DE102022100186A1 - Process for laser welding a bipolar plate for a fuel cell, with several passes over the weld seam - Google Patents

Abstract

A method for laser welding a bipolar plate (1) for a fuel cell, in which two plate parts (1a, 1b) are welded to one another along at least one weld seam (2, 2a, 2b, 2c), is characterized in that the laser welding of the at least one weld seam ( 2, 2a, 2b, 2c) with at least two passes of the at least one weld seam (2, 2a, 2b, 2c), that in each pass at least one laser beam (22) is guided along a welding curve (6) that runs along the weld seam (2, 2a, 2b, 2c), with the at least one laser beam (22) generating at least one molten pool (7, 7a, 7b) in the plate parts (1a, 1b), with a partial weld seam (8, 8a , 8b, 8c) is generated, and wherein the melt pools (7, 7a, 7b) of the laser beams (22) of different passes are separate from each other, and that the partial weld seams (8, 8a, 8b, 8c) of the different passes in one direction ( QR) transverse to the welding direction (SR) at least partially overlap. The invention provides a method by which better fluid tightness, in particular gas tightness, of weld seams of a bipolar plate for fuel cells is made possible.

Description

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 is from the subsequently published German patent application DE 10 2021 113 834.5 known.

Fuel cells, such as hydrogen fuel cells or direct methanol fuel cells, are currently becoming increasingly important in the field of sustainable technologies and can be used to drive vehicles, for example. The individual fuel cells are layered to form a stack (also referred to as a stack) and thus form a fuel cell system.

The fuel cell system typically includes a large number of membrane-electrode units that are arranged in a stack and whose electrical power is added up. A bipolar plate is arranged between each two membrane-electrode units of the stack of the fuel cell system.

As integrated components of fuel cells, bipolar plates fulfill different tasks. They establish the electrical contact between the anode and the cathode of adjacent cells. Furthermore, the bipolar plate ensures that the reaction gases (typically hydrogen and oxygen) are fed into the reaction zone (i.e. to the electrodes). For this purpose, the bipolar plates have flow profiles through which hydrogen flows on one side of the bipolar plates and through which oxygen flows on the other side of the bipolar plates. Bipolar plates are also used to dissipate water vapor and thermal energy. Bipolar plates fulfill a sealing function, in particular in the gas-tight separation between adjacent fuel cells, in the sealing to the outside and the sealing of the cooling channels that run inside the bipolar plates.

Such bipolar plates can be made from two metallic plate parts that are welded together. The weld seams should be fluid-tight, in particular gas-tight, in order to direct the gases and the water in defined paths. Furthermore, the weld seams are used for the electrical and mechanical connection of the two metal plate parts.

The post-published German patent application mentioned at the outset DE 10 2021 113 834.5 proposes a method in which two metal plate parts are connected to one another by laser welding to form a bipolar plate. For this purpose, at least one peripherally closed, gas-tight first weld seam is produced. At least one second weld seam is also produced. The second weld has a larger seam width than the first weld.

When welding a bipolar plate from the two plate parts, isolated defects can occur during the welding process, in particular microcracks and pores in the weld seam, or also ejections. This can cause the weld to leak and become irregular. Furthermore, a weld seam can look good on the surface, but the connection can be locally interrupted in the area of the joining zone between the two joining partners (so-called "false friends"). As a result, the weld seam can also leak, and the mechanical and electrical connection between the plate parts is weakened.

object of the invention

It is the object of the present invention to provide a method by which better fluid tightness, in particular gas tightness, of weld seams of a bipolar plate for fuel cells is made possible.

Description of the invention

This object is achieved according to the invention by a method of the type mentioned at the outset, which is characterized in that
that the laser welding of the at least one weld seam takes place with at least two passes of the at least one weld seam,
that in each pass at least one laser beam is guided along a welding curve that runs along the weld seam,
wherein the at least one laser beam generates at least one melt pool in the plate parts,
whereby a partial weld seam is produced with each pass,
and wherein the melt pools of the laser beams of different passes are separate from each other,
and that the partial welds of the different passes at least partially overlap in a direction transverse to the welding direction.

The invention proposes running a laser beam at least twice over the at least one weld seam, with which the two plate parts (joining partners) are connected to form the bipolar plate for the fuel cell, during laser welding. A single pass over the weld seam with a laser beam can lead to isolated defects, in particular microcracks, pores, defects throw or connection errors that are not visible from the outside in the area of the joining zone ("false friends"). Such flaws in the weld seam from a previous pass can be repaired or completely eliminated by at least one further pass over the weld seam. As a result, a weld seam with improved fluid tightness, in particular gas tightness, and also improved mechanical and electrical connection between the plate parts can be produced.

In each pass, at least one laser beam (with its central axis) follows a welding curve. The weld curve runs along the at least one weld seam. The weld curve can run on a centerline of the weld, or it can run parallel to the centerline of the weld. The welding curves of the laser beams of different passes can be made on identical or different welding curves. The welding curves of the laser beams of the different passes can therefore deviate from one another in a direction transverse to the welding direction.

At least one melt pool is generated in the two plate parts by the at least one laser beam during a respective pass. When the at least one molten bath solidifies, a partial weld seam that is associated with the respective crossing remains. With each additional pass, another partial weld is created. The weld pools from different passes of the laser beams are separate from each other. Depending on the welding situation, both the course of the welding curves and the welding conditions can be adjusted and precisely controlled.

The partial weld seams at least partially overlap in the direction transverse to the welding direction. Correspondingly, a partial weld seam produced first is at least partially melted again as part of the production of a partial weld seam produced later, it being possible for defects in the partial weld seam produced first to be repaired. In addition, partially non-overlapping partial welds allow a weld seam formed by at least two partial welds from two passes to be widened compared to a weld seam from a single pass with a laser beam. The widened weld seam allows the sealing between the plate parts and the mechanical and electrical connection between the plate parts to be improved.

Laser welding is preferably carried out as deep welding. A particularly high feed rate can be achieved in this way. The laser welding of the plate parts can be done by welding in or welding through. The individual partial welds (in a plane perpendicular to the welding direction) preferably have an aspect ratio AV=T/B with AV≧1.5, preferably AV≧2, particularly preferably AV≧3, with T: welding depth and B: width of the individual partial welds the workpiece surface.

The metallic plate parts of the bipolar plate are arranged to overlap 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, on the outside, ducts for reaction water and/or gases, in particular hydrogen and oxygen, are formed between the plate parts. 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, closed welds are typically made at the outer edge of the plate parts and around all openings, and non-closed welds are also made distributed over the surface 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

In a preferred variant of the method according to the invention, it is provided that the crossings take place individually one after the other. The crossings are separated in time and a further crossing only takes place when a previous crossing has been completed. The crossings can then be made with a single laser beam that is the same for all crossings; a corresponding welding arrangement only needs to generate a single laser beam and is then particularly simple and inexpensive.

In an alternative preferred variant of the method according to the invention, it is provided that the crossings take place at least partially overlapping in time. The crossings coincide at least partially in time. Another pass may begin if a previous pass has not yet completed. For this purpose, at least two laser beams are used at the same time. Due to the temporal overlapping of the crossings, the total part time for the production of a bipolar plate can be reduced, which can reduce the production costs.

Also preferred is a variant in which the partial welds of the different passes in the direction transverse to the welding direction at least partially do not overlap, so that the weld, which is formed by all of the partial welds, in the direction transverse to the welding direction has an overall width GB that is greater than a width B of each of the partial welds. Due to the partial non-overlapping of the partial welds in the transverse direction, the weld seam is widened compared to the width of a single partial weld. Widening the weld seam can in turn improve the tightness of the weld seam and also the mechanical and electrical connection between the plate parts. Typically, the following applies to the total width GB of the weld: GB≥1.1*Bmax, preferably also GB≥1.5*Bmax or also GB≥N*Bmax*0.66, with N: number of partial welds that form the weld, and Bmax: Width of the partial weld seam with the greatest width (each measured on the workpiece surface, perpendicular to the welding direction). For a lateral offset b of adjacent partial welds, b<(B1+B2)/2 applies, with B1, B2: widths of the adjacent partial welds (measured in each case in the cross section perpendicular to the welding direction with respect to the central axes of the partial welds, and the width is determined on the workpiece surface) , mostly with b≤0.5*[(B1+B2)/2]. Subsequent partial welds in the direction transverse to the welding direction are preferably produced in correspondingly successive passes.

Also preferred is a variant in which the width of the partial weld seam from at least one later pass is greater than the width of the partial weld seam from at least one earlier pass. Typically, the partial weld of the earlier pass is completely within the partial weld of the later pass; due to the at least one later pass, a wider partial weld seam can be placed over a less wide partial weld seam of the earlier pass. This enables a particularly reliable and comprehensive repair of defects from a respective earlier pass with the later pass. In this way, false friends in particular can be repaired particularly easily and the tightness of the weld seam can be improved. The width of the partial weld seams preferably increases from pass to pass over all passes. Typically, the welding curves of the passes are on top of each other (i.e. they are perpendicular to the welding direction without any lateral offset).

Also preferred is an alternative variant in which the widths of the partial weld seams of all passes are the same size. In practice, this is particularly easy to implement, in particular with the same at least one laser beam in each pass. In particular, any cumbersome adjustment of laser optics between the passes, with which the laser beams are directed onto the workpiece surface, in order to adjust the diameter of the laser beams used in a respective pass (and thus the width of the partial weld seams) is eliminated. Alternatively, the widths of the partial welds (and correspondingly the diameters of the laser beams used in a respective pass) can also differ, typically with the width B3 of the partial weld of a first pass and the width B4 of the partial weld of a subsequent pass being: 0.1*B4 ≤B3≤10*B4.

Also preferred is a variant in which only one laser beam is used per pass. This can be implemented particularly cost-effectively and with little effort.

• - mittels mehrerer Laserquellen und mehrerer Laseroptiken, oder
• - mittels mehrerer Laserquellen und einer Laseroptik, oder
• - mittels einer Laserquelle und mehreren Laseroptiken (beispielsweise unter Verwendung einer Laserquelle mit mehreren Lichtwegen, unter denen der Laserstrahl geschaltet werden kann, wobei an jedem Lichtweg via Lichtleitkabel eine andere Laseroptik angeschlossen ist), oder
• - mittels einer Laserquelle und einer Laseroptik und einem optischen Element zum Aufteilen eines Urlaserstrahls auf mehrere Laserstrahlen, z. B. Multifokallinse, optische Keilplatte, diffraktives optisches Element (DOE) oder refraktives optisches Element (ROE).

Also preferred is an alternative variant in which at least two laser beams are used simultaneously for each crossing. As a result, the total parts time can be shortened or the quality of the partial weld seam of the corresponding pass can be improved. If the at least two laser beams are transversally offset from one another (in the welding direction), defects (eg cracks, pores, ejecta or false friends) can already be repaired in the corresponding crossing by the at least second laser beam. If the at least two laser beams are lateral to each other (transverse to the welding direction), the partial weld seam can already be widened in the corresponding pass by the at least two laser beams of this pass compared to a partial weld seam based on only one laser beam and thus the tightness can be improved. Provision can be made for the laser beams used for each pass to be offset transversely (in the direction of welding) relative to one another, but not laterally (transverse to the direction of welding). Typically, two laser beams are used per pass. The laser beams used in a pass are typically used at the same time and have a constant mutual distance along the weld seam. Preferably, the laser beams used at the same time per pass have the same beam diameter on the workpiece surface, typically with the laser power of the laser beams also being the same. Alternatively, the beam diameters can also differ, typically in which case the following applies to the beam diameter dw1 of a laser beam that is foremost in the welding direction and the beam diameter dwx of a following laser beam: 0.1*dwx≦dw1≦10*dwx. In general, the laser beams used at the same time for each pass usually have the same feed rate speed. The multiple laser beams can be generated (in particular for a simultaneous application).

• - 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 element for dividing a Urlaserstrahl to several laser beams, z. B. Multifocal lens, optical wedge plate, diffractive optical element (DOE) or refractive optical element (ROE).

A further development of this variant is advantageous, which provides that the laser beams used simultaneously in one pass partially overlap in the welding direction.

A particularly high laser intensity can be achieved in the overlapping area of the laser beams used at the same time. In this way, welding can be carried out with high efficiency and speed.

A<(dw1+dwx)/2 then applies for a distance a in the welding direction of the (neighboring) laser beams used at the same time per pass; the distance a is measured with respect to the beam axes on the workpiece surface. The overlapping laser beams used at the same time create a common weld pool.

An alternative further development of this variant is also advantageous, which provides that the laser beams used simultaneously in one pass do not overlap in the welding direction. As a result, high laser intensities can be avoided and welding can be carried out with high efficiency and precision.

A≥(dw1+dwx)/2 then applies for a distance a in the welding direction of the laser beams used simultaneously in each pass. The simultaneously used, non-overlapping laser beams can create a common melt pool or alternatively create separate melt pools.

It is preferably provided that the molten pools of the laser beams used simultaneously in one pass do not overlap one another. In this way, the separate melt pools can be better controlled and possible effects that arise from a common melt pool and, for example, change the melt pool dynamics in such a way that pores or cracks form when the common melt pool solidifies, can be avoided. Good control over the welding process can be gained.

Also preferred is a further development that provides for the laser beams used simultaneously in a pass to be guided on identical welding curves. In other words, the weld curves have no lateral offset. This ensures that the laser beams used in the passage (at least in a central portion) process the same panel material. This can reduce defects in the partial weld. In addition, the procedure is particularly simple.

Also preferred is a variant in which the at least one weld seam comprises one or more self-contained weld seams. Closed weld seams in bipolar plates require a high level of fluid tightness, which the invention can provide in a simple and reliable manner, so that the invention is particularly advantageous here. Due to the closed weld seams, in particular partial areas between the plate parts can be sealed in a fluid-tight and in particular gas-tight manner.

Equally advantageous 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. In this way, an interior area between the two plate parts can be fluid-tight (in particular tight for cooling water and tight for reaction gases such as oxygen and hydrogen). The sealing of the circumferential weld seam on the outside of the plate parts is particularly important in the case of bipolar plates and can be provided simply and reliably with the invention; This weld can be used in particular to prevent a coolant from flowing into the reaction chambers of a stack of fuel cells.

In a preferred variant, a respective laser beam is designed as a multifocal laser beam, comprising a plurality of partial beams lying next to one another on the workpiece surface, which together generate the molten pool of the laser beam. With a multifocal laser beam, a comparatively low-defect and wide partial weld seam can be obtained in just one pass. The quality of the weld seam and thus the tightness and connection of the plate parts can then be further improved by the further passes. The partial beams lie side by side on the workpiece surface with their central axes; the associated partial laser spots can (preferably) be separate from one another or also overlap. With an increasing number of partial beams and with an appropriate arrangement (in particular on the corners of a regular polygon), the directional dependency can be reduced.

In a preferred alternative variant, a respective laser beam is in the form of a superimposed laser beam, comprising at least two partial beams which lie one inside the other on the workpiece surface and which together generate the molten pool of the laser beam. Typically, a lower local laser power density is established in a radially outer part of the heterodyne laser beam and a locally higher power density is established in a radially inner part of the heterodyne laser beam. This allows the vapor capillary to be stabilized during deep penetration welding and the weld pool dynamics to be reduced. The superimposed laser beam typically has concentric partial beams on the workpiece surface.

A preferred further development of this alternative variant is characterized in that
that the superimposed laser beam comprises a core beam and a ring beam surrounding the core beam,
or that the superimposed laser beam comprises a larger sub-beam and a smaller sub-beam lying at the workpiece surface within the larger sub-beam. This has proven itself in practice and leads to a good quality of the weld. A heterodyne 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 preferred is a variant which provides that 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, z. B. type 1.4404. The sheet metal thickness is preferably BLD=75 μm. Such plate parts are particularly suitable for the production of bipolar plates. The sheet thicknesses BLD enable a small use of material with a good mechanical stability at the same time. The metallic plate parts are inexpensive to manufacture and well suited to the requirements placed on a bipolar plate of a fuel cell (e.g. laser welding processability or resistance to corrosion).

• - Die Laserstrahlen werden mit einem Infrarotlaser erzeugt und weisen eine mittlere Wellenlänge zwischen 800 nm und 1200 nm, bevorzugt 1030 nm oder 1070 nm, auf, oder die Laserstrahlen werden mit einem VIS-Laser mit einer mittleren Wellenlänge zwischen 400 nm und 450 nm oder zwischen 500 nm und 550 nm erzeugt; und/oder
• - Das jeweilige Strahlparameterprodukt SPP der Laserstrahlen liegt zwischen 0,38 mm*mrad und 16 mm*mrad, bevorzugt mit SPP≤0,6 mm*mrad; und/oder
• - Der Strahldurchmesser dw1 des in Schweißrichtung zuerst eingesetzten Laserstrahls auf dem Werkstück liegt zwischen 10 µm und 300 µm, bevorzugt mit 30µm≤dw1≤70µm im Single Mode oder 50µm≤dw1≤170pm im Mulit-Mode, und der Strahldurchmesser dwx aller anderen Laserstrahlen ist gewählt mit 0,1*dw1≤dwx≤10*dw1, bevorzugt mit dw1=dwx; und/oder

• - Die Laserleistung P pro Laserstrahl liegt zwischen 10W und 2000W , bevorzugt mit 50W≤P≤700W; und/oder
• - Ein Vorschub VS der Laserstrahlen liegt zwischen 100 mm/s und 5000 mm/s, bevorzugt mit 300 mm/s ≤ VS ≤ 2000 mm/s; und/oder
• - Ein Abbildungsverhältnis AV einer Laseroptik, mit der die Laserstrahlen auf das Werkstück abgebildet werden, liegt zwischen 1:1 und 5:1, bevorzugt mit 1,5:1 ≤ AV ≤ 2:1.

Also preferred is a variant which is characterized in that:

• - The laser beams are generated with an infrared laser and have a mean wavelength between 800 nm and 1200 nm, preferably 1030 nm or 1070 nm, or the laser beams are generated with a VIS laser with a mean wavelength between 400 nm and 450 nm or between 500 nm and 550 nm generated; and or
• - The respective beam parameter product SPP of the laser beams is between 0.38 mm*mrad and 16 mm*mrad, preferably with SPP≤0.6 mm*mrad; and or
• - The beam diameter dw1 of the laser beam used first in the welding direction on the workpiece is between 10 µm and 300 µm, preferably with 30 µm≤dw1≤70 µm in single mode or 50 µm≤dw1≤170 pm in multi-mode, and the beam diameter dwx of all other laser beams chosen with 0.1*dw1≤dwx≤10*dw1, preferably with dw1=dwx; and or

• - The laser power P per laser beam is between 10W and 2000W, preferably with 50W≤P≤700W; and or
• A feed VS of the laser beams 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 a laser optics, with which the laser beams are imaged onto the workpiece, is between 1:1 and 5:1, preferably with 1.5:1≦AV≦2:1.

These parameters have proven themselves in practice for the production of the bipolar plates.

Furthermore, the present invention comprises bipolar plates for a fuel cell, produced by welding two plate parts according to a method according to the invention and described above. Bipolar plates that are manufactured according to the invention have good fluid tightness, in particular gas tightness, of the at least one weld seam, and achieve a good mechanical and electrical connection of 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.

character list

• 1 zeigt in einer schematischen Aufsicht eine erfindungsgemäße Bipolarplatte mit zwei Plattenteilen, die durch mehrere umlaufend geschlossene Schweißnähte sowie mehrere sich geradlinig erstreckende Schweißnähte miteinander verbunden sind;
• 2 zeigt in einer schematischen Aufsicht das Verschweißen von zwei Plattenteilen einer Bipolarplatte gemäß einer ersten Variante der Erfindung, mit einem Laserstrahl;
• 3 zeigt in einer schematischen Aufsicht das Verschweißen von zwei Plattenteilen einer Bipolarplatte gemäß einer zweiten Variante der Erfindung, mit zwei Laserstrahlen und zeitlich teilweise überlappenden Überfahrten;
• 4 zeigt in einer schematischen Aufsicht das Verschweißen von zwei Plattenteilen einer Bipolarplatte gemäß einer dritten Variante der Erfindung, mit zwei in einer Überfahrt zeitgleich eingesetzten Laserstrahlen, deren Schmelzbäder einander nicht überlappen;
• 5 zeigt in einer schematischen Aufsicht das Verschweißen von zwei Plattenteilen einer Bipolarplatte gemäß einer vierten Variante der Erfindung, mit zwei in einer Überfahrt zeitgleich eingesetzten Laserstrahlen, die teilweise überlappen;
• 6 zeigt in einer schematischen Aufsicht das Verschweißen von zwei Plattenteilen einer Bipolarplatte gemäß einer fünften Variante der Erfindung, mit einem Überlagerungs-Laserstrahl;
• 7 zeigt in schematischen Ansichten Querschnitte durch mittels Laserschweißen gefertigte Schweißnähte einer Bipolarplatte, mit den Teilbildern a) gemäß einem fiktiven Stand der Technik mit einer Überfahrt eines einzelnen Laserstrahls und einer fehlerhaften Anbindung, b) gemäß der Erfindung mit zwei Überfahrten wenigstens eines Laserstrahls, wobei der wenigstens eine Laserstrahl bei der zweiten Überfahrt eine breitere Teilschweißnaht erzeugt hat als in der ersten Überfahrt, c) gemäß einem fiktiven Stand der Technik mit einer Überfahrt eines einzelnen Laserstrahls und Rissen und Poren in der Schweißnaht, d) gemäß der Erfindung mit zwei Überfahrten wenigstens eines Laserstrahls mit überlappenden Teilschweißnähten, und e) gemäß der Erfindung mit drei Überfahrten wenigstens eines Laserstrahls mit überlappenden Teilschweißnähten.

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

• 1 shows a schematic top view of a bipolar plate according to the invention with two plate parts which are connected to one another by a plurality of circumferentially closed weld seams and a plurality of weld seams extending in a straight line;
• 2 shows in a schematic top view the welding of two plate parts of a bipolar plate according to a first variant of the invention, with a laser beam;
• 3 shows in a schematic top view the welding of two plate parts of a bipolar plate according to a second variant of the invention, with two laser beams and temporally partially overlapping passes;
• 4 shows a schematic top view of the welding of two plate parts of a bipolar plate according to a third variant of the invention, with two laser beams used simultaneously in one pass, the weld pools of which do not overlap;
• 5 shows a schematic top view of the welding of two plate parts of a bipolar plate according to a fourth variant of the invention, with two laser beams that are used simultaneously in one pass and partially overlap;
• 6 shows in a schematic top view the welding of two plate parts of a bipolar plate according to a fifth variant of the invention, with a superimposed laser beam;
• 7 shows schematic views of cross sections through weld seams of a bipolar plate produced by means of laser welding, with the partial images a) according to a fictitious prior art with one pass of a single laser beam and a faulty connection, b) according to the invention with two passes of at least one laser beam, wherein the at least a laser beam produced a wider partial weld seam during the second pass than in the first pass, c) according to fictitious prior art with one pass of a single laser beam and cracks and pores in the weld seam, d) according to the invention with two passes of at least one laser beam with overlapping partial welds, and e) according to the invention with three passes of at least one laser beam with overlapping partial welds.

The 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 1a, 1b of the bipolar plate 1 are arranged one above the other. The two plate parts 1a, 1b 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 1a, 1b are cooling fluid channels (typically for cooling water). The channels on the outer surfaces of the two plate parts 1a, 1b are guide channels for gas (such as oxygen or hydrogen) and water (which occurs as reaction water in the fuel cell). The two plate parts 1a, 1b are made of a metallic material, e.g. B. stainless steel. A sheet metal thickness of the plate parts 1a, 1b is 75 μm here; Sheet thicknesses of between 50 μm and 150 μm are generally preferred.

The two plate parts 1a, 1b 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 phantom using their center lines. A circumferential, 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 rectilinear) weld seams 2c also run on the bipolar plate 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 well-conducting connection between the two plate parts 1a, 1b. The present inventive method was applied to all welds 2a, 2b, 2c. The method according to the invention is particularly advantageous for the weld seams 2a, 2b, since a particularly high level of tightness is achieved.

2 shows a first variant of the method according to the invention, which is applied to plate parts 1a, 1b of a bipolar plate 1 (as in 1 explained) is applied. The first variant is explained using weld seam 2a as an example. The weld seam 2a that is being produced is shown schematically in broken lines on the basis of its edge lines on the workpiece surface 4 (top side of the plate part 1a). The weld seams 2b, 2c are shown schematically in dot-dash lines based on their center lines.

The production of the weld seam 2a takes place in 2 illustrated variant by means of a single laser beam from above (perpendicular to the plane of the drawing) is directed onto the upper plate part 1a. The laser beam generates a laser spot 5 on the workpiece surface 4 of the two plate parts 1a, 1b Feed moves (relative to its center / the associated central axis of the laser beam) on a welding curve 6 shown here dotted. The welding curve 6 runs along the weld seam 2a (here on the center line of the weld seam 2a). The feed direction is marked with an arrow. The feed runs along a (local) welding direction SR.

Around the laser spot 5, the laser beam generates a melt pool 7 of melted plate material. The melt pool 7 is widest in the area of the laser spot 5 . Contrary to the welding direction SR, the melt pool 7 becomes narrower.

In the variant shown, the laser beam used to produce the weld seam 2a crosses the weld seam 2a twice (two "passes"). The laser beam completely covers the welding contour 6 twice; the welding contour 6 is the same here in the first and second pass. Each pass creates a partial weld 8. In 2 the laser beam is already in the second pass. In the first pass, a first partial weld 8a was produced (area within the dashed lines of weld 2a). In the second pass, part of a second partial weld seam 8b has already been produced (hatched area within the dashed lines of weld seam 2a). The partial weld seams 8a, 8b completely overlap here and form (after completion of the second pass) the weld seam 2a in their entirety. It should be noted that here the second partial weld 8b essentially replaces (takes the place of) the first partial weld 8a in the context of the second pass. The first and second crossings take place at different times (ie without a time overlap), in this case directly one after the other.

After the laser beam has completely passed over the second time along the welding curve 6, the weld seam 2a is completely finished (more on this in 7 ). The second pass here runs like the first pass on the welding curve 6. Alternatively, the second pass can also be made on another welding curve, which is slightly offset in a direction QR transverse to the (local) welding direction SR (not shown in detail). In a similar way to the variant of 2 three or more crossings can also be made.

Further variants of the method according to the invention are presented in the following figures. Only the essential differences are explained.

3 shows a second variant of the method according to the invention. In this variant, a first laser beam and a second laser beam are used, which generate a first laser spot 5a and a second laser spot 5b and a first melt pool 7a and a second melt pool 7b, which are separate from one another.

The first laser beam performs the first pass along the welding curve 6, producing the first partial weld 8a (white area within the dotted lines of weld 2a). Before the first pass is completely completed, the second pass along the welding curve 6 is started with the second laser beam and the second partial weld seam 8b (hatched area within the dashed lines of weld seam 2a) is produced. The first partial weld seam 8a is essentially melted again by the laser spot 5b. The first and second crossings take place simultaneously for a certain period of time. In this way, the method according to the invention can be accelerated.

4 shows a third variant of the method according to the invention. In this variant, two laser beams are again used, which generate two laser spots 5a, 5b and two melt pools 7a, 7b, which are separate from one another.

In contrast to the in 3 variant shown, here the first pass and the second pass are each carried out with both laser beams and the two laser beams are used at the same time. The laser beams are both guided along the identical welding curve 6 during the passes. The first partial weld seam 8a (white area within the dashed lines of weld seam 2a) was produced in the area on the welding curve 6 over which the two laser beams and the associated molten pools 7a, 7b had already passed in the first pass. In the second pass, the second partial weld seam is produced with the laser spots 5a, 5b and the molten baths 7a, 7b, which essentially replaces the first partial weld seam 8a. With each pass, the weld seam 2a is passed over with the two laser spots 5a, 5b and the associated two molten pools 7a, 7b. The plate material of the weld seams is melted a total of four times during the two passes provided here. As a result, defects can already be repaired during the production of the partial weld seams and thus a particularly good quality of the weld seam 2a can be achieved.

5 shows a fourth variant of the method according to the invention. In this variant, too, two laser beams are used at the same time. The laser spots 5a, 5b of the two laser beams overlap in the welding direction SR in an overlap area 9 and generate a common molten pool 7. A distance a between the laser spots 5a, 5b in relation to their central axes is therefore selected in such a way that a<(dw1+dw2)/2, with dw1: spot diameter of the front laser spot 5a, and dwx: spot diameter of the second laser spot 5b.

The laser beams are both guided along the identical welding curve 6 during the passes. The first partial weld seam 8a (white area within the dashed lines of weld seam 2a) was produced in the area on the welding curve 6 over which the two laser beams and the associated molten pool 7 passed in the first pass. With each pass, the two laser beams with the two laser spots 5a, 5b and the melt pool 7 pass over the weld seam 2a. In the overlapping area 9 of the laser spots 5a, 5b, the intensity of the two laser beams is particularly high compared to the non-overlapping areas of the laser spots 5a, 5b. A vapor capillary generated in the molten bath 7 in this overlapping area is particularly stable and thereby enables a higher welding speed.

6 shows a fifth variant of the method according to the invention. A single laser beam is used, which is designed as a superimposed laser beam. The superimposed laser beam generates a superimposed laser spot 10. The superimposed laser beam generates the melt pool 7 around the superimposed laser spot 10. The superimposed laser beam has a core beam and a ring beam. The ring ray surrounds the core ray in a ring. The core ray and the ring ray both have a common optical axis.

Accordingly, the superimposed laser spot 10 also has a core portion 11 (hatched circle) and a ring portion 12 (white ring). The ring portion 12 surrounds the core portion 11 in a ring.

The heterodyne laser beam is located in 6 in the first crossing. The first partial weld seam 8a (white area within the dashed lines of weld seam 2a) was produced in the area on the welding curve 6 over which the superimposed laser beam and the associated molten pool 7 passed in the first pass.

Typically, a heterodyne laser beam is generated using a 2-in-1 fiber (not shown). The 2-in-1 fiber has a core fiber and a ring fiber. The ring fiber surrounds the core fiber in a ring shape.

The vapor capillary at the superimposed laser spot 10 can be stabilized with the aid of the superimposed laser beam. In this way, the melt pool dynamics can be reduced. In this way, for example, the formation of spatter can be reduced.

7 shows in schematic cross-sectional views (with the sectional plane perpendicular to the welding direction) different welds of bipolar plates that are manufactured according to the prior art (partial images a, c) or according to the invention (partial images b, c, and d). Only a small section of the bipolar plate is shown.

Partial image a) shows an example of a narrow weld seam 13 according to a fictitious prior art, with which the upper plate part 1a and the lower plate part 1b were apparently welded to one another by welding. Welding was carried out with a single laser beam in the deep welding regime. The weld seam 13 represents the melted and resolidified material of the plate parts 1a, 1b. If only one pass is carried out during welding, there may be an incomplete or missing connection in the area of a joining zone 14 (between the plate parts 1a, 1b) (also known as referred to as "false friends"). The weld 13 consists of an upper weld portion 13a in the upper plate portion 1a and a lower weld portion 13b in the lower plate portion 1b, which are not connected to each other. However, if the weld seam 13 is only viewed from an upper side 15 of the upper plate part 1a, one could erroneously come to the conclusion that the plate parts 1a, 1b were properly welded to one another. Due to the incomplete or missing connection, there is only little or no mechanical strength and electrical conductivity of the weld. There is also no fluid tightness. It should be noted that in part a) and in part b) the gap between the plate parts 1a, 1b is exaggerated for clarity.

Part b) shows the weld seam 2 through the plate parts 1a, 1b, which was produced according to the method according to the invention. The entire weld seam 2 is composed here of the two partial weld seams 8a, 8b. The partial welds 8a, 8b lie one above the other, the first partial weld 8a being deeper and slimmer than the second partial weld 8b. A depth T ₁ of the partial weld 8a is approximately twice as large as a width B ₁ of the partial weld 8a. An aspect ratio AV=T/B of the partial weld seam 8a is therefore approximately 2. The partial weld seam 8a was produced in a first pass and extends into the lower plate part 1a, here up to its underside.

As already indicated in part a), “false friends” can occur during a single crossing. In order to achieve a good mechanical and electrical as well as fluid-tight and in particular gas-tight connection of the plate parts 1a, 1b, the partial weld seam 8a is passed over again in a second pass. During the second pass, a laser beam is selected that generates a larger laser spot on the upper side 15 of plate part 1a. In this way, the second partial weld seam 8b is produced during the second pass. The second partial weld seam 8b here has a depth T ₂ which is approximately one and a half times the width B ₂ . The aspect ratio AV of the partial weld 8b is therefore approximately 1.5. The depth T ₂ is sufficient to melt the plate material through the joining zone again into the area of the lower plate part 1b. A faulty connection of the plate parts 1a, 1b can be repaired by the second pass.

The weld seam 2 in part b) can be produced with all variants presented above. This requires the second pass to produce a wider partial weld seam 8b in each case, for example by using two laser beams that generate laser spots of different sizes on the upper side 15 or by adjusting a single laser beam during a second pass so that it produces a larger laser spot the top 15 generated, such as by changing the focal position of the laser beam.

The sheet metal thickness BLD of the plate parts 1a, 1b is 75 μm in each case here.

Part c) shows an example of a wide weld seam 16 according to a fictitious prior art, with which the upper plate part 1a and the lower plate part 1b were welded to one another by welding. Welding was carried out with a single laser beam in the deep welding regime. The weld seam 16 represents the melted and resolidified material of the plate parts 1a, 1b. A width B of the weld seam 16 is approximately the same size as a depth T of the weld seam 16. The aspect ratio AV=T/B is approximately 1 here Wide weld 16, numerous pores 17 and other defects, such as microcracks 18, occur in the weld 16. As a result, the connecting effect of the weld seam 16 with regard to the mechanical strength and the electrical conductivity of the connection is possibly impaired. Pores 17 and microcracks 18 can also impair the fluid tightness, in particular the gas tightness, of the weld seam 16 .

Partial image d) shows the weld seam 2 through the plate parts 1a, 1b, which was produced using the method according to the invention. The entire weld seam 2 is composed here of the two partial weld seams 8a, 8b. The partial weld seams 8a, 8b partially overlap in the QR direction transverse to the (local) welding direction. The welding direction is perpendicular to the plane of the drawing. The direction QR also runs parallel to the top 15 of plate part 1a.

The production of the partial weld seams 8a, 8b took place here in the deep welding regime in welding. The partial welds 8a, 8b have the same width B and the same depth T. The individual partial welds 8a, 8b each have an aspect ratio of approximately AV=1 here. With such an aspect ratio, pores 17 and microcracks 18 can form in the weld seam 2, among other things. The partial weld seams 8a, 8b partially overlap due to the multiple passage of the laser beams over welding curves offset in the direction QR. Furthermore, there are areas of the partial welds 8a, 8b that do not overlap. In a weld seam overlapping area 21, welding defects are corrected as a result of the further crossing. By reducing pores 17 and microcracks 18, the fluid tightness, in particular the gas tightness, of the weld seam 2 can be improved.

A total width GB of the weld seam 2 is greater than the respective individual widths B of the partial weld seams 8a, 8b. In the example shown here, GB has a value of approximately GB=1.3*B.

The weld seam 2 in part d) can be produced with all variants presented above. To do this, it is necessary for the second pass to be carried out along a second welding curve that is offset from the first welding curve.

Partial image e) shows the weld seam 2 through the plate parts 1a, 1b, which was produced using the method according to the invention. The entire weld seam 2 is composed here of the two partial weld seams 8a, 8b and a third partial weld seam 8c. The partial weld seams 8a, 8b, 8c partially overlap in the QR direction transverse to the (local) welding direction. The welding direction is perpendicular to the plane of the drawing. The QR direction also runs parallel to the upper side 15 of plate part 1a..

The production of the partial welds 8a, 8b, 8c took place here in the deep welding regime in welding. The partial welds 8a, 8b, 8c have the same width B and the same depth T. The individual partial welds 8a, 8b, 8c each have an aspect ratio of approximately AV=1.75 here. The partial weld seams 8a, 8b, 8c partially overlap due to the multiple passage of the laser beams over welding curves offset in the direction QR. Furthermore there are areas of the partial welds 8a, 8b, 8c that do not overlap.

The overall width GB of the weld seam 2 is greater than the individual widths B of the partial weld seams 8a, 8b, 8c. In the example shown here, GB has a value of approximately GB=1.6*B. Adjacent partial welds (e.g. the partial welds 8b, 8c) have a distance b in the transverse direction QR (relative to the central axes of the partial welds in the cross section), for which b=0.3* [(B+B)/2] applies here.

The weld seam 2 in part d) can be produced with all variants presented above. This requires the second pass to be made along a weld curve offset from the first weld curve and a third pass to be made along a weld curve offset from the first weld curve and the second weld curve. A laser beam 22, which has produced the partial weld seam 8c, is indicated schematically above the partial weld seam 8c. The diameter of the laser beam 22 essentially corresponds to the width B of the associated partial weld seam 8c, or is slightly smaller than this.

Reference List

1

bipolar plate

1a

upper part of the plate

1b

lower part of the plate

2

Weld

2a

closed weld seam (on the outside)

2 B

closed weld seam (at the breakthroughs)

2c

open weld

3

breakthrough

4

Workpiece surface (facing the at least one laser beam)

5

laser spot

5a

first laser spot

5b

second laser spot

6

welding curve

7

melt pool

7a

first melt pool

7b

second weld pool

8th

partial weld

8a

first partial weld

8b

second partial weld

8c

third partial weld

9

overlap area

10

Overlay Laser Spot

11

core portion

12

ring share

13

(narrow) weld seam

13a

upper part of the weld

13b

lower part of the weld

14

joining zone

15

top (of the upper part of the plate)

16

(wide) weld seam

17

pores

18

microcracks

21

weld overlap area

22

laser beam

b

Distance between adjacent partial welds (in the transverse direction)

B

width of the weld

B1

width of the weld

B2

width of the weld

BLD

sheet thickness

GB

overall width

SR

welding direction

QR

Transverse direction / direction transverse to the welding direction

T

depth of the weld

T1

depth of the weld

T2

depth of the weld

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• DE 1020211138345 [0002, 0007]

Claims (15)

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 of the at least one weld seam (2, 2a, 2b, 2c) with at least two passes of the at least one weld seam (2, 2a, 2b, 2c), in each pass at least one laser beam (22) is guided along a welding curve (6) that runs along the weld seam (2nd , 2a, 2b, 2c), wherein the at least one laser beam (22) generates at least one melt pool (7, 7a, 7b) in the plate parts (1a, 1b), wherein a partial weld seam (8, 8a, 8b , 8c) is generated, and wherein the weld pools (7, 7a, 7b) of the laser beams (22) of different passes are separate from one another, and that the partial weld seams (8, 8a, 8b, 8c) of the different passes in one direction (QR) at least partially overlap transverse to the welding direction (SR). procedure after claim 1 , characterized in that the crossings take place individually one after the other. procedure after claim 1 , characterized in that the crossings are at least partially overlapping in time. Procedure according to one of Claims 1 until 3 , characterized in that the partial weld seams (8, 8a, 8b, 8c) of the different passes in the direction (QR) transverse to the welding direction (SR) at least partially do not overlap, so that the weld seam (2, 2a, 2b, 2c), which is formed by the totality of the partial welds (8, 8a, 8b, 8c), in the direction (QR) transverse to the welding direction (SR) has a total width GB that is greater than a width B of each of the partial welds (8, 8a, 8b, 8c). Procedure according to one of Claims 1 until 4 , characterized in that the width (B, B ₁ , B ₂ ) of the partial weld seam (8, 8a, 8b, 8c) from at least one later pass is greater than the width (B, B ₁ , B ₂ ) of the partial weld seam (8 , 8a, 8b, 8c) from at least one previous crossing. Procedure according to one of Claims 1 until 4 , characterized in that the widths (B, B ₁ , B ₂ ) of the partial welds (8, 8a, 8b, 8c) of all crossings are the same. Procedure according to one of Claims 1 until 6 , characterized in that only one laser beam (22) is used per crossing. Procedure according to one of Claims 1 until 6 , characterized in that at least two laser beams (22) are used simultaneously for each crossing. procedure after claim 8 , characterized in that the laser beams (22) used simultaneously in one pass partially overlap in the welding direction (SR). procedure after claim 8 , characterized in that the molten pools (7, 7a, 7b) of the laser beams (22) used simultaneously in one pass do not overlap one another. Procedure according to one of Claims 8 until 10 , characterized in that the laser beams (22) used simultaneously in one pass are guided on identical welding curves (6). Method according to one of the preceding claims, characterized in that the at least one weld seam (2, 2a, 2b, 2c) comprises one or more closed weld seams (2a, 2b). Procedure according to one of Claims 1 until 12 , characterized in that a respective laser beam (22) is designed as a superimposed laser beam, comprising at least two partial beams, which lie in one another on the workpiece surface (4) and which together form the melt pool (7, 7a, 7b) of the laser beam (22) generate. procedure after Claim 13 , characterized in that the superimposed laser beam comprises a core beam and a ring beam which surrounds the core beam, or that the superimposed laser beam comprises a larger partial beam and a smaller partial beam which lies on the workpiece surface (4) within the larger partial beam . Bipolar plate (1) for a fuel cell, produced by welding two plate parts (1a, 1b) according to a method according to one of the preceding claims.

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