Enhancement of surface quality and properties of NAK80 mold steel through hybrid laser polishing

Laser polishing is an advanced surface processing technique. However, with the development of modern manufacturing, single laser polishing often fails to meet the stringent requirements for surface quality and performance in high-end manufacturing. This study aims to enhance the surface polishing quality as well as the mechanical and chemical properties of NAK80 mold steel by employing a hybrid laser polishing technique that combines continuous wave and pulsed lasers. The surface morphology, microstructure, microhardness, wear resistance, and corrosion resistance of unprocessed NAK80, single continuous wave laser polishing (CWP), and sequential continuous wave and pulsed laser hybrid polishing (CPP) samples were systematically investigated. The results revealed that the original surface roughness was 0.232 μm Ra, which decreased to 0.155 μm Ra and 0.094 μm Ra after CWP and CPP processing, respectively. Neither CWP nor CPP processing altered the phase composition of NAK80, which remained as α-Fe. Both processes resulted in the formation of a remelted layer that exhibited significant grain refinement, particularly in the CPP processed samples, where the smallest grain size (0.15–0.75 μm) was observed, along with a large number of low-angle grain boundaries (LAGBs, <15°) enriched near the surface. Due to the grain refinement and increased presence of LAGBs, the hardness of CWP and CPP processed surfaces increased by 41.2 % and 51.9 %, respectively, compared to the original surface. Furthermore, both wear resistance and corrosion resistance were significantly improved. These enhancements in key performance indicators are expected to extend the service life of molds. This study underscores the significant potential of hybrid laser polishing technology, which is likely to find broader applications in the future, further advancing the manufacturing industry.