Microdefects formation during the twin-roll casting of Al-Mg-Mn aluminium alloys

Twin-roll casting is an established route to produce aluminium sheet (10-0.5 mm thick) directly from the melt. Despite an enormous potential, various defects may arise during casting which limit the range of operating conditions suitable for use commercially. A range of casting conditions has been investigated producing aluminium alloys sheet 1.5-5 mm thick. Internal and surface defects have been studied. Despite reasonable good surface quality and microstructure, the high productivity strip showed an abundance of surface bleeds as well as central segregates. Various techniques (electron probe microanalysis, X-ray diffraction, optical and electronic microscopy, etc.) have been used to investigate the composition, distribution and identity of the different phases. Surface bleeds are formed at higher temperature than “centre segregates” and contain more primary α-Al. Flow of liquid in the semi-solid into small surface buckles is proposed as the mechanism for bleed formation. In the strip centre, the build up in rolling pressure leads to increased rate of heat extraction; thus, trapped solute rich liquid can rapidly solidify and form complicated eutectic structure observed on composition maps.