On the Lower Limit of Misorientation of Grain Boundaries in Hot Forging of AA7050

The goal of this work is to find a reasonable norm to measure the area of low angle grain boundaries (LABs) using EBSD data, which is useful for the metal forming community involved in developing appropriate constitutive models for dynamic recovery. Typically, LABs are defined as the grain boundaries with misorientation between 2° and 10°. However, it is found in this study that reasonable variations of area per volume of LABs (Slab) with strain level (ε), strain rate (έ) and temperature is not be observed for AA7050 with this definition. There is a competition among different processes, including sub-grain formation and coarsening, rotation of sub-grain/cell boundary and migration of LABs and high angle grain boundaries (HABs) during recovery and continuous dynamic recrystallisation. By analysing the evolution of Kernal Angle Misorientation (KAM), it is shown that recovery has more effect on the lower angle part (< 1°) of local misorientation. If the lower limit of LABs is shift to 0.5°, the evolution of Slab with ε and έ shows clear pattern and increasing trend of Slab with temperature is consistent with temperature effect on sub-grain formation process. With such definition, we can have a better understanding of the microstructure evolution in the dynamic restoration of AA7050.