Surface and subsurface characterisation in nano-cutting of polycrystalline magnesium: Molecular dynamics studies

This study utilises molecular dynamics simulations to investigate the evolution of surface morphology and subsurface damage during nano-cutting of polycrystalline magnesium. During the machining process, grain boundary expansion occurs, with grain boundaries serving as barriers to dislocation movement. Surface formation is associated with the generation and annihilation of subgrains, as well as the elastic recovery of the grain structure; and grain boundary steps are formed during the nano-cutting process. As the cutting speed increases from 40 to 100 m/s, the number of atoms undergoing phase transformation in the workpiece rises from 7737 to 14,230, while the total dislocation line length decreases from 2,642.89 to 1,679.87 Å. A similar trend is observed with increasing cutting depth. Additionally, when the cutting speed rises from 40 to 100 m/s, the surface roughness parameters Sa and Sq decrease from 3.186, 3.942, 2.385, and 2.805 Å, respectively. Conversely, as the cutting depth increases from 2 to 5 nm, Sa and Sq increase from 1.832, 2.152, 3.638, and 4.658 Å, respectively. This improvement is attributed to the fact that higher cutting speeds facilitate the annihilation of subgrains and suppress the formation of burrs, thereby yielding a smoother surface finish. Conversely, a large cutting depth enhances the elastic recovery between adjacent grains, leading to the formation of prominent grain boundary steps and deteriorating surface quality.