JP5200528B2 - Laser welding method and welded joint - Google Patents

Description

  The present invention relates to a laser welding method and a welded joint.

  In recent years, laser welding has been adopted instead of spot welding in vehicle assembly processes. In laser welding, for example, steel plates on which galvanized layers are formed by rust prevention treatment are joined together.

As a technique for performing laser welding of steel plates on which galvanized layers are formed, a laser welding method shown in Patent Document 1 below is known. The laser welding method of Patent Document 1 is to irradiate the two steel plates with laser light while ensuring a gap for degassing between the two steel plates to be overlapped. With this configuration, the vaporized gas of the galvanized layer that evaporates due to the heat of the laser light is discharged to the outside from the gap for degassing, so that the porosity of the weld bead is generated by the vaporized gas of the galvanized layer. Can be prevented.
JP 2004-283835 A

  However, in the above laser welding method, there is a problem that adjustment of the gap between the two steel plates is complicated and workability is poor in order to ensure the welding quality of the welded joint. In particular, if the gap is made too large in order to improve the gas-gas exhaustability, welding failure may occur. Conversely, if the gap is made too small, the vaporized gas in the galvanized layer will be exhausted sufficiently. There is a risk that porosity may occur.

  The present invention has been made to solve the above-described problems. Accordingly, an object of the present invention is to provide a laser welding method capable of preventing the occurrence of porosity and ensuring the weld quality of a welded joint.

  Another object of the present invention is to provide a welded joint that is laser welded by the laser welding method.

  The above object of the present invention is achieved by the following means.

The laser welding method of the present invention is a laser welding method in which a plurality of plate materials including a plate material having a surface layer formed by surface treatment are overlapped and laser-welded, and a step of forming gas discharge holes and a plate material are joined. And a process. The step of forming the gas discharge hole is a gas that communicates outward from the mating surface of the plate member on which the surface layer is formed and the other plate member, and discharges the vaporized gas of the surface layer that is vaporized by laser light irradiation. A discharge hole is formed in the plate material stacked on at least one side of the mating surfaces. The step of joining the plate members irradiates the vicinity of the gas discharge holes with a laser beam to join the plurality of stacked plate members. The step of forming the gas discharge hole includes forming a circular gas discharge hole, and forming a protruding portion that forms a space for guiding the vaporized gas of the surface layer in the gas discharge hole on the outermost plate material, The step of joining the plate members includes a step of moving the laser beam along the protruding portion so as to surround the circular gas discharge hole.

The welded joint of the present invention is a welded joint formed by laminating a plurality of plate materials including a plate material having a surface layer formed by surface treatment and performing laser welding. The welded joint of the present invention has a protrusion that forms a space for inducing vaporized gas of the surface layer in the outermost plate material, and communicates outwardly from the mating surface of the plate material on which the surface layer is formed and another plate material. to the vicinity of the circular gas discharge hole for discharging the vaporized gas of the surface layer of vaporized by irradiation of the laser beam, the weld bead along the projecting portion so as to surround the circular gas discharge hole formed Has been.

  According to the laser welding method of the present invention, since the vaporized gas in the surface layer is discharged to the outside through the gas discharge hole, the generation of porosity due to the vaporized gas in the surface layer is prevented. Therefore, the welding quality of the welded joint can be ensured.

  Moreover, according to the welded assembly of the present invention, the welding quality is ensured.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the embodiment described below, a case where two steel plates (hereinafter referred to as plate members) on which a galvanized layer is formed by rust prevention treatment is laser-welded will be described as an example. Moreover, the same code | symbol was used for the same member in the figure.

(First embodiment)
First, a laser welding apparatus to which the laser welding method according to the first embodiment of the present invention is applied will be described with reference to FIGS. 1 and 2. FIG. 1 is a diagram showing a schematic configuration of a laser welding apparatus to which the laser welding method in the present embodiment is applied.

  As shown in FIG. 1, the laser welding apparatus 100 of the present embodiment includes a processing head 10, a robot 20, a laser oscillator 30, and a control device 40.

  The processing head 10 irradiates the two plate members 51 and 52 to be overlapped with the laser beam 60. The processing head 10 includes therein an optical system including a plurality of lenses and mirrors, and can move the laser beam 60 while adjusting the focal point of the laser beam 60.

  The robot 20 moves the machining head 10 in the multi-axis direction. The robot 20 of the present embodiment is an articulated arm robot, and the machining head 10 is attached to the tip of the arm. The robot 20 can move the laser beam 60 by moving the machining head 10.

  The laser oscillator 30 supplies laser light to the machining head 10. The laser oscillator 30 is, for example, a YAG laser oscillator, and supplies a laser beam 60 to the processing head 10 via the optical fiber cable 35.

  The control device 40 controls the machining head 10, the robot 20, and the laser oscillator 30.

  FIG. 2 is a view for explaining a machining head in the laser welding apparatus shown in FIG.

  As shown in FIG. 2, the processing head 10 of the present embodiment includes a plurality of lenses 11 a to 11 d that transmit laser light incident from an optical fiber cable 35 and mirrors 12 a to 12 c that reflect the laser light. .

  One lens 11b among the plurality of lenses 11a to 11d is driven by the drive motor 13a and moves in the optical axis direction. The focal length of the laser beam 60 is adjusted by moving the lens 11b. The drive motor 13a is controlled by the control device 40.

  Of the plurality of mirrors 12a to 12c, the first and second mirrors 12b and 12c are driven by drive motors 13b and 13c, and rotate about different axes. By rotating the first and second mirrors 12b and 12c, the emission direction of the laser beam 60 is freely distributed. The mirrors 12b and 12c are controlled by the control device 40.

  In the laser welding apparatus 100 configured as described above, the laser beam 60 can be freely moved on the plate members 51 and 52 by controlling the operations of the mirrors 12b and 12c inside the machining head 10. Hereinafter, the laser welding method of the present embodiment using the laser welding apparatus 100 will be described with reference to FIGS.

  FIG. 3 is a flowchart showing laser welding processing by the laser welding apparatus shown in FIG. In the laser welding method of the present embodiment, in order to prevent the generation of porosity due to the vaporized gas in the galvanized layer on the surface of the plate material, the gas discharge hole 55 communicating outward from the mating surface of the plate materials 51 and 52 is formed in the plate material 51. Then, the laser beam 60 is irradiated in the vicinity of the gas discharge hole 55.

  As shown in FIG. 3, in the laser welding method of the present embodiment, first, gas discharge holes are formed in the plate material (step S101). In the present embodiment, the gas discharge hole 55 is formed in the upper plate member 51 located on the side irradiated with the laser beam 60 among the two plate members 51 and 52 to be overlaid. The gas discharge hole 55 of the present embodiment is circular and penetrates the plate material 51. The gas discharge hole 55 is formed by press work when the plate material 51 is press-formed.

  Next, the plate material on which the gas discharge holes are formed is overlaid (step S102). In the present embodiment, as shown in FIG. 4, the upper plate material 51 in which the gas discharge holes 55 are formed in the process shown in step S <b> 101 is overlapped with the lower plate material 52. The overlapped plate members 51 and 52 are set in the laser welding apparatus 100.

  Next, a laser beam is irradiated on the overlapped plate material (step S103). In the present embodiment, as shown in FIG. 5, the laser beam 60 is irradiated from above the plate members 51 and 52 superposed in the process shown in step S102. More specifically, the laser beam 60 is irradiated from the processing head 10 of the laser welding apparatus 100 to the vicinity of the gas discharge hole 55 of the upper plate member 51.

  Next, the laser beam is moved (step S104). In the present embodiment, as shown in FIG. 6, the laser beam 60 irradiated to the vicinity of the gas discharge hole 55 of the upper plate member 51 is moved so as to surround the gas discharge hole 55. More specifically, the operations of the mirrors 12 b and 12 c inside the machining head 10 are controlled, and the laser beam 60 is moved so as to surround the gas discharge hole 55. At this time, the plate materials 51 and 52 are melted by the heat of the laser beam 60, and the galvanized layer covering the surfaces of the plate materials 51 and 52 is evaporated. The vaporized gas of the evaporated galvanized layer is discharged to the outside through the gas discharge holes 55 communicating outward from the mating surfaces of the plate members 51 and 52.

  Then, the laser beam irradiation is stopped (step S105), and the process is terminated. A laser-welded welded joint is formed with a C-shaped weld bead 56 so as to surround the gas discharge hole 55. According to the weld bead 56 having such a shape, even when stress is applied to the outer peripheral portion of the weld bead 56, the stress is transmitted to the gas discharge hole 55 inside the weld bead 56 due to the stress of the weld bead 56. Therefore, external stress does not act on the gas discharge hole 55.

  As described above, according to the process of the flowchart shown in FIG. 3, first, the gas discharge hole 55 that communicates outward from the mating surface of the two plate members 51 and 52 is formed in one plate member 51 to be laser welded. The Then, the laser beam 60 is irradiated so as to surround the gas discharge hole 55, and the two overlapped plate members 51 and 52 are joined. At this time, the vaporized gas of the galvanized layer that is vaporized by the heat of the laser beam is discharged to the outside through a gas discharge hole provided in the vicinity of the laser beam irradiation site. Therefore, it is possible to prevent porosity from being generated in the weld bead due to the vaporized gas of the galvanized layer.

  Next, the effect in the laser welding method of this Embodiment is demonstrated, referring FIG. 7 and FIG.

  FIG. 7 is a cross-sectional view taken along line VII-VII in FIG. 6, and FIG. 8 is a diagram showing a case of a general laser welding method as a comparative example.

  In the general laser welding method shown in FIG. 8, the plate material is irradiated with laser light without forming a gas discharge hole, and the plate material is joined (see FIG. 8B). When the plate materials 51 ′ and 52 ′ on which the galvanized layers 53 ′ and 54 ′ are formed are irradiated with the laser beam 60, the plate materials 51 ′ and 52 ′ are melted by the heat of the laser beam 60 and the galvanized layer 53. ', 54' evaporates. Therefore, according to a general laser welding method, the vaporized gas 57 of the galvanized layers 53 ′ and 54 ′ passes through the molten pool of the plate material and is discharged to the outside, so that the weld bead 56 ′ is called a porosity 59. Micropores are generated (see FIG. 8A). Such porosity 59 not only deteriorates the appearance quality of the welded joint, but also decreases the welding strength.

  On the other hand, as shown in FIG. 7, according to the laser welding method of the present embodiment, even if the galvanized layers 53 and 54 evaporate due to the heat of the laser beam 60, The vaporized gas 57 of the galvanized layers 53 and 54 is discharged to the outside through the hole 55. Therefore, since the vaporized gas 57 of the galvanized layers 53 and 54 is discharged outside without passing through the molten pool, the generation of porosity due to the vaporized gas of the galvanized layers 53 and 54 is prevented.

  As described above, the described embodiment has the following effects.

  (A) The laser welding method of the present embodiment is a laser welding method in which two plate materials on which a galvanized layer is formed by rust prevention treatment are overlapped and laser-welded, and a step of forming a gas discharge hole; And joining the plate materials. The step of forming the gas discharge hole is a gas discharge hole that communicates outwardly from the mating surface of the two plate members on which the galvanized layer is formed, and discharges the vaporized gas of the galvanized layer that is vaporized by laser light irradiation. Is formed on the plate material on the upper side of the mating surface. In the step of joining the plate materials, laser light is irradiated in the vicinity of the gas discharge hole to join the two plate materials that are overlapped. Therefore, since the vaporized gas of the galvanized layer is discharged to the outside through the gas discharge hole, the generation of porosity due to the vaporized gas of the galvanized layer is prevented. As a result, the welding quality of the welded joint is ensured. In addition, by changing the shape of the gas discharge hole, various shapes of weld beads can be handled.

  (B) The step of forming the gas discharge hole includes forming the circular gas discharge hole, and the step of joining the plate members includes a step of moving the laser beam so as to surround the circular gas discharge hole. Therefore, even when stress is applied to the outer periphery of the weld bead, external stress acts on the gas discharge hole to prevent the weld bead from receiving stress and transmitting stress to the gas discharge hole inside the weld bead. do not do. As a result, it is possible to reduce the weight of the welded joint by forming the gas discharge holes without reducing the bonding strength due to the formation of the gas discharge holes. Moreover, it can prevent that an excessive stress concentrates on the specific location of a weld bead.

  (C) The welded joint of this embodiment communicates outward from the mating surface of the two plate members on which the galvanized layer is formed, and discharges the vaporized gas of the galvanized layer that is vaporized by laser light irradiation. A weld bead is formed in the vicinity of the gas discharge hole. Therefore, since the vaporized gas of the galvanized layer is discharged to the outside through the gas discharge hole, the generation of porosity due to the vaporized gas of the galvanized layer is prevented. As a result, welding quality is ensured.

(Second Embodiment)
In the first embodiment, the case where the circular gas discharge holes are formed in the plate material has been described. In the present embodiment, a case where rectangular gas discharge holes are formed in a plate material will be described.

  FIG. 9A is a diagram for explaining a laser welding method according to the second embodiment of the present invention, and FIG. 9B is a cross section taken along line IX-IX in FIG. 9A. FIG. In the laser welding method of the present embodiment, a rectangular gas discharge hole 55 is first formed in the plate material 51. Then, as shown in FIG. 9A, the laser beam 60 is linearly moved so as to surround the rectangular gas discharge hole 55. Since the laser welding method of the present embodiment is the same as that of the first embodiment except that the rectangular gas discharge hole 55 is formed in the plate material 51, the detailed description is omitted.

  With such a configuration, as shown in FIG. 9B, the vaporized gas 57 of the galvanized layers 53 and 54 evaporated by the heat of the laser beam 60 is a rectangular gas provided in the vicinity of the irradiated portion of the laser beam. It is discharged to the outside through the discharge hole 55. Therefore, it is possible to prevent porosity from being generated in the weld bead due to the vaporized gas of the galvanized layer.

  As described above, the described embodiment has the following effects in addition to the effects of the first embodiment.

  (D) The step of forming the gas discharge hole includes forming the rectangular gas discharge hole, and the step of joining the plate members includes a step of linearly moving the laser beam along the rectangular gas discharge hole. Therefore, the laser beam can be moved by controlling the operation of the robot to which the machining head is attached without controlling the operation of the mirror inside the machining head.

(Third embodiment)
Next, a third embodiment of the present invention will be described with reference to FIG. In the present embodiment, a gas discharge hole is formed, and a projecting portion that forms a space for inducing a vaporized gas of the galvanized layer is formed in the gas discharge hole.

  FIG. 10A is a view for explaining a laser welding method according to the third embodiment of the present invention, and FIG. 10B is a cross section taken along line XX in FIG. 10A. FIG.

  In the laser welding method of the present embodiment, a gas discharge hole is formed in the plate material, and a protrusion that forms a space for inducing vaporized gas of the galvanized layer is formed in the gas discharge hole. More specifically, a protruding portion 58 that protrudes upward is formed on the upper plate member 51, and a gas discharge hole 55 is formed at the tip of the protruding portion 58. The protrusion 58 is formed by press molding together with the gas discharge hole 55.

  Next, the upper plate member 51 and the lower plate member 52 are overlapped to form a space between the inner surface of the protrusion 58 and the upper surface of the lower plate member 52. Then, as shown in FIG. 10A, the laser beam 60 is moved so as to surround the protruding portion 58 of the upper plate member 51, and the plate members 51 and 52 are joined. Since the laser welding method of the present embodiment is the same as that of the first embodiment except that the protrusions 58 are formed on the plate material 51, detailed description thereof is omitted.

  With such a configuration, as shown in FIG. 10B, a space in which the vaporized gas 57 of the galvanized layers 53 and 54 evaporated by the heat of the laser light is defined by the protruding portion 58 and the lower plate material 52. Through the gas exhaust hole 55. Therefore, the vaporized gas 57 of the galvanized layers 53 and 54 is reliably discharged through the gas discharge hole 55.

(Modification)
FIG. 11A is a diagram for explaining a modification of the laser welding method according to the third embodiment of the present invention, and FIG. 11B is a line XI-XI in FIG. 11A. FIG. In the present modification, the protruding portion 58 is formed so as to cross the plurality of gas discharge holes 55.

  As shown in FIG. 11A, in this modification, a gas discharge hole 55 is formed, and a protrusion 58 that crosses the plurality of gas discharge holes 55 is formed. A laser beam is formed along the side of the protrusion 58. 60 is moved linearly.

  With such a configuration, as shown in FIG. 11B, a space in which the vaporized gas 57 of the galvanized layers 53 and 54 evaporated by the heat of the laser beam 60 is defined by the protruding portion 58 and the lower plate material 52. Through the gas exhaust hole 55.

  As described above, the described embodiment has the following effects in addition to the effects of the first and second embodiments.

  (E) The step of forming the gas discharge hole is to form the gas discharge hole, and to form a protruding portion that forms a space for inducing the vaporized gas of the galvanized layer in the gas discharge hole on the upper plate material, and join the plate materials The step includes a step of moving the laser light along the protruding portion. Therefore, since the vaporized gas of the galvanized layer is reliably guided to the gas discharge hole through the space defined by the protrusions, the vaporized gas of the galvanized layer is reliably discharged through the gas discharge hole.

(Fourth embodiment)
In the first to third embodiments, the gas discharge holes are formed in the plate material by press molding. However, the gas discharge hole may be formed in the plate material by irradiating the laser beam.

  FIG. 12 is a diagram for explaining a laser welding method according to the fourth embodiment of the present invention. In the present embodiment, gas discharge holes are formed in the plate material by the laser light emitted from the laser welding apparatus 100.

  Specifically, as shown in FIG. 12A, the laser beam 60 is irradiated from the laser welding apparatus 100, and the gas discharge hole 55 is formed in the plate material 51. In the present embodiment, high-power laser light is irradiated from above the plate material 51 held by the welding jig 70 such as a magnet. At this time, the laser beam 60 is irradiated with an output that is 20 to 30% higher than when laser welding the plate material. Note that unlike this embodiment, the gas discharge hole 55 may be formed by supplying oxygen gas to the irradiated portion of the laser beam 60.

  Next, the board | plate material in which the gas exhaust hole was formed is piled up. In the present embodiment, as shown in FIG. 12B, the lower plate 51 is placed under the upper plate 51 while maintaining the position of the upper plate 51 in which the gas discharge hole 55 is formed by irradiation with the laser beam 60. The plate material 52 is pressed.

  And the laser beam 60 is irradiated to the vicinity of the gas exhaust hole 55, and the board | plate materials 51 and 52 are joined. In the present embodiment, as shown in FIG. 12C, the laser beam 60 is moved so as to surround the gas discharge hole 55 formed by the irradiation of the laser beam 60. At this time, since the position of the upper plate 51 in which the gas discharge hole 55 is formed is maintained, the laser beam 60 is emitted with reference to the position coordinates of the laser welding apparatus 100 used when the gas discharge hole 55 is formed. The plate materials 51 and 52 can be irradiated with high accuracy.

  With such a configuration, the gas discharge hole 55 can be formed using the laser welding apparatus 100 used for laser welding. Therefore, the formation of the gas discharge holes and the joining of the plate materials can be realized in the same apparatus.

  As described above, the described embodiment has the following effects in addition to the effects of the first to third embodiments.

  (F) In the step of forming the gas discharge hole, the gas discharge hole is formed in the plate by irradiating the laser beam. Therefore, the pressing process for forming the gas discharge hole can be omitted.

  (G) The laser beam for forming the gas discharge hole and the laser beam for joining the plate materials are irradiated from the same laser oscillator. Therefore, since the formation of the gas discharge holes and the joining of the plate materials can be realized in the same apparatus, the position of the gas discharge holes and the welding position can be managed with the same coordinates. As a result, misalignment of the welding position is prevented, and the plate materials can be joined with high accuracy.

(Example)
Hereinafter, embodiments of the present invention will be described in detail using examples. However, the present invention is not limited at all by this example.

  First, as Example 1, two steel plates (panels) on which a galvanized layer was formed were joined using the laser welding method in the first embodiment. Specifically, first, a circular gas discharge hole having a diameter of 6 mm was formed in the upper steel plate by press working. Then, the two steel plates were overlapped and moved so as to draw a C-shape having a diameter of 6.5 mm while irradiating the vicinity of the gas discharge hole with laser light. It was confirmed that no porosity was generated in the welded assembly thus laser welded even when the distance between the steel plates on which the galvanized layer was formed was 0 mm.

  Next, as Example 2, two steel plates were joined on the steel plate on which the galvanized layer was formed by using the laser welding method in the fourth embodiment.

  Specifically, as shown in FIG. 13, first, out of the upper two steel plates, two gas discharge holes were formed in one steel plate and one gas discharge hole was formed in the other steel plate by laser fusing. Next, the upper and lower steel plates were overlapped. Each of the three gas discharge holes was moved so as to draw a C-shaped trajectory while irradiating the vicinity of the gas discharge holes with laser light. In the laser-welded welded body as described above, it was confirmed that there was no displacement between the position of the gas discharge hole and the welding position, and no porosity was generated.

  As described above, in the first to fourth embodiments, the laser welding method and the welded assembly of the present invention have been described. However, it goes without saying that the present invention can be appropriately added, modified, and omitted by those skilled in the art within the scope of the technical idea.

  For example, in the first to fourth embodiments, two plate members are laser welded to each other. However, the number of plate materials to be laser welded is not limited to two, and the plate materials may be overlapped in three or more layers and laser welded.

  In the first to fourth embodiments, the gas discharge hole is formed only in the upper plate. However, the plate material on which the gas discharge holes are formed is not limited to the upper plate material, but may be formed on the lower plate material, or may be formed on both plate materials.

  Moreover, in the 1st-4th embodiment, the steel plates in which the zinc plating layer was formed were laser-welded. However, the object to be joined is not limited to a steel plate, and may be another metal plate material or a resin plate material. Alternatively, one of the two plates may be a bare material that has not been rust-proofed. Furthermore, the surface layer covering the plate material surface is not limited to the zinc plating layer, and may be an aluminum plating layer or a chrome plating layer, and may be various coating layers covering the plate material surface.

It is a figure which shows schematic structure of the laser welding apparatus with which the laser welding method in the 1st Embodiment of this invention is applied. It is a figure for demonstrating the process head in the laser welding apparatus shown in FIG. It is a flowchart which shows the laser welding process by the laser welding apparatus shown in FIG. It is a figure for demonstrating the superimposition process in the flowchart of FIG. It is a figure for demonstrating the irradiation start process of the laser beam in the flowchart of FIG. It is a figure for demonstrating the movement process of the laser beam in the flowchart of FIG. It is sectional drawing along the VII-VII line of FIG. It is a figure which shows the case in a general laser welding method as a comparative example. FIG. 9A is a diagram for explaining a laser welding method according to the second embodiment of the present invention, and FIG. 9B is a cross section taken along line IX-IX in FIG. 9A. FIG. FIG. 10 (A) is a diagram for explaining a modification of the laser welding method in the third embodiment of the present invention, and FIG. 10 (B) is taken along line XX in FIG. 10 (A). FIG. FIG. 11A is a diagram for explaining a modification of the laser welding method according to the third embodiment of the present invention, and FIG. 11B is a line XI-XI in FIG. 11A. FIG. It is a figure for demonstrating the laser welding method in the 4th Embodiment of this invention. It is a figure for demonstrating the Example corresponding to the laser welding method shown in FIG.

Explanation of symbols

51,52 board,
53, 54 Zinc plating layer (surface layer),
55 Gas exhaust holes,
56 weld bead,
57 vaporized gas,
58 protrusions,
60 Laser light.

Claims (4)

A laser welding method in which a plurality of plate materials including a plate material having a surface layer formed by surface treatment are overlapped and laser-welded,
At least one of the mating surfaces has a gas discharge hole that communicates outward from the mating surface of the plate material on which the surface layer is formed and the other plate material, and exhausts the vaporized gas of the surface layer that is vaporized by laser light irradiation. A step of forming the plate material on the side,
Wherein by irradiating a laser beam in the vicinity of the gas discharge holes, possess a step, a joining a plurality of plate materials superimposed,
The step of forming the gas discharge hole forms a circular gas discharge hole, and forms a protruding portion that forms a space for inducing the vaporized gas of the surface layer in the gas discharge hole on the outermost plate material,
Process, laser welding method characterized by have a step along said protrusion moving said laser beam so as to surround the circular gas discharge hole for joining the plate.   The laser welding method according to claim 1, wherein the step of forming the gas discharge hole forms a gas discharge hole in the plate by irradiating a laser beam. 3. The laser welding method according to claim 2 , wherein the laser beam for forming the gas discharge hole and the laser beam for bonding the plate members are irradiated from the same laser oscillator. A welded joint formed by laminating and welding a plurality of plate materials including a plate material having a surface layer formed by surface treatment,
A protrusion that forms a space for inducing vaporized gas in the surface layer on the outermost plate member;
From mating surfaces of the surface layer was formed plate and the other plate member in communication with the outside, in the vicinity of the circular gas discharge hole for discharging the vaporized gas of the surface layer of vaporized by laser beam irradiation, the A welded joint , wherein a weld bead is formed along the protrusion so as to surround a circular gas discharge hole .

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