Modelling of stagnant zone formation on diamond cutting tool when turning titanium alloy

Abstract

Diamond tools are increasingly used in advanced manufacturing because of their unique properties such as high hardness and thermal conductivity, and low thermal expansion unbeatable by any other known material. Despite their importance in machining of precision parts, extensive research on diamond tool wear is still underway. Particularly, maintaining the cutting edge sharpness and prolonging tool life are very important for machining difficult-to-cut materials. It is believed that the existence of stagnant zone on the tool rake face during cutting can protect the cutting edge from rapid wear. In this research, a new analytical stagnant zone model is derived based on boundary layer theory in fluid mechanics. The stagnant zone model predicts the stagnant zone length on diamond tools when machining titanium alloy. The cutting tool stress and chip velocity distributions required for the analysis of the stagnant zone model were taken from Finite Element (FE) modelling and simulation of orthogonal machining. Machining of Ti-6Al-4V with polycrystalline diamond and diamond-coated tools were experimentally carried out to obtain cutting forces and stagnant zone information at various cutting conditions. Used tools and postprocessed chips were investigated under scanning electron (SEM), field-emission scanning electron (FE-SEM) and optical microscopes. In FE modelling, input data such as material properties and the constants required in Johnson-Cook constitutive model were taken from the literature, and the friction data were calculated from experimentally measured forces. FE simulations were carried out for various cutting speeds, feeds, edge radius and surface roughness of the tool. The developed FE model is validated by comparing the predicted and experimental cutting forces. The predicted cutting forces are agreeable with the experiments. MATLAB is used to fit the equations that represent velocity and stress distribution as well as to calculate the stagnant zone length in the developed stagnant zone model. It is found that the predicted values are closed to the experimental results for both diamond tools. The average stagnant zone lengths formed at certain cutting conditions are between 0.1mm and 0.3mm. The used tool geometry, chip morphology and simulated shear strain, temperature, and velocity plots are also consistent with stagnant zone formation. The proposed stagnant zone model indicates that formation of stagnant zone is a function of cutting condition, tool geometry, the tool surface finish and material properties.