Improvements in ultrasonically assisted turning of TI 15V3Al3Cr3Sn

Titanium alloys have outstanding mechanical properties such as high hardness, a good strength-to-weight ratio and high corrosion resistance. However, their low thermal conductivity and high chemical affinity to tool materials severely impairs their machinability with conventional techniques. Conventional machining of Ti-based alloys is typically characterized by low depth of cuts and relatively low feed rates, thus adversely affecting the material removal rates (MRR) during the machining process. Ultrasonically assisted turning (UAT) is an advanced machining technique, in which ultrasonic vibration is superimposed on a cutting tool. UAT was shown to improve machinability of difficult-to-machine materials, such as ceramics, glass or hard metals. UAT employment in the industry is, however, currently lacking due to imperfect comprehensive knowledge on materials‘ response and difficulties in obtaining consistent results. In this work, significant improvements in the design of a UAT system were performed to increase dynamic and static stiffness of the cutting head. Concurrent improvements on depth-of-cut controls allowed precise and accurate machining operations that were not possible before. Effects of depth of cut and cutting speed were investigated and their influence on the ultrasonic cutting process evaluated. Different cutting conditions -from low turning speeds to higher recommended levelwere analysed. Thermal evolution of cutting process was assessed, and the obtained results compared with FE simulations to gain knowledge on the temperatures reached in the cutting zone. The developed process appeared to improve dry turning of Ti-15-3-3-3 with significant reduction of average cutting forces. Improved surface quality of the finished work-piece was also observed. Comparative analyses with a conventional turning (CT) process at a cutting speed of 10 m/min showed that UAT reduced the average cutting forces by 60-65% for all levels of ap considered. Temperature profiles were obtained for CT and UAT of the studied alloy. A comparative study of surface and sub-surface layers was performed for CT- and UAT-processed work-pieces with notable improvements for the UAT-machined ones. Two- to three-fold reductions of surface roughness and improvements of other surface parameters were observed for the UAT- machined surfaces. Surface hardness for both the CT- and UAT-machined surfaces was investigated by microindentation. The intermittent cutting of the UAT-process resulted in reduction of hardening of the sub-surface layers. Optical and electronic metallographic analyses of cross-sectioned work-pieces investigated the effect of UAT on the grain structure in material‘s sub-surface layers. Backscatter electron microscopy was also used to evaluate the formation of α-Ti during the UAT cutting process. No grain changes or α-precipitation were observed in both the CT- and UAT-machined work-pieces.