WO1992013668A1

WO1992013668A1 - Method of irridiating coated workpieces with laser radiation, and a device for carrying out the method

Info

Publication number: WO1992013668A1
Application number: PCT/DE1991/000105
Authority: WO
Inventors: Alexander Drenker; Reinhard Kramer; Konrad Wissenbach
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 1989-08-11
Filing date: 1991-02-08
Publication date: 1992-08-20
Also published as: DE3926540C2; DE3926540A1

Abstract

The invention concerns a method of irradiating coated workpieces with laser radiation, in which the surface temperature of the irradiated workpiece, in particular the zone being worked (20) is measured. In order to determine the temperature ($g(U)2?) of the workpiece in the zone being worked (20), even underneath a coating (12), the power of the laser radiation is modulated with a degree of modulation which is low in comparison with the power of the radiation (PL?) and, to determine the temperature ($g(U)2?) of the workpiece surface (13) at a point where it is covered by at least one coating layer (12), a temperature ($g(U)b?) is subtracted from the temperature ($g(U)0?) at the surface of the coating layer (12), the value of ($g(U)b?) being calculated by multiplying together the laser power (PL?) absorbed by the workpiece (10) and the thermal resistance (Rth02?) of the heat flow through the layer (12) into the workpiece (10), determined from the equivalent heat-flow diagram for the workpiece (10) concerned.

Description

Method of laser irradiation of coated workpieces and device for carrying out the method

description

Technical field

The invention relates to a method

Laser irradiation of coated workpieces, in which the

Surface temperature of the irradiated workpiece area, in particular a processing area is measured. Such methods are known as machining methods.

State of the art

When machining workpieces with laser radiation, it is generally known to measure the surface temperature of the irradiated machining region in order to record the effect of the laser radiation on the workpiece. For this, the surface temperature e.g. can be measured without contact with a pyrometer.

In the case of workpieces with bare surfaces, for example in the case of bare metal workpieces, 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.

Representation of the invention,

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.

It is important for the invention to recognize that 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. As a result, 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. Depending on its type, 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, that is to say superimposed on a laser power that is constant over time, 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.

In a large number of applications, 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. For example, for temperature determination on a workpiece with a heat equivalent circuit model modeled solely by its heat capacity, 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. However, 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 physical relationship between the above-mentioned measurable quantities is explained below.

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.

In one embodiment of the method, it is carried out during the processing of a point-wise or continuously relatively displaced workpiece and for this purpose 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. As a rule, it will suffice to consider a single heat replacement circuit when carrying out the method. This is the case, for example, when welding a single spot

Workpiece geometry. If this geometry changes, then every change must be taken into account at least by adapting the parameters of the heat equivalent circuit diagram. If this is not enough, the equivalent circuit diagram must be adapted yourself. The thermal time constant must be taken into account, for example, if the heating of the workpiece shows a corresponding temporal behavior. The influences described above on the heat replacement circuit diagram are taken into account when calculating the heat resistance. Accordingly, the more complicated the heat equivalent circuit diagram, or the more often its parameters are changed in the course of a method, the greater the computing power required. In order to simplify the method, the aim is therefore to carry out the calculations approximately, that is to say using simple heat substitute circuit diagrams, the latter being determined as far in advance as possible, ie not during the method itself. These measures minimize the computing power required for the calculation. 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

v _0g = _constant proportion of the surface temperature,

v _0w (t) = alternating _{proportion of} the surface temperature, and

P _Lw (t) = alternating part of the laser radiation power applies.

The aforementioned measuring devices are known per se. For example, a pyrometer can be used as the first measuring device. A 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.

Brief description of the drawings

The invention is explained further below. In the

Drawings is an embodiment of the invention

Device shown. It shows :

1 shows a schematic representation of a device according to the invention,

1a the detail A of FIG. 1, 2 shows a heat equivalent circuit diagram for FIG. 1a,

3 to 6 different heat equivalent circuit diagrams and FIG. 7 an analog arithmetic circuit for calculation according to the heat equivalent circuit diagram of FIG. 4.

Best ways to carry out the invention

The workpiece 10 shown schematically in FIG. 1 is to be subjected to transformation hardening. For this purpose, 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. Furthermore, 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.

In the above-described measurements of the radiation power PL, the radiation fraction absorbed by the workpiece 10 is measured or taken into account. For example done by 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.

In Figure 1 it is also shown that 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 ustelltmod 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 evaluator 17 determines the temperature v ₂ by subtracting a temperature value v _b from the measured surface temperature v _{0 of} the layer 12. The following therefore applies: v ₂ = v ₀ - v _b

The above relationship results from the following considerations: 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:

C _w1 ≡ 0

This results in a simplification of the equivalent circuit diagram in FIG. 2 in accordance with _{FIG. 4} , where R _{th02 combines} the two resistance _values R _th01 and R _th12 by addition.

Furthermore, it has been assumed in the simplification in FIG. 4 that the thermal resistance to the surroundings is infinitely large, so that no heat can flow out of the workpiece circumference 13. This is the case when the workpiece circumference 13 is equal to the total thickness of the workpiece, for example a sheet. Accordingly, FIG. 4 shows on the right that modulated power P _L = P _L (t) results in a temperature v ₂ = v ₂ (t) which increases continuously. The temperature # ₂ (t) remains unaffected due to the large heat capacity C _w2 by the modulations of the radiation power P _L (t). However, 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.

In comparison to FIG. 4, 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.

While 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.

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.

It can be derived from the equivalent circuit diagram in _FIG . ₄ that the sought temperature v _{2 must be determined} from the three variables P _L , v ₀ and R _th02 . For this determination, 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:

Q _H = c · (v _a - v _b ) / R _th

In this regard, 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 and 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

Q _H ~ P _L = P _L (t) = (v ₀ (t) - v ₂ (t)) / R _th02 .

In the above relationship, it was taken into account that 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 _L (t) = P _Lg + P _Lw (t)

v ₀ (t) = v _0g + v _0w (t)

v ₂ (t) = v _2g + 02w (t)

With these relationships follows

P _Lg + P _Lw (t) = (v _0g - v _2g ) / R _th02 + (v _0w (t) - v _2w (t)) / R _th02 .

The temperature v ₂ can only change slowly over time due to the large heat capacity C _{w2 of} the workpiece 10 or the workpiece _{circumference} 13 in comparison with the negligible heat capacity C _{w1 of} the layer 12. As a result, v ₂ (t) = v _2g = C or v _2w (t) ≡ 0 To solve the above equation for laser power by superposition, separate consideration of constant and alternating variables is carried out. The following applies:

I. P _Lg = (v _0g - v _2g ) / R _th02

II. P _Lw (t) = v _0w (t) / R _th02

From II follows:

R _th02 = v _0w (t) / P _Lw (t)

This results in:

P _Lg = (v _0g - v _2g ) P _LW (t) / v _0w (t)

respectively.

v _2g = v _0g - P _Lg / R _th02 = v ₂

It can be seen from the above relationship that 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).

In order to eliminate the high-frequency fluctuations of the alternating components, an average of the aforementioned quotient is to be formed, the time for averaging being outside the order of magnitude of the oscillation time, but being short enough to enable sufficiently frequent v ₂ determinations. In addition, the frequency range of the alternating components must be selected so that changes in temperature of the workpiece 10 or the machined workpiece circumference 13 in relation to changes in temperature of the layer 12 can be neglected. So v ₂ is determined according to the following relationship:

In this respect, the following applies: v _0g - _DC component of the surface temperature v ₀ , v _0w (t) = _AC component of the surface temperature v ₀ , and P _Lw (t) - AC component of the laser radiation power P _L.

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 radiation power P _L ⁼ P _L (7) generated by the laser 24 acts on the workpiece 10 and is measured by means of rapid laser power measurement 15, 16, which gives a corresponding signal u _L (t) to the evaluator 17. In parallel, the pyrometer 14 generates a measurement voltage value u _vo (t). This

Signal is given to a low-pass filter 28, so that the _DC component u _vog reaches an adder 30 as a result of its filter effect. This adder 30 is also supplied to a voltage value u _vb, with a negative sign, so that the output of the adder 30 a voltage _VOG u - u _Vb, that is, a searched temperature v ₂ proportional voltage u _alternate. This is the manipulated variable already described for FIG. 1 for the controller 25 to act on the laser 24.

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 cos2πf _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

34 applied with a signal ê (t). 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 | û _vow | acted upon. 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 .

The signals generated by the integrator 35, which are proportional to the thermal resistance R _th02, are fed to a multiplier 41 which receives signals u _Lg obtained from the voltage uι, (t) by filter action via a low-pass filter 42 and thus by multiplication in accordance with the relationship given above for v _b = -P _Lg / R _{th02 generates} the electrical signal u _vΔ , which is _fed into an input of the adder 30, which determines the manipulated variable u _stell proportional v ₂ already mentioned above.

According to the analog arithmetic circuit described above, 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.

Commercial recyclability

The method according to the invention is used to measure the surface temperature of the workpiece area irradiated with laser radiation,

Claims

Expectations:

1. Method of laser irradiation of coated workpieces, in which the surface temperature of the irradiated

Workpiece area (20), in particular a machining area, is characterized in that the laser radiation is power-modulated with a low degree of modulation with respect to its radiation power (P _L ), and that for determining the temperature (v ₂ ) of that of at least one layer (12) covered workpiece circumference (13) from the measured surface temperature (v ₀ ) of the layer (12), a temperature value (v _b ) is subtracted from the product of the laser radiation power (P _L ) absorbed by the workpiece (10) and that of the heat equivalent circuit diagram of the respective one Workpiece (10) determined thermal resistance (R _th02 ) of the heat flow through the layer (12) into the workpiece (10) is calculated.

2. The method of claim 1, d a d u r c h g e k e n n z e i c h n e t that for temperature determination on a workpiece (10) with only by its heat capacity

(C _w2 ) modeled heat _{equivalent circuit diagram} , a temperature value (v _b ) is subtracted from the measured surface temperature (v ₀ ) of the layer (12), which is obtained from the direct component (P _Lg ) of the laser radiation power (P _L ) by multiplication by an average quotient of alternating components (v _0w (t)) of the surface temperature (v ₀ ) and alternating components (P _Lw (t)) of the laser radiation power (P _L ) is determined.

3. The method of claim 1 or 2, d a d u r c h g e k e n n z e i c h n e t that the laser radiation power

(P _L ) is externally modulated in a frequency range adapted to the requirements during processing or measurement and / or that the natural fluctuations in the radiation power caused by the laser process

(P _L ) can be used as modulation signals.

4. The method according to one or more of claims 1 to 3, characterized in that it is carried out during the machining of a point-wise or continuously relatively displaced workpiece (10) and at least the parameters of the heat equivalent circuit diagram are adapted accordingly and / or that the thermal time constant of the layer (12) covered

Workpiece (10) in the area of the workpiece circumference (13) is taken into account by means of the parameters of the heat replacement circuit diagram.

5. Apparatus for performing the method according to claim 2, characterized in that a first measuring device (14) is able to detect the surface temperature (v ₀ ) as a function of time, that a second, the direct component (P _Lg ) of the laser radiation power (P _L ) and a third, alternating components (P _Lw (t)) of the laser radiation power (P _L ) measuring device (15, 16) are present, and that the temperature (v ₂ ) of the workpiece circumference (13) covered by a layer (12) has a Evaluator (17) after the relationship

is determinable, whereby

v _0g = _constant component of the surface temperature (v ₀ ),

v _0w (t) = alternating component of the surface temperature (v ₀ ), and

P _Lw (t) = alternating portion of the laser radiation power (P _L ) applies.

Apparatus according to claim 5, so that the evaluator (17) connected to the measuring devices (14 to 16) is an electronic analog arithmetic circuit or a digitized measured value for calculating the temperature (v2) of the workpiece (10).