Development of an ultrasonic vibration-assisted MQL device and its effects on the milling performance of ultra-high strength steel

Minimum quantity lubrication (MQL) technology offers cooling and lubrication for material cutting by atomizing cutting oil through high-pressure gas and injecting it into the cutting zone. The improvement in cutting performance is often associated with the droplet size, density, and velocity generated by the MQL device. Ultrasonic vibration-assisted MQL (UVMQL) is an innovative and enhanced MQL technique that exhibits excellent atomization properties for superior cooling and lubrication during machining. In this study, an UVMQL device was developed, and Fluent simulation was employed to enhance the inflow and ejection states of the droplets while optimizing the structural parameters within the nozzle. The spray characteristics of the device were evaluated using silicon wafer droplet analysis combined with image processing technology. Results demonstrate that applying ultrasonic vibration reduces the droplet size of MQL by 35.6%, leading to a more concentrated size distribution and higher velocity reaching the cutting zone. As gas pressure increases, there is a decrease in the difference between UVMQL and MQL droplet sizes from 3.4 to 1.2 µm, indicating a diminishing effect of ultrasonic vibration on droplet size improvement over time. Finally, milling experiments were conducted on ultra-high strength steel to investigate how parameter variations affect milling performance under both MQL and UVMQL conditions. It was observed that increasing inlet pressure by 300 kPa resulted in reductions of main cutting forces under MQL conditions by 15 N, whereas under UVMQl conditions, these forces decreased by 20.3 N due to improved spray characteristics facilitated by ultrasonic vibration effects. When increasing the flow rate of cutting oil, surface topography improvements were more pronounced under UVMQL conditions, as evidenced by a decrease in R_a value of 19.4%, thus highlighting how ultrasonic vibration indirectly enhances material milling performance.