Laser-beam welding method

Abstract

The invention relates to a method for laser-beam welding of at least two parts (1, 3) which are to be joined and which are placed one on top of the other in a lap joint, the two parts (1, 3) to be joined being welded to one another by a line seam to form a preferably closed-surface connection region (5). According to the invention, in order to form the connection region (5), the laser beam (9) is guided in the welding process in any desired path planning strategy along a target welding track (13), namely by forming a weld seam path (19), of which adjacent path portions build up the preferably closed-surface connection region (5).

Description

[0001] Laser beam welding process

[0002] DESCRIPTION:

[0003] The invention relates to a method for laser beam welding according to the preamble of claim 1.

[0004] The battery system of an electrically powered vehicle consists of several battery modules, which in turn are made up of a large number of individual battery cells. The terminals of the battery cells are interconnected by cell connectors (busbars). Both the terminals and the cell connectors can be made of aluminum. In addition to identical connections made of aluminum, combinations with, for example, copper can be used as a identical or mixed connection. In the case of round cells, it can also be steel with a corresponding coating, such as diffusion-annealed steel strip with an electrolytic nickel coating. The invention is applicable to the contacting of battery cells of any cell format (i.e., round cells, prismatic cells, or pouch cells).

[0005] Laser beam welding technology is used to electrically connect the terminals to the cell connectors. The main requirements for this connection are the electrical contact resistance and mechanical strength. To achieve the lowest possible contact resistance and high mechanical strength, a correspondingly large connection area (hereinafter referred to as the large-area connection zone) must be created between the cell connector and the terminal. Furthermore, the geometry of the weld seam pattern is crucial for the further optimization of these parameters. To achieve the associated high process quality, laser beam oscillation is used in the state-of-the-art laser beam welding process. A frequent lateral or circular oscillation (or alternatively any combination thereof, such as a Lissajous pattern) is superimposed on a target welding track along which the laser beam is guided.This type of process control is also a technology used in car body construction. During oscillation, the processing optics must typically use scanner mirrors to achieve a frequency and amplitude adjusted to the feed rate in order to generate a closed bonding surface or bonding zone.

[0006] Due to the inertia of the moving masses of the scanner mirror and the functionality of the drive motor Z-unit, this amplitude can no longer be achieved as the feed speed and thus the scanning frequency increase. Consequently, the achievable seam width in the joining plane decreases with the amplitude, and the requirements for electrical resistance and strength are no longer met.

[0007] In the current state of the art, the achievable feed rate and thus the productivity of the system are limited by the selected process strategy using beam oscillation.

[0008] As previously described, the current state of the art does not allow for arbitrary upward scaling of process speeds to produce a uniformly developed weld joint. The scanner system is the limiting factor in terms of achievable path fidelity with superimposed beam oscillation (scan frequency and amplitude). This non-continuous scaling consequently reduces the functionality of the joint (e.g., resistance and strength).

[0009] In addition to beam oscillation, a large weld width can be achieved by using a large focus diameter. Due to their process design, both processes are unsuitable for scaling in process speed. Oscillation cannot maintain the required path fidelity in terms of amplitude, and the large focus diameters require very high laser power. Such high laser power for the large focus diameter leads to overheating of the joining zone and thus to the decomposition of electrically insulating plastic layers and seals.

[0010] DE 10 2013 215 362 A1 discloses a method for determining a penetration depth during laser welding. DE 10 2010 039 893 A1 discloses a method for producing a joining body.

[0011] The object of the invention is to provide a laser beam welding method that enables high process speed while still achieving a secure weld seam with minimal contact resistance and high mechanical strength. At the same time, the limit temperature for decomposition of electrically insulating plastic layers and seals must not be exceeded.

[0012] The object of the invention is solved by the features of claim 1. Preferred developments of the invention are disclosed in the subclaims.

[0013] The invention is based on a method for laser beam welding at least two joining partners. These are placed one above the other in an overlap joint and are welded together by a linear seam, forming a preferably closed-surface connection zone or connection surface. The closed-surface connection zone forms an electrical contact between the two joining partners. In order to achieve reduced contact resistance at the connection zone, it is therefore particularly important that the two joining partners are welded to one another in the connection zone, preferably with a closed surface. According to the characterizing part of claim 1, to form the connection zone, the laser beam is guided along a desired welding track during the welding process using any path planning strategy, specifically forming a weld seam path whose adjacent path sections constitute the connection zone.In a preferred embodiment, to which the invention is not limited, the welding process is carried out with a path planning strategy in which the laser beam is guided along a meandering and / or spiral-shaped target welding track.

[0014] An increase in process speed can be achieved, in particular, by reducing the focus diameter. With a smaller focus diameter, a required welding depth can be achieved with less laser power than with a larger focus diameter. At the same time, to achieve the required welding depth, the feed rate can be scaled upwards via the laser power (conservation of energy: constant input energy per unit length = laser power per feed rate). However, as a result of the reduced focus diameter, the achievable seam width in the welding process decreases. To achieve the required total seam width for the resulting connection surface, according to the characterizing part of claim 1, several path sections of the weld seam path are placed next to one another to build up the total seam width.It is important that the welding process should preferably not be interrupted when generating the entire weld seam geometry, which has a meander shape, a spiral shape or a combination thereof.

[0015] In order to better control the process as the feed rate increases, superimposed beam shaping is a better option than a simple round laser beam spot. This smoothes the seam surface of the generated weld seam geometry and simultaneously influences the track spacing of the individual tracks. With superimposed beam shaping, the weld seam can be widened in the flanks, allowing the same seam width to be produced with fewer adjacent tracks. The power ratio of the radiation surfaces is crucial here. To ensure directionality, these are preferably divided into a radially inner circular surface and a radially outer annular or lateral surface, which are concentrically aligned with one another, with or without an intermediate geometric gap.The welding depth is created via the inner circle, and the seam width is influenced via the surrounding circular ring.

[0016] As a result of the juxtaposition of multiple tracks / paths, heat accumulates in the joining zone, depending on the weld seam geometry to be welded. This causes the welding depth to increase with each pass in proportion to the stored heat. To counteract this and achieve a constant welding depth, a targeted reduction in laser power during the construction of the closed-surface connection zone is recommended. For example, a reduction in the laser power for the subsequent individual track.

[0017] A key aspect of the invention is that any desired weld seam geometry can be generated to ensure a resistance- and strength-optimized joint for a high-voltage battery cell contact that is adapted to the overall system. The continuous increase in feed rate is subject only to the available power, and the feed rate can be increased as desired. This allows the resulting total time required to create a weld seam to be reduced several times over compared to the approaches described in the prior art.

[0018] In addition to the electrical cell contacts described, for example, preferably made of aluminum or copper or a mixed combination thereof, or of steel (for round cells), this concept can also be applied to other weld seam circumferences in the drive train, power electronics, or body construction. The advantages of the invention lie in particular in the increased process speed when laser beam joining materials with small focus diameters to generate a weld seam connection by placing individual tracks next to each other. To determine the beam geometry, a balanced consensus must be defined between focus diameter, beam shaping, intensity distribution, and feed rate. As indicated above, a small focus diameter is advantageous for achieving high speeds. Superimposed beam shaping helps to adjust the ratio of seam width to weld depth and to smooth the seam surface.

[0019] Beam shaping can be achieved by adjusting or increasing the number of optical components in the equipment line in the laser beam source, via the beam guide in the fiber optic cable, or directly in the processing optics. The design of the optical elements defines the geometry of the beam arrangement. For the most direction-independent surface processing possible, these elements are preferably arranged concentrically. The superimposed power distribution (intensity = power / area) can be individually determined by the applied beam shaping principle before and / or during the processing process. Furthermore, in addition to round beam shapes, other shapes such as ellipses, rectangles, or combinations thereof can be used.In addition to fibers, all beam configurations can be generated by optical elements such as a prism, a diffractive or refractive optical element or other features in the processing optics, preferably in the collimated beam path between the collimating lens and the focusing lens.

[0020] A narrow seam width in the joining plane results in a very long weld seam to create the required connection zone. To minimize this seam length, the entire seam length is projected onto a defined (e.g., meandering and / or spiral) weld seam geometry and built up by juxtaposing / winding the seam. One possible strategy is to create a target weld track in a meander shape, which has any number, for example, five, adjacent longitudinal track sections (their individual length is determined by the required weld seam geometry) connected with, for example, right-angled joints. Alternatively, the joints can be semicircular or have any other geometry. The number of longitudinal track sections depends on the required seam width (1 ... n). The respective track spacing is constant and is determined depending on the individual track width.For example, a track spacing of 0.5 mm can be set, which ensures an overlap with a single seam width of 0.55 mm. By selecting the processing optics, fiber diameter, beam shaping, power, and feed rate, the track spacing can be individually set and is an adaptable scaling parameter.

[0021] When welding with a constant laser power, a heat field forms depending on the weld seam geometry, which leads to a corresponding accumulation. Consequently, this preheating leads to a greater weld depth in the subsequent track. To counteract this heat accumulation-induced increase in weld depth and the simultaneous overall increase in temperature in the joint, it is advisable to reduce the laser beam power for each individual track. For example, the laser beam power can be reduced by a predefined amount per individual track. This power reduction depends on the length and geometry of the tracks and thus on the resulting heat field. This is directly related to the process parameters.

[0022] The scanner's flexibility allows weld seam geometries to be constructed using all conceivable path planning strategies. Symmetrical weld track shapes, such as a meander or a spiral, are preferred. The spiral can be designed in any geometry. Preferably, the spiral can be rectangular with parallel longitudinal track sections. Alternatively, the spiral can extend radially outward in a circular pattern with a continuously increasing radius, starting from the radially inner side.

[0023] Aspects of the invention are highlighted in more detail below: The preferably closed-surface connection zone can be formed with precisely one weld seam path that extends uninterrupted along the entire desired weld track. Alternatively, the desired weld track can also be provided with interruptions, for example, if two seam areas are constructed alternately in one geometry or adjacent geometries. Furthermore, the cell contact between the cell terminal and the cell connector can also be formed, for example, by two or more independent connection zones. In this case, each connection zone can be assigned a weld seam geometry with, for example, an uninterrupted desired weld track.

[0024] It is preferred if the laser beam is guided along the target weld track during the welding process without an overlaid oscillating pendulum motion. In this case, the path traveled by the laser beam on one of the joining partner surfaces corresponds to the length of the target weld track. In contrast, the prior art involves spatial and / or temporal laser beam oscillation, in which a frequent lateral or circular oscillation (or alternatively any hybrid of these, such as a Lissajous figure) is superimposed on the laser beam's traverse along the target weld track. In this case, the path traveled by the laser beam is significantly longer than the actual length of the target weld track. During laser beam oscillation, the processing optics in the laser beam welding device must be guided via a scanner mirror at a frequency and amplitude adapted to the feed rate in order to generate the closed-surface connection zone.Due to the inertia of the moving masses of the scanner mirror and the functionality of the drive motor Z-unit, this amplitude can no longer be achieved, especially at higher feed rates and thus increasing scan frequencies. As a result, the feed rate is limited by the inertia of the processing optics (scanner mirror).

[0025] With regard to a perfect weld seam geometry, the setting of an appropriate track spacing between adjacent track sections of the target weld track is relevant. It is preferred if the track spacing is dimensioned such that the corresponding track sections of the weld seam path formed during the welding process overlap with an overlap dimension. As mentioned above, a small focus diameter in particular leads to a correspondingly reduced seam width of the weld seam path. To increase the seam width on the weld seam path surface, it is preferred if beam shaping is carried out in which the laser beam is split into at least a first partial beam and a second partial beam, each of which has a different power per radiation area.

[0026] To achieve laser beam processing that is as direction-independent as possible, it is preferable to generate a radially inner core beam and a concentric, radially outer annular beam, with or without an intermediate geometric gap, by means of beam shaping. In this case, the power distribution between the core and annular beams can be dimensioned such that the welding depth is adjusted by the core beam and the annular beam is adjusted by the weld width.

[0027] In a specific embodiment, the laser beam power can be deliberately reduced as the welding process progresses during the formation of the closed-surface connection zone. Such a reduction in laser beam power counteracts a heat buildup-induced increase in the welding depth and process temperature. For example, the laser beam power can preferably be reduced by a predefined amount during the formation of each adjacent section of the weld seam path.

[0028] According to the invention, the desired welding track can run in any desired shape to form the closed-surface connection zone. According to a first embodiment, the desired welding track can be designed in a meandering shape with mutually parallel longitudinal track sections. The longitudinal track sections can be connected to one another in series via transverse track sections. The transverse track sections can have any desired geometry. For example, the transverse track sections can be straight, thereby providing a right-angled connection between adjacent longitudinal track sections. Alternatively, the transverse track sections can run in a circular or arcuate shape.

[0029] For example, in the welding process, the laser beam power can be reduced by a predefined amount for each longitudinal track section during the laser beam passage in the sequence from the first longitudinal track section to the last longitudinal track section.

[0030] In an alternative embodiment, the target welding track can have a spiral shape, extending from a radially inner starting point in a spiral radially outward direction, in a circular motion sequence with a continuously increasing radius of motion. In another alternative embodiment, the target welding track can be designed as a hybrid of a spiral and a meandering shape. In this case, the target welding track can have a radially inner longitudinal track section and additional longitudinal track sections arranged parallel to both sides of it. All longitudinal track sections are connected to one another in a spiral shape via transverse track sections.

[0031] Exemplary embodiments of the invention are described below with reference to the accompanying figures. They show:

[0032] Fig. 1 a welded joint of two metallic joining partners with partial elevation;

[0033] Fig. 2 a laser beam welding device for producing the

[0034] Welded joint in Figure 1 ;

[0035] Figs. 3 to 9 are views relating to different embodiments of the invention;

[0036] 10 and 11 each show views according to a comparative example not encompassed by the invention. Figure 1 shows a component arrangement comprising two joining partners 1, 3 which are placed one above the other in an overlap joint and welded together by a linear seam. Between the two joining partners 1, 3, a closed-surface connection zone 5 is formed, in which the two joining partners 1, 3 are welded together. The lower joining partner 1 can preferably be a cell terminal of a battery cell of a high-voltage battery system of a vehicle, while the upper joining partner 3 is a component of an electrical cell connector. The closed-surface connection zone 5 forms an electrical contact between the two joining partners 1, 3. It is important that the two joining partners 1, 3 are welded to one another over a closed surface in the connection zone 5 in order to keep electrical contact resistance low.

[0037] The welded joint shown in Figure 1 is created by means of the laser welding system indicated in Figure 2. This is shown only to the extent necessary for understanding the invention. Accordingly, the laser beam welding device has an electronic control unit 10, by means of which a laser beam generation unit 7 can be controlled to generate a laser beam 9. This laser beam is fed to a scanner unit 11. During the welding process, the laser beam 9 is guided over the surface of the upper joining partner 3 with the aid of movably or rotatably mounted scanner mirrors 12 of the scanner unit 11. According to the invention, the two scanner mirrors 12 are used solely to create the weld seam path 19. A superimposed laser beam oscillation, as shown in the comparative example in Figures 10 and 11, does not occur.

[0038] During the welding process, the laser beam 9 moves along a meandering target welding track 13, which is indicated in bold and dash-dotted lines in Figure 1. The target welding track 13 extends from a starting point S to an end point E. In Figure 1, the target welding track 13 has five adjacent longitudinal track sections 15, which are connected to one another in series via transverse track sections 17. The transverse track sections 17 are designed to be straight, by way of example, in Figure 1, so that a right-angled connection is provided between adjacent longitudinal track sections 15. Alternatively, the transverse track sections 17 can have any other geometry, for example be circular or arcuate.

[0039] During the welding process, the laser beam 9 moves along the target welding track 13, forming a corresponding weld seam path 19 having a predefined welding depth t and a predefined seam width b. The adjacent path sections of the weld seam path 19 build up the closed-surface connection zone 5, which is indicated by a dashed line in Figure 1. According to Figure 1, the track spacing a between adjacent longitudinal track sections 15 is set such that corresponding path sections of the weld seam path 19 formed during the welding process overlap with an overlap dimension Am. Alternatively to Figure 1, the overlap dimension Am can also be zero (Am = 0). In this case, the adjacent path sections of the weld seam path 19 merge into one another without overlap.In a further alternative, the adjacent path sections of the weld seam path 19 can also be arranged with a path spacing from one another (Am < 0).

[0040] In Figure 1, the laser beam welding device generates a laser beam 9 during the welding process, which forms a circular radiation area 23 (round spot) on the upper joining partner 3. In Figure 1, this is located at the end point E of the target welding track 13. In contrast, in the laser beam welding system, beam shaping can preferably take place via fiber / optic cable, in which the laser beam 9 is split into at least a first partial beam 25 and a second partial beam 27. As can be seen from Figures 3 to 5, the laser beam 9 has, for example, a radially inner core beam as the first partial beam 25 and a radially outer, concentrically arranged annular beam with the same center point as the second partial beam 27. The power distribution between the core beam 25 and the cladding beam 27 is dimensioned such that the welding depth t can be adjusted using the core beam 25, while the seam width b can be adjusted using the radially outer annular beam 27.In an exemplary weld seam formation shown in Figure 3, the power ratio is 40% in the core and 60% in the ring with a diameter ratio d2 / d1 of 4. The feed rate in process direction 8 is 500 mm / s with a laser power of 5.5 kW.

[0041] Examples of beam shaping in glass fibers are fibers with a concentric arrangement with or without a geometric distance (i.e., gap) between the core and ring. Variable quantities in the concentric arrangement are the diameter ratio d2 / d1, where d2 > d1 (d2: outer diameter of the ring, d1 : outer diameter of the core). Figure 4 shows the condition for the geometric distance ds - d1 = 0. This does not exist and occurs in fibers with a refractive index difference at the interface. In Figure 5, the geometric distance 2s is described as ds - d1 > 0 and d2 > ds. In the case d2 = ds, a simple round spot is present, as in Figure 4, provided d1 = d2.

[0042] Figure 6 shows a further embodiment in which only the target welding track 13 for generating the closed-surface connection zone 5 according to Figure 1 is indicated in isolation. A laser beam 9 (not shown) with a core beam 25 and a concentric annular beam 27, or alternatively a circular beam, moves along the target welding track 13. In a welding process with consistently constant laser power between the starting point S and the end point E, a heat field can form, leading to a corresponding heat accumulation. The preheating can lead to a disadvantageously large welding depth t when passing over a subsequent longitudinal track section 15.In order to counteract such a heat build-up-related increase in the welding depth t and a simultaneous increase in the process temperature in the weld joint, the following measure is taken in Figure 6: The laser beam power is reduced step by step during the laser beam pass in the sequence from the first (left) longitudinal track section 15 to the last (right) longitudinal track section 15, for example by a predefined amount of 5% per longitudinal track section 15. For example, in Figure 6, when the laser beam passes over the first longitudinal track section 15, the laser beam power is still 100%. When passing over the second to fifth longitudinal track sections 15, the laser beam power is reduced by 5% in each case, so that when the laser beam passes over the last (right) longitudinal track section 15, the laser beam power is 80%.

[0043] In Figure 6, for example, the laser power is abruptly reduced in the corners by 5% of the maximum power of the first longitudinal track section 15 for each additional longitudinal track section 15. In addition to a sudden reduction in laser power at one point, the power reduction can also be achieved via preferably linear power ramps between the power levels. The connecting section (cross-track sections 17) between the longitudinal track sections 15 is preferably suitable for this purpose. However, the length of the power ramp is not limited to this length.

[0044] The design of the weld seam geometry is also unlimited due to the flexibility of the scanner. Examples of the resulting weld seam geometry in the case of high-voltage battery cell contacts are shown in Figures 7 to 9 below. Figures 7 to 9 each show target weld tracks 13 in different geometric shapes. According to Figure 7, the target weld track 13 has a spiral shape, in which the target weld track 13 extends counterclockwise from a radially inner starting point S to the outside, following a circular movement sequence with a continuously increasing movement radius r. In Figure 7, the numbers in parentheses (1 - 9) symbolize switching points for the laser power. If the laser beam traverses the entire spiral at a constant power, the weld depth t increases radially outwards.If, however, the power is reduced in sections, a constant welding depth t results over the entire weld seam geometry.

[0045] Alternatively, in Figure 8, the target welding track 13 is designed in a mixture of spiral and meander shape, forming a radially inner longitudinal track section 15 and further longitudinal track sections 15 arranged parallel on both sides thereof. All longitudinal track sections 15 are connected to one another in a spiral shape via transverse track sections 17. The target welding track 13 shown in Figure 8 has a total of five longitudinal track sections 15. According to Figure 8, the target welding track 13 is guided counterclockwise from a radially inner starting point S (as in Figure 7) in a spiral outwards to the end point E.

[0046] The target welding track 13 shown in Figure 9 is composed of a total of four longitudinal track sections 15. Instead of a right-angled transverse line connection between the longitudinal track sections 15, the transverse track sections 17 in Figure 9 run in a circular or arcuate manner. This allows for more precise maintenance of the path speed of the laser beam 9 and is preferable to a right-angled transition. Accordingly, the previously mentioned power ramp can also be applied to the circular path. A superimposed power ramp is recommended for a controlled process start and end.

[0047] In Figure 9, the intended closed-surface connection zone 5 is not to be formed as a rectangular surface, but rather, for example, as a U-shaped surface. Accordingly, the target welding track 13 has an inner longitudinal track section 15 and longitudinal track sections 15 arranged parallel on both sides thereof. In contrast to Figure 8, in Figure 9 the longitudinal track sections 15 are angled twice by 90° to create the U-shaped connection zone 5. According to Figure 9, the target welding track 13 (as in Figures 7 or 8) is guided in a counterclockwise spiral from a radially inner starting point S outwards to the end point E.

[0048] In Figures 7 to 9, the starting points S of the target welding track 13 are located inside the welding track geometry, for example, and the target welding track 13 is directed counterclockwise outwards. Alternatively, the target welding track 13 can also extend clockwise from the starting point S. As a further alternative—depending on the desired heat propagation—the end point E can also be located inside the welding track geometry, while the starting point S is outside.

[0049] The invention is not limited to a specific surface geometry of the connection zone 5. As an alternative to Figure 9, the connection zone 5 can also have, for example, an annular surface geometry.

[0050] A core of the invention is that the laser beam 9 is guided along the desired welding track 13 during the welding process without oscillating pendulum motion P (Figure 11). The path traveled by the laser beam 9 on the joining partner surface therefore corresponds to the length of the desired welding track 13.

[0051] In contrast, Figures 10 and 11 show a comparative example not covered by the invention. The basic structure and operation of the laser beam welding device shown in Figure 10 correspond to the structure and operation of the laser beam welding device shown in Figure 2. Therefore, reference is made to the previous description.

[0052] The formation of the closed-surface connection zone 5 indicated in Figure 11 between the joining partners 1, 3 takes place by means of a path planning strategy not encompassed by the invention: Accordingly, the connection zone 5 (indicated by a dashed line in Figure 11) has a total of three target welding tracks 13, which are separate from one another and parallel to one another. Each of the three target welding tracks 13 is guided by the laser beam 9 in a separate laser beam pass from the respective starting point S to the respective end point E. In addition, a lateral spatial laser beam oscillation (which can also be any other form of oscillation) is superimposed on each laser beam pass, in which the laser beam 9 is deflected in an oscillating pendulum movement P (Figure 11) around the respective target welding track 13 with an amplitude A (Figure 11) transverse to the target welding line 13.The path covered by the laser beam 9 is therefore considerably greater than the actual length of the respective target welding track 13.

[0053] During laser beam oscillation, the processing optics must generally execute a frequency and amplitude A adapted to the feed rate via scanner mirror 12 in order to generate a closed connection surface or connection zone 5. Due to the inertia of the moving masses of scanner mirror 12 and the functioning of the drive motors, the amplitude A can no longer be achieved as the feed rate and thus the scanning frequency increase. Consequently, the achievable seam width b in the connection plane decreases with the amplitude A, and the requirements for electrical resistance and strength are no longer met.

[0054] LIST OF REFERENCE SYMBOLS

[0055] 1 , 3 joining partners

[0056] 5 closed-area connection zone

[0057] 7 Laser beam generation unit

[0058] 8 Process direction

[0059] 9 Laser beam

[0060] 10 Control unit

[0061] 11 Scanner unit

[0062] 12 scanner mirrors

[0063] 13 Target welding track

[0064] 15 Longitudinal track section

[0065] 17 Cross-track section

[0066] 19 Weld seam path

[0067] 23 Radiation circle area

[0068] 25 first partial beam, for example core beam

[0069] 27 second partial beam, for example ring beam

[0070] 29 Annular gap or distance a track pitch

[0071] At the overlap dimension b seam width

[0072] S starting point

[0073] E End point r Movement radius t Welding depth

[0074] P Pendulum movement

[0075] A Amplitude

Claims

PATENT CLAIMS:

1. Method for laser beam welding of at least two joining partners (1, 3) which are placed one on top of the other in an overlap joint, wherein the two joining partners (1, 3) are welded together by a line weld, forming a preferably closed-area connection zone (5), characterized in that, for the formation of the connection zone (5), the laser beam (9) is guided in the welding process in an arbitrary path planning strategy along a predetermined weld track (13), forming a weld seam path (19) whose adjacent path sections form the preferably closed-area connection zone (5), and in particular that the welding process is carried out with a path planning strategy in which the laser beam (9) is guided along a meandering and / or spiral predetermined weld track (13).

2. Method according to claim 1, characterized in that the connection zone (5) is formed with exactly one continuous weld seam track (19) which preferably extends without interruption along the entire desired weld track (13).

3. Method according to claim 1 or claim 2, characterized in that the laser beam (9) is guided along the desired weld track (13) in the welding process without oscillating pendulum movement (P), so that the path traveled by the laser beam (9) on one of the joining partner surfaces corresponds to the length of the desired weld track (13).

4. Method according to one of the preceding claims, characterized in that the track spacing (a) between adjacent track sections (15) of the target weld track (13) is set such that the corresponding track sections of the weld seam track (19) formed in the welding process overlap with an overlap dimension (Am), or alternatively, that the overlap dimension (Am) is zero. The method is characterized in that, to increase the weld width (b), beam shaping is performed on the weld surface, in which the laser beam (9) is split into at least a first partial beam (25) and a second partial beam (27). The method is (Am = 0), such that the adjacent weld track sections (19) merge into each other without overlap, or alternatively, the adjacent weld track sections (19) are arranged with a track spacing from each other (Am < 0). The method according to one of the preceding claims is characterized in that, to increase the weld width (b), beam shaping is performed on the weld track surface, in which the laser beam (9) is split into at least a first partial beam (25) and a second partial beam (27).A method according to claim 5, characterized in that a radially inner core beam (25) and a radially outer ring beam (27) concentric to it, with or without an intermediate geometric gap (29), are generated by means of beam shaping, and in particular that the power distribution between the core and ring beams (25, 27) is dimensioned such that the weld depth (t) can be adjusted by means of the core beam (25) and the weld width (b) can be adjusted by means of the ring beam (27). A method according to any of the preceding claims, characterized in that, during the welding process, the laser beam power is selectively reduced during the formation of the closed-surface bonding zone (5) as the process duration progresses, in order to counteract a heat-induced increase in the weld depth (t) and the process temperature, and in particular that the laser beam power is reduced by a certain amount during the formation of each adjacent path section of the weld seam path (19).Method according to one of the preceding claims, characterized in that, for the formation of the closed-surface bonding zone (5), the desired weld track (13) is meandering with mutually adjacent. a method according to one of the preceding claims, characterized in that, to form the closed-surface bonding zone (5), the desired weld track (13) has a spiral shape, in which the desired weld track (13) extends spirally from a radially inner starting point (S) to radially outward in a circular motion with a continuously increasing radius of movement (r), or the desired weld track (13) extends spirally from a radially outer starting point (S) to radially inward in a circular motion with a continuously decreasing radius of movement (r).Method according to one of the preceding claims, characterized in that, for the formation of the closed-surface bonding zone (5), the intended weld track (13) is formed in a mixture of spiral shape and meander shape, forming a radially inner longitudinal track section (15) and further longitudinal track sections (15) arranged parallel to it on both sides, all of which are connected to each other in a spiral fashion via transverse track sections (17).

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