Surface polishes of the SKD 61 tool steel by a femto pulse laser operating in a wide range of powers

[Display omitted] SKD 61 tool steel workpieces without prior heat treatment are adopted for pulse femto laser polish with the power in a range of 0.1–20 mW. Single-pass tracks are provided first to evaluate the width (W) and depth (D) of groove's lateral profile. With the characteristics exhibited in the nearly constant D/W data and the noticeable changes in the crystal species and the Fe content in this powers range, the three power subregions and the powers required for the solid-(plasma) ablation and the ablation-evaporation plus plasma breakdown transitions can be identified clearly. Surfaces polished with 0, 50, and 70 % as the overlap ratio are presented to investigate the mean areal roughness (Sa) and the highest roughness amplitude (HA) affected by the laser fluence (F) in these three subregions and the overlap ratio. As power is applied below the solid-ablation transition (9–10 mW, F: 1.3777–1.5239 J/cm2), (Sa)0% >> (Sa)70 % > (Sa)50% is obtained; if it is applied higher than this transition, (Sa)0% >> (Sa)50 % > (Sa)70% is presented. When the power is applied beyond the threshold (14 mW, 1.7224 J/cm2) for material evaporation and plasma breakdown, the HA value is reduced by increasing overlap ratio. As the laser power is sufficient low (