CN115605313A

CN115605313A - Method for analyzing a laser welding process and laser machining system

Info

Publication number: CN115605313A
Application number: CN202180030186.3A
Authority: CN
Inventors: A·博科; D·埃施; T·温内曼; M·斯特雷贝尔; A·唐克; J·诺尔特; W·杜翁; S·格劳; R·莫泽; S·比尔曼斯; J·赖泽尔; T·洛伦茨
Original assignee: Precitec GmbH and Co KG
Current assignee: Precitec GmbH and Co KG
Priority date: 2020-04-23
Filing date: 2021-04-22
Publication date: 2023-01-13
Also published as: WO2021214238A1

Abstract

The invention discloses a method for analyzing a laser welding process, which comprises the following steps: forming a weld by irradiating a laser beam onto at least one of the workpieces to be welded; irradiating the measuring beam onto the weld; sensing a measurement signal for measuring a reflected portion of the beam from the weld; and determining whether a welding defect exists based on the measurement signal.

Description

Method for analyzing a laser welding process and laser machining system

Technical Field

The present invention relates to a method for analyzing a laser welding process, in particular for analyzing the result of a laser welding process or a weld seam produced by a laser welding process, and a laser processing system for carrying out the method.

Background

In laser processing systems for processing workpieces by means of laser beams, the laser beam emerging from a laser source or one end of a laser fiber is focused or focused by means of beam-guiding and focusing optics onto the workpiece to be processed in order to locally heat the workpiece to a melting temperature. The processing may include laser welding, among others. The laser machining system may comprise a laser machining device, for example a laser machining head, in particular a laser welding head.

To ensure the processing quality, the results of the laser welding, in particular the welded connections between the welded workpieces, need to be analyzed or checked. This includes the identification of weld defects. Based on the analysis results, the welded workpieces may be marked as "good" or "good welds" (i.e., suitable for further processing or sale) or "bad" or "bad welds" (i.e., defective). In particular, if there is no weld defect, the welded workpiece may be marked as "good", and if there is a weld defect, the welded workpiece may be marked as "bad". Welding defects prove to be a significant challenge when laser welding workpieces, especially sheet metal.

Application DE 10 2019 122 047 describes a sensor module for monitoring a laser welding process, which has a plurality of detectors or sensors, which detect different parameters of the process radiation and output them as measurement signals.

Application DE 10 2020 104 462 describes a method for analyzing a welded connection when laser welding workpieces. The method is carried out during laser welding and is based on the sensing and evaluation of plasma radiation or temperature radiation in addition to the laser radiation reflected by the workpiece. It is thus possible to detect whether gaps exist between the joined workpieces and whether a soldered connection, in particular an electrical contact, is formed between the workpieces.

However, this approach reaches its limits depending on the material composition and sheet thickness. Furthermore, this method does not allow reliable detection of certain defect images, for example gaps having a very small size.

Batteries play a central role in the field of electric vehicles. The individual cells, also referred to as "cells", are connected to one another, i.e., are in contact. The combination of a plurality of battery cells is referred to as a "battery module". Typically, the joining is performed by laser welding. In this case, for example, the electrical conductors (Ableiter) of the battery cells are welded to one another, usually in an overlapping manner. The weld seam has, for example, the geometry of a so-called "I-seam". The materials are typically aluminum and copper. Typical connections or material combinations are copper-copper, aluminum-aluminum and copper-aluminum or aluminum-copper. In order to connect the battery cells to the battery module and thus to successfully construct the module, it is necessary that there be an electrical contact between the welded parts of the battery cells, for example electrical conductors, i.e. current can flow between the welded parts or through the weld seam. Only in this case is the contacting successful.

Typical defect images or machining defects occur when welding workpieces by laser welding, in particular at the overlap with an I-seam. These include gaps between welded workpieces. This defect can have different effects on the quality of the weld between the workpieces. Even small gaps can reduce the mechanical stability of the weld seam or of the connection between two workpieces. Gaps between the welded workpieces may also result in electrical contact between the welded workpieces no longer being ensured.

For some soldering applications, this defect can be tolerated if the gap is small and there is still a sufficiently stable mechanical or electrical connection between the soldered workpieces. In other welding tasks, for example, defects can be tolerated if the gap is bridged by the molten material of the workpieces, i.e. despite the gap, a welded connection is present, in particular an electrical contact is present between the welded workpieces. This case is also referred to as "welding with gap bridging" or "gap with (electrical) connection".

Another typical defect image is called a "false friend". Here, there is a gap between the workpieces, wherein the gap is not bridged and therefore there is no (electrical) contact between the workpieces to be welded. This is also referred to as "gap-free bridging welding" or "gap without (electrical) connection".

The identification of the processing defects is therefore decisive for the quality of the welded workpieces or weld seams. Welding without machining defects may also be referred to as "good welding". A weld with (intolerable) processing defects may be referred to as a "bad weld". When a pure visual inspection of the welded seam or the welded workpiece is carried out after the laser welding process is carried out, it is not possible to identify from a top view whether a machining defect is present and which type of defect is present, or to distinguish between a good weld and a poor weld.

Disclosure of Invention

The object of the present invention is to provide a method and a laser processing system for carrying out the method, by means of which the results of a laser welding process, in particular a laser welding process, can be analyzed or evaluated in a simple and rapid manner.

A further object of the present invention is to provide a method and a laser processing system for carrying out the method, by means of which it can be easily and quickly determined whether a welding defect is present or whether a welded connection, in particular an electrical contact, is present between two workpieces. In particular, the object of the invention is to provide a method and a laser processing system for carrying out the method, which allow good welds and poor welds to be classified or distinguished in a simple and rapid manner.

These objects are solved by the subject matter of the independent claims. Advantageous configurations and embodiments are the subject matter of the dependent claims.

The invention is based on the idea of evaluating the quality of a weld and the presence of weld defects by analysing the retro-reflection or back-scattering of a measuring beam from the weld. By recording and evaluating the retroreflection of the measuring beam, the topography or surface properties of the weld can be analyzed, for example by means of a photodiode, in order to determine whether a weld defect is present or to distinguish between a weld with a connection or an electrical contact, i.e. a weld connection, and a weld without a connection or an electrical contact (a fake friend), so that a good weld and a bad weld can be distinguished. A good weld can be defined as a welded connection or weld seam without weld defects. A poor weld may be defined as a welded connection or weld having a weld defect.

The weld defects may include at least one of the following: defective welded connections between workpieces, gaps between workpieces, and fake friends. A solder connection may produce or represent a contact, in particular an electrical contact, between the workpieces. It is possible to identify and differentiate between a welded connection or weld seam with a gap and a gap bridge and a welded connection or weld seam with a gap but without a gap bridge (pseudo friend).

According to one aspect of the invention, a method for analyzing a laser welding process, in particular for analyzing a weld seam produced by a laser welding process, is specified. The method comprises the following steps: forming a weld by irradiating a laser beam having a first laser power onto at least one workpiece to be welded; irradiating a laser beam with a second laser power onto the weld joint, wherein the second laser power is less than the first laser power; sensing a measurement signal for a portion of the laser beam irradiated at the second laser power reflected or backscattered from the weld; and determining whether a welded connection exists between the workpieces based on the measurement signal. The laser beam having the first laser power and the laser beam having the second laser power may be from the same laser source. Therefore, the irradiation of the laser beam having the second laser power is preferably performed after the irradiation of the laser beam having the first laser power or after the formation of the weld bead. In particular, the irradiation of the laser beam with the second laser power can take place on the cooled and/or solidified region of the weld seam.

According to a further aspect of the invention, a method for analyzing a laser welding process, in particular for analyzing a weld seam produced by a laser welding process, is specified. The method comprises the following steps: forming a weld by irradiating a laser beam having a first laser power onto at least one workpiece to be welded; irradiating the measuring beam onto the formed weld seam or onto an (already) formed region of the weld seam; sensing a measurement signal for a portion of the measurement beam reflected or backscattered from the weld; and determining whether a welding defect exists based on the measurement signal. The irradiation of the measuring beam is therefore preferably carried out after the irradiation of the laser beam with the first laser power or after the (at least partial) formation of the weld seam. In particular, the irradiation of the measuring beam can take place on the cooled and/or solidified region of the weld seam. This method, which may be referred to as a post-treatment method, is preferably directly connected to the laser welding process for forming the weld seam. The measuring beam may be, in particular, a laser beam. The laser beam having the first laser power and the measuring beam may be from the same laser source or have the same wavelength. In this case, the second laser power of the measuring beam is preferably lower than the first laser power.

The measuring beam can have any wavelength, in particular in the infrared range or in the visible green or blue range. In particular, the measuring beam may be a laser beam, for example a laser beam from the same laser source as the laser beam used for machining the workpiece (also called machining laser beam), or from a pilot laser source with a wavelength of about 630nm or about 530 nm. Alternatively, the measuring beam may also comprise or be generated by an LED light. Preferably, at least a part of the beam path of the measuring beam is coaxial with the beam path of the machining laser beam during laser machining.

The method therefore includes irradiating a laser beam having a first laser power onto one or more workpieces to weld the workpieces to one another. In this case, a weld seam is formed on at least one workpiece in order to join the workpieces. In a next step, a measuring beam, for example a laser beam with a second laser power, is irradiated onto the produced weld seam. Here, the second laser power may be lower or smaller than the first laser power. Alternatively, the measuring beam can also be LED light, as described above. Meanwhile, the measurement beam sensed for the portion of the measurement beam reflected from the weld may also be referred to as a "sensor signal". In other words, the intensity of the part of the measuring beam reflected from the weld seam or the measuring beam reflected from the weld seam is detected or measured by means of a sensor (e.g. a photodiode) and a measuring signal is generated on the basis thereof. It is then determined whether a welding defect is present based on the measurement signal. The laser beam irradiated at the first laser power may also be referred to as a processing laser beam.

According to the invention, the measurement radiation reflected from the weld seam, in particular the reflected laser radiation, can be distinguished or classified between good welds and bad welds by measurement or detection and evaluation. For this purpose, after the actual laser welding process has been completed, in particular after the weld seam has cooled or solidified, the measuring beam can be moved over or past the weld seam again, i.e. the laser beam is again directed or irradiated onto the weld seam, wherein the laser beam used as measuring beam, i.e. the re-irradiated laser beam, has a lower laser power than the laser power in the actual laser welding process, so that the laser beam or the laser power is only reflected or scattered back and is coupled into the workpiece or the weld seam as little or as little as possible. The measuring beam is substantially reflected or backscattered and couples into the workpiece or weld seam as little or no as possible. Too strong a coupling may result in re-modification or melting of the material. The intensity of the measuring radiation reflected from the weld can then be used for analyzing the weld.

As the inventors have found, the topology of a weld bead or bead for good welding or welding without welding defects is different from the topology of a weld bead or bead for poor welding or welding with welding defects: the roughness of the bead is high in good welding and low in poor welding. Alternatively or additionally, good welds and bad welds may differ in the shape of the curvature of the weld surface. For example, it is simply stated that the curvature of the weld surface may be convex in a good weld without a gap, and may be planar or even concave in a bad weld with a gap. The portion of the measuring beam reflected from the weld seam or weld bead, also referred to as "retro-reflection" or "reflected measuring radiation", is therefore lower in the case of good welds than in the case of poor welds. In other words, the intensity of the reflected measuring radiation is lower when there is a good weld than when there is a poor weld. Based on these findings, it is possible to distinguish between a good weld and a bad weld by detecting the intensity of the portion of the irradiated measurement beam reflected from the weld.

Thus, the method according to the invention makes it possible to distinguish between a good weld and a bad weld. In the case of good welding, the previously mentioned welding defects are not present. In particular, in the case of good welding, there can be a welded connection between the workpieces being connected. The welded connection may represent an electrical and/or mechanical (i.e. physical) welded connection, i.e. there is electrical or mechanical contact between the workpieces. Good welds may include those where there is no gap between the welded workpieces (so-called "zero gap") and those where there is a gap, but is bridged (welds with a joint). Poor welding may include a situation where there is a gap between the welded workpieces, but there is no bridging (no connected weld). The method can therefore be used for analyzing a weld seam produced by a laser process or an electrical connection welded by means of a laser beam for identifying whether an electrical contact is missing between the connected workpieces, for example when connecting a battery cell to a battery module.

Since the determination of the presence or absence of a weld defect is preferably carried out after the actual laser welding, the method according to the invention can be referred to as "post-processing method" or simply "post-method".

Preferably, the method may further comprise: when it is determined that no weld defects are present or that a welded connection is present, the workpieces joined by laser welding are evaluated or marked as "good" or "pass; when it is determined that there is a welding defect or there is no welding connection, it is evaluated or marked as "bad" or "bad piece". On the basis of this, the subsequent laser welding process can be adjusted or controlled. For example, processing parameters such as the supplied laser power, the distance of the laser processing device from the workpiece, the focal position and/or the focal position of the laser beam used for the laser welding, etc., can be adapted or adjusted for the next laser welding process. The method may further include outputting a defect for the workpiece when the welding defect is determined to be present and/or outputting a warning for the workpiece when the welding defect is determined to be present.

The measurement signal may correspond to the radiation intensity of the reflected part of the measurement beam. In other words, the measurement signal may be based on measuring or detecting the radiation intensity of the reflected portion. Thus, the measuring beam and the reflected part of the measuring beam sensed as a measuring signal may have the same wavelength. In particular, the laser beam irradiated with the first laser power, the measuring beam and the reflected part of the measuring beam sensed as the measuring signal may have the same wavelength. The measurement or detection of the radiation intensity of the reflected portion can be carried out by means of at least one photodiode. The photodiode may have spectral sensitivity in a wavelength range including the wavelength of the measurement beam. The photodiode may have a maximum spectral sensitivity at the wavelength of the measuring beam. The measurement signal may be digital. The measurement signal may comprise a plurality of measured values which are each associated with a time point and/or a position on the workpiece or the welding surface. The measurement signal may be variable in time.

The irradiation of the measuring beam and/or the sensing of the measuring signal may be performed along a measuring path. The irradiation with the laser beam can be carried out along a predetermined path, a so-called machining or welding path, with respect to the workpiece, in particular with respect to the surface of one or more workpieces. The irradiation of the measuring beam and/or the sensing of the measuring signal can be carried out along the same predetermined path. In other words, the welding path and the measurement path may be the same. The irradiating of the measuring beam may comprise tracking the weld by the measuring beam. In other words, the measuring beam is irradiated onto the weld seam while tracing the weld seam. The irradiation of the measuring beam and/or the sensing of the measuring signal can take place over the weld seam or from a point of the weld seam or at least partially along and/or transversely to the weld seam. The irradiation of the measuring beam along the measuring path or the tracking of the weld by the measuring beam can be carried out at the same or different speeds, also referred to as "feed speeds", with respect to the workpiece or the weld as the laser beam irradiation is used for forming the weld.

If the weld defect is a missing weld connection, and if the irradiation of the measuring beam and/or the sensing of the measuring signal takes place at least partially along the weld seam, it is thereby possible to distinguish regions along the weld seam where weld seams are present from regions along the weld seam where weld seams are absent. Thus, it is possible according to embodiments to determine whether a weld connection is present at least in the region along the weld seam between the workpieces on the basis of the measurement signal. If it has been determined that there is a welded connection at least in the region along the weld seam between the workpieces, it can therefore be determined that there is a welded connection between the workpieces. In other words, determining whether a weld connection exists between the workpieces based on the measurement signals, determining a first region along the weld where a weld connection exists, and determining a second region along the weld where a weld connection does not exist may be included. Thus, an inhomogeneous weld seam can be analyzed, wherein there are regions with welded connections and regions without welded connections, in particular without electrical contact. Furthermore, with the method according to the invention, other welding defects, in particular "fake friends", can be located along the weld seam. By analytically evaluating the parts with and without connections, physical characteristic values, such as strength or absolute conductivity, can be estimated.

Preferably, the measuring beam is irradiated onto the weld seam region where it should be determined whether a welding defect exists. For example, the measuring beam may be irradiated onto the weld seam in an overlap region where the workpieces overlap. Thus, the measurement path may include an overlap region in which the workpieces overlap. The measurement path may also include regions outside the overlap region and/or outside the weld. The measurement signal from this region can be used, for example, to determine a reference value for the intensity of the measurement radiation reflected from the workpiece or weld seam. Preferably, the measurement path comprises a first region on a first workpiece, a weld, and a second region on a second workpiece, the second workpiece having been or to be welded to the first workpiece. This is preferably the case when the measuring path extends along the weld seam. Alternatively, the measurement path may include a first region on the workpiece, a weld, and a second region on the workpiece. This is preferably the case when the measuring path extends transversely to the weld seam.

The laser beam and/or the measuring beam for processing and/or irradiation with the first laser power may comprise a wavelength in the infrared spectral range, in particular between 1030nm and 1070nm, or a wavelength in the visible green spectral range, in particular between 500nm and 570nm, preferably 515nm, and/or a wavelength in the visible blue spectral range, in particular between 400nm and 500nm, or between 440nm and 460nm, preferably 450 nm. In other words, a laser source emitting in the infrared spectrum or wavelength range may be used. Alternatively, a laser source emitting in the green or blue spectrum or wavelength range may be used. In particular, the laser beam irradiated with the first laser power and the measuring beam may be from or generated by the same laser source. The measuring beam may also be a pilot laser beam or an LED lamp. The laser beam irradiated with the first laser power and the measuring beam may thus have the same or different wavelengths.

If the measuring beam is a laser beam, the second laser power can be selected such that the measuring beam is substantially completely reflected by the workpiece or weld seam. Preferably, the second laser power is less than the laser power for machining the workpiece and/or the feed speed of the measuring beam is equal to or greater than the feed speed of the laser beam irradiated at the first laser power to form the weld. The second laser power and/or the feed speed of the measuring beam can be selected such that the measuring beam is not coupled into the workpiece. With a suitable choice of the second laser power, the feed speed of the measuring beam can also be smaller than the feed speed of the laser beam irradiated with the first laser power, for example in order to collect sufficient data for analysis. In other words, the power density of the measuring beam at the workpiece surface or weld seam surface can be adjusted such that it is below the threshold value at which the measuring beam is coupled into the workpiece or the workpiece melts. In particular, the second laser power and the cross section of the measuring beam can be selected such that the power density generated at the workpiece surface or weld surface is below a threshold value at which the measuring beam is coupled into the workpiece or melts the workpiece. In other words, the local input of the measuring beam into the workpiece surface can be so low that it does not lead to melting of the workpiece and/or to the laser beam not being coupled into the workpiece. The second laser power may be between 1W and 1KW, preferably between 5W and 300W, particularly preferably between 5W and 200W, in particular 200W. If the measuring beam is from a pilot laser source, the second laser power may be less than 20W, in particular less than 1W, even between 1mW and 10 mW. Preferably, the measuring beam is irradiated in a Continuous Wave (CW) operating mode of the laser source. The first and/or second laser power may be given an average laser power.

In one embodiment, based on the measurement signals from the weld, it is determined whether there is a weld defect or whether there is a weld connection between the workpieces. In other words, the region of the measurement signal corresponding to the weld seam or the overlap region is preferably used for evaluating the weld seam. For example, whether a weld defect is present may be determined based on the noisy portion of the measurement signal from the weld.

In one embodiment, the presence of a welding defect is determined if the measurement signal or a noise part of the measurement signal is higher than a reference value or reference trend (also referred to as "reference curve"). This may also be referred to as an increase in the measured signal or noise fraction. The reference value or reference curve may be predefined based on the material and/or thickness of the workpiece, in other words, the reference value or reference curve may depend on the material or thickness of the workpiece. The reference curve may be, in particular, a predetermined temporal or spatial reference curve. The reference curve may be a predefined lower envelope curve. In particular, the reference curve can be predefined along the weld seam and/or along the measuring path.

Determining whether a weld defect is present may include integrating the measurement signal or a noise portion of the measurement signal. In one embodiment, the presence of a weld defect is determined if the integral exceeds a predetermined integral limit value, and the absence of a weld defect is determined if the integral is below a predetermined integral limit value. The region of the measurement signal or noise portion corresponding to the weld or including a portion of the weld may be integrated. In a further embodiment, at least one region of the measurement signal or noise portion in which the measurement signal or noise portion is only above and/or only below a predefined integration reference value can be integrated. The at least one region of the measurement signal or noise portion may comprise all or a local extremum, i.e. a maximum or a minimum, of the measurement signal or noise portion and may also be referred to as a "peak" of the measurement signal or noise portion. According to an embodiment, the presence of a weld defect can be determined if the sum of the integrals over the area in which the measurement signal or noise portion is only above and/or only below a predefined integral reference value exceeds a predefined integral limit value. According to an embodiment, one of these integrals may also be determined as having a maximum value, i.e. a maximum integral. It can then be determined that a weld defect is present if this maximum integral exceeds a predefined integral limit.

Alternatively or additionally, determining whether a weld defect is present may include averaging the measured signal or noise portion. At least a portion of the measurement signal or noise portion may be averaged, for example, a portion corresponding to an overlap region or weld. In this case, it can be determined that a welding defect is present if the average value exceeds a predefined average value limit.

Alternatively or additionally, determining whether a weld defect is present may include finding an outlier frequency of the measurement signal. The outlier frequency may also be referred to as "defect frequency". In this case, if the frequency of the outliers of the measurement signal exceeds a predetermined outlier limit, it can be determined that a weld defect is present. The outlier frequency can be found for at least one region of the measurement signal. The outlier frequency may be defined as the frequency of the measurement signal or the frequency of the values lying outside a predetermined envelope of the measurement signal. The outlier frequency can be given as a percentage of the considered and/or predefined time interval or measurement interval or as a percentage of the considered and/or predefined area of the measurement signal. Alternatively, the outlier frequency may be given in absolute value.

The reference values, reference curves and extreme values mentioned above can be predefined as a function of or on the basis of the welding defect or welding task to be determined. For example, a different reference value than the one used for determining whether a gap-free bridge is present, i.e., a fake friend, may be predefined for determining whether a gap is present.

According to an embodiment, the method comprises a pre-processing of the measurement signal, in particular a smoothing and/or filtering of the measurement signal, in particular a filtering of noise parts in the measurement signal.

The method according to the invention can be used in particular for laser welding in a lap joint or parallel butt joint. In other words, the workpieces may be arranged in a lap joint or parallel butt joint, i.e. in an overlapping geometry, when the laser beam is irradiated with the first laser power. The seam geometry is preferably an I-seam or lap seam.

At least one of the workpieces may comprise or consist of aluminium, steel,

Copper, copper and/or nickel (preferably electroplated) with a nickel coating or consist thereof.

The at least one workpiece may have a thickness of between 0.05mm to 5mm, between 1mm to 5mm, between 0.1mm to 1mm, between 0.05mm to 0.5mm, about 0.07mm, between 0.2mm to 0.4mm, or about 0.3 mm.

Preferably, the method according to the invention is usedTo the battery contacts to determine if an electrical connection exists. The workpiece may comprise a part, in particular an electrical conductor, of the first battery cell and a part, in particular an electrical conductor, of the second battery cell. Thus, welds may be formed between the cell connectors and/or between the bus bars. The electrical contact of the welding between the parts of the battery cells, in particular between the electrical conductors, can be evaluated as a welded connection. The first battery cell and/or the second battery cell may be configured as a pouch-shaped battery cell, a prismatic battery cell, or a cylindrical battery cell, or may include at least one of these battery cells. In particular, the battery cell may be constructed as a pouch-shaped battery cell. In this case, part of one cell may comprise aluminum, while part of the other cell may comprise copper, wherein the latter may be selectively coated with nickel (layer thickness of e.g. 8 μm). Alternatively, portions of one cell and portions of another cell may both be constructed of the same material, such as copper or aluminum. The coating may be applied galvanically. The thickness of the portion of the battery cell may be between 0.2mm and 0.4 mm. Furthermore, the battery cell may be configured as a prismatic battery cell. In this case, portions of one cell and portions of the other cell may both be constructed of the same material, such as aluminum. The thickness of the portions of the two battery cells may be between 1mm and 5 mm. Furthermore, the battery cell may be configured as a cylindrical battery cell. In this case, a portion of one battery cell may be formed of

Or aluminum and a portion of another cell may be made of

Aluminum or copper, wherein the latter can optionally be coated with nickel (layer thickness, for example, 8 μm). The coating may be applied galvanically. The thickness of the portions of the two battery cells may be between 0.4mm and 0.5 mm.

According to another aspect of the present invention, a laser machining system for analyzing a weld formed by a laser welding process is presented. The method according to the invention described above can be carried out by a laser processing system comprising a laser processing device, in particular a laser welding head, for processing a workpiece by means of a laser beam, and a sensor module.

The laser processing system may include, for analyzing a weld formed by the laser welding process: a laser processing head, in particular a laser welding head, for irradiating a laser beam and/or a measuring beam onto a workpiece; a sensor module for sensing a portion of the measurement beam reflected from the weld; and a control device arranged for controlling the laser machining system so as to perform a method according to an embodiment of the present disclosure.

The laser welding head can be configured as a so-called solid-state optical laser welding head or as a so-called scanning laser welding head. The scanning laser welding head can have a deflection unit for deflecting the laser beam on the workpiece. The deflection unit can have a scanning optics, a scanning system, a scanning mirror and/or a Galvano-Scanner. In solid-state optical laser welding heads, the laser beam can be moved relative to the workpiece by the movement of the laser welding head itself, or the workpiece can be moved relative to the laser welding head.

The sensor module comprises at least one detector for detecting the reflected laser radiation. The detector may be arranged to detect intensity in a particular wavelength range. Furthermore, the detector may be arranged for outputting a measurement signal based on the detection. The detector may comprise at least one photodiode and/or photodiode array and/or a camera, for example a CMOS or CCD based camera. The detector may be sensitive only at a specific wavelength or in a specific wavelength range. For example, the detector may be sensitive in the wavelength range of the measuring beam or in the laser emission wavelength range of the laser processing device. According to one embodiment, the sensor module comprises a detector with at least one photodiode, which is sensitive in the wavelength range of the measuring beam or in the laser emission wavelength range in order to detect retro-reflected measuring radiation. The method according to the invention can therefore be carried out with the aid of a laser processing system. In particular, the measurement signal can be sensed by the sensor module.

Depending on the embodiment, the sensor module or detector may be coupled to the laser processing apparatus. The laser processing apparatus may have a beam splitter for decoupling the reflected radiation from the beam path of the laser beam. The laser machining device may comprise an optical output for decoupling radiation, and the sensor module may comprise an optical input for coupling radiation decoupled by the laser machining device. The radiation may comprise or may be the portion of the measuring beam reflected from the weld. The sensor module or the detector or the photodiode can be arranged along or coaxial with the optical axis of the laser processing device or the propagation direction of the laser beam. In other words, the beam path of the reflected measuring radiation between the workpiece and the detector extends at least partially within the laser processing device and/or is coaxial with the beam path of the laser beam. Alternatively, the sensor module or the detector is arranged such that the beam propagation direction between the workpiece and the detector or the beam path of the reflected measuring radiation extends completely outside the laser processing device.

Depending on the embodiment, the sensor module may alternatively or additionally have at least one detector in the laser processing device, i.e. in the interior or optical space of the laser processing device. For this purpose, a beam splitter or a scanning mirror can deflect the reflected measuring radiation onto the detector. Alternatively, the beam path of the machining laser beam may be angled, while the beam path for the reflected measuring radiation may be rectilinear.

According to a further embodiment, the laser processing system may comprise a laser source for generating a laser beam having the first and/or second laser power and/or a measuring beam source for generating a measuring beam. According to further embodiments, the sensor module may alternatively or additionally comprise at least one detector in the laser source. In other words, the sensor module may have a detector inside the laser. In this case, the reflected measuring radiation can be coupled into the optical fiber by means of optical elements of the laser processing device for guiding the laser beam generated by the laser source and acting on the detector in the laser source.

Furthermore, the laser machining system may comprise a control unit. The control unit may be arranged for receiving the analogue measurement signal from the at least one detector. The control unit may be provided for carrying out a method according to one of the embodiments of the invention in order to analyze a laser welding process or a weld seam. The control unit can also be provided for regulating or controlling the laser machining system, in particular the laser machining device, as described above, on the basis of the analysis result, in particular the presence or absence of a welding defect.

The measuring beam can have any wavelength, in particular a wavelength in the infrared range or a wavelength in the visible green or blue range. The laser processing system may include a laser source for a laser beam (also referred to as a processing laser beam) used to process the workpiece. The laser source may be arranged for generating a measuring beam. In this case, the measuring beam may be a laser beam having a lower power than the laser beam used for the material processing. The measuring beam may be a pilot laser beam. In this case, the laser processing system may include a pilot laser source, for example, for generating a pilot laser beam having a wavelength of about 630nm or about 530 nm. Alternatively or additionally, the laser machining system may comprise an LED source for generating the measuring beam. The measuring beam or LED light generated by the LED source can be coupled into the beam path of the processing laser or into the laser processing device, for example by means of a beam splitter. Preferably, the measuring beam is coupled into the beam path of the laser processing device in the direction of propagation of the measuring beam before the deflection unit.

Drawings

Embodiments of the present invention are described in detail below with reference to the accompanying drawings.

Fig. 1A shows a schematic illustration of a laser processing system for processing a workpiece by means of a laser beam for carrying out a method for analyzing a laser processing process or for analyzing a weld seam produced by the laser processing process according to an embodiment of the invention;

fig. 1B shows a schematic illustration of a laser processing system for processing a workpiece by means of a laser beam for carrying out a method for analyzing a laser processing operation or for analyzing a weld seam produced by the laser processing operation according to a further embodiment of the invention;

FIG. 2 shows a flow diagram of a method according to an embodiment of the invention;

3A-3C illustrate measurement paths of a method according to an embodiment of the invention;

4A-4D illustrate a weld being analyzed by a method for analyzing a laser machining process according to an embodiment of the present invention;

5A-5D illustrate exemplary plots of measurement signals sensed by a method according to an embodiment for the weld shown in FIGS. 4A-4D;

FIG. 6 illustrates exemplary integrated values for a plurality of measurements including good welds and bad welds, in accordance with an embodiment of the present invention;

FIG. 7 shows a weld seam analyzed by a method for analyzing a laser machining process according to an embodiment of the invention;

FIG. 8 illustrates an exemplary plot of a measurement signal sensed by a method according to an embodiment for the weld shown in FIG. 7; and

fig. 9 illustrates an exemplary integrated value for the measurement signal illustrated in fig. 8 according to an embodiment of the present invention.

In the following, elements having the same or similar functions are provided with the same reference numerals, unless otherwise specified.

Detailed Description

Fig. 1 shows a schematic illustration of a laser processing system for processing a workpiece by means of a laser beam for carrying out a method for evaluating a laser processing process, in particular for analyzing or evaluating a weld seam produced by the laser processing process, according to an embodiment of the invention. In this detailed description, an embodiment is described in which the measuring beam is a laser beam. The measuring beam can be derived from a laser source for generating the machining laser beam or from a pilot laser source for generating the pilot laser beam. However, the present disclosure is not limited thereto. The measuring beam can originate without problems from an LED source or LED light in the path of a machining laser beam coupled into the laser machining device 10.

The laser processing system 1 comprises a laser processing device 10, a sensor module 20 and a control unit 40.

The laser machining device 10, which may be embodied, for example, as a laser machining head, in particular as a laser welding head, is provided for focusing or focusing a laser beam (not shown) emerging from a laser source or one end of a laser fiber onto a

workpiece

30a,30b to be machined by means of beam guidance and focusing optics (not shown) in order to carry out a machining or machining operation therefrom. The laser processing device 10 is therefore provided for providing a processing beam for laser processing a workpiece. The processing may include laser welding, among others. Furthermore, the laser machining device 10 is provided for aligning the measuring beam with the site to be machined. When the measuring beam impinges on the

workpieces

30a,30b, a portion of the measuring beam is reflected onto the workpiece surfaces of the

workpieces

30a,30 b. The reflected measuring radiation 11 enters the laser processing device 10 and is decoupled there from the beam path of the laser beam by the beam splitter 12. The reflected measuring radiation 11 is guided into the sensor module 20 and there acts on a detector (not shown). The detector is arranged for detecting or measuring the intensity of the reflected measuring radiation 11 and for generating or outputting a measuring signal on the basis thereof. The detector may comprise a photodiode or an array of pixels. The photodiode has a spectral sensitivity in the wavelength range of the irradiated measuring beam or of the reflected measuring radiation 11.

The control unit 40 is connected to the sensor module 20 and receives the measurement signals of the detector. The control unit 40 may be arranged for recording the measurement signal. The control unit 40 is provided for determining and/or evaluating the machining result of the laser machining, and in particular for evaluating the laser welding process or the result of the laser welding process, in particular the weld seam. Furthermore, the control unit 40 may be arranged for controlling the laser machining apparatus 10 on the basis of the analysis result.

According to an embodiment of the invention, the laser machining system 1 can be provided for carrying out and/or regulating a laser machining process, in particular a laser welding process, and for carrying out a method for analyzing the laser machining process, in particular for analyzing or evaluating a weld seam produced by the laser machining process.

In the embodiment of the laser processing system 1 shown in fig. 1A, the beam path of the reflected measuring radiation 11 between the

workpieces

30a,30b and the detector is at least partially located within the laser processing device 10 and/or is coaxial with the beam path of the laser beam.

FIG. 1B shows a schematic view of a laser processing system according to further embodiments of the present invention. The embodiment shown in fig. 1B is similar to the embodiment of the laser processing system shown in fig. 1A, and therefore only the differences are described below. As shown in fig. 1B, the sensor module 20 or the detector is arranged such that the beam path of the reflected measuring radiation 11 between the workpieces 30a,30B and the detector is located completely outside the laser processing device 10. In this case, the beam splitter 12 shown in fig. 1A and the interface between the sensor module 20 and the laser processing apparatus 10 are not necessary.

Fig. 2 shows a flow chart of a method for analyzing a laser machining process, in particular for analyzing or evaluating a weld seam produced by a laser machining process, according to an embodiment of the invention.

The method begins by forming a weld by irradiating a laser beam having a first laser power onto at least one of two or more workpieces to be welded (step S1). Subsequently, the measuring beam is irradiated onto the weld bead formed in step S1 (step S2). For example, the measuring beam may be a laser beam having a second laser power lower than the first laser power. Here, the second laser power is lower than the first laser power to prevent coupling into the material. The measurement signal for the portion of the measurement beam reflected from the weld is sensed (step S3). As a final step, it is determined whether there is a welding defect based on the sensed measurement signal (step S4). In other words, it may be determined whether there is electrical or mechanical contact between the workpieces, or whether there is a gap between the workpieces.

According to the invention, the presence of a weld defect can be determined by measuring or detecting and evaluating the measurement radiation reflected from the weld seam. The analytical evaluation may be based on the strength or signal level of the sensed measurement signal and/or on the degree to which the measurement signal changes with respect to a time base (also referred to as "noise", "noise portion" or "noise signal" or "variance" of the measurement signal).

The weld defect may include at least one of: missing welded connections between workpieces, gaps between workpieces, and "fake friends". A solder connection may produce or represent a contact, in particular an electrical contact, between the workpieces. In one example, a weld or bead without a gap (i.e., zero gap) and a weld or bead with a gap may be identified and differentiated. In another example, welds or welds with gaps and gap bridges and welds or welds with gaps but without gap bridges ("fake friends") may be identified and distinguished.

For this purpose, for example, after the actual laser welding process in step S1, in particular after cooling and/or solidification of the weld seam, the weld seam is passed over again with the laser beam, i.e. the laser beam is again directed or irradiated onto the weld seam (step S2), in this example the laser beam being used as a measuring beam. However, the re-irradiated laser beam has a lower laser power than in the actual laser welding process, so that the laser beam or laser power is coupled into the workpiece or weld seam as little as possible or even not at all. The tracking of the weld by the laser beam having the second laser power can be performed at the same or different feed rate as the laser beam having the first laser power is irradiated to form the weld. The intensity of the laser radiation reflected by the weld seam is detected and a corresponding measurement signal is sensed (step S3). According to an embodiment, the second laser power is between 5W and 300W, preferably between 5W and 200W or even between 1mW and 10 mW. The intensity of the reflected measurement or laser radiation is higher in the presence of a welding defect than in the case of a good weld, i.e. in the absence of a welding defect. For example, the surface roughness may be lower in the case of a weld defect than in the case of a weld without a weld defect. Accordingly, in step S4, it may be determined whether there is a welding defect based on the sensed measurement signal.

The determination of whether a weld defect is present may be based on an integration of the measurement signal and/or an integration of a noise portion of the measurement signal in the at least one region. For this purpose, in step S4 for determining whether a welding defect is present, at least one region of the measurement signal or noise portion may be integrated. In this case, it can be determined that a welding defect is present if the integral exceeds a predetermined integral limit value. The "integral" can here denote the area of the measurement signal or noise portion in the region under consideration. Alternatively or additionally, the determination of whether a welding defect is present may be based on at least one of an amplitude or an amplitude average of the measurement signal, an area between the measurement signal and a reference curve or on an area of the measurement signal above a reference value and a defect frequency of the measurement signal in at least one region thereof. "amplitude" may be defined herein as the magnitude or magnitude of the measurement signal overshoot compared to the reference trajectory. The defect frequency can be regarded as a measure for the amount of overshoot of the measurement signal.

According to an embodiment, the measurement signal is based on a measurement or detection of the radiation intensity of the reflected portion or reflected measurement radiation. The measurement or detection of the radiation intensity of the reflected portion can be carried out by means of a photodiode. The photodiode may have a spectral sensitivity in a wavelength range including the wavelength of the measurement beam. According to an embodiment, the photodiode may have a maximum spectral sensitivity at the wavelength of the measurement beam. For example, the measurement signal can be sensed by a detector of the sensor module 20 in the embodiment shown in fig. 1. However, the present invention is not limited thereto.

The measurement signal output by the photodiode can be a temporally variable voltage signal, in particular an analog voltage signal. According to an embodiment, the measurement signal may be preprocessed. In particular, the measurement signal can be converted into a digital voltage signal, which comprises the voltage values assigned to the respective time points. Furthermore, the measurement signal may be smoothed and/or filtered. The measurement signal can therefore correspond to a time profile of the output voltage of the photodiode.

According to an embodiment, the irradiation of the measuring beam (step S2) can take place along a predefined measuring path 302, as shown in fig. 3A to 3C. According to an embodiment, the irradiation of the measuring beam (step S2) may comprise tracking the weld seam 301 by the measuring beam. In other words, the measuring beam is guided on the weld seam 301 along the weld seam itself. In this example, the measurement path 302 may include an area on the surface of the first workpiece 30a, an area on the weld 301, and an area on the surface of the first workpiece 30a or the second workpiece 30b (refer to fig. 3A and 3C). According to a further embodiment, the irradiation of the measuring beam can also take place at least partially transversely to the weld seam 301. The weld seam 301 can extend, for example, with a measuring beam perpendicularly to the weld seam 301. In this example, the measurement path 302 may include an area on the surface of the first workpiece 30a, an area on the weld 301, and another area on the surface of the first workpiece 30a (refer to fig. 3B). If the irradiation of the measuring beam and/or the sensing of the measuring signal is at least partly along the weld seam 301 or if the weld seam 301 is tracked with the measuring beam, it is possible, for example, to distinguish between areas along the weld seam 301 where there is a weld connection and areas along the weld seam 301 where there is no weld connection (i.e. where there is a weld defect). Thus, according to embodiments it may be determined, based on the measurement signal, whether a weld connection is present at least in the region along the weld seam 301 between the workpieces. If it has been determined that there is a weld connection at least in the region between the workpieces along the weld seam 301, it can be determined that there is a weld connection for the welded workpieces as a whole. Thus, an inhomogeneous weld seam 301 can be analyzed, in which regions with solder connections and regions without solder connections, in particular regions without electrical contact, are present. Furthermore, defects can be localized along the weld seam 301 by means of the method according to the invention. From the parts with and without welded connections, the absolute value of a physical characteristic quantity, such as conductivity or strength, can be estimated.

If the measurement signal is a temporally variable voltage signal with a voltage value associated with the respective time point, as described above, the measurement signal can be converted as a function of the known feed speed and the known weld seam position or the known extension of the measurement path, such that the voltage value is associated with the position of the measurement path corresponding to the respective time point. The measurement signal thus converted can therefore be a locally variable, time-independent voltage signal.

The irradiation of the laser beam with the first laser power for producing the weld seam (step S1) can be carried out along a predefined machining path with respect to the

workpieces

30a,30b, in particular with respect to the surface of one or more workpieces. For example, the processing path may include a zigzag pattern or a zigzag pattern in order to produce a straight weld seam having a predetermined width. According to an embodiment, the irradiation of the measuring beam and/or the sensing of the measuring signal (steps S2, S3) can be performed along the same predefined processing path. In other words, the measurement path may be the same as the machining path.

According to an embodiment, the laser beam irradiated at the first laser power and the measurement beam may have the same wavelength. The laser beam irradiated at the first laser power and the measuring beam may be from or generated by the same laser source. In particular, the same laser processing device 10 in the embodiment shown in fig. 1 can be used for the irradiation of the laser beam with the first laser power and for the irradiation of the measuring beam. If the measuring beam is from a different source than the machining laser beam, the measuring beam can either be coupled into the beam path of the machining laser beam, for example by a beam splitter, or the beam path of the irradiated measuring beam can extend completely outside the laser machining device 10. In the latter case, the measuring beam source may be mounted outside the laser machining apparatus 10.

According to an embodiment of the invention, the method can be used in particular for analyzing welds, in particular I-welds, when laser welding overlapping or parallel-butted workpieces.

The at least one workpiece may comprise a battery, a battery module and/or a battery cell or at least a part thereof, in particular an electrical conductor, a housing, a battery cell cover, a battery cell connector or a terminal. The battery cell may be configured as a pouch-shaped battery cell, a prismatic battery cell, or a cylindrical battery cell, or may include at least one of these battery cells. The laser welding in step S1 may be used for hermetic welding of the battery cell case, welding of the separator of the battery cell cover, welding of the terminal in the battery cell cover, and welding of the rupture disk of the battery cell cover. According to further embodiments, the electrical conductors of two or more cells are welded or otherwise in contact with each other. In these cases, the method according to the invention can be used for analyzing the electrical contact of the weld between the workpieces.

For example, in a pouch-shaped battery cell, the parts to be welded of the battery cell, in particular the electrical conductors, may consist of aluminum Al, and the parts to be welded of another battery cell, in particular the electrical conductors, may comprise copper Cu, wherein the latter may optionally be coated with nickel (layer thickness, for example, 8 μm). The coating may be applied galvanically. Alternatively, the two portions to be welded of the pouch-shaped battery cell may be made of aluminum or copper. The thickness of the portion to be welded may be between 0.2mm and 0.4 mm. Thus, a solder connection may be formed between copper and aluminum (Cu-Al) or aluminum and aluminum (Al-Al) or copper and copper (Cu-Cu) or aluminum and copper (Al-Cu).

In the prismatic battery cell, two portions of the battery cell to be welded may be composed of aluminum. The thickness of the portion to be welded may be between 1mm and 5 mm. In this case, therefore, the welded connection is formed between aluminum and aluminum Al — Al.

In a cylindrical battery cell, the part of the battery cell to be welded, in particular the battery cell housing, may be formed from

Or of aluminium, while the part to be welded of another cell, in particular the conductor/cell connector, may consist of

Aluminum copper, wherein the latter can optionally be coated with nickel (layer thickness, for example, 8 μm). The coating may be applied galvanically. Alternatively, the parts to be welded of the battery cells, in particular the electrical conductors, may be formed from

The parts to be welded of the other cell, in particular the current conductors, can be made of Al aluminum. In both examples, the parts to be welded are preferably welded from copper or aluminium. The thickness of the portion to be welded may be between 0.4mm and 0.5 mm.

Fig. 4A-4D show a weld seam analyzed by a method according to an embodiment of the invention. Fig. 4A-4D show top views of I-shaped welds formed at the overlap during laser welding in the upper row ("camera"), a cross-sectional view of each weld in the middle row, and a schematic view of the cross-sectional view in the lower row, respectively. In a plan view of the individual weld seams, it is not possible to distinguish between a good weld and a poor weld. According to embodiments, a good weld may be defined as a weld or seam without gaps between the welded workpieces, or as a weld with gaps and with gap bridges, while a bad weld may be defined as a weld with gaps and without gap bridges.

FIG. 4A shows a weld without a gap in the first gap ("gap: 0 μm"). The welded

workpieces

30a,30b, shown here as sheets, have no gap between them and current can flow through the weld. Thus, the resulting weld may be referred to as a "good weld". As shown, the bead is relatively rough and convexly shaped.

Fig. 4B shows a weld seam with a gap S between the two workpieces 30a,30B in a second gap ("gap: 100 μm"). This gap S can be tolerated because the gap S is bridged (gap bridge "B" in fig. 4B). Thus, despite the presence of the gap S, there is still a welded connection, i.e. an electrical contact, between the workpieces. Therefore, the welding may also be referred to as "good welding". As shown, the weld bead is relatively rough and concavely shaped.

Fig. 4C-4D show exemplary defect images or machining defects that may be identified by means of a method according to an embodiment of the invention. Fig. 4C and 4D show typical defect images at the third and fourth gaps ("gap: 150 μm" and "gap: 200 μm"), also referred to as "fake friends". Here, there is a gap S between the

workpieces

30a,30b that is not bridged, so that there is no electrical contact between the workpieces, i.e. there is no solder connection. Accordingly, these welds may be referred to as "poor welds". As shown, the weld bead in both cases was relatively smooth and had little or no protrusions or depressions relative to the workpiece surface.

Fig. 5A to 5D show exemplary profiles of the measurement signals, which are sensed along the measurement path in a method for analyzing a laser machining process according to an embodiment. The curves of the measurement signals shown in fig. 5A to 5D respectively sense the weld or weld seam shown in fig. 4A to 4D using a method according to an embodiment of the invention. The measuring path 302 can extend either along the weld seam 301 (see fig. 3A and 3C) or transversely to the weld seam 301 (see fig. 3B), in the example shown including not only the region on the workpiece surface but also the region on the weld seam 301.

The measurement signal may be sensed by detecting the intensity of power reflected when the measurement beam is illuminated. A higher intensity corresponds to a larger measured signal value. Three regions of the measurement signal are shown in fig. 5A-5D, respectively. The first and third regions "workpiece" correspond to the regions of the measuring beam applied to the surface of the

workpieces

30a,30b, in particular the raw surface, i.e. the surface outside the weld seam 301. Since the surface of the

workpieces

30a,30b is smooth compared to the weld seam 301, the intensity of the reflected measuring radiation is high. A second region "weld bead" located between the first region and the third region corresponds to the region in which the measuring beam acts on the weld seam 301 or weld bead. The intensity of the reflected measuring radiation is low compared to the raw surface of the

workpieces

30a,30b, since the surface of the weld seam 301 or weld bead is rough. It can be seen in fig. 5A-5D that the average value of the measurement signal in the "weld" region is always lower than the average value of the measurement signal in the "workpiece" region. The measurement signal region of the gray background in fig. 5A to 5D corresponds to an evaluation region in which the measurement signal is evaluated for determining the presence of a weld defect. Reference values or reference curves for the measurement signals are also shown in fig. 5A-5D. According to an embodiment of the invention, the presence of a weld defect is determined if the integral of the measured signal sensed is greater than a predetermined extreme value or the area of the measured signal above the reference value is greater than a predetermined extreme value.

The curves of the sensed measurement signals shown in fig. 5A and 5B correspond to the welds of fig. 4A and 4B. In fig. 5A, the measurement signal is located below the reference value. In fig. 5B, the measurement signal is also mostly located below the reference value, and in the region in which the measurement signal exceeds the reference value, the area enclosed between the integral or reference curve and the measurement signal is less than a predetermined extreme value. Thus, it was determined in the method according to the invention that no weld defects were present. I.e. in this case there is a soldered connection between the workpieces or an electrical contact between the workpieces.

The curves of the sensed measurement signals shown in fig. 5C and 5D correspond to the welds of fig. 4C and 4D. In fig. 5C and 5D, in the region in which the measurement signal exceeds the reference value, the area enclosed between the integral or reference curve and the measurement signal is greater than a predetermined extreme value. In the method according to the invention, therefore, it is determined that a weld defect is present or that no weld connection, in particular an electrical contact, is present between the workpieces.

Therefore, whether there is a good weld or a bad weld can be determined from the integral or the area value. If the integral value is less than a predefined integral limit value, the respective weld can be identified as a good weld. Accordingly, if the integral value is greater than a predefined integral limit value, the respective weld can be identified as a bad weld. This is shown visually in fig. 6 for a plurality of different welds or weld seams as an example. The multiple measurements of the different weld seams and the correspondingly determined integral values are shown visually. As shown in fig. 6, the integral extremum separates good welds from bad welds.

The formation of the integrals explained with reference to fig. 5A-5D may comprise an integration of the measurement signal in the analysis evaluation region, i.e. in the region of the measurement signal comprising at least a part of the weld seam. The integration formation explained with reference to fig. 5A-5D may alternatively comprise integrating in one or more regions of the measurement signal, wherein the measurement signal only exceeds the reference value. These regions of the measurement signal may comprise global or local maxima of the measurement signal, which may also be referred to as "peaks" of the measurement signal. For example, the measurement signal shown in fig. 5C includes five such regions or six such regions on the weld in the analysis evaluation region against a gray background. According to an embodiment, the presence of a welding defect can be determined if the sum of the integrals in at least one region of the measurement signal exceeding the reference value exceeds a predefined extreme value. According to an embodiment, the integral with the largest value among the integrals may also be determined. Subsequently, if the maximum integrated value exceeds a predetermined extreme value, it is determined that a welding defect exists.

Fig. 7 shows microscopic images of a weld seam or weld with different gap sizes, i.e. different distances between the workpieces in the region of the weld seam. A significant difference in the curvature of the weld profile can be seen. "gap" gives the gap size between the welded workpieces. The "three-dimensional view" represents a three-dimensional view of each weld. The "microtome" shows a cross section of the weld and the workpiece. The "two-dimensional view" represents a two-dimensional microscopic top view of each weld. The "bead and measurement signal" represents a schematic cross section of the surface shape of each bead and the corresponding sensed measurement signal value corresponding to the number of arrows. In a plan view of each weld, it is not possible to distinguish between good welding and poor welding.

Fig. 8 shows a detailed graph of the measurement signal sensed for the weld shown in fig. 7. In this case, a plurality of measurement signals are respectively sensed for the weld shown in fig. 7, the curves of which are shown superimposed in fig. 8. The measurement signal may be sensed by detecting the intensity of the reflected laser power when the measurement beam is illuminated. In this case, a higher intensity corresponds to a larger measurement signal value.

The topology of the welded seam or bead without welding defects can be distinguished significantly, in particular when no gaps are present between the welded workpieces, from the topology of the welded seam or bead with welding defects, in particular when gaps are present between the welded workpieces. For example, in fig. 7, in one column of "two-dimensional view" (which shows a microscopic image of each weld), the difference in topology between a weld with zero gap (0 μm) and a weld with a gap of 10 μm is clearly visible. In other words, the surface topology of the weld or bead depends on the gap size between the welded workpieces. In the case of zero clearance, the surface of the weld seam is convexly shaped. As the gap size (5 μm to 10 μm) between the welded workpieces increases, the convex curvature becomes smaller or the weld surface becomes flatter. At a certain gap size, in this case, for example, greater than 20 μm, the surface of the weld seam is concavely shaped, i.e. the weld seam surface has a concave shape.

The inventors have recognized that these very small gap sizes, in particular in the range from 0 μm to 20 μm, or in the range from 0 to 1/10 of the thickness of one of the welded workpieces, in particular the upper workpiece or the workpiece closer to the laser machining apparatus, can indicate that the intensity of the measurement signal depends on the gap size. For example, in the case of a convex curvature of the weld seam surface, a small amount of measuring radiation is reflected back or the measuring radiation is reflected back offset from the direction of incidence of the measuring beam. Therefore, the strength of the measurement signal decreases. As the curvature decreases or the gap size increases up to about 20 μm, more measurement radiation is reflected back due to the smaller weld curvature. Therefore, the strength of the measurement signal increases. The detection of welding defects, in particular the presence and/or size of gaps between the welded workpieces, can thus be carried out on the basis of an evaluation of the measurement signals. The measurement signal allows to unambiguously classify the weld as a good weld or a bad weld. For example, it can be defined that if the gap size is equal to or greater than 5 μm or equal to or greater than 20 μm, there is a welding defect and thus poor welding.

According to an embodiment of the invention, the determination of whether a welding defect is present is based on an integration of the measurement signal in at least one region in the measurement signal. In this case, if the integrated value of the sensed measurement signal is greater than a predetermined extreme value, it may be determined that the welding defect exists. This is shown visually in connection with fig. 9. Fig. 9 shows the number of measurements of the measurement signals sensed for the measurement signals shown in fig. 8 and for the welds 0 μm, 5 μm, 10 μm, 20 μm, 30 μm on the horizontal axis and the corresponding integrated values on the vertical axis. As shown in fig. 8, the integral extremum separates a good weld with zero clearance from a bad weld with a clearance dimension of 5 μm or more.

According to the invention, a method for analyzing a weld seam, in particular for identifying weld defects or distinguishing between good welds and bad welds, and a laser processing system for carrying out the method are provided. After the actual laser welding process has been carried out, a laser beam having a lower laser power than the laser welding process is irradiated onto the weld seam, and the measurement radiation reflected from the weld seam is detected and evaluated. The method according to the invention is independent of the thickness and/or the type of material of the workpiece. The invention is based on the following recognition: due to the different topographies or surface properties of the good-and poor-welded weld seams, conclusions can be drawn about the presence of welding defects on the basis of the reflected measuring radiation. The processing quality and the weld quality of the laser welding process can be evaluated easily and quickly.

Preferred embodiments:

1. a method for analyzing a weld (301) formed by a laser welding process, comprising:

-forming (S1) a weld seam (301) for welding at least two workpieces (30a, 30b) by irradiating a laser beam having a first laser power onto one of the workpieces (30a, 30b);

-irradiating (S2) a laser beam with a second laser power onto the weld seam (301), wherein the second laser power is lower than the first laser power;

-sensing (S3) a portion of the laser beam irradiated at a second laser power reflected from the weld seam (301) and generating a corresponding measurement signal; and

-determining (S4) whether a welded connection is present between the workpieces (30a, 30b) based on the measurement signal.

2. The method of example 1, wherein the intensity of the reflected portion is sensed.

3. Method according to one of the preceding examples, wherein the irradiation (S1) of the laser beam with the first laser power takes place along a predefined machining path with respect to the workpiece (30a, 30b), and wherein the irradiation (S2) of the laser beam with the second laser power takes place along the same measurement path (302) as the machining path.

4. Method according to one of the preceding examples, wherein the irradiation (S2) of the laser beam with the second laser power takes place at one point of the weld seam (301) or along a measurement path (302) which extends along and/or transversely to the weld seam (301).

5. The method according to one of the preceding examples, wherein the laser beam irradiated with the first laser power and/or the laser beam irradiated with the second laser power has a wavelength in the infrared spectral range or in the visible green and/or blue spectral range.

6. The method according to one of the preceding examples, wherein the second laser power is selected such that the laser beam irradiated with the second laser power is substantially completely reflected by the workpiece (30a, 30b).

7. The method according to one of the preceding examples, wherein the second laser power is between 1W and 1KW, preferably between 5W and 300W, particularly preferably between 5W and 200W.

8. Method according to one of the preceding examples, wherein it is determined that a welded connection exists between the workpieces (30a, 30b) if the measurement signal is below a reference value or reference curve for the measurement signal.

9. The method according to one of the preceding examples, wherein determining (S4) whether there is a welded connection between the workpieces (30a, 30b) comprises:

integrating the measurement signals of the welding seam (301), wherein a welded connection is determined to be present between the workpieces (30a, 30b) if the integral falls below a predefined integral limit value; and/or

Averaging the measurement signals of the welding seam (301), wherein a welded connection is determined to be present between the workpieces (30a, 30b) if the average value is below a predefined maximum average value; and/or

The area of the measurement signal of the weld seam (301) above the reference value is determined, and the presence of a welded connection between the workpieces (30a, 30b) is determined if the area is less than a predetermined extreme value.

10. The method according to one of the preceding examples, wherein the solder connection is a contact between the workpieces (30a, 30b), in particular an electrical contact between the workpieces (30a, 30b).

11. The method according to one of the preceding examples, wherein the workpieces (30a, 30b) are arranged in a lap joint or parallel butt joint while the laser beams are irradiated at the first laser power.

12. The method according to one of the preceding examples, wherein the material of the at least one workpiece (30a, 30b) has or consists of aluminum, steel, copper, nickel-coated copper and/or nickel.

13. The method of one of the preceding examples, wherein the at least one workpiece has a thickness of between 0.1mm and 5mm, between 11mm and 5mm, between 0.1mm and 1mm, between 0.2mm and 0.4mm, or a thickness of 0.3 mm.

14. The method according to one of the preceding examples, wherein the workpiece (30a, 30b) comprises a current conductor of a battery cell and/or a terminal of a battery cell.

15. A laser machining system (1) for analyzing a weld (301) formed by a laser welding process, comprising

A laser processing head (10) for irradiating a laser beam onto the workpieces (30a, 30b);

a sensor module (20) for sensing reflected laser radiation; and

control means (40) arranged for performing a method according to one of the preceding examples.

Claims

-forming (S1) a weld seam (301) for welding at least two workpieces (30a, 30b) by irradiating a laser beam with a first laser power onto one of the workpieces (30a, 30b);

-irradiating (S2) a measuring beam onto the formed weld seam (301);

-sensing (S3) the portion of the measurement beam reflected from the weld seam (301) by means of at least one photodiode and generating a corresponding measurement signal; and

-determining (S4) whether a welding defect is present based on the measurement signal.

2. The method of claim 1, wherein the weld defect comprises at least one of: a missing welded connection between the workpieces (30a, 30b), and a gap between the workpieces (30a, 30b).

3. The method according to claim 1 or 2, wherein the measuring beam is a laser beam, a pilot laser beam or an LED light.

4. The method according to any of the preceding claims, wherein the irradiation (S1) of the laser beam is performed along a predefined machining path with respect to the workpiece (30a, 30b), and wherein the irradiation (S2) of the measurement beam is performed along the same measurement path (302) as the machining path.

5. The method according to any of the preceding claims, wherein the measuring beam is irradiated (S2) onto a cooled and/or solidified region of the weld seam (301).

6. Method according to any of the preceding claims, wherein the measuring beam irradiation (S2) is performed onto a point of the weld seam (301) or along a measuring path (302) extending along and/or transverse to the weld seam (301).

7. Method according to any one of the preceding claims, wherein the laser beam and/or the measuring beam have a wavelength in the infrared spectral range, in particular in the range between 1030nm and 1070m, or in the visible green spectral range, in particular in the range between 500nm and 570nm, preferably 515nm, and/or in the blue spectral range, in particular in the range between 400nm and 500nm or in the range between 440nm and 460nm, preferably 450 nm.

8. The method according to any of the preceding claims, wherein the measuring beam is a laser beam having a second laser power, the second laser power being smaller than the first laser power.

9. The method according to claim 8, wherein the second laser power is between 1mW and 10mW or between 1W and 1kW, preferably between 5W and 300W, particularly preferably between 5W and 200W.

10. Method according to any of the preceding claims, wherein the presence of a welding defect is determined if the measurement signal or a noise part of the measurement signal is higher than a reference value or reference curve.

11. The method according to any one of the preceding claims, wherein determining (S4) whether a welding defect is present comprises:

integrating the measurement signal or a noise portion of the measurement signal, wherein a weld defect is determined to be present if the integration exceeds a predefined integration limit; and/or

Averaging the measurement signal or a noise component of the measurement signal, wherein a weld defect is determined to be present if the average exceeds a predefined mean limit; and/or

Determining an area of the measurement signal or of a noise part of the measurement signal above a reference value, wherein the presence of a welding defect is determined if the area is greater than a predetermined extreme value.

12. The method according to any of the preceding claims, wherein the workpieces (30a, 30b) are arranged in a lap joint or parallel butt joint while laser beam irradiation is performed.

13. The method according to any of the preceding claims, wherein the material of at least one of the workpieces (30a, 30b) has or consists of aluminium, steel, copper, nickel-coated copper and/or nickel.

14. The method of any of the preceding claims, wherein at least one of the workpieces has a thickness of between 0.05mm and 5mm, a thickness of between 1mm and 5mm, a thickness of between 0.1mm and 1mm, a thickness of between 0.05mm and 1mm, a thickness of between 0.2mm and 0.4mm, or a thickness of 0.3mm or 0.07 mm.

15. The method according to any of the preceding claims, wherein the workpiece (30a, 30b) comprises an electrical conductor of a battery cell and/or a terminal of a battery cell.

16. A laser machining system (1) for analyzing a weld (301) formed by a laser welding process, comprising:

a laser processing head (10) for irradiating a laser beam and a measuring beam onto a workpiece (30a, 30b);

a sensor module (20) having at least one photodiode for sensing the portion of the measuring beam reflected from the weld seam (301); and

control means (40) arranged for controlling the laser processing system (1) so as to perform the method according to any of the preceding claims.