Effects of multi-pulses on material removal threshold of superalloy irradiated by ultra-short pulsed lasers

The effects of femtosecond laser (wavelength 1035nm, pulse duration 400 fs, pulse repetition rate 1kHz) on the material removal threshold of superalloy was experimentally investigated, specifically the influence of pulse number irradiated on the threshold of superalloy. Irregular bulge structures were found at the bottom of ablation holes. Two sets of material removal thresholds are derived from the value of hole diameter and depth, which are 0.506 J/cm² , 0.245 J/cm² , 0.124 J/cm² , 0.102 J/cm² , 0.096 J/cm² and 0.141 J/cm² , 0.130 J/cm² , 0.031 J/cm² , 0.026 J/cm² , 0.091 J/cm² for diameter derivation and depth derivation, respectively, for 1, 10, 100, 500 and 1000 pulses. The deduced two sets of thresholds show similar behavior that the threshold sharply decreases with the pulse number and levels out as the pulse number exceeds 100. The phenomenon is due to incubation effects of multi-pulse irradiation on the surface of superalloy and the incubation coefficient for the thresholds calculated by diameter and depth is fitted to be ~0.72 and ~0.77.