DE102024001048B4 - Method for assessing a weld seam produced on a workpiece during a laser welding process using a laser beam and welding device for carrying out the method - Google Patents

Abstract

Method (1) for assessing a weld seam (5) produced on a workpiece (4) during a laser welding process (2) by means of a laser beam (3),
- wherein a welding device (6) with a laser beam optics (7) and with a measuring beam optics (8) designed to carry out a continuous optical coherence tomography and a workpiece (4), two or more joining partners (9, 10) stacked on top of one another, are provided in a predeterminable relative position to the welding device (6),
- wherein a laser welding process (2) is established, in which the welding device (6) is moved in a surface-parallel welding direction (11) relative to the workpiece (1), or vice versa, and a weld seam (5) is produced on the workpiece (4) by means of a laser beam (3) provided by means of the laser beam optics (7), wherein
- during the laser welding process (2), a continuous optical coherence tomography is carried out by means of the measuring beam optics (8), in the context of which capillary depth measurement values (12) of a vapor capillary (13) forming on the workpiece (4) during the laser welding process (2) and/or of a welding influence zone (14) surrounding the vapor capillary (13) and surface depth measurement values (17) serving as a reference for the capillary depth measurement values (12) of two surface sections (18) of a surface (19) of the workpiece (4) facing the measuring beam optics (8) are determined, and wherein
- continuously dividing the determined capillary depth measurement values (12) into separate capillary depth information blocks (20) and the determined surface depth measurement values (17) into separate surface depth information blocks (21), wherein a block size of these blocks (20, 21) is determined by a a predeterminable interval size (22), a predeterminable time period and/or a predeterminable weld seam length,
- wherein a statistical capillary characteristic (23) is derived from each of the capillary depth information blocks (20),
- wherein a statistical surface characteristic (24) is derived from each of the surface depth information blocks (21),
- wherein a ratio of the surface characteristic (24) to the capillary characteristic (23) is used as an evaluation criterion for assessing the weld seam (5) produced on the workpiece (4) in an interval (22) during the laser welding process (2) by means of the laser beam (3), characterized in that
- at least one or each limit value (25) is derived from the surface depth information blocks (21),
- wherein the derived at least one limit value (25) or the limit values (25) are continuously applied at intervals to the capillary depth information blocks (20), and those capillary depth measured values (12) of a capillary depth information block (20) which exceed the at least one limit value (25) or the limit values (25) serve as an evaluation criterion for assessing the weld seam (5) produced on the workpiece (4) in an interval (22) during the laser welding process (2) by means of the laser beam (3).

Description

The present invention relates to a method for assessing a weld seam produced on a workpiece during a laser welding process by means of a laser beam according to the preamble of claim 1. The invention further relates to a welding device according to the subject matter of claim 3, which is designed to carry out the proposed method.

A procedure of the type mentioned above is described in the document DE 10 2019 006 282 A1 described, whereby optical coherence tomography (so-called OCT measuring method) is used to assess a laser welding process, by means of which a 2- or 3-dimensional image of a surface of a workpiece can be generated as an imaging measuring method by emitting a measuring beam in a scanning movement onto the surface of the workpiece, detecting a return beam reflected from the surface and then comparing the return beam with a reference beam.

Furthermore, the scripture DE 10 2021 002 218 A1 A method for determining a distance in a machining process using a laser beam is known.

The PCT application WO 2014 / 138 939 A1 discloses a method and apparatus for monitoring and characterizing the dynamics of a phase change region generated during laser welding, particularly during keyhole welding using low-coherence interferometry. By directing a measurement beam to multiple locations within the phase change region and overlapping it with it, the spatial and temporal properties of the weld are determined in real time, such as keyhole depth, length, width, shape, and whether the keyhole weld is unstable, closing, or collapsing.

This information is important for determining the quality and material properties of a finished weld. It can also be used with feedback to modify the material modification process in real time.

The DE 10 2016 005 021 A1 In a method for measuring the depth of the vapor capillary during an industrial machining process using a high-energy beam, an optical measuring beam is directed at the bottom of a vapor capillary. An optical coherence tomograph generates interference spectra or other raw measurement data from reflections of the measuring beam. An evaluation device generates noise-reduced measurement data by jointly processing raw measurement data generated at different times using a mathematical operation. This operation can, in particular, be subtraction or division. This allows slowly changing interference to be eliminated. From the noise-reduced measurement data, a final value for the distance to the bottom of the vapor capillary is calculated using a filter, e.g., a quantile filter. From this, the depth of the vapor capillary can be determined, knowing the distance to a part of the workpiece surface not exposed to the high-energy beam.

DE 10 2017 117 413 A1 discloses a method for measuring the penetration depth, particularly during welding, drilling, or ablation using a working laser beam, in which a measuring light beam from a sensor system is coupled into a processing beam path of the working laser beam in a laser processing head and bundled or focused by focusing optics of the processing beam path into a measuring light spot on a surface of a workpiece. The measuring light beam reflected from the surface of the workpiece is then returned to a measuring and evaluation unit of the sensor system in order to obtain information about the distance of the workpiece surface from the laser processing head. In order to obtain a surface profile of the workpiece in the region of the vapor capillary, from which the position of the vapor capillary relative to the point of impact of the working laser beam can be determined, the position of the measuring light spot on the surface of the workpiece is guided over the vapor capillary both in the welding direction and transversely thereto. The measuring light spot is then moved to the determined position of the vapor capillary to measure the penetration depth during laser processing.

With the methods known from the state of the art, a laser welding process, in particular with regard to the achievable seam quality and certain achievable quality parameters, can only be assessed indirectly and without process reliability, which has an adverse effect on the quality of the manufactured welded products and is particularly undesirable in series production, since quality fluctuations in the manufactured welded products can result in high costs for scrap and rework.

The object of the invention is therefore to provide an improved or at least a different embodiment of a method for assessing a weld seam produced on a workpiece during a laser welding process using a laser beam. Furthermore, a welding device is to be specified by means of which the proposed method can be carried out.

In the present invention, this object is achieved by the subject matter of the independent claims. Advantageous embodiments are the subject matter of the dependent claims, the description, and the drawings.

The invention proposes a method for assessing a weld seam produced on a workpiece during a laser welding process using a laser beam. Within the scope of this method, a welding device with laser beam optics and with measuring beam optics configured to carry out continuous optical coherence tomography and a workpiece, in particular two or more joining partners stacked on top of one another, are initially provided in a predeterminable position relative to the welding device. Furthermore, it is provided that a laser welding process is established in which the welding device is moved relative to the workpiece in a surface-parallel welding direction, or vice versa, and a weld seam is produced on the workpiece using a laser beam provided by the laser beam optics. It is essential to the invention that continuous optical coherence tomography is carried out during the laser welding process using the measuring beam optics. Within the scope of or by means of optical coherence tomography, capillary depth measurements are determined for a vapor capillary that forms on the workpiece during the laser welding process, in particular on a surface of the workpiece facing the measuring beam optics, and/or for a welding influence zone surrounding the vapor capillary. The welding influence zone can have a first region section upstream of the vapor capillary in the welding direction and/or a second region section downstream of the vapor capillary in the welding direction, wherein the capillary depth measurements can be determined for the first region section and/or the second region section and/or the vapor capillary. Furthermore, within the scope of or by means of optical coherence tomography, surface depth measurements serving as a reference for the determined capillary depth measurements are determined for two different surface sections of the surface of the workpiece facing the measuring beam optics, said surface sections being located outside the vapor capillary and outside the welding influence zone.

The proposed method improves the assessability of the laser beam welding process, allowing the quality of the resulting weld seam to be assessed independently of the welding duration of individual welding process steps (e.g., ramp at the beginning of the welding process) and the welding position of the laser beam used for the laser welding process within the detection field of the measuring beam optics. Furthermore, the proposed method can be executed in real time, allowing quality improvement strategies to be implemented during the laser welding process. This allows, for example, immediate response to potential quality defects and the laser welding process to be controlled accordingly, e.g., by readjusting relevant physical parameters of the laser beam. Furthermore, the proposed method can reliably detect undesired joining gaps between the joining partners as well as the presence or absence of a connection between the joining partners, allowing the seam quality to be reliably assessed over the entire weld length.

In the context of the method, the measuring beam optics expediently emits at least one measuring beam or preferably a plurality of measuring beams onto the workpiece when carrying out the continuous optical coherence tomography, wherein the measuring beam or the plurality of measuring beams can each be realized as a laser beam with a wavelength different from the laser beam of the laser beam optics.

The said capillary depth measurements are, in particular, capillary or welding depths of the vapor capillary relative to the measuring beam optics. These are expediently the distances between a base of the vapor capillary and the measuring beam optics that can be determined during the laser welding process. Furthermore, the said surface depth measurements are, in particular, reference depths of the surface section of the workpiece relative to the measuring beam optics. These are expediently the distances between a surface section of the workpiece surface and the measuring beam optics that can be determined during the laser welding process.

The said capillary depth measurement values are expediently position-dependent via the weld seam position and/or dependent on the weld seam position in the detection field of the measuring beam optics.

It should also be noted that optical coherence tomography can be performed, in particular, in the form of line scans. Furthermore, it can be provided that said first surface section is positioned upstream of the first area section and/or the vapor capillary in the welding direction. Furthermore, said second surface section can be arranged transversely to the welding direction with respect to the first area section and/or the vapor capillary.

Conveniently, within the scope of the proposed method, it can further be provided that the determined capillary depth measurements are continuously divided into separate capillary depth information blocks and the determined surface depth measurements are continuously divided into separate surface depth information blocks. A block size or a block length of these blocks is determined by a predeterminable interval size, wherein such an interval size can in particular be a predeterminable time period and/or a predeterminable weld seam length. Furthermore, it is provided that a statistical capillary characteristic is then derived from each of the capillary depth information blocks, in particular percentiles, in particular 75% percentiles, mean values, or medians, and that a statistical surface characteristic is derived from each of the surface depth information blocks, in particular percentiles, in particular 75% percentiles, mean values, or medians. It is further provided that, to assess the weld seam created on the workpiece by means of the laser beam in an interval during the laser welding process, a ratio of the surface characteristic to the capillary characteristic is formed and used as an evaluation criterion. The present invention thus provides a novel approach for evaluating depth measurement data obtained using optical coherence tomography. Depending on the resulting ratio, the assessment of the weld seam can be output, for example, as "weld seam condition acceptable" or "weld seam condition not acceptable."

Furthermore, it can be provided that at least one limit value or limits are derived from the surface depth information blocks, which can in particular be a statistical limit value such as percentiles, in particular 75% percentiles, mean values, or medians. The derived at least one limit value or the derived limit values are then continuously applied at intervals to the capillary depth information blocks. Those capillary depth measured values of a capillary depth information block which exceed the at least one limit value or limits are used as an evaluation criterion for assessing the weld seam created on the workpiece during the laser welding process in an interval using the laser beam. By applying the derived limit value or limits to the capillary depth information blocks, a type of filtering of the determined capillary depth measured values is achieved, in the sense that only the capillary depth measured values relevant for assessing the weld seam in a respective interval are used as an evaluation criterion for assessing the created weld seam. In this way, an undesirable joint gap between the workpiece's joining partners can be detected. Furthermore, a sufficient or insufficient connection between the workpiece's joining partners can be detected.

It can expediently be provided that at least one physical characteristic of the laser beam provided by the laser beam optics is controlled during the laser welding process as a function of an assessment, provided by the method, of the weld seam produced on the workpiece during the laser welding process. This enables real-time control of the laser welding process based on an assessment, provided by the proposed method, of the weld seam produced during the laser welding process. This allows fluctuations in the quality of the weld seam to be corrected in situ, so to speak. Such a physical characteristic of the laser beam can be, for example, the laser beam intensity (W/ ^m² ) of the laser beam.

According to a further basic idea of the invention, a welding device is provided which has a laser beam optics and a measuring beam optics configured to carry out continuous optical coherence tomography, wherein the welding device is configured to carry out the method according to the preceding description.

Further important features and advantages of the invention emerge from the subclaims, from the drawings and from the associated description of the figures based on the drawings.

It is understood that the features mentioned above and those to be explained below can be used not only in the respective combinations specified, but also in other combinations or on their own, without departing from the scope of the present invention. Components mentioned above and those to be mentioned below of a higher-level unit, such as a device, a device, or an arrangement, which are designated separately, may form separate parts or components of this unit or be integral areas or sections of this unit, even if this is shown differently in the drawings.

Preferred embodiments of the invention are illustrated in the drawings and are explained in more detail in the following description, wherein the same reference numerals refer to the same or similar or functionally identical components.

• 1 schematisch einen Laserschweißvorgang an einem Werkstück, bei dem zwei aufeinandergestapelte Fügepartner des Werkstücks miteinander verschweißt werden,
• 2 eine Draufsicht auf das Werkstück aus 1 in einer dort durch einen Pfeil II angedeuteten Blickrichtung,
• 3 schematisch ein Diagramm von ermittelten Kapillartiefenmesswerten und Oberflächentiefenmesswerten,
• 4 schematisch ein Histogramm von Kapillartiefenmesswerten und Oberflächentiefenmesswerten,
• 5 schematisch ein weiteres Histogramm von Kapillartiefenmesswerten und Oberflächentiefenmesswerten,
• 6 schematisch ein Diagramm von ermittelten Kapillartiefenmesswerten und Oberflächentiefenmesswerten.

Showing schematically:

• 1 schematically shows a laser welding process on a workpiece in which two stacked joining partners of the workpiece are welded together,
• 2 a top view of the workpiece 1 in a direction indicated there by an arrow II,
• 3 schematic diagram of measured capillary depth values and surface depth values,
• 4 schematically a histogram of capillary depth measurements and surface depth measurements,
• 5 schematically another histogram of capillary depth measurements and surface depth measurements,
• 6 schematic diagram of determined capillary depth measurements and surface depth measurements.

The 1 and 2 show a preferred embodiment of a method 1 according to the invention, by means of which an assessment of a weld seam 5 produced on a workpiece 4 during a laser welding process 2 by means of a laser beam 3 can be carried out. First, a welding device 6 with laser beam optics 7 and with measuring beam optics 8 configured to carry out continuous optical coherence tomography, as well as a workpiece 4 in a predeterminable position relative to the welding device 6, are provided, wherein the workpiece 4 comprises, purely by way of example, two joining partners 9, 10 stacked on top of one another. Subsequently, a laser welding process 2 is established, in which the welding device 6 is moved relative to the workpiece 1 in a surface-parallel welding direction 11, which is indicated by an arrow. Alternatively, the workpiece 4 could be moved relative to the welding device 6. It is further provided that a weld seam 5 is produced on the workpiece 4 using a laser beam 3 provided by the laser beam optics 7, and the two stacked joining partners 9, 10 of the workpiece 4 are welded together. In order to reliably assess the seam quality of the produced weld seam 5 and certain quality parameters of the laser welding process 2, the method 1 provides for continuous optical coherence tomography to be carried out during the laser welding process 2 using the measuring beam optics 8. In this process, measuring beams 26 are projected onto the surface 19 of the workpiece 4, and capillary depth measurements 12 are determined for a vapor capillary 13 formed on the workpiece 4 during the laser welding process 2 and for a welding influence zone 14 surrounding the vapor capillary 13. The welding influence zone 14 has a first region section 15 upstream of the vapor capillary 13 in the welding direction 11 and a second region section 16 downstream of the vapor capillary 13 in the welding direction 11. In the present case, the capillary depth measurement values 12 are determined at the first region section 15, the second region section 16 and the vapor capillary 13, in particular linearly in the direction of the welding direction 11. Furthermore, surface depth measurement values 17 serving as a reference for the determined capillary depth measurement values 12 are determined from two surface sections 18a, 18b of the surface 19 of the workpiece 4 facing the measuring beam optics 8, said surface sections being arranged outside the vapor capillary 13 and outside the welding influence zone 14. The first surface section 18a can be arranged in the welding direction 11 in front of the welding influence zone 14 and/or the vapor capillary 13, and the second surface section 18b can be arranged transversely to the welding direction 11 next to the welding influence zone 14 and/or the vapor capillary 13. The determined capillary depth measurement values 12 and surface depth measurement values 17 are purely exemplary in the 3 illustrated diagram, where the abscissa of the diagram represents a seam position in mm and the ordinate of the diagram represents a depth in mm.

Furthermore, within the scope of method 1, it is provided that the determined capillary depth measurement values 12 are continuously divided into separate capillary depth information blocks 20 and the determined surface depth measurement values 17 are divided into separate surface depth information blocks 21, wherein a block size or a block length of these blocks 20, 21 is determined by a predeterminable interval size 22, in this case a weld seam length, which in 3 is indicated by vertical interval separators. Furthermore, what is 4 As indicated, a statistical capillary characteristic 23 is derived from each of the capillary depth information blocks 20, in this case a 75th percentile. Furthermore, a statistical surface characteristic 24 is derived from each of the surface depth information blocks 21, in this case also a 75% percentile. To assess the weld seam 5 produced on the workpiece 4 in an interval 22 during the laser welding process 2 by means of the laser beam 3, a ratio of the surface characteristic 24 to the capillary characteristic 23 is used as an evaluation criterion. Furthermore, additionally or alternatively, as in 5 is indicated, at least one limit value 25, in particular a 75% percentile, is derived from the surface depth information blocks 21, wherein the derived at least one limit value 25 is continuously applied at intervals to the capillary depths in information blocks 20 is applied in such a way that those capillary depth measurement values 12 of a capillary depth information block 20 which exceed the at least one limit value 25 and in 5 marked by an arrow A. The remaining capillary depth measurements 12, which are 6 are entered in a diagram whose abscissa represents a seam position in mm and whose ordinate represents a depth in mm, can then serve as an evaluation criterion for assessing the weld seam 5 produced on the workpiece 4 in an interval 22 during the laser welding process 2 by means of the laser beam 3.

Claims (3)

Method (1) for assessing a weld seam (5) produced on a workpiece (4) during a laser welding process (2) by means of a laser beam (3), - wherein a welding device (6) with a laser beam optics (7) and with a measuring beam optics (8) configured to carry out a continuous optical coherence tomography and a workpiece (4) and two or more joining partners (9, 10) stacked on top of one another in a predeterminable position relative to the welding device (6) are provided, - wherein a laser welding process (2) is established in which the welding device (6) is moved relative to the workpiece (1) in a surface-parallel welding direction (11), or vice versa, and a weld seam (5) is produced on the workpiece (4) using a laser beam (3) provided by means of the laser beam optics (7), - during the laser welding process (2) a continuous optical coherence tomography is carried out by means of the measuring beam optics (8), in the context of which capillary depth measurement values (12) of a Laser welding process (2) on the workpiece (4) forming a vapor capillary (13) and/or from a welding influence zone (14) surrounding the vapor capillary (13) and surface depth measurement values (17) serving as a reference for the capillary depth measurement values (12) are determined from two surface sections (18) arranged outside the vapor capillary (13) and outside the welding influence zone (14) of a surface (19) of the workpiece (4) facing the measuring beam optics (8), and wherein - the determined capillary depth measurement values (12) are continuously divided into separate capillary depth information blocks (20) and the determined surface depth measurement values (17) are continuously divided into separate surface depth information blocks (21), wherein a block size of these blocks (20, 21) is determined by a predeterminable interval size (22), a predeterminable time period and/or a predeterminable weld seam length, - wherein from the A statistical capillary characteristic (23) is derived from each of the capillary depth information blocks (20), - wherein a statistical surface characteristic (24) is derived from each of the surface depth information blocks (21), - wherein a ratio of the surface characteristic (24) to the capillary characteristic (23) is used as an evaluation criterion for assessing the weld seam (5) produced on the workpiece (4) in an interval (22) during the laser welding process (2) by means of the laser beam (3), characterized in that - at least one or in each case one limit value (25) is derived from the surface depth information blocks (21), - wherein the derived at least one limit value (25) or the limit values (25) are continuously applied at intervals to the capillary depth information blocks (20), and those capillary depth measured values (12) of a capillary depth information block (20) which exceed the at least one limit value (25) or the limit values (25) exceed, serve as an evaluation criterion for assessing the weld seam (5) produced on the workpiece (4) by means of the laser beam (3) in an interval (22) during the laser welding process (2). Method (1) according to one of the preceding claims, characterized in that at least one physical characteristic of the laser beam (3) provided by means of the laser beam optics (7) is controlled during the laser welding process (2) as a function of an assessment, provided by the method (1), of the weld seam (5) produced on the workpiece (4) during the laser welding process (2). Welding device (6) comprising a laser beam optics (7) and a measuring beam optics (8) configured to carry out continuous optical coherence tomography, wherein the welding device (6) is configured to carry out the method (1) according to one of the preceding claims.