The present invention relates to a laser welding method and a welded joint.
Laser welding is used in various fields, because energy density of a laser beam as a heat source is high so as to obtain a welded joint of low distortion, high speed and high precision. In the automotive field, there are many products in which a plurality of workpieces are welded by overlapping or butting them with steel materials such as stainless steels and carbon steels, or metal materials such as aluminum alloys and nickel alloys. A welding process using a continuous wave or pulsed wave laser beam is used for producing, for example, a vehicle body, a fuel pump and an injector (a fuel injection valve).
Further, a joining device or process for joining a resin material by a laser beam has been developed and used to produce products such as stress/strain sensors and air flow sensors using a nonmetallic material such as a resin.
The laser welding generally uses a deep penetration type (keyhole mode) welding method. In this method, when power density (laser power per unit area) of the laser beam irradiated to a surface of the workpiece is equal to 106 W/cm2 or more, temperature of a surface of the metal is equal to or higher than a boiling point of the metal, metal vapor violently jumps out from a laser irradiation point along with generation of plasma, the surface of the molten metal is recessed by reaction force of the metal vapor, and the laser beam enters the metal while repeating reflection in a recessed portion, to perform deep narrow keyhole mode welding. Theoretically, it is possible to form a laser keyhole by maintaining a balance between a pressure of the metal vapor inside the keyhole and a surface tension and static pressure of the molten metal around the keyhole. However, as shown in
For example, Patent Document 1 discloses a device and a method for adjusting laser beam spot diameter, which can adjust the laser beam spot diameter, in particular, can enlarge the diameter by using a scanner of the laser beam.
{Patent Document 1}
Japanese Patent Application Publication No. 2012-152822
However, the device and the method for adjusting the laser beam spot diameter described in Patent Document 1 has a problem that since the laser beam spot diameter is enlarged, an evaporation area of the molten metal is increased at a moment when the laser beam is irradiated to the surface of the molten pool, and a size of the spatter is increased.
An object of the present invention is to reduce spatter generation caused by instability of the keyhole.
The above object is achieved by the present invention described in claims.
According to the present invention, it is possible to reduce spatter generation caused by instability of the keyhole.
Hereinafter, embodiments of the present invention will be described in detail.
A welding method of the present embodiment is as follows. A welded joint produced by the welding method of the present embodiment is, for example, a lap joint of stainless steel having a thickness of 1.0 mm.
In the present embodiment, for example, a fiber laser having a wavelength of 1070 to 1080 nm can be used, but a laser beam of another wavelength may be used. Further, the laser beam is generated from a laser oscillator (not shown), and is condensed by a beam scanner and a condenser lens (not shown) through a transport channel, to be irradiated to a surface of the lap joint of stainless steel.
In order to prevent oxidation of the molten metal, deep penetration type laser welding of the present embodiment can use nitrogen as a shielding gas. Note that, the shielding gas is not limited to nitrogen, and Ar (argon), He (helium) or a mixture thereof may be used.
As welding conditions, for example, laser power is 200 W to 1000 W, beam spot diameter is 0.04 mm to 0.2 mm, number of repetitions of beam rotation 60 Hz to 500 Hz, and beam rotation diameter is 3.0 mm or less. In addition, it can be appropriately set such that welding speed is 10 mm/s to 100 mm/s, and flow rate of shielding gas is 5.0 l/min to 30.0 l/min.
Hereinafter, a keyhole, molten pool behavior and bead shape after welding under the welding conditions used in the present embodiment will be described with reference to
Further, since the rotation speed of the laser beam is much faster than the forward speed in the welding direction, it is possible to reduce an amount of the molten pool in front of the keyhole with respect to a direction (spiral direction of a combination of a linear welding direction and a rotation direction) of movement of the laser beam, and thus the surface width “a” of the molten pool is not much wider than the rotation diameter of the keyhole. That is, width variation from the surface to the bottom of the molten pool is smaller than that of the conventional welding method.
Further, since the rotation speed of the laser beam is very high, a time that the molten pool behind the keyhole flows back to an opening of the keyhole is short, and the opening is hardly closed. Further, since the keyhole does not move linearly but moves rotationally with respect to a traveling direction of the laser beam, the molten pool also flows in accordance with a rotation direction of the keyhole. However, a flow of the molten pool stays for a certain time due to its inertia, and receives stirring effect of the keyhole formed by irradiation with the laser beam, As a result, a concave is formed in the opening of the keyhole in a rearward. direction of movement of the keyhole, and it is possible to avoid direct irradiation of the laser beam to a surface of the molten pool and to directly irradiate the laser beam to a lower portion of the keyhole, thereby preventing intense vaporization of the molten metal around the opening of the keyhole, and thereby preventing spatter generation.
In particular, shape of the weld bead welded under the welding conditions of the present embodiment is such that (i) when the maximum penetration depth of the weld bead cross section is h, a relationship between the surface width “a” of the weld bead and the penetration width “b” at the position where the penetration depth is h/2 is b/a>0.6, (ii) a relationship between the maximum penetration depth h and the penetration depth d at a center position of the weld bead is h/d>1.0, and (iii) a relationship between the bead surface width “a” and the maximum penetration depth h of the weld bead cross section is h/a<3.0.
As a comparative example, welding is performed using the conventional welding method which does not use the beam scanner. Material and size of a test piece used in a welding test is the same as that used in Embodiment 1. The welding conditions are as follows: the laser power is 200 W to 1000 W; the beam spot diameter is 0.04 mm to 0.2 mm; the welding speed is 10 mm/s to 100 mm/s; and the flow rate of the shielding gas is 5.0 l/min to 30.0 l/min.
An example of a cross-sectional shape of the bead welded under the welding conditions described above is shown in FIG, 4. The weld bead shape is such that the surface width “a” is at least twice the penetration width “b”, a position where the penetration depth is deepest is the central portion of the weld bead, and the weld bead has a wine cup shape. Further, a large amount of spatter is generated in a welding process.
The welding method of the present embodiment is as follows. A lap welded joint according to the present embodiment is, for example, a butt joint (not shown) of copper plate having a thickness of 1.0 mm.
In a laser welding of the present embodiment, for example, a visible light and near-infrared laser having a wavelength of 500 nm to 880 nm can be used, but a laser beam of another wavelength may also be used. Further, the laser beam is generated from the laser oscillator (not shown), and is condensed by the beam scanner and the condenser lens (not shown) through the transport channel, to be irradiated to a. surface of the butt joint of copper plate described above.
As with the welding method shown in Embodiment , while rotating the laser beam using the beam scanner, the welding is performed by advancing the laser welding head including the beam scanner in the welding direction. Further, in order to prevent oxidation of the molten metal, it is possible to use Ar (argon) as the shielding gas. Note that, the shielding gas is not limited to Ar, and He (helium) or a mixture thereof may be used.
As welding conditions, for example, the laser power is 200 W to 800 W, the beam spot diameter is 0.04 mm to 0.2 mm, the number of repetitions of beam rotation is 300 Hz to 1000 Hz, and the beam rotation diameter is 0.2 mm to 3.0 mm. In addition, it can be appropriately set such that the welding speed is 10 mm/s to 100 mm/s, and the flow rate of the shielding gas is 5.0 l/min to 30.0 l/min.
Further, spatter generation is not observed during the welding.
A summary of the above results is shown in
Filing Document | Filing Date | Country | Kind |
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PCT/JP2014/050731 | 1/17/2014 | WO | 00 |