The use of soft/lubricating coatings when dry drilling BS L168 aluminium alloy

Abstract

The paper briefly reviews the development/implementation of soft/lubricating coatings and details published machinability data for dry/near dry machining applications. Following on from this, experimental work is presented relating to the performance of PVD coated WC cutting tools (Graphit-iC™, Dymon-iC™ and MoST™, with and without a hard coating underlayer) when dry drilling BS L168 aluminium alloy. Machinability was assessed in terms of tool life/wear, cutting force/torque, chip morphology, hole diameter, cylindricity and out of roundness. Despite the use of soft/lubricating coatings, there was a strong tendency for the aluminium to adhere to the drill, which contributed to low tool life. The least amount of build up was observed for carbon based Graphit-iC. Varying degrees of coating damage were evident. ZrN+MoST and MoZrN+MoST both failed due to delamination of the coatings from the cutting tool. The use of CrTiAlN as an underlayer was shown to improve the durability of the MoST topcoat. MoST deposited directly on to the WC drill receded ∼500 μm from the cutting edge compared with <10 μm for CrTiAlN+MoST, after drilling 100 holes. Regardless of the coating combination employed, little or no difference was found in hole quality. Thrust force and torque values ranged from 1000 to 1600 N and 200–800 N cm, respectively. High torque was correlated with high cyclindricity values indicating problems with chip evacuation. Considerable variation in hole diameter was also observed over the duration of each test. Initial holes drilled in test discs were oversize, however, as more holes were machined in the workpiece, hole size tended to decrease to less than the drill diameter. It is likely that the coefficient of thermal expansion of the workpiece material governed hole diameter rather than coating performance.