Numerical simulation method of composite laser ablation

Abstract

The present application belongs to the field of numerical simulation of the interaction between laser and matter, and in particular, relates to a method for numerical simulation of composite laser ablation. The method uses finite element software to establish a basic model for numerical simulation of the interaction between continuous laser and matter, and treats the change in the absorption rate of the material to the continuous laser due to manufacturing defects on the surface of the ultrafast laser ablation as equivalent to nonlinear absorption, so as to simulate the absorption coefficient change process formed by manufacturing defects on the surface of the ultrafast laser ablation material; uses spectral detection combined with Sedov-Taylor theory to construct a pressure calculation model for ultrafast laser-excited plasma shock waves, adds plasma shock wave pressure to the basic model for numerical simulation of the interaction between continuous laser and matter with the corresponding absorption coefficient, and establishes a numerical model of the ablation process of continuous-ultrafast composite laser, so as to analyze the change process of the composite laser ablation molten pool, and the influence of ultrafast laser pulse shock wave pressure on the sputtering and temperature change of the molten pool.

Description

Technical Field

[0001] The present application relates to the technical field of numerical simulation of laser-matter interaction, and in particular to a composite laser ablation numerical simulation method. Background Art

[0002] Composite laser can create defects on the sample surface through short pulse laser, thereby enhancing the absorption efficiency of continuous laser by the material, and can achieve efficient damage to difficult-to-ablate materials and high-reflective / high-transmittance materials. The existing laser ablation mechanism is mainly based on thermal ablation. Based on the continuous output of high-energy continuous laser, the target material melts, vaporizes, or even ionizes and is destroyed. If you want to improve the efficiency of the laser, you must increase the laser power, which will undoubtedly significantly increase the equipment cost. Composite laser has received widespread attention at home and abroad since it was proposed by American scholar Fox in 1975, especially in recent years It has become a domestic research hotspot. Many studies have shown that the assistance of short pulse laser can greatly reduce the ablation threshold of the target material under the action of continuous laser or long pulse laser (about one order of magnitude). Composite laser ablation technology can achieve high damage efficiency while reducing the power demand of high-energy laser, which can provide a new entry point for the development of existing high-energy laser ablation technology.

[0003] By using composite laser ablation technology, the ablation and preheating of the auxiliary pulse laser can enhance the mutual coupling between the target material and the laser energy, improve laser absorption and thus reduce the power demand for high-energy lasers; on the other hand, the high-pressure plasma shock wave excited by the auxiliary pulse laser can directly sputter and remove the molten target material. The high-energy laser only needs to heat the target material to the melting temperature without further vaporizing or ionizing the material to achieve the purpose of efficient destruction, which greatly avoids the consumption of laser energy in physical processes such as latent heat of melting, latent heat of vaporization, and plasma.

[0004] When laser interacts with materials, it is accompanied by a dynamic process of coupling multiple physical changes such as intense melting, vaporization, fluid flow, and material splashing or removal. The entire process of laser-material interaction can be dynamically simulated through numerical simulation methods, including the flow of the molten pool, vaporization and sputtering, and the energy coupling mechanism in continuous-ultrafast composite lasers can be theoretically analyzed. Studying the numerical simulation of continuous-ultrafast composite lasers can achieve an overall grasp of the ablation process, which is conducive to understanding the energy coupling mechanism of pulsed lasers and continuous lasers in the ablation process, and provides a theoretical basis for studying the ablation of composite lasers.

[0005] However, continuous lasers, which are mainly melting and sputtering, and ultrafast lasers, which are mainly ionization, are completely different physical change processes and cannot be directly coupled. At the same time, the interaction between continuous lasers and matter and ultrafast lasers differ by at least 12 orders of magnitude in the time domain, and direct coupling cannot be solved in the time domain. Therefore, a corresponding solution model is urgently needed to solve the cross-scale solution problem of the interaction process between continuous-ultrafast composite lasers and matter. Summary of the invention

[0006] The embodiment of the present application provides a method for numerical simulation of composite laser ablation, which realizes numerical simulation of continuous-ultrafast composite laser by performing equivalent treatment on the surface of ultrafast laser to induce material to enhance absorption of continuous laser in the early stage of ablation and plasma shock wave generated during ablation to enhance splashing of molten pool.

[0007] The technical solution of the present invention is as follows:

[0008] A composite laser ablation numerical simulation method comprises the following steps:

[0009] 1) Use finite element software to establish a basic model for numerical simulation of the interaction between continuous laser and matter;

[0010] 2) Regarding the change in the absorption rate of the material to the continuous laser caused by the ultrafast laser manufacturing defects, the change in the absorption rate of the material surface to the continuous laser is treated equivalently as nonlinear absorption in the numerical simulation process, so as to simulate the change process of the absorption coefficient formed by the ultrafast laser ablation of the material surface manufacturing defects;

[0011] 3) Spectral detection combined with Sedov-Taylor theory to construct a pressure calculation model for ultrafast laser-excited plasma shock waves:

[0012] 3-1) Build a spectrum detection system;

[0013] 3-2) The spectrum of plasma generated during the ablation of the material by ultrafast laser is collected by the spectrum detection system, and the light intensity distribution curve of the plasma intensity in the Z direction is extracted. The distance from the peak value of the plasma intensity to the center of the plasma eruption in the curve is the propagation radius R of the shock wave;

[0014] 3-3) Substitute the shock wave propagation radius R into the formula The energy E of the shock wave is obtained, and the energy E of the shock wave is substituted into the formula The shock wave pressure P is calculated from, where t is the propagation time, ρ is the ambient density, and ε 0 is a dimensionless quantity that depends on the formula The heat capacity ratio γ is given in , and the parameter β represents the size of the spread, β = 3;

[0015] 3-4) By fitting function R = at 0.4 The curve of shock wave pressure P changing with time t is fitted, where The pressure change value of the curve in the change curve diagram is expressed in the form of volume force.

[0016] Add it to the basic model of numerical simulation of the interaction between continuous laser and matter in step 1) and solve it.

[0017] Optionally, in step 2), the model of ultrafast laser ablation of material surface defects leading to nonlinear absorption of the material is α=a(T / T m ) b , where T is the molten pool temperature, T m It refers to the melting point of the material. Based on the experiment, the numerical simulation is made close to the experiment by adjusting the numerical parameters a and b. When the error is less than 1%, the optimization is stopped.

[0018] Optionally, the specific process of optimization is as follows:

[0019] Set the initial parameters a and b, where 0 <a<1,3<b<5;

[0020] Compared with the experiment, when the error is less than 1%, output a and b; when the error is greater than or equal to 1%, return to the previous process.

[0021] Optionally, in step 3-1), the spectral detection system includes a motion platform, a sample, an ultrafast laser, a lens assembly, a lens, an imaging system, a filter, an ICCD, a spectrometer and an analysis and processing unit, the sample is arranged on the motion platform, the laser emitted by the ultrafast laser is focused onto the sample through the lens assembly, a shutter is provided in the lens assembly, the imaging system collects an image of a portion of the sample focused by the laser through the lens, the image in the imaging system enters the spectrometer through the filter and the ICCD in sequence, and the spectrometer is electrically connected to the analysis and processing unit.

[0022] Optionally, the motion platform adopts a five-degree-of-freedom motion platform, the sample adopts alumina ceramics, the ultrafast laser adopts an 800nm ​​femtosecond laser, the shutter time is up to 2ns, the filter is an 800nm ​​filter, the spectrometer model is Shamrock SR-750, and the ICCD model is iStar DH-334T.

[0023] The beneficial effect of the composite laser ablation numerical simulation method provided by the present application is that: compared with the prior art, in the composite laser ablation numerical simulation method of the present application, finite element software is used to establish a basic model for numerical simulation of the interaction between continuous laser and matter, and the change in the absorption rate of the material to the continuous laser caused by the manufacturing defects on the surface of the ultrafast laser ablation is equivalently treated as nonlinear absorption, so as to simulate the absorption coefficient change process formed by the manufacturing defects on the surface of the ultrafast laser ablation material; spectral detection is combined with Sedov-Taylor theory to construct a pressure calculation model for the plasma shock wave excited by an ultrafast laser, and the plasma shock wave pressure is added to the basic model for numerical simulation of the interaction between continuous laser and matter with the corresponding absorption coefficient as the main body, and a numerical model of the ablation process of a continuous-ultrafast composite laser is established, so that the change process of the composite laser ablation molten pool and the influence of the ultrafast laser pulse shock wave pressure on the sputtering and temperature change of the molten pool can be analyzed, thereby forming a new method for analyzing the interaction between laser and matter across time scales. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0025] in:

[0026] Figure 1 It is an optimization flow chart of a material nonlinear absorption model caused by ultrafast laser manufacturing defects in a composite laser ablation numerical simulation method shown in an embodiment of the present application;

[0027] Figure 2 It is a schematic diagram of a spectrum detection system constructed in a composite laser ablation numerical simulation method shown in an embodiment of the present application;

[0028] Figure 3 It is a spectrum diagram of plasma eruption caused by ultrafast pulses acting on the surface of a material monitored by a spectrum detection system;

[0029] Figure 4 It is a graph showing the movement of the plasma center along the Z direction as time changes;

[0030] Figure 5 It is the Rt diagram of the Z-direction movement of the plasma shock wave;

[0031] Figure 6 This is a schematic diagram of the pressure variation law of plasma shock wave.

[0032] Description of main component symbols:

[0033] 10. Sample; 20. Ultrafast laser; 30. Lens assembly; 31. Shutter; 32. Focusing lens; 40. Lens; 50. Imaging system; 60. Filter; 70. ICCD; 80. Spectrometer; 90. Analysis and processing unit. DETAILED DESCRIPTION

[0034] In order to facilitate the understanding of the present application, the present application will be described more comprehensively with reference to the relevant drawings below. The preferred embodiments of the present application are provided in the drawings. However, the present application can be implemented in many other different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present application more thorough and comprehensive.

[0035] It should be noted that when an element is referred to as being "fixed to" or "disposed on" another element, it can be directly on the other element or indirectly on the other element. When an element is referred to as being "connected to" another element, it can be directly connected to the other element or indirectly connected to the other element.

[0036] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used herein in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0037] As described in the background technology, studying the numerical simulation of continuous-ultrafast composite lasers can achieve an overall understanding of the ablation process, which is conducive to understanding the energy coupling mechanism of ultrafast lasers and continuous lasers in the ablation process and provides a theoretical basis for studying the ablation of composite lasers.

[0038] However, continuous lasers mainly characterized by melting and sputtering and ultrafast lasers mainly characterized by ionization belong to completely different physical change processes and cannot be directly coupled. At the same time, the interaction between continuous lasers and matter and that between ultrafast lasers and matter differ by at least 12 orders of magnitude in the time domain, and direct coupling in the time domain cannot achieve a solution.

[0039] Among them, ultrafast laser refers to a pulse laser with a pulse width shorter than the molecular relaxation time, covering the range of picoseconds and femtoseconds.

[0040] In order to solve the above problems, an embodiment of the present application provides a composite laser ablation numerical simulation method, comprising the following steps:

[0041] 1) Use finite element software (such as Comsol Multiphysics) to establish a basic model for numerical simulation of the interaction between continuous laser and matter;

[0042] 2) Regarding the change in the absorption rate of the material to the continuous laser caused by the ultrafast laser manufacturing defects, the change in the absorption rate of the material surface to the continuous laser is treated equivalently as nonlinear absorption in the numerical simulation process, so as to simulate the change process of the absorption coefficient formed by the ultrafast laser ablation of the material surface manufacturing defects;

[0043] 3) Spectral detection combined with Sedov-Taylor theory to construct a pressure calculation model for ultrafast laser-excited plasma shock waves:

[0044] 3-1) Build a spectrum detection system;

[0045] 3-2) The spectrum of plasma generated during the ablation of the material by ultrafast laser is collected by the spectrum detection system, and the light intensity distribution curve of the plasma intensity in the Z direction is extracted. The distance from the peak value of the plasma intensity to the center of the plasma eruption in the curve is the propagation radius R of the shock wave;

[0046] 3-3) Substitute the shock wave propagation radius R into the formula The energy E of the shock wave is obtained, and the energy E of the shock wave is substituted into the formula The shock wave pressure P is calculated from, where t is the propagation time, ρ is the ambient density, and ε 0 is a dimensionless quantity that depends on the formula The heat capacity ratio γ is given in , and the parameter β represents the size of the spread, β = 3;

[0047] 3-4) By fitting function R = at 0.4 The curve of shock wave pressure P changing with time t is fitted, where The pressure change value of the curve in the change curve diagram is added to the basic model of numerical simulation of the continuous laser and material interaction in step 1) in the form of volume force, and solved by using COMSOL Multiphysics solver.

[0048] In the embodiment of the present application, in the numerical simulation method of composite laser ablation, finite element software is used to establish a basic model for numerical simulation of the interaction between continuous laser and matter, and the change in the absorption rate of the material to the continuous laser caused by manufacturing defects on the surface of the ultrafast laser ablation is equivalently treated as nonlinear absorption, so as to simulate the absorption coefficient change process caused by manufacturing defects on the surface of the ultrafast laser ablation material; spectral detection is combined with Sedov-Taylor theory to construct a pressure calculation model for ultrafast laser-excited plasma shock waves, and the plasma shock wave pressure is added to the basic model of numerical simulation of the interaction between continuous laser and matter with the corresponding absorption coefficient as the main body to establish a numerical model of the ablation process of continuous-ultrafast composite laser, so as to analyze the change process of the composite laser ablation molten pool and the influence of the ultrafast laser pulse shock wave pressure on the sputtering and temperature change of the molten pool, thereby forming a new method for analyzing the interaction between laser and matter across time scales.

[0049] In one embodiment, in step 2), the model of ultrafast laser ablation of material surface defects leading to nonlinear absorption of the material is α=a(T / T m ) b , where T is the molten pool temperature, T m It refers to the melting point of the material. Based on the experiment, the numerical simulation is made close to the experiment by adjusting the numerical parameters a and b. When the error is less than 1%, the optimization is stopped.

[0050] Specifically, if Figure 1 As shown in the figure, the specific process of optimization is as follows:

[0051] Set the initial parameters a and b, where 0 <a<1,3<b<5;

[0052] Compared with the experiment, when the error is less than 1%, output a and b; when the error is greater than or equal to 1%, return to the previous process.

[0053] The following uses alumina ceramics as a sample and femtosecond laser as an example to explain in detail the corresponding parts of the composite laser ablation numerical simulation method:

[0054] Femtosecond laser has a pulse width of 10 -12 ~10 -15s, which is at least 12 orders of magnitude different from the continuous laser. Because the time dimension of the two is too different, it is not feasible to realize the direct coupling simulation of continuous laser and femtosecond laser. As analyzed above, the main role of femtosecond laser pulse in composite laser ablation is manifested in two aspects. The first is to improve the absorption efficiency of continuous laser energy by creating defects on the sample surface. The second is the plasma shock wave generated by the interaction between the femtosecond pulse and the material, which can effectively enhance the sputtering of the molten pool, thereby facilitating the removal of the material. In the process of numerical simulation, the improvement of continuous laser energy absorption rate mentioned in the first point can be achieved by changing the continuous laser absorption coefficient of ceramics in the model. For the plasma shock wave mentioned in the second point, it can be equivalent to shock wave pressure for simulation. Many studies have shown that the duration of plasma shock wave is at least in the order of tens of nanoseconds, which can be coupled with continuous laser for solution. The calculation of shock wave pressure in this application is based on Sedov-Taylor theory.

[0055] The Sedov-Taylor self-similar model is widely used to describe the propagation of shock waves caused by laser pulses. The propagation radius of this model is a function of the propagation time and the shock wave energy. The relationship is described as follows:

[0056]

[0057]

[0058] In formula (1-1), R is the propagation radius of the shock wave, t is the propagation time, E is the energy of the shock wave, ρ is the ambient density, and ε 0 is a dimensionless quantity that depends on the heat capacity ratio γ given in formula (1-2). The parameter β represents the size of propagation (β = 3 for spherical propagation, β = 3 for cylindrical propagation, and β = 1 for planar propagation). In the experiment of this application, the shock wave meets the requirements of spherical propagation (β = 3).

[0059] According to formula (1-1) and formula (1-2), the shock wave energy expression (1-3) is calculated as follows:

[0060]

[0061] The shock wave pressure is calculated as follows:

[0062]

[0063] Taking the continuous laser ablation model of alumina ceramics as the main body, the plasma shock wave pressure is added to establish a numerical model of the ablation process of alumina ceramics by continuous-femtosecond composite laser, and the change process of the composite laser ablation molten pool is analyzed, as well as the influence of the femtosecond laser pulse shock wave pressure on the sputtering and temperature change of the molten pool. In fact, because the solution of the dual-temperature equation differs from the above-mentioned continuous laser ablation model of alumina ceramics by at least 12 orders of magnitude in the time domain, the two cannot be combined to construct a composite laser ablation model. Taking the continuous laser ablation model of alumina ceramics with an absorption coefficient of 0.2 as the main body, the femtosecond laser pulse shock wave pressure is added for simulation analysis to explore the evolution law of the molten pool with different shock wave pressures and spot sizes.

[0064] When studying the ablation mechanism of ceramics by ultrafast laser with large pulse energy, the plasma and the shock wave it generates are the focus of the research process. Figure 2 The spectral detection system shown in the figure includes a motion platform (not shown in the figure), a sample 10, an ultrafast laser 20, a lens assembly 30, a lens 40, an imaging system 50, a filter 60, an ICCD 70, a spectrometer 80 and an analysis and processing unit 90 (such as a computer). ICCD is an enhanced charge coupled device, i.e., the abbreviation of Intensified CCD. The sample 10 is set on the motion platform. The laser emitted by the ultrafast laser 20 is focused onto the sample 10 through the lens assembly 30. The lens assembly 30 is provided with a shutter 31 and a focusing lens 32. The imaging system 50 collects an image of the portion of the sample 10 focused by the laser through the lens 40. The image in the imaging system 50 enters the spectrometer 80 through the filter 60 and the ICCD 70 in turn. The spectrometer 80 is electrically connected to the analysis and processing unit 90.

[0065] Among them, the motion platform adopts a five-degree-of-freedom motion platform, the sample 10 adopts alumina ceramics, the ultrafast laser 20 adopts an 800nm ​​femtosecond laser, the shutter 31 time is up to 2ns, the filter 60 is an 800nm ​​filter, the spectrometer 80 and ICCD70 are both from Andor Company of the United States, the model of the spectrometer 80 is Shamrock SR-750, and the model of ICCD70 is iStar DH-334T.

[0066] according to Figure 2 The spectrum detection system built in the experiment collects the plasma spectrum generated by femtosecond laser ablation of alumina ceramics, with a collection frequency of one image every 20ns. Figure 3 The plasma spectrum collected at the middle of the filament with a femtosecond laser pulse energy of 1 mJ and a focusing lens focal length of 50 mm. Because of the presence of air pressure, the plasma expansion in the air lasts for about 80 seconds.

[0067] from Figure 3 It can be seen that the plasma expands along the Z direction. The light intensity distribution of the plasma intensity in the Z direction is extracted as shown in the figure. Figure 4 As shown in Figure 2, it can be seen that the peak value of plasma intensity decreases rapidly with time and moves toward the Z direction, which is the process of plasma expansion.

[0068] by Figure 4 The distance from the peak value to the center of the plasma eruption determines the plasma propagation radius (shown as R in the figure). The energy of the shock wave can be calculated by fitting the shock wave propagation radius data at different times. The fitting function is:

[0069] R=at 0.4

[0070] in:

[0071]

[0072] The variation of plasma propagation radius with time after fitting is as follows: Figure 5 shown.

[0073] Finally, the relationship between the pressure of the plasma shock wave and time at different pulse energies was calculated according to formula (1-4). After the femtosecond laser filamentation, the power density of the filament is limited to 10 13 ~10 14 W / cm 2 However, studies have shown that the greater the pulse energy, the higher the power density after filamentation, the stronger the plasma intensity generated during ablation of the alumina ceramic surface, and the plasma shock wave pressure generated at the end increases with the increase of pulse energy. For a femtosecond laser with a pulse energy of 1mJ, the plasma shock wave pressure generated is between 10 and 30MPa, and the duration is about 80ns. Figure 6 shown.

[0074] The technical features of the above embodiments may be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0075] The above embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the application. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the attached claims.

Claims

1. A composite laser ablation numerical simulation method, characterized in that: The following steps are involved: 1) Use finite element software to establish a basic model for numerical simulation of the interaction between continuous laser and matter; 2) Regarding the change in the absorption rate of the material to the continuous laser caused by the ultrafast laser manufacturing defects, the change in the absorption rate of the material surface to the continuous laser is treated equivalently as nonlinear absorption in the numerical simulation process, so as to simulate the change process of the absorption coefficient formed by the ultrafast laser ablation of the material surface manufacturing defects; 3) Spectral detection combined with Sedov-Taylor theory to construct a pressure calculation model for ultrafast laser-excited plasma shock waves: 3-1) Build a spectrum detection system; 3-2) The spectrum of plasma generated during the ablation of the material by ultrafast laser is collected by the spectrum detection system, and the light intensity distribution curve of the plasma intensity in the Z direction is extracted. The distance from the peak value of the plasma intensity to the center of the plasma eruption in the curve is the propagation radius R of the shock wave; 3-3) Substitute the shock wave propagation radius R into the formula The energy E of the shock wave is obtained, and the energy E of the shock wave is substituted into the formula The shock wave pressure P is calculated from the equation: t is the propagation time, ρ is the ambient density, and ε0 is a dimensionless quantity that depends on the formula The heat capacity ratio γ is given in , and the parameter β represents the size of the spread, β = 3; 3-4) By fitting function R = at 0.4 The curve of shock wave pressure P changing with time t is fitted, where The pressure change value of the curve in the change curve diagram is added to the basic model of numerical simulation of the continuous laser and material interaction in step 1) in the form of volume force to obtain a solution.

2. The composite laser ablation numerical simulation method according to claim 1, characterized in that: In step 2), the model of nonlinear absorption of the material caused by ultrafast laser ablation of surface defects is α = a(T / T m ) b , where T is the molten pool temperature, T m It refers to the melting point of the material. Based on the experiment, the numerical simulation is made close to the experiment by adjusting the numerical parameters a and b. When the error is less than 1%, the optimization is stopped.

3. The composite laser ablation numerical simulation method according to claim 2, characterized in that: The specific process of optimization is as follows: Set the initial parameters a and b, where 0 <a<1,3<b<5; Compared with the experiment, when the error is less than 1%, output a and b; when the error is greater than or equal to 1%, return to the previous process.

4. The composite laser ablation numerical simulation method according to claim 1, characterized in that: In step 3-1), the spectral detection system includes a motion platform, a sample, an ultrafast laser, a lens assembly, a lens, an imaging system, a filter, an ICCD, a spectrometer and an analysis and processing unit. The sample is set on the motion platform, the laser emitted by the ultrafast laser is focused onto the sample through the lens assembly, a shutter is provided in the lens assembly, the imaging system collects an image of the portion of the sample focused by the laser through the lens, the image in the imaging system enters the spectrometer through the filter and the ICCD in sequence, and the spectrometer is electrically connected to the analysis and processing unit.

5. The composite laser ablation numerical simulation method according to claim 4, characterized in that: The motion platform adopts a five-degree-of-freedom motion platform, the sample adopts alumina ceramics, the ultrafast laser adopts an 800nm ​​femtosecond laser, the shutter time is up to 2ns, the filter is an 800nm ​​filter, the model of the spectrometer is Shamrock SR-750, and the model of the ICCD is iStar DH-334T.

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