Anodizing defects: stripes from microstructure

Defects of anodized aluminum profiles can have their reasons at any stage of their production:

  • when casting ingots-pillars - pressed ingots,
  • during pressing and
  • at the aluminum anodizing.

(Cm. more about that - anodizing defect classification). Under “aluminum anodizing”, as usual, We understand “anodizing aluminum sulfate”

Unfortunately, completely Russian terminology defects anodised aluminum profiles It is in flux, however for uniqueness for each defect will exert an English term corresponding.

One of the most common defects in the anodized aluminum profiles – banding is due to the low quality of ingot, from which they were pressed, as well as a violation of press technology. "Culprits" of these defects may or presser, manufacturer profiles , or caster, manufacturer of ingots for pressing, or both.

Strips of segregation ingot

English term

Shell-Zone Streaking

defect Source

Ingot (blank) – Pressing

The manifestation of the defect

Anodizing

Details of the defect

Dark or light stripes on a surface of anodized aluminum profile. Arranged parallel to the direction of pressing. They have more or less vague border of varying width, which are not associated with any geometric features of the cross-section of the profile (figure 1.1).

The reason for these bands – grip the outer layer of segregation in the ingot compressible aluminum profile. These bands are due to differences in the content of iron particles have a different sensitivity to etching and anodization.


Figure 1.1 - Banding from the segregation layer of the ingot [B. R. Ellard]

Preventing defect

Strips of heterogeneity of the chemical composition of the material of the segregation layer of the ingot can not be eliminated any processing, for example, polishing or buffing defective aluminum profiles.

This defect can be avoided, if a:

  • use for pressing high-quality ingots with a minimum thickness of the surface segregation (inverse) layer and
  • applied molding technology, which ensures the prevention of ingress of metal this segregation zone compressible profile.

band coarse grain

English term

Сoarse Grain Banding

defect Source

Ingot (blank) – Pressing

The manifestation of the defect

Travlenye – Anodizing

Details of the defect

Bands of grains of various sizes are revealed during etching and anodizing (Figure 1.2). Usually found on aluminum profiles from 6xxx series alloys (for example, AD31), as well as aluminum alloys of type 3003 (AMc).

Differences in grain size may be due to homogenization of aluminum extrusion ingots (billets), the geometry of the cross section of the aluminum profile, the type and structure of the matrix, pressing ratio (drawing), speed and temperature of the pressing.

zernoFigure 1.2 - Stripes of coarse grain (increased) [B. R. Ellard]

Preventing defect

Correct homogenisation of ingots and pressing technology minimize occurrence of defects, related inequigranular.

Strips of mixing alloys

English term

Сontamination with other alloys

defect Source

Pressing

The manifestation of the defect

Travlenye – Anodizing

Details of the defect

Different zones of anodized surfaces of aluminum profiles (usually hollow) show different degrees of gloss or matt. for instance, one half of the profile looks matte (it is made of an aluminum alloy 6063), and the other half is shiny (aluminum alloy 6463). This is an extreme case, and that, which happens more often - this is when the press after the rafting 6063 It is an alloy 6060 or vice versa.

This may be the case, when different aluminum alloys compressed through the same matrix without its intermediate etching. This defect has an appearance very rough lateral weld, which often becomes rough longitudinal. Moreover, if during one shift at multiple presses of aluminum alloys, there is a possibility of contamination by a thin layer of "old" alloy wall container when pressing the "new" alloy.

smeshivanie-splavovFigure 1.3 - Banding due to mixing of alloys [B. R. Ellard]

Preventing defect

at change change alloy matrix and apply a cleaning unit for removing the previous alloy shell, the remainder of the container wall. This defect can not be eliminated in the anodizing step.

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

See. also
Anodizing aluminum profiles: defects 1.4-1.6
Anodizing aluminum profiles: defects 3.1-3.4
Anodizing aluminum profiles: defects 3.5-3.7
Anodizing aluminum profiles: defects 3.8 and 3.9