Anodizing defects: stripes from structure, geometry and welds

 

See. also: Classification anodization defects

Strips coarse structure

English term

Surface roughness streaks

defect Source

Pressing

The manifestation of the defect

Travlenye – anodizing

Details of the defect

Apparently, the most common type of "streaky" defects anodized aluminum profiles. They appear as "tapes" or small groups of tracks at the pressed matrix aluminum profile and can not always be removed by alkaline etching,. they are not usually associated with the location of the profile, or changes in the profile geometry, such as "T" -connection.

These defects are a more serious problem for thin wall sections and tend to worsen with increasing press speed. Эти дефекты проявляются в результате оптических эффектов от изменений шероховатости поверхности или ее текстуры (микроструктуры).

Preventing defect

  • For solid matrices main cause "banding" – a sharp change in the length of the belt, as well as the presence / absence and size of the feeder.
  • Increasing the overall quality of matrices, including, optimum design, workmanship and thermal treatment, proper operation, including heating modes and nitriding.

The bands on the profile geometry

English term

Profile Dependent Streaks

defect Source

Pressing

The manifestation of the defect

Travlenye – anodizing.

Details of the defect

These defects appear as bands of tape in the pressing direction, usually on the broad, flat surfaces in areas of sharp changes in wall thickness or the presence of walls or ribs on the opposite side of the aluminum profile.

On the image 1.5 shows the strip from opposite edges of scratches profile stiffness. Polishing or grinding, these bands are at the same place. These defects are attributable to changes in surface topography, grain structure or texture changes in the place of aluminum profile thickness. After grinding or polishing of these layers of material with different metallurgical structure have a different sensitivity to etching. Therefore, they have different reflectivity and clearly visible in the form of strips after anodizing.

In addition to the metallurgical inhomogeneity, appearance of these defects often associated with topographical changes due to the sharp differences in the flow of aluminum matrix. Это может приводить к образованию выступов или впадин напротив «Т»-соединений профиля (ребер, внутренних стенок), are not compensated belt matrix. After etching and anodizing profile of these small local changes in the coal surface may lead to a change in haze, блеске или цвете (из-за различий в отражательной способности).

poloschatost-ribFigure 1

Preventing defect

  • Correcting belts matrix remains the primary measure to eliminate these defects. The corresponding local acceleration or deceleration perimeter girdle matrix can minimize the occurrence of these defects.

Traces of longitudinal welds

English term

Longitudinal Weld Streaking.

defect Source

Pressing

The manifestation of the defect

Travlenye – anodizing.

Details of the defect

Все полые алюминиевые профили и некоторые полузамкнутые профили (в зависимости от конструкции матрицы) в той или иной степени имеют видимые продольные сварные швы, so they are trying to place in the corners of the section.

The longitudinal welds are joining two aluminum flows through adjacent ports and therefore often seen after anodization. These seams are compounded oxides and intermetallics, associated with the rear end of the workpiece by defects, however at the end of the preform longitudinal welds maximally manifest.

Similarly, V-shaped tracks extending transverse welds, until they reach the longitudinal seam. Therefore, poor transverse seam further degrades the appearance of a longitudinal seam. When the metal flows through the matrix type "porthol" metal flows past the "bridge" and guided in the longitudinal seal, which it is associated with a larger study of metal in this zone, in comparison with other metal. This more intensive deformation leads to a different sensitivity to metal etching and anodization and, Consequently, the formation of a longitudinal defect.

poloschatost-prodolnyy-shovFigure 2

Preventing defect

  • Reducing the pressing speed.
  • Creating a good weld requires, to a pressing tool was clean and well-heated, and the temperature of the preform and pressing were optimal speed.
  • Modifications to the matrix to optimize the location and size of welding chambers.

Source: Barry R. Ellard, Aluminum Extrusion Technology, 2000