Flow instabilities and flow localization in alloys

Industrial processes such as forging and rolling to form metallic materials, involve high temperatures, large deformations and large forces. All these processes are not only used to give a form to the metallic part, but also to modify its characteristics at the microscale. This last point is relevant for the final performance of the part. On the other hand, during these processes, undesirable damage at different size scales can occur. Many efforts have been done to describe, explain, model and predict these damage caused mainly by a strong localization of the deformation. A well-established approach to model the flow localization is given by the so-called processing maps, in which the formability of the material is predicted at different deformation temperatures and velocities. These maps are simply built using experimental data obtained from hot compression tests done at the laboratory scale. Although to the intense use of the model, the thermodynamic and physical bases to describe it, as well as its prediction potential were questionable. In this proposal we describe a complete and detailed analysis of the phenomena of localization of the deformation, to develop a new criteria combining a large amount of experimental results, simple flow modelling and irreversible thermodynamics. The new criteria can be used for optimization of industrial forming processing of metallic parts, and its potential will be tested using examples of commercial metallic materials. The use of this method will impact directly in the quality, and thus in the performance, of mechanical and structural parts. Finally, the optimization of processing routes and the better performance of structural parts are both indirectly related to decrease the energy involved in the processing and during service, respectively. To reach our goals we will develop models, use computer simulation and carry out experiments of hot deformation in the laboratory scale.

Metal forming processes such as extrusion, hot rolling and forging is used extensively in the manufacture of metal goods around the world. These processes influence the shape and the local microstructures of the end product and thus its mechanical and physical properties. Important aspects in the forming process are flow instabilities and flow localizations, undesirable phenomena that can damage the workpiece during processing. For several decades, process maps have been used by both scientists and engineers to find an optimal temperature-strain rate combination to form metallic parts without defects. Despite the intensive use of the approach, the model lacks a physical basis and predictability. Therefore, our work focused on the description, modelling and characterization of flow instabilities and flow localization to predict their occurrence. In this project we have: a) conceptualized theoretical distinctions between flow instability and flow localization. Flow instability is characterized by fluctuations in stress or force during hot forming and is explained by atomistic interactions within the material. The flow localization creates microdamage and localized deformations and is not only related to the material, but also the geometry of the workpiece, friction conditions, etc. b) developed physically based models to predict both flow instability and the occurrence of flow localization. While flow instabilities are likely to occur at slow deformation rates, flow localization occurs through fast and heterogeneous deformation. c) coupled both phenomena with the changes on the microstructural level. Flow localization, in particular, was prone to formation if the local microstructure could not recover quickly enough during the forming process d) characterized the flow localization and flow instabilities in titanium alloys and aluminium alloys with thermal deformation experiments on the laboratory scale and metallography. The new criteria can be used to optimize the industrial forming of metal parts. The model was implemented in finite element modelling software, to analyze these types of features and damage as well as the microstructure in industrial workpieces. Fianlly, the optimization of the thermomechanical routes and the better performance of structural parts are indirectly related to the energy reduction during processing and operation.