Strengthening of alloy AA6022-T4 by continuous bending under tension

This paper studies the evolution in strength of alloy AA6022-T4 sheets that have been pre-deformed by a continuous-bending-under-tension (CBT) process. Significant improvements in strength are observed only after a few CBT cycles. Less appreciable improvements in strength are observed with more CBT cycles and with every cycle the ductility reduces. These observations are rationalized by characterizing microstructural evolution using transmission electron microscopy and electron backscattered diffraction. It is found that evolution of texture and grain shape during CBT slightly differ from those in simple tension (ST). Also, the precipitates do not change their shape during CBT or ST. It is, therefore, concluded that these microstructural features have only a secondary effect on the strength behavior of the alloy. Consistent with earlier observations in the literature, we find that dislocation structures form within grains during monotonic ST and that they are disorganized and not as well defined. In contrast, cellular substructures are observed to form very early during CBT processing, even after the first cycle and to evolve from loose tangles of dislocations to well-defined walls during subsequent cycles. These dislocation patterns are found responsible for the observed behavior of the alloy. Therefore, the strength of the material is determined not only by the achieved effective strain level but also by achieved microstructure.