Some considerations on tool wear and workpiece surface quality of holes finished by reaming or milling in a nickel base superalloy

Higher-strength heat-resistant superalloys are continually being developed to meet the engineering requirements for increased efficiency of gas turbine engines. In addition, work on machinability assessment is needed to fill the gap between the material development and manufacturing capabilities before considering the implementation of such materials in production lines. The present paper presents selected results on optimization of cutting conditions for hole-making processes of a new aerospace nickel (RR1000)-based superalloy manufactured via a powder route. The mechanical properties and the chemistry of RR1000 alloy indicate lower machinability when compared with the current disc alloys (e.g. Inconel, Waspaloy, Udimet). The hole-making processes, involving a succession of roughing (drilling) and finishing (normal/special reaming or plunge milling) operations, were evaluated through a multicriteria procedure consisting of the following output measures: tool life, hole accuracy, surface roughness, workpiece surface integrity, and level of cutting torque. It was found that simultaneous fulfilment of multicriteria quality measures is more dependent on the selection of tooling/cutting edge preparation involved in the finishing step rather than the relatively narrow intervals of operating parameters. While drilling and normal reaming lead to surface overheating (white layers) and material dragging, the change of edge preparation (e.g. double-relief angle) on the special reamers or the use of alternative cutting strategies (e.g. plunge milling) can generate finished holes within the required surface integrity standards. An original relationship between a derived measure from output sensory signals (i.e. specific cutting energy per tooth per revolution) and workpiece surface integrity (i.e. depth of strained material) has been investigated to assist further the selection of the 'best' finish machining operation. Additionally, this approach opens the avenue towards online surface integrity inspection via 'key sensory measures'.