Bendability enhancement of an age-hardenable aluminum alloy: Part I — relationship between microstructure, plastic deformation and fracture

In this study, the bendability of two 1 mm thick sheets of monolithic AA6016 and composite AA6016X alloys is investigated using a series of wrap-bend tests, with emphasis on understanding the relationship between microstructure, the nature of plastic deformation and fracture behavior. The composite AA6016X alloy consists of a central core of AA6016 sandwiched between 100 μm thick clad layers of AA8xxx series aluminum alloy, processed using thermomechanical roll-bonding. It is shown that the bendability of monolithic AA6016 alloy is limited due to the formation of severe surface undulations and surface cracking, which is associated with the heterogenous nature of slip that concentrates into 5°–15° misoriented coarse slip bands of very high dislocation content in the order of 1014/m2, and intense shear bands originating from surface low cusps in the form of mutually orthogonal transgranular bands. The micro-cracks initiate at the surface and propagate along these intensely sheared regions, primarily consisting of grains with near S texture component. Grain boundary decohesion occurs along boundaries that are highly misoriented with misorientations ranging from 40° up to as high as 60°, further assisting crack propagation. The composite AA6016X alloy exhibits unlimited bendability with no signs of fracture even after a maximum bend angle of 180°. The extremely high bending strains in AA6016X are accommodated in a homogeneous manner through a grain subdivision process within the AA8xxx clad layers, promoting the formation of deformation induced high angle boundaries. This homogeneous accommodation of strain within the clad layers reduces surface roughness and further enhances the bendability of the alloy.