Origin of streaks on anodised aluminium alloy extrusions

This work investigates the root causes of streak defects, appearing as narrow, bright bands, on an anodised AA 6063-T6 extrusion using optical and electron microscopies. It was found that the appearance of etched and anodised aluminium alloy is associated with grain boundary grooves, etching steps and surface scallops which are determined by the distributions of grain size and crystallographic orientations. It was also revealed that the bright streaking regions had relatively large grains with a relatively strong texture. Such microstructure resulted in reduced light scattering by grain boundary grooves, surface scallops and etching steps with, consequently, brighter appearance.