Formation mechanisms of surface blistering in AA6xxx rolled products: microstructure characterisation, ultrasonic analysis, and rolling tests results

ABSTRACT The source of surface blister defect in 6xxx series Al rolled products was studied. A blister defect, aligned with the rolling direction, was observed on the surface of the AA6xxx Al rolled sheets. It was indicated that the porosities and small microstructural inhomogeneities inside the cast ingots, identified by ultrasonic tests and metallography analysis, were not the origin of these surface defects. Pilot scale rolling trials indicated that the blister defects were correlated with the longitudinal internal casting cracks. The short side cracks in the cast ingots, covered by a thin Al oxide layer, were not healed and remained partially open after the rolling process. The unhealed cracks served as the nucleation sites for H2 precipitation during the later stages of the thermomechanical process, which were subsequently resulted from surface blisters on the AA6xxx Al rolled sheets.