Influence of laser beam fluence on surface quality, microstructure, mechanical properties, and tribological results for laser polishing of SKD61 tool steel

In the present study, the surface polishing of SKD61 tool steel specimens was carried out using a microsecond fiber laser system. The operating conditions of laser controlling factors and fluence for the minimization of the areal average surface roughness (Sa), wear rate and friction coefficient are determined through the planned arrangements of three stages including the experimental design method. The Sa value of the polished surface obtained from the measurements in the x direction perpendicular to the laser scanning direction was effectively reduced. However, that from the measurements parallel to the scanning direction strongly depends on the overflow probability of the melt flow. In the x direction, the highest peak after polishing was shifted to have a frequency different to that of the as-received specimen. The ripple frequency associated with the highest peak is affected by the laser beam scanning velocity and pulse frequency only. In the y direction, the amplitude of the highest peak after polishing is possibly greater than that of the as-received specimen if the melt overflowing the grinding mark stems left behind after polishing was solidified locally. However, its frequency is kept the same as that of the residual stems. A small Sa value can be obtained if the polished surface has small residual stems and lacks significant melt overflow. The thicknesses of the melt zone (MZ) and the heat-affected zone (HAZ) are strongly dependent on both the applied fluence and single-style controlling factor. Increases in laser power and pulse duration increase the MZ thickness, whereas increases in scanning velocity and pulse frequency decrease the MZ thickness. The formations of MZ and HAZ have the hardness (H) and the reduced modulus (Er) lower than those of the as-received specimen. This characteristic is proved to be consistent with the iron phases and their volume fraction formed in these two zones. The wear rate of a specimen depends on the total thickness and the microstructures of these two zones. An increase in laser power and decrease in scanning velocity can decrease the wear rate. The friction coefficient decreases with decreasing Sa of a polished surface.