Micro lubrication (MQL) processing involves the application of a small amount of lubricant in the processing process in an attempt to replace the traditional flood coolant system. Understanding the correlation between fluid performance and MQL performance can help select lubricants from a variety of choices without extensive processing tests. This study compares the physical properties, wettability, tribological properties (lubricity and extreme pressure (EP) properties), fog properties and machinability of nine different MQL fluids to determine the correlation between the measured properties and MQL drilling and reaming properties. The results show that low fluid viscosity, high fog concentration, large droplet diameter and high wettability are most related to good processability. Although it is difficult to have a strong relationship, under mild cutting conditions, the best processing of low-viscosity fluids may also have the highest fog concentration, the maximum titer and the best wettability.
Various micro-lubrication (MQL) lubricants have very different properties (each supplier creates its own formula). Understanding fluid properties and micro-lubrication (MQL) properties can help select lubricants for future micro-lubrication (MQL) processing without extensive processing tests. Therefore, the purpose of this study is to test commercial micro-lubrication (MQL) lubricants to try to determine which properties or bench tests are important for predicting the processing properties of these fluids. The evaluation includes checking thermal conductivity, wettability, lubricity, extreme pressure (EP) performance and fog characteristics. Then, these results are compared with the power consumption, surface roughness and hole diameter when machining the transmission valve body to determine whether these tests can be used as prediction indicators of actual machining performance. Microlubrication (MQL) lubricant and its evaluation method Nine micro-lubrication (MQL) lubricant test samples named A to I were obtained from six suppliers, and their known physical properties are listed in Table 1. The viscosity (8.8 to 69cSt) and flash point (182 to 280C) of lubricants vary widely. Lubricant E is the same as lubricant D, except that vulcanized EP is added. Table 1 Tested MQL lubricating oil (in order of viscosity rise)
Experimental setup and results Physical properties - thermal conductivity
In the micro-lubrication (MQL) process, the heat generated is the same as that in the traditional process, but the fluid that can take away the heat is much less. Therefore, the thermal characteristics of the fluid can be an indicator of its heat removal capacity. In order to consider the effect of temperature on micro-lubrication (MQL) lubricants, the thermal performance analyzer KD2Pro (ThermoTest Company, Texas) was used to measure the thermal conductivity at 25, 50, 75 and 90C. A water-based liquid, Trimsol (marked as WB in all tests), was also tested at a concentration of 5% to compare it with a trace lubricant (MQL) lubricant. Each lubricant sample is measured in a thermal isolation box with temperature control to ensure the reliability of the results. Three measurements were carried out for each case, and the change was found to be less than 0.003W=m-K. The results in Table 2 show that the thermal conductivity of micro-lubrication (MQL) lubricant (A-I) is much lower than that of water or water-based fluid. This means that the effective heat removal rate of MQL fluid is lower than that of traditional water-based fluid. Poor heat removal will cause thermal damage to the workpiece and tool during processing. In addition, in the measurement range of 25~90C, the thermal conductivity of micro-lubrication (MQL) lubricant is not affected by temperature, while the thermal conductivity of water and water-based fluid increases with temperature. The thermal conductivity range is 0.138~0.160W=m-K, and it tends to increase with the increase of fluid viscosity. Table 2 Thermal conductivity of micro-lubrication (MQL) lubricant at different fluid temperatures (W=m-K)
Bench test Wettability Wettability is a term used to describe the ability of fluid to diffuse, penetrate and cover tools and workpieces (Sillman, 1992). The wettability of a fluid is defined as the heat balance between the liquid droplet and the solid surface and the contact angle between the liquid droplet and the gas phase. The smaller the contact angle, the higher the wettability of the fluid. As shown in Figure 1, contact angle θ The Young equation of is: 
Where, S, L and G represent solid, liquid and gas respectively, and ⋎ is the interfacial tension vector. The liquid drop measuring system developed by KRU (Germany KRU ¨ SS) is used to measure DAS10 using the fixed liquid drop method. The droplet is imaged (as shown in Figure 1), and the computer automatically fits the contour of the droplet and calculates the contact angle. The contact angle was measured on polished aluminum (Al) 6061 and tungsten carbide (WC) surfaces to simulate the aluminum base workpiece material and tool material. The surface of the sample was washed with ethanol and dried between tests. The three measured values in each case are averaged, as shown in Figure 2. The contact angle of micro-lubrication (MQL) lubricant is smaller than that of water and water-based fluid, which means that micro-lubrication (MQL) lubricant can wet the surface more thoroughly, which means that micro-lubrication (MQL) lubricant has better wettability. The contact angle between all micro-lubricating (MQL) lubricants is 8.0~20.6 on aluminum and 7.6~26.5 on WC. Since wettability is usually directly related to the surface tension (cLG) of the fluid, the surface tension is measured by using a surface tension meter (Model 21, Fisher Scientific). Water and acetone were tested to ensure the accuracy of the measurement. As shown in Table 3, the measured surface tension (⋎ LG) is similar. Therefore, the difference in contact angle between fluids may be due to the difference in the interfacial tension (⋎ SL) between them and the solid surface, because ⋎ SG is always the same in the equation (1). In addition, the research results also show that the micro-lubrication (MQL) lubricant is usually more effective than WC when wetting aluminum, which is also related to the ⋎ SL produced by different contact surfaces. Lubricity Tapping test is a standard screening method for evaluating the cutting performance of lubricants (Zimmerman et al., 2003). A self-tapping torque machine (Michigan, USA) was used in this study. The workpiece is a pre-drilled 6061 aluminum plate. Pre-drilled holes are filled with lubricant, and then tapped with M8 tool steel rotating at 1200rpm. Torque data is recorded during tapping, as shown in Figure 3. The average value of the platform area is used to represent the torque generated in a specific fluid.
