Strain localization and damage development during elevated temperature deformation of AA7075 Aluminum sheet

Elevated temperature forming of AA7075 aluminum alloy is of considerable interest in the automotive industry due to its potential for light weighting and several other environmental benefits. This study presents elevated temperature uniaxial tensile deformation and damage behavior of AA7075-W sheet through a set of experiments at the microstructural scale. A multi-scale experimental methodology consisting of in-situ SEM tensile testing, use of micro digital image correlation (µ-DIC) method for obtaining strain field at the microstructural scale and post-test X-ray computed tomography was adopted with temperature held constant at 300°C and 400°C. The deformation temperature affected the precipitate formation and their distribution and subsequently the precipitate-induced void damage development in the material. At 300°C, precipitates as well as precipitate free zones were formed at the grain boundaries during heating and following deformation. Intergranular void initiation and catastrophic void coalescence with relatively small void growth occurred along the grain boundaries during plastic straining resulting in intergranular fracture and poor ductility at this temperature. In contrast, at 400°C, fewer intergranular precipitates were formed during heating and deformation resulting in delayed void nucleation during plastic straining. The diffuse necking occurred earlier with significant void growth occurring during the long regime of diffuse necking with very little evidence of void coalescence leading to considerably higher ductility at fracture. The final fracture occurred due to transgranular microcrack formation and propagation at this temperature.