On understanding the influence of microstructure on pure tungsten machinability: A micro-end milling case

One of the crucial challenges regarding the applications of pure tungsten and its alloys as structural materials for next-generation fusion reactors is their high brittle-to-ductile transition (BDT) temperature. While BDT is an inherent property, it is strongly affected by microstructures. This work systematically investigates the effect of microstructural transformation on subsequent processibility. During preparation, the sintered tungsten was pre-deformed to refine its microstructure, resulting in a reduction of grain size to about 1/3 of its original value. The influences of different microstructures on surface and subsurface responses, as well as cutting forces and tool damage characteristics, were identified. Results showed that the grain and subgrain boundaries introduced by the pre-deformation process were insufficient to significantly enhance the material plasticity in milling, resulting in up to a 138.5 % increase in surface roughness. In contrast, dislocations nucleated during pre-deformation effectively reduced penetrative surface damages when the depth of cut remained below the average grain size. Meanwhile, the refined tungsten specimen developed continuous equiaxed ultrafine-grain layer, potentially improving irradiation resistance. Chipping, abrasive wear, and adhesive wear were identified as the primary contributors to tool wear. In small-grain samples, the Hall-Petch hardening effect induced a substantial increase in cutting forces, accelerating tool damage and resulting in up to 304.5 % and 38 % increases in rake wear width and flank wear width, respectively. These findings clearly highlight the necessity of optimizing microstructure to balance machinability and application performance, especially for hard-brittle metals.