Study of hybrid nanofluids of TiO₂ and montmorillonite clay nanoparticles for milling of AISI 4340 steel

In this work, hybrid nanofluids composed of and montmorillonite clay (MMT) and TiO₂ nanoparticles (NPs) were prepared at varying proportions and added to a cutting fluid for milling of an AISI 4340 steel. Due to their semi-spherical shape and small size NPs of TiO₂ fill surface valleys reducing friction; MMT, being a multilayer flake-like nanomaterial reduces friction and wear through the exfoliation of their weakly-bonded layers. Laboratory experiments were performed in a four-ball tribotester to determine the various combinations of TiO₂ and MMT in the cutting fluid that could provide a synergistic effect. Milling experiments were performed in a Haas VF2 CNC equipment with varying process parameters of feed rate, depth of cut, cutting speed, and different hybrid nanofluids. In these experiments, bars of AISI 4340 steel were milled with cemented carbide cutting inserts. A 3-level fractional experimental design with four factors was performed in order to optimize the milling input parameters and NP combinations that provided the lowest values of response parameters of surface roughness of steel bars, spindle power and wear of cutting inserts. Tribological testing showed that while both TiO₂ and MMT NPs provide enhancements separately, when combined in proportions of 80% MMT-20% TiO₂, and 40% MMT-60% TiO₂ their positive effect was enhanced, attributed to the combination of both their tribological mechanisms, exfoliation and filling of valleys, and ball-bearing effect by TiO₂ between MMT layers, that reduced friction between the cutting inserts and the workpiece. The optimized milling parameters that provided the lowest insert wear and Ra were a cutting speed of 1090 rpm, depth of cut of ~0.015 in, and feed rate of ~8 in/min.