Method for carrying out a laser welding process with determination of the welding depth

Abstract

The invention relates to a method for carrying out a laser welding process with determination of a welding depth (17), comprising the following steps: (a) carrying out a line scan in the longitudinal direction (X) of a keyhole (12) formed on the workpiece (10) during the laser welding process using a measurement beam (15) of an optical coherence tomograph, (b) determining a position (16.X) of a lowest point (16) in the longitudinal direction (X) of the keyhole (12) on the basis of the line scan, and defining the position of the lowest point (16) of the keyhole (12) as a measurement position (16.X) in the longitudinal direction (X) in order to determine the welding depth (17), and (c) determining the welding depth (17) by directing the measurement beam (15) at the defined measurement position (16.X) in the longitudinal direction (X).

Description

[0001] Method for performing a laser welding process with determination of the weld penetration depth

[0002] Background of the invention

[0003] The invention relates to a method for carrying out a laser welding process with determination of the weld penetration depth.

[0004] In laser welding processes, measuring the weld penetration depth is essential to guarantee a high-quality weld. If the penetration depth is insufficient, the weld will lack the required strength. Conversely, excessive penetration depth can lead to perforation. Several methods have been proposed for measuring the weld penetration depth in laser welding processes.

[0005] When using an optical coherence tomograph for weld penetration depth measurement, setting the correct measurement position of the measuring beam is essential. The measuring beam is typically positioned at the lowest point of the vapor capillary created by the laser beam in the workpiece to reliably measure the weld penetration depth. The weld penetration depth is typically defined as the distance between the top of the workpiece and the lowest point of the vapor capillary. However, it is also possible to determine a relevant or effective weld penetration depth, the value of which depends, in a known manner, on the distance between the top of the workpiece and the lowest point of the vapor capillary.

[0006] Since the lowest position of the vapor capillary can change depending on the process parameters, a setup process is usually performed before the laser welding process to determine the measuring position for the measuring beam. However, the known setup processes are complex and require a second measurement step to determine the penetration depth. Furthermore, the position of the lowest point of the vapor capillary can change during the welding process, for example, due to positioning tolerances or changes in the vapor capillary itself, which is not taken into account in the setup process. It is also known to perform a subsequent destructive test, for example, by cross-sectioning, to determine geometric parameters such as the penetration depth, weld width, or weld reinforcement. It is also known to determine the weld position offline, for example, using image processing.

[0007] German patent DE 102018124208B4 describes a device for monitoring a laser processing process on a workpiece. The device comprises a computing unit configured to determine a dynamic processing position relative to the point of impact of the laser beam for a set of process parameters of the laser processing process using a model. The device also comprises an observation unit configured to determine at least one monitoring parameter of the laser processing process at the dynamic processing position, for example, the depth of the vapor capillary. The observation unit may include an optical coherence tomograph and be configured to direct an optical measuring light beam to a measuring position on the surface of the workpiece that corresponds to the dynamic processing position.

[0008] Object of the invention

[0009] The invention is based on the objective of proposing a method for carrying out a laser welding process in which dynamic changes of an ideal measuring position are taken into account to determine the weld penetration depth.

[0010] Description of the invention

[0011] This task is solved by a method of the type mentioned above, comprising: (a) performing a longitudinal line scan of a vapor capillary formed on the workpiece during the laser welding process using a measuring beam of an optical coherence tomograph, (b) determining the position of a lowest point in the longitudinal direction of the vapor capillary based on the line scan and defining the position of the lowest point of the vapor capillary as the measuring position for determining the penetration depth, and (c) determining the penetration depth by aligning the measuring beam with the defined measuring position in the longitudinal direction.

[0012] In the method described here, before determining the penetration depth using the measuring beam, the position of the deepest point of the vapor capillary in the longitudinal direction of the vapor capillary is determined (or redefined if necessary) and defined as the measurement position for determining the penetration depth. By defining the measurement position for the penetration depth measurement on the workpiece being processed, the influence of local disturbances is minimized, and the measurement of the penetration depth becomes more robust than when the measurement position is defined by a measurement performed before the laser welding process. In this way, a dynamic adjustment of the OCT measurement position to the current welding situation can be achieved, thereby correcting changes in the geometry of the vapor capillary during the welding process.The weld penetration depth is determined by a (quasi-)static measurement at the measuring position, which corresponds to the lowest point in the longitudinal direction of the vapor capillary along the line section.

[0013] The longitudinal direction of the vapor capillary, along which the line scan is performed, typically runs parallel to the (instantaneous) feed direction of the welding process, along which the weld is formed. The position of the line scan perpendicular to the feed direction usually corresponds to the processing position or the point of impact of the laser beam perpendicular to the feed direction, which is generally located in the center of the weld, since the lowest point of the vapor capillary is usually in the center of the weld.

[0014] If a periodic movement of the laser beam is superimposed on the feed motion, the longitudinal direction of the vapor capillary may, depending on the selected process parameters, not run parallel to the feed direction, but rather follow the path of the periodic movement of the laser beam superimposed on the feed motion. In this case, the longitudinal direction of the vapor capillary corresponds to the direction of the instantaneous velocity vector of the laser beam's trajectory on the workpiece. The method described here can also be applied in this case.

[0015] In one variant, the measuring beam is directed at the measuring position for a period of at least 1 millisecond, preferably at least 5 milliseconds and no more than 100 milliseconds, when determining the penetration depth. A period on the order of a few milliseconds is generally sufficient to determine the penetration depth at the measuring position with sufficient accuracy. It is not necessary to measure a height profile of the vapor capillary along multiple longitudinal line scans in the static determination of the penetration depth described here.

[0016] In another variant, the procedure involves defining a processing position of the laser beam transversely to the feed direction of the laser welding process as a transverse measurement position for determining the weld penetration depth. In addition to defining the measurement position in the longitudinal direction of the vapor capillary, it is also necessary to define the measurement position in the transverse direction. As described above, the (instantaneous) longitudinal direction of the vapor capillary usually coincides with the (current) feed direction of the laser welding process. The vapor capillary typically has its deepest point at a position located in the center of the weld seam, and the processing position of the laser welding process, i.e., the point of impact of the laser beam, is also located in the center of the weld seam.By defining the measurement position as described here, which is typically carried out before steps (a)-(c), the line scan is therefore performed in the longitudinal direction in the middle of the weld seam, where the lowest point of the vapor capillary is usually located.

[0017] In one variant, the sequence of steps (a), (b), (c) is repeated several times during the laser welding process, preferably at predetermined time intervals. If the positional accuracy of the joining situation is particularly high, i.e., if the workpiece / fixture tolerance is low, it is not strictly necessary to perform seam position control as described below. In this case, the local target processing position set or predetermined before the laser welding process does not need to be corrected during the laser processing. The time intervals at which the weld penetration depth is determined in the manner described above are comparatively short and are less than one second, particularly on the order of a few milliseconds.

[0018] In another variant, the procedure additionally includes: (a') determining the position of a joint in the transverse direction perpendicular to a feed direction of the laser welding process by means of a line scan of the measuring beam perpendicular to the feed direction, and (b') defining the position of the joint in the transverse direction as the processing position of the processing laser beam in the transverse direction. It is understood that the line scan transverse to the feed direction is performed in advance of the processing position of the processing laser beam in order to determine the position of the joint transverse to the feed direction. In the area of ​​the joint, the line of the line scan is usually interrupted or displaced, which can be detected by the optical coherence tomograph and identified with the position of the joint transverse to the feed direction.

[0019] Seam tracking can be achieved by defining the (new) processing position determined in this way. For this purpose, the sequence of steps (a') and (b') is repeated several times during the laser welding process, or the sequence of steps (a') and (b') is performed (quasi-)continuously. By combining seam tracking and line scanning along the length of the vapor capillary, both the processing / welding position and the measurement position can be redefined practically continuously, thus creating optimal measurement conditions. It goes without saying that the sequence of steps (a') and (b') can be omitted if the positional accuracy of the joining process is particularly high, as described above.In a further development of this variant, the sequence of steps (a'), (b'), (a), (b), (c) during the laser welding process is repeated several times, preferably at predetermined time intervals, and particularly preferably at intervals of less than one second. In this case, the measuring position in the longitudinal direction of the vapor capillary is repeatedly determined throughout the entire laser welding process and corrected as necessary.

[0020] For the purposes of this application, a laser welding process is understood to be the formation of a (continuous) weld seam. In the case described here, the measuring position for determining the weld penetration depth is thus determined quasi-continuously during the formation of the weld seam and corrected if necessary. After steps (a') and (b') have been carried out, the correction of the measuring position in the transverse direction described above can be performed, i.e., the measuring position in the transverse direction is adjusted to the corrected processing position.

[0021] In a further development of this variant, the sequence of steps (a'), (b'), (a), (b), (c) is executed during time intervals in which the sequence of steps (a'), (b'), (a), (b), (c) is not being carried out. In this case, the sequence of steps (a'), (b'), (a), (b), (c) is generally performed at predetermined intervals of one or more seconds to check and, if necessary, correct the measurement position using the line scan in the longitudinal direction of the vapor capillary. During the periods between the line scans or the (re)establishment of the measurement position, the weld penetration depth is measured at the measurement position that was established in a previous line scan. The further development described here is particularly suitable for welds that are subject to tight component tolerances and high reproducibility.Increasing the time intervals between successive line scans makes it possible to increase the measurement accuracy of the weld penetration depth by using a higher number of measurement points from the vapor capillary, as well as to reduce the amount of data to be evaluated.

[0022] In an alternative variant, the laser welding process involves performing steps (a'), (b'), (a), (b), (c) once, followed by the repetition of the sequence of steps (a'), (b'), and (c), preferably at predetermined time intervals. Between the execution of steps (a') and (b') and step (c), the correction of the processing position in the transverse direction described above can be carried out; that is, the measuring position in the transverse direction is adjusted to the corrected processing position.

[0023] In the variant described here, the line scan and the determination of the measurement position in the longitudinal direction of the vapor capillary are only performed at the beginning of the welding process. Subsequently, the measurement position in the longitudinal direction of the vapor capillary is no longer corrected during welding, and the penetration depth is determined quasi-continuously at the measurement position in the longitudinal direction of the vapor capillary that was defined at the beginning of the welding process.

[0024] In another variant, the weld penetration depth determined in step (c) and / or the defined measurement position is compared with a reference value, and a deviation from the reference value is used to conclude that there is a disturbance in the laser welding process. With the help of repeated line scans or repeated determinations of the weld penetration depth, possible external process disturbances such as contamination, a shift in the focus position, or disturbances related to the workpiece clamping situation can be detected.

[0025] For the procedure described above, an optical coherence tomograph is used, which has a scanner optic for guiding or deflecting the measurement beam. The evaluation of the measurement data acquired by the optical coherence tomograph can be carried out in an evaluation unit, which is implemented as suitable hardware and / or software.

[0026] Further features and advantages of the invention will become apparent from the description, the claims, and the drawings. According to the invention, the features mentioned above and those further elaborated can each be used individually or in any suitable combination. The embodiments shown and described are not to be understood as an exhaustive list, but rather serve as examples for illustrating the invention.

[0027] Detailed description of the invention and drawing

[0028] Fig. 1 shows a cross-section through a workpiece in the area of ​​a vapor capillary generated by a laser beam during a laser welding process; Fig. 2a, b show a top view of the workpiece during and after a line scan for seam position control.

[0029] In the following description of the drawings, identical reference symbols are used for identical or functionally equivalent components.

[0030] Fig. 1 shows the production of a weld 14 in a workpiece 10. The workpiece 10 is irradiated by a laser beam 11. The laser beam 11 creates a vapor capillary 12 in the workpiece 10, in which the material of the workpiece 10 is in a gaseous or plasma state. The laser beam 11 moves across the stationary workpiece 10 in the X-direction (feed direction). Therefore, a melt pool 13 forms behind the vapor capillary, in which the previously gaseous material of the workpiece 10 has already transitioned into a liquid state due to cooling in the vapor capillary 12. Further away from the vapor capillary 12, the weld 14 is formed, in which the material of the workpiece 10 has already solidified.

[0031] To determine a weld penetration depth 17, the following procedure is performed: In step (a), a line scan in the longitudinal direction X of the vapor capillary 12 is performed using a measuring beam 15 of an optical coherence tomography (OCT) scanner (not shown). In step (b), based on the line scan and / or the depth information from the line scan, a position 16.X of the lowest point 16 in the longitudinal direction X of the vapor capillary 12 is determined. Also in step (b), the position 16.X of the lowest point 16 of the vapor capillary 12 is defined as the measuring position 16.X in the longitudinal direction X for determining the weld penetration depth 17. In step (c), the weld penetration depth 17 is then determined by aligning the measuring beam 15 with the defined measuring position 16.X in the longitudinal direction X of the vapor capillary 12. The distance of the measured depth in the Z-direction from the surface 10.1 of the workpiece 10 corresponds in the example shown to the penetration depth 17 of the weld seam 14. The measuring position 16.Y in the transverse direction Y perpendicular to the longitudinal direction X of the vapor capillary 12, which corresponds to the feed direction X in the production of the straight weld seam 14 of Fig. 1, corresponds here to a processing position 18 of the laser beam 11, more precisely a processing position 18. Y of the laser beam 11 in the transverse direction Y perpendicular to the feed direction X on the workpiece 10, as shown in Fig. 2b.

[0032] To determine the weld penetration depth 17, the measuring beam 15 is directed at the measuring position 16 in the example shown for a period of at least 1 millisecond or 5 milliseconds and no more than 100 milliseconds. With such a static measurement, the weld penetration depth 17 can be determined with sufficient accuracy.

[0033] If the weld position 14 does not need to be controlled because the positional accuracy of the joining situation is high, weld position control can be omitted during the laser welding process, i.e., during the formation of the weld 14. In this case, the sequence of steps (a), (b), (c) can be repeated several times during the laser welding process, usually at predetermined time intervals, to establish or correct the measuring position 16.X in the longitudinal direction X and to determine the penetration depth 17 at the respective, possibly corrected, measuring position 16.X.

[0034] In the event that seam position control is required, the following procedure can be used: In step (a'), a position 19.Y of a joining point 19 in transverse direction Y, perpendicular to the feed direction X of the laser welding process, is determined by means of a line scan of the measuring beam 15 transverse to the feed direction X, as shown in Fig. 2a. If a deviation of the position 18.Y' of the processing position in transverse direction Y from the position 19.Y of the joining point 19 in transverse direction Y is detected, the position 19.Y of the joining point 19 in transverse direction Y can be defined in step (b') as the (new) processing position 18.Y of the laser beam 11 in transverse direction Y. In this way, a lateral offset between position 19.Y of the joining point 19 in transverse direction Y and the machining position 18.Y' in transverse direction Y, indicated by an arrow in Fig. 2a, can be corrected, as shown in Fig. 2b.

[0035] If steps (a'), (b') are performed prior to steps (a), (b), (c), the line scan in the longitudinal direction X of the steam capillary 12 is performed in step (a) at the position of the joining point 19 on the workpiece 10, which has proven to be advantageous.

[0036] There are various ways to determine the temporal sequence of the steps (a'), (b'), (a), (b), (c) described above during the laser welding process. The temporal sequence can be determined, among other things, depending on the length of the weld seam 14 to be formed.

[0037] It is possible to repeat the sequence of steps (a'), (b'), (a), (b), (c) during the laser welding process at predetermined time intervals. In this case, after correcting the measurement position 16.Y in the transverse direction Y, the measurement position 16.Y in the longitudinal direction X is also corrected by performing a respective line scan in the longitudinal direction X of the vapor capillary 12.

[0038] The time interval in which the sequence of steps (a'), (b'), (a), (b), (c) is repeated can be less than one second. In this case, the sequence of steps (a'), (b'), (c) can be executed during time intervals in which the sequence of steps (a'), (b'), (a), (b), (c) is not performed. The sequence of steps (a'), (b'), (c), i.e., the seam position control and the measurement of the weld penetration depth 17, is performed more frequently in this case than the sequence of steps (a'), (b'), (a), (b), (c), in which the measuring position 16. X in the longitudinal direction X of the vapor capillary 12 is additionally determined and corrected if necessary. Such a procedure is particularly useful when a long weld seam 14 is formed during the laser welding process.

[0039] As an alternative to the procedure described above, the laser welding process can be carried out by performing steps (a'), (b'), (a), (b), (c) once, and then repeating the sequence of steps (a'), (b'), and (c) multiple times, usually at predetermined intervals. In this case, the measuring position 16.X in the longitudinal direction X of the weld seam 14 is determined or set once at the beginning of the welding process by a line scan. During the subsequent welding process, the seam position control is performed repeatedly, and the weld penetration depth 17 is repeatedly determined at the previously defined measuring position 16.X in the longitudinal direction X. This approach has proven particularly effective when forming a weld seam 14 on a workpiece 10 that has a high component tolerance.

[0040] The weld penetration depth 17 determined in step (c) and / or the measurement position 16 X in longitudinal direction X determined in step (b) can be compared with a reference value. A deviation from the reference value indicates a disturbance in the laser welding process, for example, contamination, a shift in the focus position of the laser beam 11, etc.

Claims

Patent claims 1. Method for performing a laser welding process with determination of a weld penetration depth (17), comprising the steps: (a) Performing a longitudinal line scan (X) of a vapor capillary (12) formed on the workpiece (10) during the laser welding process with a measuring beam (15) of an optical coherence tomograph, (b) Determining a position (16. X) of a lowest point (16) in the longitudinal direction (X) of the vapor capillary (12) using the line scan, defining the position of the lowest point (16) of the vapor capillary (12) as the measurement position (16. X) in the longitudinal direction (X) to determine the weld penetration depth (17), and (c) Determining the weld penetration depth (17) by aligning the measuring beam (15) with the specified measuring position (16. X) in the longitudinal direction (X).

2. Method according to claim 1, wherein the measuring beam (15) is directed towards the measuring position (16) for a period of at least 1 millisecond, preferably at least 5 milliseconds and not more than 100 milliseconds when determining the weld penetration depth (17).

3. Method according to claim 1 or 2, further comprising: defining a processing position (18. Y) of the laser beam (11) in the transverse direction (Y) to a feed direction (X) of the laser welding process as a measuring position (16. Y) in the transverse direction (Y) for determining the weld penetration depth (17).

4. Method according to one of the preceding claims, wherein during the execution of the laser welding process the sequence of steps (a), (b), (c) is repeated several times, preferably at predetermined time intervals.

5. Method according to any one of the preceding claims, further comprising the steps of: (a') Determining a position (19. Y) of a joining point (19) in the transverse direction (Y) perpendicular to a feed direction (X) of the laser welding process by means of a line scan of the measuring beam (15) perpendicular to the feed direction (X), as well as (b') Determining the position (19. Y) of the joining point (19) in the transverse direction (Y) as the processing position (18. Y) of the laser beam (11) in the transverse direction (Y).

6. Method according to claim 5, wherein the sequence of steps (a'), (b'), (a), (b), (c) is repeated several times during the laser welding process, preferably at predetermined time intervals, particularly preferably at intervals of at least one second.

7. The method of claim 6, wherein the sequence of steps (a'), (b'), (a), (b), (c) is performed during time intervals in which the sequence of steps (a'), (b'), (a), (b), (c) is not performed.

8. Method according to claim 5, wherein, during the execution of the laser welding process, steps (a'), (b'), (a), (b), (c) are performed once and subsequently the sequence of steps (a'), (b'), and (c) is repeated several times, preferably at predetermined time intervals.

9. Method according to one of the preceding claims, wherein the weld penetration depth (17) determined in step (c) and / or the specified measuring position (16. X, 16. Y) is compared with a reference value and a deviation from the reference value is used to infer a disturbance of the laser welding process.

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