Performance and wear mechanisms of PCD and pcBN cutting tools during machining titanium alloy Ti6Al4V

The need for increased productivity in difficult-to-machine titanium alloys has pushed manufacturers to examine the potential of ultrahard cutting tool materials such as polycrystalline diamond and polycrystalline cubic boron nitride as alternative solutions to conventional cemented carbide tools. This study examines the performance of such advanced tool materials in high speed finishing machining of Ti6Al4V, with attained PCD superiority compared to pcBN. Wear mechanisms are experimentally investigated based on in-depth microscopic analyses using techniques such as scanning electron microscopy, transmission electron microscopy, electron diffraction and X-ray energy-dispersive spectroscopy. Main wear morphologies were flank wear and cratering in both tool materials. Flank fracture caused by micro-cracking was an additional deterioration mechanism of pcBN tooling. Diamond burn-out, likely in combination with graphitization of diamond, was causing channel like wear morphology. The PCD wear mechanism was diffusion dissolution of carbon in Ti6Al4V. (Ti,V)C diffusional barrier or Tool Protection Layer (TPL) was formed due to reaction of workpiece and tool materials in presence of cobalt. Controlled grain size and increased cobalt content resulted in higher performance as protective caps of (Ti,V)C merged to form a continuous TPL. Similarly for pcBN, (Ti,V)B-2 and (Ti,V,Cr)B-2 reaction products acted as TPLs which reduced the tool deterioration rate.