DE102008053397B4 - Method for the fusion cutting of workpieces with laser radiation - Google Patents

Abstract

Process for fusion cutting of workpieces with laser radiation, in which a maximum absorption of the laser radiation can be achieved for a given workpiece thickness, a laser beam with an angle of incidence φIN strikes a cutting front formed in the interaction zone of the laser beam, a cutting gas beam and material with an inclination angle φC and an angle of incidence φIN and angle of inclination φC form a complementary angle pair to 90 °, characterized in that the angle of inclination φC of the cutting front is permanently changed by a movement of the focal point of the laser beam superimposed on the feed movement so that the angle of incidence φIN of the laser beam is kept within an interval around the Brewster angle φBr becomes.

Description

The invention relates to a method for the thermal separation of materials by means of laser radiation, in which the cutting process is modified.

Laser beam cutting is an established and industrially established method for the thermal cutting of workpieces along a predetermined cutting contour. Depending on the effective separation mechanism, the laser beam fusion cutting and the laser beam evaporation cutting are basically differentiated. Laser beam fusion cutting is the preferred technology for separating metallic materials in a large sheet thickness range.

The material separation takes place in laser beam fusion cutting by the combined use of a focused laser beam and a cutting gas jet, which usually act coaxially on the machined workpiece. The principle of the method is based on the fact that the base material is melted in the area of the kerf and driven out of the kerf by the simultaneously realized momentum transfer of the cutting gas jet, whereby a kerf is formed along the cutting contour (see FIG. 1 ).

The minimum value E _{MIN of} the process energy to be supplied results in the laser beam fusion cutting from the requirement to melt the volume of material to be expelled from the cutting gap: E _MIN = ρ · L · _K b · _K d _K · [c _p · (θ _mp - θ ₀₎ + .DELTA.h _m]

The energy demand E _MIN is a function of the thermal material properties density ρ, specific heat capacity c, melting temperature θ _mp , and specific enthalpy of fusion Δh _m and the temperature θ _{0 of} the base material and the kerf dimensions width b _K , depth d _K and length l _K.

The minimum value P _{MIN of} the process output to be supplied results in the laser beam fusion cutting from the requirement to melt the volume of material to be expelled from the cutting gap within a predetermined time interval, the process duration t _C , or at the generally constant process speed v _C :

For a specific cutting task, the material characteristics via the specification of the material as well as the cutting gap depth d _{K are} fixed quantities via the specification of the material thicknesses to be cut. In contrast, the kerf width b _{K is} a function of the process parameters. The inherent loss of material m _v is given by m _v = ρ · l _k · b _k · d _K

Efficiently executed cutting processes with regard to energy requirement P _MIN and material _loss m _v are designed to keep the kerf width b _K as small as possible.

For reasons of energy balance, a fusion cutting process can only be carried out if the power P _DISS transmitted during the cutting process, ie the dissipated power, in the area of the interaction zone between laser beam, cutting gas jet and material is greater than the required power P _MIN for melting the _{kerf volume} .

The dissipated power P _DISS during the cutting process must be greater than the power P _MIN to melt the _{kerf volume} due to unavoidable energy losses during the cutting process. These are due in particular to the heat conduction in the base material adjoining the cut front and the necessary overheating of the melt film surface at the cut front to temperatures above the melting temperature. The latter is necessary to achieve a melt film of certain thickness, which is then expelled in interaction with the cutting gas jet from the kerf.

The ratio of the minimum value P _{MIN of} the power necessarily to be supplied to the dissipated power P _DISS is referred to as melting efficiency η _S :

The melting efficiency η _S is, in particular, a function of the thermal material parameters and can not be directly influenced by process engineering measures. However, due to the fact that the material is driven out of the kerf simultaneously to the reflow process, it can be assumed that energy losses with respect to the dissipated power in the laser beam fusion cutting remain small and have no dominating influence on the overall efficiency of the process.

The ratio of dissipated power P _DISS to total supplied power P _ZU is referred to as energy transfer efficiency η _T :

maßgeblich durch den Energieübertragungswirkungsgrad η_T bestimmt.Due to the fact that the melting efficiency η _{S is} usually high, the overall efficiency η _{C of} a laser beam cutting process becomes corresponding

significantly determined by the energy transfer efficiency η _T.

Efficiently executed cutting processes with regard to energy and power requirements must be geared towards ensuring the highest possible energy transfer efficiency η _T.

The variant of laser inert gas welding allows oxide-free cutting of metals and is therefore the preferred variant for cutting high-quality materials such as stainless and other high-alloy steels, as well as aluminum and titanium alloys. The cutting gas used are mainly nitrogen and argon. The energy requirement of the cutting process for melting the material is covered solely by the incident laser radiation: P _DISS = η _TL · P _ZU = η _TL · P _L

The energy transfer efficiency η _{T, L} in the laser beam inert gas cutting is significantly determined by the absorption capacity A of metallic materials with respect to the irradiated laser power.

If a laser beam with the power P _{L impinges} on the surface of a material, only a part of the laser radiation P _{L, T} penetrates into the material, while the other part P _{L, R is} reflected.

In the case of metallic materials, the transmitted power component P _{L, T is} absorbed in the laser beam sources commonly used within a very thin layer near the surface. It is permissible to regard the transmitted power component P _{L, T} as being absorbed on the surface of the material, ie the absorbed power component P _{L, A} corresponds to the power component P _{L, T} and the power balance generally applies P _L = P _{L, R} + P _{L, A} = R × P _L + A × P _L = (R + A) × P _{L where} R + A = 1 where R is the ratio of reflected power P _{L, R} to the total radiated power P _{L is} the reflectance and A as the ratio of absorbed power P _{L, A} to the irradiated power P _L absorptance.

Given the plausible assumption that multiple reflections at the cutting front do not significantly contribute to the total energy input, the energy transfer efficiency η _{T, L} corresponds to the absorption coefficient A: η _{T, L} ≡ A = 1 - R

Consequently, the process efficiency of the inert gas laser beam cutting is significantly dependent on the degree of absorption A or reflectance R.

in denen R_P den Reflexionsgrad parallel polarisierter Strahlung und R_S den Reflexionsgrad senkrecht polarisierter Strahlung bezeichnet.The reflectance R or degree of absorption A of a metallic material for radiation of the beam sources commonly used in laser material processing is generally a function of the polarization state, the angle of incidence φ _In , and the optical material properties refractive index n and extinction coefficient k, which in turn depends on the temperature T and in particular depending on the wavelength λ of the incident laser radiation. A theoretical calculation of the reflectance is possible via the Fresnel equations. For metallic materials, the following approximations result

in which R _P denotes the reflectance of parallel polarized radiation and R _S denotes the reflectance of perpendicularly polarized radiation.

The reflectance R _{UP of} circularly polarized or unpolarized radiation results as an arithmetic mean of the reflectances R _P and R _S of parallel and perpendicularly polarized radiation. For the absorptance A _UP follows

Calculated dependencies of the degree of absorption A of iron at melting temperature shows as a function of the angle of incidence φ _IN for different wavelengths and polarization states. For parallel and circularly polarized laser radiation, there is a pronounced maximum in the absorbance A _P and A _UP for a particular angle of incidence which corresponds to the Brewster angle φ _Br . The concrete value of the Brewster angle φ _Br is of the optical properties of the irradiated material, depending on the laser wavelength and the surface temperature of the irradiated material.

For the laser beam inert gas cutting of contour cuts, the use of circularly polarized or unpolarized radiation is customary in order to avoid a directional dependence in the absorption behavior.

Efficient cutting processes with regard to energy and power requirements must be geared towards irradiating the laser power at an angle φ _IN ≈ φ _Br to the material to be machined.

Laser beam fusion cutting is carried out using various high-power lasers. These include in particular CO ₂ , fiber, disc and diode laser systems. Differences in the achievable efficiency of the cutting process and the quality of the cutting results result from differences in the respective available output powers, the laser wavelength and the beam quality.

The emitted laser radiation is usually emitted as a beam with a circular beam cross section. For fusion cutting, this beam is focused on small focus radii r _{0 in} order to achieve the intensities required for the local melting of metallic materials.

abgeschätzt werden. In dieser Gleichung bezeichnet f die Brennweite der Fokussieroptik, D den Strahldurchmesser vor der Fokussierung und λ die Laserwellenlänge. M² ist ein Maß für die Strahlqualität. Für einen idealen, d. h. beugungsbegrenzten, Strahl ist M² = 1. Der erreichbare Strahlradius ist dem Produkt aus Laserwellenlänge λ und Strahlqualitätszahl M² direkt proportional.The focus radius achievable using optical components can be determined by the relationship between the focusing conditions characteristic of laser material processing

be estimated. In this equation, f denotes the focal length of the focusing optics, D the beam diameter before focusing, and λ the laser wavelength. M ² is a measure of the beam quality. For an ideal, ie diffraction-limited, beam, M ² = 1. The achievable beam radius is directly proportional to the product of laser wavelength λ and beam quality number M ² .

The beam radius r ₀ is reached only at the focal point of the focusing optics used with the focal length f. Starting from the position z = 0 of the focal point, the beam diverges as a function of the spatial coordinate z in the beam propagation direction according to the relationship

bestimmt werden. Die Rayleighlänge z_R ist dem Produkt aus Strahlqualitätszahl M² und Laserwellenlänge λ indirekt proportional.The Rayleigh length z _R is the distance between the focal point with z = 0 and the point in the beam propagation direction, where the beam radius is r ₀ grown to the value r (z _R) = 1:41 r · _0th The Rayleigh length can be about the relationship

be determined. The Rayleigh length z _R is indirectly proportional to the product of beam quality number M ² and laser wavelength λ.

The beam geometry, ie the radial extent r (z) of the laser beam in beam propagation direction z, has a decisive influence on the width b _{K of} the generated kerf. The width b _{K of} the kerf is proportional to the mean radius r _{m of} the laser beam over the sheet metal thickness d _K to be cut.

Efficient cutting processes with regard to energy demand and material loss must be geared towards generating a beam with the smallest possible mean beam radius r _m over the sheet metal thickness d _K to be cut.

Small mean beam radii r _m can be realized using laser beam sources which have a short wavelength λ with simultaneously high beam quality M ² ≈ 1.

The criteria of a short emission wavelength combined with high beam quality are met in particular by fiber and disk lasers, which have only been available as high-power lasers in the kW range for a few years. Both systems emit laser radiation at a wavelength of λ ≈ 1 μm. The output power P _L diffraction-limited laser aggregates with M ² 1 is increasingly increasing.

CO ₂ lasers have been available as high-performance systems for some time now and are the dominant laser beam source for cutting metallic materials. CO ₂ lasers emit laser radiation with a wavelength of λ = 10.6 μm with simultaneously high beam quality with M ² ≈ 1 in a large power range.

Laser beam fusion cutting is conventionally carried out with constant laser, cutting gas and process parameters. The laser parameters include the laser power, the realized beam geometry and the laser wavelength. The cutting gas parameters relate in particular to the cutting gas composition, the cutting gas pressure and the geometry of the cutting gas nozzle. The process parameters include the type of Material, the sheet thickness to be cut and the process speed.

At constant laser, cutting gas and process parameters, a quasi-stationary cutting front at a certain angle of inclination φ _C is formed in the interaction zone between laser beam, cutting gas jet and material. As a result, the laser radiation does not impinge perpendicularly, but at a certain angle of incidence φ _IN on the interface front face (see FIG. 3 ).

Accordingly, the angle of incidence φ _IN and the angle of inclination φ _{C form} a complementary angle pair when a common reference frame is selected, ie φ _IN + φ _C = 90 ° or φ _IN = 90 ° - φ _C

The angle of inclination φ _C , which depends on the process parameters, consequently has a significant influence on the absorption of the irradiated laser power and thus on the overall efficiency η _C.

abschätzen, in der d_K die zu schneidende Werkstoffdicke (Blechdicke), und r₀ den Fokusradius bedeutet. Für die Abschätzung ist angenommen, dass die Fokussierung auf die Werkstückoberseite erfolgt (s. 3).The angle of inclination φ _{C of} the cutting front, which adjusts as a function of the process parameters, is a function of the beam geometry or the radiation caustic r (z). As an approximation, the mean tilt angle of the cutting front can be determined by the relationship

estimate, in the d _K the material thickness to be cut (sheet thickness), and r _{0 means} the focus radius. For the estimation, it is assumed that the focus is on the workpiece top side (s. 3 ).

For some cutting applications, a beam focusing takes place in the material to be cut, ie the focus position is in the workpiece. Such a shift in the focal position has only a slight effect on the value of the inclination angle φ _{C of} the cutting front (see FIG. 4 ). With a beam focusing in the center of the workpiece, the mean inclination angle is calculated to

The self-adjusting value of the inclination angle φ _C is determined by the achieved focus radius r ₀ and the Strahlkaustik r (z), but in particular by the thickness of the material to be cut (s. 5 ). In particular, when using laser beam sources with a short wavelength λ ≈ 1 microns, the inclination of the cutting front will not have an optimum absorption angle with respect to the absorption.

In conventional process control of laser beam cutting with uniform relative movement between the laser beam and the workpiece to be cut adjusts the angle of inclination according to the process and material parameters.

For a given beam caustic r (z), there is no immediate possibility of deliberate influence on the size of the tilt angle φ _{C of} the cutting front. In particular, when using laser beam sources with a short wavelength and high beam quality at the same time angle of inclination φ _{C can be} adjusted, the amount of which is significantly greater than the Brewster angle φ _Br . This leads to an uncontrolled reduction of the absorbed power component P _{L, A} and can significantly degrade the process efficiency of the laser beam cutting.

That's how it is DE 42 26 620 A1 a method for laser beam cutting of workpieces known in which maintained to maintain a vapor capillary, a hydrogen content in the cutting gas and the cutting gas is supplied to the laser beam enveloping.

In DE 10 2004 052 323 A1 A method for separating materials with a laser beam is described, in which the axis of the laser beam is to be moved with fixed orientation along a parting line.

In JP 06 234 090 A and JP 07 185 856 A It is proposed to deflect a laser beam oscillating in the feed axis direction.

The problem to be solved by the invention is to increase the efficiency and the degree of absorption compared with the known methods during the fusion cutting of workpieces with laser radiation.

According to the invention, this object is achieved by a method having the features of claim 1. Advantageous embodiments and further developments of the invention can be achieved with features described in the subordinate claims.

In the method according to the invention, the procedure is such that maximum absorption of the laser radiation is achieved for a given workpiece thickness by changing the inclination angle φ _{C of} the cutting front by a movement of the focal point of the laser beam superimposed on the advancing movement. In addition, an adaptation of the geometry of a laser beam by means of beam shaping can take place.

To achieve a high process efficiency η _C , it is necessary for the laser radiation to impinge on the cutting front in such a way that the angle of incidence φ _{IN of} the laser beam lies on the cutting front in the region of the Brewster angle φ _Br and an absorption coefficient close to the optimum is ensured. The procedure is such that the angle of inclination φ _{C of} the cutting front is permanently changed so that the angle of incidence φ _{IN is} within an interval around the Brewster angle φ _Br . The Brewster angle should be within the interval. He does not necessarily have to be in the middle of it.

A reduction in the inclination angle of the cutting front can also be achieved by increasing the focus radius r ₀ . However, such a procedure also leads to a reduction in intensity and energy density. At the same time, the power requirement P _{MIN increases} overall, since the kerf width increases with a larger jet radius and consequently more material has to be melted.

In. the additional possibility of influencing the geometry by beam shaping of the laser beam, it should be noted that the additional power requirement due to a widened kerf can be avoided if beam expansion is realized only in the feed axis direction, see 6 , For this purpose, laser beams with rectangular or elliptical beam cross sections are suitable. In this case, the dimensions b _up = b ₀ on the upper side of the sheet metal and the extension b _down = b ₀ + b _{(z) of} the jet in the feed direction enter as characteristic beam dimensions.

A targeted influence on the size of the angle of inclination with constant intensity can also be achieved by a uniform relative movement between the laser beam and workpiece superimposed non-linear oscillating deflection of the laser beam and a related movement of the focal point, see 7 , The oscillating deflection superimposed on the feed movement should preferably take place in feed axis direction. For example, it can be sinusoidal (harmonic) or triangular.

By the oscillating beam movement of the inclination angle φ _{C of} the cutting front by choice or oscillation shape, the oscillation amplitude a and the oscillation frequency f can be influenced. As a result, the inclination for each given workpiece thickness d _K , and the other process variables such as Strahlkaustik r (z), laser power P _L and cutting speed v _{C can} be adjusted so that a maximum absorption. the laser radiation is achieved on the material or workpiece.

The superimposed oscillatory deflection between the two reversal points of this movement should reach a minimum distance which is greater than the average diameter of the cross section of the laser beam at its focal point. In this case, this path (amplitude) should increase with increasing thickness d _{K of} the workpiece. It can thereby be covered paths of the focal point between the reversal points, which may be a few millimeters. In this case, as already indicated, a frequency for the oscillation movement can be maintained, which should be in the range between 10 Hz and 1000 Hz.

The oscillating deflection of the laser beam with resulting movement of the focal point can take place in different forms. In a longitudinal oscillation in feed axis direction different deflection functions, such as. B. harmonic, triangular and sawtooth are used. In addition to one-dimensional oscillation forms, elliptical and eight-shaped laser beam movements can also be performed.

Both the beam shaping in the feed axis direction and the oscillating deflection of the laser beam are limited when using conventional cutting nozzles with circular passage openings for laser beam and cutting gas. A simple enlargement of the free cross section of the outlet opening reduces the cutting gas effect and increases its consumption, which is therefore to be avoided, but at least to be kept within limits.

Advantageously, cutting gas nozzles can be used, which are adapted to the respective of the two alternatives shown according to the invention, namely the shaping of the laser beam cross section and the oscillating deflection of the laser beam. The cross sections of the outlet opening can thus have a rectangular or elliptical shape, see 8th ,

With changing feed axis directions, the cutting nozzle can be rotated synchronously and their alignment with the geometric shape of the outlet opening can be adapted to the respective instantaneous feed axis direction.

The achievable improvement in process efficiency depends on the respective jet and process parameters.

The invention will be explained in more detail by way of example with the partially self-explanatory figures.

Showing:

1 in schematic form, the process principle of laser beam fusion cutting;

2 a graph of the absorption coefficient of iron at melting temperature as a function of the angle of incidence of the laser beam for different wavelengths and polarization states;

3 in schematic form, the formation of a quasi-stationary cutting front in laser beam fusion cutting with constantly set laser, cutting gas and process parameters, with focus position on the workpiece top;

4 in schematic form, the formation of a quasi-stationary cutting front in laser beam fusion cutting with constantly set laser, cutting gas and process parameters, with focus position in the center of the workpiece;

5 A diagram of the mean cutting angle of the cutting front as a function of the sheet thickness and the laser wavelength. The focus radius r ₀ is 50 μm in each case;

6 in schematic form, a mean inclination angle of the cutting front in laser beam fusion cutting with beam cross-section widened in the cutting direction;

7 in schematic form, mean cutting angles of the cutting front in laser beam fusion cutting using an oscillating beam;

8th conventional and adapted nozzle geometries for cutting processes with beam oscillation or beam forming;

9 a diagram of the dependence of the oscillation amplitude on the inclination angle φ _{C of} the cutting front and

10 a diagram of the dependence of the oscillation frequency on the inclination angle φ _{C of} the cutting front.

With the method according to the invention investigations were carried out on a workpiece made of stainless steel with a workpiece thickness of 8 mm. It worked with a fiber laser that emitted laser radiation with a wavelength of 1.07 microns at a laser power of 2.5 kW. The laser beam was focused so that the diameter at the focal point was 110 μm. The focal plane was 6 mm deep in the workpiece to be cut. The diameter of the laser beam at the workpiece surface could be calculated as 400 μm. The normal feed rate, excluding the superimposed oscillating deflection, was 0.6 m / min. It was worked with nitrogen as the cutting gas that had been directed into the kerf via the cutting nozzle.

With the in the 9 and 10 The diagrams shown can be used to illustrate how changing the parameters of a longitudinal harmonic oscillatory deflection can influence the angle of inclination.

Thus, it was deflected once at a constant oscillation frequency of 200 Hz and only the path traveled by the focal point between the reversal points (oscillation amplitude 2a) varies (see FIG. 9 ). It becomes clear that with larger paths the inclination angle φ _{C of} the cutting front also becomes larger. In this diagram, the specific values are indicated by circles. For a distance traveled of 2 mm, the comparison at a frequency of 800 Hz has also been plotted in the diagram.

With the in 10 As shown in the diagram, the relationship of an altered oscillation frequency with a constant distance of 2 mm, which has been covered by the focal spot between the reversal points, clearly. Again, the inclination angle φ _{C of} the cutting front can be increased by increasing the oscillation frequency.

As expressed in the general part of the description, the angle of incidence φ _IN can also be influenced in this way.

Claims (16)

A method for the fusion cutting of workpieces with laser radiation, wherein for a given workpiece thickness maximum absorption of the laser radiation is achievable, while a laser beam with an angle of incidence φ _IN on a formed in the interaction zone of laser beam, a cutting gas jet and material cutting front with an inclination angle φ _C and hits Incident angle φ _IN and inclination angle φ _{C form} a pair of complementary angles to 90 °, characterized in that the inclination angle φ _{C of} the cutting front is permanently changed by a movement of the focal point of the laser beam superimposed on the advancing movement so that the angle of incidence φ _{IN of} the laser beam within an interval Brewster angle φ _{Br is} held. A method according to claim 1, characterized in that a uniform feed movement is superimposed on an oscillating deflection of the laser beam. A method according to claim 2, characterized in that the oscillating deflection is performed in the feed axis sinusoidal. A method according to claim 2 or 3, characterized in that in the oscillating deflection of the focal point between the two reversal points covers a path which is greater than the average diameter of the cross section of the laser beam at the focal point. Method according to one of claims 2 to 4, characterized in that the oscillating deflection is performed at a frequency in the range 10 Hz to 1000 Hz. Method according to one of Claims 2 to 5, characterized in that the oscillating deflection in the feed axis direction is performed taking into account a sawtooth function. Method according to one of claims 2, 4 or 5, characterized in that the oscillating deflection in Vorschubachsrichtung a triangular function is performed considering. Method according to one of claims 2, 4 or 5, characterized in that an elliptical or achtförmige oscillating deflection is performed. Method according to one of the preceding claims, characterized in that a laser beam with a rectangular laser beam cross section is used. Method according to one of claims 1 to 8, characterized in that a laser beam is used with ellipsoidal laser beam cross-section. Method according to one of the preceding claims, characterized in that the laser beam cross section and / or the oscillation direction is adapted to the respective feed axis direction. Method according to one of the preceding claims, characterized in that a cutting nozzle, which is adapted to the respective laser beam cross-section and / or the oscillating deflection of the laser beam is used. A method according to claim 12, characterized in that a cutting nozzle is used with a passage opening for a laser beam with a rectangular cross-section. A method according to claim 12, characterized in that a cutting nozzle is used with a passage opening for a laser beam with elliptical cross-section. Method according to one of claims 12 to 14, characterized in that the cutting nozzle is rotated taking into account the respective feed axis direction. Method according to one of the preceding claims, characterized in that a high-performance, fiber, disc, CO ₂ - or diode laser is used.

Images