Flow Curve Evolution During Cyclic Processing of Ultrafine-Grained Aluminum Alloy by Multiaxial Forging†

A model to describe the evolution of flow curves during multiaxial forging is presented. Using the Gleeble physical simulator, forging experiments are carried out on EN AW-6082 aluminum alloy. Three forging schedules with different applied strain amplitudes are performed at room temperature, all subjected to the same level of total accumulated strain. The flow curves are calculated using the previously developed mechanical model. The resultant microstructures are investigated by TEM studies. The flow curves are monotonic. However, the flow stress drops if the direction of the deformation is changed, indicating that the flow curves are cyclic in character. The evolution of flow stress is quantified, as well as the upper and lower limiting curves and the equivalent flow curves are determined. As the applied strain per pass increases, the cyclic flow stress varies from 100 to 150 MPa, from 105 to 170 MPa, and from 115 to 200 MPa for strain amplitudes of 0.2, 0.285, and 0.4, respectively. Comparing the effect of the different strain paths on the resultant microstructure, it becomes finer and more homogeneous with increasing strain amplitude. The average grain size decreased from 1 to 0.5 μm, when the applied strain per pass was doubled.