Anti-adhesive performance of CNT enriched nanouid lubrication during turning of Aluminum 7075 alloy using textured tools

This paper presents the effect of surface texturing and CNT enriched nanouid lubrication on the cutting performance of cemented carbide cutting inserts during the turning of Aluminum 7075 alloy. This alloy is widely used in various industries due to its excellent properties such as high corrosion resistance, high strength to weight ratio, and good formability. Although aluminum alloys have good cutting properties because of their low strength, they adhere between tool and chip on the rake face in machining processes, and this results in the low surface nish of machined components. To solve this problem, the use of textured tools under nanouid lubrication is proposed in this study. Firstly, different shape of textures was fabricated on the rake face, and effective texture was determined using experimental tests. Then, turning tests were carried out using the selected textured tool under CNT enriched nanouid lubrication to enhance the cutting performance of the machining process. Results showed that the linear microgrooves perpendicular to chip ow direction have better performance in comparison to other shapes of texture. Our ndings revealed that the main cutting force, built-up edge, and surface nish were reduced up to 32%, 37%, and 19% using the selected textured tool under CNT enriched nanouid lubrication compared to dry turning condition.


Introduction
Aluminum alloys are widely used in the aviation, automobile, marine, and machinery manufacturing industries [1] due to their excellent properties such as high corrosion resistance, high strength and stiffness to weight ratio, high electrical and heat conductivity and good formability [2]. Although aluminum alloys have good cutting properties because of their low strength, they adhere between tool and chip on the rake face in machining processes [3]. Cutting uids are conventionally used to lessen friction and adhesion at the cutting zone in the machining process; conversely, cutting uids are harmful to the machine operators, ruin the machine tool rails, and pollute the environment. For these reasons, there is a need to eliminate or minimize cutting uids and shift towards dry machining to have an environment-friendly machining process. Several sustainable methods in manufacturing have been adopted for green and cleaner production to lighten the drawbacks of dry machining [4].
Surface texturing of cutting tools is recognized as a strategy for enhancing dry machining applications [12]. This strategy is used to enhance the tribological conditions of the mating surfaces [13]. Surface textured cutting tool helps to reduce friction coe cient through increasing lubrication capacity and decreasing tool-chip contact length. Different types of textures have been created on the cutting tool surfaces and successfully used in turning [14], milling [15], drilling [16], and thread turning [17,18] processes by researchers. and cutting temperature [9,10].
Although several studies have been conducted to investigate the effect of surface texturing on the cutting performance of cemented carbide inserts, a few studies have examined the coupling effect of nano uid lubrication and surface texturing on the cutting performance of turning tools, and further research in this eld is required. Therefore, to further improve the cutting performance of turning tools, the turning of Aluminum 7075 alloy with micro-textured tools under CNT enriched nano uid lubrication was proposed in this study. To achieve this, four types of textures with linear and circular arrays were engraved on the rake face of the carbide cutting inserts using laser micromachining. Turning tests were carried out on aluminum 7075 bars with the non-textured, textured tools, and optimum texture was determined. Then, comparative turning tests were carried out using the selected textured tool under dry and different concentrations of CNT enriched nano uid lubrication.

Methods
The cutting tools in the experimental tests were cemented carbide CNMAl20408 inserts. An Nd: YAG laser (Jinan Xinchu laser Inc.) with a wavelength of 1064 nm, repetition rate of 20 kHz, pulse duration of 10 nSec was used for the fabrication of microtextures on the rake face of the tools. Figure 1 illustrates the SEM images of the plane and textured tools. The width, distance, and depth of the microgrooves are 50 µm, 150 µm, and 10 µm, respectively. The tool with linear textures perpendicular to chip ow direction, linear textures parallel to the chip ow direction, circular textures, and linear crosshatched 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 pro le of an individual microgroove created on the rake face of the carbide insert.
Turning tests were conducted on the TB50NR Lathe equipped with cemented carbide CNMA120408 inserts and a Kennametal MCGNR164C tool holder. Cold rolled Al 7075 alloy bars with a diameter of 30 mm and length of 450 mm were used as workpieces. In turning tests cutting speed (V c ) considered 33, 47, and 66 m/min, depth of cut (a p ) xed to 0.75 mm, and feed rate (a f ) xed to 0.14 mm/rev. Cutting force was measured using a Kistler 9272 type piezoelectric quartz dynamometer. The surface roughness (Ra) of machined workpieces was measured with a PCE RT 2200 portable pro lometer. The built-up edge (BUE) size was examined by utilizing a Meiji MT4000 optical microscope. Texture patterns on the rake face were examined by SEM. The experimental setup of the cutting tests was shown in Figure 3.
In order to prepare nano uid, rstly CNT particles were added to conventional water-based coolant. Then an ultrasonic stirrer was used to disperse and homogenize the mixture of nanoparticles and base uid.
The properties of nanoparticles are presented in Table 1.  Figure 4 shows the main cutting force at different cutting speeds for non-textured tool and different textured tools. In this chart, each bar represents the mean cutting force measured during the turning process of Al 7050 alloy. The chart shows that the cutting speed has a signi cant effect on the main cutting force. It was found that the main cutting force decreased with increasing cutting speed. This is related to the thermal softening of material at the high level of cutting speed which leads to a drop in shear strength in the shear zone [26]. In addition, shear angle rise with cutting speed. The main cutting force could be reduced with the increasing cutting speed in the cutting process as discussed below: During the turning process total cutting force F R is obtained from Eq. (1): (1) where A c is the area of the shear plane, τ w is the shear strength of the workpiece material, a f is the uncut chip thickness, and φ is the shear angle.
Main cutting force F c in the turning process are related to total cutting force as the following equation: (2) where β is the friction angle, and α is the rake angle. Substituting Eq (1) in Eq (2) results main cutting force relation Eq (3): (3) Thus, according to Eq (3) decreasing in shear strength (τ w ) and increasing in shear angle (φ) leads to a decrease in main cutting force.
As revealed in Figure 4, textured tools slightly reduced the main cutting force and the T-Pe with the linear micro-grooves perpendicular to chip ow direction had the lowest cutting force in comparison to the other textured tools. The results show that the average main cutting force of T-Pe at the speed of 33, 47, and 66 m/min reduced 10, 7, and 14% compared with the non-textured tool. It was found that the performance of T-Pe in force reduction was better than T-Pa, T-CH, and T-C. This can be related to the more plastic deformation of chip material in parallel, cross-hatch, and circular micro-grooves, which cause more adhesion and thus higher cutting force. While micro-grooves perpendicular to the chip ow direction reduces the more contact area and therefore leads to a reduction in cutting force.
The reduction of main cutting force during using of micro-textured tools can be explained as follows: The friction force between chip and rake face during turning process is according to Eq (4): (4) A w = la w Eq (5) where A w is the tool and chip contact area, τ c is the shear strength of tool and chip interface, l is the tool and chip contact length, and aw is the chip width. As shown in Figure 5, the contact length of tool and chip is equal to: l e = l − nw g ≈ np g Eq (6) where l denotes the contact length, l e denotes the effective contact length, w g is the width of microgrooves, p g is the distance of microgrooves, and n denotes the number of grooves in the contact area. According to Eq (6), effective contact length reduces by generating microgrooves on the rake face, therefore, contact area A w and friction force F f decreases. On the other hand, the main cutting force is related to the friction force as follows: Eq (9) Thus, it can be deduced that the main cutting force F c reduces by decreasing friction force F f . Figure 6 shows the surface roughness Ra for four types of textured tools and non-textured tool. As revealed in this chart, the surface roughness of machined parts improved with increasing cutting speed. This is due to the fact that the built-up edge size decreases with increasing cutting speed and the machining condition becomes more stable, hence, the surface nish improves [27].

Surface Roughness
Results show that the fabrication of the microtextures on the rake face of the cemented carbide tools has no signi cant effect on the surface roughness, while T-Pe has better performance in comparison with other textured tools. Figure 7 demonstrates the size of the built-up edge for different tools. It was evident that the BUE for textured tools is lower than the non-textured tool. As shown in this gure, the height of BUE for the nontextured tool (T0) was 567 µm, while it was 460, 363, 326, and 284 µm for T-C, T-Pa, T-Ch, and T-Pe tools, respectively, which was reduced by 19, 36, 43, and 50%, respectively, Therefore, adhesion of work material on the rake face reduced using micro-textured tools. As discussed above, the friction force between chip and tool reduces by surface texturing of rake face, and therefore, heat generation decreases, and this results in low adhesion of work material on the rake face.

Built-up edge
The results of dry turning tests with different textured tools and traditional tool showed that the T-Pe tool with linear microgroove perpendicular to chip ow direction improved performance of cutting process. In order to evaluate the effect of nano uid lubrication on the performance of the cutting process, experimental turning tests were performed with the selected tool under nano uid lubrication, and the results are presented below.

Main cutting force
The effect of CNT enriched nano uid lubrication on the main cutting force is presented in Figure 8. The results of experiments showed that the F c was decreased up to 21% and 32% by using 1% and 3%, CNT nano uid, respectively, compared to dry cutting with the T-Pe textured tool. Therefore, an increase in the nanoparticle concentration enhanced nano uid lubrication capability. As shown schematically in Figure  9, carbon nanotubes that are dispersed in nano uid, penetrate into the chip and tool contact area and can act as nano-bearings, hence, relative motion between chip and tool approaches from slipping to rolling. Indeed, reduction in friction and cutting force can be attributed to the nano-bearing effect that is based on the roiling of carbon nanotubes.

Surface Roughness
The surface roughness Ra of machined workpieces with T-Pe textured tools under different lubrication conditions is shown in Figure 10. As mentioned previously, surface texturing of the rake face has no signi cant effect on the surface nish, but it was improved using the T-Pe textured tool under CNT enriched nano uid lubrication condition. As presented in this chart, Ra was improved 15% and 19% by using 1% and 3% concentration nano uids in comparison to dry machining with the T-Pe textured tool. It can be related to the stable cutting condition during the turning process with T-Pe under nano uid lubrication. Dynamic force plots for different tools are shown in Figure 11. As shown in this gure, the uctuation of cutting force in the T-Pe tool with CNT nano uid lubrication was smaller than in dry condition, and this led to a more stable cutting condition, hence better surface nish was obtained.

Built-up edge
The effect of surface texturing and nano uid lubrication on the built-up edge size is shown in Figure 12.
Results revealed that the signi cant reduction in BUE size was observed by using CNT enriched nano uid coolant during the turning process with T-Pe textured cutting tool. As mentioned previously, adding CNT nanoparticles to base coolant improves the tribological performance of mating surfaces, hence, the friction coe cient between chip and tool decreases and, in turn, friction force at the rake face. Therefore, by decreasing friction force, heat generation reduces and this can alleviate the adhesion of work material on the rake face. It was shown that by increasing the nanoparticle concentration from 1-3%, compared to dry cutting with the textured tool T-Pe, the decrease in BUE size increased from 22-37%. This can be attributed to the fact that nanoparticle concentration affects the thermal characteristics of nano uids.
Thermal conductivity (k) and convection coe cient (h) of nano uids increase with increasing nanoparticles concentration [28,29], thus, extra heat can be moved from the cutting zone, therefore adhesive wear reduces.

Conclusion
In this study, the in uence of surface texturing and CNT enriched nano uid lubrication on the cutting performance of cemented carbide cutting inserts during the turning of Al 7075 alloy was investigated using experimental tests. According to ndings, CNT enriched nano uid has the capability of reducing cutting force and built-up edge size. Based on the results of this study, the following conclusions could be drawn.
The T-Pe tool with the linear microgrooves perpendicular to the chip ow direction had the lowest cutting force in comparison to the other textured tools and non-textured tools. The average main cutting force reduced up to 14% compared with the non-textured tool using the T-Pe tool under dry cutting condition.
Results show that the fabrication of the microtextures on the rake face of the cemented carbide tools has no signi cant effect on the surface roughness, while T-Pe has better performance in comparison with other textured tools.
Height of BUE for T-C, T-Pa, T-Ch, and T-Pe tools reduced 19, 36, 43, and 50%, respectively, in comparison to the non-textured tool under dry cutting condition.
An increase in the nanoparticle concentration enhanced nano uid lubrication capability; by increasing the concentration of CNT nanoparticles in the base cutting uid from 1-3%, Fc reduced 21% and 32% as compared to dry cutting with the textured tool T-Pe.
Increasing the nanoparticle concentration from 1-3%, decrease in BUE size increased from 22-37%, as compared to the dry cutting condition with T-Pe textured tool.
Surface nish was improved up to 20% and 30% by using 1% and 3% concentration CNT nano uids. This is related to the stable cutting condition during the turning process with T-Pe under nano uid lubrication.

Data availability
All data generated or analyzed during this study are included in this published article.   Schematic of chip tool contact length for textured tool Figure 6 Surface roughness for ve type of cutting tools at different cutting speeds Main cutting force with the cutting speed for different lubrication condition Figure 9 Page 14/14 Nano-bearing effect of carbon nanotubes BUE size for different lubrication condition