Direct powder forging (DPF) is an innovative powder metallurgy (PM) process that aims to manufacture nickel-based superalloy components within a very short time, compared with current hot isostatic pressing (HIPing) and other processes, by applying high temperature and pressure to metal powders. DPF process has been proposed to reduce the microstructural defects, e.g. prior particle boundaries (PPBs), possessed by the conventional HIPing process and achieve considerably high production efficiency with low cost and energy saving. The aim of this project is to study the powder consolidation and microstructure evolution of a nickel-based superalloy, FGH96, during DPF process. The work in this thesis is divided into two parts: experimental studies and numerical modelling.
The experimental studies reported in this thesis concentrate on the material characterisation of FGH96 and investigate the products of DPF process under different process conditions. Firstly, hot compression tests were conducted using Gleeble 3800 test station to obtain the mechanical properties of FGH96. Secondly, small size powder forging tests were designed to study the powder consolidation. Samples acquired from small size powder forging tests were examined with material density. Tensile tests were then carried out to evaluate the mechanical properties of FGH96 superalloy produced by DPF process, and microstructure observation was used to identify the microstructure features. Lastly, the DPF process was tested under both laboratory and industrial environment. Tests were conducted on a laboratory-based 250 kN hydraulic press machine and an industrial 20,000 kN hydraulic press machine to consolidate FGH96 powder which was encapsulated with a stainless steel container. Material density, hardness and microstructure of the FGH96 components were then examined to evaluate the feasibility of implementing DPF process to manufacture fully dense FGH96 superalloys with desired material properties and microstructure features.
The numerical modelling was used to model the material behaviours of FGH96 and investigate the powder density evolution during DPF process. Firstly, a unified viscoplastic constitutive model was developed for fully dense FGH96 based on the results obtained from the hot compression tests, and calibration was carried out to achieve good agreement between numerical integrations and experimental results. Secondly, the constitutive model was modified with powder densification parameters and variables, providing a method to numerically describe the powder density evolution during DPF process. Thirdly, the modified constitutive model, i.e. the powder material model, was implemented into the commercial software DEFORM 2D/3D via user defined subroutines. Results were analysed in terms of stress state and powder density distribution, and the correlation between the material properties of DPFed components and process parameters were discussed. Lastly, future work suggestions were proposed to improve the DPF process and modelling technique.