WO2023131468A1

WO2023131468A1 - Method for laser welding a flow field plate of a fuel cell, having a weld pool produced using a plurality of laser spots

Info

Publication number: WO2023131468A1
Application number: PCT/EP2022/085024
Authority: WO
Inventors: Oliver BOCKSROCKER; Nicolai Speker; Tim Hesse; Philipp Scheible
Original assignee: Trumpf Laser- Und Systemtechnik Gmbh
Priority date: 2022-01-05
Filing date: 2022-12-08
Publication date: 2023-07-13
Also published as: DE102022100187A1

Abstract

The invention relates to a method for laser welding a flow field plate (1) for a fuel cell, two plate parts (1a, 1b) being welded to one another along at least one weld seam (2, 2a, 2b, 2c). According to the invention, the laser welding is carried out along the at least one weld seam (2, 2a, 2b, 2c) using a laser beam ensemble (34) comprising at least three individual beams (33), the individual beams (33) each producing a laser spot (5a-5e, 5a'-5c') on a surface (4) of the plate parts (1a, 1b), and in that the at least three individual beams (33) of the laser beam ensemble (34) produce a common weld pool (9) in the plate parts (1a, 1b). The invention provides a method which can achieve good fluid-tightness of weld seams of a flow field plate with high reliability.

Description

Process for laser welding a bipolar plate of a fuel cell, with a weld pool generated with several laser spots

The invention relates to a method for laser welding a bipolar plate for a fuel cell, two plate parts being welded to one another along at least one weld seam.

Such a method has become known from the subsequently published German patent application 10 2021 113 834.5.

In fuel cells with several cells layered to form a stack, bipolar plates are used for the distribution of gases, in particular hydrogen and oxygen, the removal of water (water of reaction), the gas-tight separation between adjacent cells and the seal to the outside and cooling. In addition, the bipolar plate on the hydrogen side absorbs the electrons that are released and feeds them back to the oxygen side.

Such bipolar plates can have two metallic plate parts that are welded together. On the one hand, weld seams must be fluid-tight lead to guide gases and water in defined paths. On the other hand, weld seams serve to connect the two plate parts electrically and mechanically. It has become known from the subsequently published German patent application 10 2021 113 834.5 to produce at least one peripherally closed first weld seam with a first seam width and at least one second weld seam with a second seam width when laser welding plate parts of a bipolar plate, the second seam width being larger than the first seam width.

With conventional laser welding of the weld seams of a bipolar plate with a laser beam, there can be noticeable fluctuations in the seam quality, for example with regard to the welding depth and the seam width, and these fluctuations can also affect the tightness of the weld. Microcracks can appear in the weld seam, which can weaken the mechanical and electrical connection of the weld, and the gas-tightness can also be impaired as a result. The mechanical and electrical connection of the weld and its tightness can basically be improved by a wider weld seam that is made with a larger laser spot; however, the use of large laser spots can lead to additional pore formation in the weld seam, which in turn degrades the quality of the weld seam. object of the invention

It is the object of the invention to provide a method with which a good fluid tightness of weld seams of a bipolar plate can be achieved with high reliability.

Description of the invention This object is achieved according to the invention by a method of the type mentioned at the beginning, which is characterized in that the laser welding takes place along the at least one weld seam with a laser beam ensemble comprising at least three individual beams, the individual beams each generating a laser spot on a surface of the plate parts, and that the at least three individual beams of the laser beam ensemble produce a common melt pool in the plate parts. According to the invention, a molten bath, with which the at least one weld seam of the plate parts of the bipolar plate is produced, is not produced with a single laser beam, but with a laser beam ensemble comprising several individual beams (individual laser beams). These individual beams are directed onto the surface of the plate parts (in other words, onto the workpiece surface facing the individual beams). The laser spots generated by the individual beams are close enough together to create a common (continuous) melt pool in the plate parts. Since, within the scope of the method according to the invention, a plurality of laser spots jointly produce a single, common molten pool, the procedure according to the invention is also referred to as multi-spot welding with a mono molten pool.

The individual beams of the laser beam ensemble are applied at the same time and are guided together (synchronously) along the weld seam relative to the plate parts as part of a feed.

Due to the simultaneous action of the multiple individual beams or multiple laser spots simultaneously on the surface of the plate parts or the melt pool, a qualitatively better and, above all, more precise weld can be achieved, particularly in the weld-in regime. The weld can be made with fewer defects, in particular a significant reduction in microcracks in the weld can be achieved, especially at high ones Feed speeds, for example 1500 mm/s or more, or even 2000 mm/s or more.

In addition, it is possible to also produce comparatively wide weld seams using the method according to the invention without a relevant increase in porosity or other defects occurring. A wider weld seam can contribute to an improved mechanical and electrical connection between the plate parts and, above all, to increased fluid tightness. Typically, at least some of the laser spots of the individual beams are at least partially offset from one another in the direction transverse to the welding direction, and the width B of the weld seam produced with the laser beam ensemble (in one pass of the laser beam ensemble) is typically at least 2 times as large, preferably at least 3 times as large , such as the largest laser spot diameter GDL in the laser beam ensemble (i.e. B>2*GDL or preferably B>3*GDL), in the direction transverse to the (local) welding direction.

Overall, a molten pool that is stable under the applied feed can be achieved, which solidifies in a uniform manner, so that the weld seam obtained has only few fluctuations in the seam quality over its length (in particular in the seam width or the welding depth). Laser welding is carried out with high precision, especially in the welding regime. Accordingly, good (minimum) fluid tightness, and in particular gas tightness, of the weld seams of the bipolar plate can be ensured with high reliability.

The laser spots of the laser beam ensemble pass over the at least one welding seam completely (at least) once. If desired, additional crossings of the weld seam can be carried out with one or more laser beams, in particular again with an ensemble of laser beams, for example with the same ensemble of laser beams. Note that

Welding curves of different passes, on which the laser beam(s) of the individual passes are guided during the respective passes, one another (with no lateral offset), or offset from one another in a direction transverse to the direction of welding (with lateral offset), while lying on a centerline of the weld, or offset from a centerline of the weld in a direction transverse to the direction of welding could be. The location of one or more laser beams (in particular a laser beam ensemble) of a pass can be determined based on the center of gravity of the laser power applied (on the workpiece surface). If only a single pass is used for laser welding the weld seam, the associated welding curve of the laser beam ensemble typically lies on the center line of the weld seam. Within the scope of the invention, all crossings of the weld seam are preferably carried out with an ensemble of laser beams when producing a weld seam.

The several individual laser beams of the laser beam ensemble can be generated

- by means of several laser sources and several laser optics, or

- by means of several laser sources and laser optics, or

- by means of a laser source and several laser optics (e.g. using a laser source with several light paths, among which the laser beam can be switched, with a different laser optic being connected to each light path via a fiber optic cable), or

- by means of a laser source and a laser optics and an optical one

Element for dividing an original laser beam into several laser beams, e.g. B. Multifocal lens, optical wedge plate, diffractive optical element (DOE) or refractive optical element (ROE).

Within the scope of the invention, the laser welding typically takes place as deep welding (that is to say with at least one vapor capillary; the vapor capillaries produced by the individual beams typically merge into one another at least in an upper region); in this way, in particular, a high feed rate can be achieved. The laser welding of the plate parts arranged in an overlapping manner can be carried out as welding in or through welding. Preferred the weld seam produced with the laser beam ensemble (in a plane perpendicular to the welding direction) has an aspect ratio AV=T/B with AV>1.5, particularly preferably AV>2.0, and in some cases even AV>3, with T: welding depth and B: width of the individual weld seam (on the workpiece surface). If the weld seam is passed several times, the previously mentioned aspect ratios are present for the weld seam of a respective individual pass (with the laser beam ensemble).

The laser spots of the laser beam ensemble are preferably aligned in such a way that the orientation of the laser spots in relation to the feed direction can be arbitrary without noticeably influencing the laser welding (in particular with an equal power distribution between the laser spots); then cornering along the weld seam is particularly easy. Alternatively, a specific orientation of the laser spots to the feed direction can also be specified (in particular with an unequal power distribution of the laser spots); when cornering along the weld seam, the laser beam ensemble must then also be rotated (for example by rotating a corresponding optical element for beam splitting into the individual beams). The laser spots of the individual beams of the individual beams on the workpiece surface are preferably separate from one another, as a result of which intensity peaks due to superimposition of laser spots in the melt pool are avoided and the melt pool dynamics can be calmed down. However, it is also possible for laser spots of individual beams to overlap, in particular partially overlap, as a result of which the (common) vapor capillary can be stabilized in some applications. In general, the individual beams produce adjacent laser spots on the workpiece surface (with respect to their laser spot centers).

The metallic plate parts of the bipolar plate are arranged overlapping for welding (typically with a congruent, aligned edge contour). The plate parts of the bipolar plates typically have a profiling through which channels for cooling water and external guides can be formed between the plate parts. ments for water of reaction and/or gases, in particular hydrogen and oxygen, are formed. In addition, the bipolar plates typically have one or more openings with which gas can be transported in the stack direction in the fuel cell stack. Within the scope of the invention, ty-

5 randomly closed welds are made on the outer edge of the plate parts and around all penetrations, and non-closed welds are also placed across the face of the bipolar plates. The fluid tightness, in particular the gas tightness, is particularly important for the closed weld seams, and the mechanical and electrical connection is particularly important for the open weld seams. The invention can be applied to both closed welds and non-closed welds.

Preferred variants of the invention 5

In a preferred variant of the method according to the invention, it is provided that at least three individual beams of the laser beam ensemble form a beam set, and that the laser spot centers of the laser spots of the beam set are arranged in a ring formation. The beam set can include three, four, five or six laser spots, for example. The laser spot centers on the workpiece surface in a ring formation contribute to a smooth weld pool; In addition, a directional dependency of the laser welding can be reduced with the laser beam ensemble. Note that the beam set may include all of the individual beams of the laser beam ensemble, or alternatively in addition to that

Radiation set, the laser beam ensemble can include one or more other individual beams.

A further development of this variant is particularly preferred, in which the laser spot centers of the radiation set form a regular polygon on the surface of the plate parts. As a result, the molten pool can be calmed further and the directional dependency can be further reduced. A further development in which the laser spots of the radiation set are separate from one another on the surface of the plate parts is also advantageous. The laser spots of the beam set therefore do not overlap. This avoids local peaks in laser intensity, which means that the weld pool can be kept particularly quiet.

A further development is also preferred in which the laser spots of the radiation set have the same diameter and/or the individual beams of the radiation set have the same laser power. This also contributes to calming the weld pool and further reducing the directional dependency.

A further development is also advantageous which provides that for a distance a between the laser spot centers of adjacent laser spots in the ring formation: a<10*GDR, with GDR: largest laser spot diameter in the ring formation. This stabilizes the vapor capillary and thus the energy input into the weld pool. The desired welding depth can be set particularly precisely over the entire course of the weld seam, so that a high level of gas tightness is achieved. A common, continuous melt pool can be easily ensured and the melt pool dynamics are kept low. In many cases, a<5*GDR also applies.

In another preferred further development of the above variant of the method, it is provided that the laser beam ensemble has a further individual beam, with a laser spot center of the further laser spot being arranged in a center of the ring arrangement of the beam set. By doing this, an intensity minimum in the middle of the radiation set can be minimized, which can contribute to the stabilization of the (common) vapor capillary. It should be noted that the further individual beam can have a different laser spot diameter (in particular a smaller laser spot diameter) than the laser spots of the beam set, and/or that the further individual beam can have a different laser power (in particular a smaller laser power) than the individual beams of the beam set. sets.

In an alternative, particularly preferred development of the above variant of the method, it is provided that at least three further individual beams of the laser beam ensemble form an additional beam set, and that the laser spot centers of the laser spots of the additional beam set are arranged in an additional ring formation, with the additional ring formation is arranged concentrically with the ring formation, and wherein a respective distance of the laser spot centers of the laser spots of the additional ring formation to a center of the ring formation is smaller than a distance of the laser spot centers of the laser spots of the ring formation to the center of the ring formation. With this procedure, a low laser intensity in the area of the center of the ring formation of the radiation set can be minimized, which can contribute to the stabilization of the (common) vapor capillary. It should be noted that the other individual beams can have a different laser power (in particular a lower laser power) than the individual beams of the beam set. The laser spot centers of the additional ring formation typically form a regular polygon. Typically, the diameters of the laser spots and the laser powers for the other individual beams of the additional beam set are all the same. The laser spots of the additional beam set preferably do not overlap one another, but preferably they overlap with the laser spots of the beam set.

A sub-variant of this further development is preferred, in which the following applies for a maximum diameter DZ of the laser spots of the additional set of rays and for a maximum diameter DS of the laser spots of the set of rays: DZ<DS. Typically, DZ<0.5*DS or DZ<0.3*DS also applies. The additional laser power of the additional beam set can then be better concentrated on the area or areas of the beam set with low laser intensity, and the laser welding can take place with particularly high precision.

A variant of the method according to the invention is preferred, in which case for a maximum laser spot diameter GDL in the laser beam ensemble and for a smallest laser spot diameter KDL in the laser beam ensemble applies: KDL>l/10*GDL. This has proven itself in practice, especially in order to keep the expenditure on equipment low and to make a noticeable contribution to the heating of the melt pool even with small laser spots, without generating strong intensity peaks.

Within the scope of the invention, diameters of laser spots can generally be determined using the 86% criterion, according to which the diameter of a laser spot is determined in such a way that 86% of the total laser power of the associated individual beam (on the workpiece surface) is within a circle with it

diameter, with the circle having its center at the central axis of the single beam.

A variant is particularly preferred in which the laser spots of the individual beams of the laser beam ensemble form an arrangement on a surface of the plate parts which has an N-fold rotational symmetry, with N>3. As a result, the directional dependency of the laser welding with the laser beam ensemble can be minimized. An optical element for dividing a primary laser beam into the individual beams then generally does not need to be rotated along the weld seam when the laser beam ensemble travels around curves, which considerably simplifies the process. Note that as N increases, the directional dependence decreases. N>5 is particularly preferred and N=5 or N=6 is particularly preferred. Note that in the case of a laser beam ensemble, the number N is determined in such a way that the N indicates the highest number (largest N) that is given with the laser beam ensemble; are for example six

Laser spots arranged in a regular hexagon, the multiplicity is determined to be N=6 (six-fold rotational symmetry/rotary symmetry), although there is also a 2-fold and a 3-fold rotational symmetry at the same time. In the case of an N-fold rotational symmetry, the laser spot arrangement is converted into itself with a rotation of 360°/N in each case.

Also advantageous is a variant in which the laser beam ensemble is divided by at least one optical element for dividing at least one primary laser beam is produced. This is a simple possibility in terms of apparatus to generate the laser beam ensemble or its individual beams.

In a preferred variant it is provided that at least some of the individual beams are in the form of superimposed individual laser beams, each comprising at least two partial beams which lie one inside the other on the surface. Typically, a lower local laser power density is set up in a radially outer part of the superimposed individual laser beam and a locally higher power density is set up in a radially inner part of the superimposed individual laser beam. As a result, a (local) vapor capillary in the area of the superimposed single laser beam can be stabilized during deep-penetration welding and the weld pool dynamics can be reduced. The superimposed single laser beam typically has concentric partial beams on the workpiece surface. For the (outer) diameters dwinner (e.g. in a core jet with core diameter KD) and dwoutside (e.g. in a ring ray with ring diameter RD) of the two partial jets lying inside one another, the ratio dwinnemdwoutside (or KD: RD) is typically in the range of 1:1.1 to 1:6, preferably 1:3 to 1:5, particularly preferably 1:4. A further development of this variant is preferred, which provides that the superimposed individual laser beam comprises a core beam and a ring beam that surrounds the core beam, or that the superimposed individual laser beam has a larger partial beam and a smaller partial beam that is located on the workpiece surface within the larger partial beam lies, includes. This procedure is particularly simple and has proven itself in practice. A heterodyne single laser beam with core beam and ring beam is typically generated with a 2-in-1 fiber, and the core beam and ring beam have a common optical axis.

Also advantageous is a further development of the above variant, which provides that an original laser beam is guided in a multiclad fiber, comprising a core fiber and a ring fiber surrounding the core fiber, and that one from the Multiclad fiber exiting original laser beam is divided by means of an optical element on at least part of the individual beams. This is a simple procedure in terms of equipment to set up a core beam and a ring beam in several individual beams.

A variant is also preferred in which the plate parts each have a sheet metal thickness BLD of between 50 μm and 150 μm. The plate parts are preferably made of stainless steel, e.g. type 1.4404. The sheet thickness BLD is preferably 75 μm. Corresponding metallic plate parts can be produced inexpensively and are particularly well suited to the requirements in a bipolar plate of a fuel cell, in particular with regard to corrosion resistance, electrical conductivity and workability during laser welding. Sheet thicknesses between 50 pm and 150 pm combine sufficient robustness with a light and material-saving construction. 5

A variant of the method according to the invention is also preferred, where:

- The individual beams are generated with an infrared laser and have a mean wavelength of between 800 nm and 1200 nm, preferably 1030 nm or 1070 nm, or the individual beams are produced with a VIS laser with a mean wavelength of between 400 nm and 450 nm or between 500 nm and 550 nm generated; and or

The respective beam parameter product SPP of the individual beams is between 0.38 mm*mrad and 16 mm*mrad, preferably with SPP<0.6 mm*mrad, particularly preferably with SPP<0.4 mm*mrad; and or

- The beam diameter dwl of the first used in the welding direction

The individual beam on the workpiece is between 10 pm and 300 pm, preferably with 30 pm<dwl<50 pm in single mode or 50 pm<dwl<170 pm in multi-mode, and the beam diameter dwx of all other individual jets is chosen to be 0.1*dwl<dwx <10*dwl, preferably with dwl=dwx; and or - The laser power P per individual beam is between 10W and 2000W, preferably with 50W<P<700W; and or

- A feed VS of the laser beam ensemble is between 100 mm/s and 5000 mm/s, preferably with 300 mm/s<VS<2000 mm/s; and or

- An imaging ratio AV of laser optics, with which the individual beams are imaged onto the workpiece, is between 1:1 and 5:1, preferably with 1.5:1<AV<2:1.

In practice, these parameters have proven particularly useful for the production of the bipolar plates according to the invention.

A variant is also preferred in which the at least one weld seam comprises one or more self-contained weld seams. Closed weld seams are usually used to seal off fluids in the fuel cell (cooling water, reaction water, reaction gases, e.g. hydrogen, oxygen). Within the scope of the invention, closed weld seams can be manufactured with improved tightness, which makes the invention particularly advantageous here. Also preferred is a variant in which the at least one weld seam comprises at least one closed weld seam running around the outside of the plate parts. The closed weld seam running around the outside ensures in particular that no coolant (cooling water) escapes into the reaction chambers of a fuel cell stack, and no reaction gases (such as hydrogen and oxygen) can get between the plate parts or even mix in an uncontrolled manner. Accordingly, the improved tightness that can be achieved with the invention is of particular advantage here.

Also within the scope of the present invention is a bipolar plate for a fuel cell, produced by welding two plate parts according to the method according to the invention described above. manufactured in such a way Te bipolar plates are distinguished by good fluid tightness at the at least one weld seam, and by a good mechanical and electrical connection between the plate parts. Further advantages of the invention result from the description and the drawing. Likewise, the features mentioned above and those detailed below can be used according to the invention individually or collectively in any combination. The embodiments shown and described are not to be understood as an exhaustive list, but rather have an exemplary character for the description of the invention.

Detailed description of the invention and drawing

The invention is illustrated in the drawing and will be explained in more detail using exemplary embodiments.

1 shows a schematic plan view of a bipolar plate according to the invention with two plate parts which are connected to one another by a plurality of welded seams which are closed all the way round and a plurality of welded seams which extend in a straight line;

2 shows a schematic top view of the welding of two plate parts of a bipolar plate according to a first variant of the invention, with a laser beam ensemble comprising three individual beams that form a beam set with three laser spots;

3 shows three laser spots of a radiation set of a laser beam ensemble on a surface for welding two plate parts of a bipolar plate according to a second variant of the invention;

4 shows three laser spots of a radiation set and three further laser spots of an additional radiation set of a laser beam ensemble on a surface for welding two plate parts of a bipolar plate according to a third variant of the invention;

5 shows five laser spots of a radiation set of a laser beam ensemble on a surface for welding two plate parts of a bipolar plate according to a fourth variant of the invention; 6 shows three laser spots that are generated by three individual beams of a laser beam ensemble, the individual beams being designed as superimposed individual laser beams, for welding two plate parts of a bipolar plate according to a fifth variant of the invention; 7 shows a schematic representation of a forming arrangement for the invention in cross section, with which an exiting original laser beam can be provided for the method according to the invention;

FIG. 8 shows an exemplary welding optics for dividing a primary laser beam into a laser beam ensemble for the method according to the invention. 0

1 shows a schematic top view of a bipolar plate 1 according to the invention for a fuel cell not shown in detail here; the bipolar plate 1 shown was produced within the scope of a method according to the invention.

The bipolar plate 1 is made up of an upper plate part 1a and a lower plate part 1b. The two plate parts la, lb of the bipolar plate 1 are arranged one above the other. The two plate parts la, lb have a profile (not shown). The profiling forms a system (e.g. a meandering or double meandering system) of different channels. The channels between the two plate parts la, lb are cooling fluid channels (typically for cooling water). The channels on the outer surfaces of the two plate parts la, lb are ducts for gas (such as oxygen or hydrogen) and water (which occurs as reaction water in the fuel cell). The two plate parts la, lb are made of a metallic material, e.g. B. stainless steel. A sheet thickness BLD of the plate parts 1a, 1b is 75 pm here; Sheet thicknesses of between 50 μm and 150 μm are generally preferred.

The two plate parts 1 a , 1 b are connected to one another by a large number of weld seams 2 (after application of the method according to the invention). The weld seams 2 are shown schematically in dashed lines based on their center lines. A circumferentially closed weld seam 2a runs on the outside of the two plate parts 1a, 1b. Two closed weld seams 2b run around two openings 3, which extend through the bipolar plate 1. Several open (here straight) welds 2c also run on the bipolar plate te 1. The weld seams 2a, 2b are designed to be fluid-tight, in particular gas-tight. The weld seams 2c are used for the mechanical and electrically highly conductive connection between the two plate parts 1a, 1b. The present inventive method was applied to all welds 2a, 2b, 2c.

FIG. 2 shows a first variant of the method according to the invention, which is applied to the plate parts 1a, 1b of a bipolar plate 1 (as explained in FIG. 1). The first variant is explained using weld seam 2a as an example. The weld seam 2a in production is shown schematically in broken lines using its center line on a surface 4 ("workpiece surface"; upper side of the plate part 1a). The weld seams 2b, 2c are also shown schematically in broken lines using their center lines.

The welding seam 2a is produced in the variant shown in FIG. 2 by means of a laser beam ensemble with three individual beams which are directed from above (perpendicular to the plane of the drawing) onto the upper plate part 1a. The three individual beams of the laser beam ensemble together form a beam set. The three individual beams of the beam set generate three laser spots 5a, 5b, 5c on the surface 4 of the two plate parts 1a, 1b. The three laser spots 5a, 5b, 5c each have a laser spot center 6a, 6b, 6c. In the variant shown, the laser spots 5a, 5b, 5c overlap slightly in their edge regions.

The laser spots 5a, 5b, 5c of the radiation set are moved along a welding curve 8 as part of a feed (relative to a common center of gravity 7 of the laser spots 5a, 5b, 5c). The welding curve 8 runs along the welding seam 2a (here on the center line of the welding seam 2a, the welding curve 8 is here identical to the center line of the welding seam 2a). The direction of feed is marked with an arrow; the feed runs along a (local) welding direction SR.

Around the laser spots 5a, 5b, 5c, the individual beams produce a common melt pool 9 of melted plate material. In the area of the laser spots 5a, 5b, 5c, the common melt pool 9 is widest. In contrast to the welding direction SR. the common molten pool 9 gradually becomes narrower.

According to the invention it is provided that for the production of the weld seam 2a

5 used, at least three individual beams of the laser beam ensemble (or the individual laser spots 5a, 5b, 5c) generate a common melt pool 9. In other words, at no time during the process do the individual beams of the laser beam ensemble produce separate molten pools 9 that are separated from one another by plate material that has not been melted. 0

After a complete circuit of the laser beam ensemble and thus of the common melt pool 9, the weld seam 2a is completely produced in the variant shown. The simultaneous action of the three individual jets allows for a better quality weld and a more precise weld;

The finished weld 2a can be made with fewer defects, and in particular, the weld 2a with few microcracks (compared to using only a single beam) can be obtained. In addition, a comparatively wide weld seam 2a is produced, through which an improved mechanical and electrical connection between the plate parts la, lb

20 can be obtained and good fluid tightness can be obtained.

The individual beams of the laser beam ensemble can be generated with an optical element from a common original laser beam (see FIG. 8); the three individual beams are preferably also guided via a common scanning device. In order to keep the orientation of the laser beam ensemble constant with respect to the local welding direction, the optical element can also be pivoted in curves of the weld seam 2a according to the curve, which enables a very compact construction (not shown in detail). In the variant shown, the laser spot 5a runs ahead and the laser spots 5b, 5c run behind. 0

Fig. 3 shows a schematic representation of three laser spots 5a, 5b, 5c of a beam set of three individual beams of a laser beam ensemble on a surface. According to a second variant of the invention, two plate parts of a bipolar plate can be welded with the three laser spots 5a, 5b, 5c. The three laser spots 5a, 5b, 5c are all the same size here, and the diameter dw of the laser spots 5a, 5b, 5c on the workpiece is 100 μm in each case. The three laser spot centers 6a, 6b, 6c are in the center of the three laser spots 5a, 5b, 5c. The laser spots 5a, 5b, 5c are separate from one another here; they do not touch or overlap each other. The laser power of the individual beams is chosen to be the same here. Correspondingly, an average power density of the three laser spots 5a, 5b, 5c on the surface is also the same here. In this way, a melt pool generated on the surface by the lasers 5a, 5b, 5c can be kept particularly still, since local peaks in the laser intensity of the laser beam ensemble are avoided.

The laser spots 5a, 5b, 5c are arranged in a ring formation 10 which encloses an inner surface. In the variant shown, the corner points of a regular polygon 11, here a regular triangle 11a (shown as a dotted line), can be defined by the laser spot centers 6a, 6b, 6c. The distances a between the laser spot centers 6a, 6b, 6c from adjacent, i. H. in the ring formation 10 adjacent laser spot centers 6a, 6b, 6c are here each 108 pm. In the variant shown here with three laser spots 5a, 5b, 5c in the ring formation 10, all the laser spot centers 6a, 6b, 6c are next to one another.

The laser spots 5a, 5b, 5c of the individual beams of the laser beam ensemble have a three-fold rotational symmetry here, since the ring formation 10 is formed by the three laser spots 5a, 5b, 5c, which are formed by rotating through 360°/3=120° around a center 12 ( Focus) of the ring formation 10 can be converted into each other. The laser spot centers 6a, 6b, 6c of the laser spots 5a, 5b, 5c of the ring formation 10 lie here on a circular line 36 (shown in dashed lines) around the common center 12.

The ring arrangement 10 can calm the melt pool dynamics and reduce the directional dependency. 4 shows a schematic representation of three laser spots 5a, 5b, 5c of a set of three individual beams and three laser spots 5a', 5b', 5c' of an additional set of three additional individual beams of a laser beam ensemble on a surface. With the three laser spots 5a, 5b, 5c and the three laser spots

According to a third variant of the invention, two plate parts of a bipolar plate can be welded 5 5a′, 5b′, 5c′.

The three laser spots 5a, 5b, 5c of the beam set are all the same size here, and the diameters dw of the laser spots 5a, 5b, 5c on the workpiece are each 100 μm here. The diameter dw is also the maximum

Diameter DS of the laser spots 5a, 5b, 5c of the beam set. The three laser spot centers 6a, 6b, 6c are in the center of the three laser spots 5a, 5b, 5c. The laser spots 5a, 5b, 5c are separate from one another here; they do not touch or overlap each other. 5

The three laser spots 5a', 5b', 5c' of the additional beam set are also all the same size here, and the diameter dw' of the laser spots 5a', 5b', 5c' on the workpiece is 20 μm here. The diameter dw' is also the maximum diameter DZ of the laser spots 5a', 5b', 5c' of the

20 additional ray sets. In the center of the three laser spots 5a', 5b', 5c' are three laser spot centers 6a', 6b', 6c'. The laser spots 5a', 5b', 5c' touch each other exactly here. In addition, laser spot 5a partially overlaps laser spot 5a', laser spot 5b partially overlaps laser spot 5b', and laser spot 5c partially overlaps laser spot 5c'. With the laser beam ensemble shown, the low laser intensity in the

25 rich of the center 12 of the ring formation 10 of the laser spots 5a, 5b, 5c are compensated.

The distances a between the laser spot centers 6a, 6b, 6c of the adjacent laser spot centers 6a, 6b, 6c in the ring formation 10 are all the same size and are each 108 μm here.

The laser spots 5a', 5b', 5c' are arranged in an additional ring formation 10'.

The additional ring formation 10 ′ is arranged concentrically with the ring formation 10 .

The distances a' between the laser spot centers 6a', 6b', 6c' from the adjacent The laser spot centers 6a', 6b', 6c' lying next to one another, ie neighboring in the additional ring formation 10', are all of the same size and are each 20 μm here. The distances a′ of the laser spot centers 6a′, 6b′, 6c′ of the laser spots 5a′, 5b′, 5c′ adjacent in the additional ring formation 10′ therefore correspond to the sum of the respective half diameters dw′ of the laser spots 5a′, 5b ', 5c'.

The laser spots 5a, 5b, 5c, 5a', 5b', 5c' of the individual beams of the laser beam ensemble have a three-fold rotational symmetry here, since the ring formation 10 and the additional ring formation 10' are rotated by 360°/3=120° overall the center 12 can be converted into each other.

The laser spot centers 6a, 6b, 6c of the laser spots 5a, 5b, 5c of the ring formation 10 lie here on a circular line 36 (shown in broken lines) around the common center 12, corresponding to a regular polygon. The laser spot centers 6a′, 6b′, 6c′ of the laser spots 5a′, 5b′, 5c′ of the additional ring formation 10′ also lie on a circular line (shown in dashed lines) around the common center 12 and correspond to the corners of a regular polygons.

The distances az of the laser spot centers 6a, 6b, 6c of the laser spots 5a, 5b, 5c of the ring formation 10 to the center 12 of the ring formation 10 are all the same size and are each 70 μm here. The distances az′ of the laser spot centers 6a′, 6b′, 6c′ of the laser spots 5a′, 5b′, 5c′ of the additional ring formation 10′ to the center 12 of the ring formation 10 are also all the same and are each 12.5 μm .

FIG. 5 shows a schematic representation of five laser spots 5a, 5b, 5c, 5d, 5e of a beam set of five individual beams of a laser beam ensemble on a surface. According to a fourth variant of the invention, two plate parts of a bipolar plate can be welded with the five laser spots 5a-5e.

The five laser spots 5a-5e are all the same size here, and the diameter dw of the laser spots 5a-5e on the workpiece is 100 μm here. The five laser spot centers 6a, 6b, 6c, 6d, 6e are located in the center of the five laser spots 5a-5e. The laser spots 5a-5e are separate from each other here; they do not touch or overlap each other.

The laser spots 5a-5e are arranged in the ring formation 10, which includes an inner surface before. In the variant shown, the corner points of the regular polygon 11, here a regular pentagon 11b (shown as a dotted line), can be defined by the laser spot centers 6a-6e. The distances a between the laser spot centers 6a-6e of the adjacent, i. H. in the ring formation 10 adjacent laser spot centers 6a-6e (z. B. the laser spot centers 6a and 6b or 6d and 6e) are each 140 pm here.

The laser spots 5a-5e of the individual beams of the laser beam ensemble have a five-fold rotational symmetry here, since the ring formation 10 is formed by the five laser spots 5a-5e, which are converted into one another by rotating through 360°/5=72° around the center 12 of the ring formation 10 can become. The laser spot centers 6a-6e of the laser spots 5a-5e of the ring formation 10 lie here on a circular line 36 (shown in dashed lines) around the common center 12. With this variant, the directional dependency is reduced even further compared to variants with fewer laser spots 5a-5e. In this variant, the laser beam ensemble is typically not rotated when cornering along the weld seam. Furthermore, a large width of the weld seam can be produced particularly easily.

Fig. 6 shows a schematic representation of three laser spots 5a, 5b, 5c of a beam set of three individual beams of a laser beam ensemble on a

Surface. The three individual beams are designed here as three superimposed individual laser beams. According to a fifth variant of the invention, two plate parts of a bipolar plate can be welded with the three laser spots 5a, 5b, 5c.

The superimposed individual laser beams here include a first partial beam and a second partial beam (not shown in detail). The first sub-beam is designed as a core beam and the second sub-beam is designed as a ring beam; Core beam and ring beam can be generated by means of a multiclad fiber, for example. The ring ray surrounds the core ray in a ring.

The three laser spots 5a, 5b, 5c, which are generated on the surface by the superimposed individual laser beams, each have a core portion 13a, 13b, 13c and a ring portion 14a, 14b, 14c. The ring portions 14a, 14b, 14c surround the core portions 13a, 13b, 13c in a ring shape. The core parts 13a, 13b, 13c are all the same size and have a core diameter KD of 100 μm here. The ring parts 14a, 14b, 14c are also all the same size and have a ring diameter RD of 400 μm here. A respective ring diameter RD is four times larger than a respective core diameter KD.

For a respective power portion LK of the core portions 13a, 13b, 13c of the individual laser spots 5a, 5b, 5c of the total power of the individual laser spots 5a, 5b, 5c, LK=50% can be selected, for example. In the variant shown here, the respective ring portion 14a, 14b, 14c has an area that is approximately 15 times larger than the respective core portion 13a, 13b, 13c. An average power density in the respective core portion 13a, 13b, 13c is then about 15 times greater than an average power density in the respective ring portion 14a, 14b, 14c. In this way, a (local) vapor capillary generated by the superimposed individual laser beams can be stabilized in the deep welding regime and the dynamics of the melt pool can be reduced.

The three laser spots 5a, 5b, 5c are all the same size here, and the diameters dw of the laser spots 5a, 5b, 5c (which are equal to the ring diameters RD here) on the workpiece are each 400 μm here. The three laser spot centers 6a, 6b, 6c are in the center of the three laser spots 5a, 5b, 5c. The laser spots 5a, 5b, 5c touch each other here. The laser power of the individual beams is chosen to be the same here.

The laser spots 5a, 5b, 5c are arranged in a ring formation 10. The distances a between the laser spot centers 6a, 6b, 6c from the laser spot centers 6a, 6b, 6c (in the variant shown here, all laser spot centers 6a, 6b, 6c are next to each other) are 400 μm in each case.

The laser spots 5a, 5b, 5c of the individual beams of the laser beam ensemble have a three-fold rotational symmetry here, since the ring formation 10 is formed by the three laser spots 5a, 5b, 5c, which are formed by rotating 360°/3=120° around the center 12 of the Ring formation 10 can be converted into each other. The laser spot centers 6a, 6b, 6c of the laser spots 5a, 5b, 5c of the ring formation 10 lie here on a circular line 36 (shown in broken lines) around the common strand 12 and correspond to the corner points of a regular polygon.

7 shows a schematic longitudinal sectional view of a forming arrangement 37 for the invention, comprising a splitting device 15 and a multiclad fiber 16, which is designed here as a 2-in-1 fiber 16a. A primary laser beam 17 is reshaped with the reshaping arrangement 37 . The original laser beam 17 propagates along an axis Z.

The multiclad fiber 16 has a core fiber 18 (inner dotted area) and a cladding layer 19 surrounding the core fiber 18 (inner two dotted areas). Adjoining the cladding layer 19 radially outward is a ring fiber 20 (two outer dotted areas), which is surrounded by a further cladding layer 21 (two outer dotted areas).

The original laser beam 17 is generated by a laser source (not shown in detail). Here, the original laser beam 17 propagates in a collimated manner along the Z axis and is directed onto the splitting device 15 . The splitting device 15 here comprises an optical wedge 15a and a focusing lens 15b. A part of the original laser beam 17 is deflected with the optical wedge 15a and focused on the ring fiber 20 at a fiber input 22 of the multiclad fiber 16 with the focusing lens 15b. Another part of the original laser beam 17 is guided past the optical wedge 15a and focused onto the core fiber 18 at the fiber input 22 of the multiclad fiber 16 with the focusing lens 15b. At the fiber input 22, the original laser beam 17 is coupled into the multiclad fiber 16 and at a fiber end 23 of the multiclad fiber 16 there is then an emerging original laser beam 17' with an original laser core beam 24 (continuous line at the fiber end 23) and an original laser ring beam 25 (dashed line at fiber end 23) provided for the method according to the invention. The exiting, reshaped original laser beam 17' can now be divided into individual beams (cf. FIG. 8).

8 shows a schematic longitudinal sectional view of an exemplary welding optics 26, with which a division of a original laser beam 17 (also a shaped original laser beam, cf. FIG. 7) into individual beams 33 for the method according to the invention can be carried out.

The welding optics 26 here includes a laser light cable 27 (for example a multiclad fiber as described in Fig. 7), a collimating lens 28, an optical element 29, which is formed here with two bifocal inserts 30a, 30b, and a focusing lens 31. The bifocal inserts 30a , 30b, which are designed here as glass wedges, are arranged one behind the other and rotated by 90° to one another.

The original laser beam 17 is provided via the laser light cable 27 and exits at a fiber end of the laser light cable 27 . The end of the fiber is in the focus of the collimating lens 28, and the exiting original laser beam 17 is collimated by the collimating lens 28, as a result of which the original laser beam 17 becomes a collimated laser beam 32. The collimated laser beam 32 is directed to the bifocal inserts 30a, 30b. The bifocal inserts 30a, 30b each take up about half of a cross section of the collimated laser beam 32 here. As a result, the collimated laser beam 32 can be divided into four individual beams 33 in the exemplary welding optics 26 shown here (two of the four individual beams 33 can be seen in the perspective shown here). The four individual beams 33 form a laser beam ensemble 34 for laser welding a bipolar plate according to the invention. In the variant shown, the four individual beams 33 together form a beam set 35 of the laser beam ensemble 34. The individual beams 33 are directed through the focusing lens 31 onto the surface of the plate parts of the bi- polar plate (not shown in detail), whereby four laser spots of equal size are generated on the surface. Depending on the optical element 29 used, fewer or more laser spots can also be generated on the surface (not shown in detail).

Reference List

1 bipolar plate la upper plate part lb lower plate part

2 weld

2a circumferential, closed weld seam

2b closed weld seam

2c open (straight line) weld 3 breakthrough

4 surface

5a-5e laser spot (individual beams of the beam set)

5a'-5c' laser spot (additional individual beams of the additional beam set)

6a-6e laser spot center (of the laser spot) 6a'-6c' laser spot center (of the additional beam set)

7 common focus

8 welding curve

9 common weld pool

10 ring formation 10' additional information

11 regular polygon

11a equilateral triangle lib equilateral pentagon

12 center (of ring formation) 13a-13c core portion

14a-14c ring portion

15 dividing device

15a optical wedge

15b focusing lens 16 multiclad fiber

16a 2-in-1 fiber

17 Primal Laser Beam

17' Emerging, reshaped original laser beam

18 core fiber 19 cladding layer

20 ring fiber

21 more coat layer

22 fiber input

523 fiber end

24 Primal Laser Core Beam

25 Primal Laser Ring Beam

26 welding optics

27 Laser light cable io 28 Collimation lens

29 optical element

30a, b bifocal insert

31 focusing lens

32 collimated laser beam

1533 single beam

34 laser beam ensemble

35 Ray Set

36 circle line (ray set)

36' circle line (additional radiation set)

2037 Forming arrangement a Distance from adjacent, adjacent laser spot

Centers (of the laser spots of the radiation set) a' Distance from adjacent, adjacent laser spots

25 centers (of the laser spots of the additional beam set) az distance of the laser spot center from the ring formation to the center of the ring formation az' distance of the laser spot center from the additional ring formation to

center of the ring formation so BLD sheet thickness

DS maximum diameter of the laser spots of the beam set

DZ maximum diameter of the laser spot of the additional beam set dw diameter of the laser spot (of the beam set) on the workpiece dw' diameter of the laser spot (of the additional beam set) on the

workpiece

KD core diameter

RD ring diameter SR welding direction

Z axis

Claims

Method for laser welding a bipolar plate (1) for a fuel cell, two plate parts (1a, 1b) being welded to one another along at least one weld seam (2, 2a, 2b, 2c), characterized in that the laser welding is carried out along the at least one weld seam ( 2, 2a, 2b, 2c) with a laser beam ensemble (34) comprising at least three individual beams (33), the individual beams (33) each having a laser spot (Säße, 5a') on a surface (4) of the plate parts (la, lb). -5c') generate, and that the at least three individual beams (33) of the laser beam ensemble (34) generate a common melt pool (9) in the plate parts (la, lb). Method according to Claim 1, characterized in that at least three individual beams (33) of the laser beam ensemble (34) form a beam set (35), and that laser spot centers (6a-6e) of the laser spots (5a-5e) of the beam set (35) in a ring formation (10) are arranged. Method according to Claim 2, characterized in that the laser spot centers (6a-6e) of the beam set (35) form a regular polygon (11) on the surface (4) of the plate parts (1a, 1b). Method according to Claim 2 or 3, characterized in that the laser spots (5a-5e) of the beam set (35) on the surface (4) of the plate parts (1a, 1b) are separate from one another. Method according to one of Claims 2 to 4, characterized in that the laser spots (5a-5e) of the beam set (35) have the same diameter (dw) and/or the individual beams (33) of the beam set (35) have the same laser power. Method according to one of Claims 2 to 5, characterized in that at least three further individual beams (33) of the laser beam ensemble (34) form an additional beam set, and that laser spot centers (6a'-6c') of the laser spots (5a'-5c ') of the additional beam sets are arranged in an additional ring formation (10'), the additional ring formation (10') being arranged concentrically with the ring formation (10), and with a respective distance (az') of the laser spot centers (6a'-6c') of the laser spots (5a'-5c') of the additional ring formation (10') to a center (12) of the ring formation (10) is smaller than a distance (az) of the laser spot centers (6a- 6c) the laser spots (5a-5c) of the ring formation (10) to the center (12) of the ring formation (10). Method according to Claim 6, characterized in that the following applies for a maximum diameter DZ of the laser spots (5a'-5c') of the additional beam set and for a maximum diameter DS of the laser spots (5a-5e) of the beam set (35): DZ<DS . Method according to one of the preceding claims, characterized in that the laser spots (5a-5e, 5a'-5c') of the individual beams (33) of the laser beam ensemble (34) form an arrangement on a surface (4) of the plate parts (1a, 1b). , which has N-fold rotational symmetry, with N>3. Method according to one of the preceding claims, characterized in that at least some of the individual beams (33) are in the form of superimposed individual laser beams, each comprising at least two partial beams which lie one inside the other on the surface.

10. The method according to claim 9, characterized in that the superimposed single laser beam comprises a core beam and a ring beam surrounding the core beam.

5 or that the superimposed single laser beam comprises a larger sub-beam and a smaller sub-beam which lies on the workpiece surface (4) within the larger sub-beam.

11. The method according to claim 9 or 10, characterized in that a primary laser beam (17) is guided in a multiclad fiber (16), comprising a core fiber (18) and a ring fiber (20) surrounding the core fiber (18), and that a primary laser beam (17') emerging from the multiclad fiber (16) is split up into at least some of the individual beams (33) by means of an optical element (29). 5

12. The method according to any one of the preceding claims, characterized in that the plate parts (1a, 1b) each have a sheet thickness BLD between 50 μm and 150 μm.

20 13. The method according to any one of the preceding claims, characterized in that the at least one weld seam (2, 2a, 2b, 2c) comprises one or more self-contained weld seams (2a, 2b).

14. The method according to any one of the preceding claims, characterized in that the at least one weld (2, 2a, 2b, 2c) comprises at least one on an outside of the plate parts (1a, 1b) circumferential, self-contained weld (2a).

15. Bipolar plate (1) for a fuel cell, produced by welding

30 of two plate parts (la, lb) according to a method according to any one of the preceding claims.