Method for determining the welding depth in a laser welding process

Abstract

The invention relates to a method for determining the welding depth in a laser welding process by means of an optical coherence tomography apparatus, comprising the following steps: – directing a processing laser beam (11) onto a component (10) in order to produce a keyhole (12); – coupling a measurement beam (15) into the beam path of the processing laser beam (11); – carrying out a plurality of line scans over the component (10) using the measurement beam (15) in the region of the keyhole (12) and capturing the light portions of the measurement beam (15) that are reflected off the component (10) using the optical coherence tomography apparatus, and generating a height profile of the component (10) in the region of the keyhole (12); – from the height profile, determining the height of the component surface (10.1) of the lateral position of the deepest point (16, 16', 16'') of the keyhole (12) on the component (10) and also the depth of the deepest point (16) of the keyhole (12); – calculating the keyhole depth (17) from the difference between the height of the component surface (10.1) and the depth of the deepest point (16) of the keyhole (12), and ascertaining the welding depth therefrom.

Description

[0001] Method for determining the welding depth in a laser welding process

[0002] The invention relates to a method for determining the welding depth in a laser welding process using an optical coherence tomograph.

[0003] In laser welding processes using a laser beam, measuring the penetration depth is essential to ensure a high-quality weld. If the penetration depth is insufficient, the weld will not have the required strength. If the penetration depth is too great, however, penetration can occur. Therefore, various methods have already been proposed for measuring the penetration depth in laser welding processes.

[0004] When using an optical coherence tomography system for welding depth measurement, setting the correct measurement position of the measuring beam is essential. The measuring beam must be positioned at the deepest point of the vapor capillary created by the processing laser beam in the component to reliably measure the welding depth. Since the deepest position of the vapor capillary can change depending on the process parameters, a setup process must be performed before the laser welding process to determine the measurement position for the measuring beam. However, the existing setup processes are complex and require a second measurement step to determine the welding depth.

[0005] For example, in the method known from DE 102017 117 413 B4, the measurement position of the measuring beam for welding depth measurement is determined by performing several scans with the measuring beam in the feed direction of the processing laser beam and perpendicular to it. Using an optical coherence tomograph, these scans can be used to create a height profile of the component in the area of ​​the vapor capillary created by the processing laser beam. The lateral position of the lowest point of the vapor capillary can be determined from the height profile. The measuring beam is then directed to the lowest point of the vapor capillary, and the actual measurement of the welding depth is performed.

[0006] A similar method for determining the measuring position of a measuring beam is also known from DE 10 2019 103 734 A1.

[0007] DE 10 2016 005021 A1 describes a method for measuring the welding depth during a laser processing process by directing the measuring beam to the lowest point of a vapor capillary and carrying out measurements of the depth of the lowest point at several different times from the data of these different measurements when the height position of the component surface is known.

[0008] The invention is based on the object of proposing a more efficient method for measuring the welding depth in laser processing processes.

[0009] This object is achieved according to the invention by a method for determining the welding depth in a laser welding process by means of an optical coherence tomograph with the following steps:

[0010] - Directing a processing laser beam onto a component, which creates a vapor capillary in it;

[0011] - Coupling a measuring beam into the beam path of the processing laser beam;

[0012] - Performing several line scans over the component with the measuring beam in the area of ​​the vapor capillary and recording the light components of the measuring beam reflected by the component with the optical coherence tomograph and generating a superimposed height profile of the component in the area of ​​the vapor capillary from the line scans;

[0013] - Determination of the height of the component surface, the lateral position of the lowest point of the vapor capillary on the component, and the depth of the lowest point of the vapor capillary from the height profile; - Calculation of the vapor capillary depth from the difference between the height of the component surface and the depth of the lowest point of the vapor capillary and, from this, determination of the welding depth.

[0014] The subclaims relate to preferred embodiments.

[0015] In the method according to the invention, all parameters necessary for determining the welding depth are recorded from the measurement data of the multiple temporally and spatially spaced scans with the measuring beam. From this measurement data and the resulting height profile of the component in the area of ​​the vapor capillary, the height of the component surface, the lateral position of the lowest point of the vapor capillary, and the depth of the vapor capillary at this point can be determined. The welding depth can then be calculated from this depth and the height of the component surface. In contrast to known methods, two separate measurements do not have to be carried out here: a first measurement to determine the lateral position of the lowest point of the vapor capillary and, in a second measurement, after directing the measuring beam to the lowest point of the vapor capillary, to determine the depth of the vapor capillary at this point.

[0016] The line scans with the measuring beam can preferably be performed in the feed direction of the processing laser beam and / or perpendicular to it. This allows the entire area of ​​the vapor capillary as well as the component surface not melted by the processing laser beam to be captured with the scans.

[0017] Further advantages arise when the measurement data from several or all scans performed with the measuring beam are superimposed. By forming a moving intensity-weighted average of the measurement data, a height profile of the component in the area of ​​the vapor capillary is created and the lateral position of the lowest point of the vapor capillary is determined. Similarly, the measurement data from several or all scans performed with the measuring beam can be superimposed. By forming a moving intensity-weighted average of the measurement data, a height profile of the component in the area of ​​the vapor capillary is created and the height of the component surface is determined.

[0018] From the height profile determined by this averaging, the height of the component surface and / or the lateral position of the lowest point of the vapor capillary can be determined with sufficient accuracy. The measurement of the depth of the vapor capillary at its lowest point can also be read from the height profile.

[0019] However, a higher accuracy of the depth measurement of the vapor capillary can be achieved if the measurement data generated by the scans with the measuring beam in the area of ​​the deepest point of the vapor capillary are filtered using a percentile filter with a sliding temporal process window or by means of a histogram evaluation in order to determine the depth of the vapor capillary at its deepest point and from this the welding depth.

[0020] When evaluating the measurement data histogram, a statistically significant histogram class, in particular the one with the most measurement points below the height position of the component surface, can directly indicate the welding depth.

[0021] The method according to the invention can be carried out for setting up a laser processing machine before a production process and / or continuously or at time intervals during the production process.

[0022] If the method is only used to set up a production process, the average welding depth is already known during setup. For ongoing monitoring of the welding depth after carrying out the procedure for setting up a laser processing machine, the measuring beam can be directed at the lateral position of the lowest point of the vapor capillary and the depth of the vapor capillary at its lowest point can be continuously measured, from which the welding depth can be calculated. In this case, the method is only used for the efficient setup of the laser processing machine before the production process. The welding depth is then continuously measured by holding the measuring beam over the lowest point of the vapor capillary at the measuring position determined during setup of the laser processing machine.When producing welds under constant boundary conditions, this use of the method may be sufficient. However, the method according to the invention is preferably repeated during the production process, either continuously or at temporal or spatial intervals, in order to compensate for any shifts in the lowest point of the vapor capillary and still be able to accurately determine the penetration depth. This is particularly important in continuous welding processes such as pipe welding.

[0023] Further features and advantages of the invention will become apparent from the description, the claims, and the drawings. According to the invention, the above-mentioned and further-described features can be used individually or in combination in any convenient way. The embodiments shown and described are not intended to be exhaustive, but rather are exemplary in nature for describing the invention.

[0024] Detailed description of the invention and drawing

[0025] Fig. 1 shows a cross-section through a component in the region of a vapor capillary generated by a processing laser beam;

[0026] Fig. 2 shows a distribution of measurement data from several OCT scans over the area of ​​the vapor capillary of the component from Fig. 1; Fig. 3a, 3b show the creation of a height profile of two vapor capillaries from measurement data from several OCT scans;

[0027] Fig. 4a, 4b shows histogram evaluations of measurement data of the depth of two vapor capillaries.

[0028] Fig. 1 illustrates the production of a weld seam 14 in a component 10. For this purpose, the component 10 is irradiated by a processing laser beam 11. The processing laser beam 11 creates a vapor capillary 12 in the component 10, in which the material of the component 10 is in a gaseous or plasma state. The processing laser beam 11 moves in the x direction across the component 10. Therefore, a molten pool 13 forms behind the vapor capillary, in which the previously gaseous material of the component 10 has already transformed into a liquid state due to cooling in the vapor capillary 12. Further away from the vapor capillary 12, a weld seam 14 forms, in which the material of the component 10 has already solidified.

[0029] To measure the penetration depth 17 of the weld seam 14, a measuring beam 15 of an optical coherence tomograph (OCT) (not shown) is directed at the deepest point 16 of the vapor capillary 12, and the depth of the vapor capillary 12 in the z-direction at this point 16 is determined from the reflected radiation. The distance of this measured depth from the surface 10.1 of the component 10 corresponds to the penetration depth 17 of the weld seam 14.

[0030] Fig. 2 illustrates how, according to a method according to the invention, a plurality of depth measurement points are generated in the z-direction over a measurement line extending in the feed direction x of the processing laser beam 11 by lateral scans with the measurement beam 15 over a component such as the component 10 from Fig. 1. The measurement points are created by reflections of the measurement beam 11 at the component surface 10.1, the vapor capillary 12 and the melt pool 13. From this plurality of measurement points, a height profile 18, 19 of the scanned component is created according to the two examples in Fig. 3a, 3b by forming a moving intensity-weighted average from the measurement data. For this purpose, a window M of length b is shifted in the x-direction along the measurement line formed by the positions of the measurement beam (OCT measurement positions), and the average is calculated from all the measurement data contained in this window.From the resulting height profiles 18, 19, the lateral position of the lowest point 16', 16" of the height profiles 18, 19 can be determined. Subsequently, the measurement data in the vicinity of these lowest points 16', 16" can be evaluated using histograms to determine the depth of the vapor capillary 12', 12", as illustrated in Fig. 4a, 3b. The histogram class 20, 20' with the highest number of measurement points that lie below the height position of the component surface 10.T, 10.1" indicates the desired weld penetration depth 17. The height position of the component surfaces 10.T, 10.1" can be determined from the height profiles 18, 19. In the examples shown, the welding depth 17 is 2.70 mm in Fig. 4a and 0.80 mm in Fig. 4b.

Claims

Patent claims 1 . Method for determining the penetration depth in a laser welding process using an optical coherence tomograph, comprising the following steps: - directing a processing laser beam (11) onto a component (10, 10', 10") to produce a vapor capillary (12, 12', 12"); - coupling a measuring beam (15) into the beam path of the processing laser beam (11); - performing several line scans over the component (10, 10', 10") with the measuring beam (15) in the area of ​​the vapor capillary (12, 12', 12") and detecting the light components of the measuring beam (15) reflected by the component (10, 10', 10") with the optical coherence tomograph and generating a superimposed height profile (18, 19) of the component (10, 10', 10") in the area of ​​the vapor capillary (12, 12', 12") from the line scans; - from the height profile (18, 19) determination of the height of the component surface (10.1, 10.T, 10.1"), the lateral position of the lowest point (16, 16', 16") of the vapor capillary (12, 12', 12") on the component (10, 10', 10") and the depth of the lowest point (16, 16', 16") of the vapor capillary (12, 12', 12"); - Calculation of the vapor capillary depth (17) from the difference between the height of the component surface (10.1, 10.T, 10.1") and the depth of the lowest point (16, 16', 16") of the vapor capillary (12, 12', 12") and from this determination of the welding depth.

2. Method according to claim 1, characterized in that the line scans are carried out with the measuring beam (15) in the feed direction (x) of the processing laser beam (11) and / or transversely thereto.

3. Method according to claim 1 or 2, characterized in that the measurement data of several or all scans carried out with the measuring beam (15) are superimposed and by forming a moving intensity-weighted average from the measurement data, a height profile (18, 19) of the component (10, 10', 10") in the region of the vapor capillary (12, 12', 12") is created and the lateral position of the lowest point (16, 16', 16") of the vapor capillary (12, 12', 12") is determined.

4. Method according to claim 3, characterized in that in the image evaluation device the measurement data of several or all scans carried out with the measuring beam (15) are superimposed and by forming a sliding intensity-weighted average from the measurement data a height profile (18, 19) of the component in the region of the vapor capillary (12, 12', 12") is created and the height of the component surface (10.1, 10.1 ', 10.1") is determined.

5. Method according to one of the preceding claims, characterized in that the measurement data generated by the scans with the measuring beam (15) in the region of the lowest point (16, 16', 16") of the vapor capillary (12, 12', 12") are filtered by means of a percentile filter with a sliding temporal process window (M) or by means of a histogram evaluation in order to determine the depth of the vapor capillary (12, 12', 12") at its lowest point (16, 16', 16") and therefrom the welding depth (17).

6. Method according to claim 5, characterized in that in a histogram evaluation of the measurement data, a statistically significant histogram class (20, 20'), in particular the one with the most measurement points lying below the height position of the component surface (10.1, 10.T, 10.1"), indicates the welding depth (17).

7. Method according to one of the preceding claims, characterized in that it is carried out for setting up a laser processing machine before a production process and / or continuously or at time intervals during the production process.

8. Method according to one of the preceding claims, characterized in that after carrying out the method for setting up a laser processing machine before a production process, the measuring beam (15) is directed onto the lowest point (16, 16', 16") of the vapor capillary (12, 12', 12") and the depth of the vapor capillary (12, 12', 12") at its lowest point (16, 16', 16") is continuously measured and the welding depth (17) is calculated therefrom.

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