Experimental assessment of structure and property predictions during hot working
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I.
INTRODUCTION
PROCESS modeling
is being used increasingly to design, optimize, and control fabrication processes, tHu Process modeling is often applied to existing processes where model predictions are frequently adjusted to experimental results. A reason for this is that some of the input data to the model are not known with sufficient accuracy or generality. Examples are accurate models for interface heat conductance, friction, flow stress, and structure evolution. Process optimization should include both the mechanical design of the processing equipment and the metallurgical response of the product being produced. However, microstructural effects are often measured empirically in the actual process and then correlated to model calculations of the strain, strain rate, and temperature fields. A potentially more useful approach would be to have the process model make explicit predictions of microstructural variables. Process modeling could then be more confidently applied to the design of new processes. During large strain plastic deformation of metals, the dislocation density can be altered by the processes of work hardening, dynamic recovery, and dynamic recrystallization. These processes are affected by strain, strain rate, and temperature, which are neither constant in time nor uniform throughout a workpiece in most commercial fabrication operations. This can be due to the effects of die chilling, interface friction, deformation heating, or microstructural changes. Consequently, the deforming metal may never reach an equilibrium structure as is sometimes observed in mechanical testing under constant temperature and strain rate hot-working conditions.m-~s] L.A. LALLI, Manager, is with Alcoa Laboratories, Fabricating Technology Division, Alcoa Center, PA 15069. A.J. DeARDO, Professor, is with the Department of Materials Science and Engineering, Basic Metals Processing Research Institute, University of Pittsburgh, Pittsburgh, PA 15261. Manuscript submitted February 28, 1990. METALLURGICALTRANSACTIONSA
A constitutive model for flow stress that depends on microstructural variables as well as on strain rate and temperature is one means of coupling the calculated deformation history to the resulting microstructural response. Such a constitutive model must be capable of accounting for the effect of arbitrary thermomechanical histories on overall dislocation density and, therefore, the resistance to plastic flow. t161Models of this type utilize internal state variables, t~7-22] These models are operationally attractive because the complexities of the dislocation network are not explicitly considered. If a single internal state variable is used, it is often assumed to be related to the total dislocation density and is a measure of the net resistance to plastic deformation. Consequently, it is sometimes referred to as hardness. 1t8,2~ The structure evolves with continued straining at a rate which is a function of strain rate and temperature. Single internal variable models can describe the effects of multiple deform
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