DE102022121053A1 - Process for welding zinc-coated sheet metal - Google Patents

Abstract

The invention relates to a method for welding a first welding surface (12a) of a first metal sheet (10a) to a second welding surface (12b) of a second metal sheet (10b), the first welding surface (12a) having a zinc coating (16a). The welding surfaces (12a, 12b) face each other and are spaced apart from each other. A laser beam (18) is guided in a wobbling motion along the first welding surface (12a) and the second welding surface (12b) to weld the welding surfaces (12a, 12b). The wobbling motion has a directional component in a feed direction (FD) of the laser beam (18) and a directional component perpendicular to the feed direction (FD). The path (24) of the laser beam (18) on the weld faces (12a, 12b) has a pattern that repeats with a period (30) in the feed direction (FD). The width of the path (24) perpendicular to the feed direction (FD) is greater than the period (30) of the path (24) in the feed direction (FD). The laser beam (18) is divided radially from its beam axis (BA) into an inner profile (20) and an outer profile (22), the intensities of which differ, with the outer profile (22) surrounding the inner profile (20). .

Description

Background of the Invention

The invention relates to a method for welding zinc-coated metal sheets.

EP 0 327 320 A1 relates to a method for welding two metal sheets, in which an outgassing path for the gases produced during welding is formed in the weld seam.

U.S. 10,118,249 B2 discloses a method of welding two metal sheets in which a laser beam is swept along a helical path on the metal sheets to achieve a heat distribution suitable for a smooth weld.

US 2020/0047285 A1 relates to a method for welding two overlapping metal sheets, in which, in order to improve an outgassing process in the metal sheets, in particular of zinc, a preliminary molten bath is produced by a laser beam with a relatively low power density. A laser beam with a higher power density is then used to create a main weld pool with a larger extent.

US2020/0171603A1 describes a method for welding two metal sheets in a stack of metal sheets, in which a laser beam is scanned along a first path in a first direction and then along a second path in a second direction opposite to the first direction, the first path and the second way overlap. The repeated irradiation of the welding path reduces the defects generated during welding.

US2020/0254562A1 discloses a method of welding sheet metal using a central laser beam and an annular concentric laser beam, the annular concentric laser beam radially surrounding the central laser beam. The ring-shaped, concentric laser beam and the central laser beam have different power densities, which are varied in order to achieve a smoothing of the weld area.

aim of the invention

It is an object of the invention to provide a method for further reducing defects in the welding of two metal sheets by a laser beam, in particular defects caused by zinc coating on the metal sheets.

Description of the invention

This aim is achieved by a method according to claim 1. Dependent claims relate to advantageous embodiments.

1. a) Bereitstellen eines ersten Blechs mit einer ersten Schweißfläche und eines zweiten Blechs mit einer zweiten Schweißfläche, wobei das erste Blech eine Zinkbeschichtung auf der ersten Schweißfläche aufweist;
2. b) Anordnen der ersten Schweißfläche und der zweiten Schweißfläche, derart, dass die erste Schweißfläche und die zweite Schweißfläche einander zugewandt sind, mit einem Spalt zwischen der ersten Schweißfläche und der zweiten Schweißfläche;
3. c) Bestrahlen der ersten Schweißfläche und der zweiten Schweißfläche mit einem Laserstrahl, wobei der Laserstrahl einen inneren Kern und einen äußeren Ring aufweist, deren Intensitätsprofile unterschiedlich sind, wobei der Laserstrahl mit einer Taumelbewegung in einer Vorschubrichtung entlang einer imaginären Vorschublinie und senkrecht zur Vorschublinie bewegt wird, wobei der Weg des Laserstrahls entlang der Vorschublinie eine periodische Form aufweist, wobei die Breite des Weges senkrecht zur Vorschublinie größer ist als die Länge der Periode des Weges entlang der Vorschublinie.

The method according to the invention comprises the following steps:

1. a) providing a first sheet having a first weld surface and a second sheet having a second weld surface, the first sheet having a zinc coating on the first weld surface;
2. b) arranging the first weld face and the second weld face such that the first weld face and the second weld face face each other with a gap between the first weld face and the second weld face;
3. c) irradiating the first weld surface and the second weld surface with a laser beam, the laser beam having an inner core and an outer ring whose intensity profiles are different, the laser beam being moved with a wobbling motion in a feed direction along an imaginary feed line and perpendicular to the feed line , wherein the path of the laser beam along the feed line has a periodic shape, the width of the path perpendicular to the feed line being greater than the length of the period of the path along the feed line.

The invention is particularly advantageous for vehicle body panels that consist to a certain extent of cold-formed standard steels or steels of the 3rd generation. The surface has an anti-corrosion layer with a certain zinc content. Before laser welding, this coating must be removed in the welding area to avoid welding cracks.

Typically, some risk of cracking arises as the zinc (from the coating) fuses with the molten steel. This leads to intermetallic phases. These phases lead to embrittlement of the base material and thus to cracks in the seams. These crack events can occur immediately or with a time delay.

Due to the low vaporization temperature of zinc (~907 °C) compared to the melting point of steel (~1500 °C), strong vapor pressure forces are created that force the molten steel out of the melting zone of the process, weakening the weld or even rendering it unusable.

Expressed more generally, the invention relates to the welding of at least partially coated sheets containing metal, preferably steel, in a workpiece arrangement with a gap, in particular a degassing gap, between the sheets. More particularly, the invention relates to the welding of sheet metal having a coating on one or both sides of the sheet metal, the coating comprising zinc. Degassing of the zinc from the zinc coating is improved by the gap, which also reduces defects caused by gas pockets in the weld seam of the sheets.

The fact that the width of the path of the laser beam used for welding perpendicular to the feed line is greater than the length of the path period along the feed line allows for uniform heat distribution on the welded surfaces. This promotes even evaporation of the zinc in the zinc coating before the material in the sheets is melted to form the weld. In this way, only a small amount of gaseous zinc penetrates into the molten material of the sheets, which prevents or at least reduces defects in the weld of the sheets caused by gas inclusions from the zinc vapor generated during the welding process.

Beam shaping, in which a laser beam has an inner core and an outer ring with different intensity profiles, reduces the risk of spatter during welding and thereby also improves the quality of the sheet metal weld. The intensity profiles of the inner core and the outer ring of the laser beam can be modified during the welding process in order to adapt the heat distribution generated by the laser beam to local changes in the sheet metal along the path of the laser beam, which in particular defines the weld path during the welding process.

It is advantageous that the welding strategy can be designed by wobbling in such a way that the welding process has a certain gap-bridging ability. This is possible by expanding the melt zone by oscillating the laser beam in harmonic patterns in the transverse and longitudinal directions from a midpoint of the laser beam axis, in particular a nominal laser beam axis.

The tumble welding strategy is designed such that the resulting welding speed allows the elements of the anti-corrosion layer to be heated to the point of vaporization of these elements while the volume of steel under the hot coating section is still solid. This allows vaporized elements to escape, preferably along the gap.

In some embodiments, the invention relates to the welding of at least two metal sheets in an overlap situation, where the weld seam can be performed as a partial penetration weld or as a root weld, a so-called full penetration weld.

In some embodiments of the invention, beam shaping welding technology may be used in which the laser beam does not conform to the typically used Gaussian or cylindrical intensity profiles.

In some embodiments, the invention relates to the use of a 2-in-1 fiber technology, where the optical fiber is a beam guiding fiber or an optical fiber as part of a fiber laser source. The fiber diameters are core/ring = 50 µm/200 µm or 100 µm/400 µm or 200 µm/700 µm.

In particular, the following applies to the beam parameter product BPP of the laser beam in the core fiber: BPP>1 mm*mrad and BPP<12 mm*mrad, preferably BPP>1.8 mm*mrad and BPP<8 mm*mrad, in particular BPP≧2 mm*mrad and BPP ≤ 4mm*mrad.

The laser power is preferably in the range from 1 kW to 24 kW and in particular in the range from 4 kW to 8 kW, preferably 6 kW to 8 kW.

In some embodiments of the invention, the beam shaping can be done with a DOE (diffractive optical element). With the help of scanning optics, the laser beam can be moved along a path to be welded.

In particular, the workpiece can be positioned in a controlled manner in order to achieve a distance between the first and the second sheet. The clearance can be established by a clamp that can be opened in a controlled manner by 0.1mm to 0.3mm after the clamp has pressed the parts together to full contact. The spacing can be accomplished by creating bumps (“laser pits”) that can be created by impinging laser pulses in the area where welding takes place. Another way to achieve spacing is to use shims.

In the wobble used, the preferred pattern is an eight (8) shaped pattern.
In particular, the main axis of the pattern is in front preferably in the feed direction of the laser beam, aligned transversely to the welding direction.

Preferably, the eight (8)-shaped pattern has a dimension of 0.5 mm to 3 mm in total in the major axis direction, preferably in a direction perpendicular to the advancing direction of the laser beam in welding. This expansion can be in the range from 0.8 mm to 2 mm, in particular in the range from 1 mm to 1.5 mm.

The frequency of the vibration to create the wobble pattern can range from 100 Hz to 1000 Hz. The frequency of the vibration is preferably higher than 300 Hz, in particular higher than 333 Hz.

The wobble curve of the laser beam can be circular, with the transverse movement of the process zone of the welding operation creating a spiral pattern along the path to be welded. The spiral pattern can have a dimension of 0.5 mm up to a total of 3 mm in the direction of the main axis, preferably in a direction perpendicular to the direction of advance of the laser beam during welding. This enlargement or expansion is preferably in the range from 0.8 mm to 2 mm, particularly advantageously in the range from 1 mm to 1.5 mm.

The wobble pattern can be oval, with a spiral pattern created by the transverse movement of the process zone along the path to be welded. The main axis of the oval figure is preferably oriented transversely to the direction of welding. The oval figure can have a dimension of 0.5 mm up to a total of 3 mm in the direction of the main axis. This expansion is preferably in the range from 0.8 mm to 2 mm, particularly preferably in the range from 1 mm to 1.5 mm. The length of the short axis of the oval figure is preferably in the range of 5% to 80% of the length of the major axis of the oval figure, more preferably 10% of the length of the major axis, particularly in a direction perpendicular to a feed direction of the laser beam during welding.

In particular, the tumbling speed for forming the tumbling pattern is 800 mm/s to 2000 mm/s, preferably 1500 mm/s.

Typical welding speed is 2 m/min up to 8 m/min and preferably 4 m/min.

In particular, the start ramp of the laser power lasts 225 ms, covering a distance of 15 mm. The laser power preferably increases from 0 W to 6000 W. The welding speed and the tumbling speed can be kept constant.

The end ramp of the laser power preferably lasts 150 ms, covering a distance of 10 mm. The laser power can drop from 6000 W to 1200 W. In particular, the welding and tumbling speeds can be kept constant.

The scanning optics for welding can preferably be operated with an optical magnification in the range from 1:1 to 4:1, preferably in the range from 1.5:1 to 3.26:1, particularly preferably 1.7:1.

In particular, a 2in1 fiber with a core/ring diameter ratio of 50 μm/200 μm or 100 μm/400 μm or 200 μm/700 μm is used, with the diameters being able to vary by +/- 50 μm. The fiber can also consist of more than two sections, in particular the inner core (section) and at least two rings (sections) arranged concentrically around the inner core.

IR solid-state lasers with a wavelength η in the range of 800 nm<η<2000 nm are preferably used, preferably in the range of 1000 nm<η<1100 nm, particularly preferably 1030 nm, 1063 nm and 1070 nm.

In particular, a laser beam with a power in the range from 1 kW to 24 kW, preferably in the range from 4 kW to 8 kW, is used.

In particular, a beam shaping device is used to generate a centrally aligned and/or positioned beam with high brilliance and an annularly aligned and/or positioned ring beam with lower brilliance than the central beam.

The central laser beam and the annular laser beam can have different wavelengths. The central laser beam and the annular laser beam can originate from the same laser source. Alternatively, they can also come from separate laser sources.

According to an advantageous embodiment of the invention, the second welding surface is coated with zinc. As a result, the second metal sheet in particular is protected against corrosion on the second welding surface.

According to a further embodiment of the invention, the wobbling movement has the same amplitude on both sides of the feed line, the amplitude in particular being greater than that Period of the path of the laser beam along the imaginary feed line. The heat generated by the laser beam is distributed over a comparatively large area. This helps the zinc vaporize evenly before the metal of the sheets melts.

According to some embodiments of the invention, the first panel and the second panel are arranged as a lap joint. In particular, the first weld surface and the second weld surface are located on the longer side surfaces of the first panel and the second panel. The degassing gap is located between the longer side surfaces of the first sheet and the second sheet.

In some embodiments of the invention, the first panel and the second panel are arranged as a lap joint.

In further embodiments of the invention, the panels are joined by a fillet weld or an I-weld.

Preferably, the path of the laser beam is in the form of a sine function, a zigzag function, a meander, consecutive eights or infinities. A weld path having any of the above shapes promotes the even distribution of heat generated by the laser beam in the weld surfaces during welding.

It is advantageous that the power of the laser beam is varied along the path of the laser beam. By modulating the laser beam power along the path of the laser beam, the energy load on the welding zone generated by the laser beam can be varied in order to control the heat input in the direction of advance of the laser beam, with the anti-corrosion layer being removed by the laser beam without melting the base material, in particular the steel sheet or to soften.

In a specialized version of the above embodiment of the invention, the power of the laser beam during the welding process is increased or decreased in a predetermined portion at the end of the laser beam in the direction of advance of the laser beam from the average power of the laser beam on the path of the laser beam. In particular, the local laser power can be adjusted in such a way that it is higher or lower than the average laser power on the welding path when the laser approaches sections of the metal sheets that have not yet been melted in the feed direction. This helps to degas the zinc before the sheet material is melted at the end of the weld paths. The laser beam can be used in continuous wave (CW), pulsed or modulated mode.

In some embodiments of the invention, the laser power may be constant along the path of the laser beam.

In a further preferred embodiment of the invention, the laser beam is guided several times along the same segment of the feed line, the amplitude of the wobbling movement in the direction perpendicular to the imaginary feed line being adjusted differently for each pass through the segment. The heat input of the laser beam into the sheets is gradually increased along the segment, especially near the weld path. In this way, the degassing of the zinc can be better controlled. Remaining local brittle intermetallic phases are melted again and the brittle elements are better distributed in the molten iron-based material of the sheets.

In some embodiments of the invention, the first sheet and/or the second sheet comprises a steel containing ferrite, martensite, bainite and/or austenite, the steel being in particular a multi-phase steel. Preferably, the first sheet and/or the second sheet contain a 3rd generation steel. Third-generation steel is usually a multi-phase steel that consists at least partially of the aforementioned components. In particular, the 3rd generation steel contains residual austenite in a bainite and/or martensite matrix. Generation 3 steels are advanced high-strength steels with high tensile strength and good ductility.

Advantageously, the method according to the invention allows coated steels and steels of the 3rd generation to be welded without removing the anti-corrosion layer. In addition, the method according to the invention facilitates the welding of coated steels and steels of the 3rd generation, avoiding embrittlement of the weld zone due to incorporated coating elements in an unreasonably high amount.

(Alloys made from 3rd generation steel grades are known, for example, from manufacturers Arcelor Mittal and USS Steel. They include Fortiform®980, Fortiform®1050, Fortiform®1180, 980 XG3, HDGI 980, XG3 AHSS.)

Preferably, the imaginary feed line is sinusoidal, circular, spiral, zigzag or C-shaped. In particular, a way of the laser Jet along a helical imaginary feed line promotes comparatively even heat distribution, thereby improving even degassing and welding. A path of the laser beam along a C-shaped imaginary feed line with open ends also improves the degassing process when evaporating the zinc coating.

In an advantageous embodiment of the invention, the width of the gap between the first welding surface and the second welding surface is 0.05 mm to 0.5 mm, in particular 0.1 mm to 0.2 mm. A gap width in this range is particularly suitable for smooth degassing and welding caused by wobbling of the laser beam according to the invention.

The wobble strategy allows for the welding of sheets with a gap of the above width. In principle, tumbling is also suitable for welding in gap-free arrangements. The gap can have a circular shape. The escape of steam radially in such a gap is advantageous because otherwise the steam can escape directly through a tap hole formed when the sheet material is melted, thereby forcing the molten steel out of the process zone and possibly rendering the weld unusable.

In preferred embodiments of the invention, the proportion of the power of the inner core of the laser beam is 20% to 80% of the total power of the inner core and the outer ring of the laser beam, in particular 59% to 61% of the total power. A power distribution in this range results in a hot zone in the sheets that promotes degassing of the zinc coating before the sheet material is melted. In some embodiments, the laser beam may include more than one outer ring section, with each ring section being concentric with each other and the inner core of the laser beam. In this embodiment, the proportion of the power of the inner core of the laser beam can be 20% to 80% of the total power of the laser beam (i.e. comprising the inner core and all outer rings), in particular 59% to 61% of the total power.

According to a further embodiment of the invention, the speed of the laser beam along the feed line during the tumbling movement is 2 m/min to 8 m/min, in particular 3 m/min to 5 m/min. Here the speed of the laser beam is limited along the feed line to keep the amount of vapor per unit time low, the vapor being generated by degassing the zinc in the zinc coating. In this way, the escape of steam occurs almost without recoil and molten steel is not thrown out of the process zone where welding takes place.

Further advantages of the invention can be seen from the description and the drawings. Likewise, the features mentioned above and those detailed below can each be used individually or 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 shows schematically a method for welding two metal sheets;
• 2 shows schematically a cross section through a laser beam used in the welding process;
• 3 Fig. 12 schematically shows an enlarged view of the path of the laser beam.

1 1 schematically shows a method for welding a first metal sheet 10a with a first welding surface 12a and a second metal sheet 10b with a second welding surface 12b, the metal sheets 10a, 10b being shown in cross section. The metal sheets 10a, 10b are cuboid and are arranged as an overlap connection. The first welding surface 12a and the second welding surface 12b are located on two of the larger side surfaces of the plates 10a, 10b and face each other. A gap 14 spatially separates the first welding surface 12a and the second welding surface 12b from one another. The first weld surface 12a and the second weld surface 12b are covered with zinc coatings 16a, 16b, with further zinc coatings 16c, 16d being applied to side surfaces of the sheets opposite the first weld surface 12a and the second weld surface 12b. The zinc coatings 16a-16d serve to protect the metal sheets 10a, 10b from corrosion. The gap 14 between the weld faces 14a, 14b allows for outgassing of vaporized zinc from the zinc coatings 16a, 16b during the welding of the first sheet 10a and the second sheet 10b.

The first weld face 12a and the second weld face 12b are irradiated with a laser beam 18 to create a fusion zone 36 for the purpose of welding the sheets 10a, 10b. The laser beam 18 is moved along a wobble curve in a feed direction FD, the wobble curve defining a path 24 of the laser beam 18 . Perpendicular to the feed direction FD, the path 24 of the laser beam 18 has an imaginary path on both sides ren feed line 28, which runs in the feed direction FD, the same amplitude 26. The path 24 of the laser beam 18 has a periodic meandering shape. The amplitude 26 of the path 24 of the laser beam 18 is greater than the length of the period 30 (see 3 ) of the path 24 of the laser beam 18 along the imaginary feed line 28.

2 shows schematically a cross section through the laser beam 18 with an inner core 20 and an outer ring 22 of different intensities in the radial direction from a beam axis BA (here into and out of the plane of the paper) of the laser beam 18. The inner core 20 and the outer ring 22 help prevent spatter during the welding process. The ratio of the intensities of the inner core 20 and the outer ring 22 of the laser beam 18 along path 24 (see FIG 1 ) of the laser beam 18 can be changed in order to adapt the welding process to the local properties of the sheets 10a, 10b.

3 12 schematically shows an enlarged view of the path 24 of the laser beam 18 (cf. 1 ). In a predetermined portion 32 at an end of the path 24 of the laser beam 18 in the feed direction FD of the laser beam 18, the power of the laser beam 18 during the welding process is increased or decreased compared to the average power of the laser beam 18 on the path 24 of the laser beam 18. The imaginary feed line 28 is divided into several segments 34a, 34b. The laser beam 18 can be scanned several times along the same segments 34a, 34b of the feed line 28, with the amplitude 26 of the path 24 of the laser beam 18 in the direction perpendicular to the imaginary feed line 28 being adjusted differently with each pass through the segments 34a, 34b.

With all figures of the drawings in mind, the invention relates to a method of welding a first weld surface 12a of a first sheet 10a to a second weld surface 12b of a second sheet 10b, the first weld surface 12a having a zinc coating 16a. The welding surfaces 12a, 12b face each other and are spaced apart from each other. A laser beam 18 is guided in a wobbling motion along the first welding surface 12a and the second welding surface 12b to weld the welding surfaces 12a, 12b. In particular, the laser beam is simultaneously guided along the first welding surface 12a and the second welding surface 12b. The wobbling movement has a directional component in a feed direction FD of the laser beam 18 and a directional component perpendicular to the feed direction FD. The path 24 of the laser beam 18 on the weld faces 12a, 12b has a pattern that repeats with a period 30 in the feed direction FD. The width of the path 24 perpendicular to the feed direction FD is greater than the period 30 of the path 24 in the feed direction FD. The laser beam 18 is divided in the radial direction from its beam axis BA into an inner profile 20 and an outer profile 22 whose intensities are different, with the outer profile 22 surrounding the inner profile 20 .

Reference List

10a - b

sheets

12a - b

welding surfaces

14

gap

16a - d

zinc coatings

18

laser beam

20

inner core

22

outer ring

24

path of the laser beam

26

amplitude

28

feed line

30

period

32

Predetermined section

34a-b

segments

36

melting zone

FD

feed direction

B.A

beam axis

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

• EP 0327320 A1 [0002]
• US10118249B2 [0003]
• US 2020/0047285 A1 [0004]
• US 2020/0171603 A1 [0005]
• US 2020/0254562 A1 [0006]

Claims (14)

Method for welding two metal sheets (10a, 10b), the method comprising the following steps: a) providing a first metal sheet (10a) with a first welding surface (12a) and a second metal sheet (10b) with a second welding surface (12b), the first metal sheet (10a) having a zinc coating (16a) on the first welding surface (12a) having; b) arranging the first weld surface (12a) and the second weld surface (12b) such that the first weld surface (12a) and the second weld surface (12b) face each other with a gap (14) between the first weld surface (12a) and the second welding surface (12b); c) irradiating the first welding surface (12a) and the second welding surface (12b) with a laser beam (18), the laser beam (18) having an inner core (20) and an outer ring (22) whose intensity profiles are different, wherein the laser beam (18) is moved with a wobbling motion in a feed direction (FD) along an imaginary feed line (28) and perpendicular to the feed line (28), the path (24) of the laser beam (18) along the feed line (28) having a periodic shape, the width of the path (24) perpendicular to the feed line (28) being greater than the length of the period (30) of the path (24 ) along the feed line (28). procedure after claim 1 , wherein the second weld surface (12b) is coated with zinc. Method according to one of the preceding claims, in which the wobbling movement has the same amplitude (26) on both sides of the feed line (28), the amplitude (26) being in particular greater than the period (30) of the path (24) of the laser beam (18 ). Method according to one of the preceding claims, in which the first panel (10a) and the second panel (10b) are arranged as a lap joint. Method according to one of the preceding claims, in which the metal sheets (10a, 10b) are joined by a fillet weld or an I-weld. A method according to any one of the preceding claims, wherein the path (24) of the laser beam (18) is in the form of a sine function, a meander, a zigzag function, consecutive eights or infinities. A method according to any one of the preceding claims, wherein the power of the laser beam (18) is varied along the path (24) of the laser beam (18). procedure after claim 7 , wherein the power of the laser beam (18) during the welding operation in a predetermined section (32) at an end of the path (24) of the laser beam (18) in the direction of advance (FD) of the laser beam (18) compared to the average power of the laser beam (18) is increased or decreased on the path (24) of the laser beam (18). A method according to any one of the preceding claims, wherein the laser beam (18) is swept along the same segment (34a, 34b) of the scan line (28) multiple times, the amplitude (26) of the wobbling motion in the direction perpendicular to the imaginary scan line (28) on each pass of the segment (34a, 34b) is set differently. Method according to one of the preceding claims, wherein the first sheet (10a) and/or the second sheet (10b) comprises a steel containing ferrite, martensite, bainite and/or austenite, the steel being in particular a multiphase steel. A method according to any one of the preceding claims, wherein the imaginary feed line (28) is sinusoidal, circular, spiral, zigzag or C-shaped. Method according to one of the preceding claims, wherein the width of the gap (14) between the first welding surface (12a) and the second welding surface (12b) is 0.05 mm to 0.3 mm, in particular 0.1 mm to 0.2 mm. amounts to. Method according to one of the preceding claims, wherein the proportion of the power of the inner core (20) of the laser beam (18) is 20% to 80% of the total power of the inner core (20) and the outer ring (22) of the laser beam (18), in particular 59% to 61% of the total output. Method according to one of the preceding claims, in which the speed of the laser beam (18) along the feed line (28) during the tumbling movement is 2 m/min to 8 m/min, in particular 3 m/min to 5 m/min.

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