The ultra-high peak power and ultra-short pulse time characteristics of the femtosecond laser allow it to act quickly on a specific area during machining, and the material in the area of the laser beam is removed in the form of a plasma eruption to achieve cold machining with almost no thermal impact on the surrounding material. Laser cold machining technology is widely used because of its high processing accuracy, small heat-affected zone and high machining efficiency. One of the important applications is the machining of air film holes in aero-engine turbine blades. Due to the existence of hollow cavities and the close distance between the opposite walls of the aero-engine turbine blades, the hole making process is prone to contramural damage. Therefore, in order to avoid contramural damage and improve the quality of air film hole machining, a real-time detecting system needs to be developed to judge the hole penetration status during the hole machining in real time. Laser Induced Breakdown Spectroscopy(LIBS) is a kind of atomic emission spectroscopy technology, which has a fast response time and can achieve real-time detection and analysis of the target under test. In the LIBS detection system, a spectrometer is used to collect the emission spectra excited during the plasma cooling process of femtosecond laser machining, and the elements corresponding to each spectral line can be identified according to the characteristic wavelength of atomic or ion emission spectra in a specific wavelength band, and quantitative analysis can be achieved by measuring the intensity of emission spectra captured at a specific wavelength. Aiming at the problem of contramural damage in the machining of aero-engine turbine blade with femtosecond laser, a hole penetration detection scheme based on laser-induced breakdown spectroscopy detection technology of femtosecond laser hole making is proposed. Based on the elemental analysis of the material and the information of the LIBS spectrogram obtained in the wavelength 520 similar to 560 nm during processing, the emission spectrum of Cr(I) 521.531 nm is selected as the characteristic spectral line, and the change in intensity of the characteristic spectral line is used to realize the judgment of the hole machining process. In this paper, we design a laser cold machining hole penetration spectroscopy detection system consisting of an ultrashort pulse laser, a grating spectrometer, a focusing optical path and an upper computer for data processing and analysis. The effect of important machining parameters on the detection performance of LIBS is studied experimentally, and the effect of machining parameters on the spectral intensity is analyzed, and the optimized machining parameters are selected as laser energy of 32 W, the distance of the part to be processed from the focus of 150 mm, and the laser spin speed of 2 400 r/ min. The designed hole penetration detection system is used to continuously collect and analyze the plasma spectra during the actual hole machining, and the correspondence between the specific spectral intensity and the penetration state of the sample to be processed in the hole machining is obtained. The intensity of the spectrum peaks at the beginning of the hole punching because of the high percentage of upward reflection of the atomic emission spectra during the laser exit stage;the overall intensity of the atomic emission spectra gradually decreases as the hole depth increases; the detected intensity of the atomic emission spectra starts to decrease significantly when the hole starts to penetrate gradually;when the hole is completely penetrated, there are no reflected back characteristic spectral lines and only background noise with spectral line intensity less than 230. The experimental results verify the feasibility of the method in solving the problem of preventing damage to the opposite wall during femtosecond laser hole machining, and can provide real-time feedback on the completion of hole processing to help the development of the feedback system, which is conducive to the further optimization of laser cold machining technology in terms of accuracy and efficiency.
