Effect of flow rate in minimum quantity lubrication (MQL) in micro-drilling of Ti-6Al-4V

Minimum quantity lubrication (MQL) is an environment-friendly alternative to traditional flood cooling techniques and has great potential to be successful in micro-machining. The current research work aims toinvestigate the influence of MQL flow rates (6, 50, and 100 ml/h) and cutting parameters such as spindle speed and feed on various micro-drilling characteristics of Ti-6Al-4 V. Thrust force, specific cutting energy, and hole surface roughness were found to be lower under the MQL flow rate of 6 ml/h as compared to higher flow rates. An increase in the MQL flow rate from 6 to 100 ml/h at a spindle speed of 30,000 rpm resulted in a maximum of 40% increase in the thrust force. The surface roughness and oversize error increased by a maximum of up to 162 and 66%, respectively, under a similar change in the MQL parameters. The flow rate of 100 ml/h exhibited inferior micro-drilling characteristics primarily owing to its inability to effectively penetrate the narrow machining zone. Material adhesion under the MQL environment increased with an increase in cutting velocity due to the rise in the cutting temperature. The phenomenon of the "size effect" was evident under the lowest feed of 1 mu m/rev owing to the regime of rubbing and ploughing. Elevation of feed effectively promoted shear deformation in micro-drilling. Overall, the present study highlighted the superiority of the lowest flow rate of 6 ml/h owing to effective atomization during micro-machining under the MQL environment.