Characterization and modelling the flow localization in titanium alloys during hot forming

Titanium alloys are used for aerospace applications due to their high specific mechanical properties. However, at given forming conditions, flow localization limits their hot workability and leads to undesired shear bands, voids, cracks and fracture. A physical-based model is implemented as a subroutine and used in FE simulations to predict the microstructure and effective stress evolutions during hot deformation of Ti alloys. Additionally, a phenomenological model based on the state variables evolution was used to predict the susceptibility of flow localization in specific regions of the workpiece. The physically-based model assumes a microstructure composed of three distinct populations of dislocations named mobile, immobile, and wall dislocations. Constitutive equations correlate the flow stress with the microstructure evolution and the flow softening in the α+β field is considered a result of the change in load partitioning. The grain sizes are related to the high angle grain boundary density. A subgrain is surrounded by low and high angle grain boundaries and is the representative microstructure entity. During deformation in the α+β domain, an initial α-lamellar structure suffers dynamic globularisation due to the formation of new boundaries within the α-platelet, and the model also predicts the evolution of this phenomena. For validation of the model, the FE simulations were compared with the experimental results in terms of grain size measured from EBSD maps after hot compression, temperature evolution in two different regions on the surface of the workpiece, load vs displacement curves and final shape of the sample after deformation. The results show that the occurance of flow localization is related to a fast growth of the wall dislocation density and of the fraction of high angle grain boundaries.