Techniques for predicting joint defects during laser welding

Abstract

The present invention relates to a method for laser welding a bipolar plate (10), the method comprising the steps of: laser welding the mating plates (12, 14) along a designated welding path to create an overlapping joint between two at least partially overlapping mating plates (12, 14); observing during the welding process whether at least one undulating structure (242) is formed in the weld pool (24) following a process zone (22); and, in response to the observation, initiating measures to prevent or bypass an incomplete joint (152) between the mating plates (12, 14) along at least a portion of the welding path. The present invention also relates to a laser welding system and a computer program product for carrying out the method.

Description

Technical Field

[0001] The present invention relates to the field of laser welding. Specifically, the present invention relates to a technique for detecting certain irregularities during laser welding of bipolar plates, which indicate the occurrence of joint defects during the welding process.

Background Art

[0002] Some methods for optically monitoring the welding process are already known from the prior art. As a mere example, in this context, reference is made to DE102018220342A1, which discloses such a monitoring method.

[0003] To fabricate bipolar plates for fuel cells, metal plates or metal foils are welded together in pairs. At least some of the weld seams connecting the plate pairs must be liquid-tight so as not to compromise the subsequent functionality of the bipolar plates. Conventionally, pairs of bipolar plates are welded together using overlapping connections. First, the mating partners are positioned relative to each other and fixed in a predetermined position. The laser welding beam then traces a specified contour on the surface of one of the mating partners, and the laser beam passes through the mating partner and penetrates into the adjacent mating partner, creating a weld seam that extends at least partially into the "hidden" mating partner. However, in this case, for example, due to insufficient fixation of the mating partners, a gap exceeding the critical clearance may be formed between the mating partners, and there is a possibility that it cannot be completely filled by the weld seam. Those skilled in the art refer to this as a "joint defect". When welding bipolar plates, a distance of even 15 μm between the mating partners can lead to the formation of joint defects.

[0004] When welding overlapping connections, it is crucial to avoid joint defects. One challenge is that joint defects in welded products are often not visible to the naked eye, because the weld bead forms on the upper side of the workpiece, and there are often no abnormalities in the weld route on the lower side of the workpiece. In the case of such joint defects that are not visible from the outside, those skilled in the art also refer to them as "false friends."

[0005] Therefore, inspecting welded joints for defects is time-consuming and expensive.

[0006] This invention aims to improve the reliability of airtightness when welding overlapping connections. In particular, it aims to enable reliable evaluation of joint defect formation during the welding process in order to derive appropriate countermeasures.

[0007] Summary of the Invention The fundamental object of the present invention is achieved by the subject matter of the independent claims. Further possible embodiments of the present invention are specified in the dependent claims, specification, and drawings. Features, advantages, and possible embodiments described in the specification for one of the objectives of the independent claims shall be considered at least similarly as features, advantages, and possible embodiments for each of the other independent claims, and, optionally, for any possible combination of the objectives of the independent claims in combination with one or more of the dependent claims.

[0008] According to a first aspect, a method for laser welding bipolar plates is provided. The method comprises laser welding two at least partially overlapping mating plates along a designated welding path to create an overlapping connection between them.

[0009] In the context of the present invention, an overlapping joint is understood as a welded joint in which the weld seam extends entirely across the thickness of the first mating joint (i.e., the upper mating joint) and at least partially into the second (lower) mating joint. For this purpose, a laser beam is directed along the weld path onto the surface of the first mating joint to melt the mating joint into the second mating joint, or to melt across the entire thickness of both mating joints. In this context, the laser welding is preferably performed in deep welding mode, in which the laser beam forms vapor capillaries within a process zone in which it interacts with the mating joint, and these vapor capillaries extend into the depths of the mating joint, facilitating an efficient welding process. The welding process is preferably performed using a computer-controlled method.

[0010] For the laser welding process, for example, a solid-state laser (e.g., a disk laser or fiber laser) or a diode laser can be used. For example, a single-mode laser with a laser output of 500W to 2kW or a multi-mode laser with a laser output of 2kW to 5kW can be used. When welding bipolar plates, the preferred power output of the laser processing beam can be in the range of 10W to 2000W, particularly 50W to 700W. When using a single-mode laser, the laser beam can have a beam parameter product in the range of 0.36mm*mrad to 16mm*mrad, particularly about 0.6mm*mrad. When using a multi-mode laser, the laser beam can have a beam parameter product of at least 3mm*mrad. In particular, an infrared laser using wavelengths in the range of 800nm ​​to 1200nm, particularly 1030nm or 1070nm, can be used as the laser. Alternatively, for example, a VIS laser using wavelengths in the blue spectral range of 400nm to 450nm or the green spectral range, particularly 515nm, can be used as the laser.

[0011] The (focused) processing laser beam can have a beam diameter in the range of 10 μm to 300 μm, particularly in the range of 30 μm to 70 μm (single mode) or 50 μm to 170 μm (multimode), in the plane of the workpiece surface, i.e., the surface of the upper bonding partner.

[0012] The feed rate at which the laser beam moves along the welding path relative to the workpiece surface can range from 100 mm / second to 5000 mm / second, particularly from 300 mm / second to 2000 mm / second.

[0013] The laser beam is directed onto the workpiece, and in particular, the focusing optical unit can have an imaging ratio of 1:1 to 5:1, preferably 1.5:1 to 2:1.

[0014] The method according to the present invention further includes observing during the welding process whether at least one undulating structure following the process zone forms a weld pool, preferably extending laterally with respect to the feed direction.

[0015] The process zone refers to the region where the processing laser beam interacts with at least one of the mating materials during the welding process. In deep welding, the process zone is typically characterized by the formation of vapor capillaries that extend substantially throughout the entire depth of the subsequent weld joint. The mating material is melted within and around the process zone. The relative feed of the laser beam across the workpiece surface along the weld path creates an elongated weld pool. The temperature within the weld pool decreases with distance from the process zone. Following the process zone, the mating material remains initially molten until it solidifies at the weld joint. In this region between the process zone and the already solidified weld joint, visually apparent changes in weld pool dynamics may occur during the welding process. In this case, the inventors recognized that a ridged structure is formed on the surface of the weld pool following the process zone, immediately before a joint defect is formed. The term “ridged structure” here refers to at least one bulge or depression on the surface of the weld pool. This at least one bulge or depression may specifically have an elongated extension aligned substantially laterally with respect to the feed direction or the length of the weld pool. Observation of changes in the weld pool can preferably be performed by sequentially recording individual images of the weld pool. This makes it possible to detect changes during the welding process.

[0016] Structural changes in the weld pool typically manifest as a localized increase or decrease in process emissions (or process radiation) within the weld pool region. This can be detected visually.

[0017] The method according to the present invention further includes, in response to observation, initiating measures along at least one section of the welding path to prevent or address an incomplete connection of the mating body.

[0018] In this context, "incomplete connection" is understood to be synonymous with a joint defect. According to the inventors' observations, when an incomplete connection (i.e., areas of non-contact) begins to form, the structural changes following the process zone are already visible in the weld pool during the welding process. Therefore, at the start of detection of the corresponding changes in weld pool dynamics, a joint defect has not necessarily occurred yet. However, if the welding process continues without modification, the likelihood of a joint defect occurring increases. Depending on the situation, detecting structural changes in the weld pool can enable measures to prevent or address joint defects.

[0019] One measure to prevent incomplete connections may include, for example, modifying welding parameters so that the welding process can continue without joint defects occurring. Alternatively or additionally, the welding process may be interrupted, parameters modified, and the fixing may also be improved to prevent gaps between the joining parts. One measure to address incomplete connections may include, for example, re-welding the weld path, classifying the welded workpiece as scrap, or re-fixing the joining parts in areas where joint defects are suspected. Improvements to the fixing of joining parts may include, in particular, correcting or supplementing the clamping position so that the joining parts are pressed together by appropriate clamping means.

[0020] The observation process during the welding process according to the present invention makes it possible to efficiently select components (especially bipolar plates) that are very unlikely to meet the requirements for airtightness of the welded joint. Furthermore, the formation of joint defects can be prevented during or for subsequent welding processes (for example, by improving the fixing of the mating parts), thereby improving the overall welding result.

[0021] When observing the weld pool extending beyond the process zone, it can also be confirmed that the weld pool is prolonged. The extension of the weld pool beyond the process zone can be used as a further indicator of the formation of a joint defect.

[0022] Preferably, each of the joining components can be made of a metallic material and may have a thickness in the range of 5 μm to 500 μm, particularly in the range of 50 μm to 300 μm. The joining components can typically be metal foils based on iron, copper, or aluminum. For example, each of the joining components can be made of stainless steel, for example, type 1.4404. For example, a bipolar plate for use in a fuel cell can be made by welding the joining components together. However, the applications of the present invention are not limited to the manufacture of bipolar plates.

[0023] The method according to the present invention may further include determining the location of changes observed in the weld pool along the weld path. For this purpose, the elapsed time from the start of the welding process can be continuously obtained while observing the weld pool. The location of changes in the weld pool along the weld path can be determined by comparing the feed rate of the welding process with the geometric shape of the weld path (both known). This location then indicates the occurrence (or near-start) of a joint defect along the formed weld seam. Knowing the location where the changes in the weld pool have occurred makes it possible to take targeted measures to address (suspected) joint defects. For example, the clamping positions to which two mating pieces are fixed relative to each other can be corrected and / or supplemented to prevent the formation of critical clearances between mating pieces. For example, this makes it possible to take into account any geometric specificities (e.g., distortion) of the mating pieces for a batch of mating pieces being welded. Alternatively or additionally, welding parameters can be adjusted at “critical” locations along the weld path to counteract the formation of joint defects. Process adjustments can be particularly useful when joint defects consistently occur in the same location during multiple consecutive welds of a batch.

[0024] A further aspect of the present invention provides a system for laser welding. The system comprises a laser welding system for laser welding two at least partially overlapping mating bodies along a designated welding path to create an overlapping connection between them. The system further comprises an observation device for optically monitoring the welding process, which is designed to detect the formation of an uneven structure in the weld pool following a process zone. The system further comprises a control device which is designed to initiate measures to prevent or address an incomplete connection of mating bodies along at least one segment of the welding path when the observation device detects the formation of at least one wave in the weld pool following a process zone.

[0025] The laser welding system can be a conventional laser welding system suitable for welding bipolar plates. As of the time of this application, the applicant sells various versions of such laser welding systems. The control device can be a computer, in particular, designed to control the laser welding system. Processing of the optical signals captured by the observation device and control of the entire process can be performed on a separate computer (or computing device) to control the entire system, or on a shared computer (particularly the control device).

[0026] The observation device can, for example, be equipped with a camera. The camera can use, for example, a CMOS sensor, a CCD sensor, or an InGaAs-based sensor (InGaAs = indium gallium arsenide). The exposure time of the camera can be in the range of 1 μs to 20,000 μs, particularly in the range of 1 μs to 1000 μs. The capture rate of the camera can be at least 100 Hz, particularly at least 1000 Hz. The camera can be designed to observe wavelengths particularly in the range of 300 nm to 2000 nm, particularly in the range of 600 nm to 1000 nm. For example, it can be 1030 nm or 1070 nm. The wavelength of the processing laser beam should not be detectable by the camera. For example, the camera can use a band-pass filter for the wavelength range of 600 nm to 1000 nm, or a broadband filter having a spectral width of at least 200 nm, and the wavelength of the processing laser beam (e.g., 1030 nm or 1070 nm) is blocked. One advantage of using a band-pass filter is good contrast.

[0027] The camera can preferably be aligned with the workpiece surface through the beam path of the processing laser beam, specifically, coaxially with the processing laser beam or with an angular offset of up to 15% with respect to the processing laser beam. In particular, the field of view of the camera can be coupled to the beam path of the processing laser beam via a partially transparent mirror. By integrating the observation device into the laser welding device in this way, it becomes possible to ensure that the observation device is accurately aligned with the process zone or the welding pool at every stage of the process.

[0028] One advantage of using the camera as an observation device is that it can be relatively easily integrated into the processing optical unit (substantially) coaxially with the laser beam.

[0029] In addition to or as an alternative to a camera, the observation device can comprise at least two photodiodes having different measurement positions. In particular, the at least two photodiodes can each be aligned with measurement positions offset from one another in the feed direction following the welding pool. The formation of undulations in the welding pool can be inferred from the difference in the intensity of the process emission detected by each of the photodiodes. One advantage of using photodiodes is that the data rate is low and the sampling rate is high compared to using a camera.

[0030] In addition or alternatively, the observation device can comprise an optical coherence tomography (OCT) device. As an alternative to OCT, a laser interferometer can also be used. OCT or the laser interferometer has a spatial resolution in the welding direction following the process zone. One advantage of using OCT is that the resolution is high compared to using photodiodes.

[0031] It is possible to combine some of the aforementioned detection means and / or further detection means in the observation device. By combining these, the advantages of the individual detection means can be combined to achieve better observation results.

[0032] Preferably, the observation device does not comprise external illumination. Illumination is possible and can be used according to an alternative variant. However, by not intentionally providing an illumination solution for the observation device, better detection of the process emission can be achieved. The reflection of the illumination can overlap with the process emission and make it more difficult to detect structural changes following the welding pool.

[0033] The observation device may further include an evaluation unit that uses a neural network trained to recognize wave patterns (i.e., undulating structures) within the weld pool. The neural network can be a convolutional neural network (CNN) particularly suited to spatial resolution (2 pixels or more). The neural network may have a U-Net architecture and can evaluate the optical signal provided by the observation device based on semantic segmentation. The use of a neural network can improve the reliability and efficiency of wave pattern detection.

[0034] A further aspect of the present invention provides a computer program product comprising computer-readable instructions for performing a method according to any one of the above-described modifications in a laser welding system according to any one of the above-described modifications. The computer program product can be executed, for example, on a control device of the laser welding system according to the present invention. [Modes for carrying out the invention]

[0035] The following description of preferred exemplary embodiments, in conjunction with the drawings, will help to illustrate the invention in more detail. [Brief explanation of the drawing]

[0036] [Figure 1] This diagram shows a schematic top view of a bipolar plate, illustrating the process zone and surrounding weld pool for the laser welding process. [Figure 2] Figures 2a and 2b show schematic cross-sectional diagrams of the overlapping connections between two joining partners in each case, with Figure 2b showing an image of an incomplete connection defect (joining defect). [Figure 3] Figures 3a and 3b show schematic diagrams of the geometric shape of the weld pool between two mating parts, where laser welding creates an overlapping connection. [Figure 4] This image shows a welded joint with a bonding defect illuminated from the side. [Figure 5]Figures 5a–5c show process emissions of the laser welding process recorded at various points in time while creating the welded joint shown in Figure 4.

[0037] In the drawings, identical or functionally identical elements are given the same reference numeral.

[0038] Figure 1 shows a schematic top view of the bipolar plate 10. The bipolar plate 10 consists substantially of two flat, structured mating bodies, which essentially function as the respective half-shells of the bipolar plate 10, connected to each other by several weld seams 15, forming fluid channels (not shown in Figure 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 mating bodies are precisely positioned and fixed to each other, and the processing laser beam is directed in a feed direction 30 over the upper mating bodies of the mating bodies along the welding path (corresponding to the contours of the weld seams 15 shown) to create an overlapping connection. In this process, the material of the mating bodies is liquefied by the action of the laser beam in the process zone 22, mixed in a common weld pool 24 following the process zone, and solidified while finally forming the weld seams 15.

[0039] Figures 2a and 2b show overlapping connections between a first mating partner 12 and a second mating partner 14 of a component 10 (e.g., a bipolar plate), respectively. In Figure 2a, mating partners 12 and 14 are connected to each other by a welded joint 15. According to Figure 2b, the welded joint 15 extends through both mating partners 12 and 14. However, the gap 13 between mating partners 12 and 14 is not completely filled by the welded joint 15. The defect 152 in the welded joint 15 in Figure 2b results in an incomplete connection (joint defect) between mating partners 12 and 14. The connection is not firm at the location of the joint defect 152. From the outside, the defect 152 in component 10 is not visible because the upper and lower parts of the welded joint 15 do not show any suspicious irregularities.

[0040] Figures 3a and 3b schematically show the geometric shape of the weld pool 24 during laser (beam) welding. Figure 3a shows a cross-sectional top view of the workpiece 10, or the upper mating body 12 of the workpiece 10 facing the laser welding optical unit used. Figure 3b shows a cross-sectional side view of the same weld pool 24 extending parallel to the feed direction 30 of the processing laser beam. During laser welding, the materials of the mating bodies 12 and 14 are heated to a high temperature in the process zone 22, where the processing laser beam directly interacts with the workpiece. During deep welding, vapor capillaries are formed in the process zone 22 and extend over most of the depth of the weld. The materials of the mating bodies 12 and 14 are melted around the vapor capillaries. In the weld pool 24, the materials of both mating bodies 12 and 14 are mixed and solidified in the process zone 22, forming a joint weld seam 15. When creating such overlapping connections, for example, if the gap 13 between mating parts 12 and 14 exceeds a critical dimension, the resulting welded joint 15 may not be able to completely fill the gap 13 between mating parts 12 and 14, potentially resulting in the partial formation of a joint defect 152. The inventors of this invention have confirmed that at least one undulating structure 242 is formed on the surface of the weld pool 15 following the process zone 22 immediately before the formation of the joint defect 152 during the welding process. The undulating structure 242 is generally oriented perpendicular to the feed direction 30. Based on this knowledge, it is possible to take measures to prevent or address the joint defect 152 during the welding process.

[0041] Figure 4 shows an image of the welded seam 15 illuminated from the side. The image shows the regions of the lower (second) mating joint 14 and the upper (first) mating joint 12, which are separated from each other by a narrow gap region (see 13). At the level of the gap 13, bright lines can be partially seen, each of which indicates a joint defect 152 in the welded seam 15. The illustrated welded seam 15 extends through the mating joints 12 and 14 of the bipolar plate and was produced using a 200W laser output at a feed rate of 500 mm / second.

[0042] Figures 5a, 5b, and 5c show camera images 100-X1, 100-X2, and 100-X3 of the weld pool 22 along with the process zone 24 during the welding process to create the weld joint 15 shown in Figure 4, respectively. The images were captured through the beam path of the processing laser beam, i.e., perpendicular or substantially perpendicular to the surface of the upper mating joint 14, without the use of additional illumination. In images 100-X1, 100-X2, and 100-X3, process emission at determined time points X1, X2, and X3 during the welding process can be seen as bright areas in each case. In addition, diagrams 200-X1, 200-X2, and 200-X3 show sets of curves indicating the intensity of process emission within the observed area, respectively. The curve with the greatest deflection indicates the intensity along a line extending through the center of the weld pool 24 in the feed direction, and is where the greatest process emission occurs in each case. The flatter lines in the set of curves indicate the strength gradient in the edge region of the weld pool 24 in each case.

[0043] Figure 5a shows process emission during the laser welding process at the time point corresponding to position X1 of the weld joint 15 in Figure 4. The joint defect 152 is not present at position X1 (see Figure 4). The bright area in image 100-X1 indicates process emission within process zone 22, which is also reflected in intensity curve I-22 in diagram 200-X1.

[0044] Figure 5b shows the process emission associated with the location X2 of the weld joint 15 (see Figure 4), i.e., immediately before the formation of the joint defect 152. Image 100-X2 shows, on the one hand, the process emission of the process zone 22 within the weld pool 15, with a keyhole clearly visible in the center as an intensity gap. On the other hand, Image 100-X2 shows a further bright area 242 following the process zone 22. In Figure 200-X2, this area is indicated by the deflection of the intensity curve I-242. The shape of the process emission following the process zone 22 is caused by the wave portion 242 formed within the weld pool 22 during the welding process.

[0045] Finally, Figure 5c shows process emission associated with location X3 (see Figure 4) of the weld joint 15 where the joint defect 152 is present. Image 100-X3 and the corresponding intensity curve 200-X3 again show high process emission in the region of process zone 22 with a recognizable keyhole (see also I-22 in Figure 200-X3). No further intensity peaks are likely to be detected following the process zone.

[0046] According to the inventors' observations, the irregularities in the weld pool 24 shown in Figure 5b were visible only immediately before their formation or at the onset of subsequent joint defects 152. Due to the irregularities in process release in the process zone 22 region, it was not possible to draw any conclusions regarding the occurrence or risk of joint defects at the time of the present invention.

Claims

1. A method for laser welding a bipolar plate (10), wherein the method is The steps include laser welding the two joining partners (12, 14) along a designated welding path in order to create an overlapping connection between two joining partners (12, 14) that at least partially overlap, The steps include observing whether, during the welding process, at least one undulating structure (242) is formed in the weld pool (24) following the process zone (22), A method comprising the step of taking measures to prevent or address an incomplete connection (152) of the mating partners (12, 14) along at least one section of the welding path in response to the observation.

2. The method according to claim 1, wherein it is further observed that the weld pool (24) extends beyond the process zone (22).

3. The method according to claim 1 or 2, wherein each of the joining partners (12, 14) is made of a metallic material and has a thickness in the range of 5 μm to 500 μm, particularly in the range of 50 μm to 300 μm.

4. The method according to any one of claims 1 to 3, further comprising the step of determining the location of the observed change in the weld pool (24) along the welding path.

5. A system for a laser welding method, A laser welding system for laser welding two at least partially overlapping joining partners (12, 14) along a designated welding path to create an overlapping connection between them, An observation device for optically monitoring a welding process, wherein the observation device is designed to detect the formation of an uneven structure (242) in a weld pool (24) following a process zone (22), A system comprising: a control device, which is designed to initiate measures to prevent or address incomplete connection (152) of the mating parts (12, 14) along at least one section of the welding path when the observation device detects the formation of at least one wave section (242) in the weld pool (24) following the process zone (22).

6. The observation device comprises a camera, according to claim 5.

7. The observation device comprises at least two photodiodes having different measurement positions, according to claim 5 or 6.

8. The observation device comprises an optical coherence tomography (OCT) device, as described in any one of claims 5 to 7.

9. The system according to any one of claims 5 to 8, wherein the observation device comprises an evaluation unit that uses a neural network trained to recognize wave portions (242) within the weld pool (24).

10. A computer program product comprising computer-readable instructions for performing the 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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