Modelling of the bonding process in powder forging

Abstract

Bonding between powders is a key factor that has a crucial influence on the mechanical performance of a component formed by powder forging. Consequently, modelling the bonding process is important for controlling the quality of the forged component and predicting the optimum process parameters (i.e. forming temperature, applied load and holding time). However, few numerical models have been developed to investigate this process. In the present study, a theoretical bonding model was established to predict the extent of bonding for a contact region with constant temperature and interfacial pressure, also to identify the bonding conditions for this region (i.e. a combination of temperature, pressure and time). The bonding time for a miniature component manufactured by direct powder forging (DPF) using FGH96 powders was determined at 1150 °C in terms of two applied pressures (103.5 and 198.9 MPa). The numerical predictions from the theoretical model were compared with the obtained experimental results and a good agreement was found, indicating that the model is applicable for predicting bonding between powder particles. A methodology was proposed to implement the analytical equations derived in the theoretical bonding model into finite element (FE) simulation via user subroutines, thus real-time bonding results can be predicted. Also, a micromechanical model was established for modelling the powder densification process in commercial FE software, Abaqus, which employs the hexagonal close-packed (HCP) and face-centred cubic (FCC) structures as a representative volume element (RVE) of the powder aggregate. Since the RVE model simulated the actual interaction between powders, realistic contact pressures were obtained during powder compaction. Finally, a multiscale model was developed to predict the bonding time (i.e. critical holding time) for a specified location within a forged component, which consists of a continuum FE model, the micromechanical (RVE) model, and the theoretical bonding model. The use of this multiscale model was demonstrated via a practical DPF process, which indicates promise in determining the critical value of an industrial process parameter, i.e. holding time, through which the mechanical strength of the forged components can be guaranteed.