Laser welding method

Abstract

To provide a laser welding method which allows for determining the quality of the laser welding correctly.SOLUTION: A laser welding method comprising the steps of: causing two conductive welding target objects to overlap with or butt on each other; irradiating the overlapping or butting portion with a laser beam to weld the two conductive welding target objects together; and measuring, during the welding by using alternating current voltage or alternating current, direct current resistance of the laser welding portion where the two conductive welding target objects are being welded.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to laser welding methods. [Background technology]

[0002] Laser welding, which involves welding conductive workpieces together by irradiating them with a laser beam, is widely used in various manufacturing processes. In laser welding, it is desirable to check the welding state in order to maintain accurate welding quality, and the following techniques are known.

[0003] For example, Patent Document 1 describes a laser welding method in which conductive workpieces are overlapped or butted together and irradiated with a laser beam to weld the overlapped or butted parts, the method including the following steps: a) a step of setting the conductive workpieces overlapped or butted together, b) a step of measuring the electrical resistance between the workpieces set in the step, determining the difference from a first preset value, and resetting the workpieces if the difference is a positive value, c) a step of irradiating with a laser beam to perform laser welding if the difference is 0 or a negative value in the step, and d) a step of measuring the electrical resistance between the workpieces during the step, determining the difference from a second preset value, and irradiating with a laser beam until the difference becomes 0 or a negative value. According to the laser welding method of Patent Document 1, before the start of laser welding, the adhesion of the workpieces is ensured so that welding is normally performed, and quality control is possible even in the laser welding work process in which the progress of welding can be confirmed during laser welding.

[0004] Patent Document 2 describes a laser welding inspection method for irradiating an inspection laser beam to a welded portion of a workpiece, receiving return light of the inspection laser beam from the welded portion, and judging the presence or absence of a weld defect in the welded portion based on the intensity of the return light, in which the diameter of an area in which the return light from the welded portion is recognized is set to 1.5 times or less the diameter of the focused laser beam irradiated to the welded portion. The laser welding inspection method of Patent Document 2 is said to be able to improve the accuracy of judging weld defects.

[0005] Patent Document 3 describes a method for inspecting a laser welded portion after welding, in which two pieces of metal to be inspected are joined together and a laser is spot-irradiated onto one surface of the welded portion, the method comprising: a first step of photographing the laser welded portion, which has been cooled to the ambient temperature after welding, with an infrared camera, and detecting a welded portion region by regarding a difference in brightness in the obtained infrared image as an index representing a difference in infrared emissivity of the surface of the inspected portion; and a second step of judging the quality of the welded portion based on at least one of the shape of the welded portion region, the distribution pattern of the brightness value of the infrared image in the welded portion region, and the feature amount extracted based on the distribution pattern. It is said that the laser welded portion inspection method of Patent Document 3 can improve the reliability of the inspection and can increase the accuracy of the quality judgment. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2012-206153 A [Patent Document 2] JP 2015-182092 A [Patent Document 3] JP 2003-065985 A Summary of the Invention [Problem to be solved by the invention]

[0007] Since there is a certain probability of defective welding in laser welding, it is necessary to accurately judge the quality of the laser welding during the laser welding process.

[0008] Because the contact condition of laser welding varies from one laser weld to another and the temperature during welding is not stable, it is difficult to accurately determine the quality of the weld simply from the absolute resistance value, and therefore improvements are required.

[0009] Therefore, an object of the present disclosure is to provide a laser welding method that can accurately determine whether the laser welding is good or bad. [Means for solving the problem]

[0010] The present disclosure achieves the above object by the following means.

[0011] <Aspect 1> A laser welding method comprising overlapping or butting two conductive workpieces together, and irradiating the overlapping or butting portion with laser light to weld the two conductive workpieces together, and measuring, during the welding, the DC resistance of the laser weld where the two conductive workpieces are welded together using an AC voltage or AC current. Aspect 2: The method according to aspect 1, wherein the frequency of the AC voltage or AC current is 900 Hz to 1100 Hz. <Aspect 3> The method described in aspect 1, in which the weld is determined to be good when the DC resistance of the laser weld decreases by a magnitude greater than a first predetermined value. <Aspect 4> The method of aspect 1, wherein the weld is determined to be good when the DC resistance of the laser weld decreases by a width greater than a first predetermined value, and then the DC resistance of the laser weld increases by a width greater than a second predetermined value. <Aspect 5> The method according to any one of Aspects 1 to 4, wherein the two conductive objects to be welded are an electrode terminal and a bus bar of a lithium ion battery, respectively. <Aspect 6> A method for manufacturing a battery, comprising the method described in aspect 5. Effect of the Invention

[0012] According to the laser welding method of the present disclosure, it is possible to accurately determine whether the laser welding is good or bad during the welding process. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic diagram for explaining the laser welding method of the present disclosure. [Diagram 2]Figure 2 shows the Nyquist plot of a lithium-ion battery. The black plot shows the complex impedance at 921.5 Hz. [Diagram 3] FIG. 3 is a schematic diagram for explaining the laser welding method of the present disclosure. [Figure 4] 4 is a Nyquist diagram of the electrode terminal and the bus bar of the lithium ion battery of the present disclosure before and after laser welding. The white plot shows the Nyquist diagram before laser welding, and the gray plot shows the Nyquist diagram after laser welding. The black plot shows the complex impedance at 921.5 Hz. [Diagram 5] FIG. 5 is a schematic diagram for explaining a case in which the laser welding is determined to be good in the laser welding method of the present disclosure. [Figure 6] FIG. 6 is a schematic diagram for illustrating a case where laser welding is determined to be defective in the laser welding method of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, the embodiments of the present disclosure will be described in detail. Note that the present disclosure is not limited to the following embodiments, and various modifications can be made within the scope of the gist of the present disclosure. In addition, in the description of the drawings, the same elements are given the same reference numerals, and duplicated descriptions will be omitted.

[0015] <Laser welding method> The laser welding method of the present disclosure includes: The method includes overlapping or butting two conductive workpieces together, and irradiating the overlapped or butted portion with a laser beam to weld the two conductive workpieces together, and measuring, during the welding, a DC resistance of a laser welded portion where the two conductive workpieces are welded together by using an AC voltage or an AC current.

[0016] According to the laser welding method of the present disclosure, it is possible to accurately determine whether the laser welding is good or bad during the welding process.

[0017] Since the temperature of a laser welded portion obtained by laser welding two conductive workpieces is not stable during welding, it is difficult to accurately judge welding by a simple absolute resistance value. Therefore, the present inventor has focused on measuring the DC resistance of a laser welded portion in a laser welding process using an AC voltage or an AC current. Specifically, for example, when the frequency of the AC voltage or the AC current is 900 Hz to 1100 Hz, the DC resistance (real component) of the laser welded portion via a lithium ion battery is 0 because the imaginary component of the complex impedance of the lithium ion battery is 0.

[0018] When two conductive workpieces are overlapped or butted together, the parts to be laser welded are in surface contact. On the other hand, immediately after laser welding, the surface contact of the parts to be laser welded is released at the same time as the laser welding, and the parts are in a welded state. Therefore, it is considered that the DC resistance immediately after laser welding will be significantly lower than the DC resistance in the surface contact state (first predetermined value), and in this case, it can be determined that the laser welding is good. On the other hand, when the DC resistance immediately after laser welding does not significantly decrease compared to the first predetermined value, it can be determined that the laser welding is poor.

[0019] Furthermore, during laser irradiation, the heat of the laser welding heats the laser welded portion, increasing the metal resistance, and the DC resistance during laser welding increases by a larger amount compared to the DC resistance immediately after laser welding (the second predetermined value). Therefore, when the DC resistance of the laser welded portion decreases by a larger amount than the first predetermined value and then increases by a larger amount than the second predetermined value, it can be determined that the laser welding is good. On the other hand, when the DC resistance of the laser welded portion does not decrease by a larger amount than the first predetermined value or does not increase by a larger amount than the second predetermined value, it can be determined that the laser welding is poor.

[0020] The method of the present disclosure is described in detail below.

[0021] <Laser welded part> A laser weld is where two electrically conductive work pieces are welded together during welding.

[0022] <Conductive workpiece> The conductive workpieces to be welded are not particularly limited, but may be known ones that are melted by a laser beam to electrically join the objects. The conductive workpieces to be welded are not particularly limited, but are preferably electrode terminals and bus bars of a lithium ion battery.

[0023] FIG. 1 is a schematic diagram showing one embodiment of a laser welded portion welded by the method of the present disclosure, but is not limited to this embodiment.

[0024] The two conductive objects to be welded are an electrode terminal 11 of a lithium ion battery and a bus bar 12. The bus bar 12 is placed on top of the electrode terminal 11 of the lithium ion battery, and a probe 13 of an AC impedance measuring device is electrically connected to the bus bar 12. A laser beam 14 is irradiated from above the bus bar 12 to form a laser weld 15. A known technique can be used for the laser beam.

[0025] <DC resistance> The laser welding method of the present disclosure includes measuring the DC resistance of the laser weld using an AC voltage or current.

[0026] According to the AC impedance method, for example, a combination of a frequency response analyzer (FRA) and a potentiogalvanostat (PGS) can be used. A sine wave signal of an arbitrary frequency is output from an oscillator constituting the FRA, and a current signal I(t) and a voltage signal V(t) of a reference secondary battery corresponding to the sine wave signal are input from the PGS to the FRA. Then, in the FRA, the current signal I(t) and the voltage signal V(t) are converted into frequency domain data by a discrete Fourier frequency transform, and the complex impedance (Z=Re(Z)+Im(Z)) at a frequency f=(ω / 2π) is measured. The real component Re(Z) represents the DC resistance component, and the imaginary component Im(Z) represents the reactance component. In this disclosure, the value of Re(Z) is taken as the DC resistance of the laser weld.

[0027] FIG. 2 shows an example of a Nyquist plot showing the results of measuring the complex impedance Z of a lithium-ion battery. The horizontal axis is the real part Re(Z) of the complex impedance Z, and the vertical axis is the imaginary part -Im(Z) of the complex impedance Z. In the -Im(Z)>0 region, the larger Re(Z) is, the lower the frequency of the complex impedance Z is represented. The value of Re(Z) when -Im(Z)=0 corresponds to the transfer resistance in the electrolyte of the lithium-ion battery. The radius of curvature of the approximately semicircular part in the -Im(Z)>0 region corresponds to the charge transfer resistance of the lithium-ion battery.

[0028] In the present disclosure, from the viewpoint of reducing the influence of Im(Z) (reactance component) of complex impedance, the frequency of the AC voltage or AC current is not particularly limited, but is preferably 900 Hz to 1100 Hz.

[0029] The AC impedance can be measured using analog methods such as the Lissajous method and the AC bridge method, or digital methods such as the digital Fourier integration method and the fast Fourier transform method using noise injection.

[0030] <Judging the quality of laser welding> The laser welding method of the present disclosure may include determining that the weld is good when the DC resistance of the laser weld drops by an amount greater than a first predetermined value.

[0031] The first predetermined value is not particularly limited, but specifically, for example, it is the DC resistance measured when two conductive workpieces are overlapped or butted together before laser welding and the laser weld is simply in surface contact.

[0032] If the laser weld is determined to be good, the DC resistance immediately after the laser weld decreases by a larger amount than the first predetermined value, whereas if the laser weld is determined to be poor, the DC resistance immediately after the laser weld does not decrease by a larger amount than the first predetermined value.

[0033] One embodiment of laser welding between an electrode terminal and a bus bar of a lithium ion battery will be described with reference to Figures 3 and 4, but the present invention is not limited to this embodiment. Figure 3 is a schematic diagram showing one embodiment of a method for laser welding between an electrode terminal and a bus bar of a lithium ion battery, and Figure 4 is an example of a Nyquist diagram of a lithium ion battery before and after laser welding.

[0034] In FIG. 3, a lithium ion battery 31 has two electrode terminals 11 and 32, and a bus bar 12 is placed on the electrode terminal 11 in an overlapping manner. A bus bar 12' is similarly placed on the electrode terminal 32 of the opposite electrode in an overlapping manner. The bus bars 12 and 12' are electrically connected via an AC impedance measuring device 33. A laser beam 14 is irradiated from the bus bar 12 to form a laser welded portion 15. The DC resistance during the laser welding process is measured by the AC impedance measuring device 33, and the laser welded portion is inspected. For example, as shown in FIG. 4, the laser welding may be determined to be good when the DC resistance Re(Z) at 900 to 1100 Hz before and after the laser welding is significantly reduced.

[0035] The laser welding method of the present disclosure may include determining that the weld is good when the DC resistance of the laser weld decreases by an amount greater than a first predetermined value, and thereafter the DC resistance of the laser weld increases by an amount greater than a second predetermined value.

[0036] The second predetermined value is not particularly limited, but specifically, for example, a DC resistance measured immediately after laser welding. Immediately after laser welding is not particularly limited, but specifically, for example, a state in which the surface contact of the parts to be laser welded is released at the same time as the laser welding, and the laser welded part is welded.

[0037] During laser irradiation, the heat of the laser welding heats the laser welded portion, increasing the metal resistance, and the DC resistance during laser welding increases by a large amount compared to the second predetermined value. Therefore, when the DC resistance of the laser welded portion decreases by a larger amount than the first predetermined value and then increases by a larger amount than the second predetermined value, it can be determined that the laser welding is good. On the other hand, when the DC resistance of the laser welded portion does not decrease by a larger amount than the first predetermined value or does not increase by a larger amount than the second predetermined value, it can be determined that the laser welding is poor.

[0038] FIG. 5 is an example of a schematic diagram showing the laser welding steps and the DC resistance in each step in a case where the laser welding is judged to be good, but is not limited to this case.

[0039] The two conductive welded parts are an electrode terminal 11 and a bus bar 12 of a lithium ion battery, respectively. Before welding, the electrode terminal 11 and the bus bar 12 of the lithium ion battery are in surface contact, and the DC resistance shows a high value (5A). Then, immediately after laser welding, the surface contact between the electrode terminal 11 and the bus bar 12 of the lithium ion battery is released, and a laser weld 15 is formed. As a result, the DC resistance of the laser weld decreases by an amount greater than a first predetermined value (5B). Then, during laser welding, the temperature of the laser weld increases, and the DC resistance of the laser weld increases by an amount greater than a second predetermined value (5C). After laser welding, the DC resistance of the laser weld decreases due to natural cooling, and becomes equivalent to the DC resistance immediately after laser welding (5D).

[0040] FIG. 6 is an example of a schematic diagram showing laser welding steps and DC resistance in each step in a case where the laser welding is judged to be defective, but is not limited to this case.

[0041] The two conductive welded objects are the electrode terminal 11 and the bus bar 12 of a lithium ion battery, respectively. Before welding, the electrode terminal 11 and the bus bar 12 of the lithium ion battery are in surface contact, and the DC resistance shows a high value (6A). If the penetration of the bus bar 12 is insufficient immediately after laser welding, that is, if a laser weld is not formed to a sufficient depth, no change in the DC resistance of the laser weld is observed, and no drop in resistance is observed in a range greater than the first predetermined value (6B). Next, during laser welding, the temperature of the laser weld increases, and the DC resistance of the laser weld increases in a range greater than the second predetermined value (6C). After laser welding, the DC resistance of the laser weld decreases due to natural cooling, but becomes equivalent to the DC resistance when the terminal of the lithium ion battery and the bus bar are in surface contact (6D).

[0042] <Lithium-ion battery> The lithium ion battery of the present embodiment is not particularly limited, and any known lithium ion battery can be used. For example, a lithium ion battery in which the following positive electrode active material, negative electrode active material, and electrolyte solution are combined can be used.

[0043] The material of the positive electrode active material is not particularly limited. For example, the positive electrode active material may be lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4), LiCo 1 / 3 Ni 1 / 3 Mn 1 / 3 O2, Li 1+x Mn 2-x-y Li-Mn spinel substituted with different elements, which has a composition represented by MyO4 (M is one or more metal elements selected from Al, Mg, Co, Fe, Ni, and Zn), and lithium titanate (Li x TiO y ), lithium metal phosphate (LiMPO4, M is one or more metals selected from Fe, Mn, Co, and Ni), and the like, but are not limited thereto.

[0044] The material of the negative electrode active material is not particularly limited. The negative electrode active material is not particularly limited as long as it can absorb and release metal ions, such as lithium ions. The negative electrode active material may include metal particles, such as Li, Sn, Si, or In; lithium alloy particles, such as lithium alloy particles of lithium and titanium, magnesium, or aluminum; and carbon materials, such as carbon, hard carbon, soft carbon, and graphite; and combinations thereof.

[0045] The electrolyte preferably contains a supporting salt and a solvent.

[0046] Examples of supporting salts (lithium salts) for electrolytes having lithium ion conductivity include inorganic lithium salts such as LiPF6, LiBF4, LiClO4, and LiAsF6, and organic lithium salts such as LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(FSO2)2, and LiC(CF3SO2)3.

[0047] Examples of the solvent used in the electrolytic solution include cyclic esters (cyclic carbonates) such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC), and chain esters (chain carbonates) such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC). The electrolytic solution preferably contains two or more kinds of solvents. [Explanation of symbols]

[0048] 11 Electrode terminal 12, 12' busbar 13 Probe 14 Laser light 15 Laser welded section 31 Lithium-ion battery 32 Counterelectrode terminal 33 AC impedance measuring device

Claims

1. Two conductive workpieces are overlapped or butted together, and the overlapped or butted parts are irradiated with a laser beam to weld the two conductive workpieces; and During the welding, a DC resistance of a laser weld where the two conductive workpieces are welded is measured using an AC voltage or an AC current. Laser welding method.

2. 2. The method of claim 1, wherein the frequency of the alternating voltage or current is between 900 Hz and 1100 Hz.

3. 2. The method of claim 1, wherein the weld is determined to be good when the DC resistance of the laser weld drops by more than a first predetermined value.

4. 2. The method of claim 1, wherein the weld is determined to be good when the DC resistance of the laser weld drops by more than a first predetermined value and thereafter the DC resistance of the laser weld increases by more than a second predetermined value.

5. The method according to any one of claims 1 to 4, wherein the two conductive workpieces are an electrode terminal and a bus bar of a lithium ion battery, respectively.

6. A method for manufacturing a battery comprising the method of claim 5.

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