Surface oxide fracture in cold aluminium rolling

Aluminium strips are coated by anodic oxidation with a thin aluminium oxide film and rolled in an experimental mill. Micro-cracks are formed in the oxide film near the entry to the roll bite, running perpendicular to the rolling direction. The spacing between these cracks increases with the oxide film thickness. If the oxide film is below a critical thickness, the oxide fragments are further broken into smaller pieces soon after the entry region in the bite. The final aspect ratio of the oxide fragments is independent of the oxide film thickness. It is found that metal extrudes through the micro-cracks within the roll bite. A theoretical analysis is developed for the oxide fracture and metal extrusion processes. The predicted oxide segment length is in good agreement with the experimental measurements. The theoretical prediction, that the extruded metal will not penetrate through the oxide layer and reach the roll surface below a critical reduction of about 13%, is also supported by the observations.