Temporal evolution of hole geometry and influences of energy deposition in ultra-short pulse helical drilling

This paper presents a static investigation on temporal evolution of hole geometry and influence of laser energy deposition on hole quality in helical drilling process. By using a rotating dove prism a circular oscillation of the laser spots is performed and holes are drilled at intervals in 1 mm thick stainless steel (1.4301) by ultra-short laser pulses of 7 ps at 515 nm. The formation of hole and the behavior of energy deposition differ from other drilling strategies due to the helical revolution. The temporal evolution of the hole shape is analyzed by means of SEM techniques from which three drilling phases can be distinguished. The first phase is characterized by a highest drilling rate and the formation of a sharp-edged circular groove with a pin inside the workpiece. In the following phase, the molten and vaporized material is ejected out from the hole and a funnel-like borehole with a slim tip deepens to the backside of workpiece, growth of hole depth slows down in this period. The exit is broadened to the final shape in the final phase. Laser scanning microscope (LSM) measurements of structure details on the hole wall demonstrate that the quality of the helical-drilled hole is determined by a correlation between the pulse energy applied and the overlapping rate of laser pulses, which is described mainly by helical path and the rotation speed of laser beam.