Longitudinal–torsional ultrasonic vibration–extrusion tapping of small blind holes in Ti-6Al-4V with large length-to-diameter ratio.

Extrusion tapping (ET) poses a technical difficulty when processing internal threads with large depth, small diameter, and high-precision blind hole. In addition, conventional ET (CET) of Ti-6Al-4V small internal blind-hole threads with large length-to-diameter ratio is challenging given its small elastic modulus, large deformation resistance and easy cracking in cold forming. To this end, a novel longitudinal–torsional ultrasonic vibration–extrusion tapping (LT-UVET) is proposed to address the technical problem of these threads. On the basis of ultrasonic longitudinal–torsional vibration characteristics, the motion trajectory equations of LT-UVET were deduced. The motion model of LT-UVET was established, and the intermittent extrusion law and force state between the tap extrusion edge and the processed material were explored and studied. Furthermore, tapping experiments were conducted to compare the differences in torque, thread quality, microhardness, and tap wear obtained by CET and LT-UVET. The influence of different tapping speeds on the thread was studied through singlefactor experiment. According to experimental findings, the maximum tapping torque was reduced by 9 %–26 % compared with CET. Moreover, the surface integrity and the profile of the thread were improved. The overall microhardness of threads increased by 3 %–17 %. The wear area of the tap lobes in LT-UVET is reduced by 65 % compared with that of CET. Therefore, the introduction of longitudinal–torsional ultrasonic vibration into the extrusion zone enhances the machinability of Ti-6Al-4V, which in turn improves the quality of extruded threads and increases the efficiency of thread machining. [ABSTRACT FROM AUTHOR]