7, wherein a depth of the welding portion of the bonding surface in a cross section orthogonal to the first bonding surface or the second bonding surface is equal to or greater than the dimension of the second member in the direction orthogonal to the second bonding surface.

9. A welding structure comprising:

a first member having a plate shape;

a second member having a plate shape; and

a welding portion where the first member and the second member are bonded, wherein a dimension of the first member in a direction orthogonal to bonding surfaces of the first member and the second member is greater than a dimension of the second member in the direction,

the welding portion has a first welding portion and a second welding portion divided by a surface including the bonding surfaces,

a keyhole welding portion formed by laser with which the first member is irradiated mainly constitutes the first welding portion and the first welding portion has a tip part of the keyhole welding portion, and

a heat conduction- welding portion formed by laser with which the second member is irradiated mainly constitutes the second welding portion and the second welding portion includes at least a part of a site of the keyhole welding portion other than the tip part and at least a part of the heat conduction welding portion.

10. The welding structure according to claim 9, wherein

a depth of the welding portion of the bonding surface in a cross section orthogonal to the bonding surface is equal to or greater than a dimension of the second member in a direc tion of the cross section.