Static Recovery of a Clad Aluminum Alloy After Cold Rolling

The tailoring of cold-deformation to promote recovery and recrystallization processes upon subsequent heat-treatment is well understood in rolled monolithic alloys. In contrast, clad aluminum systems, which feature two or more different alloy systems bonded to a core alloy, contain an inter-alloy region with recovery and recrystallization behaviours which could be different from the individual clad and core alloys. Understanding these behaviours is important in optimizing the final properties of the clad product. This study examines the static recovery behaviour of a clad aluminum alloy manufactured by Novelis Fusion™ technology after 72% cold rolling. The clad alloy system studied consisted of a core AA6XXX alloy clad on one side with AA3003. The Recovery at the inter-alloy region is compared with the recovery of core AA6XXX at the same depth from the rolling surface. Sample coupons from the inter-alloy region and core AA6XXX were heated isochronally and isothermally, at different temperatures and times, respectively, to probe the recovery kinetics of the X-ray peak broadening, X-ray macro-texture and micro-hardness from the cold rolled state. The recovery of the {220} and {31 1} X-ray line profiles were observed between the anneals. A pseudo-Voigt fit function was fit to the profile to obtain the defect related information. Recovery in the peak broadening began by 100°C and correlated to a decrease in the hardness. Sharpening of X-ray profiles during recovery is attributed to the microstructural evolution resulting from preferred release of the stored energy due to dislocation rearrangement and annihilation. Kinetic behaviour of the recovery is measured by observing the evolution of X-ray profiles and hardness during isothermal annealing at two different temperatures. Recovery behaviour in the inter-alloy region is measured to be relatively slower than the recovery of the core AA6XXX after same macroscopic pre-strain. Activation energy for recovery is calculated from the isothermal data to deduce a recovery mechanism. The activation energy calculated in core AA6XXX, 1.7eV, is close to the activation energy value for diffusion of Mg in Al (1.3-1.7eV). This indicates a possible role of Mg diffusion in the recovery of AA6XXX. The relatively higher activation energy for recovery of 2.9eV measured in the inter-alloy region may be due to pinning by nano-scale Al-Mn precipitates. The X-ray broadening data is deconvoluted to determine the apparent dislocation content using a modified Williamson-Hall model. The dislocation density measured in the AA6XXX and inter-alloy regions in the deformed and recovered conditions indicates that dislocation density is a suitable parameter that represents the stored energy that drives subsequent structural evolution during recovery.