METHODS AND LASER WELDING DEVICES FOR DEEP WELDING A WORKPIECE

Abstract

The disclosure relates to methods and systems for deep welding a workpiece, a surface of the workpiece being irradiated by a first laser beam and a second laser beam. In a workpiece surface plane (OE) a first beam width B1 of the first laser beam is larger than a second beam width B2 of the second laser beam and in at least the workpiece surface plane (OE) the second laser beam lies inside the first laser beam. The intensity of the first laser beam alone is sufficient to produce a keyhole in the workpiece. The keyhole produced in the workpiece has a width KB in the workpiece surface plane (OE), KB substantially equaling B1, and B2≤0.75*KB. The methods and systems provide good seam quality, high penetration depth, and high welding speed.

Claims

1. (canceled)

2. A method for deep welding a workpiece, the method comprising: irradiating a surface of the workpiece using a first laser beam having a first laser power L1 and a first beam width B1 and a second laser beam having a second laser power L2 and a second beam width B2; wherein L1 is at least 1 kW; wherein B2<B1; wherein the second laser beam is directed onto an area of the workpiece surface plane within an area of the workpiece surface plane onto which the first laser beam is directed, and wherein a beam parameter product (BPP) of the first laser beam (BPP1) is greater than a beam parameter product of the second laser beam (BPP2), wherein a high BPP represents a low beam quality; guiding the first and second laser beams in a multicore fiber having at least one core fiber and a ring fiber, wherein the first laser beam is guided in the ring fiber and the second laser beam is guided in the core fiber, and the first and second laser beams exit from a fiber end of the multicore fiber before the workpiece; directing the first and second laser beams by a common optical unit onto the workpiece, wherein the common optical unit comprises a collimation lens and a focusing lens; and imaging the fiber end with the common optical unit in a common focus on or in the workpiece.

3. The method of claim 2, wherein the first laser beam and the second laser beam have a same focal position or have focal positions spaced apart from one another by less than 1 mm in a direction perpendicular to the workpiece surface plane.

4. The method of claim 2, wherein the method is used to produce a keyhole in the workpiece, and wherein focal positions of the first and second laser beams are located in the workpiece surface plane or below the workpiece surface inside a keyhole.

5. The method of claim 2, wherein B2≤0.75*B1.

6. The method of claim 2, wherein B2≤0.5*B1.

7. The method of claim 2, wherein BPP1≥2*BPP2.

8. The method of claim 2, wherein BPP1≥4*BPP2.

9. The method of claim 2, wherein L1 is less than or equal to L2.

10. The method of claim 2, wherein L2≥2*L1.

11. The method of claim 2, wherein L1 is greater than L2.

12. The method of claim 2, wherein L1 and L2 are each 2.5 kW.

13. The method of claim 2, wherein a welding speed feed rate is from 1 to 10 meters per minute.

14. The method of claim 2, wherein the first laser beam has a first focus diameter of about 300 microns to about 440 microns, and the second laser beam has a second focus diameter of about 110 microns to about 150 microns.

15. The method of claim 2, further comprising aligning the first laser beam and the second laser beam coaxially to have a common beam axis.

16. The method of claim 15, wherein the common beam axis extends substantially perpendicularly to the workpiece surface.

17. The method of claim 2, further comprising generating an original laser beam from a common laser source, coupling a first part of the original laser beam into the ring fiber to form the first laser beam, and coupling a second part of the original laser beam into the core fiber to form the second laser beam.

18. The method of claim 2, further comprising generating the first laser beam with a first laser source, generating the second laser beam with a second laser source, and superimposing the first laser beam and the second laser beam with a beam splitter element.

19. The method of claim 18, further comprising directing the superimposed laser beams onto the workpiece using a common focusing optical unit.

20. A laser welding device for deep welding a workpiece, comprising: one or more laser sources that generate a first laser beam having a first laser power L1 and a first beam width B1 and a second laser beam having a second laser power L2 and a second beam width B2, wherein L1 is at least 1 kW, B1 is greater than B2, and a beam parameter product (BPP) of the first laser beam (BPP1) is greater than a beam parameter product of the second laser beam (BPP2), wherein a high BPP represents a low beam quality; a multicore fiber having at least one core fiber and a ring fiber, wherein the first laser beam is guided in the ring fiber and the second laser beam is guided in the core fiber, and wherein the first and second laser beams exit from a fiber end of the multicore fiber before a workpiece; and a common optical unit arranged to direct the first laser beam and the second laser beam in a common focus onto or into the workpiece.

21. The system of claim 20, wherein the common optical unit comprises a collimation lens and a focusing lens.

Description

DESCRIPTION OF DRAWINGS

[0041] The invention is illustrated in the drawings and will be explained in greater detail on the basis of exemplary embodiments.

[0042] FIG. 1 shows a schematic longitudinal section through a part of a laser welding device and a workpiece.

[0043] FIG. 2 shows experimental polished cross-sections through a weld seam (at the top) and top views of a weld seam (at the bottom) for deep welding using a) a laser beam of high beam quality, b) two laser beams, and c) a laser beam of low beam quality, at low welding speed.

[0044] FIG. 3 shows experimental side views of the weld pool during welding (at the top), top views of the weld seam (in the middle), and cross sections through the weld seam (at the bottom) using a) two laser beams, and b) one laser beam, at high welding speed.

[0045] FIG. 4 shows experimental diagonal views of the weld pool during welding using a) one laser beam and b) two laser beams.

[0046] FIG. 5 shows a schematic view of a first embodiment of a laser welding device in twin-spot technology using a beam splitter element.

[0047] FIG. 6 shows a schematic longitudinal sectional view through a two-in-one fiber (top), and an associated beam generating construction (bottom). FIG. 7 shows a schematic view of a dual-core fiber arrangement.

[0048] FIG. 8 shows a schematic illustration of the beam caustics of first and second laser beams having approximately equal aperture angles.

[0049] FIG. 9 shows a schematic illustration of the beam caustics of first and second laser beams having unequal aperture angles.

DETAILED DESCRIPTION

[0050] In deep laser beam welding, a vapor capillary (keyhole) is moved through a liquid weld pool by the relative movement between laser beam and workpiece. However, at excessively high feed rates, a plurality of weld spatters detach and the quality of the weld seam is reduced due to the formation of notches.

[0051] In conventional deep laser beam welding, the largest part of the laser energy is absorbed in the upper region of the keyhole. The laser beam is first incident on the workpiece in this region. The focal plane is usually also positioned here. The location of the greatest power density is thus located here.

[0052] The melt flow dynamics increase with rising absorbed energy, and upon reaching critical values, the above-mentioned negative effects arise, causing a mass loss in the seam and/or pronounced cyclic melt flow behavior.

[0053] If a laser beam having very good beam quality is used, the resulting welding penetration depth is increased (with equal introduction of energy) in comparison to a laser of low beam quality. However, the limit of good weld seam quality shifts toward lower feed rates. A particularly high surface-proximal absorbed power density may be the cause of this. Moreover, if a laser beam having high beam quality is used, a narrower keyhole arises, which threatens to collapse faster at high speeds than a wider keyhole generated by a laser beam of lower beam quality.

[0054] As described herein, in the new laser systems and methods, the energy injection is intentionally displaced into lower regions of the keyhole in parts, so that high welding penetration depths are achieved with good seam quality for the largest possible speed range.

[0055] FIG. 1 shows a schematic longitudinal section through the region of the weld pool 1 in a workpiece 2, which is welded using a variant of the deep welding method using a laser welding device 3 as described herein (only partially shown in FIG. 1). The (relative) welding direction SR is directed to the left here, for example, by the workpiece 2 being moved to the right (with otherwise fixed laser welding device 3).

[0056] A first laser beam 11 and a second laser beam 12 are guided coaxially along a common beam axis 4 and are focused by a focusing optical unit 5 of the laser welding device 3 on the workpiece 2, for example, to weld two workpiece parts, the contact surface of which is located in the plane of the drawing of FIG. 1, to form the workpiece 2. The beam axis 4 extends perpendicularly to the workpiece surface plane OE in the illustrated example. Both laser beams 11, 12 have an essentially rotationally-symmetrical (“circular”) intensity profile here (in relation 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.

[0057] In the example shown, the two laser beams 11, 12 have the focus thereof in the workpiece surface plane OE, so that the laser beams 11, 12 each have the smallest diameter thereof (“waist”) in this location. The diameter (FWHM) of the first laser beam 11 in the workpiece surface plane OE, denoted first beam width B1, is significantly greater than the diameter (FWHM) of the second laser beam 12 in the workpiece surface plane OE, denoted second beam width B2, where approximately B2=0.25*B1 here. Overall, the caustic of the second laser beam 12 is smaller than the caustic of the first laser beam 11.

[0058] The first laser beam 11 alone has sufficient intensity to produce a keyhole 6 in the workpiece 2. The capillary width KB of the keyhole 6 in the workpiece surface plane OE is accordingly approximately as large as the first beam width B1; in the example shown, KB is slightly larger than B1, where approximately KB=1.15*B1. Molten material (molten workpiece material) is arranged around the keyhole 6, wherein the largest part of the weld pool 1 is located behind the keyhole 6 (in relation to the welding direction SR).

[0059] 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 the opening 7 of the keyhole 6 without being incident on the molten material of the weld pool 1. In the example shown, the first partial absorption 8 of the second laser beam 12 at the boundary surface of the keyhole 6 first occurs in a middle third (in relation to the direction z into the workpiece 2) of the keyhole 6, and the unabsorbed part of the radiation is reflected, see the reflected second laser beam 12 shown by dashed lines. Accordingly, energy is hardly introduced into the weld pool 1 by the second laser beam 12 close to the surface. It is to be noted that the first laser beam 11 is partially absorbed and partially reflected for the first time already significantly farther above, see the reflected first laser beam 11 shown by dotted lines.

[0060] The greatest width (diameter) G2 of the second laser beam 12 (at the lower edge of the upper quarter 9) and the smallest width (diameter) KD of the keyhole 6 (also at the lower edge of the upper quarter 9) can be determined in the upper quarter 9 of the keyhole 6. The smallest width (diameter) K1 of the first laser beam 11 (at the upper edge of the quarter 9 in the workpiece surface plane) can also be determined. In the example shown, approximately G2=0.46*KD applies, and approximately G2=0.5*K1. Because G2 is significantly less than KD or K1, respectively, the injection of the energy of the second laser beam 12 can be conveyed further above all in the lower region of the keyhole 6, and especially not in the upper quarter 9, into the weld pool 1.

[0061] Because of the energy injection of the second laser beam 12 into the lower-lying region of the keyhole 6, the weld pool dynamics can be reduced, e.g., close to the surface. Turbulence is prevented or reduced. The detachment of weld spatters is thus decreased and the waviness of the resulting weld seam is reduced.

[0062] In the embodiment shown, the beam parameter product BPP1 of the first laser beam is approximately 2.7 times as large as the beam parameter product BPP2 of the second laser beam 12, i.e., the second laser beam 12 has the better beam quality. A multimode laser beam is typically selected as the first laser beam (having lower beam quality). The beam parameter products BPP1, BPP2 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 (thus half of the beam widths B1, B2). In the example shown, W1<W2, with approximately W1=0.68*W2.

[0063] To produce a high-quality weld seam at high feed rate (welding speed), the first laser beam 11 initiates the formation of the vapor capillary (keyhole) 6 with sufficiently large diameter, e.g., of the upper opening 7. At the same time, the second laser beam 12 is directed or focused (coaxially) into the keyhole 6 formed, wherein 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 first absorbed/coupled in a lower zone of the keyhole 6. The absorbed energy in the upper region of the keyhole is reduced in relation to the prior art and increased in the lower region; as a result, the welding penetration depth is increased, wherein high feed rates are nonetheless possible.

[0064] For the images of FIG. 2, workpieces made of structural steel were subjected to blind welding, with a) a laser beam having BPP=2 mm*mrad, focus diameter 150 μm, laser source TruDisk® 5000.5, with b) two coaxial laser beams as described herein, with BPP=2 mm*mrad and BPP=8 mm*mrad, focus diameter 150 μm and 300 μm, laser source TruDisk® 5000.5 and TruDisk® 16002, and with c) a laser beam with BPP=8 mm*mrad, focus diameter 300 μm, laser source TruDisk® 16002. In all cases, the feed rate was 1 m/min and the total laser power was P=2 kW, in case b) with 1 kW per laser beam. The focal position was at −4 mm in the workpiece in each case.

[0065] As is apparent from the respective top cross-sectional images of FIG. 2, it was possible to achieve a welding penetration depth of 6.1 mm using one laser beam having BPP=2 mm*mrad, and only a welding penetration depth of 4.0 mm at equal laser power using one laser beam having BPP=8 mm*mrad. By combining two lasers having BPP=2 mm*mrad and BPP=8 mm*mrad, with overall equal power, a welding penetration depth of 4.6 mm could be achieved which is significantly more than using the one laser beam having BPP=8 mm*mrad.

[0066] At the comparatively low feed rate of 1 m/min, an acceptable seam quality was achieved in all workpieces, see the respective lower top view images.

[0067] If a higher feed rate is selected, the seam quality is influenced more strongly. For FIG. 3, workpieces made of structural steel were again welded, on the one hand, with a) two coaxial laser beams as described herein, having BPP=12 mm*mrad and BPP=3 mm*mrad and, on the other hand, with b) one laser beam, having BPP=8 mm*mrad. The total laser power was 5 kW in each case (in the case of two laser beams, 2.5 kW each), the focal position was −2 mm, and the feed rate was 8 m/min.

[0068] It is apparent from the side views recorded during the welding process shown at the top in each case that when using two laser beams a) a calm weld pool is obtained, while substantial ejection of molten material droplets (welding spatters) occurs using the single laser beam b). The seam quality is very good upon use of the two laser beams a), as is apparent from the top views in the middle of FIG. 3, while an uneven weld seam with numerous beads is obtained if only one laser beam b) is used. It is also apparent in the cross-sectional views at the bottom in FIG. 3 that in b) the weld seam forms a noticeable depression as a result of material loss and has uneven surface beads, while in a) no depression and a symmetrical, central weld bead are recognizable.

[0069] Larger image details during the welding of workpieces made of structural steel are shown in a diagonal view in each case in FIG. 4, in a) using one laser beam, focus diameter 440 μm, and in b) using two laser beams as described herein, focus diameters 110 μm and 440 μm. The feed rate was 10 m/min, the total laser power was 5 kW (in the case of two laser beams, 2.5 kW each), and the focal position was −2 mm, in each case.

[0070] If only the single laser beam a) is used, a substantial tail-like ejection of weld spatters occurs to the rear, while if the two laser beams b) are used, only a minimal ejection of weld spatters is observed.

[0071] An embodiment of a laser welding device 3 as described herein is shown in a schematic drawing in FIG. 5. The illustrated laser welding device 3 operates using the so-called twin-spot technology having a beam splitter element 20.

[0072] The laser welding device 3 comprises a first laser source 21 that generates a first laser beam 11 typically having 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 generates a second laser beam 12 typically having 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 optical unit 25 and directed onto the beam splitter element 20 (typically a semitransparent mirror). The second laser beam 12 is collimated (parallelized) via a second collimator optical unit 26 and also directed onto the beam splitter element 20. The second laser beam 12 is rotated by 90° in relation to the first laser beam 11, and the beam splitter element 20 is arranged at a 45° angle in relation to the laser beams 11, 12.

[0073] A so-called beam dump 16 is arranged in extension of the beam axis of the second laser beam 12.

[0074] The welding station 27 is configured in the extension of the beam axis of the first laser beam 11. The overall beam 28 made of components of first laser beam 11 and second laser beam 12 after the beam splitter element 20 is focused using a common focusing optical unit 29 for the first and second laser beams 11, 12 and directed onto a workpiece 2 to be welded. The (common) focus 14 is located in the workpiece surface plane OE.

[0075] The workpiece 2 is arranged on a positioning table 13, using which the workpiece 2 can be moved along the beam axis 4 (z direction) of the overall beam 28, e.g., to set the focal position, and also in the directions perpendicular thereto (x direction and y direction), to travel down a desired weld seam.

[0076] The top of FIG. 6 shows a schematic longitudinal section of a two-in-one fiber 30, which can be used as part of laser welding devices as described herein.

[0077] The two-in-one fiber 30 comprises a central core fiber 31 and a surrounding ring fiber 32, in each of which laser light can be guided. 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 at the top left). During the coupling, the first and second laser beams 11, 12 each have a circular cross section (see the cross-sectional view of the two-in-one fiber 30 in the top middle).

[0078] 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 that generates an original laser beam 34, from which a first radiation component 38 and a second radiation component 37 are generated using a wedge plate 35, for example (shown at the bottom in FIG. 6). The farther the wedge plate 35 is moved into the beam path of the original laser beam 34, the larger the radiation component 38, which is deflected thereby, becomes. That is, the components of the distribution onto the two laser beams 11, 12 are controllable.

[0079] Depending on which of the fibers 31, 32 the respective generated laser beam 11, 12 is coupled into, it exits having different laser beam characteristics at the fiber end 30a. For the coupling into the fiber 30, a focusing optical unit (not shown in greater detail) is typically provided between the wedge plate 35 and the fiber 30. The first radiation component 38 of the original laser beam 34 (for the first laser beam 11) is eccentrically coupled into the outer ring fiber 32, and the second radiation component 37 of the original laser beam 34 (for the second laser beam 12) is centrically coupled into the inner core fiber 31.

[0080] The second laser beam 12 from the core fiber 31, which is decoupled at the fiber end 30a (see top right in FIG. 6), has a narrow filled circular profile corresponding to the inner core fiber 31, and also a comparatively high beam quality having a sharp focus. The decoupled first laser beam 11 from the ring fiber 32 has a ring profile corresponding to the outer ring core having homogeneous power distribution, and also a comparatively lower beam quality caused by the eccentric coupling.

[0081] In summary, the core fiber 31 has a narrower diameter than the ring fiber 32, whereby the second laser beam 12 intrinsically receives a better beam quality than the first laser beam 11. Upon decoupling from the fiber end 30a, the first laser beam 11 is distributed over the entire ring fiber 32, that is, around the core fiber 31 or the exiting first laser beam 11, respectively. The exiting laser beams 11, 12 can be focused using a common optical unit on the workpiece (not shown in greater detail, but see FIG. 7 in this regard).

[0082] FIG. 7 schematically shows a part of a laser welding device 3 having a dual-core fiber 40. The dual-core fiber comprises two optical fibers (laser fibers) 41, 42 extending in parallel to one another and typically arranged at a fixed spacing from one another by a common cladding (not shown). The first laser beam 11 (shown by dashed lines) and the second laser beam 12 (shown by solid lines) exit at the fiber ends 41a, 42a and are directed and focused via a common optical unit 43, comprising a collimation lens 44 and a focusing lens 45, onto a workpiece 2. The laser beams 11, 12 are focused at the same location, e.g., at the same depth (z position) in the workpiece 2, see focus 14, but do not extend coaxially. The two optical fibers 41, 42 are formed from an optical fiber 41 having large diameter (for example, 440 μm) for the first laser beam 11 having low beam quality and an optical fiber 42 having a smaller diameter (for example, 110 μm) for the second laser beam 12 having high beam quality. The laser beams 11, 12 can be provided either from different laser sources having different beam qualities, or can originate from a single, common laser source, wherein the beam quality of the first laser beam 11 is then worsened, for example, by eccentric coupling into the optical fiber 41 having large diameter or other known methods.

[0083] It is to be noted that using a common optical unit 43 as shown in FIG. 7, the laser beams 11, 12 which exit from a fiber end 30a of the two-in-one fiber 30 (see FIG. 6) can be focused on or in a workpiece 2 (not shown here).

[0084] Typical caustics of laser beams 11, 12 are shown in FIG. 8. 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 (at z=0, in the workpiece surface plane) is approximately three times as large as the radius R2 of the second laser beam 12 at the narrowest point. The first laser beam 11 accordingly has an approximately three times greater beam parameter product BPP1 than the second laser beam 12 having beam parameter product BPP2.

[0085] FIG. 9 shows further caustics of laser beams 11, 12 that can be used. The first laser beam 11 has a smaller focusing angle W1 than the second laser beam 12 having focusing angle (aperture angle) W2, with approximately W2=1.3*W1. The radius R1 of the first laser beam 11 at the narrowest point (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 has an approximately 4.5 times greater beam parameter product BPP1 than the second laser beam 12 having beam parameter product BPP2.

OTHER EMBODIMENTS

[0086] A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.

LIST OF REFERENCE NUMERALS

[0087] 1 weld pool [0088] 2 workpiece [0089] 3 laser welding device [0090] 4 beam axis [0091] 5 focusing optical unit [0092] 6 vapor capillary (keyhole) [0093] 7 opening [0094] 8 first absorption/reflection (second laser beam) [0095] 9 top quarter (keyhole) [0096] 11 first laser beam [0097] 12 second laser beam [0098] 13 positioning table [0099] 14 focus [0100] 16 beam dump [0101] 20 beam splitter element [0102] 21 first laser source [0103] 22 optical fiber (laser fiber) [0104] 23 second beam source [0105] 24 optical fiber (laser fiber) [0106] 25 first collimating optical unit [0107] 26 second collimating optical unit [0108] 27 welding station [0109] 28 overall beam [0110] 29 focusing optical unit [0111] 30 two-in-one fiber [0112] 30a fiber end (two-in-one fiber) [0113] 31 core fiber [0114] 32 ring fiber [0115] 33 common laser source [0116] 34 original laser beam [0117] 35 wedge plate [0118] 36 beam axis (original laser beam) [0119] 37 second radiation component (part of the original laser beam) [0120] 38 first radiation component (part of the original laser beam) [0121] 40 dual-core fiber [0122] 41 optical fiber (laser fiber) [0123] 41a fiber end [0124] 42 optical fiber (laser fiber) [0125] 42a fiber end [0126] 43 common optical unit [0127] 44 collimating lens [0128] 45 focusing lens [0129] B1 first beam width [0130] B2 second beam width [0131] G2 greatest width of the second laser beam in the top quarter [0132] K1 smallest width of the first laser beam in the top quarter [0133] KB capillary width [0134] OE workpiece surface plane [0135] SR welding direction [0136] W1 first focusing angle [0137] W2 second focusing angle