Ultrashort Pulse Laser Machining Of Film Holes In Nickel-Based Single Crystal Superalloy Blades

The accuracy and quality of film hole processing is the key to ensure the reliability and stability of turbine blades.The emergence of new processing technology based on ultra-short pulse laser provides a solution for high-quality film hole processing.In order to further explore the ablation mechanism and micro-processing technology of film hole prepared by ultra-short pulse laser,the following research work has been done in this paper:(1)The threshold behavior of DD6 single crystal superalloy under femtosecond laser irradiation was studied by means of fixed-point impact experiment.Based on the relationship between ablation diameter and pulse energy,the energy threshold of DD6 single crystal superalloy with different number of pulses was calculated.Based on the two-temperature field model,the deposition process of laser energy in the material and the energy coupling process between electrons and lattices are described.The ablation morphologies under different pulse energies are plotted by using the obtained temperature field information.The ablation threshold,the geometric characteristics of etching pits and the thickness of remelting layer are predicted.(2)Based on the molecular dynamics method and coupled with a two-temperature field model,the thermodynamic behavior of nano-thickness nickel coating irradiated by femtosecond laser was systematically simulated.The deposition and transmission of laser energy in the simulated system were described at atomic scale.On this basis,the physical phenomena such as temperature rise,melting,fracture and stress wave propagation of materials under intense laser irradiation were analyzed.The melting,bubble nucleation and mechanical fracture processes of nano-nickel at different pulse energy densities were investigated.Atomic images of nano-nickel ablated by femtosecond laser were constructed.Based on these images,the ablation mechanism of nano-nickel at different energy densities was revealed.The simulation results show that the existence of stress wave will cause bubble nucleation in the material,which will lead to photomechanical fracture.In this study,stress-induced melting of the lower surface is dominant at low energy density(400J/m2),while stress-induced melting of the material is dominant at high energy density(1000J/m2),while at medium energy density(700J/m2),the melting of the material is caused by the rapid heating of the laser energy absorbed by the upper surface,the local melting of the internal unmelted zone caused by the propagation and superposition of stress waves and the melting of the stress waveguide on the lower surface.(3)Based on the electron rate equation,a plasma excitation model with avalanche ionization is constructed.The excitation morphology and spatio-temporal evolution of plasma under different pulse widths are described in detail.The results show that the duration of plasma excitation increases with the increase of pulse widths,but the excitation region decreases.The change of pulse widths makes the laser energy in time scale and space.There are differences in the interscale distribution,which affects the excitation of plasma.At the same time,the geometric characteristics of 304 stainless steel micro-machining under different pulse energy are predicted based on energy density.With the increase of pulse energy,the depth and width of micro-machining grooves increase.The calculated results are consistent with the experimental results.However,the neglect of electron diffusion and recombination in the model leads to the larger numerical results.