Method for determining the penetration depth in a laser welding process

Abstract

The invention relates to a method for determining the weld penetration depth in a laser welding process, comprising the steps of: moving a laser beam and a workpiece (10) relative to each other along a feed direction to form a weld seam, and aligning a measuring beam of an optical coherence tomograph to a measuring position (16) on the workpiece (10) to determine the weld penetration depth. In this method, the movement of the laser beam and the workpiece relative to each other is superimposed with a periodic movement of the laser beam along a path of motion that extends at least in a transverse direction (Y) perpendicular to the feed direction (X), and the measuring beam is aligned to a measuring position (16.Y) on the workpiece (10) that is constant in the transverse direction (Y) perpendicular to the feed direction (X).

Description

Background of the invention

[0001] The invention relates to a method for determining the weld penetration depth in a laser welding process, comprising the steps of: moving a laser beam and a workpiece relative to each other along a feed direction to form a weld seam, and aligning a measuring beam of an optical coherence tomograph to a measuring position on the workpiece to determine the weld penetration depth.

[0002] In laser welding processes, measuring the weld penetration depth is necessary to guarantee a high-quality weld. If the weld penetration depth is insufficient, the weld will lack the required strength. Conversely, excessive weld penetration can lead to perforation. Several methods have already been proposed for measuring the weld penetration depth in laser welding processes.

[0003] 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 whose value depends, in a known manner, on the distance between the top of the workpiece and the lowest point of the vapor capillary, but deviates slightly from this distance.

[0004] If there is no relative movement between the laser beam and the workpiece, the point of impact of the laser beam typically corresponds to the lowest position of the vapor capillary. However, when the laser beam and the workpiece move relative to each other along a feed direction, the lowest position of the vapor capillary no longer corresponds to the instantaneous point of impact of the laser beam on the workpiece, but lies behind the point of impact. This means there is an offset between the point of impact of the laser beam and the lowest position of the vapor capillary, which is usually the point to which the measuring beam should be directed. The magnitude of this offset depends, among other things, on the feed rate and feed speed of the laser beam and is typically on the order of micrometers for linear welds.

[0005] 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.

[0006] It can be advantageous if a periodic or oscillating movement of the laser beam along a predetermined path is superimposed on the movement of the laser beam and the workpiece relative to each other, i.e. the feed movement, as is described, for example, in DE102017211982A1. Object of the invention

[0007] The invention is based on the objective of proposing a method for measuring a weld penetration depth representative of the weld seam in a laser welding process in which the laser beam performs a movement superimposed on the feed movement. Description of the invention

[0008] This task is solved, according to a first aspect, by a method of the type mentioned above, in which a periodic movement of the laser beam along a path extending at least in one transverse direction perpendicular to the feed direction is superimposed on the movement of the laser beam and the workpiece relative to each other, and in which the measuring beam is directed to a fixed measuring position on the workpiece that is constant in the transverse direction. The feed direction and transverse direction are understood to be the instantaneous feed direction and the instantaneous transverse direction, which, in the case of a non-straight weld seam, depend on the position along the weld seam and on the geometry of the weld seam, respectively.

[0009] During the periodic movement of the laser beam along its path, the penetration depth, or the depth of the resulting vapor capillary, is not necessarily constant. This is because, for example, in a path consisting solely of a oscillating or pendulum motion in the transverse direction, the local velocity of the laser beam at the reversal points in the transverse direction is lower than the average velocity along the path. This results in a variation in the fluence of the laser beam and, consequently, the penetration depth along the path.

[0010] Therefore, a weld penetration depth representative of the weld seam cannot be readily determined if the measuring beam or the measuring position of the measuring beam follows the periodic movement along the path of the laser beam in addition to the feed movement.

[0011] In the aspect of the invention described here, it is therefore proposed to align the measuring beam to a measuring position on the workpiece that is constant in the transverse direction perpendicular to the feed direction, i.e. the measuring beam does not follow the superimposed periodic movement of the laser beam along the path of motion, but the measuring position is constant or fixed with respect to the lateral position of the weld seam on the workpiece, but follows the feed movement in the feed direction.

[0012] In this way, the measuring beam can be aligned to a measuring position on the workpiece that is representative of the entire weld seam. It is understood that the measuring position is chosen so that it lies both within the transverse path of the laser beam and within the vapor capillary generated by the laser beam during laser welding. In the context described here, the width of the vapor capillary corresponds at least to the diameter of the set laser focus, and if the transverse path is small, e.g., on the order of the laser focus, then to the width of the transverse path. It is understood that the measuring position is also chosen so that it lies within the vapor capillary in the feed direction. In particular, the measuring beam can be aligned to a measuring position that corresponds to a predefined position along the path of the laser beam.

[0013] In one variant, the measuring beam is aligned to a measuring position in the transverse direction at which the laser beam exhibits maximum fluence as it moves along the path of motion. The fluence H of the laser beam depends on the local path velocity v of the laser beam along the path of motion and the beam diameter d. w of the laser beam on the workpiece and the local laser power P of the laser beam on the workpiece. The following applies: H = P / v / d w The greatest penetration depth occurs at the point of greatest fluence H, which is why the measuring beam should be positioned there. A constant laser power P is preferred when moving the laser beam along its path. av used, but this is not absolutely necessary.

[0014] In one variant, the laser beam exhibits a minimum path velocity at the measurement position along its trajectory. Assuming a constant laser power and a constant beam diameter, the fluence depends solely on the path velocity along the trajectory, with a minimum path velocity resulting in maximum fluence, provided that the same point is not traversed multiple times during the periodic movement along the trajectory (su).

[0015] In another variant, the measurement position in the transverse direction corresponds to the position of a lateral reversal point in the laser beam's path of movement in the transverse direction. At this lateral reversal point, the laser beam's path of movement in the transverse direction reverses. With a linear path of movement in the transverse direction, which, when combined with the feed motion, creates a sinusoidal or zigzag-shaped trajectory on the workpiece, the path speed decreases to a minimum at each lateral reversal point. Therefore, if welding is performed with constant laser power, the fluence is greatest at the lateral reversal points. Due to the low path speed, the weld penetration depth can be measured particularly accurately at these reversal points.

[0016] In another variant, the path of motion has at least one node, and the measurement position in the transverse direction corresponds to the position of the node in the transverse direction. In this variant, the path of the laser beam is two-dimensional, meaning it also extends in the feed direction. Nodes are positions along the path of motion where the path intersects itself. During the periodic movement, the laser beam passes over nodes of the path of motion multiple times, which increases the measurement frequency of the weld penetration depth. Due to the resulting heat buildup at the node and the multiple passes, the greatest energy input can accumulate at the node, thus resulting in the greatest weld penetration depth.The path of motion with the node can, for example, be a figure eight, where the node connects the two circles of the figure eight, which typically extend transversely from the node, so that the node forms the center of gravity of the path of motion.

[0017] In another variant, the superimposed movement on the workpiece forms a path of the laser beam with intersection points, and the measuring beam is aligned to a measuring position in the transverse direction that corresponds to an intersection point of the path. Even if the periodic path of the laser beam has no nodes, for example, because it is a circular path, the superposition with the feed movement of the laser beam on the workpiece can create a path of the laser beam with intersection points. This is the case, for example, when the path repeats endlessly at high speed and a slow relative movement (welding feed) is performed simultaneously. With a periodic path in the form of a circular path, for example, a spiral path is created on the workpiece with intersection points.

[0018] In another variant, the measurement position in the transverse direction corresponds to the center position of the weld seam, whereby in this variant the fluence of the laser beam is preferably kept constant along the path of motion of the laser beam. In this case, the measurement position in the transverse direction generally corresponds to the line of symmetry of the path of motion and is positioned in the center of the emerging weld seam, while the laser beam that creates the weld seam performs an oscillating movement between a left or right boundary in the transverse direction or a corresponding lateral reversal point.

[0019] In this particular case—though not exclusively—it is advantageous to maintain a constant fluence along the path of motion, for example, by appropriately modulating the laser beam power and adjusting it to the path speed, thereby producing a comparable weld penetration depth at all points along the path. With a constant fluence along the path of motion, the transverse measurement position does not necessarily have to correspond to the center position of the weld, but this is generally advantageous.

[0020] In another variant, the measuring beam is aligned to a transverse measuring position where the laser beam exhibits minimum fluence as it moves along its path. It can be advantageous to perform a measurement at the point of minimum fluence when measuring weld penetration depth to ensure that the minimum weld penetration depth requirement is met.

[0021] In a further aspect of the invention, in the method described above, a periodic movement of the laser beam along a path of motion is superimposed on a movement of the laser beam and the workpiece relative to each other, which extends at least in a transverse direction perpendicular to the feed direction, and the measuring beam is directed to a measuring position on the workpiece which follows the path of the superimposed movement of the laser beam on the workpiece and thus also the periodic movement of the laser beam and which is preferably offset by a constant amount from an impact position of the laser beam on the workpiece.

[0022] In this aspect of the invention, the vapor capillary typically follows the path of the superimposed movement of the laser beam on the workpiece. The vapor capillary is therefore smaller than in the first aspect of the invention and does not extend across the entire width of the laser beam's path in the transverse direction, but is limited to a comparatively small area of ​​the workpiece around the point of impact of the laser beam. With this aspect of the invention, the weld penetration depth can be measured at all locations along the laser beam's path on the workpiece, and a resulting weld cross-section can be reconstructed. The result corresponds to the cross-sectional analysis obtained through destructive weld testing.

[0023] In this aspect of the invention, the measuring beam follows the laser beam and describes the same trajectory as the laser beam, but at a typically constant, defined distance. However, it is also possible for the measuring position of the measuring beam to coincide with the point of impact of the laser beam on the workpiece. As a rule, the constant amount by which the measuring position is offset from the point of impact of the laser beam is greater than zero. The distance of the offset between the point of impact and the measuring position is measured along the trajectory of the superimposed motion. In the case that no abrupt changes in the direction of the trajectory occur, as is the case, for example,In the case of a zigzag-shaped trajectory, the direction of the offset between the measuring position and the impact position on the workpiece generally corresponds essentially to the direction of the velocity vector of the impact point on the workpiece along the trajectory of the superimposed motion.

[0024] Whether a large or small vapor capillary forms on the workpiece depends on the process parameters and is known in advance, i.e., before the welding process is carried out. Typically, with a comparatively large focus diameter of the laser beam on the workpiece, a large vapor capillary is formed that only follows the feed motion, while with a comparatively small focus diameter of the laser beam on the workpiece, a small vapor capillary is formed that also follows the periodic motion of the laser beam.

[0025] The constant value or distance is typically chosen so that the measuring position coincides with the position of the maximum weld penetration depth or corresponds to the lowest point of the vapor capillary. This procedure corresponds to defining the measuring position for a weld seam produced without the superimposed periodic movement of the laser beam.

[0026] Both aspects of the invention typically require that the measuring beam or the measuring position of the measuring beam can be determined or selected independently of the laser beam's point of impact, even when the laser beam is superimposed. The laser beam and the measuring beam are typically guided and deflected separately via a scanner optic of the process optics and a scanner optic of the measuring optics, respectively, and then merged after passing through their respective scanner optics, so that they together pass through one or more optical elements, e.g., a common focusing optic. For the independent alignment of the laser beam and the measuring beam, the scanner optics of the process optics and the scanner optics of the measuring optics must be independently programmable or controllable. Data exchange between the control systems of the process optics and the measuring optics must also be possible.

[0027] In the second aspect of the invention, the control or movement of the scanner optics of the measuring optics follows the movement of the scanner optics of the welding optics. In the first aspect of the invention, the movement of the scanner optics of the measuring optics generates a movement in the opposite direction to the movement of the scanner optics of the process optics, so that an almost complete compensation of the periodic movement of the laser beam occurs and the measuring beam does not follow the periodic movement of the laser beam along the path of motion.

[0028] The measuring beam and the laser beam typically have different wavelengths. Therefore, either the focusing optics, for example, an F-theta lens, which focuses the laser beam and is usually also traversed by the measuring beam, should be chromatically corrected, or the chromatically induced positional deviation of the measuring beam and the laser beam relative to each other should be compensated for by the movement of the measuring beam. The measuring beam and the laser beam should also be able to be positioned relative to each other using an adjustment device. The diameter of the measuring beam is typically less than or equal to the diameter of the laser beam at the focus or point of impact on the workpiece. The diameter of the measuring beam is less than or equal to the diameter of the vapor capillary.

[0029] It is understood that a deep penetration welding process with a vapor capillary (keyhole) formed on the workpiece is required for the execution of the process. The aspect ratio between the depth of the vapor capillary and its diameter should be greater than 0.1, preferably greater than 0.5. For generating the periodic movement of the laser beam along the path of motion, the process optics, which guide the laser beam onto the workpiece, incorporate a scanner optic, as described above. It is possible to use a robot system on a Cartesian machine tool to generate the superimposed motion. For system solutions with a moving workpiece, stationary process optics can also be used. In this case, the feed motion can be achieved solely by the movement of the workpiece. The laser beam should have a beam quality of M 2The laser beam must have a coefficient of amplitude greater than 1 and a power output between 0.5 and 20 kW. The intensity profile of the laser beam can be a Gaussian or a tophat distribution, but it is also possible to perform laser beam shaping, as in BrightLine Weld.

[0030] 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. Detailed description of the invention and drawing Fig. Figure 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. Figures 2a-c show the trajectories of a periodic movement of the laser beam superimposed on a feed motion; Fig. Figures 3a and 3b show the fluence of the laser beam on the workpiece when the feed motion is superimposed on the motion path of Fig. 2a; Fig. Figures 4a and 4b show the fluence of the laser beam on the workpiece when the feed motion is superimposed on the motion path of Fig. 2b; Fig. Figures 5a-c show the point of impact of the laser beam on the workpiece when the feed motion is superimposed on the motion path of Fig. 2a and a measuring position of a measuring beam that is constant in the transverse direction; Fig. 6a-c show representations analogous to Fig. 5a-c, where the measuring position of the measuring beam coincides with the impact position of the laser beam; Fig. 7a-c show representations analogous to Fig. 6a-c, in which the measuring position of the measuring beam follows the point of impact of the laser beam at a constant distance.

[0031] In the following description of the drawings, identical reference symbols are used for identical or functionally equivalent components.

[0032] Fig. Figure 1 illustrates 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 within 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 within the vapor capillary 12. Further away from the vapor capillary 12, the weld 14 forms, in which the material of the workpiece 10 has already solidified.

[0033] 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 lowest 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 lowest point of the vapor capillary 12 thus corresponds to a measuring position 16 of the measuring beam 15. In the example shown, the distance of the measured depth from the surface 10.1 of the workpiece 10 corresponds to the penetration depth 17 of the weld seam 14.

[0034] At the in Fig. In the example shown, a straight weld seam 14 is produced which extends in the feed direction, i.e. in the X direction, whereby the laser beam 11 is moved only in the feed direction X and not in a transverse direction Y perpendicular to the feed direction X.

[0035] It is also possible to move the laser beam 11 not only in the feed direction X over the workpiece 10 when producing a straight weld seam 14, but also to superimpose a periodic movement of the laser beam 11 along a path of motion 18, the geometry of which can be designed in different ways.

[0036] Fig. Figure 2a shows a linear motion path 18 of the periodic motion, which extends only in the transverse direction Y perpendicular to the feed direction X, but not in the feed direction X. During the periodic motion, the laser beam 11 oscillates along the linear motion path 18 between two lateral reversal points U1, U2. Fig. Figure 2b shows a circular path of motion 18, which extends in addition to the transverse direction Y also in the feed direction X. Fig. Figure 2c shows a motion path 18 in the shape of a figure eight, which has a node K at which the motion path 18 intersects itself.

[0037] During the periodic motion of the laser beam 11 along the path 18, the local orbital velocity of the laser beam 11 can vary. For example, the orbital velocity during the Fig. The rectilinear path of motion 18 shown in Figure 2a is minimal at the turning points U1 and U2. This results in the fluence H of the laser beam 11 varying along the path of motion 18 for constant power and constant beam diameter.

[0038] Fig. Figures 3a and 3b show a three-dimensional representation and a contour line representation, respectively, of the position-dependent distribution of the fluence H on the workpiece 10 during the movement of the laser beam 11 along the in Fig. The trajectory curve 18 shown in Figure 2a is superimposed with a feed movement at a constant speed in the feed direction X, which is 10 m / min. The zero point in the transverse direction Y corresponds to the center position Y. M of the straight weld seam 14 in the transverse direction. Regarding the units of the representations of Fig. 3a,b both in the X and Y directions are in micrometers. The reversal points U1, U2 are each approximately 0.4 mm away from the center of the weld seam 14, the frequency of the periodic movement is 600 Hz, and the power of the laser beam 11 along the path of movement 18 was set to be constant.

[0039] As particularly evident in the presentation of Fig. As can be clearly seen in Figure 3b, the laser beam 11 follows an essentially sinusoidal or zigzag-shaped path on the workpiece 10. As shown in Figure 3b, the laser beam 11 follows an essentially sinusoidal or zigzag-shaped path on the workpiece 10. Fig. As can be clearly seen in 3a,b, the laser beam 11 has the maximum fluence H at the reversal points U1, U2 of the periodic motion.

[0040] To determine the weld penetration depth 17, the following remains in the Fig. In the example shown in 3a,b, the measuring position 16.Y of the measuring beam 15 in the transverse direction Y is constant or fixed to the weld seam 14 and corresponds to the position of one of the two reversal points U1, U2 of the movement path 18 in the transverse direction Y, as shown in Fig. 3b is indicated.

[0041] Fig. 4a,b show representations analogous to Fig. 3a,b, in which the laser beam 11, during the periodic motion, follows the linear path of motion 18 from Fig. 3a,b of the circular path of motion 18 of Fig. 2b follows. The frequency of the periodic motion is that shown in Fig. The example shown in 4a,b is at 300 Hz; otherwise, the parameters match the values ​​of Fig. 3a,b agree. As in particular in Fig. As can be seen in 4a, the fluence H is approximately in the middle of Y. M The measurement position 16.Y of the measuring beam 15 is maximal at the weld seam 14 (i.e., at Y = 0), since the superimposed movement forms an intersection point of the laser beam 11's path on the workpiece 10 there. The measurement position 16.Y of the measuring beam 15, which is fixed in the transverse direction Y, therefore corresponds approximately to the center position Y. M the weld seam 14 matches. Also in the Fig. In the movement path 18 shown in 2c, it is advantageous if the measuring position 16.Y of the measuring beam 15 coincides with the position of the node K in the transverse direction Y, which also corresponds to the center position Y. M which corresponds to weld 14.

[0042] Fig. Figures 5a-c show a spiral trajectory 19 of an impact position 20 of the laser beam 11 on the workpiece 10, which is formed when the linear feed movement in feed direction X is superimposed with the in Fig. The periodic motion shown in 2b along the circular path of motion 18 is created. The outline of the vapor capillary 12 is also visible; in this example, it is slightly wider in the transverse direction Y than the width of the superimposed path of motion 18 in the transverse direction Y. In the Fig. In the example shown in 5a-c, the coordinate of measuring position 16 in the feed direction X corresponds to the coordinate of the impact position 20 in the feed direction X. The measuring position 16.Y in the transverse direction Y corresponds to the center position Y. M of weld 14 in transverse direction Y. In the case of the Fig. In the example shown in 5a-c, the fluence H of the laser beam 11 is kept constant along the circular path of motion 18 by modulating the power of the laser beam 11 as a function of position so that the change in the path velocity of the laser beam 11 along the path of motion 18 is compensated and the fluence H along the path of motion 18 is essentially constant.

[0043] Alternatively, the measuring position 16.Y in the transverse direction Y can be selected such that it corresponds to the position of an intersection point 22 of the intersecting spiral path 19 on the workpiece 10, since the fluence H is at its maximum at each intersection point 22 due to the laser beam 11 repeatedly passing over it. Alternatively, the measuring beam 15 can be aligned to a constant, fixed measuring position 16.Y in the transverse direction Y, at which the laser beam 11 exhibits a minimum fluence H when moving along the path 18. This ensures that the minimum weld penetration depth requirement 17 is met.

[0044] Fig. 6a-c and Fig. Figures 7a-c show the trajectory 19 of a laser beam 11 during a welding process in which the vapor capillary 12 is significantly smaller than in the one described in Figure 7a-c. Fig. The example described in 5a-c illustrates this. In this case, the movement of the measuring beam 15 follows the movement of the laser beam 11 along the circular path of motion 18. The measuring position 16 can be selected such that it coincides with the point of impact 20 of the laser beam 11 on the workpiece 10, as described in 5a-c. Fig. Figures 6a-c are shown. Preferably, the measuring position 16 is selected such that it is offset by a constant amount A relative to the impact position 20 of the laser beam 11 on the workpiece 10, or is positioned at a constant distance A from the impact position 20, as shown in Figure 6a-c. Fig.Figures 7a-c illustrate this. Since the measuring beam 15 follows the path of motion 18 of the laser beam 11, the measuring position 16 always remains within the vapor capillary 12. The constant distance A is typically chosen such that the measuring position 16 corresponds to the lowest point of the vapor capillary 12. The direction of the offset typically corresponds essentially to the instantaneous direction of the velocity vector of the point of impact 20 on the workpiece 10.

[0045] The laser beam 11 and the measuring beam 15 are typically deflected by means of two different scanner optics. Their movement and control must be coordinated so that the measuring position 16 and the impact position 20 on the workpiece 10 can be set independently of each other, even if the laser beam 11 and the measuring beam 15 are guided onto the workpiece 10 via at least one common optical element. For this purpose, it is generally necessary to take the periodic movement of the laser beam 11 into account when controlling the measuring position 16, or to synchronize both movements, or to compensate for the movement of the laser beam 11 completely or partially. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 102018124208B4

[0005] DE 102017211982A1

[0006]

Claims

[1] Method for determining a weld penetration depth (17) in a laser welding process, comprising the steps: Moving a laser beam (11) and a workpiece (10) relative to each other along a feed direction (X) to form a weld seam (14), as well as Aligning a measuring beam (15) of an optical coherence tomograph to a measuring position (16) on the workpiece (10) to determine the weld penetration depth (17), characterized by , that the movement of the laser beam (11) and the workpiece (10) relative to each other is superimposed with a periodic movement of the laser beam (11) along a path of motion (18) which extends at least in a transverse direction (Y) perpendicular to the feed direction (X), and that the measuring beam (15) is aligned to a measuring position (16.Y) constant in the transverse direction (Y) on the workpiece (10). [2] Method according to claim 1, wherein the measuring beam (15) is directed to a measuring position (16.Y) in the transverse direction (Y) at which the laser beam (11) has a maximum fluence (H) when moving along the path of motion (18). [3] Method according to claim 1 or 2, wherein the laser beam (11) has a minimum path velocity along the path of motion (18) at the measuring position (16.Y). [4] Method according to one of the preceding claims, wherein the measuring position (16.Y) in the transverse direction (Y) corresponds to the position of a lateral reversal point (U1, U2) of the path of movement (18) of the laser beam (11) in the transverse direction (Y). [5] Method according to one of the preceding claims, wherein the path of motion (18) has at least one node (K) and wherein the measuring position (16.Y) in the transverse direction (Y) corresponds to the position of the node (K) in the transverse direction (Y). [6] Method according to one of the preceding claims, wherein during the superimposed movement on the workpiece (10) a path (19) of the laser beam (11) is formed which has intersection points (22), and wherein the measuring beam (15) is directed to a measuring position (16.Y) in the transverse direction (Y) which corresponds to a position of an intersection point (22) of the path (19) in the transverse direction (Y). [7] Method according to one of the preceding claims, wherein the measuring position (16.Y) is in the transverse direction (Y) of a center position (Y M ) of the weld seam (14), wherein preferably the fluence (H) of the laser beam (11) is kept constant along the path of motion (18) of the laser beam (11). [8] Method according to claim 1, wherein the measuring beam (15) is directed to a measuring position (16.Y) in the transverse direction (Y) at which the laser beam (11) has a minimal fluence when moving along the path of motion (18). [9] Method according to the preamble of claim 1, characterized by , that the movement of the laser beam (11) and the workpiece (10) relative to each other is superimposed with a periodic movement of the laser beam (11) along a path of motion (18) which extends at least in a transverse direction (Y) perpendicular to the feed direction (X), and that the measuring beam (15) is directed to a measuring position (16) on the workpiece (10) which follows the path of the superimposed movement of the laser beam (11) on the workpiece (10) and which is preferably offset by a constant amount (A) from an impact position (20) of the laser beam (11) on the workpiece (10).

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