Method for laser welding with a stable melt pool and laser processing device

Abstract

A method for laser welding with a stable weld pool and uniform penetration depth, wherein a laser beam (16) is guided onto workpieces (11) to be welded at a joint by means of a processing head (17) comprising at least one oscillation device (6), a focal spot (15) is generated on at least one of the workpieces (11), and the focal spot (15) is moved along the joint relative to the workpieces (11) under a spatially oscillating oscillation movement defined by oscillation parameters with oscillation movement components both parallel and perpendicular to a feed direction (x) of the processing head (17), characterized in that: - the oscillation parameters include both an oscillation amplitude (A) and an oscillation velocity; - the focal spot (15) is moved with a periodically varying oscillation speed, the period of which corresponds to the period of a change in the oscillation amplitude (A), whereby the focal spot speed (v2) on the workpiece surface, which results from the superposition of the oscillation speed and a feed speed (v1) in the feed direction (x) of the machining head (17) relative to the workpieces (11), is kept constant; - the focal spot (15) on the workpiece surface is moved along a predetermined, non-intersecting focal spot motion curve (1), wherein the focal spot (15) on the workpiece surface describes a continuous track (2) with a track width defined by the focal spot cross-section (d), the track segments of which are essentially non-overlapping with each other; and - the focal spot velocity (v2) is at least twice the feed rate (v1).

Description

[0001] The invention relates to a method and a device for the process-reliable welding of workpieces using laser radiation, preferably of materials that have a high surface tension and low viscosity in the melt, such as copper.

[0002] It is known that increasing the feed rate can improve the weld quality, e.g. with copper, but this leads to a lower penetration depth with the same laser power.

[0003] Deep penetration welding requires very high power densities of approximately 1 megawatt per square centimeter. The laser beam not only melts the metal but also generates vapor. A deep, narrow, vapor-filled hole then forms in the molten metal: the so-called vapor capillary – also known as a keyhole. The vapor capillary is the result of an equilibrium between the pressure of the vaporizing material, the surface tension acting on the melt, and gravity, which counteracts the vapor pressure to close the vapor capillary.

[0004] To improve process reliability with regard to weld penetration depth and spatter formation in deep penetration welding processes, DE 10 2016 204 578 B3 discloses the use of power modulation, while DE 10 2014 117 157 B4 proposes spatial oscillation of the laser spot. The oscillation can be uniaxial (longitudinal or transverse to the feed direction), biaxial (e.g., in the form of cycloids), or triaxial, i.e., including defocusing, although defocusing has only a relatively minor influence. In addition to harmonic oscillation patterns, compound, periodic movements are also possible. With materials of low viscosity and high thermal conductivity, the welding speed often does not reach the required value for spatter-free welding due to the limited laser power, or the weld penetration depth is insufficient.Superimposed oscillation patterns increase the energy input per weld length, thereby increasing the weld penetration depth or the connection cross-section.

[0005] From DE 10 2014 015 094 A1 it is known to carry out a harmonic oscillation movement of the laser beam superimposed on the feed movement, resulting in a trace of the focal spot on the workpiece that is comparable to a spiral path.

[0006] DE 10 2015 009 664 B3 describes a method for welding dissimilar metallic joining partners, wherein a laser beam intended for melting the joining partners is moved in a pendulum-like or oscillating manner transverse to the feed direction of the workpieces. Several spot welds of the two dissimilar joining partners are produced by melting only the joining partner with the higher melting point at a single point.

[0007] DE 10 2012 212 278 A1 discloses an arrangement for producing holes or welds using a laser beam or several laser beams in or on a 2-D and / or 3-D topology, wherein the arrangement comprises at least one fiber laser source, a laser beam deflection device comprising two X / Y scanners and a diagnostic unit.

[0008] A disadvantage of welding processes known from the prior art is that the welding spot – due to the superposition of the oscillatory movement with the path movement along the weld seam to be produced – further overheats and displaces the molten weld pool with its vapor capillary (keyhole) traversing it during movement. This leads to more or less significant weld pool movement as well as instability of the vapor capillary with heavy spatter, ejecta, and trapped gas bubbles in the form of pores.

[0009] Due to the superposition of the feed and oscillation movements, the speed of the weld spot on the workpiece surface varies. DE 10 2014 015 094 A1 proposes increasing the weld spot speed at the reversal points of the oscillation movement to address this. However, a disadvantage is that the resulting weld spot trace on the workpiece overlaps or intersects itself multiple times and extensively. Since the overlap areas are asymmetrical, the weld penetration depths differ along the weld spot path.

[0010] The object of the invention is to achieve a uniform welding depth with a stable melt pool, whereby the passage of the melt through the vapor capillary is to be avoided in order to reduce the tendency of the melt pool to unstable states, in particular to spatter formation, by means of a precisely defined melt front opposite the solid edge.

[0011] The problem is solved by a laser welding method with the characterizing features of claim 1 and a laser processing device with the characterizing features of claim 5; advantageous embodiments of the invention are found in the dependent claims. A particularly advantageous use of the invention is shown in claim 11.

[0012] According to the invention, laser welding is carried out by moving a focal spot – and thus also the vapor capillary generated by the action of the laser beam – generated on a workpiece surface by a laser beam guided by a processing head across the workpiece surface at a high but constant local relative velocity with respect to the workpiece surface, hereinafter referred to as the focal spot velocity. A focal spot motion curve generated in this process, i.e., the path described by the focal spot on the workpiece surface, is defined such that the vapor capillary preferably always occupies a position at the edge of the weld pool, or at least moves only minimally through the weld pool.

[0013] According to the method, the oscillation movement of the welding spot in the reference system of the processing head, which includes an oscillation device for generating this oscillation movement, and thus the movement of the actuators associated with the oscillation device (e.g., galvanometer scanner), is converted in such a way that the instantaneous velocity resulting on the workpiece, i.e., the welding spot velocity, remains constant throughout the entire welding process.

[0014] The melt pool and its extent are primarily determined by the focal spot cross-section, but also by the focal spot motion curve and the focal spot velocity.

[0015] To minimize the movement of the vapor capillary through the melt bath, the combustion spot motion curve is defined according to the invention such that it is, firstly, continuous, i.e., uninterrupted, and, secondly, has no intersections with itself, i.e., is free of crossings. Preferably, the combustion spot motion curve is also differentiable, i.e., it has no kinks in its course.

[0016] In the case that a focal spot motion curve with kinks in its course is chosen, the deflection angle between the curve sections at each kink is a maximum of 90°, i.e., the two curve sections on either side of the kink enclose an obtuse angle.

[0017] The flame spot movement curve is further defined such that a large proportion of its path segments are largely parallel to adjacent path segments, with these individual, parallel path segments running essentially perpendicular to the weld direction. In addition, the individual, parallel path segments are spaced apart from each other in such a way as to achieve an optimum between melting of the entire weld cross-section and minimizing spatter and porosity.

[0018] Thus, the continuous track described by the burn mark on the workpiece surface, whose track width is defined by the burn mark cross-section, is essentially free of overlap. That is, only in the case of a "kinked" track do relatively short sections of this track exhibit small overlaps in the area of ​​the kinks in the burn mark's movement curve.

[0019] To optimize path accuracy, kinks in the path can be rounded, although the limited dynamics of the beam deflection actuators can smooth out the kinks. This further reduces the overlapping areas of the tracks.

[0020] Preferably, the ratio of oscillation amplitude perpendicular to the feed direction to period length in the feed direction is large; that is, the oscillation amplitude is greater than the period length (e.g., twice as large). Here, the period length is defined as the distance between two points on the surface of the workpieces in the feed direction or along the joint that the flame spot travels relative to them during one oscillation period. Under these conditions, heat conduction acts, causing the melt front to propagate continuously in the feed direction while the flame spot moves toward the other flank of the weld. By angling the transverse tracks, it can be prevented that the flame spot moves through the melt in a way that would impair the process.

[0021] In particular, it is possible to selectively influence the process by varying the amplitude, period length, oscillation frequency and / or laser power depending on the material of the workpieces, the joint geometry, a gap formed at the joint between the workpieces and / or the workpiece temperature.

[0022] The laser processing device according to the invention for laser welding of workpieces comprises a laser beam generation unit, a processing head, a feed unit, and a control unit. The processing head includes a collimation unit for beam collimation that is small compared to the prior art, an oscillation unit with a first and a second scanner unit for guiding the laser beam, a beam expansion unit arranged behind the oscillation unit in the beam propagation direction with at least partially movable optical elements, and a focusing unit for focusing the laser beam onto the surface of the workpieces. The control unit is connected at least to the feed unit and the oscillation unit.

[0023] The machining head can be stationary or attached to another guide machine.

[0024] The laser beam generation unit preferably has a multimode laser, although single-mode lasers can also be used in principle.

[0025] According to the invention, the oscillation device has a smaller aperture than the focusing unit at its lens output. In particular, the laser beam within the oscillation device has a smaller diameter than at the lens output of the focusing unit. Furthermore, the oscillation device is highly dynamic and biaxial, meaning that the two scanner devices of the oscillation device can each be moved about a rotational axis at frequencies of up to 100 kHz, allowing the laser beam to be moved back and forth at correspondingly high frequencies, or the focal spot on the workpiece surface to be oscillated.

[0026] In particular, the rotation axes of the first and second scanner devices are perpendicular to each other to allow free movement of the scanning spot in the plane of the workpiece surface – i.e., in the x and y directions, where the x direction is the direction of the weld seam and the y direction is the direction perpendicular to it. Thus, at each point on the scanning spot's path, the feed movement of the machining head can be subtracted from the scanning spot's movement, so that the scanning spot can always trace a path independent of the feed movement.

[0027] The control and regulating device allows the oscillation device to be controlled in such a way that the focal spot velocity, i.e. the relative velocity of the focal spot with respect to the workpiece surface, always remains constant.

[0028] The velocity vector v2 of the focal spot velocity results from the vector sum of the velocity vector v1 of the relative velocity between the machining head and the workpiece, hereinafter referred to as feed rate, and the velocity vector v0 of the relative velocity between the machining head and the focal spot.

[0029] The relative velocity v0 between the processing head and the focal spot, which is to be generated by the oscillation device, is thus derived from the vector equation: v0=v2−v1

[0030] The control device is therefore set up to determine, at any given time, the components of the relative velocity v0 to be generated by the oscillation device in the x and y directions, and to control the oscillation device accordingly, given a given focal spot velocity - where the focal spot velocity is defined as the magnitude of the velocity vector v2 - and a given path of the focal spot on the workpiece surface.

[0031] For this purpose, it can be provided that the control and regulating device reads out an actual value of the feed rate or the expected actual speed of the feed device and, based on this, outputs the target path of the focal spot to the actuating axes of the oscillator device after appropriate coordinate transformation.

[0032] Alternatively, the control and regulating device can also perform a calculation and subsequent specification of the position for the oscillator device, as well as additionally specifying the feed rate for the feed device.

[0033] The advantages of the invention are the consistent design of the laser processing device to handle high average laser power and to generate a small, rapidly moving focal spot at a sufficiently large working distance. Furthermore, the use of cost-effective multimode lasers instead of the previously common basic-mode lasers reduces manufacturing costs.

[0034] Further advantages result from the increase in the consistency of the weld penetration depth according to the invention, the avoidance of spatter, ejecta and pores during welding, which are unacceptable, for example, in electronic or electrical engineering products, as well as an increase in the quality of the weld seam with large connection cross-sections.

[0035] Furthermore, the relative overall focus shift of the device is reduced, thus also reducing its impact on the processing process compared to the use of fundamental mode lasers with high cw power, which, due to the small spots, require oscillation with significantly higher frequencies or smaller raw beam diameters for larger spots.

[0036] According to one embodiment of the method, the laser power and / or the relative velocity v0 between the processing head and the focal spot are varied during the oscillation movement, e.g., synchronously with the oscillation movement. This variation can be dependent on the y-displacement, i.e., perpendicular to the seam.

[0037] The laser processing device can be further advantageously designed as follows: The laser beam generation unit can include a multimode laser, in which case the lenses of the collimation device have a particularly low absorption in the wavelength range of the laser light emitted by the multimode laser, i.e. the entire collimation device has an absorption coefficient of less than 1%, preferably less than 0.1%, in this wavelength range.

[0038] The laser beam generation unit can emit a free beam that is coupled into the processing head via a small aperture. Alternatively, the laser beam generated by the laser beam generation unit can be coupled into the processing head via an optical fiber and a small aperture, with a collimating lens positioned at the optical fiber's light exit point.

[0039] According to one embodiment of the two-axis oscillation device, whose aperture is smaller than that of the lens, the first scanner device, which, for example, comprises a mirror pivotable about an axis of rotation, is arranged with its axis of rotation essentially parallel to the plane of incidence of the laser beam. Preferably, the angle of incidence (e.g., with respect to the mirror normal) is less than 35 degrees; most preferably, the angle of incidence is 22.5 degrees. This embodiment allows for rapid oscillation due to smaller moments of inertia.

[0040] The second scanner device, e.g. also a mirror that can be pivoted about an axis of rotation, can be aligned with its axis of rotation essentially perpendicular to the plane of incidence of the laser beam.

[0041] In an advantageous embodiment of the oscillation device, its input beam axis and its output beam axis form an angle of approximately 90 degrees, i.e., the two beam axes are arranged essentially perpendicular to each other.

[0042] The beam expansion unit, arranged behind the oscillation device in the direction of beam propagation, may include a lens with a negative focal length and a lens with a positive focal length that can be moved in the direction of beam propagation. This allows the focus position to be adjusted to obtain a greater working distance and / or a smaller focus.

[0043] According to one embodiment of the laser processing device, the focusing unit is arranged behind the beam expansion unit in the direction of beam propagation.

[0044] It may also be provided that the feed device includes a linear axis, e.g. with a linear drive, or is part of an industrial robot, i.e., the industrial robot generates the feed movement.

[0045] Alternatively, the feed mechanism can be implemented using a beam shifting unit comprising optical elements, e.g., galvo axes, which laterally shifts the oscillating laser beam before it strikes the workpiece. The beam shifting unit can be positioned between the beam expansion unit and the focusing unit, or in the convergent beam path after the focusing unit.

[0046] One advantageous application of the laser processing device is, for example, laser welding of lap joints, joining workpieces with different material thicknesses, or welding workpieces made of different materials. The laser processing device allows for adjustment of the weld penetration depth, gap bridging, and additional edge rounding of the weld, e.g., in lap joints.

[0047] The invention is explained in more detail below with reference to exemplary embodiments, wherein identical or similar features are designated with the same reference numerals. The following are shown schematically: Fig. 1: a trace of the burning spot on the workpiece from the prior art; Fig. 2: a trace of the flame spot on the workpiece according to the laser welding process; Fig. 3: a first embodiment of the laser processing device; and Fig. 4: a second embodiment of the laser processing device.

[0048] Fig. Figure 1 shows an example from the prior art. The trace 2 of the focal spot 15 on the workpiece during a harmonic oscillation movement results from a superposition of a harmonic oscillation of the focal spot 15 in the x-direction and a harmonic oscillation in the y-direction, both of which are phase-shifted in the reference system of the machining head and each with amplitude A, with the feed rate v1, i.e. the relative speed between the machining head and the workpiece.

[0049] In Fig. Figure 1 shows the resulting focal spot velocity v2 as a vector arrow, where the length of the arrow corresponds to the magnitude of the focal spot velocity v2. It is evident that the focal spot velocity v2 varies along the course of the focal spot motion curve 1. The checkered areas 3 are areas of crossed tracks 2. These areas 3 are asymmetrical and occupy a large portion of the area of ​​track 2. Therefore, the weld penetration depths differ along the course of the focal spot motion curve 1.

[0050] Fig. Figure 2 shows approximately two periods of a seam weld in one embodiment of track 2 on the workpiece according to the invention. Track 2 essentially consists of track sections extending longitudinally and transversely to the feed direction, wherein the length of the track sections extending transversely to the feed direction, with twice the amplitude A, is significantly greater than the length (d + c) of the track sections extending longitudinally to the feed direction. The overlapping areas 3 of track 2 are, in comparison to the example from Fig. 1 significantly reduced in area.

[0051] The focal spot movement curve 1 shown melts approximately the area according to contour 4.

[0052] The focal spot velocity v2, and thus the movement velocity of the vapor capillary, remains constant along the entire course of the focal spot motion curve 1. For this purpose, the oscillation motion of the focal spot 15 in the reference system of the processing head and thus the movement of the associated actuators, in this example two galvanometer scanners (not shown), is converted in such a way that the instantaneous velocity resulting on the workpiece, i.e. the focal spot velocity v2, remains constant.

[0053] The period T of an oscillation cycle is calculated from the amplitude A, the track width d, the distance c between the tracks and the focal spot velocity v2 according to T=4A+2(d+c)v2

[0054] In this example, the spacing c of the tracks is chosen so that the spaces between them are sufficiently melted, but the tendency for spattering and porosity is minimal.

[0055] The period length P, as the interval between repeating oscillation cycles, is derived from P=2⋅(d+c)

[0056] The feed rate v1, as the relative speed between the machining head and the workpiece, is calculated from v1=v2⋅c+d2A+c+d

[0057] By appropriately selecting the parameters amplitude A, track width d and distance c, the feed rate v1 is thus up to, for example, ten times smaller than the focal spot velocity v2.

[0058] Fig. Figure 3 shows a first embodiment of a laser processing device. The laser beam 16, generated by the laser beam generation unit 14 (shown here only as an optical fiber), is coupled into the collimation unit 5 of the processing head 17 via a small aperture and directed onto the surface of the workpiece 11 by the oscillation unit 6, the beam expansion unit 9, and the focusing unit 10. The oscillation unit 6 includes the scanner devices 7 and 8, which each cause the mirror 7.2 or 8.2 to oscillate about the axis of rotation 7.3 or 8.3 by means of the galvanometer drive 7.1 or 8.1, respectively.

[0059] The control and regulating device 13 controls both the oscillation device 6, and thus the focal spot velocity v2, and the feed device 12, and thus the feed velocity v1.

[0060] In this example, the feed device 12 includes a linear axis, the machining head 17 is stationary, i.e. fixed in position.

[0061] Fig. Figure 4 shows an embodiment of the laser processing device with integrated scanner axes as a feed unit 12 for spatial processing. In this example, the processing head 17 is attached to a further guide machine (not shown). List of reference symbols used 1 Focal spot motion curve 2 Trace of the burn mark on the workpiece 3 Crossed path of the burn mark on the workpiece 4 Contour of the molten area 5 Collimation device 6 Oscillation device 7 First scanner device 7.1 Galvanometer drive 7.2 Mirrors 7.3 Rotation axis of the first scanner device 8 Second scanner device 8.1 Galvanometer drive 8.2 Mirror 8.3 Rotation axis of the second scanner device 9 Beam expansion unit 10 Focusing units 11 Workpiece 12 Feed device 13 Control and regulating device 14 Laser beam generation unit 15 Burning spot 16 Laser beam 17 Processing head A Amplitude c Distance between the lanes d diameter of the focal spot P Period length of the oscillation cycles T Period of the oscillation cycles V1 Feed rate / Relative speed between machining head and workpiece v2 Focal spot velocity / Instantaneous velocity of the focal spot relative to the workpiece x coordinate in feed direction y coordinate perpendicular to the feed direction

Claims

[1] A method for laser welding with a stable molten pool and a uniform welding depth, wherein a laser beam (16) is guided onto workpieces (11) to be welded at a joint by means of a processing head (17) comprising at least one oscillation device (6), a focal spot (15) is generated on at least one of the workpieces (11), and the focal spot (15) is moved along the joint in a spatially oscillating oscillation movement defined by oscillation parameters, with oscillation movement components both parallel and perpendicular to a feed direction (x) of the processing head (17) relative to the workpieces (11), characterized in that: - the oscillation parameters include both an oscillation amplitude (A) and an oscillation speed; - the focal spot (15) is moved at a periodically varying oscillation speed, the period of which corresponds to the period of a change in the oscillation amplitude (A), wherein the focal spot speed (v2) on the workpiece surface, which results from the superposition of the oscillation speed and a feed speed (v1) in the feed direction (x) of the machining head (17) relative to the workpieces (11), is kept constant; - the focal spot (15) is moved on the workpiece surface along a predetermined, intersection-free focal spot movement curve (1), wherein the focal spot (15) describes a continuous track (2) on the workpiece surface with a track width defined by the focal spot cross-section (d), the track sections of which are essentially non-overlapping; and - the focal spot speed (v2) is at least twice the feed speed (v1). [2] A method for laser welding according to claim 1, characterized by that the focal spot (15) is moved in a meandering manner on the workpiece surface. [3] Method according to one of the preceding claims, characterized by that an energy input caused by the focal spot (15) is varied by adjusting the laser power of the laser beam (16) or by changing the focus position relative to the workpiece surface. [4] Method according to claim 3, characterized by that the energy input caused by the focal spot (15) is varied depending on the component of the oscillating movement perpendicular to the feed direction (x). [5] Laser processing device for laser welding workpieces (11) by means of a laser beam (16), comprising a laser beam generation unit (14), a processing head (17) comprising a collimation device (5), an oscillation device (6) with a first (7) and a second scanner device (8) for guiding the laser beam (16), a beam expansion unit (9) arranged behind the oscillation device (6) in the beam propagation direction and having at least partially movable optical elements, and a focusing unit (10) for focusing the laser beam (16) onto the surface of the workpieces (11), a feed device (12), and a control and regulating device (13) connected to the feed device (12) and the oscillation device (6), wherein the rotation axes (7.3, 8.3) of the first scanner device (7) and the second scanner device (8) are perpendicular to one another, characterized by , that - the oscillation device (6) has a smaller aperture than the focusing unit (10) at its objective exit, wherein the diameter of the laser beam (16) within the oscillation device (6) is smaller than at the objective exit of the focusing unit (10); and - the control and regulation device (13) is designed to achieve a stable melt pool and a uniform welding depth, to set a constant focal spot speed (v2) of the focal spot (15) on the workpiece surface during laser welding by regulating an oscillation speed of the oscillation movement of the focal spot (15) in the reference system of the processing head (17) by controlling the first scanner device (7) and the second scanner device (8) in accordance with a focal spot movement curve (1) of the oscillation movement of the focal spot (15). [6] Laser processing device according to claim 5, characterized bythat the laser beam generating unit (14) is a multimode laser and the collimation device (5) has an absorption coefficient of less than 0.1% in the wavelength range of the laser light emitted by the multimode laser [7] Laser processing device according to one of claims 5 or 6, characterized by that an angle formed between the input beam axis and the output beam axis of the oscillation device (6) is substantially 90°. [8] Laser processing device according to one of claims 5 to 7, characterized by that the beam expansion unit (9) comprises a lens with a negative focal length that can be displaced in the beam propagation direction and a fixed lens with a positive focal length. [9] Laser processing device according to one of claims 5 to 8, characterized by that the feed device (12) comprises a linear axis. [10] Laser processing device according to one of claims 5 to 9, characterized bythat the feed device (12) has a beam displacement unit arranged between the beam expansion unit (9) and the focusing unit (10) or behind the focusing unit (10) in the beam propagation direction. [11] Use of the laser processing device according to one of claims 5 to 10 for joining two workpieces (11) made of different materials and / or with different workpiece thicknesses at an overlap joint.

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