Femtosecond laser layered ring trepanning of stainless steel sheets with and without transverse magnetic assistance

A novel femtosecond laser layered ring trepanning (LLRT) technology assisted by external transverse magnetic fields is reported for improving laser trepanning efficiency and quality while reducing hole defects. The influence of key operating parameters on blind-hole geometry trepanned by altering magnetic flux density was discussed. The element compositions were compared and analyzed for the corresponding domains near the entrances, middles and bottoms of the blind-hole sidewalls trepanned with and without magnetic assistance. The surface morphology and roughness together with local microscale irregularities and defects of the blind-hole sidewalls trepanned with and without magnetic assistance were also analyzed based on corresponding comparisons via altering scanning speed, magnetic flux density, pulse repetition rate and feed distance. It was demonstrated that the magnetic field-enhanced plasma motion induced by Lorentz force scoured hole sidewall and weakened the multi-shock effect on the hole wall. A stronger transverse magnetic field was more effective to reduce/avoid the local hole-wall defects such as micro cracks together with micro irregularities for improving the formation uniformity/quality of the hole wall. With the increase of magnetic flux density, the hole depth and aspect ratio increased while the hole diameter and taper angle decreased. A smaller feed distance was more helpful for reducing hole-wall surface roughness, while the addition of an external magnetic field was useful for weakening hole-wall oxidation. The oxidation difference of the oxidized layers on the hole wall mainly resulted from the transient uncertainty/instability of further oxidation, which was caused by the thermal multi-shock effect induced by the transiently-changeable motion of laser-induced plasma at a high temperature. Despite the uncertainty normally encountered during the complex LLRT process, it was indicated that the repeatability of this work was acceptable based on the repeated experiments and measurements.