Optical and Microstructural Origins of Thermomechanical Streaking Defects in Hot Extruded AA6060

Thermomechanical streaking is a common optical surface defect that affects architectural 6xxx series aluminum extrusions, and can be cause for rejection of the product. AA6060 profiles were extruded at a range of ram speeds using a specialized die with internal geometry purposely designed to produce thermomechanical variation throughout the profile. Subsequently, the extrudates underwent an industrial anodization pretreatment process that revealed the presence of streaks throughout three designated regions. The optical appearance, microstructure and surface topography of streaked and surrounding regions of the extrudate surface were analyzed using colorimetry, electron backscattered diffraction, and optical profilometry. Differences in perceived lightness, roughness, and grain size were observed between streaked and surrounding regions. Changes in appearance of the surface directly correlated with the surface roughness, with rougher surfaces yielding an increase in the perceived lightness. The surface roughness was determined to be primarily dependent on the size and distribution of grain etching steps as related to the surface grain size. A difference in grain size in the regions surrounding the streaks was determined to be the microstructural origin of the visual defect.