Suppression of Hopf bifurcation in metal cutting by extrusion machining

Hopf bifurcation is a common phenomenon during the metal cutting process, which results in poor surface finish of the workpiece and inhomogeneous grain structure in materials. Therefore, understanding and controlling Hopf bifurcation in metal cutting are necessary. In this work, the systematic low-speed extrusion machining experiments were conducted to suppress Hopf bifurcation phenomenon. It is found that the suppression of Hopf bifurcation is achieved with the increasing constraint extrusion degree. In order to reveal the mechanism of the suppression of Hopf bifurcation, a new nonlinear dynamic model for extrusion machining is developed where the convection, the diffusion, the extrusion of constraint, the thermal-softening deformation and the fracture-type damage are included. The theoretical predictions are in agreement with the experimental results; therefore, the present theoretical model is effective to characterize the suppression of Hopf bifurcation in metal cutting. Based on the numerical calculation of the theoretical model, the mechanisms underlying in extrusion machining are further revealed: Fracture-type deformation is more important than the thermal-softening-type deformation in low-speed extrusion machining; however, the thermal-softening-type deformation is the primary deformation mode for high-speed extrusion machining.