Experiment And Mechanism Study Of Femtosecond Laser Trepanning Drilling Of Inconel 718 Super-alloy

Femtosecond laser has the advantages of high peak power,short pulse time,and small heat-affected zone during processing.It is widely used in various precision machining.Inconel 718 Super-alloy has good mechanical properties at high temperature,and has great application prospects in the manufacture of hot-end components such as aero engine turbine blades.In this paper,a femtosecond laser with a wavelength of 1064 nm and a pulse width of300 fs was used to perform Inconel 718 Super-alloy trepanning drilling.The effect of various processing parameters on the quality of micro holes was systematically analyzed,thereby optimizing the process of femtosecond laser trepanning drilling.In addition,for the influence of the heat accumulation on the quality of micro hole at high pulse repetition rate,this paper has carried out experimental research and mechanism analysis.The classification of laser drilling and the basic principle of laser trepanning drilling are expounded.The physical process and mechanism of femtosecond laser processing of metal are analyzed.Based on the Active Fiber femtosecond laser,the experiment of femtosecond laser trepanning drilling is carried out.The effects of feed distance,scanning speed,pulse energy,pulse repetition rate,and scanning cycles on the quality of femtosecond laser trepanning drilling were studied.The surface morphology,diameter,taper and roundness of the micro hole were compared and analyzed.In the trepanning drilling experiment,it was found that the feed distance has a great influence on the taper of the micro hole during the machining process,and the scanning speed has a great influence on the roundness of the micro hole exit.Few scanning cycles is not conducive to the formation of the through hole,and the pulse energy has an effect on the micro hole exit The pulse repetition rate has little effect on the entrance and exit aperture and roundness of the micro hole.Through experimental and theoretical analysis,the optimization of the femtosecond laser trepanning drilling process was achieved:the pulse energy was 110?J,the pulse repetition rate was 100-120 k Hz,the feed distance was-0.9 mm,and the scanning speed was30 mm/s.When number of layers reach 30 and the scanning cycles reach 100,the quality of femtosecond laser trepanning drilling is better.The mechanism of heat accumulation effect of trepanning drilling under high pulse repetition rate of femtosecond laser is analyzed.The boundary between the pulse repetition rate and pulse energy subjected to the heat accumulation was experimentally studied.It was found that when the pulse repetition rate reaches 250 k Hz and the single pulse energy reaches about 50?J.Due to the heat accumulation effect,the obvious oxide layer and colored oxidation area are generated at the entrance and exit of the micro hole.As the pulse repetition rate increases,the pulse energy corresponding to the heat accumulation effect gradually decreases,and finally stabilizes at about 8?J.Under the effect of high-pulse repetition rate heat accumulation,defects in the heat-affected zone such as recast layer and micro-cracks appeared in the micro hole inner wall,and the roughness of the inner wall increased with the increase of the pulse repetition rate,reducing the quality of the micro hole.Finally,the effect of 1000 femtosecond laser pulses on the material ablation threshold at different pulse repetition rate was studied experimentally.When the laser pulse repetition rate increased from 200 k Hz to 500 k Hz,the ablation threshold of the Inconel 718Super-alloy dropped from 0.768 J/cm~2 to 0.308 J/cm~2.When the femtosecond laser pulse repetition rate reach to 1000 k Hz,the downward trend of the material ablation threshold decreases,and stabled at about 0.2 J/cm~2.The ablation mechanism of the femtosecond laser at high pulse repetition rate is analyzed,and it is verified that the thermal accumulation effect generated by the femtosecond laser drilling at low pulse energy and high pulse repetition rate can improve the micro hole taper to a certain extent and improve the processing effectiveness.