Machinability of green titanium powder compacts during drilling using HSS twist drills

Abstract

The poor machinability of titanium-based alloys has until now been addressed by either choosing a suitable tool material, management of the heat generated during machining or by thermally assisted machining. This article reports on a new technique, called green machining, defined as the machining of compacted powders. Commercially pure titanium powder compacts possessing different strengths, called green strengths, were generated by uniaxial compaction and their drilling machinability characterized using the size of breakouts, surface finish of the drilled holes, and cutting forces. The cutting force for drilling sintered compacts was also determined for comparison purposes. The study found that all the green powder compacts could be drilled at high speeds regardless of their green strength. While high green strength, high speeds, and feed rates all affected machinability, feed rate was the main controlling factor. Also, depending on the binder used, cutting speed and feed rate could interact and worsen the machinability. However, compared to sintered compacts, the twist drills used on the green powder compacts did not catastrophically fail, and cutting forces were an order of magnitude less. It was concluded that green machining of titanium is a viable strategy to improve the poor machinability of the alloys.