Method for monitoring a laser beam welding process and monitoring device

Abstract

The invention relates to a method for monitoring a laser beam welding process, wherein an optical signal is recorded from at least one area of ​​the liquid melt, in particular at least one area of ​​the vapor capillary (14), and signals (24, 25) of different optical polarization directions are generated from the optical signal (29), characterized in that the brightness of the optical signals (24, 25) is evaluated taking into account reference information in such a way that a disturbance of the laser beam welding process is detected if there is a predetermined deviation from the reference information.

Description

State of the art

[0001] From the article "Adaptation of a topographical measurement system for beam welding processes with high temporal and spatial resolution," Journal of Advanced Joining Processes 1 (2020), Reisgen et al., a measurement system for an electron beam welding system is already known. A goniometer is used to capture an image of the process zone and divide it into different polarization directions. High-speed cameras capture image data of the melt pool in four different polarization directions of light and, by evaluating the image information, enable an interpretation of the electron beam welding process. Advantages of the invention

[0002] The method according to the invention for monitoring a laser beam welding process has the advantage that the brightness of light beams with different polarization directions, which are recorded from an observed work area of ​​a laser beam welding process, in particular from the vapor capillary of the welding process, is evaluated. Compared to processing a recorded image, evaluating only the brightness has the advantage that a significantly smaller amount of data needs to be provided and processed for evaluation. However, by comparing it with already available reference information, it is still possible to identify anomalies in a laser beam welding process and to detect malfunctions or errors in the welding process.

[0003] Process monitoring is not primarily about precisely measuring and characterizing the vapor capillary's geometry, but rather simply detecting deviations or anomalies. It has been found that an evaluation based on a camera image is not necessary for this purpose; a simpler evaluation can achieve the same result. This allows for a quick yet surprisingly accurate evaluation with smaller data volumes.

[0004] Compared to a pure evaluation of brightness when observing the vapor capillary, the evaluation of light beams divided according to polarization directions has the advantage that changes in the surface influence the optical properties, and in particular the intensity of the reflected beam for individual polarization directions, more than the total brightness across all polarization directions. Therefore, it is significantly easier to determine deviations from reference values ​​by evaluating the brightness for individual polarization directions when the laser beam welding process is disrupted. However, compared to image evaluation using high-speed cameras, significantly less computing power is required to compare brightness information in recorded measurement signals with stored reference information.This allows for reliable detection of anomalies in laser welding processes, especially at high feed rates, such as when welding bipolar plates for fuel cells or copper components for electric motors and power electronics, and, if necessary, timely remediation by initiating countermeasures. This makes it possible to detect changes in the flow behavior of the melt pool, fluctuations in the vapor capillary, melt pool ejections, or spatter.

[0005] By evaluating different polarization directions, it is also possible to reliably rule out defects caused by scattering, which regularly occurs in this type of laser welding process. For example, it is possible to detect vapor capillary deformation in the case of partial or complete ejection of the melt from the vapor capillary by evaluating different polarization directions. However, with an integral brightness evaluation across all polarization directions, it may not yet be possible to detect a defect due to the hot vapor capillary walls. By evaluating the brightness for individual polarization directions, it is possible to evaluate the surface, taking into account the influences of cutting front angles or other surface topologies.This welding defect detection method assumes that for every weld defect that can be detected, the capillary shape must also change beforehand. This is because the energy is introduced into the component via the vapor capillary. If too little or too much energy is introduced, the vapor capillary changes, disrupting the welding process and the melt flow, resulting in defects such as spatter, ejections, or process voids.

[0006] The arrangement according to the invention can detect typical welding defects because the vapor capillary geometries change significantly in these cases. These include, for example, ejections, where large melt areas are ejected from the melt strip during the process. This can leave holes or defects in the seam. Furthermore, spatter is also possible, which detaches from the vapor capillary and in particular from the back wall. This can contaminate the component or the surrounding area or lead to melt loss in the seam. Furthermore, inadequate connections between gaps can be detected if the gap between the sheets cannot be properly bridged, thus leading to insufficient or no connection. Furthermore, weld reinforcements or under-bulging on the weld seam, which can occur at high welding speeds, can also be detected.In particular, this can lead to fluctuations in the geometry of the vapor capillary and the flow conditions in the melt strip wake. Welding depth fluctuations can also be detected, where a decrease or increase in the welding depth is associated with fluctuations in the geometry of the vapor capillary. If a failure occurs near the weld or a process interruption occurs, such a defect leads to a massive change in the geometry of the vapor capillary. A change in the geometry of the vapor capillary is particularly visible in the different polarization directions and leads to correspondingly different changes in intensity.

[0007] Further advantages arise from the dependent claims. It is advantageous to evaluate exactly two signals with different polarization directions, as this allows for a reliable conclusion and limits the computational effort required for the evaluation. It is particularly advantageous if the other different polarization directions are chosen to be orthogonal to each other in order to obtain a particularly reliable evaluation.

[0008] Furthermore, it is advantageous to evaluate signals in a wavelength range between 400 nm and 2000 nm. In this wavelength range, interference can be particularly clearly observed in the melting process. A wavelength range between 400 nm and 1100 nm is particularly advantageous for evaluation, as sensitive, fast, and inexpensive silicon diodes can be used in this range.

[0009] Furthermore, it is advantageous to capture each signal using a photodiode. Photodiodes have the advantages of being cost-effective, easily adaptable to the desired wavelength range by selecting the diode material, fast and sensitive, and reliably providing an integral signal from the laser interaction zone.

[0010] Furthermore, it is advantageous to combine signals one or more times for analysis and evaluate these combinations with reference information. This allows interference and measurement errors to be eliminated or at least minimized. In particular, errors caused by natural scattering can be more easily excluded.

[0011] Furthermore, it is advantageous to first trace a trajectory on a target for the laser beam welding process. From the recorded images, assuming the laser beam welding process was performed correctly, reference information is empirically obtained and stored in order to calibrate the monitoring device with this recorded information. This advantageously exploits the fact that the recorded signals differ depending on the direction of movement of the laser across the target, and thus the trajectory of the welding process influences the expected recorded intensities. The corresponding, movement-dependent calibration can thus improve the reliability of an evaluation of the laser beam welding process.

[0012] Furthermore, it is advantageous that if a fault is detected in the laser beam welding process, the laser power for the laser beam welding process is directly adjusted depending on the detected deviation. This adjustment makes it possible to limit the extent of a fault so that the fault or welding defect remains unimportant for the welding process itself. In particular, the laser power can be reduced if excessive material is ejected from the process zone. This prevents process changes from only being implemented when there has been a significant increase in temperature in the molten pool or spatter has already flown away. With the present method, however, it is possible to detect initial changes in the capillary shape - i.e. changes in the process before a fault such as spatter even occurs or before the process and energy coupling are disrupted.This earlier detection of anomalies opens up an efficient way to control the process without causing welding defects.

[0013] Furthermore, it is advantageous to provide the reference information from a machine-trained neural network. This allows deviations in the intensity curves to be reliably detected in a well-trained neural network.

[0014] Corresponding advantages arise for a monitoring device for carrying out the method according to the invention.

[0015] In a further embodiment, it is also possible to use cameras for brightness detection, which may also be additionally designed to record image information, for example for documentation purposes.

[0016] Furthermore, it is advantageous to provide an additional laser in addition to the laser provided for the laser beam welding process, which serves as a measuring laser beam, since with this laser a measuring laser beam can be provided in a different wavelength than the working laser beam and thus a particularly favorable wavelength range for monitoring is provided by the suitable wavelength selection of the measuring laser and influence by the processing laser can be excluded.

[0017] It is advantageous if the additional laser is circularly polarized and the reflected portion of the laser is evaluated in at least two polarization directions. To improve the evaluation, additional polarization directions (linear combinations of these polarization directions) can also be evaluated. Alternatively, two additional lasers with different polarization directions and / or wavelengths could be used. drawing

[0018] Embodiments of the invention are illustrated in the drawing and explained in more detail in the following description.

[0019] They show: Fig. 1 a monitoring device according to the invention with a laser for carrying out the laser beam welding program, Fig. 2 an exemplary optical structure of the monitoring device of the Fig. 1 Fig. 3a and Fig. 3b and the Fig. 4a and Fig. 4b two examples of exemplary recorded reference information for two different laser trajectories during laser welding processes, Fig. 5 an example of a recorded intensity curve of a brightness curve for a polarization direction with exemplary disturbances of the laser beam welding process shown. Embodiments of the invention

[0020] In the Fig. 1 shows a laser 10 having a laser unit 11 that emits laser light 12 onto a material 13. The laser light strikes the material 13 in the area of ​​the so-called vapor capillary 14, forming a melting zone 15 with a higher melting temperature around the vapor capillary 14. This allows the material 13 to be welded to an underlying material 16 at the location of the vapor capillary 14. The laser unit 11 is controlled by a control device 17 such that the laser beam 12 moves in the direction of the arrow 18 across the surface of the material 19.

[0021] Furthermore, a monitoring device 20 is provided, the optics 21 of which are directed onto the surface 19 of the material 13, with a monitoring region 22 being aligned such that it encompasses the melting region 15 and in particular a surface region of the vapor capillary 14. In a further embodiment, the monitoring region 22 can be aligned such that it is directed exclusively onto a surface of the vapor capillary 14. A received light beam is guided by the optics unit 21 onto a beam splitter 23, with the received beam 29 being split into a first beam 24 with a first polarization direction and a second beam 25 with a second polarization direction different from the first polarization direction. In one embodiment, the two polarization directions are rotated by 90 degrees and are thus orthogonal to one another.In further embodiments, it is also possible to select even further divisions of the polarization directions, for example, 45 degrees, 90 degrees, and 0 degrees, and thus generate more than two beams with a respective polarization direction. The beams 24, 25 are each guided to a first detection unit 26 and a second detection unit 27, respectively, which are each designed as intensity measuring devices for a light intensity of the light beams 24, 25. The measured intensities are forwarded via a data connection 28 to an evaluation device 30, in which a computing unit 31 compares the received data with reference information stored in a memory 32.

[0022] The receiving units 26, 27 are preferably designed as photodiodes or cameras. Photodiodes are more cost-effective and also allow for accurate brightness determination. Using cameras, an intensity value can be easily determined for different, preferably disjoint image areas. The selection of photodiodes can be adapted, in particular by selecting the diode material, to the respective wavelength depending on the radiation used in the process or to the radiation of a measuring laser. For example, silicon, indium, gallium, arsenite (also sulfur), or germanium photodiodes can be used.

[0023] When selecting different regions, you can focus on the vapor capillary. Depending on the process, it may also be advantageous to also examine the edge regions of the vapor capillary or just sections of a vapor capillary.

[0024] To select the appropriate reference information, the control unit 17 of the laser 10 can transmit information about an intensity of the laser beam 12 and / or a direction of movement 18 of the laser to the evaluation unit 30 via a second data connection 33. If the computing unit 31 detects a deviation of the intensity values ​​received from the monitoring unit 20 from the reference information that exceeds a predetermined level, a warning is output via an output unit 34 in a first embodiment. This allows a faulty laser weld connection to be documented and the error can be manually rectified if necessary, or the corresponding component can be completely rejected as defective or for manual re-inspection. The welding process can either be completed initially or, alternatively, aborted immediately in the event of an error.

[0025] In a further embodiment, the evaluation unit 30 transmits corresponding information about a deviation via the second data connection 33, for example, an intensity that is too low due to evaporation of molten material from the vapor capillary 14 due to excessive laser power. The information about excessive laser power is received by the control unit 17 of the laser 10 and causes the control unit to reduce the laser power by, for example, 10%. If necessary, a repetition of the welding process at the previously scanned location can be initiated automatically afterwards.

[0026] In a first embodiment, the values ​​for the different polarization directions can be compared separately with reference information for the respective polarization directions. In a first embodiment, an assessment of a defective laser weld can be made if one of the two intensity values ​​deviates from an expected reference value by a predetermined amount. In a further embodiment, however, it is also possible to provide a fault if both values ​​deviate from a predetermined reference value.

[0027] In a further embodiment, it can also be taken into account that the corresponding intensity values ​​in a laser welding program do not occur constantly, but that scattering occurs even during a properly performed welding process. With fixed limit values, this can lead to either critical events being lost in the scatter or false alarms being generated if the setting is too narrow. To improve this, a machine-trained neural network is therefore preferably provided in the memory 32, in which not only absolute values ​​are used as reference information, but the corresponding patterns of the intensity measurement are compared with predetermined patterns. If a deviation from a predetermined pattern occurs, a fault in the welding process is detected.

[0028] On the one hand, reference information can be determined mathematically, but on the other hand, it is advantageous to determine the reference information experimentally, especially with regard to the respective material to be welded.

[0029] In a further embodiment, alternatively or additionally, the different intensity values ​​can also be evaluated in a combined mathematical manner. In one embodiment, for example, a quotient of the two intensities can be determined and compared with a reference value for the quotient. In this case, it is possible to cancel out any interference, so that significant deviations can be more easily detected. Other mathematical functions for merging the signals or for normalization can also be used. The corresponding function can be determined experimentally for the respective material and the intended application of the laser welding process, if necessary. For example, it can be advantageous to use a sum of the intensities. Furthermore, for example, the intensity of the first polarization direction can be determined as a fused signal using the sum of the intensities of both polarization directions and can be used further.In another embodiment, it is also possible to divide the intensity signals by the laser power to obtain a standardized signal. False alarms can be avoided by only detecting a deviation when two or more of the calculated values ​​exceed the respective reference values.

[0030] The reference values ​​are preferably determined by performing various laser processes that are classified as optimal or acceptable. The intensities measured during these laser processes are used as reference information for the assessment of further processes. During evaluation, deviations from the expected signals or anomalies are then determined. Such intensity deviations indicate that the vapor capillary or the melt pool did not correspond to the reference at the corresponding point in the laser welding process. This allows melt pool ejections and other instabilities in the welding process to be detected as early as possible and, if necessary, eliminated in a timely manner by controlling the laser and, in particular, the laser intensity accordingly. Furthermore, it is also possible to adapt the laser beam profile accordingly or to optimize its focus on the weld point.

[0031] In laser processes in which geometric patterns such as curves, rectangles or other geometries such as waveforms are traced as a trajectory on the material, the orientation of the vapor capillary and thus the corresponding proportion of light polarized in different directions that emanates from the vapor capillary changes due to the trajectory of the laser beam over the component. In this respect, the intensities of the light for different polarization directions depend on the direction in which the laser is guided over the surface of the material to be processed. In a first embodiment, this dependency can be neglected for the evaluation. In a further development, however, it is also possible to take this dependency into account. Therefore, to improve the detection of defects, a calibration is carried out for a corresponding welding geometry to be carried out.In a first embodiment, the dependence of the different polarized light components on a motion vector is determined, starting from a zero point of a coordinate system. For each motion vector or for a selected set of motion vectors, the expected signals for the different polarization directions can be determined. In a further embodiment, the laser is applied to specific patterns on the material, and the intensity components are determined.

[0032] An example of this is shown in the Fig. 3A. With respect to a virtual coordinate system 40, the laser moves a circular path 42 over the surface at a respective angle 41. In the Fig. 3b, the intensities for both a first polarization direction 44 and the intensities for a second polarization direction 45 are shown on a y-axis 43 over the angle 41 of the laser along the circular path 42, which is plotted on an x-axis 46.

[0033] Such a determination of intensities, essentially a calibration, can be performed additionally or alternatively for different feed rates or different laser powers. This is advantageous when the path geometries are processed with locally different parameters.

[0034] In the Fig. 4A shows a second embodiment of a rectangular circular path 50, again above the virtual coordinate system 40, with the laser describing a rectangular pattern on the surface of the material. On the Y-axis 43, a first intensity 51 and a second intensity 52 are shown relative to a position 53 on the rectangular path 50, which is shown on the X-axis 56, with a position 54 being singled out, for example.

[0035] In the Fig. Figure 2 shows the optical beam path of the laser light and the analysis in detail. The light from the laser 10 is collimated by a lens 60, passes through a semi-transparent mirror 61, and strikes a focusing lens 62, which focuses the laser light onto the vapor capillary 14. Light emitted by the vapor capillary 14, in turn, reaches the lens 62 and the transparent mirror 61, which then redirects the corresponding light into a beam path 63 for analysis. A filter 64 can be inserted into the beam path 63 to limit the light for analysis to a desired wavelength range. Furthermore, the filter can also include an attenuation that reduces excessively high intensity of the received light to prevent damage to the downstream analysis units or to avoid overloading the sensor.The light then strikes a beam splitter 65, which creates a beam path 66 with a first polarization direction and a beam path 67 with a second polarization direction. The light is then focused onto the first evaluation unit 26 and the second evaluation unit 27 by corresponding lenses 68, 69.

[0036] If necessary, additional filters 70, 72 can be introduced into the beam paths 66, 67. In an alternative embodiment, the second evaluation unit 27 is omitted at the initially provided location. The beam 67 is then first guided to a second mirror 70, through which the light reaches the converging lens 69' and the second evaluation unit 27'. The second mirror 70 is semi-transparent, with a laser unit 71 located on the side facing away from the beam splitter 65. This laser unit emits laser light through the semi-transparent mirror 70 and the beam splitter 65 and via the first mirror 61 onto the vapor capillary 14 and serves as a measuring laser. The wavelength of the measuring laser 71 preferably differs from the wavelength of the laser 10, with the receiving units 26, 27' being designed to measure an intensity in the wavelength range of the measuring laser 71.This allows a measurement to be carried out independently of the wavelength of the laser 10 or of ambient light, since the intensity measurement is specifically tuned to the wavelength of the measuring laser 71.

[0037] In the Fig. Figure 5 shows an example of an intensity measurement for a quotient of the two intensity values ​​for the different polarization directions. Intensity values ​​81 are plotted on a y-axis above a weld seam path plotted on an x-axis 80. The intensity values ​​of the quotient are generally within a range 82; this may result in short outliers 83, drifting 84, or strong fluctuations 85. Drifting, for example, indicates insufficient laser power, strong fluctuations indicate excessive laser power, and the short deviation 83 indicates a welding defect that may require manual re-inspection. QUOTES CONTAINED IN THE DESCRIPTION

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

[0000] “Adaptation of a topographical measurement system for beam welding processes with high temporal and spatial resolution”, Journal of Advanced Joining Processes 1 (2020), Reisgen

[0001]

Claims

[1] Method for monitoring a laser beam welding process, wherein an optical signal is recorded at least from a region of the liquid melt, in particular at least from a region of the vapor capillary (14), and signals (24, 25) of different optical polarization directions are generated from the optical signal (29), characterized by that the brightness of the optical signals (24, 25) is evaluated taking into account reference information in such a way that a disturbance of the laser beam welding process is determined in the event of a predetermined deviation from the reference information. [2] Method according to claim 1, characterized by that exactly two signals with different polarization directions are generated. [3] Method according to claim 2, characterized by that the different optical polarization directions are chosen to be orthogonal to each other. [4] Method according to one of the preceding claims, characterized bythat the two optical signals are detected in an infrared spectrum, in particular in a wavelength range between 400nm and 2000nm, in particular 400nm-1100nm. [5] Method according to one of the preceding claims, characterized by that the signals are each recorded by a photodiode. [6] Method according to one of the preceding claims, characterized by that the signals are mathematically combined once or several times for evaluation and the combinations are evaluated with regard to the reference information. [7] Method according to one of the preceding claims, characterized by that the laser beam welding process is carried out according to a predetermined path on a component (19) and that the reference information assigned to the path is used for the evaluation. [8] Method according to one of the preceding claims, characterized bythat if a disturbance in the laser beam welding process is detected, the laser power for the further laser beam welding process is adjusted depending on the deviation detected. [9] Method according to one of the preceding claims, characterized by that the reference information is provided by a machine-trained neural network. [10] Monitoring device for a laser beam welding process, in particular for carrying out the method according to one of the preceding claims, characterized byan optical detection device (20) for detecting an optical signal at least from one region of the liquid melt, in particular at least from one region of the vapor capillary (14) of a laser beam welding process, with a beam splitter device (23) for generating signals (24, 25) of different optical polarization directions from the signal, with two detection devices (26, 27) for detecting the two signals and with an evaluation device (30) for evaluating the detected signals taking reference information into account in such a way that a disturbance in the laser beam welding process is determined in the event of a predetermined deviation from the reference information. [11] Monitoring device according to claim 10, characterized by that the detection devices (26, 27) comprise cameras or photodiodes. [12] Monitoring device according to one of claims 10-11, characterized bya laser (71) which, in addition to the laser (10), directs a measuring laser beam at least onto the region of the liquid melt, in particular at least onto the region of the vapor capillary (14) for carrying out the laser beam welding process.

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