Method for estimating feasibility of substance removal

Abstract

In this method for estimating the feasibility of substance removal, the equation of heat conduction in solids is used to estimate a temperature rise in the vicinity of a laser irradiation spot for when a laser beam irradiates paint, rust, extraneous matter, etc. on the surface of a metal or other material, a hypothetical substance that is solid over the entire temperature range is introduced, and the temperature rise due to laser irradiation is determined on the basis of the temperature obtained by dividing the energy per unit mass required to vaporize the substance by the heat capacity per unit mass of the substance in the solid state.

Description

Method for estimating whether substances can be removed

[0001] The present invention relates to a method for estimating whether a substance formed on the surface of a metal or the like can be removed when the substance is irradiated with a laser.

[0002] Laser cleaning technology, which removes rust, paint, and other deposits formed on the surface of materials such as metals by irradiating them with laser light, has been attracting attention. Patent Document 1 proposes a method for removing paint films from the surface of structures by rotating and scanning light in a circular pattern. Also, Non-Patent Document 1 reports that rust and salt can be removed by irradiating corroded steel plates with a laser.

[0003] Patent No. 5574354

[0004] Koji Onoue et al., "Applicability of a new laser-based surface preparation method to steel structures," Journal of Structural Engineering, Japan Society of Civil Engineers, Vol. 63 A, pp. 476-482, 2017.

[0005] The laser to be irradiated can be a continuous wave (CW laser) as in Patent Document 1, or a pulsed laser as in Non-Patent Document 1. In order to remove surface paint, rust, deposits, etc., it is important to set the irradiation conditions of these lasers appropriate for the object to be removed. Laser irradiation conditions (parameters) include laser power, laser beam diameter at the irradiation point, beam scanning speed, beam movement distance in the direction perpendicular to the scan, and number of irradiations when irradiation is repeated. In addition, in the case of a pulsed laser, parameters such as the pulse period and the length of one pulse are also used.

[0006] As described above, because there are many parameters related to laser irradiation, when trying to experimentally determine the appropriate parameter values ​​for removing surface paint, rust, deposits, etc., it is necessary to experimentally change many parameters and repeat trial and error, which is a time-consuming process.

[0007] The present invention has been made to solve the above problems, and has as its object to make it possible to determine laser irradiation conditions in a shorter time without experimental trial and error.

[0008] The method for estimating whether a substance can be removed according to the present invention is a method for estimating whether a substance formed on the surface of a metal can be removed when irradiated with a laser, and estimates the temperature rise due to laser irradiation in the vicinity of the laser irradiation point using the heat conduction equation for solids shown in the following equations (1), (2), and (3).If the temperature rise due to laser irradiation is higher than the temperature calculated by dividing the energy per unit mass required to vaporize the substance by the heat capacity per unit mass of the substance in a solid state, it is determined that the substance can be removed, and if it is lower, it is determined that the substance cannot be removed.

[0009] r: Position near the laser irradiation point t: Time (point in time) J: Thermal energy passing through unit time / unit area ρ: Density of material c: Heat capacity per unit mass of material T: Temperature rise of material A: Energy generated in material by laser irradiation per unit time / unit volume K: Thermal conductivity of material f(r): Spatial dependence of the intensity of the irradiated laser (0≦f(r)≦1) q(t): Time dependence of the intensity of the irradiated laser (0≦q(t)≦1) A p : Peak power density of the laser on the surface of the material R: Reflectance of the laser irradiated onto the material α: Laser absorption coefficient of the material z: Depth position from the surface of the material

[0010] As explained above, according to the present invention, a virtual material is assumed to be solid over the entire temperature range, the temperature rise in the vicinity of the laser irradiation point is calculated using the heat conduction equation for solids, and removal is determined based on whether this temperature rise is higher or lower than the temperature calculated by dividing the energy per unit mass required to vaporize the material by the heat capacity per unit mass of the material in a solid state. In this way, according to the present invention, it is possible to estimate whether or not a material on a surface can be removed by simple calculation, and it is possible to determine laser irradiation conditions in a shorter time without experimental trial and error.

[0011] FIG. 1 is a characteristic diagram showing the temperature dependence of rust energy. FIG. 2 is a characteristic diagram showing the temperature dependence of rust energy (a) and the temperature dependence of hypothetical rust energy (b). FIG. 3 is an explanatory diagram showing the state of irradiating rust with a laser. FIG. 4 is a characteristic diagram showing the time dependence of the temperature on the surface of a material (rust) being irradiated with a pulse. FIG. 5 is a photograph showing the appearance of rusted steel after irradiating it with a laser under various conditions. FIG. 6 is a characteristic diagram showing the calculation results of the position dependence of the temperature rise on the rust surface when irradiating rusted steel with CW laser light under various conditions. FIG. 7 is a photograph showing the appearance of rusted steel after irradiating it with a CW laser under various conditions.

[0012] A method for estimating whether a substance can be removed according to an embodiment of the present invention will be described below. In the method for estimating whether a substance can be removed according to an embodiment of the present invention, when a laser beam is irradiated onto paint, rust, or deposits on the surface of a material such as metal, the temperature rise in the vicinity of the laser irradiated point is estimated using the heat conduction equation of a solid.

[0013] In general, when a solid material is irradiated with a laser, the temperature rise of the solid in the vicinity of the laser irradiated area can be estimated by solving the heat conduction equation (1) (Reference 1).

[0014]

[0015] In equation (1), r is the position near the laser irradiation point, T is the temperature rise of the material to be estimated, and is a function of the position r near the laser irradiation point and the time t. J is the thermal energy passing through per unit time / unit area, and is a function of the position and the time t. ρ is the density of the material. c is the heat capacity per unit mass of the material. Therefore, ρc is the heat capacity of the material per unit volume. A is the energy generated in the material by laser irradiation per unit time / unit volume, and is a function of the position and the time t.

[0016] Furthermore, the following Fourier's law holds between J and T: In equation (2), K is the thermal conductivity.

[0017]

[0018] The heat source term A (energy generated in a substance per unit time and per unit volume) is expressed by the following equation (3):

[0019]

[0020] Here, f(r) is the spatial dependency of the intensity of the irradiated laser, normalized to be greater than or equal to 0 and less than or equal to 1. Also, q(t) is the time dependency of the intensity of the irradiated laser, normalized to be greater than or equal to 0 and less than or equal to 1. p is the peak power density of the laser at the surface of the material [W / m 2 ]. R is the reflectance of the laser irradiated onto the material. α is the laser absorption coefficient of the material. z is the position in the depth direction from the surface of the material.

[0021] By providing appropriate initial and boundary conditions, equations (1), (2), and (3) can be solved numerically to determine the temperature rise T of the material near the point irradiated with the laser.

[0022] However, if the temperature of the substance rises due to laser irradiation and exceeds the melting point or boiling point, equation (1) no longer holds. To avoid this problem, in the present invention, a hypothetical substance that is solid over the entire temperature range is introduced, and the energy per unit mass required to vaporize the substance is divided by the heat capacity per unit mass of the substance in a solid state, and this temperature is used as a reference temperature. This temperature is compared with the temperature rise due to laser irradiation to determine whether the substance can be removed.

[0023] The method for estimating whether a substance can be removed according to an embodiment of the present invention will be described in more detail below, taking rust on a steel surface as an example and irradiating the rust with a laser. Steel is iron containing 0.02% to 2.1% carbon.

[0024] Figure 1 shows the temperature dependence of rust energy. The rust composition was assumed to be Fe2O3. The energy at room temperature (298K) was set to 0. The heat capacity per unit mass, c, is a physical quantity that is inherently dependent on temperature, but for simplicity, it was set to a constant value of 0.9 J / (gK). The melting point of Fe2O3 was set to 1800K, and the boiling point was set to 2973K.

[0025] Since I was unable to find literature values ​​for the heat of fusion and heat of vaporization of Fe2O3, I substituted the literature values ​​for FeO listed in Reference 2, adopting a heat of fusion of 430 J / g and a heat of vaporization of 3200 J / g. Therefore, the energy required (per unit mass) to vaporize Fe2O3 at room temperature (298 K) is (2973 K - 298 K) x 0.9 J / (gK) + 430 J / g + 3200 J / g = 6037.47 J / g. Here, I introduce hypothetical rust, which is solid across the entire temperature range. Furthermore, I assume that the heat capacity c is a constant value of 0.9 J / (gK) across the entire temperature range.

[0026] Figure 2(b) shows the temperature dependence of hypothetical rust energy. The slope of the line showing the temperature dependence of hypothetical rust energy shown in Figure 2(b) is the heat capacity of hypothetical rust [c = 0.9 J / (gK)]. Figure 2(a) shows the temperature dependence of the actual rust energy shown in Figure 1.

[0027] The energy (per unit mass) required to vaporize Fe2O3 at room temperature is 6037.47 J / g, as shown by the dashed line in Figure 2(c), and the temperature at which the above-mentioned hypothetical rust has this energy will be called the hypothetical boiling point. The difference between the hypothetical boiling point of rust and room temperature is calculated by dividing the energy per unit mass required to vaporize Fe2O3 (substance) by the heat capacity per unit mass of Fe2O3 (substance) in the solid state, which is 6037.47 J / g ÷ 0.9 J / (gK) = 6708.3 K.

[0028] Therefore, if we consider irradiating a virtual rust, which is solid across the entire temperature range, with a laser and solve the heat conduction equation for solids, and the temperature of the virtual rust rises from room temperature by 6708.3 K or more, we can estimate that the rust is removable. Similarly, if the temperature is less than 6708.3 K, we can estimate that the rust is not removable.

[0029] Figure 3 shows how a laser is irradiated onto rust. Figure 3 is a cross-section of the rust and steel viewed from the y-axis direction. The z-axis is taken as the depth direction, with z = 0 at the surface of the rust. The direction of laser scanning is taken as the x-axis, and its speed is taken as v. Furthermore, if a Gaussian beam is used as the spatial dependence f(r) of the laser beam intensity, irradiation begins at time 0, and the laser irradiation position at time 0 is (x, y) = (0, 0), the heat source term A can be written as follows:

[0030]

[0031] Here, d is the 1 / e radius of the laser beam on the rust surface (the radius at which the power density is 1 / e of the peak power density). p : laser peak power density, R: laser reflectivity of rust, α: light absorption coefficient of rust. p , R, and α are constants that do not depend on temperature.

[0032] In addition, assuming that thermal radiation from the rust surface can be ignored, the following boundary conditions are adopted:

[0033]

[0034] If we assume that the temperature of the rust is the ambient temperature (room temperature) regardless of location before the laser light irradiation begins, the temperature change (temperature rise T) of the rust from room temperature at time t and location (x, y, z) after laser irradiation can be written as follows (Reference 1):

[0035]

[0036]

[0037] Here, ρ is the density of rust, c is the heat capacity per unit mass of rust, and κ is the thermal diffusivity of rust (= K / (ρc), where K is the thermal conductivity of rust), which are constants independent of temperature.

[0038] In the following evaluation, ρ = 5.2 × 10 3 kg / m 3 , c = 9.0 × 10 2 J / (kgK), κ=9.2×10 -7 m 2 / s, R = 0.3, α = 4.5 × 105 Let's say.

[0039] Next, the case where the laser is a pulsed laser and the case where the laser is a CW laser will be described in detail.

[0040] [1. When the laser is a pulsed laser] Consider a pulsed laser that outputs a pulse train at a certain period. When performing laser cleaning, pulses are usually emitted while moving (scanning) the beam, rather than repeatedly irradiating the same spot with pulses. For this reason, it is necessary for the substance (rust) to be removed with a single pulse irradiation. Therefore, when the start time of the first pulse irradiation is 0 and the pulse width is τ, it can be estimated that if the temperature of the substance (rust) exceeds the boiling point at time τ, the substance (rust) will be removed, and if it does not exceed the boiling point, it will not be removed.

[0041] Figure 4 shows the time dependence of the temperature on the surface of a substance (rust) being irradiated with pulses. Under condition A, the temperature has not yet reached the virtual boiling point at time τ when one pulse irradiation ends. Therefore, it can be assumed that the substance (rust) will not be removed. On the other hand, under condition B, the temperature has already reached the virtual boiling point at time τ when one pulse irradiation ends. Therefore, it can be assumed that the substance (rust) will be removed.

[0042] Table 1 below shows the results of model calculations of the temperature rise on the surface when a laser was irradiated on rusted steel under different conditions. For the model calculations, the pulse width τ was 100 ns, the beam diameter d was 30 μm, the scan speed v was 2.5 m / s, and the peak power density A was changed in five ways. Laser irradiation was initiated at time 0, centered on coordinates x = 0, y = 0, and the temperature rise on the surface at time τ = 100 ns and coordinates x = 0, y = 0 was calculated using equations (4), (6), and (7). Since the temperature exceeded 6708.3 K under conditions D and E, it can be assumed that rust can be removed under these two conditions.

[0043]

[0044] Figure 5 shows photographs (experimental results) of the appearance of rusted steel after laser irradiation under various conditions. The pulse width τ, beam diameter d, scan speed v, and peak power density are the same as those in the model calculations. The pulse period is 100 kHz, the movement distance in the vertical direction of the scan is 25 μm, and the number of cleanings is four. Visual inspection revealed that rust was not removed under conditions A and B, rust removal was insufficient under condition C, and rust was removed under conditions D and E.

[0045] Comparing the model calculation results with the experimental results suggests that it is possible to determine whether or not rust can be removed using model calculations.

[0046] [2. When the laser is a CW laser] Unlike pulsed lasers, there is no concept of pulse width when using a CW laser. Therefore, by calculating the positional dependency (xy dependency) of the temperature on the surface at a certain time, it can be estimated that if the maximum temperature exceeds the boiling point of the material, the material will be removed, and if it does not, it will not be removed.

[0047] Figure 6 and Table 2 show the results of model calculations of the position dependency (x dependency) of the temperature rise on the rust surface when rusted steel is irradiated with CW laser light under various conditions. p The scanning speed (v) was changed in six ways. Laser irradiation began at time 0, centered on coordinates x = 0, y = 0, and the temperature distribution at time τ = 100 μs was calculated using equations (4), (6), and (7). Conditions B and D exceeded 6708.3 K, so it can be assumed that rust can be removed under these two conditions. Furthermore, under condition F, the temperature rise was almost equal to 6708.3 K. Therefore, it can be assumed that rust removal will be insufficient under these conditions.

[0048]

[0049] Figure 7 shows the appearance of rusted steel when irradiated with a CW laser under different conditions (beam diameter d, scan speed v, peak power density A). pis the same as the model calculation. The movement distance in the vertical direction of the scan is 20 μm, and the number of cleanings is three. Visual inspection shows that rust was not removed in conditions A, C, and E, rust removal was insufficient in condition F, and rust was removed in conditions B and D.

[0050] Comparing the model calculation results with the experimental results suggests that it is possible to determine whether or not rust can be removed using model calculations.

[0051] As explained above, according to the present invention, a virtual material is assumed to be solid over the entire temperature range, the temperature rise in the vicinity of the laser irradiation point is calculated using the heat conduction equation for solids, and removal is determined based on whether this temperature rise is higher or lower than the temperature calculated by dividing the energy per unit mass required to vaporize the material by the heat capacity per unit mass of the material in a solid state. In this way, according to the present invention, it is possible to estimate whether or not material on the surface will be removed by simple calculation, and it is possible to determine laser irradiation conditions in a shorter time without experimental trial and error.

[0052] It should be noted that the present invention is not limited to the above-described embodiments, and it is clear that many modifications and combinations can be implemented by a person skilled in the art within the technical spirit of the present invention. For example, although the above-described embodiments describe the removal of rust formed on the surface of steel, the present invention is not limited to the above-described embodiments. For example, phenomena occurring when a laser is irradiated onto substances such as paint, deposits, and oxides present on the surface of metal, etc., can also be included in the scope of the present invention by calculating using appropriate values ​​for the physical properties of the substances.

[0053] Some or all of the above-described embodiments may also be described as, but are not limited to, the following supplementary notes.

[0054] [Supplementary Note 1] A method for estimating whether a substance formed on the surface of a metal can be removed when the substance is irradiated with a laser, the method estimating whether the substance can be removed by estimating the temperature rise due to laser irradiation in the vicinity of the laser irradiation point using the heat conduction equation for solids shown in the following formulas (1), (2), and (3), and determining that the substance can be removed if the temperature rise due to laser irradiation is higher than the temperature calculated by dividing the energy per unit mass required to vaporize the substance by the heat capacity per unit mass of the substance in a solid state, and determining that the substance cannot be removed if the temperature rise due to laser irradiation is lower.

[0055] r: Position near the laser irradiation point t: Time (point in time) J: Thermal energy passing through unit time / unit area ρ: Density of the substance c: Heat capacity per unit mass of the substance T: Temperature rise of the substance A: Energy generated in the substance by laser irradiation per unit time / unit volume K: Thermal conductivity of the substance f(r): Spatial dependence of the intensity of the irradiated laser (0≦f(r)≦1) q(t): Time dependence of the intensity of the irradiated laser (0≦q(t)≦1) A p : Peak power density of the laser on the surface of the material R: Reflectance of the laser irradiated onto the material α: Laser absorption coefficient of the material z: Position in the depth direction from the surface of the material

[0056] [Appendix 2] In the method for estimating whether a substance can be removed as described in Appendix 1, the temperature rise due to laser irradiation is the temperature rise on the surface at the point (x, y) where the laser irradiation started, at the time when one pulse irradiation ends.

[0057] [Supplementary Note 3] In the method for estimating whether a substance can be removed as described in Supplementary Note 1, the temperature rise due to laser irradiation is calculated based on the position dependency of the temperature rise on the surface at a certain time, and the maximum value of the temperature rise is used.

[0058] [Appendix 4] In the method for estimating whether a substance can be removed as described in any one of Appendices 1 to 3, the density ρ of the substance, the heat capacity c of the substance, the thermal conductivity K of the substance, the reflectance R of the substance, and the absorption coefficient α of the substance are constants independent of temperature, and the heat conduction equation of the solid satisfies the boundary condition shown in the following equation (4).

[0059]

[0060] [Supplementary Note 5] The method for estimating whether a substance can be removed according to any one of Supplementary Notes 1 to 4, wherein the substance is Fe2O3.

[0061] [Reference 1] JH Bechtel, "Heating of solid targets with laser pulses", Journal of Applied Physics, vol. 46, pp. 1585-1593, 1975. [Reference 2] B. Koroglu et al., "Gas Phase Chemical Evolution of Uranium, Aluminum, and Iron Oxides", Scientific Reports, vol. 8, Article number 10451, 2018.

Claims

1. A method for estimating whether a substance formed on a metal surface can be removed when the substance is irradiated with a laser, comprising the steps of: estimating the temperature rise due to laser irradiation in the vicinity of the laser irradiated area using the heat conduction equation for solids shown in the following equations (1), (2), and (3); and determining that the substance can be removed if the temperature rise due to laser irradiation is higher than the temperature calculated by dividing the energy per unit mass required to vaporize the substance by the heat capacity per unit mass of the substance in a solid state, and determining that the substance cannot be removed if the temperature rise is lower. r: Position near the laser irradiation t: Time (point in time) J: Thermal energy passing per unit time and unit area ρ: Density of the substance c: Heat capacity per unit mass of the substance T: Temperature rise of the substance A: Energy generated in the substance by laser irradiation per unit time and unit volume K: Thermal conductivity of the substance f(r): Spatial dependence of the intensity of the irradiated laser (0≦f(r)≦1) q(t): Time dependence of the intensity of the irradiated laser (0≦q(t)≦1) A p : Peak power density of the laser on the surface of the material R: Reflectance of the laser irradiated to the material α: Laser absorption coefficient of the material z: Position in the depth direction from the surface of the material 2. A method for estimating whether a substance can be removed as described in claim 1, wherein the temperature rise due to laser irradiation is the temperature rise on the surface at the point (x, y) where the laser irradiation started, at the time when one pulse irradiation ends.

3. A method for estimating whether a substance can be removed as described in claim 1, wherein the temperature rise caused by laser irradiation is calculated based on the position dependency of the temperature rise on the surface at a certain time, and the maximum value of the temperature rise is determined.

4. A method for estimating whether a substance can be removed as described in any one of claims 1 to 3, wherein the density ρ of the substance, the heat capacity c of the substance, the thermal conductivity K of the substance, the reflectance R of the substance, and the absorption coefficient α of the substance are constants independent of temperature, and the heat conduction equation of the solid satisfies the boundary condition shown in the following equation (4).

5. In the method for estimating whether a substance can be removed according to claim 1, the substance is Fe. 2 O 3 A method for estimating whether a substance can be removed.

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