Direct powder forging of PM nickel-based superalloy: densification and recrystallisation

Abstract

Powder metallurgy nickel-based superalloys have been widely used in high temperature applications. For these materials, a fully dense and fine-grained microstructure is important. Full densification can be achieved by a suitable processing technique, while the latter can be achieved through recrystallisation for which valuable guidance is provided by the information about recrystallisation nuclei. In this study, a fully dense powder metallurgy nickel-based superalloy component has been produced by a new manufacturing method—direct powder forging—using a single acting hydraulic press under normal atmosphere. Boundary misorientation of the produced material has been analysed to determine the degree of recrystallisation nucleation. A finite element model for direct powder forging has been developed in DEFORM-2D/3D and validated by comparing experimental and simulated load curves. The relationship of stress and strain state with densification and recrystallisation nucleation degree has been analysed. It was found that the direct powder forged FGH96 alloy has a much higher recystallisation nucleation degree and more recrystallised sub-grains, compared with those of the hot isostatic pressed material. Within the forged component, a higher recystallisation nucleation degree resides in the material near the container wall where greater values of shear strain rate have operated.