JP2017148829A - Ultra-short pulsed laser processing apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ultra-short pulse laser processing apparatus capable of forming a high-accuracy, high-efficiency and arbitrary pattern even in such a state that a distance from each processing point and a condenser lens does not necessarily get to constant when processing a certain processing area simultaneously according to beam branching in order to perform high-speed processing.SOLUTION: An ultra-short pulse laser processing apparatus includes: an ultra-short pulse laser beam source device composed of a seed laser and a regenerative amplifier; a high-speed shutter which opens and closes ultra-short pulse beam; beam branching means which branches the ultra-short pulse beam to a plurality of beams; beam scanning means; a condenser lens; scanning means which retains an object to be processed and performs three-dimensional translational movement and rotational movement; and vision means which observes characteristics of a processed face of the object to be processed before processing or during processing.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an ultrashort pulse laser processing apparatus for texturing the surface of a sliding part of a machine part with an ultrashort pulse laser, and in particular, an ultrashort pulse capable of forming various dimple patterns having a friction reducing effect on a workpiece surface. The present invention relates to a laser processing apparatus.

  Mankind has evolved through industry and has grown by burning the fuel buried in the earth and converting it into power. However, along with this, the amount of global warming gas emissions has increased, which has a great adverse effect on the global environment. In the automobile industry, one of the world's important industries, internal combustion engines are directly involved in the creation of harmful substances, so improved technology has been applied to their functions and components. In the sense that the total amount of discharged substances can be reduced, it is one of the important issues to increase the conversion efficiency from thermal energy to kinetic energy in addition to the viewpoint of energy saving. Mechanical friction loss accounts for 60% of the total energy loss during load operation in an internal combustion engine. The main thing exists in a piston system, a crank system, and a valve system. Friction reduction of sliding parts including not only these power parts but also power transmission parts leads to reduction of global warming gas emissions and energy saving, and research and development has been actively conducted all over the world.

  As a processing method for reducing the surface frictional resistance of the sliding portion of the internal combustion engine in a reciprocating motion or a rotational motion to the limit, there is a processing technology for applying a fine uneven shape on the surface by a mechanical external force or a laser. This texturing process brings about the effect of improving the sliding surface function such as retaining the lubricant between the two sliding surfaces, modulating the surface pressure distribution, and changing the contact area. Laser processing machines that use a femtosecond laser to form a fine periodic structure or dimple shape on the surface of a workpiece are provided. For example, see Patent Documents 1 to 4.

  Friction loss in sliding has a characteristic that depends on the sliding speed. For example, dimples in texturing are the optimum processing area, shape, size, depth, and area occupancy for each operating speed. Called parameters). Therefore, a combination of various processing parameters is required. For example, see Non-Patent Document 1.

  Surface texturing technology is expected to improve the function of the sliding surface and reduce the deterioration rate of the workpiece and extend the service life. In addition to the internal combustion engine, there are innumerable sliding parts in all mechanical devices, so a technique for reducing friction loss by surface texturing is important.

JP 2008-200698 A JP 2008-200699 A JP 2008-200700 A JP 2009-028766 A

I. Etsion, “Improving tribological performance of mechanical components by laser surface texturing”, Tribology Lett. 17, 733 (2004).

  In the above prior art, the texturing apparatus using the femtosecond laser is characterized in that beam scanning is performed so that the relative distance between the surface of the workpiece in the irradiation region and the processing point of the focused beam is always constant.

  The present invention has been made in view of the above-described problems of the prior art, and the purpose thereof is not limited to each processing particularly when a certain region is simultaneously processed by branching a beam for high-speed processing. It is an object of the present invention to provide an ultrashort pulse laser processing apparatus capable of forming an arbitrary pattern with high accuracy, high efficiency even when the distance between the point and the condenser lens is not constant.

  In order to solve the above-described problems and achieve the object, an ultrashort pulse laser processing apparatus of the present invention includes an ultrashort pulse laser light source device including a seed laser and a regenerative amplifier, a high-speed shutter that opens and closes ultrashort pulse light, and A beam branching means for splitting ultrashort pulse light into a plurality of beams, a beam scanning means, a condenser lens, a scanning means for holding a workpiece and performing a three-dimensional translation operation and a rotation operation, and a workpiece And a vision means for observing the characteristics of the processed surface before or during the processing.

  With the ultrashort pulse laser processing apparatus of the present invention, an arbitrary texturing pattern can be formed on a workpiece at high speed, high accuracy, and high efficiency. As a result, the sliding surface obtained by texturing the workpiece can reduce the frictional resistance to the utmost limit, thereby improving the fuel consumption of the internal combustion engine to the maximum. Not only the internal combustion engine, but also applied to sliding parts of various mechanical devices such as scanning stages and mechanical seals, the sliding resistance can be reduced.

It is a figure which shows the structure of the ultrashort pulse laser processing apparatus concerning this Embodiment. It is the observation photograph of the dimple formed in the piston ring sliding outer peripheral surface by the ultrashort pulse. It is a figure which shows the structure of a beam branching means. It is a figure explaining the irradiation method of the shape of a piston ring and the branched ultrashort pulse beam. It is the observation photograph of the piston ring sliding outer peripheral surface which performed the dimple process with the ultrashort pulse laser processing apparatus concerning this Embodiment.

  The best mode of an ultrashort pulse laser processing apparatus according to the present invention will be described below in detail with reference to the accompanying drawings. In the drawings shown below, for convenience of explanation, the scales of the members of the drawings may be described differently.

  FIG. 1 is a diagram showing a configuration of an ultrashort pulse laser processing apparatus according to the present embodiment. The ultra-short pulse laser processing apparatus includes an ultra-short pulse laser light source device 1, a high-speed shutter 3 that opens and closes a single beam 2.1 that is a single light beam, a beam branching means 4 that branches a laser beam into an arbitrary pattern, and a high-speed laser beam. A beam scanning means 5 that scans with high accuracy, a condenser lens 6 that condenses the ultrashort pulse laser beam, a chuck 8 that grips the workpiece 9, and a workpiece 9 that moves and positions in a two-dimensional direction X Axis stage 7.1, Y-axis stage 7.2, rotor 7.3 that rotates workpiece 9 about the cylindrical axis, workpiece 9 in the optical axis direction of the ultrashort pulse laser beam condensing point, and And a vision device 10 for observing the relative distance between the processing position of the branched ultrashort pulse beam 2.2 and the surface of the workpiece 9. Is done. The vision apparatus 10 observes the surface of the workpiece 9 with the observation wavelength light 11 different from the wavelength of the ultrashort pulse laser. The observation wavelength light 11 does not matter whether it is coaxial or non-coaxial with the ultrashort pulse beam 2.2.

  Although the case where the workpiece 9 is a piston ring mounted on the piston portion of the internal combustion engine will be described in detail later, the workpiece is not limited to the cylindrical outer peripheral portion, and for example, the inner wall of a cylinder liner may be processed. Further, the workpiece 9 is not limited to a cylindrical shape, and may be a free-form bulk shape.

  The X-axis stage 7.1, the Y-axis stage 7.2, the rotor 7.3, and the Z-axis stage 7.4 are controlled by CNC technology. Using the observation result of the vision apparatus 10, the outer peripheral surface of the piston ring is positioned with high accuracy, and the speed control of the rotation operation is performed.

  The piston ring to be processed is made of, for example, metal, and the outer peripheral portion may be coated to improve wear resistance and scuff resistance. Examples of the coating include a Cr—N-based film, a Cr—B—N-based film, a Ti—N-based film, a W—Ni-based hard carbon film, and a Si-based hard carbon film. There are generally three types of piston rings, a top ring, a second ring, and an oil ring. In the present invention, the effects of the invention have been demonstrated mainly for the top ring, but the application of the device according to the present invention is not limited thereto. . The outer peripheral surface, which is the sliding surface of the piston ring, includes a barrel type (barrel-shaped curved surface) and an inclined type.

  The ultrashort pulse laser light source device 1 includes a seed laser 1.1 and a regenerative amplifier 1.2. The seed laser 1.1 is a light source device including a similariton laser that generates a chirp pulse or a gain-switched laser diode (LD) and a preamplifier thereof.

  The seed pulse light of these seed lasers 1.1 injected into the regenerative amplifier 1.2 is linearly chirped from several tens of picoseconds to several hundreds of picoseconds. A high output with an average power of 10 W or more can be obtained in seconds to 10 picoseconds. Even if the pulse width of the ultrashort pulse laser beam is 1 to 10 ps (picoseconds) and the material of the workpiece 9 is a dielectric, a semiconductor, or a metal, non-thermal processing is performed at the laser irradiation portion boundary.

  An apparatus configured to generate a mode-locked similariton pulse with a fiber laser can be used as the seed laser 1.1. Although the output pulse width is 20 ps, it is linearly chirped, compressed with a fiber Bragg grating, and injected into the regenerative amplifier 1.2 in a state of 1 ps. The spectrum width is narrowed by the gain width of the Yb: YAG crystal, and the pulse width after reproduction amplification is output at 3 picoseconds. The seed laser 1.1 is equipped with a carbon nanotube saturable absorption modulator to stabilize the mode lock, and supplies a stable seed pulse.

  On the other hand, a gain-switched laser diode can be used for the seed laser 1.1, in which case the pulse width at the output of the regenerative amplifier 1.2 is 6 picoseconds.

  A Yb: YAG crystal is used as the gain medium of the regenerative amplifier 1.2 and generates pulsed light having a pulse width of 1 to 10 picoseconds. The center wavelength is 1030 nm, the average power is about 20 W to 100 W, the pulse generation repetition frequency is about 10 kHz to 1000 kHz, and the pulse energy is about 100 μJ to 1 mJ. The direction of light polarization of the laser pulse is linear polarization.

  FIG. 2 shows an example of dimple processing formed by irradiating the outer periphery sliding portion of the piston ring with an ultrashort pulse laser of 3 picoseconds. The processing conditions were that an ultrashort pulse beam generated at 50 kHz was irradiated for 20 milliseconds, that is, 1000 pulses were applied to one dimple. 2A is an optical micrograph, FIG. 2B is a contour map (color-coded) by a laser microscope, and FIG. 2C is a sectional view. The dimple size has a diameter of 23 μm, a depth of 10 μm, a pitch between dimples of 40 μm, and an area filling rate of 49%. The piston ring is provided with a chromium nitride (CrN) surface coating as a layer having a thickness of about 20 μm. In FIG. 2 (c), debris deposition and melting-derived burrs (projection shape) generated during conventional laser processing are extremely small in the processing peripheral portion, and non-thermal processing has been demonstrated. When protrusions are generated around the dimples by machining, the height is desirably 1 μm or less, but in this machining, the height of the protrusions was 0.1 μm or less.

  The pulse beam of the ultrashort pulse laser can be set to irradiate or not irradiate the workpiece 9 for each pulse by the high-speed shutter 3. The high-speed shutter 3 uses an electro-optic modulator or an acousto-optic modulator. The electro-optic modulator will be described in detail. The electro-optic modulator includes a Pockels cell mounted with a nonlinear optical crystal beta-barium borate crystal (BBO) and a polarizer. When a pulse voltage is applied to the Pockels cell, the direction of polarized light is changed and guided to the beam trajectory that irradiates the workpiece 9 by the polarizer. For example, TFP (Thin Film Polarizer) can be used as the polarizer.

  The beam branching unit 4 includes a DOE (Differential Optical Elements) or a spatial phase modulator (for example, Liquid crystal on silicon: LCOS) and a beam shaping optical element disposed before and after the spatial phase modulator (LCOS). FIG. 3 is a diagram showing the configuration of the beam branching means 4. Specifically, an apparatus configuration example of the beam branching unit 4 that splits the laser light into four by using DOE as the beam branching unit 4 is shown.

  Single beam (single beam ultrashort pulse beam) 2.1, which is a single beam, corrects aberrations etc. with correction optical system 4.1, and uses beam expander 4.2 as the beam for the axis. Adjust to 10 mm. Since the beam transmitted through DOE 4.3 is branched into four while being diffracted, they are transmitted to the subsequent beam scanning means 5 by the image relay optical system 4.4. FIG. 3A shows an optical microscope image of dimple processing formed by condensing and irradiating four-branched ultrashort pulse beam 2.2. The dimple diameter is 40 μm and the pitch between the dimples is 100 μm.

  An LCOS that is a liquid crystal type among spatial phase modulators (SLMs) can be mounted on the beam branching unit 4. A structure in which liquid crystal is sandwiched between a transparent substrate opposite to a silicon substrate. A liquid crystal driving circuit and a pixel electrode are provided on the silicon substrate side, and laser light that has passed through the transparent substrate and the liquid crystal layer is reflected by the pixel electrode. Is done. The reflected laser light is spatially phase-modulated, and is shaped into an arbitrary pattern at the laser irradiation point by passing through the condenser lens. A spatial phase modulation pattern is determined by inverse Fourier transform of the target laser irradiation pattern. For example, a GS (Gerber-Saxton) algorithm can be used for optimization of the inverse Fourier transform. In LCOS, the divergence angle of the branched beam can be set for each beam, and when the branched beam is condensed by the same fθ lens, the position of the condensing point can be arbitrarily changed.

  The beam branching unit 4 controls the phase of the beam wavefront according to the relative position data of one or more branched beam condensing points with the processing surface. Each of the branched beams appropriately irradiates the processing surface and simultaneously performs a plurality of multi-point processing. A vision means (vision device 10) is mounted in order to obtain information on the relative position between the processing point of the focused beam and the surface. When utilizing physical data acquired at the time of manufacturing a workpiece, it may not be necessary to obtain position information for each workpiece.

  When the beam branching means 4 is a DOE, one beam array is formed by one DOE. When the beam branching means 4 is a spatial phase modulator, the beam arrangement and phase control can be programmed, so that various dimple arrangements can be formed on different surface shapes. Only a limited area of the processing surface is processed by moving the workpiece in cooperation with the beam branching means 4 and the chuck 8 (fixing tool).

  As the beam scanning means 5, a biaxial galvano scanner capable of two-dimensional laser beam scanning is used. The galvano scanner is preferably digitally controlled with an encoder in order to operate with high accuracy. By using a telecentric fθ lens as the condenser lens 6, the plurality of branched ultrashort pulse beams 2.2 have substantially parallel beam axes and irradiate the surface of the workpiece 9 at a designed interval. To do.

It is as a mirror material of the galvano scanner, SiC, although SiO ₂ and the like, since the mirror with the use of high power beams if surface accuracy of the mirror is a high SiO ₂ is not deformed, the fθ lens condenser can be irradiated It is possible to form a substantially uniform dimple shape having a high shape stability up to the limit of visual field.

  FIG. 4 is a diagram illustrating the shape of the piston ring and the irradiation method of the branched ultrashort pulse beam. FIG. 4 shows the positional relationship between the sliding surface of the piston ring 30 and the irradiation point where the branched ultrashort pulse beam is condensed. The sliding surface of the piston ring 30 is a barrel-shaped curved surface having a convex center, and a plurality of ultrashort pulse beams 2.2 branched from the ultrashort pulse beam 2.1 by the SLM are condensed by an fθ lens. Irradiated. The size of the piston ring 30 is approximately 90 mm at the outermost diameter, and the thickness of the ring is 1.2 mm. The height difference of the barrel-type sliding surface 31 is about 200 μm, and the divergent beam is corrected for the divergence angle of each beam by the SLM so as to compensate for the height difference. When the SLM is LCOS, the pattern can be changed at about 50 Hz. Therefore, when the entire sliding surface is filled with dimples, the processing parameters of the branch beam are changed by the LCOS while changing the processing trajectory for each rotation. Can be changed. The state of the processing is shown in FIG. The number of orbits from the branch beam 32 (orbit 1) to the branch beam 33 (orbit N) in FIG. 4 can be designed according to the friction reducing function. N represents 1 or more, and N = 1 indicates that the dimple processing is performed by rotating the piston ring 30 only once.

  The throughput efficiency until the single beam 2.1 output from the ultrashort pulse laser device 1 passes through the high-speed shutter 3, the beam branching means 4, the beam scanning means 5, and the fθ lens and is irradiated onto the piston ring 30 is approximately 50%. When the total amount of pulse energy irradiated for processing was 200 μJ, dimple processing could be performed with an SLM of 7 × 7 and an irradiation energy per branch beam of about 4 μJ.

  FIG. 5 is an observation photograph of the piston ring sliding outer peripheral surface that has been dimple processed by the ultrashort pulse laser processing apparatus according to the present embodiment. Specifically, an optical micrograph of the sliding outer peripheral surface of the piston ring 30 subjected to dimple processing is shown. A dimple array is formed with different processing parameters in the peripheral portion of FIG. 5A and the barrel central portion of FIG. 5B, the area occupancy is 20% and 4%, respectively, and the dimple diameter is 20 μm and 10 μm, respectively. is there. This is the result of changing the SLM program for each lap. In this dimple processing, the processing time per piston ring 30 is 30 seconds or less, and high accuracy, high efficiency, and arbitrary pattern formation, which are the problems of the present invention, have been demonstrated.

  The piston ring 30 is not subjected to dimple processing on the upper part of the sliding surface that adversely affects oil consumption, and the area ratio of the dimple at the center of the barrel where load capacity is required is reduced to reduce load capacity and oil consumption. Has a small area, and the dimple processing is performed at a high area ratio on the lower part (peripheral part) of the sliding surface, which is considered to have a friction reducing effect.

  Table 1 shows the measurement results of the frictional force between the piston ring before dimple processing and the piston ring after processing. A friction mean effective pressure (FMEP) that directly affects fuel consumption was used as a measurement target of the friction force. FMEP is a value obtained by dividing the total friction work of the piston by the stroke volume (displacement), and represents the magnitude of piston friction loss. The advantage is that the friction loss can be compared regardless of the displacement, and it can be compared with the average effective pressure, which is the value obtained by dividing the work due to the in-cylinder pressure by the stroke volume. did. At three rotation speeds of 1500, 2000 and 2500 rpm, the effect of the texturing process this time was obtained on the low speed side. A suitable friction reduction state was demonstrated from the boundary lubrication region to the mixed lubrication region.

  The relative distance between the laser irradiation point and the piston ring outer peripheral surface in the laser optical axis direction can be adjusted by changing the positions of the condenser lens and the galvano scanner. Processing operations that complement each other with DOE and SLM are possible. The Z stage 7.4 in FIG. 1 is used for adjusting the relative distance. Furthermore, the height of the laser irradiation point in the laser optical axis direction can be adjusted by changing the divergence angle of the laser light incident on the galvano scanner by installing a collimator composed of two or more lenses. . Which method is adopted is determined in consideration of the processing time by the branch beam, which is the unit processing time for the workpiece.

  The beam spot is not limited to a circle but can be given any shape. If a high-speed galvano scanner is used as the beam scanning means and complementary operation is performed with DOE and SLM, dimple processing such as an ellipse or cross shape is possible. is there.

  The embodiments of the present invention have been described above. Obviously, modifications may be made to the invention without departing from the scope of the invention as set forth in the appended claims.

  The present invention can be applied to surface texturing of a sliding part for the purpose of reducing friction at the sliding part.

DESCRIPTION OF SYMBOLS 1 Ultrashort pulse laser apparatus 1.1 Seed laser 1.2 Reproduction amplifier 2.1 Single beam 2.2 Ultrashort pulse beam 3 High speed shutter 4 Beam branching means 4.1 Correction optical system 4.2 Beam expander 4.3 DOE (diffractive optical element)
4.4 Image relay optical system 5 Beam scanning means 6 Condensing lens 7.1 X-axis stage 7.2 Y-axis stage 7.3 Rotor 7.4 Z-axis stage 8 Chuck
9 Workpiece 10 Vision device 11 Wavelength light for observation 30 Piston ring 31 Barrel type sliding surface 32 Branch beam (orbit 1)
33 Branch beam (orbit N)

Claims (8)

An ultrashort pulse laser light source device comprising a seed laser and a regenerative amplifier;
A high-speed shutter that opens and closes ultra-short pulse light,
Beam branching means for branching the ultrashort pulse light into a plurality of beams;
Beam scanning means;
A condenser lens;
Scanning means for holding a workpiece and performing three-dimensional translation and rotation;
An ultrashort pulse laser machining apparatus capable of patterning a free surface of a workpiece at high speed, comprising vision means for observing characteristics of a machining surface of the workpiece before or during machining.   The pulse width of the ultrashort pulse light is 1 to 10 ps, and is in a pulse width region in which a thermal influence is reduced at a laser irradiation portion boundary to the workpiece. Short pulse laser processing equipment.   3. The ultrashort pulse laser processing apparatus according to claim 1, wherein the workpiece is a part having a sliding friction surface in an internal combustion engine.   The ultrashort pulse laser processing apparatus according to any one of claims 1 to 3, wherein the free surface is inclined with respect to the focused beam.   The ultrashort pulse according to any one of claims 1 to 4, wherein the beam branching unit includes a diffractive optical element or a spatial phase modulator and a beam shaping optical system suitable for either of them. Laser processing equipment.   6. The ultrashort according to claim 1, wherein the seed laser is a seed light generation unit including a similariton laser or a gain-switched laser diode and a preamplifier thereof. Pulse laser processing equipment.   The part having the sliding friction surface is a piston ring, and a dimple process that is not necessarily the same and is not necessarily circular is performed on a part or all of the surface area of the sliding outer peripheral surface. The ultrashort pulse laser processing apparatus according to claim 3.   The piston ring does not have dimple processing on the upper part of the sliding surface that adversely affects oil consumption, the barrel center where load capacity is required, load capacity, and less impact on oil consumption, and friction 8. The ultrashort pulse laser processing apparatus according to claim 7, wherein dimple processing is applied to a lower portion of the sliding surface that is considered to have a reduction effect.