Elsevier

Tribology International

Volume 151, November 2020, 106508
Tribology International

Experimental investigation on the performance of MQL drilling of AISI 321 stainless steel using nano-graphene enhanced vegetable-oil-based cutting fluid

https://doi.org/10.1016/j.triboint.2020.106508Get rights and content

Highlights

  • Nano-graphene was mixed into vegetable-oil based cutting fluid in MQL drilling.

  • The performance of different lubrication strategies on the drilling characteristics was evaluated.

  • The sufficient amount of nano-graphene can improve cooling/lubrication performances.

  • This investigation can reduce environmental-impact and improve drilling efficiency.

Abstract

The present work aims to examine the drilling performance of AISI 321 stainless steel using vegetable-oil-based cutting fluid under different minimum quantity lubrication (MQL) strategies (with and without graphene nanoparticles). Experimental results indicated that MQL drilling with 1.5 wt% of graphene nanoparticles considerably reduced the thrust force (27.4%), torque (64.9%), surface roughness (33.8%) and coefficient of friction (51.7%) at the 30th hole as compared to pure MQL condition and also improved the tool life. In brief, the sufficient quantity of graphene nanoparticles in nanofluid MQL drilling can boost the lubrication performance and increase the stability of lubrication film, which improves the drilling characteristics.

Introduction

The main aim of the industries involved in machining operations is to improve quality and productivity at a low machining cost. To achieve this goal, machining has to be performed at the highest speed without influencing the tool-life. The machining performance is affected by various process parameters [1]. Metal working-fluids (MWFs) have been widely utilised and perform a significant role in cutting operations. MWFs improve the production rate, tool-life and surface quality of the product and also reduce the temperature at tool-workpiece interface. The use of cutting fluid helps in achieving higher machining speed and feed rates [2].

A machining fluid can be described as a matter which is used to a tool during a machining process to enable chip transport easier from the cutting zone. Initially, the machining fluids were obtained from simple oils and were applied for lubricating the cutting tools with the help of brushes. Nowadays, according to the demand of the machining operations, the preparation of the cutting fluids became more difficult. Today's metalworking fluids are generally classified into five categories, i.e. straight fluids, synthetic, semi-synthetic fluids, soluble fluids and vegetable-oil based cutting fluids (VBCFs) [3]. The main functions of MWFs in machining operations are to provide lubrication effect and to make it easier for the transportation of chips from the cutting zone. Additionally, they may perform secondary functions, such as providing limited safety against oxidation and corrosion [4]. The use of MWFs also improves the tribological properties of the machining process, which occur at the tool-workpiece interface [5,6]. Machining fluids act as a lubricant to the cutting interface by fluid film formation, and the effectiveness of fluids depends on their capability to an impingement in the machining zone and forms a thin lubrication film in short machining time. This lubrication film is produced either by a chemical process or physical absorption and must protect the tool-workpiece interface. Cutting fluid reduces the friction generated between the tool-workpiece interface, and therefore the cutting temperature also decreases [7]. Generally, 20% of the total manufacturing cost increases with the use of machining fluid only [8]. Use of traditional machining fluid (flood lubricant) is blameworthy for problems regarding ecological problems, health-related issues to operators, water pollution, etc. [9]. At present, investigators are striving to attain eco-friendly and economical ways of manufacturing [10].

Owing to issues related to the environment and workers’ health, demand for green manufacturing is increasing in the manufacturing industry. Green manufacturing leads to improved working conditions for the operator, along with savings in the purchase of cutting fluids and their disposal [11]. To address the issues related with conventional cooling techniques, the researchers have explored the alternate means of machining such as dry cutting, machining with a coated tool, MQL and cryogenic cooling techniques, etc. [12]. Currently, the MQL technique has reflected its considerable improvements over dry and flood drilling [13].

As reported by Malkin and Guo [14], MQL is an environment friendly process, in which a small amount of cutting oil and gas with certain pressure is targeted to the cutting area as coolant and lubricant.

For better performance of the MQL process, it must be ensured that the oil mist should sufficiently cover the contact surfaces, for example, tool-chip and tool-work interface. MQL parameters such as droplet size, the velocity characteristics of the spray and nozzle distance are to be considered for better efficiency of MQL system. Further thermo-chemical stability of the lubricant and wet-ability on the tool surface can also influence cutting performance of the MQL technique [15]. As reported by Liao and Lin [16] protective oxide layer is formed at the tool-chip interface due to extra oxygen provided by the MQL process. This layer acts as a diffusion barrier, which helps in retaining the strength and wear resistance of the cutting tool, leading to improved tool-life.

MQL not just decreases the utilization of MWFs but also offers better cutting performance due to its superior cooling characteristics. Besides, many researchers have developed highly efficient new cutting fluids like biodegradable oils or mineral oils free from chlorine for MQL applications. This new generation of cutting fluids not only reduces the environmental hazards of traditional lubricants but also increases the machining efficiency [17]. In this technique, a mist of air and coolant provides both cooling and lubrication function along with diminishing the adhesion tendency of the materials by minimizing the friction in the machining zone. Compressed air ensures the cooling function and oil provides the necessary lubrication [18]. A lubricant must have high heat transfer capability but lower thermal conductivity [19]. In this regard, nanofluid MQL (NFMQL) technology is an attractive and innovative solution to enhance the heat transfer characteristics of base oil [9].

Based on the heat transfer improvement theory [20], researchers believe that adding a nano-sized (>100 nm) metallic or non-metallic or nanofiber particles in MQL base oil effectively improves its thermal conductivity and reduces the cutting temperature [21]. Barczak et al. [22] also discussed that the new nanofluid MQL technique is increasingly resolving heat exchange capability in the cutting zone, thereby improving the machining efficiency during the operation. So, a great combination of oil base and nano-additives is the way to improve this operation.

Zhang et al. [20] investigated the effect of MoS2 nanoparticles with different VBCFs in MQL grinding. Results revealed that the application of palm oil containing MoS2 nanoparticles showed the excellent lubricating property in the MQL strategy in comparison to other vegetable based oils, including rapeseed oil, soybean oil and liquid paraffin. This is due to the fact that the carboxyl groups in palm oil have high saturated fatty acid and high film forming property. Li et al. [23] investigated the performance of MQL with graphene-mixed VBCFs in the milling of TC4 alloy and reported that the milling characteristics were improved by the selection of optimal MQL parameters. This is because MQL appropriate parameters can boost the lubrication-cooling capabilities of the lubrication film. In addition, Sridharan and Malkin [24] studied the effect of NFMQL grinding in terms of machining characteristics using MoS2 and CNT nanoparticles. They detected that nanofluids could efficiently decrease the specific grinding energy and also enhance the surface quality of the machined workpiece. Li et al. [25] examined the effect of various nanofluids (MoS2, SiO2, nano-diamond (ND), ZrO2, CNTs (carbon nano-tubes) and Al2O3) in MQL grinding of Ni-alloy GH4169. It was reported that spherical nature nanoparticles provided better lubrication performances in comparison with water-soluble liquid and pure palm oil. This is mainly because the spherical nanoparticles are easily settled down into the gap between abrasive-grains on grinding wheel and produced antifriction effect between wheel and workpiece. They also reported that the lubricating property of the various nanofluids in the following order: Al2O3 > SiO2 > MoS2 > ND > CNTs > ZrO2. Furthermore, Jia et al. [26] also reported that MoS2 nanoparticles achieved the better lubricating performance, followed by diamond nanoparticles.

Moreover [[27], [28], [29], [30]], researchers have examined the machining characteristics of different nanoparticles mixed in cutting fluids with an aim to improve the machining performance. However, the role of graphene nanoparticles in the vegetable-based oil is not explored much, especially in the drilling operation.

Graphene is the latest two-dimensional material which is synthesized from synthetic graphite powder [31]. Graphene has a same layered structure as graphite, the multi-layer of graphene has surpassed the others because of its high thermal transmission and outstanding cooling effect in cutting fluids [32]. The highest thermal conductivity of graphene nanoparticles is up to 5500 W/m-K [33]. Thus graphene is considered as a heat transfer inter-mediator, which can be used as MWFs in cutting of hard to machine materials. Graphene nanoparticles mixed in VBCFs are a good substitute solution due to its good biodegradability, lubrication performance and less manufacture cost [34].

It has been established from the literature that vegetable-based oil with MQL results in better performance with regard to the machining characteristics when compared to dry and flood conditions [35]. The sunflower oil with MQL method is beneficial not only because it is eco-friendly and cost-effectiveness but is also technically superior [25]. Therefore, in order to develop new machining conditions with improved cooling and lubrication functions with the use of nano-additives combined with the MQL technique considering environmental and economic facets, the goal of this research is to compare the performance of lubrication strategies viz. pure MQL and NFMQL with respect to drilling characteristics namely drilling forces (thrust force and torque), surface roughness, coefficient of friction (COF) and drill wear mechanism in drilling of AISI 321 stainless steel. For nanofluid condition, three weight concentrations (0.5 wt%, 1.0 wt% and 1.5 wt%) of graphene were selected.

Section snippets

Experimental details

The drilling of AISI 321 stainless steel was carried out to evaluate the machining characteristics including thrust force and torque, surface roughness, COF between tool-workpiece interface and drill wear mechanism under the aforementioned MQL strategies.

Variation in drilling forces

The variation in thrust force (N) and torque (Nm) values during drilling of AISI 321 stainless steel (321 SS) was measured under different cooling environments (pure MQL and NFMQL with 0.5 wt%, 1.0 wt% and 1.5 wt% nanoparticles concentrations). In Fig. 3, Fig. 4, the average drilling forces are shown.

According to the obtained results, the highest values of thrust force (1475 N) and torque (25.4 Nm) were observed during pure MQL drilling when compared to NFMQL strategies. Compared with pure MQL,

Conclusions

In this research work, the drilling characteristics of the AISI 321 stainless steel using HSS drill tools in the four lubrication strategies (pure MQL, NFMQL (0.5 wt%), NFMQL (1.0 wt%) and NFMQL (1.5 wt%)) are evaluated experimentally. Particularly, the effect of graphene nanoparticles on the drilling forces, surface roughness, coefficient of friction and drill wear mechanism has been examined. The main conclusions which are obtained from the results of the present research are summarized as

CRediT authorship contribution statement

Amrit Pal: Writing - original draft. Sukhpal Singh Chatha: Writing - review & editing, Formal analysis. Hazoor Singh Sidhu: Writing - review & editing, Formal analysis.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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