Anodized aluminium defectsUpdated

Strips-like aluminium anodizing defects

 

Defects of anodized aluminium profiles

Based on their appearance, defects in anodized extruded materials are usually divided into four main categories [1, 2]:

  • Corrosion defects
  • Traces of mechanical processing of the profile surface (grinding, brushing, etc.), as well as traces of damaged or contaminated die stripes
  • Strip heterogeneity of the surface structure of the profile metal
  • Heterogeneity in appearance (spots, transverse stripes, etc.)

Strips-like anodizing defects

Defects in anodized profiles, which are associated with the heterogeneity of the material structure, are apparently the most common among the types of other defects. Below are the five most common banded defects in anodized profiles, the causes of which are violations of the pressing technology, as well as ingot casting technology:

  • Stripes associated with profile design features
  • Stripes from longitudinal welds
  • Stripes from transverse welding seams at the joint of two billets
  • Stripes from a sharp change in the length of the bearings of the die
  • Stripes from getting into the profile of the segregation surface layer of the ingot.

All these defects appear as a result of the fact that different metallurgical structures of the profile are subjected to different etching, usually alkaline, when preparing the surface for anodizing.

Stripes associated with profile design

These defects appear as light, wide stripes in the direction of extruding, usually in places where there is 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 – Strips in places of walls or ribs on the opposite side of the profile [2]

 Longitudinal welds

Hollow profiles made from alloys 6060/6063, as well as other medium-alloyed alloys of the 6xxx series, are usually extruded on dies that have special welding chambers. Such dies, for example, are the so-called porthole dies.

The joining of adjacent metal flows in the welding chambers of the die occurs under conditions of high pressure, large shear deformations and at high temperatures. This process – the extrusion welding process – occurs without the participation of liquid phases, that is, the joining of materials occurs exclusively in the solid state.

Figure 2 –  Schematic of welding chamber (porthole) die hollow extrusion [3].
(a) Cross section showing metal flow into port streams
and around the mandrel.
(b) Billet entrance face of the die set

 

Transverse welds

Transverse welds are formed at the junction of two successive billets. Like longitudinal seams, transverse seams are the result of extrusion welding.

Unlike longitudinal welds, this transverse weld is formed as a result of contact between the oxidized and possibly contaminated surface of the billets. The presence of oxides and other contaminants adversely affects the quality of extrusion welding over the entire contact area of two workpieces.

Strips from the bearings of the die

In the direct extrusion process, friction at the billet-container surface forces the metal flow near the billet surface to move more slowly. Therefore, the center of billet moves faster than areas near its outer surface. To equalize the flow of metal, the length of the die bearings must be inversely proportional to the distance from the center of the billet [3].

Figure 3 – The sharp change of bearing length distribution.
The (1), (2) and (3) variants are more optimal [3]

Stripes from the segregation surface layer of ingot

This defect occurs when intermetallic particles in an extrusion ingot – a billet – are unevenly distributed and differ significantly in size. The segregation of primary intermetallic particles in a cast aluminum billet usually takes the form of reverse segregation.

Figure 4 – Surface segregation layer of extrusion ingot goes into the surface of the extruded 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.