Plastic deformation mechanism of negative rake angle tools in low-speed cutting

Negative rake angle cutting can significantly enhance tool impact resistance and machining stability; however, the complexity of its plastic deformation mechanism hinders the optimization of tool geometry and cutting parameters. In this study, Ti6Al4V titanium alloy was selected as the workpiece material, and a finite element simulation approach was adopted to systematically investigate the influences of cutting speed (5–20 m/min), feed rate (0.1–0.4 mm/r), rake angle (−15° to 0°), and tool nose radius (0.01–0.04 mm) on the plastic deformation behavior. The evolution of temperature, strain rate, stress, and strain during the elastic, plastic, and fracture states was examined, and the shear band width, degree of chip serration, and damage distribution characteristics were analyzed. The results indicate that there exists a critical transition point of cutting speed at 15 m/min: below this value, strain-rate hardening predominates; above it, thermal softening intensifies and triggers adiabatic shear instability. Increasing the feed rate promotes a transition from localized shear to more uniform flow deformation, yet the cutting force rises linearly, while the damage evolution is stage-dependent and non-monotonic. A more negative rake angle and a larger tool nose radius enhance the extrusion effect, whereas the shearing effect becomes dominant under the opposite conditions. Accordingly, an extrusion-shear composite deformation factor ξ was proposed to quantitatively characterize the deformation mode (ξ 1: shear-dominated; ξ ≈ 1: optimal range). Furthermore, a critical cutting-speed formula for adiabatic shear instability was established and validated experimentally. The findings provide a theoretical basis for matching process parameters and optimizing tool geometry in negative-rake-angle cutting of difficult-to-machine materials.