Wear behaviour of PVD coated and cryogenically treated tools for Ti-6Al-4V turning

Titanium alloys, mainly because of their poor thermal conductivity, need to be cut at relatively low cutting speeds, with obvious negative consequences on the profitability of machining. An important amount of research has been done in order to increase productivity in titanium machining operations: high performance coatings and innovative technologies to improve insert resistance to wear represent promising solutions. In this work, a highly performing cutting insert (coated with a TiAlN layer obtained by Physical Vapor Deposition (PVD) magnetron sputtering) has been tested against the option of applying a Deep Cryogenic Treatment (DCT), when used for rough turning of aerospace titanium. The effects of the DCT have been experimentally investigated with two different experimental plans at low and high cutting speeds (respectively v_c ≤ 50 m/min, v_c ≥ 60 m/min). Statistical analyses of the results have been performed. The results show that at low cutting speed, the DCT treatment does not increase the tool life. At higher values of v_c, flank wear vs. time curves of coated tools have been determined, with and without DCT, and they clearly show that cryogenically treated tools present better wear resistance at higher cutting speeds. The wear mechanisms on the rake face and the flank for these two TiAlN coated tools have been analysed using a scanning electron microscope. The adhesion of titanium on the tool surface is lower for a DCT treated insert. The results indicate that the hardening of tools induced by the cryogenic treatment improves their useful life in high rate machining of titanium.