A study of thermomechanical behaviour and grain size evolution of AA7050 under hot forging conditions

A series of compression tests have been carried out using Gleeble (3800) thermomechanical simulator to investigate the viscoplastic behaviour of AA7050 as well as its microstructural characteristics under hot forging conditions. The thermomechanical tests were conducted at the temperatures of 350–450 °C with strain rates of 0.0005–0.5 s−1, which covers the range of hot forging conditions for AA7050. Interrupted tests have also been carried out to track the microstructural evolution under various thermomechanical deformation conditions. Particular attention has been focused on the criteria of dynamic recrystallisation in AA7050 during the uniaxial compression at high temperatures, which could help to obtain regular homogeneous grain structures for hot forged components. It was found that AA7050 has a strong viscoplastic behaviour and flow stress is more than halved from 350 to 450 °C. A suitably low strain rate and high temperature are required for significant dynamic recrystallisation (0.05 s−1 and 400 °C in current deformation level). The average grain size decreases with increasing strain and deformation temperature due to dynamic recrystallisation, but the relationship between average grain size and strain rate is not monotonic. Present findings provide a guideline for the selection of hot forging parameters so that quality components could be achieved with low forging force.