Experimental study on the effect of CNT-enriched nanofluid lubrication on the performance of textured cutting tool in the turning of aluminum 7075 alloy

This paper investigates the impact of surface texturing and the use of CNT-enriched nanofluid lubrication on the cutting performance of cemented carbide cutting tools during the turning process of aluminum 7075 alloy. Aluminum 7075 is widely utilized in various industries due to its exceptional properties, including high corrosion resistance, a favorable strength-to-weight ratio, and good formability. However, this alloy tends to excessively adhere to the cutting tool at the tool-chip interface, which negatively affects the machining process. Previous research has proposed different solutions, but the current study focuses on implementing the two most effective approaches to minimize adhesion phenomena. The first approach involves modifying the contact area by creating a pattern on the tool's rake face, while the second approach utilizes CNT-enriched nanofluid lubrication to reduce friction in the tool-chip interface. Various types of surface textures were fabricated on the rake face, and experimental tests were conducted to identify the most effective texture. The findings showed that using textured tools with micro-grooves perpendicular to the chip flow direction, with CNT-enriched nanofluid lubrication, resulted in significant reductions in main cutting force, built-up edge, and surface finish. The decreases were up to 32%, 37%, and 19%, respectively, compared to dry turning conditions.

proposes the implementation of micro-textured tools, along with CNT-enriched nanofluid lubrication, for the turning process of Aluminum 7075 alloy.Laser micromachining was employed to engrave four different types of textures, including linear and circular arrays, on the rake face of the carbide cutting inserts.Turning tests were subsequently conducted on Aluminum 7075 bars, utilizing both non-textured and textured tools, to determine the optimal texture.Subsequently, a comparative analysis was conducted by employing the selected textured tool under dry conditions and varying concentrations of CNT-enriched nanofluid lubrication.

Materials and experimental procedures
The cutting tools in the experimental tests were cemented carbide CNMA120408 inserts.To engrave microtextures, an Nd: YAG laser from Jinan Xinchu Laser Inc. was employed, operating at a wavelength of 1064 nm, a repetition rate of 20 kHz, and a pulse duration of 10 ns.In order to generate microtextures on the rake face of the tools, the cutting insert was fixed on a computer-controlled translation table, and the laser beam was focused on the rake face perpendicularly and scanned to generate microgrooves.After the laser micro-machining, ultrasonic cleaning was applied to clean the inserts.Following the laser micro-machining, ultrasonic cleaning was performed to ensure the cleanliness of the inserts.In this research, a Fiber laser with a maximum power output of 30 W was utilized for the purpose of texturing the tools.Figure 1 provides a visual representation of the tools in both their plain and textured states, as observed through SEM images.The dimensions of the textures were determined based on a thorough review of relevant literature sources 22,23 , as well as a series of preliminary experiments.Previous research conducted by other scientists has demonstrated that micro-grooves with depths ranging from 10 to 100 μm, widths between 20 and 50 μm, and spacing of 100-300 μm have yielded favorable outcomes in terms of cutting performance.Hence, the selected dimensions for the microgrooves were as follows: a width of 50 μm, a spacing of 150 μm, and a depth of 10 μm.The tool configurations used in this study included linear textures perpendicular to the chip flow direction, linear textures parallel to the chip flow direction, circular textures, and linear cross-hatch textures, referred to as T-Pe, T-Pa, T-Ch, and T-C, respectively.The non-textured plane tool was denoted as T0. Figure 2 provides a depiction of the cross-section profile of a single microgroove generated on the rake face of the carbide insert.The tool with linear textures perpendicular to the chip flow direction, linear textures parallel to the chip flow direction, circular textures, and linear cross-hatch textures were nominated T-Pe, T-Pa, T-Ch, and T-C, respectively, while, the non-textured plane tool was named T0.Figure 2 shows the cross-section profile of an individual microgroove created on the rake face of the carbide insert.
The turning experiments were carried out using a TB50NR Lathe equipped with cemented carbide CNMA120408 inserts.Workpieces made of cold-rolled Al 7075 alloy bars with a diameter of 30 mm and a length of 450 mm were employed.The cutting tests were conducted at three cutting speeds (V c ), 33 m/min, 47 m/min, and 66 m/min, with a fixed depth of cut (a p ) of 0.75 mm and a feed rate (a f ) of 0.14 mm/rev.To measure cutting Figure 1.SEM images of microtextures generated at the rake face of carbide tools.forces, a KISTLER Company turning dynamometer 9272-type with three components was employed.A piezoelectric dynamometer was mounted on the tool post and a tool holder was attached to it.As the cutting operation took place, mechanical forces exerted on the cutting tool were converted into electrical signals by the piezoelectric sensor and then transferred to the charge amplifier.After amplification, the signals were transmitted to the data acquisition system and subsequently transferred to a PC for analysis using the Dyno-Ware software.In every experiment, the PCE-RT Roughness Tester was utilized to measure the surface roughness (Ra).To accomplish this, three specific areas on the machined surface were selected for measurement.The measurements were taken in these regions, and the average of the three measurements was recorded as the Ra value.The tracing velocity and the sampling length were consistently maintained at 0.5 mm/s and 0.8 mm, respectively.The height of the built-up edge was measured using an optical Dino-Lite AM-413ZT microscope with DinoCapture 2.0 software.Tool images were captured, and measurements were taken using calibration tools for precise evaluation of the built-up edge size.SEM analysis was employed to examine the texture patterns on the rake face.The experimental setup for the cutting tests is depicted in Fig. 3.
To create the nanofluid, the process involved introducing CNT particles into an emulsion-type cutting fluid (AMIX: combined ratio with water of 5%).The concentration of CNT particles in the base fluid was set to 1% and 3% based on previous studies on nanofluid applications in machining processes 26,29,[33][34][35][36] .Nanofluids were prepared by dispersing a specific amount of CNTs in the base fluid by using an ultrasonic processor for 6 h generating pulses of 400 W at 24 kHz.This method guarantees the stability of the nanofluid for 24 h 37 .Table 1 provides an overview of the properties of the nanoparticles.www.nature.com/scientificreports/ was observed that as the cutting speed increased, the main cutting force decreased.This phenomenon can be attributed to the thermal softening of the material at higher cutting speeds, resulting in a reduction in shear strength within the shear zone 38 .Additionally, the shear angle tends to increase with cutting speed.Consequently, the main cutting force can be effectively reduced by increasing the cutting speed during the cutting process, as further discussed below:

Textured tools
The total cutting force, denoted as F R , is determined during the turning process using Eq. ( 1) 39,40 : In this equation, A s represents the shear plane area, A c denotes uncut chip cross-section area, τ s signifies the shear strength of the workpiece material, and φ represents the shear angle.
The relationship between the main cutting force, denoted as F c , in the turning process and the total cutting force F R can be expressed by the Eq. ( 2) 39,40 : where β is the friction angle, and α is the rake angle.By substituting Eq. (1) into Eq.( 2), we obtain the main cutting force relation expressed in Eq. (3) 39,40 : According to Eq. ( 3), a decrease in shear strength (τ s ) and an increase in shear angle (φ) result in a reduction in the main cutting force.
As depicted in Fig. 4, the utilization of textured tools led to a slight reduction in the main cutting force.Among the various textured tools, the T-Pe tool with linear micro-grooves perpendicular to the chip flow direction exhibited the lowest cutting force.The results indicate that, on average, the main cutting force of the T-Pe tool was reduced by 7%, 10%, and 14% at cutting speeds of 33 m/min, 47 m/min, and 66 m/min, respectively, compared to the non-textured tool.The performance of the T-Pe tool in reducing cutting forces was observed to be superior to that of the T-Pa, T-CH, and T-C tools.This can be attributed to the fact that in the T-Pa, T-CH, and T-C tools, there was a higher degree of plastic deformation in the chip material compared to the T-Pe tool, which led to greater adhesion of the work material on the rake face, resulting in higher cutting forces.
Furthermore, the micro-grooves positioned perpendicular to the chip flow direction serve as chip breakers, facilitating the chip's swift and effortless detachment from the rake face.Consequently, the tool-chip contact area is diminished.According to Eq. ( 4), a decrease in the contact area results in a reduction of the frictional force, thereby leading to a decrease in the total cutting force.As a result, the micro-grooves oriented perpendicular to the chip flow direction exhibited a smaller tool-chip contact area compared to other tools, resulting in a greater reduction in cutting force.
The decrease in the main cutting force when using micro-textured tools can be elucidated as follows: The frictional force between the chip and the rake face during the turning process can be described by Eqs. ( 4) and ( 5) 39,40 : In Eq. ( 4), A w represents the contact area between the tool and chip, τ c denotes the shear strength of the toolchip interface, l represents the contact length between the tool and chip, and a w represents the chip width.As illustrated in Fig. 5, the effective contact length of the tool chip can be determined as follows 19,39 : In the Eq. ( 6), the contact length is represented by l, the effective contact length is denoted as l e , w g signifies the width of the microgrooves, p g represents the spacing of the microgrooves, and n indicates the number of grooves in the contact area.According to Eq. ( 6), the effective contact length decreases with the generation of microgrooves on the rake face.Consequently, the contact area (A w ) and friction force (F f ) decrease.On the other hand, there is a correlation between the main cutting force and the friction force, which can be expressed as follows 39,40 : Therefore, it can be inferred that the main cutting force, F c , decreases when the friction force, F f , is reduced.
In the case of the cross-hatch pattern, significant chip deformation occurs as the chip traverses the rake face of the tool.This leads to the bending of the chip towards the micro-grooves.Consequently, the actual contact area between the rake face and chip back increases, diminishing the effectiveness of micro-texturing in reducing forces.
In Fig. 6, the surface roughness (Ra) of workpieces machined using various textured and non-textured tools is presented.The chart demonstrates that the surface roughness of the machined workpieces improves as the cutting speed increases.This improvement can be attributed to the reduction in built-up edge size and the increased stability of the machining conditions at higher cutting speeds.As a result, the surface finish is enhanced 40 .( 6)  The results indicate that the fabrication of microtextures on the rake face of cemented carbide tools does not have a substantial impact on the surface roughness.
Figure 7 illustrates the size of the built-up edge (BUE) for different tools.It is evident that the textured tools exhibit a lower BUE compared to the non-textured tools.The height of the BUE for the non-textured tool (T0) was measured as 567 μm, whereas it was reduced to 460 μm, 363 μm, 326 μm, and 284 μm for the T-C, T-Pa, T-Ch, and T-Pe tools, respectively.This reduction corresponds to a decrease of 19%, 36%, 43%, and 50%, respectively.Therefore, the use of micro-textured tools results in a reduction in the adhesion of work material on the rake face.As discussed earlier, surface texturing of the rake face reduces the friction force between the tool and chip, leading to decreased heat generation and subsequently minimizing the adhesion of work material on the rake face.
The performance of the cutting process was enhanced by using the T-Pe tool with linear microgrooves arranged perpendicular to the direction of chip flow, as indicated by the outcomes of dry turning tests comparing various textured tools and a conventional tool.To assess the impact of nanofluid lubrication on cutting performance, experimental turning tests were conducted using the chosen tool while employing nanofluid lubrication.The subsequent results are provided below.

Nanofluid effect
Figure 8 illustrates the impact of using CNT-enriched nanofluid lubrication on the main cutting force.The experimental results revealed that compared to dry cutting with the T-Pe textured tool, the main cutting force decreased by up to 21% and 32% when utilizing 1% and 3% CNT nanofluid, respectively.Consequently, an increase in the concentration of nanoparticles improved the lubrication capability of the nanofluid.Figure 9 schematically demonstrates that carbon nanotubes dispersed in the nanofluid penetrate between the tool and chip, functioning as nano-bearings, as discussed in several articles [41][42][43][44] .This alteration in the relative motion between the tool and chip, from sliding to rolling, accounts for the reduction in friction and cutting force.In essence, the decrease in friction and cutting force can be attributed to the nano-bearing effect, which is based on the rolling motion of carbon nanotubes.
Figure 10 displays the surface roughness (Ra) of machined workpieces using T-Pe textured tools under various lubrication conditions.As previously mentioned, surface texturing on the rake face had an insignificant impact on surface finish.However, when the T-Pe textured tool was utilized with CNT-enriched nanofluid lubrication,  www.nature.com/scientificreports/an improvement in surface finish was observed.The chart illustrates that Ra was enhanced by 15% and 19% when employing 1% and 3% concentration nanofluids, respectively, compared to dry machining with the T-Pe textured tool.This improvement can be attributed to the stable cutting conditions achieved during the turning process with the T-Pe tool under nanofluid lubrication.Figure 11 exhibits dynamic force profiles for different tools.It is evident from the figure that the fluctuation of cutting force was reduced when using the T-Pe tool with CNT nanofluid lubrication compared to dry conditions.This reduction in fluctuation resulted in a more stable cutting environment, leading to a superior surface finish.Figure 12 presents the impact of surface texturing and nanofluid lubrication on the size of the built-up edge (BUE).The results demonstrate a significant reduction in BUE size when CNT-enriched nanofluid coolant is used during the turning process with the T-Pe textured cutting tool.As previously mentioned, the addition of CNT nanoparticles to the base coolant enhances the tribological performance of mating surfaces.Consequently, the friction coefficient between the chip and tool decreases, leading to a reduction in friction force at the rake face.This decrease in friction force results in reduced heat generation, which helps mitigate the adhesion of work material to the rake face.
The experimental findings indicate that increasing the nanoparticle concentration from 1 to 3% leads to a greater decrease in BUE size compared to dry cutting with the T-Pe textured tool.Specifically, the decrease in BUE size increased from 22 to 37%.This can be attributed to the influence of nanoparticle concentration on the thermal characteristics of nanofluids.The thermal conductivity (k) and convection coefficient (h) of nanofluids increases with higher nanoparticle concentrations 33,34 .Hence, additional heat can be effectively transferred away from the cutting zone, thereby reducing adhesive wear.
The main limitation of the study is the current delivery method of the nanofluid to the cutting zone, which is in the splash mode.We acknowledge that better results could be achieved by designing a more effective nozzle that ensures the efficient delivery of the fluid directly to the cutting area.

Conclusion
In conclusion, this study aimed to investigate the impact of surface texturing and the application of CNT-enriched nanofluid lubrication on the cutting performance of cemented carbide cutting inserts during the turning of Al 7075 alloy.The results clearly demonstrated that the use of CNT-enriched nanofluid has significant potential in reducing cutting force and surface roughness and minimizing the size of the built-up edge.Specifically, when comparing the non-textured tool to the application of the T-Pe tool under dry cutting conditions, a decrease of up to 14% in the average main cutting force was observed.Among the investigated textures, the T-Pe texture exhibited superior performance in terms of surface roughness.The implementation of surface textures led to a reduction in friction force, resulting in a decreased height of the built-up edge (BUE).In the absence of lubrication, the height of the BUE was reduced by 19%, 36%, 43%, and 50% for the T-C, T-Pa, T-Ch, and T-Pe tools respectively, in comparison to the non-textured tool.
Furthermore, the experimental findings using the T-Pe textured tool revealed that increasing the concentration of CNT nanoparticles in the base cutting fluid within the range of 1-3% resulted in significant reductions in the main cutting force (Fc) by approximately 21% and 32%, reductions in built-up edge (BUE) sizes of approximately 22% and 37%, and surface roughness reductions of approximately 15% and 19% respectively.
In summary, the findings of this study demonstrate that the creation of appropriate micro textures on the rake face of the turning tool can significantly enhance the machining performance of Al 7075 alloy.Moreover, the use of CNT nanofluid further enhances the performance of the turning process by reducing cutting force, minimizing the size of the built-up edge, and improving surface roughness.These results highlight the potential of surface texturing and the application of CNT-enriched nanofluid lubrication as effective strategies for optimizing cutting processes and achieving improved machining outcomes.
For future studies, it would be beneficial to investigate the online application of ultrasonic probe homogenizer during the turning process to enhance the stability of the nanofluids.This could provide more control and consistency in the lubrication process, leading to further improvements in cutting performance.

Figure 4
Figure 4 illustrates the main cutting force observed at various cutting speeds for both non-textured and textured tools.Each bar in the chart represents the average cutting force measured during the turning of Al 7075 alloy.The chart clearly demonstrates that cutting speed exerts a significant influence on the main cutting force.It

Figure 2 .
Figure 2. The cross-sectional profile of a micro-groove.

Figure 3 .
Figure 3.The configuration used for conducting the cutting tests.

Figure 4 .
Figure 4. Main cutting force for five types of cutting tools at different cutting speeds.

Figure 5 .
Figure 5. Schematic of tool-chip contact length for a textured tool (l e = l−n.wg = n.pg ).

Figure 6 .
Figure 6.Surface roughness (Ra) for textured and non-textured tools at different cutting speeds.

Figure 7 .
Figure 7. Built up edge height for different tools.

Figure 8 .Figure 9 .
Figure 8.The main cutting force with the cutting speed for different lubrication conditions.

Figure 10 .
Figure 10.Surface roughness of machined workpiece under different lubricating conditions.

Figure 11 .Figure 12 .
Figure 11.Dynamic main cutting force for different tools at the cutting speed of 66 m/min.