Face milling of nickel-based superalloys with coated and uncoated carbide tools
Face milling machinability investigation of two difficult-to-machine nickel-based superalloys, namely Inconel 718 and Waspaloy, has been carried out with four different types of tungsten carbide tools under various cutting conditions. The tools comprised of one double-layer CVD-TiCN+Al2O3 coated (KC994M), two PVD-TiN coated (KC720 and KC730) and one uncoated (KMF) tungsten carbide tools. The objectives of the study include investigation of tool performance, failure modes and wear mechanisms under the cutting conditions employed. In addition, surface integrity of the machined surfaces, with regard to surface finish, subsurface microhardness and metallographic examination of the subsurface microstructure, was investigated. CVD-coated KC994M gave the best overall performance in terms of tool life at low and high cutting conditions on both workpieces. The second best-performing tool was the uncoated KMF grade which gave as high tool lives as KC994M at lower cutting speeds. However at higher cutting speeds, KMF was generally outperformed by PVD-TiN coated tools. Short tool lives were obtained at higher cutting speeds of 75 and 100 m/min due to premature failure by chipping. Tool wear at low cutting speed range was due to a combination of progressive microchipping and plucking through a fracture/attrition related wear mechanism associated with cyclic workpiece adhesion and detachment and abrasion/diffusion-related flank wear. Plucking and microchipping were the dominant wear mechanisms. Coating layers on the rake face of both CVD and PVD coated tools were almost completely removed within the first few seconds of cutting at all cutting speeds tested, thus becoming ineffective. On the flank face, however, they remained intact for a longer period and hence increasing tools performance at the medium cutting speed range. Analysis of the subsurface microstructures and microhardness measurements showed that plastic deformation was the predominant effect induced onto the machined surface, the degree of which influenced by the cutting speed, tool wear and prolonged machining. In addition surface irregularities in the form of tearing and embedded hard particles were found to occur which was mainly associated with the chipping dominated wear mode.