The mechanism of irregular hole-shape formation during ultrafast laser micro-drilling of metals

The micro-drilling of metals using ultrafast laser is of great interest for a wide range of industrial applications. To date, high drilling quality remains a grand challenge due to the elusive mechanism of hole shaping, especially the origin of the ellipticity and bulges that emerged at hole exits in the micro-drilling of metals. Here we reveal that the emergence of the hole ellipticity is jointly determined by the ablation threshold of the material and the polarization-dependent energy absorption on the hole wall. Besides, a special hole geometry, which is formed during the evolution of hole shape from round to ellipse, has a converging effect when the laser beams are reflected by the hole wall, and the beam-converging effect induces the formation of bulges at hole exits. These results contribute to the understanding of the mechanism of hole-shape formation in the micro-drilling of metals with ultrafast laser and provide a basis for the hole-shape control in high-quality micro-drilling.