Kevin Gutzeit, Georgis Bulun, Gerhard Stelzer, Benjamin Kirsch, Jörg Seewig, Jan C. Aurich

• Due to an excellent ratio of high strength and low density, Ti-6Al-4V is suitable for many industrial applications, especially in the aerospace industry. However, Ti-6Al-4V is also characterized by a very low thermal conductivity and high chemical reactivity which is why the titanium alloy is considered to be a hard-to-cut material. Machining Ti-6Al-4V leads to high cutting temperatures, which leads to a rapidly progressing thermo-chemical induced tool wear. To reduce the thermal load and to enhance the cutting performance, suitable cooling strategies are a necessity. A novel, highly efficient cooling approach is to apply sub-zero metalworking fluids (MWF) at liquid state but at supply temperatures well below 0 °C. These sub-zero MWF inhibit high cooling effects due to their low supply temperature in superposition with a beneficial wetting behavior. In this work, the application of a sub-zero cooling strategy is investigated when milling Ti-6Al-4V. The influence of both down milling and up milling is investigated under a systematic variation of the cutting speed and feed per tooth. For comparison, the experiments are also conducted using a cryogenic CO2 cooling. The performance of both cooling strategies in dependence of the milling process is described on the basis of the occurring forces, the resulting tool wear, and the surface quality achieved. The results show that the sub-zero cooling can successfully improve the machinability of Ti-6Al-4V even at elevated cutting parameters and unfavorable cutting conditions. As a result, sub-zero milling clearly outperforms the cryogenic CO2 cooling, since less tool wear and an overall lower surface roughness are observed. Consequently, when using a sub-zero cooling strategy, higher metal removal rates, longer tool life, and better surface qualities are achievable.