Mechanical Properties and Microstructures of Commercial-Purity Aluminum Processed by Rotational Accelerated Shot Peening Plus Cold Rolling

Commercial-purity aluminum with 99.7% purity is processed by rotational accelerated shot peening (RASP) and cold-rolling. RASP samples are rolled at room temperature to a thickness reduction of 20% and 30%, for the purpose of surface roughness reduction and strengthening. Detailed microstructural characterization and hardness tests reveal that cold-rolling cause grain growth at the surfaces of RASP samples from ≈472 to ≈1000 nm. Moderate cold-rolling is effective in smoothing the surface of RASP samples, while improving strength and maintaining ductility. However, cold-rolling to more than 30% thickness reduction eventually diminishes the gradient nanostructure. During cold-rolling of RASP samples, a transition zone with strong strain incompatibility is noticed by a sharp rise in hardness at some point of the hardness-distribution curve from the surface to the core of the sample. This transition zone is a result of quick generation of geometrically necessary dislocations. Although the transition zone with strong strain incompatibility is short-lived, it is anticipated to be common among gradient-nanostructured materials under cold-rolling condition.