Precision Laser Processing Of Large Area Regular Subwavelength Periodic Ripples And Cooling Holes In Turbine Blade

Femtosecond laser fabrication has demonstrated the advantages of high quality and high precision.When the flux density of femtosecond laser is much higher than the ablation threshold,the materials in the focal area are rapidly excited to form intense ablation effects such as plasma eruption and phase explosion.When the laser energy density is near the ablation threshold,the plasma and surface plasmon polariton are induced on the surface of the material,and the surface periodic nanostructures are induced.Aiming at these two problems of femtosecond laser interaction with materials,the large area periodic stripe structure and the cooling gas holes in turbine blades are studied.In this study,we design and build a system of femtosecond laser interference via two cylindrical lenses and a femtosecond/nanosecond integrated machining system,and studied respectively in the two fields: the efficient fabrication of homogeneous and extremely straight subwavelength periodic ripples,and cooling holes in turbine blades.The main research and innovations are as follows:1.This study proposes a coherent resonance-enhanced(CRE)surface plasmon polariton(SPP)to fabricate regular and uniform nanogratings in large area on silicon by the interference of two femtosecond laser beams via cylindrical lens.The scanning method of two femtosecond laser interference via cylindrical lens and the method of fabricating large area regular and uniform nanogratings by direct writing on the surface of silicon are described in detail.This method has the following four advantages:(1)Inhomogeneity and low efficiency are two important factors that hinder the wide application of laser-induced periodic surface structures.This study proposes a coherent resonance-enhanced(CRE)surface plasmon polarization(SPP)to fabricate regular and uniform subwavelength nanogratings(RUSNGs)in a large area efficiently,which overcomes well these two problems.(2)To excite a CRE SPP,the interference period is adjusted to be an integer multiple of the SPP wavelength.The CRE SPP improves and greatly constraints the growth of periodic ripples,and circumvents the inhomogeneity arising from the interference pattern of light field,and the disturbances originating from the irregular microstructures caused by the previous pulse ablation.(3)The line focus of the cylindrical lenses improved the processing efficiency greatly.A stripe of 5 mm wide covered with RUSNGs was fabricated by single scanning.(4)Complex cross-scale patterns with RUSNGs in different directions were fabricated when the wafer was mounted on a x-y-z-? 4-axis translation stage.The structural colors of RUSNGs are much pure and brighter compared with the spaced nanogratings and the ripples induced by single laser beams.2.This paper presents a new strategy for efficiently fabricating unlimited large-area subwavelength periodic ripples with commercial grating quality on a silicon surface.The CRE SPP efficiently induced homogeneous and extremely straight subwavelength periodic ripples through gentle ablation,which avoids substantial debris and reduces the roughness of the subwavelength periodic ripples.The SPR nanogratings were fabricated on a silicon wafer with a diameter of 80 mm by scanning at a speed of 2 mm/s and a spacing of 1.55 mm.The quality of subwavelength periodic grating is similar to the gold gratings.Compared with the alternating amorphous-crystalline ripples induced by a single laser beam,the fabrication efficiency was enhanced by 2-3 orders.The diffraction resolution of the subwavelength periodic ripples nanogratings was nearly as narrow as that of commercial gratings.3.Comprehensive utilization of the advantages of high precision and high quality machining of femtosecond laser and high efficiency machining of nanosecond laser,an experimental system 400 nm femtosecond/532 nm nanosecond laser processing of cooling gas holes in turbine blades is designed and built.The self-designed and customized optical lens has solved the problem of excessive energy loss in the simultaneous transmission of dual wavelengths.A composite optical transmission module and optical switching module have been designed to solve the technical difficulties of the dual wavelength common path and transmission.The independent innovation design has completed the dual CCD laser focus monitoring system,and has completed the dual wavelength focusing and common point.By studying the laser processing technology based on stainless steel circular through-holes and shaped holes,we have mastered the influence of laser processing technology parameters on the quality of air film holes,realizing the high precision and high aspect ratio(>15:1)of the film cooling holes.Blade clamping,positioning,attitude adjustment,multi-dimensional detection of on-line cooling holes are developed,and high precision laser machining of cooling holes is realized on stainless steel turbine blades.CT detection results show that there is no obvious crack in the inner wall of the hole,and the aperture error-34~10 ?m.