Analysis of volume ratio of castor/soybean oil mixture on minimum quantity lubrication grinding performance and microstructure evaluation by fractal dimension

As a non-edible, degradable, and environment-friendly crop product, castor oil can be used as lubricating oil in material processing. However, its high viscosity results in poor flowability, thereby limiting its industrial applications. Although experiments have revealed the excellent lubrication performance of castor/soybean oil mixture, the volume ratio of castor oil and soybean oil can significantly influence their minimum quantity lubrication (MQL) grinding performance. In the current study, seven castor/soybean oil mixtures under different volume ratios ranging between 1:0.5 and 1:4 were prepared for MQL grinding of Ni-based alloy. The mixed oils were compared with pure castor oil (1:0) in terms of viscosity and tribological behaviors. The grinding experiment involved the evaluation of specific grinding force, surface roughness, surface microtopography, and grinding debris, conluding that the preferred castor/soybean volume ratios were found to be 1:0.5, 1:1, 1:1.5, and 1:2. The surface microtopographies of selected workpieces were analyzed further in terms of their fractal dimension and scale coefficient, finding that the maximum fractal dimension (D = 1.31) and minimum scale coefficient (G = 0.30) are achieved when the volume ratio is 1:2. Hence, the optimal volume ratio is determined.