Effect of Initial Microstructure on Surface Appearance of Anodized Aluminum Extrusions

Surface defects such as streaks are often present on anodized extrusions of 6xxx series alloys. Microstructural examination and extrusion trials were conducted to investigate the microstructural effect on the surface appearance and the formation of streak defects. It was found that the formation of an inhomogeneous distribution of grain boundary grooves is a critical factor for the formation of die streaks on anodized aluminum extrusions. This is influenced by various microstructural features. Fe-rich intermetallic particles reduced the difference in severity of grain boundary grooves on the same extrusion surface through their effect on other microstructure features, reducing the tendency for streaking. Also, grain size was found not to be a direct cause for the formation of grain boundary grooves, but it does influence the distribution and amount of grooves. Furthermore, variation of grain orientations caused the formation of grain etching steps, which influenced the morphology of grain boundary grooves, but these effects were not significant in the formation of the streak defects. However, Mg2Si precipitates influenced the difference in dissolution rates between grain interiors and grain boundaries during etching, dramatically influencing the formation and severity of grain boundary grooves. Therefore, it is important to reduce any inhomogeneous distribution of precipitates on the surface of extrusions to prevent die streaking and improve the surface appearance of the anodized product.