Surface quality, microstructure, mechanical properties and tribological results of the SKD 61 tool steel with prior heat treatment affected by the deposited energy of continuous wave laser micro-polishing

Abstract

SKD 61 tool steel specimens hardened by heat treatment are subjected to continuous wave laser micro-polishing (CWLμP). A total of 46 specimens are classified into three groups in order to find the operating conditions to minimize the areal average surface roughness (Sa) via a three-stage process. Deposited energy per area (DE, unit: J/mm2) is defined as a composite parameter of laser power (P), laser beam scanning velocity (V), and focused spot diameter (SS). The parameters of Sa and the sum of the thickness sum of the melt zone (MZ) and heat-affected zone (HAZ), as well as the mechanical properties of hardness (H) and reduced modulus (Er), are then expressed as a function of DE. Appropriate choices of laser operating conditions allow the Sa corresponding to the DE value at about 70J/mm2 to be the smallest for these specimens. Due to the complex behavior involved in the thermocapillary flow, the only controlling parameter that Sa increases monotonically with is the hatch distance. Decreases in focal offset (FO) and laser power (P) cause a decrease in the highest amplitude of the polished surface in the spatial frequency analyses. The highest amplitude cannot decrease if the scanning velocity is excessively high or low. The core roughness depth (Sk) and the reduced peak height (Spk) and valley depth (Svk) are found to be linearly proportional to Sa. The H and Er values of the specimens are lowered by increasing the applied deposited energy, although DE is not the sole governing parameter. The mean values of H and Er in the MZ, HAZ, and base material are found to increase by increasing the depth beneath the top surface of a specimen. The smaller wear rate in the specimens of CWLμP compared to that of the as-received specimen can be attributed to the lubrication effect arising in the tribo-contacts of specimens with a grain size greater than the critical value. The differences in several properties between the as-received specimens with and without heat treatment are compared and discussed.