Comparative investigation of tool wear mechanism and corresponding machined surface characterization in feed-direction ultrasonic vibration assisted milling of Ti–6Al–4V from dynamic view

Abstract This paper mainly investigated the wear mechanism of universal coated carbide tools in feed-direction ultrasonic vibration-assisted milling (UVAM) of Ti–6Al–4V alloy through theorical analysis and experimental approaches, where the external ultrasonic vibration excitation was imposed on the workpiece. The rapid intermittent machining features, the generation mechanism of ultrasonic-frequency pulse cutting force and alternating friction, ultrasonic-frequency dynamic impact effect of resonant workpiece and considerable changes in tool trajectory and relative cutting speed in UVAM were analyzed from perspective of tool-workpiece dynamics and kinematics. Due to the ultrasonic-frequency friction between the tool and the workpiece, the minimum quantity lubrication (MQL) with biodegradable cutting fluid was firstly introduced in UVAM to improve the lubrication/cooling performance between tool and workpiece/chips interfaces. Three different machining methods, general milling, feed-direction UVAM and feed-direction UVAM&MQL, were sequentially carried out to comparatively investigate the tool wear mechanism and characterize tool wear and machined surface features. The corresponding machined surface quality including surface topography, surface profile and surface roughness under different tool wear states were presented to evaluate the tool wear condition. The results indicated that the tool wear mechanism in workpiece feed-direction UVAM system was dominated by ultrasonic-frequency dynamic load, where ultrasonic-frequency cutting force, ultrasonic-frequency friction, ultrasonic-frequency vibration impact and cutting inflection points of tool-tip trajectory worked together on the cutting edges. And the tool wear morphology in feed-direction UVAM was mainly characterized by tool-tip fracture, impact crack, friction traces and fatigue crack. The results of machined surfaces indicated that compared with the surfaces obtained in CM and general UVAM, the UVAM&MQL method could significantly improve the finished surface quality, which can also improve the tool wear to a certain extent.