Picosecond laser trepan drilling of vertical sidewalls micro-holes for 631 stainless steel

Ultrashort-pulse laser is widely used in drilling micro-hole (diameter<100μm) because of its higher energy density and less thermal effect. However, ultrashort-pulse laser drilling still has some defects, such as taper, poor entrance edge quality, etc. In this paper, the method of trepanning drilling was used to process micro-holes (the entrance diameter was about 55μm) on the stainless steel sheet (the thickness was 80μm) with a picosecond laser. The taper (7.63º ~ -1.8º) of the micro-hole was successfully adjusted by changing the laser angle of incidence (AOI) and laser energy. The experimental results showed that the entrance roundness increased from 85.2% to 94.1% with the increase of the AOI. In addition, the entrance roundness decreased with the rise of the laser fluence. Observing the hole wall surface showed that the remelts and micro-cracks on the hole wall surface increased with the increase of the AOI. The remelts and micro-cracks on the hole wall surface were obviously reduced when the laser fluence was increased. In summary, AOI and laser energy are essential parameters to adjust the geometric size of the hole. By optimizing the AOI and fluence parameters, the taper of the hole can be controlled (from positive taper to negative taper), and the morphology of the entrance and exit of the hole can be improved.