Machinability based cooling/lubrication strategies in tapping within small holes of Ti6Al4V alloy

Tapping within small holes (0.112 in.) of Ti6Al4V alloy is considered as a challenging issue, mainly manifest in excessive tap wear, low tap life and poor thread quality, resulted by severe thermal-mechanical coupling. In this study, the tapping machinability of Ti6Al4V (taps: M42 HSS-E-PM) including torque characteristics as well as the thread forming mechanism, wear characteristics, and chip formation mechanism in dry, oil and LN2 2 environments is illustrated for the first time. Multiple indicators, such as torque/thrust force, temperature, as well as wear characteristics, surface quality, microhardness, and chip formation are introduced to make a comprehensive evaluation on tapping performance. The results show that the tap wear mechanism is same under these three conditions. However, cooling and lubrication effects on tapping performance of small holes are different from literatures of Ti6Al4V alloy machining. Compared with dry tapping, oil lubrication significantly increases tap life by 200 %. LN2 2 improves the thread quality, but has no benefit to tap life. Lubrication induced low friction and cooling induced embrittlement are the dominated mechanism in tap life and thread quality, respectively. The research provides new sight into high quality and long life tapping within small holes of Ti6Al4V alloy.