MoS2 solid lubricant application in turning of AISI D6 hardened steel with PCBN tools

MoS2 solid lubricant application in turning of AISI D6 hardened steel with PCBN tools

Journal of Manufacturing Processes 47 (2019) 337–346 Contents lists available at ScienceDirect Journal of Manufacturing Processes journal homepage: ...

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Journal of Manufacturing Processes 47 (2019) 337–346

Contents lists available at ScienceDirect

Journal of Manufacturing Processes journal homepage: www.elsevier.com/locate/manpro

MoS2 solid lubricant application in turning of AISI D6 hardened steel with PCBN tools

T

Mauro Paipa Suareza, Armando Marquesb, Denis Boingc, Fred Lacerda Amorimd,⁎, Álisson Rocha Machadoa,d a

Universidade Federal de Uberlândia – UFU, Faculty of Mechanical Engineering – FEMEC, Av. João Naves de Ávila, 2121, Uberlândia, MG, 38.400 902, Brazil Federal Institute of Espírito Santo, Av. Rio Branco, 50 - Santa Lucia, 29056-255, Vitória, ES, Brazil c Brusque University Center, Department of Mechanical Engineering, Technology, Innovation and Manufacturing Center, R. Dorval Luz, 123 – Santa Terezinha, 88352 400, Brusque, SC, Brazil d Pontifícia Universidade Católica do Paraná - PUCPR, Mechanical Engineering Graduate Program - PPGEM, Av. Imaculada Conceição 1155 Prado Velho, 80.218 901, Curitiba, PR, Brazil b

ARTICLE INFO

ABSTRACT

Keywords: Hard turning PCBN MoS2 solid lubricant Minimum quantity of fluid – MQF Tool life

This research investigates the effect of the application of MoS2 mixed with oil in the hard turning of AISI D6 cold work tool steel with PCBN tools. A 33 statistical factorial planning was used to run the tests, variables: cutting speed, feed rate; lubri-cooling conditions: dry cutting, pure oil applied by MQF (minimum quantity of fluid), oil mixed with solid lubricant (MoS2) applied by MQF. The main tool wear mechanisms were abrasion, attrition (or adhesion), and diffusion. The EDS of the PCBN confirmed the presence of MoS2, indicating that solid lubricant particles took place in the lubrication process at the cutting region. The results suggest that use of solid lubricant in hard turning may be a viable alternative to tackling the challenge of machining difficult-to-cut materials.

1. Introduction For over thirty years, the hard turning process has been proved as a feasible method for manufacturing quenched and tempering steel alloys (hardness above 45 HRC) in finishing operations. The turning of hardened steels is typically carried out using PCBN or oxide ceramic as the tool material. The process flexibility, material removal rate, set up time and environmental compatibility are the hard turning benefits when it is compared to the grinding process [1–3]. Furthermore, the high precision hard turning tends to achieve ISO tolerance IT2 when combined with roughness Rz below 1 μm [4]. The hard turning prerequisites involve high stiffness and dynamic stability of the machining system [5,6]. During the hard turning process, as a result of the chip formation mechanism, the austenitization temperature of the steel is easily achieved at the chip-tool interface [3,5,7]. The common practice of the hard turning is to machine without the application of coolant or lubricant fluids in order to keep the cutting region warm enough to maintain the strength of the work material in a level that facilitates material shearing [5], which is one of the greatest benefits of the hard turning based on the environmental compatibility of the production line. Furthermore, when using oxide ceramics, the

application of a cutting fluid (i.e., wet/flooded cooling) may lead to early tool fracture due to the low thermal conductivity and fracture toughness of them [8]. On the other hand, in case of low volume of lubricant fluid application (i.e., minimum quantity of fluid – MQF, also known as MQL – minimum quantity of lubricant and NDM – Near dry machining), the friction force on the clearance surface and rake surface can be reduced, diminishing the heat generated and the temperature in the chip-tool interface in appropriate levels, increasing the tool performance [6,9,10]. Although not usual in an industrial environment, some successful application of cutting fluids, including the cryogenic cooling, in specific hard turning cases have been reported [11,12]. Near Dry Machining (NDM) or Minimum Quantity of Fluid (MQF) is placed between dry machining and machining with flood cooling, and it uses a spray of the mixture of compressed air and droplets of oil. When the MQF is compared to dry and flood cooling, the literature has shown positive results in machining regarding the surface roughness, cutting forces, cutting temperature and tool wear [13]. The hard turning process of AISI 4340 (55 HRC) was experimented by Chinchanikar e Choudhury [14] using cemented carbide tools with several nanocoatings (AlTiN, multi-layer TiAlN/TiSiN, and AlTiCrN) under dry and MQF technique. Besides pointing AlTiCrN as the best coating, the results

Corresponding author. E-mail addresses: [email protected] (M.P. Suarez), [email protected] (A. Marques), [email protected] (D. Boing), [email protected] (F.L. Amorim), [email protected] (&.R. Machado). ⁎

https://doi.org/10.1016/j.jmapro.2019.10.001 Received 29 January 2019; Received in revised form 26 August 2019; Accepted 1 October 2019 1526-6125/ © 2019 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.

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Table 1 Experimental results according to the cutting condition tested. f [mm/rev]

vc [m/min]

Lubri-cooling condition

Tool life (average) [min]

VMR (average) [cm3]

Fu (average) [N]

Surface roughness Ra (average) [μm]

0.15 0.10 0.05 0.15 0.10 0.05 0.15 0.10 0.05 0.15 0.10 0.05 0.15 0.10 0.05 0.15 0.10 0.05 0.15 0.10 0.05 0.15 0.10 0.05 0.15 0.10 0.05

300 300 300 225 225 225 150 150 150 300 300 300 225 225 225 150 150 150 300 300 300 225 225 225 150 150 150

Dry Dry Dry Dry Dry Dry Dry Dry Dry MQF MQF MQF MQF MQF MQF MQF MQF MQF MQF + MoS2 MQF + MoS2 MQF + MoS2 MQF + MoS2 MQF + MoS2 MQF + MoS2 MQF + MoS2 MQF + MoS2 MQF + MoS2

0.5 0.9 1.4 2.0 1.2 3.8 7.5 12.8 13.2 0.5 2.3 2.7 3.3 4.0 5.1 8.4 12.0 17.0 0.9 2.4 3.9 1.3 4.4 7.9 7.9 11.9 18.0

17.8 21.5 16.7 54.3 22.2 34.6 134.8 153.3 78.8 19.7 54 32.5 88.8 72.5 45.4 151.5 143.6 102 33.1 58 47.2 34.2 78.4 71.1 142.2 142.6 108

241 198 138 241 194 140 254 192 138 217 179 138 239 193 142 277 218 153 242 193 138 243 197 143 258 198 139

0.51 0.30 2.07 0.54 0.28 0.34 0.48 0.27 0.33 0.60 0.61 0.25 0.59 0.55 0.33 0.49 0.31 0.21 0.45 0.49 0.34 0.47 0.38 0.21 0.38 0.30 0.19

of AISI 304 stainless steel with the MQF technique. The nanofluid mixed with graphene nanoplatelets have presented an improved performance considering machining force and surface roughness as compared to alumina base and base fluid application. Addition of micron size particles of the graphite and molybdenum disulfide in the oil to increase the lubricity of the cutting fluid was tested using MQF technique by Marques et al. [18] in turning the superalloy - Inconel 718 (annealed condition - 214 HV30) with cemented carbide tools. The output parameters considered were the tool life, machining force, and surface integrity. Overall, the results showed better performance for the addition of MoS2 compared to graphite and pure oil applied by MQF, and flood cooling. When the same lubricooling conditions and the cutting tool was used in tests with Inconel 718 in the aged condition (40 HRC) [19], the addition of graphite in the oil improved the tool life. However, the same did not occur with MoS2; it promoted negligible effect on the surface integrity. In the context of the hard turning process, the volume fraction of primary carbides in the machined material microstructure have a strong impact on the tool performance and wear behavior [20]. Poulachon et al. [21] showed that coarse carbides in the steel microstructure promote large grooves on the flank face (flank wear) of the PCBN tools. On the other hand, steels free of primary carbides promote smooth grooves on the flank face. Boing, Schroeter and Oliveira [22] showed that the high volume fraction of carbides in the steel microstructures intensifies the abrasion wear mechanism, which promotes an exponential effect on the tool wear rate and also impacts on the tool wear topography. Furthermore, in the cutting process, the carbides collision against the tool edge, beyond the abrasive wear scratches, promotes instability in the cutting process, which can lead to the cutting edge chipping and damage. In general terms, steel alloys with the high volume fraction of carbides in the microstructure (i.e., abrasion resistance tool steels) promote an extra challenge in the hard turning process, and an improvement in the tribological conditions in the tool-chip and toolworkpiece interfaces can be useful to improve the tool and process performance. Although the existence of many research works attempting to

Fig. 1. Setup and MQF nozzle positions.

showed improved performance when the MQF spray technique was used as compared to dry cutting. To improve further the performance of the MQF technique, many researchers have added lubricants to the oil and experimented with them in hard machining. Kursuncu and Yaras [15] have added borax and boric acid additives in ethylene glycol of base fluid and applied the mixture by MQF in the hard milling of AISI O2 cold work steel. For comparisons, MQF with commercial boron oil and dry milling were also tested. Superior performance was found when both borax and boric acid were added to the oil spray. The best tool life was achieved with the borax, and the best surface roughness with the boric acid and the cutting forces decreased with the addition of both compounds. Sayuti, Sarhan e Salem [16] have added SiO2 nanoparticles to mineral oil and sprayed them during hard turning of AISI 4140 steel (52 HRC). They used Taguchi optimization method to determine the best nano-lubricant concentration, nozzle angle, and air carrier pressure besides proving the effectiveness of the nanofluid in hard machining regarding tool wear and surface quality. Singh et al. [17] tested alumina-based nanofluid mixed with graphene nanoplatelets, which promote a fluid with better thermal and tribological properties, in turning 338

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Fig. 2. PCBN Tool life and volume of material removed from the workpiece.

improve process performance in hard turning, application of a cutting fluid in hard turning is still dubious because even if applied by MQF, they may reduce the cutting temperature to a level that does not favor the shearing process of chip formation. In this process, the lubricant action of the cutting fluid is very important and addition of MoS2 solid lubricant to the oil applied by MQF technique in hard turning with PCBN tools was not found in the literature. In this context, this research aims to discuss the impact of the lubricant environment (dry, MQF and MQF + MoS2) on the tool performance and tool wear mechanisms, in the hard turning process with PCBN tools of the AISI D6 cold work tool steel (high abrasion resistance steel).

DSSNL 2525 M12 – manufactured by Sandvik Coromant®. A 3k statistical factorial planning approach was used in the tests. The input variables were: Cutting speed (vc): 150 m/min; 225 m/min; 300 m/min. Feed rate (f): 0.05 mm/rev; 0.10 mm/rev; 0.15 mm/rev. Lubri-cooling conditions: dry cutting; pure oil applied by MQF; and a mixture of pure oil and solid lubricant (MoS2) also applied by MQF. Twenty seven test condition were experimented following the mentioned factorial planning (Table 1). Each test was replicated twice. Two fluid nozzles were used directed towards to rake surface (a) and clearance surface (b) simultaneously, each providing a flow rate of 30 mL/h (Fig. 1). The applied fluid was the vegetable-based synthetic oil LB2000, produced by ITW Chemical Products Ltd. The added solid lubricant was MoS2 sphere shape particles (average of 6 μm of diameter), which were mixed with the oil, homogenized, and applied in a 20% mass concentration. Previous work in turning of Inconel 718 with cemented carbide tools using MQF [18] was decisive for the choice of this concentration. The constant depth of cut (ap) was set to 0.2 mm. The tool life tests were performed using an end-of-tool-life criterion based on a maximum tool flank wear VBBmax =0.3 mm, or tool breakage, as recommended by the ISO 3685 standard [23]. Tool wear was measured each 100 mm of machined length intervals. Along with the experiments, the flank wear was measured using a stereomicroscope (SC61, Olympus®) equipped with an image acquisition camera Evolution LC and Image Pro Express software. After de experiments, the cutting edges were evaluated by a Scanning Electron Microscopy (SEM), model TM3000 manufactured by Hitachi. A chemical attack was performed on the tools using an HNO3 solution in ultrasound for 5 min

2. Experimental procedures External cylindrical turning tests on quenched and tempered AISI D6 steel (2.1%C + 11.5%Cr + 0.15%V + 0.70% W + Fe – balance) with 60 HRC of hardness were performed. The AISI D6 steel has a high volume fraction of primary carbides in the microstructure (high abrasion resistance cold work tool steel). The workpiece has a diameter of 126 mm and length of 250 mm. The machine tool used in the tests was a CNC ROMI Multiplic 35D lathe, with 11 kW of power and 3000 rpm maximum spindle rotation. The workpiece fixture system consists of clamping chuck with machined jaws (to increase the contact area between the workpiece and the jaws), and a tailstock. The PCBN tools grade CB7015, ceramic binder (TiCN + Al2O3), TiN-coated by PVD process, geometry SNGA 120412 S01030A were used with a tool holder 339

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Fig. 3. Machining Force (Fu) and surface roughness (Ra).

into the piezoelectric platform with an overhang length of 50 mm, following the recommendations of the dynamometer manufacturer. Workpiece roughness surface was measured using a Mitutoyo® roughnesmeter SJ-201, and it was computed as an average of three measurements along the machined surface. The cut-off value used was 0.8 mm.

Table 2 ANOVA of the volume of material removed. Statistical Parameters*

SS

DF

MS

F

p-value

(1) Lubri-cooling condition (2) cutting speed - vc [m/ min] (3) feed rate - f [mm/rev] 1*2 1*3 2*3 Error Total SS

385.99 12013.84

2 2

192.994 6006.922

0.73129 22.76117

0.510885 0.000499

2537.12 626.11 1131.29 2966.67 2111.29 56289.39

2 4 4 4 8 26

1268.558 156.527 282.824 741.669 263.911

4.80677 0.59311 1.07166 2.81030

0.042557 0.677617 0.430557 0.099718

3. Results and discussion 3.1. Tool life, cutting force and surface roughness Table (1) presents the results of all the output parameters investigated in all the twenty-seven cutting condition combinations. The results are also presented graphically in Figs. 2 and 3. The surface response was generated by a cubic interpolation of the points using the software MATLAB. Fig. (2) shows the results for tool life [min] and the volume of material removed from the workpiece [cm³]. The longest tool life achieve with the experiments variables was 18 min, using 150 m/min of cutting speed (vc), 0.05 mm/rev of feed rate (f) and the fluid mixed with solid lubricant (MoS2) applied by MQF, Fig. (2c). The Fig. (2a–c) shows the influence of the cutting conditions (cutting speed and feed rate) on the tool life when applying MQF and MQF + MoS2 compared to dry machining, respectively. The tool life with dry machining was 28.7% shorter compared when MQF applied the pure oil, and 36.4% shorter than the tool using solid lubricant (MQF + MoS2) for the cutting speed of 150 m/min and feed rate of 0.05 mm/rev. The lubricant action on the tool-workpiece and tool-chip interfaces promoted by MQF and MQF + MoS2 conditions improved the tool life.

R-sqr = .96249; Adj:.8781; MS Residual = 263.9109. * SS = square sum; DF = degree of freedom; MS = mean square; F = F-test; p = probability value.

to remove the material adhering to the tool, making possible to observe the wear mechanisms. A piezoelectric Kistler 9265-B dynamometer and a Kistler 5070A amplifier were used to measuring the machining force components (cutting force - Fc; feed force - Ff; and passive force - Fp), which were used to calculate the resultant machining force, Fu. The machining force measurements were conducted with new tools to avoid the influence of tool wear on the results. Measurements were performed in a frequency of 6 kHz, during 20 mm of machining length for each factorial planning combination and average values of the data were calculated. Amplified signals were sent to a PowerDAQ A/D acquisition board (National Instruments USB DAQPad-6251 Pinout, 1.25 MS/s). The LabView 6.0 was used to process the acquired signals. The tool holder was mounted 340

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Fig. 4. PCBN worn cutting edge: (vc) 150 m/min; (f) =0.05 mm/rev; dry machining.

With the increases in cutting speed (vc) from 150 m/min to 300 m/ min, the tool life is drastically dropped, Fig. (2a–c). For the worse scenario, the cutting tool failure occurs in 30 s of machining time (vc =300 m/min; f = 0,15 min; dry machining). In this situation, the tool flank wear achieves 0.3 mm or occurs the catastrophic failure of the cutting tool. Base on the tested variables, considering the tool life results and looking for the feasibility of the industrial application, the recommendation is to apply the cutting speed (vc) =150 m/min, feed rate (f) =0.05 mm/rev with MQF + MoS2 solid lubricant. Results of the machining force and Ra surface roughness parameter are presented in Fig. (3) for the three lubri-cooling condition tested. The increase in the cutting speed (vc) promotes a smooth reduction in the machining force (Fu), regardless the lubri-cooling aplication system Fig. (3a–c), which is related to the reduction of the workpiece hardness around the cutting region because of the high temperature generated [5]. Although the benefits of the higher temperature in the cutting process regarding the material shearing and chip formation, it can accelerate the degradation of the CBN grains by chemical wear in PCBN tools [24,25], dropping the tool lives.

The increase in the feed rate (f) from 0.05 to 0.15 mm/rev, also dropped the cutting tool life, Fig. (2 a–c), which is proportional to the chip thickness and the load generated on the cutting tool, as shown in Fig. (3a–c). Although the impact on the tool life, Fig. (2a–c), mainly with cutting speed (vc) =150 m/min and feed rate (f) =0.05 mm/rev, the lubri-cooling condition (dry machining; MQF; MQF + MoS2) do not have a statistically significant effect on the machining force, Fig. (3a–c). In the machining process, to select the cutting parameters adopted, the cutting tool life needs to be associated with the volume of machined material (cm³), Fig. (2d–e), and the variables regarding the quality control of the process, i.e., the roughness surface, which are shown in Fig. (3d–e) for the three lubri-cooling condition teted. Among the input variables in the experiments, the analysis-of-variance (ANOVA) with 95% of confidence interval identified that the cutting speed (vc) and feed rate (f) (p-value <0.05) has statistically significant effect in the volume of material removed. The lubri-cooling condition was not statistically significant, even in interaction form. The p-value of the lubricooling system was 0.5108, indicating that there is only approximately 50% of chance for the influence on the volume of material removed, 341

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Fig. 5. PCBN worn cutting edge: (vc) 150 m/min; (f) =0.05 mm/rev; pure oil applied by MQF.

Table (2). The volume of material removed (VMR) has the same trend as the tool life, as shown in Fig. (2). Higher values of cutting speed promote smaller volume of material removed in function of the lower tool life. However, for the cutting speed (vc) =150 m/min, the feed rates (f) =0.10 mm/rev and (f) =0.15 mm/rev promoted a greater volume of material removed compared to the feed rate (f) 0.05 mm/rev, Fig. (2). The higher volume of removed material, despite the tool life, represents benefits to the production line considering the number of machined components. The application of the higher values of feed rate (f) in the production line needs to be considered in function of the kinematics of the process, and its interaction with the machined roughness surface. As shown in Fig. (3d–f), the roughness surface is below Ra =0.8 μm, which represents the grade N8 according to ISO 1302:2002 [26]. Regarding the surface roughness evaluation, an exception is in the condition: vc =150 m/min; f =0.05 mm/rev; dry machining. Which promoted roughness surface Ra ∼ 2.0 μm, and it is related to the instability (vibration) along the experiments for the specific condition. Table 1 and Fig. 3 show that in general, the application of a cutting fluid by MQF,

either pure oil or with the addition of MoS2, improved the surface roughness in relation to dry cutting and this is more pronounced in the smaller feed rate of 0.05 mm/rev. Regarding the tool life, the best result was found with the combination: vc =150 m/min; f =0.05 mm/rev; with the lubri-cooling condition MQF + MoS2 (18 min). Considering the volume of material removed (VMR), better results were found with the combination: vc =150 m/min; f = 0.10 and 0.15 mm/rev; independent of the lubricooling condition. The increase in the cutting speed (vc) increases the cutting temperature; furthermore, the increase in the feed rate (f) increase the chip thickness, which leads to the increase in the machining force. Both situations combined with the microstructure of the machined material achieve a region of the wear instability of the PCBN tool, reducing the tool life as shown in Fig. (2). The AISI D6 is a high abrasion resistance tool steel and has a high volume fraction of carbides in the microstructure (similar to AISI D2), which promote an extra challenge to the hard turning process because it intensifies the tool wear rate compared with steels with free of primary carbides in the microstructure [22]. 342

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Fig. 6. PCBN worn cutting edge: (vc) 150 m/min; (f) =0.05 mm/rev; Oil + MoS2 applied by MQF.

The results shown in Figs. (2 and 3) suggest that the tribological conditions imposed by the higher values of cutting parameters applied (cutting speed and feed rate) in the tool-chip and tool-workpiece interfaces suppress the lubricious action of the lubri-cooling condition applied by MQF and MQF + MoS2. It is import to note, based on the variables adopted, regarding the tool life and process feasibility, that only the results from the cutting speed (vc) =150 m/min can be extrapolated to the industrial application. Furthermore, for the process with small chip thickness, the use of lubri-cooling by MQF technique can be a viable alternative for increasing the tool life, either pure or when blended with MoS2.

pattern on the PCBN tools: crater and flank wear. Based on analyses of the worn areas of the tools after machining and characteristic details observed [27,28], it can be concluded that the wear mechanisms involved were abrasion, attrition (or adhesion) and diffusion. When the cutting speed (vc) of 300 m/min was applied, the tool life was below 5 min, and some times as short as 30 s, as in the case of dry cutting and feed rate of 0.15 mm/rev (Fig. 2) which are not suitable for industrial application (costs and process reliability). The better results were achieved applying the cutting speed (vc) 150 m/min, which will be the focus of the tool wear mechanisms discussion. The scanning electron microscopy (SEM) images with details of the worn tools used in dry machining with cutting speed (vc) =150 m/min and feed rate (f) =0.05 mm/rev are shown in Fig. (4). The main wear mechanisms for all tools were abrasion and attrition identified by parallel grooves and the rough aspect of worn surfaces composed of exposed grains, respectively. This latter wear mechanism has been previously reported for this type of cutting tool by Dosbaeva et al. [29]. Other investigations on tool wear mechanisms in machining of steels corroborate with our findings and support the present wear

3.2. Tool wear mechanisms Tool wear mechanism analyses were performed based on SEM images of the worn surfaces of the tools, as done by many authors [27–32], without performing additional tests (such as X ray or other test – e.g. EDS) in the sub-surface of the tools to confirm diffusion, for example. The three lubri-cooling conditions generated the same wear 343

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Fig. 7. (a) General view of the wear on the tool after machining with MQF + MoS2; identification of the elements by BSE (backscattering electrons) analyses: (b) sulphur; (c) titanium; (d) molybdenum. Table 3 Energy dispersive spectroscopy, Area 1 Fig. (7).

Table 6 Energy dispersive spectroscopy, Area 4 Fig. (7).

Element

Weight %

Atomic %

Element

Weight %

Atomic %

Nitrogen Oxygen Sulfur Titanium Chromium Tungsten

27.125 28.078 0.266 39.584 1.656 3.292

42.320 38.352 0.181 18.060 0.696 0.391

Carbon Oxygen Silicon Sulfur Titanium Chromium Manganese Iron Molybdenum

5.128 48.890 5.811 0.032 18.520 9.706 3.317 8.382 0.215

9.538 68.261 4.621 0.022 8.637 4.170 1.349 3.353 0.050

Table 4 Energy dispersive spectroscopy. Area 2 Fig. (7). Element

Weight %

Atomic %

Oxygen Silicon Phosphorus Sulfur Potassium Titanium Chromium Iron Molybdenum

29.333 1.031 2.268 1.153 0.666 3.817 2.849 37.198 19.216

59.259 1.186 2.367 1.162 0.550 2.575 1.771 21.528 6.474

mechanism analyses [30–32]. Abrasion marks of different sizes were observed, Fig. (4), indicating abrasive wear on the tool caused by different phenomena. The largest marks, named “abrasive wear,” oscillated between 25 and 30 μm widths and were probably produced by the microstructure components of the machined material by the intimate contact of the tool clearance surface with the workpiece, as also discussed by Poulachon et al. [21]. The AISI D6 tool steel is a material with high fraction volume of primary carbides (chromium carbides) in the microstructure, high-hardness martensite phases and vanadium, and tungsten carbide precipitates. Micro-chipping of mechanical origin was observed in the worn cutting edge of the tool, detail D in Fig. (4). This may occur at any stage during the development of the tool wear, and it is a function of the mechanical weakening of the cutting edge due to the formation of the crater wear. Based on the analysis of the tool wear morphology in Fig. (4) mainly based on the random distribution of the abrasive marks width, one hypothesis can be elaborated for the abrasive wear largest marks on the flank face of the PCBN tools. At the beginning of the tool life, one micro-chipping (similar to the one in detail D of Fig. (4)) can be generated on the tool edge radius, which promotes a preferential direction to the wear of PCBN, and a located increase in the machining

Table 5 Energy dispersive spectroscopy, Area 3 Fig. (7). Element

Weight %

Atomic %

Carbon Nitrogen Oxygen Titanium Chromium Iron

6.445 8.919 12.504 36.791 2.509 32.833

15.973 18.956 23.267 22.865 1.436 17.502

344

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force, leading to the large abrasive marks as shown in Fig. (4). The smaller marks observed in detail B, located in the flank coating region and denoted in the image as “micro abrasive wear,” was probably produced by hard grains of PCBN lost by attrition which rubs the flank surface, producing abrasion marks ten times smaller than those on the substrate. This process of wear is reported by Machado and Diniz [27] in hard machining and is probably responsible for all the abrasive wear mechanism prevailing in the present investigation. By observing more sharply the details A and C of Fig. (4), the regions close to the already-worn cutting edge have smooth topographies, also indicating the presence of diffusive wear promoted by the high cutting temperature. On the other hand, if one travels above the crater, in the opposite direction to the cutting edge, a grained and irregular surface is seen, typical of attrition wear. When machining with pure oil applied by MQF, Fig. 5, the wear mechanisms on the PCBN tools were very similar to what was seen in dry condition, with the crater size suggesting high temperatures and the presence of a microcrack of mechanical origin on the cutting edge. The images also reveal abrasive wear, with a slight trend toward a reduction of the grooves depth, probably due to the use of lubricant that reduces the contact severity. Another interesting detail on the tool wear analysis is the micro chipping of the coating on the rake face of the tool, Detail A, Fig. (5). In Fig. (6), the only perceived difference between the previously analyzed tools and that evaluated using a solid lubricant is the absence of mechanical chipping. With this cutting condition, the studied tool with solid lubricant reached the longest tool lives. The MoS2 particles contributed to changes in the tribological phenomenon, lengthening the tool life for these cutting conditions. The predominance of the attrition wear mechanisms on both crater and flank surfaces are more evident on the PCBN tools used with MQF + MoS2 lubri-cooling condition. Fig. (7) show an SEM analysis with BSE (backscattering electrons), identifying the elements sulfur, titanium, and molybdenum found in the worn region of the same tool shown in Fig. (6). In the Fig. (7), it is possible to identify the presence of the solid lubricant, mainly in the regions around the tool wear, with small amounts of sulfur and molybdenum particles present in the crater, noting that the image was obtained after ultrasound cleaning with HNO3 and acetone. Titanium is part of the tool binder and of the tool coating, but molybdenum and sulphur must have come from the lubricant. From the images, it is likely that solid lubricant material (MoS2) reached the sliding zone of the chip/tool contact area and also on the workpiece-flank interface and may have been key to the best results observed in tools that used solid lubricant. How they exactly work at the chip-tool-workpiece interfaces is not clear, but their good lubricating properties are indication that they improve the process of chip and new work surface formation. When performing a punctual analysis of the elements present in the wear of the tool, Fig. (7) and Tables (3–6), the presence of MoS2 was confirmed in several regions, which is a conspicuous confirmation that the solid lubricant particles took place in the lubrication process in the cutting region. Even after the acid cleaning, the work material adhered strongly on the tool surface, since several regions remained rich in iron and chromium, chemical elements of the AISI D6 alloy.

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MQF technique could be a viable alternative for increasing the tool life, either pure or when blended with MoS2; Considering the volume of material removed (VMR), best results were found with the combination: vc =150 m/min; f = 0.10 and 0.15 mm/rev; independent of the lubri-cooling condition. The increase in cutting speed (vc) from 150 m/min to 300 m/min, and in the feed rate (f) from 0.05 to 0.15 mm/rev drastically dropped the tool lives. The tribological conditions in the tool-chip and tool-workpiece interfaces imposed by the higher values of cutting parameters used (cutting speed and feed rate) suppress the lubricating action of the lubri-cooling condition applied by MQF and MQF + MoS2; The main wear mechanisms for all tools were abrasion, attrition, and diffusion. Energy dispersive spectroscopy of the PCBN worn tool after machining with MQF + MoS2 (vc =150 m/min; f =0.05 mm/ rev) confirmed the presence of MoS2 in several regions of the cutting edge, which is a conspicuous confirmation that the solid lubricant particles took place in the lubrication process in the cutting region; The results of the present investigation suggest that use of solid lubricant in hard turning may be a viable alternative to tackling the challenge to machining difficult-to-cut materials, i.e., materials with high fraction volume of carbides in the microstructure.

Funding No funding was received for this work. Declaration of Competing Interest No conflict of interest exists. Acknowledgments This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) Finance Code 001. The authors are grateful to Villares Metals S.A. and Sandvik Coromant for donation of the work material and the tools, respectively; they also thank CNPq, and FAPEMIG for financial support. References [1] König W, Berktold A, Koch K-F. Turning versus grinding – a comparison of surface integrity aspects and attainable accuracies. CIRP Ann Manuf Technol 1993;42:39–43. https://doi.org/10.1016/S0007-8506(07)62387-7. [2] Tönshoff HK, Arendt C, Ben Amor R. Cutting of hardened steel. CIRP Ann Manuf Technol 2000;49:547–66. https://doi.org/10.1016/S0007-8506(07)63455-6. [3] Klocke F, Brinksmeier E, Weinert K. Capability profile of hard cutting and grinding processes. CIRP Ann Manuf Technol 2005;54:22–45. https://doi.org/10.1016/ S0007-8506(07)60018-3. [4] Byrne G, Dornfeld D, Denkena B. Advancing cutting technology. CIRP Ann Manuf Technol 2003;52:483–507. https://doi.org/10.1016/S0007-8506(07)60200-5. [5] Bartarya G, Choudhury SK. State of the art in hard turning. Int J Mach Tools Manuf 2012;53:1–14. https://doi.org/10.1016/j.ijmachtools.2011.08.019. [6] Paulo Davim J. Machining of hard materials. 2011. https://doi.org/10.1017/ CBO9781107415324.004. [7] Poulachon G, Moisan A. A contribution to the study of the cutting mechanisms during high speed machining of hardened steel. CIRP Ann Manuf Technol 1998;47:73–6. https://doi.org/10.1016/S0007-8506(07)62788-7. [8] Ávila RF, Abrão AM. The effect of cutting fluids on the machining of hardened AISI 4340 steel. J Mater Process Technol 2001;119:21–6. https://doi.org/10.1016/ S0924-0136(01)00891-3. [9] Diniz AE, Ferreira JR, Filho FT. Influence of refrigeration/lubrication condition on SAE 52100 hardened steel turning at several cutting speeds. Int J Mach Tools Manuf 2003;43:317–26. https://doi.org/10.1016/S0890-6955(02)00186-4. [10] Sharif MN, Pervaiz S, Deiab I. Potential of alternative lubrication strategies for metal cutting processes: a review. Int J Adv Manuf Technol 2017. https://doi.org/ 10.1007/s00170-016-9298-5. [11] Biček M, Dumont F, Courbon C, Pušavec F, Rech J, Kopač J. Cryogenic machining as an alternative turning process of normalized and hardened AISI 52100 bearing steel. J Mater Process Technol 2012;212:2609–18. https://doi.org/10.1016/j. jmatprotec.2012.07.022. [12] Liew PJ, Shaaroni A, Sidik NAC, Yan J. An overview of current status of cutting fluids and cooling techniques of turning hard steel. Int J Heat Mass Transf

4. Conclusions Based on the results obtained in these experiments, which aimed at investigating the impact of the lubricant environment (dry, MQF and MQF + MoS2) on the tool performance and tool wear mechanism, in the hard turning process with PCBN tools of the AISI D6 cold work tool steel, it can be concluded: - Best results regarding the tool life were obtained using the variables combination: cutting speed (vc) =150 m/min; feed rate (f) =0.05 mm/rev; and the fluid mixed with solid lubricant (MoS2) applied by MQF. The results showed that the use of lubri-cooling by 345

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