defects anodizing 

Anodized aluminum strips

A well-known expert in aluminum anodizing A.W. Brace in his classic book "Defects anodic coating. Their causes and prevention ”[1] leads more 100 different types of defects in the anodized aluminum products - for the compressed profiles, rolled sheets and molded parts.

Defects anodized profiles

In appearance, defects in anodized profiles are usually divided into four main categories [1, 2]:

  • corrosion defects
  • Traces of machining the profile surface (grinding, brushing, etc.. P.), as well as traces of damaged or contaminated belts matrix
  • Banded heterogeneity of the surface structure of the metal profile
  • Inhomogeneity of appearance (spots, cross stripes, etc.)

Striped defective anodizing

Defects anodized profiles, are associated with the structure of the material inhomogeneity, are, apparently, the most common among other types of defects. Below are the five most frequent streaky defects anodized profiles, which reasons are disorders pressing technology, as well as ingot casting technology:

  • stripes, Profile features associated with structure
  • Strips of longitudinal welds
  • Strips of transverse welds at the juncture of two workpieces
  • Bands from a sharp change in the length of working belts matrix
  • The bands from entering the profile of the surface layer of the ingot segregational.

All of these defects occur as a result of, that different metallurgical structure is etched differently, usually, alkaline, during surface preparation by anodizing.

stripes, relating to the construction Profile

These defects appear as bright, wide bands in the pressing direction, usually in places of a sharp change in wall thickness or in the presence of walls or ribs on the opposite side of the profile (figure 1).


Figure 1 - bands, associated with the design features of the profile [2]

Longitudinal welds

Hollow profiles of alloy 6060/6063, as well as other 6xxx series alloys srednelegirovannoj, usually compressed on matrices, which have a special welding chamber. these matrices, for example, The so-called matrix porthol.

 

Figure 2 - Scheme of the longitudinal welds
in a hollow aluminum profile [3]:
a) metal flow through ports and around the mandrel;
b) matrix mirror, through which it enters the workpiece

Compound of adjacent metal chambers flows in welding matrix occurs at high pressure, high shear deformations and at a high temperature. This process - the process of extrusion welding - occurs without liquid phases, ie the compound materials takes place exclusively in the solid state.

Transverse welds

The transverse welding seams are formed at the junction of two consecutive billets. Like the longitudinal seams, transverse seals are the result of an extrusion welding.

Unlike longitudinal welds transverse welds are the result of two separate compounds, oxidized and possibly contaminated surfaces during the transient change of the workpiece.

This transverse seal is formed by contacting the oxidized and, perhaps, contaminated, workpiece surface. The presence of oxides and other contaminants adversely affects the extrusion welding quality over the entire area of ​​contact of the two workpieces.

Strips of working belts matrix

In a direct compression process on the friction surfaces causes the container preform during the metal near the surface of the workpiece to move more slowly. Therefore, the workpiece center moves faster, than portions near its outer surface. To align the flow of metal, the length of the matrix flanges must be inversely proportional to the distance from the center of the workpiece [3].

 

Figure 3 - Abrupt transitions between the operating matrix belts
various lengths [3]

Strips of the segregation of the surface layer of the ingot

This defect occurs, when in the extrusion ingot - blank - intermetallic particles are unevenly distributed and are significantly different in size. Segregation of primary intermetallic particles in an aluminum cast preform typically has inverse segregation type.

Figure 5 - Scheme entrance surface segregation layer
extrusion ingot (pillar) in profile [3]

Sources:

1.Brace A. W. Anodic Coating Defects. Their Cause and Cure – Interall S.r.l., Modena, Italy, 2000.

2. Barry R. Ellard, Aluminum Extrusion Technology Seminar, Chicago, 2004.

3. Saha P. Aluminum Extrusion Technology – ASM International, 2000.