Effect of cryogenic cooling and tool wear on surface integrity of turned Ti-6Al-4V

The aim of this work is to investigate the influence of flank wear and cutting fluid (emulsion vs. liquid nitrogen) on surface integrity in turning of titanium alloy Ti-6Al-4V. Longitudinal turning tests with pre-worn uncoated cemented carbide inserts are performed, after which the surface and subsurface layer of machined workpieces is studied. Results for residual stresses on the surface as well as in depth profiles, obtained by X-ray diffraction, are also presented. Scanning electron microscopy (SEM) is used to investigate the microstructure of the workpieces. The same tool holder was used for both cooling conditions, with the same nozzle configuration. The flow rate of liquid nitrogen was therefore limited and as a result, tool wear development was observed to be faster for cryogenic cooling than emulsion-based flood cooling. However, the results show limited differences in terms of achievable residual stresses when comparing cryogenic and conventional cooling at similar levels of tool wear. Despite an increase in tool wear rate, the cryogenic cooling conditions thus provide similar surface integrity results as emulsion cooling. The results suggest that the consumption of the cryogenic coolant can be reduced or optimized without a significant impact on surface quality.