JP2007229739A - Laser brazing method of high-strength steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser brazing method for a high-strength steel sheet, a method capable of preventing defects of deposited metal and improving strength of a joint, in forming a lap fillet weld joint by laser brazing. <P>SOLUTION: In the laser brazing method for forming a lap fillet weld joint of a high-strength steel sheet using a laser beam, a gap of 0.05-0.2 mm is provided between the lower face of the upper sheet and the upper face of the lower sheet in an overlapped part, while a gap of 0.2-0.5 mm is set between a wire to be used as a filler metal and the end face of the upper sheet. After a laser beam is emitted from above the upper sheet, heating the upper and the lower sheet of the overlapped part and also fusing the wire, the part is left in the air for cooling so as to form a lap fillet weld joint. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for forming a lap fillet joint by brazing (so-called brazing) a high-strength thin steel sheet using a laser beam. Here, brazing using a laser beam as a heat source is referred to as laser brazing.

Brazing is a technique for forming a metal plate joint via a bonding material (hereinafter referred to as a brazing material) having a melting point lower than the melting point of the metal plate (for example, a steel plate or the like) to be joined. That is, a joint can be formed without melting the metal plate by heating only to a temperature higher than the melting point of the brazing material and lower than the melting point of the metal plate, melting only the brazing material, and then allowing to cool.
As a method of melting the brazing material when brazing, a method of charging by heating in a heating furnace or a method of heating using a gas flame or an arc has been conventionally employed. However, when these heat sources are used, it takes a long time for the brazing material to melt. Therefore, various techniques have been studied in order to increase the speed of brazing.

  For example, Patent Document 1 discloses a technique for performing brazing (that is, laser brazing) using a laser beam. In this technique, the surface of two automotive steel plates is irradiated with a laser beam to heat the wire, and a wire-like brazing material is supplied to the irradiation position of the laser beam to melt the brazing material. Since the irradiation position of the laser beam continuously moves along the edge of the upper plate, the brazing material after the laser beam has passed is cooled in the atmosphere (so-called cooling) and solidified. Join steel plates for automobiles. A copper alloy is disclosed as a brazing material.

Laser brazing uses a laser beam with high energy density.
(a) the joint can be heated locally,
(b) can increase the speed of brazing,
(c) There are advantages such as reduction of thermal strain of the joint. Therefore, laser brazing is employed to form lap fillet joints for joining structural members such as automobile bodies and railway vehicles.

In recent years, there has been an increasing demand for weight reduction of automobile bodies and railway vehicles from the viewpoint of energy saving, and the use of high-strength steel sheets (so-called high-tensile steel sheets) has increased in automobile bodies in order to achieve weight reduction. Yes.
However, as already explained, laser brazing uses a brazing material (for example, a copper alloy, a silver alloy, etc.) having a melting point lower than that of a high-strength thin steel plate, as in general brazing. A weld metal obtained by such a brazing material being heated and melted by a laser beam and then allowed to cool and solidify has a lower strength than a high-strength thin steel plate. Moreover, in addition to the difference in strength between the high-strength thin steel sheet and the weld metal, stress is concentrated on the weld metal in the lap fillet joint, so that even if the high-strength thin steel sheet does not deform when a load is applied, There is a risk of breaking. In particular, when a defect such as a crack or a hole exists in the weld metal, the weld metal is easily broken.

As described above, laser brazing has the advantages (a) to (c) because the brazing material is melted in a short time using a laser beam having a high energy density. However, in laser brazing, since the brazing material is melted in a short time, it becomes difficult to supply the brazing material in a molten state uniformly and sufficiently to the joint portion, and as shown in FIG. Perforated defects 4 are likely to occur at the corners formed in
JP 2005-59009 A

  The present invention provides a laser brazing method for a high-strength thin steel sheet capable of solving the above problems and preventing weld metal defects and improving joint strength when forming a lap fillet joint by laser brazing. Objective.

In order to develop a technique for preventing perforated defects in laser brazing, the present inventors first investigated in detail the mechanism by which perforated defects are formed. In laser brazing, as shown in FIG.
(1) The upper surface 1 and the end surface of the upper plate 1 and the upper surface of the lower plate 2 are irradiated with a laser beam 6 to heat the high-strength thin steel plate at the joint.
(2) The wire-like brazing material 5 is supplied to the irradiation position of the laser beam 6, and the brazing material 5 is melted by the laser beam 6.
(3) The molten brazing material becomes droplets and spreads in contact with the end surface of the upper plate 1 and the upper surface of the lower plate 2;
(4) After the laser beam 6 passes, the lap fillet joint is formed by the procedure in which the molten brazing material 5 is allowed to cool and solidify. The upper plate 1 refers to a high-strength thin steel plate positioned above the lap fillet joint, and the lower plate 2 refers to a high-strength thin steel plate positioned below the lap fillet joint.

Of the above procedures (1) to (4), the perforated defects are formed in the step (3). That is, the cause of the perforated defect is that a sufficient amount of droplets is supplied to the corner formed at the meeting portion between the end surface of the upper plate 1 and the upper surface of the lower plate 2 due to the surface tension of the molten brazing material. It becomes difficult (that is, wettability decreases).
Various techniques for improving the wettability of the droplets of the brazing material 5 have been conventionally studied. For example, the wettability of the droplets of the brazing material can be improved by heating the upper plate 1 and the lower plate 2 to raise the temperature. However, since laser brazing is performed by locally irradiating a laser beam with a high energy density, if the irradiation amount is increased in order to raise the temperature of the upper plate 1 and the lower plate 2 of the overlapping portion, the upper plate 1 and the lower plate 2 are melted, and laser brazing is hindered (for example, fusing, etc.).

Moreover, if a flux to which a component that improves the wettability of the droplets of the brazing material is used is used, it is possible to prevent perforation defects. However, the flux containing a specific component causes an increase in the laser brazing construction cost.
As described above, in the lap fillet joint formed by laser brazing, a perforated defect is likely to occur in the weld metal 3 at the meeting portion between the upper plate 1 and the lower plate 2. In particular, in a lap fillet joint of high-strength thin steel sheets, the weld metal 3 may be broken even if the high-strength thin steel sheets are not deformed when a load is applied. Therefore, in order to reduce the weight of structural members (for example, automobile bodies, railway vehicles, etc.) by making full use of the mechanical properties of high-strength steel sheets, it is possible to prevent perforation defects and increase the strength of lap fillet joints. Need to increase.

  Therefore, the inventors examined a technique for preventing a hole defect in a lap fillet joint formed by laser brazing. As a result, it has been found that the arrangement of the upper plate 1 and the lower plate 2 and the position where the brazing material 5 is supplied have a great influence on the wettability of the droplets of the brazing material 5. In other words, by providing a gap between the lower surface of the upper plate 1 and the upper surface of the lower plate 2 and providing a gap between the end surface of the upper plate 1 and the wire used as the brazing material 5, We have found that it is possible to prevent perforated defects.

The present invention has been made based on these findings.
That is, the present invention provides a laser brazing method for forming a lap fillet joint of a high-strength thin steel sheet using a laser beam, and a gap of 0.05 to 0.2 mm is provided between the lower surface of the upper plate and the upper surface of the lower plate. The distance between the wire used as the brazing material and the end surface of the upper plate is 0.2 to 0.5 mm, and the upper plate and the lower plate are heated by irradiating a laser beam from above the upper plate and the wire is used. Forming a lap fillet joint by allowing it to cool after melting is a laser brazing method for a high strength thin steel sheet.

  In the laser brazing method of the present invention, the wire is preferably a wire made of a copper alloy having a diameter of 0.6 to 1.8 mm.

  According to the present invention, when forming a lap fillet joint by laser brazing, it is possible to prevent weld metal defects and improve joint strength.

FIG. 1 is a cross-sectional view schematically showing an example of the arrangement of a high-strength thin steel sheet, a brazing material, and a laser beam in laser brazing of the present invention.
As shown in FIG. 1, in the present invention, a gap D ₁ is provided between the lower surface of the upper plate 1 of the high-strength thin steel plate and the upper surface of the lower plate 2. In the clearance D ₁ is less than 0.05 mm, since the amount of heat given to the lower plate 2 by the laser beam 6 is distributed by conducting to the upper plate 1, the temperature of the overlapping portion does not rise. Therefore, it is difficult for the droplets of the brazing material 5 to be supplied to the corners formed at the meeting portion between the end surface of the upper plate 1 and the upper surface of the lower plate 2. On the other hand, when the gap D ₁ is more than 0.2 mm, droplet of brazing material 5 becomes difficult between the upper plate 1 and lower plate 2 and bridged with sufficient thickness. Thus, the gap D ₁ of the lower surface and the upper surface of the lower plate 2 of the upper plate 1 needs to be in the range of 0.05 to 0.2 mm. If the gap D ₁ is out of this range, the weld metal 3 of the lap fillet joint has a hole in the corner (when the gap D ₁ is less than 0.05 mm) or poorly welded (when the gap D ₁ exceeds 0.2 mm). ) Is likely to occur.

Further, a gap D ₂ is provided between the end face of the upper plate 1 and the wire-like brazing material 5. When the gap D ₂ is less than 0.2 mm, the droplets of the brazing material 5 are welded in a shape that bridges the position where the end surface of the upper plate 1 and the upper surface of the lower plate 2 are separated. As a result, a cavity 7 as shown in FIG. 4 is formed at the corner formed at the meeting portion between the end surface of the upper plate 1 and the upper surface of the lower plate 2. On the other hand, when the gap D ₂ exceeds 0.5 mm, the droplets of the brazing material 5 are difficult to weld to the end surface of the upper plate 1. Thus, the gap D ₂ between the end surface and the wire-like brazing material 5 of the upper plate 1 needs to be in the range of 0.2 to 0.5 mm. The gap D ₂ indicates the shortest distance between the end face of the upper plate 1 and the wire-like brazing material 5 as shown in FIG.

The distance between the upper surface of the lower plate 2 and the wire-like brazing material 5 is not limited to a specific value. Even if the brazing material 5 comes into contact with the lower plate 2, laser brazing can be performed without hindrance.
If the diameter of the wire-like brazing material 5 is less than 0.6 mm, it is necessary to feed the brazing material 5 at a high speed in order to obtain sufficient adhesion, and the feeding of the brazing material 5 tends to become unstable. On the other hand, if the diameter exceeds 1.8 mm, it is necessary to increase the diameter of the laser beam 6 in order to melt the brazing filler metal 5 and to heat the upper plate 1 and the lower plate 2. It will be difficult to ensure. Therefore, the diameter of the wire-like brazing material 5 is preferably in the range of 0.6 to 1.8 mm.

  In this way, the overlapping portion is configured, and the laser beam 6 is irradiated from above the upper plate 1. While the laser beam 6 moves along the end surface of the upper plate 1 together with the brazing material 5, the upper plate 1 and the lower plate 2 are heated and the brazing material 5 is melted. As shown in FIG. 2, the molten brazing material 5 flows into the gap between the lower surface of the upper plate 1 and the upper surface of the lower plate 2, and is allowed to cool and solidify after the laser beam 6 and the brazing material 5 have passed. The weld metal 3 is obtained.

  As described above, when laser brazing is performed by applying the present invention, a perforated defect of a weld metal in a lap fillet joint can be prevented. Therefore, if the present invention is applied when forming a lap fillet joint of a high strength thin steel plate, the lap fillet joint can be prevented from being broken and the mechanical properties of the high strength thin steel plate can be fully utilized.

As shown in FIG. 1, a cold-rolled steel plate having a thickness of 1.2 mm and a tensile strength of 390 N / mm ² is used as the lower plate 2, and a cold-rolled steel plate having a thickness of 0.7 mm and a tensile strength of 590 N / mm ² is used as the upper plate 1. An overlapped part was formed. Clearance D ₁ of the lower surface and the upper surface of the lower plate 2 of the upper plate 1, the gap D ₂ between the end surface and the wire-like brazing material 5 of the upper plate 1 is as shown in Table 1. The overlap margin was 15 mm. As the brazing material 5, a copper alloy wire (Cu-3.5 mass% Si-1 mass% Mn) having a diameter of 1.2 mm was used.

  The overlapping portion was irradiated with a laser beam 6 to form an overlapped fillet joint. The laser used was an Nd: YAG laser with a laser output of 4 kW and a beam spot diameter of 2.4 mm. The laser beam 6 was moved along the end face of the upper plate 1 together with the brazing material 5 at a speed of 3 m / min. Since the brazing material 5 is melted by the irradiation of the laser beam 6, it was always supplied at a speed of 3 m / min.

The weld metal 3 of the lap fillet joint thus obtained was visually observed and the appearance was evaluated. The results are shown in Table 1 as good if the width of the weld metal 3 is stable, and defective if there is a portion where the weld metal 3 is narrow or there is an unwelded portion.
In addition, the cross section of the lap fillet joint was observed to investigate the presence of perforated defects and cavities. The results are as shown in Table 1.

Further, a specimen having a width of 40 mm was taken from the lap fillet joint and subjected to a tensile shear test. As a result, all the test pieces were broken by the weld metal 3. The tensile shear strength is as shown in Table 1.
As is apparent from Table 1, the lap fillet joints of the invention examples (that is, the joints 1 to 7) were all good in appearance, and no perforated defects or cavities were observed.
On the other hand, in the lap fillet joint of the comparative example (that is, the joints 8 to 13), all the perforated defects or cavities were recognized. Further, the appearance of the joints 9, 12, and 13 was evaluated as defective because there were portions where the brazing material 5 was not sufficiently welded or the welding width was unstable.

  The tensile shear strength of the lap fillet joint was 11.4 to 13.8 kN in the inventive example, whereas it was 4.6 to 8.5 kN in the comparative example. In other words, the weld metal of the lap fillet joint of the invention example has no perforated defects or cavities, so that the tensile shear strength can be improved.

It is sectional drawing which shows typically the example of arrangement | positioning of the high intensity | strength thin steel plate, brazing material, and a laser beam in the laser brazing of this invention. It is sectional drawing which shows typically the example of the overprinted ink joint formed by applying this invention. It is sectional drawing which shows typically the example of the overlap fillet joint by the conventional laser brazing. It is sectional drawing which shows typically the example of the lap fillet joint which produced the cavity.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Upper plate 2 Lower plate 3 Weld metal 4 Perforated defect 5 Brazing material 6 Laser beam 7 Cavity

Claims (2)

  In a laser brazing method for forming a lap fillet joint of high-strength thin steel sheets using a laser beam, a gap of 0.05 to 0.2 mm is provided between the lower surface of the upper plate and the upper surface of the lower plate, and brazing material The distance between the wire to be used and the end surface of the upper plate is 0.2 to 0.5 mm, and a laser beam is irradiated from above the upper plate to heat the upper and lower plates of the overlapping portion and to melt the wire. After that, a laser brazing method for a high-strength thin steel sheet, wherein a lap fillet joint is formed by allowing to cool. 2. The laser brazing method for a high-strength thin steel sheet according to claim 1, wherein the wire is a wire made of a copper alloy having a diameter of 0.6 to 1.8 mm.

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