Laser welding method, terminal joint structure, and power conversion device

Abstract

To suppress sputter in laser welding.SOLUTION: A laser welding method, in which a first terminal and a second terminal which are formed of metal materials composed mainly of copper or aluminum are overlapped with each other and the first terminal and the second terminal are fusion-joined by laser, includes a laser emitting step of emitting laser, along an edge of the first terminal, to a first upper surface which is the opposite surface of a surface overlapped with the second terminal in the first terminal and to a second upper surface which is a surface at the same side of a surface overlapped with the first terminal in the second terminal, where in the laser emitting step, laser is emitted plural times under a condition for welding in a thermal conduction mode.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a laser welding method, a terminal joint structure, and a power conversion device. [Background technology]

[0002] Power conversion devices, engine control units, motors, batteries, and other devices use conductors called bus bars to achieve electrical connections. For example, an inverter is a device that generates alternating current from a direct current power source such as a battery, and is composed of a power module equipped with switching elements, a smoothing capacitor, a bus bar, a control circuit, and other components. The bus bar is electrically connected to the power module and smoothing capacitor, and the alternating current generated by the power module is supplied to the motor via the bus bar. Because bus bars must be conductive, they are generally made of copper or aluminum, and TIG (Tungsten Inert Gas) welding is often used for the connection method.

[0003] TIG welding often uses a butt joint, known as a lap joint, in which bus bars are butted together. However, lap joints have low spatial efficiency for erecting the bus bars, making them unsuitable for miniaturization and space-saving. Therefore, lap joints are being considered for welding bus bars. Lap joints are a method of joining bus bars by overlapping them one above the other, resulting in superior spatial efficiency. For lap joints, laser welding, which is highly energy-efficient, is typically used. Laser welding melting modes are broadly divided into keyhole mode and conduction mode. Keyhole mode welding is often used, which achieves deep penetration. However, keyhole mode welding generates molten metal spatter, known as spatter. Spatter adheres to the weld area and can cause problems such as short circuits. Conduction mode welding is a melting mode that does not generate spatter, but has the disadvantage of a shallow penetration depth of 0.2 to 0.3 mm. The thickness of the bus bar is often 0.3 mm or more, and the thicker ones are as thick as about 0.5 mm to 1.5 mm, so it is difficult to weld the first terminal and the second terminal together using heat conduction mode welding.

[0004] Patent Document 1 discloses a method for welding objects to be processed with a laser, which includes the steps of placing the object to be processed in an area irradiated with laser light from a laser device, irradiating the laser light from the laser device toward the object to be processed while moving the laser light and the object to be processed relatively, and sweeping the laser light over the object to be processed to melt the irradiated portion of the object to be processed, wherein the laser light is composed of a main beam and sub-beams at least a part of which is located forward in the sweep direction, and the power density of the main beam is equal to or greater than the power density of the sub-beams. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2018 / 159857 Summary of the Invention [Problem to be solved by the invention]

[0006] The invention described in Patent Document 1 leaves room for improvement in terms of suppressing sputtering. [Means for solving the problem]

[0007] A laser welding method according to a first aspect of the present invention is a laser welding method for directly overlapping a first terminal and a second terminal made of a metal material containing copper or aluminum as a main component, and melt-joining the first terminal and the second terminal with a laser, the method including a laser irradiation step of irradiating a first upper surface of the first terminal, which is the surface opposite to the surface that is overlapped with the second terminal, and a second upper surface of the second terminal, which is the surface on the same side as the surface that is overlapped with the first terminal, with the laser along an edge of the first terminal, and the laser irradiation step includes irradiating the laser multiple times under conditions that result in heat conduction mode welding. The first terminal has a low heat capacity region at a tip side overlapping with the second terminal, the low heat capacity region having a relatively smaller heat capacity than a region not overlapping with the second terminal. do. A terminal joint structure according to a second aspect of the present invention is a terminal joint structure including a welded portion formed by directly overlapping a first terminal and a second terminal made of a metal material containing copper or aluminum as a main component and melt-joining the overlapped portion with a laser, wherein the welded portion includes: a first region in which columnar crystal grains extend toward a region along an edge of the first terminal on an upper surface of the first terminal, which is the surface opposite to the overlapping surfaces of the first terminal and the second terminal; and a second region in which columnar crystal grains extend toward a region along the edge of the first terminal on an upper surface of the second terminal, which is the surface on the same side as the overlapping surfaces. The first terminal has a low heat capacity region at a tip side where the first terminal overlaps the second terminal, the low heat capacity region having a relatively smaller heat capacity than a region not overlapping the second terminal. . A power converter according to a third aspect of the present invention has the above-described terminal joint structure. [Effects of the Invention]

[0008] According to the present invention, spattering during laser welding can be suppressed. [Brief explanation of the drawings]

[0009] [Figure 1] Side view of power conversion device [Figure 2] Schematic diagram showing a welding method for a first terminal and a second terminal [Figure 3] Schematic diagram of welding equipment [Figure 4] Cross-section of welding position [Figure 5] Cross-sectional view of a welding position in a comparative example [Figure 6] FIG. 10 shows laser light irradiation in Example 2. [Figure 7] FIG. 10 shows a welding method according to Modification 1. [Figure 8] FIG. 10 is a diagram illustrating the effect of Modification 1. [Figure 9] FIG. 10 is a diagram showing the shape of a first terminal in Modification 2. DETAILED DESCRIPTION OF THE INVENTION

[0010] --Embodiment-- Hereinafter, embodiments of a power converter and a welding method according to the present invention will be described with reference to FIGS.

[0011] (composition) FIG. 1 is a side view of a power conversion device 1 according to the present invention. For ease of explanation, FIG. 1 depicts mutually orthogonal X, Y, and Z axes. The power conversion device 1 includes a semiconductor element 2, multiple heat sinks 3, a terminal block 4, and a capacitor 5. The semiconductor element 2 has multiple connection terminals 2A. The terminal block 4 includes a semiconductor-side end 41 and an external-side end 42. Each heat sink 3 sandwiches the semiconductor element 2 from both sides, i.e., from the positive and negative sides of the Z axis. The heat sink 3 has fins, which appear as protrusions in the side view, enabling efficient heat dissipation.

[0012] The semiconductor-side end 41 overlaps the connection terminal 2A of the semiconductor element 2 in the Z direction, and is welded to the connection terminal 2A at this overlapping portion by a method described below. The external-side end 42 is connected to the outside of the power conversion device 1. The capacitor 5 overlaps the connection terminal 2A in the Z direction, and is welded to the connection terminal 2A at this overlapping portion by a method described below. The welding method between the semiconductor-side end 41 and the connection terminal 2A is the same as the welding method between the capacitor 5 and the connection terminal 2A. Hereinafter, the members to be welded will be generally referred to as the "first terminal" and the "second terminal."

[0013] (welding method) FIG. 2 is a schematic diagram illustrating a welding technique for the first terminal 21 and the second terminal 22. The first terminal 21 and the second terminal 22 are arranged so as to overlap in the Z-axis direction. In the example shown in FIG. 2, the laser beam 9 is irradiated from the positive direction of the Z-axis. Hereinafter, the surface of the first terminal 21 on the positive side of the Z-axis will be referred to as the first upper surface 21U, and the surface of the first terminal 21 on the negative side of the Z-axis will be referred to as the first lower surface 21D. The surface of the second terminal 22 on the positive side of the Z-axis will be referred to as the second upper surface 22U, and the surface of the second terminal 22 on the negative side of the Z-axis will be referred to as the second lower surface 22D. The first terminal 21 is located on the positive side of the Z-axis, and the second terminal 22 is located on the negative side of the Z-axis. Therefore, the first lower surface 21D and the second upper surface 22U overlap each other. The first upper surface 21U is the surface of the first terminal 21 opposite the first lower surface 21D that overlaps with the second terminal 22. The second upper surface 22U is the surface of the second terminal 22 on the same side as the surface that overlaps with the first terminal 21.

[0014] The first terminal 21 and the second terminal 22 are made of a highly conductive metal. for example The material is copper, aluminum, or a copper or aluminum alloy. Note that the first terminal 21 and the second terminal 22 are not limited to a flat plate shape and may be a rectangular pillar or a cylindrical pin. In this embodiment, the first upper surface 21U and the second upper surface 22U are irradiated with laser light 9 multiple times along the edge of the first terminal 21 on the negative side of the X axis under conditions that result in a thermal conduction mode. By irradiating the end portion, which is prone to heat buildup, with laser light multiple times under conditions that result in a thermal conduction mode, a locally preheated state is achieved, and deep penetration can be achieved even in the thermal conduction mode.

[0015] If the laser irradiation position is too close to the end face, the laser will not be sufficiently irradiated onto the first terminal 21, and conversely, if it is too far away, it will be difficult to join the molten portion of the second terminal 22. Therefore, the laser beam 9 is preferably irradiated from a position 0.1 mm to 5 mm from the end face, more preferably 0.2 mm to 3 mm, and even more preferably 0.3 mm to 1.5 mm. The order in which the laser beam 9 is irradiated onto the first terminal 21 and the second terminal 22 is arbitrary, and either may be irradiated first, or when the first terminal 21 and the second terminal 22 are irradiated multiple times, they may be irradiated alternately.

[0016] Heat conduction mode welding is a type of welding characterized by a small penetration ratio, in which the incident laser energy is transmitted to the weld root only by thermal conduction and convection in the molten metal, and is sometimes called heat conduction welding.The opposite of heat conduction mode welding is keyhole welding, a welding method that forms a keyhole (a hole surrounded by molten metal that is formed when a laser beam with an energy density high enough to evaporate the material) near the tip of the molten pool to achieve a high penetration ratio.

[0017] However, simply irradiating the end of the first terminal 21 with laser light multiple times under conditions that result in thermal conduction does not melt the second terminal 22, and the first terminal 21 and the second terminal 22 are not sufficiently bonded together. Therefore, laser light is also irradiated multiple times along the end face of the top surface of the second terminal 22, which is the surface on the same side as the overlapping surfaces, under conditions that result in thermal conduction. Even when irradiating the top surface of the second terminal 22 with laser light, the distance from the end face must be neither too close nor too far, preferably 0.1 mm to 5 mm, more preferably 0.2 mm to 3 mm, and even more preferably 0.3 mm to 1.5 mm. By irradiating the second terminal 22 with laser light 9, the second top surface 22U, which is the top surface of the second terminal 22 near the end face, melts and connects to the first terminal 21, thereby bonding them together.

[0018] (Terminal welding equipment) Fig. 3 is a schematic diagram of a welding apparatus 100 that performs the welding described with reference to Fig. 2. The welding apparatus 100 includes a shielding gas nozzle 11 that supplies shielding gas 12, a laser processing head 13 that irradiates laser light 9, a laser oscillator 14 that generates the laser light 9, an optical fiber 15 that connects the laser oscillator 14 and the laser processing head 13, and an XY stage 16 that moves an object to be welded.

[0019] The wavelength of the laser light 9 generated by the laser oscillator 14 is not particularly limited, but to ensure stable heat conduction mode welding, the wavelength is preferably 600 nm or less, and more preferably 500 nm or less. Specifically, a green laser with a wavelength of 532 μm or a blue laser with a wavelength of 450 μm is preferred. The shielding gas 12 is, for example, argon gas. The welding device 100 irradiates the laser generated by the laser oscillator 14 onto the first terminal 21 and the second terminal 22 through the optical fiber 15 and the laser processing head 13. The XY stage 16 is operated during laser irradiation to weld any desired position. Laser Light 9 By irradiating the first upper surface 21U and the second upper surface 22U along the end of the first terminal 21 multiple times, the end of the first terminal 21 and the second terminal 22 melt and mix, and a joint between the first terminal 21 and the second terminal 22 is formed.

[0020] (Welding method) Welding of the first terminal 21 and the second terminal 22 using the welding device 100 is divided into a laser irradiation process and a weld formation process. First, in the laser irradiation process, the welding device 100 is used to irradiate the first terminal 21 and the second terminal 22 with laser light 9. In the subsequent weld formation process, the area irradiated with the laser light 9 gradually cools to form a weld. The weld will be described with reference to FIG. 4 below. Heat dissipation in the weld formation process may be natural heat dissipation, or the ambient temperature or air flow may be controlled to promote or suppress cooling.

[0021] (Cross-section) Figure 4 shows a cross-sectional view of the welding position. The upper part of Figure 4 is a cross-sectional photograph, and the lower part is a schematic diagram illustrating the cross-sectional photograph. The detailed procedure for obtaining the cross-sectional photograph will be described later. The length of the arrow in the upper part of Figure 4 indicates dimensions for reference. As shown in the schematic diagram in the lower part of Figure 4, laser light 9 was irradiated from above. Columnar crystal grains 801 were formed as the laser light 9 irradiated the welding position. Hereinafter, the region where these crystal grains 801 exist in the first terminal 21 will be referred to as the first crystal grain region 810, and the region where these crystal grains 801 exist in the second terminal 22 will be referred to as the second crystal grain region 820. The first crystal grain region 810 and the second crystal grain region 820 have structural morphologies unique to regions that have been irradiated multiple times with laser light in the thermal conduction mode, resulting from the extremely slow cooling rate during the melting and solidification process. Generally, because grain boundaries hinder electrical conduction, a larger crystal grain size is preferred in joints where electrical conduction is rate-determining, preferably 45 μm or greater. A grain size of 55 μm or greater is more preferable, and a grain size of 65 μm or greater is even more preferable.

[0022] Hereinafter, the region of the first terminal 21 and the second terminal 22 that is melted by the laser beam 9 and then joined by cooling will be referred to as a welded portion 800. The welded portion 800 includes a first crystal grain region 810 and a second crystal grain region 820.

[0023] Figure 5 is a cross-sectional view of the welding position in keyhole mode welding, a comparative example. The upper part of Figure 5 is a cross-sectional photograph, and the lower part of Figure 5 is a schematic diagram explaining the cross-sectional photograph. In Figure 5, the first terminal 21 and the second terminal 22 are also stacked vertically in the figure. Because keyhole mode welding is used, the laser beam 9 reaches a deep position in the center. In keyhole mode welding, the cooling rate is relatively fast, so the crystal grain size is fine. In addition, the crystal shape is also different, with isotropic fine crystal grains called equiaxed crystals 851 occurring in the center of the bead, and columnar crystals 852 extending toward the center of the bead.

[0024] (Example) The laser welding method of this embodiment will be specifically described below.

[0025] Example 1 A first terminal measuring 8 mm wide and 1 mm thick was overlapped with a second terminal measuring 8 mm wide and 1.5 mm thick, and the overlapping portion was laser welded. A green laser with a wavelength of 532 nm was used, and conditions were adopted in which the energy density was 1500 kW / cm2 or less to ensure thermal conduction mode welding. Two patterns of laser irradiation were performed: one in which the first upper surface 21U was irradiated with the laser and then the second upper surface 22U was irradiated with the laser, and one in which this process was performed ten times.

[0026] The laser trajectory followed the end face of the first terminal 21, regardless of whether it was irradiating the first terminal 21 or the second terminal 22. The distance from the end face to the laser irradiation position was 0.5 mm. During the laser welding experiment, the molten pool was photographed using a high-speed camera to check for spatter. The high-speed camera was equipped with a 950 nm ± 50 nm bandpass filter and the frame rate was set to 500 fps. The results of the laser welding experiment showed that no spatter occurred in either the one-time or ten-time laser irradiation process. However, the first terminal 21 and the second terminal 22 were not joined when only the one-time process was performed, while the first terminal 21 and the second terminal 22 were joined when the ten-time process was performed.

[0027] Next, the joined welded sample was cut perpendicular to the laser scanning direction to obtain a cross-sectional sample. The obtained cross-sectional sample was first polished with waterproof emery paper #1000. It was then mirror-polished using 9 μm, 3 μm, and 1 μm diamond abrasives. It was then etched with an aqueous solution of iron chloride dissolved in hydrochloric acid and ethanol, and the cross-section was observed using an optical microscope. The upper part of Figure 4 shows the results. Figure 4 shows that coarse columnar crystal grains 801 extend toward the position where the laser beam 9 was irradiated.

[0028] Next, the cross-section samples were mirror-polished, and surface distortion was removed using an active oxide polishing suspension. The grain size of the joint was then measured using electron backscatter diffraction (EBSD). The results showed that the average grain size was 67.2 μm and the maximum grain size was 312.2 μm. The grain size was calculated as the equivalent circle diameter.

[0029] Example 2 Fig. 6 is a diagram showing the irradiation of laser light 9 in Example 2. In Example 2, the first terminal 21 and the second terminal 22 were overlapped as in Example 1, and laser welding was performed by an irradiation method called wobbling, while the laser was irradiated on the top surfaces of both the first terminal 21 and the second terminal 22 in an elliptical orbit as shown by the dotted line in Fig. 6. Wobbling refers to scanning the irradiation position of the laser light so that it moves in a predetermined direction while drawing a circle.

[0030] The lengths of the major and minor axes of the ellipse were 7 mm and 1 mm, respectively. The wobbling conditions were a width of 0.8 mm, a pitch of 0.2 mm, and a rotation frequency of 250 Hz. In Example 2, a green laser with a wavelength of 532 nm was used, and the energy density was set to 1500 kW / cm2 or less to achieve thermal conduction welding. Each revolution of the ellipse was counted as one cycle, and the process was repeated once, five times, and eight times. When performing the process five or eight times, the laser irradiation was performed continuously without stopping after each revolution. As a result, no spattering occurred in either case, but no welding was achieved when the laser was irradiated only once. When performing the process five times, some welding was achieved, but unwelded areas were observed. On the other hand, when performing the process eight times, welding was confirmed without any unwelded areas.

[0031] Next, the eight samples were cut perpendicular to the laser scanning direction to obtain cross-sectional samples. The cross-sectional samples were polished using waterproof emery paper #1000 and then mirror-polished using 9 μm, 3 μm, and 1 μm diamond abrasives. Furthermore, the strain on the sample surface was removed using an active oxide polishing suspension, and the grain size of the joint was measured using the EBSD method. The results showed that the average grain size was 55.7 μm and the maximum grain size was 198.7 μm.

[0032] (Comparative Example 1) First terminal 21 and second terminal 22, which had the same dimensions as those in Example 1, were overlapped, and fillet welding was performed at the overlapping portion under conditions that would result in keyhole welding, such as an energy density of 1500 kW / cm or greater. The same green laser with a wavelength of 532 nm as in Example 1 was used, and the welding was performed once. As a result, although the joint was successfully joined with only one welding, a large amount of spatter was confirmed to be generated. Next, the joined welded sample was cut perpendicular to the laser scanning direction, and the cross-sectional sample was taken. It was then mirror-polished and the grain size of the joint was measured using the EBSD method in the same manner as in Example 1. The average grain size was found to be 30.1 μm, and the maximum grain size was 92.8 μm.

[0033] (Comparative Example 2) First terminal 21 and second terminal 22, which had the same dimensions as those in Example 1, were overlapped and lap welding was performed under conditions that resulted in keyhole welding. Note that an IR laser with a laser wavelength of 1064 nm was used, and welding was performed twice along the same trajectory. As a result, although first terminal 21 and second terminal 22 were joined, it was confirmed that a large amount of spatter was generated, as in Comparative Example 1. Next, the joined welded sample was cut in a direction perpendicular to the laser scanning direction to obtain a cross-sectional sample. Then, mirror polishing was performed in the same manner as in Example 1, and the grain size of the joint was measured using the EBSD method. The average grain size was found to be 24.4 μm, and the maximum grain size was 101.4 μm.

[0034] (Comparative Example 3) First terminal 21 and second terminal 22, which had the same dimensions as those in Example 1, were overlapped and lap welding was performed under conditions that resulted in keyhole welding. Note that wobbling welding was performed using an IR laser with a laser wavelength of 1064 nm. As a result, first terminal 21 and second terminal 22 were joined, and although spatter was suppressed compared to Comparative Examples 1 and 2, spatter was still confirmed to have occurred. Next, the joined welded sample was cut in a direction perpendicular to the laser scanning direction to obtain a cross-sectional sample. Then, mirror polishing was performed in the same manner as in Example 1, and the grain size of the joint was measured using the EBSD method. The average grain size was found to be 42.2 μm, and the maximum grain size was found to be 159.2 μm.

[0035] According to the above-described embodiment, the following effects can be obtained. (1) The laser welding method used in the power conversion device 1 involves overlapping a first terminal 21 and a second terminal 22 made of a metal material primarily composed of copper or aluminum, and melting and joining the overlapping area with a laser. This laser welding method includes a laser irradiation step of irradiating a first upper surface 21U, which is the surface of the first terminal 21 opposite to the surface that is overlapped with the second terminal 22, and a second upper surface 22U, which is the surface of the second terminal 22 on the same side as the surface that is overlapped with the first terminal 21, with a laser along the edge of the first terminal 21. In the laser irradiation step, the laser is irradiated multiple times under conditions that result in thermal conduction mode welding. This makes it possible to suppress the generation of spatter due to welding.

[0036] (2) The method includes a weld formation process in which the overlapping region melted in the laser irradiation process is solidified to form a weld 800 with an average crystal grain size of 50 μm or more. This improves the electrical conductivity of the weld 800. The details are as follows: Generally, in metallic materials, crystal grain boundaries are an obstacle to electrical conduction. In the welding method of this embodiment, by irradiating the laser in thermal conduction mode multiple times, the crystal grains in the joint become coarse crystal grains that grow toward the laser irradiation point, as shown in FIG. 4. This reduces the grain boundary density and improves electrical conductivity. Furthermore, improved electrical conductivity can suppress heat generation and energy loss.

[0037] (3) The power conversion device 1 includes a terminal joint structure having a weld 800 formed by overlapping a first terminal 21 and a second terminal 22 made of a metal material primarily composed of copper or aluminum and fusing the first terminal 21 and the second terminal 22 together with a laser. The weld 800 includes a first crystal grain region 810 in which columnar crystal grains extend toward a region along the edge of the first terminal 21 on a first upper surface 21U, which is the surface opposite the overlapping surfaces of the first terminal 21 and the second terminal 22, and a second crystal grain region 820 in which columnar crystal grains extend toward a region along the edge of the first terminal 21 on a second upper surface 22U, which is the surface on the same side as the overlapping surfaces. This joint structure facilitates coarsening of the crystal grains, thereby reducing the grain boundary density and improving electrical conductivity.

[0038] (4) In the terminal joint structure included in the power converter 1, the average crystal grain size in the welded portion 800 is 50 μm or more. Therefore, the welded portion 800 can have good electrical conductivity.

[0039] (5) The power converter 1 has the above-described terminal joint structure, which allows the power converter 1 to have good electrical conductivity.

[0040] (Variation 1) In the above-described embodiment, the laser light 9 is irradiated along the edge on the negative side in the X-axis direction of the first terminal 21. However, a through-hole may be provided in the first terminal 21, and the laser light 9 may be irradiated along the edge of the through-hole.

[0041] FIG. 7 is a diagram showing a welding method in Modification 1. A through hole 21H is provided in the first terminal 21, and the second upper surface 22U is exposed to the positive side of the Z axis through the through hole 21H. In this modification, laser light 9 is irradiated onto the edges of the through hole 21H on the first upper surface 21U and the second upper surface 22U. The positional relationship between the position where the laser light 9 is irradiated and the edges is the same as in the embodiment described above. This modification has the effect of suppressing the adverse effects of reflection of the laser light 9. This effect will be described with reference to FIG. 8.

[0042] FIG. 8 is a diagram illustrating the effect of Modification Example 1. When the first terminal 21 is irradiated with laser light multiple times along the end face, if the end face melts and changes shape as shown in FIG. 8, or if the laser is defocused, increasing the spot diameter and causing a portion of the spot to extend beyond the end face, laser reflection 9R may occur at the end face. Laser reflection 9R may damage surrounding components, such as modules and circuits. Therefore, by providing a through hole 21H in the first terminal 21 as shown in FIG. 7 and irradiating laser light 9 along the edge of the through hole 21H, even if laser reflection 9R occurs, the laser light remains inside the through hole 21H. Therefore, the effects of laser reflection 9R can be suppressed, and damage to the surrounding area can also be reduced.

[0043] Example 3 As shown in FIG. 7 , a through hole 21H was formed in the first terminal 21, and the second terminal 22 was placed on top of it. Laser irradiation was performed on the first upper surface 21U and the second upper surface 22U along the edge of the through hole 21H. The dimensions of the first terminal 21 and the second terminal 22, the wavelength of the laser light 9, and the irradiation conditions for the laser light 9 were the same as those in Example 1, and the laser irradiation position was 0.5 mm from the edge. The process of irradiating the first upper surface 21U and the second upper surface 22U with the laser light 9 was counted as one cycle, and two patterns were performed: one cycle and one cycle. As a result, no spattering occurred in either case, but the first terminal 21 and the second terminal 22 were not bonded when irradiated only once, while bonding of the first terminal 21 and the second terminal 22 was confirmed when irradiated eight times.

[0044] Next, the joined welded sample was cut in a direction perpendicular to the laser scanning direction to obtain a cross-sectional sample, which was then mirror-polished and the grain size of the joint was measured by the EBSD method in the same manner as in Example 1. As a result, it was found that the average grain size was 88.7 μm and the maximum grain size was 415.4 μm.

[0045] According to the first modification, the following effects can be obtained. (6) The edge of first terminal 21 refers to the edge of through hole 21H provided in first terminal 21. In the laser irradiation process, a laser is irradiated onto first upper surface 21U and second upper surface 22U along the edge of through hole 21H, and weld 800 is formed along the edge of through hole 21H. This makes it possible to suppress the influence of laser reflection 9R as shown in FIG. 8 and also reduce damage to the surrounding area.

[0046] (Variation 2) In the above-described embodiment, first terminal 21 has a flat plate shape, but first terminal 21 may be processed so that its temperature is easily increased by irradiation with laser beam 9. For example, the heat capacity of the region of first terminal 21 that is irradiated with laser beam 9 may be reduced, or the heat transfer path may be reduced so that heat is less likely to transfer to regions that are not irradiated with laser beam 9.

[0047] FIG. 9 is a diagram showing the shape of a first terminal 21 in Modification 2. FIG. 9 shows six examples, designated by symbols "21-1" to "21-6." In all examples, laser light 9 is irradiated to the right side of the figure. The first terminals 21 designated by symbols "21-1" and "21-2" have reduced heat capacity in the area irradiated with laser light 9, indicated by the dotted line. The first terminals 21 designated by symbols "21-3" to "21-6" have the same area irradiated with laser light 9 as in the embodiment, but the heat transfer path is reduced in the area surrounded by the dashed line so that heat is less likely to transfer to areas not irradiated with laser light 9. In all of symbols "21-1" to "21-6," the area surrounded by the dashed line is a low-heat capacity area, where the heat capacity is relatively small compared to other areas.

[0048] According to the second modification, the following effects can be obtained. (7) First terminal 21 has a low heat capacity region at the tip side where it overlaps with second terminal 22, the low heat capacity region having a relatively smaller heat capacity than the region not overlapping with second terminal 22. This allows laser welding with a small heat input, which has the advantages of facilitating welding and reducing heat damage to surrounding members.

[0049] The above-described embodiments and modifications may be combined with each other. Although various embodiments and modifications have been described above, the present invention is not limited to these. Other embodiments conceivable within the scope of the technical concept of the present invention are also included within the scope of the present invention. [Explanation of symbols]

[0050] 1: Power conversion device 2: Semiconductor elements 2A: Connection terminal 9: Laser light 10: Laser 21 :First terminal 21H: Through hole 21U: First top surface 22:Second terminal 22U: Second top surface 800: Welded section 801: Grain 810: First grain region 820:Second grain region

Claims

1. A laser welding method in which a first terminal and a second terminal made of a metal material containing copper or aluminum as a main component are directly overlapped with each other and the first terminal and the second terminal are fusion-joined with a laser, a laser irradiation step of irradiating a first upper surface of the first terminal, which is the surface opposite to the surface that is to be superimposed on the second terminal, and a second upper surface of the second terminal, which is the surface on the same side as the surface that is to be superimposed on the first terminal, with the laser along an edge of the first terminal; In the laser irradiation step, the laser is irradiated multiple times under conditions that result in thermal conduction mode welding, The first terminal has a low heat capacity region at a tip side overlapping with the second terminal, the low heat capacity region having a relatively smaller heat capacity than a region not overlapping with the second terminal.

2. 2. The laser welding method according to claim 1, the edge of the first terminal is an edge of a through hole formed in the first terminal, In the laser irradiation step, the laser is irradiated along the edge of the through hole onto the top surface of the first terminal and the top surface of the second terminal.

3. 2. The laser welding method according to claim 1, a weld forming step of forming a weld having an average crystal grain size of 50 μm or more by solidifying the overlapping portion melted in the laser irradiation step.

4. A terminal joint structure in which a first terminal and a second terminal made of a metal material containing copper or aluminum as a main component are directly overlapped with each other, and the overlapping portion is melt-joined with a laser to form a welded portion, the welded portion includes a first region in which columnar crystal grains extend toward a region along an edge of the first terminal on an upper surface of the first terminal, the region being the opposite side to the overlapping surfaces of the first terminal and the second terminal, and a second region in which columnar crystal grains extend toward a region along the edge of the first terminal on an upper surface of the second terminal, the surface being the same side as the overlapping surfaces, The first terminal has a low heat capacity region at a tip side where the first terminal overlaps the second terminal, the low heat capacity region having a relatively smaller heat capacity than a region not overlapping the second terminal.

5. The terminal joint structure according to claim 4, The terminal joint structure has an average crystal grain size of 50 μm or more in the welded portion.

6. The terminal joint structure according to claim 4, the edge of the first terminal is an edge of a through hole formed in the first terminal, The welded portion is formed along the edge of the through hole.

7. A power converter having the terminal joint structure according to any one of claims 4 to 6.

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