3.1. Textured tools
3.1.1. Main cutting force
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 significant 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 FR is obtained from Eq. (1):
\({F}_{R}={A}_{w}{\tau }_{c}=\frac{{a}_{f}}{\text{sin}\phi }{\tau }_{w}\) Eq (1)
where Ac is the area of the shear plane, τw is the shear strength of the workpiece material, af is the uncut chip thickness, and φ is the shear angle.
Main cutting force Fc in the turning process are related to total cutting force as the following equation:
\({F}_{c}={F}_{R}\text{sin}\left(\beta -\alpha \right)\) Eq (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):
\({F}_{c}=\frac{{a}_{f}}{\text{sin}\phi }{\tau }_{w}\text{sin}\left(\beta -\alpha \right)\) Eq (3)
Thus, according to Eq (3) decreasing in shear strength (τw) 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 flow 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 flow 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):
\({F}_{f}={A}_{w}{\tau }_{c}\) Eq (4)
\({A}_{w}=l{a}_{w}\) Eq (5)
where Aw is the tool and chip contact area, τc 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:
\({l}_{e}=l-n{w}_{g}\approx n{p}_{g}\) Eq (6)
where l denotes the contact length, le denotes the effective contact length, wg is the width of microgrooves, pg 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 Aw and friction force Ff decreases. On the other hand, the main cutting force is related to the friction force as follows:
\({F}_{f}={F}_{R}\text{sin}\left(\beta \right)\) Eq (7)
\({F}_{c}={F}_{R}\text{cos}\left(\beta -\alpha \right)\) Eq (8)
\({F}_{c}={F}_{f}\frac{\text{cos}\left(\beta -\alpha \right)}{\text{sin}\left(\beta \right)}\) Eq (9)
Thus, it can be deduced that the main cutting force Fc reduces by decreasing friction force Ff.
3.1.2. Surface Roughness
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 finish improves [27].
Results show that the fabrication of the microtextures on the rake face of the cemented carbide tools has no significant effect on the surface roughness, while T-Pe has better performance in comparison with other textured tools.
3.1.3. Built-up edge
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 figure, the height of BUE for the non-textured 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.
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 flow direction improved performance of cutting process. In order to evaluate the effect of nanofluid lubrication on the performance of the cutting process, experimental turning tests were performed with the selected tool under nanofluid lubrication, and the results are presented below.
3.2. NANOFLUID EFFECT
3.2.1. Main cutting force
The effect of CNT enriched nanofluid lubrication on the main cutting force is presented in Figure 8. The results of experiments showed that the Fc was decreased up to 21% and 32% by using 1% and 3%, CNT nanofluid, respectively, compared to dry cutting with the T-Pe textured tool. Therefore, an increase in the nanoparticle concentration enhanced nanofluid lubrication capability. As shown schematically in Figure 9, carbon nanotubes that are dispersed in nanofluid, 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.
3.2.2. 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 significant effect on the surface finish, but it was improved using the T-Pe textured tool under CNT enriched nanofluid lubrication condition. As presented in this chart, Ra was improved 15% and 19% by using 1% and 3% concentration nanofluids 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 nanofluid lubrication. Dynamic force plots for different tools are shown in Figure 11. As shown in this figure, the fluctuation of cutting force in the T-Pe tool with CNT nanofluid lubrication was smaller than in dry condition, and this led to a more stable cutting condition, hence better surface finish was obtained.
3.2.3. Built-up edge
The effect of surface texturing and nanofluid lubrication on the built-up edge size is shown in Figure 12. Results revealed that the significant reduction in BUE size was observed by using CNT enriched nanofluid 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 coefficient 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 nanofluids. Thermal conductivity (k) and convection coefficient (h) of nanofluids increase with increasing nanoparticles concentration [28,29], thus, extra heat can be moved from the cutting zone, therefore adhesive wear reduces.