| Femtosecond laser ablation of solid materials shows the advantages of small thermal effect,high processing accuracy,and no obvious material selectivity.Femtosecond laser processing has been widely used in aerospace,microelectronics,biomedical,and other related fields.With increasing the laser fluence,annealing(or phase change),periodic structures,and drilling appear in the laser focal spot in sequence.This doctoral dissertation studied systematically laser-induced periodic surface structures(LIPSS)and the ultra-fast laser processing of cooling holes in turbine blades.Experimental systems of femtosecond laser direct writing and femtosecond laser interference via double-cylindrical lens were built,and the fabrication of periodic surface structures,structure color,and birefringence effect on the glass surface coated with indium tin oxide(ITO)film and the bare glass surface were studied.The femtosecond/nanosecond composite precision machining system was established and the key issues related to high-quality machining of turbine blade cooling holes were also studied in detail.The following research results have been achieved:1.A femtosecond laser(1030 nm,250 fs,1 m J,1 k Hz)is used to efficiently produce a large area regular low spatial frequency LIPSS(LSFL)on the ITO film through a cylindrical lens.When there is only a few accumulated laser pulse,the absorptivity of ITO film is very low,which demonstrates mainly as a dielectric.The short periodic structures parallel to the laser polarization on the surface are mainly caused by the non-uniform absorption caused by the interference of the scattered light and incident laser.When there are many accumulation pulses,the absorption of the ITO film is increased by 2 times,which leads to transient metallization.The excitation of surface plasmons induces regular LSFL perpendicular to the laser polarization.The use of cylindrical lenses can not only improve the processing efficiency,but also improve the regularity of LIPSS.LSFL produced on the surface of ITO film shows bright structural color.At the same time,the production of nanostructures on ITO film surface can adjust its photoelectronic characteristics,such as increasing the transmittance of infrared light and high conductivity parallel to the LSFL direction.This regular LSFL and its preparation method have great application potential in preparing liquid crystal photonic devices with low loss and forming the directional conductance layer.A femtosecond laser(1030 nm,250 fs,1 m J,1 k Hz)is used to efficiently produce a large area regular LSFL on the glass surface coated with the ITO film through a cylindrical lens by energy transfer.Compared with the direct processing on the glass surface,the laser fluence used on the glass surface coated with the ITO film is only one-fifth,which greatly reduces the residual heat effect during laser ablation,resulting in more straight and regular LSFL.The effects of laser fluence and scanning velocity on the LSFL periodicity and regularity are studied in detail.When femtosecond laser fluence is 1.09 J/cm2 and scanning velocity is 3 mm/s,large-area regular LSFL with a period of 930 nm is obtained on the glass surface.2.The femtosecond laser(800 nm,50 fs,1 kHz)is focused by a single cylinder lens to efficiently produce high spatial frequency LIPSS(HSFL)with birefringence effect on the glass surface coated with indium tin oxide(ITO)film.The laser fluence on the coated glass was only one-tenth of that on bare glass,which significantly reduced the thermal effect,and increased the thickness of surface layer with nanostructures.A stripe of 4 mm wide covered with HSFL was produced by single scanning,and the machining efficiency is 100 times that of using a circular lens.HSFL with a birefringence effect with a period as short as 100 nm is efficiently produced by using this method,and the retardance is as high as 44 nm,which is 8 times that of bare glass.The glass coated with ITO film was cut to observe the cross section,from which it was found that the LIPSS layer is 1.6 ?m thick,whereas it is invisible on the bare glass.This is the main reason for the high birefringence of ITO coated glass.The content of indium ions on the cross section is measured by energy dispersion spectrometer(EDS).The results indicate that indium ions penetrate the glass during laser ablation and strongly absorb the subsequent laser pulse energy to excite the nanoplasma.The nanoplasmas naturally grow into nanoplanes irradiated by the linearly polarized laser because of the inhomogeneous local field enhancements,forming LIPSS layer.3.The femtosecond laser beam(800 nm,50 fs,1 k Hz)is focused and interfered through two cylindrical lenses,and two types of regular,straight large-area LIPSS are efficiently produced on the surface of fused silica: grating-like structures and spaced nanostructures.This method has the following advantages:(1)Inhomogeneity and low efficiency are two important factors that limit the application of laser-induced periodic surface structures(LIPSS),especially on the surface of dielectric materials with high damage threshold.This paper proposes a two-beam interference method using cylindrical lenses,which can effectively prepare large-area regular and uniform low spatial frequency LIPSS.(2)The linear focus of cylindrical lens can greatly improve the processing efficiency and the regularity of LIPSS.A stripe of 4 mm wide covered with nanostructures was fabricated by single scanning.(3)Graphic patterns of falling petals and two types of flowers with grating-like LIPSS oriented in different directions are processed by mounting the fused silica on an x-y-z-? four-axis translation stage.Compared with the spaced nanostructure,the color of the grating-like structures are purer and brighter.4.Based on the advantages of femtosecond laser machining,such as high precision,small thermal effect,no material selective characteristics,and combined with the advantages of nanosecond laser,such as high power,high stability,and high ablation efficiency,a composite machining system with a 400 nm femtosecond and a 532 nm nanosecond laser was designed and built.The key issues related to high precision and high speed laser machining of cooling holes were sudided in detail on the nickel-based alloy plates and nickel-based alloy plates with thermal barrier coatings.Several types of cooling holes(including circular holes and fan-holes)were fabricated with a high aspect ratio(>15:1)and very high precision.High precision laser machining of cooling holes has been realized on stainless steel turbine blade.CT detection results showed that there was no obvious crack in the inner wall,and the aperture error is less than ±30 ?m. |
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