Graphene Concentration Effects in MQL-Assisted Ultrasonic Drilling of CFRP

Drilling of carbon fiber-reinforced polymers is often accompanied by high cutting forces and heat accumulation, which negatively affect the quality of the final product. The thermo-mechanical correlation between cutting force and machining temperature during ultrasonic-assisted vibration drilling combined with minimum quantity lubrication using a graphene-enhanced biodegradable safflower oil nanofluid was investigated. Graphene as an additive was dispersed at concentrations of 0.25 mg/mL and 0.5 mg/mL with 1 wt.% oleic acid to ensure stable dispersion. Drilling experiments were conducted at spindle speeds of 1000, 3000, and 5000 min−1 and feed rates of 0.01, 0.02, and 0.03 mm/rev. The obtained hole walls were investigated using an optical microscope. The cutting force and the machining temperatures at the full tool–workpiece engagement stage were studied and compared with the results obtained by other authors. These measures allowed us to achieve a reduction in machining temperatures (T) by ~17.5%, in cutting forces (F) by 26.21–26.79%, and the roughness parameter Rz by 38.8% at the cutting conditions, where n = 5000 min−1 and f = 0.02 mm/rev at a graphene concentration (C) of 0.25 mg/mL.