Techniques for predicting connection defects during laser welding

Abstract

The invention relates to a method for laser welding a bipolar plate (10), comprising the following steps: laser welding two at least partially overlapping connection partners (12, 14) along a predefined welding path in order to produce an overlapping connection between the connection partners (12, 14); at least one undulation (242) is formed in the weld pool (24) in the tail region of the treatment zone (22) as viewed during the welding process; in response to the observation, a measure is initiated to prevent or avoid the use of an incomplete connection (152) between the connection partners (12, 14) along at least a portion of the weld path. The invention also relates to a laser welding system and a computer program product for implementing the method.

Description

Technical Field

[0001] This invention relates to the field of laser welding. In particular, this invention relates to techniques for identifying specific irregularities during laser welding of bipolar plates, wherein these irregularities indicate the presence of join defects during the welding process. Background Technology

[0002] Several methods for optical monitoring of welding processes are known from the prior art. Reference is made in this context, by way of example only, to DE102018220342A1, which discloses such a monitoring method.

[0003] To manufacture bipolar plates for fuel cells, metal plates or foils are welded together in pairs. At least some of the welds connecting the plates must be fluid-tight to avoid compromising the subsequent function of the bipolar plates. Traditionally, an overlapping connection is used to weld a pair of bipolar plates together. First, the connecting mating pieces are positioned relative to each other and secured in place. Then, a laser welding beam moves along a pre-defined profile on the surface of one of the connecting mating pieces, penetrating the connecting mating piece into the adjacent connecting mating piece and creating a weld that extends at least partially into the “hidden” connecting mating piece. However, in this case, gaps exceeding a critical distance and unable to be fully bridged by the weld may form between the connecting mating pieces, for example, due to insufficient securing of the connecting mating pieces. This is referred to by those skilled in the art as a “connection defect.” Even a distance of 15 µm between the connecting mating pieces can lead to the formation of connection defects when welding bipolar plates.

[0004] When welding overlapping joints, joint defects must be avoided. One difficulty is that joint defects on welded products are often invisible to the naked eye because the resulting weld beads on the top side of the workpiece and the weld roots on the bottom side of the workpiece usually do not show any abnormalities. In cases of such joint defects that are not visible from the outside, those skilled in the art also refer to them as "false friends" (falschen Freunden).

[0005] Therefore, inspecting welds for connection defects is time-consuming and costly.

[0006] This invention aims to improve the reliability of tightness in welded overlapping joints. In particular, it should be possible to reliably assess the formation of joint defects early in the welding process so that appropriate measures can be taken. Summary of the Invention

[0007] The fundamental object of this invention is achieved by the subject matter of the independent claims. Further possible embodiments of the invention are described in detail in the dependent claims, the description, and the drawings. Features, advantages, and possible embodiments set forth in the description with respect to one of the subjects of the independent claims are considered to be at least similar to the features, advantages, and possible embodiments of the corresponding subjects of the other independent claims, as well as the features, advantages, and possible embodiments of any possible combination of the subjects of the independent claims, which may be features, advantages, and possible embodiments combined 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 includes laser welding two at least partially overlapping connecting mating parts along a pre-given welding path to create an overlapping connection between the connecting mating parts.

[0009] In the context of this invention, overlapping connections should be understood as welded connections, wherein the weld extends completely through the thickness of the first connecting pair (=upper connecting pair) and at least partially into the second (lower) connecting pair. For this purpose, a laser beam is guided along the welding path to the surface of the first connecting pair and melts it into the second connecting pair, or even penetrates the entire thickness of both connecting pairs. In this context, laser welding is preferably performed in a deep welding mode, in which the laser beam forms a vapor cavity in its processing zone interacting with the connecting pairs, wherein the vapor cavity extends deep into the connecting pairs and facilitates efficient welding. The welding process is preferably performed in a computer-controlled manner.

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

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

[0012] The feed rate of the laser beam moving along the welding path relative to the workpiece surface can be in the range of 100 mm / s to 5000 mm / s, particularly in the range of 300 mm / s to 2000 mm / s.

[0013] The processing optical unit through which the laser beam is guided to the workpiece and, in particular, focused, can have an imaging ratio of 1:1 to 5:1, preferably 1.5:1 to 2:1.

[0014] The method according to the invention further includes observing the formation of at least one wavy structure in the weld pool in the tail region (Nachlauf) of the processing zone during the welding process, the wavy structure preferably extending transversely to the feed direction.

[0015] The treatment zone refers to the area in which the laser beam interacts with at least one of the connecting mating parts during the welding process. In deep welding, the treatment zone is typically characterized by the formation of a vapor cavity that extends substantially over the entire depth of the subsequent weld. The material of the connecting mating parts melts within and around the treatment zone. The relative feed motion of the laser beam along the welding path relative to the workpiece surface produces an elongated weld pool. The temperature in the weld pool decreases with distance from the treatment zone. In the tail region of the treatment zone, the material of the connecting mating parts initially remains molten and subsequently solidifies into the weld. In this region between the treatment zone and the already solidified weld, optically perceptible changes in the weld pool dynamics may occur during the welding process. In this example, the inventors have recognized that a wavy structure is formed on the surface of the weld pool in the tail region of the treatment zone immediately before welding defects form. The term "wavy structure" here refers to at least one protrusion or depression on the surface of the weld pool. This at least one protrusion or depression may in particular have an elongated extension that is substantially transverse to the feed direction or aligned with the length of the weld pool. Changes in the weld pool can preferably be observed based on a single, continuously recorded image 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 radiation (or process beam) within the weld pool region. This can be detected optically.

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

[0018] In this context, "incomplete connection" should be understood as synonymous with connection defect. According to the inventors' observations, during the welding process, structural changes are already visible in the weld pool in the tail region of the processing area when incomplete joints (i.e., loose points) begin to form. Therefore, a connection defect may not have already occurred when the corresponding dynamic changes in the weld pool are first identified. However, if the welding process continues unchanged, a connection defect may occur. Depending on the circumstances, measures to prevent or address connection defects can be initiated by identifying structural changes in the weld pool.

[0019] For example, one measure to prevent incomplete joins may involve changing the welding parameters so that the welding process can continue without a join defect. Alternatively or additionally, the welding process may be interrupted and continued with the modified parameters, wherein the fastening may also be improved to prevent gaps between the joined parts. A measure to address incomplete joins may include, for example, re-welding the weld path, classifying the welded workpiece as scrap, or re-fastening the joined parts in areas suspected of having join defects. Improvements to the fastening of the joined parts may in particular include correcting or supplementing the clamping position, at which the joined parts are pressed against each other using appropriate clamping devices.

[0020] The observation process during welding according to the invention enables efficient classification of components (especially bipolar plates) that are highly likely to fail to meet weld tightness requirements. Furthermore, it improves the overall welding result by preventing the formation of connection defects during welding or for subsequent welding processes (e.g., by improving the fixation of the connecting mating parts).

[0021] When observing the weld pool at the tail end of the processing area, it can also be determined that the weld pool will extend. The extension of the weld pool at the tail end of the processing area can serve as another indication of the formation of connection defects.

[0022] Preferably, each of the connecting mating members can be made of a metallic material and has a thickness ranging from 5 µm to 500 µm, particularly from 50 µm to 300 µm. The connecting mating members can typically be metal foil based on iron, copper, or aluminum. For example, each of the connecting mating members can be made of stainless steel, such as 1.4404 type stainless steel. For example, bipolar plates used in fuel cells can be produced by welding the connecting mating members together. However, the application of the invention is not limited to the production of bipolar plates.

[0023] The method according to the invention may further include determining the location of the observed change in the weld pool along the welding path. For this purpose, the time elapsed since the start of the welding process can be continuously monitored while observing the weld pool. The location of the weld pool change along the welding path can be determined by comparing the feed rate of the welding process and the geometry of the welding path (both of which are known). This location indicates the appearance (or approximate start) of a join defect along the forming weld. Knowing the location of the observed weld pool change allows for targeted measures to address the (presumed) join defect. For example, clamping positions can be corrected and / or supplemented to prevent the formation of a critical gap between the joined mating parts, at which the two joined mating parts are fixed relative to each other. For example, this allows any geometric characteristics (e.g., deformation) of the joined mating parts to be welded to be taken into account for a batch of joined mating parts. Alternatively or additionally, welding parameters can be adjusted at “critical” locations along the welding path to counteract the formation of a join defect. Such adjustments can be particularly helpful if the join defect consistently appears in the same location during multiple consecutive welding operations of a batch of joined mating parts.

[0024] According to another aspect of the invention, a system for laser welding is provided. The system includes a laser welding system for laser welding two at least partially overlapping joining mating parts along a pre-given welding path to create an overlapping connection between the joining mating parts. The system also includes an observation device for optically monitoring the welding process, wherein the observation device is configured to identify the formation of a wavy structure in the weld pool in a region at the tail of the processing area. The system further includes a control device configured to activate measures to prevent or respond to incomplete connection of at least a portion of the joining mating parts along the welding path when the observation device identifies at least one wave formation in the weld pool in the region at the tail of the processing area.

[0025] The laser welding system can be a conventional laser welding system suitable for welding bipolar plates. At the time of this application, the applicant is marketing such a laser welding system in various embodiments. 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 entire processing can be performed on a separate computer (or computing device) or on a common computer (in particular a control device) used to control the entire system.

[0026] The observation device may include, for example, a camera device. For example, the camera device may use a CMOS sensor or a CCD sensor, or an InGaAs-based sensor (InGaAs = Indium Gallium Arsenide). The exposure time of the camera device may be in the range of 1 µs to 20000 µm, particularly in the range of 1 µs to 1000 µs. The image frame rate of the camera device may be at least 100 Hz, particularly at least 1000 Hz. The camera device may be specifically designed to observe wavelengths in the range of 300 nm to 2000 nm, particularly in the range of 600 nm to 1000 nm. Wavelengths of the processed laser beam, such as 1030 nm or 1070 nm, should not be detectable by the camera device. For example, the camera device may use a bandpass filter with a wavelength range of 600 nm to 1000 nm, or a broadband filter with a spectral width of at least 200 nm, wherein the wavelength of the processed laser beam (e.g., 1030 nm or 1070 nm) is blocked. One advantage of using a bandpass filter is good contrast.

[0027] The camera device can preferably be aligned with the workpiece surface via the beam path of the processing laser beam, specifically coaxial with the processing laser beam or with an angular offset of up to 15% relative to the processing laser beam. In particular, the field of view of the camera device 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 apparatus in this way, it can be ensured that the observation device is precisely aligned with the processing area or weld pool at every stage of the process.

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

[0029] In addition to, or as an alternative to, a camera device, an observation device may include at least two photodiodes with different measurement positions. Specifically, the at least two photodiodes may be aligned with measurement positions offset from each other in the feed direction at the tail region of the solder pool. Based on the intensity difference of process radiation detected by each photodiode, the formation of waves in the solder pool can be inferred. One advantage of using photodiodes is the lower data rate and higher sampling rate compared to using a camera device.

[0030] Alternatively, the observation device may include an optical coherence tomography (OCT) device. As an alternative to OCT, a laser interferometer can also be used. OCT or laser interferometers offer spatial resolution in the welding direction at the tail end of the processing area. One advantage of using OCT is its higher resolution compared to using photodiodes.

[0031] In the observation device, several of the aforementioned identification devices and / or additional identification devices can be combined. By combining them, the advantages of each identification device can be combined to obtain even better observation results.

[0032] Preferably, the observation device does not include any external lighting. Illumination is possible and can be used depending on alternative variations. However, by intentionally avoiding providing an illumination solution for the observation device, better identification of process radiation can be achieved. Illumination reflections will overlap with process radiation and may worsen the detectability of structural changes in the tail region of the weld pool.

[0033] The observation device may also include an evaluation unit that uses a neural network trained to identify waves (i.e., wavy structures) in the solder pool. The neural network may, in particular, be a convolutional neural network (CNN) with spatial resolution (≥2 pixels). This neural network may have a U-Net architecture and evaluate the optical signal provided by the observation device based on semantic segmentation. By using a neural network, the reliability and efficiency of identifying wavy structures can be improved.

[0034] According to another aspect of the invention, a computer program product is provided comprising computer-readable instructions for performing the methods of any of the above-described variations in a laser welding system according to any of the variations. This computer program product can be executed, for example, on the control device of the laser welding system according to the invention.

[0035] Example The following description of preferred embodiments, in conjunction with the accompanying drawings, will help to explain the present invention in more detail. Attached Figure Description

[0036] Figure 1 A schematic top view of a bipolar plate with a drawn processing area and a surrounding weld pool for laser welding processing is shown. Figure 2a -b schematically illustrates the overlapping connection between two mating connectors using cross-sectional views, where... Figure 2b The image shows a defect image of incomplete connection (connection defect); Figure 3a -b schematically illustrates the geometry of the weld pool during the creation of an overlapping connection between two mating components by means of laser welding; Figure 4 The image shown is a transmissive photograph of a weld with connection defects (Aufnahme); and Figure 5a -c indicates the generation Figure 4 The process radiation of laser welding treatment recorded at various time points during the weld seam shown.

[0037] In the accompanying drawings, the same elements or elements with the same function are provided with the same reference numerals. Detailed Implementation

[0038] Figure 1 A schematic top view of a bipolar plate 10 is shown. The bipolar plate 10 generally comprises two flat, structured connecting mating parts that essentially function as respective half-shells of the bipolar plate 10. These two connecting mating parts are connected to each other by a plurality of welds 15 and form fluid channels within the bipolar plate 10. Figure 1 (Not shown). The bipolar plate 10 halves are welded together by means of laser welding. For this purpose, the two connecting mating parts are precisely positioned and fixed relative to each other, and a processing laser beam is guided along the welding path (corresponding to the outline of the weld 15 shown) in the feed direction 30 to the upper connecting mating part to create an overlapping connection. In this process, the material of the connecting mating parts is liquefied by the action of the laser beam in the processing zone 22, mixed in the common weld pool 24 in the tail region of the processing zone, and finally solidified to form the weld 15.

[0039] Figure 2a and Figure 2b The overlapping connection between the first connecting mating member 12 and the second connecting mating member 14 of component 10 (e.g., bipolar plate) is shown respectively. Figure 2a In the middle, the connecting mating parts 12 and 14 are connected to each other by weld 15. According to Figure 2b Weld 15 extends through the two mating members 12 and 14. However, the gap 13 between the mating members 12 and 14 is not fully bridged by weld 15. Figure 2b Defect 152 in weld 15 results in an incomplete connection (connection defect) between mating parts 12 and 14. The connection is not tight at the location of connection defect 152. From the outside, defect 152 is not visible on part 10 because no suspicious irregularities are shown at the top and bottom of weld 15.

[0040] Figure 3a and Figure 3b The geometry of the weld pool 24 during laser (beam) welding is schematically shown. Figure 3a A top cross-sectional view of workpiece 10 or the upper connecting mating member 12 of workpiece 10 facing the laser welding optical unit used is shown. Figure 3bA cross-sectional side view extending parallel to the feed direction 30 of the processing laser beam shows the same weld pool 24. During laser welding, the materials of the connecting mating parts 12, 14 are heated to high temperatures in the processing zone 22 where the processing laser beam directly interacts with the workpiece. During deep welding, a dampkapillare extending over a significant portion of the weld depth is formed in the processing zone 22. The materials of the connecting mating parts 12, 14 melt around the dampkapillare. In the weld pool 24, the materials of the two connecting mating parts 12, 14 mix and solidify in the tail region of the processing zone 22 to form a connecting weld 15. When such an overlapping connection is formed, for example, if the gap 13 between the connecting mating parts 12, 14 exceeds a critical size, the resulting weld 15 may not be able to completely bridge the gap 13 between the connecting mating parts 12, 14, resulting in a partial connection defect 152. The inventors of this invention have determined that, immediately before the formation of the weld defect 152, at least one wavy structure 242 is formed on the surface of the weld pool 15 in the tail region of the processing zone 22. The wavy structure 242 is generally oriented perpendicular to the feed direction 30. Based on this knowledge, measures can be taken to prevent or address the weld defect 152 during the welding process.

[0041] Figure 4 A side view of a perspective image of weld 15 is shown. The image shows the area of ​​the lower (second) connecting mating member 14 and the upper (first) connecting mating member 12, separated from each other by a narrow gap region (see 13). Bright lines are partially visible at the height of gap 13, each indicating a connection defect 152 in weld 15. The weld 15 shown extends through the bipolar connecting mating members 12 and 14 and was generated with a laser power of 200 W and a feed rate of 500 mm / s.

[0042] Figure 5a , Figure 5b and Figure 5c The following are examples of methods used to generate... Figure 4Images 100-X1, 100-X2, and 100-X3 are captured by a camera device on the weld pool 22 and processing area 24 during the welding process of weld 15. These images are captured along the beam path of the processing laser beam—that is, a beam path perpendicular or substantially perpendicular to the surface of the upper connecting mating member 14—without additional illumination. In images 100-X1, 100-X2, and 100-X3, the process radiation at defined time points X1, X2, and X3 during the welding process can be identified as bright areas. Additionally, graphs 200-X1, 200-X2, and 200-X3 show sets of curves indicating the intensity of the process radiation in the observation area. The curve with the maximum deflection shows the intensity along a line extending in the feed direction and penetrating the center of the weld pool 24, where the maximum process radiation occurs. The flatter lines in each set of curves indicate the intensity gradient in the edge regions of the weld pool 24.

[0043] Figure 5a This shows the situation during laser welding process corresponding to Figure 4 The process radiation at position X1 of weld 15 in the diagram. No connection defect 152 exists at position X1 (see...). Figure 4 The bright areas in the captured image 100-X1 show the process radiation in the processing area 22, which is also reflected in the intensity curve I-22 in the graph 200-X1.

[0044] Figure 5b The location X2 of weld 15 is shown (see Figure 4 This refers to the process radiation shortly before the formation of the weld defect 152. On one hand, image 100-X2 shows the process radiation in the processing zone 22 of the weld pool 15, where the keyhole is clearly visible as an intensity gap at the center. On the other hand, image 100-X2 shows another bright area 242 in the tail region of the processing zone 22. In graph 200-X2, this area is shown by the deflection of the intensity curve I-242. The shape of the process radiation in the tail region of the processing zone 22 is caused by the wave 242 formed in the weld pool 22 during the welding process.

[0045] at last, Figure 5c The location X3 of the connection defect 152 in weld 15 is shown (see Figure 15). Figure 4 The process radiation is shown in image 100-X3 and the corresponding intensity curve 200-X3 again, indicating high process radiation in the region of processing area 22 with a identifiable keyhole (see also I-22 in figure 200-X3). No additional intensity peaks were identified in the tail region of the processing area.

[0046] According to the inventor's observations Figure 5bThe irregularities in the weld pool 24 shown are only visible shortly before the formation of subsequent connection defects 152 or at the beginning of their formation. At the time of invention, no conclusion could be drawn about the occurrence or risk of connection defects from any irregularities in the process radiation in the area of ​​the processing zone 22.

Claims

1. A method for laser welding a bipolar plate (10), the method comprising the following steps: Laser welding is performed on two at least partially overlapping connectors (12, 14) along a pre-defined welding path to create an overlapping connection between the connectors (12, 14); During the welding process, at least one wavy structure (242) was observed to form in the weld pool (24) in the tail area of ​​the treatment zone (22). In response to the observation, measures are initiated to prevent or address incomplete connection (152) of at least one portion of the connecting mating parts (12, 14) along the welding path.

2. The method according to claim 1, in, It was also observed that the weld pool (24) extends into the tail region of the processing area (22).

3. The method according to any one of the preceding claims, in, Each of the connecting mating parts (12, 14) is made of a metallic material and has a thickness in the range of 5µm to 500µm, particularly in the range of 50µm to 300µm.

4. The method according to any one of the preceding claims further includes the following steps: Determine the location of the observed changes in the weld pool (24) along the welding path.

5. A system for a laser welding method, the system comprising: A laser welding system for performing laser welding on two at least partially overlapping connectors (12, 14) along a pre-given welding path to create an overlapping connection between the connectors (12, 14); An observation device for optically monitoring the welding process, wherein the observation device is configured to identify the formation of a wavy structure (242) in the weld pool (24) in the tail region of the processing area (22); and A control device is configured to activate measures to prevent or respond to incomplete connection (152) of the connecting mating parts (12, 14) along at least a portion of the welding path when the observation device detects that at least one wave (242) is formed in the weld pool (24) in the tail region of the processing area (22).

6. The system according to claim 5, in, The observation device includes a camera.

7. The system according to claim 5 or 6, in, The observation device includes at least two photodiodes with different measurement positions.

8. The system according to any one of claims 5 to 7, in, The observation device includes an optical coherence tomography (OCT) device.

9. The system according to any one of claims 5 to 8, in, The observation device includes an evaluation unit that uses a neural network trained to identify waves (242) in the weld pool (24).

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