Investigation on millijoule femtosecond laser spiral drilling of micro-deep holes in thermal barrier coated alloys

Abstract

Ultra-short laser drilling of micro-holes with high quality and high processing efficiency in the aviation industry is urgently needed. This study reported the fabrication of micro-deep holes on thermal barrier coated alloys through millijoule femtosecond laser beam irradiation. The self-focusing effect (Kerr effect induced by the intense beam itself) and waveguide effect (reflections induced by the sidewall) on beam propagation behavior inside the micro-deep hole were investigated. The experimental results showed the number of spiral orbits required for through holes has little change within a certain defocus range. Combined with the simulation analyses, it reveals that self-focusing can reinforce the laser intensity in the deep of micro-holes, preventing the beam energy attenuation as the depth of hole increases. Additionally, the guidance and reflection of the sidewall will also make the beam self-converge and enhance its intensity. The enhancement of the laser intensity in the hole cavity can help shorten the processing time. Furthermore, as the aperture increases, the self-focusing still has the noteworthy effect on the intensity enhancement inside the cavity, while the waveguide effect of the sidewall on the intensity enhancement will be weakened. The section profile of holes was analyzed, delamination-free was achieved, and no recast layer was found.