Microstructural changes induced by ultrashort pulsed lasers in microdrilling of fuel nozzles

Microholes for gasoline direct injection (GDI) nozzles were obtained in martensitic stainless steel AISI 440C with conventional micro-EDM and two laser based processes: water-jet guided s-laser and fs-laser. Since the analyzed drilling methods heavily differ for their thermal input on materials, the three processes were compared from the perspective of the microstructural changes induced in the bulk material after drilling. Moreover, the sharpness of the edges was taken into account as distinctive feature for a comparison among the three processes, being the spray atomization maximized by a cavitation process inside the microhole. A tailored procedure was optimised to prepare the samples for Scanning Electron Microscopy (SEM) and metallographic analyses. The cross sections of micro-EDM drilled samples revealed the presence of a recast (white) layer of 1-2 µm in thickness even using the lowest spark energy in the tested range. Samples drilled by water-jet guided s-laser are affected by the same phenomenon, with an even more pronounced effect. Moreover, they showed the extrusion of the melt material along the hole axis under the action of the water-jet conveying the beam. The extremely fast cooling of this layer also makes the machined surfaces prone to cracking. Conversely, metallographic analysis of cross sections of ultrashort pulsed laser drilled samples, showed no modification of the base metal microstructure in the sub-surface regions, thus testifying that the fs-pulsed laser drilling was an almost pure ablation process and not affected by a remarkable liquid phase as for the other two thermal processes. Periodicity and dimensions of laser induced periodic surface structures (LIPSS) generated by ultrashort pulsed laser were characterized by means of Scanning Electron Microscopy. SEM analysis of the microhole edges revealed burrs for the water-jet guided s-laser while radii of 3-4 µm for micro-EDM and less than 1µm for fs-lasers were measured.