DE102020209692B4 - Method and device for controlling a laser welding process for welding two glass workpieces - Google Patents

Abstract

Method for monitoring a laser welding process for welding two workpieces (2, 3) that are transparent to the laser wavelength, with a melt volume (7) being created in the workpieces (1, 2) by a pulsed laser beam (4) in the area of the interface (5) of the both workpieces (2, 3) is melted in order to produce a weld seam (8), and the intensity of the process radiation (10) emitted by the melting volume (7) being detected and evaluated, characterized in that the detected intensity profile with regard to at least one of the the following characteristics are evaluated: - the intensity of the process radiation (10) integrated via an intensity peak (23, 24), - full width at half maximum (ΔI) of an intensity peak (23, 24) of the process radiation (10), - time interval (Δt) between two intensity peaks (23) the process radiation (10) with an intensity drop (22) defined as vertical, in which the intensity drops by 90% within a maximum of 0.01 ms,- number of between two intensity peaks (23) of the process radiation (10) with a vertical drop in intensity (22) located intensity peaks (24) of the process radiation (10) without a vertical drop in intensity, and time interval (δt) between the beginning (t0) of an intensity increase and a vertical drop in intensity (22) or an intensity peak (25) of an intensity peak (23) of the process radiation (10), and in that at least one welding parameter is adjusted or regulated on the basis of the at least one evaluated feature.

Description

The present invention relates to a method and a device for monitoring a laser welding process for welding two workpieces, preferably made of glass, which are transparent to the laser wavelength, a melt volume in the workpieces being melted in the area of the interface between the two workpieces by a pulsed laser beam, in particular an ultra-short pulsed laser beam. to produce a weld seam, and wherein the intensity of the process radiation emitted by the melt volume is detected.

Such a monitoring method is, for example, by the DE 10 2018 128 377 A1 or DE 10 2018 128 368 A1 known.

The quality control of two laser-welded glass workpieces is usually carried out in a work step downstream of the welding process. The welded end workpiece is often subjected to a complex manual test. Whether or not a gap has been bridged is determined using a microscope. The position and the size of the gap have so far been examined under a microscope after the end of the process using cross sections. Strength measurements are also carried out.

From the DE 10 2018 128 377 A1 discloses a method for monitoring a welding process for welding two glass workpieces, a weld seam being formed in the workpieces in a process zone subjected to a pulsed ultra-short pulse laser beam. The intensity of the process radiation emitted by the process zone is detected in a time-resolved manner and the periodicity, the frequency and the frequency spectrum of intensity fluctuations of the detected process radiation are determined in order to draw conclusions about the quality of the weld seam produced. Process parameters, such as power or feed rate of the laser beam, are controlled on the basis of localized cracks, imperfections and defects.

DE 10 2018 128 368 A1 discloses a spatially resolved method for detecting process or laser radiation that is reflected or scattered at defects. On the basis of areas flashing outside the process zone, a specific adjustment of the processing parameters can be made in order to prevent this flashing.

Laser welding processes for transparent materials have so far been carried out in a controlled manner. To improve the quality of the weld seam, the average laser power is modulated and/or beam shaping is used.

In order to correct the tilting of a workpiece and to optimally position the weld seam, multi-stage processes have been used up to now (see e.g. Elke Kaiser, Laser Welding of Glass Replaces Glueing Procedure Glass welding with a femtosecond laser brings economic advantages and new design options, Laser Technik Journal, and Simone Russ, Strong connection: welding of different kinds of glass using femtosecond laser pulses, Lasers in Manufacturing). In the first step, the position of the workpiece is measured along the weld seam contour or using several fixed points. The measurement often takes place optically via the reflection of a measuring beam at the interface or the first glass surface. In a second step, the workpiece is aligned or the focus position is adjusted in a controlled manner. The last step is the welding process.

The object of the present invention is to further develop the monitoring method mentioned at the outset such that welding defects can be detected at an early stage by monitoring the welding process in order to enable countermeasures or regulation.

• - Maximalintensität eines Intensitätspeaks der Prozessstrahlung,
• - die über einen Intensitätspeak integrierte Intensität der Prozessstrahlung,
• - Halbwertsbreite eines Intensitätspeaks der Prozessstrahlung,
• - zeitlicher Abstand zwischen zwei Intensitätspeaks der Prozessstrahlung mit senkrechtem Intensitätsabfall,
• - Anzahl der zwischen zwei Intensitätspeaks der Prozessstrahlung mit senkrechtem Intensitätsabfall befindlichen Intensitätspeaks der Prozessstrahlung ohne senkrechten Intensitätsabfall, und
• - zeitlicher Abstand zwischen dem Beginn eines Intensitätsanstiegs und einem senkrechten Intensitätsabfall oder einer Intensitätsspitze eines Intensitätspeaks der Prozessstrahlung,

und dass mindestens ein Schweißparameter anhand des mindestens einen ausgewerteten Merkmals angepasst oder geregelt wird.In the monitoring method mentioned at the outset, this object is achieved according to the invention in that the detected intensity profile is evaluated with regard to at least one of the following features:

• - maximum intensity of an intensity peak of the process radiation,
• - the intensity of the process radiation integrated over an intensity peak,
• - FWHM of an intensity peak of the process radiation,
• - Time interval between two intensity peaks of the process radiation with a vertical drop in intensity,
• - number of intensity peaks of the process radiation without a vertical drop in intensity located between two intensity peaks of the process radiation with a vertical drop in intensity, and
• - time interval between the beginning of an intensity increase and a vertical decrease in intensity or an intensity peak of an intensity peak of the process radiation,

and that at least one welding parameter is adjusted or regulated based on the at least one evaluated feature.

Preferably, the average laser power of the laser beam is modulated to produce a single melt volume within a modulation period.

According to the invention, the process radiation, which emanates from an interaction area of the laser beam with the workpiece, is used to monitor seam formation and position and to detect welding defects, such as weld seam irregularities, and to enable countermeasures or regulation. The adaptation of the welding parameter or parameters takes place using features in the intensity profile of the emitted process radiation. The distance between two intensity peaks of the process radiation with a vertical drop in intensity can be determined in many ways, but preferably from maximum peak intensity to maximum peak intensity.

Various approaches are being pursued to produce weld seams in glass with ultra-short laser pulses. Classically, a Gaussian beam is focused into the glass volume near the interface and a single melt volume is generated with each modulation sequence. In this case, the absorption is pulled in the direction of the incident wave front, in particular over several pulses, since there is a preferred absorption due to the increased temperature. The longitudinal position of the local energy input thus varies over several pulses due to shielding. Under feed, many melt volumes lined up in a row form a continuous or periodically interrupted weld seam. The resulting weld shows a periodic signature. Modulation of the laser power represents an extended approach to welding. Processing parameters are selected in such a way that a single/independent melt volume is generated within a modulation period.

The interaction zone typically emits radiation with a continuous spectrum, the intensity of which correlates with the volume of the interaction zone. Corresponding to the change in the volume of the interaction zone, radiation is thus emitted for each individual melt volume. With modulated laser power, it has been shown that the course of intensity during the formation of the melt volume is reproducible through the laser power modulation and follows a fixed course. In addition, the course of the intensity is related to any weld seam irregularities that occur. In this way, characteristic features in the signal curve can be used to draw conclusions about changes in the extent of the seam, process interruptions or the position of the gap within a melt volume. Since the changes in intensity affect the entire spectral range in the same way, the monitoring can be broadband over a large wavelength range or wavelength-selective.

• - Regelung der Nahthöhe durch Anpassung des Schmelzvolumenabstandes/mittlere Leistung/Modulationsmusters, z.B. für kleine Radien.
• - Regelung des Schmelzvolumenabstandes bei einem nicht überbrückten Spalt. Dadurch werden für jedes Schmelzvolumen gleichbleibende Bedingungen für Spaltüberbrückung gewährleistet, sowie Schmelzvolumen, welche die Grenzfläche nicht erreichen und damit nicht zum Schweißergebnis beitragen, und Risse, die die Einkopplung und damit den Prozess stören, vermieden.
• - Regelung der Schweißnahtposition, z.B. bei einem verkippten Werkstück. Die Nahtposition wird anhand eines überbrückten oder nicht überbrückbaren Spalts erkannt. Weicht die Position vom Soll ab, wird gegengesteuert.

According to the invention, for example, the following control variants are possible for the welding of glass:

• - Control of the seam height by adjusting the melt volume distance/average power/modulation pattern, e.g. for small radii.
• - Regulation of the melt volume distance in the case of an unbridged gap. This ensures constant conditions for gap bridging for each melt volume, as well as melt volumes that do not reach the interface and therefore do not contribute to the welding result, and cracks that disturb the coupling and thus the process are avoided.
• - Regulation of the welding seam position, eg with a tilted workpiece. The seam position is detected based on a bridged or non-bridgeable gap. If the position deviates from the target, countermeasures are taken.

The at least one welding parameter is preferably the average laser power or the feed rate of the laser beam. The average laser power of the laser beam can be adjusted or regulated, for example, by changing the maximum pulse energy or by changing the distance between two modulation sequences of the laser beam.

Advantageously, the at least one welding parameter is only adjusted or regulated when the at least one welding parameter falls below or exceeds a predetermined threshold value. If the welding parameter is adjusted or regulated based on the distance between two intensity peaks of the process radiation with a vertical drop in intensity, the welding parameters are advantageously only adjusted or regulated if there is at least one intensity peak of the process radiation without a vertical drop in intensity between two intensity peaks of the process radiation with a vertical drop in intensity. This case relates to the regulation when the process is terminated. The distance between the melt volumes should be minimal or kept minimal. It may be necessary not to wait for a melting volume that does not contribute to the welding result, but to automatically reduce the distance between two intensity peaks of the process radiation with a vertical drop in intensity until no melting volume is generated that does not contribute to the welding result .

• - Maximalintensität eines Intensitätspeaks der Prozessstrahlung,
• - die über einen Intensitätspeak integrierte Intensität der Prozessstrahlung,
• - Halbwertsbreite eines Intensitätspeaks der Prozessstrahlung,
• - zeitlicher Abstand zwischen zwei Intensitätspeaks der Prozessstrahlung mit senkrechtem Intensitätsabfall,
• - Anzahl der zwischen zwei Intensitätspeaks der Prozessstrahlung mit senkrechtem Intensitätsabfall befindlichen Intensitätspeaks der Prozessstrahlung ohne senkrechten Intensitätsabfall, und
• - zeitlicher Abstand zwischen dem Beginn eines Intensitätsanstiegs und einem senkrechten Intensitätsabfall oder einer Intensitätsspitze eines Intensitätspeaks der Prozessstrahlung.

The invention also relates to a device for monitoring a laser welding process for welding two workpieces that are transparent to the laser wavelength, preferably made of glass, by means of a pulsed laser beam with a laser beam generator for generating the laser beam, which melts a melting volume in the workpieces in order to produce a weld seam processing optics, which images the laser beam in the area of the interface between the two workpieces, with a detector for detecting the process radiation emitted by the process zone and with an evaluation unit for evaluating the detected intensity profile, the evaluation unit being programmed to analyze the detected intensity profile with regard to at least one of the evaluate the following characteristics:

• - maximum intensity of an intensity peak of the process radiation,
• - the intensity of the process radiation integrated over an intensity peak,
• - FWHM of an intensity peak of the process radiation,
• - Time interval between two intensity peaks of the process radiation with a vertical drop in intensity,
• - number of intensity peaks of the process radiation without a vertical drop in intensity located between two intensity peaks of the process radiation with a vertical drop in intensity, and
• - Time interval between the start of an intensity increase and a vertical decrease in intensity or an intensity peak of an intensity peak of the process radiation.

The monitoring device according to the invention preferably has a control device for controlling at least one welding parameter of the welding process based on the at least one evaluated feature, a display unit for displaying the evaluated intensity profile and/or a data memory for storing the detected intensity profile. The at least one welding parameter is advantageously the average laser power or the feed rate of the laser beam.

Further advantages and advantageous configurations of the subject matter of the invention can be found in the description, the drawings and the claims. Likewise, the features mentioned above and those listed below can each be used individually or together in any combination. The embodiments shown and described are not to be understood as an exhaustive list, but rather have an exemplary character for the description of the invention.

• 1 eine schematische Darstellung einer Vorrichtung zur Regelung eines Laserschweißprozesses zum Verschweißen zweier Werkstücke aus Glas; und
• 2a, 2b gemessene zeitliche Intensitätsverläufe der emittierten Prozessstrahlung beim Laserschweißen zweier Werkstücke aus Alkali-Aluminosilikat-Glas ohne Regelung (2a) und mit Regelung ( 2b) des zeitlichen Abstands zwischen den Modulationssequenzen des Laserstrahls; und
• 3a, 3b gemessene zeitliche Intensitätsverläufe der emittierten Prozessstrahlung beim Laserschweißen zweier Werkstücke aus Quarzglas bei einem Nullspalt (3a) und mit einem überbrückten Spalt (3b).

Show it:

• 1 a schematic representation of a device for controlling a laser welding process for welding two workpieces made of glass; and
• 2a , 2 B Measured temporal intensity curves of the process radiation emitted during laser welding of two workpieces made of alkali aluminosilicate glass without control ( 2a) and with regulation ( 2 B) the time interval between the modulation sequences of the laser beam; and
• 3a , 3b Measured temporal intensity curves of the process radiation emitted when laser welding two workpieces made of quartz glass with a zero gap ( 3a) and with a bridged gap ( 3b) .

In the 1 The device 1 shown schematically serves to control a laser welding process for welding two superimposed, plate-shaped workpieces 2, 3 made of a material that is transparent to the laser wavelength, e.g. glass, by means of a pulsed laser beam 4. The pulsed laser beam 4 is preferably an ultra-short pulsed laser beam with pulses in the femto - or picosecond range and with frequencies of repetition rates from 100kHz up to several MHz.

The bottom of the in 1 upper workpiece 2 and the upper side of the lower workpiece 3 abut one another and form an interface 5 in which the welding is to be carried out or has been carried out. The laser beam 4 is focused through the upper workpiece 2 into the lower workpiece 3 near the interface 5 by processing optics 6, which in principle can also have beam-shaping elements, in order to produce a melting volume 7 there. The average power of the ultra-short pulse laser is modulated over time, as a result of which a single melting volume 7 is generated within a modulation period.

Due to the very high intensities that can be achieved in the focus F, non-linear absorption effects occur in the glass material. With suitable repetition rates of the pulsed laser beam 4, heat accumulation effects occur in the glass material, which leads to local melting of the glass material. The melting volume 7 is accordingly placed in the workpieces 2 , 3 in such a way that it is arranged close to the interface 5 or encompasses the interface 5 . When the previously melted material solidifies again, the workpieces 2, 3 instead of. The laser beam 4 is moved together with the processing optics 6 in a feed direction X, Y relative to the workpieces 2, 3 at a feed rate v in order to draw in a weld seam 8 in the workpieces 2, 3. Alternatively or additionally, the workpieces 2 , 3 can also be moved relative to the processing optics 6 .

In particular over several pulses, the absorption in the lower workpiece 3 is pulled in the direction of the incident laser beam 4, since preferred absorption occurs on the upper side of the interaction zone/process zone due to the increased temperature. The longitudinal position of the local energy input thus varies over several pulses due to shielding. If the absorption area is shifted too far into the convergent beam, the energy input breaks down and the process starts again, starting from the focus F. Typically this leads to modulated weld seams 8 with a periodic signature. Modulation initiates and terminates the melt volume formation in a targeted manner. The relocation of the absorption into the convergent beam is canceled or slowly ended by reducing or stopping the laser power. With the next modulation sequence, the absorption then starts again in the focus and another melt volume is created. This periodic generation of individual melt volumes then leads to melt volumes lined up one after the other, which can overlap (periodic signature).

If the interface 5 lies within the melted melt volume 7, there is a material connection. If there is a gap 9 between the workpieces 2, 3 to be joined, melt is expelled in pulses into the gap 9 by the prevailing pressure within the process zone. If the gap 9 between the workpieces 2, 3 is completely filled with material, so that the absorption can continue in the upper workpiece 2, a bonded connection occurs. However, if the melt spreads widely in the gap 9, the absorption cannot be maintained. The gap 9 is not bridged and there is no material connection. The position and the size of the gap 9 affect the strength and visibility of the welded joint and also affect the intensity of the process radiation 10 emitted during the process from the interaction zone of the laser beam 3 in the workpiece 2, 3.

The intensity of the emitted process radiation 10 is determined by a detector preferably arranged coaxially to the laser beam 4, here in the form of a photodiode 11. Alternatively, the detector can also be attached next to the processing optics 6 or below the workpieces 2, 3 (“off-axis”). Instead of using a photodiode, the emitted process radiation 10 can also be detected using a fast line sensor or a camera. In addition to the visible wavelength range, the process radiation 10 can also be detected in the near infrared range.

The laser beam 4 is guided to the processing optics 6 with a dichroic mirror 12, which is, for example, highly reflective for the laser wavelength and transmits radiation in the visible wavelength range from 300 to 800 nm, and is focused in the lower workpiece 3 just below the interface 5. Alternatively, the dichroic mirror 12 can also be transmissive for the laser wavelength and reflective for the process radiation 10 . Instead of or in addition to the visible wavelength range, the process radiation 10 can also be observed in the infrared wavelength range. The process radiation 10 emitted by the process zone of the workpieces 2, 3 is captured with the aid of the processing optics 6 and decoupled from the laser beam beam path by the dichroic mirror 12. An optical filter element 13 is used to select a desired wavelength range and/or to suppress reflected laser radiation from the process radiation 10 . With the aid of an evaluation unit 15, the time course of the intensity of the emitted process radiation 10 is evaluated for a change in the radiation intensity for each melt volume produced.

As in 1 shown further, the laser beam 4 is generated in a laser beam generator (ultra-short pulse laser) 16, the mean power of which is preferably modulated over time. The laser beam 4 is guided from the laser beam generator 16 to the processing optics 6 with the aid of a beam guidance system 17 . The workpieces 2, 3 to be joined are fixed with a holding device 18 on an axis system 19 that can be moved in the X, Y direction. The laser beam generator 16 and the axis system 19 are controlled by a machine control 20 . As described above, the process radiation 10 detected by the photodiode 11 is evaluated for each melt volume produced for features that suggest a change in the extent of the seam, a process termination or the position of the gap within a melt volume 7 . The data determined are transferred to a control device 21, which in turn controls the laser beam generator 16 with regard to the laser power and the maximum pulse energy and the axis system 19 with regard to the feed rate of the laser beam. Alternatively or additionally, the laser beam 4 together Processing optics 6 are moved in the X, Y direction.

Complex contours with small radii are often required when welding transparent workpieces 2, 3 with USP lasers. These areas are particularly prone to cracking. Due to the axis inertia, the feed rate v cannot be kept constant. Due to the reduced feed rate v, the distance between the individual melt volumes 7 decreases, resulting in a change in the inner seam structure. In addition, if the energy input remains the same, more heat is accumulated, which changes the expansion of the melted volumes. This is accompanied by an increase in thermal stresses, which can ultimately lead to cracking. The greater expansion of the melted volume results in an increase in the emitted process radiation 10. According to the invention, at least one welding processing parameter (e.g. feed rate v, average laser power of the laser beam 4) is adjusted or .regulated. The adaptation can take place, for example, when the integrated intensity is above a threshold value.

In an advantageous variant, the welding parameter is adjusted or controlled by the control device 21 in such a way that the increase in the extent of the weld seam is counteracted by reducing the average laser power of the laser beam 4 . The average laser power can be reduced by reducing the maximum pulse energy. Alternatively, the extent of the seam can be reduced by increasing the time interval between the modulation sequences of the laser beam 4 . This leads to the distance between the melting volumes 7 increasing.

In a further variant, the internal structure of the weld seam 8 and the extent of the seam are kept constant. The adjustment or regulation of the welding parameters is carried out as a function of the feed speed v of the axes and the evaluated maximum intensity of a melting volume 7 or of the radiation intensity integrated over a melting volume 7 . The adjustment or regulation can take place, for example, if the feed rate v is below a threshold value. The welding parameter is preferably adjusted in such a way that a change in the inner seam structure is counteracted by increasing the time interval between the modulation sequences of the laser beam 4 . This leads to the distance between the melting volumes 7 increasing and at the same time reducing the extent of the seam. A combination of the two variants is conceivable and sensible.

If, when welding transparent workpieces 2, 3 with USP lasers, there is a gap 9 between the workpieces 2, 3 to be joined that cannot be bridged, the process is aborted. A process termination is characterized in that melt is periodically ejected into the gap 9 and the formation of further melt volumes 7 is impaired by solidified melt on the glass surface and cavities. The melt volumes 7 formed between the melt ejections do not reach the interface and therefore do not contribute to the process result and reduce the process efficiency. Melt ejections lead to a sudden termination of the radiation emission. In the case of small melted volumes 7 which do not reach the interface 5, the radiation emission is reduced.

2a , 2 B show exemplary intensity curves I(t) over time of the emitted process radiation 10 during laser welding of two workpieces 2, 3 made of alkali aluminosilicate glass without control ( 2a) and with regulation ( 2 B) the time interval between the modulation sequences of the time-modulated laser beam 4. Intensity peaks of the process radiation 10 with a vertical intensity drop 22 are denoted by 23 and intensity peaks of the process radiation 10 without a vertical intensity drop are denoted by 24.

2a shows the intensity profile of a process termination during welding with a distance Δt between the individual melting volumes 7.

• - Maximalintensität eines Intensitätspeaks 23, 24,
• - die über einen Intensitätspeak 23, 24 integrierte Intensität,
• - Halbwertsbreite ΔI eines Intensitätspeaks 23, 24,
• - zeitlicher Abstand Δt zwischen zwei Intensitätspeaks 23 mit senkrechtem Intensitätsabfall 22,
• - Anzahl der zwischen zwei Intensitätspeaks 23 mit senkrechtem Intensitätsabfall 22 befindlichen Intensitätspeaks 24 ohne senkrechten Intensitätsabfall, und
• - zeitlicher Abstand δt zwischen dem Beginn t₀ eines Intensitätsanstiegs und dem Zeitpunkt t_a eines senkrechten Intensitätsabfalls 22 oder einer unmittelbar vorangehenden Intensitätsspitze 25 eines Intensitätspeaks 23 (3b).

After a melt is ejected, which is characterized by an abrupt or vertical drop 22 in the intensity of the process radiation 10 emitted, smaller signal deflections 24 can be observed. These characterize increasingly larger melting volumes 7. According to the invention, at least one welding parameter is adjusted or regulated depending on one of the following features:

• - maximum intensity of an intensity peak 23, 24,
• - the intensity integrated over an intensity peak 23, 24,
• - FWHM ΔI of an intensity peak 23, 24,
• - Time interval Δt between two intensity peaks 23 with a vertical drop in intensity 22,
• - number of intensity peaks 24 located between two intensity peaks 23 with a vertical drop in intensity 22 without a vertical drop in intensity, and
• - Time interval δt between the start t ₀ of an intensity increase and the time t _a of a vertical intensity drop 22 or an immediately preceding intensity peak 25 of an intensity peak 23 ( 3b) .

The adaptation or regulation can take place, for example, if melt volumes occur between two melt ejections that do not contribute to the welding result, and thus the radiation intensity does not collapse completely for two consecutive melt volumes 7, i.e. if between two intensity peaks 23 with a vertical intensity drop 22 at least one intensity peak 24 without vertical drop in intensity.

In an advantageous variant, at least one welding parameter is adjusted in such a way that each melt ejection is followed by a further melt ejection. Melt volumes in between, which do not contribute to the welding result, can be counteracted by increasing the time interval between the modulation sequences. This means that the distance Δt between the melt volumes 7 increases and the seam formation is no longer influenced by ejected melt at the interface or cavities.

2 B shows the intensity curve during welding with an adapted, larger distance Δt between the modulation sequences. Each melt volume 7 formed leads to the ejection of melt. Due to the increased distance .DELTA.t between the melting volumes 7, the extent of the seam is reduced. This can also be counteracted by adjusting the maximum pulse energy.

When welding transparent workpieces 2, 3 with USP lasers, the position of the weld seam has a major influence on the strength of the welded joints, the ability to bridge a gap 9 and the susceptibility to cracking. However, a tilting of the workpiece 2, 3 can lead to the seam position to the gap 9 varying along the component geometry. If a gap 9 is bridged, this can lead to an emission peak and/or a brief dip in the radiation emission. The position of this feature in the detected intensity curve provides information about the gap position within weld seam 8.

3a , 3b show exemplary intensity curves I(t) over time of the emitted process radiation 10 when laser welding two workpieces 2, 3 made of quartz glass with a zero gap ( 3a) and with a bridged gap ( 3b) . In 3a the interface is not visible; in 3b the position of the gap 9 within the melt volume 7 can be inferred based on the point in time t _a of the intensity drop 22 or an immediately preceding intensity peak 25 . The later the melt is ejected, the higher the gap 9 is. According to the invention, at least one welding parameter is adjusted or regulated as a function of the time t _a of the brief intensity drop 22 or the intensity peak 25 . The adjustment can take place, for example, when the point in time t _a of the short-term drop in intensity 22 lies outside a predefined time window. In an advantageous variant, at least one welding parameter is adjusted in such a way that the change in position of the gap 9 is counteracted by the focal position F being adjusted accordingly.

As in 1 shown, the monitoring device 1 also has a data memory 26 for storing the detected intensity profile and a display unit 27 for displaying the evaluated intensity profile.

Claims (11)

Method for monitoring a laser welding process for welding two workpieces (2, 3) that are transparent to the laser wavelength, with a melt volume (7) being created in the workpieces (1, 2) by a pulsed laser beam (4) in the area of the interface (5) of the both workpieces (2, 3) is melted in order to produce a weld seam (8), and the intensity of the process radiation (10) emitted by the melting volume (7) being detected and evaluated, characterized in that the detected intensity profile with regard to at least one of the the following characteristics are evaluated: - the intensity of the process radiation (10) integrated via an intensity peak (23, 24), - the full width at half maximum (ΔI) of an intensity peak (23, 24) of the process radiation (10), - the time interval (Δt) between two intensity peaks (23) the process radiation (10) with an intensity drop (22) defined as vertical, in which the intensity drops by 90% within a maximum of 0.01 ms, - number between two intensity peaks (23) of the process radiation (10) with a vertical drop in intensity (22) located intensity peaks (24) of the process radiation (10) without a vertical drop in intensity, and - time interval (δt) between the beginning (t ₀ ) of an intensity increase and a vertical drop in intensity (22) or an intensity peak (25) of an intensity peak (23) of the process beam ment (10), and that at least one welding parameter is adjusted or regulated based on the at least one evaluated feature. procedure after claim 1 , characterized in that the average laser power of the laser beam (4) is modulated in order to produce a single melting volume (7) within a modulation period. procedure after claim 1 or 2 , characterized in that the at least one welding parameter is the average laser power or the feed rate (v) of the laser beam (4). procedure after claim 3 , characterized in that the average laser power of the laser beam (4) is adjusted or regulated by changing the maximum pulse energy or a modulation pattern or by changing the distance between two modulation sequences of the laser beam (4). Method according to one of the preceding claims, characterized in that the at least one welding parameter is only adjusted or regulated when the at least one welding parameter falls below or exceeds a predetermined threshold value. Method according to one of the preceding claims, characterized in that the adjustment or regulation of the at least one welding parameter only takes place when at least one intensity peak (24) of the process radiation (10 ) without a vertical drop in intensity. Device (1) for monitoring a laser welding process for welding two workpieces (2, 3) which are transparent to the laser wavelength, by means of a pulsed laser beam (4), with a laser beam generator (16) for generating the laser beam (4) which is deposited in the workpieces (2 , 3) a melting volume (7) melts in order to produce a weld seam (8), with processing optics (6) which images the laser beam (4) in the area of the interface (5) of the two workpieces (2, 3), with a detector (11) for detecting the process radiation (10) emitted by the melting volume (7), and with an evaluation unit (15) for evaluating the detected intensity curve, characterized in that the evaluation unit (15) is programmed to analyze the detected intensity curve with regard to at least evaluate one of the following characteristics: - the intensity of the process radiation (10) integrated via an intensity peak (23, 24), - half-width (ΔI) of an intensity peak (23, 24) of the process radiation (10), - time interval (Δt) between two intensity peaks (23) of the process radiation (10) with an intensity drop (22) defined as vertical, in which the intensity drops by 90% within a maximum of 0.01 ms , - Number of between two intensity peaks (23) of the process radiation (10) with vertical intensity drop (22) located intensity peaks (24) of the process radiation (10) without vertical intensity drop, and - time interval (δt) between the beginning (t ₀ ) of a Intensity increase and a vertical drop in intensity (22) or an intensity peak (25) of an intensity peak (23) of the process radiation (10). device after claim 7 , characterized by control device (21) for controlling at least one welding parameter of the welding process based on the at least one evaluated feature. device after claim 8 , characterized in that the at least one welding parameter is the average laser power or the feed rate (v) of the laser beam (4). Device according to one of Claims 7 until 9 , characterized by a display unit (27) for displaying the detected or evaluated intensity profile. Device according to one of Claims 7 until 10 , characterized by a data memory (26) for storing the detected intensity profile.

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