The role of diamond-like carbon coated drills on minimum quantity lubrication drilling of magnesium alloys

Minimum quantity water lubrication (H2O-MQL) and dry drilling behavior of a cast magnesium alloy (AZ91) were investigated using non-hydrogenated diamond-like carbon (NH-DLC) coated HSS drills and by measuring cutting torque as well as the temperature generated in the workpiece. Due to adhesion of magnesium to the drills that caused rapid drill failure neither uncoated HSS nor NH-DLC coated drills could be used in dry drilling of AZ91. In drilling with NH-DLC coated tools flank type drill wear was also observed as a result of coating degradation induced by high temperature generated (271 °C) during contact. Application of H2O-MQL at a rate of 30 ml/h increased tool life of NH-DLC coated drills and also reduced drill torque as well as the temperature to levels comparable to the conventional flooded drilling using mineral oil. AZ91 workpiece experienced recrystallization during dry drilling using both uncoated HSS and NH-DLC coated drills but while drilling with NH-DLC under H2O-MQL condition no evidence of recrystallization was found. The low coefficient of friction of NH-DLC under H2O-MQL condition limited the temperature increase to below 55 °C and hence both drill wear and magnesium adhesion were effectively reduced.