METHOD FOR ANALYSING A WELD DURING LASER WELDING OF WORKPIECES

Abstract

A method of analyzing a welded connection during laser welding of workpieces includes acquiring a first measurement signal for a process radiation generated during laser welding, acquiring a second measurement signal for a laser radiation reflected by the workpieces, determining whether there is a gap between the workpieces based on the first measurement signal, and when it is determined that there is a gap, determining based on the second measurement signal whether there is a welded connection.

Claims

1. A method of analyzing a welded connection during laser welding of workpieces, said method comprising: acquiring a first measurement signal for a process radiation generated during laser welding; acquiring a second measurement signal for a radiation reflected by the workpieces; determining based on said first measurement signal whether there is a gap between the workpieces; and when it is determined that there is a gap, determining based on said second measurement signal whether there is a welded connection.

2. The method according to claim 1, wherein the reflected radiation comprises at least one of: reflected laser radiation of the machining laser beam, reflected radiation of LED light radiated into a machining area, and reflected laser radiation of a pilot laser beam radiated into a machining area.

3. The method according to claim 1, wherein said first measurement signal and/or second measurement signal is based on a detection of a radiation intensity.

4. The method according to claim 1, wherein said first measurement signal is acquired in a first wavelength range above a wavelength of a machining laser beam used for laser welding and/or above a wavelength of the reflected radiation; and/or wherein said first measurement signal is acquired in a second wavelength range below the wavelength of the machining laser beam used for laser welding and/or below the wavelength of the reflected radiation.

5. The method according to claim 1, wherein the process radiation acquired as said first measurement signal is thermal radiation in an infrared spectral range and/or plasma radiation in a visible spectral range.

6. The method according to claim 1, wherein the reflected radiation acquired as said second measurement signal is in an infrared spectral range or in a visible green or blue spectral range.

7. The method according to claim 1, wherein determining whether there is a gap between the workpieces comprises determining a gap width based on the first measurement signal, and wherein it is determined that there is a gap when the gap width is greater than a predetermined gap width limit value.

8. The method according to claim 1, wherein determining whether there is a gap between the workpieces comprises determining whether said first measurement signal is or falls below a reference value or a reference curve, wherein it is determined that there is a gap between the workpieces when said measurement signal is or falls below the reference value or the reference curve.

9. The method according to claim 1, wherein determining whether there is a gap between the workpieces comprises taking a first integral over said first measurement signal and/or a first mean value of said first measurement signal, wherein it is determined that there is a gap between the workpieces when the first integral falls below a predetermined first integral limit value and/or when the first mean value falls below a predetermined first mean value limit value.

10. The method according to claim 1, wherein said first measurement signal is acquired in a first wavelength range above a wavelength of the reflected radiation or above a wavelength of a machining laser beam used for laser welding and in a second wavelength range below the wavelength of the reflected radiation or below the wavelength of the machining laser beam used for laser welding, and determining whether there is a gap between the workpieces comprises taking a first integral over the first measurement signal acquired in the first wavelength range and taking a second integral over the first measurement signal acquired in the second wavelength range; and wherein it is determined that there is a gap between the workpieces when the first integral falls below a predetermined first integral limit value and/or when the second integral falls below a predetermined second integral limit value.

11. The method according to claim 1, wherein the determining wherein there is a welded connection comprises determining based on a noise of said second measurement signal whether there is a welded connection.

12. The method according to claim 11, wherein it is determined that there is no welded connection, when an outlier frequency of the noise of said second measurement signal exceeds a predetermined first noise limit value; and/or when an integral over the noise of said second measurement signal exceeds a predetermined second noise limit.

13. The method according to claim 1, wherein at least one of the workpieces comprises or consists of aluminum and/or copper and/or nickel.

14. The method according to claim 1, wherein at least one of the workpieces has a thickness of 0.10 mm to 0.50 mm, or 0.15 mm to 0.35 mm, or 0.20 mm to 0.30 mm.

15. The method according to claim 1, wherein the workpieces comprise a diverter of a first battery and a diverter of a second battery, and wherein a welded electrical contact between the diverters of the batteries is analyzed as the welded connection.

16. The method according to claim 1, wherein the workpieces are arranged in a lap joint or parallel joint during laser welding.

17. A method for laser welding a first workpiece and a second workpiece, said method comprising the steps of: arranging the workpieces such that a first surface of the first workpiece and a first surface of the second workpiece lie on top of each other; laser welding the workpieces to form a welded connection between the workpieces by radiating a machining laser beam onto a second surface of said first workpiece, said second surface of said first workpiece being opposite said first surface of said first workpiece, and/or by radiating a machining laser beam onto a second surface of said second workpiece, said second surface of said second workpiece being opposite said first surface of said second workpiece; performing the method of analyzing the welded connection according to claim 1.

18. The method according to claim 17, wherein the workpieces are arranged in a lap joint or parallel joint.

19. The method according to claim 17, wherein the first surfaces of the workpieces touch in at least one region and/or wherein a gap is present in another region between the first surfaces of the workpieces.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] The invention is described in detail below with reference to figures.

[0051] FIG. 1 shows a schematic diagram of a laser machining system for machining a workpiece by means of a laser beam for performing a method of analyzing a welded connection according to embodiments of the present disclosure;

[0052] FIG. 2 shows a detailed schematic diagram of a sensor module of the laser machining system shown in FIG. 1;

[0053] FIG. 3 shows a flowchart of a method of analyzing a welded connection during laser welding according to embodiments of the present disclosure;

[0054] FIGS. 4A-4D show welded connections analyzed with a method of analyzing a welded connection during laser welding of workpieces according to embodiments of the present disclosure;

[0055] FIGS. 5A-5D show examples of time curves of measurement signals acquired by a method of analyzing a welded connection during laser welding of workpieces according to embodiments; and

[0056] FIG. 6 shows, by way of example, a determination of gap widths by a method of analyzing a welded connection during laser welding of workpieces according to embodiments of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

[0057] Unless otherwise noted, the same reference symbols are used in the following for the same elements and elements with equivalent effect.

[0058] FIG. 1 shows a schematic diagram of a laser machining system for machining a workpiece by means of a (machining) laser beam according to embodiments of the present disclosure. FIG. 2 shows a detailed schematic diagram of the sensor module of the laser machining system shown in FIG. 1.

[0059] The laser machining system 1 comprises a laser machining device 10, a sensor module 20, and a control unit 40.

[0060] The laser machining device 10, which may be configured, for example, as a laser machining head, in particular as a laser welding head, is configured to focus or collimate a laser beam (not shown) by means of beam guiding and focusing optics (not shown) onto the workpieces 30a, 30b to be machined, in order thereby to carry out machining or a machining process. Machining may in particular comprise laser welding. During machining, process radiation 11, which enters the laser machining device 10 and is coupled out of the beam path of the laser beam by a beam splitter 12, is generated. The process radiation is guided into the sensor module 20 and is incident on at least one detector D1, D2, D3 there.

[0061] For machining, the workpieces 30a, 30b may be arranged in such a way that they overlap. The workpieces 30a, 30b may in particular be arranged in a parallel joint or lap joint.

[0062] For example, as in FIG. 1, a lower surface of the workpiece 30a is opposite an upper surface of the workpiece 30b and the laser beam is radiated onto an upper surface of the workpiece 30a. The upper surfaces and the lower surfaces of the workpieces 30a, 30b may also be referred to as the main faces or main surfaces of the workpieces 30a, 30b.

[0063] As shown, the laser beam is radiated onto the upper surface or the upper main surface of the workpiece 30a, preferably substantially perpendicular to the main surfaces of the workpieces 30a, 30b. Accordingly, the laser beam is not radiated onto the edges or edges or in parallel to the main surfaces of the workpieces 30a, 30b.

[0064] Accordingly, the resulting process radiation 11 is emitted from the upper surface or from the upper main surface of the workpiece 30a. The process radiation 11 is thus preferably acquired from the upper surface of the workpiece 30a. Likewise, reflected radiation is preferably acquired from the upper surface of workpiece 30a. In an exemplary embodiment that is not shown, the laser machining system may include an LED lighting unit for radiating LED light into a machining area on the workpiece. In this case, the reflected radiation detected by the sensor module includes reflected LED radiation or reflected LED light. In a further exemplary embodiment that is not shown, the laser machining system may include a pilot laser unit for radiating a pilot laser beam into a machining area on the workpiece. In this case, the reflected radiation detected by the sensor module includes reflected pilot laser radiation or reflected LED light. The pilot laser unit may include a pilot laser beam source.

[0065] In particular for laser welding of the workpieces 30a, 30b, the workpieces 30a, 30b should be arranged in a lap or parallel joint such that there is no gap between the workpieces 30a, 30b arranged in this way or that the gap is as small as possible. As shown, there is an (undesirable) gap between the workpieces 30a, 30b, i.e. between the upper surface of the workpiece 30b and the lower surface of the workpiece 30a. In a plan view of the workpieces 30a, 30b, in particular in a plan view of the upper surface of the workpiece 30a or a plan view of the lower surface of the workpiece 30b, it cannot be seen whether there is a gap between the workpieces 30a, 30b.

[0066] As shown in FIG. 2, the sensor module 20 preferably includes a plurality of detectors or sensors D1, D2, D3 configured to detect various parameters, such as an intensity, of the process radiation 11 and to output a measurement signal based thereon. Each of the detectors D1, D2, D3 may comprise a photodiode or a photodiode or pixel array. The detectors preferably include a photodiode or a sensor for the visible spectral range, a photodiode or a sensor for the infrared spectral range and a photodiode or a sensor for a wavelength range of the laser beam or the radiated pilot laser beam or the radiated LED light. Furthermore, the sensor module 20 may include a plurality of beam splitters 221, 222 in order to split the process radiation 11 and to direct it to the corresponding detectors D1, D2, D3. The beam splitters 221, 222 may be configured as partially transparent mirrors and may be wavelength-selective according to embodiments.

[0067] The control unit 40 is connected to the sensor module 20 and receives the measurement signals from the detectors D1, D2, D3. The control unit 40 may be configured to record the measurement signals from the detectors D1, D2, D3. The control unit 40 is configured to determine and/or analyze a machining result of the laser machining and is in particular configured to analyze welded joints. The control unit 40 may be further configured to control the laser machining device 10 based on a result of the analysis.

[0068] The laser machining system 1 may be configured to carry out laser machining processes, in particular laser welding, and to carry out methods for analyzing a welded connection during laser welding of workpieces according to embodiments of the present disclosure.

[0069] FIG. 3 shows a flowchart of a method of analyzing a welded connection during laser welding of workpieces according to embodiments of the present disclosure.

[0070] The method starts by acquiring a first measurement signal for a process radiation generated during laser welding (step S1). The method further includes acquiring a second measurement signal for radiation reflected by the workpieces (step S2). According to embodiments, acquiring the first measurement signal and acquiring the second measurement signal may be carried out simultaneously. Subsequently, it is determined based on the first measurement signal whether there is a gap between the workpieces (step S3). When it is determined that there is a gap, it is determined on the basis of the second measurement signal whether there is a welded connection or gap bridging between the two workpieces (step S4). In other words, it is determined whether there is electrical or mechanical contact between the workpieces.

[0071] Therefore, the method makes it possible to detect whether there is a gap between the connected workpieces. The method also makes it possible to identify whether there is a gap bridging, i.e. a welded connection, in particular an electrical and mechanical welded connection. In particular, the method may be used to analyze a welded electrical connection, for example to recognize a lack of electrical contact between joined workpieces. It is therefore possible to distinguish between a proper weld, i.e. a weld without a gap, also referred to as “good weld” or “0 gap weld”, or a weld with a gap and with gap bridging so that an electrical contact between the joined workpieces exists, or a weld with a gap but no gap bridging so that there is no electrical contact between the joined workpieces.

[0072] The first measurement signal is preferably acquired in two different wavelength ranges. For example, the first measurement signal may be acquired based on a detection of radiation intensity of the process radiation in a first wavelength range above the wavelength of the reflected radiation or above the wavelength of the laser beam used for laser welding, in particular in an infrared range, and on a detection of radiation intensity of the process radiation in a second wavelength range below the wavelength of the reflected radiation or below the wavelength of the laser beam, especially in a visible range. The first measurement signal acquired in the first wavelength range may correspond to thermal radiation and may be referred to as a “thermal signal”. The first measurement signal acquired in the second wavelength range may correspond to a plasma radiation and may be referred to as a “plasma signal”. However, it is also possible to acquire or evaluate only the first measurement signal in only one of these wavelength ranges. As mentioned above, the reflected radiation may include reflected laser radiation of a radiated pilot laser beam or reflected laser radiation of the (machining) laser beam used for the welding process or reflected laser radiation of a radiated LED light.

[0073] In the exemplary embodiment of FIGS. 1 and 2, the plasma signal may be acquired by the detector 1, which is sensitive in a wavelength range below the wavelength of the reflected radiation or the laser beam, in particular in the visible wavelength range of light, in order to detect the intensity of plasma process emissions. The thermal signal may be acquired by the detector D2, which is sensitive in a wavelength range above the wavelength of the reflected radiation or the laser beam, in particular in an infrared wavelength range of the light, in order to detect the intensity of process emissions in the infrared or thermal spectral range, i.e. of thermal radiation. The second measurement signal may be acquired by the detector D3, which is sensitive in the wavelength range of the reflected radiation or the laser beam, in order to detect back reflections of the laser of the laser machining device.

[0074] According to embodiments, determining whether there is a gap between the workpieces (step S3) may include taking a first integral over the plasma signal and taking a second integral over the thermal signal. In this case, it may be determined that there is a gap between the workpieces when the first integral falls below a predetermined first integral limit value and/or when the second integral falls below a predetermined second integral limit value.

[0075] According to embodiments, the determination of whether there is a welded connection or a gap bridging (step S4) may be based on a noise of the second measurement signal. In this case, it may be determined that there is no welded connection or no gap bridging when an outlier frequency of the noise of the second measurement signal exceeds a predetermined first noise limit value and/or when an integral over the noise of the second measurement signal exceeds a predetermined second noise limit value. The noise may be defined as a deviation from a mean value of the second measurement signal, preferably in a predetermined time interval or measurement signal, and in particular amplified by a predetermined factor. The mean value may be predetermined or may be determined based on the second measurement signal.

[0076] According to embodiments, at least one of the steps S1 to S4 may be carried out during the laser welding of the welded connection.

[0077] Preferably, one of the workpieces includes a battery, a battery module and/or a battery cell and another one of the workpieces includes a diverter. In this case, the method according to embodiments of the present disclosure may be used to analyze a welded electrical contact between the diverter and the battery, the battery module or the battery cell. In particular, one of the workpieces may consist of aluminum and another one of the workpieces may comprise copper and be coated with nickel. The coating may be applied galvanically. At least one of the workpieces may have a thickness of 0.10 mm to 0.50 mm, preferably a thickness of 0.15 mm to 0.35 mm, particularly preferably a thickness of 0.20 mm to 0.30 mm.

[0078] In an embodiment, diverters from two or more batteries are welded or contacted to one another. The diverters may be made of copper Cu or aluminum Al. In particular, a diverter of a first battery may be made of aluminum or copper and a diverter of a second battery may be made of aluminum or copper, so that the welded connection is formed between aluminum and aluminum Al—Al, or between copper and copper Cu—Cu, or between aluminum and copper Al—Cu.

[0079] Laser welding may include gas-tight welding of cell housings of battery cells, welding membranes of cell lids of battery cells, welding connections in the cell covers of battery cells and welding a bursting plate of cell lids of battery cells.

[0080] In particular, the method according to embodiments of the present disclosure may be used for analyzing a welded connection during laser welding of workpieces in lap or parallel joints, and in particular in I-weld seams.

[0081] FIGS. 4A-4D show welded connections analyzed with a method of analyzing a welded connection during laser welding of workpieces according to embodiments of the present disclosure.

[0082] FIGS. 4A-4D each show a top view of I-weld seams created during laser welding in lap joint in the upper row (“camera”) and each show a sectional view of the respective weld seam in the middle row. A schematic view of the sectional view is shown in each case in the bottom row. In the plan view of the respective workpieces 30a, 30b or the respective weld seams, it is not possible to distinguish whether there is a weld without a gap, a weld with a gap and gap bridging, or a weld with a gap but without gap bridging. The plan view is of the upper surface of workpiece 30a as discussed with reference to FIG. 1.

[0083] In the first column (“Gap: 0 μm”), FIG. 4A shows a proper weld seam, also referred to as a “good weld”, which was recognized using the method of analyzing welded connections during laser welding of workpieces according to embodiments of the present disclosure. The welded workpieces 30a, 30b, shown here as metal sheets, have no gap between them and current can flow via the weld seam. The resulting welded connection is marked as “good weld” or “0-gap”.

[0084] FIGS. 4B-4D show typical defect patterns recognized using the method of analyzing welded connections during laser welding of workpieces according to embodiments of the present disclosure.

[0085] FIG. 4B shows a gap S between the two welded workpieces 30a, 30b in the second column (“gap: 100 μm”). This gap S can be tolerated because the gap S is bridged (gap bridging “B” in FIG. 4B). Thus, despite the existing gap S, there is still electrical contact between the welded workpieces, i.e. there is a welded connection. This is also referred to as “welding with gap bridging” or “gap with (electrical) connection or (electrical) contact”.

[0086] In the third and fourth columns (“Gap: 150 μm” and “Gap: 200 μm”), FIGS. 4C and 4B show another typical defect pattern, also referred to as “false friend”. There is a gap S between the welded workpieces 30a, 30b that is not bridged so that there is no electrical contact between the workpieces. This is also referred to as “welding without gap bridging” “gap without (electrical) connection or (electrical) contact”. That is, there is no welded connection.

[0087] FIGS. 5A to 5D show examples of time curves of measurement signals acquired by a method of analyzing a welded connection during laser welding of workpieces according to embodiments.

[0088] In the embodiment shown in FIGS. 5A to 5D, the first measurement signal was acquired in the first and in the second wavelength range and includes the plasma signal P1 and the temperature signal P2. The second measurement signal for the reflected laser light is referred to as the back reflection signal P3. FIGS. 5A-5D show exemplary curves of the measurement signals P1, P2 and P3, each for one laser welding process. In addition, the curve of noise of the measurement signal P3 is shown as “P3 noise”.

[0089] The method according to embodiments of the present disclosure includes acquiring the plasma signal P1 and the temperature signal P2. It is determined that there is a gap between the workpieces when, for example, the plasma signal P1 and/or the temperature signal P2 falls, i.e. lies at or below or falls below a respective lower envelope. This may be determined, for example, by taking a first integral over the plasma signal P1 and a second integral over the temperature signal P2. When the first integral falls below a predetermined first integral limit value and/or when the second integral falls below a predetermined second integral limit value, a gap exists. When a gap exists, it is determined based on the back reflection signal P3 whether there is a welded connection or gap bridging. There is no welded connection or gap bridging when an outlier frequency of the noise of the back reflection signal P3 exceeds a predetermined first noise limit value and/or when an integral over the noise of the back reflection signal P3 exceeds a predetermined second noise limit value. Otherwise there is a gap with gap bridging, i.e. a welded connection.

[0090] On the one hand, the method may be used to distinguish between good welds, i.e. welds without a gap between the workpieces, and welds with a gap. On the other hand, the method can distinguish between welds with a gap but with gap bridging and welds with a gap but without gap bridging.

[0091] In FIG. 5A, the integrals of the plasma signal P1 and the temperature signal P2 exceed the respective limit values. The weld created during the laser welding process is marked as “good weld”. A welded connection with a 0 gap is present between the workpieces joined in this way. In particular, there is an electrical contact or an electrical connection between the connected workpieces. This corresponds to the welded connection shown in FIG. 4A.

[0092] In FIGS. 5B-5D, the plasma signal P1 and the thermal signal P2 have fallen relative to the respective predetermined reference values or envelope curves. In other words, the integrals of the plasma signal P1 and of the thermal signal P2 fall below the respective limit values. The welds created during the respective laser welding processes are marked as welds with a gap.

[0093] According to embodiments, it is sufficient when either the integral of the plasma signal P1 or the integral of the temperature signal P2 falls below the respective limit value. According to further embodiments, it may be determined that a gap is only present when both the integral of the plasma signal P1 and the integral of the temperature signal P2 fall below the respective limit value.

[0094] In FIG. 5B, there is a gap of 100 μm between the workpieces, in FIG. 5C there is a gap of 150 μm between the workpieces, and in FIG. 5D there is a gap of 200 μm between the workpieces. The welds shown in FIGS. 5B-5D correspond to the welds shown in FIGS. 4B-4D. The gap width may be determined based on the integral value of the plasma signal P1 and/or the thermal signal P2. When the integral value is in a first range, a gap width of a first value or range of values may be assigned to the corresponding weld. Correspondingly, an integral value that lies in a second range may be assigned a gap width of a second value or range of values, etc. This is illustrated by way of example in FIG. 6 for the plasma signal P1.

[0095] For the corresponding welds in FIGS. 5B-5D, it is now determined whether there is nevertheless a welded connection between the workpieces and, accordingly, an electrical contact or an electrical connection. For this purpose, the noise of the back reflection signal P3, P3 noise, is analyzed.

[0096] In FIG. 5B, an outlier frequency of the noise of the rear reflection signal P3 is below a predetermined first noise limit value. Therefore, it is determined that, despite the existing gap, there is a weld connection between the workpieces or a gap bridging.

[0097] In FIGS. 5C and 5D, an outlier frequency of the noise of the back reflection signal P3 is greater than the predetermined first noise limit value. It is therefore determined that there is no welded connection or gap bridging, and thus no electrical contact, between the workpieces.

[0098] The present invention is based on the finding that in laser welding in lap joint, a good weld can be distinguished from welds with a gap by a drop of the intensity of a plasma signal and a drop of the intensity of a thermal signal of the laser welding process. Furthermore, the present invention is based on the finding that a weld with a gap and with gap bridging can be distinguished from a weld with a gap but without gap bridging by a significant increase of the noise of a back reflection signal of the radiation reflected back from the workpieces in the latter case. Accordingly, a combination of the plasma signal and the thermal signal with the back reflection signal provides unambiguous information about the presence or absence of a welded connection, in particular an electrical contact, between the workpieces. Here “gap is present” may be considered as a necessary condition, and excessive noise as a sufficient condition for the gap not being bridged. Accordingly, it can be recognized unambiguously whether a false friend is present.