TRANSFORMATION OF MICROSTRUCTURE AND TEXTURE IN ALUMINUM 7475 ALLOY DURING SUPERPLASTIC FORMING

The process of superplastic forming in 7475 aluminum alloy was analyzed using orientation imaging microscopy (OIM), X-ray diffraction (XRD) and scanning electron microscopy (SEM). These techniques were used to determine the effect of the superplastic forming on texture, grain boundary character distribution (GBCD), grain size, residual stress and microstructure at different stages of the deformation of specimens deformed in tension. Results indicate that a microstructure is transformed mainly by the grain boundary sliding process that is responsible for rapid randomization of texture. There is also much evidence for crystallographic slip occurring in conjunction with grain boundary sliding. Accommodation of superplastic flow is linked to increased dislocations density in the lattice. At a threshold level, the dislocation density reaches certain saturation level and the nucleation of voids starts. At this threshold strain, the deformation mechanism is altered and superplastic flow proceeds, however, cavities continue to be produced and coalesce due to the grain boundary sliding process. When the dislocation based accommodation mechanism switches from dislocation based and cavities are formed, the lattice strains begin to recover and no further plastic deformation is introduced into the lattice. The Kernel average misorientation function of OIM was used to indicate the level of strain within the grains to explain the formation of cavities