Thermomechanical processing of metals at moderate and large strains

The hot deformation of metallic materials up to large deformations provokes changes in the microstructure that cannot be achieved at smaller strains. Despite the efforts of the scientific community to explain and model the developed microstructure, the panorama is still quite unclear. Not only has the description of the mechanism itself been a topic of discussions and controversies, but also the reliability of the experimental data. It is general accepted that at large deformations the structure of high stacking fault energy metals can be refined by continuous recrystallization processes. In the literature, two possible processes have been reported: (1) continuous dynamic recrystallization by progressive lattice rotation (cDRX) and (2) geometric dynamic recrystallization (gDRX) by high angle grain boundary (HAGB) pinching off. Interesting is that this classification was used to describe either the same phenomenon at different conditions sometimes, or to describe two different phenomena (interrelated or not) which appear depending on the deformation conditions. We propose that the combination of reliable data acquisition, physical modelling and finite element simulations over three different materials are the key to understand, explain and predict the deformation mechanisms taking place at large strains. The chosen materials are: wrought aluminium age-hardenable alloy (AA6082), a near ß-titanium alloy (Ti5553) and one magnesium alloy (MgAl4Ba2Ca2). The aluminium alloy will be set as the starting material, and the preliminary results of torsion tests, microstructure characterization and microstructure and flow modelling up to moderate strains will be used. Additionally, MgAl4Ba2Ca2 alloys were partially characterized. Large deformations under excellent control of the temperature, atmosphere, strain and cooling/heating rates will be obtained with torsion tests in a Gleeble3800 machine. Furthermore, a method for in-situ hot torsion tests will be adapted to carry out x-ray diffraction by means of high energy synchrotron source. Metallography of post-mortem samples carried out by means of light optical microscopy, scanning electron microscopy and electron backscattered diffraction will allow the microstructure characterization. The microstructure will be described using one and two types of dislocations models. The subgrain and the flow evolution will be described firstly in the low/moderate strain ranges. By increasing the strain, the model will take into account the lattice rotation and the HAGB formation vs the subgrain boundary consumption. In a further step, the effect of second phases will be analysed: (1) drag forces pinning boundaries and dislocations in AA6082 and MgAl4Ba2Ca2 (2) and the influence on deformation of hard-deformable alpha-phase in Ti5553. Additionally, phenomenological models will be implemented to determine the relationship between grain size, strain, strain rate and temperature. The developed physical models will be coupled to finite element simulations by means of subroutines to predict the microstructure as a function of the deformation parameters.

In this project we dealt with the behaviour of aluminium-, titanium- and magnesium-based materials when are thermo-mechanically processed. Thermomechanical processes involve the use of high temperatures to permanently deform a material in order to obtain a given geometry and performance. Typical processes are forging, rolling and extrusion. Specially in this last, the materials are subjected to very large permanent (plastic) deformations, generating a unique structure of the material in the microscopic scale, followed by high performance. We used experiments at the laboratory scale to deform the above-mentioned metals at different high temperatures. The obtained samples were characterized with microscopy, and read the force required to achieve permanent deformation. We found a description of the phenomena that are responsible of the modifications of the microstructure, simplified it, and developed a model to mathematically account for these modifications with the temperature and the velocity of deformation. The here obtained results and models will be used in the future to optimize thermomechanical processes and/or the performance of these aluminium, titanium and magnesium based-materials.