A method for laser welding two thin-walled workpieces in an overlapping region.

Abstract

The present invention relates to a method for laser welding two workpieces (W1, W2) along a weld seam (4), comprising: A first workpiece (W1) having a thickness D1 and a second workpiece (W2) having a thickness D2 are stacked so as to overlap at least in an overlapping region (UeB), The thicknesses D1 and D2 of the two workpieces (W1 and W2) are each 400 μm or less, By means of a laser beam (2) guided along the weld seam (4), from the side of the first workpiece (W1) in the overlap region (UeB), the material of the first workpiece (W1) is melted over the entire thickness D1, and the material of the second workpiece (W2) is melted only over a partial thickness TD of the entire thickness D2; The present invention relates to a method in which laser welding is performed such that the laser beam (2) generates a vapor capillary (1) that extends into a first workpiece (W1) or into a first and a second workpiece (W1, W2) to a capillary depth KT, where 0.33*EST≦KT≦0.67*EST and the weld depth EST=D1+TD. The present invention makes it possible to achieve high seam quality at high feed rates when welding thin-walled workpieces.

Description

Technical Field

[0001] The present invention relates to a method for laser welding two workpieces along a welding seam, wherein a first workpiece having a thickness D1 and a second workpiece having a thickness D2 are stacked so as to overlap at least in an overlapping region, and the thicknesses D1 and D2 of the two workpieces are each 400 μm or less.

Background Art

[0002] Laser welding (also called laser beam welding) is used to permanently connect meltable, usually metallic, workpieces to each other. In this case, laser welding can be realized at a relatively high speed, high precision (especially with a narrow welding seam), and with little thermal deformation of the workpiece.

[0003] Depending on the beam intensity of the laser beam used, laser welding can be realized as conduction welding or as deep penetration welding.

[0004] In deep penetration welding, the laser beam generates a significant vapor capillary (keyhole) in the workpiece material, and the vapor capillary extends along the beam direction within the workpiece material. As a result of multiple reflections of the laser beam on the wall of the vapor capillary, absorption in the workpiece material increases. The material can also be melted in a large volume in the depth direction. Deep penetration welding can be realized at a relatively high feed rate (welding speed). However, spatter and porosity often occur during deep penetration welding, and it is frequently observed that the welding depth along the welding seam becomes irregular (spikes). Therefore, when welding thin workpieces, local connection problems such as the welding seam becoming mechanically unstable or the desired airtightness or the desired electrical contact connection quality not being achieved may occur.

[0005] In thermal conduction welding, the workpiece material is melted by a laser beam placed close to the surface, without generating significant vapor capillaries. The weld depth is basically determined by the thermal conduction of the workpiece material. Spatter or pores are minimal, and the weld seam is relatively smooth. However, its drawbacks include relatively low feed rates, shallow weld depths, and the potential for significant thermal deformation. [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] The objective of this invention is to achieve high seam quality at high feed rates during the welding of thin-walled workpieces. [Means for solving the problem]

[0007] The above objective is achieved by a method for laser welding two workpieces along a welding seam, according to the present invention, wherein a first workpiece having a thickness D1 and a second workpiece having a thickness D2 are stacked on top of each other so as to overlap at least in an overlapping region, the thicknesses D1 and D2 of the two workpieces are 400 μm or less, and a laser beam guided along the welding seam melts the material of the first workpiece over its entire thickness D1 in the overlapping region, starting from the side of the first workpiece, and melts the material of the second workpiece over only a partial thickness TD of its entire thickness D2, and the laser welding is performed such that the laser beam generates a vapor capillary in the first workpiece or in the first and second workpieces extending to a capillary depth KT, where 0.33*EST ≤ KT ≤ 0.67*EST and the welding depth EST = D1 + TD.

[0008] This invention proposes performing laser welding of lap joints of two thin-walled workpieces in a transition mode between thermal conduction welding and deep penetration welding ("transition mode welding"). This makes it possible to greatly utilize the advantages of both processes and largely avoid the disadvantages of both processes. In particular, a highly accurate and sufficient weld depth can be observed, and a hermetically sealed connection exhibiting particularly good electrical conductivity can also be established in a reliable manner. At the same time, manufacturing can be carried out at relatively high feed rates.

[0009] Under the conditions of the capillary depth KT (spread of vapor capillaries into the workpiece material) related to the weld depth EST (spread of molten pool into the workpiece material) according to the present invention, welding is achieved in a desired transition region between heat conduction welding and deep penetration welding, and high weld seam quality can be achieved at relatively high feed rates.

[0010] In relation to the method according to the present invention, a vapor capillary is generated, but the vapor capillary is relatively short (in the direction into the workpiece material or in the direction of the laser beam) compared to conventional deep penetration welding. The weld depth is basically determined by both heat conduction and the depth of the vapor capillary, and the ratio of the two is approximately equal. This makes it possible to achieve a greater weld depth than in the case of heat conduction welding, which is particularly suitable for welding thin-walled workpieces such as metal plates. However, at the same time, the dynamics of the molten pool remain low, especially since the total amount of material melted remains relatively small. Energy absorption from the laser beam into the workpiece material is less pronounced than in the case of deep penetration welding because the reflection of the laser beam within the vapor capillary is minimal due to the shallow capillary depth. In addition, in contrast, the melting by heat conduction of the workpiece material, which is substantially synchronized with the feed rate, greatly compensates for the more rapid dynamic movement in the molten pool.

[0011] The weld depth EST can be measured during the welding process, for example, by ultrasonic waves reflected at the interface between the liquid and solid workpiece materials. The capillary depth KT of a vapor capillary can be measured during the welding process, for example, by the reflection of a measuring laser beam at the bottom of the capillary. Other parameters are usually known (e.g., the focal diameter of the laser beam) or can be easily confirmed by other sensors during the welding process. As an example, several parameters, particularly the width B of the weld seam / molten area, or the capillary width KB on the workpiece surface perpendicular to the feed direction, which roughly corresponds to the focal diameter FDQ perpendicular to the feed direction, can be optically measured by a camera during the welding process. Thus, compliance with the conditions according to the present invention can be confirmed as desired during the welding process and readjusted where appropriate.

[0012] A molten region with a molten width SB is generated almost uniformly in all directions around the vapor capillary (in a plane perpendicular to the feed direction). If the focal diameter FDQ on the front surface of the first workpiece W1 facing the laser beam, perpendicular to the welding direction of the laser beam, is known, and the focal diameter roughly corresponds to the local width of the vapor capillary KB, then the molten width SB can be easily determined using the width B of the weld seam on the front surface of the workpiece, and SB = (B - FDQ) / 2 can be obtained. It is also possible to approximately determine the capillary depth KT in the cross section based on the difference between the weld depth EST, which can be easily confirmed in the cross section (transverse plane), and the molten width SB thus obtained, and KT = EST - SB can be obtained. Therefore, compliance with the conditions according to the present invention can also be easily confirmed later in the welded workpiece, and where appropriate, the process parameters can be repeatedly used to comply with the conditions according to the present invention in future workpieces.

[0013] It should be noted that the melt width SB related to the present invention generally corresponds to a capillary depth KT that is preferably 0.67*SB ≤ KT ≤ 1.33*SB, and particularly preferably 0.80*SB ≤ KT ≤ 1.20*SB.

[0014] The thickness and depth (particularly KT, EST, D1, D2) are determined perpendicular to the surface of the first workpiece facing the laser beam, respectively. Preferably, in connection with the present invention, an unstretched laser beam (with an aspect ratio FDQ / FDL of about 1, typically 0.8 ≤ FDQ / FDL ≤ 1.2, and preferably 0.9 ≤ FDQ / FDL ≤ 1.1) is used for laser welding. The focal point of the laser beam on the workpiece surface is generally circular (isotropic laser beam).

[0015] Preferred Modification of the Invention In a preferred modification of the method according to the present invention, 0.40*EST ≤ KT ≤ 0.60*EST, and preferably 0.45*EST ≤ KT ≤ 0.55*EST. This parameter range has proven to be particularly good in practice. This allows for a particularly good balance between the ratio of heat conduction in the weld depth and the capillary depth.

[0016] Furthermore, in a preferred variant, 0.25*D2≦TD≦0.75*D2, preferably 0.33*D2≦TD≦0.67*D2, and particularly preferably 0.40*D2≦TD≦0.60*D2. This makes it possible to achieve a particularly reliable connection of the second workpiece to the first workpiece. On the other hand, a sufficient portion thickness of the second workpiece is melted to ensure a mechanically minimal connection. At the same time, an excessively large portion thickness is not melted, thereby reducing the risk of through-welding. In through-welding, the connection can be mechanically weakened due to the loss of material. In addition, when the portion thickness is large, especially when TD > 0.5*D2, the mechanical connection usually does not improve further, but the energy requirements of the welding process and, at the same time, the risk of undesirably high molten pool dynamics increase.

[0017] In a particularly preferred modification, the laser welding is performed such that the width KB of the vapor capillary on the laser beam-facing surface of the first workpiece, measured perpendicular to the direction of travel of the weld seam, is 0.50 ≤ KT / KB ≤ 2.00, preferably 0.75 ≤ KT / KB ≤ 1.50, and in particular, the focal diameter FDQ of the laser beam perpendicular to the feed direction and the focal diameter FDL of the laser beam along the feed direction, measured respectively on the laser beam-facing plane of the surface of the first workpiece, are 0.8 ≤ FDQ / FDL ≤ 1.2, preferably 0.9 ≤ FDQ / FDL ≤ 1.1. The desired transfer welding and its associated advantages, in particular, a uniform weld depth EST and the fastest possible feed rate, are best achieved with the specified aspect ratio of KT / KB. These aspect ratios of KT / KB are particularly suitable when FDL ≥ FDQ. In addition, the use of lasers with non-elliptical focal profiles, such as approximate point focals, where the aspect ratio FDQ / FDL is approximately 1, has been found to be particularly good for keeping the molten pool dynamics low. It is also often applicable that 0.50 ≤ EST / B ≤ 1.50, and preferably 0.75 ≤ EST / B ≤ 1.25.

[0018] Furthermore, in a preferred modified form, the laser beam has an average wavelength λ, where λ ≤ 1200 nm, and preferably, a) 900nm ≤ λ ≤ 1100nm, and in particular, λ = 1030nm, 1064nm, or 1070nm, or b) 500nm ≤ λ ≤ 600nm, and in particular, λ = 515nm, or c) 400nm ≤ λ ≤ 500nm, and in particular, λ = 450nm. These average laser wavelengths are well-suited for welding thin workpieces such as metal plates.

[0019] Furthermore, in one modified configuration, it is advantageous that the laser beam has an average laser power P such that 60W ≤ P ≤ 1200W, and preferably 100W ≤ P ≤ 500W. These laser powers allow the transition mode laser welding according to the present invention to be easily performed on many types of workpieces.

[0020] Furthermore, in a preferred modified form, the laser beam has a focal diameter FD in the plane of the surface of the first workpiece facing the laser beam, where 10 μm ≤ FD ≤ 100 μm, preferably 14 μm ≤ FD ≤ 60 μm, and particularly preferably 25 μm ≤ FD ≤ 39 μm. These diameters can be readily used in practice to weld thin-walled workpieces in transition mode in relation to the present invention. Here, the focal diameter FD is assumed to be the maximum focal diameter, generally 0.8 ≤ FDQ / FDL ≤ 1.2, and preferably 0.9 ≤ FDQ / FDL ≤ 1.1.

[0021] Furthermore, in a preferred modification, the width B of the molten material of the first workpiece on the laser beam-facing surface of the first workpiece, measured perpendicular to the direction of the weld seam, is set to 60 μm ≤ B ≤ 600 μm, preferably 80 μm ≤ B ≤ 400 μm, and particularly preferably 100 μm ≤ B ≤ 200 μm. Within this range, good mechanical connection can be achieved with thin-walled workpieces.

[0022] In particularly preferred modified forms, D1 ≤ 250 μm and D2 ≤ 250 μm, preferably 50 μm ≤ D1 ≤ 200 μm and 50 μm ≤ D2 ≤ 200 μm, and especially preferably 75 μm ≤ D1 ≤ 100 μm and 75 μm ≤ D2 ≤ 100 μm. With these workpiece thicknesses, extremely good weld seam quality has been achieved at high welding speeds in practice. In many applications, at least in the weld seam region, D1 = D2 or 0.8 * D1 ≤ D2 ≤ 1.2 * D1.

[0023] In a preferred variant form, 50 μm ≤ EST ≤ 600 μm, preferably 60 μm ≤ EST ≤ 400 μm, and particularly preferably 75 μm ≤ EST ≤ 225 μm. In connection with the present invention, these welding depths are very easily achievable and, in particular, are very constant over the length of the weld seam. In the case of the welded joints according to the present invention, the welding depth EST generally varies by less than 20%, usually less than 10%, and often less than 5% from its average value.

[0024] In one variant form, the laser beam is moved relative to the workpiece at a feed rate v, with v ≥ 5 m / min, and preferably v ≥ 10 m / min being advantageous, and in particular the laser beam is deflected by a laser scanner. In connection with the present invention, the specified high feed rate (welding speed) can generally be established without problems with good weld seam quality and can enable high manufacturing efficiency.

[0025] Also, in a preferred variant form, the two workpieces are in the form of curved metal sheets that are oriented such that the metal sheets are generally parallel in plane and are pressed against each other by the convex outer side that is curved during laser welding so as to press against each other at the contact part by elastic deformation, and the laser beam welds the two metal sheets along the weld seam in the region of this contact part. In particular, the two curved metal sheets are made of steel. This procedure enables a particularly robust connection of the workpieces. By elastic deformation, the gap (empty space) between the workpieces during the welding process is avoided or minimized, and a welded joint is obtained over the same width as in the case of flat workpieces even if the workpieces are curved in a relaxed state.

[0026] A variant form in which the two workpieces are in the form of flexible metal foils is also advantageous. When welding flexible metal foils, the present invention makes it possible to form a very reliable and robust mechanical connection. Generally, the foils are pressed against each other by rams during welding.

[0027] The scope of the present invention also includes the use of the above method for welding conductors and / or gas seals formed by two workpieces. According to the present invention, a highly reliable welded connection of two workpieces that meets high requirements regarding airtightness (or liquid tightness) can be achieved, and a low electrical (or thermal) contact resistance between the workpieces can be ensured. Therefore, use in conductors and gas seals is particularly advantageous.

[0028] In a preferred variant of the use according to the invention, the two workpieces are bipolar plates of a fuel cell. The bipolar plates of a fuel cell generally have to be connected airtight (usually airtight with respect to oxygen) and have to have a good electrical connection so that the current generated by the fuel cell can be transported without loss. In addition, the bipolar plates have a thickness that can be easily connected by the method according to the invention.

[0029] Further advantages of the present invention will become apparent from the specification and the drawings. Similarly, according to the present invention, the above-described features and the features to be detailed hereinafter can be used individually in each case or together in any desired combination. The illustrated and described embodiments should not be understood as an exhaustive enumeration, but rather as an exemplary and exact enumeration for summarizing the present invention.

Brief Description of the Drawings

[0030] [Figure 1a] A schematic cross-sectional view through two workpieces welded by the method according to the invention at the height of the vapor capillary perpendicular to the feed direction of the laser beam is shown. [Figure 1b] A schematic perspective view of the workpiece of FIG. 1a is shown. [Figure 2a] A schematic cross-sectional view through two workpieces welded by heat conduction welding in a manner deviating from the present invention is shown. [Figure 2b] A schematic cross-sectional view through two workpieces welded according to the present invention in the transition mode between heat conduction welding and deep penetration welding is shown. [Figure 2c]A schematic cross-sectional view is shown through two workpieces welded by deep penetration welding in a manner that deviates from the present invention. [Figure 3a] A schematic cross-sectional view is shown, passing through two convexly curved workpieces intended to be welded according to the present invention. [Figure 3b] Figure 3a shows a schematic cross-sectional view passing through the workpiece that is welded according to the present invention while pressed together and elastically deformed. [Modes for carrying out the invention]

[0031] An exemplary variant of the method for laser welding two thin-walled workpieces W1 and W2 according to the present invention is shown in a schematic cross-sectional view in Figure 1a (perpendicular to the feed direction VR and in the center of the vapor capillary 1) and in a schematic perspective view in Figure 1b. For simplicity, the workpieces W1 and W2 are shown only in partial regions. The workpieces W1 and W2 may be, for example, in the form of flexible foils.

[0032] The first workpiece W1 and the second workpiece W2 are stacked on top of each other so as to overlap in an overlapping region UeB, and for this purpose, a suitable holding tool may be used (e.g., a robotic arm or ram, although not shown in further detail). Workpieces W1 and W2 have thicknesses D1 and D2 in the overlapping region UeB, respectively, where the thicknesses are selected such that D1 = D2 = 100 μm. Workpieces W1 and W2 are typically manufactured from metallic material. Thicknesses D1 and D2 are measured perpendicular to the surface 3 of the first workpiece W1.

[0033] A laser beam 2 is directed towards the surface 3 of the first workpiece W1 in order to overlap and weld the workpieces W1 and W2 together. In this case, the laser beam 2 is moved relative to the workpieces W1 and W2 along the feed direction VR by a laser scanner (not shown), which is generally configured to include a mirror that can be moved, for example, by piezoelectric drive. The laser beam 2 is generated by an IR laser having a wavelength of 1030 nm. As a result, the laser beam 2 generates a weld seam 4 having a travel direction VLR corresponding to the feed direction VR.

[0034] Here, the laser beam 2 generates a vapor capillary 1 in the material of the first workpiece W1 (note that in other variations where the second workpiece is considerably thicker than the first workpiece, the vapor capillary may also reach the second workpiece, although these are not shown). The vapor capillary 1 has a (maximum) capillary width KB on the surface 3 of the first workpiece W1 that corresponds very precisely to the (maximum) focal diameter FDQ of the laser beam 2, and the focal diameter is measured in the transverse direction QR. The transverse direction QR runs perpendicular to the feed direction VR and within the plane of the surface 3 of the first workpiece W1 facing the laser beam 2.

[0035] Here, the laser beam 2 has the form of a circular point focal point such that the (maximum) focal diameter FDL (also called the longitudinal focal diameter) along the feed direction VR is equal to the focal diameter FDQ (also called the transverse focal diameter) in the transverse direction QR. Here, the laser beam 2 has a direction-independent uniform focal diameter FD, which corresponds to a preferred variant.

[0036] In this case, the steam capillary 1 reaches a capillary depth KT within the material of the first workpiece W1. In the illustrated modified form, KT is approximately 3 / 4 of the thickness D1, or approximately 75 μm.

[0037] The materials of workpieces W1 and W2 are melted around the steam capillary 1, thereby forming a molten pool 5. Starting from the steam capillary 1, the material is uniformly melted in all directions over a generally uniform molten width SB (in a cross-sectional plane perpendicular to the feed direction VR shown in Figure 1a). In this case, the molten width SB is approximately 65 μm. Therefore, in this case, the material of the second workpiece W2 is melted over a partial thickness TD of approximately 40 μm. In this case, the weld depth EST = D1 + TD over which the materials of workpieces W1 and W2 are melted overall, starting from the surface 3, is approximately 140 μm. Therefore, KT = 0.54 * EST is approximately applied here.

[0038] In the illustrated modified configuration, the steam capillary 1 also has a capillary width KB of approximately 50 μm, measured in the transverse direction QR on the plane of the workpiece surface 3. Note that the capillary width KB corresponds very precisely to the focal diameter FDQ in the transverse direction QR. Therefore, the capillary depth KT is approximately 1.5 times the capillary width KB, i.e., KB / KT = 1.50. In this case, the weld seam 4 has a width B of approximately 180 μm (measured in the transverse direction QR), which corresponds to the sum of KB + 2 * SB. The thickness TD of the portion where welding is performed into the second workpiece W2 is, in this case, approximately 40% of the total thickness D2, i.e., TD = 0.40 * D2.

[0039] In particular, the laser power of the laser beam 2, the focal diameter FD of the laser beam 2 on the workpiece surface 3, and the feed rate (welding rate) of the laser welding are selected so that the ratio of the steam capillary 1, molten pool 5, and workpiece geometry shown herein is set to enable laser welding in a transition mode between heat conduction welding and deep penetration welding.

[0040] Figures 2a, 2b, and 2c show an overview of the ratio of capillary depth KT to weld depth EST, and the ratio of capillary width KB to capillary depth KT, during laser welding in different welding modes in a cross section perpendicular to the feed direction (similar to Figure 1a). In the illustrated examples, it is assumed that the focal geometry of laser beam 2 is not elliptically spread (FDQ = FDL, e.g., due to a circular, round point focus / isotropic laser beam). Figure 2a shows a typical heat conduction welding operation, Figure 2b shows a typical transition mode laser welding operation according to the present invention, and Figure 2c shows a typical deep penetration laser welding operation.

[0041] In heat conduction welding, as shown in Figure 2a, the laser beam 2 is extremely small, generating a shallow vapor capillary 1 with a shallow capillary depth KT (or, although not shown, a vapor capillary that is not noticeable at all). The resulting weld depth EST is substantially based on the width of the molten pool 5, i.e., the melt width SB, and according to EST = KT + SB, KT is considerably smaller than SB. In practice, the melt width SB* at the lowest point of the vapor capillary 1 corresponds very accurately to the melt width SB** on the workpiece surface 3, so please note that in the following text, the melt width will be uniformly denoted as SB. In the illustrated example, KT is approximately 0.23*EST. In relation to the present invention, the range KT < 0.33*EST is assigned to an undesirable heat conduction mode. The weld depth EST reaches only a minimal extent within the second workpiece W2, where TD is approximately 0.08*D2.

[0042] In addition, in the heat conduction mode (where the isotropic laser beam 2 is used), the capillary depth KT is also considerably smaller than the capillary width KB. In Figure 2a, KT / KB is approximately 0.30. In relation to the present invention, the range KT / KB < 0.50 is assigned to an undesirable heat conduction mode.

[0043] Figure 2b shows a transition mode laser welding according to the present invention. The laser beam 2 generates a medium-sized vapor capillary 1. The welding depth EST is based on the capillary depth KT of the vapor capillary 1 and the melt width SB of the molten pool 5 being approximately equal. In the illustrated example, KT = 0.5 * EST. In relation to the present invention, the range 0.33 ≤ KT / EST ≤ 0.67 is assigned to a desirable transition mode. The welding depth EST reaches sufficiently into the second workpiece W2, where TD = 0.6 * D2.

[0044] In transition mode, the capillary depth KT is also additionally similar in size to, or only slightly larger than, the capillary width KB. In Figure 2b, KT / KB is approximately 1.0. In relation to the present invention (when an isotropic laser beam 2 is used, or when at least FDQ ≤ FDL), the range 0.50 ≤ KT / KB ≤ 2.00 is assigned to the desired transition mode laser welding.

[0045] Finally, Figure 2c shows deep penetration laser welding. The laser beam 2 generates an extremely large and deep vapor capillary 1. The weld depth EST is substantially based on the capillary depth KT of the vapor capillary 1. In the illustrated example, KT is approximately 0.88*EST. In relation to the present invention, the range KT > 0.67*EST is assigned to an undesirable deep penetration welding mode. In this case, the weld depth EST is approximately TD = 0.96*D2, as it almost penetrates the entire second workpiece W2 (although not illustrated, in many cases the deep penetration welding mode can even be a through-weld, i.e., TD = D2).

[0046] In the deep penetration welding mode (where an isotropic laser beam 2 is used), the capillary depth KT is also considerably larger than the capillary width KB. In Figure 2c, KT / KB is approximately 2.1. In relation to the present invention, the range KT / KB > 2.0 is assigned to undesirable deep penetration laser welding.

[0047] If the focal diameter FDQ in the transverse direction QR is known (or the capillary width KB is known), the capillary depth KT can be easily determined from the weld seam width B and weld depth EST. B and EST can also be easily determined in cross-section (cross-section as shown in Figures 2a-2c), or they can be easily observed in situ with a camera and ultrasound. Based on B and FDQ (the latter corresponding to KB), SB is, SB = (B - FDQ) / 2 It can be calculated as shown above, and furthermore, KT = EST - SB.

[0048] In the examples in Figures 2a and 2c, D2 is somewhat larger than D1, but often D1 = D2.

[0049] Figure 3a schematically shows two workpieces W1 and W2, which are curved metal plates, particularly steel plates, in a schematic cross-sectional view perpendicular to a desired weld seam. In this case, the two workpieces W1 and W2 are bipolar plates for a fuel cell. Note that Figure 3a (as with Figure 3b) shows only a portion of the workpieces W1 and W2 intended to achieve the welding according to the present invention. Also note that the two workpieces W1 and W2 may optionally yield multiple weld seams (not shown).

[0050] The two workpieces W1 and W2 have opposing convex curved outer surfaces 31 and 32. When the two workpieces (metal plates) W1 and W2 are placed in contact with each other by these curved outer surfaces, contact occurs only along a narrow contact line 30, and in the cross-section shown in Figure 3a perpendicular to this contact line 30, this contact line 30 appears as a point.

[0051] Welding workpieces W1 and W2 along this contact line 30 is extremely difficult. This is because the area that is normally melted when planes are in parallel contact is partially located in one or more V-shaped gaps 33 between the outer edges 31 and 32 of the workpieces. As a result, gaps or at least weak areas can easily form in the weld seam.

[0052] According to the present invention, in order to weld, the workpieces W1 and W2 are pressed toward each other by their convex outer surfaces 31 and 32 (see pressing direction 34), resulting in elastic deformation of the outer surfaces 31 and 32 (see Figure 3b). In this case, the convex outer surfaces 31 and 32 are pressed so as to become slightly flat, and a contact portion 35 is formed around the original contact line, extending perpendicular to the pressing direction, so that the workpieces W1 and W2 or the metal plate are oriented so as to be roughly parallel to each other and press against one another.

[0053] In this elastically deformed state of workpieces W1 and W2, laser welding according to the present invention is achieved by a laser beam 2 directed toward the workpiece surface 3 of the first workpiece W1. In this case, the direction of the laser beam 2 is perpendicular to the plane of the paper in Figure 3b. The laser beam 2 melts the material of the first workpiece W1 over its total thickness D1 and the material of the second workpiece W2 over approximately half of its thickness D2 (see Figure 2b for, for example, the conditions of the transition mode according to the present invention). The melting of the workpiece material takes place within the contact area 35, which simultaneously represents the overlapping region UeB of workpieces W1 and W2 stacked on top of each other.

[0054] It should be noted that the elastic deformation or pressing force of workpieces W1 and W2 is selected to a significant extent that the contact width KOB of the contact portion 35 is greater than the width B of the weld seam 4. This makes it possible to obtain a particularly high-quality weld seam 4, comparable in quality to a weld formed by stacking two planar workpieces (as shown in Figure 1a). The weld seam obtained in Figure 3b can be manufactured particularly airtight and with low electrical resistance between workpieces W1 and W2.

[0055] After laser welding of workpieces W1 and W2 and sufficient cooling, the pressing force is released again, and workpieces W1 and W2 generally spring back to the elastically relaxed state shown in Figure 3a. However, workpieces W1 and W2 remain welded to each other with good seam quality across width B.

[0056] In a preferred modification of the method according to the present invention, welding can be achieved with the following parameters in particular. -Workpiece thickness D1=D2=75μm, - Welding depth EST = 112.5 μm, -FD=KB=31.5μm, -KT = 47.5 μm, -SB=65.25μm, -B = 162 μm. In this case, since KT / KB = 1.5 and KT = 0.42 * EST, TD = 0.5 * D2 is applied.

[0057] In another preferred variation of the method according to the present invention, welding can be achieved with the following parameters in particular: -Workpiece thickness D1=D2=75μm, - Welding depth EST = 112.5 μm, -FD=KB=31.5μm, -KT=63μm, -SB=49.5μm, -B = 130.5 μm. In this case, since KT / KB=2.0 and KT=0.56*EST, TD=0.5*D2 is applied. [Explanation of symbols]

[0058] 1. Steam capillary 2 laser beams 3 Workpiece surface 4. Weld seam 5. Melting pool 30 Contact lines 31. Outer surface (first workpiece) 32. Outer (Second Workpiece) 33. Empty space 34. Direction of pressure 35 Contact area B. Width of the weld seam D1 Thickness of the first workpiece D2 Thickness of the second workpiece EST weld depth FD (maximum) focal diameter (Maximum) focal diameter in the FDL feed direction FDQ (Maximum) focal diameter in the transverse direction KB capillary width KOB contact width KT Capillary Depth QR Cross Direction SB melting width SB* (Melting width measured at the center under the steam capillary) SB** (Melt width measured on the workpiece surface) TD partial thickness UeB overlapping region Direction of VLR weld seam VR feed direction W1 First Workpiece W2 Second Workpiece

Claims

1. A method for laser welding two workpieces (W1, W2) along a welding seam (4), A first workpiece (W1) having a thickness D1 and a second workpiece (W2) having a thickness D2 are stacked on top of each other so as to overlap at least in an overlapping region (UeB). The thicknesses D1 and D2 of the two workpieces (W1 and W2) are 50 μm or more and 200 μm or less, A laser beam (2) guided along the welding seam (4) melts the material of the first workpiece (W1) over its entire thickness D1 in the overlapping region (UeB) from the side of the first workpiece (W1), and melts the material of the second workpiece (W2) over only a partial thickness TD of the entire thickness D2. The laser welding is performed such that the laser beam (2) generates a vapor capillary (1) extending to a capillary depth KT within the first workpiece (W1) or within the first and second workpieces (W1, W2), where 0.33*EST ≤ KT ≤ 0.67*EST, and the welding depth EST = D1 + TD. With respect to the width B of the molten material of the first workpiece (W1) on the surface (3) of the first workpiece (W1) facing the laser beam (2), measured perpendicular to the running direction (VLR) of the welding seam (4), 60 μm ≤ B ≤ 600 μm, Preferably, 80 μm ≤ B ≤ 400 μm, Particularly preferably, the condition 100 μm ≤ B ≤ 200 μm is applied. A method where 75 μm ≤ EST ≤ 225 μm.

2. 0.40*EST ≤ KT ≤ 0.60*EST, Preferably, 0.45*EST ≤ KT ≤ 0.55*EST. The method according to claim 1.

3. 0.25 * D2 ≤ TD ≤ 0.75 * D2, Preferably, 0.33*D2 ≤ TD ≤ 0.67*D2, Particularly preferred is 0.40*D2 ≤ TD ≤ 0.60*D2. The method according to claim 1.

4. The width KB of the vapor capillary (1) on the surface (3) of the first workpiece (W1) facing the laser beam (2), measured perpendicular to the running direction (VLR) of the welding seam (4), is as follows: 0.50 ≤ KT / KB ≤ 2.00, Preferably, the condition 0.75 ≤ KT / KB ≤ 1.50 is applied. In particular, the focal diameter FDQ of the laser beam (2) perpendicular to the feed direction (VR) of the laser beam (2) and the focal diameter FDL of the laser beam (2) along the feed direction (VR), which are measured on the plane of the surface (3) of the first workpiece (W1) facing the laser beam (2), are as follows: 0.8 ≤ FDQ / FDL ≤ 1.2, The method according to any one of claims 1 to 3, wherein the laser welding is carried out such that preferably 0.9 ≤ FDQ / FDL ≤ 1.1 applies.

5. The laser beam (2) has an average wavelength λ, λ ≤ 1200 nm, Preferably, a) 900 nm ≤ λ ≤ 1100 nm, and in particular, λ = 1030 nm, 1064 nm, or 1070 nm, or b) 500 nm ≤ λ ≤ 600 nm, and in particular λ = 515 nm, or c) The method according to any one of claims 1 to 4, wherein 400 nm ≤ λ ≤ 500 nm, and in particular λ = 450 nm.

6. The laser beam (2) has an average laser power P. 60W ≤ P ≤ 1200W, Preferably, the method according to any one of claims 1 to 5, wherein 100W ≤ P ≤ 500W.

7. The laser beam (2) has a focal diameter FD in the plane of the surface (3) of the first workpiece (W1) that faces the laser beam (2), 10 μm ≤ FD ≤ 100 μm, Preferably, 14 μm ≤ FD ≤ 60 μm. Particularly preferred is the method according to any one of claims 1 to 6, wherein 25 μm ≤ FD ≤ 39 μm.

8. The method according to any one of claims 1 to 7, wherein 75 μm ≤ D1 ≤ 100 μm and 75 μm ≤ D2 ≤ 100 μm.

9. The laser beam (2) is moved relative to the workpiece (W1, W2) at a feed rate v. v ≥ 5 m / min, Preferably, v ≥ 10 m / min. The method according to any one of claims 1 to 8, in particular, wherein the laser beam (2) is deflected by a laser scanner.

10. The two workpieces (W1, W2) are oriented so that their planes are roughly parallel, and are pressed against each other by their convexly curved outer surfaces (31, 32) during laser welding, so that they press against each other at the contact portion (35) due to elastic deformation, and the laser beam (2) welds the two metal plates along the welding seam (4) in the region of the contact portion (35). In particular, the method according to any one of claims 1 to 9, wherein the two curved metal plates are made of steel.

11. The method according to any one of claims 1 to 9, wherein the two workpieces (W1, W2) are in the form of flexible metal foils.

12. Use of the method according to any one of claims 1 to 11 for welding a conductor and / or gas seal formed by the two workpieces (W1, W2).

13. The use according to claim 12, wherein the two workpieces (W1, W2) are bipolar plates of a fuel cell.

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