Minimum quantity lubrication drilling of aluminium–silicon alloys in water using diamond-like carbon coated drills

The dry drilling of aluminium alloys (without using cutting fluids) is an environmentally friendly machining process but also an exceedingly difficult task due to aluminium's tendency to adhere to the drills made of conventional materials such as the high-speed steel (HSS). Diamond-like carbon (DLC) coatings improve the dry drilling performance due to their adhesion mitigating properties. In this work, improvements that are possible when DLC coated tools are used under minimum quantity lubrication (MQL) condition in drilling of an Al–6%Si (319 Al) alloy were examined. Results were compared with drilling using conventional flooded coolant. The experimental approach consisted of the evaluation of the cutting performance of DLC coated HSS drills in a distilled water spray (30 ml/h) used as the MQL agent. Two types of DLCs (non-hydrogenated and hydrogenated) with different tribological responses in H2O testing environment were considered. The H2O-MQL cutting of 319 Al using either type of DLC coated drills reduced the drilling torque compared to dry drilling to a level similar to the performance under the flooded condition. An added advantage of the H2O-MQL over dry drilling was that the process was more stable; a smaller percentage of drilled holes exhibited “torque spikes”, i.e., an abrupt increase in torque, indicative of adhesion. H2O-MQL cutting using non-hydrogenated DLC was preferred to hydrogenated DLC because it resulted in smaller built-up edge (BUE) formation and also less amount of drill flute aluminium adhesion resulting in less torque and thrust force being required during drilling.