Discover the different types of streaking defects in anodised extrusion profiles, including segregational streaking. Learn how to identify and address these issues.

Streaking anodising defects

Streaking is one of the major cause of non-uniformity on anodised extrusion profiles. Streaking is related to non-uniform metallurgical structure on the defect aluminium surface. This non-uniform metallurgical structure results in variations in the etching response of the material. These variations are appeared as a difference in brightness or reflectivity on various areas of the aluminum product [1, 2].

The following types streaking defects of anodised aluminium are distinguished [1, 2]:

• Segregational streaking.
  Non-uniform metallurgical macrostructure due the entrainment of extrusion ingot inverse segregation into extruded profile.
• Shell-zone streaking.
  Non-uniform metallurgical macrostructure due the entrainment of the ingot oxide shell-zone into the extruded profile. The analogue of segregational streaking.
• Back-end streaking.
  Non-uniform metallurgical macrostructure due the entrainment of segregate or oxide from the billet back into the extrudate.
• Die design streaking.
• Die line streaking
• Longitudinal weld streaking
• Transverse weld streaking
• Carbon mark streaking
• Intermetallic particles streaking

Segregational streaking

Description

Segregation on specific areas of the profile often appears as dark streaks (Figures 1 and 2). Segregational streaking is not visible in the extruded condition. It appears after anodizing.

Cause

Segregational streaking on anodised surface is caused by the preferential etching of exposed layers having different metallurgical characteristics due to extrusion ingot segregation.

Remedy

The defect can be avoided by attention to casting practices designed to minimize extrusion ingot inverse segregation [1-3].

Figure 1 – Segregational streaking [4]

Figure 2 – Conventional appearance of the “Segregational streaking” defect [4]

Extrusion ingot inverse segregation

Macrosegregation and microsegregation

Macrosegregation is chemical inhomogeneity that occurs on the scale of the grain. Macrosegregation cannot be eliminated during downstream processing, unlike microsegregation, which can be removed relatively easily by high-temperature anneal (homogenization). Diffusion lengths are measured in micrometers for microsegregation and in centimeters for macrosegregation [5].

Normal macrosegregation

• The advancing solidification front pushes the liquid enriched in the solute toward the hotter part of the casting, e.g., the center.
• As a result, after solidification, the center of the casting will contain more solute than the periphery of the casting.
• The driving force behind such transport is either convection or shrinkage-driven flow.

Inverse macrosegregation

• “Iinverse” macrosegregation is quite typical of billets and ingots from DC-cast aluminum alloys (Figure 2).
• The ultimate form of “inverse” segregation is liquation or exudation at the casting surface, when the solute-rich liquid penetrates through the outer shell of a casting and solidifies at the surface as liquates or eutectic exudates (see Figure 3).
• In this case the solute-rich liquid is transported in the direction opposite to the solidification-front movement.
• The inverse segregation is manifested as negative centerline segregation and positive surface segregation, as illustrated in Figure 4.

Figure 5 shows the microstructure of a 6xxx alloy including inverse macrosegregation zone of 6060 alloy ingot.

Figure 2 – Casting an ingot method Hot-Top ]6]

Figure 3 [6]

Figure 4 – The microstructure of the ingot-pillar made of aluminum alloy 6xxx series [5]

Figure 5 – Inverse segregation zone in the 6060 alloy ingot
cast by Hot-Top method [5]

Formation of a “segregational streaking” defect

In case of violations of extrusion technology, the metal from this segregated layer can fall into the finished aluminum profile (Gigures 6 and 7). Because of its chemical composition is significantly different from the chemical composition of the base metal, it causes all sorts of heterogeneity profile properties, including, heterogeneity anodized surface appearance.  This defect is  called “Segregational streaking on anodised aluminium”.

Figure 6 – Schematic picture showing normal metal flow in container during aluminium extrusion [7]

Figure 7 – Segregation shell is collected in a rear portion of billet.
With too thin extrusion butt (less 15 % billet length)
material inverse segregation zone can enter the profile [6]

Sources:

1. Barry R. Ellard, Aluminum ET Seminar, 2000

2. Tedd

3. Qualanod – Anodizing defect catalogue

4. Merkmale

5. Eskin

6. https://lukeeng.com/wp-content/uploads/2017/04/InfluenceofAlMicrostructure.pdf

7. FE-Analysis of Inward Flow of Surface Materials ay the back End of Billet During Al-Extrusion / J. Hou, B. Bengtsson, U. Shtalberg, H. Lindqvist – Aluminum ET Seminar 2000