Surface integrity analysis of Ti6Al4V after semi-finishing turning under different low-temperature cooling strategies

During machining of Difficult-To-Cut (DTC) alloys, the rapid tool wear as a consequence of the high values of cutting forces and temperatures significantly affects the surface integrity of the machined surfaces reducing the product in-service performances. The use of conventional cutting fluids limits the tool deterioration, but provides several environmental issues to deal with, such as the exhausted fluid disposal, cleaning of the parts after machining, health hazards for the machines operators. In recent years, several researches have proposed to use environmental-friendly cryogenic coolants, whose very low operating temperatures have demonstrated to be able to inhibit the thermally activated tool wear mechanisms leaving the machined surface almost free from pollutants. In this framework, the aim of this research work is to analyse the surface integrity of the machined surfaces produced in semi-finishing turning of the Ti6Al4V titanium alloy under different low temperature cooling techniques, namely Liquid Nitrogen (LN2), Carbon Dioxide (CO2) and gaseous nitrogen (N2) cooled at different temperatures, using the dry and wet cutting conditions as baseline. The surface integrity was evaluated considering both the surface finish (surface defects, roughness and topography) and the surface microstructural and mechanical alterations (altered layer, nano-hardness and residual stresses). The results showed that the best performances were obtained using N2 cooled at -150°C, which determined both the absence of surface defects, as wrinkles or feed marks irregularities typical of the surface generated under LN2, and high compressive residual stresses that made the surface suitable for biomedical and aerospace applications