The plastic flow stability of chip materials in metal cutting process

This study investigates the chip formation mechanism and relevant plastic flow stability in the orthogonal cutting process (OCP) through high-speed cutting experiments and theoretical modelling for four types of metals. The chip morphology transitions from continuous to serrated and to continuous again are observed with critical speeds depending on the work material properties and cutting conditions. To exam the influence of two-dimensional (2D) effects on plastic flow stability, a complete theoretical framework under plane strain state is established to model the 2D orthogonal cutting process. Based on the new framework, a set of governing equations with three dimensionless parameters are used to analytically derive a universal instability criterion, the approximate velocity fields, and stress fields in the expanding chip formation zone (CFZ). It is shown that the plastic flow of continuous chip may become unstable once the cutting speed reaches a critical value. In contrast to the shear localization deformation in the serrated chip, we found a new instability mechanism occurring in the continuous chip which undergoes the uniform but severe shear deformation due to the plane strain loadings. A new dimensionless parameter therefore is proposed to describe the plastic instability in continuous chip and the shear banding instability in serrated chip. The difference of two instability modes is further investigated in terms of dissipation mechanism of cutting energy, and the plastic instability of continuous chip is shown as the best instability mode regarding tool vibration and surface machining quality. These findings provide practical insights into improving modern cutting technology by controlling the plastic flow instability.