Cryogenic and hybrid induction-assisted machining strategies as alternatives for conventional machining of refractory tungsten and niobium

Abstract

Tungsten (W) and niobium (Nb) belong to the refractory metal group and are used as alloying elements in steels and superalloys for high demand products. These metals are also used in a high purity single-phase state for products within nuclear, space, military and research facilities.In the current study, tool wear development and surface quality at different cutting conditions have been evaluated when machining high purity W 99.95% and Nb 99.7% in longitudinal turning. Cryogenic LN2 cooling and hybrid induction-assisted machining strategies were applied in order to alter W and Nb material state with respect to the ductile-to-brittle transformation temperature, and thus to attempt controlling their machinability. These advanced machining strategies were benchmarked against the more conventional machining solutions of dry, flood and high pressure cooling.It was found that the use of LN2 cooling demonstrated the best result in terms of tool life when machining W followed by induction-assisted heating, dry and flood. All machining strategies provided similar surface quality which is related to strong build-up edge formation and respective surface alteration. When machining Nb, high pressure coolant results in best performance in terms of tool life and surface quality, then followed by flood and cryogenic strategies. Overall, substantial improvement of surface quality (Ra = 0.4–0.6 μm) was observed for high cutting speeds vc ≥ 225 m/min.