Method for making very small holes in a workpiece

专利摘要:
In a method of making very small holes on the order of 100 micrometers in a workpiece by means of a laser, which workpiece is stainless steel AISI 316L, a laser source is used which emits a pulsed laser sample with a wavelength of 1030 nm, an average power of 40 W, a maximum pulse energy of 200 micro Joules, a pulse duration of 900 fs, and a repetition frequency of 10 between 200 and 800 kHz.
公开号:NL2026675A
申请号:NL2026675
申请日:2020-10-14
公开日:2021-12-01
发明作者:Van Bussel Joost；Hogenstein Peter；Van Grootel Bas
申请人:Aae B V；
IPC主号:

专利说明:

DESCRIPTION Field of the Invention The invention relates to a method for making very small holes of the order of 100 micrometers in a workpiece by means of a laser, which workpiece made of stainless steel AISI 316L.
Stainless Steel AISI 316L is an 18/8 austenitic stainless steel reinforced with an addition of 2.5% molybdenum to provide superior corrosion resistance to type 304 stainless steel. 316L has improved resistance to pitting corrosion and has excellent resistance to sulfates, phosphates and other salts. 316L has better resistance than standard 18/8 grades to seawater, reducing acids and dissolution of chlorides, bromides and iodines. State of the art A method according to the preamble of claim 1 is generally known. The drawback of the known method is that contamination of the treated surface, recast layer and microcracking, takes place. Heat transfer to adjacent material causes damage to the surface.
Summary of the invention An object of the invention is to provide a method of the type described in the preamble, wherein the above-mentioned drawbacks do not occur. To this end, the method according to the invention is characterized in that a laser source is used which emits a pulsed laser sample with a wavelength between 900 and 1100 nm, an average power between 35 and 45 W, a maximum pulse energy of 200 micro Joules, a pulse duration between the 800 and 1000 fs, and has a repeat frequency between 200 and 800 kHz. Machining with laser ablation has major advantages over regular, traditional and conventional machining techniques, such as: no contamination, no recast layer, no microcracking, no melting zone, no shock waves in melt, no heat transfer to adjacent material, and no damaged surface. Preferably, the laser source has a wavelength of 1030 nm, an average power of 40 W, and a pulse width of 900 fs.
An embodiment of the method according to the invention is characterized in that an inert process gas is injected during the lasering. Due to the injection of inert process gas, the formed plasma plume is “blown away” and the inter-pulse oxidation is counteracted. Brief description of the drawings The invention will be further elucidated below on the basis of an exemplary embodiment of the method according to the invention shown in the drawings. Hereby: Figure 1 shows schematically the process of material removal by means of a laser with an extra Short wavelength. Detailed description of the drawings Figure 1 shows in detail the process of material removal by means of an extra short wavelength laser. Machining with (especially physical cold) laser ablation has major advantages compared to regular, traditional and conventional machining techniques. A few advantages at a glance: e No pollution e No recast layer e No microcracking e No melting zone e No shock waves in the melt e No heat transfer to adjacent material e No damaged surface
All this as shown in figure 1. A plasma plume 5 is briefly formed in the hole 3 each time a short laser pulse 7 hits the material 1. The laser pulses 7 are focused by a lens 9. The surface 11 next to the formed hole is not damaged in this case.
Machining (ablation) with laser pulses is based on the following principle / foundation: A “cloud” of photons with a certain high energy / at a certain wavelength is sent towards an atom. The photons temporarily push the electrons out of their atomic orbit (result: ionization / plasma) so that the piece of material leaves the mother material. We see this as machining.
In physical cold laser ablation, the sum of the energy in the photon cloud will have to be equal to the sum of the energy with which the electrons bond to their atomic nucleus. This is a single model which does not occur in practice. In practice we are dealing with: - On the light/pulse side, the sum of the energy is determined by: o Wavelength o Pulse length o Spot diameter o Energy intensity - On the material side, the sum of the energy is determined by: o Reflectivity of the irradiated surface o Penetration depth (wavelength is characteristic) o Chemical composition of the irradiated material volume ... this determines the cold ablation energy threshold value.
This means in advance that with physical cold laser ablation the material must be very consistent in terms of chemical composition, homogeneity, grain structure, etc.
So if the energy balance of the light pulse is smaller than the energy balance of the material then 100% physical cold laser ablation is not possible. Instead, a “warm” laser is ablated, a portion of the light pulse (first oncoming portion) performs cold ablation, and the rest of the energy from the light pulse performs warm ablation.
With warm ablation, the photons only have the power to enter the electron orbits, causing temporary disruption of the energy balance in the atom. Photons take higher orbits in the atom and the electrons present experience friction, consequence: heat development, consequence: material is released from mother material (eventually also gas formation and plasma).
The inertia due to the lack of momentum ensures that surrounding material is also heated up.
For cold ablation to be applied to an AISI 316L workpiece, it is then strictly necessary that the material, its surface and homogeneity are extremely sharp; that the cold ablation threshold is known and constant with respect to the energy supplied in the light pulse.
Means that the allowable variation in AISI 316 L in terms of chemical composition and homogeneous distribution creates an ablation threshold window so that the energy balance cannot be made exactly cold. Variations in chemically permissible composition: Table 1: Composition AIST 316L Ee ee [eea [LE] So the proportion of iron in AISI 316L may simply be MAX 72% or MIN 62%. In addition, the element nickel specifically has unpleasant properties with regard to (cold) ablation and may vary between 10% and 14%.
Specifically in the outokumpu material, the roll direction and its effects on structure are still present. Solution annealing/1120°C (if processed in the material by outokumpu ex factory) does not resolve the rolling direction.
Apropos annealing: The behavior of the material AISI 316L after our current dual annealing steps around 900°C leads to a change in grain structure and the possible trigger for sentisitation. Also for the "lukewarm" laser ablation, this annealing is deadly and will therefore be reset to a conservative value between 450°C and 650°C.
By definition, physical cold laser ablation on an alloy such as AISI 316L is virtually impossible. What may come close is an LVM quality that achieves homogeneity due to remelting and is controlled in its melt for exact chemical composition with a then known cold ablation threshold. The ultra short laser pulse generator 5 The most commonly used US laser pulse generator is constructed as follows. A solid-state seed laser makes constant low power pulses of a fixed fs pulse length, in a frequency (depending on manufacturer / device) 200 KHz — 1 MHz. In software mode (i.e. adjustable), a pulse is selectively taken from the SSS strand by means of a pulse Picker.
Each pulse is optically pulled apart, filled with energy by an amplifier (oscillator), after which the filled pulse is compressed again. After the compressed pulse, the pulse may still be converted from Gausian distribution to rectangular in order to arrive as evenly as possible as a rectangle at the target material.
Creating a high-intensity pulse in the range 0-250 fs is still very difficult, while amplifying the energy in the amplifier, this high-intensity pulse has to be transferred several times between the mirror and oscillator and the reflection loss (%) is converted to heat causing this mirror/oscillator to heat up. Ultimately, these parts will wear out and break after the window shift process. Cannot be used for stable IH Mk 6.3 production.
A high-energy pulse in the 150-250 fs range also has such a high energy density that the (fused silica) optics between source and material must be very large (= expensive) in order not to deform / melt. These lens costs are recouped per hole made.
The material removal rate of femto lasers around 150-250 fs is very low based on a very expensive machine, as a result the costs per hole get unnecessarily out of hand. The Trumpf Trumicro series 5000 Femto edition The applied source has a fixed wavelength of 1030 nm , pulse length of 900 fs , max pulse energy of 200u4J and a max repetition rate of 800 KHz. Means that lukewarm laser ablation is applied by definition; so part cold and part warm. This is a good choice industrially as it is a stable 24/7 source.
About the only variable that can be controlled by software is the pulse repetition rate, the selective taking of pulses from the SSS pulse generator by means of the pulse picker. In addition, there is so much material present in the workpiece around the machining (heatsink) that the warm part of the ablation need not have any effect on the material; since the repetition rate is not too high. The pulse source is further provided with module internal measurements around the pulse energy. The permissibility of the tepid ablation also ensures that the focal intensity need not be spot-on on the material surface to be ablate; there is no steering on the exact energy balance. This means that the expensive equipment is not necessary for this, and that the time loss of stage adjustment in between is not included in the machining time. Could be that off focus causes the balance to shift cold — warm, causing hole geometry to change.
The Pulse Repetition Rate is the most important variable. The number of pulses per unit time is decisive in the balance between permissible thermal load (without material damage) versus the MRR (the costs per hole). Furthermore, the plasma plume formed must have time to clear before the next pulse arrives. If the plasma plume is not gone, the next pulse loses energy intensity before arriving at the material.
Although the invention has been elucidated in the foregoing with reference to the drawings, it should be noted that the invention is by no means limited to the embodiment shown in the drawings. The invention also extends to all embodiments deviating from the embodiment shown in the drawings within the framework defined by the claims.

权利要求:
Claims (3)
[1]
Method for making very small holes of the order of 100 micrometers in a workpiece by means of a laser, which workpiece is made of stainless steel AISI 316L, characterized in that a laser source is used which emits a pulsed laser sample with a wavelength between 900 and 1100 nm, an average power between 35 and 45 W, a maximum pulse energy of 200 micro Joules, a pulse duration between 800 and 1000 fs, and a repetition frequency between 200 and 800 KHz.
[2]
Method according to claim 1, characterized in that the laser source has a wavelength of 1030 nm, has an average power of 40 W, and has a pulse duration of 900 fs.
[3]
Process according to Claim 1 or 2, characterized in that inert process gas is injected during the lasering.

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