On the mechanism of tool crater wear in titanium alloy machining

摘要

Today the aerospace industry spends hundreds of millions of dollars on the machining of titanium alloy components. And with increasing aircraft orders, there is pressure to machine at higher production rates and develop more machinable alloys (e.g. TIMETAL® 54M, TMETAL® 407) without compromising titanium’s excellent mechanical properties. Increasing the tool life by a factor of minutes can have a dramatic effect on machining cost. Unlike steels, the same tool grade is used for all titanium alloy types from alpha to beta rich, with the latter being more difficult to machine. Diffusion dominated crater wear is the primary tool wear phenomena which has yet to be fully understood. This thesis demonstrates the application of a low cost diffusion couple technique which gives a strong indication of the complex reaction mechanisms occurring at the tool-chip interface during the machining of titanium alloys. These small scale tests have been validated with large scale dynamic machining trials and strong agreement has been observed. The results have allowed for hypotheses to be made over the reaction mechanisms behind tool crater wear underpinned by key observations in the literature. Such a testing regime can be incorporated into alloy design approaches to inform the industry e.g. TIMET and Rolls-Royce about the ‘machinability’ qualities at a much earlier stage before costly machining trials. Such a method will also aid tool manufacturers to tailor tool carbide grades as well as new coatings to specific alloy chemistries.ududThis is the first time that small scale testing such as this has shown why different alloy chemistries exhibit different tool wear characteristics. The technique is now being developed further by the aerospace manufacturing supply chain including tool manufacturers and titanium alloy producers. It will be used to; (a) develop more machinable alloys at an earlier stage in the alloy design development and (b) match different titanium alloys to more appropriate tool materials and new coatings. As such this thesis should be of interest to a broad readership including mechanical engineers and materials scientists as well as the machining and manufacturing community.