LASER WELDING FOR CORNER JOINS OF WORKPIECE PARTS

A method for laser welding of a workpiece includes welding at a corner joint of two workpiece parts of the workpiece by a welding laser beam to create an aluminum connection between the two workpiece parts, and feeding an output laser beam into a first end of a multiclad fiber to generate the welding laser beam. The multiclad fiber comprises at least a core fiber and a ring fiber surrounding the core fiber. A first portion LK of a laser power output of the output laser beam is fed into the core fiber, and a second portion LR of the laser power output of the output laser beam is fed into the ring fiber. A second end of the multiclad fiber is reproduced on the workpiece. The method further includes welding the workpiece by deep welding.

FIELD

Embodiments of the present invention relate to a laser welding method for corner joins of workpiece parts.

BACKGROUND

US 2017/0334021 A1 discloses a laser welding system used in the production of electronic devices such as batteries, comprising a laser source for generating a laser beam having a beam profile. To modify the beam profile, the laser welding system comprises beam-shaping means, for example optical elements for diffraction of the laser beam, and shielding components, which make it possible to shield at least part of the laser beam. The targeted beam shaping is intended to bring about a reduction in the power of the laser beam that is required for the welding and in undesired side effects.

DE 10 2010 003 750 A1 discloses a method for modifying the profile of a laser beam. The laser beam is coupled into the one fiber end of a multiclad fiber and is coupled out of the other end of the multiclad fiber. In the process, the incident laser beam is coupled at least into the inner fiber core of the multiclad fiber and/or into an outer ring core of the multiclad fiber. This brings about a modification of the profile of the laser beam after it is coupled out compared to the laser beam before it is coupled in.

In the case of welding together corner joins of workpieces by methods previously used for this, instabilities, for example in the form of pores, may be produced in the workpiece and eject spatters of the molten material.

SUMMARY

Embodiments of the present invention provide a method for laser welding of a workpiece. The method includes welding at a corner joint of two workpiece parts of the workpiece by a welding laser beam to create an aluminum connection between the two workpiece parts, and feeding an output laser beam into a first end of a multiclad fiber to generate the welding laser beam. The multiclad fiber comprises at least a core fiber and a ring fiber surrounding the core fiber. A first portion LK of a laser power output of the output laser beam is fed into the core fiber, and a second portion LR of the laser power output of the output laser beam is fed into the ring fiber. A second end of the multiclad fiber is reproduced on the workpiece. The method further includes welding the workpiece by deep welding.

DETAILED DESCRIPTION

Embodiments of the present invention provide a laser welding method for stable formation of corner joins of workpiece parts without the production of spatters of the metal melt, as are valued for battery housings.

Embodiments of the present invention provide a method for laser welding a workpiece, with a plain butt joint weld being created by welding at the corner joint of two workpiece parts of the workpiece by means of a welding laser beam, as a result of which an aluminum connection is created between the workpiece parts, with, to generate the welding laser beam, an output laser beam being fed into a first end of a multiclad fiber, in particular a 2-in-1 fiber, with the multiclad fiber comprising at least a core fiber and a ring fiber surrounding the latter, with a first portion LK of the laser power output of the output laser beam being fed into the core fiber and a second portion LR of the laser power output of the output laser beam being fed into the ring fiber, with a second end of the multiclad fiber being reproduced on the workpiece, and with the workpiece being laser welded by deep welding.

The welding method according to embodiments of the present invention, in particular the combination according to embodiments of the present invention of workpiece geometry, workpiece material, beam shaping at the laser beam and procedure, brings about stable weld connections combined with a reduction in spatters. The weld in the form of a plain butt joint weld is distinguished by a small notch effect and an undisrupted force flow through the weld. This results in high stability of the plain butt joint weld. Aluminum as a material has a comparatively low weight along with high strength and durability, and therefore the stability of the welded connection is also increased as a result. The deep welding obtains high welding-in depths. The laser beam exiting the multiclad fiber has a beam cross section having a core beam, which is emitted by the core fiber, and a ring beam, which is emitted by the ring fiber. This minimizes spatter formation when deep welding the plain butt joint weld in the aluminum material. Moreover, a weld is created that has a smooth weld upper bead and high gas tightness, this having proved well suitable for the manufacture of battery housings.

At a corner joint, in particular two workpiece parts bear against one another by way of their ends at an angle, preferably at a right angle or an approximately right angle of 75° to 105°. When there is a plain butt joint weld at the corner joint, the workpiece parts are in particular arranged in such a way that an extension of the longitudinal axis of a first workpiece part passes through an end of a second workpiece part, with the plain butt joint weld extending over the entire width of the first workpiece part transversely, in particular perpendicularly, with respect to the longitudinal axis.

The abutting area of the workpiece parts is in particular parallel or virtually parallel to the beam direction (beam propagation direction) of the welding laser beam. In particular, the abutting area of the workpiece parts is aligned at a maximum angle of 15° to −15°, preferably 5° to −5°, in relation to the beam direction of the laser beam. Typically, one of the workpiece parts extends perpendicularly away from the abutting area, and one of the workpiece parts extends parallel to the abutting area. At the entrance side of the welding laser beam on the workpiece, the workpiece parts are in line typically with respect to the beam direction. The workpiece parts consist substantially of aluminum and may comprise a plastics coating, for instance for electrical insulation purposes.

In the case of the welding method according to embodiments of the present invention, in the deep welding mode lasers with a comparatively high power output density are used, which results in the laser creating vapor during the welding. The vapor displaces the melt produced during the welding. This results in the formation of a deep, vapor-filled hole, the vapor capillary. The metal melt flows around the vapor capillary and solidifies on the rear side.

In the course of laser welding without a multiclad fiber for generating the laser beam, an excess pressure often builds up in vapor capillaries in the workpiece, this resulting in protuberances of the vapor capillaries. These protuberances increase in size and open explosively, melt being ejected in the form of spatters. The fluctuations of the metal melt on the side of the vapor capillaries that faces the laser beam also often result in spatters of the metal melt. Sharp edges, which prevent the flow of the metal melt and therefore promote the occurrence of spatters, can form in the vapor capillaries. The protuberances may moreover result in pores in the workpiece.

The multiclad fiber used according to embodiments of the present invention for the purpose of beam shaping has at least a core fiber (solid-profile fiber) and a ring fiber (hollow-profile fiber), which surrounds the core fiber. The ring fiber is in particular in the form of a peripherally closed fiber with a recess. The core fiber and the ring fiber may have any desired cross-sectional profiles, for example in a square shape. The core fiber and the ring fiber preferably have a cross section in the shape of a circle or a circular ring. The multiclad fiber is preferably in the form of a 2-in-1 fiber having the core fiber and a ring fiber. The laser beam exiting the multiclad fiber has a beam cross section having a core beam, which is emitted by the core fiber, and a ring beam, which is emitted by the ring fiber. The intensities of the core beam and the ring beam are determined by the first portion LK and the second portion LR, respectively, of the laser power output of the output laser beam that is fed in.

The beam profile of the welding laser beam is modified in comparison with the output laser beam to the effect that the interaction of a determined ring intensity with a determined core intensity modifies the coupling of energy into the workpiece such that the manifestation of the vapor capillary and the weld pool dynamics are influenced. In this way, welding with beam shaping, in particular low-spatter deep welding, with very high advancement rates and weld upper bead quality is made possible, as is the case in heat conduction welding.

The ring beam may in particular have the effect that the opening of the vapor capillary on that side of the workpiece that is irradiated by the laser beam is increased and the emergence of gases from the vapor capillary is facilitated. The ring intensity thus further opens the vapor capillary in the upper part, with the result that the metal vapor can flow out without obstruction or virtually without obstruction. This largely suppresses the formation of protuberances in the vapor capillary and the production of spatters. Spatter formation is minimized, since the pressure of the gas in the vapor capillary and a corresponding action on the weld pool are reduced. The ring beam additionally transmits a pulse from above (in the propagation direction of the laser beam) into the weld pool, the direction of which pulse is counter to the acceleration of molten material on the rear side of the vapor capillary and as a result also reduces spatter formation. Fluctuations favoring the production of spatters are suppressed by the ring beam. The heat conduction in the weld results in a further expansion of the weld. A weld which has a smooth weld upper bead (comparable with heat conduction weldings) and high gas tightness is created.

By means of high-speed recordings, the inventors have observed that n it is possible to achieve a reduction in spatter formation by up to 90% in comparison with the prior art (without the beam shaping according to embodiments of the present invention). The inventors have additionally observed this significant reduction in spatter formation at advancement speeds that were higher by a factor of 7.5 (approx. 30 m/min) than in the case of the prior art (approx. 4 m/min). The inventors have also established that the use of the technology according to embodiments of the present invention can achieve significantly smoother weld upper beads than in the case of welds that were welded using other welding methods.

The method according to embodiments of the present invention is suitable for the production of stable corner joins in battery housings with a low risk of short circuiting and high gas tightness by virtue of the use of the plain butt joint weld at the corner joint, the use of aluminum and the deep welding with a multiclad fiber.

In an embodiment of the method according to embodiments of the present invention, the first portion LK of the laser power output for the core fiber and the second portion LR of the laser power output for the ring fiber are selected with 0.15≤LK/(LK+LR)≤0.50, preferably 0.25≤LK/(LK+LR)≤0.45, particularly preferably LK/(LK+LR)=0.35.

These respective fractions of the laser power output for the core fiber and the ring fiber effect a welding process with a large penetration depth combined with avoidance of spatters and have proven suitable in the manufacture of battery housings. In the case of a lower fraction of the laser power output for the core fiber, the fraction of the laser power output for the ring fiber dominates, with the result that the laser welding process is again comparable with the case of laser welding using a homogeneous fiber. This also applies for a larger fraction of the laser power output for the core fiber as specified above, it then being the case that the fraction of the laser power output for the core fiber dominates in comparison with the fraction for the ring fiber.

An embodiment in which the laser welding is effected at an advancement speed v with v≥7 m/min, in particular with v≥10 m/min, preferably v≥20 m/min, particularly preferably v≥30 m/min, is preferred. These advancement speeds can be readily realized with low spatter in the case of typical laser power outputs of 2 to 6 kW, a wavelength of 1030 nm, along with a typical workpiece thickness at the joint (of the workpiece part that is smaller in the beam direction) of 0.5 mm-2 mm.

Further preferable is an embodiment in which the second end of the multiclad fiber is reproduced on the workpiece enlarged by an enlargement factor VF, with VF>1.0, in particular with VF≥1.5, preferably VF≥2.0. With such an enlargement, it is possible to obtain a comparatively small divergence angle of the laser beam; the reflection of the laser beam at the workpiece is minimized. A small divergence angle also makes it possible to better avoid the combustion of insulating material. The welding process can be carried out with a greater tolerance in terms of the distance of the focus of the welding laser beam from the surface of the workpiece.

Likewise preferred is an embodiment in which the output laser beam is generated by means of a solid-state laser, in particular a disk laser. Solid-state lasers are cost effective and have proven successful. Disk lasers are distinguished by good possible ways of cooling the laser crystal during operation, this having a positive effect on the focusability of the laser beam.

Also preferred is an embodiment in which the multiclad fiber is selected such that, for a diameter DK of the core fiber and a diameter DR of the ring fiber, the following holds true: 2.5≤DR/DK≤6, preferably 3≤DR/DK≤5. Typically, for the core fiber it holds true that 50 μm≤DK≤250 μm or 100 μm≤DK≤200 μm. Typically, for the ring fiber it furthermore holds true that 100 μm≤DR≤1000 μm or 150 μm≤DK≤900 μm or 150 μm≤DR≤500 μm. With these diameter ratios, it is possible to carry out the welding process with comparatively short process times.

Furthermore advantageous is an embodiment in which the welding laser beam with its focus in the beam propagation direction has a maximum height offset MHO with respect to the surface of the workpiece, with |MHO|≤1.5 mm, preferably |MHO|≤1.0 mm, particularly preferably |MHO|≤0.5 mm. Within this range of the height offset, a local annular minimum of the intensity distribution manifests between the core beam and the ring beam. In particular, this has a positive effect on the prevention of spatters during the welding process.

Likewise advantageous is an embodiment in which the welding laser beam has a maximum lateral offset MLO on the workpiece with respect to an abutting area of the workpiece parts, with |MLO|≤0.2 mm, preferably |MLO|≤0.1 mm. Within this range of the lateral offset MLO, a comparatively smooth weld upper bead, which is in particular round over the resulting corner, is obtained during the welding process.

An embodiment in which the workpiece parts are clamped to one another over their surface area during the laser welding is advantageous, with a maximum gap width MS between the workpiece parts being maintained, with MS≤0.1 mm. At these gap widths, a homogeneous distribution, with few pores, of the material in the weld is created during the welding.

Additionally preferred is a variant in which the workpiece parts at the corner joint in the beam propagation direction of the welding laser beam are arranged in relation to one another in line or with a step having a step height SH,

with SH≤0.3 mm,
preferably SH≤0.2 mm,
particularly preferably SH≤0.1 mm. Such a maximum step height has made it possible to manufacture welds of good quality.

Embodiments of the present invention also includes the use of the method according to one of the preceding embodiments for manufacturing a battery housing, the workpiece parts being parts of the battery housing, in particular one of the workpieces being a cap which closes off the battery housing. The parts of the battery housing can be connected by the method in a particularly stable and quick manner without spatters. The manufactured housings are reliably tight, in particular gastight.

Embodiments of the present invention relate to the creation of a plain butt joint weld at the corner joint with aluminum connections, a joining situation as is typically present in battery housings (what are known as “can caps”). When battery cell housings are being laser welded, on account of the high advancement speeds required during the welding process it is often possible for weld spatters and non-uniform weld upper beads to be produced. Embodiments of the present invention, which provides a beam profile created by means of a multiclad fiber, smooths the weld and minimizes spatter formation, this resulting in a lower risk of short circuiting and higher tightness. As a result, an increase which is reliable in terms of the production in the advancement speeds compared to the prior art is clearly possible.

FIG. 1ashows a workpiece1having a first corner joint2a, which comprises a first workpiece part3aand a second workpiece part3b. The first workpiece part3ain this instance has a larger width B1than a width B2of the second workpiece part3b. The workpiece parts3a,3bbear against one another by way of a right-angled inner corner4. A blunt end5aof the first workpiece part3ais arranged at an end6aof the second workpiece part3b. The extension of the longitudinal axis7aof the first workpiece part3apasses through the end6aof the second workpiece part3b. In order to create a weld in the form of a plain butt joint weld8, the surface9of the workpiece1is irradiated by means of a welding laser beam11at the abutting area10, at which the workpiece parts3a,3bbear against one another. The welding laser beam11is coupled out of a multiclad fiber (seeFIG. 2). The abutting area10of the workpiece parts3a,3blies parallel to the beam propagation direction12of the welding laser beam11. The first (wider) workpiece part3aextends perpendicularly away from the abutting area10, and the second (narrower) workpiece part3bextends parallel to the abutting area10. At the entrance side of the welding laser beam11on the workpiece1, the workpiece parts3a,3bare in line with respect to the beam propagation direction12. The welding laser beam11is radiated in from that side of the workpiece1comprising the two workpiece parts3a,3bthat faces away from the right-angled inner corner4of the workpiece1. The workpiece parts3a,3bare respectively manufactured from an aluminum-containing material, in particular aluminum3003, and typically have a width B1, B2of 0.5 mm to 2.0 mm. The arrangement of the workpiece parts3a,3bconstitutes in particular a schematic representation of a corner join of a battery housing.

The welding situation shown inFIG. 1bis similar to the welding situation shown inFIG. 1a. By contrast to the arrangement fromFIG. 1a, the second (narrower) workpiece part3bextends perpendicularly away from the abutting area10, and the first (wider) workpiece part3aextends parallel to the abutting area10. The thinner, second workpiece part3bextends in particular perpendicularly to the thicker, first workpiece part3a. The thinner, second workpiece part3bis in particular welded to the side of the thicker, first workpiece part3a. The extension of the longitudinal axis7bof the second workpiece part3bpasses through the end5bof the first workpiece part3a.

FIG. 1cshows by way of example a schematic view of a first and a second workpiece part3a(I),3b(I)arranged in a corner shape in a welding situation according to embodiments of the present invention, which workpiece parts are arranged with a maximum gap width MS of 0.1 mm between the workpiece parts3a(I),3b(I), and with a welding laser beam11. It should be noted that the gap width MS and the beam divergence of the welding laser beam11are greatly exaggerated (the same also applies correspondingly for the other figures).

FIG. 1dshows by way of example a schematic view of a welding situation with a first and a second workpiece part3a(I),3b(I)in a corner-shaped arrangement, the welding laser beam11being offset along the surface of the workpiece parts3a(I),3b(I)in relation to the abutting area10of the workpiece parts3a(I),3b(I). The welding laser beam11has in particular a maximum lateral offset MLO of 0.2 mm with respect to the abutting area10of the workpiece parts3a(I),3b(I).

FIG. 1eshows by way of example a schematic view of a welding situation with a first and a second workpiece part3a(I),3b(I)in a corner-shaped arrangement, the focus F of the welding laser beam11being spaced apart upwardly from the surface of the workpiece parts3a(I),3b(I). The focus F of the welding laser beam11for welding the workpiece parts3a(I),3b(I)together has in particular a maximum height offset MHO in relation to the surface of the workpiece of 1.5 mm. It should be noted that a height offset MHO may also be set up in that the focus F lies below the surface of the workpieces (not illustrated in more detail).

Figure if shows by way of example a schematic view of a welding situation, as frequently arises when welding a battery housing as a result of joining tolerances. The first workpiece part3ais aligned vertically in this instance and sits on a base39; it is formed by the can of the battery housing or by a part of the can. This can should be closed by means of a cap. The second workpiece part3bforms this cap or part of it. The workpiece parts3a,3bshould be welded to one another in a gastight manner at the abutting area10thereof.

In the direction of the (in this instance) vertical beam propagation direction12, in which the welding laser beam11propagates, the workpiece parts3a,3bare in this instance arranged slightly offset; correspondingly, a step40is formed adjacent to the abutting area10. The step height SH of the step40in the direction R is typically at most 0.3 mm, preferably at most 0.1 mm.

The focus F of the welding laser beam11is typically oriented at the edge of the surface41, facing the incident welding laser beam11, of the first workpiece part3a, which is standing on the base39, in this instance without a height offset. The focus F can be seen here by virtue of the constriction of the lateral boundary lines of the welding laser beam11.

FIG. 2shows a laser system17for generating a welding laser beam11by means of a multiclad fiber18. An output laser beam19is coupled into a first fiber end20aof the multiclad fiber18, which in the radial direction has layers21a,21b,21c,21dwith different refractive indices n1, n2, n3, n4. Furthermore, the welding laser beam11is coupled out of a second end20bof the multiclad fiber18. The beam profile of the exiting welding laser beam11is modified by coupling the output laser beam19at least into an inner core fiber25, having the diameter DK, of the multiclad fiber18and into an outer ring fiber26, having the diameter DR, of the multiclad fiber18by virtue of a deflection optical unit24, in this instance configured as an optical wedge24a, having different power output fractions LK, LR. In this case, in the center position of the optical wedge24a, a beam fraction StrA1of the output laser beam19is deflected by the optical wedge24aand fed into the layer21c, whereas a second beam fraction StrA2of the output laser beam19propagates, still undistorted, in a straight line upstream of the optical wedge24awith respect to the beam direction of the output laser beam19and is fed into the layer21a. In the embodiment shown, the multiclad fiber18is in the form of a 2-in-1 fiber with the inner core fiber25and the outer ring fiber26. The inner core fiber25is formed in particular by the layer21aand the ring fiber26is formed in particular by the layer21c. The other layers21b,21dserve as linings in order to prevent the beam fractions StrA1, StrA2from passing through between the inner core fiber25and the outer ring fiber26. In particular, the refractive index n1of the layer21aand the refractive index n3of the layer21care greater than the refractive index n2of the layer21band the refractive index n4of the layer21d. The second end20bof the multiclad fiber18is reproduced enlarged on the workpiece (seeFIG. 1a), in particular with an enlargement factor VF of greater than 2.0 (not illustrated in any more detail).

FIG. 3ashows the profile of the intensity27of a welding laser beam11coupled out of a multiclad fiber18(see e.g.FIG. 2) in a direction x transverse with respect to the propagation direction of the welding laser beam11close to the surface9of the workpiece (seeFIG. 1a) with the focus within the preferred maximum height offset MHO with respect to the surface9of the workpiece of 1.5 mm (seeFIG. 1c). The intensity27aof the ring beam28, that is to say of the laser beam from the ring fiber26(seeFIG. 2), drops in this instance in the direction of the core beam29, that is to say of the laser beam from the core fiber25(seeFIG. 2), and outwardly, in the radial direction away from the core beam29. In between, the intensity27aof the ring beam28is approximately constant. The intensity27bof the core beam29is higher than that of the ring beam28. There is therefore a local minimum27cof the intensity27between the ring beam28and the core beam29.

FIG. 3bschematically shows an areal cross-section of the welding laser beam11, coupled out of the multiclad fiber18, ofFIG. 3atransverse with respect to the propagation direction of the welding laser beam11close to the surface of the workpiece within the preferred maximum height offset MHO with respect to the surface of the workpiece of 1.5 mm, with the ring beam28, the core beam29and the local minimum of the intensity27cbetween the ring beam28and the core beam29. The profiles of ring beam28and core beam29may each also have a different shape, for example be quadrilateral. The integration of the intensity by way of the core beam29results in the beam power output of the core beam29, which is in particular 25% to 50% of the overall power output of the welding laser beam11.

FIG. 4shows a schematic illustration of the weld pool and the vapor capillary30located therein during the welding method according to embodiments of the present invention by means of a laser beam coupled out of the multiclad fiber (seeFIG. 3). The core beam29substantially determines the depth31of the vapor capillary30. The melt flows at the front side of the vapor capillary30, downwardly with respect to the advancement direction33bof the welding laser beam11, toward the base of the vapor capillary30. At the rear side of the vapor capillary30, the melt flows upwardly and then rearwardly away from the welding laser beam11. The ring beam28enlarges the opening32in the vapor capillary30and facilitates the emergence of gases from the vapor capillary30. The dynamic pressure of the gas produced during the welding method and therefore the respective pulse transmitted by the gas particles to the melt is lower. This reduces the flow velocity in the melt. Less spatters of the melt are ejected during the welding method. Moreover, the ring beam28acts on the melt by way of a pulse, which is directed counter to the flow direction of the melt toward the surface of the vapor capillary30and likewise counteracts the ejection of spatters. The flow direction33aof the molten material is indicated schematically by non-solid arrows. The gas flow in the vapor capillary30is marked by arrows33c.

FIG. 5ashows a sectional view through a welded workpiece1awith a plain butt joint weld8a, which was created by a laser beam that was coupled out of a single-core fiber. The width34aand depth31aof the weld8ais illustrated by bars. The weld8ahas a comparatively small width34a(cf.FIG. 5b) of 1.41 mm along with a depth31aof the weld8aof 1.22 mm. The vapor capillary that created the weld had a corresponding relatively small width, with the result that gases produced during the laser welding could escape only comparatively slowly from the vapor capillary. An edge35formed by the beam profile makes it more difficult for molten material to flow away from the laser beam further outward. The excess pressure generated during the welding forms protuberances of the vapor capillaries that are part of the weld8aand spatters are ejected. The weld8aadditionally has a comparatively high degree of humping.

FIG. 5bshows a sectional view through a welded workpiece1bwith a plain butt joint weld8b, which was created by a laser beam that was coupled out of a multiclad fiber (seeFIG. 2). The weld8bhas a larger width34bof 1.56 mm than the weld8ashown inFIG. 5a, along with a depth31bof the weld8bof 1.34 mm. The vapor capillary that created the weld had a corresponding larger width, with the result that gases produced during the laser welding could escape comparatively easily from the vapor capillary. This prevents excess pressure in the vapor capillary that is part of the weld8band suppresses spatters. The weld8badditionally has a comparatively low degree of humping.