Method for laser welding of workpieces susceptible to hot cracking

Abstract

The invention relates to a method and a device for laser welding a workpiece (12) susceptible to hot cracking, wherein a laser beam (16) directed at the workpiece (12) forms a melt pool (30) on the workpiece (12), wherein a laser power (32) of the laser beam (16) is at least partially modulated with a modulation frequency (34) and a modulation amplitude.

Description

[0001] The invention relates to a method for laser welding of workpieces susceptible to hot cracking.

[0002] From DE 10 2016 204 578 B3 a method for laser welding of steel with power modulation to avoid hot cracking is known.

[0003] Aluminum-zinc alloys (7000 series) are considered unsuitable for laser welding due to their high susceptibility to hot cracking. Initial approaches focus on the use of titanium-doped filler wire, which refines the microstructure. While this reduces hot cracking, it also leads to an increase in process porosity.

[0004] The invention is based on the objective of providing a safe and reproducible method for laser welding of workpieces susceptible to hot cracking.

[0005] The problem underlying the invention is solved by a method with the features of claim 1. The invention relates to a method for laser welding at least one workpiece made of an aluminium-zinc alloy, wherein a laser beam directed at the workpiece forms a melt pool on the workpiece, wherein a laser power of the laser beam is at least partially modulated with a modulation frequency and a modulation amplitude such that the melt pool preferably undergoes movement in the process along the z-direction.

[0006] Due to the modulation of the laser power, the geometric shape and dynamics of the weld pool are influenced, and in particular reduced. The vapor capillary experiences a characteristic oscillation frequency in the z-direction (laser beam direction), which reduces uncontrolled collapse of the vapor capillary. This minimizes the formation of process pores. Furthermore, the z-movement of the melt facilitates the outgassing of any pores that do form. The targeted discontinuous solidification of the weld, combined with a stabilized vapor capillary, counteracts hot cracking.

[0007] An advantageous aspect of the invention provides that the modulation frequency lies in a range between 50 Hz and 500 Hz, particularly between 50 Hz and 250 Hz, preferably at 100 Hz. The modulation frequency is preferably lower than the cracking frequency of the hot cracks that form, which is, for example, 200 Hz. The range of the modulation frequency further counteracts hot cracking.

[0008] An advantageous aspect of the invention provides that the modulation amplitude is greater than 50%, in particular greater than 60%, preferably greater than 70%, and more preferably greater than 80%. This ensures sufficient movement of the melt pool for discontinuous solidification and outgassing.

[0009] An advantageous aspect of the invention provides that a normalized frequency L of the power modulation lies in a range between 0.2 and 1. The normalized frequency L is defined according to the following formula: L=f∗dfv

[0010] An advantageous aspect of the invention provides that the laser beam has a core region with a core intensity and a core diameter and / or a ring region surrounding the core region with a ring intensity and a ring diameter.

[0011] An advantageous aspect of the invention provides that the core diameter is in a range between 10 µm and 200 µm, preferably between 50 µm and 100 µm, and / or the ring diameter is in a range between 200 µm and 800 µm, preferably between 200 µm and 400 µm. Preferably, in one embodiment, the core diameter is 50 µm and the ring diameter is 200 µm. This allows for a high welding intensity. The core intensity is at least 10%, in particular at least 30%, preferably at least 50%, smaller or larger than the ring intensity. In the case of a higher core intensity, targeted energy input into the workpiece is ensured.

[0012] An advantageous aspect of the invention provides that the laser beam is divided into at least two, and in particular four, partial laser beams (BrightLine Weld) by means of a beam splitter. The generated partial laser beams jointly create the weld pool. The multifocus enlarges and stabilizes the vapor capillary, so that the welds exhibit fewer hot cracks and pores.

[0013] An advantageous aspect of the invention is that no filler wire is used to create the weld. This simplifies the process and significantly improves the quality of the welds.

[0014] An advantageous aspect of the invention provides that the laser beam feed rate is in a range between 1 m / min and 20 m / min, in particular between 2 m / min and 15 m / min, preferably between 3 m / min and 12 m / min. In this range, the crack frequency is in the range of 200 Hz. Therefore, the power modulation in the aforementioned range is optimally adjusted to the crack frequency.

[0015] The problem underlying the invention is also solved by a method with the features of claim 9. The invention relates to a method for laser welding a workpiece made of a heat-treatable wrought aluminum alloy, in particular aluminum-magnesium alloys or aluminum-zinc alloys, of a non-heat-treatable wrought aluminum alloy, in particular aluminum-manganese alloys or aluminum-magnesium alloys, and / or of steel, wherein a laser beam directed at the workpiece forms a melt pool on the workpiece, wherein a laser power of the laser beam is at least partially modulated with a modulation frequency and a modulation amplitude. The laser beam has a core region with a core intensity and a ring region surrounding the core region with a ring intensity (2in1 LLK). The core intensity is lower or higher than the ring intensity.Furthermore, the laser beam is divided into at least two, in particular two or four, partial laser beams by means of a beam splitter (BrightLine Weld), wherein the partial laser beams form a common melt pool.

[0016] The power modulation and / or the 2in1 laser beam and / or the multifocus can be configured as described above.

[0017] The problem underlying the invention is also solved by a laser welding device for laser welding of workpieces according to claim 10. The laser welding device comprises at least one laser source for generating a laser beam, at least one beam shaper for adjusting a core intensity and a ring intensity of the laser beam, at least one beam splitter for dividing the shaped laser beam into at least two partial laser beams, and a control device for controlling the laser source and / or the beam shaper and / or the beam splitter, which is configured to carry out a previously described method.

[0018] Further advantages, features, and details will become apparent from the following description, in which various embodiments of the invention are illustrated with reference to the drawing. The features mentioned in the claims and the description can each be essential to the invention individually or in any combination.

[0019] They show: Fig. 1 a laser welding device; Fig. 2 a modulated power profile of a laser beam; and Fig. 3 the beam profile of four laser partial beams, each with a core and a ring region.

[0020] The laser welding device 10 is set up for laser welding of workpieces 12 made of aluminium-zinc alloys and has according to Fig. 1. A laser source 14 for generating a laser beam 16, a beam shaper 18, in particular a wedge-shaped splitter, for splitting the laser beam 16 into a core region 20 and a ring region 22, and a beam splitter 24 for splitting the generated and shaped laser beam 16 into at least two partial laser beams 26. The laser welding device 10 further comprises a control device 28 for controlling the laser source 14, the beam shaper 18, and the beam splitter 24. The laser beam 16 is directed towards the workpiece 12 to form a melt pool 30.

[0021] The control device 28 is configured such that the laser power 32 of the laser beam 16 is modulated with a modulation frequency 34 and a modulation amplitude 35. The modulation frequency 34 lies in a range between 50 Hz and 500 Hz, in particular between 50 Hz and 250 Hz, preferably at 100 Hz. The modulation frequency 34 is thus lower than the crack frequency of incipient hot cracks, which is 200 Hz. The modulation amplitude is preferably greater than 50%, in particular greater than 60%, preferably greater than 70%, and more preferably greater than 80%.

[0022] In Fig. Figure 2 shows an example power curve of a modulated laser power 32. The power curve P(t) is calculated according to the following formula: P(t)=Pampl∗sin(2∗p∗fm)+Pav

[0023] Power modulation influences the weld pool dynamics. This imparts a characteristic oscillation frequency to the vapor capillary in the z-direction, thereby reducing uncontrolled collapse. This, in turn, minimizes the formation of process pores. Furthermore, the z-movement of the weld pool facilitates the outgassing of any pores that do form. The targeted discontinuous solidification of the weld, combined with a stabilized vapor capillary, counteracts hot cracking. This enables the production of high-quality welds.

[0024] To further improve the welds, in particular to reduce hot cracking and porosity, the laser beam 16 is divided into a core area 20 and a ring area 22 according to Fig. 3 divided. The core area 20 has, according to the beam profile in Fig. 3. The core intensity is greater than the ring intensity of the ring region 22, in particular by 30%, and preferably by 50%. The core diameter 36 of the core region 20 is 50 µm. The ring diameter 38 of the ring region 22 is 200 µm.

[0025] Furthermore, the laser beam 16 is according to Fig. The laser beam is divided into four partial laser beams 26 by means of the beam splitter 24, each of which has a core region 20 and a ring region 22. This allows the melt pool 30 and the vapor capillary to be enlarged, which further improves the formation of hot cracks and pores.

[0026] In a preferred method, a laser beam 16 is first generated, the laser power 32 of which is modulated. The laser beam 16 is then divided into a core region 20 with a core intensity and a ring region 22 with a ring intensity, the core intensity and the ring intensity being of different values. The modulated and shaped laser beam 16 is then split into at least two partial laser beams 26. These partial laser beams 26 are subsequently directed onto the workpiece 12 to form a laser spot and a melt pool 30, and to create a weld seam. Reference symbol list 10 Laser welding device 12 workpieces 14 Laser source 16 Laser beam 18 beam shapers 20 Core area 22 Ring area 24 beam splitters 26 Laser partial beam 28 Control device 30 Melt bath 32 laser power 34 Modulation frequency 35 Modulation amplitude 36 Core diameters of the core area 38 Ring diameter of the ring area QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2016 204 578 B3

[0002]

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