Survey of inclusions in twin roll casting of wrought aluminium alloys

Spent filters employed in twin roll casting of wrought Al alloys were thoroughly analysed to identify the type of inclusions responsible for a variety of quality problems in Al sheet and foil. The inclusions are predominantly of the TiB(2), Al(2)O(3) and MgAl(2)O(4) variety. The former are invariably in the form of clusters and occupy a volume fraction relatively larger than that of oxides. The oxides entrapped inside the spent filter are mainly films with a lacy morphology and are accounted for by poor foundry practices. The angular oxide particles, 10 to 20 mm in size, are believed to be inherited from the electrolytic cells where primary Al is produced and from eroded refractory linings. There are occasionally dispersed carbide and halide salt particles retained inside the spent filter. Their volume fractions, however, are too small to generate Bragg reflections in the XRD analysis.