ON THE FEASIBILITY OF MQL USING A MIXTURE OF SUPERCRITICAL CO 2 WITH CUTTING FLUID FOR GREEN MACHINING

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1 ON THE FEASIBILITY OF MQL USING A MIXTURE OF SUPERCRITICAL CO 2 WITH CUTTING FLUID FOR GREEN MACHINING Thai-Son Le 1a, Phi-Hai Dau 1b, Quoc-Thang Vuong 2, Bich-Thuy Vuong 3, and Van-Cuong Nguyen 1c,* 1 Vinh University of Technology Education, Vinh city, Vietnam 2 Vietnam National University, Hanoi, Vietnam 3 Dongnam Economic Zone, Vinh city, Vietnam ABSTRACT Recently, the peanut oil MQL has revealed its cooling and lubrication roles in hard turning process. In this on going research, the post supercritical-fluid-mixed solutions was evaluated in order to determine the remaining CO 2 gas in relevant aqueous electrolyte. The mixtures of 30 vol.% of carbon dioxide at supercritical state with different peanut oil concentrations were first formed with a duration of 30min. then exposed to the atmosphere and the release of CO 2 from these correlative mixtures were measured accordingly. As a result, the maximum released CO 2 concentration was up to 7% then gradually decreased to 0.04% within an hour under the condition of 500 rpm agitation. Those maintained supercritical gaseous substance in the aqueous cutting solution promises the high penetrating potential in the minimum quantity lubrication (MQL) as the green cooling and lubricant technique. KEYWORDS: supercritical CO 2 ; MQL; CO 2 release; peanut oil; high pressure spray 1. INTRODUCTION There appears to be a general agreement, that the minimum quantity lubrication (MQL) is the efficient and green cooling and lubricant technique which uses very small amount of oil mixed with copious amount of high pressure air in machining. This novel technique has been applied in machining processes (grinding, hard turning, hard milling, etc.) and has achieved better products [1, 2]. Recently, the peanut oil MQL applied in hard turning has been implemented in confirming the advantages of MQL over its dry cutting counterpart. The obtained results showed that the tool life was increased 31% and the surface integrity was improved when the peanut oil MQL was applied [3]. Meanwhile, the supercritical carbon dioxide (Sc-CO 2 ), which formed at the initial condition of a pressure of 7.4 MPa and a temperature of 31.1 o C, has been stated as a novel technique for dry cleaning [4], applications in pharmaceutics, and deposition of nano-material products [5-8]. The special Sc-CO 2 properties, with a combination of liquid-like density and gas-like viscosity, possess high penetrating facility in tiny cavities [8]. Carbon dioxide is nontoxic, nonflammable, and unreactive under most conditions, leaves no liquid waste, and is not surprisingly the leading candidate for a more environmentally responsible replacement for organic and aqueous solvents in many microelectronic applications. The hybrid use of MQL together with the spraying CO 2 has been applied in grinding and the desired cooling and lubricant efficiencies are achieved accordingly [9]. In that case, CO 2 was employed because of its low temperature with respected to its cooling role. However, in this work, the CO 2 will be used with expectations of its high dissolubility into the cutting solution under the supercritical state, thus, high penetrating efficiency when introduced to the cutting area. The advantages from this new MQL technique will also promise a green technique for 435

2 machining processes. Thus, the investigation on the feasibility of using the MQL process under the condition of the Sc-CO 2 has potential in cooling and lubrication for green machining. 2. EXPERIMENTAL PROCEDURE Shown in Figure 1 is the proposed process of the new MQL technique used in this work. The supercritical-fluid mixing part has already depicted in our previous work [7]. Herein, the made in cutting fluid will first be mixed inside a high pressure chamber with the Sc-CO 2, for 30 min. with respect to enhance the homogeneity of the emulsion. The CO 2 volume fraction can be adjusted for a desired mixture herein. The proposed spray section for MQL process depicted herein was also used previously [3]. The complete MQL system shown in Figure 1 is the combination from these two separate sections. For this purpose, by controlling the magnetic valve, the mixture was introduced into the post Sc-CO 2 container, and then be delivered to the machining process areas through the spray nozzle. The optimal set of peanut oil MQL has been revealed [3], as a pre-spray pressure of 0.5 MPa (~5atm) accompanied with speed of 1.0 ml/min. The fact that this proposed supercritical fluid MQL was designed for applying in every machining process with most of cutting fluid type. In order to exploit its advantage, one should examine the optimal mixing parameters under the Sc-CO 2 condition for the specific cutting fluid. In this case, the CO 2 fraction, Sc-CO 2 condition (pressure and temperature), mixing duration, agitation speed, and maybe the addition of additives would be some of the adjustable parameters for a desired mixture. The higher pressure, addition of surfactant, and sufficient CO 2 fraction and mixing duration may result in smaller size and dense CO 2 bubbles. CO 2 cooler High pressure pump Supercritical CO 2 forming and mixing with cutting fluid (T 32 o C, P 1 7.4MPa) Magnetic valve Post Sc-CO 2 solution (P 2 >1atm) Spray to working zone Liquid CO 2 tank Circulating pump Hot bath Figure 1. Schematic of the supercritical CO 2 MQL system In this preliminary examination, the supercritical state mixing parameters are listed in Table 1. When introducing CO 2 into the reaction chamber, the outlet valve was closed with a short delay for releasing the trapped air in the reaction chamber in terms of purifying working CO 2. After mixing, pressure was released by controlling the magnetic valve in order to maintain all solution in chamber. Subsequently, the remaining CO 2 concentration in the mixture was examined through the evaluation of the release of 436 dissolved CO 2 in the solution. The CO 2 released measurements were carried out in a total exposed air volume of 340 cm 3, monitoring by using an exhausted gas analyzer, the HORIBA MEXA-584L, Japan, within 60 min. continuously agitating. Table 1. Sc-CO 2 mixing condition Condition Value CO 2 fraction, vol.% 30 Pressure, MPa 14

3 Temperature, o C 40 Agitation, rpm 500 Sc-CO 2 mixing duration, min 30 The measurement configuration was depicted in Figure 2. The exposed air volume was then continuously pumped out and the relevant CO 2 concentration was evaluated accordingly. The measured result was simultaneously recorded at each 15sec interval for the beginning 10min and each 60sec for the rest duration. The examined solutions were prepared with peanut oil concentration of 0 vol.%, 1 vol.%, 10 vol.%, and 100 vol.%. mixture, the release of oversaturated CO 2 in the corresponding peanut oil solutions was measured and the results are depicted in Figure 2, accordingly. Figure 3. Evaluation of CO 2 concentrations released from supercritical-fluid mixtures with different solution concentrations. Figure 2. Schematic of released CO 2 concentration measurement 3. PRELIMINARY RESULTS AND DISCUSSIONS 3.1 Release CO 2 concentration from supercritical-fluid mixture In a number of reports about the applications of Sc-CO 2 in material processing, it is known that under the supercritical condition the CO 2 dissolved in the aqueous electrolyte and then plays an important role in the processes. Considering this occurrence, in this new MQL process, the nano-sized bubbles [10] may formed inside machining solution then reduce the viscosity of cutting fluid, hence, assist its penetration into the tiny gaps of the cutting zone. Those would be difficult to be penetrated by the conventional fluid, even by the conventional MQL ones. In order to determine the remaining CO 2 concentration in the post supercritical-fluid 437 As the results shown, the released CO 2 concentration from the oversaturated post Sc-CO 2 solutions under continuous agitation, at the atmospheric pressure, decreased distinctly. The released concentration from the 1 and 10 vol.% peanut oil solutions are lower and shorter than that of the pure DI water one. However, the value recorded from the pure peanut oil mixture is predominant. The CO 2 concentration, in this case, decreased from 7 vol.% to 0.15 vol.% within first 10 min, and still with 0.04 vol.% after 60 min. Herein, it is clearly that the released CO 2 from agitating post Sc-CO 2 solutions reduced gradually as soon as they were exposed to atmosphere. This would be because during the supercritical-fluid mixing duration, the gas substance was introduced into aqueous solution already. As soon as this mixture exposed at the atmosphere and the agitation started, because of its oversaturated gas concentration as well as different pressure, the fizzing bubbles coalescences one another for enlarging their size then escaped out from mixture. The high concentration and long released duration imply high maintained CO 2 concentration in aqueous peanut oil mixture.

4 Moreover, the release of CO 2 from the pure deionized (DI) water showed the CO 2 concentration dropped to zero after an 50 min. of agitation; whereas, those from a mixture with pure peanut oil still apparently keep maintaining to 0.04% after 60 min. exposed. Thus, it is suspected that the residual of dissolved CO 2 bubbles in the post Sc-CO 2 mixed solution would probably play important roles in improving the penetration of the cutting fluid into the cutting interfaces over its conventional MQL counterpart. The solubility of vegetable oil in the Sc-CO 2 can be evaluated by the derived equation by Del Valle et al. [11], as shown in following: c = T + ρ T ± exp( ) ) 2.7 (1) 2 where c is gram of oil per litter of CO 2, T is temperature (K), and ρ is density of oil (g/ml). This applied equation has been validated through the condition of pressure between 150atm to 880atm (~15-89 MPa). By applying Equation (1), with T=313K (~40 o C), ρ=0.91g/ml for peanut oil, the solubility of peanut oil in Sc-CO 2 in this work can be estimate approximately to 6.74± 2.7 g/l CO 2 for the case of 100 vol.% oil used. 3.2 The feasibility of MQL using supercritical mixed fluid In supercritical fluid application, some type of surfactant have been used for enhancing the Sc-CO 2 mixture [5, 12], the supercritical CO 2 droplets have been observed in nano scale (~10 nm) and in some cases the stability can exist over 24 hr. [10]. Owing these special characteristics, Sc-CO 2 easily penetrates into nano-scale gaps of the products. In this circumstance, peanut oil is known as the surfactant-like solution, thus its role would be somehow similar to those advantages achieved. Herein, after mixing with Sc-CO 2, a foam peanut oil solution was observed implying the well combination between gaseous phase and liquid phase had been recently formed. When exposing to the atmospheric condition, those CO 2 bubbles in the supercritical-fluid-mixed solution will getting larger in size and scatter in density. However, as the results shown in Figure 3, the remaining period would be sufficiently long for MQL application. Furthermore, one can lengthen this duration for desired process by optimizing all the supercritical-fluid-mixing parameters as mentioned previously. Figure 4. Schematic of the hybrid use of MQL and CO 2, reprinted from [9] Referring to those use in the work of Sanchez et al. [9], with schematic shows in Figure 3, the role of CO 2 was to low the cutting temperature and to accelerate the removal of cutting chips; however, in this work, the Sc-CO 2 s role would be expected to improve the penetration into the tiny cutting gaps because of its low viscosity. In previous work [3], Le has confirmed the cooling and lubrication role of the peanut oil used in MQL in hard turning as a green machining applications. Therefrom, the peanut oil MQL, which was formed at the condition of pressure from 0.4 to 0.6 MPa, would able to penetrate to the cutting gaps for taking part in cooling and lubrication processes resulted in smoother surface and longer cutting tool life. According to the special characteristics of supercritical CO 2, high solubility and high penetration abilities, the efficiency of this new MQL technique would be probably higher. 4. CONCLUSION This is the first time study on the feasibility of the supercritical fluid MQL process. The remained Sc-CO 2 in the cutting fluid 438

5 mixture would have the potential in enhancing the cooling and lubrication in machining process. This new MQL process is worth to apply for every cooling and lubrication process in machining as the green technique. The details investigation on this process is on going. 5. REFERENCES [1] Liao, Y. S., Lin, H. M., and Chen, Y. C., "Feasibility study of the minimum quantity lubrication in high-speed end milling of NAK80 hardened steel by coated carbide tool," International Journal of Machine Tools and Manufacture, Vol. 47, No. 11, [2] Khan, M. M. A., Mithu, M. A. H., and Dhar, N. R., "Effects of minimum quantity lubrication on turning AISI 9310 alloy steel using vegetable oil-based cutting fluid," Journal of Materials Processing Technology, Vol. 209, No , 2009, pp [3] Le, T. S., "An investigation of effects of MQL parameters in hard turning 9XC steel," Ph.D, Thai Nguyen University, Vietnam, [4] Keagy, J. A., Zhang, X., Johnston, K. P., Busch, E., Weber, F., Wolf, P. J., and Rhoad, T., "Cleaning of patterned porous low-k dielectrics with water, carbon dioxide and ambidextrous surfactants," The Journal of Supercritical Fluids, Vol. 39, No. 2, 2006, pp [5] Chang, T. F. M., and Sone, M., "Function and mechanism of supercritical carbon dioxide emulsified electrolyte in nickel electroplating reaction," Surface and Coatings Technology, Vol. 205, No , 2011, pp [6] Nguyen, V. C., Lee, C. Y., Chang, L., Chen, F. J., and Lin, C. S., "The Relationship between Nano Crystallite Structure and Internal Stress in Ni Coatings Electrodeposited by Watts Bath Electrolyte Mixed with Supercritical CO 2," Journal of the Electrochemical Society, Vol. 159, No. 6, 2012, pp. D393-D399. [7] Nguyen, V. C., Lee, C. Y., Chen, F. J., Lin, C. S., and Liu, T. Y., "Study on the internal stress of nickel coating electrodeposited in an electrolyte mixed with supercritical carbon dioxide," Surface and Coatings Technology, Vol. 206, No. 14, 2012, pp [8] Shinoda, N., Shimizu, T., Chang, T. F. M., Shibata, A., and Sone, M., "Filling of nanoscale holes with high aspect ratio by Cu electroplating using suspension of supercritical carbon dioxide in electrolyte with Cu particles," Microelectronic Engineering, 2012, pp. Article in press. [9] Sanchez, J. A., Pombo, I., Alberdi, R., Izquierdo, B., Ortega, N., Plaza, S., and Martinez-Toledano, J., "Machining evaluation of a hybrid MQL-CO2 grinding technology," Journal of Cleaner Production, Vol. 18, No. 18, 2010, pp [10] Dhanuka, V. V., Dickson, J. L., Ryoo, W., and Johnston, K. P., "High internal phase CO2-in-water emulsions stabilized with a branched nonionic hydrocarbon surfactant," Journal of Colloid and Interface Science, Vol. 298, No. 1, 2006, pp [11] Del Valle, J. M., and Aguilera, J. M., "An improved equation for predicting the solubility of vegetable oils in supercritical carbon dioxide," Industrial & Engineering Chemistry Research, Vol. 27, No. 8, 1988, pp [12] Da Rocha, S. R. P., Psathas, P. A., Klein, E., and Johnston, K. P., "Concentrated CO2-in-water emulsions with nonionic polymeric surfactants," Journal of Colloid and Interface Science, Vol. 239, No. 1, 2001, pp Contact Van-Cuong Nguyen 439

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