Technique for predicting joining defects during laser welding

Abstract

The invention relates to a method for laser welding bipolar plates (10), the method comprising the steps of: laser welding two at least partly overlapping joining partners (12, 14) along a specified welding path in order to produce an overlapping connection between the joining partners (12, 14); observing, during the welding process, whether, following the process zone (22), at least one undulating structure (242) forms in the weld pool (24); initiating, in response to the observation, a measure for preventing or bypassing using an incomplete join (152) between the joining partners (12, 14) along at least one portion of the welding path. The invention also relates to a laser welding system and to a computer program product for implementing the method.

Description

[0001] Technique for predicting bonding defects in laser welding

[0002] Field of the invention

[0003] The present invention relates to the field of laser welding. In particular, the invention relates to a technique for detecting certain irregularities during the laser welding of bipolar plates, which irregularities indicate the formation of bonding defects during the welding process.

[0004] State of the art

[0005] Several methods for optical monitoring of welding processes are already known from the state of the art. Reference is made in this context to DE 10 2018 220 342 A1, which discloses such a monitoring method, as an example only.

[0006] To manufacture bipolar plates for fuel cells, metallic plates or foils are welded together in pairs. At least some of the weld seams connecting the plate pairs must be fluid-tight to avoid compromising the subsequent functionality of the bipolar plate. Traditionally, a pair of plates of a bipolar plate is welded together using lap joints. First, the joining partners are positioned relative to each other and secured. A laser welding beam then traces a predetermined contour on the surface of one of the joining partners, penetrating through the joining partner into the adjacent joining partner, creating a weld seam that extends at least partially into the "hidden" joining partner.However, due to insufficient fixation of the joining partners, for example, a gap can develop between the joining partners that exceeds a critical gap size and is not completely bridged by the weld seam. Experts refer to this as "bonding defects." When welding a bipolar plate, a gap of as little as 15 μm between the joining partners can lead to the formation of bonding defects.

[0007] When welding lap joints, joining defects must be avoided at all costs. One difficulty is that joining defects on the welded product are often not visible to the naked eye, as the resulting weld bead on the top side of the workpiece and the weld root on the underside of the workpiece often do not exhibit any abnormalities. Experts also refer to such joining defects, which are not visible from the outside, as "false friends."

[0008] Checking weld seams for connection defects is therefore complex and costly.

[0009] The present invention is based on the objective of improving the reliability of sealing during the welding of overlap joints. In particular, it should be possible to reliably determine the formation of joint defects during the welding process in order to derive appropriate measures.

[0010] The invention

[0011] The problem underlying the invention is solved by the subject matter of the independent claims. Further possible embodiments of the invention are specified in the subclaims, the description, and the drawings. Features, advantages, and possible embodiments presented in the description for one of the subject matter of the independent claims are to be regarded at least analogously as features, advantages, and possible embodiments of the respective subject matter of the other independent claims, as well as any possible combination of the subject matter of the independent claims, optionally in conjunction with one or more of the subclaims.

[0012] According to a first aspect, a method for laser welding bipolar plates is provided. The method comprises laser welding two at least partially overlapping joining partners along a predetermined welding path to create an overlap joint between the joining partners.

[0013] In the context of the present invention, an overlap joint is to be understood as a welded joint in which the weld seam extends completely across the thickness of a first of the joining partners (= upper joining partner) and at least partially into the second (lower) joining partner. For this purpose, the laser beam is directed along the welding path onto a surface of the first joining partner and melts the joining partners up to the second joining partner or even across the entire thickness of both joining partners. Laser welding is preferably carried out in the deep welding regime, in which the laser beam forms a vapor capillary in the process zone in which the laser beam interacts with the joining partners. This vapor capillary extends deep into the joining partners and promotes an efficient welding process. The welding process is preferably computer-controlled.

[0014] For laser welding, for example, a solid-state laser (e.g., a disk laser or a fiber laser) or a diode laser can be used. For example, a single-mode laser with a laser power of 500 W to 2 kW or a multi-mode laser with a laser power of 2 kW to 5 kW can be used. A preferred power of the laser processing beam when welding bipolar plates can be in the range of 10 W to 2000 W, in particular from 50 W to 700 W. When using a single-mode laser, the laser beam can have a beam parameter product in the range of 0.36 mm*mrad to 16 mm*mrad, in particular of approximately 0.6 mm*mrad. When using a multi-mode laser, the laser beam can have a beam parameter product of at least 3 mm*mrad. In particular, an infrared laser with a wavelength in the range of 800 nm to 1200 nm, in particular 1030 nm or 1070 nm, can be used as the laser.Alternatively, a VIS laser with a wavelength in the blue spectral range of, for example, 400 nm to 450 nm, or in the green spectral range, in particular 515 nm, can be used as the laser. The (focused) processing laser beam can have a beam diameter in the region of the plane of the workpiece surface, i.e., the surface of the upper joining partner, in the range of 10 pm to 300 pm, in particular in the range of 30 pm to 70 pm (single-mode) or in the range of 50 pm to 170 pm (multi-mode).

[0015] A feed rate at which the laser beam is moved along the welding path relative to the workpiece surface can be in the range of 100 mm / s to 5000 mm / s, in particular in the range of 300 mm / s to 2000 mm / s.

[0016] A processing optics through which the laser beam is directed and in particular focused onto the workpiece can have an imaging ratio of 1:1 to 5:1, preferably of 1.5:1 to 2:1.

[0017] The method according to the invention further comprises observing, during the welding process, that at least one wave-shaped structure forms in the melt pool in the wake of the process zone, which structure preferably extends transversely to the feed direction.

[0018] The process zone is the area in which the processing laser beam interacts with at least one of the joining partners during the welding process. In deep penetration welding, the process zone is generally characterized by the formation of a vapor capillary that extends essentially over the entire depth of the subsequent weld seam. The material of the joining partners is melted in and around the process zone. The feed movement of the laser beam relative to the workpiece surface along the weld path creates an elongated molten pool. The temperature in the molten pool decreases with the distance to the process zone. In the downstream part of the process zone, the material of the joining partners initially remains molten before solidifying to form the weld seam. In this area between the process zone and the already solidified weld seam, changes in the molten pool dynamics can occur during the welding process that are visually perceptible.In the present case, the inventors have recognized that a wave-like structure forms on the surface of the molten pool in the downstream part of the process zone, immediately before a bonding defect occurs. The "wave-like structure" here refers to at least one elevation or depression on the surface of the molten pool. This at least one elevation or depression can, in particular, have an elongated extension that is oriented essentially transversely to the feed direction or the length of the molten pool. The observation of changes in the molten pool can preferably be carried out using sequentially acquired individual images of the molten pool. In this way, the change during the welding process can be recorded.

[0019] The structural change in the melt pool typically manifests itself as a local increase or decrease in process emissions (or process radiation) in the melt pool area. This can be detected optically.

[0020] The method according to the invention further comprises initiating, in response to the observation, a measure for preventing or dealing with an incomplete connection of the joining partners along at least a portion of the weld path.

[0021] In this context, an "incomplete bond" is to be understood as synonymous with a bonding defect. According to the inventors' observations, a structural change in the melt pool during the welding process can already be detected in the wake of the process zone when an incomplete bond (i.e., a leak) is imminent. Thus, at the beginning of the detection of a corresponding change in the melt pool dynamics, a bonding defect has not necessarily yet occurred. However, if the welding process continues unchanged, the occurrence of a bonding defect can be expected. Depending on the circumstances, the detection of structural changes in the melt pool can lead to measures for preventing or dealing with bonding defects. A measure for preventing an incomplete bond can, for example, include changing welding parameters with which the welding process is continued to avoid a bonding defect.Alternatively or additionally, the welding process can be interrupted and continued with modified parameters, whereby the fixation can also be improved to avoid a gap between the joining partners. Measures to deal with an incomplete connection can, for example, include passing over the weld path again, classifying the welded workpiece as scrap, or improving the fixation of the joining partners in the area of ​​the suspected connection defect. In particular, improving the fixation of the joining partners can include correcting or supplementing the clamping positions at which the joining partners are pressed against each other using appropriate clamping devices.

[0022] By processing observations during the welding process according to the invention, components (especially bipolar plates) that are highly unlikely to meet the weld seam tightness requirements can be efficiently sorted out. Furthermore, the formation of connection defects can be prevented either during the welding process or in subsequent welding processes (e.g., by refining the fixation of the joining partners), thus improving the overall welding result.

[0023] When observing the melt pool in the downstream section of the process zone, it can also be observed that the melt pool is lengthening. An elongation of the melt pool in the downstream section of the process zone can be used as a further indication of the formation of a bonding defect.

[0024] Preferably, each of the joining partners can consist of a metallic material and have a thickness in the range of 5 μm to 500 μm, in particular in the range of 50 μm to 300 μm. The joining partners can typically be metallic foils based on iron, copper, or aluminum. For example, each of the joining partners can consist of stainless steel, for example, type 1.4404. For example, by welding the joining partners, a bipolar plate for use in a fuel cell can be produced. However, the application of the invention is not limited to the production of bipolar plates.

[0025] The method according to the invention can further comprise determining a position of the observed change in the molten pool along the weld path. For this purpose, the time since the start of the welding process can be continuously recorded while observing the molten pool. By comparing this with the feed rate of the welding process and the geometry of the weld path (which are both known), the position of the molten pool change on the weld path can be determined. This position then indicates the formation (or the approximate beginning) of a bonding defect along the developing weld seam. By knowing the position at which the molten pool change occurred, targeted measures can be initiated to deal with (suspected) bonding defects.For example, the clamping positions at which two joining partners are fixed to each other can be corrected and / or supplemented to prevent the formation of critical gaps between the joining partners. For example, for a batch of joining partners to be welded, possible geometric peculiarities (e.g.

[0026] Distortion) of the joining partners must be taken into account. Alternatively or additionally, welding parameters can be adjusted at the "critical" positions along the weld path to counteract the formation of bonding defects. Adjustments in the process can be particularly helpful if bonding defects always occur at the same locations during several consecutive welds in a batch.

[0027] According to a further aspect of the invention, a system for laser welding is provided. The system comprises a laser welding system for laser welding two at least partially overlapping joining partners along a predetermined welding path to create a lap joint between the joining partners. The system further comprises an observation device for optically monitoring the welding process, wherein the observation device is configured to detect the formation of wave-shaped structures in the molten pool in the downstream section of the process zone. The system also comprises a control device configured to initiate a measure for preventing or dealing with an incomplete connection of the joining partners along at least one section of the welding path if the observation device detects the formation of at least one wave in the molten pool in the downstream section of the process zone.

[0028] The laser welding system can be a conventional laser welding system suitable for welding bipolar plates. Such laser welding systems are, for example, marketed by the applicant in various designs at the time of this application. The control device can, in particular, be a computer configured to control the laser welding system. The processing of the optical signals detected by the observation device and the control of the overall process can take place either on separate computers (or computing devices) or on a shared computer (in particular the control device) for controlling the entire system.

[0029] The observation device can comprise a camera, for example. The camera can use a CMOS sensor or a CCD sensor, for example, or an InGaAs-based sensor (InGaAs = indium gallium arsenide). An exposure time of the camera can, for example, be in the range from 1 ps to 20,000 pm, in particular in the range from 1 ps to 1,000 ps. A recording rate of the camera can be at least 100 Hz, in particular at least 1,000 Hz. The camera can, in particular, be designed to observe wavelengths in the range from 300 nm to 2,000 nm, in particular in the range from 600 nm to 1,000 nm. The wavelength of the processing laser beam, which can, for example, be 1,030 nm or 1,070 nm, should not be detectable by the camera.For example, the camera can use a bandpass filter for a wavelength range from 600 nm to 1000 nm, or a broadband filter with a spectral width of at least 200 nm, blocking the wavelengths of the processing laser beam (e.g., 1030 nm or 1070 nm). One advantage of using a bandpass filter is good contrast.

[0030] The camera can preferably be aligned with the beam path of the processing laser beam, particularly coaxially or with an angular offset of up to 15% to the processing laser beam, toward the workpiece surface. In particular, the camera's field of view can be coupled into the beam path of the processing laser beam via a semi-transparent mirror. By integrating the observation device into the laser welding device in this way, precise alignment of the observation device to the process zone or melt pool can be ensured at all times during the processing process.

[0031] An advantage of using a camera as an observation device is the comparatively simple integration into the processing optics (essentially) coaxial to the laser beam.

[0032] In addition to or as an alternative to a camera, the observation device can comprise at least two photodiodes with different measuring positions. In particular, at least two photodiodes can each be aligned to a measuring position offset from one another in the feed direction in the wake of the melt pool. The difference in the intensity of the process emission detected by each of the photodiodes can be used to determine the wave formation in the melt pool. One advantage of using photodiodes is the lower data rate and higher sampling rate compared to using a camera.

[0033] Additionally or alternatively, the observation device can include an optical coherence tomography (OCT) device. A laser interferometer can also be used as an alternative to OCT. The OCT or laser interferometer has a spatial resolution in the welding direction in the downstream part of the process zone. One advantage of using an OCT is the higher resolution compared to photodiodes. A combination of several of the aforementioned and / or other detection devices in the observation device is possible. By combining them, the advantages of the individual detection devices can be combined to achieve even better observation results.

[0034] Preferably, the observation device should not include external lighting. While lighting is possible and can be used in alternative ways, deliberate omission of the observation device's lighting can result in better detection of process emissions. The reflection of lighting would interfere with the process emissions and potentially complicate the detection of structural changes in the melt pool's wake.

[0035] The observation device can further comprise an evaluation unit that uses a neural network trained to detect waves (i.e., wave-like structures) in the melt pool. The neural network can, in particular, be a convolutional neural network (CNN) qualified for spatial resolution (> 2 pixels). The neural network can have a U-Net architecture and evaluate the optical signals provided by the observation device based on semantic segmentation. Using a neural network can improve the reliability and efficiency of detecting the wave structure.

[0036] According to a further aspect of the invention, a computer program product is provided that contains computer-readable instructions for carrying out a method according to one of the variants described above in a laser welding system according to one of the variants described above. The computer program product can be executed, for example, on the control device of a laser welding system according to the invention. Embodiments

[0037] The following description of preferred embodiments, in conjunction with the drawings, serves to explain the invention in more detail.

[0038] They show:

[0039] Fig. 1 Schematic of a bipolar plate in a plan view with the process zone and surrounding melt pool of a laser welding process indicated;

[0040] Figs. 2a-b Schematically, an overlap connection of two joining partners in a sectional view, whereby Figure 2b shows the defect pattern of an incomplete connection (connection defect);

[0041] Fign. 3a-b Schematic representation of the geometry of a melt pool during the creation of an overlap joint between two joining partners by laser welding;

[0042] Fig. 4 A photograph of a laterally illuminated weld seam with connection defects; and

[0043] Figs. 5a-c Process emissions of a laser welding process, recorded at different times during the creation of the weld seam from Figure 4.

[0044] Identical or functionally equivalent elements are provided with the same reference numerals in the figures.

[0045] Figure 1 schematically shows a top view of a bipolar plate 10. The bipolar plate 10 essentially consists of two flat, structured joining partners, which – as respective half-shells of the bipolar plate 10 – are connected to one another via several weld seams 15 and form fluid channels (not shown in Fig. 1) inside the bipolar plate 10. The half-shells of the bipolar plate 10 are welded together by laser welding. For this purpose, the two joining partners are precisely positioned and fixed relative to one another, and a processing laser beam is directed in a feed direction 30 along a welding path (corresponding to the contour of the weld seams 15 shown) onto the upper of the joining partners to create an overlap connection.The material of the joining partners is liquefied by the effect of the laser beam in a process zone 22, mixes in a common melt pool 24 in the downstream part of the process zone and finally solidifies to form the weld seam 15.

[0046] Figures 2a and 2b each show an overlap joint between a first joining partner 12 and a second joining partner 14 of a component 10 (e.g., a bipolar plate). In Figure 2a, the joining partners 12, 14 are connected to one another by a weld seam 15. According to Figure 2b, the weld seam 15 extends through both joining partners 12, 14. However, a gap 13 between the joining partners 12, 14 is not completely bridged by the weld seam 15. The defect 152 in the weld seam 15 according to Figure 2b creates an incomplete connection (connection defect) between the joining partners 12, 14. The connection is not tight at the location of the connection defect 152. From the outside of the component 10, the defect 152 cannot be seen, since the top and bottom of the weld seam 15 do not have any suspicious irregularities.

[0047] Figures 3a and 3b schematically illustrate the geometry of a molten pool 24 during laser (beam) welding. Figure 3a shows a partial top view of a workpiece 10 or of the upper joining partner 12 of the workpiece 10 facing a laser welding optics used. Figure 3b shows the same molten pool 24 in a lateral sectional view extending parallel to the feed direction 30 of the processing laser beam. During laser welding, the material of the joining partners 12, 14 is strongly heated in the process zone 22, in which the processing laser beam directly interacts with the workpiece. During deep welding, a vapor capillary is formed in the process zone 22, which extends over a large part of the welding depth. The material of the joining partners 12, 14 is melted around the vapor capillary.In the molten pool 24, the material of both joining partners 12, 14 mixes and solidifies in the downstream part of the process zone 22 to form the common weld seam 15. When creating such lap joints, it can happen - for example, if a gap 13 between the joining partners 12, 14 exceeds a critical dimension - that the created weld seam 15 does not completely bridge the gap 13 between the joining partners 12, 14, thus leading to the formation of connection defects 152 in certain sections. The inventors of the present invention have determined that at least one wave-shaped structure 242 forms on the surface of the molten pool 15 immediately before the formation of connection defects 152 during the welding process, in the downstream part of the process zone 22. The wave-shaped structure 242 is generally oriented perpendicular to the feed direction 30.Based on this knowledge, it is possible to initiate measures to avoid or deal with a connection error 152 during the welding process.

[0048] Figure 4 shows a side view of a transilluminated weld seam 15. The image shows the areas of the lower (second) joining partner 14 and the upper (first) joining partner 12, which are separated from each other by a narrow gap (see 13). At the level of the gap 13, bright lines can be seen in sections, each of which indicates a connection defect 152 in the weld seam 15. The weld seam 15 shown extends through the joining partners 12, 14 of a bipolar plate and was created at a feed rate of 500 mm / s at 200 W laser power.

[0049] Figures 5a, 5b, and 5c each show a camera image 100-X1, 100-X2, 100-X3 of the molten pool 22 including the process zone 24 during the welding process to create the weld seam 15 shown in Figure 4. The images were taken along the beam path of the processing laser beam—i.e., perpendicular or essentially perpendicular to the surface of the upper joining partner 14—without additional illumination. In images 100-X1, 100-X2, 100-X3, the process emissions at a specific time XI, X2, X3 during the welding process can be seen as bright areas. In addition, diagrams 200-X1, 200-X2, 200-X3 each show sets of curves indicating the intensity of the process emissions in the observed area. The curves with the largest deflection each show the intensity along a line that runs in the feed direction through the center of the melt pool 24, where the largest process emissions occur.The flatter lines of the sets of curves indicate intensity curves in the edge areas of the melt pool 24.

[0050] Figure 5a shows the process emissions during the laser welding process at the time corresponding to point XI of the weld seam 15 in Figure 4. There is no bonding defect 152 at point XI (see Fig. 4). The bright area in image 100-X1 shows the process emissions in process zone 22, which are also reflected in the intensity curves 1-22 in diagram 200-XI.

[0051] Figure 5b shows the process emissions with reference to point X2 of the weld seam 15 (cf. Fig. 4), i.e., shortly before the formation of a connection defect 152. Image 100-X2 shows, on the one hand, the process emissions of process zone 22 in the melt pool 15, in the center of which the keyhole can be clearly seen as an intensity gap. On the other hand, image 100-X2 shows another bright area 242 in the wake of process zone 22. In diagram 200-X2, this area is shown by the deflection of the intensity curves I-242. The shape of the process emissions in the wake of process zone 22 results from a wave 242 that forms in the melt pool 22 during the welding process.

[0052] Finally, Figure 5c shows the process emissions related to point X3 of weld 15 (see Fig. 4), where a connection defect 152 is present. Image 100-X3 and the corresponding intensity curve 200-X3 again show high process emissions in the area of ​​process zone 22 with a recognizable keyhole (see also 1-22 in diagram 200-X3). No further intensity peaks are detectable in the wake of the process zone.

[0053] According to the inventors' observations, the irregularities in the melt pool 24 according to Figure 5b were only detectable shortly before the formation or at the beginning of a subsequent bonding defect 152. Any irregularities in the process emissions in the area of ​​the process zone 22 did not allow any conclusions to be drawn about the formation or risk of bonding defects at the time of the invention.

Claims

Patent claims 1. A method for laser welding bipolar plates (10), the method comprising the steps: Laser welding two joining partners (12, 14) which at least partially overlap one another along a predetermined welding path to produce an overlap connection between the joining partners (12, 14); Observing, during the welding process, that at least one wave-shaped structure (242) forms in the melt pool (24) in the wake of the process zone (22); Initiating, in response to the observation, a measure to prevent or deal with an incomplete connection (152) of the joining partners (12, 14) along at least a portion of the weld path.

2. The method according to claim 1, wherein it is further observed that the melt pool (24) is lengthened in the downstream part of the process zone (22).

3. Method according to one of the preceding claims, wherein each of the joining partners (12, 14) consists of a metallic material and has a thickness in the range from 5 pm to 500 pm, in particular in the range from 50 pm to 300 pm.

4. Method according to one of the preceding claims, further comprising the step: Determining a position of the observed change in the melt pool (24) along the weld path.

5. System for a laser welding process, comprising: A laser welding system for laser welding two joining partners (12, 14) which at least partially overlap one another along a predetermined welding path to produce an overlap connection between the joining partners (12, 14); An observation device for optically monitoring the welding process, wherein the observation device is designed to detect the formation of wave-shaped structures (242) in the melt pool (24) in the wake of the process zone (22); and A control device which is designed to initiate a measure for preventing or dealing with an incomplete connection (152) of the joining partners (12, 14) along at least one section of the welding path if the formation of at least one wave (242) in the molten pool (24) in the downstream part of the process zone (22) has been detected by the observation device.

6. The system of claim 5, wherein the observation device comprises a camera.

7. System according to claim 5 or 6, wherein the observation device comprises at least two photodiodes having different measuring positions.

8. System according to one of claims 5 to 7, wherein the observation device comprises an optical coherence tomography (OCT) device.

9. System according to one of claims 5 to 8, wherein the observation device comprises an evaluation unit which uses a neural network which has been trained to detect waves (242) in the melt pool (24).

10. A computer program product containing computer-readable instructions for carrying out a method according to any one of claims 1 to 4 in a laser welding system according to any one of claims 5 to 9.

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