WO2019020741A1

WO2019020741A1 - Method for laser drilling or laser cutting of a workpiece with a protective liquid

Info

Publication number: WO2019020741A1
Application number: PCT/EP2018/070279
Authority: WO
Inventors: Csaba Laszlo SAJTI; Andreas Schwenke; Claudia UNGER; Jürgen Koch
Original assignee: Laser Zentrum Hannover E.V.
Priority date: 2017-07-26
Filing date: 2018-07-26
Publication date: 2019-01-31
Also published as: DE102017116943B4; EP3658326A1; WO2019020741A9; DE102017116943A1

Abstract

The invention relates to a method for laser drilling or laser cutting of a workpiece (2), wherein electromagnetic radiation (10) emitted by a laser strikes the workpiece (2) and a liquid is situated on the side of the workpiece (2) facing away from the laser, said liquid containing particles such that the electromagnetic radiation (10) emitted by the laser strikes the particles once the electromagnetic radiation (10) has penetrated through the workpiece (2), wherein the particles contain nanoparticles of a first material, which are adsorbed on microparticles, wherein the microparticles are aggregated nanoparticles of a second material.

Description

METHOD FOR LASER DRILLING OR LASER CUTTING ONE

WORKPIECES WITH A PROTECTIVE LIQUID

The invention relates to a method for laser drilling or laser cutting a workpiece, wherein emitted by a laser electromagnetic radiation strikes the workpiece and is located on a side facing away from the laser of the workpiece, a liquid in which particles are contained, so that emitted by the laser electromagnetic radiation hits the particles when the electromagnetic radiation has penetrated the workpiece.

To produce holes with high aspect ratio and very good edge quality by laser in an economical process time, pulsed lasers with high pulse energy are required. After the electromagnetic radiation emitted by these pulsed lasers has penetrated the workpiece, it constitutes a danger for further materials and / or persons located in the beam path. In particular if further material is located only a short distance from the borehole outlet side, this material will be in the Usually damaged by the emerging laser radiation. The drilling or cutting process can not be stopped before the onset of damage, since the through hole in the workpiece after opening must still be brought to the required shape. The damage that occurs is unacceptable for many applications, such as laser drilling injectors or creating cooling holes in turbine blades. Even when cutting tubing with a small inner diameter, for example, in the manufacture of medical stents, the problem may occur.

In order to protect the material located behind the borehole or cutting hole, different approaches are known from the prior art. In all, a mate rial is arranged in the beam direction of the electromagnetic radiation behind the workpiece in order to prevent the electromagnetic radiation from causing damage. In principle, solids, circulating liquids and fluids or particle suspensions come into question. From US Pat. No. 6,303,901 B1 it is known, for example, to arrange a monoatomic or molecular gas in the intermediate space between the material to be drilled or cut and a further material behind, which absorbs the photons of the laser radiation and thus forms a high-density plasma. In another embodiment, the gap is filled with a solid or high viscosity liquid.

In particular, the use of solids to trap the electromagnetic radiation has some disadvantages. In order to use such solids, such as ceramic rods or plates, the cavity between the workpiece to be cut on the back must be easily accessible from the outside. In addition, the intermediate solid is removed and must either be pushed or renewed. In addition, the removal particles of the solid, which are replaced by the electromagnetic radiation of the laser must be easily removed from the cavity at the end of the drilling process.

From WO 2007/089469 A2 it is known to fill the intermediate space with a dry, stable powder, for example aluminum oxide powder. The size of the individual particles is chosen between 10μηη and 1 .ΟΟΟμιτι. Even with this solution, however, the powder has to be removed from the cavity after drilling and, if necessary, fresh powder must be replenished for the next drilling.

On the other hand, WO 2000/69594 A1 and US Pat. No. 6,365,891 B1 suggest using a liquid in which dyestuffs are present. These dyes can be chosen so that they absorb in particular photons having the wavelength of the laser light. It comes in the dye molecules to an electronic excitation, in which electrons are raised to higher energy levels. The disadvantage, however, is that such dyes fade relatively quickly and become transparent to the electromagnetic radiation of the laser. In addition, they usually have only a relatively small absorption cross-section.

In addition to the dyes, microparticles may be contained in the liquid which scatter the incident laser light and so reduce the energy density of the electromagnetic radiation, so that it is no longer sufficient, in particular in combination with the absorption by the dye to remove material in undesirable places. For this, however, the particle concentration in the liquid must be very high. This leads to a high viscosity, which prevents a high flow velocity in narrow cavities. This has the disadvantage, on the one hand, that the highly viscous liquid is difficult to remove from, in particular, narrow cavities, and, on the other hand, if the flow velocity is low, there is a risk that the electromagnetic radiation locally vaporizes the liquid and thus no longer provides protection in this area lying materials. It creates bubbles through which the laser radiation can pass almost unhindered.

DE 10 2013 212 665 proposes, therefore, to use as back-room protection a liquid in which nanoparticles are contained, in which the predominant part of the electromagnetic radiation is absorbed in collective excitations, for example plasmons.

For example, spherical gold nanoparticles having a particle size of 3 nm to 10 nm excellently absorb in the laser wavelength range of 515 nm to 532 nm, but do not provide any backspace protection for near-infrared laser wavelengths such as 1030 nm and 1064 nm. However, classic laser wavelengths for cutting and drilling are in the near-infrared range. Larger gold nanoparticles with a diameter of about 100 nm to 150 nm have a collective absorption of electromagnetic radiation with a wavelength of about 1000 nm to 1 100 nm. These gold nanoparticles, however, are fragmented by the absorbed energy into smaller nanoparticles with a diameter of one nm to 20 nm, so that the larger gold nanoparticles with their diameter of 100 nm to 150 nm do not provide stable backspace protection.

The invention is therefore based on the object, a method according to the preamble of claim 1 such as known from DE 10 2013 212 665, so further develop that for electromagnetic radiation in the near infrared region at wavelengths of, for example, 1030 nm and 1064 nm as well as electromagnetic radiation in the green visible region at wavelengths of, for example, 515 nm and 532 nm, a time-stable back space protection is possible. This is advantageous because, for example, pulsed near-infrared lasers are converted by additional structures to a frequency-doubled laser. For industriel-le applications, it is therefore of great advantage to use a material as back space protection, which covers both wavelength ranges stable over time.

The invention achieves the stated object by a method according to the preamble of claim 1, characterized in that the particles contain nanoparticles of a first material which are adsorbed on microparticles, wherein the microparticles are aggregated nanoparticles of a second material.

The interaction of microparticles and nanoparticles in a single suspension is known in the art. The nanoparticles are responsible for the absorption of the electromagnetic radiation and the microparticles for the scattering of the radiation. The arrangement of nanoparticles on scattering micro-particles can already be found in DE 10 2013 212 665 A1. Unlike this prior art solution, however, microparticles that are aggregated nanoparticles are used in the present invention. This solution has some surprising advantages. First of all, it is surprising that such aggregated nanoparticles, which form microparticles that are hit by the incident electromagnetic radiation of the pulsed laser, survive this radiation unscathed. In addition, it is surprisingly found that the previously known from the prior art division of the tasks in the absorption of electromagnetic radiation by the nanoparticles and scattering of the electromagnetic radiation by the microparticles by the inventive design does not appear to be maintained. Rather, it has been found, for example, that nanoparticles of gold, which exhibit a plasmon resonance at an energy that corresponds to an electromagnetic radiation of a wavelength of less than 600 nm, in a method according to the invention have excellent backspace characteristics. protective properties even with electromagnetic laser radiation having a wavelength of 1 .030 nm shows. Consequently, laser radiation can be absorbed for which the nanoparticles alone would not be suitable.

Advantageously, the first material is different from the second material.

Preferably, the electromagnetic radiation has a wavelength between at least 500 nm and at most 1 100 nm, preferably between at least 1030 nm and at most 1064 nm.

In a preferred embodiment, the first material is a metal, preferably gold, silver, platinum, palladium or copper, in particular in the form of copper selenide, or carbon, in particular in the form of carbon black, which is also referred to as "carbon", or a Selenide, especially copper selenide, or sulfite. The material may contain or consist of all or a combination thereof. The optimal material can be matched to the shape, in particular the geometric contour of the nanoparticles and, of course, the laser wave length to be absorbed.

In a preferred embodiment, at least a predominant proportion of the nanoparticles of the first material, preferably all nanoparticles of the first material, have a particle size between 1 nm and 99 nm, preferably between 2 nm and 50 nm.

Advantageously, the second material contains titanium dioxide, aluminum dioxide, silicon dioxide, silicon carbide, barium sulfate, calcium phosphate or zinc oxide, or consists thereof or of a combination of several of the substances mentioned.

Preferably, the liquid is water or oil, especially a mineral oil. It is also possible to use natural, for example vegetable oils or mixtures of different oils. In particular, for static processes in which the liquid is not or hardly moved and thus long influenced by the electromagnetic radiation, the use of an oil has proven to be advantageous because water evaporates through the incident laser energy which greatly reduces or completely destroys the protective effect.

Advantageously, the microparticles have an average size of between 0.1 and 10 μm, preferably between 0.2 μm and 0.4 μm, preferably of 0.3 μm, and preferably have the nanoparticles of the second material have a particle size between 1 nm and 99 nm.

Advantageously, the concentration of microparticles in the liquid is between 50 mg / ml and 500 mg / ml, preferably between 100 mg / ml and 200 mg / ml.

Preferably, a proportion of the first material to a total amount of the particles contained in the liquid less than 30 mass percent, preferably between 3 and 15 percent by mass, particularly preferably 5 percent by mass.

In a particularly simple embodiment of the method, the microparticles are produced by dispersing aggregated, powdery nanoparticles of the second material by the use of ultrasound in the liquid and preferably present as nanoparticle aggregates having an average particle size in the range of 0.3 μm.

The invention moreover achieves the stated object by a liquid for use in such a method, which is characterized in that it contains particles which contain nanoparticles of a first material which are adsorbed on microparticles, the microparticles being aggregated nanoparticles of the second material are.

The inventive configuration of the method and the liquid ensures that the liquid contains a large number, preferably only, of microparticles, and hardly or preferably no nanoparticles are contained which are not adsorbed on microparticles. As a result, a better separability of the particles contained after drilling or cutting is optionally achieved. The preferably used aggregated nanoparticles which form the microparticles of the second material are preferably composed of nanoparticles having a primary particle size between 1 nm and 99 nm. The term "aggregate" describes firmly interconnected primary particles, which can not be broken by the use of known methods, such as ultrasound.

With the aid of the accompanying drawings, an embodiment of the present invention will be explained in more detail below. It shows:

FIG. 1 shows the schematic representation of back wall damage during laser drilling;

FIG. 2 shows the transmitted laser power as a function of time when carrying out a method according to a first exemplary embodiment of the present invention,

Figure 3 - the transmitted laser power as a function of time for different fluids used, and

Figure 4 - the transmitted laser power as a function of time for different test liquids and.

Figure 5 - the schematic representation of different particles in different liquids.

Figure 1 shows a workpiece 2, inside which a cavity 4 is located. In the workpiece 2, a through hole 6 is to be drilled, for which purpose a front side 8 of the workpiece 2 is irradiated with laser radiation 10. In the stage of the method shown in FIG. 1, the through-hole 6 is already opened, so that the laser radiation 10 penetrates into the workpiece 2. In the process, the laser radiation 10 strikes a back space material 12 in the rear space and leads to damage 14. This is to be avoided with the method according to the present invention. FIG. 2 shows an irradiation time in seconds on the x-axis. It is the duration that the liquid in which the particles are contained is irradiated with the laser's electro-magnetic radiation. On the vertical axis or y-axis, the transmitted power is plotted in mW. A picosecond laser with a wavelength of 1 .030nm, a repetition rate of 200kHz and a pulse duration of 7ps was used. The beam of the laser has a diameter of 3mm, the maximum power of the laser is 50W. The diagrams shown were produced at a laser power of 35%, ie an irradiated power of 17.5W. The laser beam focused (focal length 100mm) passed through a cuvette with 2mm path length. The cuvette stood at a distance of 50mm to the focal point.

The illustration shown in FIG. 2 shows the transmitted laser power as a function of the irradiation duration. The liquid used was water. The first material of the nanoparticles was copper selenide, the second material titanium dioxide.

It can be seen that in the first 8 seconds the transmitted laser power is close to 0 mW and the liquid offers excellent backspace protection. After about 10 seconds, the transmitted laser power increases sharply. Since the fluid can not move in the test cuvette, it is therefore a static experimental setup. Therefore, the liquid, so the water begins to evaporate in this temporal range, whereby the back space protection is greatly weakened and therefore the transmitted laser power increases sharply.

To prepare the test liquid with the particles contained therein, colloids of the nanoparticles of the first material, in the present case copper selenide, are first mixed with water. Preferably, these have an average particle size of about 10 nm. In a second starting liquid, which is also water, are aggregates, which are composed of titanium dioxide nanoparticles and have a mean aggregate size of 0.3 μιτι used. This second part-liquid is added to the first part-liquid, which contains the copper selenide particles contained in water, and shaken. It comes without further action to adsorb the copper selenide nanoparticles on the surface of the Titanium dioxide. After adsorption, preferably no free nanoparticles are found in the solution. The adsorption is advantageously 100%. The particle concentration is 100 mg / nnl.

Figure 3 also shows the transmitted laser power on the y-axis in mW as a function of the irradiation time in seconds, when irradiated with a picosecond laser having a wavelength of 1030 nm. The particles used are gold nanoparticles adsorbed on microparticles of titanium dioxide are. A solid line shows the result of a first test liquid in which the liquid is water. The dashed line represents a second test liquid in which the liquid is oil. It can be clearly seen that with short irradiation times, a similarly good, namely almost complete absorption of the laser power is ensured. Only after about 10 seconds, the liquid, which has water as a base liquid, begins to fade, as it evaporates due to the strong absorption of the irradiated laser power. Oil has a much higher boiling point than water, so the spillway protection will last longer. It should be noted that in all the embodiments shown, the evaporation of the liquid water can be avoided when a dynamic system is used, that is, the water is in motion. This is by the low viscosity quite possible and of great advantage.

FIG. 4 again shows the transmitted laser power in mW as a function of the irradiation time in seconds, in the case of irradiation with a picosecond laser having a wavelength of 1030 nm. The result of a test fluid which is not nanoparticles of the first material but exclusively microparticles is shown by a solid line which are aggregated nanoparticles of titanium dioxide. Not shown is the result of exclusively gold nanoparticles in water with an average particle size of 10 nm and a concentration of 4 mg / ml. In this liquid, the transmitted laser power is over one watt, so that no effective backspace protection is achieved. A dashed line shows the result for a test liquid containing gold nanoparticles in addition to these microparticles. However, the test fluid was supplemented with a relatively large amount of sodium citrate, so that adsorption of the gold nanoparticles on the surface of the titanium dioxide was prevented. Only with the dotted line, the result for a liquid is shown, which can be used in a method according to the invention. It contains 50 mg / ml particles, which consist of 1 .5 mg / ml gold nanoparticles with a mean size of 10 nm, which are arranged on 48.5 mg / ml microparticles with an average particle size of 0.3 μιτι of aggregated titanium dioxide nanoparticles ,

FIG. 5 schematically shows different particle configurations. In the left-hand area, three microparticles 16 are shown, each of which is aggregated from nanoparticles 18 of a second material. In the middle region, additional nanoparticles 20 of a first material have been added to this microparticle 16, but these are arranged side by side in isolation. In the right-hand illustration, a liquid is shown schematically, as it can be used in a method according to an embodiment of the present invention. The nanoparticles 20 of the first material are absorbed on the microparticles 16 of nanoparticles 18 of the second material.

LIST OF REFERENCE NUMBERS

2 workpiece

4 cavity

6 through hole

8 front side

10 laser radiation

12 back space material

14 damage

16 microparticles

18 nanoparticles of a second material

20 nanoparticles of a first material

Claims

claims

1 . Method for laser drilling or laser cutting a workpiece (2), wherein

- Exposed by a laser electromagnetic radiation (10) hits the workpiece (2) and

a liquid, in which particles are contained, is located on a side of the workpiece (2) facing away from the laser;

- So that the electromagnetic radiation emitted by the laser (10) hits the particles when the electromagnetic radiation (10) has penetrated the workpiece (2),

characterized in that

the particles contain nanoparticles of a first material which are adsorbed on microparticles, the microparticles being aggregated nanoparticles of a second material.

2. The method according to claim 1, characterized in that the first material and the second material are different.

3. The method according to claim 1 or 2, characterized in that the electromagnetic radiation (10) has a wavelength of at least 500 nm and at most 1 100 nm, preferably at least 1030 nm and at most 1064 nm.

4. The method of claim 1, 2 or 3, characterized in that the first material is a metal, preferably gold, silver, platinum, palladium or copper, in particular in the form of copper selenide, or carbon, soot ("carbon black"), or Selenide, in particular copper selenide or sulfides contains or consists of or a combination of several thereof.

5. The method according to any one of the preceding claims, characterized in that at least a predominant proportion of the nanoparticles of the first Matenals, preferably all nanoparticles of the first material have a particle size between 1 nm and 99 nm, preferably between 2 nm and 50 nm.

6. The method according to any one of the preceding claims, characterized in that the second material contains or consists of titanium dioxide, alumina, silica, Siliziunnkarbid, barium sulfate, calcium phosphate or zinc oxide or a combination of several thereof.

7. The method according to any one of the preceding claims, characterized in that the liquid is water or an oil, in particular a mineral oil.

8. The method according to any one of the preceding claims, characterized in that the microparticles have a mean size between 0.1 μιτι and 10 μιτι, preferably between 0.2 μιτι and 0.4 μιτι, preferably of 0.3 μιτι have and preferably the nanoparticles of the second material have a particle size between 1 nm and 99 nm.

9. The method according to any one of the preceding claims, characterized in that a concentration of the microparticles in the liquid between 50 mg / ml and 500 mg / ml, preferably between 100 mg / ml and 200 mg / ml.

10. The method according to any one of the preceding claims, characterized in that a proportion of the first material to a total amount of the particles contained in the liquid is less than 30 percent by mass, preferably between 3 and 15 percent by mass, particularly preferably 5 percent by mass.

1 1. Liquid for use in a method according to any one of the preceding claims, characterized in that the liquid contains particles containing nanoparticles of a first material adsorbed on microparticles, the microparticles being aggregated nanoparticles of a second material.