Mechanism of Regulating Surface Morphology and Microstructure in Laser Polishing of Mold Steel Through Pulse Spatiotemporal Distribution

In this work, nanosecond pulsed laser was employed to polish the rough surface of mold steel. The critical quasi-continuous melting criterion (C-Q) was proposed to reflect the coupling relationship between pulse time and space distribution. The three laser-polished samples with C-Q = 1 exhibited similar surface morphology, with a reduction in roughness of over 75 %. The minimum surface roughness achieved is Sa 0.49 mu m. Numerical simulations were conducted to model the temperature evolution of laser-polished samples, and the model exhibited excellent agreement with experimental results. The simulations explained that when C-Q remained consistent, the same surface temperature of the molten pool led to uniform thermal capillary forces on the surface, thereby achieving similar morphological changes. Additionally, when C-Q was consistent, the microstructure exhibited similar phase distributions. The austenite content in the remelted zone is below 5 %, while in the remelted + austenitized zone, the austenite content reaches a maximum of 19.9 %. After laser polishing, the average grain size in the remelted layer decreases from 0.28 mu m to 0.24 mu m. Based on temperature field results at different depths, the mechanism of pulse time-space distribution regulation on microstructure was revealed: the remelted surface formed martensite due to rapid cooling, while residual austenite was generated below the remelted region with increasing cooling time. When C-Q > 1, the surface temperature exceeds 3300 K, surpassing the vaporization temperature. This leads to the formation of a fully austenitized region on the surface due to the depletion of Fe. For polished samples, the overall surface hardness of mold steel decreased due to the thickness of the austenitic region and HAZ being greater than that of the grain-refined remelted region. This work provides new insights into controlling the surface morphology after pulsed laser polishing.