Effectiveness of lubricating coatings in dry drilling of aluminium alloys

Aluminium alloy stack materials offer good strength-to-weight ratio and are used for high strength airframe structures. For the panels to be joined, numerous holes have to be drilled and due to structure size, mobile drilling heads are used. The drilling process releases lubricant oil which becomes airborne or collects on the shop floor and presents a health and safety hazard. Avoiding this oil release is therefore a major driver for developing dry drilling processes. However, drilling aluminium alloys in the absence of cutting fluids is a challenging task due to its tendency to adhere to the cutting tool, and the high thermal conductivity of the workpiece. Owing to their low coefficient of friction, carbon-based coatings are an option to improve the machinability of aluminium alloys. This paper presents an industrial collaboration study on the performance of carbon-based coatings in dry drilling aluminium alloy 2024-7150 stacks. A chemical vapour deposition (CVD) diamond, hard diamond-like carbon (DLC), doped molybdenum disulphide (MoS2) and doped amorphous carbon (doped a-C) were evaluated in comparison with an uncoated tungsten carbide (WC) drills. Coating performance was assessed in terms of tool wear, hole diameter and surface roughness. The results revealed that CVD Diamond-coated drills outperformed other coatings in terms of tool wear and hole quality. The coating enabled lower aluminium pickup on the drills as well as minimised variations in diameter deviation and investigated area hole surface roughness. The work shows the capability for dry drilling of stacked aluminium alloys and hence eliminating the health and safety risk associated with use of oil in mobile drilling heads.