Optical and transmission electron microscopical study of the evolution of surface layer on recycled aluminium along the rolling mills

A deformed near-surface region, also referred as ‘surface layer’, is always present on rolled aluminium alloy sheets. This paper studies in detail the evolution of the surface layer along the production line on a ‘recycled’ commercial alloy (AA5050). A microscopic investigation of a step-by-step formation of the surface layer has been carried out in the roll-bite. The effect of thermo-mechanical processing on the microscopic and optical characteristics of the surface layer has been studied. The distribution of the alloying elements within the surface layer has been investigated along the production line. The importance of the Total Reflectance (TR) method has been demonstrated for accurate estimation of the average thickness of the surface layer. Based on these results, a correlation has been drawn between the microscopic characteristics of the surface layer and the TR profile which in turn shows that the TR can be used in lieu of TEM for estimation of the surface layer thickness at different production stages.