Manufacture and microstructural characterisation of bimetallic gas turbine discs

Abstract

A number of model Ni based superalloy bimetallic discs have been manufactured, each having a cast and wrought U720Li hub bonded to a cast IN738LC ring by means of hot isostatic pressing augmented by isothermal forging. The microstructures of both the as hipped and HIP plus forged model discs showed a complex precipitation reaction at the interface between the two alloys. Different types of precipitate were identified at the bond line, which had formed as a consequence of interdiffusion and recrystallisation at the interface during hipping. Residues from a pre-HIP acid cleaning process were also detected. Preliminary forging trials conducted on the model discs found poor control of the deformation and shape of the interface due to temperature and die lubrication issues and resulted in strong recrystallisation of the IN738LC and internal cracking. Subsequent full scale disc manufacture was able to overcome these problems, although further work is continuing in the microstructural and mechanical characterisation of these parts.