Laser processing method

Abstract

The present invention addresses a problem in which a metal is easily corroded by rust when water is used as a plasma confinement layer in a laser peening method utilized as a metal surface modification method.　This laser processing method comprises a step of applying or affixing a gel-like substance to a processing region of an object to be processed, and a step of repeatedly irradiating the processing region with a laser. Because the laser processing method allows use of a gel-like substance containing no water as the plasma confinement layer, corrosion of the metal, which is the object to be processed, due to rust can be avoided. The gel-like substance is suitably used as a physical gel.

Description

Laser processing method

[0001] The present invention relates to a laser processing method for irradiating a surface of a workpiece with a pulsed laser beam through a plasma confinement layer to increase the hardness of the workpiece.

[0002] Laser peening is a cold working technique for metal surfaces and is industrially used for parts that require high reliability, such as aircraft parts, nuclear power plants, and bridge girders, as improvement and prevention measures against metal fatigue and stress corrosion cracking. The principle is to generate ablation plasma on the surface of a sample by focusing and irradiating a high-power pulsed laser with a time width of 10 －９ seconds to 10 －８ seconds on a metal material in water, and form a compressive residual stress layer and a hardened layer near the metal surface by suppressing the expansion of the plasma with water.

[0003] Here, the medium that suppresses the generated plasma is called a plasma confinement layer. Since a high plasma pressure is required to generate a shock wave energy large enough to plastically deform the metal, the plasma confinement layer is responsible for suppressing the expansion of the plasma generated on the metal surface.

[0004] Generally, water is used for the plasma confinement layer. However, since the target of laser peening is mostly metal materials, there is a problem that rust occurs and the corrosion of the metal progresses when water is used. Also, for this reason, it is considered more beneficial to use a solid medium such as glass for the plasma confinement layer. However, a fine gap occurs between the glass and the metal, resulting in an impedance mismatch, and the effect of laser peening is reduced.

[0005] Also, when water is used for the plasma confinement layer, in order to suppress the progress of corrosion as described above, a process such as drying must be added, and problems such as an increase in processing costs also occur. Furthermore, when water is used for the plasma confinement layer, problems such as the need for dimensional limitations and the immersion of areas not requiring construction also occur.

[0006] Patent Document 1 discloses a laser processing apparatus that, as a method for solving these problems, comprises a laser oscillator for emitting laser light to irradiate a workpiece, a ball lens for focusing the laser light and guiding it to the workpiece, and a media holding mechanism for holding a medium that is pressed by the ball lens and placed between the ball lens and the workpiece.

[0007] This laser processing device uses a transparent resin tape such as silicon as the plasma confinement layer (medium), and irradiates the surface to be processed with a pulsed laser while pressing the medium against it with a ball lens for laser irradiation.

[0008] Japanese Patent Publication No. 2017-042774

[0009] The apparatus described in Patent Document 1 has, to some extent, solved the problems associated with using water in the plasma confinement layer. However, because the silicon tape (medium) used in the plasma confinement layer must always be pressed against the surface of the workpiece, the processing area per shot is very small, and in laser peening methods where coverage (overlapping shots) must be performed while changing the irradiation position, a problem arises in that the processing speed decreases.

[0010] Furthermore, since irradiation cannot be performed from a point far from the irradiation site, there are geographical limitations on the areas that can be treated.

[0011] This invention was conceived in view of the above-mentioned problems, and provides a laser processing method that can confine the plasma, does not corrode the metal material to be processed, and achieves laser peening without pressing the laser against the processing area.

[0012] More specifically, the laser processing method according to the present invention is characterized by comprising the steps of applying or attaching a gel-like substance to a processing area of ​​an object to be processed, and repeatedly irradiating the processing area with a laser.

[0013] Physical gels are particularly suitable as gel-like substances.

[0014] In the laser processing method according to the present invention, a gel-like substance is used as the plasma confinement layer, which adheres closely to the surface of the metal material to be processed, confining the plasma produced by the pulsed laser and improving the surface hardness of the workpiece.

[0015] Furthermore, if a physical gel is used, even with a gel-like substance, the treated object does not come into contact with water, thus eliminating the problem of corrosion due to rust.

[0016] Furthermore, it eliminates the need for additional steps such as subsequent washing and drying.

[0017] This figure shows the implementation state of the laser processing method according to the present invention. This graph shows the results of the embodiment when the type of plasma confinement layer is changed. This graph shows the results of dynamic viscoelasticity measurement (storage modulus) of various gel-like materials used in the plasma confinement layer. This graph shows the results of dynamic viscoelasticity measurement (loss modulus) of various gel-like materials used in the plasma confinement layer. This graph shows the results of dynamic viscoelasticity measurement (loss tangent) of various gel-like materials used in the plasma confinement layer. This graph shows the measurement results of light transmittance at a thickness of 2 mm of various gel-like materials used in the plasma confinement layer.

[0018] The laser processing method according to the present invention will be described below with reference to drawings and examples. The following description illustrates one embodiment of the present invention and one example, and the present invention is not limited to the following description. The following description may be modified without departing from the spirit of the present invention.

[0019] Furthermore, embodiments and examples obtained by appropriately combining the technical means disclosed in different embodiments and examples are also included within the technical scope of the present invention. In addition, all references cited herein are incorporated herein by reference. In this specification, when a numerical range is described as "A to B", such description is intended to mean "A or greater and B or less".

[0020] Figure 1 shows the state of implementation of the laser processing method according to the present invention. A processing area 14 is provided on the surface of the workpiece 100. Then, a plasma confinement layer 12 is arranged to cover the processing area 14. In the present invention, since the plasma confinement layer 12 is a gel-like substance, a coating or attachment area 12w with a larger area than the processing area 14 can be obtained by coating or attaching it.

[0021] The thickness 12t of the plasma confinement layer 12 is preferably 2 ± 0.5 mm. If the thickness 12t is too thin, the plasma cannot be confined, and the plasma confinement layer 12 will break. Conversely, if the plasma confinement layer 12 is too thick, the laser power will be absorbed. Furthermore, a thickness of 2 ± 0.5 mm allows for the desired improvement in surface hardness to be obtained.

[0022] The laser device 10 includes a laser oscillator 10d, a waveguide 10c, a lens unit 10b, and a lens 10a. The high-power pulsed laser LP generated by the laser oscillator 10d passes through the waveguide 10c and is irradiated from the lens 10a of the lens unit 10b. The laser device 10 or the workpiece 100 is moved (for example, in the MD direction) by a moving device (not shown), and the high-power pulsed laser LP is irradiated onto the processing area 14. The distance SD between the laser lens 10a and the workpiece 100 is not particularly limited as long as the focal point can be brought to the surface of the processing area 14.

[0023] The laser processing method according to the present invention can suitably be used to process materials 100 such as iron, copper, aluminum, titanium, and carbon steel. It is particularly suitable for easily oxidized metal materials. It can also suitably be used for rust-resistant materials such as stainless steel. This is because even rust-resistant metal materials such as stainless steel may lose the properties of the base material in areas where surface modification such as laser peening occurs.

[0024] The laser device 10 that can be used in the present invention has a pulse width of 1 ns (1 × 10⁻¹⁶). ―９ The time interval is 10 ns (1 x 10 seconds), and the output is 1 GW (1 x 10⁻¹⁰ seconds). ９ watts) / cm² ２ ~7GW / cm ２A suitable material of a certain degree can be used. The wavelength is not particularly limited.

[0025] The plasma confinement layer 12 usable in the laser processing method according to the present invention is a gel-like material having a three-dimensional network structure. This gel-like material includes those in which polymer chains or particles form a network structure through non-covalent interactions. It may be fluid when applied, but preferably has thixotropy to the extent that it becomes a gel after application. Furthermore, it is desirable that it has sufficient mechanical strength to maintain the plasma confinement function by plasma from a high-power pulsed laser LP when in the gel state.

[0026] A physical gel is a gel in which polymer chains or particles form a three-dimensional network structure through physical crosslinking points based on non-covalent interactions. More specifically, the gel-like material that can be used in the laser processing method according to the present invention may be any so-called physical gel. A physical gel is a material that has at least the following properties (a), and further has properties (b) and (c).

[0027] (a) Multiple polymer chains and / or particles form a three-dimensional network structure by physical crosslinking points based on non-covalent interactions, and hold liquid or mobile molecular components within the network structure.

[0028] (b) Under no-load conditions, it does not exhibit macroscopic flow due to its own weight and maintains a solid or semi-solid form that retains its shape for a predetermined period of time.

[0029] (c) In response to at least one external stimulus selected from changes in temperature, concentration, solvent composition, or mechanical stress, the state of the network structure reversibly changes, accompanied by the formation and dissociation of the physical crosslinking points, and the viscoelastic properties change reversibly accordingly (meaning that when the external stimulus is removed, the network structure or viscoelastic properties recover toward the state before the stimulus was applied). Due to the properties of physical gels, the numerical value of the viscoelastic properties may change over time due to aging, etc., but as long as the crosslinking and dissociation properties in response to external stimuli are not lost, it shall be included in the physical gel of the present invention.

[0030] These characteristics can be easily determined by the following relatively simple tests: (1) Confirm fluidity under no load. The gel-like substance to be tested is placed in a container or on a flat substrate and observed for a predetermined time under no load without applying any external force. If the gel-like substance does not flow due to its own weight and maintains its shape macroscopically, it is determined to have the characteristics specified in (b) above.

[0031] (2) Evaluation of changes in viscoelastic properties in response to external stimuli At least one external stimulus selected from temperature changes, concentration changes, solvent composition changes, or mechanical stress is applied to the gel-like substance, and dynamic viscoelasticity measurements are performed. At this time, changes in the storage modulus (G') and / or loss tangent (tanδ) are measured. If a change in the storage modulus and / or loss tangent is observed due to the application of external stimuli, it is determined that the three-dimensional network structure or the state of intermolecular interactions of the gel-like substance has changed.

[0032] (3) Confirmation of reversibility After removing the external stimulus and returning the gel-like substance to the conditions before the stimulus was applied, dynamic viscoelasticity measurement is performed again. At this time, if the storage modulus and / or loss tangent recover toward the state before the stimulus was applied, the change is considered reversible and is judged to be a change based on the formation and dissociation of the physical crosslinking points. On the other hand, if the viscoelastic properties do not recover even after removing the external stimulus and show an irreversible change, it is highly likely that the gel-like substance does not qualify as a physical gel in the present invention.

[0033] (4) Handling of the effects of aging, etc. Due to the properties of physical gels, the values ​​of the storage modulus or loss tangent may change over time due to aging, etc. However, even if such changes over time are observed, as long as a reversible change in viscoelastic properties based on the formation and dissociation of physical crosslinking points can continue to be observed by applying and removing the external stimuli, the gel-like substance shall be included in the physical gel of the present invention.

[0034] Based on the above evaluation, the present invention can objectively determine whether the target gel-like substance is a physical gel that has physical crosslinking points based on non-covalent interactions and whose three-dimensional network structure and viscoelastic properties reversibly change in response to external stimuli.

[0035] Furthermore, it is preferable that the gel-like material has a transmittance of 35% or more when it is the thickness of the plasma confinement layer 12. This transmittance refers to the light transmittance at the wavelength of the laser used. This is because the laser needs to reach the surface of the workpiece 100. If the transmittance is high, the laser can reach the surface of the workpiece 100 without loss, and a peening effect can be expected in a short time.

[0036] Furthermore, an organogel using a non-aqueous solvent or dispersion medium is more preferable as the plasma confinement layer 12. This is because, since the solvent or dispersion medium does not contain water, it can effectively suppress the occurrence of oxidation and corrosion (rust) on the surface of the workpiece 100.

[0037] The physical gel constituting the plasma confinement layer 12 in the present invention contains a solvent or dispersion medium and a gel-forming component that forms reversible physical crosslinks in the solvent or dispersion medium. The following can be suitably used as the gel-forming component. However, it is not limited to these, and any material that forms physical crosslinks based on non-covalent interactions and has the property of reversibly forming and dissociating the crosslinks in response to external stimuli is also included. (1) Low molecular weight gelling agents: 12-hydroxystearic acid, amino acid derivatives, naphthamide derivatives, sugar derivatives, etc. (2) Physically crosslinking polymers: polyvinyl alcohol (PVA), agar, gelatin, etc. (3) Block copolymers: polystyrene-block-polyisoprene-block-polystyrene (SIS), etc.

[0038] The solvent or dispersion medium is selected from aqueous or non-aqueous media depending on the chemical properties of the workpiece. The combination of gel-forming component and solvent or dispersion medium is appropriately selected based on the solubility and self-assembly ability of each component so as to form a physical network structure (for example, a non-aqueous medium is selected for 12-hydroxystearic acid, and an aqueous medium for agar).

[0039] Examples of the present invention are shown below. The object to be processed 100 was made of stainless steel (SUS316L). The second harmonic of a Nd:YAG laser (wavelength 532 nm, pulse width 4 ns, repetition frequency 10 Hz) was used as the laser light source. The laser spot diameter was 200 μm. The laser irradiation intensity was fixed at 3 GW / cm ２ and the coverage (laser overlapping rate) was changed to 100, 300, 500, 700, 900, 1100, 1300, 2000, 5000, 10000%.

[0040] An organogel (physical gel using an organic solvent) and a urethane gel were used as the plasma confinement layer 12. The organogel used was prepared by mixing 1.0 mass% of 12-hydroxystearic acid (12-HSA) in mineral oil (SMR-100 manufactured by Albac Kiko Co., Ltd.) as a solvent. Also, the urethane gel used was "Gel Stock Solution for Human Skin, Transparent, Hardness 7" manufactured by EXCEL Co., Ltd.

[0041] The treatment area 14 was 2 mm × 2 mm (4 mm ２ ), and the plasma confinement layer 12 was attached so as to completely cover the treatment area 14. The thickness 12t of the plasma confinement layer 12 was 2 mm.

[0042] The treatment area 14 after laser peening was measured by Vickers hardness. The results are shown in FIG. 2. Referring to FIG. 2, the horizontal axis is the coverage (number of irradiations at the same location (%), and the vertical axis is the amount of hardness increase (HV). The triangular marks indicate the case where the plasma confinement layer 12 is an organogel, and the square marks indicate a urethane gel.

[0043] The urethane gel faced the peak of the hardness increase at a coverage of 1300%, and the hardness decreased rapidly at subsequent coverages. In the case of the urethane gel, the coverage showing the effect of hardness increase was in the range of 1300% or more and 3000% or less.

[0044] On the other hand, in the case of the organogel, as the coverage increased, the amount of increase in hardness also increased. Since the organogel is less susceptible to damage by laser irradiation, the hardness of SUS316L also increased even in the high-energy region. That is, in the case of the organogel, the coverage showing the hardness increase effect was 1300% or more.

[0045] Note that the upper limit of the coverage showing the hardness increase effect in the case of the organogel has not been determined in this experiment, but it may be at least 20,000% or less from the perspective of approximating the graph in Fig. 2 and from a practical implementation perspective.

[0046] Next, the dynamic viscoelasticity of the gel-like substance was measured. In addition to the above-mentioned organogel and urethane gel, samples were used that were prepared by mixing 1.5% by mass of 12-hydroxystearic acid (12-HSA) with mineral oil (SMR-100 manufactured by Albak Kiko Co., Ltd.) as a solvent. To distinguish the organogel, the one mixed with 1.0% by mass of 12-hydroxystearic acid (12-HSA) was designated as "organogel 1", and the one mixed with 1.5% by mass was designated as "organogel 2". The measurement conditions are as shown in Table 1.

[0047]

[0048] The dynamic viscoelasticity measurement was carried out using a rheometer under the conditions of a frequency of 1 Hz, a strain amplitude of 0.1% within the linear viscoelastic region, and a heating rate of 2 K / min.

[0049] The measurement results are shown in Figs. 3, 4, and 5. Fig. 3 shows the storage modulus, Fig. 4 shows the loss modulus, and Fig. 5 shows the loss tangent. The storage modulus is an index representing the energy component that is temporarily stored as elastic energy in the material when a periodic strain is applied from the outside and can be recovered after the strain is removed. The higher the value of the storage modulus, the more the material shows spring-like elastic behavior and solid properties that are easy to maintain its shape.

[0050] The loss modulus is an index representing the energy component that is dissipated as heat or the like due to molecular motion and internal friction inside the material among the energy applied from the outside. The higher the value of the loss modulus, the more the material shows liquid-like viscous behavior.

[0051] The loss tangent is the value obtained by dividing the loss modulus by the storage modulus (tanδ), and it is an index that represents the relative contribution of the viscous component and the elastic component in a material. A larger loss tangent value indicates that viscous behavior is dominant, while a smaller value indicates that elastic behavior is dominant.

[0052] Figures 3 and 4 show sample temperature (°C) on the horizontal axis and (storage or loss) modulus (Pa) on the vertical axis. On the other hand, Figure 5 shows sample temperature on the horizontal axis and loss tangent (tanδ (dimensionless)) on the vertical axis. Furthermore, throughout Figures 3 to 5, the solid black circles represent the measured values ​​for urethane gel, the dashed black triangles represent the measured values ​​for organogel 1, and the dotted black squares represent the measured values ​​for organogel 2.

[0053] Furthermore, the transmittance was measured for urethane gel, organogel 1, and organogel 2 at a thickness of 2 mm. The results are shown in Figure 6. In Figure 6, the horizontal axis represents the wavelength of light (nm), and the vertical axis represents the transmittance (%). The laser used in Figure 2 was a YAG laser (second harmonic) with a wavelength of 532 nm.

[0054] In Figure 6, the solid line represents urethane gel, the dashed line represents organogel 1, and the dotted line represents organofel 2.

[0055] Referring to Figures 3 and 4, the elastic modulus of the urethane gel changed linearly as the temperature increased. The storage modulus showed an increasing trend, while the loss modulus showed a decreasing trend. On the other hand, both the storage modulus and loss modulus of organogel 1 and organogel 2 decreased sharply between 65°C and 75°C.

[0056] Referring to Figure 6, the urethane gel of the example had a transmittance of 70% at a laser wavelength (532 nm) with a thickness of 2 mm. On the other hand, organogel 1 (1.0 mass%) of 12-hydroxystearic acid had a transmittance of 51%, and organogel 2 (1.5 mass%) of 12-hydroxystearic acid had a transmittance of 44%.

[0057] Below, we will consider the results shown in Figure 2 based on the results described above. This organogel is a physical gel that reversibly changes state between a sol phase (liquid state) that exhibits fluidity and a gel phase (solid state) that retains its shape. Therefore, as the temperature rises, the state changes from the gel state to the sol state above a certain temperature.

[0058] In other words, a gel-sol transition occurs in the organogel within a predetermined temperature range. As a result, the storage modulus decreases sharply with increasing temperature, and the loss modulus increases gradually before decreasing (see Figures 3 and 4). Consequently, the relative contribution of the loss modulus to the storage modulus increases, and the loss tangent shows a sharp increase (Figure 5).

[0059] On the other hand, in the urethane gel, both the storage modulus and the loss modulus changed gradually with increasing temperature (see Figures 3 and 4), and the loss tangent (tanδ) showed a decreasing behavior with increasing temperature (Figure 5). In other words, the urethane gel did not undergo a clear gel-sol transition, and a strong crosslinked structure was maintained within the measured temperature range. This characteristic does not involve the dissociation of the network structure due to thermal stimulation, and therefore does not contribute much to the shock mitigation that is the objective of the present invention.

[0060] In laser peening, the temperature rise caused by the plasma is an extremely localized and instantaneous phenomenon. When a liquid such as water is used as the plasma confinement layer, heat is rapidly diffused by convection and thermal diffusion, so the overall temperature rise of the confinement layer is thought to be suppressed to a relatively small level.

[0061] In contrast, in solid or semi-solid materials such as gels, thermal diffusion is suppressed compared to liquids, which can lead to localized temperature increases near the plasma. Furthermore, the role of the plasma confinement layer is to suppress plasma expansion and generate extremely high pressure states (around tens of GPa), and gel-like materials are directly subjected to this high-pressure shock.

[0062] At this time, the organogel undergoing the gel-sol transition is thought to temporarily transition to a sol state (liquid state) due to a localized temperature increase, thereby mitigating strong impacts and suppressing plasma expansion. As a result, as shown in Figure 2, the amount of hardness increase increased as the coverage increased.

[0063] On the other hand, since urethane gel does not undergo a gel-sol transition, repeated localized high-pressure impacts do not easily induce energy relaxation through solification. As a result, localized stress tends to concentrate on the structure of the gel-like material. Therefore, structural deterioration and damage are more likely to occur, and it is understood that the amount of hardness increase tends to decrease with increasing coverage.

[0064] Furthermore, the relatively high increase in hardness when urethane gel was used as a containment layer at low coverage levels may be due to the high transmittance of urethane gel to laser light, which allowed the laser peening effect to manifest before the structure of the gel-like material changed. Additionally, the small loss tangent (tanδ) of the urethane gel, which allowed it to respond elastically to the initial impact without viscous energy dissipation, is also considered a contributing factor. However, a small loss tangent implies a poor ability to internally mitigate impact energy as a viscous component.

[0065] Therefore, with repeated irradiation, impact energy accumulates in the mesh structure, resulting in increased structural deterioration and damage. In contrast, the organogel of the present invention can efficiently mitigate and disperse impact energy through the development of appropriate viscosity (increase in tanδ) accompanied by a gel-sol transition, thus maintaining a stable peening effect even under high coverage conditions.

[0066] Based on the above results, an organogel that exhibits a gel-sol transition in which the loss tangent increases sharply when dynamic viscoelasticity measurements are performed under at least the conditions shown in Table 1 can be suitably used as the gel-like substance of the present invention. Specifically, it is desirable to have a temperature range within any 5°C in which the increment of the loss tangent (tanδ) is 0.05 or more.

[0067] Furthermore, even without exhibiting a gel-sol transition, a certain degree of peening effect could be obtained if the transmittance was 60% or more with respect to the laser light used, at a plasma confinement layer thickness of 2 mm ± 0.5 mm.

[0068] Furthermore, the physical gel used as the plasma confinement layer 12 of the present invention also has a significant advantage in terms of ease of removal (cleanability) after processing. Because the physical gel has thermally reversible gel-sol transition properties, the residue after processing can be easily dissociated from its physical network structure by heating or washing with an appropriate solvent (such as a non-aqueous medium).

[0069] The plasma confinement layer 12 of the present invention provides a stable peening effect against high-pressure shocks during laser irradiation through a viscoelastic relaxation mechanism, while being removable in a short time after processing through sol formation. This high level of balance between "structural stability during processing" and "structural dissociability after processing" is a technical effect unique to physical gels that cannot be achieved with chemically crosslinked gels.

[0070] The laser processing method according to the present invention can be suitably used to improve the hardness of metal surfaces. In particular, its use can be expanded to surface modification of materials that are easily corroded by water.

[0071] 10 Laser device 10a Lens 10b Lens unit 10c Waveguide 10d Laser oscillator 12 Plasma confinement layer 12w Coating or attachment area (of the plasma confinement layer) 12t Thickness (of the plasma confinement layer) 14 Processing area 100 Workpiece LP High-power pulsed laser SD Distance between lens and workpiece MD Relative movement direction between workpiece and lens

Claims

1. A laser processing method comprising the steps of applying or attaching a gel-like substance to a processing area of ​​an object to be processed, and repeatedly irradiating the processing area with a laser.

2. The laser processing method according to claim 1, wherein the gel-like substance is a physical gel, and the number of repetitions is 1300 or more.

3. The laser processing method according to claim 2, wherein the gel-like substance is an organogel.

4. The laser processing method according to claim 2 or 3, wherein the coated or attached gel-like substance has a thickness of 2 ± 0.5 mm.

5. The laser processing method according to claim 4, wherein the transmittance of the laser in the thickness direction of the coated or attached gel-like substance is 35% or more.

6. The laser has a power output of 1 GW / cm². ２ ~7GW / cm ２ The laser processing method described in claim 4.

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