Understanding the surface polishability and hardness-softening mechanisms of martensitic mould steel in multi-mode nanosecond laser polishing

Laser polishing is an efficient, reliable, and environmentally friendly surface-finishing technique aimed at improving the surface quality. However, its application to martensitic mould steel is limited by surface-softening issues induced by the pronounced thermal effects of conventional continuous-wave laser polishing. This study proposes a multi-mode nanosecond laser polishing approach that employs a millimetre-diameter beam with top-hat energy distribution to mitigate the thermal effects and alleviate surface softening. A key challenge is achieving high-quality polishing at shallow melting depths. Further, the proposed process may alter the surface-softening mechanism when considering the unique phase-transformation behaviour of martensitic mould steel. To clarify these aspects, the characteristics of multi-mode nanosecond laser polishing, related to the process and properties, are investigated via finite-element simulations and experiments. The results demonstrate that, unlike the multi-directional melt flow induced by the intense melt pool reaction in continuous-wave laser polishing, multi-mode nanosecond laser polishing drives long-range horizontal melt flow and simultaneously induces multiple convex peaks to fill concave valleys, thereby achieving high-quality surface smoothing (Sa = 0.23 μm) with a minimal melting depth (<2 μm). Moreover, a novel surface-softening mechanism involving the synergistic induction of residual austenite enrichment (up to 90 %) in the fusion zone and martensite tempering effects in the heat-affected zone is presented, which contrasts with the traditional mechanism that relies solely on tempering-induced softening in the heat-affected zone. This study presents a low-thermal-effect, high-quality, and high-efficiency polishing solution for metal components, while advancing the theoretical understanding of hardness-softening mechanisms in the laser manufacturing of martensitic steel.