Modelling of hot deformation for microstructural control

Abstract

Bulk metal working processes are carried out at elevated temperatures where the occurrence of simultaneous softening processes would enable the imposition of large strains in a single step or multiple steps. Hot working also causes a significant change in the microstructure of the material and this contributes to one part of workability generally referred to intrinsic workability, which is sensitive to the initial conditions of the material and the process parameters. The other part, related to the state of stress existing in the deformation zone, is specific to a metal working process and may be controlled by changing the applied state of stress and/or the geometry of the deformation zone. An understanding of the constitutive behaviour of the material is essential for the optimisation of the intrinsic workability and control of microstructural evolution during hot working. In recent years, modelling techniques have been developed for this purpose and these are critically reviewed. These include: (1) the kinetic model, (2) the atomistic model, (3) the dynamic materials model, and (4) other models including activation energy maps with stability criteria and polar reciprocity model. Since the first two models do not lead to workability optimisation directly and are not useful for commercial alloys, the dynamic materials model has been developed. This model forms a bridge between the continuum mechanics and the microstructural mechanisms occurring during hot deformation, and consists of principles of irreversible thermodynamics of large plastic flow. The dissipated power is related to the rate of entropy production due to metallurgical processes obtained by partitioning the total power between that due to the temperature rise and the microstructural change. In view of the viscoplastic nature of hot deformation, the strain rate sensitivity of flow stress has been found to be the power partitioning factor. The development of processing maps using this model, their interpretation and application for optimising hot workability and control of microstructure are reviewed. The development of activation energy maps and their use in the control of microstructure are useful for obtaining robust process control for microstructural optimisation, while the use of the stability criteria puts undue restrictions on the processing regimes. Several mechanisms of hot deformation, with special reference to dynamic recrystallisation, are discussed in the light of results from processing maps obtained on a wide range of materials. Finally, the application of the processing maps in designing bulk metal working processes is briefly explained.