Micrometric orthogonal cutting can lead to sideway chip flow: Uncovering crystallographic orientation and grain boundary effects

Micrometric cutting has attracted great research interest both in academic and industrial areas due to its vital role in microfabrication. Different from macro-cutting where many grains are involved in the process that averages/minimises the crystallographic effects, in micrometric cutting localised crystallographic deformation mechanisms can highly affect the machining results. However, experimental challenges of micrometric level cutting have confined most investigations to simulations or less-ideal tests (e.g. micro-scratching and single-point diamond turning), leaving the detailed interplay between chip flow and microstructure largely unexplored. In an apparent paradox, in orthogonal micrometric level cutting conditions, expected to yield forward (2D) chip flow, can produce pronounced sideway (3D) chip flow when grain-orientation anisotropy and boundary-induced kinematic constraints dominate; such aspects cannot be captured in macro-cutting. Based on these, when cutting at micrometric level it is important to understand how the slip system of the grains will be activated, what will happen when the grain boundary is encountered, as well as why the specific chip form and flow direction is generated under different conditions of these. To resolve this, grain orientations and boundaries were pre-characterised on a Ni-based superalloy sample, on which micrometric boss features were subsequently fabricated. Orthogonal grain-level cutting tests were then conducted on these structures, effectively isolating the deformation region, eliminating constraints from adjacent material, and allowing the chip to flow freely on both sides. Chip morphology, flow direction, and local deformation mechanisms were examined via advanced material characterisation technologies. Key findings that are specifically manifested at micro level include: The formation of serrated chips is influenced by the Schmid factor, resulting in variations in segment morphology across different crystallographic orientations. Sideway chip flow can be generated in micrometric orthogonal cutting process due to the selective activation of the slip-systems and inclined grain boundary guided sliding. Furthermore, when twin boundary exists, periodic extrusion-shear dominated material deformation cycle can happen in chip formation process due to the alternated stress. Therefore, we reveal for the first time that when cutting at grain levels, although geometrically defined orthogonal cutting was performed, the chip follows crystallographic rules imposed by slip planes and grain boundary conditions. These insights provide a new mechanistic framework for understanding micrometric level cutting anisotropy and the boundary-driven paradox of sideways chips, offering guidelines to optimise micrometric machining strategies in microfabrication applications. • Schmid factor governs chip serration: low factors cause coarse serrations, while high factors yield smooth chips. • Orthogonal micro-cutting can produce 3D sideway chip flow due to crystal anisotropy and grain boundary effect, a key paradox revealed. • Grain boundary orientation dictates chip formation: inclined boundaries can cause separation, while twin boundaries create periodic extrusion-shear cycles. • Cross-sections directly link observed cutting behaviours to microstructural features like activated slip systems and grain refinement.