ENHANCEMENT OF Ti6Al4V MACHINABILITY THROUGH THE USE OF INNOVATIVE COOLING/LUBRICATING TECHNOLOGIES

The machining operations of Difficult-to-cut materials such as Titanium and its alloys, has an important role in the development of new techniques with the aim to improve both the process productivity and the final product quality. The performances of the cutting fluids have recently been under investigation to drive machining operations towards cleaner and more sustainable targets. Several efforts are being made to test new formulations of coolants and to implement cooling/lubricating strategies alternative to standard flooding. As reported in literature the low-temperature cooling seems to be an efficient solution to enhance the process sustainability, among its several advantages, no contaminants are left on the chips and workpiece, hence reducing the chip disposal costs and limiting skin and breath diseases for the machine tool operators. Furthermore, in case of surgical prosthesis, it can help reducing the cleaning steps before the final sterilization. However its industrial application is still limited due to the cryogenic coolant extremely low temperatures that could determine a loss of geometrical accuracy of the machined components as a consequence of the thermal contraction induced mechanical components of the CNC machine tool. Another interesting solution that minimizes the consumption of cutting fluid consumption without determine the above mentioned problems typical of the pure cooling techniques, consists in the use of Minimum Quantitative Lubrication (MQL) technologies. Nevertheless the low lubricating capacity of the cutting fluid does not allow to efficiently inhibit the tool activated wear mechanisms responsible to cutting edge degradation. In this PhD thesis, several cooling, lubricating and hybrid technologies have been developed and implemented to improve the machinability of Ti6Al4V alloys, as promising alternatives to standard cooling methods applied in semi-finishing turning operations. To reduce the main critical issues of the cryogenic cooling strategies different solutions have been proposed: (i) For the first time an innovative cooling approach based on the use of gaseous Nitrogen (N2) cooled by LN2 in a range of temperature between 0° and -150°C. (ii) Tool holder re-designed in order to make competitive the use of cryogenic coolant (LN2) reducing the thermal distortions. As concerning the strategies to improve the minimal lubricating technologies both the Solid Lubricant assisted technologies and the hybrid ones (Minimum Quantitative Cooling Lubrication) were investigated. Two SL-assisted strategies were considered, namely an MQL vegetable oil enriched with PTFE particles and an aqueous solution added with graphite at different percentages, while for the MQCL technology the MQL system was implemented together with Liquid Nitrogen (LN2) and Carbon Dioxide (CO2) distribution systems designing the position of the nozzles to optimize the lubrication and cooling effects. The Ti6Al4V machinability has been firstly investigated through an experimental approach, evaluating both the tool wear mechanisms and the surface integrity, using thedry cutting and the wet strategies as a reference. Lastly the effects onto the machinability of Ti6Al4V alloy of different microstructurevariants produced though both conventional and Additive Manufacturing technologiesand different insert coatings were evaluated.