Evaluation and prediction of material response during superplastic forming at various strain rates

Current trends in the automotive industry include replacing steel body components by superplastically formed aluminum alloys. The application of aluminum alloys is largely limited by the slow forming rates required for the superplastic forming process. In order to overcome the difficulties associated with this process, considerable research is being done to study the behavior of AA5083 at high strain rates. However, the microstructure evolution under these deformation conditions has not been characterized previously. In this study we have studied the microstructure evolution in tensile specimens of AA5083 deformed at a wide range of strain rates. Strain rates between 0.0005 and 3/s were used for the tests, which were performed with the samples at 450°C. The results showed that a strong crystallographic texture develops and the most highly strained regions of the samples recrystallize dynamically at strain rates greater than 0.01/s. For strain rates at or below 0.01/s, no recrystallization occurred. This recrystallization is stimulated by the constituent particles. The recrystallized grain size was smallest at the highly-strained, fracture point and increased in size as one moved away from the fracture end towards the grip of the sample. Below a critical strain, no further recrystallization was observed. This critical strain decreased with increasing strain rate and with an increase in the diameter of the largest constituent particles in the sample. Analysis of the results showed that both a critical strain rate and critical strain were required to achieve dynamic recrystallization. A model for critical strain rate by Humphreys and Kalu gave qualitative agreement with the experimental results. Tests were also conducted at room temperature followed by annealing at 450°C. Study of these specimens enabled a comparison of dynamic recrystallization with the critical strain driven static recrystallization process.; Finite element simulations were used to predict the constitutive response of the material. The simulations included grain boundary diffusion, grain boundary sliding and dislocation creep mechanisms. The predictions of simulations are in good agreement with the experimental results. The results agree with the transition in the mechanism of deformation with increasing strain rate, from grain boundary sliding to dislocation creep.