JP7781626B2 - Laser welding method and laser welding device - Google Patents

Description

The present invention relates to laser welding and a laser welding device that uses a laser to continuously weld a first base material and a second base material placed on top of the first base material.

The outer panels that make up the exterior surface of railway vehicles have frame members such as vertical ribs that extend along the vehicle height and horizontal ribs that extend along the track joined to the inner surface (hereinafter referred to as the back surface) of the railway vehicle. Joining of these frame members to the outer panels is generally done by laser welding.

Specifically, for example, as shown in FIG. 5 , a cross frame 52 (an example of a frame member) with a hat-shaped cross section extends from the back surface 51a of the outer plate 51, and the flange portion 52a of the cross frame 52 is joined to the back surface 51a of the outer plate 51 by laser welding. This laser welding is a continuous weld along the longitudinal direction of the cross frame 52, as shown by weld 57, and is performed by irradiating a laser from the cross frame 52 side.

To ensure the strength of the weld 57, it is necessary to achieve a penetration depth that reaches all the way to the outer plate 51. However, if the laser is irradiated from the back side of the outer plate 51 so that it reaches the surface opposite the back surface 51a of the outer plate 51, weld marks will remain on the outer surface of the railway vehicle, compromising the design of the railway vehicle. Therefore, welding must be performed in a way that ensures a sufficient penetration depth, while preventing the laser from reaching the surface of the outer plate 51.

However, if the laser does not reach the surface of the outer panel 51, it is impossible to determine from the outside whether sufficient penetration depth has been achieved. In such cases, whether sufficient penetration depth has been achieved is determined by conducting non-destructive testing or by using optical coherence tomography.

One known method for determining whether sufficient penetration depth has been achieved using optical coherence tomography is the method disclosed in Patent Document 1. Here, the method for determining whether sufficient penetration depth has been achieved using the method disclosed in Patent Document 1 will be explained using Figures 6 and 8. Figure 6 is a diagram showing how a laser beam L11 emitted from a laser welding device 200 according to conventional technology is used to weld an outer plate 51 to a cross frame 52 (flange portion 52a) placed on top of the outer plate 51. Figure 8 is a cross-sectional view taken along the line B-B in Figure 6.

The outer plate 51 and cross frame 52 are both made of austenitic stainless steel. The outer plate 51 is approximately 2 mm thick, and the cross frame 52 (flange portion 52a) is approximately 1.5 mm thick. Arrow Y in Figure 6 indicates the direction of continuous welding. The laser welding device 200 irradiates the outer plate 51 and cross frame 52 with laser light L11 from the cross frame 52 side. This laser light L11 melts the outer plate 51 and cross frame 52, creating molten material 55. The molten material 55 then evaporates, creating a keyhole 54. A weld bead 56, where the molten material 55 solidifies, is formed on the opposite side of the keyhole 54 in the direction of travel (arrow Y).

The method disclosed in Patent Document 1 uses measurement beams L31 and L32 emitted from a laser welding device 200 using optical coherence tomography to measure the position (distance d1) of the bottom 54a of the keyhole 54 and the position (distance d2) of the surface 52b of the cross bone 52 (flange portion 52a). The penetration depth d21 of the laser beam L11 is then calculated from the difference between the position (distance d1) of the bottom 54a and the position (distance d2) of the surface 52b. Since this laser penetration depth d21 can be considered the same as the penetration depth d31 in Figure 8, calculating the penetration depth d21 of the laser beam L11 and verifying this value makes it possible to determine whether a sufficient penetration depth d31 has been achieved.

Special Publication No. 2016-538134

If non-destructive testing were to be performed to determine whether sufficient penetration depth had been achieved, it would have to be carried out as a separate process from the welding process. This would mean that the testing would be costly and time-consuming, raising concerns that this could increase railway vehicle manufacturing costs and reduce manufacturing efficiency.

On the other hand, the technology using optical coherence tomography disclosed in Patent Document 1 makes it possible to determine whether sufficient penetration depth has been achieved while welding is taking place. This is desirable in that it can resolve issues such as increased manufacturing costs and reduced manufacturing efficiency for railway vehicles.

However, the inventors of the present application have discovered that the technology disclosed in Patent Document 1 has the following problem. When joining the outer plate 51 and the cross frame 52 (flange portion 52a) by laser welding, what is important for ensuring the strength of the weld is not the total penetration depth d31 (laser penetration depth d21) between the outer plate 51 and the cross frame 52, but the penetration depth d41 in the outer plate 51 (see Figure 8).

As shown in Figure 6, it is possible to calculate the laser penetration depth d11 in the outer plate 51 by subtracting the known thickness t of the cross frame 52 (flange portion 52a) from the penetration depth d21 of the laser L11, which can be calculated using the technology disclosed in Patent Document 1. Therefore, by equating this laser penetration depth d11 with the penetration depth d41 in the outer plate 51 shown in Figure 8, it is possible to determine the penetration depth d41 in the outer plate 51. However, the penetration depth d11 calculated by subtracting the thickness t of the cross frame 52 (flange portion 52a) from the penetration depth d21 will only match the actual penetration depth d41 when the outer plate 51 and the flange portion 52a are in tight contact with each other without any gaps, as shown in Figures 6 and 8.

Because warping and undulation occur in the outer plate 51 and the cross frame 52, when the cross frame 52 is placed on the outer plate 51, the outer plate 51 and the cross frame 52 (flange portion 52a) do not adhere to each other throughout, and a gap g occurs between the outer plate 51 and the cross frame 52 (flange portion 52a), as shown in Figures 7, 9, and 10. Figure 7 shows how the outer plate 51 and the cross frame 52 (flange portion 52a) placed on top of the outer plate 51 with the gap g remaining, are welded together using a laser L11 emitted from a laser welding device 200 according to conventional technology. Figure 9 is a cross-sectional view C-C of Figure 7, showing a state where the gap g is 0.2 mm. Figure 10 is a cross-sectional view C-C of Figure 7, showing a state where the gap g is 0.4 mm.

When performing laser welding, measures are taken to minimize the gap g by pressing down the outer plate 51 and cross frame 52, but depending on the rigidity of the materials used to make the outer plate 51 and cross frame 52, it may be difficult to completely eliminate the gap g.

When a gap g exists between the outer plate 51 and the flange portion 52a, the laser penetration depth d51 calculated by subtracting the known thickness t of the cross frame 52 (flange portion 52a) from the laser penetration depth d21 (penetration depth d31) calculated using the technology disclosed in Patent Document 1 is the actual laser penetration depth d11 plus the gap g (i.e., the penetration depth d41 in the outer plate 51 (see Figures 9 and 10) plus the gap g). Therefore, the technology disclosed in Patent Document 1 does not accurately determine the penetration depth d41, and therefore does not allow for an accurate determination of whether the strength of the weld 57 is sufficient.

Furthermore, as described above, if an attempt is made to calculate penetration depth d41 by subtracting thickness t from laser penetration depth d21, and the gap g becomes even larger, the erroneous result will be that penetration depth d41 has been obtained, even though in reality penetration depth d41 has not been obtained at all.

For example, Figure 11 is a cross-sectional view taken along the line C-C of Figure 7, showing a state where the gap g is 0.6 mm. The state shown in Figure 11 is one in which no penetration depth d41 (laser penetration depth d11) has actually been achieved in the outer panel 51. However, using the technology disclosed in Patent Document 1, the position of the rear surface 51a of the outer panel 51 is measured by measurement light L31, and the laser penetration depth d21 is calculated from the position of the rear surface 51a and the position of the front surface 52b of the cross frame 52 (flange portion 52a) measured by measurement light L32. Subtracting the thickness t of the cross frame 52 (flange portion 52a) from this laser penetration depth d21 yields the penetration depth d51, which corresponds to the size of the gap g. If this penetration depth d51 were to be considered the same as the penetration depth d41, the erroneous result would be that penetration depth d41 has been achieved, even though in reality it has not.

As described above, when attempting to calculate the penetration depth d11 (penetration depth d41) in the outer plate 51 using the technology disclosed in Patent Document 1, the calculated value (penetration depth d51) is greater than the actual penetration depth d11 (penetration depth d41) by the amount of gap g. This makes it impossible to accurately determine the penetration depth d41, which poses the problem of making it impossible to determine whether sufficient weld strength has been ensured.

The present invention is intended to solve the above-mentioned problems, and aims to provide a laser welding method and laser welding apparatus that can accurately determine the penetration depth in a first base material when welding a first base material and a second base material placed on top of the first base material by irradiating a laser from the side of the second base material.

In order to solve the above problems, the laser welding method and laser welding apparatus of the present invention have the following configuration.
(1) A laser welding method for continuously welding a first base material and a second base material placed on top of the first base material using a laser, the method comprising: irradiating the first base material and the second base material with the laser from the side of the second base material to generate a keyhole and a weld bead on the side of the keyhole opposite to the direction of travel of the continuous welding; irradiating the bottom of the keyhole with measurement light using optical coherence tomography to measure the position of the bottom; irradiating the surface of the second base material with the measurement light to measure the position of the surface; irradiating the weld bead with the measurement light to measure the height of the weld bead; calculating a gap between the first base material and the second base material based on the height of the weld bead; and calculating a penetration depth of the laser into the first base material based on the position of the bottom, the position of the surface, the thickness of the second base material, and the gap.

(4) A laser welding apparatus that uses a laser to continuously weld a first base material and a second base material placed on top of the first base material, comprising a laser light source, an optical interferometer, a scanning means, and a control device having a control program for controlling the laser welding apparatus, the control program generating the laser from the laser light source and irradiating the laser onto the first base material and the second base material from the side of the second base material, thereby generating a keyhole and a weld bead on the side of the keyhole opposite the direction of progression of the continuous welding, and The interferometer generates measurement light using optical coherence tomography, and the scanning means sequentially irradiates the measurement light onto the bottom of the keyhole, the surface of the second base material, and the weld bead, thereby measuring the position of the bottom of the keyhole, the position of the surface of the second base material, and the height of the weld bead. The gap between the first base material and the second base material is calculated based on the height of the weld bead, and the penetration depth of the laser into the first base material is calculated based on the position of the bottom, the position of the surface, the thickness of the second base material, and the gap.

According to the laser welding method described in (1) or the laser welding apparatus described in (4), the position of the bottom of the keyhole, the position of the surface of the second base material, and the height of the weld bead are measured using optical coherence tomography, and the gap between the first base material and the second base material is calculated based on the height of the weld bead. The penetration depth into the first base material is then calculated based on the position of the bottom of the keyhole, the position of the surface of the second base material, the gap between the first base material and the second base material, and the known thickness of the second base material. Because the gap between the first base material and the second base material is taken into account, which was not taken into account in conventional technology, it is possible to accurately calculate the penetration depth of the laser into the first base material. Furthermore, the penetration depth of the laser into the first base material can be considered identical to the penetration depth in the first base material. In other words, it is possible to accurately determine the penetration depth in the first base material.

Here, the gap between the first and second base materials (hereinafter simply referred to as the gap) is calculated based on the measured value of the weld bead height measured using a measurement beam. This is due to the inventor's discovery that there is a correlation between the height of the weld bead and the size of the gap. The inventor conducted an experiment to measure the height of the weld bead by laser welding a first test piece corresponding to the first base material with a second test piece corresponding to the second base material, with the gap set to 0 mm, 0.1 mm, 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, and 0.6 mm. As a result, an empirical formula expressing the relationship between the height of the weld bead and the size of the gap was derived. Therefore, by substituting the height of the weld bead measured using the measurement beam into the empirical formula, it became possible to accurately calculate the size of the gap.

(2) In the laser welding method described in (1), the gap is calculated based on the height of the weld bead, a gap calculation history showing the gap calculated within a predetermined time immediately before the gap is calculated, and the standard deviation of the height of the weld bead measured within the predetermined time.

(5) In the laser welding apparatus described in (4), the control program calculates the gap based on the height of the weld bead, a gap calculation history that shows the gap calculated within a predetermined time immediately before the gap is calculated, and the standard deviation of the height of the weld bead measured within the predetermined time.

The gap between the first and second base materials is not constant in the direction of continuous welding, but fluctuates due to waviness or warping of the material. However, this fluctuation is gradual, not sudden, except when there are defects such as dents on the material surface. Therefore, the accuracy of the calculated gap can be improved by looking at the amount of change in the gap up to the time the gap is calculated. In this regard, with the laser welding method described in (2) or the laser welding device described in (5), when calculating the gap between the first and second base materials, the gap calculation history, which is the gap calculated within a predetermined time period immediately prior to calculating the gap, is taken into account. This makes it possible to see how the size of the gap has changed over time, and the amount of change, allowing for more accurate calculation of the gap between the first and second base materials.

Furthermore, through the above-mentioned experiments, the inventors of the present application discovered that there is a correlation between the size of the gap between the first and second base materials and the standard deviation of the height of the weld bead. According to the laser welding method described in (2) or the laser welding apparatus described in (5), the standard deviation of the height of the weld bead is also taken into account when calculating the gap between the first and second base materials, making it possible to calculate the gap between the first and second base materials more accurately.

(3) The laser welding method described in (1) or (2) is characterized in that the reliability of the calculated penetration depth is evaluated based on whether the height of the weld bead exceeds a predetermined threshold.

(6) In the laser welding apparatus described in (4) or (5), the control program is characterized in that it evaluates the reliability of the calculated penetration depth based on whether the height of the weld bead exceeds a predetermined threshold.

The inventors of the present application discovered that the correlation between the height of the weld bead and the size of the gap between the first and second base materials holds when the gap between the first and second base materials is within a certain range. In other words, if the gap between the first and second base materials is outside this range, the gap cannot be accurately calculated from the above-mentioned empirical formula. "Within this certain range" varies depending on the material and thickness of the base materials to be welded, welding conditions, etc. For example, if the first base material is austenitic stainless steel with a thickness of 2 mm and the second base material is austenitic stainless steel with a thickness of 1.5 mm, the gap is approximately 0 to 0.3 mm.

The present inventors have discovered that it is possible to determine whether the gap between the first and second base materials is within a certain range based on the height of the weld bead. Specifically, when the height of the weld bead exceeds a predetermined threshold, the gap between the first and second base materials is within the certain range, and the penetration depth in the first base material calculated using the above-mentioned empirical formula can be determined to be highly reliable. On the other hand, when the height does not exceed the predetermined threshold, the gap between the first and second base materials is outside the certain range, and the penetration depth in the first base material calculated using the above-mentioned empirical formula can be determined to be unreliable. Therefore, the laser welding method described in (3) or the laser welding apparatus described in (6) can evaluate the reliability of the calculated penetration depth to determine whether the height of the weld bead exceeds a certain threshold. The predetermined threshold varies depending on the material and thickness of the base materials to be welded, welding conditions, etc. Therefore, the gap size and weld bead height are measured using a test piece corresponding to the base materials to be welded, and the threshold is appropriately set based on the actual measurements.

The laser welding method and laser welding apparatus of the present invention make it possible to accurately determine the penetration depth in the first base material when welding a first base material and a second base material placed on top of the first base material by irradiating a laser from the side of the second base material.

FIG. 1 is a diagram showing a schematic configuration of a laser welding device. 1 is a diagram showing how an outer panel and a cross frame are welded together using a laser welding device according to the present embodiment. FIG. 10 is a graph showing the results of an experiment in which laser welding was performed using a first test piece corresponding to an outer panel and a second test piece corresponding to a cross beam, and the height of the weld bead was measured. 1 is a graph summarizing the relationship between the height of a weld bead and the standard deviation of the height of the weld bead. FIG. 10 is a diagram showing a state in which a cross frame is joined to an outer panel. FIG. 1 is a diagram showing how an outer plate and a cross frame are welded together by a laser emitted from a laser welding device according to the prior art. 1 is a diagram showing how a skin plate and a cross frame placed on top of the skin plate with a gap therebetween are welded together by a laser irradiated from a laser welding device according to the prior art. FIG. 7 is a cross-sectional view taken along the line BB in FIG. 6. 8 is a cross-sectional view taken along the line CC in FIG. 7, showing a state where the gap is 0.2 mm. FIG. 8 is a cross-sectional view taken along the line CC in FIG. 7, showing a state where the gap is 0.4 mm. FIG. 8 is a cross-sectional view taken along the line CC in FIG. 7, showing a state where the gap is 0.6 mm.

Embodiments of the laser welding method and laser welding apparatus according to the present invention will be described in detail with reference to the drawings.

First, the configuration of the laser welding apparatus 1 according to this embodiment will be described. FIG. 1 is a diagram illustrating a schematic configuration of the laser welding apparatus 1. The laser welding apparatus 1 is used, for example, to join an outer plate 51 (an example of a first base material) and a cross frame 52 (an example of a second base material) of a railway vehicle. Both the outer plate 51 and the cross frame 52 are made of austenitic stainless steel. The outer plate 51 is approximately 2 mm thick, and the cross frame 52 is approximately 1.5 mm thick. As shown in FIG. 5, the back surface 51a of the outer plate 51 faces the interior of the railway vehicle. A cross frame 52 with a hat-shaped cross section extends from this back surface 51a. The flange portion 52a of this cross frame 52 is joined to the back surface 51a of the outer plate 51 by laser welding. This laser welding is a continuous welding along the longitudinal direction of the cross frame 52. As shown in FIG. 1, the cross frame 52 is placed on top of the outer plate 51, and laser L11 is applied from the cross frame 52 side.

The laser welding device 1 includes a laser incident unit 11. A laser light source 2 is connected to the laser incident unit 11 via an optical fiber 27. The laser beam L11 output from the laser light source 2 is incident on the laser welding device 1 from the laser incident unit 11 via the optical fiber 27. The incident laser beam L11 is collimated by a first collimator lens 12. The collimated laser beam L11 is reflected by a first dichroic mirror 13, and its direction of travel is redirected 90 degrees. The laser beam L11 is then focused by a focus lens 15 and output from the laser welding device 1, where it is irradiated onto the outer panel 51 and cross beams 52. The focus lens 15 can be moved axially (vertically in FIG. 1). This allows the focal position of the laser beam L11 to be changed axially (vertically in FIG. 1).

The laser welding apparatus 1 includes a measurement light input section 19. An optical interferometer 3 is connected to the measurement light input section 19 via an optical fiber 29. Measurement light L21 output from the optical interferometer 3 is input to the laser welding apparatus 1 from the measurement light input section 19 via the optical fiber 29. The input measurement light L21 is collimated by a second collimator lens 21. The collimated measurement light L21 is reflected by a second dichroic mirror 23, and its direction of travel is redirected approximately 90 degrees. The measurement light L21 then passes through a first dichroic mirror 13, is focused by a focus lens 15, and is output from the laser welding apparatus 1. As a result, the measurement light L21 is irradiated onto the measurement target portions of the outer panel 51 and cross frame 52. The measurement target portions are the bottom 54a of the keyhole 54, the surface 52b of the cross frame 52, and the weld bead 56, as shown in FIG. 2. Details of each of these portions will be described later. The irradiated measurement light L21 is then reflected by the area to be measured, returns to the measurement light incident section 19 via the first dichroic mirror 13 and the second dichroic mirror 23, and enters the optical interferometer 3 via the optical fiber 29.

The optical interferometer 3 uses, for example, Swept Source Optical Coherence Tomography (SS-OCT), a type of optical coherence tomography, to measure the optical path length of the measurement light L21 and determine the position of the area to be measured based on the measured optical path length. Note that the optical interferometer 3 is not limited to those using SS-OCT technology, and may also use technologies such as Spectral Domain Optical Coherence Tomography (SD-OCT) or Time Domain Optical Coherence Tomography (TD-OCT).

The laser welding device 1 also includes a scanning means 25 for oscillating the second dichroic mirror 23. The scanning means 25 oscillates the second dichroic mirror 23 to scan the measurement light L21 and vary the irradiation position. This allows the measurement light L21 to be sequentially irradiated onto the three measurement target locations (the bottom 54a of the keyhole 54, the surface 52b of the crossbeam 52, and the weld bead 56).

In addition, the laser welding device 1 is fitted with a protective lens 17 to protect the inside of the laser welding device 1 from spatter and fumes generated when laser welding the outer panel 51 and cross frame 52.

Next, the condition of the area where continuous welding of the outer plate 51 and cross frame 52 is performed will be described in detail with reference to Figure 2. Figure 2 is a diagram showing how the outer plate 51 and cross frame 52 are welded together using the laser welding device 1 according to this embodiment. The arrow Y in the figure indicates the direction in which continuous welding proceeds.

The laser beam L11 output from the laser welding device 1 melts and vaporizes the outer plate 51 and cross frame 52 around the laser beam L11, forming a keyhole 54 in the outer plate 51 and cross frame 52. The keyhole 54 is surrounded by a molten material 55 formed by melting the outer plate 51 and cross frame 52, and this molten material 55 solidifies as the distance from the focus of the laser beam L11 increases. Therefore, a weld bead 56 formed by solidifying the molten material 55 is formed on the side opposite the direction of progression of the continuous welding (the direction of arrow Y).

As already mentioned, the laser welding device 1 can use the scanning means 25 to sequentially irradiate the measurement light L21 onto three measurement target locations (the bottom 54a of the keyhole 54, the surface 52b of the crossbar 52, and the weld bead 56). Here, the measurement light irradiated onto the bottom 54a of the keyhole 54 is referred to as the first measurement light L21a, the measurement light irradiated onto the surface 52b of the crossbar 52 is referred to as the second measurement light L21b, and the measurement light irradiated onto the weld bead 56 is referred to as the third measurement light L21c.

As shown in FIG. 2, the first measurement light L21a is output coaxially with the laser L11. As a result, the first measurement light L21a is irradiated onto the bottom 54a of the keyhole 54 formed near the focus of the laser L11. The first measurement light L21a is then reflected by the bottom 54a of the keyhole 54 and returns to the optical interferometer 3. This makes it possible to measure the position of the bottom 54a of the keyhole 54 (e.g., the distance d1) based on the SS-OCT technology already described.

When the second measurement light L21b is irradiated onto the surface 52b of the crossbar 52, it is reflected by the surface 52b of the crossbar 52 and returns to the optical interferometer 3. This makes it possible to measure the position of the surface 52b of the crossbar 52 (e.g., the distance d2) based on the SS-OCT technology already described. The position at which the second measurement light L21b is irradiated is forward of the laser L11 in the direction of progression of the continuous welding, but this is not particularly limited, and it is sufficient that the second measurement light L21b is irradiated onto the surface 52b of the crossbar 52 near the keyhole 54 and not melted by the laser L11.

The third measurement light L21c is also irradiated onto the center of the weld bead 56 in the width direction (depth direction in Figure 2). When irradiated onto the weld bead 56, it is reflected by the weld bead 56 and returns to the optical interferometer 3. This makes it possible to measure the height (e.g., spacing d3) of the weld bead 56 based on the SS-OCT technology already described. Note that by irradiating the center of the weld bead 56 in the width direction with the third measurement light L21c, the height (spacing d3) of the weld bead 56 is measured at the highest point if the weld bead 56 is convex from the surface 52b of the cross frame 52, and the height (spacing d3) is measured at the lowest point if the weld bead 56 is concave from the surface 52b of the cross frame 52.

The laser welding apparatus 1 described above is controlled as follows by a control program in a control device (not shown).

First, a laser beam L11 is generated from the laser light source 2, and the laser is irradiated onto the outer plate 51 and cross frame 52 from the cross frame 52 side. At this time, the focus position of the laser beam L11 is adjusted by the focus lens 15 according to a predetermined target value for the penetration depth d11 of the laser beam L11. Then, continuous welding is performed between the outer plate 51 and cross frame 52 along the direction of travel (arrow Y). Irradiation of the laser beam L11 creates a keyhole 54 and molten material 55 in the outer plate 51 and cross frame 52. Furthermore, as the continuous welding progresses, a weld bead 56 is formed on the opposite side of the direction of travel (arrow Y), where the molten material 55 solidifies.

The control program performs continuous welding using the laser L11, while simultaneously causing the optical interferometer 3 to generate measurement light L21 using optical coherence tomography, and causes the scanning means 25 to sequentially irradiate the measurement light L21 (first measurement light L21a, second measurement light L21b, third measurement light L21c) onto the bottom 54a of the keyhole 54, the surface 52b of the cross frame 52, and the weld bead 56. The order in which the measurement light L21 is irradiated does not matter, and it may be irradiated from any of the bottom 54a of the keyhole 54, the surface 52b of the cross frame 52, or the weld bead 56. Then, by irradiating the measurement light L21 (first measurement light L21a, second measurement light L21b, third measurement light L21c), the position of the bottom 54a of the keyhole 54 (distance d1), the position of the surface 52b of the cross beam 52 (distance d2), and the height of the weld bead 56 (distance d3) are measured.

Next, the control program calculates the gap g between the outer plate 51 and the cross frame 52 based on the height (distance d3) of the weld bead 56 measured using the measurement light L21 (third measurement light L21c).

Furthermore, after calculating the gap g, the control program calculates the penetration depth d11 of the laser L11 into the outer plate 51 based on the position of the bottom 54a of the keyhole 54 (distance d1), the position of the surface 52b of the cross frame 52 (distance d2), the calculated gap g, and the known thickness t of the cross frame 52 (1.5 mm in this embodiment). Specifically, the penetration depth d11 is calculated by subtracting the distance d2, the calculated gap g, and the thickness t from the distance d1. Note that the calculation of the penetration depth d11 is performed continuously, for example, at intervals of 10 milliseconds while welding is being performed. However, this measurement interval is not limited to 10 milliseconds and can be set appropriately based on the welding speed and the weld length over which the penetration depth d11 to be detected falls short.

As described above, the penetration depth d11 of the laser L11 into the outer plate 51 is calculated taking the gap g into account, making it possible to accurately calculate the penetration depth d11. Furthermore, the penetration depth d11 can be considered the same as the penetration depth in the outer plate 51 (e.g., penetration depth d41 in Figures 8 to 10). In other words, by being able to accurately calculate the penetration depth d11, it is possible to accurately determine the penetration depth in the outer plate 51. Note that Figure 2 shows a case where a gap g exists between the outer plate 51 and the cross frame 52. However, if no gap g exists, the gap g is calculated to be, for example, 0 mm based on the height (spacing d3) of the weld bead 56. Therefore, the laser welding device 1 can accurately calculate the penetration depth d11 regardless of whether or not a gap g exists.

Here, we will explain how to calculate the gap g based on the height (distance d3) of the weld bead 56. The reason why the gap g between the outer plate 51 and the cross frame 52 is calculated based on the height (distance d3) of the weld bead 56 is that the inventors of the present application discovered through experiments that there is a correlation between the height (distance d3) of the weld bead 56 and the size of the gap g.

The inventors conducted an experiment using a first test piece corresponding to the outer panel 51 and a second test piece corresponding to the cross frame 52. The second test piece was placed on top of the first test piece with a gap g between them, and laser welding was performed to measure the height of the weld bead 56. The height of the weld bead 56 was measured for pieces welded with the gap between the first and second test pieces set to 0 mm, 0.1 mm, 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, and 0.6 mm.

Figure 3 is a graph showing the results of the above experiment. Note that "weld bead height" in Figure 3 refers to the amount of protrusion of the weld bead 56 from the surface of the second test piece (equivalent to the amount of protrusion of the weld bead 56 from the surface 52b of the cross frame 52 in Figure 2). As shown in this graph, when the gap between the first and second test pieces is in the range of 0 to 0.3 mm, the height of the weld bead 56 decreases as the size of gap g increases. In other words, it can be seen that there is a negative correlation between the size of gap g and the height of the weld bead 56. Based on this correlation, the applicant has derived an empirical formula that expresses the relationship between the height of the weld bead 56 and the size of gap g. Therefore, the control program of the laser welding apparatus 1 uses this empirical formula to calculate gap g based on the height of the weld bead 56 (distance d3). As mentioned above, the correlation between the size of gap g and the height of weld bead 56 holds when gap g is in the range of 0 to 0.3 mm. However, this range applies when the first test piece (i.e., outer plate 51) is made of austenitic stainless steel with a thickness of 2 mm and the second test piece (i.e., cross beam 52) is made of austenitic stainless steel with a thickness of 1.5 mm. The range of gap g for which this correlation holds varies depending on the material and thickness of the base material to be welded, the welding conditions, etc.

The correlation between the size of gap g and the height of the weld bead 56 does not hold if the gap is outside the range of 0 to 0.3 mm. In other words, if gap g is outside the range of 0 to 0.3 mm, gap g cannot be accurately calculated using the above empirical formula. As a result, the penetration depth d11 in the outer plate 51 cannot be accurately calculated.

The graph in Figure 3 shows that the height of weld bead 56 increases when gap g exceeds 0.3 mm, but the increase is smaller between 0.5 and 0.6 mm compared to between 0.4 and 0.5 mm. In other words, even if gap g increases further from this point, the height of weld bead 56 is likely to converge to a certain value. From this, the applicant discovered that by checking whether the height of weld bead 56 exceeds a predetermined threshold value X, it is possible to determine whether the size of gap g is within a range in which a correlation exists between the size of gap g and the height of weld bead 56.

In other words, when the height of the weld bead 56 exceeds the threshold X, the gap g between the outer plate 51 and the cross frame 52 is within the range of 0 to 0.3 mm, and therefore the penetration depth d11 in the outer plate 51 calculated using the above empirical formula can be determined to be highly reliable. On the other hand, when the height of the weld bead 56 does not exceed the threshold X, the gap g between the outer plate 51 and the cross frame 52 is outside the range of 0 to 0.3 mm, and therefore the penetration depth d11 in the outer plate 51 calculated using the above empirical formula can be determined to be unreliable.

As described above, by checking whether the height of the weld bead 56 exceeds a predetermined threshold X, it is possible to evaluate the reliability of the calculated penetration depth d11. Note that, according to the threshold X in Figure 3, even a gap of 0.3 mm would be deemed unreliable, but this is due to consideration of a safety factor. Furthermore, the predetermined threshold X varies depending on the material and thickness of the base material to be welded, welding conditions, etc. Therefore, the size of the gap and the height of the weld bead are actually measured as in the above experiment, and the threshold is set appropriately based on these measured values.

In the above-described embodiment, gap g is calculated based on the height (spacing d3) of weld bead 56, but when calculating gap g, in addition to the height (spacing d3) of weld bead 56, it is also possible to take into account the gap calculation history, which is the gap g calculated within a predetermined time immediately before calculating gap g, and the standard deviation of the height (spacing d3) of the weld bead measured within the predetermined time.
Specifically, for example, the gap g calculated based on the height (distance d3) of the weld bead 56 is corrected using the gap calculation history and the standard deviation. This correction may be performed using an empirical formula derived through experiments by the present invention or by machine learning. Note that the above-mentioned "predetermined time" is, for example, approximately 100 milliseconds, but is not limited to this and is set appropriately based on the welding speed and the weld length over which the penetration depth d11 to be detected is continuously insufficient.

The gap g between the outer plate 51 and the cross frame 52 is not constant in the direction of continuous welding, but fluctuates due to waviness or warping of the material. However, this fluctuation is gradual, not sudden, except in cases where there are defects such as dents on the material surface. Therefore, the accuracy of the calculated gap g can be improved by looking at the amount of change in gap g up to the time when gap g is to be calculated. Therefore, by considering the gap calculation history described above when calculating gap g, it is possible to see how the size of gap g has changed over time and the amount of change, making it possible to more accurately calculate the gap g between the outer plate 51 and the cross frame 52.

Furthermore, through the above-mentioned experiments, the inventors of the present application discovered that there is a correlation between the size of gap g and the standard deviation of the height of weld bead 56. Figure 4 is a graph summarizing the relationship between the height of weld bead 56 measured through the above-mentioned experiments and the standard deviation of the height of weld bead 56. The vertical axis represents the standard deviation value, and the horizontal axis represents the height of weld bead 56.

The graph shown in Figure 4 shows that when the gap g is 0.4 mm, the standard deviation is significantly larger than when the gap g is 0 to 0.3 mm or 0.5 to 0.6 mm. Because the gap g range in which a correlation between the size of the gap g and the height of the weld bead 56 is 0 to 0.3 mm, as described above, the value of 0.4 mm is the boundary value within the experimental range of 0 to 0.6 mm at which the correlation is established. The graph in Figure 4 also shows that, with the exception of this boundary value, the larger the gap g, the smaller the standard deviation of the weld bead height (distance d3). Based on these experimental results, when calculating the gap g, it is possible to more accurately calculate the gap g between the outer plate 51 and the cross frame 52 by taking into account that the standard deviation is significantly larger when the gap g is within the boundary value and that the standard deviation is smaller with the larger gap g, with the exception of this boundary value.
In this embodiment, the boundary value is 0.4 mm, but this is merely an example. As already mentioned, the range of gap g, within which a correlation exists between the size of gap g and the height of weld bead 56, varies depending on the material and thickness of the base material to be welded, the welding conditions, and the like, and therefore the boundary value also varies in accordance with the variation in the range of gap g.

As described above, according to the laser welding method and laser welding apparatus 1 of this embodiment,
(1) In a laser welding method for continuously welding a first base material (outer plate 51) and a second base material (cross frame 52) placed on top of the first base material (outer plate 51) using a laser L11, the laser L11 is irradiated onto the first base material (outer plate 51) and the second base material (cross frame 52) from the side of the second base material (cross frame 52), thereby generating a keyhole 54 and a weld bead 56 on the side of the keyhole 54 opposite to the direction of progression (arrow Y) of the continuous welding; measuring light L21 (first measuring light L21a) by optical coherence tomography is irradiated onto a bottom 54a of the keyhole 54, thereby measuring the position (distance d1) of the bottom 54a; the position (distance d2) of the surface 52b of the second base material (cross frame 52) is measured by irradiating the surface 52b of the second base material (cross frame 52) with measurement light L21 (third measurement light L21c); the height (d3) of the weld bead 56 is measured by irradiating the weld bead 56 with measurement light L21 (third measurement light L21c); the gap g between the first base material (outer plate 51) and the second base material (cross frame 52) is calculated based on the height (d3) of the weld bead 56; and the penetration depth d11 of the laser L11 into the first base material (outer plate 51) is calculated based on the position (distance d1) of the bottom 54a, the position of the surface 52b (distance d2), the thickness t of the second base material (cross frame 52), and the gap g.

(4) A laser welding apparatus 1 that continuously welds a first base material (outer plate 51) and a second base material (cross frame 52) placed on top of the first base material (outer plate 51) using a laser L11 includes a laser light source 2, an optical interferometer 3, a scanning means 25, and a control device having a control program for controlling the laser welding apparatus 1. The control program generates a laser L11 from the laser light source 2 and irradiates the first base material (outer plate 51) and the second base material (cross frame 52) with the laser L11 from the side of the second base material (cross frame 52), thereby creating a keyhole 54 and a weld bead 56 on the side of the keyhole 54 opposite the direction of progression of the continuous welding (arrow Y). The optical interferometer 3 generates measurement light L21 using optical coherence tomography, and the measurement light L21 (first measurement The scanning means 25 sequentially irradiates the bottom 54a of the keyhole 54, the surface 52b of the second base material (cross frame 52), and the weld bead 56 with the first measurement light L21a, the second measurement light L21b, and the third measurement light L21c, thereby determining the position (distance d1) of the bottom 54a of the keyhole 54, the position (distance d2) of the surface 52b of the second base material (cross frame 52), and the height (d3) of the weld bead 56. and calculate the gap g between the first base material (outer plate 51) and the second base material (cross frame 52) based on the height (distance d3) of the weld bead 56, and calculate the penetration depth d11 of the laser L11 into the first base material (outer plate 51) based on the position of the bottom 54a (distance d1), the position of the surface 52b (distance d2), the thickness t of the second base material (cross frame 52), and the gap g.

According to the laser welding method described in (1) or the laser welding apparatus 1 described in (4), the position of the bottom 54a of the keyhole 54, the position (distance d2) of the surface 52b of the second base material (cross frame 52), and the height (d3) of the weld bead 56 are measured using optical coherence tomography. Then, the gap g between the first base material (skin plate 51) and the second base material (cross frame 52) is calculated based on the height (d3) of the weld bead 56. The penetration depth d11 into the first base material (skin plate 51) is calculated based on the position of the bottom 54a of the keyhole 54, the position (distance d2) of the surface 52b of the second base material (cross frame 52), the gap g between the first base material (skin plate 51) and the second base material (cross frame 52), and the known thickness t of the second base material (cross frame 52). By taking into account the gap g between the first base material (skin plate 51) and the second base material (cross frame 52), which was not taken into account in conventional technology, it is possible to accurately calculate the penetration depth d11 of the laser L11 into the first base material (skin plate 51). Furthermore, the penetration depth d11 of the laser L11 into the first base material (skin plate 51) can be considered to be the same as the penetration depth d41 in the first base material (skin plate 51). In other words, it is possible to accurately determine the penetration depth d41 in the first base material (skin plate 51).

Here, the gap g between the first base material (skin plate 51) and the second base material (cross frame 52) is calculated based on the height (distance d3) of the weld bead 56 measured using measurement light L21 (third measurement light L21c). This is because the inventors discovered a correlation between the height (distance d3) of the weld bead 56 and the size of the gap g. The inventors conducted experiments to measure the height (distance d3) of the weld bead 56 by laser welding a first test piece corresponding to the first base material (skin plate 51) with a second test piece corresponding to the second base material (cross frame 52) while setting the gap g to 0 mm, 0.1 mm, 0.2 mm, 0.3 mm, 0.4 mm, 0.5 mm, and 0.6 mm. As a result, they were able to derive an empirical formula expressing the relationship between the height (distance d3) of the weld bead 56 and the size of the gap g. As a result, by substituting the height (spacing d3) of the weld bead 56 measured using measurement light L21 (third measurement light L21c) into the empirical formula, it is possible to accurately calculate the size of the gap g.

(2) In the laser welding method described in (1), the gap g is calculated based on the height (distance d3) of the weld bead 56, the gap calculation history, which is the gap g calculated within a predetermined time immediately before calculating the gap g, and the standard deviation of the height (distance d3) of the weld bead 56 measured within the predetermined time.

(5) In the laser welding apparatus 1 described in (4), the control program calculates the gap g based on the height (spacing d3) of the weld bead 56, the gap calculation history, which is the gap g calculated within a predetermined time immediately before calculating the gap g, and the standard deviation of the height (spacing d3) of the weld bead 56 measured within the predetermined time.

The gap g between the first base material (skin plate 51) and the second base material (cross frame 52) is not constant in the direction of continuous welding, but fluctuates due to waviness and warping of the material. However, this fluctuation is gradual, not sudden, except when there are defects such as dents on the material surface. Therefore, the accuracy of the calculated gap g can be improved by looking at the amount of change in gap g up to the time when gap g is calculated. In this regard, with the laser welding method described in (2) or the laser welding apparatus 1 described in (5), when calculating the gap g between the first base material (skin plate 51) and the second base material (cross frame 52), the gap calculation history, which is the gap g calculated within a predetermined time period immediately prior to calculating the gap g, is taken into account. This makes it possible to see how the size of gap g has changed over time, and the amount of change. This enables more accurate calculation of the gap g between the first base material (skin plate 51) and the second base material (cross frame 52).

Furthermore, through the above-mentioned experiments, the inventors of the present application discovered that there is a correlation between the size of the gap g between the first base material (skin plate 51) and the second base material (cross frame 52) and the standard deviation of the height (distance d3) of the weld bead 56. According to the laser welding method described in (2) or the laser welding apparatus 1 described in (5), when calculating the gap g between the first base material (skin plate 51) and the second base material (cross frame 52), the standard deviation of the height of the weld bead 56 is also taken into consideration, making it possible to more accurately calculate the gap g between the first base material (skin plate 51) and the second base material (cross frame 52).

(3) The laser welding method described in (1) or (2) is characterized in that the reliability of the calculated penetration depth d11 is evaluated based on whether the height (spacing d3) of the weld bead 56 exceeds a predetermined threshold value X.

(6) In the laser welding apparatus 1 described in (4) or (5), the control program is characterized in that it evaluates the reliability of the calculated penetration depth d11 based on whether the height (spacing d3) of the weld bead 56 exceeds a predetermined threshold value X.

The inventors of the present application discovered that the correlation between the height of the weld bead 56 (spacing d3) and the size of the gap g holds when the gap g is within a certain range. In other words, if the gap g is outside this range, the gap g cannot be accurately calculated from the above-mentioned empirical formula. The "within this certain range" varies depending on the material and thickness of the base material to be welded, the welding conditions, etc. For example, if the first base material (outer plate 51) is austenitic stainless steel with a thickness of 2 mm and the second base material (cross frame 52) is austenitic stainless steel with a thickness of 1.5 mm, the gap g is approximately 0 to 0.3 mm.

In light of this, the present inventors have discovered that it is possible to determine whether gap g is within the specified range based on the height (distance d3) of the weld bead 56. Specifically, when the height (distance d3) of the weld bead 56 exceeds a predetermined threshold X, gap g is within the specified range, and the reliability of the penetration depth d11 in the first base material (skin plate 51) calculated using the above empirical formula can be determined to be high. On the other hand, when gap g does not exceed the specified threshold X, gap g is outside the specified range, and the reliability of the penetration depth d11 in the first base material (skin plate 51) calculated using the above empirical formula can be determined to be low. Therefore, the laser welding method described in (3) or the laser welding apparatus 1 described in (6) can determine whether the height (distance d3) of the weld bead 56 exceeds the specified threshold X, thereby enabling a reliability evaluation of the calculated penetration depth d11. The specified threshold X varies depending on the material and thickness of the base material to be welded, welding conditions, etc. Therefore, the size of the gap and the height of the weld bead are measured using a test piece equivalent to the base material to be welded, and the settings are appropriately made based on these measurements.

The above-described embodiment is merely illustrative and does not limit the present invention in any way. Naturally, various improvements and modifications to the present invention are possible without departing from the spirit and scope of the present invention. For example, in the above-described embodiment, the scanning means 25 sequentially irradiates the measurement light L21 onto the areas to be measured (the bottom 54a of the keyhole 54, the surface 52b of the cross-section 52, and the weld bead 56). However, a beam splitter may be used to split the measurement light L21 into three beams, which are then irradiated onto the areas to be measured.

1 Laser welding device 51 Outer plate (an example of a first base material)
52 cross bone (an example of the second base material)
54 Keyhole 54a Bottom 56 Weld bead L11 Laser L21 Measurement light

Claims (4)

A laser welding method for continuously welding a first base material and a second base material disposed on top of the first base material using a laser, comprising:
irradiating the first base material and the second base material with the laser from the second base material side to generate a keyhole and a weld bead on the opposite side of the keyhole to a direction in which the continuous welding proceeds;
measuring the position of the bottom of the keyhole by irradiating the bottom with measurement light by optical coherence tomography;
measuring a position of the surface by irradiating the surface of the second base material with the measurement light;
measuring a height of the weld bead by irradiating the measurement light onto the weld bead;
Calculating a gap between the first base material and the second base material based on the height of the weld bead;
calculating a penetration depth of the laser into the first base material based on the position of the bottom portion, the position of the surface, the thickness of the second base material, and the gap;
the gap is calculated based on the height of the weld bead, a gap calculation history that is the gap calculated within a predetermined time immediately before the gap is calculated, and a standard deviation of the height of the weld bead measured within the predetermined time;
A laser welding method characterized by: A laser welding method for continuously welding a first base material and a second base material disposed on top of the first base material using a laser, comprising:
irradiating the first base material and the second base material with the laser from the second base material side to generate a keyhole and a weld bead on the opposite side of the keyhole to a direction in which the continuous welding proceeds;
measuring the position of the bottom of the keyhole by irradiating the bottom with measurement light by optical coherence tomography;
measuring a position of the surface by irradiating the surface of the second base material with the measurement light;
measuring a height of the weld bead by irradiating the measurement light onto the weld bead;
Calculating a gap between the first base material and the second base material based on the height of the weld bead;
calculating a penetration depth of the laser into the first base material based on the position of the bottom portion, the position of the surface, the thickness of the second base material, and the gap;
evaluating the reliability of the calculated penetration depth based on whether or not the height of the weld bead exceeds a predetermined threshold;
A laser welding method characterized by: A laser welding apparatus for continuously welding a first base material and a second base material disposed on top of the first base material using a laser, comprising:
a laser light source, an optical interferometer, a scanning means, and a control device having a control program for controlling the laser welding device;
The control program
The laser is generated from the laser light source, and the laser is irradiated onto the first base material and the second base material from the second base material side, thereby generating a keyhole and a weld bead on the opposite side of the keyhole from the direction in which the continuous welding proceeds;
generating measurement light by optical coherence tomography from the optical interferometer;
the scanning means sequentially irradiates the measurement light onto the bottom of the keyhole, the surface of the second base metal, and the weld bead, thereby measuring the position of the bottom of the keyhole, the position of the surface of the second base metal, and the height of the weld bead;
Calculating a gap between the first base material and the second base material based on the height of the weld bead;
calculating a penetration depth of the laser into the first base material based on the position of the bottom portion, the position of the surface, the thickness of the second base material, and the gap;
the control program calculates the gap based on the height of the weld bead, a gap calculation history that is the gap calculated within a predetermined time immediately before calculating the gap, and a standard deviation of the height of the weld bead measured within the predetermined time;
A laser welding device characterized by: A laser welding apparatus for continuously welding a first base material and a second base material disposed on top of the first base material using a laser, comprising:
a laser light source, an optical interferometer, a scanning means, and a control device having a control program for controlling the laser welding device;
The control program
The laser is generated from the laser light source, and the laser is irradiated onto the first base material and the second base material from the second base material side, thereby generating a keyhole and a weld bead on the opposite side of the keyhole from the direction in which the continuous welding proceeds;
generating measurement light by optical coherence tomography from the optical interferometer;
the scanning means sequentially irradiates the measurement light onto the bottom of the keyhole, the surface of the second base metal, and the weld bead, thereby measuring the position of the bottom of the keyhole, the position of the surface of the second base metal, and the height of the weld bead;
Calculating a gap between the first base material and the second base material based on the height of the weld bead;
calculating a penetration depth of the laser into the first base material based on the position of the bottom portion, the position of the surface, the thickness of the second base material, and the gap;
the control program evaluates the reliability of the calculated penetration depth based on whether or not the height of the weld bead exceeds a predetermined threshold;
A laser welding device characterized by: