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/cm(2)), (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/cm(2)) 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/cm(2)), 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.