Method for laser drilling or laser cutting of workpiece

Abstract

The present invention relates to a method (100) for laser drilling or laser cutting of a workpiece (1), wherein material of the workpiece (1) is removed (110) by means of pulsed laser radiation (2), wherein a fluid (4) is arranged in a rear space (3) located behind a region (1a) to be penetrated of the workpiece (1) along the beam direction, wherein when the laser radiation has penetrated the region (1a), the laser radiation (2) enters the rear space, and the fluid at least partially blocks the passage of the laser radiation (2), wherein the fluid (4) comprises particles (4a) dispersed in a solvent (4b), and is characterized in that particles (4a) are selected (121) to form a plasma (5) under the action of the laser radiation (2) entering the rear space (3), so that the plasma (5) at least partially blocks the passage of the laser radiation (2).

Description

Technical Field

[0001] The invention relates to a method for laser drilling or laser cutting of a workpiece, in which method the rear space behind the point where the laser beam penetrates the workpiece is better protected from the focusing effect of the laser beam. Background Art

[0002] During laser drilling or laser cutting of workpieces, the workpiece material is irradiated with very brief and, for this reason, very intense laser pulses. When the instantaneous intensity is high enough, the electron shells of the material atoms are directly ionized, so that the material can be removed without introducing too much heat into the workpiece.

[0003] If the laser radiation has penetrated the workpiece at one point, it may, without adequate protective measures, undesirably remove material from other areas of the workpiece or escape uncontrolled into the surrounding environment. Therefore, EP 3 013 517 B1 proposes a method for laser drilling or laser cutting, in which a fluid loaded with nanoparticles is introduced into the rear space behind the workpiece. The energy of the laser radiation penetrating into the rear space is mostly converted into accumulated excitation of the nanoparticles and dissipated in this way. Summary of the invention

[0004] Within the scope of the invention, a method for laser drilling or laser cutting of a workpiece has been developed. In the method, workpiece material is removed by means of pulsed laser radiation. A fluid is arranged in a rear space behind the area of ​​the workpiece to be penetrated in the beam direction, into which the laser radiation penetrates when the laser radiation has penetrated this area. The fluid at least partially blocks the passage of the laser radiation. The fluid comprises particles dispersed in a solvent.

[0005] The particles which form the plasma under the effect of the laser radiation entering the rear space are selected such that the plasma at least partially blocks the passage of the laser radiation.

[0006] The term "particles" itself is not limiting here with regard to their size. The particle size for forming the desired plasma depends, for example, on the material of the particles and on the intensity of the laser radiation entering the back space.

[0007] Furthermore, the term "particles" should not be construed as limiting the corresponding material to be in a solid aggregate state at room temperature. For example, metal-containing droplets that are liquid at room temperature can also be used as particles. For example, gallium or a gallium-indium-tin alloy can be dispersed in a solvent under the action of ultrasound.

[0008] The plasma is generated by a very high instantaneous intensity, which first removes electrons from the electron shells of the particle atoms and thereby ionizes these atoms. Afterwards, the remaining ions are also separated from the particles, so that the ions and electrons form a quasi-neutral plasma together. The plasma is largely impermeable to further laser radiation. The energy that appears with further laser pulses is partially diffusely reflected and / or scattered and partially absorbed. In particular, the diffusely reflected and / or scattered parts are deflected at a large spatial angle and can therefore be particularly effectively derived. The energy flux density of the laser pulse in the original beam direction can thus be reduced to below a threshold value, from which other materials of the workpiece will be removed or there is a risk of otherwise causing thermal damage to the workpiece.

[0009] When the irradiation is over, part of the separated ions is deposited on the particles again and can be used again to form the plasma. However, the ions will eventually enter the fluid and are thus "lost". Therefore, the particles are lost over time. However, this is also the case in other particle-based solutions for back space protection. When the plasma is formed in the described manner, the total amount of laser energy that prevents each formula unit of the particle material from passing through the back space until the point in time when it can no longer be used is the largest.

[0010] In addition, the plasma increases the interaction volume in the fluid in which the propagation of the laser radiation along the beam path is interrupted. Accordingly, the energy flux density is reduced even more in the original beam direction.

[0011] In principle, if the instantaneous intensity of the laser radiation is large enough, it can cause particles of any material to form plasma. However, what is decisive for the protection of the rear space is whether the formation of plasma has already occurred at the instantaneous intensity generated at the particle position just after the laser radiation penetrates the workpiece, or whether this specific instantaneous intensity is too low to cause the specific particle to form plasma. The instantaneous intensity is derived from the parameters of the laser radiation, that is, in particular from the pulse energy, pulse duration, repetition rate and the time trend of the pulse. The minimum instantaneous intensity required for the formation of plasma is determined in turn by the properties of the corresponding material of the particles in combination with the particle size. Therefore, there is a required minimum instantaneous intensity for each material and each particle size. Conversely, this means that in a specific application case where certain instantaneous intensities will enter the rear space behind the workpiece, a person skilled in the art can selectively select particles that form plasma at this instantaneous intensity, and thus prevent the laser radiation from propagating uncontrolled in the shape of a sharp bunch.

[0012] The expression "selected particles" should generally not be understood in a restrictive manner as meaning that the fluid only contains particles that satisfy the given conditions. It is only required that particles that satisfy the conditions exist, but it cannot be ruled out that other particles exist in the fluid.

[0013] For example, a threshold value for the instantaneous intensity can be determined based on the intensity distribution of the pulse over time. The threshold value can be, for example, the average value of the intensity during the pulse, a certain percentage of the maximum intensity, or an intensity exceeded during a certain portion of the pulse duration. The particles that form the plasma at the latest at the instantaneous intensity corresponding to the threshold value are selected.

[0014] In addition, a limit temperature can also be determined, and particles can be selected which form a plasma at the latest at a momentary intensity corresponding to a threshold value and at a temperature corresponding to the limit temperature. The underlying idea is that the momentary intensity required to form a plasma from certain particles is greatest in the cooled solid state of the particles and decreases significantly when the particles have been heated and / or softened. If the limit temperature is determined, more particle material can be used for a specific application. Materials that already form a plasma at room temperature at the available momentary intensity are combined with materials that only form a plasma at higher temperatures up to the limit temperature at the same momentary intensity.

[0015] The limiting temperature may be, for example, the melting point or boiling point of the particles. When the particles melt, for example, the thermal conductivity of the particle material gradually decreases. Therefore, under the action of the laser radiation, the average surface temperature of the particles over time increases compared to their solid state, which in turn facilitates the formation of plasma near the particles.

[0016] In a particularly advantageous configuration, the particles can be selected from materials that have a transmittance, reflectance or absorption of 0.9 or more for the wavelength of the laser radiation when no plasma is present. The idea is that materials that are optically particularly good in any way with respect to the wavelength of the laser radiation are good candidates for being caused to form a plasma by the laser radiation. When the plasma is present, the material behaves completely differently than the original material.

[0017] Droplets made of gallium indium tin alloys, which can be used as particles, have the advantage that gallium indium tin alloys have a significantly lower boiling point than solid materials such as carbon. Accordingly, nonlinear scattering effects associated with plasma formation can already be expected at significantly lower instantaneous intensities.

[0018] Gallium indium tin alloys can be dispersed very finely in solvents, for example, by means of ultrasound. If the intensity of the ultrasound source is sufficient to overcome the limit given by the surface tension of the gallium indium tin alloy, nanometer-scale droplets can also be achieved. This is advantageous for surface fractions that absorb or accumulate under sweeping incidence and for phase separation due to density differences.

[0019] In order to finely atomize the metal-containing droplets, for example, the Rayleigh instability of the droplets can be utilized. For this purpose, for example, a metal-containing liquid can be pressed into a solvent through a conductive capillary at a sufficiently high negative potential, so that smaller droplets are formed under the same ultrasound intensity. In addition, the electrostatic repulsion between the metal-containing droplets inherently resists the condensation of these droplets, without the need for special functionalization of the surface of the droplets or other particles for this purpose.

[0020] For example, particles made of barium sulfate BaSO4, calcium sulfate CaSO4, aluminum oxide Al2O3, titanium oxide TiO2, zirconium oxide ZrO2 and / or boron nitride BN can be selected. These materials appear white or transparent when illuminated with white visible light. Therefore, they have good reflectivity or good transmittance for many wavelengths.

[0021] Alternatively or in combination, for example, particles consisting of carbon, silicon carbide SiC, boron carbide B4C and / or tungsten carbide WC can be selected. These materials appear black when irradiated with white visible light. Therefore, they have good absorption for many wavelengths. In this case, carbon can be present in various forms, for example as diamond, as graphite or as carbon black.

[0022] In particular, the combination of a material that is reflective or transparent in a non-plasma state and a material that absorbs in a non-plasma state is advantageous. The more diverse or different the interaction complexes in the fluid, the more difficult it is for the laser pulse that invades the back space to penetrate there, and if necessary, propagate in the original beam direction according to the number density of the interaction complexes. The wavefront of the laser pulse can be dephased (dephasieren) in combination with different interaction complexes, because the particles are randomly distributed in the radiation field, and the interaction with the laser depends on the material. Therefore, different materials require different instantaneous intensities to form plasma, for example. The lifetime of internal excitation also depends on the material. However, the high energy flux density in the original beam direction depends largely on the wavefront of the laser radiation being in phase. The idea is that different interaction complexes locally change the speed of light, wherein the change is different for different types of interaction complexes, especially in terms of the (local) intensity and the correlation with the wavelength used by the laser.

[0023] In another particularly advantageous embodiment, the solvent is selected to contain a surfactant. In this way, a greater number of particles can be dispersed in the solvent, so that a greater portion of the laser radiation can be prevented from being propagated by the plasma in the original beam direction.

[0024] In another particularly advantageous embodiment, a solvent containing hydrocarbons is selected. The hydrocarbons do not attack the workpiece material and are not converted into substances that attack the workpiece material under the action of the laser radiation. In particular, pure hydrocarbons do not contain any oxygen, which would be released during molecular decomposition and could damage the workpiece material. In addition, a wider variety of particles is dispersed in hydrocarbons than in aqueous solvents.

[0025] Alternatively or also in combination therewith, the solvent may contain alcohol. Compared with aqueous solvents, more types of particles can also be dispersed in alcohol. For example, an emulsion consisting of a polar liquid such as alcohol, a nonpolar liquid such as alkane and a surfactant can also be selected as a solvent.

[0026] For example, a solvent having an alkane or a mixture of alkanes may be selected. Depending on the chain length, alkanes have a higher boiling point. An example of an alkane-based solvent is paraffin oil. It is a solvent having a molecular formula of C n H 2n+2 A mixture of higher alkanes with n between 18 and 32.

[0027] In another particularly advantageous configuration, a solvent with a hydrocarbon is selected, which contains a carbon chain that is end-capped and / or functionalized on at least one end. End-capping or functionalization can help disperse certain particles in the solvent. Then, only a lower concentration of additional surfactant is needed to disperse the particles, or even this additional surfactant can be completely eliminated. Therefore, for example, a benzene ring as a functionalized substance helps disperse the carbon particles.

[0028] With dodecylbenzene C 18 H 30 The pulverized carbon can be dispersed particularly well in the substance used, for example, as transformer oil.

[0029] In order to better disperse the particles in the solvent, the particles can, for example, be charged. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Further measures for improving the invention are shown in more detail below together with the description of preferred exemplary embodiments of the invention with reference to the drawings.

[0031] The accompanying drawings show:

[0032] Figure 1 : An embodiment of method 100;

[0033] Figure 2 : The laser radiation 2 irradiates the particles 4a in the rear space 3 after penetrating the region 1a of the workpiece 1;

[0034] Figure 3: Formation of plasma 5 and diffuse scattering 2a of laser radiation 2 on plasma 5. DETAILED DESCRIPTION

[0035] Figure 1 is a schematic flow chart of an embodiment of the method 100. In step 110 of the method 100, material of the workpiece 1 is removed by means of pulsed laser radiation 2 in order to perform laser drilling or laser cutting on the workpiece 1. In step 120, a fluid 4 is arranged in the rear space 3 behind the area to be penetrated 1a of the workpiece 1 in the beam direction, and the fluid 4 at least partially blocks the passage of the laser radiation 2. The geometric arrangement of the workpiece 1, the laser radiation 2, the rear space 3 and the fluid 4 is as follows: Figure 2 and 3 is shown in more detail in .

[0036] The fluid 4 comprises particles 4a dispersed in a solvent 4b. According to block 121, the particles 4a forming a plasma under the effect of the laser radiation 2 incident in the back space 3 are selected such that the plasma 5 at least partially blocks the passage of the laser radiation 2.

[0037] Various exemplary possibilities and criteria for selecting particles 4 a are shown in block 121 .

[0038] According to block 121a, a threshold value for the instantaneous intensity can be determined based on the intensity distribution over time of the pulses of the laser radiation 2. According to block 121b, particles 4a can then be selected which form the plasma 5 at the latest at an instantaneous intensity corresponding to the threshold value.

[0039] According to block 121c, this can be further refined by additionally determining a limit temperature. According to block 121d, particles 4a can then be selected which form the plasma 5 at the latest at an instantaneous intensity corresponding to the threshold value and at a temperature corresponding to the limit temperature.

[0040] According to block 121 e , the particles 4 a may be selected from materials which, in the absence of a plasma 5 , have a transmittance, a reflectance or an absorbance of 0.9 or more for the wavelength of the laser radiation 2 .

[0041] According to box 121f, the particles 4a can be selected from barium sulfate BaSO4, calcium sulfate CaSO4, aluminum oxide Al2O3, titanium oxide TiO2, zirconium oxide ZrO2 and / or boron nitride BN.

[0042] According to block 121 g , particles 4 a consisting of carbon, silicon carbide SiC, boron carbide B 4 C and / or tungsten carbide WC may be selected.

[0043] According to block 121h, metal-containing droplets can be selected as particles 4a. According to block 121i, these particles can contain gallium or a gallium-indium-tin alloy, for example.

[0044] Furthermore, several exemplary possibilities and criteria for selecting the solvent 4 b are described in block 120 .

[0045] According to block 122, the solvent 4b with the surfactant may be selected so that more particles 4a can be dispersed in a given amount of solvent 4b.

[0046] According to block 123 , a solvent containing hydrocarbons can be selected which does not attack the material of the workpiece 1 and which, under the action of the laser radiation 2 , is not converted into a substance that attacks the material of the workpiece 1 .

[0047] In this case, a solvent 4 b may be selected which comprises an alkane or an alkane mixture, in particular, for example, according to block 123 a .

[0048] Alternatively or in combination therewith, according to block 123b, a solvent 4b having a hydrocarbon may be selected, the hydrocarbon comprising a carbon chain capped and / or functionalized at at least one end. For example, according to block 123c, the solvent 4b may in particular comprise dodecylbenzene C 18 H 30 .

[0049] Figure 2 An exemplary application of the method 100 is schematically shown in a cross-sectional view. In this example, the workpiece 1 is a tube having a hole in a region 1a of its wall, while the region 1b opposite the hole is not damaged. For this purpose, the rear space 3 is filled internally with a fluid 4 containing particles 4a dispersed in a solvent 4b. The concentration and size of the particles 4a are Figure 2 It is shown in a very exaggerated way.

[0050] exist Figure 2 In the transient image shown, the laser radiation 2 has just penetrated the region 1 a of the workpiece 1 and has thus produced the desired hole. The laser radiation 2 now impinges on the particles 4 a in the fluid 4 .

[0051] As in Figure 3 As shown in , this has the effect that the particles 4a form a plasma 5. In this case, the plasma 5 is preferably formed on the side of the particles 4a that respectively faces the laser radiation 2, although this does not exclude that very small particles 4a are completely converted into plasma within a short time.

[0052] If the laser radiation 2 now continues to impinge on the plasma 5, it is at least partially reflected and / or scattered and thus converted into diffuse light 2a that propagates in all directions. This diffuse light 2a is no longer so intense in the region 1b of the workpiece 1 that this region 1b is undesirably damaged.

[0053] Whether the plasma regions 5 of the particles 4a which form the plasma respectively overlap or not depends largely on the (local) number density of the particles 4a, the respective material and the properties of the individual laser pulses and of the entire laser radiation 2. In addition to the plasma regions 5, for example, vapor bubbles of the solvent 4b may form. Thus, the energy of the laser radiation 2 may be dissipated (mainly) into thermal processes by various processes which occur on different time scales.

Claims

1. A method (100) for laser drilling or laser cutting of a workpiece (1), wherein: Material of the workpiece (1) is removed by means of pulsed laser radiation (2), wherein a fluid (4) is arranged in a rear space (3) located behind a region (1a) to be penetrated of the workpiece (1) along the beam direction, and when the laser radiation has penetrated the region (1a), the laser radiation (2) invades the rear space, and the fluid at least partially blocks the passage of the laser radiation (2), wherein the fluid (4) comprises particles (4a) dispersed in a solvent (4b), characterized in that particles (4a) are selected to form plasma (5) under the action of the laser radiation (2) entering the rear space (3), so that the plasma (5) at least partially blocks the passage of the laser radiation (2).

2. The method (100) according to claim 1, wherein: A threshold value for the instantaneous intensity is determined based on the intensity distribution of the pulses of the laser radiation (2) over time, and particles (4a) are selected which form the plasma (5) at the latest at an instantaneous intensity corresponding to the threshold value.

3. The method (100) according to claim 2, wherein: In addition, a limit temperature is determined, and particles (4a) are selected which form a plasma (5) at the latest at a momentary intensity corresponding to the threshold value and at a temperature corresponding to the limit temperature.

4. The method (100) according to any one of claims 1 to 3, wherein: The particles (4a) are selected from the following materials, which have a transmittance, reflectance or absorption of 0.9 or more for the wavelength of the laser radiation (2) under the condition that the plasma (5) is not yet present.

5. The method (100) according to any one of claims 1 to 3, wherein: The metal-containing droplets are selected as particles (4a).

6. The method according to claim 5, wherein: A droplet (100) having gallium or a gallium-indium-tin alloy is selected as the particle (4a).

7. The method (100) according to any one of claims 1 to 3, wherein: The particles (4a) are selected from barium sulfate BaSO4, calcium sulfate CaSO4, aluminum oxide Al2O3, titanium oxide TiO2, zirconium oxide ZrO2 and / or boron nitride BN.

8. The method (100) according to any one of claims 1 to 3, wherein: The particles (4a) are selected from carbon, silicon carbide SiC, boron carbide B4C and / or tungsten carbide WC.

9. The method (100) according to any one of claims 1 to 3 and 6, wherein: A solvent (4b) is selected that has a surfactant.

10. The method (100) according to any one of claims 1 to 3 and 6, wherein: The solvent (4b) is selected to include hydrocarbons which do not attack the material of the workpiece (1) and which are not converted into substances that attack the material of the workpiece (1) under the action of the laser radiation (2), and / or an alcohol.

11. The method (100) according to claim 10, wherein: A solvent (4b) is selected which has an alkane or a mixture of alkanes.

12. The method (100) according to claim 10, wherein: The solvent (4b) is selected to have a hydrocarbon compound comprising a carbon chain that is capped and / or functionalized on at least one end.

13. The method (100) according to claim 12, wherein: Select dodecylbenzene C 18 H 30 of solvent (4b).

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