Sustainable cooling/lubrication induced thermo-mechanical effects on ultrasonic vibration helical milling of CFRP/Ti–6Al–4V stacks

Sustainable cooling/lubrication strategies including dry, minimum quantity lubrication (MQL), cryogenic (LN₂) and hybrid (MQL and LN₂) were used in ultrasonic vibration helical milling (UVHM) machining to improve the performance of hole-making for CFRP/Ti–6Al–4V stacks. The machining temperatures and forces were measured to characterize the thermo-mechanical effects on UVHM with different cooling/lubrication conditions. The machining temperatures at cryogenic conditions were −146 °C, −170 °C and −53 °C at CFRP layer, interface and Ti–6Al–4V layer, respectively. Axial and radial resultant forces at different conditions were highly related to the cutting temperature. Fiber removal mechanism at different conditions was analyzed according to the cutting temperatures, forces and the kinematic analysis in UVHM. Effects of sustainable cooling strategies and ultrasonic vibration on the hole surface texture of Ti–6Al–4V alloy were discussed. The amplitudes at different conditions varied approximately from 3.5 to 7 μm due to the variation of the forces. High precision of the exit geometry was achieved, as the height of hole exit burrs at Ti–6Al–4V layer were less than 40 μm except for the cryogenic condition. Diameters at the MQL and hybrid conditions were closer to the target diameter (ϕ10 mm), and the precision of the cylindricity of the machined holes of the stacks with the MQL and hybrid cooling conditions was higher than those at other conditions. Tool wear at different conditions were analyzed according to the SEM and EDS results. This work provided the fundamental understand of the hybrid process with sustainable cooling/lubrication strategy in UVHM machining. High quality of holes in CFRP/Ti–6Al–4V stacks were achieved by the hybrid processes.