A study of viscosity and thermal conductivity of vegetable oils as base cutting fluids for minimum quantity lubrication machining of difficult-to-cut metals

The properties of environmentally friendly, sustainable, and renewable cutting fluids are very important to effective machining of difficult-to-cut materials under minimum quantity lubrication (MQL). This paper presents the experimental investigation of viscosity and thermal conductivity of three vegetable oils for use as base cutting fluids: modified high oleic soybean oil (HOSO), refined low oleic soybean oil (LOSO), acculube LB2000 (LB2000) oil, and mineral oil–based emulsion cutting fluid (EC) as benchmark and at temperature from 25 to 70 °C. The results show that viscosity of all vegetable oils decreases exponentially with temperature, and are significantly higher than that of EC. HOSO shows an enhanced viscosity of 6.03% compared to LB2000, 25.69% compared to LOSO, and 4688.30% compared to EC at 30 °C temperature. Thermal conductivity of soybean oils and LB2000 is similar, decreases gradually from 0.192 to 0.179 W/mK, not significantly affected by temperature and very low compared to EC. EC increases from 0.698 to 1.857 W/mK between 25 and 50 °C and decreases from 1.857 to 0.762 W/mK between 50 and 70 °C. Critical temperature of EC is around 50 °C. Shear stress–shear rate plots of vegetable oils show linear relation typical of Newtonian fluids. The R-square of HOSO was greater than 0.99 at all investigated temperature and better than the observed values for LB2000, LOSO, and EC. HOSO with higher oleic fatty acid composition shows the most significant viscosity enhancement and oxidation stability, and is recommended as alternative to EC and based-cutting fluid for MQL machining.