JP5061670B2 - Laser welding method - Google Patents

Description

  The present invention relates to a laser welding method in which a butt portion of a steel plate is welded by forming a keyhole.

  The laser beam has a very strong light condensing property and becomes a concentrated heat source with an extremely high energy density. Therefore, when applied to welding, the penetration depth is deep and high-speed welding can be performed.

  In laser welding, a thin plate is butted and fixed, a laser beam is irradiated to the butted portion, and welding is performed by penetrating the back side of the thin plate beam irradiation by keyhole welding.

  In the case of laser welding, the focused diameter of the laser beam is as small as about 0.2 mm, and in the case of butt welding, the allowable gap is as narrow as 0.08 mm, so the butt surface of the thin plate is machined or filler is fed. Various welding methods have been proposed, such as welding.

  Patent Document 1 relates to a laser welding method for improving the reliability of welding, and a laser beam is collected by a condensing lens, reflected by a rotating mirror, and the workpiece or the laser beam is made relative to the workpiece joint direction. The laser beam is scanned conically, and there is a gap between the mating surfaces, and it is described that good welding quality can be obtained even if the relative positioning accuracy between the laser spot and the mating surface is poor.

  Patent Document 2 relates to a method of performing at least one of preheating and postheating using a laser beam for welding when laser welding is performed, and a laser beam is scanned with respect to a groove line without scanning the laser beam with respect to a groove line. It is described that the laser spot is moved in the vertical direction along the axis, and the laser spot is in-focused and out-focused on the groove surface.

  Patent Document 3 relates to laser butt welding of a thin metal plate, in which at least one thin plate is subjected to plastic deformation by a mash roller before or within the welding zone to reduce the plate thickness and reduce the gap between the thin plates. It is described that good workability can be obtained by secondary processing such as press working after welding by appropriately defining the width and depth of the plastic deformation portion according to the thickness of the thin plate.

Also, in Patent Document 4, when laser welding a thin plate, the stability of welding is determined by the presence or absence of the generation of a blue frame (plasma), and welding is repeatedly performed until the light beam of the blue frame is detected. Are listed.
JP 54-116356 A JP 6-304772 A JP 2006-218499 A

  By the way, when high-speed welding or thick steel plates are welded using a high-power laser exceeding 5 kW, the key plate becomes unstable as the welding speed increases or the plate thickness increases. This is limiting the thickness.

  That is, if the keyhole becomes unstable: Molten metal seals the keyholes, creating defects such as blowholes. In addition, 2. The weld bead width on the back side of the material to be welded becomes narrower and off-seam defects are more likely to occur, resulting in poor weld quality.

  When the material to be welded is irradiated with the laser beam, the material to be welded melts and evaporates, and a keyhole is formed, but there is an anti-tension of the evaporated particles as the force to expand the keyhole, and the molten metal There is gravity and surface tension, and the keyhole is held by the balance between the two.

  As a factor that makes the keyhole unstable, interference between a laser beam and plasma (referred to as laser-induced plasma) generated by ionizing a part of the evaporated particles and the evaporated particles has been pointed out.

  When the laser beam is applied to the evaporated particles or laser-induced plasma ejected from the keyhole, the laser is reflected or absorbed by the evaporated particles or plasma.

  When the laser energy is reflected / absorbed, the energy reaching the bottom of the keyhole is reduced, so that no vaporized particles or plasma is generated.

  Therefore, the laser energy increases again, the laser beam reaches the bottom of the keyhole, and vaporized particles and plasma are generated.

  In this way, the vaporized particles and plasma are repeatedly generated and disappeared intermittently, making the keyhole depth non-uniform, inducing welding defects and making the penetration depth non-uniform.

  As a technique to stabilize the keyhole and suppress welding defects, when welding using a pulsed laser, the frequency of the evaporated particles or plasma that is intermittently ejected from the keyhole and the pulse frequency of the laser are synchronized, A technique has been proposed in which the laser is turned off when evaporated particles or plasma is generated, and the laser is turned on when they disappear.

  The fact that the weld bead width on the back side of the material to be welded is narrowed and off-seam defects are likely to occur is effective to reduce the welding speed and increase the amount of heat per unit weld length.

  However, pulsing the laser reduces the average energy and reduces the welding speed to obtain the same penetration depth compared to a continuous wave laser.

  Moreover, although the method of increasing the heat per unit weld length by slowing the welding speed contributes to improving the quality of welding, it cannot be adopted in consideration of productivity in actual operation.

  Therefore, an object of the present invention is to provide a laser beam welding method capable of stable keyhole welding by butt welding of thin metal plates.

The object of the present invention can be achieved by the following means.
1. A laser welding method in which a butt joint of a thin metal plate is keyhole welded with a laser beam in which a beam spot is focused on a butt portion of the butt joint , wherein the laser beam has at least an amplitude in the front-rear direction of the welding progress direction. The frequency at which the laser beam vibrates as the beam spot diameter vibrates and the laser beam vibrates in advance is the laser output of the laser beam and the frequency of the laser-induced plasma generated by the laser beam. A laser welding method characterized by having the same frequency as the laser-induced plasma frequency obtained from the relationship .
2. 2. The laser welding method according to 1, wherein the amplitude is at least 1 and less than 3 times the diameter of the beam spot.
3. The laser welding method according to 1 or 2, wherein a frequency at which the laser beam vibrates is 100 to 300 Hz.
4). When the laser beam is radiated to the center of the amplitude, the laser beam is radiated perpendicularly or rearwardly with respect to the welding progress direction, and when the laser beam is radiated to the center of the amplitude, the normal of the metal plate surface The laser welding method according to any one of 1 to 3, wherein an angle between the two is in a range of 0 to 15 deg.

  INDUSTRIAL APPLICABILITY According to the present invention, stable keyhole welding is possible by butt welding of a metal thin plate using a laser beam, which is frequently used in a vehicle body structure of an automobile, which is extremely useful industrially.

  In the present invention, the beam spot of the laser beam focused on the abutting portion is vibrated in the front-rear direction of the welding line direction, and at the same frequency as the plasma frequency obtained in advance from the relationship between the laser output and the plasma frequency. Keyhole welding while scanning.

  Evaporated particles or plasma is generated by laser beam irradiation, but these are generated from the portion of the laser beam irradiated with the beam: the portion corresponding to the spot diameter, and there is a time lag from irradiation to generation. When oscillated and irradiated in the welding direction, the laser beam is not irradiated to the evaporated particles or plasma.

  Generally, the laser beam diameter applied for welding is 0.1 mm to 1.0 mm. Therefore, the keyhole diameter is approximately equal to the laser beam diameter of 0.1 mm to 1.0 mm. Therefore, it is preferable that the amplitude of the laser beam is in the range of about 1 time or more and less than 3 times the beam spot diameter.

  That is, if the laser beam amplitude is less than 1 times the beam spot diameter, the laser beam is always irradiated with evaporated particles or plasma ejected from the keyhole, so that the keyhole becomes unstable and welding defects are likely to occur.

  On the other hand, when the laser beam amplitude exceeds three times the beam spot diameter, the laser beam does not irradiate the vaporized particles or plasma ejected from the keyhole, so the generation of welding defects can be suppressed, but the laser beam amplitude is too large. The average laser energy becomes lower and the penetration depth becomes shallower. In order to obtain the same penetration depth as in the case without scanning, the welding speed has to be reduced and the productivity is lowered.

  The laser beam is scanned in advance at the same frequency as the plasma frequency obtained from the relationship between the laser output and the plasma frequency.

  Since the frequency of the plasma ejected from the keyhole varies depending on the laser output, the laser beam diameter, etc., it is determined by a preliminary test using the same welding conditions as in actual operation.

  FIG. 1 shows the relationship between the laser output and the frequency of the generated laser-induced plasma. The figure shows the result of obtaining the laser-induced plasma frequency by changing the output of the CO2 laser with the laser wavelength of 10.6 μm and the semiconductor laser with the laser wavelength of 800 nm from 1 kW to 10 kW. To do.

  Also, the longer the laser frequency, the more easily the laser energy is absorbed by the reverse bremsstrahlung action, the laser-induced plasma is amplified, and at the same time the degree of ionization increases and expands. It can be seen that the frequency of increases.

  As shown in FIG. 1, in ordinary laser welding, a carbon dioxide laser with a laser wavelength of 10.6 μm or a semiconductor laser with a laser wavelength of 800 nm is often used, so the scanning frequency of the laser beam is the same as the plasma frequency that changes according to the laser output. 100 Hz or more and less than 300 Hz is preferable because there is no interference between the laser beam and the plasma.

In the present invention, when the material to be welded 3 is irradiated with the laser beam 1, the angle θ ₂ formed between the center of the scanning amplitude of the laser beam 1 and the normal of the plate surface of the material to be welded 3 is relative to the welding direction 2. It is preferable to set to 0 to 15 deg so that the laser beam 1 tilts backward. FIG. 2 illustrates the irradiation angle of the laser beam.

If the amplitude center of the scanning of the laser beam is inclined at an angle θ _{2 with} respect to the normal direction of the metal plate surface to the rear side in the welding direction, welding defects are suppressed. However, when the angle theta ₂ is too large depth penetration decreases. Therefore, the angle theta ₂ formed by the normal of the center direction and the metal plate surface in the amplitude of the scanning of the laser beam is preferably set to 0～15Deg.

Examples of the material of the metal thin plate applicable to the present invention include carbon steel, aluminum, aluminum alloy, magnesium, magnesium alloy and the like. As the laser, any laser system that can be used for thermal processing, such as CO ₂ laser, CO laser, slab laser, Nd-YAG laser, glass laser, excimer laser, fiber laser, and semiconductor laser can be applied.

A carbon steel plate having a thickness of 10 mm was irradiated with a laser beam and subjected to bead-on-plate welding. Welding focuses the beam spot on the plate surface and sets the welding speed to 2 m / min and 3 m / min. At each welding speed, the beam spot moves in the welding direction and the beam spot moves in the welding line direction (scanning). The scanning frequency was changed. Tables 1 and 2 show the welding conditions.

  The bead-on-plate welded portion was inspected using an X-ray nondestructive inspection method, and a blow hole having a diameter of 0.15 mm or more was detected. The number of blow holes per 100 mm weld line length was measured, and the case of 30 or less was judged as “good quality”. Next, the depth of penetration was measured by cutting in the direction perpendicular to the weld line.

  From Table 1, when the laser output, laser wavelength, welding speed, and beam frequency are constant, there are few blow holes and “good quality” in the region where the beam amplitude is 0.5 mm or more and 3.0 mm or less. It is recognized that the penetration depth also increases (Test Nos. 3 to 6).

  From Table 2, when the laser wavelength, welding speed, and beam amplitude are constant, when the laser output is 5 kW, there are few blowholes in the region where the beam frequency is 200 Hz or more and 225 Hz or less, and “good quality”. It is recognized that the penetration depth also becomes deep (Test Nos. 3 and 4).

  When the laser output is 10 kW, there are few blowholes in the region where the beam frequency is 250 Hz or more and 300 Hz or less, “quality is good”, and in addition, the penetration depth becomes deep (Test No. 8.9).

  From the above test results, it is clear that the beam frequency at which the generation of blowholes is suppressed and the quality is good differs depending on the laser output.

  Table 3 shows the effect of the beam irradiation angle (θ1 or θ2 shown in FIG. 2) on the welding quality, the laser beam is tilted backward (the beam irradiation angle is θ2) with respect to the welding progress direction, and In the case of 0 to 15 ° (Test Nos. 4 to 7), it is recognized that there are few blow holes and “good quality”, and in addition, the penetration depth becomes deep.

The figure which shows the relationship between the laser output and the frequency of the laser induced plasma which generate | occur | produced. The figure explaining the irradiation angle of a laser beam.

Explanation of symbols

1 Laser beam 2 Welding direction 3 Material to be welded

Claims (4)

A laser welding method in which a butt joint of a thin metal plate is keyhole welded with a laser beam in which a beam spot is focused on a butt portion of the butt joint , wherein the laser beam has at least an amplitude in the front-rear direction of the welding progress direction. The frequency at which the laser beam is vibrated while oscillating as the diameter of the beam spot and the laser beam oscillates in advance is the laser output of the laser beam and the frequency of the laser-induced plasma generated by the laser beam. A laser welding method characterized by having the same frequency as the laser-induced plasma frequency obtained from the relationship . The laser welding method according to claim 1, wherein the amplitude is not less than 1 and not more than 3 times the diameter of the beam spot. The laser welding method according to claim 1 or 2, wherein a frequency at which the laser beam vibrates is 100 to 300 Hz. When the laser beam is radiated to the center of the amplitude, the laser beam is radiated perpendicularly or rearwardly with respect to the welding progress direction, and when the laser beam is radiated to the center of the amplitude, the normal of the metal plate surface The laser welding method according to any one of claims 1 to 3, wherein the angle formed is in a range of 0 to 15 deg.

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