Molecular dynamics modelling and mechanics analysis on the mechanism of brittle to ductile cutting mode transition in ultra-precision machining

Abstract

Brittle-ductile cutting mode transition (BDCMT) is an important phenomenon in ultra-precision machining of brittle materials. Though many studies have been conducted to investigate the mechanism of BDCMT, a number of fundamental issues remain unclear or controversial. Four such issues are identified and investigated in this study. The first issue is the atomicscale details of BDCMT under practical undeformed chip thickness (UCT). Molecular dynamics (MD) codes based on graphics processing units are developed to enable large-scale simulations with an advanced interaction potential. MD modelling of BDCMT is realized for the first time. It is found that the tensile stress in the cutting zone increases with UCT, and finally induces brittle fracture. In the simulations, the direction of crack propagation varies with UCT, and agrees with the surface morphologies produced by plunge cutting experiments. The second issue is the relationship between the tensile and compressive stresses in BDCMT. The mechanisms of BDCMT are analyzed from the perspective of the mechanics involved. The finite element method is employed to verify the proposed mechanics on silicon and 6H SiC, and the critical UCT (CUCT) for BDCMT are estimated accordingly. The estimated values of CUCT are verified in plunge cutting experiments. It is found that, to induce the same magnitude of compressive stress, the tool has to advance a larger distance for larger UCT, and thereby induces larger tensile strain and consequently larger tensile stress below and behind the tool. The third issue is whether high pressure phase transformation (HPPT) is the origin of ductility in BDCMT for 6H SiC. MD simulations are conducted to investigate the mechanism of ductile deformation in BDCMT for 6H SiC. The HPPT and dislocation activities in the machining of 6H SiC are visualized for the first time. Micro Raman spectroscopy is conducted on a ductile-cut 6H SiC surface, with no peaks detected for amorphous SiC. These results indicate that the ductility in BDCMT of 6H SiC is due to both HPPT and dislocation activities, with dislocation plasticity playing a major role. The fourth issue is whether the change of effective rake angle to be highly negative accounts for BDCMT. MD simulations show that BDCMT also occurs under a positive rake angle for silicon and SiC, though the CUCT are much smaller than that under negative rake angles. This indicates that negative rake angle is a more favorable condition for BDCMT than positive rake angle, rather than an enabling factor for BDCMT. The originality and significance of this study can be summarized as (i) MD modelling of BDCMT is realized for the first time, revealing important details of BDCMT; (ii) an explicit relationship between the tensile and compressive stresses in BDCMT is revealed by mechanistic analysis; (iii) the mechanism of ductile deformation in BDCMT of 6H SiC is visualized by MD simulations for the first time, resolving the controversy regarding the role of HPPT; (iv) it helps to achieve more comprehensive understanding on the role of rake angle in BDCMT.