Machining performance evaluation of Ti6Al4V alloy with laser textured tools under MQL and nano-MQL environments

Journal of Manufacturing Processes 53 (2020) 174–189

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Machining performance evaluation of Ti6Al4V alloy with laser textured tools under MQL and nano-MQL environments

T

Sarvesh Kumar Mishra*, Sudarsan Ghosh, Sivanandam Aravindan Department of Mechanical Engineering, Indian Institute of Technology- Delhi, Hauz Khas, New Delhi, India

ARTICLE INFO

ABSTRACT

Keywords: Textured tools Ti6Al4V machining nanoMQL Al2O3nanofluids Sustainable machining Derivative cutting

The study aims to conduct experimental investigations towards an integrated coolant and cutting tool based strategies for sustainable machining of Ti6Al4V alloy. Laser textured cutting tools were used under vegetable oil based MQL, and alumina suspended DI water based nMQL environments. The results were compared with plain and textured tools under dry condition with the same machining parameters. A detailed experimental plan was executed to conduct the series of experiments considering machining parameters, texture parameters and machining environment collectively. The MQL and nMQL parameters were selected based on spreadability of droplets at varying air pressure and flow rate. The results have shown a reduction in cutting forces, apparent friction coefficient, contact length, tool wear and chip adhesion over the rake face with textured tools under MQL environment. Nano fluid based cooling has a limited advantage over the range of cutting speeds and feeds as alumina nanoparticles get accumulated in the textured space. Reduced curling radius is obtained with textured tools due to the intense heat generated and associated interface multipoint micro-cutting (IMP-μC) mechanism.

1. Introduction Conventionally cutting fluids have been used in machining industries due to the ability to reduce cutting forces, dissipate heat and chip breaking. The cutting fluids not only contain coolant and lubricants but also added extreme pressure additives, germicides, antifoaming, antibacterial agents, humectants, and other chemical additives to maintain its use over time [1,2]. The fluids combining all these additives have been a potential challenge to human and the ecosystem. The recycling of these fluids can be considered as a possible option to favor the application of the cutting fluids, but that also costs too high for manufacturing industries. In a nutshell, the use and attempts to reuse the cutting fluids for machining have been a big challenge for industries, and an alternative is sought in this direction. In order to cater to the need and fulfill the regulatory obligations imposed by government bodies, dry or near dry machining should be performed. The recent approaches towards environment-friendly techniques (Fig. 1) can be considered as a possible direction to the dry or near dry machining process. Many researchers have castigated flood cooling as it causes skin and respiratory diseases, e.g., chest bronchitis, dermatitis, and skin disorder. Minimum quantity lubrication is an integration of cutting fluid and compressed air to form a coolant mist that can improve the ability

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of fluids to reduce heat and friction in machining. In MQL, evaporative heat transfer removes the generated heat compared to convective heat transfer in flood cooling and thus better heat transfer is achieved causing reduced temperature and chip adhesion [3]. Interestingly, although MQL minimizes the amount of fluid required to ≤ 250 ml/h [4], respiration-related health problems remain the same or sometimes elevated. For better penetration of MQL fluids in cutting zones, phosphorus/sulfur/calcium sulphonate based extreme pressure additives and zinc-based anti-wear additive are used [2]. When mineral oils or synthetic oils are used in MQL as cutting fluid, the aerosol formed as the mist contains very tiny particles that can lead to severe health hazards. The effect of aerosol due to atomization leads to serious risks of colon, pancreas, rectum, esophagus and prostate cancers [1]. So, biodegradable oils are suggested to avoid the aforementioned health problems and offer better lubrication compared to mineral oils. Triglyceride chains present in vegetable oils help in improving boundary lubrication as the long and fatty acid chains form a high-strength lubricating film over the contact pairs [5]. Su and co-workers [6] studied the machinability of AISI1045 steel with vegetable oil and ester oil based nano-MQL conditions. Cutting forces and temperature were found to be reduced by 11 %–26 % and 12 %–21 % respectively at different machining conditions. In grinding of Si3N4 with three different laminar nanofluids (graphite, MoS2, and

Corresponding author. E-mail address: [email protected] (S. Kumar Mishra).

https://doi.org/10.1016/j.jmapro.2020.02.014 Received 29 August 2018; Received in revised form 30 January 2019; Accepted 8 February 2020 1526-6125/ © 2020 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.

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Nomenclature MQL nMQL DI f vc ap

de EDM IMP-μC FEM µ Fx Fy Fz

Minimum quantity lubrication Nanofluids based MQL Deionized Feed (mm/rev) Cutting velocity (m/min) Depth of cut (mm)

WS2), MoS2 based nanofluids have shown reduced cutting force and improved surface topography [7]. The reason for the same is attributed to enhanced wettability and friction reduction due to ball bearing effect. MoS2 suspended palm oil was used under MQL environment for grinding of 440 with corundum grinding wheel [8]. The least friction coefficient was obtained for nanoMQL condition compared to flood and MQL conditions. In another study for grinding of Yttrium-stabilized tetragonal zirconium, the critical maximum undeformed equivalent chip thickness for ductile to brittle transition has been correlated with different lubrication conditions [9]. The high viscosity of palm oil, excellent toughness, and ductility offered by MoS2 nanoparticles resulted in the slippage of nanoparticles into the grinding zone and the formation of a stable thin lubricating film. For grinding of IN718, carbon nanotube (CNT) and MoS2 (comparable mean diameter ∼30 nm), based nanofluids were used under MQL and hybrid MQL grinding [10]. Lowest grinding force ratio and roughness have been obtained by hybrid nanoMQL owing to the physical collaboration of CNT-MoS2 nanoparticles. The similar research has been conducted for grinding of IN718 focusing on the effect of different nanoparticle concentration MoS2/CNT (1:1, 1:2, 1:3, 2:1) [11]. The high modulus, high strength of nanotubes and lower friction offered by MoS2 resulted in the physical synergistic effects which improved the grinding performance under hybrid nanoMQL environments. Laser textured cutting tools are getting attention as a sustainable approach for machining under dry cutting environment. Different geometric patterns (channel, circular, and rectangular shaped) fabricated over the rake face of textured tools were used for machining aluminum [12]. Reduction in cutting forces, friction coefficient, and

distance of textures from cutting edge (μm) Electro-discharge machining Interface multipoint micro cutting Finite element method Apparent friction coefficient Axial thrust force (N) Main cutting force (N) Radial thrust force (N)

Table 1 Machining and texture parameters for experiments. Run

Dry MQL

1 2 3 4 5 6 7 8 9

D1 D2 D3 D4 D5 D6 D7 D8 D9

M1 M2 M3 M4 M5 M6 M7 M8 M9

nMQL nM1 nM2 nM3 nM4 nM5 nM6 nM7 nM8 nM9

vc (m/min) 60 60 60 90 90 90 120 120 120

f (mm/rev) 0.1 0.15 0.2 0.1 0.15 0.2 0.1 0.15 0.2

de (μm) 50 100 150 100 150 50 150 50 100

Fig. 2. Experimental design for machining experiments under dry, MQL and nMQL environments.

chip adhesion was obtained with textured tools. Dimple textured cutting tools fabricated on the rake surface with different texture geometric parameters (depth, edge width, and array orientation) were used for face milling of steel under dry, flood and paste cutting [13]. The improved performance of textured tools was attributed to micro-reservoir and micro trap actions. The closed shape structures such as micro dimples and microholes are suggested to exhibit superior performance under severe lubrication compared to open shape structures (grooved textures). The lubricant retention and storage capability under severe lubrication make microholes (closed shapes) effective while the open structures were found to be advantageous for flood lubrication. Grooved channels have been fabricated on carbide tools for machining of Ti6Al4V [14]. Reduction in cutting force, feed force, and friction angle is reported, and the importance of texture edge distance was signified. Using finite element simulations, the influence of textured and coated textured tools has been studied for residual stresses and temperature generation at the rake face [15]. It was revealed that the TiAlN coated textured tools were helpful to reduce the average cutting tool temperature and maximum rake surface temperature. FEM based analysis of textured tools with different texture shapes, area density, and textured depth shows that the texture shapes do not influence the

Fig. 1. Different environment-friendly machining processes. 175

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Fig. 3. Laser textured rake face of cutting tools at (a)100X, (b) texture dimensions, (c) laser-induced layers around microholes and textured tools at varying distance from cutting edges (d)-(f).

Fig. 4. Machining set up for MQL and nMQL experiments.

machining performances of titanium alloy under dry cutting [16]. The developed contact length model for textured tools incorporating the chip serration parameters at varying machining conditions confirms the limited applicability of textured tools. Most of the studies conducted on machining with textured tools are either filled with solid lubricants or dry/flood cutting. The effectiveness of solid lubricants for high cutting speeds vanishes as solid lubricants get oxidized at temperature range 5000C 6500C [17] and also the retention of the solid lubricants over longer machining time is debatable. To improve the utilization of textured tools for steels at high cutting speeds (200 m/min), TiAlN coated textured tools under flood and starved lubrication were used [18]. Reduction in cutting temperature, cutting forces, and surface roughness was observed under flood conditions. The reason for the same was attributed to capillary action and wear debris entrapment in textures. Machining and adhesion tests of TiAlN coated textured tools were performed for cutting of SAE1045 steels [19]. The improved adhesion of deposited coatings over the textured tools improved the machinability under dry cutting. Dry cutting of Ti6Al4V was attempted using chevron shaped textured tools

with AlTiN and AlCrN coatings [20]. Better adhesion of coating with WC substrate was achieved due to mechanical interlocking and the formation of laser induced stable cobalt oxide phases. The textured coated tools reduced cutting forces, flank wear, and improved wear resistance than that of plain coated tools. The effect of different texture shapes (circular, rectangular, triangular and chevron shape) on wettability and metal cutting response has been studied for Ti6Al4V machining [21]. Under MQL conditions, chevron shaped textured tools yielded in better lubrication mechanism compared to other texture shapes. The continuous spread of single chevron units over the tool surface offered texture guided liquid flow which improved the surface wettability and reduced cutting forces and tool wear. Niketh and Samuel [22] studied the effect of texturing on drill bits for Ti6Al4V drilling under dry, wet and MQL environment. MQL environment was suggested as a better option to reduce frictional heat and chip adhesion for textured drills. Zhang et al. [23] developed a composite coated tools depositing WS2 soft coatings on laser textured TiAlN coated tools. The three step procedure was followed and initially PVD-TiAlN coatings were deposited on WC/Co cutting tools which further were 176

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Table 2 Droplet distribution at varying air pressure and liquid flow rates at constant stand-off distance for MQL conditions.

Table 3 Droplet distribution at varying air pressure and liquid flow rates at constant stand-off distance for nMQL conditions.

nanotextured using laser. Soft WS2 coatings deposited on the textures improved the adhesion of coatings and retained the solid lubricants for longer durations. The available studies attempted the use of textured tools with dry, flood or solid lubricants based cutting except a few are focused on hard coatings and MQL to sustain high machining temperature. Although the application of minimum quantity lubrication is widely accepted amongst the researchers, still there are some concerns about the penetration of fluid into the secondary cutting zone. Texturing the tool rake face helps in providing lubricants as textures act as a local

lubricant reservoir, and trapped liquids can be available for better heat transfer and friction reduction. This may further assist in imparting the ability of MQL fluids to better access the secondary zone due to capillary suction and improve machining performance. With the help of the present study, an attempt is made to analyze different mechanisms associated with MQL and nMQL namely Laidenfrost effect [24], Rebinder or mending effect [25], ball bearing effect [26], and tribo-film formation [27] in the context of textured tools. The study is motivated towards combining/integrating the two dimensions of coolant based and cutting tool based sustainable 177

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Fig. 5. (a) Droplet distribution in the impact zone, (b) different zones over the surface, and (c) contact angle variation for plain and textured tools.

Fig. 6. Main cutting forces for different experimental runs for various machining environments.

strategies for machining. To the best of author’s knowledge, experimental investigation in this direction is not available. Hence, the present study aims to investigate the strategy combining these sustainable techniques as the motive of an integrated sustainable approach.

the combined effect of cooling and lubrication methodologies have been investigated. Machining, as well as texture parameters, were considered to understand the effects of both the factors for integrated sustainable machining of Ti6Al4V alloy. For textured tools, the selection of texture parameters is deemed to be important for improved machining performances. Texture dimensions (diameter of microholes, width/length of microgroove, and depth of texture), the pitch of textures, and the distance of textures from the principal cutting edge are considered to be essential factors defining the performance of textured tools. The distance of textures from cutting edge is considered as a significant factor to avoid catastrophic fracture of the textured cutting

2. Materials and method 2.1. Experimental parameters and design The machining experiments were conducted to evaluate the effectiveness of textured tools compared to the plain tools. Apart from that, 178

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Fig. 7. (a) Mechanism of capillary suction for plain tools and (b) enhanced capillary suction for textured tools (Adapted and modified from Godlevski et al [31].).

Fig. 8. Schematic representation of mechanism of aerodynamic lubrication for (a) plain tools, (b) textured tool under dry condition, and capillary suction for textured tools under (c) MQL and (d) nMQL modes. 179

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Fig. 9. Variation of thrust force for experimental runs under different lubrication.

Fig. 10. Variation of (a) contact area and (b) contact length for different experiments.

tools. If the distance of textures is close enough to the principal cutting edge, the stresses and chip load will be too high on weakened cutting edge, and it’ll cause sudden failure. If the textures are distant enough from the cutting edge, the effective textured zone to reduce chip-tool contact length will be lower, hence decreasing the effectiveness of textured tools. To combine the texture and machining parameters together for titanium machining, vc , f, and de were selected as the major parameters. Three levels of low, medium and high values are taken for each input parameters as shown in Table 1 according to hybrid Taguchi (L9 × 3) orthogonal design. To integrate the two different methods of tool and lubricant based sustainable techniques, different machining environments are selected as shown in the Fig.1 with colored blocks. The experiments were conducted under dry, MQL, and nMQL environments and replicated thrice to maintain the repeatability of cutting tests. Fig. 2 shows the

representation of the experimental plan as elaborated in Table 1. The machining speed and feed values are considered according to initial screening experiments under three different conditions for maximum speed and feed at constant ap = 1 mm . Each experiment has been performed for 30 s cutting time. 2.2. Laser surface texturing and machining performance tests To use the tool based sustainable approaches, the rake face of cutting tools was textured using laser micromachining. Texturing on cutting tools was performed by nanosecond pulsed laser (Nd: YAG Laser, Make: Lee Laser, TQ9005) at different distance from cutting edges. Laser micromachining is used to fabricate textures at the rake face (Fig. 3a), and laser parameters are kept at wavelength 1064 nm, pulse duration 20 ns, pulse energy 20 mJ, and frequency 20 kHz. The laser 180

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Fig. 11. Variation of apparent friction coefficients for different experimental runs.

Fig. 12. (a) The rake surface analysis for experimental conditions and (b) corresponding flank surface.

fabricated cutting tools are shown in Fig. 3(b) with a high magnification micrograph to show the dimensions of textured microholes. Fig. 3(c) shows sideway deposition of ablated material due to laser surface interaction which is known as the recast layer. Optical microscopic images of different textured tools are presented in Fig. 3(d)-(f) to show the variation of distance of texture fabrication from the cutting edge. Machining experiments were conducted on CNC turning center (Leadwell CNC: Fanuc Oi Mate-TD). Cylindrical Ti6Al4V (Ti-grade 5) bar of ϕ 80mm × 300 mm has been used for turning experiments.

Uncoated WC-6 %Co carbide inserts (CNMA120408-THM-F: WIDIA) were used in combination with PCLNL2020-K12 tool holder with 20 mm tool overhang. The resultant clearance angle and rake angle are 50 and 60 respectively. Cutting forces were recorded by 3-component piezoelectric dynamometer (9129AA: Kistler) with charge amplifier (5070A). The cutting forces were analyzed by using Dynoware software (2825-D03, Version 3.0.9). Tool chip contact length, chip underside study and contact area have been measured by an optical microscope (SteReo Discovery V20: Carl Zeiss, Germany). Scanning electron 181

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Fig. 13. The rake face analysis for (a) plain dry, (b) textured dry, (c) textured MQL and (d) textured nMQL.

micrographs (Zeiss EVO 50), and electron dispersive X-ray spectroscopy (Bruker-ASX QuanTax200) were used for tool wear, and adhesion analysis over cutting tools.

droplets are enveloped by oil that help in carrying and spreading the droplet over the tool surface due to inertia [28]. Water-based nanofluid for nMQL was prepared by mixing 0.5 wt. % Al2O3 (spherical shaped, average particle size 40 nm,) in DI water based on its excellent thermal, adsorption, and dispersion properties [29]. To stabilize the nanoparticles’ suspension in fluid, 0.05 wt. %. SDBS (Sodium dodecyl benzene sulphonate) is added to the mixture. The solution is ultrasonicated for 150 min in bath sonicator and 30 min in magnetic stirrer respectively. Trial experiments have been performed with base fluid (water) and base fluid added with SDBS (0.05 wt. %) to understand any possible influence of surfactant alone in titanium machining. The results reveal that the surfactant alone has no significant effect on cutting forces under the selected machining conditions (vc = 90 m/min, f = 0.1 mm,

2.3. Coolant preparation and MQL parameters selection Both plain and laser textured tools were used under dry, MQL and nMQL environments. For MQL, sunflower oil is selected in ratio 1:10 with deionized water due to its proven application as environmentally friendly fluid. The resulting fluid is continuously stirred for 90 min to prepare a biodegradable emulsion of oil in water. The selected MQL fluid offers the advantage of cooling by water due to high heat capacity and high thermal conductivity and lubrication by oil due to better lubricity and oleophilicity. For oil in water-based cutting fluids, the water 182

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Fig. 14. EDS scans for (a) plain tool under dry conditions and textured tools under (b) dry, (c) MQL, and (d) nMQL conditions. Table 4 Shape of different machining chips formed under different machining environment. vc

f

Textured tools

de

Plain Tool Dry

Environment

60 60 60 90 90 90 120 120 120

0.1 0.15 0.2 0.1 0.15 0.2 0.1 0.15 0.2

50 100 150 100 150 50 150 50 100

Dry

MQL

nMQL

Spiral Spiral Spiral Spiral with low radius Spiral Spiral Spiral with low radius Spiral Spiral

Helical/tubular with very low radius Helical with very low radius Helical with very low radius Helical with very low radius Helical with very low radius Helical with very low radius Helical with very low radius Spiral with high curling frequency Spiral with high curling frequency

Helical/tubular with low radius Spiral with high curling frequency Spiral with high curling frequency Spiral Spiral with high curling frequency Helical/tubular with low radius Spiral Spiral with high curling frequency Spiral with high curling frequency

ap =1 mm, air flow rate = 6 bar). The variation in forces at different MQL flow parameters remains inappreciable (within the range of force measurement errors) which shows the influence of SDBS itself in base oil can be neglected for machining of Ti6Al4V. The MQL set up consists of indigenous coolant delivery system for MQL integrated with twin siphon nozzle and external mixing chamber. Compressed air and liquid delivery inputs are fed into the chamber that breaks the fluid droplets and results in mist lubrication as shown in Fig. 4. The distribution and particle size in MQL based cooling are the main factors which govern the ability of the liquid droplets to penetrate the secondary cutting zone. The droplet distributions depend upon nozzle stand-off distance, air supply pressure and liquid delivery rate [2]. For flow parameter selection under both MQL and nMQL environments, the droplets are collected at air pressure (4 bar–7 bar), and liquid flow rates (100 ml/h - 250 ml/h) and the distribution is shown in Tables 2 and 3. In order to simulate the impingement of MQL droplets over the cutting tool surface, the droplet impact and distribution

Ribbon Ribbon Ribbon Ribbon Ribbon Ribbon Ribbon Ribbon Ribbon

with with with with with

high high high high high

radius radius radius radius radius

analysis are performed. The MQL nozzle set up and optical microscope have been used to analyze the droplet distribution on a surface. An acrylic plate of 30 mm × 30 mm dimension is placed on tool holder at the same orientation of the cutting tool, and the MQL jet is allowed to fall over the plate. Proper care has been taken to orient the plate surface in the direction of flow in such a way that the flow should not be obstructed, and the jet falls parallel to the plate surface except at the edge of the plate. The MQL jet impacts over the plate edge and forms a droplet distribution cone over the plate surface. The method of droplet collection and droplet distribution analysis is shown in Fig. 5. The droplets falling over the surface are categorized into three distinct zones namely the zone of impact, the zone of droplet sliding and the zone of flaring (Fig. 5b). The fluid stream as mist impacts at the focused surface and gets dispersed in these different zones. The zone of impact has a maximum number of tiny droplets due to breakage of liquid drops in air stream and impact on the surface. MQL jet should be arranged in such a manner that the zone of impact always fall on tool 183

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Fig. 15. (a) Ribbon, (b) ribbon with high curvature, (c) spiral, (d) spiral with high curling, helical chips with (e–f) low curling radius and (g) chip entanglement in machining with plain tools.

tip or just before tool tip/cutting edge. The droplet density in the zone of impact depends on the liquid flow rate and air pressure. Owing to high air pressure, tiny droplets accelerate at the surface, and there remains a zone in which a reduced number of droplets are seen. A visible evidence of droplet sliding is seen in the outward direction of the zone of impact that reflects as track formed due to high-speed droplet movement. The droplets further decelerate and coalesce to form bigger droplets at the end of the motion. This zone of larger droplets collected at the enveloping boundary of droplet sliding zone is considered as a zone of flare. The collection of droplets and formation of bigger drops will help in reducing the heat of the machined surface after striking the workpiece. The distance of the nozzle from cutting edge is kept at 52 mm to allow the liquid jet to form a fully developed mist cone over the tool rake face and it remains constant for all the cases. The developed mist at the rake face helps in better dispersion of lubricant at the tool surface and penetration of liquid droplets in the secondary cutting zone. The contact angle variation over the plain and textured cutting tools is shown in Fig. 5(c). MQL fluid (oil in water) shows better spreading behavior compared to nanoMQL fluid on both plain and textured tools. The oleophilic nature of oil is attributed to the spreading of the MQL fluid. Tables 2 and 3 reveal that at low air pressure larger droplets are formed and vice versa. High air pressure breaks the liquid drops into tiny droplets and spread it over the surface. With the increased flow rates, the droplet size and its spreading increases. The increase in droplet size at high flow rates is due to the availability of more liquid to mix with the incoming air jet and droplets’ coalescence. The maximum droplet density and uniform distribution were obtained at 150 ml/h and 6 bar pressure for MQL. For nMQL, better distribution and tiny droplets are seen at 200 ml/h and 6 bar pressure. Trial experiments were conducted with plain cutting tools under MQL and nMQL environments at vc = 100 m/min, f = 0.1 mm/rev, and ap = 1 mm . The main cutting forces were compared and the least cutting forces were obtained for 6 bar – 7 bar, and 150 ml/h. Hence, these parameters were used for further machining studies. For comparison purpose, both MQL and nMQL parameters are selected as 150 ml/h and 6 bar for all the experiments.

for textured MQL conditions, and the reduction ranges from 1.9 % (for M1) to 16.78 % (for M5). Textured tools under dry and MQL condition reduce main cutting forces for D3-D6 and M3-M9. Under MQL conditions, forces reduce due to effective penetration of fluid droplets into the tool-chip interface. The tiny droplets size are comparable to the texture dimensions, and it helps for the droplets to form a layer of fluid separating the tool and chip surfaces. For plain cutting tools, the friction at the interface generates heat that is transferred to tool and chip materials. The share of heat carried away by titanium chips reduces due to the lower thermal conductivity of the material. Under dry cutting of aluminum and steels with textured tools, aerodynamic lubrication was considered as a possible lubrication mechanism due to available textured space at the interface. Due to the reduced asperity contact area of textured surfaces, higher heat is available for the tool materials, and high thermal loads will be present over the textured tools compared to plain tools. The increased thermal load causes the chip material to soften, and the textures act as microcutters to undercut the chip underside surface. The phenomenon of chip undercutting due to microholes is considered as derivative cutting or IMP-μC and responsible for heavy adhesion in textured spaces under dry conditions. For titanium machining, the aerodynamic lubrication seems not to have much effect on the performance of textured tools as evident by increased forces for dry conditions at increased feed and speed. Lubricant model by Childs [30] proposed that the existence of surface roughness at the exit end of chip sliding contact zone decreases the real area of contact between chip and tool. The reduced area due to roughness results as microchannel that can promote lubricant penetration in the contact zone. During machining, the penetration of liquid into the sticking contact zone of the secondary interface is supported by the capillary suction mechanism. The fluid penetration mainly works on the principle of lubrication through dynamic capillary networks in the secondary zone. The major factors affecting the capillary suction are (a) different behavior of a lubricant in its liquid and gaseous phases, (b) pressure difference in the capillary zone (sliding-sticking interface), (c) fluid properties (density, viscosity, vapor pressure), (d) influence of contact surfaces at tool-chip interface, and (e) additives in cutting fluids (surfactant, solid particles to enhance surface tension and thermal conductivity, etc.) [31]. The capillary suction mechanism through interface channels is explained in Fig. 7(a) for plain cutting tools. It is divided into three main steps: (a) progression of fluid penetration, (b) droplet explosion initiation due to equilibrium of liquid pressure and vapor pressure, and (c) vapor generation and formation of thin tribo-layer at the capillary outer zone. The time required to finish the mentioned three stages should always be less than the capillary life for effective lubrication in the interaction zone [31]. The concept is extended for textured tools

3. Results and discussion 3.1. Cutting forces 3.1.1. Main cutting forces Fig. 6 shows the variation of main cutting forces for different experimental runs under different machining environments. P refers to experimental results of plain tools under dry conditions and used for comparison with textured tools. Reduction in cutting forces was found 184

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Fig. 16. Underside images for machined chips with (a) plain, (b) laser textured tools under dry condition, and laser textured tools under (c) MQL and (d) nMQL condition..

(Fig. 7b), and it is proposed the reduction in the actual area of contact due to texturing can be helpful in promoting the lubrication similar to roughness induced micro-channels [30]. Also, the textures can create high vacuum due to available space (volume created due to texturing) at the interface. Under MQL

condition, the tiny droplets formed due to breakage of liquid at high air pressure help in reducing filling of the capillary zone and shorten aggregate time to complete all above stages. Tiny MQL and nMQL droplets have high velocity and better chances to penetrate the interface compared to flood conditions. Once the liquid pressure reaches the vapor 185

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Fig. 16. (continued)

Fig. 17. SEM micrograph of chip underside with plain and laser textured tools under different environments.

pressure, heating and evaporation of the MQL and nMQL droplets start simultaneously, and this step is considered as the initiation of the micro-droplet explosion. The explosion breaks out the droplets and forms a vapor front which fills the capillary channel to complete depth and creates a thin tribo-film of vapor layer at the entrance of lubricants. The formation of thin tribo-layer and vapor film above the surface are attributed to the underlying mechanism of Laidenfrost effect. The micro-droplet/vapor layer formation and tribo-layer formation helps in improving the heat transfer to the coolant and reducing friction respectively (Fig. 8). The microdroplets can penetrate the cracks formed by plastic deformation during machining and progress inside the microcracks unless retarded by a chemical reaction caused by atomic arrangements. The microdroplets compress the air trapped in the microcracks and augment in the stress concentration apart from applied stress due to plastic deformation (machining load due to the cutting tool). It is considered as filling up of the crack edges or healing the crack formed due to plastic deformation during cutting. This physicochemical interaction of tiny fluid droplets of surface active cutting fluids (MQL and nMQL) can cause embrittlement of chips and improve the chip formation mechanism owing to the Rebinder effect [32] and reduce cutting forces. For nMQL environment, the forces remained unchanged from plain tools at higher cutting speeds. The reason is the puncturing of the nanofluids droplets and inability of nanofluids to form an effective layer for heat transfer. One of the important mechanisms responsible for

reduced machining forces is ball bearing effect. The nanoparticles may get accumulated in the textures as the dimensions of textures are much higher than nanoparticle size. The entrapped nanoparticles are unable to be withdrawn from the textures and hence do not contribute to force reduction effectively. Hence, with textured tools, the chance of ball bearing mechanism to reduce cutting forces is limited due to entrapment of nanoparticles in the microtextures available at the rake face. The base fluid for nanofluids is DI water, so the formation of tribo-layer and its chances to sustain at higher speeds are less compared to MQL. Only at lower speeds, reduced forces under nMQL environment are obtained, and maximum reduction is achieved for the nM2 condition. 3.1.2. Thrust forces The increase in axial forces is obtained for the dry condition with textured tools (Fig. 9). The reduction in forces was obtained for MQL conditions higher speeds. The thrust forces are an indication of frictional characteristics over the tool rake face. The reason behind the reduction in the thrust forces are the mechanisms mentioned in the previous sections. The formation of vapor film and thin tribo-layer due to enhanced capillary suction can be suggested as a possible reason for reducing thrust forces in MQL conditions. The forces under nMQL conditions are higher as compared to plain tools under dry environment. Hence, no improvement in frictional forces is seen for nMQL conditions. 186

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Fig. 18. Serrated chip cross section micrograph for (a) plain dry, (b) textured dry, (c) textured MQL and (d) textured nMQL conditions.

3.2. Contact length and contact area

dry, MQL and nMQL in Fig. 11. Apparent friction increased for textured tools under dry cutting in nearly all conditions except (D2, D4, and D5).The higher increase in friction is not a good indicator of the performance of textured tools for dry cutting of titanium alloy. At de = 50 µm from cutting edge, increased friction is evident whereas for higher de values, the friction is high only at higher cutting speeds and feeds (D6-D9). Thus, the apparent friction under dry cutting is greatly affected by texture parameter (de ). For MQL and nMQL conditions, reduced apparent friction is observed from experimental results and 4 %–19.6 % friction reduction is obtained for textured tools under MQL conditions. Maximum reduction in friction coefficient for nMQL environment is 13.76 % under nM6 condition.

Sticking contact length and contact area are plotted in Fig. 10(a) and (b) which indicate the decrease in the contact area and contact length for MQL conditions. Higher contact area for textured tools under dry environment is due to high heat and derivative cutting mechanism. High heat and chip adhesion will cause catastrophic breakage for textured tools as evident from fracture tool edge in the case of D2 in Fig. 12(b). The de values should be kept optimum and tool edge breakage is visible at vc = 60 m / min , f = 0.15 mm/rev and de = 100 μm. Thermal loads at low speed are high at the interface due to reduced contact area available for textured tools. The high thermal load and stresses over the tool due to texturing results in catastrophic edge fracture as seen under dry textured condition. Also, severe adherence on tool edge is visible at vc = 120 m / min and de = 150 μm (D7 condition) due to the high heat generated at higher cutting speeds. The high textured area is available in this condition that offers an increase in the amount of chip undercutting and hence, heavy adhesion at the textured surface. Compared to dry cutting, textured tools do not suffer from tool breakage or heavy chip adhesion in MQL or nMQL conditions. The availability of lubricant in the textured space and the resulting formation of vapor film during machining can be considered as a possible reason for the improved performance of textured tools under MQL condition. Similarly, less severe tool conditions are seen for nMQL environments except at de = 50 μm and vc = 120 m/min.

3.4. Tool wear and wear mechanism analysis The catastrophic edge fracture is seen for D2 condition (dry textured) at speed vc = 60 m / min , f = 0.15 mm /rev , and de = 100 µm . The distance of textures from tool edges (de ) affects the health of the flank surface and the cutting edge at higher speeds as high flank wear and chipping fracture is seen for D6 and D8 for de = 50 µm . Hence, de is considered as a factor affecting the tool edge fracture and flank wear for textured tools under dry cutting. At higher speeds and feeds, textured tools under dry conditions do not seem to offer better flank wear resistance than plain tools. However, improvement is seen for MQL and nMQL conditions as compared to textured tools under dry conditions (Fig. 12). Flank wear is reduced under these environments except nM8 and nM9 conditions where nearly similar flank wear is observed compared to the plain cutting tool (D8 condition). SEM images of tool rake face are shown in Fig. 13(a)-(d) for high speed and feed conditions (Run 9) under all environments. Severe adhesion and micro-chipping are evident for textured tools under dry (D9) condition. The reason for increased cutting forces for textured tools is heavy adhesion, and edge dulling (Fig. 13b). The microparticles formed as wear debris are filled in the textures, and thus textures can act as a possible space for wear debris entrapment. The entrapment of wear debris in textures helps in reducing abrasive wear over the tool rake face (Fig. 13b). Adhesion wear is a dominant

3.3. Apparent coefficient of friction Friction generated over the rake face has a great influence on the machining performance of the tools and the concerned environments. Friction over the rake face in machining can be obtained by the apparent coefficient of friction (Eq. 1).

µ=

{(Fx )2 + (Fz ) 2} Fy

(1)

The variation of friction coefficients is shown for experiments under 187

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mechanism for textured tools and abrasive wear is seen only for plain tools (Fig. 13a). So, it can be considered that high thermal load is generated on textured tools under dry cutting that causes heavy chip adhesion. The EDS scan of plain and textured tools compares the adhesion of titanium, and the higher elemental percentage is observed on the textured tool under dry cutting (Fig. 14a, b). This justifies the selection of a better lubri-coolant strategies for machining titanium with textured tools. Under MQL and nMQL environments for M9 and nM9 experimental run, wear images are shown in Fig. 13(c) and 13(d) respectively. The microholes are completely clean up to some distance away from cutting edge under MQL cooling condition. The effect of high-pressure air can be considered as a possible reason to sweep microparticles away from the secondary interface and hence effective in avoiding abrasive wear under MQL environment. The similar results have been obtained for nMQL environment except for the textures in machining zone is not completely unfilled by wear debris. The abrasion over the cutting edge is also absent, but textures are filled with micro-particles. For the analysis of trapped particles in nMQL conditions, EDS spectra of a particular filled texture is taken. The point EDS results in the case of D9, M9 and nM9 conditions are shown in Fig. 14. The EDS spectra (Fig. 14d) confirms the presence of alumina nanoparticles in the textures that are also visible from Fig. 13(d) as the agglomerated nanoparticles are observed at the rake face. The increased percentage of aluminum and oxygen for textured tools under nMQL condition (nM9) in Fig. 14(d) clearly suggests the presence of nanoparticles in the textured space. The size of a particle on the rake face is in the submicron range that suggests the agglomeration of nanoparticles due to the collection in the textured spaces. The water droplets carrying nanoparticles gets punctured after impacting on the tool surface, and nanoparticles get trapped in the textures (Fig. 8d). Some of the agglomerated particles get out of the textures during chip flow and may rub over the tool and chip interface. This helps in reducing frictional forces by changing the sliding friction to rolling friction/boundary friction to some extent due to ball bearing effect but only limited to low cutting speeds. The highest adhesion is found for textured tools under dry cutting, and the percentage adhesion of titanium is reduced by MQL and nMQL based lubri-coolant applications. Also, the filling of textures due to wear debris and resulting abrasion wear are reduced for MQL (M9) and nMQL (nM9) conditions compared to dry condition (D9).

cutting marks are evident. The patterns on the chip underside are termed as derivative cutting or IMP-μC caused by cutting of softened chips. The derivative cutting mechanism with textured tools under dry cutting is present irrespective of the shape of textures. The similar observation is also obtained for chevron textured tools under dry cutting conditions [20,33]. The tracks under the chips are also observed for MQL and nMQL environments [Fig. 16(c, d)], but sideway material deposition or groove formation is reduced. Reduced derivative cutting in MQL is due to better spreading of coolant, improved lubrication offered by vegetable oils and its ability to penetrate in the interface due to enhanced capillary suction. Under nMQL conditions, at lower cutting speeds (nM1-nM3), only striation and friction marks are visible with some indentation marks over the chip surface. The indentation marks over the chip underside suggest the presence of agglomerated nanoparticles in the zone and thus reducing friction to some extent. At increased speeds, derivative cutting becomes dominant, and the effect of nanoparticles vanishes for textured tools due to the discussed mechanism in Fig. 8(d). The chip underside interaction with plain and textured cutting tools under different environments are further discussed in Fig. 17(a–d) and supported with chip cross section images shown in Fig. 18. The heavy lamellae deformation of chips formed under dry cutting using plain tools (Fig. 17a) is shown by intense shear localized fracture of chips in Fig. 18(a). Both plain and textured tools under dry cutting environment reflected intense shearing as evident by serration peak height in Fig. 18(a, b). Comparatively, the serration peak height is reduced as well as no shear localized fracture is visible at serration valley for chips under textured MQL conditions (Fig. 18c). Under nMQL conditions, some incipient cracks were seen at the depth of valleys in Fig. 18(d). This shows that there are chances for heavy lamella deformation and chip fracture when machining with nMQL at higher cutting speeds. The chip fracture initiation for nMQL conditions shows that with textured tools, the heat transfer mechanism of nMQL is not more effective at higher speed compared to MQL conditions. 4. Conclusions An integrated sustainable approach has been investigated for machining of Ti6Al4V with textured tools. The coolant and tool based techniques are integrated to study the effects of machining performance. The cutting forces, friction coefficient, contact length, and contact area were measured as output responses. The tool wear mechanisms and chip undercutting are also studied to understand the influence of different machining and texturing parameter on chip adhesion and tool-chip contact area. From the experimental results following points can be concluded:

3.5. Chip formation and morphology study The analysis of chip underside gives a better understanding of the chip formation mechanics and underlying theories related to textured tools. Nature of different chips formed under various machining conditions is given in Table. 4. With plain untextured tools, chips are long and uncurled and caused entanglement with machined surface (Fig. 15g). The problem is severe during machining at higher speeds and results in poor surface quality and surface damage. Compared to plain untextured tools, textured tools resulted in decreasing the curling radius and spiraling the chips. The chip curling frequency and curling radius increase for textured tools under MQL and nMQL conditions as shown in Fig. 15. The low curling radius of MQL and nMQL chips with textured tools suggests better lubrication and liquid penetration in the sliding zone. Thus MQL and nMQL conditions also affect the chip formation during cutting and effectively solve the challenge of dry cutting with plain tools. For analysis of chip rubbing in contact with the tool, optical images of chip underside are shown in Fig. 16(a)-(d). With plain cutting tools under dry conditions, the chips underside is plain, and striation marks are visible. The striation marks are formed due to rubbing over the tool rake face as fractured chip microparticles act as a possible source of third body abrasion. At increased cutting speed, highly deformed lamellae are seen due to high heat and friction resulting in adiabatic shear banding. With textured tools, under dry conditions continuous grooves and micro-

(1) MQL and nMQL droplets are finely dispersed in the zone of impact at an intermediate flow rate (150 ml/h) and high air pressure (6 bar). The dispersion of MQL and nMQL droplets in the zone of impact should be maximum compared to flare or sliding zones. Maximum reduction in main cutting force was 16.8 % for M5 condition (Run 5, MQL) and maximum thrust force reduced by 21.4 %, 28.6 % and 20.8 % at 0.1 mm/rev, 0.15 mm/rev and 0.2 mm/ rev (M4, M5 and M9) respectively. (2) Aerodynamic lubrication in textured tools under dry cutting cannot be considered for titanium machining due to heavy chip adhesion, high friction, and generation of IMP-μC. (3) The formation of high vacuum due to textured space and reduction in capillary suction time improves the formation and explosion of microdroplets. This enhances the heat transfer in secondary interface due to the promotion of evaporative heat transfer. Improved spreadability of MQL fluid has been obtained over the textured surface. (4) The nMQL environment is helpful in machining at lower speeds. Impact of nMQL droplets on the tool surface punctures the droplet 188

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film, and nanoparticles get deposited/agglomerated in the textured space. (5) Integrating texturing and lubri-coolant strategies helps in reducing the apparent friction coefficient, sticking contact length, and contact area compared to plain tools. Compared to plain cutting tools under dry conditions, the maximum reduction in apparent friction coefficient for MQL and nMQL are 19.61 % and 13.7 % respectively. (6) Friction coefficient is profoundly influenced by cutting environments, and MQL can be considered as the best option for integrating with textured tools for machining titanium alloy. (7) Reduced lamellae deformation and chip fracture tendency are observed when machining with textured tools under MQL conditions.

[12] [13] [14]

[15]

[16]

Declaration of Competing Interest

[17]

The authors at this moment declare that no conflict of interest exists in the publication of this article.

[18]

Acknowledgments [19]

The authors acknowledge Central Research Facility (CRF-IIT Delhi) for SEM/EDS analysis and Mr. S. Prabhakaran to provide help for nanosecond laser fabrication.

[20]

References [21]

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