SEM and TEM characterization of micro-holes produced in steel by helical drilling with laser pulses in the ns to fs range

Electron microscopic methods (SEM and TEM) were used to characterized the precision and quality of micro through-holes produced in 0.7 mm thick steel sheet by applying the pulsed laser helical drilling technique. The primary aim of the present work is to investigate how the mechanisms of material removal and re-deposition and consequently the precision and quality of drilled holes change when the pulse width is systematically varied from nanosecond to femtosecond range (8 ns, 10 ps, 160 fs). Furthermore the influence of the pulse energy (0.2 mJ 0.6 mJ) on the drilling result is studied. The best drilling results, i. e. smooth holes being nearly free of recast and free of thermally driven structural damage, are achievable by using laser pulses with a width of 10 ps and a pulse energy in the range from 0.2 mJ to 0.3 mJ. On the contrary, drilling with ns pulses is associated with thick melt re-depositions and a high thermal load, while processing with ps and fs pulses of high pulse energies suffers from the formation of irregular hole shapes.