Investigation of hard turning performance of eco-friendly cooling strategies: Cryogenic cooling and nanofluid based MQL

Investigation of hard turning performance of eco-friendly cooling strategies: Cryogenic cooling and nanofluid based MQL

Tribology International 144 (2020) 106127 Contents lists available at ScienceDirect Tribology International journal homepage: http://www.elsevier.co...

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Tribology International 144 (2020) 106127

Contents lists available at ScienceDirect

Tribology International journal homepage: http://www.elsevier.com/locate/triboint

Investigation of hard turning performance of eco-friendly cooling strategies: Cryogenic cooling and nanofluid based MQL �rı Vakkas Yıldırım Çag Department of Airframes and Powerplants, Erciyes University, Kayseri, Turkey

A R T I C L E I N F O

A B S T R A C T

Keywords: Nanofluid Cryogenic cooling Graphene Liquid nitrogen

In hard turning, the chip removal process is very difficult. One of the preferred auxiliary methods to reduce cost and improve quality is the use of cutting fluid. However, the use of conventional cutting oil has several disad­ vantages, such as polluting the environment, damaging employee health, and increasing production costs. Therefore, alternative methods are very important. In this study, nanoadditive-based cutting fluid and cryogenic cooling by liquid nitrogen, are compared in terms of machining performance in hard turning processes. Hardened AISI 420 was selected as the experiment material. 0.5 vol% graphene nanoplatelet (GnP) was added to the cutting oil to obtain nanofluid. The experiment variables were listed as three cutting speeds, three feed rates and two cooling conditions. Tool-chip interface temperature, surface roughness/topography, tool life, tool wear characterization and chip morphology were used to evaluate the experimental results. The results showed that cryogenic cooling was better in tool-chip interface temperature, tool life, tool wear and chip morphology, whereas nanofluid was better in average surface roughness and surface topography.

1. Introduction Cutting fluids are helpful factors that perform critical tasks such as cooling, lubrication, and removing the chip from the cutting zone and significantly increase machining efficiency due to this feature. Because of these positive properties, preferred cutting fluids also bring some disadvantages with themselves. So much so that the environmental concern over cutting fluids is increasing due to the mixing of cutting fluid into the soil, fluid drag in workpiece and chips, and the use of hazardous substances and mist [1]. Because of all these concerns, many governments are making decisions to ensure that the cutting fluids used comply with the ISO 14000 Standard [2]. Some countries have put the Ecomark label into effect and moved the environmental legislation to a specific point. European Ecolabel or the German “Blue Angel” can give as examples of these regulations [3]. In addition to the damage they cause to the environment, conventional cutting fluids cause serious concerns for the health of the worker. Because it contains chemicals such as secondary amines, sodium nitrate, phenols, chlorinated paraffin, boric compounds, polycyclic aromatic hydrocarbons (PAHs) and biocide products, shows that these concerns are justified [4]. Boric compounds (like boric acid), which are involved in cutting fluid to perform tasks such as corrosion protection, pH buffer capacity and bacterial growth inhibition, can cause reproductive problems. Moreover, nitrosamine

which occur as a result of the reaction of nitrite such as diatenolamin with secondary amines and also many other chemicals in cutting fluids have carcinogenic effects and threaten working health [5–7]. For these reasons, ECHA (European Chemicals Agency) is listed in class of haz­ ardous chemicals. As a result, it is possible to both reduce the amount of cutting fluid and improve machining efficiency by using different sus­ tainable methods instead of conventional cutting fluid. This situation was examined in many studies and positive results were reported. For example, Yıldırım et al. [8] in their study, they compared dry, conven­ tional cutting fluid and the minimum quantity lubrication (MQL) system and claimed that better machining performance was achieved with the reduction of cutting fluid. In another study, Mia et al. [9] compared dry, conventional cooling, MQL and solid lubricant & compressed air when turning hardened steel. At the end of the research, the authors stated the MQL system was environmentally friendly, made cleaner production and improved machining efficiency In other studies, studies such as conventional cutting fluid, high pressure cutting fluid, etc. were compared with MQL, nMQL and cryogenic cooling and they stated that cutting fluid reduction is more accurate for sustainable environmentally friendly manufacturing [10–12]. While studies are underway on many alternatives to reduce cutting fluid, nanofluid and cryogenic cooling are two of them. The term of nanofluid is the name given to a type of coolant and

E-mail address: [email protected] https://doi.org/10.1016/j.triboint.2019.106127 Received 18 October 2019; Received in revised form 29 November 2019; Accepted 16 December 2019 Available online 17 December 2019 0301-679X/© 2019 Elsevier Ltd. All rights reserved.

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lubrication obtained by adding nano-sized solid particles into the cutting oils in recent years. According to another definition, a nanofluid is a type of coolant and lubrication produced by dispersing metallic or nonmetallic nanoparticles of less than 100 nm or nanofibers in a base liquid [13]. Nanofluids have higher heat transfer coefficient and specific surface area compared to pure cutting oils. Therefore, they improve the heat efficiency in the cutting zone and machining efficiency. Further­ more, the solid lubricants contained in the nanofluid help to improve the oil film thickness between the cutting tool and the workpiece. Thus, they increase the lubricating performance and tribological properties of the coolant [14]. Nanofluids are applied using the MQL system. By nozzle and compressed air, cutting fluid is moved to the cutting area in this process. In many literature studies, it is seen that with the addition of nano-sized solid particles, the efficiency of the cutting fluid is improved. For example; In turning AISI 420 martensitic stainless steel with tung­ sten carbide, Uysal et al. [15] compared nanofluid with dry and pure MQL. The researchers used molybdenum disulfide (MoS2) as nano ad­ ditive. The results showed that in terms of tool wear and surface roughness, nanofluid was better. In another study, S¸irin and Kıvak [16] compared dry and pure MQL with different proportions of hexagonal boron nitride (hBN), MoS2 and graphite doped nanofluids during machining of Inconel X-750 superalloy. As parameters for machin­ ability, researchers have tool life, wear, cutting temperature, cutting force and surface roughness. According to experimental results, nano­ fluid was better than dry and pure MQL. They also claimed that nano­ fluid containing 0.5 vol% hBN in the highest efficiency nonfluid. Darshan et al. [17] in their study, they investigated the effects of dry cutting and MoS2-doped nano cutting fluid on machinability as well as cutting parameters during machining of Inconel 718. The researchers who examined the results of the experiment said that nano cutting fluid gave better results in critical subjects such as tool wear, cutting tem­ perature, surface roughness, chip morphology and cutting force. In some other studies, it was said that nano cutting fluid gave better results than dry and MQL system [18–24]. Nano additives that are usually included in the vegetable-based cutting fluid can also be added to deionized water, mineral ester or other pure cutting fluid [25]. As additive, generally, carbon nanotubes (CNT), zinc peroxide (ZnO2), hexagonal boron nitride (hBN), graphite, copper-oxide (CuO), aluminum oxide (Al2O3), slicon-oxide, molybdenum disulfide (MoS2), diamond etc. many substances like can be used [13]. One of these additives is gra­ phene. Graphene, when added to cutting oils, especially at high cutting temperatures, increases the lubricants by increasing the stability of the cutting fluid and allowing it to hold on to the surface. Moreover, it helps to increase tool life by reducing notch formation in the cutting tool. Thus, cutting temperature helps that it is improved cutting force and surface quality. Park et al. [26] in their study, they investigated the effects of dry, conventional cooling and both pure MQL and graphene doped nano cutting fluid on tool wear. The results of the experiment claimed that graphene-based nano fluid provided improvement over other methods. In another study, Samuel et al. [27] compared pure MQL, multiwalled carbon nanotubes (MWCNT) based and single-walled carbon nanotubes (SWCNT) cutting fluids with graphene based nano fluid with different concentration ratios (0.1, 0.2 and 0.5% by weight). Since the study’s main focus was on investigating the efficacy of cutting liquids, parameters such as cutting tool material, feed rate, cutting speed and cutting depth were kept constant. The results of the experiment showed that 0.5 wt % graphene-doped nano liquid gave better results. Singh et al. [28] when turning AISI 304 steel, they examined the effect of dry, pure MQL, Al2O3-based nanofluid and Al2O3-GnP-based hybrid nanofluid in terms of surface roughness, feed force, thrust force and cutting force. The researchers also studied the thermal conductivity and viscosity effect of the nano additive. The results showed that nano ad­ ditive gave better results than dry and pure MQL. Furthermore, the re­ searchers claimed that the properties of nano cutting fluid improved much more with the addition of GnP to the Al2O3-based additive. Failure to control the high heat generated during chip removal

significantly reduces the machining efficiency. One of the preferred cooling methods for heat control is cryogenic cooling. For more than a century, the science and technology generally referred to as cryogenics to produce low-temperature environments. Although there is a lot of inconsistent information about the term cryogenic cooling in the liter­ ature, the research and standards generally mention temperatures below 150 � C (123 K; 238 � F) [29]. In general, cryogenic cooling is defined as the sending of a low temperature refrigerant gas in liquid form, through the nozzle, pressurized to the tool-workpiece interface. Today, as coolant, liquid carbon dioxide (CO2) and liquid nitrogen (LN2) are mostly used. Although CO2 is used first in machining operations, it is the most preferred LN2. The biggest reason for this condition is that there is too much in the air (79%), non-toxic, non-hazardous, colorless, non flammable and odorless. It is also lighter than air and spreads into the air after application. In this way, it reduces maintenance, post-machining cleaning and disposal requirements [29]. Cryogenic cooling provides satisfactory improvements in key issues such as reducing cutting tem­ perature, increasing tool life, improving surface quality, and reducing tool wear. In addition, cryogenic cooling is always regarded as green production technology and is one of the leading cooling methods used for sustainable manufacturing [30]. Several literature studies also concentrate on the impact of cryogenic cooling on production perfor­ mance and sustainable manufacturing. For instance, Aramcharoen and Chuan [31], machined Inconel 718 and enquired the effectiveness of cooling strategies on machinability depending on machining time. Re­ searchers who chose dry, conventional and LN2 as cooling methods claimed that cryogenic cooling gave the best results in cutting temper­ ature, cutting force, chip formation, surface quality and tool wear. Danish et al. [32] compared dry machining and cryogenic cooling dur­ ing turning of the AZ31C magnesium alloy. The results showed that cryogenic cooling, when compared to dry machining, resulted in a 35% decrease in chip surface and 29% temperature reduction in tool surface. In another study, Mia and others [33] examined the effect of dry machining with mono-jet LN2 and dual-jet LN2 and cutting parameters on machining efficiency during Turning of Ti-6AL-4V alloy. The re­ searchers who analyzed the results of the experiment claimed that dual-jet LN2 gave better results in all output parameters. In their study, Leadebal and others [34] investigated the effect of dry machining and cryogenic cooling on tool wear and chip morphology during hard turning of AISI D6 steel. The researchers examined only the effect of the cooling method by keeping parameters such as cutting speed, progress and cutting depth constant during the study. According to the results of the experiment, LN2 which is sent from rank and flank faces at the same time gave better results than other cooling methods. When the literature is examined, it is seen that both cooling methods have some advantages and disadvantages. However, which one will increase production efficiency more is still an important research topic for today’s researchers. Therefore, the researches that compare with each other still continue today. In their study, Chetan and others [35] compared cryogenic cooling, cryogenic treatment and nanoMQL when turning the Nimonic 90 alloy. Researchers who chose 0.5 vol% Al2O3 as the nano-additive opted LN2 for cryogenic cooling. The researchers who analyzed the results of the experiment claimed that cryogenic cooling technique was more effective than other techniques. In another study, Jamil and others [36] compared cryogenic cooling with nanofluid dur­ ing turning of TI–6AL–4V alloy. Researchers who selected a mixture of alumina (Al2O3) and multi-walled carbon nanotubes (MWCNTs) as nano additives were preferred for cryogenic cooling. The test results showed that nanofluid compared to cryogenic cooling, while the average surface roughness decreased by 8.72% and cutting force decreased by 11.8%, tool life increased by 23%. In contrast, cryogenic cooling at the cutting temperature gave better results and reduced 11.2% compared to nano cutting fluid. As can be seen from the literature researches, although there are many studies on cryogenic cooling and nanofluid, the studies that are compared to each other is much less. The aim of this study was to 2

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Fig. 1. Experimental flow chart.

compare the graphene doped nano coolant (0.5 vol%) with cryogenic cooling. However, the investigation of the effect of cooling methods on different cutting parameters is one of the main focus of this study. Therefore, three different cutting speeds and three different pro­ gressions were included in the experimental design. Experimental pa­ rameters, experimental setup and analysis results of the study are given in sections following.

2. Methodology 2.1. Preparation of workpiece material Hard turning is a commonly used turning method for finishing op­ erations of hard materials. Because they are easily hardened by quenching and tempering, stainless steel alloys such as AISI 420 and especially martensitic stainless steels are the material groups that hard turning is very common. Martensitic stainless steels are materials with high corrosion resistance, which are designed to meet the needs of the industry, which are respond to heat hardening [37]. Martensitic 3

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were used as the machining parameters for the turning tests, the cutting depth was kept constant (1 mm). Factors and levels of the machining parameters are given in Table 1.

Table 1 Machining parameters and their levels for hard turning. Parameters

Notation

Level 1

Level 2

Level 3

Units

Cooling Conditions Cutting speed Feed rate

CC Vc f

nanoMQL 75 0.05

LN2 100 0.10

– 125 0.15

– m/min mm/rev

2.3. Cooling/lubrication methods In this study, cryogenic cooling and nanofluid were used as cooling and lubrication method. The process of preparation of these methods is detailed below. Preliminary tests, literature and manufacturer’s rec­ ommendations were taken into consideration during the preparation phase. Graphene nanoplatelet (purity: 99.9þ%, length: 5 nm, specific surface area: 170 m2/g and diameter: 30 μm) which is given in SEM and TEM photographs of Fig. 2 was preferred as nano additive. The nano additive was added to the Fuchs PlantoCut 10 SR (viscosity: 10 mm2/s at 40 � C, density: 860 kg/m3 at 15 � C, flash point: 205 � C) ester-based cutting oil. SKF brand and LubriLean-Vario model MQL system were used to transmit nano coolant to the cutting zone. 80 ml/h flow rate, 8 bar air pressure, 25 mm nozzle distance, 2 mm nozzle diameter and 30� nozzle angle were selected as MQL parameters. Preliminary experiments and similar studies in the literature were effective during the selection of MQL parameters [38,39]. Graphene nanoplatelet was added into the cutting fluid at 0.5 vol% and a homogeneous mixture was obtained [19]. A two-step mixing process was applied to obtain homogeneous mixture. In the first step, solid nanographene particles added to the cutting oil were mixed with DAIHAN HS-100D mechanical stirrer for 60 min at 750 rpm. In the second step, TERMAL brand N11150 M model magnetic stirrer was mixed with 1500 rpm for 60 min and homogeneous distri­ bution was obtained (see Fig. 3). In cryogenic cooling trials, liquid nitrogen ( 196 � C) was used. Liquid nitrogen was processed using a 133 kb liquid nitrogen tank (XL45 HP) self-pressurized Taylor Watson. During the transmission of liquid

stainless steels are materials with high corrosion resistance, which are designed to meet the needs of the industry, which are respond to heat hardening [37]. The AISI 420 steel which will be used during Hard turning experiments was first kept in Schmetz brand vacuum furnace at 1030 � C for 480 min and then prepared by tempering for 120 min at 170 � C. Thus, the hardness of the material was increased from 178 HV to 624 HV. Metkon brand and Duroline-M model microhardness tester was used to measure the hardness value of the material before and after hard­ ening. The AISI 420 martensitic stainless-steel used during the experi­ ments was prepared with a diameter of 70 mm and a length of 300 mm. 2.2. Hard turning tests During hard turning experiments, ACCUWAY JT-150-8 CNC (Power: 7.5 kW; made in Taiwan) lathe which has a maximum speed of 4000 rpm was used. In the turning experiments, M grade CVD Ti(C,N)þAl2O3þTiN coated carbide inserts, manufactured by Sandvik Coromant (2020 quality and ISO code SNMG 120404-MF) were used. Cutting insert properties; inscribed circle: 12.7 mm, insert thickness: 4.763 mm, nose radius: 0.397 mm, relief angle: 0� . Also, PSBNR 2525 M12 coded tool holder (manufactured by Sandvik Coromant) was used as the tool holder. The experimental setup is given in Fig. 1. While three different cutting speeds, three different feeds and two different cooling conditions

Fig. 2. The SEM and TEM images of graphene nanoplatelet.

Fig. 3. Preparation of nanofluid. 4

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Fig. 4. Effect of machining parameters on tool-chip interface temperature.

Fig. 5. Effect of cooling conditions and cutting velocity on tool life.

nitrogen to the cutting zone, heat losses during transmission were minimized by using a 4-m vacuum stainless steel tube. So that the cooling conditions are equal, as in the MQL system, a 2 mm diameter nozzle was sprayed to the cutting zone at a distance of 25 mm and an angle of 30 � C 15 bars were used as spray pressure.

Optris PI 450 infrared camera, capable of thermographic imaging and real-time measurement, was used to measure the tool-chip interface temperature in the cutting zone at the time of chip removal. Infrared thermography provides reliable recordings with spatial and temporal resolutions that are much more accurate than thermocouples commonly used in machining processes [41]. The emissivity value is one of the thermal camera’s most critical setting parameters. The emissivity value was set to Ɛ ¼ 0.40 according to the literature and company catalog and calibrated in the measured temperature range [42]. Interpretation of the temperature values obtained by infrared camera was done through Optris PIX Connect software. The measurement range of the infrared camera was selected at 0–250 � C. Preliminary experiments were con­ ducted for the placement of the camera and the selection of parameters and were decided according to the results of the experiments. Dino-Lite (AM 4113 ZT) brand digital microscope with 1.3�-megapixel resolution and 200� optical magnification was used to measure tool wear. Maximum flank wear (Vbmax) value of 0.3 mm is determined as tool life criterion. To Measure tool life/wear, the experiment was stopped for certain periods and the measurement values were controlled, illustrated

2.4. Measurements For the measure of the surface roughness results obtained from the tests, portable Mahr (Marsurf PS 10) brand instrument was used. Roughness measurements were made according to ISO 4287 [40]. Average of surface roughness values (Ra) were used as measurement criteria. During the measurement, measurements were made from four different points by randomly turning the workpiece in the direction of machining and the arithmetic average of these values was accepted as the result of the measurement. The measurement was done immediately so that oxidation does not occur on the machined surface and does not affect the results. In addition, the Phase View optical profilometer was used to obtain the 3D surface topography of the machined surfaces. The 5

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Fig. 6. SEM image and EDS spectrum of the insert for Vc ¼ 75 m/min and LN2.

and recorded by using DinoCapture 2.0 software. In all tool life exper­ iments, this process was repeated with certain periods and it was assumed that the cutting tool completed its life when flank wear was found to have reached 0.3 mm. Each experiment was conducted with a previously unused insert, and the insert was retained at the end of the experiment. After the experiments were completed, ZEISS brand Gem­ iniSEM 500 model scanning electron microscope equipped with energy dispersive X-ray spectrometer (EDS) was used to examine the wear characteristic in the inserts.

interface temperature is quite important. In this part of the study, an experimental examination of the temperature at the tool-chip interface was conducted according to cutting speed, progress rate and cooling conditions and the results were given in Fig. 4. The effect of machining parameters on the tool-chip interface tem­ perature was shown in Fig. 4. When the figure is examined, it is seen that cryogenic cooling provides better cooling than nanofluid. Such that cryogenic cooling gave better result in all cutting parameters. According to the average of the cutting parameters, cryogenic cooling achieved a 31.05% improvement in tool-chip interface temperature compared to nanofluid. This situation was associated with the cooling attribute of liquid nitrogen. The effective cooling provided by liquid nitrogen offered a more positive effect on the tool-chip and workpiece-tool interface, and led to that cryogenic cooling gave better results. The liquid nitrogen also absorbs heat, evaporates rapidly and creates a fluid gas shield between the chip and tool’s head, which serves as a lubricant [44]. Another reason why cryogenic cooling give better results is related to the use of a very small amount of coolant during nanofluid use. As a result of the use of a very small amount of cutting oil, which is the general principle of the MQL system, cutting oil evaporates in a much shorter time and loses its effectiveness [18]. Some studies in the litera­ ture, cryogenic cooling has also been reported to reduce the reduction temperature between 10% and 35% depending on cutting parameters, tool geometry and experimental material [45–47]. The results of this study were similar to those in the literature. When examined in terms of cutting speed and cooling condition in Fig. 4, it is observed that although cryogenic cooling results better at low and medium cutting speeds, the actual improvement occurs at high cutting speed (125 m/min). This situation has been associated with the emergence of more heat at high

3. Results and discussion This study’s main purpose is to equate nanofluid based on graphene with cryogenic cooling. Tool-chip interface temperature, tool life, wear characterization, surface roughness, surface topography, and chip morphology were selected as comparison criteria. During the compari­ son, by using three cutting speeds and three feed rates, the behavior of cooling methods under different cutting parameters was examined. Thus, a more comprehensive evaluation was possible. A total of 18 ex­ periments were conducted during the experimental study. 3.1. Tool-chip interface temperature Tool-chip interface temperature is an important quality indicator that directly affects tool life and dimensional accuracy. Moreover, the surface integrity of the workpiece is also directly related to the tool-chip interface temperature [43]. In other words, tool-chip interface temper­ ature is an important parameter that directly affects the quality level of machining efficiency. Therefore, accurate analysis of the tool-chip 6

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Fig. 7. SEM image and EDS spectrum of the insert for Vc ¼ 75 m/min and nanofluid.

cutting speeds and the use of cryogenic cooling more effectively. In­ crease of the cutting speed generally means increase of the friction and hence increase of the tool-chip interface temperature. An effective cooling/lubrication may cause the tool-chip interface temperature to be controlled [48]. When examined terms of feed rate, it is observed that the tool-chip interface temperature increases with increase of feed rate (Fig. 4). This situation especially is observed much more clearly in ex­ periments performed at 125 m/min cutting speed. Increase of progress increases friction and hence tool-chip interface temperature [47]. As a result, the lowest tool-chip interface temperature was obtained with 0.05 mm/rev feed rate, 75 m/min cutting speed and cryogenic cooling parameters.

possibility that the chip will be welded to the cutting tool. In addition, easier chip evacuation from the cutting zone and prevention of built-up-edge formation by cryogenic cooling are also reasons that in­ crease tool life [50]. Bordin and others [51], stated that cryogenic cooling delays various wear types in the turning process with carbide tools and helps increase tool life. In Fig. 5, it is one of the clear situations that the efficiency level of cooling methods varies depending on the cutting speed. For example, cryogenic cooling compared to nanofluid at 75 m/min cutting speed gave 28.66% more tool life while this ratio was 209.43% at 100 m/min cutting speed and 852.68% at 125 m/min cut­ ting speed. As can be seen clearly from the ratios, the efficiency of cryogenic cooling increased with the increase of cutting speed. The reason for this situation is the increase of temperature in cutting zone along with the increase of the cutting speed, and hence cryogenic cooling is thought to perform the cooling task more effectively. The product used for the workpiece is stainless, with poor thermal conduc­ tivity relative to plain carbon steels. As a result, at high cutting speeds, more heat is produced and the temperature in the cutting area rises to very high levels. This situation was also proved by the results of the experiment in which the tool-chip interface temperature was measured (section 3.1). Therefore, the heat generated from the cutting zone is difficult to expel and affects the workability in a negative direction. In such an environment, the application of the cryogenic cooler may generate more heat from the machining zone [52]. There are very few studies compared cryogenic cooling and nanofluid in the literature. However, these studies give different results. For example; Chetan and others [35] conducted a similar study and obtained parallel results with this study. However, in another study Jamil and others [36] claimed that nanofluid in gave better results in surface roughness, shear strength and

3.2. Tool life The effect of machining parameters on the life of the tool is examined in this part of study. However, the number of experiments was reduced due to constraints such as time and cost during tool life and wear ex­ periments. Accordingly, in the tool life/wear tests, the cutting depth and feed value was kept constant (0.1 mm/rev). Thus, 6 experiments were conducted in which three different cutting speeds and two different cooling conditions participated. Flank wear was selected as the team life evaluation criterion. According to ISO 3685, the amount of time the flange wear value reaches 0.3 mm was accepted as the effective life of the cutting tool [49]. The results of the experimental study were pre­ sented in Fig. 5. When Fig. 5 is examined, it is seen that cryogenic cooling method provides longer tool life than nanofluid. This situation has been associated with the high cooling capability of cryogenic cool­ ing. Thanks to its cooling capability, cryogenic cooling reduces the 7

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Fig. 8. SEM image and EDS spectrum of the insert for Vc ¼ 100 m/min and LN2.

tool life, while said that cryogenic cooling at tool-chip interface tem­ perature was more efficient.

methods in reducing heat. However, in this study, during the turning experiments performed with nanofluid, it was observed that the chip was always formed in a type of chip (Fig. 15). Therefore, it is thought that the aerosol has difficulty reaching the cutting zone. On the contrary, in cryogenic cooling, the chips exhibited a t broken chip type and the cooling was more effective. The fact that cryogenic cooling results more effectively in tool wear can be attributed to this difference. However, in order to evaluate the effect of the cooling method more accurately, it is necessary to examine its effect on wear at equal cutting speeds. For example; At a cutting speed of 75 m/min, while abrasion, attrition and chipping were seen as the dominant type of wear under cryogenic cooling, edge fracture emerged under nanofluid (Figs. 6–7). In other words, under the nanofluid, fracture in the cutting insert occurred. Although notch wear and attrition were observed under cryogenic cooling, they did not cause a fracture in the cutting insert. Therefore, it can be said that cryogenic cooling is effective in issue of protection the cutting inserts even at low cutting speeds. In the literature are also in quality which support this situation [48]. Furthermore, the tool life experiment results in section 3.2 are in quality which confirm this sit­ uation. Although the effect difference between cryogenic cooling and nanofluid reduced at low temperatures, the superiority of cryogenic cooling was clearly observed. Another type of wear seen in both cooling methods is also adhesion. Adherence level is clearly seen in EDX analysis results. This situation can be associated with be low of lubricant prop­ erties of cryogenic cooling. This situation is more obvious, especially at low temperatures. However, as can be seen in Fig. 7, in experiments performed with nanofluid, fracture of the cutting insert occurred and this situation prevented a healthy adhesion analysis. When the SEM photos of the experiments with cutting speed of 100 m/min are

3.3. Wear characterization Tool wear directly affects all quality values that have a significant impact on machinability, such as tool life, spent power, cutting force, cutting temperature and surface quality. Hence, evaluating tool wear in depth and making wear recommendations is very relevant. In this study, the effect of three cutting speeds and three cooling conditions on wear was examined. Parameters such as progress (0.1 mm/rev), cutting depth (1 mm), cutting tool material and nose radius were kept constant in order to better analyze the effect of cooling conditions on experimental results. There are several studies in the literature in which progress and cutting depth under cryogenic cooling and nanofluid are analyzed [40, 47,53]. SEM photographs showing wear characterizations under different cutting speeds and cooling conditions are given in Figs. 6–11 In Wear experiments, the cutting tool mostly failed due to maximum flank wear, notch and nose wear. Adhesion, attribution and abrasion are the wear mechanisms seen. As can be seen clearly from the SEM photos, cryogenic cooling at all cutting speeds helped create a more regular wear compared to nanofluid. It was more pronounced, especially at a high cutting speed of 125 m/min. This situation may be attributed to the effect of cryogenic cooling on the tool-chip interface temperature. The size or progression of wear is related to the increase of the temperature and friction at the tool-workpiece interface [54,55]. In the machining of hard materials, high heat generation is an expected situation and the tool life directly affects the machinability criteria such as cutting force [56–59]. Both cryogenic cooling and nanofluid are effective cooling 8

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Fig. 9. SEM image and EDS spectrum of the insert for Vc ¼ 100 m/min and nanofluid.

examined, it is observed that a more stable maximum flank wear is observed under cryogenic cooling while it is observed that a deeper wear is occurred in the experiments with nanofluid (Figs. 8–9). Although edge fracture also occurred at the cutting insert in both cooling conditions, it was observed that this value is higher in nanofluid. During the wear experiments, cryogenic cooling followed a more balanced maximum flank wear profile while the experiment with nanofluid experienced a very fast wear and 0.3 mm wear value was reached in a very short time. This situation is clearly seen in section 3.2. Thus, in the tool life, cryo­ genic cooling showed a better performance by 209.43% ratio. In the experiments performed at a cutting speed of 125 m/min, cryogenic cooling caused in a much clearer improvement. The tool life results are also quality that confirm this situation. Especially at high cutting speeds, cryogenic cooling has led to a considerable improvement in tool life values. This is because it performs better the cooling task at high cutting speeds. As a result, have yielded better result than cryogenic cooling nanographene doped cutting oil in tool life and tool wear. This improvement became much more a pronounced, especially at high cutting speeds.

parameters on surface roughness was examined. Test parameters and measurement results are presented in Fig. 12. When Fig. 12 is examined in terms of cooling conditions, it is observed that nanofluid gives better results than cryogenic cooling. For example, when the feed rate of 0.05 mm/rev was examined by getting reference, at 75 m/min cutting speeds, nanofluid provided improvement 60.27% compared to cryogenic cooling. This ratio is at 100 m/min and 125 m/min cutting speeds, 50.2% and 68.43% respectively. When the feed rate of 0.1 mm/rev was examined by getting reference, at 75 m/min cutting speeds, nanofluid provided improvement 20.78% compared to cryogenic cooling. This ratio is at 100 m/min and 125 m/min cutting speeds, 47.38% and 14.66% respectively. When the feed rate of 0.15 mm/rev was examined by getting reference, at 75 m/min cutting speeds, nanofluid provided improvement at 2.18% compared to cryogenic cooling. This ratio is at 100 m/min and 125 m/min cutting speeds, 14.36% and 9.77% respectively. The fact that nano-doped cutting oil gives better results than cryogenic cooling was attributed to the lubri­ cating quality of the cutting oil. This situation became more apparent through solid lubricants. Even at high temperatures, the normal lubri­ cation properties of solid lubricants are attributable to the layered structure of solid lubricants which act as lubricants. In other words, cutting oils have better wettability than cryogenic cooling and therefore form a film layer that reduces friction [36]. Moreover, nano additives coated with cutting oil in nanofluids containing graphene nanoplatelets atomize a thin layer of fog along the nozzle. Thus, it enters the tool-workpiece interface more easily and significantly increase tribo­ logical properties by forming a thin film layer in that region [28]. Another situation clearly seen in Fig. 12 is that there is a dramatic in­ crease in surface roughness with an increase in feed rate. It is known

3.4. Surface roughness/analysis A surface quality is one of the most critical performance measure­ ment variables in metal removal operations. The best parameter for measuring surface quality is surface roughness. Surface roughness should be measured to assess the performance of the machined com­ ponents and to determine if the desired requirements are met under the specified cutting and cooling conditions [48]. In this part of the study, a series of experiments were conducted and the effect of machining 9

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Fig. 10. SEM image and EDS spectrum of the insert for Vc ¼ 125 m/min and LN2.

situation that the most effective cutting parameter on surface roughness is progress and that surface roughness increases sharply with progress [60]. This situation is thought as a roughness caused by signs of progress. A more anisotropic surface appears in the hard-turning process. A symmetrical and regular pattern of peaks and valleys characterizes the anisotropic turned layer. Surface topography study is very important because it directly affects component structures such as friction, wear, fatigue and sealing behavior [61]. In this part of the Study, 6 experi­ ments were conducted by using three cutting speeds and two cooling methods and the effect of these parameters on surface topography was examined. Since it is known that the most effective parameter on surface quality is progress [47], it has been kept constant and its effect has been ignored. 2D pictures of surfaces obtained under different machining parameters were given in Fig. 13 while 3D surface topography graphics were given in Fig. 14. Both figures are quality that confirm the results given in section 3.4. If compared in the progress value of 0.1 mm/rev, in the experiments with nanofluid, the surface roughness was high at 75 m/min cutting speed and a decrease in surface roughness was observed with increasing cutting speed to 100 m/min (Figs. 12 and 14). This situation has been associated with a more comfortable cutting with in­ crease of heat in the cutting zone. However, with the cutting speed of increase to 125 m/min, the surface quality deteriorated again. It is thought that the cutting tool is enter the process of wear and therefore cannot cut properly. As seen in Figs. 13 and 14, the surfaces were more irregular in experiments with cryogenic cooling. This situation is a clear example of the relationship of surface quality to lubrication. Especially cryogenic cooling is a cooling method that stands out with its cooling feature and has low lubrication ability. Therefore, the surfaces obtained

by cryogenic cooling were more irregular, rougher and lower quality than nanofluid. Lubrication significantly reduces residue, exaggerated feed lines and crack formation on the treated surface [62]. In experi­ ments with nanofluid, peak-to-valley height was in a situation more regular, while the surfaces obtained by result of cryogenic cooling became a situation in which more complex, irregular, and surface in­ juries were observed. In the cryogenic process, in the experiment per­ formed with only a cutting speed of 125 m/min, a more uniform peak-to-valley structure was observed than a cutting speed of 15 m/min and 100 m/min. This situation has been associated with very high tool-chip interface temperature. It is thought that effects of cryogenic cooling and the temperature generated by high cutting speed balance the cutting environment and provide appropriate cutting condition. 3.5. Chip morphology In chip removal operations, the form of the produced chip gives important clues about the level of machining efficiency [63]. In Fig. 15, the chip morphology of AISI 420 stainless steel under different cutting speeds and cooling conditions during hard turning is given compara­ tively. In the first column, chip pictures showing the size and curves of the chips are given, while in the second column SEM photographs showing the lamellated structure of the chip curl regions are given. In the third column, SEM images of back surfaces are given. In both cryogenic cooling and nanofluid, it is seen that the chip form de­ teriorates as the cutting speed increases. While continuous cutting ten­ dency was observed at low cutting speeds, especially in experiments with nanofluid, both color and form of chip became undesirable with increase of cutting speed (Fig. 15c). Cutting speed is an important factor 10

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Fig. 11. SEM image and EDS spectrum of the insert for Vc ¼ 125 m/min and nanofluid.

Fig. 12. Effect of machining parameters on surface roughness.

in the machining of hardened steels and directly affects chip morphology [34]. Especially in experiments performed with nanofluid, increase of cutting speed severely affected chip morphology. This situ­ ation has been associated with the immediate evaporation of nanofluids

at high cutting speeds. Thus, nonfluids are unable to perform the lubricating/cooling task and the cutting tool is worn. In their study, Chetan et al. [35] found a similar result and claimed that nano fluids at high cutting speeds were not effective on chip morphology. 11

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Fig. 13. 2D surface pictures at f ¼ 0.1 mm/rev.

Furthermore, in experiments performed with nanofluid, micro-adhesions were observed on the chip surface with increase of cutting speed (Fig. 15b and c). These also confirm that the nano-doped cutting fluid evaporates at high temperatures and is unable to perform the cooling/lubrication task. However, in cryogenic cooling experi­ ments, chip surfaces are smoother. Although the chip notch tooth rate increases with the increase of cutting speed in cryogenic cooling, it is seen to be much more efficient compared to nanofluid (Fig. 15f). This situation has been associated with the cooling ability of the cryogenic cooling. Cryogenic cooling, which provides effective cooling especially at high speeds, delayed the wear of the cutting tool and helped to make a more effective cutting. Section 3.2. and section 3.3 clearly shows the effect of cryogenic cooling on tool life/wear. This situation is similar to previous studies. For example, in their study, Bermingham and others [64], they claimed that cryogenic coolant is effective on chip morphology and tool-chip contact length. The researchers also said that

cryogenic cooling affects the primary cutting band angle. In another study, Ross and Manimaran [65] claimed that better chip was broken due to cryogenic cooling and smaller notches was formed in chips. In addition, both cooling systems operate by different parameters due to their nature. For example; while nanofluid was sprayed with 8 bar, LN2 was sprayed with 15 bar. The use of high pressure in the Cryogenic system may have had somewhat positive effect on the easier break of the chip. 4. Conclusions During hard turning of hardened AISI 420, the effects of cryogenic cooling and nanofluid on different machinability outputs were investi­ gated in this research. The results of the study are given following;

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Fig. 14. 3D surface topography at f ¼ 0.1 mm/rev.

� While the quality of the machined surface nanofluid gave better re­ sults, cryogenic cooling gave better results in tool-chip interface temperature, tool life, chip morphology and tool wear. Although the thermal conductivity of the nanofluid is high, the excellent cooling property of liquid nitrogen has been the main reason for it being more effective. Especially at high cutting speed which very high temperatures are observed, the effect of cryogenic cooling was further increased.

� Compared to nanofluid, cryogenic cooling significantly reduced the temperature of the chip-tool interface. At a cutting speed of 75 m/ min, the tool-chip interface temperature decreased by 29.65% (for 0.0.5 mm/rev feed rate), 27.53% (for 0.1 mm/rev feed rate) and 26.63% (for 0.15 mm/rev feed rate. This was 27.18%, 24.57% and 27.66% respectively for the cutting speed of 100 m/min while the cutting speed of 125 m/min was 27.10%, 28.36% and 31.72 respectively.

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Fig. 15. Chip morphologies at 0.1 mm/rev feed rate; A)75 m/min, nanofluid, B)100 m/min, nanofluid, C) 125 m/min, nanofluid, D) 75 m/min, LN2, E) 100 m/min, LN2, F) 125 m/min, LN2.

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� In surface roughness, nanofluid gave better results than cryogenic cooling. Cutting surface roughness of 75 m/min improved by 60.27% (for 0.0.5 mm/rev feed rate), 20.78% (for 0.1 mm/rev feed rate) and 2.19% (for 0.15 mm/rev feed rate). This was 50.20%, 47.38% and 14.36% respectively for the cutting speed of 100 m/min while the cutting speed of 125 m/min was 68.43%, 14.66% and 9.77% respectively. � Cryogenic cooling provided higher tool life than nanofluid. Cryo­ genic cooling at a cutting speed of 75 m/min gave 28.66% more tool life, while this rate was 209.43% at a cutting speed of 100 m/min and 852.68% at a cutting speed of 125 m/min. � In Wear experiments, the cutting tool mostly failed due to cheek wear, notch and nose wear. Adhesion, attribution and abrasion are the wear mechanisms observed. At all cutting speeds, cryogenic cooling performed more regular wear compared to nanofluid. Espe­ cially welding and adhesion of chip such as BUL and BUE was seen less under cryogenic cooling. � Chip morphology deteriorated dramatically with increase of cutting speed. Given the effect of cooling on chip morphology, cryogenic cooling provided a better chip structure. In the experiments per­ formed with nanofluid, it was observed that at low cutting speeds, the chip engulfed the workpiece and the cutting tool and decreased the machining efficiency while at high cutting speeds the size and color of the chip deteriorated significantly. � In future studies, different machining parameters, nano additives with different ratio and structure, different cooling gases can be compared. Thus, literature knowledge can be enriched in the name of environmentally friendly machining.

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