Tapping of Al–Si alloys with diamond-like carbon coated tools and minimum quantity lubrication

The deep hole drilling and tapping of automotive powertrain components made of hypoeutectic Al–Si alloys are of considerable importance. This work investigates the dry and minimum quantity lubricated (MQL) tapping of Al–6.5%Si (319 Al) alloys as alternatives to conventional flooded tapping. Two types of tests were done in comparison with flooded tapping. In the first set dry tapping experiments were performed using diamond-like carbon (DLC) coated and uncoated HSS taps. HSS-dry tapping caused immediate tool failure within less than 20 holes due to aluminum adhesion, resulting in high forward and backward torques. DLC-dry tapping improved tool life considerably and exhibited small torques. The second set of tapping experiments used MQL and only uncoated HSS taps. The use of MQL at the rate of 80 ml/h produced similar average torques to flooded tapping, and a high thread quality was observed. DLC coatings’ low COFs against 319 Al limited the temperature increase during DLC-dry tapping to 75 °C. The low COF of DLC against aluminum was responsible for preventing built-up edge (BUE) formation and thus, instrumental in improving thread quality. The use of MQL reduced the tapping temperature to 55 °C. The mechanical properties of the material adjacent to tapped holes, evaluated using hardness measurements, revealed a notable softening in the case of HSS-dry tapping, but not for MQL tapping. The presence of sulphur and phosphorus-based additives in MQL fluids proved beneficial in preventing aluminum adhesion.