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Mechanisms of Streaking on Anodized 6xxx Series Extrusions

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Mechanisms of Streaking on Anodized 6xxx Series Extrusions

Xinquan Zhang, Rio Tinto Aluminium Technology, Australia
Hanliang Zhu, Materials Engineering, University of Queensland, Australia
Arne K. Dahle, Materials Engineering, University of Queensland, Australia
Malcolm J. Couper, Rio Tinto Aluminium Technology, Australia

Track: VALUE-ADDED PROCESSES

ABSTRACT --- Streaking is a common surface defect on anodized extrusions of
6xxx series soft alloys. Very often, the defect only becomes apparent after
anodizing, which makes it difficult to identify its origin. This paper describes
the current understanding of formation mechanisms and preventive measures of
streaking on the basis of a literature review and experimental results. Special
attention has been paid to die streaks that cannot be removed by mechanical
treatment of the extrusion. The origin of streaking is a difference in intensity
and diffuse nature of the reflected light. Surface imperfections in the anodized
extrusion increase the diffuse part of reflected light, and thus any
inhomogeneous distribution of the surface imperfections results in the
occurrence of streak defects. Surface imperfections can include etching pits,
grain boundary grooves, and/or grain etching steps and are created in the
extrusion surface during etching. The severity of the surface imperfections
depends on the microstructure variations such as inhomogeneous intermetallic
particle distribution, grain size, and texture. In the case of die streaks, a
heterogeneous microstructure is produced by localization of deformation due to
non-uniform metal flow, which is dependent on die design and influenced by
extrusion process conditions. Good die design is critically important for
preventing die streaks.

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