Numerical Simulation And Experimental Study On Ablation Kinetics Of Copper Films Irradiated By Ultrafast Laser

Ultrafast laser has been widely used in precision processing because of its non-contact properties,high peak power,short operating time,and small heat-affected zone.Ablation caused by ultrafast lasers is different from that caused by conventional long pulsed lasers.For long-pulse lasers,material ablation is mainly induced by melting and vaporization.As for ultrafast lasers,material ablation may be induced by thermal stress,hot-electron blast force,phase explosion,etc.which is complicated.Therefore,it is necessary to study the mechanism of ultrafast laser ablation of metal materials.In this thesis,pure copper is used to study the thermomechanical response and damage prediction under low fluence and the ablation process of copper film under high fluence from two aspects of simulation and experiment.The research results can help to understand the ablation mechanism of metal materials under the ultrafast laser,which has academic significance and engineering application value.The specific content and conclusions are as follows:(1)Based on the ultrafast thermoelastic model,the dynamic thermophysical properties and optical properties were introduced to study the thermomechanical response of the thin copper film irradiated by femtosecond laser with different fluence and pulse duration.The results show that at the early stage of ultrafast laser irradiation,the reduction in reflectivity(R)and absorption coefficient(?)results in more laser energy absorption and deeper deposition in the film.In particular,when the laser fluence is high and the pulse duration is short,the degree of thermomechanical response is greatly underestimated compared to studies using constant R and?.High thermal stress can be induced in the film and possible non-thermal ablation can be caused by that.Therefore,it is necessary to use dynamic optical properties to more accurately describe the ultrafast process associated with the interaction of lasers with metal materials.(2)The thermomechanical response of the copper film under multiple ultrafast laser pulses based on dynamic optical properties was simulated.The difference in the thermomechanical response of the target under different laser pulses and pulse separation time is analyzed.The results show that as the number of pulses increases,the lower the maximum temperature reached by the electron and lattice,the smaller the maximum tensile stress,and the smaller the degree of stretching of the hot-electron blast force on the film when the total energy is the same.The shorter pulse separation time induces smaller R and?,which allows more energy to be absorbed and deposited in the film.In addtition,the electron and lattice do not have enough time to transfer heat to the deeper portion of the material before the next pulse arrives,which results in a higher surface temperature of the material and a higher tensile stress near the upper surface region.Therefore,for a multi-pulse laser with a short pulse separation time,the cumulative effect is more significant.(3)Based on the TTM and phase explosion theory,a theoretical model of single-pulse femtosecond laser ablation copper foil was established,and the relationship between fluence and ablation depth was obtained.At the same time,the ablation experiments of copper foil with different energy femtosecond lasers show that the thermal ablation phenomenon is more obvious with the increase of laser energy.By fitting the relationship between ablation diameter and energy,the laser copper ablation threshold is 2.24 J/cm~2and the effective spot radius is 14.6?m.By comparing the ablation depths of the simulations and experiments,it is found that the theoretical model can describe the ultrafast ablation process of the material relatively well.