On the Dynamics of Chip Formation in Machining Hard Metals
Matthew A. Davies, Timothy J. Burns, Christopher J. Evans
The results of orthogonal cutting tests on electroplated Nickel-Phosphorus (15% phosphorus) and AISI 52100 bearing steel are presented and compared. For both materials, chips become segmented at relatively low cutting speeds (0.3 m/s to 2 m/s) due to the onset of an oscillation in the material flow that is manifested in the repetitive formation of localized shear bands. The average spacing between the shear bands increases monotonically with cutting speed and asymptotically approaches a limiting value that is determined by the cutting conditions and the properties of the material being cut. The similarity in the behavior of the two materials (which have significantly different microstructure) and the regularity of the shear band pattern observed in the chips provides strong evidence for a continuum mechanics model of the process. A simplified one-dimensional thermo-mechanical model of a continuous, homogeneous material being sheared by an impinging rigid wedge is developed to explain the observed behavior. Numerical simulations of this model show that at low wedge speeds, material deformation reaches a thermo-mechanical equilibrium, in which material flow is homogenous and the stress, strain-rate and temperature fields reach a steady state behavior that is constant in time (when viewed from a tool-fixed reference frame). As the wedge speed is increased, the stress, strain-rate and temperature fields become oscillatory, and the material flow becomes inhomogeneous. As the speed of the wedge is increased further, the material shows repetitive shear localization, with the distance between shear zones increasing monotonically to some limiting value, as was observed in experiments.