Study on white layer formation during machined surface evolution in high-speed machining of rail steel

Abstract

White layer formation at the machined surface in high-speed machining process is inevitably induced by the propagation effect of adiabatic shear behavior, which unavoidably produces various impacts on the machinabilities. To investigate the characteristics of white layer formation under relatively high cutting speeds, large feeds, and negative rake angles, the high-speed machining experiments of high-manganese rail steel and its quick-stop tests were carried out. The evolution of machined surface layer and the process of machined surface formation were analyzed through the microscopic observations. The three-stage physical model of machined surface formation was proposed. Based on the propagation theory of thermo-plastic shear wave under plane strain state, the theoretical model of machined surface layer energy in high-speed machining was deduced, the critical energy criterion of white layer formation was proposed, and the surface layer thickness and the corresponding surface energy were calculated and verified with the experimental results. The influences of the thermal and mechanical properties of the rail steel on the white layer formation were revealed and discussed. It was shown that there existed a critical cutting speed above which the machined surface completely transformed into a white layer. The formation and transformation of machined surface layer were closely related to the thermo-plastic localization effects. The energy dissipation of machined surface layer was effectively assessed through the proposed surface layer energy model.

Appendix

Notation

$t$ :

time (s)

$x,y$ :

coordinate axis (m)

$v$ :

cutting velocity (m^.s⁻¹)

a :

rate-related gradient factor

${a}_{\mathrm{c}}$ :

uncut thickness (m)

${a}_{\mathrm{ch}}$ :

chip thickness (m)

$\phi$ :

shear angle ($\circ$)

${\sigma }_{b}$ :

tensile strength (MPa)

${\sigma }_{s}$ :

yield strength (MPa)

$U$ :

upper side displacement of shear band region (m)

$S$ :

shear bandwidth ($\mathrm{\mu m}$)

$\zeta$ :

deformation coefficient of serrated segment

$\xi$ :

boundary location of shear band region (m)

$\tilde{\sigma }$ :

effective normal stress (MPa)

$\overline{\varepsilon }$ :

effective normal strain

$\dot{\overline{\varepsilon }}$ :

effective normal strain rate (s⁻¹)

$\left[{\sigma }_{i}\right]$ :

principal stress components

$\left[{\varepsilon }_{i}\right]$ :

principal strain components

$\left[{\dot{\varepsilon }}_{i}\right]$ :

principal strain rate components

${\delta }_{ij}$ :

Kronecker’s delta function

$\sigma$ :

normal stress (MPa)

$\tau$ :

shear stress (MPa)

$\overline{\tau }$ :

effective shear stress (MPa)

$\overline{\gamma }$ :

effective shear strain

$\dot{\overline{\gamma }}$ :

effective shear strain rate (s⁻¹)

${\overline{\tau }}_{P}$ :

equivalent peak stress (MPa)

$\overline{\tau }\left(\gamma ,\dot{\gamma },\theta \right)$ :

pre-peak constitutive relation (MPa)

$\overline{\tau }\left({\overline{\tau }}_{P},\theta ,D\right)$ :

post-peak constitutive relation (MPa)

$A,B,C,m,n$ :

constitutive parameters

$\gamma$ :

shear strain

$\dot{\gamma }$ :

shear strain rate (s⁻¹)

${\dot{\overline{\gamma }}}_{0}$ :

mean strain rate (s⁻¹)

${\gamma }_{0}$ :

rake angle ($\circ$)

$\theta$ :

temperature (K)

${\theta }^{*}$ :

characteristic temperature

${\theta }_{0}$ :

initiate temperature (K)

${\theta }_{M}$ :

melt point (K)

$\rho$ :

mass density (kg^.m⁻³)

$c$ :

thermal specific capacity (J^.kg^−1.K⁻¹)

$D$ :

degenerating coefficient

${D}_{\mathrm{WL}}$ :

transformation degree

$\chi$ :

thermal diffuse coefficient (m^2.s⁻¹)

$\alpha$ :

thermal weakening factor

$\beta$ :

Taylor and Quinney coefficient

${W}_{c}$ :

critical energy dissipation (J m⁻²)

${G}_{SL}$ :

surface layer energy (J m⁻²)

${G}_{c}$ :

critical energy (J m⁻²)

${G}_{o}$ :

pre-peak energy (J m⁻²)

${G}_{P}$ :

post-peak energy (J m⁻²)