JP2638379B2 - Laser welding method and cooling head used therefor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser welding method used for fusion joining metal materials.

[0002] As one of the fusion welding methods for metal materials, there is a laser welding method using a laser beam. Since this is welding using a coherent high-energy-density light heat source, a high-quality joint with high speed, low heat input, and deep penetration can be obtained. The welding process in laser welding is described as follows.

[0003] A laser beam oscillated by a laser oscillator is condensed by a condenser lens or a condenser mirror, and is applied to a portion to be welded as a high energy density beam. At this time, the to-be-welded part is instantaneously heated and melted, and at the same time accompanied by severe evaporation. The reaction force of this evaporation pushes the molten pool downward, and forms a narrow and deep hole called a keyhole at the laser beam irradiation point. When the keyhole is formed, the laser beam applied to the keyhole repeatedly hits the wall of the keyhole and repeatedly reaches the bottom of the keyhole while maintaining a high energy density. As described above, since high-density heat input is directly applied to the inside of the material to be welded in laser welding, a high-quality joint having low heat input that is deep and narrow is obtained.

[0004] However, laser welding has a problem caused by plasma as described below. In laser welding, metal vapor generated at the time of welding blows out above a welded part and absorbs a laser beam to form metal plasma. Since the plasma has a high laser absorptivity, once the plasma is formed, it grows larger while further absorbing the laser. The plasma thus formed absorbs and scatters the laser, resulting in a large energy loss.

[0005] During welding, Ar is usually used as a shielding gas for the welded portion in order to prevent oxidation and nitridation of the welded portion. Since the ionization voltage of Ar is not so large, under welding conditions in which the heat input density is particularly large, the shield gas is also turned into plasma and a larger plasma is formed, so that the energy loss is further increased and the penetration depth is increased. Decrease. As described above, the plasma generated at the time of laser welding involves a large energy loss and lowers the penetration depth, which is a serious problem in practical use.

Factors that affect the amount of plasma generated include the ionization voltage of the shielding gas and the cooling capacity of the shielding gas. If a gas with a high ionization voltage is used as the shielding gas, the shielding gas is difficult to turn into plasma.
The plasma cannot grow too large. Further, when the cooling capacity of the shielding gas is high, the shielding gas efficiently absorbs the thermal energy of the metal plasma, so that the plasma growth is suppressed. Since He has a high ionization voltage and a large thermal conductivity, it has a great effect in suppressing the growth of plasma. He is an inert gas like Ar, and can be used as a shielding gas for preventing contamination of a welded portion.

For these reasons, He is used instead of Ar.
Is used as a shielding gas to suppress plasma. However, since He is a very expensive gas, it is disadvantageous in terms of cost to use He gas alone. Therefore, a method of substituting a mixed gas of He and Ar has been proposed in Japanese Patent Application Laid-Open No. Sho 61-232320.

[0008]

However, in this method, if the mixing ratio of He is less than 30%, the effect of suppressing the plasma growth is lost, so that the mixing ratio of He is 3%.
It must be kept at 0% or more. Therefore, even when this method is used, a significant increase in welding cost is unavoidable, and it is difficult to put it to practical use except for special applications. On the other hand, a method has been proposed in which generated plasma is blown off by a high-speed Ar gas that is blown from a side called a side gas. Is difficult.

An object of the present invention is to use He as a shielding gas.
Inexpensive A without the use of
An object of the present invention is to provide a laser welding method that can suppress the growth of plasma only by using r gas and can increase the penetration depth. Another object of the present invention is to provide a cooling head that is particularly effective in suppressing plasma growth.

[0010]

The cause of the decrease in the penetration depth in laser welding is, as described above, that the plasma generated immediately above the welded portion absorbs or scatters the laser beam. Due to its physical properties, plasma is more likely to grow by absorbing energy at higher temperatures and pressures. Conversely, when cooled or reduced in density, the plasma state cannot be maintained and disappears.

The present inventors have paid attention to such physical properties of plasma, and have conducted experiments to find a method of cooling plasma and a method of diffusing plasma to reduce the density.
Researched. As a result, if the plasma generated immediately above the weld is cooled by a cooling head such as a water-cooled copper plate that has been passed through and cooled, this can be eliminated, and in the middle of the path of injecting the shielding gas toward the weld, It has been found that the plasma can be reduced by forming an outward gas flow radially spreading toward the outer peripheral side.

The laser welding method of the present invention has been developed on the basis of such knowledge, and uses a focused laser beam as a light source .
In the laser welding method of irradiating a welded part of a material to be welded from a workpiece, a cooling head which is forcibly cooled and provided with a beam passage hole having a diameter of less than 1.8 mm for passing a laser beam irradiated to the welded part is provided. With the torch and the material to be welded.
Distance and from the material to be welded between the performs welding to hold the position of less than 2.2 mm, during the welding, cold from the torch
A first shielding gas is injected toward the cooling head, and a second shielding gas is injected from the cooling head toward the workpiece.

Further, the cooling head of the present invention is made of a metal plate having a high thermal conductivity through which a cooling liquid flows, in which a beam passage hole penetrating between both surfaces is provided, and a shielding gas is introduced into the plate. A gas path is provided, and an outlet of the gas path is opened around the beam passage hole on one surface of the plate material.

[0014]

FIG. 1 is a side view schematically showing one embodiment of the laser welding method of the present invention.

The torch 1 irradiates a focused laser beam B toward a welded portion 2a of a workpiece 2 below, and also ejects Ar as a first shield gas G1. A cooling head 3 made of a copper plate is held between the torch 1 and the workpiece 2.

As shown in FIGS. 2 and 3, the cooling head 3 has a circular beam passage hole 3a penetrating the upper and lower surfaces. The cooling head 3 is provided with a U-shaped water passage 3b surrounding the beam passage hole 3a over a half circumference,
A gas passage 3c is provided so as to be surrounded by the water passage 3b. The distal end of the gas passage 3c is formed in a quadrangle surrounding the beam passage hole 3a, and the corners are opened on the lower surface of the cooling head 3 to form four downward nozzles 3d. In addition, a second nozzle 3e facing the welded portion 2b is provided near the tip of the gas passage 3c.

The arrangement of the water passage 3b and the nozzles 3d and 3e is not limited to the illustrated example.
b denotes a plurality of paths, a nozzle 3 surrounding the laser through hole 3a.
Needless to say, five or more nozzles d may be provided on a concentric circle centered on the laser through hole, and two or more nozzles 3e opposed to the welding part 2b may be provided other than on the circumference. .

At the time of welding, the center of the beam passage hole 3a of the cooling head 3 is made to coincide with the beam axis of the laser beam B,
The laser beam B is applied to the welded portion 2a of the workpiece 2 through the beam passage hole 3a. Cooling water is circulated through the water passage 3b of the cooling head 3, and Ar as the second shield gas G2 is introduced into the gas passage 3c.

By irradiating the laser beam B to the welded portion 2a, the metal is melted and evaporated, and metal vapor explosively blows upward. However, since the metal vapor is cooled by the cooling head 3 made of a water-cooled copper plate and loses energy, no plasma is generated.

Further, the first shield gas G1 ejected from the torch 1 spreads around the upper surface of the cooling head 3 and forms an outward gas flow. During welding, the amount of evaporation of the metal suddenly increases, and the metal vapor passes through the beam passage hole 3a.
May blow out above the cooling head 3 through the
Also in this case, no plasma is generated because the energy of the ejected steam is taken away by the outward gas flow.

From these synergies, the laser beam B is applied to the welded portion 2a without loss of energy even though Ar is used as the shielding gas G1 and G2 and the side gas is not used together. The penetration depth is greatly increased as compared with the conventional method. Here, the shield gas G1 also has a function of protecting the optical system. The shielding gas G2 is injected from the plurality of nozzles 3d of the cooling head 3 to the workpiece 2 so as to surround the laser beam B, thereby preventing contamination such as oxidation and nitriding of the welded portion. Also, the second nozzle 3e guarantees sufficient shielding properties even when the molten pool extends rearward during high-speed welding.

As described above, the laser welding method of the present invention
The effect of suppressing the generation of plasma is obtained without e and the side gas. However, if the diameter a of the laser passage hole is too large, or if the distance b from the workpiece to the cooling head is too large, the ability to cool the plasma is reduced, so that plasma is generated and the penetration depth is reduced. Therefore, there are appropriate values for a and b.

FIG. 4 shows a laser output of 5 kW, a welding speed of 1 m / min, and a condensing mirror having a focal length of 254 mm.
The result of investigating the relationship between the penetration depth and the diameter a of the laser passage hole when welding is performed with b = 1 mm is shown.
As is clear from FIG. 4, when the value of a becomes 1.8 mm or more, the penetration becomes sharply shallow. FIG. 5 shows the results of an investigation on the effect of the b value on the penetration depth. Here, the diameter a of the laser passage hole was 1 mm, and the other conditions were the same as those in FIG. As is clear from FIG. 5, the b value is 2.2 m.
The penetration depth is sharply reduced at m or more. Similar results were obtained by changing the focal length of the condenser mirror, the laser output, and the welding speed. From the above results, the appropriate value of a is 1.
It is clear that the appropriate value of b is less than 8 mm and the appropriate value of b is less than 2.2 mm.

The material of the cooling head is preferably a metal having a high thermal conductivity, and copper is most preferable from this point, but stainless steel, brass and the like can also be used.

The size of the cooling head is 12 m from the center of the beam passage hole in the welding progress direction and in both the left and right directions.
It is desirable to secure a size of 20 mm or more (plane area: about 770 mm ² or more) in the direction opposite to the welding progress direction. The reason is that if the diameter is smaller than this, the gas flows of the shield gases G1 and G2 may be disturbed, and the protection (shielding) of the welded portion may not be sufficiently performed. The upper limit of the size is not limited at all. However, if it is too large, it becomes difficult when it is necessary to observe the welded portion, etc., so that the size is 20 mm in both the welding direction and the left and right directions, and 30 mm (flat) in the direction opposite to the welding direction. Area: about 2000
mm ² ).

It is desirable that the flow rates of the shield gases G1 and G2 are respectively 10 liter / min or more. The reason for this is that if the flow rate is less than 10 liters / minute, the plasma generated during welding may not be sufficiently diffused / dissipated with the shielding gas G1, and the shielding gas G2 may not be sufficient. Otherwise, the welded portion may not be sufficiently protected (shielded). The upper limit of the flow rate of any of the gases G1 and G2 is not limited at all.

[0027]

Embodiments of the present invention will be described below.

In laser welding a steel sheet having the chemical composition shown in Table 1, a cooling head shown in FIGS. 2 and 3 was used. Table 2 shows the specifications and operating conditions of the cooling head. Laser welding was performed using a carbon dioxide laser oscillator with a rated output of 5 kW, setting the output to 5 kW and setting the focus on the surface of the steel plate. FIG. 6 shows the penetration depth when the welding speed was changed, in comparison with the case of the conventional method without using a cooling head. By the practice of the present invention, the penetration depth was improved by 1.2 to 1.5 times. Even when the laser output was changed, the same effect as in the case of 5 kW was obtained.

[0029]

[Table 1]

[0030]

[Table 2]

[0031]

As is clear from the above description, the laser welding method of the present invention can suppress the generation of plasma and achieve a great increase in the penetration depth. In addition, since there is no need to use He, the welding cost is substantially the same as in the past, and since no side gas is used, no complicated control mechanism is required.

Further, since the cooling head of the present invention is made of a metal plate having a high thermal conductivity through which a cooling liquid flows, the plasma generated immediately above the welded portion can be sufficiently cooled, and is injected from the beam entrance side. The first shield gas is effectively rectified on its entry side into an outward gas flow. Since the second shield gas is ejected from around the beam passage hole, contamination of the welded portion is sufficiently prevented, and even when the molten land extends long backward, the second shield gas is discharged from the gas outlet provided at the position opposed to the welded portion. Sufficient shielding is ensured by the shielding gas of 2.

[Brief description of the drawings]

FIG. 1 is a side view schematically showing one embodiment of the laser welding method of the present invention.

FIG. 2 is a plan view showing an example of a cooling head of the present invention.

FIG. 3 is a vertical sectional side view of the cooling head of FIG. 2;

FIG. 4 is a table showing a relationship between a diameter of a laser passage hole and a penetration depth.

FIG. 5 is a table showing a relationship between a distance from a workpiece to a cooling head and a penetration depth.

FIG. 6 is a table showing the relationship between welding speed and penetration for the method of the present invention and the conventional method.

[Explanation of symbols]

 Reference Signs List 1 torch 2 material to be welded 2a welded portion 2b welded portion 3 cooling head 3a beam passage hole 3b water passage 3c gas passage 3d, 3b nozzle B laser beam G1, G2 shielding gas

Claims (2)

(57) [Claims] 1. A laser welding method of irradiating the welded portion of the workpieces a focused laser beam from the torch, strong
A cooling head provided with a beam passage hole having a diameter of less than 1.8 mm, which is controlled to be cooled and allows a laser beam irradiated to a portion to be welded to pass, is provided between the torch and the material to be welded. Welding is performed with the distance from the welding material being less than 2.2 mm, and during the welding, from the torch to the cooling head
Laser welding, characterized in that with injecting a first shield gas toward injects a second shield gas toward the cooling head to workpieces. 2. A high heat conductive metal plate through which a cooling liquid flows, wherein a beam passage hole penetrating between both surfaces is provided in the plate, and a gas path for introducing a shielding gas into the plate is provided. A cooling head, wherein an outlet of a gas passage is opened around a beam passage hole on one surface of a plate material.