Efficiency of power dissipation and instability criterion for processing maps in hot forming

The processing maps are a superimposition of iso-efficiency contour map and flow instability map, which are used to design hot working processing conditions in a wide variety of materials. In order to construct the processing maps, the efficiency of power dissipation and an instability criterion taking into account the contribution of strain and microstructure evolution are proposed based on a set of microstructure-based viscoplastic constitutive equations. In viscoplastic constitutive equations, the grain size of matrix phase and the dislocation density are taken as internal state variables. And, the material constants in present equations can be identified by a genetic algorithm (GA)-based objective optimization technique. Isothermal compression of Ti-6Al-4V alloy is conducted on a Thermecmaster-Z simulator with different deformation temperatures, strain rates and height reductions so as to establish the processing maps by using the present model. The primary α grain size is measured at an OLYMPUS PMG3 microscope with the quantitative metallography image analysis software. Based on the experimental results, the processing maps of Ti-6Al-4V alloy are constructed at different strains. The processing maps show that the instability domains and the efficiency of power dissipation vary as the strain increases. The comparison between the processing maps of present study and that based on Prasad's theory shows that the present processing maps can more efficiently describe the deformation behavior and provide more appropriately physical interpretation and optimize processing conditions accurately.