MQL machining: from mist generation to tribological behavior of different oils

Minimum Quantity Lubrication (MQL) is a solution towards cutting fluids reduction. This technology requires a deeper understanding of local mechanisms (in particular tribology) occurring at micrometer levels. A previous study showed that two oils with similar lubricating performance (full lubrication) could differ when applied through MQL. The following study aims at closing the loop between local tribologigal results and mist generation phenomenon by observing the mist generated (particle size distribution, oil flow rate) for both oils. The following conclusions can be drawn: an oil with low viscosity will be easily pumped into the venturi nozzle leading to a higher quantity of oil which can be sent to the pipeline. Different sizes of droplets are observed at the generator outlet: 8, 100 and 450 mu m. Even though big particles appear sporadically, they do represent approximately 70 % of the volume sprayed. Neither the air pressure nor the oil properties seem to influence the size of the droplets produced. Droplets coagulation/break up and film breakup are not observed inside the pipeline for the chosen configuration. However, when traveling through the pipeline, the smallest droplets interact with the inner wall by forming an oil film and a spatter flow (laminar) is generated in addition to the spray flow (turbulent). The mist at the pipeline outlet is mainly composed of spray droplets (85 % for the first oil and 66 % for the second oil). Furthermore, the spray for both oils is composed of similar types of particles with similar proportions. As a consequence, the main differentiation between oils is in terms of number of particles in the sprayed mist. The MQL performance is therefore explained from the mist generation to the tribological behavior of the cutting interface.