Grain refinement-promoted diamond turning performance of polycrystalline copper: ex-situ versus in-situ

While grain refinement is promising for suppressing the anisotropic deformation behavior of polycrystalline materials, the coupling approach between grain refinement and subsequent cutting has a strong impact on the machinability of fine-grained polycrystalline metals. In this work, we investigate the comparative effects of ex-situ heat treatment and in-situ ultrasonic vibration-assisted cutting (UVAC) on the grain refinement and subsequent ultra-precision diamond cutting of polycrystalline copper. Cross-sectional microstructural characterization of fine-grained polycrystalline copper reveals that heat treatment induces static recrystallization with moderate grain refinement, while UVAC promotes continuous dynamic recrystallization to form a gradient fine-grained layer beneath machined surface. Further analyses of subsurface microstructure confirm that the UVAC intensifies dislocation accumulation and stabilizes refined grains, resulting into a 27.5% reduction in grain size compared with the heat treatment. Subsequent diamond cutting of fine-grained polycrystalline copper demonstrates a surface roughness Sa of 1.83 nm obtained for the in-situ approach, which is 37.3% lower than that for the ex-situ one, accompanied with enhanced mechanical properties of machined surface. The findings provide a feasible coupling approach for promoting grain refinement-based cutting performance of polycrystalline metals. • Heat treatment and ultrasonic vibration-assisted cutting (UVAC) for grain refinement of polycrystalline Cu are conducted • Dense gradient fine-grained layer with micrometer grain size accompanied by continuous dynamic recrystallization in UVAC is revealed • Finer grain size by UVAC leads to 37.3% reduction in surface roughness and enhanced mechanical properties than heat treatment.