Performance and wear mechanisms of TiAlN-NbN coated cemented carbide in milling Ti6Al4V with different cooling and lubrication approaches

Titanium alloys are difficult-to-machine materials given their high strength, the high temperatures generated, and the naturally short contact length that combined result in rapid tool wear. Different machining strategies can be employed to cope with the low machinability. This study investigates the machining performance using different cooling and lubrication methods during down face milling of the most employed Ti alloy Ti6Al4V. The conditions include a dry environment, flood coolant, and sustainable vegetable oil applied at minimum quantity lubrication (MQL) condition. The tool life at dry conditions and with MQL applied vegetable oil are similar but MQL shows promising potential in prolonging tool life. Flood coolant provided no beneficial effect and tool life was about 50 % shorter as to dry and MQL conditions. The tool wear evolution on as-worn tools and their cross-sections is studied in scanning electron microscope using X-ray energy-dispersive spectroscopy. The PVD applied TiAlN-NbN coated WC-12%Co tools are worn by mechanical fracturing which is additionally accelerated by the outward diffusion of C from WC and Co binder that facilitate WC grain pull-out. Oxidation of Co binder further weakens the tool material mechanically.