Experimental characterization of the inner surface in micro-drilling of spray holes: A comparison between ultrashort pulsed laser and EDM

In this research, the inner surface characteristics of micro-drilled holes of fuel injector nozzles were analyzed by Shear Force Microscopy (SHFM). The surface texture was characterized by maximum peak-to-valley distance and periodicity whose dimensions were related to the adopted energy. 180 mu m diameter holes were drilled using ultrashort pulsed laser process using pulse energies within the range of 10-50 mu J. Laser ablated surfaces in the tested energy range offer a smooth texture with a peculiar periodic structure with a variation in height between 60 and 90 nm and almost constant periodicity. The Scanning Electron Microscopy (SEM) photograph of the Laser Induced Periodic Surface Structure (LIPSS) showed the co-existence of Low Spatial Frequency LIPSS (LSFL) and High Spatial Frequency LIPSS (HSFL). A comparative analysis was carried out between the highest laser pulse energy in the tested range energy laser drilling which enables the shortest machining time and micro-Electrical Discharge Machining (mu-EDM). On the contrary, results showed that surfaces obtained by electro-erosion are characterized by a random distribution of craters with a total excursion up to 1.5 mu m with a periodicity of 10 mu m. The mean-squared surface roughness (R-q) derived from the scanned maps ranges between 220 and 560 nm for mu-EDM, and between 50 and 100 nm for fs-pulses laser drilling