Experimental Evaluation of the Lubrication Performance of Milling Al7075 with Different Nano-fluids MQL

The addition of nano-fluids with different nano-particles into the biodegradable vegetable oil base has an effect on the wetting contact angle of Al7075, the cutting force and the lubrication performance, and the surface morphology of the workpiece and chip. In Milling Al7075, the lubrication performance of six different nano-fluids was evaluated. In this paper, the contact angle, cutting force and SEM morphology of aircraft Al7075 were evaluated under the lubrication conditions of Al2O3, MoS2, SiO2, CNTs, SiC and Graphite. The experimental results show that the cutting force in x direction of milling under so 2 nano-fluid lubrication is obviously smaller than that under so 2 nano-fluid lubrication. The cutting force in x direction under Al2O3 nano-fluid lubrication is 109.6% larger than that under so 2 nano-fluid lubrication, the cutting force of other nano-fluids in x direction is 2%-5% higher than that of Al2O3 nano-fluids in x direction. The rolling of SO2 spherical micro-particles can reduce the wear and the cutting force, improve the surface quality. The results show that the SO2 nano-fluid has good lubrication and wear-reducing performance.

The tool wear is very fast when milling high strength and high toughness aeronautical aluminum alloy whose tensile strength is over 600MPA, so it is necessary to study the lubrication and wear reduction of milling high strength aluminum alloy. The nano-fluid is sprayed into the cutting zone after atomization to lubricate and reduce friction [1],Reducing Lubrication deficiency in milling high strength aluminum alloy and Solving the problem of tool failure too fast [2,3].
The results of high speed milling of Al7050 by Aixing et al show that the main effective mechanisms are adhesive wear, diffusion wear and oxidation wear [4] .He Ning, Li Liang, Su Yu [5,6] et al mainly conducted research on systematic low-temperature MQLM of aviation materials, and concluded that MQL effectively improves the frictional contact state between the tool and the workpiece. Malaysian scholar Sayuti organized [7,15,16] to study the performance parameters of SiO 2 NMQLM Al6061-T6, and found that the surface roughness is the smallest when the air pressure  [8,14] used MoS 2 NMQLM aviation Al6061-T6, and the results showed that the surface quality of the workpiece was optimal when the mass fraction of Mos 2 nano-particles was 0.5%. Professor Li Changhe et al. [9][10][11][12][13] studied the MQL cutting mechanism, and the results showed that nano-particle MQL improved the heat transfer capacity and the surface quality of the workpiece was significantly improved. In this thesis, the aviation Al7075 material is milled processing using six different nano-particles and cottonseed oil MQL methods, testing different nano-fluid infiltration contact angles, measuring and analyzing the x, y, and z direction forces of the cutting process, scanning the surface of the workpiece and the surface of the chip by electron microscopy to conduct surface analysis, and comparatively discuss and analyze the lubrication performance of Al7075 milling under different NMQL.

Experimental Equipment
Experimental milling machining center ML1060B, MQL system KS-2106 and experimental measurement equipment (force gauge JR-YDCL-05B, contact angle meter JGW-360A, scanning electron microscope, stylus surface roughness meter) are shown in Figure 1.

Experimental Plan
In the experiment, six kinds of nanoparticles and cottonseed oil were used to mill aviation Al7075, with an average particle size of 70 nm, AL 2 O 3 , MOS 2 , SiO 2 , CNTs, SiC, and Graphite, with a 0.5% mass fraction ratio of cottonseed oil. For nano-fluid, the experimental plan of milling processing is shown in Table 2. 0.4 To verify the lubrication performance of different NMQL under the same milling parameter mode to process aviation Al7075, the experimental data is analyzed from the surface quality and integrity.

Analysis of the Influence of Different Nano-Fluids on the Contact Angle of Al7075
Milling processing lubricating fluid affects the wettability of aviation Al7075 contact angle. Different nano-fluid droplets drop on the surface of aluminum alloy, due to the difference of the surface tension of the droplets, and the wettability characteristics of the solid surface are different, adhere to the solid surface to form different convex lens-like adhesion and wetting. The angle between the solid interface and the liquid interface is the contact angle. As shown in Figure 2, the smaller the contact angle, the liquid is easier to wet the solid, and the surface of the wetted part is easy to form an oil film. The three-phase interfacial tension satisfies the Young equation:   Figure 2. Contact angle measurement model The comparison results of multiple sets of measurement of the contact angle of different nano-fluids on the aluminum alloy processing surface show that the addition of SiO 2 nano-fluid has the smallest contact angle on the aluminum alloy surface compared with other nano-fluids (Figure 3), which shows that the SiO 2 nano-particle lubricant has better wettability, and is conducive to the formation of oil film and improve lubricity.

Analysis of Milling Force under Different NMQL Milling Al7075
Under the same processing parameters (as shown in Table 3), the changes in the cutting force of milling aluminum alloys reflect the pros and cons of different nano-fluid lubricants. The effect of lubrication affects the wear of cutting tools and the quality of the processed surface. The milling processing model is established as shown in Figure 4, using the feed along the x-axis direction and the tool on the y-axis direction.    Figure 6. The force in the x, y, and z directions is compared. The cutting force in the x direction is much greater than the y, z, which has relatively small difference in cutting force under different nano-fluid lubrication. Therefore, we mainly analyze the force in x direction. Milling under SO 2 nano-fluid lubrication is significantly smaller in x direction cutting force. The x-direction cutting force under Al 2 O 3 nano-fluid lubrication is increased by 109.6% compared with SO 2 nano-fluid lubrication, and the x-direction cutting force of other nano-fluids was increased by 2%-5% compared with that of Al 2 O 3 nano-fluid. The reason is analyzed. The nano-fluid lubricating fluid is added with SO 2 nano-particles, and its microscopic shape is spherical, which makes it transform the chip removal from sliding mode to rolling mode during cutting, further increasing lubricity and reducing friction. The AI element in Al 2 O 3 nano-fluid has a certain affinity with aluminum alloy, and the rolling effect is not obvious.

Analysis of Milling Surface and Chip Surface Morphology of Milling Al7075 under Different Nano-Fluid Lubrication Methods
The milling surface of Al7075 is milled under different nano-fluid lubrication methods. The different lubrication methods have different effects on the surface quality of the workpiece. The surface quality of the workpiece is analyzed by scanning the electron microscope to observe the workpiece surface. The morphology of Al7075 was observed by scanning electron microscopy to analyze the influence of cutting force by milling Al7075 under different lubrication methods. As shown in Figure 7, the cutting surface of Al7075 was milled under different nano-fluid lubrication methods. SEM (×500), Graphite NMQL milling can form larger tool marks. SiC, MOS 2 , CNTs (carbon nano-tubes) NMQL milling aluminum alloys are prone to form harder built-up tumors, which have large scratches on the surface of the workpiece, but the SiO 2 , AL 2 O 3 nano-fluid lubrication has rather small tool mark, which also shows that the nano-fluid has better lubricity with small cutting force.  Figure 7. SEM of cutting surface of milling Al7075 under different NMQL methods (×500) When the cutting force is large, the milling chips will change sharply or tear. As shown in Figure 8, milling Al7075 chips under different nano-particle lubrication methods SEM (×100), Graphite, MOS 2 , CNTs, NMQL milling Al7075, the cutting force is relatively large, the two sides of the generated chips are not uniformly torn, and one side is severely torn. SiO 2 , Al 2 O 3 , SiC NMQL milling Al7075 produces symmetrical tears on both sides of the chip, and the surface of the aluminum alloy chip with Al 2 O 3 NMQL milling has ripple changes.

Conclusion
Through six different NMQL methods, milling, analysis and comparison, and experimental research on Al7075 specimens have concluded as follows: (1) The order of the contact angle: Graphite> MOS 2 > AL 2 O 3 > SiC> CNTs> SiO 2 , the size of the infiltration contact angle affects the affinity of aluminum alloy, the area of lubricating workpiece, and the effect of lubricating speed.
(2) The order of the main milling force F(x): MOS 2 >CNTs >SiC >Graphite >AL 2 O 3 >SiO 2 , NMQL milling Al7075, the x-direction cutting force under SO 2 nano-fluid lubrication is significantly smaller. The x-direction cutting force under AL 2 O 3 nano-fluid lubrication is increased by 109.6% compared with SO 2 nano-fluid lubrication. The x-direction cutting force of other nano-fluids is increased by 2%-5% compared with the AL 2 O 3 nano-fluid. The smaller the cutting force of SO 2 nano-fluid lubrication, the better the lubrication effect. The small cutting force will affect the wear of the tool and the machine tool, which can appropriately improve the cutting processing parameters, thereby improving the production efficiency.
(3) Through the analysis of the milling surface and chip surface morphology of milling aluminum 7075 under different nano-fluid lubrication methods, the surface quality of SiO 2 , AL 2 O 3, SiC nano-fluid milling aluminum 7075 with MQL is better, so the machining accuracy is improved, so as the production efficiency.
Through the analysis and summary of the above experimental conclusions, the lubricating fluid on the macroscopically affects the cutting force of the aluminum alloy clear and moisturizing contact angle, thereby affecting the apparent morphology. On the microscopically, the addition of nano-particles and the rolling of SO 2 spherical micro-particles have the effect of reducing wear and rolling, reduce cutting force and improve surface quality. It shows that SO 2 NMQL has good lubrication and anti-wear performance in cutting Al7075.