Towards simulating built-up-edge formation in the machining of steel

Abstract

Finite element simulations of the machining of carbon steels have been carried out over the cutting speed range 0.1–150 m/min. This range spans the ≈1–60 m/min within which built-up-edge occurs in practice. The material models used have included strain hardening varying with cutting speed. They have also included peaks in flow stress and reductions in ductility over temperature ranges associated with blue-brittle effects. The reductions in ductility have been introduced by means of a Johnson–Cook type of damage law which includes an influence of hydrostatic pressure on damage accumulation and a failure strain varying with temperature. The simulations are compared with previously published experimental results. They show that the damage law is more important than the thermal hardening associated with blue-brittleness in initiating built-up-edge formation. Secondary shear zone strain-rate fields have been obtained in which the surfaces of largest strain-rate are separated from the rake face by a stagnant region that may be considered to be an incipient built-up-edge. However, the variations of thrust force with cutting speed within the built-up-edge range that have been obtained from the simulations are not quantitatively realistic. In the modelling, material flow stress falls to zero once damage accumulation exceeds unity. It is speculated that a more realistic damage modelling, in which the shear failure surfaces between the chip and built-up-edge experience a residual friction resistance to slip between them, will lead to improved agreement with experiments. This will be investigated in future work.