The surface defect "banding" very often occurs when anodizing aluminum profiles из:
- alloys 6060 and 6063 в международном (или европейском) обозначении или
- alloy AD31 – domestic analogue of alloys 6060/6063.
About the reasons for the formation of this defect, associated with the ingress of contaminated metal from the surface zone of the ingot into the profile, look. article Полосы на анодированных профилях: роль матрицы и оптические эффекты. Here we will consider the reasons, related to the design of the extrusion die.
Anodizing defect “foliation”
optical properties, for example, dullness or gloss of the surface of an aluminum profile after alkaline etching depends on the initial microstructure of its surface layer, including distribution of grain boundaries and grain texture. This microstructure is determined by the degree of plastic deformation and its rate, as well as temperatures in the deformation zone at the exit of the profile and during its subsequent cooling.
Press deformation is inherently inhomogeneous, since this is the only way to form a profile of a given complex section from a massive round blank. The parameters of plastic deformation differ greatly at points at different distances from the center of the section of the profile or workpiece..
The degree of shear deformation increases from the center to the surface of the extruded profile, reaching there maximum. The larger the deformation gradient, the greater the microstructural changes along the profile section. However, is, не любая гетерогенность микроструктуры по поперечному сечения профиля приводит к дефекту «полосчатость» (англ. streaking).
Impact of matrix design
Streaks along the longitudinal welds of hollow profiles are difficult to completely eliminate, however, the intensity of their manifestation can be reduced by optimizing the design of the matrix. Longitudinal seams of hollow sections are usually characterized by a small grain size and texture that is different from adjacent areas due to high local shear deformation in the weld.
Although the flow of metal in the matrix is always inhomogeneous, a well-designed matrix is able to correct non-uniform metal flow so, to make it nearly uniform on the output from the matrix. Therefore, good die design is key to prevent streak defect..
The source: X. Zhang et al, Aluminium Extrusion Technology Seminar, 2008.