Given the increasing attention to environmental and health problems caused by machining, the development of an environmentally friendly grinding fluid has become an urgent task. In this study, seven typical vegetable oils (i.e., soybean, peanut, maize, rapeseed, palm, castor, and sunflower oil) were used as the minimum quantity lubrication (MQL) base oil to conduct an experimental evaluation of the friction properties of the grinding wheel/workpiece interface. With nickel-based alloy GH4169 as workpiece material, the flood grinding and MQL grinding were selected. Experimental results indicated that castor oil MQL grinding had a friction coefficient and specific grinding energy of 0.30 and 73.47 J/mm3, which decreased by 50.1% and 49.4%, respectively, compared with flood grinding. Moreover, maize oil had the highest G-ratio of 29.15. Peanut, sunflower, and soybean oil with more saturated fatty acids, castor oil with more castor acids, and palm oil with numerous palmitic acids were suitable as lubricating fluids.
Top11.1 Introduction
To obtain a higher precision and surface quality of the workpiece, the grinding process is indispensable during machining. The grinding process is a random integration of scratching, ploughing, and cutting using abundant irregular abrasive particles that are scattered on the grinding wheel/workpiece interface (Chen, Rowe, & W, 1996; Xun & Rowe, 1996). Grinding employs a higher unit grinding force and grinding speed than other cutting processes and therefore involves a significantly higher grinding energy. Most of this energy is converted into thermal energy. This thermal energy has different distribution patterns from other cuttings. Most grinding heat (approximately 80%) is transferred into the workpiece (Komanduri & Hou, 2001). Aerospace materials are widely used, especially nickel-based alloy. However, their hardness is great, and their thermal conductivity is poor, so that more heat is consumed during the machining process of these hard materials; these shortcomings severely influence the surface quality, machining precision, and machining efficiency of the workpiece (Guy Albert, 2010). A high temperature in the grinding process also has significant influences on the grinding property of abrasive particles, which may directly decrease the service life of the grinding wheel.
To decrease the temperature in the grinding zone and to improve the machining quality of the workpiece and the life of the cutter, different cooling lubrication technologies are applied in the grinding process. In machining, flood cooling technology is the most common cooling lubrication method, which requires about 60 L/h of grinding fluid flow. The grinding fluid can cool the lubrication grinding wheel and workpiece and remove debris. Flood cooling grinding results in better workpiece surface quality than dry grinding; however, the grinding wheel at a high speed forms a layer of gas barrier, which makes entry into the grinding wheel and workpiece interface difficult for numerous grinding fluids. Thus, an effective flow rate between the grinding wheel and workpiece interface is only 5%–40%. Moreover, in the machining process, the volatilization, leakage, or overflow of the grinding fluid may cause significant damage to the health of workers, and may induce water pollution primarily because of the grinding fluid. Direct contact between the smog caused by the heat and volatilization of the grinding fluid and the human body poses threat to human health and induces various diseases of the skin, respiratory tract, and lung. Mineral oil is also used as a cooling lubricating liquid. It has excellent lubrication properties, but its low cooling property and high cost limit its application in machining. To protect the environment and decrease costs, dry grinding is adopted because it is significantly advantageous in environment protection. However, given that removing materials per unit volume through the grinding process consumes more energy than other processes, more heat gathers in the grinding zone (Malkin & Guo, 2007). Only 10% heat is removed by the grindings in the grinding process (Mao et al., 2013). The gathered heat leads to high temperature, and without the lubrication and cooling effect of the grinding fluid, the grinding wheel wears seriously, and the workpiece precision and surface integrity deteriorate. Furthermore, the process of dry grinding has relatively high requirements in terms of the grinding wheel, workpiece materials, and machine tool. The grinding wheel should have high hardness, toughness, and wear resistance. These specific conditions required by dry grinding limit its wide application in machining.