DE102021111879A1 - Laser welding of joining partners with a curved surface - Google Patents

Abstract

A method for forming a welded joint between a first joining partner (14) and a second joining partner (15) using a laser beam (3) is disclosed. The welded joint welds a first joining surface (14B) of the first joining partner (14) to a second joining surface (15B) of the second joining partner (15). At least the first joining partner (14) has a curved surface (14A) onto which the laser beam is radiated. In order to form an in particular elongated focus zone (7), the laser beam (3) is shaped with an arrangement of diffractive, reflective and/or refractive optics, the beam shaping comprising a focus-forming beam shaping and a phase-correcting beam shaping. The phase-correcting beam shaping counteracts the curved surface (14A) influencing the propagation of the laser beam (3) in the material of the first joining partner (14), so that the focal zone (7) can be formed on the first joining surface (14B).

Description

The present invention relates to a method for forming a welded joint between joining partners using a laser beam. Furthermore, the invention relates to a laser processing system with an optical system.

In laser welding, laser radiation, which includes, for example, high-intensity, ultra-short laser pulses, is focused in the area of the joining surfaces of two or more joining partners. Intensities are generated in the focus of the laser radiation, which lead to melting of the material of the joining partners. The formation of a defined focal zone causes localized melting of the material in an area delimited by the focal zone. To form a melting zone that extends beyond the focal zone, a relative movement can be undertaken between the focal zone and the joining partners. The melting zone is thus dependent on the geometry of the focal zone and the relative movement. Their extent defines a spatial extent of the welded joint.

The heating required for melting takes place through the interaction of the laser radiation with the material. The heating can be caused, for example, by linear absorption or non-linear absorption (such as multiphoton absorption in the case of ultrashort high-intensity laser pulses) of the laser radiation in the material. As the melted material cools down, there is an inseparable, materially bonded connection between the joining partners in the area of the melting zone.

One aspect of this disclosure is based on the task of carrying out a laser welding process for joining partners, at least one of which has a curved surface such as the surface of a glass tube or a glass cylinder. In particular, beam shaping approaches, such as those developed for the laser processing of flat workpieces, should also be used in a laser welding process for joining partners with at least one curved surface.

At least one of these objects is achieved by a method for forming a welded joint according to claim 1 and a laser processing system according to claim 20. Further developments are specified in the dependent claims.

• Lagern des ersten Fügepartners und des zweiten Fügepartners derart, dass die erste Fügefläche und die zweite Fügefläche räumlich zueinander fest ausgerichtet sind. Ein Abstand der Fügeflächen ist insbesondere derart gewählt, dass aufgeschmolzenes Material der Fügepartner den Spalt überbrücken kann.
• Strahlformen des Laserstrahls zum Ausbilden einer Fokuszone an der ersten Fügefläche, wobei das Strahlformen mit einer Anordnung von diffraktiven, reflektiven und/oder refraktiven Optiken durchgeführt wird. Das Strahlformen umfasst eine fokusbildende Strahlformung und eine phasenkorrigierende Strahlformung. Die phasenkorrigierende Strahlformung wirkt für eine vorgegebene Ausrichtung einer Strahlachse des Laserstrahls bei Eintritt des Laserstrahls in den ersten Fügepartner einer Beeinflussung der Propagation des Laserstrahls im Material des ersten Fügepartners durch die gekrümmte Oberfläche entgegen. Weisen insbesondere beide Fügepartner eine gekrümmte Oberfläche auf, kann die phasenkorrigierende Strahlformung eine Beeinflussung der Propagation von entsprechenden Strahlbereichen des Laserstrahls durch diese gekrümmten Oberflächen entgegenwirken.
• Einstrahlen des Laserstrahls auf die gekrümmte Oberfläche derart, dass zumindest ein Teil des Laserstrahls durch die gekrümmte Oberfläche in den ersten Fügepartner eintritt, in einem Material des ersten Fügepartners zu der ersten Fügefläche propagiert und die Fokuszone zumindest teilweise mit dem Material des ersten Fügepartners und/oder des zweiten Fügepartners überlappend erzeugt wird. Allgemein kann die Fokuszone zum Aufschmelzen von Material des ersten Fügepartners, des zweiten Fügepartners oder beider Fügepartner im Bereich mindestens einer der Fügeflächen positioniert werden. Die Ausbildung der Fokuszone - und damit das Aufschmelzen - basiert dabei auf Laserstrahlung des Laserstrahls, die durch die gekrümmte Oberfläche in das Material des ersten Fügepartners eingetreten ist. Zusätzlich kann Laserstrahlung des Laserstrahls durch das Material des zweiten Fügepartners propagieren und zur Ausbildung der Fokuszone beitragen.
• Einstellen von Strahlparametern des Laserstrahls derart, dass das Material des ersten Fügepartners und/oder des zweiten Fügepartners in der Fokuszone aufgeschmolzen wird. Das Aufschmelzen kann allgemein durch lineare und/oder nichtlineare Absorption der Laserstrahlung bewirkt werden.
• Ausbilden der Schweißverbindung zwischen dem ersten Fügepartner und dem zweiten Fügepartner durch Abkühlen des aufgeschmolzenen Materials. Im Zuge der Abkühlung des aufgeschmolzenen Materials tritt eine Verfestigung der Schmelze ein, die zur Ausbildung der Schweißverbindung führt.

In one aspect, a method for forming a welded joint between a first joining partner and a second joining partner using a laser beam is disclosed. The laser beam is in particular an ultra-short pulse laser beam. The welded connection welds a first joining surface of the first joining partner to a second joining surface of the second joining partner. At least the first joining partner has a curved surface. The procedure includes the steps:

• Storing the first joining partner and the second joining partner in such a way that the first joining surface and the second joining surface are spatially firmly aligned with one another. A distance between the joining surfaces is selected in particular in such a way that melted material from the joining partners can bridge the gap.
• Beam shaping of the laser beam to form a focus zone on the first joining surface, the beam shaping being carried out with an arrangement of diffractive, reflective and/or refractive optics. Beam shaping includes focus-forming beam shaping and phase-correcting beam shaping. The phase-correcting beam shaping counteracts an influencing of the propagation of the laser beam in the material of the first joining partner by the curved surface for a predetermined alignment of a beam axis of the laser beam when the laser beam enters the first joining partner. In particular, if both parts to be joined have a curved surface, the phase-correcting beam shaping can counteract the influence of the propagation of corresponding beam regions of the laser beam through these curved surfaces.
• Radiation of the laser beam onto the curved surface in such a way that at least part of the laser beam enters the first joining partner through the curved surface, propagates in a material of the first joining partner to the first joining surface and the focal zone at least partially with the material of the first joining partner and/or of the second joining partner is generated overlapping. In general, the focus zone for melting material of the first joining partner, the second joining partner or both joining partners can be positioned in the area of at least one of the joining surfaces. The formation of the focus zone - and thus the melting - is based on the laser radiation of the laser beam, which has entered the material of the first joining partner through the curved surface. In addition, laser radiation from the laser beam can propagate through the material of the second joining partner and contribute to the formation of the focal zone.
• Setting beam parameters of the laser beam in such a way that the material of the first joining partner and/or the second joining partner is melted in the focal zone. The melting can generally by linear and/or non-linear absorption of the laser radiation can be effected.
• Forming the welded connection between the first joining partner and the second joining partner by cooling the melted material. As the melted material cools, the melt solidifies, leading to the formation of the welded joint.

• Bewegen der Fokuszone entlang der ersten Fügefläche unter Beibehalten einer Ausrichtung des Laserstrahls mit Bezug zu einer Krümmung der gekrümmten Oberfläche. Optional kann der Laserstrahl derart auf die gekrümmte Oberfläche eingestrahlt werden, dass die Strahlachse des Laserstrahls in einem Einfallswinkelbereich von ±10° zu einer Normalenrichtung der gekrümmten Oberfläche an einem Auftreffpunkt der Strahlachse auf der gekrümmten Oberfläche ausgerichtet ist.

In some embodiments, the method may further include:

• moving the focal zone along the first mating surface while maintaining an alignment of the laser beam with respect to a curvature of the curved surface. Optionally, the laser beam can be irradiated onto the curved surface in such a way that the beam axis of the laser beam is aligned in an incident angle range of ±10° to a normal direction of the curved surface at an impingement point of the beam axis on the curved surface.

In some embodiments of the method, the focal zone can be moved by rotating the first joining partner and/or the second joining partner about an axis associated with the curvature of the curved surface and/or by rotating the laser beam about the first joining partner and the second joining partner.

In some specific embodiments of the method, the curved surface can be designed at least in sections as a cylinder jacket surface with respect to a cylinder axis. The laser beam can be irradiated in a radial direction on the cylinder axis and the rotation can be made around the cylinder axis.

In some embodiments of the method, the first joining partner can at least partially have a cylindrical or hollow-cylindrical shape with respect to a cylinder axis and the first joining surface can be a butt joint surface of the cylindrical or hollow-cylindrical shape.

In some embodiments of the method, the second joining partner can have a cylindrical basic shape, which extends along the cylinder axis or at an angle to the cylinder axis of the first joining partner. Optionally, the focus zone can also be formed on the second joining surface and at least partially overlap with the material of the second joining partner.

In some embodiments of the method, the first joining partner can have, at least in sections, a hollow-cylindrical shape with respect to a cylinder axis, and the first joining surface can be an inner surface of the hollow-cylindrical shape. An outer surface of the second joining partner can have a section whose course at least partially follows a course of the inner surface of the first joining partner. Optionally, moving the focal zone can include a relative translational movement between the laser beam and the first joining partner. In particular, the first joining partner can be a hollow cylinder and the second joining partner can have a cylindrical basic shape.

In some embodiments of the method, the focal zone can be moved by a relative translational movement between the laser beam and the first joining partner and the second joining partner.

In some embodiments of the method, the curved surface can be configured at least in sections as a cylinder jacket surface with respect to a cylinder axis and the first joining partner can have an azimuthal section of a cylindrical or hollow-cylindrical shape and the first joining surface can be a side surface of the azimuthal section extending in the direction of the cylinder axis. The laser beam can be irradiated in the radial direction onto the cylinder axis and the translational movement can take place along the cylinder axis.

In some embodiments of the method, the focus-forming beam shaping can be set up to allow beam portions of the laser beam to arrive at an angle of incidence onto a beam axis of the laser beam to form an elongated focus zone along a beam axis at least partially in the material of the first joining partner and/or in the material of the second To effect joining partners by interference. Furthermore, the method can include setting the focal length to be less than or equal to a predetermined joint length in a propagation direction of the laser beam, in particular setting the focal length in a range between 100 μm and 500 μm.

• - Einstellen einer Fokuslänge der Fokuszone in einem Bereich zwischen 20 µm und 500 µm und/oder auf maximal die Wanddicke, insbesondere auf maximal 80% der Wanddicke. Optional kann die Fokuszone an der gekrümmten Oberfläche des ersten Fügepartners beginnen. Die Schweißverbindung kann zwischen dem ersten Fügepartner und dem zweiten Fügepartner eine Mehrzahl von Schweißnähten umfassen, die sternförmig angeordnet sind und die optional ineinander übergehen.

In some embodiments of the method, the first joining partner can be designed as a hollow body, in particular a glass tube, with a wall that delimits an interior space and has a wall thickness, and the method can also have the step:

• - Setting a focal length of the focal zone in a range between 20 microns and 500 microns and / or to a maximum of the wall thickness, in particular other to a maximum of 80% of the wall thickness. Optionally, the focal zone can start at the curved surface of the first joining partner. Between the first joining partner and the second joining partner, the welded connection can comprise a plurality of weld seams which are arranged in a star shape and which optionally merge into one another.

• - Einstellen einer Fokuslänge der Fokuszone in einem Bereich zwischen 20 µm und 500 µm und/oder auf maximal einen halben Durchmesser des Zylinders.

In some embodiments of the method, the first joining partner can be a solid cylinder, in particular a glass cylinder, and the method can also have the step:

• - Setting a focal length of the focal zone in a range between 20 µm and 500 µm and/or to a maximum of half the diameter of the cylinder.

The focal zone can optionally start at the curved surface of the joining partner. The welded connection between the first joining partner and the second joining partner can comprise a plurality of weld seams which are arranged in a star shape and which optionally merge into one another.

In some embodiments of the method, the focus-forming beam shaping can be set up to generate a Gaussian focus.

In some embodiments of the method, moving the focal zone can also include a transverse oscillating movement of the focal zone perpendicular to the first joining surface and/or a translational movement of the focal zone in the direction of the beam axis.

In some embodiments of the method, the material of the second joining partner can be transparent, partially transparent or non-transparent with respect to the laser beam and can be melted near the surface of the second joining surface of the second joining partner. The material of the second joining partner can be heated in particular via linear absorption of the laser beam and the material of the first joining partner can be heated in particular by non-linear absorption of the laser beam and/or indirectly by the heated material of the second joining partner.

In some embodiments of the method, the curved surface may be curved in one direction. The beam shaping of the laser beam can include impressing at least one two-dimensional phase distribution on the laser beam, wherein the at least one two-dimensional phase distribution for the phase-correcting beam shaping can include phase contributions that cancel out an entry phase locally accumulated by the laser beam when it enters the first joining partner, and optionally for the Focus-forming beam shaping can include phase contributions that cause beam components to arrive at an angle of incidence and, in particular, to generate a non-diffracting beam for the formation of an elongated focus zone along the beam axis in the area of the joint surfaces.

• Ausrichten der achsensymmetrische Phasenverteilung und des ersten Fügepartners derart zueinander, dass die Symmetrieachse unter Berücksichtigung eines Strahlengangs zwischen einem Ort des Aufprägens der achsensymmetrischen Phasenverteilung und dem ersten Partner orthogonal zu einer Ebene verläuft, in der ein Krümmungsradius der Oberfläche definiert ist.

In some embodiments of the method, the phase contributions can form a phase distribution that is axisymmetric to an axis of symmetry, the second phase contributions being constant in particular parallel to the axis of symmetry and changing perpendicularly to the axis of symmetry, and the method can also include the step:

• Aligning the axisymmetric phase distribution and the first joining partner to one another in such a way that the axis of symmetry runs orthogonally to a plane, taking into account a beam path, between a location where the axisymmetric phase distribution is impressed and the first partner, in which a radius of curvature of the surface is defined.

In some embodiments of the method, the phase-correcting beam shaping can be generated by a cylindrical lens, which is positioned in a beam path of the laser beam before or after an optical system that brings about the focus-forming beam shaping.

• eine Laserstrahlquelle, die einen, insbesondere gepulsten, Laserstrahl ausgibt,
• ein optisches System zum Strahlformen des Laserstrahls für eine Ausbildung einer Fokuszone in dem ersten Fügepartner und/oder dem zweiten Fügepartner, mit einer fokusbildenden Optik, die für eine Ausbildung der Fokuszone ohne Berücksichtigung der gekrümmten Oberfläche ausgebildet ist, wobei die Fokuszone optional entlang einer Strahlachse des Laserstrahls langgezogen ausgebildet ist, und
• eine Werkstückhalterung zur Lagerung des ersten Fügepartners und/oder des zweiten Fügepartners.

Another aspect includes a laser processing system for forming a welded joint between a first joining partner and a second joining partner with a laser beam, in particular an ultra-short pulse laser beam, the welded joint welding a first joining surface of the first joining partner to a second joining surface of the second joining partner and with at least the first joining partner has a curved surface. The laser processing equipment can include:

• a laser beam source that emits a particularly pulsed laser beam,
• an optical system for beam shaping of the laser beam to form a focal zone in the first joining partner and/or the second joining partner, with focus-forming optics that are designed to form the focal zone without taking the curved surface into account, with the focal zone optionally being along a beam axis of the Laser beam is elongated, and
• a workpiece holder for storing the first joining partner and/or the second joining partner.

In this case, a phase correction, which counteracts an influencing of the formation of the focus zone due to the curved surface, is provided with phase-correcting optics or is integrated into the focus-forming optics.

In summary, according to the concepts disclosed herein, a process for laser welding joining partners with at least one curved surface can be implemented. The joining partners can consist of transparent, partially transparent or opaque materials, such as glasses, crystals, polymers, semiconductors, ceramics, metals such as aluminum, and different materials can also be welded together. In particular, a combination of a transparent/partially transparent material and an opaque material is possible.

The underlying optics concept allows, for example, the welding of glass tubes with radii in the range of 0.5 mm to e.g. 25 mm, e.g. with radii of a few millimeters. In particular, laser welding processes can be carried out with elongated focal zones from a few 10 μm to a few 100 μm in length. The diameter of the beam impinging on the parts to be joined or the areas of the curved surface through which the beam passes are, for example, in the range from 0.5 mm to 15 mm. Aberrations caused by the curved surface can be pre-compensated or evened out with the phase-correcting beam shaping in order to ensure a focus formation that is aimed for in the context of the focus-forming beam shaping.

For example, the focus-forming beam shaping can bring about the focusing of the laser radiation in a Gaussian focus or in an elongated focal zone. In this way, a non-diffracting beam can be generated for the formation of an elongated focal zone along the beam axis in at least one of the joining partners.

Concerning non-diffracting rays disclose WO 2016/079062 A1 , WO 2016/079063 A1 and WO 2016/079275 A1 the applicant that in transparent materials an elongated focus zone and corresponding elongated modifications can be generated in a material. Non-diffracting beams can be formed, for example, due to interference from beam components arriving from the outside. This is the case, for example, with Bessel-like rays. Beam-shaping elements and optics structures for laser processing are mentioned, for example, in WO 2016/079275 A1 described. The concepts presented here enable the use of such non-diffracting beams (designed for irradiation through a flat workpiece surface) for laser welding of joining partners with at least one curved surface.

The connection of joining partners using ultra-short laser pulses can be based in particular on non-linear absorption and heat accumulation in the material of the joining partners. The focal zones of successive laser pulses usually overlap. The locally melted and cooled material leads to the connection of the joining partners when the cooled melt (weld seam) is localized in the interface between the joining partners.

In order to connect joining partners (e.g. pipes) to one another at the front (in a butt joint), the laser pulses can be focused through the upper material forming the curved surface into the interface in order to initiate the laser welding process. If necessary, the position of the laser focus can be modulated along the laser propagation direction. A circumferential weld seam can be created by rotating the pipes to be connected.

Alternatively, the laser pulses can be irradiated and focused parallel to the interface of the joining partners and next to the interface. The joining partners are welded together by scanning/wobbling the focal zone across the interface. The scanning/wobbling of the focal zone takes place in particular at the same time as a rotary movement of the pipes to be connected.

In addition to butt-joining, further joining constellations are explained below in connection with the figures, whereby, for example, the optional approaches mentioned above, such as wobbling, can be implemented in an appropriately adapted manner.

The surfaces of the joining partners to be welded are preferably clean and polished. This means that they have sufficient optical quality for the laser radiation to enter the material with as little disruption as possible. Likewise, the surfaces of the abutting surfaces/joining surfaces to be welded preferably have an evenness that leads to a joint gap that preferably has a maximum width of a few micrometers.

For the welding process, the parts to be joined are pressed together, e.g. with a device in the area of the joining surfaces, or clamped with a defined gap (e.g. less than 15 µm) between the joining surfaces.

In general, allow the concepts proposed herein, laser radiation through a curved surface of a joining partner in the To irradiate the area of a joining zone so that the laser radiation melts the material there of at least one of the joining partners in order to form the weld seam after cooling.

• 1 Abbildungen zur Verdeutlichung einer Fokuszone eines Gauß-Strahls und von nicht-beugenden Strahlen,
• 2 eine schematische Skizze einer Laserbearbeitungsanlage zum Laserschweißen,
• 3A bis 4B schematische Skizzen zu Stumpfstoßschweißvorgängen,
• 5 eine schematische Skizze einer Laserbearbeitungsanlage zum Laserschweißen mit einer langgezogenen Fokuszone,
• 6A und 6B schematische Skizzen eines optischen Systems zur Strahlformung,
• 7A und 7B Skizzen zur Verdeutlichung der Auswirkung einer gekrümmten Oberfläche auf die Ausbildung einer Bessel-Strahl-Fokuszone,
• 8A und 8B Intensitätsverteilungen einer Bessel-Strahl-Fokuszone, die für einen Eintritt in einen planen Fügepartner bzw. für eine Korrektur hinsichtlich einer gekrümmten Oberfläche eines Fügepartners simuliert wurde,
• 9A und 9B Intensitätsverteilungen einer Bessel-Strahl-Fokuszone ohne Korrektur hinsichtlich einer gekrümmten Oberfläche,
• 10A bis 10F Phasenverteilungen für fokusbildende Strahlformungen und phasenkorrigierende Strahlformungen und diese kombinierende Phasenverteilungen,
• 11 ein Flussdiagramm zur Verdeutlichung eines beispielhaften Verfahrens zum Ausbilden einer Schweißverbindung bei einem Fügepartner mit einer gekrümmten Oberfläche und
• 12A bis 12D schematische Skizzen zu weiteren beispielhaften Schweißvorgängen.

Concepts are disclosed herein that allow at least some aspects of the prior art to be improved. In particular, further features and their usefulness result from the following description of embodiments with reference to the figures. From the figures show:

• 1 Figures showing a focal zone of a Gaussian beam and non-diffracting rays,
• 2 a schematic sketch of a laser processing system for laser welding,
• 3A until 4B schematic sketches of butt welding processes,
• 5 a schematic sketch of a laser processing system for laser welding with an elongated focal zone,
• 6A and 6B schematic sketches of an optical system for beam shaping,
• 7A and 7B Sketches to illustrate the effect of a curved surface on the formation of a Bessel ray focal zone,
• 8A and 8B Intensity distributions of a Bessel beam focus zone that was simulated for entry into a planar joining partner or for a correction with regard to a curved surface of a joining partner,
• 9A and 9B Intensity distributions of a Bessel beam focal zone without correction for a curved surface,
• 10A until 10F Phase distributions for focus-forming beam formations and phase-correcting beam formations and phase distributions combining these,
• 11 a flowchart to illustrate an exemplary method for forming a welded joint in a joining partner with a curved surface and
• 12A until 12D schematic sketches of further exemplary welding processes.

Aspects described herein are based in part on the knowledge that when joining partners are welded, at least one of which has a curved surface, laser radiation enters the material of the joining partner through the curved surface in order to form the focus therein. The curved surface affects the formation of the focal zone because aberrations (especially spherical aberrations) can deform the focal zone or even prevent the formation of the focal zone. As a result, the laser radiation can no longer be absorbed to the extent necessary for the welding process. The inventors propose phase-correcting beam shaping which counteracts an influencing of the propagation of the laser beam through the curved surface for a predetermined alignment of a beam axis of the laser beam when the laser beam enters the joining partner with the curved surface. The inventors have also recognized that aberration correction depends on the shape/curvature of the curved surface, for example the radius of curvature of a tube surface, and the beam diameter, generally the extent of the laser radiation impinging on the surface.

genügen und eine klare Separierbarkeit in eine transversale (d.h., in x- und y-Richtung) Abhängigkeit und eine longitudinale (d.h., in z-Richtung/Ausbreitungsrichtung) Abhängigkeit der Form $$U\left(x,y,z\right)=U^t\left(x,y\right)\text{exp}\left(\iota k_{z}z\right)$$

aufweisen.The aspects described herein generally relate to focus-forming beam shaping and thus in particular to the use of non-diffracting beams in laser welding. Non-diffractive beams can be formed by wave fields that correspond to the Helmholtz equation $${\nabla }^{2}u\left(\mathrm{right}\right)+k^{2}u\left(\mathrm{right}\right)=0$$

suffice and a clear separability into a transverse (ie, in the x and y direction) dependency and a longitudinal (ie, in the z direction/propagation direction) dependency of the shape $$u\left(x,y,\mathrm{e.g}\right)=u^t\left(x,y\right)\text{ex}\left(\iota k_{\mathrm{e.g}}\mathrm{e.g}\right)$$

exhibit.

und U^t (x, y) ist eine beliebige komplexwertige Funktion, die nur von den transversalen Koordinaten x und y abhängt. Da die z-Abhängigkeit in Gleichung 2 eine reine Phasenmodulation aufweist, ist eine Intensität I(x, y, z) einer die Gleichung 2 lösenden Funktion propagationsinvariant und wird als „nicht-beugend“ bezeichnet: $$I\left(x,y,z\right)={\left|U\left(x,y,z\right)\right|}^2=I\left(x,y\mathrm{,0}\right)$$

Here k = ω/c is the wave vector with its longitudinal/axial and transverse components $k^{}$${}^2=k_{\mathrm{e.g}}^2+{\mathrm{<figure-callout\; id="2"\; label="k\; t"\; filenames="DE102021111879A1\_0020.png,DE102021111879A1\_0021.png"\; state="\{\{state\}\}">k</figure-callout>}}_t^{}$

and U ^t (x, y) is any complex-valued function that depends only on the transverse coordinates x and y. Since the z-dependence in Equation 2 shows a pure phase modulation, an intensity I(x, y, z) of a function solving Equation 2 is propagation-invariant and is called “non-diffractive”: $$I\left(x,y,\mathrm{e.g}\right)={\left|u\left(x,y,\mathrm{e.g}\right)\right|}^2=I\left(x,y\mathrm{,0}\right)$$

This approach provides different solution classes of the Helmholtz equation in different coordinate systems, such as so-called Mathieu rays in elliptic-cylindrical coordinates or so-called Bessel rays in circular-cylindrical coordinates.

See also J. Turunen and AT Friberg, "Propagation-invariant optical fields", in Progress in optics, 54, 1-88, Elsevier (2010) and M. Woerdemann, "Structured Light Fields: Applications in Optical Trapping, Manipulation, and Organisation", Springer Science & Business Media (2012).

A large number of types of non-diffracting beams can be realized to a good approximation. These realized non-diffractive beams will continue to be referred to herein as "finitely limited non-diffractive beams", "non-diffractive beams", or also as "quasi-non-diffractive beams" for the sake of simplicity. In contrast to the theoretical construct, they lead to a finite performance. A length L of a propagation invariance assigned to them is also finite.

1 shows in comparison with intensity representations of a conventional Gaussian focus (see the propagation behavior of a Gaussian focus in figure (a) of the 1 ) the propagation behavior of non-diffracting rays based on intensity plots in Figures (b) and (c). Figures (a), (b) and (c) each show a longitudinal section (xz-plane) and a transverse section (xy-plane) through the focus of a Gaussian beam and non-diffracting rays, respectively, directed in the z-direction propagate.

Figure (b) refers to a rotationally symmetrical, non-diffracting beam, in this case a Bessel-Gaussian beam. Figure (c) refers to a non-asymmetric non-diffracting beam as an example. For a Bessel-Gaussian beam, Figures (d) and (e) show the 1 further details of a central intensity maximum. So shows figure (d) of the 1 an intensity profile in a transversal section plane (XY plane) and a transversal intensity profile in the X-direction. The figure (e) of 1 shows details of the central intensity maximum in a slice in the direction of propagation.

des Gauß-Fokus definiert, wobei der Gauß-Fokus über die zweiten Momente festgelegt wird. Ferner wird eine zugehörige charakteristische Länge über die Rayleigh-Länge $z_{\text{R}}=\pi {\left(d_0^{GF}\right)}^2/4\lambda$

definiert, die als eine Distanz ausgehend von der Fokusposition festgelegt wird, bei der der Strahlquerschnitt um einen Faktor 2 zugenommen hat. Ferner wird für einen quasi nicht-beugenden Strahl ein transversaler Fokusdurchmesser $d_0^{\text{ND}}$

als die transversale Dimension eines lokalen Intensitätsmaximums definiert, wobei der transversaler Fokusdurchmesser $d_0^{\text{ND}}$

durch die kürzeste Distanz direkt angrenzender, gegenüberliegender Intensitätsminima (z.B. Intensitätsabfall auf 25 %) gegeben ist. Siehe hierzu z.B. die Abbildungen (b) und (d) der 1. Die longitudinale Ausdehnung des nahezu propagationsinvarianten Intensitätsmaximums kann als eine charakteristische Länge L des quasi nicht-beugenden Strahls angesehen werden. Sie ist definiert über einen Intensitätsabfall auf 50 %, ausgehend vom lokalen Intensitätsmaximum, jeweils in positive und negative z-Richtung, siehe Abbildungen (c) und (e) der 1.A focus diameter is used for comparison ${\mathrm{i.e}}_0^{\text{GF}}$

of the Gaussian focus, where the Gaussian focus is defined via the second moments. Furthermore, an associated characteristic length over the Rayleigh length ${\mathrm{e.g}}_{\text{R}}=\pi {\left({\mathrm{i.e}}_0^{Gf}\right)}^2/4\lambda$

defined as a distance from the focal position at which the beam cross-section has increased by a factor of 2. Furthermore, for a quasi-non-diffracting beam, a transverse focus diameter ${\mathrm{i.e}}_0^{\text{ND}}$

defined as the transverse dimension of a local intensity maximum, where the transverse focus diameter ${\mathrm{i.e}}_0^{\text{ND}}$

is given by the shortest distance of directly adjacent, opposite intensity minima (e.g. intensity drop to 25%). See, for example, Figures (b) and (d) of the 1 . The longitudinal extent of the almost propagation-invariant intensity maximum can be viewed as a characteristic length L of the quasi-non-diffracting beam. It is defined by an intensity drop to 50%, starting from the local intensity maximum, in the positive and negative z-direction, see Figures (c) and (e) of the 1 .

die charakteristische Länge L des nicht-beugenden Strahls die Rayleigh-Länge des zugehörigen Gauß-Fokus deutlich überragt, insbesondere wenn L > 10z_R.Herein a quasi-non-diffracting beam is assumed if for similar transverse dimensions, e.g ${\mathrm{i.e}}_0^{\text{ND}}\approx {\mathrm{i.e}}_0^{\text{GF}},$

the characteristic length L of the non-diffracting ray significantly exceeds the Rayleigh length of the associated Gaussian focus, especially when L > 10z _R .

(Quasi) Bessel rays, also known as Bessel-like rays, are examples of a class of (quasi) non-diffracting rays. In such beams, the transverse field distribution U ^t (x, y) near the optical axis obeys a Bessel function of the first kind of order n to a good approximation. A subset of this class of beams are the so-called Bessel-Gauss beams, which are widespread due to their ease of generation. A Bessel-Gaussian beam can be formed, for example, by illuminating an axicon of refractive, diffractive or reflective design with a collimated Gaussian beam. An associated transverse field distribution in the vicinity of the optical axis in the area of an associated elongated focal zone obeys, to a good approximation, a Bessel function of the first kind of order 0 (in a good approximation), which is enveloped by a Gaussian distribution, see figure (d) and (e) the 1 , where the intensity distribution shown corresponds to the square of the absolute value of a Bessel function (to a good approximation).

auf. Beim Laserschweißen liegt die Länge L zum Beispiel im Bereich von 20 µm bis 500 µm (z.B. im Bereich von 100 µm bis 500 µm), siehe Abbildungen (b) und (e) der 1. (Bei nicht-beugenden Strahlen kann die Länge allgemein auch 1 mm übersteigen.) Ein Fokus eines Gauß-Strahls mit $d_0^{\text{GF}}\approx d_0^{\text{ND}}=\mathrm{2,5}\mathrm{\mu }\text{m}$

zeichnet sich hingegen durch eine Fokuslänge in Luft von lediglich z_R ≈ 5 µm bei einer Wellenlänge λ von 1 µm aus, siehe Abbildung (a) der 1. In diesen für das Laserschweißen relevanten Fällen gilt demnach für die zugehörige Länge L eines nicht-beugenden Strahls: L >> 10z_R, beispielsweise das 20-fache oder mehr oder sogar das 100-fache oder mehr der Rayleigh-Länge.Typical Bessel-Gauss beams, which can be used to weld a transparent/partially transparent material to a transparent/partially transparent/opaque material, have a central intensity maximum diameter on the optical axis in the range of 0.5 μm and 10 μm, in particular between 1 μm and 5 µm like e.g ${\mathrm{i.e}}_0^{\text{ND}}=2.5\mathrm{\mu }\text{m}$

on. In laser welding, for example, the length L is in the range from 20 µm to 500 µm (eg in the range from 100 µm to 500 µm), see figures (b) and (e) of the 1 . (For non-diffracting beams, the length can also generally exceed 1 mm.) A focus of a Gaussian beam with ${\mathrm{i.e}}_0^{\text{GF}}\approx {\mathrm{i.e}}_0^{\text{ND}}=2.5\mathrm{\mu }\text{m}$

is characterized by a focus length in air of only z _R ≈ 5 µm at a wavelength λ of 1 µm, see figure (a) of 1 . In these cases, which are relevant for laser welding, the following applies to the associated length L of a non-diffracting beam: L >> 10z _R , at for example 20 times or more or even 100 times or more the Rayleigh length.

However, the elongated focal zone is associated with a larger irradiated area when entering the material in which the elongated focal zone is to be formed. Exemplary ring beam diameters for typical Bessel-Gaussian beams range from 1 mm to 15 mm, for example.

Those skilled in the art will appreciate that the advantage of a non-diffracting beam, compared to a Gaussian beam, is the increased tolerance in positioning the elongated focal zone with respect to the mating surfaces. In the case of an overlap joint in particular, only a small area of the focal zone in the direction of propagation will contribute to the welding. In particular, this is the area that contributes to the melting of the material near the joining surfaces. A larger focus length can more easily compensate for/tolerate changes in the thickness of the joining partners. Furthermore, the non-diffracting beam allows a larger gap between joining surfaces to be bridged than a Gaussian focus, which is particularly suitable for contacting joining surfaces due to its short extent. The focal position tolerance of a non-diffracting beam is, for example, in the range of the length of the elongated focal zone, i.e. in the range of, for example, 20 µm to 500 µm (e.g. in the range of 100 µm to 500 µm). An increased tolerance is particularly advantageous for laser welding of curved joining surfaces.

Aspects described herein are also based in part on the knowledge that if joining partners with a curved surface are to be welded with a laser beam, in particular a non-diffracting beam such as is formed, for example, in an interference-based focal zone of a Bessel-Gaussian beam the curved surface can affect the formation of the focal zone, in the case of a non-diffracting beam e.g. an underlying interference. Accordingly, beam shaping/focusing, as used to process flat workpieces, is no longer effective. This is particularly the case when the curvature is not rotationally symmetrical but is one-dimensional, as in the case of a tube or cylinder to be machined made of glass or ceramic, for example.

According to the invention, this influencing of the e.g.

In order to maintain the formation and properties of a desired focus zone, e.g. a non-diffracting beam with a beam profile similar to a Bessel beam, it is proposed here to counteract the aberrations that occur when entering the joining partner with a phase correction. The phase correction in the beam path is preferably carried out in the area in which there is still a Gaussian or almost Gaussian lateral laser beam profile. In the case of beam shaping of a non-diffracting beam, the phase correction can take place in the area of a phase imprint, such as is used to form the non-diffracting beam for welding planar joining partners. In the case of a curved surface that is curved square and aligned symmetrically to the pulsed laser beam, the phase correction can be effected with simple optical components. Even with tilted surfaces, the geometries of the optical components required for the correction can become very complex.

Despite the aberrations that occur when entering a joining partner, a non-diffracting beam can be generated with almost undisturbed propagation in the material of a tube or cylinder, as explained below. If the phase correction is carried out, a pulsed laser beam can be used to produce, for example, elongated melt zones even in a joint partner with a curved surface, with parameters such as pulse energy, pulse duration and focus zone geometry being set accordingly.

2 shows a schematic representation of a laser processing system 1 with a laser beam source 1A and an optical system 1B. The optical system 1B is designed for beam shaping of a pulsed output laser beam 3' of the beam source 1A and emits a laser beam 3 for a laser welding process.

The laser processing system 1 is set up for welding two parts to be joined 4, 5, with at least one of the parts to be joined 4, 5 having a curved (outer) surface 4A, 5A. In 2 the joining partners 4, 5 are indicated schematically in three dimensions with a basic cylindrical shape.

In general, the beam shaping of the optical system 1B serves to provide laser radiation in a focal zone 7 with the pulsed laser beam 3 . In the area of the focus zone 7 , energy can be introduced into the material of the joining partners 4 , 5 for welding the joining partners 4 , 5 . In order to control the position of the energy input, the optical system 1B can be moved, for example, in the three spatial directions (arrow 6A). This allows, for example, a transverse oscillating movement (wobbling) (eg perpendicular to the joint surface in a butt joint configuration or along the joint surface in a lap joint configuration). Furthermore, an oscillating movement/translational movement can be carried out in the direction of the beam axis, with the latter being able to affect the phase-correcting beam shaping and possibly requiring its adaptation.

Furthermore, the position of the energy input can be varied by pivoting the optical system 1B about the joining partners 4, 5 (arrow 6B).

For the energy input, the focus zone 7 overlaps with at least one of the joining partners 4, 5. Since the energy is to be introduced into the interior of at least one of the joining partners 4, 5, the laser radiation will at least partially propagate in the material. Correspondingly, at least one of the joining partners 4, 5 consists of a (largely for the laser wavelength of the laser beam 3 used) transparent or partially transparent material in e.g. ceramic or crystalline design such as glass, sapphire, transparent ceramic, glass ceramic or a polymer. Transparency of a material herein refers to linear absorption. For light below the threshold fluence/intensity of nonlinear absorption, a “substantially” transparent material may absorb or scatter, e.g., less than 20% or even less than 10% of the incident light over a length of 1 mm. A “substantially” partially transparent material can, for example, absorb or scatter 20% to 80% of the incident light over a length of 1 mm. An opaque material essentially absorbs all of the laser radiation near the surface.

In general, the output laser beam 3' and thus the laser beam 3 is determined by beam parameters such as the formation of individual laser pulses or groups of laser pulses, wavelength, spectral width, temporal pulse shape, pulse energy, beam diameter and polarization. For laser welding, the laser pulses have, for example, pulse energies that lead to pulse peak intensities that cause volume absorption in the material of the joining partners and thus the formation of a melting area (melting pool) in a desired geometry.

2 also shows a workpiece holder 19 for mounting the joining partners 4, 5. In the illustrated example of a butt joint welding process, the end faces represent the joining surfaces of the joining partners. The workpiece holder 19 can bring the joining partners 4, 5 together at the end faces (arrow 6C) and in particular set a predetermined dimension for a maximum gap width. As an alternative or in addition to pivoting the optical system 1B, the parts to be joined 4, 5 can be rotated with the aid of the workpiece holder 19 to produce a circumferential welded joint.

In general, the weld seam can be produced continuously or azimuthally in sections. in 2 illustrated case of an elongated focal zone 7, the weld extends in the radial direction over the length of the focal zone.

the inside 2 witnessed exemplary butt welding process of two cylindrical base bodies is in connection with 3A and 3B for welding two pipes 14, 15 and in connection with 4A , and 4B for two solid cylinders 16, 17 illustrated. Generally show the 2 until 4B Examples of the curved surface being designed at least in sections as a cylinder jacket surface with respect to a cylinder axis, the laser beam being radiated onto the cylinder axis in a radial direction and the rotation taking place around the cylinder axis.

The tubes 14, 15 (as an example of a hollow cylindrical shape with respect to a cylinder axis like a glass tube) and solid cylinders 16, 17 (like a glass rod) were axially aligned and brought into contact at joining surfaces 14B, 15B and 16B, 17B, respectively, or in a predetermined one positioned at a distance from each other. Joining surfaces are the one butt joint surface of the cylindrical or hollow-cylindrical shape.

The laser beam 3 enters the material (at least for the most part) through the curved surfaces 14A, 15A or 16A, 17A. An example is in 4A an incident ring ray is indicated to form a Bessel-Gaussian beam, the ring ray extending over a surface 18 upon entry into the solid cylinders 16,17. In the area of the irradiated area 18, there are incidence conditions (angle of incidence) which depend on the respective location and which can lead to aberrations in the formation of the focal zone 7A, 7B.

For the formation of a desired weld it is in the example 3A the goal, despite these aberrations, is to form the focal zone 7A with the desired geometry and beam intensity in the region of the tube wall on the joint surface 14B of the tube 14. Analogous is in the example 4A proposes the goal of forming the focal zone 7B with the desired geometry and beam intensity in such a way that the focal zone 7B overlaps both the solid cylinder 16 and the solid cylinder 17 in the area of the joining surfaces 16B, 17B over a predetermined length.

To form these focal zones, the laser beam 3' in the optical system 1B is provided with a Arrangement of diffractive, reflective and / or refractive optics shaped, wherein the beam shaping comprises a phase-correcting beam shaping in addition to a focus-forming beam shaping. The phase-correcting beam shaping is designed in such a way that, for a predetermined alignment of the beam axis 8 of the laser beam 3 when it enters the tubes 14, 15 or the solid cylinders 16, 17, the propagation of the laser radiation in the material of the joining partners is influenced by the curved surfaces 14A , 15A or 16A, 17A.

In the case of the tubes 14, 15, a focal length of the focal zone can be set, for example, in a range between 20 µm and 500 µm. The focus length is preferably set to a maximum of the wall thickness, in particular to a maximum of 80% of the wall thickness. Optionally, the focal zone starts at the curved surface of the joining partner. In the case of solid cylinders 16, 17, a focal length of the focal zone can be set, for example, in a range between 20 μm and 500 μm. The focal length is preferably set to a maximum of half the diameter of the cylinder, with the focal zone optionally starting again at the curved surface of the joining partner.

3B FIG. 12 further illustrates that a continuous circumferential melt zone 21 can be formed in the tube wall on the mating surface 14B by rotating the tubes 14,15. For this purpose, the welded connection between the joining partners can comprise a plurality of weld seams which are arranged in a star shape around the cylinder axis and merge into one another.

As another example clarifies 4B that discrete fusion zones 21A, 21B distributed azimuthally on the mating surface 16B can be formed by incrementally rotating the solid cylinders 16, 17, thereby forming a plurality of welds arranged in a star (spaced) pattern about the cylinder axis.

The in the 3A until 4B The welding processes shown are examples of welding processes with a pure rotational movement around a longitudinal axis of the tubes 14, 15/solid cylinders 16, 17, the scanning trajectory of the laser beam 3 running in a plane of maximum curvature of the surface. In addition, a translational movement in the direction of the longitudinal axis of the tubes 14, 15/solid cylinders 16, 17 can take place to bridge the gap. Accordingly, in 3B both joining surfaces 14B, 15B are also melted.

Other exemplary constellations in the formation of welds are in connection with the 12A until 12D explained.

5 shows another example of a laser processing system 1 '. Regarding 2 Similar/comparable features are identified with corresponding reference numbers. The laser beam source 1A (for example a UKP high-power laser system) generates a collimated Gaussian beam with a transverse Gaussian intensity profile as the output laser beam 3'. The output laser beam 3' is irradiated into an optical system 1B'.

In the optical system 1B', on the one hand, the output laser beam 3' is shaped in such a way that the focus zone 7 is formed by the laser beam 3 in the form of a non-diffracting beam. The focus zone 7 is elongated along the beam axis 8 of the laser beam 3 .

For example, a Bessel-Gaussian beam with an ordinary or inverse Bessel-beam-like beam profile is generated with the aid of a beam-shaping element 11 of the optical system 1B'. The beam-shaping element 11 is designed to impress a transverse phase curve on the incident output laser beam 3'. The beam shaping element 11 is, for example, a (hollow cone) axicon, a (hollow cone) axicon lens/mirror system, a reflective axicon lens/mirror system or a diffractive optical beam shaping element. The diffractive optical beam-shaping element can be a programmable or permanently written diffractive optical beam-shaping element, in particular a spatial light modulator (SLM spatial light modulator). For exemplary configurations of the optical system and in particular of the beam-shaping element 11, reference is made to that mentioned at the outset WO 2016/079275 A1 referred. The transversal phase curve is designed in particular in such a way that an elongated focal zone is formed without radiation passing through a curved surface.

5 FIG. 12 schematically shows further beam-guiding components 20 as part of the optical system 1B', such as a telescope arrangement 20A, mirrors, lenses, filters and control modules for aligning the various components.

5 shows a tube 9 as an example of a joining partner, which has an outer radius Ra and an inner radius Ri and a wall thickness Ra-Ri. In 5 the beam axis 8 is directed onto the surface 9A along a normal direction N of the surface 9A and strikes it at an impingement point P.

The optical system 1B' focuses the pulsed laser beam 3 into the tube 9, here into the wall of the tube 9, so that the elongated focus zone 7 is formed there. The elongated focus zone 7 refers here to a three-dimensional intensity distribution that determines the spatial extent of the interaction for the melting of the material with the laser pulse/the laser pulse group in the joining partner. The elongated focus zone 7 defines an elongated volume area in which there is a fluence/intensity. If the fluence/intensity is above the threshold fluence/intensity relevant for the required heating, an elongated melt pool 21C is formed along the elongated focus zone 7 .

One speaks of elongated focal zones when a three-dimensional intensity distribution with regard to a target threshold intensity is characterized by an aspect ratio (expansion in the direction of propagation in relation to the lateral extent transverse to the axis of the focal zone (diameter of the on-axis maximum)) of at least 10:1, for example 20:1 and more or 30:1 and more, or 1000:1 and more. Such an elongated focal zone (without relative movement) can lead to a melt pool with a similar aspect ratio. In general, with such aspect ratios, a maximum change in the lateral extension of the intensity distribution, which causes the melt pool, over the focal zone 7 can be in the range of 50% and less, for example 20% and less, for example in the range of 10% and less. The same applies accordingly to a maximum change in the lateral extent of the weld pool.

In general, when creating a melt pool by means of elongated volume absorption, it applies that as soon as absorption takes place, this absorption/the melt pool can influence the propagation of laser radiation. Therefore, it is advantageous to direct the beam components that lead further down the beam to the melt pool at an angle to the focal zone/to the axis of the interaction zone. An example of such an energy input is the non-diffracting beam, e.g. a (conventional) Bessel-Gaussian beam (see 1 ), in which there is a ring-shaped far-field distribution whose ring width is typically small compared to the radius. In the case of a rotationally symmetrical Bessel-Gaussian beam, the interaction zone/focus zone axis is supplied with radial beam components essentially with this angle in a rotationally symmetrical manner. The same applies to the inverse Bessel-Gauss beam and to modifications such as homogenized, asymmetric or modulated (inverse) Bessel beams.

An example of beam shaping that leads to such an elongated focal zone is in the 6A until 9B clarified. The figures show an example of the arrival of (radial) beam components 3T in a sectional plane through the beam path (see in particular 7B ) at an angle of incidence δ in air or δ' in the material on the beam axis 8 of the laser beam 3. Thus, the elongated focal zone 7 along the beam axis 8 in the material can be caused by interference of the beam components 3T (over a length Z, see 1 ) be formed.

The formation of an elongated (e.g. Bessel beam-based) focal zone presupposes that energy can be supplied laterally over the entire length L of the focal zone and that the conditions for constructive interference are present.

To weld two joining partners, a relative movement takes place between the optical system 1B′ and the joining partners, so that the focal zone 7 can be irradiated at different positions to melt the material. The relative movement is controlled in such a way that molten pools line up along a scanning trajectory T. the inside 5 The arrow illustrating the scanning trajectory T is representative of a movement of the impact point P over the surface of the pipe 9. In particular, during laser welding, a relative movement can be effected in which the focal zone 7 is repeatedly positioned along the scanning trajectory T (at least partially) in the material of the pipe 9 becomes.

For a circumferential weld seam (circular scanning trajectory T) is stored in 5 the workpiece holder 19 rotates the tube 9 about a longitudinal axis A of the tube 9 . Support rollers 19A are indicated as an example. Furthermore, a base unit 19B of the workpiece holder 19 can allow the tube 9 to be displaced along the longitudinal axis A or can adjust the distance from the optical system 1B'.

Alternatively or additionally, a relative movement between the joining partners and the optical system 1B′ can be brought about by moving the optical system 1B′ (or components thereof). In 5 a linear displacement unit 19' of the optical system 1B' is shown by way of example, with which the focal zone 7 can be positioned along the beam axis 8. Further processing axes can be provided, which allow the exiting laser beam 3 and thus the axis of the focal zone to be spatially aligned.

The laser processing system 1' (similar to the laser processing system 1) also has a control unit 23, which in particular has an interface for inputting operating parameters by a user. In general, the control unit 23 includes electronic control components such as a processor for controlling electrical, mechanical and optical components of the laser processing system 1′. For example, operating parameters of the laser beam source 1A such as pump laser power, parameters for setting an optical element (e.g. an SLM) and parameters for the spatial alignment of an optical element of the optical system 1B' and parameters of the workpiece holder 19 (for traversing the scanning trajectory T) can be set (indicated by arrows 23A).

In 5 A distance sensor 25 is also indicated schematically, which is arranged, for example, on the optical system 1B′. The distance sensor 25 is set up to detect the distance between the joining partners and the optical system 1B′. In particular, the distance sensor 25 can determine a position of a surface 9A of the tube 9 along the beam axis 8 in relation to a target position with regard to the elongated focal zone. The target position is specified specifically for a welding process, the respective joining partners and the respective beam formation. For example, the target position can be specified by a desired displacement Δz (in or against the propagation direction of the laser beam) between the beginning of the elongated focal zone and the surface of the joining partner/tube 9 . For example, the beginning of the elongated focal zone can be at the location in the Z-direction at which the intensity has increased to 50% of the maximum intensity. In addition to the beginning of the elongated focus zone meeting the surface of the joining partner, a displacement Δz>0 can be set as the target position, in which the elongated focus zone is already formed in front of the tube 9 and then extends into the material of the tube 9 . Likewise, a shift Δz < 0 can be set, in which case the elongated focal zone forms only in the material of the pipe or at least beginning in the material of the pipe 9 (as in 5 indicated).

The distance sensor 25 outputs distance data to the control unit 23, which can regulate the distance, for example, via the workpiece holder 19 or the linear displacement unit 19' in relation to a specified target position. Distance sensors can be designed, for example, as confocal white-light sensors, white-light interferometers (such as optical coherence tomographs) or capacitive sensors.

In particular, the distance sensor 25 can be used to measure the position of the joining partners with respect to processing optics and/or the geometry of the joining partners in a pre-measurement step or during the welding process. For example, it can happen that the joining partners are mounted in the workpiece holder 19 in a wobbling manner during a rotation. In such a case, the distance between the surface and the specified target position can be measured before the actual processing and the position data of the surface can be saved. For this purpose, the control unit 23 can cause the joining partners to be traversed once along the trajectory to be processed without the laser beam 3 being irradiated. During this pre-measurement, the distance sensor 25 detects the distance data and outputs it to the control unit 23 in which the distance data is stored.

Furthermore, the distance sensor 25 can be designed to measure a curvature of the surface of at least one of the joining partners in a pre-measurement step or during processing. The curvature can be calculated from the distance data, for example. For example, the curvature of the surface of a joining partner can vary along the scanning trajectory, so that the scanning trajectory is traversed in a pre-measurement in order to save data about the curvature of the surface along the scanning trajectory and to use it for later adjustment of a phase-correcting beam shaping. A corresponding preliminary measurement can in turn be controlled by the control unit 23 and the recorded curvature data can be stored in the control unit 23 .

• Laserpulsenergien/Energie einer Laserpulsgruppe (Burst): z.B. im mJ-Bereich und mehr, beispielsweise im Bereich zwischen 20 µJ und 5 mJ (z.B. 1200 µJ), typischerweise zwischen 100 µJ und 1 mJ
• Wellenlängenbereiche: IR, VIS, UV (z.B. 2 µm > λ > 200 nm; z.B. 1550nm, 1064 nm, 1030 nm, 515 nm, 343 nm)
• Pulsdauer (FWHM): einige Pikosekunden (beispielsweise 3 ps) und kürzer, beispielsweise einige hundert oder einige (zehn) Femtosekunden, insbesondere ultrakurze Laserpulse/Laserpulsgruppen
• Anzahl der Laserpulse in einem Burst: z.B. 2 bis 4 Pulse (oder mehr) pro Burst mit einem zeitlichen Abstand im Burst von einigen Nanosekunden (z.B. 20 ns)
• Anzahl der Laserpulse pro Modifikation: ein Laserpuls oder ein Burst für eine Modifikation Repetitionsrate: üblicherweise größer 0.1 kHz, z.B. 100 kHz Länge der Fokuszone im Material: größer 20 µm, bis zu einigen Millimetern
• Durchmesser der Fokuszone im Material: größer 1 µm, bis zu 20 µm und mehr (sich ergebende laterale Ausdehnung der Schmelzzone im Material (ohne Relativbewegung): größer 100 nm, z.B. 300 nm oder 1 µm, bis zu 20 µm und mehr)

Furthermore, the control unit 23 can set parameters of the laser beam 3 . Exemplary parameters of the laser beam 3 that can be used within the scope of this disclosure for the laser welding process are:

• Laser pulse energies/energy of a laser pulse group (burst): eg in the mJ range and more, for example in the range between 20 μJ and 5 mJ (eg 1200 μJ), typically between 100 μJ and 1 mJ
• Wavelength ranges: IR, VIS, UV (e.g. 2 µm > λ > 200 nm; e.g. 1550 nm, 1064 nm, 1030 nm, 515 nm, 343 nm)
• Pulse duration (FWHM): a few picoseconds (e.g. 3 ps) and shorter, for example a few hundred or a few (tens) of femtoseconds, in particular ultra-short laser pulses/laser pulse groups
• Number of laser pulses in a burst: e.g. 2 to 4 pulses (or more) per burst with a time interval in the burst of a few nanoseconds (e.g. 20 ns)
• Number of laser pulses per modification: one laser pulse or one burst for a modification Repetition rate: usually greater than 0.1 kHz, eg 100 kHz Length of the focal zone in the material: greater than 20 µm, up to a few millimeters
• Diameter of the focal zone in the material: greater than 1 µm, up to 20 µm and more (resulting lateral expansion of the melting zone in the material (without relative movement): greater than 100 nm, e.g. 300 nm or 1 µm, up to 20 µm and more)

The pulse duration refers to a single laser pulse. Accordingly, an exposure time refers to a group/burst of laser pulses that lead to the formation of a melt pool at a location in at least one of the joining partners. If the duration of action and the duration of the pulse are short with regard to a given feed rate, one laser pulse and all laser pulses of a group of laser pulses contribute to the formation of a melt pool at one location. With lower feed rates or repeated scanning, continuously melted areas can also occur along the joining surfaces.

The aforementioned parameter ranges can allow the formation of a melt pool that extends up to, for example, 1 mm (typically 20 μm to 500 μm) in the beam propagation direction along a joint surface.

For the formation of elongated focal zones, the focus-forming beam shaping can be set up to cause beam portions of the laser beam to arrive at an angle of incidence onto a beam axis of the laser beam at least partially in the material of the first joining partner and/or in the material of the second joining partner through interference. Thus, the focus length (given essentially by the extent of the molten pool) can be set to be less than or equal to a predetermined joint length in the direction of propagation of the laser beam. In particular, the focal length can be adjusted in a range between 20 µm and 500 µm (e.g. between 100 µm and 500 µm). Furthermore, focal zones with asymmetrical intensity distributions, such as a flattened beam, can be used in laser welding in order to produce correspondingly shaped molten pools (and thus, if appropriate, correspondingly shaped weld seams).

The beam shaping is done in 5 implemented by way of example with a flat diffractive optical beam-shaping element 27 .

The concepts disclosed herein for phase correction with regard to the curved surface of a joining partner is described in 5 implemented by a cylindrical lens 29. A cylinder axis associated with the cylinder lens extends in 5 along the axis A of the tube 9. Furthermore, in 5 schematically indicated that the cylindrical lens 29 can be formed as a phase distribution of an (adjustably deformable) cylindrical mirror or a diffractive optical beam-shaping element 29'. In this case, the diffractive optical beam-shaping element 29' can be embodied as a permanently inscribed diffractive optical beam-shaping element. Furthermore, the diffractive optical beam-shaping element 29' can be an adjustable diffractive optical beam-shaping element. Furthermore, the diffractive optical beam-shaping element 27 and the diffractive optical beam-shaping element 29' can be combined in one diffractive optical beam-shaping element. An adjustable diffractive optical beam-shaping element or a deformable cylinder mirror can be controlled and adjusted by the control unit 23 with regard to the phase-correcting beam shaping to be carried out (see arrow 23A in 5 ).

In general, a diffractive optical beam-shaping element is designed to impose a phase contribution on a transverse beam profile of the output laser beam 3', the diffractive optical beam-shaping element adjoining surface elements (see, for example, for the beam-shaping element 27 in 5 indicated surface elements 27A). The surface elements 27A can build up a surface lattice structure in which a phase shift value is assigned to each surface element 27A. With the help of specially selected phase shift values, an axicon or an inverse axicon, for example, but also a cylindrical lens can be diffractively simulated.

Here, diffractive optical beam-shaping elements and corresponding refractive optics as well as reflective optics implementations are regarded as essentially equivalent optical means with regard to the phase correction to be carried out.

the 6A and 6B show orthogonal sectional views of the beam path (only schematically, not physically) in an optical system to clarify the beam shaping.

6A shows a sectional view in a ZY plane and 6B shows a sectional view in a ZX plane.

The processing optics is used for (Gaussian) Bessel beam generation and includes an axicon 31 with a cone angle γ, so that radial beam components are each at an angle δ on the Converge beam axis 8 and form a first real Bessel beam focal zone (interference zone 33 over a length 10). The Axicon 31 is embedded in two telescopes. A telescope (not shown) positioned upstream matches the beam diameter of the output laser beam 3' to the axicon 31, generally the beam shaping element. Explicitly in the 6A and 6B the telescope arrangement 20A positioned downstream with telescope lenses L1 and L2 and focus lengths f1 and f2 is shown, with which the axicon tip 31_S is imaged in the joining partners in a reduced manner (reduction factor M=f1/f2). The general aim of the illustration is that the interference zone 33 is reduced to form the elongated focus zone 7 (of the non-diffracting laser beam) in the material of at least one of the joining partners. The non-diffractive laser beam hits the curved surface at an impingement point P. In general, the point of impact is understood here as the point of intersection of the optical axis of the laser beam with the arrangement of joining partners. When the axicon tip 31_S is imaged onto the curved surface, there is no shift Δz of the beginning of the non-diffractive laser beam to the entrance surface (ie, Δz=0). In the example of 6A the start of the non-diffracting laser beam lies before the surface by an amount Δz > 0. Alternatively, the start of the non-diffracting laser beam lies behind the surface by an amount Δz < 0 (see Fig 5 and 8A ), ie, in the material so that the surface is not affected by the welding process. For example, |Δz| in the range of 100 µm to 200 µm or more.

After imaging and entering the material, the radial ray fractions in the Z-Y plane in the material extend towards the ray axis 8 at, for example, an angle δ'.

this is in 7A (ideal propagation; without aberrations) for a flat surface 37 shown. The interference of the radial beam components 3T takes place over the entire length.

An exemplary intensity curve I(x, z) is along the beam axis 8 (in the Z direction) in 8A shown how it can be generated with a Bessel beam. An associated transverse intensity curve I(x, y) shows 8B . The intensity curves correspond to those in figure (b) of the 1 .

The goal is to achieve such an intensity curve for a joining partner with a curved surface 9A. However, this is not possible without correcting the optical path in the plane of curvature.

7B (disturbed propagation; with aberrations) clarifies the problem for the entry of a Bessel beam-shaped laser beam into a material through a curved surface 39. Due to the curvature (ie, a locally inclined incidence), the radial beam components in the material run at varying angles δ (r) towards the beam axis 8. In the case of the Bessel beam, the interference conditions are only given at the beginning (in the case of the inverse Bessel beam only at the end) due to the still approximately flat surface in the central area around the beam axis. For example, this is the situation for a surface with a radius of curvature R of 5 mm and a diameter D of the impinging laser beam 3 of, for example, 250 μm to 2 mm.

An exemplary intensity profile I (x, z) is in the 9A and 9B for this situation of entry through a curved surface. 9A shows an intensity profile I(x, z). It can be seen that high intensities are only present over a limited area along the beam axis 8 (in the Z-direction), zones of somewhat higher intensity then form at a distance from the beam axis 8 . In the associated transversal intensity profile I(x, y) of 8B one can see that these off-axis zones are arranged in the X and Y directions.

The wave front aberrations when passing through the curved surface 39 thus result in a focus distribution with a significant loss of intensity in the direction of propagation, so that optical processing of, in particular, deeper-lying areas is no longer possible. Similar effects can also exist with regard to a Gaussian focus.

coming back on 6B Due to the curved surface 9A of the tube 9 in the ZX plane, a non-diffracting beam extending over the entire intended length (ie constructive interference in particular in lower-lying areas) is no longer formed without compensation in the ZX plane , since the conditions regarding the interference in the joining partner differ from those in the interference zone 33 . The desired intensity distribution is therefore no longer generated in the joining partner without compensation.

Assuming the phase compensation concept proposed here has been implemented, the correction phase influences the course of the laser beam in the material in such a way that the radial beam components also in the Z-X plane also essentially run towards the beam axis 8 at the angle δ′.

For phase compensation is in the construction of the 6A and 6B a cylindrical lens 35 is positioned in front of the axicon 31, the refractive effect of which lies in the cross-sectional plane of the tube 9. To illustrate an alternative arrangement, a cylindrical lens 35 'in 6B indicated by dashed lines, which is positioned directly downstream of the axicon 31 in the beam path in the processing optics. The cylindrical lens 35, 35' represents the place where an axisymmetric phase distribution is impressed.

The cylindrical lens has a refractive index nz, a cylinder radius Rz and a focal length fz to compensate for aberrations due to the radius of curvature Ra of the surfaces and a refractive index nw.

Except for the cylindrical lens 35, the optics are in the structure 6A and 6B to be understood as rotationally symmetrical about the beam axis 8 in the case of a rotationally symmetrical axicon 31 .

Because of the cylindrical lens 35, the interference following the axicon 31 will no longer form in a rotationally symmetrical manner, since, for example, the conditions in the interference zone 33 in the Z-Y plane differ from those in the Y-X plane.

In the 5 and in the 6A and 6B The optical systems shown are examples of an optical system for beam shaping of a laser beam for forming a focal zone in a joining partner/joining partners with a curved surface, the focal zone being elongated along a beam axis of the laser beam. On the one hand, the optical systems have focus-forming optics, which allow beam components to enter the beam axis of the laser beam at an angle of incidence for forming the elongated focus zone along the beam axis (ie, for forming a non-diffracting beam) in the material by interference causes. On the other hand, a phase correction is provided in the optical system, which counteracts the interference caused by the laser beam entering the joining partners. The phase correction can generally be implemented as diffractive, refractive and/or reflective. It can, for example, be provided as phase-correcting (separate) optics or be integrated into the focus-forming optics.

For the use of a cylindrical lens 35, 35' as compensation optics, the cylindrical lens 35, 35' should be as close as possible in the plane of the axicon or the diffractive optical element (beam shaping element 27) (if possible directly in front of or behind it).

The selection of the radius of curvature Rz of the surface of the cylindrical lens depends on the relative position of the non-diffracting beam with respect to the joining partner, in particular from the beginning of the elongated focal zone with respect to the entrance surface of the joining partner.

mit

Rz

Krümmungsradius der Zylinderlinse,

nz

Brechungsindex der Zylinderlinse,

Rw

Krümmungsradius der Oberfläche des Fügepartners,

nw

Brechungsindex des Materials des Fügepartners und

M

M = f1 / f2 - Abbildungsfaktor des Strahlengangs zwischen einem Ort des Aufprägens der Phasenverteilung und dem Fügepartner.

In the event that there is no shift Δz of the beginning of the elongated focal zone to the entrance surface (Δz = 0), the radius of curvature Rz of the cylindrical lens is calculated to a good approximation from fz ≈ Rw M ² / (n-1) and fz ≈ Rz / ( nz-1) $$R_{\mathrm{e.g}}\approx \left(n_{\mathrm{e.g}}-1\right)\cdot \left[-\frac{{\mathrm{<figure-callout\; id="2"\; label="M"\; filenames="DE102021111879A1\_0020.png,DE102021111879A1\_0021.png"\; state="\{\{state\}\}">M</figure-callout>}}^2\cdot R_w}{n_w-1}\right]$$

With

para

radius of curvature of the cylindrical lens,

na

refractive index of cylindrical lens,

Rw

radius of curvature of the surface of the joining partner,

nw

Refractive index of the material of the joining partner and

M

M = f1 / f2 - imaging factor of the beam path between a location where the phase distribution is impressed and the joining partner.

If one takes into account that the elongated focal zone begins in front of the surface of the joining partner (Δz greater than 0) or falls into the joining partner (Δz less than 0), the following approximate definitions of the radius of curvature R _z of the surface of the cylindrical lens result, each for Δz = 0 of the meets the above condition.

Δz > 0 (with the additional parameters a = 5060 mm ^-2 and b = 9645 mm ^-1 ): $$R_{\text{e.g}}\left(\Delta \text{e.g}\ge \mathrm{0,}{R}_{\text{w}}\right)\approx \left(n_{\text{e.g}}-1\right)\cdot \left[\left(b-a\cdot R_{\text{w}}\right)\cdot {\left(\Delta \mathrm{e.g}\right)}^2-\frac{M^2\cdot R_{\text{w}}}{n_{\text{w}}-1}\right]$$

Δz ≤ 0 (with the additional parameters c = 284 mm ^-1 and d = 590): $$R_{\text{e.g}}\left(\Delta \text{e.g}\le \mathrm{0,}{R}_{\text{w}}\right)\approx \left(n_{\text{e.g}}-1\right)\cdot \left[\left(\mathrm{i.e}-c\cdot R_{\text{w}}\right)\cdot \Delta \mathrm{e.g}-\frac{{\mathrm{<figure-callout\; id="2"\; label="M"\; filenames="DE102021111879A1\_0020.png,DE102021111879A1\_0021.png"\; state="\{\{state\}\}">M</figure-callout>}}^2\cdot R_{\text{w}}}{n_{\text{w}}-1}\right]$$

An approximately linear curve can be seen for Δz < 0 and an approximately quadratic curve for Δz > 0.

As in 5 In the case of a "positive" curved joining partner (convex in the section plane), a "negative" cylindrical lens convex is indicated exercise (concave) to be provided. In other words, a phase distribution caused by the cylindrical lens has a scattering effect (and not a collecting effect, as occurs when it enters the joining partner). Those skilled in the art will appreciate that with the concepts disclosed herein, mating partners having a concavely curved surface (e.g., for laser welding along a rod having a trough) can also be welded by providing a "positive" cylindrical lens curvature (convex).

Radii of curvature are considered herein to be generally in a sectional plane transverse to the longitudinal axis of the curved-surface join partner to be welded. A radius of curvature is the inverse of a concave shape for a joining partner with a convex shape in the section plane (round tube surface). The curvature of the correcting optics (or a “curvature” that can be assigned to the corresponding phase curves) is inverted in accordance with the curvature of the joining partner. This is indicated in the above formula for by the factor (-1). A radius of curvature R _z of less than zero/negative cylindrical lens can be seen for a concave shape of the joining partner. Accordingly, a radius of curvature R _z greater than zero/positive cylindrical lens results for a convex shape of the joining partner. Those skilled in the art will appreciate that in addition to plano-convex or plano-concave cylindrical lenses (see 5 ) corresponding lenses curved on both sides with the appropriate refractive behavior can be used.

The following are based on the 10A until 10F Phase distributions with diffractive optical beam shaping elements explained.

Exemplary phase distributions for a realistic laser welding process based on phase progressions as they correspond to a 2° axicon and a 1000 mm cylindrical lens are given in the 10A until 10C shown for center cutouts of eg 1 inch diameter DOEs. Since the phase distribution for the 2° axicon dominates the combined phase distribution, the 10D until 10F to illustrate an example case based on phase curves, as they can be assigned to a 0.5° axicon and a 200 mm cylindrical lens.

10A shows a two-dimensional phase distribution PHI_BESSEL(x, y) [in rad] for a diffractive optical element that brings about focus-forming beam shaping. In particular, the phase distribution PHI_BESSEL(x,y) can impose a symmetric Bessel beam phase distribution on an incident Gaussian beam (to produce a Bessel-Gaussian beam). In the phase distribution PHI_BESSEL(x, y) you can see ring-shaped constant phase shift values that run radially in a sawtooth shape between -PI and +PI. The phase shift values represent first phase contributions 25A of the beam formation and cause beam components to arrive at a beam axis of the laser beam at an angle of incidence for formation of an elongated focus zone along the beam axis in the material by interference. The elongated focus zone roughly corresponds to the focus zone created with a 2° axicon (γ = 2°).

10B shows a two-dimensional phase distribution PHI_ZYL(x, y) for a diffractive optical element, which causes a phase-correcting beam shaping, as is the case when welding pipes with an outer radius of 5 mm with an elongated focal zone, as is the case with the phase distribution PHI_BESSEL(x, y) is generated can be used. The phase curve roughly corresponds to that of a 1000 mm cylindrical lens with a cylinder radius of "Rz ≈ -500 mm".

One recognizes constant phase shift values in the Y direction, which run (increase) squarely in the X direction in a sawtooth-like form between −PI and +PI in the X direction and represent second phase contributions 25B of the beam shaping. The second phase contributions 25B form a phase distribution that is symmetrical to an axis of symmetry S, the second phase contributions 25B being constant parallel to the axis of symmetry S (in the y-direction) and changing perpendicularly to the axis of symmetry S.

Assuming an imaging factor M = 10 and correspondingly usual refractive indices for the cylindrical lens and the tube, the second phase contributions 25B can cancel out the entry phase, which occurs locally, i.e., when a laser beam enters the tube with the outer radius of 5 mm. on a surface element on the curved surface.

The two-dimensional phase distribution PHI_BESSEL(x, y) and the phase distribution PHI_ZYL(x, y) can be generated together with a diffractive optical element. 10C shows a corresponding superimposed two-dimensional phase distribution PHI total (x, y), in which the two-dimensional phase distribution PHI_BESSEL(x, y) dominates in appearance.

To clarify the appearance of a superimposition of a point-symmetrical phase curve for Bessel beam generation of an axisymmetric phase curve for phase correction 10D a phase distribution PHI_BESSEL(x,y) to generate an elongated focal zone that roughly corresponds to the focal zone generated with a 0.5° axicon. 10E indicates a two-dimensional phase distribution PHI_ZYL(x, y), the phase curve of which roughly corresponds to that of a 200 mm cylindrical lens with a cylinder radius of approx. -100 mm.

In 10F a corresponding deformation of the phase distribution PHI_BESSEL(x, y) can now be seen in the superimposed two-dimensional phase distribution PHI total(x, y). With the phase distribution PHI_BESSEL(x, y) of 10F pipes with an outer radius of 1 mm (at M = 10) could be welded. It is noted again that the 10D until 10F underlying parameters serve purely to clarify the phase progression and are not intended to represent a realistic example.

11 shows a flowchart of a method for forming a welded joint between a first joining partner and a second joining partner using a laser beam, in particular an ultra-short pulse laser beam, the welded joint welding a first joining surface of the first joining partner to a second joining surface of the second joining partner.

In a step 100, 2 parts to be joined are spatially mounted in such a way that the associated joining surfaces are spatially firmly aligned with one another. The joining surfaces usually contact each other or have a distance/gap of a maximum size of a few micrometers (less than 15 μm, for example).

In a step 101, a laser beam is shaped to form a (in particular elongated) focal zone in the area of the joining surfaces of the joining partners. Beam shaping is performed with an array of diffractive and/or refractive and/or reflective optics. Step 101 includes the sub-steps of focus-forming beam shaping 101A and phase-correcting beam shaping 101B.

The focus-forming beam shaping 101A leads, for example, to the formation of the focal zone in the event that no curved surface would have to be traversed. For example, in the area of the joining surfaces, it causes the formation of a Gaussian focus or that beam components run in at an angle of incidence onto a beam axis of the laser beam for the formation of an elongated focus zone along the beam axis by interference.

The phase-correcting beam shaping 101B counteracts an influencing of the interference by the entry of the laser beam into the joining partner with the curved surface or into the joining partner with the curved surfaces.

Steps 101A and 101B can also be carried out in combination in a common phase imprinting step. In step 101, a two-dimensional phase distribution can be applied to the laser beam to form the focus zone, with the phase distribution for the focus-forming beam shaping comprising first phase contributions, which e.g. cause beam components to arrive at the angle of arrival, and/or for the phase-correcting beam shaping comprising second phase contributions , which cancel an entry phase locally accumulated by the laser beam when it enters the joining partner(s). The accumulated entry phase is determined for an alignment of the beam axis along a normal direction of the surface at a point of impact of the beam axis on the surface. It takes into account the angle of incidence (δ'), the radius of curvature Rw of the surface of the joining partner at the point of impact and the refractive index nw of the material of the joining partner (especially the material in the area where the beam enters).

In a step 103, for example, the symmetrical phase distribution and the joining partners can be aligned in such a way that an axis of symmetry of the phase distribution of the second phase contributions, taking into account the beam path, runs orthogonally to a plane between a location where this axisymmetric phase distribution is impressed and the joining partners, in which a ( maximum) radius of curvature of the surface is defined.

In a step 105, beam parameters of the laser beam are set in such a way that the material of at least one of the joining partners is melted.

In a step 107, the laser beam is radiated onto the curved surface of the joining partner in such a way that at least part of the laser beam enters the joining partner through the curved surface. The beam axis of the laser beam can be aligned to a normal direction of the surface (step 107A) such that the beam axis impinges on the surface in an angular range of 10° around the normal direction and preferably along the normal direction. In the material of the joining partner(s), laser radiation propagates to the first joining surface(s) and creates the focal zone. This at least partially overlaps with the material of one of the joining partners.

In a step 109, a relative movement is effected between at least one of the joining partners and the focal zone, in which the focal zone is positioned along a scanning trajectory in the material of at least one of the joining partners. Ent accordingly, a plurality of melt zones or a continuous melt zone can be formed in the area of the joining surfaces along the scanning trajectory.

For example, the focal zone is moved along the first joining surface while maintaining an alignment of the laser beam with respect to a curvature of the curved surface. For example, the focus zone can be brought about by a rotation of one or both joining partners about an axis assigned to the curvature of the curved surface. Alternatively or additionally, the laser beam (of the beam-shaping optics) can be rotated about the first joining partner and/or the second joining partner.

The relative movement can be controlled in step 109 as a rotational movement of at least one of the joining partners, in which the beam axis of the laser beam runs in particular through a longitudinal axis of the joining partner. In the case of a purely rotational movement, the scanning trajectory of the laser beam can run in a plane with the maximum curvature of the surface of the joining partner. Alternatively or additionally, a translational movement can be controlled, for example transversely, in order to expand the area of the melt pool laterally.

In a step 111, a position of the surface along the beam axis can optionally be monitored and regulated to a target position (target distance from the optical system). The monitoring and regulation is carried out in particular when a rotational axis of the rotational movement deviates from an axis of rotational symmetry of the surface and/or the surface deviates at least in sections from a rotationally symmetrical surface profile.

In particular, the use of elongated focal zones can (at least partially) replace a translational movement in the direction of propagation. It is noted that a change in distance can affect the phase-correcting beamforming to be carried out. Accordingly, if the distance changes, the phase-correcting beam shaping would have to be changed or the possible deviation in the focus geometry would have to be worked with.

Similarly, in a step 113, a phase-correcting beam shaping 101B′ can be adjusted in its compensation effect on the curvature of the surface that is present in each case if the trajectory runs in a region with varying curvature. In this way, for example, an influence on the interference caused by the entry of the laser beam into the material can be counteracted for the existing curvature of the surface. The phase-correcting beam shaping is adapted, for example, by taking into account the curvature that is present in each case in the two-dimensional phase distribution of the beam-shaping element. For example, the second phase contributions 25B provided for such a phase correction can be adjusted accordingly in an adjustable SLM. Alternatively, for example, the curvature of a deformable mirror can be adjusted.

The adjustment can be based, for example, on measurements that are carried out during the welding process. A correspondingly fast analysis unit for the geometry of the joining partners must be provided. Alternatively or in addition, in a step 115, a preliminary measurement of the geometry of the joining partners can be carried out along the scanning trajectory. For the preliminary measurement, the laser processing system can, for example, follow the scanning trajectory to be traversed for the welding process without activating the laser beam source for measuring the geometry of the joining partners.

12A clarifies, as a further example of a welding process, the constellation of an angled butt joint 51 when welding two pipes 53A, 53B as joining partners. That is, each of the joining partners has a cylindrical basic shape. However, in this case, the cylinder axes 55A, 55B extend at an angle. In contrast to the arrangement of 3A , in which the cylinder axes are aligned, the tubes 53A, 53B have a different geometry of incidence on the curved outer surfaces with respect to the laser beam 3: orthogonal incidence on the cylinder axis 55A in the case of the tube 53A and oblique incidence on the cylinder axis 55B in the case of the tube 53B.

For example, the focus zone 7 overlaps with the material of the tubes 53A, 53B on both tube end faces (as joining faces).

One recognizes in 12A that phase-correcting beam formations for beam components 57A, 57B, which each propagate through one of the joining parts, can differ. For example, the beam portion 57A as in the example of FIG 3A be compensated for, whereas the beam component 57B could be adjusted depending on the rotation angle due to the oblique incidence.

Because if a rotation (arrow 59) takes place around the cylinder axis 55A during the welding process, it should be noted that the ideal phase-correcting beam shaping for the beam component 57B changes as the angle of incidence on the cylinder axis 55B changes during rotation.

12B clarifies, as a further example of a welding process, the constellation of a butt joint 61 when welding a transparent glass tube 63A (first joining partner) to an opaque rod 63B (second joining partner). The material of the rod 63B is essentially opaque to the laser radiation of the laser beam 3. As in the arrangement of FIGS 3A the cylinder axes 65A, 65B of the glass tube 63A and the rod 63B are aligned with one another, so that the curved outer surfaces of the glass tube 63A and the rod 63B merge into one another.

The material of the opaque rod 63B already absorbs an incident beam portion 67B of the laser radiation of the laser beam 3 with linear absorption on the surface of the opaque rod 63B, so that the beam portion 67B cannot be used to form the melt pool in the area of the joining surfaces.

However, due to the phase-correcting beam shaping, a beam component 67A spreads out in the direction of the joining surface of the opaque rod 63B in such a way that a focal zone, in particular also an elongated one, can be formed there. In view of the losses, sufficiently high intensities are to be generated for the remaining beam portion 67A. In particular, the material of the opaque rod 63B can be heated via linear absorption of the laser beam 3 . The material of the glass tube 63A can also be heated by non-linear absorption and/or indirectly by the heated material of the opaque rod 63B in order to form a common molten pool of the joining partners in the focus zone 7.

12C clarifies as a further example of a welding process the constellation of an overlap joint 71 when welding a transparent glass tube 73A (first joining partner: hollow cylinder) into which a glass rod 73B (second joining partner: cylindrical basic shape) was partially inserted. That is to say, the glass tube 73A has, at least in sections, a hollow-cylindrical shape with respect to a cylinder axis 75, along which the glass rod 73B also extends.

The first joining surface is a portion of an inner surface 77A of the transparent glass tube 73A. A section of an outer surface 77B of the glass rod 73B forms a second joining surface, the course of the second joining surface essentially following a course of the first joining surface.

In 12C a melting of material of the glass tube 73A and material of the glass rod 73B with an elongated focus zone 7 is effected. The use of the elongated focus zone 7 can bridge a possible gap between the joining partners. In addition to a rotational movement (arrow 79A) of the focal zone 7 around the cylinder axis 75, the focal zone 7 can also be moved with a relative translational movement (79B) between the laser beam 3 and the joining partners along the cylinder axis 75, for example to produce a spiral weld seam.

12D FIG. 12 illustrates, as a further example of a welding process, the constellation of a longitudinal weld 81 of a pipe that is butt-welded from two half-shells 83A, 83B as joining partners. The half-shells 83A, 83B are formed along a direction of extension (cylinder axis 85 of the tube to be produced) and in 12D shown spaced apart for clarity. Curved surfaces 87A, 87B of the half-shells 83A, 83B together form a cylinder surface around the cylinder axis 85 at least in sections in the area of the weld seam. In general, the joining partners include an azimuthal section of a cylindrical or hollow-cylindrical basic shape - in 12D in the form of a half shell.

Side surfaces 88A, 88B of the half-shells 83A, 83B form the joining surfaces and extend in the radial direction with respect to the cylinder axis 85. The laser beam 3 is radiated in the radial direction onto the cylinder axis 85, with phase-correcting beam shaping compensating for aberrations due to the curved surfaces 87A, 87B. A step-by-step translational movement (arrow 89) of the focal zone 7, which is elongated (e.g. over 80% of the wall thickness), takes place along the cylinder axis 85, so that the half-shells 83A, 83B are joined together. In the process, spatially separated melting zones 91 are formed, which run towards the cylinder axis 85 in the radial direction.

Joining partners such as pipes, cylinders or sections of a pipe or cylinder, such as a half-pipe or half-cylinder, can be welded with the welding methods described here.

In the optics systems exemplified herein, separate optics are shown for beam shaping and phase compensation. However, these optics can also be implemented in a single optic (e.g. as a refractive/reflective free-form element or as a diffractive optical element) or in a hybrid optical element (cylindrical lens on the input side, axicon on the output side; "Zaxicon").

For the sake of completeness, it is pointed out that, in addition to a symmetrical intensity distribution in a focal zone, there is also a phase imprint e.g. with a diffractive optical element, which leads to an intensity distribution in the focal zone that is asymmetrical (e.g. flattened in one direction) or produces several intensity ranges running parallel to one another (see figure (c) of the 1 ). Exemplary phase impressions and intensity distributions are, for example, in the German patent application 10 2019 128 362.0 , "Segmented beam-shaping element and laser processing system", filed October 21, 2019 by the applicant and in K. Chen et al., "Generalized axicon-based generation of nondiffracting beams", arXiv:1911.03103v1 [physics.optics] 8 Nov 2019 disclosed. In other words, beam shaping, which is to be carried out for such asymmetric intensity distribution, can be combined with phase correction, which can correct the influence on the phase distribution when entering the material.

Another aspect discloses optical systems for beam shaping of a pulsed laser beam, in particular ultra-short laser pulses, for forming a focal zone in at least one joining partner with a curved surface, the focal zone being elongated along a beam axis of the laser beam. The optical system includes focus-forming optics, which causes beam components to arrive at an angle of incidence onto a beam axis of the laser beam for formation of the elongated focus zone along the beam axis in the material of the joining partner by interference. A phase correction, which counteracts the interference caused by the laser beam entering the joining partner, is provided with phase-correcting optics or is integrated into the focus-forming optics. In other words, the optical systems comprise phase-correcting optics which effect phase correction which counteracts the interference being influenced by the laser beam entering the material, or such a phase correction is integrated into the focus-forming optics.

In some developments of the method, the locally accumulated entry phase for an alignment of the beam axis along a normal direction of the surface at a point of impact of the beam axis on the surface can be determined and the angle of arrival, a radius of curvature of the surface at the point of impact and - a refractive index of the material of a joining partner can be taken into account.

In some developments of the method, phase contributions can be impressed on a transverse beam profile of the laser beam with a diffractive optical beam-shaping element, the diffractive optical beam-shaping element having surface elements which adjoin one another and build up a surface grating structure in which each surface element is assigned a phase shift value, and the phase shift values cause the phase contributions.

In some developments of the method, the phase-correcting beam shaping can be generated by a cylindrical lens, which is positioned in a beam path of the laser beam before or after an optical system that effects the focus-forming beam shaping. The cylindrical lens may have a radius of curvature that matches a radius of curvature of the surface.

• - einem diffraktiven optischen Strahlformungselement, das festeingestellte oder einstellbare Phasenwerte in einer zweidimensionalen Anordnung aufweist; oder
• - einer Kombination einer Zylinderlinse mit einem Axicon; oder
• - einer Kombination einer Zylinderlinse mit einem diffraktiven optischen Strahlformungselement, das festeingestellte oder einstellbare Phasenwerte in einer zweidimensionalen Anordnung aufweist, die zum Aufprägen einer einen nicht-beugenden Strahl bewirkenden Phasenverteilung, insbesondere einer Bessel-Strahl-artigen Phasenverteilung, für die Ausbildung der langgezogenen Fokuszone ausgebildet ist.

In some developments of the method, the beam shaping of the laser beam can be carried out by impressing a two-dimensional phase distribution on a transverse beam profile of an output laser beam with:

• - a diffractive optical beam-shaping element having fixed or adjustable phase values in a two-dimensional array; or
• - a combination of a cylindrical lens with an axicon; or
• - a combination of a cylindrical lens with a diffractive optical beam-shaping element, which has fixed or adjustable phase values in a two-dimensional arrangement, which is designed to impress a phase distribution causing a non-diffracting beam, in particular a Bessel beam-like phase distribution, for the formation of the elongated focal zone is.

• Überwachen und Regeln einer Position der Oberfläche des Fügepartners entlang der Strahlachse auf eine Soll-Position.

In some developments, the methods can further include the step:

• Monitoring and controlling a position of the surface of the joining partner along the beam axis to a target position.

The monitoring and regulation can be carried out in particular if, during a rotational movement, a rotational axis deviates from an axis of rotational symmetry of the surface of the joining partner and/or the surface of the joining partner deviates at least in sections from a rotationally symmetrical surface profile.

In some developments, the methods may further include adjusting the phase-correcting beam shaping to a change in curvature of the curved surface along the scanning trajectory of the laser beam. For example, a control signal to adjust sen the phase-correcting beam shaping based on a pre-measurement of a curvature of the curved surface along the welding trajectory and / or based on an online measurement of a curvature of the curved surface during a relative movement between the joining partner / the joining partners and the focal zone along the scanning trajectory. The control signal can then be output to an adjustable beam-shaping element, such as a spatial light modulator or deformable mirror, to adjust the phase contributions for phase-correcting beam-shaping.

In some developments, the optical systems can be set up to impress a two-dimensional phase distribution on the laser beam and to output it as a laser beam that forms a non-diffracting beam, in particular a real or virtual Bessel-like one, with the focus-forming optics generating first phase contributions to the phase distribution and the phase-correcting optics or the phase correction can generate second phase contributions to the phase distribution, which cancel out an entry phase locally accumulated by the laser beam when it enters the joining partners.

In some developments of the optical systems, the focus-forming optics and/or the phase-correcting optics for the phase imposition of a two-dimensional phase distribution can be designed as a diffractive optical beam-shaping element that is set up to impress the first phase contributions and/or the second phase contributions on a transverse beam profile of the laser beam . The diffractive optical beam-shaping element can have surface elements adjoining one another, which build up a surface grating structure in which each surface element is assigned a phase shift value, and the phase shift values bring about the first phase contributions and/or the second phase contributions. Alternatively or additionally, the focus-forming optics can be designed as an axicon that generates the focus-forming phase contributions. Furthermore, as an alternative or in addition, the phase-correcting optics can be designed as a cylindrical lens, which generates the second phase contributions and is positioned directly before or after the focus-forming optics in the beam path of the laser beam. Alternatively or additionally, the focus-forming optics can be designed as a refractive free-form element that generates the first phase contributions and the second phase contributions. Alternatively or additionally, the focus-forming optics and the phase-correcting optics can be embodied as hybrid optics that generate the first phase contributions and the second phase contributions and are embodied in particular as a combination of an input-side cylindrical lens and an output-side axicon.

In some developments of the optical systems, the phase-correcting optics for the phase imposition of a two-dimensional phase distribution can be designed as an optical element that can be adjusted in the two-dimensional phase distribution. For example, it can be designed as a diffractive optical beam-shaping element such as a spatial light modulator or a deformable cylinder mirror. Furthermore, the adjustable optical element can be designed for an adjustment of the phase corrections when there is a change in a curvature of the curved surface that is to be corrected, as a function of a control signal.

• eine Teleskopanordnung zum Verkleinern einer realen oder virtuellen Fokuszone, die der fokusbildenden Optik zugeordnet ist, und/oder
• einen Abstandssensor, der dazu eingerichtet ist, eine Position einer Oberfläche des Fügepartners entlang der Strahlachse zu bestimmen.

In some developments, the optical systems can also include:

• a telescope arrangement for reducing a real or virtual focal zone, which is assigned to the focus-forming optics, and/or
• a distance sensor that is set up to determine a position of a surface of the joining partner along the beam axis.

• - die Strahlachse des Laserstrahls zu einer Normalenrichtung der Oberfläche derart auszurichten, dass die Strahlachse in einem Winkelbereich von 5° oder 10° um die Normalenrichtung, bevorzugt entlang der Normalenrichtung, auf die Oberfläche auftrifft, und/oder
• - eine Relativbewegung zwischen dem Fügepartner und der Fokuszone zu bewirken, bei der die Fokuszone entlang einer Abtasttrajektorie im Material positioniert wird, wobei die Ausrichtung der Strahlachse zur Normalenrichtung an den Verlauf der Oberfläche angepasst wird.

In some developments of the laser processing system, the optical system and/or the workpiece holder can be set up to:

• - aligning the beam axis of the laser beam to a normal direction of the surface in such a way that the beam axis impinges on the surface in an angular range of 5° or 10° around the normal direction, preferably along the normal direction, and/or
• - bring about a relative movement between the joining partner and the focal zone, in which the focal zone is positioned along a scanning trajectory in the material, with the alignment of the beam axis to the normal direction being adapted to the course of the surface.

In some developments, the laser processing system also includes
a distance sensor which is arranged and set up to determine a position of a surface of the joining partner along the beam axis, and
a controller that is set up to monitor a position of the surface of the joining partner along the beam axis with the distance sensor and to regulate it to a target position.

In some developments of the laser processing system, the optical system can have phase-correcting optics for phase imprinting of a two-dimensional phase distribution, which is adjustable in the two-dimensional phase distribution. For this purpose, the laser processing system can also include a controller that is set up to output a control signal to the adjustable optical element that adapts the two-dimensional phase distribution to a curvature of the curved surface of the joining partner that is to be corrected. The control signal can be provided, in particular by the control unit, based on a pre-measurement of a curvature of the curved surface along a scanning trajectory or based on an online measurement of a curvature of the curved surface during a relative movement between the joining partner and the focus zone along a scanning trajectory.

In some developments, the laser processing systems can also have a distance sensor that is arranged and set up to determine a position of a surface of the joining partner along the beam axis. Furthermore, the laser processing systems can have a controller that is set up to monitor a position of the surface of the joining partner along the beam axis with the distance sensor and to regulate it to a target position.

It is explicitly emphasized that all features disclosed in the description and/or the claims are to be regarded as separate and independent from each other for the purpose of original disclosure as well as for the purpose of limiting the claimed invention independently of the combinations of features in the embodiments and/or the claims must. It is explicitly stated that all indications of ranges or groups of units disclose every possible intermediate value or subgroup of units for the purpose of original disclosure as well as for the purpose of limiting the claimed invention, in particular also as a limit of a range indication.

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• WO 2016/079062 A1 [0030]
• WO 2016/079063 A1 [0030]
• WO 2016/079275 A1 [0030, 0077]
• DE 102019128362 [0166]

Claims (20)

Method for forming a welded joint between a first joining partner (14) and a second joining partner (15) using a laser beam (3), in particular an ultra-short pulse laser beam, the welded joint having a first joining surface (14B) of the first joining partner (14) with a second Joining surface (15B) of the second joining partner (15) is welded and at least the first joining partner (14) has a curved surface (14A), with the steps: Storing the first joining partner (14) and the second joining partner (15) in such a way that the first joining surface (14B) and the second joining surface (15B) are spatially firmly aligned with one another, Beam shaping of the laser beam (3) to form a focus zone (7) on the first joining surface (14B), the beam shaping being carried out with an arrangement of diffractive, reflective and/or refractive optics and comprising: - a focus-forming beam shaping and - a phase-correcting beam shaping which, for a predetermined alignment of a beam axis (8) of the laser beam (3) when the laser beam (3) enters the first joining partner (14), influences the propagation of the laser beam (3) in the material of the first joining partner (14 ) counteracted by the curved surface (14A), Radiating the laser beam (3) onto the curved surface (14A) in such a way that at least part of the laser beam (3) enters the first joining partner (14) through the curved surface (14A), in a material of the first joining partner (14). the first joining surface (14B) and the focal zone (7) is generated at least partially overlapping with the material of the first joining partner (14) and/or the second joining partner (15), Setting beam parameters of the laser beam (3) in such a way that the material of the first joining partner (14) and/or the second joining partner (15) is melted in the focal zone (7), and Forming the welded connection between the first joining partner (14) and the second joining partner (15) by cooling the melted material. procedure after claim 1 , further comprising moving the focal zone (7) along the first joining surface (14B) while maintaining an orientation of the laser beam (3) with respect to a curvature of the curved surface (14A), and optionally wherein the laser beam (3) is so directed onto the curved surface (14A) that the beam axis (8) of the laser beam (3) is in an angle of incidence range of ±10° to a normal direction (N) of the curved surface (14A) at an impingement point (P) of the beam axis (8) on the curved Surface (14A) is aligned. procedure after claim 2 , the movement of the focal zone (7) by rotating - the first joining partner (14) and/or the second joining partner (15) about an axis associated with the curvature of the curved surface (14A) and/or - the laser beam (3). the first joining partner (14) and the second joining partner (15). procedure after claim 3 , wherein the curved surface (14A) is formed at least in sections as a cylinder jacket surface with respect to a cylinder axis and wherein the laser beam (3) is radiated onto the cylinder axis in a radial direction and the rotation takes place around the cylinder axis. procedure after claim 3 or 4 , wherein the first joining partner (14) has at least partially a cylindrical or hollow-cylindrical shape with respect to a cylinder axis and the first joining surface (14B) is a butt joint surface of the cylindrical or hollow-cylindrical shape. procedure after claim 4 or 5 , wherein the second joining partner (15) has a cylindrical basic shape that extends along the cylinder axis or at an angle to the cylinder axis of the first joining partner (14), and wherein the focal zone (7) is optionally also formed on the second joining surface (15B). and at least partially overlaps with the material of the second joining partner (15). Procedure according to one of Claims 1 until 4 , wherein the first joining partner has at least in sections a hollow-cylindrical shape with respect to a cylinder axis (50) and the first joining surface is an inner surface (77A) of the hollow-cylindrical shape and wherein an outer surface (77B) of the second joining partner has a section whose course at least partially corresponds to a course follows the inner surface (77A) of the first joining partner, and wherein optionally the movement of the focal zone (7) comprises a relative translational movement between the laser beam (3) and the first joining partner. procedure after claim 7 , wherein the first joining partner is a hollow cylinder and the second joining partner has a cylindrical basic shape. procedure after claim 2 , wherein the movement of the focal zone (7) is effected by a relative translational movement between the laser beam (3) and the first joining partner and the second joining partner. procedure after claim 9 , wherein the curved surface at least partially is designed as a cylindrical lateral surface with respect to a cylinder axis (85) and the first joining partner has an azimuthal section of a cylindrical or hollow-cylindrical shape and the first joining surface is a side surface (88A) of the azimuthal section extending in the direction of the cylinder axis (85) and the laser beam (3) is irradiated in the radial direction onto the cylinder axis and the translational movement takes place along the cylinder axis (85). Method according to one of the preceding claims, wherein the focus-forming beam shaping is set up to allow beam portions (3T) of the laser beam (3) to arrive at an incidence angle (δ') onto a beam axis (8) of the laser beam (3) to form an elongated effecting a focal zone (7) along a beam axis (8) at least partially in the material of the first joining partner (14) and/or in the material of the second joining partner (15) by interference, and the method further comprises: Setting the focus length (L) to be less than or equal to a predetermined gap length in a propagation direction of the laser beam (3), in particular setting the focus length (L) in a range between 100 µm and 500 µm. Method according to one of the preceding claims, wherein the first joining partner (14) is designed as a hollow body, in particular a glass tube, with a wall delimiting an interior space with a wall thickness, and wherein the method further has the step: - Setting a focus length (L) of the focus zone (7) in a range between 20 µm and 500 µm and/or to a maximum of the wall thickness, in particular to a maximum of 80% of the wall thickness, with the focus zone (7) optionally on the curved surface (14 A) of the first joining partner (14), and wherein the welded connection between the first joining partner (14) and the second joining partner (15) comprises a plurality of weld seams which are arranged in a star shape and which optionally merge into one another. Method according to one of the preceding claims, wherein the first joining partner (14) is a solid cylinder, in particular a glass cylinder, and wherein the method further has the step: - Setting a focal length (L) of the focal zone (7) in a range between 20 µm and 500 µm and/or to a maximum of half the diameter of the cylinder, with the focal zone (7) optionally being on the curved surface (14A) of the joining partner (14 ) begins, and wherein the welded connection between the first joining partner (14) and the second joining partner (15) comprises a plurality of weld seams which are arranged in a star shape and which optionally merge into one another. Procedure according to one of Claims 1 until 10 , wherein the focus-forming beam shaping is set up to generate a Gaussian focus. Procedure according to one of claims 2 until 14 , wherein the movement of the focal zone (7) further comprises a transverse oscillating movement of the focal zone (7) perpendicular to the first joining surface (14B) and/or a translational movement of the focal zone (7) in the direction of the beam axis (8). Method according to one of the preceding claims, wherein the material of the second joining partner (15) is transparent, partially transparent or non-transparent with regard to the laser beam (3) and is melted close to the surface on the second joining surface (15B) of the second joining partner (15), the material of the second joining partner (15) is heated in particular via linear absorption of the laser beam (3) and the material of the first joining partner (15) is heated in particular by non-linear absorption of the laser beam (3) and/or indirectly by the heated material of the second joining partner (15). becomes. Method according to one of the preceding claims, wherein the curved surface (14A) is curved in one direction, and the beam shaping of the laser beam (3) comprises imposing at least one two-dimensional phase distribution on the laser beam (3), the at least one two-dimensional phase distribution comprising: - phase contributions (25B) for the phase-correcting beam formation, which cancel out an entry phase locally accumulated by the laser beam (3) when entering the first joining partner (14) and - optional phase contributions (25A) for the focus-forming beam shaping, which cause the arrival of beam components (3T) at an arrival angle (δ') and in particular a non-diffracting beam for the formation of an elongated focus zone (7) along the beam axis (8) in the Create the area of the joining surfaces (14 B, 15B). procedure after Claim 17 , wherein the phase contributions (25B) form a phase distribution that is axisymmetric to an axis of symmetry (S), the second phase contributions being constant parallel to the axis of symmetry (S) and changing perpendicularly to the axis of symmetry (S), further with the step: aligning the axisymmetric phase distribution and of the first joining partner (14) to one another in such a way that the axis of symmetry (S), taking into account a beam path, is between a location where the axisymmetric phases are impressed distribution and the first partner (14) is orthogonal to a plane in which a radius of curvature of the surface (14A) is defined. Method according to one of the preceding claims, wherein the phase-correcting beam shaping is generated by a cylindrical lens (35, 35') which is positioned in a beam path of the laser beam (3) before or after an optical system (27, 31) causing the focus-forming beam shaping. Laser processing system (1) for forming a welded joint between a first joining partner (14) and a second joining partner (15) using a laser beam (3), in particular an ultrashort pulse laser beam, the welded joint having a first joining surface (14B) of the first joining partner (14). a second joining surface (15B) of the second joining partner (15) and wherein at least the first joining partner (14) has a curved surface (14A), with: a laser beam source (1A) which emits a particularly pulsed laser beam (3'), an optical system (1B) for shaping the laser beam (3) to form a focus zone (7) in the first joining partner (14) and/or the second joining partner (15), with focus-forming optics (27, 31) which are the focal zone (7) is formed without taking the curved surface (14A) into account, the focal zone (7) optionally being elongated along a beam axis (8) of the laser beam (3), and a workpiece holder (19) for mounting the first joining partner (14) and/or the second joining partner (15), and wherein a phase correction, which counteracts an influencing of the formation of the focus zone (7) due to the curved surface (14A), is provided with phase-correcting optics (29, 29', 35, 35') or in the focus-forming optics (27, 31) is integrated.

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