EP4326480A1

EP4326480A1 - Laser beam joining method

Info

Publication number: EP4326480A1
Application number: EP22724047.0A
Authority: EP
Inventors: Markus GRETZER; Jan-Philipp Weberpals
Original assignee: Audi AG
Current assignee: Audi AG
Priority date: 2021-04-22
Filing date: 2022-04-21
Publication date: 2024-02-28
Also published as: US20240131627A1; DE102021110241A1; WO2022223654A1; CN117083143A

Abstract

The invention relates to a method for joining at least two joint partners (1, 3) using a laser beam, wherein a laser beam device (13) produces a continuous welding seam (5) along an application web (11) which has a length that is preferably very long. According to the invention, irregularities in the welding seam (5) due to high process speeds (v) are prevented as follows: in a first process step (I), at least two welding seam sections (S1,S3, S5, S7, S9) which are mutually spaced in the web longitudinal direction are produced with respective interposed welding seam interruptions (15); and in a second process step (II), the laser beam device (13) produces an additional welding seam section (S2, S4, S6, S8) in each welding seam interruption (15) so that all of the welding seam sections (S1 to S9) transition into one another without interruptions, thereby forming a continuous welding seam (5).

Description

Process for laser beam joining

DESCRIPTION:

The invention relates to a method for laser beam joining according to the preamble of claim 1.

For example, bipolar plates of a fuel cell can be made from a component composite of two sheet metal parts that have a very thin material thickness. To prepare for the welding process, the sheet metal parts are placed one on top of the other and joined together by laser beams. The weld seams formed can extend over a length of several meters.

In a generic method for joining at least two joining partners with a laser beam, a laser beam device produces a continuous, uninterrupted weld seam along a predefined application path.

In the case of laser beam joining known from the prior art, the laser beam moves continuously along the application path at a process speed, as a result of which the weld seam is formed. At a process speed above a critical limit value and depending on other process parameters and the physical and geometrical material properties, periodic irregularities in the course of the weld seam occur after the start of the welding process. This effect is known as humping because it takes on a structure of pearls or small accumulations. In such a welded seam structure, material accumulations and material deficits periodically form, which weaken the welded connection and thus lead to a higher likelihood of leakage. Accordingly, in the prior art, the critical process speed limit value above which this effect occurs represents a process limitation.

Against this background, in the state of the art, the process speeds in laser beam joining cannot be scaled up as desired to produce a uniformly shaped weld seam connection.

The regulation due to the occurrence of irregularities (humping) sets limits to scaling, as this reduces the functionality of the connection (e.g. leakage when gas-tightness is required for bipolar plates due to the partial material deficit). Due to the limited process speed, the number of welding systems must be increased in large-scale production. In addition, in the prior art there is a high risk of rejects or an additional expense resulting from component distortion due to heat input caused by sweat.

So far, there is no implemented large-scale system that requires such a process speed. With the previous concept ideas, a very large number of systems and devices with the associated ho hen investment volume is necessary to reach the required quantities. The increase in process speed is directly related to the reduction in cycle time and thus productivity.

Due to the process limits described above, the process speed cannot currently be further increased by adjusting the process parameters. Current speeds during deep laser beam welding of sheet steel, for example, amount to values in the range of 1 m/s, which in the case of thin foils leads to irregularities in the seam surface and leaks in the weld seam.

A method and a device for pulsed high-energy welding are known from EP 0 143450 B1. US Pat. No. 5,595,670 A discloses a method for high-speed welding. From the DE 10 2019 006217 A1 discloses a method for connecting at least two workpieces by means of a laser beam.

The object of the invention is to provide a method for laser beam joining in which, compared to the prior art, the process speed can be increased and at the same time periodic irregularities in the course of the weld seam can be avoided.

The object is solved by the features of claim 1. Preferred developments of the invention are disclosed in the dependent claims.

The invention is based on a method for laser beam joining of at least two joining partners, in which a laser beam device produces a continuous weld seam along an application path with a preferably very long path length.

The invention is based on the following finding: With conventional laser beam joining, the laser beam moves continuously along the application path at a process speed, as a result of which the continuous weld seam is produced. Especially at a process speed above a critical limit value, periodic irregularities with material accumulations and material deficits occur after a welding process start phase. In the prior art, therefore, the weld seam produced in the welding process start phase is still flawless, while the weld seam produced in the further course of the welding process is afflicted with periodic irregularities. Against this background, according to the characterizing part of claim 1, the weld seam is not produced continuously over the entire length of the application web. Rather, the method according to the invention for avoiding irregularities in the weld seam due to excessive process speeds has two process steps: In the first process step, at least two weld seam sections that are spatially distant from one another in the length of the application path over a weld seam distance are separated with an intermediate weld seam interruption generated. In the second process step, the welding seam Interruption produces another weld section, so that the weld sections merge into one another without any interruptions, thereby forming the continuous weld seam.

It should be emphasized that the method according to the invention is not limited to the laser beam joining of two joining partners. Rather, the method according to the invention is also suitable for the production of a composite component from several joining partners. It should also be emphasized that the method according to the invention can be used regardless of the material thickness. This means that the process can cover both applications with thicker materials, for example in body construction, and applications with thinner materials, for example approx. 50pm to 200pm, such as occur when laser beam joining of electrochemical components of an electrochemical system occurs, for example in the case of bipolar plates in a fuel cell, in the case of battery cell components or in the case of components of a battery module, an overall battery system, an electrolyzer, a hydrogen compressor and the like.

The idea is based on the idea of creating a joining process using laser beam welding, which enables a high process speed and yet reliably avoids the occurrence of humping in the weld seam to be produced. The idea for the implementation is characterized by the fact that a long, continuous and gas-tight weld seam is to be created step by step from a large number of weld seam sections. The aim is that the individual weld seam pieces can be joined at a significantly higher process speed than a long continuous seam.

The idea is that at the beginning of a weld seam, the welding process develops as a function of the process parameters - primarily the process speed - and the physical and geometric material properties. During this starting phase, in addition to the melting of the components, the flow fields around the capillary and in the resulting melt pool must first build up. In this starting phase occurs periodic formation of humps. The length of this start phase corresponds to the length of the individual weld sections. It depends on the process parameters and mainly depends on the process speed.

As a result, the process speed with regard to the weld seam pieces can be significantly increased. Laser optics used in laser beam welding can precisely align the laser beam between two weld seam sections (end of seam section 1 to start of seam section 2) in the millisecond range. This makes it possible to successively produce weld seam sections that are spatially distant from one another without any significant loss of time. Depending on the strategy for the sequential order of the individual weld seam sections, these are preferably carried out in an overlapping manner, since a continuous, materially bonded and gas-tight connection is created between the weld seam sections. In order to ensure the safety of the tightness between the weld seam sections, in addition to an overlapping line-up, the overlapping of the two weld seam sections can be designed as crossing or intersecting path geometries. The length of the area to be overlapped is determined by the process-relevant parameters and the physical and geometrical material properties. In addition, the strategy for the spatial generation of the weld seams can be determined via the process control using the laser optics (e.g. a large field scanner with gravimetrically driven mirrors). Conceivable are, for example, weld seam sections placed one after the other (example sequence: 1, 3, 2, 5, 4, 7, 6, etc. or first even then odd sections), seam nests, parallelize weld seams or build up alternately. This also allows the heat field that is created to be influenced and the resulting thermal distortion to be kept to a minimum or controlled.

The main technical benefit of the invention consists in increasing the process speed when joining materials with a laser beam in order to avoid the humping effect by dividing the weld seam into weld seam pieces. Relevant aspects of the invention are highlighted again in detail below: It is preferred if the length of the respective weld seam section is less than or equal to the starting phase length of the weld seam produced in conventional laser beam joining. The length of the respective weld section can therefore be determined in a series of tests in which the starting phase length of the weld is determined in a comparative welding process.

In a technical implementation, to increase the weld seam tightness, adjacent weld seam sections can merge into one another with an overlap.

In a first embodiment variant, all of the weld seam sections can be aligned completely in longitudinal alignment with the application path. In this case, the weld seam end of one weld seam section can overlap with the weld seam start of the adjacent weld seam section in the overlapping area.

As an alternative to this, the weld seam overlap can be implemented as follows: the adjacent weld seam sections can have overlapping sections that are positioned at an angle to one another. The overlapping sections may cross or intersect at an overlap point.

The finished weld seam can be subdivided, starting with a first weld seam section at the edge with ascending numbering, into a second, third, fourth and further weld seam sections. Within the scope of the invention, the weld seam sections can be placed in different strategies: for example, in a laser beam joining process in a lateral process sequence, first the first, then the third, then the second, fifth, fourth, seventh, etc. weld seam -Part are generated. In an alternative welding strategy, in one of the two process steps (e.g. the first Process step) first the weld seam sections are created with even numbering. In the other process step (e.g. the second process step), the weld sections can then be produced with odd numbering.

According to the invention, the laser beam joining is implemented in particular as a laser beam deep welding, in which sheet metal parts lying one on top of the other and preferably having an extremely thin material thickness are connected to one another as joining partners. The material thickness can be in a range of 75 μm, for example. However, it should be emphasized that the invention is not only limited to thin material thicknesses, but can be used with any material thickness and/or shape of the joining partners.

The method can be used in particular for the laser beam joining of components in an electrochemical system, such as battery cell components, or for components of a battery module, a battery overall system, a fuel cell, an electrolyser, a hydrogen compressor or the like. In this case, sheet metal parts lying one on top of the other with a material thickness in particular in the range of, for example, 50 pm to 250 pm, or in the range of, for example, 250 pm to 500 pm, can be connected to one another. As an alternative to this, other applications are also possible, for example in the laser beam joining of sheet metal parts lying one on top of the other with a material thickness in the range of, for example, 250 μm to 500 μm.

The method can also be used in the laser beam joining of components in body construction. In this case, sheet metal parts lying one on top of the other with a material thickness of, for example, greater than 0.5 mm, in particular in the range from 0.5 mm to 5 mm, particularly preferably in the range from 0.5 mm to 3 mm, can be connected to one another as joining partners.

Exemplary embodiments of the invention are described below with reference to the attached figures. Show it:

FIGS. 1 and 2 each show a completed welded connection according to the invention in different views;

FIGS. 3 and 4 each show views based on which laser beam joining according to the invention is illustrated;

FIGS. 5 and 6 each show different views, on the basis of which a weld seam produced in a comparative welding process is illustrated; and

FIG. 7 shows a welded seam connection according to a second exemplary embodiment of the invention.

The method according to the invention is used to produce a composite component of two or more sheet metal parts. In principle, the process can be used regardless of the material thickness. This means that in addition to an application, for example in body construction, applications with thin material thicknesses in the range from, for example, approx. Battery cell components, com ponents of a battery module or battery overall system, an electrolytic seurs, a hydrogen compressor or the like.

In the figures 1 and 2, a welded joint between two superimposed sheet metal parts 1, 3 is shown. The two sheet metal parts 1, 3 have extremely thin material thicknesses of 75 μm, for example. The sheet metal parts 1, 3 are, for example, components of an electrochemical system, such as a battery cell or a fuel cell, a battery module or an overall battery system.

It should be emphasized that the invention is not limited to the material thickness of 75 μm specified above. Rather, they can superimposed sheet metal parts 1, 3 also have a material thickness in particular in the range of, for example, 50 pm to 250 pm, or in the range of, for example, 250 pm to 500 pm. As an alternative to this, other applications are also possible, for example in the laser beam joining of sheet metal parts lying one on top of the other with a material thickness in the range of, for example, 250 μm to 500 μm.

In addition, the method is not limited to the laser beam joining of components of an electrochemical system. Rather, the process can be used in any application, for example in laser beam joining of components in body construction. In this case, superimposed sheet metal parts 1, 3 with a material thickness of, for example, greater than 0.5 mm, in particular in the range from 0.5 mm to 5 mm, particularly preferably in the range from 0.5 mm to 3 mm, can be connected to one another as joining partners.

In the exemplary embodiment of FIGS. 1 and 2, the two sheet metal parts 1, 3 are connected to one another via a straight weld seam 5, which extends along an application path 11 indicated by dot-dash lines. The weld seam 5 is produced using a laser beam joining method, which will be described later and is implemented specifically as a deep laser beam welding process.

As can also be seen from FIGS. 1 and 2, the weld seam 5 is continuous, that is to say without interruption. The weld seam 5 is divided in FIGS. 1 and 2 into individual weld seam sections S1 to S9, beginning with a first weld seam section S1 at the edge with increasing numbering and into a second to ninth weld seam section S2 to S9. Adjacent weld sections S1 to S9 merge into one another with an overlap 9 . In FIGS. 1 and 2, all weld seam sections S1 to S9 are aligned longitudinally to the application path 11, along which a laser beam head 13 of a laser beam device is guided at a process speed v in the joining process. In the overlapping areas 9 is the end of the weld a weld seam section S1 to S9 overlaps with the weld seam start of the adjacent weld seam section S1 to S9.

The laser beam joining according to the invention for producing the weld seam 5 shown in Figures 1 and 2 is described below with reference to Figures 3 and 4: Accordingly, the method has a first process step I (Fig. 3) and a second process step II (Fig. 4 ) on. In the first process step I, the weld sections S1, S3, S5, S7, S9 are produced with odd numbering over a weld seam distance a from each other spatially along the path length of the application path 11 with an intermediate weld seam interruption 15 in each case. In the second process step II (FIG. 4), the laser beam device produces the weld seam sections S2, S4, S6, S8 with even numbering in the respective weld seam interruptions 15. After completion of the second process step II, therefore, all weld sections S1 to S9 merge into one another without interruption, specifically with the formation of the continuous weld seam 5.

As an alternative to FIGS. 3 and 4, any other welding strategy can be used within the scope of the invention. For example, in laser beam joining, the first, then the third, then the second and then the fifth weld seam section can be produced in a chronological process sequence. Irrespective of this, the weld seam sections S1 to S9 can be placed in any order within the scope of the invention. The welding sequence can be designed in such a way, particularly in the case of joining partners with a low material thickness, that thermal component distortion that occurs during the welding process is avoided.

Furthermore, the invention is not limited to the number of weld seam sections shown in the figures. Rather, the invention is applicable to any number of weld sections. Likewise, the weld seam is not limited to the linear course of the weld seam shown in the figures. Rather, the weld and/or the weld seam sections can be realized in any free form, such as curved, circular, rectangular or the like.

The essence of the invention is the knowledge that with conventional laser beam joining, in which the laser beam head 13 is guided along the application path 11 at a continuous process speed v, the following problem arises: This occurs at a process speed v above a critical limit value a welding process start phase leads to periodic irregularities with material accumulations 17 and material deficits 19. Correspondingly, the weld seam 21 (FIG. 5) produced in the start phase of the welding process is still flawless, while the humping effect occurs over the further course of the welding process, i.e. the further Welding process course generated weld 23 (Figure 5) with the periodic irregularities 17, 19 is afflicted.

In order to avoid the humping effect in the weld seam 5 according to the invention, a comparison laser beam joining indicated in FIGS. 5 and 6 is first carried out in a series of tests. In the comparative laser beam joining (FIGS. 5 and 6) and in the laser beam joining according to the invention (FIGS. 1, 2, 3 and 4), the process speeds v are selected to be identical. In contrast to the invention, in comparison laser beam joining, the laser beam head 13 is guided along the application path 11 at a continuous process speed v. After the comparative laser beam joining has taken place, the starting phase length ls of the starting phase weld seam 21 produced in the comparative laser beam joining is determined. The length lt of the respective weld seam section S1 to S9 is dimensioned to be less than or equal to the starting phase length _ls of the starting phase weld seam 21 produced in the comparison laser beam joining.

FIG. 7 shows a weld seam geometry according to a second exemplary embodiment. As a result, the weld seam sections S1 to S5 are not aligned completely in longitudinal alignment with the application path 11 . Rather, the adjacent weld seam sections S1 to S5 each have overlapping sections 25 that are inclined relative to one another. This cross or intersect at an overlap point 27 in order to produce a gas-tight weld seam 5 .

REFERENCE LIST:

1, 3 joining partners

5 Weld 5' Comparative weld

9 overlap

11 application panel

13 Laser beam head of a laser beam device

15 Weld seam interruption 17 Material accumulations

19 Material deficits 21 Initial phase weld seam 23 Weld seam a Weld seam distance ls Length of initial phase weld seam It Length of weld seam sections

S1 to S9 weld sections v process speed

I, II process steps

Claims

PATENT CLAIMS:

1. Method for laser beam joining of at least two joining partners (1, 3), in which a laser beam device (13) produces a continuous weld seam (5) along an application path (11) with a preferably very long path length, characterized in that to avoid irregularities in the weld seam (5), due to high process speeds (v), the method is divided into a first process step (I), in which at least two weld seam sections (S1, S3,

S5, S7, S9) are generated with an intermediate weld seam interruption (15), and in a second process step (II), in which the laser beam device (13) cuts a further weld seam section ( S2, S4, S6, S8) generated, so that all weld sections (S1 to S9) merge into one another without interruption, to be precise with the formation of the continuous weld seam (5).

2. The method according to claim 1, characterized in that to increase the tightness of the weld seam, adjacent weld seam sections (S1 to S9) merge into one another with an overlap (9).

3. The method according to claim 1 or 2, characterized in that all weld seam sections (S1 to S9) are aligned in longitudinal alignment with the application web (11) and that in particular in the overlapping area (9) the weld seam end of one weld seam Section (S1 to S9) overlaps the start of the weld seam of the adjacent weld section (S1 to S9).

4. The method according to claim 2, characterized in that for the weld seam overlap the adjacent weld seam sections (S1 to S5) have mutually inclined overlapping sections (25) which cross or intersect one another at an overlap point (27). Method according to one of the preceding claims, characterized in that the finished weld seam (5) can be subdivided, starting from a first weld seam section (S1) on the edge with increasing numbering, into a second, third, fourth ff. Weld seam section (S2 to S9), and that in particular in the laser beam joining process the first, third, second, fifth, fourth, seventh, etc. weld section (S1 to S9) is produced in a chronological process sequence.

Method according to claim 5, characterized in that in the laser beam joining process in a process step, for example the first process step (I), first the weld sections (S1, S3, S5,

S7, S9) are generated with odd numbering, and in the other process step, for example the second process step (II), the weld sections (S2, S4, S6, S8) are generated with even numbering.

Method according to one of the preceding claims, characterized in that in a particularly conventional comparative laser beam joining, the laser beam device (13) is moved at a process speed (v) along the application path (11), specifically with the formation of a comparative weld seam (5' ), and that particularly at a process speed (v) above a critical limit value after a welding process start phase, periodic irregularities with material accumulations (17) and material deficits (19) occur, so that the start phase weld seam (21 ) is still flawless, while the weld seam (23) produced in the further course of the welding process is afflicted with periodic irregularities (17, 19).

Method according to claim 7, characterized in that the length of the respective weld seam section (S1 to S9) in the weld seam (5) is less than or equal to the starting phase length (l _s ) of the weld seam (5' produced in the comparison laser beam joining ) is dimensioned.

9. The method according to any one of the preceding claims, characterized in that the laser beam joining is realized as a laser beam deep welding Shen, in which as joining partners (1, 3) superimposed sheet metal parts with a material thickness in particular in the range of for example 50 pm to 250 pm, preferably 75 pm, or in the range of, for example, 250 pm to 500 pm can be connected to one another.

10. The method according to any one of the preceding claims 1 to 8, characterized in that the laser beam joining is realized as a deep laser beam welding, in which the joining partners (1, 3) are overlying sheet metal parts with a material thickness of, for example, greater than 0.5 mm, in particular in the range from 0.5mm to 5mm, particularly preferably in the range from 0.5mm to 3mm.