ROLE OF MICROSTRUCTURAL SOFTENING EVENTS IN METAL CUTTING

Abstract

Oxley's model for predicting equilibrium shear angle turns out to be the most comprehensive approach that incorporates both the mechanics of metal cutting and dynamic behavior of metal during metal cutting. Oxley's prediction of equilibrium shear angle for flow chip morphology is validated in metal cutting at low cutting speeds. However, the domain of flow chip is limited by major microstructural softening events that occur at high cutting speeds particularly if the matrix is hardened by heat treatment or there is a large volume fraction of second phase particles. Dynamic recrystallisation and phase transformation are identified as major microstructural softening events occurring in the hardened matrix that cause shear localisation. Incompatibility of deformation between the matrix and second phase particles causes shear localisation due to geometric softening. Quantitative modeling to predict the critical speed for the onset of shear localised chip morphology requires quantitative database on the dynamic flow stress behavior of materials that duly incorporates the microstructural softening events, which is the critical path. Quantitative analysis of phenomenological database in model alloys has shown that shear localisation can be suppressed by engineering glassy oxide inclusions that lubricate in-situ the tool-chip interface. This concept underlies the development of self-lubricating steel designed to suppress chemical wear in high speed machining.