Improved machinability of titanium alloy TI-6AL-4V through workpiece preheating

Abstract

Titanium alloys are generally regarded as difficult-to-cut materials and plentiful research works have been conducted on their machinability in the past few decades with several diverse objectives, such as improvement on tool life, surface finish and surface integrity, investigation on chip and tool wear morphology, cutting force and vibration/chatter as well as reducing cost of machining. This study introduces induction preheating as one of the new variable parameters to improve the machinability of titanium Ti-6Al-4V alloy. In the current work the influence of heating parameter i.e. preheating temperature, and the cutting variable, i.e. cutting speed, feed and axial depth of cut were investigated during end milling operation on a vertical machining center (VMC). The inserts used were uncoated tungsten-cobalt (WC-Co) carbide and polycrystalline diamond (PCD) attached to a 20 mm diameter end mill tool. The study comprehensively investigated the effect of preheating temperature on machinability parameters i.e. tool life, surface finish and cutting force. The effect of preheating on vibration/chatter, surface integrity, chip-tool contact length and chip serration morphology were also investigated. Central composite design (CCD) of response surface methodology (RSM) coupled with Design-Expert 6.0.8 software was used to develop empirical models of tool life, cutting force and surface roughness both for room temperature and preheated machining. The software was further used to maximize the tool life and concurrently minimize the surface roughness by optimising the cutting parameters and preheating temperature. As a result, in preheated machining, the performance of uncoated WC-Co was tremendously improved (almost three times compared to room temperature machining), even the values of tool life are much higher than those cutting with PCD under room temperature machining. Preheated machining substantially contributes to the reductions of vibration/ chatter and resultant cutting force, facilitating increased tool life. Furthermore, preheated machining facilitates increased chip-tool contact length, stable chip serrations resulting in reduction of tool wear rate.