Machinability variations in Alloy 718 —With focus on machining of turbine components

Abstract

In gas turbine engine, nickel base alloy, such as Alloy 718, accounts for almost half of the total material requirement. Nickel-base superalloys are used for gas turbine components mainly because of their outstanding strength and resistance to oxidation at high temperatures (> 550°C). The massive cost involved with the machining of nickel alloys has driven continued research and development of cutting tool materials as well as of cutting techniques that ensure higher metal removal rate with minimum surface and sub-surface damage on the machined components. In machining of Alloy 718, one of major issues is the machinability variations as the result of variation in the mechanical properties and microstructures from workpiece. Sometimes it was difficult to machine different regions of the same component and when that happened, machining issues arose with chip entanglement and tool failures. The aim of the research addressed in this thesis is to explain machinability variation in Alloy 718 turbine discs with respect to material parameters such a grain size, hardness, deformation and work hardening; all of which are believed to have great influence on tool wear. The investigation was based on the large amount of production data retrieved from the production scene and the experimental data from laboratory. With this work it is expected to refine and adjust the gap between the theory and the production practice to optimize the manufacturing process in general and machining process in particular. For Alloy 718 machined with Al2O3-SiCW ceramic tools the main types of wear are mainly notch wear, flank wear and flaking wear. From retrieving used round C670 tools in production, it was found that an optimum effective cutting angle range existed in terms of insert wear. This means that if the effective cutting edge length is controlled through programming for each feed rate lower wear rates should be obtained. A polar diagram method for describing and evaluating the machinability of Alloy 718 was developed. Five key parameters of the work material, representing the mechanical and physical properties which have strongest influence on its machinability, were employed in the construction of polar diagrams. Work materials of Alloy 718 in which the polar diagrams of machinability were similar in size and shape exhibited very similar behavior during the cutting process. A tool life model, named “ShortCut-Wear-Model”, was developed with consideration of work-hardening effect in the machining of Alloy 718 for tool life prediction. In addition, the statistic based model, Weibull model, was also used for tool life predication makes it possible to derive an optimal replacement strategy which will minimize the unit production cost and other costs associated with machining of Alloy 718