Formation of pinch marks on pack rolled aluminium foil

The matte side of the pack rolled aluminium foils was found to contain frequent pinch marks distributed over the surface randomly with no periodicity along the rolling direction. They were judged to be material-related since only an intermittent fraction of the aluminium coils pack rolled under exactly the same process conditions has suffered this problem. Metallographic investigation of pack rolled foil sections has shown these pinch marks to be invariably associated with eutectic colonies across the section of either one of the packed webs. These lamellar eutectic colonies form due to centreline segregation typical of the twin roll casting process and have apparently survived the homogenization treatment and the downstream processing cycle. They are harder than the solid solution aluminium matrix and resist plastic deformation during cold rolling. They make an impression on the surfaces of both webs while the stacked webs pass through the roll gap. The polishing effect thus produced is retained on the inner matte surfaces that are not in contact with the work rolls producing bright spots.