Laser device and method for machining at least one workpiece by means of at least one laser beam

Abstract

The invention relates to a laser device (10) for machining at least one workpiece (12) by means of at least one laser beam (14), in particular laser welding at least one workpiece, having the features of claim 1 and to a method for machining at least one workpiece (12) by means of at least one laser beam (14), in particular laser welding at least one workpiece, having the features of the dependent claims.

Description

Technical Field

[0001] The present invention relates to a laser apparatus having the features of claim 1 for processing at least one workpiece by means of at least one laser beam, and a method having the features of the parallel independent claims for processing at least one workpiece by means of at least one laser beam. Background Technology

[0002] When laser welding at least two parts, deep penetration welding is typically performed using a CW (continuous wave) laser with constant power throughout the duration of the laser welding process. In particular, when performing deep penetration welding on metals with low viscosity, instabilities can occur in the weld zone, especially in the so-called keyhole. This can lead to spatter, porosity, and / or uneven weld penetration, for example. To reduce such instabilities, multiple laser beams can be stacked. However, this does not eliminate all instabilities, especially in metals with low viscosity.

[0003] Purpose of the invention Therefore, the object of the present invention is to provide a laser apparatus and method for processing at least one workpiece by means of at least one laser beam, particularly for laser welding at least one workpiece, wherein the aforementioned disadvantages are corrected. Summary of the Invention

[0004] The aforementioned objective is achieved by a laser device having the features of claim 1 for processing at least one workpiece by means of at least one laser beam. The processing may involve laser welding.

[0005] The laser device is configured to generate a laser beam and focus it onto a workpiece, thereby creating a laser spot on the workpiece. The laser spot has a core region, particularly a circular core region, and a ring-shaped region, particularly an annular region. The average laser power density in the core region is higher than that in the ring-shaped region.

[0006] The laser device is also designed to vary the average laser power density in the core region by a variation of at least 10% of the average laser power density in the core region and a variation frequency of at least 50 kHz (kilohertz). Alternatively or additionally, the laser device is configured to vary the average laser power density in the annular region by a variation frequency of at least 50 kHz and a variation of at least 10% of the average laser power density in the annular region. The average laser power density in the annular region may correspond to a maximum of 5% of the (total) laser power.

[0007] Here, the variation can be positive or negative. In other words, the average laser power density (in the core region and / or the annular region) can periodically increase (in the case of a positive variation) and / or periodically decrease (in the case of a negative variation).

[0008] This allows for (additional) stabilization of the keyhole, particularly in workpieces with lower viscosity, thereby reducing spatter and porosity and achieving the most consistent weld penetration possible. Keyhole instability can be avoided, or at least reduced, to prevent keyhole collapse, especially by rapidly increasing and decreasing the steam rate and steam pressure. In other words, keyhole stabilization can be achieved by selectively influencing the steam pressure (or steam rate), resulting in less spatter, less porosity, and a constant weld penetration.

[0009] According to one extension of the laser device, the laser device can be configured to maintain a constant average laser power density in the core region and / or the average laser power density in the annular region. Using multiple laser beams or multiple laser spots, the average laser power density of at least one core region and / or at least one annular region can be kept constant. Alternatively or additionally, the average laser power density in the core region and / or the average laser power density in the annular region can be varied at a certain frequency. Using multiple laser beams or multiple laser spots, the average laser power density of at least one core region and / or at least one annular region can be varied at a certain frequency.

[0010] According to one extension of the laser device, the laser device can be configured to change the average laser power density in the core region and the average laser power density in the annular region at different changing frequencies and / or different changing amplitudes, respectively. It is also conceivable that the average laser power density in the core region and the annular region can be changed at the same changing frequency and / or changing amplitude, respectively.

[0011] This allows for selective adjustment and optimization of the laser spot in specific areas, thereby enabling further stabilization of the keyhole.

[0012] According to one extension of the laser device, the laser device may include at least one CW laser for generating a laser beam, particularly for generating a core region and / or annular region. In this regard, the laser device may be configured such that the core region and / or annular region of the laser spot are modulated with a specific modulation frequency and modulation amplitude, respectively. Here, the modulation frequency may correspond to a variable frequency, and the modulation amplitude may correspond to a variable amplitude. The CW laser may have an average power output of at least 0.2 kW, preferably at least 4 kW.

[0013] This makes it possible to generate laser beams in a simple way, especially to generate core and / or ring-shaped regions.

[0014] It is also conceivable that the laser device may include at least one pulsed laser for generating a laser beam, particularly for generating a core region and / or a ring region. In this respect, the pulse repetition rate of the pulsed laser may correspond to a varying frequency. The pulsed laser may have an average power of at least 0.2 kW, preferably at least 4 kW, and / or a pulse energy of at least 10 mJ, preferably at least 50 mJ.

[0015] It can be envisioned that the core region of the laser spot is generated using a pulsed laser (especially a nanosecond pulsed laser), and the annular region of the laser spot is generated using a CW laser, or vice versa.

[0016] Pulsed lasers and / or CW lasers can each produce a maximum of 8 (millimeters) The product of beam parameters (in milliradians). Pulsed lasers and / or CW lasers can each be designed as (multimode) NIR (near-infrared) lasers. It is also conceivable that pulsed lasers and / or CW lasers can each be designed as lasers in the visible light range with wavelengths in the blue or green wavelength range (VIS lasers). Pulsed lasers and / or CW lasers can each be designed as solid-state lasers.

[0017] The laser device may include a control unit. The control unit may be configured to program and / or control (high-frequency) modulation or pulses with varying frequencies and amplitudes. The control unit may be configured to control, particularly separately and independently, the core region and the annular region of the laser spot generated by the beam, or to modulate or adjust the corresponding average laser power density.

[0018] The laser device may have a nozzle for supplying a protective gas (cutting gas or welding gas) to the workpiece. This nozzle may have a Laval internal geometry.

[0019] According to one extension of the laser device, the laser device may include at least two laser sources, particularly different laser sources, for generating a laser beam, particularly for generating a core region and / or a ring-shaped region. For example, at least one first laser may be used to generate the core region, and at least one second laser may be used to generate the ring-shaped region.

[0020] This allows for the selective generation and adjustment of laser spots in specific areas, thereby enabling further stabilization of the keyhole.

[0021] According to one extension of the laser device, the laser device for generating a laser beam and / or laser spot may include an optical fiber (2-in-1 fiber) having a core region and a ring region. The outer diameter of the core region of the fiber may be at most 30 µm (micrometers), particularly at most 20 µm, preferably at most 10 µm (e.g., in fundamental mode or single-mode). Alternatively, the outer diameter of the core region of the fiber may be larger, particularly at least 50 µm or 100 µm (e.g., in multimode). Alternatively or additionally, the outer diameter of the ring region may be 200 µm or 400 µm. The core region may be arranged within the ring region. The ratio between the outer diameter of the core region and the outer diameter of the ring region may be in the range of 1:2 to 1:9, particularly 1:3, 1:4, or 1:6.

[0022] The core region may have a cladding. The cladding may be disposed between the core region and the annular region. The maximum thickness of the cladding may be 10 µm. The refractive index of the cladding may be lower than that of the core region and / or the annular region. The optical fiber may be configured such that light within the core region is "enclosed" due to total internal reflection at the boundary between the core region and the cladding, particularly at the cladding of the core region. Therefore, light within the annular region that can undergo total internal reflection between the cladding of the core region and the additional cladding of the annular region can be "enclosed".

[0023] This allows for further stabilization of the weld area, especially the keyhole. This can lead to a further reduction in instability (within the keyhole) and results in less spatter, less porosity, and a constant weld penetration. Due to the coverage of the core and / or annular regions, light can be guided in the core and annular regions in a way that isolates them from each other.

[0024] According to one extension of the laser device, the laser device can be configured to generate at least two laser beams and focus said at least two laser beams onto a workpiece. In particular, at least four laser beams can be generated and focused onto the workpiece. Specifically, all laser beams can be designed to be identical. Therefore, all laser spots generated on the workpiece by each laser beam can be designed to be identical. To generate multiple laser beams, the laser device can include optical elements for beam splitting, particularly wedge switches.

[0025] This allows for further stabilization of the weld area, especially the keyhole. This can lead to a further reduction in instability (within the keyhole) and result in less spatter, less porosity, and a constant weld penetration.

[0026] According to one extension of the laser device, the laser device can be configured such that the rise time and / or fall time of the amplitude variation is at most 10 µs (microseconds).

[0027] In this invention, rise time refers to the time required for the average laser power density to increase to its maximum value (e.g., a positive change). Correspondingly, fall time refers to the time required for the average laser power density to decrease to its minimum value (e.g., a negative change).

[0028] According to one extension of the laser device, the laser device may include a scanner optical unit for moving a laser beam across a workpiece. The aspect ratio of the scanner optical unit can be in the range of 1:1 to 5:1, preferably 1.9:1 or 3.4:1.

[0029] This makes it possible to optimally focus the laser beam onto the workpiece and / or move the laser beam across the workpiece in a simple manner.

[0030] According to one extension of the laser device, the laser device may include an optical sensor for detecting the position of the laser beam and / or the workpiece. The optical sensor may be designed as a sensor based on a camera device or a camera device itself. It is also conceivable that the optical sensor may be designed as a sensor system based on an interferometer. The laser beam and the optical sensor can be calibrated against each other.

[0031] This allows for simple position control of the laser beam and / or workpiece. This enables the most precise possible machining of the workpiece, especially laser welding.

[0032] The aforementioned objectives are further achieved by a method having the features of the parallel independent claims for processing at least one workpiece using at least one laser beam. The processing may involve laser welding. The method includes the following steps: Provide the workpiece. The workpiece can be made of metal, especially aluminum and / or copper. The viscosity of the workpiece in the molten state (melting temperature) can be up to 5. (millipascal) (seconds). The maximum thickness of the workpiece can be 4 mm.

[0033] At least one laser beam is generated and focused onto a workpiece such that the laser beam produces a laser spot on the workpiece, wherein the laser spot has a core region, particularly a circular core region, and an annular region, particularly a ring-shaped annular region. Here, the average laser power density in the core region is higher than the average laser power density in the annular region.

[0034] The average laser power density, particularly in the pulsed or modulated core region and / or the annular region, is varied with a frequency of at least 50 kHz and an amplitude of at least 10% of the corresponding average laser power density. Here, the rise time and / or fall time of the amplitude variation can be less than 10 µs (microseconds). The amplitude variation can be positive or negative. In other words, the average laser power density can periodically increase (in the case of a positive amplitude variation) and / or periodically decrease (in the case of a negative amplitude variation).

[0035] This allows for (additional) stabilization of the keyhole, particularly in workpieces with lower viscosity, thereby reducing spatter and porosity and achieving the most consistent weld penetration possible. Keyhole instability can be avoided, or at least reduced, to prevent keyhole collapse, especially by rapidly increasing and decreasing the steam rate and steam pressure. In other words, keyhole stabilization can be achieved by selectively influencing the steam pressure (or steam rate), resulting in less spatter, less porosity, and a constant weld penetration.

[0036] The method may include the following steps: Supplying a protective gas (cutting gas or welding gas) to the workpiece, especially to the area in which the workpiece is being processed. This can be achieved using nozzles, especially nozzles with Laval internal geometry.

[0037] According to an extension of this method, the method may include the following steps: The laser beam and / or workpiece are moved to generate a feed of the laser beam focused onto the workpiece. In this regard, a scanner optics unit can be used to move the laser beam across the workpiece. The movement of the laser beam and / or workpiece can be a relative movement between the laser beam and the workpiece. In other words, either the laser beam or the workpiece can be moved, or both can be moved.

[0038] This makes it possible to perform continuous machining of workpieces, such as welds, in a simple way.

[0039] According to an extension of this method, the method may include the following steps: Determine the position of the laser beam and / or the workpiece. This can be achieved using an optical sensor.

[0040] If necessary, readjust the laser beam and / or the workpiece's processing position.

[0041] This enables simple position control and allows for corresponding adjustments to the laser beam and / or workpiece in the event of deviation.

[0042] According to an extension of this method, the method may include the following steps: The average laser power density in the pulse or modulation core region and the average laser power density in the annular region are changed by different frequency changes and / or different amplitude changes of the corresponding average laser power density.

[0043] This allows for targeted adjustments and optimizations of the energy input through the core or annular regions as needed.

[0044] According to an extension of the method, the method can be performed using a laser device according to the embodiments mentioned above.

[0045] Regarding the achievable advantages, refer to the relevant embodiments relating to laser devices. The method can be further improved using the measures described in conjunction with the laser device and / or the measures explained below. Attached Figure Description

[0046] Further features, details, and advantages of the invention will become apparent from the wording of the claims and from the following description of exemplary embodiments with reference to the accompanying drawings. In the drawings: Figure 1 A schematic representation of a laser device for processing at least one workpiece is shown; Figure 2 The workpiece and from according to Figure 1 A schematic top view of the laser beam spot of the laser device; Figure 3 It shows according to Figure 1 A schematic cross-section of the optical fiber in a laser device; Figure 4 It shows according to Figure 1 A schematic diagram illustrating the variation of average laser power density in the core and annular regions of the laser beam of a laser device. Detailed Implementation

[0047] In the following description and accompanying drawings, corresponding parts and elements have the same reference numerals. For clarity, not all reference numerals are reproduced in all drawings.

[0048] Figure 1 A laser device 10 is schematically shown for processing at least one workpiece 12 by means of at least one laser beam 14. In this invention, the processing type is laser welding.

[0049] The laser device 10 is configured to generate a laser beam 14 and focus the laser beam onto the workpiece 12, such that the laser beam 14 produces a laser spot 16 on the workpiece 12, wherein the laser spot 16 has a core region 18, particularly a circular core region, and an annular region 20, particularly an annular region (see...). Figure 2The average laser power density 22 in the core region 18 is higher than the average laser power density 24 in the annular region 20 (see...). Figure 4 ).

[0050] The laser device 10 is also configured to vary the average laser power density 22 in the core region 18 and / or the average laser power density 24 in the annular region 20 by a variation amplitude 26 of at least 10% of the respective average laser power densities 22 and 24 and a variation frequency of at least 50 kHz (see Figure 4 ).

[0051] The laser device 10 can be configured such that the rise time and / or fall time of the variation amplitude 26 is at most 10 µs.

[0052] The laser device 10 can be configured to change the average laser power density 22 in the core region 18 and the average laser power density 24 in the annular region 20 with different variation frequencies and / or different variation amplitudes 26, respectively.

[0053] The laser device 10 may include a CW laser for generating a laser beam 14, particularly a core region 18 and / or annular region 20. The laser device 10 may be configured such that the core region 18 and / or annular region 20 of the laser spot 16 are modulated with a specific modulation frequency and modulation amplitude. Here, the modulation frequency may correspond to a variable frequency, and the modulation amplitude may correspond to a variable amplitude 26. The CW laser may have an average power output of at least 1 kW, particularly at least 4 kW.

[0054] In this invention, the laser device 10 includes a scanner optical unit 34 for moving the laser beam 14 on the workpiece 12. The aspect ratio of the scanner optical unit 34 can be in the range of 1:1 to 5:1, preferably 1.9:1 or 3.4:1.

[0055] In this invention, the laser device 10 further includes an optical sensor 36 for position detection of the laser beam 14 and / or the workpiece 12.

[0056] The laser device 10 can be configured to generate at least two laser beams 14, and in particular at least four laser beams, and focus these laser beams onto the workpiece 12. Here, all laser beams 14 (or laser spots 16 generated by each laser beam 14) can be designed to be identical.

[0057] Figure 2 Workpiece 12 and according to Figure 1 A schematic top view of the laser beam 14 and the laser spot 16 of the laser device 10.

[0058] In this invention, the laser device 10 includes an optical fiber 28 having a core region 30 and an annular region 32 for generating a laser beam 14 and / or a laser spot 16.

[0059] Figure 3 It shows according to Figure 1 A schematic cross-section of the optical fiber 28 of the laser device 10.

[0060] In this invention, the core region 30 of the optical fiber 28 is arranged within the annular region 32 of the optical fiber 28. The outer diameter of the core region 30 of the optical fiber 28 can be 50 µm or 100 µm. The outer diameter of the annular region 32 of the optical fiber 28 can be 200 µm or 400 µm.

[0061] In this invention, the laser device 10 has at least two laser sources for generating a laser beam 14. The laser sources can be designed in different ways. In this invention, light from the first laser source is coupled into the core region 30 of the optical fiber 28. For this purpose, the core region 30 of the optical fiber 28 can be coupled to a first optical waveguide 38 (first optical fiber). Figure 3 The first optical waveguide 38, which is coupled to the core region 30, is indicated by a dashed circle.

[0062] In this invention, light from at least one second laser source is coupled into the annular region 32 of the optical fiber 28. For this purpose, the annular region 32 of the optical fiber 28 can be coupled with a plurality of second optical waveguides (second optical fibers), in this case, six second optical fibers. Figure 3 The second optical waveguide 40 coupled to the annular region 32 of the optical fiber 28 is indicated by a dashed circle. Light can be supplied to the six second optical waveguides 40 by means of a common second laser source. It is also conceivable that light can be supplied to the six second optical waveguides 40 separately by means of individual second laser sources (i.e., a total of six second laser sources).

[0063] Figure 4 It shows according to Figure 1 A schematic diagram illustrating the variation of the average laser power density 22, 24 in the core region 18 and the annular region 20 of the laser spot 16 of the laser beam 14 of the laser device 10.

[0064] The figure shown depicts the laser beam 14 and laser spot 16 of the laser device 10 in terms of W / cm². 2 The average laser power density 42, expressed in watts per square centimeter, varies with time 44, expressed in seconds. In other words, the x-axis represents time 44 in seconds, and the y-axis represents the average laser power density in W / cm². 2 The average laser power density is 42 units.

[0065] In this invention, the average laser power density 22 of the core region 18 of the laser spot 16 remains constant. In other words, the average laser power density 22 of the core region 18 of the laser spot 16 is not modulated in this invention. Here, the average laser power density 24 of the annular region 20 is modulated with a certain modulation frequency and modulation amplitude. In other words, the average laser power density 24 of the annular region 20 varies periodically by at least 10% (modulation amplitude). Therefore, the average laser power density 24 of the annular region 20 of the laser spot 16 changes the modulation amplitude or variation amplitude 26 at regular intervals of up to 20 µs (because the modulation frequency or variation frequency is at least 50 kHz). In the example shown, the average laser power density 24 of the annular region 20 decreases the variation amplitude 26 at intervals of up to 20 µs and then increases it back to the initial value. In this invention, the variation amplitude is greater than 10% and less than 100% of the average laser power density 24 of the annular region 20 of the laser spot 16.

[0066] The following is for reference Figures 1 to 4 A method for processing at least one workpiece 12, particularly for laser welding at least one workpiece, is described. The method includes the following steps: Workpiece 12 is provided. Workpiece 12 may be made of metal, particularly aluminum and / or copper. The viscosity of workpiece 12 in the molten state may be a maximum of 5. .

[0067] At least one laser beam 14 is generated and focused onto a workpiece 12 such that the laser beam 14 produces a laser spot 16 on the workpiece 12, wherein the laser spot 16 has a core region 18, particularly a circular core region, and an annular region 20, particularly an annular region. The average laser power density 22 in the core region 18 is higher than the average laser power density 24 in the annular region 20.

[0068] The average laser power density 22 in the pulsed or modulated core region 18 and / or the average laser power density 24 in the annular region 20 are varied by a frequency of at least 50 kHz and an amplitude of at least 10% of the corresponding average laser power densities 22 and 24, respectively. The rise time and / or fall time of the amplitude of variation 26 can be less than 10 µs.

[0069] The method may include the following steps: The laser beam 14 and / or the workpiece 12 are moved to generate a feed of the laser beam 14 or laser spot 16 focused onto the workpiece 12. The movement of the laser beam 14 or laser spot 16 can be achieved using the scanner optical unit 34.

[0070] The method may also include the following steps: The positions of the laser beam 14 and / or the workpiece 12 are determined. This can be achieved using the optical sensor 36.

[0071] The method may include the following steps: The average laser power density 22 in the pulse or modulation core region 18 and the average laser power density 24 in the annular region 20 are changed by different frequency changes and / or different amplitude changes 262 of the corresponding average laser power densities 22 and 24, respectively.

[0072] To perform this method, a laser device 10 according to the embodiments mentioned above can be used, especially... Figure 1 The laser device 10 shown.

Claims

1. A laser apparatus (10) for processing at least one workpiece (12) by means of at least one laser beam (14), particularly for laser welding at least one workpiece, wherein, The laser device (10) is configured to generate the laser beam (14) and focus the laser beam onto the workpiece (12) such that the laser beam (14) generates a laser spot (16) on the workpiece (12), wherein the laser spot (16) has a core region (18), particularly a circular core region and an annular region (20), particularly an annular annular region, wherein the average laser power density (22) in the core region (18) is higher than the average laser power density (24) in the annular region (20), wherein the laser device (10) is further configured to change the average laser power density (22) in the core region (18) and / or the average laser power density (24) in the annular region (20) with a variation frequency of at least 50 kHz and a variation amplitude (26) of at least 10% of the corresponding average laser power density (22, 24).

2. The laser device (10) according to claim 1, characterized in that, The laser device (10) is configured to change the average laser power density (22) in the core region (18) and the average laser power density (24) in the annular region (20) at different frequency and / or different amplitude (26), respectively.

3. The laser device (10) according to claim 1, characterized in that, The laser device (10) is configured to keep the average laser power density (22) in the core region (18) and / or the average laser power density (24) in the annular region (20) constant.

4. The laser device according to any one of the preceding claims, characterized in that, The laser device (10) includes at least one CW laser for generating the laser beam (14), particularly the core region (18) and / or the annular region (20), wherein the laser device (10) is configured such that the core region (18) and / or the annular region (20) of the laser spot (16) are modulated with a certain modulation frequency and modulation amplitude, wherein the modulation frequency corresponds to the variation frequency and the modulation amplitude corresponds to the variation amplitude (26), and in particular, wherein the CW laser has an average power of at least 0.2 kW, preferably at least 4 kW.

5. The laser device (10) according to any one of the preceding claims, characterized in that, The laser device (10) includes at least two laser sources, particularly different laser sources, for generating the laser beam (14), particularly generating the core region (18) and / or the annular region (20).

6. The laser device (10) according to any one of the preceding claims, characterized in that, The laser device (10) includes an optical fiber (28) for generating the laser beam (14) and / or the laser spot (16), the optical fiber having a core region (30) and an annular region (32).

7. The laser device (10) according to any one of the preceding claims, characterized in that, The laser device (10) is configured to generate at least two laser beams (14), in particular at least four laser beams, and to focus the laser beams onto the workpiece (12), wherein all laser beams (14) and / or laser spots (16) are designed to be identical.

8. The laser device (10) according to any one of the preceding claims, characterized in that, The laser device (10) is configured such that the rise time and / or fall time of the variation amplitude (26) is at most 10 µs.

9. The laser device (10) according to any one of the preceding claims, characterized in that, The laser device (10) includes a scanner optical unit (34) for moving the laser beam (14) on the workpiece (12), and in particular, the aspect ratio of the scanner optical unit (34) is in the range of 1:1 to 5:1, preferably 1.9:1 or 3.4:

1.

10. The laser device (10) according to any one of the preceding claims, characterized in that, The laser device (10) includes an optical sensor (36) for position detection of the laser beam (14) and / or the workpiece (12).

11. A method for processing at least one workpiece (12) by means of at least one laser beam (14), particularly for laser welding at least one workpiece, the method comprising the steps of: - Provide the workpiece (12), in particular wherein the workpiece (12) is made of metal, preferably aluminum and / or copper, and the viscosity of the workpiece in the molten state is at most 5. ; - Generate at least one laser beam (14) and focus the at least one laser beam onto the workpiece (12) such that the laser beam (14) generates a laser spot (16) on the workpiece (12), wherein the laser spot (16) has a core region (18), particularly a circular core region and an annular region (20), particularly an annular annular region, wherein the average laser power density (22) in the core region (18) is higher than the average laser power density (24) in the annular region (20). - The average laser power density (22) in the core region (18) and / or the average laser power density (24) in the annular region (20) are changed, in particular pulsed or modulated, by a change frequency of at least 50 kHz and a change amplitude (26) of at least 10% of the corresponding average laser power density (22, 24), in particular, wherein the rise time and / or fall time of the change amplitude (26) is at most 10 µs.

12. The method according to claim 11, characterized in that, The method includes the following steps: - Move, especially by means of the scanner optical unit (34), the laser beam (14) and / or the workpiece (12) to produce a feed of the laser beam (14) focused on the workpiece (12).

13. The method according to claim 11 or 12, characterized in that, The method includes the following steps: - To determine, in particular by means of an optical sensor (36), the position of the laser beam (14) and / or the workpiece (12), and - If necessary, readjust the processing position of the laser beam (14) and / or the workpiece (12).

14. The method according to any one of claims 11 to 13, characterized in that, The method includes the following steps: - The average laser power density (22) in the core region (18) and the average laser power density (24) in the annular region (20) are changed, in particular pulsed or modulated, by different frequency and / or different amplitude (26) of the corresponding average laser power density (22, 24).

15. The method according to any one of claims 11 to 14, characterized in that, The method is performed using the laser device (10) according to any one of claims 1 to 10.

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