Study on thermo-plastic instability characteristics of machined surface localization in high-speed machining of rail steel

Machined surface localization tends to occur due to the impact of thermo-plastic propagation effect under high cutting speed, leading to the machined surface evolution from deformed layer to white layer. To investigate the thermo-plastic characteristics of machined surface localization in high-speed machining, the formation model of machined surface layer was further developed through considering the propagation effect of thermo-plastic shear wave. The continuum governing equations and the constitutive relations of the rigid-plastic and thermo-plastic zones were established. Considering the energy dissipation states in the rigid-plastic and thermo-plastic zones, the propagation angle of thermo-plastic shear wave, the critical surface energy of white layer formation, and its corresponding surface layer thickness were yielded and verified through the high-speed machining experiment of rail steel and the microscopic examinations. On this basis, both stable and unstable solutions for the dependent variables were explicitly solved through the perturbation analysis method. The thermal and mechanical coupling mechanisms of the dependent variables which involve deformation, stress, temperature, and energy in the time-spatial domain during machined surface localization were further discussed with the numeral solutions. The numeral solutions and the perturbation solutions reasonably revealed that the machined surface localization experienced a thermo-plastic transformation process from stability to instability in the whole time-spatial domain.