Experimental comparison between traditional and cryogenic cooling conditions in rough turning of Ti-6Al-4V

Titanium alloys, mainly because of their poor thermal conductivity, need to be cut at relatively low cutting speeds to avoid a severe diffusion wear, with obvious negative consequences on the profitability of machining. An important amount of research activities has been done in order to increase productivity in titanium machining operations and one of the most promising solutions is represented by the use of liquid nitrogen as a coolant during the machining operation. The aim of this paper is to compare traditional and cryogenic turning of Ti6Al4V in a region of cutting parameters particularly relevant to the aerospace industry where no previous data are available. The cutting parameters are those typical of titanium alloys rough machining which is considered, cost-wise, the most important operation because, for aerospace components, the so-called Buy-To-Fly ratio can reach values up to 20:1. The experiments have been performed using a full factorial design in order to statistically evaluate, using ANOVA and regression analyses, the significance of the input factors on the process most interesting outputs. The considered input factors are: type of cooling method, cutting speed and feed rate. The main analysed responses are: tool wear, surface roughness, cutting forces, coefficient of friction and chip morphology. The results show the significance of the cooling method on the tool life and that cryogenic machining is able to increase the tool life with respect to wet cutting. On the other hand, the beneficial effect of the liquid nitrogen cooling is reduced at high cutting speed and feed rate. Besides, the results showed that a small but significant reduction can be achieved for both the repulsion force and the coefficient of friction at the tool-workpiece interface.