The Machinability of Ultrafine-grained Grade 2 Ti Processed by Equal Channel Angular Pressing

All processes of severe plastic deformation (SPD) are known to improve the mechanical strength of metals and alloys through microstructural refinement. Although the literature contains a large number of investigations on the effect of fine-grained microstructures on mechanical behaviour, data on the machinability is almost non-existent. This lack of information motivated the present work, in which the machinability of severely-deformed Grade 2 Ti is assessed in terms of cutting forces and the resulting product surface roughness. The SPD process here employed is Equal Channel Angular Pressing (ECAP), and the results are compared with those obtained on Ti and Ti-6% aluminum-4% vanadium (Ti6-4) alloy, both in the annealed condition. It was observed that the machining of ultrafine-grained Ti in the as-deformed state, requires larger cutting forces than the necessary for the annealed material, whilst for the alloy, the forces are of the same order. Due to continuous chip generation taking place in commercially pure Ti but not in Ti6-4, the passive component of the cutting force and the average surface roughness of the fine grained material are higher. Finally, whilst both annealed and fine grained Ti wear the tool by an attrition mechanism, titanium alloy machining promotes only adhesion over the tool edge.