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

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 (S_a) 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), (S_a)_0% >> (S_a)_{70 %} > (S_a)_50% is obtained; if it is applied higher than this transition, (S_a)_0% >> (S_a)_{50 %} > (S_a)_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 (<5 mW, F: 0.8252 J/cm²), the hardness (HD) and reduced modulus (E_r) of polished surface are dominated by areal surface roughness (S_a), they are lowered by increasing in S_a.