Influence of oxygen content on the machinability of Ti-6Al-4V alloy

Titanium alloys are widely employed in many aerospace components due to their high strength-to-weight ratio, good corrosion resistance and fatigue properties, maintained at relatively high temperatures. Nevertheless, machining these materials efficiently has become a challenge in a sector that demands high productivity rates and minimal manufacturing times. This work analyses the machinability of the alpha + beta Ti-6Al-4V alloy in rough turning. Since oxygen is one of the elements that most affects the mechanical properties of titanium alloys, the aim is to understand its influence on the machinability of these materials. To do so, two different oxygen contents of 1200 and 2000 ppm are tested. A comprehensive material characterisation of both materials is carried out in order to clearly establish their differences: chemical composition, microstructure and mechanical properties (yield strength and hardness of the phases by nanoindentation). Machining trials consist of (i) tool life tests, which include a tool wear analysis, and (ii) cutting fundamental turning tests, in which the cutting forces and the chip form are studied. The work concludes that Ti-6Al-4V alloy with the highest oxygen content has the worst machinability (similar to 15 % lower compared to lowO), due to its higher volume fraction of beta + alpha(s) phase, slightly harder alpha(p) and greater yield strength, which generates higher mechanical and thermal tool wear. Thus, oxygen is a key composition element that should be controlled in machining titanium alloys, if robust results in tool life are expected to be obtained.