Performance evaluation of aluminium 6063 drilling under the influence of nanofluid minimum quantity lubrication
Introduction
In manufacturing, the process of removing unwanted segment of metal work piece in the form of chips is known as machining (Nagendra Parashar and Mittal, 2007). The machining process will shape the workpiece as desired and it is usually done using machine and cutting tools (Yusup et al., 2012). The growing demands for high productivity of machining require high material removal rates, which require high cutting speed and feed rate. Such high material removal rates inherently produce high cutting temperature (Meena and Mansori, 2011). High temperature and forces on a cutting tool drastically reduces tool life and surface finish (Park et al., 2010). Thus cutting conditions is identified as a critical factor leading to producing an optimal surface finish with minimal form error and is featured by high-frequency tool-tip vibration and the formation of regularly spaced shear bands appeared on chip surfaces (Chan et al., 2013). During cutting operations one of the most important problems is tool wear, caused by the normal load generated by the interaction between tool and workpiece and by the relative motion between tool-chip and tool-workpiece (Kendall, 1998).
Heat generation and friction between tool and chip usually limit machining performance in drilling operations (Tasdelen et al., 2008), but this can be reduced by lubrication. Coolants and lubricants are therefore used in high quantities to reduce the temperature, surface roughness and friction in the cutting area (Barros et al., 2013). There are several types of metal cutting fluids (MCFs) that can be extensively classified as straight fluids, synthetic and semi-synthetic fluids, soluble fluids and vegetable based cutting fluids (VBCFs). Petrochemical and synthetic based cutting fluids create many negative effects (Ozcelik et al., 2011). Due to the several negative effects they cause, when inappropriately handled, cutting fluids may damage soil and water resources because of their toxic effects. As machine operators are constantly at the same ambient with cutting fluids, they are primarily under impact of toxic fluids, which cause serious health problems like lung cancer, respiratory diseases and genetic diseases (Dhar et al., 2007). Several researchers state that the costs related to cutting fluids are frequently higher than those related to cutting tools, so cost also effected with cutting fluids (Dhar et al., 2006). All costs involved with cutting fluids (purchasing, recycling, chip drying, etc.) represent 7.5–17% of the manufacturing costs of a part, which in many cases is even higher than tool costs (Bragaa et al., 2002, Sharma et al., 2015). Conservation of materials, energy and environment is becoming a very important issue. Enormous efforts to reduce the use of lubricant in metal cutting are being made from the viewpoint of cost, ecological and human health issues. Minimal quantity lubrication (MQL) can be considered as one of the solutions to reduce the amount of lubricant or cutting fluid (Itoigawa et al., 2006). The use of cutting fluid is important in a machining process to cool and lubricate tool and workpiece. Besides that, in some operations such as drilling, for example, use of cutting fluid helps to remove the chips from inside the holes, thus preventing drill breakage (Nandi and Davim, 2009).
It is important to find a way to accomplish machining processes without or with little cutting fluid and at the same time, promoting long tool life and good workpiece quality (Bragaa et al., 2002). Biodegradability is the most important aspect with regard to the environment and human health. If biodegradability should be considered, esters and vegetable oils are more indicate to formulate cutting fluids (Davoodi and Tazehkandi, 2014).
Vegetable based cutting fluids (VBCFs) can be considered environmentally friendly because these types of fluids are renewable, less toxic and possess high biodegradability. They have also good lubrication capability as compared to straight fluids, synthetic fluids, soluble fluids (John et al., 2004). Studies about VBCFs on various mechanical processes have been focused on drilling, turning, tapping and reaming (Sreejith, 2008). Rahim and Sasahara (2011), found in high speed drilling of Ti-6Ai-4v, MQL palm oil produced lower cutting forces and workpiece temperatures than MQL synthetic ester, almost equal to the flood condition. This was due to the formation of thin boundary lubrication film and the reduction of friction at the tool–workpiece interfaces, which reduced the heat. Stronger environmental concerns and growing regulations over contamination and pollution will increase the need for renewable and biodegradable lubricant (Fox and Stachowiak, 2007).
Dry operations would be the best solution for the environmental issues involving metal machining. In reality, they are sometimes less effective when higher machining efficiency, better surface finish quality and severe cutting conditions are required (Sutherland et al., 2000). For these situations, semi dry operations utilizing very small amounts of cutting lubricants, such as MQL systems are expected to become a powerful tool (Heisel, 1994). MQL is a method of supplying lubrication in machining to achieve both environmental and economic benefits. Cutting fluids (CFs) have so far been selected mainly on the basis of their primary characteristics, i.e., their cutting performance (Klocke and Eisenblatter, 1997). In semi-dry operations with MQL, however, the secondary characteristics of lubricants, such as their safety properties, biodegradability, oxidation stability and storage stability, are also very important because the lubricants must be compatible with the environment and resistant to long-term usage caused by very low consumption (Sreejith, 2008).
Typically, an MQL system supplies 0.3–0.5 ml/min of a MWF (Metal working fluids), with pressurized air or other supplemental gases, where as a conventional system supplies about several thousand ml/min of MWF. The conventional flood supply system demands more resources for operation, maintenance, and disposal, and results in higher environmental and health problems. MQL machining has many advantages in this regard (Weinert et al., 2004). In MQL, mist and dangerous vapours are reduced and the mixture setting is very easy to control (Attanasio et al., 2006).
Typical oil-based lubricants used in MQL do not have the heat capacity of water-based coolants. Accordingly, the most important function provided by the lubricants in MQL is to reduce the friction between tool and work material (Park et al., 2010). So, a good combination between oil base and additives is the key to improve this process.
The reduction of tool wear, surface roughness and improving dimensional accuracy was successfully conducted using clean machining processes with minimum quantity lubricant (MQL). MQL shows superior performance compared to dry and wet turning. However, the usage of MQL in conjunction with a nano-lubrication system would be a noteworthy advantage to the manufacturing process due to its effect on product quality. Nowadays, several nano-lubricants have been identified by the advancement in modern technology, making it possible to sustain and provide lubricity over a wide range of temperatures (Sayuti et al., 2014b).
Among various techniques to reduce full usage of MWF (Metal working fluids), nanofluid MQL was such an environment-friendly one, which has been recently introduced. Nano technology research has primarily focused on man-made particles, but naturally occurring nano-particles have been present for millions of years (Stanley, 2014).
Nanofluid is a fluid containing nano-particles such as carbon Nano-tube (CNT), C60, TiO2, Al2O3, MoS2, diamond and so forth. In a nanofluid MQL, the nanofluid is supplied to the machining area as a form of mist mixed with highly pressurized compressed air through a nozzle. Nano-particles are an excellent media to increase the thermal conductivity of the base fluid. Besides, nano-particles enhance tribological and wear characteristics significantly (Nam et al., 2011). Since the discovery of these materials in the form of nano-particles, especially fullerene like nano-particles and nano-tubes they have been found to be suitable for adding to lubrication fluids (Kogovsek et al., 2013). A physical analysis of nano-lubricant showed that nano-particles can easily penetrate into the rubbing surfaces and have considerable elasto hydrodynamic lubrication effect (Rahmati et al., 2014). Researchers discovered that the frictional coefficient of nanofluid is lower than that of pure oil and confirmed that the extreme pressure of nanofluid is two times higher than that of pure oil. Hence, it can be concluded that nanofluid improved lubrication performance by increasing viscosity and preventing contact between the metal surfaces (Sayuti et al., 2014a). In this regard, nano-particles suspended in lubricant make an attractive and innovative solution effect (Rahmati et al., 2014). As reported by Sayuti et al. (2014a) and Zhang et al. (2011) handling the nanoparticle itself without mixing with oil is dangerous to human's health due to nanoparticle would become as airborne nanoparticle in the air. However, by mixing the nanoparticle with lubricant oil, the nanoparticle is hardly to become as airborne nanoparticle in the air.
From the above literature review it may be noted that MQL with vegetable based cutting fluids (VBCFs) have good lubrication, environmentally friendly, renewable, less toxic and possess high biodegradability as compared to straight fluids, synthetic fluids, soluble fluids. Hence, the present study investigate the effect of nanofluid minimum quantity lubrication (NFMQL) compare with the pure MQL, dry and flooded/wet condition in terms of cutting forces, surface roughness and tool wear.
Section snippets
Workpiece material and cutting tool
The work-piece used in the drilling experiments was Aluminium (6063) with the size of 25 mm thick aluminium plate. A plate held in a vice mounted on top of a dynamometer, which measured the thrust force and torque data after every 15 holes. The chemical composition of Aluminium (6063) workpiece measured by Spectro Metal Analyzer in weight% is as follows: 0.047C, 0.450 Mg, 0.547Si, 0.307Fe, 0.012Zn, 0.007 Pb, 0.008Sn, 0.018Ti, 0.004Cr and the balance Al.
The machining tests were conducted using
Drilling thrust force and torque
The initial step of the experimental methodology consisted of measuring thrust force and torque coming into action during drilling of aluminium 6063 under the flooded, MQL, nanofluid MQL and dry conditions. The drilling thrust forces and torques during drilling experiments is measured by the force dynamometer. Three measurements are taken for each case and the average is plotted to obtain the graphs. The measured drilling thrust forces (in N) and torques (in Nm) during the whole drilling
Conclusions
This research describes the lubricating mechanisms of advanced nano-lubricants, consisting of aluminium oxide nano-particles (20 nm), in MQL drilling with vegetable oil on aluminium 6063. MQL is a technique that could reduce many problems coming from high consumptions of lubricant, like high machining costs or environmental effects and worker health problems. So, it is important to know all advantages and limits of this technique.
On the basis of results obtained after the drilling of aluminium
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