A method for suppressing wall damage in ultrafast laser processing

Abstract

The application provides a wall damage inhibition method in ultrafast laser processing, aiming at the wall damage situation in the processing, and executing the subsequent steps; selecting a filling material as a typical sample, correcting the damage threshold and the material band gap width relationship formula, quickly generating a multi-pulse damage threshold table of all filling materials; using the multi-pulse damage threshold table, selecting a material with a multi-pulse damage threshold greater than the laser pulse energy density selected in the processing as the filling material in the processing; filling the selected filling material in the cavity of the to-be-processed sample, using the optimal processing parameters to complete the sample processing, pouring the filling material after finishing, checking the sample quality, if not satisfied, selecting other materials meeting the conditions as the filling material according to the requirements, if satisfied with the processing requirements, outputting the result for the formal workpiece manufacturing.

Description

Technical Field

[0001] This invention relates to the field of laser processing technology, and more specifically to a processing method for suppressing damage to the rear wall during laser processing of enclosed cavities. Background Technology

[0002] Hole machining is a crucial machining process that constitutes a significant portion of parts manufacturing, with important and widespread applications in aerospace, aviation, electronics, instrumentation, automatic control, and medical device manufacturing, among other scientific and technological and industrial production fields. Statistics show that hole machining accounts for approximately one-third of the total machining volume and one-quarter of the machining time. Machining precision micro-circular holes or irregularly shaped holes on closed cavity surfaces, such as the film cooling holes of turbine blades in aerospace engines and the fuel injection holes in ramjet engines, has always been a technical challenge in the field of machining. This is because, firstly, due to the extremely small hole diameter—typically less than 0.2 mm—micro-hole machining almost entirely results in deep holes with a large length-to-diameter ratio (usually greater than 5). Secondly, the materials of parts requiring micro-hole machining are generally high-strength, high-hardness, and difficult-to-machine materials, such as high-temperature alloys, titanium alloys, cemented carbide, ceramics, and diamond. Therefore, conventional machining methods (CNC machining, EDM, electrochemical machining) are extremely difficult to use for machining these precision hole structures.

[0003] Laser manufacturing, with its advantages in flexibility, efficiency, precision, and material applicability, is an ideal means of processing micro-hole structures. Laser drilling is not limited by the mechanical properties of materials such as hardness, rigidity, strength, and brittleness, and can process almost any material. The energy of the laser beam is easy to adjust and the adjustment process is very rapid. Laser drilling parameters are easier to optimize than other drilling methods, so a much larger aspect ratio can be obtained than electrical discharge machining (EDM) and mechanical drilling. Furthermore, the diameter of the focused laser beam (especially ultrafast lasers) can be controlled to the micrometer or even nanometer scale, enabling the processing of extremely small structures. However, when laser processing is used to machine precision micro-circular holes or irregularly shaped holes on the surfaces of closed cavities, such as the film cooling holes of engine turbine blades and the fuel injection holes of ramjet engines, the laser beam can easily cause wall damage when penetrating one side of the cavity, leading to workpiece failure.

[0004] Therefore, there is an urgent need to develop a laser drilling method that can achieve precision drilling in narrow cavity structures without damaging the opposite wall structure. Summary of the Invention

[0005] The technical problem solved by this invention is that, in the existing technology, ultrafast lasers are prone to causing wall damage after processing one side of the material during the processing of thin, closed cavities. This invention proposes a laser processing method that overcomes the above difficulties by quickly selecting an appropriate material to fill the cavity.

[0006] The solution of this invention is: a method for suppressing wall damage in ultrafast laser processing, comprising:

[0007] Step 1: Determine the multi-pulse damage threshold F of the material to be processed under the pulse width and wavelength conditions of the laser used. th_1 When the optimal processing parameters for the material are determined, the laser power density F_z corresponding to the cavity depth as the defocus distance Z is calculated to determine whether the material's multi-pulse damage threshold is reached, i.e. whether wall damage will occur during the processing; if wall damage occurs, subsequent steps are executed.

[0008] Step 2: Select a filling material as a typical sample and determine the single-pulse threshold F of the material under the laser pulse width and wavelength conditions. th_1 (A) and multi-pulse damage threshold F th_N (A), and using it as a benchmark, solve the undetermined coefficients in the relationship between the damage threshold and the material bandgap width, and quickly generate a table of all multi-pulse damage thresholds that can be used as filling materials.

[0009] The relationship between the damage threshold and the material bandgap is F. th (N)=(kE g +b)τ 0.3 N S-1 Where k is a unit of mass in J / cm -2 fs -0.3 eV -1 The constant, b is -0.14 Jcm -2 fs -0.3 τ is the laser pulse width (fs), N is the in-situ equivalent number of knocks, and S is the material hatching coefficient;

[0010] Step 3: Using the multi-pulse damage threshold table, select a material with a multi-pulse damage threshold greater than the laser pulse energy density used during processing as the filling material during processing;

[0011] Step 4: Fill the cavity of the specimen with the selected filling material, complete the specimen processing using the optimal processing parameters, pour out the filling material after completion, check the specimen quality. If it does not meet the requirements, jump to step 3, and select other materials that meet the requirements as filling materials. If the processing requirements are met, the output result is used for the manufacturing of the formal workpiece.

[0012] Furthermore, the material to be processed is a solid material.

[0013] Furthermore, when the focused spot in step 2) is a Gaussian spot, the original equivalent number of impacts N is estimated using the following formula:

[0014]

[0015] Where: d x Let v be the laser spot diameter parallel to the scan line direction, f be the laser scanning speed, f be the pulse repetition frequency, and d be the laser beam diameter. y Ly represents the spot diameter parallel to the scan line direction, and Ly represents the spot spacing perpendicular to the scan line direction.

[0016] Furthermore, the band gap width E of the filling material g The relationship between the laser wavelength λ and the wavelength λ satisfies hc / λ<E g / 2, where h and c are Planck's constant and the speed of light in a vacuum, respectively.

[0017] Furthermore, the laser emits pulsed laser light with a single pulse lasting less than 20 ps.

[0018] Furthermore, when the focused spot in step 1 is a Gaussian spot, the cavity depth is used as the defocus distance Z, and the corresponding laser power density F th _z is calculated using the following formula:

[0019]

[0020] Where P is the laser power, Z R Let f be the Rayleigh length, f be the pulse repetition frequency, w be the spot radius at the beam waist, and 1 / e 2 Type definition.

[0021] Furthermore, the filling material in step 2 includes, but is not limited to, wide bandgap materials: titanium oxide, silicon nitride, zirconium oxide, aluminum oxide, aluminum nitride, gallium nitride, and sodium chloride.

[0022] Furthermore, in step 4, the filling material is a solid particle with a diameter that is 0.1-0.5 times the cavity height.

[0023] Furthermore, the measured single-pulse threshold F of the material is... th_1 (A) and multi-pulse damage threshold F th_N (A) Substitute the relationship between the damage threshold and the material bandgap width into the equation and solve for the unknown coefficients k and s in the equation.

[0024] A laser processing apparatus for performing the aforementioned method for suppressing wall damage in ultrafast laser processing.

[0025] The advantages of this invention compared to the prior art are:

[0026] (1) The present invention provides a method for suppressing wall damage in ultrafast laser processing. Compared with the traditional method of reducing laser pulse energy density or using short focal length and small spot laser equipment to process thin cavity closed cavities, it can significantly improve processing efficiency and reduce the difficulty of laser parameter control. It is an economical and easy-to-operate processing method.

[0027] (2) The present invention provides a method for suppressing wall-to-wall damage in ultrafast laser processing, which involves pre-filling the cavity with microparticles made from selected fillers. During processing, the filler particles block the penetrating laser pulses, thus suppressing wall-to-wall damage. Specifically, firstly, it is determined whether the selected laser pulse parameters will cause wall-to-wall damage; if wall-to-wall damage will occur, then readily available filler material A is used as a typical sample, and the multi-pulse damage threshold F of this material under the laser pulse width and wavelength conditions is determined. th_N (A), and use it as a reference value, then use this reference value to correct the relationship between the damage threshold and the material bandgap width, and based on the material bandgap width E g The damage threshold of various optional filling materials under the pulsed laser is quickly generated; a material with a multi-pulse damage threshold greater than the laser pulse energy density is selected as the filling material during processing, and the cavity is filled with near-circular microparticles of the selected material. After further processing, the filling material is poured out to suppress wall damage.

[0028] (3) This invention provides a method for suppressing wall damage in ultrafast laser processing, and gives a rapid estimation formula for the multi-pulse damage threshold of the filler material under the action of the pulsed laser. According to this formula, the damage threshold of various filler materials under the action of the pulsed laser can be quickly estimated, providing great convenience for the rapid selection of filler materials. The proposed method is applicable to various laser subtractive manufacturing methods such as laser cutting of internal cavity structures, laser milling, and laser rotary cutting for hole making. Moreover, the wall damage suppression method has universality for material types, and the degree of damage can be flexibly adjusted as needed. Therefore, this method is also a flexible and universal method. Attached Figure Description

[0029] Figure 1 This document describes the process of a method for suppressing wall damage in ultrafast laser processing according to the present invention. Detailed Implementation

[0030] The present invention will be further described below with reference to the embodiments.

[0031] A method for suppressing wall damage in ultrafast laser processing, employing Figure 1 The processing flow shown is as follows:

[0032] Step 1: Turn on the ultrafast laser of the processing system and adjust the relative positions and orientations of the components. Use the classical area epitaxy method to determine the multi-pulse removal threshold of the material being processed, and obtain the optimal processing parameters (including laser pulse energy density, in-situ equivalent impact count, corresponding laser power, and spot overlap rate) using conventional methods. Calculate whether the processing parameters will cause wall damage. If no contrast damage is caused, proceed directly to the manufacturing of the final workpiece. If wall damage occurs, proceed to the next step.

[0033] When the focused spot is a Gaussian spot, with the cavity depth as the defocus distance Z, the corresponding laser power density F th _z is calculated using the following formula:

[0034]

[0035] Where P is the laser power, Z R Let f be the Rayleigh length, f be the pulse repetition frequency, and w be the beam radius at the beam waist (1 / e). 2 (Type definition).

[0036] When the focused spot is of other types, the laser power density at defocus Z is calculated according to the conventional algorithm in the field.

[0037] Step 2: Select an easily obtainable filler material A as a typical sample and determine the single-pulse damage threshold F of this material under the laser pulse width and wavelength conditions. th_1 (A) and multi-pulse damage threshold F th_N (A), and using it as a benchmark, solve for the undetermined coefficients k and s in the relationship between the damage threshold and the material bandgap:

[0038] F th (N)=(kE g +b)τ 0.3 N S-1

[0039] Where k is a dimensionless quantity in J / cm -2 fs -0.3 eV -1 The constant, b is -0.14 Jcm -2 fs -0.3 τ is the laser pulse width (fs), N is the in-situ equivalent number of impacts, and S is the material incubation coefficient.

[0040] When the focused spot in step 2 is a Gaussian spot, the original equivalent number of impacts N is estimated using the following formula:

[0041]

[0042] Where: d xLet v be the laser spot diameter parallel to the scan line direction, f be the laser scanning speed, f be the pulse repetition frequency, and d be the laser beam diameter. y Ly represents the spot diameter parallel to the scan line direction, and Ly represents the spot spacing perpendicular to the scan line direction. The filler material bandgap width E... g The relationship between the laser wavelength λ and the wavelength λ satisfies hc / λ<E g / 2, where h and c are Planck's constant and the speed of light in a vacuum, respectively.

[0043] Then, based on the bandgap width of different materials, a multi-pulse damage threshold table of all materials that can be used as filler materials is quickly generated.

[0044] Filler material A should possess characteristics such as opacity, high melting point, chemical stability, and a high removal threshold for the laser pulse used. This includes, but is not limited to, wide bandgap materials: titanium oxide, silicon nitride, zirconium oxide, aluminum oxide, aluminum nitride, gallium nitride, sodium chloride, etc.

[0045] Step 3: Using the multi-pulse damage threshold table of filling materials generated in Step 2, select materials with multi-pulse damage thresholds greater than the laser pulse energy density used during processing as filling materials during processing.

[0046] Step 4: Fill the cavity of the specimen with the near-circular microparticles of the filling material selected in Step 3 (the diameter should be 0.1-0.5 times the cavity height; if the diameter is too large, it will cause through-gap, but the diameter should not be too small, causing it to be adsorbed to the inner wall of the cavity and difficult to be discharged). Complete the specimen processing using the optimal processing parameters. After completion, pour out the microparticles of the filling material and check the quality of the specimen using an endoscope or non-destructive testing methods. If it does not meet the requirements, skip to Step 3 and select a material with a higher processing threshold as the filling material according to the requirements. If the processing requirements are met, the output result is used for the manufacturing of the formal workpiece.

[0047] The following is a further explanation with reference to specific embodiments:

[0048] The workpiece being processed is an oil injection hole for a stamping engine. The material is a nickel-based high-temperature alloy, the hole diameter is 0.3 mm, the material wall thickness is 1 mm, and the cavity height is 2 mm. Ultrafast laser processing must ensure no visible damage to the wall surface.

[0049] The laser emits light at a center wavelength of 1030 nm and can generate a single pulse sequence with a duration of 300 fs and a pulse frequency adjustable from 0.1 MHz to 2 MHz. The maximum single pulse energy is 1 mJ, the laser spot diameter is about 40 μm, and the half-focal depth is 2 mm.

[0050] First, the ultrafast laser of the machining system was activated, and the relative positions and orientations of the various components of the system were adjusted. The multi-pulse damage threshold of the machined nickel-based superalloy was determined to be 0.23 J / cm² using the classical area epitaxy method. 2 The optimal processing parameters (laser pulse energy density of 1.60 J / cm²) were obtained using conventional methods. 2 The in-situ equivalent number of impacts is 9, corresponding to a laser power of 10.15W and a pulse frequency of 1MHz. Using formula (1), the laser energy density on the wall when the cavity height is 2mm is calculated to be 0.436J / cm². 2 The multi-pulse removal threshold of this material is greater than 0.23 J / cm. 2 This will cause contrast damage. Further steps are required.

[0051] Secondly, titanium oxide (Eg = 3.3 eV) was selected as a typical sample of the filler material, and the single-pulse threshold F of this material was determined under the pulse width and wavelength conditions of the laser. th_1 (TiO2) is 0.75 J / cm 2 The multi-pulse damage threshold F th_9 (TiO2) is 0.48 J / cm 2 And using this as a benchmark, the relationship between the damage threshold and the material bandgap is corrected:

[0052] F th (N)=(kE g +b)τ 0.3 N S-1

[0053] Where k is 0.078 J / cm -2 fs -0.3 eV -1 S is 0.8.

[0054] Then, based on the bandgap width of different materials, a multi-pulse damage threshold table of all materials that can be used as filler materials is quickly generated.

[0055] Serial Number Material <![CDATA[E g (eV)]]> <![CDATA[F th_1 (J / cm 2 )]]> <![CDATA[F th_9 (J / cm 2 )]]> 1 TiO2 3.3 0.80 0.52 2 SiC 3.2 0.75 0.48 3 ZrO2 6.2 2.34 1.51 4 <![CDATA[Al2O3]]> 6.5 2.50 1.61 5 <![CDATA[SiO2]]> 8.3 3.46 2.23 6 NaCl 8.5 3.56 2.30

[0056] Next, using the generated multi-pulse damage threshold table for filling materials, materials with multi-pulse damage thresholds greater than the laser pulse energy density used during processing are selected as filling materials for processing. Specifically, Al2O3, SiO2, and NaCl are selected as optional filling materials. Initially, Al2O3 spherical particles with a diameter of 0.5-1 mm are selected as the filling material. These particles are filled into the cavity of the workpiece to be processed. The workpiece is then processed using optimal processing parameters. After completion, the tiny particles of the filling material are removed, and an endoscope is used to inspect the wall damage at the processing location. No visible wall damage was observed, meeting the product manufacturing requirements, and no further iteration is needed. If wall damage occurs, another material is selected from the above optional filling materials, and the above operation is repeated until no wall damage occurs. Therefore, the output results are used for the manufacturing of the final workpiece.

[0057] As a common method, the wall-to-wall damage suppression method provided by this invention in ultrafast laser processing obviously has good universality: it uses the relationship between damage threshold and bandgap width to quickly estimate the damage threshold of various suitable fillers, which provides convenience for filler selection. It can be seen that this invention also has the characteristics of simplicity and good operability. It can directly use existing general equipment conditions, and only the filler material needs to be added to eliminate the wall-to-wall damage problem of narrow cavity workpieces in laser processing. Moreover, the filler material can be reused, which is an economical and easy-to-implement method.

[0058] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make possible changes and modifications to the technical solutions of the present invention by utilizing the methods and techniques disclosed above without departing from the spirit and scope of the present invention. Therefore, any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solutions of the present invention shall fall within the protection scope of the technical solutions of the present invention.

Claims

1. A method for suppressing wall damage in ultrafast laser processing, characterized in that... include: Step 1: Determine the multi-pulse damage threshold F of the material to be processed under the pulse width and wavelength conditions of the laser used. th_1 When the optimal processing parameters for the material are determined, the laser power density F_z corresponding to the cavity depth as the defocus distance Z is calculated to determine whether the material's multi-pulse damage threshold is reached, i.e. whether wall damage will occur during the processing; if wall damage occurs, subsequent steps are executed. Step 2: Select a filling material as a typical sample and determine the single-pulse threshold F of the material under the laser pulse width and wavelength conditions. th_1 (A) and multi-pulse damage threshold F th_N (A), and using it as a benchmark, solve the undetermined coefficients in the relationship between the damage threshold and the material bandgap width, and quickly generate a table of all multi-pulse damage thresholds that can be used as filling materials. The relationship between the damage threshold and the material bandgap is F. th (N)=(kE g +b)τ 0.3 N S-1 Where k is a unit of mass in J / cm - 2 fs -0.3 eV -1 The constant, b is -0.14 Jcm -2 fs -0.3 τ is the laser pulse width (fs), N is the in-situ equivalent number of knocks, and S is the material hatching coefficient; Step 3: Using the multi-pulse damage threshold table, select a material with a multi-pulse damage threshold greater than the laser pulse energy density used during processing as the filling material during processing; Step 4: Fill the cavity of the specimen with the selected filling material, complete the specimen processing using the optimal processing parameters, pour out the filling material after completion, check the specimen quality. If it does not meet the requirements, jump to step 3, and select other materials that meet the requirements as filling materials. If the processing requirements are met, the output result is used for the manufacturing of the formal workpiece.

2. The method for suppressing wall damage in ultrafast laser processing according to claim 1, characterized in that: The material to be processed is a solid material.

3. The method for suppressing wall damage in ultrafast laser processing according to claim 1, characterized in that: When the focused spot in step 2) is a Gaussian spot, the original equivalent number of impacts N is estimated using the following formula: Where: d x Let v be the diameter of the laser spot parallel to the scan line direction, f be the laser scanning speed, f be the pulse repetition frequency, and d be the laser beam diameter. y Ly represents the spot diameter parallel to the scan line direction, and Ly represents the spot spacing perpendicular to the scan line direction.

4. The method for suppressing wall damage in ultrafast laser processing according to claim 1, characterized in that: Filler material band gap width E g The relationship between the laser wavelength λ and the wavelength λ satisfies hc / λ<E g / 2, where h and c are Planck's constant and the speed of light in a vacuum, respectively.

5. The method for suppressing wall damage in ultrafast laser processing according to claim 1, characterized in that: The laser emits pulsed laser light, with each pulse lasting less than 20 ps.

6. The method for suppressing wall damage in ultrafast laser processing according to claim 1, characterized in that: When the focused spot in step 1 is a Gaussian spot, the cavity depth is used as the defocus distance Z, and the corresponding laser power density F th _z is calculated using the following formula: Where P is the laser power, Z R Let f be the Rayleigh length, f be the pulse repetition frequency, w be the spot radius at the beam waist, and 1 / e 2 Type definition.

7. The method for suppressing wall damage in ultrafast laser processing according to claim 1, characterized in that: The filling materials in step 2 include, but are not limited to, wide bandgap materials: titanium oxide, silicon nitride, zirconium oxide, aluminum oxide, aluminum nitride, gallium nitride, and sodium chloride.

8. The method for suppressing wall damage in ultrafast laser processing according to claim 1, characterized in that: In step 4, the filling material is a solid particle with a diameter that is 0.1-0.5 times the cavity height.

9. The method for suppressing wall damage in ultrafast laser processing according to claim 1, characterized in that: The measured single-pulse threshold F of the material th_1 (A) and multi-pulse damage threshold F th_N (A) Substitute the relationship between the damage threshold and the material bandgap width into the equation and solve for the unknown coefficients k and s in the equation.

10. A laser processing device, characterized in that, The laser processing equipment is used to perform a method for suppressing wall damage in ultrafast laser processing as described in any one of claims 1 to 9.

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