Lathe turning of titanium using pulsed laser deposited, ultra-hard boride coatings of carbide inserts

Despite several years of research and development, titanium machining remains a challenging task that is currently carried out by the use of straight WC/Co and polycrystalline diamond (PCD) tools. Commercially available coated tools tend to react chemically with titanium, while ceramic tools suffer from chipping and notching. Advancements in cutting tools, particularly coated carbides, are needed to reduce tool wear in machining of titanium alloys. In this work, a recently developed, ultra-hard AlMgB14-20%TiB2 composite material was applied as a coating on WC/6%Co tool inserts by a pulsed laser (excimer) deposition technique. The coating was smooth, continuous, and fairly uniform in thickness. The average coating thickness was 0.7 mum for a deposition rate of 0.08 nm per pulse. Nanoindentation tests revealed that the hardness of the coating was approximately twice that of the WC/6%Co substrate. Dry machining tool wear tests, conducted with a CNC lathe by turning bar stocks of heat-treated Ti-6Al-4V alloy, showed that the coated tools outperformed uncoated tools by about two times in flank and nose wears and performed nearly same as that of the commercially available TiAlN coated tool. Detailed analysis of worn tools revealed that the wear mechanisms are quite different in coated tools and are similar to those observed in PCD tools. Results agree well with the general observation that a stable, strong adherent layer forms at the interface between the tool and the chip and minimizes the dissolution-diffusion wear mechanism.