| The peripheral-coarse gain (PCG) defect of aluminum extrusion may originate in an unusually fine-gained microstructure evolved during deformation. This microstructure manifests in a region referred to as the "dead-metal zone", so-called because it is considered stagnant during deformation. Furthermore, it consists of equiaxed high-angle boundaries reminiscent of recrystallization. As recrystallization requires significant stored energy, the question arises---if this material is stagnant, what generated the energy to induce recrystallization?; A modeling effort was undertaken to simulate the forming conditions and the dynamic and static microstructural evolution. If a similar fine-grained microstructure is predicted, perhaps the mechanisms modeled are indicative of the actual evolution occurring in the material.; As many static recrystallization models exist, the dynamic modeling effort was focused upon. Most dynamic models employ a single evolution mechanism; few studies model multiple mechanisms. That a more realistic model may be achieved by combining multiple mechanisms is fueled by evidence that individually, two of the dynamic microstructure evolution mechanisms proposed in the literature, continuous and geometric dynamic recrystallization (CDRX and GDRX), underpredict the high-angle gain boundaries formed. Perhaps both modeled in concert can closer predict the experimental observations.; A Finite Element Modeling simulation was evaluated by comparison to visioplastic experiments, and subsequently used to provide state-variables for the microstructure evolution model. A dynamic model was written that blends a CDRX model from the literature with a new GDRX model. The results generated were used to produce two-dimensional simulations of microstructures, upon which a simple static recrystallization simulation was performed. Each stage of the model was evaluated by comparison to experimental results.; The conclusions reached are (1) the dead-metal zone in fact experiences some of the most deformation in the indirect extrusion process; (2) the new GDRX model proposed here can account for the "missing" high-angle grain boundaries under-predicted by other models; (3) a steep microstructure refinement reminiscent of the experimental observations is predicted; (4) FEM simulations suggest that this material, with high-stored energy as a result of high strain, subsequently flows into the region of the PCG defect. |
