A novel method for investigating drilling machinability of titanium alloys using velocity force maps

Abstract

During the manufacture of critical aerospace components from metastable β titanium alloys, excessive tool wear and machining cost can be significant obstacles. Although tools are highly optimised for drilling the α+β alloy Ti–6Al–4V (Ti-64), insufficient information is available to efficiently improve tools for use on higher strength alloys due to the cost intensive processes required to design and optimise new drills. To reduce this cost, a novel method is presented which enables rapid comparisons of machinability between different alloys, (Ti-64; Ti–6Al–2Sn–4Zr–6Mo (Ti-6246); and Ti–5Al–5Mo–5V–3Cr (Ti-5553)), to quickly assess the performance of specific tool characteristics using industrially relevant drilling parameters. Rapid assessment and comparison of microstructural damage at selected forces was made possible through the development of 3D tool velocity force maps (VFM) for coated TiAlN and uncoated WC/Co tools at different cutting feeds and speeds. Microstructural damage depth has been shown to decrease with increasing alloy strength even for cases where the thrust force between the work piece and the tool was the same ~1600 ​N. In addition, a lower damage depth was measured when using uncoated tools. The machinability evaluation of tool coating and its interaction with force, torque, and material properties showed that TiAlN coated tools responded differently in the case of each alloy, displaying significantly different torque profiles, thus indicating different optimum cutting speeds and feeds. Abrasive and adhesive wear mechanisms were identified in the coated tools, while the majority of wear in uncoated tools was abrasive.