Microstructural evolution and mechanical property correlation of machined Ti–6Al–4V governed by initial structure and cooling strategy

Intense thermo-mechanical coupling in titanium machining induces drastic microstructural evolution, altering surface/subsurface mechanical properties. However, the combined influence of the material's initial microstructure and the applied cooling conditions on this evolution mechanism remains insufficiently understood. This study addresses this gap by investigating Ti–6Al–4V through a dual comparative approach: dry cutting (DC) versus cryogenic cooling cutting (CCC), and equiaxed structure (ES) versus Widmanstätten structure (WS). Results demonstrate that while CCC increases surface hardness magnitude by enhancing plastic deformation and grain refinement, the hardness distribution pattern is governed by the microstructure. WS Ti–6Al–4V uniquely develops a beneficial gradient hardness distribution under both cooling conditions, contrasting with subsurface softening in conventional difficult-to-machine materials (including ES Ti–6Al–4V). This WS advantage stems from lamellar-mediated “interfacial slip”. Crucially, WS Ti–6Al–4V exhibits exceptional “strong yet deformable” characteristics, with its α-lath/β-phase microstructure allowing the β phase to function as a “plastic bridge” that facilitates dislocation transfer across α/β interfaces, enhancing deformation compatibility far beyond ES Ti–6Al–4V counterparts. Surface microstructure evolution is driven by synergistic interactions: dislocation slip (dominant) and micro-twinning (assistive) promote plastic deformation and grain refinement, while martensitic transformation induces specific grain boundary misorientations. Regarding surface evolution mechanisms, thermo-mechanically induced β→α′ transformation consistently generates a universal 60°–63° misorientation peak—irrespective of initial microstructure or cooling—originating from Burgers-oriented martensitic variant interfaces forming self-accommodating structures to minimize strain energy. This research confirms that WS under CCC provides superior surface integrity by combining gradient hardening with refined microstructure, providing a theoretical basis for high-performance manufacturing optimization.