WO1992013668A1 - Procede d'exposition de pieces revetues a un faisceau laser et dispositif pour l'execution de ce procede - Google Patents

Procede d'exposition de pieces revetues a un faisceau laser et dispositif pour l'execution de ce procede Download PDF

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Publication number
WO1992013668A1
WO1992013668A1 PCT/DE1991/000105 DE9100105W WO9213668A1 WO 1992013668 A1 WO1992013668 A1 WO 1992013668A1 DE 9100105 W DE9100105 W DE 9100105W WO 9213668 A1 WO9213668 A1 WO 9213668A1
Authority
WO
WIPO (PCT)
Prior art keywords
workpiece
temperature
laser radiation
radiation power
layer
Prior art date
Application number
PCT/DE1991/000105
Other languages
German (de)
English (en)
Inventor
Alexander Drenker
Reinhard Kramer
Konrad Wissenbach
Original Assignee
Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. filed Critical Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Publication of WO1992013668A1 publication Critical patent/WO1992013668A1/fr

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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/02—Positioning or observing the workpiece, e.g. with respect to the point of impact; Aligning, aiming or focusing the laser beam
    • B23K26/03—Observing, e.g. monitoring, the workpiece
    • B23K26/034—Observing the temperature of the workpiece
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K26/00—Working by laser beam, e.g. welding, cutting or boring
    • B23K26/70—Auxiliary operations or equipment
    • B23K26/702—Auxiliary equipment
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/06—Surface hardening
    • C21D1/09—Surface hardening by direct application of electrical or wave energy; by particle radiation

Definitions

  • the invention relates to a method
  • the surface temperature of the irradiated machining region in order to record the effect of the laser radiation on the workpiece.
  • the surface temperature e.g. can be measured without contact with a pyrometer.
  • the absorption of the radiation is often not large enough to ensure sufficient energy coupling.
  • the conversion is an example hardened with laser radiation. It is known here to apply a radiation-absorbing coating to the workpiece surface, with which the energy coupling is improved. As a result of the coating, however, it is no longer possible to directly measure the temperature of the workpiece or workpiece circumference covered by the layer.
  • the conventional measurement of the surface temperature with a pyrometer only gives the temperature at the surface of the layer applied to the workpiece.
  • the invention is therefore based on the object of improving a method of the type mentioned at the outset so that the workpiece temperature in the irradiated region can also be determined by coating the workpiece surface.
  • This object is achieved in that the laser radiation is power-modulated with a low degree of modulation with respect to its radiation power, and in that to determine the temperature of that covered by at least one layer
  • Workpiece circumference is subtracted from the measured surface temperature of the layer, a temperature value which is calculated from the product of the laser radiation power absorbed by the workpiece and the thermal resistance of the heat flow through the layer into the workpiece determined using the heat replacement circuit diagram of the respective workpiece.
  • power-modulated laser radiation can be used to approximately calculate a temperature value to be subtracted from the measured surface temperature. Because every brief change in the laser radiation power leads to a change in the surface temperature of the coating of the workpiece, but not to a change in the temperature of the workpiece to be determined, because this has a large heat capacity compared to the coating, which allows only a slow change in the workpiece temperature. Under this condition, the correlation coefficient, according to which every short-term change in the laser radiation power is correlated with the change in the surface temperature of the coating, can be equated with the thermal resistance of the heat flow through the coating become.
  • the temperature value to be subtracted from the measured surface temperature of the layer can be determined by calculating it using the thermal resistance of the layer and the absorbed laser radiation power.
  • the latter can be determined by measurement and the thermal resistance results from the heat equivalent circuit diagram of the respective workpiece.
  • the calculation of the temperature value to be subtracted is essentially based on a correlation coefficient or thermal resistance, which can be determined by the heat equivalent circuit diagram corresponding to the respective workpiece.
  • the heat equivalent circuit diagram can be evaluated analog and / or digitally for the calculation.
  • the power modulation to be selected for the determination of the temperature value to be subtracted is to be matched in terms of its amplitude and its frequency spectrum to the coating and the design of the workpiece. It is therefore understood that the measured surface temperature of the layer and also the measurement of the absorbed laser radiation power have to be time-resolved. As a result, the calculations required to calculate the temperature value to be subtracted from the measured surface temperature of the layer must also be carried out accordingly online, which applies both to the analog arithmetic circuits and to any digital arithmetic circuits.
  • the heat equivalent circuit diagrams used to calculate the thermal resistance essentially depend on the material properties and the geometry of the workpiece, e.g. of heat capacity, thermal conductivity, density and thickness. However, machining parameters are also included, such as feed speed and beam radius, namely via the thermal time constant of the irradiated workpiece area.
  • the resulting heat substitute circuit diagrams can in any case be determined using conventional calculation methods for which there are numerous known theories.
  • the method according to the invention is advantageously used when machining workpieces, for example when Hardening. However, it can also be used as a measuring method, for example to determine the thickness of a coated workpiece.
  • the heat equivalent circuit diagrams to be used are comparatively simple in nature. Accordingly, simplifications result in particular when calculating the temperature value to be subtracted from the measured surface temperature of the layer.
  • a temperature value is subtracted from the measured surface temperature of the layer, which is determined from the direct component of the laser radiation power by multiplication with an average quotient of alternating components of the surface temperature and alternating components of the laser radiation power.
  • the three last-mentioned variables can be determined by measurement and their taking into account when calculating the temperature value to be subtracted from the surface temperature is therefore correspondingly simple.
  • the averaging of the alternating components or the quotient of the alternating components can be carried out over a predetermined period of time which is large enough so that the high-frequency fluctuations or modulations of the laser power and thus the surface temperature of the irradiated area do not lead to excessively fluctuating values of the workpiece temperature.
  • the time for the aforementioned averaging must also not be too long so that measurements can be carried out sufficiently quickly, for example in order to use the measurement result to influence process parameters during processing, for example the laser radiation power.
  • the modulations of the laser radiation power required in the method have a low degree of modulation of, for example, a few percent.
  • the frequencies required for this modulation depend on the workpiece and, for example, on the type of processing. For hardening, for example, 10 to 200 Hz are mentioned. It is therefore advantageous to carry out the method in such a way that the laser radiation power is externally modulated in a frequency range adapted to the requirements during machining or measurement and / or that the natural fluctuations in the radiation power caused by the laser process are used as modulation signals.
  • the use of natural fluctuations in laser power as modulation signals can be considered in particular if no special requirements are imposed on the frequency range.
  • the parameters of the heat replacement circuit diagram are adapted accordingly and / or the thermal time constant of the one covered by the layer
  • the workpiece in the area of the workpiece circumference is taken into account by means of the parameters of the heat replacement circuit diagram.
  • a single heat replacement circuit when carrying out the method. This is the case, for example, when welding a single spot
  • a device for carrying out the method in which the temperature determination on a workpiece is carried out using a heat substitute circuit diagram modeled only by its heat capacity, is characterized in that a
  • the first measuring device is able to detect the surface temperature as a function of time, that a second measuring device is available, which detects the direct component of the laser radiation power and a third, alternating component of the laser radiation power, and that the temperature of the workpiece circumference covered by a layer can be determined with an evaluator according to the relationship , in which
  • thermopile and, for example, a pyroelectric detector, such as are used, for example, in German patent application P 38 20 619, are used as second and third measuring devices. 6 are described.
  • any device that calculates the above-mentioned relationship is suitable as an evaluator.
  • This device is advantageously designed such that the evaluator connected to the measuring devices is an electronic analog arithmetic circuit or a digitized measured value for calculating the temperature of the workpiece utilizing computer.
  • Such evaluators can easily process the electrical signals made available by the measuring devices in electrical circuits with low time constants.
  • FIG. 1 shows a schematic representation of a device according to the invention
  • FIG. 1a the detail A of FIG. 1, 2 shows a heat equivalent circuit diagram for FIG. 1a
  • FIG. 7 an analog arithmetic circuit for calculation according to the heat equivalent circuit diagram of FIG. 4.
  • the workpiece 10 shown schematically in FIG. 1 is to be subjected to transformation hardening.
  • a laser beam 18 is used, which is generated, for example, by a carbon dioxide laser.
  • the laser beam 18 is focused by a focusing lens 19 on the machining area 20, in which the workpiece 10 is covered by a coating 12.
  • the laser beam 18 is guided by the laser 24 via a splitter plate 21 onto the focusing optics 19.
  • the divider plate 21 is partially transparent to the laser radiation and allows approx. 1% of the laser radiation power which is available for measuring devices 15, 16 to pass through.
  • the corresponding partial beam 18 ' reaches a further splitter plate 22, which fades out 50% of the power of the partial beam 18' to a second measuring device 15 and transmits the other 50%, which reach a third measuring device 16 via a mirror 23.
  • a first measuring device 14 is present, which monitors the machining area 20 of the workpiece 10.
  • This measuring device 14 is, for example, a pyrometer, with which the surface temperature v 0 on the surface of the coating 12 is measured in exact dependence on time, that is to say it is time-resolved.
  • the second measuring device 15 is, for example, a thermopile with which the laser radiation power is measured, specifically its mean value or direct component P Lg including the low-frequency fluctuations.
  • the measuring device 16 is, for example, a pyroelectric detector with which the higher-frequency fluctuations of the laser radiation power are measured, that is to say the alternating component P Lw (t) of the laser radiation power P L.
  • the radiation fraction absorbed by the workpiece 10 is measured or taken into account.
  • a two-color pyrometer not shown, which is used in a known manner.
  • All measuring devices 14, 15, 16 are connected to an evaluator 17, which determines the temperature v 2 of the workpiece circumference 13. This determination is described below.
  • the temperature v 2 (t) determined by the evaluator 17 is passed on to a controller 25, which evaluates it as an actual value in comparison with a setpoint value and generates a manipulated variable u stell to influence the laser 24.
  • the controller 25 is, for example, a PID controller that controls the laser power in accordance with the setpoint.
  • the manipulated variable is modulated ustell with a modulation signal Urnod.
  • This modulation variable u mod is generated with the signal generator 26.
  • the modulation is carried out by an adder 27 whose output variable u safeguardmod controls the laser radiation power P L (t) of the laser 24 accordingly.
  • Such external modulation of the laser radiation power is required if the natural fluctuations in the radiation power caused by the laser process are insufficient or unsuitable as modulation signals because, for example, they are not in the frequency range required for processing.
  • the surface temperature v 0 is greater than the sought workpiece temperature v 2 because a temperature drop takes place within the absorption layer and when the heat transfers from the absorption layer to the workpiece.
  • This drop in temperature up to the heat-affected workpiece circumference 13 covered by the layer 12 can be determined, for example, using the equivalent circuit diagram 2.
  • 2 schematically shows the processing area 20 in which the laser beam 18 acts with the power P L. At this point the surface temperature v 0 prevails.
  • the coating or layer 12 of the workpiece 10 opposes the heat flow passing through it, a resistance which is essentially determined by the thermal resistance R th01 of the layer 12.
  • the temperature V 1 prevails in the middle of the layer 12 and the layer 12 has the heat capacity C w1 .
  • the thermal resistance from the layer 12 into the heated workpiece circumference 13 is characterized by the thermal resistance R th01 .
  • the temperature v 2 prevails in the workpiece circumference 13.
  • the heat flow flows from the workpiece circumference 13 into the non-machined environment formed by the workpiece 10 in accordance with the thermal resistance R th2u .
  • the workpiece 10 has a heat capacity C w2 .
  • the heat equivalent circuit diagram described above can be simplified if, for example, it is assumed that the heat capacity C w2 is considerably greater than the heat capacity C w1 of the layer 12. In the following, the assumption is made that the heat capacity of the layer 12 can be neglected. The following therefore applies:
  • the temperature # 2 (t) remains unaffected due to the large heat capacity C w2 by the modulations of the radiation power P L (t).
  • there is a temperature influence in the course of the surface temperature v 0 (t), which is associated with the short-term i changes in radiation power are directly correlated.
  • FIG. 3 shows a simplified heat circuit diagram, according to which the workpiece acts as a heat sink with a constant temperature. Therefore, the temperature v 0 (t) on the surface of the layer 12 is directly proportional to the absorbed radiation power P L (t). The time course of the modulation signal or the radiation power can be arbitrary. In this case too, the temperature v 2 (t) remains unaffected by the modulation.
  • FIG. 3 is a simplified model compared to FIG. 4, it is assumed according to the equivalent circuit diagram of FIG. 5 that a thermal resistance from the workpiece circumference 13 into the surroundings of the workpiece 10 must be taken into account. Accordingly, at constant absorbed radiation power P L (t), temperatures v 2 (t) and v 0 (t) adjust to their stationary end values with corresponding thermal time constants.
  • the equivalent circuit diagram could, for example, be used for an approximately 100 mm thick workpiece.
  • FIG. 6 shows an example of a heat replacement circuit diagram for multi-layer workpieces, be it that the workpiece is coated several times and / or that it itself consists of several layers, so that corresponding resistance values R th12 to R thn-1, n have to be taken into account, possibly not shown heat capacities between the capacities C w1 and C wn .
  • Analog and / or digital arithmetic circuits within the evaluator 17 are used for the calculation.
  • Such arithmetic circuits are in any case largely known for the above-described heat equivalent circuit diagrams, so that only the heat equivalent circuit diagram according to FIG. 4 is used here for explanation.
  • the sought temperature v 2 must be determined from the three variables P L , v 0 and R th02 .
  • the heat flow flowing through the layer 12 is assumed, for which for a layer with a constant thickness and a certain area, the generally known relationship applies:
  • c is a constant taking into account the area and the thickness of the layer
  • v a , v b are the temperatures on both sides of the layer
  • R th is the thermal resistance. It is assumed that the coating 12 constantly absorbs the irradiated laser power on its surface, so that there is a constant heat flow through the layer 12. The above relationship then results for the equivalent circuit according to FIG. 4
  • the laser power basically has time-dependent components or is power-modulated and consequently the temperatures v 0 and v 2 generated as a result are also time-dependent.
  • the following relationships apply to the quantities P L (t), v 0 (t) and 0 2 (t):
  • P Lg + P Lw (t) (v 0g - v 2g ) / R th02 + (v 0w (t) - v 2w (t)) / R th02 .
  • the temperature v 2 can be determined from the quantities of the direct component v 0g of the surface temperature v 0 and the direct component P Lg of the laser power P L to be measured, namely by forming the difference if the direct component P Lg is given by the quotient is divided from the alternating component P Lw (t) and the alternating component v 0w (t).
  • the evaluator 17 can determine the direct component v 0g and the alternating component vow (t) of the surface temperature vo on the basis of the measurement result of the measuring device 14 for the respective measuring period or measuring time, while the direct component P Lg and the alternating component P Lw (t) of the laser radiation power P L the measuring devices 15, 16 are determined.
  • 7 shows an example of an analog arithmetic circuit of the evaluator 17 for determining v 2 .
  • the pyrometer 14 generates a measurement voltage value u vo (t). This
  • the voltage u vo (t) is given to a high-pass filter 29, so that the alternating component u vow (t) comes to an input of an adder 31, the other input of which is used for band conversion for the purpose of selective frequency analysis with a variable influenced by the function cos2 ⁇ f o ⁇ t is applied.
  • the output variable of the multiplier 31 passes through a low-pass filter 32 and an absolute value generator 33, for example a rectifier, in the form of to an adder 34, which is an integrator
  • the integrator 35 supplies the required resistance value R tho2 due to its arrangement in a follow-up circuit shown in FIG. 7 with a multiplier 36 which supplies the adder 34 with a negative voltage value
  • This value results from the multiplier 36 in that the voltage value u L (t), analogous to the voltage value u ⁇ o (t), a cooking pass 37, a bandum Setter 38, a low-pass filter 39 and an absolute value generator 40 is supplied, the absolute value is fed to the multiplier 36 and from this the value I û vow I is determined using a signal corresponding to the resistance R th02 .
  • digital arithmetic circuits can also determine the workpiece temperature using the same principle.
  • the determination of the thermal resistance R th02 can also be determined using the methods for system identification known from control engineering , eg with Fourier analysis, cross-correlation and a minimal square error method for parameter estimation. With known material properties, such as thermal conductivity, density and heat capacity, additional information about the machining process can be obtained with these methods, for example about the depth of hardening.
  • the method according to the invention is used to measure the surface temperature of the workpiece area irradiated with laser radiation

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  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Chemical & Material Sciences (AREA)
  • Mechanical Engineering (AREA)
  • Optics & Photonics (AREA)
  • Plasma & Fusion (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Laser Beam Processing (AREA)

Abstract

Procédé pour l'exposition de pièces revêtues à l'action d'un faisceau laser, dans lequel on mesure la température de surface de la zone (20) exposée au rayonnement laser, en particulier celle d'une zone à usiner. Pour pouvoir déterminer la température de la pièce (2) dans la zone à usiner (20), même lorsque la surface de la pièce comporte un revêtement (12), le procédé est mis en oeuvre de telle manière que le rayonnement laser soit modulé en puissance avec un niveau de modulation faible par rapport à la puissance (PL) du rayonnement et que, pour déterminer la température (2) de la périphérie (13) de la pièce recouverte par au moins une couche (12), on déduise de la température superficielle mesurée (0) de la couche (12) une température (b) qui est calculée sur la base du produit de la puissance (PL) du rayonnement laser absorbée par la pièce (10) et de la résistance thermique (Rth02) du flux de chaleur pénétrant dans la pièce (10) à travers la couche (12), dans un flux thermique calculé au moyen du schéma d'équivalence thermique.
PCT/DE1991/000105 1989-08-11 1991-02-08 Procede d'exposition de pieces revetues a un faisceau laser et dispositif pour l'execution de ce procede WO1992013668A1 (fr)

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
DE3926540A DE3926540A1 (de) 1989-08-11 1989-08-11 Verfahren einer laserbestrahlung beschichteter werkstuecke und vorrichtug zur durchfuehrung des verfahrens

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1994026459A1 (fr) * 1993-05-19 1994-11-24 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. Procede permettant de travailler des materiaux par rayonnement emis par des diodes

Families Citing this family (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE3926540A1 (de) * 1989-08-11 1991-02-14 Fraunhofer Ges Forschung Verfahren einer laserbestrahlung beschichteter werkstuecke und vorrichtug zur durchfuehrung des verfahrens
DE4234339A1 (de) * 1992-10-12 1994-04-14 Manfred Prof Dr Ing Geiger Verfahren zum Laserstrahlschweißen überlappender Bleche und Vorrichtung zu dessen Durchführung
DE19618045C2 (de) * 1996-05-06 1998-03-26 Fraunhofer Ges Forschung Verfahren zum Bearbeiten von Werkstücken mit Laserstrahlung
DE102012221218A1 (de) 2011-11-22 2013-05-23 Leibniz-Institut Für Festkörper- Und Werkstoffforschung Dresden E.V. Vorrichtung zur Qualitätssicherung von mittels Laserstrahlbearbeitung hergestellten Produkten

Citations (3)

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WO1989012806A2 (fr) * 1988-06-17 1989-12-28 Fraunhofer-Gesellschaft Zur Förderung Der Angewand Dispositif pour mesurer la puissance de rayonnement de lasers
EP0309973B1 (fr) * 1987-10-01 1990-05-23 Messer Griesheim Gmbh Procédé de traitement thermique de pièces en phase solide par un faisceau laser
DE3926540A1 (de) * 1989-08-11 1991-02-14 Fraunhofer Ges Forschung Verfahren einer laserbestrahlung beschichteter werkstuecke und vorrichtug zur durchfuehrung des verfahrens

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US4579463A (en) * 1984-05-21 1986-04-01 Therma-Wave Partners Detecting thermal waves to evaluate thermal parameters
DE3424825A1 (de) * 1984-07-06 1986-02-06 Gerd Prof. Dr.-Ing. 6101 Roßdorf Herziger Verfahren und einrichtung zum bearbeiten von werkstuecken mittels laserstrahl
DE3701013A1 (de) * 1987-01-15 1988-07-28 Messerschmitt Boelkow Blohm Verfahren zum mikroloeten

Patent Citations (3)

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Publication number Priority date Publication date Assignee Title
EP0309973B1 (fr) * 1987-10-01 1990-05-23 Messer Griesheim Gmbh Procédé de traitement thermique de pièces en phase solide par un faisceau laser
WO1989012806A2 (fr) * 1988-06-17 1989-12-28 Fraunhofer-Gesellschaft Zur Förderung Der Angewand Dispositif pour mesurer la puissance de rayonnement de lasers
DE3926540A1 (de) * 1989-08-11 1991-02-14 Fraunhofer Ges Forschung Verfahren einer laserbestrahlung beschichteter werkstuecke und vorrichtug zur durchfuehrung des verfahrens

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1994026459A1 (fr) * 1993-05-19 1994-11-24 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. Procede permettant de travailler des materiaux par rayonnement emis par des diodes
US5705788A (en) * 1993-05-19 1998-01-06 Fraunhofer-Gesellschaft Zur Forderung Der Angewandten Forschung E.V. Process for treatment of materials with diode radiation

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DE3926540A1 (de) 1991-02-14
DE3926540C2 (fr) 1992-12-24

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