Thermal Confinement Effects in Laser-Polished AM 316L Slots: The Role of Geometry in Internal Surface Response

Laser polishing (LP) is widely used to improve the surface quality of additively manufactured (AM) metals; however, its behaviour within deep or narrow internal geometries remains insufficiently understood. Many high-performance AM components including biomedical implants, turbine cooling channels, and metal microfluidic devices contain confined internal features where heat-transfer conditions differ substantially from those at open surfaces. In this study, LPBF-fabricated 316L stainless steel specimens containing ~10 mm deep slots with widths ranging from 1 to 5 mm were laser polished to examine how internal geometry influences microstructural evolution and mechanical response. A clear depth-dependent microhardness gradient was observed along the slot wall, with hardness decreasing from approximately 270 HV in the lower region to about 210 HV toward the slot opening. The gradient was more pronounced in narrower slots. Microstructural characterization revealed finer grains near the slot base and progressively coarser grains toward the upper regions. These variations are consistent with differences in conductive coupling to the surrounding bulk substrate along the slot depth, which influence local cooling conditions during solidification. The results provide quantitative evidence that internal geometric boundary conditions can affect microstructure and hardness development during laser polishing, even when nominal processing parameters are held constant. This work highlights the importance of considering feature geometry in the post-processing of AM components containing confined internal structures and offers guidance for achieving more predictable local mechanical performance.