Method for deep welding of a workpiece and associated laser welding device

Abstract

Method for deep welding of a workpiece (2), wherein a first laser beam (11) and a second laser beam (12) are used to irradiate a surface of the workpiece (2), wherein in a workpiece surface plane (OE) a first beam width B1 of the first laser beam (11) is larger than a second beam width B2 of the second laser beam (12), and wherein, at least in the workpiece surface plane (OE), the second laser beam (12) lies within the first laser beam (11), characterized by that the intensity of the first laser beam (11) alone is sufficient to create a vapor capillary (6) in the workpiece (2), and that the vapor capillary (6) generated in the workpiece (2) has a capillary width KB in the workpiece surface plane (OE), where KB is essentially the same size as B1, and furthermore B2 ≤ 0.75*KB.

Description

[0001] The invention relates to a method for deep welding a workpiece, wherein a surface of the workpiece is irradiated with a first laser beam and a second laser beam, wherein in a workpiece surface plane a first beam width B1 of the first laser beam is greater than a second beam width B2 of the second laser beam, and wherein at least in the workpiece surface plane the second laser beam lies within the first laser beam.

[0002] Such a procedure has become known from US 2006 / 0157457 A1.

[0003] Laser beam welding allows workpieces to be joined together at a relatively high speed with a narrow weld seam. Only a relatively small amount of heat energy is introduced into the workpieces, resulting in little or no thermal distortion.

[0004] In deep laser welding, the workpiece material is not only melted on the surface, but a vapor capillary forms in the molten pool in the direction of the beam. This allows the laser radiation to penetrate to greater depths, creating stronger bonds between workpieces.

[0005] With high-quality laser beams, i.e., a small beam parameter product (defined as the aperture angle * the radius of the laser beam at its thinnest point, often abbreviated to SPP), a particularly deep weld penetration can be achieved. On the other hand, with low-quality laser beams, i.e., a large beam parameter product, good weld seam quality can be achieved at higher feed rates.

[0006] The quality of the weld is particularly compromised by the formation of weld spatter. Liquid workpiece material is ejected from the molten pool, creating notches in the weld and leading to material loss in the weld, which weakens it mechanically and also contaminates the surrounding area.

[0007] In the prior art, attempts were made to influence the quality of the welding process by using multiple laser beams.

[0008] US 2006 / 0157457 A1 discloses the superposition of a first and a second laser beam on a workpiece. The spot of the second laser beam is smaller than the spot of the first laser beam, with the spot of the second laser beam located within the spot of the first laser beam. The second laser beam is directed onto the vapor capillary being formed. This process is intended to enable high welding depth and width, as well as high welding speed, for workpieces with high reflectivity.

[0009] Similar processes in which a laser beam with a small spot creates a vapor capillary and is superimposed by another laser beam with a larger spot in order to act on the surrounding weld pool are known, for example, from DE 10 2006 011 064 B4, EP 1 007 267 B1, DE 197 511 95, DE 198 59 243 A1 and JP 45 80065 A.

[0010] US 7,807,939 B2 describes a laser welding process in which two lasers irradiate a workpiece alternately.

[0011] JP 2004 358 521 A also discloses a laser welding process in which laser beams from different laser sources or the same laser source are superimposed and focused at different depths in the workpiece. A similar process using laser sources with different wavelengths is disclosed in JP 2006 263 771 A.

[0012] JP 2003 340 582 A proposes splitting a laser beam into a main beam with a small diameter and high energy and a secondary beam with a larger diameter and lower energy, and focusing them differently. A vapor capillary is created using the main beam.

[0013] JP 2007 319 878 A discloses a laser welding method with a first beam source and a second beam source, in which a first laser beam of the first beam source and a second laser beam of the second beam source are superimposed.

[0014] Another laser welding process for deep welding is known from JP 2004 154 813 A. Object of the invention

[0015] The invention is based on the object of providing a laser deep welding process in which good seam quality can be achieved with a large welding depth and high feed rate. Brief description of the invention

[0016] This object is achieved by a method of the type mentioned above, which is characterized in that that the intensity of the first laser beam alone is sufficient to create a vapor capillary in the workpiece, and that the vapor capillary generated in the workpiece has a capillary width KB in the workpiece surface plane, where KB is essentially the same size as B1, and further B2 ≤ 0.75*KB.

[0017] According to the present invention, the intensity of the first laser beam is selected to be sufficiently high that it is capable of generating a vapor capillary in the workpiece (or at the interface between workpiece parts that are to be joined to form the workpiece). Accordingly, the first laser beam already creates an opening in the surface plane of the workpiece.

[0018] The second laser beam, smaller than the first laser beam, can be directed through this opening so that the second laser beam does not hit the melt (or even solid workpiece material) in the area near the surface of the workpiece, usually comprising at least the upper third of the workpiece (relative to the depth within the workpiece). Only at a certain distance from the workpiece surface, usually in a middle third and lower third of the vapor capillary (relative to the depth within the workpiece), does absorption and reflection of the second laser beam occur at the interface between the vapor capillary and the surrounding melt pool, releasing energy.

[0019] The second laser beam thus uses the opening of the vapor capillary created by the first laser beam to reach a deeper zone of the vapor capillary without (or with only minimal) energy loss in order to release its energy there.

[0020] The capillary width KB of the vapor capillary in the process according to the invention essentially corresponds to the first beam width B1 of the first laser beam (viewed in the workpiece surface plane); usually |KB-B1|≤0.2*B1, preferably |KB-B1|≤0.15*B1, and often also |KB-B1|≤0.1*B1. In many cases, B1≤KB also applies.

[0021] Due to the lower energy input in the upper area of ​​the workpiece compared to a single laser beam, the melt flow dynamics in the upper part of the melt pool decrease. Melt flow velocities are reduced, and the tendency toward turbulence is reduced. This, in turn, reduces spatter formation and can be used to increase feed rates during the laser welding process. At the same time, the second laser beam can deliver a high level of energy at greater depths, allowing the weld penetration to be increased compared to a single laser beam of low beam quality, and also achieves a slender weld seam shape.

[0022] Within the scope of the invention, a specified welding depth can be achieved with good seam quality using lower laser power than with the prior art. This increases energy efficiency, thus reducing operating and investment costs. Furthermore, the more targeted energy coupling leads to lower heat input into the workpiece (component), thus reducing residual stresses and workpiece distortion.

[0023] The intensity of the first laser beam is typically just sufficient to create a (stable) vapor capillary in the workpiece with the first laser beam alone; typically, the intensity of the first laser beam is a maximum of twice as high as necessary to create a stable vapor capillary with the first laser beam alone.

[0024] The specified width ratios (e.g., B1, B2, KB) apply at least along the feed direction (welding direction) of the laser welding process, and typically also in any direction within (or parallel to) the workpiece surface plane. Within the scope of the invention, B2 ≤ 0.5*KB or B2 ≤ 0.4*KB or even B2 ≤ 0.3*KB also preferably applies. B2 ≤ 0.5*B1 or B2 ≤ 0.4*B1 also preferably applies.

[0025] The first and second laser beams can be generated with a common laser source or with separate laser sources. Typically, the first laser beam and the second laser beam are coaxial. Note that (in the workpiece surface plane) the second laser beam can overlap with the first laser beam, or the first laser beam can enclose the second laser beam in a ring. Preferred variants of the invention

[0026] In a preferred variant of the method according to the invention, the first laser beam and the second laser beam have the same focal position with respect to a z-direction perpendicular to the surface of the workpiece, or have focal positions spaced apart by less than 1 mm. In this case, a particularly simple, in particular shared, imaging optics can be used for both laser beams. Furthermore, the energy of the second laser beam can usually be introduced at a particularly great depth; the beam profile of the second laser beam can optimally utilize the free space in the vapor capillary created by the first laser beam.

[0027] A preferred embodiment is one in which the focus positions of the first and second laser beams are located in the workpiece surface plane or below the workpiece surface within the vapor capillary. This optimizes the seam quality. Common foci, or the deeper focus, are generally located at a maximum depth corresponding to half the depth of the vapor capillary or the workpiece thickness (with respect to the z-direction).

[0028] Particularly preferred is a variant in which the second laser beam has a better beam quality (also called beam quality) than the first laser beam, in particular with SPP1 ≥ 2*SPP2, preferably SPP1 ≥ 4*SPP2, where SPP1: beam parameter product of the first laser beam, and SPP2: beam parameter product of the second laser beam. This made it possible to achieve both large welding depths and high feed rates with good seam quality. Within the scope of the invention, the advantage of a low SPP, i.e. high beam quality, namely a large welding depth, can be combined with the advantages of a high SPP, i.e. low beam quality, namely better seam quality or a higher selectable feed rate.

[0029] Another advantageous variant is one in which the following applies to a first laser power (or average laser power) L1 of the first laser beam and a second laser power (or average laser power) L2 of the second laser beam: L2 ≥ L1, preferably L2 ≥ 2*L1. This has proven successful in practice to achieve a high energy input at a greater depth of the vapor capillary. In particular, good welding results have also been achieved with the same laser powers (or average laser powers) L2=L1. The intensity of the second laser beam alone would also be sufficient to create a vapor capillary in the workpiece. In most cases, L2 ≥ 5*L1 also applies. Alternatively, L2 < L1 can also be selected. In general, the intensity of the second laser beam at the focus is at least as high as the intensity of the first laser beam at the focus.

[0030] A further preferred variant is one in which the following applies to a first focusing angle W1 of the first laser beam and a second focusing angle W2 of the second laser beam: W2 ≥ W1 . Particularly with a common focus position below the workpiece surface, unwanted contact of the second laser beam with the melt near the workpiece surface can then generally be avoided. Furthermore, a particularly narrow beam caustic of the second laser beam (compared to the first laser beam) can often be achieved. Alternatively, W2 < W1 can also be provided.

[0031] In a preferred variant, the following applies for a smallest width KD of the vapor capillary in the upper quarter of the vapor capillary, close to the workpiece surface, and for a largest width G2 of the second laser beam in the upper quarter of the vapor capillary: 0.8*KD ≥ G2, preferably 0.6*KD ≥ G2. This allows energy input into the melt pool in the near-surface quarter of the vapor capillary to be reduced, and particularly good seam quality or a particularly high feed rate can be achieved. Preferably, 0.8*K1 ≥ G2 or 0.6*K1 ≥ G2 also applies with regard to a smallest width K1 of the first laser beam in the upper quarter of the vapor capillary.

[0032] A preferred method variant is one in which the first laser beam and the second laser beam are aligned coaxially with a common beam axis. This coaxial alignment allows the second laser beam to penetrate particularly deep into the workpiece without first encountering the melt. Furthermore, the welding device can have a simple design.

[0033] A preferred further development is one in which the common beam axis runs essentially perpendicular to the workpiece surface. This optimizes the penetration depth. In particular, the beam axis can be selected with a deviation from the perpendicular to the workpiece surface of 5° or less, and preferably 0°. Depending on the type of joint and weld seam, other angles of incidence are also applicable, particularly with a deviation of more than 5° from the perpendicular to the workpiece surface.

[0034] Preferably, the first and second laser beams are guided together in a single optical fiber, as this allows for a particularly simple structure for light transport and imaging onto the workpiece. A common fiber can be used, e.g., by using different laser beam coupling angles, different coupling locations on the fiber entry surface (e.g., centric and eccentric), and / or different laser beam wavelengths. Alternatively, multiple fibers or multiple-core fibers (i.e., multiple fiber cores positioned adjacent to one another or coaxially within one another) can be used.

[0035] A further preferred variant provides that the first and the second laser beam are guided in a 2-in-1 fiber with a core fiber and a ring fiber, in particular wherein the first laser beam is guided in the ring fiber and the second laser beam in the core fiber, that the first and second laser beams emerge from one end of the 2-in-1 fiber in front of the workpiece, and that the fiber end with ring fiber and core fiber is imaged onto or into the workpiece by means of a common optic in a common focus, whereby the first and second laser beams are directed onto the workpiece by the common optic, In particular, the common optics comprise, preferably exclusively, a collimating lens and a focusing lens. This variant allows for a particularly simple structure for laser light transport and imaging onto the workpiece.

[0036] In an advantageous further development of this variant, a common laser source generates a primary laser beam, with a first part of the primary laser beam being eccentrically coupled into the ring fiber to form the first laser beam, and a second part of the primary laser beam being coupled into the core fiber to form the second laser beam. This makes it possible to easily and automatically provide a partial beam of high beam quality and a partial beam of low beam quality with a single laser source (beam source) by (jointly) coupling them into the 2-in-1 fiber. Depending on which fiber the primary laser beam part is coupled into, it exits the other fiber end with different laser beam characteristics. To control the beam components on the core and ring fibers, a wedge plate that can be moved into the beam path of the primary laser beam can be provided.2-in-1 fibers, which have a core fiber with the smallest possible core fiber diameter ≤150µm and a ring fiber with an outer diameter larger than three times the core fiber diameter, have proven particularly advantageous for generating different laser characteristics.

[0037] In an alternative variant, a first laser source generates the first laser beam and a second laser source generates the second laser beam, and the first and second laser beams are superimposed using a beam splitter element. With this so-called twin-spot technology, the laser sources for the first and second laser beams can be freely selected. The construction of a corresponding laser welding device is generally simple and cost-effective.

[0038] In a further development of this variant, the superimposed laser beams are directed onto the workpiece using a common focusing optics, particularly a focusing lens. This is particularly simple and proven in practice.

[0039] Another alternative embodiment provides that the first and second laser beams are guided in a dual-core fiber with two adjacent, parallel laser fibers, that the first and second laser beams emerge from the fiber ends of the dual-core fiber in front of the workpiece, and that the fiber ends of the laser fibers are imaged via a common optic in a common focus on or in the workpiece, whereby the first and second laser beams are directed from the common optics onto the workpiece, In particular, the common optics comprise, preferably exclusively, a collimating lens and a focusing lens. This enables a comparatively simple design of a laser welding device for light transport and the imaging of the laser beams onto the workpiece.

[0040] The present invention also includes a laser welding device designed to carry out a method according to the invention described above. The laser welding device can be used to weld workpieces, particularly steel workpieces, with a large penetration depth and at high feed rates while maintaining good seam quality.

[0041] Further advantages of the invention will become apparent from the description and the drawings. Likewise, the above-mentioned and further-described features can be used individually or in combination in any desired manner. The embodiments shown and described are not intended to be exhaustive, but rather are exemplary in nature for describing the invention. Detailed description of the invention and drawing

[0042] The invention is illustrated in the drawing and explained in more detail using exemplary embodiments. They show: Fig. 1 a schematic longitudinal section through part of a laser welding device for the invention and a workpiece in a variant of the deep welding method according to the invention; Fig. 2 experimental cross-sections through a weld seam (each above) and top views of a weld seam (each below) during deep welding with a laser beam of high beam quality a), with two laser beams according to the invention b) and with a laser beam of low beam quality c), at low welding speed; Fig. 3 experimental side views of the molten pool during welding (each top), top views of the weld seam (each middle) and cross sections through the weld seam (each bottom) for a variant according to the invention with two laser beams a) and with one laser beam b), at high welding speed; Fig. 4 experimental oblique views of the molten pool during welding using one laser beam a) and in a variant according to the invention with two laser beams b); Fig. 5 a schematic view of a first embodiment of a laser welding device for the invention, in twin-spot technology with beam splitter element; Fig. Figure 6 is a schematic longitudinal sectional view through a 2-in-1 fiber (top) for use with the invention, and an associated beam generating assembly (bottom); Fig. Figure 7 is a schematic view of a dual-core fiber assembly for use with the invention; Fig. 8 a schematic representation of the beam caustics of the first and second laser beam in a variant of the laser welding method according to the invention, with approximately the same opening angles; Fig. 9 a schematic representation of the beam caustics of the first and second laser beam in a variant of the laser welding method according to the invention, with unequal opening angles. introduction

[0043] In deep laser welding, the relative motion between the laser beam and the workpiece moves a vapor capillary (keyhole) through a molten molten pool. If the feed rate is too high, a large amount of molten spatter is released, reducing the quality of the weld seam due to the formation of notches.

[0044] In conventional deep-penetration laser welding, the majority of the laser energy is absorbed in the upper region of the vapor capillary. This is where the laser beam first hits the workpiece. The focal plane is usually positioned here. Accordingly, this is the location of the highest power density.

[0045] With increasing absorbed energy, the melt flow dynamics increase, and when critical values ​​are reached, the above-mentioned negative effects occur, causing mass loss in the weld and / or pronounced cyclic melt flow behavior.

[0046] If a laser beam with very good beam quality (also called beam grade) is used, the resulting weld penetration depth increases (at the same energy input) compared to a laser with low beam quality. However, the limit for good weld seam quality shifts to lower feed speeds. The inventors assume that this is due to a particularly high absorbed power density near the surface. In addition, the use of a laser beam with high beam quality creates a narrower vapor capillary, which is more likely to collapse at high speeds than a wider vapor capillary caused by a laser beam with lower beam quality.

[0047] With the present invention, the energy coupling is partially shifted to deeper regions of the vapor capillary so that high welding depths with good seam quality are achieved for the largest possible speed range. Inventive process control

[0048] The Fig. 1 shows in a schematic longitudinal section the area of ​​the melt pool 1 in a workpiece 2, which is just being welded with a variant of the deep welding process according to the invention using a laser welding device 3 (in Fig. 1 only partially shown). The (relative) welding direction SR is directed to the left here, for example by moving the workpiece 2 (with the laser welding device 3 otherwise stationary) to the right.

[0049] A first laser beam 11 and a second laser beam 12 are guided coaxially along a common beam axis 4 and focused onto the workpiece 2 by means of a focusing optics 5 of the laser welding device 3, for example in order to weld two workpiece parts whose contact surface is located in the plane of the drawing from Fig. 1, to the workpiece 2. The beam axis 4 here runs perpendicular to the workpiece surface plane OE. Both laser beams 11, 12 have an essentially rotationally symmetric ("circular") intensity profile (relative to the beam axis 4). The locations where the intensity has dropped to half of the maximum intensity can be used as the boundaries of the laser beams 11, 12.

[0050] In the variant shown, the two laser beams 11, 12 are focused in the workpiece surface plane OE, so that the laser beams 11, 12 each have their smallest diameter ("waist") here. The diameter (FWHM) of the first laser beam 11 in the workpiece surface plane OE, called the first beam width B1, is significantly larger than the diameter (FWHM) of the second laser beam 12 in the workpiece surface plane OE, called the second beam width B2, here approximately B2=0.25*B1. Overall, the caustic of the second laser beam 12 is smaller than the caustic of the first laser beam 11.

[0051] The first laser beam 11 alone has sufficient intensity to create a vapor capillary 6 in the workpiece 2. Accordingly, the capillary width KB of the vapor capillary 6 in the workpiece surface plane OE is approximately the same as the first beam width B1; in the variant shown, KB is slightly larger than B1, here approximately KB=1.15*B1. Melt (molten workpiece material) is arranged around the vapor capillary 6, with the largest part of the melt pool 1 (relative to the welding direction SR) located behind the vapor capillary 6.

[0052] Since the second beam width B2 of the second laser beam 12 is significantly smaller than the first beam width B1 of the first laser beam 11, the second laser beam 12 can pass through the opening 7 of the vapor capillary 6 without encountering the melt of the melt pool 1. In the variant shown, the first partial absorption 8 of the second laser beam 12 at the interface of the vapor capillary 6 only occurs in a middle third (relative to the direction z into the workpiece 2) of the vapor capillary 6, and the non-absorbed portion of the radiation is reflected; see the reflected second laser beam 12 shown in dashed lines. Accordingly, hardly any energy is introduced into the melt pool 1 near the surface by the second laser beam 12. Note that the first laser beam 11 is partially absorbed and partially reflected for the first time significantly higher up; see the reflected first laser beam 11 shown in dotted lines.

[0053] In the upper quarter 9 of the vapor capillary 6, the greatest width (diameter) G2 of the second laser beam 12 (here at the lower edge of the upper quarter 9) and the smallest width (diameter) KD of the vapor capillary 6 (here also at the lower edge of the upper quarter 9) can be determined. The smallest width (diameter) K1 of the first laser beam 11 (here at the upper edge of quarter 9, i.e. in the workpiece surface plane) can be determined in the same way. In the variant shown, approximately G2 = 0.46 * KD and also approximately G2 = 0.5 * K1 apply. Because G2 is significantly smaller than KD or K1, the irradiation of the energy of the second laser beam 12 can be conveyed further into the melt pool 1, primarily in the lower area of ​​the vapor capillary 6, and not in the upper quarter 9.

[0054] Due to the energy irradiation of the second laser beam 12 in the deeper region of the vapor capillary 6, the melt pool dynamics can be reduced, especially near the surface. Turbulence is reduced or even eliminated. This reduces the detachment of weld spatter and the waviness of the resulting weld seam.

[0055] In the embodiment shown, the beam parameter product SPP1 of the first laser beam is approximately 2.7 times the beam parameter product SPP2 of the second laser beam 12, i.e., the second laser beam 12 has the better beam quality. Typically, a multimode laser beam is selected as the first laser beam (with low beam quality). The beam parameter products SPP1, SPP2 can be determined as the product of the respective focus angles (aperture angles in the far field) W1, W2 with the radii of the respective laser beams 11, 12 at the narrowest point of the caustic (here, the halved beam widths B1, B2). In the variant shown, W1 < W2, here with approximately W1=0.68*W2.

[0056] In order to produce a high-quality weld seam at high feed speeds (welding speeds), the first laser beam 11 initiates the formation of the vapor capillary (keyhole) 6 with a sufficiently large diameter, particularly the upper opening 7. At the same time, the second laser beam 12 is directed or focused (here coaxially) into the formed vapor capillary 6, whereby the caustic of the second laser beam 12 is smaller than that of the first laser beam 11. The energy of the second laser beam 12 is only absorbed / coupled in a lower zone of the vapor capillary 6. The absorbed energy in the upper region of the vapor capillary is reduced compared to the prior art and increased in the lower region; as a result, the welding depth is increased, while high feed speeds are still possible. Experimental results

[0057] For the recordings of the Fig. In 2, structural steel workpieces were blind welded using a) a laser beam with SPP=2 mm*mrad, focus diameter 150 µm, TruDisk 5000.5 laser source, b) two coaxial laser beams according to the invention, with SPP=2 mm*mrad and SPP=8 mm*mrad, focus diameters 150 µm and 300 µm, TruDisk 5000.5 and TruDisk 16002 laser sources, and c) a laser beam with SPP=8 mm*mrad, focus diameter 300 µm, TruDisk 16002 laser source. In all cases, the feed rate was 1 m / min and the total laser power P=2 kW, in case b) with 1 kW per laser beam. The focus position was -4 mm in the workpiece in each case.

[0058] As can be seen from the upper cross-sectional images of the Fig. As can be seen in Figure 2, a laser beam with SPP 2 mm*mrad achieved a welding depth of 6.1 mm, while a laser beam with SPP 8 mm*mrad achieved a welding depth of only 4.0 mm at the same laser power. By combining two lasers with SPP 2 mm*mrad and SPP 8 mm*mrad, with the same overall power, a welding depth of 4.6 mm was achieved, significantly greater than with the single laser beam with SPP 8 mm*mrad.

[0059] At the comparatively low feed rate of 1 m / min, an acceptable seam quality was achieved on all workpieces, see the respective lower top view images.

[0060] If a higher feed rate is selected, the seam quality is more strongly influenced. Fig. 3, structural steel workpieces were again welded, firstly with a) two coaxial laser beams, with SPP=12 mm*mrad and SPP=3 mm*mrad, and secondly with b) one laser beam, with SPP=8 mm*mrad. The total laser power was 5 kW in each case (with two laser beams, both 2.5 kW each), the focus position was -2 mm, and the feed rate was 8 m / min.

[0061] From the side views taken during the welding process above, it is clear that with the two laser beams a) a steady molten pool is obtained, while with the single laser beam b) there is a significant ejection of melt droplets (welding spatter). The weld quality is, as can be seen from the top views in the middle of the Fig. 3, when using two laser beams a) the weld is very good, while when using only one laser beam b) an uneven weld with numerous beads is obtained. In the cross-sectional images below in Fig. 3 it is also evident that in case b) the weld seam forms a noticeable depression due to material loss and has uneven surface bulges, whereas in case a) no depression and a symmetrical, central weld bead is visible.

[0062] In Fig. Figure 4 shows larger image sections of the welding of structural steel workpieces in an oblique view, in a) with one laser beam, focus diameter 440 µm, and in b) with two laser beams according to the invention, focus diameters 110 µm and 440 µm. The feed rate was 10 m / min in each case, the total laser power was 5 kW in each case (with two laser beams, both 2.5 kW each), and the focus position was -2 mm in each case.

[0063] When using only one laser beam a), a significant, tail-like ejection of weld spatter to the rear occurs, whereas when using two laser beams b) only minimal ejection of weld spatter is observed. Laser welding devices

[0064] In the Fig. Figure 5 shows a schematic drawing of an embodiment of a laser welding device 3 for the invention. The laser welding device 3 shown operates using the so-called twin-spot technology with a beam splitter element 20.

[0065] The laser welding device 3 comprises a first laser source 21, which generates a first laser beam 11 with typically low beam quality. If necessary, the first laser beam 11 can be partially transported via an optical fiber 22. Furthermore, a second laser source 23 is provided, which generates a second laser beam 12 with typically high beam quality. If necessary, the second laser beam 12 can also be partially transported via an optical fiber 24. The first laser beam 11 is collimated (parallelized) via a first collimator optics 25 and directed onto the beam splitter element 20 (usually a semi-transparent mirror). The second laser beam 12 is collimated (parallelized) via a second collimator optics 26 and also directed onto the beam splitter element 20.The second laser beam 12 is rotated by 90° relative to the first laser beam 11, and the beam splitter element 20 is arranged at a 45° angle to the laser beams 11, 12.

[0066] In extension of the beam axis of the second laser beam 12, a so-called beam trap 16 is arranged here.

[0067] The welding station 27 is located here in the extension of the beam axis of the first laser beam 11. The total beam 28, consisting of portions of the first laser beam 11 and the second laser beam 12 behind the beam splitter element 20, is focused with a common focusing optics 29 for the first and second laser beams 11, 12 and directed onto a workpiece 2 to be welded. The (common) focus 14 lies in the workpiece surface plane OE.

[0068] The workpiece 2 is arranged here on a traversing table 13, with which the workpiece 2 can be moved along the beam axis 4 (z-direction) of the total beam 28, in particular in order to adjust the focus position, and also in the directions perpendicular thereto (x-direction and y-direction) in order to traverse a desired weld seam.

[0069] The Fig. 6 shows at the top in a schematic longitudinal section a 2-in-1 fiber 30 which can be used as part of a laser welding device for the invention.

[0070] The 2-in-1 fiber 30 comprises a central core fiber 31 and a surrounding ring fiber 32, each of which can guide laser light. A first laser beam 11 is coupled into the ring fiber 32, and a second laser beam 12 is coupled into the core fiber 31 (shown top left). Upon coupling, the first and second laser beams 11, 12 each have a circular cross-section (see the cross-sectional view of the 2-in-1 fiber 30 in the center above).

[0071] The first laser beam 11 to be coupled in and the second laser beam 12 to be coupled in can originate from a common laser source 33, which generates a primary laser beam 34, from which a first radiation component 38 and a second radiation component 37 are generated using, for example, a wedge plate 35 (in Fig. 6 below). The further the wedge plate 35 is moved into the beam path of the primary laser beam 34, the greater the radiation component 38 that is deflected by it, ie the proportions of the distribution between the two laser beams 11, 12 are controllable.

[0072] Depending on which of the fibers 31, 32 the respective generated laser beam 11, 12 is coupled into, it exits the fiber end 30a with different laser beam characteristics. For coupling into the fiber 30, a focusing optics (not shown in detail) is typically provided between the wedge plate 35 and the fiber 30. The first radiation component 38 of the primary laser beam 34 (for the first laser beam 11) is coupled eccentrically into the outer ring fiber 32, and the second radiation component 37 of the primary laser beam 34 (for the second laser beam 12) is coupled centrally into the inner core fiber 31.

[0073] The fiber end 30a (cf. Fig. The second laser beam 12 coupled out of the core fiber 31 (see Figure 6, top right) exhibits a narrow, solid circular profile corresponding to the inner core fiber 31, as well as a comparatively high beam quality with a sharp focus. The first laser beam 11 coupled out of the ring fiber 32 exhibits a ring profile corresponding to the outer ring core with a homogeneous power distribution, as well as a comparatively lower beam quality, which is caused by the eccentric coupling.

[0074] In summary, the core fiber 31 has a narrower diameter than the ring fiber 32, whereby the second laser beam 12 intrinsically has a better beam quality than the first laser beam 11. Upon coupling out of the fiber end 30a, the first laser beam 11 is distributed over the entire ring fiber 32, i.e., around the core fiber 31 or the emerging first laser beam 11. The emerging laser beams 11, 12 can be focused onto the workpiece using a common optics (not shown in detail, but see Fig. 7).

[0075] In Fig. Figure 7 schematically shows part of a laser welding device 3 for the invention with a dual-core fiber 40. The dual-core fiber comprises two parallel optical fibers (laser fibers) 41, 42, which are typically arranged at a fixed distance from one another by a common cladding (not shown). The first laser beam 11 (shown in dashed lines) and the second laser beam 12 (shown in solid lines) emerge from the fiber ends 41a, 42a. These beams are directed and focused onto a workpiece 2 via a common optics 43, here comprising a collimation lens 44 and a focusing lens 45. The laser beams 11, 12 are focused here at the same location, in particular at the same depth (z-position) in the workpiece 2, see focus 14, but are not coaxial. The two optical fibers 41, 42 are made of an optical fiber 41 with a large diameter (e.g.440µm) for the first laser beam 11 with low beam quality and an optical fiber 42 with a small diameter (e.g., 110µm) for the second laser beam 12 with high beam quality. The laser beams 11, 12 can either be provided from different laser sources with different beam qualities or originate from a single, common laser source, wherein the beam quality of the first laser beam 11 is then degraded, for example, by eccentric coupling into the large-diameter optical fiber 41 or other known methods.

[0076] Note that with a common optic 43 as in Fig. 7 also shows the laser beams 11, 12 emerging from the fiber end 30a of a 2-in-1 fiber 30 (see Fig. 6), can be focused on or into a workpiece 2 (not shown in detail). Beam caustics

[0077] In Fig. Figure 8 shows typical caustics of laser beams 11, 12, as they can be used in the invention. The first laser beam 11 and the second laser beam 12 have approximately equal focusing angles (aperture angles) W1, W2. However, the radius R1 of the first laser beam 11 at the narrowest point (here at z=0, in the workpiece surface plane) is approximately 3 times as large as the radius R2 of the second laser beam 12 at the narrowest point. Accordingly, the first laser beam 11 has a beam parameter product SPP1 that is approximately 3 times larger than the second laser beam 12 with beam parameter product SPP2.

[0078] Fig.9 shows further caustics of laser beams 11, 12 as can be used in the invention. The first laser beam 11 here has a smaller focusing angle W1 than the second laser beam 12 with focusing angle (aperture angle) W2, with approximately W2=1.3*W1. The radius R1 of the first laser beam 11 at the narrowest point (here at z=0, in the workpiece surface plane) is approximately 5.8 times as large as the radius R2 of the second laser beam 12 at the narrowest point. The first laser beam 11 here has a beam parameter product SPP1 that is approximately 4.5 times larger than the second laser beam 12 with beam parameter product SPP2. List of reference symbols 1 melt bath 2 Workpiece 3 Laser welding device 4 beam axis 5 Focusing optics 6 steam capillary 7 Opening 8 first absorption / reflection (second laser beam) 9 upper quarter (vapor capillary) 11 first laser beam 12 second laser beam 13 Travel table 14 Focus 16 beam trap 20 beam splitter element 21 first laser source 22 optical fiber (laser fiber) 23 second beam source 24 optical fibers (laser fibers) 25 first collimating optics 26 second collimating optics 27 welding station 28 total beam 29 Focusing optics 30 2-in-1 fiber 30a fiber end (2-in-1 fiber) 31 core fiber 32 ring fibers 33 common laser source 34 original laser beam 35 wedge plate 36 Beam axis (original laser beam) 37 second radiation component (part of the original laser beam) 38 first radiation component (part of the original laser beam) 40 dual-core fiber 41 Optical fiber (laser fiber) 41a Fiber end 42 optical fiber (laser fiber) 42a Fiber end 43 common optics 44 Collimating lens 45 focusing lens B1 first beam width B2 second beam width G2 largest width of the second laser beam in the upper quarter K1 smallest width of the first laser beam in the upper quarter KB Capillary width OE workpiece surface level SR welding direction W1 first focusing angle W2 second focusing angle

Claims

[1] Method for deep welding a workpiece (2), wherein a surface of the workpiece (2) is irradiated with a first laser beam (11) and a second laser beam (12), wherein in a workpiece surface plane (OE) a first beam width B1 of the first laser beam (11) is greater than a second beam width B2 of the second laser beam (12), and wherein at least in the workpiece surface plane (OE) the second laser beam (12) lies within the first laser beam (11), characterized by , that the intensity of the first laser beam (11) alone is sufficient to create a vapor capillary (6) in the workpiece (2), and that the vapor capillary (6) generated in the workpiece (2) has a capillary width KB in the workpiece surface plane (OE), where KB is substantially the same size as B1, and furthermore B2 ≤ 0.75*KB. [2] Method according to claim 1, characterized bythat the first laser beam (11) and the second laser beam (12) have the same focus position or focus positions spaced apart by less than 1 mm with respect to a z-direction which is perpendicular to the surface of the workpiece (2). [3] Method according to one of the preceding claims, characterized by that the focus positions of the first and second laser beams (11, 12) lie in the workpiece surface plane (OE) or below the workpiece surface within the vapor capillary (6). [4] Method according to one of the preceding claims, characterized by that the second laser beam (12) has a better beam quality than the first laser beam (11). [5] Method according to claim 4, characterized by that SPP1 ≥ 2*SPP2, with SPP1: beam parameter product of the first laser beam (11), and SPP2: beam parameter product of the second laser beam (12). [6] Method according to one of the preceding claims, characterized bythat for a first average laser power L1 of the first laser beam (11) and a second average laser power L2 of the second laser beam (12) the following applies: L2 ≥ L1. [7] Method according to one of the preceding claims, characterized by that for a first focusing angle W1 of the first laser beam (11) and a second focusing angle W2 of the second laser beam (12) the following applies: W2 ≥ W1. [8] Method according to one of the preceding claims, characterized by that for a smallest width KD of the vapor capillary (6) in the upper quarter (9) of the vapor capillary (6) close to the workpiece surface and for a largest width G2 of the second laser beam (12) in the upper quarter (9) of the vapor capillary (6) the following applies: 0.8*KD ≥ G2. [9] Method according to one of the preceding claims, characterized by that the first laser beam (11) and the second laser beam (12) are aligned coaxially with a common beam axis (4). [10] Method according to claim 9, characterized by that the common beam axis (4) runs essentially perpendicular to the workpiece surface. [11] Method according to one of claims 1 to 10, characterized by that the first and the second laser beam (11, 12) are guided in a 2-in-1 fiber (30) with a core fiber (31) and a ring fiber (32), that the first and the second laser beam (11, 12) emerge from a fiber end (30a) of the 2-in-1 fiber (30) in front of the workpiece (2), and that the fiber end (30a) with ring fiber (32) and core fiber (31) is imaged onto or into the workpiece (2) by means of a common optics (43) in a common focus (14), whereby the first and the second laser beam (11, 12) are directed onto the workpiece (2) by the common optics (43). [12] Method according to claim 11, characterized by that the first laser beam (11) is guided in the ring fiber (32) and the second laser beam (12) in the core fiber (31). [13] Method according to claim 11 or 12, characterized by that a common laser source (33) generates a primary laser beam (34), wherein a first part (38) of the primary laser beam (34) is coupled eccentrically into the ring fiber (32) and forms the first laser beam (11), and a second part (37) of the primary laser beam (34) is coupled into the core fiber (31) and forms the second laser beam (12). [14] Method according to one of claims 1 to 10, characterized by that a first laser source (21) generates the first laser beam (11) and a second laser source (23) generates the second laser beam (12), and that the first laser beam (11) and the second laser beam (12) are superimposed by means of a beam splitter element (20). [15] Method according to claim 14, characterized by that the superimposed laser beams (11, 12) are directed onto the workpiece (2) using a common focusing optics (29). [16] Method according to one of claims 1 to 8, characterized bythat the first and the second laser beam (11, 12) are guided in a dual-core fiber (40) with two adjacent, mutually parallel laser fibers (41, 42), that the first and the second laser beam (11, 12) emerge from the fiber ends (41a, 42a) of the dual-core fiber (40) in front of the workpiece (2), and that the fiber ends (41a, 42a) of the laser fibers (41, 42) are imaged via a common optics (43) in a common focus (14) on or in the workpiece (2), whereby the first and the second laser beam (11, 12) are directed by the common optics (43) onto the workpiece (2). [17] Laser welding device (3) designed to carry out a method according to one of the preceding claims.

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