Comparative evaluation of soybean oil–based MQL flow rates and emulsion flood cooling strategy in high-speed face milling of Inconel 718

The increasing environmental and health concerns of conventional emulsion flood coolants have motivated the use of vegetable oil in the form of minimum quantity lubrication (MQL) in machining. This paper presents comparative evaluation of high oleic soybean oil (HOSO)–based MQL flow rates at 10, 30, 50, 70, and 90 ml/h with a mineral oil–based emulsion flood coolant as a benchmark in face milling of Inconel 718 using AlTiN/TiN-coated carbide inserts. Cutting forces, tool wear, and surface roughness were measured and analyzed. The results show that MQL oil flow rate at 70 ml/h gave the longest tool life comparable to that of mineral oil–based emulsion flood cooling, while 10 ml/h flow rate gave the shortest tool life. Also, 70 ml/h flow rate gave the lowest resultant cutting force among all MQL oil flow rates and conventional emulsion cooling at tool life. Increasing HOSO-based MQL flow rate improves surface roughness and reduces tool wear by providing enough thin lubrication film but also leads to an increase in chip affinity and formation of large built-up edges (BUEs) as the MQL flow rate reaches 90 ml/h. At lower HOSO-based MQL flow rate, tool wear mechanism is predominantly abrasion due to large surface friction, while at higher HOSO-based MQL flow rate, tool wear mechanism is adhesion leading to excessive BUEs. HOSO-based MQL flow rate of 70 ml/h and air pressure of 4.14 bar are recommended when face milling Inconel 718 and are demonstrated to be a potential replacement of mineral oil–based conventional emulsion flood cooling strategy for machining of difficult-to-cut metals.