DE102020209705A1 - Method for monitoring a laser welding process for welding two workpieces with regard to the seam height of a weld seam - Google Patents

Abstract

In a method for monitoring a laser welding process for welding two workpieces (2, 3) that are transparent to the laser wavelength, preferably made of glass, a melting volume (7) is melted in the area of the boundary surface (5) of the two workpieces (2, 3) in order to produce a weld seam (8), and the intensity of the process radiation (10) emitted by the melting volume (7) being detected, according to the invention the detected intensity curve evaluated with regard to intensity fluctuations and local seam height fluctuations of the weld seam (8) are determined on the basis of this evaluation.

Description

The present invention relates to a method for monitoring a laser welding process for welding two workpieces, preferably made of glass, which are transparent to the laser wavelength, wherein a melting volume in the area of the interface between the two workpieces is melted in the workpieces by a pulsed laser beam, in particular an ultrashort pulse laser beam, in order to form a weld seam to generate, 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 known.

Since the seam width changes with the seam height when welding transparent materials, local seam height differences can reduce the strength of a welded joint. In addition, increased stresses can occur in areas where the seam height changes abruptly. Stresses encourage cracking, which weakens the load-bearing capacity of the connection in this area.

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. Defects along the weld seam are mostly identified optically by means of a microscopic examination of the permanent weld seams after the end of the process (top view). Strength measurements are also carried out. The assessment of the overall welding result is based on the experience of a skilled worker.

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.

The object of the present invention is to further develop the monitoring method mentioned at the outset such that local seam height changes within a weld seam can be detected when transparent materials are joined.

In the monitoring method mentioned at the outset, this object is achieved according to the invention in that the detected intensity curve is evaluated with regard to intensity fluctuations and local seam height fluctuations of the weld seam are determined on the basis of this evaluation.

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 melt volume is generated. 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. If the absorption range is shifted too far into the convergent beam, the energy input breaks down and the process starts again, starting from the focus, whereby another melt volume is formed. Typically, this leads to modulated weld seams with periodically lined up, possibly overlapping melt volumes (periodic signature). If the interface lies within the melted volume, a material bond is created.

There are two types of weld formation. Material is melted either continuously or discretely. In a continuous melting process, an interaction zone is created, the longitudinal extent of which roughly corresponds to that of the final seam extent. With feed, the material is melted homogeneously over the entire length of the path and leads to a continuous weld seam. Meanwhile, the interaction zone emits radiation whose intensity correlates with the volume of the interaction zone. If the height of the weld seam correlates with the volume of the interaction zone, conclusions can be drawn about the weld seam height based on the radiation intensity. In the case of discrete melting, many melt volumes lined up in a row form a coherent or periodically interrupted weld seam, with each melt volume being produced separately. The resulting weld seam thus exhibits a periodic signature, with the seam height ideally being determined by the height of the individual melt volumes. In the first case, the final height of the melt volume is determined purely by the maximum extent of the interaction zone. In the second case, the change in extension of a movement of the interaction zone is in Direction of the incident laser radiation is superimposed, whereby the position at which the interaction zone is greatest can vary. In both cases, process radiation is emitted for each individual melt volume, corresponding to the change in the volume of the interaction zone. From a comparison of the emitted radiation of successive melt volumes, it is possible to draw conclusions about existing height differences based on the emitted maximum intensity or the integrated radiation intensity of a melt volume.

In order to detect changes in the seam height during laser welding, changes in the seam height are inferred according to the invention based on the detected intensity of the process radiation emitted. In this way, the quality inspection of welded joints on an industrial scale can be carried out directly during processing, which means that there is no need for a time-consuming manual inspection downstream of the process. Since the intensity fluctuations can be observed equally over the entire emission spectrum, detection is possible over a large wavelength range or wavelength-selectively.

An intensity tolerance range with a lower and an upper intensity limit value is preferably defined for a predetermined seam height of a weld seam to be produced, so that the weld seam produced maintains the predetermined seam height if the detected intensity fluctuations are within the intensity tolerance range and if the detected intensity fluctuations are outside the intensity tolerance range , does not comply with the specified seam height.

Depending on how the seam is formed, changes in seam height are localized based on changes in a continuously monitored radiation intensity or changes in a local maximum intensity or an integrated intensity.

If material is continuously melted during the laser welding process, the analysis can be based on the detected intensity profile. If, on the other hand, material is melted discretely in individual melting volumes during the laser welding process, the evaluation is advantageously carried out using the maximum intensities of the respective melting volumes or using the intensities integrated over a melting volume in each case.

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 Überwachung eines Laserschweißprozesses zum Verschweißen zweier Werkstücke aus Glas;
• 2a, 2b gemessene zeitliche Intensitätsverläufe der emittierten Prozessstrahlung beim Laserschweißen zweier Werkstücke aus Quarzglas in einem Schweißprozess, bei dem Material kontinuierlich aufgeschmolzen wird, für eine Schweißnaht konstanter Nahthöhe ( 2a) und eine Schweißnaht mit Nahthöhenschwankungen (2b); und
• 3a, 3b gemessene zeitliche Intensitätsverläufe der emittierten Prozessstrahlung beim Laserschweißen zweier Werkstücke aus Quarzglas in einem Schweißprozess, bei dem Material diskret aufgeschmolzen wird, für eine Schweißnaht konstanter Nahthöhe (3a) und eine Schweißnaht mit Nahthöhenschwankungen (3b).

Show it:

• 1 a schematic representation of a device for monitoring a laser welding process for welding two workpieces made of glass;
• 2a , 2 B Measured temporal intensity curves of the process radiation emitted during laser welding of two workpieces made of quartz glass in a welding process in which material is continuously melted, for a weld seam of constant seam height ( 2a) and a weld with seam height variations ( 2 B) ; and
• 3a , 3b Measured temporal intensity curves of the process radiation emitted during laser welding of two workpieces made of quartz glass in a welding process in which material is melted discretely, for a weld seam of constant seam height ( 3a) and a weld with seam height variations ( 3b) .

In the 1 The device 1 shown schematically is used to monitor 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. made of glass, by means of a pulsed laser beam 4 Pulses in the femtosecond or picosecond range and with frequencies of repetition rates from 100kHz to several MHz are used.

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 by processing optics 6 through the upper workpiece 2 into the lower workpiece 3 near the interface 5 in order to produce a melting volume 7 there. An ultra-short-pulse laser is preferably used, the mean power of which is modulated over time. The processing parameters are preferably selected in such a way that a single melting volume 7 is produced for each 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 it occurs to heat accumulation effects 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 are welded together in a material-to-material manner. The laser beam 4 is moved together with the processing optics 6 in a feed direction X relative to the workpieces 2, 3 in order to pull in a weld seam 8 in the workpieces 2, 3.

In particular over several pulses, the absorption in the lower workpiece 3 is pulled in the direction of the incident laser beam 4, since preferential absorption occurs at the upper side of the interaction 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. If the interface 5 lies within the melted melt volume 7, there is a material connection.

During the process, a process radiation 10 is emitted from the interaction zone of the laser beam 4 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.

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. Following the filter element 13, the process radiation 10 is focused onto the photodiode 11, e.g. with a lens 14.

A monitoring unit 15 is used to evaluate the temporal intensity curve of the emitted process radiation 10 in order to detect local seam height fluctuations based on the detected process radiation 10 . Changes in the intensity of the emitted radiation process radiation 10 indicate seam height changes. To monitor the seam height, a lower and an upper limit value 11, 12 is used to define a permissible tolerance range within which the intensity of the process radiation 10 may vary. If the detected intensity is less than the lower limit value 11, the permissible height of the weld seam 8 is not reached. If the detected intensity is greater than the upper limit value 12, the permissible height of the weld seam 8 is exceeded.

2a , 2 B show measured intensity curves I(t) over time of the detected process radiation 10 during laser welding of two workpieces 2, 3 made of quartz glass in a welding process in which material is continuously melted. Areas in which the specified seam height is maintained are marked with an "o". Areas in which the seam height deviates from the target are marked with an "x". 2a shows an intensity curve I(t) with a constant intensity within the permissible tolerance range, so that a permissible weld seam 8 with a constant seam height can be concluded. 2 B In contrast, FIG. 1 shows an intensity profile I(t) with intensity changes outside the permissible tolerance range, so that a welded seam 8 with impermissible seam height fluctuations can be inferred.

For weld seams 8 that are produced discretely, the evaluation is based on the individual melt volumes. For this purpose, the maximum intensity or the intensity integrated over a melt volume is used. 3a , 3b show the intensity curve I(t) of the emitted process radiation for a welding process in which material was melted discretely. The measurement signal is made up of a large number of intensity peaks 16, each of the intensity peaks 16 corresponding to a melt volume produced for itself. Changes in seam height are indicated by changes in the maximum intensity of the intensity peaks (curve I'(t)) and in the integrated intensity of the individual melt volumes (curve I"(t)). For a weld seam 8 with constant height are the maximum of the intensity peaks 16 and the intensity integrated over a melt volume constant and are within the permissible tolerance range ( 3a) . 3b In contrast, FIG. 1 shows an intensity profile I′(t), I″(t) with intensity changes outside the permissible tolerance range, ie for a weld seam 8 with impermissible seam height fluctuations.

With the help of the method described, the quality inspection of welded joints on an industrial scale can be carried out directly during processing, which means that there is no need for a time-consuming manual inspection downstream of the process.

QUOTES INCLUDED IN DESCRIPTION

This list of documents cited by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent Literature Cited

• DE 102018128377 A1 [0002, 0005]

Claims (4)

Method for monitoring a laser welding process for welding two workpieces (2, 3) that are transparent to the laser wavelength, preferably made of glass, with a melting volume (7) being produced in the workpieces (1, 2) by a pulsed laser beam (4), in particular an ultra-short pulsed laser beam the boundary surface (5) of the two 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, characterized in that the detected intensity curve with regard to intensity fluctuations, is evaluated and local seam height fluctuations of the weld seam (8) are determined on the basis of this evaluation. procedure after claim 1 , characterized in that for a predetermined seam height of a weld seam (8) to be produced, an intensity tolerance range with a lower and an upper intensity limit value (11, 12) is defined and in that the weld seam (8) produced if the detected intensity fluctuations lie within the intensity tolerance range, maintains the specified seam height and, if the detected intensity fluctuations lie outside the intensity tolerance range, does not maintain the specified seam height. procedure after claim 1 or 2 , characterized in that material is continuously melted during the laser welding process and the evaluation is based on the detected intensity profile. procedure after claim 1 or 2 , characterized in that during the laser welding process material is melted discretely in individual melting volumes and the evaluation is based on the maximum intensities of the respective melting volumes or on the basis of the intensities integrated over a melting volume in each case.

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