Optimization of MQL Parameters using Computational Fluid Dynamics for Enhanced High Speed Machining of AISI 4340 Steel

This study addresses the challenge of thermal management in high-speed machining of hardened AISI 4340 steel by investigating the effectiveness of Minimum Quantity Lubrication (MQL) as a sustainable alternative to conventional cooling. Although MQL offers environmental and safety benefits, its restricted cooling capacity can limit its effectiveness for hard-to-cut alloys. Using computational fluid dynamics (CFD) simulation, this work systematically explores the influence of MQL parameters specifically oil flow rate, F (50–150 ml/h), nozzle distance, D (20–40 mm), and nozzle angle, A (30–60°) on temperature generated at the cutting zone. Temperature boundary conditions from finite element analysis (FEA) at a cutting speed, V  = 400 m/min; feed, f  = 0.1 mm/rev; and depth of cut, d  = 0.2 mm were applied. Experimental turning tests validated the simulation results, with cutting temperature deviations of 8% and 10% for dry and MQL conditions, respectively. The optimal MQL configuration (50 ml/h oil flow, 20 mm nozzle distance, 30° nozzle angle) was identified, with nozzle angle exerting the greatest influence on temperature control. Compared to dry cutting, this optimal MQL setup reduced cutting temperature by 32%, cutting force by 28%, and surface roughness by 11.9%, resulting in an estimated 22% increase in tool life. These findings affirm that optimizing MQL parameters can substantially improve cooling efficiency and machinability in high-speed machining and hard turning of AISI 4340 steel, supporting further advances in sustainable, high-performance manufacturing.