Methods for evaluating laser-welded hybrid joints

Abstract

Method for evaluating laser-welded hybrid joints during laser beam welding of an upper joining partner (11) with at least one lower joining partner (12), wherein the upper joining partner and the at least one lower joining partner consist of different materials, and wherein depth information in a keyhole (22) formed by the laser beam welding and / or in an area surrounding the keyhole (22) is evaluated using optical coherence tomography, wherein during a welding process in which a welding laser beam (34) of a laser welding device (30) is moved in a welding feed direction x (51), at least one OCT measuring beam (41) of an OCT measuring beam unit (40) continuously and repeatedly scans the area of ​​the keyhole (22) at at least two different depth levels (53) and thereby obtains depth z measurements in the depth direction (53) from at least one reflected OCT measuring beam (42, 43).the intensity (I) and the position x in the welding feed direction (51) are jointly determined in a positionally accurate manner, characterized in that the materials of the upper joining partner (11) and at least one lower joining partner (12) as well as a degree of mixing of the materials are classified from the determined measured values.

Description

The invention relates to a method according to the features of claim 1 for evaluating mixed laser welds.It is known that when connecting components by means of laser beam welding methods, in particular in the case of two overlapping joining plates, reliable quality assurance is essential for the quality of the weld. It is also known that with previous quality assurance methods, no direct conclusion can be generated on the seam connection in the joining plane during laser beam welding. Thus, a sufficient quality statement about the connection quality and connection strength of welded seams cannot be made disadvantageously.It is also known that in laser beam welding in mass production, the connection of two components, for example in an overlapping arrangement, in particular in the case of components of different materials, cannot be determined during the joining process in a disadvantageous manner in a completely reliable manner.Conventional inline QS methods (such as acoustics, process lights,... ) cannot ensure reliable detection. The result is increased process and quality fluctuations and reject rates.The OCT technology (OCT: Optical Coherence Tomography, dt.: Optical coherence tomography) provides conclusions about the depth of the vapor capillary during the joining process with the aid of the capillary depth measurement. However, the pure depth signal obtained in this way is disadvantageously not sufficient for process-safe detection of the binding in mixed compounds.A downstream seam test (offline QS), such as e.g. visual test (coloring of the upper bead of the seam), destructive material test (ground sections, chisel testing) and CT, is known from the prior art. However, pure capillary depth measurements with OCT are often not sufficient in mixed compounds.Camera-based systems are known from the prior art; these provide findings on the basis of the measurement of the upper seam bead and / or the capillary width at the height of the component surface, but disadvantageously no information about the joining plane between the at least two components to be joined and overlapping. A further disadvantage of camera-based systems is the often lack of coaxiality, as a result of which distortion occurs in the image plane and leads to misinterpretations.DE 10 2019, 006 282 A1 discloses a method for evaluating the process during laser beam welding of an upper joining partner to at least one lower joining partner, wherein height information is evaluated by means of optical coherence tomography in a keyhole formed by the laser beam welding and / or in a surrounding region of the keyhole, wherein the height information signals of the optical coherence tomography are evaluated, which originate from a characteristic depth which is to be assigned to an upper side of the at least one lower joining partner.DE 10 2024 001 028 A1 discloses a method for quality assurance of a weld joint produced in a laser welding method between at least two joining partners, wherein during the production of a weld joint a penetration depth of the laser beam is measured by means of optical coherence tomography using a measurement beam, whereby depth information is evaluated in a keyhole formed by the laser beam and in a surrounding region of the keyhole. According to the invention, measurement data determined on the basis of the measurement beam are assigned to at least one time interval and / or weld seam length interval during the generation of the weld seam, the assigned measurement data are divided at least into depth measurement data acquired within the keyhole, and the depth measurement values obtained from the depth measurement data are divided into an equal number of depth intervals and a characteristic feature and / or a characteristic depth in the region of the weld seam is determined and / or it is determined whether the generated weld seam connection has a defect by means of its associated signal intensity, an associated number of measurement signals and / or from a product of the respective signal intensity and the respectively associated number of measurement signals in the same depth interval.It is an object of the invention to provide a device and a method for reliably evaluating mixed laser welding joints, i.e. joints of components of different materials.The object is achieved by a method for evaluating mixed laser welding joints during laser beam welding of an upper joining partner with at least one lower joining partner, wherein the upper joining partner and the at least one lower joining partner consist of different materials and wherein depth information is evaluated by means of optical coherence tomography in a keyhole formed by the laser beam welding and / or in a surrounding region of the keyhole. The keyhole which is formed by the welding laser beam in the region of incidence of the welding laser beam on the joining partners, in particular by melting a base material of the joining partners and at least partially evaporating and / or displacing a melt produced thereby, is also referred to as a capillary or vapor capillary.In such a method, it is proposed that during a welding process, in which a welding laser beam of a laser welding device is moved in a welding feed direction x, at least one OCT measurement beam of an OCT scanner continuously repeatedly scans the region of the keyhole at at least two different depth levels and in the process measurement values of the depth z in the depth direction, the intensity I and the position x in the welding feed direction are determined together with accurate positioning.Advantageous embodiments of the invention are the subject matter of the dependent claims.The method according to the invention is advantageously designed in that the materials of the upper joining partner and at least one lower joining partner and a degree of intermixing of the materials are classified from the measured values determined. A classification is to be understood here in particular to identify the materials of the joining partners or / and to distinguish them from one another and / or to identify the intensity of the thorough mixing of the materials or / and to derive a statement regarding the quality of the connection of the joining partners to one another or / and to assign the statements mentioned above to a depth of the measurement or / and to a position of the measurement along the welding feed direction x.The classification can advantageously be carried out with the aid of statistical evaluations and / or AI algorithms. It can furthermore advantageously be provided that statistical evaluations are carried out on the basis of a histogram of the measurement point accumulation as a function of the measurement position in the region of the joining plane to be expected.It can further advantageously be provided that process parameters of the weld are influenced by control technology depending on the classification of the measured values. For example, a control of various process parameters for increasing or reducing the joint connection can be carried out when a specific critical state is detected (e.g. too low or high intermixing).In certain embodiments of the method according to the invention, it can be provided, for example, that measured values in the region of the keyhole are determined at two and / or more discrete depth levels. This can be realized, for example, by directing a first OCT measurement beam to a first height level and directing at least one second OCT measurement beam to at least one second height level within the keyhole, and acquiring a measurement value of the intensity from the respective reflected OCT measurement beam with an associated measurement value of the depth and a measurement value of the position in the welding feed direction x and storing it, for example, and / or classifying it immediately. Alternatively, a single OCT measurement beam can be directed back and forth between two or more depth levels in such a way that a measurement value of the intensity of the reflected OCT measurement beam is recorded from each of these height levels with an associated measurement value of the depth and a measurement value of the position in the welding feed direction x and is stored or classified immediately, for example.In certain embodiments of the method according to the invention, it can be provided, for example, that measured values in the region of the keyhole are ascertained continuously along a scanning direction. For this purpose, for example, an OCT measurement beam can be guided back and forth between a minimum and a maximum depth level within the keyhole in a plane spanned by the depth direction z and the welding feed direction x, and in the process measurement values of the intensity of the reflected OCT measurement beam can be continuously recorded with associated measurement values of the depth and measurement values of the position in the welding feed direction x and stored, for example, or classified immediately.For carrying out the method according to the invention, a laser welding device is advantageously suitable which comprises a welding laser for generating a welding laser beam and a welding optics for aligning the welding laser beam, wherein the device further comprises an OCT measurement beam unit which generates at least one OCT measurement beam and continuously repeatedly deflects in a linear manner or in a circular manner (for example in the case of curved seam geometries) for the projection of measurement points or a measurement line, and the OCT measurement beam unit is arranged such that the OCT measurement beam is guided through the welding optics, through which the welding laser beam is also guided.With the method according to the invention, an approach can be implemented by OCT technology in order to distinguish the two materials from one another and / or to detect the intensity of the intermixing of the two materials with the aid of the intensity of a depth measurement value of an emitted OCT measurement beam and detected measurement signal from a reflected OCT measurement beam in the region of the vapor capillary depending on the measurement position.In this case, the different reflection properties of the melt-shaped materials can be exploited. Thus, for example, copper has a high reflection, while steel (e.g. HILUMIN® a diffusion-annealed strip steel with electrolytic nickel coating) has a lower reflection and consequently a lower intensity of the reflected measurement signal.An exemplary process engineering procedure can be described as follows:OCT measurements are carried out in the region of the vapor capillary during the joining process with or without scanning movement of the measurement beam around the tool center point.The measurement data (for example depth, intensity and measurement position) thus determined are determined with true position during the welding.On the basis of measured values and / or the distribution of the intensity as a result of the back reflection of the OCT measurement beam, the materials and / or the level of the intermixing are classified with the aid of algorithms.The evaluation of the measurement results determined is effected statistically and / or with the aid of Kl-based algorithms (e.g. machine learning).If necessary, various process parameters are controlled to increase or reduce the joint connection when a specific critical state is detected (e.g. when mixing is too low or too high).The method according to the invention has in particular the following advantages:The method enables the detection of different process states and an existing or non-existing joint connection in mixed connections (e.g. copper and steel (hilumin), copper and aluminum, etc.).It enables a targeted influencing of the bond strength by a lower or higher mixing of the different materials.The method also opens up the possibility of online process control, by means of which online repair solution becomes possible, and a reduction of waste and a higher flexibility of the OCT measurement data evaluation compared to mixed connections.The method can be carried out using current system technology and is particularly suitable for systems having a scanner optical unit.It has particular suitability for producing connections of electrical / current-carrying components (e.g. contacting battery cell to cell connector).With the aid of the method according to the invention, an improvement in the connection quality can be achieved, since it has a high flexibility with respect to material thicknesses (both in the case of thin and thick sheet metal connections) and with respect to joint joints (overlap joint, butt joint, fillet joint etc.).Exemplary embodiments of the invention are explained in more detail below with reference to drawings.The following are shown: FIG. 1 shows a schematic illustration of an exemplary laser welding device, FIG. 2 shows a schematic illustration of the welding point, and FIG. 3 shows exemplary histograms of the measured values determined in a method according to the invention.FIG. 1 is a schematic diagram of the laser welding apparatus 30 used, wherein the x-axis is a welding feed direction 51 and the z-axis is a depth direction 53. The y-axis is a transverse direction 52; FIG. 2 shows a vertical longitudinal section in an x-z plane through the ongoing weld.An upper joining partner 11 and a lower joining partner 12 lie overlapping on one another. To generate a welding laser beam 34, the laser welding device 30 comprises a welding laser 31 which generates laser light and feeds this laser light to a welding optics 32 arranged above the arrangement of the two joining partners 11, 12. By deflecting mirrors 33 arranged in the welding optics 32, a welding laser beam 34 is directed onto the joining partners 11, 12. The high-energy radiation melts the material of the joining partners 11, 12. A bath of melt 21 is formed around the point of impingement of the welding laser beam 34, which subsequently solidifies to form a weld seam 20. In the narrowest region around the point of incidence of the welding laser beam 34, a keyhole 22 is formed, which is also referred to as a vapor capillary.The welding optics 32 and / or the entire robot- or gantry-axis-controlled welding system is continuously moved further in the welding feed direction 51 during welding, so that a weld seam 20 running in the welding feed direction 51 is formed.The laser welding apparatus 30 further includes an OCT measurement beam unit 40 that generates an OCT measurement beam 41 that performs depth measurements at multiple discrete height levels within the keyhole 22. For this purpose, the OCT measurement beam 41 is continuously moved repeatedly back and forth, as shown by the double arrows in FIGS. 1 and 2. As a result, the OCT measurement beam 41 is reflected by the melt 21 at at least two depth levels. In FIGS. 1 and 2, this is illustrated by way of example by the first reflected OCT measurement beam 42 and the second reflected OCT measurement beam 43.FIG. 3 shows exemplary histograms of the measured values determined in a method according to the invention. The values determined by the measurements are shown in the left histogram as value pairs of the scan position x 51 and the depth z 53 and in the right histogram as value pairs of the intensity I and the depth z 53. The data determined in this way can advantageously be used to influence process parameters of the weld by means of regulation technology depending on the classification of the measured values.List of reference characters11 Upper joining partner 12 Lower joining partner 20 Weld seam 21 Melt 22 Keyhole 30 Laser welding device 31 Welding laser 32 Welding optical unit 33 Deflection mirror 34 Welding laser beam 40 OCT measurement beam unit 41 OCT measurement beam 42 First reflected OCT measurement beam 43 Second reflected OCT measurement beam 51 Welding feed direction 52 Transverse direction 53 Depth direction I Intensity

Claims

Method for evaluating mixed laser welding joints during laser beam welding of an upper joining partner (11) with at least one lower joining partner (12), wherein the upper joining partner and the at least one lower joining partner consist of different materials and wherein depth information is evaluated by means of optical coherence tomography in a keyhole (22) formed by the laser beam welding and / or in a surrounding region of the keyhole (22), wherein during a welding in which a welding laser beam (34) of a laser welding device (30) is moved in a welding feed direction x (51), At the same time, at least one OCT measurement beam (41) of an OCT measurement beam unit (40) continuously repeatedly scans the region of the keyhole (22) at at least two different depth levels (53) and, in the process, measured values of the depth z in the depth direction (53), of the intensity (I) and of the position x in the welding feed direction (51) are ascertained together with accurate positioning from at least one reflected OCT measurement beam (42, 43), characterized in that the materials of the upper joining partner (11) and of at least one lower joining partner (12) and also a degree of intermixing of the materials are classified from the ascertained measured values.Method according to claim 1 characterised in that the classification is carried out with the aid of statistical evaluations and / or AI algorithms.Method according to claim 2, characterised in that statistical evaluations are carried out on the basis of measurement point clusters using the intensity I as a function of the measurement depth and / or measurement position.Method according to one of Claims 1 to 3, characterized in that process parameters of the weld are influenced by control technology as a function of the classification of the measured values.Method according to one of Claims 1 to 4, characterized in that measured values in the region of the keyhole (22) are determined at two or more discrete depth levels (53).Method according to one of Claims 1 to 4, characterized in that measured values in the region of the keyhole (22) are ascertained continuously along the depth direction (53).

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