Investigation of ionic liquids as metalworking fluids in minimum quantity lubrication machining

Abstract

Sustainable manufacturing aims to develop methods to convert materials into finished products while reducing consumption of natural resources and energy, reducing environmental emissions and contamination, reducing health and safety risks, and retaining the goal of better performance and economy to the users. Machining, which is a highly energy intensive process, is a key manufacturing process for many products. The energy consumed in machining process is nearly completely converted into heat and traditionally a cutting fluid or coolant is employed as a flood to remove the heat generated during machining. The negative ecological aspects of conventional cutting fluids include waste disposal, release of harmful substances into the environment and hazardous working conditions for the operators. Skin contact and inhalation of MWFs loaded with microorganisms, biocides and toxic metal particles is the main cause of occupational diseases of industrial workers in the metalworking industry. Stringent environmental regulations have necessitated research into the reduction or elimination of harmful cutting fluids from the machining processes. Various researchers have investigated many different approaches for reduction or elimination of cutting fluids, which include the use of high pressure water jet, cold water mist, chilled compressed air, cryogenic liquid nitrogen, carbon dioxide and super-critical CO 2 spray, and Minimum Quantity Lubrication (MQL) machining. In MQL machining systems a very small amount of lubricant is supplied to the cutting point along with air or inert gas stream and the lubricant gets consumed in the cutting process itself and is not re-circulated, hence there are no serious disposal or ecological issues, when compared with coolant assisted machining. Mineral oils, synthetic chemicals, and vegetable oils have been extensively applied as lubricants in MQL machining. Many researchers have also explored adding nano- particles to the lubricating oil. However, the above-mentioned categories of MWFs for MQL machining have many limitations and a better alternative is needed which should exceed the performance of all these systems and be environment friendly. Ionic liquids are a relatively new family of eco-friendly chemicals being actively researched upon by scientists for chemical, pharmaceutical and engineering applications including many challenging tribological areas involving relatively high contact stresses and temperatures. The ionic liquids, therefore, can be suitable green and effective candidates as lubricants in MQL machining and their potential needs to be explored critically. In the present research work, conventional oils used in MQL machining have been replaced with ionic liquids applied as additives to a vegetable (Canola) oil. This research work aims to investigate the effectiveness and fundamental action mechanisms of ionic liquids when employed as lubricants in interrupted orthogonal machining under MQL conditions. Machining experiments were carried out under dry, flood and different MQL conditions in order to investigate the influence of machining parameters and cutting conditions on the tribological performance of different types of lubricant systems prepared by combining different ionic liquids with vegetable (canola) oil and poly ethylene glycol (PEG). The ionic liquids selected hexafluorophosphate for the (BMIMPF 6 ), study were 1-methyl 1-methyl 3-butyl 3-butyl imidazolium imidazolium tetrafluoroborate (BMIMBF 4 ), 1-methyl 3-butylimidazolium bis(trifluoromethyl-sulfonyl)imide (BMIMTFSI), and tributyl(nonyl)phosphonium bis(2-ethylhexyl) phosphate. These ionic liquids were used as additives to the base vegetable oil. Canola oil was used as the base vegetable oil. Among the ionic liquids, BMIMPF 6 , BMIMBF 4 and BMIMTFSI were hydrophilic in nature and non- soluble in oil while ionic liquid tributyl(nonyl)phosphonium bis(2-ethylhexyl) phosphate was oil miscible. Another lubricant used in study was Poly Ethylene Glycol (PEG) with average molecular wt. of 400. PEG has hydrophilic nature and was able to dissolve BMIMPF 6 . The three main categories of lubrication combinations investigated in this study included: (i) vegetable oil with non-miscible hydrophilic ionic liquid added via sonication, (ii) vegetable oil in solution with oil-soluble hydrophobic ionic liquid, and (iii) hydrophilic ionic liquid in solution with another hydrophilic base lubricant (PEG). The analysis of force signals recorded in machining revealed that in light machining, the machining force values obtained during MQL machining using vegetable oil with ionic liquids remained consistently lower in magnitude than the force values observed in case of machining under dry, flood, air jet, or MQL with neat vegetable oil conditions. Additionally, the workpiece surface roughness obtained in machining under MQL condition with vegetable oil containing ionic liquids was significantly lower than the values observed under dry, compressed air jet, flood cooling, and MQL machining with neat vegetable oil without any additives. This further confirmed the generally superior tribological conditions existing during MQL machining with vegetable oil containing ionic liquids as additives. The element dot maps of Iron (Fe) obtained with Energy Dispersive Spectroscopy (EDS), for the area near the cutting edge of the tool used in MQL machining with oil and ionic liquid BMIMBF 4 , and BMIMPF 6 , clearly demonstrated the presence of workpiece material adhering to the tool rake face, since this element was not present in detectable quantities either in the cutting tool material or in any of the applied MWFs, over most of the tool-chip contact zone. The element map for Fluorine (F) showed much higher density in the areas where significant Iron deposits were present. This suggests that at the temperatures and pressures prevailing at the tool chip interface in cutting at higher speeds the fluorine containing ionic liquids disintegrated and liberated fluorine, which, being more electronegative than any oxygen present in the environment, readily combined with freshly exposed surfaces of the workpiece material and formed fluorides of iron. This is hypothesized to be the dominant mechanism through which ionic liquids used in this study work as lubricants in metal cutting at high cutting speeds and loads. The other tribologically active elements present in the selected ionic liquids included sulphur and phosphorous. The element maps for these elements and the EDS spectra show sulphur and phosphorous in trace quantities only, suggesting that they did not bind effectively with the tool or workpiece material. The thermal decomposition temperature showed a strong positive correlation with cutting forces and workpiece surface roughness at higher cutting speeds and loads. The specific heat capacity (Cp), product of density and specific heat capacity (ρ.Cp), and the heat absorbed during decomposition (ΔH - decomposition), all of which influence the cooling capacity of the lubricant, however, did not seem to have any consistently strong relation with machining forces, workpiece surface roughness and tool-chip contact length. The heat extraction by the lubricants appears to be a transient phenomenon at the beginning of the cut, as the Cp, ρ.Cp and ΔH (decomposition) show significant correlations only with the peak cutting forces observed during tool entry in finish machining condition. While machining under light cutting conditions at higher cutting speeds, the fluorine containing ionic liquids, when used as additives to vegetable oil in a small quantity, yielded lower cutting forces as compared to those observed with neat vegetable oil. Such ionic liquids may be suitable for high speed machining applications where the cutting forces are low, but the cutting speeds are high enough to raise the cutting temperatures well above the decomposition temperatures of the ionic liquids. Under heavy cutting conditions, the stresses and temperatures are high enough even at low cutting speed, hence showing beneficial effects of these ionic liquids. At higher cutting speeds however, the effect of ionic liquids in heavy machining seemed to diminish. Similarly, while evaluating the effect of addition of ionic liquids on the workpiece surface roughness, the improvement was more significant at lower cutting speed than at higher cutting speed where there was no significant difference. When an oil miscible ionic liquid was added to the vegetable oil it showed lower cutting forces as compared to those of machining with neat vegetable oil at lower cutting speed under both light and heavy cutting conditions. Again, the benefit disappeared at higher cutting speeds. The decomposition temperature of the oil soluble ionic liquid used in the study was designed to be lower than those of oil non-soluble ionic liquids. That is why the effect was visible even at lower cutting speed under light machining conditions. Additionally, it was observed that the solution in which ionic liquid was added in 1% concentration performed better than the solution with 0.5% concentration under heavy cutting conditions, suggesting that a higher concentration may give better results in more severe machining applications. When a hydrophilic lubricant (Poly Ethylene Glycol – PEG) was used as the MQL fluid, it showed a significant improvement in the cutting forces at lower cutting speed under light machining conditions. At higher speeds and higher loads it was seen that the addition of a hydrophilic ionic liquid, which was completely soluble in PEG, produced lower machining forces as compared to machining with PEG alone. The workpiece surface roughness, however, was better while machining with neat PEG under most of the cutting conditions. A thermal model has been developed for estimation of cutting temperatures utilizing two dimensional finite difference method for transient heat conduction with inverse heat transfer procedure. The estimated tool-chip interface temperatures were used to gain insight into whether ionic liquids and other lubricants used in the studies would reach their decomposition temperature or not during different cutting and lubricating conditions and, thus, explain the mechanism of working of the applied MWFs under different conditions. The results indicate that when the temperature remains below thermal degradation temperature the MWF’s physical properties, such as viscosity and density may influence its efficacy to a greater extent. When the temperature goes beyond the degradation temperature the lubricant will decompose and the decomposition products define the mechanism of action. From the thermal model it was observed that in light machining at low cutting speed the cutting temperatures were estimated to be below thermal decomposition temperatures of the lubricants employed. In this condition the viscosity of the lubricant played a dominant role and lubricants with higher viscosity performed better in terms of yielding lower measured machining forces. At higher cutting speed the temperatures increased beyond decomposition temperature. In this case fluorine containing ionic liquid lubricants decomposed and released fluorine, which readily bonded with freshly machined surfaces and reduced the intensity of adhesion between the tool and chip, and between the tool and the new workpiece surface, resulting in lower cutting forces. Decomposition of other ionic liquids, which did not contain fluorine as the tribo-active element released upon decomposition, however, did not yield the same extent of improvement in measured machining forces or workpiece surface roughness. Several directions for future work involving application of next generation ionic liquid-based MWFs for MQL machining are identified based on the insight gained from experimental observations and numerical modelling of thermal conditions in machining.