Effects of Process Parameters on Hole Quality during Femtosecond Laser Micro-hole Drilling of Stainless Steel foil

Abstract

Currently, laser drilling processes are preferred in many industries areas, and the demand for thin metal foil processes has increased, especially for the miniaturization of products and components. The percussion technique is adopted to realize high aspect ratio holes and fast machining times. Femtosecond laser (PH1-sp-1.5, Light conversion; Yb:KGW laser, wavelength = 1030 nm, pulse width = 190 fs) is used to minimize heat affected zone (HAZ) and spatter formation that affect hole quality. In percussion drilling, much research has been done on hole radius, hole circularity, and hole taper angle, and these have been focused on the evaluation of geometrical characteristics according to process parameters because they affect performance in specific applications of laser drilling. We found the optimal conditions for drilling micro holes in stainless steel depending on the processing parameters and the sample thickness (50 μm, 100 μm). The laser spot size that directly affects hole quality was determined quantitatively by ablation experiment on silicon wafer. To analyze the hole morphology according to the energy change, we defined the number of pulses that do not change the hole diameter, and then found an energy section where the hole taper angle is relatively good in the 10 to 20 μm hole diameter range. To improve hole circularity, hole morphology was analyzed by drilling through a circular polarized beam using a quarter waveplate. In addition, the gas nozzle coaxially mounted with the focused beam path was used for high aspect ratio and high speed processing. We confirmed that we can achieve high aspect ratio hole with less the number of pulses by blowing ablated materials and debris as an assistant gas during the drilling processing. The HAZ and spatter formation around the drilled hole surface were analyzed by SEM (Scanning electron microscope) according to the sample thickness and ambient characteristics.