Fundamental mechanisms of chip formation in orthogonal cutting of medical grade cobalt chromium alloy (ASTM F75)

Abstract

This paper reports on research into the fundamental mechanisms of chip formation in cutting of biomedical grade cobalt chromium molybdenum (Co-Cr-Mo) alloy conforming to the ASTM F75 standard. A full factorial orthogonal cutting experiment was undertaken to examine the effect on chip morphology of the basic control parameters: undeformed chip thickness (h) and cutting speed (vc) over ranges from 20 to 140μm and 20 to 60m/min respectively. The morphological characteristics considered include: segmentation spacing, frequency, segmentation and deformation angle where all were found to show a statistically significant dependence on the control parameters. This dependence was evident notwithstanding the “within sample” variation of these characteristics associated here with crystallographic anisotropy due to grain size far exceeding the maximum undeformed chip thickness (h) and giving rise to conditions normally found in micro-cutting. Analysis of the morphological and microstructural characteristics of the chip has led to a hypothesis that the heat treated ASTM F75 Co-Cr-Mo exhibits bifurcation in work hardening rates, possibly due to strain-induced martensitic transformation (SIMT). It is further hypothesised that an increase in cutting energy due to work-hardening depends on temperature and therefore vc and h but also, strain rate, grain size and orientation. Moreover, it is proposed that SIMT dominates over thermal softening and decreases the amount of plastic deformation prior to the onset of crack initiation and propagation, a mechanism identified to result in saw tooth morphology of Co-Cr-Mo chip.