Mechanism and Surface Morphology Evolution in Nanosecond Laser Polishing of the Inner Surface of Conical Holes

Ball-end milling of 440C stainless steel inevitably leaves machining marks, and traditional polishing methods struggle to refine small conical holes effectively. Nanosecond pulsed laser polishing offers a promising alternative, preserving dimensional accuracy while effectively improving surface quality. To investigate this process, a numerical simulation model was developed to analyze laser polishing on conical hole interiors. The study examined temperature distribution, melt pool dynamics, and surface morphology under two scanning approaches: top-down and bottom-up trajectories. The results revealed that gravity plays a critical role in melt pool behavior, leading to distinct melting patterns depending on the scanning direction. Initial optimization of laser and scanning parameters was conducted through planar experiments. Subsequent single-factor tests on conical hole interiors demonstrated a significant roughness reduction, achieving a 76% decrease in Ra (from 3.65 to 0.874 μm) and a 76.3% reduction in Rz (from 21.7 to 5.13 μm). These findings confirm that nanosecond laser polishing is a viable solution for enhancing the surface finish of complex internal geometries, effectively eliminating machining marks while maintaining precision.