Modeling of dynamic material behavior in hot deformation: forging of Ti-6242

Abstract

A new method of modeling material behavior which accounts for the dynamic metallurgical processes occurring during hot deformation is presented. The approach in this method is to consider the workpiece as a dissipator of power in the total processing system and to evaluate the dissipated power co-content J=∫_o^σε^.⋅dσ from the constitutive equation relating the strain rate (ε^.) to the flow stress (σ). The optimum processing conditions of temperature and strain rate are those corresponding to the maximum or peak in J. It is shown that J is related to the strain-rate sensitivity (m) of the material and reaches a maximum value (J_max) when m=1. The efficiency of the power dissipation (J/J_max) through metallurgical processes is shown to be an index of the dynamic behavior of the material and is useful in obtaining a unique combination of temperature and strain rate for processing and also in delineating the regions of internal fracture. In this method of modeling, no a priori knowledge or evaluation of the atomistic mechanisms is required, and the method is effective even when more than one dissipation process occurs, which is particularly advantageous in the hot processing of commercial alloys having complex microstructures. This method has been applied to modeling of the behavior of Ti-6242 during hot forging. The behavior of α+β and β preform microstructures has been exam-ined, and the results show that the optimum condition for hot forging of these preforms is obtained at 927°C (1200 K) and a strain rate of 10^-3 s^-. Variations in the efficiency of dissipation with temperature and strain rate are correlated with the dynamic microstructural changes occurring in the material.