Method and apparatus for laser processing, in particular for laser welding, of a workpiece

Abstract

The invention relates to a method for laser processing, in particular laser welding, of a workpiece (2), comprising: providing a laser beam (4) by means of an optical fiber (3), collimating the laser beam (4) by means of a collimation optic (5), focusing the collimated laser beam (4) onto the workpiece (2) by means of a focusing optic (6), and forming the laser beam (4) by means of a forming optic into the form of an axicon (7), wherein the laser beam (4) is provided by means of an optical fiber in the form of a multi-clad fiber, in particular in the form of a double-clad fiber (3).The invention also relates to a device for laser processing, preferably for laser welding, of a workpiece (2), in particular for carrying out the method described above, comprising: an optical fiber (3) for providing a laser beam (4), a collimation optic (5) for collimating the laser beam (4), a focusing optic (6) for focusing the laser beam (4) onto the workpiece (2), and a forming optic (7) in the form of an axicon for forming the laser beam (4). In the device (1), the optical fiber is designed as a multi-clad fiber, in particular as a double-clad fiber (3).

Description

[0001] The present invention relates to a method for laser processing, in particular laser welding, of a workpiece, comprising: providing a laser beam by means of an optical fiber, collimating the provided laser beam by means of collimation optics, focusing the collimated laser beam onto the workpiece by means of focusing optics, and forming the laser beam into the shape of an axicon by means of forming optics. The invention also relates to a device for laser processing, in particular laser welding, of a workpiece, in particular for carrying out a method as described above, comprising: an optical fiber for providing a laser beam, collimation optics for collimating the laser beam, focusing optics for focusing the laser beam onto the workpiece, and forming optics for forming the laser beam into the shape of an axicon.

[0002] Such a method and such a device are known from DE 10 2019 217 754 A1.

[0003] German patent application DE 10 2019 217 754 A1 refers to DE 10 2010 003 750 A1, which describes coupling a laser beam into a double-clad fiber to obtain a laser beam at the output of the double-clad fiber with a filled circular profile corresponding to an inner fiber core and a ring profile corresponding to an outer fiber core. DE 10 2019 217 754 A1 asserts that providing a multi-clad fiber, such as a double-clad fiber, and coupling the laser beam into a multi-clad fiber are comparatively complex and expensive. Therefore, DE 10 2019 217 754 A1 proposes to use a comparatively inexpensive axicon, which is used with a simple optical fiber, to generate a focused laser beam with the core focus component and at least a ring focus component.

[0004] CN115453767A describes a laser system comprising a diffractive optical element, a focusing lens, and a working plane. An incident light beam is modulated by the diffractive optical element and focused by the focusing lens onto the working plane, where a dot-ring type light spot is formed. The diffractive optical element may have axicon phase-etched regions. The laser system may include a collimating lens, and the diffractive optical element may be positioned in the beam path upstream of the collimating lens.

[0005] DE 10 2021 206 488 A1 describes a method for welding at least two aluminum-containing components, in which an initial laser beam is split into several partial beams directed at the components, so that multiple laser spots are generated on one surface of the components, which trace a welding contour on the surface of the components. The laser spot centers of at least three laser spots are arranged in a ring formation. The initial laser beam is generated using a multi-fiber, in particular a 2-in-1 fiber, such that the multiple laser spots on the surface of the components each have a core component and a ring component. In this way, a media-tight weld joint is to be created, especially when welding aluminum materials. Object of the invention

[0006] The invention is based on the objective of providing a method and a device with which the machining result can be improved when machining the workpiece and, in particular, a media-tight welded joint can be produced. Subject matter of the invention

[0007] According to a first aspect, this task is solved by a method of the type mentioned at the outset, in which the laser beam is provided by means of an optical fiber in the form of a multi-clad fiber, in particular in the form of a double-clad fiber.

[0008] The inventors propose combining a forming optic, which includes or consists of an axicon, with an optical fiber in the form of a multi-clad fiber, e.g., a double-clad fiber, to generate a laser beam with a rotationally symmetrical beam profile on the workpiece. In a laser processing process such as laser welding, this allows for the creation of a stable and direction-independent vapor capillary in all welding directions.

[0009] The inventors have recognized that such a rotationally symmetrical beam profile is advantageous compared to the multispot beam profile described in DE 10 2021 206 488 A1, in which the laser spot centers of at least three laser spots are arranged in a ring formation, particularly in cases where the local welding direction changes abruptly, e.g., when a corner is formed at the weld seam: With rapid changes in direction, the use of a non-rotationally symmetrical multifocus beam profile can potentially lead to an unstable vapor capillary. By using a rotationally symmetrical, appropriately designed beam profile of the focused laser beam on the workpiece, a maximally stable vapor capillary can be generated even during changes in direction during welding, and a completely direction-independent welding result can be obtained.This is particularly advantageous for creating a media-tight welded joint with minimal process pores in workpieces made of aluminium materials.

[0010] In one embodiment, the beam profile of the laser beam focused on the workpiece has a first ring region containing a radiation component from an inner light-guiding core of the multi-clad fiber. A multi-clad fiber according to the present application has an inner, circular light-guiding core and at least one light-guiding annular core surrounding the inner light-guiding core. The multi-clad fiber can, for example, be a so-called double-clad fiber (or a 2-in-1 fiber) with exactly one light-guiding, annular core surrounding the central fiber core. However, a multi-clad fiber according to the invention can also have more than one annular fiber core and thus be configured as a 3-in-1 fiber, 4-in-1 fiber, or more generally as an n-in-1 fiber. Each of the annular fiber cores is preferably arranged concentrically with the central fiber core.The axicon, or more precisely, a suitably designed axicon, transforms the (rotationally symmetric) intensity distribution of the laser beam at the exit end of the multiclad fiber into an annular intensity distribution. The axicon causes the intensity distribution at the exit end to rotate around the center of the beam profile, so that, for example, a Gaussian rotationally symmetric beam profile at the exit end is transformed into a Gaussian annular beam profile on the workpiece. Similarly, the (rotationally symmetric) intensity distribution in the inner light-guiding core of the multiclad fiber is transformed by the axicon into an annular intensity distribution.

[0011] The collimation and focusing optics image the exit end of the optical fiber onto the workpiece surface, whereby the circular beam profile of the laser beam at the exit end of the optical fiber is transformed by the axicon into an annular beam profile on the workpiece. This imaging can be performed at a 1:1 scale or at another scale.

[0012] In a further development, the beam profile of the laser beam focused on the workpiece has a second ring region containing a radiation component from a light-guiding annular core of the multi-clad fiber. The first ring region is located between an inner and an outer ring region of the second ring region. Typically, only a radiation component from the light-guiding annular core of the multi-clad fiber is present in the inner and outer ring regions of the second ring region; that is, the radiation component from the inner light-guiding core is limited to the first ring region. In the case of a triple-clad fiber with two light-guiding annular cores, the second ring region of the beam profile is surrounded by an inner and an outer ring region of a third ring region; in the case of a quadruple-clad fiber, by an inner and an outer ring region of a fourth ring region, and so on.

[0013] In a further development of this variant, the beam profile of the focused laser beam exhibits an intensity maximum in the first ring region. This has proven advantageous for welding. The intensity of the focused laser beam in the first ring region can be determined by distributing the power of the laser beam coupled into the multi-clad fiber between the inner light-guiding core and the at least one annular light-guiding core. If more power is coupled into the inner annular core, the intensity of the focused laser beam in the first ring region increases. The distribution of the laser beam power across the cores of a multi-clad fiber in the form of a double-clad fiber can be achieved, for example, as described in DE 10 2010 003 750 A1.It can be advantageous for the welding process if at least 15%, preferably at least 30%, of the laser beam power is coupled into the inner light-guiding core. It is understood that the distribution of the laser power across the cores of the multi-clad fiber also depends on the welding task and the ratio of the cross-sectional areas of the fiber cores to each other.

[0014] In another variant, the beam profile of the focused laser beam has a diameter of no more than 1000 µm, preferably no more than 800 µm and no less than 300 µm. To achieve optimal welding results, the diameter of the focused laser beam's beam profile on the workpiece should be on the order of the outer diameter of the multi-focus profile described above. The beam profile diameter can be defined by a decrease in intensity to 10% of the laser beam's maximum intensity.

[0015] In another variant, the laser beam profile exhibits a minimum intensity at its center. Specifically, the laser beam profile can have a zero point or a circular region at its center where the intensity is minimal or zero. However, it is also possible that the laser beam intensity is not minimal at the center of the beam profile.

[0016] In another variant, the laser beam is reshaped by a refractive axicon that has a cone-shaped surface. The material of the refractive axicon is transparent to the laser beam. The cone-shaped surface of the axicon can protrude from a substrate or base, or it can be machined into a substrate as a recess (so-called "inverted axicon"). The axicon is rotationally symmetrical and typically occupies a circular area.

[0017] In a further development of this variant, the refractive axicon has exactly one cone-shaped surface. For generating the beam profile described above, which has a first ring region and a second, third, ... ring region, an axicon with a single cone-shaped surface is sufficient due to the use of the multi-clad fiber.

[0018] In another variant, the cone-shaped surface of the axicon has a cone angle of 0.5° or less, preferably 0.4° or less, and particularly 0.2° or less. To generate a laser beam profile on the workpiece with a comparatively small diameter, which is advantageous for the welding process, it is necessary that the cone angle of the axicon is not too large. Besides the cone angle of the axicon, the refractive index of the axicon material is also relevant for its refractive power. Typically, the axicon is made of a conventional glass material, for example, fused silica or optical glass, whose refractive index at the wavelength of the laser beam can be, for example, on the order of 1.3–1.7.

[0019] In another variant, the deflecting optics or axicon are positioned in the laser beam path between the optical fiber and the collimating optics, in the laser beam path between the collimating optics and the focusing optics, or in the laser beam path between the focusing optics and the workpiece. While the deflecting optics or axicon can be positioned at various points in the laser beam path, it has proven advantageous to place the axicon in the collimated beam path of the laser beam.

[0020] Another aspect of the invention relates to a device of the type mentioned above, in which the optical fiber is designed as a multi-clad fiber, in particular as a double-clad fiber. With the aid of this device, the advantages described above with regard to the quality of the welding result can be realized. The device can, for example, be implemented as a standard welding optic in the form of a fixed optic or a scanner optic, which includes a forming optic in the form of a beam-shaping insert that has an axicon or is designed as an axicon.

[0021] In one embodiment, the axicon is a refractive axicon having a cone-shaped surface, in particular exactly one cone-shaped surface. As described above, a rotationally symmetric beam profile with the intensity distribution described above in connection with the method can be generated using a refractive axicon.

[0022] In another embodiment, the cone-shaped surface of the axicon has a cone angle of 0.5° or less, preferably 0.4° or less, and particularly 0.2° or less. As described above, in order to generate a beam profile on the workpiece that has the desired dimensions or size for carrying out a laser processing process in the form of a welding process, the cone angle of the axicon must not be too large.

[0023] In another embodiment, the deflecting optics are arranged in the beam path of the laser beam between the optical fiber and the collimating optics, in the beam path of the laser beam between the collimating optics and the focusing optics, or in the beam path of the laser beam between the focusing optics and the workpiece. The deflecting optics, in the form of an axicon, are preferably arranged in the beam path between the collimating optics and the focusing optics.

[0024] Further advantages of the invention will become apparent from the description and the drawing. Likewise, the features mentioned above and those listed below can be used individually or in any combination. The embodiments shown and described are not to be understood as an exhaustive list, but rather serve as examples illustrating the invention.

[0025] They show: Fig. 1a-c schematic representations of a device for laser welding a workpiece, which has a forming optic designed in the form of an axicon, Fig. 2a,b schematic representations of the axicon of Fig. 1a-c, which is formed as a refractive axicon or as an inverse refractive axicon, Fig. 3a-c schematic representations of beam profiles of the laser beam focused on the workpiece, as well as Fig. 4 a schematic representation of the intensity profile of the beam profile of Fig. 3b.

[0026] In the following description of the drawings, identical reference symbols are used for identical or functionally equivalent components.

[0027] Fig. Figure 1 shows a device 1 for laser processing, more precisely for laser welding, of a workpiece 2, which has an optical fiber 3 for providing a laser beam 4 that exits the optical fiber 3 at an exit end 3a. The laser beam 4 is generated by a laser source (not shown), which is also part of the device 1. The device 1 has a collimation optic 5 for collimating the laser beam 4, a focusing optic 6 for focusing the laser beam 4 onto the workpiece 2, and a forming optic for shaping the laser beam 4, which is designed as an axicon 7.

[0028] The optical fiber 3 is a multi-clad fiber in the form of a double-clad fiber 3. The double-clad fiber 3 has an inner light-conducting core 8a, which is circular in shape, and a light-conducting annular core 8b, which surrounds the inner circular core 8a, as shown in Fig. Figure 1a-c on the left indicates the double clad fiber 3 in a cross-sectional view.

[0029] At the in Fig. In 1a-c, the plate-shaped axicon 7 is a refractive axicon, as shown in Fig. The axicon 7 can be formed as shown in 2a,b. It has a planar beam entry surface 7a and a conical surface 7b, which serves as the beam exit surface for the laser beam 4. The conical surface 7b is rotationally symmetric about a central axis of the axicon 7, which is shown in Fig. 2a,b is shown with a dash. In the case of the Fig. In the example shown in 2a, the cone-shaped surface 7b extends from the flat, circular beam entry surface 7a of the axicon 7. In the example shown in Fig. The example shown in 2b is an inverse axicon 7, in which the cone-shaped surface 7b is worked into the material of the axicon 7 as a depression.

[0030] The cone angle α of the axicon is 0.5° or less, 0.4° or less, ideally 0.2° or less. The cone angle α is determined in the Fig. 2a example shown, measured from the tip of axicon 7 and at the point shown in Fig. The example shown in Figure 2b relates to a surface that runs parallel to the beam entry surface 7a through the outer edge of the cone-shaped surface 7b. The comparatively small cone angle α makes it possible to generate a beam profile 9 (focus spot) on the workpiece 2, which has a comparatively small diameter D on the order of no more than approximately 1000 µm, ideally no more than 800 µm and no less than 300 µm, cf. Fig. 3a-c. The axicon 7 is made of a material transparent to the laser beam 4, for example quartz glass or optical glass.

[0031] As in Fig. As can be seen in Figures 3a-c, the beam profile 9 of the focused laser beam 4 on the workpiece 2 has a first ring region 10, which contains a radiation component from the inner light-guiding core 8a of the multi-clad fiber 3, and a second ring region 11a, 11b, which has a first, inner ring part 11a and a second, outer ring part 11b, each containing a radiation component from a light-guiding annular core 8b of the multi-clad fiber 3. The first ring region 10 is located between the inner ring part 11a and the outer ring part 11b of the second ring region 11a, 11b. The radiation component of the laser beam 4 from the inner light-guiding core 8a of the double-clad fiber 3 is confined to the first ring region 10. By changing the distribution of the power of the laser beam 4 entering the optical fiber 3 onto the inner light-guiding core 8a and onto the ring-shaped light-guiding core 8b, which e.g.In the manner described in DE 10 2010 003 750 A1, the location-dependent intensity I(x,y) of the rotationally symmetric beam profile 9 of the laser beam 4 on the workpiece 2 can be changed. In the example shown, the light-guiding inner core 8a has a diameter of approximately 50 µm and the light-guiding annular core 8b has a diameter of approximately 200 µm.

[0032] Fig. Figure 4 shows, by way of example, the intensity I of the laser beam 4 along the X-direction of an XY coordinate system of the beam profile 9 of Fig. 3b. As in Fig. As can be seen in Figure 4, the intensity I or the intensity distribution I(x) is rotationally symmetric or mirror-symmetric about the center Z of the beam profile 9. At the center Z, the beam profile 9 exhibits an intensity minimum I Min On and within the first ring area 10, the beam profile 9 exhibits an intensity maximum I Max on.

[0033] All three rotationally symmetric beam profiles 9 of Fig. Beam profiles 3a-c have a zero point or a circular region at center Z where the intensity I of the beam profile 9 is minimal or zero. However, this is not mandatory; that is, the beam profile 9 can also have an intensity I other than zero at center Z. The differences between the three beam profiles 9k result from differently selected parameters of the axicon 7 (cone angle α or refractive index) and from a differently selected imaging scale when imaging the exit end 3b of the optical fiber 3 onto the surface of the workpiece 2 to be processed.

[0034] It has been shown that with the help of the in Fig.The rotationally symmetrical beam profiles shown in Figures 3a-c enable the production of high-quality welds, particularly in laser welding of aluminum-containing workpieces, because a stable and direction-independent vapor capillary can be generated in all welding directions. The formation of process pores is avoided, allowing for media-tight welding in all directions on aluminum-containing workpieces. Furthermore, in a device 1 that incorporates scanner optics, a highly dynamic axis kinematics can be dispensed with, resulting in a cost-effective system design. QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2019 217 754 A1 [0002, 0003] DE 10 2010 003 750 A1 [0003, 0013, 0031] CN 115453767A

[0004] DE 10 2021 206 488 A1 [0005, 0009]

Claims

[1] Method for laser processing, in particular for laser welding, of a workpiece (2), comprising: Providing a laser beam (4) by means of an optical fiber (3), Collimating the laser beam (4) by means of collimation optics (5), Focusing the collimated laser beam (4) onto the workpiece (2) by means of a focusing optics (6), and Reshaping the laser beam (4) by means of a reshaping optic in the form of an axicon (7), characterized by , that the laser beam (4) is provided by means of an optical fiber in the form of a multiple clad fiber, in particular in the form of a double clad fiber (3). [2] Method according to claim 1, wherein a beam profile (9) of the laser beam (4) focused on the workpiece (2) has a first ring region (10) which contains a radiation component from an inner light-conducting core (8a) of the multi-charge fiber (3). [3] Method according to claim 2, wherein the beam profile (9) of the laser beam (4) focused on the workpiece (2) has a second ring region (11a, 11b) which contains a radiation component from a light-conducting ring-shaped core (8b) of the multi-clad fiber (2), the first ring region (10) being located between an inner ring part (11a) and an outer ring part (11b) of the second ring region (11a, 11b). [4] Method according to claim 2 or 3, wherein the beam profile (9) of the focused laser beam (4) in the first ring region (10) has an intensity maximum (I max ). [5] Method according to one of the preceding claims, wherein the beam profile (9) of the focused laser beam (4) has a diameter (D) of not more than 1000 µm, preferably not more than 800 µm and not less than 300 µm. [6] Method according to one of the preceding claims, in which the beam profile (9) of the focused laser beam (4) has a minimum intensity (I min ). [7] Method according to one of the preceding claims, in which the laser beam (4) is reshaped on a refractive axicon (7) which has a conical surface (7b). [8] Method according to claim 7, wherein the refractive axicon (7) has exactly one cone-shaped surface (7b). [9] Method according to claim 7 or 8, wherein the conical surface (7b) of the axicon (7) has a cone angle (α) of 0.5° or less, preferably of 0.4° or less, in particular of 0.2° or less. [10] Method according to one of the preceding claims, in which the deflection optics (7) are arranged in the beam path of the laser beam (4) between the optical fiber (3) and the collimation optics (5), in the beam path of the laser beam (4) between the collimation optics (5) and the focusing optics (6) or in the beam path of the laser beam (4) between the focusing optics (6) and the workpiece (2). [11] Device for laser processing, preferably for laser welding a workpiece (2), in particular for carrying out a method according to one of the preceding claims, comprising: an optical fiber (3) for providing a laser beam (4), a collimation optics (5) for collimating the laser beam (4), a focusing optics (6) for focusing the laser beam (4) onto the workpiece (2), and a forming optics (7) in the form of an axicon for forming the laser beam (4), characterized by , that the optical fiber is designed as a multiple clad fiber, in particular as a double clad fiber (3). [12] Device according to claim 11, wherein the axicon is a refractive axicon (7) having a cone-shaped surface (7b), in particular exactly a cone-shaped surface (7b). [13] Device according to claim 11 or 12, wherein the conical surface (7b) of the axicon (7) has a cone angle (α) of 0.5° or less, preferably of 0.4° or less, in particular of 0.2° or less. [14] Device according to one of claims 11 to 13, in which the deflection optics (7) are arranged in the beam path of the laser beam (4) between the optical fiber (3) and the collimation optics (5), in the beam path of the laser beam (4) between the collimation optics (5) and the focusing optics (6) or in the beam path of the laser beam (4) between the focusing optics (5) and the workpiece (2).

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