Environmental sustainability during machining of hardened steel using nanofluid: A case study

. The positive effects of nanofluid-assisted minimum quantity lubrication include improved performance during machining and environmental sustainability. In the present study two cutting parameters levels such as feed rate of 0.05 (mm/rev)- depth of cut of 0.1(mm)-cutting speed of 80(m/min) and feed rate of 0.15(mm/rev)-depth of cut of 0.3(mm)-cutting speed of 200(m/min) has been used in the hard turning of D2 steel. It was determined that the 0.3% ZrO 2 wt% observed to be improved machinability as compared to dry and Minimum Quantity Lubrication (MQL) environment. Finally, the sustainability assessment through the Pugh Matrix Assessment (PMA) presented the potential of nanofluid-MQL for improvement of machinability of hardened D2 steel for cleaner production.


Introduction
Sustainable machining processes have yielded significant results for all materials, particularly at higher cutting speeds, but complexity reduction and structuring for continuous improvement are concerns that must be addressed in order to steer the manufacturing process towards perfection. Sustainable machining demands tool efficiency and low energy consumption for improved machining performance and quality in modern manufacturing system [1]. Modern organisations prefer inexpensive, sustainable, and ecologically responsible machining technologies for high-quality manufacturing. Depending on the type of machining, the heat produced by the tool and work piece varies. The coolant flow approach is less cost-effective and environmentally friendly. Nano fluids may be improved machining fluids and heat conductors [2]. The interest in process sustainability has boosted study in sustainable manufacturing engineering. Long-term viability requires quality improvement and manufacturing process optimisation [3]. The researchers suggested that the overall energy need of environmentally friendly turning process cutting activities be estimated and computed. [4]. Metal cutting now makes use of nanotechnology to improve cooling efficiency. Traditional coolants with nanoparticles improve heat transmission [5]. Because of the complex-dynamic performance of the hard turning process and its close association with multiple variables, high output is required both technologically and economically. High-speed machining of hard part materials is now possible because to new tool materials, surface coatings, and cooling systems. [6]. Metal working fluid is used extensively in milling, drilling, stamping, turning, and other metal cutting processes. Excessive usage of metal working fluid, on the other hand, causes occupational dangers, environmental contamination, and greater machining costs. Minimum quantity lubrication is a revolutionary way for overcoming the shortcomings of metal working fluids. Traditional cutting fluids are improved by nanofluids for low-volume lubrication. Base oils with nanometer-sized particles suspended in them improve heat conductivity and lubrication. [7]. Dry machining is prohibited due to the strict machining techniques. The reasons include increased heat, tool-workpiece interaction, and chip removal. Sustainability is also driving new cutting fluid formulation [8]. Because nanofluids improve cooling efficiency, metal and alloy cutting is perfect. Rolling nanoparticles reduce frictional wear and workpiece-tool interactions. Nanofluids extend the life of machining machines. To improve workpiece quality, Nano MQL eliminates chips. [9]. Sustainable production is a globally acknowledged environmental requirement. MQL and MQL based on nanofluids are two modern, ecologically friendly lubrication systems. Machinists have attempted to turn AISI D2 steels for difficult items. Because of its high material removal rate, low cost, and flexibility, single-point turning has largely replaced cylindrical grinding [10]. Nanofluid extends tool life and reduces wear. Nanofluid coats the workpiece, reducing cutting tool wear. Nanofluid may replace coolant in machining. [11]. Machining uses a lot of energy; therefore sustainability research reduces waste, power, and environmental impact. Nanofluids improve lubrication, wettability, and heat transfer, extending tool life and reducing workpiece hardening. [2]. Effect of alumina nanofluid concentration on minimal lubricant quantity for sustainable hard machining has been illustrated by researchers [13].

Literature Review
Researchers [14] studied how machining conditions affect EN-24 steel turning performance. MQL, Al2O3 nanofluids, CuO nanofluids, and Al-CuO hybrid nanofluids were investigated under dry, wet, and cryogenic cooling conditions. Al-CuO hybrid nanofluids with MQL outperformed other turning cutting conditions. Energy generation, as a result of consumer demand, is a significant contribution to climate change and carbon dioxide emissions. As a result, reducing energy usage is an essential component of the sustainable manufacturing process [15]. Product quality, energy and resource efficiency, environmental preservation, waste reduction, and worker health and safety are all improved by sustainable production [16]. Das et al. [17] claimed that while turning hardened AISI 4340 steel, employing nano fluid as a metal working fluid can accomplish the required machining properties within a certain range of cutting variables. Herein, the focus of research is to find out the best possible sustainable environment between dry, MQL and nFMQL during hard part turning of AISI D2 steel. The evaluated parameters are tool flank wear, surface roughness, cutting temperature and cutting power. In addition to investigating the aforesaid technological factors, environmental aspects of a manufacturing process such as environmental impact, operator health, lubricant price, and recycling are evaluated. The PMA was utilised in this work to evaluate sustainability in dry, MQL, and nFMQL cutting circumstances.

Experimental Details
Hardened D2 steel with diameter 45 mm and length 150 mm is utilized as the work material. The machining experiments are performed with a CVD (Al2O3-TiCN) multilayer coated carbide insert, manufactured by WIDIA. Furthermore the tool holder with ISO designation PCLNR 2525M12 is utilized. Hard turning operations are conducted under three different environment such as dry, MQL and nanofluid MQL (nFMQL) condition in two different cutting level parameters. Inaddition, Each trial is repeated three times with a new cutting tool tip and the average value has taken for the further analysis. In this current experiment, a ZrO2 nano particle-based cutting fluid is used, and LRT 30 (Mineral oil) is used as the base oil as well as a cutting fluid in MQL conditions. The experimental cutting conditions and the experimental scheme is demonstrated in Table 1 and Fig. 1 respectively.

Results and Discussion
Dry cutting, or machining without the use of cutting fluids, can have beneficial and detrimental effects. On the plus side, it helps save money, is safer for the environment, and does away with the need for cutting fluids. However, dry cutting can cause more tool wear, a lower quality surface finish because of increased friction, and a shorter tool life due to higher temperatures. When using MQL, a small amount of lubricant is applied to the cutting area. The cooling effects help dissipate heat, and it reduces tool wear and improves surface finish. However, MQL has restrictions on the amount of lubrication it can provide, which could slow down chip evacuation and necessitate careful optimisation for challenging hard turning tasks. Incorporating nanoparticles into the lubricant, nanofluid MQL is a variant of MQL. The added lubrication and cooling are secondary goals. Although research into the precise impacts of nanofluid MQL on hard turning is still ongoing, advantages could include further decreased tool wear, increased surface quality, and enhanced cooling compared to conventional MQL.

Tool wear
Tool wear influences other machining outputs, such as tool life, cutting temperature, and the quality of the machined surface, so it is essential to inspect it thoroughly. In the present study, the effect of two cutting parameters and three cutting environments such as dry , MQL and nFMQL conditions on wear was examined. Fig. 2a Fig. 3 depicts flank wear photographs at two distinct cutting levels and three different cutting environments. There are three apparent wear mechanisms such as abrasion, adhesion and chipping are observed in all environments. Particularly at a high cutting speed of 200 m/min, regular deterioration was more apparent. Due to high friction and temperature generated at the cutting zone results in rapid tool wear.

Surface Roughness
The application of lubrication in the form of MQL or nanofluid MQL tends to improve surface roughness by lowering friction and promoting lubrication, whereas dry cutting often produces higher surface roughness. With the addition of nanoparticles, nanofluid MQL may provide additional advantages such as improved lubrication and cooling effects to produce even smoother surface finishes. Fig. 2b demonstrate a comparison graph of surface roughness in two different cutting level parameters under three environments such as dry, MQL and NFMQL. According to the result ZrO2 nanofluid with concentration of 0.3% Wt. produces superior surface finish compared to MQL and dry environments in both cutting level parameters. According to increase in feed rate surface roughness goes high in all environments. Due to high heat tool coating degrades rapidly which leads to rapid tool wear, results in degradation of surface of the workpiece.

Cutting Temperature
A critical factor that affects tool life and dimensional accuracy is the cutting temperature. The temperature rises anytime the cutting speed rises as a result of the increased friction. A comparison graph of the cutting power outcome in relation to changes in cutting environments at two different cutting parameter levels is shown in Fig. 2c. Cutting temperature was found to be greater with higher speed and feed rates combined with dry environment. When compared to other environments such as dry and MQL, 0.3% wt. ZrO2 nanofluid showed better results in terms of cutting temperature, with temperature variations between Level 1 and Level 2 ranging from 73.3 °C to 125.3 °C. Highest temperature was observed at dry environment 247.3 °C at high speed, feed and depth of cut conditions. Figure 5 shows thermal images taken by an Infra-Red thermal camera at various concentrations and cutting levels.

Cutting Power
The use of lubrication in the form of MQL or nanofluid MQL tends to minimise cutting power by providing lubrication and reducing friction, whereas dry cutting often requires more cutting power due to increased friction. The cutting power of nanofluid MQL may be much lower than that of conventional MQL. Nanoparticles can be added to lubricants to improve lubrication, reduced friction, lessen cutting forces, and ultimately need less cutting power. According to the results, MQL consumes less power than dry hard turning. However, because ZrO2 nanofluid has strong lubricious properties, it reduces friction at the cutting zone, resulting in the lowest cutting power usage. As the speed and depth of cut rise,

ICSTCE 2023
https://doi.org/10.1051/e3sconf /202340504042 E3S Web of Conferences 405, 04042 (2023) correspondingly rises in the cutting power. The lowest cutting power was obtained in a nanofluid MQL environment at Level 1 cutting parameters with low speed, feed, and depth of cut parameters. Cutting power consumption in level 2 is not substantially high, which is recommended for high speed turning of hardened AISI D2 steel in a ZrO2 nanofluid cutting environment. Fig. 2d demonstates the cutting power usage graph in both cutting parameter (Level 1 and Level 2) under dry , MQL and nFMQL cutting environmrnts.

Sustainability assessment
In hard turning, sustainability refers to the technique of reducing environmental impact and fostering resource efficiency. Utilising recycling and waste minimization practises, implementing cutting parameter optimisation to lower energy consumption, choosing ecofriendly cutting tool materials, and using cutting fluids and lubricants in controlled quantities (like MQL or nanofluid MQL) are all strategies for sustainable hard turning. Sustainable hard turning seeks to strike a balance between output and quality while cutting back on resource use, emissions, and waste production, making a positive difference in the manufacturing process's environmental impact.

Pugh Matrix Approach
The Pugh Matrix Approach (PMA) was utilised to measure the sustainability of several cutting conditions in hard turning of AISI D2 steel, such as dry, MQL, and nFMQL. The weightage between −2 to +2 is allocated to the defined factors for the comparison analysis. [18,19,20]. In this context, some weightages are assigned to parameters for various cutting conditions ranging from -2 to 2 for each better or inferior outcome. Similarly, for worse and better outcomes, this might be assigned a value ranging from 1 to + 1 In this study, disparate parameters such as environmental impact, safety of machine operator, Lubricant recycling and disposal, Part cleaning, coolant cost, Output responses (Surface quality, Tool wear, Cutting temperature), Noise level, were selected for sustainability evaluation between Dry, MQL, nFMQL machining conditions. Table 2 presents the comparison between selected cutting situations. In terms of environmental impact, dry machining gets a "+2," MQL obtains a "+1," and nFMQL get a "1." Dry machining is a "2" weighted operation for operator safety. MQL and NFMQL have a "+2" weighting due to effective cooling. The research findings show nFMQL "+2" and MQL "+1" for surface quality and tool wear. Based on the results of the experiments, the weightage of dry machining is "2". The Kiviat radar plot is presented in Fig. 5.  It can be summarized from Kiviat radar plot that cutting with nFMQL cutting environment is economically viable and socio-technologically beneficial considering sustainability.

Conclusion
The 0.3% ZrO2 wt% i.e. nano fluid Minimum Quantity Lubrication (nFMQL) cutting environment was shown to have improved machinability when compared to dry and MQL cutting environments. The nFMQL system exceeded both dry and MQL conditions in terms of sustainability. In comparison to dry and MQL conditions, nFMQL delivers superior cooling and lubrication methods, reducing friction between the workpiece and the cutting tool, resulting in a better surface finish and lower power consumption. ZrO2 nanoparticles improve the thermal conductivity of the base oil, which helps to reduce cutting zone temperature and tool flank wear when compared to a dry and solely MQL method. Overall, it is suggested to machinists and the industry to use nanofluid as a cooling environment, due to the aforementioned benefits of not only enhancing machinability performance but also for a green and sustainable cleaner production of hardened steels during turning operations.