Enhancement of viscosity and thermal conductivity of soybean vegetable oil using nanoparticles to form nanofluids for minimum quantity lubrication machining of difficult-to-cut metals

Sustainable use of vegetable oil as a base fluid in minimum quantity lubrication (MQL) strategy for machining advanced materials is promising but limited due to their low thermal conductivity and viscosity. This paper presents the results of experimental investigation for enhancing viscosity and thermal conductivity of high oleic soybean vegetable oil (HOSO) using Al₂O₃, MoS₂, and TiO₂ nanoparticles (30 nm particle size and 0.5–4.0% wt. concentration) inclusion to form nanofluids at temperature ranging from 25 to 70 °C for use in vegetable oil-based nanofluids-MQL machining of difficult-to-cut metals. The result shows that viscosity and thermal conductivity of HOSO increase with increase in nanoparticle weight concentration, but there is a decrease in suspension stability of the nanofluid. Also, viscosity of HOSO nanofluids decreases with increase in temperature, but thermal conductivity increases with increase in temperature, while for the base HOSO, it decreases with increase in temperature. This is a very significant positive observation especially for difficult-to-cut materials that generate high heat that need to be conducted away from the cutting zone. Thermal conductivity and viscosity were enhanced up to 55% (using MoS₂ at 70 °C and 4% wt. concentration) and 11.5% (using TiO₂ at 50 °C and 3.5% wt. concentration), respectively. The Brownian motion of the nanoparticles and liquid-solid interlayer interfaces are responsible for this behavior of the nanofluid thermal conductivity, while nanoparticle thickening and entangle mechanism were responsible for the behavior of the nanofluid viscosity. This implies that lower oil flow rate can be applied during machining of Inconel-718 due to increased viscosity and thermal conductivity to obtain optimal machining performance, lower power consumption, and reduce negative impact on the environment.